Ken Mingard

Nucleation control for large, single crystalline domains of monolayer hexagonal boron nitride via Si-doped Fe catalysts.

The scalable chemical vapor deposition of monolayer hexagonal boron nitride (h-BN) single crystals, with lateral dimensions of ∼0.3 mm, and of continuous h-BN monolayer films with large domain sizes (>25 μm) is demonstrated via an admixture of Si to Fe catalyst films. A simple thin-film Fe/SiO2/Si catalyst system is used to show that controlled Si diffusion into the Fe catalyst allows exclusive nucleation of monolayer h-BN with very low nucleation densities upon exposure to undiluted borazine. Our systematic in situ and ex situ characterization of this catalyst system establishes a basis for further rational catalyst design for compound 2D materials.

Factors affecting the accuracy of high resolution electron backscatter diffraction when using simulated patterns

High resolution EBSD directly compares electron backscattering patterns (EBSPs), generated in a scanning electron microscope, to measure relative strain and rotation to a precision of ∼ 10(-4) in strain and 10(-4)rad (0.006 °) in rotation. However the measurement of absolute strain and rotation requires reference EBSPs of known strain and orientation (or a far field region of known strain). Recent suggestions of using simulated EBSPs with known strain show much promise. However precise measurement of the experimental geometry (pattern centre) is required. Common uncertainties of 0.5% in pattern centre result in uncertainty of ∼ 10(-3) in strain state. Aberrations in the compact lenses used for EBSP capture can also result in image shifts that correspond to strains/rotations of ± 10(-3) between experimental and simulated EBSPs. Simulated EBSPs can be generated using dynamical or kinematic models (or a combination of the two). The choice in simulation model has a significant effect on the measured shifts, particularly at zone axis and high structure factor bands, due to large intensity variations, and for simple kinematic simulations can result in the measurement of rogue shifts and thus erroneous strain measurements. Calibrant samples of known strain provide a method of measuring the experimental geometry but imprecise stage movement combined with the high depth of field in the SEM could also result in uncertainties in strain of ∼ 10(-3).

Towards high accuracy calibration of electron backscatter diffraction systems

For precise orientation and strain measurements, advanced Electron Backscatter Diffraction (EBSD) techniques require both accurate calibration and reproducible measurement of the system geometry. In many cases the pattern centre (PC) needs to be determined to sub-pixel accuracy. The mechanical insertion/retraction, through the Scanning Electron Microscope (SEM) chamber wall, of the electron sensitive part of modern EBSD detectors also causes alignment and positioning problems and requires frequent monitoring of the PC. Optical alignment and lens distortion issues within the scintillator, lens and charge-coupled device (CCD) camera combination of an EBSD detector need accurate measurement for each individual EBSD system. This paper highlights and quantifies these issues and demonstrates the determination of the pattern centre using a novel shadow-casting technique with a precision of ∼10μm or ∼1/3 CCD pixel.

Grain size measurement by EBSD in complex hot deformed metal alloy microstructures

The measurement of grain size by EBSD has been studied to enable representative quantification of the microstructure of hot deformed metal alloys with a wide grain size distributions. Variation in measured grain size as a function of EBSD step size and noise reduction techniques has been assessed. Increasing the EBSD step size from 5% to 20% of the approximate mean grain size results in a change in calculated arithmetic mean grain size of approximately 15% and standard noise reduction techniques can produce a further change in reported size of up to 20%. The distribution of measured grain size is found not to be log‐normal, with a long tail of very small sizes in agreement with a computer simulation of linear intercept and areal grain size measurements through randomly oriented grains. Comparison of EBSD with optical measurements of grain size on the same samples shows that, because of the ability of EBSD to distinguish twins and resolve much smaller grains a difference of up to 50% in measured grain size results.

Investigation of slice thickness and shape milled by a focused ion beam for three-dimensional reconstruction of microstructures

Three-dimensional reconstructions of microstructures produced by focused ion beam (FIB) milling usually assume a uniform slice thickness with flat and parallel surfaces. Measurement of the actual slice thickness and profile is difficult, and is often simply ignored. This paper reports the use of artificial 3D structures of known geometry to enable the full 3D profile of a sequence of slices produced by FIB to be measured for the first time. A transient period at the beginning of a milling process is observed in which the actual slice thickness varies by as much as ±50% from the target thickness (with significantly greater error near the base of the slice), before settling to a ±20% variation as the milling progresses. Although SEM images appear to show flat milled surfaces perpendicular to the top surface, the development of a curved, tapering milled surface is also observed. This profile is then maintained through the milling process with the bottom of the slice lagging the top by up to three slice thicknesses.

