Effects Of Deposition Parameters On Microstructure, Interface And Hardness Of Ti/TiN And CrN/Si₃N₄ Nanolayered Coatings And Their Thermal Stability

There has been increasing interest in protective coatings for cutting tools under dry machining conditions. This is mainly due to environmental and economic concerns about the use of coolants/lubricants in the machining process. Nanolayered coatings can play an important role in this area, as they can provide high hardness and a low friction coefficient at room temperature. However, machining at high speeds without coolants can easily raise tool temperatures to 800-1000 ℃. In many cases, nanolayered coatings are unstable due to inter-diffusion or other degradation at elevated temperatures, leading to a decrease in hardness and coating degradation. The key to minimizing such instability is to use immiscible components with low energy interfaces in nanolayered coatings.In the past decades, TiN-based films deposited by physical vapor deposition (PVD) or chemical vapor deposition (CVD) method have found widespread applications in wear resistant situations such as cutting tools or machine parts because of their high hardness, wear resistance, chemical inertness, and high temperature stability. Veprek reported superhard Ti-Si-N nanocomposite films with a hardness of 80-105 GPa, which is higher than that of diamond (70-90 GPa), and he has proposed a two-phase structure model to interpret the superhardness mechanism of nanocomposite films. According to the model, because Si₃N₄ is immiscible with TiN and can wet the growing surface of TiN grains, Ti-Si-N nanocomposite films form a structure of nanocrystalline (nc) TiN surrounded by amorphous Si₃N₄, known as nc-TiN/a-Si₃N₄. This concept is based on a strong, thermodynamically driven, and diffusion rate-controlled (spinodal) phase segregation that leads to the formation of a stable nanostructure by self-organization.