Eigenstrain boundary layer modelling of the yttria-partially stabilised zirconia–porcelain interface in dental prostheses

The exceptional strength and appealing aesthetics of porcelain veneered yttria partially stabilised zirconia (YPSZ) dental prostheses, has led to the widespread adoption of these materials. However, near-interface chipping of the porcelain remains the primary failure mode. Advanced experimental techniques have recently revealed significant variations in residual stress and YPSZ phase distribution at the YPSZ–porcelain interface. Therefore, in order to improve existing understanding and effectively optimise the production of these devices, an enhanced model of the YPSZ coping that includes these newly discovered phenomena is presented in this study. Macroscale stresses are shown to arise through the uneven temperatures within the coping during the sintering process and the coefficient of thermal expansion mismatch with the porcelain during veneering. In contrast, microscale stresses are driven by the YPSZ phase transformation and the associated volumetric expansion. The eigenstrain approach proposed here was found to demonstrate a good match between the phase variation determined experimentally, and the corresponding residual stress distribution showed an effective comparison with the empirical measurements. The proposed technique is a straightforward but powerful method for simulating this dominant mechanical behaviour, with significant potential to combine the resulting expressions into existing models. These enhanced simulations are the only viable approach for the precise, reliable and systematic optimisation of prosthesis production parameters that are needed to significantly reduce prosthesis failure rates.</p

Intragranular residual stress evaluation using the semi-destructive FIB-DIC ring-core drilling method

Titanium aluminide (TiAl) is a lightweight intermetallic compound with a range of exceptional mid-to-high temperature mechanical properties. These characteristics have the potential to deliver significant weight savings in aero engine components. However, the relatively low ductility of TiAl requires improved understanding of the relationship between manufacturing processes and residual stresses in order to expand the use of such components in service. Previous studies have suggested that stress determination at high spatial resolution is necessary to achieve better insight. The present paper reports progress beyond the current state-of-the-art towards the identification of the near-surface intragranular residual stress state in cast and ground TiAl at a resolution better than 5 μm. The semi-destructive ring-core drilling method using Focused Ion Beam (FIB) and Digital Image Correlation (DIC) was used for in-plane residual stress estimation in ten grains at the sample surface. The nature of the locally observed strain reliefs suggests that tensile residual stresses may have been induced in some grains by the unidirectional grinding process applied to the surface. © (2014) Trans Tech Publications, Switzerland

A review of micro-scale focused ion beam milling and digital image correlation analysis for residual stress evaluation and error estimation

In the past decade several versions of micro-scale residual stress analysis techniques have been developed based on Focused Ion Beam (FIB) milling at sample surface followed by Digital Image Correlation (DIC) for the determination of the resulting strain relief. Reliable and precise estimation of the error bounds on these measures is critical in determining the usefulness and accuracy of residual stress evaluation. Here we present an overview of the steps necessary for effective outlier removal, error propagation and estimation in order to provide reliable confidence limits for the stress value obtained. Error propagation analysis begins with DIC marker tracking errors that depend on imaging contrast and magnification, and can be improved with sub-pixel tracking and marker shift averaging. We demonstrate how the outliers and poorly tracked markers ought to be removed from the data set using correlation coefficient thresholding and/or correlation peak confidence intervals. Markers showing large displacements relative to their neighbours can also be identified as aberrant, and removed. By performing careful error propagation throughout the analysis chain we quantify the displacement and strain fields, and qualify them with the associated confidence intervals. These values, in combination with the elastic modulus confidence limits, are then used to provide the final confidence intervals for the determined residual stress values. The generic nature of the methodology presented ensures its suitability for all residual stress analysis techniques based on FIB milling and image correlation analysis. An example of implementation is presented for the micro-scale ring-core FIB-DIC approach.</p

Investigating the machining of tungsten (W) using finite element analysis

Tungsten is extensively used as a plasma facing material in fusion energy reactors. A finite element model was created to simulate the machining of tungsten for the first time by estimating the cutting forces and observing the impact of the variation in tool rake angle. The model was validated through machining experiments involving a specially designed single flute fly cutter which indicated errors of 6% – 34%, depending on the rake angle. This investigation is the first step in understanding the impact of cutting parameters on machining of tungsten. However, the model is affected by the unpredictable impact of tungsten’s deformation behaviour and especially the effects of its brittle nature and low fracture toughness

An electron microscopy study of sintering in three dental porcelains

In the manufacture of yttria partially stabilised zirconia dental prostheses, layers of porcelain veneer are sintered onto zirconia copings in order to reduce surface hardness and to produce an aesthetically pleasing finish. The stress of this interfacial bond and of the near-interface porcelain layers is crucial for reducing the likelihood of chipping during use. An improved understanding of the sintering behavior and the resulting microstructure is therefore required to ensure good prosthesis performance. In this study we use scanning electron microscopy in combination with energy dispersive spectroscopy mapping to examine the impact of vacuum sintering on the microstructure and elemental distribution of three types IPS e.max® Ceram dental porcelain (Incisal, BI shade and C4 shade) over 400 x 400 pm2 regions. It was found that the powder samples showed distinct differences in the average grain size (7 - 16 pm), maximum grain size (12 - 33 pm) and elemental composition prior to sintering. Following the application of the recommended sintering regimes, clear differences could be observed between the three samples. The Incisal porcelain demonstrated a uniform surface with limited numbers of grains and no evidence for porosity. In contrast, large numbers of sodium, aluminum and calcium rich grains were observed on the surface of the shaded porcelains, along with clear evidence of voiding.".</p

