Weak adhesion and delamination of hydrogenated diamond like carbon coating on a rough surface of tappet valve substrate

Abstract

DLC(a-C:H) film are known for their excellent tribological properties however, possess poor adhesion on metallic substrate causing film delamination. Introduction of adhesive silicon interlayer has showed improvement in the adhesion of DLC-H on metallic substrate. In the present study DLC-H was deposited on tappet valve and silicon wafer using PECVD. The as-deposited DLC-H was characterized using XRD, EDS and Raman spectroscopy. SEM was also used to measure the thickness of the DLC-H coating and the average surface roughness (Ra) was measured using 3D Profilometry. Scratch test was used to determine the COF, adhesion and hardness of the DLC-H coating. The DLC-H coating on tappet valve had weak adhesion compared with DLC-H coating deposited on silicon substrate, which had good adhesion. The result of the Raman spectroscopy revealed a sp3 fraction of 0.43 for the DLC-H deposited on silicon substrate. The thickness of the as-deposited DLC-H film was 1.78 µm, which was measured using SEM to view the cross-section of the as-deposited DLC-H coating on the silicon substrate. The surface roughness of the as-deposited DLC-H coating on silicon substrate was 26.59 nm. Silicon substrate coated with DLC-H had the lowest COF, ranging from 0.06 to 0.09.