MoSi2 Composites Research Articles

Fabrication and optimization process of mechanical properties of MoSi2 composites reinforced by carbon nanotubes (CNT) using Taguchi method

This experimental research aims to produce and study the properties of MoSi2 reinforced with CNT composites using the SPS sintering method. First and before any laboratory operations, using the Minitab 14 software, experimental samples were designed under L16 arrays. Four levels were set for defining parameters, such as the amount of reinforcing phase (CNT), sintering temperature (°C), and holding time (T). XRD, FESEM, density, micro-hardness, bending strength, and fracture toughness tests were used in the study for in-depth investigations. According to the signal-to-noise analysis, the parameter with the greatest influence is the reinforcement rate. Based on the obtained results, the composite with 5 wt% of carbon nanotubes, a sintering temperature of 1300 °C, and a holding time of 10 min was found to have the highest results.

Sintering of Si3N4–SiC–MoSi2 composites additively manufactured by Multi Material Jetting

AbstractThe aim of this work was to investigate the sintering behavior of Si3N4–SiC–MoSi2 composites manufactured by the Multi Material Jetting (MMJ) process. Therefore, electrically conductive and insulating Si3N4–SiC–MoSi2 composites were printed using MMJ. Samples were sintered in a gas‐pressure furnace at 6 and 31 atm, respectively. The density, microstructure, and crystalline phase composition were investigated, and the results discussed regarding the phase stability of the Si3N4–SiC–MoSi2 composites. The results show that the nitrogen pressure must be set carefully to avoid the formation of Mo5Si3 or Mo4.8Si3C0.6 on the one hand and to prevent the decomposition of the phases into a Si‐containing melt on the other.

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.

Laser ablation mechanism of ZrC-SiC-MoSi2 ternary ceramic modified C/C

C/C composites were prepared by chemical vapor infiltration (CVI), and then were subjected to Si, Zr, and MoSi2 reactive melt infiltration (RMI) to obtain C/C-SiC, C/C-SiC-ZrC, and C/C-SiC-ZrC-MoSi2 composites. The ablation behavior of these three composites was evaluated by high-energy CO2 laser irradiation. The surface temperature distribution of composite materials was simulated by finite element analysis. The results show that the ablation resistance mechanisms of the three materials are entirely different. The C/C-SiC-ZrC-MoSi2 composite showed the best ablation performance among them. It is attributed to the lower oxygen permeability and richer heat dissipation mechanism of the C/C-SiC-ZrC-MoSi2 composite within the total temperature threshold.

Investigation of the Effect of Molybdenum Silicide Addition on the Oxidation Behavior of Hafnium Carbonitride

In this study, the oxidation stability up to 1000 °C in air of the Hf(C,N)-MoSi2 composites was explored under non-isothermal and isothermal conditions. Composites with 1, 5, 10, and 20% volume fractions were produced by low-energy ball milling and subsequent spark plasma sintering. Differential scanning calorimetry (DSC) and thermogravimetric (TG) coupled with mass spectrometry were used to reveal the staging of the oxidation process depending on the additive content. It was found that samples containing 1 and 5 vol% MoSi2 had the lowest weight gain and the best oxidation behavior. The results of this study were supported by microstructural and phase analyses of the samples after isothermal treatment in a furnace. The samples with the lowest molybdenum disilicide content had a dense and thin protective oxide film on the surface, consisting of hafnium orthosilicate and monoclinic HfO2. The increase in the amount of MoSi2 contributed to the formation of a loose and porous oxide layer due to the increase in the concentration of volatile MoO3. However, all samples exhibited higher oxidation resistance compared to the pure Hf(C,N).

Thermal shock behavior of ZrB2-MoSi2-SiCw composites

Effect of SiCw volume fraction on thermal shock resistance (TSR) of ZrB2-20 MoSi2 composite has been studied in the present work. Three composites namely ZMSw0 (ZrB2-20 MoSi2-0 SiCw), ZMSw5 (ZrB2-20 MoSi2 - 5SiCw) and ZMSw20 (ZrB2-20 MoSi2-20 SiCw) were prepared by multi-stage spark plasma sintering at 1700 °C. TSR was measured using indentation quench technique. Results indicate that an increase in silicon carbide percentage decrease the crack growth rate and increase the critical temperature differential ΔTc, which in turn continuously raises the TSR of the composites. Development of thermal residual stresses due to thermal shock leads to excellent crack shielding and higher TSR of the ZMSw20 composite. The reduction in hardness and fracture toughness due to thermal shock for the ZMSw20 composite is lesser than that of both the ZMSw0 and ZMSw5 composites.

