Synchrotron X-ray spectromicroscopy analysis of wear tested graphene-containing alumina coatings

Thermally sprayed Al2O3 coatings containing graphene nano platelets (GNP) have been shown to exhibit improved wear resistance. To understand the positive influence of GNP on wear properties, scanning transmission X-ray microscopy (STXM) analyses were performed to determine the structural and chemical changes that occur on the GNP and alumina matrix after wear tests. STXM results acquired at the C K-edge showed that the GNP are aligned parallel to the specimen surface in the as-sprayed coatings. GNP flakes are also observed at the tribo-surface of the wear tested sample. The results obtained at the Al K-edge show that the aluminum oxide below the wear track becomes amorphous during the wear test and carbon is dissolved in it. Wear tests performed on thermally sprayed pure alumina samples (without GNP) and on sintered bulk alumina prove that neither the presence of GNP nor the porosity in the coating are responsible for the amorphization of the alumina matrix. The results discussed in this work advance the fundamental understanding of graphene-containing composites, which is considered very important to exploit the unique advantages of graphene in technological applications.