Carburization Behavior of 310 Stainless Steel in CH4/H2 Gas Mixture with Trace Amount of Oxygen

The carburization behavior of 310 stainless steel has been studied after cyclic exposures to carburizing gas mixtures at elevated temperatures for 500hr exposures. A thermodynamic analysis indicated that 1000°C was an approximate critical temperature, below which the environment should result in mixed oxidizing/carburizing behavior while above this temperature, reducing-carburizing behavior should occur. The experimental results agree well with the thermodynamic analysis, at 800°C in 2% CH₄/H₂ for 310SS which suffers both external oxidation, carburization, and internal carburization. At 1100°C in 10% CH₄/H₂ external carburization occurs and internal carburization becomes less pronounced. Schematics are illustrated to show corrosion mechanisms in various exposure environments.     

Stability of MnCr2O4 spinel and Cr2O3 in high temperature carbonaceous environments with varied oxygen partial pressures

In this investigation, Cr₂O₃ and MnCr₂O₄ were comparatively tested at 1050 °C in carbonaceous environment with varied oxygen partial pressures. MnCr₂O₄ exhibits much better resistance to carbonaceous attack than Cr₂O₃. The carburization rate of MnCr₂O₄ decreases sharply with increasing oxygen partial pressures. The oxygen partial pressures have less effect on the carburization resistance of Cr₂O₃. The increased resistance of MnCr₂O₄ to carburization is attributed to the dissolution of MnO into Mn-Cr-O spinel lattices with elevated oxygen partial pressures, which retards the decomposition and carburization of Mn-Cr-O spinel. The thermodynamic equations defining the carburization stability of MnCr₂O₄ and Cr₂O₃ are modified.     

Oxidation behavior of micro-sized Al2O3–TiC–Co composites prepared from cobalt-coated powders

The oxidation behavior of hot-pressed Al₂O₃–TiC–Co composites prepared from cobalt-coated powders has been studied in air in the temperature range from 200 °C to 1000 °C for 25 h. The oxidation resistance of Al₂O₃–TiC–Co composites increases with the increase of sintering temperature at 800 °C and 1000 °C. The oxidation surfaces were studied by XRD and SEM. The oxidation kinetics of Al₂O₃–TiC–Co composites follows a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. The mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations.     

Impact of oxyfuel atmospheres H2O/CO2/O2 and H2O/CO2 on the oxidation of ferritic–martensitic and austenitic steels

In future power plant technologies, oxyfuel, steels are subjected to steam rich and carbon dioxide rich combustion gases. The effect of simulated combustion gases H₂O/CO₂/O₂ (30/69/1 mol%) and H₂O/CO₂ (30/70 mol%) on the corrosion behavior of low alloyed steels, 9–12% chromium steels and an austenitic steel were studied. It was discovered that the formation of protective chromium rich oxides is hampered due to the carburization of the base material and the formation of chromium rich carbides. The kinetics of corrosion and carburization are quantified. The effect of temperature and the effect of gas pressure are analyzed statistically.     

High-temperature carburization behaviour of HASTELLOY X in CH4 gas

Ni-base wrought alloy HASTELLOY X tube was exposed to Ar–CH₄ at 800 and 1000 °C in order to understand the carburization kinetics of the alloy used for fuel injection nozzles of micro-gas turbine combustors. Three different internal carbides, (Cr,Mo)₃C₂, (Cr,Mo)₇C₃ and (Cr,Mo)₂₃C₆ were observed in this order from the surface, and the partial damage to the outer surface of the specimen tube appeared similar to metal-dusting. The internal carburization kinetics on both the inner and outer parts of tube followed the parabolic rate law. The carbon permeability in HASTELLOY X was obtained, and was slightly smaller than that of Ni–20%Cr.     

