Abstract
Tungsten-based cermets are well-known engineering materials finding applications in aerospace, nuclear equipment, and many other fields. Plasma spraying is an interesting industrial process to manufacture those refractory materials. Original plasma sprayed hard coatings for wear protection composed of a stainless steel matrix and inclusions of tungsten carbide (WC) nanoparticles were developed. To built-up the coatings, two precursors were injected separately in the plasma jet : a stainless steel micrometric powder was classically injected into the plasma jet using a carrier gas whereas WC nanoparticles were injected with a liquid carrier, like in the so-called process suspension plasma spraying. One of the challenges is to maintain the WC phase stoichiometry in the deposit, without decomposing the carbide into brittle W2C, W3C, and metallic tungsten, phenomenon usually occurring with thermal spraying techniques. Another issue is to succeed in including homogeneously the carbide nanoparticles in a sufficiently dense stainless steel matrix. Coatings with different WC contents were deposited on stainless steel substrates and investigated with respect to their microstructure by optical and scanning electron microscopy, porosity level using the Archimedean method, phase composition by X-ray diffraction and Vickers micro-hardness. Results have shown that coatings consisting of a stainless steel matrix containing inclusions of carbide nanoparticles can be produced by plasma spraying. The phase composition analysis indicated that nanoparticles are largely composed of the WC phase and contain a small amount of WC1-x phases. A slight increase of the porosity level was measured for coatings containing nanoparticles, compared to the pure matrix, probably due to the cooling effect of the WC carrier liquid on the in-flight characteristics of the stainless steel particles. Micro-hardness measurements gave similar values for with or without nano-sized particles, showing that the amount of WC included in the samples was insufficient to improve the hardness property.
