The fatigue properties of ultrathin protective coatings on silicon thin films were investigated. The cohesive and delamination fatigue properties of 22 nm-thick atomic-layered-deposited (ALD) titania were characterized and compared to that of 25 nm-thick alumina. Both coatings were deposited at 200 °C. The fatigue rates are comparable at 30 °C, 50% relative humidity (RH) while they are one order of magnitude larger for alumina compared to titania at 80 °C, 90% RH. The improved fatigue performance is believed to be related to the improved stability of the ALD titania coating with water compared to ALD alumina, which may in part be related to the fact that ALD titania is crystalline, while ALD alumina is amorphous. Static fatigue crack nucleation and propagation was not observed. The underlying fatigue mechanism is different from previously documented mechanisms, such as stress corrosion cracking, and appears to result from the presence of compressive stresses and a rough coating–substrate interface.
In this paper, we present the results of mechanical testing that reveals the onset crack strain and fatigue crack growth of TiO2 and Al2O3 films deposited by atomic layer deposition. Data show that both films have a strong thickness dependence of the onset crack strain, with the strain reducing with increasing film thickness. Additionally, TiO2 films have a lower onset crack strain and strain for fatigue damage than Al2O3. However, TiO2 films are more resilient in harsh environments where fatigue damage in Al2O3 films grows faster.
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