PROPERTIES AND STRUCTURE OF TUNGSTENCARBIDE – COBALT COATINGS DEPOSITED BY THE APS – PLASMA SPRAY PROCESS

The aim of this study was to optimize the parameters of the plasma spray and to deposit WC17Co layers with optimal structural mechanical characteristics. The powder was deposited by the plasma spraying process at the atmospheric pressure (APS). When choosing the parameters, the flow of the He plasma gas was taken as the basic parameter. In relation to other gases, helium does not react with the powder,it produces a denser plasma with a lower heat content and it incorporates less ambient air into the plasma jet which reduces decarburization of the powder. The study shows three groups of samples obtained with three plasma gas flows of 12, 22 and 32 l/min He. The coating with the best properties was deposited on the shaft sleeve of the main rotor of the Gazelle H42 helicopter, in order to reduce the influence of vibrations and bearings on sleeve wear up to 500°C. The estimates of the WC17Co layers of the coating were made on the basis of their structural mechanical properties. The surface morphology of the WC17Co powder particles was examined on the SEM. The mechanical properties of the deposited coatings were tested in accordance with the ‘TURBOMECA’standard. The estimate of the mechanical properties of layers was done by examining microhardness with the method HV0.3 and bond strength with tensile testing. Metallographic assessment of the pore proportion in the layers of the WC17Co coating (image analysis) was performed with the technique of light microscopy in accordance with the ‘Pratt & Whitney’ standard. Studies have shown that the rate of the plasma gas flow significantly affects the mechanical properties and the structure of coatings.


PROPERTIES AND STRUCTURE OF TUNGSTENCARBIDE -COBALT COATINGS DEPOSITED BY THE APS -PLASMA SPRAY PROCESS
The aim of this study was to optimize the parameters of the plasma spray and to deposit WC17Co layers with optimal structural -mechanical characteristics.The powder was deposited by the plasma spraying process at the atmospheric pressure (APS).When choosing the parameters, the flow of the He plasma gas was taken as the basic parameter.In relation to other gases, helium does not react with the powder,it produces a denser plasma with a lower heat content and it incorporates less ambient air into the plasma jet which reduces decarburization of the powder.The study shows three groups of samples obtained with three plasma gas flows of 12, 22 and 32 l/min He.The coating with the best properties was deposited on the shaft sleeve of the main rotor of the Gazelle H42 helicopter, in order to reduce the influence of vibrations and bearings on sleeve wear up to 500°C.The estimates of the WC17Co layers of the coating were made on the basis of their structural -mechanical properties.The surface morphology of the WC17Co powder particles was examined on the SEM.The mechanical properties of the deposited coatings were tested in accordance with the 'TURBOMECA'standard.The estimate of the mechanical properties of layers was done by examining microhardness with the method HV 0.3 and bond strength with tensile testing.Metallographic assessment of the pore proportion in the layers of the WC17Co coating (image analysis) was performed with the technique of light microscopy in accordance with the 'Pratt & Whitney' standard.Studies have shown that the rate of the plasma gas flow significantly affects the mechanical properties and the structure of coatings.

