Masayuki Yoshiba

Microstructural features of mechanical failure in thermal barrier coating systems under static loadings

Abstract In order to clarify qualitatively and quantitatively the failure mechanism of plasma-sprayed thermal barrier coating (TBC) systems from the microstructural viewpoint, in situ observation of the mechanical failure behavior was conducted for TBC systems under the static loadings at ambient temperature; as the most fundamental aspect, by means of an optical microscopy. Several kinds of TBC systems were prepared by using different sorts of ceramic coating materials. Mechanical tensile loading or compressive loading was gradually applied to the plate shape of TBC specimen using a four-point bending test methodology. It was found that the tensile failure behavior of TBC systems depends strongly on the top-coat microstructures as well as heat treatment after the plasma spraying. The compressive failures were also found rather incidental and depended on the strength of top-coat at the interfacial region. Among different TBC systems, those with the finely segmented top-coat exhibited a good spalling resistance.

In Situ Observation of Mechanical Failure Behavior for Plasma Sprayed Thermal Barrier Coating Systems at Ambient and Elevated Temperatures

1 緒 言 発電プラントをはじめとする化石燃料使用によるエネ ルギー変換機器の高温・高効率化への要請は近年ますま す強くなり,その中核となる大型の複合火力発電システ ムのトッピングサイクルに運用される先進型高効率ガス タービンでは,クリティカルな熱的・機械的負荷を受け る動静翼部などにおいても遮熱性に優れたセラミックス を高強度超合金基材の表面に被覆させた遮熱コーティン グ (TBC) システムの実用化導入が不可避の状況となっ ている.1)~ 3) 先進型ガスタービン動静翼においては,内部冷却構造 の採用により,実機作動条件下で受ける熱機械的損傷要 因が翼の部位によって著しく変化することが知られてい る.4), 5)すなわち,最大引張負荷が高温域で作用する Inphase型の部位だけでなく,例えばリーディングエッジ のように最大引張負荷がむしろ低温域で作用するOut-ofphase型の部位も存在する.それゆえ最近では,このよ うな複雑で多様な負荷様式を試験条件に反映させた熱機 械的疲労 (TMF) 損傷に基づく TBCの性能評価が実施 されつつあるが,6), 7)未だ十分な損傷解析がなされていな いのが現状である.したがって,このような複雑な熱機 械的負荷要因に対する TBCシステムの損傷メカニズム を解明するためには,高温域のみならず,低温域におけ る TBCシステムの基本的強度特性や損傷き裂の生長挙 動を考慮した損傷メカニズムの解明も重要課題である. このような観点から著者らは,コーティング微構造が 異なる種々の TBCシステムを対象として,静的曲げ様 式による機械的損傷挙動のその場観察を室温で実施し, き裂発生・成長挙動を TBC微構造と関連づけて詳細に 検討してきている.8)~ 10)本研究では,これまでの研究成 果を含め,種々の熱機械的損傷要因に対する TBCシス テムの損傷劣化メカニズムを体系的に把握するために,軸 負荷様式による TBCシステム損傷挙動のその場観察装置 を開発し,室温ならびに高温下での各種 TBCシステムの 引張あるいは圧縮負荷における損傷挙動をコーティング 微構造と関連づけて綿密に検討した. 2 供試材および実験方法 2・1 各種 TBCシステムの構築 基材合金として高強度 Ni基超合金Mar-M247(主要組 成:Ni-9.9Co-8.3Cr-5.5Al-9.9W-3Ta-1.5Hf (mass%),平均 プラズマ溶射遮熱コーティングシステムの室温ならびに 高温における機械的損傷挙動のその場観察 橋 智 吉 葉 正 行 原 田 良 夫 In Situ Observation of Mechanical Failure Behavior for Plasma Sprayed Thermal Barrier Coating System at Ambient and Elevated Temperatures

THE ROLE OF APPLIED CREEP STRESS ON HOT CORROSION BEHAVIOR OF A NICKEL-BASE SUPERALLOY

In order to clarify the role of an applied creep stress on the hot corrosion behavior of nickel-base superalloys, the creep rupture tests were performed of the wrought nickel-base alloy 751 at 800°C in the hot corrosive environment simulated both by coating with a given amount of 90%Na 2 SO 4 −10%NaCl synthetic salt mixture and by controlling the oxygen partial pressure (pO 2 ) of gaseous atmosphere in a wide range from O to 1 atm. The metallographic surface loss as a measure of a general corrosion was found to increase in accordance with a quasi-parabolic kinetics, independently of an applied stress. On the contrary, a preferential intergranular attack was much stimulated under the creep stress, particularly in the higher pO 2 atmospheres. Then the simultaneous action of an applied stress and the hot corrosion associated with high pO 2 atmosphere tends to bring about the most significant reduction of the creep rupture life.

Micro-Area Analysis of the Corrosion Products Formed on Alloy 825 Exposed to Simulated Waste Incineration Boiler Conditions

An analytical study of high-temperature corrosive failure in a waste incineration plant was conducted using a new type EPMA equipped with a Schottky type field emission gun. For sample preparation, a cross-section polisher that uses an argon ion beam was applied. Both the element distribution through the alloy substrate to the scale and the chemical states of sub-micron sized corrosion products were revealed, leading to inferences about the corrosion mechanism.

V 2 O 5 -Na 2 SO 4 -NaCl MOLTEN SALT CORROSION BEHAVIOR OF VARIOUS HIGH TEMPERATURE STRUCTURAL CERAMICS

In order to investigate the molten salt corrosion behavior of various typical structural ceramics; Al 2 O 3 , mullite and ZrO 2 (PSZ) as oxide ceramics, Si 3 N 4 and SiC as non-oxide ceramics, the corrosion tests were performed for them in the V 2 O 5 -Na 2 SO 4 -NaCl systems of molten salts at 900 and 1200°C. It was found that the corrosion resistance of different ceramics was affected by the impurities and the additives such as the stabilizer and the sintering aids as well as their major constituents, depending on the environment and temperature conditions. It was also revealed that some additives such as Y 2 O 3 reacted directly with the corrosive species in the V 2 O 5 -Na 2 SO 4 -induced V-attack environment. On the other hand, non-oxide ceramics such as Si 3 N 4 and SiC were found to be severely corroded at 1200°C in the Na 2 SO 4 -NaCl-induced hot corrosive environment, probably because of an enhanced reaction of the SiO 2 oxide film with the corrosive species such as Na. For the sintered Si 3 N 4 , in particular, the corrosion resistance appears to be affected also by the microstructural factor such as a porosity developed in the processing of ceramics.

Introduction: High-Temperature Corrosion in Energy-Related Systems

Materials used at elevated temperatures are required to have both excellent mechanical properties and oxidation/corrosion resistance. These tend to be opposing properties from the standpoint of alloying, which presents technical challenges to the extent that there is an ever-increasing need to understand better the fundamental issues affecting optimal design of high-temperature materials. Indeed, recent demands for improved energy savings, reduced environmental pollution, and decreased emissions of CO2 and other greenhouse gases impose the need for further advancements in high-temperature materials. A series of specialized meetings entitled, International Symposium on HighTemperature Oxidation and Corrosion (ISHOC), have been held regularly in Japan since 2000. The fourth of these meetings was ISHOC-2014, which was held in the northern city of Hakodate on July 23–27, 2014. The main objective of the ISHOC series is to address current issues of high-temperature materials used in harsh service environments. This includes providing further elucidation of fundamental

Technological Archives of R & D Activities about Corrosion-Resistant Materials and Protective Coating Systems at High Temperatures in Japan - Historical Retrospection and Future Prospect -

In order to search for the future technological strategy about the high-temperature materials including the coating systems with superior environmental performance, historical archives have been made on the research reports of the JSPS 123rd Committee; about Heat-Resisting Materials and Alloys, for more than 50 years. According to the historical and economic backgrounds associated with a variety of the fuel utilization, the environment-resistant materials have been developed to apply skillfully against different kinds of aggressive environments. Furthermore, future prospects of the corrosion-resistant materials and coating systems were attempted from the viewpoints of both the global environment protection and the energy saving and/or new energy development which must contribute for a prevention of the global warming.

In Situ Observation of Failure Behavior for Plasma Sprayed Thermal Barrier Coating Systems under Static Loadings

In situ observation of the mechanical failure behavior was conducted for different kinds of the plasma sprayed thermal barrier coating (TBC) systems by means of an optical microscopy under the static loadings at room and elevated temperatures; as the fundamental aspect, in order to clarify the thermomechanical failure mechanism of TBC system in connection with various coating characteristics. Mechanical tensile or compressive loading was applied progressively to the TBC specimen by an axial loading mode. It was found that the failure behavior of TBC system depends strongly on the testing temperature under both the tensile and compressive loadings. At the elevated temperature which is higher than the ductile-brittle transition temperature (DBTT) of metallic bond-coat (BC), in particular, the ceramic top-coat (TC) spallation can be prevented by virtue of the stress relief induced by the enhanced plastic flow in the BC layer. At the room temperature which is lower than the DBTT of BC, on the contrary, the TC spalling was inevitably induced, but the initiation site of TC spalling is closely related with the magnitude of local plastic deformation in the alloy substrate. Furthermore, an influence of thermally grown oxides (TGO) layer developed at the TC / BC interface on the crack initiation and propagation behavior was investigated in some detail.

Environmental Effect of Hot Corrosion on Creep and Fatigue Failure of Thermal Barrier Coating Systems

In order to clarify the failure behavior of plasma sprayed thermal barrier coating (TBC) systems under the complicated modes of thermal-mechanical-chemical loadings, the stress rupture property evaluation and failure analysis were conducted for Y2O3-ZrO2 (YSZ) and CaO-SiO2-ZrO2 (C2S-CZ) TBC systems in air and two kinds of high-temperature corrosive environments. Static creep loading was found to bring about the typical creep failure for TBC systems even in the aggressive environment so called hot corrosion almost in similar manner to the case in air. On the contrary, it was revealed that the dynamic fatigue loading tends to cause a significant failure life reduction of TBC systems both in air and in corrosive environments. For YSZ TBC system, the penetration crack preexisting through the top-coat layer tends to provide a nucleation site for the fatigue crack even in air, and more significantly a short circuit path for the corrosive species in hot corrosive environment. For C2S-CZ system, on the contrary, the top-coat / bond-coat interface tends to provide easily the nucleation site for a main crack to propagate thereafter toward both the alloy interior and outer surface. Under lower stress level at 950°C, however, the oxide-induced crack closure together with crack tip blunting attributed mainly to the high reactivity of Ca compounds as a major constituent of the TC is effective to suppress substantially the crack propagation, so as to cause the prolonged failure life as compared to YSZ system even in aggressive gaseous environment.

CORROSION PERFORMANCE OF ULTRA-HIGH-PURITY CHROMIUM SHEET IN HIGH TEMPERATURE AGGRESSIVE ENVIRONMENTS

In order to assess utility of the ultra high purity Cr (UHP Cr) sheet developed by the improved processing technique with a powder metallurgy for a high temperature corrosion resistant material, both the high temperature gas corrosion test in the 10 5 Pa-O 2 or the 10 5 Pa-SO 2 atmosphere, and the molten salt corrosion test by the coating method using the V 2 O 5 -Na 2 SO 4 -NaCl systems of salt mixtures were performed, and the corrosion behaviour was investigated from different aspects. The hot gas corrosion behaviour of the UHP Cr was found to be according to parabolic rate law at temperature up to 1273K, exhibiting much higher corrosion resistance than the other heat resisting materials. Furthermore the UHP Cr showed an excellent corrosion resistance against the V 2 O 5 -Na 2 SO 4 and Na 2 SO 4 -rich Na 2 SO 4 -NaCl salt systems as compared with the commercial superalloys. These results suggest the formation of the protective Cr 2 O 3 layer on the UHP Cr which leads to a diffusion controlled corrosion process even in such aggressive environments.