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INFLUENCE OF A POWDER FEED RATE ON THE PROPERTIES OF THE PLASMA SPRAYED CHROMIUM CARBIDE- 25% NICKEL CHROMIUM COATING
Mihailo R. Mrdak
Research and Develoopment Center IMTEL, Communications a.d., Belgrade
DOI: 10.5937/vojtehg62-3793
FIELD: Chemical Technology ARTICLE TYPE: Original Scientific Paper
Summary:
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 decarbu-rization 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.
Key word: property; powders; plasmas; feed rate; coatings; chromium.
ACKNOWLEDGEMENT: The author is thankful for the financial support from the Ministry of Education and Science of the Republic of Serbia (national projects OI 174004, TR 34016).
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Introduction
Thermal 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, Bacova, 2009, pp.359-366), (Mann, Arya, 2003, pp.652-667), (Monticelli, et al., 2004, pp.1225-1237), (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-Cr3C2 and Fe-CrAlY-Cr3C2. In the thermal spray technology, Cr3C2-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.392408). Cr3C2-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 Cr3C2 carbide (10.3 x 10"6°C_1) is almost the same as the coefficient of the thermal expansion of iron (11.4 x 10"6°C-1) and nickel (12.8 x 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, pp.2684-2688). 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 75Cr3C2-25Ni(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 ^m. (Material Product Data Sheet, 2012, Woka 7203 Chromium Carbide - 25% Nickel Chromium Powders, DSMTS-
0031.1, Sulzer Metco). The high energy level of the plasma causes the decomposition of the initial Cr3C2 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 Cr23C6, Cr7C3 and Cr3C2 (Kajihara, Hillert, 1990, pp.2777-2787). At 1534 ± 10°C, the first eutectic reaction L = (Cr) + Cr23C6 occurs and the solubility of C in the Cr-C solid solution is increased to 0.07 wt% C. The first peritectic reaction L + Cr7C3 _ Cr23C6 occurs at 1576 ± 10°C. Moreover, at 1727 ± 7°C, there is another eutectic reaction L = Cr7C3 + Cr3C2 and the melting temperature of the Cr7C3 carbide is 1756 ± 10°C. Another peritectic reaction occurs at 1811 ± 10°C when Cr3C2 carbide is formed by the reaction of L + C = Cr3C2 (ASM HANDBOOK VOLUME 3.Alloy Phase Diagrams, ASM International, Printed in the United States of America). In a series of carbides (Cr23C6, Cr7C3and Cr3C2), Cr3C2 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, pp.1004-1013). Therefore, coatings should be carefully sprayed and the spraying parameters should be carefully chosen prior to deposition. For example, 75Cr3C2-25NiCr 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 75Cr3C2-25NiCr sprayed coatings produced by different systems were tested in previous studies. Virojanu-patump published the influence of the 75Cr3C2 - 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 micro-hardness - 910HV0 .3, the microhardness of the coating deposited from a mixture of powders is 650HV0.3, and the value of the microhardness of the coating deposited by composite powder is 820HV0.3. He and Manish found that in the coating sprayed by agglomerated 75Cr3C2 - 25Ni(Cr) powder there was present only the Cr3C2 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 75Cr3C2 - 25Ni(Cr) powder, Matthevs also found significant amounts of the Cr3C2 phase (Matthews, et al., 2009, pp.1086-1093), (Matthews, et al., 2009, pp.10941100). The concentration of Cr7C3 carbide was too low. Similar work was
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done by Suegama, who used the XRD analysis for the agglomerated 75Cr3C2 - 25Ni(Cr) powder and for the coating sprayed with the HVOF system and found that the powder consists of Cr3C2 carbide, Cr7C3 and a basis based on Ni (Suegama, et al., 2006, pp.434-445). In the 75Cr3C2 -25Ni(Cr) coatings, deposited by plasma spraying, there are present the particles of carbide types Cr3C2, Cr7C3 and Cr23C6. The phenomenon of a significant degradation of Cr3C2 carbide particles has been published in the literature (Ji, et al., 2006, p.6749) (Picas, et al., 2003, pp.1095). Cr7C3 and Cr23C6 carbides are formed by the decomposition of the primary Cr3C2 carbide. Due to the decomposition of the Cr3C2 carbide, in accordance with the Pratt & Whitney standard,, an acceptable value of the microhardness of the 75Cr3C2 - 25Ni(Cr) coating is in the range of 450-850HV03 (Turbojet Engine - Standard Practices Manual (PN 582 005), 2002, Pratt & Whitney, East Hartford, USA ). Verdon C. et al. found that higher values of hardness of Cr3C2-NiCr coatings can be obtained because of high density and cohesive strength as result of high influence of high-speed of particles during deposit (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, NiCr2O4, Cr2O3 and CrO3, unmelted and semi-melted particles in the coating structure (Brossard, et al., 2010, pp.16081615), (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 re-hardening 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 75Cr3C2 - 25Ni (Cr) coatings, there are three different zones. The dark zone indicates the presence of primary Cr and C, revealing the Cr3C 2 phase. The second zone is gray, which indicates the presence of Cr7C3 and Cr23C6 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 75Cr3C2 - 25Ni (Cr) coatings .The main
goal was to apply the cermet 75C3C2 - 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 Cr3C2chromium carbides and 25% of a nickel-chromium alloy (80% / 20%). The 75Cr3C2 -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 C3C2 (br-own)and the particles of the 25Ni(Cr) alloy (white) can be seen.
