Properties of TiB2–PSO modified SiHfBCN-based adhesive through polymer-derived-ceramic route

High temperature SiHfBCN-based ceramic adhesives are fabricated by polymer derived ceramic route with SiHfBCN precursors, TiB₂ and polysiloxane (PSO). The phase composition and microstructure were investigated by X-ray diffraction and scanning electron microscopy, respectively and the evolution of pores was analyzed by Micron X-ray 3D Imaging System and VG Studio MAX 3.0.2 software. After heat-treating at 80 °C and curing at 170 °C in air, the adhesion strength detected in air of SiHfBCN adhesives is 3.22 MPa at room temperature (RT) and can rise to 5.47 MPa at 1000 °C after pyrolysis at 1000 °C in air for 2 h with a universal testing machine. By modifying SiHfBCN with TiB₂–PSO, the adhesion strength can be enhanced to 9.49 MPa at RT and 6.37 MPa at 1000 °C. The results indicate that the formation of SiO₂–B₂O₃–TiO₂ ternary glasses play an important role in improving the adhesion strength. The present study broaden the high temperature adhesive family suitable for large-scale complex ceramic components in harsh environments.     

The improvement in thermal and mechanical properties of TiB2 modified adhesive through the polymer-derived-ceramic route

An air stable high temperature adhesive synthesized via the polymer-derived-ceramic route had received increased attention in the last two decades. To improve the thermal stability and adhesion strength of a polysilazane (PSNB) adhesive, TiB₂ was added as active filler to join SiC ceramic discs. The thermal stability, phase composition and microstructure were investigated by using TGA, XRD, FT-IR, BSE and SEM measurements. Effects of the pyrolysis temperature and active filler TiB₂ on the microstructure and adhesion strength have been investigated. After curing and heat-treating at 120?°C and 1000?°C in air for 2?h, respectively, the adhesion strength of the modified adhesive reached up to 10.07?MPa (3 times higher than that of pure PSNB) at room temperature, and, more importantly, retained a strength of 8.0?MPa at 800?°C in air. It should be noted that the formation of a glass comprised of SiO₂-B₂O₃-TiO₂ and the emergence of the hexagonal and granular TiB₂ in the adhesive layer are mainly responsible for the enhanced high temperature strength.     

Interfacial microstructure evolution and mechanical properties of B4C-based composite joints bonded with Ti foil

The interfacial microstructure and mechanical properties of B₄C-SiC-TiB₂ composite joints diffusion bonded with Ti foil interlayer were investigated. The joints were diffusion bonded in the temperature range of 800–1200?°C with 50?MPa by spark plasma sintering. The results revealed that robust joint could be successfully obtained due to the interface reaction. B₄C reacted with Ti to form nanocrystalline TiB₂ and TiC at the interface at 800–1000?°C. Both the reactions between SiC and Ti and between TiB₂ and Ti were not observed during joining. A full ceramic joint consisted of micron- and submicron-sized TiB₂ and TiC, accompanied with the formation of micro-crack, was achieved for the joint bonded at 1200?°C. Joint strength was evaluated and the maximum shear strength (145?±?14.1?MPa) was obtained for the joint bonded at 900?°C. Vickers hardness of interlayer increased with increasing the joining temperature.     

Temperature dependent hardness and strength properties of TiB2 with TiSi2 sinter-aid

From the perspective of high temperature structural applications, it is important to evaluate temperature dependent mechanical properties of titanium diboride (TiB₂) ceramics. The present study reports the effect of TiSi₂ content (up to 10 wt.%) and temperature on hardness and strength of TiB₂. The hardness properties were measured from room temperature (RT)—900 °C in vacuum; the four-point flexural strength properties were evaluated at selected temperatures in air up to 1000 °C. An attempt has been made to discuss the difference in hardness and strength properties with sinter-aid amount and microstructure. Our experimental results clearly indicated that the addition of 2.5 wt.% TiSi₂ to TiB₂ resulted almost full densification at a lower hot pressing temperature of 1650 °C without compromising on the high temperature strength and hardness properties. The hot pressed TiB₂–2.5 wt.% TiSi₂ ceramic could retain moderate strength of more than 400 MPa and hardness of 9 GPa at 1000 °C and 900 °C, respectively.     

