Depth sensing indentation of organic-inorganic hybrid coatings deposited onto a polymeric substrate

PEO-Si/SiO2 hybrid coatings deposited onto a PVC substrate were micromechanically characterized using depth sensing indentation. The effect of curing time and coating thickness was investigated. Elastic moduli of coated systems determined by the Oliver–Pharr approach displayed a continuous decreasing trend with increasing indentation depth, reflecting that the hybrids are stiffer than the substrate. Aiming to extract coating-only elastic modulus a simple method based on FE simulations was developed. The method was applied to evaluate the moduli of the hybrid coatings and the values were compared with those obtained by applying different approaches available in literature. The elastic modulus of PEO-Si/SiO2 hybrids was proven to be practically independent of curing time after 24 h. However, large curing times resulted in coatings being more prone to failure.Fil: Fasce, Laura Alejandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Seltzer, Rocío. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; ArgentinaFil: Frontini, Patricia Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones en Ciencia y Tecnología de Materiales. Universidad Nacional de Mar del Plata. Facultad de Ingeniería. Instituto de Investigaciones en Ciencia y Tecnología de Materiales; Argentin

Improved micro-contact resistance model that considers material deformation, electron transport and thin film characteristics

This paper reports on an improved analytic model forpredicting micro-contact resistance needed for designing microelectro-mechanical systems (MEMS) switches. The originalmodel had two primary considerations: 1) contact materialdeformation (i.e. elastic, plastic, or elastic-plastic) and 2) effectivecontact area radius. The model also assumed that individual aspotswere close together and that their interactions weredependent on each other which led to using the single effective aspotcontact area model. This single effective area model wasused to determine specific electron transport regions (i.e. ballistic,quasi-ballistic, or diffusive) by comparing the effective radius andthe mean free path of an electron. Using this model required thatmicro-switch contact materials be deposited, during devicefabrication, with processes ensuring low surface roughness values(i.e. sputtered films). Sputtered thin film electric contacts,however, do not behave like bulk materials and the effects of thinfilm contacts and spreading resistance must be considered. Theimproved micro-contact resistance model accounts for the twoprimary considerations above, as well as, using thin film,sputtered, electric contact

Réduction des contraintes secondaires en tension dans les pales en céramique de rotors de turbine en configuration renversée

