MoSi2 Matrix Research Articles

On the high temperature oxidation of MoSi2 particles with boron addition

Boron doped MoSi2 particles have been envisioned as sacrificial particles for self-healing thermal barrier coatings (TBCs) but their oxidation behaviour is yet not well understood. In this work, oxidation of MoSi2 based particle is studied in the temperature range of 1050–1200 °C. The oxidation proceeds from a transient to a steady-state oxidation stage. The kinetics during steady-state oxidation is captured with a thermal diffusion-based model. As compared to the oxidation of pure MoSi2 particles, the addition of boron strongly enhances the silica formation. Also, a finer dispersion of MoBx in the MoSi2 matrix accelerates the formation of silica. The oxide growth rate constant increases proportional with the boron content of the MoSi2 particles. This enhanced oxidation is related to the microstructure of the oxide scale. Upon oxidation, boron yields B2O3, which promptly merge with SiO2 to form amorphous borosilicate, hindering the formation of crystalline SiO2. Consequently, the migration of oxygen in the borosilicate oxide scale is faster than in the silica oxide scale on pure MoSi2 particles.

Oxidation resistance of Cr-modified MoSi2 composites at high temperature

Alloying is an important measure to improve the oxidation resistance of MoSi2. Cr-modified MoSi2 composites were prepared by hot pressing sintering in this study, and their oxidation behaviors at different temperatures were examined by XRD, SEM and EDS. The results show that the MoSi2 matrix forms CrSi2 compound with the increase of Cr content, which is conductive to the formation of surface oxide scale. Besides, the microstructure study shows that the modified element Cr promotes the formation of SiO2 protective scale at 500 °C–900 °C. When oxidized at 1300 °C, Cr2O3 in the form of Cr oxidation dissolved in MoSi2 is dispersed in the SiO2 oxide scale. The high temperature oxidation resistance of MoSi2 composites is improved by adding Cr modified element.

Dense SiCf/MoSi2 composite via liquid silicon infiltration

AbstractSilicon carbide fiber reinforced MoSi2 matrix composite (SiCf/MoSi2) is prepared by liquid silicon infiltration at 1450°C. SiC fiber preform is first impregnated with phosphomolybdic acid (PMA) solution in ethyl alcohol. After calcinations, the PMA is converted into MoO3. Following the heating in hydrogen atmosphere, the MoO3 is reduced into metallic Mo, leading to a porous SiCf/Mo. The porous preform is then infiltrated with liquid silicon above silicon melting point to produce SiCf/MoSi2. The microstructure evolution and the underlying mechanism are studied. It is found that MoSi2 is formed by dissolution‐precipitation. Through multiple impregnation‐calcination cycles, a fully dense SiCf/MoSi2 can be obtained with MoSi2 as the continuous matrix phase. The presence of Mo is found to significantly reduce the attack of liquid silicon the silicon carbide fiber reinforcements.

Refractory metal silicides reinforced by in-situ formed Nb2O5 fibers and mullite nanoclusters

There is keen interest in the use of refractory metal silicides as structural materials or thermal barrier coatings for a high temperature environment. However, a long-standing problem for these materials is their poor thermal shock property. To address this challenge, Nb-Al-SiC elements were introduced into the MoSi2 matrix and consolidated by in-situ hot pressing. We find that this treatment leads to improved performance of MoSi2 composites in high temperature thermal shock resistance and bending strength. After in-situ HPing, the Nb, Al2O3 particles, and SiC nanoclusters were uniformly dispersed in the MoSi2 matrix and inhibited the movement of dislocation, resulting in a strengthening effect. During the thermal shock process, the fragmentized oxide layer present in the surface of the pure MoSi2 alloy disappeared completely, and a dense multi-component oxide layer was formed in-situ on the surface of the MoSi2 composites. The dense multi-component oxide layer was composed of SiO2 glass, fiber-structured Nb2O5, and nano-sized mullite phases. The fiber structured and nano-sized oxide phases play an important role in strengthening the oxide layer.

