PROPERTIES OF THE ZrO 2 MgO / MgZrO 3 NiCr / NiCr TRIPLE-LAYER THERMAL BARRIER COATING DEPOSITED BY THE ATMOSPHERIC PLASMA SPRAY PROCESS

This paper presents the results of the examinations of TBC ZrO2MgO / MgZrO3NiCr / NiCr thermal barrier layers deposited by the plasma spray process at the atmospheric pressure on substrates of Al alloys. In order to obtain the structural and mechanical properties of layers, which will provide a good heat and abrasion protection of the tail elevators of aircraft J-22 when firing "Lightning" and "Thunder" rockets, the deposition of three powder types was performed on 0.6 mm thick Al alloy substrates. This study describes a procedure of using triple-layer TBC coatings as a good combination among many available ones, which gives a good compromise between thermal protection and resistance to abrasion for protecting aircraft tail elevators. The study is mainly based on the experimental approach. The evaluation of the mechanical properties of layers was done by the examination of microhardness by method HV0.3 and bond strength on the tensile machine. The structure of layers was examined by the method of light microscopy while the surface of ZrO2MgO ceramic layers was examined by the method of scanning electron microscopy (SEM).The thermal protection of TBC layers and resistance to abrasion were tested in the tunnel of the MilitaryTechnical Institute, Zarkovo. The obtained characteristics of the surface layers and the rocket firing simulations have proven the triple-layer system of TBC coatings reliable.


Introduction
TBC -thermal barrier plasma spray coatings are widely used to protect parts of turbo jet engines and other engine parts exposed to high temperatures, oxidation, corrosion and erosion of gas particles.Plasmadeposited TBC ceramic coatings are a good solution for the thermal protection of diesel engine parts such as pistons, valves, etc. (Çelik, et al., 1997, pp.361-365), (Demirkiran, Avci, 1999, pp.292-295), (Miyamoto, et al., 1999, p.100).Oxide ZrO 2 was selected because of its high strength and fracture toughness compared to other oxides, and physical characteristics such as a thermal conductivity of λ ≈ 1.7 W/mK, a coefficient of thermal expansion of α ≈ 9 × 10 -6 1/K and a melting point of 2710°C (Boutz, et al., 1994, pp.89-102), (Chevalier, et al., 2009(Chevalier, et al., , pp.1901(Chevalier, et al., -1920)).The polymorphism of pure ZrO 2 is an important feature (Johner, Schweitzer, 1984, pp.301-315).At the atmospheric pressure, there are three crystallographic phases: the monoclinic, the tetragonal and the cubic one.When alternating heating and cooling, the thermal fatigue of the ZrO 2 material occurs due to the volume changes caused by the phase transformation.As a result of the reversible transformation of the monoclinic phase into the tetragonal one, the occurrence of microcracks spreading and converting into macro crevices was observed in the temperature range of 950°C-1170°C (Garvie, et al., 1975, pp.703-704), (Garvie, 1970, pp.117-166).For this reason, pure ZrO 2 is not suitable for the preparation of the TBC coating.In order to reduce the effect of the tetragonal transformation into the monoclinic one, other oxides such as MgO, CaO, Y 2 O 3 , CeO 2 , HfO 2 , and In 2 O 3 are added to pure ZrO 2 .These additives stabilize the ceramic layer partially or in full by forming a cubic structure stable from the room temperature up to more than 2000°C.ZrO 2 with addition of magnesium oxide MgO is often used as a TBC due to its high coefficient of linear expansion that is 11 × 10 -6 1/K, a coefficient of thermal conductivity of 1.5 W/mK, high resistance to thermal cycles, resistance to corrosion, and easy preparation of the coating by plasma spray spraying.When the TBC is subjected to elevated temperatures, this induces mechanical degradation that involves the stratification and cracking of ceramics as a result of the factors such as stresses due to thermal expansion conflicts related to the changes in the microstructure because of thermal cycles.TBC coatings consist of at least two layers.The outer, generally thicker