Preparation of TiB2 sintered compacts by hot pressing

A sintered compact of titanium diboride (TiB₂) was prepared by hot pressing of the synthesized TiB₂ powder, which was obtained by a solid-state reaction between TiN and amorphous boron. Densification of the sintered compact occurred at 20 MPa and 1800° C for 5 to 60 min with the aid of a reaction sintering, including the TiB₂ formation reaction between excess 20 at % amorphous boron in the as-synthesized powder (TiB₂ + 0.2B) and intentionally added 10 at % titanium metal. A homogeneous sintered compact of a single phase of TiB₂, which was prepared by hot pressing for 30 min from the starting powder composition [(TiB₂ + 0.2B) + 0.1 Ti], had a fine-grained microstructure composed of TiB₂ grains with diameters of 2 to 3 m. The bulk density was 4.47 g cm^–3, i.e. 98% of the theoretical density. The microhardness, transverse rupture strength and fracture toughness of the TiB₂ sintered compact were 2850 kg mm^–2, 48 kg mm^–2 and 2.4 MN m^–3/2, respectively. The thermal expansion coefficient increased with increasing temperature up to 400° C and had a constant value of 8.8 x 10^–6 deg^–1 above 500° C.     

Silicon carbide fibre reinforced glass-ceramic matrix composites exhibiting high strength and toughness

Silicon carbide fibre reinforced glass-ceramic matrix composites have been investigated as a structural material for use in oxidizing environments to temperatures of 1000° C or greater. In particular, the composite system consisting of SiC yarn reinforced lithium aluminosilicate (LAS) glass-ceramic, containing ZrO₂ as the nucleation catalyst, has been found to be reproducibly fabricated into composites that exhibit exceptional mechanical and thermal properties to temperatures of approximately 1000° C. Bend strengths of over 700 MPa and fracture toughness values of greater than 17 MN m^–3/2 from room temperature to 1000° C have been achieved for unidirectionally reinforced composites of 50 vol% SiC fibre loading. High temperature creep rates of 10^–5 h^–1 at a temperature of 1000° C and stress of 350 MPa have been measured. The exceptional toughness of this ceramic composite material is evident in its impact strength, which, as measured by the notched Charpy method, has been found to be over 50 times greater than hot-pressed Si₃N₄.     

In situ reacted TiB2-reinforced mullite

In situ formation of TiB₂ in mullite matrix through the reaction of TiO₂, boron and carbon has been studied. In hot-pressed and pressureless-sintered samples, in addition to TiB₂, TiC was also found to be dispersed phases in mullite matrix. However, in the case of pressurelesssintered samples, mullite/TiB₂ composite with 98% relative density can be obtained through a preheating step held at 1300 °C for longer than 3 h and then sintering at a temperature above 1600 °C. Hot-pressed composite containing 30 vol% TiB₂ gives a flexural strength of 427 MPa and a fracture toughness of 4.3 MPam^1/2. Pressureless-sintered composite containing 20 vol% TiB₂ gives a flexural strength of 384 MPa and a fracture toughness of 3.87 MPam^1/2.     

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

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

Reaction control of TiB2 formation from titanium metal and amorphous boron

TiB₂ powder was synthesized by a controlled formation reaction from titanium metal and amorphous boron. Precursory TiB₂ formed by the pretreatment of the mixed powder (mole ratio: B/Ti=2.0) at 600° C for 60 min in an argon stream. Hollow TiB₂ powder with an average grain size of 15m was obtained by subsequent heat treatment above 900° C for more than 60 min in an argon stream. The formation reaction of TiB₂ powder was further controlled by pretreatment of the mixed powder at 600° C for 60 min in a hydrogen and argon stream and subsequent heat treatment at 1000° C for 360 min in an argon stream, when hollow-free TiB₂ powder was formed by a milder formation reaction between amorphous boron and the reformed titanium metal with hydrogen diffused lattice.     

Chemical vapour deposition of tungsten and TiN on SiC fibres

Handling of uncoated SiC/W (Sigma) fibres resulted in a drop in their strengths. Coating with TiB₂ or boron at about 1000 °C by chemical vapour deposition resulted in a further reduction in strength. Coating with tungsten or TiN by lower temperature chemical vapour deposition processes did not degrade the fibre and the coated fibres also retained their strengths after heating in hydrogen at 1040 °C.     

