Isothermal Oxidation Resistance Research Articles

Laser shock processing effects on isothermal oxidation resistance of GH586 superalloy

The oxidation is one of the main failure mode of Ni-based alloy at high temperature, laser shock processing not only can improve the mechanical properties but also the oxidation resistance. So the study on laser shock processing effects on oxidation resistance of this alloy is necessary. The aim of this paper is to investigate the effects of laser shock processing on microstructure, micro-hardness and isothermal oxidation resistance of GH586 superalloy. Scanning electron microscopy, energy-dispersive spectrum, transmission electron microscope, and X-ray diffraction technique were used to analyze the microstructure changes and the surface morphologies of the oxide scales. In addition, micro-hardness of LSP-treated samples was measured. The results show that the average grains size on the surfaces of LSP specimen was found to be significantly finer compared to the untreated one (33.3μm vs. 18.5μm). Highly tangled and dense dislocation arrangements and a high amount of twins have been observed. After the oxidation, the defects density (dislocations and twins) in the specimen decreased. The oxidation kinetics approximately followed a parabolic oxidation law at 800°C and 900°C. The oxidation layer was composed of Cr2O3, NiCr2O4, TiO2, and Al2O3, which generated more quickly on the surface treated by LSP during initial oxidation. The average oxidation rate was lower after LSP due to the dense, tiny and homogeneous oxidation layer. The results show that the specimens treated by LSP have a better high temperature oxidation resistance.

Progress on the formation of ceramics and ceramic-based composites through geopolymer precursors

Geopolymers have the advantages of being low cost, environmentally friendly, fabricated at low temperature, and possessing sound heat resistant and tunable thermal properties. They have extensive potential applications in the aerospace, nuclear, and chemical industries. As geopolymers are prepared at room temperature, diverse kinds of reinforcement, such as ceramic fillers, carbon nanotubes, and short or continuous fibers, can be easily applied. Therefore, together with proper high-temperature heat treatment, the geopolymer technique provides a novel route for the preparation of high-performance ceramics and their composites. This would be coupled with the advantages of being environmentally friendly and low cost. Upon heating, the geopolymer underwent evaporation of water, dehydroxylation, particle rearrangement, viscous sintering, nucleation and crystallization, and a new kind of glass ceramic, leucite, with high-strength, high melting point, and adjustable coefficient of thermal expansion, was obtained. On increasing the cesium substitution in place of the potassium, the amount of stabilized leucite increased and with 20 at% cesium substitution, leucite was fully stabilized in the cubic phase and the thermal expansion coefficient decreased sharply. After proper heat treatment, the Cf/geopolymer composite could be directly converted into the carbon fiber-reinforced leucite ceramic matrix composites, and the mechanical properties were greatly enhanced. For the carbon-fiber-reinforced K-based geopolymer composite heat treated at 1100℃, flexural strength, work of fracture and Youngs modulus reach their highest values, increasing by 102.3%, 29.1%, and 84.7%, respectively, relative to their original state before heat treatment. The increased strength was attributed to the cracks blunding and release of residual strength based on the integrity of the carbon fiber. The impregnated composites also showed considerable isothermal oxidation resistance, and after being oxidized at 900℃ for 60 min, the composites still retained approximately 50% of the original value. Therefore, geopolymer technology together with high-temperature heat treatment provides a new method to fabricate composites with low cost and high specific strength, which might be attractive for applications at elevated temperature. We report the progress on the thermal evolution, crystallization kinetics, and evolution of the microstructure and properties of geopolymers and carbon-fiber- reinforced geopolymer composites. Future research directions are also presented.

HfC nanowire-toughened TaSi2–TaC–SiC–Si multiphase coating for C/C composites against oxidation

To prevent carbon/carbon (C/C) composites from oxidation at high temperature, a HfC nanowire-toughened TaSi2–TaC–SiC–Si multiphase coating was prepared by a two-step technique including chemical vapor deposition (CVD) and pack cementation. The microstructure and oxidation resistance of the coating were investigated. The HfC nanowires can effectively reduce the size of microcracks and avoid the formation of through-thickness microcracks in the multiphase coating. Due to the toughening mechanisms including nanowire pullout and debonding, the cyclic and isothermal oxidation resistance of the coating are both improved.

