B Goswami

Thermal Barrier Coating System for Gas Turbine Application - A Review

Ceramic coatings are refractory metal compounds deposited on substrates to reduce thermal loss and to protect components from high temperature. Thermal barrier coatings (TBC) are composite overlay of bond coat and ceramic coat oil a superalloy substrate. Atomised deposition or splat deposition of fine semi-molten particle technique deposits thin coatings of brittle ceramic. Thermal and mechanical strains arising from service exposure require structural compliance tolerances. This is facilitated by brittle constituent deposition over a ductile substrate. Electron beam, physical vapour deposition and plasma spray technique lead to a tortuous intergranular network of coating Porous deposition technique is applied in all cases instead of cementation or continuous section thickness. Thermal barrier coating is inevitable in aerospace engine sections operating at limiting conditions of strains. Thermal barrier coatings help in protection of high temperature components for maximum utilisation of component lives, and maximum utilisation of energy by operating at optimum allowable temperature limits. Thermo mechanical behaviour of TBC is optimised by in-silu formation and transformation mechanisms of alumina from aluminium of substrate/bond coat and metastable tetragonal zirconia to stable tetragonal zirconia respectively at temperature of service. While the former produces a volumetric contraction, the latter produces volumetric expansion. In service the composite system provides auto-toughening effects in due course. An intergranular tortuous network of coating forms cracks on exposure of strain and the crack tip blunting forms cubic allotropy from metastable tetragonal phase, resulting in an increase in toughness due to elimination of c/a ratio. However, a prolonged exposure forms, localised spallation zones, which are initiated by volumetric expansion stresses associated with nickel enrichment of thermally grown oxides (TGO) at bond coat/ceramic coat interface, and auto-sintering. Bond coat is applied to produce mechanical adherence and stress relaxation effects. Generally M-CrAlY families of bond coating alloys are used for this purpose. Exposure to operating/test temperature produces thermally grown oxides (TGO) at interface. This occupies an intermediate zone in response to property interactions. TGO mainly consists of alumina being catalyzed by chromia and adhered by yttria. Active research is going oil to study the mechanisms of auto sintering and auto-toughening of TBC. Work is in progress to explore how to decrease thermal expansion mismatch stresses by application of composite. coatings made from functionally graded materials, microlaminated, and multilayered ceramic/ceramic or metallic/ceramic or metallic/metallic coatings. The application of laser scaling or remelting to reduce porosity of free surface and to increase glaze are other avenues to reduce diffusion of reactive gases and to increase internal heat, transfer respectively. The former increases life of bond coat/substrate, whereas the latter increases energy efficiency by maximum utilisation of heat. The main unsolved problem is spallation of ceramic coating, which is cohesively induced, in either side of interface and spread out to interfaces of adhesion. TBC increases. life more than two-fold for cases of aerospace engines. However localised spallation may rise by high temperature corrosion of bond coat/substrate, TGO stresses, gaseous/liquid contaminant diffusion/impregnation through tolerance networking of voids, and erosion.

Assessment of service exposed boiler tubes

Boiler tubes in power plants have finite life because of prolonged exposure to high temperature, stress and aggressive environment. Service-exposed platen superheater and reheater tubes (148,900 h) made of 2.25Cr-1 Mo steels in a 120 MW boiler of a thermal power plant were evaluated for remnant life. The investigation included hot tensile tests, hardness measurement, dimensional measurement, microscopy and creep tests. Experimentally determined yield and ultimate tensile strength, and estimated 10,000–100,000 h rupture strength in the temperature range 520–580 °C, exhibited a decreasing trend with increasing temperature. Microstructural study did not reveal any significant degradation in terms of creep cavities, cracks, graphitization, etc. Analysis of tensile and stress rupture data revealed that although there was degradation of the tubes due to prolonged service exposure in terms of the ultimate tensile strength values, stress rupture plots showed that the service exposed superheater and reheater tubes could remain in service for a length of more than 10 years at the operating hoop stress of 40 MPa/540 °C, provided no localised damage in the form of cracks or dents develop.

High temperature mechanical properties of thermal barrier coated superalloy applied to combustor liner of aero engines

High temperature load controlled fatigue, hot tensile and accelerated creep properties of thermal barrier coated (TBC) Superni C263 alloy used as a candidate material in combustor liner of aero engines are highlighted in this paper. Acoustic emission technique has been utilised to characterise the ductile-brittle transition temperature of the bond coat. Results revealed that the DBTT (ductile to brittle transition temperature) of this bond coat is around 923 K, which is in close proximity to the value reported for CoCrAlY type of bond coat. Finite element technique, used for analysing the equivalent stresses in the bond coat well within the elastic limit, revealed the highest order of equivalent stress at 1073 K as the bond coat is ductile above 923 K. The endurance limit in fatigue and the life of TBC coated composite under accelerated creep conditions are substantially higher than those of the substrate material. Fractographic features at high stresses under fatigue showed intergranular cleavage whereas those at low stresses were transgranular and ductile in nature. Delamination of the bond coat and spallation of the TBC at high stresses during fatigue was evident. Unlike in the case of fatigue, the mode of fracture in the substrate at very high stresses was transgranular whereas that at low stresses was intergranular in creep.

Creep and Fatigue Behavior in Micro-alloyed Steels – A Review

Abstract This is a study of microalloyed steels for power plants and reactors. Components operate at coal dust fire temperature or thermal states of reactors, prone to creep during its service. This is to assess remaining life after passage of valuable life by variation in microstructure, e.g. cavity formation. Precipitation at the sub-grain boundaries and grain interior has increased high temperature strength. Coarsening of these appears at the end of life. Variation of heat treatment like spheroidising in place of solutionizing has been responsive to deteriorate performance. Dislocation interplay with precipitate has been acceptable while interaction among dislocations to forest dislocation has been unacceptable. Dislocation assisted nucleation of precipitates of fine size has been found to strengthen steel by thermo-mechanical control process with in greater heating temperature and lower finish rolling temperature. High temperature performance of materials has been assessed by creep, accelerated creep, creep-fatigue and fatigue performances. Increasing temperature for increasing efficiency has correlated the phase transformation of steel. Fatigue performances have been included in creep properties of materials when intermittent shut down–shut up schedules are operated, e.g. peaking power plants.

Fatigue damage of a thermal barrier coated Ni-base superalloy

High temperature force controlled fatigue testing of thermal barrier coated (TBC), bond coated only and bare Superni C263 superalloy were conducted in air. Results reveal that the endurance limits for the TBC and bond coated substrate were substantially higher than that of the base alloy, while the opposite was found for high stress, low cyclic life times. It appears that the increase in endurance limit for the TBC and bond coated superalloy is due,to load shifting to the bond coat, interdiffusion of A] from coating to substrate and the premature failure for these two materials is possibly due to high stress crack imitation/growth in the TBC/bond coat layers. The mode of fracture in the substrate at very high fatigue stress was intergranular whereas that at low stress was transgranular. Spallation of the ceramic layer was evident at very high fatigue stress and also at low fatigue stress where the TBC composite specimen failed after 5400107 cycles during fatigue testing at 1073 K in air, due to a continuous alumina scale growth at the top coat (TBC) / bond coat interface.

Damage Analysis of Service Exposed Reformer Tubes in Petrochemical Industries

Abstract Accelerated creep or stress rupture data is used for remaining life assessment for life management studies of elevated temperature components, e.g. for reformer tubes where packed nickel catalysts are used for synthesis of hydrogen, ammonia etc. This research has become a regular task because of large range of time for failure (3 to 15 years) compared to designed life (11.4 years or 100,000 hours) and huge loss associated to damage, production and safety hazards. Utilization of appropriate inspection during plant shut down has been strategic short term life assessment. Tests have been typically done by high temperature mechanical properties, microstructure analysis and accelerated creep. Inspection of micro-cracks, hot spot formation, carburization/metal dusting for inner wall and oxidation, tube diameter increment for outer wall inspection have been traditional symptoms of expiry of tubes after service exposure. Aim of this review has been to study damage analysis of reformer tube in response to so wide time frame for failures and accidents involved, even after stipulation of designed time schedules.

Application of thermal barrier coatings on combustion chamber liners - A review

Thermal barrier coatings (TBC) help to reduce the temperature of combustion chamber liner by 473-573K during operation on being aided by swirls of film cooling air. Versatility and low production cost make air plasma spray (APS) TBCs more attractive on liners. However spinal formation at the top to bond coat interface induces ceramic sintering rate and forms thermally grown oxide (TGO) growth. Lifing of engines is attempted to utilize existing design and remnant life within design constrains by giving emphasis on coating philosophies. A proper and efficient substitute with multimetallic bond coat and ceramic topcoat yields longer hours of exposure. Parallel removal of harmful elements in the liner material, restriction on the use of poor quality fuels, and atmospheric effects increase life. Studies of remnant life assessment have been found to be based on control of parameters that check TGO growth, increase adhesion of thicker TGO and restrict ceramic top coat sintering. For example, ceria or lanthana stabilized zirconia transform at comparatively higher temperature than yttria stabilized zirconia. The current scenarios of protection have been changed to replacements by continuous fiber ceramic composite (CFCC), and ceramic matrix composites (CMC) component; e.g. Sylramic, Nicalon, Naxtal, and Ceracurb. Non-destructive examination of ceramic translucence based on Parkers optical property produces in-situ information about ceramic degradations.

Damage Resistance of a Thermal Barrier Coated Superalloy for Combustor Liners in Aero Turbines During Fatigue and Creep

Abstract This paper deals with an evaluation of the lifetime of a thermal barrier coated (TBC) C263 superalloy under fatigue and creep loading. Results revealed that both TBC and bond-coated substrate had higher endurance limits than the base alloy, while the opposite was found for high stress, low cyclic lifetimes. At high stress, the premature failure for these two materials is possibly due to high stress crack initiation/growth in the TBC/bond coat layers. Oxidation is the cause of the reduced life of the bare substrate as compared to the coated substrate while fatigue and creep experiments are carried out in an oxidizing environment. During 800 °C fatigue, the bare specimens behave differently from the coated specimens, but both the bond-coated only and bond coat + TBC specimens seem to exhibit very similar results that are within experimental scatter. Delamination of the bond coat, oxidation of the substrate and spallation of the ceramic layer were evident at very high fatigue and creep stresses. Lateral cracks that grew in the ceramic layer parallel to the stress axis were responsible for spallation of the top coat (TBC) at a very high fatigue stress, whereas, at low creep stress, spallation of the top coat was due to the growth of alumina scale (of thickness >3μm) at the top coat (TBC)/bond coat interface.

Influence of thermally grown oxide scale on fatigue resistance of a thermal barrier coated superalloy

Abstract The life of thermal barrier coating prior to spallation is dominated by micro-cracking in both the thermally grown oxide and the yttria stabilized zirconia top coat. The damage generated by this micro-cracking is expected to be a primary life limiting factor. High temperature force controlled fatigue testing of thermal barrier coated (TBC), bond coated only and bare Superni C263 superalloy was conducted in air. It was observed that the coated materials had higher endurance limits than the bare superalloy and the premature failure for these two materials is possibly due to high stress crack nucleation and growth in the TBC/bond coat layers. Oxidation is also the cause of the reduced life of the bare substrate as compared to the coated substrate while fatigue testing is carried out in an oxidizing environment. Spallation of the ceramic layer was evident at very high fatigue stress and also at low fatigue stress where the TBC composite specimen failed after 5,400,107 cycles during fatigue testing at 800 °C in air due to a continuous alumina scale growth (thickness >3 μm) at the bond coat/TBC (top coat) interface. La vie d’un revêtement thermique avant l’écaillage est dominée par la micro fissuration tant de la calamine développée thermiquement que de la couche supérieure de zircone stabilisée à l’oxyde d’yttrium. On s’attend à ce que le dommage engendré par cette micro fissuration soit un facteur primaire de limite de vie. On a effectué un essai de fatigue de force contrôlée, à haute température, à l’air, du superalliage Superni C263 revêtu d’une barrière thermique (TBC), avec seulement une couche d’ancrage et de l’alliage à nu. On a observé que les matériaux avec revêtement avaient des limites d’endurance plus élevées que le superalliage nu. La défaillance prématurée de ces deux matériaux est possiblement due à la nucléation et à la croissance élevées de fissures de contrainte dans les couches TBC/couche d’ancrage. L’oxydation est également la cause de la durée de vie réduite du substrat nu par rapport au substrat avec revêtement lorsque l’essai de fatigue est effectué dans un environnement oxydant. L’écaillage de la couche de céramique était évident à des contraintes très élevées de fatigue et également à de faibles contraintes de fatigue où l’échantillon composite TBC s’est détérioré après 5400107 cycles lors de l’essai de fatigue à 800 °C à l’air. La détérioration est attribuée à la croissance continue d’une écaille d’alumine (épaisseur > 3 μ m) à l’interface couche d’ancrage/TBC.

Performance and Life Assessment of Reformer Tubes in Petrochemical Industries

Abstract High temperature mechanical properties of service exposed reformer tubes are compared and revalidated with those of virgin material for the sake of health assessment studies. Technical interpretations of creep and stress rupture (accelerated creep) data usually are in close proximity in absence of defects. Results of extrapolation of accelerated creep data were optimized. This review article is aimed at studying life prediction methodology of reformer tubes operating at high temperature and aggressive chemical environment, in response to the designed time frame for preventing failures and accidents, even before stipulated design time schedules in some cases.