layer is made of a ceramic material or a mixture of ceramics and fire-resistant metals, the primary purpose of which is to provide thermal insulation and resistance to thermal shocks.The material of the ceramic layer which is in a direct contact with the working fluid, has a drop of temperature per cross section even to 400-500°C (Mrdak, et al., 2013, pp.559-567), (Mrdak, et al., 2015, pp.337-343).That layer should also have resistance to erosive effects and good bonding with the substrate material.The inner, thinner layer provides protection from the oxidative degradation of the base material, but also provides a good bond between the base metal and the outer ceramic layer.In order to reduce stresses, triple-layer systems of TBC coatings are often produced; they consist of the bonding NI20%Cr layer, the transitional MgZrO 3 35%NiCr cermet inter-layer and the top ZrO 2 24%MgO ceramic layer.This study describes a method of using the triple-layer TBC coatings as a selection of a good combination among many available options, which provides a good compromise between the thermal protection and the resistance to erosion of the Al alloy substrate for protecting the tail elevators of aircraft J-22.The study is mainly based on the experimental approach.The properties of the deposited materials are generally functions of their microstructures.According to previous studies, plasma deposited ceramic deposits show a lamellar structure with limited inter -lamellar bonding (Li, Ohmori, 2002, pp.365-374) (Mrdak, et al., 2013, pp.559-567).Because of this, micro pores are present in the deposit as volume errors.
This paper presents the examination of a ZrO 2 MgO/MgZrO 3 NiCr/NiCr triple-layer system of TBC coatings deposited by the atmospheric plasma spraying (APS) process on the substrates of Al alloys, which serve as the thermal abrasive barriers of the tail elevators of aircraft J-22.The aim of the study was to produce the TBC coatings of such structural and mechanical properties of the layers which will provide a good heat and abrasion protection on the aircraft tail elevators when firing "Lightning" and "Thunder" rockets.The microhardness and bond tensile strength of the triple system of TBC coatings and layer microstructures were examined.The obtained characteristics of the TBC layers and rocket firing simulations have proven the triple-layer system of TBC coatings reliable.

Materials and experimental details
The material on which layers of the ZrO 2 MgO/MgZrO 3 NiCr/NiCr triple-layer TBC coating were deposited was aluminum alloy ENAW-AlMg1(C)(ENAW-5005).For the production of the top ceramic coating layers, the ZrO 2 24%MgO powder of the Sulzer Metco company, labelled Metco 210NS-1, was used.The powder is produced by the method of casting into blocks and subsequent grinding of these blocks to obtain a specific granularity.The melting point of powder is 2140°C.The powder with a range of granules of 10-53μm (Metco 210NS-1 Powder Magnesium Zirconate, 2000.Sulzer Metco.Technical Bulletin 10-289) was used for the experiment.Figure 1 shows a (SEM) scanning electron photomicrography of the morphology of ZrO 2 24%MgO powder particles.The powder particles are of an irregular angular shape.MgZrO 3 35%NiCr) and 210NS-1 (ZrO 2 24%MgO).The bond strength of the ZrO 2 MgO/MgZrO 3 NiCr/NiCr triple system of TBC coatings was tested as well.Five test pieces were examined for all types of coatings, out of which two extreme values were rejected.The average bond strength value is shown for the three remaining values.The morphology of the ZrO 2 24%MgO powder particles and that of the deposited coating surface were determined by using scanning electron microscopy (SEM).The microstructure of the deposited layers was examined on the optical microscope (OM).The analysis of the share of micro pores in the coating layers was done on five photos at the 200X magnification.Over tracing paper, micro pores were labelled and shaded and their total area was counted in relation to the total surface of the micrographs.This paper presents the average value of the shares of micro pores in the TBC coating layers.
