Oxidation Resistance In Air Research Articles

Oxidation protective behavior of SiC/Si–MoSi2 coating for different graphite matrix

A SiC/Si–MoSi2 oxidation protective coating was prepared on different graphite matrix, and the oxidation behavior of SiC/Si–MoSi2 coated on different graphite matrix was also studied. Results showed that the porosity and pore radius of graphite materials had marked effect on the oxidation behavior ability for carbon materials. Appropriate porosity and pore radius are the necessary factors to realize the excellent oxidation resistance in air, and the reason for the different oxidation behavior of coating on different carbon matrix was also analyzed.

Reactive magnetron sputtering of hard Si–B–C–N films with a high-temperature oxidation resistance

Based on the results obtained for C–N and Si–C–N films, a systematic investigation of reactive magnetron sputtering of hard quaternary Si–B–C–N materials has been carried out. The Si–B–C–N films were deposited on p-type Si(100) substrates by dc magnetron co-sputtering using a single C–Si–B target (at a fixed 20% boron fraction in the target erosion area) in nitrogen-argon gas mixtures. Elemental compositions of the films, their surface bonding structure and mechanical properties, together with their oxidation resistance in air, were controlled by the Si fraction (5–75%) in the magnetron target erosion area, the Ar fraction (0–75%) in the gas mixture, the rf induced negative substrate bias voltage (from a floating potential to −500V) and the substrate temperature (180–350°C). The total pressure and the discharge current on the magnetron target were held constant at 0.5Pa and 1A, respectively. The energy and flux of ions bombarding the growing films were determined on the basis of the discharge characteristics measured for the rf discharge dominating in the deposition zone. Mass spectroscopy was used to show composition of the total ion fluxes onto the substrate and to explain differences between sputtering of carbon, silicon and boron from a composed target in nitrogen-argon discharges. The films, typically 1.0–2.4μm thick, possessing a density around 2.4gcm−3, were found to be amorphous in nanostructure with a very smooth surface (Ra⩽0.8nm) and good adhesion to substrates at a low compressive stress (1.0–1.6GPa). They exhibited high hardness (up to 47GPa) and elastic recovery (up to 88%), and extremely high oxidation resistance in air at elevated temperatures (up to a 1350°C substrate limit).

Liquid phase sintered silicon carbide (LPS-SiC) ceramics having remarkably high oxidation resistance in wet air

Numerous studies on the oxidation characteristics of common silicon carbide ceramics have revealed very good oxidation resistance in dry atmospheres, provided that the oxygen partial pressure is sufficiently high. On the contrary, as other SiO 2-passivated ceramics, SiC shows poor oxidation resistance in moist atmospheres. In this contribution, the oxidation behaviour at 1400 °C of fully dense SiC samples with Lu 2O 3–AlN and Lu 2O 3–Ho 2O 3 sintering additives is investigated. Similar to results recently reported for a novel Si 3N 4 material, it is demonstrated that Lu 2O 3 yields greatly improved oxidation behaviour as compared to other liquid phase sintered SiC materials. In particular, the Lu 2O 3–Ho 2O 3 additive leads to passive oxidation under a 0.01 MPa partial pressure of water.

Oxidation Resistance of Mo Coated with Mo(Si,Al)<SUB>2</SUB> Layer Prepared by Dipping into Liquid of Al-25 mass%Si alloy

A homogeneous layer of molybdenum alumininosilicide Mo(Si,Al) 2 with C(40) structure was made on the sample surface of molybdenum by dip-coating technique using Al-25 mass%Si liquid at 1123 K. Adherent Al-Si was removed by soaking the dipped samples in the NaOH saturated hydraulic solution. The Mo samples coated with the Mo(Si,Al) 2 layer wew found to have excellent oxidation resistance in air even at 1473 K due to formation of a dense alumina film on the sample surface during oxidation. The formation of Mo 5 (Si,Al) 3 layer was also observed at the interface between the Mo substrate and the Mo(Si,Al) 2 layer.

