Effect of SiC whiskers on the oxidation protective properties of SiC coatings for carbon/carbon composites

In order to effectively employ the unique high temperature mechanical properties of carbon/carbon composite substrates, SiC coatings reinforced by SiC whiskers were prepared by pack cementation method. The effect of SiC whiskers on the oxidation resistance properties of the single-layer coating and double-layer coating was investigated. SiC whiskers in the single-layer SiC coating have little effect on the anti-oxidation property but obviously improve the thermal shock property. The double-layer coating with inner-layer reinforced coating exhibits more perfect anti-oxidation ability than the double-layer coating with SiC inner-layer coating.

Synergistically Toughening Effect of SiC Whiskers and Nanoparticles in Al_2O_3-based Composite Ceramic Cutting Tool Material

In recent decades, many additives with different characteristics have been applied to strengthen and toughen Al2O3-based ceramic cutting tool materials. Among them, SiC whiskers and SiC nanoparticles showed excellent performance in improving the material properties. While no attempts have been made to add SiC whiskers and SiC nanoparticles together into the ceramic matrix and the synergistically toughening effects of them have not been studied. An Al2O3-SiCw-SiC np advanced ceramic cutting tool material is fabricated by adding both one-dimensional SiC whiskers and zero-dimensional SiC nanoparticles into the Al2O3 matrix with an effective dispersing and mixing process. The composites with 25 vol% SiC whiskers and 25 vol% SiC nanoparticles alone are also investegated for comparison purposes. Results show that the Al2O3-SiCw-SiCnp composite with both 20 vo1% SiC whiskers and 5 vol% SiC nanoparticles additives have much improved mechanical properties. The flexural strength of Al2O3-SiCw-SiCnp is 730＋ 95 MPa and fracture toughness is 5.6 ± 0.6 MPa.m1/2. The toughening and strengthening mechanisms of SiC whiskers and nanoparticles are studied when they are added either individually or in combination. It is indicated that when SiC whiskers and nanoparticles are added together, the grains are further refined and homogenized, so that the microstructure and fracture mode ratio is modified. The SiC nanoparticles are found helpful to enhance the toughening effects of the SiC whiskers. The proposed research helps to enrich the types of ceramic cutting tool and is benefit to expand the application range of ceramic cutting tool.

Effect of whisker alignment on microstructure,mechanical and thermal properties of Mg-SiC_(w)/Cu composite fabricated by a combination of casting and severe plastic deformation(SPD)

In this research,microstructure evaluation,mechanical properties and thermal conductivity of the Mg-SiC_(w)/Cu composite with laminar structure were investigated.For this purpose,SiC whiskers were added to magnesium alloy by using stir-casting,then the Mg-SiC_(w)composite was bonded to copper layers by warm accumulative roll bonding(ARB).Based on the results of optical microscopy(OM)and scanning electron microscopy(SEM),SiC whiskers were well distributed in the magnesium matrix and they were aligned parallelly when the composites were plastically deformed at higher rolling passes.Furthermore,all layers remained continuous with localized necking sites.Also,no intermetallic compounds and phases were detected by XRD and EDS analyzes.Apart from the significant effect of severe plastic deformation on mechanical properties,the findings of mechanical tests point to the usefulness of reinforcements in improving up to 60%microhardness,Young’s modulus,yield,and up to 41%tensile strengths.Further,thermal conductivities of composites increased by adding reinforcement and above all by increasing the number of rolling passes.This growth is attributed to the higher thermal diffusivity of copper and whiskers as well as the increased number of conductive layers within composite.

Synthesis and characteristics of SiC whiskers with "rosary bead" morphology

SiC whiskers with "rosary bead" morphology were synthesized using suitable silicon source and carbon source through solid reaction at the temperature above 1537 K. The diameter and length of the SiC whiskers were about 0.1-1.0 μm and 20-100 μm, respectively. The largest diameter of their enlarged ends of the whiskers was about 0.2-1.0μm, and it gradually and smoothly decreased to the size of the plain part of the whiskers. The results of X-ray diffraction analysis show that the crystalline structure of the obtained SiC whiskers is β-SiC. It is considered that the SiC whiskers grow via a vapor-solid mechanism.

