MoSi2 Content Research Articles

ZrB2-MoSi2 ceramics: A comprehensive overview of microstructure and properties relationships. Part II: Mechanical properties

The mechanical behavior of ZrB2-MoSi2 ceramics made of ZrB2 powder with three different particle sizes and MoSi2 additions from 5 to 70 vol% was characterized up to 1500 °C. Microhardness (12–17 GPa), Young’s modulus (450–540 GPa) and shear modulus (190–240 GPa) decreased with both increasing MoSi2 content and with decreasing ZrB2 grain size. Room temperature fracture toughness was unaffected by grain size or silicide content, whilst at 1500 °C in air it increased with MoSi2 and ZrB2 grain size, from 4.1 to 8.7 MPa m½. Room temperature strength did not trend with MoSi2 content, but increased with decreasing ZrB2 grain size from 440 to 590 MPa for the largest starting particle size to 700–800 MPa for the finest due to the decreasing size of surface grain pullout. At 1500 °C, flexure strength for ZrB2 with MoSi2 contents above 25 vol% were roughly constant, 400–450 MPa, whilst for lower content strength was controlled by oxidation damages. Strength for compositions made using fine and medium ZrB2 powders increased with increasing MoSi2 content, 250–450 MPa. Ceramics made with coarse ZrB2 displayed the highest strengths, which decreased with increasing MoSi2 content from 600 to 450 MPa.

ZrB2-MoSi2 ceramics: A comprehensive overview of microstructure and properties relationships. Part I: Processing and microstructure

This study systematically correlates processing with quantitative microstructural information over an extended compositional range for ZrB2-MoSi2 ceramics, with MoSi2 contents ranging from 5 to 70 vol% and diboride starting particle sizes ranging from 3 to 12 μm. Fifteen different ceramics were hot pressed between 1750 and 1925 °C. Plastic deformation of MoSi2 contributed to initial densification, but some of the MoSi2 decomposed during the later stages of hot pressing. Finer diboride particles required lower temperatures to densify (1750 to 1850 °C) compared to coarser diboride particles (1900 °C). Increasing MoSi2 content led to a decrease in sintering temperature. As MoSi2 content increased, ZrB2 grain size decreased and MoSi2 cluster size increased. Starting powders with lower impurity contents and isothermal vacuum holds contributed to lower oxide impurity contents in the final ceramics. A diboride core-shell microstructure involving (Zr1-x,Mox)B2 solid solutions formed in all compositions with Mo contents in the solid solution shells ranging from 3 to 6 at%.This work identified specific relationships between starting composition, processing conditions and final microstructure, showing how microstructure and properties could be tailored by processing. The outcomes of this extensive study will serve as guidelines for the design of other structural ceramics that have to attain determinate thermo-mechanical properties for targeted applications.

New insights into linear electrical properties of pressureless sintered SiC-MoSi2-AlN composites

Highly conductive SiC-MoSi2-AlN composites were fabricated by β-SiC, AlN and MoSi2 powders with Y2O3 additive via pressureless sintering. The effect of MoSi2 content on the microstructure, mechanical and electrical properties of SiC-MoSi2-AlN composites was systematically investigated. A finer microstructure was obtained and electrical conductivity was enhanced with increasing MoSi2 content. The impedance spectroscopy and potential-current measurements were implemented to figure out the electrical conduction mechanism. The introduction of MoSi2 effectively reduced the Schottky barrier height at the grain boundary, and subsequently the U-I curves changed from nonlinear to linear electrical characteristics. The notable decrease in electrical resistivity was owing to the breakdown of grain boundaries and the formation of percolation paths. The percolation threshold was in the range of 0–5.44 vol% MoSi2, much lower than the reference value. The composites with 10 wt% MoSi2 exhibited an electrical resistivity of about 60 Ω cm, suitable for infrared source element applications.

Combustion synthesis and mechanical properties of MoSi<sub>2</sub>–ZrB<sub>2</sub>–SiC ceramics

MoSi2ZrB2SiC ceramics were synthesized using Mo, Zr, Si and B4C powders by self-propagating high-temperature synthesis and densifying by spark plasma sintering. The effects of MoSi2 content on the ...

