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PROPERTIES OF THE ZrO2MgO/MgZrO3NiCr/NiCr TRIPLE-LAYER THERMAL BARRIER COATING DEPOSITED BY THE ATMOSPHERIC PLASMA SPRAY PROCESS
Mihailo R. Mrdak
Research and Development Center IMTEL Communications a.d.,
Belgrade, Republic of Serbia
e-mail: miki@insimtel.com,
ORCID iD: ©http://orcid.org/0000-0003-3983-1605
DOI: 10.5937/vojtehg64-9612
FIELD: Chemical Technology ARTICLE TYPE: Original Scientific Paper ARTICLE LANGUAGE: English
Summary:
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 HV03 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.
Key words: substrates, protective, property, layers, coatings, barriers, alloys.
ACKNOWLEDGMENT: The author is thankful for the financial support from the Ministry of Education and Science of the Republic of Serbia (national projects OI 174004, TR 34016).
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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. Plasma-deposited 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 ZrO2 was selected because of its high strength and fracture toughness compared to other oxides, and physical characteristics such as a thermal conductivity of X ~ 1.7 W/mK, a coefficient of thermal expansion of a ~ 9 x 10-61/K and a melting point of 2710°C (Boutz, et al., 1994, pp.89-102), (Chevalier, et al., 2009, pp.19011920). The p olymorphism of pure ZrO2 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 ZrO2 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 ZrO2 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, Y2O3, CeO2, HfO2, and In2O3 are added to pure ZrO2. 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. ZrO2 with addition of magnesium oxide MgO is often used as a TBC due to its high coefficient of linear expansion that is 11 x 10-61/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 MgZrO335%NiCr cermet inter-layer and the top ZrO224%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 ZrO2MgO/MgZrO3NiCr/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 ZrO2MgO/MgZrO3NiCr/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 ZrO224%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 10-53^m (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 ZrO224%MgO powder particles. The powder particles are of an irregular angular shape.
Figure 1 - (SEM) Scanning electron micrography of ZrO2 24%MgO powder particles Slika 1 - (SEM) Skening elektronska mikrografija cestica praha ZrO2 24%MgO Рис. 1 - (SEM) Электронная микрография частиц порошка ZrO2 24%MgO
For the production of inter-layer TBC coatings, MgZrO335%NiCr cermet powder of the Sulzer Metco company, labelled Metco 303NS - 1, was used. The powder is a mechanical mixture of ZrO2MgO powder and NiCr in relation 35%(80Ni20%Cr) + 65%(ZrO224%MgO). Powder with a range of granules of 11-90 |jm (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 ^m (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 ZrO2MgO/MgZrO3NiCr/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 025x50mm. The investigation of the microhardness of the layers was done by the method HV03 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
bond strength testing was done at room temperature with a speed of tensile testing of 1cm/60s. The bond strength testing was performed for each individual coating 43F-NS (80Ni20%Cr), 303NS-1 (MgZrO335%NiCr) and 210NS-1 (ZrO224%MgO). The bond strength of the ZrO2MgO/MgZrO3NiCr/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 ZrO224%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 Gl 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 60-80^m, the cermet layers with a thickness of 40-60^m and the top ceramic layer with a thickness of 280-300^m.
Table - 1 Parameters of the deposition of powders Tabela 1 - Parametri depozicije prahova Таблица 1 - Параметры напыления порошка
Parameters 43F-NS 303NS-1 210NS-1
Electric Current, I (A) 700 900 900
Arc voltage, U (V) 30 43 43
Primary plasma gas, l/min 50 50 50
Secondary plasma gas, l/min 12 12 12
Carrier gas powder, l/min 7 7 7
Powder feed rate, g/min 50 50 50
Distance of plasma guns, mm 90 100 100
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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.
Results and discussion
The values of microhardness and bond strength of TBC coating systems are shown in Figures 3 and 4. The metal bonding coating 43F-NS (NI20%Cr) had the lowest values of microhardness of 238-254HV03, 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 (MgZrO335%NiCr) had higher values of microhardness, in a range of 293-330HV0.3 and in accordance with the Pratt & Whitney standard(Turbojet Engine-Standard Practices Manual (PN 582005) 2002. Pratt & Whitney, East Hartford, USA).
