Fabrication and mechanical properties of SiCw/MoSi2–SiC composites by liquid Si infiltration of pyrolyzed rice husk preforms with Mo additions

Dense SiC ceramic matrix composites containing SiC whiskers (SiC_w) and MoSi₂ phase (SiC_w/MoSi₂–SiC) are fabricated by a liquid Si infiltration (LSI) method. Pyrolyzed rice husks (RHs) containing SiC whiskers, particles and amorphous carbon are mixed with different amounts of Mo powder to form preforms for the infiltration. Microstructure and mechanical properties of the composites are studied. Fracture mode of the composites is discussed. Results show that the SiC whiskers and fine particles in the pyrolyzed RHs were preserved in the composites after the LSI process. The amorphous carbon and Mo powder in the preforms reacted with molten Si, forming SiC and MoSi₂ in the composites. The presence of MoSi₂ in the composite increases the elastic modulus but lowers the flexure strength. Content of MoSi₂ of ca. 20 wt.% provides an enhanced fracture toughness of 4.1 MPa m^1/2 for the composite. But too large amount of MoSi₂ caused crack formation in the composite. The compressive residual stress introduced by the formation of MoSi₂ and SiC, and the de-bonding of the fine SiC particles and SiC whiskers from the residual Si phase are considered to favor the fracture toughness of the composites.     

Effects of Y2O3 on SiC/MoSi2 composite by mechanical-assistant combustion synthesis

MoSi₂ based materials are considered as a potential high temperature structural parts. In this work, a 0.5 wt% Y₂O₃–20 vol% SiC/MoSi₂ composite was successfully prepared by pressureless sintering from mechanical-assistant combustion synthesized powders. Adding a small amount of Y₂O₃ to the SiC/MoSi₂ composite decreased the apparent activation energy of sintering by 10.4%, resulting in a denser composite with finer grains. The relative density, flexural strength, Vickers hardness and fracture toughness of 0.5 wt% Y₂O₃–20 vol% SiC/MoSi₂ increased by 5.3%, 27.7%, 27.2% and 35.8% as compared to 20 vol% SiC/MoSi₂, respectively. The oxidation mass gain of Y₂O₃–20 vol% SiC/MoSi₂ at 1200 °C was higher than that of 20 vol% SiC/MoSi₂ for 16.9%, while it still exhibited very good oxidation resistance at this temperature.     

Microstructure and mechanical properties of TiB2–SiC ceramic composites by Reactive Hot Pressing

TiB₂–SiC ceramic composites with different contents of Ni as additive were prepared by the Reactive Hot Pressing (RHP) process at 1700 °C under a pressure of 32 MPa for 30 min. For comparison, a monolithic TiB₂ ceramic and TiB₂–SiC ceramic composite were also fabricated under the identical temperature, pressure and holding time by the Hot Pressing (HP) process. The effects of the fabrication process and Ni on the microstructure and mechanical properties of the composites were investigated. About 8 vol.% of elongated TiB₂ grains with an aspect ratio of 3–6 and a diameter of 0.5–1 μm were produced in the composite prepared by the RHP process. The improvement of the fracture toughness was attributed to the toughening and strengthening effects of SiC particles and the elongated TiB₂ grains such as crack deflection. The TiB₂–SiC–5 wt.% Ni ceramic composite had the optimum mechanical properties with a flexural strength of 858 ± 87 MPa, fracture toughness of 8.6 ± 0.54 MPa·m^1/2 and hardness of 20.2 ± 0.94GPa. The good mechanical properties were ascribed to the relatively fine and homogeneous microstructure and the strengthening effect of Ni. Ni inhibited the anisotropic growth of TiB₂.     

Structure and yield strength of directionally solidified Ni3Al intermetallic premelted with MoSi2 phase

Ni₃Al and MoSi₂ intermetallic phases were arc melted in Ni₃Al/MoSi₂ molar proportion of ten. Pure binary Ni₃Al and the quaternary alloy were subjected to directional solidification using the floating zone method at growth rates of 10–50 mm h⁻¹. The phase composition and structure of the crystals were analyzed using optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffractometry. The yield strength was studied in compression at 293–1293 K and in tension at room temperature. Compared to the binary Ni₃Al crystals, the quaternary Ni–Al–Mo–Si crystals revealed up to 5 times higher compressive yield strength at 400–800 K. Ni–Mo–Si C14 Laves phase precipitation in the L1₂ Ni₃Al+L1₀ NiAl matrix duplex phase was found in quaternary crystals. This precipitation is assumed to cause the observed mechanical behaviour.     

