Evaluation of Titanium Carbide Thin Film Coatings on Surface Microstructure Controlled WC-Co

Abstract

Chemical vapor deposited (CVD) or physical vapor deposited (PVD) hard coating technique is widely used the cemented-tungsten carbide (WC-Co) for molds and cutting tools, which plays an important role in a lot of manufacturing industry. For example, titanium carbide (TiC), titanium nitride (TiN) and titanium carbonitride (TiCN) is one of the candidates to control surface properties in order to increase lifetime or to decrease friction coefficient of molds, tools and so on. The film prepared by thermal CVD carried out around 1000°C has good uniformity and mechanical properties, which restricts substrates having low melting points, and also causes deformation of substrates. High temperature sometimes results in decarbonization of WC-Co surface which causes a significant decrease in toughness and transverse rupture strength. Plasma enhanced chemical vapor deposition (PECVD) or PVD has some merits as lower deposition temperature (< 500°C) than thermal CVD and high through-put, however, the film usually has low adhesion strength. Then, life of molds and cutting tools obtained PVD are sometimes shorter than that with CVD. In this study, Several surface pretreatment methods were investigated to remove Co from the WC-Co surface and to increase surface roughness to enhance adhesion strength. The procedure include, Dry etching by CF4 plasma for 15 to 120min in the substrate temperature range from R.T. to 500°C. Chemical treatment with aqua regia (3HCl:HNO3) for 5 min at 25 to 60°C. After pretreatment, TiC coatings were formed by sputtering with TiC target or by PECVD from TiCl4/CH4. Surface roughness was evaluated by AFM. Crystallographic structure of WC-Co and TiC was evaluated by XRD. Hardness was evaluated by dynamic nanoindenter. Atomic concentration was determined by XPS. Figure 1 shows maximum height roughness (Rz) and arithmetic average roughness (Ra) of WC-Co substrates after pretreatment. Rz and Ra got larger as increasing temperature of treatment conditions. In particular, substrate surface with aqua regia treatment at 60°C disappear Co peak. Scratch test for PVD TiC coating on pretreated substrate revealed that critical load was larger than that with the film on non-pretreated substrate. When Ra was 150 nm, critical load was over 30N. The pretreated substrates surface by CF4 plasma and aqua regia have similar Ra value as 150 nm, however, the film with CF4 plasma exhibited no delamination and the film aqua regia exhibited delamination. The opposite trend could be due to the micro-level morphologies. The differences between PVD and CVD films will be discussed. Figure 1