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Sliding Wear Properties of HVOF Sprayed WC-10Co4Cr Coatings With Conventional Structure and Bimodal Structure Under Different Loads

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Abstract

The WC-10Co4Cr coatings with conventional structure and bimodal structure were sprayed by high velocity oxygen fuel (HVOF) technology. The phase compositions and morphologies of the WC-10Co4Cr powders and coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The microhardness, porosity, bonding strength, elastic modulus and indentation fracture toughness of the conventional coating (Conventional) and the bimodal coating (Bimodal) were also studied. The sliding wear properties of the Conventional and the Bimodal against Si3N4 counterballs under different loads at room temperature (~25 °C) were investigated using a friction and wear tester. Compared with the Conventional, the Bimodal has denser microstructure, lower porosity, more excellent mechanical properties, and the Bimodal has better wear resistance than the Conventional under different loads. The two coatings under 15 N and 30 N only exhibit abrasive and slightly adhesive wear mechanism, while in the load application of 45 N, additional mechanism which is fatigue is detected and causes flaking of the coating.

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In this work, the spray parameters of the bimodal WC–12Co powder are optimized by Taguchi experiment program by using liquid fuel JP-8000 HVOF spray system with mass flow meter controlling the flux of the medium. The phase composition, microstructure, hardness, porosities, fracture toughness, per-pass deposited thickness and the abrasive wear mechanism of the coatings have been studied in detail. The results indicate that the varying of the spraying parameters shows little effect on the phase composition of the WC–12Co coatings but great influence on their other performance such as hardness, porosity, fracture toughness and the per-pass thickness of coatings. High hardness of the coatings usually means low porosity, low fracture toughness, low per-pass thickness and high wear resistance for the HVOF WC–12Co coatings. The bimodal coating deposited under the optimal spray parameter exhibits excellent performance, which can be attributed to its small mean free path of the cobalt binder resulting from the bimodal distribution of WC particles. As last, three groups of spray parameters, named as economic, moderate and high level spray parameters, are suggested according to the results of Taguchi experiment program and the applied situation of the coated parts as well as their preparation cost.
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Conventional and nanostructured WC–12Co coatings were deposited on 1Cr18Ni9Ti substrate using high velocity oxy-fuel spraying. The surface roughness and microhardness of as sprayed coatings were measured, while the phase compositions and microstructures of WC–12Co feedstock powders and corresponding coatings were respectively analysed by means of X-ray diffraction and scanning electron microscopy. Moreover, the microsliding wear behaviour of both coatings sliding against Si3N4 ball from room temperature to 500°C was evaluated under unlubricated condition with an oscillating SRV tribotester. Results reveal that nanostructured WC–12Co coating not only had better wear-resistance than the conventional WC–12Co coating but also led to reduced wear of the Si3N4 counterpart under the same test condition, which could be attributed to the fine microstructure and improved mechanical properties of the nanostructured coating. The detailed wear mechanisms for both coatings have also been discussed.
Article
Conventional and nanostructured WC–12%Co coatings were deposited on 1Cr18Ni9Ti stainless steel substrate using air plasma spraying. The hardness of the coatings was measured, while their friction and wear behavior sliding against Si3N4 at room temperature and elevated temperatures up to 400 °C was comparatively studied. The microstructures and worn surface morphologies of the coatings were comparatively analyzed as well by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDXA). It was found that the as-sprayed WC–12%Co coatings were composed of WC as the major phase and W2C, WC1−x, and W3Co3C as the minor phases. The plasma sprayed nanostructured WC–12%Co coating had much higher hardness and refined microstructures than the conventional WC–12%Co coating. This largely accounted for the better wear resistance of the nanostructured WC–12%Co coating than the conventional coating. Besides, the two types of WC–12%Co coatings showed minor differences in friction coefficients, though the nanostructured WC–12%Co coating roughly had slightly smaller friction coefficient than the conventional coating under the same sliding condition. Moreover, both the conventional and nanostructured WC–12%Co coatings recorded gradually increased wear rate with increasing temperature, and the nanostructured coating was less sensitive to the temperature rise in terms of the wear resistance. The worn surfaces of the conventional WC–12%Co coating at different sliding conditions showed more severe adhesion, microfracture, and peeling as compared to the nanostructured WC–12%Co coating, which well conformed to the corresponding wear resistance of the two types of coatings. The nanostructured WC–12%Co coating with a wear rate as small as 1.01 × 10−7 mm3/Nm at 400 °C could be promising candidate coating for the surface-modification of some sliding components subject to harsh working conditions involving elevated temperature and corrosive medium.
