Fabrication and characterization of lithium metasilicate/disilicate glass ceramics and yttria tetragonal zirconia polycrystals for dental restorations

Alao, Abdur-Rasheed (2016) Fabrication and characterization of lithium metasilicate/disilicate glass ceramics and yttria tetragonal zirconia polycrystals for dental restorations. PhD thesis, James Cook University.

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Lithium disilicate glass-ceramics (LDGC) and yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) are state-of-the-art materials for monolithic dental restorations. This is due to their excellent mechanical, chemical, optical and biocompatible properties. These ceramics are shaped either in pre-sintered or sintered states by computer aided design/manufacturing (CAD/CAM) abrasive machining which inevitably induces surface/sub-surface damage to the ceramic structures increasing their susceptibility to property degradation and shortening their lifespans. Addressing this fundamental fabrication issue to minimize machining-induced damage requires a comprehensive understanding of their mechanical behavior which, in turn, provides scientific insights into their responses to machining and mechanical loadings. Nanoindentation tests were conducted on lithium metasilicate glass-ceramic (LMGC), sintered and pressable LDGC, pre-sintered and sintered Y-TZP at a peak load of 10 mN and 0.1–0.2 mN/s loading rates to probe the rate effect on their mechanical properties and behavior. The Oliver-Pharr model was used to extract their contact hardness values and Young's moduli. Indentation responses of these ceramics from their force-displacement curves were related to different mechanically-induced deformations assisted by the in situ scanning probe microscopy and contact mechanics models including strain rate sensitivity, pressure-sensitive idealized yield criterion and continuum models.

Compared to other materials investigated, the highest and the lowest intrinsic contact hardness values were revealed by LMGC and pre-sintered Y-TZP respectively. Also, pre-sintered Y-TZP showed the smallest Young's modulus while sintered Y-TZP was the stiffest. In addition, several mechanisms of plasticity were revealed including compaction and kink bands for presintered Y-TZP; densification, shear bands and strain hardening for LMGC, sintered and pressable LDGC, and strain- and pressure-hardening, and dislocations for sintered Y-TZP. Based on these deformations, different mechanisms were proposed to minimize brittle fractures during their abrasive machining. The deformations were further partitioned into elasticity and plasticity using Sakai and Sakai-Nowak models to reveal the dominant deformation mechanisms. Resistances to plasticity, normalized indentation absorbed energies and resistances to machining-induced cracking were also extracted providing a quantitative basis to rank their machinability. Pre-sintered Y-TZP exhibited the most quasi-plastic behavior ranking it more machinable than others; LMGC was least resistant to machining-induced cracking.

Fabrication of LDGC restorations is accomplished by CAD/CAM machining of LMGC followed by sintering, glazing and polishing processes conducted in an arbitrary manner. This research also investigated the surface quality of CAD/CAM-milled and subsequent surfacetreated LMGC/LDGC with respect to phase transformation, surface roughness and morphology, and removal mechanisms. CAD/CAM machining induced extensive brittle cracks and crystal pulverization indicating the dominant fracture mode material removal mechanism for LMGC. Subsequent polishing and sintering respectively improved the surface roughness after milling while polishing and glazing did not improve the roughness after sintering. To have a smooth surface on the milled LMGC, it was proposed that polishing must be applied after milling before sintering (i.e. CAD/CAM-polished-sintered process). The improved surface quality from this procedure was lower than the threshold surface roughness for bacterial plaque retention.

Fabrication of Y-TZP restorations is carried out by CAD/CAM machining of pre-sintered YTZP followed by sintering and polishing. Sandblasting is also applied to roughen the cementation surface for improved adhesion with the luting cement. This research investigated the surface quality of CAD/CAM-milled pre-sintered Y-TZP which subsequently underwent sintering, polishing and sandblasting processes with respect to phase transformation, surface roughness and morphology, and removal mechanisms. CAD/CAM milling induced both partial ductile and brittle fracture modes as the dominant material removal mechanism in pre-sintered Y-TZP. Subsequent polishing and sintering processes could not improve the surface roughness after milling respectively. Polishing after sintering did not improve the roughness. However, the simultaneous application of polishing and sintering processes after the CAD/CAM milling significantly produced the surface roughness that met the bacterial plaque retention surface roughness threshold and was therefore recommended (i.e. CAD/CAM-polished-sintered process). In addition, sandblasting the sintered Y-TZP with 110 μm and 250 μm alumina particles produced similar surface roughness but less severe damage was induced by the former than the latter. Therefore, sandblasting with 110 μm was recommended for sintered Y-TZP restorations.

Finally, low-cycle-high-load Hertzian cyclic spherical indentations simulating teeth grinding and clenching in the posterior region where the highest concentrating stresses occur were conducted to study the fatigue behavior of treated LDGC and Y-TZP surfaces. Maximum contact stresses were evaluated as functions of number of cycles and surface treatments using the Hertzian model. The fatigue damage of treated LDGC and Y-TZP surfaces after cyclic indentations was viewed using SEM to understand the relationships among microstructures, surface asperities and crack propagation.

