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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.

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.

A high burn-up Zircaloy-2 cladding is characterised in order to correlate its microstructure and composition to the change of oxidation and hydrogen uptake behaviour during long term service in the reactor. After 9 cycle of service, the chemical analysis of the cladding segment shows that most secondary phase particles (SPPs) have dissolved into the matrix. Fe and Ni are distributed homogenously in the metal matrix. Cr-containing clusters, remnants of the original Zr(Fe, Cr)2 type precipitates, are still present. Hydrides are observed abundantly in the metal side close to the metal-oxide interface. These hydrides have lower Fe and Ni concentration than that in the metal matrix. The three-dimensional (3D) reconstruction of the oxide and the metal-oxide interface obtained by Focused Ion Beam (FIB) tomography shows how the oxide microstructure has evolved with the number of cycles. The composition and microstructural changes in the oxide and the metal can be correlated to the oxidation kinetics and the H-uptake. It is observed that there is an increase in the oxidation kinetics and in the H-uptake between the third and the fifth cycles, as well as during the last two cycles. At the same time the volume fraction of cracks in the oxide significantly increased. Many fine cracks and pores exist in the oxide formed in the last cycle. Furthermore, the EPMA results confirm that this oxide formed at the last cycle reflects the composition of the metal at the metal-oxide interface after the long residence time in the reactor.

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...

Intergranular oxide intrusions formed ahead of a crack in a Ni-based superalloy at 650 °C have been examined using three-dimensional atom probe tomography. Of two scans undertaken, a thin (∼5 nm) alumina layer was observed next to the alloy substrate in one, overlaid by a chromia layer. In the second, chromia penetrated to the alloy interface. The chromia layer contained Ti in solution as well as discrete particles of TiO 2 .

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.

A comparison of literature data indicates that intergranular oxide intrusions formed in the nickel-based superalloy RR1000 ahead of stationary cracks grow at a rate higher than that observed on flat surfaces. This is attributed to the effect on vacancy equilibria of the tensile stress that exists at the tip of the intrusion. A stress-aided oxidation rate parameter is defined that is stress dependent and varies with the nature of the oxide. The threshold condition for crack advance is also discussed.

Oxide intrusions which formed ahead of the crack tip in an advanced nickel-based superalloy after exposure to air at 650 °C have been investigated by (scanning) transmission electron microscopy equipped with energy-dispersive X-ray spectrometry. Three different cases were considered: (i) a fast growing crack; (ii) a stationary crack exposed to a 5 h period under load; and (iii) a stationary crack exposed to a 5 h period but held at zero mechanical applied load. The intrusions formed during both hold periods were much longer than those present ahead of the (growing) dynamic crack (∼1.5 μm) and consisted of layered oxides in the thermodynamic sequence of CoO, NiO, Cr 2 O 3 , TiO 2 and Al 2 O 3 . The last three of these oxides appeared to form protective layers along the flanks and, for the stationary cracks, at the intrusion tip. Calculations of the oxygen partial pressure both across the width and along the length of the oxide intrusions have been undertaken assuming such protective conditions. Significant quantities of oxygen are thus not expected to exist in the alloy ahead of intrusions which are sealed by protective oxide layers.

The deleterious effect of sulphur contamination on oxide spallation has been well demonstrated experimentally. The present paper attempts to account for this using finite-element modelling of crack growth along a sulphur-contaminated interface during cooling. Sulphur reduces the intrinsic work of adhesion, Wad, of this interface and, it is suggested, this is equivalent to reducing the characteristic fracture stress for that interface. It is shown that this approach does predict the typical trends in S-bearing alloys of spallation after shorter exposure times. A significant result is that the effective fracture energy for spallation, γeff, reduces with increasing sulphur contamination but is always 1–2 orders of magnitude larger than Wad. This high value for γeff arises because of creep relaxation within the alloy. Sulphur does not affect this process directly but reduces the extent of creep relaxation in a cooling transient by initiating spallation at a smaller value of temperature drop.