The layers of the ceramic coating Metco 210NS-1 (ZrO224%MgO) had the highest microhardness values of 478-519HV03 that are characteristic for this type of the coating. These layers had the highest share of micro pores because ceramic particles create a weaker inter-lamellar contact in comparison to metal particles. Figure 3 shows the minimum and maximum values of the microhardness of TBC coatings.
Figure 3 - Microhardness of ZrO224%MgO, MgZrO3 35%NiCr and Ni20%Cr coatings Slika 3 - Mikrotvrdoca ZrO224%MgO, MgZrO3 35%NiCr i Ni20%Cr prevlaka Рис. 3- Микротвердость ZrO224%MgO, MgZrO335%NiCr i Ni20%Cr покрытия
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 (MgZrO335%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 (ZrO224%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 ZrO2MgO/MgZrO3NiCr/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 ZrO2MgO/MgZrO3NiCr/NiCr system of of TBC coatings were in correlation with the microstructure of deposited layers.
Figure 4 - Bond strength of ZrO224%MgO, MgZrO3 35%NiCr and Ni20%Cr coatings Slika 4 - Cvrstoca spoja ZrO224%MgO, MgZrO3 35%NiCr and Ni20%Cr prevlaka Рис. 4 - Микропрочность соединений ZrO224%MgO, MgZrO3 35%NiCr and Ni20%Cr покрытия
Figures 5 and 6 shows the microstructure of the triple system of thermal barrier coatings TBC - ZrO224%MgO/MgZrO335%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.
Figure 5 - Microstructure of the triple layer of ZrC^MgO/MgZrOaNiCr/NiCr coatings Slika 5 - Mikrotvrdoca troslojne prevlake ZrO2MgO/MgZrO3NiCr/NiCr Рис. 5- Микротвердость трехслойного покрытия ZrO2MgO/MgZrO3NiCr/NiCr
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 (MgZrO335%NiCr) was 7%, and the layers of ceramic coatings 210NS-1 (ZrO224%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 y - Ni(Cr). Between the lamellas of the solid solution in the coating layers there are light gray oxides: NiO, NiCr2O4, C2O3 and CrO3 (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 Cr2O3 is present and, in rare cases, oxide CrO3, formed in a thin layer on the surface of NiCr lamellae (Brossard, et al., 2010, pp.1608-1615). 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.
Figure 6 - Microstructure of the triple layer of ZrO2MgO/MgZrO3NiCr/NiCr coatings Slika 6 - Mikrotvrdoca troslojne prevlake ZrO2MgO/MgZrO3NiCr/NiCr Рис. 6 - Микротвердость трехслойного покрытия ZrO2MgO/MgZrO3NiCr/NiCr
Figure 7 shows a SEM photomicrograph of the surface of a ZrO2MgO molten particle. The SEM analysis of the morphology of the surface of the deposited ceramic ZrO2MgO powder particle shows a complete melting and casting of ceramic particles on the previously deposited ceramic layer.
Figure 7 - (SEM) Morphology of the ZrO2MgO coating surface Slika 7 - (SEM) Morfologija povrsine ZrO2MgO prevlake Рис. 7 - Морфология поверхности ZrO2MgO покрытия
The surface of the molten ZrO2MgO 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.426432), (Mrdak, et al., 2015, pp.337-343). In the microstructure, there are fine precipitates of irregular shapes with a size up to 5 ^m, circled in yellow. On the SEM micrograph, micro pores of irregular shapes in black with a size up to 5 ^m 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 - Zr0224%Mg0/MgZr0335%NiCr/Ni20%Cr. The system of the deposited coatings consisted of the Ni80%Cr bonding layer, the intermediary MgZr0335%NiCr inter-cermet layer and the top Zr0224%Mg0 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 238-254HV03, the intermediary cermet layer microhardness values of 293-330HV03 and the top ceramic layer microhardness values of 478-519HV03. 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
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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 y - Ni(Cr). There are light gray NiO, NiCr2O4, Cr2O3 and CrO3 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 Cr2O3 is present and, in rare cases, oxide CrO3, 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.
