Spark plasma sintering and high-temperature strength of B6O–TaB2 ceramics

Tantalum diboride – boron suboxide ceramic composites were densified by spark plasma sintering at 1900 °C. Strength and fracture toughness of these bulk composites at room temperature were 490 MPa and 4 MPa m^1/2, respectively. Flexural strength of B₆O–TaB₂ ceramics increased up to 800 °C and remained unchanged up to 1600 °C. At 1800 °C a rapid decrease in strength down to 300 MPa was observed and was accompanied by change in fracture mechanisms suggestive of decomposition of boron suboxide grains. Fracture toughness of B₆O–TaB₂ composites showed a minimum at 800 °C, suggestive a relaxation of thermal stresses generated from the mismatch in coefficients of thermal expansion.Flexural strength at elevated temperatures for bulk TaB₂ reference sample was also investigated.Results suggest that formation of composite provides additional strengthening/toughening as in all cases flexural strength and fracture toughness of the B₆O–TaB₂ ceramic composite was higher than that reported for B₆O monoliths.     

Strength of Zirconium Diboride to 2300°C

The strength of zirconium diboride (ZrB₂) ceramics was measured up to 2300°C, which are the first reported measurements above 1500°C since 1970. ZrB₂ ceramics were prepared from commercially available powder by hot pressing. A mechanical testing apparatus capable of testing material in the ultra‐high temperature regime with atmosphere control was built, evaluated, and used. Four‐point bend strength was measured as a function of temperature up to 1600°C in air and between 1500°C and 2300°C in argon. Strength between room temperature and 1200°C was ~390 MPa, decreasing to a minimum of ~170 MPa between 1400°C and 1500°C, with strength increasing to ~220 MPa between 1600°C and 2300°C.     

High‐temperature strength and plastic deformation behavior of niobium diboride consolidated by spark plasma sintering

Bulk niobium diboride ceramics were consolidated by spark plasma sintering (SPS) at 1900°C. SPS resulted in dense specimens with a density of 98% of the theoretical density and a mean grain size of 6 μm. During the SPS consolidation, the hexagonal boron nitride (h‐BN) was formed from B₂O₃ on the powder particle surface and residual adsorbed nitrogen in the raw diboride powder. The room‐temperature strength of these NbB₂ bulks was 420 MPa. The flexural strength of the NbB₂ ceramics remained unchanged up to 1600°C. At 1700°C an increase in strength to 450 MPa was observed, which was accompanied by the disappearance of the secondary h‐BN phase. Finally, at 1800°C signs of plastic deformation were observed. Fractographic analysis revealed a number of etching pits and steplike surfaces suggestive of high‐temperature deformation. The temperature dependence of the flexural strength of NbB₂ bulks prepared by SPS was compared with data for monolithic TiB₂, HfB₂ and ZrB₂. Our analysis suggested that the thermal stresses accumulated during SPS consolidation may lead to additional strengthening at elevated temperatures.     

Influence of sintering temperature on the crystallization and mechanical properties of BN-MAS composites

A type of boron nitride–magnesium aluminum silicate (BN-MAS) composite ceramics was fabricated by hot-press sintering at different sintering temperatures. The relationship between the sintering temperature and microstructure was investigated by analyzing the interaction between hexagonal boron nitride (h-BN) and the MAS phase. The main MAS phase in the composite ceramics is the α-cordierite phase at a sintering temperature of 1300°C. At temperatures above 1400°C, the inhibitory effect of h-BN on the crystallization of the MAS system is significant, and MAS mainly exists in the form of an amorphous phase. The composite sintered at 1700°C exhibited the highest bending strength of 218MPa. h-BN and MAS were co-enhanced. MAS can be used as an effective liquid-phase sintering aid to assist in the sintering of h-BN, whereas h-BN can absorb the fracture energy of the composite ceramics through the pull-out and bridging effect of the particles.     

