Cyclic Oxidation Experiments Research Articles

Selection of high temperature materials for concentrated solar power systems: Property maps and experiments

Concentrated solar power systems are receiving increasing attention as electricity generating systems, whereby the sun’s heat is focused onto a receiver heated to high temperatures. Systems operating today are generally limited to temperatures below about 600°C, but there is considerable interest in increasing operating temperatures, with heat receivers generally constructed using refractory metallic alloys or ceramics. The present study compares the behaviour of a range of metallic or ceramic materials with advanced intermetallic alloys which have been developed for high-temperature aeronautic or power-generating systems. A few experiments are reported using intense solar heating to demonstrate the damage that may be sustained, depending on the material that is considered. Accelerated cyclic oxidation experiments further emphasize the sensitivity of the various materials to oxidation and thermal damage accumulation. The important characteristics required of the material to be used for the receiver are described and used to generate property maps where the suitability of different classes of materials (metal, intermetallic, ceramic) may be compared. Depending on the precise conditions of where the receiver will operate, and whether creep, fracture or oxidation/spalling is the most important damaging process, either refractory Ni-base alloys, Mo-silicides, or ceramics may be the best material for the application.

The cyclic oxidation behaviour of Ni-based superalloy GTD-111 with sulphur impurities at 1100 °C

The effect of sulphur on the cyclic oxidation of Ni-based superalloy GTD-111 was investigated. A cyclic oxidation experiment was performed for up to 200 cycles of 1h exposure at 1100°C. Even at the initial stage of 15 cycles, porosity increased with increasing sulphur content. The formation of pores along Cr2O3 layer grain boundaries was particularly pronounced. Increased porosity leads to an increase in spallation, which results in accelerated depletion of Cr and Al. As a result, continuity of an Al2O3 layer and cyclic oxidation resistance reduced with increasing sulphur content.

Oxidation resistance of low-velocity oxy fuel-sprayed Al2O3-13TiO2 coating on nickel-based superalloys at 800 °C

Oxidation of metals and alloys has been recognized as a serious problem for high-temperature applications such as boilers, internal combustion engines, gas turbines, fluidised bed combustion and industrial waste incinerators. Superalloys are used in these applications because of their excellent strength and creep resistance at high temperature, but these alloys lack resistance to oxidation. In the present investigation, Al2O3-13%TiO2 coating is deposited on nickel-based Superni 718 and AE 435 superalloys using a low-velocity oxy fuel (LVOF) process. The coating has been characterised for scanning electron microscopy (SEM), X-ray diffraction (XRD) and surface roughness. Cyclic oxidation experiments were conducted for 50cycles, with each cycle consisting of keeping the samples at 800°C for 60min followed by 20min of cooling in air. The weight change measurements were taken to establish the oxidation kinetics of the coated and uncoated superalloys. The LVOF sprayed Al2O3-13%TiO2 coating has shown good oxidation resistance as well as adherence to the substrates under the tested environment. The AE 435 superalloy has shown lower oxidation rates in comparison to Superni 718 superalloy.

Characterization of Oxidation Protection Coatings for High Temperature Applications by Means of Nanoindentation and Scanning Electron Microscopy Methods

AbstractOxidation protection coatings are required for thermally highly stressed components such as turbine blades in aircraft engines. Cyclic oxidation experiments were performed on a NiCoCrAlY protective coating of a nickel-based superalloy and hardness and modulus of elasticity (mechanical properties) were determined by nanoindentation before and after the experiments. Microstructure and chemical composition were characterized by means of scanning electron microscopy. Here, the focus is on the phase identification by combining electron backscatter diffraction and energy dispersive X-ray spectroscopy. Findings indicate that the chemical composition strongly influences the mechanical properties.

