Crack and wear behavior of SiC particulate reinforced aluminium based metal matrix composite fabricated by direct metal laser sintering process

In this investigation, crack density and wear performance of SiC particulate (SiCp) reinforced Al-based metal matrix composite (Al-MMC) fabricated by direct metal laser sintering (DMLS) process have been studied. Mainly, size and volume fraction of SiCp have been varied to analyze the crack and wear behavior of the composite. The study has suggested that crack density increases significantly after 15 volume percentage (vol.%) of SiCp. The paper has also suggested that when size (mesh) of reinforcement increases, wear resistance of the composite drops. Three hundred mesh of SiCp offers better wear resistance; above 300 mesh the specific wear rate increases significantly. Similarly, there has been no improvement of wear resistance after 20 vol.% of reinforcement. The scanning electron micrographs of the worn surfaces have revealed that during the wear test SiCp fragments into small pieces which act as abrasives to result in abrasive wear in the specimen.     

Influence of grain size on wear behavior of SiC coating for carbon/carbon composites at elevated temperatures

To improve the wear performance of SiC coating for C/C composites at elevated temperatures, the grain was refined by adding small amounts of titanium, in the raw powders for preparing this coating. The related microstructure and mechanical characteristics were investigated by scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy and nano-indention. The results show that the grain size of SiC coating decreased from ∼30 μm to ∼5 μm due to the addition of grain refiner. TiC formed by reacting titanium with graphite, can act as perfect heterogeneous nucleus for the nucleation and growth of β-SiC. The wear resistance and fracture toughness of SiC coating was improved by grain refinement. However, the increasing interfaces increased the friction resistance and resulted in the high friction coefficient of fine-grained coating at room temperature. As the temperature rose, oxides layer formed on the surface of fine-grained coating, which can reduce the adhesive wear and decrease the friction coefficient. The fine-grained coating exhibited relative low friction coefficient of ∼0.41 owing to a compact silica film formed on the worn surface at 600 °C, and the wear was dominated by plastic deformation and shear of silica film. The wear of coarse-grained coating was controlled by the fracture of SiC at high temperature.     

Dry sliding wear behavior of heat treated hybrid metal matrix composite using Taguchi techniques

Dry sliding wear behavior of zinc based alloy and composite reinforced with SiCp (9 wt%) and Gr (3 wt%) fabricated by stir casting method was investigated. Heat treatment (HT) and aging of the specimen were carried out, followed by water quenching. Wear behavior was evaluated using pin on disc apparatus. Taguchi technique was used to estimate the parameters affecting the wear significantly. The effect of HT was that it reduced the microcracks, residual stresses and improved the distribution of microconstituents. The influence of various parameters like applied load, sliding speed and sliding distance on wear behavior was investigated by means and analysis of variance (ANOVA). Further, correlation between the parameters was determined by multiple linear regression equation for each response. It was observed that the applied load significantly influenced the wear volume loss (WVL), followed by sliding speed implying that increase in either applied load or sliding speed increases the WVL. Whereas for composites, sliding distance showed a negative influence on wear indicating that increase in sliding distance reduces WVL due to the presence of reinforcements. The wear mechanism of the worn out specimen was analyzed using scanning electron microscopy. The analysis shows that the formation and retention of ceramic mixed mechanical layer (CMML) plays a major role in the dry sliding wear resistance.     

界面对颗粒增强金属基复合材料强化性能的影响

为提高颗粒增强金属基复合材料的力学性能,采用基于微观组织的胞元模型建模方法,并利用有限元软件ABAQUS着重分析了界面层厚度以及界面层强度对复合材料性能的影响,通过对复合材料中各组成部分的应力、应变云图的获取,形象地说明了各部分的变形规律.研究结果表明,在弱界面层下,随着界面层厚度的增加,复合材料的强化效果并不显著,而在强界面层下,随着界面层厚度的增加,强化效果非常明显;就界面层强度来说,界面越强,所表现出的强化效果就越明显,但当界面层强度比基体大得多时,随着界面层强度的增加,虽然复合材料的强化呈递增趋势,但是递增的幅度已逐渐降低.     

