Fabrication and Characterization of Nanostructured Functionally Graded Ni-P Electroless Coating

In this research, novel functionally graded Ni-P coating was deposited via electroless process. The content of phosphorus was controlled to change gradually through the thickness of the coating. During the plating, bath temperature and pH changed at specified intervals to obtain a functionally graded structure. To compare the properties of functionally graded coating with the Ni-P single-layer coatings, three types of coatings with different phosphorus contents were also deposited. Microstructure and phase composition of the coatings were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractometry (XRD). The mechanical properties and tribological behavior of the coatings were also investigated. Low phosphorus and medium phosphorus single-layer coatings had fully crystalline and amorphous-crystalline structures, respectively. While high phosphorus coating appeared to have a fully amorphous structure. TEM images showed that low phosphorus coating had a nanocrystalline structure. Results of nano-indentation test showed gradual changes in hardness profile in the cross-section of functionally graded coating due to the gradual changes of the phosphorus content in the thickness of this coating. According to the wear test data, medium phosphorus coating had minimum wear resistance. Functionally graded coatings had better wear resistance than single-layer coatings.