A new possibility in tailoring microstructures and properties of silicon nitride ceramics is demonstrated by the use of fine beta -Si3N4 powders. Fine powders were produced by a plasmachemical synthesis and by crystallisation of amorphous Si3N4 in the presence of an oxide nitride liquid. A wide range of microstructures can be obtained depending on sintering and powder characteristics. At low sintering temperatures microstructures consisting of nanosized, equiaxed grains were obtained, whereas at high temperatures or long sintering times microstructures consisting of fine needle like grains were formed. Microstructural analysis revealed a high anisotropic grain growth of the beta grains in the fine grained materials.

AbstractA new possibility in tailoring microstructures and properties of silicon nitride ceramics is demonstrated by the use of fine b-Si3N4powders. Fine powders were produced by a plasmachemical synthesis and by crystallisation of amorphous Si3N4in the presence of an oxide nitride liquid. A wide range of microstructures can be obtained depending on sintering and powder characteristics. At low sintering temperatures microstructures consisting of nanosized, equiaxed grains were obtained, whereas at high temperatures or long sintering times microstructures consisting of fine needle like grains were formed. Microstructural analysis revealed a high anisotropic grain growth of the b grains in the fine grained materials.

The use of silicon nitride materials in wear applications such as ball bearings or seals is often limited by the relatively high friction coefficient under nonlubricating conditions. Very fine homogeneous materials were produced using fine-grained B-Si3N4 powders. The microstructures of the materials were observed and correlated with the wear behaviour. The dry friction coefficients and wear were measured for self-mated sliding couples. The materials produced have a grain size of 0.1 mum and exhibit friction coefficients of up to 0.1, whereas conventional materials have friction coefficients in the range of 0.5 under the same conditions. Reduced wear rates were also observed.

Concepts for the microstructure development of ultrafine Si3N4 materials were developed. During unlubricated sliding of Si3N4 against Si3N4, coefficients of friction 0,1...0,15 (fretting wear) could be measured on beta-Si3N4 materials with special microstructures, which are characterised by anisotropic beta-Si3N4 grains with grain diameters d(50) < 0,2 mu m, maximal grain lengths < 10 mu m, and special compositions of glassy phase. The manufacture of the special microstructures is only successful when Si3N4 powders produced using the plasma method, or mixtures from heat treated plasma powder and ultrafine commercial alpha-Si3N4 powders are used.

The improvement of the properties of beta-Si3N4-ceramics is connected with the optimization of sintering and microstructure The sintering behaviour of Si3N4-materials synthesized from different powders was investigated by dilatometric measurements, by thr analysis of the phase content and of distribution of pores and sintering additives during the sintering process. The shrinkage can be correlated with the characteristics of powders, the microstructure of the green bodies,the content of the sintering additives and the interaction with the sintering atmosphere. Decisive factors for the formation of the microstructure are, in addition to the sintering cycle and the beta-Si3N4-content of the initial powder, the grain size of the beta-Si3N4-grains.