Comparison between oxidation kinetics of HVOF sprayed WC–12Co and WC–10Co–4Cr coatings

In this study, the high temperature oxidation behavior of HVOF-sprayed WC–12Co and WC–10Co–4Cr coatings were investigated. To explore the oxidation mechanism, thermo-gravimetric analysis (TGA) was applied for isothermal treatments in the range of 500–800 °C for 3 h. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate the structural changes and microstructural evolutions during oxidation tests. The TGA experiments showed negligible oxidation mass gains at 500 °C for both coatings. At higher temperatures, i.e. 700 and 800 °C, the oxidation mass gains of WC–12Co were found to be much higher than those for WC–10Co–4Cr coating, respectively. The higher oxidation resistance of WC–10Co–4Cr coating probably results from the formation of compact chromium oxide layers and higher MWO₄ type tungstate (M: Co and/or Cr) to tungsten trioxide (WO₃) ratios which provide lower porosity and consequently more efficient passivation effect against oxidation. The time dependent mass gain of WC–12Co coating obeys the linear law within temperature range of 600–800 °C with apparent oxidation activation energy of ~ 104 kJ/mol. As for the oxidation of WC–10Co–4Cr coating, a negligible deviation from linear law was observed possibly due to the presence of chromium oxide and higher tungstate to tungsten trioxide ratio which hinders the diffusion process through the scales compared with WC–12Co coating. The apparent activation energy for oxidation of the WC–10Co–4Cr coating was found to be ~ 121 kJ/mol.     

Electrodeposition of high performance multilayer coatings of Zn–Co using triangular current pulses

Abstract

Compositionally modulated alloy (CMA) coatings of Zn–Co were electrodeposited on to mild steel from an acid chloride bath containing thiamine hydrochloride, as an additive. Electroplating was carried out galvanostatically from a single bath containing Zn²⁺ and Co²⁺ ions. Gradual change in composition in each layer was effected by triangular current pulses, cycling between two cathode current densities. Compositionally modulated alloy coatings were developed under different conditions of cyclic cathode current density and number of layers, and their corrosion resistances were evaluated by potentiodynamic polarisation and electrochemical impedance spectroscopy. The formation of multilayer and corrosion mechanism was analysed using scanning electron microscopy. The corrosion resistances of CMA and monolithic alloy coatings were compared with that of the base metal. Compositionally modulated alloy coating at optimal configuration, represented as (Zn–Co)_{2·0/4·0/300}, was found to exhibit ～80 times better corrosion resistance compared with monolithic (Zn–Co)_3·0 alloy, deposited for the same length of time from the same bath. Improved corrosion resistance was attributed to the formation of n-type semiconductor film at the interface, supported by Mott–Schottky plots. Decrease in corrosion resistance at high degree of layering was found, and is due to lower relaxation time for redistribution of solutes in the diffusion double layer, during plating.     

Quantitative characterization of the microstructure and properties of nanocrystalline WC–Co bulk

Microstructure of the nanocrystalline WC–Co cermet bulk was quantitatively described by transmission electron microscopy based precession electron diffraction technology. It is discovered that the fraction of the Σ2 grain boundaries increases with the decrease of WC grain size. The effect of microstructure on mechanical properties depends on Co distribution, Σ2 boundaries fraction and WC grain contiguity.

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Correlation between residual stress and abrasive wear of WC–17Co coatings

This investigation had been conducted to determine the influence of residual stresses on the abrasive wear resistance of HVOF thermal spray WC–17 wt.% Co coatings, as well as to derive stress relaxation after cutting by wire electric discharge machining (EDM). The abrasive wear properties of the coatings were characterised using an ASTM-G65 three body abrasive wear machine with silica sand as the abrasive. The residual stress was measured by means of X-ray diffraction techniques, on the coated samples before and after the abrasive wear tests. Compressive residual stresses were observed in the surface layer of the large coated samples. However, stress relaxation results after cutting into small sizes were distinctly different. There was strong correlation between residual stresses in the surface layer and abrasive wear resistance, as well as yield strength of a material.     

