Analysis on failures of protective-oxide layers and cyclic oxidation

A condition of oxide-scale spalling is derived from the concept of strain energy proposed by Evans. By considering the probabilistic distribution of fracture strain energy across the metal oxide interface, the fraction of oxide scale spalled is described as a function of the weight of oxide before cooling and temperature change. The cyclic-oxidation behavior is modeled with this result. The prediction by the model is shown to be in good agreement with data on the oxidation behavior under thermal cycling.     

Resistance of Ni-Cr-Al alloys to cyclic oxidation at 1100 and 1200°C

A series of Ni-rich alloys in the Ni-Cr-Al system were cyclically oxidized in still air for 500 1 -hr heating cycles at 1100°C and 200 1 -hr heating cycles at 1200° C. The specific sample weight-change data for each sample were then used to determine both a scaling constant k₁ and a spalling constant k₂ for each alloy, using the regression equation w/A=k ₁ ^1/2 t^1/2 – k₂t±.These in turn were combined to form an oxidation attack parameter K_a,where K_a= (k ₁ ^1/2 + 10 k₂).Log K_a was then fitted to a fourth-order regression equation as a function of the Cr and Al content at the two test temperatures. The derived estimating equations for log K_a were presented graphically as iso-attack contour lines on ternary phase diagrams at each temperature. At 1100°C compositions estimated to have the best cyclic oxidation resistance were Ni-45 at. % Al and Ni-30 at. % Cr-20 at. % Al, while at 1200°C compositions estimated to have the best cyclic oxidation resistance were Ni-45 at. % Al and Ni-35 at. % Cr-15 at. % Al. In general, good cyclic oxidation resistance is associated with Al₂O₃ and/or NiAl₂O₄ formation. The analysis also indicated that alloys prepared by zirconia crucible melting, compared to other types of melting, had tramp Zr pickup, which significantly improved the cyclic oxidation resistance. The nature of the improvement in oxidation due to tramp Zr pickup, however, is not yet understood.     

Comparison of isothermal and cyclic oxidation behavior of twenty-five commercial sheet alloys at 1150°C

Twenty-five commercial nickel-, iron-, and cobalt-base sheet alloys incorporating chromium or chromium and aluminum additions for oxidation resistance were tested at 1150°C in air for 100 hr in both isothermal and 1-hr cyclic furnace exposures. The alloys were evaluated by sample specific weight change, by type of scale formed, by amount and type of spall, and by sample thickness change and microstructure. In isothermal steady-state oxidation, four types of controlling oxides were observed depending on alloy composition: NiO, Cr₂O₃-chromite spinel, ThO₂-blocked Cr₂O₃, and Al₂O₃-aluminate spinel. The latter three types are considered protective. In the Cr₂O₃-forming alloys, however, scale vaporization is a critical factor in determining the parabolic scaling rate based on paralinear oxidation. In cyclic oxidation the alloys which form Cr₂O₃-chromite spinel scales were degraded severely when sufficient chromite spinel developed to trigger spalling. The cyclic behavior of the other three types of alloys does not differ greatly from their isothermal behavior. If chromite spinel formation is minimal, the thinner the oxide formed, the less the tendency to spall. Factors contributing to a thin scale are low isothermal scaling rates; reactive element additions, such as thorium, lanthanum, and silicon; and scale vaporization. Scale vaporization may, however, lead to catastrophic oxidation at high gas velocities or low pressures or both. A tentative mass-balance approach to scale buildup, scale vaporization, and scale spalling was used to calculate the critical oxidation parameter—the effective metal thickness change. In general, this calculated thickness change agrees with the measured change to within a factor of 3 if a correction is made for grain boundary oxidation. The calculated thickness change parameter was used to rate the oxidation resistance of the various alloys under isothermal or cyclic conditions. The best alloys in cyclic furnace oxidation tests were either Al₂O₃-aluminate spinel formers or Cr₂O₃ formers with ThO₂ blockage.     

High-temperature oxidation and spalling behavior of incoloy 825

Incoloy-825-sheet specimens were exposed to four different atmospheres; steam, synthetic air, CO₂, and CO₂ diluted with argon. The duration of exposure was varied from 2 min to 100 hr in the temperature range of 600–1300°C. A comparison of the results in these four atmospheres showed the maximum weight gain in specimens exposed under steam, while the minumum value was obtained in specimens subjected to diluted CO₂. The alloy obeyed similar reaction kinetics while exposed to all the atmospheres under consideration. The overall domination of parabolic rates was observed at 800–1000°C. For still higher temperatures, a transition from the parabolic to the cubic rate law was observed. Thermal cycling did not show any appreciable effects on the reaction kinetics of the alloy when specimens were cycled between test temperatures and 500°C, and finally cooled to ambient temperature.     

