Title of Invention	A PROCESS FOR PREPARING NICKEL BASED NANOCOMPOSITE COATING CONTAINING PARTICLES OF YTTRIA STABILIZED CUBIC XIRCONIA-ALUMINA (NI-YZA) USEFUL FOR TRIBOLOGICAL APPLICATIONS AND NI-YZA NANOCOMPOSITE COATING PREPARED THEREBY
Abstract	The present invention provides a novel nickel based nanocomposite coating containing embedded particles of 5 to 20 wt%, 8mol% yttria stabilized cubic zirconia, 80 to 95 wt% alumina (YZA) and a process for preparing the same. Nanosized YZA powder in the present invention is prepared by solution combustion process. Further, this nanoceramic powder is co-deposited with Ni for preparing nanocomposite coating from a nickel sulfamate bath on metal substrates. In the present invention, YZA particles are embedded in the nickel matrix, which have resulted in the improved tribological properties. Present invention also reveals synergistic combination of Ni-YZA, which gives nanocomposite coating having higher microhardness, lower friction coefficient and higher corrosion resistance when compared to Nickel coating.

Title of Invention

A PROCESS FOR PREPARING NICKEL BASED NANOCOMPOSITE COATING CONTAINING PARTICLES OF YTTRIA STABILIZED CUBIC XIRCONIA-ALUMINA (NI-YZA) USEFUL FOR TRIBOLOGICAL APPLICATIONS AND NI-YZA NANOCOMPOSITE COATING PREPARED THEREBY

Abstract

The present invention provides a novel nickel based nanocomposite coating containing embedded particles of 5 to 20 wt%, 8mol% yttria stabilized cubic zirconia, 80 to 95 wt% alumina (YZA) and a process for preparing the same. Nanosized YZA powder in the present invention is prepared by solution combustion process. Further, this nanoceramic powder is co-deposited with Ni for preparing nanocomposite coating from a nickel sulfamate bath on metal substrates. In the present invention, YZA particles are embedded in the nickel matrix, which have resulted in the improved tribological properties. Present invention also reveals synergistic combination of Ni-YZA, which gives nanocomposite coating having higher microhardness, lower friction coefficient and higher corrosion resistance when compared to Nickel coating.

