Titanium diboride composite with improved sintering characteristics

Titanium diboride (TiB₂) and its ceramic composites were prepared by hot pressing process. The sintering process, phase evolution, microstructure and mechanical properties of TiB₂ ceramics prepared by using different milling media materials: tungsten carbide (WC/Co) or SiAlON was studied. It was found that the inclusion of WC/Co significantly improved the sinterability of the TiB₂ ceramics. A core/rim structure with pure TiB₂ as the core and W-rich TiB₂, i.e. (Ti,W)B₂ as the rim was identified. Microstructure analysis revealed that this core/rim structure was formed through a dissolution and re-precipitation process. In addition, silicon carbide (SiC) was also introduced to form TiB₂–SiC composites. The addition of SiC as the secondary phase not only improved the sinterability but also led to greatly enhanced fracture toughness. The optimum mechanical properties with Vickers hardness ~ 22 GPa, and fracture toughness ~ 6 MPa m^1/2 were obtained on TiB₂–SiC composites milled with WC/Co.     

Preparation of Si3N4-TaC and Si3N4-ZrC composite ceramics and their mechanical properties

Si₃N₄-TaC and Si₃N₄-ZrC composite ceramics with sintering additives were consolidated in the sintering temperature range of 1500–1600 °C using a resistance-heated hot-pressing technique. The addition of 20–40 mol% carbide improved the sinterability of the ceramics. The ceramics were densely sintered under 0–40 mol% TaC or ZrC at 1500 °C, 0–80 mol% TaC at 1600 °C, and 0–60 mol% ZrC at 1600 °C. In ceramics sintered at 1500 °C, the proportion of α-Si₃N₄ was larger than that of β-SiAlON; α-Si₃N₄ transformed mostly to β-SiAlON at 1600 °C. Carbide addition was effective in inhibiting α-Si₃N₄-to-β-SiAlON phase transformation. Young's modulus for the dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics increased with the carbide amount, and the hardness of dense Si₃N₄-ZrC and Si₃N₄-TaC ceramics increased from 14 GPa to 17 GPa with increasing α-Si₃N₄ content. Dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics, with larger quantities of α-Si₃N₄ sintered at 1500 °C, exhibited high hardness; the fracture toughness of these ceramics decreased with increasing α-Si₃N₄ proportion. Both the hardness and fracture toughness of the dense Si₃N₄-TaC and Si₃N₄-ZrC ceramics were strongly related to the proportion of α-Si₃N₄ in the sintered body.     

Fabrication and mechanical properties of TiB2/ZrO2 functionally graded ceramics

Five layers were considered for the present TiB₂/ZrO₂ functionally graded ceramics and TiB₂/ZrO₂ functionally graded ceramics were prepared by hot-pressing. The first layer, marked as L₁, was composed of TiB₂–15vol%SiC. The L₂ layer was composed of TiB₂–15vol%SiC–10vol%ZrO₂. The L₃ layer was composed of TiB₂–25vol%ZrO₂ without addition of SiC, because 25vol%ZrO₂ was enough to densify TiB₂. The last two layers, L₄ and L₅, were composed of TiB₂–35vol%ZrO₂ and TiB₂–45vol%ZrO₂, respectively. Denser graded microstructures as well as the stronger interfaces were achieved for TiB₂/ZrO₂ functionally graded ceramics hot-pressed at 1900 °C. Due to the phase transformation of ZrO₂, the toughness of TiB₂/ZrO₂ functionally graded ceramics increased from ~ 4.3 MPa m^1/2 for the TiB₂-rich layer to ~ 8.9 MPa m^1/2 for the ZrO₂-rich layer. The effect of hot-pressing temperature on the toughness of TiB₂/ZrO₂ functionally graded ceramics was further investigated in detail.     

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%.     

Ti (C,N)-based cermets sintered under high pressure

In this study, high pressure and high temperature sintering (HPHT) is adopted in the cermet fabrication process, and the microstructure and mechanical properties of cermets with TiC_0.5N_0.5–15WC–10Mo₂C–5TaC–10Ni–10Co (wt%) sintered under 5 GPa and different temperatures (900–1600 °C) using 6 × 14 MN cubic press are investigated. Results show that the densities of samples can reach up to 7.00 g/cm³. Vickers hardness and fracture toughness of the products are over 1727 HV30 and 7.2 MPa m^1/2 respectively. In addition, the sintering results are compared with the data that obtained from commercial samples which produced via conventional sintering technique. The conclusion is that high density and high hardness cermets can be obtained through HPHT sintering.     

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.     

Superhard and tougher SiC/diamond-like-carbon composite films produced by electron beam physical vapour deposition

SiC/diamond-like carbon (DLC) composite films have been produced on metal substrates via electron beam physical vapour deposition process with various substrate temperatures. The films deposited at 700 °C contain a DLC matrix and nanocrystalline 3CSiC. However, the films deposited at 900 °C contain a 3CSiC matrix and DLC plus nanocrystalline diamond. Both nanoindentation and Hysitron testing have shown that the Young’s moduli and hardnesses of the films increased with the substrate temperature. The hardness could reach ～60 GPa in some parts of the films produced at 900 °C. Meanwhile, the fracture toughness, measured using a micro-beam bending technique, reached 9.2 ± 2 MPa^1/2 for such a composite film. Both high hardness and toughness could be explained by the unique microstructure of the composite film.     

