Hardness influence on the adhesion of thin protective coatings for aerospace applications

Ti-Al-V alloys are mostly used in the aerospace and automotive industries as light material with good strength and elasticity. However, it shows some drawbacks, such as high friction coefficient especially at elevated temperatures, as well as fast oxidation and deterioration of its chemical and crystal structure. Thus, it turns improper its use where the surface properties are of importance. The present study aims to cover this alloy with a protective layer of thickness up to 3 microns with especially pronounced compositional gradient character in order to minimize these drawbacks. The gradient structure is essential to assure a good adherence, especially required at high temperature and stress. The chosen protective coatings are based on Ti-carbo-nitride multilayers (TiCN), providing outstanding hardness, exceeding 30 GPa, while keeping low friction coefficient and excellent adhesion properties even under extreme conditions. The coatings were deposited by Physical Vapor Deposition (PVD). The adhesion is an important quality and the present study shows that near the interface, the both Young's modulus at the adjacent materials are close.