Effect of heat-treatment on stress relaxation behavior of plasma-sprayed 7 wt.% Y2O3-ZrO2 stand-alone coatings

The present work studied the effect of heat-treatment temperature (1000 °C and 1200 °C) and time (10, 50, and 100 h) on the compressive stress relaxation behavior of plasma-sprayed stand-alone 7 wt.% Y₂O₃-ZrO₂ (YSZ) coatings at test temperatures of 1000 °C, 1100 °C, and 1200 °C, from stresses of 60 and 20 MPa. As-sprayed coatings were also stress relaxed in the baseline condition at room and elevated temperatures. All coatings demonstrated a two-stage relaxation behavior: fast relaxation (stage I) in the first 10 min and much slower relaxation in the final 170 min of the test (stage II). Stage I relaxation, as measured by percentage of the original stress relaxed, accounts for at least 50% of the total stress relaxed despite occurring in only 5-10 min and was attributed to lamella sliding and compaction, and permanent intralamellar crack closure (for tests conducted at higher temperatures). Stage II relaxation behavior is dominated by diffusion creep mechanisms, where prior densification at 1200 °C resulted in reduced relaxation rates compared to coatings heat treated at 1000 °C and in the as-sprayed condition. The 1200 °C test temperature greatly influenced the percentage of relaxation in the coating, more so than the prior coating heat-treatment conditions.     

Dielectric and microwave properties of ZrO2 doped CaO-SiO2-B2O3 ceramic matrix composites

The behavior of dielectric and microwave properties against sintering temperature has been carried out on CaO-SiO₂-B₂O₃ ceramic matrix composites with ZrO₂ addition. The results indicated that ZrO₂ addition was advantageous to improve the dielectric and microwave properties. X-ray diffraction (XRD) patterns show that the major crystalline β-CaSiO₃ and a little SiO₂ phase existed at the temperature ranging from 950 °C to 1050 °C. At 0.5 wt% ZrO₂, CaO-SiO₂-B₂O₃ ceramic matrix composites sintered at 1000 °C possess good dielectric properties: ?_r = 5.85, tan δ = 1.59 × 10⁻⁴ (1 MHz) and excellent microwave properties: ?_r = 5.52, Q · f = 28,487 GHz (11.11 GHz). The permittivity of Zr-doped CaO-SiO₂-B₂O₃ ceramic matrix composites exhibited very little temperature dependence, which was less than ±2% over the temperature range of −50 to 150 °C. Moreover, the ZrO₂-doped CaO-SiO₂-B₂O₃ ceramic matrix composites have low permittivity below 5.5 over a wide frequency range from 20 Hz to 1 MHz.     

Phase transformations in face centered cubic (Al0.32Cr0.68)2O3 thin films

Face centered cubic (Al_0.32Cr_0.68)₂O₃ thin films have been annealed in the temperature range of 500–1000 °C during 2–8 h. The fcc structure of the film remains intact when annealed at temperatures up to 700 °C for 8 h. X-ray diffraction and transmission electron microscopy show the onset of phase transformation to corundum phase alloys in the sample annealed at 900 °C for 2 h, where annealing at 1000 °C for 2 h results in complete phase transformation to α-(Al_0.32Cr_0.68)₂O₃. In-plane and out-of-plane line scans performed in EDX TEM and θ/2θ XRD patterns did not show any phase separation into α-Cr₂O₃ and Al₂O₃ prior and after the annealing. The apparent activation energy of this process is 380–480 kJ/mol as determined by the Johnson–Mehl–Avrami model.     

Preparation of thermally stable anatase TiO2 photocatalyst from TiOF2 precursor and its photocatalytic activity

Preparation of anatase TiO₂ with high themal stability is of great importance for its environmental application. In this work, TiOF₂ was first synthesized by a simple microwave-assisted hydrothermal route using tetrabutyl titanate and hydrofluoric acid as precursors at 200 °C for 20 min. Then the resulted precipitates were calcined at different temperatures (300-1000 °C) for 2 h. The as-prepared samples were characterized by X-ray diffraction, Raman spectrum, scanning electron microscopy, N₂ adsorption-desorption isotherms and X-ray photoelectron spectroscopy. The photocatalytic activity was evaluated using Brilliant Red X3B, an anionic azo dye, as the target organic molecule under UV light irradiation. The results showed that the prepared TiOF₂ exhibited weak or no photocatalytic activity. The phase transformation of TiOF₂ to anatase TiO₂ occurred at about 300 °C. The prepared anatase TiO₂ from TiOF₂ showed very high thermal stability and the anatase-to-rutile phase transformation temperature was up to 1000 °C. Fluoride ions played an important role in the improvement of thermal stability of anatase TiO₂ by strongly adsorbing on the crystal planes of anatase to stabilize the anatase structure. The 700 °C-calcined sample showed the highest photocatalytic activity due to its relative good crystallization and high specific surface areas.     

