High-temperature oxidation of silicon nitride-based ceramics by water vapour

Hot-pressed Si₃N₄, sintered Si₃N₄ and three kinds of sialon with different compositions were oxidized in dry air and wet nitrogen gas atmospheres at 1100 to 1350° C and 1.5 to 20 kPa water vapour pressure. All samples were oxidized by both dry air and water vapour at high temperature, and formed oxide films consisting of SiO₂, Y₂Si₂O₇ and Y4A1209. The oxidation rate was in the order sialon > sintered Si₃N₄ > hot-pressed Si3N4. The oxidation rate of sialon increased with increasing Y₂O₃ content, and oxidation kinetics obeyed the usual parabolic law. The oxidation rates in dry air and wet nitrogen were almost the same: the rate in wet nitrogen was unaffected by water vapour pressure above 1.5 kPa. The activation energy was about 800 kJ mol^–1.     

Oxidation behaviour of the sintered Si3N4-Y2O3-Al2O3 system

The oxidation behaviour of silicon nitride composed of Si₃N₄, Y₂O₃, Al₂O₃, AlN and TiO₂ was investigated in dry and wet air at 1100–1400 °C. The oxidation rates were confirmed to obey the parabolic law. An activation energy of 255 kJ mol^–1 was calculated from the Arrhenius plots of the results of oxidation in an air flow. In still air the oxidation rate was larger than that in an air flow, but the oxidation rate in flowing air was not affected by the air flow rate. -cristobalite and Y₂O₃·2SiO₂ were formed in oxidized surface layers. These crystal phases increased with increasing oxidation temperature. In particular, a higher content of -cristobalite was obtained in still air oxidation. The existence of water vapour in flowing air greatly promoted the oxidation.Concurrent with Kanagawa Academy of Science and Technology.     

Oxidation of silicon carbide in a wet atmosphere

The effects of water vapour on oxidation of pressureless-sintered silicon carbide containing alumina as a densifying aid were studied in a wet air flow with 10, 20, 30 and 40vol% H₂O at 1300° C for 100h. The oxidation kinetics were determined in a wet air flow with 20 vol % H₂O and in a dry air flow at 1300° C for times up to 360 h. The weight gain on oxidation showed an increasing tendency with increasing water vapour content. Water vapour in the atmosphere strongly influenced oxidation and accelerated the reaction. Oxidation in a wet atmosphere proceeded in a diffusion-controlled manner, in the same process as that for the dry atmosphere. No remarkable differences were noticed in the microstructure of the oxide layer and the surface roughness between the samples oxidized under the four wet conditions, but the surface roughness increased with increasing oxidation time. Water vapour evidently accelerated the devitrification of amorphous silica and promoted oxidation. Oxidation in a wet atmosphere (10 to 40 vol % H₂O for 100 h and 20 vol % H₂O up to 360 h) had a slight degrading effect on the flexural strength. The microstructure or surface roughness of the oxide layer formed during oxidation presumably had very little effect on the room-temperature strength.     

Oxidation resistance of silicon nitride ceramics with various additives

Five silicon nitride ceramics with various additives were evaluated for their oxidation resistance at 1300 ° C in flowing dry and wet air. In a dry atmosphere, the oxidation of all types of Si₃N₄ except one proceeded in two stages with different reaction rates nearly following the parabolic law. In a wet atmosphere, all types showed a linear relationship between weight gain by oxidation and water varpour content. The water vapour in the atmosphere slightly accelerated the oxidation. The influence of oxidation on room-temperature strength was complex, but there was no observed distinguishable difference of the effect on the flexural strength between dry and wet oxidation.     

Effect of water vapor on the mechanical behaviors of hot isostatically pressed silicon nitride containing Y2O3

The effect of water vapor on the mechanical behavior of Si₃N₄ ceramics was studied. Strength measurements by flexural dynamic fatigue tests were made at temperatures from 1038°C to 1350°C and at actuator speeds of 8.4 × 10^–2 and 8.4 × 10^–5 mm/s (200 and 0.2 MPa/s). Step stress rupture tests were also performed at 1288°C and 1150°C. Water vapor had a beneficial effect on the flexural strength due to flaw healing, and/or blunting mechanisms. Dynamic fatigue results demonstrated that the beneficial effects of water vapor on the strength increases as temperature increases and/or loading rate decreases. Time-to-failure was always longer in wet air during step stress rupture testing. Creep crack growth by formation and coalescence of cavities ahead of the crack tip generated from the oxidation pits or subsurface pores were the primary mechanism for slow crack growth for NT 164 Si₃N₄.     

