Effect of spinels on the mechanical and tribological behavior of plasma sprayed alumina based coatings

Plasma sprayed ceramic coatings are widely used to protect metallic substrates when high temperature and/or friction are developed in service.In particular, alumina based coatings have been extensively used.Examples of applications are pistons in pumps or internal combustion engines and steam valve spindles.Alumina is hard, it shows a very high oxidation resistance and it is not permeable to aggressive gases.However, the alumina main drawback is its low fracture toughness.Brittle fracture is one of the most common wear mechanisms reported for ceramics.It is well known that the fracture toughness and the operative conditions are the key parameters controlling the crack propagation throughout the material.As a consequence, transition from mild to severe wear in ceramic coatings is mainly controlled by the fracture toughness of the coating.The typical solutions to improve the fracture behavior of alumina coatings are based on mixing alumina powders with other ceramic powders with superior toughness.Alumina -Zircona systems are used in this way.When a mixture of ceramics is thermally sprayed, a very complex microstructure is attained in the coating.Nevertheless, a common feature can be pointed out for different systems: formation of spinels from both oxides is commonly reported.These phases seem to play an important role in the fracture behavior of the coatings and subsequently in their tribological behavior.This paper presents a summary of the work done in the microstructural, mechanical and tribological characterization of plasma sprayed alumina based coatings (Al2O3 -TiO2, Al2O3 -ZrO2 and Al2O3 -Cr2O3).Special attention is paid on the role played by spinels formed during deposition.