STRUCTURE AND PROPERTIES OF PLASMA SPRAYED APS-NI20AL COATINGS

The aim of this study is to obtain, by optimizing the deposit parameters, optimal structural and mechanical properties of Ni20Al layers to be applied on a radar station part. The powder was deposited by the process of atmospheric plasma spraying (APS), with a plasma spray (PS) distance of 80, 90 and 100 mm. The coating with the best characteristics was deposited on the hardener of the GTD 2PV8 turbo gas engine for a radar station, in order to reduce the effect of oxidation and vibration on the wear at 450 500°C. The quality of Ni20Al coating layers was assessed on the basis of their microstructure, microhardness and tensile strength. The best structure and mechanical properties were found in the layers deposited with a distance of plasma spray of 80 mm. The surfaces morphology of the deposited coatings and the fracture morphology of the best layers were examined with the SEM (scanning electron microscope). The microstructure of the deposited layers of coatings was examined by light microscopy. The microstructural analysis of the deposited layers was performed according to the PrattWhitney standard. The evaluation of the mechanical properties of the layers was done by examining microhardness with the HV0.,3 method and tensile testing was used for the bond strength. Studies have shown that the plasma spray distance significantly affects the mechanical properties and the microstructure of coatings. The effect of Ni20Al coating application has been tested on a hardener of the GTD 2PV8 turbo gas engine on a radar station.


STRUCTURE AND PROPERTIES OF PLASMA SPRAYED APS-NI20AL COATINGS
The aim of this study is to obtain, by optimizing the deposit parameters, optimal structural and mechanical properties of Ni20Al layers to be applied on a radar station part.The powder was deposited by the process of atmospheric plasma spraying (APS), with a plasma spray (PS) distance of 80, 90 and 100 mm.The coating with the best characteristics was deposited on the hardener of the GTD 2PV8 turbo gas engine for a radar station, in order to reduce the effect of oxidation and vibration on the wear at 450 -500°C.The quality of Ni20Al coating layers was assessed on the basis of their microstructure, microhardness and tensile strength.The best structure and mechanical properties were found in the layers deposited with a distance of plasma spray of 80 mm.The surfaces morphology of the deposited coatings and the fracture morphology of the best layers were examined with the SEM (scanning electron microscope).The microstructure of the deposited layers of coatings was examined by light microscopy.The microstructural analysis of the deposited layers was performed according to the Pratt-Whitney standard.The evaluation of the mechanical properties of the layers was done by examining microhardness with the HV 0.,3 method and tensile testing was used for the bond strength.Studies have shown that the plasma spray distance significantly affects the mechanical properties and the microstructure of coatings.The effect of Ni20Al coating application has been tested on a hardener of the GTD 2PV8 turbo gas engine on a radar station.

Introduction
he composite Ni20Al nickel -aluminide powder was developed for aviation industry.Its important characteristic is a good deposition by plasma spray at the atmospheric pressure APS.NiAl is an important intermetallic material used in aviation industry and other industries.This material has good resistance to oxidation, corrosion and erosion.The material can be produced as an alloy or as clad powder.In the second case, clad powder produces intermetallic compounds using various sources of heat such as plasma (Moshksar, Mirzaee, 2004), (Hashemi, et al., 2009), (Sampath, et al., 1990), (Kumar, Selvarajan, 2006).The most important feature of nickel -aluminide is high strength and toughness with excellent resistance to high temperatures.These characteristics were the basis for the development and a wider application of NiAl-based coatings.Plasma spray technology is used widely in the production of intermetallic surface layers.NiAl coatings deposited by plasma spray have found wide application in the protection of areas exposed to elevated and high temperatures (Liu, White, 1985, pp.365-371), (Cahn, 1991, pp.18-25), (Chen, et al., 1993, pp.357-363), (Liu, Sikka, 1986, pp.13-16).In particular, these coatings have a potential demand in aviation industry and other industries of high-performance (Chen, et al., 1993, pp.357-363), (Liu, Sikka, 1986, pp.13-16).Nickel -aluminide plasma spray coatings are used as bonding coatings, where their function is to minimize thermo -mechanical stresses at the substrate/coating interface, as well as to increase coating adhesion (Hiemann, 1986).The coefficient of thermal expansion of these alloys is intermediate between the thermal expansion coefficients of ceramics and metals, and therefore can reduce stress at the interface.Moreover, the exothermic reaction between Ni and Al leads to better coating adhesion.In addition, it is mainly used as a bonding layer for ceramic materials (Lee, et al., 1996, pp.3003-3009).Nickel-based coatings are used when there is a need for good wear resistance combined with oxidation or hot corrosion resistance (Rosso, Bennani, 1998, pp.524-530).The Ni20Al coating is typically used as a bonding layer to improve adhesion of the subsequently deposited top layer, as a middle layer for mitigating non-complance materials with different