METHOD FOR PRODUCING CERAMIC COMPOSITE MATERIALS CONTAINING SILICON OXYNITRIDE AND ZIRCONIUM OXIDE

Abstract: A new group of ceramic composite materials containing silicon oxynitride (Si2N2O, O'-phase) and zirconium oxide (ZrO2) can be prepared using a thermal reaction between zirconium silicate (ZrSiO4) and silicon nitride powder (Si3N4) supplied completely or partly as silicon powder (Si) which is nitrided to Si3N4 in a nitrogen-containing atmosphere. By adding aluminium oxide or the like, composite compositions with ZrO2, O'-phase and sialons (silicon aluminium oxynitrides) as main components can be obtained. In the method according to this invention, the raw powder material is prepared, e.g. by concurrent milling and blending to submicron particle size, formed by known technique into a compact, nitrided and sintered to the desired density in nitrogen-containing atmosphere.

Description: METHOD FOR PRODUCING CERAMIC COMPOSITE MATERIALS CON¬ TAINING SILICON OXYNITRIDE AND ZIRCONIUM OXIDE The present invention relates to a method for producing ceramic composite materials containing si¬ licon oxynitride and zirconium oxide, starting from ceramic powder materials containing silicon and/or silicon nitride powder and zirconium silicate powder. In the method of preparation, use is made of a high- temperature reaction between silicon nitride and zir¬ conium silicate which are reacted to form silicon oxy¬ nitride and zirconium oxide in connection with the sintering of compacts to the desired density- Composite materials containing silicon oxynitride and zirconium oxide according to this invention should primarily be considered to belong to the group of high- performance ceramics which are distinguished inter alia by high strength, fracture toughness, wear, corrosion and thermal shock resistance also at high temperatures. However, advanced applications in the refractory field are also conceivable for this type of composite mate¬ rials. An increase and extension of the use of the above- mentioned composite materials is however affected/hamper¬ ed to a significant extent by the relatively high costs for the powder raw material and the costs for so-called sintering aids, which must be supplied to achieve the desired sintering qualities and qualities of the mate- rial. This applies especially to rare earth metal oxides. Another limiting factor is the need for an advanced furnace equipment to be able to sinter powder compacts to the desired density." The generally high degree of shrinkage in connection with sintering (densification) is another process engineering drawback. For composites containing true non-oxidic components, such as silicon nitride, there is often the additional problem of chemical incompatibility with respect to zirconium oxide which may result in the formation of secondary phases, such as zirconium nitride or zirconium oxynitride, which have an adverse effect on the thermomechanical properties. By using the method according to this invention, several of the above-mentioned drawbacks will be overcome in a composite material where the raw material powder consists to an essential part of ZrSiO. and Si powder of a particle size substantially within the submicron range. As will be described further on, such a powder mixture can be obtained by concurrent milling and blending of the components, and the cost of this powder mixture is estimated at about one tenth of the current cost. The amount of necessary sintering aids from the group of rare earth oxides can be reduced by half at the same time as the sintering temperature can be below about 1650°C, which thus makes it possible to use a more conven¬ tional furnace equipment. Further, the linear shrinkage in connection with sintering (densification) to almost full density can also be minimized to about 12% or less because of the increase of the internal volume on account of nitridation of Si to Si-.N. and the reac¬ tion between ZrSiO. and Si-.N. to ZrO- and Si 2 N„0. By the nitridation step, a bond is also produced in the material, resulting in fully sufficient strength to permit machin- ing the nitrided material, if desired, before the final sintering step. After completed sintering, the included main components ZrO„ and Si p N„0 will exhibit good com¬ patibility with respect to the formation of undesirable secondary phases. In the scientific/technical litera- ture, ZrSiO. and Si„N~0 materials are described several¬ ly and do not seem to have been tested as components in a composite material. However, ZrSiO. is mentioned by Rice et al (Refs. 1,2) as a desirable grain boundary phase in connection with the sintering of Si-.N.. This is so because of its high liquidus temperature, high E-modulus and low thermal expansion coefficient. The latter two properties are close to those of Si-.N., but also for the so-called sialons (β'-phase) or Si N„0 (O'-phase), such as represented in the known compati¬ bility diagram Si^N.