Title of Invention

PROCESS FOR THE PRODUCTION OF SHAPED ARTICLES FROM A COMPOSITE CERAMIC STRUCTURE

Abstract

A process is proposed for the production of shaped articles from a composite ceramic structure, in particular a combination of trisilicon tetranitride and a metal silicide, in which gas pressures of up to 100 bar are used and the sintering additive content can be reduced to below 10 MA%. This sintering and compression process using an inert gas makes it possible to obtain larger structural spaces with more complex geometrical structures of the shaped articles than known processes. Furthermore, it is possible to adjust the electrical properties of this composite structure by setting a particular nitrogen partial pressure.

Full Text

Process for the production of shaped articles from a composite ceramic structure Prior Art The invention relates to a process for the production of shaped articles from a composite ceramic structure, in particular a trisilicon tetranitride/metal silicide composite structure, according to the generic type of the independent claim. Ceramic materials are significantly superior to comparable materials, for example metal alloys, in their potential regarding higher working temperatures. In this context, trisilicon tetranitride materials, which are distinguished by outstanding mechanical as well as (in corresponding composite structures in combination with electrically conducting compounds) electrical properties, are particularly suitable for a variety of applications, in particular at high temperatures. Non-oxide ceramic materials based on silicon can withstand significant thermal and mechanical loading, and substantially resist oxidation and corrosion even in high temperature ranges of up to, for example, 1300°C. A further important aspect of non-oxide ceramic materials resides in the controlled adjustment of their electrical properties, in particular in material combinations and composite structures. DE 37 34 274 C2 describes ceramic materials based on silicon nitride, aluminium nitride and β-sialon in combination with secondary phases of various silicides, carbides, borides and nitrides of transition metal elements. Depending on the secondary phase content, these materials have controllably adjustable electrical properties. The specific values which can be adjusted for the electrical resistance of these materials are, at room temperature, between 1*1013 to 1*10-4 Ώ cm and exhibit a positive dependence on temperature (PTC effect). The strength level of the composite materials produced in this way is not less than 200 MPa. The process used in this context for the production of high temperature-stable composite materials is referred to as uniaxial hot pressing, which in particular has disadvantages in terms of the shaping of articles made from these composite materials, may have anisotropic material properties, on account of the pressing direction, and is only usable as a batch process, i.e. not as a continuous process. Furthermore, this process requires high temperatures and pressures. It is furthermore known to prodvice high temperature-stable electrically insulating composite materials based on trisilicon tetranitride with metal silicides of formula MSi2 and M5Si3, M being a transition metal or a main group metal, by gas pressure sintering at a pressure of 100 bar N2 (DE 195 00 832 Al and EP 0 721 925 A2) . Amongst other things, gas pressure sintering is expensive and elaborate on account of the high pressures, of up to 400 MPa, which are required. Hot pressing likewise requires axial pressures of up to 3 0 MPa, is expensive and can be used only for simple components. Advantages of the Invention The process according to the invention has the advantage over the hitherto described processes that it is possible to obtain larger structural spaces for shaped articles subjected to a high degree of thermal and mechanical loading, in combination with controlled variation of their electrical properties. Furthermore, it is possible to make complex geometrical structures very substantially more favourable processing in the green state. This is achieved in that, in a multistage pressing process, the shaped article, is firstly cold-pressed isostatically and subsequently brought into the desired shape. This avoids elaborate hard processing, for example after hot sintering, as is required in the case of a uniaxial pressing process. Further advantageous refinements and developments of the process according to the invention are given in the subclaims. In a preferred embodiment, a first sintering process is carried out at atmospheric pressure under an inert gas after the cold isostatic compression moulding step. This further consolidates the shaping which is carried out. Advantageously, the last sintering process is carried out under a pairtial pressure of shielding gas, preferably nitrogen, or from 2 to 10 bar. The sintering temperature of this sintering process is in this