SiC whisker reinforced MoSi_2 composite prepared by spark plasma sintering from COSHS-ed powder

SiC whisker reinforced MoSi2 composite powder was synthesized by a novel process, referred to the literature as chemical oven self-propagating high temperature synthesis(COSHS). The as-prepared SiCw/MoSi2 composite powder was rapidly sintered by spark plasma sintering(SPS) process. The sintering temperature and pressure were 1 723 K at heating rate of 100 K/min and 40 MPa, respectively. The microstructure and mechanical properties of the composite were investigated. Relative densities of the monolithic material and composite are 95% and 99.3%, respectively. SEM micrographs of SiCw/MoSi2 composite show that SiC whiskers homogeneously distribute in MoSi2 matrix. The composite containing SiC whisker has higher Vicker hardness than monolithic MoSi2. Especially the room-temperature fracture toughness of the composite is higher than that of MoSi2, from 3.6 MPa·m1/2 for MoSi2 to 7.7 MPa·m1/2 for composite with 15% SiC(volume fraction), increased by 113.9%. The morphology of propagation of crack and fractured surface of composite reveal the mechnaism to improve fracture toughness of MoSi2 matrix. The results show that the in-situ SiCw/MoSi2 composite powder prepared by COSHS technique can be successfully sinterded through SPS process and significant improvement of low temperature fracture toughness can be achieved.

Study on Microstructures of Micron Si_3N_4 Composite Ceramic with Addition of Nano SiC and SiC Whisker

The microstructures and crack propagating characteristic of Si 3N 4 (μ)/SiC (n, w) composite ceramic were studied with AEM. The Si 3N 4 (μ)/SiC (n, w) composite ceramic consists of β Si 3N 4, β SiC, a small amount of α Si 3N 4 and α SiC, and amorphous phase. Most of Si 3N 4 grains were equiaxed crystal and also there were some bulky columnar ones. Most of SiC particles and SiC whiskers distributed at the Si 3N 4 grain boundaries and a few of smaller SiC particles in the Si 3N 4 grains. Most of amorphous structure was in the junction of several Si 3N 4 grains and thin amorphous layer was observed only at a few of Si 3N 4 boundaries. Failured cracks propagated mainly along the boundaries of the Si 3N 4 grains and partially passed through Si 3N 4 grains. The path of crack propagating might change, branching and twisting of the cracks might occur when the expanding crack meet the SiC particle and/or SiC whisker. Effect of the microstructure on strength and toughness of the composite ceramic was briefly discussed.

温度和催化剂种类对稻壳粉合成SiC晶须的影响

以稻壳粉为原料,硝酸钴、硝酸铁为催化剂,采用催化法合成SiC晶须,研究了热处理温度和催化剂种类对合成SiC晶须含量和形貌的影响。研究结果表明,在硝酸镍或硝酸铁的催化作用下,1200℃是稻壳合成SiC的温度转变点。热处理温度的升高,有利于促进稻壳中SiO_(2)和C反应,提高SiC晶须生成量和晶粒尺寸。相较于添加硝酸钴,硝酸铁的添加有利于提高SiC晶须生成量,并且SiC晶须长径比增大,SiC晶须主要遵循VS生长机制。

FRACTURE BEHAVIOUR OF SiC WHISKER REINFORCED Al COMPOSITES

The dynamic tensile processes of silicon carbide whisker reinforced Al composite(SiC_w/Al) with 22% V_f have been in situ observed with a scanning electron microscope equipped with a tensile stand.The results show that a number of microcracks have formed as the tensile stress increases to 180～190 MPa.Microcracks form in most cases at the whisker ends.The frac- ture behaviour of SiC_w/Al composite observed has been explained theoretically.

THE EFFECT OF SiC PARTICLE AND WHISKER ON THE STRENGTHOF ALUMINUM ALLOY MATRIX COMPOSITES

The effects of SiC particle and whisker on the strength of aluminum alloy matrix composites have been discussed. The effects of the characteristics of discontinuons reinforcements on the strength of compsites was revealed on the basis of a review of the elastic-plastic finite element method which was applied to predict the stress-strain constituent relationship for particle and whisker reinforeed MMCs. It is snggested that the reinforcement shape or morphology, aspect ratio and distribution in the matrix should be considered in order to get more accumte numerical results. Theoretical strength analysis models predicted the trend of the strength for particle MMCs, but the theoretical results are generally lower compared with the experimental results, especially for the lower strength aluminum alloy. This is becaase strength theories did not consider the change of microstructure of reinforced matrix after particle incorpomtion. We have proposed a combined microstructure strengthening analysis, and the prediction is consistent with the eperimental results.

