Contact fatigue in an alumina microcontact: A confocal laser scanning microscope study

Abstract

Using a confocal laser scanning microscope the development of stress in a micromechanical contact could be measured for ruby with a resolution of ∼1 μm. Ruby (α-Al2O3: Cr3+) spheres with radii of 75 μm were compressed quasi-statically between two sapphire (α-Al2O3) plates. While applying an increasing uniaxial load, confocal microscopy was used to record the fluorescence spectra at the contact region. The peak positions of the fluorescence spectrum shift to longer wavelengths with increasing stress. By detecting the shift in wavelength the local stress could be measured. We adopted two-photon excitation process (800 nm wavelength) to reduce background fluorescence. Loading–unloading cycles were applied with the maximal loading force increased subsequently for each of the next cycle. Progressive fatigue was observed when the load exceeded 1.1 N. As long as the load did not exceed 4 N stress-versus-load curves were still continuous and could be described by Hertz’s law with a reduced Young’s modulus or increasing damage. Once the load exceeded 4 N, spikelike decreases of the stress were observed. This indicates the formation of microcracks on the 10 μm length scale.