Microstructure evolution of SiC/SiC joints during ultrasonic-assisted air bonding using a Sn–Zn–Al alloy

Direct soldering of SiC ceramic in air at 230 °C was achieved using a Sn–9Zn–2Al alloy assisted by ultrasonic wave within seconds. Experimental results indicated that a sound metallurgical bond was formed between the SiC ceramic and Sn–9Zn–2Al alloys. The dependence of interfacial microstructure evolution on ultrasonic action duration time was investigated. Two types of interfacial structures at the interface were observed as the ultrasonic action duration time increased. An amorphous SiO₂ layer was identified at the interface for ultrasonic exposures of 1 s, which was the oxide layer formed on the SiC ceramic surface during heating. A layer of amorphous alumina with a thickness of ~ 6.8 nm formed at the interface under ultrasonic action for over 4 s. The shear strength of joints could reach up to 44 MPa. The formation of the alumina layer at the interface was attributed to the redox reaction of Al from the filler metal and SiO₂ on the SiC ceramic surface under the action of ultrasonic waves. The rapid interfacial reaction was principally induced by the acoustic cavitation and streaming effects at the liquid/solid interface.     

Structural characterization of YSZ/Al2O3 nanostructured composite coating fabricated by electrophoretic deposition and reaction bonding

Suspension of YSZ and Al particles in acetone in presence of 1.2 g/l iodine as dispersant was used for electrophoretic deposition of green form YSZ/Al coating. Results revealed that applied voltage of 6 V and deposition time of 3 min were appropriate for deposition of green composite form coating. After deposition, a nanostructured dense YSZ/Al₂O₃ composite coating was fabricated by oxidation of Al particles at 600 °C for 2 h and subsequently sintering heat treatment at 1000 °C for 2 h. Melting and oxidation of Al particles in the green form composite coating not only caused reaction bonding between the particles but also lowered the sintering temperature of the ceramic coating about 200 °C. The EDS maps confirmed that the composition of fabricated coating was uniform and Al₂O₃ particles were dispersed homogenously in YSZ matrix.     

Interface controlled micro- and macro- mechanical properties of aluminosilicate fiber reinforced SiC matrix composites

Novel Nextel™ 440 aluminosilicate fiber reinforced SiC matrix composites, with/without chemical vapor deposited carbon interphase were fabricated by polymer derived ceramic process, and they were studied by a combination of micro- and macro- mechanical techniques such as nanoindentation, micropillar splitting, fiber push-in, digital image correction and high temperature three point bend tests. Specifically, micropillar splitting test was firstly employed to measure in-situ the localized fracture toughness. The results revealed that the carbon interphase can effectively hinder the interfacial reactions between Nextel™ 440 fiber and SiC matrix, thus remarkably weakening the composite interfacial shear strength from ∼293 MPa to ∼42 MPa, and enhance the composite fracture toughness from ∼1.8 MPa√m to ∼6.3 MPa√m, respectively. This is mainly a consequence of weak interface that triggers crack deflection at the fiber/interphase interface. Finally, this novel composite showed stable mechanical properties in vacuum at temperature range from 25 °C to 1000 °C.     

Crystallization of SiC and its effects on microstructure,hardness and toughness in TaC/SiC multilayer films

A series of TaC/SiC multilayer films with different SiC thicknesses (t_SiC) have been prepared by magnetron sputtering and their microstructure, hardness and toughness investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy (AFM), scanning electron microscopy (SEM) and nanoindentation. Results show that SiC crystallized and grew coherently with TaC layers at low t_SiC (≤ 0.8 nm), resulting from the template effect of TaC layers. Maximum hardness and toughness of 46.06 GPa and 4.21 MPa m^1/2 were achieved at t_SiC = 0.8 nm with good coherent interface. With further increasing of t_SiC, SiC layers partially transformed to an amorphous structure and gradually lost their coherent interface, leading to a rapid drop in hardness and toughness. The crystallization of SiC layers and the coherent growth are required to achieve superhardness and high toughness in the TaC/SiC multilayers.     

