Brazing SiC ceramic using novel B4C reinforced Ag–Cu–Ti composite filler

The development of new composite fillers is crucial for joining ceramics or ceramics to metals because the composite fillers exhibit more advantages than traditional brazing filler metal. In this research, novel B₄C reinforced Ag–Cu–Ti composite filler was developed to braze SiC ceramics. The interfacial microstructure of the joints was characterized by scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The effect of B₄C addition and brazing temperature on the microstructure evolution and mechanical properties of the joints was analyzed. The results revealed that TiB whisker and TiC particles were simultaneously synthesized in the Ag-based solid solution and Cu-based solid solution due to the addition of B₄C particles. As the brazing temperature increased, the thickness of Ti₃SiC₂+Ti₅Si₃ layers adjacent to SiC ceramic increased. Desirable microstructure similar to the metal matrix reinforced by TiB whisker and TiC particles could be obtained at brazing temperature of 950 °C. The maximum bending strength of 140 MPa was reached when the joints brazed at 950 °C for 10 min, which was 48 MPa (~52%) higher than that of the joints brazed using Ag–Cu–Ti filler.     

Effects of oxidation cross-linking and sintering additives (TiN,B) on the formation and heat-resistant performance of polymer-derived SiC(Ti,B) films

Effects of oxidation cross-linking and sintering additives (TiN, B) on the microstructure formation and heat-resistant performance of freestanding SiC(Ti, B) films synthesized from Ti, B-containing polycarbosilane (TiB-PCS) precursor were investigated. TiB-PCS green films were first cross-linked for 1 h, 2 h, 3 h and 4 h, respectively, and then pre-sintered at 950 °C. Finally, they were sintered at 1800 °C to complete the conversion from organic films to inorganic SiC(Ti, B) films. The results reveal that curing time has a great impact on the uniformity and density of SiC(Ti, B) films. TiB-PCS films cured for 3 h yield the best quality SiC(Ti, B) films, which are composed of β-SiC crystals, C clusters, α-SiC nano-crystals, a small amount of TiB₂ and B₄C. TiB₂ and B₄C are both steady phases which can inhibit abnormal growth of β-SiC, effectively reduce sintering temperature and help consume excess C from decomposition of amorphous SiOxCy. After high temperature annealing at 1500 °C, 1600 °C and 1700 °C in argon, SiC(Ti, B) films still keep excellent mechanical properties, which makes them attractive candidate materials for microelectromechanical systems (MEMS) used at ultra-high temperatures (exceeding 1500 °C).     

Ultra-high elevated temperature strength of TiB2-based ceramics consolidated by spark plasma sintering

Spark plasma sintering of TiB₂–boron ceramics using commercially available raw powders is reported. The B₄C phase developed during reaction-driven consolidation at 1900 °C. The newly formed grains were located at the grain junctions and the triple point of TiB₂ grains, forming a covalent and stiff skeleton of B₄C. The flexural strength of the TiB₂–10 wt.% boron ceramic composites reached 910 MPa at room temperature and 1105 MPa at 1600 °С. Which is the highest strength reported for non-oxide ceramics at 1600 °C. This was followed by a rapid decrease at 1800 °C to 480–620 MPa, which was confirmed by increased number of cavitated titanium diboride grains observed after flexural strength tests.     

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.     

Reaction Synthesis and Mechanical Properties of TiB2–AlN–SiC Composites

TiB₂–AlN–SiC (TAS) ternary composites were prepared by reactive hot pressing at 2000°C for 60 min in an Ar atmosphere using TiH₂, Si, Al, B₄C, BN and C as raw powders. The phase composition was determined to be TiB₂, AlN and β-SiC by XRD. The distribution of elements Al and Si were not homogeneous, which shows that to obtain a homogeneous solid solution of AlN and SiC in the composites by the proposed reaction temperatures higher than 2000°C or time duration longer than 60 min are needed. The higher fracture toughness (6·35±0·74 MPa·m^1/2 and 6·49±0·73 MPa·m^1/2) was obtained in samples with equal molar contents of AlN and SiC (TAS-2 and TAS-5) in the TAS composites. The highest fracture strength (470±16 MPa) was obtained in TAS-3 sample, in which the volume ratio of TiB₂/(AlN+SiC) was the nearest to 1 and there was finer co-continuous microstructure. ©     

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.     

