Abrasive wear behaviour of conventional and nanocomposite HVOF-sprayed WC–Co coatings

Microstructural evolution in thermally sprayed WC-Co coatings: Comparison between nanocomposite and conventional starting powders

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

Effect of carbide grain size on microstructure and sliding wear behavior of HVOF-sprayed WC–12% Co coatings

In order to protect machining parts against wear and corrosion, they are coated by cermet coatings. The coatings consist of WC or Cr3C2 particles in a metal binder, which can be a pure metal or a mixture consisting of Ni, Cr, Co. The examined coatings were produced by high velocity oxy-fuel (HVOF) spraying and were investigated with regard to erosion and corrosion resistance. The combined erosion corrosion tests were carried out at ambient temperature in 0.1 M NaOH and 0.1 M H2SO4 solutions containing sand. Information about the corrosion resistance was gained from electrochemical polarization measurements and salt spray test (NaCl solution). The results of combined wear and corrosion tests showed that the corrosion properties of the sprayed coatings strongly affect the materials loss rate under wear corrosion conditions. Coatings with a less corrosion resistant matrix present enhanced erosion, also. The erosion mechanism of the carbide coatings seams to be controlled by the skeletal network of the carbides. For comparison, flame and plasma sprayed hard Cr2O3 coatings were examined. Due to the low electrical conductivity the corrosion rate of these coatings was very low. Under erosion conditions the hard Cr2O3 coatings exhibited a high erosion rate and the erosion mechanism seems to be that of the brittle erosion due to grain-by-grain removal of oxide grains during impact.

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Wear and corrosion behaviour of thermally sprayed cermet coatings

Sand erosion tests were performed on WC-Co and WC-CoCr coatings deposited by the high velocity oxy-fuel spraying method. Several analytical techniques, including X-ray diffraction, Auger electron spectroscopy and energy-dispersive spectroscopy in a transmission electron microscope were used to characterize the microstructures formed during powder processing and spraying. It was found that a substantial fraction of WC decomposed into W2C or reacted with the cobalt matrix to form ternary carbides such as Co3W3C and other mixed compounds. In both cases the binder phase had a nanocrystalline structure of size 4-8 nm containing tungsten, cobalt, carbon and chromium elements. The addition of chromium inhibits to a large extent the decomposition of WC and avoids the formation of metallic tungsten. In addition, chromium improved the erosion resistance by several times compared with the WC-Co coating. Scanning electron microscopy showed that the CoCr matrix binds carbides better than the cobalt matrix, thereby inhibiting carbide loss at the spray particle boundaries. The hydroabrasive wear behaviour of coatings and the mechanisms for material removal are discussed with respect to the microstructures formed during spraying.

Microstructure and hydroabrasive wear behaviour of high velocity oxy-fuel thermally sprayed WCCo(Cr) coatings

Slurry erosion behaviour of thermally sprayed WC-M coatings

The microstructure of High Velocity Oxy-Fuel (HVOF) thermally sprayed WC-Co coatings was quantitatively evaluated in an attempt to describe the transformations that take place during thermal spraying. Slurry erosion tests were performed to measure the weight loss as a function of flow velocity and incident angle. It was found that an optimum amount of transformation is required to ensure a maximum of erosion resistance.

Microstructural and analytical study of thermally sprayed WC-Co coatings in connection with their wear resistance

Hydroabrasive wear tests were conducted on the coatings of tungsten carbide-metal matrix cermets of type WC-M, where M≡Co, CoCr and Ni. The coatings were deposited using the high velocity oxyfuel (HVOF) spraying process. Cermets of the cobalt binder phase showed 50%–200% higher strength than those of the nickel matrix depending on the erosion conditions, and the addition of chromium improved the erosion resistance several times. Analytical techniques including X-ray diffraction and energy-dispersive spectroscopy (EDS) as well as transmission electron microscopy were used to characterize microstructures formed during powder processing and spraying. Owing to overheating of powder particles within the spray torch, the HVOF cermets develop complex microstructures. The hard phase consists basically of tungsten monocarbide (WC) as well as ditungsten carbide (W2C), while the binder phase obtains a nanocrystalline structure of the size between 4 and 8 nm, containing ternary carbides and mixed compounds. The erosion results are discussed with respect to the microstructural features developed during spraying.

Ch. Verdon

Papers

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

Microstructure and hydroabrasive wear behaviour of high velocity oxy-fuel thermally sprayed WCCo(Cr) coatings

Slurry erosion behaviour of thermally sprayed WC-M coatings

Microstructural and analytical study of thermally sprayed WC-Co coatings in connection with their wear resistance

Hydroabrasive wear behaviour of high velocity oxyfuel thermally sprayed WC-M coatings