Development of Composite Stainless Steel and Nano-Tungsten Carbide Coatings by Plasma Spraying

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.

In the present work, original hard coatings formed by a stainless steel matrix containing tungsten carbide (WC) nanoparticles were developed by plasma spraying. Stainless steel was chosen because it is the material of the substrate to be protected and WC for its unique set of properties including high melting point, high hardness and erosion resistance.

To produce the coatings, two precursors were injected separately in the plasma jet: a stainless steel micrometric powder was classically injected with a carrier gas and WC nanoparticles were injected with a carrier liquid, like in the so-called process suspension plasma spraying (SPS).

One of the challenges is to maintain the WC phase in the deposit, without decomposing the carbide into brittle W₂C, W₃C, and metallic tungsten, phenomenon usually obtained with the thermal spraying techniques. Another one is to be able to include homogeneously the carbide nanoparticles in the stainless steel matrix.

Coatings with different WC contents were deposited on stainless steel substrates and investigated with respect to microstructure by optical and scanning electron microscopy, porosity level using the Archimedean method, phase composition by X-ray diffraction and Vickers micro-hardness.

Results showed 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 with the WC phase and also with a small amount of WC_1-x phases. A slight increase of the porosity level was measured for coatings containing nanoparticles, compared to the 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 coatings containing or not the nano-sized particles, showing that the amount of WC including in the samples was insufficient to improve the hardness property.