Microstructure and Properties of Thermal Electrode Material Si₃N₄⁻MoSi₂ Composite Ceramics.

With good high temperature and corrosive resistance performance, ceramic based composites can be used as promising materials to replace metal thermocouple materials. In this study, Si3N4–MoSi2 composites were prepared via hot pressing technology. X-ray diffraction (XRD), optical microscopy (OM), and scanning electron microscopy (SEM) were used to analyze the microstructure of the composites. The mechanical properties and electrical conductivity were tested. The results showed that the composites were composed of β-Si3N4, MoSi2, a small amount of Mo5Si3, and an amorphous glassy phase. The MoSi2 phase was evenly distributed in the matrix. The percolation network was formed with increasing MoSi2 content. The strength of the composites reached its maximum value when the MoSi2 content reached a critical point. The electrical conductivity behaved like a typical percolation phenomenon. The percolation threshold was about 30% to 45%.

Effect of MoSi2 Content on Dry Sliding Tribological Properties of Zr-Based Bulk Metallic Glass Composites

Zr55Cu30Al10Ni5 bulk metallic glass and its composites were prepared by suction casting into a copper mold. The effect of MoSi2 content on the tribological behavior of Zr55Cu30Al10Ni5 BMG was studied by using a high-speed reciprocating friction and wear tester. The results indicate that the friction coefficient and wear resistance of the BMGs can be improved by a certain amount of crystalline phase induced by MoSi2 content from 1 to 3% and deteriorated with MoSi2 content of 4%. The wear mechanism of both the metallic glass and its composite is abrasive wear. The mechanism of crystalline phase-dependent tribological properties of the composite was discussed based on the wear track and mechanical properties in the present work. The wear behavior of Zr55Cu30Al10Ni5 BMG and its composite indicates that a good combination of the toughness and the hardness can make the composite be well wear resistant.

In situ fabrication and properties of 0.4MoB-0.1SiC-xMoSi2 composites by self-propagating synthesis and hot-press sintering

Mo, Si and B4C powders were used to fabricate 0.4MoB-0.1SiC-xMoSi2 composites by self-propagating high-temperature synthesis (SHS) and hot pressing (HP). The effects of MoSi2 content (x = 1, 0.75, 0.5 and 0.25) on phase composition, microstructure and properties of the composites were investigated. The results showed that the 0.4MoB-0.1SiC-xMoSi2 composite exhibited Vickers hardness of 10.7–15.2GPa, bending strength of 337–827MPa and fracture toughness of 4.9–7.0MPam1/2. The fracture toughness increased with the increasing volume fraction of MoB and SiC particles which were promoted by the toughening mechanisms, such as crack bridging, cracks deflection and crack branching. Moreover, the electrical resistivity showed an increasing trend with decreasing volume fraction of MoSi2.

Preparation and thermal shock resistance of high emissivity molybdenum disilicide- aluminoborosilicate glass hybrid coating on fiber reinforced aerogel composite

To develop a flexible reusable surface insulation for thermal protection system, MoSi2-aluminoborosilicate glass hybrid coatings have been prepared on Al2O3 fiber reinforced Al2O3-SiO2 aerogel composite by slurry dipping and rapid sintering method. The effect of MoSi2 content on radiative property and thermal shock behavior was investigated. The total emissivity values of all the coatings exceeded 0.85 in the wavelength of 0.8–2.5μm. The M10 and M50 coatings were up to 0.9, which was due to the highest amorphous glass content of the M10 coating and the largest surface roughness of the M50 coating. The M30 coated composite showed the best thermal shock resistance with only 0.023% weight loss after 20 thermal shock cycles between 1473K and room temperature, which was attributed to the similar thermal expansion coefficients between the coating and the substrate and the appropriate viscosity of aluminoborosilicate glass at 1473K. The cracks resulted from CTE mismatch stress with different sizes formed and grew on the surface of M10, M40 and M50 coated samples, leading to the failure of the composites.