Processing–structure–property correlation and decarburization phenomenon in detonation sprayed WC–12Co coatings

In this paper, we demonstrate how the microstructure evolution in terms of nature/extent of decomposition of WC as well as properties of WC–12wt.% Co coatings is critically dependent on variation of oxygen–fuel (OF) ratio. The coating deposition was carried out over a wide range of OF ratios using a detonation spray technique, and particle velocity was measured using a high-speed particle diagnostics system. The presence of free W and increased W₂C phase is observed under deposition at higher OF ratios of 1.5 or 2.0, and this has been confirmed using XRD, EBSD and SEM-EDS elemental mapping. In order to obtain representative hardness and modulus of the as-deposited coatings, careful measurements and analysis of indentation response (cross-section and surface) were carried out using nanoindentation and Vickers hardness testers. Based on our experimental results, a major emphasis has been put forward to establish a processing–structure–property correlation for detonation sprayed WC–12Co coatings.     

On the reaction sequence of WC–Co formation using an integrated mechanical and thermal activation process

A systematic study on the reaction sequence of WC–Co composite formation by annealing high energy ball milled WO₃, CoO and graphite powder mixtures in a hydrogen atmosphere has been conducted. X-ray diffraction has been used as the main tool to analyze the phase transformation of the powder mixture during processing. It was observed that WO₃ is reduced to W phase by passing through the intermediate W₂₀O₅₈ and WO₂ phases and the subsequent carburization sequence appears as W→Co₆W₆C→Co₃W₃C→W₂C→WC. The intermediate Co₃W has been found in the reduction stage, which can be subsequently carburized at higher temperatures.     

Phase transformation of NiCrBSi–WC and NiBSi–WC arc sprayed coatings

The coating microstructures of electric arc sprayed NiCrBSi–WC and NiBSi–WC coatings were investigated. It was found that, for NiCrBSi–WC coating, the resulting microstructure consists of NiCr, NiCrW solid solutions and WC/W₂C as major phases. For NiBSi–WC coating, the major phases are Ni, NiW solid solution and WC/W₂C. Some B is present within the coatings as inclusions. The amount of WC/W₂C was reduced in both coatings due to the formation of the NiCrW and NiW solid solutions containing a large amount of W. Precipitation of W-rich phase from NiCrW and NiW solutions was also observed but the transformation is restricted by the fast cooling rate characteristic of the spraying process. There is more dissolution of WC/W₂C into the matrix in NiBSi–WC coating resulting in a greater reduction in the microhardness of this coating. Wear test results, however, show that even though the NiBSi–WC coating possesses lower microhardness, it is more effective against dry sliding wear than the NiCrBSi–WC coating, owing to better metallurgical bonding between the matrix and the carbide as a result of WC/W₂C dissolution.     

Mechanical properties of WC–Co coatings with different decarburization levels

In this study, two kinds of WC–Co coatings with different decarburization levels were deposited by high-velocity oxy-fuel(HVOF) spraying using the ultrafine WC–Co composite powder and commercial micronsized powder, respectively. The hardness and elastic modulus were measured on the top surface and cross sections of the prepared coatings by the nanoindentation method. The results show that the ultrafine-structured coating has much higher density and inhibited decarburization than the conventional coating, which thus results in higher hardness and elastic modulus values than the micronsized coating. The wear resistance of thermal-sprayed cermet coatings greatly depends on the cross-sectional hardness and elastic modulus which reflects the bond strength between splats to some extent. Based on the analysis, a better understanding of the microstructure and properties in cermet coating materials was obtained.     

Microwave sintering of nanocrystalline WC–12Co: Challenges and perspectives

Microwave sintering of microcrystalline and nanocrystalline WC–12Co powder compacts was carried out employing different time–temperature schedules. The microcrystalline powder compacts were made from powders with particle sizes ranging from 5 to 45 μm by using methyl cellulose as the lubricant. The nanocrystalline powder compacts were made from powders having a mean WC grain size of 38 nm, without employing any lubricant. The sintered samples were characterized with respect to their densities, Vickers hardness, fracture toughness and microstructures and the challenges encountered during microwave sintering of the WC–12Co powders are discussed.     

