The Bonding Formation during Thermal Spraying of Ceramic Coatings: A Review.

Thermal spraying is the most important coating technology for depositing advanced ceramic coatings which have been widely applied to different industrial fields for materials protection and various physical–chemical functions. The adhesion and cohesion are of primary importance for the successful applications of ceramic coatings. Three bonding mechanisms contribute to the enhancement of the adhesion and cohesion, including mechanical interlocking, physical bonding and chemical bonding. It is still challenging to achieve chemical bonding in thermally-sprayed coatings. In this paper, the main factors influencing the bonding formation during thermal spraying of ceramic coatings, including spray particle parameters and substrate parameters, are examined from splat formation to coating formation to find solutions to the above challenge. The research progress on splat formation revealing characteristic dynamic parameters relating to the bonding formation kinetics will be briefly presented for the key factors determining splat shape, flattening time, solidification time, cooling rate, interface temperature, and transient dynamic contact pressure during flattening. The typical coating lamellar structure features with limited intersplat bonding less than one-third for refractory ceramics, which dominate the coating properties and performance based on theoretical relationships between the microstructure and properties, are presented. The effects of spray particle parameters on the intersplat bonding reveal that the bonding ratio is increased with increasing particle temperature, but decreased with increasing particle velocity which benefits only the mechanical bonding. Most importantly, recent studies have revealed that the liquid splat–substrate interface temperature higher than the glass transition temperature of spray materials is a necessary and sufficient condition for splat bonding formation. A critical bonding temperature concept is proposed to control the intersplat bonding formation by controlling the substrate preheating temperature. The critical bonding temperature is related to the melting point of spray materials. A model is proposed to understand the effect of the interface temperature on the bonding formation of impacting liquid splat and the bonding mechanisms. The condition for certain ceramic spray materials to form a bulk-like dense coating with the intersplat interface completely bonded becomes well understood. Moreover, the effect of metal substrate oxide scale control on the adhesion reveals that an adhesive strength higher than 100 MPa can be achieved for plasma-sprayed ceramic coatings. The excellent bonding at the interface between the splat and the oxide scale pre-oxidized on the metal substrate can be also explained by the bonding formation model. It becomes possible that, through both the controls of the pre-oxidation and the deposition temperature, all the interfaces in the ceramic coating with the metal/oxide-scale/splat/splat system can be bonded by chemical bonding to achieve an excellent load-bearing ceramic-coating system. [ABSTRACT FROM AUTHOR]