**Al2O3** + **Graphene Low-Friction Composite Coatings Prepared By Sol–Gel Method**

#### **Bo ˙zena Pietrzyk 1,\*, Sebastian Miszczak 1, Ye Sun 2 and Marcin Szyma ´nski 1**


Received: 15 July 2020; Accepted: 2 September 2020; Published: 4 September 2020

**Abstract:** In this work, Al2O3 + graphene coatings were prepared using the sol–gel method. The aim of the study was preliminary determination of the influence of size and amount of graphene nanoplatelets on morphology, chemical structure, and basic tribological properties of Al2O3 +graphene composite coatings. Two types of reduced graphene oxide (rGO) nanoplatelets with different lateral size and thickness were used to prepare the coatings. To characterize them, scanning electron microscope (SEM), glow discharged optical emission spectrometer (GDOES), Fourier-transform infrared (FTIR), reflectance spectrometer, and ball-on-disk tribological tests were used. It was found that the presence of graphene in the Al2O3 + graphene coatings did not fundamentally change the chemical transformation of ceramic Al2O3 matrix. Morphology examinations of coatings containing larger graphene nanoplatelets revealed a tendency to their parallel arrangemen<sup>t</sup> in relation to the coated surface. The tribological properties of Al2O3 + graphene coatings turned out to be strongly dependent on the size of graphene nanoplatelets as well as on the heat treatment temperature. The friction coefficient as low as 0.11 and good durability were obtained for the Al2O3 + graphene coating with larger nanoplatelets and heat-treated at 500 ◦C. The results of conducted research indicate the potential use of Al2O3 + graphene composite coatings prepared by the sol–gel method as low-friction ceramic coatings.

**Keywords:** ceramic coating; alumina coating; sol–gel; composite coating; graphene oxide; graphene nanoplatelets (GNP); rGO
