Oxidation behavior of silicon carbide at 1200 °C in both air and water–vapor-rich environments

Oxidation of SiC in both air and water–vapor–rich environments was carried out at 1200 °C to examine the effects of different oxidation conditions on the early-stage oxidation behavior of SiC. Two different types of SiC oxidation behavior were found, passive or active, depending on the oxidation environment. All the samples possessed amorphous oxide layers, regardless of the oxidation environment. Three Si oxidation states (SiO, Si₂O₃, and SiO₂) were observed in this layer. The amorphous Si₂O₃ state was dominant, and the ratio of the three different states changed with the test conditions.     

Oxidation resistance of SHS Fe–Al–Si alloys at 800 °C in air

Alloys formed by Fe–Al intermediary phases have lower density than common metallic high-temperature alloys and good high-temperature oxidation resistance. Previously it was proved that silicon additions to these alloys enable to produce them efficiently by reactive sintering and improves the wear resistance. In this work, the oxidation resistance of the novel Fe–Al–Si alloys containing 10–30 wt. % of aluminium and 5–30 wt. % of silicon produced by the reactive sintering technology was tested. Cyclic and isothermal oxidation tests were carried out at 800 °C in air. Tested alloys exhibit excellent oxidation resistance, which increases with silicon content up to 20 wt. %. The reasons are discussed in terms of the phase composition of the oxide layer and the changes in chemical composition under the oxide layer during oxidation.     

Oxidation behavior of a Zr–Cu–Al–Ni amorphous alloy in air at 300–425 °C

The oxidation behavior of Zr–30Cu–10Al–5Ni bulk metallic glass and its crystalline counterpart was studied over the temperature range of 300–425 °C in dry air. In general, the oxidation kinetics of both amorphous and crystalline alloys followed a two- or three-stage parabolic rate law at T⩾350 °C, while at 300 °C the amorphous alloy oxidized following a linear behavior. The oxidation rate constants for the amorphous alloy are slightly higher than those for the crystalline alloy at 350–400 °C. The scale formed on the amorphous alloy consists of mainly tetragonal-ZrO₂ at 300 °C, while a mixture of monoclinic-ZrO₂ (m-ZrO₂) and tetragonal-ZrO₂ (t-ZrO₂) and some CuO were detected at higher temperatures. The scale formed on the crystalline alloy, on the other hand, consists of mainly Al₂O₃, some tetragonal-ZrO₂, and a slight amount of monoclinic-ZrO₂ at 300 °C. At higher temperatures, the crystalline alloy consists of mainly monoclinic-ZrO₂, some CuO and Cu₂O, and limited tetragonal-ZrO₂. It is suggested that the formation of Al₂O₃ (at 300 °C) and CuO/Cu₂O (at 350-400 °C) on the crystalline alloy is responsible for the reduced oxidation rates as compared with those of amorphous alloy.     

Thermal expansion of silicon at temperatures up to 1100 °C

Thermal expansion of CVD single crystal silicon was measured with a push-rod dilatometer up to 1100 °C for different crystallographic orientations of the specimen. Thermal analysis, Laue analysis and X-ray diffraction were used to verify silicon crystal orientation and absence of possible phase transformations. Coefficients of technical thermal expansion have been calculated in this temperature range and their variations with temperature have been demonstrated. These differences might cause anisotropy in thermal stresses, which has been calculated and compared with experimental values of dry-oxidised silicon wafers.     

Isothermal oxidation of Zr2[Al(Si)]4C5–SiC composites at 1000–1300 °C in air

The effect of SiC on the oxidation performance of Zr₂[Al(Si)]₄C₅–SiC composites at 1000–1300 °C was explored. The present results demonstrated that scale growth on Zr₂[Al(Si)]₄C₅ ceramics obeyed a nearly linear rate law during oxidation, while the oxidation rate of Zr₂[Al(Si)]₄C₅–20SiC ceramics obeyed a nearly parabolic law, increasing rapidly during the initial stages of the reaction and then becoming slower. The oxidation resistance of Zr₂[Al(Si)]₄C₅–20SiC was improved remarkably; this was likely due to the formation of more protective oxide scales on the ceramic consisting of viscid aluminosilicate glass.     