In-situ preparation of nano MoSi2[sbnd]SiC composite by melt infiltration of silicon method

The dual-phase and in-situ MoSi2SiC composite were prepared by melt silicon infiltration method, in which the SiC phase dispersed in the MoSi2 matrix. The results show that the matrix and reinforcement phase of this composite are all in nanometer scale, and the composite is dense and no oxygen content, also the matrix MoSi2 is even occurred preferring orientation in the (211) lattice plane. Furthermore, there are lots of stacking faults and some dislocations in this composite. And this in-situ composite possesses higher strength owing to its transgranular fracture morphology. It is clear that the resistivity of composite is increased with the increasement of SiC content and its resistivity simulation also confirmed that the composite own very fine and dispersed SiC phase. After oxidation at 1300 °C for 3 h, the mass gain of this composite would keep unchanged; this means a passivation stage is gained. When the oxidized samples are used to oxidize again at low temperature as 500 °C, the “PEST” is no longer observed.

Synthesis and Properties of MoSi2–MoB–SiC Ceramics

MoB and SiC particulate reinforced MoSi2 matrix composites were synthesized in situ from Mo, Si, and B4C powder mixtures by self‐propagating high‐temperature synthesis (SHS). The SHS MoSi2–MoB–SiC products were vacuum hot‐pressed (HPed) at 1400°C for 90 min to fabricate high‐density (> 97.5% relative density) bulk composites. Microstructure refinement and improvements in the Vickers hardness and fracture toughness of the HPed composites were observed with increasing B4C content in the reaction mixture. The HPed composite of composition MoSi2–0.4MoB–0.1SiC exhibited grain size of 1–5 μm, Vickers hardness of 12.5 GPa, bending strength of 537 MPa, and fracture toughness of 3.8 MPa.m1/2. These excellent mechanical properties indicate that MoB and SiC particulate reinforced MoSi2 composites could be promising candidates for structural applications.

Orientation relationships, interface structures, and mechanical properties of directionally solidified MoSi2/Mo5Si3/Mo5Si3C composites

The microstructures, orientation relationships, and high-temperature deformation behavior of directionally solidified (DS) ingots of MoSi2/Mo5Si3/Mo5Si3C three-phase eutectic composites were investigated. The DS three-phase composites exhibit a fine and homogeneous script lamellar microstructure. Mo5Si3 and Mo5Si3C individually form interconnected networks that extend in the growth direction (GD) of the MoSi2 matrix and maintain [11¯0]MoSi2, [001]Mo5Si3, and [0001]Mo5Si3C approximately parallel to the GD. High-temperature strength is higher in the DS three-phase composite than in the binary two-phase counterpart, especially below 1200 °C. This is mainly because of MoSi2 matrix refinement and the presence of the strengthening phase Mo5Si3C. The observed orientation relationship between component phases is discussed in relation to lattice misfits.

Effects of Raw Materials on Synthesis, Microstructure and Properties of MoSi2-10 Vol% SiC Composites

ABSTRACTMoSi2-10 vol% SiC composites have been prepared in situ by self propagating combustion synthesis from three different composite systems: Mo-Si-C, Mo-Si-Mo2C and Mo-Si-SiC. The combustion synthesis temperatures were all above 1800 K and the products were composed of MoSi2, SiC and trace Mo5Si3. MoSi2 and MoSi2-10 vol% SiC composites were consolidated by vacuum hot-pressing at 1400°C to produce monoliths with high density. A uniform dispersion of SiC particles was obtained in MoSi2 matrix composites. MoSi2-10 vol% SiC composites prepared by Mo-Si-SiC raw materials exhibited excellent mechanical properties: Vickers hardness 12.21 GPa, bending strength 773.4 MPa and fracture toughness 3.65 MPa m1/2.

