STUDY OF THE CHARACTERISTICS OF PLASMA SPRAY SEALING ALUMINUM-SILICON-POLYESTER COATINGS

This study shows the homologation of the plasma spray parameters of soft abrasive AlSi – Polyester seals so that they can be applied on the TV2 117A compressor engines. The research has aimed at substituting existing sealants with a new class of materials in order to increase the sealing effect under the highest levels of pressure and to provide the air flow temperature of 100-125°C through the compressor. The Metco 601NS material and plasma spray technology were applied on the air labyrinth ring as a part of the TV2-117A turbojet engine compressor in order to obtain soft sealing. The deposit parameters were carefully selected in order to obtain coatings with the best characteristics depending on their application.The flow of helium was taken as a basic parameter in the parameter selection procedure. The coating with the best mechanical and structural properties was deposited on the air labyrinth ring to examine the effect of the coating application in an assembly. The microstructures of deposited layers were estimated with a light microscope and a (SEM) Scanning Electron Microscope. The microstructural analysis of deposited layers was performed according to the Pratt Whitney standard. The assessment of the mechanical properties of the coatings was done by examining the macrohardness of the sealing layers with the HR15Y method. The coating bond strength was tested by tensile testing. The effect of the air labyrinth ring sealing was tested inside the TV2-117A engine compressor on the test station for a period of 42 hour.


Introduction
he development of new generations of turbo-jet engines has led to the development of new technologies and sealing materials.Nowadays, soft wear coatings, resistant to erosion and resulting in low-energy friction conditions, are applied on low and high pressure compressor components.During exploitation, soft-sealing coatings are exposed to wear instead of causing the wear of blades or labyrinth seams.Improved sealing between rotating and stationary parts in aircraft gas turbines significantly increases engine performances by improving thermal efficiency.This objective has been achieved by applying soft and hard seals in turbo-jet engines.Blades in low and high pressure compressors and gas turbines act on soft seals as lathe knives, thus keeping the gap between the rotor and the coating at the minimum.These coatings are mostly multiphase materials produced by thermal spray technology procedures.Powders for the production of soft wear coatings consist of a metal base and an additional phase which controls the amount of porosity in layers.Sealing efficiency requires a combination of properties usually optimized by a composition and a phase ratio in the mixture.Development of turbo jet engines has resulted in a growing demand for improved efficiency and greater sealing power of engines.This has led to extensive efforts to improve the performance of various components in turbo-jet engines, industrial gas turbines, compressors and generators.One of the critical areas is the clearance between the blade tip and a casing which should be reduced to a minimum in order to minimize leakage between the engine components [1][2][3].In recent years, the application of modern seals on turbo-jet engines has significantly improved engine performances and sealability.Soft-wear coatings have been developed for various locations in turbines [4].The plasma spray process is the most popular technological procedure for the productin of soft seals [4].During operation, very complex mechanisms occur during the consumption of coating materials.This includes scraping sealant materials by blade tips, their smearing and sticking to blades, crushing, melting, erosion and oxidation [5].During operation, sealing coatings exposed to wear must not be damaged and after being scrapped by blades they must remain smooth.Good coating wear in exploitation involves appropriate coating softness as well as sufficient coating hardness in order to resist to gas erosion due to high gas velocity and particles present in the gas.Resistance to oxidation and corrosion and resistance to thermal shock are also the key criteria which coatings exposed to wear must satisfy in engine systems.In order to meet all these requirements and allow the control of leakage between components, coatings are often composed of two or more components.Sealing coating consists of a metal base and the second phase as a solid lubricant to control wear and porosity [6].Polyester is often used as a means to control porosity and as a solid lubricant surrounded by and enclosed in the metal base.This helps to reduce the transfer of the coating onto the blades, to reduce chipping and to prevent coating erosion.
Sealing materials used in gas turbine compressors and turbo-jet engines are subjected to a wide temperature range, from the lowest to the hig-hest engine operating temperatures.Among the most successful materials for sealing low and medium pressure compressors at relatively low temperatures is AlSi-polyester wear coating.The development of compositions and structures of soft sealing materials is mainly based on studies of tribological properties of coatings [7][8][9]10], which allow to determine under what conditions coatings have the best wear characteristics [11,12].This information allows an appropriate choice of a coating and a prediction of its behavior in exploitation [13].Soft sealing coatings are suitable for eccentric assemblies and they compensate for tolerance changes caused by machining.Coatings with gaps adjustable to the minimum value improve engine performances [14].
The Metco 601NS powder is a mixture of AlSi alloys and polyester which has been specially developed to control the clearance of mechanical part elements.Tests on aircraft engines have confirmed that Metco 601NS coatings have high wear resistance without causing the compressor blade tip wear, excellent resistance to oxidation up to 345°C and good resistance to thermal shock.The powder is designed to be deposited by the atmospheric plasma spray process (APS) using argon / hydrogen or argon / helium as plasma gases.The Metco 601NS powder is certified as a product that meets the requirements of reputable firms such as Pratt & Whitney specification PWA1349 and Rolls Royce MSRR 9507/15, etc.The powder is recommended for sealing compressor parts where air of a certain temperature flows through and it serves as a replacement for black rubber.It can be applied to internal and external surfaces as a seal on low and high pressure compressors.Depending on the plasma spray equipment, the Metco 601NS powder produces coatings of macro hardness HR15y: 73 ± 5, 70 ± 10 and 70 ± 5. Generally, the coating macro hardness is within the limits of 60-80 HR15y.The adhesion of coatings ranges from 6.9 to 14 Mpa, depending on the depositing parameters.Due to low bond strength values, sealing coatings are always deposited in a dual system with nickel-aluminide coating (Metco 450NS powder).Coatings contain tiny pores with a share of 3% to 5% depending on the type of equipment and plasma depositing parameters.The coatings paired with tempered steel have the coefficient of friction of 0.47 [15].
The aim of this study was to optimize the parameters of plasma spray depositing AlSi -polyester powder.The research and results achieved in the world in this area were the starting point for us to homologate and apply, for the first time, the material produced in Western countries on the air labyrinth ring of the TV2-117A turbojet engine compressor produced in the East.The aim of this study was to examine the effect of sealing gaskets by plasma spray technology and a new material and to substitute existing materials.A new generation of materials was expected to provide better sealing for higher compressor efficiency with lower fuel consumption fuel.The expected effects were confirmed after 42-hour engine testing on a test stand.

