ZIRCONIA CERAMIC MATERIALS

Abstract: Reaction bonded composite of silicon nitride and zirconia using unstabilised zirconia. Materials according to the invention have useful strength, and shrinkage during manufacture has been shown to be small thus requiring little finishing machining. The compositional range of these binary ceramics is from 10 % to 80 % of silicon nitride. These materials can overcome problems previously encountered with adhesion of zirconia coatings to a bulk ceramic material. Silicon and zirconia powder are mixed together and then compacted, and finally the compacted powder is heated in an atmosphere of nitrogen. The nitriding step is a multistage process wherein the compacted powder is successively heated: a) for 10 h at 1150 DEG C; b) for 5 h at 1200 DEG C; c) for 5 h at 1250 DEG C; d) for 10 h at 1300 DEG C; e) for 60 h at 1350 DEG C and then f) for 10 h at 1425 DEG C. Advantageously the composite ceramic has at least 18-20 % of silicon nitride since it has been discovered that these materials do not shrink on reaction bonding the silicon nitride in the nitriding process step. In addition to the useful strength and the dimensional stability within the above mentioned range of silicon nitride proportions, these materials have much lower thermal conductivities than reaction-bonded silicon nitride or sintered and hot pressed silicon nitride ceramics, making them useful alternatives to zirconia ceramics where such low thermal conductivity properties are required.

Description: Zirconia Ceramic Materials The invention relates to ceramic materials and in particular to the toughening of zirconia ceramics. It is known that zirconia ceramics can be transformation toughened by developing a dispersion of fine tetragonal phase zirconia particles within a cubic phase matrix. Matrix con¬ straint then prevents the high temperature stable tetragonal phase zironia from transforming to the lower density monoclinic phase. In the stress field of an advancing crack, or at a free surface (eg a machined one) the zirconia particle is able to transform from its metastable state, and the volume expansion compresses the surrounding matrix, making it more difficult for the crack to propagate through it, giving a toughening effect. A similar toughening effect has been shown by making disper¬ sions of zirconia in other ceramics, for example aluminia. Trans¬ formation toughened zirconia materials are also available produced by the recently developed fine-grained all-tetragonal zirconias- • etragonal zirconia polycrystal) . In these materials the grains mutually restrain neighbouring tetragonal phase grains from trans¬ forming until an advancing fracture or surface machining releases the restraint. Zirconia (ZrO ? ) ceramics having appreciable monoclinic phase at room temperature are not very strong, and when temperature cycled through the monoclinic/tetragonal transition (approximately 950 -1220 * C) crack spontaneously and may even fragment due to the severe volume change (up to 9.7Ϊ). The addition of several (3 upwards) per cent of certain oxides (eg MgO, CaO or Y-C ) forms a cubic phase, which is stable over a wide temperature range, and such materials are known as 'stabilised' zirconias. The transformation toughened zirconias (TTZ) are either partially stablised zirconias (PSZ), with moderate additions of stabilising oxides, or metastable all-tetragonal materials as mentioned above. The enhanced toughness of TTZs has been found to diminish progressively as the temperature is raised, and this tendency is apparent at temperature appreciably below the transformation range. This effect is causing some reser¬ vations about their applicability in heat engines, where their low thermal conductivity (eg 1-5 W/mK) , high strength and enhanced toughness suggest they should