Mechanical and thermal stability of hard nitride coatings

Abstract

Hard coating 's thermal stability is essential due to the high temperature environment of high-speed cutting applications, while the induced phase and microstructure evolution affects the mechanical properties. In this thesis, the mechanical stability of arc-evaporated hard nitride coatings annealed at high temperature is analyzed and connected to the phase evolution. In addition to hardness, fracture toughness is evaluated by surface and cross-sectional investigations by scanning/transmission electron microscopy of damage events by mechanical tests. The crack resistance of Ti1-xAixN with a range of Al content (x = 0.23-0.82) is studied by contact fatigue tests, where the difference in the microstructure plays a major role. Superior mechanical properties are found in annealed Ti0.63AI0.37N at 900 oC due to the spinodal decomposition. The mechanical and high-temperature properties of hard coatings can be enhanced by alloying or multi-layering. Quaternary Ti-Al-X-N (X = Cr, Nb and V) alloys are studied, and superior toughness is found in TiAI(Nb )N in both the as-deposited and annealed (1100 oC) states. The h-AIN formation in TixAI0.37Cr1-0.37-xN (x = 0.03 and 0.16) is analyzed by in-situ x-ray scattering during annealing. The kinetic energy for h-AIN formation is found to be dependent on the microstructure evolution during annealing, which varies with coating composition. High Al content h-ZrAIN/c-TiN and h-ZrAIN/c-ZrN multilayers are investigated through scratch tests followed by focused ion-beam analysis of the crack propagation. A c-Ti(Zr)N phase forms in h-ZrAIN/c-TiN multilayers at high temperatures and that contributes to enhanced hardness and fracture toughness by keeping the semi-coherency at the sub-interfaces. Finally, an in-situ analysis of coatings by x-ray scattering during a turning process is carried out. lt demonstrates the possibility of observation of stress evolution and thermal expansion of the coatings or the work piece material during machining. This experiment provides real-time information on the coating behavior during cutting.