Details of the experiment
The Metco 601NS powder, Sulzer Metco, was used for the experiments.This powder is a mixture of polyester and AlSi alloy (AlSi 60% -40% polyester) with a melting temperature of 425°C and the range of grain size of powder particles from 10 microns to 106 microns [15].The deposited AlSipolyester powder bonds well with the base.The powder manufacturer prescribes that the powder be used with its Metco (3M, 7M, 9M and 10M) plasma spray systems with prescribed parameters of powder deposition; therefore, if other plasma spray systems are used, the parameters must be optimized and tested.Due to the powder low melting temperature, helium was applied since the Ar/He combination gives a shorter and denser plasma jet of lower specific entalphy as compared to Ar / H 2 .This allows less powder component evaporation and combustion.Also, denser plasma such as Ar-He pumps in the ambient air to a lower extent, thus avoiding powder oxidation reduction and the powder deposits with higher density packaging in the coating [16].Due to the low melting temperature, powder coatings were also deposited at a lower intensity of electrical current and a plasma gas flow variable.The experiment used the SG -100 Plasmadyne plasma gun with appropriate robotic control spray conditions.The plasma gun consists of: K 1083 -129 cathode, A 2083 -129 anode and GI 2083-130gas injector.In selecting the depositing parameters, the helium plasma gas flow was taken as the basic parameter.The helium gas flow must be optimal to ensure complete melting of powder particles and thereby minimize the evaporation of powder components.On the other hand, plasma gas flow rate should provide a uniform deposit of powder particles to provide continuous deposition of layers in the form of AlSi mesh that will take soften and molten polyester particles into it thus providing a uniform and homogeneous structure of the deposited layers throughout the coating cross-section.In this study, during the deposition of AlSi-polyester powder particles, two He plasma gas flows of 6 l / min and 12 l / min were used while other parameters were of constant values.The list of the used parameters is shown in Table 1.Three groups of coatings were deposited.The first group of coatings was deposited with the He flow of 6 l / min and 12 l / min with a thickness of 0.5 mm to determine the effect of He flow on the bond macro hardness and strength.The second group of coatings was deposited with the He flow of 12 l / min with a layer thickness of 0.10, 0.20 and 0.30 mm to determine the influence of the coating thickness on the bond strength.In the third group, coatings with thicknesses from 1.6 to 2.2 mm prescribed for the air labyrinth ring were deposited to examine the layer microstructure.In order to analyze the fracture morphology, the coating with the best structural and mechanical properties was fractured.
The substrate material on which AlSi -polyester layers were deposited was X15Cr13 steel (AMS 5504).Before deposition, the substrate was not preheated and the surface was roughened with white electrocorundum of the grain size from 0.7 to 1.5 mm.The roughening aimed to remove the thin oxide film from the substrate surface in order to obtain higher bond strength of the coating substrate.The testing and characterization of the coatings were done according to the Pratt & Whitney standard [17].
Macro hardness was measured using the Rockwell steel balls of 12.7 mm in diameter with a weight of 15 kg (HR15y).The coating macro hardness was measured along the layers.In order to evaluate the homogeneity of the layers, the measurement was carried out in three areas: in the middle, on the left side and on the right side of the samples.The samples for the macro hardness measurement as well as for the microstructure analysis had the dimensions of 70 × 20 × 1.5 mm.The shown macro hardness results are the average of multiple measurements.
The bond strength between the coating and the base was determined by tensile testing.Two samples were paired, with dimensions of Ø25 × 50 mm, and the coating was deposited on only one of them.The samples were glued together and left to bind, fixed in an appropriate tool.Adapters, used in the testing, were constructed to ensure the elimination of shear forces.The tests were performed on a universal hydraulic tensile testing machine at room temperature at a constant speed of 1 mm / min [17].Three pairs of samples were used for each group of coatings, and the obtained values were averaged.
The microstructure of coatings and the quality of the coating bond and the base were analyzed using a light microscope.The morphology of the fracture of the coating with the best characteristics was examined on the Scanning Electron Microscope (SEM).

