Enhancing amorphous carbon wear resistance at high contact stresses through Ag incorporation

Abstract

High load capacity is desirable in various industrial applications, such as gudgeon pins, cam followers, gears and bearings. Amorphous carbon (a-C) coatings, known for their minimal friction and low wear rates, have found extensive application in automotive components. However, the a-C films often fail at high contact stress (>2 GPa). Extensive efforts have been devoted for the application of a-C under high contact stress, and a few groups can prevent the sudden failure of a-C in high load condition up to now. Nevertheless, the wear rate is still high, which is in the range of ∼10−7 mm3/Nm, at least ten times greater than that of films under low stresses (<2 GPa). To tackle this issue, Ag-C supra-nano-dual-phase composite (SNDP) was fabricated using magnetron sputtering (‘supra-nano-dual-phase’ means the diameter of each phase is smaller than 10 nm). Various methods were employed to examine the structural features, mechanical attributes, and tribological performance of Ag-C SNDP. The research revealed that Ag nanocrystals, ranging in size from 2 to 4 nm, were uniformly incorporated within the amorphous carbon matrix. The Ag-C SNDP film with 4.0 at.% Ag shows an order of magnitude lower than amorphous carbon (a-C) at wear rate. It also pushes the load-bearing capacity as high as 4.0 GPa, which has rarely been reported before. The superior tribological behavior of Ag-C SNDP composite can be explained in three factors: (1) the ‘supra-nanometer-sized dual-phase’ structure provides adequate strength; (2) the surface graphitization within several nanometers from the surface enables low friction coefficient and prevents sudden failure; (3) incorporating Ag into the films resulted in a reduction in residual stress and an increase in film toughness.