In situ reacted TiB2-reinforced alumina

In situ formation of TiB₂ in Al₂O₃ matrix through the reaction of TiO₂, boron and carbon has been studied. In hot-pressed samples, in addition to TiB₂, TiC and Al₂TiO₅ were also found to be dispersed phases in Al₂O₃ matrix. However, in the case of pressureless-sintered samples, pure Al₂O₃/TiB₂ composite with > 99% relative density can be obtained through a preheating step held at 1300°C for longer than 30 min and then sintering at a temperature above 1500°C. Pressureless-sintered composite containing 20vol% TiB₂ gives a flexural strength of 580 MPa and a fracture toughness of 7.2 MPa m^1/2.     

Synthesis of TiB2 powder from a mixture of TiN and amorphous boron

TiB₂ powder was synthesized by solid state reaction using amorphous boron and TiN as a source of titanium. The TiB₂ formation did not occur at all in a nitrogen atmosphere even at 1400° C. TiB₂ formed above 1100° C in argon and hydrogen atmospheres. The only crystalline phase of TiB₂ powder was favourably synthesized at 1400° C for 360 min in an argon atmosphere from a starting powder with a composition containing excess boron (B/Ti = 2.2). The synthesized powder was well dispersed and had a particle size of 0.5 to 2 µm. The powder activity was evaluated by sintering at 4 G Pa and 1300 to 1600° C for 15 min.     

Development and characterisation of SiAlON composites with TiB2, TiN, TiC and TiCN

30 vol% of TiB₂, TiCN, TiN or TiC was added to a sialon matrix with an X-phase sialon (Si₁₂Al₁₈O₃₉N₈) and an Al₂O₃–Si₃N₄ (77/23 wt%) starting powder composition and hot pressed at 1650°C in vacuum. The microstructures of the obtained composites were characterised by means of X-ray diffraction and electron microscopy, and the mechanical properties; E-modulus, hardness, bending strength and fracture toughness were measured and evaluated.Fully dense composites with an X-phase sialon or a polyphase Al₂O₃–-sialon–X-sialon matrix with 30 vol% of TiB₂, TiN and TiCN were obtained. TiC, added as a dispersed phase, however reacts with the nitrogen from the Si₃N₄ during liquid phase sintering, with the formation of TiC_1–x N _x , SiC and a changed sialon matrix composition. In the case of the X-phase sialon starting composition, a mullite matrix is obtained after sintering. The microstructural observations with respect to the sialon-TiC composites are found to be in agreement with the thermodynamic calculations.     

Microstructure and properties of the sintered composite prepared by hot pressing of TiN-coated alumina powder

Alumina powders (average grain size: 50 m) coated with TiN film of thickness 0.5 and 1.2 m were prepared by rotary powder-bed chemical vapour deposition for 15 and 90 min, respectively. These Al₂O₃-TiN composite powders were hot-pressed at 1800 °C and 40 MPa for 30 min. The microstructure of the Al₂O₃-TiN sintered composite was composed of a TiN network homogeneously distributed on the grain boundaries of alumina. The mechanical properties (hardness, bending strength and fractured toughness) and thermal conductivity of the sintered composite were found to depend on the composition and microstructure of the sintered composite, even with a small content (3–7 wt%) of TiN. The resistivity of the sintered composite was 10^–1-10^–3 cm. The relatively high electrical conductivity of the Al₂O₃-TiN composite was caused by the grain boundary conduction of TiN.     

Microstructure and mechanical properties of pulsed electric current sintered B4C-TiB2 composites

Monolithic B₄C, TiB₂ and B₄C-TiB₂ particulate composites were consolidated without sintering additives by means of pulsed electric current sintering in vacuum. Sintering studies on B₄C-TiB₂ composites were carried out to reveal the influence of the pressure loading cycle during pulsed electrical current sintering (PECS) on the removal of oxide impurities, i.e. boron oxide and titanium oxide, hereby influencing the densification behavior as well as microstructure evolvement. The critical temperature to evaporate the boron oxide impurities was determined to be 2000 °C. Fully dense B₄C-TiB₂ composites were achieved by PECS for 4 min at 2000 °C when applying the maximum external pressure of 60 MPa after volatilization of the oxide impurities, whereas a relative density of 95-97% was obtained when applying the external pressure below 2000 °C. Microstructural analysis showed that B₄C and TiB₂ grain growth was substantially suppressed due to the pinning effect of the secondary phase and the rapid sintering cycle, resulting in micrometer sized and homogeneous microstructures. Excellent properties were obtained for the 60 vol% TiB₂ composite, combining a Vickers hardness of 29 GPa, a fracture toughness of 4.5 MPa m^1/2 and a flexural strength of 867 MPa, as well as electrical conductivity of 3.39E+6 S/m.     