Oxidation kinetics of hexagonal boron nitride powder

The isothermal oxidation of hexagonal boron nitride powders was carried out at 900–1050°C in dry oxygen and air. The oxidation kinetics were found to obey linear rate law and were described by the surface chemical reaction-controlled shrinking cylindrical model. The apparent activation energies were found to be 298 and 330 kJ mol^–1in dry oxygen and air, respectively. The oxygen partial pressure dependence of the oxidation rate constant at 1000 °C is well represented by a Langmuir-type equation. Microscopic examination of the oxidized sample after removal of the oxide scale with water suggested that the rate of attack by oxygen was determined by an anisotropy due to the crystallographic direction, similar to oxidation in graphite. The volatilization of B₂O₃ was observed only in dry oxygen and obeyed a linear rate law, and was found not to affect the oxidation reaction.     

Oxidation of aluminium nitride

The oxidation kinetics of polycrystalline aluminium nitride substrates in air at temperatures in the range 1150 to 1750°C have been studied by measuring the weight increase in the oxidized samples. At the lowest temperature, the oxide layer was not continuous on the AlN surface and the oxidation kinetics followed a linear rate law with an activation energy of 175 kJ mol^–1. At all the higher temperatures, the growth kinetics followed a parabolic rate law with an activation energy of 395 kJ mol^–1. Samples oxidized at these higher temperatures were covered with a dense oxide layer having a fine-grained microstructure.     

Effect of TiO2 addition on oxidation behavior of sintered bodies composed of Si3N4-Y2O3-Al2O3-AlN

The effect of TiO₂ content on the oxidation of sintered bodies from the conventional Si₃N₄-Y₂O₃-Al₂O₃-AlN system was investigated. Sintered specimens composed of Si₃N₄, Y₂O₃, Al₂O₃, and AlN, with a ratio of 100 : 5 : 3 : 3 wt% and containing TiO₂ in the range of 0 to 5 wt% to Si₃N₄, were fabricated at 1775 °C for 4 h at 0.5 MPa of N₂. Oxidation at 1200 to 1400 °C for a maximum of 100 h was performed in atmospheres of dry and wet air flows. The relation between weight gain and oxidation time was confirmed to obey the parabolic law. The activation energies decreased with TiO₂ content. In the phases present in the specimens oxidized at 1300 °C for 100 h in dry air, Y₃Al₅O₁₂ and TiN, which had existed before oxidation, disappeared. Alpha-cristobalite and Y₂O₃·2TiO₂ (Y2T) appeared in their place and increased with increasing TiO₂ content. In those oxidized at 1400 °C, -cristobalite was dominant and very small amounts of Y₂O₃·2SiO₂ and Y2T were contained. There was a tendency for more -cristobalite to form in oxidation in wet air than in dry air. Therefore, moisture was confirmed to affect the crystallization of SiO₂ formed during oxidation. Judging from the lower activation energy, the crystallization, and the pores formation, we concluded that the addition of TiO₂ decreases oxidation resistance.     

Oxidation of silicon nitride in a wet atmosphere

The effect of water vapour on oxidation was studied with hot-pressed silicon nitride containing both yttria and alumina as sintering aids in wet air flow with 10, 20, 30, and 40 vol% H₂O at 1300°C for 100 h. The oxidation kinetics were determined in a wet air flow with 20 vol% H₂O and in a dry air flow at 1300°C for oxidation times up to 360 h. The water vapour in the atmosphere slightly influenced the oxidation and accelerated the reaction, and the weight gained on oxidation in a wet atmosphere had an increasing tendency with increasing water vapour content. The oxidation proceeded in a diffusion-controlled manner in both wet and dry atmospheres. The values of weight gained in wet oxidation varied to a greater degree than in dry oxidation. Water vapour had a strong effect on the devitrification of the amorphous oxide. This process was presumed to promote the rate of oxidation more than in dry atmosphere. The water vapour also had a strong roughening effect on the surface oxide layer grown during oxidation. The flexural strength at room temperature was degraded by oxidation in a wet atmosphere and it is presumed to be degraded by wet oxidation slightly and consecutively with time.     