Whisker Orientation Function and Elastic Modulus of the as-cast 20% SiC_W/Mg Composite

The relationship between the plane-orientation function and the space-orientation function of whiskers in whisker-reinforced metal matrix composites was analyzed theoretically. The actual orientation of whiskers in the as-cast 20%SiCw/Mg composite (SiCw content in volume fraction) were investigated, and the elastic modulus of the composite was measured with an ultrasonic velocity analyzer. Results show that there is an evident difference between the plane-orientation function and the space-orientation function of whiskers and the space-orientation function can represent the actual condition of the composite. Only by using the space-orientation function of whiskers, the difference of elastic modulus of the as-cast composite in different directions can be explained reasonably.

Microstructure of SiC Whiskers with Different Cross-sections Perpendicular to the Whiskers Axis

Microstructure of β-SiC whiskers with differ- ent cross-sections perpendicular to their growing direction was studied in detail by transmission elec- tron microscopy (TEM).It was indicated that there were three types of cross-sections:round, hexagonal and trigonal.The whiskers with round and hexagonal cross-sections had a high density of planar faults lying on the (111) close packed planes perpendicular to the whisker axis.There existed a few stacking faults on the other {111} planes in some hexagonal whiskers.The whiskers with bicrystals were also found in hexagonal whiskers. The microstructure of trigonal SiC whiskers was basically perfect but there were a few intrinsic stack- ing faults on the (11) planes (mostly) and (111) planes.The characters of electron diffraction pat- terns of β-SiC whiskers with different cross-sec- tions were reasonably analyzed using a reciprocal space model with continuous diffraction streaks along the [111] reciprocal direction.

Study on Preparation and Separation of SIC Whiskerand Particle from Rice Hulls

The growth mechanism of paramorphic SiC whisker (SiCw) and SiC particle (SiCp) produced from rice bulls under the catalytic actions is studied. The results indicate that the growtb mechanism of the SiCw is vapour-liquid-solid (VLS) , while the crystalline structure of the SiCw is β-SiC. Some SiCp will be formed during the production of the SiCw. Under the pilot-plant condition, the grade of SiCw is over 20%, and SiCp is over 50%. The micron-grade of SiCw and SiCp are separated successfully by flotation and other traditional mineral separation and purification technology. So the grade of the high purity SiCw is over 95 %.

Comparison of K_(IC) Values for SiC Whisker Reinforced Ceramic Composites Obtained by Using Various Methods

The fracture toughness (KIC) values determined by indentation microfracture method (IM ) for SiC whisker reinforced Al2O3 and ZrO2 based composites were calculated with different IM equations and compared with those obtained by singte edge notched beam (SENB) technique. Experimental results show that the KIC (IM) values calculated with different equations are quite different one from another. For composites without phase transformable components the KIC (IM) and KIC (SENB) values are practically on the same level, but for composites with phase transformable components (partially stabilized zirconia) the KIC (SENB) values are always higher than KIC (IM). This is because that the IM method can not reveal sensitively the toughening effect due to dynamic t-m transformation of ZrO2 as the SENB method does. The accuracy of the IM method depends on the Suitability of the IM equations and was evaluated for the materials used in this investigation. Two new IM equations are suggested with which the KIC (IM ) values can be obtained very close to KIC (SENB) values for composites having phase transformable components.

Study on the thermodynamics and the growth kinetics of synthesis of the β-SiC whiskers

The thermodynamics and the growth kinetics of synthesis of the SiC whiskers (SiC w ) from rice hulls are studied in this paper. The results show that the intimate contact of SiO 2 with C in the rice hulls resulted in the formation of SiC particle (SiC p ) at lower temperature, and the external ash of the hulls (w (SiO 2 )>98%) is the main silicon source for SiCw growth. The metallic composite catalyst increases the selectivity for SiCw growth and the reaction rate. The growth mechanism of the SiCw can be characterized as the VLS (vapour liquid solid) with the presence of the whisker forming catalyst: from SiC nucleation through enlargement and growing with the <1 1 1> crystallographic orientation in a certain diameter, then the SiC w is a complete single crystal of β SiC. The generation reaction of SiO is the rate determing step for synthesis of SiC w .

SYNTHESIS OF SiC WHISKER AND ITS APPLICATION IN COMPOSITE MATERIALS

The SiC whiskers (SiCw) synthesized from rice hulls is studied in this paper. The properties of the application in the composite materials are tested, and also compared with the SiCw produced in US and Japan, the results indicate that the SiCw produced by this method is mainly straight crystals with multi nodes on face, the main type is P-SiC. lt has many advantages such as high strength and excellent oxidation resistance to high temperature. The Si3N4 ceramic composite materials reinforced and toughened with the SiCw, δfRT of the material is (856±22)MPa, δf300℃ is approximately (418. 5±14. 2) MPa and Klc is approximately (11. 3±1. 0) MPa m1/2. Besides, the application of the SiCw in the engineering materials of mining is forecasted.

原位自生SiC晶须对原料预氧化SiC耐磨材料显微结构和性能的影响

以Si粉、活性炭、预氧化的SiC颗粒为原料,于埋炭条件下经1500℃保温3h烧制后,制备出了原位自生SiC晶须复合SiC耐磨材料,并研究了SiC晶须对原料预氧化SiC耐磨材料性能的影响,并分析了这些性能和显微结构间的关系。结果表明,原位自生SiC晶须可提高SiC耐磨材料的致密度和力学性能,并在Si粉和活性炭共同添加量为12 wt.%时改善效果较佳,对应试样的抗冲击强度和硬度分别为29.91 kJ·m^(-2)和98(HRB),磨耗比为4.49。这是因为原位自生的SiC晶须充填在SiC颗粒间的孔隙中,形成了均匀的网络结构,且其伴生的SiO_(2)和SiC原料预氧化的产物SiO_(2),均可与α-Al_(2)O_(3)添加剂反应生成莫来石,增强了晶须与基体间的结合力。当SiC耐磨材料受到外力作用时,这些与基体结合较好的SiC晶须,可通过拔出、裂纹的偏转和桥联等途径改善SiC耐磨材料的力学性能。

Grinding Characteristics Of Directionally Aligned SiC Whisker Wheel - Comparison With A12O3 Fiber Wheel

A unique SiC whisker wheel was invented,in which the whiskers were aligned normally to the grinding wheel surface.In this paper,grindabilities of the SiC whisker wheel are investigated and compared with those of other wheels of SiC grains,Al2O3 grains,as well as Al2O3 long and short fibres which were also aligned normally to the grinding wheel surface,respectively.The main research contents concern grinding characteristics of a directionally aligned SiC whisker wheel such as material-removal volume,wheel-wear rates,integrity of the ground surfaces,grinding ratios and grinding efficiency.Furthermore,grinding wheels of whiskers and fibres have a common disadvantage:they tend to load easily.The authors have proposed a simple method of loading-free grinding to overcome this propensity and investigate some related grinding characteristics under loading-free grinding conditions.

Synthesis and characteristics of SiC whiskers with "rosary bead" morphology

SiC whiskers with 'rosary bead' morphology were synthesized using suitablesilicon source and carbon source through solid reaction at the temperature above 1537 K. Thediameter and length of the SiC whiskers were about 0.1-1.0 μm and 20-100 μm, respectively. Thelargest diameter of their enlarged ends of the whiskers was about 0.2-1.0μm, and it gradually andsmoothly decreased to the size of the plain part of the whiskers. The results of X-ray diffractionanalysis show that the crystalline structure of the obtained SiC whiskers is β-SiC. It isconsidered that the SiC whiskers grow via a vapor-solid mechanism.

原位生长SiC晶须对C/C-SiC复合材料摩擦磨损性能影响

采用聚合物浸渍热解(PIP)技术向C/C-SiC复合材料纤维束内引入了SiC晶须(SiC_(W)),然后结合化学气相渗透(CVI)和液硅渗透(LSI)法制备了C/SiCW-C-SiC复合材料,系统研究了原位生长SiCW的物相组成及典型微结构特征,在不同制动初速度下测试了C/SiC_(W)-C-SiC复合材料的摩擦磨损性能,结果表明:引入SiC_(W)后,对平均摩擦系数和瞬时摩擦系数影响较小,摩擦性能未发生明显变化,中高速制动时,C/SiC_(W)-C-SiC的磨损率相比于C/C-SiC可降低20%~60%。

SiC晶须增强铝基复合材料热膨胀行为与内应力关系的研究

本文作者研究了 6 0 0℃水淬和 6 0 0℃退火处理的碳化硅晶须增强铝基复合材料的热膨胀行为 ,阐述两者热膨胀行为与内应力的内在关系。结果表明 :淬火后复合材料基体的位错密度、内应力、及材料的有效屈服强度较高 ;而退火后复合材料基体的位错密度、内应力、及材料的有效屈服强度较低。当材料在 6 0 0℃淬火后 ,升温过程中材料的热膨胀系数曲线在 80℃和 2 4 5℃各出现一个峰值 ,且后者明显高于前者 ;而 6 0 0℃退火后材料的热膨胀系数曲线只在 80℃出现一个波峰 ,且其峰值低于淬火材料相应的峰值。分析表明 :材料热膨胀系数曲线出现的第一个峰是基体内拉应力释放的结果 ;