Investigation On Microstructures And Properties Of Electrodeposited Ni-xAl-yTi Composite Coatings

The ternary nickel composite coatings synthesized by electrodeposition are widely researched due to their superior properties.The superior properties of ternary nickel coatings are greatly controlled by the microstructures.Thus,the importance of researching evolution rules of microstructures and properties should attract much attention.In this study,the Ni—x Al—y Ti composite coatings are synthesized.The influences of electrodeposition parameters including current density,Al and Ti mixed particles concentration?A/Ti concentration?and weight rate of Al and Ti particles?Al/Ti rate?in electrolyte on microstructure,internal stress and properties of Ni—x Al—y Ti composite coatings are researched.The evolution rules of microstructure and internal stress with electrodeposition parameters are analyzed by X-ray diffraction?XRD?.An important XRD profile analysis method,Rietveld refinement,is utilized to evaluate the evolution of crystallite size and texture of Ni—x Al—yTi composite coatings with electrodeposition parameters.The accuracy of calculated results obtained from Rietveld method are supported by the transmission electron microscope?TEM?and XRD measured pole figures.The results show that the crystallite size decreases and texture diminishes with decreasing current density,increasing Al/Ti concentration and decreasing Al/Ti rate.The microstructure evolution of Ni—x Al—y Ti composite coatings are greatly influenced by incorporation of Al and Ti particles.The interface conditions of between Al,Ti particles and nickel matrix crystallites are analyzed by high resolution transmission electron microscope?HR-TEM?.An amorphous layer of alumina covering Al particle contributes to the“surrounding growth mechanism”of nickel deposits near Al particle.However,the nickel deposits grow directly on the surface of Ti particle,namely“direct growth mechanism”.This two different growth mechanism of nickel deposits cause the different influences on microstructure evolution of Ni—x Al—y Ti composite coatings.Comparing with Al particles,the influences of Ti particles on growth of crystallite size,texture and micro-strain of Ni—x Al—y Ti composite coatings are stronger.The relationships between internal stress and electrodeposition parameters of Ni—x Al—y Ti composite coating have been researched by the X-ray analysis.The internal stress in surface layer of Ni—x Al—y Ti composite coating shifts from compressive stress to tensile stress and the value of tensile stress increases with decreasing current density?under 100 g/L and Al/Ti=1/1?and increasing Al/Ti concentration?under 5 A/dm² and Al/Ti=1/1?.With decreasing Al/Ti rate under 1 A/dm² and 200 g/L,the tensile stress of composite coating increases.Meanwhile,the internal stress in composite coating transforms from compressive stress to tensile stress and the value of tensile stress increases with Ni—x Al—y Ti composite coating growing.By HR-TEM,it can be found that some nickel twin crystals prefer to grow near the Ti particles rather than Al particles,which is caused by the higher stress in nickel deposits near Ti particles.The microhardness and corrosion resistance of Ni—x Al—y Ti composite coatings are investigated.The results show that the microhardness of Ni—xAl—yTi composite coating increases with decreasing current density,increasing Al/Ti concentration and decreasing Al/Ti rate.The microhardness of Ni—x Al—y Ti composite coating?electrodeposited under 1 A/dm²,100 g/L,Al/Ti=1/1,and 5 A/dm²,200 g/L,Al/Ti=1/1,and 1 A/dm²,200 g/L?can be more than 400 HV_0.025,even more,521.4 HV_0.025?under Al/Ti=0/1,1 A/dm² and 200 g/L?,whereas only ca 211.7 HV_0.025 for pure nickel coating.The localized corrosion prefers to appear on the surface of pure nickel coating.The incorporation of Al/Ti particles can improve the corrosion resistance of Ni—xAl—y Ti composite coating?electrodeposited under low current densities,100g/L,Al/Ti=1/1 and 5 A/dm²,200 g/L,Al/Ti=1/1 and 1 A/dm²,200 g/L,different Al/Ti rates?by one magnitude compared with pure nickel coating.The incorporation of Al/Ti particles and microstructure evoluation of Ni—xAl—y Ti composite coatings are responsible for the improvement of corrosion resistance.The incorporation of Al/Ti particles can change corrosion paths and inhibiting localized corrosion.