Influence of whisker-aspect-ratio on densification,microstructure and mechanical properties of Al2O3 whiskers-reinforced CeO2-stabilized ZrO2 composites

A novel composite of 12 mol% CeO₂-stablized tetragonal ZrO₂ reinforced with Al₂O₃ whiskers (designated as Ce-TZP/A_w) has been prepared and studied in this work. The objective of this investigation was to systematically study the influence of whisker-aspect-ratio on the densification behaviors, microstructure evolution, and mechanical properties of Ce-TZP/A_w composite. Results showed that the sintered density of composite increased and the grain growth tended to diminish with the decrease in whisker aspect radio. Both the fracture toughness and flexural strength reached maximum values of 475 ± 12 MPa and 11.4 ± 0.2 MPa m^1/2, respectively at a whisker aspect ratio of about 12. It was also observed that the fracture toughness, flexural strength and tetragonal to monoclinic ZrO₂ transformation of the dual-phase composite exhibited similar variation trend as a function of the whisker-aspect-ratio, which suggested that the stress-induced phase transformation should be the main toughening and strengthening mechanism in the Ce-TZP/A_w composite.     

Microstructure characteristic and its influence on the strength of SiC ceramic joints diffusion bonded by spark plasma sintering

The interfacial microstructure evolution and shear strength of SiC joints for high temperature applications diffusion bonded by spark plasma sintering with a Ta-5W interlayer in the temperature range of 1500 °C to 1700 °C were investigated. The interfacial microstructure analysis indicated that (Ta,W)C phase formed initially and (Ta,W)-Si intermetallic compounds subsequently at SiC/Ta-5W interface. Bonding temperature had a significant effect on the reaction layer thickness, which increased with increasing the bonding temperature, and holding time also has an influence on reaction layer thickness. Calculation of diffusion kinetics for the SiC/Ta-5W interface showed that the diffusion constant was about two orders of magnitude larger than that obtained by hot-pressing bonding, and the activation energy was almost one-tenth that of hot-pressing bonding. Both the reaction layer thickness and the interfacial defects had a great effect on the robustness of the joint, and the maximum shear strength of 122 ± 15 MPa was obtained for the joint bonded at 1600 °C for 5 min.     

A case study of mechanical properties of perovskite-structured Ba0.5Sr0.5Co0.8Fe0.2O3−δ oxygen transport membrane

This study has investigated mechanical properties of perovskite-structured Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) oxygen transport membrane. The Young’s modulus and fracture toughness are determined by both macroscopic-scale and microscopic-scale methods. Both three-point and ring-on-ring bending tests as macroscopic-scale methods produce broadly similar results with a Young’s modulus, which is lower than that measured from micro-indentation method under a 10 N load. Young’s modulus and fracture toughness of BSCF show strongly dependent of the porosity. However, the fracture toughness of BSCF is independent of grain size. The fracture toughness determined by macroscopic-scale method is similar with that measured by microscopic-scale method. The crack shape of BSCF under a 10 N load is determined to be a median-radial mode. The intrinsic Young’s modulus and fracture toughness are determined to be 105.6 GPa and 1.49 MPa m^0.5, respectively, according the Minimum Solid Area (MSA) model. Annealing decreases the fracture toughness of BSCF between RT and 800 °C.     

Enhancing the sinterability and fracture toughness of Al2O3–TiN0.3 composites

We introduce a new and effective method for improving the fracture toughness of Al₂O₃-based composites through the addition of a nonstoichiometric material. Al₂O₃–TiN_0.3 composites were sintered by spark plasma sintering with different TiN_0.3 content at temperatures between 1300 and 1600 °C for 10 min and a micro-region diffusion phenomenon was observed at the Al₂O₃–TiN_0.3 interface. Ti atoms from TiN_0.3 diffused into Al₂O₃ to occupy Al sites, which led to the formation of Al vacancies that enabled the transport of aluminum by a vacancy mechanism. The optimal densification temperature of the Al₂O₃–30vol% TiN_0.3 composite was approximately 1400 °C. The maximum fracture toughness measured was 6.91 MPa m^1/2, from the composite with 30 vol% TiN_0.3 sintered at 1500 °C.     