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Figure 1 - (SEM) Scanning electron micrograph of 75Cr3C2 - 25Ni(Cr) powder particles Slika 1 - (SEM) Skening elektronska mikrografija cestica praha 75Cr3C2 - 25Ni(Cr)
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The substrate material on which the coatings for testing microhard-ness are deposited and which is used for the evaluation of the microstructure in the deposited state is made of steel C.4171 (X15Cr13 EN10027), thermally unprocessed, with the dimensions of 70x20x1.5mm. (Turbojet Engine - Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA). The bases for testing the bond strength are also made of steel C.4171(X15Cr13EN10027), thermally unprocessed, with the dimensions of 025x50 mm (Turbojet Engine - Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA).
The examination of the microhardness of coating layers was done by the method HV03. 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.
Table 1 - Plasma spray parameters
Tabela 1 - Plazma sprej parametri
Deposition parameters Values
Plasma current, I (A) 700 700 700
Plasma Voltage, U (V) 36 36 36
Primary plasma gas flow rate, Ar (l/min) 47 47 47
Secondary plasma gas flow rate, He (l/min) 12 12 12
Carrier gas flow rate, Ar (l/min) 5 6 7
Powder feed rate (g/min) 30 45 60
Stand-off distance (mm) 100 100 100
Results and discussion
The values of the microhardness and bond strength of the deposited 75Cr3C2 - 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 75Cr3C2 - 25Ni(Cr) coating layers deposited with the lowest powder feed rate of 30g/min have a microhardness value of 515-798HV03.
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Figure 2 - Microhardness of 75Cr3C2 - 25Ni(Cr) layers Slika 2 - Mikrotvrdoca 75Cr3C2 - 25Ni(Cr) slojeva
With a lower powder feed rate than the optimum one, primary Cr3C2 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 670-845HV03, which is consistent with the values (450-850 HV03) 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 438-695HV03. 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.
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Figure 3 - Bond strength of 75Cr3C2 - 25Ni(Cr) layers Slika 3 - Cvrstoca 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 75Cr3C2-25Ni(Cr) layers showed that the substrate / coating interface has a negligible share of corundum Al2O3 particles due to roughening.
Figure 4 - 75Cr3C2 - 25Ni(Cr) coating microstructure deposited
with the powder feed rate of 30 g/min Slika 4 - Mikrostruktura 75Cr3C2 - 25Ni(Cr) prevlake deponovane sa brzinom dovoda praha 30 g/min
Figure 5 - 75Cr3C2 - 25Ni(Cr) coating microstructure deposited
with the powder feed rate of 30 g/min Slika 5 - Mikrostruktura 75Cr3C2 - 25Ni(Cr) prevlake deponovane sa brzinom dovoda praha 30 g/min
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 HV0.3 prescribed by the Pratt & Whitney Pratt & Whitney standard (Turbojet Engine - Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA).
Figs. 6 and 7 show the microstructures of the 75Cr3C2-25Ni(Cr) coating layers deposited with the highest powder feed rate of 60g/min., which had the worst microstructure and mechanical properties.
