Preparation and characterization of ZrB<Subscript>2</Subscript>-SiC ultra-high temperature ceramics by microwave sintering

ZrB₂-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB₂-SiC composites and almost fully dense ZrB₂-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB₂-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m^1/2, respectively.     

Effects of VC additives on densification and elastic and mechanical properties of hot-pressed ZrB<Subscript>2</Subscript>–SiC composites

In this study, highly dense ZrB₂-20 vol% SiC composites with 3–10 wt% VC additives were prepared by hot-pressing at 1750 °C for 1 h under a pressure of 20 MPa in a vacuum. The densification behavior and elastic and mechanical properties of the obtained composites were examined, and the effect of the VC content on the densification and the properties is analyzed. The addition of VC promotes the activation of densification mechanism at a lower temperature and inhibits the growth of ZrB₂ and SiC grains during the sintering. In addition, the elastic moduli, hardness and fracture toughness that measured in the obtained composites are constant and independent of the VC content, with a shear modulus of ~ 220 GPa, Young’s modulus of ~ 500 GPa, hardness of ~ 20 GPa and fracture toughness of ~ 4.4 MPa m^1/2. On the other hand, the flexural strength of the composites decreased as the VC content increased from 3 to 7 wt% and then it increased with further increasing the VC content to 10 wt%, with strength values of 620–770 MPa.     

Preparation and characterization of ZrB 2 -SiC ultra-high temperature ceramics by microwave sintering

ZrB₂-SiC ultra-high temperature ceramic composites reinforced by nano-SiC whiskers and SiC particles were prepared by microwave sintering at 1850°C. XRD and SEM techniques were used to characterize the sintered samples. It was found that microwave sintering can promote the densification of the composites at lower temperatures. The addition of SiC also improved the densification of ZrB₂-SiC composites and almost fully dense ZrB₂-SiC composites were obtained when the amount of SiC increased up to 30vol.%. Flexural strength and fracture toughness of the ZrB₂-SiC composites were also enhanced; the maximum strength and toughness reached 625 MPa and 7.18 MPa·m^1/2, respectively.     

Preparation of B<Subscript>4</Subscript>C-ZrB<Subscript>2</Subscript>-SiC eutectic ceramics by arc melting method

The B₄C-ZrB₂-SiC ternary composites with super hard and high toughness were obtained by arc melting in argon atmosphere. Microstructures were observed by SEM, and phase compositions were analyzed by XRD. The hardness and fracture toughness of ternary composites are 28 GPa and 4.5 MPa·m^1/2. The eutectic mole composition is 0.39B₄C-0.25ZrB₂-0.36SiC, and the eutectic lamellar microstructure is composed of B₄C matrix with the lamellar ZrB₂ and SiC grains.     

Preparation of B4C-ZrB2-SiC eutectic ceramics by arc melting method

The B₄C-ZrB₂-SiC ternary composites with super hard and high toughness were obtained by arc melting in argon atmosphere. Microstructures were observed by SEM, and phase compositions were analyzed by XRD. The hardness and fracture toughness of ternary composites are 28 GPa and 4.5 MPa·m^1/2. The eutectic mole composition is 0.39B₄C-0.25ZrB₂-0.36SiC, and the eutectic lamellar microstructure is composed of B₄C matrix with the lamellar ZrB₂ and SiC grains.     

Preparation and properties of 2D C/ZrB2-SiC ultra high temperature ceramic composites

Two-dimensional C/ZrB₂-SiC composites were fabricated by chemical vapor infiltration (CVI) process combined with slurry paste (SP) method. ZrB₂ was introduced in the matrix by stacking the pasted carbon cloth with ZrB₂-polycarbosilane slurry. After heat-treated at 900 °C, the stacked carbon cloth preform was infiltrated SiC by CVI process to obtain 2D C/ZrB₂-SiC composites. Mechanical properties such as flexural strength and interlaminar shear strength were investigated. The ablation tests were carried out on an oxyacetylene torch flame. The small linear erosion rates indicate that the composites have good ablation resistance properties. These results demonstrate that CVI combined with SP method is a useful way to fabricate 2D C/ZrB₂-SiC composites.     

Microstructure and mechanical properties of BAS/SiC composites sintered by spark plasma sintering

High-density BAS/SiC composites were obtained from β-SiC starting powder by the spark plasma sintering technique. Various physical properties of the BAS/SiC composites were investigated in detail, such as densification, phase analysis, microstructures and mechanical properties. The results demonstrated that the relative density of the BAS/SiC composites reached over 99.4% at 1900 °C. The SiC grains were uniformly distributed in the continuous BAS matrix which is probably because of complete infiltration of the SiC particles in BAS liquid-phase formed during sintering. The pull-out of SiC particles, crack deflection and bridging were observed as the major toughening mechanism. The flexural strength and fracture toughness of the BAS/SiC composites sintered at 1900 °C were up to 560 MPa and 7.0 MPa·m^1/2, respectively.     

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.     

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.     

Preparation of Si3N4-SiC composites by microwave route

Si₃N₄-SiC composites have been microwave sintered using β-Si₃N₄ and β-SiC as starting materials. Si₃N₄ rich compositions (95 and 90 vol.% Si₃N₄) have been sintered above 96% of theoretical density without using any sintering additives in 40 min. A monotonic decrease in relative density is observed with increase in SiC proportion in the composite. Decrease in relative density has manifested in the reduction of fracture toughness and microhardness values of the composite with increase in SiC content although the good sintering of matrix Si₃N₄ limits the decrease of fracture toughness. Highest value of fracture toughness of 6.1 MPa m^1/2 is observed in 10 vol.% SiC composite. Crack propagation appears to be transgranular in the Si₃N₄ matrix and the toughening of the composites is through crack deflection around hard SiC particles in addition to its debonding from the matrix.     

Processing and characterization of ZrB2-based ultra-high temperature monolithic and fibrous monolithic ceramics

Zirconium diboride (ZrB₂) based ultra-high temperature ceramics either unmodified or with SiC particulate additions of 10, 20, or 30 volume percent were prepared by conventional hot pressing. The ZrB₂-SiC compositions had improved four-point bend strength compared to the ZrB₂ prepared in our laboratory as well as other reported ZrB₂ or ZrB₂-SiC materials. Strength and toughness increased as the amount of SiC increased. Measured strengths ranged from 550 MPa for ZrB₂ to over 1000 MPa for ZrB₂-30% SiC. Likewise, toughness increased from 3.5 MPa to more than 5 MPa over the same composition range. The addition of SiC also improved oxidation resistance compared to pure ZrB₂.Co-extrusion processing was used to produce ZrB₂-based ultra-high temperature ceramics with a fibrous monolithic structure. Samples had dense ZrB₂-30 vol% SiC cells approximately 100 m in diameter surrounded by porous ZrB₂ cell boundaries approximately 20 m thick. ZrB₂-based fibrous monoliths had four point bend strength of 450 MPa, about half of a conventional ZrB₂-SiC ceramic with the cell composition. Preliminary analysis of fracture behavior found that ZrB₂-based fibrous monoliths did not exhibit graceful failure because the difference in strength between the cell and cell boundary of the current materials was not sufficient.     

Effect of BN grain size on microstructure and mechanical properties of the ZrB2–SiC–BN composites

ZrB₂–20 vol.%SiC composites containing 10 vol.% h-BN particles (ZSB) with average grain sizes ranging from 1 μm to 10 μm were hot-pressed. The fracture toughness of the ZSB composites was higher than reported results of monolithic ZrB₂ (2.3–3.5 MPa m^1/2) and SiC particle reinforced ZrB₂ composites (4.0–4.5 MPa m^1/2). The improvement in the fracture toughness of the ZSB composites was due to the high aspect ratio of h-BN and weaker interface bonding, which could enhance crack deflection and stress relaxation near the crack-tip. Compared with the flexural strength of the ZrB₂–SiC composites, the reduction in the flexural strength of the ZSB composites was attributed to the weaker interface bonding and the lower relative density. Furthermore, improvement in toughness and the reduction in the strength were valuable to improve the thermal shock resistance of the ZSB composites. The ΔT_c of ZSB5 material is 400 °C which is higher than ZrB₂–20%SiC and ZrB₂–15%SiC–5%AlN.     

Effect of graphite flake on the mechanical properties of hot pressed ZrB2-SiC ceramics

A ZrB₂ ceramic containing 20 vol.% SiC and 10 vol.% graphite flake (ZrB₂-SiC-G) was fabricated by hot pressing. It was shown that the fracture toughness was improved due to the introduction of graphite flake, whereas the flexure strength and hardness decreased slightly. The fracture toughness of ZrB₂-SiC-G composite was 6.1 ± 0.3 MPa·m^1/2, which was much higher than that of monolithic ZrB₂, ZrB₂-SiC composite and similar ZrB₂-SiC-C composite. The toughening mechanisms are crack deflection and branching as well as stress relaxation near the crack tip. The results here pointed to a potential method for improving fracture toughness of ZrB₂-based ceramics.     

Preparation and mechanical properties of laminated zirconium diboride/molybdenum composites sintered by spark plasma sintering

Laminated ZrB₂/Mo composites, alternately consisting of matrix layers of 80 vol.% ZrB₂ + 10 vol.% nano-SiC whiskers + 10 vol.% SiC particles and Mo interlayers, with the addition of Si and B as interlayer adjusting agent, were prepared by roll-compaction and spark plasma sintering (at 1600°C) process. XRD and SEM techniques were used to characterize the phases and microstructure of the obtained composites. The results showed that without the addition of Si and B in the interlayer, interfacial debonding between the matrix layer and interlayer often occurred due to the thermal mismatch between the two kinds of layers. However, the interfacial mismatch could be effectively inhibited by the addition of Si and B to the Mo interlayers. The laminated ZrB₂/Mo composites with 6 at.% Si and 4 at.% B in the interlayers showed the highest bending strength at (451±20) MPa and the highest fracture toughness at (7.52±0.12) MPa·m^?. MoB, ZrB and Mo5SiB2 were formed by the reactions among ZrB₂, Mo and the additions.     