Structure and cavitation erosion behavior of HVOF sprayed multi-dimensional WC–10Co4Cr coating

A new kind of multi-dimensional WC–10Co4Cr coating which is composed of nano, submicron, micron WC grains and CoCr alloy, was developed by high velocity oxy-fuel (HVOF) spraying. Porosity, microhardness, fracture toughness and cavitation erosion resistance of the multi-dimensional coating were investigated in comparison with the bimodal and nanostructured WC–10Co4Cr coatings. Moreover, the cavitation erosion behavior and mechanism of the multi-dimensional coating were explored. Results show that HVOF sprayed multi-dimensional WC–10Co4Cr coating possesses low porosity (≤0.32%) and high fracture toughness without obvious nano WC decarburization during spraying. Furthermore, it is discovered that the multi-dimensional WC–10Co4Cr coating exhibits the best cavitation erosion resistance which is enhanced by approximately 28% and 34%, respectively, compared with the nanostructured and bimodal coatings in fresh water. The superior cavitation resistance of multi-dimensional WC–10Co4Cr coating may originate from the unique micro–nano structure and excellent properties, which can effectively obstruct the formation and propagation of cavitation erosion cracks.     

Effect of nanostructuring and Al alloying on friction and wear behaviour of thermal sprayed WC–Co coatings

Nanostructured WC–Co and WC–Co–Al coatings, with about 300-μm as-deposited coating thickness, were deposited by high velocity oxy-fuel (HVOF) spraying. Agglomerated nanostructured cermet powders produced by the Mechanomade® process was used for HVOF spraying. Dense and well-adherent coatings with crystal sizes below 30 nm were deposited on stainless steel 304 substrate. Porosity was less than 5% and the bond strength with the substrate was around 60 MPa. Experimental data on friction, wear, and abrasion resistance revealed that nanostructured WC–Co based coatings containing some Al as alloying element, exhibit improved tribological characteristics in comparison to nanostructured and micron-sized WC–Co coatings. This was attributed to a carbide particle distribution within the coating revealed by SEM, the absence of brittle W₂C-like phases revealed by XRD, and the presence of Al at particle/matrix boundaries revealed by TEM.     

Optimization of process parameters for fabricating functionally gradient WC–Co composites

Functionally gradient WC–Co hardmetals not containing η phase with a Co depleted surface layer can be produced by heat treating WC–Co in a carburizing atmosphere. This study investigates the effect of process parameters of carburizing heat treatment, such as time, temperature, volume fraction of methane in carburizing atmosphere, and flow rate of mixed gases on cobalt gradient structure in functionally gradient WC–Co. Taguchi method is used to formulate the experiment layout, and the range analysis and the analysis of variance are employed to determine the optimal process parameter levels and to analyze the effect of parameters on cobalt gradient structure. The results indicate that the order of significance of the parameters on both thickness and amplitude of cobalt gradient is temperature followed by volume fraction of methane, time and flow rate of mixed gases. Both the thickness and the amplitude of the gradients increase with increasing temperature and time, and decrease with increasing volume fraction of methane, and first increase then decrease with increasing flow rate of mixed gases. Finally, the optimal combination of process parameters for fabricating functionally gradient WC–Co composites is determined.     

Influence of laser cladding process on the magnetic properties of WC–FeNiCr metal–matrix composite coatings

Metal–matrix composite (MMC) coatings were deposited by laser cladding technique with direct injection of WC–FeNiCr powder onto N1310 nonmagnetic steel matrix. Laser cladding was conducted using a Trumpf6000 CO₂ laser. The morphology of WC–FeNiCr MMC coatings was characterized using scanning electron microscopy (SEM). Magnetic properties of WC–FeNiCr MMC coatings were examined by vibrating sample magnetometer (VSM) at room temperature. The influence of laser cladding process on the magnetic properties of coatings was investigated. It was found that the content of tungsten carbide and laser power have significant effect on the magnetic properties of composite coatings. The evolution of phase constitution at different laser power was identified by X-ray diffraction (XRD). The presence of an austenitic γ-(Fe, Ni), Cr_0.19Fe_0.7Ni_0.11, Fe₃W₃C, WC and W₂C phases were confirmed by the XRD analysis in the laser clad layer.     