Effect of Water Vapor and Hydrogen on the Oxidation of Metallic Interconnect Materials for Solid Oxide Fuel Cells

One of the critical requirements for interconnect materials in solid oxide fuel cells (SOFCs) is oxidation resistance in both air and fuel environments. Chromium oxide (Cr 2 O 3 ) is stable and grows in both environments, but differences in the hydrogen and water vapor contents between the electrodes affect the defect equilibria, which in turn affects the growth mechanism and thus the scale properties. In this paper, the effects of water vapor and hydrogen on the oxidation behavior of chromia-forming alloys are discussed. In addition to the oxidation in either air or fuel environments, the behavior in a dual atmosphere (simultaneous exposure to air on one side and fuel on the other) is also discussed.

Magnetic fine particles of Fe and Co encapsulated by carbon layers

Fine particles of Fe and Co encapsulated by carbon (C) nanolayers were synthesized through reduction of the metal oxides by C. They were ∼400nm in diameter, and the shell of the C layers was ∼5nm in thickness. The Fe particles were composed of mixture of body-centered cubic (BCC), α, and face-centered cubic (FCC), γ-phase, and the Co particles were composed of a FCC, α-phase. Maximum saturation magnetization of the Fe was 101Am2/kg and that of the Co was 136Am2/kg. Those C-encapsulated particles showed excellent soft magnetic properties and oxidation resistance in air.

Polymer derived Si–C–B–N ceramics via hydroboration from borazine derivatives and trivinylcyclotrisilazane

2,4,6-Trimethyl-2,4,6-trivinylcyclotrisilazane (VSZ) and borazine (B 3N 3H 6) were used as starting materials to synthesize a preceramic polymer for a Si–C–B–N ceramic. The product was obtained as a soluble gel polymer by hydroboration between vinyl and B–H groups at 0 °C to room temperature for 30 h by the standard Schenk technique. The polymer-to-ceramic conversion at a ceramic yield of 70–75% was investigated by simultaneous TG/DTA, 13C and 29Si MAS-NMR, IR, XRD, TEM and XPS. The polymer-derived SiCBN ceramic remained as an amorphous solid up to 1400 °C, and crystallized slightly at 1800 °C to show mixed XRD patterns for Si 3N 4, SiC and BN. The ceramic product obtained at 1000–1400 °C exhibited excellent oxidation resistance in air. In addition, it was demonstrated that dense and smooth ceramic films were readily prepared by spin-coating the liquid polymer.

Crystal growth and some properties of REMn 2Si 2 (RE=Y, Tb, Dy, Ho)

Single crystals of REMn 2Si 2 (RE=Y, Tb, Dy, Ho) were grown from a high-temperature lead metal solution in an argon atmosphere. Single crystals of REMn 2Si 2 were obtained in the form of thin plates with well-developed (001) faces. The as-grown REMn 2Si 2 crystals were used for measurements of micro-Vickers hardness at room temperature, oxidation resistance in air and magnetic susceptibility at low temperatures. The values of the micro-Vickers hardness for the (001) faces of REMn 2Si 2 crystals are about 5.5 GPa. The oxidation process of REMn 2Si 2 crystals was studied at a temperature below 1473 K by TG-DTA analyses. The TG curves show that the oxidation of YMn 2Si 2, TbMn 2Si 2, DyMn 2Si 2 and HoMn 2Si 2 crystals starts at about 811, 807, 844, and 849 K, respectively. Weight gains of the compounds after TG determination were measured to be in the range of 14.7–19.7 mass%. The results of magnetic susceptibility measurements at low temperatures of the compounds are discussed.

