Interferometric studies on the directional hardness of some single metal crystal

A directional effect has been observed when indentation hardness tests were made with a Double-Cone indenter on single crystals of Tin and Bismuth, The distortion around the impressions made during, the teats was studied by multiple beam interferometric techniques. The surface contours shown by the fringes revealed deformation of an unsymmetrical nature, with "piling-up" and "sinking-in occurring in directions mutually at right angles. The fringes also revealed slip traces, of step heights of the order 200 A.u. or less, crossing the hill formations. As the direction of indentation on the surface was changed, the observed interference patterns varied continuously, the amount of deformation passing from a maximum to a minimum and returning to the original form when the indenter had rotated through 180&deg;. Because of the presence of the slip traces an attempt was made to explain the directional effects observed, in terms of the plastic deformation properties of the crystals and the direction of the forces exerted on the crystal during indentation. As the Indenter was rotated with respect to the crystal, it was found that the value of the shear stress t component on the activated slip planes varied in a similar manner as the measured lengths of the indentations. It is suggested that the hardness variation is a measure of the change in yield stress in the crystal, as the, deformation takes place under strainless conditions. <p

The plasticity of diamond

Aspects of the crystal structure of diamond, and its associated defects, have been considered with reference to the effect such characteristics might have on its mechanical properties. Also, established resolved shear stress models, which account for anisotropy in conventional Knoop indentation hardness of all single crystals, have been reviewed. Particular attention has been given to the role of microplasticity and the nature of crack formation in the deformed zone formed beneath the indenter. It is then shown that a similar approach can be applied to the case where a cone, made from a softer material, replaces the conventional rigid indenter. By using different materials covering a range of hardness, impressions can be formed beneath which there is a controlled density and depth of dislocations.In this work, the 'soft' indenter technique has been extended to high temperatures and applied to study the plasticity of various types of natural and synthetic diamond. Consequently, the effect of temperature on the critical resolved shear stress of synthetic type Ib, and natural type Ia and type IIa has been established.Above a critical threshold temperature for the onset of plasticity, time dependent growth of the impression volume occurs whilst the mean contact pressure is decreasing. It is shown that geometrical similarity, i.e. the ratio of the impression size to dislocated volume, is maintained whilst the critical mean pressure continues to be exceeded during this process of 'impression creep'. Activation energies of about 2.9 eV and 1.2 eV were determined, from rates of volume change, for natural (both type I and II) and synthetic type Ib respectively.Whilst no significant differences were observed between 98.9% 12C (natural abundance) and 99.9% 12C (isotopically enriched) synthetic diamonds, their behaviour was most like that of a type IIb diamond. Finally, by studying type la diamonds with a nitrogen concentration ranging from 14 - 750 ppm, evidence is obtained which supports the suggestion that this element reduces the intrinsic resistance to dislocation movement and encourages the initiation of cracks in the diamond structure

Rolling contact fatigue failures in silicon nitride and their detection

The project investigates the feasibility of using sensor-based detection and processing systems to provide a reliable means of monitoring rolling contact fatigue (RCF) wear failures of silicon nitride in hybrid bearings. To fulfil this investigation, a decision was made early in the project to perform a series of hybrid rolling wear tests using a twin disc machine modified for use on hybrid bearing elements.The initial part of the thesis reviews the current understanding of the general wear mechanisms and RCF with a specific focus to determine the appropriate methods for their detection in hybrid bearings. The study focusses on vibration, electrostatic and acoustic emission (AE) techniques and reviews their associated sensing technologies currently deployed with a view of adapting them for use in hybrids. To provide a basis for the adaptation, an understanding of the current sensor data enhancement and feature extraction methods is presented based on a literature review.The second part describes the test equipment, its modifications and instrumentation required to capture and process the vibration, electrostatic and AE signals generated in hybrid elements. These were identified in an initial feasibility test performed on a standard twin disc machine. After a detailed description of the resulting equipment, the thesis describes the calibration tests aimed to provide base data for the development of the signal processing methods.The development of the signal processing techniques is described in detail for each of the sensor types. Time synchronous averaging (TSA) technique is used to identify the location of the signal sources along the surfaces of the specimens and the signals are enhanced by additional filtering techniques.The next part of the thesis describes the main hybrid rolling wear tests; it details the selection of the run parameters and the samples seeded with surface cracks to cover a variety of situations, the method of execution of each test run, and the techniques to analyse the results.The research establishes that two RCF fault types are produced in the silicon nitride rolling element reflecting essentially different mechanisms in their distinct and separate development; i) cracks, progressing into depth and denoted in this study as C-/Ring crack Complex (CRC) and ii) Flaking, progressing primarily on the surface by spalls. Additionally and not reported in the literature, an advanced stage of the CRC fault type composed of multiple and extensive c-cracks is interpreted as the result of induced sliding in these runs. In general, having reached an advanced stage, both CRC and Flaking faults produce significant wear in the steel counterface through abrasion, plastic deformation or 3-body abrasion in at least three possible ways, all of which are described in details