coefficients of thermal expansion, as a bonding layer resistant to oxidation for operating temperatures up to 650°C, as well as for the protection and revitalization of parts damaged by wear, oxidation and vibration.To obtain a functional layer, it is necessary to choose carefully a combination of plasma spraying parameters.For the control of plasma spray processes, the correlations between the parameters must be known well, as well as the individual effects of the parameters on the coating characteristics.One of important atmospheric plasma spray parameters is the dis-T tance of plasma spraying, which significantly affects the oxidation of melted particles during deposition on the surface as well as the pore size and the content.The oxide content increases with the distance increasing between the substrate and the plasma gun.In addition, with the increasing plasma spray distance, the content of pores increases, which significantly affects the mechanical properties of coatings (McPherson, Cheang, 1989).During the plasma spray process, nickel and aluminum react chemically above 660°C and form nickel -aluminide with an exothermic reaction which further transmits heat to the melted particles and to the surface.The products of this reaction are formed intermetallic compounds Ni 3 Al, Ni 2 Al 3 and NiAl which additionally increase the strength and the adhesion of coatings.These intermetallic compounds show superior properties at elevated temperatures.The key feature of the powder is to form coatings which are self -bonding for a wide range of metal substrates.Self-bonding materials require minimal surface preparation for good adhesion of the coating, which is suitable for thin substrates.Coatings deposited with optimal parameters are dense with a metallurgic bond at the interface with the base material.The properties of plasma deposited coatings are influenced by the microstructure of the layers.The microstructure and the phases that are formed when NiAl is the deposited depend of raw materials, technological processes and parameters used.NiAl is available as alloyed and as powder clad.For this work, the clad of Ni20Al powder was of interest.The clad of NiAl powder consists of the core of Ni and the outer shell of Al (Deevi, et al., 1997, pp.335-344).Aluminum is a chemical element that protects the coating of Ni from oxidation by forming Al 2 O 3 oxide and an intermetallic compound γ'-Ni 3 Al in the basic alloy α-Ni.At the atmospheric pressure, the Ni20Al powder deposited by the plasma spraying process produces a lamellar coating which consists of the α-Ni solid solution which includes γ'-Ni 3 Al phases and the NiAl phase in traces (Knotek, Lugscheider, 1976, pp.244-251), (Knotek, et al., 1980, pp.282-286).In accordance with the dual diagram of Ni-Al, the γ'-Ni 3 Al phase is built with the aluminum content of 12.5-14 wt.% Al, while the NiAl phase is built with the aluminum content of over 17 wt.%Al(ASM Handbook, 1992).The NiO oxides and γ-Al 2 O 3 and pores are between the base lamellas at the interfaces.
This paper presents the results of experimental investigations of the influence of the plasma spray distance (PS) on the microstructure and the mechanical properties of the Ni20Al coating layers.The main objective was to attest the quality of coating and to apply it on the hardener of the GTD 2PV8 turbo gas engine on the radar station.Three groups of samples were made with three different plasma spraying distances: 80, 90 and 100 mm.The microstructure and the mechanical properties of coating layers were analyzed and studied in order to select the best coa-ting quality.The coating with the best properties was tested on the hardener of the GTD 2PV8 turbo gas engine on the radar station at a time period of over 100 hours.

Materials and experimental details
The substrate material on which the coating layers were deposited was X15Cr13 stainless steel (EN 1.4024) in the thermally unprocessed state.For the production of coatings, the powder of the 'Sulzer Metco' company labeled Metco 404NS was used.The Ni20Al powder is composite powder with 80 wt.% Ni and 20 wt.% Al with a grain powder in a particle size range from 53μm to 90μm.The powder is well deposited and bonded for the Ni and Fe-based basis.The SG -100 plasma gun of the atmospheric plasma spray system (APS) by 'Plasmadyne' was used for powder deposition.The SG-100 plasma spray gun consisted of the cathode type K 1083-129, the anode type A 2084-145 and the gas injector type GI 2083-130.The Argon gas was used in the combination with the Helium and the power of supply of 40 KW.The plasma spray distance was the main parameter for the powder deposition.In this experiment, three different plasma spray distances (80mm, 90mm and 100mm) were used.An optimal distance of the plasma gun from the substrate allows the coating layers to have the smallest content of non-melted particles, oxides and pores.The pores, as volume errors, together with non-melted particles and oxides in the layers, significantly influence the strength of the coating and the protective effect of the coating in exploitation.The detailed values of plasma spray parameters have been shown in Table 1.The substrate surfaces were roughened with the white electrical corundum Al 2 O 3 with particles sizes of 0.7 -1.5mm before depositing the powder.The coatings were deposited with a thickness of 0.15 mm.The coating layers with the best structural and mechanical properties are deposited on the hardener of the GTD 2PV8 turbo gas engine.