Figure 6 - 75Cr3C2 - 25Ni(Cr) coating microstructure deposited with the powder feed rate
of 60 g/min
Slika 6 - Mikrostruktura 75Cr3C2 - 25Ni(Cr) prevlake deponovane sa brzinom dovoda
praha 60 g/min
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, NiCr2O4, Cr2O3 and CrO3 (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, pp.1608-1615), (Mrdak, 2011, pp.9 - 14). In the coating there are dispersed Cr3C2 carbides in dark gray, located in the light gray area of Cr23C6 and Cr7C3 carbides formed in plasma by the temperature decomposition of the primary Cr3C2carbide (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).
Figure 7 - 75Cr3C2 - 25Ni(Cr) coating microstructure deposited
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 75Cr3C2-25Ni(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 Cr23C6 and Cr7C3 carbides (light gray) containing dispersed Cr3C2 carbides (dark gray). In the coating layers there are present fine black micropores and thin oxide layers of NiO, NiCr2O4 and Cr2O3 (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 Cr23C6 and Cr7C3 carbides, white light lamellae of Ni(Cr) alloy are clearly visible.
Figure 8 - 75Cr3C2 - 25Ni(Cr) coating microstructure deposited with the powder
feed rate of 45 g/min Slika 8 - Mikrostruktura 75Cr3C2 - 25Ni(Cr) prevlake deponovane sa brzinom dovoda praha 45 g/min
Conclusion
75Cr3C2-25Ni(Cr) coatings were deposited by the atmospheric plasma spraying (APS) process with three powder feed rates of 30, 45 and 60g/min. This paper analyzes the mechanical properties and the microstructures of the deposited layers by light microscopy. The morphology of the powder particles was examined on a Scanning Electron Microscope (SEM). The analysis led to the following conclusions.
The morphologies of the agglomerated powder particles of 75Cr3C2-25Ni(Cr) are spherical in shape, consisting of sintered Cr3C2carbides and particles of 25Ni(Cr) alloy.
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 (670-845HV03) and the tensile bond strength of 47MPa, which are within the limits of 450-850 HV03 and min.35MPa prescribed by the Pratt & Whitney standard. The micro-hardness and tensile bond strength values were correlated with their microstructures.
The structure of the deposited 75Cr3C2-25Ni(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 Cr23C6 and Cr7C3 carbides created by the decomposition of the primary Cr3C 2 carbide. In the light gray fields of the Cr23C6 and Cr7C 3 carbides, there is a dispersed phase of the primary non-decomposed Cr3C2 carbide (dark gray) . In the light gray zone of the Cr23C6 and Cr7C3 carbides, there are light white lamellae of the Ni(Cr) alloy. In the coating layers, there are also present thin NiO, NiCr2O4 and Cr2O3 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.
The coating deposited with the powder feed rate of 45g/min., which showed the best mechanical properties and the microstructure, has been tested and homologated on the inlet flange parts of the turbo jet engine TV2-117A at the test station for 45 hours in VZ "Moma Stanojlovic" , -Batajnica.
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Kamal, S., Jayaganthan, R., Prakash, S., & Kumar, S. 2008. Hot corrosion behavior of detonation gun sprayed Cr3C2-NiCr coatings on Ni and Fe-based superalloys in Na2S04-60% V2O5 environment at 900°C. Journal of Alloys and Compounds, 463(1-2), pp. 358-372. doi:10.1016/j.jallcom.2007.09.019
Kamal, S., Jayaganthan, R., & Prakash, S. 2009. Evaluation of cyclic hot corrosion behaviour of detonation gun sprayed Cr3C2-25%NiCr coatings on nickel- and iron-based superalloys. Surface and Coatings Technology,203(8), pp. 1004-1013. doi:10.1016/j.surfcoat.2008.09.031
Kajihara, M., & Hillert, M. 1990. Thermodynamic evaluation of the Cr-Ni-C system. Metallurgical Transactions A,21(10),pp. 2777-2787. doi:10.1007/BF02646072
Mann, B.S., & Arya, V. 2003. HVOF coating and surface treatment for enhancing droplet erosion resistance of steam turbine blades. Wear, 254(7-8), pp. 652-667. doi:10.1016/S0043-1648(03)00253-9
Matthews, S.J., James, B.J., & Hyland, M.M. 2007. Microstructural influence on erosion behaviour of thermal spray coatings. Materials Characterization, 58(1), pp. 59-64. doi:10.1016/j.matchar.2006.03.014
Matthews, S., James, B., & Hyland, M. 2009. The role of microstructure in the mechanism of high velocity erosion of Cr3C2-NiCr thermal spray coatings: Part 1 — As-sprayed coatings. Surface and Coatings Technology, 203(8), pp. 1086-1093. doi:10.1016/j.surfcoat.2008.10.005
Matthews, S., James, B., & Hyland, M. 2009. The role of microstructure in the mechanism of high velocity erosion of Cr3C2-NiCr thermal spray coatings: Part 2 — Heat treated coatings. Surface and Coatings Technology, 203(8), pp. 1094-1100. doi:10.1016/j.surfcoat.2008.10.013
Material Product Data Sheet, Woka 7203 Chromium Carbide - 25% Nickel Chromium Powders, DSMTS-0031. 12012. Sulzer Metco.