be prime candidates for ceramic insulative or wear resistant components. There is also considerable interest in the use of both ZrO coated metallic partβ and mono¬ lithic ZrO parts in diesel engines because the low thermal conduc¬ tivity of zirconia (1-5 W/mK) offers a thermal insulation capability. This property can be used to enable waste heat to be usefully employed, leading to an improvement in efficiency. In a military application, diminution of cooling requirements by improved insu¬ lation leads to reduced infra red signatures and consequent reduc¬ tions in vulnerability. Silicon nitride (Si N, ) and silicon carbide (Si C) ceramics -6 have lower thermal expansion (about 3 and 4.5 x W /K respectively, RT - 1000 β C) than monoclinic Zr02 (7.5 x W~ /K at temperatures below about 1050 β C) and consequently have much better thermal stress resistance. Attempts have been made to combine the advantages of silicon compound and zirconia ceramics by coating silicon nitride for example with zirconia. These materials must overcome problems of adhesion of the thermal barrier coating to the bulk ceramic mat¬ erial. Investigations have been made of the silicon nitride-zirconia system. These have been concerned with the role of zirconia as a sintering additive (eg "Zyttrite" - yttria - stabilised zirconia) or mechanical property enhancer, relying on the known toughening effect of fine zirconia particles. The object of the present invention is to provide a ceramic material having bulk properties of low thermal expansion and thermal shock resistance comparable to silicon nitride and low thermal conductivity comparable to zirconia. The invention comprises in one form a reaction bonded composite of silicon nitride and zirconia. Preferably the zirconia is unstabilised. Materials according to the invention have useful strength and shrinkage durin -manufacture has been shown to be small. The range of binary ceramics produced spans the composi¬ tional range from 10% to 80$ of silicon nitride. Materials accor¬ ding to the present invention can overcome the problems previously encountered with adhesion of zirconia coatings to a bulk ceramic material. The invention also provides a method of making a silicon nitride - zirconia ceramic composite comprising the steps of mixing together silicon and zirconia powders, compacting the powder mix¬ ture, and nitriding the compacted powder by heating in an atmosphere of nitrogen. Advantageously the nitriding step is a multistage process wherein the compacted powder is successively heated: a) for 10 h at 1150 β C b) w 5 h at 1200 β C c) " 5 h at 1250 * C d) rt 10 h at 1300"C e) " 60 h at 1350"C and then f) " 10 h at 1425'C Advantageously the composite ceramic has at least 18-20% of silicon nitride since it has been discovered that these materials do not shrink on reaction bonding the silicon nitride in the nitriding process step. In addition to the useful strength and the dimen¬ sional stability within the above-mentioned range of silicon nitride proportions, these materials have much lower thermal conductivities than reaction-bonded silicon nitride or sintered and hot pressed silicon nitride ceramics, making them useful alternatives to zirco¬ nia ceramics where such low thermal conductivity properties are required. The composite powder mixture advantageously comprises a minimum of 6.25 /0 (weight percentage) silicon powder and unstabilised. zirconia powder of 8 micron mean particle size. Such materials have been found to have a useful strength and small thermal shrinkage. Advantageously the composite powder mixture is ball milled with tungsten carbide balls in propan-2-ol (iso propyl alcohol) to obtain effective mixing. The composite powder mixture may be die pressed in a double-acting die at pressures up to about 100 lbf/inch or may be