The triple-layer system of thermal barrier coatings - ZrO224%MgO / MgZrO335%NiCr / Ni20%Cr, deposited on the Al alloy substrate as the thermal abrasive protection of the tail elevators of aircraft J-22, proved to be reliable protection against the jet temperature and jet abrasive particles during firing of "Lightning" and "Thunder" rockets.
Literature
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ХАРАКТЕРИСТИКИ ТРЕХСЛОЙНЫХ ТЕРМОБАРЬЕРНЫХ ПОКРЫТИЙ ггС^МдО/ МдггОЗЫЮг/ ЫЮг, НАНЕСЕННЫХ ВОЗДУШНО-ПЛАЗМЕННЫМ НАПЫЛЕНИЕМ
Михаило Р. Мрдак
Центр исследований и развития А.О. «ИМТЕЛ коммуникации», Белград, Республика Сербия
ОБЛАСТЬ: химические технологии
ВИД СТАТЬИ: оригинальная научная статья
ЯЗЫК СТАТЬИ: английский
Резюме:
В данной статье представлены результаты испытаний термобарьерных покрытий ТБС ZrO2MgO/MgZrO3NiCr/NiCr, нанесенных воздушно-плазменным напылением при атмосферном давлении на субстраты сплавов Al.
Испытания проводились с целью получения структурных и механических характеристик слоев, обеспечивающих качественную и абразивную защиту задних крыльев самолета J-22 при выпуске ракет, а также от грома и молний. Нанесено напыление трех типов порошков на субстраты сплавов М, толщиной 0,6мм.
В данном исследовании представлен метод применения трехслойного ТБС покрытия, которое обладает лучшими защитными свойствами, когда речь идет о термоизоляции и защите от абразивного износа задних крыльев самолета.
Анализ механических характеристик покрытия проведен на основании испытаний микротвердости методом HV03 и прочности соединений методом растяжения. Структура слоев испытана методом оптической микроскопии, а поверхность ZrO2MgO испытана методом электронной микрографии (SEM).
Испытания теплоизоляционных ТБС слоев и сопротивления абразивному износу были проведены в аэродинамической трубе Военно-технического института Жарково. На основании полученных характеристик поверхности слоев и моделирования выпуска ракет, можно утверждать, что трехслойные системы ТБС являются надежным способом покрытия.
Ключевые слова: субстраты, защита, характеристика, слои, покрытие, барьер, сплавы.
SVOJSTVA TROSLOJNE TERMOBARIJERNE PREVLAKE ZrO2MgO/ MgZrO3NiCr/NiCr DEPONOVANE ATMOSFERSKIM PLAZMA SPREJ PROCESOM
Mihailo R. Mrdak
Istrazivacki i razvojni centar IMTEL Komunikacije a. d., Beograd, Republka Srbija
OBLAST: hemijske tehnologije VRSTA CLANKA: originalni naucni clanak JEZIK CLANKA: engleski
Sazetak:
U radu su prikazani rezultati ispitivanja termobarijernih slojeva TBC - ZrO2MgO/MgZrO3NiCr/NiCr koji su deponovani plazma sprej procesom na atmosferskom pritisku na substratima od legure Al. Radi dobijanja strukturnih i mehanickih osobina slojeva, koji ce obezbediti dobru toplotnu i abrazivnu zastitu zadnjim krilima aviona J-22 pri ispa-ljivanju raketa munje i groma, izvrsena je depozicija tri tipa praha na substratima od legure Al debljine 0,6 mm. Ova studija opisuje postupak koriscenja troslojne TBC prevlake kao izbor dobre kombinacije od mnogo raspolozivih mogucnosti, koja predstavlja kompromis izmedu toplotne zastite i otpornosti na abraziju za zastitu zadnjih krila aviona. Studija se, uglavnom, zasniva na eksperimentalnom pristupu. Procena mehanickih osobina slojeva uradena je ispitivanjem mikrotvrdoce me-
todom HV03 i cvrstoce spoja ispitivanjem na zatezanje. Struktura sloje-va ispitana je metodom svetlosne mikroskopije i povrsina ZrO2MgO ke-ramickih slojeva metodom skenirajuce elektronske mikroskopije (SEM). Toplotna zastita TBC slojeva i otpornost na abraziju ispitana je u tunelu Vojnotehnickog instituta iz Zarkova. Na osnovu dobijenih ka-rakteristika povrsinskih slojeva i simuliranja ispaljivanja rakete, troslojni sistem TBC prevlake pokazao se pouzdanim.