Roy, M., Pauschitz, A., Polak, R., & Franek, F. 2006. Comparative evaluation of ambient temperature friction behaviour of thermal sprayed Cr3C2-25(Ni20Cr) coatings with conventional and nano-crystalline grains. Tribology International, 39(1), pp. 29-38. doi:10.1016/j.triboint.2004.11.009
Monticelli, C., Frignani, A., & Zucchi, F. 2004. Investigation on the corrosion process of carbon steel coated by HVOF WC/Co cermets in neutral solution. Corrosion Science, 46(5), pp. 1225-1237. doi:10.1016/j.corsci.2003.09.013
Mrdak, M.R. 2010. Uticaj brzine depozicije praha na mehanicke karakter-istike i strukturu APS-NiCr/Al prevlake. Vojnotehnicki glasnik, 58(4), pp. 5-16. Taken from http://scindeks.ceon.rs/article.aspx?artid=0042-84691004005M0
Mrdak, M., & Vencl, A. 2011. Uticaj parametara nanosenja NiCr prevlake plazma sprej postupkom u atmosferskim uslovima na njene mehanicke karakter-istike i strukturu. Tehnicka dijagnostika, 10(3), pp. 9-14. Taken from http://scindeks.ceon.rs/article.aspx?artid=1451-19751103009M
Mrdak, M. 2012. Study of the properties of plasma deposited layers of nickel-chrome-aluminium-yttrium coatings resistant to oxidation and hot corrosion. Vojnotehnicki glasnik, 60(2), pp. 182-201. doi:10.5937/vojtehg1202182M
Mrdak, M. 2013. Characterization of sealing nickel - graphite coating in the system with bonding of nickel-aluminum coating. Vojnotehnicki glasnik, 61(1), pp. 68-88.
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Sukhpal, Singh, C., Hazoor, S., Buta, S., & Sidhu, S. 2012. Characterisation and Corrosion-Erosion Behaviour of Carbide based Thermal Spray Coatings. Journal of Minerals & Materials Characterization & Engineering, 11(6), pp. 569-586.
Suegama, P.H., Espallargas, N., & Guilemany, J.M. 2006. Electrochemical and structural characterization of heat-treated Cr3C2-NiCr coatings. Journal of the Electrochemical Society, 153, pp. 434-445.
Tillmann, W., Vogli, E., Baumann, I., Kopp, G., & Weihs, C. 2010. Desirability-Based Multi-Criteria Optimization of HVOF Spray Experiments to Manufacture Fine Structured Wear-Resistant 75Cr3C2-25(NiCr20) Coatings. J. Therm. Spray Technol, 19(1-2), pp. 392-408.
Verdon, C., Karimi, A., & Martin, J. 1998. A study of high velocity oxy-fuel thermally sprayed tungsten carbide based coatings. Part 1: Microstructures. Materials Science and Engineering: A, 246(1-2), pp. 11-24. doi:10.1016/S0921-5093(97)00759-4
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Wirojanupatump, S., Shipway, P.H., & Mccartney, D.G. 2001. The influence of HVOF powder feedstock characteristics on the abrasive wear behaviour of CrxCy-NiCr coatings. Wear, 249, pp. 829-837.