A combinatorial investigation of palladium and platinum additions to β-NiAl overlay coatings

A combinatorial approach was used to investigate the effects of higher-order additions to NiAl on the oxidation, phase stability and interdiffusion behavior of the coatings. Overlay coatings with compositions (in at.%) in the range Ni–38Al–5Cr–(0.1–0.5)Hf–(2–8)Pd and Ni–36Al–5Cr–(0.1–0.5)Hf–(2–8)Pt were deposited on René N5 single crystal substrates via ion plasma deposition (IPD). Cyclic oxidation experiments on ten different Pd+Hf- or Pt+Hf-modified NiAlCr coatings at 1100°C revealed similar weight gain and oxide scale thickness. While the thickness of alumina scales formed on Pd- and Pt-modified coatings were comparable, a higher density of oxide spalls were observed in the former. High-temperature transformations from β→γ′→γ+γ′ occurred during extended exposure in both Pd- and Pt-modified coatings owing to Al depletion to substrate and surface oxide. On cooling, Al-depleted β-NiAl transformed to two martensite phases, 3R and fine-twinned 7R. Rumpling or surface roughening occurred for some coating compositions. Microprobe measurements showed that the difference in penetration depths of Pd into the substrate was not as large as expected from the higher interdiffusion coefficient of Pd in β-NiAl and γ-Ni.

The effects of the minor alloying elements Al, Si and Mn on the cyclic oxidation of Ni–Cr–W–Mo alloys

The effect of the addition of less than 1wt% of Al, Si and Mn on the cyclic oxidation of the Ni–22Cr–14W–2Mo alloy is investigated. The cyclic oxidation experiment is performed between 1150°C and 400°C. Al and Si addition reduced the oxidation rate by suppressing the formation of transient oxides. In particular, Al enhanced the adhesion of the oxide scale by forming oxide pegs and a (Cr,Al)2O3 layer. Mn is detrimental to the oxidation resistance by enhancing the formation of NiO, NiCr2O4, and NiWO4. However, the detrimental effect of Mn is suppressed by the addition of Al and Si.

Sewage sludge ash as an alternative low-cost oxygen carrier for chemical looping combustion

In this paper, novel low-cost oxygen carriers containing Fe2O3 are evaluated for use in chemical looping combustion. Sewage sludge ashes and reference samples were prepared and used in cyclic reduction and oxidation experiments in a thermogravimetric analyzer (TG). A gaseous (3 % H2) fuel and a solid fuel (hard coal) were tested. Three-cycle CLC tests were carried out in the 600–800 °C temperature range and long-term testing was performed at 950 °C. A reactivity study showed that the natural sewage sludge ash sample was stable during the cycling TG tests when hydrogen was used as a fuel at all of the temperatures investigated. Strong temperature effects on the oxygen transport capacity were observed. An one-cycle test at 900 °C showed also that the sewage sludge ash successfully reacted with coal. The oxygen released was fully used for coal combustion, with appreciable reaction rate at temperature of ~750–800 °C, that is significantly lower than that obtained for pure Fe2O3-based oxygen carrier. The oxidation reaction was much faster than the reduction reaction. Moreover, the sewage sludge ash showed a low tendency toward agglomeration in the cyclic test, which was superior to the behavior of synthetic materials. The sewage sludge ash exhibited also high mechanical strength, an attrition index of 1 % and a high-temperature resistance of 1,170 °C in a reducing atmosphere. We conclude that sewage sludge ash can be effectively used as a low-cost, valuable oxygen carrier in practical application in chemical looping combustion technology for power generation.

Modelling high temperature oxidation behaviour of Ni–Cr–W–Mo alloys with Bayesian neural network

Abstract High temperature oxidation property of Ni–Cr–W–Mo alloys is modelled as a function of alloy composition. A database has been constructed from cyclic oxidation experiments performed between 1150 °C and 400 °C of 27 alloys having various contents of Cr, W, and Mo. The Bayesian neural network technique was used for the modelling of cyclic oxidation experiments. With a 3-17-1 neural network architecture, the model shows a precise prediction of oxidation property of Ni–Cr–W–Mo alloys ( R = 0.999). Automatic relevance determination (ARD) analysis reveals that the influence of alloying elements on the output is in the order of Cr, Mo, and W.