Stress-dependence and time-dependence of the post-fatigue tensile behavior of carbon fiber reinforced SiC matrix composites

The major objective of this paper is to phenomenally report the stress-dependence and time-dependence of fatigue damage to C/SiC composites, and to tentatively discuss the effects of the fatigue stress levels and the fatigue cycles on the post-fatigue tensile behavior. Results show that compared with the virgin strength of the as-received C/SiC specimens, the tensile strengths of the as-fatigued specimens after 86,400 cycles were increased by 8.47% at the stresses of 90 ± 30 MPa, 23.47% at 120 ± 40 MPa, and 9.8% at 160 ± 53 MPa. As cycles continued, however, the post-fatigue strength of the composites gradually decreased after the peak of 23.47%, at which the optimal strength enhancement was obtained because the mean fatigue stress of 120 MPa was the closest to thermal residual stress (TRS), and caused TRS relieve largely during the fatigue. Most interestingly, there was a general inflexion appeared on the post-fatigue tensile stress-strain curves, which was just equal to the historic maximum fatigue stress acted upon the as-fatigued specimens. Below this inflexion stress the tensile curves revealed the apparent linear behavior with little AE response, and above that nonlinearity with new damage immediately emitted highly increase rate of AE activities. This ‘stress memory’ characteristic was strongly relevant to damaged microstructures of the as-fatigued composites in the form of the coating/matrix cracks, interface debonding/wear, and fiber breaking, which resulted undoubtedly in reduction of modulus but in proper increase of strength via TRS relief.     

短切碳纤维导电复合材料渗流和PTC行为的唯象分析

为研究短切碳纤维含量和温度对导电复合材料渗流行为的影响,通过Landau相变理论导出了复合材料电导率与填料体积分数及温度的方程,并用该方程分析了短切碳纤维/乙烯基酯树脂复合材料的渗流和PTC行为。结果表明:复合材料的渗流阈值随温度升高而增加,PTC温度随碳纤维含量增加而上升。当温度由24.6℃上升到108.4℃时,其渗流阈值的理论值由1.06%增加到1.60%;当碳纤维体积分数由3.1%增加到4.6%时,其PTC温度的理论值由120℃上升到170℃。复合材料渗流阈值和PTC温度理论值与实验值符合得很好。      

Fatigue-crack propagation behavior in monolithic and composite ceramics and intermetallics

We study microstructural mechanisms of fatigue crack growth in advanced monolithic and composite ceramics and intermetallics. Much attention is devoted to the contribution of cycling loading to the hindrance of mechanisms that lead to a considerable increase in toughness (crack-tip shielding) of these materials. For example, in intermetallics with a ductile phase, such as -TiNb-reinforced -TiAl or Nb-reinforced Nb₃Al, a significant increase in toughness caused by the presence of uncracked ductile phase inside a crack is retarded under cyclic loading because ductile particles immediately fail by fatigue. Similarly, in monolithic ceramics, e.g., in alumina (aluminum oxide) or silicon nitride, the significant plasticization appearing under monotonic loading is greatly diminished under cyclic loading due to gradual wear at the grain-matrix interface. In fact, the nature of fatigue in such low-plasticity materials differs essentially from the well-known mechanisms of fatigue in metals and is governed, first of all, by a decrease in shielding, which depends on the loading cycle and time. The susceptibility of intermetallics and ceramics to fatigue degradation under cyclic loading affects seriously the possibility of structural use of these materials in practice. In particular, in this case, it is difficult to apply strength calculation methods that take into account the presence of defects and to implement life-prediction procedures.Center for Advanced Materials, Lawrence Berkeley Laboratory, University of California, USA. Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 3, pp. 7–35, May–June, 1994.     

Non-destructive testing of satellite nozzles made of carbon fibre ceramic matrix composite,C/SiC

Carbon fibre ceramic matrix composite materials, C/SiC, are excellent candidates as lightweight structural materials for high performance hot structures such as in aerospace applications. Satellite nozzles are manufactured from C/SiC, using, for instance, the Liquid Polymer Infiltration (LPI) process.In this article the applicability of different non-destructive analysis methods for the characterisation of C/SiC components will be discussed. By using synchrotron and neutron tomography it is possible to characterise the C/SiC material in each desired location or orientation. Synchrotron radiation using tomography on small samples with a resolution of 1.4 μm, i.e. the fibre scale, was used to characterise three dimensionally fibre orientation and integrity, matrix homogeneity and dimensions and distributions of micro pores. Neutron radiation tomography with a resolution of about 300 μm was used to analyse the over-all C/SiC satellite nozzle component with respect to the fibre content. The special solder connection of a C/SiC satellite nozzle to a metallic ring was also successfully analysed by neutron tomography. In addition, the residual stress state of a temperature tested satellite nozzle was analysed non-destructively in depth by neutron diffraction. The results revealed almost zero stress for the principal directions, radial, axial and tangential, which can be considered to be the principal directions.     

