Microstructure of Cermets

Doctoral thesis, 1995

The microstructure of several TiC-TiN-Mo2C-WC-Co/Ni model alloys and a commercial (Ti,Ta,W,Mo)(C,N)-(Co,Ni) type cermet was studied using x-ray diffraction, optical microscopy, scanning electron microscopy, analytical electron microscopy and atom probe field ion microscopy. In addition thermodynamical calculations were made in order to understand the development of the microstructure during sintering. Some alloys were performance tested to study their properties in cutting tool applications. All alloys contained carbonitride grains with a core-rim structure. Ti(C,N) and (Ti,W)(C,N) cores were remnants of undissolved raw material powders. The rims were formed during sintering and contained both Ti and heavy elements. The nitrogen content of the carbonitride phases increased linearly with increasing nitrogen content in the material. Mo and W increased the activity of N in the carbonitride. The grains were embedded in the continuous Co/Ni binder phase. Its Ti content could be controlled by the Co/Ni ratio, while its Mo content was more dependent on nitrogen content in the material. The C and N content of the binder was very low. An increasing content of WC in the raw materials gave an increased volume fraction of inner rim and an increased W content in the outer rim. For high WC contents undissolved WC remained after sintering. Mo2C was completely dissolved during sintering. With increasing Mo2C content in the raw materials the volume fraction of inner rim and the Mo content in both inner and outer rim increased. Thermodynamical calculations suggested that the inner rim is formed during solid state sintering at a temperature of approximately 1000oC when there is an open porosity and thus the nitrogen activity can be assumed to be very low. Cermets manufactured from pre-alloyed raw material powders had a more homogeneous microstructure. Performance testing suggested that this improved wear resistance, whereas a high volume fraction of inner rims/W-rich cores and Ti(C,N) cores promoted toughness. The microstructure of the commercial cermet was very complicated but could be well explained using the results obtained from the model alloys.