In situ pressureless sintering of SiC/MoSi2 composites

SiC-reinforced MoSi₂ composites have been successfully prepared by in situ pressureless sintering from elemental powders of Mo, Si and C. Meanwhile, the evolutions of the samples’ microstructure and phase at different temperatures were investigated by using X-ray diffraction (XRD) and scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDS). It can be seen that at the temperature of 1100 °C, the main phases were Mo and Si, accompanying with a small amount of rich molybdenum products Mo₅Si₃ and Mo₃Si. Then the main phases changed to MoSi₂ and SiC when the sintering temperature reached 1300 °C. Finally we obtained MoSi₂/SiC composites with well-dispersed SiC particles after sintering at the temperature of 1550 °C for 120 min. The evolution of porosity in these composites fits the porosity reduction model well developed by Pines and Bruck, which revealed the particle agglomeration in the composites. The flexural strength and fracture toughness of 10% SiC/MoSi₂ composites were up to 274.5 MPa and 5.5 MPa m^1/2, increased by approximately 40.8% and 30.6% compared with those of monolithic MoSi₂, respectively.     

Self-propagating high-temperature synthesis of advanced ceramics MoSi2–HfB2–MoB

This study focuses on the investigation of the macrokinetic features of SHS (combustion synthesis) of elemental mixtures Mo–Hf–Si–B, in particular the mechanisms of structure and phase formation in the combustion front as well as the structure and properties of consolidated ceramics. Two routes for the fabrication of the composite SHS powder in system MoSi₂–HfB₂–MoB were used: (1) synthesis using Mo–Si–B and Hf–B mixtures followed by mixing of the combustion products and (2) synthesis using the four-component Mo–Hf–Si–B mixture. Dense ceramic samples with a homogeneous structure and low residual porosity (0.8–3.6%) were prepared by hot pressing of SHS powders. Although the particles size distribution and phase composition of SHS powders are similar for both synthesis routes, the structure and properties of both the composite SHS powders and hot-pressed ceramics differ considerably. Synthesis using the four-component Mo–Hf–Si–B mixture allows one to produce hierarchically ordered nanocomposite material with improved mechanical properties: hardness up to 17.6?GPa and fracture toughness up to 7.16?MPa?m^1/2.     

Sintering behavior and mechanical properties of Ta4HfC5-based composites with ZrB2 additive

Ta₄HfC₅ powder was synthesized using TaCl₅, HfCl₄ and phenolic resin as raw materials. Then, Ta₄HfC₅–10 vol% MoSi₂ ceramics and Ta₄HfC₅–10 vol% MoSi₂ with different proportions of ZrB₂ (10 – 30 vol%) ceramics were sintered by spark plasma sintering. Zr atoms substituted Ta and Hf atoms in Ta₄HfC₅ during the sintering process at 2000 °C. The sintering behavior and microstructure evolution upon the ceramics are discussed. The mechanical properties of the composites were improved compared to the pure Ta₄HfC₅ ceramics. The hardness of Ta₄HfC₅–MoSi₂ with 30 vol% ZrB₂ increased from around 10 GPa to almost 13 GPa, the flexural strength increased from around 245–435 MPa, and the fracture toughness increased from 2.56 ± 0.12 MPa?m^1/2 to 4.46 ± 0.20 MPa?m^1/2.     

Advantages and disadvantages of graphite addition on the characteristics of hot-pressed ZrB2–SiC composites

ZrB₂–SiC–graphite composites with 0–35 vol% graphite flakes were densified via hot-pressing route at the temperature of 1800 °C under the uniaxial pressure of 40 MPa for 1 h. Consolidation, mechanical properties, and microstructure of hot-pressed composites were investigated by variation of graphite content. By the addition of graphite, the relative density of composites increased, and at this hot pressing condition, fully densified composites were fabricated. The highest flexural strength of 366 MPa was measured for composite containing 7.5 vol% graphite, while the maximum Vickers hardness resulted in 2.5 vol% graphite doped one, and its value was equal to 20.8 GPa. Phase analysis of hot-pressed samples revealed the formation of the Zr₃C₂ and B₄C phases besides the main existing ZrB₂, SiC, and graphite phases. The newly carbide phases formed at the surface of ZrB₂ grains. The addition of graphite into the ZrB₂–SiC composites improved the sintering process and caused a fine-grained microstructure.     

