Investigation On Microstructures?Texture And Internal Stress Of Electrodeposited Ni-Al And Ni-Zr Composite Coatings

The Ni based composite coatings have been widely used in the fields of electronic packaging,chemical engineering,precise instrument and weapon industry due to their superior properties.The existence of stress and texture in the composite coatings play an important effect on the properties of the coating.Addition of the second phase metal particles and alloying with Co element resulted in significant difference in microstructure,stress and texture of the composite coatings compared with the pure Ni coating.In this study,the Ni-Al and Ni-Zr composite coatings reinforced with Al and Zr metal micro-particles and NiCo-Al and NiCo-Zr composite coatings alloyed with Co element were prepared by using electro-deposition technique.The effects of deposition parameters such as current densities,particle loadings and Co²⁺concentrations on the microstructure,texture and stress of the composite coatings were investigated by using the XRD analysis methods,and the relationship between the formation and evolution of microstructure,texture and internal stress were systematically investigated.In addition,the hardness and corrosion resistance were also studied and the relationship between hardness and corrosion resistance and microstructures and texture was investigated.The detail results are listed as follows:XRD line profile analysis methods including Voigt and whole pattern fitting method were used to examine the micro-structures of the coatings.The pure Ni electrodeposited coating showed the larger grain size?173 nm?.Addition of Al and Zr metal particles could significantly refine the grain size of the composite coatings,and the grain size decreased with increasing metal particle contents in the deposits.The grain size was 72 nm and 55nm at Al and Zr contents of 10.9 wt%and 13.6 wt%,respectively.The reason for refining grain size was that the incorporated particles could act as new nucleus,which promoted the nucleation while inhibited the growth of the grain.The microstrain of the Ni-Al and Ni-Zr composite coatings increased with increasing particle contents;Addition of Co²⁺could further decrease the grain size of the NiCo-Al and NiCo-Zr composite coatings,and the grain size was 22 nm and 21 nm at Co contents of 60.5 wt%and 58 wt%,respectively.And the microstrain and lattice parameter also increased with increasing Co contents;Annealing experiment showed that,with both increasing annealing temperatures and times,the grain size increased while microstrain decreased for the Ni-Al and Ni-Zr composite coatings.According to the regression calculation,the Ni-Al and Ni-Zr composite coatings showed the smaller activation energy of 79.4 and 53.2KJ/mol,respectively.Different texture characterization methods including pole figure,simulated pole figure and texture coefficients were used to analyze the texture of the composite coating.Both the pole figure and simulated pole figure results showed the typical firber characteristic for the electrodeposited coatings.The pure Ni coating showed the stronger?200?texture,and addition the Al and Zr metal particles could inhibit the?200?texture and promote the growth of?111?planes.The main reason was that the Al and Zr particles could adsorb the ions in the bath,which inhibited the?200?planes and promoted the formation of other planes.The schematic diagram of grain refining and texture evolution caused by the incorporation of metal particles was also proposed.Addition of the Co²⁺ions could further promote the growth of the?111?planes;Research also showed that the?200?texture of the coating could be inhibited by controlling the depositing parameters such as increasing the metal particle loadings or decreasing the current densities.Combining the Voigt and Reuss models,the geometric method was applied to calculate the X-ray diffraction elastic constants of the Ni coating with fiber texture,and the calculated results were applied to the stress characterization of the Ni based composite coatings.Addition of Zr particles increased the internal stress of Ni-Zr composite coating and the internal stresses of the Ni-Zr coatings increased with increasing Zr contents;Alloying with Co elements further increased the internal stresses of NiCo-Al and NiCo-Zr composite coatings,and the stresses of the composite coatings increased with increasing Co contents.The increase of internal stresses in the composite coatings could be due to the decreasing grain and texture evolution from?200?plane to?111?plane;Stress tuning methods were also investigated and it could be found that annealing treatment and applying ultrasonic in the depositing process could decrease the stresses of the coatings.The properties such as hardness and corrosion resistance of the compositing coatings were also investigated.The results showed that alloying with Co could increase the hardness of the NiCo-Al and NiCo-Zr composite coatings,and the hardness increased with increasing Co contents and obtained 444 Hv and 498 Hv at Co contents of 42 wt%and 43wt%,respectively.This was ascribed to the grain refining and solid solution strengthening.However,the hardness of the coatings decreased with further increasing the Co content,which was due to the formation of softer HCP-Co phase.Pure Ni coating showed the lower corrosion resistance with the I_corr of 7.206?A/cm².The corrosion resistance of the composite coating could be significantly improved by the addition of Al and Zr metal particles,and the corrosion resistance increased with increasing particle contents and showed the best corrosion resistance at Al content of 23.9 wt%and Zr content of 13.6wt%,respectively,and the corresponding corrosion current densities I_corr for Ni-Al and Ni-Zr coating decreased to 0.207?A/cm² and 0.296?A/cm²,respectively.The improvement in corrosion resistance could be ascribed to three main factors:first,the Al₂O₃ and ZrO₂ passive film could be easily formed on the coating surface in corrosion medium,which decreased the further corrosion of the Ni matrix;second,the decreasing grain size increased the“channel”from surface to the substrate of the corrosive medium;final,the?111?textured composite coating possessedd the higher corrosion resistance than the?200?textured composite coating.Alloying with Co element decreased the corrosion resistion of the NiCo-Al composite coating,but was still more than that of the pure Ni coating.However,addition of Co increased the corrosion resistance of NiCo-Zr composite coating,which could be ascribed to the decreasing surface roughness and increasing compactness.