View of REVIEW PAPER ON WC-12Co AND WC-20Cr₂C₃-7Ni HVOF SPRAYED COATINGS

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Vol. 06, Issue 07,July 2021 IMPACT FACTOR: 7.98 (INTERNATIONAL JOURNAL)

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REVIEW PAPER ON WC-12Co AND WC-20Cr₂C₃-7Ni HVOF SPRAYED COATINGS

Rasmita Mishra, T K Mishra, P. K Sahu 1 INTRODUCTION

Techniques for the modification of surfaces are evolving rapidly. For modern industries, modification of surface and its optimization to increase the resistance against wear, corrosion is a challenging aspect. Surface modification is the process of modifying the surface of a material by bringing physical, chemical characteristics different from the original surface of the material on which it is done. The most effective way to enhance wear resistance is surface coating, which is defined as a thin film of material deposition on the base metals to improve the surface properties of the base metal.Various industrial application of surface coating are cylinder and valves, turbine, aerospace, oil refinery and rollers in paper mills etc.[1,2].Several processes which are used for surface coating are – Physical Vapor Deposition(PVD), Chemical Vapor Deposition(CVD), Thermal Spray Coating (TSC), etc.[2]. Coating by thermal spray technique include flame spray, High Velocity Oxy Fuel( HVOF) coating, Plasma spray, HVAF and Detonation gun, among which HVOF coating offers good mechanical and microstructural properties fulfilling the modern industrial requirement[1]. It can bear harsh conditions like moisture, penetration of abrasive and erosive particles. It is extremely used for the deposition of hard metal powder and metal powder composite [1]. Normally HVOF coating is preferred for tungsten carbide (WC), Chromium carbide and their matrix. It consist of assorted mixture of Cobalt, Nickel or Chromium [4]. The HVOF process involves a supersonic jet flame produced by combustion of a mixture of oxygen and fuel to decrease the decarburization and oxidation resulting to low flash temperature, high spray velocity, minimize the porosity thereby excellent wear resistance, toughness and bond strength [1,3]. Depending upon the coating to be deposited, several types of fuel can be used on HVOF gun. Hydrogen, propane, kerosene (aviation grade) and Liquefied Petroleum Gas(LPG). As the liquid fuels have several advantages over gaseous fuel, so liquid fuels are used in

latest technology the most. The speed of the sprayed particles reaches 500m/s and temperature of up to 2300K [5]. The increase in oxygen flow rate or fuel rate leads to an increase in both the particle temperature and speed. But, if the oxygen flow rate is too high, a decrease in particle temperature is observed as it leads to a combustion with low efficiency. And increase in fuel flow rate decreases the particle temperature because of the non – stoichiometric combustion and the cooling produced by excess fuel [6].

Mechanical and microstructural properties of surface coating are depend upon parameters like feedstock powder, residual stress, resultant porosity, binder fraction and grain size of powder [7].

2. HIGH VELOCITY OXYGEN-FUEL (HVOF) COATING

In HVOF system, coating powder will be injected into a stream of burning gases.

Then the coating material melts and deposited on the substrate. HVOF guns are designed in such a way that they can withstand the shock existed in the flame.

Once the flame exited from the nozzle, it extended freely and produce a supersonic jet. The metal and structure of HVOF guns differ from each other based on the gases used, nozzle space required, weight, type of particles to be injected, efficiency, etc. One of the popularly used HVOF gun is the Diamond Jet Hybrid gun in which oxygen is mixed with propylene, propane and hydrogen after which the outlet flame is blended in a syphon system and accelerated further by a CD nozzle to hypersonic speeds [8, 9].The major benefits of this coating is high kinetic energy and good density. It can be coated with the required thickness and obtain a good surface finish. HVOF coatings have excellent wear and corrosion resistance properties and also it improves hardness and toughness of the base metal [2].

3 COATING PROPERTIES 3.1 Tribological properties

WC-Co and WC-Co-Cr coating retains excellent sliding and abrasive wear resistance, better hardness and

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toughness [10, 11]. WC-Co shows a very

low wear rate as a pullout of single carbide particle provide less damage to finer coating and the debris consisting of finer carbides are less effective. So, the sliding wear rate decreases with decreasing carbide size in the coating[12].

