W Dressler, R. Riedel nuclei followed by p-Si, N4 grain coarsening" mined by X-ray analysis 2)from 6 5N um and (iii) the coalescence of ystals, (undoped)to 9 3 and 12. 1 N um. Under the which is of limited importance due to the low assumption that B-Si, N4 nucleation is negligible volume diffusion in si this was explained by a higher number of grow In this context beneath others particularly ing grains during aB-transformation if B-Si, N two contradictory requirements are to be taken doping is applied. The influence of seeding of into account. On the one hand optimal densifi- fine grained a-rich UBE SN-E10(E10, mean cation by using high volume fractions of addi- crystallite size r=0-06 um) with B-Si, NA Denka tives having low melting temperatures and low (mean crystallite size r=0. 14 um) is shown in iscosities. On the other hand high temperature Fig. 6. The quantitative analysis of the micro- resistance, which is deteriorated by the soften- structure was performed by measuring the two ing of a secondary phase. Additionally, the dimensional size shape distribution of more microstructural development has to be control- than 2000 grains on polished and plasma etched led with respect to high toughness and specimen and by the subsequent stereological strength. In general, it is impossible to meet computation of the three-dimensional grain size all requirements and therefore silicon nitride shape distributions. The size-shape distribu- ceramics have to be designed for specific appli- tions (weighted by volume)reveal that the cations. Some basic principles used to control B-doping leads to a decrease of the volume frac- the microstructural development and to tion of grains having a length smaller than improve the mechanical properties of silicon 0.5 um from 11. 5 vol%(E10)to 0. 5 vol% nitride ceramics are presented below (Denka). Simultaneously, the mean grain length and grain diameter increases from 0-36to 4.1.2 Microstructural design 0-80 um and from 0- 12 to 0- 45 um, respectively The tailoring of Si,NA microstructure can be Additionally, low B-doping(Denka/E10 4/96) used to produce high fracture toughness com- leads to an enhanced grain growth in the length bined with high reliability and strength if con- direction, but to a decrease in aspect ratio of trolled grain growth can be achieved. Otherwise the coarser grains owing to the large grain abnormally grown B-Si3N4-crystals act as crack width of the added B-particles. Higher amounts initiation sites and thus reduce the strength of of B-nuclei(Denka/E10 20/80) results in a the material. Herewith it has to be taken into reduction of the maximum grain length and account that the microstructural development aspect ratio USing pure B-Si3N4(Denka)start- of Sia na depends on the starting powder charac- ing powders an equiaxed microstructure is pro- teristics, the used additive system and the sin- duced possessing a low mean aspect ratio of 1. 8 tering conditions. Additionally, the morphology in comparison to specimens sintered from a-rich of the B-Si, N4-crystals in the final microstruc- powder(E10) having a mean aspect ratio of 2.7 ture is determined by their growth anisotropy. Moreover, an overall grain coarsening was The preferred growth direction is perpendicular observed. From this investigation it was con- to the basal plane())of the formed hex- cluded that, if the B-Si3N4 nuclei density agonal prisms. The influence of the intrinsic reaches a certain value, depending on the grain powder properties on the final microstructure size distribution of the starting powder, a dis- was shown by several authors in different addi- solution of the smaller B-Si3 N, particles in the tive systems. 3 B3, 85- Particularly, the phase liquid secondary phase occurs at an early stage ratio and crystallite size of a-and B-phase in the of a/B-transformation resulting in a coarsening starting powder have been found to be the key of the final microstructure. 3 Additionally, it factors in the microstructural development. was deduced that the often observed abnormal On the one hand the doping of coarse a-Si3N4- grain growth in Siana is due to a kinetic and powders (UBE SN-ESP, mean crystallize size energetic growth advantage of B-Si3N4-crystals r=0-10 um) with coarse B-Si3N2-nuclei(Denka, having a large basal plane(1001)). .These mean crystallite size r=0 14 um) led to a grain results show that tailoring of the final Si3 na refinement in the sintered ceramics. The grains microstructure becomes possible by controlling er unit area increased from 0 56N um for the B-Si3Na-nuclei density, morphology and size the undoped material to 0 72 and 0-86N um distribution in the starting powder. In order to by increasing the B-Si3Na-nuclei density(deter- optimize mechanical properties of the final20 W. Dressier, R. Riedel nuclei ~° followed by//-Si3N4 grain coarsening "3 and (iii) the coalescence of //-Si3N4 crystals, which is of limited importance due to the low volume diffusion in Si3N 4. In this context beneath others particularly two contradictory requirements are to be taken into account. On the one hand optimal densifi￾cation by using high volume fractions of addi￾tives having low melting temperatures and low viscosities. On the other hand high temperature resistance, which is deteriorated by the soften￾ing of a secondary phase. Additionally, the microstructural development has to be control￾led with respect to high toughness and strength."