Full Text	The present invention relates a process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications and Ni-YZA nanocomposite coating prepared thereby. In the present invention, electrodeposited Ni based composite containing particles of yttria stabilized cubic zirconia alumina (YZA) is prepared. YZA particles when incorporated in the Ni matrix not only enhance the micro hardness but also significantly improve the corrosion resistance and wear resistance of Ni-YZA coating. Ni-YZA Nanocomposite coating of the present invention can find application in the area of reciprocating engine systems, diesel and gas turbine engines, which are subjected to high temperature corrosion and wear. In general, wherever engineering components have to combat both friction and corrosion problems, Ni-YZA coating will find usage. Composite coatings constitute a new class of materials, which are mostly used for mechanical and tribological applications. Among these materials, nickel deposits with incorporation of hard ceramic particles such as Silicon Carbide, diamond, AI2O3, MoS2, graphite has been most widely reported. Depending on the type of application, different particles are incorporated in the Ni matrix. For example, wherever, wear resistance is required, SiC particles are incorporated. Similarly for lubrication, MoS2 and graphite particles are incorporated in the Ni matrix. Nanocomposites often exhibit useful physical properties, such as strength, temperature resistance, chemical inertness, and lower gas permeability. Because of these properties they are useful as sensors, catalysts, coating materials and miniaturization of devices. In recent years, research for development of a metal matrix composite coating containing ceramic particles has been growing in importance due to its high hardness, better anti-corrosion and anti-wear properties than that of the metal without any particles. However, these properties depend on the contributions from the distributed and matrix phases of a composite coating. One of the continuing goals of the nanocomposite coatings is the production of coatings with enhanced properties such as higher microhardness, corrosion resistance and wear resistance. The combination of metals and ceramics wherein the strengths of each is used to compensate the weaknesses of the other is well known in the art. The most widely known form of such combination has been ceramic particles reinforced metal, which is comprised of a metal matrix, loaded with solid ceramic particles produced by electrodeposition. The coatings of this nature are being widely used for surface protection of various metal articles, including internal combustion engine cylinders. The most widely used metal matrix is nickel and ceramic particles used are SiC, AI2O3, ZrO2, TiO2, diamond, BC, Si3N4 and BN. Electroplating is an effective method to prepare composite coatings through the co-deposition of metallic particles, or non-metallic particles, even polymer particles with metal, and the properties such as wear-resistance, lubrication, or corrosion resistance can be improved remarkably. Recently, with the progress of nanotechnology, a new type of composite plating technology called nanocomposite plating technology has been developed. This technique has received increasing attention in view of the interesting possibilities that it offers. These composite coatings possess enhanced properties such as wear, corrosion and oxidation resistance, dispersion hardening or self-lubrication relative to pure metal, so that they can protect the metal substrates more effectively against severe environments during operation. Electrodeposition is a low-temperature process to fabricate nanocomposite coatings in a single step without secondary treatment. The nickel matrix prepared by electrodeposition, has uniquely high density, minimum porosity and has been widely studied. Metal matrix composites will find applications as wear resistant coatings, self-lubricating films and thermal barrier coatings. Prior art search was made for nickel based nanocomposites and we did not find any report on the preparation of Ni-YZA nanocomposite coating. The other works, which are related to the field of the present invention, are discussed below. Reference may be made to F. Hou et al [F. Hou, W. Wang, H. Guo, Applied Surface Science 252 (2006) 3812], wherein they have reported improved mechanical property (529 HV) for Ni-ZrO2 nanocomposite coating having well dispersed zirconia particles by electrodeposition. However, the corrosion and wear properties of the composite coating are not reported. Another reference may be made to Li et al [J. Li, Y Sun, X. Sun and J. Qiao, Surface and Coatings Technology 192 (2005) 331], wherein Li et al. have reported the mechanical and corrosion resistance of electrodeposited Ni-TiO2 composite coatings. They found that nanocomposite coatings containing the anatase nanoparticles have lower corrosion rate than those containing the rutile microparticles. Results indicate that the corrosion resistance of the nanocomposite coatings is improved with increasing Wtitania or decreasing grain size of the titania particles. The wear resistance of Ni-TiO2 has not been reported. Reference may also be made to Aruna et al [ST. Aruna, C.N. Bindu, V. Ezhil Selvi, V.K. William Grips and K.S. Rajam, Surface and coatings Technology, 200 (2006) 6871], wherein they have reported the synthesis and improved corrosion resistance, lower initial friction coefficient and higher microhardness for Ni-CeO2 composite coatings when compared to Ni. However, the wear loss was higher for Ni-CeO2 when compared to Ni. Reference may be drawn to Japanese Patent no.57-71812, in which a method of preparing silicon carbide particles having improved dispersibility has been disclosed and the process involves, washing of the silicon carbide particles by an acid solution, removing suspended impurities and the residue acid solution on the silicon carbide particles by a hot alkaline solution. In the hot alkaline solution treatment, ammonia water is under a temperature of 90°C to 100°C. This causes evaporation of ammonia water and a bad odour. Another observation made by us is that the nanosize SiC (procured from Pred materials, USA) contains some organic groups attached to them and these nanoparticles of SiC in a nickel bath undergo lot of frothing and cannot be electrodeposited along with Ni. Reference may also be made to US patent no. 5,34,2502, which discloses a method for preparing SiC particles having excellent dispersibility in which the step of addition of a dispersant can be eliminated to avoid bad odour so produced. However, this process also involves steps like washing the SiC particles with an organic solvent followed by washing with an inorganic acid; grinding the SiC particles and heating the SiC particles in a solution containing nickel at a boiling temperature for predetermined period of time. As referred and described herein above, SiC nanocomposites are prominently featured in the prior art and widely used for various engineering applications. However, the processes used for the preparation of Ni-SiC composites are energy and capital intensive, time consuming since the commercially available SiC has some limitations regarding its purity and thus it involves a purification step before carrying out electrodeposition. Reference may be made to Biamino et al [S.Biamino, P.Fino, M.Pavese, C.Badini, Ceramic International 32 (2006) 509], wherein Biamino et al have reported the synthesis of nanosize 20 vol% of t-zirconia partially stabilized with 3 mol % of yttria dispersed in alumina matrix by solution combustion synthesis using urea. They have not reported on the preparation of Ni matrix containing those particles. Also, the compositions prepared in this patent are different. The hitherto known prior art, which are related to the field of the present invention, have been detailed herein above. Prior art search was made for nickel based nanocomposites; however no report on the preparation of Ni-YZA nanocomposite coating could be found. Hence there is definite need to provide a process for the preparation of Ni based nanocomposites containing embedded particles of yttria stabilized cubic zirconia alumina (YZA), which can obviate the drawbacks of the hitherto known prior art while retaining the tribological properties of nanocomposites. The main object of the present invention is to provide a process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications and Ni-YZA nanocomposite coating prepared thereby, which obviates the drawbacks of earlier discussed prior art. Another object of the present invention is to provide a process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA), wherein YZA powder is prepared by solution combustion process. Still another object of the present invention is to provide an electrodeposition process for the preparation of nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina. Yet another object of the present invention is to incorporate nanosized YZA powder in the Ni matrix for enhancing the microhardness of the Ni matrix. Still yet another object of the present invention is to provide a nickel-based nanocomposite coating having better corrosion and wear resistance compared to Ni. A further object of the present invention is to provide nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina having improved tribological properties, which obviates the drawbacks of earlier discussed prior art. In the present invention there is provided a process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications and Ni-YZA nanocomposite coating prepared thereby, wherein electrodeposition method has been adopted for the preparation of Ni-YZA nanocomposites; as it offers several advantages over other fabrication methods like liquid metal infiltration, powder metallurgy or hot pressing. Like these methods, electrodeposition does not require high temperature or pressure The present invention provides a novel nickel based nanocomposite coating containing embedded particles of 5 to 20 weight % of 8mole % yttria stabilized cubic zirconia, 80 to 95 weight % of alumina (YZA) and a process for preparing the same. Nanosized YZA powder in the present invention is prepared by solution combustion process. Further, this nanoceramic powder is co-deposited with Ni for preparing nanocomposite coating from a nickel sulfamate bath on metal substrates. In the present invention, YZA particles are embedded in the nickel matrix, which have resulted in the improved tribological properties. Present invention also reveals synergistic combination of Ni-YZA, which gives nanocomposite coating having higher microhardness, lower friction coefficient and higher corrosion resistance when compared to Nickel coating. The novelty of the present invention lies in providing nickel based nanocomposite coating having improved tribological properties. Novel features of the present invention have been achieved by the non-obvious inventive steps of selection of 8mol % of yttria stabilized cubic zirconia-alumina (YZA) powder and its incorporation in the nickel matrix by electrodepositing from nickel sulfamate bath. Accordingly, the present invention provides a process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications, which comprises dissolving yttrium oxide in dilute nitric acid to get yttrium nitrate; preparing an aqueous redox mixture containing stoichiometric amounts of said yttrium nitrate, zirconium nitrate, aluminum nitrate and fuel such as urea; heating the said aqueous redox mixture at a temperature of around 400 °C; crushing the resultant foamy mass to obtain YZA powder; characterized in that dispersing of 100 to 150 gm/litre of said YZA powder in nickel sulfamate bath under constant stirring for a duration of 16 hours; subjecting the said bath mixture maintained at a temperature below 60°C, to electrodeposition on a substrate at a current density below 5.4 A/dm2for a duration of 1.5 to 20 hours. In an embodiment of the present invention, the stoichiometric amount of fuel such as urea in aqueous redox mixture corresponds to oxidizer to fuel ratio of 1. In another embodiment of present invention, the aqueous redox mixture is