CORRECTION OF THE POST – NECKING TRUE STRESS – STRAIN DATA USING INSTRUMENTED NANOINDENTATION

The study of large plastic deformations has been the focus of numerous studies particularly in the metal forming processes and fracture mechanics fields. A good understanding of the plastic flow properties of metallic alloys and the true stresses and true strains induced during plastic deformation is crucial to optimize the aforementioned processes, and to predict ductile failure in fracture mechanics analyzes. Knowledge of stresses and strains is extracted from the true stress-strain curve of the material from the uniaxial tensile test. In addition, stress triaxiality is manifested by the neck developed during the last stage of a tensile test performed on a ductile material. This necking phenomenon is the factor responsible for deviating from uniaxial state into a triaxial one, then, providing an inaccurate description of the material’s behavior after the onset of necking The research of this dissertation is aimed at the development of a correction method for the nonuniform plastic deformation (post-necking) portion of the true stress- strain curve. The correction proposed is based on the well-known relationship between hardness and flow (yield) stress, except that instrumented nanoindentation hardness is utilized rather than conventional macro or micro hardness. Three metals with different combinations of strain hardening behavior and crystal structure were subjected to quasi- static tensile tests: power-law strain hardening low carbon G10180 steel (BCC) and electrolytic tough pitch copper C11000 (FCC), and linear strain hardening austenitic stainless steel S30400 (FCC). Nanoindentation hardness values, measured on the broken tensile specimen, were converted into flow stress values by means of the constraint factor from Tabor’s, the representative plastic strain and the post-test true plastic strains measured. Micro Vickers hardness testing was carried out on the sample as well. The constraint factors were 5.5, 4.5 and 4.5 and the representative plastic strains were 0.028, 0.062 and 0.061 for G101800, C11000 and S30400 respectively. The established corrected curves relating post-necking flow stress to true plastic strain turned out to be well represented by a power-law function. Experimental results dictated that a unique single value for and for is not appropriate to describe materials with different plastic behaviors. Therefore, Tabor’s equation, along with the representative plastic strain concept, has been misused in the past. The studied materials exhibited different nanohardness and plastic strain distributions due to their inherently distinct elasto-plastic response. The proposed post- necking correction separates out the effect of triaxiality on the uniaxial true stress-strain curve provided that the nanohardness-flow stress relationship is based on uniaxial values of stress. Some type of size effect, due to the microvoids at the tip of the neck, influenced nanohardness measurements. The instrumented nanoindentation technique proved to be a very suitable method to probe elasto-plastic properties of materials such as nanohardness, elastic modulus, and quasi-static strain rate sensitivity among others. Care should be taken when converting nanohardness to Vickers and vice versa due to their different area definition used. Nanohardness to Vickers ratio oscillated between 1.01 and 1.1

Development of Eco-Friendly Composite Foam Boards for Thermal Insulation and Packaging Purposes Using Cellulose Nanofibrils (CNF)

Reducing energy consumption is a high priority in the United States and throughout the world. Energy used to heat and cool occupied constructed facilities is of particular concern, and one of the most effective strategies is insulating the building envelope. Historically, builders used whatever material was available to fill the void between interior and exterior walls, including wool fibers, paper, and even corn cobs. Today, homes are built using foam insulation that harden when applied, blown-in loose insulation, fiberglass mats or rigid foam boards usually composed of polystyrene. Rigid foam boards are used in a variety of applications despite the fact that they typically contain non bio-based materials, require substantial amount of energy to produce, and are not easily recycled. A new “green” insulation material is needed that uses a new raw material and a new process to create its structure. In this study cellulose nanofibrils (CNF) were used as the raw material and industrial corn-starch was used as a binder that uses hydrogen bonding for cross linking to create a successful thermal insulation foam board. Cellulose, one of the most ubiquitous and abundant renewable polymers on the planet, can be obtained from a variety of sources including trees, agricultural crops, bacteria, and even from animals. The material’s abundance and properties have increased research on cellulose and its derivatives in recent years. Cellulose nanofibrils are organic polymers that can be obtained through chemical or mechanical methods. The CNF used in this study was produced by the mechanical breakdown of softwood cellulose fibers. Starch is an abundant green polymer and is a promising raw component for the development of novel materials. However, starch has low mechanical properties. In this research, industrial corn starch was reinforced with CNF suspensions through a unique freeze-drying technique. The research showed significant improvement in the mechanical properties and micromechanical models were created to understand the role of CNF in the composite foam boards. In addition to the theoretical modeling, practical investigation was performed to determine the nanomechanical properties of CNF using an Atomic Force Microscope (AFM) equipped with a Nanoindenter (NI). This study resulted in successful development of eco-friendly composite foam boards that could be used for thermal insulation and packaging purposes. The nanomechanical properties of CNF were determined, the knowledge and information is a contribution to our understanding of the role of CNF in composite structures. The results of this study show a significant opportunity for using CNF and the data on nanomechanical properties of CNF will provide crucial information to other researchers and industry experts who work on nanocellulose composites and on understanding the role of CNF in the composites

