EFFECT OF HELIUM PLASMA GAS FLOW RATE ON THE PROPERTIES OF WC-12 wt.%Co COATINGS SPRAYED BY ATMOSPHERIC PLASMA

The cermet coatings of WC-12wt.%Co are extensively used to improve the wear resistance of a wide range of technical components. This paper analyses the influence of the plasma gas flow of helium on the microstructure and mechanical properties of WC-12wt.%Co coatings deposited by plasma spraying at atmospheric pressure (APS). In order to obtain homogeneous and denser coatings, three different flows of He (8 l/min., 16 l/min. and 32 l/min.) were used in the research. With the application of He, coatings achieved higher values of hardness due to less degradation of the primary WC carbides. The main goal was to deposit dense and homogeneous layers of WC-12wt.%Co coatings with improved wear resistance for different applications. The test results of the microstructure of the layers were evaluated under a light microscope. The analysis of the microstructure and the mechanical properties of the deposited layers was made in accordance with the standard of Pratt-Whitney. The morphology of the powder particles and the microstructure of the best coating was examined on the SEM (scanning electron microscope). The evaluation of the mechanical properties of the layers was done by applying the HV0.3 method for microhardness testing and by applying tensile testing to test the bond strength. The research has shown that the flow of He plasma gas significantly affects the microstructure, the mechanical properties and the structure of WC-12 wt.%Co coatings.

The cermet coatings of %Co are extensively used to improve the wear resistance of a wide range of technical components.This paper analyses the influence of the plasma gas flow of helium on the microstructure and mechanical properties of WC-12wt.%Cocoatings deposited by plasma spraying at atmospheric pressure (APS).In order to obtain homogeneous and denser coatings, three different flows of He (8 l/min.,16 l/min. and 32 l/min.)were used in the research.With the application of He, coatings achieved higher values of hardness due to less degradation of the primary WC carbides.The main goal was to deposit dense and homogeneous layers of WC-12wt.%Cocoatings with improved wear resistance for different applications.The test results of the microstructure of the layers were evaluated under a light microscope.The analysis of the microstructure and the mechanical properties of the deposited layers was made in accordance with the standard of Pratt-Whitney.The morphology of the powder particles and the microstructure of the best coating was examined on the SEM (scanning electron microscope).The evaluation of the mechanical properties of the layers was done by applying the HV 0.3 method for microhardness testing and by applying tensile testing to test the bond strength.The research has shown that the flow of He plasma gas significantly affects the microstructure, the mechanical properties and the structure of WC-12 wt.%Co coatings.

Introduction
C-Co cermet coatings are groups of coatings designed for wide-spectrum abrasion resistance.Thermal spray processes such as plasma spraying APS, VPS and HVOF processes are commonly used for their deposition.These technological processes have proven to be good for the production of WC-Co cermet coatings as a replacement for electrolytic hard chromium, especially in the aerospace industry (Berger, et al., 1996, pp.89-96), (Dorfman, et al., 2000, pp.471-478), (Sartwell, et al., 2002), (Savarimuthu, et al., 2000(Savarimuthu, et al., , pp.1095(Savarimuthu, et al., -1104)).Thermally sprayed WC-Co coatings are used extensively to improve the abrasion resistance of technical components.WC-Co coatings deposited with thermal spraying are widely used in the cases when high resistance to abrasion and erosion resistance are requested.These coatings exhibit higher phase microstructures which are formed from the WC primary carbides such as W 2 C, W and amorphous phases of Co-based binders (Li, et al., 1996, p.785), (Verdon, et al., 1998, p.11).The microstructure of WC-Co coatings has three crystalline phases (WC, W 2 C and W).The WC carbide phase is present in the initial powder but the other two phases are formed during the spraying process through decarburization of WC carbideparticles.The share of the secondary phases (W 2 C and W) was higher in the coatings sprayed using hydrogen H 2 as plasma gas.Hydrogen has a high heat content -enthalpy and therefore there is greater decarburization of WC carbide