isostatically pressed to give higher pressed density. An organic binder may first be added to the powder mixture to yield crack-free die pressings. The invention will now be described by way of ex-ample only with reference to the accompanying drawings of which: Figure 1 shows graphs of the temperature dependence of the thermal diffusivities of known silicon nitride and zirconia ceramics; Figure 2 shows graphs comparing reaction-bonded silicon nitride/zirconia composities of the invention with the known ceramics of Figure 1; and Figure 3 shows graphically the variation of strength of silicon nitride/zirconia materials with zirconia content. Table ]_ shows four different mixtures giving the proportions by weight of silicon nitride and zirconia powders used to produce reaction bonded silicon nitride- zirconia ceramic materials. 8 W/O % O3 - zirconia (Magnesium Elektron SC 10Y8 grade) powder and silicon powder (8 micron mean particle size) were milled by tungsten carbide balls in propan-2-ol for 4 hours to obtain effective mixing. The powders were die pressed in a double acting steel die without binder at pressures up to 1000 ibf/inch^, into discs 18.9 mm diameter and about 1 mm in thickness, and reaction bonded in nitrogen in stages over the temperature range 1150 - 1425°C in the normal manner for producing RBSN (10 h 1150°C, 5 h 1200°C, 5 h 1250°C, 10 h 1300°C, 60 h 1350°C, 10 h 1425°C) . Disc strengths were measured by a disc flexure method, in which the 18.9 mm as-nitrided disc was loaded centrally with a 20 mm steel ball, and supported underneath on three steel equispaced 4 mm ball bearings on a pitch wide diameter of 12 mm. Strength data as well as densities are given in Table 1. For comparison purposes the bottom row of Table 1 gives the measured results for reaction bonded silicon nitride (RBSN) . The RBSN was isostatically pressed, argon sintered, machined and then nitrided. The Table shows that the RB (SN/Z) strength increases as the silicon nitride proportion increases from a sample average of 38.4 MPa at 21.8% SN to 227.6 HPa at 81.7% SN compared to 265 MPa for the RBSN samples. The results also show that the measured strengths are substantially independent of disc thickness. TABLE 1 S13N4/Zr02 Whether Disc Strength Ratio Zirconia was thickness Density As Nitrided SN Za Stabilised TTΪTT1 (BL)» 21.8 78.2 Y203 - stab 3.25 3.034 37.6 2.92 3.056 36.9 3.03 3.068 39.2 3.45 3.046 37.8 3.051 38.4 +0.014 +.1.35 42.7 57.3 Y203 - stab 2.91 2.596 62.4 3.51 2.640 75.9 2.51 2.706 76.1 3.04 2. " 646 72.5 2.647 71.1 +0.045 +.6.4 62.6 87.4 Y203 - stab 3.63 2.389 109.1 3.11 2.376 145.6 3.25 2.352 124.6 2.80 2.366 159.7 2.371 134.8 +0.016 i 22.3 81.7 18.3 Y203 - stab 3.12 2.180 221.6 2.81 2.212 248.0 3.28 2.193 195.8 2.35 2.205 245.1 227.6 +_ 24.8 100 BAR Isostatically - 4.5 2.476 217.1, 297.7, 308, .5 pressed argon X 2.469 285.1 , 269.8, 266. .8 sintered 4.5 2.446 262.4, 206.2, 270 .4 machine and X 2.462 282.4, 278.5, 245 .6 nitrided RBSN 50.6 265.9 + 30.0 *BL = single ( 19mm) ball loaded Table 2 si ύ * Si/ZrO Whether Disc Diametral Strength MPa (No of Samples) Oxidatio Ratio Zirconia was t Shrinkage Density As (BL)After 100 h wt gain SN Za Used Stabilised mm Nitrided 1000 * C Oxidation g/nr/ 3.86 116.4 (6) 85.6 I [5* 0.13 (6) 10 90 6:90 Unstabilised 0.88 3. 