Amorphous self-glazed,chopped basalt fiber reinforced,geopolymer-based composites

Geopolymer composites reinforced with refractory, chopped basalt fibers, and low melting glass were fabricated and heat treated at higher temperatures. K₂O·Al₂O₃·4SiO₂·11H₂O was the stoichiometric composition of the potassium-based geopolymer which was produced from water glass (fumed silica, deionized water, potassium hydroxide), and metakaolin. Addition of low melting glass (T_m ~815°C) increased the flexure strength of the composites to ~5 MPa after heat treatment above 1000°C to 1200°C. A Weibull statistical analysis was performed exhibiting how the amorphous self-healing and self-glazing effect of the glass frit significantly improved the flexure strength of the geopolymer and ceramic composites after exposure for 1 hour to high temperatures. At 950-1000°C, the K-based geopolymer converted to primarily a crystalline leucite ceramic, but the basalt fiber remained intact, and the melted glass frit flowed out of the surface cracks and sealed them. 1150℃ was determined to be the optimum heat treatment temperature, as at ≤1200°C, the basalt fibers melt and the strength of the reinforcement in the composites is significantly reduced. The amorphous self-healing and amorphous self-glazing effects of the glass frit significantly improved the room temperature flexure strength of the heat-treated geopolymer and ceramic composites.     

Joining of Si3N4/Si3N4 with in-situ formed Si-Al-Yb oxynitride glasses interlayer

Si₃N₄ ceramic was successfully joined to itself with in-situ formed Yb-Si-Al oxynitride glass interlayer. The joints were composed of three parts: (I) Si₃N₄ matrix, (II) oxynitride glass interlayer in which hexagonal or fine elongated β-sialon grains and a few ball-like β-Si₃N₄ grains exist, and (III) diffusion zone in Si₃N₄ matrix containing a thin dark layer and a ~ 25?µm thick bright layer. The seam owned similar microstructure to matrix and was inosculated with the matrix as a whole. The strength of the joint tended to increase with the increase of bonding temperature and reached the value of 225?MPa, when the joints were prepared at 1600?°C for 30?min under a pressure of 1.5?MPa. The high-temperature strength remained 94.7% and 75.2% of R.T. strength when the joints were tested at 1000?°C and 1200?°C, respectively. It may be contributed to the high softening temperature of the Yb-Si-Al oxynitride glass phase formed in the seam. Even suffered to the air exposure for 10?h at 1200?°C, the residual strength of the joints was still 143?MPa, attributed to the existence of YbAG phase.     

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.     

Amorphous self-healed,chopped basalt fiber-reinforced,geopolymer composites

Geopolymer composites containing refractory, chopped basalt fibers and low-melting glass were made and systematically heat-treated at higher temperatures. Potassium-based geopolymer of stoichiometric composition K₂O·Al₂O₃·4SiO₂·11H₂O was produced by high shear mixing from fumed silica, deionized water, potassium hydroxide, (i.e., water glass) and metakaolin. With the addition of low-melting glass (T_m ~815°C) the flexure strengths of the composites increased to ~6 MPa after heat treatment above 900°C to 1100°C. A Weibull statistical analysis was performed showing how the amorphous self-healing effect of the glass frit significantly improved the flexure strength of the geopolymer and ceramic composites after high-temperature exposure. At temperatures up to 900°C, the geopolymer-basalt composite remained amorphous and the low-melting glass frit flowed into the dehydration cracks in the geopolymer matrix. This type of composite could be described as amorphous self-healed geopolymer (ASH-G). At ~1000°C, the geopolymer converted to primarily a crystalline leucite ceramic, but the basalt fiber remained intact, and the melted glass frit flowed and sealed the cracks developed at that temperature. This type of composite could then be described as amorphous self-healed ceramic (ASH-C). A temperature of 1150°C was determined to be optimum as at 1200°C the basalt fibers melted and the strength of the reinforcement was lost in the composites. The amorphous self-healing effect of the glass frit significantly improved the room temperature flexure strength of the heat-treated geopolymer-based composites.     

Fracture and property relationships in the double diboride ceramic composites by spark plasma sintering of TiB2 and NbB2

Bulk titanium diboride–niobium diboride ceramic composites were consolidated by spark plasma sintering (SPS) at 1950°C. SPS resulted in dense specimens with a density exceeding 98% of the theoretical density and a multimodal grain size ranging from 1 to 10 μm. During the SPS consolidation, the pressure was applied and released at 1950 and 1250°C, respectively. This allowed obtaining a two-phase composite consisting of TiB₂ and NbB₂. For these ceramics composites, we evaluated the flexural strength and fracture toughness and room and elevated temperatures. Room-temperature strength of thus produced bulks was between 300 and 330 MPa, at 1200°C or 1600°C an increase in strength up to 400 MPa was observed. Microstructure after flexure at elevated temperatures revealed the appearance of the needle-shape subgrains of NbB₂, an evidence for ongoing plastic deformation. TiB₂–NbB₂ composites had elastic loading stress curves at 1600°C, and at 1800°C fractured in the plastic manner, and strength was ranged from 300 to 450 MPa. These data were compared with a specimen where a (Ti,Nb)B₂ solid solution was formed during SPS to explain the behavior of TiB₂–NbB₂ ceramic composites at elevated temperatures.     

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.     

New Sintered Li2O–Al2O3–SiO2 Ultra‐Low Expansion Glass‐Ceramic

We developed a new Li₂O–Al₂O₃–SiO₂ (LAS) ultra‐low expansion glass‐ceramic by nonisothermal sintering with concurrent crystallization. The optimum sintering conditions were 30°C/min with a maximum temperature of 1000°C. The best sintered material reached 98% of the theoretical density of the parent glass and has an extremely low linear thermal expansion coefficient (0.02 × 10^?6/°C) in the temperature range of 40°C–500°C, which is even lower than that of the commercial glass‐ceramic Ceran^® that is produced by the traditional ceramization method. The sintered glass‐ceramic presents a four‐point bending strength of 92 ± 15 MPa, which is similar to that of Ceran^® (98 ± 6 MPa), in spite of the 2% porosity. It is white opaque and does not have significant infrared transmission. The maximum use temperature is 600°C. It could thus be used on modern inductively heated cooktops.     

Influence of TiN dopant on microstructure of TiB2 ceramic sintered by spark plasma

In this study, the impact of TiN as a sintering aid on the relative density and microstructure of TiB₂ ceramic was investigated. Monolithic TiB₂ and TiB₂ doped with 5?wt% TiN were sintered at 1900?°C for 7?min dwell time under the pressure of 40?MPa by spark plasma. The addition of TiN affected the microstructure of TiB₂-based sample considerably depicting the finer grains in the as-sintered ceramic. X-ray diffraction evaluation indicated that no interaction occurred between the initial materials. However, detail investigation by the map analysis and energy dispersive spectroscopy results revealed the formation of in-situ nano-sized hBN secondary phase in the TiN-doped TiB₂. In addition, TiN played a remarkable role on increasing the relative density of TiN-doped TiB₂ ceramic producing a nearly fully dense ceramic with relative density of 99.9% in comparison with the monolithic ceramic having 96.7% relative density.     

Joining of titanium diboride-based ultra high-temperature ceramics to refractory metal tantalum using diffusion bonding technology

