Temperature-variant scratch deformation response of automotive paint systems

Painted systems are complex and most automotive car paints consist of up to five different paint layers. The way in which the paint system responds to contact can be strongly influenced by the temperature, which, in service, can vary between −20 °C and +50 °C depending on the weather conditions. This paper compares the scratch response of an automotive system at temperatures ranging from −80 °C to +80 °C made using a specially designed scratch test apparatus. The changes in the deformation modes at various temperatures have been investigated using environmental scanning electron microscopy. It was found that the deformation mechanism at all temperatures was ductile tensile cracking behind the stylus. At temperatures below room temperature the response of the paint was less ductile in nature, and the coefficient of friction was low. Above room temperature the penetration of the stylus into the sample increased rapidly and the coefficient of friction also increased dramatically. The scratch test critical load and scratch hardness were both found to decrease as the temperature increased. The results show the importance of testing at a range of temperatures and the use of scanning electron microscopy in assessing the scratch damage in order to fully evaluate the scratch resistance of paint systems.