A multilayer coating of dense SiC alternated with porous Si-Mo for the oxidation protection of carbon/carbon silicon carbide composites

In order to improve the oxidation behavior of carbon/carbon silicide carbide composites prepared by liquid silicon infiltration of carbon/carbon porous preforms, a multilayer coating of dense SiC alternated with porous Si-Mo was prepared by chemical vapor deposition combined with slurry painting. Oxidation test showed that weight loss of the coated sample was only 0.25% after 150 h oxidation in air at 1673 K. And the coated sample gained weight in the course of 46 cycles of thermal shock test between 1673 K and 373 K. The coating remained intact during the two kinds of tests and no obvious failure was found. The excellent oxidation protective ability and thermal shock resistance of the SiC/Si-Mo coating can be attributed to the alternated structure.     

Development of a Self-Forming Ytterbium Silicate Skin on Silicon Nitride by Controlled Oxidation

A dense and uniform polycrystalline ytterbium silicate skin on silicon nitride ceramics was developed by a controlled oxidation process to improve the hot corrosion resistance of silicon nitride. The process consists of purposely oxidizing the silicon nitride by heating it at high temperatures. It was found that the ytterbium silicate phase was formed as an oxidation product on the surface of the silicon nitride when it was exposed to air at temperatures above 1250°C. The volume fraction of ytterbium silicate compared with that of SiO₂ on the silicon nitride surface increased with increasing oxidation time and temperature. The formation and growth of ytterbium silicate on the surface of silicon nitride is attributed to a nucleation and growth mechanism. Ultimately, a dense and uniform ytterbium silicate skin with 3–4 μm of skin thickness was obtained by oxidation at 1450°C for 24 h. The ytterbium silicate layer, formed by oxidation of the silicon nitride, is associated with the reaction of SiO₂ on the surface of silicon nitride with Yb₂O₃ introduced in the silicon nitride as a sintering additive. Preliminary tests showed that the ytterbium silicate skin appears to protect silicon nitride from hot corrosion. No observable evidence of a reaction between the skin and molten Na₂SO₄ was found when it was exposed to molten Na₂SO₄ at 1000°C for 30 min.     

Fracture of Yttria-Doped, Sintered Reaction-Bonded Silicon Nitride

Flexural strength of an yttria-doped, slip-cast, sintered reaction-bonded silicon nitride was evaluated as a function of temperature (20° to 1400°C in air), applied stress, and time. Static oxidation at 700o to 1400°C was investigated in detail; in tests at 1000°C in air, the material showed anomalous weight gain. Flexural stress-rupture testing a 800° to 1200°C in air indicated that the material is susceptible to stress-enhanced oxidation and early failure. Fractographic evidence for time-dependent and -independent failures is presented.     

Crack-healing in ytterbium silicate filled with silicon carbide particles

A study on the influence of the silicate composition on the self-healing ability of a promising environmental barrier coating (EBC) material is reported. The EBC material consists of silicon carbide particles dispersed in a ytterbium silicate matrix. The composition of the silicate was varied from pure monosilicate to pure disilicate including a mixture of 50 vol-% of both phases. Pre-cracked specimens were mechanically tested before and after thermal annealing in air at 1400 °C accompanied by microstructural investigations. It is shown, that the self-healing mechanism is based on the application of compressive surface stresses due to graded oxidation of the SiC particles. With increasing amount of disilicate strength and self-healing ability increase.     

Dry air cyclic oxidation of mixed Y/Yb disilicate environmental barrier coatings and bare silica formers

Mixed Y and Yb disilicate coatings (Y/Yb)DS have been proposed as dual function thermal and environmental barrier coatings (EBCs) for protecting SiC-based ceramic matrix composites in gas-turbine environments. As an initial step, the 1350 °C dry air cyclic oxidation of atmospheric plasma sprayed (Y_1.2/Yb_0.8)DS and ytterbium disilicate/ytterbium monosilicate (YbDS/YbMS) EBCs deposited onto Si bond coatings was compared. As a baseline for evaluating EBC oxidant permeability, the dry air cyclic oxidation scale growth rates for bare silica formers (SiC, Si) were also measured and were consistently higher than rates previously measured after isothermal oxidation. Regarding Si bond coat oxidation rates underlying (Y/Yb)DS and YbDS/YbMS EBCs, the thinner silica scale formed under the thinner and denser (Y/Yb)DS coatings suggested a lower oxidant permeability than YbDS/YbMS. After 500 1-h cycles, the (Y/Yb)DS coating was comprised of only the β-polymorph disilicate and minor amounts of the X-2 phase monosilicate phase. Negligible differences in oxidation kinetics for (Y/Yb)DS coatings over the 90 – 240 µm thickness range were observed.     

