FABRICATION OF CERAMIC MICROPATTERNS AND THEIR IMPACT ON BONE CELLS

The main objective of this study is to elucidate possible methods of producing ceramic calcium phosphate micropatterns ranging from 5 to 100 µm. Today, micropatterned ceramic surfaces are of great interest for fundamental materials research as well as for high-end industrial processes, whereas the fabrication of these patterns in the sub-100 µm range is still a challenge. Therefore, six different patterning techniques have been applied in order to generate ceramic patterns: Microtransfer molding (µTM), modified micromolding (m-µM), Aerosol-Jet® printing, CNC-micromachining, laser ablation and direct laser interference patterning (DLIP). The patterning techniques have been evaluated concerning their capability of fabricating ceramic patterns smaller than 100 µm. Another objective of this study has been the investigation of the influence of ceramic patterns on human osteoblasts (HOB). This investigation has revealed that ceramic hydroxyapatite-based patterns ranging from 16 to 77 µm in widths have a strong influence on the contact guidance of the HOB, whereas the cells showed distinct orientations between 0°-15° in reference to the pattern direction

In-situ Micro and Nanomechanical Characterization and Ultrasonic Machining of Zirconia Ceramics

Zirconia ceramics are popular load-bearing ceramics exhibiting superior mechanical, chemical, optical, and biocompatibility properties, suitable for dental applications. These ceramic properties are available in distinct microstructures under pre-sintered and sintered states, respectively. Their mechanical properties, behavior, and deformation are influenced by their distinct microstructures. Zirconia product failure rates are a concern and understanding of the material general properties associated with distinct microstructures at small-scale contact can provide insight into their load-bearing functions. Shaping of these ceramics structures are conducted using dental CAD/CAM diamond machining processes involving micro and nanoscale diamond tools and material contacts, resulting in severe machining-induced damage. Addressing these matters requires a fundamental understanding of the influence of the ceramics distinct microstructures on indentation mechanics, which lays the foundation of generic insight into indentation-induced deformation and material removal mechanisms. Further, a review of the literature has revealed that conventional machining induces severe machining damage; whereas ultrasonic machining is an emerging technology with the capability to reduce such damage, possessing superior machining responses. A comparison of the ceramics machining responses can provide an alternative to using ultrasonic technology for the improvement of product longevity. This thesis has pursued an understanding of the microstructure-property-processing relations of zirconia materials. Hence, a thorough investigation was made into the influence of distinct microstructures in zirconia materials in terms of their micro and nanomechanical responses at small scale length and conventional and ultrasonic machining processes. The first objective of this thesis is an investigation of zirconia materials with distinct microstructures under external load at small-scale contact volume, providing the critical micromechanical properties and behaviors for load-bearing functions. In-situ micropillar compression tests were conducted on pre-sintered and sintered zirconia materials. The two zirconia materials revealed micropillar-induced plastic behaviors with severe buckling occurred more frequently in pre-sintered zirconia than in sintered zirconia. Presintered zirconia showed lower Young’s moduli, strength properties (yield, compression, and fracture) and energy absorption properties (toughness and resilience) but higher ductility, in comparison with sintered zirconia. In addition, different quasi-brittle failure mechanisms were revealed including mushrooming buckling damage with microcracks and severe compaction for pre-sintered zirconia. Plastic crushing damage with microcracks and microfractures in sintered zirconia was also observed. The second objective of this thesis is an examination of the microstructure responses associated with indentation mechanics and behavior of zirconia materials at small-scale contact using sharp diamond indenters simulating tool-sample contact mechanics in dental abrasive machining. In-situ nanoindentation tests combined with an in-situ technique were also conducted on pre-sintered and sintered zirconia materials. The nanoindentation revealed quasi-brittle behavior for both zirconia materials but at the microstructural level different quasi-plastic mechanisms were identified for the two materials. Weak pore interface boundaries in the pre-sintered zirconia resulted in compression, fragmentation, pulverization, and microcracking of zirconia crystals. Shear bands with localized microfractures were induced in sintered zirconia. Pre-sintered zirconia had a lower rank in quasi-plasticity than sintered zirconia, predicting that it is more susceptible to abrasive machining-induced damage than sintered zirconia. The higher indentation volume in pre-sintered zirconia compared with sintered zirconia indicates the pre-sintered state has higher machining efficiency than the sintered state. The third objective of this thesis is the cyclic nanoindentation of the zirconia materials. To further understand diamond machining of zirconia materials, experiments to help with understanding of material responses under repetitive indentation mechanics, which more closely represents the machining process, were conducted. In-situ cyclic nanoindentation tests were performed with 10 repeated loading and unloading cycles. Cyclic nanoindentation induced quasi-plastic deformation for the two zirconia materials with distinct mechanisms of quasi-plasticity. Agglomeration of zirconia crystals, cracks, compresses, and pulverized crystals were revealed in pre-sintered zirconia cyclic indentation imprints. Shear band, edge pile-ups, and microfractures were revealed in sintered zirconia indentation imprints. Advanced analysis of the zirconia materials deformation mechanisms revealed zirconia microstructures determined their cyclic nanoindentation induced deformation, predicting the ease of machining for pre-sintered zirconia but also revealing they may potentially suffer more severe abrasive machining damage than sintered zirconia. The fourth objective of this thesis is to investigate the zirconia materials responses to edge chipping damage induced in conventional and ultrasonic vibration-assisted diamond machining processes. The edge chipping damage observed in zirconia materials largely depends on microstructure and the applied vibration amplitude during machining. Edge chipping damage was more severe in pre-sintered zirconia with weak pore interface boundaries and a higher brittleness index than in dense zirconia with a tightly packed microstructure and a lower index. Ultrasonic machining at an optimum vibration amplitude led to different material removal mechanisms reducing the brittle fracture induced during machining, hence significantly decreasing edge chipping damage and fracture in both zirconia materials. The fifth objective of this thesis is an examination of the microstructural influence of damage-induced surface asperities produced by conventional and ultrasonic vibration-assisted diamond machining processes. The machining-induced surface damage, removal mechanisms, and surface asperities in the processing of pre-sintered and sintered zirconia depend on microstructure and ultrasonic vibration amplitudes. Conventional and ultrasonic milling induced mixed ductile and brittle fracture modes in pre-sintered and sintered zirconia materials. Milled pre-sintered zirconia showed dominant brittle fracture removal mechanism whereas ductile deformation was the dominant mode for sintered zirconia. Milled pre-sintered zirconia surfaces had more fractures and cracks and higher surface asperities than milled sintered zirconia surfaces. At an optimized vibration amplitude in both zirconia materials, ultrasonic machining enabled the minimization of brittle fracture at the micro-scale to result in more ductile deformation, with reduced surface damage and asperities than conventional machining.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 202

Multi-Scale Modeling for Assessment of Sub-Surface Damage in Iron-Titanium Carbide Metal Matrix Composites

This study is concerned with investigating the effect of laser assisted machining on sub-surface damage during turning of iron-titanium carbide metal matrix composite (MMC) manufactured through laser direct deposition. A high-fidelity 3D multi-scale computational model is presented to predict the macroscopic and micromechanical response of the metal matrix composites undergoing laser assisted turning. Implementation of the multi-scale model has been realized through a hierarchical multi-scale modeling methodology where the results of interfacial mechanics for Fe-TiC composites determined from MD calculations have been used to parameterize the cohesive zone model in the finite element simulations. The 3D nose turning simulation model is capable of predicting the mechanics of cutting of composites, tool-particle interaction, cutting forces and sub-surface damage, hence providing a holistic framework for investigation of machinability of metal matrix composites. With the help of the simulation model, it has been discovered that the particles plough through the matrix material due to their increased concentration ahead of the cutting tool. This contributes to the dynamic loading of the machined workpiece in addition to loading from the secondary cutting edge of the tool. These two mechanisms collectively contribute towards sub-surface damage in the machined metal matrix composites. Damage analysis of Fe-TiC MMC revealed three-different types of damage mechanisms, namely particle pullout/fracture, interfacial debonding leading to void nucleation in matrix and coalescence of voids in the matrix. Localized heating of the Fe-TiC workpiece via a CO2 laser ahead of the cutting tool has been shown to be effective in reducing sub-surface debonding by approximately 20%. An experimental evaluation of the machinability of MMCs revealed a reduction in specific cutting energy by approximately 19%, a 20% improvement in tool life when using carbide tool at an optimum material removal temperature of 300 °C as compared to conventional machining after full annealing of the MMC

Laser Machining of Structural Ceramics: Computational and Experimental Analysis

Outstanding mechanical and physical properties like high thermal resistance, high hardness and chemical stability have encouraged use of structural ceramics in several applications. The brittle and hard nature of these ceramics makes them difficult to machine using conventional techniques and damage caused to the surface while machining affects efficiency of components. Laser machining has recently emerged as a potential technique for attaining high material removal rates. Major focus of this work is to understand the material removal mechanisms during laser machining of structural ceramics such as alumina (Al2O3), silicon nitride (Si3N4), silicon carbide (SiC) and magnesia (MgO). A 1.06 μm wavelength pulsed Nd:YAG laser was used for machining cavities of variable dimensions in these ceramics and an ab-initio computational model was developed to correlate attributes of machined cavities with laser processing conditions. Material removal in Al2O3, Si3N4 and SiC takes place by a combination of melting, dissociation and evaporation while dissociation followed by evaporation is responsible for material removal in MgO. Temperature measurement at high temperatures being difficult, thermocouples were used to measure temperatures in the low temperature regime (700- 1150K). A thermal model was then iterated to obtain trends in absorptivity variation below phase transition temperature for these ceramics. Following this, measured machined depths were used as a benchmark to predict absorptivity transitions at higher temperatures (\u3e 1150K) using the developed thermal model. For temperatures below phase transition, due to intraband absorption, the absorptivity decreases with increase in temperature until the surface temperature reaches the melting point in case of Al2O3, Si3N4 and SiC and the vaporization temperature in case of MgO. The absorptivity then continues to follow increasing trend with increasing temperature due to physical entrapment of laser beam in the cavity evolved during machining of certain depth in the ceramic. Rate of machining was predicted in terms of material removed per unit time and it increased with increase in heating rate. Such a composite study based on comput ational and experimental analysis would enable advance predictions of laser processing conditions required to machine cavities of desired dimensions and thus assist in controlling the laser machining process more proficiently

