Thermal and tensile properties of polysialate composites

Polysialate, or geopolymer, composites have gained interest due to their inherent high temperature resistance, low density and ease of manufacturing. These characteristics also suggest that polysialate composites have significant potential as materials in high temperature structures, although little is known about their thermal and mechanical properties. This study aimed to determine relevant thermal and mechanical properties over a representative temperature range. The results show that polysialate composites can exhibit stable thermal properties up to 1000 °C. Tensile properties up to 760 °C highlight a significant reduction in stiffness, but a retention of strength, at these temperatures. The thermal and mechanical results achieved provide strong evidence that polysialate composites can be suitable for use in high temperature structures, whilst subsequently providing an understanding of their limitations. In addition to this, the values ascertained also provide the data required for the design and modelling of next generation high temperature structures.     

Environmental effects on the strength and impact damage resistance of alumina based oxide/oxide ceramic matrix composites

In this study the effects of high temperature and moisture on the impact damage resistance and mechanical strength of Nextel 610/alumina silicate ceramic matrix composites were experimentally evaluated. Composite laminates were exposed to either a 1050°C isothermal furnace-based environment for 30 consecutive days at 6 h a day, or 95% relative humidity environment for 13 consecutive days at 67°C. Low velocity impact, tensile and short beam strength tests were performed on both ambient and environmentally conditioned laminates and damage was characterized using a combination of non-destructive and destructive techniques. High temperature and humidity environmental exposure adversely affected the impact resistance of the composite laminates. For all the environments, planar internal damage area was greater than the back side dent area, which in turn was greater than the impactor side dent area. Evidence of environmental embrittlement through a stiffer tensile response was noted for the high temperature exposed laminates while the short beam strength tests showed greater propensity for interlaminar shear failure in the moisture exposed laminates. Destructive evaluations exposed larger, more pronounced delaminations in the environmentally conditioned laminates in comparison to the ambient ones. External damage metrics of the impactor side dent depth and area directly influenced the post-impact tensile strength of the laminates while no such trend between internal damage area and residual strength could be ascertained.     

Carbon fiber reinforced ceramic matrix composites with an oxidation resistant boron nitride interface coating

Toughening a ceramic in a ceramic matrix composite (CMC) depends on an ability of the composite to tolerate an accumulation of matrix cracks. When the reinforcement phase is carbon fiber, these cracks leave the fiber susceptible to destructive oxidation by ingress of air during high temperature exposure. Generally, a graphitic carbon interface coating is applied to carbon fibers because it provides for a weak bond between fiber and matrix that is required to promote toughening. This investigation seeks to utilize a BN coating instead of a C coating in order to promote oxidation resistance. Like graphitic carbon, BN is soft and easily cleavable. Preliminary observations that C/BN/SiC CMC's using Toray T300 carbon fibers were highly brittle and of low strength lead to a requirement of heat treating the fibers prior to the CVD of BN for toughened composites to be fabricated. It is likely heat treating removed reactive functionalities from the fiber surface to yield a weakly adhered and compliant interface.     

MgO-SiC-C复合材料力学性能和抗热震性能研究

对MgO-SiC-C复合材料的力学性能和抗热震性能的研究结果表明SiC含量增加,材料的强度和抗热震性能提高。升温过程中结合剂结构的变化对MgO-SiC-C复合材料强度变化起重要作用。     

炭/陶复合材料的抗热震性能研究

对含石墨的炭／陶复合材料优良的抗热震性能进行了讨论。这种性质与石墨的导热系数大、断裂功高、热膨胀和弹性模量小密切相关。     

Ablation behavior and mechanism of SiCf/Cf/SiBCN ceramic composites with improved thermal shock resistance under oxyacetylene combustion flow

The ablation properties and mechanisms (under oxyacetylene combustion) together with thermal shock behavior of SiC_f/C_f/SiBCN ceramic composites were investigated. The solid ablation products are primarily amorphous SiO₂ and cristobalite. The primary ablation mechanisms include fiber and ceramic matrix oxidation, evaporation of B₂O₃ (l) and SiO₂ (l), and mechanical exfoliation. SiC_f/C_f/SiBCN has a significantly low mass ablation rate and a desirable linear ablation rate. The combination of crack deflection caused by SiC and carbon fibers, fiber pull-out and debonding improves thermal shock resistance and thus leads to the absence of surface macrocracks.     