Introduction
he plasma spray process is applied in many industries with the aim to improve the characteristics of components or to extend their working life.This process has found a wide application in the aviation industry and it is one of the most applicable processes for thermal coatings.The process configuration can vary so that particle jets can have a wide range of temperature and velocity values.The resource of coatings can be influenced by the characteristics of the deposited material, its phase composition, proportion of pores and unmelted particles as well as by cohesion and adhesion strength.These properties are closely related to the conditions of the deposition and a significant improvement can be expected when optimizing the deposition parameters (Vencl, et al., 2006, pp.151-157), (Dorfman, 2002, pp.47-50), (Brossard, et al., 2010(Brossard, et al., , pp.1599(Brossard, et al., -1607)).WC17Co coatings deposited with thermal spray processes always show change in the phase composition in relation to the composition of the starting powder.This is something that cannot be avoided.Changes of the phase composition in deposited carbide coatings were described by Saha, G.C., et al. (Saha, et al., 2010, pp. 592-595) who explained the cause of phase changes.The examinations of the phase of the nanocrystalline WC17Co coatings deposited by the HVOF thermal process and polycrystalline coatings deposited by the process of the plasma spraying have shown that, in both coatings, the starting phase WC decomposes, more or less, into the phase of W 2 C and W.This decomposition is more pronounced in plasma due to higher temperatures.In the structure of the polycrystalline coating deposited by plasma, besides the phase WC, the phases W 2 C, W, W 3 C and mixed carbide ή -Co 3 W 3 C (Saha, et al., 2010, pp.592-595) are always present.The type of the applied plasma gas, flow and the power of supply are important parameters affecting the quality of the deposited carbide coatings.In the case of the application of H 2 as a plasma gas, the WC-Co powder is more exposed to decomposition because of the combination of decarburization, oxidation and the reactions between WC and cobalt, which leads to the formation of hard and brittle phases such as W 2 C, CoxWyCz, and even WO 3 and W (de Villiers Lovelock, 1998, pp.357-373).The decarburization of one powder particle takes place as follows.As temperature increases, cobalt melts and WC also passes to the liquid state.Carbon is removed from the melted particle by some reaction with oxygen on the liquid/gas boundary or through the diffusion of oxygen into the melted particle, which leads to the formation of carbon monoxide (Stewart, et al., 2000(Stewart, et al., , pp.1593(Stewart, et al., -1604)).Due to the incorporation of air into the plasma jet and T this influence on the carbon reduction from carbide, the powder deposition is done with an average power of supply of the plasma gun and with Helium as the plasma gas.Helium, as a noble gas, has great advantages as a plasma gas in comparison to other gases.Experimental investigations of plasma jet characteristics have shown that the isotherms near the anode exit have a smaller diameter for the Ar-He plasma than for the Ar-H 2 .The isotherms are also shorter in the Ar-He plasma because of less specific enthalpy and because of higher viscosity when compared to Ar-H 2 .Denser plasma, such as Ar-He, can substantially reduce the incorporation of ambient air into the plasma jet.Mixing of the plasma jet with the ambient air increases with the increasing intensity of the electric arc and with the plasma gas flow (Roumilhac, et al., 1988, pp.105-119), (Roumilhac, Fauchais, 1988, pp.121-126).Helium, because of these characteristics, allows deposition of carbide coatings with a reduced process of decarburization and with a lower content of pores.The properties of coatings are directly related to the deposition parameters.The WC17Co powder was developed for the aviation industry.The WC17Co-based coatings are resistant to: wear, abrasion, erosion, corrosion and cavitation up to 500˚C (Material Product Data Sheet, 2011).Cubic monokarbid WC contains 6.13%C and has the microhardness of about 2700HV 0.3 , while W 2 C contains 3.16%C and has the microhardness of about 3000HV 0.3 while being more brittle than WC.The W 2 C brittle phase degrades the coating and reduces the coating resistance to abrasive wear (Valiev, Nature, 2002, pp.887-889).The highest wear resistance of carbide phases can be retained in coatings through the optimization of the spray process parameters (Li, et al., 1996, pp.785-794).Despite the fact that W 2 C is metastable below 1250°C, it is often present in WC-Co, even after slow cooling.The metastable WC 1-x phase, however, can be found only at room temperature when the material is quickly quenched (de Villiers Lovelock, 1998, pp.357-373), (Mrdak, et al., 2004, pp.407-421).The decomposition of WC carbide is incomplete because of a short retention of powder particles in the plasma jet.Depending on the heat quantity exchanged between the plasma jet and the powder, decomposition of carbides occurs to a higher or lower extent.Therefore, the coating has the microhardness in a range of 800-1300 HV 0.3 , since the composition of the coating deviates from the initial composition of the powder In the coating, besides carbide WC, W 2 C, there is a complex carbide W 3 Co 3 C whose microhardness is in a range of 600-1300 HV 0.3 .Depending on the deposition parameters, W with the microhardness of about 400 HV 0.3 and Co with the hardness below 200 HV 0.3 may appear in the coating.In order to understand the metallurgical processes that take place when carbide WC is deposited, it is important to know the W-C binary system, shown in Fig. 1 (ASM Handbook, 1992).Tungsten and carbon build W 2 S and WC hexagonal carbides as well as β-WC cubic carbide.Tungsten carbide, with the WC hexagonal structure, has the melting temperature of 2785 ± 10°C and, in the cooling process, builds eutectic with tungsten at 2715 ± 5°C.Stability of the carbide decreases with the decreasing temperature and at 1250°C it decomposes to pure tungsten and WC hexagonal monocarbide.This carbide has a very narrow area of stability and is not used for deposition.Consequently, WC cubic monocarbide which has a wide area of stability (Mrdak, et al., 2004, pp.407-421), (Mrdak, 2010, pp.43-52), (Mrdak, et al., 2003, pp.125-128).is used for deposition.This paper shows the results of experimental research on the influence of the plasma gas flow (He) on the mechanical properties and on the microstructure of WC17Co layers.The main aim was to replace the old technology of shaft sleeve cementation with depositing WC17Co coatings using the plasma spray technology.The mechanical properties and the microstructure of coatings were analyzed in order to select a coating with the best properties.Coatings with the best mechanical and structural characteristics were tested and homologated on the shaft sleeve of the main rotor of the Gazelle H42 helicopter, in flight tests in the period of 50 hours in the VZ "Moma Stanojlović" -Batajnica.