Isothermal oxidation behavior of reactive hot-pressed TiN–TiB2 ceramics at elevated temperatures

Isothermal oxidation behavior of reactive hot-pressed TiN–TiB₂ ceramics with various TiN/TiB₂ molar ratios of 2/1, 1/1 and 1/2 was evaluated in the temperature range of 500–800 °C in air. TiN–TiB₂ ceramics have a relative density of 97–98.6%. The oxidation weight gains of TiN–TiB₂ ceramics depend upon the composition, oxidation temperature and exposure time. The structure and morphology of oxidized layers of TiN–TiB₂ ceramics were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). During isothermal oxidation of TiN–TiB₂ ceramics, anatase and rutile-TiO₂ form as the oxidized products at 500 °C. However, phase transformation from anatase to rutile occurs at temperatures between 500 and 600 °C, and therefore rutile-TiO₂ becomes the only crystalline phase after oxidation at temperatures of 600–800 °C for 10 h. The oxidation mechanism was proposed with reference to thermodynamically feasible oxidation reactions. The influence of composition on oxidation behavior of TiN–TiB₂ ceramics varies with temperature.     

SiC matrix composites containing transition metal boride particulates internally synthesised by carbothermal reaction

SiC matrix composites reinforced with the various borides of the transition metals in group IV a-VI a, which were synthesized from the transition metal oxide, boron carbide and carbon mixed with SiC powder. Dense composites containing boride particulates of titanium, zirconium, niobium and chromium were prepared through reactive hot-pressing. The morphology of the internally synthesized boride particles reflected that of the starting oxide powders. SiC-NbB₂ composites with four-point flexural strength of 500 to 600 MPa and better oxidation resistance than SiC-TiB₂ were prepared even through pressureless sintering process. Pressureless-sintered and HIPed SiC-20 vol% NbB₂ exhibited the four-point flexural strength of 760 MPa at 20 °C and 820 MPa at 1400 °C.     

AEM investigation of ceramic/lncoloy 909 diffusional reactions after joining by HIP

Diffusion bonding by hot isostatic pressing (HIP) was performed between Incoloy 909 and five different ceramics. Two of the ceramics were composites made from powder mixtures of Si₃N₄ and either 60 vol% TiN or 50 vol% TiB₂, while three were monolithic materials, namely Si₃N₄ with 2.5 wt% Y₂O₃ as a sintering additive, Si₃N₄ without additives, and Si₂ N₂O without additives. A diffusion couple geometry was developed to facilitate the preparation of thin-foil specimens for examination by analytical electron microscopy (AEM). Diffusion bonding was performed by HIP at 927°C (1200K) and 200 MPa for 4 h. The formation of reaction layers was very limited, being less than 1 m in total layer thickness. Two reaction products were found by AEM; a continuous, very thin, (100 nm) layer of fine TiN crystals at the initial ceramic/metal interface, and larger grains extending about 100–500 nm into the superalloy and forming a semi-continuous layer of a G-phase suicide containing mainly nickel, silicon and niobium.     

Ceramic composites: TiB2-TiC-SiC

The composition 20% TiB₂-55% TiC-25% SiC (mol%) was selected and the hot pressing parameters were optimized using optimal design. The optimized hot-pressed material had a bend strength > 1200 M Pa, a thermal shock resistance of 300 °C, and a bend strength at 1200 °C of 456 MPa. The pressureless sintering of the selected composite was optimized, with and without additives. Powders sintered without additives (2200 °C, 2 h) had a low density (92.2%) and strength (480 MPa). Carbon and boron additives led to higher densities (97.5%) but did not improve the strength.     

Fabrication and mechanical properties of silicon carbide–silicon nitride nanocomposites

A powder mixture of ultrafine –SiC–35 wt% –Si₃N₄ containing 6 wt% Al₂O₃ and 4 wt% Y₂O₃ as sintering additives were liquid–phase sintered at 1800°C for 30 min by hot–pressing. The hot–pressed composites were subsequently annealed at 1920°C under nitrogen–gas–pressure to enhance grain growth. The average grain–size of the sintered bodies were ranged from 96 to 251 nm for SiC and from 202 to 407 nm for Si₃N₄, which were much finer than those of ordinary sintered SiC–Si₃N₄ composites. Both strength and fracture toughness of fine–grained SiC–Si₃N₄ composites increased with increasing grain size. Such results suggested that a small amount of grain growth in the fine–grained region (250 nm for SiC and 400 nm for Si₃N₄) was beneficial for mechanical properties of the composites. The room–temperature flexural strength and fracture toughness of the 8–h annealed composites were 698 MPa and 4.7 MPa · m^1/2, respectively.     

