Understanding surface properties
From water droplet erosion to environmental corrosion, understanding what alters the surface properties of a material and how it performs in harsh environments is critical to lifetime management, failure analysis and confidence in the selection of materials for fit-for-purpose products and components.
Our tribology research focuses on the development of measurement methods that provide improved understanding of how degradation mechanisms depend on the operating conditions and the microstructure and makeup of the materials that are being examined. The development of new and improved measurement methods is complemented by the use of a range of microstructural characterisation techniques including optical and SEM microscopy and FIB topography.
For sliding wear contacts the emphasis is on developing test methods that give information on the mechanisms of damage as they occur, such as :
- Development of systems for the visualisation of damage to wear surfaces as it accumulates, for example using an optical line-scan camera or scanning electron microscope (SEM) based in situ imaging.
- Methods for real time measurement of the depth of wear using both contact and non-contact (optical) probes.
- The challenges of accurately measuring the low friction properties of modern carbon coatings are being addressed.
- Measurement of small wear volumes by relocation profilometry is also being explored.
For abrasion and erosion the focus is on developing measurement methods for exposure under demanding environment.
- The NPL water droplet erosion test system continues to be developed to provide better control of the test variables and so decrease variability in the results and improve understanding of the erosion process
- a high temperature solid particulate erosion test system is being tested that has the capability to measure wear volume by laser triangulation, and mass loss in situ giving major efficiency in the effectiveness of measurement.
Characterising the mechanical properties of engineered surfaces at the appropriate length scale is key to understanding how performance may be controlled for demanding environments. New methods are being developed and validated, including:
- high temperature nanoindentation is being developed to understand surface properties at the operating temperatures of many processes
- micro-pillars and cantilevers can be produced by focused ion beam milling and their properties measured while the test is observed in an electron microscope
- micro-scratch tests, with an NPL designed apparatus and carried out in an electron microscope, can provide a wealth of information on the durability of a coating and the failure mechanisms
Other measurements have been developed and used to support our work, including laser surface acoustic wave measurements, micro mechanical measurements of adhesion and fracture toughness for coatings.