Mechanical properties and microstructure evolution of the nano WC–Co coatings fabricated by detonation gun spraying with post heat treatment

Abstract

Effects of post heat treatment of thermally sprayed nano WC–Co coatings on their mechanical properties were studied. The thermal behavior of WC particles in the coatings was also investigated. WC–Co coatings containing nano carbide particles in the 100–200 nm range were fabricated by detonation gun spraying. Considerable phase decomposition of WC to W 2C and amorphous phase was detected, which degrades the mechanical properties of coatings. In order to improve the mechanical properties of the coatings by recovery of dissociated carbide phases, post heat treatment was conducted in an Ar environment in the temperature range of 400–900 °C. Microhardness and fracture toughness were measured by Vickers indentation testing and wear resistance was also evaluated by using a scratch tester. Phase evolution and microstructural changes due to post heat treatment were investigated by optical microscopy, X-ray diffractometry and field-emission scanning electron microscopy. After heat treatment in all temperature ranges, microhardness increased. Fracture toughness and wear resistance of coatings were increased by increasing temperature to 800 °C but decreased after heat treatment at 900 °C. Amorphous phase disappeared and other carbide phases such as W 3Co 3C and W 6Co 6C formed during heat treatment above 700 °C. The improved properties were elucidated and discussed in terms of microstructural changes and the relationship between mechanical properties and carbide phase was also discussed.