Oxidation of Super 304H and shot-peened Super 304H austenitic stainless steel was conducted over a temperature range of 600 – 700°C in steam at atmospheric pressure and compared with oxidation in laboratory air. It was observed that the mass gain was higher in steam compared with air. It should be noted that the shot-peening treatment improved the oxidation resistance in both air and steam considerably. Furthermore, spallation was only observed on the pickled Super 304H specimens and only in steam after exposures at 650°C and 700°C. The protective films found at the base of the duplex oxide grown on pickled Super 304H in air and steam were found to differ in that after exposure to air, a chromium rich film was present whereas after exposure to steam, a less protective Cr–Mn spinel was present. The oxidation kinetics were assumed to be parabolic for purposes of comparison and parabolic rate constants, Kp, were calculated for oxide growth under the different oxidation conditions. The Kp value for th...

The spallation of thermal barrier coatings can be life-limiting but its prediction has proven to be a difficult problem. The final spallation event can often occur by buckling and is driven by the release of strain energy within the ceramic top coat and within the underlying thermally-grown oxide (TGO) layer if the delamination interface is at the TGO/bond-coat interface. Prior to this event, substantial sub-critical damage must develop at one or both of the TGO interfaces. It is argued in this paper that it is only the strain energy within the TGO produced during cooling that contributes significantly to this damage development and not that within the top coat. A critical strain energy within the TGO layer is suggested as a possible pragmatic method of predicting spallation. A critical assessment of proposed mechanisms which implicate bond coat oxidation in the failure process is also undertaken in the paper. Attention is given to: the role of phase changes in the bond coat; the influence of the mechanical constraint imposed by the top coat on the mechanical stability of the bond coat interface; the effect of the growth of the TGO on a non-planar interface on stress development; the importance of localised Al depletion in nucleating a fast-growing non-protective TGO.

The wear behaviour of Stellite 6 was studied during rotational sliding in a bespoke bearing rig at 600 °C for times between 2 min and 12 h. Six stages of wear were identified: (i) formation of a mixed oxide ‘glaze’, (ii) cobalt and chromium elemental diffusion to the ‘glaze’ surface forming chromium- and cobalt-dominated oxide layers, (iii) oxygen diffusion into the ‘glaze’ leading to a chromium-dominated oxide layer at the ‘glaze’/substrate interface, (iv) spallation of the ‘glaze’ through chemical failure, (v) re-formation of the ‘glaze’ and (vi) elemental diffusion within the ‘glaze’, again resulting in discrete oxide layer formation.

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.

The wear behaviour of a cobalt-based superalloy, Tribaloy 400C, was studied during self-mating rotational sliding in a specially constructed bearing rig at 600 °C between 2 min and 12 h at a rotation speed of 3 ms−1. Stable wear protective oxide films often referred to as ‘glazes’ were formed with their coverage of the wear scar increasing with time. Six stages were identified in the formation of the wear protective oxide films: (i) transient oxidation to form a mixed oxide layer, (ii) preferential oxidation of the chromium to form a chromium oxide layer at the substrate interface, (iii) removal of this layer and compaction onto the surface, (iv) diffusion of mainly Co and Mo cations to the surface through the chromium oxide forming an oxidised layer above it, (v) slow breakdown and formation of the oxide films with further diffusion of mainly Co and Mo to the surface to form Co and Mo dominated layers on top a mixed Co, Cr, Si and Mo oxide film, and (vi) further breakdown and formation of the oxide film to form a uniform distribution of elements within the wear protective oxide film.

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.

SmCo alloys form the basis of excellent permanent magnets with potential service applications up to 550 °C. It is suspected that their oxidation behaviour may limit their usefulness but this is a relatively unstudied subject. In this work, two grades of Sm2(Co,Fe,Cu,Zr)17-based alloys were oxidised in air at temperatures between 300 and 600 °C, and the oxidation morphology characterised by post-test SEM examination. Both alloys formed a thin external oxide scale composed of Cu and mixed Fe/Co oxides and an extensive internal oxidation layer consisting of Sm oxide within a (Co,Fe) solid solution. The depth of the internal oxidation zone (IOZ) was typically an order of magnitude thicker than the surface oxides and increased parabolically with time. The internal oxidation is an important degradation phenomenon because the transformation to (Co,Fe) causes a loss of magnetic properties proportional to the volume consumed. The morphologies of the oxidation zones, nature and mechanisms of various precipitated phases and the differences between the two alloys’ oxidation behaviour 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.

