An investigation of adhesion strength between layers of electron-beam–deposited hafnia and silica

Dielectric coatings are ubiquitous in the field of laser optics, and serve a wide range of needs including beam transport and polarization control. While much attention is paid to their optical properties and interaction with light, their mechanical properties are of importance in understanding modes of failure and environmental stability, and are an integral part of material modifications following laser-induced damage. In fact, the damage morphology of coatings encompasses information about the mechanical properties of coatings that can reveal failure mechanisms. In this work, we study nanosecond laser-damage sites in electron-beam–deposited coatings with alternating layers of hafnia and silica. The results show a consistent pattern of separation between layers of silica deposited on hafnia, indicating that the adhesion strength of hafnia deposited on silica is higher to that of silica deposited on hafnia. A number of mechanical strength testing methods have been employed in order to help understand this discrepancy between adhesive strengths between two interface types in the same pair of materials. Pull-off tests performed on bilayer coatings show adhesion strengths of all interfaces are in excess of 6 MPa, and adhesion between a number of liquid-based adhesives and coatings is shown to be greater when applied to hafnia. The effect of each coating type on the fracture strength of silica substrates is demonstrated to be minimal through a series of ring-on-ring stress tests. Results of indentation and scratch tests covering a range of applied stresses will be covered as well. We postulate that the origin of this behavior is related to the columnar structure of the as-deposited hafnia layers and the resulting surface roughness.