Silicon-based non-oxide structural ceramics gas pressure sintering. In contrast to that, the The temperature dependence of the thermo- strength and reliability of ceramics produced mechanical properties of Si3 N4-ceramics is the from a gas phase derived Si, n4-powder(BGSN: key factor for high temperature engine and gas GP 15/4, Bayer AG, Germany) deteriorate turbine applications. In this context the sinter- during prolonged high temperature gas pressure ing aids necessary for full densification are sintering. Fractographic investigations showed unfavourable due to decomposition or softening that pores act as crack initiation sites in the at higher temperatures leading to enhanced case of the materials sintered at 1835.C and creep and oxidation as well as to strength that mainly large grains of about 100 um cause reduction. In order to overcome these serious failure after heat treatment at 1900C pointing problems different methods have been out that their grain size distribution is respon- developed: (i reduction of impurity and sible for the strength variation. It was found secondary phase amount, 0.41, 102(ii) using addi from quantitative microstructural analysis that tives with high softening temperatures and visc the size distribution of large grains( fracture osities, Io.(ii) reduction of secondary phase origin)in the case of the UBE E10(1900C, amount by incorporation of Al2O into B-Si, N4 360 min) ceramics is narrower in comparison to leading to the formation of B-Si6-AL,ON, the BGSN(1900C, 360 min)derived material during sintering 4. los and,(iv) devitrification of generating the higher reliability(m=46)of the residual secondary phase after densification in UBE E10 ceramic. These findings led to the order to achieve a more rigid secondary phase conclusion that the control of abnormal grain skeleton. 42 106, 10 The densification of cera- growth described in the previous section is mics according to (i)and () has to be indispensable for the optimization of both enhanced by applying external stresses by strength and reliability means of hot pressing and HIPing, respectively UBE E-10 BGSN oH1835℃30 0H1835℃ m=13,5±3.5 m=198士72 0=1134±115MPa a=1105±101MPa 9001.1001301500500 9oo1.1001.3001.500 Strength [] MPa UBE E-10 BGSN oH 1900C, 360 min oH1900c,300mn m=46,0士132 m=184±53 G=902士21MPa =814±57MPa 500 9001.10013001.50 4 7009001.10013001.500 Strength [MPal Strength PAl Fig. 8. Four-point-bending strength distributions of gas pressure sintered(10 MPa N2, 10.7 wt%Y,O,+3 6 wt% Al,OJ) Si Na-ceramics with fine (1835C, 30 min ) and coarse(1900C, 360 min)microstructures
Silicon-based non-oxide structural ceramics 23 gas pressure sintering. In contrast to that, the strength and reliability of ceramics produced from a gas phase derived Si3N4-powder (BGSN: GP 15/4, Bayer AG, Germany) deteriorate during prolonged high temperature gas pressure sintering. Fractographic investigations showed that pores act as crack initiation sites in the case of the materials sintered at 1835°C and that mainly large grains of about 100 gtm cause failure after heat treatment at 1900°C pointing out that their grain size distribution is responsible for the strength variation. It was found" from quantitative microstructural analysis that the size distribution of large grains (fracture origin) in the case of the UBE El0 (1900°C, 360 min) ceramics is narrower in comparison to the BGSN (1900°C, 360 min) derived material generating the higher reliability (m=46) of the UBE El0 ceramic. These