The use of ultra-high temperature ceramics (UHTCs) requires effective methods to overcome the problems associated with manufacturing parts with complex shapes. In this study, a titanium diboride (TiB₂)-based ultra-high-temperature ceramic, TiB₂-20 vol.%TiC-20 vol.%SiC (TTS), was joined to refractory metal tantalum (Ta) using titanium (Ti) interlayer. The interface microstructure and mechanical properties of joints obtained at different bonding temperatures were investigated. The bonding mechanism of the joint was discussed based on TEM analysis and theoretical calculation. The results revealed that a (Ti, Ta)B + TiC + Ti₅Si₃ reaction layer formed adjacent to the TTS ceramic substrate while a β-(Ta, Ti)+β-(Ti, Ta)+α-Ti layer formed adjacent to the Ta substrate. The α-Ti was gradually replaced by β-(Ta, Ti) and β-(Ti, Ta), and the reaction layer of the ceramic side became thicker as the bonding temperature increased. The maximum joint shear strength of room temperature was 176 MPa when the joint was bonded at 1200 °C for 60 min under 20 MPa, and cracks propagated in the ceramic. The shear strength of the joint tested at 800 °C was 86 MPa, and fracture occurred at the β-(Ti, Ta).     

Effect of temperature on the structure and mechanical properties of SiC–TiB2 composite ceramics by solid-phase spark plasma sintering

SiC composite ceramics have good mechanical properties. In this study, the effect of temperature on the microstructure and mechanical properties of SiC–TiB₂ composite ceramics by solid-phase spark plasma sintering (SPS) was investigated. SiC–TiB₂ composite ceramics were prepared by SPS method with graphite powder as sintering additive and kept at 1700 °C, 1750 °C, 1800 °C and 50 MPa for 10min.The experimental results show that the proper TiB₂ addition can obviously increase the mechanical properties of SiC–TiB₂ composite ceramics. Higher sintering temperature results in the aggregation and growth of second-phase TiB₂ grains, which decreases the mechanical properties of SiC–TiB₂ composite ceramics. Good mechanical properties were obtained at 1750 °C, with a density of 97.3%, Vickers hardness of 26.68 GPa, bending strength of 380 MPa and fracture toughness of 5.16 MPa m^1/2.     

Oxidation-induced crack-healing behavior of SiC-Al2O3-TiB2 composites at 600°C–800°C

It is necessary to give self-healing function to ceramic materials because of their notch sensitivity. In the past, studies on self-healing ceramics have mainly focused on the high-temperature stage, and less research has been done below 1000°C. In this study, SiC/Al₂O₃/TiB₂ ceramic composites were prepared by spark plasma discharge sintering, and cracks were introduced on the surface of the polished specimens. Crack healing at 600°C–800°C was investigated, and the recovery of macroscopic bending strength and the change of microscopic crack morphology after heat treatment were used to evaluate the crack-healing effect. It was found that the surface cracks of the material were completely filled and healed by oxidation products after heat treatment at 700°C for 60 min, and the highest healing efficiency exceeded 95% for both specimens with different crack lengths, and the main mechanism of crack by Si-Al-B-Na-Ca-O type glass produced by the reaction of TiB2 and a small amount of SiC with oxygen to produce oxides and glass powder. Good healing effect and fast healing speed effectively improve the service life and reliability of ceramic materials, which has very far-reaching significance for the practical application with ceramic materials.     

Preparation and performance of ceramizable heat-resistant organic adhesive for joining Al2O3 ceramics

Ceramizable heat-resistant organic adhesive (CHA) was prepared by using preceramic polymer polysiloxane as matrix, TiB₂ ceramic powder and low melting point glass powder as additives. The curing mechanism, thermal stability properties, phase composition after pyrolysis, structural evolution of bonding layer and bonding mechanism of the adhesive were investigated by FTIR, TGA-DSC, XRD, SEM, FESEM and bonding strength tests. Results of bonding tests showed the maximum shear strength of the joints was 21 MPa when heat treatment at 1200 °C for 2 h in air. Polysiloxane resin acted as crosslinking adhesive at low temperatures and tended to convert to ceramic bonding layer at high temperatures, resulting from ceramization reaction with active fillers. The formation and growth of ceramic phase after heat treatment enhanced the thermal stability and bonding performance of the adhesive at high temperature.     