-" In general, it is impossible to meet all requirements and therefore silicon nitride ceramics have to be designed for specific appli￾cations. Some basic principles used to control the microstructural development and to improve the mechanical properties of silicon nitride ceramics are presented below. 4.1.2 Microstructural design The tailoring of SigN4 microstructure can be used to produce high fracture toughness com￾bined with high reliability and strength if con￾trolled grain growth can be achieved. Otherwise abnormally grown //-Si3Na-crystals act as crack initiation sites and thus reduce the strength of the material." Herewith it has to be taken into account that the microstructural development of Si3N 4 depends on the starting powder charac￾teristics, the used additive system and the sin￾tering conditions. Additionally, the morphology of the //-Si3Na-crystals in the final microstruc￾ture is determined by their growth anisotropy. The preferred growth direction is perpendicular to the basal plane ({001}) of the formed hex￾agonal prisms. "4 The influence of the intrinsic powder properties on the final microstructure was shown by several authors in different addi￾tive systems.' "' 3. ~3. ~5-,,, Particularly, the phase ratio and crystallite size of a- and//-phase in the starting powder have been found to be the key factors in the microstructural development.' "'3 On the one hand the doping of coarse a-Si3N a￾powders (UBE SN-ESP, mean crystallize size r=0"10 pm) with coarse //-Si3N4-nuclei (Denka, mean crystallite size r=0"14 #m) led to a grain refinement in the sintered ceramics. The grains per unit area increased from 0.56 N pm 2 for the undoped material to 0.72 and 0.86 N #m 2 by increasing the fi-Si3N4-nuclei density (deter￾mined by X-ray analysis '2) from 6-5 N #m -3 (undoped) to 9.3 and 12.1 N pm 3. Under the assumption that fl-Si3N4 nucleation is negligible this was explained by a higher number of grow￾ing grains during a/fl-transformation if fl-Si3N4 doping is applied. The influence of seeding of fine grained a-rich UBE SN-E10 (El0, mean crystallite size r=0.06 #m) with fl-Si3N4 Denka (mean crystaUite size r=0.14/~m) is shown in Fig. 6.'3 The quantitative analysis of the micro￾structure was performed by measuring the two￾dimensional size shape distribution of more than 2000 grains on polished and plasma etched specimen and by the subsequent stereological computation of the three-dimensional grain size shape distributions. 92 The size-shape distribu￾tions (weighted by volume) reveal that the //-doping leads to a decrease of the volume frac￾tion of grains having a length smaller than 0.5pm from 11.5vo1% (El0) to 0.5vo1% (Denka). Simultaneously, the mean grain length and grain diameter increases from 0.36 to 0.80 pm and from 0"12 to 0-45 #m, respectively. Additionally, low //-doping (Denka/E10 4/96) leads to an enhanced grain growth in the length direction, but to a decrease in aspect ratio of the coarser grains owing to the large grain width of the added//-particles. Higher amounts of //-nuclei (Denka/E10 20/80) results in a reduction of the maximum grain length and aspect ratio. Using pure //-Si3N 4 (Denka) start￾ing powders an equiaxed microstructure is pro￾duced possessing a low mean aspect ratio of 1.8 in comparison to specimens sintered from a-rich powder (El0) having a mean aspect ratio of 2.7. Moreover, an overall grain coarsening was observed. From this investigation it was con￾cluded that, if the fl-Si3N 4 nuclei density reaches a certain value, depending on the grain size distribution of the starting powder, a dis￾solution of the smaller fl-Si3N 4 particles in the liquid secondary phase occurs at an early stage of a///-transformation resulting in a coarsening of the final microstructure.'" ,3 Additionally, it was deduced that the often observed abnormal grain growth in Si3N 4 is due to a kinetic and energetic growth advantage of //-Si3Na-crystals having a large basal plane ({001}). ''''3 These results show that tailoring of the final Si3N4 microstructure becomes possible by controlling the//-Si3Nn-nuclei density, morphology and size distribution in the starting powder. In order to optimize mechanical properties of the final