Cyclic oxidation method for studying scale-resistant metallic materials

A cyclic method has been developed to measure the spalling resistance of metals. The method involves the determination of sample weight as a function of the number of heating cycles. The shape of the curve provides a quick evaluation and, in some cases, the scale resistance can be ascertained after just a few cycles. The technique is simpler than present methods and eliminates the need to remove the scale to determine metal recession. Minimum thickness requirements must be met, however, to avoid complications from the depletion of alloying elements from the surface. The technique has been applied to various steels and to Fe-Cr-Al alloys.     

Accelerated cyclic oxidation testing protocols for thermal barrier coatings and alumina‐forming alloys and coatings

The oxide spallation resistance of oxide scales and ceramic thermal barrier coatings is a key design factor for developing high‐temperature alloy systems. Determination of the lifetimes of such alloy and coating systems is highly desirable. However, as improved systems are developed, lifetimes become so long that the time required to test a system to failure becomes prohibitive. Therefore, reliable protocols for accelerated testing and lifetime prediction are needed. This paper describes two attempts at developing such protocols. The first involves modification of the NASA COSP model to predict cyclic oxidation behavior of alloys and metallic coatings and the incorporation of acoustic emission data into this model. The second involves use of an indentation technique to induce spalling of thermal barrier coatings (TBCs) after short‐term thermal exposures. The first effort involves using the COSP Model, developed at NASA, as the basis for the prediction of oxide spallation. Acoustic emission measurements are used in an attempt to obtain critical parameters in the model from short‐time experiments for a variety of alloys and coatings which rely on alumina scales for oxidation resistance. The model is then used to predict the lives of these alloys and coatings when subjected to cyclic oxidation at 1100°C. A principal concern with ceramic thermal barrier coatings (TBCs) used in gas turbines is their loss of adhesion during service, leading to coating spallation. In this paper, an overview is given of an indentation test for brittle coatings on ductile substrates which is used to quantify decreases in interfacial toughness of TBC systems due to cyclic high‐temperature exposures. The indentation test involves penetration of the TBC and the oxide layer below it, inducing plastic straining in the underlying metal bond coat and superalloy substrate. The indentation strains cause an axisymmetric delamination of the TBC and oxide layers. Measurement of the extent of the delamination, coupled with finite‐element modeling, provides a measure of the adherence of the coating. Test results are presented tracking the loss of interfacial toughness for EBPVD TBC systems cyclically exposed at 1100°C.     

Variation in cyclic oxidation testing practice and data: The European situation before COTEST

A survey of existing testing practice and pre‐existing data was conducted to provide a starting point for the COTEST project on cyclic oxidation testing. The main parameters within the test that need to be controlled were identified as: control of test environment; temperature stability during hold periods; heating and cooling rates; specimen preparation; and post test evaluation. Existing experimental facilities were surveyed to establish the full range of variability within these parameters.     

COSP: A computer model of cyclic oxidation

A computer model useful in predicting the cyclic oxidation behavior of alloys is presented. The model considers the oxygen uptake due to scale formation during the heating cycle and the loss of oxide due to spalling during the cooling cycle. The balance between scale formation and scale loss is modeled and used to predict weight change and metal loss kinetics. A simple uniform spalling model is compared to a more complex random spall site model. In nearly all cases, the simpler uniform spall model gave predictions as accurate as the more complex model. The model has been applied to several nickel-base alloys which, depending upon composition, form Al₂O₃ or Cr₂O₃ during oxidation. The model has been validated by several experimental approaches. Versions of the model that run on a personal computer are available.     

Influence of cycling parameter variation on thermal cyclic oxidation testing of high temperature materials (COTEST)

The effect of cycling parameter variation (i.e. oxidation temperature, upper and lower dwell time, humidity in test gas) of the oxidation/spallation kinetics on four alloys was investigated within the framework of an EC funded research project (COTEST). For this purpose, specimens of AISI 441, Alloy 800H, CM 247 and P91 were subjected to thermocyclic testing in dry or humidified air. It was found that a minimum of 300 h accumulated hot dwell time is required for meaningful test results. Detailed characterisation of the corrosion products was performed using OM, SEM/EDX and XRD. The net weight change curves were evaluated with regards to the characteristic quantitative parameters describing oxide growth rate, time to onset of spallation/breakaway and weight of spalled oxide. Analysis of these values was made by the ANOVA method, which allows assessment of the significance of the roles of different test parameters and parameter variations on the oxidation/spallation kinetics from a limited number of experiments.     