8 (12) 1.0.13 117.5 119.5 10.05 3.28 83.5 (6) 66.3 < [6) 0.30 (6) 20 80 12:80 Unstabilised 0.77 0 (12) 10.11 111.3 15.4 10.05 2.34 16.4 ( 3 ) 13.1 < 13) 0.66 (3) 20 80 12:80 Stabilised 1.51 0 10.06 12.0 11.2 10.04 0.82 0 3.06 140.9 (5) 107.6 (6) 0.480 (6 40 60 24:60 Unstabilised (12) 10.17 136.5 122.0 io.11 0.86 2.86 212.1 154.8 (6) 0.67 60 40 36:40 Unstabilised (12) 0 10.04 130.7 113.7 10.09(6) 7 2.35 143.0 89.9 ( 3 ) 2.00 (3) 77 23 40:20 Sta γ b2i°l3 1.9 ised (6)- 0.05 io.oi 11-3 12.0 10.08 0.83 0 2.55 207.9 148.8 (2) 0.76 (6) 80 20 48:20 Unstabilised (12) 10.05 130.3 iθ.03 2.10 0 2.27 141.0 (6) 112.4 ( 3 ) 1.71 (3) 80 20 48:20 Stabilised (9) 40.02 In.3 128.3 10.20 RBSN •Jt Gain Strength MPa Oxidation * Density As After 100 h wt gain Nitrided 1000 * C Oxidation g/πβ/h Silicon Powder, Unmilled, Unsieved 1.28 61.75 (12) 2.28 357.5 (6) 227.1 (^) 0.66 Silicon Powder, Unmilled, Sieved 2.22 328.8 231.7 (6) 0.92 (6) 5.5 urn iθ.05 156.0 Ϊ28.8 10.20 Silicon Powder, Milled 1.28 60.9 2.27 237.3 152.0 (3) 0.94 (3) 10.04 128.9 in .9 10.06 Table 2 shows the sample average results of measurements on both stabilised (Y 0 ) and unstabilised zirconia employed in the SN/Z compounds. The numbers of samples used are shown in the strength columns. In addition to the Table 1 measurements of disc thickness, density and strength there are also given measurements of disc shrinkage, strength after 100 hours oxidation at 1000 * C and the oxidation weight gain. In addition there are given the starting relative weights of silicon and zirconia (column 2) needed to give the nitrided proportions shown in column 1 (2/3 increase in weight of silicon when nitrided) . The silicon and zirconia powders were milled in propan-2-ol with tungsten carbide balls for 3 days. Pressed discs were then multistage reaction bonded in nitrogen as before, after burnout of an organic binder which was added to yield crack-free die pressings. The percenta-ge disc shrinkage figures were obtained from measurements of the disc diameterβ before and after reaction bonding. The results show a significant shrinkage in the zirconia rich composition (10,1 SN, 90J Za) . The measurements show that oxidation, under the test conditions used, reduces the ceramic strength by the order of 20-25J. The compositional variation of shrinkage which was shown in Table 2 to vary from a mean value of about 3-4? for 90J zirconia compounds to zero for zirconia proportions of less than 80J was further investigated by the sample measurements shown in Table _3_. Twelve discs were used for each of the five compositions shown with half the number being tested as nitrided and half after oxid¬ ation as for the Table 2 samples. The results show that shrinkage on nitriding is eliminated at a threshold ratio of 18 parts silicon nitride to 82 parts of zirconia and the compound ceramic is dimen- sionally stable during consolidation for higher proportions of silicon nitride. Thus with only a minimum of 18% by weight of silicon nitride in the binary composition the dimensional stability of the ceramic is equivalent to that of reaction-bonded silicon nitride ceramics. Table 3 HIGH ZIRCONIA CONTENT MATERIALS - SHRINKAGE ELIMINATION THRESHOLD EXPERIMENT Si3N4/Zr02 Si/Zr Wt Diameter Strength Strength (BL) Ratio Ratio Gain Change Density As Fired Oxidised SN Za Used % ■ % 100h 1000 β C 10 90 6:90 3.01 -3.13 3.60 89.1 81.6 10.10 10.20 £0.0212 £5.65 £4.56 12 88 7.2:88 4.07 -2.18 3-45 ' 77.4 75.7 10.08 £0.12 £0.0312 £5.66 £5.16 14 " 86 8.4:86 4.78 -0.79 3.32 • 70.8 54.7 10.12 £0.12 £0.0312 £5.36 £4.26 16 84 9.6:84 5.57 -0.41 3-31 74.9 68.5 10.10 £0.04 £0.0212 £5.66 £2.76 18 82 10.8:82 5.83 0 3.21 68.9 60.0 1010 - £0.0112 £2.96 £2.56 A batch of 12 discs was prepared in each of five compositions. Subscript = number tested Table 4 Variation with Temperature of the Thermal Diffusivity for Various Silicon Nitride and Binary Silicon Nitride/Zirconia Cera¬ mics. C p __ρ p p Material(density) Thermal Diffusivity (10~ m /s _=_1θ " cm /s=mm /s 25 100 200 400 600 800 1000"C 10SN 90Za 1.014 0.932 0.833 0.669 0.552 0.481 0.457 (3-70) 20SN 80Za 0.851 0.781 0.698 0.565 0.480 0.441 0.449 (3.3D 40SN 60Za 0.946 0.885 0.813 0.695 0.614 0.568 0.559 (3.08) 60SN 40Za 1.047 0.995 0.932 0.826 0.749 0.699 0.678 (2.77) 80 SN 20Za 1.227 1.196 1.154 1.076 1.002 0.932 0.867 (2.43) 20SN 80Za Stab 0.241 0.236 