The testing of the structural and mechanical properties of the coatings was done in accordance with the 'Pratt & Whitney' standard (Turbojet Engine, 2002, Pratt & Whitney).
The samples with the dimensions 70 × 20 × 1.5 mm were used for the microhardness measurement and for the microstructure analysis.The microhardness of coatings was measured along the coating layers with the Vickers method (HV 0.3 ) with a load of 300 g.The measurement was done in the middle and at the ends of the samples.The presented results of microhardness were the averaged values.
The bond strength testing was done with the tensile testing method.The paired samples of dimensions Ø25 × 50 mm, one of which was with the deposited coating, were used for measuring the strength of the bond between the coating and the substrate.The testings were performed at room temperature at a speed of 1 mm/min.Five samples were done for each plasma spraying distance, and the measured values were averaged.
The microstructural analysis of the coating layers was done on the light microscope.The surface morphology of deposited coatings and the fracture morphology of the best layers were examined with the SEM.

Results and discussion
The measured values of the microhardness and the bond strength of deposited Ni20Al coatings depending on the plasma spray (PS) distance are shown in Figs. 1 and 2. The values of the microhardness of the coating layers are directly related to the plasma spraying distance.The plasma spray distance significantly affects the microhardness values and the bond strength of the deposited layers.The layers of Ni20Al coating, deposited with the smallest distance of 80 mm have a microhardness value of 220HV 0.3 that is within the limits prescribed by the powder manufacturer (190 -230HV 0.3 ) (Material Product Data Sheet, 2011).
These layers have shown the best microstructure with a denser packing of the melted particles and with the lowest proportion of the interlamellar oxides and pores.The highest value of the microhardness of 273HV 0.3 was found in the layers with the highest proportion of oxides that were deposited with a plasma spray distance of 100 mm.A higher microhardness value than 243HV 0.3 was also found in the layers deposited with a plasma spray at a distance of 90 mm.The layers of coatings deposited at a larger distance had the microhardness values above the prescribed values.The values of the microhardness coatings were in accordance with the proportion of the oxide in the deposited layers.Larger distances of the plasma spray affected a higher degree of the oxidation of melted powder particles, followed up by a larger propotion of interlamellar oxides.These were confirmed by the metallographic examination of the coating layers.
Tensile bond strength is directly related to the plasma spray distance and to the proportion of inter-lamellar oxides and pores in the microstructure of Ni20Al coatings.The measurements of the values of the tensile bond strength showed that, for all three plasma spray distances, the obtained values were higher than 20.7MPa, which is prescribed by the powder manufacturer (Material Product Data Sheet, 2011).All deposited coatings had good values of the tensile bond strength resulting from the exothermic reaction of Al and Ni that occurs during the deposition of powder (McPherson, Cheang, 1989), (Deevi, et al., 1997, pp. 335-344), (Material Product Data Sheet, 2011).The exothermic reac-tion enabled obtaining a good adhesion strength with the substrate as well as obtaining a good inter-lamellar cohesive strength.The highest value of the bond strength of 39MPa was found in the layers deposited by plasma spraying with the smallest distance.These layers had the lowest proportion of oxides NiO and Υ -Al 2 O 3 and pores (Knotek, Lugscheider, 1976, pp. 244-251), (Knotek, et al., 1980, pp. 282-286).
The tensile bond strength testing showed that, for all deposited coatings, the mechanism of destruction occured at the interface between the substrate and the coating.This indicates a good melting of powder particles and their bonding to the substrate for all three plasma spray distances.Since the proportion of oxide, unmelted particles and pores is directly related to the microhardness values and the coating bond strength, the measured values for the deposited coating with the lowest plasma spray distance indicate that their proportion is the smallest in this coating.Also, these values are verified by the analysis of the coating microstructure on the light microscope.