Scribe Marks at Fuselage Joints: Determination of Residual Stress and Effects of Fatigue Loading Using Nanoindentation and Snychrotron X-Ray Diffraction

Empirical understanding of fatigue crack growth from small defects is of tremendous importance and of significant concern for structural integrity of aerospace structures. In fatigue, a crack initiates from a stress concentration location and causes premature failure. In principle fatigue life for scribes and scratches is function of the stress concentration around the root which depends upon the depth and root radius of the scribe, the associated microstructure, the residual stress field, work hardening from plastic deformation during scribing and relaxation or redistribution of these residual stresses in fatigue. The scope of the present work is on determination of the residual stress field around scribe marks of different geometries and the effect of fatigue loading on the residual stress field. The determination of a local residual stress field in a small area of 100 μm x 100 μm around shallow scribes (<150 μm deep) is a very challenging engineering problem. Additionally, the large grain size of A1 2024-T351 (-20 μm), anisotropy between grains and texture makes further difficulties. The residual stress field associated with scribes of different geometries produced by different tools has been measured using synchrotron X-ray diffraction. It was found that some tools produce a severe tensile stress field and work hardening around the root of scribe as compared to the other tools which also produce a tensile residual stress field but without work hardening. Additionally, a method of extraction of residual stress has been developed using the nanoindentation load-displacement data. The method has been applied to extract residual stresses while taking into account the plastic deformation around scribe roots. The residual stress field of twenty nine scribes of different geometries produced from different tools has been determined from the nanoindentation method. Depending upon the tool and root radius, residual stresses were different in magnitude and were found in the range of +100MPa to +200MPa. The effect of fatigue loading on the relaxation of pre-existing residual stresses was studied. It was found that stress field around scribes is not affected by fatigue loading. It was concluded that fatigue life of these scribes should be examined with consideration of residual stresses. Any crack initiation and propagation model without consideration of residual stress fields may predict more conservative lives in fatigue

