Oxidation behaviour of particle reinforced MoSi2 composites at temperatures up to 1700°C. Part I: Literature review

In the first part of this paper the results of a literature review are presented. An overview of the oxidation behaviour in air and in combustion environments of both pure MoSi₂ and MoSi₂ composites in the temperature range from 400 to 1650°C is given. The second part of this paper reports about our results from oxidation tests with selected MoSi₂ composites (containing 15 vol.‐% Al₂O₃, Y₂O₃, ZrO₂, HfO₂, SiC, TiB₂, ZrB₂, or HfB₂, respectively) from different development stages at temperatures in the pest region as well as up to 1700°C. The third part describes the oxidation behaviour of the optimised MoSi₂ composites developed on the basis of the results from part II.     

Oxidation behaviour of particle reinforced MoSi2 composites at temperatures up to 1700°C.

Part III: Oxidation behaviour of optimised MoSi₂ composites The topic “Oxidation behaviour of particle reinforced MoSi₂ composites at temperatures up to 1700°C” is discussed in a three part publication. In the first part of this paper a literature survey on the oxidation behaviour of MoSi₂ and MoSi₂ composites has been given. In the second part an initial screening at 1600°C revealed those composites which may be suitable for high temperature applications. The low temperature oxidation behaviour of selected composites in the “pest” region was examined as well. Additionally, the effect of iron impurities on the high temperature behaviour of the composites was explained. The present part deals with a detailed investigation of an optimised MoSi₂/HfO₂ composite. These investigations include high temperature oxidation at 1500 to 1700°C and low temperature oxidation at 400 and 500°C.     

Oxidation of SiOC/MoSi2/SiC Composites Prepared by Polymer Pyrolysis

From the pre-ceramic polymer of polymethylsiloxane (PMS) and powders of MoSi₂, SiC and Si, new ceramic composites that consisted primarily of an amorphous SiOC matrix containing dispersed particles of MoSi₂ and SiC were synthesized. The composites displayed superior oxidation resistance at both high and low temperatures by forming SiO₂ on the surface. The thin, amorphous SiO₂ layer that formed initially gradually to crystallized during oxidation between 1000 and 1300°C. An outer highly porous and an inner superficial SiO₂ layer that formed from the initial stage of oxidation between 400 and 500°C protected the composites from pesting.     

MoSi2 oxidation resistance coatings for Mo5Si3/MoSi2 composites

In order to improve the oxidation resistance properties of 30 at.% Mo₅Si₃/MoSi₂ composite at high temperature in air, a molybdenum disilicide coating was prepared on its surface by a molten salt technology. XRD and SEM analysis showed that only tetragonal MoSi₂ phase existed in the coating after being siliconized for 5 h at 900°C. The oxidation film formed on the uncoated sample was not dense, so that oxygen diffused easily through it. The volatilization of MoO₃ resulted in the oxidation film separating from the substrate. The MoSi₂ coating was proved to be an effective method to prevent 30 at.% Mo₅Si₃/MoSi₂ composites from being oxidized at 1200°C. A dense glassy SiO₂ film was formed on the MoSi₂ coating surface, which acted as a barrier layer for the diffusion of oxygen atoms to the substrate. The 30at.% Mo₅Si₃/MoSi₂ composites with a MoSi₂ coating showed much better oxidation resistance at high temperature.     

Oxidation Behavior of ZrO2 Reinforced MoSi2 Composite Coatings Fabricated by Vacuum Plasma Spraying Technology

In this work, MoSi₂, MoSi₂-20 vol.% ZrO₂, MoSi₂-40 vol.% ZrO₂ (denoted, respectively, as MZ0, MZ2, and MZ4) coatings were fabricated by vacuum plasma spraying technology. The oxidation behavior of the coatings was examined at 500, 1200, and 1500 °C, respectively. Some basic properties of the coatings, including microhardness, porosity, and surface roughness were characterized. The tests at 500 °C showed that the pest oxidation phenomenon of MoSi₂ coatings was restrained by the addition of ZrO₂. The MZ2 coating exhibited excellent oxidation-resistant behavior both at 1200 and 1500 °C. However, the MZ4 coating presented the impaired oxidation-resistant behavior at 1500 °C, though the comparable oxidation property at 1200 °C was still obtained.     

Oxidation behavior of ZrB2 composites doped with various transition metal silicides

This paper deals with the oxidation behavior of ZrB₂-based composites sintered with different additives, namely ZrSi₂, MoSi₂, TaSi₂ and WSi₂. The oxidation mechanisms were investigated between 1200 and 1800 °C for 15 min in a bottom loading furnace. The scope of this study is to draw a classification of goodness for the 4 composites depending on the temperature range and understand how each cation influences the oxidation behavior of ZrB₂ by acting either on glass or on ZrO₂ modification. MoSi₂ was the best additive for improving the oxidation resistance of ZrB₂, even up to 1800 °C.     

