Jay C. Hanan

Mapping the tetragonal to monoclinic phase transformation in zirconia core dental crowns

OBJECTIVE Chipping failures observed clinically in bilayer systems of porcelain and zirconia restorations should be coupled with a monoclinic to tetragonal phase transformation in the zirconia layer due to the high compressive stress. METHODS Phase transformations were mapped using 2D micro X-ray diffraction of 1083 frames at 100μm×100μm spacing automatically positioned along the core layer of a sectioned fractured crown. RESULTS Yttria-zirconia tetragonal phase transformations to monoclinic zirconia and monoclinic yttria were observed, mostly at the impacted area. A simple map of (101) tetragonal d-spacing strain reveals stress relaxation during phase transformation was detected at inner section of lingual side, because the initial state of compressive residual stress assists this phase transformation at the inter section of lingual side of the core while initial tensile stress at the outer sides under the veneer relaxes under compression and initially prevents phase transformation. SIGNIFICANCE This study implements an experimental method to map the phase transformation, after applying local compressive load until fracture. Such fractures resemble clinically observed chipping failure.

Residual thermal stress simulation in three-dimensional molar crown systems: a finite element analysis

PURPOSE To simulate coefficient of thermal expansion (CTE)-generated stress fields in monolithic metal and ceramic crowns, and CTE mismatch stresses between metal, alumina, or zirconia cores and veneer layered crowns when cooled from high temperature processing. MATERIALS AND METHODS A 3D computer-aided design model of a mandibular first molar crown was generated. Tooth preparation comprised reduction of proximal walls by 1.5 mm and of occlusal surfaces by 2.0 mm. Crown systems were monolithic (all-porcelain, alumina, metal, or zirconia) or subdivided into a core (metallic, zirconia, or alumina) and a porcelain veneer layer. The model was thermally loaded from 900°C to 25°C. A finite element mesh of three nodes per edge and a first/last node interval ratio of 1 was used, resulting in approximately 60,000 elements for both solids. Regions and values of maximum principal stress at the core and veneer layers were determined through 3D graphs and software output. RESULTS The metal-porcelain and zirconia-porcelain systems showed compressive fields within the veneer cusp bulk, whereas alumina-porcelain presented tensile fields. At the core/veneer interface, compressive fields were observed for the metal-porcelain system, slightly tensile for the zirconia-porcelain, and higher tensile stress magnitudes for the alumina-porcelain. Increasingly compressive stresses were observed for the metal, alumina, zirconia, and all-porcelain monolithic systems. CONCLUSIONS Variations in residual thermal stress levels were observed between bilayered and single-material systems due to the interaction between crown configuration and material properties.

Residual thermal stress simulation in three-dimensional molar crown systems

PURPOSE To simulate coefficient of thermal expansion (CTE)-generated stress fields in monolithic metal and ceramic crowns, and CTE mismatch stresses between metal, alumina, or zirconia cores and veneer layered crowns when cooled from high temperature processing. MATERIALS AND METHODS A 3D computer-aided design model of a mandibular first molar crown was generated. Tooth preparation comprised reduction of proximal walls by 1.5 mm and of occlusal surfaces by 2.0 mm. Crown systems were monolithic (all-porcelain, alumina, metal, or zirconia) or subdivided into a core (metallic, zirconia, or alumina) and a porcelain veneer layer. The model was thermally loaded from 900°C to 25°C. A finite element mesh of three nodes per edge and a first/last node interval ratio of 1 was used, resulting in approximately 60,000 elements for both solids. Regions and values of maximum principal stress at the core and veneer layers were determined through 3D graphs and software output. RESULTS The metal-porcelain and zirconia-porcelain systems showed compressive fields within the veneer cusp bulk, whereas alumina-porcelain presented tensile fields. At the core/veneer interface, compressive fields were observed for the metal-porcelain system, slightly tensile for the zirconia-porcelain, and higher tensile stress magnitudes for the alumina-porcelain. Increasingly compressive stresses were observed for the metal, alumina, zirconia, and all-porcelain monolithic systems. CONCLUSIONS Variations in residual thermal stress levels were observed between bilayered and single-material systems due to the interaction between crown configuration and material properties.

Residual stress delaying phase transformation in Y-TZP bio-restorations

Engineering favorable residual stress for the complex geometry of bi-layer porcelain-zirconia crowns potentially prevents crack initiation and improves the mechanical performance and lifetime of the dental restoration. In addition to external load, the stress field depends on initial residual stress before loading. Residual stress is the result of factors such as the thermal expansion mismatch of layers and compliance anisotropy of zirconia grains in the process of sintering and cooling. Stress induced phase transformation in zirconia extensively relaxes the residual stress and changes the stress state. The objective of this study is to investigate the coupling between tetragonal to monoclinic phase transformations and residual stress. Residual stress, on the surface of the sectioned single load to failure crown, at 23 points starting from the pure tetragonal and ending at a fully monoclinic region were measured using the micro X-ray diffraction sin2 ψ method. An important observation is the significant range in measured residual stress from a compressive stress of −400 MPa up to tensile stress of 400 MPa and up to 100% tetragonal to monoclinic phase transformation.

Residual stress and quantitative phase mapping on complex geometries

(Received 21 February 2014; accepted 20 March 2014)As a consequence of substantial advances in computer-aided design and manufacturing technology,engineering parts areno longer restricted to combination of simple geometrical shapes. Implementingcomplex curved surfaces in engineering components in combination with finite-element geometryoptimization has become a prevalent means of designing a part. Measuring residual stresses usingX-ray diffraction (XRD) on complex curved surfaces requires further development of currentmeasurement methods. Here we investigate how a laboratory XRD system equipped with a five-axis stage and two-dimensional detector can execute sin

Three-dimensional x-ray diffraction detection and visualization

A new method of sensing and analyzing three-dimensional (3D) x-ray diffraction (XRD) cones was introduced. Using a two-dimensional area detector, a sequence of frames was collected while moving the detector away from the sample with small equally spaced steps and keeping all other parameters constant. A 3D dataset was created from the subsequent frames. The 3D x-ray diffraction (XRD3) pattern contains far more information than a one-dimensional profile collected with the conventional diffractometer and 2D x-ray diffraction (XRD2). The present work discusses some fundamentals about XRD3, such as the data collection method, 3D visualization, diffraction data interpretation and potential applications of XRD3.

Validating the Critical Areal Density for a New Ballistic Armor Exhibiting Blunt Trauma Reduction

Today there are several armor technologies for reducing penetration injuries. Some of the best technologies are light weight, but alone the trauma to the wearer remains significant. The areal density of a new Hybrid Composite Armor (HCA) maintaining compliance with level III NIJ 0101.06 standards was evaluated using ballistic testing and FEM. This new HCA has a multilayered composite design tailored for Behind Armor Blunt Trauma (BABT) reduction. Three areal densities of HCA were ballistic tested using 7.62 FMJ Lead Core projectiles and measurements of Back Face Signature (BFS) and V50 velocities were collected. For comparison, baseline monolith inserts of similar areal densities were also tested. A critical areal density was found to qualify for the standard while demonstrating a 29.4% reduction in BFS (and hence BABT) in comparison to its baseline. This armor design showed the greatest known propensity to reduce injury both due to the light weight and improved trauma performance compared to existing commercial designs. Numerical simulations in finite element code were carried out to validate with the experimental results. A method for evaluating change-in-velocity and volumetric stress distribution plots was presented using a mock HCA-P2 model. In the future, a similar FEA scheme can be used for predicting the ballistic limit and estimating improvements possible in BABT reduction for HCA concepts with relative design changes.Copyright