Two-step Cr and Al diffusion coating on TiAl at high temperatures

The formation of an oxidation resistive coating layer on a TiAl alloy was investigated with a two- step Cr (at 1573 K for up to 72 ks ) and then Al (at temperatures between 1273 and 1573 K for 36 ks) pack diffusion process at high temperatures. The coated TiAl was oxidized in air at 1173 K for up to 1252.8 ks under a thermal cycling condition. The Cr diffusion coating layer consisted of γ, β, and Laves phases, which were transformed during cooling from the β-phase formed at 1573 K. The coating layer formed by the Al diffusion at 1473 and 1573 K consisted of an outermost TiAl2, outer Al-rich γ-TiAl, intermediate γ, β, and Laves phases, and a diffusion zone; there is little Al-diffusion at 1273 and 1373 K. The TiAl coated by the two-step Cr and Al diffusion process at 1573 K showed very good oxidation resistance in air at 1173 K due to the formation of a protective α-Al2O3 scale. After oxidation for up to 1,252.8 ks the coating layer maintained a structure with three phases γ, Laves, and β, which acts as a diffusion barrier to Al and Ti.

Influence of siliconizing on the oxidation behavior of a γ-TiAl based alloy

Ti–48Al–1.3Fe-1.1V–0.3B (at.%) alloy was siliconized to improve its oxidation resistance by burying it in Si powder and heating at 1073, 1123, 1173 and 1273 K for 18 ks in a vacuum. The isothermal oxidation behavior of all the treated specimens was tested at 1173 K for 349.2 ks (97 h) in air using a thermobalance. To evaluate the practical performance of this measure, the cyclic oxidation behavior of the specimen, siliconized at 1273 K, was also examined at 1123 K for 1260 ks (350 h) in a simulated exhaust gas. The element distribution, phase composition, and morphology of the Si-modified layer, and the oxide scale were characterized by AES, XRD, GAXRD and SEM. The results indicated that the TiAl alloy siliconized above 1173 K shows excellent isothermal and cyclic oxidation resistance in air or simulated exhaust gas. When siliconizing temperature is below 1123 K, however, this beneficial effect was limited to the early stage of oxidation. A Si-rich layer which was identified as Ti 5Si 3 and the following an Al- rich zone are the major constituents in the Si-modified layer on the TiAl substrate if the siliconizing temperature is above 1173 K. The thickness of the above constituent layers increases with rising siliconizing temperature. Existence of Si and Al with high concentrations is detected in the oxide scale even after long-term oxidation. It is concluded that the existing Si-rich layer, probably amorphous silica, and large amounts of Al 2O 3 in the oxide scale along with the remaining of the newly formed phase such as Ti 5Si 3 are responsible for the significant improvement in the oxidation resistance.

The effect of Cr addition on mechanical and chemical properties of Ni 3Si alloys

The alloying behavior and microstructure of the Ni–Si–Cr ternary alloys were firstly characterized by optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) with electron probe analysis. The microstructures of the Ni–Si–Cr ternary alloys consisted of largely dispersed Ni 5Si 2 phase and finely precipitated Ni 3Si phase in Ni solid solution. Then, the high-temperature mechanical properties, three-point bend test, oxidation and corrosion properties were investigated. The Ni–Si–Cr ternary alloys showed significant strengthening over a wide range of temperatures, and also large compressive plastic deformation at high temperatures. The strength and fracture toughness at ambient temperatures were correlated with the volume fraction of the Ni 5Si 2 phase. The Ni–Si–Cr ternary alloys showed substantially improved oxidation resistance in air at 1173 K, comparing with the Ni 3Si and Ni 3(Si,Ti) alloys. Also, the Ni–Si–Cr ternary alloys showed corrosion resistance comparable to the Ni 3Si and Ni 3(Si,Ti) alloys.

Mechanical and oxidation properties of Ti– xFe– ySi alloys

New Ti-based alloys having reasonable mechanical properties and good oxidation resistance were designed by alloying with low cost elements of iron and silicon. From tensile tests at room temperature and 400 °C, Ti–4 wt.% Fe–(0.5–2)wt.% Si alloys were found to have the optimum combination of strength and ductility, which are comparable to the Ti–6Al–4V alloy. The precipitation of fine Ti-silicides and the reduction in α/β colony sizes are primarily responsible for the obtained mechanical behavior. The air oxidation resistance of Ti–4 wt.% Fe–(0.5–2)wt.% Si alloys between 700 and 1000 °C is far superior to that of the Ti–6Al–4V alloy, and even better than that of TiAl intermetallics. The formation of amorphous silica within the TiO 2-rich oxide layers increased the oxidation resistance significantly.