SiC-TiC and SiC-TiB2 composites densified by liquid-phase sintering

Particle-reinforced SiC composites with the addition of TiC or TiB₂ were fabricated at 1850 °C by hot-pressing. Densification was accomplished by utilizing a liquid phase formed with added Al₂O₃, Y₂O₃, and surface SiO₂ on SiC. Their mechanical and electrical properties were measured as a function of TiC or TiB₂ content. Adding TiC or TiB₂ to the SiC matrix increased the toughness, and decreased the strength and electrical resistivity. The fracture toughnesses of SiC-50 wt% TiC and SiC-50 wt% TiB₂ composites were approximately 60% and 50%, respectively, higher than that of monolithic SiC ceramics. Microstructural analysis showed that the toughening was due to crack deflection, with some possible contribution from microcracking in the vicinity of TiC or TiB₂ particles.     

Chemical reactivities of hafnium and its derived boride, carbide and nitride compounds at relatively mild temperature

MB₂/SiC composites are materials of choice for ultra-high-temperature structural applications, primarily in the aerospace arena. These composites are processed in a hot-press operation at a temperature range of 1900 to 2200°C. This article assesses potential mild-temperature (below 1500°C) chemical reactions that may lead to structures and coatings made of HfB₂/SiC under pressureless or mild-pressure conditions. The reactions are anticipated to be involved in reactive and shape-forming processes, where ceramic precursors and/or reactive powders are incorporated. This article pays special attention to exothermic reactions as well as to formers of a liquid phase; both can aid the desired phase formation, microstructure development, and sintering of the composite under milder conditions than currently practiced. Reactions between loosely mixed powders with melting points significantly above 1500°C were detected by X-ray diffraction (XRD) analyses. Significant solid-phase reactions of the loose powder mixtures were observed at this mild temperature in powder form. Preliminary microstructural studies using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray Spectroscopy (EDX) techniques have confirmed the presence of unique reaction mechanisms between the loosely connected particles.Good examples are the reactions between Hf powder and powders of BN or B₄C, all having melting points above 2200°C, which form at 1500°C, or below HfB₂/HfN and HfB₂/HfC crystalline domains, respectively. These reactions are less intuitive than the reaction with B₂O₃, which forms HfB₂/HfO₂, potentially via molten or gaseous phases of boron oxide.     

Pressure-pulsed chemical vapour infiltration of TiN to SiC particulate preforms

SiC particulate preforms were infiltrated by TiN matrix from a gas mixture of TiCl₄ (5%), nitrogen (30%) and hydrogen using a repeating pressure pulse between 760 and about 1 torr. SiC particle sizes of 5 and 20 m were used. For matrix packing into deep level, optimum temperature was determined between 800 and 850 °C, and the maximum packing ratio reached 67% after 4 × 10⁴ pulses at 850 °C. The increase of TiCl⁴ concentration to 10% resulted in higher deposition rate and packing ratio. The decrease of nitrogen concentration led to slower deposition, that is, a similar effect to temperature lowering. The maximum flexural strength measured was 140 MPa.     

Reactive and dense sintering of reinforced-toughened B4C matrix composites

Reactive hot-press (1800-1880 °C, 30 MPa, vacuum) is used to fabricate relatively dense B₄C matrix light composites with the sintering additive of (Al₂O₃ +Y₂O₃). Phase composition, microstructure and mechanical properties are determined by methods of XRD, SEM and SENB, etc. These results show that reactions among original powders B₄C, Si₃N₄ and TiC occur during sintering and new phases as SiC, TiB₂ and BN are produced. The sandwich SiC and claviform TiB₂ play an important role in improving the properties. The composites are ultimately and compactly sintered owing to higher temperature, fine grains and liquid phase sintering, with the highest relative density of 95.6%. The composite sintered at 1880 °C possesses the best general properties with bending strength of 540 MPa and fracture toughness of 5.6 MPa m^1/2, 29 and 80% higher than that of monolithic B₄C, respectively. The fracture mode is the combination of transgranular fracture and intergranular fracture. The toughening mechanism is certified to consist of crack deflection, crack bridging and pulling-out effects of the grains.