Flexural strength of lapped and oxidized siliconized silicon carbide

A siliconized silicon carbide composite has been microstructurally characterized, oxidized in air at 1350 °C for times up to 1079 h, air-cooled, and tested in four-point bending in the lapped condition at various temperatures up to 1425 °C, and in the pre-oxidized condition at room temperature and at 1300 °C. The strength of the lapped specimens increased by 25% at temperatures up to 1350 °C. Oxidation always decreased the strength of the material. After 315 h oxidation, the strength at room temperature and 1300°C was reduced by 50% and 40%, respectively. Preferential oxidation of the inter-grain regions formed pits up to 50 m deep. Hot salt corrosion increased the amount of oxidation by nearly 800%, and formed pits about 100 m deep. Microstructural details of the oxidation and fracture processes are presented, and the possible mechanisms of failure discussed.     

Sintering and crystallization of mullite powder prepared by sol-gel processing

Mullite powder with the stoichiometric composition (3Al₂O₃.2SiO₂) was synthesized by a sol-gel process, followed by hypercritical drying with CO₂. Within the limits of detection by X-ray diffraction, the powder was amorphous. Crystallization of the powder commenced at 1200 °C and was completed after 1 h at 1350 °C. In situ X-ray analysis showed no intermediate crystalline phases prior to the onset of mullite crystallization and the pattern of the fully crystallized powder was almost identical to that of stoichiometric mullite. The synthesized powder was compacted and sintered to nearly theoretical density below 1250 °C. The microstructure of the sintered sample consisted of nearly equiaxial grains with an average size of 0.2 m. The effect of heating rate (1–15 °C min^–1) on the sintering of the compacted powder was investigated. The sintering rate increased with increasing heating rate, and the maximum in the sintering curve shifted to higher temperatures. The sintering kinetics below 1150 °C can be described by available models for viscous sintering.     

Oxidation behavior of hot isostatically pressed silicon nitride containing Y2O3

Oxidation in the presence of air and water vapor at high temperatures wasstudied for Si₃N₄ ceramics containing Y₂O₃ and Al₂O₃ as sintering aids.The test environments for this study consisted of air with 0, 1.2,and 6.4 v/o H₂O at temperatures from 1000°C to 1350°C. Theoxidation exposure times were up to 500 hours. The presence of water vaporenhances oxidation and crystallization of the oxidation phases. Weight losswas observed for the oxidation in air or dry air because of Na contaminationduring the fabrication processing. The effect of applied stress on the growthof oxide scale is minimal, however, the applied stress resulted in deeperpenetration of oxygen and pit formation in the oxide phase.     

Effect of manganese on the oxidation of Fe-Mn-Al-C alloys

The effects of manganese on the oxidation of alloys with the chemical composition (wt%) Fe-5AAl-1.5Mn-0.58C and Fe-5.3Al-3.5Mn-0.53C at 600, 800 and 1000° C in dry air were investigated. Kinetic curves were determined by thermogravimetric analyses. Optical metallography and electron probe microanalysis were used to examine the oxide scales. The kinetic curves of Fe-5.4Al-1.5Mn-0.58C alloy oxidized at 600, 800 and 1000° C had simple, three- and two-stage parabolic rate laws, respectively. On the other hand, two stages of linear rate law were observed in Fe-53Al-3ZMn-0.53C alloy when oxidized at 600° C, while two distinct parabolic rate laws were found in the same alloy oxidized at 800 and 1000° C. Oxidation behaviours and the oxide formation mechanisms of the alloys at different temperatures are discussed in this paper.     