Preparation of Al2O3/DLC/Au/MoS2 chameleon coatings for space and ambient environments

We have investigated the tribological properties of nanocomposite “chameleon” coatings, which adapt their low friction behavior with the surrounding environmental humidity and temperature. The material system of interest included alumina (Al₂O₃) in an Au matrix with diamond-like carbon (DLC) and MoS₂ nanoparticle inclusions. The coating design included formation of nanocrystalline hard oxide particles for wear resistance, embedding them into an amorphous matrix for toughness enhancement, and inclusion of nanocrystalline and/or amorphous solid lubricants for friction adaptation to different environments.Chemical analysis was used to ascertain a correlation between chemical bonding of species and frictional properties. Friction measurements were studied in cycling between humid air and dry nitrogen conditions at room temperature and during heating in air to 500 °C. It was observed that both graphitic carbon and MoS₂ worked together to give low friction in variable humidity environment, while Au was valuable for a low friction at elevated temperatures. Friction coefficients were found to be 0.02-0.03 in dry nitrogen, 0.1-0.15 in humid air, and 0.1 in air at 500 °C. Thus the tribological property results have shown that the system provides “chameleon” type adaptation behavior in different environments relevant for aerospace systems.     

High temperature scaling of Ni-xSi-10 at.% Al alloys in 1 atm of pure O2

The oxidation of Ni-xSi-10Al alloys (with x = 0, 2, 4 and 6 at.%), has been studied at 900 and 1000 °C in 1 atm of pure O₂ to examine the effect of different silicon additions on the behavior of ternary Ni-Si-10Al alloys. The kinetic curves of Ni-10Al are approximately parabolic at both 900 and 1000 °C. Conversely, the kinetics of the ternary alloys at both temperatures correspond generally to a rate decrease faster than predicted by the parabolic rate law, except for the oxidation of Ni-6Si-10Al at 1000 °C, which exhibits a single nearly-parabolic stage. Oxidation of the binary alloy formed at both temperatures an internal oxidation zone beneath a layer of NiO. Oxidation of Ni-2Si-10Al at both temperatures and of the other two alloys at 900 °C formed initially a zone of internal oxidation of Al + Si. However, a layer of alumina forming at the front of internal oxidation after some time blocked the internal oxidation and produced a gradual conversion of the metal matrix of this region into NiO, with a simultaneous decrease of the oxidation rate. Conversely, the oxidation of Ni-4Si-10Al and Ni-6Si-10Al at 1000 °C did not produce an internal oxidation, but formed an alumina layer directly on the alloy surface after an initial stage when also Ni was oxidized. Therefore, silicon exerts the third-element effect by reducing the critical Al content needed for the transition from its internal to its external oxidation with respect to the corresponding Ni-Al alloy. This result is interpreted by means of an extension to ternary alloys of Wagner’s criterion for the same transition in binary alloys based on the attainment of a critical volume fraction of internal oxide.     

High-temperature corrosion and creep of Ni-Cr-Fe alloys in carburizing and oxidizing environments

The carburization of NiCr 32 20 and NiCrSi 60 16 has been studied in CH₄-H₂ mixtures in the temperature range 900–1100°C. The methods included thermogravimetric measurements and studies on reacted specimens by X-ray diffraction, metallographic, and chemical analysis. Upon carburization internal carbides M₇C₃ and M₂₃C₆ are formed (M=mainly Cr); the rate of carburization is determined by carbon diffusion in the Fe-Ni matrix with carbide precipitations. The effect of the alloying elements Ni and Si on the carburization resistance of austenitic alloys is explained. By the same methods the oxidation and carburization in CO-H₂O-H₂ mixtures have been studied. The important role of a stable chromium oxide layer for the carburization resistance was confirmed. Creep tests at 1000°C in a CO-H₂O-H₂ atmosphere where Cr₂O₃ is stable showed carburization occurring through cracks in the oxide layer. At high strain rates premature failure occurs by carburization, which is followed by internal oxidation and formation of cracks, voids, and holes.     