Crack-healing behaviour of MoSi2 dispersed Yb2Si2O7 environmental barrier coatings

MoSi2 doped Yb2Si2O7 composites were designed to extend the lifetime of Yb2Si2O7 environmental barrier coatings (EBCs) via self-healing cracks during high-temperature applications. Yb2Si2O7–Yb2SiO5–MoSi2 composites with different mass fractions were prepared by applying spark plasma sintering. X-ray diffraction results confirmed that the composites consisted of Yb2Si2O7, Yb2SiO5, and MoSi2. The thermal expansion coefficients (CTEs) of the composites increased with an increase in the MoSi2 content. The average CTE of the 15 wt% MoSi2 doped Yb2Si2O7 composite was 5.24 × 10−6 K−1, indicating that it still meets the CTE requirement of EBC materials. After being pre-cracked by using the Vickers indentation technique, the samples were annealed for 0.5 h at 1100 or 1300 °C to evaluate the crack-healing ability. Microstructural studies showed that cracks in 15 wt% MoSi2 doped Yb2Si2O7 composites were fully healed during annealing at 1300 °C. Two mechanisms may be responsible for crack healing. First, the cracks were filled with SiO2 glass formed by MoSi2 oxidation. Second, the formed SiO2 continued to react with Yb2SiO5 to form Yb2Si2O7, which can cause cracks to heal owing to volumetric expansion. The Yb2Si2O7 formation with smaller volume expansion is more beneficial.

Microstructures and mechanical properties of reactive spark plasma-sintered ZrB2–SiC–MoSi2 composites

Ternary ZrB2–SiC–MoSi2 composites with different nominal volume fractions of SiC (10–25 vol%) and MoSi2 (5–20 vol%) were sintered by reactive spark plasma sintering (RSPS) at 1650 °C and 35 MPa from the starting powders of ZrB2, Si, Mo and activated charcoal. The densification behaviour, microstructures and mechanical properties were comprehensively investigated. SiC clusters and MoSi2 were formed by in-situ reactions and the improved sintering efficiency was attributed to the mass transfer by liquid Si. The formation of a core–shell (Zr, Mo)B2 solid solution was confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). With the addition of the nominal MoSi2 content up to 15 vol%, the molar ratio of MoB2 in the solid solution shell increased owing to the increase of Mo; the densification and mechanical properties decreased owing to the softness of MoSi2, the cracks, and the large size of SiC clusters. The relative density, Vickers hardness, and fracture toughness of ZrB2–SiC–MoSi2 composites were significantly better than those of bulk ZrB2 owing to the combined effect of phase enhancement and solid solution strengthening.

Densification behavior of ZrB2–MoSi2–SiCw composite processed by multi stage spark plasma sintering

This study reports the densification behavior and novel design of core-rim microstructure for different ZrB2–MoSi2-SiCw (ZMSw) composites. Four ZrB2–MoSi2 composites containing various amounts of SiCw (0, 5, 10, and 20 vol%) were densified by multistage spark plasma sintering. Optimal combinations of sintering schedule, temperature, and composition are recognized for producing dense ZrB2-based ceramic-matrix composite with core-rim grain structure in the boride matrix, where ZrB2 core is surrounded by a solid solution of (Mo,W)Si2. Fragmentation, rearrangement, plastic deformation, and grain boundary diffusion mechanisms are responsible for densification of composites sintered by multistage SPS conditions at 1700 °C. Addition of SiCw significantly improves the density and mechanical properties of ZrB2–MoSi2–SiCw composites due to the formation of a core-rim structure.

Effect of in situ SiC and MoSi2 phases on the oxidation behavior of HfB2-based composites