Residual strain mapping through pair distribution function analysis of the porcelain veneer within a yttria partially stabilised zirconia dental prosthesis

OBJECTIVE: Residually strained porcelain is influential in the early onset of failure in Yttria Partially Stabilised Zirconia (YPSZ) - porcelain dental prosthesis. In order to improve current understanding it is necessary to increase the spatial resolution of residual strain analysis in these veneers. METHODS: Few techniques exist which can resolve residual stress in amorphous materials at the microscale resolution required. For this reason, recent developments in Pair Distribution Function (PDF) analysis of X-ray diffraction data of dental porcelain have been exploited. This approach has facilitated high-resolution (70μm) quantification of residual strain in a YPSZ-porcelain dental prosthesis. In order to cross-validate this technique, the sequential ring-core focused ion beam and digital image correlation approach was implemented at a step size of 50μm. This semi-destructive technique exploits microscale strain relief to provide quantitative estimates of the near-surface residual strain. RESULTS: The two techniques were found to show highly comparable results. The residual strain within the veneer was found to be primarily tensile, with the highest magnitude stresses located at the YPSZ-porcelain interface where failure is known to originate. Oscillatory tensile and compressive stresses were also found in a direction parallel to the interface, likely to be induced by the multiple layering used during fabrication. SIGNIFICANCE: This study provides the insights required to improve prosthesis modelling, to develop new processing routes that minimise residual stress and ultimately to reduce prosthesis failure rates. The PDF approach also offers a powerful new technique for microscale strain quantification in amorphous materials.</p

An investigation of residual stress gradient effects in FIB-DIC micro-ring-core analysis

In many cases residually stresses surface layers that are obtained by surface treatment or coating deposition contain significant stress gradients. These gradients affect the performance of component surfaces under the conditions of contact loading in service, such as impact, scratch and abrasion, wear, erosion, fretting fatigue, etc. The determination of residual stress in the close vicinity of sample surfaces, at the depths ranging from sub-micron to a few microns, is a challenging task that cannot be accomplished routinely using existing techniques. A challenging aspect of the problem of stress evaluation concerns the nature of weighting of the contributions from different depths below the surface within the gauge volume. In the present study we focus our attention on this issue, specifically, on the effects of sub-surface stress gradients on the apparent stress value obtained by FIB-DIC micro-ring-core analysis. The implications of this investigation are discussed in the concluding section.</p

The effect of eigenstrain induced by ion beam damage on the apparent strain relief in FIB-DIC residual stress evaluation

FIB milling using Ga ions is known to be accompanied by implantation, multiplication of material defects, material property modification (e.g. amorphisation), inelastic shrinking/swelling and residual stress generation. These processes affect the reliability of the micro-ring-core method for residual stress evaluation. Safe use of this technique requires formulating approaches that provide quantitative criteria of the method's validity. In the present study this task is accomplished by proposing a numerical model based on eigenstrain. Parametric simulations were performed to identify the extent to which the FIB-DIC micro-ring-core measurements are affected. As an example of a real and relevant material system, the procedure was applied to silicon material. The curvature of an AFM cantilever due to FIB damage was monitored, and the eigenstrain magnitude determined by matching the model to observations. Using the resulting eigenstrain profile, parametric analysis was performed in terms of the pillar radius, and the elastic strain field calculated at the pillar surface that is monitored in the FIB-DIC micro-ring-core method. An important property of the model is its versatility that allows it to be adapted to different milling conditions and geometries to determine the ultimate spatial resolution limits of the FIB-DIC method.</p

Microscale resolution fracture toughness profiling at the zirconia-porcelain interface in dental prostheses

The high failure rate of the Yttria Partially Stabilized Zirconia (YPSZ)-porcelain interface in dental prostheses is influenced by the micro-scale mechanical property variation in this region. To improve the understanding of this behavior, micro-scale fracture toughness profiling by nanoindentation micropillar splitting is reported for the first time. Sixty 5 μm diameter micropillars were machined within the first 100 μm of the interface. Berkovich nanoindentation provided estimates of the bulk fracture toughness of YPSZ and porcelain that matched the literature values closely. However, the large included tip angle prevented precise alignment of indenter with the pillar center. Cube corner indentation was performed on the remainder of the pillars and calibration between nanoindentation using different tip shapes was used to determine the associated conversion factors. YPSZ micropillars failed by gradual crack propagation and bulk values persisted to within 15 μm from the interface, beyond which scatter increased and a 10% increase in fracture toughness was observed that may be associated with grain size variation at this location. Micropillars straddling the interface displayed preferential fracture within porcelain parallel to the interface at a location where nano-voiding has previously been observed and reported. Pure porcelain micropillars exhibited highly brittle failure and a large reduction of fracture toughness (by up to ∼90%) within the first 50 μm of the interface. These new insights constitute a major advance in understanding the structure-property relationship of this important bi-material interface at the micro-scale, and will improve micromechanical modelling needed to optimize current manufacturing routes and reduce failure.</p