Toughness and R-curve behaviour of laminated Si3N4/SiCw ceramics

Laminated Si₃N₄/SiC_w ceramics were successfully prepared by tape casting and hot-pressing. Its mechanical properties were measured and the impact resistance was discussed. The toughness of the laminated Si₃N₄/SiC_w ceramics was 13.5 MPa m^1/2, which was almost 1.6 times that of Si₃N₄/SiC_w composite ceramics, namely 8.5 MPa m^1/2. Moreover, the indentation strength of laminated Si₃N₄/SiC_w ceramics was not sensitive to increasing indentation loads and exhibited a rising R-curve behaviour, indicating that the laminated Si₃N₄/SiC_w ceramics had excellent impact resistance. The improved toughness and impact resistance of laminated Si₃N₄/SiC_w ceramics was attributed to the residual stress caused by a thermal expansion coefficient mismatch between the different layers, resulting in crack deflection and bridging of SiC whiskers in the interface layer, thus consuming a large amount of fracture work.     

Low-temperature sintered (Ti,Zr, Nb,Ta, Mo)C-based composites toughened with damage-free SiCw

The high sintering temperature would have a great tendency to damage the morphology and thus properties of the silicon carbide whisker (SiC_w) in high entropy carbide-silicon carbide whisker (HEC-SiC_w) composites, which, in turn, would impact the effectiveness of the operative toughening mechanisms. The objective of this study was to achieve full contributions to the toughening effects of SiC_w by preparing (Ti, Zr, Nb, Ta, Mo)C-SiC_w composites at low temperature (1600 ℃) using cobalt as additives. Results showed that the fracture toughness of the (Ti, Zr, Nb, Ta, Mo)C bulk reinforced with 20 vol% SiC_w and 5 vol% Co was 7.2 MPa?m^1/2, which was much higher than that of the (Ti, Zr, Nb, Ta, Mo)C bulk only sintered with 5 vol% Co (3.4 MPa?m^1/2). Meanwhile, it was also higher than that of the reported HEC-20 vol% SiC_w composite sintered at 2000 ℃ (4.3 MPa?m^1/2). For the fracture toughness of HEC-SiC_w composites, it was significantly increased by the introduction of damage-free SiC_w.     

Effects of SiC amount and morphology on the properties of TiB2-based composites sintered by hot-pressing

This investigation intended to assess the influence of SiC morphology on the sinterability and physical-mechanical features of TiB₂-SiC composites. For this aim, different volume percentages of SiC particles and SiC whiskers were introduced to TiB₂ samples hot-pressed at 1950 °C for 2 h under an external pressure of 25 MPa. The characterization of as-sintered specimens was carried out using X-ray diffraction, optical microscopy, and scanning electron microscopy. The relative density studies revealed that SiC_w had a more significant impact on the sinterability of TiB₂-based composites. The XRD investigation confirmed the production of an in-situ TiC phase during the hot-pressing; however, some peaks related to the graphitized carbon also appeared in the patterns of SiC_w-doped ceramics. The addition of 25 vol% SiC_p halved the average grain size of TiB₂ while introducing the same content of SiC_w decreased this value by just around 20%. Finally, the highest Vickers hardness and fracture toughness were obtained for the sample reinforced with 25 vol% SiC_w, standing at 29.3 GPa and 6.1 MPa m^1/2, respectively.     

Microstructure and fracture toughness of SiAlCN ceramics toughened by SiCw or GNPs

Mechanical alloying and spark plasma sintering (SPS) were used to prepare dense SiAlCN ceramic and SiAlCN ceramic toughened by SiC whiskers (SiC_w) or graphene nanoplatelets (GNPs). The influences of different reinforcements on the microstructure and fracture toughness were investigated. The SiAlCN ceramic exhibited a fracture toughness of 4.4 MPa m^1/2 and the fracture characteristics of grain bridging, alternative intergranular and transgranular fracture. The fracture toughness of SiC_w/SiAlCN ceramic increased to 5.8 MPa m^1/2 and toughening mechanisms were crack deflection, SiC_w bridging and pull-out. The fracture toughness of GNP/SiAlCN ceramic increased significantly, which was up to 6.6 MPa m^1/2. GNPs played an important role in grain refinement, which resulted in the smallest grain size. Multiple toughening mechanisms, including crack deflection, crack branch, GNP bridging and pull-out could be found. The better toughening effect could be attributed to the larger specific surface area of GNPs and the appropriate interface bonding between GNPs and matrix.     

