Indentation fracture and toughness assessment for thin optical coatings on glass

The different indentation fracture mechanisms of coated glass caused by a sharp tip (cube corner) at low loads (<4 mN) and a blunt tip (Berkovich) at high loads (up to 500 mN) have been analysed in this study. Existing indentation methods to estimate fracture toughness are unsuitable for very thin coatings (<500 nm). An alternative energy-based method can successfully be applied in the assessment of the ultra-small cracks produced in coated glass based on excursions in the load–displacement curve caused by indentation with a sharp tip. However, it was found that no excursions were observed associated with picture-frame cracks produced by a blunt tip in the same coatings; this is not unusual in other coated systems and makes the existing energy-based models invalid. Therefore, a new energy-based model is developed here to solve this problem and good results have been obtained. The existing models cannot be applied to the analysis of the indentation-induced interfacial failure in this study; therefore, new approaches have also been developed to assess the interfacial toughness and again reasonable results are obtained.