Document Type

Thesis

Date of Degree

Spring 2016

Degree Name

MS (Master of Science)

Degree In

Oral Science

First Advisor

Isabelle L. Denry

Abstract

Objective: The goal of the present study was to investigate the effect of chairside surface treatments on biaxial flexural strength and subsurface damage of monolithic zirconia ceramics.

Methods: Specimens (15x15x1.2 mm3) were prepared by sectioning from commercially available zirconia blanks (BruxZirTM) and sintering according to manufacturer's recommendations. Fully dense specimens were randomly divided into five groups (n=30) and treated as follows; 1) as-sintered (AS) 2) air abraded with 50 μm alumina fine particles (AAF), 3) air abraded with 250 μm coarse alumina particles (AAC), 4) ground (G), and 5) ground and polished (GP) to mimic chairside and dental laboratory treatments. Microstructural changes were thoroughly characterized by optical and scanning electron microscopy, surface profilometry and atomic force microscopy. Crystalline phases and their depth profile were investigated by x-ray diffraction (XRD) and grazing incidence x-ray diffraction (GIXRD). Results were analyzed by Kruskal-Wallis test and Tukey's adjustment for multiple comparisons. A 0.05 level of significance was used. Reliability was evaluated by Weibull analysis.

Results: All treatment groups exhibited a significant difference in mean surface roughness (Rq) compared to the as-sintered group (p<0.05). The AAC group showed the highest surface roughness at 1.08 ± 0.17 μm, followed by the G, AAF and AS groups. The GP group exhibited the lowest surface roughness. The group air abraded with fine particles showed the highest mean biaxial flexural strength (1662.62 ± 202.58 MPa), but was not different from the ground and polished group (1567.19 ± 209.76 MPa). The groups air abraded with coarse particles or ground with diamond bur exhibited comparable mean biaxial flexural strength at 1371.37 ±147.62 MPa and 1356.98 ±196.77 MPa, respectively. The as-sintered group had the lowest mean biaxial flexural strength at 1202.29 ±141.92 MPa. The depth of compressive stress layer, measured by GIXRD was approximately 50 μm in the AAF group, followed by the AAC group with ~35 μm, ~10 μm for the ground group and ~5 μm for the ground and polished group. Deep subsurface cracks were observed in the AAC group (~80 μm in depth) and G group (~25 μm in depth), whereas shallower flaws were present in the AAF and GP groups at 10 and 3 μm, respectively. Weibull analysis represented a greater reliability in zirconia specimens treated with air abrasion groups.

Conclusions: Surface treatments induced the t-m transformation in 3Y-TZP and associated development of compressive stresses to a depth that varied with the severity of the treatment performed. GIXRD revealed that AAF led to the thickest compressive stress layer, followed by AAC, G and GP. SEM revealed that subsurface damage was most severe with AAC, followed by G, AAF and GP. We propose that the flexural strength results can be explained by the difference between the depth of the compressive stress layer induced by the transformation and the depth of the subsurface flaws.

Public Abstract

This study investigated the effect of chairside surface treatments on bending strength and subsurface damage of a commercially available zirconia ceramic used for all-ceramic dental restorations. Zirconia samples were grouped according to treatment modality as follows; 1) air abraded with fine particles, 2) air abraded with coarse particles, 3) ground and 4) ground plus polished. Untreated, as-sintered zirconia was used as control. Subsurface damage, toughening layer resulting from structural changes, and bending strength were investigated.

Our results showed that groups air abraded with fine particles or ground plus polished had the highest bending strength. The group air abraded with fine particles led to the thickest toughening layer followed by groups air abraded with coarse particles, ground, and ground plus polished, respectively. The subsurface damage was most severe with the group air abraded with coarse particles followed by groups ground, air abraded with fine particles and ground plus polished, respectively. Overall this research indicates that the bending strength of zirconia can be explained based on the difference between the depth of the toughening layer and the deepest flaws from chairside surface treatments. Air abrasion with fine particles and grinding followed by fine polishing are acceptable chairside or laboratory treatments for the commercially available zirconia dental ceramic investigated in this study.

Keywords

publicabstract, Air abrasion, Chairside surface treatments, Compressive stress layer, Grinding, Phase transformation, Zirconia

Pages

ix, 50 pages

Bibliography

Includes bibliographical references (pages 45-50).

Copyright

Copyright 2016 Kan Wongkamhaeng

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