Mechanical and thermal properties of silicon-carbide composites fabricated with short Tyranno® Si-Zr-C-O fibre

Silicon carbide (SiC) composites reinforced with 10–50 mass% (10.5–51.2 vol%) of short Tyranno® Si-Zr-C-O fibre (average length 0.5 mm) and 0–10 mol% of Al₄C₃as a sintering aid were fabricated using the hot-pressing technique. Firstly, the effect of Si-Zr-C-O fibre addition on the relative density (bulk density/true density) of the SiC composite hot-pressed at 1800 °C for 30 min was examined by fixing the amount of Al₄C₃to be 5 mol%. Although the relative density was reduced to 87.4% for 10 mass% of Si-Zr-C-O addition, further increases in the amount of Si-Zr-C-O fibre increased density to a maximum of 92.8% at 40 mass% of fibre addition. Secondly, the effect of varying the amount of Al₄C₃addition on the relative density was examined by fixing the amount of Si-Zr-C-O fibre to be 40 mass%. The optimum amount of Al₄C₃addition for the fabrication of dense SiC composite was found to be 5 mol%. The fracture toughness of the hot-pressed SiC composites with 20–40 mass% of Si-Zr-C-O fibre addition (amount of Al₄C₃: 5 mol%) was 3.2–3.4 MPa · m^1/2and approximately 1.5 times higher than that (2.39 MPa · m^1/2) of the hot-pressed SiC composite with no Si-Zr-C-O fibre addition. SEM observation showed evidence of Si-Zr-C-O fibre debonding and pull-out at the fracture surfaces. The hot-pressed SiC composite with 5 mol% of Al₄C₃and 40 mass% of Si-Zr-C-O fibre additions showed excellent heat-resistance at 1300 °C in air due to the formation of a SiO₂layer at and near exposed surfaces.     

Hot-pressed ZrB2–SiC–YSZ composites with various yttria content: Microstructure and mechanical properties

ZrB₂–10 vol%SiC–20 vol%YSZ composites were prepared by hot-pressed sintering with yttria content ranging from 2 mol% to 8 mol% in YSZ. The phase constitution, microstructure and mechanical properties of the composites were found to be strongly dependent on the yttria content. The average grain size became bigger for the composites with higher yttria content. When the yttria content was below 3 mol%, there is no cubic zirconia in the polished surface of composites, and the flexural strength of the composites was above 740 MPa. With the increase in yttria content, the fracture toughness fell down from 6.4 MPa m^1/2 to 5.6 MPa m^1/2. Vickers’ hardness of the hot-pressed composites varied above 18 GPa without obvious effect of the yttria content.     

Microstructure and properties of an electroconductive SiC-based composite

In this work, an SiC-based electroconductive composite is obtained through simultaneous addition of MoSi₂ and ZrB₂ particles. The composite material is fully densified by hot pressing at 1860 °C and the microstructure is investigated by SEM-EDS analysis. Microstructural features and mechanical properties are compared to those of a monolithic hot-pressed SiC material. The MoSi₂ and ZrB₂ particles, besides increasing the electrical conductivity of the silicon carbide matrix, also act as reinforcement for the material. Room-temperature strength reaches the value of 850 MPa and the fracture toughness is 4.2 MPa m^0.5. The composite electrical resistivity is of the order of 10⁻³ Ω cm.     

Mechanical properties of the solid Li-ion conducting electrolyte: Li0.33La0.57TiO3

