Engineering topics

Bhatt, R. T.

Publications and source records attributed to Bhatt, R. T..

At least 19 records

NASA NTRS2017

Creep and Cyclic Fatigue Durability of 3D Woven SiC/SiC Composites with (CVI+PIP) Hybrid Matrix

SiC-SiC composites are potential candidate materials for turbine components such as combustor liners, nozzle vanes and blades because of their low density, high temperature capability, and tailorable mechanical properties. The first generation of SiC-SiC composites fabricated by melt infiltration are being introduced in current engines. These materials are limited to 1315 degrees Centigrade applications because of the presence of residual silicon in the SiC matrix. Currently there is an increasing interest in developing SiC-SiC composites without silicon for structural aerospace applications above 1315 degrees Centigrade. The developmental 3D woven SiC-SiC composites with Chemical Vapor Infiltration plus Polymer Impregnation and Pyrolysis (CVI+PIP) hybrid matrix show potential for 1482 degrees Centigrade applications. In this study 3D woven SiC-SiC composite specimens were creep tested at 1482 degrees Centigrade at different stress levels until failure or for 1000 hours without failure. The failed specimens were analyzed for under a scanning electron microscope to assess the damage mechanisms. The creep data of 3D woven SiC-SiC composites with (CVI+PIP) hybrid matrix were compared with those of full CVI SiC-SiC composites and sintered SiC from the literature. The potential benefits limitations and durability of 3D woven SiC-SiC composites for turbine applications will be discussed.

creep durability↗

NASA NTRS2017

Creep/Stress Rupture Behavior and Failure Mechanisms of Full CVI and Full PIP SiC/SiC Composites at Elevated Temperatures in Air

SiC/SiC composites fabricated by melt infiltration are being considered as potential candidate materials for next generation turbine components. However these materials are limited to 2400 F application because of the presence of residual silicon in the SiC matrix. Currently there is an increasing interest in developing and using silicon free SiC/SiC composites for structural aerospace applications above 2400 F. Full PIP or full CVI or CVI + PIP hybrid SiC/SiC composites can be fabricated without excess silicon, but the upper temperature stress capabilities of these materials are not fully known. In this study, the on-axis creep and rupture properties of the state-of-the-art full CVI and full PIP SiC/SiC composites with Sylramic-iBN fibers were measured at temperatures to 2700 F in air and their failure modes examined. In this presentation creep rupture properties, failure mechanisms and upper temperature capabilities of these two systems will be discussed and compared with the literature data.

Creep Durability↗

NASA NTRS2017

Creep/Stress Rupture Behavior of 3D Woven SiC/SiC Composites with Sylramic-iBN, Super Sylramic-iBN and Hi-Nicalon-S Fibers at 2700F in Air

To determine the influence of fiber types on creep durability, 3D SiC/SiC CMCs were fabricated with Sylramic-iBN, super Sylramic-iBN and Hi-Nicalon-S fibers and the composite specimens were then tested under isothermal tensile creep at 14820C at 69, 103 and 138 MPa for up to 300hrs in air. The failed specimens were examined by scanning electron microscopy (SEM) and computed tomography (CT) for fracture mode analysis. The creep data of these composites are compared with those of other SiC/SiC composites in the literature. The results of this study will be presented.

failure modes↗

NASA NTRS2015

Creep Behavior and Durability of Cracked CMC

To understand failure mechanisms and durability of cracked Ceramic matrix composites (CMCs), Melt Infiltration (MI) SiCSiC composites with Sylramic-iBN fibers and full Chemical vapour infiltration SiCSiC composites with Sylramic-ion bombarded BN (iBN) and Hi-Nicalon -S fibers were pre-cracked between 150 to 200 megapascal and then creep and Sustained Peak Low Cycle Fatigue (SPLCF) tested at 13150 C at stress levels from 35 to 103 megapascal for up to 200 hours under furnace and burner rig conditions. In addition creep testing was also conducted on pre-cracked full Chemical vapour infiltration SiCSiC composites at 14500 C between 35 and 103 megapascal for up to 200 hours under furnace conditions. If the specimens survived the 200 hour durability tests, then they were tensile tested at room temperature to determine their residual tensile properties. The failed specimens were examined by Scanning electron microscope (SEM) to determine the failure modes and mechanisms. The influence of crack healing matrix, fiber types, crack density, testing modes and interface oxidation on durability of cracked Ceramic matrix composites (CMCs) will be discussed.