Monolithic HfB2 and HfB2-15vol%SiC-15vol%MoSi2 composite samples were oxidized by a conventional electric furnace at 1700 °C for 5 h. Microstructural and phase analysis of the oxidized samples were performed by X-ray diffraction (XRD) analysis and field emission scanning electron microscope (FESEM) equipped with energy-dispersive spectroscopy (EDS). Besides, for analyzing the oxidation mechanism of the samples, thermodynamic calculations were also accomplished by HSC software. The changes in weight and thickness of the oxide scale were measured and the oxide growth rate of the oxidized samples was subsequently calculated. The results showed that HfB2-15vol%SiC-15vol%MoSi2 composite was much more resisted than that monolithic HfB2 due to the formation of a thin Si-rich glass layer on the surface of the composite sample. By acting to fill the porosities between HfO2 grains, Si-based glass phase enhanced the oxidation resistance of HfB2-15vol%SiC-15vol%MoSi2 composite. Conversely, the oxidized monolithic HfB2 had only a thick porous oxide layer (HfO2) which led to considerably lower oxidation resistance. On the other side, three layers containing HfO2 and Si-based glass phases were formed on the oxidized HfB2-15vol%SiC-15vol%MoSi2 composite. Moreover, no porosities and no porous layers were also detected on the oxidized composite sample. Consequently, HfB2-15vol%SiC-15vol%MoSi2 composite had a parabolic behavior owing to its diffusion-controlled oxidation under the isothermal oxidation process.

Synthesis, Characterization and Dry Sliding Wear Behaviour of Al 7075–MoSi2 Composites Prepared by Stir Casting Technique

Present work aims to develop Al composite reinforced by intermetallic particles. Such composites provide good wear resistance along with less harm to the counterpart in comparison with ceramic-reinforced composites and thereby performance of tribosystem is improved. In the present research work, intermetallic reinforcement molybdenum disilicide (MoSi2) has been used for making Al composites having different particle contents (2, 3, 4 and 5 wt%). The composites were synthesized by stir casting process and characterized by SEM and XRD. Dry sliding wear tests were carried out in a pin-on-disc wear testing machine at different applied loads of 30 N, 60 N and 90 N. Effects of particle content, applied load and sliding distance on density, hardness, volume loss, wear rate and mode of material removal/wear mechanisms have been analysed critically. It has been found that with increase in applied load, mode of material removal changes as ploughing, delamination, crater formation and plastically deformed layers. Particle content and frictional heating affect the wear characteristics. With increase in applied load, performance (wear resistance) of composite has been found to be improved when compared with that of the monolith.

Effect of Nb–Al–SiC elements combined with pre-oxidation treatment on the pesting resistance of MoSi2

Abstract Nb, Al, SiC elements were introduced into the matrix to improve the pest phenomenon of the MoSi2 composites. The results showed that although the oxidation rate of the composite decreased, the pest phenomena eventually occurred at the oxidation temperature of 500 °C and 750 °C. And after 120 h of oxidation, the bending strength of the composite decreased dramatically from 472 MPa to 258 MPa (500 °C) and 225 MPa (750 °C). In order to solve this problem, pre-oxidation has been adopted. During pre-oxidation at 1200 °C via exposure to air, a dense fibrous layer was in-situ formed, which had excellent adhesion to the matrix. The substrate which adjoins the interface of the oxide layer and matrix was oxidized into nano-sized grains. This layer and the nano-sized substrate played important roles in eliminating the pest phenomenon of the MoSi2 composite.

Microstructure dependent physical and mechanical properties of spark plasma sintered ZrB2-MoSi2–SiCw composites

A comparative evaluation has been carried out off the physical and mechanical properties of ZrB2–20 vol% MoSi2 composites reinforced with 5, 10 and 20 vol% SiC whiskers (SiCw). The abovementioned hybrid composites have been densified by spark plasma sintering (SPS) at 1600 °C for 10 min. The present study focuses on the effect of the quantity of SiCw on the densification, electrical conductivity, hardness, and fracture toughness ZrB2–MoSi2–SiCw composites in detail. Microstructural characterization suggests that SiCw restricts the grain growth of ZrB2 during densification. The electrical conductivities of the ZrB2–MoSi2–SiCw composites have been found to vary in the range ~2.4–3.5 × 106 S/m, and it reduce with increasing SiCw content. Results exhibit that the indentation fracture toughness of ZrB2–20 vol% MoSi2 improves ~70%, ~112% and ~130% with an addition of 5, 10 and 20 vol% SiCw, respectively. An increase in SiCw content leads to more amount of crack deflection and crack arrest that successively improves the fracture toughness of the composites.

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%.