Depuis quelque temps, un groupe de chercheurs du Groupe d'innovation Createk de l'Université de Sherbrooke travaille sur une nouvelle configuration de rotor de turbine qui utilise un carénage rotatif structurel pour tenir des pales en céramique sous compression, sous l’action du champ centrifuge. Cette configuration de rotor, baptisée Inside-Out Ceramic Turbine (ICT) vise à augmenter la température d'entrée de la turbine (TIT) pour les petites turbines, pour lesquelles le refroidissement et les techniques de fabrication complexes sont d'un coût prohibitif. Plusieurs prototypes et itérations du rotor ICT ont été testés au cours des dernières années, augmentant le temps de fonctionnement et la température d’opération atteignables. Au fur et à mesure que la confiance de l'équipe dans le rotor ICT s'améliore, les problèmes auxquels il est confronté deviennent importants à identifier et à résoudre. Cette thèse vise à étudier et à améliorer la fiabilité des pales en céramique dans des conditions normales de fonctionnement. Ceci est essentiel à l'adoption de la technologie ICT dans un grand nombre de moteurs. La thèse est découpée en trois parties : (1) isoler la cause la plus probable de défaillance des pales grâce à l'observation des résultats expérimentaux et numériques passés, (2) quantifier l'influence relative des principaux paramètres de conception sur la fiabilité des pales à l’aide d’un modèle numérique, et (3) tester des solutions potentielles qui ciblent les variables de conception les plus critiques, pour obtenir une meilleure fiabilité des pales ICT. Les pales étant en céramique technique monolithique, actuellement en nitrure de silicium, elles supportent mal les contraintes en tension. Le niveau de contrainte le plus élevé dans les pales s’avère être à l'interface avec les composants métalliques de support, car les pales sont maintenues en place par friction sous une force normale importante, et des contraintes élevées en tension se manifestent à l'interface. Cela demande de réduire d'abord le nombre d'interfaces au minimum, c'est-à-dire uniquement au niveau du bout de pale en appui contre le carénage structurel tournant. Deuxièmement, à cette interface subsistante, les efforts pour réduire le coefficient de frottement, ainsi que pour réduire la différence de dilatation thermique et de déformation tangentielle entre la pale et le carénage, sont essentiels pour obtenir de faibles contraintes en tension à l'extrémité de la pale. La validation expérimentale des revêtements a été menée avec succès : le revêtement de barrière thermique (TBC) appliqué entre l'extrémité de la pale et le carénage pourrait augmenter la température de l'extrémité de la pale et diminuer la température du carénage, conduisant ainsi à une meilleure correspondance de la dilatation thermique ; le nitrure de bore hexagonal (hBN) est un lubrifiant solide à haute température qui réduit considérablement le coefficient de frottement, même sous une charge extrême, et pourrait conduire à au moins doubler la tolérance à la différence de déformation. Ainsi, un double revêtement de TBC et de hBN devrait réduire la charge en bout de pale. Les niveaux de contrainte réels dépendent de la géométrie de la pale et de la conception globale de la turbine, mais un outil numérique simple a été développé qui permet au concepteur de déterminer quel niveau de réduction de frottement et d'ajustement de dilatation thermique est nécessaire pour obtenir une fiabilité adéquate dans la pale.Abstract : For some time now, a group of researchers in the Createk Innovation Group at Université de Sherbrooke have been working on a novel turbine rotor configuration which uses a structural, rotating shroud to compress ceramic blades under centrifugal load. This rotor configuration, dubbed Inside-Out Ceramic Turbine (ICT) aims at increasing turbine inlet temperature (TIT) for small turbines, for which intricate cooling and complex manufacturing techniques are prohibitively costly. Several prototypes and iterations of the ICT rotor have run over the last years, achieving consistently better run times and firing temperatures. As the team’s confidence in the ICT rotor improves, the issues facing it become important to identify and address. This thesis aims at investigating and improving reliability of the ceramic blades under normal operating conditions. This is central to the adoption of ICT technology in mainstream engines. The thesis is cut into three parts: (1) isolating the most probable cause of failure in the blades through observation of past experimental and numerical results, (2) quantifying the relative influence of the main design parameters on blade reliability through a numerical model, and (3) testing potential solutions which target the most critical design variables, to achieve a better reliability in the ICT blades. As the blades are made of monolithic technical ceramic, currently silicon nitride, they do not tolerate tensile stress fields well. The highest stress level in the blades was found to be at the interface with supporting metallic components, as the blades are maintained in place through friction under large normal force, and a strain mismatch is present at the interface. This leads to first reduce the number of interfaces to a minimum, i.e., only at the blade tip pressing up against the rotating structural shroud. Second, at this remaining interface, efforts to reduce coefficient of friction especially, as well as reduce thermal expansion mismatch and hoop strain between the blade tip and the shroud, are key to achieving low tensile stresses at the blade tip. Experimental validation of coatings was successfully conducted: thermal barrier coating (TBC) applied between the blade tip and the shroud, could increase blade tip temperature and decrease shroud temperature, thus leading to a closer match in thermal expansion; hexagonal boron nitride (hBN) is a high temperature solid lubricant which significantly reduces coefficient of friction, even under extreme load, and could lead to at least double the amount of strain mismatch tolerance. Thus, a double coating of TBC and hBN is expected to reduce the load at the blade tip. Actual stress levels depend on blade geometry and overall turbine design, but a simple numerical tool was developed which could allow the designer to determine what level of friction reduction and thermal expansion fit is required to achieve an adequate reliability in the blade