Microstructural development and mechanical properties of reactive hot pressed nickel-aided TiB2-SiC ceramics

TiB₂-SiC composites with different amounts of Ni (0, 2 and 5 wt.%) added as sintering aid were fabricated by reactive hot pressing (RHP). The mechanical properties were assessed under ambient conditions and the flexural strength was further tested in the temperature range of 700–1000 °C. The microstructures of the composites were characterized by a scanning electron microscope (SEM), transmission electron microscope (TEM) and energy-dispersive spectrometer (EDS). The flexural strength degradation mechanism occurring at elevated temperatures was studied. Addition of a moderate amount of Ni led to an improvement of the mechanical properties at room temperature. For the investigated ceramic composites, TiB₂-SiC-5 wt.% Ni sample showed significantly enhanced mechanical properties, i.e., a flexural strength of 1121 ± 31 MPa, a fracture toughness of 7.9 ± 0.58 MPa·m^1/2, a hardness of 21.3 ± 0.62 GPa, and a relative density of 98.6 ± 1.2%. Ni distributed along grain boundaries improved the interface strength. The improved fracture toughness was ascribed to crack deflection, grain rupture and crack shielding effect of Ni. A substantial strength degradation occurred at elevated temperatures, which was attributed to softening of the grain boundaries, surface oxidation and sliding of grain boundaries. The elastic modulus was found to decrease with increasing temperature.     

ZrB2-ZrSi2-SiC composites prepared by reactive spark plasma sintering

ZrSi₂ and SiC are good candidates to improve both sinterability and mechanical properties of ZrB₂ ceramics, which were synthesized simultaneously by an in-situ reaction of ZrC and Si additives during the sintering processing in this work. The ZrB₂ ceramic composites with different amount of ZrSi₂ and SiC were fabricated by reactive spark plasma sintering (RSPS) method. X-ray diffraction, scanning microscopy and Archimedes's method are used to characterize the phase, microstructure and density of the composites. Meanwhile, fracture toughness and flexural strength of the obtained composites were investigated too. It's found that a fully dense composite can be achieved at 1500 °C by SPS. Both fracture toughness and flexural strength of ZrB₂ ceramics increased with increasing the concentration of ZrSi₂ and SiC additives and reached a maximum of 7.33 ± 0.24 MPa·m^1/2 and 471 ± 15 MPa, respectively, with the ZrSi₂ + SiC content of 30 wt%.     

Effect of Ni content on the compression properties and abrasive wear behavior of the (TiB2–TiCxNy)/Ni composites

The (TiB₂–TiC_xN_y)/Ni composites were fabricated by the method of combustion synthesis and hot press consolidation in a Ni–Ti–B₄C–BN system. The effect of Ni content on the microstructure, hardness, compression properties and abrasive wear behavior of the composites has been investigated. The results indicate that with the increase in Ni content from 30 wt.% to 60 wt.%, the average size of the ceramic particles TiB₂ and TiC_xN_y decreases from 5 μm to ≤ 1 μm, while the hardness and the abrasive wear resistance of the composites decrease. The composite with the Ni content of 30 wt.% Ni possesses the highest hardness (1560.8 Hv) and the best abrasive wear resistance. On another hand, with the increase in the Ni content, the compression strength increases firstly, and then decreases. The composite with 50 wt.% Ni possesses the highest compression strength (3.3 GPa). The hardness and fracture strain of the composite with 50 wt.% Ni are 1251.2 Hv and 3.9%, respectively.     

Microstructure and mechanical properties of ZrB2–SiC composites prepared by gelcasting and pressureless sintering

ZrB₂–SiC ceramic composites were prepared through water-based gelcasting and pressureless sintering. Effects of the pressureless sintering temperature (1500–2000 °C), heating rate (5–15 °C/min) and soaking time (0.5–2 h) on the relative density, microstructure and mechanical properties of the ZrB₂–SiC composites were investigated in detail. A sintering temperature of 2000 °C, a heating rate of 5 °C/min and a soaking time of 2 h were found to be the optimal pressureless sintering procedure. The relative density, flexural strength and fracture toughness of the ZrB₂–SiC composite prepared under the optimum condition were 97.8%, 403.1 ± 27.8 MPa and 4.05 ± 0.42 MPa·m^1/2, respectively.     