-SiO ? -AlN-Al 0-, (Ref. 3). However, the literature is silent on the use of a reaction be¬ tween ZrSiO. and Si-.N 4 and/or the component combina¬ tion Zr0 2 /Si-,N,.0 in these contexts. As mentioned in the introduction to this specifi¬ cation, the method for producing composite materials according to the present invention relies on a reac¬ tion between ZrSiO. and Si,N. to give ZrO^ and Si-N^O. The following text describes the essential process steps of this method and the properties of the material obtained thereby, the method being thereafter illustrat¬ ed by an Example. The starting raw material in the preparation of the composite material according to this invention should exclusively consist of ZrSiO. and Si and/or Si-.N. powder for optimum utilisation of the reaction giving ZrO„ and Si„N 2 0. Depending on the desired appli- cations of the composite material where priority is given e.g. to such properties as high strength, low thermal conductivity or the like, an excess of either component may however be supplied. Similarly, the con¬ tent of components resulting from the reaction, i.e. Zr0 2 and Si„N 2 0, can be altered by separate addition of any of the substances to affect the properties of the composite material as desired. In this context, an increased addition of Si and/or Si-.N. should espe¬ cially be noted for obtaining a composite in the Si-^O/ Si N range (Ref. 3) or an increased addition of Si and/or Si,N. , combined with an adequate amount of Al 2 0-,, for obtaining a composite material in the β'-phase/O'- phase range within the phase diagram earlier mentioned. In addition to the components appearing within the system of ZrSi0 4 -Si-.N.-Si 2 N 2 0-Zr0 2 , other components may be used as well (e.g. silicon carbide, aluminium nitride etc.) for affecting the thermomechanical , elec- trical or other properties of the composite material produced. In the use of conventional (pressureless) sin¬ tering, sintering aids must be supplied. The sintering aids can be selected from those which are known e.g. in the sintering of Si_,N. and sialons. Oxides, such as Al„0.,, MgO, BaO etc. or oxides of rare earth metals, can thus be used. The amount added depends on the desired sintering properties or properties of the final composite. If a sintering technique with the applica¬ tion of mechanical pressure is used (gas overpressure sintering, hot pressing, HIP), sintering aids can be dispensed with or their content reduced. In order to completely or partly stabilize the formed Zr0 2 in its tetragonal form and, thus, use the strengthening effect obtained by martensitic transformation of the tetragonal form into the monoclinic one according to known principles for high-performance ZrO- materials/ composites, one or more of the known oxid ' ic stabilizers, such as Y O-. , MgO Λ CaO, CeO p etc., should be used. Since the monoclinic/tetragonal transformation is also linked with the size of the ZrO_ grains in the composite matrix, the addition of oxidic stabilizers can be avoided by adequate powder preparation according to known principles. The same applies, i.e. no addition of stabilizer, if the strengthening mechanism by micro- cracking in connection with spontaneous tetragonal/ monoclinic phase transition of the ZrO„ grains is utilized. in addition to the ZrSiO. powder and other com¬ ponents as stated above, the starting material may consist of Si-,N. , Si powder or a mixture of Si-.N. and Si powder. These powders should have submicron size for preparing high-performance composite mate- rials of fine-grained microstructure, while for re¬ fractory applications a coarser particle size, i.e. an average particle size of above 1 μ , can be used. The desired particle size of the starting powder mate¬ rial can be obtained by blending separately obtained powders or by concurrent milling and blending in a suitable milling equipment. It has then been found that if submicron ZrSiO. powder is milled together with Si powder having a particle size substantially exceeding 1 μm, a submicron ZrSiO./Si powder mixture can be ob¬ tained, the specific surface of the concurrently milled and blended powders becoming larger than if each powder ia, milled separately. A similar synergistic milling effect is thus obtained as disclosed in Swedish Patent Applica¬ tion SE 8103269-0 for submicron Si,N. powder and Si powder as above. However, milling Si powder together with ZrSiO. powder of a particle size of above 1 μm is not unlikely to give a submicron powder mixture. The sintering aids/stabilizers, if any, are supplied in connection with the concurrent milling and blending of the main components, but can also be added by impregna¬ tion or in any other adequate manner after the concur- rently milled and blended powder material has been formed by known methods, such as slip casting, injection mold¬ ing, pressing, extrusion, stamping etc., into a compact of the desired shape. After burning-off any process aids supplied in connection with the forming operation, the nitridation step takes place where the added Si powder is reacted in a nitrogen-containing atmosphere to form Si^N.. To avoid overheating in connection with the exothermal nitridation reaction, the nitridation step should be carried out with a slow temperature in- crease or, for