Design of hard surfaces with metal (Hf/V) nitride multinanolayers

Physical properties as mechanical and tribological evolution on 4140 steel surfaces coated with hafnium nitride/vanadium nitride [HfN/VN]n multinanolayered systems deposited in various bilayer periods via magnetron sputtering has been exhaustively studied in this work. The coatings have been characterized in terms of structural, chemical, morphological, mechanical, and tribological properties by X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, scanning and transmission electron microscopies, nanoindentation, pin-on-disc and scratch tests. Moreover, the failure mode mechanisms were observed via scanning electron microscopy. The preferential growth in the face-centered cubic (111) crystal structure for [HfN/VN]n multilayered coatings have been shown by X-ray diffraction results. The best enhancement of the mechanical behavior has been obtained when the bilayer period was 15 nm (n = 80), yielding the highest hardness (37 GPa) and elastic modulus was (351 GPa). The values of the hardness and elastic modulus were 1.48 and 1.32 times higher than the coating with n = 1, respectively, as well as the lowest friction coefficient (~ 0.15) and the highest critical load (72 N). These results indicated significant enhancements in mechanical, tribological, and adhesion properties, compared to HfN/VN multilayered systems with bilayer period of 1200 nm (n = 1). The hardness and toughness enhancement in the multilayered coatings could be attributed to the different mechanisms that produce the layer formation with nanometric thickness due to the number of interfaces acting as obstacles for crack deflection and dissipation of crack energy. Due to the emergent characteristics of the synthesized multinanolayered material, the developed adaptive coating could be considered as higher ordered tool machining systems, capable of sustaining extreme operating conditions for industrial applications.Фізичні властивості як механічні і трибологічні зміни на поверхні сталі марки 4140 з покриттям із нанобагатошарових систем нітриду гафнію/нітриду ванадію [HfN/VN]n, нанесених магнетронним розпиленням з різними проміжками між двома шарами, були ретельно вивчені в цій роботі. Структурні, хімічні, морфологічні і трибологічні властивості покриттів визначали дифракцією рентгенівських променів, фотоелектронною рентгенівською спектроскопією, атомно-силовою мікроскопією, растровою та просвічувальною електронною мікроскопією, наноіндентуванням, методом “штифт на крузі” і випробуванням дряпанням. Крім того, механізми відмов спостерігали за допомогою растрової електронної мікроскопії. Результати дифракції рентгенівських променів показали краще зростання (111) граніцентрованої кристалічної структури для багатошарових покриттів [HfN/VN]n. Максимальне підвищення механічних характеристик було досягнуто при товщині бішару HfN/VN, що дорівнював 15 нм (число шарів 80), твердість складала 37 ГПа, а модуль пружності – 351 ГПа. Ці значення твердості і модуля пружності були вище, ніж у покриття з n = 1 (в 1,48 і 1,32 рази відповідно), також це багатошарове покриття мало найнижчий (~ 0,15) коефіцієнт тертя і найвище (72 Н) критичне навантаження. Ці результати показали значне поліпшення механічних, трибологічних і адгезійних властивостей порівняно з HfN/VN багатошарової системою з товщиною бішару 1200 нм (n = 1). Підвищення твердості і в’язкості руйнування багатошарових покриттів може бути пояснено різними механізмами утворювання шарів нанометричної товщини, зумовлених кількістю меж розділу, що діють як перешкоди для відхилення тріщини і розсіювання її енергії. Завдяки покращеним характеристикам синтезованого багатошарового матеріалу, розроблене адаптивне покриття можна розглядати як більш високо впорядковану інструментальну систему обробки, здатну підтримувати екстремальні робочі умови при використанні в промисловості.Физические свойства как механические и трибологические изменения на поверхности стали марки 4140 с покрытием из наномногослойных систем нитрида гафния/нитрида ванадия [HfN/VN]n, нанесенных магнетронным распылением с различными промежутками между двумя слоями, были тщательно изучены в этой работе. Структурные, химические, морфологические и трибологические свойства покрытий определяли дифракцией рентгеновских лучей, фотоэлектронной рентгеновской спектроскопией, атомно-силовой микроскопией, растровой и просвечивающей электронной микроскопией, наноиндентированием, методом “штифт на круге” и испытания царапаньем. Кроме того, механизмы отказов наблюдали посредством растровой электронной микроскопии. Результаты дифракции рентгеновских лучей показали предпочтительный рост (111) гранецентрированной кристаллической структуры для многослойных покрытий [HfN/VN]n. Максимальное повышение механических характеристик было достигнуто при толщине бислоя HfN/VN равной 15 нм (число слоев 80), твердость была равна 37 ГПа, а модуль упругости – 351 ГПа. Эти значения твердости и модуля упругости были выше, чем у покрытия с n = 1 (в 1,48 и 1,32 раза соответственно), также у этого многослойного покрытия был самый низкий (~ 0,15) коэффициент трения и самая высокая (72 Н) критическая нагрузка. Эти результаты показали значительное улучшение механических, трибологических и адгезионных свойств по сравнению с HfN/VN многослойной системой с толщиной бислоя 1200 нм (n = 1). Повышение твердости и вязкости разрушения многослойных покрытий может быть объяснено различными механизмами образования слоев нанометрической толщины, обусловленных количеством границ раздела, действующих как препятствия для отклонения трещины и рассеяния ее энергии. Благодаря улучшенным характеристикам синтезированного многонанослойного материала, разработанное адаптивное покрытие можно рассматривать как более высоко упорядоченную инструментальную обрабатывающую систему, способную поддерживать экстремальные рабочие условия при использовании в промышленности