Fracture toughness of thermally sprayed MoSi2 composite with different melting indices

Abstract Whether the in-situ multiphase doping, especially the metastable phase, makes the thermally sprayed materials tough or brittle is still unknown. In this sense, plasma sprayed MoSi2 composite was accordingly fabricated to investigate the effect of in-situ multiphase doping on the fracture toughness by melting index (M.I.) tuning. The fracture toughness of plasma sprayed MoSi2 composite was improved with M.I. increasing. The maximum fracture toughness can reach to 5.64 MPa m1/2 at a M.I. of 0.59, which was the highest among currently reported single-phase-derived thermally sprayed MoSi2 composites. A large number of metastable β generated in plasma sprayed MoSi2 composite were responsible for such remarkable improvement. The fracture toughness of the plasma sprayed MoSi2 composite, however, would decline as M.I. value exceeded 0.59. Over-heating derived crystallization, which decreased the positive effect of β, caused the fracture toughness deterioration.

Thermo – mechanical properties of SPS produced self-healing thermal barrier coatings containing pure and alloyed MoSi2 particles

Yttria – partially stabilised zirconia (YPSZ) MoSi2 composites have been designed to prolong the lifetime of the matrix by self – healing cracks during thermal cycling. The healing reaction at high temperatures is based on the decomposition of MoSi2, leading to a volumetrically expanding reaction product, which seals the crack. In this work, coefficient of thermal expansion (CTE) and the fracture toughness of composites containing MoSi2 particles, produced by spark plasma sintering (SPS) have been compared to conventional YPSZ. The CTE mismatch between YPSZ and MoSi2 was found to be small, implying that thermally induced mismatch stresses will be small and the composites have a similar CTE to conventional YPSZ. Fracture toughness was found not to be affected by the particles and showed similar values to unreinforced YPSZ. Cracks introduced by indentation have been shown neither to prefer, or avoid, the particles suggesting that such a composite system is capable of autonomously activating the self – healing reaction.

Dry sliding wear response of ZrB2-20vol.% MoSi2 composite

Abrasive wear behavior of ZrB2- 20 vol.% MoSi2 composite has been studied using electroplated diamond disk as the counter facing body. The investigated composite has been prepared by hot pressing and it has been found to possess 97% of the theoretical density. The microstructure of the composite has been characterized using scanning electron microscopy (SEM), and results show that MoSi2 particles are uniformly distributed in the ZrB2 matrix. The Young’s modulus, Poisson’s ratio and fracture toughness of the investigated composite are 455 GPa, 0.11 and 4.7MPa√m, respectively. The effect of experimental conditions including applied load, sliding velocity and sliding distance on the tribological behavior of ZrB2- 20 vol.% MoSi2 composite have been investigated. Experimental results show that applied load and sliding velocity plays an important role on specific wear rate and surface roughness of worn surfaces. SEM is used to analyze the wear surface morphology and crack propagation to study the wear mechanism. The dominant mode of wear was identified as abrasion. Grain pullout and micro cracking control the prevailing wear mechanism.

Scratch Testing of Hot-Pressed Monolithic Chromium Diboride (CrB2) and CrB2 + MoSi2 Composite

The tribological performance of hot-pressed monolithic CrB2 and a newly developed CrB2 + 20 vol.% MoSi2 composite was investigated by using scratch test. The test was carried out under progressive loading ranging from 0.9 to 30 N over a scratch distance of 3 mm. In situ values of coefficient of friction (COF), depth of penetration and acoustic emission were recorded. The wear volume and fracture toughness were also calculated. COF of both materials is increased with increasing the scratch length and progressive load. COF of the composite was observed to be slightly higher compared to the monolithic CrB2. The wear volume of the composite is 60% higher compared to monolithic CrB2. Fracture toughness values of ~2.48 and ~2.81 MPa m1/2 were calculated for monolithic CrB2 and CrB2 + 20 vol.% MoSi2 composite, respectively. Microstructural characterization indicates that the abrasive wear is the dominant wear mechanism in both the materials.

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.

Preparation and Characterization of (Mo, W)Si2-SiC Composites by In Situ Microwave Reaction Sintering

MoSi2-based materials are considered potentially useful for making high-temperature structural parts. In this work, (Mo, W)Si2-SiC composites have been successfully prepared in situ by microwave reaction sintering of the elemental reactants. Benefiting from the lower sintering temperature and faster densification procedure, the as-prepared composites have finer microstructures compared to the analogs prepared by conventional reaction sintering, resulting in a significant improvement of mechanical properties. The flexural strength, Vickers hardness and fracture toughness of 10%SiC/10%WSi2/80%MoSi2 are as high as 302.81 MPa, 8.55 GPa and 8.90 MPa m1/2, respectively, which are also better than those of monolithic MoSi2 and 10%SiC/90%MoSi2 composite due to the synergistic strengthening and toughening effects of W alloying and SiC second phase.