High temperature flexural strength,microstructure, phase evolution and anti-oxidation mechanism of Al-coated carbon fiber/boron phenolic resin ceramizable composite modified with TiB2 and B4C

Carbon fiber/phenolic resin composites (CF/Ph) have attracted great interests in the field of thermal protection materials for their characteristics of high specific strength and easy manufacturing. However, CF/Ph are inherently susceptible to oxidation failure at elevated temperatures. In this study, a novel Al-coated carbon fiber/boron phenolic resin ceramizable composite modified with TiB₂ and B₄C was fabricated by an impregnating and compression molding route. Thermal stability, flexural strength, microstructure and phase evolution of the resulting ceramizable composite were studied. The residue yield at 1400 °C and flexural strength after treated at 1400 °C for 15min was 90.4% and 53.1 MPa, respectively, which was increased by 15.9% and 532.1% than that without ceramizable fillers. Surface defects generated by matrix pyrolysis were well healed, and PyC and carbon fibers were covered with dense ceramic layers while the fracture surface was covered with relatively continuous ceramic layers without visible pores. Multiphase ceramics composed of TiB₂, TiO₂, TiC and PyC were identified. Furthermore, oxidation failure and anti-oxidation mechanism was revealed based on the aforementioned characterizations and thermodynamic calculation results. Oxidation resistance got enhanced markedly for synergistic effects of oxygen consuming, carbon fixation, oxygen barrier and endothermic effect, which were derived from ceramization reactions between TiB₂, B₄C, O₂, Al and PyC.     

High temperature deformation of non-directionally solidified Al2O3/YAG/ZrO2 eutectic bulk ceramic

Excellent high temperature mechanical properties of melt-grown Al₂O₃-based eutectics have previously been demonstrated in samples prepared by directional solidification methods. In this study, the deformation behaviour of melt-grown Al₂O₃/YAG/ZrO₂ eutectic bulk prepared by a non-directional solidification method was investigated by means of compressive tests in a temperature range of 1200–1700 °C. The non-directionally solidified eutectic bulk ceramic has a colony structure and is polycrystalline. It begins to show ductility and has a compressive strength of 320 MPa at 1500 °C, which is much higher than that of the sintered ceramic with the same composition. However, its plastic deformability is insufficient, even at 1700 °C (just below the melting point of 1715 °C), and cracking occurs during compressive deformation.     

Thermal shock resistance of the porous boron nitride/silicon oxynitride ceramic composites

The thermal shock resistance of the porous boron nitride/silicon oxynitride (BN/Si₂N₂O) ceramic composites were tested by the quenching‐strength method with temperature differences of 600‐1400°C. The residual flexural strength of the composites decreased with increasing temperature difference from 600°C to 900°C. This weakening in flexural strength was attributed to the formation of microcracks in the matrix caused by thermal stress damage. Afterward, as the formation of a dense oxidized layer sealed the surface and hindered further oxidation, the residual flexural strength increased with the further increase of temperature difference from 900°C to 1100°C. Finally, when the temperature differences were above 1100°C, the residual flexural strength gradually decreased with increasing temperature difference, which was attributed to the further oxidation and large thermal stress damage. And the thermal shock resistance of the porous BN/Si₂N₂O ceramic can be improved by the introduction of high contents of sintering aids and h‐BN.     

Fatigue behavior and oxidation resistance of carbon/ceramic composites reinforced with continuous carbon fibers

The aim of this work was to compare fatigue behavior and oxidation resistance of pitch-derived CC (carbon) composite with CC/ceramic (carbon/ceramic) composites obtained by impregnation of CC composite with polysiloxane-based preceram and their subsequent heat treatment. Two types of CC/ceramic composites were studied; CC/SiCO composite obtained at 1000 °C, and CC/SiC composite obtained at 1700 °C. Both types of composites show much better fatigue mechanical performance in comparison to pure CC composite. CC/SiCO composite had 3 times better fatigue properties, and CC/SiC composite 4.5 times better fatigue properties than the reference CC composite. After a fatigue test composites partially retain their mechanical properties, and normalized residual modulus in the direction perpendicular to laminates exceeds 50% for CC and CC/SiCO composites. In the other directions normalized residual modulus is higher than 80% for all composites. Oxidative tests led at 600 °C in air atmosphere indicated oxidation resistance of CC/SiC composites.     