High Temperature Oxidation Studies of Al Coating Deposited by Magnetron Sputtering

Al coating was deposited on 0Cr18Ni10Ti steel by magnetron sputtering, and then diffusing annealing at 950°C for 2h. The isothermal oxidation behaviors of the Al coating and 0Cr18Ni10Ti steel were tested at 900 °C for 24 h and 800 °C for 24 h using Thermo gravimetric analyzer. The morphology was studied by SEM, the phase composition was characterized by EDS. The result of isothermal oxidation test showed that isothermal oxidation resistance of Al coated steel is better than 0Cr18Ni10Ti steel at 900 °Cand 800 °C, due to continuous and compact Al2O3scales. Al clusters of particles appears on Al coated steel during deposition. The larger particles peeling off was observed at 900°C after 18h, due to thermal and growth stresses, while no spallation appeared at 800°C for 24h.

PtAl 2 单相涂层的高温抗氧化性能及失效机制研究

Pt-modified aluminide coating has attracted great attention due to its advantage of the integrated property in resisting both high temperature oxidation and hot corrosion. By the presence of Pt, the spallation trend of the grown oxide scale and the detrimental effect of S can be restrained at a very low level. Besides, Pt could promote α-Al2O3 formation and stabilize β-NiAl phase. Thus Pt-modified aluminide (Pt-Al) coating has been widely used in some crucial applications requiring reliability and extended service life. There are mainly PtAl2, β-(Ni, Pt) Al and γ/γ ′-NiPtAl phases existing inside Pt-Al coating. In this work, a single phase PtAl2 coating was prepared on a Ni-based K38G superalloy through pulse-electroplating of Pt and pack aluminization under stepped heating mode. At 1100 °C , the isothermal oxidation behavior of the single phase PtAl2 coating was evaluated by thermogravimetric analysis (TGA). Cyclic oxidation test of the PtAl2 coating was performed within a vertical muffle furnace at the same temperature. The results indicate that the singular PtAl2 coating possesses quite good isothermal oxidation resistance. However, its resistance against cyclic oxidation is very poor. The cyclic stress induced by repeated heating and cooling has caused visible detachment of PtAl2 coating layer, and the spallation of PtAl2 in further would lead to a premature failure of the whole coating system. Partial spallation of PtAl2 layer, including unde*国家自然科学基金项目51301184,国家重点基础研究发展计划项目2012CB625100和国家高技术研究发展计划项目2012AA03A512

High Temperature Oxidation of the Haynes 282© Nickel-Based Superalloy

Nickel-based superalloys are used in applications where corrosion and oxidation resistance at high temperatures are required together with microstructural stability. Superalloys with different metallurgical characteristics are therefore currently being developed; the high temperature behaviour of these systems must be evaluated. In this investigation, the isothermal oxidation resistance of a Haynes 282© nickel-based superalloy was studied by gravimetric means in the temperature range 800–1,000 °C for relatively short exposure times up to 150 h. The results from the tests suggest that the parabolic rate law describes the oxidation kinetics of the alloy. The chemical composition of the oxides present in the scale comprised an outer TiO2 layer and an inner Cr2O3 layer, with the latter located at the metal/oxide interface. In addition, the formation of an internal oxidation zone of Al2O3 and TiO2 was also observed at all temperatures. The role of oxide formation on microstructural changes experienced by the alloy is discussed.

Influence of cobalt on the oxidation resistance of Al2O3–TiC composites at 800–1000 °C in air

The isothermal oxidation resistance of Al2O3–TiC composites with 3wt.% and 5wt.% cobalt is investigated at 800–1000°C in air for 25h. The oxidation resistance of Al2O3–TiC composites with 5wt.% cobalt is better than that of ones with 3wt.% cobalt. The general rate constant k and oxidation exponent n depend on the cobalt content and oxidation temperature. The oxidation kinetics of Al2O3–TiC composites with 3wt.% and 5wt.% cobalt both followed a rate that is slightly faster than the parabolic-rate law at 800–1000°C. The activation energy of the Al2O3–TiC composites with 5wt.% cobalt is larger than that of ones with 3wt.% cobalt at 800–1000°C.