The deposition of powders was done with the atmospheric plasma spray system by the Plasmadyne company and the SG-100 plasma gun with controlled plasma spray parameters.The SG-100 plasma gun consisted of the cathode type K 1083A-129, the anode type A 1083-165 and the gas injector type GI 1083-113.Ar as an arched gas was used in combination with He and the power of supply of 40 KW.The plasma spray parameters of the deposition powders are shown in Table 1.Before the depositing process, the surface of the test samples and the surface of the substrate of the thermal abrasive barrier for the aircraft tail elevators were not roughened, due to the small thickness of the substrate of 0.6 mm.The bonding layers were deposited with a thickness of 60-80μm, the cermet layers with a thickness of 40-60μm and the top ceramic layer with a thickness of 280-300μm.

Results and discussion
The values of microhardness and bond strength of TBC coating systems are shown in Figures 3 and 4 The layers of the ceramic coating Metco 210NS-1 (ZrO 2 24%MgO) had the highest microhardness values of 478-519HV 0.3 that are characteristic for this type of the coating.These layers had the highest share of micro pores because ceramic particles create a weaker interlamellar contact in comparison to metal particles.Figure 3 shows the minimum and maximum values of the microhardness of TBC coatings.The tensile bond strength of coatings was directly related to the powder type.The highest bond strength values of 31 MPa were found in the metal bonding layers of 43F-NS(NI20%Cr) coating.The layers of cermet coating 303NS-1 (MgZrO 3 35%NiCr) had a tensile bond strength of 22 MPa, while a minimum value of 17 MPa was found in the ceramic layers of Metco 210NS-1 (ZrO 2 24%MgO).For all coatings, the bond strength values were good because the coatings were deposited on the non-roughened substrates of Al alloy which reduce the tensile bond strength in relation to roughened substrates based on Fe or Ni alloys.
The average tensile strength value of the ZrO 2 MgO/MgZrO 3 NiCr/NiCr system of TBC coatings was 30 MPa.The TBC coating was destroyed at the substrate/coating interface, which was expected due to two different materials.The measured values of microhardness and tensile bond strength of the ZrO 2 MgO/MgZrO 3 NiCr/NiCr system of of TBC coatings were in correlation with the microstructure of deposited layers.6 shows the microstructure of the triple system of thermal barrier coatings TBC -ZrO 2 24%MgO/MgZrO 3 35%NiCr/Ni20%Cr.The photomicrographs clearly show the boundaries of the interface between the bonding coating layers and the substrate, the bonding coating and the cermet coatings, as well as between the cermet coatings and the ceramic coatings.The interface between the substrate and the bonding coating layers is very clean, which indicates a good bond between the coating layers with the substrate.At the interface between the substrate and the bonding coating layers there are no defects such as discontinuities of deposited layers, microcracks, macrocracks, coating peeling and separation from the substrate.Generally, the layers are uniformly deposited on the substrate.Along the following interfaces: the substrate / the bonding coatings, the bonding coatings / the cermet coatings and the cermet coatings / the ceramic coatings, there are no microcracks and macrocracks present.The bond between all layers is good.In the layers of deposited coatings, there were no unmelted powder particles observed, which indicates that the powders were deposited with the optimal deposition parameters.The analysis of the photomicrographs showed that in the layers of bonding coatings 43F-NS (NI20%Cr) there were micro pores with an average share of 2.6%.The share of micro pores in the layers of cermet coatings 303NS-1 (MgZrO 3 35%NiCr) was 7%, and the layers of ceramic coatings 210NS-1 (ZrO 2 24%MgO) had the content of micro pores of 12%.