heated using a pre-heated hot plate maintained at a temperature of around 400 °C. In yet another embodiment of the present invention, the bath containing YZA particles is subjected to ball milling for reducing the agglomeration of YZA particles. In still another embodiment of present invention, the nickel sulfamate bath used for dispersing and electrodepositing YZA particles consists of 300 - 450 gm per litre of nickel, 20- 45 gm per litre of boric acid, 3 - 30gm per litre of nickel chloride and 0.1-0.5 gm per litre of sodium lauryl sulfate. In still yet another embodiment of the present invention, the electrodeposition of YZA particles from nickel sulfamate bath is carried out by maintaining the pH of bath in the range of 3.5 to 4.5. In a further embodiment of the present invention, the electrodeposition of YZA particles from nickel sulfamate bath is carried out at a bath temperature below 60°C, preferably between room temperature to 50°C. In a still further embodiment of the present invention, the electrodeposition of YZA particles from nickel sulfamate bath is carried out at a current density below 5.4 A/dm2, preferably in the range of 0.23 A/dm2 to 3.1 A/dm2 for duration of 1.5 to 20 hours. In another embodiment of present invention, the electrodeposition of YZA particles along with nickel is carried out on a substrate, such as brass, mild steel, and nickel sheets. Accordingly, the present invention provides nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications, prepared by the process as described herein above, which comprises: nickel matrix containing 4 to 8 volume % of YZA powder, said YZA powder containing 5 to 20 weight % of 8mole % yttria stabilized cubic zirconia, 80 to 95 weight % of alumina. In an embodiment of the present invention, the particle size of zirconia and alumina in YZA powder is between 10 to 60 nm and 10 to 40 nm, respectively. In yet another embodiment of the present invention, the Ni-YZA nanocomposite coating is having Ni grains of size between 17 to 40 nm, microhardness in the range of 350 to 600 HK (applied load 50gf), corrosion rate in the range of 3.94x10"3 to 14.9 x 10"3 mm/yr, wear coefficient in the range of 5.04 x 10-8 to 2.77 x 10"7 and friction coefficient in the range of 0.457 to 0.85. Ceramics possess a number of physical and chemical properties that seem to predestine them for excellent performance in service where high temperatures, chemical inertness and high resistance to wear are important. Zirconia has been widely used in various applications like catalysts, catalyst support, high-performance ceramics, thermal barrier coatings and so forth. Alumina is one of the most widely used ceramic materials due to its high elastic modulus, high wear resistance, high-temperature stability and so on. Alumina is relatively inert in the normal atmosphere, non-toxic, hard and cheap to produce and it has a high thermal conductivity. Alumina-based ceramics are effectively used as cutting tools, dies or prosthesis components. The addition of AI2O3 to zirconia has been reported to improve the properties of zirconia, such as suppressing grain growth during sintering and greatly increasing its mechanical properties [N. Mori, M. Yoshikawa, H. Itoh, T. Abe, J. Am. Ceram. Soc., 77 (1994) 2217 & O. V.Y. Sakka, V.V. Skorokhod, J. Am. Ceram. Soc., 86 (2003) 299]. Failure under such service related conditions is often associated with low to moderate fracture toughness exhibited by alumina ceramics. Zirconia particles (ZrO2) are highly refractory materials. Zirconia possesses excellent chemical inertness and corrosion resistance at temperatures well above the melting point of alumina and is expensive compared to alumina. A promising strategy to overcome this is to combine both alumina and zirconia in such a way that the drawbacks of each are overcome. Alumina/zirconia nanocomposite material has attracted increasing interests in recent years due to the further-enhanced performance relative to either single phase AI2O3 or ZrO2 materials. To prevent macro cracking, zirconia is usually stabilized with calcia or magnesia. These compounds go into the solid solution in the zirconia grains and prevent or reduce the transformation from tetragonal to monoclinic by forming a stable cubic zirconia phase. Both magnesia and calcia do not work well in the presence of alumina as they come out of solid solution and react with alumina rather than the zirconia [ Bosomworth et al US Patent 5045511]. In view of these factors, zirconia-alumina composite powder containing cubic zirconia phase stabilized with yttria was selected. If too much yttria is used, the ceramic bodies will have a low thermal shock resistance and if too little yttria is present, large parts will crack during the firing cycle. So in the present invention 8-mol% yttria was used to get fully stabilized cubic zirconia. The novelty of the present invention for the process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (YZA) useful for tribological applications and Ni-YZA nanocomposite coating prepared thereby, wherein the nickel matrix consists of 4 to 8 volume % of YZA powder, said YZA powder containing 5 to 20 weight % of 8mole % yttria stabilized cubic zirconia, 80 to 95 weight % of alumina, resides in providing nickel based nanocomposite (Ni-YZA) coating having improved tribological properties such as : microhardness in the range of 350 to 600 HK (applied load 50gf), corrosion rate in the range of 3.94x10-3 to 14.9 x 10-3 mm/yr, wear coefficient in the range of 5.04 x 10~8to2.77 x 10"7 and friction coefficient in the range of 0.457 to 0.85. The novelty of the present invention in providing nickel based nanocomposite coating having improved tribological properties has been realized by the non-obvious inventive steps of: 1. Dispersing yttria stabilized cubic zirconia alumina (YZA powder) in a nickel sulfamate bath with constant stirring for a duration of 16 hours. 2. Electrodepositing dispersed YZA particles along with nickel from nickel sulfamate bath for duration of 1.5 to 20 hours, by maintaining the bath mixture at a temperature below 60°C, pH of bath in the range 3.5 to 4.5 and at a current density below 5.4 A/dm2. The process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications and Ni-YZA nanocomposite coating prepared thereby, involves the following main steps and is explained with the subsequent examples: 1. Dissolving yttrium oxide in dilute nitric acid to get yttrium nitrate. 2. Preparing aqueous redox mixture containing stoichiometric amounts of yttrium nitrate, zirconium nitrate, aluminum nitrate and fuel such as urea, wherein the stoichiometric amount of fuel such as urea in aqueous redox mixture corresponds to oxidizer to fuel ratio of 1. 3. Heating the aqueous redox mixture on a pre-heated hot plate (~400 °C). 4. Crushing the resulting foamy mass in a pestle and mortar to get YZA powder. 5. Preparing nickel sulfamate bath containing 300 - 450 gm/litre of nickel, 3 - 30 gm/litre of nickel chloride, 20 - 45 gm/litre boric acid, 0.1- 0.5 gm/litre of sodium lauryl sulfate. 6. Dispersing 100 to 150 gm of YZA powder per litre of said nickel sulfamate bath and stirring over night. 7. Electrodepositing dispersed YZA particles along with nickel from nickel sulfamate bath maintained at a temperature below 60°C, preferably between room temperature to 50°C, at a pH in the range 3.5 to 4.5 and at a current density below 5.4 A/dm2, preferably in the range of 0.23 A/dm2 to 3.1 A/dm2, for a duration of 1.5 to 20 hours. The following examples are given by way of illustration of the present invention in actual practice and therefore should not be construed to limit the scope of the present invention in any way. Example 1 In a typical example, YZA powder containing 90 wt% AI2O3,10 wt% (8 mol% Y2O3) ZrO2 was prepared as follows: 0.24425 gm of Y2O3 was dissolved in dilute nitric acid to which 50 gm of aluminum nitrate (Al (NO3) 3.9H2O), 4.21935 gm of zirconium nitrate, 22.6446 gm of urea and distilled water (150 ml) was added to get a clear solution. This aqueous redox mixture was then introduced on a preheated hotplate at a temperature of 400°C. The solution boils foams and catches fire to give white foamy mass of about 10 gm, which is X-ray crystalline. The resultant foamy mass was crushed with the help of a pestle and mortar. Fig. 1 is a transmission electron micrograph of this YZA powder and it is clearly seen in the micrograph that the particles are almost spherical and are of nanosize. The XRD pattern of YZA powder showed peaks corresponding to a-alumina and cubic zirconia. The crystallite sizes calculated from X-ray line broadening using Scherrer formula for alumina and zirconia particles are 27 nm and 23 nm in size respectively. Nickel bath was prepared by mixing 300 gm/litre of nickel, 10 gm/litre of nickel chloride, 30 gm/litre boric acid, 0.2 gm/litre of sodium lauryl sulfate. 100 gm of YZA powder was added to 1 L of nickel bath and stirred over night and electrodeposition of YZA particles along with nickel was carried out on brass substrate. It can be carried out on other substrates like mild steel and nickel sheets. The pH of nickel sulfamate bath was maintained at 4 before carrying out the electrodeposition at room temperature (35 °C). The electrodeposition was carried out for 3 hrs at a current density of 1.55 A/dm2. The coating thickness was about 60 μm on brass substrate. The optical micrograph of the resultant Ni-YZA composite coating is illustrated in Fig. 2. The optical micrograph shows that the particles are having irregular shapes. Even though the YZA particles were of nanosize, they agglomerated during electrodeposition and are seen as thread like particles The microhardness was 550 HK (50gf load) for this nanocomposite coating. For wear testing, semicircular brass pins of radius 6 mm was coated with Ni and Ni-YZA at a current density of 1.55 A/dm2 for 3 hours. The coating thickness was 60 urn. The wear tests were conducted on a pin-on-disc tribometer (DUCOM, India) under ambient conditions of temperature and humidity (30°C, 50%RH) using a load of 9.8N. All tests were conducted at a wear track radius of 30 mm at 200 rpm at a sliding speed of 0.628 m/s and a constant distance of 4525 m. The disc used was hardened EN 31 steel with a Vickers hardness of 750 HV. Since the experimental conditions used by different authors are different, a comparison between the reported and the present results cannot be made. So in order to compare, Ni matrix without particles was chosen and it was electrodeposited under similar conditions and all the tests were performed under similar experimental conditions for both Ni and Ni-YZA. Table-1 and Table-2 illustrates the wear results of Ni and Ni-YZA. TABLE 1 : Wear properties of Ni and Ni-YZA composite coatings (Table Removed) From Table -1, it is apparent that Ni-YZA exhibits a lower wear loss and lower coefficient of friction when compared to Ni. The lower coefficient of friction for Ni-YZA is attributed to higher micro hardness of the composite, which reduces the true contact. Also YZA particles may have features of easy slide, thus resulting in the decrease of frictional force during sliding. TABLE-2: Wear track width and wear volume of Ni and Ni-YZA composite coatings (Table Removed) From the above Table-2, it is clear that the wear volume is significantly reduced in case of Ni-YZA. The wear rate of Ni and Ni-YZA were 12.19x10"11 m'1 and 18.686x1012 m"1. The wear resistance of Ni-YZA is 6.5 times that of Ni. Ni-YZA exhibited wear coefficient value of 0.8456x10"7 as against 5.516x10"7 of Ni. The surface roughness of the disk was 0.09 urn before wear test. After the wear, the wear track on the disk for Ni shows a surface roughness of 0.22 urn and Ni-YZA shows a surface roughness value of 0.14 urn (Fig. 3). This also confirms the very low values of wear coefficient for Ni-YZA confirming that the composite coating is undergoing burnishing type wear resulting in polishing. Thus Ni-YZA exhibits better anti wear performance compared to Ni when tested under similar conditions. For evaluating the corrosion properties, electrochemical experiments were performed using an Autolab PGStat 30 (Ecochemie, Netherlands) system. A platinum counter