Oxidation behavior of Ni-based superalloy GH738 in static air between 800 and 1000℃

The oxidation behavior of a nickel-based superalloy GH738 was studied by isothermal oxidation tests in still air at different temperatures,with exposure time up...     

Oxidation behavior of oxidation protective coatings for C/C-SiC composites at 1 500 °C

Porous carbon/carbon preforms were infiltrated with melted silicon to form C/C-SiC composites. Three-layer Si-Mo coating prepared by slurry painting and SiC/Si-Mo multilayer coating prepared by chemical vapor deposition(CVD) alternated with slurry painting were applied on C/C-SiC composites, respectively. The oxidation of three samples at 1 500 °C was compared. The results show that the C/C-SiC substrate is distorted quickly. Three-layer Si-Mo coating is out of service soon due to the formation of many bubbles on surface. The mass loss of coated sample is 0.76% after 1 h oxidation. The sample with SiC/Si-Mo multilayer coating gains mass even after 105 h oxidation. SiC/Si-Mo multilayer coating can provide longtime protection for C/C-SiC composites and has excellent thermal shock resistance. This is attributed to the combination of dense SiC layer and porous Si-Mo layer. Dense SiC layer plays the dual role of physical and chemical barrier, and resists the oxidation of porous Si-Mo layer. Porous Si-Mo layer improves the thermal shock resistance of the coating.     

Mechanical properties of as-fabricated and 2300 °C annealed tungsten wire tested up to 600 °C

Recent efforts dedicated to the mitigation of tungsten brittleness have demonstrated that tungsten fiber-reinforced composites acquire pseudo ductility even at room temperature. Crack extension and fracture process is basically defined by the strength of tungsten fibers. Here, we move forward and report the results of mechanical and microstructural investigation of different grades of W wire with a diameter of 150 μm at elevated temperature up to 600 °C. The results demonstrated that potassium doping to the wire in the as-fabricated state does not principally change the mechanical response, and the fracture occurs by grain elongation and delamination. Both fracture stress and fracture strain decrease with increasing test temperature. Contrary to the as-fabricated wire, the potassium-doped wire annealed at 2300 °C exhibits much lower fracture stress. The fracture mechanism also differs, namely: cleavage below 300 °C and ductile necking above. The change in the fracture mechanism is accompanied with a significant increase of the elongation to fracture being ~ 5% around 300 °C.     

The oxidation behaviour of uniaxial hot pressed MoSi2 in air from 400 to 1400 °C

MoSi₂ samples were prepared by hot uniaxial pressing from a 2 μm grain-size powder of commercially available MoSi₂. The oxidation behaviour of MoSi₂ was systematically studied from 400 °C to 1400 °C, which includes the pest-oxidation temperature range. It was observed that the rate and mechanism for oxidation of MoSi₂ change significantly with increasing temperature. Five temperature regimes have to be considered regarding both kinetic results and cross-sections: i) 400 < T < 550 °C; ii) 550 ≤ T < 750 °C; iii) 750 ≤ T < 1000 °C; iv) 1000 ≤ T < 1400 °C; v) T ≥ 1400 °C. In the first range, pesting did not occur in samples that were free of cracks and residual stresses and the oxidation kinetics were governed by surface or phase boundary reactions. Above 550 °C, there was a change in the physical properties of the oxidation products due to the evaporation of MoO₃. The formation of Mo₅Si₃ was observed above 800 °C showing that the thermodynamic previsions were satisfied above this temperature. At higher temperatures (>1000 °C), the oxide scale became very protective and transport in the silica scale (amorphous and β cristobalite) governed the oxidation kinetics. The Mo₅Si₃ phase did not appear anymore at 1400 °C, indicating that another oxidation mechanism has to be proposed.     

