Comparison of Nanometric and Micrometric Alumina COATINGS on Wear Resistance

Two different structure scales of alumina coatings have been elaborated by plasma spraying (APS) in order to compare the micro- and nano-structure effects on the tribological properties and especially on the wear resistance. The micrometric scale structure coatings has been elaborated with micrometric fused and crushed powder (22 – 45 µm) and the nanometric scale ones with nanometric powder (particles: 200 to 500 nm) agglomerated into micrometric particles (size distribution between 20 and 80 µm). Both types of coatings have been sprayed onto steel substrates (stainless steel 304L and low carbon XC38). Several plasma spraying parameters have been tested (current intensity, plasma forming gas, powder injection of the anode nozzle and powder carrier gas flow rates were adapted to spray conditions). Coatings were characterized by SEM, EDS and XRD for their structure and morphology. The tribological properties have been studied by measuring the micro-hardness, the friction coefficient by using a ball on disk tribometer and the wear rate by profilometry of the track made by the friction. Results have shown that the nanometric structure coatings have a lower hardness than the micrometric structure ones, but the friction coefficient decreased below 0.2 (against 0.85 for micrometric structure coatings) and the wear could not be measured compared to the wear rate (4,90·10^-4 mm³/m/N) of micrometric structure coatings.