New approach to MoSi2/SiC intermetallic-ceramic composite with B4C

The effects of SiC and B₄C additives in the MoSi₂ matrix on the microstructures and mechanical properties at room temperature were investigated. Their coefficients of thermal expansion (CTE) were also evaluated up to 1200°C by a thermal mechanical analysis (TMA). The experimental results show that the Mo₂B₅ reinforced phase was formed in situ in the hot-pressed MoSi₂/SiC/B₄C composites. Both the Mo₂B₅ phase and the SiC phase significantly improved the mechanical behavior of MoSi₂. Besides, the SiC with a high content up to 40 vol% could be added into the MoSi₂ composite with the B₄C additive. As a result, a dense and homogenous MoSi₂/SiC/B₄C composite was obtained, which possessed a relatively high bending strength and fracture toughness. Meanwhile, the CTE of the MoSi₂/SiC/B₄C composites linearly decreased with the increasing SiC content, which dropped to 21% at 1200°C in comparison with the pure MoSi₂ when adding 40 vol% SiC. This MoSi₂/SiC/B₄C composite system is very important for developing new applications at elevated temperature, particularly for high-temperature coating applications.     

Investigation of dynamic Young's modulus for molybdenum disilicide-titanium trisilicide (MoSi2-Ti5Si3)

Molybdenum disilicide-titanium trisilicide (MoSi₂-Ti₅Si₃) is a ceramic matrix composite (CMC) created for the purpose of improving the mechanical properties of molybdenum disilicide. Five alloys of this CMC were prepared by varying the ratio of volume per cent Ti₅Si₃ to volume per cent MoSi₂. Evaluation of the specimens using the piezoelectric ultrasonic composite oscillator technique (PUCOT) determined the values of Young's modulus to range from 303 to 378 GPa at room temperature increasing with volume fraction of MoSi₂. The values of Young's modulus for temperatures up to about 400 °C for each alloy decrease linearly with respect to increasing temperature. Damping within the specimens is independent of strain amplitude at room temperature, but shows some strain amplitude dependence at higher temperatures. The values of density for the alloys, determined using Archimedes' method, range from about 5200 to 5900 kg m^–3, and compare favourably with the values determined by the rule of mixtures.     

Novel synthesis of MoSi2 and MoSi2-Al203 ultrafine powders

Abstract

Molybdenum disilicide (MoSi₂) is a promising high temperature material with a high melting point (2032°C), low density (6.24 Mg m^-3 ), excellent high temperature oxidation resistance (up to about 1900°C), and metal like thermal and electrical conductivities. For structural applications, the formation of composites can be benejicial. Alumina (Al₂O₃) has a thermal expansion coefficient close to that of MoSi₂ in a wide temperature range from 200°C to 1400°C. Improvement in elevated temperature strength has been reported by the formation of a composite between MoSi₂ and A1₂0₃. However, the brittle nature of MoSi₂ based materials at medium to low temperatures remains an obstacle for their applications. One possible solution is to develop microstructures with ultrajine grains so that ductility can be improved through the grain boundary sliding mechanism. The present work gives preliminary results related to the synthesis ofultrajine MoSi₂-Al₂0₃ composite powders. The MoSi₂ powders (0.1–4 μm) were produced by an electric arc discharge process under varying voltage and capacitance. Three powder generating mechanisms have been identified. The influence of processing parameters (voltage, capacitance, and energy input) on the powder characteristics is discussed. Finally, the synthesis of MoSi₂-A1₂0₃ composite powders by coating individual MoSi₂ particles with an alumina gel through a sol-gel process is also demonstrated.     

Preparation and mechanical properties of ZrB2-based ceramics using MoSi2 as sintering aids

ZrB₂ (zirconium diboride)-based ceramics reinforced by 15vol.% SiC whiskers with high density were successfully prepared using MoSi₂ as sintering aids. The effects of sintering condition and MoSi₂ content on densification behavior, phase composition, and mechanical properties of SiC_w/ZrB₂ composites were studied. Nearly, fully dense materials (relative density >99%) were obtained by hot-pressing (HP) at 1700°C–1800°C in flow argon atmosphere. The grain size of ZrB₂ phase in the samples sintered by HP at 1700°C–1800°C were very fine, with mean size below 5 μm. Mechanical properties (such as flexural strength, fracture toughness, and Vickers hardness) of the sintered samples were measured. The sample with 15vol.% MoSi₂ addition sintered by HP at 1750°C displayed the best mechanical properties.     