In Situ Graphene Growth Dynamics on Polycrystalline Catalyst Foils

The dynamics of graphene growth on polycrystalline Pt foils during chemical vapor deposition (CVD) are investigated using in situ scanning electron microscopy and complementary structural characterization of the catalyst with electron backscatter diffraction. A general growth model is outlined that considers precursor dissociation, mass transport, and attachment to the edge of a growing domain. We thereby analyze graphene growth dynamics at different length scales and reveal that the rate-limiting step varies throughout the process and across different regions of the catalyst surface, including different facets of an individual graphene domain. The facets that define the domain shapes lie normal to slow growth directions, which are determined by the interfacial mobility when attachment to domain edges is rate-limiting, as well as anisotropy in surface diffusion as diffusion becomes rate-limiting. Our observations and analysis thus reveal that the structure of CVD graphene films is intimately linked to that of the underlying polycrystalline catalyst, with both interfacial mobility and diffusional anisotropy depending on the presence of step edges and grain boundaries. The growth model developed serves as a general framework for understanding and optimizing the growth of 2D materials on polycrystalline catalysts.

Mapping microstructure inhomogeneity using electron backscatter diffraction in 316L stainless steel subjected to hot plane strain compression tests

Abstract The microstructure inhomogeneity in 316L stainless steel subjected to hot plane strain compression tests has been assessed using electron backscatter diffraction (EBSD). Two variables were investigated: the effect of strain rate and the effect of friction at the tool/specimen interface. Tests were performed isothermally at 950°C at nominal equivalent tensile strain rates of 0·01 and 1 s−1. Low and high friction conditions have been simulated by applying both a glass based lubricant and a boron nitride spray respectively. Results suggest that friction causes a variation in microstructure from the surface to the midplane of the deformed specimen. Several methods used to quantify and represent this inhomogeneity are presented in the present paper. Electron backscatter diffraction measurement issues are discussed. A grain size mapping method using a two-dimensional moving average has been developed to overcome the difficulties associated with the visualisation of measurement results over large areas on EBSD maps. It has proved to be a powerful tool for the spatial statistics of large quantity data obtained by EBSD.

Metrological challenges for reconstruction of 3-D microstructures by focused ion beam tomography methods.

The development of combined focused ion beam and scanning electron microscopes has enabled significant advances in the characterization of the 3‐D structure of materials. The repeated removal of thin layers or slices with an ion beam and imaging or mapping the chemical or crystallographic structure of each slice enables a 3‐D reconstruction from the images or maps. The accuracy of the reconstruction thus depends on the accuracy with which the slice thickness is measured and maintained throughout the process, and the alignment accuracy of the slices achieved during acquisition or by postacquisition corrections. A survey of papers published in this field suggests that the reconstruction accuracy is not often considered or reported. Using examples from examination of the 3‐D structure of hardmetals, issues affecting the accuracy of slice thicknesses and image realignments are examined and illustrated and potential errors quantified by the use of fiducial markers and the expected isotropy of the hardmetal structure itself.

Recent developments in two fundamental aspects of electron backscatter diffraction

Two very different aspects of electron backscatter diffraction (EBSD) are considered in this paper. Firstly, the use of the technique for the measurement of grain size is discussed with particular reference to the development of international standards to help ensure reproducible and repeatable measurements. In particular the lessons learnt for both calibration of the complete SEM-EBSD system and in choice of the correct data acquisition and processing parameters from an international round robin are summarized. Secondly, extending the capability of EBSD through development of new detectors is discussed. New shadow casting methods provide a means to achieve better accuracy in definition of sample-pattern geometry, while increased detail can be obtained by larger cameras and ultimately direct electron detection.

Wear of Hardmetals

This chapter describes the principal mechanisms of wear and reviews the main categories of hardmetal wear and how the wear depends on their makeup and structure. Abrasion is the first category examined where the volume of wear has been found to be related to the hardness. The effects of using different abrasives and other parameters such as the size of abrasive, the applied load, and the chemical environment are also described. The next category is erosion where the wear of hardmetals shows features characteristic of the classical classes of erosion behavior representative of both ductile and brittle materials. The microstuctural basis of hardmetal wear is then explored in some detail. Common features of both abrasion and erosion are removal of the binder phase in the early stages of wear, accumulation of plastic strain in the tungsten carbide (WC) grains, fracture and fragmentation of the WC grains and removal of grains. A feature that is sometimes seen is reembedment of small WC fragments into binder to form wear-resistant surface layers. The chapter also discusses other modes of wear such as sliding wear, impact loading and thermal fatigue.