EFFECT OF ANNEALING TEMPERATURE ON HARDNESS AND WEAR RESISTANCE OF ELECTROLESS Ni-B-Mo COATINGS

Formation of nickel-boron-molybdenum (Ni-B-Mo) coating on steel by electroless plating and evaluation of their morphology, hardness and tribological properties post heat treatment at different temperatures for 1 h is investigated. The 25 mu m thick coating is uniform and adhesion between the substrate and coating is good. Ni-B-Mo coating was amorphous-like structure in their as-plated condition and by 400 degrees C heat-treated coating, nickel fully crystallized and nickel borides and molybdenum carbide were formed. All coatings exhibited higher hardness than the substrate steel. Hardness values of all coatings up to 400 degrees C did not change distinctively but decreased partly beyond 400 degrees C. Friction coefficient reached lowest value post heat treatment at 300 degrees C but later increased with increasing tempering temperature. Wear resistance was lowest in as-plated coating; however it reached the highest value at 300 degrees C. Worn surface of the coatings showed the abrasive wear as the dominant wear mechanism. An additional adhesive wear mechanism was detected in coating tempered at 550 degrees C. Moreover, our results confirmed that the molybdenum addition improved the thermal stability of the resulting coating. Therefore, Ni-B-Mo coating has potential for application in precision mould, optical parts mould or bipolar plates, where thermal stability is essential.