CERES :: Browsing by Author "Nicholls, John R."

Browsing by Author "Nicholls, John R."

Now showing 1 - 20 of 58

Open Access
Analysis of combined static load and low temperature hot corrosion induced cracking in CMSX-4 at 550°C
(Elsevier, 2019-10-18) Brooking, Laurie; Gray, Simon; Dawson, K.; Nicholls, John R.; Simms, Nigel J.; Sumner, Joy; Tatlock, G. J.
A CMSX-4 3-point bend specimen was statically loaded under hot corrosion conditions and SEM, (S)TEM and EDX techniques were used to analyse the cracking generated. Sulphur, chlorine, sodium and oxygen were found at the crack tip, and an influence of loading on the corrosion mechanism’s preference to interact with either the γ or γʹ was observed. The microscopy analysis is in support of the corrosive mechanism being a combined stress and electrochemical corrosion linked with low temperature hot corrosion, where crack propagation occurs as a result of localised corrosion enhanced material degradation. High magnification EDX mapping identified W as segregating to the γʹ at room temperature.
Open Access
Analysis of high temperature steam oxidation of superheater steels used in coal fired boilers
(Springer, 2015-10-30) Dudziak, T.; Lukaszewicz, Mikolaj; Simms, Nigel J.; Nicholls, John R.
The present work compares the behaviour of four steels: (T23, T92, T347HFG, Super304H) in the temperature range 600–750 °C. This study focuses on the analysis of the oxidation kinetics in terms of mass change, metal loss and thickness change of the selected materials. In order to understand the differences in oxidation rates between the selected steels, the impact of chromium and the alloying elements were considered in this work. The obtained results show that the impact of alloying elements differs with exposure conditions and importance of the synergy effect.
Open Access
Bioinspired metal–polymer thin films with varying hydrophobic properties
(Springer, 2017-08-14) Rao, Jeff; Anjum, Sadaf Saad; Craig, M.; Nicholls, John R.
Nanocomposites involve the inclusion of one material into the layers of another material at a nanoscale level. Inspired by nature, nanocomposites material systems offer functionalities over their bulk forms which in some cases have evolved over millions of years. Here, thin film coatings have been fabricated by PVD sputtering, comprising a soft PTFE phase which is combined with a hard metallic NiTi phase. A series of coatings with PTFE ranging from 10 to 75 vol% have been prepared, and their surface energies and microstructures investigated. The surface energy of the nanocomposite films changes with the PTFE content, falling in the range between PTFE and NiTi with water contact angles between 80° and 102° for a thin film with 25 and 75 vol% of PTFE, respectively. Here, both TEM and EDX reveal PTFE forming along NiTi column boundaries. Coatings with PTFE content greater than 50 vol% failed due to a build-up of intrinsic stress. The degree of hybridization between NiTi and PTFE was found to be dependent on the PTFE layer thickness. SEM analysis of this coating reveals PTFE at the surface embedded within the NiTi matrix.
Open Access
Carbon and titanium diboride (TiB2) multilayer coatings.
(2004-11-01T00:00:00Z) Rao, Jeff; Cruz, R.; Lawson, K. J.; Nicholls, John R.
Titanium Diboride, (TiB2) is a metal-based refractory ceramic material that has been investigated in industrial applications ranging from, cutting tools to wear parts and for use in the aerospace industry. The unique properties which make this material so fascinating are, its high hardness, high melting point and its corrosion resistance. TiB2 is prevented from wider mainstream use because of its inherent brittle nature. With a view to overcome this in coating form and with the aim of providing in addition inherent lubricity, in this study 50 layer TiB2/C multilayer stacks have been fabricated, with varying volume fractions of ceramic, whereby the interfaces of the layers limit crack propagation in the TiB2 ceramic. TiB2 has been multilayered with carbon, to make use of the unique and hybrid nature of the bonding in carbon coatings. DC magnetron sputtering with substrate bias was the preferred route for the fabrication of these coatings. AISI tool steel has been used as the substrate material. By varying the amount of TiB2 ceramic from 50% to 95%, the Hardness of the coating is seen to increase from 5 GPa to 17GPa. The Hardness is observed to decrease as a function of increasing carbon content, agreeing with other studies that the carbon layers are not load-bearing. The graphitic nature of the sp2 bond, however, acts as a lubricant layer.
Open Access
Carbon Brainprint Case Study: ceramic coatings for jet engine turbine blades
(2011-07-31T00:00:00Z) Parsons, David J.; Chatterton, Julia C.; Nicholls, John R.