Testing Materials and Samples
The Metco 73F-NS-1 powder by the 'Sulzer Metco'company was used for the production of coatings.For powder, the technique of dry dispersion/sintering with a content of 83 wt.% WC and 17 wt.%Co was used.The powder used in the experiment had a granulation span from 11 μm to 53 μm (Material Product Data Sheet, 2011).Fig. 2 shows the scanning electron micrography (SEM) of the morphology of WC17Co powder particles.The powder particles are porous and spherical.
The bases on which coatings for assessing the structure and for testing microhardness were deposited were made of Č.4171 (X15Cr13 EN10027) steel in the thermally unprocessed state with the dimensions 70x20x1.5mm(Turbojet engine-standard practices manual TURBOMECA).Also, the bases for testing bond strength were made of Č.4171 (X15Cr13EN10027) steel in the thermally unprocessed state with the dimensions Ø25x50mm (Turbojet engine-standard practices manual TURBOMECA).Examination of microhardness, bond strength and microstructure Mechanical characterizations of WC17Co coatings were made in accordance with the TURBOMECA standard (Turbojet engine-standard practices manual TURBOMECA).For microhardness testing and metallographic tests, 70 × 20 × 1.5mm samples were used, while for tensile strength tests, Ø25 x 50mm samples were used.
The measurements of microhardness were done by using the Vickers diamond pyramid indenter and 300 grams of load (HV 0.3 ).The mea-surements waere done along the lamellar structure, in the middle and at the ends of the samples.Five readings were performed at three places and the results were averaged.
The tensile strength tests were done at room temperature on the hydraulic equipment with a speed of 10 mm/min.Two samples were used in pairs, but the coating was deposited only on one of them.For each group of WC17Co coatings five samples were tested, and the results were averaged.
The metallographic evaluation of the pore proportion (image analysis) in the WC17Co coating layers was made using the light microscopy technique in accordance with the 'Pratt & Whitney' standard (Turbojet Engine, 2002).The morphology of the powder particles was done under the SEM (scanning electron microscope).

Powder deposition
The process of depositing layers on the metal matrix was done by the plasma spraying process at atmospheric pressure (APS).The coatings were deposited on the steel bases roughened with white corundum Al 2 O 3 with the granulation sizes of 0.7-1.5 mm.The 'Plasmadyne' atmospheric plasma spray (APS) system was used for making coatings.The powder was deposited with the plasma gun 'SG -100' which consisted of a cathode type K 1083A-129, an anode type A 2083-175 and a gas injector type GI 1083-113.Argon was used as a gas in combination with Helium and the power of supply was 40 KW.The plasma gas flow of Helium was the main parameter for the powder deposition.Three different flows of Helium were used (12 l/min, 22 l/min and 32 l/min).The optimum flow of Helium enables minimum decarburazition, oxidation and a reaction between WC and cobalt, which leads to the formation of hard and brittle phases.The detailed values of plasma spray parameters are shown in Table 1.The layers are deposited on the substrates of total thickness of 0.020 to 0.025mm, with the plasma gun speed of 500 mm/s.The WC17Co coating with the best structural and mechanical properties was deposited on the shaft sleeve of the main rotor of the Gazelle H42 Helicopter.

