Microstructural influence on the R-curve behavior of a 70%Si3N4-30% barium aluminum silicate self-reinforced composite

Fracture toughness and crack growth resistance (R-curve) behavior of several microstructures of 70%Si₃N₄-30%BAS composite are characterized using the indentation and modified compact tension methods, at room temperature. Both the whisker morphology and matrix properties contribute to the fracture resistance behavior. Crack deflection, whisker bridging and pullout are regarded as the dominant toughening mechanisms.     

Preparation and mechanical properties of self-reinforced in situ Si3N4 composite with La2O3 and Y2O3 additives

Self-reinforced in situ Si₃N₄ composite material was prepared with high amount of La₂O₃ and Y₂O₃ additives by two-step hot pressing, and the optimum amount of additives was determined. The volume fraction of boundary glass phase was calculated based on the equilibrium of equivalent number in chemical reaction. For material with 15 mol% additives, flexural strength and fracture toughness at room temperature were 960 MPa and 12.3 MPa m^1/2, respectively. At temperature of 1350°C, flexural strength was maintained to 720 MPa and fracture toughness was significantly increased to 23.9 MPa m^1/2 because of the high refractory of oxynitride glass containing compositions of La and Y. Self-reinforced mechanism was mainly responsible for crack deflection along the elongated β-Si₃N₄ grains.     

Preparation and dielectric properties of Si3N4/SiCw composite ceramic

Gelcasting was employed to fabricate Si₃N₄/SiC whisker (SiCw) composite ceramics, and the effects of heat-treatment temperature on the length-to-diameter ratio of the whiskers and SiCw content on microwave dielectric properties were studied. Compared with pure SiCw of spherical structure obtained at temperature of 1,750 °C(Ar), pure SiCw treated at 1,600 °C(Ar) showed rod-like structure, higher dielectric properties and more evenly distribution in Si₃N₄/SiCw composite ceramics. Both the real (ε′) and imaginary (ε″) permittivity of Si₃N₄/SiC whisker (SiCw) composite ceramics decreased with increasing frequency and increased as the whisker content raised owing to the interface and SiCw playing a role of dipole in the frequency range of 8.2–12.4 GHz. In addition, comparing the ceramics with lower content of SiCw, the reflectivity of the composite ceramics moved to a lower frequency; the maximum absorption peak reached ?22.4 dB at the whisker content of 15 wt%.     

Surface modification of hydroxyapatite to introduce interfacial bonding with polyactiveTM 70/30 in a biodegradable composite

A method was developed to improve the interfacial bonding between hydroxyapatite and a biodegradable copolymer Polyactive^TM 70/30. Hydroxyapatite was first surface modified by the polyelectrolytes polyacrylic acid or poly(ethylene-co-maleic acid) in aqueous solutions. Subsequently the surface-modified hydroxyapatite was used as filler in composites with Polyactive^TM 70/30. The strength, elongation at break and elastic modulus of the composite in aqueous environment were significantly improved by this method. Based on these experimental results, it is believed that the interface improvement is due to hydrogen bonding and/or dipole interactions formed between polyelectrolyte molecules and polyethylene glycol segments in the polymer matrix. Due to the introduction of interfacial bonding by using such method, a new biodegradable bone-bonding composite can be made.     

Interface structure and mechanical properties of Al2O3-20 vol% ZrO2-20 vol% SiCW ceramic composite

The microstructure, mechanical properties, fracture behaviour and toughening mechanisms of Al₂O₃-20 vol% ZrO₂ (2 mol% Y₂O₃)-20 vol% SiC_W ceramic matrix composite were investigated by X-ray diffraction, scanning and transmission electron microscopies, energy dispersive analysis of X-rays, high-resolution electron microscopy techniques and three-point bending tests. The results show that the Al₂O₃ matrix is simultaneously strengthened and toughened by both ZrO₂ particles and SiC whiskers. The interfacial amorphous layers between SiC whiskers and ZrO₂, and Al₂O₃ grains were observed by both TEM dark-field and high-resolution electron microscopy techniques.     