Silicide–carbide composites obtained from alloyed melt infiltration

The development of a C_f/(Mo, Ti)Si₂–SiC composite using melt infiltration technique was investigated. C/C preforms and also C_f-felts were infiltrated with an alloyed melt of Si, Ti and MoSi₂. The amount of each element was selected so that the melting point of the alloy was lower than 1600 °C. It was then possible to prevent the melt from reacting heavily with the carbon fibers and preserve their reinforcing effect in case of the C/C preforms. After infiltration no residual silicon could be detected in the matrix of the infiltrated C/C composites. The infiltrated C/C samples reached a maximum bending strength of 210 MPa at room temperature. At 1600 °C there is even an increase in their bending strength to 250 MPa. Infiltrated felts showed monolithic and brittle characteristics. Their bending strength at room temperature was not higher than 150 MPa. Because of softening of the residual silicon, the strength of the infiltrated felts was reduced at high temperatures. The felt samples which were infiltrated with an alloyed melt showed higher mechanical strength than pure silicon infiltrated felts both at room temperature and at 1600 °C.     

Electrical and thermal conductivities of porous SiC/SiO2/C composites with different morphology from carbonized wood

Porous SiC/SiO₂/C composites exhibiting a wide range of high thermal and electrical conductivities were developed from carbonized wood infiltrated with SiO₂. As a pre-treatment, the samples were either heated at 100 °C or kept at room temperature followed by sintering in the temperature range 1200–1800 °C. The microstructure, the morphology, and the electrical and thermal conductivities of the composites were investigated. Pre-treatment at room temperature followed by sintering up to 1800 °C produced composites exhibiting a greater size of carbon crystallites, a higher ordering of the microstructure of carbon and β-SiC and a smaller amount of SiO₂, resulting in electrical and thermal conductivities of 1.17 × 10⁴ Ω⁻¹ m⁻¹ and 25 W/mK, respectively. The thermal conductivity could be further improved to 101 W/mK by increasing the density of the composite to 1.82 g/cm³. In contrast, the pre-treatment at 100 °C produced composites possessing a lower thermal conductivity of 2 W/mK.     

Deposition and characterisation of MoSi2 films

Deposition of MoSi₂ films on silicon and tantalum substrates applying pulsed laser deposition technique has been performed. Crystalline, hexagonal symmetry, MoSi₂ films were prepared directly from stoichiometric MoSi₂ tetragonal target on room temperature and heated substrates (500 °C). Textured MoSi₂ films having privileged (110) and (115) orientations and average crystallite size of about 105 nm were grown on Si(111) substrates with a good degree of axial texture (rocking curve full width half maximum of 1.5°). MoSi₂ films grown on Ta(211) substrates, instead, turned out to be polycrystalline, with an average crystallite size of about 100 nm and 50 nm on substrates kept at room temperature and at 500 °C, respectively. Vickers hardness for 1.2 μm thick MoSi₂ films on Si(111) substrates resulted to be 15 GPa both at room temperature and 500 °C, while for 0.4 μm thick MoSi₂ films on Ta(211) substrates — 26 GPa at room temperature and 30 GPa at 500 °C.     

Room and high temperature tensile tests on the AA6061/10vol.%Al2O3p and AA7005/20vol.%Al2O3p composites

Aluminium alloy based Metal Matrix Composites (MMCs), reinforced with ceramic particles such as Al₂O₃ or SiC, have a number of advantages over conventional aluminium alloys, primarily enhanced stiffness and increased wear resistance. In order to improve the fields of application, fundamental understanding of the relationship between microstructural features and mechanical properties is however required. In this work, the tensile behaviour of two composites based on 6061 and 7005 aluminium alloys, reinforced with Al₂O₃ particles, at room temperature, at 100°C and at 150°C was studied. The ductility of the composites was found to be much lower than that of the unreinforced alloys, while a significative increase of the elastic modulus and tensile strength was found. Both materials showed a slight decrease of the tensile strength with temperature, while elongation increased. Large particles and clusters of the reinforcement were found to be locations prone to failure in the composite, due to the high stress concentrations, mainly at room temperature. With increasing temperature, the fracture surfaces showed a dimpled appearance with a large amount of plastic deformation of the matrix, indicating that void nucleation, growth and coalescence is the main fracture mechanism.     

Microstructure and mechanical properties of RB-SiC/MoSi2 composite

Microstructure, high temperature strength and oxidation behaviour of reaction bonded silicon carbide, RB-SiC/17 wt% MoSi₂ composite prepared by infiltrating a porous RB-SiC bulk (after removal of free silicon) with molten MoSi₂ were investigated. There was good bonding between the SiC and MoSi₂ particle, without a significant reaction zone and microcracking caused by the thermal mismatch stresses. A thin (2 nm) layer, however, was observed at the SiC/MoSi₂ interfaces. At room temperature, the composite exhibited a bending strength of 410 MPa, which is 20% loss in comparison to that of RB-SiC alone (containing 10 wt% free silicon). However, the composite strength increased to a maximum of 590 MPa in the temperature range 1100 and 1200° C and dropped to 460 MPa between 1200 to 1400° C, after which the strength remained constant. The passive oxidation of the composite in dry air in the temperature range 1300 to 1400° C was found to follow the parabolic rate law with the formation of a protective layer of cristobalite on the surface.     

Room temperature mechanical properties and high temperature oxidation behavior of MoSi2 matrix composite reinforced by adding La2O3 and Mo5Si3

MoSi₂ matrix composites containing 0.8 wt.%La₂O₃ and different volume fractions of Mo₅Si₃ were synthesized by self-propagating high temperature synthesis. The room temperature mechanical properties and high temperature oxidation behavior at 1200 °C were studied. Results showed that La₂O₃ and Mo₅Si₃ caused the grain size to decrease of the MoSi₂ matrix composite. The flexure strength and fracture toughness are improved compared with pure MoSi₂. The strengthening mechanism of La₂O₃–Mo₅Si₃/MoSi₂ is fine-grain strengthening, and its toughening mechanisms are fine-grain toughening, crack deflection, crack branching and crack bridging. With an increase in Mo₅Si₃ content, the oxidation resistance gradually decreased. This is attributed to the poor oxidation resistance of Mo₅Si₃, grain refinement and relative density decrease of the composites. In this experiment, a La₂O₃–Mo₅Si₃/MoSi₂ composite was found to have optimal mechanical properties and high temperature oxidation resistance after adding 0.8 wt.%La₂O₃ and 16.3 wt.% Mo₅Si₃.     

Thermal history induced variable relaxor behavior in the high TC ternary ferroelectric 0.6Bi(Mg1/2Ti1/2)O3–0.05Bi(Zn1/2Ti1/2)O3–0.35PbTiO3

Studies carried out on a perovskite-structured rhombohedral 0.6Bi(Mg_1/2Ti_1/2)O₃–0.05Bi(Zn_1/2Ti_1/2)O₃–0.35PbTiO₃ (xBZT–yBMT–zPT) ceramic quenched from temperatures below 1000 °C show that the dielectric properties are dramatically altered by the thermal history. Samples quenched from temperatures 650 °C–900 °C show classical ferroelectric switching behavior that is not observed on either side of this temperature range. The quenched states lose their switchable ferroelectric properties when heated to temperatures as low as 400 °C. The results demonstrate for the first time that the dielectric and electromechanical response, as observed at room temperature, can be varied between normal to relaxor behavior by changing thermal quenching conditions.     