Oxidation of P92 steel weldment between 600 and 800 °C in air

P92 alloy with a composition of Fe-9.1Cr-0.5Mo-1.7W (wt.%) was welded, and its oxidation behavior was studied at 600, 700 and 800 °C for up to 6 months in air. The oxidation resistance decreased in the order of the base metal, weld metal, and the heat affected zone. The morphology and the composition of the scales formed on these samples were similar. The scales were either uniform in thickness or nodular. The scales consisted mainly of Fe₂O₃. As oxidation progressed, thick, nodular oxide scales formed. The alloying elements such as Cr, W, and Mn tended to incorporate in the lower part of the oxide scale.     

Evolution of phases in Al55Ni30Pd15 alloy at temperatures up to 600 °C

The originally as-cast Al₅₅Ni₃₀Pd₁₅ alloy was investigated during continuous heating from room temperature to 600 °C and after annealing at 600 °C for respective 455, 2650, and 4050 h. In the investigation the synchrotron X-ray diffraction and the high-resolution scanning electron microscopy inclusive of the energy-dispersive X-ray spectroscopy were used. In all the investigated conditions β-(Ni,Pd)Al (Pm3m) and Al₃(NiPd)₂ (P3m1) phases were identified. After 2650 h of annealing the former phase was found to be separated into two isostructural modifications differing from one another in Pd- and Ni-contents. The annealing for 455 h contributed to the decrease of lattice parameters in the Al₃NiPd phase compared to the original as-cast condition.     

Oxidation behavior of single crystal TMS-82＋ superalloy in air

The oxidation behavior of the single crystal Ni-based superalloy TMS-82＋ was studied under cyclic condition at 900 and 1 000℃ for 200 h in air. The oxidation kinetics for the superalloy at both exposure temperatures was explained by the subparabolic relationship. The results show that increasing the exposure temperattire from 900 to 1 000℃, the amounts of α-Al2O3, （Ni, Co）Al2O4 and NiCr2O4 increase, resulting in a higher mass gain. The oxides consist of （Ni, Co）O, CrTaO4, AITaO4, Cr2O3, θ-,_ α-Al2O3 and （Ni, Co）Al2O4 in the specimen at 900 ℃, but NiCr204 spinel forms for the specimen exposed at 1 000℃ except the above mentioned oxides. A continuous α-Al2O3 layer is responsible for a slow growth rate of the scale after an initial rapid oxidation of NiO.     

Oxidation behavior of three commercial ODS alloys at 1200°C

The isothermal-oxidation behavior of three oxide-dispersion-strengthened (ODS) alloys, viz., MA 956, ODM 751, and PM 2000, has been examined in air at 1200°C for exposure times up to 4800 hr. During exposure all the alloys formed an external scale of alpha alumina (-Al₂O₃). The growth rate of alumina on MA 956 was significantly faster than that formed on ODM 751 resulting in an oxide layer which was about twice as thick after 4800 hr. The oxide-grain morphology on MA 956 was essentially equiaxed containing irregularly shaped, titanium-rich particles, whereas the oxide formed on ODM 751 was slightly finer, distinctly columnar and contained elongated yttrium-rich particles. Spalling of the oxide layer occurred after approximately 2400 hr on MA 956, whereas only slight spalling occurred on ODM 751 even after the longest exposure time. Experiments revealed that the initial surface roughness of PM 2000 can contribute significantly to spalling by enabling the growth of highly convoluted scale layers which are mechanically unstable under compressive stresses (buckling). Internal porosity is also observed in all three alloys after exposure. The pores were generally spherical with small Ti-, Al-, Y-rich particles distributed over their internal surfaces. The amount of porosity increases to a maximum and then slowly decreases.     