The oxidation characteristics of NdFeB alloys, modified by additions of Co up to 10 at.% and with minor additions of Dy, Zr and V, have been examined in this article over the temperature range 250–500 °C. The surface oxide consisted of an outer surface layer of polycrystalline haematite, regardless of Co content, and an inner layer of Co-modified magnetite. Underlying this two-layer surface oxide was an extensive internal oxidation zone (IOZ) formed as a result of preferential reaction with oxygen of the magnetic phase, Nd2(Fe, Co)14B. The IOZ consisted of a bcc solid solution of Fe and Co and contained a fine dispersion of Nd-rich oxide particles. The zone grew by inward oxygen diffusion, principally along high-angle grain boundaries within the Fe-rich matrix. The thickening of the IOZ occurred with parabolic kinetics but at a rate which decreased with increasing alloy Co content by a consistent factor of 2–3 in parabolic rate constant at all test temperatures. The activation energy for the growth of the IOZ lay in the approximate range 91–104 kJ/mol for the various alloys but there did not appear to be a systematic variation with alloy composition.

This study has used thermogravimetric, acoustic emission and finite-element modelling techniques to examine the tensile cracking of a chromia layer on a 20Cr25Ni steel during thermal cycling from an oxidation temperature of 900 °C. It is found that the process is highly sensitive to the temperature amplitude of the thermal cycle and that hold periods at the bottom temperature result in enhanced oxide cracking whereas hold periods at the peak temperature can inhibit the onset of cracking. For standard saw-tooth cycles, the finite-element analysis shows that these effects can be explained by a creep hysteresis effect. This arises from stress relaxation during the cooling half-cycle, which results in the development of in-plane tensile stresses within the oxide layer on the return to temperature. Low-temperature hold periods in the creep regime increase the in-plane tensile stress in the oxide on the return to temperature. High-temperature hold periods tend to reduce the peak tensile stress produced in subsequent cycles but, for the example examined here, the effect is small.

An overview is given of existing models of oxide spallation with particular emphasis on their relevance to the mechanical integrity of steam-grown oxide layers on alloys used in power generating systems. Although the bulk of the experimental and modelling work over the last decade has been undertaken on alumina-forming high-temperature alloys, the understanding obtained can be used advantageously to identify likely key factors affecting the mechanical response of alloys at lower temperatures. Various aspects are addressed, such as the mechanisms of cracking and spallation, but a central issue is whether creep rates in the multi-layered scale formed on ferritic alloys can ever be high enough to relax possible growth stresses and to permit the Critical Strain Energy Criterion to be used as a method for predicting oxide spallation.

Fecralloy foil specimens of 50 μm thickness have been oxidised isothermally in air, using thermo-balances, at both 900 and 925°C for exposure periods up to 70 hours. The oxidation kinetics, determined from the mass gain measurements, show the expected very rapid initial rates, associated with transitional oxide formation, but these rates decrease to low values as α-alumina forms. It is estimated that the extent of this conversion is significant after approximately 10 hours at 925°C and after 30 hours at 900°C. These kinetics, in each case, could be fitted to a hyperbolic function which was used to generate input data for the 2-dimensional ODIN finite-difference code. This code was then used to predict aluminium depletion profiles in the specimens as a function of time and temperature, both under planar surfaces and at 2-dimensional corners. It was predicted that depletion at corners would be worse than under planar surfaces but that this difference would reduce with increasing exposure time. The p...

The oxidation kinetics in air of a commercial NdFeB magnet have been investigated over the temperature range 335–500°C. The oxide microstructure has been characterized by SEM, XRD and cross-sectional TEM. The results show that the external scale formed consists of an outer layer of Fe2O3 and an inner layer of Fe3O4 but that the principal degradation process is the formation of an extensive zone of internal oxidation. HREM has been used to show that this zone contains NdO particles embedded in an α-Fe matrix. These particles are discrete and very small, approximately 2 nm in diameter, and have an amorphous structure. The α-Fe matrix has a columnar grain structure with a grain width of approximately 100 nm. It is argued that the high rates of internal oxidation arise because the external-oxide layers are not protective at the oxidation temperature, and oxygen penetrates to the zone front by fast diffusion along the columnar α-Fe grain boundaries.

