Development of Cermet Microstructures during Sintering and Heat-Treatment
Abstract: The work presented in this thesis is concerned with the microstructure of Ti(C, N) based cermet materials used in cutting tool applications. Several techniques have been used to study the microstructure of cermets, in particular transmission electron micro-scopy in combi-na-tion with energy dispersive X-ray analysis and electron energy-loss spectroscopy, energy-filtered transmission electron micro-scopy, atom-probe field-ion microscopy and scanning electron microscopy.The carbon content of (Ti, W, Ta, Mo)(C, N)-(Co, Ni) cermet materials was found to control the dissolution of metal carbides and carbonitrides in the binder. The forma-tion of heavy (Ti, W, Ta)(C, N) cores was favoured by a high carbon con-tent, whereas a high amount of undissolved Ti(C, N) cores were found in the low carbon material. The perfor-mance of cermet materials was found to depend on the detailed microstructure in a complex way. Both a solid solution hardened binder phase and a high volume fraction of heavy cores seemed to be advan-ta-geous for a high wear resistance. Energy-filtered transmission electron microscopy was found to be a very powerful technique for cermet investigations. Elemental maps provide the elemental distribu-tion and can be used to extract quantitative phase information. A comparison between atom-probe analysis and electron energy-loss spectroscopy showed that quantitative microanalysis of carbon and nitrogen in cermet carbonitride phases can be accurately performed in the transmission electron microscope. The formation of the microstructure of a Ti(C, N)-TiN-WC-Co cermet was studied from interrupted sintering experiments. The binder acts as the transport medium both in its solid and liquid state. Before melting the binder was found to be nanocrystalline. The inner rim repre-cipitates from the binder onto titanium carbonitride cores already during solid-state sintering, whereas the outer rim forms during liquid phase sintering. At the sintering temperature, grain growth is mainly due to outer rim formation enabled by preferen-tial dissolution of small carbonitride grains in contact with the binder phase. Post-sintering heat-treatments in a nitrogen atmosphere can be used to create gradient structured cermet materials. The high nitrogen activity causes dissolution of tungsten-rich phases in the surface zone, as well as diffusion of tungsten and cobalt inwards and of titanium towards the surface. During the heat-treatment, a nitrogen-rich titanium carbonitride phase reprecipitates onto carbonitride grains in the surface zone, often as irregularly shaped surrounding layers. This phase is responsible for the dramatically im-proved wear resistance of the nitrided cermet, probably by obstructing grain boundary sliding and thereby restricting plastic deformation.
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