findings led to the conclusion that the control of abnormal grain growth described in the previous section is indispensable for the optimization of both strength and reliability. The temperature dependence of the thermomechanical properties of Si3N4-ceramics is the key factor for high temperature engine and gas turbine applications. In this context the sintering aids necessary for full densification are unfavourable due to decomposition or softening at higher temperatures leading to enhanced creep and oxidation as well as to strength reduction. In order to overcome these serious problems different methods have been developed: (i) reduction of impurity and secondary phase amount, 4'''4'' IO2 (ii) using additives with high softening temperatures and viscosities,4,,. ,,,3 (iii) reduction of secondary phase amount by incorporation of A1203 into fl-Si3N4 leading to the formation of fl-Si6_~/zOzNs_z during sintering '''4- ,o5 and, (iv) devitrification of residual secondary phase after densification in order to achieve a more rigid secondary phase skeleton. 3"42" '''6" '''8 The densification of ceramics according to (i) and (ii) has to be enhanced by applying external stresses by means of hot pressing and HIPing, respectively. 2 0 13. • i,.- ' - 2 v v _c 4 c -6 5OO UBE E-IO 1835"(3, 30min J 0=1134 _ 115 MPa I I I I I I 700 900 1.100 1.3001.500 Strength [MPa] 2 0 13.- • 1-" ' - 2 -...... '1-- c -~-4 -6 5OO 1835"C, 30 min [] [] 7,2 , /' I°=11 105 -+ 101, MPa 700 900 1.100 1.3001.500 Strength [MPa] 2 0 n T-- ' - 2 v "l-'. v c_ 4 t.-- Fig. 8. -6 5OO UBE E-IO 19oo'c, 360 rnin i/ [] m = 46,0 __. 13,2 0=902 __. 21 MPa I I I I 700 900 1.100 1.3001.500 Strength [MPa] 0 13_ .i.. ' - 2 v "l"- v r" ~-4 -6 5OO m = 18,4 __. 5,3 o=814 __. 57 MPa I I I I 700 900 1.100 1.3001.500 Strength [MPa] Four-point-bending strength distributions of gas pressure sintered (10 MPa N2, 10.7 wt% Y20~ + 3'6 wt% A1203) Si,N4-ceramics with fine (1835°C, 30 min) and coarse (1900°C, 360 min) microstructures. '~
W Dressler R. riedel The temperature dependent three-point erties of various sintered silicon nitrides includ bending strength of a B-SiAION-ceramics with ing two commercially available products and a z=0. 5 is shown in Fig. 9. 9 The strength of SiAlON-ceramics are summarized in Table 3 about 830 MPa remains constant up to 1300.C pointing out that nearly no softening of grain 4.2 Silicon carbide ceramics boundaries occurs. The effect of crystalli the amorphous grain boundary phase of a 4.2. 1 Sintering of Sic-ceramics Y203/Al2O doped Si,N, ceramics on the high In order to densify Sic ceramics various sinter temperature strength is depicted in Fig. 10. The ing methods like pressureless sintering, hot increase of mellilite(Y Si,O, NA) crystallized in pressing, HIPing and sinter HIPing are used the grain boundary from 20 to 90% results in an Except for pressureless sintering these methods increase of high temperature strength from 650 have the disadvantage of being cost intensive to 1020 MPa. 7 The thermo-mechanical prop- 1400 1000 800 a.1000 乏 800 600 600 400 400 200H·sAN(2=0.5) 90 Mellilite 0 0200400600800100012001400 0200400600800100012001400 Temperature [c] Temperature [c] Fig. 10. Influence of secondary phase crystallization ig.9. Temperature dependent three-point-bending (metllilite: YSi, O, N, )on high temperature strength of strength of B-sis.sALsOI-SN Y2OvAl,O doped si, N,-ceram Table 3 processes materials S(SN-SA8, of thermo-mechanical properties of Si N,and SiAIon ceramics produced by different densification to Komeya and Matsui. 