case between 1700o and 1900°C, From a phase diagram of the components A and B which are used, it can be gathered under these conditions that only the pure phases A and B are present, and possible mixed phases do not occur. This prevents, in particular, for example non-conducting phases or phase transitions or poorly conducting phases from preventing or critically impairing the desired electrical properties. In a further particularly advantageous embodiment, sintering takes place in a range of the shielding gas partial pressure log p(N2) which is defined by an upper limit Y1 and a lower limit ¥3. In this case, the nitrogen partial pressures Y1 and Y2 and the sintering temperature T are related to one another in the following way: Y1 = 7.1566 • ln(T)-52.719 Y2 = 9.8279 • ln(T)-73.988 This ensures that, in the phase diagram of the system of the the chosen materials, likewise only the pure phases occur in this range. The variable relationship between temperature and pressure thus defines an optimum range which makes it possible, by varying the pressure and the temperature, to preserve the optimum process parameters in a broader range, without this leading, for example, to thermal decomposition, in particular of a less thermally stable component, for example a nitride. Furthermore, this defined relationship between pressure and temperature avoids mixed phases with undesired property profiles in a simple way. Preferably, trisilicon nitride is used as component A in the composite ceramic structure, and a metal silicide is used as component B. Relevant silicides include the most commonly used metal silicides, for example MoSi;,. In comparison with its boron and nitrogen homologues, trisilicon tetranitride is harder and has better sinterability. Advantageously, nitrogen or a mixture of nitrogen and a noble gas, for example argon, is used as the shielding gas, so as to Suppress substantially a possible decomposition reaction of the nitride which is used, in particular Si3N4, according to the following equilibrium reaction: Si3N4 > 3Si+ 2N2 Applying Le Chatelier's principle, increasing the concentration of one component of the equilibrium thus makes it possible in simple fashion to increase the thermodynamic stability of Si3N4, Even sintering temperatures which are above the decomposition point of Si3N4 can therefore be used for the sintering process. Furthermore, it is therefore possible for the content in Si3N4 of sintering additives, for example aluminium oxide or yttrium oxide, which often impair the electrical properties, to be reduced to a value of less than 10% by weight. It is also advantageous that the total pressure can be adjusted and controlled in simple fashion by adding a second inert gas, for example argon. This has a particularly advantageous effect on the sintering result in relation with the material density achieved in the two sintering variants, without altering the electrical properties. Drawing The process according to the invention will be explained below in the description and by the drawings, in which: Figure 1 shows the essential steps in the process according to the invention, Figure 2 shows a simplified phase diagram for the components A and B as a function of temperature, on the ordinate, and the logarithm of the nitrogen partial pressure, on the abscissa. Description of the Illustrative Embodiments Figure 1 shows the essential steps in the process according to the invention. This being the case, a powder mixture is prepared in Step 1, for example from the raw materials Si-iN4 and corresponding additives, for example AlnO3 and Y2O3 or other known sintering additives, and MSi2, M standing for Mo, Nb, W, Ti for example, as well as, optionally, organic pressing or binding aids, by mixing and grinding in an attritor mill with an organic solvent. Step 2 comprises drying the "attrited" suspension in a rotary evaporator- Step 3 represents the cold isostatic pressing of the dried powder to form the shaped articles. Step 4 comprises the presintering or the removal of the binder under an inert gas atmosphere with a pressure of about 1 bar at a temperature of up to 900°C. Step 5 comprises the "main sintering" under a defined partial pressure of shielding gas, for example nitrogen, the N2 partial pressure in the sintering gas being no more than 10 bar, and at the same time the sintering temperature being no more than 1900°C. Instead of N2, it is also possible to use a nitrogen/noble gas mixture. Step 5' , which is an alternative to Step 5, comprises the main sintering with variable pressure and variable temperature. The N2 partial pressure is varied with the temperature in such a way that the partial pressure lies within a range bounded by the following dependencies: The upper limit obeys the following equation: Y1 = log p(N2) = 7.1566 • ln(T)-52.719 and the lower limit obeys the equation: Y2, = log plN2) = 9.8279 • In (T)-73 . 