Nanocomposite Hard Coatings: Deposition Issues and Validation of their Mechanical Properties

Abstract

The limitations of conventional coatings due to inferior hardness or poor oxidation stability can be overcome by nanocomposite hard coatings such as nc-TiN/a-SiNx, which consists of nanocrystalline TiN and a non-crystalline tissue phase of SiNx which are mutually immiscible. The properties of nanocomposite coatings, especially their increased hardness, can be explained by their nanostructure, which leads to a maximum hardness at typically 80 atomic percent of the crystalline phase. We show that enhanced hardness can only be attained when the silicon nitride phase is sufficiently nitrided. The accurate and reliable measurement of the hardness and elastic modulus requires the use of appropriate nanoindentation equipment and a careful tip correction with periodical validation. It is shown that for a correct hardness determination of a few microns thick nanocomposite coatings, an indentation depth of 100 nm is sufficient. The maximum hardness of our nc-TiN/a-SiNx coatings deposited by a hybrid UBM/arc-PVD process is about 40 GPa. This value represents a global hardness value, due to the nanocomposite structure there may be a local hardness variation of about ±10 %.