Effect of ZrB2 and SiC addition on TiB2-based ceramic composites prepared by spark plasma sintering

TiB₂-based ceramic composites with different amounts of ZrB₂ and SiC were prepared by spark plasma sintering at 1700 °C with an initial pressure of 40 MPa and a holding time of 10 min. The (Ti_xZr_y)B₂ solid solution was found in the sintered TiB₂/ZrB₂/SiC composites by XRD. The microstructural and mechanical properties of the prepared samples were investigated. The composite with the addition of 30 vol.% ZrB₂ shows better comprehensive performances, and the bending strength and the fracture toughness of the composite are 780.5 MPa and 7.34 MPa m^1/2, respectively. The generation of the (Ti_xZr_y)B₂ solid solution makes the microstructures of the composites finer and more homogeneous, which has played a very important role in grain refinement and interface fusion.     

Processing and properties of sintered reaction-bonded silicon nitride with Y2O3–MgSiN2: Effects of Si powder and Li2O addition

The effects of Si powder and Li₂O addition on the processing, thermal conductivity and mechanical properties of sintered reaction-bonded silicon nitride (SRBSN) with Y₂O₃–MgSiN₂ sintering aids were studied. Addition of Li₂O provides a less-viscous liquid phase that results in a more uniform and finer pore structure in RBSN with the coarser Si powders, but the pore structure plays a less important role in the densification of RBSN. The thermal conductivity of SRBSN without porosity decreases with increased Al impurity content and also decreases with the Li₂O addition regardless of the Si purity. The impurest coarse Si powder produces the lowest thermal conductivity (93 W m⁻¹ K⁻¹) but the highest four-point bending strength (∼700 MPa) and a higher fracture toughness (∼10 MPa m^1/2). However, the purer fine Si powder produces the highest thermal conductivity (119 W m⁻¹ K⁻¹) and highest toughness (∼11 MPa m^1/2) but the lowest strength (∼500 MPa).     

ZrB2-ZrSi2-SiC composites prepared by reactive spark plasma sintering

ZrSi₂ and SiC are good candidates to improve both sinterability and mechanical properties of ZrB₂ ceramics, which were synthesized simultaneously by an in-situ reaction of ZrC and Si additives during the sintering processing in this work. The ZrB₂ ceramic composites with different amount of ZrSi₂ and SiC were fabricated by reactive spark plasma sintering (RSPS) method. X-ray diffraction, scanning microscopy and Archimedes's method are used to characterize the phase, microstructure and density of the composites. Meanwhile, fracture toughness and flexural strength of the obtained composites were investigated too. It's found that a fully dense composite can be achieved at 1500 °C by SPS. Both fracture toughness and flexural strength of ZrB₂ ceramics increased with increasing the concentration of ZrSi₂ and SiC additives and reached a maximum of 7.33 ± 0.24 MPa·m^1/2 and 471 ± 15 MPa, respectively, with the ZrSi₂ + SiC content of 30 wt%.     

Stainless steel bonded NbC matrix cermets using a submicron NbC starting powder

The microstructure and mechanical properties of 316 L and 430 L stainless steel bonded NbC cermets were assessed. NbC starting powder mixtures with 15 and 30 vol% steel binder were pressureless vacuum sintered for 1 h at 1420 °C. The liquid forming temperature and shrinkage behaviour of the green powder compacts were investigated by differential scanning calorimetry and dilatometry. Microstructural and compositional analysis were conducted by electron probe microanalysis (EPMA) and XRD to investigate the effect of the steel binder on NbC grain growth and Cr-rich carbide precipitation. Rapid NbC grain growth was observed and the average NbC grain size decreased with increasing binder content. The residual Cr-rich carbide located at NbC grain boundaries can be eliminated by the addition of carbide forming metal precursors such as TiH₂ or by a thermal annealing process of the sintered NbC cermets at 1200 °C. The hardness and fracture toughness of the NbC-steel cermets was influenced by the steel binder type and content. A maximum hardness of 13.6 GPa was measured for the NbC-15 vol% 430 L cermet, combined with a modest fracture toughness of 7.3 MPa m^1/2.     