Ceramic thermal barrier coatings (TBCs) are applied to jet turbine blades to protect them from the high temperature gases leaving the combustion chamber and to increase the efficiency of the engine. Professor John Nicholls of the Surface Science and Engineering Group, Cranfield University has been working with Rolls- Royce plc for about 17 years to improve the insulating performance of TBCs. As a result, the TBCs used in the current generation of aircraft turbofan jet engines achieve a temperature drop about 80ºC greater than at the start of the work, with an estimated fuel saving of about 1%. This case study considered two engine types: Trent 700, used on about half the Airbus A330 aircraft currently in service, and Trent 500, used on all Airbus A340-500 and A360-600 aircraft. The greenhouse gas emissions considered were, in order of magnitude, carbon dioxide from combustion of the fuel, emissions during extraction and refining of the fuel, and emissions of other greenhouse gases during combustion. Emissions associated with transport of the fuel were found to be negligible compared with these, and all emissions not related to fuel consumption, for example manufacture of the coating, were also assumed to be insignificant or excluded from the assessment because they were unaffected by the change in the TBC. The baseline fuel consumption during each flight phase (landing and take-off cycle and cruise) was estimated from publicly available data. Airline activity data for A330 and A340 models from European operators was taken to represent typical patterns of use, enabling annual emissions per aircraft to be calculated. Data on current operating aircraft and orders were then used to estimate the total current and projected future emissions. From these, the higher emissions that would have occurred in the past if the improved TBCs had not been used, and the corresponding future emissions, were estimated. The best estimates of the current emissions (the retrospective brainprint) for individual aircraft were 1016, 1574 and 1646t CO2e/year for A330, A340-500 and A340-600 respectively, giving 568 kt CO2e/year for the total fleet. Including all the aircraft on order, the prospective emissions reduction was 833kt CO2e/ year. Assuming a service life of 20 years, the total brainprint was approximately 17MtCO2e. An uncertainty analysis was performed with assumed uncertainties for aircraft activity, fuel consumption and the efficiency change. The 95% confidence interval for the current annual emissions reduction was 429-721kt CO2e/year excluding the efficiency change uncertainty, and 258-1105 if it was included. The relative changes in the other output measures were similar. Assuming that older engines do not and will not benefit from the improvement, reduced the total brainprint to 14MtCO2e. The assessment did not include an adjustment for the effect of emissions at high altitude, which would increase all the outputs by a factor of 1.9.
Open Access
Carbon brainprint – An estimate of the intellectual contribution of research institutions to reducing greenhouse gas emissions
(Elsevier, 2015-05-07) Chatteron, Julia C.; Parsons, David J.; Nicholls, John R.; Longhurst, Philip J.; Bernon, Mike; Palmer, Andrew; Brennan, Feargal P.; Kolios, Athanasios; Wilson, Ian; Ishiyama, Edward; Clements-Croome, Derek; Elmualim, Abbas; Darby, Howard; Yearly, Tom; Davies, Gareth J.
Research and innovation have considerable, currently unquantified potential to reduce greenhouse gas emissions by, for example, increasing energy efficiency. Furthermore, the process of knowledge transfer in itself can have a significant impact on reducing emissions, by promoting awareness and behavioural change. The concept of the ‘carbon brainprint’ was proposed to convey the intellectual contribution of higher education institutions to the reduction of greenhouse gas emissions by other parties through research and teaching/training activities. This paper describes an investigation of the feasibility of quantifying the carbon brainprint, through six case studies. The potential brainprint of higher education institutes is shown to be significant: up to 500 kt CO2e/year for one project. The most difficult aspect is attributing the brainprint among multiple participants in joint projects.
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. CalciumÃ¢Â Â magnesiumÃ¢Â Â 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 1240Ã¢Â Â 1260 Ã Â°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 tÃ¢Â Â² 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 erosionÃ¢Â Â corrosion p
Open Access
Corrosion fatigue testing: the combined effect of stress and high temperature corrosion
(Taylor & Francis, 2017-10-20) Chapman, Neil; Brooking, Laurie; Sumner, Joy; Gray, Simon; Nicholls, John R.
A corrosive environment can have a detrimental effect on the fatigue life of a material due to a change in failure mechanism. Attempts have been made to replicate this change on nickel-base superalloy CMSX-4 cast in the <001> orientation. Fatigue testing in air, of this material typically produces a fracture on an angle of approximately 55° which is consistent with the fracture having propagated on a {111} slip plane. The aim of the research was to fatigue test in a corrosive environment with the purpose of producing a crack/fracture which deviated from the typical angle and thus confirm that the corrosive environment had affected the fatigue mechanism. It was concluded that the change in mechanism to high temperature corrosion fatigue was associated with a reduced load application rate together with precorroding the test specimens to trigger the initiation of the corrosion fatigue mechanism.
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 90Ã Â° 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 Ã¢Â Â featheredÃ¢Â Â 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 eng
Open Access
Effect of plasma processing parameters on the surface modification of fibre reinforced composites by atmospheric pressure plasma treatment
(euspen, 2021-06-10) Frumosu, Lydia; Bishop, Alex; Castelli, Marco; Bennett, Adam; Nicholls, John R.; Huang, Zhaorong
This report presents a study on the effect of microwave plasma sources on the surface modification of carbon fibre reinforced composites. The adhesion property of polymer composites is an important consideration in manufacturing processes. A key challenge in polymer adhesion is the need for adhesive pre-treatment to improve the wettability properties of the surface. Often three sequential steps are needed: contaminant removal, physically induced surface modification and chemical treatment. Atmospheric plasma treatment (APT) has been shown to either achieve, or eliminate the need for, one or more of these steps. The effects of APT on the surface properties of thermoplastics and thermosetting polymers have been reported recently, but the report on the effect of different plasma sources on the APT of polymers is limited. 50% carbon fibre epoxy samples were subjected to a microwave plasma source with varied flow rate, power, and samples’ distance from source, under atmospheric conditions. The surface energy of the composites was determined using liquid contact angle analysis. A coherence scanning interferometer was used to characterise the surface changes. The underlying mechanisms for the observed change of surface properties will be discussed.
Open Access
Effect of stress state and simultaneous hot corrosion on the crack propagation and fatigue life of single crystal superalloy CMSX-4
(Elsevier, 2018-05-03) Brooking, Laurie; Gray, Simon; Sumner, Joy; Nicholls, John R.; Marchant, G.; Simms, Nigel J.
Operating conditions within industrial gas turbines are changing in response to pressures to reduce environmental impact and enable use of renewable sources. This is driving an increase in the operational temperatures and pressures of combustion in turbine systems. Additionally, diverse operating environments can result in higher sulphur and trace metal contaminant levels, exacerbating hot corrosion in GT systems. Low cycle fatigue (LCF) cycling can also be intensified as a result of increased start/stop shutdowns. The combined effects of hot corrosion and stress are experimentally studied on CMSX-4 single crystal (SC) γ/γ' system under both fatigue and static stress conditions, with either a multi-axial bending or uniaxial stress state. The associated stress intensity thresholds (KTH) under the various stress conditions were evaluated using finite element analysis (FEA). Cracking was observed both under static and fatigue stress conditions in a hot corrosion environment. Crack morphologies were analysed using SEM techniques. Bending stresses and fatigue cycles demonstrated increased crack propagation in the presence of hot corrosion with static uniaxial stresses showing the longest nucleation times and lowest propagation rates.
Open Access
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.
Open Access
The effect of trepanning speed of laser drilled acute angled cooling holes on the high temperature low cycle corrosion fatigue performance of CMSX-4 at 850 °C
(Elsevier, 2017-05-04) Morar, Nicolau; Roy, Rajkumar; Mehnen, Jorn; Nicholls, John R.; Gray, Simon
The effect of laser trepanning speed and, as a result, recast layer thickness on the high temperature corrosion fatigue behaviour of CMSX-4 superalloy acute angled holes was investigated. The experimental test results show that an increasing laser drilling speed caused a reduction in corrosion fatigue life by 35–50% at 850 °C, under low cycle fatigue regime. This reduction was found to correlate directly with the recast layer thickness and surface anomalies within the recast layer produced during the laser drilling process. Corrosion had a smaller effect on the overall life of the laser drilled specimens under the conditions tested. The results presented show that laser trepanning speed is influential in limiting the life performance of laser drilled components in service.
Open Access
The effect of trepanning speed of laser drilled acute angled cooling holes on the high temperature low cycle corrosion fatigue performance of CMSX-4 at 850°C
(Elsevier, 2017-05-04) Morar, N.; Roy, Rajkumar; Mehnen, Jorn; Nicholls, John R.; Gray, Simon
The effect of laser trepanning speed and, as a result, recast layer thickness on the high temperature corrosion fatigue behaviour of CMSX-4 superalloy acute angled holes was investigated. The experimental test results show that an increasing laser drilling speed caused a reduction in corrosion fatigue life by 30-50% at 850°C, under low cycle fatigue regime. This reduction was found to correlate directly with the recast layer thickness and surface anomalies within the recast layer produced during the laser drilling process. Corrosion had a smaller effect on the overall life of the laser drilled specimens under the conditions tested. The results presented show that laser trepanning speedis influencial in limiting the life performance of laser drilled components in service.
Open Access
Effects of oxidation and hot corrosion in a nickel disc alloy
(2008-12-31T00:00:00Z) Encinas-Oropesa, Adriana; Drew, G. L.; Hardy, M. C.; Leggett, A. J.; Nicholls, John R.; Simms, Nigel J.
This paper describes work to study oxidation and hot corrosion damage in nickel disc alloy RR1000. Oxidation damage has been characterised via mass change data from thermogravimetric analyses at temperatures from 700 to 800 ºC and cyclic oxidation testing at 700 and 750 ºC. Thin oxide scales rich in Cr and Ti have been found to grow in a parabolic dependence with time. Oxidation rate constants have been correlated with temperature using an Arrhenius equation. Through the use of simplifying assumptions, mass change data have been converted to oxide thickness values. These values have been compared with observed values of oxide scale and the depth of near-surface damage, which have been measured using focused ion beam (FIB) microscopy. Hot corrosion of RR1000 has been studied in the laboratory by ‘deposit' recoat experiments at 700ºC. In these tests, samples were coated with deposits of Na2SO4/NaCl salt in a gas stream of air-300 vpm SO2. Corrosion damage has been quantified by dimensional metrology of samples before and after exposure. This enabled metal losses, i.e. the depth of corrosion pits, to be determined. Distributions of metal loss data were then generated to produce median damage values and exceedance cumulative probabilities. Finally, the effect of contaminants on the hot corrosion resistance of RR1000 has also been evaluated.
Open Access
Electrodeposition of aluminium film on P90 Li-Al alloy as protective coating against corrosion
(Elsevier Science B.V., Amsterdam., 2009-02-28T00:00:00Z) Bardi, U.; Caporali, S.; Craig, M.; Giorgetti, A.; Perissi, I.; Nicholls, John R.
In this paper we report on the electrodeposition of thin aluminium layers on P90 lithium–aluminium alloy at room temperature from a chloroaluminate ionic liquid (1-butyl-3-methyl imidazolium heptachloroaluminate [BMIm]Al2Cl7). We found that the treatment of the P90 sample's surface is a key point to obtain good quality coatings. On freshly mechanically polished surfaces, thin (about 24 µm), homogeneous and dense aluminium layers were obtained at 10 µm h− 1 deposition rate. Finally, the corrosion resistance of the uncoated and Al-coated samples was investigated by means of electrochemical techniques such as open-circuit voltage curves, impedance spectroscopy and potentiodynamic polarization in 3.5 wt.% NaCl aqueous solution. We found that the protective aluminium layer significantly reduces the corrosion rates of the P90 Li–A
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, Ansart
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- 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.
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 Ã Â°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 impaction
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 calciumÃ¢Â Â magnesiumÃ¢Â Â aluminaÃ¢Â Â silicates (CMAS) which degrades the YSZ both chemically and micro-structurally. CMAS has a melting point of around 1240 Ã Â°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 erosionÃ¢Â Â 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 EB
Open Access
Evolution of stress and morphology in thermal barrier coatings
(Elsevier Science B.V., Amsterdam., 2010-08-25T00:00:00Z) Wang, Xin; Atkinson, Alan; Chirivi, Laura; Nicholls, John R.
Residual stress in the thermally grown oxide (TGO) in thermal barrier coatings (TBCs) was measured by photoluminescence piezospectroscopy (PLPS) and stress maps created to track local stress changes as a function of thermal cycling. The local stress images were observed to be correlated with morphological features on the metal surface that were purposely introduced during specimen preparation. Local stress relaxation and morphological evolution with thermal cycling were studied using the stress maps combined by post-mortem SEM examination. It was found that the morphology in the specimen having an initial polished surface was quite stable, while that in the specimen with a rough surface was unstable. The average residual stress in the specimen with the unstable morphology decreased with thermal cycling and it eventually failed along TGO/YSZ interface. The specimen with stable morphology maintained a high TGO stress throughout the thermal cycling process and failed along TGO/bond coat interface. The rough surface was also found to give rise to the formation of transition alumina (theta-Al2O3) in the TGO which was correlated with a reduced TGO stress. (C) 2010 Elsevier B.V. All rights reserved.