Browsing by Author "Wellman, Richard G."

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    The effect of TBC morphology on the erosion rate of EB PVD TBCs
    (Elsevier Science B.V., Amsterdam., 2005-01-31T00:00:00Z) Wellman, Richard G.; Deakin, M. J.; Nicholls, John R.
    Since thermal barrier coatings (TBCs) have been used in gas turbines most of the research conducted on them has involved the bond coat and the growth of the thermally grown oxide (TGO) as failure of the bond coat and the TGO were considered to be the primary causes of failure. Erosion of TBCs has been considered as a secondary problem and as such received less attention. Most of the initial work on the erosion of TBCs covered the effects of velocity and impact angle on the erosion rates of both plasma sprayed (PS) and electron beam physical vapour deposited (EB PVD) TBCs and compared the differences between the two deposition systems. Most of the tests were conducted on coatings in the as-received condition. This paper aims at expanding the understanding of the erosion of EB PVD TBCs by examining the effects of TBC morphology, column diameter, column inclination angle and the effects of aging and sintering on the erosion rates of EB PVD TBCs. Monte Carlo modelling and mapping of EB PVD TBCs is also briefly discussed along with the associated mechanisms. It was found that, all else being equal, erosion rate decreases with a decrease in the column diameter, while aging results in an increase in the erosion rate, dependent on the aging temperature and time. A decrease in the inclination angle of the columns with respect to the substrate increases the erosion rate, when the inclination angle is less than 60degrees the erosion rate increases catastrophically. These effects are all discussed and explained in terms of erosion mechanisms and mechanical properties in the paper. (C) 2004 Elsevier B.V. All rights reserved.
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    Microstructural damage of thermal barrier coatings due to CMAS
    (Cranfield University, 2013-10) Ndamka, Ngunjoh Lawrence; Nicholls, J. R.; Wellman, Richard G.; Craig, M.
    Over recent years, due to a constant desire for higher efficiency engines and hence increased turbine entry temperatures and a proportional reduction in [Carbon dioxide] emissions, there is a need to understand how molten slags (CMAS: Calcia magnesia alumina-silicate), including volcanic ash, affect engine life. Thermal barrier coatings (TBC) are employed together with cooling technology to protect engine hardware from the high temperature seen within the turbine and combustion zones. At current operating temperatures, CMAS can adhere to the TBC surface resulting in premature degradation of the coating. The columnar, high porosity microstructure of electron beam physical vapour deposited (EB-PVD) TBCs make them particularly susceptible to CMAS/molten deposit attack. CMAS attack of PYSZ is reported in literature to be characterised by penetration of the melt along the columnar structure, chemically attacking the TBC whereupon yttria is leached from PYSZ and into the melt, creating an yttria depleted interaction zone. A new approach for classifying and reporting CMAS attack on TBCs is introduced in this thesis and a degradation map is created to acknowledge that the mechanism and severity of CMAS damage is related to variation in the CMAS compositions. CMAS degradation of EB- PVD has been extensively studied by previous authors, all reporting similar degradation mechanism with varying degree of severity. In this study, this category of CMAS degradation mechanism is termed “classic” CMAS attack. The primary aim of this study has been to investigate the damage caused by volcanic ash and CMAS to materials used within an aerospace gas turbine engine. The thesis investigates two aspects. It is recognised that, debris ingested by the engine will cause erosion damage to components in the cooler section of the engine (compressor), thus the first part examines this issue. A series of erosion tests with Eyjafjallajokull volcanic ash and similar sized MIL spec silica sand have been undertaken with two compressor-typical materials (Ti-6Al-4V and Inconel 718). The results were consistent with volcanic ash behaving like fine silica sand both at room and at compressor operating temperatures. The measured erosion rates are consistent with a ductile erosion mechanism with peak rates of material loss at lower impact angle. The results would appear to fit classical ductile erosion models where the material loss depends on particle velocity and follows a power with an exponent close to 2.4.
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    Phosphor thermometry in an EB-PVD TBC
    (Cranfield University, 2008-01) Steenbakker, Remy; Nicholls, J. R.; Wellman, Richard G.
    Thermal Barrier Coatings (TBCs) are used to reduce the actual working temperature of the high pressure turbine blade surface. Knowing the temperature across a TBC and at the interface with the thermally grown oxide (TGO) under realistic conditions is highly desirable. As the major life#controlling factors for TBC systems are linked with temperature, this would provide useful data for a better understanding of these phenomena and to assess the remnant life#time of the TBC. This would also enable the design of advanced cooling strategies in the most efficient way using a minimum amount of air. Further the integration of a sensor coating into an on#line temperature detection system will enable the full potential of TBCs to be realised due to improved precision in temperature measurement and early warning of degradation. This in turn will increase fuel efficiency and reduce CO2 emissions. The concept of sensing TBCs was patented by Choy et al. [114] in 1998 and consists of locally modifying the composition of the TBC so that it acts as a thermographic phosphor. As a result, the temperature dependence of the lifetime of the laser induced phosphorescence process can be used for temperature measurements. The purpose of this work was to develop a multilayer sensing TBC deposited by electron beam physical vapour deposition (EB#PVD) which could be used to remotely measure the temperature at different depths in the coating. In this study, the reader is introduced to the theory of luminescence sensing and its TBC application. Several yttria partially stabilised zirconia TBCs, co#doped with rare earth oxides (YSZ:RE) phosphors, were studied and it was shown that dysprosia doped YSZ has the highest temperature sensitivity. The influence of dopant concentration, layering and high temperature aging on the phosphorescence process were also researched. During the project, a novel, non#destructive, method to monitor the high temperature degradation of the TBC using phosphorescence measurements was found. Alternative phosphor compositions, based on yttrium aluminium garnet (YAG) material, were successfully deposited by EB#PVD and it was shown that doped YAG TBC compositions could further improve the maximum temperature measurement capability of current sensing TBCs. A multilayer EB#PVD coating comprising of two different phosphor layers was deposited and tested in order to demonstrate that such systems could be used to remotely measure the temperature at two different depths in the TBC simultaneously and therefore to monitor the thermal gradient in the coating, permitting the direct measurement of heat flux under thermal gradient conditions, for example in service.