Investigation of the adhesion strength and deformation behaviour of in situ fabricated NbC coatings by scratch testing

Niobium carbide (NbC) coatings on the surfaces of grey cast iron (HT300) substrates were produced by in situ synthesis combining casting with heat treatment. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses reveal that the coating consists of dense NbC grains and small amounts of α-Fe. The grain size of NbC is 350 nm on average. Due to the improvement of the microstructure by ultrafine-grained NbC, the hardness and elastic modulus of the coating are 21.1 GPa and 376.1 GPa, respectively. The critical load in the micro-scratch test is used as a measure of scratch adhesion. The average value of adhesion strength at the coating/substrate interface is 87.8 N, at which the NbC coating (thickness of 21.3 μm) is delaminated from the substrate. The results indicate that the coating exhibits high adhesion strength on the substrate. The deformation behaviour studied by micro-scratch tests demonstrates that cracks propagate along the grain boundaries and that the failure of the coating is caused by the interaction of radial and lateral cracks.