Amy Crabtree from Cambridge-based Blue Scientific, a company that provides market-leading scientific laboratory instruments for research, development and process control applications in materials science, life science and energy research, looks at how using nanoscale analysis can give a deeper understanding of the makeup of polymers.
The development of polymer and rubber materials involves testing mechanical properties including hardness, modulus, elasticity, cracking and delamination. These properties are also important in quality assurance, to ensure the quality of end products. Nanomechanical testing analyses materials at an incredibly small scale, with high resolution and small forces. This opens the door to new insights and a better understanding of the true make-up ofthe surfaces of polymers, paints and coatings. This is useful in industrial R&D, as well as in academic materials research and surface science testing.
Cracking, Defects and Critical Toughness
Using nanomechanical testing systems, forces high enough to initiate the formation of cracks are applied to polymers and hard thin films. Precise nanoscale measurement of the force required and displacement provides information about critical toughness, crack formation and delamination.
Testing at nanoscale level provides results that are far more precise than at micro-level, where information is averaged over the surface of the test probe. At the nanoscale, much smaller diamond-tipped probes are used, along with specially-developed force transducers, to provide precise quantitative results and surface property information with very high spatial resolution.
Smaller forces provide data about modulus, hardness, cracking forces, adhesion and delamination on thin films and engineered polymer materials. Displacements are used to determine hardness (or compliance) results from the sum of the indent depth and the displacement, caused by the shape and area of the probe tip. For an accurate nanoindentation measurement,it is vital that the contact area of the probe tip (load frame) is well described and defined. This can vary between individual probes, so the indent shape and area of each tip is measured by the manufacturer prior to shipment. A low noise floor is also vital, so data is not lost in system noise.
Modulus and Hardness
Modulus and hardness are fundamental properties of plastics, rubber, paints and coatings that can be measured accurately through nanoindentation. Indenters make accurate load-to-displacement measurements by monitoring and controlling the position of the indenter relative to the surface of the specimen (and the depth of the indent) during the indentation process, with nano-Newton sensitivity. While microindenters can measure bulk and large area properties using forces as low as ~100 mN, thin film polymers and polymer blends with domains in the micron size prove more difficult. These advanced engineered materials require indent forces between 1 µN and 30 mN, with force sensitivity in the 100 nN range. Nanoindenters are able to image features in the nanometer range. The indenter probe can be positioned precisely over a specific region, or even non homogenous regions of blends and composites.
Polymers, paints and coatings applied to substrates pose special problems for manufacturers. Many automotive parts are made from polymer coatings on chrome or other metals and paints are also essentially coatings on substrates. Interfacial properties are of key importance. Studying delamination, adhesive properties, adhesion defects and coating integrity is invaluable in the development and control of coating processes. Delamination and coating problems can often be detected by nanomechanical testing systems as unexpected force displacement results. Adhesive properties and their defects can be measured by examining the pull-off or snap-from-surface forces exerted on the probe tip, during the retraction portion of the force-displacement curves rendered by the system.
Marring and Scratch Healing
Nano-scratching with various shaped probes is used to evaluate properties related to marring, scratch-toughness and scratch-healing. Nanomechanical testing systems precisely control forces and x, y distances, to provide quantitative information related to these properties. Marring and scratching is relevant in the development of automotive exterior and interior products, paints, coatings, flooring and home products.