INFLUENCE OF A POWDER FEED RATE ON THE PROPERTIES OF THE PLASMA SPRAYED CHROMIUM CARBIDE-25 % NICKEL CHROMIUM COATING

The plasma spray process is a leading technology of powder depositing in the production of coatings widely used in the aerospace industry for the protection of new parts and for the repair of worn ones. Cermet 75Cr3C2 25Ni(Cr) coatings based on Cr3C2 carbides are widely used to protect parts as they retain high values of hardness, strength and resistance to wear up to a temperature of 850°C. This paper discusses the influence of the parameters of the plasma spray deposition of 75Cr3C2 25Ni(Cr) powder on the structure and mechanical properties of the coating. The powder is deposited using plasma spraying at atmospheric pressure (APS). The plasma gas is He, which is an inert gas and does not react with the powder; it produces dense plasma with lower heat content and less incorporated ambient air in the plasma jet thus reducing temperature decomposition and decarburization of Cr3C2 carbide. In this study, three groups of coatings were deposited with three different powder feed rates of: 30, 45 and 60 g/min. The coating with the best properties was deposited on the inlet flange parts of the turbo jet engine TV2-117A to reduce the influence of vibrations and wear. The structures and the mechanical properties of 75Cr3C2 25Ni(Cr) coatings are analyzed in accordance with the Pratt & Whitney standard. Studies have shown that powder feed rates have an important influence on the mechanical properties and structures of 75Cr3C2 25Ni(Cr) coatings. *

The plasma spray process is a leading technology of powder depositing in the production of coatings widely used in the aerospace industry for the protection of new parts and for the repair of worn ones.Cermet 75Cr 3 C 2 -25Ni(Cr) coatings based on Cr 3 C 2 carbides are widely used to protect parts as they retain high values of hardness, strength and resistance to wear up to a temperature of 850°C.This paper discusses the influence of the parameters of the plasma spray deposition of 75Cr 3 C 2 -25Ni(Cr) powder on the structure and mechanical properties of the coating.The powder is deposited using plasma spraying at atmospheric pressure (APS).The plasma gas is He, which is an inert gas and does not react with the powder; it produces dense plasma with lower heat content and less incorporated ambient air in the plasma jet thus reducing temperature decomposition and decarburization of Cr 3 C 2 carbide.In this study, three groups of coatings were deposited with three different powder feed rates of: 30, 45 and 60 g/min.The coating with the best properties was deposited on the inlet flange parts of the turbo -jet engine TV2-117A to reduce the influence of vibrations and wear.The structures and the mechanical properties of 75Cr 3 C 2 -25Ni(Cr) coatings are analyzed in accordance with the Pratt & Whitney standard.Studies have shown that powder feed rates have an important influence on the mechanical properties and structures of 75Cr 3 C 2 -25Ni(Cr) coatings.
Introduction hermal spray coatings belong to a developing field of surface engineering.These high-quality functional coatings are applied to new parts in basic industry and for the renovation of parts, mainly because of their excellent characteristics, characterized by high resistance to wear, erosion, abrasion, corrosion resistance and resistance to high temperatures (Berget, et al., 2007, pp.7619-7625), (Jankura, Bačová, 2009, pp.359-366), (Mann, Arya, 2003, pp.652-667), (Monticelli, et al., 2004(Monticelli, et al., , pp.1225(Monticelli, et al., -1237)), (Wheeler, Wood, 2005, pp.526-536).Coatings must provide effective protection against wear and oxidation as well as have high thermal conductivity in order to secure proper and efficient functioning of parts in service (Bala, et al., 2007, pp.201-218).The primary aim of coatings is to be stable in operation and provide good protection (Fernandez, et al., 2005, pp.1-7).Cermet coatings are a combination of hard ceramic phases embedded in tough metal matrices.Typical coating systems are WC-Co, NiCr-Cr 3 C 2 and Fe-CrAlY-Cr 3 C 2 .In the thermal spray technology, Cr 3 C 2 -NiCr cermet coatings have been used extensively to mitigate erosion and abrasive wear at high temperatures up to 850°C.