Structure of B-Modified MoSi<sub>2</sub> Coating on C103 Alloy Prepared through Glow Plasma Deposition and HAPC Combined Method

Glow plasma deposition (GPD) and halide activate pack cementation (HAPC) methods were combined to prepare B-modified MoSi2 coatings on C103 alloy. The pure Mo layer was firstly deposited on the substrate through GPD process, and then Si and B were co-deposited into the Mo layer through HAPC process to form the B-modified MoSi2 coating. Experimental results revealed that the Mo layer exhibited metallurgical bonding with the substrate. Two typical coatings were produced through the above combined method. One consisted of the MoSi2 outer layer, the MoSi2+MoB two-phase middle layer and the Mo5Si3 inner layer, the other consisted of an outer deposits layer composed of MoSi2 matrix and MoB dispersions, and an inner diffusion layer composed of NbSi2, NbB2 and Nb5Si3 phases. The latter coating was oxidized at 1250 °C, and it has been found that the protective scale can form on the coating surface, which indicates the good oxidation resistance of such B-modified MoSi2 coating prepared through the combined method.

Quantitative Analysis of Heterogeneous Microstructure and Diversified Strengthening Mechanisms in Spark Plasma Sintered Molybdenum Disilicide

Molybdenum disilicide (MoSi2) was in situ synthesized by spark plasma sintering into fully dense bulk material. Electron backscatter diffraction revealed that over the range from the center to periphery of the cylindrical sample, the microstructure is heterogeneously constituted by gradually refined MoSi2 grains and increasing amounts of Mo5Si3 impurity phase. The diversified strengthening mechanisms are primarily attributed to the Peierls–Nabarro stress in MoSi2 matrix, the Hall–Petch strengthening effect, and thermal mismatch stress between Mo5Si3 and MoSi2.

Microstructures and Mechanical Properties of MoSi2 / Mo5Si3 / Mo5Si3C Ternary Eutectic Composite

ABSTRACTMicrostructures and mechanical properties of directionally solidified (DS) MoSi2 / Mo5Si3 / Mo5Si3C ternary eutectic composites were investigated. Ternary eutectic microstructure of a script-lamellar type that is characterized by rod-shaped Mo5Si3 and Mo5Si3C phases extending along the growth direction in the MoSi2 single crystalline matrix was developed simply by directional solidification at a growth rate of 10 mm/h. Compression tests along $[{\rm{1}}\mathop 1\limits^- 0]_{MoSi_2 }$ nearly parallel to the growth direction revealed that the DS ternary eutectic composites were plastically deformed above 1000 °C. Yield stresses of the DS ternary eutectic composites were much higher than those of binary composites mainly because of a smaller average thickness of MoSi2 matrix.

Preparation and Characterization of MoSi<sub>2</sub>/WSi<sub>2</sub> Composites from MASHSed Powder

MoSi2 based materials are considered to have potential for use in high temperature structural part. In this work, MoSi2/WSi2 composites have been successfully prepared by pressureless sintering from mechanically-assisted combustion synthesized powders. The size of green powders shows great impact on the properties of sintered samples. The sample obtained from smaller powders has finer microstructure, and its Vickers hardness, flexural strength and fracture toughness were as high as 10.78GPa, 327.21MPa and 7.32MPam1/2, respectively. The morphologies of the fractured surface of the composites revealed the mechanism to improve the mechanical properties of MoSi2 matrix. Moreover, this composite exhibits good oxidation resistance at low temperature. A continuous SiO2 layer was formed after exposure to air at 500°C for 120 hours, which could prevent further oxidation of the composite. [doi:10.2320/matertrans.M2014370]

Low Temperature Oxidation Behaviors of CNTs/MoSi<sub>2</sub> Composites

Molybdenum disilicide (MoSi2) matrix composites with various contents of carbon nanotubes (CNTs) were fabricated by sintering in vacuum at 1550 °C for 1 h. The oxidation behaviors of CNTs/MoSi2composites at 400 °C and 500 °C for 200 h in air were studied. Results showed that the weight loss of CNTs/MoSi2composites increased with the increase of CNTs content. “Pest” phenomenon happened at 400 °C but not at 500 °C. Phase identification and microstructure of the samples were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that many MoO3whiskers and microcracks only occurred on the surface of CNTs/MoSi2composites when oxidized at 400 °C in air, which leaded to the catastrophic disintegration of CNTs/MoSi2composites.

