Laser Surface Melting of Carbon Coated Aa7075 Aluminum Alloy: Structural Transformations and Tribological Behavior

Abstract

Carbon is one of the most common solid lubricants and has often been used to achieve superior tribological properties. In this investigation, a CO2 laser in continuous wave mode was used to develop diamond-like carbon (DLC)-rich coatings on 7075 alloys. A detailed investigation on the effect of the in-situ transformed DLC and graphitic nanoparticles on the tribological properties of the 7075 alloys was performed. A unique relationship between the surface hardness and wear rate was noted, wherein the wear rate decreased with decrease in surface hardness after the laser surface engineering. An analysis of wear track and counterbody using EDS and Raman spectroscopy confirmed the formation of metastable DLC-rich graphitic tribolayer during the wear process. A `reverse' transformation of the apparently more stable graphitic carbon to metastable DLCs in the tribolayer during frictional loadings was also noticed. The tribolayer was noted to generate significant lubricating action, and therefore, improve the wear resistance. Also, unlike the loss in lubricating action with increasing time usually exhibited by non-hydrogenated DLCs, the increase in nano-carbon rich melt-zone was observed to partially mitigate this effect by enabling presence of lubricating particles even at greater depths from the surface.