Bionic Coupling of Hardness Gradient to Surface Texture for Improved Anti-wear Properties

This work investigates the potential of combining hardness gradient with surface texture (an example of bionic coupling) to improve anti-wear properties. The bionic coupling of hardness gradient and Hexagonal Texture (HT) was achieved by laser heat treatment on steel specimens with pre-engraved hexagonal surface texture. The successful establishment of decreasing hardness from surface to internal bulk was verified by hardness measurements along the depth of cross-sectioned specimens and correlated with the observations from metallurgical microscopy. The tribological performance of bionic coupling specimens (HT-L) was examined under dry contact condition, together with respective control specimens of individual bionic features, e.g. HT-H (of similar surface hardness generated by conventional heat treatment but without hardness gradient) and SS-L (of smooth surface treated by the same laser processing as for HT-L). It is found that HT-L not only exhibits lower friction coefficient and less friction fluctuation than HT-H and SS-L, but also demonstrates a >50% reduction of wear loss compared to HT-H and SS-L (0.0343 g for HT-L vs. 0.0723 g for HT-H, P<0.001; 0.0343 g for HT-L vs. 0.0817 g for SS-L, P<0.001). Corroboratively, observations with scanning electron microscopy revealed a relatively smooth surface for worn HT-L specimen, contrasting with the rugged and grooved surfaces of worn HT-H and SS-L specimens. These results indicate that the bionic coupling of hardness gradient to hexagonal texture can indeed improve anti-wear properties, affording a new strategy to wear and friction management.