The microstructures of two tungsten carbide–cobalt (WC–Co) coatings, deposited using high velocity oxy-fuel (HVOF) thermal spraying method in different conditions, are studied. They are compared with that of the WC–Co powder grains injected in the flame, in an attempt to understand the transformations that occur during deposition. For this purpose, various imaging and analytical techniques in electron microscopy are used, in addition to global characterization methods such as X-ray diffraction and fluorescence. These methods reveal that the coatings are made of distinct islands, elongated along the substrate direction, which exhibit a nano-crystalline matrix containing tungsten, cobalt and carbon. The fraction of WC grains in the coating is smaller than that in the powder and fluctuates throughout the coating. A net loss in carbon is evidenced in the coatings as compared to the powder grains. New phases, W2C and W, appear in specific locations in the microstructure in relation with the local composition of the matrix. Very little metallic cobalt is retained. The extent of the transformation is related to the spraying conditions. Some processes that account for the change in microstructure and composition during spraying are proposed.

A study of high velocity oxy-fuel thermally sprayed tungsten carbide based coatings. Part 1: Microstructures

http://www.asbindustries.com/documents/thinsolidfilms_nano_wc%20.pdf?origin=publication_detail

Microstructural characteristics of cold-sprayed nanostructured WC-Co coatings

Thermally sprayed coatings based on tungsten carbide are widely used but not yet fully understood, particularly with regard to the chemical, microstructural, and phase changes that occur during spraying and their influence on properties such as wear resistance. The available literature on thermally sprayed WC-Co coatings is considerable, but it is generally difficult to synthesize all of the findings to obtain a comprehensive understanding of the subject. This is due to the many different starting powders, spray system types, spray parameters, and other variables that influence the coating structures and cause difficulties when comparing results from different workers. The purpose of this review is to identify broad trends in the powder/processing/structure relationships of WC-Co coatings, classified according to powder type and spray method. Detailed comparisons of coating microstructures, powder phase compositions and coating phase compositions as reported by different researchers are given in tabular form and discussed. The emphasis is on the phase changes that occur during spraying. This review concerns only WC-12% Co and WC-17% Co coatings, and contrasts the coatings obtained from the cast and crushed, sintered and crushed, and agglomerated and densified powder types. Properties such as hardness, wear, or corrosion resistance are not reviewed here.

Powder/processing/structure relationships in WC-Co thermal spray coatings: A review of the published literature

We have deposited a series of WC–Co coatings by the high velocity oxy fuel (HVOF) process, using three different powders, and different spray conditions. The powders are a nanocrystalline (Nanocarb), a near-nanostructured powder (Infralloy) containing a proprietary additive aimed at retarding grain growth, and multimodal (mixed micro and nano) powder (Nanomyte). HVOF spray conditions were stoichiometric, fuel rich and oxygen rich. ‘In-flight’ feedstock powder temperature and velocity were measured. The hardness and toughness of the coatings are found to depend on WC-binder adhesion and adhesion between splats. High flame temperatures increase WC-binder adhesion but increase decarburization. The latter is found to decrease adhesion between the splats. Decarburization is most pronounced for nanostructured powders because of their high specific surface. The additive in Infralloy decreases adhesion between WC grains and binder, but it also reduces decarburization. The wear resistance of the coatings increases with hardness and decreases with increasing decarburization. Sliding wear occurs by a attrition of the WC grains and the lifting of entire splats; abrasive wear occurs by ductile cutting, grain loss and lifting of splats; the low wear rate in sliding leads to splat-boundary weakening by fatigue. The effect of decarburization predominates in sliding wear and is less pronounced in abrasion; the high abrasive wear removes material before fatigue becomes important. The coating deposited at high temperature, from the multimodal powder Nanomyte, presents outstanding sliding and abrasive wear resistance but inflicts large wear on the opposing silicon nitride surface in sliding. Coatings deposited with the near nanostructured powder containing an additive present high sliding wear resistance, independent of the deposition parameters, and cause low wear of the opposing silicon nitride. Coatings deposited with spray-dried nanostructured powders offer comparatively low wear resistance, in agreement with previous reports.

The effects of fuel chemistry and feedstock powder structure on the mechanical and tribological properties of HVOF thermal-sprayed WC–Co coatings with very fine structures

WC–17 wt%Co powders have been sprayed by the HVOF method to form coatings approximately 200 μm thick on steel substrates. The microstructure of the coatings has been analysed and found to consist of WC, W2C and an amorphous binder phase. Some of the carbide particles are found to decarburise and dissolve in the liquid metal binder during spraying, resulting in a brittle binder phase in the coating. The coatings also exhibit tensile stresses in the as-sprayed condition. Coatings have been heat-treated at a range of temperatures between 250 and 1100 °C. Heat treatment above 600 °C results in significant phase changes within the coating. However, heat treatment at all the temperatures examined resulted in changes in the integrity and residual stress state of the coating due to thermal expansion coefficient mismatch between the coating and substrate. The abrasive wear behaviour of the as-sprayed and heat-treated coatings demonstrates that heat treatment improves the wear behaviour. Indeed, heat treatment at temperatures as low as 250 °C has resulted in improvements in wear resistance of up to 35%. The abrasive wear behaviour has been correlated with the residual stress state, coating integrity and microstructure.

Influence of heat treatment on the abrasive wear behaviour of HVOF sprayed WC–Co coatings

A WC-Co coating was sprayed by the high-velocity oxyfuel process using a feedstock of tungsten carbide clad with cobalt. The structure of the sprayed coating was characterized by x-ray diffraction (XRD), differential scanning calorimetry (DSC), and differential thermal analysis (DTA). It was found that an amorphous phase of Co-W-C ternary alloy observed as a large, broad peak in the XRD pattern can be formed in the as-sprayed WC-Co coating. The DSC, DTA, and XRD analyses revealed that the amorphous phase crystallized at a temperature of around 873 K to metallic cobalt, Co6W6C, and tungsten with appreciable precipitation of free carbon. The heat treatment of as-sprayed WC-Co coating at a high temperature of 1173 K suggests that annealing at a temperature higher than about 1104 K will promote the reaction of tungsten and cobalt with carbon to form the complex carbide C06W6C.

Y. Harada

Papers

Formation of an amorphous phase in thermally sprayed WC-Co