Preparation and properties of high toughness RBAO macroporous membrane support

Reaction bonding of aluminum oxide (RBAO) is a novel technique for preparing porous alumina. By adapting this manufacturing route, macroporous Al₂O₃ supports with high fracture toughness are prepared for ceramic membrane. The effects of sintering temperatures and aluminum (Al) content on mechanical properties of macroporous Al₂O₃ supports are investigated, especially for the improvement of fracture toughness. When the sintering temperatures increase from 1200 °C to 1600 °C, increments of fracture toughness and bending strength are observed. Sintered at 1600 °C, when Al content is 16 wt%, the maximum value of fracture toughness and bending strength of macroporous Al₂O₃ supports are 2.0 MPa m^1/2 and 137 MPa, respectively, which are 2.0 and 2.6 times than that of the supports without adding any additives. By SEM analysis, many fine Al₂O₃ particles form a network which is beneficial to the improvements of fracture toughness and bending strength. After corroded in nitric acid and sodium hydroxide solutions of 1 mol L^?1 at 80 °C for 168 h, respectively, the mass loss percentage is lower than 1 wt%. And the bending strength keeps at the level of ～40 MPa which is strong enough to apply in industry. Simultaneously, the toughening mechanism of RBAO macroporous support is also discussed.     

Reactive sintering cBN-Ti-Al composites by spark plasma sintering

When synthesizing polycrystalline cubic boron nitride (PcBN) at normal pressure, cBN had a trend of hexagonal transformation, which reduces the hardness and strength of PcBN. The cBN-Ti-Al composite was prepared by spark plasma sintering with introducing Ti and Al to absorb hexagonal boron nitride (hBN) transformed from cBN. By the results of X-ray diffraction (XRD), Ti and Al reacted with BN and forming TiN, TiB₂, and AlN, which combined cBN as the binder by chemical bonding. The mechanical properties of the prepared composite increased as the increment of sintering temperature. The threshold temperature for preparing composite without hBN phase was at 1400 °C. The composite with optimal mechanical properties was prepared at 1400 °C, and the relative density, the bending strength, hardness, and fracture toughness were 98.9 ± 0.1%, 390.7 ± 4.4 MPa, 14.1 ± 0.5 GPa, and 7.6 ± 0.1 MPa·m^0.5, respectively.     

High temperature interfacial phase stability of a Mo/Ti3SiC2 laminated composite

A Mo/Ti₃SiC₂ laminated composite is prepared by spark plasma sintering at 1300 °C under a pressure of 50 MPa. Al powder is used as sintering aid to assist the formation of Ti₃SiC₂. The fabricated composites were annealed at 800, 1000 and 1150 °C under vacuum for 5, 10, 20 and 40 h to study the composite's interfacial phase stability at high temperature. Three interfacial layers, namely Mo₂C layer, AlMoSi layer and Ti₅Si₃ solid solution layer are formed during sintering. Experimental results show that the Mo/Ti₃SiC₂ layered composite prepared in this study has good interfacial phase stability up to at least 1000 °C and the growth of the interfacial layer does not show strong dependence on annealing time. However, after being exposed to 1150 °C for 10 h, cracks formed at the interface.     

Effect of multi-walled carbon nanotubes on microstructure and fracture properties of carbon fiber-reinforced ZrB2-based ceramic composite

Multi-walled carbon nanotubes (MWCNTs) were used to optimize the microstructure and improve the fracture properties of hot-pressed carbon fiber-reinforced ZrB₂-based ultra-high temperature ceramic composites. Microstructure analysis indicated that the introduction of MWCNTs effectively reduced the carbon fiber degradation and prevented fiber-matrix interfacial reaction during processing. Due to the presence of MWCNTs, the matrix contained fine ZrB₂ grains and in-situ formed nano-sized SiC/ZrC grains. The fracture properties were evaluated using the single edge-notched beam (SENB) test. The fracture toughness and work of fracture of the C_f/ZrB₂-based composite with MWCNTs were 7.0±0.4 MPa m^1/2 and 379±34 J/m², respectively, representing increases of 59% and 87% compared to those without MWCNTs. The excellent fracture properties are attributed to the moderate interfacial bonding between the fibers and matrix, which favour the toughening mechanisms, such as fiber bridging, fiber pull-out and crack deflection at interfaces.     

A novel design to produce high-strength and high-toughness alumina self-lubricated composites with enhanced thermal-shock resistance—Part I: Mechanical properties and thermal shock behavior of Al2O3/Mo-Al2O3 laminated composites

This paper proposes a new strategy to design the high-performance Al₂O₃/Mo self-lubricated composites with excellent practical value and durability. The relationships among the relevant structural parameters, interfacial compositions, mechanical and thermal properties of the materials were analyzed. Results show that the apparent toughness, bending strength and work of fracture of the optimal Al₂O₃/Mo-Al₂O₃ laminated materials could reach 8.1 MPa m^1/2, 634 MPa and 330 J m⁻². Moreover, the new-developed materials exhibited a good self-lubricating property on every surface and thermal shock resistance. The friction coefficients of all the surfaces can be as low as 0.45 at 800 °C, and the retention rates of strength and toughness after thermal shock between 25 °C and 1000 °C for 50 cycles could reach 98.8% and 85.3%, respectively. The new strategy is based on a combination strong interfacial bonding and with accelerated formation of a reasonable residual stress and enhanced grain-interlocking among particles during fatigue.     

High temperature mechanical properties of Al2O3/TiC micro–nano-composite ceramic tool materials

A type of Al₂O₃-based composite ceramic tool material simultaneously reinforced with micro-scale and nano-scale TiC particles was fabricated by the hot-pressing technology with different contents of cobalt additive. The effects of cobalt on the ambient temperature mechanical properties and high temperature flexural strength were investigated. The flexural strength and fracture toughness of the composite with 3 vol% cobalt as a function of temperature were investigated. Cobalt greatly enhanced the ambient temperature flexural strength and fracture toughness, while further increasing the content of cobalt led to a dramatic strength degradation, especially at high temperature. The flexural strength of the composite containing 3 vol% cobalt decreased as the temperature increased from 20 to 1200 °C, and the fracture toughness decreased as a function of the temperature up to 1000 °C but increased at 1200 °C. The degradation of high temperature flexural strength was ascribed to the change of the fracture mode, the grain and grain boundary oxidation, the decrease of elastic modulus and the grain boundary sliding.     

In situ fabrication and properties of 0.4MoB-0.1SiC-xMoSi2 composites by self-propagating synthesis and hot-press sintering

Mo, Si and B₄C powders were used to fabricate 0.4MoB-0.1SiC-xMoSi₂ composites by self-propagating high-temperature synthesis (SHS) and hot pressing (HP). The effects of MoSi₂ content (x = 1, 0.75, 0.5 and 0.25) on phase composition, microstructure and properties of the composites were investigated. The results showed that the 0.4MoB-0.1SiC-xMoSi₂ composite exhibited Vickers hardness of 10.7–15.2 GPa, bending strength of 337–827 MPa and fracture toughness of 4.9–7.0 MPa m^1/2. The fracture toughness increased with the increasing volume fraction of MoB and SiC particles which were promoted by the toughening mechanisms, such as crack bridging, cracks deflection and crack branching. Moreover, the electrical resistivity showed an increasing trend with decreasing volume fraction of MoSi₂.     

Pressureless sintering of boron carbide

The effect of small Al addition on pressureless-sintering and mechanical properties of B₄C ceramic was analyzed. Different amounts of aluminium powder, from 0% to 5 wt%, were added to the base material and pressureless-sintering was conducted at 2050 and 2150 °C under argon atmosphere. Microstructure, crystalline phases, density evolution, fracture strength, elastic modulus, hardness and fracture toughness were analyzed and correlated to Al additions and firing temperature. Density and grain size of sintered samples increased significantly with Al load while the effect of sintering temperature was less evident; 94% dense material was obtained by adding 4 wt% Al. Bending strength, hardness and fracture toughness of sintered B₄C samples were shown to increase for Al content up to 4 wt% while further additions resulted in a decrease of the mechanical resistance. Conversely, elastic modulus showed an increase with Al load especially between 1 and 3 wt%.     