0.230 0.219 0.211 0.204 0.200 (2.41) 66SN 34Za Stab 0.994 0.968 0.935 0.874 0.818 0.768 0.724 (2.20) 80SN 20Za Stab 1.239 1.187 1.123 1.015 0.932 0.874 0.842 (2.24) 100 SN 3.022 2.834 2.607 2.229 1-956 1.786 1.720 die pressed (2.3D 100 SN 5.509 5.119 4.646 3.861 3.291 2.937 2.798 isostatically pressed (2.49) Sintered SN 13.06 11.77 10.22 7.711 5.989 5.053 4.904 (8Y203 1Mg0) (3.17) Thermal diffusivity measurements using a laser flash diffu¬ sivity method are given in Table 4 for a number of different silicon nitride and binary silicon nitride/zirconia ceramics. The thermal diffusivity was measured at a number of temper¬ atures from 25 β C to 1000 β C. Included in the table are thermal diffusivity measurements for the following three silicon nitride materials: a) sintered silicon nitride (SSN) (8 Ya/Ma - 8Y 0 iMgO, density 3.17); b) isostatically pressed (IP), density 2.49; and c) die pressed (DP), density 2.31. The results show that thermal diffusivity is lower for the binary ceramics, with both stabilised and unstabilised zirconia, than for silicon nitride alone throughout the temperature and composition ranges. The results for the three silicon nitride compositions are also presented in Figure 1, where the curves 10, 12 and 13 represent SSN, RBSN-IP and RBSN-DP respectively. In addition, there are shown data curves 11 and 14 for a reaction bonded silicon nitride and a calcium oxide partially stabilised zirconia respectively. Figure 2 shows a comparison of the thermal diffusivity measure¬ ments for various compositions of silicon nitride/unstabilised zirconia (SN Za) and silicon-nitride/stabilised zirconia (SN Za ST) and data for two typical zirconia ceramics. The numerical prefixes represent the proportions by weight. The two zirconias, 5 CaO PSZ and 7.6 CaO PSZ are represented by the curves 20 and 21. Three stabilised binary zirconia ceramics are represented by curves 22-24 having respective percentages by weight of silicon nitride of 20, 66 and 80. The remaining curves 25-29 are for unstabilised zirconia mixtures as indicated. These data show that the thermal diffusivity of the strongest silicon nitride zirconia material (60 SN 40 Za) represented by curve 28 is only slightly greater than that for typical zirconia ceramics (curves 20, 21). Thermal conductivity, which is the product of thermal diffu¬ sivity, specific heat and density is calculated in the following table for 7.6 CaO PSZ and 60 SN 40 Za at 100 β C: Thermal Specific Density Thermal Diffusivity heat kg/m 3 Conduct 10 "6 m 2 /s J/kg W/mK 7.6 CaO PSZ . 0.89 0.51 5.5 2.5 60 SN 40Za 1.0 0.48» 2.77 1.3 •interpolated on the basis of 0.51 for zirconia and 0.8 for Si ^ (at 100 * C) giving 0.684 for a fully dense material and the measured density of 2.77 divided by the theoretical density of 3.95 3 3 x 10 J for a 60? mixture of Tetra ZrO- (6.1 x 10 J ) and Si_N. (3.2 x 10 J ), giving a theoretical density of 70.1?. Even lower diffusivities and conductivities are possible with lower density materials (curves 25-27). Of these three materials 20 SN 80 Za and 40 SN 60 Za retain the negligible dimensional change aspect characteristic of reaction bonded silicon nitride ceramics, however their strengths are not so great as for the 60 SN 40 Za material. Discussion Material made from a 6.25w/o silicon powder - unstabilised zirconia powder disc pressed material according to the invention gives a ceramic of 10 Si N - 90 ZrO (10SN 90Za) with useful w strength and only 3-5? shrinkage. By adding as little as 13 /o silicon powder to unstablised zirconia to yield a 20 SN 80 Za ceramic shrinkage was found to have been eliminated and the material also had a useful strength level. The limiting silicon nitride content above which no shrinkage occurs was determined to be in the range 18-20?. Shrinkages of 15? and above are