Effect of plasma treatments on the adhesion properties of PACVD a-SiC:H coatings on TA6V alloy

摘要

Recently, hard a-SiC:H coatings, have received considerable attention for tribological applications, in particular in the aeronautical field where titanium alloys (TA6V) are widely used. However, this increase in tribological properties is often hindered by poor adhesion of the coating due to the growth of natural oxide layers on the titanium alloy surface. This work is aimed at investigating the use of plasma pre-treatments to remove these oxide layers prior to a-SiC:H deposition. Pre-treatment and a-SiC:H deposition from tetramethylsilane are both carried out in a hybrid microwave/low frequency plasma device (dual PACVD microwave [v=2.45 GHz]/low frequency [v=50 KHz]). X-ray Photoelectron Spectroscopy analysis (XPS), coupled with in situ Ar~+ sputtering, reveals complex (TiO_2/Ti_xO_y/TiO) oxide layers. As determined from TRIM simulations, the argon ion energies required to sputter these oxide layers lie in the range 200 to 400 eV. These values are experimentally checked in the PACVD reactor in a pure argon plasma medium at variable surface DC bias. Optical Emission Spectroscopy (OES) of the atomic oxygen species, carried out in the vicinity of the sputtered surface is compared to the oxygen content measured at the interface between the film and substrate surface by post-mortem Secondary Ion Mass Spectrometry (SIMS) analysis. Finally, Atomic Force Microscopy (AFM) topographic images show that TA6V surface roughness and film adhesion as evaluated by scratch-test, are strongly correlated. These results reveal counter influence of interfacial oxygen content and surface roughness. The sputtering parameters are eventually optimised to enhance the adhesion of a-SiC:H coatings on TA6V.