Effects of carbide size and Co content on the microstructure and mechanical properties of HVOF-sprayed WC–Co coatings (original) (raw)
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Effect of Co Content on the Properties of Hvof Sprayed Coatings Based on Tungsten Carbide
2012
Thermal sprayed coatings based on tungsten carbide are the most durable materials in terms of wear resistance. Although they are not suitable for high temperature applications, they can be applied in many areas of industry due to the combination of very hard carbides and tough matrix. The good wettability of carbides WC in Co matrix contributes to the high cohesive strength of WC-Co cermets. The hardness and toughness rate of WC-Co coatings is, in the case of thermal sprayed coatings, determined by mutual proportion of carbide phase and matrix, and by spraying parameters. Depending on the application, either hardness of coating (abrasive wear) or higher level of toughness (erosive wear) can be preferred for different types of wear. The presented study was conducted to determine the effect of cobalt matrix content on the resulting coating mechanical properties. Samples were prepared using the HVOF (high velocity oxy-fuel) spraying equipment HV-50. Three powder types with different co...
Journal of Thermal Spray Technology, 2017
The use of nanoscale WC grain or finer feedstock particles is a possible method of improving the performance of WC-Co-Cr coatings. Finer powders are being pursued for the development of coating internal surfaces, as less thermal energy is required to melt the finer powder compared to coarse powders, permitting spraying at smaller standoff distances. Three WC-10Co-4Cr coatings, with two different powder particle sizes and two different carbide grain sizes, were sprayed using a high velocity oxyair fuel (HVOAF) thermal spray system developed by Castolin Eutectic-Monitor Coatings Ltd., UK. Powder and coating microstructures were characterized using XRD and SEM. Fracture toughness and dry sliding wear performance at three loads were investigated using a ball-on-disk tribometer with a WC-Co counterbody. It was found that the finer powder produced the coating with the highest microhardness, but its fracture toughness was reduced due to increased decarburization compared to the other powders. The sprayed nanostructured powder had the lowest microhardness and fracture toughness of all materials tested. Unlubricated sliding wear testing at the lowest load showed the nanostructured coating performed best; however, at the highest load this coating showed the highest specific wear rates with the other two powders performing to a similar, better standard.
The Mechanical Properties and Wear Resistance of HVOF Sprayed WC-Co Coatings
Acta Physica Polonica A, 2016
In this work, the Woka 5810 powders (88% tungsten carbide-12% cobalt) were used to produce coating by high velocity oxy-fuel spraying. WC-Co is widely used as a tribological coating material providing a combination of high toughness, high hardness, and good strength. The treated samples were characterized by using optical micrograph, stereo microscope and scanning electron microscopy, X-ray diffractometry, and microhardness tests. Also the wear performance of the coatings was investigated. The results indicated that the coating shows slight higher microhardness and better abrasive wear resistance than the conventional counterpart. The friction coefficient of coating was low. The scanning electron microscopy and energy dispersive spectroscopy analyses were applied to worn surfaces.
EFFECT OF POWDER PARTICLE SIZE ON THE STRUCTURE OF HVOF WC-Co SPRAYED COATINGS
2009
Tungsten carbide coatings were deposited on steel substrate using a high velocity oxy-fuel (HVOF) spraying method. WC-Co coatings were prepared from commercially available WC-12% Co powders with different powder particle size distribution. Three different nominal powder particle size distributions were used:-45+20 μm,-45+11 μm and-20+5 μm. The microstructures of the coatings were analyzed by light microscopy, scanning electron microscopy and phase composition by Xray diffraction. Powder particle size distribution was found to be an important parameter, strongly affecting the coating quality. Powder particle size influences porosity and morphology of the splats, as well as phase composition. The smallest porosity and visible lamellar microstructure was achieved using the smallest powder particles. The coating sprayed from powder with the coarsest particle had a higher amount of retained WC. In coating sprayed from powder with the smallest particles, a considerable amount of WC probably has been transformed on W 2 C and W and part of WC has reacted with cobalt forming amorphous complex phases of Co x W y C z .
Surface & Coatings Technology, 2009
Micron-sized WC-Co powder (powder) was coated onto an 420J2 steel substrate and the bond coats (BCs) of Ni, NiCr, and Ni/NiCr using high-velocity oxy-fuel thermal spraying to study the surface properties, friction behavior, and tensile bond strength of the WC-Co coating (WC-Co) on the 420J2 substrate (sub) and the BCs of Ni, NiCr, and Ni/NiCr. During the spray coating, a small portion of WC decomposed to the less-hard W 2 C, W, and free carbon above its decomposition temperature of 1250°C, decreasing hardness and increasing porosity. The surface hardness of 1120 ± 100 Hv (10,980 ± 980 MPa) depended strongly on the spray parameters. It was three to four times harder than metals and alloys, but less than one-half the hardness of binder-less pure WC (2400 Hv). Free carbon reacted with the sprayed oxygen gas and formed carbon oxide gases, resulting in a coating of 4.3 ± 1.0% porosity. The friction coefficient of the coating increased about 17% with increasing surface temperature: 0.65 ± 0.03 at 25°C to 0.76 ± 0.06 at 500°C because of the increased local cold-welding of the asperities at the higher temperature of 500°C. Sub/WC-Co, sub/Ni/WC-Co, sub/NiCr/WC-Co, and sub/Ni/ NiCr/WC-Co had tensile bond strengths of 9600 ± 300 psi (66.2 ± 3.4 MPa), 6300 ± 200 psi, 6000 ± 200 psi, and 7500 ± 200 psi, respectively. The fracture locations of all coatings were at interfaces with the WC-Co coating, indicating that the adhesion of the WC/Co inside coating was higher than 9600 ± 500 psi and that the adhesion of WC-Co on the substrate (9600 ± 500 psi) was much higher than the adhesion on the BCs.
