Microstructure and mechanical characterization of HVOF sprayed boride-based cermet coatings

Abstract

Summary form only given. Sintered Co(Cr)-MoB materials have a good combination of high modulus, high wear resistance, high corrosion resistance and adequate fracture toughness, making them suitable for many industrial applications such as metal-cutting tools and wear resistant components. In many applications, they are superior to oxide and carbide coatings. Recently, boride- based cermet surface coatings have been used to enhance the corrosion and wear resistance of various engineering components such as sink rolls and supporting bearings on a continuous galvanizing line (CGL). Many thermal spraying techniques such as air plasma spraying (APS) and high velocity oxy fuel (HVOF) spraying can be used to deposit the boride-based coatings. However, the coating properties depend strongly on the spraying technique. Compared to other spraying techniques, HVOF spraying is one of the best methods to deposit conventional boride-based cermet powders due to the very high velocities thereby flattening the particles into a denser and more wear-resistant coating. In this work, the microstructures and mechanical properties of the deposited Co(Cr)-MoB cermet coatings are investigated via SEM, TEM, XRD, MIP (Mercury intrusion porosimetry), tensile test and Vickers microhardness. The results show that the as-sprayed cermet coating main consists of ternary transition metal borides phases such as CoMo2B2 and CoMoB. The microstructure of the coating keeps unchanged compared with the starting powders and no reaction and interdiffusion occurs at the coating/substrate interface during HVOF spraying. The surface connected porosity of the as-sprayed cermet coating shows a typical bimodal pore size distribution. The adhesive strength of the Co(Cr)-MoB cermet coating decreases with increasing thickness of the as-sprayed cermet coatings, which is attributed to the residual stress induced during thermal spraying. The distributions of the microhardness values of the coatings are conducted by Weibull statistical means and the results reveal a bimodal distribution of Vickers microhardness values. Such distribution can be attributed to the presence of melted and unmelted phases in the resultant coating produced from the microstructured powder feedstock. The anisotropy in the mechanical properties between the cross section and the top surface of the cermet coatings is examined.