Lantan zirkonat esaslı termal bariyer kaplamaların üretimi ve karakterizasyonu
Production and characterization of lanthanum zirconate based thermal barrier coatings
Yazar: MUHAMMET KARABAŞ
Danışman: PROF. DR. İSMAİL YILMAZ TAPTIK
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Doktora, 2016
Konu: Metalurji Mühendisliği = Metallurgical Engineering ; Mühendislik Bilimleri = Engineering Sciences ; Seramik Mühendisliği = Ceramic Engineering
Dizin: Seramik kaplama malzemeleri = Ceramic covering equipments ; Termal bariyer kaplama = Thermal barrier coating ; Termal püskürtme = Thermal spraying

Özet:
Termal bariyer kaplamalar (TBK), enerji ve uçak-uzay sanayisine ait gaz türbinlerinde yapısal bileşenlerin yüksek sıcaklığa karşı korunmasında çok geniş uygulama alanına sahiptir. Bununla birlikte, güç üretici motorların yüksek verimliliğe sahip olması operasyon sıcaklığının artması olarak düşünülebilir. Termal bariyer kaplama kavramı; soğutulan metalik malzeme ile sıcak gaz katmanı arasına, iş parçasına ısı transferini engelleyecek\yavaşlatacak termal yalıtım yeteneği yüksek malzeme katmanını içermektedir. Günümüzde geliştirmiş en iyi ve yaygın Termal Bariyer Kaplama malzemesi %8 İtriyum ile stabilize edilmiş zirkonyum (YSZ) dur. TBK, seramik bir kaplama olup, oda sıcaklığından 1200 °C'ye kadar kullanılabilir ve bu kaplamalar düşük termal iletkenliğe, yüksek termal şok dayanımına sahiptir. Zirkonya oda sıcaklığından 1170 °C'ye kadar monoklinik kristal yapıdadır ve artan sıcaklıklarda tetragonal (1170-2370°C) yapıdan kübik kristal yapıya (>2700°C ) allotropik dönüşümler gösterir. Bu dönüşümler % 3-5'lik hacim değişimini de beraberinde getirdiğinden kaplamaların bütünlüğü ve servis ömrü kısalır. Kullanımda olan mevcut kaplama malzemeleri yerine termal, mekanik ve kimyasal özellikleri daha üstün malzemeler; türbinlerin daha güvenli şekilde çalışmasını, türbin veriminin artmasını, türbin bakım aralıklarının uzamasını ve son derece pahalı olan sıcak kısım parçalarının ömürlerinin artmasını sağlayacaktır.
Bu çalışma havacılık endüstrisinde kullanılan gaz türbin motorların bileşenlerinden olan türbin kanatlarının ve yanma odalarının yüksek sıcaklıkta oksidasyon ve korozyondan korumak amacıyla kaplanmasında kullanılan Y2O3 (yttria) ile stabilize edilmiş ZrO2 (YSZ)  yerine, daha üstün termofiziksel özellikleri ile kullanılması muhtemel malzemelerden olan La2Zr2O7 (LZ) ve molar olarak %30 Gd2O3, Yb2O3 gibi nadir toprak elementleri ile modifiye edilmiş LZ esaslı kaplamalar amorf ve kristallin tozlar kullanılarak literatürde ilk defa üretilmiştir. 
Kaplamaların üretilmesi amacıyla paslanmaz çelik, aluminyum ve Inconel super alaşım altlık malzemeler üzerine öncelikle HVOF (High Velocity Oxygen Fuel) işlemi ile bağ katman kaplamalar (NiCoCrAlY) üretilmiş, daha sonra seramik üst katman kaplamalar APS (Atmospheric Plasma Spray) işlemi kullanılarak kaplamaların üretim işlemi yapılmıştır.
Karakterizasyon çalışmaları kapsamında kaplamaların mikroyapısal özellikleri, yapışma mukavemetleri, termal iletkenlik, termal çevrim, termal şok ömürleri ve optimum lazer yüzey modifikasyon parametreleri belirlenmiştir. Termal çevrim ve termal şok testleri sonrasında kaplamalarda meydana gelen hasarların sebepleri araştırılmıştır. Lazer yüzey modifikasyonu için 4 farklı parameter belilenmiş ve bu parametreler ile modifiye edilen kaplama yüzeylerindeki mikroyapısal değişimler SEM ile incelenerek intersept metodu ile tane boyutu hesaplanmış ve bu yüzeylerin sertlik değerleri ölçülmüştür.
Karakterizasyon çalışmaları neticesinde tüm kaplamaların başarılı şekilde üretildiği, termal bariyer kaplamaların karakteristik mikroyapı özelliklerini gösterdiği, tüm kaplamaların faz yapısının düzensiz florit yapı olduğu, yapışma mukavemeti ve termal iletkenlik değerleri acısından ise YSZ ve CYSZ' ya göre üstün özelliklere sahip olduğu görülmüştür. Ayrıca Gd ve Yb ile modifiye edilmiş kaplamaların termal çevrim ömürlerinin YSZ ve CYSZ'ya yaklaştığı belirlenmiştir. Termal şok testlerinde ise tüm kaplamalar YSZ ve CYSZ kaplamalara nazaran daha kötü performans göstermiştir. Termal çevrim ve termal şok testleri neticesinde oluşan hasarların sebepleri incelendiğinde ise seramik üst katman kaplamaların sinterlenmesi ile mikroyapının değişmesi, termal genleşme katsayısı uyumsuzlukları ve üst katman ile bağ katman arasında termal olarak büyüyen oksit tabakası (TGO) oluşumu sonucu oluşan gerilmeler ve bu gerilmelerin oluşturduğu kılcal çatlaklar gibi etmenlerin kaplamaların hasar görmesine sebep oldukları sonucuna varılmıştır. Termal çevrim ve termal şok testleri sonrasında kaplamalar faz stabilitelerini korumuştur.
En yüksek termal çevrim performansına sahip olan LYZA ve LYZC kaplamalar için optimum lazer yüzey modifikasyon parametreleri ile yapılan işlem neticesinde 25-40 µm kalınlığında porozitesiz, az pürüzlü ve yoğun bir tabaka başarı ile elde edilmiştir. Lazer ile modifiye edilmiş bölgelerin ortalama tane boyutunun artması ile sertlik değerinin azaldığı sonucuna varılmıştır.

Abstract:
Thermal Barrier Coatings (TBC) find a large application as a protection shield against high temperature for the structural components in stationary and aerospace gas turbines. The Thermal Barrier Coating (TBC) concept involves placing a thermally insulating layer between a cooled metallic component and the hot working gas to reduce heat transfer to the component. However, , increasing the operation temperature to achieve higher efficiencies in power engines is envisaged by state of the art TBCs based on yttria stabilized zirconia (YSZ) which have the ability of low thermal conductivity and high thermal shock resistance and may be used at temperatures up to 1200°C,. Pure zirconia exists in three crystal phases at different temperatures. At very high temperatures (> 2370°C) the material has a cubic structure. At intermediate temperatures (1170 to 2370°C), it has a tetragonal structure while at low temperatures (below 1170°C), it transforms to the monoclinic structure. The tetragonal-to-monoclinic phase transformation is martensitic and can be observed in heating and cooling periods. The transformation occurs while cooling from high temperatures and involves a 3-5% volume increase. The volume change induces a significant shear strain in the structure, affecting the integrity and the service life of the coating.
Improvements on new thermal barrier coating materials, instead of  currently used coatings, will allow the turbines work more reliably, increase the turbine efficiency, extend the turbine maintenance periods and increase the lives of hot section parts that are of high cost. Until today, various researchers have studied Pyrochlores, Fluorite Hexaaluminate, Perovskites and complex form ceramic materials for high temperature service conditions instead of YSZ.  Lanthanum zirconate (LZ), a new candidate ceramic materials for next generation TBC applications, has attracted much interest due to its low thermal conductivity, high melting point (2280 °C), and high phase stability.  However, LZ has a lower thermal expansion coefficient compared to metallic materials, which leads to higher stress levels in a TBC system. For this reason, increasing thermal expansion coefficient of LZ to a value similar to that of YSZ without influencing the pyrochlore structure has always been an important research area.