NANOINDENTATION OF A ZINC METAL SOAP MIXTURE FOR USE IN A LASER PRINTER

At the start of this project, the possible choices of metal soaps had already been narrowed to include some of the zinc soaps used in this project. These zinc soaps are mixtures of zinc stearate and zinc palmitate of varying ratios purchased from a supplier. Zinc soap was chosen as result of its common use in various industries as a lubricant and mold release, which implied potential benefits in an electrophotographic printing system. These potential benefits include, but are not limited to, a more efficient transfer from a photoconductive drum and protection of the drum from mechanical and chemical degradation. Nanoindentation of these soaps was implemented in an effort to characterize each soap mixture and compare how the soap types differed from one another. Each sample was indented under a variety of different maximum loads and at different holding times to observe effects on the modulus, hardness, and, creep. The mechanical properties measured were then used to help distinguish differences between each type and provide an insight as to how or why one mixture may be preferable over another. The data could be utilized in conjunction with further testing to be used in a simulation of an interface of interest

A Study of Indentation Cracking in Brittle Materials Using Cohesive Zone Finite Elements

Cohesive zone finite element simulations of pyramidal indentation cracking in brittle materials have been carried out in order to: (1) critically examine indentation cracking models that relate fracture toughness to indentation data; (2) determine the underlying physical mechanisms of indentation crack growth from a continuum view and their relationship to material properties; (3) explore the influence of indenter geometry on crack extension; and (4) provide a platform from which future simulations can add more complex material behavior as well as guidance for experimental measurements of fracture toughness. Standard fracture toughness geometries in addition to simplified indentation geometries were simulated in order to assess the advantages and limitations of using cohesive zone finite element simulations to study indentation cracking in brittle materials. Simulation results were found to be consistent with linear-elastic fracture mechanics when crack lengths approximately 10 times larger than process zone sizes. Results from Vickers indentation cracking simulations showed deviations from standard models and additional material dependencies not considered in therein. A transition in cracking behavior from median type cracks to Palmqvist type cracks was observed as the ratio of elastic modulus to hardness increased and plasticity played a more prominent role in the deformation response. Separate stress intensity factor solutions were derived for the two cracking regimes by applying simple scaling relationships and observations from the finite elements. Simulations of different indenter geometries were found to correlate well with the stress intensity factors. In addition, the indentation cracking response could be tailored to a specific behavior by changing the indenter centerline-to-face angle. Cohesive zone finite element simulations were found to be well suited to exploring, improving, and studying the materials science of indentation cracking