electrode and an Ag/AgCI (207 versus SHE) reference electrode were also used. Three-electrode electrochemical cell was used with the platinum counter electrode of 1 cm2 area and Ag/AgCI as reference electrode. The substrate (14.8 mm diameter and 6 mm thickness) used was mild steel (MS) consisting of 0.37 wt% C, 0.28 wt% Si, 0.66 wt% Mn and 98.69 wt% Fe. The substrate was mechanically polished with SiC paper of decreased grit size. The nickel sulfamate bath with and without YZA particles were used and electrodeposition was carried out at 1.55 A/dm2 for 36 min. Total thickness of the coating was about 12μm. The electrodeposited samples were ultrasonically cleaned and degreased prior to electrochemical testing. The sample was loaded in a PVDF sample holder and the surface area exposed to the corrosive medium was 0.785 cm2. All the electrochemical tests were performed under free air condition in 3.5 % NaCI (pH 5.6) solution at room temperature. After exposure, corroded samples were examined by means of SEM of the surface. A qualitative judgment concerning the probable corrosion was made using EDAX analysis in the vicinity of revealed pinholes in the coating, on particles and the surface. The corrosion potential, corrosion rates and Tafel slopes calculated from potentiodynamic diagrams of Ni-YZA and pure Ni coatings and the Rct (Charge transfer resistance) values calculated from Nyquist plots are shown in Table-3. TABLE- 3: Corrosion potential, corrosion rates calculated from potentiodynamic diagrams for pure Ni and Ni-YZA coatings and the electrochemical impedance analysis data (Table Removed) From Table - 3, it is pointed out that the corrosion potential of Ni-YZA coated on mild steel is nobler than Ni (-0.261 vs. -0.304 mV (SCE)). Ni-YZA exhibited lower corrosion current density and higher polarization resistance compared to Ni. In Fig.4 the Tafel plots of Ni and Ni-YZA are shown. From the potentiodynamic polarization curves it is clear that Ni-YZA is nobler than Ni. This shows that Ni-YZA is more corrosion resistant than Ni. A higher Rct (charge transfer resistance) value was observed for Ni-YZA composite. This confirms better corrosion resistant nature of Ni-YZA. The SEM of the corroded Ni and Ni-YZA coatings are shown in Fig,5 and Fig. 6. From the scanning electron micrographs it is clear that Ni has undergone corrosion to a large extent showing pits and the EDX results for Ni (Table-4) also show the presence of Fe content (~ 3.5 wt%) which is significantly higher than that of Ni-YZA (0.6 wt%). The Ni-YZA has undergone uniform corrosion and there were no visible pits in the microstructure (Fig. 6) after corrosion. The EDX results for Ni-YZA also substantiate (Table -4) this observation. Table-4: Energy Dispersive X-ray analysis data of Ni coating (Fig. 5) and Ni-YZA coating (Fig. 6) after potentiodynamic polarization Sample (Table Removed) Example - 2 In another typical example, YZA powder containing 90 wt% AI2O3 10 wt% (8 mol% Y2O3) Zr02 was prepared as explained in example 1. A nickel bath was loaded with 150 g/L of YZA particles and electrodeposition was carried out. Ni-YZA coating prepared from 150-gm/litre nickel bath at 1.55 A/dm2 exhibited a microhardness value of 545 HK, an average coefficient of friction of 0.411 and thus proving improved wear behaviour of Ni-YZA compared to Ni. The other properties exhibited by this coating are icorr of 0.2457 μA/cm2 and Ecorr value of -0.256 V thus proving improved corrosion resistance of Ni-YZA compared to Ni. Example - 3 In another example, the applied current density and deposition time was varied during electrodeposition for maintaining a constant thickness of the coating. Composition of YZA powder and all the parameters were kept constant as mentioned in example 1. The current densities and the duration used for deposition were as follows: 0.23 A/dm2 for 20h, 0.77 A/dm2 for 6h 1.55 A/dm2 for 3h, and 3.1 A/dm2 for 1 5h. The microhardness values varied with the current density. It was found that the composite coating prepared at 0.77 A/dm2 current density exhibited a higher microhardness value (Table -5). Table-5: Effect of current density on microhardness of Ni-YZA coating (Table Removed) Example 4 In another example, 100 grams of YZA powder as prepared in example 1 was added to bath and bath solution was subjected for ball milling for 2 days using zirconia balls as the grinding media. Electrodepositions of YZA particles were carried out at various current densities: 0.23 A/dm2 for 20h, 0.77 A/dm2 for 6h 1.55 A/dm2 for 3h, and 3.1 A/dm2 for 1.5h. The ball-milled solution gave a Ni-YZA coating with higher microhardness of 600 HK when 0.23 A/dm2 current density was used for electrodeposition. Also, there was a reduction in the agglomerated particle size of the YZA particles as seen by the optical micrograph as shown in figure-7 of the drawings accompanying this specification. The effect of the current density and ball milling on coating microhardness is shown in Table -6 below. Table-6: Effect of ball milling and current density on microhardness of Ni-YZA coating (Table Removed) Example - 5 In another example, YZA powder containing 80 wt% AI2O3, 20 wt% (8 mol% Y2O3) ZrO2 was prepared as follows: 0.88 gm of Y2O3 was dissolved in dilute nitric acid to which 50 gm of aluminum nitrate (Al (NO3) 3 .9H2O), 10.15 gm of zirconium nitrate, 26.56 gm of urea and distilled water (150 ml) were added to get a clear solution. This aqueous redox mixture was then introduced on a preheated hotplate at a temperature of 400°C. The solution boils, foams and catches fire to give white foamy mass, which is X-ray crystalline. The resultant foamy mass was crushed with the help of a pestle and mortar. The XRD pattern of YZA powder showed peaks corresponding to a-alumina and cubic zirconia. The crystallite sizes calculated from X-ray line broadening using Scherrer formula for alumina and zirconia particles are 11.90 nm and 10.91 nm in size respectively. Nickel bath was prepared and electrodeposition was conducted as in example-1. The microhardness was 415 HK (50gf load) for this nanocomposite coating. Electrodeposition of nickel along with YZA particles was carried out by keeping the other parameters constant as mentioned in Examplel. Estimated values of properties of Ni-YZA coating are given in Table 7. TABLE-7: Tribological properties of coating having composition containing 80 wt% AI2O3,20 wt% (8 mol% Y2O3) ZrO2 (Table Removed) Example - 6 In another example, YZA powder containing 95 wt% AI2O3, 5 wt% (8 mol% Y2O3) ZrO2 was prepared as follows: 0.116315 gm of Y2O3 was dissolved in dilute nitric acid to which 50 gm of aluminium nitrate (Al (NO3) 3 .9H2O), 2.01583 gm of zirconium nitrate, 21.25 gm of urea and distilled water (150 ml) was added to get a clear solution. The combustion reaction was similar to example-1. The XRD pattern of YZA powder showed peaks corresponding to a-alumina and cubic zirconia. The crystallite sizes calculated from X-ray line broadening using Scherrer formula for alumina and zirconia particles are 11.58 nm and 10.54 nm in size respectively. Electrodeposition of nickel along with YZA particles was carried out by keeping the other parameters constant as mentioned in Example! Estimated values of properties of Ni-YZA coating aregiven in Table 8. TABLE-8: Tribological properties of coating having composition containing 95 wt% AI2O3,5 wt% (8 mol% Y2O3) ZrO2 (Table Removed) In the present invention 5 to 20 wt% of 8 mol% yttria stabilized cubic zirconia containing 80 to 950 wt% alumina (YZA) was prepared by solution combustion process using urea as fuel. Urea is used as a fuel for preparation of YZA powder as it is cheaper, readily available and easy to handle. Cost of YZA powder prepared by solution combustion process is much lower when compared to other processes. Additionally, solution combustion process is simple and rapid for the preparation of nanosized YZA. The prepared YZA powder is crystalline in nature does not require any pretreatment or calcination steps and can be used directly in the bath. However, in case of Ni-SiC the purity of particles, purification of the particles and pretreatment of the particles influence the co-deposition of SiC particles. Further, the electrodeposition does not require high temperature or pressure. The main advantages of the present invention are as follows: 1. YZA powder is prepared by very simple and fast solution combustion process using metal nitrates and a cheaper fuel like urea. 2. One can prepare instant powders at the work spot itself. 3. The cost of one Kg of YZA powder prepared by solution combustion process is less when compared to powders prepared from other techniques. 4. YZA powder does not need all the tedious pretreatment procedures involved for SiC to get dispersion. 5. Electrodeposition does not require high temperature or pressure. 6. Ni-YZA exhibits a synergetic combination of higher microhardness; higher wear resistance and higher corrosion resistance than Ni alone. We claim: 1. A process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications, which comprises dissolving yttrium oxide in dilute nitric acid to get yttrium nitrate; preparing an aqueous redox mixture containing stoichiometric amounts of said yttrium nitrate, zirconium nitrate, aluminum nitrate and fuel such as urea; heating the said aqueous redox mixture at a temperature of around 400 °C; crushing the resultant foamy mass to obtain YZA powder; characterized in that dispersing of 100 to 150 gm/litre of said YZA powder in nickel sulfamate bath under constant stirring for a period of 16 hours; subjecting the said bath mixture maintained at a temperature below 60°C, to electrodeposition on a substrate at a current density below 5.4 A/dm2 for a duration of 1.5 to 20 hours. 2. A process as claimed in claim 1, wherein the stoichiometric amount of fuel such as urea in aqueous redox mixture corresponds to oxidizer to fuel ratio of 1. 3. A process as claimed in claim 1-2, wherein the aqueous redox mixture is heated using a pre-heated hot plate maintained at a temperature of around 400 °C. 4. A process as claimed in claim 1-3, wherein the bath containing YZA particles is subjected to ball milling for reducing the agglomeration of YZA particles. 5. A process as claimed in claim 1-4, wherein the nickel sulfamate bath used for dispersing and electrodepositing YZA particles consists of 300-450 gm per litre of nickel, 20-45 gm per litre of boric acid, 3-30gm per litre of nickel chloride and 0.1-0.5 gm per litre of sodium lauryl sulfate. 6. A process as claimed in claim 1-5, wherein the electrodeposition of YZA particles from nickel sulfamate bath is carried out by maintaining the pH of bath in the range of 3.5 to 4.5. 7. A process as claimed in claim 1 -6, wherein the electrodeposition of YZA particles from nickel sulfamate bath is carried out at a bath temperature below 60°C , preferably between room temperature to 50°C. 8. A process as claimed in claim 1-7, wherein the electrodeposition of YZA particles from nickel sulfamate bath is carried out at a current density below 5.4 A/dm2, preferably in the range of 0.23 A/dm2 to 3.1 A/dm2 for a duration of 1.5 to 20 hours. 9. A process as claimed in claim 1-8, wherein the electrodeposition of YZA particles along with nickel is carried out on a substrate, such as brass, mild steel, nickel sheets. 10. Nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications, prepared by the process as claimed in claims 1-8, which comprises: nickel matrix containing 4 to 8 volume % of YZA powder, said YZA powder containing 5 to 20 weight % of 8mole % yttria stabilized cubic zirconia, 80 to 95 weight % of alumina. 11. Nickel based nanocomposite coating as claimed in claim 10, wherein the particle size of zirconia and alumina in YZA powder is between 10 to 60 nm and 10 to 40 nm, respectively. 12. Nickel based nanocomposite coating as claimed in claim 10-11, wherein the Ni-YZA nanocomposite coating is having Ni grains of size between 17 to 40 nm, micro hardness in the range of 350 to 600 HK (applied load 50gf), corrosion rate in the range of 3.94x10-3 to 14.9 x 10-3 mm/yr, wear coefficient in the range of 5.04 x 10-8 to 2.77 x 10-7 and friction coefficient in the range of 0.457 to 0.85.