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.m^1/2. These excellent mechanical properties indicate that MoB and SiC particulate reinforced MoSi₂ composites could be promising candidates for structural applications.     

Pressureless sintering of high-entropy boride ceramics

High-entropy boride ceramics were densified by pressureless sintering. Green densities of the ceramics varied by composition with the highest green density of 53.6 % for (Hf, Nb, Ta, Ti, Zr)B₂. After pressureless sintering, relative densities up to ∼100 % were obtained for (Cr, Hf, Ta, Ti, Zr)B₂ and (Hf, Ta, Ti, V, Zr)B₂. Two compositions, (Hf, Ta, Ti, W, Zr)B₂ and (Hf, Mo, Ti, W, Zr)B₂ contained secondary phases and did not reach full density. All compositions had average grain sizes less than 10 µm and less than 2 vol % of residual B₄C. This is the first report of conventional pressureless sintering of high-entropy boride ceramics powder compacts without evidence of liquid phase formation.     

Dynamic compression behavior of reactive spark plasma sintered ultrafine grained (Hf,Zr)B2–SiC composites

This paper reports the dynamic compression behavior of ultrafine grained (Hf, Zr)B₂–SiC composites, sintered using reactive spark plasma sintering at 1600 °C for 10 min. Dynamic strength of~2.3 GPa has been measured using Split Hopkinson Pressure Bar (SHPB) tests in a reproducible manner at strain rates of 800–1300 s⁻¹. A comparison with competing boride based armor ceramics, in reference to the spectrum of properties evaluated, establishes the potential of (Hf, Zr)B₂–SiC composites for armor applications.     

Effect of temperature on the structure and mechanical properties of SiC–TiB2 composite ceramics by solid-phase spark plasma sintering

SiC composite ceramics have good mechanical properties. In this study, the effect of temperature on the microstructure and mechanical properties of SiC–TiB₂ composite ceramics by solid-phase spark plasma sintering (SPS) was investigated. SiC–TiB₂ composite ceramics were prepared by SPS method with graphite powder as sintering additive and kept at 1700 °C, 1750 °C, 1800 °C and 50 MPa for 10min.The experimental results show that the proper TiB₂ addition can obviously increase the mechanical properties of SiC–TiB₂ composite ceramics. Higher sintering temperature results in the aggregation and growth of second-phase TiB₂ grains, which decreases the mechanical properties of SiC–TiB₂ composite ceramics. Good mechanical properties were obtained at 1750 °C, with a density of 97.3%, Vickers hardness of 26.68 GPa, bending strength of 380 MPa and fracture toughness of 5.16 MPa m^1/2.     

In situ reactive spark plasma sintering of WSi2/MoSi2 composites

MoSi₂ based materials have the potential for use in high temperature structural parts. In this work, WSi₂ reinforced MoSi₂ composites were successfully prepared by mechanical activation followed by in situ reactive spark plasma sintering of Mo, Si, and W elemental powders. Benefiting from the high energy raw materials prepared through ball milling, these mechanically activated reactants started to transform into MoSi₂ at 1000 °C. Full density composites were obtained at a low sintering temperature (1200 °C) within 5 min. The addition of W to the reactants led to a finer microstructure than that obtained using pure MoSi₂, resulting in a significant improvement of mechanical properties. The Vicker's hardness of 20 vol% WSi₂/MoSi₂ was as high as 16.47 GPa.     