SYNTHESIS AND CHARACTERIZATION OF TANTALUM AND DIAMOND-LIKE CARBON THIN FILMS ON CoCrMo ALLOY SHEETS

In the present research study, Tantalum (Ta) and Diamond-like Carbon (DLC) thin films were deposited on a biomedical Cobalt-Chromium-Molybdenum alloy (CoCrMo alloy) and investigated to improve the surface functionality of this alloy as femoral heads for artificial hip joints. Ta thin films were deposited on the CoCrMo alloy sheets by magnetron sputtering and the effect of deposition parameters on the formation of different phases of Ta was studied using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), and X-ray Absorption Spectroscopy (XAS). By choosing appropriate processing conditions, adherent α- and β-Ta thin films were developed on the CoCrMo alloy sheets and their adhesion, mechanical, and surface properties were characterized using Rockwell C indentation, nanoindentation, optical profilometry, and a contact angle goniometer. The tribological and corrosion behavior of the Ta coated and uncoated CoCrMo alloy sheets were studied using a ball-on-disk tribo tester and a potentiostat. The results demonstrate that adherent Ta thin films (α-Ta or β-Ta) can be applied to improve corrosion and wear behavior of the CoCrMo alloy, and possibly the performance of the alloy in orthopedic implant applications. Specifically, fcc Ta thin film formation, its structure and stability were investigated and its XRD pattern was obtained and reported for the first time. DLC thin films were deposited on the CoCrMo alloy sheets using Ta interlayers by ion beam deposition and characterized using Raman spectroscopy, XPS, and SEM. Severe delamination of DLC coatings was observed on the samples. The results show that the delamination is not just related to the energy level of ion bombardment, which induces intrinsic stress into the film during DLC deposition, but also related to the interfacial layer formation between Ta and DLC films. Furthermore, a simple nondestructive method was evaluated for DLC stress measurement. For this purpose, DLC thin films on Si wafers with different shapes and sizes were synthesized and the internal stress of the thin films were measured with the aid of Zygo optical profiler using the Stoney equation. The results show that this simple method is appropriate and reliable for DLC stress determination

Surface engineering through low temperature deposition of wear resistant layers by reactive magnetron sputter ion plating

The aim of this investigation was the deposition of hard» wear resistant titanium nitride (TiN) thin films, produced from a reactive magnetron sputter ion plating device, at high deposition rates and low substrate temperatures. An allied objective vas the understanding and development of experimental methods which would permit the deposition of titanium nitride-type layers on plastic. The early part of the work deals with the formation of TiN layers on high speed steel, at 500°C deposition temperature Modifications in equipment design and deposition procedures aided the formation of adherent TiN layers. The routine deposition of stoichiometric titanium nitride vas facilitated by a control feedback network The use of graded interfaces between the film and the substrate improved adhesion. Total gas pressure and the level of substrate bias affect film hardness and wear resistance. The next stage of the development process vas the deposition of TiN at approximately 250°C substrate temperature. The main source of substrate heating, in the case of an indirectly cooled magnetron, was identified as the heat liberated from the target. The use of a directly cooled magnetron configuration resulted in lover substrate temperatures. With this device, TiN films vere formed on high speed steel at high deposition rates and with good adhesion. The increased ion current to the substrate is, tentatively, attributed to an extended plasma region associated with the directly cooled configuration Metastable T^N phases are formed from the combination of high deposition rates, low substrate temperatures (250°C) and increased ion bombardment to the substrate. These TiN films, however, are softer and less wear resistant than those produced at 500°C. The final part of the investigation centred around the deposition of TiN type layers onto plastic T1 -T1 N and AI-T1 -T1N layered structures were deposited onto polycarbonate plastic at 100°C. An experimental design approach was employed to develop adhering coatings. A slight partial pressure of oxygen during the initial Ti deposition improves film adhesion. The use of the aluminium interface improves film reflectivity, cosmetic appearance and adhesion. This aluminium interface makes the multilayer structure more susceptible to physical and chemical attack. The wear resistance of the coated plastic is 2 to 4 times greater than the base plastic material

Monitoring and characterization of abnormal process conditions in resistance spot welding