Paralinear Oxidation of Cr-Si-C-Coated C/SiC at 1300°C in Wet and Dry Air Environments

JOM - The oxidation kinetics of a Cr3Si-Cr7C3/SiC/SiC-coated C/SiC were comparatively investigated in dry and wet air at 1300°C under 1&nbsp;atm. After oxidation for 10&nbsp;h, Cr2O3...     

Oxidation of ZC63 Mg alloys reinforced with SiC particles between 390 °C and 500 °C in air

The ZC63 magnesium alloys reinforced with 10 wt.% of SiC particles with an average particle size of 50 μm were cast. The fabricated SiC_p/ZC63 composite consisted of an α-Mg matrix, unreacted α-SiC particles, and an intergranularly formed CuMgZn compound. It was oxidized at 390 °C to 500 °C up to 5 h in air. The oxide scales were thin and compact below 430 °C, but became porous and loose above 450 °C. They consisted primarily of MgO and a small amount of Mg₃N₂. SiC particles were stable over the temperature range explored.     

High-Temperature Oxidation Behavior and Mechanism of a New Type of Wrought Ni–Fe–Cr–Al Superalloy up to 1300°C

The oxidation behavior of a new type of wrought Ni–Fe–Cr–Alsuperalloy has been investigated systematically in the temperature range of1100 to 1300°C. Results are compared with those of alloy 214, Inconel600, and GH 3030. It is shown that the oxidation resistance of the newsuperalloy is excellent and much better than that of the comparisonalloys. Scanning electron microscopy (SEM), electron probe microanalysis(EPMA), and X-ray diffraction (XRD) experiments reveal that the excellentoxidation resistance of the new superalloy is due to the formation of adense, stable and continuous Al₂O₃ and Cr₂O₃ oxide layer at hightemperatures. Differential thermal analysis (DTA) shows that the formationof Cr₂O₃ and Al₂O₃ oxide layers on the new superalloy reaches a maximum at1060 and 1356°C, respectively. The Cr2O3 layer peels off easily, and thesingle dense Al₂O₃ layer remains, giving good oxidation resistance attemperatures higher than 1150°C. In addition, the new superalloypossesses high mechanical strength at high temperatures. On-site testsshowed that the new superalloy has ideal oxidation resistance and can beused at high temperatures up to 1300°C in various oxidizing andcorrosion atmospheres, such as those containing SO₂, CO₂ etc., for longperiods.     

Oxidation protection of 20Cr-25Ni-Nb stainless steel at 1300°C

Enhanced environmental protection of chromia-forming advanced metallic alloys at normal operating temperature, typically 900°C, may be provided by two well-established approaches—incorporation of reactive elements into the protective oxide scale or an amorphous ceramic coating acting as a diffusion barrier. The continued effectiveness of such approaches, namely by cerium and yttrium ion implantation and with a vapor deposited amorphous silica coating, in reducing oxidation of 20Cr-25Ni-Nb stainless steel in a carbon-dioxide-based environment has been examined during 0.5 and 1 hr transients to 1300°C. The influence of pre-oxidation of the ion-implanted, silica-coated, and uncoated steel for extended periods in the same environment at 825–900°C has also been established.     

Thermophysical properties of TiB2SiC ceramics from 300 °C to 1700 °C

In the present work, high-frequency induction heating is used to fabricate TiB₂SiC ceramics and the relative density was more than 97%, and then the thermophysical properties of TiB₂SiC ceramics were investigated in detail. The specific heat showed the weak dependence on the test temperature due to the presence of the interface gap because the relative density was not 100%. As the sintering temperature increased, the thermal diffusivity of TiB₂SiC ceramics increased, which was due to the increase of relative density and grain growth. The thermal conductivity of TiB₂SiC ceramics showed a marked increase with increasing grain size and relative density. This could be attributed to a reduction in the number of grain boundaries that interrupt the heat flow path, resulting in an increase in the mean free path of the phonons. Larger grains led to an increase of mean free path of the phonons and thus contributed to a further increase in thermal conductivity.