Research On The Preparation And Properties Of Nano-particles Enhanced Brush Plating Cr Composite Coatings

In this paper, Cr-CNTs and Cr-n-Al₂O₃composite coatings have been prepared onsteel45and gray cast iron substrates by brush plating. Scanning ElectronMicroscope（SEM） were employed to investigate the surface and cross section morphologyof the coatings. Energy Dispersive Spectrometer （EDS） were used to analyze thedistribution of the nano particle CNTs and n-Al₂O₃in the coatings. X-ray diffraction（XRD） were applied to analyse the structure of the coatings, at the same time, the grainsize of the coatings can be calculated by Scherrer Equation. In addition, hardness, wearresistance and corrosion resistance of the coatings were tested with micro-hardness tester,wear tester and electrochemical workstation. The wear and friction mechanism andelectrochemical corrosion mechanism of the coatings have been studied deeply. Also theaffect of the properties of coatings by the addition of nano particle were discussed asfollow.Both the steel45and gray cast iron substrated composite coatings present compactstructure without obvious defects in moderate doping levels of nano particle. When dopedtoo much amounts of nano particle, the structures of the composite coatings became loosebecause of the agglomeration of nano particle. The result of EDS indicated that CNTsdisperse in the coatings and concentrated on the boundaries of the cell structure, n-Al₂O₃distributes and disperses evenly in the coatings.XRD show that both the Cr-CNTs Cr-n-Al₂O₃composite coatings and Cr coating hadcrystal structure; the lord strong pea of all coatings were Cr（110）. From this, we can seethe existence of nano-particles did not change the preferentially growth orientation ofgrain. The calculations result of Scherrer Equation indicated the composite coatings hadsmaller grain size,which result in high hardness.The abrasive resistance of both the steel45and gray cast iron substrated compositecoatings were better than Cr coating.The main reason can be listed as follow: first the hardnano particles led to dispersion-strengthening in coatings; second the hard nano particlesrestrained the degree of plough by defriction components and stopped the growth andpropagation of the microcracks in the process of wear; third hard nano particles canprovide protection for sureface of coatings. All these factors caused the improvement of the abrasive resistance. At the same time, the property of nano particles also can influencethe abrasive resistance of coatings. CNTs have good toughness properties and goodself-lubricating which determined by its perfect bonds and unique morphology. Comparedto Cr-n-Al₂O₃composite coatings, CNTs have stronger adhesion stress in coatings. Forthis reason, the abrasive resistance of Cr-CNTs composite coatings was better thanCr-n-Al₂O₃composite coatings. The wear morphology show that the main wearmechanisms of Cr coatings and Cr-CNTs Cr-n-Al₂O₃composite coatings were abrasivewear.The results of polarization curves showed that moderate doping of nano particles canimprove the corrosion-resistance of coatings in sulfuric acid solution, but have little effectin the akali resistance to corrosion of coatings. Beacuse the electrochemical corrosionmechanism of coatings in sulfuric acid solution is different from it in aqueous alkali. Tafelfitting results of the polarization curves show that the doping amounts of nano particles isthe major factor to affect the corrosion-resistance of coatings in sulfuric acid solution.What is more, the corrosion-resistance of coatings in sulfuric acid solution is much betterthan it in aqueous alkali. The analysis result of the corrosion morphology of the coatingsindicated that Cr cotings strippinged a lot after being corrupted in sulfuric acid. But thecorrosion morphology of the coatings in aqueous alkali is not so serious than it in sulfuricacid. The XRD of coatings after corrosion have also illustrated this fact. So we can see, Crcoatings can be use in the field of alkali corrosion.