particles.The porosity of the coatings produced is about 10%.In these coatings, the presence of a matrix rich in Co was identified.This Co-rich phase represents the areas with different composition.The bright fields belong to the matrix areas with a high percentage of W. WC grains are within the splats where the temperature achieved is not high enough to produce the collapse of grains.The W 2 C carbide phase was identified around WC grains.The metal W was discovered in the outer part of the splats where decarburization is higher.The reason for this may be a larger share of W 2 C and higher strenghtening of the Co matrix with the desintegration of WC carbides.When He is used as plasma gas, coatings achieve higher hardness values due to a higher content of primary non decomposed WC carbide grains.The He gas has a lower enthalpy value than H 2 and makes denser plasma which absorbs less oxygen (Mrdak, 2012, pp.71-89), (Mrdak, 2013, pp.68-88), which results in the smaller decomposition of WC grains; therefore,the coating in the structure retains a higher number of primary WC grains.The increase of the coating microhardness is expected when the proportion of WC grains in the microstructure is higher.The highest values of hardness and toughness of the coating are achieved when He is used as plasma gas.These values are similar to those obtained in the coatings sprayed W with the HVOF process (Khameneh Asl, Heydarzadeh Sohi, 2006, pp.1203-1208).With the development of the cold spray, there is a possibility for carbide decarburization to be eliminated, completely or partially, since the spraying of particles takes place at lower temperature (Jodoin, et al., 2006, pp.4424-4432), (Kim, et al., 2005, pp.243-248), (Lima, et al., 2002), (Li, et al., 2007(Li, et al., , pp.1011(Li, et al., -1020)), (Papyrin, 2001, pp.49-51), (Stoltenhoff, et al., 2002, pp.542-550).Using powders with a reduced WC grain size improves the properties such as hardness and toughness, even resistance to sliding (Jia, Fischer, 1996, pp.206-214) and resistance to wear (Jia, Fischer, 1997, pp.310-318).A significant effort has been devoted to studying the effect of the percentage of binder phase and spraying conditions on the properties of coatings (Dent, et al., 2002, pp.551-558), (Qiao, et al., 2003, pp.24-41), (Marple, Lima, 2003, pp.273-282).In these studies, it was found that the adhesion and the level of decarburization affect significantly the microstructure and properties of coatings.Decarburization affects not only hardness, but also wear, as suggested by Qiao et al.; therefore, the optimization of parameters it is very important (Qiao, et al., 2003, pp.24-41).The reaction of the WC carbide decomposition takes place during thermal spraying (Guilemany, et al., 1999(Guilemany, et al., , pp.1913(Guilemany, et al., -1921)), by the reaction: (Eq 2) W 2 (CO)↔2W + CO (Eq 3) Chemical reactions take place in WC carbide primary grains that interact with oxygen.Also, WC carbide primary grains can be degraded in the atmosphere without oxygen in accordance with equation 4: According to some authors (Guilemany, et al., 1999(Guilemany, et al., , pp.1913(Guilemany, et al., -1921)), (Verdon, et al., 1998, pp.11-24), two types of W 2 C carbide can be formed.The first type is formed when the primary grains of WC carbides decompose in accordance with equations Eq 1 or Eq 4. The second mechanism occurs during the solidification of the Co-rich matrix , which leads to the precipitation of the W 2 C phase on the WC grain boundaries.Later,W 2 C phases appear as globular edges around WC grains.As a result of the decomposition reactions, some carbon is dissolved in the matrix, and some reacts with oxygen from the surface to form CO/CO 2 , thus losing a part of carbon from the starting powder.The retained C in the matrix, together with W present in the liquid, Eeriches the Co matrix forming amorphous compounds and nanocrystalline regions (Verdon, et al., 1998, pp.11-24), (He, Schoenung, 2002, pp.274-319).Depending on the decarburization degree, metal W can be deposited near the lamella boundaries (Qiao, et al., 2003, pp.24-41)where the carbon envelope is,due to the reaction with oxygen.Also, depending on the degree of decarburization, the precipitation of the η -phase can also occur in the following way (Guilemany, et al., 1999(Guilemany, et al., , pp.1913(Guilemany, et al., -1921)) (Eq 7) This experimental observation has been confirmed in the coatings with a higher content of decarburization and cracks (Sobolev, et al., 2004).