Microstructure and bend strength of WC–Co and steel joints

Abstract

Tungsten inert gas arc welding of WC–30 wt-%Co cemented carbide and steel was carried out using a variety of filler alloys. Precipitation of Fe₃W₃C occurred at the joint interface and this reduced the bend strength and toughness of the welded joint. The problem could be alleviated by adjusting the carbon concentration of the filler metal.     

Structure–property relationship of a spray formed Al–Y–Ni–Co alloy

The structure–property relationship of a spray formed Al–Y–Ni–Co alloy with two sets of processing conditions was investigated. Significant differences in tensile strength, yield strength, and high temperature ductility were observed with respect to the microstructural changes. Fracture toughness values were determined for both sets of specimens and found to be 7.5 and 5.8 MPa·m^1/2, respectively. Three intermetallic phases were observed in the matrix and constitute a volume fraction of approximately 75%. It is believed that the specimens failed during fracture toughness testing by the mechanism of cleavage, observed in the Al₃Y intermetallic particles.     

Microstructure and anodic dissolution mechanism of brazed WC–Ni composite coatings

WC–Ni composite coatings were prepared on the mild steel substrates by high temperature sinter brazing. Potentiodynamic polarisation was performed on the coated sample immersed in 3%NaCl solution. Two limited current plateaus appear in the anodic polarisation curve. The first limited current plateau is attributed to the formation of Ni and Cr oxide, while the second corresponding to W oxide. The surface morphology observation and composition analysis after anodic polarisation were investigated by the scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) spectroscopy. The elemental concentrations in the immersion solution were studied with inductively coupled plasma (ICP) spectroscopy. It shows that the selective dissolution of Ni base binder alloy as well as the electrochemical dissolution of WC exists during the anodic polarisation process.     

Effects of Ni addition and cyclic sintering on microstructure and mechanical properties of coarse grained WC–10Co cemented carbides

Coarse grained WC–10(Co, Ni) cemented carbides with different Ni contents were fabricated by sintering-HIP and cyclic sintering at 1450 °C. The effects of Ni addition and cyclic sintering on the microstructures, magnetic behavior and mechanical properties of coarse grained WC–10(Co, Ni) cemented carbides have been investigated using scanning electron microscope (SEM), magnetic performances tests and mechanical properties tests, respectively. The results showed that the mean grain size of hardmetals increases from 3.8 μm to 5.78 μm, and the shape factor Pwc decreases from 0.72 to 0.54, with the Ni content increases from 0 to 6 wt.%. Moreover, the W solubility reaches the highest value of 10.33 wt.% when the Ni content is 2 wt.%. The hardness and transverse rupture strength of WC–8Co–2Ni are 1105 HV₃₀ and 2778 MPa, respectively. The cyclic sintering is conducive to increase the WC grain size of WC–10(Co, Ni) and improves the transverse rupture strength of WC–10Co without compromising the hardness of alloys.     

“In-situ” mechanical characterisation of WC–Co hardmetals using microbeam testing

Microbeam testing is proposed as a new method for analysing the mechanical properties of individual microstructural features in WC–Co hardmetals; i.e. portions of WC grains or a single metallic ligament. Firstly, cantilever microbeams with dimensions below the microstructural scale of the material are machined by means of a focused ion beam (FIB). Afterwards, these beams are bended to fracture by means of an instrumented nanoindenter. In this way, both portions of WC grains and binder phase ligaments are broken while simultaneously recording the load and the vertical displacement of the nanoindenter tip. These cracking events are detected as sudden steps in the load vs. displacement curves. Afterwards, a scanning electron microscope is used to measure the distance from the main crack to the beam clamping. From these data, the stresses at which portions of cobalt ligaments and WC grains fail are estimated from linear elastic theory and FEM models.     

Influence of O and Co on the early stages of sintering of WC–Co: a surface study by AES and STM

The influence of oxygen on the sintering behavior of WC–Co has been investigated by Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). Deposition of Co on the WC(0001) surface and subsequent annealing at 650°C results in a 2×2 reconstructed pre-cursor layer on top of which Co grows in weakly bound islands which can be moved on the surface by the STM tip. Annealing at 850°C removes excess Co and leaves only the 2×2 surface. Oxygen exposure of the 2×2 surface results in a “clustered” cobalt oxide overlayer which on annealing at 750°C breaks up and restores the 2×2 structure as the metallic Co wets the surface.