Tribological Behaviour of Electroless Ni-P Deposits Under Elevated Temperature

Abstract

Electroless Ni-P (EN) coatings have already proven their aptness for improving the tribological performance of the base material. This is possible due to their high hardness, good wear resistivity and corrosion resistance. However, the performance evaluation of the EN coatings under high temperature or the assessment of their thermal stability is yet to be conducted. The present work deals with the study of tribological characteristics viz. friction and wear of EN coatings at elevated temperatures (100 °C, 300 °C and 500 °C) by varying the tribological testing parameters viz. applied load and sliding velocity. A detailed study of the tribological behaviour of the coating is undertaken individually for the as-deposited and heat treated samples. The results obtained are compared among each other and also with that of the room temperature (RT) tests of the coating. It is found that the friction coefficient (COF) and wear rate of EN coatings mostly increases with increase in load for all the test temperatures. However, for variation in sliding velocity, both COF and wear rate show a reverse trend. The as-deposited samples show lesser wear rate particularly at high temperature, which may be because of the in situ heat treatment received during the test. The asdeposited coatings yield better results especially when the test temperature is kept above or near the phase transformation temperature of the coating. The surface morphology, composition of coatings and crystalline structure are studied with the help of scanning electron microscopy, energy dispersed X-ray analysis and X-ray diffraction analysis respectively. The coating displays a nodular morphology and is amorphous in the as-deposited phase.With heat treatment, the coating turns crystalline. A mixed adhesive and abrasive wear mechanism is observed for the EN coatings tested at elevated temperature. Adhesive wear is accompanied by micro-cracks. Tribo-oxidation is confirmed from energy dispersive X-ray spectrometry.