Figs. 3 and 4 show the microstructures of the layers deposited by plasma spray (PS) at a distance of 80mm, which had the best microstructure and mechanical properties.The qalitative analysis of the deposited Ni20Al layers (Fig. 3) showed that, at the substrate/coating interface, there was an insignificant proportion of particles of the Al 2 O 3 electric corundum due to roughening.Along the substrate/coating interface, there are no microcracks and macrocracks.The coating/substrate bond is uniform without the separation of the coating layers from the substrate.The structure of the coating is lamellar.The coating layers were continuously deposited without the presence of microcracks and macrocracks.There were no unmelted powder particles present in the layers.The coating substrate consists of a solid solution of α -Ni, which is light gray, and an intermetallic phase of γ'-Ni 3 Al and NiAl (Fig. 4).In the lamellar base of the solid solution and intermetallic phases, the thin inter-lamellar oxide NiO and Υ -Al 2 O 3 films are clearly observed, derived from the oxidation of Ni and Al in the process of cooling and solidification of melted powder particles (Knotek, Lugscheider, 1976, pp.244-251), (Knotek, et al., 1980, pp. 282-286).Dark inter-lamellar and spherical pores are clearly seen through the substrate layers.Due to a lower plasma spray distance, melted particles are more shortly retained in the plasma jet so rough oxides are not seen in the coating layers .Also, the coating layers do not show unmelted particles, microcracks and macrocracks.Precipitates of a spherical shape can be seen in a smaller proportion in the coating layers.The precipitates are a result of the impact of melted powder particles with the substrate and with the previously deposited layer.After the impact with the substrate, the melted particles chip at the ends, solidify and remain in the coating as residue.
Fig. 5 shows a scanning electron microphotography (SEM) of the surface of the Ni20Al coating deposited with the plasma spray at a distance of 80mm.The microphotography shows that the melted powder particles are uniformly distributed.
Unmelted particles are not present in the microstructure on the coating surface.Precipitates of a spherical shape can be seen in a small proportion.Along the boundaries of deposited particles,inter-lamellar pores and pores of irregular shape are observed.The microstructure of the Ni20Al coating surface is typical for atmospheric plasma spray (APS) coatings.Fig. 6 shows a microphotography of the fracture of the Ni20Al coating layers deposited with plasma spray at a distance of 80mm.The fracture morphology of the NiAl coating is seen at the fracture.The coating fracture is ductile.The microphotography clearly shows inter-lamellar pores and pores of irregular shapes present throughout the whole cross section of the coating; they did not affect the cohesion and adhesion strength significantly.Microcracks and macrocracks cannot be seen through the coating layers.

Conclusion
Ni20Al coatings were deposited by the procedure of atmospheric plasma spraying with three different plasma spraying (PS) distances of 80, 90 and 100 mm.The mechanical and microstructural characteristics of the deposited layer were researched and analyzed with the light microscope and the scanning electron microscope (SEM), which led to the following conclusions.
The coating microhardness, tensile bond strength and microstructure were in accordance with the terms of the plasma spray powder deposition.The quality of the coatings was directly dependent on the plasma spray distance.
The values of the coating microhardness increased with the increase of the plasma spraying distance due to the oxidation of melted particles and the formation of NiO and Υ -Al 2 O 3 oxides.The coating layers deposited with the highest plasma spraying distance of 100 mm had the highest microhardness values These layers had the highest proportion of oxides.The layers deposited with the plasma spray distance of 80 mm had the microhardness values within the prescribed limits by the powder manufacturer.These layers had the lowest proportion of oxides.
The values of the tensile bond strength of the coatings for all three plasma spray distances were higher than the minimum value prescribed by the powder manufacturer.The highest value of the tensile bond strength is shown in the layers with the lowest plasma spray distance and with the smallest proportion of oxides in the microstructure.The fracture occured along the coating/substrate interface in all investigated coatings.The values of the microhardness and the tensile bond strength were in correlation with their microstructures.
The structure of the layers of the deposited coatings is lamellar.The substrate consists of the solid solution of α -Ni and the intermetallic phases γ'-Ni 3 Al and NiAl.In the lamellar base of the solid solution there are thin inter-lamellar NiO and Υ -Al 2 O 3 oxide films, derived from the oxidation of Ni and Al in the process of cooling and solidification of the melted powder particles.Inter-lamellar pores and spherical pores are present in the coating layers.Rough oxides, unmelted particles, microcracks and macrocracks are not present In the coating layers while precipitates of a spherical shape are seen in a smaller proportion.
The obtained results have shown that the plasma spray distance significantly affects the structure and the mechanical properties of the coating layers.The testings of the coatings have confirmed that the best layers were deposited with a plasma spray distance of 80 mm.
The Ni20Al coatings, deposited with a plasma spray distance of 80 mm that showed the best microstructure and mechanical properties were deposited on the hardener of the GTD 2PV8 turbo gas engine on a radar station.
The application of the coating have significantly improved the efficiency of the hardener and the operational reliability.The effects of oxidation, vibration and wear have been significantly reduced on the part in the temperature range of 450 -500°C.The effect of the Ni20Al coating has been tested and confirmed on the hardener of the GTD 2PV8 turbo gas engine on the radar station over a period of 100 hours in the "Moma Stanojlović" Aeronautical Plant -Batajnica.
Mihailo R. Mrdak, Research and Development Center IMTEL Communications a. d., Belgrade Summary: