Papers by Keyword: Spallation

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: Several Nb based alloys (Nb-20Mo-15Si-25Cr, Nb-20Mo-15Si-25Cr-5B, Nb-20Si-20Cr-5Al, and Nb-20Cr-20Si-5Hf) have been prepared to evaluate the oxidation resistance from 700 to 1400°C in air. The phase identification was determined by calculating the isothermal sections in this temperature range using PANDATTM software. The experiments involve static heating for 24 hours (short term oxidation, STO) or 7 cycles of 24 hour heating (long term oxidation, LTO). Weight gain per unit area as a function of either temperature (STO) or time (LTO) has been used to determine their oxidation resistance. However, SEM, EDS on SEM, x-ray mapping, and XRD have been used to evaluate the oxide scale characterization and the influence of various microconstituent effects have been determined. It appears that B addition may be beneficial while Al is advantageous in comparison to Hf addition. The problem of pesting, typically, in a range of temperature from 900 to 1100°C needs to be controlled through minor additions since the alloys exhibit fairly good resistance at lower and higher temperatures up to 1400°C.

Abstract: During service, TBC can suffer degradation by CMAS, FOD, erosion or spallation. Whereas the first three are due to foreign particles, the last one is related to thermal cycling. When subjected to high temperature exposures followed by rapid coolings under oxidizing conditions, a TBC system undergoes morphological changes and stress development. This will initiate cracks which propagate and finally lead to failure by spallation. Consequently, the aim of the present study is to understand better the mechanisms responsible for such spallation events. Two kinds of TBC systems with different bond coatings (NiCoCrAlYTa or Pt-modified nickel aluminide bond coatings) are thermally cycled. Subsequently, SEM investigations on TBC systems after spallation concentrate on failure path, defect, morphological and microstructural changes to propose way for improving TBC system lifetime.

Abstract: Alumina scale adhesion on high temperature alloys is known to be affected primarily by sulfur segregation and reactive element additions. However adherent scales can become partially compromised by excessive strain energy and cyclic cracking. With time, exposure of such scales to moisture can lead to spontaneous interfacial decohesion, occurring while the samples are maintained at ambient conditions. Examples of this Moisture-Induced Delayed Spallation (MIDS) are presented for NiCrAl and single crystal superalloys, becoming more severe with sulfur level and cyclic exposure conditions. Similarly, delayed failure or Desk Top Spallation (DTS) results are reviewed for TBC’s, culminating in the water drop failure test. Both phenomena are discussed in terms of moisture effects on bulk alumina and bulk aluminides. A mechanism is proposed based on hydrogen embrittlement and is supported by a cathodic hydrogen charging experiment. Hydroxylization of aluminum from the alloy interface appears to be the relevant basic reaction.

Abstract: Thermally grown oxide (TGO) spallation increases the degradation rate of aluminide protective coatings during thermomechanical cycling. Thermal expansion misfit between TGO, bond coat and substrate, applied mechanical load in the system, and local instabilities are known triggers for spallation. Mechanical tests have been performed on coated and oxidised AM1 superalloy. In situ and post mortem study including digital image analysis and SEM were performed in order to characterise strain fields and associated damage field. Good correlation is found between oxide strain and damage extent.

Abstract: Ceramic Thermal Barrier Coatings (TBCs) have been developed for advanced gas turbine engine components to improve the engine efficiency and reliability. The integrity and reliability of these coatings is of paramount importance. Accurate prediction of service lifetimes for these components relies upon many factors, and is not straightforward as knowledge of the service conditions and accurate input data for modelling are required. The main cause of failure of coatings is through debonding which develops as a consequence of thermally induced strains between the metallic bondcoat and the alumina TGO layers due to the differences in the thermal expansion coefficients of the individual layers. Thermal transients due to the power cycles of turbines will then cause these fractures to grow between the TGO and the bondcoat. When these fractures reach a critical size they can grow rapidly and cause the TBC to spall off. Thermal cycling of TBCs is used therefore to evaluate and rank TBC performance. Within the laboratory this is often conducted under isothermal conditions. Whilst this test method has performed adequately in the past it does not fully simulate service conditions. Work has been underway therefore to develop a more complex test method, which better simulates the service conditions experienced by the TBC. The approach here employs a gas torch to heat the operating face of the TBC whilst cooling the rear of the substrate with compressed air, thereby imparting a heat flux on the specimen. The specimen is then cycled by removing the gas torch and cooling with compressed air on the front and rear faces. Tests have been conducted on a TBC system consisting of an IN738 substrate with a CN334 bondcoat and EBPVD TBC. Thermal cycling tests have been performed under both isothermal and heat flux conditions. During the course of the tests the samples were examined non-destructively using a thermal camera to identify early indications of spallation. This paper reports on the performance of the flame rig equipment and the results from the exposures on the TBC system.

Abstract: The spallation of thermal barrier coatings (TBCs) is promoted by thermally grown oxide (TGO). To improve TBCs, it is very important to understand the influence of TGO on the spalling stress. In this study ,the TBCs were oxidized at 1373 K for four diferent periods: 0, 500,1000 and 2000 h. The distribution of the in-plane stress in oxidized TBCs, s1, was obtained by repeating the X-ray stress measurement with low energy X-rays after successive removal of the surface layer. The distribution of the out-of-plane stress, s1− s3, was measured with hard synchrotron X-rays, because high energry X-rays have a large penetration depth. From the results by the low and high energy Xrays, the spalling stress in the oxidized TBCs, s3, was evaluated. The evaluated value of the spalling stress for the oxidized TBC was a small tension beneath the surface, but steeply increased near the interface between the top and bond coating. This large tensile stress near the interface is responsible for the spalling of the top coating.