High-temperature oxidation resistance of spent MoSi2 modified ZrB2–SiC–MoSi2 coatings prepared by spark plasma sintering

The spent MoSi₂ modified ZrB₂–SiC–MoSi₂ coatings were prepared on carbon matrixes by spark plasma sintering. A continuous metallurgical bonding was formed at the interface between the coating and matrix, and no obvious defects such as pores and cracks were observed inside. The effects of spent MoSi₂ content and trace doping in the spent powder on the oxidation behavior of the coatings in air at 1700 °C were investigated. During the active oxidation stage, the spent MoSi₂ promoted the densification of the coating and enhanced the structural oxygen barrier properties. With the increase of service time, during the inert oxidation stage, doping an appropriate amount of spent MoSi₂ helped to increase the fluidity of the rich-SiO₂ protective layer so that the Zr oxides fully dispersed in the generated Zr–B–Si–O–Al multiphase glass layer, which could impede the penetration of oxygen and enhance the oxidation protection efficiency. However, excessive spent MoSi₂ exacerbated the volatilization of gas by-products, forming pores and cracks in the glass layer and rising the oxidation loss. When the content of spent MoSi₂ was 20 vol%, the glass layer is dense and uniform, with few defects and the best oxygen resistance property. Moreover, compared with commercial powders, spent MoSi₂ contained Al₂O₃ and SiO₂. Al₂O₃ had an excellent modification effect, while SiO₂ glass can promote liquid phase sintering and seal the defects in the coatings. By adding spent MoSi₂, the modified ZrB₂–SiC–MoSi₂ composite coatings could inhibit the formation of defects and improve the dynamic stability of the coatings effectively.     

High Hardness and Ductile Mosaic SiC/SiO2 Composite by Spark Plasma Sintering

SiC powder was coated with SiO₂ layer by chemical vapor deposition, and the SiC(core)/SiO₂(shell) composite powder was consolidated to a SiC/SiO₂ composite with a mosaic microstructure by spark plasma sintering (SPS) at 1923 K for 1.8 ks. The SiC(core)/SiO₂(shell) powder with a 80–100 nm thick SiO₂ layer resulted in a SiC/SiO₂ composite with a relative density of 97% and hardness and fracture toughness of 17.1 GPa and 8.4 MPa m^1/2, respectively.     

Preparation and properties of MoSi2 based composites reinforced by carbon nanotubes

Molybdenum disilicide (MoSi₂) based composites with various contents of carbon nanotubes (CNTs) were made by sintering in vacuum at 1500 °C for 1 h. Mechanical properties of these composites at room temperature revealed the addition of CNTs to have good hardening and toughening effect on the matrix. Especially when adding 6.0% CNTs by volume, the hardness and fracture toughness were improved respectively by about 25.3% and 45.7% compared to pure MoSi₂. Phase identification and microstructure of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HTEM). Multi-walled CNTs were found in the powders synthesized by self-propagating high temperature synthesis (SHS) and SiC phase existed in the sintering samples. Fine grain and the favorable effect of dispersed SiC particles resulted in a high hardness of the CNTs/MoSi₂ composite. The toughening mechanisms for the CNTs/MoSi₂ composites included crack deflection, crack micro-bridging, crack branching, crack bowing and fine-grain pullout.     

Effects of mechanically alloying Al2O3 and Y2O3 additives on the liquid phase sintering behavior and properties of SiC

In order to improve the sintering of SiC, mixtures of Al₂O₃ and Y₂O₃ powders are commonly included as sintering additives. The aim of this work was to use mechanically alloyed Al₂O₃–Y₂O₃ mixtures as sintering additives to promote liquid phase sintering of SiC using spark plasma sintering. The results showed that milling reduced the particle size of the powders and led to the formation of complex oxide phases (YAP, YAM, and YAG) at low temperatures. As the ball milling time increased, the mass loss of specimens sintered with mechanically alloyed Al₂O₃–Y₂O₃ mixtures decreased, and accordingly the relative density increased. However, the hardness and flexural strength of sintered SiC specimens first increased and then decreased. Because the specimens prepared with oxides milled for a long time contained too much YAG/YAP and accordingly too much liquid at sintering temperature. This negatively affected the mechanical properties of the SiC specimens because of the increased volume of the complex oxide phases, which have inferior mechanical properties to SiC, in the sintered specimens. When the ball milling time was 6 h, the hardness (24.02 GPa) and flexural strength (655.61 MPa) of the SiC specimens reached maximum values.     

