Corrosion resistance and lubricated sliding wear behaviour of novel Ni–P graded alloys as an alternative to hard Cr deposits

Alternative process to hexavalent chromium, substitute materials and new designs are urgently needed owing to the requirement of ‘‘clean’’ manufacture. This comparative study was conducted to systematically investigate the corrosion resistance and lubricated sliding wear behavior of graded Ni–P alloy deposits produced from a single plating bath by electrodeposition and hard Cr deposits, using potentiodynamic polarization and reciprocating ball-on-disc tribometer. Results showed that Ni–P deposits heat-treated at 400 8C with maximum hardness exhibited more than two orders of magnitude higher corrosion resistance than hard Cr deposits in 10 wt.% HCl solution. The Stribeck curves for the heat-treated Ni–P gradient deposits and hard Cr under lubrication conditions were obtained with accurate control of normal load and sliding speed during the wear process, three main different regimes corresponding to different lubrication mechanism were identified. Heat-treated Ni–P gradient deposits showed relatively poor wear resistance than hard Cr deposits under the lubrication conditions, which may be attributed to superior oil-retaining surface structure and the unique ‘‘nodular’’ effect of hard Cr in wear process.