Polystyrene/Polyolefin Elastomer Blends Loaded with Halloysite Nanotubes: Morphological,Mechanical, and Gas Barrier Properties

Herein, a simple melt-blending method is utilized to disperse of halloysite nanotubes (HNTs) in polystyrene/polyolefin elastomer (PS/POE) blends. Based on morphological studies, the PS/POE/HNT nanocomposite containing up to 3 phr HNTs shows excellent nanofiller dispersion, while those filled with 5 phr HNTs exhibit nanofiller aggregation. To overcome the nanofiller aggregation issue, the polypropylene-grafted-maleic anhydride (PP-g-MA) compatibilizer is added to the PS/POE/HNT nanocomposite, which results in improved mechanical properties for the nanocomposite sheets. Furthermore, the addition of compatibilized HNTs to the PS/POE blends leads to decreased O₂ and N₂ gas permeabilities. Besides, incorporating POE, HNTs, and PP-g-MA leads to a decrease in water vapor transmission of PS. In the end, the experimentally-determined mechanical properties and gas permeabilities of the nanocomposite sheets are compared to those predicted by prevalent theoretical models, revealing a good agreement between the experimental and theoretical results. Molecular-dynamics simulations are also carried out to calculate the gas diffusion coefficients in the different sheets to further support the experimental findings in this study. Overall, the PS/POE/HNT/PP-g-MA nanocomposite sheets fabricated in this work demonstrate excellent mechanical and gas barrier properties; and hence, can be used as candidate packaging materials. However, the strength of the resulting PS/POE blend may be inferior to that of the virgin PS.     

How Phosphorous Flame Retardant Additives Affect Benzoxazine-Based Monomer and Polymer Properties

The phosphorous-based flame retardant additives poly(m-phenylene methylphosphonate) (PMP) and resorcinol bis(diphenyl phosphate) (RDP) are reacted with bisphenol F and aniline–based benzoxazine (BF-a). DSC, rheological analysis, FT-IR, and soxhlet extraction reveal the covalent incorporation of both FR additives—initiating phenols in PMP structure as well as free phenols generated via transesterification reaction in the case of RDP. In contrast to PMP, RDP elongates the processing window but decreases the thermo–mechanical properties. Both additives increase the resistance in reactions against small flames with solely a phosphorous loading of 0.3 wt%, resulting in a V-0 rating and an improvement in the OI value by up to 2% for RDP and 4% for PMP. Both FRs reduce the heat release rate but increase the smoke production and the smoke toxicity in the case of RDP.     

Fatigue resistance and damage mechanisms of 2D woven SiC/SiC composites at high temperatures

Fatigue resistance and damage mechanisms of 2D woven SiC/SiC composites at high temperatures were investigated in this research. Fatigue behavior tests were performed at 1200℃ and 1000°C at 10 Hz and stress ratio of 0.1 for maximum stresses ranging from 80 to 120 MPa, and the fatigue run-out could be defined as 10⁶ cycles. Evolution of the cumulative displacement and normalized modulus with cycles was analyzed for each fatigue condition. Fatigue run-out was achieved at 80 MPa and 1000°C. It could be found that the cycle lifetimes of the composites decreased sharply with the increasing maximum stress and temperature conditions significantly affected the fatigue performance under matrix cracking stress. The cumulative displacement showed no noticeable increase before 1000 cycles and the modulus of the failed specimens decreased before fracture. The retained properties of composites that achieved fatigue run-out, as well as the microstructures, were characterized in order to understand the fatigue behavior and failure mechanisms. The composites exhibited similar fracture morphology with matrix crack extension and glass phase oxidation formation under different conditions. In general, the high-temperature fatigue damage and failure of composites could be affected by combination of stress damage and oxidative embrittlement.     

ZrB2-based solid solution ceramics and their mechanical and thermal properties

Zirconium diboride ceramics as one of the main members of ultrahigh-temperature ceramics are capable of being used as structural components at ultrahigh temperatures. Entropy adjusting is a newly developed approach to improving the properties of ceramics. In this work, a series of ZrB₂-based solid solution ceramics with different mixing entropies, formulated (Zr_xTi_yNb_yTa_y)B₂ (x = .25, .85, .925, .9625, 1; x + 3y = 1), were prepared by adjusting the content of other diborides. Diboride solid solution powders were synthesized by boro/carbothermal reduction process and then densified by spark plasma sintering. The results show that the formation of a single-phase solid solution is independent of the mixing entropy in (Zr_xTi_yNb_yTa_y)B₂ system. The addition of other diborides into ZrB₂ is beneficial to reduce the particle size of the synthesized powder and promote the densification process. The dense sintered samples with higher mixing entropy have finer grain size, higher hardness, and modulus. The (Zr_0.25Ti_0.25Nb_0.25Ta_0.25)B₂ ceramic has the highest hardness of 31 GPa and a modulus of 682 GPa. Severe lattice distortion in samples with higher mixing entropy will result in increased phonon scattering and lower thermal conductivity.     

Preparation of Si3N4 ceramic based on digital light processing 3D printing and precursor infiltration and pyrolysis

The emergence of digital light processing (DLP) 3D printing technology creates favorable conditions for the preparation of complex structure silicon nitride (Si₃N₄) ceramics. However, the introduction of photosensitive resin also makes the Si₃N₄ ceramics prepared by 3D printing have low density and poor mechanical properties. In this study, high-density Si₃N₄ ceramics were prepared at low temperatures by combining DLP 3D printing with precursor infiltration and pyrolysis (PIP). The Si₃N₄ photocurable slurry with high solid content and high stability was prepared based on the optimal design of slurry components. Si₃N₄ green parts were successfully printed and formed by setting appropriate printing parameters. The debinding process of printed green parts was further studied, and the results showed that samples without defects and obvious deformation can be obtained by setting the heating rate at .1°C/min. The effect of the PIP cycle on the microstructure and mechanical properties of the Si₃N₄ ceramics was studied. The experimental results showed that the mass change and open porosity of the samples tended to be stable after eight PIP cycles, and the open porosity, density, and bending strength of the Si₃N₄ ceramics were 1.30% (reduced by 97%), 2.64 g/cm³ (increased by 43.5%), and 162.35 MPa.     

Cutting performance improvement of Si3N4 ceramic cutting tools by adding boride

Cutting performances of silicon nitride (Si₃N₄) ceramic cutting tools with and without boride additive (2.5 vol% ZrB₂ or TiB₂) prepared by hot-pressing at 1500°C were investigated. Due to the α- to β-Si₃N₄ phase transformation and low densification temperature, boride-containing Si₃N₄ ceramics with high hardness and high toughness were obtained. The turning tests showed that the effective cutting lengths of the Si₃N₄–2.5 vol% TiB₂ ceramic (∼2480 m) and Si₃N₄–2.5 vol% ZrB₂ ceramic (∼2200 m) were higher than the monolithic Si₃N₄ ceramic (∼1780 m). As the toughness was improved while maintaining relative high hardness, the cutting performances of the boride-containing Si₃N₄-based inserts were improved by adding 2.5 vol% ZrB₂ or TiB₂. The improved cutting performance indicated that the boride-containing Si₃N₄ ceramics are expected to be used in the field of ceramic cutting tools.     

Spark plasma sintering of boron nitride micron tubes reinforced boron carbide ceramics with excellent mechanical property

In this paper, the novel boron nitride micron tubes (BNMTs) were used to reinforce commercial boron carbide (B₄C) ceramics prepared via spark plasma sintering technology. The effects of the sintering parameters, sintering temperature, the holding time, and the BNMTs content on the microstructure and mechanical properties of B₄C/BNMTs composite ceramics were studied. The results indicated that adding a proper amount of BNMTs could inhibit the grain growth of B₄C and improve the fracture toughness of the B₄C/BNMTs composite ceramics. The prepared composite ceramic sample with 5 wt% BNMTs at 1850°C, 8 min and 30 MPa displayed the best mechanical properties. The relative density, hardness, fracture toughness, and bending strength of the samples were 99.7% ± .1%, 35.62 ± .43 GPa, 6.23 ± .2 MPa m^1/2, and 517 ± 7.8 MPa, respectively. Therein, the corresponding value of hardness, fracture toughness, and bending strength was increased by 10.3%, 43.59%, and 61.5%, respectively, than that of the B₄C/BNMTs composite ceramic without BNMTs. It was proved that the high interface binding energy and bridging effect between boron carbide and BNMTs were the toughening principle of BNMTs.     

Mechanical properties of B4C–CrB2 binary eutectic composites prepared by arc-melting

B₄C–CrB₂ composites were prepared by arc-melting using B₄C and CrB₂ powders as raw materials. The eutectic composition of B₄C–CrB₂ system was 30B₄C–70CrB₂ (mol%) with a labyrinth-like irregularly layered eutectic microstructure, composed of B₄C phase about 1–2 μm in thickness dispersing in CrB₂ matrix, much smaller than raw powders. The interface of the eutectic composite was well bonded, and there were edge dislocations at the interface to alleviate the interface mismatch. The eutectic temperature of B₄C–CrB₂ composites was approximately 2200 K. At the eutectic composition, the B₄C–CrB₂ composites showed the maximum Vickers hardness (24.6 GPa) and fracture toughness (4.3 MPa m^1/2) at room temperature.     

The effect of Ti and Si on the mechanical and electrochemical behaviors of the AlCrN coating deposited by CAPVD on 304SS

This study aimed to investigate the effect of adding titanium (Ti) and silicon (Si) elements on the mechanical and electrochemical properties of the AlCrN-based coating. For this purpose, a cathodic arc physical vapor deposition machine was used. Scanning electron microscopy, X-ray diffraction, and nanoindentation tests were utilized for morphological, microstructural, and mechanical characterization of the coatings. The hardness value and plastic deformation index of CrAlN-based coating increase with the presence of Si element. The mechanical properties improvement is attributed to the reduction of crystallite size as well as to the tendency of the coating structure to become amorphous. The specimens were subjected to 3.5 wt% NaCl solution to electrochemical impedance corrosion and potentiodynamic polarization tests. The results showed that by increasing the coatings’ titanium content, the coatings’ corrosion resistance improved. Moreover, by adding 3% and 5% of Si elements to the coatings’ composition, the corrosion resistance of the AlCrTiSiN coatings was enhanced by 35% and 78%, respectively. Improving the corrosion resistance of the AlCrN-based coatings by adding the Si element is attributed to the change in the microstructure and reduction in the porosity of the coatings.     

Effect of BN nanoparticle content in SiC matrix on microstructure and mechanical properties of SiC/SiC composites

BN-nanoparticle-containing SiC-matrix-based composites comprising SiC fibers and lacking a fiber/matrix interface (SiC/BN + SiC composites) were fabricated by spark plasma sintering (SPS) at 1800°C for 10 min under 50 MPa in Ar. The content of added BN nanoparticles was varied from 0 to 50 vol.%. The mechanical properties of the SiC/BN + SiC composites were investigated thoroughly. The SiC/BN + SiC composites with a BN nanoparticle content of 50 vol.%, which had a bulk density of 2.73 g/cm³ and an open porosity of 5.8%, exhibited quasiductile fracture behavior, as indicated by a short nonlinear region and significantly shorter fiber pullouts owing to the relatively high modulus. The composites also exhibited high strength as well as bending, proportional limit stress, and ultimate tensile strength values of 496 ± 13, 251 ± 30, and 301 MPa ± 56 MPa, respectively, under ambient conditions. The SiC fibers with contents of BN nanoparticles above 30 vol.% were not severely damaged during SPS and adhered to the matrix to form a relatively weak fiber/matrix interface.