Mechanical and thermal properties of hot pressed ZrB2 system with TiB2

ZrB₂-TiB₂-based ceramics with varying amount of TiB₂ (up to 30 wt%) were hot pressed at 2200 °C in Ar atmosphere, and the effect of the TiB₂ addition on mechanical properties like hardness, fracture toughness, scratch resistance, wear resistance and thermal conductivity of the system was compared to monolithic ZrB₂ ceramic. It was found from X-ray diffraction that TiB₂ completely entered into the structure and formed solid solution with ZrB₂. Addition of TiB₂ in ZrB₂ system improves the mechanical and wear resistance properties. ZrB₂-TiB₂ (30 wt%) ceramic, for example, showed highest hardness of 22.34 GPa, fracture toughness 3.01 MPa(m)^1/2 and lowest coefficient of friction (0.398 at 10 N load). The addition of TiB₂ in ZrB₂ system showed lower thermal conductivity than monolithic ZrB₂ by increasing grain boundary thermal resistance.     

On the toughness enhancement in hydroxyapatite-based composites

Among various biologically compatible materials, hydroxyapatite (HA) has excellent bioactivity/osteointegration properties and therefore has been extensively investigated for biomedical applications. However, its inferior fracture toughness limits the wider applications of monolithic HA as a load-bearing implant. To this end, HA-based biocomposites have been developed to improve their mechanical properties (toughness and strength) without compromising biocompatibility. Despite significant efforts over last few decades, the toughness of HA-based composites could not be enhanced beyond 1.5–2 MPa m^1/2, even when measured using indentation techniques. In this perspective, the present work demonstrates how spark plasma sintering can be effectively utilized to develop hydroxyapatite–titanium (HA–Ti) composites with varying amounts of Ti (5, 10 and 20 wt.%) with extremely high single edge V-notch beam fracture toughness (4–5 MPa m^1/2) along with a good combination of elastic modulus and flexural strength. Despite predominant retention of HA and Ti, the combination of critical analysis of X-ray diffraction and transmission electron microscopy investigation confirmed the formation of the CaTi₄(PO₄)₆ phase with nanoscale morphology at the HA/Ti interface and the formation of such a phase has been discussed in reference to possible sintering reactions. The variations in the measured fracture toughness and work of fracture with Ti addition to the HA matrix were further rationalized using the analytical models of crack bridging as well as on the basis of the additional contribution from crack deflection. The present work opens up the opportunity to further enhance the toughness beyond 5 MPa m^1/2 by microstructural designing with the desired combination of toughening phases.     

Comparison of ZrB2–ZrO2f ceramics prepared by hot pressing and pressureless sintering

Zirconium diboride based ultra-high temperature ceramics toughened by zirconia fiber (ZrB₂–ZrO_2f) were prepared by hot pressing and pressureless sintering, and the effect of two sintering techniques on the phase composition, microstructures and mechanical properties of ZrB₂–ZrO_2f ceramics were studied in detail. The densification behavior was investigated through the analysis of the density curves. The microstructures and mechanical properties of ZrB₂–ZrO_2f ceramics were analyzed and compared in order to research the influence of the two sintering techniques. Results indicated that the hot-pressing process was more suitable for preparing ZrB₂–ZrO_2f ceramics than pressureless sintering process. The comprehensive properties of ZrB₂ plus 30 vol.% ZrO_2f ceramics obtained at temperature 1950 °C by hot pressing for 2 h were optimal, the flexural strength and fracture toughness reached 633 MPa and 5.6 MPa·m^1/2, respectively. The higher flexural strength was attributed to the smaller size of grains and higher relative density, furthermore, the toughening mechanisms were fiber debonding, fiber pull-out, crack deflection and transformation toughening.     

Enhanced fracture toughness in carbon-nanotube-reinforced amorphous silicon nitride nanocomposite coatings

Multiwalled-carbon-nanotube (MWCNT)-reinforced silicon nitride coatings were grown to evaluate the toughness contribution of nanotubes in a ceramic coating. An MWCNT array was first grown using catalytic chemical vapor deposition of acetylene on a silicon substrate. This aligned MWCNT preform was then infiltrated with an amorphous silicon nitride matrix by low-pressure chemical vapor deposition of dichlorosilane (DCS) and ammonia (NH₃). The fracture toughness of this material was determined by generating cracks using nanoindentation and then employing finite-element analysis to estimate the bridging toughness contribution of the MWCNTs. The MWCNT bridging toughness of the composites is determined to be ～5.6 MPa m^1/2, which is seven times higher than that of the matrix. The interfacial frictional stress is also estimated and ranges from 7 to 20 MPa.     

