Wear behaviour of flame sprayed NiCrBSi coating remelted by flame or by laser

Abstract

Flame spray coatings are widely used in industry because of low cost and process simplicity. However, high porosity and poor adherence to the substrate means that quality is poor, though it can nevertheless be improved by subjecting coatings to a remelting using a technique that usually involves an oxyacetylene flame. The study that follows is an attempt to evaluate a laser technique as an alternative to the more traditional flame remelting of flame spray layers, using grey cast iron (DIN GG30) as the substrate and a NiCrBSi alloy as the coating material. Coatings obtained by laser remelting exhibited a practically pore-free microstructure with good adherence to the substrate. The limited control of process parameters during flame remelting led in some cases to incomplete melting of the full thickness of the layers. Hardness of the remelted coatings was very similar in both cases, with values that were slightly lower than for flame-spray layers. The tribological behaviour of both coating types was then compared in dry sliding wear tests (block-on-ring tests) at various loads (30–100 N) and sliding speeds (0.65–2.62 m/s). Both coatings wear rates and wear rate coefficients k (mm 3/Nm) were calculated. No significant differences in wear performance between the two coatings were found. A severe wear regime with adhesion as the principal component was observed at the higher test loads. The predominant wear mechanism at the lower test velocities was oxidation.