Study On Preparation Process And High Temperature Oxidation Resistance Of Ni-W-P Alloy Coating

Due to the fine thermal stability, excellent corrosion resistance and wear resistance,Ni-W-P alloy was used many fields such as bearing, roller, piston ring, hot forging die, clock movement, oil containers and internal-combustion engine cylinders. The metal compositions of plating solution and technological conditions had complex influence on the composition of the deposited layer, which resulted from the induced codeposition of Ni-W alloy. Besides, the content of each element in the Ni-W-P alloy coating had important influence on the microstructure and properties of the coating.In this paper, Ni-W-P series alloy coatings were prepared using different process parameters and composition of plating solution, which based on the Watts solution. The surface morphology, microstructure, composition and deposition rate of Ni-W-P coatings were characterized by SEM, XRD and EPMA. In order to determine the optimum composition of the plating solution, the weight gain method was used to measure the high temperature oxidation resistance of the coating. Under these conditions, the Ni-W-P-Al₂O₃ composite coating was prepared by adding Al₂O₃ particles in the bath, and the effect of Al₂O₃ content on the high temperature oxidation resistance and the bonding strength of these coatings were studied. The results were obtained as follows:The effects, including current density, pH value and bath temperature, had significantly influence on the deposition rate and morphology of Ni-W-P coating. The Ni-W-P alloy coating with good surface quality could be obtained under the condition of pH value of 6,current density 12 A·dm-2, bath temperature 60 ℃ and plating time 20 min. Besides, the concentration of H₃PO₃ and Na₂WO₄·2H₂O in the bath had a great influence on the content of each element in the coating. When the concentration of Na₂WO₄·2H₂O reached 80 g/L, the coating contained higher W content and the grain size arrived to the minimum value. When the concentration of H₃PO₃ increased to 20 g/L, the content of P in the coating increased to3.9 at.% and the surface quality became better. During the deposition process, W and P elements would be competitive precipitation in the coating, leading to the interaction of the two elements. The coating with W 9.1 at.% and P 3.9 at.% exhibited the best high temperature oxidation resistance. Adding a certain concentration of nano-Al₂O₃ particles can refine the microstructure of the Ni-W-P alloy coating, and the refinement effect of the nanoparticles was obviously superior to that of the micron grade. The experimental results showed that the addition of nano-Al₂O₃ could improve the oxidation resistance of the coating. When the concentration of Al₂O₃ reached 15 g/L, the average size of the cellular tissue arrived to the minimum value and the resistance to high temperature oxidation was better than others.According to the experimental results of grid method, the adhesion of Ni-W-P-Al₂O₃ composite coatings achieved the first level standards. Therefore, the optimum conditions that prepared the Ni-W-P-Al₂O₃ composite coatings, including the process parameters and the main plating solution composition, could be obtained. The process parameters: pH was 6,current density was 12 A·dm-2, plating liquid temperature was 60℃ and plating time was 20 min. The main plating solution composition: NiSO4·6H2O was 80 g/L, H₃PO₃ was 20 g/L,Na₂WO₄·2H₂O was 80 g/L, Na3C6H5O7·H2O was 70 g/L and nano-Al₂O₃ particles was 15 g/L.