Sintered and crushed powder of WC-12wt.%(88WC/12Co) is most often deposited by atmospheric plasma spraying -APS process or by HVOF for the production of very dense WC coatings.These coatings provide excellent resistance to most forms of abrasive wear at temperatures ≤ 500°C (Material Product Data Sheet, 2012, Metco 72F-NS Tungsten Carbide -12% Cobalt Sintered and Crushed Powders, DSMTS-0115.0,Sulzer Metco).These coatings contain fine WC carbide grains for abrasive resistance against abrasive impact of hard particles, hard surfaces, erosive particles and mechanisms of wear by friction.Coatings are intended for use in dry non-corrosive environments.In comparison with the WC coatingswith 17 wt% of cobalt, the reduced Co content in the coating reduces toughness while increasing hardness and resistance to friction and wear.WC-12wt.%(88WC/12Co)coatings are used on parts such as: conveyor screws, compressor stators, impeller shafts, fan blade midspan supports, exhaust fans, etc. (Material Product Data Sheet, 2012, Metco 72F-NS Tungsten Carbide -12% Cobalt Sintered and Crushed Powders, DSMTS-0115.0,Sulzer Metco).Mann and al. showed that WC-12wt.%Cocoatings significantly increase the erosion resistance of components in the oil industry, such as valves and valve rings (Mann et al., 2006, pp.75-82).
The main objective of this study was to deposit, by atmospheric plasma spraying -APS, dense and homogeneous layers of WC -12wt.%Cocoatings with high resistance to wear and erosion for different applications.When choosing the parameters, He was used as plasma gas; unlike H 2 , He does not react with the powder.It produces denser plasma with a lower heat content which reduces the temperarture of decomposition and decarburization of WC carbide.Three groups of samples were made with a secondary plasma gas flow rate He of 8 l/min., 16 l/min.and 32 l/min.The microstructure and mechanical properties of coating layers were analysed and studied.The best performance was found in layers deposited with a flow rate of He 32 l/min.The mechanical properties of the layers were evaluated by testing hardness using the HV 0.3 method and by testing their bond strength using tensile testing.Microhardness measurement was performed in the direction along the lamellae, in the middle and at the ends of the samples.Five values were obtained and averaged.Bond strength was tested by tensile testing.The testing was done at room temperature with a tensile speed of 1cm/60s.For each group of samples, three test specimens were tested, and in this paper the mean values are presented.

Materials and experimental details
The microstructural analysis of the coatings was performed under a light microscope.The morphology of the powder particles and the best coating microstructure were done on the SEM (Scanning Electron Microscope).
Layers were applied on the metal substrate using the APS method.The coatings were deposited on steel bases roughed with white electocorundum with a grain size of 0.7 mm to 1.5 mm.For the production of coatings, the atmospheric plasma spray (APS) system of the company Plasmadyne was used.The system consists of: cathode K1083 -129, anode A 2083 -129 and gas injector GI 2083 -130.The main parameter when choosing powder deposition parameters was the plasma gas flow of He (l/min.).Helium was used in a combination with argon gas and an arc power supply of 40 kW.Three groups of samples with three He flows of 8 l/min., 16 l/min.and 32 l/min were made.The layers were deposited on the substrates of a total thickness of 0.15 to 0.18 mm.The detailed values of the plasma spray deposition parameters are shown in Table 1.

