Papers by Author: Yi Hong Liu

Abstract: The purpose of this study was to study the surface characteristics of the dental 3Y-TZP (3 mol% yttria stabilized tetragonal zirconia) ceramic treated by machining process and analysis what impact on the mechanical properties of the 3Y-TZP dental restorations. Dense core ceramic discs were made of 3Y-TZP. The samples were partially sintered to get part strength after uniaxial pressure moulding. The partially sintering green body could be treated by grinding, incising and milling and incising to simulate the CAD-CAM (computer aided design/computer aidedmanufacturing) process in dentistry. The partially sintering samples were then sintered to achieve full density. The dense sintered ceramics were also treated by grinding and incising to simulate the dental restorations adjusting process in dental clinic. The both surface of thespecimens were studied with an optical microscope and field emission SEM. Bulges were observed on the surface of the sample after uniaxial pressure moulding process. The defects due to partially sintered green body milling process were different from incising process in size and shape. Groves are observed on the surfaces of the samples after grinding process. The conclusions were that (i) machining of the final sintered dental 3Y-TZP ceramic brought more critical damage than the machining of partly-sintered green-bodies; (ii) the milling process ofthe partially sintered green-body brought less damage to the 3Y-TZP samples than the incising process.

Abstract: The residual strength of dental porcelain discs were test after cycling fatigue compressive load in this study. Moreover, the effects of stress intensity and cycle frequency to the damage of porcelain were explored. The porcelain specimens were bonded to the epoxy resin blocks, with 12.5mm height. The cycling fatigue contact loads were subjected on the porcelain surfaces of the bi-layer composites with silicon nitride spheres of radius 2.5mm. The crosshead speed was 6mm/min. The top load value was varied from 10N-20N in every load cycles. The cycling number was from 50-300. After cycling test, the residual strength of all porcelain samples was measured by bi-axial bending test. Fractured pieces of the specimens were collected and examined with optical microscope and SEM. On the fracture surfaces of porcelain discs after bi-axial bending test, Hertz cone cracks were presented due to cycling fatigue load. The result showed that the residual strength of dental porcelain had negative correlation with the fatigue load values and cycles. When the fatigue load cycles were lower than 100, the residual strength of porcelain presented no significant different under 10 or 20N fatigue load. In contrary, when the fatigue load cycles were over 100, the residual strength of porcelain was much lower under 20N fatigue load than 10N fatigue load. Based on the limited results of this study, the conclusion could be drawn that the residual strength of porcelain under fatigue compressive stress was main determined by load cycles in the range of small load values. The porcelain could be destroyed after very few cycles if the load is up to a critical value.

Abstract: The conventional method of producing porcelain standard disc specimens is based on the dental lab method for bilayer ceramic crowns, in order to reflect the clinical using condition. However heavy task is facing to the researchers, which is producing a large amount of the specimens efficiently. The press molding method is used to produce dental porcelain disc specimens in this study. Then, the basic mechanical properties of two groups of porcelain specimens were tested respectively, comparing with handmade and press molding method. The density was measured by Archimedes' method, the flexural strength was tested by biaxial bending test, and the hardness was checked by microhardness tester. The micro features of the specimens before and after bending test were observed under optical microscope and SEM. The results showed that there was no significant difference in the flexural strength, microhardness and density of porcelain discs made by two methods. Air pores were found in all specimens with size between 5-50 micrometers. The size and distribution of pores in the specimens by press molding method are more homogeneous than handmade method. Producing dental porcelain discs for laboratory testing specimens by press molding method was much easy than handmade method. Furthermore, the mechanical properties and microstructures of the specimens made by handmade method and press molding method were almost same.

Abstract: Bi-layer all-ceramic dental crowns comprise an inner core made of alumina or zirconia and an outer porcelain shell. The veneer safe from damage is mainly depended on the properties of core materials due to the poor strength and toughness of porcelain. The fracture modes and crack transformation of zirconia/porcelain and alumina/porcelain bi-layer composites reveal the potential feasibilities of improving the fatigue life of all-ceramic restorations. The failure modes of bi-layer composites were confirmed under cyclic spherical fatigue loading by dynamic microcosmic observation. Crack modes and evolvement of bi-layered composites were explored inflecting with fatigue load, cycles, frequency and water environment. Microcosmic morphology of the samples was observed by high depth of field stereomicroscope to determine degree and modes of fatigue damage. The appearance of cone cracks of porcelain surface was almost consistent in both of zirconia and alumina bi-layer composites. However, the pattern and process of the damage are different under cycle fatigue load. Furthermore, process of fatigue damage of bi-layer composites were observed to failure rapidly under water environment in both of zirconia and alumina bi-layer composites.

Abstract: Glass/ zirconia laminated material is the most commonly used material in the field of dental restoration, although the biocompatibility of such material has been subject to lots of research and the mechanical compatibility and durability have not been deeply researched. This paper, by the research on the contact stress of static and dynamic surfaces of the ceramic material, reveals the physical meaning of damage and fatigue at the interface of material; through contacting with the sphere indentation test and by the investigation of fatigue and damage of the surface and interface of such material in the corresponding time scale as well as by the use of acoustic emission monitoring for detecting the damage process and crack propagation process within the detection layer and the laminated ceramic surface, the time of occurrence and corresponding load can be obtained, which plays an importantly practical role in resolving the forecasting of the changes in material damage and the analysis of fatigue.

Abstract: Glass-alumina bi-layer composite was widely used as dental materials, the reliability and mechanical properties are very important for its practical application. In this work, residual stresses in dental glass-alumina composite and failure mechanism were investigated through finite element analysis and 3-point and ball-on-ring bending tests, respectively. The mechanical behavior of the dental material was studied using the bending tests and indentation method. Small bi-layer disk specimens were prepared for the axis-symmetrical tests using a set of small device in a universal tester. The results indicate that the difference in thermal expansion coefficients between two layers lead to residual stresses in both layers, and the stresses distribution depends on the thickness ratio and temperature difference. Furthermore, a proof testing method was proposed to estimate the workability and reliability of the dental ceramics.

Abstract: Surface and subsurface contact damage in dental ceramics and the influence of microstructure on damage tolerance were investigated in this study through indentation methods. The dependence of the damage tolerance on common materials properties was discussed. A simple expression to estimate quantitatively the damage tolerance via basic material parameters, based on the features of damage tolerance and fracture mechanics is confirmed. And the residual strength of damaged specimen was measured via three-point bending tests.