Investigations on reducing friction coefficient of MAO coating fabricated on 7075 Al alloy

Abstract

Aluminum (Al) and its alloys have attracted attention in a wide range of application areas thanks to their high specific strength-to-weight ratio, excellent formability, and ease of fabrication. Also, in the automotive industry, they reduce CO2 emissions by lowering vehicle weight due to their low density and high specific strength. These crucial advantageous have made Al alloys a very attractive material for the aerospace and automotive industry, especially. In addition to these advantages of Al alloys; they have some limitations such as low wear resistance due to low hardness, and high and unstable friction coefficient. Thus, Al alloys cannot exhibit sufficient strength and tribological properties in rotating and reciprocating dry sliding conditions. Among the Al alloys, the 7xxx series have been used for high-strength applications. These series are suitable for surface treatment when various material properties have been expected. Also, they have resistance to stress corrosion cracking. In these series, 7075 Al alloys which are known as high-strength Al alloys have significant advantages such as high toughness and good fatigue strength. Although it has these advantages; it cannot exhibit sufficient tribological properties because of its high and unstable coefficient of friction, low hardness and wear resistance. To tackle these limitations, different surface modification techniques have been developed. In this way, different functional properties can be gained from the alloy. In this regard, the micro-arc oxidation (MAO) process, also called as plasma electrolytic oxidation (PEO) process have been become crucial considering numerous advantages such as coating producibility for different applications (wear, corrosion, optical etc.), no deterioration of substrate, environmentally-friendly. MAO process is based on the fabrication of oxide-based ceramic coating on light metals (Al, Ti, and Mg) by using an electrolyte resulting in improvement in wear and corrosion properties, especially. With particle addition into the electrolyte, porosities formed during the MAO process can be minimized; both complex-oxide ceramic coatings that can improve different properties can be obtained and the friction coefficient can be reduced with the solid lubricant additives settled in the micro-pores in the structure. Thus, further improvement in tribological properties can be achieved.