STUDY OF THE PROPERTIES OF PLASMA DEPOSITED LAYERS OF NICKEL-CHROME-ALUMINIUM-YTTRIUM COATINGS RESISTANT TO OXIDATION AND HOT CORROSION

The aim of this study was to examine the properties of Ni22Cr10Al1Y layers in order to obtain optimal structural mechanical properties with the optimization of depositing parameters. Powder was deposited by the atmospheric plasma spray (APS) process with the current intensity of 600, 700 and 800A, with a corresponding plasma gun power supply of 22KW, 34KW and 28KW. The evaluation of the Ni22Cr10Al1Y coating layers was made on the basis of their microhardness, tensile strength and microstructure performance. The best performance was obtained in the layers deposited with 800A and the 34KW plasma gun power supply. The coating with the best characteristics was tested to oxidation in the furnace for heat treatment without a protective atmosphere at 1100°C for one hour. The examination of the morphology of Ni22Cr10Al1Y powder particles was carried out on the SEM (Scanning Electron Microscope) as well as the EDS analysis of the best layers. The microstructure of the deposited coating layers was examined with a light microscope. The microstructure analysis was performed according to the TURBOMECA standard. The mechanical properties of layers were evaluated by the method HV0.3 for microhardness and by tensile testing for bond strength. The research has shown that plasma gun power supply significantly affects the mechanical properties and microstructure of coatings that are of crucial importance for the protection of components exposed to high temperature oxidation and hot corrosion.


Introduction
eCrAlY type alloys (Me = Ni, Co and / or Fe) are most often used for protecting the parts of jet engines from the influence of high temperatures, especially from the influence of oxidation and hot corrosion.Selection and composition of materials are largely determined by optimizing the relations of physical -chemical properties of applied materials and substrates, operating temperature and a deposition method [1].For NiCrAlY alloys, yttrium is usually added in certain quantities.Yttrium addition is essential, because it significantly increases the adhesion of Al 2 O 3 and Cr 2 O 3 oxides formed in the base coating thus preventing cracking and separation of the protective surface oxide layer due to the effects of thermal fatigue [2].This prevents and minimizes the further development of high-temperature corrosion.It is especially important to ensure a uniform oxide distribution in metal-based coatings, in order to use fully the potential of these types of protective coatings.The idea to extend the life span of coatings regarding oxidation led to the development of Ni22Cr10Al1Y coatings.Coatings are commonly used in aerospace industry on the parts such as blades and other parts of gas turbines to protect them from high-temperature hot corrosion and oxidation up to 1100°C [3].Selecting a right MeCrAlY coating depends on the operating conditions of the coating and the substrate to which it is applied.A complex interaction between the working environment, the coating and the substrate makes coating design and selection difficult.Generally, it is necessary to find a compromise between the required mechanical strength, resistance to hot corrosion, oxidation and adhesion.Years of experience in modeling the behavior of coating layers, together with improved techniques of deposit, led to the development of multicomponent coatings.The parameters of depositing have a very important influence on the quality of coating layers.For atmospheric plasma spray (APS) technique, one of the important parameters is the current intensity which affects: the degree of plasma gas ionization, the plasma temperature, the temperature of melting powder particles, their rate during depositing and their sagging on the substrate.