Resistance spot welding (RSW) is extensively used for sheet metal joining of body-in-white (BIW) structure in the automobile industry. Key parameters, such as welding current, electrode force and welding time, are involved in the RSW process. Appropriate welding parameters are vital for producing good welds; otherwise, undersized weld and expulsion are likely to be caused. For a specific type of sheet metal, an acceptable nugget is produced when an appropriate combination of welding parameters is used. However, undersized welds and expulsion are still commonly seen in the plant environment, where some abnormal process conditions could account for the production of the poor quality welds. Understanding the influence of abnormal process conditions on spot weld quality and other RSW related issues is crucial. A range of online signals, strongly related to the nugget development history, have attracted keen interest from the research community. Recent monitoring systems established the applied dynamic resistance (DR) signal, and good prediction of nugget diameter was made based on signal values. However, the DR curves with abnormal process conditions did not agree well with those under normal condition, making them less useful in detecting abnormal process conditions. More importantly, none of the existing monitoring systems have taken these abnormal process conditions into account. In addition, electrode degradation is one of the most important issues in the plant environment. Two major electrode degradation mechanisms, softening and intermetallic compound (IMC) formation, are strongly related to the characteristics of welding parameters and sheet metals. Electrode misalignment creates a very distinct temperature history of the electrode tip face, and is believed to affect the electrode degradation mechanism. Though previous studies have shown that electrode misalignment can shorten electrode life, the detailed mechanism is still not understood. In this study, an online-monitoring system based on DR curve was first established via a random forest (RF) model. The samples included individual welds on the tensile shear test sample and welds on the same sheet, considering the airgap and shunting effect. It was found that the RF model achieved a high classification accuracy between good and poor welds. However, the DR signals were affected by the shunting distance, and they displayed opposite trends against individual welds made without any shunting effect. Furthermore, a suitable online signal, electrode displacement (ED), was proposed for monitoring abnormal process conditions such as shunting, air gap and close edged welds. Related to the thermal expansion of sheet metal, ED showed good consistency of profile features and actual nugget diameters between abnormal and normal welds. Next, the influence of electrode misalignment on electrode degradation of galvannealed steel was qualitatively and quantitatively investigated. A much-reduced electrode life was found under the angular misalignment of 5°. Pitting and electrode softening were accelerated on the misaligned electrodes. δ Fe-Zn phase from the galvannealed layer that extends electrodes was found non-uniformly distributed on the worn electrode. Furthermore, electron backscatter diffraction (EBSD) analysis was implemented on the worn electrode, showing marked reduction in grain diameter and aspect ratio. The grain deformation capacity was estimated by the distribution of the Taylor factor, where the portion of pore grain was substantially weakened in the recrystallized region compared to the base metal region

A tribological and mechanical study of ion assisted diamond-like carbon thin films

Amorphous hydrogenated carbon (a-C:H), diamond & diamond-like (DLC) thin films are some of the many terms used when referring to the generic group of coatings based on hard carbon. They are an emerging technological area within the surface coating discipline and are being increasingly used to improve the efficiency of a wide range of engineering components. In addition, the unique and extreme characteristics of these films result in unequalled material properties, such that in many cases a wide range of new and superior performance devices have only recently begun to be realised.This study focuses on hydrogenated & non-hydrogenated diamond-like thin films deposited by various plasma based, hybrid and beam deposition techniques. The wear resistant and low friction properties of these films are of great importance in many of the potential application areas and has attracted particular interest in recent years. Therefore the major thrust of this research has been on the tribological aspect, particularly in relation to other advanced ceramic coatings, and to highlight the applicability of endurance wear tests used to evaluate diamond-like films. The main findings have been:-a} That carbon can be deposited by several techniques in a hard amorphous phase, the properties of which depend heavily upon the conditions, substrate choice and method of deposition. For a particular technique, material properties can be made to be repeatable by a good understanding of deposition process control.b} The use of plasma based hybrid PVD and beam methods have resulted in a considerably improved structural performance of the films over those produced by the direct evaporation of graphite. The introduction of a hydrocarbon gas into the plasma at the synthesis stage has also been shown to provide further improvements in the physical properties which has correspondingly led to an enhancement in the tribological behaviour. The levels of hydrogen, whether in an unbonded or bonded form, included in the film after deposition has been demonstrated to affect the mechanical and optical properties of the considerably.c) The wear resistant and frictional performance of these coatings has been shown to be variable, depending upon the method and conditions of deposition as well as test parameters such as humidity, surface roughness, film structure, adhesive strength and oxide/impurity formation. In some cases the tribological performance was found to be excellent. The presence of the diamond-like carbon coating has been shown to be beneficial in reducing wear between contacting bodies experiencing relative movement by encouraging the formation of a carbon transfer layer on the surface of the counterface material which acts as a zone of low shear and provides a physical barrier to tribo-chemical interactions. Under certain conditions, such tribo-chemical interactions can occur readily at the interface, facilitating the formation of strong interfacial bonding and increased wear.d) The inclusion of metallic elements into the carbon matrix has been shown to enhance the wear resistant properties of the film to only a small extent, although at the expense of a deterioration in the friction coefficient. The most beneficial effect of doping carbon films with metal species has been the improved resistance to thermal degradation.e) Thin intermediate layers of titanium nitride have also been shown to produce a remarkable improvement in both wear resistance and frictional performance of the diamond-like carbon films to an extent which appears to be related to the level of stoichiometry of the titanium nitride. The main mechanism behind this increased performance appears to be due largely to an enhancement in adhesive strength at the diamond-like carbon/titanium nitride junction, with an increase in load support being provided as a secondary benefit.f) A critical assessment of the available techniques and methodology available for testing hard carbon films has been made and in some cases methods have been found to be either entirely inappropriate or appropriate only when suitable precautionary measures have been taken. These difficulties largely stem from the exacting demands of thin, hard layers of diamond-like carbon due to its unique and extreme mechanical, electrical and optical properties