Pressureless sintering ZrB2-SiC ceramics at low temperatures

ZrB₂-SiC ceramics are prepared by pressureless sintering using ZrB₂ powders and liquid polycarbosilane (LPCS) precursors. The LPCS can effectively reduce the sintering temperature. The phases of the sintered ceramics are characterized by X-ray diffraction, and their morphologies are observed by scanning electron microscopy. From these results, it is learned that LPCS can provide free carbon and silicon at high temperatures. Therefore, the oxides on the ZrB₂ surface can be removed by free carbon, and the densification process can be promoted by silicon. These coupled effects make it possible to pressureless sinter the ZB₂-SiC ceramics at low temperatures.     

Influence of the microstructure evolution of ZrO2 fiber on the fracture toughness of ZrB2–SiC nanocomposite ceramics

ZrB₂–SiC nanocomposite ceramics toughened by ZrO₂ fiber were fabricated by spark plasma sintering (SPS) at 1700 °C. The content of ZrO₂ fiber incorporated into the ZrB₂–SiC nanocomposites ranged from 5 mass% to 20 mass%. The content, microstructure, and phase transformation of ZrO₂ fiber exhibited remarkable effects on the fracture toughness of the ZrO_2(f)/ZrB₂–SiC composites. Fracture toughness of the composites greatly improved to a maximum value of 6.56 MPa m^1/2 ± 0.3 MPa m^1/2 by the addition of 15 mass% of ZrO₂ fiber. The microstructure of the ZrO₂ fiber exhibited certain alterations after the SPS process, which enhanced crack deflection and crack bridging and affected fracture toughness. Some microcracks were induced by the phase transformation from t-ZrO₂ to m-ZrO₂, which was also an important reason behind the improvement in toughness.     

Preparation of YAG gel coated ZrB2–SiC composite prepared by gelcasting and pressureless sintering

In this paper, gelcasting and pressureless sintering of YAG gel coated ZrB₂–SiC (YZS) composite were conducted. YAG gel coated ZrB₂–SiC (YZS) suspension was firstly prepared through sol–gel route. Poly (acrylic acid) was used as dispersant. YZS suspension had the lowest viscosity when using 0.6 wt.% PAA as dispersant. Gelcasting was conducted based on AM–MBAM system. The gelcast YZS sample was then pressureless sintered to about 97% density. During sintering, YAG promoted the densification process from solid state sintering to liquid phase sintering. The average grain sizes of ZrB₂ and SiC in the YZS composite were 3.8 and 1.3 μm, respectively. The flexural strength, fracture toughness and microhardness were 375 ± 37 MPa, 4.13 ± 0.45 MPa m^1/2 and 14.1 ± 0.5 GPa, respectively.     

Effect of a weak fiber interface coating in ZrB2 reinforced with long SiC fibers

This study reports the microstructural analysis and mechanical properties of a ZrB₂ ceramic containing long BN-coated Hi-Nicalon SiC fibers. A composite was produced and thoroughly characterized by transmission electron microscopy to study the interfaces at the nanoscale level. Full densification was accomplished by hot pressing at 1450 °C. The fiber in the sintered material retained its pristine aspect, confirming that the coating was effective in preventing degradation due to interactions with the matrix. Pull-out was observed on fractured surfaces, but toughness values were about 4.5 MPa√m, which was comparable to those of ZrB₂ materials with SiC additions in the form of particles or short fibers. However, the composites exhibited a controlled fracture behavior, as confirmed by a notably higher work of fracture, 140 J/m², compared with 20–30 J/m² of unreinforced ZrB₂ or ZrB₂ containing chopped fibers.     

Fabrication and microstructural evaluation of ZrB2/ZrC/Zr composites by liquid infiltration

The microstructure of ZrB₂/ZrC/Zr composites was examined using scanning electron microscopy, optical microscopy, and X-ray diffraction techniques. Dense ZrB₂/ZrC/Zr composites could be fabricated by the reaction sintering of molten zirconium with ZrB₂ preform. The composites were made by infiltration of molten zirconium into ZrB₂ preform, which contained 0–40 vol% B₄C, at 1900C for 10 min. The average grain size of ZrB₂ in the reaction-sintered composites decreased slightly with an increase in the volume fraction of the B₄C. The volume fraction of the solid increased with further increase of B₄C contents. The mechanical properties were measured in accordance with B₄C contents. The composites exhibited a four-point bending strength of up to 570 MPa and a fracture toughness of up to 11.5 MPa m^1/2.