Polymer derived silicon oxycarbide ceramic monoliths: Microstructure development and associated materials properties

Polymer derived SiOC and SiCN ceramics (PDCs) are interesting candidates for additive manufacturing techniques to develop micro sized ceramics with the highest precision. PDCs are obtained by the pyrolysis of crosslinked polymer precursors at elevated temperatures. Within this work, we are investigating PDC SiOC ceramic monoliths synthesized from liquid polysiloxane precursor crosslinked with divinylbenzene for fabrication of conductive electromechanical devices. Microstructure of the final ceramics was found to be greatly influenced by the pyrolysis temperature. Crystallization in SiOC ceramics starts above 1200?°C due to the onset of carbothermal reduction leading to the formation of SiC and SiO₂ rich phases. Microstructural characterisation using ex-situ X-ray diffraction, FTIR, Raman spectra and microscopy imaging confirms the formation of nano crystalline SiC ceramics at 1400?°C. The electrical and mechanical properties of the ceramics are found to be significantly influenced by the phase separation with samples becoming more electrically conducting but with reduced strength at 1400?°C. A maximum electrical conductivity of 10¹ S?cm^?1 is observed for the 1400?°C samples due to enhancement in the ordering of the free carbon network. Mechanical testing using the ball on 3 balls (B3B) method revealed a characteristic flexural strength of 922?MPa for 1000?°C amorphous samples and at a higher pyrolysis temperature, materials become weaker with reduced strength.     

Effect of pre-oxidation on the microstructure,mechanical and dielectric properties of highly porous silicon nitride ceramics

Highly porous Si₃N₄ ceramics have been fabricated via freeze casting and sintering. The as-sintered samples were pre-oxidized at 1200–1400 °C for 15 min. The effect of pre-oxidation temperature on the microstructure, flexural strength, and dielectric properties of porous Si₃N₄ ceramics were investigated. As the pre-oxidation temperature increased from 1200 °C to 1400 °C, firstly, the flexural strength of the pre-oxidized specimens remained almost constant at 1200 °C, and then decreased to 14.2 MPa at 1300 °C, but finally increased to 25.6 MPa at 1400 °C, while the dielectric constant decreased gradually over the frequencies ranging from 8.2 GHz to 12.4 GHz. This simple process allows porous Si₃N₄ ceramics to have ultra-low dielectric constant and moderate strength, which will be feasible in broadband radome applications at high temperatures.     

Elevated Temperature Strength Enhancement of ZrB2–30 vol% SiC Ceramics by Postsintering Thermal Annealing

The mechanical properties of dense, hot‐pressed ZrB₂–30 vol% SiC ceramics were characterized from room temperature up to 1600°C in air. Specimens were tested as hot‐pressed or after hot‐pressing followed by heat treatment at 1400°C, 1500°C, 1600°C, or 1800°C for 10 h. Annealing at 1400°C resulted in the largest increases in flexure strengths at the highest test temperatures, with strengths of 470 MPa at 1400°C, 385 MPa at 1500°C, and 425 MPa at 1600°C, corresponding to increases of 7%, 8%, and 12% compared to as hot‐pressed ZrB₂–SiC tested at the same temperatures. Thermal treatment at 1500°C resulted in the largest increase in elastic modulus, with values of 270 GPa at 1400°C, 240 GPa at 1500°C, and 120 GPa at 1600°C, which were increases of 6%, 12%, and 18% compared to as hot‐pressed ZrB₂–SiC. Neither ZrB₂ grain size nor SiC cluster size changed for these heat‐treatment temperatures. Microstructural analysis suggested additional phases may have formed during heat treatment and/or dislocation density may have changed. This study demonstrated that thermal annealing may be a useful method for improving the elevated temperature mechanical properties of ZrB₂‐based ceramics.     

Elevated temperature strength and thermal shock behavior of hot-pressed carbon fiber reinforced TiC composites

In order to improve the elevated strength and thermal shock resistance of TiC materials, 20vol% short carbon fiber-reinforced TiC composite (C_f/TiC) was produced by hot pressing. With carbon fiber addition, the strength and fracture toughness of TiC is increased remarkably, and the elastic modulus and thermal expansion coefficient are decreased. The strength value of C_f/TiC composite is 593 MPa at room temperature and 439 MPa at 1400°C, and the fracture toughness value at room temperature is 6.87 MPa m^1/2. The thermal stress fracture resistance parameter, R, thermal stress damage resistance parameter, R^IV, and thermal stress crack stability parameter, R_st, are all increased. The residual strength decreases significantly when the thermal shock temperature difference, ΔT, is higher than 900°C, and the residual strength is 252 MPa when ΔT is 1400°C. Carbon fiber reinforced-TiC composite exhibits superior resistance to thermal shock damage compared with monolithic TiC. The catastrophic failure induced by severe thermal stresses can be prevented in C_f/TiC composite.     

Mechanical properties of ZrB2–SiC(ZrSi2) ceramics

Mechanical properties of ZrB₂–SiC and ZrB₂–ZrSi₂–SiC ceramics in the temperature range from 20 to 1400 °C were studied. It was found that the introduction of zirconium silicide resulted in pore-free ceramics having bending strengths of 400–500 MPa over a wide range of boride–carbide compositions. Zirconium silicide additive did not lead to significant strength and hardness changes at low temperature, but essentially increased Weibull modulus, and, therefore, the reliability of the ceramics. However, zirconium silicide additions resulted in noticeably reduced bending strength in ZrB₂–SiC based composites at 1400 °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.     

High-temperature flexural strength of SiC ceramics prepared by additive manufacturing

Silicon carbide (SiC) ceramics, as a kind of candidate material for aero-engine, its high-temperature performance is a critical factor to determine its applicability. This investigation focuses on studying the high-temperature properties of SiC ceramics fabricated by using additive manufacturing technology. In this paper, SiC ceramics were prepared by combining selective laser sintering (SLS) with precursor infiltration and pyrolysis (PIP) technique. The microstructure, phase evolution, and failure mechanism after high-temperature tests were explored. SiC ceramic samples tested at room temperature (RT), 800°C, 1200°C, 1400°C, and 1600°C demonstrated bending strengths of 220.0, 226.1, 234.9, 215.5, and 203.7 MPa, respectively. The RT strength of this material can be maintained at 1400°C, but it decreased at 1600°C. The strength retention at 1400°C and 1600°C were 98% and 92%, respectively. The results indicate that the mechanical properties of SiC ceramics prepared using this method have excellent stability. As the temperature increases, the bending strength of the specimens increased slightly and reached the peak value at 1200°C, and dropped to 203.7 MPa at 1600°C. Such an evolution could be mainly due to the crack healing, and the softening of the glassy phase.     

Bending strength and microstructure of Al2O3 ceramics densified by spark plasma sintering

Al₂O₃ ceramics were superfast densified using spark plasma sintering (SPS) by heating to a sintering temperature between 1350 and 1700°C at a heating rate of 600°C/min, without holding time, and then fast cooling to 600°C within 3 min. High-density Al₂O₃ ceramics could be achieved at lower sintering temperatures by SPS, as compared with that by conventional pressureless sintering (PLS). The bending strength of Al₂O₃ superfast densified by SPS in the range of sintering temperature between 1400 and 1550°C reached values as high as 800 MPa, almost twice that obtained by the PLS. SEM observations indicated that intragranular fracture was the preponderant fracture mode in these samples, resulting in these excellent bending strength values.     

A Novel Way to Prepare Hollow Sphere Ceramics

Recent developments in the fabrication of hollow spheres have allowed our group to prepare a new type of macroporous ceramics: hollow sphere ceramics (HSCs). Alumina hollow spheres were first produced by centrifugal spray‐drying of particle‐stabilized foam slurry. The obtained hollow spheres were sintered together to form HSC at high temperatures. The effect of the sintering temperature on the linear shrinkage, porosity and compressive strength of HSC samples was investigated. When the sintering temperature was increased from 1400°C to 1600°C, the samples shrunk increasingly and the porosity decreased from 59% to 42%, which lead to an increase in the strength of the alumina foams from 6.9 (at 1400°C) to 100.0 MPa (at 1550°C). The mechanical strength of the HSC highly depends on the contact area between the hollow spheres, which could be increased by increasing the sintering temperature, decreasing the size of hollow spheres or by slurry infiltration.     

Preparation and study of the mechanical and optical properties of infrared transparent Y2O3–MgO composite ceramics

In this study, fine Y₂O₃–MgO composite nanopowders were synthesized via the sol–gel method. Dense Y₂O₃–MgO composite ceramics were fabricated by pre-sintering the green body in air at different temperatures for 1 h and then subjecting the sintered bodies to hot isostatic pressing at 1300°C for 1 h. The effects of pre-sintering temperature on the microstructural, mechanical, and optical properties of the resulting ceramics were studied. The average grain size of the ceramics was increased, whereas their hardness and fracture toughness were decreased with increasing pre-sintering temperature. A maximum fracture toughness of 1.42 MPa·m^1/2 and Vickers hardness of 10.4 GPa were obtained. The average flexural strength of the ceramics was 411 MPa at room temperature and reached 361 MPa at 600°C. A transmittance of 84% in the 3–5 µm region was obtained when the composite ceramics were sintered at 1400°C. Moreover, a transmittance of 76% in the 3–5 µm region was obtained at 500°C.     

Potential of the Sinter-HIP-technique for the Development of High-temperature Resistant Si3N4-ceramics

Silicon nitride ceramics were densified with the sintering additives Y₂O₃ and SiO₂ by a two-step sinter-HIP-process. Three compositions with additive contents between 2 and 7 wt% Y₂O₃ were prepared to study the influence of the processing conditions on the mechanical properties. The minimum additive content required for nearly complete densification (>98.5%) was only 2 wt% Y₂O₃. However, densification was limited to certain Y₂O₃/SiO₂ ratios. The additive-rich samples revealed a mean strength at room temperature up to 800 MPa which degrades at 1400°C. The material with only 2 wt% Y₂O₃ has a room temperature strength of ∼500 MPa, but no strength degradation up to 1400°C. The lower strength correlates with a pronounced increase in brittleness with a decreasing additive content indicated by a fracture toughness of only 2·5 MPam^1/2 for composition 2/0. The investigated materials exhibit a relatively high creep resistance at 1400°C with creep rates down to 1·5×10⁻⁹ s⁻¹.     

High-toughness mullite ceramics fabricated by hot-press sintering of pyrophyllite and AlOOH at low temperatures

High-toughness mullite ceramics were fabricated through hot-press sintering (HPS) of pyrophyllite and AlOOH, which were wet-milled and well mixed using a planetary ball mill. The impacts of sintering temperatures and contents of AlOOH on mullite phase formation, densification, microstructure and mechanical properties in ceramic materials were investigated through XRD, SEM and mechanical properties determination. The results indicated that high-toughness mullite ceramics could be successfully prepared by HPS at temperatures higher than 1200°C for 120 min. Increasing the sintering temperature from 1000 to 1300°C significantly enhanced the flexural strength and fracture toughness of samples. The highest flexural strength of 297.97±25.32 MPa and fracture toughness of 4.64±0.11 MPa⋅m^1/2 were obtained for samples sintered at 1300°C. Further increase of temperature to 1400°C resulted in slight decrease of flexural strength and fracture toughness. Compared with the mullite ceramics prepared only using pyrophyllite as raw material, incorporation of AlOOH into raw material significantly increased the mechanical properties of final mullite ceramics. And stoichiometric AlOOH and pyrophyllite as starting material gave the best performance in fracture toughness. The high-toughness of mullite ceramics were ascribed to the high mullite phase content, fine mullite whiskers and in situ formed, intertwined three-dimensional network structure obtained through HPS at a low temperature of 1300°C.     

Reduced-graphene-oxide-reinforced boron carbide ceramics fabricated by spark plasma sintering from powder mixtures obtained by heterogeneous co-precipitation

Reduced graphene oxide (rGO) sheets were uniformly dispersed in boron carbide ceramics by a heterogeneous co-precipitation method. This approach was used to improve the fracture toughness of boron carbide ceramics and to address the problem of agglomeration of graphene in the boron carbide matrix. Cetyltrimethyl ammonium bromide was used as a heterogeneous co-precipitation reaction initiator to prepare a homogeneously dispersed graphene oxide/boron carbide (GO/B₄C) mixture. Reduced graphene oxide/boron carbide (rGO/B₄C) powder mixtures with good dispersion were obtained by high temperature heat treatment. Dense rGO/B₄C composite ceramics were fabricated by spark plasma sintering at 1800 °C and 50 MPa. The fracture toughness and flexural strength of the rGO/B₄C with an rGO content of 2 vol% composite increased by 42% (from 3.43 to 4.88 MPa·m^1/2) and 28% (from 372 to 476 MPa) compared with those of pure B₄C, respectively. The markedly improved fracture toughness and flexural strength of the boron carbide ceramics were attributed to the effect of crack bridging and crack deflection by graphene sheets, graphene interface sliding, and pulling out of graphene.     

Zirconium diboride laminates for improved damage tolerance at elevated temperatures

The mechanical response was studied for dense laminates containing layers of ZrB₂ (~145 µm) and graphite—10 vol% ZrB₂ (~20 µm). Individual layers were formulated by mixing starting powders with thermoplastic polymers and pressing into sheets. Laminates were produced by stacking and warm pressing the sheets, debinding, and hot pressing at 2050°C, 32 MPa, in Ar. The laminates were fractured at temperatures up to 2000°C in Ar. Laminates exhibited room temperature flexure strength of 260 MPa, increasing to 300 MPa at 1600°C, and then decreasing to 160 MPa at 2000°C. Inelastic work of fracture was 0.6 kJ/m² at room temperature, reached a maximum of 1.3 kJ/m² at 1400°C, and reverted to linear elastic failure at 2000°C. During fracture, cracks were deflected at the interfaces between the strong ZrB₂ layers and the relatively weak C-ZrB₂ layers, which led to an increased inelastic work of fracture by more than an order of magnitude compared to conventional ZrB₂ ceramics. This study demonstrated that laminate architectures are a promising approach for improving the damage tolerance of ZrB₂-based ceramics at elevated temperatures.     

Microstructure and mechanical properties of fine-grained boron carbide ceramics fabricated by high-pressure hot pressing combined with high-energy ball milling

Dense and fine-grained boron carbide (B₄C) ceramics were fabricated via high-pressure hot pressing (100?MPa) using powders, which are prepared by high-energy ball milling. These powders were sintered at a low temperature (1800?°C) without any sintering aid. The dense and fine-grained B₄C ceramics demonstrate super high hardness, outstanding fracture toughness and modern flexure strength. The milled powders were characterised by disordered crystal structure and ultrafine particle size that ranges from a few nanometres to a few hundred nanometres. The combined contributions of high pressure and the characteristic of the milled powders guaranteed that the dense fine-grained microstructure was achieved at only 1800?°C. The grain size distribution of the ceramics was inhomogeneous and ranged from 70?nm to 1.6?µm. However, the average grain size was fine at only 430?nm, which partially contributed to the super high hardness of the B₄C ceramics. The locally concentrated areas of the small grains changed the fracture mode of the B₄C ceramics from the complete transgranular fracture to a mixture of transgranular and intergranular fractures, thereby enhancing the toughness of the B₄C ceramics. The relative density, Vickers hardness, flexure strength and fracture toughness of the obtained B₄C ceramics reached up to 99.5%, 41.3?GPa, 564?MPa and 4.41?MPa?m^1/2, respectively.     

Effect of boron addition on microstructure,mechanical properties and oxidation resistance of TaC ceramics

TaC ceramics with 0.03–0.60?wt% of boron additions were prepared by hot pressing at 2100?°C for 1?h under a pressure of 40?MPa. Effects of boron content on densification, phase composition, microstructure, mechanical properties and oxidation resistance of the TaC ceramics were investigated. When the boron content was 0.12?wt% and above, full density was obtained due to reactions between boron and oxygen impurity at presence of TaC. Minor phases of TaB₂ and C were formed in the 0.24 and 0.60?wt% B compositions after gas-out of the oxygen impurity. Microstructure of the TaC ceramics was refined with increasing in boron content. The TaC ceramic with 0.24?wt% of boron showed the best mechanical properties with a Vickers hardness, flexural strength and fracture toughness of 17.7?GPa, 534?MPa and 4.6?MPa?m^1/2, respectively. When more boron was added, interfacial bonding of the TaC grains was strengthened causing a decrease in fracture toughness. Oxidation resistance of the TaC ceramics increased with boron content. Particularly, the 0.60?wt% B composition showed a weight gain of 0.0018?g/cm² after oxidization at 800?°C in air for 3?h.