Oxidation behavior of Ti3AlC2 at 1000-1400 °C in air

The isothermal oxidation behavior of bulk Ti₃AlC₂ has been investigated at 1000-1400 °C in air for exposure times up to 20 h by means of TGA, XRD, SEM and EDS. It has been demonstrated that Ti₃AlC₂ has excellent oxidation resistance. The oxidation of Ti₃AlC₂ generally followed a parabolic rate law with parabolic rate constants, k_p that increased from 4.1×10⁻¹¹ to 1.7×10⁻⁸ kg² m⁻⁴ s⁻¹ as the temperature increased from 1000 to 1400 °C. The scales formed at temperatures below 1300 °C were dense, adherent, resistant to cyclic oxidation and layered. The inner layer of these scales formed at temperatures below 1300 °C was continuous α-Al₂O₃. The outer layer changed from rutile TiO₂ at temperatures below 1200 °C to a mixture of Al₂TiO₅ and TiO₂ at 1300 °C. In the samples oxidized at 1400 °C, the scale consisted of a mixture of Al₂TiO₅ and, predominantly, α-Al₂O₃, while the adhesion of the scales to the substrates was less than that at the lower temperatures. Effect of carbon monoxide at scale/substrate was involved in the formation of the continuous Al₂O₃ layers.     

Synthesis and characterization of Y3Al5O12 and ZrO2-Y2O3 thermal barrier coatings by combustion spray pyrolysis

Y₃Al₅O₁₂ and ZrO₂-Y₂O₃ (8 mol% YSZ) coatings for potential application as thermal barrier coatings were prepared by combustion spray pyrolysis. Thermal cycling of as deposited coatings on stainless steel and FeCrAlY bond coat substrates was carried out at 1000 °C and 1200 °C to determine the thermal fatigue response. Structural and morphological studies on Y₃Al₅O₁₂ and 8 mol% YSZ coatings before and after thermal cycling have been carried out. It has been noted that the coatings on FeCrAlY substrates remain intact after 50 cycles between room temperature and 1200 °C, whereas the coatings on stainless steel show some minor damage such as peeling off near the periphery after 50 cycles at 1000 °C. Thermal diffusivity values of Y₃Al₅O₁₂ and 8 mol% YSZ films were measured by using photo thermal deflection spectroscopy and the values are lower than those of coatings produced by conventional techniques such as EBPVD and APS.     

Unusual oxidation behaviour of Zr50Cu50 alloy at high temperatures

The oxidation of Zr₅₀Cu₅₀ alloy at 500-700 °C is characterized by preferential oxidation of zirconium, while the excess of copper is accumulated at the alloy-oxide interface forming the Zr₁₄Cu₅₁ phase. The strong reaction at 800 and 850 °C resulted in the total corrosion of the specimens in 21 and 15 h, respectively. The oxidation at elevated temperatures showed an anomalous decrease of the oxygen consumption rate in the temperature range 930-1000 °C, corresponding to the preferentially oriented crystallites of ZrO₂ in the oxide scale at 900 and 1000 °C. The oxide layer consists of ZrO₂ and CuO in the whole temperature interval of the oxidation. The reaction kinetics obeys a parabolic rate law. An activation energy of 92.0 ± 0.3 kJ/mol has been estimated.     

Hot corrosion of Ti3SiC2-based ceramics coated with Na2SO4 at 900 and 1000 °C in air

The hot corrosion behavior of polycrystalline Ti₃SiC₂ under thin films of Na₂SO₄ was studied at 900 and 1000 °C in air. The microstructure and composition of the scales were investigated by scanning electron microscope/energy dispersive spectroscope and X-ray diffraction. The results demonstrated that Ti₃SiC₂ suffered from serious attack during hot corrosion at 900 and 1000 °C. The corroded scale had a duplex microstructure, the outer layer consisted of coarse grains with pores, the inner layer consisted of fine grains and was compact. The whole corroded layer consisted of a mixture of TiO₂ and SiO₂ after hot corrosion attack, which was different from the scale formed during the oxidation of Ti₃SiC₂ in air.     

