Tribological performance of functionally gradient structure of graphene nanoplatelets reinforced Ni3Al metal matrix composites prepared by laser melting deposition

In order to meet more rigorous tribological requirements for aerospace and automobile applications, there is a need to improve the wear and frictional performance of Ni3Al-based metal matrix composites. This investigation sought to improve performance by designing gradient-type Ni3Al metal matrix, self-lubricating composites (GNMMCs), in which each layer contained a different content of graphene nanoplatelets (GNPs). Dry sliding tests of a novel material are conducted as a basic assessment for the reference of the specific applications. Rotating sliding tests of GNMMCs against Ni3Al balls were performed under different loading conditions to compare with Ni3Al metal matrix without graphene nanoplatelets (NA) and with homogeneous graphene nanoplatelets (NMCs). The results show that the friction coefficients and wear loss of GNMMCs under different loading conditions are lower than those of NA and NMCs. It is attributed to the facts that together, GNPs and oxides on the worn surface can form a tribo-layer with excellent anti-friction and wear performance, and GNPs accumulations in the worn subsurface can further enhance the strength of the tribo-layer to resist deformation. Additionally, the deposited layers with a gradient GNP content of GNMMCs present a grain gradient structure transiting from the fine grains to coarse grains, which is characteristic of a gradient hardness change. When GNMMCs with high strain accommodating ability are compressed under heavy loading, plastic deformation propagates from coarse grains to fine grains progressively to accommodate the stresses, and that process effectively maintains the wear resistance of tribo-layer.