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

Abstract

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.

Corrosion resistance of CrN/NbN superlattice coatings grown by various physical vapour deposition techniques

“…3b. Previous work [19] has shown that this is indicative of local epitaxy between coating and substrate, which imparts excellent adhesion between coating and substrate. In both cases, however, the voids developed in the CrN base layer do not propagate further in the superlattice structured CrN/NbN coating (Fig.…”

CrN/NbN superlattice structured coatings with enhanced corrosion resistance achieved by high power impulse magnetron sputtering interface pre-treatment

“…Etching pre-treatment in a Nb cathodic arc (CA) plasma is characterised by intense ion bombardment of the substrate with highly charged Nb 3+ ions providing both efficient surface sputter cleaning and low energy ion implantation [6,7]. As a consequence, the adhesion of subsequently deposited films is enhanced due to the removal of contaminating oxides [8].…”

Optimization of in situ substrate surface treatment in a cathodic arc plasma: A study by TEM and plasma diagnostics

“…The exposure to ion bombardment lasted for 20 min. 3,5 This long exposure time was used to ensure a clean surface. However, particularly for small tools and parts this long etching time may lead to local substrate overheating depending on the tool geometry causing detrimental softening effects.…”