Effect of water vapour on cyclic oxidation of Fe–Cr alloys

Model Fe–Cr alloys containing 9, 17 or 25 wt% Cr were subjected to repeated 1 h cycles of exposure at 700 °C to flowing gas mixtures of Ar‐20O₂, Ar‐20O₂‐5H₂O and Ar‐5O₂‐20H₂O (all in volume %) for up to 400 cycles. The kinetics and morphological development of these reactions were compared with those found during isothermal exposure to the same gases. Under isothermal conditions, all alloys developed thin protective chromium‐rich scales in dry oxygen. Addition of 5% H₂O induced breakaway for Fe‐9Cr within 48 h, but had little effect on higher chromium alloys. Isothermal chromia scale growth on Fe‐17Cr and Fe‐25Cr was accelerated by the addition of 20% H₂O, but breakaway did not result. Under cyclic conditions in dry oxygen, Fe‐9Cr quickly entered breakaway, oxidising according to fast, linear kinetics, but the higher chromium alloys exhibited protective behaviour. When 5% H₂O was added to the oxygen, the 17% Cr alloy also underwent fast breakaway oxidation, but Fe‐25Cr continued to be protected by a chromia scale. In the 20% H₂O gas, all alloys failed under cyclic conditions, producing thick, iron‐rich oxide scales. The synergistic effects of water vapour and temperature cycling are discussed in terms of alloy chromium depletion and the effects of H₂O(g) on oxide transport properties.     

NiCoCrAlYHf/EB-PVD热障涂层的热循环氧化行为

采用电子束物理气相沉积方法(EB-PVD)在NiCoCrAlYHf粘结层上沉积YSZ热障层,研究了该热障涂层1 100℃的循环氧化行为(每个循环为:1 100℃保温30 min、空冷5 min),分析了粘结层和热生长氧化物的演化过程.结果表明:NiCoCrAlYHf粘结层氧化初期由β-NiAl和γ固溶体组成,186次循环氧化后β相完全转变为γ固溶体.NiCoCrAlYHf/EB-PVD热障涂层中的热生长氧化物包含Al2O3层和靠近热障层的尖晶石薄层.该热生长氧化物生长速度较快,在粘结层的一些富Hf区域优先生长而呈现出明显的不均匀性;但其具有较大的失效临界热生长氧化物厚度,失效时热生长氧化物均匀处的厚度约为10 μm.     

The influence of thermal cycling on the oxidation and oxide spallation of a 1%Cr–0.5%Mo low-carbon steel

The oxidation and oxide spallation of 1%Cr–0.5%Mo low carbon steel disks in dry oxygen was studied isothermally at 800°C (1073 K) and in thermal cycling between 800 and 600°C (1073 and 873K) followed by cooling at rates from 3 to 100°C/min. Mostly parabolic oxidation kinetics were observed. Thin scales (10 ) were more prone to spalling than thicker scales (20 ). The thickness and growth imperfections of an inner scale layer enriched in chromium, molybdenum, and silicon strongly influenced the probability of cohesive failure exceeding that of adhesive failure of the scale. Cohesive failures in the bulk scale during thermal cycling were probably nucleated at voids and microcracks produced in the initial isothermal period of scale growth. The number of segmented scale layers that became detached during cycling was governed by the number of parallel rows of voids in the scale and not necessarily by the number of cycles.     

Static and dynamic cyclic oxidation of 12 nickel-, cobalt-, and iron-base high-temperature alloys

Twelve typical high-temperature nickel-, cobalt-, and iron-base alloys were tested by 1 hr cyclic exposures at 1038, 1093, and 1149°C and 0.05 hr exposures at 1093°C. The alloys were tested in both a dynamic burner rig at Mach 0.3 gas flow and in static air furnace for times up to 100 hr. The alloys were evaluated in terms of specific weight loss as a function of time, and X-ray diffraction analysis and metallographic examination of the posttest specimens. A method previously developed was used to estimate specific metal weight loss from the specific weight change of the sample. The alloys were then ranked on this basis. In general the burner-rig test was more severe than a comparable furnace test and resulted in an increased tendency for oxide spalling due to volatility of Cr in the protective scale and the more dratic cooling due to the air-blast quench of the samples. Increased cycle frequency also increased the tendency to spall for a given test exposure. The behavior of the alloys in both types of tests was related to their composition, particularly their Cr and Al contents and their tendency to form four types of scales: NiO or CoO, Cr₂O₃-chromite spinel, -Al₂O₃-aluminate spinel, or ThO₂-blocked Cr₂O₃. The alloys with the best overall behavior formed -Al₂O₃-aluminate spinels.     