Uvod
TBC termobarijerne plazma sprej prevlake uveliko se koriste kao zastita delova turbomlaznih motora i za druge delove motora izlozene visokim temperaturama, oksidaciji, gasnoj koroziji i eroziji cestica. Pla-zmom deponovane TBC keramicke prevlake dobro su resenje za to-plotnu zastitu delova dizel motora, kao sto su klipovi, ventili, itd. (Qelik, et al., 1997, pp.361-365), (Demirkiran, Avci, 1999, pp.292-295), (Miyamoto, et al., 1999, p. 100). Izbor oksida ZrO2 izvrsen je zbog viso-ke cvrstoce i zilavosti loma u odnosu na druge okside i fizickih karakte-ristika, kao sto je toplotna provodljivost X ~ 1.7 W/mK, koeficijent ter-micke ekspanzije a ~ 9x10-61/K i temperature topljenja 2710°C (Boutz, et al., 1994, pp.89-102), (Chevalier, et al., 2009, pp.1901-1920). Na at-mosferskom pritisku postoje tri kristalografske faze: monoklinicna, te-tragonalna i kubna. Prilikom naizmenicnog zagrevanja i hladenja dolazi do toplotnog zamora ZrO2 meterijala usled zapreminskih promena uzrokovanih faznom transformacijom. Kao posledica reverzibilne tran-sformacije monoklinicne faze u tetragonalnu u temperaturnom opsegu od 950° do 1170°C uocen je nastanak mikropukotina koje se sire i pre-tvaraju u makropukotine (Garvie, et al., 1975, pp.703-704), (Garvie, 1970, pp.117-166). Zbog toga cisti ZrO2 nije pogodan za izradu TBC prevlaka. Radi smanjenja efekta tetragonalne transformacije u mono-klinicnu, cistom ZrO2 dodaju se drugi oksidi, kao sto su: MgO, CaO, Y2O3, CeO2 HfO2 i In2O3. ZrO2 sa dodatkom magnezijumoksida MgO cesto se koristi kao TBC zbog velikog koeficijenta linearnog sirenja koji je 11 x 10 61/K, koeficijenta toplotne provodljivosti 1,5 W/mK, velike ot-pornosti na toplotne cikluse, otpornosti na koroziju i lake izrade prevlake plazma sprej prskanjem. TBC prevlake sastoje se najmanje od dva sloja: spoljasnjeg, po pravilu debljeg sloja, sacinjenog od keramickog materijala ili mesavine keramike i vatropostojanih metala, koji pre sve-ga treba da obezbedi toplotnu izolaciju i otpornost na termosokove. Materijal keramickog sloja koji se nalazi u direktnom kontaktu sa rad-nim fluidom ima pad temperature po preseku i do 400-500°C (Mrdak, et al., 2013, pp.559-567), (Mrdak, et al., 2015, pp.337-343). Da bi se smanjili naponi cesto se proizvode troslojni sistemi TBC prevlaka koji se sastoje od veznog sloja Ni20%Cr, prelaznog kermet medusloja MgZrO335%NiCr i gornjeg keramickog sloja ZrO224%MgO. Ova studija
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opisuje postupak koriscenja troslojne TBC prevlake kao izbor dobre kombinacije, od mnogo raspolozivih mogucnosti, koja daje dobar kom-promis izmedu toplotne zastite i otpornosti na eroziju substrata od le-gure Al za zastitu zadnjih krila aviona J-22. Studija se uglavnom zasni-vala na eksperimentalnom pristupu. Svojstva deponovanih materijala su generalno funkcije njihovih mikrostruktura.