Li_0.33La_0.57TiO₃ (LLTO) is a potential Li-ion conducting membrane for use in aqueous Li-air batteries. To be in this configuration its mechanical properties must be determined. Dense LLTO was prepared using a solid-state (SS) or sol–gel (SG) procedure and was hot-pressed to yield a high relative density material (>95 %). Young’s modulus, hardness, and fracture toughness of the LLTO-SS and sol–gel LLTO-SG materials was determined and compared to other solid Li-ion conducting electrolytes. The Young’s modulus for LLTO-SG and LLTO-SS was 186 ± 4 and 200 ± 3 GPa, respectively. The Vickers hardness of LLTO-SG and LLTO-SS was 9.7 ± 0.7 and 9.2 ± 0.2 GPa, respectively. The fracture toughness, K _IC, of both LLTO-SG and LLTO-SS was ~1 MPa m^1/2; the fracture toughness of LLTO-SG was slightly higher than that of LLTO-SS. Both LLTO-SG and LLTO-SS have a Young’s modulus and hardness greater than the other possible solid Li-ion conducting membranes; Li₇La₃Zr₂O₁₂ and Li_1+x+y Al _x Ti_2−x Si _y P_3−y O₁₂. Based on modulus and hardness hot-pressed LLTO exhibits sufficient mechanical integrity to be used as a solid Li-ion conducting membrane in aqueous Li-air batteries but, its fracture toughness needs to be improved without degrading its ionic conductivity.     

A high-strength SiCw/SiC–Si composite derived from pyrolyzed rice husks by liquid silicon infiltration

A high-strength SiC composite with SiC whiskers (SiC_w) as reinforcement has been fabricated by liquid silicon infiltration (LSI) using pyrolyzed rice husks (RHs) as raw material. RHs were coked and pyrolyzed subsequently at high temperature to obtain a mixture containing SiC whiskers, particles, and amorphous carbon. The pyrolyzed RHs were then milled and modeled to preforms, which were then used to fabricate biomorphic SiC_w/SiC–Si composites by liquid silicon infiltration at 1,450, 1,550, and 1,600 °C, respectively. Dense composite with a density of 3.0 g cm⁻³ was obtained at the infiltration temperature of 1,550 °C, which possesses superior mechanical properties compared with commercial reaction-sintered SiC (RS-SiC). The Vickers hardness, flexure strength, elastic modulus, and fracture toughness of the biomorphic SiC_w/SiC–Si composite were 18.8 ± 0.6 GPa, 354 ± 2 GPa, 450 ± 40 MPa, and 3.5 ± 0.3 MPa m^1/2, respectively. Whereas the composites obtained at the other two infiltration temperatures contain unreacted carbon and show lower mechanical properties. The high flexure strength of the biomorphic composite infiltrated at 1,550 °C is attributed to the dense structure and the reinforcement of the SiC whiskers. In addition, the fracture mechanism of the composite is also discussed.     

Strength and fracture toughness of in situ-toughened silicon carbide

Fine β-SiC powders either pure or with the addition of 1 wt % of α-SiC particles acting as a seeding medium, were hot-pressed at 1800 °C for 1 h using Y₂O₃ and Al₂O₃ as sintering aids and were subsequently annealed at 1900 °C for 2, 4 and 8 h. During the subsequent heat treatment, the β → α phase transformation of SiC produced a microstructure of “in situ composites” as a result of the growth of elongated large α-SiC grains. The introduction of α-SiC seeds into the β-SiC accelerated the grain growth of elongated large grains during annealing which led to a coarser microstructure. The sample strength values decreased as the grain size and fracture toughness continued to increase beyond the level where clusters of grains act as fracture origins. The average strength of the in situ-toughened SiC materials was in the range of 468–667 MPa at room temperature and 476–592 MPa at 900 °C. Typical fracture toughness values of 8 h annealed materials were 6.0 MPa m^1/2 for materials containing α-SiC seeds and 5.8 MPa m^1/2 for pure β-SiC samples. This revised version was published online in November 2006 with corrections to the Cover Date.     