Article
The performance of multimodal and conventional materials in the form of coatings deposited by high velocity oxy-fuel (HVOF) thermal spraying has been studied. WC-12Co coatings were deposited under same conditions using multimodal and conventional WC-12Co powder feedstocks. The phase composition of the feedstock powders and the coatings were analyzed by XRD. Abrasive wear resistances of coatings were carried out on wet sand rubber wheel abrasion tester. The characterizations of spraying feedstock powders, microstructure and surface micrographs of the prophase and anaphase attrition surfaces were performed by SEM. The results indicated the multimodal coating shows slight higher microhardness and better abrasive wear resistance than the conventional counterpart. Also, the thermally sprayed carbide-based coatings have excellent wear resistance with respect to the hard chrome coatings.
Article
Effects of post heat treatment of thermally sprayed nano WC–Co coatings on their mechanical properties were studied. The thermal behavior of WC particles in the coatings was also investigated. WC–Co coatings containing nano carbide particles in the 100–200 nm range were fabricated by detonation gun spraying. Considerable phase decomposition of WC to W2C and amorphous phase was detected, which degrades the mechanical properties of coatings. In order to improve the mechanical properties of the coatings by recovery of dissociated carbide phases, post heat treatment was conducted in an Ar environment in the temperature range of 400–900 °C. Microhardness and fracture toughness were measured by Vickers indentation testing and wear resistance was also evaluated by using a scratch tester. Phase evolution and microstructural changes due to post heat treatment were investigated by optical microscopy, X-ray diffractometry and field-emission scanning electron microscopy. After heat treatment in all temperature ranges, microhardness increased. Fracture toughness and wear resistance of coatings were increased by increasing temperature to 800 °C but decreased after heat treatment at 900 °C. Amorphous phase disappeared and other carbide phases such as W3Co3C and W6Co6C formed during heat treatment above 700 °C. The improved properties were elucidated and discussed in terms of microstructural changes and the relationship between mechanical properties and carbide phase was also discussed.
Article
WC–Co thermally sprayed coatings are now widely used in a range of industries to combat wear. During spraying of WC–Co coatings, there is a decomposition involving dissolution of the hard phase into the molten binder; as such, the coatings are often made up of WC along with other carbides in a matrix consisting of tungsten and carbon dissolved in the cobalt. High velocity oxy-fuel (HVOF) spraying is seen as the pre-eminent process for deposition of such coatings. There have been moves in the industry to spray coatings with liquid-fuelled systems rather than gas-fuelled guns, since the former lead to shorter residence times of the particles in the flame and lower temperatures, both of which lead to less dissolution of WC and decomposition of the coating. This paper presents work concerning the sliding wear behaviour of WC–Co coatings HVOF sprayed with both liquid-fuelled (HVOLF) and gas-fuelled (HVOGF) systems and demonstrates that with a dense powder feedstock, the HVOGF deposited coating is superior to the HVOLF deposited coating. The poorer performance of the HVOLF-sprayed coating is associated with mechanical damage to the WC–Co powder particles as they impact with the substrate resulting in carbide cracking and a reduction in the integrity of the bond between the carbide particles and the matrix phase.
Article
WC–Co coatings have been deposited by high velocity oxy-fuel thermal spraying of conventional and nanocomposite powders which contain WC grains in the size range 2–5 μm and 70–250 nm, respectively. The coatings differed not only in microstructural scale, but also in the nature and proportion of the phases present and in the overall degree of decarburization. A model describing the evolution of microstructure has been developed. As a WC–Co particle is heated in the hot gas jet, the cobalt phase melts and the WC grains begin to dissolve in it. The periphery of the semi-molten particle becomes decarburized by oxidation, promoting further WC dissolution in this region. Particle quenching on impact with the substrate results in precipitation from the melt of W2C and possibly W depending on the local melt composition. The larger surface to volume ratio of the WC in the nanocomposite material promotes more rapid dissolution and thus decarburization. Consequently, W2C is observed in both coatings, whereas W is found only in the nanocomposite deposit.
Article
Fracture by plastic indentation has been studied for a range of brittle materials, and the fractures have been characterized and quantified. An approximate stress analysis has indicated the importance of both the plastic penetration and the interface friction in the development of the observed fractures. The crack extension has been shown to depend primarily on the impression radius and the ratio of the hardness to the fracture toughness. This fracture characterization has been used to examine several important consequences of indentation fracture: notably, fracture toughness determination by indentation, material removal by abrasive processes, and material removal by low velocity solid particle impact.