Microtopography and mechanical properties of vacuum hot pressing Al/B4C composites

Al/B₄C composites with various volume contents of B₄C (5%, 10%, 15%, 20%, and 25%) reinforcing the Al matrix, have been fabricated by vacuum hot press sintering at 680 °C, with a soaking time of 90 min and external pressure of 30 MPa. Mechanical properties, phase composition, and microstructure of the Al/B₄C composites are discussed to reveal the physical properties of the composites. Field emission transmission electron microscopy and selected area electron diffraction have been employed to verify the interior structure and crystal growth direction, respectively. The Vickers hardness, fracture strength, tensile strength, and maximum force attained the optimal values of 108.45 ± 4.02 HV, 585.70 ± 23.26 MPa, 196.18 ± 2.48 MPa, and 4.44 ± 0.17 kN, respectively, for 25 vol% B₄C/Al composites. The static compression strength increased before the 15 vol% B₄C addition and then decreased, acquiring the highest value of 292.15 ± 2.09 MPa for 15 vol% B₄C/Al composites. In general, the relative density and ductility of these composites consistently increased, with an increase in the volume content of Al, achieving a maximum of 99.22% and 54.63 ± 7.34%, respectively, for 5 vol% B₄C/Al composites.     

Developing high toughness and strength Al/TiC composites using ice-templating and pressure infiltration

We prepared Al/TiC composites with different ceramic volume fractions (15, 25 and 35 vol%) using ice-templating and pressure infiltration. The thickness of the lamellar layer and the porosity in the ceramic layer of the TiC scaffolds were controlled by varying the slurry concentration. The Al/15 vol%TiC composite had a thick metal layer and a low-density ceramic layer, which effectively dissipated the stress at the crack tip and fractured in a multiple-crack-propagation mode, giving bending strength of 355±3 MPa and fracture toughness of 81±2 MPa m^1/2. However, the Al/25 vol%TiC and Al/35 vol%TiC composites had much higher bending strength (417−500 MPa) but lower fracture toughness (46−33 MPa m^1/2) as compared to the Al/15 vol%TiC composite, and they fractured in a single-crack-propagation mode. In addition, an increase in the brittle TiAl₃ phase with increasing ceramic volume at the fracture surface greatly deteriorated the toughness of the Al/TiC composites. Finally, the relationship between cracking mode and structure features in the laminated composites was discussed to account for the toughening mechanism.     

Microscopic analysis of single-fiber push-out tests on ceramic matrix composites performed with Berkovich and flat-end indenter and evaluation of interfacial fracture toughness

Single-fiber push-out tests performed with a Berkovich and a flat-end indenter tip were conducted on the same SiC/PyC/SiC ceramic matrix composite sample for comparison. Push-out measurements were stopped at different stages during the experiment for a detailed microscopic analysis of the front and back side of the sample, to investigate the progression of failure during push-out process. The microscopic analyses reveal differences from the established interpretations which are crucial for quantitative evaluation of interface properties. Based on the microscopic findings, a modified loading schedule comprising unloading–reloading cycles is proposed, which provides access to the dissipative and non-dissipative energy contributions during push-out test. A new energy-based approach is presented which allows for the determination of the interfacial fracture toughness, without assumptions regarding the stress distribution along the interface to be made. Presuming stable crack growth along the complete debonding length, the interfacial fracture toughness of the sample investigated amounts to 44 ± 9 J/m².     

Microstructure,mechanical, thermal,and oxidation properties of a Zr2[Al(Si)]4C5–SiC composite prepared by in situ reaction/hot-pressing

The microstructure, mechanical and thermal properties, as well as oxidation behavior, of in situ hot-pressed Zr₂[Al(Si)]₄C₅–30 vol.% SiC composite have been characterized. The microstructure is composed of elongated Zr₂[Al(Si)]₄C₅ grains and embedded SiC particles. The composite shows superior hardness (Vickers hardness of 16.4 GPa), stiffness (Young's modulus of 386 GPa), strength (bending strength of 353 MPa), and toughness (fracture toughness of 6.62 MPa m^1/2) compared to a monolithic Zr₂[Al(Si)]₄C₅ ceramic. Stiffness is maintained up to 1600 °C (323 GPa) due to clean grain boundaries with no glassy phase. The composite also exhibits higher specific heat capacity and thermal conductivity as well as better oxidation resistance compared to Zr₂[Al(Si)]₄C₅.