Results and discussion
The measured values of the microhardness of the WC -12 wt.% Co coating depending on the flow of helium are presented in Fig. 1.The values of the microhardness of the layers of WC-12 wt.%Co are directly related to the flow of helium.The layers deposited at a flow rate of helium of 8 l/min.had the lowest values of microhardness in the range of 598 -876HV 0.3 .The highest microhardness values of 997 -1420HV 0.3 were observed in the layers with the highest helium flow of 32 l/min.The values of the microhardness of the WC -12 wt.%Co coatings deposited with a helium flow of 16 l/min.and 32 l/min.were in accordance with the values set by the standard PWA(min.700HV 0.3 ) for this type of coatings (Turbojet Engine -Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA).The helium flow affected the density of the deposited layers.A small flow of helium resulted in less melting and lower deformation of hard particles during the impact with a previously deposited layer.Limited deformation of particles in the impact with the substrate, which is also less deformed under the impact of depositing particles due to its hardness, causes more porous coating layers.Layers deposited with a high flow of helium are denser as confirmed by the metallographic examination of the samples.Higher values of the microhardness of the WC -12wt.%Colayers deposited with a higher flow of helium indicate a higher cohesive strength of the layers, as confirmed by the results of tensile bond strength.The tensile bond strength values of WC -12wt.%Cocoatings directly depend on the flow of helium (Fig. 2).As seen in the figure, the helium flow rate affects the values of tensile bond strength.The coatings deposited with the highest helium flow rate of 32 l/min.which had the highest values of microhardness of 997 -1420HV 0.3 , had the highest tensile bond strength of 54MPa.The layers deposited with the smallest helium flow of 8 l/min.have the smallest minimum tensile bond strength of 37MPa.The tensile bond strength values of the coatings deposited with a helium flow rate of 16 l/min.and 32 l/min.were in accordance with the values set by the standard PWA (45MPa)(Turbojet Engine -Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA).During the testing of the samples, a fracture occurred along the coating / substrate interface in all samples.This suggests that the WC -12wt.%Cocoating has a good cohesive strength.
Fig. 4 shows the microstructure of WC -12wt.%Cocoatings deposited with the smallest helium flow rate of 8 l/min.The boundaries on the interface between the substrate and the coating layers are very clean, indicating a good substrate surface preparation.The bond of the coating with the substrate is uniform, without separation of coating layers from the substrate.The layers were deposited without the presence of microcracks and macrocracks.In the microstructure of the coatings, black micro pores of irregular shape were most pronounced, due to which the coating has the lowest value of microhardness of 598 -876HV 0.3 .The coating shows a structure with a limited inter-lamellar bonding, because of volume errors which, in operational conditions, can cause the appearance of micro-cracks and accelerate coating wear.Figs. 5 and 6 show the microstructures of the layers of WC-12wt.%Cocoatings deposited with a helium flow of 16 l/min.Because of the higher flow of helium, powder particles melt better and deform plastically during theimpact with the substrate, resulting in lower porosity in the coating layers.Fig. 6 clearly shows the black fields of pores.Unmelted powder particles are not present in the microstructure.In the deposited state, powder particles of WC-12wt.%Co are well melted and mutually interconnected to make continual and continuous coating layers (Fig. 6).a helium feed rate of 16 l/min Slika 6 -Mikrostruktura WC -12tež.%Coprevlake deponovane sa protokom helijuma 16 l/min Figs.7 and 8 show the layers of WC-12 wt.%Co coatings deposited with the highest helium flow of 32 l/min.These layers had the best microstructure and mechanical properties.The photomicrograph 7 shows the interface between WC-12wt.%Co layers and the substrate indicating a very good layer-substrate bond, which is consistent with the values of tensile bond strength.Through the coating layers fine micro pores are observed without the presence of coarse pores, which is consistent with the mechanical characteristics of the layers.Unmelted powder particles are not present in the microstructure.In the deposited state, the WC-12wt.