Pressure distribution in a mechanical microcontact

We verify an important prediction of Hertz theory [J. Reine Angew. Math. 92, 156 (1882)] for hard microcontacts, namely, the hemispherical pressure distribution in the contact area. Therefore, a confocal microscope was used to measure the local spectral shifts of the fluorescence lines in a ruby (Al2O3:Cr3+) sphere of R=78 mu m radius compressed between two sapphire (Al2O3) Plates. From the spectral shifts the local stress was calculated. At a load of 4.3 N the radius of the circular contact area was measured to be a=(11.2 +/- 1.0) mu m. A hemispherical pressure distribution was observed with a maximal normal pressure of P-0 = (12.7 +/- 2.2) GPa. (c) 2006 American Institute of Physics.