High temperature mechanical properties of Al2O3/TiC micro–nano-composite ceramic tool materials

A type of Al₂O₃-based composite ceramic tool material simultaneously reinforced with micro-scale and nano-scale TiC particles was fabricated by the hot-pressing technology with different contents of cobalt additive. The effects of cobalt on the ambient temperature mechanical properties and high temperature flexural strength were investigated. The flexural strength and fracture toughness of the composite with 3 vol% cobalt as a function of temperature were investigated. Cobalt greatly enhanced the ambient temperature flexural strength and fracture toughness, while further increasing the content of cobalt led to a dramatic strength degradation, especially at high temperature. The flexural strength of the composite containing 3 vol% cobalt decreased as the temperature increased from 20 to 1200 °C, and the fracture toughness decreased as a function of the temperature up to 1000 °C but increased at 1200 °C. The degradation of high temperature flexural strength was ascribed to the change of the fracture mode, the grain and grain boundary oxidation, the decrease of elastic modulus and the grain boundary sliding.     

Heat conduction mechanisms in hot pressed ZrB2 and ZrB2–SiC composites

Thermal diffusivity and conductivity of hot pressed ZrB₂ with different amounts of B₄C (0–5 wt%) and ZrB₂–SiC composites (10–30 vol% SiC) were investigated experimentally over a wide range of temperature (25–1500 °C). Both thermal diffusivity and thermal conductivity were found to decrease with increase in temperature for all the hot pressed ZrB₂ and ZrB₂–SiC composites. At around 200 °C, thermal conductivity of ZrB₂–SiC composites was found to be composition independent. Thermal conductivity of ZrB₂–SiC composites was also correlated with theoretical predictions of the Maxwell–Eucken relation. The dominated mechanisms of heat transport for all hot pressed ZrB₂ and ZrB₂–SiC composites at room temperature were confirmed by Wiedemann–Franz analysis by using measured electrical conductivity of these materials at room temperature. It was found that electronic thermal conductivity dominated for all monolithic ZrB₂ whereas the phonon contribution to thermal conductivity increased with SiC contents for ZrB₂–SiC composites.     

Boron-modified phenol formaldehyde resin-based self-healing matrix for Cf/SiBOC composites

C_f/SiBOC was fabricated from 2D carbon fabric as reinforcement and slurry-containing boron-modified phenol formaldehyde (BPF) resin with silicon as matrix resin using reaction-bonded silicon carbide method. The processing involves synthesis of (BPF) resin by reacting various amount of boric acid with phenol formaldehyde resin, polymer to ceramic transformation at 1450°C under argon atmosphere, with and without silicon, thermal transformation of the polymer matrix composite into a ceramic matrix composite and evaluation of isothermal oxidation for ceramics and its composites at 1000, 1250 and 1500°C. The ceramic studies, confirmed the formation of B₄C, SiC and SiB₄ (SiBOC) mixed phase and the role of boron as a catalyst for graphitisation of free carbon present in the ceramic. Oxidation of C_f/SiBOC composite at various temperatures leads to the formation of borosilicate glass which heals the cracks, hindering the inwards diffusion of oxygen.     

Influence of the fiber surface properties on the mechanical strength of unidirectional fiber composites

The influence of the thermodynamic adhesion between fibers and matrix on the mechanical properties of a continuous fiber reinforced composite is studied for two systems: carbon fiber reinforced poly(ether ether ketone) and glass fiber reinforced poly(ether imide). The fibers are modified chemically and characterized by measuring the contact angle formed by molten resin on the fibers. Various fiber treatments yield a wide range of contact angles, which are determined optically. Unidirectional fiber reinforced laminates are manufactured and transverse flexural strength is measured with the values reported as a function of the specific work of adhesion. It is shown that adhesion at the fiber-resin interface correlates with both the composite strength and the void morphology within the laminate after consolidation.     

纤维缠绕成型工艺中张力控制系统的研究

纤维缠绕中张力的控制是关系到产品质量的关键技术。本文主要通过对张力产生的分析，论述了分别采用磁粉离合器和力矩电机作为控制元件，对张力进行有效的两种控制方案。     

碳纤维及其复合材料在国外军民领域的应用

介绍了碳纤维（CF：carbon fiber）及碳纤维增强复合材料（CFRP：carbon fiber minfomed plastics）在国外发达国家军事领域和民用领域的应用。     

空气气氛中烧成刚玉-氮化硅复合材料的研究

对刚玉-氮化硅复合材料在氮气中埋炭、空气中埋炭和空气 3种气氛中加热时可能发生的、对烧结有影响的重要反应进行了热力学分析,然后按m(Al2O3 ) : m(Si3N4 ) =62. 5: 37. 5的配比制成试样,在上述 3种气氛中于 1600℃保温 2h烧成,并对烧成后试样的性能、相组成以及空气气氛中烧成后试样的显微结构进行了分析。结果表明:在氮气中埋炭或空气中埋炭烧成时,材料的烧结情况没有在空气气氛中的好;在氮气中埋炭和空气中埋炭烧成后试样中分别生成了少量的β SiAlON或SiC,而在空气气氛中烧成后试样中出现了莫来石、石英和X相,残留了较多的β Si3N4;在空气气氛中烧成后,试样断面明显分为氧化层和非氧化层,其间有一沉积致密层。     

Collapsed chains as models for filler particles in a polymer melt

Simulations of dense melts of coarse-grained chains have been modified so that they contain filler particles. Since the filler particles and matrix chains are constructed from the same repeat unit, all of the intermolecular energetic interactions in the system (filler-filler, filler-matrix, matrix-matrix) are identical. The collapse of individual chains to form filler particles is achieved by a simple modification in the strength of the minimum in the Lennard-Jones potential governing pair-wise intramolecular interactions within a filler particle. Even when completely collapsed, the filler particles retain mobility in their internal degrees of freedom. Their centers of mass are also mobile. The filler particles can be collapsed completely to dense, impenetrable objects, but they can also be collapsed incompletely to produce permeable filler particles.There is no evidence for spontaneous aggregation of impermeable filler particles, but sufficiently permeable filler particles can aggregate. The parameters used in the simulations insure that the aggregation cannot be energetically driven. Matrix chains that fill space within a permeable filler particle have severe restrictions placed on their available conformations. The reduction in the conformational entropy of the matrix chains can be alleviated if the permeable filler particles interpenetrate, or aggregate. Then fewer matrix chains must enter the permeable filler particles in order to maintain the density of the system. The simulation detects no aggregation of impermeable filler particles because it is not necessary for matrix chains to enter completely collapsed particles.     

A model for composites containing three-dimensional ellipsoidal inclusions

A model is developed for the mechanical properties of composites containing complex inclusions with no axes of symmetry, e.g. three dimensional ellipsoids (a₁>a₂>a₃) characterized by two aspect ratios, α=a₁/a₃ and β=a₁/a₂, by using the Eshelby's equivalent tensor with a Mori-Tanaka type model.The influences of the primary and secondary aspect ratios on the effective elastic moduli of nanocomposites containing aligned isotropic inclusions are examined. The model is limited to unidirectionally aligned inclusions where both the matrix and the inclusions have linearly elastic, homogeneous properties. The longitudinal moduli (E₁₁, E₂₂ and E₃₃) and the shear moduli (μ₁₂, μ₁₃ and μ₂₃) are calculated. The longitudinal Young's modulus E₁₁ increases, as the primary and secondary aspect ratios increase. However, the transverse Young's modulus E₂₂ and shear modulus μ₁₂ decrease, as the secondary aspect ratio increases.     

Effects of adhesive type and polystyrene concentration on the shear strength of bonded polystyrene/high-density polyethylene blends

The lap-shear strengths of adhesively bonded polystyrene (PS), high-density polyethylene (HDPE), and their blends, were studied as a function of adhesive type and blend composition. The performance of virgin and recycled polymer systems was examined. The lap-shear strength depended strongly on the amount of PS in the blend and the type of adhesive, and the acrylic adhesives demonstrated the best performance for all compositions. Bonded strengths of HDPE increased by approximately 50% when HDPE was blended with 34% PS, the co-continuous composition. The results indicate that structural elements made from PS/HDPE immiscible blends may be effectively bonded with adhesives without expensive surface treatments.