The microstructure of the NI20%Cr bonding coating is lamellar.The coating base is a solid solution of chromium in nickel γ -Ni(Cr).Between the lamellas of the solid solution in the coating layers there are light gray oxides: NiO, NiCr 2 O 4 , Cr 2 O 3 and CrO 3 (Nicoll, 1984), (Mrdak, 2015, pp.32-55) due to oxidation of powder particles in plasma during the process of coating formation.In most cases, the oxide of chromium Cr 2 O 3 is present and, in rare cases, oxide CrO 3 , formed in a thin layer on the surface of NiCr lamellae (Brossard, et al., 2010(Brossard, et al., , pp.1608(Brossard, et al., -1615)).In the middle cermet inter-layer, there are clearly visible light gray lamellae of the bonding coating, evenly distributed between ceramic lamellae in dark gray.The top ceramic layer is uniformly deposited on the cermet layer in which black micro pores can be seen.The surface of the molten ZrO 2 MgO powder particle was circled with a red line on the SEM micrograph.The molten powder particle formed an almost circular shape in the collision with the surface of the previously deposited layer.The surface of the particle shows a fine net of microcracks which cannot be avoided and which always occurs in the deposition process (Guo, et al., 2011, pp.161-174).
Microcracks are formed during the cooling of molten particles to the coating temperature.The inner coating layers, which have a higher temperature compared to that of the coating surface, are exposed to tensile stress and are opposed to the shrinkage of the particles on the coating surface.On the other hand, the particles on the coating surface while cooling and shrinking during solidification, are exposed to compression stresses.Microcracks on the particle surface are caused by tensile stresses of deposited layers which are always higher than compression stresses of the particles while cooling Guo, et al., 2011, pp.161-174), (Mrdak, et al. 2013, pp.559-567), (Mrdak, 2013, pp.426-432), (Mrdak, et al., 2015, pp.337-343).In the microstructure, there are fine precipitates of irregular shapes with a size up to 5 μm, circled in yellow.On the SEM micrograph, micro pores of irregular shapes in black with a size up to 5 μm are clearly seen and circled in green.

Conclusion
This paper describes how the APS -atmospheric plasma spray process was used to produce a triple-layer system of thermal barrier coatings TBC -ZrO 2 24%MgO/MgZrO 3 35%NiCr/Ni20%Cr.The system of the deposited coatings consisted of the Ni80%Cr bonding layer, the intermediary MgZrO 3 35%NiCr inter-cermet layer and the top ZrO 2 24%MgO ceramic layer.The coatings were deposited on the test Al alloy samples on the surfaces without roughening.The mechanical properties and the microstructures of the coating layers were analyzed in the deposited condition, which led to the following conclusions.
The triple-layer system of the thermal barrier coatings had good mechanical properties with the bonding layer microhardness values of 238-254HV 0.3 , the intermediary cermet layer microhardness values of 293-330HV 0.3 and the top ceramic layer microhardness values of 478-519HV 0.3 .The microhardness values were within the limits prescribed by the Pratt&Whitney standard.The bond strength of the deposited coatings on the non-roughened Al alloy samples had good values.The tensile bond strength was 31 MPa for the bonding layer, 22 MPa for the cermet coating and 17 MPa for the ceramics.The bond strength of the triple-layer system of TBC coatings is 30 MPa.The analysis of the photomicrographs has shown that the average share of micro pores was 2.6% in the bonding layers, 7% in the cermet layers and 12% in the ceramic layers.The microstructure of the deposited coating layers is lamellar.
The coating base consists of a solid solution of chromium in nickel γ -Ni(Cr).There are light gray NiO, NiCr 2 O 4 , Cr 2 O 3 and CrO 3 oxides between the solid solution lamellae in the coating layers due to oxidation of powder particles in plasma during the coating formation process.In most cases, the oxide of chromium Cr 2 O 3 is present and, in rare cases, oxide CrO 3 , formed in a thin layer on the surface of NiCr lamellae.The cermet inter-layer had a uniform distribution of bond coating lamellae between ceramic lamellae.The top ceramic layer is uniformly deposited on the cermet layer without the presence of unmelted particles.

