Overview

Tungsten carbides coatings are used in drills and cutting tools to provide a desirable operational lifetime of the coatings in use, as they have enhanced mechanical and wear properties. They are typically deposited using thermal spraying techniques. Tungsten carbides deposited using these techniques exhibit undesirable characteristics which are attributable to deposition processes that occur in high-temperature environments. These include tensile residual stress, thermal oxidation, phase transformations, and grain growth during deposition which have a negative impact on the durability and longevity of tools produced.

Cold spray technology is a low-temperature deposition technique that has also been used to deposit carbides. This approach does not result in the abovementioned issues associated with thermally sprayed coatings. Whilst cold spray offers a number of benefits, the resultant coatings generally experience low retention of the tungsten carbide particles, as well as erosion of the substrate together with the erosion of previously deposited layers by hard tungsten carbide particles on impact during the deposition process. These factors may lead to increased crack formation and higher porosity levels, which result in a decrease in coating hardness, strength and wear properties.

Wits researchers developed a method of preparing improved hard metal cemented carbide coatings deposited using cold spray technology.  This is achieved through the introduction of niobium ions into the carbide coating. The ions are implanted into the coating post-cold spraying using a conventional accelerator which results in a coating that is 30% better than conventional tungsten carbide coatings i.e. the useful properties of cold spraying are retained, whilst the undesirable properties are reduced.

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