This work investigates the mechanical properties and microstructure evolution of hard coatings at elevated temperature, which imitates the thermal condition of cutting tools operation. TiAlN has been widely studied due to the age hardening effect by spinodal decomposition at 800 ̊C, however, low phase stability of the c-AlN at high temperature above 1000 ̊C could lead to poor mechanical properties. By alloying with a ternary transition metal the thermal stability is expected to be improved. Three coatings, Ti_0.37Al_0.45Nb_0.18N, Ti_0.34Al_0.47V_0.19N and Ti_0.32Al_0.51Cr_0.17N, and three reference coatings, Ti_0.43A_l0.57N, Ti_0.49Al_0.51N and Ti_0.55Al_0.45N, were deposited by cathodicarc evaporation. The thermal stability was then studied by annealing at temperatures ranging from 600 ̊C to 1100 ̊C.

The composition of each coating was measured by energy dispersive x-ray spectroscopy (EDS) and the thickness, microstructure and surface morphology were studied by scanning electron microscopy (SEM). The microstructure was similar for all six coatings, but a higher macroparticle density was found in the Ti0.37Al0.45Nb0.18N coatings. The phase analysis from θ-2θ and grazing incidence X-ray diffraction indicates that Ti_0.37Al_0.45Nb_0.18N has good phase stability due to the occurrence of h-AlN at 1100 ̊C, which is 100 ̊C higher than the reference TiAlN coatings. The in-plane stress was measured by sin2ψ method of X-ray diffraction, and the hardness was measured by nanoindentation. The stress state and hardness results do not have strong correlation, but (Ti,Al,Me)N coatings have better thermal stability than the reference TiAlN at 1000 ̊C and 1100 ̊C. TEM work has been done on Ti_0.34Al_0.47V_0.19N with 800 ̊C and 1000 ̊C annealed samples. The mixture of c-AlN and h-AlN phases wasshown in 800 ̊C sample, and the h-AlN phase exists at the column boundaries for both samples. STEM z-contrast measurement on 1000 ̊C annealed sample showed the segregation of Ti-rich and Al-rich domains, where the V was primarily found in the Ti-rich domains. In summary, Ti_0.37Al_0.45Nb_0.18N, Ti_0.34Al_0.47V_0.19N and Ti_0.32Al_0.51Cr_0.17N are promising candidates for high temperature application.