Ferroelectric studies and impedance analysis of PLZT (10/52/48) electroceramics

Perovskite Pb_0.90La_0.10(Zr_0.52Ti_0.48)O₃ ceramic material was prepared through sol–gel process. Structural, phase formation and thermal properties were confirmed by X-ray diffraction, thermogravimetry and differential thermal analysis; size and microstructural study was carried out using particle size analyser and scanning electron microscope. The electrical properties of the ceramics were investigated as a function of both temperature (from room temperature to 500 °C) and frequency (from 100 Hz to 1 MHz) using complex impedance spectroscopy (CIS). The impedance spectrum results indicate the decrease in dielectric constant with increase in frequency while the dielectric loss increases with frequency. The activation energy of the sample was calculated from the slope of the Arrhenius plot as 0.129 eV from the Arrhenius’s plot of dc conductivity versus inverse of absolute temperature. The dc conductivity was obtained from CIS measurements and the activation energy. The remnant polarization (P_r) and coercive electric fields (E_c) are found out as 6.52 μC/cm² and 2.55 kV/cm from the ferroelectric loop measured at room temperature.     

Templated grain growth of textured mullite/zirconia composites

Mullite is an attractive material for advanced ceramic applications, but its low fracture toughness prevents it from widespread industrial applications. Therefore, mullite/zirconia composites were prepared from a reactive mixture of alumina and zircon with additives of TiO₂ and MgO to increase mechanical properties and densification. <001> aluminum borate templates were used to nucleate, and texture mullite in [001]. Mullite/zirconia formation started at 1350 °C and was complete at 1450 °C. Dense mullite/zirconia composites with highly textured mullite were produced after sintering at 1450 °C. A relatively constant tetragonal ZrO₂ content of 11±2 wt.% was retained at room temperature after sintering between 1350 and 1550 °C. A high quality of texture with an orientation parameter of 0.22 and a very narrow distribution of elongated mullite grains within 8.8° around [001] were successfully produced.     

Multistage homogenization process of aluminum alloy 2618

Single-, two-, and three-stage homogenization treatments of heat-resistant alloy 2618 were conducted in this study. Results reveal a low melting point Al₂CuMg phase and high melting point Al₂Cu phase in the as-cast aluminum alloy 2618. After single-stage homogenization at 495 °C for 10 h, the Al₂CuMg phase dissolves completely, but the Al₂Cu phase cannot be completely dissolved even once the homogenization time is prolonged to 18 h. After the alloy 2618 are homogenized using two stages: 495 °C for 10 h and 520 °C for 8 h, a portion of the Al₂Cu phase remains in the alloy. The Al₂Cu phase remains undissolved even after prolonged time. After the two-stage homogenization treatment at 495 °C for 10 h and 540 °C for 5 h, the high melting point Al₂Cu phase completely dissolves but overburn occurs. After the alloy 2618 are homogenized using three stages at 495 °C for 10 h, 520 °C for 5 h, and 540 °C for 3 h, the Al₂Cu phase completely dissolves and no overburn is detected. The three-stage homogenization treatment is an effective method for dissolving the high melting point Al₂Cu phase in the alloy 2618 and increasing their overburn temperature and solid solution temperature.     

Tensile Stress Rupture Behavior of a Woven Ceramic Matrix Composite in Humid Environments at Intermediate Temperature — Part I

The stress rupture strength of the SYL-iBN/BN/SiC composite was evaluated at 550 and 750 °C with moisture content levels of 0.0, 0.2, and 0.6 atm partial pressure of water vapor, pH₂O. The stress rupture strengths decreased with respect to time with the rate of decrease related to the temperature and the amount of moisture content. In all cases the degradation was more severe initially and then approached a run-out threshold level. The thresholds were reached at approximately 100+, 60, 80 h for the 550 °C with 0.0, 0.2, and 0.6 pH₂O, respectively. The thresholds were reached at approximately 40, 20, and 10 h for the 750 °C cases. The interpolated stress rupture strengths at 100 h for 0.0, 0.2, and 0.6 pH₂O at 550 °C were 82%, 68%, and 51% of the room temperature monotonic tensile strength. At 750 °C these strengths were 67%, 51%, and 50%. Analysis of Field Emission Scanning Electron Microscopy images showed evidence of embrittlement of the fiber/matrix interphase. Little to no embrittlement was observed at both temperatures with 0.0 pH₂O. At both 550 and 750 °C with 0.2 and 0.6 pH₂O, evidence of embrittlement increased with temperature and test duration with the most extensive embrittlement observed at 750 °C with 0.6 pH₂O.     

