The hardness and fracture toughness of a model two-phase composite consisting of crystalline Si3N4 particles in a matrix of amorphous Si3N4 are examined. The composite is created by heat treating high purity, partially amorphous CVD Si3N4 in N2 for various times and temperatures in order to induce crystallization of the a phase. Microindentation tests at temperatures up to 1200 °C are conducted to evaluate the high temperature hardness and fracture toughness. The role of the microstructure is examined using optical and transmission electron microscopy. Finally, the relationship between the microstructure and the mechanical properties is discussed.

Si3N4 ceramics are excellent metal cutting materials. The cutting behavior can be improved by hard particle dispersion phase hardening and heat treatment so as to reduce glass phase in the grain boundaries. Compared with cemented carbide tools, the cutting life of the composite Si3N4 cutting tool is about 10-100 times longer, and the optimum cutting speed is about 3-10 times faster. It performs well in cutting hardened tools, nickel based alloys, and other hard materials and can sustain shock loads in operations such as milling, planing and other types of interrupted cutting. Applications in various fields show that machining efficiency can be increased by 3-10 times resulting in savings of time, electricity, and machining of 30-70% or even more.

One major goal of automotive engineers actually is to reduce fuel consumption and emissions of present internal combustion engines. First promissing results have been achieved by the substitution of conventional by silicon nitride valves. The weight reduction of about 60 % per valve is corresponding with lower inertia and spring forces, which lead to improved emissions, reduced fuel consumptions as well as high performances of the engines. Applying statistical methods, two advanced silicon nitride materials have been developed, concerning their application as “cool” inlet valves and “hot” exhaust valves, respectively. Using high quality powders and combined optimum processing parameters, 4-point-bending strength of more than 1200 MPa (room temperature) and 870 MPa (1000 °C), resp., have been realized. Weibull moduli of more than 30 have been reproduced. Grinding of valves produced from these materials has economically been demonstrated by drastically reducing the machining times. Some results of road tests as well as of simulated engine tests from cooperations with different automotive companies are presented in detail.

Ceramics based on silicon nitride have long been considered promising candidates for high temperature engineering applications due to their excellent refractory properties combined with high strength and good resistance to oxidation. It is well known, however, that the performance of ceramics varies considerably with chemical composition, for example with type and amount of intergranular phase. Especially the evaluation of the high temperature behaviour of the ceramics under varying conditions is therefore important in increasing the knowledge of these materials.

In yttria densified ZrO2-sialon composites, an increase in β′:O′-sialon ratio results in a structural change in the zirconia phase because an increasing amount of nitrogen dissolves in the zirconia structure as the overall composition becomes increasingly nitrogen rich. Both ZrN and nitrogen-containing zirconia (N-ZrO2) are observed in the β′-ZrO2 composite and these phases show a poor oxidation performance above 600°C. However, the introduction of small amounts of O′ phase into the β′-ZrO2 composite has provided an effective oxygen-rich barrier to reduce the diffusion rate of nitrogen in zirconia and therefore O′-β′-ZrO2 composites showed superior oxidation resistance compared with β′-ZrO2 materials at temperatures between 600 and 1200°C.

Corrosion of silicon nitride was studied in boiling nitric acid to examine its feasibility as a drying pan material in the reprocessing of nuclear fuel. Unlike stainless steel (a conventional drying pan material), the weight loss and strength degradation were negligible in the concentration of nitric acid. The corrosion increased with decreasing concentration of nitric acid. At the concentration of 1-6N, the maximum losses of weight and strength were 0.8% and over 40%, respectively, in 200 h. Ionic species dissolved in nitric acid were determined by ICP analysis and were found to be accurately correlated to the weight loss and thickness of the corrosion layer determined by micrography. In the corrosion layer, grain boundary glassy phase was selectively dissolved. Strength loss was correlated to the weight loss and was ascribed to the reduced load bearing area due to corrosion.

Si-Al-O-N ceramics, especially β′-Sialon, are well known as a material offering a very good resistance against aggressive media combined with a good thermal shock behaviour. This combination recommends the Si-Al-O-N ceramics also as coating for products used in applications with high thermal gradients plus corrosion attack.

Therefore this paper presents new developments in the deposition of Si-Al-ON, especially in combination with the aircraft alloy TiAl6V4 as substrate. Investigations of the coating structure in dependence on the deposition parameters were done and the resistance against oxidation and attack of H2SO4 containing media were examined.