U ovom radu ispitan je troslojni sistem TBC prevlaka ZrO2MgO/MgZrO3NiCr/NiCr koji je deponovan atmosferskim plazma sprej (APS) postupkom na substratima od legure Al, koji sluze kao ter-moabrazivna barijera zadnjim krilima aviona J-22. Cilj rada je bio da se proizvedu TBC prevlake strukturnih i mehanickih osobina slojeva, koji ce obezbediti dobru toplotnu i abrazivnu zastitu zadnjim krilima aviona pri ispaljivanju raketa munje i groma. Ispitane su mikrotvrdoce i zate-zne cvrstoce spoja trojnih sistema TBC prevlaka i mikrostrukture slojeva. Na osnovu dobijenih karakteristika TBC slojeva i simuliranja ispalji-vanja rakete troslojni sistem TBC prevlake pokazao se pouzdanim.
Materijali i eksperimentalni detalji
Materijal na kojem su deponovani slojevi troslojne TBC -ZrO2MgO/ MgZrO3NiCr/NiCr prevlake bio je od legure aluminijuma ENA W-AlMg1 (C)(ENA W-5005A). Za izradu slojeva gornje kera-micke prevlake upotrebljen je prah ZrO224%MgO firme Sulzer Metco sa oznakom Metco 210NS-1. Prah je proizveden metodom livenja u blokove i naknadnim mlevenjem blokova na odredenu granulaciju. Temperatura topljenja praha je 2140°C. Za eksperiment se koristio prah koji je imao raspon granulata od 10 do 53pm, a cestice praha su nepravilnog uglastog oblika (Metco 210NS-1 Magnesium Zirconate Powder, 2000. Sulzer Metco.Technical bulletin 10-289). Za izradu srednjih slojeva TBC prevlake upotrebljen je kermet prah MgZrO335%NiCr, firme Sulzer Metco, koji nosi oznaku Metco 303 NS-1. Prah je mehanicka mesavina praha ZrO2MgO i NiCr u odnosu 35%(80Ni20%Cr)+65%(ZrO224%MgO). Za eksperiment se koristio prah koji je imao raspon granulata od 11 do 90pm (Material Product Data Sheet, 2012. Metco 303NS-1 Magnesium Zirconate - Nickel Chromium Cermet Blends. Sulzer Metco. DSMTS-0070.0). Za izradu donjeg veznog sloja koristio se prah oznake Metco 43F-NS, koji je le-gura nikla i hroma Ni20%Cr. Temperatura topljenja praha je 1400°C. Za eksperiment se koristio prah koji je imao raspon granulata od 10 do 63pm (Material Product Data Sheet, 2012. Metco 43F-NS Nickel - 20% Chromium Powders, Sulzer Metco. DSMTS-0109.0).
Ispitivanje mehanickih karakteristika slojeva TBC prevlake ZrO2MgO/ MgZrO3NiCr/NiCr radeno je prema standardu Pratt & Whitney (Turbojet Engine - Standard Practices Manual (PN 582005), 2002. Pratt & Whitney, East Hartford, USA). Osnove na kojima su deponova-
ni slojevi prevlake za ispitivanje mikrotvrdoce i za procenu mikrostrukture u deponovanom stanju izraúene su od legure aluminijuma ENAW-AlMg1(C)(ENAW-5005A)dimenzija 70x20x1,5 mm. Osno-ve za ispitivanje cvrstoce spoja takoúe su izraúene od legure aluminijuma ENAW-AlMgl(C)(ENAW-5005A), dimenzija 025x50 mm. Ispitivanje mikrotvrdoce slojeva raúeno je metodom HV03 i cvrstoce spoja ispitivanjem na zatezanje. Merenje mikrotvrdoce izvrseno je u prav-cu duz lamela. Izvrseno je pet ocitavanja vrednosti mikrotvrdoce slojeva u sredini i na krajevima uzoraka od kojih su odbacene dve krajnje vrednosti. Od tri preostale vrednosti prikazane su minimalne i maksi-malne vrednosti. Ispitivanje cvrstoce spoja raúeno je na sobnoj tempe-raturi sa brzinom zatezanja 1cm/60s. Izvrseno je ispitivanje cvrstoce spoja pojedinacno svake prevlake 43F-NS(80Ni20%Cr), 303NS-1(MgZrO335%NiCr) i 210NS-(Zr0224%Mg0), kao i ispitivanje cvrstoce spoja trojnog sistema TBC - Zr02Mg0/MgZr03NiCr/NiCr prevlaka. Is-pitano je po pet epruveta za sve tipove prevlaka od kojih su odbacene dve krajnje vrednosti. 0d tri preostale vrednosti prikazana je srednja vrednost cvrstoce spoja. Morfologija cestica praha Zr0224%Mg0 i po-vrsina deponovane prevlake uraúena je skening elektronskom mikro-skopijom (SEM). Mikrostruktura deponovanih slojeva ispitana je na op-tickom mikroskopu (0M). Analiza udela mikropora u slojevima prevlake uraúena je obradom 5 fotografija na uvelicanju 200x. Preko paus papi-ra mikropore su oznacene i osencene, a njihova ukupna povrsina racu-nala se na ukupnu povrsinu mikrofotografije. U radu je prikazana srednja vrednost udela mikropora u slojevima TBC prevlake.
Depozicija prahova uraúena je sa atmosferski plazma sprej siste-mom, firme Plasmadyne, i plazma pistoljem SG-100, sa kontrolisanim plazma sprej parametrima. Plazma pistolj SG-100 sastojao se od katode tipa K 1083A-129, anode tipa A 1083-165 i gas injektora tipa GI 1083A-113. Kao lucni gas koristio se Ar u kombinaciji sa He i snaga napajanja do 40 KW. Pre procesa deponovanja povrsine ispitnih uzoraka i povrsina substrata termoabrazivne barijere za zadnja krila avio-na nisu hrapavljene, zbog male debljine substrata od 0,6mm. Vezni slojevi su deponovani sa debljinom od 60 do 80 pm, kermet slojevi sa debljinom od 40 do 60 pm i gornji keramicki sloj sa debljinom od 280 do 300 pm.
Rezultati i diskusija
Vezna metalna prevlaka 43F-NS(Ni20%Cr) imala je najmanje vrednosti mikrotvrdoce od 238 do 254 HV0 3, koje su u granicama vrednosti koje propisuje proizvoúac praha i 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). Izmerene
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vrednosti mikrotvrdoce veznih slojeva ukazuju na to da je udeo mikro-pora u propisanim granicama, sto je potvrdila analiza udela mikropora. Zbog sadrzaja keramike slojevi kermet prevlake 303NS-1(MgZrO335%NiCr) imali su vece vrednosti mikrotvrdoce, koje su bile u rasponu 293-330HVa3 i u skladu sa standardom Pratt & Whitney (Turbojet Engine - Standard Practices Manual (PN 582005)(2002), Pratt & Whitney, East Hartford, USA). Slojevi keramicke prevlake Met-co 210NS-1(Zr0224%Mg0) imali su najvece vrednosti mikrotvrdoce od 478 do 519HV0 3 koje su karakteristicne za ovaj tip prevlake. Ovi slojevi su pokazali najveci udeo mikropora, jer keramicke cestice ostvaruju slabiji medulamelarni kontakt u odnosu na metalne cestice. Zatezna cvrstoca spoja prevlaka bila je u direktnoj vezi sa tipom praha. Najvece vtrenosti cvrstoce spoja od 31 MPa imali su metalni vezni slojevi prevlake 43F- NS (Ni20%Cr). Slojevi kermet prevlake 303NS-1(MgZr0335%NiCr) imali su zateznu cvrstocu spoja od 22 MPa, a naj-manju vrednost od 17 MPa imali su keramicki slojevi Metco 210NS-1 (Zr0224%Mg0). Za sve prevlake vrednosti cvrstoce spoja bile su do-bre, jer su se prevlake deponovale na neohrapavljenim substratima od legure Al koji umanjuju zateznu cvrstocu spoja u odnosu na hrapavlje-ne substrate na bazi legura Fe ili Ni. Srednja vrednost zatezne cvrstoce sistema TBC - Zr02Mg0/MgZr03NiCr/NiCr prevlaka bila je 30 MPa.