Activated Charcoal Versus a Chemical Whitening Agent: Effect on Human Enamel

Objective: For many years, the conventional method to achieve a white smile has been chemical regimens such as peroxide compounds. Previous studies suggest risks associated with chemical procedures, so many are seeking out alternative options such as utilizing activated charcoal. Since this new method is rapidly trending, there have not been many scientific studies on how utilizing activated charcoal can affect the mechanical/physical properties of tooth structure. The primary objective of this study was to investigate the effect of activated charcoal on enamel’s mechanical/physical properties and compare it to a conventional tooth whitening method (carbamide peroxide). Materials and Methods: Extracted adult human molars and premolars with no/minimal restorations and/or decay were collected from local dental offices and disinfected with 1% thymol solution for 48 hours. A total of 30 collected teeth were prepared and sliced in half, resulting in 60 specimens (n=60) that were stained with coffee for 72 hours. The samples were randomly assorted into six groups with 10 samples per group: Control without Brushing (CW), Control with Brushing (CWB), Activated Charcoal/Dab (ACNB), Activated Charcoal/Brush (ACB), Opalescence 20%/Brush (OPB) and Opalescence 20%/Dab (OPNB)). Each group was treated accordingly for 14 days. All groups were tested for color, hardness and surface roughness via a spectrophotometer, Vickers hardness and a 3D measuring laser microscope, respectively. Data were analyzed by a multiple variable linear regression model with a significance level at P \u3c 0.002. Results: Over a period of 14 days, OPNB produced a significantly greater color change (Eab and E00) compared to the Activated Charcoal and Control Groups. Treatment with Activated Charcoal significantly changed color more than the Control without Brushing, but not the Control with Brushing. The greatest contributor to E was a change in L* (lightening/whitening). Brushing had a negative effect on the whitening of the Opalescence treatment. No significant differences were found for microhardness and surface roughness amongst the six groups within 14 days of treatment. Conclusion: In general, chemical whitening with carbamide peroxide was more effective than using activated charcoal in treating stained enamel. Both whitening methods were not detrimental to enamel in this in vitro study as they did not alter microhardness or surface roughness

Tribological properties of structural ceramics

The tribological and lubricated behavior of both oxide and nonoxide ceramics are reviewed in this chapter. Ceramics are examined in contact with themselves, other harder materials and metals. Elastic, plastic and fracture behavior of ceramics in solid state contact is discussed. The contact load necessary to initiate fracture in ceramics is shown to be appreciably reduced with tangential motion. Both friction and wear of ceramics are anisotropic and relate to crystal structure as has been observed with metals. Grit size effects in two and three body abrasive wear are observed for ceramics. Both free energy of oxide formation and the d valence bond character of metals are related to the friction and wear characteristics for metals in contact with ceramics. Surface contaminants affect friction and adhesive wear. For example, carbon on silicon carbide and chlorine on aluminum oxide reduce friction while oxygen on metal surfaces in contact with ceramics increases friction. Lubrication increases the critical load necessary to initiate fracture of ceramics both in indentation and with sliding or rubbing. Ceramics compositions both as coatings and in composites are described for the high temperature lubrication of both alloys and ceramics

Development of instrumented indentation test model for fracture toughness evaluation and cryogenic test application