instance, according to a two-step program with the first temperature level at 1000-1300°C and the final nitridation temperature at 1150-1450°C. Generally, the final nitridation temperature should be below the lowest eutectic temperature for the optionally present sintering aids/stabilizers/main components, since the presence of a melt is likely to accelerate the exother¬ mal reaction between ZrSi0 4 and Si.,N. to form ZrO~ and Si N O. If submicron Si powder is included, a com¬ plete reaction into nitride can normally occur at a ' final nitridation temperature of 1250-1300°C and a holding time of 1-3 hours. If, for particular reasons, it is however desirable to use the released reaction heat in connection with nitridation and the exothermal reaction between ZrSiO. and Si-N., the process may op¬ tionally be performed adiabatically, such that also partial or complete sintering (densification) can be obtained in connection with nitridation and reaction of ZrSiO. and Si 3 N. into r0 2 and Si 2 2 0. In conventional (pressureless) sintering, the tem¬ perature should be below about 1750°C to minimize the decomposition of the formed Si-N-O into SiO and N„ gas. A nitrogen-containing atmosphere should be used. The powder bed technique according to known principles should be used where the powder bed used, e.g. pre- oxidized Si-.N. powder, SiO ^ /Si^N. powder mixture, SipN„0 powder or the like, produces the same type of gaseous decomposition products as the sintered compact. Further, the temperature should be maintained preferably below 1650°C to minimize grain growth, es¬ pecially grain growth of the formed Zr0 2 in the compo¬ site matrix. Heat treatment as a separate step or as a part of the sintering program can be used to complete¬ ly or partly crystallize the grain boundary glass phase formed after cooling into crystalline phases and, thus, to optimize the high-temperature strength, corrosion resistance and other related properties of the compo- site. Normally, the reaction of ZrSiO./Si-.N. into Zr0 2 /Si 2 N„0 occurs in connection with sintering and is accompanied by a molar volume increase of about 9%. When using sintering methods with mechanical pressure, the pressure must thus be applied after the reaction is substantially completed. The method for producing the composite material according to this invention will be illustrated in more detail by the following Example: Two batches of ZrSiO. powder (Ventron, BET specific area: 12.8 m 2/g, purity: 99.98%) and Si powder (KemaNord, 2 BET specific area: 2.1 m /g, purity: 99.8%) with an addi¬ tion (in per cent by weight) of 9% Y 2 0 3 /5% l 2 0 and 3% Y 2 0-,/4% A1„0-., respectively (batches 1 and 2, respective¬ ly) as stabilizer/sintering aid were used in this Example (the added stabilizer of course also assists as sintering aid), to illustrate the effect of varying amounts of stabilizer/sintering aid. The quantity ratio of ZrSiO. to Si was the stoichiometric one for the reaction. Powder preparation was effected by concurrently milling and blending the components in a ball mill in plastic con¬ tainers with Si-.N. milling balls for about 70 hours in ethanol as milling liquid. The thus prepared slurries were wet-sieved through a 10 μm screen, whereupon the liquid phase was removed in a rotating evaporator. Mea- 2 surement of the BET specific area yielded 14-21 m /g, in¬ dicating that it would have been easier to mill the added Si powder to submicron size by concurrent milling and blending. When separately milling Si powder, a specific 2 area of not more than 8-11 m /g is normally obtained. The resulting dried powder mixture was screen-granulated, prepressed uniaxially into rectangular bars and was cold isostatically pressed at 280 MPa. The resulting green density was about 55% of the theoretical one. Nitrida¬ tion of the compacts was carried out in a graphite re- sistance furnace or a thermobalance in nitrogen gas atmosphere at a pressure of 0.1 MPa. The following two- step program was used (temperature/holding time): 1150°C/0.5 hours - 1250°C/1.5 hours. For the sample size used (7x7x40 mm), this program resulted in a complete reaction of Si without any overheating effects. Minor shrinkage (linear) of about 1-1.3% was obtained after nitridation, and the green density was 67-68% of the theoretical one. X-ray diffraction analysis of nitrided material indicated the presence of ZrSiO. and Si-.N. (alpha phase). Sintering was carried out in a graphite resistance furnace with the samples embedded in pre- oxidized silicon nitride powder (preoxidation: 1200°C/0.5 hours) in nitrogen gas at 0.1 MPa. In the sintering programs used, temperature was 1600-1640°C with a hold¬ ing time of 1-4 hours. At a temperature above 1670°C, a certain decomposition took place in this case, result- ing in increased open porosity. Samples from batches 1 and 2 densified to about 99% and 95%, respectively, of the theoretical density. Because of the uncertainty about the relative amounts/exact composition of the phases formed after sintering, the theoretical density may deviate slightly from the compact density. Phase