Production of nanoscale Ba(Zn1/3Nb2/3)O3 microwave dielectric ceramics by polymerised complex method

Ba(Zn_1/3Nb_2/3)O₃ nanoparticles have been synthesized by a polymerised complex method by using precursor materials of barium nitrate, zinc acetate, niobium oxide, hydrofluoric acid and citric acid. Thermal decomposition characteristics and crystallization behavior of the powders were investigated by the thermogravimetric and differential thermal analysis, X-ray diffractometer and Fourier transform infrared spectroscopy. Ba(Zn_1/3Nb_2/3)O₃ phase started to form at low temperature of 400 °C and, single phase Ba(Zn_1/3Nb_2/3)O₃ perovskite structure was obtained at 1000 °C. Microstructural investigation revealed that the major particle size of Ba(Zn_1/3Nb_2/3)O₃ nanoparticles were in the range of 80–110 nm with spherical morphology and homogeneous size distribution. But the powders also contained some agglomeration.     

Einflüsse kleiner Zusätze von Niob oder Cer auf Korrosion und Kriechen von Incoloy 800 in Co-H2O-H2-Atmosphären

Effects of small alloying additions of niobium or cerium on the corrosion and creep of Incoloy 800 in CO-H₂O-H₂-atmospheres In oxidizing and carburizing atmospheres at high temperatures Fe-Ni-Cr-alloys are carburized under creep conditions by carbon transfer through cracks in the oxide layer. In creep experiments in CO-H₂O-H₂ atmospheres at 1000 °C with several alloys based on Incoloy 800 the carburization could be related to the strain of the specimens. Alloying additions of Nb in the range 0.2 to 1% caused changes in the creep rate, a decrease for >0.35% and an increase for >0.35% Nb. The creep resistance for the high Nb concentrations could be improved by solution annealing at high temperatures (1200 °C). Niobium strongly decreases the carburization - this effect can be explained by the formation of an internal layer of Al- and Nb-oxides beneath the outer Cr₂O₃ layer. An alloying addition of Ce (0.06%) also has beneficial effects on the creep resistance and carburization resistance of Incoloy 800.     

Synthesis, structure and optical properties of CoAl2O4 spinel nanocrystals

CoAl₂O₄ nanocrystals were synthesized by sol-gel method using citric acid as a chelating agent at low temperature. The as-synthesized samples were characterized by thermal analysis, X-ray powder diffraction, infrared spectroscopy and transmission electron microscopy. The results show that CoAl₂O₄ spinel is the only crystalline phase with a size of 10-30 nm in the temperature range 500-1000 °C. The temperature dependence of the distribution of Al³⁺ and Co²⁺ ions in the octahedral and tetrahedral sites in nanocrystals was investigated by X-ray photoelectron spectroscopy (XPS). It is observed that the inversion parameter decreases with increasing annealing temperature. Analysis of the absorption properties indicates that Co²⁺ ions are located in the tetrahedral sites as well as in the octahedral sites in the CoAl₂O₄ nanocrystals. The origin of the green color (300-500 nm absorption band) should be due to the octahedrally coordinated Co²⁺ ions.     

Epitaxial 0.65PbMg1/3Nb2/3O3-0.35PbTiO3 (PMN-PT) thin films grown on LaNiO3/CeO2/YSZ buffered Si substrates

Ferroelectric PMN-PT thin films with a thickness of 600 nm were epitaxially grown on buffered Si (0 0 1) substrates at a substrate temperature that ranged from 550 to 700 °C using pulsed laser deposition (PLD). LaNiO₃ (LNO) electrode thin films with a resistivity of ∼1900 μΩ cm were epitaxially grown on CeO₂/YSZ buffered Si (0 0 1) substrates. The PMN-PT thin films grown at 600 °C on LNO/CeO₂/YSZ/Si substrates had a pure perovskite and epitaxial structure. The PMN-PT films exhibited a high dielectric constant of about 1818 and a low dissipation factor of 0.04 at a frequency of 10 kHz. Polarization-electric-field (P-E) hysteresis characteristics, with a remnant polarization of 11.1 μC/cm² and a coercive field of 43 kV/cm, were obtained in the epitaxial PMN-PT films.