Full Text

The present invention relates a process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications and Ni-YZA nanocomposite coating prepared thereby. In the present invention, electrodeposited Ni based composite containing particles of yttria stabilized cubic zirconia alumina (YZA) is prepared. YZA particles when incorporated in the Ni matrix not only enhance the micro hardness but also significantly improve the corrosion resistance and wear resistance of Ni-YZA coating.
Ni-YZA Nanocomposite coating of the present invention can find application in the area of reciprocating engine systems, diesel and gas turbine engines, which are subjected to high temperature corrosion and wear. In general, wherever engineering components have to combat both friction and corrosion problems, Ni-YZA coating will find usage.
Composite coatings constitute a new class of materials, which are mostly used for mechanical and tribological applications. Among these materials, nickel deposits with incorporation of hard ceramic particles such as Silicon Carbide, diamond, AI2O3, MoS2, graphite has been most widely reported. Depending on the type of application, different particles are incorporated in the Ni matrix. For example, wherever, wear resistance is required, SiC particles are incorporated. Similarly for lubrication, MoS2 and graphite particles are incorporated in the Ni matrix.
Nanocomposites often exhibit useful physical properties, such as strength, temperature resistance, chemical inertness, and lower gas permeability. Because of these properties they are useful as sensors, catalysts, coating materials and miniaturization of devices. In recent years, research for development of a metal matrix composite coating containing ceramic particles has been growing in importance due to its high hardness, better anti-corrosion and anti-wear properties than that of the metal without any particles. However, these properties depend on the contributions from the distributed and matrix phases of a composite coating.
One of the continuing goals of the nanocomposite coatings is the production of coatings with enhanced properties such as higher microhardness, corrosion resistance and wear resistance. The combination of metals and ceramics wherein the strengths of each is used to compensate the weaknesses of the other is well known in the art. The most widely known form of such combination has been ceramic particles reinforced metal, which is comprised of a metal matrix, loaded with solid ceramic particles produced by electrodeposition. The coatings of this nature are being widely used for surface protection of various metal articles, including internal combustion engine cylinders. The most widely used metal matrix is nickel and ceramic particles used are SiC, AI2O3, ZrO2, TiO2, diamond, BC, Si3N4 and BN.
Electroplating is an effective method to prepare composite coatings through the co-deposition of metallic particles, or non-metallic particles, even polymer particles with metal, and the properties such as wear-resistance, lubrication, or corrosion resistance can be improved remarkably. Recently, with the progress of nanotechnology, a new type of composite plating technology called nanocomposite plating technology has been developed. This technique has received increasing attention in view of the interesting possibilities that it offers. These composite coatings possess enhanced properties such as wear, corrosion and oxidation resistance, dispersion hardening or self-lubrication relative to pure metal, so that they can protect the metal substrates more effectively against severe environments during operation. Electrodeposition is a low-temperature process to fabricate nanocomposite coatings in a single step without secondary treatment. The nickel matrix prepared by electrodeposition, has uniquely high density, minimum porosity and has been widely studied. Metal matrix composites will find applications as wear resistant coatings, self-lubricating films and thermal barrier coatings.
Prior art search was made for nickel based nanocomposites and we did not find any report on the preparation of Ni-YZA nanocomposite coating. The other works, which are related to the field of the present invention, are discussed below.
Reference may be made to F. Hou et al [F. Hou, W. Wang, H. Guo, Applied Surface Science 252 (2006) 3812], wherein they have reported improved mechanical property (529 HV) for Ni-ZrO2 nanocomposite coating having well dispersed zirconia particles by electrodeposition. However, the corrosion and wear properties of the composite coating are not reported.
Another reference may be made to Li et al [J. Li, Y Sun, X. Sun and J. Qiao, Surface and Coatings Technology 192 (2005) 331], wherein Li et al. have reported the mechanical and corrosion resistance of electrodeposited Ni-TiO2 composite coatings. They found that nanocomposite coatings containing the anatase nanoparticles have lower corrosion rate than those containing the rutile microparticles. Results indicate that the corrosion resistance of the nanocomposite coatings is improved with increasing Wtitania or decreasing grain size of the titania particles. The wear resistance of Ni-TiO2 has not been reported.
Reference may also be made to Aruna et al [ST. Aruna, C.N. Bindu, V. Ezhil Selvi, V.K. William Grips and K.S. Rajam, Surface and coatings Technology, 200 (2006) 6871], wherein they have reported the synthesis and improved corrosion resistance, lower initial friction coefficient and higher microhardness for Ni-CeO2 composite coatings when compared to Ni. However, the wear loss was higher for Ni-CeO2 when compared to Ni.
Reference may be drawn to Japanese Patent no.57-71812, in which a method of preparing silicon carbide particles having improved dispersibility has been disclosed and the process involves, washing of the silicon carbide particles by an acid solution, removing suspended impurities and the residue acid solution on the silicon carbide particles by a hot alkaline solution. In the hot alkaline solution treatment, ammonia water is under a temperature of 90°C to 100°C. This causes evaporation of ammonia water and a bad odour. Another observation made by us is that the nanosize SiC (procured from Pred materials, USA) contains some organic groups
attached to them and these nanoparticles of SiC in a nickel bath undergo lot of frothing and cannot be electrodeposited along with Ni.
Reference may also be made to US patent no. 5,34,2502, which discloses a method for preparing SiC particles having excellent dispersibility in which the step of addition of a dispersant can be eliminated to avoid bad odour so produced. However, this process also involves steps like washing the SiC particles with an organic solvent followed by washing with an inorganic acid; grinding the SiC particles and heating the SiC particles in a solution containing nickel at a boiling temperature for predetermined period of time.
As referred and described herein above, SiC nanocomposites are prominently featured in the prior art and widely used for various engineering applications. However, the processes used for the preparation of Ni-SiC composites are energy and capital intensive, time consuming since the commercially available SiC has some limitations regarding its purity and thus it involves a purification step before carrying out electrodeposition.
Reference may be made to Biamino et al [S.Biamino, P.Fino, M.Pavese, C.Badini, Ceramic International 32 (2006) 509], wherein Biamino et al have reported the synthesis of nanosize 20 vol% of t-zirconia partially stabilized with 3 mol % of yttria dispersed in alumina matrix by solution combustion synthesis using urea. They have not reported on the preparation of Ni matrix containing those particles. Also, the compositions prepared in this patent are different.
The hitherto known prior art, which are related to the field of the present invention, have been detailed herein above. Prior art search was made for nickel based nanocomposites; however no report on the preparation of Ni-YZA nanocomposite coating could be found. Hence there is definite need to provide a process for the preparation of Ni based nanocomposites containing embedded particles of yttria
stabilized cubic zirconia alumina (YZA), which can obviate the drawbacks of the hitherto known prior art while retaining the tribological properties of nanocomposites.
The main object of the present invention is to provide a process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications and Ni-YZA nanocomposite coating prepared thereby, which obviates the drawbacks of earlier discussed prior art.
Another object of the present invention is to provide a process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA), wherein YZA powder is prepared by solution combustion process.
Still another object of the present invention is to provide an electrodeposition process for the preparation of nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina.
Yet another object of the present invention is to incorporate nanosized YZA powder in the Ni matrix for enhancing the microhardness of the Ni matrix.
Still yet another object of the present invention is to provide a nickel-based nanocomposite coating having better corrosion and wear resistance compared to Ni.
A further object of the present invention is to provide nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina having improved tribological properties, which obviates the drawbacks of earlier discussed prior art.
In the present invention there is provided a process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications and Ni-YZA nanocomposite coating
prepared thereby, wherein electrodeposition method has been adopted for the preparation of Ni-YZA nanocomposites; as it offers several advantages over other fabrication methods like liquid metal infiltration, powder metallurgy or hot pressing. Like these methods, electrodeposition does not require high temperature or pressure
The present invention provides a novel nickel based nanocomposite coating containing embedded particles of 5 to 20 weight % of 8mole % yttria stabilized cubic zirconia, 80 to 95 weight % of alumina (YZA) and a process for preparing the same. Nanosized YZA powder in the present invention is prepared by solution combustion process. Further, this nanoceramic powder is co-deposited with Ni for preparing nanocomposite coating from a nickel sulfamate bath on metal substrates. In the present invention, YZA particles are embedded in the nickel matrix, which have resulted in the improved tribological properties. Present invention also reveals synergistic combination of Ni-YZA, which gives nanocomposite coating having higher microhardness, lower friction coefficient and higher corrosion resistance when compared to Nickel coating.
The novelty of the present invention lies in providing nickel based nanocomposite coating having improved tribological properties. Novel features of the present invention have been achieved by the non-obvious inventive steps of selection of 8mol % of yttria stabilized cubic zirconia-alumina (YZA) powder and its incorporation in the nickel matrix by electrodepositing from nickel sulfamate bath.
Accordingly, the present invention provides a process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications, which comprises dissolving yttrium oxide in dilute nitric acid to get yttrium nitrate; preparing an aqueous redox mixture containing stoichiometric amounts of said yttrium nitrate, zirconium nitrate, aluminum nitrate and fuel such as urea; heating the said aqueous redox mixture at a temperature of around 400 °C; crushing the resultant foamy mass to obtain YZA
powder; characterized in that dispersing of 100 to 150 gm/litre of said YZA powder in nickel sulfamate bath under constant stirring for a duration of 16 hours; subjecting the said bath mixture maintained at a temperature below 60°C, to electrodeposition on a substrate at a current density below 5.4 A/dm2for a duration of 1.5 to 20 hours.
In an embodiment of the present invention, the stoichiometric amount of fuel such as urea in aqueous redox mixture corresponds to oxidizer to fuel ratio of 1.
In another embodiment of present invention, the aqueous redox mixture is heated using a pre-heated hot plate maintained at a temperature of around 400 °C.
In yet another embodiment of the present invention, the bath containing YZA particles is subjected to ball milling for reducing the agglomeration of YZA particles.
In still another embodiment of present invention, the nickel sulfamate bath used for dispersing and electrodepositing YZA particles consists of 300 - 450 gm per litre of nickel, 20- 45 gm per litre of boric acid, 3 - 30gm per litre of nickel chloride and 0.1-0.5 gm per litre of sodium lauryl sulfate.
In still yet another embodiment of the present invention, the electrodeposition of YZA particles from nickel sulfamate bath is carried out by maintaining the pH of bath in the range of 3.5 to 4.5.
In a further embodiment of the present invention, the electrodeposition of YZA particles from nickel sulfamate bath is carried out at a bath temperature below 60°C, preferably between room temperature to 50°C.
In a still further embodiment of the present invention, the electrodeposition of YZA particles from nickel sulfamate bath is carried out at a current density below 5.4 A/dm2, preferably in the range of 0.23 A/dm2 to 3.1 A/dm2 for duration of 1.5 to 20 hours.
In another embodiment of present invention, the electrodeposition of YZA particles along with nickel is carried out on a substrate, such as brass, mild steel, and nickel sheets.
Accordingly, the present invention provides nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications, prepared by the process as described herein above, which comprises: nickel matrix containing 4 to 8 volume % of YZA powder, said YZA powder containing 5 to 20 weight % of 8mole % yttria stabilized cubic zirconia, 80 to 95 weight % of alumina.
In an embodiment of the present invention, the particle size of zirconia and alumina in YZA powder is between 10 to 60 nm and 10 to 40 nm, respectively.
In yet another embodiment of the present invention, the Ni-YZA nanocomposite coating is having Ni grains of size between 17 to 40 nm, microhardness in the range of 350 to 600 HK (applied load 50gf), corrosion rate in the range of 3.94x10"3 to 14.9 x 10"3 mm/yr, wear coefficient in the range of 5.04 x 10-8 to 2.77 x 10"7 and friction coefficient in the range of 0.457 to 0.85.
