Synthesis And Mechanical Properties Of Nanostructured WC-Co Coatings Deposited By Plasma Flame Spray

Nanomaterials are widely paid more attention in various fields including coatings owing to its unique properties. Tungsten carbide-cobalt coatings are extensively used in aeronautical and automotive applications requiring abrasion, sliding, fretting and erosion resistance. The hard WC particles in the coatings lead to high coating hardness and high wear resistance, while the metal binder Co supplies the necessary coating toughness. At present, the researches on WC-Co coatings are mainly focused on conventional coating but less on the nanostructured coating, and HVOF (high velocity oxy fuel) is generally applied to synthesize the nanostructured WC-Co coating. In this paper, the nanostructured WC-Co coating was synthesized by plasma flame spray, microstructure and mechanical properties of coatings are characterized and evaluated, respectively.In this thesis, WC-12Co nanocomposite powder was synthesized by the method of "spray drying and fixed bed technique" , nanostructured WC-12Co cermet coatings were deposited by plasma flame spray. Microstructure of cermet coatings was evaluated by using X-ray diffraction(XRD), Electron probe analyzer(EPA), Scanning electron microscopy(SEM) and Transmission electron microscopy(TEM), meanwhile, the forming mechanism of nanostructured WC-12Co cermet coatings was discussed on the basis of the microstructure of coatings. Mechanical properties of coatings such as: microhardness, abrasion resistance and adhesive strength were tested. Compare between nanosructured and conventional WC-12Co coatings was done in respect of microstructure and mechanical properties. The main results obtained are described as follows:1. WC-12Co nanocomposite powder was synthesized by the method of "spray drying and fixed bed technique" . Analyzed by using XRD SEM TEM, nanocomposite powder consist only of WC particles and Co matrix, WC grain size is about 80nm and particle size is 300nm. In order to spray thermally, WC-12Co nanocomposite powder are agglomerated into spherical agglomerates which range from 40-55 μ m.2. The lamellar-like microstructure can be apparently seen in the nanostructured WC-12Co coating. The phases W2C W and Co₃W₉C₄ Co₃W₃C, Co₃C/amorphous binder phases based on Co and nano-twins occur in coatings.3. The forming mechanism of nanostructured WC-12Co coating is as follows: (i )melting of cobalt during spraying;(ii) dissolution of WC into the molten cobalt;(iii) loss of carbon from the periphery of the particle by oxidation which promotes further WC dissolution locally;(iv) quenching of the particle on impact with the substrate which results in formation of an amorphous binder and C03C phases , and precipitation of W2C or W.4. During spraying, dissolution of WC into the molten cobalt and loss of carbon result in new phases appearing. The possible ranking order of these phases in radial direction from the core of WC to the Co is like this: WC-*W2C-*CoxWyCz(Co3W9C4> C03W3C) -?CoaC-^Co. With the diffusion of C to the outside, the W:C atomic ratios increase. W2C occur in the periphery of WC particle, CoxWyCz(Co3W9C^ C03W3C) appear subsequently, and then C03C by the action of C and Co. These phases clung to the shell of WC particle break off, smash, and disperse in the binder Cobalt because of tremendous kinetic energy when impinging into the substrate.5. Compare to the conventional coating, nanostructured WC-12Co coating is with denser microstructure, less crack, lower porosity.6. The wear properties of nanostructured WC-12Co coating were examined. The experimental results obtained show that the frictional coefficients of nanostructured coating against WC is lower than that of conventional coating and nanostructured coating exhibit higher abrasion resistance.