The thermal shock resistance of the ZrB2–SiC–ZrC ceramic

In the present work, the thermal shock resistance of the ZrB₂–SiC–ZrC ceramic was estimated by the water quenching method and the flexural strength of the quenched specimen was measured. The measured critical temperature difference of the ZrB₂–SiC–ZrC ceramic was significantly greater than that of the ZrB₂–15 vol.% SiC ceramic. The improvement in thermal shock resistance was attributed to its higher fracture toughness (6.7 MPa m^1/2) and lower flexural strength (526 MPa) relative to the ZrB₂–15 vol.% SiC ceramic (4.1 MPa m^1/2 and 795 MPa) based on Griffith fracture criterion. Furthermore, the temperature and thermal stress distributions in the specimen during instantaneous water quenching were simulated by Finite element analysis.     

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.     

Interfacial fracture characteristic and crack propagation of thermal barrier coatings under tensile conditions at elevated temperatures

Thermal barrier coatings (TBCs) have been extensively used in aircraft engines for improved durability and performance for more than fifteen years. In this paper, thermal barrier coating system with plasma sprayed zirconia bonded by a MCrAlY layer to SUS304 stainless steel substrate was performed under tensile tests at 1000°C. The crack nucleation, propagation behavior of the ceramic coatings in as received and oxidized conditions were observed by high-performance camera and discussed in detail. The relationship of the transverse crack numbers in the ceramic coating and tensile strain was recorded and used to describe crack propagation mechanism of thermal barrier coatings. It was found that the fracture/spallation locations of air plasma sprayed (APS) thermal barrier coating system mainly located within the ceramic coating close to the bond coat interface by scanning electron microscope (SEM) and energy dispersive X-Ray (EDX). The energy release rate and interface fracture toughness of APS TBCs system were evaluated by the aid of Suo–Hutchinson model. The calculations revealed that the energy release rate and fracture toughness ranged, respectively, from 22.15 J m⁻² to 37.8 J m⁻² and from 0.9 MPa m^1/2 to 1.5 MPa m^1/2. The results agree well with other experimental results.     

Microstructure and mechanical properties of ZrB2-SiC composites

A ZrB₂-based composite containing 20 vol.% nanosized SiC particles (ZSN) was fabricated at 1900 °C for 30 min under a uniaxed load of 30 MPa by hot-pressing. The microstructure and mechanical properties of the composite were investigated. It was shown that the grain growth of ZrB₂ matrix was effectively suppressed by submicrosized SiC particles located along the grain boundaries. In addition, the mechanical properties of ZSN composite were strongly improved by incorporating the nanosized SiC particles into a ZrB₂ matrix, especially for flexural strength (925 ± 28 MPa) and fracture toughness (6.4 ± 0.3 MPa•m^1/2), which was much higher than that of monolithic ZrB₂ and ZrB₂-based composite with microsized SiC particles, respectively. The formation of intragranular nanostructures plays an important role in the strengthening and toughening of ZrB₂ ceramic.     

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.     

Effect of chopped Si–Al–C fiber addition on the mechanical properties of silicon carbide composite

Silicon carbide (SiC) composites containing 0–50 mass% of chopped Tyranno^® Si–Al–C (SA) fiber (mean length: 214 μm (SA(214)), 394 μm (SA(394)), and 706 μm (SA(706)) were fabricated using the hot-pressing technique at 1800 °C for 30 min under a uniaxial pressure of 31 MPa in Ar atmosphere. The maximum flexural strength of the SiC composite was 344 MPa for 30 mass% of SA(706) fiber addition, whilst the maximum fracture toughness was 4.7 MPa m^1/2 for 40 mass% of SA(706) fiber addition. Increasing the mean fiber length from 214 to 706 μm decreased the flexural strength from 380 to 281 MPa for 30 mass% of fiber addition, whilst the fracture toughness increased from 3.4 to 4.7 MPa m^1/2 for 40 mass% of fiber addition. Through use of a treated SA(706) fiber containing an approximately 100 nm surface layer of carbon, the fracture toughness further increased to 6.0 MPa m^1/2 for 40 mass% of fiber addition; this value was more than twice that of the monolithic SiC ceramic and is believed to be the highest so far achieved for this type of SiC/SiC composite containing chopped fibers.     

Microwave sintering improves the mechanical properties of biphasic calcium phosphates from hydroxyapatite microspheres produced from hydrothermal processing

Starting from two microspherical agglomerated HAP powders, porous biphasic HAP/TCP bioceramics were obtained by microwave sintering. During the sintering the HAP powders turned into biphasic mixtures, whereby HAP was the dominant crystalline phase in the case of the sample with the higher Ca/P ratio (HAP1) while α-TCP was the dominant crystalline phase in the sample with lower Ca/P ratio (HAP2). The porous microstructures of the obtained bioceramics were characterized by spherical intra-agglomerate pores and shapeless inter-agglomerate pores. The fracture toughness of the HAP1 and HAP2 samples microwave sintered at 1200 °C for 15 min were 1.25 MPa m^1/2. The phase composition of the obtained bioceramics only had a minor effect on the indentation fracture toughness compared to a unique microstructure consisting of spherical intra-agglomerate pores with strong bonds between the spherical agglomerates. Cold isostatic pressing at 400 MPa before microwave sintering led to an increase in the fracture toughness of the biphasic HAP/TCP bioceramics to 1.35 MPa m^1/2.     

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.     

Fracture behavior and mechanism of 2D C/SiC-BCx composite at room temperature

2D C/SiC composite was modified with partial BC_x matrix by low pressure chemical vapor infiltration technique (LPCVI), which was named as 2D C/SiC-BC_x composite. The flexural fracture behavior, mechanism, and strength distribution of 2D C/SiC-BC_x composite are investigated. The results indicate that the flexural strength, fracture toughness, and fracture work are 442.1 MPa, 22.84 MPa m^1/2, and 19.2 kJ m⁻², respectively. The flexural strength of C/SiC-BC_x composite decrease about 20% than that of C/SiC composite. However, the fracture toughness and fracture work increase about 19% and 18.5%, respectively. The properties varieties between C/SiC-BC_x composite and C/SiC composite can be attributed to the weak-bonding interface between BC_x/SiC matrices according to the results of detailed microstructure analysis. The strength distribution of 2D C/SiC-BC_x composite follows as Normal distribution or Weibull distribution with σ_u = 0, and m = 8.1393. The mean value of flexural strength for 2D C/SiC-BC_x composite is 443 MPa obtained by theory calculation, which is consistent with experiment result (442.1 MPa) very well.     

Microstructure and mechanical properties of SiC<Subscript>P</Subscript>/SiC and SiC<Subscript>W</Subscript>/SiC composites by CVI

37.2 vol.% SiC_P/SiC and 25.0 vol.% SiC_W/SiC composites were prepared by chemical vapor infiltration (CVI) process through depositing SiC matrix in the porous particulate and whisker preforms, respectively. The particulate (or whisker) preforms has two types of pores; one is small pores of several micrometers at inter-particulates (or whiskers) and the other one is large pores of hundreds micrometers at inter-agglomerates. The microstructure and mechanical properties of 37.2 vol.% SiC_P/SiC and 25.0 vol.% SiC_W/SiC composites were studied. 37.2 vol.% SiC_P/SiC (or 25.0 vol.% SiC_W/SiC) consisted of the particulate (or whisker) reinforced SiC agglomerates, SiC matrix phase located inter-agglomerates and two types of pores located inter-particulates (or whiskers) and inter-agglomerates. The density, fracture toughness evaluated by SENB method, and flexural strength of 37.2 vol.% SiC_P/SiC and 25.0 vol.% SiC_W/SiC composites were 2.94 and 2.88 g/cm³, 6.18 and 8.34 MPa m^1/2, and 373 and 425 MPa, respectively. The main toughening mechanism was crack deflection and bridging.     

