Edge chipping damage in lithium silicate glass-ceramics induced by conventional and ultrasonic vibration-assisted diamond machining.

OBJECTIVES: Diamond machining of lithium silicate glass-ceramics (LS) induces extensive edge chipping damage, detrimentally affecting LS restoration functionality and long-term performance. This study approached novel ultrasonic vibration-assisted machining of pre-crystallized and crystallized LS materials to investigate induced edge chipping damage in comparison with conventional machining. METHODS: The vibration-assisted diamond machining was conducted using a five-axis ultrasonic high-speed grinding/machining machine at different vibration amplitudes while conventional machining was performed using the same machine without vibration assistance. LS microstructural characterization and phase development were performed using scanning electron microscopy (SEM) and x-ray diffraction (XRD) techniques. Machining-induced edge chipping depths, areas and morphology were also characterized using the SEM and Java-based imaging software. RESULTS: All machining-induced edge chipping damages resulted from brittle fractures. The damage scales, however, depended on the material microstructures; mechanical properties associated with the fracture toughness, critical strain energy release rates, brittleness indices, and machinability indices; and ultrasonic vibration amplitudes. Pre-crystallized LS with more glass matrix and lithium metasilicate crystals yielded respective 1.8 and 1.6 times greater damage depths and specific damage areas than crystallized LS with less glass matrix and tri-crystal phases in conventional machining. Ultrasonic machining at optimized amplitudes diminished such damages by over 50 % in pre-crystallized LS and up to 13 % in crystallized LS. SIGNIFICANCE: This research highlights that ultrasonic vibration assistance at optimized conditions may advance current dental CAD/CAM machining techniques by significant suppression of edge chipping damage in pre-crystallized LS.

Surface fractures in pre-crystallized and crystallized zirconia-containing lithium silicate glass-ceramics generated in ultrasonic vibration-assisted machining.

Machining-induced surface fractures in ceramic restorations is a long-standing problem in dentistry, affecting the restorations' functionality and reliability. This study approached a novel ultrasonic vibration-assisted machining technique to zirconia-containing lithium silicate glass-ceramics (ZLS) and characterized its induced surface fracture topographies and morphologies to understand the microstructure-property-processing relations. The materials were processed using a digitally controlled ultrasonic milling machine at a harmonic vibration frequency with different amplitudes. Machining-induced surface fracture topographies were measured with a 3D white light optical profilometer using the arithmetic mean, peak and valley, and maximum heights, as well as the kurtosis and skewness height distributions, and the texture aspect ratios. Fracture morphologies were analysed using scanning electron microscopy (SEM). The surface fracture topographies were significantly dependent on the material microstructure, the mechanical properties, and the ultrasonic machining vibration amplitudes. Larger scale fractures with higher arithmetic mean, peak and valley heights, and kurtosis and skewness height distributions were induced in higher brittleness indexed pre-crystallized ZLS than lower indexed crystallized ZLS by conventional machining. Conchoidal fractures occurred in pre-crystallized ZLS while microcracks were found in crystallized state although brittle fractures mixed with localized ductile flow deformations dominated all machined ZLS surfaces. Ultrasonic machining at an ideal vibration amplitude resulted in more ductile removal, reducing fractured-induced peaks and valleys for both materials than conventional processing. This research demonstrates the microstructure-property-processing interdependence for ZLS materials and the novel machining technique to be superior to current processing, reducing fractures in the materials and potentially advancing dental CAD/CAM techniques.

In-situ SEM micropillar compression of porous and dense zirconia materials.

