Synergistic addition of yttria-stabilized zirconia and h-BN/graphene/diamond restricts multi-scale length wear of Cr-based hybrid coatings

Abstract

In the current study, electro-deposition of hybrid coatings consisting of Cr simultaneously reinforced with nano-scale 3 mol% yttria-stabilized zirconia (YSZ) along with hexagonal boron nitride (Cr-YSZ-BN), graphene (Cr-YSZ-Gr), and diamond (Cr-YSZ-D) was performed using an environmentally-benign trivalent Cr bath. Abrasive wear with ploughing for scratch test is recognized as the dominant wear mechanism in coatings; on increasing the scratch speed, gross spallation occurs in the BN reinforced coatings. While graphene and diamond were strongly adherent to the substrate with no signs of delamination. During fretting, oxidative and three-body abrasive wear in wedge formation mode with severe crack networking was observed in Cr-YSZ-BN. Micro-ploughing was active with intermittent cracking in Cr-YSZ-Gr coatings, indicating lower wear (via solid-lubrication by graphene). Diamond added coating, on the other hand, exhibits fewer cracks with ploughing. High hardness and elastic modulus with a maximum of ~26 GPa and 245 GPa, respectively, were obtained for Cr-YSZ-D coating with a minimum wear rate of ~3.5×10−11 m3/Nm (fretting) and 3.8×10−5 mm3/Nm (scratching). Further, rigid sp3 hybridization of carbon atoms in Cr-YSZ-D leads to the strong interlocking between Cr matrix and diamond, thereby lowering the wear damage of the coating system. The unique combination and uniform dispersion of YSZ and diamond particles displayed enhanced friction and wear resistance (k ~ 1.4×10−3), primarily due to the inherent high hardness of diamond and matrix strengthening by nano and micro-scale particles. Therefore, Cr-YSZ-D coating system emerges out as a highly robust coating capable of providing enhanced damage tolerance towards wear for various tribological applications.