학위논문 (박사) -- 서울대학교 대학원 : 공과대학 재료공학부, 2020. 8. 권동일.Structural integrity assessment is to evaluate the condition of a structure or component before it is destroyed. To manage the structural integrity, an engineer must consider the presence of flaws, designed stress and material properties in the structures. But the most important factor is the mechanical properties of a material firstly, such as strength, hardness, or fracture toughness. In many cases, structural failures arise from the change of mechanical properties of the material due to degradation or embrittlement so that it is required to, if possible, measure in-situ mechanical properties of in-service structural components for structural integrity assessment. There are various mechanical properties, however, among them, fracture toughness, the resistance to crack propagation, is one of the most important mechanical properties for fracture mechanical analysis on structural integrity. But the standard fracture toughness test is a destructive method and requires complex shapes and test procedures, making it nearly impossible to measure the fracture toughness of an in-service structures. For this reason, a nondestructive tool to measure in-situ mechanical properties as well as fracture toughness has required and developed to improve the reliability of structural integrity assessment. Instrumented indentation testing can be considered one of solutions in this issue because it is developed for nondestructive testing of in-field structures. Many researchers have worked to estimate fracture toughness of metallic materials using instrumented indentation testing, trying to develop theoretical or experimental models. The study on the prediction of fracture toughness through the instrumented indentation test started from methods of generating cracks, but the study on the metallic materials that does not occur crack was expanded. In the fact that the crack does not occur in the metallic material, the study has been divided into mechanical model and fracture energy model, but in both cases, many assumptions and empirical correlations have been used. In this study, indentation fracture toughness models are introduced. Among them, flat tip fracture toughness estimation model is selected due to its simple test method and derivation of the fracture mechanics situation. Since the previous approach was focused on being somehow phenomenal in the method of determining fracture toughness, this approach was tired to determine the fracture toughness in an indentation situation by adapting fracture mechanics. According to fracture mechanisms, two distinct indentation fracture toughness models, ductile fracture model and brittle fracture model, are modified. In ductile fracture model, in order to match the stress state beneath an indenter with that ahead of a crack tip, fully plastic state at fracture in ligament of cracked round bar test specimen, crack initiation point is determined at the point which fully plastic zone is developed beneath the indenter. In brittle fracture, it is noted that brittle fracture does not involve plasticity, and the crack initiation point in the indentation test is determined using small scale yielding condition at which plastic deformation energy is minimal. By using the flat punch indenter, due to the geometry of the indenter, one normalized curve not dependent on the size of indenter radius can be obtained and this can be converted to any other sized indentation load-depth curve. Thus, for those two model, the indenter size with a radius corresponding to 1T thickness can be determined, and the fracture toughness can be calculated from the load-displacement curve of that size. To verify developed models, experimental results are compared with standard J test results and it is confirmed that these results match well within 20% error range in both two models. In addition, since, it is very important in practical structures to ensure fracture toughness at cryogenic temperatures, cryogenic indentation system was developed. The system was designed and improved by referring to the previous environmental indentation test and conventional environmental facilities. The developed system was applied to materials used in nuclear power plant structures, and compared with the fracture toughness values obtained from the master curve method.