Preparation Technology Of Wear-Resistant And High-Temperature Oxidation Coating By PN-MAIP On Titanium Alloy Surface

The valve is the engine on the important parts,it and the valve seat to form a closed space to protect the cylinder head,its performance directly affects the efficiency of the engine and life.In recent years,due to the poor wear resistance of engine valves and low oxidation resistance,the surface wear of the valve is serious,which in turn affects the tightness of the valve,reduces the sealing performance of the valve,causes waste of fuel and insufficient power,indirectly leads to increased fuel consumption,reduces engine efficiency,affects the economy of the vehicle,and the engine failure caused by valve failure increases year by year,so it is necessary to improve the oxidation resistance and wear resistance of the engine valve.Plasma nitriding（PN）technology and multi-arc ion plating（MAIP）technology were used to prepare composite coatings on the surface of TC4 valve materials,and their microstructure,mechanical properties,friction properties,and oxidation properties were investigated.In this paper,SEM,XRD,and white light interferometer were used for the inspection and analysis of the microstructure,phase composition and three dimensions of the plating;Vickers hardness tester,friction and wear testing machine and adhesion automatic scratch tester were used for the determination of the surface hardness,friction coefficient and film base bonding;High-temperature oxidation test was conducted by high temperature oxidation furnace.The results showed that:（1）TC4 was treated with ion nitriding at different nitriding temperatures.After ion nitriding,a nitriding layer and a diffusion layer are formed on the surface of TC4,and the nitriding layer is mainly composed of Ti N and Ti₂N.When the nitriding temperature is870～950℃,with the increase of nitriding temperature,the thickness of the nitriding layer first decreases and then increases,and the diffusion layer increases first and then decreases;The surface hardness of the nitriding layer first increases and then decreases,and the surface roughness and friction coefficient decrease first and then increase.When the nitriding temperature is 910℃,the surface structure of the nitriding layer is uniform and dense,the minimum surface roughness is 0.465μm,the hardness of the nitriding layer is up to 1215.8HV,the friction coefficient is 0.5,and the wear mechanism is abrasive wear and oxidation wear.After 120h oxidation from 600℃-800℃,cracks and shedding appeared on the surface of the nitriding layer,and the oxidation product was Ti O₂.The average oxidation rate was as low as0.277g·m^-2·h^-1,and TC4 was anti-oxidation at 600℃-700℃after nitridation treatment,and weak anti-oxidation at 800℃.（2）Multi-arc ion coating technology was used to prepare Cr Ti Al N coating on the TC4surface by controlling the flow of N₂,and the coating surface was mainly composed of Cr N phase.When the nitrogen flow rate is 45sccm～90sccm,with the increase of nitrogen flow volume,the surface structure of the coating is uniform and dense,and the joint between the coating and the matrix is smooth and flat;The hardness increases,the surface roughness and friction coefficient first decrease and then increase,and the bonding strength of the coating and the matrix first increases and then decreases.When the nitrogen flow rate is 75sccm,the minimum surface roughness is 0.219μm,the thickness of the film layer is 4.4μm,the interface between the coating and the matrix is smooth and flat,the highest bonding force is 64N,the lowest friction coefficient is 0.4,and the wear mechanism is abrasive wear and oxidation wear;after oxidation temperature at 600℃-800℃for 120h,the coating gradually produces a small number of cracks until the temperature rises to 800℃and falls off,and the oxidation products are Al₂O₃,Cr₂O₃and Ti O₂.The lowest average oxidation rate was 0.018g·m^-2·h^-1,and the Cr Ti Al N coating was a complete anti-oxidation at 600℃,an antioxidant at 700℃,and a secondary antioxidant at 800℃.（3）TC4 was first treated with ion nitriding,and then Cr Ti Al N coating was prepared on the surface by controlled bias,and the surface of the sample after ion nitriding was cleaned with glow.When the negative bias is-600V～-900V,increasing the bias will refine the surface grain of the specimen and increase the surface roughness,which is conducive to increasing the bonding strength of the coating and the matrix,but the bias pressure at-900V will cause defects on the surface of the specimen;The coating is mainly composed of Cr N phase,and the peak position is shifted,and Ti and Al elements replace some Cr elements in the Cr N lattice to form a face-centered cubic structure dominated by Cr N phase.With the increase of negative bias pressure,the surface hardness and bonding strength of the coating first increase and then decrease,and the surface roughness and friction coefficient decrease first and then increase.When the negative bias is-800V,the surface roughness of the coating is as low as 0.377μm,the hardness is as high as 1637HV,the bonding force is up to 72N,the friction coefficient is at least0.35,and the wear mechanism is abrasive wear and oxidation wear;after oxidation at600℃-800℃for 120h,the coating does not appear cracks,falling off,etc.,the oxidation products are Al₂O₃,Cr₂O₃and Ti O₂,and the average oxidation rate is the lowest 0.012g·m^-2·h^-1.The Cr Ti Al N coating is completely anti-oxidation at 600℃and 700℃,and anti-oxidation at800℃.（4）The plasma nitriding technology and multi-arc ion plating technology were used for preparing composite coatings on the surface of TC4.When the nitriding temperature was 910℃,the nitrogen flow rate of multi-arc ion coating was 75sccm and the negative bias voltage was-800V,the surface hardness,wear resistance and high temperature oxidation resistance of TC4were significantly improved.