2 As a comparison the properties of two industrially produced Si, N NGK Insulators Ltd, Nagoya, Japan; New Material, Kyocera Corp, Kyoto, Japan)are given Property Silicon nitride SiAION Reaction Pressureless or Hot SN-88, New material NGK Kyocera Thermal conductivity(Wm K 2 6-20 29-35 29-3·5 3-4 13-18 23-30 3·0-35 thermal expansion Young’ s modulus(GPa) Poissons ratio 024-0-26 320323 Bending strength(MPa) 150 400-1000800-1050 1000°C 23 350-1000 800-1000 770 1200°C 170-307 250-800 250-950 770 703 760 606 Fracture toughness 4-7 Critical thermal 350-600 400-800 800-900 500-600 shock temperature (C: ATo Oxidation resistance(mg cm 0-5,1400°C,014,1500°C 1000h 100h
24 W. Dressier, R. Riedel The temperature dependent three-point bending strength of a //-SiAlON-ceramics with z=0.5 is shown in Fig. 9. '0° The strength of about 830 MPa remains constant up to 1300°C pointing out that nearly no softening of grain boundaries occurs. The effect of crystallizing the amorphous grain boundary phase of a Y203/m1203 doped Si3N4 ceramics on the high temperature strength is depicted in Fig. 10. The increase of mellilite (Y2Si303N4) crystallized in the grain boundary from 20 to 90% results in an increase of high temperature strength from 650 to 1020 MPa."'7 The thermo-mechanical prop- 1000 800 t~ Q. 600 t-- t- 400 ,o 09 200 Fig. 9. iIIiIIIIIIllllIIIIIIIIIIIIIIIl~ q --o--SiAION (z = 0.5) I , I I I I I 0 200 400 600 800 1000 1200 1400 Temperature [°C] Temperature dependent three-point-bending strength of/~-Sis.~AI,,~O,,~N7 ~.'" erties of various sintered silicon nitrides including two commercially available products and a SiAION-ceramics are summarized in Table 3.' '" 4.2 Silicon carbide ceramics 4.2.1 Sintering of SiC-ceramics In order to densify SiC ceramics various sintering methods like pressureless sintering, hot pressing, HIPing and sinter HIPing are used. Except for pressureless sintering these methods have the disadvantage of being cost intensive 1400 1200 t~ 1000 800 t- ¢- 600 ,= ",- 400 CO mIim ~||II|||II |||II|~ ~ --e--20 % Mellilite I 200 ~90 % Mellilite 0 I I I I I I 0 200 400 600 800 1000 1200 1400 Temperature [°C] Fig. 10. Influence of secondary phase crystallization (metllilite: Y2Si303N4) on high temperature strength of Y203/AI203 doped Si3Nn-ceramics."'7 Table 3. Range of thermo-mechanical properties of SigN4- and SiAION ceramics produced by different densification processes according to Komeya and Matsui. ''2 As a comparison the properties of two industrially produced Si3Nnmaterials (SN-88, NGK Insulators Ltd, Nagoya, Japan; New Material, Kyocera Corp., Kyoto, Japan) are given Property Silicon nitride SiAION Reaction Pressureless or Hot SN-88, New material, sintering gas-pressure pressing NGK Kyocera sintering Density (g cm 3) Thermal conductivity (W m Coefficient of (10 "°C ') thermal expansion Young's modulus (GPa) Poisson's ratio Bending strength (MPa) RT 1000°C 1200°C 1400°C Fracture toughness (MPa.m'/2) Critical thermal shock temperature (°C: AT,.) Oxidation resistance (mg cm 2) 2' 1-2"6 2'9-3 "5 2"9-3"5 3 "5 3 "4 3"0-3" 15 K ') 2"6-20 13-18 29-32 70 59 -- 2'3-3"0 3'0-3"5 3"1-3"3 3"4 3"0 2'5-3"0 100-200 240-330 320 300 318 230 0"24-0"26 0"24-0"28 0'26 0"26 0"28 0"29 150-295 400-1000 800-1050 790 861 360-800 160-300 350-1000 800-1000 770 801 350-800 170-307 250-800 250-950 770 703 -- -- -- -- 760 606 300-800 3-4 4-7 6-7 7 6 2-4 350-600 400-800 800-900 1200 -- 500-600 0'5, 1400°C, 0"14, 1500°C, 1000 h 100 h