The effects of matrix microcracking on the oxidation behaviour of carbon-fibre/glass-matrix composites

Carbon-fibre/glass-matrix composites were fabricated using Fortafil fibres and two different glass matrices: a sodium-borosilicate glass (CGW 7740), and a calcium-aluminosilicate glass (CGW 1723). Upon cooling from the hot-pressing temperature used to fabricate the composites (approximately 1250°C), the glass matrices cracked due to differences in the coefficients of thermal expansion between the fibres and the matrix. At elevated temperatures these cracks serve as short-circuit diffusion paths for oxygen transport, and the majority of the weight loss from the cracked samples was caused by oxygen diffusing along these microcracks and reacting with the fibres. Because of the relatively large diameter of these cracks compared to the mean free path for diffusing oxygen, traditional gas kinetics can be applied to the various transport processes occurring in the oxidation reactions, and there is no need to allow for capillary size or to apply Knüdsen diffusion. The composites made of 1723 glass exhibited linear relationships between specific-mass loss ( mass/initial exposed surface area of carbon fibres) and time at all oxidation temperatures (450, 500, 550 and 600 °C). With the 7740 composites, a parabolic relationship between specific-mass loss and time was obtained. As the oxidation temperature approached or exceeded the glass-transition temperature, T _g, for the 7740 composites (560 °C), this parabolic relationship became more pronounced. Microstructural evidence revealed that at temperatures near or exceeding the T _g for the 7740 glass the microcracks in the matrix heal, thereby decreasing the amount of fibre surface area available for chemical reaction. Because the rate of oxidation is directly proportional to the amount of available fibre-surface area, the weight-loss data appear parabolic with time. Additionally, the mechanism for the oxidation of the carbon fibres does not appear to change once the fibres are placed in a glass matrix. The apparent activation energy for oxidation remained constant at approximately 174 kJ mol^–1.     

Nitriding in the high temperature oxidation of Fe-31Mn-9AI-6Cr alloy

An alloy, of Fe-31.3Mn-8.92AI-5.96Cr-0.86C composition, was heated from 800 to 1000° C in atmospheres of oxygen, nitrogen and dry air respectively. A needle-like structure was observed between the alloy matrix and the external oxidation layer in the nitrogen-containing atmosphere at temperatures higher than 800° C. The needle-like phase was identified as AIN by both X-ray and STEM diffraction methods. Nitriding first occurred in the austenitic grains adjacent to the free surface, with subsequent AIN growth towards the alloy matrix. The ferrite phase, formed due to the precipitation of chromium carbide, prevented the growth of AIN. For the alloy oxidized in air, AIN formed and the growth front of AIN was ahead of the oxides. The aluminium content of the alloy matrix in the nitrided region was depleted by the formation of AIN. Due to the rapid nitriding of AI, the formation of a protective oxide layer was retarded and the oxidation resistance became less promising.Will be on leave to the Department of Materials Science and Engineering, Cornell University, as visiting scientist, after 1 October 1987.     

Oxidation behaviour of Ti6Al4V titanium alloy in oxygen

Abstract

The isothermal oxidation behaviour of two phase (α + β) titanium base alloy Ti6Al4V (coupons) has been studied at 1050, 1150, 1250, and 1340 K in O₂ gas at atmospheric pressure for 2, 4, 6, 8, and 12 h. Investigations on kinetic behaviour followed by the metallographic examination of oxidised scale morphology was carried out. Thermogravimetric data (weight gain v time) exhibited parabolic behaviour. Below 1250 K, the rate of oxidation substantially decreased after 8 h exposure, however, at 1340 K the oxidation rate was markedly high over the whole 12 h period. Parabolic rate constants were 0.234×10^-7, 3.67×10^-7, 10.72×10 ^-7, and 31.17×10^-7 kg² m^-4 s^-1 at 1050, 1150, 1250, and 1340 K respectively. The effective activation energy of oxidation was 88 kJ mol^-1. The instantaneous rate constant k _i exhibited a marked deviation from parabolic behaviour at high temperatures e.g. 1150, 1250, and 1340 K, however, k _i at lower temperature (1050 K) remained broadly unchanged with time exhibiting no deviation from parabolic behaviour. Metallographic observation of the sample coupons treated at 1340 K revealed an identical oxide scale morphology with increased thickness over the time.     