Microstructure and mechanical properties of hot pressed ZrB2-SiCp-ZrO2 composites

The present paper investigated the microstructure and mechanical properties of ZrB₂-10 vol.%SiC_p-10 vol.%ZrO₂ composites hot pressed at three temperatures. Phase transformability from t-ZrO₂ to m-ZrO₂ during fracture was analyzed through calculating the volume fractions of m-ZrO₂ and t-ZrO₂ on polished and fracture surfaces. The densification temperature was found to have a significant effect on the microstructure, phase transformation and the properties of the composites. When the composite was hot pressed at 1950 °C, the average grain size was 9.5 µm, and the fracture toughness was 4.5 MPa·m^1/2. Comparatively, when the composite was hot pressed at 1750 °C, the average grain size was 3.4 µm, and the fracture toughness increased by ~ 50% to 6.8 MPa·m^1/2.     

Hot-pressed A12O3–SiC(–C) composites with polycarbosilane as the precursor

The processing and mechanical behavior of Al₂O₃–xSiC (–C) (x = 1, 2, 5, 10 wt.%, ASx and ASCx) composites prepared by in situ reaction synthesis SiC from polycarbosilane (PCS) were investigated. The composites were densified by hot pressing. The pyrolysis process of PCS, microstructure, phase structure and mechanical properties of sintered composites were analyzed. Fully dense structure was obtained, and it was found that the fracture toughness and strength were highly improved compared with monolithic Al₂O₃. The fracture toughness reached 5.1 MPa m^1/2 in 1 wt.%SiC composite ASC1. AS1 showed 516 MPa of flexural strength.     

Sinterability of magnesium silicon nitride powder with yttrium oxide addition coated using the homogeneous precipitation method

The sinterability of magnesium silicon nitride (MgSiN₂) powder with yttrium oxide (Y₂O₃) addition was examined using the hot-pressing technique (31 MPa and N₂ atmosphere) at 1550°C for 90 min; the MgSiN₂ powder had been coated with 0–4 mass% of Y₂O₃ addition by a (urea-based) homogeneous precipitation method. Relative densities of the hot-pressed MgSiN₂ compacts (ceramics) with and without Y₂O₃ addition were 99.6% apart for the MgSiN₂ceramic with 4 mass% Y₂O₃ addition (98.4%). The thermal conductivities of the MgSiN₂ ceramics with 0–1 mass% Y₂O₃ addition were in the range of 20–21 W · m^–1 · K^–1 whilst the Vickers hardness was 19.7 GPa for the pure MgSiN₂ ceramic and decreased slightly with Y₂O₃ addition. Average fracture toughness values were in the range of 1.2–1.6 MPa · m^1/2 with significant trend being noted with regards to the ceramic containing 0.5 mass% of Y₂O₃. It was concluded that the use of homogeneous precipitation processing resulted in significant advantages regarding the densification, homogeneous microstructure, and fracture toughness despite the amount of Y₂O₃ addition being as low as 0.5 mass%.     

Microstructure and mechanical properties of an HfB2 + 30 vol.% SiC composite consolidated by spark plasma sintering

An ultra-high-temperature HfB₂–SiC composite was successfully consolidated by spark plasma sintering. The powder mixture of HfB₂ + 30 vol.% β-SiC was brought to full density without any deliberate addition of sintering aids, and applying the following conditions: 2100 °C peak temperature, 100 °C min⁻¹ heating rate, 2 min dwell time, and 30 MPa applied pressure. The microstructure consisted of regular diboride grains (2 μm mean size) and SiC particulates evenly distributed intergranularly. The only secondary phase was monoclinic HfO₂. The incorporated SiC particulates played a key role in enhancing the sinterability of HfB₂. Flexural strength at 25 °C and 1500 °C in ambient air was 590 ± 50 and 600 ± 15 MPa, respectively. Fracture toughness at room temperature (RT) (3.9 ± 0.3 MPa √m) did not decrease at 1500 °C (4.0 ± 0.1 MPa √m). Grain boundaries depleted of secondary phases were fundamental for the retention of strength and fracture toughness at high temperature. The thermal shock resistance, evaluated through the water-quenching method, was 500 °C.