Abdur-Rasheed Alao

Nano-scale mechanical properties and behavior of pre-sintered zirconia

This paper reports on the mechanical properties and material behavior of pre-sintered zirconia using nanoindentation with in situ scanning probe microscopy. Indentation contact hardness, Hc, and Young׳s modulus, E, were measured at loading rates of 0.1-2mN/s and 10mN peak load to understand the loading rate effect on its properties. Indentation imprints were analyzed using in situ scanning probe imaging to understand the indentation mechanisms. The average measured contact hardness was 0.92-1.28GPa, independent of the loading rate (ANOVA, p>0.05). Young׳s moduli showed a loading rate dependence, with average 61.25GPa and a great deviation at a low loading rate of 0.1mN/s, which was twice the average moduli at the loading rates of 0.5-2mN/s. Extensive discontinuities and the largest maximum penetration, final and contact depths were also observed on the load-displacement curves at the lowest loading rate. These phenomena corresponded to microstructural compaction (pore closure and opening) and kink band formation, indicating the loading rate dependence for microstructural changes during nanoindentation. The in situ scanning probe images of indentation imprints show plastic deformation without fracture at all loading rates, compaction at the low loading rate and pore filling at the high loading rate. The mechanical behavior studied provides physical insight into the abrasive machining responses of pre-sintered zirconia using sharp diamond abrasives.

Surface quality of yttria-stabilized tetragonal zirconia polycrystal in CAD/CAM milling, sintering, polishing and sandblasting processes.

This paper studied the surface quality (damage, morphology, and phase transformation) of yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) in CAD/CAM milling, and subsequent polishing, sintering and sandblasting processes applied in dental restorations. X-ray diffraction and scanning electron microscopy (SEM) were used to scan all processed surfaces to determine phase transformations and analyse surface damage morphology, respectively. The average surface roughness (Ra) and maximum roughness (Rz) for all processed surfaces were measured using desk-top SEM-assisted morphology analytical software. X-ray diffraction patterns prove the sintering-induced monoclinic-tetragonal phase transformation while the sandblasting-induced phase transformation was not detected. The CAD/CAM milling of pre-sintered Y-TZP produced very rough surfaces with extensive fractures and cracks. Simply polishing or sintering of milled pre-sintered surfaces did not significantly improve their surface roughness (ANOVA, p>0.05). Neither sintering-polishing of the milled surfaces could effectively improve the surface roughness (ANOVA, p>0.05). The best surface morphology was produced in the milling-polishing-sintering process, achieving Ra=0.21±0.03µm and Rz=1.73±0.04µm, which meets the threshold for bacterial retention. Sandblasting of intaglios with smaller abrasives was recommended as larger abrasive produced visible surface defects. This study provides technical insights into process selection for Y-TZP to achieve the improved restorative quality.

Nano-mechanical behaviour of lithium metasilicate glass-ceramic.

This paper reports the first study on the mechanical behavior of lithium metasilicate glass-ceramic using nanoindentation and in situ scanning probe imaging techniques. Indentation contact hardness, Hc, and Youngs modulus, E, were measured at 10 mN peak load and 0.1-2 mN/s loading rates to understand the loading rate effect on its properties. Indentation imprints were analysed with the in situ scanning probe imaging to understand indentation mechanisms. The average contact hardness increased by 112% with the loading rate (ANOVA, p<0.05) while the Youngs modulus showed the loading rate independence (ANOVA, p>0.05). A strain rate sensitivity model was applied to determine the intrinsic contact hardness. Extensive discontinuities and largest maximum, contact and final depths were also observed at the lowest loading rate. These phenomena corresponded to inhomogeneous shear-band flow and densification leading to the material strain softening. The in situ scanning probe images of indentation imprints showed plastic deformation at all loading rates and shear band-induced pileups at the lowest loading rate. With the increase in loading rate, the induced pile-ups decreased. The continuum model predicted the largest densified shear zone at the lowest loading rate. Finally, these results provide scientific insights into the abrasive machining responses of lithium metasilicate glass-ceramic during dental CAD/CAM processes using sharp diamond abrasives.