An amphoteric water‐soluble copolymer. II. Effect of its molecular weight on the dispersion of barium titanate in water

An amphoteric water‐soluble copolymer, that is, polyacrylamide/(α‐N,N‐dimethyl‐N‐acryloyloxyethyl)ammonium ethanate (PAAM/DAAE) was synthesized and it showed the ability to disperse BaTiO₃ (BT) particles in aqueous solutions. In this work, the effect of molecular weight of this polymer on the dispersing properties was further examined. The results indicate that the effectiveness of three polymer samples with different molecular weights in the dispersion of BT particles is P2 (M_w = 1.1 × 10⁵) > P1 (M_w = 1.2 × 10⁴) > P3 (M_w = 3.0 × 10⁵). Apparently, P2 is most effective in dispersing the particles, reducing the viscosity of the suspensions, and obtaining highest green and sintered densities. This is attributed to the highest adsorption of this polymer onto BT powder, and causes strongest electrostatic and steric repulsions. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 886–891, 2006     

Seed-mediated growth of ZrC whisker and its toughening effect on ZrB2-SiC-C ceramic

ZrC whiskers (ZrC_w) hold great promise in improving the strength and toughness of ultra-high temperature ceramics (UHTCs) without reducing their high-temperature stability. However, obtaining high quality ZrC_w has been challenging. Herein, we propose a novel method for easily synthesizing catalyst-free ZrC_w by seed-mediated growth technique, in which single crystal ZrC nanoparticle (ZrC_np) was used as seed crystal and ZrO₂-C-NaF mixture was used as precursor system. The effect of ZrC_np, NaF, and, synthesis temperature on the growth of ZrC_w was studied and the reaction process was analyzed based on the experimental results. The vapor-solid (V-S) growth mechanism was proved to be the dominating growing mechanism. Subsequently, the synthesized ZrC_w were added to a ZrB₂-SiC-C ceramic. Compared with the baseline, ZrC_w reinforced ZrB₂-SiC-C exhibited a remarkable combination of high strength and high toughness (592 ± 30 MPa and 7.1 ± 0.8 MPa·m^1/2). This novel synthesis method of ZrC_w may be applicable to the synthesis of other carbide ceramic whiskers and enrich the design of high performance ultra-high temperature ceramics.     

Improvement of toughness of reaction bonded silicon carbide composites reinforced by surface-modified SiC whiskers

To improve the reliability, especially the toughness, of the reaction bonded silicon carbide (RBSC) ceramics, silicon carbide whiskers coated with pyrolytic carbon layer (PyC-SiC_w) by chemical vapor deposition (CVD) were introduced into the RBSC ceramics to fabricate the SiC_w/RBSC composites in this study. The microstructures and properties of the PyC-SiC_w/RBSC composites under different mass fraction of nano carbon black and PyC-SiC_w were investigated methodically. As a result, a bending strength of 550 MPa was achieved for the composites with 25 wt% nano carbon black, and the residual silicon decreased to 11.01 vol% from 26.58 vol% compared with the composite of 15 vol% nano carbon black. The fracture toughness of the composites reinforced with 10 wt% PyC-SiC_w, reached a high value of 5.28 MPa m^1/2, which increased by 39% compared to the RBSC composites with 10 wt% SiC_w. The residual Si in the composites deceased below to 7 vol%, resulting from the combined actively reaction of nano carbon black and PyC with more Si. SEM and TEM results illustrated that the SiC_w were protected by PyC coating. A thin SiC layer formed of outer surface of whiskers can provide a suitable whisker-matrix interface, which is in favor of crack deflection, SiC_w bridging and pullout to improve the bending strength and toughness of the SiC_w/RBSC composites.     

Processing and properties of polycrystalline cubic boron nitride reinforced by SiC whiskers

Dense polycrystalline cBN (PcBN)–SiCw composites were fabricated by a two-step method: First, SiO₂ was coated on the surface of cubic boron nitride (cBN) particles by the sol-gel method. Then, silicon carbide whisker (SiC_w)- coated cBN powder was prepared by carbon thermal reaction between SiO₂ and carbon powders at 1500°C for 2 hour. Then, cBN–SiC_w complex powders were sintered by high-pressure and high-temperature sintering technology using Al, B, and C as sintering additives. The phase compositions and microstructures of cBN–SiC_w composites were investigated by X-ray diffraction and scanning electron microscopy, respectively. It was found that the SiC_w and Al₃BC₃ had been fabricated by in situ reaction, which cannot only promote densification but also improve mechanical properties. The relative density of PcBN composites increased from 96.3% to 99.4% with increasing SiC_w contents from 5 to 20 wt%. Meanwhile, the Vickers hardness, fracture toughness and flexural strength of as-obtained composites exhibited a similar trend as that of relative density. The composite contained 20 wt% of SiC_w exhibited the highest Vickers hardness and fracture toughness of 42.7 ± 1.9 GPa and 6.52 ± 0.21 MPa•m^1/2, respectively. At the same time, the flexural strength reached 406 ± 21 MPa.     

