In order to develop protective coatings for sink rolls in continuous hot-dip galvanizing, a sprayed MoB/CoCr cermet coating was formed on a 316L stainless steel by the HVOF spraying process and its durability in the molten 55%Al-Zn- 1.5%Si bath (923 k) has been investigated by SEM and EDS. The immersion test revealed that the MoB/CoCr coating has much higher durability (640 hours) in 55%Al-Zn-1.5%Si bath than the conventional sprayed coatings (120 hours), such as WC-Co, WC/Co/Cr and ceramics. It was found that the failure of MoB/CoCr coating is mainly caused by the mismatch of coefficient of thermal expansion (CTE) between the top coating and the substrate. The failure procedure is that first crack is generated because of heat stress, then the crack proceeds and causes scaled delamination, at the same time molten Al-Zn will enter into cracks and/or Al-Zn reaches the undercoat and/or substrate, finally molten Al-Zn dissolves the substrate.

Thermal spray coatings of new MoB/CoCr cermets were developed. The mechanical behavior of HVOF-sprayed MoB/CoCr novel composite coatings was evaluated via Vickers microhardness. Microstructure of the coatings on 316L stainless steel substrates, as well as powders, were studied with optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD). X-ray microanalysis of the coatings was carried out using energy dispersive X-ray spectrometer (EDS) attached to the SEM. The distributions of microhardness values of the coatings were analyzed via Weibull statistic. Weibull analysis revealed a bimodal distribution of Vickers microhardness values. Such distribution was attributed to the presence of melted and unmelted phases in the resultant coating produced from the microstructured powder feedstock. The excellent mechanical properties of the coating are due to the MoB/CoCr powder, which results in the formation of complex ternary transition metal boride hard particles that exhibit exceptional mechanical properties.

Mechanical properties of micro/nanoscale structures are necessary to design reliable ceramic coatings. Micro/nanomechanical characterizations of novel nanocomposite coatings (NCCs) and micron composite coatings (MCCs) have been studied. Hardness, elastic modulus and creep resistance of these materials were measured by means of nanoindentation. It is found that the nanocomposite coatings exhibit higher hardness, elastic modulus and creep resistance as compared to the micron composite coatings. The nanoindentation tests used in this study can be satisfied to evaluate the mechanical properties of micro/nanoscale structures of ceramic coatings