Characteristics And Properties Of Laser Nanocrystallised Electroless Ni-W-P/Ni-P Duplex Plating

Electroless plating has been widely applied in practical production because of the simple process and large area coating. Currently, the gradient coating with specific functions,stable and reliable performance has been received attention. The properties of the electroless duplex coatings is more excellent than a single combination by selecting the right combination of coating composition. In this paper, a comparative study of the chemical deposition Ni-W-P (outer)/Ni-P (inner) duplex layers and single alloy coating with the same composition. Using XRD, SEM/EDS and other test methods to analyze the microscopic characteristics of coating composition, microstructure and surface morphology and by hardness, corrosion and friction and wear experiment to research the effect of annealing crystallization to the coating microstructure, hardness and corrosion resistance; the wear mechanism of the coating had been discussed, the main conclusions are as follows:(1) The effect of Annealing on the microstructure of duplex alloy coatingsThe out layer of the double alloy coating in as-plated Ni-4.9W-6P (wt%) is a mixture of amorphous structure,and the inner layer of Ni-9.3P (wt%) is an amorphous structure.For the heat annealing methodin furnace, the inner Ni-P player of double coatings to precipitate Ni3P phase is from 300℃ and the Ni-W-P delay to 400 ℃.The size of Ni3P both in the inner and outer lalyers is larger than Ni when beginning precipitated,and in turn to ℃600.At the same annealing temperature, the crystallization degree and the crystallization of Ni3P phase of the outer layer and inner layer of the double coatings were slightly lower than corresponding single coating.For laser annealing,as the scanning speed is reduced from 10 to 8mm/s, both the outer and inner layer of the duplex coatings precipitate Ni3P, the maximum degree of crystallization reached 71% and 83% respectively, has not yet been fully. After laser annealing, the inner layer of the grain size of Ni3P phase is larger than Ni, the grain size of Ni3P in the outer layer is less than Ni, this is mainly because the scanning speed corresponding to the heating temperature.(2) The effect of Annealing on the hardness of duplex alloy coatings.The hardness of Ni-P, Ni-W-P single alloy coating and double alloy coatings Ni-W-P/Ni-P by furnace annealing reached the maximum value of temperature at 400 ℃, the largest hardness belong to single Ni-W-P with 1151HV.And the maximum hardness value in inner and outer layers of double coatings and the single Ni-W-P alloy coating after laser annealing is in the scanning speed of 8mm/s, the same as single Ni-W-P alloy coating, hardness is up to 967HV.(3) The effect of annealing temperature on the corrosion resistance of the duplex coatingsThe corrosion features in 0.5 M H2SO4 solution of Ni-P, Ni-W-P single alloy coating and double alloy coatings Ni-W-P/Ni-P by furnace annealing is uniform corrosion and without local holes on surface. In the annealing temperature is 300℃, double coatings showed higher corrosion resistance than Ni-P and Ni-W-P single alloy coating.(4)The influence of scanning speed on wear resistance of the duplex coatingsWith the laser scanning speed is reduced from 10 to 8mm/s, the wear rate of double coating decreases, and the wear resistance is better than that of single Ni-W-P alloy coating. The wear form of the inner and outer coating as-plated is dominant by adhesive wear; and after laser annealing of duplex coatings, the inner layer shows abrasive wear behavior with main feature of plough of the outer layer, and the outer layer showed a slightly adhesive wear behavior.The wear mechanism of the coating is relate to the size of Ni3P phase, crystallization degree, hardness and other factors between the friction pairs.