Browsing by Author "Wellman, R. G."
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Item Open Access CMAS corrosion of EB PVD TBCs: Identifying the minimum level to initiate damage(2010-01-01T00:00:00Z) Wellman, R. G.; Whitman, G.; Nicholls, John R.Over the last decade a significant amount of research has been conducted into the durability of thermal barrier coatings (TBCs) focusing mainly on issues of oxidation, erosion and foreign object damage (FOD). However, as the performance and durability of TBCs has improved the temperatures at which they operate has increased. This increase in temperature has resulted in another lifing issue for EB PVD TBCs, namely that of CMAS attack. Calciumâ  magnesiumâ  alumino-silicate (CMAS) attack occurs when atmospheric dust that has deposited on the surface of turbine blades melts and wicks into the columns of the TBC. This occurs at temperatures above 1240â  1260 à °C and results in the degradation of the columnar microstructure of the TBCs. Due to the fact that TBCs operate in a temperature gradient CMAS only infiltrates part of the coating before solidifying. There are a number of issues associated with CMAS attack, both chemical and mechanical. From a chemical point of view CMAS attack of electron beam (EB) physical vapour deposited (PVD) TBCs can be considered as a form of corrosion; when there is a lot of excess CMAS on the surface of a coated component Yttria diffuses out of the TBC into the molten CMAS resulting in a tâ ² to monoclinic phase transformation in the yttria stabilised zirconia (YSZ), CMAS attack also results in localised melting and subsequent re-precipitation of the coating resulting in a loss of the defined columnar microstructure. While from a mechanical point of view the CMAS, once re-solidified, reduces the strain compliance of the EB PVD and can result in spallation of the TBC on cooling. Furthermore, current studies have indicated that small amount of CMAS infiltration significantly increases the erosion rate of EB PVD TBCs. This paper covers various aspects of CMAS attack of EB PVD TBCs, specifically looking at minimum levels of CMAS required to initiate damage, as well as investigating it from an erosionâ  corrosion pItem Open Access Effect of microstructure and temperature on the erosion rates and mechanisms of modified EB PVD TBCs(Elsevier Science B.V., Amsterdam., 2009-10-29T00:00:00Z) Wellman, R. G.; Nicholls, John R.; Murphy, K. S.Thermal barrier coatings (TBCs) have now been used in gas turbine engines for a number of decades and are now considered to be an accepted technology. As there is a constant drive to increase the turbine entry temperature, in order to increase engine efficiency, the coatings operate in increasingly hostile environments. Thus there is a constant drive to both increase the temperature capabilities of TBCs while at the same time reducing their thermal conductivities. The thermal conductivity of standard 7 wt% yttria stabilized zirconia (7YSZ) electron beam (EB) physical vapour deposited (PVD) TBCs can be reduced in two ways: the first by modification of the microstructure of the TBC and the second by addition of ternary oxides. By modifying the microstructure of the TBC such that there are more fine pores, more photon scattering centres are introduced into the coatings, which reduce the heat transfer by radiation. While ternary oxides will introduce lattice defects into the coating, which increases the phonon scattering, thus reducing the thermal conductivity via lattice vibrations. Unfortunately, both of these methods can have a negative effect on the erosion resistance of EB PVD TBCs. This paper compares the relative erosion rates of ten different EB PVD TBCs tested at 90à ° impact at room temperature and at high temperature and discusses the results in term of microstructural and temperature effects. It was found that by modifying the coating deposition, such that a low density coating with a highly â  featheredâ  microstructure formed, generally resulted in an increase in the erosion rate at room temperature. When there was a significant change between the room temperature and the high temperature erosion mechanism it was accompanied by a significant decrease in the erosion rate, while additions of dopents was found to significantly increase the erosion rate at room and high temperature. However, all the modified coatings still had a lower erosion rate than a plasma sprayed coatings. So, although, relative to a standard 7YSZ coating, the modified coatings have a lower erosion resistance, they still perform better than PS TBCs and their lower thermal conductivities could make them viable alternatives to 7YSZ for use in gas turbine engItem Open Access Erosion and high temperature oxidation resistance of new coatings fabricated by a sol-gel route for a TBC application.