High Temperature Oxidation Behaviors of CNTs/MoSi<sub>2</sub> Composites

Molybdenum disilicide (MoSi2) matrix composites with various contents of carbon nanotubes (CNTs) were fabricated by sintering in vacuum at 1550°C for 1 h. The oxidation behaviors of CNTs/MoSi2composites at 1300°C for 200 h in air were studied. Results showed that MoSi2matrix composites with no more than 8 % CNTs in volume had good oxidation resistance at 1300 °C, although addition of CNTs reduced the high temperature oxidation resistance of MoSi2. An approximate linear relationship was found between the weight gain of CNTs/MoSi2composites and the content of CNTs. The oxidation resistance of CNTs/MoSi2composites at high temperature decreased with the increasing of CNTs contents. Since the gaseous products were formed during the oxidation process and escaped from the oxide film, the protective film became loose which offered channels for the oxygen soaking into the composites. Thus the oxidation resistance of CNTs/MoSi2composites was decreased.

Microstructure Characteristics and Oxidation Behavior of Molybdenum Disilicide Coatings Prepared by Vacuum Plasma Spraying

In this study, molybdenum disilicide (MoSi2) coatings were fabricated by vacuum plasma spraying technology. Their morphology, composition, and microstructure characteristics were intensively investigated. The oxidation behavior of MoSi2 coatings was also explored. The results show that the MoSi2 coatings are compact with porosity less than 5%. Their microstructure exhibits typical lamellar character and is mainly composed of tetragonal and hexagonal MoSi2 phases. A small amount of tetragonal Mo5Si3 phase is randomly distributed in the MoSi2 matrix. A rapid weight gain is found between 300 and 800 °C. The MoSi2 coatings exhibit excellent oxidation-resistant properties at temperatures between 1300 and 1500 °C, which results from the continuous dense glassy SiO2 film formed on their surface. A thick layer composed of Mo5Si3 is found to be present under the SiO2 film for the MoSi2 coatings treated at 1700 °C, suggesting that the phenomenon of continuous oxidation took place.

Study on Sliding Wear Characteristics and Processing of MoSi

Abstract In this study, a monolithic MoSi2 matrix reinforced with 20 vol% SiC particles, a SiC/MoSi2 composite matrix reinforced with 20 vol% ZrO2 particles, and a ZrO2/MoSi2 composite were fabricated using hot press sintering at 1350 for 1 h under a pressure of 30 MPa. The Vickers hardness and sliding wear resistance of the monolithic MoSi2, ZrO2/MoSi2, and SiC/MoSi2 composite were investigated at room temperature. A wear behavior test was carried out using a disk-type wear tester with a silicon nitride ball. The ZrO2/MoSi2 composite showed an aver-age Vickers hardness value and excellent wear resistance compared with the monolithic MoSi2 and SiC/MoSi2 composite at room temperature. Keywords: Friction coefficient, Monolithic MoSi2, Scanning Electron Microscopy, Sliding wear resistance 1. Introduction MoSi 2 , Mo 5 Si 3 , and other transition metal sili-cides have drawn considerable attention as struc-tural materials in oxidizing environments at ele-vated temperatures [1,2]. MoSi 2 is known to have excellent oxidation resistance, a high melting point (2020 ), moderate density, high thermal conductivity, and thermodynamic compatibility with other potential reinforcements [3,4]. MoSi

Development of Mo and Ta Foil Reinforced (MoSi<sub>2</sub> + 20 Vol% SiC<sub>p</sub>) Matrix Laminated Composites

