Effect of ion bombardment and micro-blasting on the wear resistance properties of hard TiN coatings

Abstract

Ion bombardment (IB) is conventionally used as a pre-coating treatment process to increase surface energy of tools to improve the adhesion of coatings produced using cathodic arc Physical Vapour Deposition (CA-PVD) technique. Micro-blasting (MB) as a post-treatment process is used to improve the surface finish of the coated tools. Both these processes impact surface finish, adhesion and wear resistance of the coated tools. This article presents a systematic study to understand the effects of these two processes on the surface finish, adhesion and wear resistance properties of TiN coated HSS substrate. IB process was carried out using Ti ions for different time periods which results in the formation of Ti layer on HSS M2 grade substrate. This was followed by TiN coating deposited at −150 V bias voltage and 1.3 Pa of nitrogen gas. Coating properties evaluated were: coating micro-structure, surface morphology, surface roughness, macroparticle (droplet) density, adhesion and wear rate. TiN coating with FCC structure was confirmed using XRD measurements. With the decrease in IB time from 25 min to 5 min, the surface roughness reduces from 0.31 µm to 0.15 µm respectively. Adhesion and Calo wear tests also showed improvement in adhesion and wear resistance with the reduction in IB time. The TiN coated sample without IB process was also prepared, which exhibited lowest surface roughness and droplet density, but the coating adhesion and wear resistance properties were optimum for 5 min IB treated HSS. The Ti layer deposited during the IB process helps in improving the coating adhesion. However, as the result shows, higher Ti inter-layer thickness results in poor adhesion and higher roughness. Thus, to get better wear resistance and adhesion, ion bombardment is necessary but has to be kept as low as possible. In addition, Post-coating MB process was carried out at different pressures and time on TiN coated sample to identify the optimum pressure and time which improves wear resistance of the coating.