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Is there a critical thickness over which the solids can be considered as an elastic half-space? If it exists, what is the crucial factor determining the critical thickness? And how the specimen thickness affects the real contact area growth of the textured surface? The reply to these questions could surely provide a proper guideline for elastomer product design such as seals. With this in mind, the elastic contact of two-dimensional regular wavy surface with a flat surface are studied by using four types of blocks of silicone rubber as specimen under decompression and wet environments. The results suggest there exists the critical thickness depending on the sizes of the contact surface and the pitch of asperities of the wavy surface.

The elastic contact of two-dimensional regular wavy surface with a flat surface and its frictional behaviour are studied by using three types of blocks of silicone rubber as specimen. The friction force as well as the real contact area is markedly affected by the shape of the valley of the wavy surface. In the case of the specimens with U-shaped trough, each contact spot touches the neighbours before complete contact, resulting in the extinction of the percolation channels allowing the fluid flow. If the tests were conducted in wet environment, the rate of increase in the real contact area with the load shows sharp drops when each contact spot touches the neighbours, which inhibits the increase in the friction force.

A theoretical model for the adhesive friction between elastomers and rough solid surfaces is proposed on the basis of opening crack propagation processes at the boundary of the contact interfaces and the rate processes of formation of molecular bonds on the solid surface. This model, which is expressed as a product of the terms related to the two abovementioned processes, requires some measurable and fitted parameters such as the frictional shear strength expressed as a function of viscoelastic dissipation, rate-dependent elasticity, density of bonded molecular chains at a contact junction, critical velocity related to viscoelastic relaxation, and critical velocity related to the rate process of formation of molecular bonds on the solid surface. The friction-velocity relationship exhibits a remarkable fit to previously obtained experimental results for polymers such as engineering rubber, gels, and plastics (glassy polymers), and all fitting parameters are physically reasonable. The viscoelastic index "n" is also related to the "glass-to-rubber transition" of a nanometer-thick polymer layer for frictional behavior. Thus, from a practical viewpoint, this model can be used effectively for fitting the adhesive friction behavior of polymers.