COMPRESSIVE BEHAVIOR OF BULK METALLIC GLASS UNDER DIFFERENT CONDITIONS —— COUPLED EFFECT OF TEMPERATURE AND STRAIN RATE

Metallic glass was first reported in 1960 by rapid quenching of Au-Si alloys. But, due to the size limitation, this material did not attract remarkable interest until the development of bulk metallic glasses (BMGs) with specimen sizes in excess of 1 mm. BMGs are considered to be promising engineering materials because of their ultrahigh strength, high elastic limit and wear resistance. However, they usually suffer from a strong tendency for localized plastic deformation with catastrophic failure. Many basic questions, such as the origin of shear softening and the strain rate effect remain unclear. In this thesis, the mechanical behavior of the Zr55Al10Ni5Cu30 bulk metallic glass and a metallic glass composite is investigated. The stress-strain relationship for Zr55Al10Ni5Cu30 over a wide range of strain rate (5 × 10-5 to 2 × 103s-1) was investigated in uniaxial compression loading using both MTS servo-hydraulic system (quasi-static) and compression Kolsky bar system (dy- namic). The effect of the strain rate on the fracture stress at room temperature was discussed. Based on the experimental results, the strain rate sensitivity of the bulk metallic glass changes from a positive value to a negative value at high strain rate, which is a consequence of the significant adiabatic temperature rise during the dynamic testing. In order to characterize the temperature effect on the mechanical behavior of the metallic glass, a synchronically assembled heating unit was designed to be attached onto the Kolsky bar system to perform high temperature and high strain rate mechanical testing. A transition from inhomogeneous deformation to homoge- neous deformation has been observed during the quasi-static compressive experiments at testing temperatures close to the glass transition temperature. However, no tran- sition has been observed at high strain rates at all the testing temperatures. A free volume based model is applied to analyze the stress-strain behavior of the homoge- neous deformation. To further examine the inelastic deformation of the Zr-based bulk metallic glasses, instrumented nanoindentation experiments were performed. A tran- sition from discrete plastic deformation to continuous plastic deformation was found when strain rate is increased but still within the quasi-static strain rate region. Moti- vated by the metal matrix composite material, a tungsten reinforced BMG composite was investigated at quasi-static and dynamic strain rates. The mechanical behavior of the metallic glass matrix was improved significantly by the addition of W particles