Effect of different additives and open porosity on fracture toughness of ZrB2–SiC-based composites prepared by SPS

In this work, Taguchi experimental design technique was applied to determine the most influential additives and SPS parameters for optimizing fracture toughness of ZrB2–SiC based composites. In this case, nine factors (SiC, Cf, MoSi2, HfB2 and ZrC content, milling time of Cf and SPS parameters such as temperature, time and pressure) were examined on four different levels in order to obtain the optimum mixture. A total of 32 mixtures were prepared in accordance to the L32 array proposed by the method. Fracture toughness of all composites was measured by single edge-notch beam test. SEM was applied to evaluate microstructure. It has been concluded that the open porosity up to 10% has no significant effect on fracture toughness but in higher values, it is varied inversely with its changes. The results showed that temperature with 34.7% and SiC with 29.7% have significant effect on fracture toughness. Cf, M.t, HfB2, pressure and time with 2.3%, 3.2%, 0.05%, 0.44% and 2.3% have influence on fracture toughness, respectively. ZrC has 7.8% and MoSi2 has 6.3% on fracture toughness.

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.

Microstructures and ablation properties of Cf/C-SiC-MoSi2 composites: Influence of the SiC:MoSi2 mass ratio

In order to find out the influence of relevant content of SiC and MoSi2 on the properties of Cf/C-SiC-MoSi2 composites, SiC and MoSi2 were introduced into C/C composites by vacuum filtration infiltration (VFI) process, and the composites were densified through hydrothermal carbonization treatment using glucose as the carbon source. The composition, pore size distribution, microstructure and ablation resistance of the composites were investigated. Results showed that with the increase of SiC content, the interface bonding between carbon fibers and ceramic matrix was improved, companied with much smaller pore size in C/C-SiC-MoSi2 composites. As the SiC:MoSi2 mass ratio was 15:51, the composites exhibited compact microstructure, which was beneficial to good ablation resistance. After being exposed to an oxyacetylene torch flame at 2000°C for 60s, the mass and linear ablation rates were respectively decreased by 84% and 66% compared to those of composites without SiC. The good anti-ablation performance was attributed to the generation of SiO2 layer, which was regarded as an effective barrier between corrosive gases and inner materials.

Comparative study of MoSi2-borosilicate glass coatings on fibrous ceramics prepared by in-situ reaction method and two-step method

Two infrared radiating coatings with borosilicate glass as binder and MoSi2 as emittance agent were prepared on mullite fibrous ceramics by an in-situ reaction method and a two-step method, respectively. The in-situ reaction coating was prepared from Si, B2O3 and MoSi2, in which borosilicate glass was obtained through the in-situ reaction of Si, B2O3 and O2. By contrast, the two-step method included the preparation of borosilicate glass powders by a melt-quench technique and then coating preparation with MoSi2 and the glass powders. The structure, phase composition, thermal endurance, infrared radiation property and impact resistance of the two coatings were compared. The results showed that the in-situ reaction coating was denser, and much less MoSi2 therein was oxidized during sintering (11.3%) than that in the two-step coating (67.0%), resulted from the oxygen diffusion barrier formed by molten B2O3 at approximately 450 °C. The in-situ reaction coating possessed better thermal endurance due to its higher MoSi2 content and denser structure. In addition, the in-situ reaction coating had higher emissivity (0.787) and better impact resistance (0.051 J) than the two-step coating (0.706, 0.038 J).