MoSi2 based composites have been recognized as a potential candidate materials for high temperature structural applications. Although, MoSi2 has been reinforced by a variety of ceramic particles, ductile phase toughening of MoSi2 has received only limited attention. In the present investigation, ductile refractory metal (Mo and Ta) foils were used as reinforcements in MoSi2 matrix to prepare the laminated composites. 20 vol% SiC particles (SiCp) were also added in MoSi2 matrix to reduce the thermal expansion mismatch between MoSi2 and the ductile metal foils. The improved bonding between MoSi2-SiCp matrix layer and Mo / Ta foils due to reduced residual thermal stresses resulted in significant improvement in the room temperature fracture toughness of both the composite systems over the monolithic MoSi2.

In-situ preparation of SiC–MoSi2 composite by microwave reaction sintering

A 20% volume SiC reinforced MoSi2 composite has been successfully in-situ prepared by precisely controlling the microwave reaction sintering process. After sintering, a uniform dispersion of SiC particles was obtained in the MoSi2 matrix from the reaction of Mo, Si and C powders. The sintered product is a composition of MoSi2, Mo4.8Si3C0.6 and SiC. The relative density, flexural strength, Vicker's hardness and fracture toughness of the composite are 91.5%, 251.29MPa, 7.86GPa and 8.17MPam1/2, respectively. We found the heating procedure is critical in the microwave reaction sintering process to obtain dense SiC–MoSi2 composites.

Preparation and Characterization of SiC/MoSi<sub>2</sub> Composites from Powder Resulting from a Mechanical-Assistant Combustion Synthesis Method

SiC reinforced MoSi2 composites have been successfully prepared by pressureless sintering from mechanical-assistant combustion synthesized powders. The sintering temperatures and holding time were 1500°C~1650°C at a heating rate of 10K/min and 1 hour, respectively. The microstructure and mechanical properties of the as-sintered composites were investigated. SEM micrographs of SiC/MoSi2 composites showed that SiC particles were homogeneously distributed in MoSi2 matrix. The Vickers hardness, flexural strength and fracture toughness of the SiC/MoSi2 composites were up to 15.50GPa, 468.7MPa and 9.35MPa•m1/2, respectively. The morphologies of fractured surface of the composites revealed the mechanism to improve mechanical properties of MoSi2 matrix. At last, the cyclic oxidation behavior of the composites was discussed. The results of this work showed that in situ SiC/MoSi2 composite powder prepared by MASHS technique could be successfully sintered via pressureless sintering process and significant improvement of room temperature mechanical and anti-oxidation properties could be achieved.

Reasearch on Heat-Shocking Resistance of SiC<sub>(w)</sub>/ZrO<sub>2</sub>-MoSi<sub>2</sub> Ceramic Composites

The heat-shocking resistance of SiC(w)/ZrO2-MoSi2 ceramic composites prepared by hot-pressing sintering was studied by indentation-quench method together with the calculation of the crack propagation rate of ceramic nanocomposites. The results showed that the crack propagation rate of MoSi2 matrix ceramics samples decreased remarkably with the addition of ZrO2 and SiC nanoparticles. The crack propagation rate of SiC(w)/ZrO2-MoSi2 ceramic composites was obviously lower than that of ZrO2-MoSi2 ceramic composites, which revealed that the synergism between SiC(w) and ZrO2 was more advantageous to enhance the heat-shocking resistance of MoSi2 ceramic. Moreover, the ability of ZrO2 particles to hinder the heat-shocking crack propagation was better than that of SiC whiskers. The synergism between SiC(w) and ZrO2 changed the crack propagation path and shape in MoSi2 ceramics. The mechanisms of ZrO2 to improve the heat-shocking resistance of the MoSi2 ceramic were mainly phase transfer toughening, while that of SiC whiskers to improve the heat-shocking resistance of the MoSi2 ceramic were mainly crack deflection and bridge union.