Papers by Keyword: TGO

Abstract: MCrAlY can be used as bond coats for thermal barrier coatings (TBCs) with good ductility and excellent resistance against high temperature oxidation and hot corrosion. The behavior of the thermally grown oxide (TGO) scale formed at the MCrAlY coatings plays a key role on the oxidation resistance. In this paper, the oxidation kinetic curves of a MCrAlY coating at 900~1000 °C were obtained by measuring the thickness of the TGO scales. The curves basically conveyed parabolic laws, indicating a diffusion-controlled mechanism of the TGO growth. The thickness of TGO was positively correlated with the consumption of β phase during the early stage of the oxidation processes. After about the half-life of the β phase consumption, the depletion of the β phase significantly accelerated, which was caused by coating-substrate interdiffusion. In addition, the microstructure of the TGO was analyzed

Abstract: The bond coat plays an important role in the failure of the thermal barrier coating (TBC) system used for gas turbines ^{[1, 2]}. In this research, the CoNiCrAlY coated Ni-base superalloy specimens were used for developing evaluation method for interfacial damage in the coat. Samples were exposed at 1000°C and 1100°C for up to 1000 hours. The morphology and residual stress in the thermally grown oxide (TGO) layer on the CoNiCrAlY coating were characterized by microscopic observation and luminescence spectroscope, respectively. The microstructure and damage o\n both the coating surfaces and the cross sections were observed by optical microscope and scanning electron microscope. According to the results, the low pressure plasma sprayed CoNiCrAlY coating (LPPS) showed the thinnest TGO layer and lowest residual stress.Residual stress decreased with an increase in exposure time, depending on the morphology of TGO layer. The effects of thermal spraying methods on the oxidation of yttrium in TGO layer and BC layer and its influence on interfacial damage were discussed.

Abstract: This paper presents the results of a study of the oxidation behavior of NiAl produced by gel combustion synthesis calcined at two different temperatures. The objective is to compare the oxide growth rates, oxide scale composition, morphology and elemental composition of the sample powder subjected to isothermal oxidation and calcined at 1050 °C and 1300 °C for 1, 2, 4 and 10 hours by means of mass gain measurements, X-ray diffraction (XRD), field emission scanning electron microsocopy (FESEM) and energy-dispersive spectrometry (EDX) in order to investigate the reliability of the gel combustion synthesis method and evaluate the effect of calcination temperature on the oxidation behaviour of the powder. It was found that for the sample calcined at 1300°C the sample was made up mainly of metastable and stable alumina before oxidation and stable alpha alumina after oxidation whereas for the powder calcined at 1050°C the sample was mainly composed of detrimental mixed oxides before and after oxidation. Overall findings indicate that the oxidation behavior of the powder calcined at 1300°C is more protective compared to the powder calcined at 1050°C.

Abstract: CoNiCrAlY bond coat (BC) and top ceramic coating (TCC) was fabricated on the GH99 super alloy by high velocity oxyfuel spray (HVOF) and air plasma spray (APS), respectively. Thermal cycling treatment was applied to the thermal barrier coatings (TBCs). The cross-sectional images of crack initiation and propagation of TBCs after treatment were investigated by scanning electron micrograph (SEM), meanwhile crack initiation and propagation in TBCs were analyzed based upon ABAQUS software using extended finite element method (XFEM). The results show that, crack initiation and propagation can be easily traced via microscopy at the interface areas in TBCs; after thermal cycling treatments, the crack associated with the TCC/TGO interface morphology initiates at interface peak area and propagates along TCC/TGO interface with thermal cycles; the interface roughness affects the crack magnitude in length and width obviously, the rougher the morphology, the bigger the crack is; the XFEM is a novel and effective method to well predict the crack initiation and calculate the crack propagation, and simulation and experimental results fit well.

Abstract: In the present paper, an attempt has been made to discuss the degradation mechanism of bond coat and bond coat/top coat interface in thermal barrier coating system in the presence of corrosive salts such Na2SO4 and V2O5.These salts come from impurities in low grade fuel used in gas turbine industry. Salt mixture of Na2SO4 + V2O5 was prepared and applied on surface of thermal barrier coating (TBC) specimens. The specimens were exposed isothermally to 900oC for 200, 400 and 700 hours. SEM analysis revealed the formation of thermally grown oxide (TGO) in specimens sprayed with corrosive salts. Results revealed that there was no degradation of either bond coat or bond coat/top coat interface up to 200 hours of isothermal exposure .Interface cracking and spallation was observed after 400 hours of isothermal exposure owing to depletion of zirconia stabilizer i.e yttria and phase transformation of tetragonal zirconia to monoclinic zirconia.

Abstract: Considering the thermally-growth oxide (TGO) that grows between ceramic coating and bond coat interface and surface topography of bond coat in a TBC system, the effect of residual stresses distribution by growth of TGO and cone interface topography in thermal barrier coating was calculated. The calculating result shows that the residual stress of TGO interface is affected by interface topography unit size and topography distribution density obviously. The stress between TGO and ceramic coating interface is greater than the stress between TGO and bond coat interface. Stress concentrates in topography center and the maximal value is 870MPa.The stress decreases with the increase of topography quantity.

Abstract: Thermal barrier coatings (TBCs) were deposited by an Air Plasma Spraying (APS) technique. The TBC coating comprised of 92 wt.% ZrO2 and 8 wt.% Y2O3 (YSZ), CoNiCrAlY bond coat, and MarM247 nickel base super alloy. After APS of YSZ two batches of TBC specimens were tested, one batch of which was pre-oxidised in air for 10h at 1080 oC. Both types of the specimens were directly pushed into a combustion gas at 1150 oC for 25 min and then out to the natural air for quenching. The combustion gas was produced by burning jet fuel with high speed air in a high temperature wind tunnel, which simulates the real service conditions in an aeroengine. Results show that TBCs prepared by the APS had good thermal shock resistance in the combustion gas. The pre-oxidation treatment of the TBC had a significant effect on its thermal shock life. The as-oxidised TBCs always had worse thermal shock resistance than the as-sprayed ones after thermal shock cycles.

Abstract: It has been found that under oxygen partial pressure of ~2×10-6 kPa, the high-temperature oxidation of thermal barrier coatings (TBCs) occurred during an electron beam physical vapor deposition (EB-PVD) process for producing the TBCs top ceramic coating. In the present investigation, two modified bond coats (BCs) of NiCrAlY with Si addition, and NiCrAlY with Co and Hf additions, were developed by Arc Ion-plating technique to study the effects of the EB-PVD process on thermally grown oxide (TGO) formation and growth. The isothermal and cyclic oxidation tests were conducted and the cross-sectional morphologies of the specimens were examined to compare the high-temperature oxidation behaviors of the two TBCs. It was found that a mixed oxide layer have been developed in the as-deposited TBCs with a NiCrAlYSi BC. The mixed oxide layer mainly included Cr2O3, NiO, Al2O3 and their spinel. With the mixed oxide layer, TBCs with the NiCrAlYSi BC showed a superior high-temperature resistance on later high-temperature exposure to TBCs with NiCoCrAlYHf BC, where no mixed oxide layer was observed. The pre-formed mixed oxide layer apparently shortened the time to fully develop a protective α-Al2O3 layer and therefore restrained the TGO growth in TBCs.

Abstract: Growth behavior of thermally grown oxide (TGO) layer with thermal fatigue condition in thermal barrier coatings (TBCs) was investigated, including its effect on fracture behavior of TBCs and bonding strength of top coat. The formation of TGO layer was influenced by both temperature and time exposed. However, the TGO thickness was independent on the bond coat thickness (80, 140, and 280 µm) and the preparing method (APS and HVOF methods) of the bond coat. In Hertzian indentation tests for the TBCs before thermal fatigue, the TBCs with the bond coat prepared by APS showed fracture and/or delamination in regions of the top coat near to the interface, whereas the TBCs prepared by HVOF indicated cracks and fracture at the interface. After thermal fatigue, the fracture path passed along the TGO layer without any cracks created from Hertzian indentation within the top coat in both cases. The bonding strength of the top coat measured by adhesion tests shows lower values for the TBCs with the HVOF bond coat than those with the APS bond coat before thermal fatigue. However, the values are similar to each other after thermal fatigue. This result is attributed to the fracture path of the TBCs, depending on the TGO formation.