Sprayed WC-Co Research Articles

MACHINING OF THERMALLY SPRAYED WC-Co COATINGS

This paper reports the research results obtained from machining of thermally sprayed WC-Co coatings. WC-Co coatings are used in marine applications. The machined coatings were obtained by using two thermal spraying processes: arc spraying and high-velocity oxy-fuel (HVOF) spraying. Different techniques were tried to optimize the surface finishing of the thermally sprayed coatings based on surface finish and time required. The machining processes tried for WC-Co coatings were diamond grinding, cubic boron nitride (CBN) grinding, diamond turning, and diamond polishing. Diamond turning had the advantage of speed, however, the surface finish was not as good as that with fine grinding. Polishing should have given the best surface finish, however, this process was slow. It is suggested to reduce the machining time, first by rough grinding or diamond turning to near the final dimensions, and then by fine grinding or polishing to get the required surface finish and dimensions.

Duplex Coating of Electroless Nickel and HVOF (High-Velocity Oxygen Fuel) Sprayed WC-Co

The porosity of thermal sprayed coatings is usually a problem when coatings are used in wet corrosion application. The porosity allows media to penetrate to the surface of the base material. Corrosion spreads rapidly and the coating loses contact with the substrate and delaminates. This problem can be initiated by impregnating different polymers into the pores. An alternative approach has been tested in the present work to prevent corrosion of cemented carbide coated carbon steel in wet corrosion environments.

Wear rates and specific energies of some ceramic, cermet and metallic coatings determined in the Coriolis erosion tester

A series of ceramic, cermet and metallic coatings of known resistance to slurry jet erosion has been subjected to sliding bed wear using a 10 wt.% aqueous slurry of 80 grit crushed alumina particles in the Coriolis erosion tester. Wear scar cross-sections have been measured and specific energies for material removal calculated for these coatings as well as for some bulk solid materials for reference purposes. Thermal-sprayed coatings included high velocity oxygen-fuel (HVOF)-sprayed WC–12Co, Cr 3C 2/NiCr and 316L steel, plasma-sprayed (PS) Al 2O 3 and Al 2O 3/TiO 2 and arc-sprayed 440C steel. Pulsed laser clad coatings of two FeCrB based alloys were also tested. The Coriolis erosion tester consists of a 150 mm diameter rotor with a diametral passage in which a flat plate specimen is located on each side, equidistant from the rotation centre. Slurry, delivered to a central orifice, is constrained to flow outwards through the channel as the rotor is turned at speeds up to 7000 rpm, the erodent particles being pressed onto the test specimen surface by the Coriolis force. The cross-sectional area of the scar is measured as a function of distance from the rotation centre and, through a summation of an expression for frictional work done, a value for the specific energy for material removal (essentially a measure of erosion resistance) is calculated. The test method seeks to simulate the wear environment in slurry pumps and cyclones in which particles move rapidly over the test surface either singly or in a bed. The wear rate of a WC–Co cermet was found to be ∼1000 times lower than that of 316L stainless steel. The remarkable ability of the Coriolis erosion tester to discriminate in the erosion performance of different materials is discussed in terms of particle trajectories and the probable wear mechanism. Coriolis test results are compared with those for slurry jet erosion and the applicability of the two tests for erosion resistance ranking of hard coating materials is discussed.

Abrasive wear behaviour of detonation sprayed WC–Co coatings on mild steel

The main objective of the present investigation is to evaluate the abrasive wear behaviour of detonation sprayed WC–Co coatings and to compare them with plasma sprayed WC–Co coatings and carburised and nitrided surfaces. Mild steel was coated with WC–12Co and WC–17Co using detonation and plasma guns. The abrasive wear rates of these coatings were determined at three different loads and compared with wear rates of as received, carburised, and nitrided mild steel. The abraded surfaces and the surfaces beneath the abraded surfaces were characterised by SEM. The results indicate that the detonation sprayed WC–12Co coating has the best abrasive wear resistance at all loads. Maximum improvement in wear resistance is obtained at an intermediate load (i.e. at 5 kg). The wear of detonation sprayed coatings is by abrasion of the soft Co matrix followed by the pullout of hard WC particles.

Effect of WC Particle Size on the Abrasive Wear of Thermally Sprayed WC-Co Coatings

Four types of WC-Co powders with different WC particle sizes are sprayed to produce WC-Co coatings with a Jet-Kote gun using a gas mixture of C2H2–30% C3H6 and pure propylene as fuel gases. The carbide size in sprayed high velocity oxy-fuel (HVOF) WC-Co coatings is measured from the microstructure. The abrasive wear of the coatings is characterized using a Suga abrasive wear tester. The abrasive wear mechanism of thermally sprayed WC-Co is discussed. The relationship between the relative wear of WC-Co coatings and carbide sizes is established according to the strength theory on hard alloy. The correlation with experimental data proves that the relative abrasive wear of WC-Co coating is proportional to square root of relative carbide size.