Microstructural effects on the sliding-wear resistance of ZrC–MoSi2 triboceramics fabricated by spark-plasma sintering

The effects were individually investigated of ZrC grain size and MoSi2 content on the lubricated sliding-wear resistance of ZrC–MoSi2 triboceramics densified by spark-plasma sintering (SPS). Microstructures with different average grain sizes were obtained by varying the high-energy ball-milling time of the ZrC–MoSi2 powder mixtures and their SPS temperature, while the microstructures with different secondary phase contents were obtained by varying the relative concentrations of ZrC and MoSi2 in the powder batches. Experimentally, it was found that the ZrC–MoSi2 triboceramics are all highly wear resistant, but also that wear resistance decreases as the ZrC grain size and MoSi2 content increase. The dominant wear mode was identified as abrasion, and the prevailing wear mechanism was plastic deformation accompanied by localized fracture. Accordingly, the wear trends are explained in terms of the hardness of the ZrC–MoSi2 triboceramics. Finally, the implications are discussed of providing the electrically-conductive ZrC–MoSi2 triboceramics with greater wear resistance via controlled processing.

MoSi2-borosilicate glass coating on fibrous ceramics prepared by in-situ reaction method for infrared radiation

MoSi2-borosilicate glass coating with MoSi2 as emittance agent was prepared on mullite fibrous ceramics for enhanced superficial infrared radiation via an in-situ reaction method. The phase and structure evolutions during sintering were studied in detail. The infrared radiating property, impact resistance, thermal shock resistance and thermal endurance of the coating were investigated comprehensively. The results show that the coating was dense and flat, and it tightly adhered to the substrate by mechanical interlocking. During the sintering process, the low-temperature oxidation of MoSi2 was restrained due to the package of binder phase formed below 500°C, and the coating was fully densified via the viscous flow of molten Si. The total emissivity of the as-prepared coating was higher than 0.85 and the emissivity at low wavelength was higher than 0.9. The impact resistance was preferable attributed to the reinforced transition layer at the interface. The coating went through 30thermalcycles between 1500°C and 25°C without cracking and spalling. The total emissivity declined merely by 4% after oxidation at 1500°C for 50h, which was influenced by the MoSi2 content and the surface roughness of the coating.

High performance electromagnetic interference shielding of lamellar MoSi2/glass composite coatings by plasma spraying

MoSi2/glass composite coatings with different MoSi2 content were fabricated by low power plasma spraying. The microstructure, electrical conductivity, dielectric properties and electromagnetic interference (EMI) shielding effectiveness (SE) of MoSi2/glass composite coatings were investigated in this study. The EMI SE is found to be related with the electrical conductivity and dielectric property, which are attributed to the high aspect ratio of lamellar structure and well dispersion state of MoSi2. The average value of EMI SE is 24.2 dB in the frequency range of 8.2–12.4 GHz for the MoSi2/glass composite coating with 30wt% MoSi2 in 1.5 mm. The results indicate that the plasma sprayed MoSi2/glass composite coatings can be applied to effective EMI shielding materials.

Application of Taguchi L32 orthogonal design to optimize flexural strength of ZrB2-based composites prepared by spark plasma sintering

In this work, Taguchi L32 experimental design was applied to optimize flexural strength of ZrB2-based composites which were prepared by SPS. With this aim, batch ZrB2-based composites tests were performed to achieve targeted experimental design with nine factors (SiC, carbon fiber (Cf), MoSi2, HfB2 and ZrC content, milling time of Cf (M.t, hr) and SPS parameters such as temperature, time and pressure) at four different levels. Optimal experimental conditions were determined by calculated standard analysis. “Higher is better” approach was followed to calculate standard analysis. By using statistical analysis of variance (ANOVA), it has been concluded that the most significant effect on the densification is related to temperature with 23.8%, ZrC with 16.2%, HfB2 with 9.8%, SiC with 9.6% and Cf with 8%, respectively. MoSi2 with 2% has the least influence on flexural strength. Other factors, such as time, M.t and pressure have 6.2%, 5.7% and 3.9% portion on flexural strength, respectively.

Effect of Si3N4 content on microstructures and antioxidant properties of MoSi2/Si3N4 composite coatings on Mo substrate

To determine the effective content of Si3N4 on microstructures and antioxidant properties of MoSi2/Si3N4 composite coatings on Mo substrate, the MoSi2–xvol% Si3N4/Mo (x=0, 2.0, 3.5, 5.0, and 8.0) diffusion couples were fabricated by spark plasma sintering and annealed at a temperature of 1300°C for various periods of time up to 168h. The interfacial diffusion between MoSi2–Si3N4 phase and Mo substrate was examined. Using scanning electron microscopy, the observation of intermediate phases indicated two different mechanisms in Mo5Si3 phase growth. The degradation rates of MoSi2 phase in MoSi2–xvol% Si3N4 diffusion couples were calculated, and the expected Si3N4 content of MoSi2–Si3N4 composite coating was 3.5vol%.