High-temperature oxidation of AlN–SiC–TiB2 ceramics in air

The oxidation behaviour of AlN–SiC–TiB₂ composite materials with 2, 5 and 10 mass% TiB₂ and 3 mass% Fe additive obtained using powder metallurgy methods was studied in air up to 1500 °C by thermogravimetry (TG) and differential thermal analysis (DTA) techniques. The phase composition and structure of the oxide films formed were investigated using metallography, X-ray diffraction (XRD) and electron probe microanalysis (EPMA) methods. The two-stage character of non-isothermal oxidation kinetics (heating rate of 15 grade/min) of composites was established. During the first oxidation stage (up to 1350 °C), the formation of α-Al₂O₃, TiO₂ (rutile), B₂O₃ and β-cristobalite as well as different aluminium borates was found. They formed as a result of interaction between Al₂O₃ and melted B₂O₃. During the second stage (above 1350–1400 °C), the mullite 3Al₂O₃·2SiO₂ proved to be a main oxidation product in the scale; besides, some amounts of β-Al₂TiO₅ were formed as well. The iron additive dissolved in the mullite and aluminium titanate phases that led to the stabilization of a scale formed. It was established that for the three different TiB₂ contents, oxidation isotherms follow the parabolic or paralinear rate law. The slope change on the Arrhenius plot given by the dependence of the parabolic rate constants on the reciprocal temperature, suggests a change of the oxidation mechanism in the temperature range of 1300–1350 °C. For example, for the (AlN–SiC)–5% TiB₂ composite specimen, the calculated values of apparent activation energy are equal to 285 kJ/mol (1100–1300 °C) and 500 kJ/mol (1350–1550 °C), respectively. The AlN–SiC–TiB₂ ceramics developed here can be recommended as high-performance materials for a use in oxidizing medium up to 1450 °C.     

Wetting behavior and interfacial interactions of molten Cu50Ti alloy with hexagonal BN and TiB2 ceramics

Wetting behavior of molten Cu50Ti alloy on hexagonal BN (h-BN) and TiB₂ ceramics has been studied under vacuum using a modified sessile drop method. Final contact angles of 8° and 3° are obtained at 1000 °C on h-BN and TiB₂, respectively. Interaction occurs at the interface between the molten alloy and BN, leading to the formation of a reaction layer containing TiB and Ti nitrides. Interfacial interaction of Cu50Ti with TiB₂ results in the formation of densely packed TiB layer about 60–100 μm thick and the detachment of TiB₂ grains. Spreading wetting of liquid Cu50Ti on h-BN is mainly controlled by the reactions between Ti and BN at the triple line. For Cu50Ti/TiB₂ system, spreading is mainly limited by the interfacial reaction in the first stage, and is possibly influenced by both the diffusion of boron atoms and viscous friction of the liquid in the second stage. Finally, brazing of graphite to CuCrZr alloy has been realized using Cu50TiH₂ with ceramic additives (including BN and TiB₂) as composite fillers. The joints exhibit favorable interfacial bonding between the filler layer and the substrates. The ceramic reinforcements in the filler layer could contribute to the improvement of the shear strength.     

Synthesis and ceramic conversion of a novel processible polyboronsilazane precursor to SiBCN ceramic

Polyboronsilazane (PBSZ) precursors for SiBCN ceramics were prepared by using 9-borabicyclo-[1,3,3] nonane (9-BBN) and copolysilazanes (CPSZ) as starting materials, involving the hydroboration reaction between vinyl groups of PSZ and BH groups of 9-BBN under mild conditions. The as-synthesized PBSZ was obtained as a soluble liquid, which was characterized by FT IR and NMR. The polymer-to-ceramic conversion of PBSZ at a ceramic yield of 62.2–79.9% was investigated by means of FT IR and TGA. The crystallization behavior and microstructures of PBSZ-derived SiBCN ceramics were studied by XRD, SEM and HRTEM. The SiBCN ceramic began to crystallize at 1600 °C. Further heating at 1800 °C induced partial crystallization to give mixed XRD patterns for SiC, Si₃N₄, and BN(C). It is observed that the introduction of boron improves the thermal stability of SiBCN ceramics, especially under high temperatures of 1600–1800 °C. In addition, the introduction of boron significantly improves the ceramic density while inhibits the SiC crystallization.     

Microstructural evolution,mechanical and thermal properties of LaB6 embedded in Si-B-C-N prepared by spark plasma sintering

Si-B-C-N monoliths with 5 wt% LaB₆ additives were prepared by spark plasma sintering at 1250–2000 °C and 50 MPa using a mechanically alloyed mixture of graphite, c-Si, h-BN and LaB₆ powders as the starting materials. Microstructural evolution, mechanical and thermal properties of the as-prepared La/Si-B-C-N monoliths were investigated. The densification of the ceramics starts at 1160° and ends at 1800 °C with the formation of La-containing compounds coupled with SiC and BN(C) phases. La-containing BN(C) grains develop into a lamellar structure at 1900 °C offering improved fracture toughness and decreased Vickers hardness, flexural strength and elastic modulus. The formation of lamellar BN(C) is also responsible for a high thermal expansion coefficient of 4.2×10⁻⁶ /°C.     

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.     

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.     