Figure 1
Figure 1 -(SEM) Scanning electron micrography of ZrO2 24%MgO powder particles Slika 1 -(SEM) Skening elektronska mikrografija čestica praha ZrO2 24%MgO Рис. 1 -(SEM) Электронная микрография частиц порошка ZrO2 24%MgO For the production of inter-layer TBC coatings, MgZrO 3 35%NiCr cermet powder of the Sulzer Metco company, labelled Metco 303NS -1, was used.The powder is a mechanical mixture of ZrO 2 MgO powder and NiCr in relation 35%(80Ni20%Cr) + 65%(ZrO 2 24%MgO).Powder with a range of granules of 11-90 μm (Material Product Data Sheet, 2012.Metco 303NS-1 Magnesium Zirconate-Nickel Chromium Cermet Blends.Sulzer Metco DSMTS-0070.0) was used for the experiment.For the production of the lower bonding layer, the powder type labelled Metco 43F-NS (an alloy of nickel and chromium NI20%Cr) was used.The melting point of the powder is 1400°C.The powder with a range of granules of 10-63 μm (Material Product Data Sheet, 2012.Metco 43F-NS Nickel-20% Chromium Powders.Sulzer Metco.DSMTS-0109.0)was used for the experiment.Testing the mechanical properties of the ZrO 2 MgO/MgZrO 3 NiCr/NiCr TBC coating was done according to the Pratt & Whitney standard (Turbojet Engine -Standard Practices Manual (PN 582005), 2002.Pratt & Whitney, East Hartford, USA).The bases with deposited coating layers for the microhardness testing and the evaluation of microstructure in a deposited condition are made of ENAW-AlMg1(C)(ENAW 5005) aluminum alloy with the dimensions 70x20x1.5 mm.The bases for testing bond strength are also made of ENAW-AlMg1(C)(ENAW-5005) aluminum alloy with the dimensions Ø25x50mm.The investigation of the microhardness of the layers was done by the method HV 0.3 and the bond strength was tested on a tensile machine.Microhardness measurements were performed in the direction along the lamellas.Five readings of microhardness values of the layers were performed in the middle and at the ends of the samples while two extreme values were rejected.Out of three remaining values, minimum and maximum values are shown.The

Figure 2 -
Figure 2 -Section of the edge of the aircraft rear wing with a TBC coating Slika 2 -Sekcija ivice zadnjeg krila aviona sa TBC prevlakom Рис. 2 -Часть заднего крыла самолета с TBC покрытием Figure 2 shows one section of the edge of the aircraft tail elevators with a deposited TBC coating.
. The metal bonding coating 43F-NS (NI20%Cr) had the lowest values of microhardness of 238-254HV 0.3 , which are within the limits of values prescribed by the powder manufacturer and by the standard (Material Product Data Sheet, 2012.Metco 43F-NS Nickel-20% Chromium Powders, Sulzer Metco.DSMTS-0109.0) (Turbojet Engine-Standard Practices Manual (PN 582005) 2002.Pratt & Whitney, East Hartford, USA).The measured values of the microhardness of the bonding layers indicate that the share of micro pores is within the prescribed limits, which was confirmed by the analysis of the shares of micro pores.Due to ceramics content, the layers of the cermet coating 303NS-1 (MgZrO 3 35%NiCr) had higher values of microhardness, in a range of 293-330HV 0.3 and in accordance with the Pratt & Whitney standard(Turbojet Engine-Standard Practices Manual (PN 582005) 2002.Pratt & Whitney, East Hartford, USA).

Figure 6 -
Figure 6 -Microstructure of the triple layer of ZrO2MgO/MgZrO3NiCr/NiCr coatings Slika 6 -Mikrotvrdoća troslojne prevlake ZrO2MgO/MgZrO3NiCr/NiCr Рис. 6 -Микротвердость трехслойного покрытия ZrO2MgO/MgZrO3NiCr/NiCr Figure 7 shows a SEM photomicrograph of the surface of a ZrO 2 MgO molten particle.The SEM analysis of the morphology of the surface of the deposited ceramic ZrO 2 MgO powder particle shows a complete melting and casting of ceramic particles on the previously deposited ceramic layer.