Results and discussion
The results of testing the microhardness and bond strength of WC17Co coating layers, depending of the plasma Helium gas flow, are shown in Figs. 3 and 4.   The values of the microhardness and bond strength of the deposited layers are directly related to the flow of Helium as a plasma gas.All deposited layers have the values of microhardness within the prescribed limits of 850 -1300 HV 0.3 (Material Product Data Sheet, 2011), (Turbojet engine-standard practices manual TURBOMECA).
The most uniformly distributed microhardness was found in the layers deposited with the plasma He gas flow of 22 l/min.These layers had the smallest difference between the maximum and the minimum of the microhardness values (289HV 0.3 ).The biggest distribution of microhardness was found in the layers deposited with the highest plasma He gas flow of 32 l/min.These layers had the biggest difference between the maximum and the minimum of the microhardness values (404 HV 0.3 ).A higher distribution of microhardness was also found in the layers with the smallest plasma He gas flow of 12 l/min.The difference between the maximum and the minimum of the microhardness values for these layers was (373 HV 0.3 ).The layers of WC17Co coating deposited with the optimal plasma He gas flow of 22 l/min showed the best microstructure with the smallest proportion of lamellar pores.This was confirmed by the image analysis of the microstructure of coating layers under the light microscope.
The tensile strength of bonding is directly related to the plasma He gas flows.The values of bond tensile strength in the deposited layers, with the plasma He gas flows of 12 l/min and 22 l/min, are in the prescribed limits per standard (min.45 MPa) (Turbojet engine-standard practices manual TURBOMECA).
These layers had good adhesion strength with the substrate and good lamellar cohesive strength.The highest value of bond strength of 49MPa was found in the layers deposited with the plasma He gas flow of 22 l/min, which had the lowest distribution of microhardness.These layers had the smallest proportion of pores.The lowest value of bond tensile strength of 42 MPa was found in the layers deposited with the highest plasma gas flow of He.These layers had the greatest distribution of microhardness and the greatest propotion of micro pores.The testing of bond tensile strength showed that, for all deposited coatings, the failure mechanism was on the interface between the substrate and the coating.This indicates good melting of powder particles and their bonding to the substrate for all three plasma gas flow of He.These values were confirmed by the image analysis of the coating microstructure under the light microscope.
Figs. 5, 6 and 7 show the microstructure of WC17Co coating layers deposited with the plasma gas flows He of 12 l/min, 22 l/min and 32 l/min.The figures under (a) show the substrate/ coating interface and (b) shows the middle of the layer.The quantitative analysis of the total content of pores in WC17Co layers shows that the measured values are directly related to the plasma gas flows of He.In the shown micrographs, different proportions of pores in the deposited layers are clearly recognizable.The smallest proportion of pores was in the layers of WC17Co coating deposited with the plasma gas flow He of 22 l/min.In these layers, the total proportion of pores was 1%.In the layers deposited with the plasma gas flow He of 12 l/min, the total proportion of pores was 1.3%.The greatest proportion of pores was in the layers deposited with the plasma gas flow He of 32 l/min.In these layers, the pores are more prominent and coarser with the proportion of pores of 2.5%.In all layers, the total proportion of pores was within the prescribed limits of 0.5 -3% (Material Product Data Sheet, 2011), (Turbojet engine-standard practices manual TURBOMECA).The measured values of the total content of pores in the WC17Co layers are consistent with the measured microhardness values.