Enhanced mechanical properties,thermal shock resistance and ablation resistance of Si2BC3N ceramics with nano ZrB2 addition

The mechanical properties, thermal shock resistance, and ablation resistance of nano ZrB₂ modified Si₂BC₃N ceramics were investigated. The results show that ZrB₂ stimulated microstructure evolution obviously. Therefore, the maximum strength and fracture toughness reach 559.6 MPa and 6.77 MPa·m^1/2, which are improved by 61.0% and 29.4%, respectively. Furthermore, the residual strengths of 10 wt% ZrB₂ containing composites tested at 1000 ℃ retain 363.6 MPa, which is much higher than 97.7 MPa of pristine Si₂BC₃N ceramics. Besides, the ablation resistance of ZrB₂ modified Si₂BC₃N ceramics at 3000 ℃ is enhanced remarkably and the linear and mass ablation rates of ZrB₂-10 are only 0.009 mm/s and 1.91 mg/s, respectively. The ablation in the ultra-high temperature zone is totally dominated by the ZrB₂ component, and the thermochemical erosion is determined by the oxidation resistance of ZrB₂ in the thermal affected zone.     

Improvement on the phase stability,mechanical properties and thermal insulation of Y2O3-stabilized ZrO2 by Gd2O3 and Yb2O3 co-doping

Gd₂O₃ and Yb₂O₃ co-doped 3.5 mol% Y₂O₃–ZrO₂ and conventional 3.5 mol% Y₂O₃–ZrO₂ (YSZ) powders were synthesized by solid state reaction. The objective of this study was to improve the phase stability, mechanical properties and thermal insulation of YSZ. After heat treatment at 1500 °C for 10 h, 1 mol% Gd₂O₃–1 mol% Yb₂O₃ co-doped YSZ (1Gd1Yb-YSZ) had higher resistance to destabilization of metastable tetragonal phase than YSZ. The hardness of 5 mol% Gd₂O₃–1 mol% Yb₂O₃ co-doped YSZ (5Gd1Yb-YSZ) was higher than that of YSZ. Compared with YSZ, 1Gd1Yb-YSZ and 5Gd1Yb-YSZ exhibited lower thermal conductivity and shorter phonon mean free path. At 1300 °C, the thermal conductivity of 5Gd1Yb-YSZ was 1.23 W/m K, nearly 25% lower than that of YSZ (1.62 W/m K). Gd₂O₃ and Yb₂O₃ co-doped YSZ can be explored as a candidate material for thermal barrier coating applications.     

Microstructure, mechanical properties and thermal shock resistance of plasma sprayed nanostructured zirconia coatings

Nanostructured yttria stabilized zirconia (YSZ) coatings were deposited by Atmospheric Plasma Spraying (APS). X-ray diffraction (XRD) was used to investigate their phase composition, while scanning electron microscopy (SEM) was employed to examine their microstructure. The coatings showed a unique and complex microstructure composed of well-melted splats with columnar crystal structure, partially melted areas, which resembled the morphology of the powder feedstock, and equiaxed grains. Vickers microhardness of nanostructured zirconia coatings was similar to that of the conventional ones and strongly depended on the indentation load. Otherwise, a higher thermal shock resistance was found. This effect was addressed to the retention of nanostructured areas in coating microstructure and to the corresponding high porosity.     

Mechanical properties of graphene platelet-reinforced alumina ceramic composites

Alumina (Al₂O₃) ceramic composites reinforced with graphene platelets (GPLs) were prepared using Spark Plasma Sintering. The effects of GPLs on the microstructure and mechanical properties of the Al₂O₃ based ceramic composites were investigated. The results show that GPLs are well dispersed in the ceramic matrix. However, overlapping of GPLs and porosity within ceramics are observed. The flexural strength and fracture toughness of the GPL-reinforced Al₂O₃ ceramic composites are significantly higher than that of monolithic Al₂O₃ samples. A 30.75% increase in flexural strength and a 27.20% increase in fracture toughness for the Al₂O₃ceramic composites have been achieved by adding GPLs. The toughening mechanisms, such as pull-out and crack deflection induced by GPLs are observed and discussed.     

HfB2-CNTs composites with enhanced mechanical properties prepared by spark plasma sintering

HfB₂-x vol%CNTs (x=0, 5, 10, and 15) composites are prepared by spark plasma sintering. The influence of CNTs content and sintering temperature on densification, microstructure and mechanical properties is studied. Compared with pure HfB₂ ceramic, the sinterability of HfB₂-CNTs composites is remarkably improved by the addition of CNTs. Appropriate addition of CNTs (10 vol%) and sintering temperature (1800 °C) can achieve the highest mechanical properties: the hardness, flexural strength and fracture toughness are measured to be 21.8±0.5 GPa, 894±60 MPa, and 7.8±0.2 MPa m^1/2, respectively. This is contributed to the optimal combination of the relative density, grain size and the dispersion of CNTs. The crack deflection, CNTs debonding and pull-out are observed and supposed to exhaust more fracture energy during the fracture process.     

纳米BN复合SiC材料的抗热震性研究

将分析纯H3BO3、CO(NH2)2按物质的量比为1∶2.5溶于无水乙醇中,搅拌过程中按质量分数为80%加入平均粒径0.2μm的β-SiC,在850℃氮气中(纯度99.99%,压力为0.92~0.93 MPa)反应15 h制得纳米BN复合SiC粉体,然后在0.92~0.93 MPa N2气氛中以30 MPa轴向压力于1 750~1 800℃保压0.5~1 h热压烧结上述粉体,制成纳米BN复合SiC试样,采用三点弯曲及透射电镜、扫描电镜等方法研究了纳米BN复合SiC材料的抗热震性。结果表明:在SiC材料中引入纳米BN,一方面可以降低材料的弹性模量,有利于抗热震性的提高。另一方面由于基体SiC与第二相六方氮化硼(h-BN)的热膨胀系数相差较大,热失配作用导致h-BN晶粒发生晶间脱层,在复相陶瓷材料内产生许多微孔,这些微孔的存在可以有效缓解由于高温引起的热膨胀作用,从而极大地改善材料的抗热震性。     

低碳MgO-C材料的抗热震性研究

以电熔大结晶镁砂、天然鳞片石墨、纳米炭黑、酚醛树脂、铝粉等为主要原料制备w(C)=3%的低碳MgO-C材料,以其抗热震性为考核指标,选取颗粒级配、复合抗氧化剂、石墨粒度和复合结合剂4个因素,进行了四因素三水平正交试验。结果表明:在本试验范围内,颗粒级配是影响低碳MgO-C材料抗热震性的主要因素,复合抗氧化剂次之,石墨粒度和复合结合剂的影响基本相当;通过极差分析确定,镁砂颗粒级配(3～1、1～0.088和≤0.088mm的镁砂的质量比)采用50:23:27,复合抗氧化剂采用Al2.5+Mg-Al0.5+B4C0.5,石墨粒度采用10μm的,复合结合剂采用炭黑N220+沥青+酚醛树脂,可制备出抗热震性最佳的低碳MgO-C材料。     

Mechanical properties and thermal shock resistance of Si2BC3N ceramics with ternary Al4SiC4 additive

Dense Si₂BC₃N ceramics were prepared through SPS sintering the amorphous Si₂BC₃N and Al₄SiC₄ powders obtained from mechanical alloying. The phase compositions, microstructures, and mechanical properties, as well as the thermal shock resistance were investigated. In addition, evaluations of oxidation and the ablation resistance were also preceded. The results show that Al₄SiC₄ phase can be detected at 1200 and 1400?°C under pressureless sintering. However, Al₄SiC₄ can be decomposed to AlN and SiC phases under higher temperatures. As for the bulk Si₂BC₃N ceramics, the Al₄SiC₄ additive induce the development of turbostratic BN(C) plates and improve the relative density consequently. Besides, the Al₄SiC₄ plates are embedded in the matrix of ceramics. Therefore, the mechanical properties and thermal shock resistance are improved apparently with the addition of additive. Meanwhile, the additive containing composites have superior ablation resistance than the pristine Si₂BC₃N ceramics due to their higher relative density.