TRIBOLOGICAL PROPERTIES AND WEAR MECHANISM OF HARD COATINGS

In the modern technology, tribologically suitable components and devices are important to increase the energy efficiency. It is possible when one can reduce the friction coefficient and wear of sliding components. The economic effectiveness can be achieved by better tribological system and therefore research in tribology is aimed at minimizing the energy losses resulting from friction and wear. In this view, hard coatings deposited by physical vapor deposition (PVD) are adequate solutions for increasing the work efficiency, lifetime of tools and components. The present thesis deals with hard tribological materials γ-TiAl and coatings such as TiAlN, CrN/NbN superlattice, diamond like carbon (DLC) and nanocrystalline diamond nanowire (DNW) films. Various characterization techniques were used to study morphology, microstructure and chemical state of the materials. The thesis describes tribological properties of above mentioned hard coatings sliding against 100Cr6 steel, Al2O3 and SiC balls. It also describes friction and wear based on classified mechanisms and outlines material properties that influence the performance of sliding surfaces. Traditionally wear is associated with friction and wear mechanisms are classified as adhesion, abrasion, erosion, fatigue and oxidational. Mechanical and tribological properties of γ-TiAl alloy, TiAlN, CrN, carbon based coatings of DLC and nanocrystalline DNW were reviewed. The importance of such hard coatings and critical application in machine and industries are highlighted. Moreover, tribological properties and evaluation of wear mechanism is introduced in the respect of microstructure and chemical behavior of the sliding interfaces. Various wear mechanism with different combination of sliding surfaces such as hard coating/soft ball and soft ball/hard coating is reviewed in order to understand the wear mechanism. The fundamentals of some of the characterization techniques used to study the mechanical, tribological, morphological, structural, and chemical properties of the coating and wear track is introduced. Instrumented micro-indentation technique have been used to characterize the mechanical properties namely hardness and elastic modulus of γ-TiAl alloy. The recorded indentation curves and the related energetic properties were analysed in order to compare the Attaf energetic approach and Oliver-Pharr method. Moreover, tribological behavior of γ-TiAl alloy was studied by sliding against 100Cr6 steel, SiC and Al2O3 balls as counterbodies for friction pairs. The friction coefficient and wear rate was found to high when γ-TiAl alloy slides against Al2O3 and SiC balls. However, these values were less while sliding against steel ball. The wear mechanism is explained by the sliding combination of harder/harder system such as SiC/γ-TiAl, and Al2O3/γ-TiAl alloy. However, steel/γ-TiAl alloy acts as softer/harder sliding combination. Tribological behavior of TiAlN coating were studied by sliding against 100Cr6 steel, SiC and Al2O3 as counterbodies for friction pairs. Two distinct types of wear modes such as oxidational and plastic deformation are investigated. It is shown that wear of metal debris tribochemically reacted with moisture available in ambient atmosphere and metal oxide formation which leads oxidational wear in TiAlN/steel sliding pair. In TiAlN/SiC sliding pair, low friction coefficient is measured and this is attributed to the formation of lubricious composite tribofilm. In contrast, TiAlN/Al2O3 pair shows high friction coefficient and wear mechanism is governed by plastic deformation. CrN/NbN superlattice coating sliding against 100Cr6 steel, SiC and Al2O3 ball as counterbodies for friction pairs was investigated. The value of friction coefficient and wear rate was lowest ~0.01 and 2.6×10–7 mm3/Nm, respectively, when coating slides against Al2O3 ball. In contrast, friction coefficient and wear rate is increased while sliding with steel and SiC balls. It is observed that the deviation in friction coefficient is described by mechanical and chemical properties of these balls. In this respect, hardness of Al2O3 and SiC ball is comparable but significant deviation in friction coefficient is observed. This is related to oxidation resistance of balls which is high for Al2O3 compared to SiC as evident by Raman analysis of the wear track. However, steel ball shows oxidational wear mechanism against CrN/NbN superlattice coating. The tribological properties of DLC and nanocrystalline DNW films were investigated. A friction mechanism based on surface chemistry and mechanical properties of sliding interfaces such as DLC/100Cr6 steel, DLC/SiC and DLC/Al2O3 is studied. In DLC film, the high friction coefficient is governed by surface roughness of the sliding interfaces during initial sliding passes. However, for longer sliding cycles, the sliding interfaces get smoothened and magnitude of friction coefficient is reduced. Under these experimental conditions, carbonaceous transferlayer forms on the ball sliding surface. Nanocrystalline DNW films was deposited in N2 enriched microwave plasma enhanced chemical vapor deposition (MPECVD) system. As-deposited DNW film was treated in O2 plasma which resulted in chemical and microstructural modification. The sheath of the nanocrystalline DNW is chemically constituted by amorphous carbon (a-C) and graphite (sp2C-C) like bondings. However, nanowires transformed into ultra- small spherical grains after the O2 plasma treatments. In this condition, a-C and sp2C-C fraction get significantly reduced due to plasma etching. Oxidation and formation of functional groups increases on the surface and inside the wear track. The friction coefficient of O2 plasma treated film showed super low value of ~0.002 with exceeding high wear resistance of 2×10–12mm3/Nm. Such an advance tribological properties is explained by passivation of covalent carbon bonding and transformation of sliding surfaces by van der Waals and hydrogen bondings. High surface energy and the consequent superhydrophilic behavior of film attributed to the formation of an adsorbate layer of above mentioned functional groups which acts as a lubricant

Micromechanics of oxides - From complex scales to single crystals

Protective oxide scales shield high temperature materials from corrosion, thus ensuring safety and long material life under adverse operating conditions. Cracking and spallation of such scales can lead to fatigue crack initiation and expose the material to further oxidation. It is therefore imperative to measure the fracture properties of oxides so that they can be incorporated in the life estimation models of high temperature materials. Existing models require inputs on oxide properties such as fracture strain and elastic modulus. The established measurement methods are mainly applied for thick (several microns) scales, but for many materials such as superalloys the oxides are thinner (&lt; 1 \ub5m), and the results would be affected by the influence of substrate and residual stresses. Focused ion beam machining (FIB) enables the preparation of micro sized specimens in the size range of these scales. \ua0In this work, a modified microcantilever geometry with partially removed substrate is proposed for testing of oxide scales. Room temperature microcantilever bending of thermally grown superalloy oxide (complex oxide with an upper layer of spinel and lower layer of Cr2O3) revealed the presence of plasticity, which is attributed to the deformation of the upper cubic spinel layer and low defect density of the volume being probed. Due to difficulties in isolating Cr2O3 from the complex oxide layer, dedicated oxidation exposures are performed on pure chromium to generate Cr2O3 which is tested using the same cantilever geometry at room temperature and 600 \ub0C. Results show lower fracture strain at 600 \ub0C in comparison to room temperature and presence of cleavage type of transgranular fracture in both cases, pointing to a need for studying cleavage fracture of Cr2O3. This was analysed using microcantilever bending of single crystal Cr2O3 to identify the preferential cleavage planes. Finally, fracture toughness was also measured through microcantilever bending and micropillar splitting. \ua0Thus, it is shown that micromechanical testing is an effective tool for measuring fracture properties of oxide scales. The fracture study of Cr2O3 scales show that it is a complex process in which the crystallographic texture also plays a role. Surface energy and fracture toughness criterion was unable to explain the fracture behaviour of single crystal Cr2O3 observed from experiments. Such a comprehensive analysis can contribute towards the development of reliable models for oxidation assisted failure

