Abstract: The effect of external pressure on friction and contact with rough surfaces of the inclined plane/sliding block under different vacuum degrees is investigated. Angle of the inclined plane at the sliding block of some pairs of materials is measured at different vacuum degrees. The experimental results show that the inclination angle of the inclined plane at the block start sliding and the static friction force increase with the growth of external pressure. The friction pairs with low material hardness, light component and components with large contact area are more affected by external pressure. As the pressure of the environment increases from vacuum degree -0.095 MPa to 0 MPa, the maximum static friction force of bearing steel/bearing steel increases by 11%, polycarbonate/bearing steel goes up by 12%, while polycarbonate/polycarbonate rise by 39%. Often, the block always is subjected by the external pressure, so the friction angle should be conducted in the absolute vacuum environment, then the critical angle of inclined plane is the true friction angle, if conduct in the air will encounter error. It is indicated that, this experimental method can be used to obtain the actual contact area of two bodies. The experimental results show the actual area of contact increases when the load grow up, this is consistent with the experiment and theory of tribology.

Key words: inclined plane/sliding block, external pressure, friction coefficient, vacuum degree