Effect of Ni–Mo addition on cyclic and isothermal oxidation resistance and electrical behavior of ferritic stainless steel for SOFCs interconnect

Ni and Mo were added to ferritic stainless steel to improve the properties of the oxidation, electrical conductivity and Cr evaporation rate. The purpose of making this alloy was for application in solid oxide fuel cells (SOFCs) interconnect. Therefore, the cyclic and isothermal oxidation tests were performed and the area-specific resistance (ASR) and the Cr evaporation rate were investigated. The microstructure was discussed by scanning electron microscope (SEM). Glow discharge spectrometry (GDS) and X-ray were used for elemental and phase analysis, respectively. The addition of 3 wt% Mo reduced the oxidation rate and the Cr evaporation resistance by the formation of Laves-phase in oxide/metal interface. By reducing the activity of Fe in the steel, Mo was prevented outward diffusion of cations and anion diffusivity. Furthermore, Mo improved the electrical conductivity by filling the gaps and voids in the oxide/metal interface. The addition of 2 wt% Ni reduced the distribution of Laves-phase particles in substrate and the particles were moved to side oxide/metal interface and neighboring areas to it. Ni also improved the oxidation resistance and electrical conductivity in the steel.

Effect of different fillers on oxidation behavior of low-temperature chromizing coating

Three different chromizing coatings were produced on Ni substrate using a conventional pack-cementation method with Al2O3, Al2O3+CeO2 and CeO2 acting as filler, respectively, at a greatly decreased temperature (700 °C). Effects of different fillers on the isothermal and cyclic oxidation resistance of chromizing coating in air at 850 °C were comparably investigated. Microstructure results show that the addition of CeO2 into the filler significantly retards the grain growth of the chromizing coating. Oxidation results indicate that the chromizing coating using CeO2 as filler exhibits somewhat increased oxidation resistance than the normal chromizing coating, while the chromizing coating using Al2O3+CeO2 as filler exhibits much better oxidation resistance. The effects of different fillers on the oxidation behaviors were discussed in detail.

Effects of Grit Blasting and Annealing on the High-Temperature Oxidation Behavior of Austenitic and Ferritic Fe-Cr Alloys

Grit blasting (corundum) of an austenitic AISI 304 stainless steel (18Cr-8Ni) and of a low-alloy SA213 T22 ferritic steel (2.25Cr-1Mo) followed by annealing in argon resulted in enhanced outward diffusion of Cr, Mn, and Fe. Whereas 3 bar of blasting pressure allowed to grow more Cr2O3 and MnxCr3−xO4 spinel-rich scales, higher pressures gave rise to Fe2O3-enriched layers and were therefore disregarded. The effect of annealing pre-oxidation treatment on the isothermal oxidation resistance was subsequently evaluated for 48 h for both steels and the results were compared with their polished counterparts. The change of oxidation kinetics of the pre-oxidized 18Cr-8Ni samples at 850 °C was ascribed to the growth of a duplex Cr2O3/MnxCr3−xO4 scale that remained adherent to the substrate. Such a positive effect was less marked when considering the oxidation kinetics of the 2.25Cr-1Mo steel but a more compact and thinner FexCr3−xO4 subscale grew at 650 °C compared to that of the polished samples. It appeared that the beneficial effect is very sensitive to the experimental blasting conditions. The input of Raman micro-spectroscopy was shown to be of ground importance in the precise identification of multiple oxide phases grown under the different conditions investigated in this study.

Microstructure and oxidation resistance of SiC–MoSi2 multi-phase coating for SiC coated C/C composites

In order to improve the anti-oxidation of C/C composites, a SiC–MoSi2 multi-phase coating for SiC coated carbon/carbon composites (C/C) was prepared by low pressure chemical vapor deposition (LPCVD) using methyltrichlorosilane (MTS) as precursor, combined with slurry painting from MoSi2 powder. The phase composition and morphology were analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD) methods, and the deposition mechanism was discussed. The isothermal oxidation and thermal shock resistance were investigated in a furnace containing air environment at 1500°C. The results show that the as-prepared SiC–MoSi2 coating consists of MoSi2 particles as a dispersing phase and CVD–SiC as a continuous phase. The weight loss of the coated samples is 1.51% after oxidation at 1500°C for 90h, and 4.79% after 30 thermal cycles between 1500°C and room temperature. The penetrable cracks and cavities in the coating served as the diffusion channel of oxygen, resulted in the oxidation of C/C composites, and led to the weight loss in oxidation.