Densification and mechanical properties of CrB2+MoSi2 based novel composites

CrB₂+MoSi₂ ceramic composites with different contents of MoSi₂ (5 wt% and 15 wt%) were prepared by pressureless sintering and hot-pressing techniques. For comparison, a monolithic CrB₂ ceramic was also consolidated under the identical temperature, pressure and holding time by both pressureless sintering and hot-pressing techniques. The effects of the fabrication processes on the densification and mechanical properties of the composites were investigated. No improvement in density was observed upon addition of MoSi₂ as sinter additive. The phase analysis and microstructural characterization of the resultant composites indicate that there are no sintering reactions between the matrix (CrB₂) and the additive (MoSi₂). The hardness and fracture toughness of the composites were measured in the range of 17–19 GPa and 3–5 MPa m^1/2 respectively. The hardness was found to decrease (7% to 8%) and fracture toughness was found to increase (60%–90%) with respect to the addition of MoSi₂.     

Pressureless sintered in situ toughened ZrB2-SiC platelets ceramics

ZrB₂-SiC composite ceramics were densified by pressureless sintering with addition of Si₃N₄ or MoSi₂ at temperatures that induced SiC anisotropic growth from particles to platelets, within a ZrB₂ matrix with rounded grains. Si₃N₄ addition resulted in the formation of large amounts of liquid phase which enhanced mass transfer mechanisms in terms of matrix grain growth and homogeneous distribution of SiC platelets having an aspect ratio of 3. On the contrary, MoSi₂ helped the densification with local formation of liquid phases leading to a finer matrix with finer SiC platelets, though more agglomerated and with a lower aspect ratio (about 2). These different microstructures had very different fracture properties values, namely a toughness of 3.8 MPa m^1/2 and a strength of 300 MPa for the Si₃N₄-doped composite; toughness of 5 MPa m^1/2 and strength of 410 MPa for the MoSi₂-doped one.     

Fracture toughness of ZrO2–SiC/MoSi2 composite ceramics prepared by powder metallurgy

ZrO₂–SiC/MoSi₂ composite ceramics were prepared by powder metallurgy. The room temperature and high temperature fracture toughness of 10 vol%ZrO₂+MoSi₂, 10 vol%ZrO₂+10 vol%SiC + MoSi₂, 20 vol%ZrO₂+10 vol%SiC + MoSi₂, 10 vol%ZrO₂+20 vol%SiC + MoSi₂ composite ceramics were studied. The room temperature fracture toughness was calculated by Vickers indentation crack. High temperature fracture toughness is measured by wavefront interference principle of moire interferometry and non-contact measurement of in-plane displacement. The synergistic effects mechanism of nano-ZrO₂ and SiC particles on the fracture toughness of MoSi₂ nanocomposite ceramics was discussed.     