The reduction of failure rates of small-sized zirconia devices depends on the understanding of their micromechanical properties. This paper reports the micromechanical behaviors of porous and dense zirconia materials using in-situ micropillar compression with a flat diamond indenter in a scanning electron microscope (SEM). Porous and dense zirconia micropillars were made using focused ion beam (FIB) milling technique in the SEM. They were then subject to in-situ SEM compression to identify their Young's moduli, yield stresses, plastic deformation, compressive and fracture strengths, damage accumulations, and failure mechanisms. We found that while both porous and dense zirconia microstructures exhibited plastic behaviors, the former had much lower Young's moduli, strengths (yield, compression and fracture), resilience and toughness but higher ductility, resulting in significant buckling than the latter. In plastic regions, alternative strain softening and hardening may have caused stress variations in porous zirconia while dislocation movement contributed to strain hardening in dense zirconia. Although both zirconia materials had quasi-brittle failures, there were different damage mechanisms. The quasi-brittle failure for porous state was due to mushrooming buckling damage driven by breaking of weak interconnected pore networks, resulting in severe compaction and pulverization, microcracks and material piling. The quasi-brittle failure for dense state was identified as plastic crushing damage, involving microcrack initiation and propagation, cleavage and intergranular fractures, and delamination. The mechanical properties of porous and dense zirconia micropillars investigated contributed to the knowledge on deformation and damage mechanisms of zirconia materials at the small scale.

In-situ SEM cyclic nanoindentation of pre-sintered and sintered zirconia materials.

Efficient diamond machining of zirconia requires a comprehensive understanding of repetitive diamond indentation mechanics. This paper reports on in-situ cyclic nanoindentations of pre-sintered and sintered zirconia materials performed inside a scanning electron microscope (SEM). In-situ SEM imaging of cyclic indentation processes and high-magnification SEM mapping of indentation imprints were conducted. The elastic and plastic behaviors of pre-sintered and sintered zirconia materials were investigated as a function of the cyclic nanoindentation number using the Sakai and Sakai-Nowak models. For pre-sintered zirconia, cyclic nanoindentation induced quasi-plastic deformation, causing localized agglomeration of zirconia crystals with microcracks and large cracking along the indentation edge. Severely compressed, fragmented, and pulverized zirconia crystals and smeared surfaces were also observed. For sintered zirconia, shear bands dominated quasi-plastic deformation with the formation of edge pile-ups and localized microfractures occurred at indentation apex and diagonals. All elastic and plastic behaviors for pre-sintered and sintered zirconia materials revealed significantly microstructure-dependent. Pre-sintered zirconia yielded significantly lower contact hardness, Young's moduli, resistance to plasticity, elastic deformation components, and resistance to machining-induced cracking, and higher elastic and plastic displacements than sintered state. Meanwhile, all the behaviors for the two materials were independent from the cyclic nanoindentation number. A model was proposed for cyclic nanoindentation mechanics, revealing their cyclic indentation-induced microstructural changes in the two zirconia materials. This study advances the fundamental understanding of nanoindentation mechanics of zirconia materials.

Microstructural responses of Zirconia materials to in-situ SEM nanoindentation.

Development of optimal shaping processes for pre-sintered and sintered zirconia materials requires a fundamental understanding of damage and deformation mechanisms at small-scale contacts with diamond tools. This paper reports on responses of zirconia materials with distinct microstructures to nanoindentation associated with diamond machining using a Berkovich diamond indenter. In-situ nanoindentation was performed in a scanning electron microscope (SEM) and in-process filmed to record small contact events. Indentation morphology was SEM-mapped at high-magnifications. Although both pre-sintered porous and sintered dense zirconia materials mechanically revealed the quasi-plastic behavior in indentation, there were distinct responses of the two materials to quasi-plasticity at the microstructural level. For pre-sintered porous zirconia, the quasi-plasticity was attributed to shear faults resulting from breaking pore networks as microstructurally discrete interfaces, to lead to compression, fragmentation, pulverization and microcracking of zirconia crystals in indentation imprints. In contrast, sintered dense zirconia had shear band-induced quasi-plastic deformation, accompanied with localized tensile microfracture. A material index associated with the mechanical properties ranked the lower quasi-plasticity for pre-sintered porous zirconia than its sintered dense state, predicting more machining-induced damage in the former than the latter. Significantly higher indentation imprint volumes induced in indented pre-sintered porous zirconia than sintered dense state previses higher machining efficiency for the former than the latter. The microstructure-dependent indentation mechanisms provide the fundamental knowledge into micromechanics of abrasive machining of zirconia materials and may lead to a new microstructural design for zirconia materials to achieve a balanced machining efficiency and damage control.