Graphite, due to its combination of unique properties such as low density, anisotropy, high chemical resistance, and self-lubrication, has been regarded as an excellent option for tribological applications. In this study, nano-graphite particles with concentrations of 0.2 g/L, 0.5 g/L, and 0.7 g/L were added to the electroless nickel-phosphorus bath, and the desired composite coatings were deposited on CK45 steel substrates. The baths used were prepared with a commercial electroless solution from Schloetter, Germany, at a temperature range of 82-88 °C, a pH of 4.4-6.4, and a stirring speed of 400 rpm (using a magnetic stirrer). The reduction in hardness due to the increase in the percentage of nano-graphite in the coating, as well as the effect of increasing the concentration of graphite nanoparticles on the deposition rate of the electroless nickel-phosphorus-nano-graphite composite coating, were eva‎luated. The obtained nickel-phosphorus-nano-graphite composite coatings were heat-treated at 400 °C for one hour, and the structural changes from amorphous to crystalline, along with their hardness measurements, were compared to simple nickel-phosphorus coatings. Wear and friction tests were conducted over a distance of 400 m with forces of 1N, 3N, and 6N at a speed of 0.05 m/s to assess the impact of nano-graphite in the electroless coating and the effect of heat treatment on wear and its mechanisms. Corrosion tests were performed in a 3.5% NaCl solution at room temperature, and the effect of the presence of nano-graphite particles in the electroless coatings and their heat treatment was compared to simple electroless coatings. Optical microscopy (OM), scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) analysis were employed in this study. According to the results obtained, increasing the amount of nano-graphite in the bath from 0.2 g/L to 0.5 g/L and 0.7 g/L was associated with a reduction in hardness by approximately 3% and 20%, respectively, along with a reduction in deposition rate of about 11% and 37%. Increasing the vertical force from 1N to 6N in the wear study was accompanied by an increase in wear amount, with an increase of about 230% for the nickel-phosphorus-nano-graphite composite coatings obtained from a bath containing 0.5 g/L of nano-graphite, which decreased to less than 1% after heat treatment (based on weight loss). The average coefficient of friction for the nickel-phosphorus-nano-graphite coatings obtained from a bath with 0.5 g/L of nano-graphite showed a reduction of about 45%, and after heat treatment, approximately 85% compared to the simple nickel-phosphorus coating. Corrosion test results indicated that nano-graphite particles had a negative impact on the corrosion resistance of the electroless coatings, leading to a decline in this resistance, with the level for the Ni-P-0.5 g/L nGr sample being about one-fourth compared to the simple nickel-phosphorus coating; however, after heat treatment, this sample exhibited excellent corrosion behavior. According to the findings of this research, to simultaneously maintain deposition rate, hardness, wear resistance, friction, and corrosion, the optimal option for the electroless nickel-phosphorus-nano-graphite composite coating was obtained from a bath with 0.5 g/L of nano-graphite and underwent heat treatment (400 °C for one hour).

محمود منيرواقفي

علي اشرفي

فخرالدين اشرفي زاده , عبدالمجيد اسلامي