The main chemical element of NiCrAlY coatings, designed to protect against high temperature oxidation and hot corrosion should be compatible with the substrate material to reduce the diffusion, and reduce the chemical activity of Al.Aluminum is an element that protects the coating of the Ni alloy from oxidation at 1200°C by forming the phase α -Al The Al diffusion coefficient along the α -Al 2 O 3 grain boundaries is higher than Ni and Cr (Al: 2.8 x 10 -4 m 2 / s, Cr: 6.9 x10 -11 m 2 / s, Ni: 2.53 x 10 -10 m 2 / s) [4].Chromium is used in coatings based on Ni due to its resistance to hot corrosion and oxidation up to 900°C.Increasing the Cr content in the NiCrAlY alloy reduces the critical level of Al needed to form a continuous protective oxide α -Al 2 O 3 .Yttrium improves the adhesion of oxides α -Al 2 O 3 and Cr 2 O 3 and reduces the degree of chromium oxidation.In order to form continuous protective layers of oxides of α-Al 2 O 3 and Cr 2 O 3 types on the coating surface and to prevent the formation of NiO oxides during exploitation, the lower concentration of Cr in the coating should be about 20% and Al content should be at least 5%.Depending on the purpose of NiCrAlY coatings, the content of Cr and Al in the alloy changes.For optimal protection of substrates from hot corrosion and high temperature oxidation of the coating, MeCrAlY coatings contain 18-30 wt% Cr and 5-14 wt% Al [5,6,[7][8][9][10].The α-Al 2 O 3 oxide that forms in the coating is the thermodynamically stable phase that grows very slowly when compared to other types of oxides.Besides the presence of Ni, Cr and Al oxides, there is also a spinel phase NiCr 2 O 4 which also increases the oxidation resistance [11,12,13].Chatterjee et al. [14] explained that the spinel phase provides better protection against oxidation due to the fact that spinels of mixed oxides of the general composition AB 2 O 4 (A and B are the two metal components) have much lower diffusion coefficients of cations and anions than the starting oxides of NiO and Cr 2 O 3 [14].
According to the dual diagram Ni -Cr, in the structure of the coating there is present a solid solution γ -Ni in which the substitution is Cr [15,16].In order to obtain a solid solution of Cr in Ni, the Cr content in the alloy should be in the range of 0 to 47 wt% Cr.In the structure of the coating there is the phase γ'-Ni 3 Al which is built by the binary system Ni-Al with the Ni content over 42 wt.% and with 3.5 to 14 wt.% of Al [15,16].In NiCrAY alloys with 10 wt% of Al, a phase of Al 7 Cr (Al 13 Cr 2 ) type cannot be obtained with the dual diagram Al -Cr [15,16].
Generally, MeNiCrY powder is well deposited by the atmospheric plasma spray system and coatings have good adhesion to basic materials.The structure of the plasma spray coating deposited state presents two very important phases based on Ni (γ -Ni , γ' -Ni 3 Al) and α -Al 2 O 3 oxide.The internal structure of the coating is a heterogeneous mixture of a metal base with non-melted powder particles, precipitates, micropores and oxides of nickel NiO and Cr 2 O 3 and NiCr 2 O 3 .Exposing the coating to high temperatures leads to the oxidation of Al which forms an oxide of α -Al 2 O 3 type.This oxide is created on the coating surface due to high-speed Al diffusion.Only a small amount of Al remains in the regions rich in Ni, Cr [17].At a temperature of 1200°C, NiO destabilizes in the coating and reacts with Cr 2 O 3 forming a thin layer of an NiCr 2 O 4 spinel phase which is thermodynamically more stable than NiO and Cr 2 O 3 oxides [18,19,20].A temperature of 1200°C in NiCrAlY coatings results in layers of mixed oxides of gray color.This gray area contains mixed oxides of Al  [21].
Today, MeCrAlY coatings have been produced with different depositing processes.One of the standard processes for depositing is a plasma spray procedure.It is widely used owing to its smaller restriction in size and shape as well as to lower costs.In this paper, we presented the results of the experimental research into the effect of atmospheric plasma spray parameters on the mechanical properties and microstructure of layers of Ni22Cr10Al1Y coatings.The main objective was to homologate coatings and to apply them to the conduction garland of the gas turbine of the ASTAZOU III B turbo-jet engine.Three groups were done with samples of three different plasma gun power supplies (22KW, 34KW and 28KW), with the current intensity values of 600, 700 and 800 A. The coating with the best characteristics was tested in the oxidation furnace for a heat treatment without protective atmosphere at 1100°C for one hour.We analyzed and studied the microstructure and mechanical characteristics of selected coating layers in order to chose the highest quality coating and to homologate plasma spray parameters.

