Fabrication of Functionally Graded Electroless Ni-P Coating with Variable Thickness and Evaluating the Coating Thickness Changes and Hardness by Response Surface Method (RSM)

In this research, novel hybrid coatings were fabricated with a gradient thickness of Ni-P by the electroless process on the steel substrate, so that the coating thickness along these coatings gradually changes. To fabricate coating with variable thickness, the method of taking out the sample from the electroless bath during the process was used. For purposive experiments, response surface methodology (RSM) was utilized. Optical microscopy, SEM, EDS and micro-hardness test have been used to characterize the fabricated coatings. The results of variance analysis of the models indicated that the models are highly valued. The sum of squares showed that pH is the most important factor in coating hardness. Also, the effect of pH on the gradient of coating thickness is about 1.5 times the sample exit rate of the electroless bath and 2.5 times the temperature. SEM images and EDS analysis results showed pH and temperature are two important factors in surface morphology and phosphorus content of coatings. The phosphorous content of coatings varies from 7.8 to 10.1 wt%. The coating fabricated in pH 5.4 and 84℃ has the highest hardness about 650 HV. The coating fabricated in pH 5.2, 86℃ and the sample exit rate from the electroless bath 0.11mm/min has the highest gradient of thickness, about 3μm/mm.