The influence of phase gradient within the micro arc oxidation (MAO) coatings on mechanical and tribological behaviors

The alumina coatings of 100 μm thickness were deposited on 6061 T6 Al-alloy through micro arc oxidation (MAO) technique. The phase composition across the coating thickness was evaluated using step-wise grinding followed by X-ray diffraction analysis. The microhardness and elastic modulus were measured through micro and nano indentation techniques respectively on the mounted and polished coating cross-sections as a function of distance from substrate-coating interface. The coatings represent typical graded composite of α-Al2O3 and γ-Al2O3 phases and accompanied by the corresponding hardness and modulus gradients across the coating thickness. Tribological performance of MAO coatings was evaluated through pin-on-disc wear test under dry (un-lubricated) conditions. The results obtained suggest that the transition from γ-Al2O3 rich surface sub-layers to α-Al2O3 rich inner sub-layers is gradual. On the basis of the above experimental results, the critical interrelationships between the coating phase composition, hardness distribution, modulus distribution and the accompanied sliding wear loss have been established. It was found that the simple rule of mixture explains (ROM) the hardness and modulus distributions as a function of phase gradient across the coating thickness while the inverse rule of mixture (I-ROM) is applicable in the case of sliding wear-phase gradient relationship. Further, the relationships established were utilized to evaluate the mechanical and tribological properties of pure α-Al2O3 and γ-Al2O3 phases. The properties of gamma alumina such as hardness, elastic modulus and sliding wear resistance have been reported for the first time in the present work. Further, the worn surface examination indicates that the formation of mechanically mixed layer (MML) which subsequently undergoes abrasive wear and micro-crack induced chipping as the predominant material removal mechanism during dry sliding wear tests.