Developing small scale fracture tests for polycrystalline diamond

Opening ceramics up to a wider range of applications, where their high hardness and high strength are required, necessitates our understanding and improving of their fracture properties. In the last three decades, such improvements have been sought through developing our understanding of toughening mechanisms, typically involving microstructure control that focuses on crack deflection and grain bridging at grain boundaries and interfaces. However, these are often difficult to engineer, as changing microstructural processing (e.g. through heat treatment, chemistry or powder processing) does not result in a one-to-one correlation with performance, since the influence of microstructure on crack path is varied and complex. Recent developments on characterisation at the micro-scale therefore present an opportunity to broaden our understanding of the role of individual factors on the bulk performance. To investigate the fracture properties of individual features (i.e. individual crystallographic planes, grain boundaries or interfaces), a testing method was developed. This approach is based on the double cantilever wedging to measure the fracture energy change during stable crack growth and was successfully applied at the micron scale inside a scanning electron microscope. Direct view of the crack growth in the sample and measurement of the energy absorbed during fracture, without use of load-displacement data, is afforded through the combination of a stable test geometry with an image based analysis strategy. In addition to these precise tests, characterisation of the role of microstructure on crack paths in polycrystalline metal-ceramic composites was carried out. The focus has been on using high angular resolution electron backscatter diffraction combined with microindentation, to correlate intragranular residual stress gradients, due to thermal expansion mismatches, to crack deflection. Fracture energy of individual crystallographic planes and interfaces was measured in both brittle and brittle/ductile systems. In addition, local residual stresses and microstructure in diamond were related to fracture path.Open Acces

In situ stable fracture of ceramic interfaces

The fracture toughness of ceramics is often dominated by the structure of their grain boundaries. Our capacity to improve the performance of ceramic components depends on our ability to investigate the properties of individual grain boundaries. This requires development of new fracture testing methods providing high accuracy and high spatial resolution. Recently, several techniques have been developed using small scaled mechanical testing, based within a nanoindenter, using a variety of tip and sample geometries, including: micropillar compression, microcantilever bending and double-cantilever compression. However, the majority of the published work relies on load-displacement curves for the identification of crack initiation and the geometries can result in a complex analysis of force distribution and stress intensity factor. Our approach uses a double cantilever geometry to obtain stable crack growth and we calculate the fracture energy under a constant wedging displacement. The tests are carried out within an SEM, this has two benefits: the sample is well aligned for a controlled test and images are recorded during the test for later analysis. Crucially this allows us to use beam deflection and crack length rather than critical load to measure fracture toughness. Our tests have proved it is possible to initiate and stably grow a crack in a controlled manner in ceramic materials for several microns. This approach has been validated on SiC where it gives a good approximation of the surface energy and then extended to SiC bi-crystals along with Ni-Al2O3 interfaces where crack blunting and bridging mechanism can be observed and measure

In-situ fracture tests of brittle materials at the microscale

The fracture toughness of ceramics is often dominated by the structure of their grain boundaries. Our ability to improve life of ceramic components depends on our ability to investigate properties of individual grain boundaries. This requires development of new fracture testing methods allowing high spatial resolution and high control over the area to test. Further benefits of these ‘small scale’ approaches will enable testing of specimens for which big volumes are not available (e.g. thin films, coating, or simply samples of dimensions limited by production process). Recently, several techniques have been developed using small scaled mechanical testing, based within a nanoindenter, changing tip and sample geometries, including: micropillar compression [1]; microcantilever bending [2,3]; and double-cantilever compression [4]. However, the majority of the published works utilises complex geometries resulting into complex analysis of force distribution and stress intensity factor and rely on load-displacement curves for the identification of crack initiation, with the added complication of friction. Our approach builds upon the work of Lawn [5], who showed that a practical test geometry to obtain stable crack growth and calculate the fracture energy G is that of a double-cantilever beam (DCB) under constant wedging displacement. We replicate this configuration in our tests fabricating double-cantilever beams of micrometric dimensions by focused ion beam (FIB) milling and loading them in-situ in an SEM using a nanoindenter with a wedge-shaped tip. This has two benefits: the sample is well aligned for a controlled test; images are recorded during the test for later analysis. This allows us to use beam deflection and crack length rather than critical load to measure fracture toughness. Our tests have proved it is possible to initiate and stably grow a crack in a controlled manner in ceramic materials (fig. 1) and our fracture energy results have been validated against prior macro-scale fracture data. This approach is being extended to multi-phase materials with unknown materials properties and extends our arsenal of small-scale characterisation techniques required to generate new processing strategies for the next generation of materials design