Na uzorcima se jasno uocavaju granice meduspoja izmedu slojeva vezne prevlake i substrata, vezne prevlake i kermet prevlake i ker-met prevlake i keramicke prevlake. Medugranica izmedu substrata i slojeva vezne prevlake je izuzetno cista, ukazujuci na dobru vezu slojeva prevlake sa substratom. Na interfejsu izmedu substrata i slojeva vezne prevlake nisu prisutni defekti kao sto je diskontinuitet deponova-nih slojeva, mikropukotine, makropukotine, ljustenje i odvajanje prevlake sa substrata. Generalno, slojevi su ravnomerno deponovani na pod-logu. Duz interfejsa izmedu substrata/vezna prevlaka, vezna prevlaka/ kermet prevlaka i kermet prevlaka/keramicka prevlaka nisu prisutne mikropukotine i makropukotine. Veza izmedu svih slojeva je dobra. U slojevima deponovanih prevlaka nisu uocene neistopljene cestice praha, ukazujuci da su prahovi deponovani sa optimalnim parametrima depozicije. Analiza mikrofotografija je pokazala da su u slojevima vezne prevlake 43F-NS(Ni20%Cr) prisutne mikropore sa srednjim ude-lom od 2,6%. Udeo mikropora u slojevima kermet prevlake 303NS-1(MgZr0335%NiCr) bio je 7%, a u slojevima keramicke prevlake 210NS-1 (Zr0224%Mg0); sadrzaj mikropora bio je 12%. Analiza mikrofotografija je pokazala da su u slojevima vezne prevlake 43F-NS(Ni20%Cr) prisutne mikropore sa srednjim udelom od 2,6%. Udeo mikropora u slojevima kermet prevlake 303NS-1(MgZr0335%NiCr) bio je 7%, a u slojevima keramicke prevlake 210NS-1 (Zr0224%Mg0) sadrzaj mikropora iznosio je 12%. SEM analiza morfologije povrsine de-ponovane keramicke cestice praha Zr02Mg0 pokazuje potpuno toplje-
nje i razlivanje keramickih cestica na prethodno deponovani keramicki sloj. Istopljena cestica praha je u sudaru sa povrsinom prethodno de-ponovanog sloja formirala priblizno kruzan oblik. Na povrsini cestice vi-di se fina mreza mikropukotina koja se ne moze izbeci i uvek se javlja u procesu depozicije (Guo, et al., 2011, pp.161-174). Mikropukotine se formiraju za vreme hladenja istopljene cestice do temperature prevla-ke. Unutrasnji slojevi prevlake koji imaju vecu temperaturu u odnosu na povrsinu prevlake izlozeni su naponima na istezanje i suprotstavlja-ju se skupljanju cestice na povrsini prevlake. S druge strane, cestica na povrsini prevlake koja se hladi i skuplja tokom ocvrscavanja izloze-na je naponima na sabijanje. Naponi na istezanje deponovanih slojeva koji su uvek veci od napona na sabijanje cestice koja se hladi na povrsini prevlake uzrokuju stvaranje mikropukotina na povrsini cestice (Guo, et al., 2011, pp.161-174), (Mrdak, et al. 2013b, pp.559-567), (Mrdak, 2013a, pp.426-432), (Mrdak, et al., 2015a, pp.337-343). U mi-krostrukturi su prisutni fini precipitati nepravilnog oblika velicine do 5 pm, koji su zaokruzeni zutom bojom. Na SEM mikrofotografiji jasno se vide mikropore nepravilnog oblika crne boje, velicine do 5 pm, zaokru-zene zelenom bojom.
