The aim of this work was to investigate the sintering behavior of Si3N4-SiC-MoSi2 composites manufactured by the Multi Material Jetting (MMJ) process. Therefore, electrically conductive and insulating Si3N4-SiC-MoSi2 composites were printed using MMJ. Samples were sintered in a gas-pressure furnace at 6 and 31 atm, respectively. The density, microstructure, and crystalline phase composition were investigated, and the results discussed regarding the phase stability of the Si3N4-SiC-MoSi2 composites. The results show that the nitrogen pressure must be set carefully to avoid the formation of Mo5Si3 or Mo(4.8)Si(3)C(0.6 )on the one hand and to prevent the decomposition of the phases into a Si-containing melt on the other.

Wear-resistant, super hard ceramic composites based on cubic boron nitride (cBN) are of great interest to industry. However, cBN is metastable under sintering conditions at normal pressure and converts into the soft hexagonal BN (hBN). Therefore, efforts are being made to avoid this process. Besides short sintering times, the use of coated cBN-particles is a way to minimize this process. Therefore, the thermal stability of TiN coated cBN powders in high purity argon and nitrogen atmospheres up to temperatures of 1600 °C was investigated by thermogravimetry, X-ray phase analysis, scanning electron microscopy and Raman spectroscopy. The TiN coating was prepared by the atomic layer deposition (ALD)-method. The investigations showed that the TiN layer reacts in Ar at T ≥ 1200 °C with the cBN and forms a porous TiB2 layer. No reaction takes place in nitrogen up to temperatures of 1600 °C. Nevertheless, the 20 and 50 nm thin coatings also undergo a recrystallization process during heat treatment up to temperatures of 1600 °C.

High quality α/β-Sialon materials were reproducible prepared using cost effective silicon nitride powders and an aqueous processing route. The influence of the powder quality (SN-E10, Silzot HQ, SicoNide P95H) on sintering and phase formation was investigated. With all 3 powders dense Sialon materials with nominal composition RxSi12-(m + n)Alm + nOnN16-n and m = 0.5 and n = 1 and 4 wt% excess rare earth additives could be prepared. The material based on Silzot HQ exhibits the highest α-Sialon content and subsequently the highest hardness due to the lower oxygen content of the starting powder. The microstructural analysis using XRD, FESEM including EDX- and EBSD-mapping reveal, that the α-Sialon grains are formed on existing α-Si3N4 grains of the starting powder. Anisotropic grain growth of the α-Sialon takes place therefore the differences in the microstructure depend strongly on the oxygen content but not from the α/β-content of the starting powders.

Diamond-silicon nitride composite materials with 40 vol% of diamond with two different grain sizes (50 µm and 100 µm) were produced using the field-assisted sintering technique/spark plasma sintering (FAST/SPS). A diamond powder with a SiC coating was also used. For the pure matrix material and the composites, the microstructure and the wear behavior under dry oscillating sliding conditions were investigated with use of steel and silicon nitride balls as counter-body materials. The composite materials showed coefficients of friction (COFs) of as low as 0.1, substantially lower than the COFs (0.8–1.2) obtained for the matrix. The highest wear resistance (wear rate: 8*10-8 mm³/Nm) was observed for the composite material with the SiC-coated diamond particles due to the stronger bonding of the diamond particles to the matrix in this material.

Cubic boron nitride (c-BN) composites produced at high pressures and temperatures are widely used as cutting tool materials. The advent of new, effective pressure-assisted densification methods, such as spark plasma sintering (SPS), has stimulated attempts to produce these composites at low pressures. Under low-pressure conditions, however, transformation of c-BN to the soft hexagonal BN (h-BN) phase can occur, with a strong deterioration in hardness and wear. In the present work, the influence of secondary phases (B2O3, Si3N4, and oxide glasses) on the transformation of c-BN was studied in the temperature range between 1100 °C and 1575 °C. The different heat treated c-BN particles and c-BN composites were analyzed by SEM, X-ray diffraction, and Raman spectroscopy. The transformation mechanism was found to be kinetically controlled solution–diffusion–precipitation. Given a sufficiently low liquid phase viscosity, the transformation could be observed at temperatures as low as 1200 °C for the c-BN–glass composites. In contrast, no transformation was found at temperatures up to 1575 °C when no liquid oxide phase is present in the composite. The results were compared with previous studies concerning the c-BN stability and the c-BN phase diagram.

Superhard silicon carbide-bonded diamond materials were synthesized by liquid silicon infiltration of diamond-containing preforms. The properties of the materials were strongly influenced by the strength of the interfaces between the diamond and the silicon carbide. Interface formation was investigated through local analysis of the microstructure in the interface regions using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD) as well as X-ray diffraction (XRD). The results of these experiments revealed a pronounced orientation relationship between SiC and diamond at their interfaces and, as a result, strong bonding of the diamond particles to the ceramic matrix. There was also an orientation relationship between the nano-sized SiC grains, which were embedded in residual silicon near the diamond interfaces, and diamond. Additionally, the different morphologies and phenomena occurring in the microstructures of the diamond-SiC composites and their dependence on the infiltration temperature were studied.