Effect of SiC nanowires on the thermal shock resistance of joint between carbon/carbon composites and Li2O–Al2O3–SiO2 glass ceramics

In order to improve the bonding property of joint between SiC modified carbon/carbon (C/C) composites and Li₂O–Al₂O₃–SiO₂ (LAS) glass ceramics, SiC nanowires were attempted as the reinforcement materials in the interface region of SiC transition layer and Li₂O–MgO–Al₂O₃–SiO₂ (LMAS) gradient joining interlayer. The C/C–LAS joint with SiC nanowire-reinforced interface layer was prepared by a three-step technique of pack cementation, in situ reaction and hot-pressing. The microstructure and thermal shock resistance of the as-prepared joints were examined. The average shear strength of the joined samples with SiC nanowires increased from 24.9 MPa to 31.6 MPa after 40 thermal cycles between 1000 °C and room temperature, while that of the joined samples without SiC nanowires dropped from 21.4 MPa to 8.3 MPa. The increase of thermal shock resistance of the C/C–LAS joints was mainly attributed to the toughening mechanism of SiC nanowires by pullout, bridging and crack deflection.     

Reaction joining of SiC ceramics using TiB2-based composites

SiC ceramics were reaction joined in the temperature range of 1450–1800 °C using TiB₂-based composites starting from four types of joining materials, namely Ti–BN, Ti–B₄C, Ti–BN–Al and Ti–B₄C–Si. XRD analysis and microstructure examination were carried out on SiC joints. It is found that the former two joining materials do not yield good bond for SiC ceramics at temperatures up to 1600 °C. However, Ti–BN–Al system results in the connection of SiC substrates at 1450 °C by the formation of TiB₂–AlN composite. Furthermore, nearly dense SiC joints with crack-free interface have been produced from Ti–BN–Al and Ti–B₄C–Si systems at 1800 °C, i.e. joints TBNA80 and TBCS80, whose average bending strengths are measured to be 65 MPa and 142 MPa, respectively. The joining mechanisms involved are also discussed.     

Joining of SiC ceramics via a novel liquid preceramic polymer (V-PMS)

A novel liquid preceramic polymer (V-PMS) was synthsized by modifying polymethylsilane (PMS) with 1,3,5,7-tetramethyl-1,3,5,7-tetravinylcyclotetrasiloxane ([CH₃(CH₂CH)SiO]₄, D4Vi), for joining SiC ceramics under ambient pressure. The obtained V-PMS with a viscosity of 125 Pas at room temperature exhibits excellent thermal properties and bonding strength. The ceramic yield of V-PMS treated at 1200 °C under Ar atmosphere is 84.5%, which is 38.3% higher than the original PMS. The shear strengths of the SiC joints joined by V-PMS at 800 °C, 1000 °C and 1200 °C under N₂ atmosphere are 11.9 MPa, 34.5 MPa and 29.9 MPa, respectively. The excellent performances make the obtained V-PMS promising candidates for joining SiC ceramics in high-temperature applications.     

Microstructure and mechanical properties of SiC-SiC joints joined by spark plasma sintering

The development of reliable joining technology is of great importance for the full use of SiC. Ti₃SiC₂, which is used as a filler material for SiC joining, can meet the demands of neutron environment applications and can alleviate residual stress during the joining process. In this work, SiC was joined using different powders (Ti₃SiC₂ and 3Ti/1.2Si/2C/0.2Al) as filler materials and spark plasma sintering (SPS). The influence of the joining temperature on the flexural strength of the SiC joints at room temperature and at high temperatures was investigated. Based on X-ray diffraction and scanning electron microscopy analyses, SiC joints with 3Ti/1.2Si/2C/0.2Al powder as the filler material possess high flexural strengths of 133 MPa and 119 MPa at room temperature and at 1200 °C, respectively. The superior flexural strength of the SiC joint at 1200 °C is attributed to the phase transformation of TiO₂ from anatase to rutile.     