(Matthews, et al., 2007, pp.59-64), (Tillmann, et al., 2010, pp.392-408).Cr 3 C 2 -NiCr coatings, when compared to other cermet coatings, offer better resistance to corrosion and oxidation; they also have a high melting point and high hardness, strength and wear resistance up to 850°C.In addition to these functions, the coefficient of thermal expansion of the Cr 3 C 2 carbide (10.3 × 10 -6 °C-1 ) is almost the same as the coefficient of the thermal expansion of iron (11.4 × 10 -6 °C-1 ) and nickel (12.8 × 10 -6 °C-1 ), which are the basis of most high-temperature alloys.This reduces thermal expansion stresses during thermal cycles (Kamal, et al., 2008, pp.358-372).Thermally sprayed cermet coatings are considered to be an important option for the replacement of electro deposited chromium on many components in industries (Guilemany, et al., 2002, pp.107-113).The application of cermet coatings results in a better service life of machinery components.Coatings based on chromium carbide are often used in gas turbines, vapour turbines, and aviation engines to improve slip resistance as well as abrasive and erosive wear (Hillery, 1986(Hillery, , pp.2684(Hillery, -2688)).Parts on which this coating is applied are: hydraulic cylinders and piston rods, valve stems, turbine components, ship engine valve spindles, pump housings and others.(Material Product Data Sheet, 2012, Woka 7203 Chromium Carbide -25% Nickel Chromium Powders, DSMTS-0031.1, Sulzer Metco).
The 75Cr 3 C 2 -25Ni(Cr) powder contains 75% of hard chromium carbide resistant to abrasion and 25% of nickel-chromium alloy (80%/20%) as a carbide binder resistant to corrosion and oxidation.The powder grain size is from 11 to 45 μm.(Material Product Data Sheet, 2012, Woka 7203 Chromium Carbide -25% Nickel Chromium Powders, DSMTS-T 0031.1,Sulzer Metco).The high energy level of the plasma causes the decomposition of the initial Cr 3 C 2 carbide, so that other types of carbides are present in the coating.The Cr-C system is formed by three types of crystal structures such as Cr 23 C 6 , Cr 7 C 3 and Cr 3 C 2 (Kajihara, Hillert, 1990, pp.2777-2787).At 1534 ± 10°C, the first eutectic reaction L = (Cr) + Cr 23 C 6 occurs and the solubility of C in the Cr-C solid solution is increased to 0.07 wt% C. The first peritectic reaction L + Cr 7 C 3 = Cr 23 C 6 occurs at 1576 ± 10°C.Moreover, at 1727 ± 7ºC, there is another eutectic reaction L = Cr 7 C 3 + Cr 3 C 2 and the melting temperature of the Cr 7 C 3 carbide is 1756 ± 10°C.Another peritectic reaction occurs at 1811 ± 10°C when Cr 3 C 2 carbide is formed by the reaction of L + C = Cr 3 C 2 (ASM HAND-BOOK VOLUME 3.Alloy Phase Diagrams, ASM International, Printed in the United States of America).In a series of carbides (Cr 23 C 6 , Cr 7 C 3 and Cr 3 C 2 ), Cr 3 C 2 carbide has the best mechanical properties and a coating with a higher content of this carbide is more resistant to wear.The microstructure of the coating is important as well as the chemical composition of the coating material.In cermet coatings with different thermal spray processes considerable variations are observed in the composition and the microstructure due to the exposure of powder to high temperatures and to different gas rates in the process (Matthews, et al., 2007, pp.59-64).Besides carbide and the metal phase, thermally sprayed coatings consist of oxide and pores located at the lamella boundaries originating from the spraying process conditions (Kamal, et al., 2009(Kamal, et al., , pp.1004(Kamal, et al., -1013)).Therefore, coatings should be carefully sprayed and the spraying parameters should be carefully chosen prior to deposition.For example, 75Cr 3 C 2 -25NiCr coatings deposited by the HVOF process are dense and in their microstructure there is less porosity due to high rates and relatively low temperature.The microhardness of 75Cr 3 C 2 -25NiCr sprayed coatings produced by different systems were tested in previous studies.Virojanupatump published the influence of the 75Cr 3 C 2 -25Ni(Cr) powder processing technology on the characteristics of coatings deposited by the HVOF spray system (Wirojanupatump, et al., 2001, pp.829-837).The coating of sintered and crushed powder shows the highest value of microhardness -910HV 0 .3 , the microhardness of the coating deposited from a mixture of powders is 650HV 0 .3 , and the value of the microhardness of the coating deposited by composite powder is 820HV 0 .3 .He and Manish found that in the coating sprayed by agglomerated 75Cr 3 C 2 -25Ni(Cr) powder there was present only the Cr 3 C 2 carbide phase (He, et al., 2000, pp.555-564), (Manish, et al., 2006, pp.29-38).In the microstructure of the coatings deposited by agglomerated and sintered 75Cr 3 C 2 -25Ni(Cr) powder, Matthevs also found significant amounts of the Cr 3 C 2 phase (Matthews, et al., 2009(Matthews, et al., , pp.1086(Matthews, et al., -1093)), (Matthews, et al., 2009(Matthews, et al., , pp.1094(Matthews, et al., -1100)).The concentration of Cr 7 C 3 carbide was too low.Similar work was done by Suegama, who used the XRD analysis for the agglomerated 75Cr 3 C 2 -25Ni(Cr) powder and for the coating sprayed with the HVOF system and found that the powder consists of Cr 3 C 2 carbide, Cr 7 C 3 and a basis based on Ni (Suegama, et al., 2006, pp.434-445).In the 75Cr 3 C 2 -25Ni(Cr) coatings, deposited by plasma spraying, there are present the particles of carbide types Cr 3 C 2 , Cr 7 C 3 and Cr 23 C 6 .The phenomenon of a significant degradation of Cr 3 C 2 carbide particles has been published in the literature (Ji, et al., 2006, p.6749) (Picas, et al., 2003(Picas, et al., , pp.1095)) (Verdon et al., 1998, pp.11-14).Non uniform values of hardness along coatings have been reported by many authors.This variation in hardness values is attributed to microstructural changes along the coating cross-section.These changes may be microstructural because of the presence of porosity and oxides ofy Ni(Cr) alloy, such as: NiO, NiCr 2 O 4 , Cr 2 O 3 and CrO 3 , unmelted and semimelted particles in the coating structure (Brossard, et al., 2010(Brossard, et al., , pp.1608(Brossard, et al., -1615)), (Matthews, et al., 2007, pp.59-64), (Mrdak, 2011, pp.9-14).Thermal spraycoatings show a typical lamellar structure with carbides in the structure and with clear boundaries of lamellas, due to precipitation and rehardening of melted powder droplets.Lamellas are oriented parallelly to the substrate surface.Also, in the microstructure of coatings, unmelted particles can be found as well assemi-melted powder particles and the presence of fine particles -precipitates formed after the breaking of some powder particles during collision with the substrate.During spraying,oxides can be formed, because of oxidation during the flight of melted drops to the substrate on which they are deposited (Mrdak, 2010, pp.5-16), (Mrdak, 2011, pp.9-14), (Mrdak, 2012, pp.182-201), (Mrdak, 2013, pp.68-88).In the microstructure of 75Cr 3 C 2 -25Ni (Cr) coatings, there are three different zones.The dark zone indicates the presence of primary Cr and C, revealing the Cr 3 C 2 phase.The second zone is gray, which indicates the presence of Cr 7 C 3 and Cr 23 C 6 carbides.In addition to Cr and C, this area contains Ni.The third zone is white and consists primarily of the NiCr phase (Sukhpal, et al., 2012, pp.569-586).
