Enhanced adhesion through local epitaxy of transition-metal nitride coatings on ferritic steel promoted by metal ion etching in a combined cathodic arc/unbalanced magnetron deposition system

In situ substrate cleaning by ion etching prior to deposition in physical vapor deposition processes is a key step in achieving good film adhesion, which is essential for all coating applications. Irradiation with metal or gas ions alters substrate surface chemistry, topography, and microstructure thus affecting subsequent film growth. This study compares $Ti1−xAlxN$/ferritic steel $(x=0.54)$ interfaces formed after Cr ion bombardment at negative substrate biases, $Us,$ ranging from 600 to 1200 V during a Cr cathodic arc discharge, stabilized with a 0.06 Pa Ar background pressure. Samples biased with −1200 V in an Ar glow discharge at a pressure of 0.6 Pa were also investigated. Microstructure and microchemistry of the interfaces was studied by scanning transmission electron microscopy with energy dispersive x-ray analysis using cross-sectional samples. Cr ion etching with $Us=1200 V$ resulted in a net removal of over 100 nm of substrate material with the formation, through implantation, of a Cr-enriched near-surface region extending to a depth of ∼10 nm. As $Us$ was reduced to 600 V, Cr accumulated at the surface as a ≳5 nm thick layer. Ar was incorporated at the surface to levels of 4–6 at. % during both Cr arc and Ar glow discharge etching. The microstructure of $Ti1−xAlxN$ overlayers was dramatically affected by pretreatment procedures. Ar sputter cleaned steel surfaces $(Us=1200 V)$ promote nucleation of randomly oriented grains leading to a competitive column growth with small column size and open boundaries. In contrast, Cr irradiation at the same bias voltage results in local epitaxial growth of $Ti1−xAlxN$ on steel and lead to a superior performance in scratch testing compared to coatings deposited after Cr treatment with $Us=600 V$ or Ar ion bombardment at $Us=1200 V.$ Critical loads were 63, 47, and 27 N, respectively.