Results and discussion
Table 2 shows the obtained values for the macro hardness and the bond strength of the AlSi-polyester coating for the layer thicknesses of 0.5 mm, depending on the helium flow.The values of AlSi -polyester layer macro hard-ness are directly related to the helium flow.The layers deposited with the helium flow of 6 l / min showed lower values of macro hardness: 72HRy.The coatings deposited with the helium flow of 12 l / min showed the macro hardness value of 80HRy.The average difference of macro hardness values between the layers was HR15y.The average macro hardness values for both coatings were within the prescribed limits [15].The helium flow influenced the density of the layers and the ratio of AlSi/polyester deposited in coatings to a lower extent.The layers deposited with a higher helium flow were thicker as confirmed by the metallographic examination of the specimens.Also, higher values of the macro hardness of the AlSi-polyester layers deposited with a greater helium flow show a higher cohesive strength of the layers, as confirmed by the results of the tensile strength testing.The values of the bond strength of the AlSi -polyester layers were directly related to the helium flow.The coatings deposited with the higher throughput of helium had a higher tensile strength of bonds -13MPa, and the layers deposited with a lower helium flow rate had the tensile bond strength of 10MPa.During the tests, fractures occurred along the coating/supstrat interface in all samples.This meant that the cohesive strength of the AlSipolyester coating was good.The values of tensile bond strength were within the prescribed limits for both coatings [15].For the second group of coatings deposited with a layer thickness of 0.10, 0.20 and 0.30 mm and a helium flow of 12 l / min, the values of the tensile bond strength (Fig. 1) showed a great influence of the coating thickness on the obtained values for the bond strength.The layers of thickness at 0.1 mm showed the highest value of the tensile bond strength of 29MPa as expected.During the depositing procedure, the molten powder particles input higher voltage to the layers with the increase of the coating thickness due to a larger temperature difference between the upper layers that were being deposited and the lower layers which had been already deposited.Due to a higher proportion of residual stresses in thick coatings, tensile strength was lower.The layers with a thickness of 0.2 mm had a strength of 23MPa, and the layers with a thickness of 0.3 mm had a bond strength of 20MPa.All the layers had the tensile bond strength higher than the prescribed values [15].Another parameter resulting in high values of tensile bond strength for all the coatings is a good preparation of the surface substrate, which was confirmed by a metalographic examination of the interface between the surface substrate and the coating.The microstructures of the AlSi-polyester layers deposited with a helium flow rate of 6 l / min are shown in Figs. 2 and 3.Both photomicrographs show a uniform mesh structure of the AlSi -polyester layers.The boundaries at the interface between the substrate and the AlSi layers -polyester coatings are extremely clean, which indicates a good preparation of the surface substrate.The boundary of the AlSi alloy layer bond with the substrate cannot be seen in the photomicrographs, thus indicating a very good bond of the coating and the substrate, which is consistent with the values of tensile bond strentgh.Several places on the interface show a minor part of polyester since it is surrounded by AlSi alloy lamellae in the process of deposition.A good preparation of the surface of the substrate resulted in the absence of remains of electocorund of roughening particles in the interface, which had a good adhesion of the coating with the substrate as a result.For this reason, ther are no micro cracks and macro cracks in the interface and there is no separation of the coating layers from the substrate and no coating peeling.Generally, all the layers are uniformly deposited on the substrate with a mesh structure.In the photomicrographs, the white AlSi alloy mesh is marked with black arrows.Unmelted powder particles are not present in the microstructure.In the deposited state, the AlSi powder particles are well melted and well-bonded thus making a continuous mesh of the base coating.The continuous mesh of the AlSi base is an indicator that the powder particles melted evenly and deposi- .The extracted particles of AlSi and polyester are not seen in the microstructure.Micro cracks and rough pores cannot be seen through the coating layers, which is consistent with the mechanical properties of the layers.Fig. 3 shows the cross section of the coating with a total thickness of 1.6 mm.Figs. 4 and 5 show the microstructure of the deposited layers with a helium flow of 12 l / min, which showed the best structural and mechanical characteristics.Fig. 4 shows the cross section of the coating with a total thickness of 2.2 mm.The qualitative analysis revealed that the AlSi -polyester layers along the intersection of the coating are evenly and continuously deposited on the substrate (Fig. 4).The meshlike structure of the coating has a good bonding with the substrate.Due to the large flow of helium and a higher heat content of the plasma jet, the melted powder particles deformed smoothly and evenly during the collision with the substrate to form a mesh of AlSi and polyester (Fig. 5).

