| Ni-Mo alloy coatings are widely used in electronics,automobile,aerospace,molding and chemicals industry due to their high hardness,excellent wear resistance and corrosion resistance,and low hydrogen evolution overpotential.However,with the development of industry,the alloy coatings are difficult to meet the requirements of materials worked in extreme friction and corrosive environment.Investigations showed that the alloy matrix/second phase hard particles composite coatings can effectively improve the mechanical and anti-corrosion properties of the material.In this case,Ni-Mo/diamond coatings are prepared by adding micron-sized diamond particles into Ni-Mo matrix.The structure,hardness,wear and corrosion resistance of these coating are systematically studied.In this paper,Ni-Mo coatings with wide Mo content were prepared by adjusting electroplating parameters.The effects of Mo content on the structure,hardness and anti-wear properties of the as-deposited Ni-Mo coatings and the coatings heat-treated at 450? were systematically investigated,and the plastic deformation mechanism of the Ni-Mo coating was discussed.Then Ni-Mo/diamond coatings were prepared by adding diamond particles of different concentrations and sizes into the plating bath.The effects of particle content and size on the structure,hardness,wear resistance and corrosion resistance of Ni-Mo coatings after annealed at 450? were analyzed.Subsequently,the influence of the evolution of particle/matrix interface on the structure,mechanical and corrosion properties of Ni-Mo/diamond coatings were analyzed.The main results are as follows:Both the as-deposited and as-annealed Ni-Mo coatings consists of nanocrystalline Ni(Mo)solid solution with the average grain size less than 15 nm,and the grain size decreases with the increase in Mo content.Stress induced grain growth is happend during the friction.The TEM analysis showed that the plastic deformation mechanism of the nanocrystalline coating is controlled by grain boundary.The hardness of as-deposited Ni-Mo coatings is in the range of 5.3 GPa to 6.2 GPa,and the critical grain size of Hall-Petch relationship is about 7.9 nm.After annealed at 450?,the hardness of coatings reaches 7.7-10 GPa.In addition,the wear rate of the as-deposited and as-annealed Ni-Mo coatings is in the order of 10-4 mm3 N-1 m-1,and there is a traditional Archard law between the wear rate and the hardness.The metal matrix in the Ni-Mo/diamond coating after annealed at 450? is still composed of nanocrystalline Ni(Mo)solid solution with grain size small than 10 nm.The grain size of matrix decreases with the increase of particle content,which indicates that the addition of diamond can improve the thermal stability of nanocrystalline.In addition,diamond particles are helpful to improve the hardness and wear resistance of the coating.The hardness of the coating is between 9.6 GPa and 15.4 GPa,and the wear rate is in the range of 6.8×10-5mm3 N-1 m-1 to 2×10-6 nm3 N-1 m-1.At low diamond content,the hardness of the coating has almost no change.When the content of diamond with the size of 0.5,1,5 and 10 ?m exceeds 9,13,18 and 21 vol.%,respectively,the hardness has an obvious improvement.The wear resistance of the coating with low diamond content(<10 vol.%)first increases and then decreases with the increment of particle size.Considering that the hardness of the coating is basically the same,the change of wear resistance for the coatings mainly comes from the wear mechanism in which the abrasive wear first increases and then decreases.With the increase of particle content,Ni-Mo+Al2O3 film is easy to form in the coating containing large diamond(?5 ?m).This continuous film can prevent the direct contact between the ball and the coating,thus reducing the friction coefficient and wear rate of the coating.During annealing at 450?,the segregation of Mo at the particle/matrix interface improves the hardness and wear resistance of the coating.When the heat treatment temperature is 600?,the formation of MoNi phase consumed the enriched Mo at the particle/matrix interface,and the the growth of grains make the decrease in the hardness and wear resistance of the coating.The same reason also causes the softening of Ni-Mo/diamond coating after annealing at 750?.However,due to the formation of Mo2C and the increment in elasticity modulus,the wear resistance of the coating is improved by an order of magnitude.The corrosion properties of Ni-Mo and Ni-Mo/diamond coatings were evaluated in 3.5 wt.%NaCl solution by electrochemical methods.Results showed that the corrosion resistance of Ni-Mo coating after annealed at 450? decreases with the increase in Mo content because severe local corrosion occurres at the boundary of columnar clustersthe.The addition of particles also causes the uneven distribution of solute atoms at the particle/matrix interface,which leads to the decrease in corrosion resistance of annealed Ni-Mo/diamond coatings.Compared with the Ni-Mo/diamond coating after annealed at 450?,the solute atoms in the as-deposited coating distribute uniformly,and the coating exhibits uniform corrosion and high corrosion resistance.After composite coating annealed at 600? and 750?,Mo compounds are precipitated in the coating,which decreases the element segregation at the particle/matrix interface,thus the coating also exhibites uniform corrosion and improved corrosion resistance. |
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