(Trans Tech Publications, 2008) Viazzi, Céline; Wellman, R. G.; Oquab, Djar; Nicholls, John R.; Monceau, Daniel; Bonino, Jean-Pierre; Florence, AnsartThis 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- 25µm, 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 (100hr 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.Item Open Access Erosion of gadolinia doped EB-PVD TBCs(Elsevier Science B.V., Amsterdam., 2006-12-01T00:00:00Z) Steenbakker, R J L; Wellman, R. G.; Nicholls, John R.Gadolinia additions have been shown to significantly reduce the thermal conductivity of EB-PVD TBCs. The aim of this paper is to further the understanding on the effects of dopants on the erosion resistance of EB-PVD TBCs by studying the effects of 2 mol% Gd2O3 additions on the room and high temperature erosion resistance of as received and aged EB-PVD TBCs. Previously it has been reported that gadolinia additions increased the erosion rate of EB- PVD TBCs, this is indeed the case for room temperature erosion, however under high temperature (825 à °C) erosion conditions this is not the case and the doped TBCs have a slightly lower erosion rate than the standard YSZ EB-PVD TBCs. This has been attributed to a change in the erosion mechanisms that operate at the different temperatures. This change in mechanism was not expected under the impact conditions used and has been attributed to a change in the column diameter, and how this influences the dynamics of particle impactionItem Open Access Erosion, corrosion and erosion-corrosion of EB PVD thermal barrier coatings(Elsevier Science B.V., Amsterdam., 2008-07-01T00:00:00Z) Wellman, R. G.; Nicholls, John R.Electron beam (EB) physical vapour deposited (PVD) thermal barrier coatings (TBCs) have been used in gas turbine engines for a number of years. The primary mode of failure is attributed to oxidation of the bond coat and growth of the thermally grown oxide (TGO), the alumina scale that forms on the bond coat and to which the ceramic top coat adheres. Once the TGO reaches a critical thickness, the TBC tends to spall and expose the underlying substrate to the hot gases. Erosion is commonly accepted as a secondary failure mechanism, which thins the TBC thus reducing its insulation capability and increasing the TGO growth rate. In severe conditions, erosion can completely remove the TBC over time, again resulting in the exposure of the substrate, typically Ni-based superalloys. Since engine efficiency is related to turbine entry temperature (TET), there is a constant driving force to increase this temperature. With this drive for higher TETs comes corrosion problems for the yttria stabilised zirconia (YSZ) ceramic topcoat. YSZ is susceptible to attack from molten calciumâ  magnesiumâ  aluminaâ  silicates (CMAS) which degrades the YSZ both chemically and micro-structurally. CMAS has a melting point of around 1240 à °C and since it is common in atmospheric dust it is easily deposited onto gas turbine blades. If the CMAS then melts and penetrates into the ceramic, the life of the TBC can be significantly reduced. This paper discusses the various failure mechanisms associated with the erosion, corrosion and erosionâ  corrosion of EB PVD TBCs. The concept of a dimensionless ratio D/d, where D is the contact footprint diameter and d is the column diameter, as a means of determining the erosion mechanism is introduced and discussed for EBItem Open Access High Temperature Erosion-Oxidation Mechanisms, Maps and Models(Elsevier Science B.V., Amsterdam., 2004-05-01T00:00:00Z) Wellman, R. G.; Nicholls, John R.The aim of this paper is to provide an overview of erosion-oxidation studies by reviewing the work that has been done on mechanisms, maps and models. High temperature erosionoxidation is a major cause of wear in both fluidised bed combustors and gas turbines and much effort has been put into understanding the phenomena and reducing wear rates. A number of different erosion-oxidation mechanisms have been proposed over the years to describe the different wear regimes, some of these mechanisms are discussed in this paper, as well as the mapping techniques that have been used to quantify wastage rates. Finally, erosion-oxidation modelling is discussed, starting with models that combine oxidation kinetics with erosion rate equations that lead to predictive models before concentrating on Monte Carlo modelling methods.Item Open Access A Mechanism for the Erosion of EB PVD TBCS(Trans Tech Publications, Switzerland., 2000-07-10) Wellman, R. G.; Nicholls, John R.Since the introduction of electron beam (EB) physical vapour