Study On The Preparation And Properties Of Plasma Sprayed Nickel-coated Graphite Coatings

The losing energies and shortening machine life caused by friction,have brought huge losses in world economy every year.Toreduce friction andwear,lubrication has been used to effectively reduce energy consumption and to protect the working mechanical parts,which greatly meets the urgent demands for energy conservation and long life of parts in the modern industry.With the sharp increase of working load and rotate speed,and especially under the harsh working conditions of high temperature,high pressure,high vacuum lubrication,solid self-lubricating coating has received the widespread attentions.In this article,plasma spraying technique was used to prepare the nickel package graphite coating with two spray parameters.The structure and phase composition of the coatings were characterized by SEM and XRD,respectively.The porosity,hardness and bonding strength of the coatings were measured by weighing method,vickers hardness tester and tensile testing method,respectively.On the basis of these,the impacts of spray parameters on microstructure,porosity,hardness and bonding strength the coatings were studied.In addition,the friction and wear test was carried out for the coating with good properites to inspect the influences of load and temperature on friction and wear performance of the coating.Meanwhile,the mechanisms of wear and lubrication of nickel package graphite coating were discussed.The experimental results show that,compared with the coating prepared under the low power(25 kW),nickel package graphite coating sparyed under the larger power exhibits a good coating-substrate adhesion with no obvious coating-substrate gap.In addition,low porosity and uniform structure of the coating make the coating properties good.At room temperature and with a fixed ratate speed of 200 r/min,the friction coefficient of vermicular graphite cast iron substrate is 0.64,0.68,0.71 under the load of 4 N,8 N,20 N,respectively.With nickel package graphite coating on the substrate,the friction coefficient is 0.22,0.18,0.15,respectively.At 200?,the friction coefficient is 0.23,0.2,0.36,respectively,which are larger than those tested at room temperature,especially the friction coefficient increased significantly at high load,.Both at room temperature and 200?,after spraying nickel package graphite coating,the friction coefficient of the substrate reduced significantly.This suggests that the effect of nickel package graphite coating is antifriction.The wear rate of the substrate is larger than that of coated substrate.With the increase of load,the wear rate of the substrate increases linearly.After spraying nickel package graphite coating,both at room temperature and 200?,the wear rates evidently decreased,and the wear rate at 200? is higher than that at room temperature.At room temperature,when the load is 4 N,graphite lubricating phase spreads out gradually with exposing of graphite.At this time,a complete graphite lubricant film did not formand the wear loss is small,wear mechanism is adhesive wear and tear.When the load is 8 N,graphite lubricant film gradually formed with widening of friction traces and increasing of exposing graphites,and the antifriction effect is more remarkable than that prepared under a load of 4 N.At this moment,the wear mechanism is adhesive wear and abrasive wear.When the load is 20 N,the surface of the coating began to appear cracks,and there are a few peeling phenomena.,The wear mechanism is abrasive wear.At 200?,when the load is 4 N,graphite lubricant film gradually formed,but the graphite has been oxidized slightly,lubrication effect is weaker than that at room temperature.The wear mechanism is adhesive wear and oxidation wear.When the load is 8 N load,enhanced temperature makes graphite oxidized seriously..The wear mechanism is abrasive wear and oxidation wear.When the load is 20 N,graphite oxidation is serious as a result of increased micro area temperature,The content of graphite decreased and with the increase in stress and insufficient lubrication,the coating spalled in a large area and wear mechanism is abrasive wear and oxidation wear.The lubricating mechanism of nickel package graphite coating is the exposed graphite constantly reach the surface of the coating as friction,under the action of shear stress and compressive stress,spread out on the coating surface,gradually form a relatively complete graphite film,which separates the direct contact between the friction pairs,so as to achieve the results of lubrication and reducing friction.