Materials and details of the experiment
The material substrate is made of stainless steel X15Cr13 (EN 1.4024).The material is used in a thermally untreated state.For the production of coatings resistant to high-temperature oxidation and hot corrosion we used the AMDRY 962 powder of the company Sulzer Metco.This NiCrAlY powder is an alloy of nickel with 20 wt% Cr, 10 wt% Al and 1 wt% Y with the range of grain size of powder particles from 53 microns to 106 microns.The powder of the Ni22Cr10Al1Y alloy applies and bind well to Ni-and Fe-based substrates.It is produced by a vacuum melting technique and liquid melt atomization with inert gas.The technological process of making powder allows obtaining homogeneous spherical particles of powder.Fig. 1shows the SEM microphotography showing spherical powder particles of Ni22Cr10Al1Y.The atmospheric plasma spray (APS) was used in the experiment for the deposition of powder.For this purpose, we used a mini gun plasma spray gun of the Plasmadyne company, comprising: cathode type K: 1083 -129 A, anode type A : 2084 -F45 and of gas injector type GI : 2084 B -103.Argon gas was used in the combination with helium and electrical current from to 600.700 and 800A with the power supply of the plasma gun of 22 KW, 28 KW and 34 KW.The detailed values of the spray parameters are shown in Table 1.Before the deposition process, the surface of the substrate steel is roughened with white corundum (Al 2 O 3 ) using a particle size of 0.7 to 1.5 mm.Coatings are deposited with a thickness of 0.1 to 0.15 mm.The coating with the best characteristics was tested to oxidation in a furnace for heat treatment without a protective atmosphere at 1100°C for one hour.The mechanical and microstructural characterizations of the obtained coatings were carried out according to the TURBOMECA standard [14].For the measurement of hardness and metallographic tests, we used rectangular samples of 70 × 20 x 1.5 mm, while the tensile strength was tested on cylindrical samples of Ø25 x 50 mm.The microstructural analysis of the coatings was made with the light microscope, while the SEM (Scanning Electron Microscope) was used for the morphology and EDS analysis of powder coatings tested for oxidation at 1100°C for one hour.
Microhardness measurements were made using a Vickers diamond pyramid indenter and 300 g load (HV 0.3 ).The measurement was carried out in the direction along the lamellae, in the middle and at the ends of the sample.Three readings were conducted at three points and the results were averaged.
Tensile tests were carried out at room temperature in hydraulic equipment with a rate of 10 mm / min for all tests.The strength was calculated by dividing the breaking load by the sample cross-section.The geometry of the samples was in accordance with ASTM C633 standards.Pairs of two samples were used, but the coatings were deposited on only one of them.Samples were glued and kept under pressure in a furnace at a temperature of 180°C for 2 hours.For each group of samples three tests were done and the results were averaged.

Results and Discussion
The obtained values of microhardness and bond strength for Ni22Cr10Al1Y deposited coatings, depending on the spray parameters used, are shown in Figs. 2 and 3.For all the deposited coatings, we obtained microhardness values above the required value of 200 HV 0.3 [22,23,24].Electrical current values, directly related to the plasma gun power supply, significantly influenced the values of microhardness and the values of the layer bond strength.The highest values of microhardness of 342 HV 0.3 was found in the layers deposited on the sample C with the highest current intensity of 800A.The higher electrical current affected the higher melting and increased the rate of powder melting.This resulted in better and denser packing of particles followed by a small portion of lamellar pores and oxides.The lowest value of microhardness of 220HV 0.3 layers was deposited on the sample A and was deposited with a current intensity of 600A.The layers with the highest microhardness had the smallest share of pores.These values were confirmed for all three groups of samples by analyzing the microstructure of coatings on a light microscope.The microhardness values were in line with the share of pores in the layers of deposited coatings.Differences in hardness of the layers should be attributed to oxide share, pores and the basis-oxide share ratio.Besides the difference in the proportion of oxide phases, microstructures may vary in dispersion of oxides, which is primarily determined by the degree of oxidation and cooling rate.The microhardness values were within the expected limits.The comparison of the values of coating bond strength obtained by tensile testing showed that for all three groups of samples the obtained values were higher than 35 MPa, as required by the appropriate standard [22].The lower value of the tensile bond strength of 36MP occurred in a coating on the sample A as a consequence of lower current intensity, which resulted in a lower degree of fusion of powder particles compared to the other two layers deposited with a higher current intensity value.The best bond strength of 54MPa was found in the layers deposited with the highest amperage in the sample C.These layers had the lowest proportion of pores and coarse oxides.The tensile testing showed that in all deposited coatings the mechanism of destruction took place at the interface between the substrate and the coating phase.Since the proportion of pores, nonmelted particles and oxides directly relates to the values of microhardness and bond strength of coatings, the measured values for the coating deposited with the highest amperage in the sample C indicate that their number was minimal in this coating.These values are also confirmed by the analysis of the microstructure of coatings on a light microscope.
