Structures And Properties Of Surface Treated Ti(C,N)-based Cermets

In response to increasingly severe service conditions for cutting tool materials, the high performance materials have been investigated and developed for high speed and dry cutting application. Ti(C,N)-based cermets haved been prepared using surface treatment technology, such as nitridation at high temperature and high pressure, multi-arc ion plating and hybrid surface treatment. The effects of processing parameters, matrix materials and post-treatment process on the microstructure and properties of the coating have been explored using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive X-ray detector (EDX), and atomic force microscope (AFM).The microstructure and properties of Ti(C,N)-based cermets, the development of surface treatment for cermets, the principles and characteristics of heat isostatic pressing, and mutli-arc ion plating have been critically overviewed in the first part of the dissertation. And the purpose and significance of the dissertation have been set forth.The composition of cermet based on the higher nickel content (32%) and the vacuum sintering process have been adjusted in minute quantities. The cermets sintered at 1415 oC have the highest transverse fracture strength and fractrue toughness, and its microstucture is typical core-rim strucuture and very uniform. The vacuum sintering process for preparing Ti(C,N)-based cermet matrix is established as 1415 oC for 60 min.The nitridation for Ti(C,N)-based cermets had been performed at high temperature and pressure using the heat isostatic pressing furnace. There formed an approximate 15μm thick, TiN-riched, golden, reacted and affected area in the surface area of cermet after nitridation, which improved the hardness and fracture toughness. Furthmore, at the condition of high temperature and high pressure, the temperature should not be excessively high. Otherwise, the particles would grow up and deteriorate its properties.Ti(C,N)-based cermets had been ion plated with TiN coating for the first time. There formed a compact golden TiN phase with (111) texture, which significantly improved the surface microhardness and had no effect on the transverse fracture strength. The adhesive strength between the TiN coating and the cermet matrix was higher when the roughness of the matrix is lower. And the adhesive strength of the coating on the Ti(C,N)-based cermet was higher than that on the hardmetal. TiN has good wettability with the Ti(C,N)-based cermet and TiN coating grew in layer-by-layer mode. TiN had formed bonding with matrix atoms and TiN coatings were not only smooth and compact but also had higher adhesive strength than that on the hardmetal which grew in Volmer-Weber mode and were roughness. The titanium raindrop on the coated hardmetals had orientation.The hybrid surface treatment of multi-arc ion plating plus heat isostatic pressing post-treatment for the Ti(C,N)-based cermet have been explored for the first time. The heat isotatic pressing post-treatment improved the fracture toughness and adhesive strength of the TiN coating and had no effect on its phase structure. The mechanisms of improving the TiN coating properties were forming gradient interface through diffusion and improving the mechanical interlock role at the interface by compacting the materials.Ti(C,N)-based cermets had been ion plated with TiN/TiAlN multi-coating approximately 3μm for the first time. The coating's Knoop hardness reached 2808 HK and the coating's adhesive strength to the matrix 57.52 N, while its transverse fracture strength was seldom affected. The top layer of the coating was TiN, the middle layer mainly TiAlN, the matrix mainly Ti(C,N) and Ni. The matrix diffraction peaks were also detected in the top layer and middle layer, but their intensities were different. TiN and TiAlN phase had severe (111) texture. The aluminium content was increase from the inner coating to the top coating and showed gradient charateristic. TiN and TiAlN had good wettability with Ti(C,N)-based cermet and their growth mode were layer-by-layer. So they formed smooth and compact multilayer, which had root mean square roughness of 20.6 nm. They had poor wettability with the hardmetal matrix and grew in Volmer-Weber mode. There are many titanium raindrops on the coated hardmetals, which had orientation and the coating root mean square roughness reached 272.2 nm.The tribological properties and wear mechanism of TiN/TiAlN multi-coating had been investigated using a ball-on-disc friction and wear apparatus. The mean friction coefficients of the TiN/TiAlN multi-coating were all lower than that of the matrix. The coated cermets had higher friction coefficient under higher load and higher sliding speed. The mass changes were first increment and then decrement, that is, from severe adhesion to little adhesion. The wear mechanisms were mainly adhesive wear and particle wear. The TiN/TiAlN coating on the hardmetal had lower friction coefficient than that of Ti(C,N)-based cermet under the same load and sliding speed. Their surface composed of many titanium particls like raindrop and drop pit between the particls, which favored of minishing friction coefficient and improving the wear resistance. The wear mechanism mainly were adhesive wear, micro-peel.The coating crack densities of TiN, TiN/TiAlN coatings had been investigated. The indentation edge of TiN coating on the Ti(C,N)-based cermet showed large piece spalling and the coating was very brittle and its fracture toughness was very low. But after heat isotatic pressing the coating adhesive strength and fracture toughness had been improved. The fracture toughness of the TiN/TiAlN coating of the Ti(C,N)-based cermet was higher than that of the hardmetal. There is no spalling in the edge of indentation. The radial cracks of the coating on the cermet were very short and had deflexion phenomenon. While its lateral cracks were very long and distributed around the indentation. The radial cracks of the coating on the hardmetal were long, smooth and straightforward and the lateral cracks were very short and only existed in the direction that was perpendicular to the particle's direction.The TiN/TiAlN coated cermet milling cutters were tested by milling 0Cr18Ni9 and 45# steel. The results show that the cutting characteristic of TiN/TiAlN coated cermet excelled the uncoated cermet and hardmetal YW2 and its service life was twice that of hardmetal. The failure forms of the TiN/TiAlN coated cermet cutting tools were mainly wear and tipping, without coating spalling. The failure forms of the uncoated cermet cutting tools were wear and adhesion.