Boundary Friction and Boundary Lubrication

Friction arises at the real contact area of two contacting bodies. This area depends on the mechanical properties of the bodies, geometrical configuration, and load. The authors described a new optical method of determining the area of contact, developed in the Laboratory of Friction and Friction Materials of the Academy of Sciences, Russia. This method is based on reflection and diffusion of rays of light from transparent samples when passing out of one transparent medium into another having a different coefficient of refraction. The light ray passes through the contact surface without reflection only if it falls strictly perpendicularly on the surface. Owing to this, rough surfaces diffuse light and its brightness diminishes. When light passes through two surfaces of transparent bodies, the degree of diffusion becomes even greater. In places of contact of the two surfaces the interlayer of air disappears and the ray passes directly out of the one body into the other. If the two bodies have the same refractive index, the ray does not deflect from its path and no loss of intensity results.