Vacuum brazing of TZM alloy to ZrC particle reinforced W composite using Ti-28Ni eutectic brazing alloy

Reliable brazing of TZM alloy and ZrC particle reinforced (ZrC_p) W composite was achieved in this study by using Ti-28Ni eutectic brazing alloy. The typical interfacial microstructure of TZM/Ti-28Ni/ZrC_p-W brazed joint consisted of a Ti solid solution (Ti(s, s)) layer, a continuous Ti₂Ni layer and a diffusion layer mainly composed of W particles and (Ti, Zr)C particles. With an increase of brazing temperature, more ZrC particles and W particles entered the molten brazing alloy, which broadened the brazing seam and diminished the Ti₂Ni layer, resulting in the disappearance of the Ti₂Ni layer eventually. Meanwhile, more Ti(s, s) stripes were observed on the TZM side. The presence of continuous Ti₂Ni intermetallic phase and Ti(s, s) stripes structure in joints deteriorated the joining properties, which resulted in the formation of brittle fracture under shear test. In addition, the fracture path was related to the brazing temperature, and cracks initiate and propagate in the continuous Ti₂Ni layer at lower temperatures. However, the fracture path tended to be located at the TZM substrate close to the interface between TZM and the brazing seam when the brazing temperature exceeded 1040 °C. The optimal room temperature shear strength reached 120.5 MPa when brazed at 1040 °C for 10 min and the fracture surface exhibited cleavage fracture characteristics, and the shear strength at high temperature of 800 °C for the specimens with highest shear strength at room temperature reached 77.5 MPa.     

In situ synthesis,microstructure, mechanical properties and thermal shock resistance of (ZrB2 + SiC)/Zr2[Al(Si)]4C5 composites

Dense (ZrB₂ + SiC)/Zr₂[Al(Si)]₄C₅ composites with adjustable content of (ZrB₂ + SiC) reinforcements (0–30 vol.%) were prepared by in situ hot-pressing. The microstructure, room and high temperature mechanical and thermal physical properties, as well as thermal shock resistance of the composites were investigated and compared with monolithic Zr₂[Al(Si)]₄C₅ ceramic. ZrB₂ and SiC incorporated by in situ reaction significantly improve the mechanical properties of Z₂[Al(Si)]₄C₅ by the synergistic action of many mechanisms including particulate reinforcement, crack deflection, branching, bridging, “self-reinforced” microstructure and grain-refinement. With (ZrB₂ + SiC) content increasing, the flexural strength, toughness and Vickers hardness show a nearly linear increase from 353 to 621 MPa, 3.88 to 7.85 MPa·m^1/2, and 11.7 to 16.7 GPa, respectively. Especially, the 30 vol.% (ZrB₂ + SiC)/Zr₂[Al(Si)]₄C₅ composite retains a high modulus up to 1511 °C (357 GPa, 86% of that at 25 °C) and superior strength (404 MPa) at 1300 °C in air. The composite shows higher thermal conductivity (25–1200 °C) and excellent thermal shock resistance at ΔT up to 550 °C. Superior properties render the composites a promising prospect as ultra-high-temperature ceramics.     

Cfiber/SiCfiller/Si–B–C–Nmatrix composites with extremely high thermal stability

Precursor-derived Si–B–C–N ceramics are well known for their outstanding thermal stability up to 2000 °C. However, if they are integrated with long ceramic fiber fabrics, the thermal stability of the respective fiber–matrix composites decreases, and the associated thermomechanical properties worsen. A method of improving the thermal stability of a fiber-reinforced Si–B–C–N-based composite up to 1700 °C by the application of SiC filler particulates is reported. The mass loss of such composites is very low even after heating to 2100 °C. Remarkably, a pre-heat treatment of the SiC filler is essential in order to achieve the thermal stability of the ceramic matrix composites by removing surface SiO₂. The composite described here retained 96% of its room-temperature strength and possessed non-brittle fracture behavior after heating at 1700 °C for 10 h in Ar. The flexural creep deformation of the composite at 1400 °C was only 0.25% after 60 h under 100 MPa pressure.     

Effect of graphene platelets on tribological properties of boron carbide ceramic composites

The friction and wear behaviour of hot pressed boron carbide/graphene platelets (GPLs) composites have been investigated using the ball-on-flat technique with SiC ball under dry sliding conditions at room temperature. The hardness and fracture toughness of the investigated materials varied from 18.21 GPa to 30.35 GPa and from 3.81 MPa·m^1/2 to 4.60 MPa·m^1/2, respectively. The coefficient of friction for composites were similar, however the wear rate significantly decreased ~ 77% in the case of B₄C + 6 wt.% GPLs when compared to reference material at a load of 5 N, and ~ 60% at a load of 50 N. Wear resistance increased with increasing GPLs content in regards to the present graphene platelets, which during the wear test pulled-out from the matrix, exfoliated and created a wear protecting graphene-silicon based tribofilm.     

