Contact fatigue in an alumina microcontact: A confocal laser scanning microscope study
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Using a confocal laser scanning microscope the development of stress in a micromechanical contact could be measured for ruby with a resolution of ∼1 m. Ruby (␣-Al2O3: Cr3+) spheres with radii of 75 m were compressed quasi-statically between two sapphire (␣-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 m length scale.
I. INTRODUCTION
II. EXPERIMENTAL
The development of stress in a mechanical microcontact between hard, brittle materials is essential in processes such as friction, wear,1–6 and fracture.7 For brittle materials beneath concentrated loading forces, damage takes the form of cracking in response to tensile stresses or the form of quasi-plastic yield deformation in response to shear stresses.8–11 The aim of this paper is to gain more insight into the process of deformation of a brittle material on the micron scale. By combining laser scanning confocal spectroscopy with two-photon excitation the local stress at a given point in a ruby sphere was monitored while varying the load. We used ruby (aluminum oxide doped with a small amount of chromium) because it shows a stress-dependent shift of its fluorescence line. In addition, fatigue in alumina has been studied extensively.12–18 When excited by blue or green light, ruby shows a fluorescence spectrum with two sharp peaks at 694.2 nm (R1) and 692.8 nm (R2) of wavelength. The peak positions move to longer wavelength when ruby is subjected to compressive stress.19–23 We used this pressure-dependent wavelength shift to measure the local stress in the center of the contact region when applying a load.24,25
Therefore, a ruby sphere (75-m-radius single crystal, 98.9% Al2O3, 0.9 wt% Cr2O3, 0.1% Si2O3; Goodfellow, Bad Nauheim, Germany) was clamped between two parallel sapphire plates (LOT-Oriel GmbH & Co. KG, Darmstadt, Germany) of 0.5 mm thickness on top of a laser scanning confocal microscope during repeated loading and unloading cycles. The loading force was exerted by a spring (5500 N/m), which was compressed by a step-motor as described previously.25 Each single step corresponds to a load increment of 0.055 N. The whole setup was fixed on the stage above the objective of a confocal microscope (LSM 510–ConfoCor 2,
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