Tribological performance at high temperatures of alumina coatings applied by plasma spraying process onto a refractory material

Abstract

Two different Al2O3 coatings manufactured by plasma spraying process were tested under dry sliding contact conditions at high temperatures in order to determine their tribological behavior. At higher temperature than 800 °C, wear rate and mechanisms for Al2O3 coatings have not yet been reported. The results obtained were compared with those from both: an electro-melted Al2O3 refractory used as reference and a pressed & sintered silico-aluminous refractory used as substrate, which were tested under the same conditions. The reference was chosen due to the high performance against wear that this material shows, and the substrate was chosen due to the high potential that this material with low properties against wear shows when it is coated. Crystallographic phases, micro-hardness, Young's modulus and fracture toughness were measured before and after the wear tests, obtaining as results that both Al2O3 coatings showed better wear performance than the pressed & sintered silico-aluminous refractory used as substrate, and even than the electro-melted Al2O3 refractory used as reference. Additionally, in the plasma sprayed coatings, as well as, in the electro-melted Al2O3 refractory, the wear mechanism was controlled by a transformation produced by the increasing in temperature from ductile deformation to brittle deformation, for finally returning to ductile deformation again. The high performance of plasma sprayed coatings was mainly due to the high values of toughness, as well as due to the increasing of the α-Al2O3 levels and therefore, the hardness during the wear tests because of the high temperature, allowing to these ceramic coatings to be a possible way to protect the conventional pressed & sintered silico-aluminous refractories to replace the expensive non-structural electro-melted ones currently exposed to abrasive wear in glass, cement and other primary industries.