Multilayered thermal barrier coating for land-based gas turbines

Multilayered thermal barrier coatings (TBC) with different functions were proposed for the hot section components of land-based gas turbines. This article describes a multilayered TBC with an oxidation resistant layer. A conventional duplex TBC and a triplex TBC, in which an aluminized layer was added to the conventional duplex TBC to increase oxidation resistance, were prepared. It was confirmed by a burner rig test that the triplex TBC with the aluminized layer is resistant to oxidation and shows high durability in a thermal cycle test, compared with the conventional duplex TBC. The spalling in the thermal cycle test of each TBC specimen occurred at the same position, when the thickness of the oxidation layer was 11 to 13 μm. The mechanism of spalling of the coating in the thermal cycle test was discussed in terms of stress in the coating. Stress in the direction of spalling occurred by an uneven interface between the bond and top coat and increased with growth of the oxidation layer. It is thought that the high durability of the triplex TBC in the thermal cycle test is derived from suppressing the growth of the oxidation layer and decreasing the stress due to the addition of the aluminized layer.     

Modification of the oxidation behavior of high-purity austenitic Fe-14Cr-14Ni by the addition of silicon

The oxidation behavior of Fe-14Cr-14Ni (wt.%) and of the same alloy with additions of 1 and 4% silicon was studied in air over the range of 900-1100° C. The presence of silicon completely changed the nature of the oxide scale formed during oxidation. The base alloy (no silicon) formed a thick outer scale of all three iron oxides and an internally oxidized zone of (Fe,Cr,Ni) spinels. The alloy containing 4% silicon formed an outer layer of Cr₂O₃ and an inner layer of either (or possibly both) SiO₂ and Fe₂SiO₄. The formation of the iron oxides was completely suppressed. The oxidation rate of the 4% silicon alloy was about 200 times less than that of the base alloy, whereas the 1% silicon alloy exhibited a rate intermediate to the other two alloys. The actual ratio of the oxidation rates may be less than 200 due to possible weight losses by the oxidation of Cr₂O₃ to the gaseous phase CrO₃. The lower oxidation rate of the 4% silicon alloy was attributed to the suppression of iron-oxide formation and the presence of Cr₂O₃, which is a much more protective scale.     

Furnace cyclic oxidation behavior of multicomponent low conductivity thermal barrier coatings

Ceramic thermal barrier coatings (TBCs) will play an increasingly important role in advanced gas turbine engines due to their ability to further increase engine operating temperatures and reduce cooling, thus helping achieve future engine low emission, high efficiency, and improved reliability goals. Advanced multicomponent zirconia (ZrO₂)-based TBCs are being developed using an oxide defect clustering design approach to achieve the required coating low thermal conductivity and high-temperature stability. Although the new composition coatings were not yet optimized for cyclic durability, an initial durability screening of the candidate coating materials was conducted using conventional furnace cyclic oxidation tests. In this paper, furnace cyclic oxidation behavior of plasma-sprayed ZrO₂-based defect cluster TBCs was investigated at 1163°C using 45 min hot-time cycles. The ceramic coating failure mechanisms were studied using scanning electron microscopy (SEM) combined with x-ray diffraction (XRD) phase analysis after the furnace tests. The coating cyclic lifetime is also discussed in relation to coating processing, phase structures, dopant concentration, and other thermo-physical properties.     

The cyclic oxidation resistance of cobalt-chromium-aluminum alloys at 1100 and 1200° C and a comparison with the nickel-chromium-aluminum alloy system

A series of Co-rich alloys in the Co-Cr-Al system were cyclically oxidized in still air for 500 1 -hr heating cycles at 1100° C and 200 1 -hr heating cycles at 1200°C, The specific weight-change data for each sample were then used to determine both an oxide growth constant, k₁, and a spoiling constant, k₂, for each alloy, using the regression equation W/A=k ₁ ^1/2 t^1/2— k₂t±. These in turn were combined to form an oxidation attack parameter, K_a, where K_a=(k ₁ ^1/2 +10k₂). These attack parameters, along with X-ray diffraction results, were then compared with K_a values determined for comparable Al and Cr compositions in the Ni-rich Ni-Cr-Al system. The K_a and X-ray diffraction results indicated that initially, if the Cr and Al contents in both systems are high enough, protective -Al₂O₃ and aluminate spinel(s) are formed. However, in the long run, when the scales eventually start to fail, mainly CoO is formed in the Co-Cr-Al system and mainly NiO in the Ni-Cr-Al system. The Ni-Cr-Al system is considered more oxidation resistant since CoO leads to massive spalling and sudden drastic weight loss, while NiO fails in a more gradual, predictive manner. Both sets of alloys were melted in zirconia crucibles and the resultant Zr pickup increased the cyclic oxidation resistance of the alloys in both systems.     