Lehmann and coworkers reported that thermal expansion coefficients of the materials with a complete substitution of 30% of the rare-earth elements such as Nd, Gd, Eu, Dy were higher than that of lanthanum zirconate in the whole temperature range.
For the atmospheric plasma spray process, Crystalline and coarse powder is usually preferred as the feedstock. Coarse particles consist of nanosized grains. During the plasma spray process, powder particles transforms to semi molten form when passing through the  plasma flame. After transformation, semi molten particles hit the substrate and solidify rapidly. At the end of this process, nanosize grains grow and are distributed randomly. Chen et al. reported that average grain size distribution of LZ coating produced by amorphous feedstock (250nµ) was smaller than produced by crystalline feedstock (450nµ). In addition, thermal conductivity of LZ coating produced by amorphous feedstock lower than coating produced by crystalline feedstock in the room temperature. Also, LZ coating produced by amorphous feedstock has a high thermal expansion coefficient compared to the LZ coating produced by crystalline feedstock.   
In this study, La2Zr2O7 (LZ) and 30% Gd (LGZ), Yb  doped (LYZ) thermal barrier coatings were fabricated from amorphous (A) and crystalline(C) feedstock by atmospheric plasma spray (APS) technique. Inconel 630, AISI stainless steel and aluminium disc shaped samples were used as substrate. Commercial Sulzer Metco Amdry 997 (Ni- 23Co-20Cr-8 .5Al-4Ta-0.6Y) powders were selected to manufacture the bond coats by HVOF process.
Microstructural properties, adhesion strenght, phase structure, thermal conductivity, thermal cycle, shock performance and optimum laser surface modification parameters of TBCs was also investigated.  
The microstructure, morphology and chemical composition of the cross-section of the coatings were examined by field emission electron microscopy (JEOL JSM 7000F) which is equipped with EDS. X-ray diffraction (Rigaku Miniflex) was used to determine the crystalline structure of the powder and coatings. 
Thermal cycling tests were carried out on a burner-rig facility with a propane + oxygen flame. The sample surface was heated from room temperature to 1250±50 °C in 1 minute followed by a cool down process within 1 minute using  a compressed air jet. The cycling process was repeated until 50% of the ceramic coating area was spalled.
Thermal shock tests were performed by heating and water quenching method. The samples were put into the high temperature tube furnace at temperature 1200 °C with the dwell time 5 min, then they were thrown into the water with the temperature 20°C quickly.  The cycling process was repeated until nearly 50% of the ceramic coating area was spalled.
According to microstructural characterization, different layers of usual TBCs can be observed including NiCoCrAlY bond coat and top coat. Thickness of top coat and bondcoat is 250±25 μm and 100±25 μm, respectively.
The microstructure of top coats prepared with crystalline and amorphous powders are relatively porous, which are uniquely found in plasma sprayed coatings.
There are relatively large defects such as large pores and intersplats cracks in the TBCs microstructure which are believed to be produced from crystalline feedstock. However, pores in TBCs from crystalline feedstock are distributed irregularly in microstructure compared to TBCs which are produced from amorphous feedstock. TBCs produced from amorphous feedstock have smaller pores than coatings produced from crystalline feedstock and its pore distribution is very regular. Porosity percentage is about %12 which is close to those of TBCs which are produced from two type of feedstocks.
TBCs produced from amorphous feedstock had an average grain size of approximately 160 nm, which was smaller than the average size of 450 nm for coating produced from crystalline feedstock.
Intersplat boundary cracks of TBCs produced from crystalline feedstock are very distinctive and TBCs produced from amorphous feedstock contain less intersplats cracks and voids. Also, intersplat crack size of TBCs produced from amorphous feedstock are very short compared to TBCs produced from crystalline feedstock.
XRD patterns of as-sprayed TBCs belong to defect fluorite structure and all TBCs consist of defect fluorite phase. After spraying, La2O3 and ZrO2 peaks following the heat treatment of feedstocks disappeared.
Adhesion strenght values of coating changed between 7 and 19 MPa. Lanthanum zirconate based coating produced from crystalline powders have lower bonding strenght values than coating produced from amorphous powders.
Thermal conductivity measurement carried out temperature range between 25-85 ˚C by laser flash method. Average thermal conductvity values of coating were change between 0.46-1.05 W/mK. All of the lanthanum zirconate based coating have lower thermal conductvity value than YSZ and CYSZ based coating. Thermal conductivity value of coating increased with increasing temperature. Lanthanum zirconate based coating produced from crystalline powders have higher thermal conductivity values than coating produced from amorphous powders.
Thermal cycle tests indicate that LYZ and LGZA TBCs have a higher thermal cycling lifetime (400 cycle) than other coatings. Different sintering and solidification behaviours of two type feedstock result in different microstructures. This strongly affected thermal cycling lifetime of TBCs in parallel with thermal expansion behaviour of top coat. High intersplat crack density and size reduced inner stresses due to thermal expansion mismatch. The spallation of the coatings are due to the effect of thermal expansion mismatch and thermal stress produced during thermal cycling. The oxidation of the bond coat and sintering of the top coat are the factors for the damage. In addition, thermal expansion mismatch accelerates the spallation process. There is no observed phase transformation for the all types of TBCs following thermal cycling test. All of TBCs have fluorite peaks as in the as sprayed TBCs, but peak intensity of TBCs increases after thermal cycling tests.    
According to thermal shock test results of TBCs, LGZ-A and LGZ-C TBCs have better thermal shock cycling lifetime compared to other TBCs. The spallation of LGZ-A and LGZ-C coatings started at the middle region of the top coat after 45 and 43 cycles respectively, and then, further spallation started, which is associated with propagation of cracks to the adjacent regions on further cycling, and finally, more than 50% spallation occurred at the 55. and 54. cycles. Similar to the failure processes of LZ-A and LZ-C TBCs, the first spallation of the coatings samples were occurred after 33. and 40. cycles, and nearly 50% spallation occurred in the range of 37 and 48 cycles. 
For LYZ-A and LYZ-C TBCs, first spallation are observed at 34. and 35. cycle, respectively, at the edge of the top coats. After 42 and 43 cycles, approximately %50 spallation of top coats occurred and thermal shock test was stopped.
LGZ-A and LGZ-C TBCs have longer thermal shock lifetime. Lifetime extension of LGZ TBCs, as compared to the LZ and LYZ TBCs, can be explained with respect to increasing thermal expansion coefficient as a result of Gd doping.
The spallation of the TBCs are due to the effect of thermal expansion mismatch and thermal stress produced during thermal shock tests. Oxidation of the bond coat and sintering of the top coat are the factors for the damage.
After thermal shock tests, There is no phase transformation for the all of TBCs. All types of TBCs have a defect fuorite peaks as in the TBCs. But peak intensity of TBCs increases after thermal shock tests.
Optimum lazer surface modification parameters determined for LYZA and LYZC coating which having best thermal cyclic lifetime. LYZA and LYZC coating surface remelted by using 4 different laser modification parameters as Laser power and laser scan speed. After laser surface modification process, microstructural evaluation and microhardness of laser remelted zones investigated. 25-40 µm thickness ,dense and smooth surface with regular crack network obtained as a result of laser surface modification by using optimum parameters as laser power: 75W and scan speed 150mm/sn for LYZA coating, Laser power:100W and scan speed:200 mm/sn for LYZC coating. Hardness and grain size of laser remelted zone affected from the Laser surface modification parameters and it increase with the decreasing average grain size of remelted zone