analysis by means of X-ray diffraction showed that a reaction into ZrO„ and Si 2 N„0 had occurred during sin¬ tering. The Zr0 2 formed appeared as tetragonal/cubic phase and monoclinic/tetragonal (cubic) phase, respec- tively, in batches 1 and 2. The ratio of monoclinic to tetragonal (cubic) phase in batch 2 was estimated at about 1:5. On account of coinciding tetragonal/cubic reflexes, it was not possible to make an exact quantity calculation of the phases included. The microstructure which was examined by means of a scanning electron microscope in a polished cross-sectional surface showed the presence of small, oblong Si 2 N„0 grains (about " 1 μm long) surrounded by a glass phase as well as ZrO„ grains of a size of about 0.5-2 μm. These grains were homogenous- ly distributed in the Si-,N 2 0 matrix. Literature references: 1. Rice et al, Progr. Rep. NRL, December 1974, p. 18 2. Rice et al, J. Am. Cera . Soc. 58(5-6) (1975), p 264 3. M.B. Trigg, K.H. Jack, Proc. Int. Symp. Ceramic Comp. for Engines, 1983, Japan, p. 199

Claims: CLAIMS 1. A method for producing ceramic composite mate¬ rials containing silicon oxynitride (Si„N„0, 0' -phase) and zirconium oxide ( Zr0 2 ) , starting from pulverulent raw materials formed by known techniques, nitrided in a nitrogen-containing atmosphere and/or sintered to the desired density, c h a r a c t e r i z e d in that the raw material powder contains zirconium silicate ( ZrSiO. ) and silicon powder (Si) or ZrSiO. and silicon nitride powder (Si-.N. ) or a mixture of Si and Si,N. powder, and optionally a stabilizing oxide for forming tetragonal/cubic ZrO~ and/or a sin¬ tering aid which, after powder preparation and forming, is reacted to form ZrO- and Si 2 N„ (O'-phase) in connec¬ tion with nitridation of Si into Si- > N. and sintering or in connection with sintering only. 2. Method as claimed in claim 1, c h a r a c ¬ t e r i z e d in that the amount of added ZrSiO. and Si_N. supplied as Si-.N. powder or supplied com¬ pletely or partly as Si powder which is subsequently reacted by nitridation into Si-,N. , is the stoichiometric one for reaction into ZrO„ and Si 2 N„0 (O'-phase) as main components in the composite material produced. 3. Method as claimed in claim 1 or 2, c h a ¬ r a c t e r i z e d in that the amount of added Si-.N.. powder, alternatively supplied completely or partly as Si powder subsequently reacted by nitridation into Si-.N , varies within the composition range of Si 3 N., Si 2 N 2 0 and Zr0 2 as main components in the composite material produced. 4. Method as claimed in claims 1 and 2, c h a ¬ r a c t e r i z e d in that the amount of added Si-.N. varies according to claim 3, and that aluminium oxide (Al 0^) or the like is added in an amount correspondii to the formation of Si0 2 2 0 (O'-phase), r0 2 and a 10 sialon (silicon aluminium oxynitride) as main com¬ ponents in the composite material produced. 5. Method as claimed in claims 1-4, c h a r a c ¬ t e r i z e d in that a sintering aid is added, select¬ ed from oxides, such as A1 2 0-., MgO, BaO or oxides of rare earth metals. 6. Method as claimed in claims 1-5, c h a r a c ¬ t e r i z e d in that oxides stabilizing the tetragonal and/or cubic form of Zr0 2 are added, such as Y 2 0-,, MgO, CaO, Ce0 and others, either separately or in admixture. 7. Method as claimed in claims 1-6, c h a r a c ¬ t e r i z e d in that the raw material powder is prepared by blending and/or concurrent milling and blending of the powder components included to a sub- micron particle size, starting from powder materials where all the main components, i.e. ZrSiO., Si and/or Si-,N., have submicron particle size or at least one of them is within the submicron particle size range. 8. Method as claimed in claims 1-7, _. c h a r a c - t e r i z e d in that the powder raw material which contains Si powder as one of the components included is formed by known methods, such as slip casting, injection molding, extrusion etc., into a compact and is nitrided in a nitrogen-containing atmosphere at a maximum temperature below the lowest eutectic temperature for the phases present/formed. 9. Method as claimed in claims 1-7 in which the powder raw material contains Si powder according to claim 8, c h a r a c t e r i z e d in that nitrida- tion is carried out in the presence of a melt formed by phase reaction between the components present. 10. Method as claimed in claims 1-9, c h a r a c ¬ t e r i z e d in that nitridation is carried out in a two-step program with the first temperature level at 1000-1300°C and the second level at 1150-1450°C. 11. Method as claimed in claims 1-7 in which the compact optionally is nitrided as claimed in claims 8-10, c h a r a c t e r i z e d in that sin¬ tering is carried out without any pressure, i.e. without the use of any mechanical pressure applied with or without the use of a powder bed in a nitrogen-containing atmosphere at a temperature below 1750°C.

Inventors: POMPE ROBERT, POMPE, ROBERT
Applicants: SVENSKA SILIKATFORSKNING, SVENSKA SILIKATFORSKNINGSINSTITUTET
Application year: 1988
Main classification: C04B35
IPC codes: C04B35