Microstructure and mechanical properties of ZrB2–SiC composites prepared by gelcasting and pressureless sintering

ZrB₂–SiC ceramic composites were prepared through water-based gelcasting and pressureless sintering. Effects of the pressureless sintering temperature (1500–2000 °C), heating rate (5–15 °C/min) and soaking time (0.5–2 h) on the relative density, microstructure and mechanical properties of the ZrB₂–SiC composites were investigated in detail. A sintering temperature of 2000 °C, a heating rate of 5 °C/min and a soaking time of 2 h were found to be the optimal pressureless sintering procedure. The relative density, flexural strength and fracture toughness of the ZrB₂–SiC composite prepared under the optimum condition were 97.8%, 403.1 ± 27.8 MPa and 4.05 ± 0.42 MPa·m^1/2, respectively.     

Microstructure and some properties of TiAl-Ti2AlC composites produced by reactive processing

The TiAl–Ti₂AlC composites with and without impurities, Ni, Cl and P, were prepared by combustion reaction from the elemental powders and cast after arc melting. The resulting composites had about 18 vol% Ti₂AlC in the lamellar matrix consisting of γ-TiAl and Ti₃Al (α₂). In the homogenized specimens, the α₂ phase decomposed to γ-TiAl and Ti₂AlC. The composite material had a high strength both at ambient and elevated (1173 K) temperatures; about 800 and 400 MPa, respectively, with an ambient temperature ductility of 0.7% at bending test. The fracture toughness test also proved that the homogenized composite has higher toughness than the as cast one. The toughness value reached to 17.8 MPa m^1/2. The zigzag cracks propagated in the homogenized composite and the reinforcement Ti₂AlC particles and the finely precipitated Ti₂AlC particles were main obstacles to the crack propagation. The composite with impurities showed a marginal improvement in the oxidation resistance over the composites without impurities.     

Two-step hot pressing of bimodal micron/nano-ZrB2 ceramic with improved mechanical properties and thermal shock resistance

The densification and grain growth behaviors for micron- and nano-sized ZrB₂ particles were investigated. The densification on-set temperature (T_d-micron) and grain growth on-set temperature (T_g-micron) for micron-sized ZrB₂ particles were about 1500 °C and 1800 °C, respectively. And the densification on-set temperature (T_d-nano) and grain growth on-set temperature (T_g-nano) for nano-sized ZrB₂ particles were about 1300 °C and 1500 °C, respectively. A bimodal micron/nano-ZrB₂ ceramic was therefore prepared using a novel two-step hot pressing. A high relative density of 99.2%, an improved flexural strength of 580.2 ± 35.8 MPa and an improved fracture toughness of 7.2 ± 0.4 MPa·m^1/2 were obtained. The measured critical thermal shock temperature difference (ΔT_c) for this bimodal micron/nano-ZrB₂ ceramic was as high as 433 °C.     

Microstructure and tribological performance of NbC-Ni cermets modified by VC and Mo2C

The current study reports on the influence of the addition of 5–15 vol% VC or/and Mo₂C carbide on the microstructure and mechanical properties of nickel bonded NbC cermets, which are compared to cobalt bonded NbC cermets. The NbC, Ni and secondary carbides powder mixtures were liquid phase sintered for 1 h at 1420 °C in vacuum. The fully densified cermets are composed of a cubic NbC grains matrix and an evenly distributed fcc Ni binder. NbC grain growth was significantly inhibited and a homogeneous NbC grain size distribution was obtained in the cermets with VC/Mo₂C additions. The mechanical properties of the NbC-Ni matrix cermets are strongly dependent on the carbide and Ni binder content and are directly compared to their NbC-Co equivalents. The liquid phase sintered NbC-12 vol% Ni cermet had a modest Vickers hardness (HV₃₀) of 1077 ± 22 kg/mm² and an indentation toughness of 9.1 ± 0.5 MPa·m^1/2. With the addition of 10–15 vol% VC, the hardness increased to 1359 ± 15 kg/mm², whereas the toughness increased to 11.3 ± 0.1 MPa·m^1/2. Addition of 5 and 10 vol% Mo₂C into a NbC-12 vol% Ni mixtures generated the same values in HV₃₀ and K_IC when compared to VC additions. A maximum flexural strength of 1899 ± 77 MPa was obtained in the cermet with 20 vol% Ni binder and 4 vol% VC + 4 vol% Mo₂C addition, exhibiting a high fracture toughness of 15.0 ± 0.5 MPa·m^1/2, but associated with a loss in hardness due to the high Ni content. The dry sliding wear behaviour was established at room temperature and 400 °C from 0.1 to 10 m/s.     

Designing highly toughened hybrid composites through nature-inspired hierarchical complexity

The notion of replicating the unique fracture resistance of natural composites in synthetic materials has generated much interest but has yielded few real technological advances. Here we demonstrate how using ice-templated structures, the concept of hierarchical design can be applied to conventional compounds such as alumina and poly(methyl methacrylate) (PMMA) to make bulk hybrid materials that display exceptional toughness that can be nearly 300 times higher (in energy terms) than either of their constituents. These toughnesses far surpass what can be expected from a simple “rule of mixtures”; for a ～80% Al₂O₃–PMMA material, we achieve a K_Jc fracture toughness above 30 MPa m^1/2 at a tensile strength of ～200 MPa. Indeed, in terms of specific strength and toughness, these properties for alumina-based ceramics are at best comparable to those of metallic aluminum alloys. The approach is flexible and can be readily translated to multiple material combinations.     

Effect of silicidation pretreatment with gaseous SiO on sinterability of TiC powders

Silicidation pretreatment with gaseous SiO at 1350 °C for 30 min is employed for chemically modifying commercially available TiC powder. Phase composition and microstructural features of the pretreated powder are discussed. Densification behavior of the pretreated TiC powder during hot pressing is studied in comparison with that of non-pretreated one. Significantly improved densification behavior and sinterability of TiC powder after silicidation pretreatment are explained by the effect of Ti₃SiC₂ acting as a solid lubricant. Nearly fully dense TiC-based ceramics having flexural strength of 370 MPa, fracture toughness of 5.6 MPa m^½, and microhardness of 24 GPa is obtained by hot pressing under conditions as mild as 1600 °C and 20 MPa.     

Improved microstructure and fracture properties of short carbon fiber-toughened ZrB2-based UHTC composites via colloidal process

Ultra-high temperature ceramics are potential materials for a variety of high temperature applications because of excellent thermo-mechanical properties and oxidation resistance. To further improve their fracture properties, a novel colloidal process was proposed to fabricate the short carbon fiber-toughened ZrB₂–ZrSi₂ composites. Microstructure analysis found that the colloidal processing route could avoid the fibers' agglomeration and alleviate the fibers' damage, which minimizes the structural defects and retains the fibers' strength. The relative density of composites achieves 98.35% and the distribution of fibers in matrix is homogeneous. Mechanical tests indicate that the flexural strength is 458 MPa and the fracture toughness is 6.9 MPa·m^1/2. In comparison to the composite obtained by conventional processing route, the fracture toughness increases by 47%. The main mechanisms for improved fracture properties could be attributed to the crack deflection, fiber sliding and fiber bridging.     

Effects of ZrC and SiC addition on the microstructures and mechanical properties of binderless WC

ZrC-added WC ceramics and SiC-added WC–2 mol% ZrC ceramics were sintered at 1800 °C using a resistance-heated hot-pressing machine. Dense WC ceramics containing 0–1 mol% ZrC and WC–2 mol% ZrC ceramics containing 1–6 mol% SiC were obtained. The reaction products W₂C, ZrO₂ and ZrC-based solid solutions were formed in the ZrC-added WC ceramics during sintering. The relative amount of W₂C reached zero at 2 mol% ZrC, increased in the range of 2–6 mol% ZrC, and decreased again above 6 mol% ZrC. The average WC grain size decreased from 0.49 μm for the WC ceramic to 0.24 μm at 4 mol% ZrC. The SiC addition of 1–2 mol% to the WC–2 mol% ZrC ceramics caused abnormal growth of WC grains. The Vickers hardness of the ZrC-added WC ceramics decreased to 17 GPa at 2 mol% ZrC. The hardness of the SiC-added WC–2 mol% ZrC ceramics increased from 12.4 at 2 mol% SiC to 21.5 GPa at 6 mol% SiC. The fracture toughness of the ZrC-added WC ceramics decreased from 6.2 MPa m^0.5 for the WC ceramic to 5.2 MPa m^0.5 at 4 mol% added ZrC. The fracture toughness of the WC–2 mol% ZrC ceramics with 6 mol% SiC were relatively high at 6.7 MPa m^0.5. The addition of SiC to WC-based ceramics thus improved both hardness and fracture toughness.     

W-ZrC composites prepared by reactive melt infiltration of Zr2Cu alloy into partially carburized W preforms

W-ZrC composites without residual WC have been prepared for the first time by reactive infiltration at 1300 °C for 1 h in vacuum using a molten Zr₂Cu alloy and a newly designed partially-carburized W powder as raw materials. The as-synthesized composites consist of two major phases of W and ZrC, in which the content of W is 65 vol%. The reaction time needed to produce a fully densified W-ZrC bulk ceramic is distinctly shortened by this means, as contrasted with conventional WC/W or WC preforms. The microstructural evolution during reactive melt infiltration is investigated to obtain a better understanding of reaction mechanisms and mechanical properties of the W-ZrC composites derived by infiltrating Zr₂Cu alloy into partially carburized W preforms. The flexural strength, Young's modulus and fracture toughness for the W-ZrC composite are 554 MPa, 339 GPa and 9.7 MPa·m^1/2, respectively.     

