Effect Of TaC Content On Thermal Shock Resistance Of Ti(C,N)-based Cermets

The TiC-TiN-WC-Mo-Ni system cermets with TaC content of0~6wt.%wereprepared by powder metallurgy technology on the premise of excellent comprehensiveproperties of cermets. The microstructures and room temperature mechanical properties ofTi(C,N)-based cermets with different TaC content were observed by the opticalmicroscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy(EDS), and tested by bending strength tester, rockwell hardness tester and vickershardness tester. The effect of TaC content on the thermal shock resistance ofTi(C,N)-based cermets was investigated by an indentation-quenching method.In the first part of the dissertation, the development of cermets has been summarized,including the influence of manufacturing process and chemical composition on themicrostructures and properties of cermets. Then, present research situation of thermalshock resistance was introduced and the evaluation theory of thermal shock resistance ofcermets was concluded. The purpose and significance of the thesis has been put forwardbased on the above.The influence of TaC content on the microstructures of Ti(C,N)-based cermets. Thewhite core and grey rim structure appeared continuously with the increasing of TaCcontent, where black core-grey rim phase were well distributed in the matrix of cermets.The particle size of cermets decreased with the increasing of TaC content, while TaCcontent less than2wt.%. The cermets with TaC content of2wt.%provided the finestgrains, the most homogeneous microstructure and the moderate thickness of rim phase.The quantity and particle size of un-dissolved black hard phase increased withcontinuously increasing of TaC content.The room temperature mechanical properties of Ti(C,N)-based cermets with TaCcontent between0wt.%to6wt.%was tested. The rockwell hardness of cermets increasedfrom HRA87.5to HRA90.7with the increasing of TaC content, when the transverserupture strength(TRS) increased initially and then decreased, the fracture toughness decreased from15.2MPa·m1/2to14.3MPa·m1/2. The transverse rupture strength ofcermets with TaC content of2wt.%was2188MPa.The propagation situation of indentation crack under the different thermal shocktemperature and thermal shock cycles was tested and analyzed by anindentation-quenching method. Results showed that the indentation crack on the surface ofcermets propagated with the increasing of thermal shock cycles. The thermal stressaccumulated in the course of thermal shocking with the increasing of thermal shocktemperature, resulted in the significant increase of the propagation speed of the indentationcrack. When thermal shocked, the crack mainly propagated along the interfaces betweenhard phase and binder phase, and also propagated in binder phase, hard phase hardlyaffected, which give rise to the decrease of rockwell hardness less than1%. The residualtransverse rupture strength after several thermal shock cycles decreased significantly, anddecreased rapidly with the increasing of the thermal shock temperature. Cermets with TaCcontent of2wt.%showed the best thermal shock resistance for the best transverse rupturestrength and fracture toughness, and the crack on them propagated the slowest. Cermetswith TaC content of1wt.%came second. Cermets with TaC content between4wt.%to6wt.%showed worse thermal shock resistance because of the decrease of both thetransverse rupture strength and fracture toughness.