C2D
UTICAJ BRZINE DOVODA PRAHA NA SVOJSTVA PLAZMA NAPRSKANE HROMKARBID 25% NIKAL HROM PREVLAKE
OBLAST: hemijske tehnologije VRSTA C LANKA: originalni naucni clanak
Sazetak:
Plazma-sprej postupak je vodeca tehnologija deponovanja praha u cilju izrade prevlaka koje imaju siroku primenu u avio-industriji za zastitu novih i opravku pohabanih delova. Kermet prevlake 75Cr3C2 -25Ni(Cr) na bazi Cr3C2 karbida imaju veliku primenu za zastitu delova, jer zadrzavaju visoke vrednosti tvrdoce, Cvrstoce i otpornosti na haba-nje do temperature od 850°C. Ovaj rad razmatra uticaj parametara plazma-sprej depozicije praha 75Cr3C2 - 25Ni(Cr) na strukturu i meha-nicke karakteristike prevlake. Prah je deponovan plazma-sprej postup-kom na atmosferskom pritisku (APS). Pri izboru parametara kao pla-zma gas koristio se helijum, koji je inertan i ne reaguje sa prahom, pro-izvodi guscu plazmu sa manjim toplotnim sadrzajem i manje inkorpori-ra okolni vazduh u mlaz plazme, sto smanjuje temperarturno razlaga-nje i dekarburizaciju karbida Cr3C2. U istrazivanju su deponovane tri grupe prevlaka sa tri razlicite brzine dovoda praha od 30, 45 i 60 g/min. Prevlaka sa najboljim karakteristikama deponovana je na ulaznoj pri-rubnici dela turbo-mlaznog motora TV2-117A da bi se smanjio uticaj vibracija i habanja. Analizirane su strukture i mehanicke karakteristike 75Cr3C2 - 25Ni(Cr) prevlaka u skladu sa standardom Pratt & Whitney. IstraZivanja su pokazala da brzine dovoda praha bitno uticu na mehanicke osobine i strukture75Cr3C2 - 25Ni(Cr) prevlaka.
Uvod
Termo-sprej prevlake pripadaju razvoju oblasti inzenjerstva povrsi-na. Ove visokokvalitetne funkcionalne prevlake primenjuju se na novim delovima u baznoj industriji, kao i za renoviranje delova, uglavnom zbog svojih odlicnih karakteristika, koje odlikuje visoka otpornost na habanje, eroziju, abraziju, otpornost na koroziju i otpornost prema visokim tempe-raturama (Berget, et al., 2007, pp.7619-7625), (Jankura, Bacová, 2009, pp.359-366), (Mann, Arya, 2003, pp.652-667), (Monticelli, et al., 2004, pp. 1225-1237), (Wheeler, Wood, 2005, pp. 526-536). Kermet prevlake su kombinacija keramickih tvrdih faza ugradenih u zilavoj metalnoj osno-vi. Cr3C2-NiCr kermet prevlake se intenzivno koriste za ublazavanje abrazionog i erozionog habanja na visokim temperaturama do 850°C (Matthews, et al., 2007, pp. 59-64), (Tillmann, et al., 2010, pp. 392408). Prevlake Cr3C2-NiCr u odnosu na druge kermet prevlake nude vecu otpornost na koroziju i oksidaciju, takoúe ima visoku tacku topljenja i odrzavanje visoke tvrdoce, Cvrstoce i otpornosti na habanje do 850°C. Primenom kermet prevlaka dobio se bolji radni vek komponenti masina. Prevlake na bazi hrom karbida cesto se koriste na gasnim turbinama,
parnim turbinama i vazduhoplovnim motorima da poboljsaju otpornost na klizanje, abraziono i eroziono habanje (Hillery, 1986, pp.2684-2688). Delovi na kojima se primenjuje ova prevlaka su : hidraulicki cilindri i klip-njace, plocice ventila, turbinske komponente, sita i cunjevi, vretena i ventili brodskih motora, kucista pumpi i dr. (Material Product Data Sheet, 2012, Woka 7203 Chromium Carbide - 25% Nickel Chromium Powders, DSMTS-0031.1, Sulzer Metco). U kermet prevlakama sa razlicitim ter-mo-sprej procesima uocene su znacajne varijacije u sastavu i mikro-strukturi usled izlaganja praha visokim temperaturama i razlicitim brzi-nama gasa u procesu (Matthews, et al., 2007, pp.59-64). U prevlakama 75Cr3C2 - 25Ni(Cr) deponovanim plazma-sprej postupkom prisutne su cestice karbida tipa Cr3C2,Cr23C6 i Cr7C3. Fenomen znacajne razgradnje cestica karbida Cr3C2 publikovan je u literaturi (Ji, et al., 2006, p.6749), (Picas, et al., 2003, pp.1095). Karbidi Cr7C3 i Cr23C6 formiraju se raz-gradnjom osnovnog karbida Cr3C2. Zbog razgradnje karbida Cr3C2, po standardu Pratt & Whitney prihvatljiva vrednost mikrotvrdoce prevlake 75Cr3C2 - 25Ni(Cr) je raspona 450-850 HV03 (Turbojet Engine - Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA). Neuniformne vrednosti tvrdoce duz prevlaka prijavili su mnogi autori. Ova varijacija u vrednosti tvrdoce pripisuje se mikrostruk-turnim promena duz poprecnog preseka prevlaka. Ove promene mogu biti mikrostrukturne zbog prisustva poroznosti, oksida legure Ni(Cr) kao sto su: NiO, NiCr2O4, Cr2O3 i CrO3, neistopljenih i poluistopljenih cesti-caa u strukturi prevlaka (Brossard, et al., 2010, pp.1608-1615), (Matthews, et al., 2007, pp. 59-64), (Mrdak, 2011, pp. 9-14).