Ceramics possess a number of physical and chemical properties that seem to predestine them for excellent performance in service where high temperatures, chemical inertness and high resistance to wear are important. Zirconia has been widely used in various applications like catalysts, catalyst support, high-performance ceramics, thermal barrier coatings and so forth. Alumina is one of the most widely used ceramic materials due to its high elastic modulus, high wear resistance, high-temperature stability and so on. Alumina is relatively inert in the normal atmosphere, non-toxic, hard and cheap to produce and it has a high thermal conductivity. Alumina-based ceramics are effectively used as cutting tools, dies or prosthesis components. The addition of AI2O3 to zirconia has been reported to improve the properties of zirconia, such as suppressing grain growth during sintering and greatly
increasing its mechanical properties [N. Mori, M. Yoshikawa, H. Itoh, T. Abe, J. Am. Ceram. Soc., 77 (1994) 2217 & O. V.Y. Sakka, V.V. Skorokhod, J. Am. Ceram. Soc., 86 (2003) 299]. Failure under such service related conditions is often associated with low to moderate fracture toughness exhibited by alumina ceramics. Zirconia particles (ZrO2) are highly refractory materials. Zirconia possesses excellent chemical inertness and corrosion resistance at temperatures well above the melting point of alumina and is expensive compared to alumina. A promising strategy to overcome this is to combine both alumina and zirconia in such a way that the drawbacks of each are overcome. Alumina/zirconia nanocomposite material has attracted increasing interests in recent years due to the further-enhanced performance relative to either single phase AI2O3 or ZrO2 materials. To prevent macro cracking, zirconia is usually stabilized with calcia or magnesia. These compounds go into the solid solution in the zirconia grains and prevent or reduce the transformation from tetragonal to monoclinic by forming a stable cubic zirconia phase. Both magnesia and calcia do not work well in the presence of alumina as they come out of solid solution and react with alumina rather than the zirconia [ Bosomworth et al US Patent 5045511]. In view of these factors, zirconia-alumina composite powder containing cubic zirconia phase stabilized with yttria was selected. If too much yttria is used, the ceramic bodies will have a low thermal shock resistance and if too little yttria is present, large parts will crack during the firing cycle. So in the present invention 8-mol% yttria was used to get fully stabilized cubic zirconia.
The novelty of the present invention for the process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (YZA) useful for tribological applications and Ni-YZA nanocomposite coating prepared thereby, wherein the nickel matrix consists of 4 to 8 volume % of YZA powder, said YZA powder containing 5 to 20 weight % of 8mole % yttria stabilized cubic zirconia, 80 to 95 weight % of alumina, resides in providing nickel based nanocomposite (Ni-YZA) coating having improved tribological properties such as : microhardness in the range of 350 to 600 HK (applied load 50gf), corrosion rate in
the range of 3.94x10-3 to 14.9 x 10-3 mm/yr, wear coefficient in the range of 5.04 x 10~8to2.77 x 10"7 and friction coefficient in the range of 0.457 to 0.85.
The novelty of the present invention in providing nickel based nanocomposite coating having improved tribological properties has been realized by the non-obvious inventive steps of:
1. Dispersing yttria stabilized cubic zirconia alumina (YZA powder) in a nickel
sulfamate bath with constant stirring for a duration of 16 hours.
2. Electrodepositing dispersed YZA particles along with nickel from nickel
sulfamate bath for duration of 1.5 to 20 hours, by maintaining the bath
mixture at a temperature below 60°C, pH of bath in the range 3.5 to 4.5 and
at a current density below 5.4 A/dm2.
The process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications and Ni-YZA nanocomposite coating prepared thereby, involves the following main steps and is explained with the subsequent examples:
1. Dissolving yttrium oxide in dilute nitric acid to get yttrium nitrate.
2. Preparing aqueous redox mixture containing stoichiometric amounts of
yttrium nitrate, zirconium nitrate, aluminum nitrate and fuel such as urea,
wherein the stoichiometric amount of fuel such as urea in aqueous redox
mixture corresponds to oxidizer to fuel ratio of 1.
3. Heating the aqueous redox mixture on a pre-heated hot plate (~400 °C).
4. Crushing the resulting foamy mass in a pestle and mortar to get YZA
powder.
5. Preparing nickel sulfamate bath containing 300 - 450 gm/litre of nickel, 3 - 30
gm/litre of nickel chloride, 20 - 45 gm/litre boric acid, 0.1- 0.5 gm/litre of
sodium lauryl sulfate.
6. Dispersing 100 to 150 gm of YZA powder per litre of said nickel sulfamate
bath and stirring over night.
7. Electrodepositing dispersed YZA particles along with nickel from nickel sulfamate bath maintained at a temperature below 60°C, preferably between room temperature to 50°C, at a pH in the range 3.5 to 4.5 and at a current density below 5.4 A/dm2, preferably in the range of 0.23 A/dm2 to 3.1 A/dm2, for a duration of 1.5 to 20 hours.
The following examples are given by way of illustration of the present invention in actual practice and therefore should not be construed to limit the scope of the present invention in any way.
Example 1
In a typical example, YZA powder containing 90 wt% AI2O3,10 wt% (8 mol% Y2O3) ZrO2 was prepared as follows:
0.24425 gm of Y2O3 was dissolved in dilute nitric acid to which 50 gm of aluminum nitrate (Al (NO3) 3.9H2O), 4.21935 gm of zirconium nitrate, 22.6446 gm of urea and distilled water (150 ml) was added to get a clear solution. This aqueous redox mixture was then introduced on a preheated hotplate at a temperature of 400°C. The solution boils foams and catches fire to give white foamy mass of about 10 gm, which is X-ray crystalline. The resultant foamy mass was crushed with the help of a pestle and mortar. Fig. 1 is a transmission electron micrograph of this YZA powder and it is clearly seen in the micrograph that the particles are almost spherical and are of nanosize. The XRD pattern of YZA powder showed peaks corresponding to a-alumina and cubic zirconia. The crystallite sizes calculated from X-ray line broadening using Scherrer formula for alumina and zirconia particles are 27 nm and 23 nm in size respectively. Nickel bath was prepared by mixing 300 gm/litre of nickel, 10 gm/litre of nickel chloride, 30 gm/litre boric acid, 0.2 gm/litre of sodium lauryl sulfate. 100 gm of YZA powder was added to 1 L of nickel bath and stirred over night and electrodeposition of YZA particles along with nickel was carried out on brass substrate. It can be carried out on other substrates like mild steel and nickel sheets. The pH of nickel sulfamate bath was maintained at 4 before carrying
out the electrodeposition at room temperature (35 °C). The electrodeposition was carried out for 3 hrs at a current density of 1.55 A/dm2. The coating thickness was about 60 μm on brass substrate. The optical micrograph of the resultant Ni-YZA composite coating is illustrated in Fig. 2. The optical micrograph shows that the particles are having irregular shapes. Even though the YZA particles were of nanosize, they agglomerated during electrodeposition and are seen as thread like particles The microhardness was 550 HK (50gf load) for this nanocomposite coating.
For wear testing, semicircular brass pins of radius 6 mm was coated with Ni and Ni-YZA at a current density of 1.55 A/dm2 for 3 hours. The coating thickness was 60 urn. The wear tests were conducted on a pin-on-disc tribometer (DUCOM, India) under ambient conditions of temperature and humidity (30°C, 50%RH) using a load of 9.8N. All tests were conducted at a wear track radius of 30 mm at 200 rpm at a sliding speed of 0.628 m/s and a constant distance of 4525 m. The disc used was hardened EN 31 steel with a Vickers hardness of 750 HV. Since the experimental conditions used by different authors are different, a comparison between the reported and the present results cannot be made. So in order to compare, Ni matrix without particles was chosen and it was electrodeposited under similar conditions and all the tests were performed under similar experimental conditions for both Ni and Ni-YZA. Table-1 and Table-2 illustrates the wear results of Ni and Ni-YZA.
TABLE 1 : Wear properties of Ni and Ni-YZA composite coatings
(Table Removed) From Table -1, it is apparent that Ni-YZA exhibits a lower wear loss and lower coefficient of friction when compared to Ni. The lower coefficient of friction for Ni-YZA is attributed to higher micro hardness of the composite, which reduces the true contact. Also YZA particles may have features of easy slide, thus resulting in the decrease of frictional force during sliding.
TABLE-2: Wear track width and wear volume of Ni and Ni-YZA composite coatings
(Table Removed)
From the above Table-2, it is clear that the wear volume is significantly reduced in case of Ni-YZA. The wear rate of Ni and Ni-YZA were 12.19x10"11 m'1 and 18.686x1012 m"1. The wear resistance of Ni-YZA is 6.5 times that of Ni. Ni-YZA exhibited wear coefficient value of 0.8456x10"7 as against 5.516x10"7 of Ni. The surface roughness of the disk was 0.09 urn before wear test. After the wear, the wear track on the disk for Ni shows a surface roughness of 0.22 urn and Ni-YZA shows a surface roughness value of 0.14 urn (Fig. 3). This also confirms the very low values of wear coefficient for Ni-YZA confirming that the composite coating is undergoing burnishing type wear resulting in polishing. Thus Ni-YZA exhibits better anti wear performance compared to Ni when tested under similar conditions.
For evaluating the corrosion properties, electrochemical experiments were performed using an Autolab PGStat 30 (Ecochemie, Netherlands) system. A platinum counter electrode and an Ag/AgCI (207 versus SHE) reference electrode were also used. Three-electrode electrochemical cell was used with the platinum counter electrode of 1 cm2 area and Ag/AgCI as reference electrode. The substrate (14.8 mm diameter and 6 mm thickness) used was mild steel (MS) consisting of 0.37 wt% C, 0.28 wt% Si, 0.66 wt% Mn and 98.69 wt% Fe. The substrate was mechanically polished with SiC paper of decreased grit size. The nickel sulfamate
bath with and without YZA particles were used and electrodeposition was carried out at 1.55 A/dm2 for 36 min. Total thickness of the coating was about 12μm. The electrodeposited samples were ultrasonically cleaned and degreased prior to electrochemical testing. The sample was loaded in a PVDF sample holder and the surface area exposed to the corrosive medium was 0.785 cm2. All the electrochemical tests were performed under free air condition in 3.5 % NaCI (pH 5.6) solution at room temperature. After exposure, corroded samples were examined by means of SEM of the surface. A qualitative judgment concerning the probable corrosion was made using EDAX analysis in the vicinity of revealed pinholes in the coating, on particles and the surface. The corrosion potential, corrosion rates and Tafel slopes calculated from potentiodynamic diagrams of Ni-YZA and pure Ni coatings and the Rct (Charge transfer resistance) values calculated from Nyquist plots are shown in Table-3.
TABLE- 3: Corrosion potential, corrosion rates calculated from potentiodynamic diagrams for pure Ni and Ni-YZA coatings and the electrochemical impedance analysis data
(Table Removed)
From Table - 3, it is pointed out that the corrosion potential of Ni-YZA coated on mild steel is nobler than Ni (-0.261 vs. -0.304 mV (SCE)). Ni-YZA exhibited lower corrosion current density and higher polarization resistance compared to Ni. In Fig.4 the Tafel plots of Ni and Ni-YZA are shown. From the potentiodynamic polarization curves it is clear that Ni-YZA is nobler than Ni. This shows that Ni-YZA is more corrosion resistant than Ni.
A higher Rct (charge transfer resistance) value was observed for Ni-YZA composite. This confirms better corrosion resistant nature of Ni-YZA. The SEM of the corroded Ni and Ni-YZA coatings are shown in Fig,5 and Fig. 6. From the scanning electron micrographs it is clear that Ni has undergone corrosion to a large extent showing pits and the EDX results for Ni (Table-4) also show the presence of Fe content (~ 3.5 wt%) which is significantly higher than that of Ni-YZA (0.6 wt%). The Ni-YZA has undergone uniform corrosion and there were no visible pits in the microstructure (Fig. 6) after corrosion. The EDX results for Ni-YZA also substantiate (Table -4) this observation.
Table-4: Energy Dispersive X-ray analysis data of Ni coating (Fig. 5) and Ni-YZA coating (Fig. 6) after potentiodynamic polarization
Sample
(Table Removed)
Example - 2
In another typical example, YZA powder containing 90 wt% AI2O3 10 wt% (8 mol% Y2O3) Zr02 was prepared as explained in example 1. A nickel bath was loaded with 150 g/L of YZA particles and electrodeposition was carried out. Ni-YZA coating prepared from 150-gm/litre nickel bath at 1.55 A/dm2 exhibited a microhardness value of 545 HK, an average coefficient of friction of 0.411 and thus proving improved wear behaviour of Ni-YZA compared to Ni. The other properties exhibited by this coating are icorr of 0.2457 μA/cm2 and Ecorr value of -0.256 V thus proving improved corrosion resistance of Ni-YZA compared to Ni.
Example - 3
In another example, the applied current density and deposition time was varied during electrodeposition for maintaining a constant thickness of the coating. Composition of YZA powder and all the parameters were kept constant as mentioned in example 1. The current densities and the duration used for deposition were as follows: 0.23 A/dm2 for 20h, 0.77 A/dm2 for 6h 1.55 A/dm2 for 3h, and 3.1 A/dm2 for 1 5h. The microhardness values varied with the current density. It was found that the composite coating prepared at 0.77 A/dm2 current density exhibited a higher microhardness value (Table -5).
Table-5: Effect of current density on microhardness of Ni-YZA coating
(Table Removed)
Example 4
In another example, 100 grams of YZA powder as prepared in example 1 was added to bath and bath solution was subjected for ball milling for 2 days using zirconia balls as the grinding media. Electrodepositions of YZA particles were carried out at various current densities: 0.23 A/dm2 for 20h, 0.77 A/dm2 for 6h 1.55 A/dm2 for 3h, and 3.1 A/dm2 for 1.5h. The ball-milled solution gave a Ni-YZA coating with higher microhardness of 600 HK when 0.23 A/dm2 current density was used for electrodeposition. Also, there was a reduction in the agglomerated particle size of the YZA particles as seen by the optical micrograph as shown in figure-7 of the drawings accompanying this specification. The effect of the current density and ball milling on coating microhardness is shown in Table -6 below.