구조 건전성 평가는 구조물이나 부품이 파괴를 방지하기 위해 그 구조물이나 부품의 상태를 평가하는 것인데, 구조물의 건전성을 관리하기 위해, 공학자들은 결함의 유무, 설계 응력, 기계적 특성 등을 파악하고자 한다. 그러나 그 중에서도 가장 중요한 요인은 구조물의 기계적 특성으로 강도, 경도 또는 파괴인성 등이 이에 속한다. 구조물이나 설비의 많은 파손 케이스에 있어, 대다수의 파손은 재료의 열화나 취화에 의해 발생하기 때문에, 구조 건전성 평가 시 가동중인 구조물 재료의 기계적 특성을 평가하는 것이 요구된다. 다양한 기계적 특성 중에서도 균열에 대한 저항성의 척도로 표현되는 파괴인성이 구조 건전성 평가의 파괴 역학 분석에 있어 가장 중요한 특성이다. 그러나, 표준에서 제시하고 있는 파괴인성 시험방법은 복잡한 형상과 시험 절차를 요구하고 있기 때문에 가동중인 구조물에 대해 실험을 수행하기에는 거의 불가능하다. 이런 이유에서 비파괴적인 기법을 통해 운용중인 구조물의 기계적 특성을 평가가 요구되고 또 구조 건전성 평가의 신뢰도를 높이고자 연구가 진행되고 있다. 연속압입시험법은 비파괴적으로 가동 중인 구조물에 실험이 가능하여 다양한 기법들 중에서도 가장 유망한 시험법으로 알려져 있다. 이에, 많은 연구자들이 연속압입시험을 통한 파괴인성 예측 연구를 위해 장비와 이론을 개발하고 있다. 이러한 연구는 처음 군열을 직접적으로 발생시키는데서부터 출발하였으나, 결국 대다수의 금속소재들에서는 균열이 발생하지 않기 때문에, 금속소재들을 대상으로 연구가 확장되었다. 금속소재들에서 압입시험 중 균열이 발생하지 않기 때문에 연구는 각각 기계적인 모델링와 파괴 에너지 모델로 나뉘어져 있으나 두 모델에서 모두 실험적인 관계식이나 많은 가정을 포함할 수 밖에 없는 한계가 있었다. 본 연구에서는 압입시험을 통한 파괴인성 예측 모델을 제안하였다. 과거의 많은 연구들이 있었으나, 실험의 간단함과 파괴 역학과의 유사성을 유도할 수 있는 끝이 평평한 플랫 펀치 압입자가 채택되었다. 과거의 플랫펀치 연구에서는 다소 현상적인 측면에서 파괴인성을 예측하고자 하였기 때문에 본 연구에서는 보다 파괴역학적인 관점에서 균열 개시시점을 결정하고자 하였다. 파괴 거동에 따라 모델을 연성 파괴 모델과 취성 파괴 모델로 나누었다. 연성 파괴 모델에는 압입자 하부와 균열 팁 앞에서의 유사한 응력 상태를 연결하기 위해, 플랫 펀치 압입자로 시험을 했을 때 압입자 하부에 발생하는 완전 소성 영역과 균열 앞에서의 소성역이 발생하는 것을 관계 지어 균열 개시 시점을 결정하였다. 취성 파괴 모델에서는 소성이 고려되지 않고, 균열 앞에서의 소성 변형에너지가 최소가 되는 소성역이 소규모 항복 조건이 적용되는 것을 통해 균열 개시시점을 결정하였다. 플랫 펀치 압입자를 사용하면, 압입자의 자기 유사성에 의해 압입자 사이즈에무관한 하나의 일반화곡선을 얻을 수 있고, 그로부터 다른 반지름 사이즈의 하중-변위 곡선을 얻을 수 있다. 그러므로 두 모델에 대해 표준의 파괴인성 시험에 주로 사용되는 1T 두께에 대응하는 압입자 사이즈를 결정하여 이 때의 하중-변위 곡선을 통해 파괴인성을 결정할 수 있다. 제안된 모델을 검증하기 위해 J test 파괴인성 결과와 비교하여 두 모델 모두 20% 내외의 오차 범위를 가지는 것을 확인하였다. 또한, 파괴인성의 주요한 영향인자인 온도 영향을 확인하기 위해 극저온 압입시스템을 개발하였다. 기존의 극저온 환경 시험들을 조사하였고, 이를 바탕으로 극저온 압입시험을 도입하였고, 원자력 발전소 구조물에 쓰인 소재를 수급하여 파괴인성 마스터 커브법의 시험결과와 비교하였다.Chapter 1. Introduction 1 1.1. Object of this Thesis 2 1.2. Outline of the Thesis 6 Chapter 2. Research Background 7 2.1. Fracture Mechanics 8 2.1.1. Overview 9 2.1.2. Stress analysis of cracks 13 2.1.3. Fracture toughness parameters 23 2.1.4. Fracture toughness test 34 2.2. Instrumented Indentation Technique 45 2.2.1. Introduction 45 2.2.2. Indentation tensile properties 47 2.2.3. Evaluation of residual stress 51 2.3. Indentation fracture toughness 59 2.3.1. Introduction 59 2.3.2. Indentation cracking method 60 2.3.3. Spherical indenter model 63 2.3.4. Flat indenter model 74 Chapter 3. Theoretical Modeling 82 3.1. Introduction 83 3.2. Ductile Fracture Model 85 3.3. Brittle Fracture Model 88 3.4. Size adjustment 90 Chapter 4. Experimental Verification 99 4.1. Materials and Methods 100 4.2. Results 103 Chapter 5. Extension to cryogenic environment 119 5.1. Introduction 120 5.2. Development of cryogenic indentation system 121 5.3. Application 125 Chapter 6. Conclusions 134 Reference 137 Abstract in Korean 145 List of publications 148Docto

The Effect of Ultrasonic Power in Aluminum Wire Bonding Hardness Profiles

Ultrasonic wire bonding is a critical process used widely across the microelectronics industry. Despite its ubiquity, there is a breadth of literature and ongoing active research into the basic principles of wire bonding. In particular, the effects of ultrasonic bonding on material properties are not fully understood. This thesis presents the effects of different ultrasonic bond powers on material properties. The changes in mechanical properties were measured by collecting Vickers microhardness data and nanoindentation data. The hardness in the bonded wire varied with two parameters: the distance from the bond interface, and the applied ultrasonic power. The hardness varied 5 HV across the profile of a bond and a 5 HV difference was also measured due to change in bond power. In addition, the measured hardness of the bonds was lower by up to 10 HV than calculated hardness values based on strain hardening only. These trends were found with the microhardness data and corroborated by nanoindentation results. This work provides a method to further study the effects of additional bonding parameters on mechanical properties