Effects of Y2O3 on SiC/MoSi2 composite by mechanical-assistant combustion synthesis

MoSi₂ based materials are considered as a potential high temperature structural parts. In this work, a 0.5 wt% Y₂O₃–20 vol% SiC/MoSi₂ composite was successfully prepared by pressureless sintering from mechanical-assistant combustion synthesized powders. Adding a small amount of Y₂O₃ to the SiC/MoSi₂ composite decreased the apparent activation energy of sintering by 10.4%, resulting in a denser composite with finer grains. The relative density, flexural strength, Vickers hardness and fracture toughness of 0.5 wt% Y₂O₃–20 vol% SiC/MoSi₂ increased by 5.3%, 27.7%, 27.2% and 35.8% as compared to 20 vol% SiC/MoSi₂, respectively. The oxidation mass gain of Y₂O₃–20 vol% SiC/MoSi₂ at 1200 °C was higher than that of 20 vol% SiC/MoSi₂ for 16.9%, while it still exhibited very good oxidation resistance at this temperature.     

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 MoSi₂‐based materials or RSiC coated with a MoSi₂‐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 Mo₅Si₃ content in MoSi₂ and the influence of second phase particles in MoSi₂ consisting of ZrB₂ 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.     

Microstructure,sintering behavior and mechanical properties of SiC/MoSi2 composites by spark plasma sintering

SiC/MoSi₂ composites were synthesized at different temperatures by spark plasma sintering using Mo, Si and SiC powders as raw materials. The phase composition, microstructure and mechanical properties of the as-prepared composites were investigated and the sintering behavior was also discussed. Results show that SiC/MoSi₂ composites are composed of MoSi₂, SiC and trace amount of Mo_4.8Si₃C_0.6 phase and exhibit a fine-grain texture. During the synthesis process, there was an evolution from solid phase sintering to liquid phase sintering. When sintered at 1600 °C, the SiC/MoSi₂ composites present the most favorable mechanical properties, the Vickers hardness, bending strength and fracture toughness are 13.4 GPa, 674 MPa and 5.1 MPa·m^1/2, respectively, higher 44%, 171%, 82% than those of monolithic MoSi₂. SiC can withstand the applied stress as hard phase and retard the rapid propagation of cracks as second phase, which are beneficial to the improved mechanical properties of SiC/MoSi₂ composites.     

SiC–MoSi2/ZrO2–MoSi2 coating to protect C/C composites against oxidation

To improve the oxidation resistance of carbon/carbon (C/C) composites in air at high temperatures, a SiC–MoSi₂/ZrO₂–MoSi₂ coating was prepared on the surface of C/C composites by pack cementation and slurry method. The microstructures and phase compositions of the coated C/C composites were analyzed by scanning electron microscopy and X-ray diffraction, respectively. The result shows that the SiC–MoSi₂/ZrO₂–MoSi₂ coating is dense and crack-free with a thickness of 250–300 μm. The preparation and the high temperature oxidation property of the coated composites were investigated. The as-received coating has excellent oxidation protection ability and can protect C/C composites from oxidation for 260 h at 1773 K in air. The excellent anti-oxidation performance of the coating is considered to come from the formation of ZrSiO₄, which improves the stability of the coating at high temperatures.     

A MoSi2/SiC oxidation protective coating for carbon/carbon composites

To protect carbon/carbon (C/C) composites against oxidation, a MoSi₂ outer coating was prepared on pack-cementation SiC coated C/C composites by a hydrothermal electrophoretic deposition. The phase composition, microstructure and oxidation resistance of the prepared MoSi₂/SiC coatings were investigated. Results show that hydrothermal electrophoretic deposition is an effective route to achieve crack-free MoSi₂ outer coatings. The MoSi₂/SiC coating can protect C/C composites from oxidation at 1773 K for 346 h with a weight loss of 2.49 mg cm⁻² and at 1903 K for 88 h with a weight loss of 5.68 mg cm⁻².     

