Phase stability and formation of Ti-Al-C-N PVD coatings

Abstract Knowledge about the phase formation and stability of metastable coatings obtained by physical vapour deposition (PVD) is necessary for the right selection of a coating for practical application. Because of the metastable or amorphous state of most PVD coatings due to the deposition, a phase transition at higher temperatures has to be taken into account for the selection of a coating. This is especially important for long-term and high temperature applications. Metastable materials are used and developed because they offer some properties superior to stable materials. The best-known example for these metastable PVD materials is the (Ti, Al)N coating. The metastable Ti-Al-C-N coatings examined here are a combination of the known Ti(C, N) and the above mentioned (Ti, Al)N coatings. After the deposition by the MSIP process, the coatings were annealed in a high vacuum furnace temperature range between 600 and 1400°C, using 200°C steps. The as-deposited and annealed samples were examined with X-ray diffraction. The phases and the lattice parameters were investigated. The microhardness was also examined. A special interest of this investigation was the Al loss at high temperatures as known from (Ti, Al)N. This takes place during the transition from the metastable to the stable state. The coatings in the quaternary Ti-Al-C-N system form a metastable (Ti, Al)(C, N) phase with a phase stability which is superior to (Ti, Al)N. The Al loss of the metastable (Ti, Al)N phase is shifted to higher temperatures through the incorporation of carbon. These quaternary coatings have in the examined temperature range a higher microhardness level. The (Ti, Al)N and the (Ti, Al)(C, N) coatings have a secondary hardness maximum at 1200°C. This is caused by recrystallization effects and the previous Al loss.