MECHANICAL PROPERTY STUDY OF THE FIBER-MATRIX INTERFACE IN SIC/SIC COMPOSITES

ABSTRACT The fiber-matrix interface between ceramic fibers and ceramic matrix plays a major role in the fatigue properties and toughness of continuous fiber reinforced ceramic matrix composites (CMCs). Boron Nitride (BN) is a widely used fiber coating material that provides a weak bond between the fiber and matrix. A weak fiber-matrix interface increases the strength and toughness of the overall CMC. Single fiber push-out tests were performed to study interfacial shear strength as a main parameter defining fatigue properties and toughness of SiC/SiC composites. The fiber-matrix interfacial shear strength was studied in melt infiltrated Hi-Nicalon/BN(CVI)/SiC composites exposed to various temperature and loading conditions, similar to those that are used in actual applications. Hi-Nicalon fibers with diameters of 13-14.5 µm were pushed out from samples with thicknesses ranging from 125-280 µm using a spherical tip with a 1 µm radius and 90° conical shape. Interfacial shear strength was calculated from sliding load, fiber diameter and sample thickness. Due to significant scattering, 30 individual push tests in every sample were used to obtain the average interfacial shear strength. The virgin sample has a shear strength of 20 MPa which is higher than tensile tested samples (12 MP

Effects Of Rolling And Hot Pressing On Mechanical Properties Of Boron Carbide-Based Ceramics

A study of hot pressed B4C-based laminates, after rolling and without rolling, has been performed to elucidate the existence of fracture resistance/crack length anisotropy induced by this processing technique. While the crack lengths/fracture resistance was affected significantly by the presence of the residual stresses in B4C/B4C-ZrB2 laminates, no differences in Vickers crack lengths were observed in B 4C/B4C laminates prepared by rolling and hot pressing, as compared to the crack lengths seen in pure B4C ceramics prepared by hot pressing without rolling. X-ray diffraction analysis confirmed that no texture has been formed during the rolling and hot pressing of B4C ceramics. © 2008 Springer Science+Business Media, LLC

Phase Stability And Sintering Behavior Of 10 Mol % Sc2O 3-1 Mol %Ceo2-Zro2 Ceramics

The phase composition and sintering behavior of two commercially available 10 mol % Sc2O3-1 mol % CeO2-ZrO2 ceramics produced by Daiichi Kigenso Kagaku Kogyo (DKKK) and Praxair have been studied. DKKK powders have been manufactured using a wet coprecipitation chemical route, and Praxair powders have been produced by spray pyrolysis. The morphology of the powders, as studied by scanning electron microscopy, has been very different. DKKK powders were presented as soft ((100 (m] spherical agglomerates containing 60-100 nm crystalline particles, whereas the Praxair powders were presented as sintered platelet agglomerates, up to 30 (m long and 3-4 (μ thick, which consisted of smaller 100-200 nm crystalline particles. X-ray diffraction analysis has shown that both DKKK and Praxair powders contained a mixture of cubic (c) and rhombohedral (r) phases: 79% cubic (21% rhombohedral for DKKK powders and 88% cubic )12% rhombohedral for Praxair powders. Higher quantities of the Si impurity level have been detected in Praxair powder as compared to DKKK powder by secondary ion mass spectroscopy. The morphological features, along with differences in composition and the impurity level of both powders, resulted in significantly different sintering behaviors. The DKKK powders showed a more active sintering behavior than of Praxair powders, reaching 93-95% of theoretical density when sintered at 1300°C for 2 h. Comparatively, the Praxair powders required high sintering temperatures at 1500-1600°C. However, even at such high sintering temperatures, a significant amount of porosity was observed. Both DKKK and Praxair ceramics sintered at 1300°C or above exist in a cubic phase at room temperature. However, if sintered at 1100°C and 1200°C, the DKKK ceramics exist in a rhombohedral phase at room temperature. The DKKK ceramics sintered at 1300°C or above exhibit cubic to rhombohedral and back to cubic phase transitions upon heating at a 300-500°C temperature range, while Praxair ceramics exist in a pure cubic phase upon heating from room temperature to 900°C. However, if heated rather fast, the cubic to rhombohedral phase transformation could be avoided. Thus it is not expected that the observed phase transitions play a significant role in developing transformation stresses in ScCeZrO2 electrolyte upon heating and cooling down from the operation temperatures. Copyright © 2009 by ASME

Evolution of Microstructure and Mechanical Properties of Mg-6Al Alloy Processed by Differential Speed Rolling upon Post-Annealing Treatment

Magnesium-6 wt.% aluminum (Mg-6Al) alloy plates with a 6-millimeter thickness were processed from an initial 12-millimeter thickness by differential speed rolling (DSR), with a 0.76-millimeter thickness reduction per pass using a speed ratio of 2, preheating temperature of 315 °C, and roll temperature of 265 °C. The effects of annealing temperature of 250, 275, and 300 °C with a corresponding holding time of 15 min on the microstructure, texture, and mechanical properties were investigated. Key results show that dynamic recrystallization (DRX) occurred during the roll processing, resulting in a greatly reduced grain size. In addition, the basal pole of the as-rolled plate was inclined to the rolling direction (RD) by ~20°, due to the shear strain introduced during DSR. Subsequent annealing caused grain growth, eliminated the basal pole inclination towards the RD, and slightly increased the pole intensity. Compared with the as-rolled plate, the average of the ultimate tensile strength (UTS) and the yield strength (YS) of the annealed plates decreased, while the average elongation at fracture (εf) increased. With the annealing temperature of 275 °C, the plate achieved a good combination of mechanical properties with UTS, YS, and εf being 292.1 MPa, 185.0 MPa, and 24.9%, respectively. These results suggest that post-roll annealing is an effective way to improve the mechanical response of this Mg alloy processed by DSR

The Effect of Extrusion Temperatures on Microstructure and Mechanical Properties of Mg-1.3Zn-0.5Ca (wt.%) Alloys

The present work mainly investigated the effect of extrusion temperatures on the microstructure and mechanical properties of Mg-1.3Zn-0.5Ca (wt.%) alloys. The alloys were subjected to extrusion at 300 °C, 350 °C, and 400 °C with an extrusion ratio of 9.37. The results demonstrated that both the average size and volume fraction of dynamic recrystallized (DRXed) grains increased with increasing extrusion temperature (DRXed fractions of 0.43, 0.61, and 0.97 for 300 °C, 350 °C, and 400 °C, respectively). Moreover, the as-extruded alloys exhibited a typical basal fiber texture. The alloy extruded at 300 °C had a microstructure composed of fine DRXed grains of ~1.54 µm and strongly textured elongated unDRXed grains. It also had an ultimate tensile strength (UTS) of 355 MPa, tensile yield strength (TYS) of 284 MPa, and an elongation (EL) of 5.7%. In contrast, after extrusion at 400 °C, the microstructure was almost completely DRXed with a greatly weakened texture, resulting in an improved EL of 15.1% and UTS of 274 MPa, TYS of 220 MPa. At the intermediate temperature of 350 °C, the alloy had a UTS of 298 MPa, TYS of 234 MPa, and EL of 12.8%

Fabrication and Characterization of Magnesium Ferrite-Based PCL/Aloe Vera Nanofibers

Composite nanofibers of biopolymers and inorganic materials have been widely explored as tissue engineering scaffolds because of their superior structural, mechanical and biological properties. In this study, magnesium ferrite (Mg-ferrite) based composite nanofibers were synthesized using an electrospinning technique. Mg-ferrite nanoparticles were first synthesized using the reverse micelle method, and then blended in a mixture of polycaprolactone (PCL), a synthetic polymer, and Aloe vera, a natural polymer, to create magnetic nanofibers by electrospinning. The morphology, structural and magnetic properties, and cellular compatibility of the magnetic nanofibers were analyzed. Mg-ferrite/PCL/Aloe vera nanofibers showed good uniformity in fiber morphology, retained their structural integrity, and displayed magnetic strength. Experimental results, using cell viability assay and scanning electron microscopy imaging showed that magnetic nanofibers supported 3T3 cell viability. We believe that the new composite nanofibrous membranes developed in this study have the ability to mimic the physical structure and function of tissue extracellular matrix, as well as provide the magnetic and soluble metal ion attributes in the scaffolds with enhanced cell attachment, and thus improve tissue regeneration

SiC/SiC \u3c inf\u3e woven fabric laminates: Design, manufacturing, mechanical properties

The SiC/SiCwf laminate design has been targeted to achieve increased residual compressive stress in thin SiCwf layers and small residual tensile stress in thick SiC layers, that would lead to a significant increase of the apparent fracture toughness of the composite. The laminates have been manufactured using rolling and hot pressing techniques. The measured apparent fracture toughness of laminates showed a significant increase over the monolith SiC ceramics. Thus, while the intrinsic fracture toughness of pure SiC ceramics is in the range of 2-3 MPa m1/2, significant increase up to 6-8 MPa m1/2 was achieved for the layered ceramics. A crack shielding by compressive residual stress was a main mechanism responsible for the significant toughening of the SiC/SiCwf laminate. © 2006 Elsevier Ltd. All rights reserved

Directional-Tribological Investigation of Magnesium Alloys Under As-Cast and Hot Extrusion Conditions

ABSTRACT In recent years, magnesium (Mg) and its alloy are being studied for their potential use in orthopedic implants with the novel ability to biodegrade after the implant serves its therapeutic function. Pure Mg, by itself, would not be suitable for use in a load-bearing implant application, due to its high corrosion rate and poor tribological properties. However, through proper alloying, this degradable metal is capable of achieving good mechanical properties reasonably similar to bone, a retarded rate of corrosion and enhanced biocompatibility. Previous studies have shown that alloying Mg with aluminum, lithium, rare earth (RE), zinc (Zn), and calcium (Ca) result in lower corrosion rates and enhanced mechanical properties. Despite the growing popularity of Mg and it alloys, there is relatively little information in the literature on their wear performance. In this paper, we report on an investigation of the directional tribological properties of Mg and Mg-Zn-Ca-RE alloy fabricated via two different manufacturing processing routes: as-cast and hot-extruded after casting, with extrusion ratios of 10 and 50. Pure Mg was cast 350°C. After casting, MgZn-Ca-RE alloy was heat-treated at 510°C. Another Mg-Zn-Ca-RE alloy was hot-extruded at 400°C. Dry sliding wear tests were performed on as-cast and hot-extruded pure Mg and Mg-Zn-Ca-RE alloys using a reciprocating test configuration. Wear rate, coefficient of friction and wear coefficient were measured under applied loads ranging from 0.5 -2.5N at sliding frequency of 0.2 Hz for 120 cycles, using microtribometery. Wear properties of the extruded specimen were measured in cross-section and longitudinal section. In the longitudinal section studies, wear properties were investigated along the extrusion direction and the transverse direction. Hardness properties were evaluated using microindentation. Cross-section and longitudinal section were indented with a Vickers indenter under applied load of 2.94 N. Alloying and extrusion enhanced the mechanical properties significantly, increased hardness by 80% and wear resistance by 50% compared to pure Mg. Despite the low hardness in both Mg and the Mg alloy cross-sections, the cross-sections for both displayed higher wear resistance compared to the longitudinal section. In the longitudinal section, wear resistance was higher along the transverse direction of the longitudinal section for both Mg and the Mg alloy. The wear coefficient was used to evaluate how the wear behavior of the material varied with respect to alloying, fabrication process, and direction of wear. The wear coefficient of pure Mg decreased as the extrusion ratio increased, thus, increasing the specific wear rate. The opposite behavior was found in the Mg alloy: as the wear coefficient increases, the specific wear rate decreases. The active wear mechanisms observed on the worn surface of Mg were fatigue, abrasive, adhesive and delamination wear. The same wear mechanisms were observed in the Mg alloy except for fatigue wear. Surface microstructure and topographical characterization were conducted using optical microscopy, scanning electron microscopy mechanical stylus profilometry, and optical profilometry