High temperature rapid reactive joining of dissimilar materials: Silicon carbide to an aluminum alloy

An effective method for bonding of silicon carbide (SiC) ceramic to 5083 Aluminum metal-alloys was developed. This method employed the concept of high-temperature rapid heterogeneous combustion reaction to joint dissimilar materials. Scaled-up welding apparatus, which can accommodate specimens of up to 5″ in diameter and 6″ in height and allow a rapid (～10 ms) load application (up to 1000 MPa) on the joining stack during reaction, was designed and built. An exothermic mixture of titanium and carbon powders (molar ratio 1:0.4) was utilized as a joining reactive layer. Uniform crack-free bonding of SiC to 5083 Al-alloy was accomplished both without and with application of loads. SEM and focused ion beam (FIB) images revealed that ～10 μm (without load) and ～500 nm (with load) transitional joining layers along the interfaces were observed, respectively. Compositional and mechanical properties of joined samples were also characterized by EDS analysis and Vickers microhardness investigations.     

Flash joining of CVD-SiC coated Cf/SiC composites with a Ti interlayer

Flash joining of CVD-SiC coated C_f/SiC samples with a Ti interlayer was achieved using a Spark Plasma Sintering machine. The influence of different heating powers and discharge times were investigated. The sample flash joined at a maximum heating power of 2.2 kW (peak electric current of 370 A) within 7 s showed the highest apparent shear strength of 31.4 MPa, which corresponds to the interlaminar shear strength of the composites. A maximum joining temperature of ∼1237 °C was reached during the flash joining. An extremely rapid heating rate of 9600 °C/min combined with a very short processing time hindered any reaction between the CVD-SiC coating and the Ti interlayer. The formation of a metallic joint (Ti based) in the absence of any detectable reaction phase is proposed as a new joining mechanism. For a conventionally joined SPS sample, the formation of titanium silicide phases inhibited the formation of a bond.     

Thickness-dependent phase evolution and bonding strength of SiC ceramics joints with active Ti interlayer

A robust solid state diffusion joining technique for SiC ceramics was designed with a thickness-controlled Ti interlayer formed by physical vapor deposition and joined by electric field-assisted sintering technology. The interface reaction and phase revolution process were investigated in terms of the equilibrium phase diagram and the concentration-dependent potential diagram of the Ti-Si-C ternary system. Interestingly, under the same joining conditions (fixed temperature and annealing duration), the thickness of the Ti interlayer determined the concentration and distribution of the Si and C reactants in the resulting joint layer, and the respective diffusion distance of Si and C into the Ti interlayer differentiated dramatically during the short joining process (only 5 min). In the case of a 100 nm Ti coating as an interlayer, the C concentration in the joint layer was saturated quickly, which benefited the formation of a TiC phase and subsequent Ti₃SiC₂ phase. The SiC ceramics were successfully joined at a low temperature of 1000 °C with a flexural strength of 168.2 MPa, which satisfies applications in corrosive environments. When the Ti thickness was increased to 1 μm, Si atoms diffused easily through the diluted Ti-C alloy (a dense TiC phase was not formed), and the Ti₅Si₃ brittle phase formed preferentially. These findings highlight the importance of the diffusion kinetics of the reactants on the final composition in the solid state reaction, particularly in the joining technique for covalent SiC ceramics.     

Microstructure development,hardness, toughness and creep behaviour of pressureless sintered alumina/SiC micro–nanocomposites obtained by slip-casting

Al₂O₃–SiC micro–nanocomposites are much more resistant materials than monolithic alumina regarding some mechanical properties. In order to study the possibility of obtaining creep resistant alumina/SiC micro–nanocomposites using inexpensive forming methods, alumina 1 and 5 vol% SiC materials were produced by slip-casting and pressureless sintering. Well-densified alumina–SiC pressureless sintered materials were obtained at 1700 °C for 2 h and attained 97–99% of the theoretical density. The microstructure, hardness and toughness were examined and 4-point flexure creep tests were performed at 1200 °C and 100 MPa in air. Compared with pure alumina materials, the creep resistance, toughness and hardness were enhanced drastically in materials containing 5 vol% of SiC.     

Development of SiC–SiC joint by reaction bonding method using SiC/C tapes as the interlayer

SiC/C tapes with different compositions and thicknesses were used to join pressureless sintered silicon carbide ceramics by reaction bonding method. The microstructure of the joints and the influences of joint thickness and residual silicon content in joint layer on the 4-point flexural strength of as joined SiC ceramics have been investigated. Specimens with high flexural strength can be achieved through the control of the composition and the thickness of the joint layer. The highest flexural strength of the joined specimens with the joint thickness of 13 μm can reach 346 ± 35 MPa and 439 ± 31 MPa at room temperature and 1250 °C, respectively. The microstructure development and the reaction bonding mechanism were also studied.     

