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 respon￾sible 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 thermo￾mechanical properties of Si3N4-ceramics is the key factor for high temperature engine and gas turbine applications. In this context the sinter￾ing 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 addi￾tives with high softening temperatures and visc￾osities,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 cera￾mics 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. '~