Microstructure and tribological behaviour of nano-structured metal matrix composite boride coatings synthesized by combined laser and sol-gel technology

Hard nano-structured metal matrix composite (MMC) boride coatings have been synthesized by laser melting of pre-placed powder mixture paste of B4C + sol-gel derived nano-particulate TiO2 on AISI 1050 (EN43) medium carbon steel and AISI 316L stainless steel substrates. Different coating/processing gas conditions were employed to understand the influence of graphite and nitrogen gas interactions with the coating material at high temperatures. Laser synthesized coatings were characterized by SEM, EDX, FEGSEM, XRD and HRTEM. Results show that it is possible to synthesize nano-structured MMC coatings (with TiB2 and TiB particulates in the ranges of 5-10 nm, 20 nm and 200-500 nm) by employing the combined laser and sol-gel route. Nano-particulate and sub-micron level TiB and TiB2 are found dispersed throughout the metal matrix. Other borides and carbides are present in micro-level patches dispersed in a eutectic matrix. Hardness of the composite coatings is in the range 800-2000 HV0.1. The minimum coefficient of sliding friction obtained in a pin-on-disc set-up was 0.35 (against cemented tungsten carbide) while wear rates (against diamond) were substantially improved (up to 5 fold reduction) over that of the substrates.