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 degrees 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. (C) 2006 Elsevier B.V. All rights reserved.