Phase instability in ZrO2–TiB2 composites

2.5 mol% Y₂O₃-stabilised ZrO₂ composites with 30 vol.% TiB₂ particles, hot pressed in vacuum at 1450 °C, were investigated by means of quantitative electron energy loss spectroscopy (EELS) with a nanobeam analytical field emission gun transmission electron microscope (FEG-TEM). The experimental results revealed that B is diffusing out of the titanium boride grains, forming textured polycrystalline BN agglomerates leaving behind non-stoichiometric TiB_2−x single crystal grains. The formation of BN was calculated to be thermodynamically favourable under the actual hot pressing conditions. Boron was also detected in the amorphous intergranular phase, together with Y, Si, O, Al, but not in the ZrO₂ grains. Ti was only detected in the amorphous material in-between the TiB_2−x grains and the micro-textured BN grains, but not in the zirconia grains nor in the intergranular amorphous phase in between. There are indications that Ti forms facetted carbide precipitates inside the TiB_2−x grains. Zr was not detected in the titanium boride grains, nor in the intergranular amorphous phase in-between the zirconia grains.     

Preparation of B4C composites toughened by TiB2-SiC agglomerates

High-performance B₄C composites toughened by TiB₂-SiC agglomerates were fabricated via reactive hot pressing with B₄C, TiC and Si as raw materials. The TiB₂-SiC composite serves as a composite toughening phase formed in the B₄C matrix through an in situ reaction; its agglomerates are composed of interlocked TiB₂ and SiC, which can remarkably improve the toughness of the B₄C composites. The Vickers hardness, flexural strength and fracture toughness of the B₄C-TiB₂-SiC composite reached 35.18 ± 0.45 GPa, 567 ± 14 MPa, and 6.38 ± 0.18 MPa m^1/2 respectively. The special toughening structure of the TiB₂-SiC composite introduced into B₄C ceramics was evaluated for the first time in this study.     

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.     

Process-tolerant pressureless-sintered silicon carbide ceramics with alumina-yttria-calcia-strontia

Process-tolerant SiC ceramics were prepared by pressureless sintering at 1850–1950 °C for 2 h in an argon atmosphere with a new quaternary additive (Al₂O₃-Y₂O₃-CaO-SrO). The SiC ceramics can be sintered to a > 94% theoretical density at 1800–1950 °C by pressureless sintering. Toughened microstructures consisting of relatively large platelet grains and small equiaxed grains were obtained when SiC ceramics were sintered at 1850–1950 °C. The presently fabricated SiC ceramics showed little variability of the microstructure and mechanical properties with sintering within the temperature range of 1850–1950 °C, demonstrating process-tolerant behavior. The thermal conductivity of the SiC ceramics increased with increasing sintering temperature from 1800 °C to 1900 °C due to decreases of the lattice oxygen content of the SiC grains and residual porosity. The flexural strength, fracture toughness, and thermal conductivity of the SiC ceramics sintered at 1850–1950 °C were in the ranges of 444–457 MPa, 4.9–5.0 MPa m^1/2, and 76–82 Wm^?1 K^?1, respectively.     

Structural and thermal properties of boron nitride nanoparticles

In the present study, our main motivation was to investigate the structural and thermal stability of BN nanoparticles (B_1.0N_0.9-NPs) produced by spray-pyrolysis (SP) of borazine at 1400 °C by thermogravimetric experiments and X-ray diffraction. We observed that B_1.0N_0.9-NPs are relatively stable in air below 850 °C in which only oxidation of the NP surface proceeded. Above 850 °C, the powders started to strongly react with air due to bulk oxidation. Under nitrogen, they appeared to be less stable than plate-like BN synthesized from borazine at 1400 °C through conventional pyrolysis. This is related to the low degree of crystallization of B_1.0N_0.9-NPs that clearly affects their stability. Using a post-pyrolysis treatment at 1400 °C, B_1.0N_0.9-NPs remained stable up to 1600 °C similarly to plate-like BN. However, above 1600 °C, a relatively fast weight loss occurred for B_1.0N_0.9-NPs, whereas plate-like BN remained stable up to 1800 °C. This indicated that their lower size also affects their high temperature thermal behavior.     

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.     

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.     

Wear mechanisms of TiN–TiB2 ceramic in sliding against alumina from room temperature to 700 °C

TiN–TiB₂ ceramic was prepared by the reactive hot-pressing method using titanium and BN powders as raw materials. The friction and wear properties of TiN–TiB₂ ceramic were evaluated in sliding against alumina ball from room temperature to 700 °C in air. The TiN–TiB₂ ceramic has a relative density of 98.6%, a flexural strength of 731.9 MPa and a fracture toughness of 8.5 MPa m^1/2 at room temperature. The TiN–TiB₂ ceramic exhibits a distinct decrease in friction coefficient at 700 °C as contrasted with the friction data obtained at room temperature and 400 °C. Wear mechanisms of TiN–TiB₂ ceramic depend mainly upon testing temperature at identical applied loads. Lubricious oxidized products caused by thermal oxidation provide excellent lubrication effects and greatly reduce the friction coefficient of TiN–TiB₂ ceramic at 700 °C. However, abrasive wear and tribo-oxidation are the dominant wear mechanisms of TiN–TiB₂ ceramic at 400 °C. Mechanical polishing effect and removal of micro-fractured grains play important roles during room-temperature wear tests.