Damage behavior of atomic oxygen on CVD SiC coating-modified carbon/carbon composite in low earth orbit environment

In this work, samples of carbon/carbon (C/C) and chemical vapor deposited (CVD) SiC-coated C/C samples were investigated to understand the AO damage mechanism in low Earth orbit (LEO) environment. The ground-based simulated atomic oxygen (AO) generator was employed. Results indicate that the CVD SiC coating exhibited improved radiation resistance properties against AO radiation as evidenced by a 16% better strength retention ratio, 60% less mass ablation, and increased strength stability. The magnitude of these influences affected the surface morphology, as observed by scanning electron microscopy (SEM) and surface resistance meter test results. The variations in the surface constituents were confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results. The main products left on surface after AO exposure are SiO₂ and SiC_xO_y film. Additionally, Si atoms are found to be the preferential reacting element in the SiC coating, and this process is accompanied by graphite precipitation, grain growth, and crack necking. Also, the damage mechanism of the AO-exposed SiC coating was revealed and is discussed.     

Healing behavior of a matrix crack on a carbon fiber/mendomer composite

The cure kinetics of a thermally mendable polymer based on Diels-Alder (DA) and retro-Diels-Alder (rDA) reactions, mendomer 401, was investigated using dynamic differential scanning calorimetry (DSC). The resulting behavior was modeled using a conventional cure kinetic model for thermosetting polymers. A composite panel with two layers of carbon fabric and mendomer 401 was fabricated following the cure cycle suggested by the cure kinetics model. Micro-indentation tests were performed to investigate mechanical properties and dynamic mechanical thermal analysis (DMTA) was conducted to study thermal behavior and to find the trigger temperature for healing. Self-healing behavior of the carbon fiber/mendomer composite was demonstrated using electrical resistive heating over the glass transition temperature.     

Preparation and characterization of nano-structured monolithic SiC and Si3N4/SiC composite by hot isostatic pressing

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

切削SiC增强铝基复合材料时刀具的磨损形态及机理

为研究切削SiC增强铝基复合材料时刀具的磨损形态和机理,采用硬质合金和聚晶金刚石(PCD)刀具进行了各切削工况下的切削试验。用爆炸式快速落刀装置获取切屑根,研究了前刀面的磨损部位。借助扫描电子显微镜(SEM)和原子力显微镜(AFM),检测分析了前、后刀面的磨损形态和成分组成,并进一步研究了磨损机理。结果表明:切削刀具的主要磨损部位发生在后刀面,磨损机理是磨料磨损;前刀面临近刃口区域首先产生由SiC增强相引起的磨料磨损,该区域随后由机械镶嵌生成积屑瘤,积屑瘤脱落后导致产生黏结磨损。黏结磨损的程度较轻,没有形成月牙洼型。前刀面离刃口稍远的区域(积屑瘤尾部后面)会同时产生由切屑底层SiC增强相引起的再次磨料磨损,磨料磨损的主要机理是"微切削"。     

Dry sliding wear behavior of extruded titanium matrix composite reinforced by in situ TiB whisker and TiC particle

The dry sliding wear behavior of titanium matrix composite (TMC) reinforced by in situ TiB whisker and TiC particle was investigated. Compared to the unreinforced pure Ti matrix, the TMC exhibited a markedly improved wear resistance due to the existence of the ceramic reinforcements. The TMC showed lower friction coefficient than the pure Ti. The mean values of steady-state friction coefficient of the TMC and pure Ti against a tool steel were about 0.270–0.330 and 0.385–0.395, respectively, under the loads of 40–100 N. Meanwhile, the TMC showed lower weight loss and its surface wearing was less severe compared to that of the pure Ti. The worn surface of the TMC was covered with mild grooves and some fine wear debris, which exhibited the characteristic of both adhesive and abrasive. TiO₂ was found on the worn surface due to the oxidation behavior of the Ti matrix, which may reduce the wear tendency of the TMC. The results show that the in situ ceramic reinforcements could greatly increase the wear resistance of pure Ti.     

Fail-safe light transmitting SiC fiber-reinforced spinel matrix optomechanical composite

A light transmitting Optomechanical Composite was fabricated by incorporating SiC (SCS-6) fibers into optically transparent MgAl₂O₄ ceramics. The fibers were aligned unidirectionally in the matrix with a spacing of 1 to 4 mm to allow light transmittance between the fibers: resulting fiber volume content was from 0.75 to 0.19 vol%. The total and in-line light transmittance of the matrix and the composite was measured in the visible/near-IR wavelength region. The light transmittance of the composite was found to be decreasing with increasing fiber volume fraction, however, even the composite with the highest fiber volume fraction has a light transmittance of 25–50% in the visible wavelength region. Although the optical transparency of the matrix becomes slightly lower by the incorporation of the opaque fibers, its catastrophic failure is prevented by the bridging effect of the intact fibers, introducing the fail-safe nature to the brittle ceramic material. To obtain this fail-safe mechanism, the minimum fiber volume fraction was 0.37 vol% in the present material system.     

Fabrication of SiC particles-reinforced magnesium matrix composite by ultrasonic vibration

Magnesium matrix composites reinforced with two volume fractions (1 and 3%) of SiC particles (1 μm) were successfully fabricated by ultrasonic vibration. Compared with as-cast AZ91 alloy, with the addition of the SiC particles grain size of matrix decreased, while most of the phase Mg₁₇Al₁₂ varied from coarse plates to lamellar precipitates in the SiCp/AZ91 composites. With increasing volume fraction of the SiC particles, grains of matrix in the SiCp/AZ91 composites were gradually refined. The SiC particles were located mainly at grain boundaries in both 1 vol% SiCp/AZ91 composite and 3 vol% SiCp/AZ91 composite. SiC particles inside the particle clusters may be still separated by magnesium. The study of the interface between the SiC particle and the alloy matrix suggested that SiC particles bonded well with the alloy matrix without interfacial reaction. The ultimate tensile strength, yield strength, and elongation to fracture of the SiCp/AZ91 composites were simultaneously improved compared with that of the as-cast AZ91 alloy.     

Tensile creep behavior of notched two-dimensional-C/SiC composite

Tensile creep tests of two-dimensional-C/SiC specimens with double-edge arc notches have been carried out at 1100, 1300 and 1500 °C in vacuum. The matrix cracks on the surface and resonance frequency were examined at different creeping times. At 1100 °C, the creep strains of both smooth and notched specimens were concentrated at the transient stage and the steady creep rates were nearly zero, whereas steady creep rates of notched specimens and smooth specimens were similar at 1500 °C. It has been observed that the creep damage mainly concentrated at the area near the notches. Micro-cracks appeared in the area near the notches and on the cross-points of the woven fiber bundles, and the longitudinal fibers near the notches fractured easily. Both types of curves, namely quantity of micro-cracks vs. time and micro-crack width vs. time, were extremely similar as for the creep curves. In general, micro-cracks developed fast during the first 10 h. It has been noticed that within the first 2 h, the micro-cracks near the notches grow faster than those far from the notches, whereas the growth rate of micro-cracks far from notches was faster than those near the notches after 2 h. This phenomenon indicates the stress redistribution during creep. Damage curves at 1300 and 1500 °C have similar trend, though the damage and the quantity of micro-cracks at 1500 °C are higher than those at 1300 °C.     

Effect of frequency and environment on fatigue behavior of a CVI SiC/SiC ceramic matrix composite at 1200 °C

The fatigue behavior of a SiC/SiC CMC (ceramic matrix composite) was investigated at 1200 °C in laboratory air and in steam environment. The composite consists of a SiC matrix reinforced with laminated woven Hi-Nicalon™ fibers. Fiber preforms had boron nitride fiber coating applied and were then densified with CVI SiC. Tensile stress-strain behavior and tensile properties were evaluated at 1200 °C. Tension-tension fatigue tests were conducted at frequencies of 0.1, 1.0, and 10 Hz for fatigue stresses ranging from 80 to 120 MPa in air and from 60 to 110 MPa in steam. Fatigue run-out was defined as 10⁵ cycles at the frequency of 0.1 Hz and as 2 × 10⁵ cycles at the frequencies of 1.0 and 10 Hz. Presence of steam significantly degraded the fatigue performance. In both test environments the fatigue limit and fatigue lifetime decreased with increasing frequency. Specimens that achieved run-out were subjected to tensile tests to failure to characterize the retained tensile properties. The material retained 100% of its tensile strength, yet modulus loss up to 22% was observed. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Friction and wear behavior of polytetrafluoroethene composites filled with Ti3SiC2

In this work, polytetrafluoroethylene (PTFE) composites filled with Ti₃SiC₂ or graphite were prepared through powder metallurgy. The effects of different filling components, loads and sliding velocities on the friction performance of Ti₃SiC₂/PTFE composites were studied. Ti₃SiC₂/PTFE composites exhibit better wear resistance than graphite/PTFE composites due to the better mechanical properties of Ti₃SiC₂. The wear resistance was found to improve around 100× over unfilled PTFE with the addition of 1 wt.% Ti₃SiC₂. In addition, the 10 wt.% sample had the lowest wear rate of K = 2.1 × 10⁻⁶ mm³/Nm and the lowest steady friction coefficient with μ = 0.155 at the condition of 90 N–0.4 m/s. Ti₃SiC₂ was proved to promote the formation of a thin and uniform transfer film on counterpart surface and a protection oxide film on worn surface, which are the key roles for improving wear resistance.