An attempt was made to determine the strain and the stresses generated by the growth of an oxide film using several approaches: an experimental one by means of deflection tests and modeling using either a recently developed creep analysis or a finite-element simulation. A new deflection apparatus was developed and NiO growth studied during the early stages of oxidation of a Ni80Cr20 alloy at 900°C, since many microstructural, kinetics and mechanical data are available for this system. The comparison of experiments and modeling indicate that the oxide layers are mostly subjected to compressive stresses when NiO is growing and the stress level and evolution clearly show that viscoplastic strain occurs in both the substrate and the oxide during oxidation. The comparison between the two modeling approaches with experiment leads to good agreement and suggests that the compressive-growth stresses derive from the lateral expansion of the fraction of new oxide that is formed within the oxide layer.

This paper examines the role played by oxide layers in the deposition of carbon on two 20Cr–25Ni–Nb-stabilized austenitic steels containing either zero or 0.56 wt% Si. Selective preoxidation, at 550°C in Ar/H2/H2O, was used to produce regions covered by either chromia or silica or, in the silicon-free alloy, to be bare metal. Deposition was performed, also at 550°C, in a CO2/1% CO/1000 vppm C2H4 gas mixture having an estimated carbon activity very much greater than unity. This gas also had an oxygen potential sufficient to form magnetite, but not nickel oxide. It was found that, even at these high carbon activities, none of the oxides formed could catalyze carbon deposition and that this occurred only when the gas had direct access to the alloy substrate. The carbon filaments formed were found, by high-resolution electron microscopy, to be solid, have a turbostratic structure, and to contain at least one nickel particle at their tips. The source of these nickel particles is the alloy substrate and a mechanism is proposed for their formation.

MCrAlY overlay coatings have been successfully used as a means of improving the oxidation performance of gas turbine blades operating at elevated temperatures. However, depletion of aluminium can limit the ability of such coatings to form a protective oxide layer should spallation of the original α-Al2O3 oxide layer occur under thermal cycling conditions. It is the objective of the current research to evaluate the potential of NiAl3 as a reservoir phase for a NiCrAlY overlay coating on a IN738LC superalloy substrate at 1,100°C in air. The morphologies and microstructures of the conventional NiCrAlY and NiAl3-modified NiCrAlY overlay coatings in the as-sprayed and oxidised conditions were characterised using SEM, EDX and XRD techniques.

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 ...

The oxidation behavior in air of air-plasma sprayed (APS) overlay coatingsof Ni–25Cr–6Al–Y have been studied at 1100°C. Aprotective alumina scale developed after 5- to 10-hr exposure with, initially,parabolic growth kinetics. With protracted exposures (>100 hr),subparabolic behavior developed, associated with aluminum depletion withinthe coating caused, principally, by internal oxidation of the low-densityAPS structure. This depletion caused intrinsic chemical failure, manifestedby the formation of a layer of Cr,Al,Ni-rich oxide beneath the residualalumina layer. Associated with this process of chemical failure was theformation of a layer of porous Ni,Cr-rich oxide above the aluminalayer. Oxide spallation occurred by delamination within this layer duringcooling; the spallation sites tended to lie above protuberances in theunderlying coating. Initial spallation occurred at a critical temperaturedrop, which decreased rapidly with increasing exposure time. A nonrigorousmodel of this spallation process has been developed which envisages thatdelamination occurs by the propagation of an oxide void under the action ofout-of-plane tensile stresses developed during cooling. Agreement with thespallation data is encouraging and shows that the deterioration ofspallation resistance with exposure time arises not only because oxidethickness increases but also because the maximum void size within the porousoxide layer increases.