Contribution of oxidation to the wear of carbon-carbon composites

An investigation was made of the effects of oxidation on the wear of carbon-carbon composites for aircraft brake materials. Wear tests were performed on samples under various oxygen partial pressures, and measurements were made of wear rates in weight and thickness. Wear rate was found to increase with increasing oxygen partial pressure; the wear in air was more than 4.5 times the wear in an inert atmosphere under the conditions used in this study. At least 60% of the total weight loss was caused by oxidation, that is, direct removal by conversion of the solid to CO and CO₂. Furthermore, the oxidation weakened the surface and subsurface, which in turn facilitated material removal by mechanical action at the rubbing interfaces.     

Steam oxidation and microstructural evolution of rare earth silicate environmental barrier coatings

A primary failure mode for environmental barrier coatings (EBCs) on SiC ceramic matrix composites (CMCs) is the oxidation of the intermediate Si-bond coating, where the formation of SiO₂ at the bond coating–EBC interface results in debonding and spallation. This work compares the microstructure evolution and steam oxidation kinetics of the Si-bond coating beneath yttrium/ytterbium disilicate ((Y/Yb)DS) and ytterbium disilicate/monosilicate (YbDS/YbMS) EBCs to better understand the impact of EBC composition on oxidation kinetics. After 500 1-h cycles at 1350°C, (Y/Yb)DS displayed a decreasing concentration of the monosilicate minor phase and increasing concentration of porosity as furnace cycling time increased, whereas the YbDS/YbMS EBC displayed negligible microstructural evolution. For both EBC systems, thermally grown oxide growth rates in steam were found to increase by approximately an order magnitude compared to dry air oxidation. The (Y/Yb)DS EBC displayed a reduced steam oxidation rate compared to YbDS/YbMS.     

Degradation at 1200°C of a SiC coated 2D-Nicalon/C/SiC composite processed by SICFILL® method

The thermal stability of a 2D-Nicalon/C/SiC composite was studied through the variation of both mechanical properties and microstructure occurring during heat treating. The composite was processed by infiltration of SiC preforms according to SICFILL® method. The material toughness was enhanced by a carbon interphase put between the fibers and the matrix. In order to improve the thermal stability a CVI layer was deposited on the carbon interphase and the specimen surfaces were CVD covered by an external SiC seal coating about 165 μm thick. The aging tests were carried out at 1200°C in air or in non oxidizing environment (vacuum). Other specimens were thermally cycled between 25 and 1150°C. Three point bending tests and Charpy impact measurements were performed before and after these treatments. The composite microstructure was investigated by scanning electron microscope (SEM), electron probe microanalysis (EPMA), X-ray diffraction (XRD), reflectance infrared spectroscopy (FTIR) and surface area BET measurements. The as-processed material showed a modulus of rupture (MOR) of 483 MPa and appreciable toughness. These characteristics were retained after aging (200 h at 1200°C) under vacuum. Air thermal treatments caused heavy loss of strength and increase of brittleness. Strong oxidation occurred during these last treatments at both the carbon interlayer and the matrix, while the SiC external sample coating was not oxidized. The oxygen needed for composite bulk oxidation flowed through the SiC coating due to the occasional presence of very few structural defects.     

Degradation of ytterbium disilicate environmental barrier coatings in high temperature steam atmosphere

The use of Ceramic Matrix Composites (CMCs) in the hottest part of an aero engine promises great improvements in fuel efficiency by decreasing component weight and allowing higher gas inlet temperatures. However, an environmental barrier coating (EBC) is required to protect the CMC from the corrosive water vapour contained in the combustion environment.Here, CMC specimens were coated with a silicon bond coat and ytterbium disilicate (Yb₂Si₂O₇) layer using air plasma spraying. The specimens were subsequently exposed to a water steam environment at 1350 °C for hundreds of hours. Stress evolution and phase stability were measured throughout to observe possible degradation. Cross-sectioning of the samples revealed the occurrence of sintering, the formation of a thermally grown oxide along the silicon/EBC interface, and a reaction between the ytterbium disilicate and silica. However, no coating failure was observed, even after 750 h of isothermal exposure to the hot steam environment.     

The effect of YB4 addition in ZrB2-SiC composites on the mechanical properties and oxidation performance tested up to 2000 °C

The influence of YB₄ and Y₂O₃ on densification, mechanical properties and oxidation performance of ZrB₂-SiC (ZS) composite was studied. The oxidation tests were performed in static air up to temperature of 1650 °C for 1 h as well as under dynamic conditions of oxyacetylene torch at 2000 °C. Static oxidation of ZS led to the formation of protective silica-based glass on the surface. However, ablation tests showed absence of silica in ablation centre. Only dense zirconia layer was left on the top of ZS. Composites with Y-containing additives exhibited significantly inferior oxidation performance in static conditions, since severe spallation and deeper degradation of the material were observed. On the contrary, the depth of material degradation after ablation was comparable with ZS. Samples were covered by solid solution of zirconia and yttria. Due to very low vapor pressure, yttria-based oxidation products are of interest considering even higher application temperatures exceeding 2000 °C.     