This paper presents the results of the experimental investigation of the impact of the powder feed rate g / min on the mechanical properties and the microstructure of cermet 75Cr 3 C 2 -25Ni (Cr) coatings .The main goal was to apply the cermet 75Cr 3 C 2 -25Ni(Cr) coating deposited by the APS -atmospheric plasma spraying process on the inlet flange of the turbo-jet engine TV2 117A.Three groups of samples are made with the values of the powder feed rate of: 30, 45 and 60 g/min.The microstructure and the mechanical properties of the coatings were analyzed in order to select a coating with the best properties.The coating with the best mechanical and structural properties was tested and homologated on the part of inlet flange of the turbo -jet engine TV2-117A at the testing station for a period of 45 hours in VZ "Moma Stanojlovic" -Batajnica.

Materials and experimental details
For the production of coatings the powder of the Sulzer Metco company marked Woka 7203 was used ( Material Product Data Sheet, 2012, Woka 7203 Chromium Carbide -25% Nickel Chromium Powders, DSMTS-0031.1, Sulzer Metco).The Woka powder contains 75% of hard Cr 3 C 2 chromium carbides and 25% of a nickel-chromium alloy (80% / 20%).The 75Cr 3 C 2 -25Ni(Cr) powder particles were spheroidized by agglomeration and sintering with a range of the powder particle grain size from 11 to 45 um.Due to the content of 25Ni(Cr) alloy, the powder deposits well and bonds to the base based on Fe and Ni.Fig. 1 shows a scanning electron micrograph (SEM) of the morphology of powder particles.Spherical grains of powder Cr 3 C 2 (brown)and the particles of the 25Ni(Cr) alloy (white) can be seen.The substrate material on which the coatings for testing microhardness are deposited and which is used for the evaluation of the microstructure in the deposited state is made of steel Č.4171 (X15Cr13 EN10027), thermally unprocessed, with the dimensions of 70x20x1.The examination of the microhardness of coating layers was done by the method HV 0.3 .The measurement was done in the direction along the lamellae, in the middle and at the ends of the sample.There were five readings in three placesand the minimum and maximum values are presented in the paper.. Tests for tensile bond strength were done at room temperature on hydraulic equipment with a speed of 10 mm / min for all tests.For each group of samples there were three specimens and the average values are given in the paper.. Mechanical and microstructural characterizations of the coatings were done according to the Pratt & Whitney standard (Turbojet Engine -Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA).
The microstructural analysis of the coatings was done on a light microscope.The morphology of the powder particles was done on the SEM (Scanning Electron Microscope).
For powder deposition,the atmospheric plasma gun SG -100 of the plasma spray system (APS) Plasmadyne was used.The plasma spray gun SG -100 consisted of a cathode type K 1083 -129 A , an anode type A 2084 -145 and a gas injector type GI 2083 -130.As a gas, argon was used in a combination with helium and with a power supply of 40 kW.Before depositing, the substrates were not preheated andthe substrate surfaces awee roughened with white electro-corundum with a granulation from 0.7 to 1.5 mm.The aim of increasing the roughness of the substrate surface is removing the thin oxide layer in order to make the surface more reactive using molten powder and in order to get a better bond between the coating and the substrate.In selecting the powder deposition parameters, the powder feed rate (g/min.)was taken as the main parameter.The powder feed rate is one of important parameters that influence the stress state of the coating which is directly related to the cohesion strength, microhardness and adhesion of the coating.The share of unmelted particles, oxides and pores in the coating can be significantly controlled by controlling the powder feed rate.The powder feed rate must be optimal toenable complete melting of the powder par-ticles and to reduce the minimum percentage of unmelted particles, oxides and pores in the coating layer.In this study there were three groups of samples.In the first group of samples, the powder feed rate was 30 g/min.With a carrier gas flow rate of 5 l/min, in the second group of samples, this rate was 45 g/min.with a carrier gas flow rate of 6 l/min, and in the third group of samples, the rate was 60 g/min.with a carrier gas flow rate of 7 l/min.Other parameters of the powder deposition had the following values: plasma current of 700 A, arc voltage -36 V, the primary gas (Ar) -47 l/min, the flow of secondary gas (He) -12 l/min, and plasma arc distance -100 mm from the substrates.The coatings were formed with a thickness of 0.2 mm.The detailed values of the plasma spray deposition parameters are shown in Table 1.