In situ synthesis,microstructure, mechanical properties and thermal shock resistance of (ZrB2 + SiC)/Zr2[Al(Si)]4C5 composites

Dense (ZrB₂ + SiC)/Zr₂[Al(Si)]₄C₅ composites with adjustable content of (ZrB₂ + SiC) reinforcements (0–30 vol.%) were prepared by in situ hot-pressing. The microstructure, room and high temperature mechanical and thermal physical properties, as well as thermal shock resistance of the composites were investigated and compared with monolithic Zr₂[Al(Si)]₄C₅ ceramic. ZrB₂ and SiC incorporated by in situ reaction significantly improve the mechanical properties of Z₂[Al(Si)]₄C₅ by the synergistic action of many mechanisms including particulate reinforcement, crack deflection, branching, bridging, “self-reinforced” microstructure and grain-refinement. With (ZrB₂ + SiC) content increasing, the flexural strength, toughness and Vickers hardness show a nearly linear increase from 353 to 621 MPa, 3.88 to 7.85 MPa·m^1/2, and 11.7 to 16.7 GPa, respectively. Especially, the 30 vol.% (ZrB₂ + SiC)/Zr₂[Al(Si)]₄C₅ composite retains a high modulus up to 1511 °C (357 GPa, 86% of that at 25 °C) and superior strength (404 MPa) at 1300 °C in air. The composite shows higher thermal conductivity (25–1200 °C) and excellent thermal shock resistance at ΔT up to 550 °C. Superior properties render the composites a promising prospect as ultra-high-temperature ceramics.     

Sintering behavior and mechanical properties of spark plasma sintered β–SiAlON/TiN nanocomposites

In this study, fully dense β-SiAlON/TiN composites were produced by Spark Plasma Sintering (SPS) method. Si₃N₄, Al₂O₃, AlN and TiO₂ powders were used as precursors. Starting powders were mixed with high energy ball milling and then were sintered by SPS method (at 1750 °C under pressure of 30 MPa for 12 min.). The milled powders had an average particle size of below ~ 155 nm. The XRD patterns of SPS-ed composites showed that the entire β-SiAlON phase constituent was in the form of Si₄Al₂O₂N₆ phase and cubic TiN phase can be formed by the phase transformation of TiO2 in relation with other precursors. FESEM micrographs confirmed that TiN particles were distributed homogeneously throughout β-SiAlON matrix. Mechanical properties evaluation revealed that by adding micro sized TiO₂, optimal mechanical properties with a hardness ~ 14.6 GPa and a fracture toughness ~ 6.3 MPa m^1/2 were obtained. The improvement in the fracture toughness was attributed to the presence of the crack deflection as the dominant toughening mechanism in the SPS-ed β–SiAlON/TiN composites.     

Ultra high-pressure spark plasma sintered ZrC-Mo and ZrC-TiC composites

Ultra-high-pressure spark plasma sintering was applied to ZrC-20 wt%Mo and ZrC-20 wt%TiC composites with a pressure up to 7.8 GPa and temperatures of 1550 °C and 1950 °C. Mechanical performance of the composites was benchmarked against a plain ZrC produced by the same method. Both composites outperformed the pure ZrC with superior hardness and indentation fracture toughness of 2239 HV1 and 5.4 MPa m^1/2, and 1896 HV1 and 5.9 MPa m^1/2, respectively, for ZrC-Mo and ZrC-TiC composites. It was shown that ultra-high compaction pressure affected the ZrC-20 wt%TiC miscibility gap by lowering the temperature threshold from the usually applied 1800 °C down to 1550 °C resulting in formation of the solid state solution of (Zr,Ti)C. In contrast, the high pressure does not inhibit the carburisation of Mo with ZrC to form MoC, even when experiments were performed in a graphite free environment. The equiaxed morphology of ZrC grains along with a right-shift in XRD peaks for ZrC indicates dissolution of Mo in ZrC resulting in formation of the solid solution of (Zr,Mo)C. High-temperature X-ray diffraction analysis under oxidation conditions was performed on the samples showing degradation of ZrC-20 wt%Mo due to the oxidation of Mo at high-temperature leading to MoO₃ vaporisation. Conversely, the oxidation of ZrC-20 wt%TiC composites was characterised by formation of ZrO₂ and TiO₂ remaining stable up to 1500 °C.