Effect of Hf Content on Oxidation Resistance of Single Crystal Superalloy at 1000°C

The specimens of single crystal superalloy DD6 with 0.10% Hf and 0.47% Hf were prepared in the directionally solidified furnace. The effect of Hf content on the isothermal oxidation resistance of the second generation single crystal superalloy DD6 was studied at 1000°Cin ambient atmosphere. Morphology of oxides was examined by SEM, and their composition was analyzed by XRD and EDS. The experimental results show that the oxidation resistance of DD6 alloy with 0.47% Hf is better than that of the alloy with 0.10% Hf. The alloy with different Hf content all obeys parabolic rate law during oxidation for 100h at 1000°C. The increase of Hf content can promote the Al2O3 formation and decreases the proportion of NiO. The oxide grain size and the thickness of the oxide layer all reduce with increasing of Hf content. The oxide scale of the alloy with different Hf content is made up of an outer NiO layer with a small amount of Co3O4, inner Al2O3 and Cr2O3 layer with a small amount of TaO2.

C/SiC/Si-Mo-B/glass multilayer oxidation protective coating for carbon/carbon composites

To prevent carbon/carbon (C/C) composites from oxidation, a self-sealing multilayer oxidation resistant coating including a C/SiC gradient inner layer, a Si-Mo-B middle layer and a glass exterior layer was prepared by pack cementation and slurry method. Scanning electron microscopy and X-ray diffraction were used to analyze the microstructure and phase composition of the as-prepared coating. The isothermal and thermal shock oxidation resistance of the coating was also investigated. The results showed that the multilayer coating exhibited excellent oxidation resistance from room temperature to 1873K. It could effectively protect C/C composites for 100h at 1173K and 150h at 1873K, and endure 40 thermal cycles between 1873K and room temperature. The excellent oxidation and thermal shock resistance could be attributed to the gradient structure and the self-sealing property of the multilayer coating.

C/SiC/MoSi 2–Si multilayer coatings for carbon/carbon composites for protection against oxidation

To improve the oxidation resistance of carbon/carbon (C/C) composites, a C/SiC/MoSi 2–Si multilayer oxidation protective coating was prepared by slurry and pack cementation. The microstructure of the as-prepared coating was characterized by scanning electron microscopy, X-ray diffraction and energy dispersive spectroscopy. The isothermal oxidation and erosion resistance of the coating was investigated in electrical furnace and high temperature wind tunnel. The results showed that the multilayer coating could effectively protect C/C composites from oxidation in air for 300 h at 1773 K and 103 h at 1873 K, and the coated samples was fractured after erosion for 27 h at 1873 K h in wind tunnel. The weight loss of the coated specimens was considered to be caused by the formation of penetration cracks in the coating. The fracture of the coated C/C composites might result from the excessive local stress in the coating.

Microstructures, Thermal Shock Resistance and Isothermal Oxidation Resistance of Nanostructured Ceria Stabilized Zirconia Coatings

Microstructures, thermal shock resistance and isothermal oxidation resistance of plasma-sprayed nanostructured ceria stabilized zirconia (CSZ) coatings were investigated. The results revealed that the nanostructured CSZ coatings mainly included two kinds of typical microstructures, nanosized CSZ particles embedded in the so-called matrix and micrometer-sized columnar grain structures of CSZ similar to those of the conventional coatings. Compared with the conventional CSZ coatings, the nanostructured CSZ coatings were denser and had finer and fewer porous structures and microcracks. Further study showed that the nanostructured CSZ coatings had higher thermal shock resistance and isothermal oxidation resistance compared with conventional CSZ coatings. The improved properties of the nanostructured CSZ coatings were attributed to the better coating toughness and decreased porosities and microcracks in the coatings.