Predicting the depletion of chromium in two high temperature Ni alloys

The oxidation resistance of the high temperature Ni-Cr alloys alloy 601 and alloy 602 CA was studied. Based on microanalysis data from cyclic oxidation experiments performed in a burner test rig, the total loss of chromium in the alloy was determined. A numerical analysis using the general diffusion equation was performed to model the mass transport for the observed chromium loss. Chromium depletion profiles were calculated with an effective diffusion coefficient derived from the data of the shortest exposure time. Good agreement was found between the measured and the computed chromium concentration at the alloy-oxide interface for longer exposure.

Improving oxidation resistance and thermal insulation of thermal barrier coatings by intense pulsed electron beam irradiation

In this paper, intense pulsed electron beam was used for the irradiation treatment of 6–8% Y2O3-stablized ZrO2 thermal barrier coating prepared by electron beam-physical vapor deposition to achieve the “sealing” of columnar crystals, thus improving their thermal insulation properties and high temperature oxidation resistance. The electron beam parameters used were: pulse duration 200μs, electron voltage 15kV, energy density 3, 5, 8, 15, 20J/cm2, and pulsed numbers 30. 1050°C cyclic oxidation and static oxidation experiments were used for the research on oxidation resistance of the coatings. When the energy density of the electron beam was larger than 8J/cm2, ZrO2 ceramic coating surface was fully re-melted and became smooth, dense and shiny. The coating changed into a smooth polycrystalline structure, thus achieving the “sealing” effect of the columnar crystals. After irradiations with the energy density of 8–15J/cm2, the thermally grown oxide coating thickness decreased significantly in comparison with non-irradiated coatings, showing that the re-melted coating improved the oxidation resistance of the coatings. The results of thermal diffusivity test by laser flash method showed that the thermal diffusion rate of the irradiated coating was lower than that of the coating without irradiation treatment, and the thermal insulation performance of irradiated coating was improved.

Cyclic oxidation behavior of HVOF bond coatings deposited on La- and Y-doped superalloys

One suggested strategy for improving the performance of thermal barrier coating (TBC) systems used to protect hot section components in gas turbines is the addition of low levels of dopants to the Ni-base superalloy substrate. To quantify the benefit of these dopants, the oxidation behavior of three commercial superalloys with different Y and La contents was evaluated with and without a NiCoCrAlYHfSi bond coating deposited by high velocity oxygen fuel (HVOF) spraying. Cyclic oxidation experiments were conducted in dry O2 at 1050°, 1100° and 1150°C. At the highest temperature, the bare superalloy without La showed more attack due to its lower Al content but no difference in oxidation rate or scale adhesion was noted at lower temperatures. With a bond coating, the alumina scale was non-uniform in thickness and spalled at each temperature. Among the three coated superalloys, no clear difference in oxide growth rate or scale adhesion was observed. Evaluations with a YSZ top coat and a bond coating without Hf are needed to better determine the effect of superalloy dopants on high temperature oxidation performance.

Microstructure and thermal cycling behavior of nanostructured yttria partially stabilized zirconia (YSZ) thermal barrier coatings

Nanostructured yttria partially stabilized zirconia (YSZ) coatings were prepared by atmospheric plasma spraying (APS) using the conglomeration made by zirconia nanoparticle as the raw materials. The measurement methods, which consisted of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermal cycling behavior, were used to character the morphology, composition and thermal oxidation behavior of the powder and the coatings. From the results, it was shown that the YSZ coating was the laminar structure, and the elements distribution in the bond and top coat were well-proportioned. The YSZ coatings were composed of fine grains with size ranging from 30 to 110 nm. The laminar layers with columnar grains were surrounded with unmelted parts of the nanostructured powder and some equiaxed grains. In the as-sprayed nanostructured zirconia coatings, there existed pores that were less than 1 μm. The cracks were observed on some of the crystal border. The cyclic oxidation experiment showed that the nanostructured coating had longer thermal cycling lifetime to exhibit the promising thermal cyclic oxidation resistance. The failure of the nanostructured TBC was similar to the failure of conventional APS TBC.

