MoSi2 Al2O3 Research Articles

High temperature oxidation behavior of MoSi2–Al2O3 composite coating on TZM alloy

A novel MoSi2–Al2O3 composite coating was prepared on Mo-based TZM alloy by slurry sintering method. The oxidation behavior of the coating was evaluated at 1600 °C in static air. Microstructure and phase composition of the as-prepared and oxidized coatings were characterized, and the antioxidant mechanism of the coating at high temperature was discussed. A three-layer structure was observed in the as-prepared coating, consisting of a ∼2 μm thick Mo5Si3 diffusion layer, a ∼65 μm thick MoSi2 inner layer and a ∼36 μm thick outer layer of mixture of MoSi2 and Al2O3. After oxidation at 1600 °C for 5 h, all MoSi2 phases were completely converted to intermediate silicide Mo5Si3 by solid-state diffusion, and the formed Mo5Si3 phase would be transformed into Mo3Si phase with further extending the oxidation time. Furthermore, a dense oxide layer of SiO2-mullite was formed on the specimen surface, which can effectively protect the material to further oxidation. The MoSi2–Al2O3 coating could protect the substrate effectively at 1600 °C for 20 h without failure. The enhanced oxidation resistance of MoSi2–Al2O3 coating is due to the formation of multi-layer structure containing a SiO2-mullite composite oxide outer layer with high thermal stability and low oxygen permeability.

Stability and electrical properties of MoSi2‐ and WSi2‐oxide electroconductive composites

AbstractMoSi2‐ and WSi2‐based electroconductive ceramic composites were fabricated using 40‐80 vol% fine‐ and coarse‐Al2O3, and ZrO2 particles (refractory oxides) after sintering in argon. Their chemical and thermal stability was tested between 1400°C‐1600°C for up to 48 hours. X‐ray diffraction analysis showed the formation of secondary 5‐3 metal silicide (Mo5Si3, W5Si3) and silica phases on the grain boundaries and surface. The fraction of the W5Si3 (11.4‐38.8 vol%) was significantly higher than that of the Mo5Si3 (3.3‐7.3 vol%) in the composites after annealing at 1400°C for 48 hours. The rates of grain growth in the composites (0.013‐0.023 μm/h) were highly decreased by a grain‐boundary pinning effect. This effect was relatively better with the addition of the coarse‐grained oxides due to their more homogeneous distribution throughout the microstructure. The 20–80 vol% MoSi2‐Al2O3 (fine‐grained) composite exhibited an electrical conductivity of 8.8 S/cm at 900°C. At the 60 vol% silicide content, MoSi2–Al2O3 (coarse‐grained) and WSi2–Al2O3 (fine‐grained) showed higher electrical conductivity (126‐128 S/cm) at 900°C. The density, porosity level, particle distribution, intrinsic conductivity of silicide phase, particle size, and fraction of the secondary 5‐3 silicide phase highly influenced their electrical properties.

Combustion Synthesis of MoSi2-Al2O3 Composites from Thermite-Based Reagents

Formation of MoSi2–Al2O3 composites with a broad range of the MoSi2/Al2O3 ratio was conducted by thermite-based combustion synthesis in the SHS mode. The addition of two thermite mixtures composed of MoO3 + 2Al and 0.6MoO3 + 0.6SiO2 + 2Al into the Mo–Si reaction systems facilitated self-sustaining combustion and contributed to in situ formation of MoSi2 and Al2O3. The samples adopting the former thermite reagent were more exothermic and produced composites with MoSi2/Al2O3 from 2.0 to 4.5, beyond which combustion failed to proceed. Because of lower exothermicity of the reactions, the final products with MoSi2/Al2O3 from 1.2 to 2.5 were fabricated from the SHS process involving the latter thermite mixture. Combustion temperatures of both reaction systems decreased from about 1640 to 1150 °C with increasing MoSi2/Al2O3 proportion, which led to a phase transition of MoSi2. It was found that the dominant silicide was β-MoSi2 when the combustion temperature of the synthesis reaction exceeded 1550 °C and shifted to α-MoSi2 as the combustion temperature fell below 1320 °C. The results of this study showed an energy-efficient fabrication route to tailor the phase and content of MoSi2 in the MoSi2–Al2O3 composite.

In Situ Synthesis of MOSi2-Al2O3 Composite by Mechanical Milling and Subsequent Heat Treatment

In situ MoSi2–Al2O3 composites have been successfully synthesized by mechanical milling (MM) and subsequent heat treatment of pure molybdenum, silicone and Aluminum elemental powders. The powders were mixed and subjected to MM in an attritor ball mill for 20 h under argon (Ar) atmosphere. Mechanically milling powders consolidated and then heat-treated under Ar atmosphere in a tube furnace for 7 h at 1100˚C. The phase transformation, microstructure and morphology of mechanical milling powder particles and consolidated samples were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results did not confirm the presence of any related intermetallics after MM. However, MoSi2 and Al2O3 were successfully in situ formed during heat treatment at 1100˚C. It was also found that after heat-treating, the phase composition Mo(Si,Al)2 was formed.

