Plasma-assisted pre-treatment for PACVD TiN coatings on tool steel

Abstract

Ti-based hard coatings are widely used to protect tools from early failure caused by wear and corrosion. In recent years, plasma-assisted chemical vapor deposition (PACVD) was introduced as a suitable technique to deposit hard coatings onto tools of complex geometry. In addition, PACVD offers the possibility of combining pre-treatment steps such as sputter cleaning and nitriding with coating deposition in the same chamber. Although PACVD coatings have been successfully applied to a variety of applications, deposition on tool steels with high chromium content may cause adhesion problems. Therefore, the aim of this work was to investigate the influence of different gas compositions during pre-treatment on the adhesion of PACVD TiN coatings on a cold work tool steel containing 11.5 wt.% chromium. For plasma-assisted etching and nitriding, mixtures of H 2, N 2 and Ar were used. The efficiency of different pre-treatment steps has been characterized using TiN-coated as well as pre-treated uncoated samples. The phases formed in the near-surface region of the tool steel after pre-treatment were characterized by X-ray diffraction (XRD). The adhesion of 3.5-μm-thick TiN coatings was characterized using the VDI indentation test. The results obtained are correlated with the chemical composition at the interface characterized by secondary ion mass spectrometry (SIMS). The best adhesion quality value of HF 1 was achieved for coatings pre-treated in an atmosphere containing 40 vol.% N 2, whereas more or less nitrogen resulted in decreasing adhesion. Increasing nitrogen content promoted the formation of a CrN phase in the near-surface region of the tool steel. No compound layer was formed, but the diffusion layer varied in thickness, which can be related to the adhesion quality value. With Ar addition, adhesion generally deteriorated.