Synthesis of high temperature wear resistant WC and WN coatings

Abstract

In this study, WC and WN thin films were synthesized using reactive magnetron sputter deposition in order to develop promising alternatives to the well known wearresistant coatings such as CrN, TiN, TiAlN and TiB2 etc. For this purpose, WC and WN coatings were deposited on Si(100) and steel (100Cr6) substrates by a Direct Current (DC) reactive magnetron sputtering system. X-ray Diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) were used to uncover the atomic structure of the films and the change in the chemical bonding states of the atoms. Also, the hardness measurements were performed using a nano-indentation tester on as-deposited films and after the films were subjected to 500°C for two hours. A pin-on-disc tribometer was used to investigate the wear-rates of these coatings at room temperature (RT) and 500°C under ambient atmosphere conditions using 6 mm in diameter Al2O3 balls. Scanning electron microscopy (SEM) was used to investigate the microstructure and measure the thickness of the coatings. Also, for chemical analysis energy dispersive spectroscopy (EDS) was used. Two sets of experiments were designed and performed for the preparation of WN coatings. In the first set, effect of N2 flow rates during deposition was investigated on the structure and properties of coatings deposited. The N2 flow rates were varied between 17% - 66% of the total flow (Ar+N2). The results of these experiments exhibited a significant drop for the hardness and wear rates of WN coatings deposited with increasing N2 flow rates when tested after 500°C treatment. On the contrary, RT wear test results indicated an improvement in the wear rates with increasing N2 flow rates. XRD data for the samples subjected to 500°C. XRD analysis indicated the presence of a soft tet-WO3 layer over the coatings treated at 500°C which is found to be the main culprit for the degradation of the tribological properties. In order to prevent the formation of this soft oxide layer on WN coatings, W/WN multilayer coatings were synthesized in the second set of experiments where the W layers were used as diffusion barriers for oxygen. After the optimization of deposition parameters for synthesizing W/WN multilayer coatings, subsequent tribological examinations indicated the multilayer coatings to be comparably wear resistant at both RT and 500°C. Furthermore, hardness of the multilayer coatings with optimized parameters were found to be around 20-25 GPa at RT and 15-20 GPa at 500°C and their corresponding wear resistances were measured to be ~2.0x10-6 mm3 /Nm at RT and ~4.0x10-6 mm3 /Nm at 500oC. As the second challenge, WC thin films were synthesized within the framework of this study. The first set of synthesis experiments was done using acetylene (C2H2) as the carbon source. It was found that increasing relative amount of C2H2 flow during sputter deposition resulted in the degradation of tribological properties of coatings due to amorphous carbon build-up in the films. To overcome this degradation of properties, alternative solid sputter targets such as B4C (as an alternative C source for 2nd set), W/B4C (composite targets) and W2C targets (for co-sputtering experiments of 3rd set) were used for the deposition of WC films. The results of the structural and chemical analysis indicated the presence of well-crystallized WC phases (WC and W2C) in the coatings sputter deposited from B4C targets. Comparatively, coatings deposited using solid WC and W2C targets resulted in coatings with the same phases. Tribological testing of these coatings indicated that WC films synthesized using B4C targets to have better mechanical performance after 500oC treatment while coatings co-sputtering from compound targets worked better at RT.