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

Abstract

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.