Coating
The layers are without the presence of unmelted particles, pores, microcracking and they are denser than the layers deposited with a helium flow rate of 6 l / min.The base structure of the coating is the AlSi alloy which, as the metallic phase of the coating, provides: strength, erosion resistance, mutual bonding of the AlSi deposited particles and their bonding with the surface substrate.The AlSi alloy mesh is white marked with black arrows.The polyester phase is black surrounded and sealed with a metal mesh which is marked with white arrows on the photomicrographs.Fig. 6 shows the fracture morphology of the AlSi-polyester coatings deposited with a helium flow of 12 l / min.The photomicrograph shows the AlSi alloy fracture in light gray marked with black arrows.The AlSi alloy fracture is tough.The microstructure does not show the deposited polyester particles due to weak bonding between the AlSi alloy and polyester particles.The cohesive strength between metal grains of the AlSi alloy and polyester molecules is low due to different types of material bonding.Poliester is non-metallic polymeric material with a molecular bond, consisting of macromolecules.

Conclusion
The examination of AlSi-polyester coatings deposited with different plasma gas flows of helium has shown that the gas flow affects the mechanical properties and the microstructure.
The layers deposited with a higher plasma gas flow had higher macro hardness values.For both coatings, the macro hardness values were within the prescribed limits.
The tensile strength of the coating bond is directly related to the thickness of the deposited layers and the plasma gas flow.The bond tensile strength decreases with the increase of coating thickness due to a higher share of residual stresses in the deposited layers.The layers deposited 100µm AlSi Alloy with a higher plasma gas flow had higher values of tensile bond strength.Both coatings were deposited with the bond strength within the prescribed limits.
The qualitative analysis of the AlSi -polyester layers, for both coatings, showed that, along the entire cross-section, the coating were evenly and continuously deposited on the substrates.The interface does not show the particles of electrocorundum from grinding, micro cracks and macro cracks, which enabled the coatings to have good adhesion to substrates.
The microstructures of both coatings showed a uniform structure of AlSi alloy and polyester.The AlSi-alloy coating base in the solid state has a continuous and uninterrupted mesh.The microstructure of the AlSi alloy is white while that of polyester is black.The polyester particles are surrounded and closed by AlSi particles thus preventing the polyester particles from falling out during exploitation.All the layers are uniformly deposited on the substrate.
The comparison of the coatings shows that the layers deposited with a higher plasma gas flow are more evenly deposited due to a higher heat content of plasma jets.The coatings are thicker due to a lower share of micro pores and more homogenous along the entire cross-section.
Based on the obtained results, it can be concluded that the plasma gas flow rate can affect the mechanical and structural characteristics of the AlSi -polyester soft seal.The application of the AlSi-polyester coating on the air labyrinth ring resulted in a higher compressor efficiency with lower fuel consumption.The expected effects have been confirmed after 42-hour engine tests on the test station.

Figure 2 -Figure 3 -
Figure 2 -Microstructure of the NiAl-polyester coatings deposited with 6 l / min He Slika 2 -Mikrostruktura NiAl-poliester prevlake deponovane sa 6 l / min He in the coating layers.Interwoven with the AlSi mesh there is the black polyester mesh marked with white arrows.The polyester particles are deposited in such a way that they are surrounded and closed by the AlSi particles that prevent the polyester particles from falling out in the exploitation.Pores in the deposited polyester particles cannot be seen in the microphotographs.The absence of visible pores indicates that the deposited layers have a lower content of pores than the value set by the powder manufacturer (3 -5%)[15]

Table 2
Vrednosti makrotvrdoće i čvrstoće spoja slojeva AlSi-poliester Values of the macro hardness and the bond strength of AlSi-polyester layers