Micromechanisms of compressive failure of fibre reinforced polymers

Fibre reinforced polymers benefit from high flexural strength, corrosion resistance and low density. These qualities make them a candidate to substitute the conventional rigid steel pipelines for subsea transport of oil and gas. However, deep water pipelines are subject to high external hydrostatic compressive stresses alongside variable internal fluid pressure that can result in high compressive hoop, radial and axial stress. For aligned fibre reinforced composites, compressive strength is generally lower than the tensile strength and a design limiting factor. Therefore, failure mechanisms and conditions need to be well understood in order to design safe and cost-effective structures. Please click Additional Files below to see the full abstract

Small scale fracture of bone to understand the effect of fibrillar organization on toughness

Fracture toughness is a critical component of bone quality and derives from the hierarchical arrangement of collagen and mineral from the molecular level to the whole bone level. Molecular defects, disease, and age affect bone toughness, yet there is currently no treatment to address deficits in toughness. Toughening mechanisms occur at every length scale, making it difficult to isolate the influence of specific components. Most experimental studies on the fracture behaviour of bone use milled samples of bone or whole bones. Toughness deficits can be identified but may be caused by a multitude of parameters across length-scales, making it difficult to develop targeted therapies. Herein, we measure the toughness of bone in micropillars where porosity and heterogeneities are minimized, allowing us to determine the role of fibril anisotropy on fracture toughness. Double cantilever beam micromechanical tests were conducted in a scanning electron microscope on 4x6x15 mm pillars of mouse bone femorae produced in the longitudinal and transverse orientations. Subsequent transmission electron microscopy of the fractured pillars revealed a role of the local organization of the mineralized collagen fibrils in influencing crack propagation. We demonstrate that fibril orientation is a critical factor in deflection during crack propagation, significantly contributing to fracture toughness

Using coupled micropillar compression and micro-Laue diffraction to investigate deformation mechanisms in a complex metallic alloy Al13Co4

In this investigation, we have used in-situ micro-Laue diffraction combined with micropillar compression of focused ion beam milled Al13Co4 complex metallic alloy to study the evolution of deformation in Al13Co4. Streaking of the Laue spots showed that the onset of plastic flow occured at stresses as low as 0.8 GPa, although macroscopic yield only becomes apparent at 2 GPa. The measured misorientations, obtained from peak splitting, enabled the geometrically necessary dislocation density to be estimated as 1.1 x 1013 m-2

Light and Strong SiC Networks

The directional freezing of microfiber suspensions is used to assemble highly porous (porosities ranging between 92% and 98%) SiC networks. These networks exhibit a unique hierarchical architecture in which thin layers with honeycomb‐like structure and internal strut length in the order of 1–10 μm in size are aligned with an interlayer spacing ranging between 15 and 50 μm. The resulting structures exhibit strengths (up to 3 MPa) and stiffness (up to 0.3 GPa) that are higher than aerogels of similar density and comparable to other ceramic microlattices fabricated by vapor deposition. Furthermore, this wet processing technique allows the fabrication of large‐size samples that are stable at high temperature, with acoustic impedance that can be manipulated over one order of magnitude (0.03–0.3 MRayl), electrically conductive and with very low thermal conductivity. The approach can be extended to other ceramic materials and opens new opportunities for the fabrication of ultralight structures with unique mechanical and functional properties in practical dimensions

Data for "In situ stable crack growth at the micron scale"

<p>Data for "In situ stable crack growth at the micron scale"</p