%Co powder particles are well melted and interconnected to make continual and continuous coating layers.The WC-12 wt.% Co powder particles are evenly melted and properly deposited in the coating layers.Due to the decomposition of the primary WC carbide, which cannot be avoided in the process of deposition, the coating shows a multiphase microstructure.In the microstructure, there are angularly shaped grains of the undecomposed primary WC carbide WC (point 2).Decomposition of primary WC carbides results in grains obtaining smoother edges.The formed W 2 C carbide concentrates in the form of globules on the edges or the surface of the primary WC carbide grains (point3).Some primary WC carbide grains can be completely surrounded by W 2 C carbide (point 3).The primary WC carbide is dark gray, and the W 2 C carbide is light white (Li, et al., 1996, p.785), (Verdon, et al., 1998, p.11).Three crystalline phases (WC, W 2 C and W dissolved in Co binder base) are present in the microstructure.As a result of the decomposition reaction, some carbon C dissolves in the Co base, and some reacts with oxygen to form CO/CO 2 ; a share of carbon C from the initial powder is therefore lost.The retained C in the Co base with W enriches the Co base (He, Schoenung, 2002, pp.274-319), (Verdon, et al., 1998, pp.11-24).Depending on the decarburization, metal W can be deposited near the lamella boundary (Qiao, et al., 2003, pp.24-41).

Conclusion
WC-12wt.% Co coatings were deposited by APS -atmospheric plasma spraying with three helium flows of 8 l/min., 16 l/min.and 32 l/min.The microstructure and the mechanical properties of the deposited coatings were analysed and the following conclusions were made.
The mechanical properties and the microstructure of WC-12wt.%Cocoatings were directly dependent on the flow rate of helium plasma gas.The increased helium flow led to the deposition of the coatings with higher values of microhardness and tensile bond strength.The coatings deposited with the highest helium flow of 32 l/min.had the highest values of microhardness and tensile bond strength.In all coatings,fractures occurred at the interface between the coating and the substrate.The microhardness and tensile strength values of the bonds were consistent with their microstructures.
Black pores were present in the coating microstructures.The pores were least pronounced in the layers of the coating deposited with a helium flow of 32 l/min.In these layers, fine micro pores are present without the presence of coarse pores, which is consistent with the microstructure and mechanical properties of the layers.The most prominent and the roughest micro pores were in the layers deposited with a helium flow of 8 l/min.
In the microstructure there are the crystalline phases of WC, W 2 C and W dissolved together with C in the Co binder base.The dissolved W with C in the Co binder base enriches the Co base.The coatings deposited with a helium flow of 32 l/min.showed the best mechanical properties and microstructure.
WC-12wt.%Co coatings were made with powder Metco 72F -NS (Material Product Data Sheet, 2012, Metco 72F -NS Tungsten Carbide -12% Cobalt Sintered and Crushed Powders, DSMTS-0115.0,Sulzer Metco).The powder is produced by sintering WC mono carbide particles and metal Co particles and subsequent crushing of sintered particles to obtain specific granulation.The powder used in the experiment had a grain size range of 11 -45 um.The melting temperature of the powder is 1250°C.Fig. 1 shows a (SEM) scanning electron micrograph of the morphology of powder particles.Irregularly shaped grains of powder WC-12 wt.%Co can be seen.Slika 1 -(SEM) Skening elektronska mikrografija čestica praha WC -12 tež.%Co Figure 1 -(SEM) Scanning electron micrograph of WC -12 wt.%Co powder particles The substrates for depositing coatings for testing microhardness and assessing their structure are made of steel Č.4171 (X15Cr13 EN10027) in the thermally unprocessed state, with the dimensions 70x20x1.5mm(Turbojet Engine -Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA).The substrates for testing bond strengthare also made of steel Č.4171(X15Cr13EN10027) in the thermally unprocessed state, with the dimensions Ø25x50 mm (Turbojet Engine -Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA).