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Synchrotron grazing incidence X-ray diffraction has been used to newly reveal the heating rate dependent oxidation reactions that develop on a polycrystalline nickel-based superalloy. A continuous layer of precursor oxide was shown to form during the heating stage. Their approximate growth rates, their effect on local surface compositions of the alloy substrate, and their degree of interface planarity are considered critical in determining subsequent oxidation reactions when held for extended thermal exposures. The precursor oxides were predominantly nickel or cobalt based (NiO/CoO and Co3O4/NiCo2O4). Following the fastest heating rates (40 °C min−1 and above), the stable Cr2O3 phase formed, inhibiting Ni or Co diffusion to the surface. At slower heating rates (10–20 °C min−1), no evidence of the stable Cr2O3 was found, even after 200 h at elevated thermal exposure, instead continued growth of the precursor oxides was observed. Heating at 5 °C min−1 gave rise to an intriguing zone where sufficient precursor and favourable kinetics enabled the formation of a spinel, NiCr2O4, surface layer. Cross sections observed with electron microscopy confirmed this to be planar and continuous. Heating at the slowest tested 2 °C min−1 contrarily gives a non-protective surface layer comprising an outwardly growing NiO/CoO precursor oxide on top of an inwardly growing mixed oxide. The quantities, interfacial morphologies of oxides of the precursor oxide grown and the possible thermodynamic reactions that lead to their formation are discussed.

The results of a pre-oxidation heat treatment at 930 °C in Ar/H2/H2O environments on a Si-bearing Nb-stabilised 20Cr25Ni austenitic steel are presented. The heat treatment was conducted under low pO2, achieved by the introduction of controlled amounts of moisture into the gas. The atmosphere promoted the formation of a continuous, dense, adherent, protective surface scale composed of Cr2O3 and MnCr2O4 with a thin Si-rich oxide at the oxide alloy interface. Samples with different oxide layer thicknesses were produced and further exposed at 700 °C, to a gas of nominal composition CO2/1%CO/1000 vpm C2H4 for 4 h. This gas mixture has a carbon activity greater than unity and readily forms filamentary carbon on the non-pre-oxidised alloy. This is catalysed by nickel particles formed intrinsically from the alloy during the early stages of oxidation of the unprotected surface. The oxide layers produced, as a result of the pre-oxidation process, could suppress carbon deposition onto the alloy; a significant reduction in carbon deposit was noted with an oxide of 125 nm thickness, and no deposit was found on the sample with an oxide thickness of 380 nm. The depth of depletion of chromium from the alloy correlated with the thickness of the oxide formed during the pre-oxidation heat treatment, but the chromium concentration at the oxide/metal interface remained at ~ 15–16 wt% and considered to be sufficient to reform a protective layer in the event of mechanical damage to the original. No additional chromium depletion of the alloy occurred during the 4-h deposition stage.

The photoelectron spectrum for the two lowest ionisation bands of phenol has been simulated using quantum dynamic methods. A vibronic coupling Hamiltonian was set up consisting of seven vibrational modes and the first two ionised states. Parameters for the model are obtained by fitting adiabatic surfaces to a series of points calculated using ab initio methods. Such a model allows non-adiabatic couplings between the states to be included. CASSCF calculations used in this work provide reliable quantum chemical information for the model and the calculated photoelectron spectrum shows good agreement to experiment. The vibrational fine structure of both bands are reassessed and different assignments to those previously reported are detailed. The existence of a conical intersection between the ionised states is reported and its role in the dynamics of phenol upon ionisation is examined.

Carbon deposition on 20Cr25Ni steel at 700 °C has been investigated in CO2-based gases containing 0.7 vol%, 1.0 vol% and 1.4 vol% CO, each with ∼1000vpm C2H4, at 700 °C. Filamentary carbon deposits were formed at the two lowest CO contents but not at 1.4 vol% CO. TEM analyses have shown that the different CO levels change the propensity for deposition by altering the ratio of Ni:Fe in the nanometre sized catalytic particles produced during oxidation.

Austenitic stainless steel is used as fuel cladding in advanced gas-cooled nuclear reactors (AGR). At elevated temperatures, when the steel is exposed to CO2 based environments filamentary carbon deposits form on the surface of the steel. This filamentary carbon deposition is known to be catalysed by metallic nickel-rich particles. Adding trace amount of carbonyl sulphide (COS) into the gas mixtures suppresses the carbon deposition. In this current work, it has been shown that at 600 °C, the formation of filamentary carbon was suppressed by the addition of 215 ppb COS to a depositing gas mixture (containing approximately 1000 vppm C2H4/1% CO/bal. CO2) which was known to provide the environment suitable for carbon deposition. Samples exposed to the gas mixtures with and without 215 ppb COS were characterised using electron microscopy techniques to understand the formation mechanism of the nickel-rich particles and the inhibition mechanism due to the addition of COS. Electron diffraction study shows that the nickel-rich particles in the oxide layers assume the same crystallography as that of the austenitic metal underneath, regardless of the COS addition. The current observations also show that the metal-oxide interfaces was nickel-rich and a simple model has been proposed to explain the formation of nickel-rich particles within the subsurface oxide. Furthermore, it was found that when COS was added the surface of the nickel-rich particles in the oxide layer was found to be sulphur-rich by energy dispersive spectroscopy (EDS) on a scanning transmission electron microscope (STEM). It is believed that the surface sulphur adsorption onto the nickel-rich particles, rather than bulk sulphide formation, resulted in the inhibition of carbon deformation on the steel.

Protective metallic MCrAlY or diffusion type (NiAl) coatings enhance the oxidation and corrosion resistance of the underlying high temperature materials employed in aeroengines and industrial gas t...

The cyclic oxidation behaviour of fine-grained Type 347 stainless steel (TP347H FG) at 650 °C in air saturated steam and deoxygenated steam environments for 100–1000 h has been investigated. Electr...

Samples of RR1000 with differing grain size and surface finish have been exposed to air at different pressures. Specimens were exposed for 4000 h at 650°C with some exposed to atmospheric pressure air and some exposed to air at 40 bar pressure. Samples exposed to elevated pressure formed a surface layer of NiCr2O4 whereas that formed on samples tested at 1 bar pressure was chromia. The surface layer formed at 40 bar pressure was thinner than that a 1 bar pressure. At 1 bar pressure, some samples exhibited regions of convoluted buckled oxides but no spallation. In adjacent regions of planar oxides, spallation did occur. For the latter case, an estimate of 6 Jm−2 for the interfacial fracture energy has been made. None of the specimens tested at 40 bar pressure exhibited oxide spallation.

In this review, the role of oxidation of chromia-forming Ni-based superalloys used for rotor disc applications in gas turbines is examined. A detailed review of the oxidation performance of these alloys in air is undertaken with emphasis on oxide composition and kinetics. The actual surface condition of the component entering service is important but often overlooked during oxidation studies. The effect of shot-peening on the oxidation of Ni-based superalloys is here evaluated. The oxide growth rates and composition of these alloys was assessed with respect to their response to an applied load or stress. One area where a significant amount of research has been conducted is on the effect of oxidation on the mechanical performance, with oxidation having been shown to reduce time to crack initiation, increase fatigue crack growth rates and therefore reduce overall component fatigue life. It is argued that this enhancement in crack growth rates arises from the cracking of an oxide intrusion ahead of the crack...

Spallation of oxides grown on TP347H FG in air-saturated steam at 923 K for 50–1000 hours was investigated using a newly developed technique termed: Simultaneous Thermographic and Optical Recording Media Examination (STORME), recording the temperature disturbance resulting from delamination and spallation, during cooling, using Infra Red and video cameras. For the first time, the radius of the delamination site was measured experimentally at the point of buckle formation and followed during growth to the point of spallation. Collection of the exact spalled fragment was possible. The strain energies at each stage were calculated and a Spallation Map constructed.

The effect of shot-peening on the oxidation in air of the Ni-based superalloy RR1000 has been investigated over the temperature range 700–800 °C. The surface oxide in both peened and un-peened conditions consisted of isolated grains of rutile on the outermost surface beneath which was a protective Ti-doped chromia scale. Internal oxidation of aluminium occurred within the alloy with the formation of alumina particles within a γ ′ (nominally Ni 3 (Al,Ti)) denuded zone but the morphology of the sub-surface oxides differed between the two surface conditions examined. The kinetics of thickening of the chromia layer were sub-parabolic in most cases but closely approached parabolic behaviour for the un-peened surface condition at 800 °C. An enhancement in the rate of chromia growth was found for both surface conditions compared with a Ti-free chromia layer. This enhancement has been attributed to increased Cr ion diffusion as a result of Ti-doping of the chromia layer but the effect is reduced over time because of Ti-depletion in the alloy. At 800 °C, in the un-peened condition, the reduction in growth rate with exposure time is much less marked and this effect seems to be associated with the formation of a (Ti,Ta)O 2 phase beneath the chromia layer.

The effect of prior oxidation on the room temperature fatigue life of coarse grained Ni based superalloy, RR1000 has been performed at an R ratio of 0·1 with two pre-oxidation times: 100 and 2000 h at 700°C. These pre-exposures produce extensive oxidation damage. The room temperature high cycle fatigue life of the pre-oxidised specimens has been compared to as received specimens. At a maximum applied stress of 800 MPa a significant fatigue life deficit is observed in the pre-oxidised testpieces. This is accompanied with the observations of significant cracking of the external chromia scale and the intergranular internal oxides within the area of maximum stress. Preferential cracking of oxides may lead to early crack initiation and consequently a reduction in total fatigue life.

Isothermal oxidation of the Ni based superalloy, RR1000, has been performed at 700, 750 and 800°C for exposure periods up to 2000 h. A chromia external oxide scale with an outer layer of rutile was formed together with extensive internal oxidation of aluminium, both intragranularly and intergranularly. The internal oxidation was associated with a deeper γ′ particle free zone. Extensive metallographic measurements of both the depth of internal oxidation and of the γ′ particle free zone were made. The kinetics in each case approximated well to parabolic. Possible rate controlling processes are discussed.

Previous observations on Ni based superalloys, obtained through the use of focused ion beam (FIB) sample preparation and imaging, have reported the presence of subsurface voids after oxidation. In this present study, oxidised specimens of the Ni based superalloy, RR1000, were subjected to conventional sample preparation as well as both dual and single beam FIB preparation, with the aim of re-examining the previous observations of subsurface void formation. It is clear from FIB preparations that features previously interpreted as networks of voids have been demonstrated to be internal oxides by varying the sample tilt angles and imaging signal using either secondary electrons (SEs) or secondary ions (SIs). Conventional preparation methods illustrate the presence of subsurface alumina intrusions and the absence of voids, supporting previous evidence. The positive identification of voids and oxides by FIB can be complex and prone to misinterpretation and thus, the use of several imaging conditions and tilt angles must be used, along with conventional preparation methods, to confirm or refute the presence of ‘voids’ underneath oxides.

Current thermal barrier coating systems (TBCs) rely on an alumina-forming bond coat to provide protection against oxidation. At bond coat temperatures below 900 °C, however, optimum protection against oxidation and hot corrosion is best achieved by a chromia layer. TBC systems using this approach are currently being developed and an example is examined in this paper. The multilayered bond coat in the present TBC system consisted of: a NiCrAlY layer aluminised at its surface; a 50:50 NiCr layer (containing 1.8 wt% Si); an air plasma sprayed yttria stabilised zirconia top coat. These samples were oxidised in laboratory air at 750, 800 and 850 °C for durations up to 1,000 h. Post-test characterisation was undertaken using SEMs and energy dispersive X-ray spectroscopy to determine the growth rate of the TGO and to investigate interdiffusion within the layered coating structure. The interdiffusion process was also modelled using the ODIN finite difference code.

The oxidation kinetics are presented for a coarse-grained RR1000 alloy in the temperature range 700 – 900°C for times up to approximately 100 hours. The tests were undertaken in laboratory air in thermo-balances. The kinetics tended to sub-parabolic but were treated as parabolic to allow comparison with literature data. These parabolic rate constants were appreciably higher than expected for the growth of a protective oxide layer but were similar to those found previously on other Cr-rich Ni-based superalloys. SEM and confocal microscopy were used to show that the regions of emergent grain boundaries on the alloy surface oxidised faster than the grain centres. The surface oxide formed was Cr-rich with an enhancement of titanium in its outer regions. This is the situation at the grain boundaries and over the grain centres. Sub-surface oxidation of aluminium was observed, principally as intergranular fingers but also as intragranular particles. Ahead of this internal oxidation zone, at least for the...

In this paper the oxidation kinetics data on a TBC system where the NiCoCrAlY bond coat and the YSZ topcoat are both applied by the EB-PVD process are presented. The oxidation testing was performed at 950, 1050 and 1150 °C with times up to 1000 h. SEM analysis and modelling of the TGO growth rate show that the TGO is formed in two stages and that the initial oxide formed during the manufacture of the topcoat is non-protective. A global expression is derived for the oxidation kinetics at the three temperatures.

Conventional thermal barrier coating (TBC) systems consist of an insulating ceramic topcoat, a bond coat for oxidation protection and the underlying superalloy designed to combat the oxidising conditions in aero- and land-based gas turbines. Under high-temperature oxidation, the use of an alumina forming bond coat is warranted, thus all current TBC systems are optimised for the early formation of a dense, protective thermally grown oxide (TGO) of alumina. This also offers protection against Type I hot corrosion but a chromia layer gives better protection against Type II corrosion and intermediate temperatures, the conditions found in land-based gas turbines. In this paper the authors present the first known results for a chromia forming TBC system. Tests have been performed under oxidising conditions, up to 1000 h, at temperatures between 750 °C and 900 °C, and under Type I (900 °C) and Type II (700 °C) hot corrosion conditions up to 500 h. Under all these conditions no cracking, spallation or degradation was observed. Examination showed the formation of an adherent, dense chromia TGO at the bond coat / topcoat interface. These initial results are very encouraging and the TGO thicknesses agree well with comparable results reported in the literature.

It is known that fast-growing non-alumina scales in thermal barrier coating (TBC) systems tend to form at features where aluminium depletion is enhanced due to a high surface-area to volume ratio, a phenomenon known as breakaway oxidation. In this work, the influence of breakaway oxidation around protuberances in an MCrAlY bond coat in a TBC system during isothermal oxidation and after cooling is quantified numerically. A finite element approach is used which incorporates elastic, plastic and creep deformation of the bond coat and thermally grown oxide (TGO) and, importantly, the volumetric strains associated with oxide formation during the isothermal exposure. It is shown how volumetric strains which develop during breakaway oxidation can result in the formation of significant (>0.5 GPa) out-of-plane tensile stresses within the yttria-stabilized zirconia top coat at the oxidation temperature. These stresses are located along the flanks of the bond coat protuberances in locations where sub-critical cracks have previously been reported. The magnitude of the stresses increases with bond coat surface roughness, after the initiation of breakaway oxidation and after cooling. Results are also presented for the normal tractions across both TGO interfaces, and the influence of breakaway oxidation on these is discussed.

A series of isothermal and cyclic oxidation tests were carried out on two thermal barrier coating systems consisting of a CMSX-4 nickel-based superalloy substrate, NiCoCrAlY overlay bond coat applied by two different techniques and a yttria stabilised zirconia topcoat applied by electron beam physical vapour deposition (EBPVD). The bond coats were applied by either high velocity oxy-fuel spraying (HVOF) or EBPVD. Isothermal oxidation tests were carried out at 950°C, 1050°C and 1150°C for both coating system for up to 3000 hours. Cyclic oxidation testing was conducted at 1150°C in one hour cycles to coating failure on both coatings. A detailed examination on the oxide thickness was conducted on all specimens, along with characterisation of the bond coat and TGO. This was coupled with examination of specimen cross-sections for cracking and signs of coating degradation. Isothermal oxidation showed sub parabolic oxide growth consistent with the literature. Detailed analysis of oxide thickness showed a normal distribution but with increasing standard deviation with increasing oxidation time. Both bond coats were dual phase, β + γ. The EBPVD applied bond coat only, was found to contain yttrium rich precipitates in the bond coat and TGO. Both coatings showed no increase in surface roughness after either isothermal or thermal cycle testing. Short sub-critical cracks were observed at the TGO/topcoat interface in the HVOF applied bond coat only associated with the flanks of asperities. Coating failure in both coatings occurred at the TGO/bond coat interface on cooling and was likely driven by the thermal expansion mismatch between the TGO, topcoat and substrate. The initial mechanism of crack formation was not determined conclusively but could be a wedge cracking type mechanism.

Localised loss of aluminium from the bond coat within thermal barrier coating (TBC) systems is a critical factor determining the lifetime of these coatings. In this paper, it will be demonstrated that electroplated MCrAlY bond coats with asperities of high aspect ratios can experience premature chemical failure during isothermal exposures at 1100 °C in air. This can result in the rapid conversion of the asperity into Co- and Cr-rich oxides. The enhanced aluminium depletion within these asperities, which leads to chemical failure, is a consequence of their increased surface area coupled with a limited physical access to the reservoir of aluminium in the bulk of the bond coat. This process has been demonstrated using the 2-D finite-difference model ODIN which predicts a depletion to less than 1 at% aluminium within a typical asperity after 1 h exposure at 1100 °C. The results also show that the aluminium content of the bulk of the bond coat is sufficient to enable the formation of a continuous protective alumina underneath these regions, as observed experimentally. It is suggested that the volume increase associated with the conversion of the bond coat asperities into breakaway oxide results in out-of-plane tensile stress development at the oxidation temperature. These stresses are anticipated to be at a maximum between the oxide asperities and are thought to nucleate small, sub-critical cracks at these locations at temperature. Further delamination will occur during cooling both by the extension of these cracks and by the growth of crack-like defects within the porous breakaway oxides.

P-type transparent conductive oxides have potential applications in photovoltaics, transparent electronics, and organic optoelectronics. In this paper, results are presented on the synthesis of Cu 2 SrO 2 thin films, a p-type transparent conducting oxide by a sol–gel route. Cu(II)methoxide and Sr-metal dissolved in anhydrous isopropanol were used as precursor for the sol preparation. For potassium (K) doping, K-acetate dissolved in anhydrous isopropanol was used as the precursor. Films were spin-coated onto substrates and partially pyrolysed in air at 225°C. After partial pyrolization, a two stage annealing sequence was used to achieve the final film microstructure and composition. Although combinations of oxygen pressure, annealing time, and annealing temperature were used to obtain phase pure Cu 2 SrO 2 thin films, X-ray diffraction consistently showed the presence of Cu 2 O as a second phase with Cu 2 SrO 2 −the desired phase. Microstructural studies showed similar phase separation in the films and confirmed the microcrystalline nature. The best conductivities obtained for the undoped and 1% K-doped films were 2 × 10 − 3 and 1.2 × 10 − 2 S/cm, respectively. Both films showed a broad optical absorption edge in the visible range.

The tensile creep properties of a β-rich (Ni,Pt)Al diffusion coating have been determined at various strains over the temperature range 800–1000 °C by deconvoluting the creep response of layered composite specimens under constant stress conditions. It is found that the creep strength of the diffusion coating tends to be higher than that of a similar coating tested previously by Pan et al. [Pan D, Chen MW, Wright PK, Hemker KJ. Acta Mater 2003;51:2205]. Associated with the higher strength is a relatively high creep activation energy (∼600 kJ/mol) and a high stress dependence of creep rate which increases with creep strain from ∼6.8 at 1% strain to ∼9.4 at 9% strain. These high values and the associated high creep strength are attributed to the formation, at the creep temperatures, of a fine dispersion of second-phase particles, presumed to be γ′ and possibly α-Cr, within the β-(Ni,Pt)Al matrix. It is suggested that the extent of this transformation and, hence, the strengthening effect are related to the original aluminium and platinum content of the coating.

Nickel-based superalloys used in stressed situations at high temperatures tend to be optimized for mechanical properties and have insufficient aluminium to form a continuous surface layer in service to provide long-term protection against oxidation. This deficiency can be met through the use of coatings that have higher aluminium contents and which form a protective alumina layer during service. These can take the form of metallic overlays or diffusion coatings, both of which can also form a bondcoat layer within thermal barrier coatings (TBCs). Factors which lead to loss of aluminium through oxidation and interdiffusion are discussed in this article. A consequence of excessive aluminium depletion is the formation of non-protective oxides which can be particularly deleterious in TBCs.

Metallic overlay coatings of the MCrAlY type (where M is Ni, Co or a combination of both) are regularly used in high-temperature plant to provide resistance to oxidation and high-temperature corrosion through the formation of a protective surface layer of alumina. Such coatings are also frequently used as a bond coat in thermal barrier coating systems. For both applications, the endurance of the coating system depends on the maintenance of the protective alumina layer. In particular, a high resistance to spallation is required and also a sufficient reservoir of aluminium within the coating so that re-healing of the alumina layer will occur should spallation occur.In this paper the results of a study into the oxidation behaviour of an LPPS MCrAlY coating on a CM186LC alloy are presented. During the study extensive TGO thickness measurements were made at varying times at 1100°C from which the oxidation kinetics have been calculated up to the point where extensive spallation occurred. Over the time r...

The formation of diffusionally isolated regions in air plasma-sprayed, APS, MCrAlY bond coat or overlay coatings has been established. The ‘diffusion cells’ experience greatly accelerated depletion of aluminium and an early onset of breakaway oxidation associated with the morphology and extent of the internal oxides. In this paper the effect of internal oxides produced in low pressure plasma sprayed, LPPS, CoNiCrAlY coatings are presented. Isothermal tests in laboratory air at 1200°C were conducted with post-test characterisation performed using scanning electron microscopy and energy dispersive X-ray analysis. A parabolic correlation was found between TGO growth rate and time at temperature. Importantly, compositional profiles of cross sections through the coatings demonstrated that no barriers to diffusion occurred throughout the body of the coating. What has been found in the LPPS coatings is a mechanism whereby diffusion cell formation is possible at sites where splat boundaries containing int...

The results of a study on two thermal barrier coating systems, oxidised isothermally in thermobalances at 1000°C, show that the surface roughness of the bond coat and its density have a significant effect on the oxidation behaviour. The particular system examined is composed of the Haynes 230 alloy substrate with CoNiCrAlY bond coat and yttria stabilised zirconia ceramic outer layer. Coatings were applied either using air- plasma spraying for both layers in the sample type with the coarsest interfacial roughness or electroplating deposition for the bond coat in the sample with the smoothest interface. Weight gain per unit geometric area is higher for the plasma-sprayed bond coat due both to its larger effective surface area and because of gas access into the structure and consequent internal oxidation. This enhanced oxidation results in a much more rapid depletion of aluminium within the bond coat and an earlier transition into chemical failure. The consequences of this on the formation of delamination cracks is discussed.

It is proposed that bond coats in thermal-barrier coating (TBC) systems, particularly those deposited by plasma spraying, can contain regions which are diffusionally isolated from the bulk of the coating. This can arise through the internal formation of alumina layers as a consequence of the ingress of molecular oxygen into the relatively porous structure. Such isolated regions, termed diffusion cells, will experience enhanced depletion of aluminum as a result of the continued thickening of the alumina layer at their surface. This process has been demonstrated for a CoNiCrAlY bond coat after oxidation in air at 1100°C. A consequence of this enhanced depletion is that chemical failure will occur sooner in diffusion cells and voluminous breakaway oxides will form above them at the interface of the bond coat and the ceramic top coat. The associated spatial variation in oxidation and displacement rates across the surface of the bond coat are expected to aid delamination of the outer ceramic layer.

The oxidation dynamics of an overlay coating and the corresponding thermal barrier coating system are presented. The particular systems examined are composed of a nickel-based superalloy with an air plasma-sprayed NiCrAlY bond coat and the thermal barrier coating system consists of air plasmasprayed yttria stabilized zirconia layer. Failure can occur in these systems by crack propagation within the ceramic outer layer at the interface with the bond coat. Defects, such as microcracks and pores, are common in plasma-sprayed coatings and within the thermally grown oxide scales. These can act as initiation sites for cracks. The subsequent growth of these cracks can lead to loss of the outer protective materials. Considerable information is available by microscopic examination of sections through test specimens that have been held at temperature for varying amounts of time. By careful sample preparation it is possible to monitor the development of the oxide scale formed during high temperature testing ...

A new method is described for the evaluation of the creep properties of as-deposited overlay coatings. The method uses a composite tensile specimen consisting of a core alloy of well-characterised creep properties onto which the overlay coating is deposited. The overall strain/time response of this specimen, tested under constant stress conditions, can then be deconvoluted to obtain the creep characteristics of the coating. The method has the advantage that the coatings are evaluated for the same microstructural conditions as they will be used under, i.e. having the same porosity levels and microstructure. A limited demonstration of the method has been made for an air plasma-sprayed Ni25Cr6AlY coating deposited onto an austenitic steel core and creep tested at 900°C. The creep strength of this coating was surmised to be higher than that of equivalent monolithic samples produced by low pressure plasma spraying.

The ability to predict the onset of spallation of protective oxide layers is an important requirement in evaluating the endurance of high temperature coatings or thin-sectioned components. Oxide spallation tends to occur during cooling, when the oxide layer is usually in compression. Considerable progress has been made in modelling spallation from flat substrates under such conditions, and much of this work is reviewed in this paper. In particular, the results of finite-element modelling of oxide/metal interfacial cracking are presented for both chromia-and alumina-forming substrates. It is shown that creep relaxation in the coating or alloy can retard the growth of the interfacial crack even during fast cooling but the extent of retardation increases with lower cooling rates. When such creep relaxation is extensive, i.e. at low cooling rates or with substrates weak in creep, improved spallation resistance results. For such cases, a simple critical strain-energy approach can be used to predict the onset of spallation.

Oxidation of the Ni-based superalloy RR1000 has been undertaken in air over the temperature range 600-900 degrees C for times up to 5000 h. The surface oxide consisted of a protective Ti-doped chromia layer but with rutile forming on its outer surface. Sub-surface oxidation of Al and Ti also occurred. The thickening kinetics of the chromia layer were sub-parabolic with initial rates around two orders of magnitude higher than expected for Ti-free chromia. This enhancement and the sub-parabolic kinetics are accounted for by Ti-doping of the chromia layer. Over time the enhancement reduced because of Ti-depletion in the alloy. Funding Agencies|Engineering and Physical Sciences Research Council||Rolls-Royce plc

High temperature oxidation resistance is becoming increasingly important to component life as operating temperatures increase. Additionally, shot-peening is used to help extend the service life of high-duty components through inducing compressive stresses in the surface. The effect of shot-peening on oxidation resistance has not been examined in detail, at least on recent high-strength turbine disc alloys such as RR1000. In this study, oxidation of a coarse-grained variant of RR1000, with and without shot peening, has been characterised over a range of temperatures (700-800 o C) for periods of time extending to 2000 hours. Microstructural characterisation of the oxide and underlying material was performed using a number of techniques. A duplex surface oxide, comprised largely of Cr2O3 covered by a thin top layer of TiO2 was recorded in both surface conditions. Beneath this surface oxide was an alumina internal oxide, with the shot-peened condition exhibiting more numerous penetrations. The oxide growth kinetics in both alloy conditions were similar.

International audience; A finite-element study has been undertaken to investigate the stress development within a TBC system consisting of an EB-PVD YSZ topcoat and a Pt-aluminized diffusion bondcoat. Particular attention has been paid to the role of variables such as the elastic anisotropy within the topcoat, interface roughness, variation in creep strength of the bondcoat and the volumetric strains associated with the formation of the thermally grown oxide (TGO). Bond coat oxidation and thermal loading during cooling give rise to significant tensile stresses within the topcoat and tensile tractions at the TGO interfaces. Bondcoat creep, as distinct from yield and plastic behaviour, was the dominant stress relaxation process, and strong bondcoats (in creep) tended to show higher tensile stress levels. Another important factor determining thermal barrier coating stress levels was the level of elastic anisotropy of the topcoat: an elastic isotropic yttria-stabilized zirconia gave rise to considerably higher stresses than a transversely isotropic topcoat.

If strong and persistent misalignments of the exchange rate are caused by non-fundamental influences, such that a return to equilibrium is hampered by a coordination failure among fundamentals-based traders, then central bank intervention may act as a coordinating signal, encouraging stabilizing speculators to re-enter the market at the same time. We develop this idea in the framework of a simple microstructural model of exchange rate movements, which we then estimate using daily data on the dollar-mark exchange rate and on Federal Reserve and Bundesbank intervention operations. The results are supportive of the existence of a coordination channel of intervention effectiveness

Amorphous indium zinc oxide (a-IZO) is an increasingly important material both as a transparent conductor or semiconductor in transparent electronics and as an archetype amorphous electronic mixed metal oxide in itself. Here, a co-sputtering composition spread combinatorial approach was used to examine the conductivity and optical transparency in a-IZO thin films as a function of both In : Zn metals stoichiometry and the amount of oxygen added to the argon sputter gas. For optimising the conductivity, the percent oxygen in the sputter gas and the metals composition were found to have a strongly coupled effect. In particular, a-IZO films with conductivity σ > 2000 S/cm can be grown for a broad range of metals compositions, In content from ∼60 to ∼85 cation%, as long as the corresponding optimal oxygen level is used in the deposition. The amount of oxygen required increases with increasing indium content. When too much oxygen is used, the conductivity is reduced due to a decreased carrier concentration whereas when too little oxygen is used, the conductivity is reduced due to decreased electron mobility. Concurrent with the decrease in electron mobility, there is increasing optical absorption from 400 nm to 1000 nm which renders the oxygen deficient a IZO samples grey.

The current efficiency model of Lillebuen et al. (Electrochim. Acta25, 131, 1980) was reworked and the analytical derivation clarified. As a result, the predicted metal loss differred by nearly 9%.

The dual operating problems of anode effect and sludging, in smelting cells, highlight the central concept of this paper, that is, the electrochemical parameters which determine the rate and efficiency of the electrolysis also control the overall cell heat balance. An additional constraint is present since the material balance, and the necessity of maintaining it through bulk feeding, exerts a strong influence on those same electrochemical parameters. Low concentrations of alumina in the bath result in excessive polarization and anode effect. Overfeeding gives rise to sludging and voltage instability. Even during stable operation the conductivity, decomposition voltage and polarization potentials respond to the electrochemical depletion of the alumina. These interactions place severe limits on the controllability of the batch fed smelting cell so that its operation is essentially dynamic. New technology now emphasizes the concept of continuous or 'point feeding' of the alumina.

The hazards which may arise due to the use of radioactive foils in the gas chromatography detectors are reviewed; they are external radiation, effect of mechanical damage, emission of radioactive gases, the effect of chemical attack and the effect of heat. Overheating of the foils is the principle source of danger to the majority of users, but the other hazards particularly effect those persons who build or service their own apparatus.

An autoradiographic technique is described, and has proved to be sensitive and useful in the estimation of very low concentrations of Po210 in urine. The chemical separation of the polonium is described in detail. Although the overall time for the assay of a sample is long, the technique can be used to advantage when there are a considerable number of samples. Furthermore, for samples containing up to 1 ωωc/1 of urine, the length of time taken, in the autoradiographic method described, is no more than that required to obtain comparable accuracy by conventional α-scintillation techniques.