CoCrAlY表面改性后热障涂层高温氧化及热震性能

采用大气等离子喷涂技术（APS）在镍基高温合金表面制备了CoCrAlY粘结层,利用电子束蒸发镀膜在CoCrAlY表面蒸镀纳米铝膜,并使用强流脉冲电子束熔敷纳米铝膜进行表面改性,最后使用APS在表面改性后的CoCrAlY表面沉积陶瓷层制备了热障涂层.在空气环境中对热障涂层进行高温氧化试验和热震试验.结果表明,CoCrAlY表面改性后热障涂层经1 050℃静态空气氧化后,界面处生成的热生长氧化物（TGO）具有较高的连续性和致密性,有效阻碍了氧化的进一步发展且避免尖角型氧化物的形成,提高了热障涂层的抗氧化能力;在1 050℃高温加热后10℃水淬热震条件下,脱落率仅为2%左右.     

Isothermal and cyclic oxidation resistance of boron-modified and germanium-doped silicide coatings for titanium alloys

Since titanium alloys with an adequate balance of mechanical properties and high-temperature oxidation resistance have not been developed, protective coatings are required. In our previous paper, B-modified and Ge-doped silicide diffusion coatings grown on CP Ti, Ti–24Al–11Nb, Ti–22Al–27Nb, and Ti–20Al–22Nb by the halide-activated, pack-cementation method were described. In this study, isothermal and cyclic oxidation were used to evaluate the oxidation performance of these coatings in comparison to uncoated substrates. The rate-controlling mechanism for isothermal oxidation at high temperature was solid-state diffusion through a SiO₂ scale, while the mechanism for low-temperature oxidation involved grain-boundary diffusion through TiO₂. Both isothermal and cyclic oxidation rates for the B-modified and Ge-doped silicide coatings were much slower than for pure TiSi₂. Oxygen contamination was not detected by microhardness measurements in the coated substrates after 200 oxidation cycles at 500–1000°C for the Ti–Al–Nb alloys, or at 500–875°C for CP Ti. The excellent oxidation resistance for the optimum coating compositions is discussed.     

Rapid cyclic oxidation tests,using joule heating of wire and foil materials

As part of a programme to establish a thermal cyclic oxidation test standard for metallic materials at elevated temperatures (COTEST) a programme of work has been undertaken on the rapid cyclic oxidation of wire and foil materials, using Joule heating. By way of introduction, alternative technologies for rapid thermal cyclic tests are reviewed. Following this the benefits of adopting a modified existing ASTM standard is discussed. The aims of the project were, first, to define a suitable test matrix to evaluate the effect of critical material parameters, to undertake the test to a prescribe standard and then evaluate the performance of materials under rapid thermal cycling, using Joule heating. The performance of the testing methodology is assessed using two materials, Kanthal A1 (reference temperature 1250°C) and Alloy 800 (reference temperature 1000°C). In this paper tests on Kanthal A1 at 1200, 1250 and 1300°C are reported. These tests have been undertaken in laboratory air. Other parameters include the upper dwell time (2, 5 and 10 min), the lower dwell time (constant at 2 min) and specimen geometry. Kanthal A1 material was available as 0.4 mm diameter wire, 0.7 mm diameter wire and 70 μm × 1.25 mm ribbon. The results of these tests were analysed statistically using a 3 × 3 × 3 test matrix with triplicate repeat specimens. The lifetime of the wire or foil samples could be measured using either of two parameters: 1) the number of cycles to failure or the accumulated hot time to failure (accumulated upper dwell time). The cyclic lifetime was critically dependent on temperature, hot dwell time and sample geometry. For these rapid cycle tests on Kanthal A1 wire and foil cycle life decreased with increase in temperature and increase in hot dwell time. The wire/foil endurance (accumulated hot time to failure) decreased slightly with increase in temperature, but increased with hot dwell time (fewer cycles). The endurance of foil samples was shorter than wire samples. Thus for wire/foil endurance only hot dwell time was a statistically significant parameter, over the temperature range studied.