Current Research in Thin Film Deposition

Today, thin films are near-ubiquitous and are utilised in a very wide range of industrially and scientifically important areas. These include familiar everyday instances such as anti-reflective coatings on ophthalmic lenses, smartphone optics, photovoltaics, decorative, and tool coatings. A range of somewhat more exotic applications also exists, such as astronomical instrumentation (e.g., ultra-low loss dielectric mirrors and beam splitters in gravitational wave detectors, such as laser interferometer gravitational-wave observatory (LIGO)), gas sensing, medical devices and implants, and accelerator coatings (e.g., coatings for the large hadron collider (LHC), and compact linear collider (CLIC) experiments at European organization for nuclear research (CERN)). This Special Issue will provide a platform for researchers working in any area within this highly diverse field to share and exchange their latest research findings. The Special Issue contains novel studies encompassing material characterisation techniques, a range of thin-film coating deposition processes and applications of such technology

THE EFFECTS OF SILICA SAND (Si02) AND ZIRCONIA (Zr02) ON THE MECHANICAL AND THERMAL PROPERTIES OF THE PRESSURELESS SINTERED Al203-Si02-Zr02 COMPOSITE

The research work presents an investigation on the pressureless sintering, microstructure and mechanical properties of three component ceramic composite material consisting of alumina, silica and zirconia for structural applications. The effect of each component composition on the physical and mechanical properties was studied. Mechanical properties including fracture toughness, flexural strength, and hardness at ambient temperature were determined and thermal shock resistance properties up to 950°C were also investigated. These properties were compared among the monolithic A1203, Al203-Zr02 (AZ) and Al203-Si02-Zr02 (ASZ) composites. The microstructure of sintered, thermal shocked and fractured surface states was investigated using FESEM and AFM. A three phase microstructure was adopted with a composition of 70% by weight of alumina, 10% by weight of silica and 20% by weight of zirconia. The reinforcements of the Si02 and Zr02 contributed in improving the mechanical properties of the composite

Modelling of grinding mechanics : a review

Grinding is one of the most widely used material removal methods at the end of many process chains. Grinding force is related to almost all grinding parameters, which has a great influence on material removal rate, dimensional and shape accuracy, surface and subsurface integrity, thermodynamics, dynamics, wheel durability, and machining system deformation. Considering that grinding force is related to almost all grinding parameters, grinding force can be used to detect grinding wheel wear, energy calculation, chatter suppression, force control and grinding process simulation. Accurate prediction of grinding forces is important for optimizing grinding parameters and the structure of grinding machines and fixtures. Although there are substantial research papers on grinding mechanics, a comprehensive review on the modeling of grinding mechanics is still absent from the literature. To fill this gap, this work reviews and introduces theoretical methods and applications of mechanics in grinding from the aspects of modeling principles, limitations and possible future trendencies

Effect of solid solutions and second phases on the thermal conductivity of zirconium diboride ceramics

The research presented in this dissertation is focused on the thermal conductivity (k) of ZrB2 ceramics. The goal was to develop a better understanding of how various solid solutions and second phases affect the thermal and electrical transport in ZrB2, with a focus on the effect of C, W, and ZrC. The first study showed C additions improved densification and it was proposed that the reduction of boria was the impetus for this result. Boron carbide was formed by the reaction of excess C with reduced B and its formation was mitigated by the addition of ZrH2. This allowed the ZrB2-C binary system to be evaluated for study two. Study two showed the k of ZrB2 is reduced by C in solid solution and as a second phase due to the decrease in the electron contribution to thermal conductivity. Conductivities of 99 (25⁰C) and 76 W/m·K (2000⁰C) were obtained for the most pure ZrB2 (0.026 wt% C in solution and 0.2 vol% zirconia) produced in this study, which are the highest reported values for ZrB2 processed using commercial powders since 1980. The third study evaluated the electrical resistivity of ZrB2 up to 1860⁰C using the van der Pauw technique. Separate linear regimes were observed below and above 950⁰C, whereas, previous studies assumed a linear relation. Finally the effect of ZrC on the (Zr,W)B2 solid solution was evaluated in study four. The formation of (Zr,W)C initially increased k, but further ZrC additions resulted in decreased thermal conductivities. In the end, this research provides both: (1) usable information for the design of future ultra-high temperature ceramic systems; and (2) fundamental research that lays the groundwork for future studies aimed at understanding thermal transport in diboride based materials --Abstract, page iv