Creep↗

NASA NTRS2012

Environmental Barrier Coating (EBC) Durability Modeling; An Overview and Preliminary Analysis

A study outlining a fracture mechanics based model that is being developed to investigate crack growth and spallation of environmental barrier coating (EBC) under thermal cycling conditions is presented. A description of the current plan and a model to estimate thermal residual stresses in the coating and preliminary fracture mechanics concepts for studying crack growth in the coating are also discussed. A road map for modeling life and durability of the EBC and the results of FEA model(s) developed for predicting thermal residual stresses and the cracking behavior of the coating are generated and described. Further initial assessment and preliminary results showed that developing a comprehensive EBC life prediction model incorporating EBC cracking, degradation and spalling mechanism under stress and temperature gradients typically seen in turbine components is difficult. This is basically due to mismatch in thermal expansion difference between sub-layers of EBC as well as between EBC and substrate, diffusion of moisture and oxygen though the coating, and densification of the coating during operating conditions as well as due to foreign object damage, the EBC can also crack and spall from the substrate causing oxidation and recession and reducing the design life of the EBC coated substrate.

Abdul-Aziz, A.↗

NASA NTRS2009

Architectural Effects on Impact Resistance of Uncoated MI SiC/SiC Composites

Impact tests were conducted on uncoated 2D and 2.5D MI SiC/SiC composite specimens at room temperature and 1316 C in air. The specimens were analyzed before and after impact using optical microscopy, pulsed thermography (PT) and computed tomography (CT). Preliminary results indicate the following. Both 2-D and 2.5D composites show increase in surface and volumetric damages with increasing impact velocity. However, 2-D composites are prone to delamination cracks. In both 2D and 2.5D composites, the magnitude of impact damage at a fixed impact velocity is slightly greater at room temperature than at 1315 C. At a fixed projectile velocity and test temperature, the depth of penetration of the projectile into the substrate is significantly lower in 2.5D composites than in 2D composites. Fiber architecture plays a significant role controlling impact damage in MI SiC/SiC composites.

Bhatt, R. T.↗

NASA NTRS2008

Processing and Structural Advantages of the Sylramic-iBN SiC Fiber for SiC/SiC Components

The successful high-temperature application of complex-shaped SiC/SiC components will depend on achieving as high a fraction of the as-produced fiber strength as possible during component fabrication and service. Key issues center on a variety of component architecture, processing, and service-related factors that can reduce fiber strength, such as fiber-fiber abrasion during architecture shaping, surface chemical attack during interphase deposition and service, and intrinsic flaw growth during high-temperature matrix formation and composite creep. The objective of this paper is to show that the NASA-developed Sylramic-iBN SiC fiber minimizes many of these issues for state-of-the-art melt-infiltrated (MI) SiC/BN/SiC composites. To accomplish this, data from various mechanical tests are presented that compare how different high performance SiC fiber types retain strength during formation of complex architectures, during processing of BN interphases and MI matrices, and during simulated composite service at high temperatures.

Yun, H. M.↗

NASA NTRS2005

Foreign Object Damage Resistance of Uncoated SiC/SiC Composites

2-D woven SiC/SiC composites fabricated by melt infiltration method were impact tested at room temperature and at 1316 C in air using 1.59 mm steel-ball projectiles at projectile velocities ranging from 110 m/s to 400 m/s. The extent of substrate damage with increasing projectile velocity was imaged and analyzed using optical microscopy and nondestructive evaluation (NDE) methods. The impacted specimens were flexure and tensile tested at room temperature to determine their residual mechanical properties. Results indicate that as the projectile velocity increases, internal damage in the target material also increases and the mechanical properties degrade. At velocities > 300 m/s, the projectile penetrates through the target material, but it still retains approx. 50% ultimate strength of the as-fabricated composites and exhibits non-brittle failure.

