Mechanical Behavior at the Nanoscale VI: Contact and Fracture
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS Structural Materials Division, TMS: Computational Materials Science and Engineering Committee, TMS: Mechanical Behavior of Materials Committee, TMS: Nanomechanical Materials Behavior Committee
Program Organizers: Matthew Daly, University of Illinois-Chicago; Douglas Stauffer, Bruker Nano Surfaces & Metrology; Wei Gao, University of Texas at San Antonio; Changhong Cao, McGill University; Mohsen Asle Zaeem, Colorado School of Mines

Tuesday 2:30 PM
March 1, 2022
Room: 262B
Location: Anaheim Convention Center

Session Chair: Matthew Daly, University of Illinois-Chicago; Robert Carpick, University of Pennsylvania

2:30 PM  Invited
Visualizing Nanoscale Contacts In Situ: History-dependent Adhesion of Si-Si and MoS2-MoS2 Interfaces: Robert Carpick1; 1University of Pennsylvania
    I will discuss nanocontact experiments conducted using an in situ nanoindentation apparatus inside a transmission electron microscope (TEM). The instrument is customized to permit atomic-scale imaging of contact formation, asperity sliding, and adhesive separation of two nanoasperities with real-time TEM imaging. Forming and separating Si-Si nanocontacts without sliding revealed small adhesion forces; sliding before retraction resulted in a nearly 20 times increase in adhesion. This reversible sliding-dependent adhesion is attributed to removal of passivating species from the surfaces, followed by their re-adsorption after separation []. Molecular dynamics (MD) simulations support this idea and further elucidate processes occurring at the interface []. I will also discuss nanocontact experiments of 2D materials using this instrument. For tip-on-tip contacts of few-layer MoS2, adhesion increases with successive cycles of contact, far exceeding values expected for pure van der Waals interfacial interactions. Combined with MD, this effect is attributed to contact-induced defect formation.

3:00 PM  
NOW ON-DEMAND ONLY – Measuring and Understanding Nanoscale Adhesion and Deformation Using Iin Situ Experiments in a Transmission Electron Microscope: Tevis Jacobs1; Soodabeh Azadehranjbar1; Ruikang Ding1; Andrew Baker1; Sai Bharadwaj Vishnubhotla1; Ingrid Padilla Espinosa2; Rimei Chen2; Ashlie Martini2; 1University of Pittsburgh; 2University of California, Merced
    Nanoscale adhesion and deformation are controlled by atomic-scale processes. While scanning probes experiments measure forces precisely, fundamental understanding is typically precluded by a lack of knowledge of the shape and structure of the probe and the instantaneous nature of the contact. This talk will discuss recent progress in understanding the nature of adhesion and deformation by performing tests inside of a transmission electron microscope (TEM). The in situ TEM setup enables sub-nanonewton resolution of applied and adhesive forces, combined with Ångström-scale information about the bodies and their contact. This is coupled with electrical biasing to yield instantaneous contact conductance measurements with nanoamp-resolution. Furthermore, the knowledge of material shape, structure, and crystallographic orientation is used to generate precisely matched model tips that can be simulated using molecular dynamics. Results reveal the strong effect of composition, structure, surface chemistry, and applied stress on nanoscale adhesion and deformation.

3:20 PM  
Tribochemical Formation of Diamond-like Carbon Films on Catalytically-active Noble Alloys: Frank DelRio1; Morgan Jones1; Thomas Beechem1; Anthony McDonald1; Tomas Babuska2; Michael Dugger1; Michael Chandross1; Nicolas Argibay1; John Curry1; 1Sandia National Laboratories; 2Florida State University
    Low shear strength organic films were grown in-situ on platinum-gold thin films via cyclic sliding contact in dry nitrogen with trace concentrations of ambient hydrocarbons. Steady-state friction coefficients were found to be as low as µ ≈ 0.015 and inversely proportional to contact pressure, revealing non-Amontonian behavior. At contact pressures above 500 MPa, shear strength dropped, indicating an activated process. The regions of steady-state low friction behavior exhibited spectra similar to diamond-like carbon coatings. Raman spectroscopy identified non-uniformity in areal coverage and relative order with contact pressure. Atomic force microscopy was used to study the formation and growth of these films at the nanoscale; stress- and time-dependent measurements suggested a sublinear increase of film volume with time, and a transition from growth to wear at a contact pressure of 1.2 GPa. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

3:40 PM  
Direct Measurement of Adhesion for Noble-metal Nanoparticles Using In Situ Transmission Electron Microscopy: Andrew Baker1; Sai Bharadwaj Vishnubhotla1; Sanjana Karpe1; Yahui Yang1; Goetz Veser1; Tevis Jacobs1; 1University of Pittsburgh
    A critical aspect of metal nanoparticle technologies is how strongly the nanoparticles adhere to their substrates. This adhesion is directly related to the efficiency and lifetime of many nanoparticle applications. In the present work, we performed in situ adhesion tests inside of a transmission electron microscope (TEM) on nanoparticles of gold and platinum. This approach combines Angstrom-scale characterization of particle size, shape, and structure together with nanonewton-scale measurements of adhesive forces. Together, these capabilities allow the determination of adhesion energy on a particle-by-particle basis. First, the accuracy of the technique was assessed based on comparison to prior approaches; then the adhesion energy was measured for new material combinations that had not previously been investigated. Finally, this flexible test platform was applied to study structure-properties relationships, including the effect of particle shape and size on adhesion.

4:00 PM Break

4:20 PM  Invited
Fracture of Two-dimensional Materials: Jun Lou1; 1Rice University
    In this talk, we will report our recent effort to study fracture behaviours of 2D materials. Our combined experiment and modelling efforts verify the applicability of the classic Griffith theory of brittle fracture to graphene. Strategies on how to improve the fracture resistance in graphene, and the implications of the effects of defects on mechanical properties of other 2D atomic layers will be discussed. More interestingly, stable crack propagation in monolayer 2D h-BN is observed and the corresponding crack resistance curve is obtained for the first time in 2D crystals. Inspired by the asymmetric lattice structure of h-BN, an intrinsic toughening mechanism without loss of high strength is validated based on theoretical efforts. The crack deflection and branching occur repeatedly due to asymmetric edge elastic properties at the crack tip and edge swapping during crack propagation, which toughens h-BN tremendously and enables stable crack propagation not seen in graphene.

4:50 PM  
Competing Behavior of Slip and Fracture on the Nanomechanical Response of Pharmaceutical Materials: Sushmita Majumder1; Chenguang Wang1; Kevin Schmalbach1; Javier Garcia-Barriocanal1; Greg Haugstad1; Changquan Calvin Sun1; Nathan Mara1; 1University of Minnesota-Twin Cities
    In the pharmaceutical industry, crystal milling and tablet forming are two manufacturing processes that are strongly affected by the tendency for crystals to deform and/or fracture. The present investigation aims to utilize advancements in operando nanoindentation to study the influence of applied stress and environmental factors on nanomechanical behavior of single molecular crystals, L-alanine and succinic acid. Both Berkovich and spherical indenters have been used to elucidate the effects of loading geometry on mechanical behavior. We employ XRD and AFM to determine crystal orientation effects and slip/fracture features on the sample surface respectively. The observed nanomechanical behavior will be discussed in terms of existing models utilized to describe slip and fracture and can be further used to design pharmaceutical materials optimized for milling and tabletability.