Table-6: Effect of ball milling and current density on microhardness of Ni-YZA coating
(Table Removed)
Example - 5
In another example, YZA powder containing 80 wt% AI2O3, 20 wt% (8 mol% Y2O3) ZrO2 was prepared as follows:
0.88 gm of Y2O3 was dissolved in dilute nitric acid to which 50 gm of aluminum nitrate (Al (NO3) 3 .9H2O), 10.15 gm of zirconium nitrate, 26.56 gm of urea and distilled water (150 ml) were added to get a clear solution. This aqueous redox mixture was then introduced on a preheated hotplate at a temperature of 400°C. The solution boils, foams and catches fire to give white foamy mass, which is X-ray crystalline. The resultant foamy mass was crushed with the help of a pestle and mortar. The XRD pattern of YZA powder showed peaks corresponding to a-alumina and cubic zirconia. The crystallite sizes calculated from X-ray line broadening using Scherrer formula for alumina and zirconia particles are 11.90 nm and 10.91 nm in size respectively. Nickel bath was prepared and electrodeposition was conducted as in example-1. The microhardness was 415 HK (50gf load) for this nanocomposite coating.
Electrodeposition of nickel along with YZA particles was carried out by keeping the other parameters constant as mentioned in Examplel. Estimated values of properties of Ni-YZA coating are given in Table 7.
TABLE-7: Tribological properties of coating having composition containing 80 wt% AI2O3,20 wt% (8 mol% Y2O3) ZrO2
(Table Removed)
Example - 6
In another example, YZA powder containing 95 wt% AI2O3, 5 wt% (8 mol% Y2O3) ZrO2 was prepared as follows:
0.116315 gm of Y2O3 was dissolved in dilute nitric acid to which 50 gm of aluminium nitrate (Al (NO3) 3 .9H2O), 2.01583 gm of zirconium nitrate, 21.25 gm of urea and distilled water (150 ml) was added to get a clear solution. The combustion reaction was similar to example-1. The XRD pattern of YZA powder showed peaks corresponding to a-alumina and cubic zirconia. The crystallite sizes calculated from X-ray line broadening using Scherrer formula for alumina and zirconia particles are 11.58 nm and 10.54 nm in size respectively.
Electrodeposition of nickel along with YZA particles was carried out by keeping the other parameters constant as mentioned in Example! Estimated values of properties of Ni-YZA coating aregiven in Table 8.
TABLE-8: Tribological properties of coating having composition containing 95 wt% AI2O3,5 wt% (8 mol% Y2O3) ZrO2
(Table Removed)
In the present invention 5 to 20 wt% of 8 mol% yttria stabilized cubic zirconia containing 80 to 950 wt% alumina (YZA) was prepared by solution combustion process using urea as fuel. Urea is used as a fuel for preparation of YZA powder as it is cheaper, readily available and easy to handle. Cost of YZA powder prepared by solution combustion process is much lower when compared to other processes. Additionally, solution combustion process is simple and rapid for the preparation of nanosized YZA. The prepared YZA powder is crystalline in nature does not require any pretreatment or calcination steps and can be used directly in the bath. However, in case of Ni-SiC the purity of particles, purification of the particles and pretreatment of the particles influence the co-deposition of SiC particles. Further, the electrodeposition does not require high temperature or pressure. The main advantages of the present invention are as follows:
1. YZA powder is prepared by very simple and fast solution combustion
process using metal nitrates and a cheaper fuel like urea.
2. One can prepare instant powders at the work spot itself.
3. The cost of one Kg of YZA powder prepared by solution combustion process
is less when compared to powders prepared from other techniques.
4. YZA powder does not need all the tedious pretreatment procedures involved
for SiC to get dispersion.
5. Electrodeposition does not require high temperature or pressure.
6. Ni-YZA exhibits a synergetic combination of higher microhardness; higher
wear resistance and higher corrosion resistance than Ni alone.