Results and discussion
The values of the microhardness and bond strength of the deposited 75Cr 3 C 2 -25Ni(Cr), coatings depending on the powder feed rate, are shown in Figs. 2 and 3.The values of the microhardness of thecoating layers are directly related to the powder feed rate.The powder feed rate significantly affects the values of the microhardness of the deposited layers, which must be optimal in order to ensure complete melting of powder particles and reduction of unmelted particles, pores and oxides in the coating layers to a minimum.Non-uniform values of microhardness attributed to microstructural changes along the coating cross-section are measured in the coating layers.Microstructural changes were caused by the presence of porosity, unmelted particles, Ni(Cr) alloy oxide and decomposed carbides in the structure of the coatings, which was confirmed by metallographic examinations of coating layers.The 75Cr 3 C 2 -25Ni(Cr) coating layers deposited with the lowest powder feed rate of 30g/min have a microhardness value of 515-798HV 0.3 .With a lower powder feed rate than the optimum one, primary Cr 3 C 2 carbide powder has enough time to degrade in plasma with pores which reduce the microhardness of the coating.The layers deposited with a powder feed rate of 45g/min have the microhardness values of 670-845HV 0.3 , which is consistent with the values (450-850 HV 0.3 ) prescribed by the Pratt & Whitney standard (Turbojet Engine -Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA).These layers showed the densest and the best microstructure, which was also confirmed by metallographic examinations of the coatings.The coating layers deposited with the highest powder feed rate of 60g/min had the lowest values of microhardness of 438-695HV 0.3 .With a powder feed rate higher than the optimum rate, all powder particles do not have enough time to be completely melted, which leads to the increase of the proportion of unmelted particles and coarse pores in the coating layers.Unmelted particles together with pores decrease the coating microhardness.
Tensile bond strength is, as well as microhardness, directly related to the powder feed rate, presence of pores, unmelted particles and inter lamellar oxides.The measurements of the tensile bond strengthshowed that the powder feed ratesof 30 and 45g/mingive values of more than 35 MPa, which is prescribed by the Pratt & Whitney standard (Turbojet Engine -Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA).The highest value of tensile strength of 47MPa was found in the layers deposited with the powder feed rate of 45 g / min.These layers in the microstructure did not have coarse pores, unmelted particles or semi-molten particles.Since the presence of pores, unmelted particles and oxides is directly related to the values of the bond strength of coatings, the measured values of the coating deposited with the powder feed rate of 45g/min.indicate that their share is the lowest in this coating.These values were also confirmed by the analysis of the microstructure of the coatings under a light microscope.Along the substrate/coating interface, micro-cracks and macrocracks are not present.The coating/substrate bond is uniform, without the separation od coating layers from the substrate.The coating structure is lamellar (Fig. 5).The coating layers were deposited continuously without the presence of micro-cracks and macro cracks.Unmelted powder particles are not present in the layers.Through the coating layers,wecan clearly seen dark micro pores of irregular spherical shapes, which affected the coating to have a lower microhardness value of 850 HV0.Figs. 6 and 7 show the microstructures of the 75Cr 3 C 2 -25Ni(Cr) coating layers deposited with the highest powder feed rate of 60g/min., which had the worst microstructure and mechanical properties.Because of a high powder feed rate, all particles do not have enough time to melt completely in the plasma jet, due to which unmelted particles and coarse pores (black) are present in the coating layers (Fig. 6., and 7).The coatings show a lamellar structure with limited inter-lamellar bonding.Therefore,micro pores are present as volumetric errors with large concentrations of stress which can cause the appearance of micro-cracks and