The effect of the type of electric current on the cathodic corrosion of aluminium

A comparative experimental study on the cathodic corrosion of aluminium in 0.52 M sodium chloride distilled water solutions is carried out. The electrolysis is conducted using a single half-cycle rectified, direct or industrial frequency current. Characteristic relationships concerning the cathodic corrosion are noted. Attention is drawn to the higher rates of cathodic corrosion observed on electrolysis with single half-cycle rectified current which is combined with lower energy costs.Nomenclature w _k1 cathodic corrosion rate for direct current electrolysis (g s^–1 m^–2) - w_k2 cathodic corrosion rate for single half-cycle rectified current electrolysis (g s^–1 m^–2) - w _a anodic dissolution rate (g s^–1 m^–2) - w _F theoretical Faradaic dissolution rate (g s^–1 m^–2) - w dissolution rate for alternating current electrolysis (g s^–1 m^–2) - j electric current density (A m^–2)     

Effects of SiC whiskers on the mechanical properties and microstructure of SiC ceramics by reactive sintering

As-received and pre-coated SiC whiskers (SiC_w)/SiC ceramics were prepared by phenolic resin molding and reaction sintering at 1650 °C. The influence of SiC_w on the mechanical behaviors and morphology of the toughened reaction-bonded silicon carbide (RBSC) ceramics was evaluated. The fracture toughness of the composites reinforced with pre-coated SiC_w reached a peak value of 5.6 MPa m^1/2 at 15 wt% whiskers, which is higher than that of the RBSC with as-received SiC_w (fracture toughness of 3.4 MPa m^1/2). The surface of the whiskers was pre-coated with phenolic resin, which could form a SiC coating in situ after carbonization and reactive infiltration sintering. The coating not only protected the SiC whiskers from degradation but also provided moderate interfacial bonding, which is beneficial for whisker pull-out, whisker bridging and crack deflection.     

Room temperature living cationic polymerization of styrene with HX-styrenic monomer adduct/FeCl3 systems in the presence of tetrabutylammonium halide and tetraalkylphosphonium bromide salts

Living cationic polymerization of styrene was achieved with a series of initiating systems consisting of a HX-styrenic monomer adduct (X = Br, Cl) and ferric chloride (FeCl₃) in conjunction with added salts such as tetrabutylammonium halides (nBu₄N⁺Y⁻; Y⁻ = Br⁻, Cl⁻, I⁻) or tetraalkylphosphonium bromides [nR′₄PBr; R′ = CH₃CH₂-, CH₃(CH₂)₂CH₂-, CH₃(CH₂)₆CH₂-] or tetraphenylphosphonium bromide [(C₆H₅)₄PBr] in dichloromethane (CH₂Cl₂) and in toluene. Comparison of the molecular weight distributions (MWDs) of the polystyrenes prepared at different temperatures (e.g., −25 °C, 0 °C and 25 °C) showed that the polymerization is better controlled at ambient temperature (25 °C). The polymerization was almost instantaneous (completed within 1 min) and quantitative (yield ∼100%) in CH₂Cl₂. In CH₂Cl₂, polystyrenes with moderately narrow (M_w/M_n ∼ 1.33-1.40) and broad (M_w/M_n ∼ 1.5-2.4) MWDs were obtained respectively with and without nBu₄N⁺Y⁻. However, in toluene, the MWDs of the polystyrenes obtained respectively with and without nBu₄N⁺Y⁻/nR′₄P⁺Br⁻ were moderately narrow (M_w/M_n = 1.33-1.5) and extremely narrow (M_w/M_n = 1.05-1.17). Livingness of this polymerization in CH₂Cl₂ was confirmed via monomer-addition experiment as well as from the study of molecular weights of obtained polystyrenes prepared simply by varying monomer to initiator ratio. A possible mechanistic pathway for this polymerization was suggested based on the results of the ¹H NMR spectroscopic analysis of the model reactions as well as the end group analysis of the obtained polymer.     

