In Situ Study of Role of Microstructure on Antiwear Tribofilm Formation on AZ91 Magnesium Alloy under Zinc Dialkyldithiophosphate Containing Lubricant

Herein, role of microstructure on friction and wear properties of as‐cast and aged AZ91 alloy is investigated using atomic force microscopy (AFM) in lubricated conditions. The lubricant contains commercial antiwear additive zinc dialkyldithiophosphate (ZDDP), which reduces wear via sliding‐induced antiwear tribofilm. The tribofilm growth is found to be highly sensitive to the local microstructure of the alloy. For the case of as‐cast AZ91 alloy, the tribofilm readily forms on large Mg₁₇Al₁₂ precipitates, whereas α‐Mg matrix shows wear and absence of any tribofilm growth. Interestingly, for the aged alloy, the tribofilm formation on continuous precipitate (CP) and discontinuous precipitate (DP) regions is not observed. The stable sliding contact offered by large precipitates in the as‐cast microstructure is found to be critical for the tribofilm formation. Moreover, the results show a considerable difference in coefficient of friction (COF) for Mg₁₇Al₁₂ in the as‐cast microstructure as compared with that in the aged alloy.