Comparison of microstructures and properties of Zr-based bulk metallic glass composites with dendritic and spherical bcc phase precipitates

In this paper, the microstructures and mechanical properties of the two BMG composites with the same composition of Zr_56.2Ti_13.8Nb_5.0Cu_6.9Ni_5.6Be_12.5, which contain in situ formed dendritic and spherical bcc β-Zr (Ti, Nb) solid solutions, are compared based on micrograph observations, XRD analysis and uniaxial compression tests at room temperature. The dendritic β phase exhibits well-developed primary dendrite axes with lengths of 20–50 μm and diameters of about 1–3 μm; the spherical β phase with the same volume fraction of about 30% as the dendritic β phase has a much larger average diameter of about 18 μm as compared to the primary dendrite axes of the β phase. Compression tests demonstrated that the yield stress, strain at the yield point, ultimate fracture strength and fracture plastic strain of the composite containing dendritic β phase are 1208 MPa, 1.78%, 1757 MPa and 8.82%, respectively. For the composites containing spherical β phase, however, a much larger fracture plastic strain of about 12% was achieved, and combined with a somewhat higher yield strength of 1350 MPa and a larger strain of 2.32% at the yield point, the ultimate fracture strength was measured to be about 1800 MPa.     

Effect of small aluminum additions on microstructure and mechanical properties of molybdenum di-silicide

The effect of Al alloying on the microstructure and properties of MoSi₂ was investigated. MoSi₂–2.8 and 5.5 at% Al (1.5 and 3 wt %, respectively) alloys showed a two phase microstructure comprising of α-Al₂O₃ and MoSi₂–Al alloy, while the MoSi₂–9 at% (5 wt %) Al alloy possessed a 3 phase microstructure: MoSi₂–Al alloy, Mo(Si,Al)₂ and α-Al₂O₃. Al additions to MoSi₂ reduced the SiO₂ phase, which is detrimental for high temperature strength of MoSi₂, forming Al₂O₃. The surplus Al entered into solid solution with the MoSi₂ and altered the lattice parameters. The Al in solid solution was 0.5 at% in MoSi₂–2.8 at% Al alloy, 2.5 at% in MoSi₂–5.5 at% Al alloy and 3.1 at% in the MoSi₂–9 at% Al alloy. The fracture toughness underwent only a moderate improvement in MoSi₂–2.8 at% and 5.5 at% Al alloys, but exhibited a significant 49% rise in the MoSi₂–9 at% Al alloy. Replacement of SiO₂ by Al₂O₃ in MoSi₂–Al alloys led to a significant improvement in the high temperature yield strength between 1100°C and 1250°C.     

Effect of ZrB2 and SiC addition on TiB2-based ceramic composites prepared by spark plasma sintering

TiB₂-based ceramic composites with different amounts of ZrB₂ and SiC were prepared by spark plasma sintering at 1700 °C with an initial pressure of 40 MPa and a holding time of 10 min. The (Ti_xZr_y)B₂ solid solution was found in the sintered TiB₂/ZrB₂/SiC composites by XRD. The microstructural and mechanical properties of the prepared samples were investigated. The composite with the addition of 30 vol.% ZrB₂ shows better comprehensive performances, and the bending strength and the fracture toughness of the composite are 780.5 MPa and 7.34 MPa m^1/2, respectively. The generation of the (Ti_xZr_y)B₂ solid solution makes the microstructures of the composites finer and more homogeneous, which has played a very important role in grain refinement and interface fusion.     

Temperature dependent tensile and fracture behaviors of Mo–10 wt.% Cu composite

Uniaxial tension tests are carried out for the Mo–10 wt.% Cu (Mo–10Cu) composite under a scanning electron microscope (SEM) at a temperature range from 25 °C to 725 °C. The stress–strain curves are obtained with both the tensile strength and the fracture strain peaked at 500 °C. Further raise of temperature would reduce the tensile strength and the fracture strain. In-situ SEM observations reveal the microstructure characteristics for Mo–10Cu composite at different temperatures. The fracture is of brittle inter-granular type when uni-axially tensioned at room temperature. As the temperature increases, formation of slip bands and linkage of micro-voids via plastic shear are observed. The fracture is characterized by mixed inter-granular fracture and plastic shear. The fracture is of predominantly plastic shear when uni-axially tensioned at 500 °C. Under uniaxial tension at temperatures higher than 650 °C, Mo–10Cu composite embrittles due to the insolubility of molybdenum and copper, and the activated grain boundary diffusion of Cu. These results are of importance for the basic understanding of the microstructure–mechanical properties relationship, as well as for the evaluation of Mo–Cu composites in practical applications.     