Bhatt, R. T.↗

NASA NTRS2004

SiC/SiC Composites for 1200 C and Above

The successful replacement of metal alloys by ceramic matrix composites (CMC) in high-temperature engine components will require the development of constituent materials and processes that can provide CMC systems with enhanced thermal capability along with the key thermostructural properties required for long-term component service. This chapter presents information concerning processes and properties for five silicon carbide (SiC) fiber-reinforced SiC matrix composite systems recently developed by NASA that can operate under mechanical loading and oxidizing conditions for hundreds of hours at 1204, 1315, and 1427 C, temperatures well above current metal capability. This advanced capability stems in large part from specific NASA-developed processes that significantly improve the creep-rupture and environmental resistance of the SiC fiber as well as the thermal conductivity, creep resistance, and intrinsic thermal stability of the SiC matrices.

DiCarlo, J. A.↗

NASA NTRS2004

Advanced Constituents and Processes for Ceramic Composite Engine Components

The successful replacement of metal alloys by ceramic matrix composites (CMC) in hot-section engine components will depend strongly on optimizing the processes and properties of the CMC microstructural constituents so that they can synergistically provide the total CMC system with improved temperature capability and with the key properties required by the components for long-term structural service. This presentation provides the results of recent activities at NASA aimed at developing advanced silicon carbide (Sic) fiber-reinforced hybrid Sic matrix composite systems that can operate under mechanical loading and oxidizing conditions for hundreds of hours at 2400 and 2600 F, temperatures well above current metal capability. These SiC/SiC composite systems are lightweight (-30% metal density) and, in comparison to monolithic ceramics and carbon fiber-reinforced ceramic composites, are able to reliably retain their structural properties for long times under aggressive engine environments. It is shown that the improved temperature capability of the SiC/SiC systems is related first to the NASA development of the Sylramic-iBN Sic fiber, which displays high thermal stability, creep resistance, rupture resistance, and thermal conductivity, and possesses an in-situ grown BN surface layer for added environmental durability. This fiber is simply derived from Sylramic Sic fiber type that is currently produced at ATK COI Ceramics. Further capability is then derived by using chemical vapor infiltration (CVI) to form the initial portion of the hybrid Sic matrix. Because of its high creep resistance and thermal conductivity, the CVI Sic matrix is a required base constituent for all the high temperature SiC/SiC systems. By subsequently thermo- mechanical-treating the CMC preform, which consists of the S ylramic-iBN fibers and CVI Sic matrix, process-related defects in the matrix are removed, further improving matrix and CMC creep resistance and conductivity.

Yun, H. M.↗

NASA NTRS2004

SiC/SiC Composites for Applications Above 2600 F

Using data from panels with 2D woven architectures,t his presentation describes progress in identifying advanced constituent materials and processes for achieving a high conductivity SiC/SiC composite system with long-term structural life under oxidizing conditions at 2600 degrees F. A key factor for this progress is the development of approaches at NASA GRC that allow the constituent SiC fiber and SiC matrix to display high thermal conductivity, high creep-rupture resistance and high microstructural stability at temperatures above 3000 degree F. Another important factor is the avoidance of free silicon in the SiC matrix, thereby providing the composites with the capability for extended service above the silicon melting point (2550 degrees F). The effects of utilizing various materials and processes for the SiC matrix are presented and discussed.

