Carbon/chromium low friction surface coating for gears application

Purpose - Provide tribological information about the applicability of multi-layer carbon-chromium composite coatings to gears. Discuss the protection provided against scuffing failures, wear and the influence on gear power losses. Design/methodology/approach - Several screening tests, such as Rockwell indentations, ball cratering, pin-on-disc and reciprocating wear tests, were performed in order to evaluate the adhesion to the substrate and the tribological performance of the carbon/chromium composite coating. Afterwards, twin-disc tests were performed at high contact pressure and high slide-to-roll ratios to confirm the good adhesive and tribological properties of the coating under operating conditions similar to those found in gears. Gear tests were performed in the FZG machine in order to evaluate the anti-scuffing performance of the carbon/chromium coating using additive free gear oils. Finally, the carbon/chromium composite coating was also applied to the gearing in a gearbox and its influence on the gearbox efficiency was analysed. Findings - The C/Cr has got very good adhesion to the steel substrate, provides low friction coefficients between contacting solids in relative movement, gives excellent protection against scuffing and wear reduction in gears, and promotes a slight improvement of the gears efficiency. Research limitations/implications - The protection of this carbon/chromium coating against gear micro-pitting should be investigated. Practical implications - This study confirms the applicability of this coating to industrial gear applications, especially in two particular applications: severe applications involving high contact pressures and high sliding, frequent start-ups and inefficient lubrication; and acting as tribo-reactive material and substituting non-biodegradable and toxic additives in environmental lubricants. Originality/value - This work validates and quantifies the influence of this C/Cr multi-layer composite coating in gear applications in terms of adhesion to the substrate, anti-scuffing performance and efficiency.