The maximum contact stresses of indented LGDC surfaces reduced significantly with the number of cycles and surface treatments (ANOVA, p < 0.05). The smoothest CAD/CAM polished-sintered surfaces sustained the highest maximum contact stresses and the least fatigue damage at higher number of cycles. Furthermore, quasi-plastic deformation was dominant on all indented surfaces at a single indentation. At higher indentations, partial cone cracks were formed on all surfaces; radial and transverse cracks were formed on the roughest surfaces. In addition, ring cracks, fretting, pulverization, micro-bridges, surface smearing and wedging and edge chippings were propagated on all surfaces. Therefore, the proposed fatigue mechanism was mechanically assisted growth of surface asperities for treated LDGC surfaces and the rougher the surface, the heavier the induced mechanical damage.

The maximum contact stresses of indented Y-TZP surfaces reduced significantly with number of cycles and surface treatments (ANOVA, p < 0.05). The CAD/CAM-polished-sintered surfaces sustained the highest maximum contact stresses. The surface quality influence on the fatigue damage of treated Y-TZP surfaces was dependent on the asperities present. At a single indentation, quasi-plastic deformation was induced on all surfaces. At higher indentations, cyclic indentations led to plastic deformation-induced smoothening process which increased with number of cycles. Therefore, crack surface-roughness-induced closure was the main fatigue mechanism proposed for this material. However, cyclic indentations also led to intergranular fractures in the roughest surfaces and phase transformation in the smoothest CAD/CAM-polished-sintered surfaces. With respect to sandblasted surfaces, cyclic indentations induced more fatigue damage on surfaces abraded with 250 μm alumina grains than 110 μm alumina grains.

The fundamental research conducted in this thesis provides technical insights into the fabrication and application of LDGC and Y-TZP for durable restorations.

Item ID: 46591
Item Type: Thesis (PhD)
Keywords: CAD/CAM milling; dental ceramic metals; dental ceramics; dentistry; elastic/plastic deformation; glass-ceramics; in situ scanning probe imaging; in situ SPM; lithium metasilicate glass ceramic (LMGC); lithium silicates; loading rate; material behaviour; material removal mechanisms; mechanical behaviour; mechanical properties; nanoindentation; phase transformation; polycrystals; pre-sintered zirconia; resistance to machining-induced cracking; resistance to plasticity; surface damage; surface morphology; surface treatments; yttria-stabilized tetragonal zirconia polycrystal; zirconia
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Alao, Abdur-Rasheed, Stoll, Richard, Song, Xiao-Fei, Miyazaki, Takashi, Hotta, Yasuhiro, Shibata, Yo, and Yin, Ling (2017) Surface quality of yttria-stabilized tetragonal zirconia polycrystal in CAD/CAM milling, sintering, polishing and sandblasting processes. Journal of the Mechanical Behavior of Biomedical Materials, 65. pp. 102-116.

Alao, Abdur-Rasheed, and Yin, Ling (2016) Assessment of elasticity, plasticity and resistance to machining-induced damage of porous pre-sintered zirconia using nanoindentation techniques. Journal of Materials Science and Technology, 32 (5). pp. 402-410.

Alao, Abdur-Rasheed, and Yin, Ling (2015) Nano-mechanical behaviour of lithium metasilicate glass-ceramic. Journal of The Mechanical Behavior of Biomedical Materials, 49. pp. 162-174.

Alao, Abdur-Rasheed, and Yin, Ling (2015) Nanoindentation characterization of the elasticity, plasticity and machinability of zirconia. Materials Science and Engineering: A, 628. pp. 181-187.

Alao, Abdur-Rasheed, and Yin, Ling (2014) Loading rate effect on the mechanical behavior of zirconia in nanoindentation. Materials Science and Engineering: A, 619. pp. 247-255.

Alao, Abdur-Rasheed, and Yin, Ling (2014) Nano-scale mechanical properties and behavior of pre-sintered zirconia. Journal of the Mechanical Behaviour of Biomedical Materials, 36. pp. 21-31.

Date Deposited: 08 Dec 2016 01:32
FoR Codes: 09 ENGINEERING > 0912 Materials Engineering > 091201 Ceramics @ 30%
11 MEDICAL AND HEALTH SCIENCES > 1105 Dentistry > 110599 Dentistry not elsewhere classified @ 30%
09 ENGINEERING > 0913 Mechanical Engineering > 091399 Mechanical Engineering not elsewhere classified @ 40%
SEO Codes: 86 MANUFACTURING > 8610 Ceramics, Glass and Industrial Mineral Products > 861099 Ceramics, Glass and Industrial Mineral Products not elsewhere classified @ 60%
92 HEALTH > 9204 Public Health (excl. Specific Population Health) > 920402 Dental Health @ 40%
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