Stress Rate Dependence of Fracture Strength in Pre-Cracked Silicon Nitride Ceramics Doped with Ytterbium Oxide

High-temperature dynamic fatigue behavior has been investigated in 6 wt% ytterbium oxide and 2 wt% alumina-doped silicon nitride ceramics by nitrogen gas pressure sintering. The specimens were pre-cracked by Vickers indentation to prevent creep damage and to ensure dynamic fatigue dominating. The tests were performed in four-point flexure in air at temperatures of 1000°, 1200°, 1300°, and 1400°C and by varying the loading rate from 1, 0.5, 0.1–0.01 mm/min at each temperature. The analyses were conducted by plotting fatigue stress against loading rate at each testing temperature in double logarithm coordinates. The material was found to be the least susceptible (the highest slow crack exponent number N ) to slow crack growth at 1200°C, as reflected by the comparison of the plot slopes for the four testing temperatures. The explanation and analyses take into consideration the grain-boundary phase crystallization, crack healing, and oxidation during testing evidenced by X-ray diffraction and transmission electron microscopy. The fracture surfaces were characterized by three well-defined zones, namely zone I, II, and III, referring to the pre-cracked area, slow crack growth area, and fast fracture area, respectively.     

The oxidation of dispersed refractory metal compounds and their behavior as carbon oxidation catalysts

The oxidation behavior of a number of finely dispersed refractory metal borides, carbides, nitrides and silicides has been studied in flowing air at temperatures up to 1000°C. Most of the materials investigated were rapidly converted to oxides under these conditions, but the carbide, nitride and silicide of chromium were more oxidation resistant. Several of these compounds (particularly those of V and Mo), were very active catalysts for the oxidation of graphite in this temperature range, but the presence of boron oxide reduced gasification rates in most cases. Compounds of Zr, Si, Al, Ti, Hf and Nb had little or no catalytic activity for graphite oxidation, whereas the borides of Zr, Si, Cr and Al exerted an inhibiting effect.     

Particle-Filled PHPS Silazane-Based Coatings on Steel

Precursor-based composite coatings were developed as barrier coatings on steel against oxidation and corrosion using a silazane in the system SiN as a polymer material and BN particles as a passive filler. After thermal treatment in air up to 800°C, dense and very well adhered ceramic composite SiNO/BN coatings with a thickness of 12 μm were achieved. These were investigated by means of scanning electron microscopy, energy-dispersive spectrum, glow discharge optical emission spectroscopy, and adhesion measurements. Static oxidation tests on coated mild steel substrates up to 700°C displayed parabolic oxidation kinetics and a reduced weight gain by two orders in magnitude compared with uncoated substrates.     

Oxidation behavior of C–SiC composites with a Si–W coating from room temperature to 1500°C

Oxidation tests of carbon fiber reinforced silicon carbide composites with a Si–W coating were conducted in dry air from room temperature to 1500°C for 5 h. A continuous series of empirical functions relating weight change to temperature after 5 h oxidation was found to fit the test results quite well over the whole temperature range. This approach was used to interpret the different oxidation mechanisms. There were two cracking temperatures of the matrix and the coating for the C–SiC composite. Oxidation behavior of the C–SiC composite was nearly the same as that of the coated C–C composite above the coating cracking temperature, but weight loss of the C–SiC composite was half an order lower than that of the coated C–C composite below the cracking temperature. As an inhibitor, the SiC matrix increased the oxidation resistance of C–SiC composites by decreasing active sites available for oxidation. As an interfacial layer, pyrolytic carbon decreased the activation energy below 700°C. From 800°C to 1030°C, uniform oxidation took place for the C–SiC composite, but non-uniform oxidation took place for the coated C–C composite in the same temperature range. The Knudsen diffusion coefficient could be used to explain the relationship between weight loss and temperature below the coating cracking temperature and the matrix cracking temperature.     

Oxidation Behavior and Flexural Strength of Aluminum Nitride Exposed to Air at Elevated Temperatures

The oxidation behavior of a sintered aluminum nitride containing 3 wt% Y₂O₃ as a sintering aid was investigated. Samples were exposed to air at elevated temperatures for times up to 100 h. The weights of the samples were continuously monitored during exposure at various temperatures and humidity levels. The effects of oxidation on room-temperature flexural strength were also determined, and correlated to the observed weight changes of the samples. At temperatures 1200°C, linear weight gains were observed. However, at temperatures above 1200°C, the weight gains became parabolic with respect to exposure time. The oxidation rates were significantly increased by water vapor in the air. The oxidation products were found by X-ray analysis to be a mixture of Al₂O₃ and 5A1₂O₃·3Y₂O₃. The oxide layer formed on the surface was severely cracked because of the thermal expansion mismatch between the oxide layer and the substrate. The cracks initiated in the oxide layer and propagated into the substrate, resulting in severe reduction in the room-temperature flexural strength of the material. When exposed to ambient air for more than 50 h at temperatures greater than 1100°C, the strengths of the samples decreased to less than half that of the as-received material.     

浸渍复合磷酸盐法石墨模具的抗氧化处理

针对热压烧结金刚石工具用石墨模具,采用浸渍复合磷酸盐法进行抗氧化处理,通过氧化腐蚀性能的研究,分析讨论了石墨模具抗氧化处理前后的抗氧化性能等。结果表明：抗氧化处理后的石墨模具,基本保持石墨的电学特性,其抗氧化性和抗压强度有明显提高,700℃下空气中氧化失重率为6％左右的氧化时间从5h提高到80h以上。     

Model and Simulation of a Ported Kiln for Iron Oxide Pellet Induration

A mathematical model of a ported kiln for iron oxide pelletizing was developed to simulate the effects of under‐bed air injection on kiln fuel requirements and magnetite oxidation. A tanks‐in‐series model was used to set up material and energy balance equations for countercurrent flow of solids and gas through the kiln. The diffusion‐limited, shrinking core model was used for the magnetite oxidation reaction kinetics. The port air distribution in the model was adjusted by global optimization to minimize the fuel requirement. The simulation results show that magnetite oxidation may be completed in the kiln with significant fuel savings.     

Integrated Wet Air Oxidation and Biological Treatment of Polyethylene Glycol-Containing Wastewaters

The treatment of a model wastewater containing polyethylene glycol (PEG) of molecular weight 10000 in an integrated wet air oxidation–aerobic oxidation process was investigated. Partial wet air oxidation under mild operating conditions was capable of converting the original polymer to lower molecular weight compounds, such as oligomers and short-chain organic acids. The effect of molecular weight on the aerobic biodegradability of polyethylene glycol was assessed by performing shake flasks experiments with various polyethylene glycols in a molecular weight range from 200–35000 and it was found that biodegradability generally decreased with increasing molecular weight. Aqueous solutions of PEG 10000 were subjected to continuous wet air oxidation at a temperature of 423 K and a residence time of 30 min and the oxidised effluents were then subjected to subsequent continuous aerobic oxidation at residence times varying between 0·5 and 4 days. Simultaneously, continuous aerobic oxidation experiments on solutions of PEG 10000 were performed and used to compare the efficiency of the integrated treatment process with that of the direct biological treatment. Partial pretreatment by wet air oxidation under mild operating conditions resulted in effluents whose biodegradation rates were significantly higher than those of the original, unoxidised wastewater, as assessed by total organic carbon (TOC) removal rates. The original wastewater was practically non-biodegradable at a 0·5-day residence time with direct biological treatment, while an 80% TOC removal was achieved when biological treatment was coupled with a preceding wet air oxidation step. Conversely, with a 4-day residence time for the direct biological treatment the original wastewater was only partially degradable with 60–70% TOC removal recorded; with the integrated treatment process at a 4-day residence time in the bioreactor the overall TOC removal was greater than 90%. © 1997 SCI     

Structure and Chemistry of Oxide Films Thermally Grown on Molybdenum Silicides

The oxides grown during thermal oxidation of molybdenum silicides were studied using electron microscopy, infrared absorption, and thermogravimetric techniques. Analysis of the oxidation mechanism based on thermochemical and diffusion rate data was correlated with experimental results. Because of the higher oxidation potential of silicon, it may be preferentially depleted from the silicide substrate to form a single-phase silica layer. Whether this occurs depends also on the relative diffusion rates in the oxide and substrate layers. Alternately, a duplex oxide layer containing both silica and molybdenum oxide phases will form. The molybdenum oxide phase is a necessary condition for the silicide pest, a low-temperature failure mechanism in which oxidation results in a nonadherent powder rather than in a protective film.     

Oxidation of nitride bonded silicon carbide in wet air atmosphere

Abstract

The oxidation behaviour of porous nitride bonded silicon carbide (NBSC) ceramics at 1000 and 1200°C in air and wet air (20% water vapour by volume) was investigated. A rapid linear oxidation rate occurred during initial exposure (1-3 h) to air and water vapour and this was found to be followed by either parabolic or asymptotic scale growth. Water vapour influences the oxidation of NBSC: compared to oxidation in air, in wet air a higher weight gain is observed resulting from enhanced permeation of H₂O molecules through the scale, and devitrification of amorphous silica is also enhanced.