R.G. Wellman

Some observations on erosion mechanisms of EB PVD TBCS

Abstract Following the successful application of electron beam (EB) physical vapour deposition (PVD) thermal barrier coatings (TBCs) to moving parts of turbine engines, the erosion resistance of these coatings has been of interest among researchers. However, although there are a number of papers on the erosion rate of these coatings, little has been reported on their erosion mechanism. This paper provides observations on the erosion damage of EB PVD TBCs and discusses the type of damage caused by erosion, as well as proposing a possible mechanism of erosion. The aim of the project as a whole was to model the erosion of EB PVD TBCs, but before modelling could begin, it was necessary to determine the erosion mechanism of these coatings. It was found that in all cases examined, the erosion of the coatings proceeds through the accumulation of damage in the form of horizontal cracks in the columns of the coating and subsequent removal of the fractured sections. Since it appears as though the contact radius is important in the erosion process, the effect of varying the elastic properties of the erodent and the target on the contact radius was assessed.

A review of the erosion of thermal barrier coatings

The application of thermal barrier coatings (TBCs) to components with internal cooling in the hot gas stream of gas turbine engines has facilitated a steep increase in the turbine entry temperature and the associated increase in performance and efficiency of gas turbine engines. However, TBCs are susceptible to various life limiting issues associated with their operating environment including erosion, corrosion, oxidation, sintering and foreign object damage (FOD).This is a review paper that examines various degradation and erosion mechanisms of TBCs, especially those produced by electron beam physical vapour deposition (EB-PVD). The results from a number of laboratory tests under various impact conditions are discussed before the different erosion and FOD mechanisms are reviewed. The transitions between the various erosion mechanisms are discussed in terms of the D/d ratio (contact area diameter/column diameter), a relatively new concept that relates the impact size to the erosion mechanism. The effects of ageing, dopant additions and calcium?magnesium?alumina?silicates on the life of TBCs are examined. It is shown that while ageing increases the erosion rate of EB-PVD TBCs, ageing of plasma sprayed TBCs in fact lowers the erosion rate. Finally modelling of EB-PVD TBCs is briefly introduced.

Nano and Micro indentation studies of bulk zirconia and EB PVD TBCs

Abstract In order to model the erosion of a material, it is necessary to know the material properties of both the impacting particles as well as the target. In the case of electron beam (EB) physical vapour deposited (PVD) thermal barrier coatings (TBCs) the properties of the columns as opposed to the coating as a whole are important. This is due to the fact that discrete erosion events are on a similar scale as the size of the individual columns. Thus, nano 1 and micro indentation were used to determine the hardness and the Youngs modulus of the columns. However, care had to be taken to ensure that it was the hardness of the columns that was being measured and not the coating as a whole. This paper discusses the differences in the results obtained when using the two different tests and relates them to the interactions between the indent and the columns of the EB PVD TBC microstructure. It was found that individual columns had a hardness of 14 GPa measured using nano indentation, while the hardness of the coating, using micro indentation decreased from 13 to 2.4 GPa as the indentation load increased from 0.1 to 3 N. This decrease in hardness was attributed to the interaction between the indenter and a number of adjacent columns and the ability of the columns to move laterally under indentation.

Multilayered ruthenium-modified bond coats for thermal barrier coatings

Diffusional approaches for fabrication of multilayered Ru-modified bond coats (BCs) for thermal barrier coatings (TBCs) have been developed vialow-activity (LA) chemical vapor deposition andhigh-activity (HA) pack aluminization. Both processes yield BCs comprising two distinct B2 layers, based on NiAl and RuAl; however, the position of these layers relative to the BC surface is reversed when switching processes. The structural evolution of each coating at various stages of the fabrication process and subsequent cyclic oxidation is presented, and the relevant interdiffusion and phase equilibria issues are discussed. Evaluation of the oxidation behavior of these Ru-modified BC structures reveals that each B2 interlayer arrangement leads to the formation of α-Al2O3 TGO at 1100 °C, but the durability of the TGO is somewhat different and in need of further improvement in both cases.

Sensor Thermal Barrier Coatings: Remote In Situ Condition Monitoring of EB-PVD Coatings at Elevated Temperatures

Thermal barrier coatings (TBCs) are used to reduce the actual working temperature of the high pressure turbine blade metal surface. Knowing the temperature of the surface of the TBC and at the interface between the bondcoat and the thermally grown oxide (TGO) under realistic conditions is highly desirable. As the major life-controlling factors for TBC systems are thermally activated, therefore linked with temperature, this would provide useful data for a better understanding of these phenomena and to assess the remaining lifetime of the TBC. This knowledge could also enable the design of advanced cooling strategies in the most efficient way using minimum amount of air. The integration of an on-line temperature detection system would enable the full potential of TBCs to be realized 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 a thermal-sensing TBC was first introduced by Choy, Feist, and Heyes (1998, “Thermal Barrier Coating With Thermoluminescent Indicator Material Embedded Therein,” U.S. Patent U.S. 6974641 (B1)). The TBC is locally modified so it acts as a thermographic phosphor. Phosphors are an innovative way of remotely measuring temperatures and also other physical properties at different depths in the coating using photo stimulated phosphorescence (Allison and Gillies, 1997, “Remote Thermometry With Thermographic Phosphors: Instrumentation and Applications,” Rev. Sci. Instrum., 68(7), pp. 2615‐2650). In this study the temperature dependence of several rare earth doped EB-PVD coatings will be compared. Details of the measurements, the influence of aging, the composition, and the fabrication of the sensing TBC will be discussed in this paper. The coatings proved to be stable and have shown excellent luminescence properties. Temperature detection at ultrahigh temperatures above 1300°C is presented using new types of EB-PVD TBC ceramic compositions. Multilayer sensing TBCs will be presented, which enable the detection of temperatures below and on the surface of the TBC simultaneously. DOI: 10.1115/1.3077662

Erosion of thermal barrier coatings

Abstract Thermal barrier coatings have been used within gas turbines for over 30 years to extend the life of hot section components. Thermally sprayed ceramics were the first to be introduced and are widely used to coat combustor cans, ductwork, platforms and more recently turbine aerofoils of large industrial engines. The alternative technology, electron beam physical vapour deposition,(EB-PVD) has a more strain-tolerant columnar microstructure and is the only process that can offer satisfactory levels of spall resistance, erosion resistance and surface finish retention for aero-derivative engines. Whatever technology is used, the thermal barrier must remain intact throughout the turbine life. Erosion may lead to progressive loss of TBC thickness during operation, raising the metal surface temperatures and thus shortening component life. Ballistic damage can lead to total TBC removal. This paper reviews the erosion behaviour of both thermally sprayed and EB-PVD TBCs relating the observed behaviour to the coating microstructure. A model for the erosion of EB-PVD ceramics is presented that permits the prediction of erosion rates. The model has been validated using a high velocity erosion gas gun rig, both on test coupons and samples removed from coated components. The implications of erosion on component life are discussed in the light of experimental results and the model predictions.

Self Diagnostic EB-PVD Thermal Barrier Coatings

Thermal barrier coatings (TBCs) are an enabling materials technology to improve the efficiency and durability of gas turbines and thus through such efficiency improvements offer reduce fuel usage and an associated reduction in CO2 emission. This commercial drive is pushing both aero- and industrial turbines to be lifetime dependent on TBC performance – the TBC must be “prime reliant”. However, the prediction of the durability of the TBC system has proved difficult, with lifetimes varying from sample to sample and component to component. One factor controlling this is the inability to measure accurately the bondcoat/ceramic interface temperature when buried under a TBC. In operating engines this is further exacerbated by the fact that such TBC systems operate in strong temperature gradients due to the need to cool aerofoil components. This research examines the design and manufacture of self diagnostic thermal barrier coatings capable of accurately measuring the interface temperature under the TBC, whilst providing the requisite thermal protection. Data on the temperature sensing capability of various rare earth doped EB-PVD thermal barrier coatings will be reported. It will be shown that systems exist capable of measuring temperatures in excess of 1300oC. Details of the measurement method, the compositions and the thermal stability of such systems will be discussed in this paper. The ability to produce a sensing TBC capable of measuring interface temperature, surface temperature and heat flux will further be discussed permitting the design of thermal barrier protected components capable of in-situ performance monitoring.

Erosion and High Temperature Oxidation Resistance of New Coatings Fabricated by a Sol-Gel Route for a TBC Application

This paper examines the erosion and cyclic oxidation performance of novel thermal barrier coatings produced via the sol-gel route. The ceramic top coat, with a thickness of 5-80 m, was deposited via a sol-gel route onto standard MCrAlY and PtAl bond coats. In both the erosion and the cyclic oxidation tests it was found that the bond coat had a profound affect on the results. The erosion of the sol-gel coatings were compared to standard EB PVD and PS TBCs and were found to be significantly higher. The effect of aging (100 h at 1100°C) on the erosion rates was also evaluated and was found to increase the erosion rates. The information obtained from the erosion and cyclic oxidation tests have highlighted the need to develop and optimise the parameters for producing thicker coatings

Direct observations of erosion-induced ferroelasticity in EB-PVD thermal barrier coatings

Imaging of the presence and location of erosion-induced ferroelastic toughening was completed on 8-weight percent yttria-stabilized zirconia EB-PVD thermal barrier coatings using confocal polarized Raman spectroscopy. A combination of measurements made using ultraviolet and visible laser radiation was conducted to determine the location and extent of ferroelastic twinning present at and below the eroded surfaces as well as along fracture surfaces of the coatings. Ferroelastic twinning events were identified at three major locations within and on the coating: the erosion surface, just below the surface, and in the bulk of the coating. The results shown here reveal that not all fracture events result in a ferroelastic response. This suggests there may be an opportunity to increase the toughness of thermal barrier coatings by increasing the possibility that a crack can produce a ferroelastic twin in the coating.

Sensor TBCs: Remote In-Situ Condition Monitoring of EB-PVD Coatings at Elevated Temperatures

Thermal Barrier Coatings (TBC) are used to reduce the actual working temperature of the high pressure turbine blade metal surface. Knowing the temperature of the surface of the TBC and at the interface between the bondcoat and the thermally grown oxide (TGO) under realistic conditions is highly desirable. As the major life-controlling factors for TBC systems are thermally activated, therefore linked with temperature, this would provide useful data for a better understanding of these phenomena and to assess the remaining lifetime of the TBC. This knowledge could also enable the design of advanced cooling strategies in the most efficient way using a minimum amount of air. The integration of an on-line temperature detection system would 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 a thermal-sensing TBC was first introduced by Choy, Feist and Heyes in 1998 [1]. The TBC is locally modified so it acts as a thermographic phosphor. Phosphors are an innovative way of remotely measuring temperatures and also other physical properties at different depths in the coating using photo stimulated phosphorescence [2]. In this study the temperature dependence of several rare earth doped EB-PVD coatings will be compared. Details of the measurements, the influence of aging, the composition and the fabrication of the sensing TBC will be discussed in this paper. Temperature detection at ultra-high temperatures above 1300°C is presented using new types of EBPVD TBC ceramic compositions. Multilayer sensing TBCs will be presented, which enable the detection of temperatures below and on the surface of the TBC simultaneously.Copyright