Preparation Of Plasma Sprayed Gray Cast Iron Coatings And Its Wear Resistance

To reduce the energy consumption of automobile,lightweight aluminum alloy engine was used to replace the traditional gray cast iron engine.However,the aluminum alloy engine inner wall is easily worn during working due to the poor wear resistance of the aluminum alloy.Therefore,some surface technologies were utilized to improve the wear resistance of the inner wall of the aluminum alloy engine.Among these processes,thermal spray technology was widely used for its high efficiency and low cost.The use of thermal spraying to prepare cast iron coating on the cylinder wall surface is an effective method to improve the wear resistance of the cylinder wall of automotive aluminum alloy engine and prolong the lifetime of the engine.During plasma spraying,it is difficult to precipitate a large amount of graphite in the cast iron coating for high cooling rate of the particles.With the large difference in melting point between graphite and cast iron matrix,it is expected that gray cast iron particles maintain the solid(graphite)-liquid(gray cast iron matrix)two-phase state in plasma jet.Due to the characteristics of rapid heating and rapid cooling of the particles during plasma spraying,the loss of graphite is limited by the short residence time of the particles at high temperature.Therefore,it is entirely possible to reserve the graphite in gray cast iron powder in the coating.In view of this,we propose to prepare gray cast iron coating using a “graphite structure transplantation” method.Firstly,gray cast iron(GCI)powder and graphite-gray cast iron shell-core-structured(G-GCI)powder were prepared by graphitization annealing of cast iron(CI)powder and mechanical ball milling of GCI and graphite powders,respectively.The effects of ball milling parameters such as ball milling speed,ball milling time and ball-to-powder ratio on the surface morphology and surface coverage of graphite-gray cast iron shell-core-structured powder were studied.Then,the influence of powder structure on the microstructure,phase composition and wear resistance of plasma sprayed coatings were studied.As a result,a method of improving the wear resistance of gray cast iron coating was proposed,which provided data support and theoretical basis for the practical applications ofgray cast iron coating.It is showed that the gray cast iron powder with a graphite content of 3.12 wt.% can be obtained by graphitization annealing treatment.The optimum process parameters for preparing graphite-gray cast iron shell-core-structured powder by mechanical ball milling method are as follows: the addition of graphite is 20 wt.%,the ball milling speed is 450 r/min,the ball milling time is 120 min,and the ball-to-powder ratio is 10:1.The graphite content of this powder is 12.42 wt.%.The gray cast iron powder achieved a maximum coverage of 74.69±1.21 % under the optimal ball milling parameters.The flow property of two powders is in the range of 38-41 s/50 g,which is suitable for thermal spraying.The structural study of the coatings shows that:(1)When the cast iron powder is used as the feedstock,due to the rapid heating and rapid cooling feature of the particles,graphite-containing gray cast iron coating cannot be obtained.(2)Using gray cast iron powders as the feedstock,the graphite on the surface of the powder particle is oxidized during spraying,and the graphite in the powder particle is partially retained in the coating with a the graphite content of 1.24 wt.%.(3)when the graphite-gray cast iron shell-core-structured powder is used as the feedstock,the partial graphite-shell is oxidized and the remaining graphite-shell is reserved in the coating.The oxidative exotherm of graphite in the plasma jet increases the temperature of the particles,resulting in the dissolution of the graphite in the powder particles,while the higher particle temperature can improve the bonding between the coating layers.Therefore,the use of graphite-gray cast iron shell-core-structured powder can not only increase the content of graphite in the coating(5.23 wt.%),but also not significantly damage the interlayer bonding.The results of wear test of the coatings show that the friction coefficient and wear rate of gray cast iron coating and graphite-gray cast iron shell-core-structured coating are significantly lower than those of cast iron coating.The main reason is that the graphite in the coating acts as a solid lubricant during the wear process.Since the graphite content of the graphite-gray cast iron shell-core-structured coating is higher than that of the gray cast iron coating,and the graphite mainly distributes between the coating layers,the graphite is morelikely to form a large-area uniform lubricating film during wear process,causing a low the friction coefficient and a low the wear rate.The wear mechanism of cast iron coating and gray cast iron coating is abrasive wear and a small amount of oxidative wear.The wear mechanism of graphite-gray cast iron coating is a small amount of oxidative wear.