The microstructures of the Ni22Cr10Al1Y coating layers deposited with the current intensity of 600, 700 and 800 A are shown in Figs. 4, 5 and 6.The qualitative analysis showed that defects such as discontinuity of the deposited layers on substrates, micro cracks and macro cracks and separation of the coating from the base (Fig. 4) are not present in the interface between the substrate and the deposited coating.In addition, there was no contamination due to a roughening agent.The structure of the inner layers of all coatings is of a lamellar type, with the basis of a solid solution of chromium and aluminum in nickel -γ -Ni + γ'-Ni3Al phase.In the coating layers deposited with 600A, there were no nonmelted powder particles ranging from 53 microns to 106 microns (Fig. 4).Fig. 7 shows the SEM microstructure of the coating on the sample C tested to oxidation in a furnace for heat treatment without a protective atmosphere at 1100°C for one hour.In the Ni22Cr10Al1Y coating microstructure, there are obvious changes compared with the microstructure of the coating in the deposited state.Based upon the EDS measurements and the results shown in Fig. 8 and Table 2, some phases in the marked coating regions can be expected.In the Ni22Cr10Al1Y alloy, there has been an expected stechiometric change of the elements Al, Cr and Ni caused by diffusion at high temperature.The solid γ -Ni solution is depleted with elements such as Al and Cr (EDS 1).Only a small amount of Al remains in the regions rich in Ni and Cr [17,18,19,20].Some authors in the studies [23,24] also show that there is diffusion and oxidation of Ni from the solid γ -Ni solution.Nickel oxidizes to NiO and reacts with the already existing oxides Cr 2 O 3 which are thermodynamically unstable at 1100°C.These oxides build a stable spinel phase NiCr 2 O 4 .In Fig. 7, the EDS 2 analysis confirms that layers of dark gray mixed oxides are formed in the coating.This gray area contains mixed oxides of Al 2 O 3 type and spinels (Ni, Cr) 2 O 4 , (Cr, Al) 2 O 4 [21].

Conclusion
Three groups of samples of Ni22Cr10Al1Y coatings were deposited in the atmospheric conditions by the plasma spray procedure, using three different amperages of 800 A, 600 A and 700 A which correspond to plasma gun power supplies of 22KW, 28KW and 34KW.Mechanical and structural characteristics of the deposited layers were tested and analysed as well as the effect of oxidation on the microstructure of the deposited layers with the best characteristics.
The increase in amperage resulted in higher microhardness values.The third group of samples deposited with the amperage of 800A had the highest values of microhardness.The third group also had the highest values of the bond tensile strength, while the fracture occurred along the interface between the coating and the substrate.The tensile strength values of all three groups were higher than the minimum value required by the appropriate standard.Microhardness and tensile strength values were in correlation with their microstructures.
The structure of the deposited Ni22Cr10Al1Y coating layers is lamellar and consists of a solid solution of chromium and aluminum in nickel γ -Ni, γ' + Ni 3 Al phase, oxides α -Al 2 O 3 and Cr 2 O 3 and interlamellar micro-pores.
In the microstructure of the Ni22Cr10Al1Y coating, after temperature treatment at 1100°C for one hour, there was a change in the microstructure compared to the coating in the deposited state.Due to diffusion and oxidation of the elements at 1100°C, the structure of the primary solid solution γ -Ni is depleted of Cr and Al.Because of the thermal instability of NiO and Cr 2 O 3 oxides at 1100 0 C, the structure of the coating contains Al 2 O 3 oxides and mixed spinels (Ni, Cr) 2 O 4 , (Cr, Al) 2 O 4 .
The application of the coating with the best structural and mechanical properties during the reparation of a part of an ASTAZOU III B turbojet engine has significantly improved its efficiency and reliability.

Figure 4 -
Figure 4 -Microstructure of the NI22Cr10Al1Y coating deposited with 600A on the sample A Slika 4 -Mikrostruktura NI22Cr10Al1Y prevlake deponovane sa 600A na uzorku AIn the structure, there are extracted precipitates of a spherical shape, which are seen as black as a result of the collision fields.Precipitates are a result of the collision of melted particles with the