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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.

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...

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.

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.

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.

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.

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.

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.