Oxidation behavior of TiC-containing Ti<Subscript>3</Subscript>AlC<Subscript>2</Subscript> based material at 500–900 °C in air

The isothermal oxidation behavior of Ti₃AlC₂ based material containing 5 vol% TiC inclusion in air had been investigated at 500–900 °C by means of TGA, XRD, Raman spectroscopy and SEM/EDS. It was demonstrated that, although Ti₃AlC₂ based material exhibited good oxidation resistance at temperatures above 700 °C, anomalous oxidation with higher oxidation rate occurred at lower temperatures of 500 and 600 °C. This interesting phenomenon was due to the formation of microcracks associated with the stress developed within the scales, mainly consisting of anatase, and the volume expansion as Ti₃AlC₂ based material was directly exposed to air at those temperatures. Its oxidation, at temperatures investigated with the exception of 600 °C, generally obeyed a parabolic rate law. The weight gain data for the remaining temperatures were analyzed with an instantaneous parabolic rate constant method by assuming a parabolic rate law. The variations of instantaneous parabolic rate constant with time reflected the complexity of the oxidation behavior of Ti₃AlC₂ based material. These variations were discussed from the viewpoint of the formation of microcracks at 500 °C, and preferred oxidation of TiC inclusion in the initial oxidation and its subsequent depletion at 800 and 900 °C on the basis of X-ray diffraction, Raman spectroscopy, SEM scale morphology observation and composition analysis using EDS. In addition, the deleterious effect of TiC inclusion on the oxidation resistance of Ti₃AlC₂ based material was also investigated and discussed with comparison to monolithic Ti₃AlC₂, which was helpful to understand the discrepancies in reports on the oxidation of Ti₃AlC₂.     

Oxidation, microstructure and metallization of aluminum nitride substrates

Oxidation kinetics and the effect of oxide scale on interfacial microstructure of polycrystalline aluminum nitride substrate metallized with conductor and resistor thick-film pastes have been investigated over an oxidation temperature range of 1150–1450 °C at 50 °C intervals in ambient air. At temperatures below 1150 °C, weight gain increased linearly with the isothermal time. The grown oxide layer was porous and was not protective at the temperature, revealing an interface-controlled mechanism. By contrast, the oxidation kinetics exhibited a parabolic behavior as the temperature was raised further above 1250 °C, which is indicative of a diffusion-controlled mechanism. The oxide scale effectively reduced the population of gas bubbles located at the film-substrate interface when coated with the resistor film on the AlN substrate, while the conductor paste provided a hermetic interface even when the AlN was not pre-oxidized. Elemental analyses showed that glass composition of the pastes, e.g., lead, was crucial in determining the interface defects. The differences in interface microstructure of the resistor and conductor metallization, as well as the implication of the thick-film structures to the formation of blisters at the interface, are discussed.     

Effect of silicon content in steel and oxidation temperature on scale growth and morphology

The effect of high silicon content in steel, 1.6 wt.%Si and 3.2 wt.%Si, and high oxidation temperatures (850–1200 °C) on scale growth rate and morphology were investigated. The steels were oxidized in a 15% humid air with short isothermal oxidation times (15 min). The scale growth rate of the non-alloyed steel follows a parabolic law with time; it is an iron diffusion controlled oxidation. The presence of silicon delays scale growth by forming a silica SiO₂ barrier layer at the scale/metal interface, this effect is more important for the steel containing 3.2 wt.%Si and induces a discontinuous scale. Silicon oxides are concentrated at the scale/metal interface; their morphology depends on the oxidation temperature. For temperatures lower than 950 °C, silica is formed. Between 950 °C and 1150 °C, fayalite (Fe₂SiO₄) grains appear in the wüstite matrix close to the scale/metal interface. For temperatures higher than 1177 °C, a fayalite–wüstite eutectic is formed; this molten phase favours iron diffusion leading to high scale growth. After cooling, a continuous fayalite layer with small wüstite grains is obtained at the scale/steel interface.     

Oxidation behavior of hot-pressed MoSi2-TiC composite

The oxidation behavior of a hot-pressed MoSi₂-TiC composite was investigated in air over the temperature range of 500 °C–1400 °C. The composite exhibits parabolic oxidation kinetics between 500 °C–800 °C, where the activation energy was calculated to be 32 kJ/mol. However, above 800 °C, it shows two-step parabolic oxidation kinetics, where the first stage (step I) was proposed to be dominated by the oxidation of surface TiC particles. After all the surface TiC particles are oxidized, the oxidation of the composite turns into the second stage (step II) which was controlled by the oxidation of MoSi₂. The activation energy was determined to be 130 kJ/mol and 141 kJ/mol for step I and step II, respectively. A dense cristobalite scale with TiO₂ in lath shapes on the top was observed after oxidation at 1400 °C.