Effects of temperature and strain rate on the scratch resistance of poly-methylmethacrylate

The scratch resistance of polymers has been the subject of numerous studies that have led to the characterization of plastic and fracture phenomena during scratching. Viscoelastic and viscoplastic behavior during scratching have been related to dynamic mechanical properties that can be measured via dynamic nano-indentation testing. Yet, an understanding of the origin of the fracture phenomena in a polymer during scratching remains approximate. Parameters like tip geometry and size, scratch velocity and loading rate, and applied strain and strain rates, have been considered critical parameters for the fracture process, but no correlation has been clearly established. The goal of this work was to evaluate scratch resistance parameters as a function of temperature and strain rate, and compare them to dynamic mechanical properties obtained from nano-indentation tests over a range of temperature for poly-methylmethacrylate (PMMA). Fracture in scratch testing is modeled as resulting from tensile stresses behind the scratch tip. A new scratch fracture parameter is introduced that is related only to material properties and not to the scratch tip geometry. This study brings a new understanding to the origin of fracture mechanisms during scratch testing, and to the influences of indentation strain on the fracture strength of PMMA.