Powder/Processing/Structure Relationships in WC-Co Thermal Spray Coatings: A Review of the Published Literature

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.

Thermal Processes in HVOF Sprayed WC-Co Coating on a Copper Substrate

Mathematical modelling of the heat transfer between a WC-Co coating and a copper substrate during high-velocity oxygen-fuel (HVOF) spraying was undertaken. The modeling included the investigation of temperature variation, coating solidification, melting and solidification in the substrate interfacial region, and specific features of the substrate-coating thermal interaction.

Fatigue Cracks in HVOF Thermally Sprayed WC-Co Coatings

Initiation and early growth of fatigue cracks of a medium carbon steel with HVOF thermally sprayed WC-Co coatings prepared from two types of commercially available powders with similar total chemical composition were investigated under rotating bending conditions. The morphology of the fatigue crack is divided into two type—linear cracks and net-like cracks—depending on the types of powders and the thickness of the coatings. The fatigue cracks in thinner coatings were closer to each other than those for the thick coatings.

Fatigue Cracks in HVOF Thermally Sprayed WC-Co Coatings

Fatigue Cracks in HVOF Thermally Sprayed WC-Co Coatings

Rolling contact fatigue failure mechanisms in plasma and HVOF sprayed WC-Co coatings

The rolling contact fatigue (RCF) behaviour of thermally sprayed WC-Co coatings with nominal compositions of WC-12%Co, WC-10%Co-4%Cr and WC-17%Co was studied with a two-roll configuration roll-against-roll testing apparatus under 420–600 MPa Hertzian contact stresses in unlubricated pure rolling conditions. The coatings were prepared by atmospheric plasma spray (APS) and two high-velocity oxyfuel (HVOF) spray processes. In the APS sprayed WC-12%Co coating the RCF damage was dominated by an increased surface roughness due to spallation of flakes and a formation of a network of cracks within the coating layer. HVOF sprayed WC-12%Co and WC-10%Co-4%Cr coatings were damaged either by the formation of vertical, linear cracks or pitting of the contact surface. The formation of pits in the HVOF sprayed coatings was significantly less than that found in the APS sprayed coatings. The HVOF sprayed WC-17%Co coating showed the best RCF behaviour among the studied coatings with unchanged surface roughness, no formation of cracks and only a few pits were found on the contact surface. The good resistance of this coating against formation of failure in the RCF testing is caused by its higher ductility and fracture toughness due to a higher metallic binder content in comparison with the other coatings. Other characteristics such as a low amount of brittle Co-W-C carbides and a dense microstructure are also believed to be beneficial for a RCF resistant coating.

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

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.

Interface structures of high velocity oxy-fuel sprayed WC-Co coating on a copper substrate

Microstructures developing at the interface of a high velocity oxy-fuel (HVOF) sprayed WC-Co coating onto a copper substrate, have been studied by preparing thin foils from the interfacial region and examining them using transmission electron microscopy. Two types of substrate surfaces have been considered: smooth and grit blasted. Modelling of the development of the interface structure has also been undertaken. Both experimental and theoretical results, which agree well, show that the interface structure depends significantly on the morphology of the substrate surface. The results obtained enable new views to be put forward on the factors leading to the adhesion between a coating and a substrate material.

Effect of Carbide Grain Size on the Sliding and Abrasive Wear Behavior of Thermally Sprayed WC-Co Coatings

The carbide size and cobalt content of thermally sprayed tungsten carbide/cobalt coatings (WC-Co) can influence their microstructure, fracture strength, friction response and abrasion resistance. In this paper, these properties have been determined for one commercially available and three experimental WC-17 wt.% Co thermally sprayed coatings. The experimental coatings were processed from starting powders containing median carbide size distributions of 1.2, 3.8 and 7.9μm, respectively. All the coatings were produced using a high velocity oxy-fuel (HVOF) spray process. The present results indicate that coatings with a higher percentage of finer carbide size distribution in the starting powder display a higher degree of decomposition of the WC phase to W2C phase and, consequently, display lower fracture toughness and abrasion resistance values. Unidirectional, unlubricated sliding wear tests did not reveal major differences in the sliding wear response of the coatings as a function of carbide size. The microscopic analysis of the sliding wear-tested surfaces showed particle delamination and fracture, providing insight into wear mechanisms in these materials. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in San Francisco, California, October 13–17, 1996

Friction and Wear Behavior in Plasma Sprayed Coatings

The wear behavior and the mechanism of plasma sprayed WC12Co, Cr2 O 3 and ΑI2 O3 coatings in sliding contact with steel bronze and carbon graphite were studied with block-on-ring arrangement. The coefficient of friction and wear rate of these coatings were measured at dry friction conditions. The worn surfaces of the coatings were examined employing the ' scanning electron microscope. The results obtained that the wear mechanism of coatings changes for different kinds of layers.