Formation of Protective Intermediate Phase in Ta Addition TiAl during High Temperature Oxidation

The mechanical properties and the oxidation resistance of -TiAl at elevated temperatures have to be improved to be used in the severe environmental conditions. It has become clear that the addition of more than 4at.%Ta in TiAl demonstrates a superior oxidation to the other TiAl-X compounds, according to the weight gain results of cyclic oxidation experiments at 1173 and 1273K. Oxidation behaviors are strongly influenced by the Ta concentration in TiAl. XRD, SEM-EDS, and TEM-EDS observations have been carried out to determine the microstructures and the surface compositions of multi-layered oxide scales. It was revealed that a protective intermediate phase simultaneously formed between the substrate γ-TiAl and the oxide scale layer. The Ti53Al32Ta15 ternary compound exists as an equilibrium phase at 1373K, according to the published Ti-Al-Ta ternary phase diagram. This ternary compound can work as a barrier to some extent. It contributes to decelerating the diffusion of Ti and Al atoms and to decreasing the oxidation rate. The formation mechanism of the intermediate phase has been discussed in conjunction with diffusion in TiAl.

Effects of Lanthanum on Fe-25Cr Alloys under Cyclic Oxidation

Fe-Cr binary model alloys (Cr: 25 wt %) with additions of 0.09 wt% lanthanum were subjected to cyclic oxidation experiments at 700oC. All model alloys were exposed in five different gases; Ar-20O2, Ar-20O2-5H2O, Ar-5O2-20H2O, Ar-10H2-5H2O (pO2 = 3.64 x 10-22 atm) and Ar-10H2-20H2O (pO2 = 7.37 x 10-21 atm) all in volume %. Very low weight gains were observed in all gases of Fe-25Cr-0.09La. However, breakaway oxidation occurred on La-free alloy experienced increased weight gain in Ar-5O2-20H2O due to formation of iron-rich oxide. The addition of La (<0.1 wt%) to the Fe25Cr retarded the growth of iron-rich oxide in Ar-5O2-20H2O.

Oxidation Behavior of SiC/TiAl Composite Material

SiC fiber reinforced intermetallic is one of the promising candidate material for the next generation space plane, because of its excellent high temperature specific strength and elastic modulus. Oxidation behavior of the fiber-reinforced intermetallic (FRIM) is one of the most important properties for the practical use in the severe environment. Recently fabrication process of CVD-SiC fiber reinforced γ-TiAl matrix composite has been developed. Oxidation behavior of SiC/γ-TiAl and γ-TiAl was studied. Cyclic oxidation experiments were executed at 900°C under the dry airflow for 200 hours. Mass gains of the specimens were measured. The cross sections of specimen were observed by optical microscope. Mass gain of the SiC/γ-TiAl composite material was two times larger than that of γ-TiAl. Surface of the SiC/γ-TiAl composite was covered with a comparatively thick oxide scale. Furthermore, formation of the oxide at the vicinity of interface between SiC fiber and γ-TiAl matrix was observed. Oxidation mechanism of SiC/γ-TiAl composite was discussed.

Microstructure and cyclic oxidation behavior of hot dip aluminized coating on Ni-base superalloy Inconel 718

Ni-base superalloy In-718 was coated by hot-dipping into a molten bath containing Al–7wt.%Si. Cyclic oxidation experiments on bare substrate and aluminized alloy were conducted at 1100 °C, covering 240 cycles in static air. After hot dip treatment the coating layers consisted of two phases Al and FeAlSi were detected in the external topcoat to the aluminide/alloy substrate. After oxidation testing, a continuous alumina scale was detected on the surface of the aluminide layer. This coating shows better cyclic oxidation resistance for In-718 alloy than untreated substrate. Cr 2O 3 was found to be the primary oxide phase in the oxidation of bare In-718 alloy. The inward diffusion of Al in the aluminide layer was restricted by the interdiffusion zone. The NiAl phase constituent of the aluminide layer was similar through all of the testing. Only the γ phase could be found below the coating surface and in the subsurface region as aluminum was lost to form the oxide.

