Electron microscope loading and in situ nanoindentation of water ice at cryogenic temperatures

PLOS ONE RESEARCH ARTICLE Electron microscope loading and in situ nanoindentation of water ice at cryogenic temperatures Renelle Dubosq ID1*, Eric Woods ID1, Baptiste Gault1,2, James P. Best ID1 1 Max-Planck-Institüt für Eisenforschung GmbH, Düsseldorf, Germany, 2 Department of Materials, Royal School of Mines, Imperial College London, London, United Kingdom * Abstract a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Dubosq R, Woods E, Gault B, Best JP (2023) Electron microscope loading and in situ nanoindentation of water ice at cryogenic temperatures. PLoS ONE 18(2): e0281703. https:// doi.org/10.1371/journal.pone.0281703 Editor: Khalil Abdelrazek Khalil, University of Sharjah, UNITED ARAB EMIRATES Interest in the technique of low temperature environmental nanoindentation has gained momentum in recent years. Low temperature indentation apparatuses can, for instance, be used for systematic measurements of the mechanical properties of ice in the laboratory, in order to accurately determine the inputs for the constitutive equations describing the rheologic behaviour of natural ice (i.e., the Glen flow law). These properties are essential to predict the movement of glaciers and ice sheets over time as a response to a changing climate. Herein, we introduce a new experimental setup and protocol for electron microscope loading and in situ nanoindentation of water ice. Preliminary testing on pure water ice yield elastic modulus and hardness measurements of 4.1 GPa and 176 MPa, respectively, which fall within the range of previously published values. Our approach demonstrates the potential of low temperature, in situ, instrumented nanoindentation of ice under controlled conditions in the SEM, opening the possibility for investigating individual structural elements and systematic studies across species and concentration of impurities to refine to constitutive equations for natural ice. Received: January 16, 2023 Accepted: January 30, 2023 Published: February 10, 2023 Copyright: © 2023 Dubosq et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: The author(s) received no specific funding for this work. Competing interests: The authors have declared that no competing interests exist. Introduction Glaciers and ice sheets presently cover ~10% of Earth’s land surface in alpine and polar regions, forming an integral part of the planet’s climate system, influencing regional- and global-scale climate as well as responding to climate change [1]. Our understanding of ice flow dynamics is therefore essential for forecasting glacier and ice sheet response to global warming. For instance, variations in the net mass transport of ice to the oceans can eventually lead to sea-level changes potentially drastically affecting the global water cycle [2, 3]. The dominant component of horizontal ice flow towards the oceans is shearing between the basal layer, which has a relatively higher content of chemical impurities and rock particles, and the bedrock beneath [3]. Although the rheologic behaviour of pure ice can be generalized by the Glen flow law [4], impurities introduce an enhancement coefficient as a multiplier of the stress term [5]. Based on a compilation of deformation data and mechanical tests, impurity-rich glacial ice deforms on average 2.5× faster than impurity-poor Holocene ice in simple shear [6]. While it PLOS ONE | https://doi.org/10.1371/journal.pone.0281703 February 10, 2023 1 / 11 PLOS ONE Electron microscope loading and in situ nanoindentation of water ice at cryogenic temperatures is known that impurities affect the mechanical properties and flow behaviour of ice causing localized enhanced deformation, the effect of different impurity species at various concentrations remains ambiguous [6–8]. Therefore, simple, and systematic methods of testing the mechanical properties of ice in the laboratory while varying the species and concentration of impurities need to be developed in order to refine to constitutive equations for natural ice. Standardized testing methods for measuring the mechanical properties of ice at millimetre length-scales currently consist of laboratory creep experiments including uniaxial compression or tension experiments, flexural testing, extrusion experiments and fracture testing [9–15]. Several groups have also applied atomic force microscopy (AFM) to measure the surface properties of ice [16–19]. The low loads available to AFM, however, limits measurements to surface forces subject to strong non-contact interactions and introduces complexities due to bending of the AFM cantilever and difficulties in the accurate determination of the tip area function. In materials sciences, instrumented nanoindentation uses a nanometer-scale tip with known mechanical properties pressed into a material to probe its local mechanical properties [20]. Hardness and elastic modulus are then generally derived from load-displacement curves using the Oliver-Pharr analysis method [21]. Compared to macro-mechanical testing, nanoindentation has simpler specimen requirements (i.e., a flat surface), and the response of individual microstructural regions can be tested independently, enabling high throughput testing [22, 23]. Advances in instrumented nanoindentation also allow for testing of micro-geometries, including micro-cantilevers for fracture toughness or micro-pillars for strength measurements for instance [24]. The last few decades have seen significant developments in low temperature nanoindentation. In one type of apparatus, the specimen and indenter can be fully immersed in a cryogenic liquid contained in an insulated vessel [25–27]. For such systems, the testing temperature is limited to the natural boiling point of liquid nitrogen (LN2, 77 K) or liquid helium (LHe, 4.2 K). Temperature control is challenging, and the constant formation of gas bubbles in the liquid cells result in turbulence that affect the load measurements during indentation. In another type of apparatus, the indenter can be retrofitted with refrigeration systems (e.g., GiffordMcMahon refrigerator, Peltier coolers) and electric heaters to control the temperature of the specimen and indenter independently [22, 28–30]. To prevent frost contamination, newer setups operate inside a scanning electron microscope (SEM) which allow for in situ testing and observations under vacuum, together with precise control over the tip, sample and frame temperatures for minimised thermal drift. Herein, we introduce an experimental protocol to conduct in situ instrumented nanoindentation of water ice using an Alemnis Low Temperature Module (LTM-CRYO) installed within a SEM. While th (...truncated)