U ovom radu predstavljeni su rezultati eksperimentalnih istraziva-nja uticaja brzine dovoda praha (g/min) na mehanicka svojstva i mikro-strukturu kermet prevlake 75Cr3C2 - 25Ni(Cr). Glavni cilj bio je da se na ulaznoj prirubnici dela turbomlaznog motora TV2-117A primeni kermet prevlaka 75Cr3C2 - 25Ni(Cr) deponovana APS - atmosferskim plazma-sprej postupokom. Uradene su tri grupe uzoraka sa vrednostima brzine dovoda praha od 30, 45 i 60 g/min. Analizirane su mehanicke karakte-ristike i mikrostrukture prevlaka da bi se odabrala prevlaka najboljih ka-rakteristika. Prevlaka sa najboljim mehanickim i strukturnim karakteri-stikama je testirana i homologovana na ulaznoj prirubnici dela turbomlaznog motora TV2-117A motora na ispitnoj stanici u trajanju od 45 casova u VZ "Moma Stanojlovic" - Batajnica.
Materijali i eksperimentalni detalji
Za proizvodnju prevlaka koristio se prah firme Sulzer Metco s oznakom Woka 7203 ( Material Product Data Sheet, 2012, Woka 7203 Chromium Carbide - 25% Nickel Chromium Powders, DSMTS-0031.1, Sulzer Metco). Prah Woka sadrzi 75% tvrdog hrom karbida Cr3C2 i 25% legure nikl-hrom (80%/20%). Cestice praha 75Cr3C2 - 25Ni(Cr) su sferoidizirane aglomeracijom i sinterovanjem sa rasponom granulacije cestica praha od 11 do 45 pm.
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Materijal substrata na kojem su deponovane prevlake za ispitiva-nje mikrotvrdoce i za procenu mikrostrukture u deponovanom stanju iz-raden je od celika C.4171 (X15Cr13 EN10027) u termicki neobrade-nom stanju dimenzija 70x20x1,5mm (Turbojet Engine - Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA). Osnove za ispitivanje cvrstoce spoja takode su izradene od celika C.4171(X15Cr13EN10027) u termicki neobradenom stanju dimenzija 025x50 mm (Turbojet Engine - Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA).
Ispitivanje mikrotvrdoce slojeva prevlaka radeno je metodom HV03. Merenje je uradeno u pravcu duz lamela, u sredini i na krajevima uzorka. Uradeno je pet ocitavanja na tri mesta, a u radu su prikazane minimalne i maksimalne vrednosti.
Ispitivanja zatezne cvrstoce spoja vrsena su na sobnoj temperaturi na hidra ulicnoj opremi brzinom od 10 mm/min, za sva ispitivanja. Za svaku grupu uzoraka uradene su tri epruvete, a u radu su prikazane srednje vrednosti. Mehanicke i mikrostrukturne karakterizacije dobijenih prevlaka uradene su prema standardu Pratt & Whitney (Turbojet Engine - Standard Practices Manual (PN 582005), 2002, Pratt & Whitney, East Hartford, USA).
Mikrostrukturna analiza prevlaka uradena je na svetlosnom mikroskopu, a morfologija cestica praha na SEM-u (skening elektronskom mikroskopu).
Za depoziciju praha koriscen je plazma-pistolj SG -100 atmosfer-ski plazma-sprej sistema (APS) firme Plasmadyne. Kao gas koriscen je argon u kombinaciji sa helijumom i snaga napajanja od 40 kW. Pri iz-boru parametara depozicije praha kao osnovni parameter uzeta je br-zina dovoda praha (g/min). U ovom istrazivanju uradene su tri grupe uzoraka. Kod prve grupe uzoraka brzina dovoda praha bila je 30 g/min sa protokom noseceg gasa od 5 l/min, kod druge grupe uzoraka ova brzina bila je 45 g/min sa protokom noseceg gasa od 6 l/min, a kod trece grupe uzoraka 60 g/min sa protokom noseceg gasa od 7 l/min. Ostali parametri deponovanja praha imali su sledece vrednosti: pla-zma-struja 700 A, napon luka 36 V, protok primarnog gasa (Ar) 47 l/min, protok sekundarnog gasa (He) 12 l/min, i odstojanje mlaza pla-zme 100 mm od supstrata. Prevlake su formirane sa debljinama do 0,2 mm.
Rezultati i diskusija
Brzina dovoda praha bitno utice na vrednosti mikrotvrdoce depo-novanih slojeva, koja mora da bude optimalna da bi se obezbedilo pot-puno topljenje cestica praha i na minimum smanjio procenat neisto-pljenih cestica, pora i oksida u slojevima prevlake. Mikrostrukturne promene uzrokovane su prisustvom poroznosti, neistopljenih cestice, oksida legure Ni(Cr) i razgradenih karbida u strukturi prevlaka, sto su potvrdila metalografska ispitivanja slojeva prevlaka. Slojevi deponovani brzinom dovoda praha od 45g/min imaju vrednosti mikrotvrdoce od
670-845HV033 koje su u skladu sa vrednostima (450-850 HV033) koje propisuje standard Pratt S Whitney(Turbojet Engine - Standard Practices Manual (PN 582005), 2002, Pratt S Whitney, East Hartford, USA). Ti slojevi su pokazali najguscu i najbolju mikrostrukturu, sto su potvrdi-la metalografska ispitivanja prevlaka.
Zatezna cvrstoca spoja je, kao i mikrotvrdoca, u direktnoj vezi sa brzinom dovoda praha, prisustva pora, neistopljenih cestica i medula-melarnih oksida. _Najvecu vrednost zatezne cvrstoce spoja od 47 M Pa pokazali su slojevi, koji su deponovani sa brzinom dovoda praha od 45 g/min. Ti slojevi u mikrostrukturi nisu imali grube pore, neistopljene ce-stice i poluistopljene cestice. Posto je prisustvo pora, neistopljenih cestica i oksida u direktnoj vezi sa vrednostima cvrstoce spoja prevlaka, to izmerene vrednosti za prevlaku deponovanu sa brzinom dovoda praha od 45g/min Ukazuje na to da je njihov udeo najmanji u ovoj pre-vlaci.
Kvalitativna analiza deponovanih 75Cr3C2-25Ni(Cr) slojeva pokaza-la je da je na interfejsu supstrat/prevlaka zanamarljiv udeo cestica ko-runda Al2O3 od hrapavljenja. Duz interfejsa izmedu substrata i prevlake nisu prisutne mikropukotine i makropukotine. Veza prevlake sa substra-tom je uniformna bez odvajanja slojeva prevlake sa substrata. Slojevi prevlake su deponovani kontinualno bez prisustva mikropukotina i ma-kropukotina. Prevlaka deponovana sa brzinom dovoda praha 45g/min, koja ima najbolje mehanicke karakteristike pokazuje lamelarnu strukturu sa dobrim interlamelarnim vezivanjem. Dobro medulamelarno vezivanje lamela u depozitu povecava vrednosti mikrotvrdoce i zilavost loma, sto su potvrdila mehanicka ispitivanja prevlaka. Unutrasnji slojevi prevlake su bez prisutnih mikroprskotina i makroprskotina. U slojevima prevlake ne uocavaju se neistopljene cestice i grube pore, sto govori o dobroj is-topljenosti cestica i dobrom razlivanju cestica tokom procesa nanosenja na metalnu osnovu. Mikrostruktura prevlake je slojevita sa poduznim lamelama karbida Cr23C6 i Cr7C3 svetlosive boje u kojima se nalaze dis-pergovani karbidi Cr3C2 tamnosive boje. U slojevima prevlake prisutne su fine mikropore crne boje, kao i tanki slojevi oksida NiO, NiCr2O4 i Cr2O3 svetlosive boje. Oksidni slojevi su posledica inkorporiranja vazdu-ha u mlaz plazme i oksidacije legure Ni(Cr) tokom depozicije, tj. neizbe-zna su posledica primenom plazma-sprej postupka u atmosferskim uslovima. U svetlosivoj zoni karbida Cr23C6 i Cr7C3 jasno se vide svetlo-bele lamele legure Ni(Cr).
Zakljucak
Atmosferskim plazma-sprej postupkom (APS) deponovane su prevlake 75Cr3C2-25Ni(Cr) sa tri brzine dovoda praha 30, 45 i 60g/min. U radu su analizirane mehanicke karakteristike deponovanih slojeva i mikrostrukture na svetlosnom mikroskopu. Morfologija cestica praha ispitana je na (SEM) skening elektronskom mikroskopu. Na osnovu iz-vrsenih analiza doslo se do odredenih zakljucaka.
>
™ Morfologije aglomerisanih cestica praha 75Cr3C2 - 25Ni(Cr) su
° sfernog oblika koje se sastoje od sinterovanih cestica karbida Cr3C2 i
cestica legure 25Ni(Cr).
Vrednosti mikrotvrdoce i cvrstoce spoja deponovanih slojeva bili <5 su u direktnoj vezi sa brzinama dovoda praha (g/min). Slojevi depono-
vani sa brzinom dovoda praha 45g/min imali su najvece vrednosti mikrotvrdoce 670-845HV03 i zatezne cvrstoce spoja 47MPa, koje su u granicama od 450 do 850 HV03 i min 35MPa koje su propisane stan-¡¿ dardom Pratt & Whitney. Vrednosti mikrotvrdoce i zatezne cvrstoce
spoja bile su u korelaciji sa njihovim mikrostrukturama. =5 Struktura deponovanih prevlake 75Cr3C2 - 25Ni(Cr) je lamelarna.
g U svim prevlakama su prisutne mikropore crne boje. Slojevi depono-
vani sa brzinom dovoda praha 45 g/min nisu imali grube mikropore u o mikrostrukturi. Ti slojevi imali su najbolju mikrostrukturu. U njima nisu
prisutne neistopljene cestice praha, precipitati i interlamelarne pore. o Mikrostruktura prevlake je slojevita sa poduznim lamelama karbida.
m Svetlosiva polja su karbidi tipa Cr23C6 i Cr7C3 nastali razgradnjom pri-
> marnog karbida Cr3C2. U svetlosivim poljima karbida Cr23C6 i Cr7C3 pri-
<c sutna je dispergovana faza primarnog nerazgradenog karbida Cr3C2
tamnosive boje. U svetlosivoj zoni karbida Cr23C6 i Cr7C3 prisutne su svetlobele lamele legure Ni(Cr). U slojevima prevlake prisutni su i tanki slojevi oksida NiO, NiCr2O4 i Cr2O3 svetlosive boje. Oksidni slojevi su posledica inkorporiranja vazduha u mlaz plazme i oksidacije legure < Ni(Cr) tokom depozicije praha.
CD Prevlaka deponovana sa brzinom dovoda praha 45 g/min, koja je
2 pokazala najbolje mikrostrukture i mehanicka svojstva testirana je i
homologovana na ulaznoj prirubnici dela turbomlaznog motora TV2-x 117A na ispitnoj stanici u trajanju od 45 casova u VZ "Moma Stanojlo-
h vic" - Batajnica.
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Kljucne reci: osobine, prah, plazma, brzina dovoda, prevlaka, hrom.
Datum prijema clanka/Paper received on: 20. 04. 2013.
Datum dostavljanja ispravki rukopisa/Manuscript corrections submitted on: 24. 02. 2014. Datum konacnog prihvatanja clanka za objavljivanje/ Paper accepted for publishing on: 26. 02. 2014.