Quantitative Electron Microprobe Characterizations of Oxidized ZrB2 Containing MoSi2 Additives

This study reports the oxidation behavior of hot-pressed ZrB₂ ceramics, with 10, 20 and 40 vol.% MoSi₂ additives, exposed to dry air at 1500 °C for up to 10 h. The effect of the amount added MoSi₂ on oxidation resistance was assessed. Quantitative electron microprobe characterizations of the oxidized ZrB₂ with MoSi₂ additives were carried out with electron probe micro analysis. Parabolic oxidation behavior was observed for the three compositions. The oxidation resistance was significantly improved with MoSi₂ additives. Reaction-product phase compositions and phase distribution were thoroughly characterized from the oxidized surface through to the unreactive bulk material. It was found that the oxidation products consisted of nonstoichiometric amorphous SiO₂, stoichiometric crystalline ZrO₂, and MoB. The amounts of these phases in the oxidized reactive region were qualitatively determined.     

Materials for temperatures above 1500°C in oxidizing atmospheres. Part I: Basic considerations on materials selection

In the first part of this paper the results of a literature review are presented. Here we provide an overview of the materials that are suitable for long term service in oxidizing atmospheres at temperatures of 1500°C and higher. Criteria are a sufficient stability against the surrounding environment, low vapor pressure of the material or the reaction product formed and sufficient creep resistance. A further aspect is that reactions between contacting materials of different compositions should not lead to the formation of low melting eutectics at the contact areas. The second part reports about results from thermogravimetric measurements on the oxidation resistance of several potential materials. The third part deals with the experimental investigations concerning the “contact‐corrosion” of selected high‐temperature resistant materials (Al₂O₃, ZrO₂, CeO₂, La₂O₃, Y₂O₃, HfO₂, and the Spinel (MgO · Al₂O₃)) in direct contact with a SiO₂‐scale forming material (MoSi₂) at 1600°C in air, since most of the potential materials for such high temperatures are silica formers. Parts II and III will appear in the next 2 issues of this journal.     

The oxidation resistance of MoSi2 composites

Recent work at the Naval Air Development Center demonstrates that the oxidation resistance of MoSi₂ is not significantly reduced by XD? compositing. The MoSi₂ + SiC material exhibited an oxidation resistance equal to or better than that of base MoSi₂. This excellent oxidation resistance, in conjunction with the enhanced high-temperature strength provided by XD particles, makes these materials highly attractive for use as future gas turbine engine materials. The problem of low room-temperature ductility and fracture toughness will be approached by incorporating other types of reinforcement phases.     

The influence of the grain boundary phase on the mechanical properties of Si3N4–MoSi2 composites

The microstructure and mechanical properties of Si₃N₄–MoSi₂ composites doped with two different sintering additive systems, Y₂O₃–Al₂O₃ and Lu₂O₃, were investigated. It was found that the composite doped with Y₂O₃–Al₂O₃ had an amorphous grain boundary phase, while the grain boundary phase of the Lu₂O₃-doped composites was completely crystallized. The Si₃N₄–MoSi₂ composite containing Lu₂O₃ had higher elastic modulus and better creep resistance at elevated temperatures (>1000 °C) than the composite doped with Y₂O₃–Al₂O₃. This is attributed to the crystallization and higher softening temperature of the Lu₂O₃-doped grain boundary phase compared with that doped with Y₂O₃–Al₂O₃. However, the toughness and strength were not influenced significantly by the grain boundary phase. The inclusion of MoSi₂ particles in Si₃N₄ can improve their fracture toughness through residual stresses induced by the coefficient of thermal expansion mismatch of Si₃N₄ and MoSi₂. The strength decreased significantly at temperatures over 1000 °C due to the brittle–ductile transition of the MoSi₂ phase.     

Densification and oxidation behavior of a novel TiB2–MoSi2–CrB2 composite

Titanium diboride (TiB₂) composite with MoSi₂ and CrB₂ has been prepared and tested to possess excellent oxidation resistance. Dense composite pellets were fabricated by hot pressing of powder mixtures. Microstructural characterization was carried out by XRD and SEM. Hardness and fracture toughness values were measured. Extensive oxidation studies of the composites were also carried out. Density of ≥ 96% ρ_th was obtained by hot pressing at 1800 °C under a pressure of 35 MPa for 1 h. Hardness and fracture toughness were in the range of 18–24 GPa and 3.5–4 MPa·m^1/2 respectively. Crack branching, deflection and bridging mechanisms were observed in the crack propagation paths. Isothermal and continuous oxidation studies of these composites up to 1000 °C showed improved oxidation resistance with the formation of protective glassy layer. TiO₂, Cr₂O₃ and SiO₂ phases were identified on the oxidized surface. Diffusion controlled mechanism of oxidation was observed in the composites.     

