| dc.description.abstract | The electroless deposition has been adopted as a simple and cost-effective method to synthesize Ni-P coatings on various substrates for corrosion and wear protection. Such coatings on diamond abrasives have been used to manufacture diamond tools. Those coatings, however, if not synthesized properly, are easy to be fractured and detached from the substrate during the deposition and manufacturing processes. This research investigates the corrosion resistance, deposition kinetics, and coating failure mechanisms of Ni-P coatings electrolessly deposited on diamond particles. The major activities are summarized in the following.
One of the challenges in this research is the lack of characterization methods for coatings on such small particles with irregular shapes. To overcome this, a novel electrode was developed so that the electrochemical characterization was successfully conducted on Ni-P coated particles. The micro-CT technique was used to characterize the coating on small irregular-shaped particles to get comprehensive morphological information.
Through electrochemical characterizations, an improved electrolyte with less corrosion power to the Ni-P coating was developed for the diamond tool manufacturing process. The crucial microstructure which causes coating failure is found to be the micropores generated in the coating due to hydrogen gas evolved during the deposition process. The coating failure was found to be caused by the corrosion assisted breakthrough of close pores in the coating.
Various morphological characterizations techniques demonstrate that the coating coverage rate on diamond particles is affected by the synergistic action of the deposition time, substrate morphology, and reducing agent concentration. The two major morphological features of the coating: nodular and smooth are influenced by the deposition parameters, coating integrity, and substrate morphology.
The deposition kinetics of coatings were studied through analyzing coating thickness as a function of plating time in the plating solutions of varied reducing agent concentrations. The dependence of deposition rate on reducing agent concentration and nickel ion concentration was found at different stages of the deposition process. The change of the dependence relationship was found to be affected by the free nickel ion concentration.
Morphological characterization revealed that the failure was due to tensile residual stress produced by the coalescing of crystallites during the deposition. This failure mechanism explains the tendency of coating fracture at three morphological features of the substrate.
This research is the first comprehensive study of the coating failure mechanisms on diamond particles. The new methodologies developed in this study will be beneficial for the investigation of other coatings on the particle systems. | |