Oxidation Resistance of Ti5Si3 and Ti5Si3Zx at 1000°C (Z = C, N, or O)

The oxidation behavior of Ti5Si3+y (y=0 or 0.2) and Ti5Si3Zx (Z=C, N or O, x=0.25 or 0.5) was studied at 1000°C in air or argon–oxygen mixtures for up to 500 h. Ti5Si3 has poor oxidation resistance in air because of the formation of an oxide scale rich in rutile and subscale formation of TiN, TiSi, TiSi2 and Si. In contrast, Ti5Si3.2 has excellent oxidation resistance because of the formation of a silica scale. Samples with interstitial oxygen or nitrogen show only slight improvements in the early stages of oxidation, compared to Ti5Si3, which is in stark contrast to previous research. However, samples with interstitial carbon displayed excellent oxidation resistance at 1000°C, consistent with previous research.

Mechanical and chemical properties of Ni3Si and Ni3(Si,Ti) alloys multiphased by chromium addition

The alloying behaviour and microstructure of Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys were first characterised by optical microscopy, X-ray diffraction, and scanning electron microscopy with electron probe analysis. The microstructures of the Ni–Si–Cr ternary alloys consisted of large dispersed Ni5Si2 phase and finely precipitated Ni3Si phase in nickel solid solution, while the Ni–Si–Ti–Cr quaternary alloys consisted of finely precipitated Ni3(Si,Ti) phase and nickel solid solution. Then, the high temperature mechanical properties, bend strength, and oxidation and corrosion properties of the alloys were investigated. The Ni–Si–Cr ternary alloys showed significant strengthening over a wide range of temperatures, and also large compressive plastic deformation at high temperatures. The strength and fracture toughness at ambient temperatures were correlated with the volume fraction of Ni5Si2 phase. The Ni–Si–Ti–Cr quaternary alloys did not show increased yield strength, but exhibited improved tensile ductility and plasticity over a wide range of temperatures. Both Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys showed substantially improved oxidation resistance in air at 1173 K, compared with Ni3Si and Ni3(Si,Ti) alloys. Also, the Ni–Si–Cr ternary and Ni–Si–Ti–Cr quaternary alloys showed corrosion resistance comparable to that of the Ni3Si and Ni3(Si,Ti) alloys.

Materials for temperatures above 1500°C in oxidizing atmospheres. Part II: Experimental results on the oxidation of MoSi2 composites and coated RSiC

The topic Materials for temperatures above 1500 °C in oxidizing atmospheres is discussed in a 3 part publication. In the first part a literature survey had been performed leading to the conclusion that either particle or fiber reinforced MoSi 2 -based materials or RSiC coated with a MoSi 2 -based layer are suitable for applications at such high temperatures. In the present part such material systems are investigated experimentally at 1500 and 1600 °C with respect to their oxidation resistance in air. Particular interest was given to the influence of the Mo 5 Si 3 content in MoSi 2 and the influence of second phase particles in MoSi 2 consisting of ZrB 2 and SiC. Furthermore, the oxidation resistance of several dip-coatings on RSiC which were manufactured from a polysiloxane precursor by subsequent pyrolysis were also investigated. Part III which reports about contact corrosion between different materials at these temperatures will follow in the next issue.

Application of plasma chemical vapour deposition of TiN to HSS precision bearings

The paper reports the application of plasma chemical vapour deposition (PCVD) of TiN to high speed steel precision bearings. The surface metallurgical characteristics of the coatings were determined by employing surface and structural analytical techniques, i.e. scanning electron microscopy, Auger electron spectroscopy, and X-ray photoelectron spectroscopy. The layer adhesion of the coatings was evaluated by means of the scratch test. Experiments on oxidation and thermal shock resistance were also carried out. The results demonstrated good adhesion between TiN coating and high speed steel substrate. Test results showed that using PCVD, uniform coatings can be achieved having good oxidation resistance in air and good thermal shock resistance between 580°C and room temperature. The tests showed that bearings coated with PCVD TiN worked normally and satisfied the working conditions.

