The durability and performance of coatings in almost every application depend on their as-deposited mechanical properties such as adhesion, hardness, toughness and elastic modulus. These properties may not be the same as those of the same material in bulk form due to different microstructural and defect states arising from the deposition process. As the coating thickness is reduced it becomes increasingly difficult to measure these properties by conventional methods. Low load nano-indentation and scratch testing have been developed to enable measurements to be achieved at scales commensurate with the coating thickness. Continuously recording indentation test methods are well established for the determination of elastic Modulus and hardness. But methods for assessment of fracture toughness and adhesion are much less developed. In part the reason for this is that mechanism of fracture failure around an indentation is complex and depends on the relative properties of coating, substrate and interface and therefore a universal analysis method is unlikely to be produced. However, in some cases it is possible to identify failure modes that are amenable to analysis resulting in reasonable toughness data. Microscopy and analysis of fracture paths is a key part of this approach, particularly as the indentations get smaller. This work attempts to obtain fracture toughness of some of the thin films used at Laboratory for Laser Energetics, University of Rochester.

Optical oxide multilayers are a critical technology used at the University of Rochester’s inertial confinement fusion (ICF) facility, Laboratory for Laser Energetics (LLE), whose function is primarily as reflective or antireflective coatings for the high-intensity laser systems such as OMEGA and OMEGA EP (Extended Performance). These multilayers consist primarily of alternating low and high refractive index layers (~25-35), such as SiO2/HfO2 deposited on glass substrates. They, by themselves and/or as a part of the MLD gratings, are exposed to high fluences repeatedly, and are therefore prone to laser damage. It is essential to measure the mechanical properties of these films especially hardness (H), elastic modulus (E) and fracture toughness (Kc). Nano-indentation, among other techniques, is a promising and relatively straight forward technique for these measurements.

The samples used here were prepared by electron beam and plasma-assist electron beam deposition techniques.

Nanoindentation was done based on the standards for instrumented indentation, ASTM E2546 and ISO 14577. It uses a pre-established procedure where an indenter tip with a known geometry is pressed onto a specific location of the material to be tested, by applying a normal load. When reaching a preset maximum value, the normal load is reduced until complete relaxation occurs. The load is applied by using a piezo-actuator and is measured in a controlled loop with a high sensitivity load cell. During the experiment the position of the indenter relative to the sample surface is precisely monitored with high precision sensor that is usually capacitive. The resulting curves showing loading vs. displacement provide data specific to the mechanical nature of the material under examination. Established models are used to calculate quantitative hardness and modulus values for such data.

Differential interference contrast microscopy (DIC), also known as Nomarski Interference Contrast (NIC), is an optical microscopy illumination technique used to enhance the contrast in unstained, transparent samples. DIC works by separating a polarized light source into two orthogonally polarized mutually coherent parts which are spatially displaced (sheared) at the sample plane, and recombined before observation. The interference of the two parts at recombination is sensitive to their optical path difference (i.e. the product of refractive index and geometric path length). Adding an adjustable offset phase determining the interference at zero optical path difference in the sample, the contrast is proportional to the path length gradient along the shear direction, giving the appearance of a three-dimensional physical relief corresponding to the variation of optical density of the sample, emphasizing lines and edges though not providing a topographically accurate image.

Two samples were provided by Karan Mehrotra, Doctoral student at the Mechanical Engineering department. Two samples were used for this study. One was a multilayer diffraction grating consisting of a total of 32 alternating layers of Hafnia (HfO2) and Silica (SiO2) on a fused silica (FS) glass substrate. The total thickness was ~5 µm. The other specimen consisted of a Niobia monolayer on a FS substrate. The samples were indented at various specified loads using a MTS XP nanoindenter. The load distribution for the multilayer specimen is shown in Fig. 1a and Fig. 1b while the same for the monolayer specimen is shown in Fig. 2. The tip used was a diamond cube corner tip. This took the longest time in the work, as the indentation runs lasted about 12 hours.

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The samples then underwent a preliminary inspection using an optical microscope. This image is shown in Fig. 3. The main objective of the optical microscopy was to ensure that the indents were properly impressed onto the sample. Previous samples (excluded from this study) were shown to develop contamination squares when imaged under the electron microscope, as seen in Fig. 4. Hence these samples were plasma cleaned in an oxygen medium. They were then sputter coated using a platinum target in Argon medium.

Then, these specimens were imaged using the Scanning Electron Microscope (SEM, Zeiss Auriga) at the Institute of Optics, University of Rochester. The main interest of the initial electron microscopy was observing the indents and recording the cracking and deformation resulting from the indents.

Further, these samples were ion-milled using Gallium by the FIB (Focused Ion Beam) accessory in the above mentioned SEM. After aligning the required spot with the eucentric point, a gallium in beam was focused on the specimen according to a pre-specified pattern. Each single operation lasted 2 minutes. The beam current was kept initially at 4 nA, but later reduced to 600 pA. The main objective of the milling operation was to qualitatively understand the effect of the loading on the thin film.

It was to be seen if the cracking produced in the indentation process extended into the substrate. The milling operation was intended to cut a transverse section through the indent, so that the indentation root could be checked for the size of the plastic zone and also if there was any crack propagation through the crack, and into the substrate. Some of the images obtained were colorized using Adobe Photoshop CS5, access to which was provided at the Carlson Library, University of Rochester.