Evaluation of aluminide diffusion coatings for thermal cyclic oxidation protection of a nickel-base superalloy

Active element modified aluminide diffusion coatings on IN738 substrates were produced by a new route using continuously cast, aluminum alloy wires consisting of Al-Y, Al-Ce, Al-La and Al-Si-Y. The cast wires were used as evaporation sources for ion-vapour deposition followed by diffusion heat treatments to form nickel aluminide coatings. In order to examine the oxidation resistance of these coatings at elevated temperatures, thermal cyclic oxidation experiments were carried out in air at 1050°C. While all coatings were found to provide significant protection, the Al-La modified coatings provided the greatest resistance to cyclic oxidation. On the other hand, with coatings based on Al-Si-Y alloys, while silicon has a strong ability to reduce the outward diffusion of aluminum, the adverse effect of silicon on mechanical properties of the coating, together with the formation of volatile silicon monoxide, led to catastrophic localized oxidation of the protective coatings.

Cyclic-Oxidation Resistance of Protective Silicide Layers on Titanium

Titanium was powder siliconized and gas nitrided, in order to improve its cyclic-oxidation resistance. Siliconizing was performed in a pure-silicon powder at temperatures in the range of 800–1100° C for 3–48 h. Gas nitriding was carried out in pure N2 at 1100° C/12 h. Cyclic-oxidation experiments with the siliconized and nitrided samples were conducted in air at 850 and 950° C for up to 560 h. It was found that the siliconized layers grew according to the parabolic law with the activation energy for siliconizing E S being 47.2 kJ mol−1. Powder siliconizing at 900–1100° C/3 h produced multi-phase layers, in which Ti5Si3 silicide predominated The siliconizing temperature of 800° C/3 h appeared to be insufficient, because it led to a non-uniform surface layer with a slight protective effect. The nitrided layers were composed of titanium nitride TiN and α-Ti(N) intestitial solid solution. Measurement of the oxidation kinetics revealed that the titanium siliconized at 900–1100° C/3 h oxidized much more slowly than pure Ti, Ti–6Al–4V alloy and nitrided titanium. Microstructural investigation revealed the complex sub-structure of the scales on the siliconized samples which was composed of rutile+silica, rutile and nitrogen-rich sub-layers. The mechanism of high-temperature cyclic oxidation of the siliconized and nitrided titanium is discussed.

Application of a simple statistical spalling model for the analysis of high-temperature, cyclic-oxidation kinetics data

A statistical cyclic-oxidation model is presented. This model gives analytical formulas to assess stabilized metal consumption in cyclic-oxidation experiments. The model is first detailed, then applied to se eral Ni-base superalloys, which form an α-alumina scale during oxidation above 1000C. A new map is introduced in order to compare the cyclic-oxidation beha ior of these alloys.

Studies of the influence of surface pre-treatment on the integrity of alumina scales on MA 956

Quasi-isothermal and cyclic oxidation experiments at 1100 °C in air have been performed on a oxide dispersion strengthened (ODS) alloy MA 956 in order to investigate the effects of prior surface treatments on the integrity of the alumina scales formed. Three surface conditions: (i) as received with pre-oxidised and Al 2O 3 grit blasted, (ii) 800 grit SiC ground surface, and (iii) lathe turned surface—have been studied. Effects of these surface treatments on the growth kinetics of the scales, their morphologies, defect structures and spallation behaviour have been investigated for exposure times up to 5000 h (quasi-isothermal) and 3500 h (cyclic). The scale failure mechanisms involved have been identified and modelled. Good scale/substrate adhesion was observed for both flat and concave surfaces. Scale spallation occurred by a buckling mechanism. For the lath turned samples finite element (FE) modelling allowed the development of a predictive methodology for scale failure.