Formation of an amorphous phase in thermally sprayed WC-Co

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.

Effect of powder structure on the structure of thermally sprayed WC-Co coatings

Five representative types of WC-Co powders were selected to clarify the dependence of the structure of sprayed coatings on the structure of powders themselves. The WC-Co coatings were sprayed with the Jet-Kote process and plasma spraying as well. The structure of WC-Co coatings was primarily characterized by X-ray diffraction. The X-ray diffraction patterns of the sprayed coatings were illustrated compared with those of powders, which aims at a better understanding of the structure of thermally sprayed WC-Co coatings. The selected coating was also analysed by differential scanning calorimetry (DSC). The decarburizing process, and the effects of powder structure and spray conditions on the crystal structure of sprayed WC-Co coatings are discussed in detail.

Structure/property relationships in sintered and thermally sprayed WC-Co

Thermally sprayed WC-Co is widely used as a wear-resistant coating for a variety of applications. Although it is well established that thermal spray processes significantly affect chemistry, microstructure, and the phase distribution of WC-Co coatings, little is known about how these changes influence wear resistance. In this study, the microstructure and wear behavior of sintered and thermally sprayed WC-Co materials are examined. Powders of WC-12 wt% Co and WC-17 wt% Co were pressed and sintered, as well as thermally sprayed by high-velocity oxy-fuel (HVOF), air plasma spray (APS), and vacuum plasma spray (VPS) techniques. Results indicated considerable differences in the resulting microstructures, mechanical properties, and wear resistance. The thermally sprayed coatings showed anisotropic fracture toughness, whereas the sintered materials did not. It was also shown that a combined mechanical property/microstructure parameter, based on considerations of indentation fracture mechanisms, can be used in most cases to describe abrasive and erosive wear resistance of thermally sprayed WC-Co materials as follows: Wear resistance a % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiFj0de9sqqrpepC0xbbL8F4rrpm0dbba9-u0ddr-df9% qqFn0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-xfr-x% b9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaqcLbsacqaHXoqyju% aGdaqadaGcbaqcLbsacaWGlbqcfa4aa0baaSqaaGqaaKqzGeGaiaiG% aaab--xsaiacaciaaqW--neaaSqaaKqbaoacaciaaWX-liaameacac% iaaWX-jugibiacaciaaWX-iodaaWqaiaiGaaah-NqzGeGaiaiGaaah% --hoaaaaaaGaamisaKqbaoacaciaaWR-CaaaleqcaciaaWR-bGaGac% aa86Fcfa4aiaiGaaae-VGaaWqaiaiGaaae-NqzGeGaiaiGaaae-Jym% aaadbGaGacaaq8FcLbsacGaGacaaq8-FYaaaaaaaaOGaayjkaiaawM% caaKqbaoaabmaakeaajuaGdaWcaaGcbaqcLbsacGaGacaaKdWGwbqc% fa4aiaiGaaa5a0baaSqaiaiGaaa5aKqzGeGaamOzaaWcbGaGacaaKd% qcLbsacGaGS-3qaiacaY+FVbaaaaGcbaqcLbsacGaG48xmaiacaIZF% TaGaiaiodAfajuaGdGaG40baaSqaiaioieGajugibiacaciaaWO-+z% gaaSqaiaiojugibiacaciaaaX--neacGaGacaaq8-FVbaaaaaaaOGa% ayjkaiaawMcaaaaa!8ABD! $$\alpha \left( {K_{IC}^{{\raise0.7ex\hbox{$3$} \!\mathord{\left/ {\vphantom {3 8}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{$8$}}} H^{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 2}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{$2$}}} } \right)\left( {\frac{{V_f^{Co} }}{{1 - V_f^{Co} }}} \right)$$ whereK ic is the indentation fracture toughness,H is hardness, andV Co f is the volume fraction of cobalt. This relationship provides a means for assessing wear resistance of WC-Co coatings intended for industrial applications requiring abrasion and/or erosion resistance.