Zakljucak
U ovom radu je atmosferskim plazma sprej procesom (APS) proizveden troslojni sistem termobarijernih prevlaka TBC-ZrO224%MgO/MgZrO335%NiCr/ Ni20%Cr. Sistem deponovanih prevlaka sastojao se od veznog sloja Ni80%Cr, srednjeg prelaznog medukermet sloja MgZrO335%NiCr i gornjeg keramickog sloja ZrO2 24%MgO. Prevlake su deponovane na ispitnim uzorcima od legure Al na povrsinama bez hrapavljenja. Analizirane su mehanicke karakteri-stike i mikrostrukture slojeva prevlaka u deponovanom stanju, na osno-vu cega se doslo do odredenih zakljucaka.
Troslojni sistem termobarijernih prevlaka imao je dobre mehanicke karakteristike sa vrednostima mikrotvrdoce veznog sloja od 238 do 254HV0 3, prelaznog kermet sloja od 293 do 330HV0.3 i gornjeg keramickog sloja od 478 do 519HV03. Vrednosti mikrotvrdoce bile su u gra-nicama koje propisuje standard Pratt & Whitney. Cvrstoce spoja deponovanih prevlaka na neohrapavljenim uzorcima od legure Al imale su dobre vrednosti. Za vezni sloj zatezna cvrstoca spoja bila je 31 MPa, za kermet prevlaku 22 MPa, a za keramiku 17 MPa. Cvrstoca spoja troslojnog sistema TBC prevlaka bila je 30 MPa. Analiza mikrofotogra-fija je pokazala da je srednji udeo mikropora u veznim slojevima bio 2,6%, u kermet slojevima 7% i u keramickim 12%. Mikrostruktura deponovanih slojeva prevlaka je lamelarna. Osnova prevlake sastoji se od cvrstog rastvora hroma u niklu y - Ni(Cr). Izmedu lamela cvrstog rastvora u slojevima prevlake prisutni su svetlosivi oksidi tipa NiO, Ni-
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Сг204, Cr2O3 i CrO3, usled oksidacije cestica praha u plazmi tokom procesa izrade prevlake. U vecini slucajeva prisutan je oksid hroma Cr203, a rede oksid Cr03, koji se formiraju u vidu tankog sloja na povr-sini NiCr lamela. Kermet medusloj imao je ravnomernu raspodelu lamela vezne prevlake izmedu keramickih lamela. Gornji keramicki sloj ravnomerno je deponovan na kermet sloju bez prisustva neistopljenih cestica.
Troslojni sistem termobarijernih prevlaka
Zr0224%Mg0/MgZr0335%NiCr/ Ni20%Cr deponovan na substratu od legure Al kao termoabrazivne zastite zadnjih krila aviona J-22 pokazao se kao pouzdana zastita od temperaturnog mlaza i abrazivnih cestica raketa munje i groma.
Kljucne reci: supstrati, zastita, svojstvo, slojevi, prevlake, barijere, legure.
Datum prijema clanka / Дата получения работы / Paper received on: 28. 11. 2015. Datum dostavljanja ispravki rukopisa / Дата получения исправленной версии работы / Manuscript corrections submitted on: 10. 12. 2015.
Datum konacnog prihvatanja clanka za objavljivanje / Дата окончательного согласования работы / Paper accepted for publishing on: 12. 12. 2015.
© 2016 Autor. Objavio Vojnotehnicki glasnik / Military Technical Courier (www.vtg.mod.gov.rs, втг.мо.упр.срб). Ovo je clanak otvorenog pristupa i distribuira se u skladu sa Creative Commons licencom (http://creativecommons.org/licenses/by/3.0/rs/).
© 2016 Автор. Опубликовано в "Военно-технический вестник / Vojnotehnicki glasnik / Military Technical Courier" (www.vtg.mod.gov.rs, втг.мо.упр.срб). Данная статья в открытом доступе и распространяется в соответствии с лицензией "Creative Commons" (http://creativecommons.org/licenses/by/3.0/rs/).
© 2016 The Author. Published by Vojnotehnicki glasnik / Military Technical Courier (www.vtg.mod.gov.rs, втг.мо.упр.срб). This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/3.0/rs/).