Oxidation features of SiOC/MoSi2 composite at low temperatures

The oxidation behavior of SiOC containing dispersed MoSi₂ particles was studied. An SiOC/MoSi₂ composite was prepared by converting polymethylsiloxane into SiOC via pyrolysis after mixing MoSi₂ particles. The SiOC matrix phase oxidized to SiO₂, and MoSi₂ particles oxidized to SiQ and MoO₃. The composite displayed superior oxidation resistance at 110 0°C, because a thin SiO₂ layer formed on the surface deterred the formation of highly volatile MoO₃. However, accelerated oxidation occurred between 400 and 500°C owing to the concurrent formation of SiO₂ and MoO₃, which led to porous, nonprotective scale formation.     

Process study, microstructure, and matrix cracking of SiC fiber reinforced MoSi2 based composites

SiC fiber reinforced SiAlON-MoSi₂ composites have been manufactured by a concurrent fiber winding and low pressure plasma spraying (LPPS) technique to produce a multilayer, circumferentially fiber reinforced composite ring. The LPPS parameters for SiAlON-MoSi₂ powder were optimized by a two-level experimental design approach followed by further optimization, which provided a smooth sprayed surface, low matrix porosity, and high deposition efficiency. The microstructure of SiAlON-MoSi₂ matrix consisted of a lamellar structure built up of individual splats and a uniform distribution of discontinuous SiAlON splats throughout the MoSi₂ matrix. The spray/wind composites exhibited 2% porosity and well-controlled fiber distribution. High temperature consolidation led to the formation of a thick reaction zone at the fiber-matrix interface by a chemical reaction between C coating and MoSi₂. Matrix cracking occurred in SiC _f (15 vol.%)/MoSi₂ after cooling from 1500 to 25 °C and was attributed to the large tensile residual stresses in the matrix developed on cooling because of coefficient of thermal expansion (CTE) mismatch between matrix and fiber. The addition of 40 vol.% SiAlON into the MoSi₂ effectively eliminated the matrix cracking by reducing the matrix-fiber CTE mismatch. Predictions of matrix cracking stress on the basis of residual stresses in the composites showed that the maximum permissible fiber volume fraction to avoid matrix cracking was 6% for SiC _f /MoSi₂ and 23% for SiC _f /SiAlON(40 vol.%)-MoSi₂.     

Fabrication of Y2Si2O7 coating and its oxidation protection for C/SiC composites

Yttrium silicate (Y₂Si₂O₇) coating was fabricated on C/SiC composites through dip-coating with silicone resin + Y₂O₃ powder slurry as raw materials. The synthesis, microstructure and oxidation resistance and the anti-oxidation mechanism of Y₂Si₂O₇ coating were in–estigated. Y₂Si₂O₇ can be synthesized by the pyrolysis of Y₂O₃ powder filled silicone resin at mass ratio of 54.2:45.8 and 800 °C in air and then heat treated at 1400 °C under Ar. The as-fabricated coating shows high density and fa–orable bonding to C/SiC composites. After oxidation in air at 1400, 1500 and 1600 °C for 30 min, the coating-containing composites possess 130%–140% of original flexural strength. The desirable thermal stability and the further densification of coating during oxidation are responsible for the excellent oxidation resistance. In addition, the formation of eutectic Y–Si–Al–O glassy phase between Y₂Si₂O₇ and Al₂O₃ sample bracket at 1500 °C is disco–ered.     

Fabrication and mechanical properties of SiCw/MoSi2–SiC composites by liquid Si infiltration of pyrolyzed rice husk preforms with Mo additions

Dense SiC ceramic matrix composites containing SiC whiskers (SiC_w) and MoSi₂ phase (SiC_w/MoSi₂–SiC) are fabricated by a liquid Si infiltration (LSI) method. Pyrolyzed rice husks (RHs) containing SiC whiskers, particles and amorphous carbon are mixed with different amounts of Mo powder to form preforms for the infiltration. Microstructure and mechanical properties of the composites are studied. Fracture mode of the composites is discussed. Results show that the SiC whiskers and fine particles in the pyrolyzed RHs were preserved in the composites after the LSI process. The amorphous carbon and Mo powder in the preforms reacted with molten Si, forming SiC and MoSi₂ in the composites. The presence of MoSi₂ in the composite increases the elastic modulus but lowers the flexure strength. Content of MoSi₂ of ca. 20 wt.% provides an enhanced fracture toughness of 4.1 MPa m^1/2 for the composite. But too large amount of MoSi₂ caused crack formation in the composite. The compressive residual stress introduced by the formation of MoSi₂ and SiC, and the de-bonding of the fine SiC particles and SiC whiskers from the residual Si phase are considered to favor the fracture toughness of the composites.