988 . T is expressed ip °C. p(N2) is expressed in bar. The sintering temperature is no more than 1900°C. In Figure 2, this range, bounded by the two logarithmic dependencies, is represented in the context of a two-phase diagram. The curve between points, D and C corresponds to the upper limit Y1 and the curve between points A and B corresponds to Y2. Above and below the two functions Y1 and Y2, respectively, there are mixed phases or decomposition phases between the nitride which is used and the silicide, or the shield gas, respectively. The term MA% is used below to denote the expression "percentage by mass". A first illustrative embodiment will be described below with reference to process step 5. A composite material, consisting of 36 MA% Si2N4 and 60 MA% MoSi2, as well as the sintering additives with n .72 MA% Al2O, and 2.28 MA% Y.O,, the average particle size of the Si3N4 which is used being 0.7 μm and that of the MoSi2 being 1.8 μm, is subjected to cold isostatic compression at 200 MPa. After this, presintering takes place under a nitrogen atmosphere at a pressure of 1 bar and at a temperature of 900 °C. The subsequent main sintering, under a defined partial pressure of 10 bar and a sintering temperature of 1900°C, produces a material whose density is 97% of the material density. The electrical resistivity * is 1*10-3 Ώ cm at 25°C. A second Illustrative embodiment is produced as follows by means of process Step 5'. A composite material, consisting of 63 MA% Si3N4 and 30 MA% MoSi2, as well as the sintering additives with 3 MA% Al2O3 and 4 MA% Y2O3, the average particle size of the SisN4 which is used being 0.7 μm and that of the MoSi2 being 1.8 μm, is subjected to cold isostatic compression at 2 00 MPa. After presintering in 'accordance with Illustrative Embodiment 1, the main sintering takes place within the phase range given by points A, B, C and D in Figure 2, at variable pressure and temperature. The density achieved for the material is 97% of the material density. The electrical resistivity is 106 Ώ cm at 25°C. R. 31799 A third illustrative embodiment is produced in similar fashion to the first, but after the preparation and presintering, the main sintering takes place at 1800°C. The nitrogen partial pressure is 5 bar and the total sintering pressure is 20 bar. The sintering density achieved for the material is 98%. The resistivity is 1*10-4 Ώ cm. We Claim; 1. Process for the production of a shaped article with adjustable electrical conductivity from a composite ceramic structure which contains at least two components A and B with different electrical conductivity, characterized in that the shaped article is formed by a cold isostatic compression moulding step before sintering. 2. Process according to Claim 1, characterized in that the sintering process takes place in at least two stages. 3. Process according to Claim 2, characterized in that a first sintering process is carried out at atmospheric pressure under an inert gas. 4. Process according to Claim 3, characterized in that the sintering temperature is at most 900°C. 5. Process according to Claim 2, characterized in that the last sintering process is carried out under a nitrogen partial pressure of from 2 to 10 bar. 6. Process according to Claim 5, characterized in that argon is mixed with the nitrogen. 7. Process according to Claims 5 and 6, characterized in that the sintering temperature of the last sintering is between 1700 and 1900°C. 8. Process according to Claim 2, characterized in that the last sintering is carried out at variable temperature and/or variable nitrogen partial pressure, such that the composite ceramic structure comprises the pure phases of component A and component B in the phase diagram. 9. Process according to Claim 8, characterized in that sintering takes place in an oxygen partial pressure range with an upper limit Y1 and a lower limit Y2, where the upper limit Y1 and the lower limit Y2 are given by the following functions: Y1 = 7.1566 ln(T) - 52.719 and Y2 = 9.8279 In (T) - 73.988 where the sintering temperature T s 1900°C 10. Process according to Claim 8, characterized in that the composite ceramic structure contains trisilicon tetranitride as component A and a metal silicide as component B. 11. Process according to Claims 9 and 10, characterized in that between 3 0 and 70 MA% by mass trisilicon tetranitride, 25-65 MA% by mass MoSi2, 1.5 - 8 MA% by mass Al2O3 and 2 - 2.5 MA% by mass Y2O3 are contained as components. 12. Process for the production of a shaped article, substantially as herein described, with reference to the accompanying drawings.

Documents:

1360-mas-1998-abstract.pdf

1360-mas-1998-claims duplicate.pdf

1360-mas-1998-claims original.pdf

1360-mas-1998-correspondence othrs.pdf

1360-mas-1998-correspondence po.pdf

1360-mas-1998-description complete duplicate.pdf

1360-mas-1998-description complete original.pdf

1360-mas-1998-drawings.pdf

1360-mas-1998-form 1.pdf

1360-mas-1998-form 19.pdf

1360-mas-1998-form 26.pdf

1360-mas-1998-form 3.pdf