Challenge 4 - Particles and 2nd Phase

Friction mechanisms increase in complexity as we move from simple 2-body contacts to systems involving particles,
thin-films and liquids. Understanding the friction of systems incorporating micro- and nano-particles is a priority. The model of friction and growth of technologies including powder-based materials processing (e.g. additive manufacture/3D printing), nanostructured lubricants and abrasives. Furthermore, in addition to ‘known’ particles in a system, friction-induced damage can also generate additional ‘3rd-body’ particles by the break-up and interaction of existing components. 3rd body particles are system dependant, and can lead to complex, time-dependant and spatially-localized friction behaviour. On the micron-scale, for example, interactions between discrete micron-scale particles or surface asperity-asperity interactions there are many unknowns relating to how materials deform, react with their environment, fracture and adhere. Multiscale experimental and theoretical simulations are bringing advances in understanding of the processes that control friction at such interfaces. The information is important for the development and operation of machines with micron-sized components (e.g. MEMs and other actuation devices), but also in determining the bulk properties of granular flows and particle reinforced lubricants. Understanding surface/environment and surface/surface interactions at the small scale has important impact across the length scales in a range of applications. Friction arises from an assembly of micro- and nano-scale physical and chemical phenomena operating within a contact zone therefore we need to develop fundamental understanding and quantification these contact processes and lead to information about the energy flow between contacting bodies. Challenge 4 will focus on the in-situ, spatially- and time resolved quantification of interfacial and frictional phemonema. Specifically we will exploit the imaging resolution and spectroscopy capabilities of electron microscopes to go beyond the limitations of conventional, predominantly post-mortem, tribological imaging methodologies. We will implement dynamical real-time mechanical testing of industrially relevant tribosystems in-situ in electron microscopes to evaluate time-resolved frictional phenomena. The teams in Leeds and Sheffield have been leading in this area through their work on:

  • Linking microscale measurements to granular flow models using Discrete Element Modelling (DEM) (Ghadiri)
  • Assessing single asperity friction and associated changes in surface reactivity in tribochemistry (Mangolini/Neville/Morina)
  • In-situ TEM and SEM of dynamical tribological contacts including particles as 3rd bodies1 (Inkson)
  • Micron and sub-micron scale characterisation of tribofilms and tribologically induced layers (Brydson/Rainforth/Neville/Morina/Mangolini/Bryant/Inkson)
  • Triboelectric charging of pharmaceuticals in powder transport (Ghadiri)

The initial projects in this Challenge will develop the methodology for quantifying surface-particle and particle particle friction and associated dynamical reaction processes in the environmental SEM (Project 4: Advanced microscopy for tribological contacts), and attempt to remove some of the empiricism out of bulk solids “flow” modelling by understanding what processes affect friction/adhesion in rolling and rolling/sliding contacts (Project 8: Particle/particle wear/friction).

The core methodology here will be tribological testing of material contacts inside a Scanning Electron Microscope (SEM). A variety of electron microscopy probes have been developed over the last 10 years. One of the most promising for multi-scale frictional phenomena is in-situ SEM nanoindentation where a sharp probe is brought into contact with another surface. Such methodologies are a challenge due to requirements including instrumentation size minimisation (to fit inside the SEM chamber whilst maintaining instrumentation stiffness), geometry optimization to get coincidence of the triboprobe, sample and electron beam, positioning resolution and load range. Commercial systems have recently available optimised for SEM-scale mechanics, which this project will exploit for dynamical tribology.

In manufacturing involving particulate solids (tablet/capsule medicine production, nuclear fuel rod production) interparticle friction and adhesion cause mechanical and cohesive jamming, adversely affecting productivity to the extent that it constitutes major technology bottleneck currently subject to much effort to mitigate2. The focus of Project 8, therefore, is to understanding the role of flow-aids in controlling friction and adhesion for the transport of granular flows. There is a need to understand small scale interfacial processes and their role in affecting macroscopic/bulk flow processes. Mg stearate and silica nano-particles are commonly used as flow-aids, and whilst the latter act as spacers between particles, reducing their interactions, the full mechanisms of the former are not well-understood, hence the focus of this study. Mg stearate laminates on shearing, and in practice that there is an optimum level of shearing beyond which its positive effect is reduced/lost. There is also known to be reactivity between the Mg stearate and the particles being transported and the formation of solid lubricant reaction films has been reported. We will use a combined experimental and modelling approach to assess the mechanism of Mg stearate as a flow aid, building on the in-situ SEM capability developed as part of this Challenge, project 4.

Co-Investigators

  • Prof. Beverley Inkson – Challenge Leader
  • Prof. Mojtaba Ghadiri – Challenge Leader
  • Prof. Rik Drummond-Brydson
  • Prof. Matthew Marshall
  • Prof. Mark Rainforth

Postdoctoral Staff

  • Dr. Mingwen Bai – Project 4
  • Dr. Sadegh Nadimi  – Project 8

1K Anantheshwara, AJ Lockwood, RK Mishra, BJ Inkson, MS Bobji, Dynamical Evolution of Wear Particles in Nanocontacts, Trib Letters, 45, 2, 229-235 (2012).
2http://www.ifpri.net/events/powder-flow-workshop