DiCarlo, James A.↗

NASA NTRS2004

Foreign Object Damage Resistance of Uncoated and EBC Coated SiC/SiC Composites

SiC/SiC composites fabricated by the melt infiltration method are candidate materials for the turbine components such as combustor liners, nozzle vanes and blades because of their high temperature strength, and thermal conductivity. To avoid surface recession in a combustion environment containing moisture, these materials also require an environmental barrier coating (EBC). Under thermal loading condition alone, the EBC coated SiC/SiC components show microstructural and strength stability after -1 5000 hr exposure to 12OO0C in combustion environment. However, the stability of SiC/SiC composites under impact conditions is not fully understood. In this study, both uncoated and EBC coated SiC/SiC composite specimens were impact tested by steel-ball projectiles at room temperature and at 1300 C in air. After impact, the specimens were analyzed under a scanning electron microscope to assess coating damage, and then tensile and bend tested at room temperature to determine their residual strength. The extent of coating and substrate damage with increasing projectile velocity was imaged and analyzed using nondestructive evaluation (NDE) methods such as micro focus x-ray radiography, tomography, and thermal wave imaging. The mechanical property results of impacted specimens are correlated with the NDE results and the scanning electron microscopy (SEM) observations. Influence of projectile velocity on impact damage of the coating/substrate, and the implication coating damage on durability of the composite will be discussed.

Bhatt, R. T.↗

NASA NTRS2004

Thermo-Mechanical Properties of SiC/SiC Composites with Hybrid CVI-PIP Matrices

For long term structural service, the upper temperature capability for slurry-cast melt infiltrated (MI) SiC/SiC composites is limited to approx. 1315 C because of silicon reaction with the SiC fibers. For applications requiring material temperatures in excess of 1315 C, alternate methods of manufacturing the SiC matrices without silicon are being investigated, such as a hybrid combination of CVI and PIP. In this study, stacked fabric plies of Sylramic i-BN SiC fibers were coated with a CVI BN interface layer followed by a partial CVI SiC matrix. The remaining porosity in the SiC/SiC preforms was then infiltrated with silicon carbide matrix by PIP. Thermo-mechanical property measurements indicate that these composites are stable to 1700 C in inert environments under no load conditions for 100 h and under load conditions to 1450 C in air for 300 h. The advantages, disadvantages, and potential of this composite system for high temperature applications will be discussed.

Bhatt, R. T.↗

NASA NTRS2003

Microstructural and Defect Characterization in Ceramic Composites Using an Ultrasonic Guided Wave Scan System

In this study, an ultrasonic guided wave scan system was used to characterize various microstructural and flaw conditions in two types of ceramic matrix composites, SiC/SiC and C/SiC. Rather than attempting to isolate specific lamb wave modes to use for characterization (as is desired for many types of guided wave inspection problems), the guided wave scan system utilizes the total (multi-mode) ultrasonic response in its inspection analysis. Several time and frequency-domain parameters are calculated from the ultrasonic guided wave signal at each scan location to form images. Microstructural and defect conditions examined include delamination, density variation, cracking, and pre/ post-infiltration. Results are compared with thermographic imaging methods. Although the guided wave technique is commonly used so scanning can be eliminated, applying the technique in the scanning mode allows a more precise characterization of defect conditions.

Roth, D. J.↗

NASA NTRS1998

Mechanical Behavior and Analytical Modeling of Melt-Infiltrated SiC/SiC Woven Composite

The desirable properties in ceramic matrix composites (CMCs), such as high temperature strength, corrosion resistance, high toughness, low density, or good creep resistance have led to increased use of CMCs in high-speed engine structural components and structures that operate in extreme temperature and hostile aero-thermo-chemical environments. Ceramic matrix composites have been chosen for turbine material in the design of 21st century civil propulsion systems to achieve high fuel economy, improved reliability, extended life, and reduced cost. Most commercial CMCs are manufactured using a chemical vapor infiltration (CVI) process. However, a lower cost fabrication known as melt-infiltration process is also providing CMCs marked for use in hot sections of high-speed civil transports. Limited samples of a SiC/SiC melt-infiltrated woven composites are being investigated at room and elevated temperature below and above matrix cracking. These samples show graceful failure and toughness at room temperature with a reduction in strength and modulus at elevated temperatures. A generic finite element model is also being developed to predict monotonic and cyclic loading behavior of the woven composite. Use of the initial test data from the woven composite is being used for the development of the analytical model. This model is the first of a iterative process leading towards the development the model's capability to predict behavior at room and elevated temperature for monotonic and cyclic loading. The purpose of this paper is to report on the material and mechanical findings of the SiC/SiC melt-infiltrated woven composite and progress on the development of the finite element model.