The qualitative analysis of all deposited WC17Co layers showed that there is a negligible proportion of residual particles of corundum Al 2 O 3 left from roughening at the substrate/coating interface.The bond of coatings with the substrates is niform without the separation of coating layers from the substrate.Along the interface, between the substrate and the coating, there are no micro cracks and macro cracks.Fig. 6 shows the micrographs of WC17Co coatings whose layers had the best mechanical properties and the smallest proportion of micro pores.These layers are deposited with the plasma gas flow He of 22 l/min.The microstructure of all coatings is lamellar.The micrographs show that the melted powder particles are uniformly distributed.The coating layers were deposited continuously without present micro cracks and macro cracks through the layers.In the layers, there are no unmelted powder particles, precipitates, and inter-lamellar pores.In the deposited layers of all coatings,uniformly distributed carbide phases are clearly seen in the tough cobalt base.The light fields of metal phases and the dark-gray fields of carbide phases are clearly seen in all micrographs.Light fields contain the initial metallic phase Co and the metallic phase W which is derived from a partial degradation of the initial cubic monocarbide WC (Saha, et al., 2010, pp.592-595), (de Villiers Lovelock, 1998, pp.357-373).In gray fields, there is the initial phase of cubic monocarbide WC and the carbide phases resulting from the decomposition of cubic carbide WC into the carbides of type W 2 C and W 3 C, and the mixed carbide ή -Co 3 W 3 C (Saha, et al., 2010, pp.592-595), (de Villiers Lovelock, 1998, pp.357-373).

Conclusion
The atmospheric plasma spray (APS) process was used for depositing WC17Co coatings with the plasma gas flow He of 12 l/min, 22 l/min and 32 l/min.The mechanical properties of deposited layers were analysed as well as the microstructure under the light microscope.The morphology of powder particles was investigated under the scanning electron microscope (SEM).The performed analyses led to the following conclusions.
The morphology of WC17Co powder particles is of a spherical shape and is typical for powder particles produced with the technique of dry dispersion/sintering.The values of microhardness and bond strength of the deposited layers were directly related to the flows of Helium as a plasma gas.All deposited layers had the values of microhardness in the prescribed limits of 850 -1300 HV 0.3 .The most uniform distribution of microhardness was found in the layers deposited with the plasma gas flow He of 22 l/min.These layers had the smallest difference between the maximum and the minimum microhardness values (289 HV 0.3 ).
The values of bond tensile strength of the layers deposited with the plasma gas flows He of 12 l/min and 22 l/min were within the prescribed limits in accordance with the standard (min.45MPa).These layers had good adhesion of strength with the substrate and good lamellar cohesive strength.The highest value of the bond strength of 49MPa was found in the layers deposited with the plasma gas flow of He, which had the lowest distribution of microhardness.These layers had the smallest proportion of pores.The lowest value of bond tensile strength of 42 MPa was seen in the layers deposited with highest plasma gas flow of He.These layers had the greatest distribution of microhardness and the greatest proportion of micro pores.
The quantitative analysis of the total content of pores in the WC17Co layers showed that the measured values are directly related to the plasma gas flows of He.The smallest proportion of pores was seen in the WC17Co coating layers deposited with the plasma gas flow He of 22 l/min.In these layers, the total proportion of pores was 1%.In the layers deposited with the plasma gas flow He of 12 l/min, the total proportion of pores was 1.3%.The greatest proportion of pores was in the layers deposited with the plasma gas flow He of 32 l/min.In these layers, the pores are more prominent and coarser with the proportion of pores of 2.5%.In all layers, the total proportion of pores was within the prescribed limits of 0.5 -3% .
The microstructure of all coatings is lamellar.The coating layers were deposited continuously without present micro cracks and macro cracks through the layers.In the layers there are no unmelted powder particles, precipitates, and inter-lamellar pores.In all deposited coating layers, the uniformly distributed carbide phases are clearly seen in the tough cobalt.baseThe light fields contain the initial metallic phase Co and the metallic phase W which is derived from a partial degradation of the initial cubic monocarbide WC.In gray fields, there is the initial phase of cubic monocarbide WC and the carbide phases resulting from the decomposition of cubic carbide WC into the carbides of type W 2 C and W 3 C, and the mixed carbide ή -Co 3 W 3 C .
The obtained results showed that the plasma gas flow of He significantly affects the mechanical properties of layers and the proportion of pores in the coatings.The tests confirmed that the best layers are those deposited with the plasma gas flow He of 22 l/min.The oating with the best mechanical and structural characteristics was tested and homologated on the shaft sleeve of the main rotor of the Gazelle H42 helicopters during flight tests in the period of 50 hours in the VZ "Moma Stanojlović" -Batajnica.
Mihailo R. Mrdak, Research and Development Center IMTEL Communications a. d., Belgrade Summary: