Mrityunjay Singh

Evaluation of ultra-high temperature ceramics for aeropropulsion use

Among the ultra-high temperature ceramics (UHTC) are a group of materials consisting of zirconium diboride or hafnium diboride plus silicon carbide, and in some instances, carbon. These materials offer a good combination of properties that make them candidates for airframe leading edges on sharp-bodied reentry vehicles. These UHTCperform well in the environment for such applications, i.e. air at low pressure. The purpose of this study was to examine three of these materials under conditions more representative of a propulsion environment, i.e. higher oxygen partial pressure and total pressure. Results of strength and fracture toughness measurements, furnace oxidation, and high velocity thermal shock exposures are presented for ZrB2 plus 20 vol.% SiC, ZrB2 plus 14 vol.% SiC plus 30 vol.% C, and SCS-9a SiC fiber reinforced ZrB2 plus 20 vol.% SiC. The poor oxidation resistance of UHTCs is the predominant factor limiting their applicability to propulsion applications. # 2002 Elsevier Science Ltd. All rights reserved.

Interfacial and thermomechanical characterization of reaction formed joints in silicon carbide-based materials

Abstract The microstructure and mechanical properties of reaction formed joints of RefelTM reaction bonded SiC (RB-SiC) and HexoloyTM sintered SiC were studied in order to achieve a better understanding of the influence of base materials and joining process parameters on the high temperature strength of reaction formed joints. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and optical microscopy were used to characterize the joints prior to mechanical tests. The microstructural analysis indicated that the joints consist of silicon carbide (SiC) grains (with grain sizes ranging from 0.1 to 2 μm) and crystalline silicon as an intergranular phase. Most of the silicon carbide grains in the joint have hexagonal crystal structure with certain preferential orientations related to the silicon matrix. The high temperature strength of joints was measured by constant strain rate experiments in compression where joints were forming 45° with the compression axe. The strength of the joined Refel RB-SiC has been found to be at least equal to that of the bulk materials (550 MPa at 1235°C and 400 MPa at 1385°C). The joined Hexoloy specimens had strengths (1.4 GPa at 1290°C and 750 MPa at 1420°C) lower than the bulk material but higher than the joints of RB-SiC.

Ceramic integration and joining technologies : from macro to nanoscale

This book envisions integration in its broadest sense as a fundamental enabling technology at multiple length scales that span the macro, millimeter, micrometer and nanometer ranges. Consequently, the book addresses integration issues in such diverse areas as space power and propulsion, thermoelectric power generation, solar energy, micro-electro-mechanical systems (MEMS), solid oxide fuel cells (SOFC), multi-chip modules, prosthetic devices, and implanted biosensors and stimulators. The engineering challenge of designing and manufacturing complex structural, functional, and smart components and devices for the above applications from smaller, geometrically simpler units requires innovative development of new integration technology and skillful adaptation of existing technology.

Bonding and Integration of Silicon Carbide Based Materials for Multifunctional Applications

Robust bonding and integration technologies are critically needed for the successful implementation of silicon carbide based components and systems in a wide variety of aerospace and ground based applications. These technologies include bonding of silicon carbide to silicon carbide as well as silicon carbide to metallic systems. A diffusion bonding based approach has been utilized for joining of silicon carbide (SiC) to silicon carbide sub-elements for a micro-electro-mechanical systems lean direct injector (MEMS LDI) application. The objective is to join SiC sub-elements to from a leak-free injector that has complex internal passages for the flow and mixing of fuel and air. A previous bonding approach relied upon silica glass-based interlayers that were non-uniform and not leak free. In the newly developed joining approach, titanium foils and physically vapor deposited titanium coatings were used to form diffusion bonds between SiC materials using hot pressing. Microscopy results show the formation of well adhered diffusion bonds. Initial tests show that the bond strength is much higher than required for the component system. Benefits of the joining technology are fabrication of leak free joints with high temperature and mechanical capability.

Evaluation of Fracture Behavior of Ceramic Composite Joints by Using a New Interface Potential

In order to examine mode-I & II type fracture behavior of ceramic joints, the interface element was proposed as a simple model which represents the mechanism of failure in an explicit manner. It was applied to the analyses of four point bending test and asymmetrical four point bending test for SiC/SiC composite specimen joined by ARCJoinTTM. By using a new type interface potential, which is a coupled function of opening and shear deformations, both the bending and asymmetrical bending tests were simulated. From comparison with experiments, surface energy at the interface between the joint and composite was estimated to be about 30 N/m regardless of the fracture mode. Also, from the comparison between the predicted strength and the experiments for the tensile test of lap joint of SiC/SiC composites, it was found that the proposed method was considered to have a great potential as a tool to study the failure problems whose fracture type was a mixture of mode-I & II.

Measurement of Young’s modulus and damping for hexoloy SG silicon carbide

Stiffness and damping are important design parameters for the potential applications of silicon carbide in the aerospace industry. Thus, Young`s modulus values are needed for calculations of stress and strain, and mechanical damping determines the rate at which vibrations are dissipated in the material. In this paper, the measurements of Young`s modulus and mechanical damping of Hexoloy SG silicon carbide at high temperatures are discussed and compared with published values. These measurements were made using the Piezoelectric Ultrasonic Composite Oscillator Technique (PUCOT). The modulus values are important for calculations involving stress and strain for design purposes. The mechanical damping determines the rate at which vibrations are dissipated in the material. The relationship of the mechanical damping to the strain amplitude is discussed. Microstructural analysis and microhardness tests are discussed as well.

Microstructure and Mechanical Properties of Joints in Sintered SiC Fiber-Bonded Ceramics

Active metal brazing of a new high thermal conductivity SiC-polycrystalline fiber-bonded ceramic (SA-Tyrannohex™) has been conducted using a Ti-containing Ag-Cu active braze alloy (Ticusil®). The brazed joints were characterized using SEM-EDS and Knoop hardness scans across the interfaces. The effects of fiber orientation in the composite on the microstructure, elemental composition, and microhardness are presented. Results show that this material can be successfully joined using judiciously selected off-the shelf active braze alloys to yield metallurgically sound joints possessing high integrity.

Infiltration Processing of Ceramic-Metal Composites: The Role of Wettability, Reaction, and Capillary Flow

The infiltration of ceramics by liquid metals to fabricate ceramic-metal composites is discussed. In particular, the complexity of infiltrating ceramics by liquid metals at high temperatures due to interfacial reactions, metal oxidation, pore modulation and closure, and transient capillary forces has been highlighted. The role of these factors is discussed in the context of reactive infiltration with examples from ceramic/metal composites of practical interest. In addition to flow through porous ceramics, reactive penetration of dense ceramics via chemical dissolution and reaction is also discussed.

Numerical Analysis of Test Methods for Evaluating Shear Strength of Ceramic Composite Joints Using Interface Element

As examples of the most typical methods to determine the shear strength of SiC/SiC composite joints, the asymmetrical four point bending test of butt joined composite, the tensile test of lap joined composite and the compressive test of double-notched composite joint were analyzed by using finite element method with the interface element. From the calculation results, it was found that the shear strength in the asymmetrical bending test was controlled by both the surface energy and the shear strength at the interface regardless of their combination although the strength in the tensile test or the compressive test was governed by the surface energy when the shear strength was large. Also, it was revealed that the apparent shear strength of the composite joint obtained experimentally might be affected by the combination of the surface energy and the shear strength at the interface.

Numerical Analysis of Mechanical Test Methods for Evaluating Shear Strength of Joint by Using Interface Element

As examples of the most typical methods to determine the shear strength of SiC/SiC composite joints, the tensile test of lap joined composite and the asymmetrical four point bending test of butt joined composite were analyzed by using finite element method with the interface element. From the calculation results, it was revealed that the strength in the tensile test was strongly influenced by the residual stress as the increase of the joint layer thickness. In the case of asymmetrical bending test, it was found that the crack initiation point would move due to the residual stress and the strength was also affected by the joint layer thickness.