Effect of SiC/ZrC ratio on the mechanical and ablation properties of C/C–SiC–ZrC composites

The effect of SiC/ZrC weight ratio on the mechanical and ablation properties of carbon/carbon composites modified by SiC nanowires reinforced SiC–ZrC ceramics (C/C–SiC–ZrC) was studied. Results showed that C/C–SiC–ZrC composites with a SiC/ZrC ratio of 1:1.5 exhibited good mechanical and ablation properties. The flexural strength and modulus were 201 ± 20 MPa and 18 ± 1 GPa, respectively. After ablation for 120 s, the linear and the mass ablation rate were 0.012 mm/s and 0.0019 g/s. The good performance is attributed to a higher density, the reinforcing effect of SiC nanowires and the proper SiC/ZrC ratio.     

Effects of TaC and TiC addition on the microstructures and mechanical properties of Binderless WC

WC ceramics were sintered using a resistance-heated hot-pressing machine in the temperature range of 1600–1800 °C for TaC addition and at 1800 °C for TiC addition. Dense WC ceramics containing 0–2 mol% TaC at 1800 °C, 1–2 mol% TaC at 1700 °C, and 0–6 mol% TiC were obtained. A small addition of 1–2 mol% TaC at 1700 °C improved the sinterability of WC. The W₂C- and TaC-type solid solutions, (W, Ta)₂C and (Ta, W)C, were produced during the sintering process. The added TaC and TiC fully changed to the solid solutions of (Ta, W)C for dense TaC-added WC ceramics and (Ti, W)C for TiC-added WC ceramics. TiC inhibited the grain growth of WC in WC ceramics. The hardness of TaC-added WC ceramics sintered at 1700 °C increased from 19 GPa with no TaC addition to 25.1 GPa with 1 mol% TaC. The hardness of TiC-added WC ceramics sintered at 1800 °C differed only slightly from that without TiC addition, ~24 GPa. The relationship between the hardness of dense TaC- and TiC-added WC ceramics and WC grain size was similar to that for pure WC. The fracture toughness of the TaC-added WC ceramics at 1700 °C increased from 5.7 MPa m^0.5 without TaC addition to 6.7 MPa m^0.5 at 1 mol% TaC.     

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.     

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.     

Effect of impingement angle and WC content on high temperature erosion behavior of SiC-WC composites

Hot pressed dense SiC-(0, 10, 30 or 50 wt%)WC composites were subjected to erosion against SiC particles at 800 °C. Effects of WC content and angle of impingement (30°, 60° or 90°) on the erosion performance of composites were evaluated. Erosion rate ranged from 2.1 × 10² mm³/kg to 7.7 × 10² mm³/kg with varying WC content or angle of impingement. The erosion rate of the composites increased with increasing the impingement angle from 30° to 90°, and decreased with WC content up to 30 wt%. Minimum and maximum erosion wear rates were obtained for SiC-30 wt% WC composites at 30° and for SiC-50 wt% WC composites at normal impact, respectively. Grain fracture and pull-out were observed as major mechanisms of material removal for the composites. Decreased angle of impingement led to reduced grain fracture and pull-out, and hence reduction in material removal. Owing to increased fracture toughness with incorporation of WC particles, the composites showed less fracture and removal of WC particles up to 30 wt% reinforcement.     

Fracture toughness of carbides in tool steels evaluated by nanoindentation

In this study, the hardness and fracture toughness of different primary carbides present in tool steels have been evaluated by nanoindentation. Toughness was evaluated by the indentation method using Laugier’s equation, after experimentally assessing its applicability. The results obtained allow different types of carbides to be discerned, in terms of hardness and toughness, which ranges from 2.2 to 3.7 MPa m^1/2. It has been found that M₇C₃ carbides exhibit a marked fracture resistance anisotropy, which is lower in the direction parallel to its larger edge. This anisotropy is related to the crystalline texture. The relationship between hardness and toughness of the analysed carbides can be used as a guideline for the selection of the most appropriate type of carbide that the microstructure should contain in order to optimize the wear behaviour and the fracture resistance of a tool steel.     

Microstructure and mechanical properties of TiB2–SiC ceramic composites by Reactive Hot Pressing

TiB₂–SiC ceramic composites with different contents of Ni as additive were prepared by the Reactive Hot Pressing (RHP) process at 1700 °C under a pressure of 32 MPa for 30 min. For comparison, a monolithic TiB₂ ceramic and TiB₂–SiC ceramic composite were also fabricated under the identical temperature, pressure and holding time by the Hot Pressing (HP) process. The effects of the fabrication process and Ni on the microstructure and mechanical properties of the composites were investigated. About 8 vol.% of elongated TiB₂ grains with an aspect ratio of 3–6 and a diameter of 0.5–1 μm were produced in the composite prepared by the RHP process. The improvement of the fracture toughness was attributed to the toughening and strengthening effects of SiC particles and the elongated TiB₂ grains such as crack deflection. The TiB₂–SiC–5 wt.% Ni ceramic composite had the optimum mechanical properties with a flexural strength of 858 ± 87 MPa, fracture toughness of 8.6 ± 0.54 MPa·m^1/2 and hardness of 20.2 ± 0.94GPa. The good mechanical properties were ascribed to the relatively fine and homogeneous microstructure and the strengthening effect of Ni. Ni inhibited the anisotropic growth of TiB₂.     

