Tungsten carbide phase transformation during the plasma spray process

Abstract

Tungsten carbide coatings applied by the plasma spray process have been widely used in wear applications. In the W–C–Co ternary system, tungsten carbide can either be present as WC or W2C. Frequently WC transforms into W2C during the plasma spray process. In this study, tungsten carbide/17% cobalt coatings were applied by both the air plasma and the vacuum plasma spray processes using different power levels and plasma gases. The W2C phase was found by X-ray diffraction techniques in the air plasma sprayed coatings. Decarburizing of the WC in the presence of the oxygen took place in the plasma. In this study air plasma sprayed coating hardness and microstructure are superior to those of the vacuum plasma sprayed ones. However the vacuum plasma coatings were found to be more wear and impact resistance than the air plasma coatings. This performance difference may be attributed to the presence of hard and brittle W2C phase in the air plasma sprayed tungsten carbide coatings.

Advances in Thermally Sprayed WC-Based Wear-Resistant Coatings: Co-free Binders, Processing Routes and Tribological Behavior

“…This situation leads to an excessive level of material degradation in form of oxidation, decomposition and decarburization which would affect the wear performance of the resulting coating (Ref 20,79). It has been reported that a large portion of the WC phase can transform into W 2 C phase during the plasma spray process (Ref 12,105). Results from a study by Al-Mutairi et al (Ref 106) show that excessive phase transformation and cobalt evaporation occur during APS as compared to HVOF, which causes the wear performance of WC-based coatings processed by APS to be inferior compared to those sprayed using HVOF (Ref 107 -110).…”

Hardmetals as thermal spray coatings

“…A detailed and excellent analysis of the decomposition and oxidation of a WC-Co powder in a controlled atmosphere plasma spray process (argon) was performed by Lammer and Benesovsky. 12 Other attempts to exclude the negative influence of the atmosphere used vacuum plasma spraying (VPS), 13,14 but the negative effect of high temperature remained. Despite these measures, the quality of DGS hardmetal coatings was never attained with plasma spray processes.…”

Synthesis, characterization and physical properties of Al-Cu-Fe quasicrystalline plasma sprayed coatings

“…Plasma spraying is one of these techniques and differs from others by the formation of its heat source. The plasma gun dates back to the mid 1950s, when it was introduced for the deposition of alloys and ceramics (22). Since then, equipment improvements, process knowledge, diagnostic techniques and novel coating materials have expanded the applications of plasma spraying.…”

“…Perhaps the most well-known case is the decarburization of WC powders during air plasma spraying (22,23). Similar reactions leading to mass loss have been observed in spraying other refractory metal carbides (27).…”

“…For example, metal carbides can easily decompose and oxidize during PAS and form undesirable phases (22,23). Similar difficulties can be encountered with alloy powders if selective vaporization of a particular element occurs.…”