WC-12Co shows a higher sliding and rolling resistance than the substrate material (C45 steel). Sliding wear investigation corresponding to fine structured and nanostructured coating achieved a higher resistance and low coefficient of friction[13, 14]. Under the dry sliding condition, the average friction coefficient of WC-12Co coating under dry sliding condition is around 0.069 with 80N load. At the same load with wet condition COF is 0.052. So it is clear from the above that the wear rate of sample under dry condition is higher than the wet condition [15]. With the increase in sliding speed, the wear rate and COF of WC-12Co coatings were decreased [16].

WC-17Co has best wear resistance according to lowest wear rate when compared to WC-10Co-4Cr, WC-12Co,

&Cr₃C₂-25NiCr [17]. Corrosion rate of WC- 17Co coating is extremely high compared to WC-10Co-4Cr and Cr₃C₂-25NiCr give better corrosion rate. Corrosion rate of WC-17Co is five time higher than Cr₃C₂- 25NiCr [2, 4]. The porosity of WC-10Co- 4Cr coating is larger than WC-17Co. The large porosity can be encouraging the interconnection of porosity and increase the corrosion rate. Cr is used to improve the corrosion resistance in Cr₃C₂-NiCr and WC based coating [18]. When the development wear rate compared with respect to sliding wear rate, the wear rate of WC-10Co-4Cr coating is slightly less than WC-12Co and COF of Cr₃C₂-25NiCr coating is best when comparing WC-12Co

& WC-10Co-4Cr. Initially COF increases with a developing contact stress but when contact stress decreases the area of contact increases there by increase in sliding distance [19]. The tribological behavior of WC-Cr₃C₂-Ni feedstock powder which composition change during spraying process and designated as WC- (W, Cr) ₂C-Ni is controlled by plastic smearing of a thin and uniform NiWO₄+CrWO₄ oxide scale, preventing direct contact between the hard metal surface and the counter body and

reducing the friction coefficient of the system WC-CoCr, bycontrast, experiences catastrophic oxidation and losses its functionality [20].The corrosion resistance of WC-(W,Cr)₂ C-Ni is eight times better than WC-12Co coating in 3.5% wt. NaCl solution[29].

3.2 Mechanical Properties

The wear mechanism, working conditions, the microstructure of the surface affects the wear and friction behavior of the coating [16].The WC metal matrix combination have arrested the mechanical properties because the tungsten carbide improves hardness, and toughness by metal matrix compounds like cobalt, Cobalt-chromium and Nickel presented by Hemant Asgari et al [4].The chemical composition of WC-Co are 86- 88% WC and 6-13% Cobalt with or without an addition of 1.5-8% Chromium.

The microstructure of coated specimens was analyzed with the occurrence of WC and tungsten-containing Chromium carbide grain particles. The hardness of the material was improved after heat treatment, which limit from precipitation of auxiliary carbides and solid reinforcement by tungsten. The experimental coating improves the wear properties at elevated temperature and in room temperature too [16].

T.K. Mishra et al. [1] represented that the applied load, thickness of transfer layer and the strength of the binder phase influences wear rate. The increase in wear rate is due to the propagation of micro cracks and plastic deformation. WC-12Co exhibits dense and homogeneous microstructure due to presence of WC particles diffused in melted Co binder.WC-12Co coating exhibits a stable and thick transfer layer due to strong bonding in presence of Co binder. The stability and thickness of the transfer layer depends on loads and increase with an increase in load up to critical value then decrease with further increase in load. WC-12Co coating exhibited maximum bond strength i.e.

2.03% and 10.5% and best sliding wear resistance, maximum micro hardness higher than WC-10Co-4Cr and Cr₃C₂- 25NiCr.

Kinetic energy and hardness of enforced particle directly affect the adhesive strength which leads to

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fastening effect on the surface[21].HVOF

coating having higher particle speed during spraying promotes the enhancement of adhesion. The presence of WC makes a strong influence on adhesive strength [22].

Erwin Mayrhofer et al. [23]

concluded a superior resistance against cracking of HVOF thermally sprayed WC- 20Cr₃C₂-7Ni coating compared to Cr₃C₂- 25(Ni20Cr) by approximately 100Mpa and 0.1% of applied stress and strain respectively. And WC-20Cr₃C₂-7Ni have higher compressive residual stress but this coating also exhibit evidence for transgranualar fracture mode with branched cracks and crack path deflection around tungsten carbide.