normal in the con¬ ventional methods of consolidating oxide ceramics by sintering and these shrinkages make the processing of these materials to a precise shape expensive and difficult. Thus for SN percentages above the 18-20? limit the invention overcomes the prior art dimensional shrinkage problem. Zirconia ceramics are almost always made by adding a stabili¬ sing oxide such as calcium or magnesium oxide; recently yttrium oxide has been extensively employed for this purpose. The stab¬ ilizer produces cubic phase zirconia, which does not undergo a destructive phase change in the temperature range 950-1220 * C, and materials can therefore be cycled from high temperature to low temperature without cracking or suffering fragmentation. Surprisingly the inventor has discovered that silicon nitride reaction bonded zirconia ceramics have useful strengths despite the absence of a stabilising oxide. Indeed when yttria-stabilised zirconia powder was used instead of ordinary zirconia, material with greatly inferior properties was obtained; for example the strength of 20 SN 80 Za using stabilised zirconia was found to be only about 20? that of using unstabilised zirconia and the strength of 80 SN 20 Za percentage for these materials was measured to be 52?. Table 5 Material Mean Strength ? Degradation of . MPa Strength After 1000 β C Oxidation 10SN 90Za 116.4 BL 26.5 20SN 80Za 83.5 BL 32.1 20SN 80Za STAB 16.4 BL 20.1 40SN 60Za 140.9 BL 23-6 60SN 40Za 212.1 BL 27.1 77 SN 23Za STAB 143.0 BL 37.1 80SN 20Za 207.9 BL . 28.4 80SN 20Za STAB 141.0 BL 20.3 100SN 0M, US 357.5 BL 36.5 (die pressed) 100SN UM, S (DP) 328.8 29.5 100SN M, (DP) 237.3 35.9 10SN 90Za 89.1 8.4 12SN 88Za 77.4 2.2 14SN 86Za 70.8 8.6 16SN 84Za 74.9 8.5 18SN 82Za 68.9 12.9 Table 6 ? Proportions ? Proportions Calculated by Weight by Volume Conductivity W/mK SN Za SN Za 10 90 17.55 82.45 2.03 20 80 32.39 67.31 2.90 40 60 56.07 43.93 12.59 50 50 65.69 34.31 15.49 60 40 74.17 25.83 18.39 80 20 • 88.48 11.52 24.21 33. .41 66.59 49 51 4.39 35, .12 64.79 51 49 11.20 The oxidative degradation of the strength of reaction bonded silicon nitride is well known. It has been shown that reductions as great as 40? in room temperature strength can occur after oxidation, although this effect can be mitigated by additions of oxides which react with silicon to reduce the formation of cristobalite, the destructive phase change of which at about 270*C is responsible for the strength loss. In Table 5 the results of oxidative degra¬ dation tests (100 hours at 1000 * C) are summarised. In general the degradation is less than the extreme quoted above of 40?, with a tendency being apparent for oxidative degra¬ dation of subsequent room temperature strength to be less severe with lower Si.N j , content Si N^/ZrO binary materials. In Table 6 the calculated thermal conductivities based upon the thermal diffusivity measurements are summarised. The strength tests on the silicon materials shown in the earlier Tables were evaluated with a single ball load technique. Copending UK Patent Application No describes a novel multi- ball ring-load technique. This multi-ball ring technique gives more realistic strength measurements made over a loading circle instead of the point loading of the single ball technique previously used. In one arrangement used here for making ring loading strength measurements (RL),a ring of twelve 1 mm balls was used, giving a 3.86 mm diameter loading circle while with the ball loaded technique (BL) a ball of 20 mm diameter was used. Comparative tests for the two techniques applied to the same material show the ratio of ball loaded (BL) to ring-loaded (RL) strengths is 1.89. This ratio needs to be considered when comparing the following tabulated strength measurements. Table 7 shows ring load strength measurements on three