Preparation of high-temperature organic adhesives and their performance for joining SiC ceramic

Two kinds of high-temperature organic adhesives were prepared and successfully applied to join SiC ceramic. One adhesive was composed of preceramic polymer (V-PMS) and B₄C powder (HTA-1), and the other was composed of V-PMS, B₄C powder and low melting point glass powder (HTA-2). The properties of the obtained adhesives were investigated by TGA, XRD, bonding test and SEM analysis. The results show that the obtained adhesives exhibit outstanding heat-resistant property and excellent bonding strength. The bonding strength of HTA-1 treated at 200 °C, 400 °C, 600 °C were 26.8 MPa, 18.9 MPa, 7.3 MPa, respectively. When the temperature increased to 800 °C or even higher, the shear strengths of the joints were enhanced to over 50 MPa. Moreover, by adding glass powder as the second filler, it was found that the minimum shear strength of HTA-2 was enhanced to 16.4 MPa. The excellent performances of the obtained adhesives make them as promising candidates for joining SiC ceramics for high-temperature applications.     

Densification behaviour and mechanical properties of B4C–SiC intergranular/intragranular nanocomposites fabricated through spark plasma sintering assisted by mechanochemistry

High-performance B₄C–SiC nanocomposites with intergranular/intragranular structure were fabricated through spark plasma sintering assisted by mechanochemistry with B₄C, Si and graphite powders as raw materials. Given their unique densification behaviour, two sudden shrinkages in the densification curve were observed at two very narrow temperature ranges (1000–1040 °C and 1600–1700 °C). The first sudden shrinkage was attributed to the volume change in SiC resulting from disorder–order transformation of the SiC crystal structure. The other sudden shrinkage was attributed to the accelerated densification rate resulting from the disorder–order transformation of the crystal structure. The high sintering activity of the synthesised powders could be utilised sufficiently because of the high heating rate, so dense B₄C–SiC nanocomposites were obtained at 1700 °C. In addition, the combination of high heating rate and the disordered feature of the synthesised powders prompted the formation of intergranular/intragranular structure (some SiC particles were homogeneously dispersed amongst B₄C grains and some nanosized B₄C and SiC particles were embedded into B₄C grains), which could effectively improve the fracture toughness of the composites. The relative density, Vickers hardness and fracture toughness of the samples sintered at 1800 °C reached 99.2±0.4%, 35.8±0.9 GPa and 6.8±0.2 MPa m^1/2, respectively. Spark plasma sintering assisted by mechanochemistry is a superior and reasonable route for preparing B₄C–SiC composites.     

Characterization of silicon carbide joints fabricated using SiC particulate-reinforced Ag–Cu–Ti alloys

CVD silicon carbide was brazed to itself using two Ag–Cu–Ti braze alloys reinforced with SiC particulates to control braze thermal expansion and enhance joint strength. Powders of the braze alloys, Ticusil (composition in wt%: Ag–26.7Cu–4.5Ti, T_L: 900 °C) and Cusil-ABA (Ag–35.3Cu–1.75Ti, T_L: 815 °C) were pre-mixed with 5, 10 and 15 wt% SiC particulates (～20–30 μm) using glycerin to create braze pastes that were applied to the surfaces to be joined. Joints were vacuum brazed and examined using optical microscopy (OM), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and the Knoop hardness test. The SiC particles were randomly distributed in the braze matrix and bonded to it via reaction with the titanium from the braze alloy. Titanium together with Si and C segregated at the particle/braze interface, and promoted nucleation and precipitation of the Cu-rich secondary phase on particle surfaces. The Si–Ti–C-rich reaction layers also formed at the interface between CVD SiC substrate and the braze alloy. The loss of Ti in the reaction with SiC particulates did not impair either the bond quality or the thickness of the reaction layer on the CVD SiC substrate. Microhardness measurements showed that the dispersed SiC particulates lowered the braze hardness by depleting the braze matrix of Ti. Theoretical calculations indicated the CTE of the braze to decrease by nearly 45–60% with the incorporation of about 45 vol% SiC.     

C/C–SiC composite prepared by Si–10Zr alloyed melt infiltration

A high performance and low cost C/C–SiC composite was prepared by Si–10Zr alloyed melt infiltration. Carbon fiber felt was firstly densified by pyrolytic carbon using chemical vapor infiltration to obtain a porous C/C preform. The eutectic Si–Zr alloyed melt (Zr: 10 at.%, Si: 90 at.%) was then infiltrated into the porous preform at 1450 °C to prepare the C/C–SiC composite. Due to the in situ reaction between the pyrolytic carbon and the Si–Zr alloy, SiC, ZrSi₂ and ZrC phases were formed, the formation and distribution of which were investigated by thermodynamics. The as-received C/C–SiC composite, with the flexural strength of 353.6 MPa, displayed a pseudo-ductile fracture behavior. Compared with the C/C preform and C/C composite of high density, the C/C–SiC composite presented improved oxidation resistance, which lost 36.5% of its weight whereas the C/C preform lost all its weight and the high density C/C composite lost 84% of its weight after 20 min oxidation in air at 1400 °C. ZrO₂, ZrSiO₄ and SiO₂ were formed on the surface of the C/C–SiC composite, which effectively protected the composite from oxidation.     

Fabrication and properties of ZrB2–SiC–BN machinable ceramics

ZrB₂–SiC–BN ceramics were fabricated by hot-pressing under argon at 1800 °C and 23 MPa pressure. The microstructure, mechanical and oxidation resistance properties of the composite were investigated. The flexural strength and fracture toughness of ZrB₂–SiC–BN (40 vol%ZrB₂–25 vol%SiC–35 vol%BN) composite were 378 MPa and 4.1 MPa m^1/2, respectively. The former increased by 34% and the latter decreased by 15% compared to those of the conventional ZrB₂–SiC (80 vol%ZrB₂–20 vol%SiC). Noticeably, the hardness decreased tremendously by about 67% and the machinability improved noticeably compared to the relative property of the ZrB₂–SiC ceramic. The anisothermal and isothermal oxidation behaviors of ZrB₂–SiC–BN composites from 1100 to 1500 °C in air atmosphere showed that the weight gain of the 80 vol%ZrB₂–20 vol%SiC and 43.1 vol%ZrB₂–26.9 vol%SiC–30 vol%BN composites after oxidation at 1500 °C for 5 h were 0.0714 and 0.0268 g/cm², respectively, which indicates that the addition of the BN enhances oxidation resistance of ZrB₂–SiC composite. The improved oxidation resistance is attributed to the formation of ample liquid borosilicate film below 1300 °C and a compact film of zirconium silicate above 1300 °C. The formed borosilicate and zirconium silicate on the surface of ZrB₂–SiC–BN ceramics act as an effective barriers for further diffusion of oxygen into the fresh interface of ZrB₂–SiC–BN.     

UHTC–carbon fibre composites: Preparation,oxyacetylene torch testing and characterisation

Current generation carbon–carbon (C–C) and carbon–silicon carbide (C–SiC) materials are limited to service temperatures below 1800 °C and materials are sought that can withstand higher temperatures and ablative conditions for aerospace applications. One potential materials solution is carbon fibre-based composites with matrices composed of one or more ultra-high temperature ceramics (UHTCs); the latter are intended to protect the carbon fibres at high temperatures whilst the former provides increased toughness and thermal shock resistance to the system as a whole. Carbon fibre–UHTC powder composites have been prepared via a slurry impregnation and pyrolysis route. Five different UHTC compositions have been used for impregnation, viz. ZrB₂, ZrB₂–20 vol% SiC, ZrB₂–20 vol% SiC–10 vol% LaB₆, HfB₂ and HfC. Their high-temperature oxidation resistance has been studied using a purpose built oxyacetylene torch test facility at temperatures above 2500 °C and the results are compared with that of a C–C benchmark composite.     

Microstructure and densification of ZrB2–SiC composites prepared by spark plasma sintering

ZrB₂–SiC composites were prepared by spark plasma sintering (SPS) at temperatures of 1800–2100 °C for 180–300 s under a pressure of 20 MPa and at higher temperatures of above 2100 °C without a holding time under 10 MPa. Densification, microstructure and mechanical properties of ZrB₂–SiC composites were investigated. Fully dense ZrB₂–SiC composites containing 20–60 mass% SiC with a relative density of more than 99% were obtained at 2000 and 2100 °C for 180 s. Below 2120 °C, microstructures consisted of equiaxed ZrB₂ grains with a size of 2–5 μm and α-SiC grains with a size of 2–4 μm. Morphological change from equiaxed to elongated α-SiC grains was observed at higher temperatures. Vickers hardness of ZrB₂–SiC composites increased with increasing sintering temperature and SiC content up to 60 mass%, and ZrB₂–SiC composite containing 60 mass% SiC sintered at 2100 °C for 180 s had the highest value of 26.8 GPa. The highest fracture toughness was observed for ZrB₂–SiC composites containing 50 mass% SiC independent of sintering temperatures.     

Joining of β-SiC by spark plasma sintering

Spark plasma sintering (SPS) was employed to join monolithic β-SiC with or without titanium as intermediate joining material. Both the localized and rapid heating contributed to the inherent energy saving of electric current assisted joining technique. The effects of uniaxial pressure and surface preparation were analyzed independently with respect to the flexural strength and the morphology of the joints. In particular samples polished down to 1 μm and joined at 1900 °C for 5 min achieved the strength of the as received material. The failure occurred outside the joining interface, confirming the optimum quality of the joint. Pressure in combination with surface preparation was necessary to achieve perfect adhesion and pore free direct joining of SiC. The use of Ti foil as a joining material and pressure allowed joining of unpolished SiC.     

Physical characterization and arcjet oxidation of hafnium-based ultra high temperature ceramics fabricated by hot pressing and field-assisted sintering

For this study, HfB₂-based ultra high temperature ceramic (UHTC) samples were prepared by hot pressing and field-assisted sintering (FAS) with 10–20 vol.% SiC (baseline), 5 vol.% TaSi₂, and 5 vol.% iridium. Dense billets were tested for hardness and mechanical strength. When compared, the FAS method consistently yielded materials with a grain size 1.5–2 times finer than samples processed via hot pressing. In general, room temperature flexural strengths of these materials were found to be lower (～400 MPa) than similar fully dense HfB₂–SiC materials, with strengths between 500 and 700 MPa. Oxidation resistance testing of flat-face models was conducted in a simulated re-entry environment, at Q_{Cold Wall} ～250 W/cm² for 5 min. Samples processed by FAS had reduced oxide thickness and SiC depletion zones compared to the baseline HfB₂–20SiC material. In all cases oxide thickness was reduced by ～3× and SiC depletion zone thickness was reduced ～3× over the baseline.     

Heat conduction mechanisms in hot pressed ZrB2 and ZrB2–SiC composites

Thermal diffusivity and conductivity of hot pressed ZrB₂ with different amounts of B₄C (0–5 wt%) and ZrB₂–SiC composites (10–30 vol% SiC) were investigated experimentally over a wide range of temperature (25–1500 °C). Both thermal diffusivity and thermal conductivity were found to decrease with increase in temperature for all the hot pressed ZrB₂ and ZrB₂–SiC composites. At around 200 °C, thermal conductivity of ZrB₂–SiC composites was found to be composition independent. Thermal conductivity of ZrB₂–SiC composites was also correlated with theoretical predictions of the Maxwell–Eucken relation. The dominated mechanisms of heat transport for all hot pressed ZrB₂ and ZrB₂–SiC composites at room temperature were confirmed by Wiedemann–Franz analysis by using measured electrical conductivity of these materials at room temperature. It was found that electronic thermal conductivity dominated for all monolithic ZrB₂ whereas the phonon contribution to thermal conductivity increased with SiC contents for ZrB₂–SiC composites.     

Diffusion bonding of ZrB2–SiC/Nb with in situ synthesized TiB whiskers array

The ZrB₂–20 vol.% SiC (ZS) composites were diffusion bonded to Nb using pure Ti interlayer. Effects of joining temperature on the microstructure and mechanical properties of the joints were investigated. The results show that Ti reacted with Nb and ZS to form a typical three layers in the joint. An in situ synthesized TiB whiskers array which consisted of two types of TiB was produced in the reaction layer. The formation mechanism of TiB was analyzed. Mutual diffusion between Ti and Nb led to a ductile β-(Ti, Nb) layer on Nb side. Joining temperature influenced the thickness of reaction layers and distribution of TiB seriously. The maximum shear strength reached 158 MPa with bonding temperature at 1200 °C for 60 min.