Preparation And Tribological Properties Of The Self-lubricating Micro-arc Oxidation Composite Films On The Surface Of Aluminum Alloy

In recent years,the lightweight of automobiles has become an unstoppable trend in the development of the domestic and international automotive industry.Aluminum alloys are increasingly favored by the automotive parts processing industry due to their light weight,high specific strength and good processing performance.However,aluminum alloys generally exhibit poor tribological properties in the absence of lubrication,which limits their practical application.Based on this,a hard MAO ceramic coating was prepared in situ on the surface of 7075 aluminum alloy by micro-arc oxidation technique.Then,PTFE and UHMWPE materials with self-lubricating properties were prepared on the surface of porous ceramic coating by impregnation-sintering treatment.This solves the key problem of the high friction coefficient of the traditional micro-arc oxidation ceramic coating.Research indicates:（1）The electrolyte system affects the surface morphology,thickness,hardness,anti-wear and corrosion resistance of the micro-arc oxidation ceramic coating on the surface of the aluminum alloy.The ceramic coating prepared in the silicate electrolyte has the lowest surface porosity and the largest thickness,showing the best corrosion resistance.However,it has high surface roughness and its hardness is low,so the anti-wear performance is poor.On the contrary,the surface of the ceramic coating prepared in the phosphate electrolyte has the largest pore size and porosity and its thickness is thin,so the corrosion resistance is poor.But its surface roughness is lowest and the hardness is the highest.Compared with the aluminum alloy,the wear rate is reduced by 90%,showing the best wear resistance,but the friction coefficient is higher during the whole friction test and the average friction coefficient is about 0.5.（2）After the PTFE coating is applied on the surface of the micro-arc oxidation ceramic coating by the dipping-sintering treatment process,a large number of micropores and microcracks on the surface of the ceramic coating are filled with PTFE material and a self-lubricating film with a thickness of about 13 μm is formed on its surface,which effectively reduces the surface roughness of the ceramic coating.The porous structure of the ceramic coating is beneficial to storage and adhesion of PTFE.The static water contact angle of the MAO ceramic coating is only 41°,which has obvious hydrophilic properties,while the static water contact angle of the self-lubricating composite coating is as high as 130°,showing excellent hydrophobic properties.Compared with the MAO ceramic coating,the corrosion potential of the composite coating is moving forward by 90 m V,the corrosion current density reduced by three orders of magnitude and only a slight pitting occurs under the seawater immersion experiment for up to 15 days.In addition,the coefficient of friction of the ceramic coating under dry sliding conditions is about 0.6 and the average wear rate is 2.65×10^-4 mm³·N^-1·m^-1.However,the friction coefficient of the self-lubricating composite coating is only 0.12 and remained stable during the whole friction experiment and the average wear rate was 30% lower than that of the ceramic layer,which effectively improved the tribological properties of the ceramic coating.（3）The self-lubricating UHMWPE film with different thickness on the surface of the ceramic coating has different surface morphology and properties.The UHMWPE film with the largest thickness（12 μm）can effectively improve the rough and porous surface defects of the ceramic coating and its static water contact angle can reach 118°.The corrosion potential is increased by 160 m V and the corrosion current density is reduced by three orders of magnitude compared to the ceramic coating.In addition,the self-lubricating UHMWPE film with smaller thickness has lower bearing capacity,while the UHMWPE film with the largest thickness has smaller friction coefficient（0.99）under high load and long-term experimental conditions,and the average wear rate is 75% lower than that of the ceramic coating,showing excellent anti-friction and anti-wear properties.