Abstract
Dry sliding test were performed following ball-on-flat test geometry against an EN31 steel counterface on four different heat treated electroless Ni–B coatings: as-deposited (amorphous), 385 °C/4 h (completely crystalline), 300 °C/10 h (mix of amorphous and crystalline Ni3B), and 300 °C/48 h (minor amount of amorphous phase with crystalline Ni3B and metallic Ni) to evaluate the tribo response of the coating as a function of coating's microstructures. Hardness, wear, and friction behavior of the coatings are strongly dependent on the volume fraction and length scale of the boride phases produced upon crystallization of the as-deposited coating. Formation of nanocrystalline boride precipitates is governed by the complex phase transformation behavior of the as-deposited amorphous coating. Presence of local composition modulation in the as-deposited amorphous structure, induced by the deposition process, controls overall microstructure development of the coating. Two dominant wear mechanisms: oxidative and adhesive have been observed for the present experimental conditions, where the transition from the oxidative to adhesive wear regime depends on the coating's microstructure and contact pressure. Synergistic effects of the volume fraction of crystalline boride phases and its sizes, metallic Ni, and presence of remnant amorphous phase on determining the hardness and wear mechanism transition have been explained in relation to the worn track microstructure and chemical characterizations. A microstructure-based guideline for designing an optimal heat treatment scheme has been proposed.
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