Fracture, roughness and phase transformation in CAD/CAM milling and subsequent surface treatments of lithium metasilicate/disilicate glass-ceramics

This paper studied surface fracture, roughness and morphology, phase transformations, and material removal mechanisms of lithium metasilicate/disilicate glass ceramics (LMGC/LDGC) in CAD/CAM-milling and subsequent surface treatments. LMGC (IPS e.max CAD) blocks were milled using a chairside dental CAD/CAM milling unit and then treated in sintering, polishing and glazing processes. X-ray diffraction was performed on all processed surfaces. Scanning electron microscopy (SEM) was applied to analyse surface fracture and morphology. Surface roughness was quantitatively characterized by the arithmetic average surface roughness Ra and the maximum roughness Rz using desktop SEM-assisted morphology analytical software. The CAD/CAM milling induced extensive brittle cracks and crystal pulverization on LMGC surfaces, which indicate that the dominant removal mechanism was the fracture mode. Polishing and sintering of the milled LMGC lowered the surface roughness (ANOVA, p < 0.05), respectively, while sintering also fully transformed the weak LMGC to the strong LDGC. However, polishing and glazing of LDGC did not significantly improve the roughness (ANOVA, p > 0.05). In comparison of all applied fabrication process routes, it is found that CAD/CAM milling followed by polishing and sintering produced the smoothest surface with Ra = 0.12 ± 0.08µm and Rz = 0.89 ± 0.26µm. Thus, it is proposed as the optimized process route for LMGC/LDGC in dental restorations. This route enables to manufacture LMGC/LDGC restorations with cost effectiveness, time efficiency, and improved surface quality for better occlusal functions and reduced bacterial plaque accumulation.

A review of engineered zirconia surfaces in biomedical applications

Zirconia is widely used for load-bearing functional structures in medicine and dentistry. The quality of engineered zirconia surfaces determines not only the fracture and fatigue behaviour but also the low temperature degradation (ageing sensitivity), bacterial colonization and bonding strength of zirconia devices. This paper reviews the current manufacturing techniques for fabrication of zirconia surfaces in biomedical applications, particularly, in tooth and joint replacements, and influences of the zirconia surface quality on their functional behaviours. It discusses emerging manufacturing techniques and challenges for fabrication of zirconia surfaces in biomedical applications.

Prediction of the resistance to machining-induced cracking in zirconia by nanoindentation

Zirconia is a unique material widely used in engineering, medicine and dentistry as load-bearing structures. Zirconia structures can be shaped either in pre-sintered or sintered states by abrasive machining. However, abrasive machining inevitably induces surface and sub-surface cracks in both pre-sintered and sintered zirconia materials, resulting in poor surface quality and shortened lifespans of zirconia products. This research was undertaken to predict the resistance to machining-induced cracking for pre-sintered and sintered zirconia in nanoindentation using the Sakai model at a peak load of 10 mN and loading rates of 0.1–2 mN/s. The results show that pre-sintered zirconia yielded higher ductility indices and better resistances to machining-induced cracking than sintered zirconia. Both materials revealed the loading rate-independent resistances to machining-induced cracking. This work suggests that pre-sintered zirconia can sustain more mechanical damage and absorb more energy and therefore can be more easily machined than sintered zirconia.

Manufacturing reliable ceramic crowns: the role of abrasive machining in digital dentistry

Dental caries is a ubiquitous disease and nearly 100% of the population is affected worldwide. Consequently, reliable dental restorations are in high demand. More and more patients expect and request esthetics and biosafety, and desire metal-free prostheses. Both biocompatible and esthetic ceramics and digital processing of prostheses have been developed to meet these demands. This paper reviews the current status of abrasive machining involved in affordable digital dental ceramic restorations with regard to dental ceramic materials, dental CAD/CAM systems, and extra/intraoral dental handpiece adjustments. It highlights the importance and challenge of abrasive machining technologies in manufacturing of affordable and reliable dental restorations with cutting-edge materials.