Influence of the substrate temperature on the properties of MoSi2 thin films

Investigations of the microcrystalline structure of sputtered MoSi₂ thin films deposited at temperatures in the range 60–500 °C are described. At low temperatures the films are amorphous, and in the higher range of temperature hexagonal MoSi₂ is detected. Annealing the films at 960 °C to form the low resistivity tetragonal MoSi₂ phase results in different grain sizes depending on the substrate temperature during deposition.     

Electrical properties of Na1/2Nd1/2TiO3 Ceramics

The polycrystalline sample of Na_1/2Nd_1/2TiO₃ was prepared by a high-temperature solid-state reaction technique. The formation of the compound was confirmed by both XRD and EDX studies. Preliminary structural analysis ofNa_1/2Nd_1/2TiO₃ using X-ray diffraction data exhibits a tetragonal phase of the material at room temperature. The dielectric permittivity and the loss tangent of the pellet sample were obtained in a wide frequency range (1 kHz to 1 MHz) at different temperatures (30 °C to 425 °C). The dielectric anomaly at 114 °C, appearance of hysteresis loop and piezoelectric properties at room temperature confirmed the ferroelectric properties of the compound. Measurements of frequency and temperature dependence of impedance over a wide frequency range (100Hz–1MHz) were carried out by complex impedance spectroscopy as a non-destructive tool and indicate that the electrical properties of the material are strongly temperature dependent. Evidence of temperature dependence of electrical relaxation phenomenon as well as the negative temperature coefficient (NTC)-type of resistance behavior of the sample has also been observed. The dc conductivity graph follows the Arrhenius law. Studies of dielectric modulus suggest the non-Debye type of relaxation in the materials, which is supported by the impedance data.     

Preparation and mechanical properties of ZrB<Subscript>2</Subscript>-based ceramics using MoSi<Subscript>2</Subscript> as sintering aids

ZrB₂ (zirconium diboride)-based ceramics reinforced by 15vol.% SiC whiskers with high density were successfully prepared using MoSi₂ as sintering aids. The effects of sintering condition and MoSi₂ content on densification behavior, phase composition, and mechanical properties of SiC_w/ZrB₂ composites were studied. Nearly, fully dense materials (relative density >99%) were obtained by hot-pressing (HP) at 1700°C–1800°C in flow argon atmosphere. The grain size of ZrB₂ phase in the samples sintered by HP at 1700°C–1800°C were very fine, with mean size below 5 μm. Mechanical properties (such as flexural strength, fracture toughness, and Vickers hardness) of the sintered samples were measured. The sample with 15vol.% MoSi₂ addition sintered by HP at 1750°C displayed the best mechanical properties.     

Joining of Cf/SiBCN composite with CuPd-V filler alloy

Two compositions of CuPd-V system filler alloy were designed for joining the Cf/SiBCN composite. Their dynamic wettability on the Cf/SiBCN composite was studied with the sessile drop method. The CuPd-8 V alloy exhibited a contact angle of 57° after holding at 1170℃ for 30 min, whereas for CuPd-13 V alloy,a lower contact angle of 28°can be achieved after heating at 1200 ℃ for 20 min. Sound C_f/SiBCN joints were successfully produced using the latter filler alloy under the brazing condition of(1170-1230)℃for 10 min. The results showed that the active element V strongly diffused to the surface of Cf/SiBCN composite, with the formation of V_2 C/VN reaction layer. The microstructure in the central part of the joint brazed at 1200 ℃ was characterized by the V_2 C/VN particles distributing scatteringly in CuPd matrix. The corresponding joints showed the maximum three-point bend strength of 82.4 MPa at room temperature.When the testing temperature was increased to 600 0 C, the joint strength was even elevated to 108.8 MPa.Furthermore, the joints exhibited the strength of 92.4 MPa and 39.8 MPa at 800 ℃ and 900 ℃, respectively.     

Effect of infiltrants on the electrical resistivity of reaction-sintered silicon carbide

The effect of infiltrants on the electrical resistivity of reaction-sintered silicon carbide, at temperatures ranging from RT to 1000°C, has been studied. Electrical resistivity decreases with increase in temperature up to 1000°C in VC and MoSi₂, whereas minimum electrical resistivity is observed at ∼600°C in B₄C infiltrant.