Microstructure and mechanical properties of (Zr,Ti)B2–SiC composites obtained by pressureless sintering of ZrB2–SiC–TiO2 powder mixtures

Multiphase ceramics have been highlighted due to the combination of different properties. This work proposes to obtain the multiphase composite of (Zr,Ti)B₂–SiC based on the mixture of ZrB₂, SiC, and TiO₂ sintered without pressure. The effect of TiO₂ addition on solid solution formation with ZrB₂, densification, microstructure, and mechanical properties was investigated. For this, 2.0 wt% TiO₂ was added to ZrB₂–SiC composites with 10–30 vol% SiC and processed by reactive pressureless sintering at 2050 °C with a 2 h holding time. Sinterability, crystalline phases, microstructure, Vickers hardness, and indentation fracture toughness of these composites were analyzed and compared to the non-doped ZrB₂–SiC samples. The XRD analysis and EDS elemental map images indicated the incorporation of Ti atoms into the ZrB₂ crystalline structure with solid solution generation of (Zr,Ti)B₂. The addition of TiO₂ resulted in matrix grain size refinement and a predominant intergranular fracture mode. The relative densities were not significantly modified with the TiO₂ addition, though a higher weight loss was detected after the sample sintering process. The composites doped with TiO₂ showed an increase in fracture toughness but exhibited a slightly lower Vickers hardness compared to composites without TiO₂ addition.     

Preparation,properties and high-temperature oxidation resistance of MoSi2-HfO2 composite coating to protect niobium using spent MoSi2-based materials

Industrial spent MoSi₂-based materials and HfO₂ were recycled as raw materials to fabricate MoSi₂-HfO₂ composite coating by spark plasma sintering (SPS). The microstructural evolution of the coatings was characterized and the 1500 °C oxidation behavior was explored. Cracks penetrated through the MoSi₂ coating while no cracks can be found in the HfO₂-containing composite coating owing to the reduction of the mismatch of thermal expansion coefficient (CTE). Good metallurgical bonding was exhibited since (Mo,Nb)₅Si₃ diffusion layer was found in the HfO₂-containing coating by the diffusion of Nb and Si across the interface without gaps. After 1500 °C oxidation of 20 h, cracks appeared in the surface of SiO₂ layer on MoSi₂ coating while the HfO₂-containing composite coating possessed crack-free oxide scale. HfSiO₄ with high temperature (>2900 °C) is formed during oxidation and it inlays in the silica oxide scale to improve the stability. Compared to MoSi₂ coating, Nb coated MoSi₂-HfO₂ has thinner oxide scale with lower mass gain during oxidation, thus presenting better high-temperature anti-oxidation properties.     

Mechanical properties of nano-grain SiO2 glass prepared by spark plasma sintering

Silicon carbide (SiC) layers were deposited on silica (SiO₂) glass powder by rotary chemical vapor deposition (RCVD) to form SiO₂ glass (core)/SiC (shell) powder; this powder was consolidated by spark plasma sintering (SPS). SiO₂ glass powder with a particle size of 250 nm was coated with 5–10-nm-thick SiC layers. The resultant SiO₂ glass (core)/SiC (shell) powder was consolidated to form a nano-grain SiO₂ glass composite at a relative density above 90% by SPS in the sintering temperature range of 1573–1823 K. The Vickers hardness and fracture toughness of the SiO₂ glass composite at 1723 K were found to be 14.2 GPa and 5.4 MPa m^1/2, respectively.     

Novel (Zr,Ti)B2-(Zr,Ti)C-SiC ceramics via reactive hot pressing

Fully dense (Zr, Ti)B₂-(Zr, Ti)C-SiC ceramics were prepared by reactive hot-pressing using ZrB₂, TiC, and SiC as the initial materials for the first time. Effects of SiC addition on the microstructure evolution and mechanical properties were reported. The in-situ reaction between ZrB₂ and TiC as well as the SiC addition leads to the grain refinement. Besides, elongated (Zr, Ti)B₂ plate-like grains are obtained due to the occurrence of a transient liquid phase, which leads to the crack deflection in the matrix effectively. Mechanical properties are improved significantly due to grain-refinement and solid solution strengthening, and plate-like grains toughening effects. The ZrB₂-10 mol%TiC composite with 10 mol% SiC additional exhibits good comprehensive mechanical properties of the hardness of 20.2 GPa, the flexural strength of 803 MPa, and the fracture toughness of 5.7 MPa m^1/2.