3.3 Effect of Temprature on Wear Behaviour of Coating

HVOF cermet coating are particularly suitable for high- temperature application, since they are able to retain their hardness up to 600 ͦC or more. In these application ceramic material are always used as counterparts.Z. Geng et.

al. [24] investigated on wear behavior of WC-Co HVOF coating within the range of room temperature to 800 ͦ C in air and RT to 650 ͦC in argon using a ball on disk tribometer and concluded that the volume loss of WC-Co coating is low at RT600 ͦC which indicates that oxidation inhibits wear. But above 600 ͦC the coating oxides vigorously and its properties deteriorates rapidly which cause serious wear. I.e.

wear decreases as temperature rises because of gradual formation of oxides.

So, WC-Co coating should not be used as wear resistant coating in 0-deficient environments at room temperature or at elevated temperatures. M.Richert et.al.[25] Investigated that the annealing process cause increase of the value of micro hardness HVOF WC-Co coating.

After annealing at 500 ͦC average micro hardness was equal 1488HV with comparison to the initial state increase was about 20%. After annealing WC-Co coating in temp 5oo ͦC the voids and pores disappeared almost.

La Vecchia et.al. [26] Investigated the dry sliding behavior of WC-12Co, WC- 10Co-4Cr, Cr₃C₂-25 NiCr grounded to a surface roughness of Rₐ=0.1µm and the test were performed using block on ring

test. Both block and ring were coated and concluded that WC-12Co coating shows mild wear (with Kₐ lower than 10^- 15m^2/N, Specific wear rate defined by ratio of wear volume to the applied load and sliding distance), which is quite similar to that of WC-Co hard metals.

Mr. R. Puschmann et al. [27]

investigated that up to 600 ͦC wear of WC- (W, Cr)₂C-Ni is higher than that of WC-Co- Cr. At room temperature, abrasion of the matrix phase by Al₂O₃ counter body and some loose oxidized debris takes place. At high temperature, softening of hard metal coating results in significant build-up of oxidized transfer material on the counter body surface, increasing the COF of the tribopair and finally concluded that WC- (W,Cr)₂ C-Ni coating is capable of producing quite mild wear regimes over a large temperature range, offering a flexibility up to 750 ͦC.

3.4 Microstructure Analysis

The microstructural analysis were carried out by using a Scanning Electron Microscope, Image Analyzer, X-ray Diffractometer, and the wear test were conducted by Pin-on-disc tribometer. In general, HVOF cermet coatings display a splat-like microstructure due to the impacts of partially melted droplets followed by their rapid solidification.

Some residual porosity is found in the interlamellar regions (the typical residual porosity of HVOF coatings are below 2- 3%). This splat-like microstructure leads to the anisotropic mechanical properties of sprayed coating. A further source of anisotropy is related to the WC particles that remain in solid state during spraying.

They may retain their original angular morphology in microstructure or have a rounded appearance. The rounded morphology is a result of WC dissolution into the matrix with the formation of an external irregular-shaped W₂C phase and possibly a monocrystalline/amorphous matrix phase [28].

Mingxiang Xie et al. reported comparative study on a sprayed HVOF carbide based coating such as WC- 17Co,WC-12Co,WC-10Co-4Cr and Cr₃C₂- 25NiCr powder have most common spherical particles in which WC-17Co has the most regular particle size. The cross- sectional SEM micrograph shows that

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four coatings have similar porous

microstructure. It is commonly observed that HVOF sprayed coatings has lamellar structure due to the accumulation of the high speed molten or semi-molten particles on the substrate. The coatings have dense structure and well bond performance with the substrate. The above three WC based coatings have higher micro-hardness & indentation fracture toughness compared to Cr₃C₂- 25NiCr coating which ultimately shows higher hardness as a result of WC phase than Cr₃C₂.[17]

V. Matikainen et al. investigated that the wear resistance of Cr₃C₂-25NiCr coatings can be improved by replacing some of the Cr₃C₂ particles with harder WC particles e.g. 20Cr₃C₂-WC-NiCr. This coating Cr₃C₂-20WC- NiCrstructure contains homogeneously dispersed WC particles surrounded by darker and larger Cr₃C₂ particles. With addition of WC particles the porosity of coating is lowest

& with higherCr₃C₂ particle high material density, i. e. kinetic energy so porosity decreases. This coating suffered from the rapid oxidation of WC particles preventing the formation of uniform tribolayer at room temperature.[30].

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