High-temperature properties of hardmetals are critical to their use in many applications but a challenge to measure accurately. Creep behaviour is not well understood so this work has studied uniaxial tensile testing of small simple geometry samples to look at how modifications to the microstructure can affect creep behaviour at temperatures between 800 and 900°C. In particular, a carbon-ladder series with high, medium and low carbon contents in the 10wt-%Co binder has been investigated. Significant differences between the stress–strain curves of the different carbon contents have been observed, but the underlying microstructural mechanisms appear to be similar in detailed large area examination of samples after failure. Penetration of Co along WC-WC boundaries with ‘precipitation’ of discrete islands is seen as well as the formation of continuous thin lamellae while void formation tends to occur at WC-Co boundaries. EBSD mapping suggests Co penetration varies as a function of WC-WC misorientation.

The acoustically-driven dynamics of isolated particle-like objects in microfluidic environments is a well-characterised phenomenon, which has been the subject of many studies. Conversely, very few acoustofluidic researchers looked at coated microbubbles, despite their widespread use in diagnostic imaging and the need for a precise characterisation of their acoustically-driven behaviour, underpinning therapeutic applications. The main reason is that microbubbles behave differently, due to their larger compressibility, exhibiting much stronger interactions with the unperturbed acoustic field (primary Bjerknes forces) or with other bubbles (secondary Bjerknes forces). In this paper, we study the translational dynamics of commercially-available polymer-coated microbubbles in a standing-wave acoustofluidic device. At increasing acoustic driving pressures, we measure acoustic forces on isolated bubbles, quantify bubble-bubble interaction forces during doublet formation and study the occurrence of sub-wavelength structures during aggregation. We present a dynamic characterisation of microbubble compressibility with acoustic pressure, highlighting a threshold pressure below which bubbles can be treated as uncoated. Thanks to benchmarking measurements under a scanning electron microscope, we interpret this threshold as the onset of buckling, providing a quantitative measurement of this parameter at the single-bubble level. For acoustofluidic applications, our results highlight the limitations of treating microbubbles as a special case of solid particles. Our findings will impact applications where knowing the buckling pressure of coated microbubbles has a key role, like diagnostics and drug delivery.

The contiguity of a hardmetal is a measure of the proportion of the carbide grain boundaries that are in direct contact with other carbide grain boundaries. Recent analysis of data available in the literature shows a large scatter in results and a significant difference in values measured from scanning electron microscope (SEM) images and from electron backscatter diffraction (EBSD) mapping. An interlaboratory exercise has been carried out with the measurement of a range of WC-Co hardmetal grades. For each grade, SEM images were acquired from both an etched surface and an ion beam polished surface and EBSD maps with two different processing routes. These maps and images were provided to the participants for measurement to eliminate variability from sample preparation and image acquisition. It was shown that measurement of contiguity from EBSD maps is likely to lead to an overestimation of contiguity, largely because EBSD maps do not have the resolution of SEM images to identify small binder phase regions between WC grains. Ion beam polishing combined with backscattered electron imaging was found to provide the best images of the microstructure to underpin a confident measurement of contiguity. However, high resolution SEM images of etched surfaces gave values close to those from ion beam polished samples so it is recommended that, as etching is much more widely available, high-resolution imaging of a lightly etched WC surface should be promoted as the preferred method for measurement of contiguity, in combination with backscattered imaging where possible. Even with good images, variation between operators can give uncertainties of approximately ±10%.