Deflection tests have been used to estimate the stresses developed in the alumina layer formed during short-term oxidation of a Fe-22Cr-5Al-0.3Y Fecralloy steel at 1000°C. Elastic analysis of the deflecting specimen is inappropriate under these test conditions because of the low creep strength of the alloy. Accordingly, a recent creep analysis has been used in this work using currently determined creep properties of the alloy substrate. The results of the analysis show that for the thin oxides produced (

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.

A finite-element analysis has been performed of alumina spallation by wedge cracking from a flat Nil6Cr6A1Y coating substrate during cooling from 1100°C. The analysis allowed for creep deformation within both the coating and oxide but it was found that only creep within the coating influenced the growth of the interfacial wedge crack by relaxing stresses at the crack tip. As a result, crack growth was suppressed during the early stages of cooling. Computed values of the critical temperature drop to cause spallation are presented for various cooling rates and values of the coefficient of thermal expansion of the coating. The results show that the effective fracture energy for growth of the interfacial crack in the presence of creep is some 1–2 orders of magnitude greater than previously determined values for elastic fracture of the oxide and demonstrate the important role of coating creep. These results are used to produce a first spallation map for this system.

Mechanical damage to thin, protective oxide layers arises principally in service from differential strains produced by temperature changes. For typical alumina- or chromia-forming alloys, in-plane tensile stresses are developed within the oxide layer during upward temperature transients and compressive stresses are produced during cooling. This paper reviews the fracture of oxide scales due to these imposed stresses, under both tensile and compressive loading conditions. Imposed strain rates can vary over many orders of magnitude and, since these are applied at high temperatures, the possibility exists that stress relaxation processes (generically termed ‘creep’) will reduce the development of high stresses within the oxide or at the oxide/metal interface and, thus, have a beneficial influence on the processes of scale failure. This aspect is considered in detail both by reviewing published data and by presenting new numerical results on the influence of metal creep strength on the growth characteristics ...

During isothermal oxidation of one surface of a metal strip, growth-induced stresses will produce curvature of the metal/oxide composite sample. This observation has been used extensively elsewhere to evaluate the average stress developed within the oxide layer, but previous estimates have been based on an elastic stress analysis of the bending composite. At typical oxidation temperatures, creep deformation of both oxide and metal phases is much more likely. The present paper provides a stress analysis assuming such non-linear creep deformation. It considers the situation where the oxide experiences a net compressive stress and the metal a net tensile stress. The oxide neutral axis may be located either within the oxide layer or outside it, in the gas phase, depending on the thickness of the oxide and the relative creep strengths of the oxide layer and metal substrate. Evaluation of the stress distribution and average stress values in both the metal and oxide phases is provided for notional mechanical properties, within which the metal creep strength is varied widely. It is also shown how the assumption of elastic deformation can lead to large overestimates of stress values.

Mechanical damage to thin, protective oxide layers arises principally in service from differential strains produced by temperature changes. For typical alumina- or chromia-forming alloys, in-plane tensile stresses are produced during cooling. Imposed strain rates can vary over many orders of magnitude and, since these are applied at high temperatures, the possibility exists that diffusional relaxation processes (generally termed 'creep') will reduce the development of high stresses within the oxide or at the oxide/metal interface and, thus, have a beneficial influence on the processes of scale failure. This aspect is considered in detail in this paper both by reviewing published data and by presenting new numerical results on the influence of metal creep strength on the growth characteristics of an interfacial crack. In general, it is shown that diffusional relaxation can be important but that its contribution will depend on the intrinsic creep properties of the phase concerned and also on the values of imposed strain rate and temperature.

In this review the role of stress in the high temperature oxidation of metals is examined. In the introductory sections the Wagner theory of oxide growth and its modification to account for short circuit diffusion are outlined. The direct influence of oxide growth stresses on defect diffusion is then examined and shown to be a significant factor when constrained volume changes accompany the growth of the oxide layer. Methods for evaluating growth stresses are critically reviewed and the possible origins of such stresses considered in detail. The response of the oxide layer to thermally and mechanically induced stresses is considered in the last part of the review. Under tensile conditions, through-thickness cracks can readily be produced but may also heal with fresh oxide if imposed strain rates are relatively low. Spallation of the oxide is not usually found under tensile conditions but occurs frequently under compressive stresses by either a ‘buckling’ or ‘wedginq’ process. Detailed descriptions...

