The heat treatment effects on the structure and wear behavior of pulse electroforming Ni–P alloy coatings

The Ni–P alloy coatings with phosphorous content from 8.7 to 13.9 wt.% were prepared in this study by pulse current electroforming. The effects of operating conditions of electroformed Ni–P coatings on the phosphorous contents and micro-hardness of the deposits were investigated. The heat treatment effects on the morphology and grain size was evaluated by X-ray diffraction. The effects of heat treatment on the hardness and wear resistance were also investigated. Our results show that the internal stress within the pc Ni–P deposited coating is much lower than that of dc Ni–P deposited coating and the phosphorus content profile is also more stable than that of dc Ni–P deposited coating. The grain size of as-plated Ni–P deposited coating is smaller than 1.5 nm, but as the phosphorus content increased, both the grain size and microhardness decrease. After heat-treated at 400 °C, the hard Ni3P phase precipitates and the grain size become gross, however, the grain size is still smaller than 5 nm. The hardness of heat-treated coating can be enhanced efficiently and is well above 1000 HV. It leads to a lower wear rate for heat-treated coating. The wear resistance of heat-treated coating can be as high as 2.5 times that of as-plated coating. In addition, the wear resistance and hardness increases with the increasing of grain size for both as-plated and heat-treated coatings. It suggests that the strength and grain size of the Ni–P coating with high phosphorus content obeys the inverse Hall–Petch relationship.