Effects of mechanical properties on the reliability of Cu/low-k metallization systems

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (leaves 211-217).Cu and low-dielectric-constant (k) metallization schemes are critical for improved performance of integrated circuits. However, low elastic moduli, a characteristic of the low-k materials, lead to significant reliability degradation in Cu-interconnects. A thorough understanding of the effects of mechanical properties on electromigration induced failures is required for accurate reliability assessments. During electromigration inside Cu-interconnects, a change in atomic concentration correlates with a change in stress through the effective bulk modulus of the materials system, B, which decreases as the moduli of low-k materials used as inter-level dielectrics (ILDs) decrease. This property is at the core of discussions on electromigration-induced failures by all mechanisms. B is computed using finite element modeling analyses, using experimentally determined mechanical properties of the individual constituents. Characterization techniques include nanoindentation, cantilever deflection, and pressurized membrane deflection for elastic properties measurements, and chevron-notched double-cantilever pull structures for adhesion measurements. The dominant diffusion path in Cu-interconnects is the interface between Cu and the capping layer, which is currently a Si3N4-based film. We performed experiments on Cu-interconnect segments to investigate the kinetics of electromigration. A steady resistance increase over time prior to open-circuit failure, a result of void growth, correlates with the electromigration drift velocity. Diffusive measurements made in this fashion are more fundamental than lifetime measurements alone, and correlate with the combined effects of the electron wind and the back stress forces during electromigration induced void growth.(cont.)Using this method, the electromigration activation energy was determined to be 0.80±0.06eV. We conducted experiments using Cu-interconnects with different lengths to study line length effects. Although a reliability improvement is observed as the segment length decreases, there is no deterministic current-density line-length product, jL, for which all segments are immortal. This is because small, slit-like voids forming directly below vias will cause open-failures in Cu-interconnects. Therefore, the probabilistic jLcrit values obtained from via-above type nterconnects approximate the thresholds for void nucleation. The fact that jLcrit,nuc monotonically decreases with B results from an energy balance between the strain energy released and surface energy cost for void nucleation and the critical stress required for void nucleation is proportional to B. We also performed electromigration experiments using Cu/low-k interconnect trees to investigate the effects of active atomic sinks and reservoirs on interconnect reliability. In all cases, failures were due to void growth. Kinetic parameters were extracted to be ... Quantitative analysis demonstrates that the reliability of the failing segments is modulated by the evolution of stress in the whole interconnect tree. During this process, not only the diffusive parameters but also B play critical roles. However, as B decreases, the positive effects of reservoirs on reliability are diminished, while the negative effects of sinks on reliability are amplified.(cont.) Through comprehensive failure analyses, we also successfully identified the mechanism of electromigration-induced extrusions in Cu/low-k interconnects to be nearmode-I interfacial fracture between the Si3N4-based capping layer and the metallization/ILD layer below. The critical stress required for extrusion is found to depend not only on B but also on the layout and dimensions of the interconnects. As B decreases, sparsely packed, wide interconnects are most prone to extrusion-induced failures. Altogether, this research accounts for the effects of mechanical properties on all mechanisms of failure due to electromigration. The results provide an improved experimental basis for accurate circuit-level, layout-specific reliability assessments.by Frank LiLi Wei.Ph.D