Nano-Scratch and Micro-Scratch Properties of CrN/DLC and DLC-W Coatings

Diamond-like carbon (DLC) coatings are well known for their excellent adhesion to silicon wafers. However, they often exhibit poor adhesion properties on metallic substrates. Inter layers and metallic doping help improve the adhesion properties of DLC coatings on metallic substrates. In this study, both nano-scratch and micro-scratch were performed on chromium nitride (CrN)/DLC and tungsten doped DLC coating (DLC-W) coatings deposited on 920 HV DIN 16CrMn martensitic valve tappets. Nano-scratch was performed at 300 mN in a Hysitron nano-indenter, whereas micro-scratch was performed at 1-50 N using a CETR-UMT tribometer. The 3-D images and 2-D longitudinal and transversal profiles of the nano-scratch and micro scratch were obtained using atomic force microscopy and 3-D optical profilometry, respectively. The scratch hardness equation was used to estimate the scratch hardness of the coatings. Experimental and theoretical values for the volume removed and the specific wear rates for the micro-scratch and nano-scratch of CrN/DLC and DLC-W coatings were estimated. The coefficients of friction (COF) obtained during the micro-scratch tests were very similar for both coatings. The same happened with the COF measured during the nano-scratch. The maximum COF in both cases reached 0.14. The wider and deeper penetration of the indenter for the DLC-W coating was mainly due to the lower hardness of the multilayered coating, composed of alternating nanometric thick amorphous carbon and tungsten carbide (WC) layers. The greater wear observed for the DLC-W coating system could also be attributed to the abrasive effect of detached WC nanoparticles abrasively acting during the contact of the diamond tip with the DLC coating. The experimental and theoretical values for the volume removed and the specific wear rates indicate a lower volume removal and specific wear rate for CrN/DLC because of higher hardness and better load-carrying capacity, contrary to DLC-W, which presents higher volume removal and specific wear rate because of its lower hardness.