Shortlisted substrate ion etching in combined steered cathodic arc–ubm deposition system: effects on interface architecture, adhesion, and tool performance

Abstract

In PVD processes, substrate cleaning by ion etching prior to deposition is a key step in achieving good film adhesion, which is essential for all coating applications. The substrate surface is generally cleaned by bombarding it with metal or gas ions, extracted from a cathodic arc or glow discharge plasma respectively, by negatively biasing the substrate. This alters substrate surface chemistry, topography, and microstructure and affects subsequent film growth and adhesion. The present study involved Cr ion bombardment of steel and tungsten carbide at bias voltages U_b of -600 and -1200 V as well as ion etching in an Ar glow discharge at U_b = -1200 V. Ion etching was followed by reactive unbalanced magnetron deposition of TiAlN based coatings. The microstructure and microchemistry of the interface region was investigated by XTEM and STEM–EDX analysis. After Ar ion pretreatment the presence of a low density substrate surface region was observed. This surface promotes the nucleation of randomly oriented grains leading to competitive growth with small column size and open boundaries. Interfaces generated after Cr ion bombardment at U_b = -600 V exhibit a local Cr deposition layer which leads also to competitive growth. In contrast, Cr ion bombardment at U_b = -1200 V results in the removal of around 100 nm of substrate material, providing a clean surface, and local epitaxial growth is observed. The different interface architecture is reflected by the critical loads obtained in scratch testing of 3·5 μm thick TiAlCrYN coatings deposited on tungsten carbide, ranging from 33 N for Ar bombardment to 141 N achieved by Cr ion bombardment at U_b = -1200 V. Cr ion pretreatment at U_b = -600 V results in a critical load of 68 N. The influence on the lifetime of coated tungsten carbide tools is also discussed.