Densification and oxidation behavior of a novel TiB2–MoSi2–CrB2 composite

Titanium diboride (TiB₂) composite with MoSi₂ and CrB₂ has been prepared and tested to possess excellent oxidation resistance. Dense composite pellets were fabricated by hot pressing of powder mixtures. Microstructural characterization was carried out by XRD and SEM. Hardness and fracture toughness values were measured. Extensive oxidation studies of the composites were also carried out. Density of ≥ 96% ρ_th was obtained by hot pressing at 1800 °C under a pressure of 35 MPa for 1 h. Hardness and fracture toughness were in the range of 18–24 GPa and 3.5–4 MPa·m^1/2 respectively. Crack branching, deflection and bridging mechanisms were observed in the crack propagation paths. Isothermal and continuous oxidation studies of these composites up to 1000 °C showed improved oxidation resistance with the formation of protective glassy layer. TiO₂, Cr₂O₃ and SiO₂ phases were identified on the oxidized surface. Diffusion controlled mechanism of oxidation was observed in the composites.     

Synthesis of (Mo1−x–Crx)Si2 nanostructured powders via mechanical alloying and following heat treatment

MoSi₂–CrSi₂ nanocomposite powder was successfully synthesized by ball milling of Mo, Si and Cr elemental powders. Effects of the Cr content, milling time and annealing temperature were studied. X-ray diffraction (XRD) was used to characterize the milled and annealed powders. The morphological and microstructural evolutions were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). High temperature polymorph (HTP) of MoSi₂ begins to form after 50 h of milling and completes after 70 h of milling. MoSi₂–CrSi₂ composite powder was also prepared with a combination of short milling time (50 h) and low temperature annealing (850 °C). Annealing led to the HTP to low temperature polymorph (LTP) transformation of MoSi₂. MoSi₂–CrSi₂ nanocomposite powder with the mean grain size less than 50 nm was obtained at the end of milling. This composite maintained its nanocrystalline nature after annealing. A spherical morphology was procured for 50 h milled powder with 0.25 mole Cr.     

Measurement of scratch-induced residual stress within SiC grains in ZrB2–SiC composite using micro-Raman spectroscopy

An analytical framework for determination of scratch-induced residual stress within SiC grains of ZrB₂–SiC composite is developed. Using a “secular equation” that relates strain to Raman-peak shift for zinc-blende structures and the concept of sliding blister field model for scratch-induced residual stress, explicit expressions are derived for residual stress calculation in terms of phonon deformation potentials and Raman peak shift. It is determined that, in the as-processed composite, thermal expansion coefficient mismatch between ZrB₂ and SiC induces compressive residual stress of 1.731 GPa within the SiC grains and a tensile tangential stress of 1.126 GPa at the ZrB₂–SiC interfaces. With increasing scratch loads, the residual stress within the SiC grains becomes tensile and increases in magnitude with scratch load. At a scratch load of 250 mN, the calculated residual stress in SiC was 2.6 GPa. Despite this high value, no fracture was observed in SiC grains, which has been rationalized based on fracture strength calculations from Griffith theory.     

Microstructure and some properties of TiAl-Ti2AlC composites produced by reactive processing

The TiAl–Ti₂AlC composites with and without impurities, Ni, Cl and P, were prepared by combustion reaction from the elemental powders and cast after arc melting. The resulting composites had about 18 vol% Ti₂AlC in the lamellar matrix consisting of γ-TiAl and Ti₃Al (α₂). In the homogenized specimens, the α₂ phase decomposed to γ-TiAl and Ti₂AlC. The composite material had a high strength both at ambient and elevated (1173 K) temperatures; about 800 and 400 MPa, respectively, with an ambient temperature ductility of 0.7% at bending test. The fracture toughness test also proved that the homogenized composite has higher toughness than the as cast one. The toughness value reached to 17.8 MPa m^1/2. The zigzag cracks propagated in the homogenized composite and the reinforcement Ti₂AlC particles and the finely precipitated Ti₂AlC particles were main obstacles to the crack propagation. The composite with impurities showed a marginal improvement in the oxidation resistance over the composites without impurities.