Preparation and spectral properties of solar selective absorbing MoSi2–Al2O3 coating

A novel solar selective absorbing MoSi2–Al2O3 multilayer coating deposited on Cu or stainless steel (SS) substrate by magnetron sputtering is reported. The coating consists of an infrared reflective metal layer using Mo or Cu, an absorption MoSi2–Al2O3 bilayer, and an anti‐reflection Al2O3 layer from substrate to top. With the help of computer simulation, we obtain the optimal metal volume fraction and thickness of each layer on Cu substrate. And then a good optical performance (α/ϵ) of 0.94/0.06 for the all‐layer coating on Cu substrate is obtained. On the other hand, the influence of the deposition parameters for the infrared reflective Mo layer on the emittance is discussed. The emittance decreases with the decrease of sputtering pressure and the increase of the target power. The lowest emittance of the SS (substrate)/Mo is 0.047 after optimization. The optical performance (α/ϵ) of the all‐layer coating on the optimized SS/Mo substrate is 0.95/0.07. The thermal stability of the all‐layer coating on SS substrate is evaluated, and it is found to have a good thermal stability at 400 °C, due to the choice of MoSi2, which means a good candidate of solar selective absorbing coating for parabolic trough concentrated solar power (CSP).

Effects of the LMVF and HMVF absorption layer thickness and metal volume fraction on optical properties of the MoSi2–Al2O3 solar selective absorbing coating

A novel MoSi2–Al2O3 solar selective absorbing coating deposited on stainless steel (SS) substrate by magnetron sputtering is reported. The effects of the HMVF and LMVF absorption layers thickness and metal volume fraction on optical properties of the coatings are investigated. As the thickness or metal volume fraction of the HMVF or LMVF layers increases, it is found that the absorptance first increases and then decreases, this can be explained by the regularly changes of the reflectance spectrum. The position of the cutoff limit has a redshift as the thickness of the HMVF or LMVF layers or the metal volume fraction of the HMVF layer increases, which will induce the quick increase of the emittance with temperature. The cutoff limit is almost unchanged as the metal volume fraction of the LMVF layer increases. Most of the experimental results are consistent with the simulated results in the trends.

Preparation of MoSi2–Al2O3 nano-composite via MASHS route

In this study, MoSi2–Al2O3 nano-composite powder was prepared by mechanical alloying. Mixture of MoO3, Si and Al powders was exposed to high energy ball milling. Phase compositions and structural evolutions during milling were investigated by X-ray diffraction (XRD) analysis. The morphology of the milled powders was evaluated by scanning electron microscope (SEM). Within short milling times [15, 25 and 60minutes in ball to powder weight ratios (BPR) of 35:1, 20:1 and 10:1, respectively], a combustion process occurred within the milling powder and the process mechanism was characterized to be mechanically activated self-propagating high-temperature synthesis (MASHS) type. From XRD results it was found that during the combustion process, MoO3 was completely reduced with Al and the resulting Mo reacted with Si to produce molybdenum disilicide and eventually a MoSi2–Al2O3 nano-composite powder formed within a short period of time. Further milling resulted in reduced mean crystallite sizes and increased lattice micro strain in the product phases. After 30hours of milling in BPR of 35:1, the mean crystallite sizes were found to be 10, 9 and 11nm for Al2O3, α-MoSi2 and β-MoSi2, respectively. Also, effect of stearic acid addition as a Process Control Agent (PCA) was studied and the results showed that its usage postpones the reaction initiation and reduces mean crystallite sizes.

Molybdenum-based oxidation-resistant MoSi2–Al2O3 and WSi2–Al2O3 coatings

Various studies and comparative tests are conducted on oxidation-resistant unmodified and modified MoSi2–Al2O3 and WSi2–Al2O3 silicide coatings in air at 1800 and 1900°C. Metallographic, x-ray diffraction, electron microprobe, and chemical analyses show that the introduction of alumina into molybdenum and tungsten disilicides improves their hardness and produces fine-grained silicides with high corrosion resistance. The composition with fine alumina particles (O < 0.1 μm) with the total alumina concentration from 3 to 15 wt.% in the coating is most favorable for improving the oxidation resistance of disilicides. It is shown that the high oxidation resistance of MoSi2–Al2O3 and WSi2–Al2O3 coatings is due to the presence of alumina in disilicides, which contributes to the formation of an external high-temperature oxide layer. This layer consists of α-Al2O3 and a mullite sublayer in which Al2O3 crystals are introduced into SiO2.

Key developments in high temperature structural silicides

Significant progress has been made in the past few years in both the scientific understanding of high temperature structural silicides and in their technological development. This overview highlights key aspects of this structural silicide research and development, in the areas of materials, composites, and applications. Silicide materials discussed are MoSi2, Mo5Si3, Mo<5Si3C<1, Ti5Si3, and C40 type silicides. Results on MoSi2–Si3N4, MoSi2–SiC, MoSi2–Al2O3, and MoSi2–Si3N4–SiC composites are summarized. Finally, selected applications in glass processing, wear resistance, diesel engine glow plugs, and aerospace components are described.

Processing and properties of MoSi2–SiC and MoSi2–Al2O3

Molybdenum disilicide (MoSi 2 ) is a candidate material for high temperature structural applications due to its high melting point and excellent oxidation resistance. A major impediment to implementing this material is its low temperature fracture toughness. Earlier studies examined polycrystalline MoSi 2 for the role of grain size and SiO 2 impurity distribution on the fracture response via indentation. The goal of this study is to apply this indentation analysis to MoSi 2 containing additions of either Al 2 O 3 or SiC. These additions modify the microstructure and result in changes to the indentation cracking behavior. Indentation crack responses and crack lengths are recorded and the fracture toughness values are determined. An explanation is presented for the formation of a tortuous crack path that increases the fracture toughness for the sample with the SiC addition.