Hard and easy sinterable B4C-TiB2-based composites doped with WC

B₄C-TiB₂ composites were contaminated with WC to study the effect on densification, microstructure and properties. WC was introduced through a mild or a high energy milling with WC-6?wt%Co spheres or directly as sintering aid to 50?vol% B₄C / 50?vol%TiB₂ mixtures. High energy milling was very effective in improving the densification thanks to the synergistic action of WC impurities, acting as sintering aid, and size reduction of the starting TiB₂-B₄C powders. As a result, the sintering temperature necessary for full densification decreased to 1860?°C and both strength and hardness benefited from the microstructure refinement, 860?±?40 MPa and 28.5?±?1.4?GPa respectively. High energy milling was then adopted for producing 75?vol% B₄C/25?vol% TiB₂ and 25?vol% B₄C/ 75vol%TiB₂ mixtures. The B₄C-rich composition showed the highest hardness, 32.2?±?1.8?GPa, whilst the TiB₂-rich composition showed the highest value of toughness, 5.1?±?0.1?MPa?m^0.5.     

Fracture toughness in some hetero-modulus composite carbides: carbon inclusions and voids

Structure and mechanical characteristics of dense ceramic composites synthesised by reactive hot pressing of TiC–B₄C powder mixtures at 1800–1950°C under 30?MPa were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM and EDX). The results show that during hot pressing solid-phase chemical reaction 2TiC?+?B₄C?=?2TiB₂?+?3C has occurred with final products like TiB₂–TiC–C, TiB₂–C or TiB₂–B₄C–C hetero-modulus composite formation with around one micrometer size carbon precipitates. The fracture toughness depends on the amount of graphite precipitation and has a distinct maximum K1C?=?10?MPa?m^1/2 at nearly 7?vol.-% of carbon precipitate. The fracture toughness behaviour is explained by the developed model of crack propagation. Within the model, it is shown that pores (voids) and low-modulus carbon inclusions blunt the cracks and can increase ceramic toughness in some cases.     

Processing,Mechanical Characterization,and Alkali Resistance of SiliconBoronOxycarbide (SiBOC) Glass Fibers

A borosilicate sol–gel solution is synthesized using a mixture of methyltriethoxysilane, dimethyldiethoxysilane, and boric acid. SiBOC gel fibers are produced from the as‐synthesized sol–gel solution using a spinning apparatus. Subsequently, SiBOC glass fibers are prepared through pyrolysis under argon atmosphere at 1000°C and 1200°C. Mechanical properties of the SiBOC glass fibers are studied by measuring the tensile strength and the elastic modulus. The results show a high tensile strength ?1300 and 1058 MPa, and a high Young modulus ?79 and 95.5 GPa, for the fibers prepared at 1000°C and 1200°C, respectively. Furthermore, alkali resistance of the SiBOC fibers is investigated by measuring the tensile strength after soaking them for 20 h in NaOH and Ca(OH)₂ solutions at 100°C. For comparison, the same measurements are performed on commercial AR and E glass fibers. The SiBOC fibers show excellent alkaline resistance and perform better than commercial AR fibers. Indeed, SiBOC fibers retain 80%–90% of the initial strength after Ca(OH)₂ attack.     

Preparation,Mechanical Properties and Cyclic Oxidation Behavior of the Nanostructured NiCrCoAlY-TiB2 Coating

TiB₂-Metal composite coatings with excellent oxidation resistance become ideal candidates using at high temperature ranging from 600 to 1000?°C. In order to maintain both the superior mechanical properties and oxidation resistance in severe working conditions, the nanostructured NiCrCoAlY-TiB₂ coating was fabricated by the activated combustion high velocity air-fuel spraying (AC-HVAF) with the composite powders prepared by ball milling and plasma spheroidization. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the phase constituents and microstructure. It was found that the coating with nanostructured TiB₂ particles uniformly distributed in the NiCrCoAlY matrix has the same structure as the composite powders. The coating shows excellent mechanical properties, such as high microhardness (991.43 HV_0.3), good fracture toughness (4.12?MPa?m^?1/2) and large bonding strength (75.43?MPa), and excellent oxidation resistance with low weight gain (1.56?×?10^?6 mg² cm^?4 s^?1). The cyclic oxidation behavior is in accordance with the parabolic law.