accelerated wear during exploitation.Limited bonding of lamellae in the deposit decreases microhardness and fracture toughness.Through the coating layers, there can be clearly seen thin interlamellar films of gray oxides: NiO, NiCr 2 O 4 , Cr 2 O 3 and CrO 3 (Fig. 7) originating from the oxidation of Ni and Cr in the process of the melting of Ni (Cr) particles in plasma (Brossard, et al., 2010(Brossard, et al., , pp.1608(Brossard, et al., -1615)), (Mrdak, 2011, pp.9 -14).In the coating there are dispersed Cr 3 C 2 carbides in dark gray, located in the light gray area of Cr 23 C 6 and Cr 7 C 3 carbides formed in plasma by the temperature decomposition of the primary Cr 3 C 2 carbide (Fig. 7).Throughout the carbide layers, there are also present bright white lamellae of the Ni(Cr)alloy (Sukhpal, et al., 2012, pp.569-586).with the powder feed rate of 60 g/min Slika 7 -Mikrostruktura 75Cr3C2 -25Ni(Cr) prevlake deponovane sa brzinom dovoda praha 60 g/min Fig. 8 shows the layers of the 75Cr 3 C 2 -25Ni(Cr) coating deposited with the powder feed rate of 45g/min., which had the best microstructure and mechanical properties.The coating shows a lamellar structure with good inter-lamellar bonding.Good inter-lamellar bonding of the lamellae in the deposit increases the value of microhardness and fracture toughness, as confirmed by the mechanical testing of coatings.Micro cracks and macro cracks are not present in zhe inner lazers of the coating.In the coating layers there are no unmelted particles and coarse pores, which points to good melting of particles and good diffusion of particles during the coating process on the metal substrate.The microstructure of the coating is layered with longitudinal lamellar Cr 23 C 6 and Cr 7 C 3 carbides (light gray) containing dispersed Cr 3 C 2 carbides (dark gray).In the coating layers there are present fine black micropores and thin oxide layers of NiO, NiCr 2 O 4 and Cr 2 O 3 (light gray).The oxide layers are the result of incorporating air into the plasma jet and the oxidation of Ni(Cr) alloys during the deposition, i.e.they are an inevitable consequence of the application of the plasma spray process in atmospheric conditions.In the light gray zone of the Cr 23 C 6 and Cr 7 C 3 carbides, white light lamellae of Ni(Cr) alloy are clearly visible.The values of the microhardness and the bond strength of the deposited layers were directly related to the powder feed rate (g/min).The layers deposited with the powder feed rate of 45g/min.had the highest values of microhardness (670-845HV 0.3 ) and the tensile bond strength of 47MPa, which are within the limits of 450-850 HV 0.3 and min.35MPaprescribed by the Pratt & Whitney standard.The microhardness and tensile bond strength values were correlated with their microstructures.
The structure of the deposited 75Cr 3 C 2 -25Ni(Cr) coatings is lamellar.Micro pores (black) were present in all coatings .The layers deposited with the powder feed rate of 45g/min did not have coarse micro pores in the microstructure.These layers had the best microstructure.These layers did not show the presence of unmelted powder particles, precipitates, and inter-lamellar pores.The microstructure of the coating is layered with longitudinal lamellar carbides.Light gray fields are Cr 23 C 6 and Cr 7 C 3 carbides created by the decomposition of the primary Cr 3 C 2 carbide.In the light gray fields of the Cr 23 C 6 and Cr 7 C 3 carbides, there is a dispersed phase of the primary non-decomposed Cr 3 C 2 carbide (dark gray) .In the light gray zone of the Cr 23 C 6 and Cr 7 C 3 carbides, there are light white lamellae of the Ni(Cr) alloy.In the coating layers, there are also present thin NiO, NiCr 2 O 4 and Cr 2 O 3 oxide layers (light gray).The oxide layers are the result of incorporating air into the plasma jet and the oxidation of Ni(Cr) alloys during the powder deposition.

Figure 3 -
Figure 3 -Bond strength of 75Cr3C2 -25Ni(Cr) layers Slika 3 -Čvrstoća spoja 75Cr3C2 -25Ni(Cr) slojeva Figs. 4 and 5 show the microstructures of the layers deposited with the powder feed rate of 30 g / min.The qualitative analysis of the deposited 75Cr 3 C 2 -25Ni(Cr) layers showed that the substrate / coating interface has a negligible share of corundum Al 2 O 3 particles due to roughening.