A Microstructural Study of the Al-Cr-Ru System

Abstract Work undertaken on the Al-Cr-Ru system has shown excellent oxidation resistance in air up to 1500°C for compositions between Al20:Cr60:Ru20 and Al35:Cr32:Ru33 (at.%). Since no phase diagram data for the Al-Cr-Ru system have been published to date, an investigation was initiated. This part of the investigation is to identify the phases which form on solidification, and to derive the liquidus surface for alloys containing in excess of 50 at. % aluminium. Samples were prepared as arc-melted buttons from 99.9% pure elements, and were analysed in the as-cast state in the SEM using EDS with elemental standards, and by X-ray diffraction. The B2 phase, based on AlRu, extends to at least 25 at. % Cr. The Al3Ru2 phase has not been identified, except perhaps as a thin peritectic layer which was too fine to analyse. This is not surprising, since it exhibits very limited extension in other Al- Ru-X ternary systems, and in the Al-Ru binary system reacts peritectically forming AlRU2, and also subsequently decomposes at lower temperatures.

Sintering and physicochemical properties of compositions in the SiC–Al 2O 3–La 2O 3–Cr 2O 3 system II. Oxidation resistance of pressureless sintered compacts in the SiC–Al 2O 3–La 2O 3–Cr 2O 3 system

Compacts prepared from SiC (70–55 wt%), Al 2O 3 (10 wt%) and LaCrO 3 (20–35 wt%) by pressureless sintering in carbon at 1700°C were studied for their oxidation resistance in air at 1300, 1400 and 1500°C. The oxidation resistance of the material at 1500°C was dependent on the quantity of La 2O 3–Al 2O 3–SiO 2 melt, which diffused from inside to the surface of the compacts. Instead, chromium compounds were retained in the material and barred the melt diffusion. Nevertheless, at 1300°C, La 2O 3 of the compacts reacted with SiO 2 and gave a dependable protective layer of lanthanum silicates.

ゾルゲル法酸化物を添加した新規なプレセラミックポリマーを前駆体とするSi-M-C-(O)繊維の開発

A new SiC-based continuous fiber containing a small amount of sol-gel-derived oxide material(zircon) has been developed. The new inorganic continuous fiber has a better oxidation resistance in dry and wet air at high temperatures than those of the current SiC-based continuous fibers (Tyranno ZMI). The precursor polymer was prepared by polymerizing the current precursor polymer with a sol-gel-derived alcoholic solution. The production process of the newly developed Si-M-C-(O) fiber (Tyranno ZX) from its precursor polymer was almost the same as that of the currently available Tyranno Fiber (ZMI). Although the productivity is open to improvement, fiber diameter, tensile strength and tensile modulus of Tyranno ZX fiber were 11 μm, 3.0 GPa and 190 GPa, respectively. The oxidation resistance of Tyranno ZX at 1273-1773 K in wet air was excellent, compared to the current polymer-derived SiC-based fiber (Tyranno ZMI). Since the addition of an oxide material to the SiC-based fibers was shown to improve the fiber properties, we are currently investigating the addition of other oxide material, Al2O3, in order to achieve fibers with much higher thermal stability by using the sol-gel method.

Synthesis of Silicon Carbide Ceramics Using Chemically Modified Polycarbosilanes as a Compaction Binder

A chemically modified polycarbosilane (PC) containing organofluoric groups (PCOCF) has been synthesized from PC and fluoroalkylmethyldimethoxysilane. PCOCF acts as an efficient compaction binder for SiC powders and as a coating material with excellent oxidation resistance in wet air. PCOCF‐coated SiC powders also show excellent packing properties because of the organofluoric side chains, which give highly dense green compacts. PCOCF provides a high ceramic yield of 75% and highly dense SiC ceramics. Four‐point bending strength increases and the scatter in strength values decreases significantly by PCOCF coating.