Cyclic oxidation behavior of glass–ceramic composite coatings on superalloy K38G at 1100 °C

The SiO 2–Al 2O 3–ZrO 2–CaO–ZnO glass–ceramic composite coatings (GC), nanocrystalline NiCoCrAlY coating, and their combinations (bi-layer GC/NiCoCrAlY) were prepared on K38G specimens. The thicknesses of the glass–ceramic coatings and the NiCoCrAlY coatings were about 10 μm and 20 μm, respectively. Cyclic oxidation tests were carried out at 1100 °C for 120 cycles. Microstructures of the specimens before and after oxidation tests were characterized by SEM, EDS and XRD. The glass–ceramic coatings with or without a NiCoCrAlY intermediate layer improved the isothermal and cyclic oxidation resistance of the Ni-base superalloy K38G at 1100 °C, and performed better than the NiCoCrAlY coatings. An alumina layer formed at the glass/metal interfaces of the specimens coated by the glass–ceramic coatings with or without a NiCoCrAlY intermediate layer. The NiCoCrAlY intermediate layer was beneficial to the cyclic oxidation resistance of the glass–ceramic coatings.

Influence of Re on the Properties of a NiCoCrAlY Coating Alloy

MCrAlY can serve as stand-alone overlay coatings or bond coats in thermal barrier coating systems, and its properties play a vital role in determining the performance of these coating systems. In order to further understand the behavior of MCrAlY coatings, several NiCoCrAlY model alloys with different levels of Re (0.3 wt%, 6 wt%, and 9 wt%) were investigated. Microstructural observation showed the addition of Re promoted the precipitation of Cr-rich phases, such as α-Cr and σ. The presence of α-Cr lowered the coefficient of thermal expansion (CTE) of the coating alloys, which could reduce the CTE mismatch at the scale-metal interface. The solid solution strengthening effect of Re is responsible for an increase in Rockwell hardness of the coating alloys. But the isothermal oxidation resistance at 1100°C was deteriorated due to the precipitation of brittle α-Cr phase, a phase of inferior oxidation resistance compared with β-NiAI and γ-Ni.

Oxidation of Powder Metallurgy (PM) Ti–48%Al–2%Cr–2%Nb–(0–1%)W Alloys Between 800 and 1000°C in Air

Three powder metallurgy (PM) TiAl alloys with a fully lamellar structure were oxidized isothermally and cyclically between 800 and 1000°C in air in order to find the effect of W on the oxidation behavior of Ti–48Al–2Cr–2Nb alloys. The alloys oxidized parabolically during isothermal oxidation. Tungsten improved the isothermal and cyclic oxidation resistance. The oxide scales consisted primarily of an outer TiO2 layer, an intermediate Al2O3-rich layer, and an inner TiO2-rich layer. The alloying elements of Cr, Nb, and W tended to segregate in the lower part of the scale owing to their thermodynamic nobility. In the vicinity of the scale/matrix interface, TiN and Ti2AlN coexisted.

Improvement of the isothermal oxidation resistance of CoNiCrAlY coating sprayed by High Velocity Oxygen Fuel

Two CoNiCrAlY powders with similar chemical composition and granulometry, but with different starting reactivity toward oxygen, were sprayed by High Velocity Oxygen Fuel (HVOF) onto Hastelloy X superalloy substrates obtaining coatings of comparable thickness. After spraying, samples were maintained at 1000 °C in air for different test periods up to 3000 h. Morphological, microstructural, compositional and electrochemical analyses were performed onto the powders and the coated samples in order to assess the high temperature oxidation resistance provided by the different powders. The powder with higher starting reactivity toward oxygen sensibly improves the oxidation resistance of the coated samples by producing thinner and more adherent thermally grown oxide layers in comparison with the other powders.

Oxidation behavior of porous NiAl prepared through reactive synthesis

Porous NiAl alloys were prepared by die-pressing Ni + Al powder mixtures and vacuum sintering. The porous NiAl has a porosity of 50% with permeability of 1.1 × 10 3 m 3 m −2 kPa −1 h −1. Cyclic oxidation of porous NiAl alloys was tested in air at 400, 600, 800 and 1000 °C, respectively. The oxidation kinetic curves were determined by weight gains of samples under different cyclic conditions. Through characterizing the formation of the oxide layers formed on the external and internal surfaces, it was found that the porous NiAl alloys exhibit excellent isothermal and cyclical oxidation resistance. Only very small peak for oxide phase can be detected by XRD even when the sample was oxidized at 800 °C for 160 h. The pore structure remains stable due to the formation of very thin and incontinuous oxide films at 800 °C. The oxide scales consist primarily of α-Al 2O 3 at 1000 °C. The porous NiAl has provided a better oxidation resistance in the given environment compared to porous 316L stainless steel (316LSS), showing a promising alternative to the 316LSS in severe environment.