deposition (PVD) TBCs and their application to moving components in the hot gas stream, erosion has become a prime concern. This project has been involved in determining the erosion mechanism of EB PVD TBCs, in order to develop a computational erosion model. The unique columnar microstructure of the EB PVD TBCs precludes the use of the classical brittle erosion mechanisms. This meant that a thorough investigation into the erosion mechanism of the coatings was necessary before it would be possible to develop a model to predict material wastage.Item Open Access A Monte Carlo model for predicting the erosion rate of EB PVD TBCs(Elsevier Science B.V., Amsterdam., 2004-05-01T00:00:00Z) Wellman, R. G.; Nicholls, John R.Since the introduction of electron beam (EB) physical vapour deposition (PVD) thermal barrier coatings (TBCs) and their application to moving components in the hot gas stream, erosion has become a prime concern. EB PVD TBCs, due to their unique columnar microstructure are far more strain tolerant than their plasma sprayed (PS) counter parts and can thus be used under more exacting operating conditions. It is under these operating conditions that erosion of the coated components is of primary importance. The main aim of this project was the development of a computer model capable of predicting the erosion rate of EB PVD TBCs under various different conditions relevant to engine operation. In order to do this it was first necessary to determine the erosion mechanisms of EB PVD TBCs as well as their mechanical properties. Steady-state erosion and single impact studies together with SEM were used to determine the erosion mechanisms. While nano-indentation techniques were used to obtain the hardness and the Youngâ  s modulus of the EB PVD TBC. All these findings were then used in the development of a Monte Carlo type computational erosion model capable of predicting the erosive wear rate of EB PVD TBCs under various conditions. The model which has been developed is capable of predicting the erosion rate of EB PVD TBC to within 30%, so long as the erosion falls within the defined mechanism. This can easily be checked against an erosion map which has been developed. This paper discusses the program and reports some results from running the program under various conditionItem Open Access Multi-layered Ruthenium-modified Bond Coats for Thermal Barrier Coatings(Springer Verlag, 2006-11) Tryon, B.; Feng, Q.; Wellman, R. G.; Murphy, K. S.; Yang, J.; Levi, C.G.; Nicholls, John R.; Pollock, T. M.Diffusional approaches for fabrication of multi-layered Ru-modified bond coats for thermal barrier coatings have been developed via low activity chemical vapor deposition and high activity pack aluminization. Both processes yield bond coats comprising two distinct B2 layers, based on NiAl and RuAl, however, the position of these layers relative to the bond coat surface is reversed when switching processes. The structural evolution of each coating at various stages of the fabrication process has been and subsequent cyclic oxidation is presented, and the relevant interdiffusion and phase equilibria issues in are discussed. Evaluation of the oxidation behavior of these Ru-modified bond coat structures reveals that each B2 interlayer arrangement leads to the formation of α-Al 2 O 3 TGO at 1100°C, but the durability of the TGO is somewhat different and in need of further improvement in both cases.Item Open Access Nano and Micro indentation studies of bulk zirconia and EB PVD TBCs(Elsevier Science B.V., Amsterdam., 2004-01-01T00:00:00Z) Wellman, R. G.; Dyer, A.; Nicholls, John R.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* and micro* indentation were used to determine the hardness and the Young"s 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 3N. 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.Item Open Access Nano and microhardness testing of aged EB PVD TBCs(Elsevier Science B.V., Amsterdam., 2004-11-01T00:00:00Z) Wellman, R. G.; Tourmente, H.; Impey, Susan A.; Nicholls, John R.Previous studies on the erosion of electron beam physical vapour deposited thermal barrier coatings (EB PVD TBCs) has shown that aging the coatings at between 1100 and 1500 à °C before erosion increases the erosion rate. These changes in the erosion rate were attributed to a number of factors including changes to the nanoporosity within the coatings as well as phase changes within the coatings. Such changes in the morphology of the coatings should be measurable as changes in their hardness. Thus, it was decided to ascertain the effect that the aging had on the hardness of the coatings. Since, during erosion, the size of the interaction zone between the impacting particle and the coating is in the same range as the size of the individual columns of the coating, it was decided to measure the change in the hardness of the columns as well as the coating as a whole. It was found that the aging increased the hardness of both the coating as a whole and the individual columns of the coatings. The microhardness of the coating was found to increase from 2.5â  3.5 GPa in the as-received condition to 4.5â  6 GPa after 100 h at 1100 à °C and to 7.5â  8 GPa after 24 h at 1500 à °C. The nanohardness of the individual columns on the other hand was found to increase from 18 GPa in the as-received condition to 35 GPa after aging. This paper discusses the increases in hardness due to aging in terms of the sintering and morphological changes that occur in the coating. The difference in the nano- and microhardness results are discussed in terms of the relative size of the indents and column size and the associated interactions that occur under the different indenter heads aItem Open Access On the Effect of Ageing on the Erosion of EB-PVD TBCs(Elsevier Science B.V., Amsterdam., 2004-01-30T00:00:00Z) Wellman, R. G.; Nicholls, John R.Thermal barrier coatings have been used in gas turbine engines for some four decades, and the erosion resistance of these coatings has been well documented over the years by various different groups. However, as far as can be ascertained most of the laboratory research on erosion resistance of the coatings has been conducted on the coatings in the as-received condition. The effect of service conditions on the erosion rate of the coatings has not been addressed. This paper looks at the effects of thermal aging on the erosion rate of electron beam physical vapour deposited thermal barrier coatings (EB-PVD TBCs) by testing the erosion resistance of coatings that have received various different heat treatments and comparing them to those in the as received condition. Initially two different aging heat treatments were used 1500à °C for 24hrs, aimed at sintering the ceramic columns, and a less severe aging of 1100à °C for 100hrs. It was found that both of the heat treatments resulted in a significant increase in the erosion rates when compared to the as received samples. Subsequent to the initial tests, it was found that heat treating the TBC for 30 hours at 1100à °C resulted in an insignificant increase in the erosion rate. For the samples given the higher temperature heat treatment, the increase in the erosion rate was attributed to the fact that the columns were partially sintering together which enabled cracks to propagate into neighbouring columns, as opposed to stopping at column boundaries as occurs in samples that have not been heat treated. This results in an increase in erosion rate due to the fact that more material is removed per impact eveItem Open Access Pulsed electron beam treatment of MCrAlY bondcoats for EB PVD TBC systems part 2 of 2: Cyclic oxidation of the coatings.(Elsevier Science B.V., Amsterdam., 2007-12-01T00:00:00Z) Wellman, R. G.; Scrivani, A.; Rizzi, G.; Weisenburger, A.; Tenailleau, F H; Nicholls, John R.This paper discusses the effect of pulsed electron beam (PEB) treatment of thermal sprayed MCrAlY bondcoats on the cyclic life of thermal barrier coatings (TBCs). Standard MCrAlY bondcoats were produced via HVOF, VPS and LPPS thermal spray methods. Some of the HVOF and VPS coatings were then given a PEB treatment before all the samples were coated with an EB PVD 7-8 wt.% yttria partially stabilized zirconia topcoat. The samples were all tested under cyclic oxidation conditions at 1150 à °C with 1 h at temperature and 15 min cooling, samples were removed after 20% coating spallation and prepared for cross sectional analyses. Cyclic testing revealed that although the PEB treatment had no measurable effect on the VPS sprayed samples, the HVOF coatings showed a significant increase in the cyclic life after the PEB treatment. The effect of the PEB treatment on the various samples is discussed as well as its effect on TGO growth morphology. PEB treatment of the HVOF bondcoat was found to reduce the rate of alumina growth and to suppress the formation of oxide pegs resulting in a smoother bondcoat interfaceItem Open Access A Review of the Erosion of Thermal Barrier Coatings.(Iop Publishing Ltd, 2007-08-21T00:00:00Z) Wellman, R. G.; Nicholls, John R.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 calciumâ  magnesiumâ  aluminaâ  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 introItem Open Access Some observations on erosion mechanisms of EB PVD TBCS(Elsevier Science B.V., Amsterdam., 2000-07-10T00:00:00Z) Wellman, R. G.; Nicholls, John R.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.