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, W 2 C 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.
Microstructure and properties of flame sprayed tungsten carbide coatings
International Journal of Refractory Metals and Hard Materials, 2002
This article reports on feasibility experiments carried out with oxy-acetylene spray system with various oxygen to fuel ratios using two different tungsten carbide powders and powder feeding methods, to evaluate the newly developed fused WC, synthesised by transferred arc thermal plasma method. Transferred arc thermal plasma method is more economical and less energy intensive than the conventional arc method and results in a fused carbide powder with higher hardness. The microstructure and phase composition of powders and coatings were analysed by optical and scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Carbon content of the powders and coatings were determined to study the decarburisation of the material during spraying process. Coatings were also characterised by their hardness and abrasive wear. The effects of metallurgical transformation and phase content are related to wear performance. The results demonstrate that the powders exhibit various degree of phase transformation during the spray process depending on the type of powder, powder feeding and spray parameters. The carbon loss during the spray process in excess of 45% resulted in reduced hardness and wear resistance of the coatings. Coatings with high amount of WC and W 2 C along with FeW 3 C showed higher wear resistance. Thus, coatings of high wear resistance can be produced using fused tungsten carbide powder with WC and W 2 C phases, which can be economically synthesised by thermal plasma transferred arc method.
Tungsten Carbide (WC) coatings exhibit high wear resistance at low and high temperatures, WC - Cobalt coatings will demonstrate anti-resistive and wear characteristics better than those of conventional materials. Research in this area has shown that the service life of the WC-Co coatings depended on varying compositions of tungsten and cobalt. WC-Co coating is developed on the stainless steel AISI 304 by High Velocity Oxy-Fuel spray technique. The grain size of WC is varied in three ranges of 10-40μm, 15-63μm and 45-90μm. Microstructure, chemical composition, phases present in the coating on the steel substrate was studied by using Scanning Electron Microscope (SEM) and X-Ray Diffraction method. Microstructure shows uniform distribution of WC in the matrix. WC coatings exhibit increased in hardness and resistance to wear. The wear rate of tungsten carbide of sample C mesh size (45 to 90 μm) is less compare to two remaining two samples by considering different loads.
The Surface Properties of WC-Co-Cr Based Coatings Deposited by High Velocity Oxygen Fuel Spraying
Acta Physica Polonica A, 2017
The aim of this research was to investigate microstructural and mechanical properties of the WC-Co-Cr coatings by high velocity oxygen fuel spraying. Woka 3653 (WC10Co4Cr) powder was used as coating material. This powder is widely used as a tribological coating material providing a combination of high toughness, high hardness, and good strength. The coatings were produced for the different high velocity oxygen fuel spraying parameters. The treated samples were characterized by using scanning electron microscopy/energy dispersive X-ray spectrometry and X-ray diffractometry. Microhardness measurements were executed to evaluate the mechanical properties of the coatings. Also the wear performance of the coatings was investigated. The scanning electron microscopy and energy dispersive X-ray spectrometry analyses were applied to worn surfaces. The results indicated that the coating shows slightly higher microhardness and better abrasive wear resistance than the conventional counterpart.
Surface & Coatings Technology, 2007
In present paper the influence of the tungsten carbide (WC) particle addition on the microstructure, microhardness and abrasive wear behaviour of flame sprayed Co-Cr-W-Ni-C (EWAC 1006) coatings deposited on low carbon steel substrate has been reported. Coatings were deposited by oxy-acetylene flame spraying process. Wear behaviour of coatings was evaluated using pin on flat wear system against SiC abrasive medium. It was observed that the addition of WC particle in a commercial Co-Cr-W-Ni-C powder coating increases microhardness and wear resistance. Wear behaviour of these coatings is governed by the material parameters such as microstructure, hardness of coating and test parameters (abrasive grit size and normal load). Addition of WC in a commercial powder coating increased wear resistance about 4-9 folds. WC modified powder coatings showed better wear resistance at high load. Heat treatment of the unmodified powder coatings improved abrasive wear resistance while that of modified powder coating deteriorated the wear resistance. SEM study showed that wear of coatings largely takes place by microgroove, crater formation and scoring. Electron probe micro analysis (E.P.M.A.) of unmodified and WC modified powder coating was carried out for composition and phase analysis.