Effects of the composition and molecular weight of maleimide polymers on the dispersibility of carbon nanotubes in chloroform

We synthesized maleimide polymers (MIPs) as effective carbon nanotube (CNT) dispersants for stable CNT-dispersed solutions. The MIPs were random copolymers which consist of N-phenyl maleimide (PhMI) units that imparted strong physical adsorption to the CNT surfaces and methyl methacrylate units; we evaluated the effects of the PhMI content and molecular weight in the MIPs on the dispersibility of the CNTs in chloroform. Increasing the PhMI content in the MIPs increased the grafting amount (GA) (g m⁻²) of MIPs on the CNT surfaces and led to a large increase of CNT dispersion. In particular, when single-walled carbon nanotubes (SWCNTs) were used, MIPs having 100 mol% PhMI units (MIP100s) were imperative for a drastic increase in the GA value. Increasing the weight average molecular weight (M_w) of the MIPs resulted in no significant effect on CNT dispersibility; however, the particularly low M_w (?10,000) of the MIPs dramatically improved CNT dispersion. Therefore, MIP100s with particularly low M_w’s were extremely effective for dispersing various CNTs with different diameters. In the case of SWCNTs, much more effective exfoliation of SWCNT bundles was achieved by MIP100s with particularly low M_w’s than MIP100s with higher M_w’s.     

Mechanical properties of hybrid multilayer graphene/whisker-reinforced ceramic composites: Simulation and experimental investigation

The trial-and-error method used in ceramics research has certain limitations such as the high blindness of material component design. Moreover, calculations of the toughness of ceramics using the extended finite element method, which is the most broadly applied technique, are complicated. To overcome these issues, in this study, multilayer graphene (MLG)/Si₃N₄ whisker (Si₃N_4w)-reinforced Si₃N₄ ceramics (MWSCs) were used as the model material, and the modeling of MWSCs was conducted using Voronoi tessellation. Additionally, a more concise novel approach was applied for the prediction of the fracture toughness of MWSCs. Furthermore, the optimal MLG and Si₃N_4w contents were predicted, and then they were verified by fabricating MWSCs using spark plasma sintering (SPS). Simulation results indicated that the optimum MLG and Si₃N_4w contents to enable the toughness and hardness to reach the maximum values (9.87 MPa·m^1/2 and 23.19 GPa) were 1 wt% and 3 wt%, which were consistent with the experimental results. Consequently, the effectiveness of the proposed method was verified. Moreover, the experimental values of the maximum fracture toughness and hardness were 11.04 MPa·m^1/2 and 20.29 GPa, which were 47.20% and 12.10% higher than those of Si₃N₄ ceramics reinforced with 1 wt% MLG, respectively. The synergistic toughening effects of MLG and Si₃N_4w were significantly reflected. The load-bearing effect, bridging, and crack deflection induced by MLG and Si₃N_4w were the key reasons for the improvement in the mechanical properties of MWSCs.     

Fabrication of SiCw reinforced ZrB2-based ceramics

ZrB₂-based ceramics with SiC_w were produced by hot pressing at 1750 °C for 1 h from mixed powders after adding liquid polycarbosilane. The obtained ZrB₂-SiC_w composites had toughness up to 7.57 MPa m^1/2, which was much higher than those for monolithic ZrB₂, SiC particles reinforced ZrB₂ composites, and other ZrB₂–SiC_w composites directly sintered at high temperatures. The added liquid polycarbosilane could reduce the sintering temperatures and restrict the reaction of matrix with whisker, which led to fewer damages to the whisker and high fracture toughness.     

Determination and correction of peak broadening in size exclusion chromatography of controlled rheology polypropylene

The peak broadening in size exclusion chromatography of seven commercial polystyrene (PS) standards with narrow molar mass distribution (MMD) and of 6–9 polypropylene (PP) fractions (M?_w/M?_n = 1.34–2.10) obtained by direct extraction was determined in four different column sets. The dependence of the Gaussian peak broadening parameter, σ², on the peak elution volume was different for PS and PP. For a commercial grade of controlled rheology PP (CR-PP) (Daplen PT55) with M?_w/M?_n < 3 it was shown that disregard of the peak broadening effect leads to considerable errors in the MMD at low and higher molar mass and in the polydispersity parameter M?_w/M?_n. The same lot was studied in an Austrian National Research program for its physical propeties in processing and application and in a round-robin test of IUPAC working party IV.2.2. on molecular characterization of commercial polymers. © 1994 John Wiley & Sons, Inc.     

