Microstructural Characteristics And Wear Behavior Of Electroless Duplex Ni-P/Ni-Mo-P Alloy Coatings With Different Crystallization Processes

With the development of modern manufacturing industry,the performance requirements of materials are more and more higher and promoting the development of nickel phosphorus alloy coating.The development of duplex alloy coating of nickel phosphorus with multifunction and excellent property has become a new hot issue.In the article,The electroless deposition of Ni-P/Ni-Mo-P duplex alloy coatings were prepared by continuous plating method plating.The surface morphology,chemical composition as well as inner and outer layer structure of laser crystallizing and furnace annealing crystallizing of alloy coatings were analyzed by means of SEM/EDS,XRD and jade 5.0 software analysis and testing methods.The hardness and wear resistance as well as wear behavior of the alloy coatings before and after annealing were investigated by means of measurements of microhardness,friction and wear tests.The results show that the outer layer Ni-4.11wt%Mo-6.5wt%P is a semi-crystalline structure,and the inner layer Ni-9.19wt%P is an amorphous structure.For furnace annealing,below 400?,crystallization of only Ni phase appears for the outer layer Ni-Mo-P.Above 400?,except the Ni phase,Ni3 P and Ni-Mo phase precipitation,the mass fraction of Ni3 P phase of coating is less than that of Ni/Ni-Mo phase.Increasing to 800?,the degree of crystallization of the outer layer is about 97.13%.Precipitation of Ni3 P phase occurs for the inner layer Ni-P at 300?,and the mass fraction of Ni3 P phase of coating is larger than that of Ni phase,and the inner layer is completely crystallized at 800?.The grain size of the outer layer is larger than that of the inner layer under the Ni3 P phase is not precipitated,and the outer layer of the grain size is smaller than that of the inner layer once the Ni3 P is precipitated.In addition,the grain size of Ni3 P phase,whether for inner or outer layer,is larger than that of Ni/Ni-Mo phase at the crystallization reaction temperature of Ni3 P phase.For laser annealing,the scanning speed from 14 mm/s~6 mm/s,the inner and outer layer of Ni-P/Ni-Mo-P alloy is not completely crystallized.crystallization of Ni3 P phase appears for the outer layer Ni-Mo-P,below 10 mm/s,associated with Ni-Mo phase precipitation,the mass fraction of Ni3 P phase of coating is less than that of Ni/Ni-Mo phase;precipitation of Ni3 P phase occurs for the inner layer Ni-P at 12 mm/s,and the mass fraction of Ni3 P phase of coating is larger than that of Ni phase,and the inner layer is completely crystallized at 800?.The change of grain size of the inner and outer coating laser annealing and the furnace annealing showed the same trends before and after the Ni3 P phase precipitates.With the change of annealing temperature,the hardness of Ni-P/Ni-Mo-P duplex coating and its inner and outer layers showed the same changing trend.The maximum microhardness of the coatings appears at 400?,and the inner layer hardness is higher than the outer layer below 400?,more than 400?,two layer coating the hardness of the opposite.The highest hardness of the outer Ni-Mo-P and inner Ni-P in the laser annealing treatment appears at the scanning velocity of 8 mm/s and 10 mm/s,respectively.At 12 mm/s or more than 12 mm/s,the outer layer hardness is higher than the inner layer,less than 12 mm/s,two layer coating the hardness of the opposite.Whether for furnace annealing or laser annealing,the main wear mechanism of the outer layer is adhesive wear behavior before the Ni3 P phase precipitates,and the wear mechanism of the outer layer is related to the size of Ni3 P after the Ni3 P phase precipitates.Smaller size Ni3 P lead to wear squamous micro fracture wear behavior;After falling off larger Ni3 P lead to become grits wear like furrows of abrasive wear behavior.the smaller size of Ni3 P lead to wear surface like squamous micro fracture wear behavior;the larger size of Ni3 P shedding as abrasive particles leads to wear surface of furrow like abrasive wear behavior.For the Ni-P/Ni-Mo-P duplex coating,the wear mechanism mainly depends on the structure of the removedouter layer and the exposed inner layer and its hardness.For furnace annealing at 600?,Ni-P/Ni-Mo-P duplex coating is lower than the outer layer and show lower wear rate and the value of 2.7×10-15m3/Nm,which can provide a possibility of potential industrial applications for components required at high temperature condition to keep high wear resistance.For laser annealing at 14 or 6 mm/s scanning speed,the wear rate of the double layer is lower than that of the outer layer,which is 16.8×10-15m3/Nm and 24.9×10-15m3/Nm,respectively.and the value is much lower than that of the furnace annealing at other temperatures.Therefore,the laser annealing is more suitable for the surface modification of the substrate metal which cannot withstand the 600? furnace annealing.