Influence of porosity on the high temperature oxidation of a SiC-reinforced Si3N4 ceramic composite

In this work the oxidation kinetics of a porous (21.5% porosity) nitride bonded silicon carbide containing 20% SiC was investigated in the temperature range of 1100–1400 °C. Two oxidation stages were found which corresponded to (i) a rapid parabolic oxidation rate during short term oxidation exposure (0–10 hours), and (ii) a parabolic scale growth after some exposure time. During short term oxidation a continuous oxide film developed, but it was unable to block internal oxidation at the pore channels. An experimental activation energy of 55 kJ/mol was obtained which was attributed to intially rapid external and internal oxidation through the open pore network. After 1–10 hours depending on the actual temperature, closure of the surface pores through oxidation lead to a transition where a continuous SiO₂ scale grew on the outer sample surface through diffusion. An activation energy of 132 kJ/mol was associated with this parabolic growth suggesting that inward oxygen diffusion through the SiO₂ scale was rate limiting. Metallographic observations indicated severe cracking of the scale developed. This was attributed to the relatively large shrinkage (1%) associated with the – cristobalite transformation occurring at temperatures below 250 °C. Moreover, X-ray diffraction indicated the presence of cristobalite and tridymite, but it was unable to identify a discontinuous phase developed beneath the SiO₂ scale during oxidation.     

Physical and mechanical properties of highly textured polycrystalline Nb4AlC3 ceramic

Abstract

Highly textured polycrystalline Nb₄AlC₃ ceramic was fabricated by slip casting in a strong magnetic field followed by spark plasma sintering. Its Lotgering orientation factor was determined on the textured top and side surfaces as f_(00l) ～1.0 and f_(hk0)=0.36, respectively. This ceramic showed layered microstructure at the scales ranging from nanometers to millimeters. The as-prepared ceramic had excellent anisotropic physical properties. Along the c-axis direction, it showed higher hardness, bending strength, and fracture toughness of 7.0 GPa, 881 MPa and 14.1 MPa m^1/2, respectively, whereas higher values of electrical conductivity (0.81×10⁶ Ω^?1 m^?1), thermal conductivity (21.20 W m^?1 K^?1) and Young’s modulus (365 GPa) were obtained along the a- or b-axis direction.     

Microstructural features of sintered Si3N4/SiC-whiskers composites: mechanical integrity of whiskers

A detailed microstructural analysis of slip-cast Si₃N₄/SiC-whisker composites has been made by TEM. In spite of the gentle forming process, SiC whisker breakage constitutes a fundamental feature of this material. The breaking takes place during the sintering process and could be associated with residual thermal stresses, as revealed by the experimental evidence. The dramatic decrease in the whiskers aspect ratio translates into little or no effect of whisker addition on the mechanical properties of the Si₃N₄ matrix.     

Microstructure and mechanical properties of a Si3N4/Al2O3 nanocomposite

A nanocomposite material fabricated by hot pressing in the form of nanometre-sized Si3N4 particles dispersed in an Al2O3 matrix has been shown to exhibit enhanced mechanical properties compared with monolithic matrix material. It was observed by transmission electron microscopy (TEM) for the first time that the alumina grains were in the shape of elongated columns with aspect ratios in the range 2.5–4. The presence of liquid phase during sintering was found to be responsible for the appearance of columnar grains. Regular hexagon-shaped larger -Sialon grains formed during sintering were mainly situated at grain boundaries of the matrix material while irregular smaller dispersoids were trapped within the alumina grains. The improvement in the mechanical properties of the nanocomposite is attributed to the change in fracture mode from intergranular fracture to transgranular fracture, the self-reinforcement effect arising from the elongated columnar grains of the matrix, as well as the pinning effect due to the existence of intergranular -sialon particles. It was revealed that the trapped particles have an -Al2O3 structure with partial sites of aluminium and oxygen atoms substituted by silicon and nitrogen atoms, which is also likely to lead to the strengthening of the composite.     

The mechanical behaviour of a cross-weave ceramic matrix composite

In order to find out whether or not a cross-weave ceramic composite, graphite fibre/SiC matrix, would be prone to fatigue failure, tests in pulsating tension and pulsating compression have been carried out with the weave oriented in the 0/90 configuration. Both types of testing (at fairly high fractions of the ultimate monotonic failure load) cause creep strain, which is frequency dependent in tension, and ultimately complete failure can occur. Damage in pulsating tension is found to consist of cumulative microcracking and spalling, with the final failure mechanism broadly similar to that in monotonic deformation. Damage in pulsating compression appears dominated by delamination.     

机械振动对SHS陶瓷内衬复合管显微结构和力学性能的影响

通过在重力分离SHS法制备陶瓷内衬复合管过程中施以不同振幅与振频的单自由度上下往复式机械振动 ,研究了机械振动对SHS铝热燃烧、陶瓷凝固及复合管组织性能的影响 .研究发现 ,机械振动并相应提高振频可以提高燃烧温度、燃烧速率和反应转化率 ,促进Al2 O3 Fe液相重力分离和陶瓷致密过程 ,并使陶瓷层的凝固组织发生改变 ;性能测试结果表明机械振动并相应提高振频可以提高复合管的各项力学性能指标和内衬陶瓷层的表面质量 .     

In vitro mechanical and drug release properties of bioabsorbable ciprofloxacin containing and neat self-reinforced P(L/DL)LA 70/30 fixation screws

Osteomyelitis is inflammation of the bone caused by a pathogenic organism. Both its acute and chronic forms are difficult to heal. Antibiotics are still the basic treatment for osteomyelitis. Bioabsorbable ciprofloxacin containing bone fixation screws based on self-reinforced (SR) copolylactide P(L/DL)LA 70/30 have been developed for local treatment of bone infections. These screws gradually released ciprofloxacin during the in vitro bulk degradation of the matrix polymer and at the same time have sufficient mechanical strength. All the loaded ciprofloxacin was released from the gamma sterilized screws during 44 in vitro weeks and the concentration of the released drug per day remained between 0.06 and 8.7 g/ml after the start-up burst peak.     

SiC_W含量对SiC_W/Si_3N_4复合陶瓷力学及高温微波性能影响

以α-Si_3N_4粉、β-SiC_W为原料,Al_2O_3、Y_2O_3为烧结助剂,采用凝胶注模工艺制备了SiC_W/Si_3N_4复合陶瓷材料,烧结温度为1 650℃,保温1.5h。研究了SiC_W加入含量对SiC_W/Si_3N_4复合陶瓷的微观结构、力学及常温/高温微波吸收性能的影响。结果表明:随着SiC_W含量的增加,SiC_W/Si_3N_4复合陶瓷的抗弯强度和断裂韧性都有先増后减的趋势,当含量为10wt%时,抗弯强度达到最大值505MPa,断裂韧性达9.515MPa·m1/2。常温介电常数在SiC_W含量为10wt%时,实部达最大值12,在12GHz最大吸收值为-21dB。高温介电常数随着SiC_W含量的增加有先增后减的趋势,在含量为10wt%时,实部达到最大值12.5。相比于纯Si_3N_4陶瓷,当SiC_W含量为10wt%时,SiC_W/Si_3N_4复合陶瓷在11.7GHz左右最大吸收可达-27dB,有效吸收频带(小于-5dB)为11.2~12.3GHz。     

Interface fracture properties of a bimaterial ceramic composite

In this investigation, the interface fracture toughness is measured for a pair of ceramic clays which are joined together. The Brazilian disk specimen, which provides a wide range of mode mixity, is employed to measure these properties. Calibration equations relating the stress intensity factors to the applied load and geometry are determined by means of the finite element method and the M-integral. The effect of residual stresses is accounted for by employing a weight function to obtain the contribution to the stress intensity factors. Total stress intensity factors are obtained by superposition. These are employed to determine the critical interface energy release rate as a function of mode mixity from critical data obtained from tests carried out on the Brazilian disk specimens. An energy release rate fracture criterion is compared to the experimental results for .