Synthesis of boride coatings on steel using plasma transferred arc (PTA) process and its wear performance

Boriding of the surface of a tool steel using boron powder and the plasma transferred arc process was investigated. It was shown that this method is an easy and effective technique in producing uniform alloyed layers with a thickness of about 1.5 mm and a hardness between 1000 and 1300 HV. The microstructure of the borided surfaces consists of primary Fe2B-type borides and a eutectic mixture of borides and martensite. Some cracks are observed in the eutectic regions but they do not seem to critically affect the behaviour of the coatings in sliding wear. The wear rate of pin on disc tests is primarily affected by the applied load and it lies between 10−5 mm3/m for low loads and 10−2 mm3/m for high loads. Two distinct regimes of mild and severe wear are obtained separated by a critical load. Mild wear is due to the load supporting effect of borides and severe wear is due to their breakage above a critical load. The wear rate is not significantly affected by the sliding velocity and is consistent with the friction coefficient. The friction coefficient varies from 0.13 to 0.23 and depends strongly on the oxidation status of the wear track. The sliding velocity affects the sliding distance where the coefficient of friction reaches equilibrium.