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.

Stresses developed isothermally at temperature in both the oxide layer and metal substrate as a consequence of the oxidation process are known as ‘growth stresses’. They seem to be an indigenous fe...

Experimental results on the failure of oxide scales on chromia-forming and alumina-forming alloys are reviewed and discussed in terms of the modes, onset and kinetics of spallation. Other factors affecting the extent of spallation are listed. It is concluded that despite there existing a large body of experimental results, the understanding of the mechanisms of spallation is still not sufficient to enable effective improvement of the service properties of high temperature alloys.

Temperature changes pose a significant threat to the integrity of protective oxide layers because of the differential strains developed between oxide and substrate. These may induce either tensile or compressive in-plane stresses within the oxide layer. The former may produce cracks through the thickness of this layer but oxide spallation under such tensile conditions appears to be a difficult process. By contrast, spallation is relatively easy under compression, i.e. in most cases during cooling from the oxidation temperature. The two principal routes, buckling and wedging, leading to spallation under these conditions are described. It is shown that the wedging process is likely to be the one which occurs in protective oxide layers of industrial significance. The results of finite element analyses of this decohesion route are presented. The rate of propagation of the interfacial wedge crack is shown to be strongly influenced by creep relaxation within the metal substrate. Alloys which have low creep stre...

A method is proposed that usesthe fundamental physical differences between direct and indirect energy deposition processes to obtain accurate values of the sensitive volume (SV) thickness and critical charge of electronic devices. The intention is to calibrate a unified model capable of describing both heavy ion and proton test data through Monte Carlo (MC) simulations and therefore also suitable for performing on-board predictions. The method was applied toavailable test data from a Gallium Arsenide (GaAs) power MESFET currently on board the MetOp-A Earth-observation mission, and results were contrasted with the measured on-board time to failure period.

Finite element analyses have shown that the growth rate of a tensile crack at the oxide/metal interface can be reduced by the creep relaxation of crack-tip stresses at intermediate temperatures during cooling. This result has been used in this work to examine the possibility that hold periods imposed at an intermediate temperature will reduce spallation by similar processes of creep relaxation. The experimental tests were performed on two chromic-forming 20%Cr/25%Ni austenitic steels, differing significantly in composition only in silicon content. Hold periods, extending to 500 h, were applied at 400°C (673 K) after cooling from the oxidation temperature of 900°C (1173 K). The influence of this annealing treatment on spallation is described in this paper where it is shown that, at least for one of the alloys tested, large improvements in spallation resistance can be obtained.

Coatings are used extensively on high temperature components to provide protection against oxidation and hot corrosion. An overview is provided in this chapter of the main coating types available commercially and to the processes that lead to their degradation and ultimate failure. The coatings considered range from both metallic and oxide overlays, functionally-graded SMART coatings, diffusion coatings and thermal barrier coating (TBC) systems. A common feature of these coatings is that protection is achieved through the formation of a thin slow-growing oxide layer, usually alumina, chromia or silica. Coating failure arises when this protective layer is damaged, mechanically or chemically, and loses its ability to self-repair. Examples of these degradation processes are given in two case studies in which the breakdown and failure of alumina-forming MCrAlY overlays and also of TBC systems are considered in detail. In these examples, particular attention is given to the mechanisms of mechanical spallation of the protective layer during temperature changes and to the initiation of chemical failure as a result of aluminium depletion due to oxidation and interdiffusion.

The sub-surface region of semi-insulating gallium arsenide, after annealing at a range of temperatures, has bean investigated with a positron beam. A positron diffusion model analysis of the Doppler lineshape parameter as a function of positron energy shows significant variations in the surface region due to annealing. A comparison with infrared spectroscopy measurements suggests that the observed changes are due to positron trapping at Ga vacancies associated with the formation of GaAs antisite defects. After annealing at 900 K, a layer of positively charged As vacancies induces an electric field which in turn prevents positrons from diffusing to the surface.