We claim:
1. A process for preparing nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications, which comprises dissolving yttrium oxide in dilute nitric acid to get yttrium nitrate; preparing an aqueous redox mixture containing stoichiometric amounts of said yttrium nitrate, zirconium nitrate, aluminum nitrate and fuel such as urea; heating the said aqueous redox mixture at a temperature of around 400 °C; crushing the resultant foamy mass to obtain YZA powder; characterized in that dispersing of 100 to 150 gm/litre of said YZA powder in nickel sulfamate bath under constant stirring for a period of 16 hours; subjecting the said bath mixture maintained at a temperature below 60°C, to electrodeposition on a substrate at a current density below 5.4 A/dm2 for a duration of 1.5 to 20 hours.
2. A process as claimed in claim 1, wherein the stoichiometric amount of fuel such as urea in aqueous redox mixture corresponds to oxidizer to fuel ratio of 1.
3. A process as claimed in claim 1-2, wherein the aqueous redox mixture is heated using a pre-heated hot plate maintained at a temperature of around 400 °C.
4. A process as claimed in claim 1-3, wherein the bath containing YZA particles is subjected to ball milling for reducing the agglomeration of YZA particles.
5. A process as claimed in claim 1-4, wherein the nickel sulfamate bath used for dispersing and electrodepositing YZA particles consists of 300-450 gm per litre of nickel, 20-45 gm per litre of boric acid, 3-30gm per litre of nickel chloride and 0.1-0.5 gm per litre of sodium lauryl sulfate.
6. A process as claimed in claim 1-5, wherein the electrodeposition of YZA particles from nickel sulfamate bath is carried out by maintaining the pH of bath in the range of 3.5 to 4.5.

7. A process as claimed in claim 1 -6, wherein the electrodeposition of YZA particles from nickel sulfamate bath is carried out at a bath temperature below 60°C , preferably between room temperature to 50°C.
8. A process as claimed in claim 1-7, wherein the electrodeposition of YZA particles from nickel sulfamate bath is carried out at a current density below 5.4 A/dm2, preferably in the range of 0.23 A/dm2 to 3.1 A/dm2 for a duration of 1.5 to 20 hours.
9. A process as claimed in claim 1-8, wherein the electrodeposition of YZA particles along with nickel is carried out on a substrate, such as brass, mild steel, nickel sheets.

10. Nickel based nanocomposite coating containing particles of yttria stabilized cubic zirconia-alumina (Ni-YZA) useful for tribological applications, prepared by the process as claimed in claims 1-8, which comprises: nickel matrix containing 4 to 8 volume % of YZA powder, said YZA powder containing 5 to 20 weight % of 8mole % yttria stabilized cubic zirconia, 80 to 95 weight % of alumina.
11. Nickel based nanocomposite coating as claimed in claim 10, wherein the particle size of zirconia and alumina in YZA powder is between 10 to 60 nm and 10 to 40 nm, respectively.
12. Nickel based nanocomposite coating as claimed in claim 10-11, wherein the Ni-YZA nanocomposite coating is having Ni grains of size between 17 to 40 nm, micro hardness in the range of 350 to 600 HK (applied load 50gf), corrosion rate in the range of 3.94x10-3 to 14.9 x 10-3 mm/yr, wear coefficient in the range of 5.04 x 10-8 to 2.77 x 10-7 and friction coefficient in the range of 0.457 to 0.85.

Documents:

1524-DEL-2006-Abstract-(26-06-2012).pdf

1524-del-2006-abstract.pdf

1524-DEL-2006-Claims-(26-06-2012).pdf

1524-del-2006-claims.pdf

1524-DEL-2006-Correspondence Others-(26-06-2012).pdf

1524-del-2006-correspondence-others.pdf

1524-del-2006-description (complete).pdf

1524-DEL-2006-Drawings-(26-06-2012).pdf

1524-del-2006-drawings.pdf

1524-del-2006-form-1.pdf

1524-del-2006-Form-18 (14-02-2008).pdf

1524-del-2006-form-2.pdf

1524-DEL-2006-Form-3-(26-06-2012).pdf

1524-del-2006-form-3.pdf

1524-del-2006-form-5.pdf

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Patent Number

254523

Indian Patent Application Number

1524/DEL/2006

PG Journal Number

46/2012

Publication Date

16-Nov-2012

Grant Date

12-Nov-2012

Date of Filing

28-Jun-2006

Name of Patentee

COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH

Applicant Address

ANUSANDHAN BHAWAN, RAFI MARG, NEW DELHI-110 001, INDIA

Inventors:

#	Inventor's Name	Inventor's Address
1	VATIA KRISHNA MURTHY WILLIAM GRIPS	SURFACE ENGINEERING DIVISION NATIONAL AEROSPACE LABORATORIES BANGALORE-560 017
2	KARAIKUDI SANKARA NARAYANA RAJAM	SURFACE ENGINEERING DIVISION NATIONAL AEROSPACE LABORATORIES BANGALORE-560 017
3	SINGANAHALLI THIPPAREDDY ARUNA	SURFACE ENGINEERING DIVISION NATIONAL AEROSPACE LABORATORIES BANGALORE-560 017

PCT International Classification Number

B32B 15/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA