STUDY OF THE APPLICATION OF PLASMA SPRAYED COATINGS ON THE SECTIONS OF THE ASTAZOU III B TURBOJET ENGINE

The plasma spray process is used extensively in the aerospace industry for manufacturing key components exposed to excessively high temperatures, aggressive chemical environments, wear, abrasion, erosion and cavitation. The process covers a large field of parameters so that almost every layer can be combined with any other as well as with the base material. Coatings can be deposited uniformly; therefore, they allow worn components to be brought to final dimensions in the process of aircraft repair. This research shows an effective procedure of the application of plasma spray coatings on the parts of the Astazou III B turbo jet engine in the process of repair.The engine manufacturer,Turbomeca, has prescribed that powders should be deposited by plasma spray systems under designation Metco 3M and 7M for the prescribed parameters of powder deposition, so that during the application of other plasma spray depositing systems the parameters must be tested and optimized. The aim was to apply the Plasmadyne plasma spray system during the repair process and to optimize the parameters, which will enable producing coatings that fulfill all the criteria prescribed in the engine manufacturer standard. The optimization of the parameters was carried out with a plasma gun MINI GUN II with a large number of samples. This paper presents the optimal parameters of the deposition on the ASTAZOU III B engine casing, casing frame, duct and oil tank.The assessment of the coating


Summary: *
The plasma spray process is used extensively in the aerospace industry for manufacturing key components exposed to excessively high temperatures, aggressive chemical environments, wear, abrasion, erosion and cavitation.The process covers a large field of parameters so that almost every layer can be combined with any other as well as with the base material.Coatings can be deposited uniformly; therefore, they allow worn components to be brought to final dimensions in the process of aircraft repair.This research shows an effective procedure of the application of plasma spray coatings on the parts of the Astazou III B turbo -jet engine in the process of repair.The engine manufacturer,Turbomeca, has prescribed that powders should be deposited by plasma spray systems under designation Metco 3M and 7M for the prescribed parameters of powder deposition, so that during the application of other plasma spray depositing systems the parameters must be tested and optimized.The aim was to apply the Plasmadyne plasma spray system during the repair process and to optimize the parameters, which will enable producing coatings that fulfill all the criteria prescribed in the engine manufacturer standard.The optimization of the parameters was carried out with a plasma gun MINI -GUN II with a large number of samples.This paper presents the optimal parameters of the deposition on the ASTAZOU III B engine casing, casing frame, duct and oil tank.The assessment of the coating

Introduction
The development of jet engines and the demands for increased resistance to oxidation, hot corrosion and sulphuring of engine parts influenced the development of the thermal spray process and nickelbased powders.For the protection of parts of jet engines, NiAl, NiCr, NiCrAl, NiCrAlY, CoCrAlY, NiCoCrAlY, etc. plasma spray coatings are commonly used today.The most effective protection of substrates from oxidation at temperatures above 800°C is provided by coatings which form oxides of the α-Al 2 O 3 and Cr 2 O 3 type.In most cases, coatings forming a continuous layer of α-Al 2 O 3 are applied since this type of oxide is superior and more reliable as compared to other types of oxides (Mrdak, 2012, pp.182-201).At the beginning of the oxidation, NiO, α-Al 2 O 3 and Cr 2 O 3 oxide types are rapidly formed as well as spinel phases.The relative ratio of these phases is determined by the initial composition of the alloy.As oxidation continues, the diffusion processes are beginning to show their effects.The nature of these effects depends on the content of the chemical elements in the coating and the diffusion parameters.When the coating has a low content of chromium and aluminum, protective continuous α-Al 2 O 3 and Cr 2 O 3 oxide layers cannot be formed on the coating surface; instead, undesirable continuous NiO oxide layers are formed.The mechanism of the NiO oxide growth causes the formation of micro pores in the oxide / alloy interlayer.Micro pores grow and merge into large macro pores.The mechanism of the NiO oxide growth creates significant stress which eventually leads to cracks in the oxide layer.The coefficient of the thermal expansion of NiO oxide and that of metal vary considerably.NiO oxide is subjected to tensile stresses as a metal base, so that the elastic deformation of the metal substrate causes breakage and peeling of the oxide layer on the coating surface (Mrdak, 2012, pp.182-201).In order to build up continuous α-Al 2 O 3 and Cr 2 O 3 oxide layers on the coating surface, a minimum of 20%Cr and 5%Al should be used for nickel alloys.NiCrAl alloy is added as well as yttrium for better cohesive oxide strength and better adhesive strength of the oxide coating on the substrate.Depending on the alloy type, the content of yttrium in the alloy ranges from 0.1 to 0.5% (Mrdak, 2012, pp.182-201).In exploitation, coatings are often exposed to the influence of impurities in the fuel and air.Depending on gas impurity, coatings can be exposed to a greater or lesser influence of Na, S and V.At high temperatures, diffusion processes occur at the interface between the coating and the gaseous environment, accelerating deposit corrosion.As far as air impurities are concerned, salt sucked by a turbojet engine is in the first place.Salt has the greatest impact on the corrosion of the parts of the turbojet engine that runs on distilled fuel without vanadium content.Salt sucked into the engine reacts with sulfur in the fuel to form sodium sulfate.In gas turbines that operate in the medium where chlorine is present, sodium chloride can also occur.This concerns air vehicles with a gas turbine developing a temperature at the turbine exit of about 750 °C, stationed on aircraft carriers or in coastal areas.Vanadium can also occur as impurity originating from fuel combustion.During fuel combustion, ash with a low melting point is created and deposited on the gas turbine components.Sulfur in fuel reacts with chromium from the alloy, thus forming chromium sulfate which precipitates on grain boundaries.During oxidation, chromium bonds with oxygen, simultaneously releasing sulfur that diffuses into the depth of the surface layer.In this way, new sulfides are formed beneath chromium oxide.Sulphur never goes into the atmosphere, but still diffuses through the surface layer, causing hot corrosion (Mrdak, 2012, pp.182-201).The experience of Turbomeca company which, in the production of the Astazou III B engine, applies plasma spray coatings resistant to oxidation and hot corrosion, as well as coatings for the repair of parts made of Al alloys, enabled the usage of plasma spray technology in the process of engine overhaul.The engine manufacturer prescribes that powder is to be deposited by plasma spray systems labeled Metco 3M and 7M for the prescribed parameters of powder deposition; therefore, the parameters must be optimized when applying other plasma spray depositing systems in order to meet all the criteria set by the Turbomeca standard.For saving and repairing engine parts from oxidation and hot corrosion, the manufacturer of the Astazou III B engine uses Ni/5Al, NiCr/6Al and Ni22Cr10Al1Y powders, and, for recovery of dimensions and repair of parts from aluminum alloys, it uses Al12Si powder.Composite Ni/5Al powder, due to its exothermic reaction during deposition, provides good bonding of the coating to the substrate.The products of this reaction are intermetallic compounds NiAl 3 , Ni 2 Al 3 and NiAl which add to the strength of the coating.These are thick coatings with metallurgical bond at the interface with the base material.The coating consists of lamellae of a solid solution of aluminum in nickel α-Ni (Al), and inter-lamellar oxides NiO and γ-Al 2 O 3 uniformly distributed over the boundaries of solid solution lamellae (Knotek, et al., 1980, pp.282-286), (Mrdak, 2015, pp.32-55), (Mrdak, 2013, pp.7-22), (Svantesson, Wigren, 1992, pp.65-69).Coatings are resistant to oxidation, gas corrosion, wear, abrasion and erosion at temperatures up to 980°C.Bond strength with the substrate remains adequate to 700°C (Griffiths, et al., 1980).Coatings deposited in accordance with the Turbomeca standard have values of microhardness of min.140HV0.3 and bond tensile strength of min.35MPa.NiCrAl types of coatings in a deposited state consist of a solid solution of chromium and aluminum in nickel γ-Ni (Cr,Al).NiO, α-Al 2 O 3 , Cr 2 O 3 , and CrO 3 oxide types are present in layers as well as Ni(Cr,Al 2 )O 4 spinel phases (Badrour, et al., 1986(Badrour, et al., , p.1217)), (Brossard, et al., 2009, pp.1-9), (Mrdak, 2010, pp.5-16), (Mrdak, 2012, pp.182-201), (Mrdak, 2013, pp.7-22), (Tran, et al., 2008, p.701).Tensile bond strength of the coating stays adequate to the operating temperature of 980°C (Mrdak, 2012, pp.182-201).Coatings deposited by the Turbomeca standard have values of microhardness of min.170HV0.3 and tensile bond strength of min.35MPa.NiCrAlY alloy is used to protect parts from hot corrosion and high temperature oxidation up to 1100°C (Material Product Data Sheet, 2013, Nickel Chromium Aluminum Yttrium (NiCrAlY) Thermal Spray Powders Amdry 963, DSMTS-0102.1, Sulzer Metco).Addition of yttrium is essential because it significantly increases the adhesion of Al 2 O 3 and Cr 2 O 3 oxides that are formed in the coating with the coating base, thus preventing cracking and separation of the protective surface oxide layer at thermal fatigue (Mrdak, 2012, pp.182-201).The structure of the inner layers of the coating consists of a solid solution of chromium and aluminum in nickel γ-Ni(Cr,Al) and the intermetallic compound γ'-Ni 3 Al.NiO, α-Al 2 O 3 , Cr 2 O 3 and NiCr 2 O 3 oxides are also present in the structure (Badrour, et al., 1986, p.1217), (Leea, 2005, pp.239-242).Coatings deposited by the Turbomeca standard have microhardness values of min.200HV 0.3 and tensile bond strength of min.35 MPa.Al12Si coating is of a general purpose and is applied for the protection of new aviation parts and in the repair process to restore dimensions of aluminum and magnesium alloy parts changed due to wear (Material Product Data Sheet, 2011, Aluminum 12% Silicon Thermal Spray Powders Metco 52C-NS, DSMTS -0045.2,SulzerMetco), (Pramila Bai, Biswas, 1987, p.61).In the deposited state, the coating microstructure consists of two phases: α-Al solid solution and α-Al + Si eutectic mixture.Fine eutectic grains of α-Al + Si are uniformly formed on the boundaries of the α-Al solid solution (Laha et al. 2005, pp.5429-5438).Coatings deposited by the Turbomeca standard have microhardness values of min.70HV0.3 and tensile bond strength of min.25 MPa.For all coatings, the allowed share of micro pores in the microstructure is max.8% and that of unfused particles is up to 15% of a particle size below 60μm (Turbojet enginestandard practices Manuel, Turbomeca).
The aim of the research was to apply the plasma spray system of the Plasmadyne company in repair of the Astazou III B engine and to optimize the powder deposition parameters, in order to produce coatings that will fulfill all the criteria prescribed in the standard of the engine manufacturer.The optimization of the parameters for a MINI -GUN II plasma gun was performed on fixed samples in a special tool.A large number of samples was made to obtain the microstructures and mechanical properties of coatings that will fulfill all the criteria prescribed by the Turbomeca standard.This paper presents the optimum parameters with which coatings are deposited on turbine casing, casing frame, duct and oil tank as well as the mechanical and structural characteristics of the coatings tested on the Astazou III B turbojet engine on the test stand.The performed tests have confirmed the quality of the coatings thus allowing the application of plasma spray technology in the Astazou III B engine overhaul.

Materials and experimental details
For testing and applying coatings on the parts of the Astazou III B turbo-jet engine, four types of Sulzer Metco powders were used: Metco 450NS, Metco 443NS, Amdry 963 and Metco 52C-NS.Metco 450NS powder (Ni/5Al) based on Ni is intended to protect the turbine casing from the influence of high temperature, hot corrosion and erosion.The powder Ni/5Al particles coated with the Ni content of 95.5% and the Al content of 4.5% had a distribution of the granulate of 45-88 μm (Metco 450NS Nickel/Aluminum Composite Powder, 2000, Technical Bulletin 10-136, Sulzer Metco).For the protection of the turbine casing frame from the impact of sand at lower temperatures, Metco 443NS powder (Ni19Cr/6Al) containing 19% Cr and 6% Al was applied.The powder had a grain range of 45-120 μm (Metco 443NS Nickel-Chromium/Aluminum Composite Powder, 2000, Technical Bulletin 10-130, Sulzer Metco).To produce a coating resistant to high temperature oxidation and hot corrosion up to 1200 °C, applied to the duct, Ni22Cr10Al1Y powder alloy with a range of granulation of powder particles of 53-106 μm was used (Material Product Data Sheet, 2013, Nickel Chromium Aluminum Yttrium (NiCrAlY) Thermal Spray Powders Amdry 963, DSMTS-0102.1, Sulzer Metco).To restore the size of the opening in the Astazou III B engine oil tank, Metco 52C-NS powder was applied, which is aluminum alloy with 12% Si.The granulation of the powder particles was from 45-90 μm (Material Product Data Sheet, 2011, Aluminum 12% Silicon Thermal Spray Powders Metco 52C-NS, DSMTS -0045.2,SulzerMetco).
The investigation of the structural and mechanical characteristics of the coatings was done in accordance with the Turbomeca standard (Turbojet engine-standard practices manuel, TURBOMECA).The substrate material of the samples where Ni5Al, Ni19Cr6Al and Ni22Cr10Al1Y coating layers were deposited was stainless steel X15Cr13 (EN 1.4024) in the thermally unprocessed condition.The substrates of the samples where Al12Si coatings were deposited were made of AMS4117 aluminum alloy (AlMg1 EN5005).For microhardness testing and evaluation of the microstructure of the deposited state, 70x20x1.5 mm samples were made.The bases for examining tensile bond strength were Ø25x50mm.The investigation of the microhardness of coatings was done using the HV 0.3 method.In order to assess the homogeneity of the coating layers, the microhardness measurement was carried out in a direction along the lamellae.Five readings of microhardness values were performed, in the middle and at the ends of the samples, out of which the two extreme values were rejected.The minimum and maximum values of the three remaining values are presented.Tensile bond strength was examined using the tensile test.
The tests were performed at room temperature at a tensile speed of 10 mm / min on the hydraulic equipment.Every part of the Astazou III B engine was tested by five specimens.The engine parts samples were rotated at the same rotational speed to ensure the same conditions of coating deposition.The obtained results were averaged and the paper presents the average tensile bond strength values.
The microstructure of the deposited coating layers was examined on an optical microscope -OM.The analysis of the micro pores share in the coating was performed by treating 5 photos at 200X magnification.Through tracing paper, micro pores were labeled and shaded, with a total area of micropores calculated for the total surface of micrographs.The paper presents the mean values of the micropores share in the coatings.Table 1 shows the parts of the Astazou III B turbojet engine, the types of materials used for its parts and the operating conditions for the oparating parts on which coatings were deposited.All Astazou III B engine parts are made of special purpose aircraft materials.The oil tank is made of AG5 -EN AW-5083 aluminum alloy, the casing frame and the turbine casing of 15CDV6 -EN 1.7734 stainless steel, and the duct of AFNOR Z3NCT25 -ASTM A638 nickel alloy.Turbomeca, engine manufacturer, prescribed that on the Astazou III B engine parts powders are to be deposited with Metco 3M and 7M equipment for the prescribed parameters of powder deposition and the standards on the quality of deposited coatings.Powder deposition parameters were optimised for an atmospheric plasma spray system of the Plasmadayne company that uses a specially designed plasma spray gun MINI -GUN II with the dimensions of Ø25 X 600 mm.A large number of samples were used and the paper shows the optimal parameters with which coatings were deposited on the Astazou III B turbojet engine parts tested on the test stand.
Powder was deposited on the samples and the parts under the same conditions in specially designed and manufactured tools.Coatings were deposited on the preheated rough samples and engine parts at a temperature of 90-120 °C.The MINI -GUN II plasma gun consisted of: anode A 2084-F45, cathode K 1083-129 and gas injector GI 2084 B -103.The coating deposition was performed with the power supply of 40KW.All coatings were deposited with a plasma gas mixture of Ar-He.The layer thickness of NiAl, NiCrAl and NiCrAlY coatings with a single plasma gun pass was 25μm.The thickness of the Al12Si alloy layer with a single pass of the plasma gun was 30 um.
Figure 1 shows the APS -atmospheric plasma spray system of the Plasmadyne company used to produce coatings.The figure shows the process of powder deposition with a MINI GUN II plasma gun on the Astazou III B turbine engine in a cabin protecting from ionic radiation and noise.The deposition process is performed with a RISE robot.Table 2 shows the plasma spray parameters for depositing powders with a MINI -GUN II plasma gun.The thickness of the deposited Ni5Al coating on the turbine casing and the Ni19Cr6Al coating on the casing frame was from 0.55 to 0.6 mm.The coating thickness was increased by 0.3 mm for extra machining.The Ni22Cr10Al1Y coating thickness on the edges of the duct ranged from 1.2 -1.5 mm.It was increased by 0.3 mm for coating machining.At the opening of the oil tank, the Al12Si coating was deposited with a thickness from 0.54 to 0.6 mm with additional thickness for machining.
The investigation of the effect of the deposited coatings on the parts of the ASTAZUO III B turbojet engine was done at the test stand with the engine operation time of 42 hours.The wear of the coatings was determined on the basis of the change in the dimensions of machined surfaces after testing the engine parts.The change in dimensions was measured on a coordinate measuring machine MAUSER ML 28 at eight measuring points around the perimeter of cylindrical parts.This paper presents the mean values of coating wear in mm, compared with the values of allowed tolerances of machined parts.

Results and discussion
Figure 2 shows the turbine casing of the Astazou III B turbojet engine and the microstructure of the deposited Ni5Al coating.Red lines on the casing mark the inner surface protected by the plasma spray Ni5Al coating from hot corrosion and erosion caused by particles carried by gas.The microstructure of the Ni5Al coating is lamellar.The light blue lamellae of the coating consist of the α solid solution of aluminum in nickel α-Ni (Al).At the inter-lamellar boundaries of the α solid solution, there are evenly distributed nickel oxide NiO and aluminum γ-Al 2 O 3 marked with red arrows (Knotek, et al.,1980, pp.282-286), (Mrdak, 2013, pp.7-22), (Svantesson, Wigren, 1992, pp.65-69).Between the lamellae boundaries of the solid solution and oxide lamellae, there are irregularly shaped dark blue inter-lamellar pores.There are also spherical precipitates of a size of 18 to 25μm, which are always smaller than the granulation of deposited powders.The precipitates did not affect the mechanical properties of the coating.The layers of the deposited Ni5Al coating had the microhardness values of 155 -179HV 0.3 .The mean value of the tensile bond strength of the coating was 72MPa.The mechanism of destruction was that of adhesion on the substrate / coating boundary.The values of the microhardness and tensile bond strength of Ni5Al coating are above the minimum values prescribed by the Turbomeca standard (min.140HV 0.3 and min.35MPa) (Turbojet engine-standard practices manuel, TURBOMECA).The analysis of photomicrographs of Ni5Al coatings showed that the proportion of pores was 2.5%.The content of pores was significantly lower than the value set by the engine manufacturer Turbomeca (max.8%pores).In the microstructure, there were no unfused powder particles of 45-60 μm, whose presence is allowed in a content of up to 15% by the Turbomeca standard (Turbojet engine-standard practices manuel, TURBOMECA).The microhardness values of the coating were in the range of 278-315 HV 0.3 .The distribution of microhardness was directly related to the distribution of oxides and pores in the coating layers.The mean value of tensile bond strength of the coating was 52MPa.The character of destruction was adhesion.The structure of the coating layers is lamellar.The coating base consists of light blue lamellae of the solid solution of chromium and aluminum in nickel γ-Ni.At solid solution lamellae boundaries, there are the lamellae of oxides NiO, α-Al 2 O 3 , NiCr 2 O 3 and a small amount of Cr 2 O 3 marked with red arrows (Brossard, et al., 2009, pp.1-9), (Mrdak, 2012, pp.5-16), (Mrdak, 2012, pp.182-201).Between the boundaries of solid solution lamellae and oxide lamellae there are inter lamellar pores in dark blue.The analysis of photomicrographs showed that the Ni19Cr6Al coating layers had a share of micro pores of 3.5%.The analysis of the coating microstructure showed that the coating microstructure did not contain unfused powder particles whose presence is permitted by the Turbomeca standard in the amount up to 15% and of size under 60 μm (Turbojet engine-standard practices manuel, TURBOMECA). Figure 4 shows the duct of the Astazou III B turbojet engine and the microstructure of the deposited Ni22Cr10Al1Y coating.TURBOMECA).The analysis of the micrographs of the Ni22Cr10Al1Y coating showed that the pore share was about 3%.The content of micro pores was lower than the value set by the engine manufacturer Turbomeca (max.8% pores).Unfused powder particles up to 60μm, whose presence is allowed in the content up to 15% by the Turbomeca standard (Turbojet engine-standard practices manuel, TURBOMECA), were not found in the microstructure.Figure 5 shows the oil tank of the Astazou III B turbojet engine and the microstructure of the deposited Al12Si coating.The hole in the oil tank is marked with a red circle, the inner surface of which is protected by the plasma sprayed Al12Si coating against the effects of synthetic oils and wear.The microstructure of the Al12Si coating consists of two phases, the α-Al solid solution and the α-Al + Si eutectic mixture.At the boundaries of the α-Al solid solution, dendritic solidification resulted in α-Al + Si eutectic grains (Laha et al. 2005, pp.5429-5438), (Pramila Bai, Biswas, 1987, p.61).The content of pores in the coating was negligible, which is why the coating microhardness value was at the upper limit of 130 HV 0.3 .The mean value of tensile bond strength of 27MPa was in accordance with the coating microstructure.The mechanism of destruction was adhesion at the substrate / coating boundary.The values of microhardness and tensile bond strength of the Ni12Si coating are above the minimum value prescribed by the Turbomeca standard (min.70HV0.3 and min.25 MPa) (Turbojet engine-standard practices Manuel, Turbomeca).In the microstructure there are no unfused powder particles, although the Turbomeca standard allows their presence up to 15%, with a size below 60μm (Turbojet engine-standard practices Manuel, Turbomeca).
After the tests at the test station, the wear of the coatings was significantly lower than the allowable tolerance for engine parts.The Ni5Al coating wear on the turbine casing of 0.002 mm is significantly lower than the allowable tolerance of 0.3 mm.The Ni19Cr6Al coating wear on the casing frame was 0.0025 mm, while the allowed dimension tolerance for the casing frame is 0.3 mm.The Ni22Cr10Al1Y coating wear on the duct ridges was 0.001 mm, while the tolerance for the Ni22Cr10Al1Y coating on the duct ridges is 0.05 mm.At the opening of the oil tank, there were no changes in the size of the Al12Si coating, which is understandable because the coating is subjected to wear during opening and closing of the the tank when changing oil.The wear of the coatings on all tested parts was low.Based on the test results, plasma spray coatings have been successfully applied in the process of the general repair of the Astazou III B turbojet engine.

Figure 2 -
Figure 2 -Turbine casing of the ASTAZOU III B turbojet engine and the microstructure of the Ni5Al coating Рис. 2 -Корпус турбины турбореактивного двигателя ASTAZOU III B и микроструктура покрытия Ni5Al Slika 2 -Kućište turbine turbomlaznog motora ASTAZOU III B i mikrostruktura prevlake Ni5Al Figure 3 shows the casing frame of the Astazou III B engine and the microstructure of the deposited Ni19Cr6Al coating.The inner surface of the casing frame marked with red lines has the deposited Ni19Cr6Al coating which protects the surface from abrasion of sand particles up to 200°C.Coating layers are deposited uniformly on the inner surface, with the coating mechanical properties and its microstructure showing the quality better than that prescribed by the Turbomeca standard.The values of microhardness and tensile bond strength in the Turbomeca standard are min.170HV0.3 and 35MPa (Turbojet engine -standard practices manuel, TURBOMECA).

Table 2 -
Plasma spray parameters