Contact Behavior of Soft-Hard Contact Interface under Mechanical Stress Conditions

The big and dynamic surface deformation of soft material under mechanical stress has an important influence on its tribological properties. Different from traditional hard-hard contact interfaces, the contact mechanic behavior is more complex due to the nonlinearity of the ontological relations of soft matter materials. Since the bigger and dynamic surface deformation happens on soft material surface when subjected to the same load, compared with hard contact material, the interface contact behavior mechanism under compressive shear conditions is still unclear. Therefore, it is very necessary to study the contact behavior on soft matter surface under mechanical pressure conditions, for further insight the tribological property and lubrication mechanism of the soft materials by constructing a bionic soft contact model interface and in-situ observing the contact mechanic behavior. In view of the characteristics of bigger deformation size, high speed of surface deformation and large surface energy dissipation of soft contact interface, the optical system of the point contact was designed and built, in order to realize the observation and measurement of large-size deformation area (contact diameter of mm order), and at the same time, it can track in situ in a very short time. Considering the difficulty, variability and ethical issues in obtaining biological tissues, researchers generally choose elastomers as models to carry out related basic research. Among them, cross-linked polydimethylsiloxane (hereinafter referred to as PDMS) is widely used in biological simulations due to its mechanical properties similar to human tissue, easy manufacturing, physiological inertness, low cost, and good thermal and oxidative stability surface. In this study, the soft contact interface consists of a smooth hard ball and a transparent glass disc coated with an elastic soft cushion layer. Firstly, a high-speed camera was used to record the entire deformation process, and then the acquired image sequence was systematically analyzed to draw the change curves of the contact diameter and the indentation depth with time.The measurement results showed that the contact diameter of the deformation area increased with the increase of load and ranged from 1.01 to 2.21 mm. By comparing these curves of the measured value of contact radius α3 with the predicted value of Hertzian contact theory, it was found that the Hertz contact model no longer accurately predicted the contact radius, therefore a new contact theory model for soft-coated materials needed to be developed based on experimental results, e.g. a modified Hertzian theory model. Simultaneous measurement of contact diameter with micron-level accuracy and indentation depth with nanometer-level accuracy had been achieved during the mechanical loading, holding load and unloading processes, the visual measurement of deformation occurring on soft contact surfaces both in time and space had been realized. The change of the surface contact contour over time further described the entire deformation process the mechanical response speed of the PDMS film along the normal load direction and in the radial direction was inconsistent, and the former was faster. The results would like to contribute to the understanding of the complex contact mechanical behavior of soft-hard interacting interfaces and the tribological performance and lubrication mechanisms of soft material surfaces.