The Effect of Heat-treatment on the Structure and Hardness of Electroless Ni-B Coated Ferrous PM Parts

摘要

The Ni-B coating was deposited on ferrous powder metal (PM) compacts by electroless coating process and its morphology, structure and microhardness was evaluated. Some of the coated specimens were heat treated at different temperatures (300-500 ℃) as isothermal for 1 h. The surfaces and cross-sections of coatings were examined and characterized by scanning electron microscopy (SEM). X-ray diffraction (XRD) analysis was performed to investigate structural modifications. The phase transformation temperature of Ni-B coating was determined by differential scanning calorimetry (DSC). Microhardness of electroless as-deposited and heat-treated Ni-B coatings were evaluated. The results showed that, Ni-B coating has an amorphous structure as deposited, but, after heat treatment, its turn to crystalline. It was found that nickel borides (Ni2B, Ni3B) precipitated in the nickel matrix. It was determined that precipitation has caused the increase of hardness. The maximum hardness was provided in heat treated Ni-B coatings at 400 ℃.