Polymerization of Vinyl Acetate Initiated by a Copper Alginate Coordination Polymer Film/Na2SO3/H2O System

A hydrophobic film of copper alginate coordination polymer was prepared by soaking a sodium alginate film in a 13% CuCl₂ aqueous solution at room temperature for more than 24 h. The composition, structure, and property of copper alginate as a coordination catalyst were studied by ESR, IR, XPS, and electric conductivity methods. It was shown that a low-spin copper complex was formed as a result of the acceptance by dsp² hybrid orbitals of nonpaired electrons transferred from oxygen atoms of two carbonyl hydroxyl groups and negatively charged oxygen atoms of two deprotonated carbonyl hydroxyl groups of two alginate chains. The coordination number of the central Cu²⁺ ions in copper alginate is 4. It can be concluded that the vacant site on the active catalysis resulted from the distorted tetragonal configuration that was caused by a crimped chain of the alginate molecule. HSO ₃ ^– produced primary free-radical hydrogen by a coordination catalysis mechanism and vinyl acetate (VAc) polymerization by a free-radical mechanism that is different from polymerization initiated by a CuCl₂–Na₂SO₃–H₂O oxidation–reduction system. The induction period for VAc polymerization is 90 s and the yield is 82%. The M _w, M _n, and M _w/M _n of poly(vinyl acetate) (PVAc) were found to be 1.23×10⁶, 2.27×10⁵, and 4.51, respectively.     

Heating and cooling of flocculated and non-flocculated slurries in tubular heat exchangers

The distinctly different turbulent heat transfer characteristics of flocculated kaolin slurries are compared with nonflocculated coal slurries via modified J-factor correlation forms for tubular exchangers. Both slurry types were statistically best-fitted with the approximate solution Bingham plastic flow model in preference to the local shear power law. In addition, the first known turbulent slurry heat transfer cooling results are presented along with the heating results. No distinction was made between heating and cooling correlation forms because of a random scatter of experimental points. Experimental results were interpreted as follows A. Flocculated kaolin slurries J_H = (h/C_pG) (C/K₁)^2;3= 0.027(DVρ/μLe^B?)^0.2 11%ω < %ω Solids < 24%ω Maximum Deviations: + 19.3%, ? 17.8% B. Non-flocculated coal slurries J_w = (h/C_pG)(C_pμLe^B?/k_w)^2/3e^?B?/4) =0.0285 (DVρ/ μLe^B?)^?0.2 13%_w > %w Solids < 33.5%. Maximum Deviations: + 17.8%, - 11.1%     

Relations between Wallace plasticity and Mw for natural rubber

Wallace plasticity and the weight‐average molar mass (M_w) were measured on natural rubber samples of different origins. A sigmoidal model describes the relations between Wallace plasticity and M_w (0.872 > r² > 0.992) for given families of samples. The families of samples analyzed differed through their clonal origin, collection method (cup lumps or latex), and type of processing (CV or non‐CV). This study showed that two samples of natural rubber can be identical in terms of plasticity, but very different in terms of the average polyisoprene chain length or M_w. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 3078–3087, 1999     

Efficient multiscale strategy for toughening HfB2 ceramics: A heterogeneous ceramic–metal layered architecture

A multiscale structural design was innovatively adopted herein to increase the toughness of monolithic HfB₂ ceramics. SiC whiskers (SiC_w) and graphene oxide (GO) were used as fillers for the HfB₂ matrix, whereas a ductile W foil was introduced as an interlayer to synthesize laminated HfB₂-SiC_w-rGO/W ceramics. Monolithic HfB₂-SiC_p (particulate) and laminated HfB₂-SiC_p/W ceramics were prepared using the same routes and used as controls. Following tape casting and spark plasma sintering at 1800°C, the toughness of the prepared laminated HfB₂-SiC_w-rGO/W samples was increased to 14.2 ± 0.6 MPa·m^1/2, with minimal sacrifice in flexural strength (421 ± 16 MPa). Morphological analysis of the fracture surface revealed the synergistic effects of micro-toughening (including bridging and pullout of whiskers and rGO) and macro-toughening (including crack deflection, bifurcation, and delamination) mechanisms responsible for improving the fracture toughness of the laminated HfB₂-SiC_w-rGO/W composites.     

Fast seamless joining of SiCw/Ti3SiC2 composite using electric field-assisted sintering technique

A pair of Ti₃SiC₂ reinforced with SiC whiskers (SiC_w/Ti₃SiC₂) composites was successfully joined without any joining materials using electric field-assisted sintering technology at a temperature as low as 1090°C (T_i) and a short time of 30 s. The microstructure and mechanical properties of the obtained SiC_w/Ti₃SiC₂ joints were investigated. The solid-state diffusion was the main joining mechanism, which was facilitated by a relatively high current density (~586 A/cm²) at the joining interface. The shear strength of the sample joined at 1090°C was 51.8 ± 2.9 MPa. The sample joined at 1090°C failed in the matrix rather than at the interface, which confirmed that a sound inter-diffusion bonding was obtained. A rapid and high efficient self-joining process may find application in the case of SiC_w/Ti₃SiC₂ sealing cladding tube and end cap.