Ablation resistance of SiC–ZrC coating prepared by a simple two-step method on carbon fiber reinforced composites

SiC–ZrC ablation resistance coating was prepared on the surface of carbon fiber reinforced carbon (C/C) composites by simple pack cementation combined with low-cost slurry infiltration method. The results showed that SiC–ZrC coating could effectively protect C/C composites from ablation for 45 s at 3723 K under oxyacetylene torch. The mass and linear ablation rates (0.038 ± 0.01 mg/(s cm²) and 2.42 ± 0.15 μm/s) were largely reduced compared with that of uncoated C/C composites (0.530 ± 0.01 mg/(s cm²) and 1.75 ± 0.15 μm/s) after ablation for 20 s. The good ablation protective ability of SiC–ZrC coating is mainly attributed to the volatilization of SiO₂ and the formation of ZrO₂.     

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).     

Preparation and properties of hot-pressed NbMo-matrix composites reinforced with ZrB2 particles

The NbMo-matrix composites reinforced with (0–60 vol%) ZrB₂ were fabricated by hot-pressing at 2400 °C for 10 min under a pressure of 50 MPa in dynamic vacuum in the induction heating furnace specially designed in our institute. The optimum ZrB₂ content in NbMo solid solution was determined to be 30 vol% for excellent comprehensive mechanical property. NbMo-30 vol% ZrB₂ has the highest density of 99.63%, the most uniform microstructure, high fracture toughness of 5.75 MPa m^1/2. The highest ZrB₂ concentration that reacts with NbMo solid solution is at the range of 30 to 45 vol%. The types of the formed niobium borides were decided by the original ratio of Nb to B. The distribution of Mo and Zr was mutually exclusive in low ZrB₂ content composites, however, there was Mo₂Zr in high ZrB₂ content composite. Except for NbMo-45 vol% ZrB₂, the compressive strength increased with ZrB₂ content (from 927.09 MPa to 1635.91 MPa). The Young's modulus values were directly proportional to ZrB₂ content. The fracture toughness (from 6.34 MPa m^1/2 to 3.99 MPa m^1/2) was inversely proportional to ZrB₂ content. The big residual ZrB₂ particles in high ZrB₂ content samples such as NbMo-45 vol% ZrB₂ and NbMo-60 vol% ZrB₂ was the main reason for nonhomogeneous microstructure, low density (94.09% and 94.83%, respectively) and low fracture toughness (4.58 MPa m^1/2 and 3.99 MPa m^1/2, respectively).     

Hardness and toughness enhancement of CeO2 addition to ZTA ceramics through HIPping technique

The aim of this research is to investigate the combined effects of CeO₂ additions and hot-isostatic pressing sintering (HIPping) technique on the hardness and toughness of ZTA ceramics. Addition of CeO₂ to ZTA ceramics leads to formation of a secondary phase (CeAl₁₁O₁₈) which played a vital role in affecting the Vickers hardness and toughness. Microstructure investigations showed that HIPping had a significant role in the removal of pores, and consequently affected both hardness and toughness of the samples. The highest Vickers hardness (1838.3 HV) and toughness (8.92 MPa·√m) were obtained with the 5 wt.% CeO₂ additions that also had the highest bulk density (4.48 g/cm³) and the lowest percentage of porosity (0.37%).     

Fabrication and mechanical properties of SiCw/MoSi2–SiC composites by liquid Si infiltration of pyrolyzed rice husk preforms with Mo additions

Dense SiC ceramic matrix composites containing SiC whiskers (SiC_w) and MoSi₂ phase (SiC_w/MoSi₂–SiC) are fabricated by a liquid Si infiltration (LSI) method. Pyrolyzed rice husks (RHs) containing SiC whiskers, particles and amorphous carbon are mixed with different amounts of Mo powder to form preforms for the infiltration. Microstructure and mechanical properties of the composites are studied. Fracture mode of the composites is discussed. Results show that the SiC whiskers and fine particles in the pyrolyzed RHs were preserved in the composites after the LSI process. The amorphous carbon and Mo powder in the preforms reacted with molten Si, forming SiC and MoSi₂ in the composites. The presence of MoSi₂ in the composite increases the elastic modulus but lowers the flexure strength. Content of MoSi₂ of ca. 20 wt.% provides an enhanced fracture toughness of 4.1 MPa m^1/2 for the composite. But too large amount of MoSi₂ caused crack formation in the composite. The compressive residual stress introduced by the formation of MoSi₂ and SiC, and the de-bonding of the fine SiC particles and SiC whiskers from the residual Si phase are considered to favor the fracture toughness of the composites.     

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.