FINITE ELEMENT ANALYSIS OF THE CONTACT DEFORMATION OF PIEZOELECTRIC MATERIALS

Piezoelectric materials in the forms of both bulk and thin-film have been widely used as actuators and sensors due to their electromechanical coupling. The characterization of piezoelectric materials plays an important role in determining device performance and reliability. Instrumented indentation is a promising method for probing mechanical as well as electrical properties of piezoelectric materials. The use of instrumented indentation to characterize the properties of piezoelectric materials requires analytical relations. Finite element methods are used to analyze the indentation of piezoelectric materials under different mechanical and electrical boundary conditions. For indentation of a piezoelectric half space, a three-dimensional finite element model is used due to the anisotropy and geometric nonlinearity. The analysis is focused on the effect of angle between poling direction and indentation-loading direction on indentation responses. For the indentation by a flat-ended cylindrical indenter, both insulating indenter and conducting indenter without a prescribed electric potential are considered. The results reveal that both the indentation load and the magnitude of the indentation-induced potential at the contact center increase linearly with the indentation depth. For the indentation by an insulating Berkovich indenter, both frictionless and frictional contact between the indenter and indented surface are considered. The results show the indentation load is proportional to the square of the indentation depth, while the indentation-induced potential at the contact center is proportional to the indentation depth. Spherical indentation of piezoelectric thin films is analyzed in an axisymmetric finite element model, in which the poling direction is anti-parallel to the indentation-loading direction. Six different combinations of electrical boundary conditions are considered for a thin film perfectly bonded to a rigid substrate under the condition of the contact radius being much larger than the film thickness. The indentation load is found to be proportional to the square of the indentation depth. To analyze the decohesion problem between a piezoelectric film and an elastic substrate, a traction-separation law is used to control the interfacial behavior between a thin film and an electrically grounded elastic substrate. The discontinuous responses at the initiation of interfacial decohesion are found to depend on interface and substrate properties

Deposition and Characterization of Ceramic Thin Films and a New Experimental Approach to Evaluate the Mechanical Integrity of Film/Substrate Interfacial Layers

Due to their corrosion resistance, high temperature stability, high strength, and high hardness, refractory ceramic thin films and coatings have been utilized for surface engineering of mechanical components and mechanical fabrication tools. Adhesion between ceramic thin films and coatings and the substrate is of critical concern for performance and life time of coated systems. In this dissertation, a custom designed and constructed ultra-high-vacuum (UHV) vapor phase deposition system was used for the preparation of ceramic thin films through low-pressure high-density plasma assisted physical vapor deposition (PVD) methods. Deposited thin films were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), instrumented nanoindentation, focus ion beam (FIB) scanning electron microscope (FIB SEM), and transmission electron microscope (TEM). The effective interfacial shear strength between TiN and CrN thin films and their substrates was evaluated through a substrate- tension method and a newly introduced experimental testing method involving FIB script-milling of film/substrate specimens into micro-pillars and instrumented compression testing performed on such micro-pillars. This micro-pillar testing protocol was further used to experimentally demonstrate, for the first time to our knowledge, a size effect in the shear strength in the configuration of confined shear plastic flow of ductile thin layers. This latter experiment furnishes new and fundamental data for micron scale plasticity theories