different composition die-pressed disc silicon nitride/zir¬ conia materials. As can be seen the 50:50 and 60:40 W/0 composi¬ tions are much stranger than the 20:80 composition. Table 7 Further Comparison of 20/80, 50/50 and 60/40 Silicon Nitride/ Zirconia (unstabilised) Tested by a Multiball (12) Ring Load Method. .Composition W/0 Weight Density Strength Silicon Nitride Zirconia Gain (No of samples) 20 80 5.60 3-58 30+2.9(10) +0.07 +0.01 50 50 22.05 2.97 104/4+7.7(10) +0.34 +0.02 60 40 30.07 2.79 125.8+9.6(10) +0.38 +0.02 Whilst the strength of the strongest binary Si N j VZrO, mater¬ ials were only approximately two thirds of that of 100? Si N,, reaction bonded materials (e.g. 60SN 40Za and 80SN 20Za), these strengths may very well be suitable for some mechanically loaded applications in which an insulating material is required or for applications carrying out very small thermal stresses such as abrodable coatings for use in gas turbines eg to improve blade/ shroud gas sealing. The measured strength value of about 100 MPa is about half that obtained with many commercial types of alumina (when tested in the ring - loaded mode) . This could probably be improved usefully if isostatic pressing were used instead of die pressing to give im¬ proved density in powder compacts before reaction-bonding: all the data herein, except where otherwise indicated, is for materials compacted more conveniently (but less effectively) by die-pressing with a suitable binder addition e.g. polyethylene glycol or poly-vinyl pyrrolidon. Figure 3 is a graphical presentation of results showing how the strength of silicon nitride/zirconia reaction-bonded materials gradually diminishes with increasing zirconia content from 222.1 MPa (ref No 30) for pure silicon nitride (zero zirconia) down to 55.9 MPa for 90% zirconia content. These results were measured by biaxial flexure of test discs using ring-loaded testing with a 4 mm diameter ring of six 2 mm diameter balls. The strengths of all of these materials are greater than comparable aluminium titanate materials which have strengths up to 40 MPa when measured by the same means. In addition to the greater strength of the silicon nitride/zirconia materials, the very low contraction on manufacture for zirconia percentages of up to 80 % by weight (zero up to 60 % by weight of zirconia) make these materials very attractive as very little finishing machining is needed.

Claims: \ (mm Clai g 1. Aceramic material comprising a reaction bonded composite of silicon nitride and zirconia. 2. A ceramic material as claimed in claim 1 wherein the zirconia is unstabilised. 3. A ceramic material as claimed in claim 2 wherein there is at least 18% silicon nitride. 4. Amethod of making a ceramic material comprising the steps of mixing together silicon and zirconia powders, compacting the powder mixture, and nitriding the compacted powder by heating in an atmosphere of nitrogen. 5. Amethod as claimed in claim 4 wherein the nitriding step is a multistage process wherein the compacted powder is successively heated: a) for 10 hrs at 1150°C b) for 5 hrs at 1200°C c) for 5 hrs at 1250°C d) for 10 hrs at 1300°C e) for 60 hrs at 1350°C and then f) for 10 hrs at 1425°C. . 6. Amethod as claimed in claim 4 or 5 wherein the silicon powder has a mean particle size of about 8 microns. 7. A method as claimed in any one of claims 4 to 6 wherein the powder mixture is pressed to a pressure of about 100 Ibf/inch^. 8. A method as claimed in claim 7 wherein the mixture is pressed in a double acting die. 9. A method as claimed in claim 7 wherein the mixture is hydrostatically pressed.

Inventors: GODFREY DAVID JAMES, GODFREY, DAVID, JAMES
Applicants: SECR DEFENCE BRIT, THE SECRETARY OF STATE FOR DEFENCE IN HER MAJESTY'
Application year: 1987
Main classification: C04B35
IPC codes: C04B35