Lang, J.↗

NASA NTRS1998

The Evolution of Interfacial Sliding Stresses During Cyclic Push-in Testing of C- and BN-Coated Hi-Nicalon Fiber-Reinforced CMCs

Interfacial debond cracks and fiber/matrix sliding stresses in ceramic matrix composites (CMCs) can evolve under cyclic fatigue conditions as well as with changes in the environment, strongly affecting the crack growth behavior, and therefore, the useful service lifetime of the composite. In this study, room temperature cyclic fiber push-in testing was applied to monitor the evolution of frictional sliding stresses and fiber sliding distances with continued cycling in both C- and BN-coated Hi-Nicalon SiC fiber-reinforced CMCs. A SiC matrix composite reinforced with C-coated Hi-Nical on fibers as well as barium strontium aluminosilicate (BSAS) matrix composites reinforced with BN-coated (four different deposition processes compared) Hi-Nicalon fibers were examined. For failure at a C interface, test results indicated progressive increases in fiber sliding distances during cycling in room air but not in nitrogen. These results suggest the presence of moisture will promote crack growth when interfacial failure occurs at a C interface. While short-term testing environmental effects were not apparent for failure at the BN interfaces, long-term exposure of partially debonded BN-coated fibers to humid air resulted in large increases in fiber sliding distances and decreases in interfacial sliding stresses for all the BN coatings, presumably due to moisture attack. A wide variation was observed in debond and frictional sliding stresses among the different BN coatings.

Eldridge, J. I.↗

NASA NTRS1998

Surface Area, and Oxidation Effects on Nitridation Kinetics of Silicon Powder Compacts

Commercially available silicon powders were wet-attrition-milled from 2 to 48 hr to achieve surface areas (SA's) ranging from 1.3 to 70 sq m/g. The surface area effects on the nitridation kinetics of silicon powder compacts were determined at 1250 or 1350 C for 4 hr. In addition, the influence of nitridation environment, and preoxidation on nitridation kinetics of a silicon powder of high surface area (approximately equals 63 sq m/g) was investigated. As the surface area increased, so did the percentage nitridation after 4 hr in N2 at 1250 or 1350 C. Silicon powders of high surface area (greater than 40 sq m/g) can be nitrided to greater than 70% at 1250 C in 4 hr. The nitridation kinetics of the high-surface-area powder compacts were significantly delayed by preoxidation treatment. Conversely, the nitridation environment had no significant influence on the nitridation kinetics of the same powder. Impurities present in the starting powder, and those accumulated during attrition milling, appeared to react with the silica layer on the surface of silicon particles to form a molten silicate layer, which provided a path for rapid diffusion of nitrogen and enhanced the nitridation kinetics of high surface area silicon powder.

Bhatt, R. T.↗

NASA NTRS1997

An Investigation of SiC/SiC Woven Composite Under Monotonic and Cyclic Loading

The desirable properties in ceramic matrix composites (CMCs), such as high temperature strength, corrosion resistance, high toughness, low density, or good creep resistance have led to increased use of CMCs in high-speed engine structural components and structures that operate in extreme temperature and hostile aero-thermo-chemical environments. Ceramic matrix composites have been chosen for turbine material in the design of 21 st-century civil propulsion systems to achieve high fuel economy, improved reliability, extended life, and reduced cost. Most commercial CMCs are manufactured using a chemical vapor infiltration (CVI) process. However, a lower cost fabrication known as melt-infiltration process is also providing CMCs marked for use in hot sections of high-speed civil transports. The scope of this paper is to report on the material and mechanical characterization of the CMCs subjected to this process and to predict the behavior through an analytical model. An investigation of the SiC/SiC 8-harness woven composite is ongoing and its tensile strength and fatigue behavior is being characterized for room and elevated temperatures. The investigation is being conducted at below and above the matrix cracking stress once these parameters are identified. Fractography and light microscopy results are being studied to characterize the failure modes resulting from pure uniaxial loading. A numerical model is also being developed to predict the laminate properties by using the constituent material properties and tow undulation.

Lang, J.↗