Effect of open porosity on flexural strength and hardness of ZrB2-based composites

In this work, Taguchi L32 experimental design was applied to optimize flexural strength and hardness of ZrB2-based composites which were prepared by SPS. With this aim, batch ZrB2-based composites tests were performed to achieve targeted experimental design with nine factors (SiC, Cf, MoSi2, HfB2 and ZrC content, milling time of Cf and SPS parameters such as temperature, time and pressure) at four different levels. Flexural strength of all composite was measured by three-point bending test. Hardness measurement was done by Vickers indenter. SEM was applied to evaluate microstructure. The results showed that SiC grain size plays important role on flexural strength and correlation between flexural strength and open porosity is low while hardness strongly depends to open porosity. Grain size variation in the range of ~1.5µm to ~8µm has little effect on hardness.

Influence of MoSi2 content on the microstructure and properties of MoSi2–RSiC composites

The influence of MoSi2 content on the microstructural, mechanical, and electrical properties of MoSi2–RSiC composites was studied. Results showed that the main phases in the MoSi2–RSiC composites were SiC, MoSi2 and Mo4.8Si3C0.6. When the MoSi2 content was below 10%, most of the Mo compounds coated the surface of the coarse SiC particles. At MoSi2 contents >10%, some Mo compounds coated the surface while others filled the pores near the sintering neck between the SiC particles. MoSi2 can also accelerate the re-crystallization process of RSiC. Increases in MoSi2 content resulted in decreases in the porosity of the composite; in contrast, the flexural strength initially increased and then decreased under the same conditions. The flexural strength reached 94.0MPa when the MoSi2 content was 10%. With increasing MoSi2 content, the volume resistivity of the composites decreased steeply, the electrical conductivity mechanism of it resembled that of a two shunt-wound equivalent circuit.

Enhanced thermoelectric and mechanical performance of polycrystalline p-type Bi0.5Sb1.5Te3 by a traditional physical metallurgical strategy

In this paper, p-type Bi0.5Sb1.5Te3 polycrystalline materials have been fabricated by a traditional vacuum melting method, and the effects of cooling rate and MoSi2 addition on the microstructure, thermoelectric and mechanical performance of the polycrystalline materials have been studied detailedly. It shows that the amount of Te-rich eutectic phase increases and the lamellar microstructure has been refined with the increase of the cooling rate. Due to the combined effect of cooling rate on the carrier concentration and mobility, the air cooled sample has higher figure of merit than the furnace cooled, water cooled and liquid nitrogen cooled samples, and a maximal ZT of 1.02 at 50 °C was obtained for the air cooled polycrystalline sample. Under the same air cooling condition, the inhomogeneous nucleation sites increase with increasing the amount of MoSi2 particles, therefore the amount of Te-rich eutectic phase increases and the lamellar microstructure get refined, and the thermal conductivity of the sample decreases significantly due to the extra phonon scattering by the refined microstructure and MoSi2 particles. The resulted figure of merit ZT increases with increasing the amount of MoSi2 particles, and it decreases with further increasing the MoSi2 content after attaining the vertex of ZT = 1.33 at 100 °C at a content of 0.2 wt.% MoSi2. The flexural strength of the air cooled polycrystalline sample also increases with the amount of MoSi2 increasing from 0 to 0.3 wt.%, and a nearly 56% enhancement was achieved for the 0.2 wt.% MoSi2 sample (28.0 MPa) compared with the MoSi2 free sample. The improvement of flexural strength is in agreement with the Hall–Petch strengthening mechanism due to the lamellar microstructure refinement induced by MoSi2.