Electroless NiP coatings due to their high corrosion and wear resistance, hardness, desirable physical and mechanical properties have introduced as hard coatings. Also composite electroless coatings as new generation of electroless coatings attracted the attention of researchers in the last few decades. Corrosion and wear behavior of electroless composite have been widely studied over the past decades. Essentially, size, type and distribution of co-depoisited particles determine the corrosion and wear resistance of composite coatings. But tribocorrosion of these coatings have not studied widely. As zirconia shows outstanding mechanical properties, it was selected as second phase of NiP electroless composite coatings in this study. Therefore, NiP, NiP-ZrO 2 (5?m size of ZrO 2 particles) and NiP-ZrO 2 (submicron) composite coatings, were deposited from a commercial electroless bath with 40?m thickness. Characterization of coatings was performed by X-ray diffraction, scanning electron microscope and energy dispersive spectroscopy. Tribocorrosion experiments were conducted in 3.5wt% NaCl solution under 10 N normal load and 1800 sliding cycles at 0.5Hz against SiC counter ball at room temperature. Hardness evaluation of coationgs illustrates that hardness of NiP coating increased from 601 to 655(HV1) by co-deposition of ZrO 2 (5?m) particles and decreased to 588(HV1) by co-deposition of submicron particles. Cyclic polarization results shows that current density of NiP coatings increased from 0.28 to 0.61 ?A/cm 2 by co-deposition of ZrO 2 particles and corrosion potential decreasd from -300 to -348 mV Ag/AgCl . Also as a result of ZrO 2 particles co-deposition, pitting potential decreased and repassivation capacity of coatings increased. Corrosion behavior of the coatings was studied by cyclic potentiodynamic polarization and electrochemical impedance spectroscopy before and after tribocorrosion tests. In tribocorrosion experiments, the effect of ZrO 2 particles of the coating on open circuit potential, anodic current, friction coefficient and weight loss was evaluated. The results obtained from OCP variation and EIS confirm that the repassivation rate of composite coatings increases during tribocorrosion via separation of zirconia particles. Evaluation of the synergistic effect between corrosion and wear shows that the corrosion resistance of NiP-ZrO 2 (5?m) during sliding is more than NiP coating, but wear resistance of composite coatings is lower than NiP coating. Keywords : Corrosion, Tribocorrosion, NiP-ZrO 2 , Composite, Coating
