1,720,971 research outputs found
Fabrication and characterization of lithium metasilicate/disilicate glass ceramics and yttria tetragonal zirconia polycrystals for dental restorations
Full text linkLithium 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
A Fundamental Study of Vibration Assisted Machining
No full textConventional diamond cutting of ferrous materials is rarely economical due to the rapid tool wears which result from diffusion and graphitization of the tools. Conventional machining of hard-brittle materials like silicon and germanium results in surface and subsurface damage due to their brittle fracture. Although ductile mode machining (DMM) concept can be used to have a flawless machining on these materials but the mirror surfaces can only be realized on expensive ultraprecision machine tools because the critical depth of cut must be on the order of 1μm or less. Furthermore, there is a need to eliminate or reduce the use of cutting fluids during machining due to their attendant ecological hazards. However, grinding is one of the most difficult processes with regard to eliminating cutting fluids. Vibration assisted machining (VAM) can be used to minimize the problems enumerated above. VAM combines precision machining with small-amplitude tool vibration to improve the fabrication process. It has been applied to a number of processes ranging from turning, drilling to grinding. Therefore, this paper discusses DMM, the general overview of VAM, the basic kinematics of one-dimensional VAM; the advantages derived from using VAM and the ability of VAM to machine brittle materials in the ductile regime at increased depth of cut are described. Finally, the research directions in VAM are outlined.</jats:p
Nano-scale mechanical properties and behavior of pre-sintered zirconia
No full textThis 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 0.1–2 mN/s and 10 mN 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 0.92–1.28 GPa, independent of the loading rate (ANOVA, p>0.05). Young's moduli showed a loading rate dependence, with average 61.25 GPa and a great deviation at a low loading rate of 0.1 mN/s, which was twice the average moduli at the loading rates of 0.5–2 mN/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
Loading rate effect on the mechanical behavior of zirconia in nanoindentation
No full textThis paper reports the loading rate effect on the mechanical behavior of zirconia using nanoindentation and in situ scanning probe imaging techniques. Nanoindentation tests were performed at a peak load of 10 mN and 0.1–2 mN/s loading rates. The results show that the contact hardness increased by 31% with the loading rate while the Young׳s modulus was loading rate independent (ANOVA, p>0.05). A strain rate sensitivity model was applied to determine the strain rate sensitivity and the intrinsic contact hardness. A pressure-sensitive idealized yield criterion model was applied to analyze the pressure hardening coefficient and the intrinsic compressive yield stress. Extensive discontinuities and largest maximum and contact depths were also observed on the force–displacement curves at the lowest loading rate. These phenomena corresponded to nanoindentation-induced strain softening. The in situ scanning probe images of indentation imprints showed plastic deformation without fracture at all loading rates and dislocation-induced pileups around indentation imprints at the low loading rate. The amount of pileups decreased with increase in loading rate. Finally, these results provide scientific insight into the submicron material removal mechanisms for zirconia during sharp abrasive machining
Investigation of indentation size effect and R-curve behaviour of Li2O–SiO2 and Li2O–2SiO2 glass ceramics
Full text linkIndentation size effect (ISE) and R-curve behaviour of Li2O–SiO2 and Li2O–2SiO2 glass ceramics are investigated using micro-indentation and indentation-strength (IS) techniques, respectively. Vickers micro-indentations were applied on both materials at the load of 0.10–19.6 N to determine the load influence on the measured hardness. For the IS-measured fracture toughness, the load ranged from 1.96 to 19.6 N. The hardness decreased with increasing load by 20% and 18% on Li2O–SiO2 and Li2O–2SiO2 glass ceramics, respectively, indicating the ISE behaviour on both materials. The fracture toughness increased with the load by 27% and 59% on Li2O–SiO2 and Li2O–2SiO2 glass ceramics, respectively, signifying the R-curve behaviour. The ISE behaviour of both materials was analysed using the Meyer's, Hays–Kendall (HK), proportional specimen resistance (PSR), Nix–Gao (NG), modified PSR (MPSR) and elastic plastic deformation (EPD) models while the R-curve behaviour was analysed by the fractional power law. The Meyer's index of both materials was less than 2, strongly confirming the ISE existence. The HK, PSR and NG models were only suitable to determine intrinsic Vickers hardness for Li2O–2SiO2 glass ceramic while the MPSR and EPD models were successful for both materials. The fractional power law gave higher R-curve steepness for Li2O–2SiO2 than Li2O–SiO2 glass ceramics. Also, material and brittleness indices predicted, respectively, higher quasi-plasticity and better machinability for Li2O–2SiO2 than Li2O–SiO2 glass ceramics indicating superior performance in the former to the latter. Finally, this study presents a new significant insight into the micro-mechanisms of fracture tolerance behaviour of these glass ceramics which is critical to their functional performance as structural ceramics
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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