Calculating the Fracture Toughness of Optical Thin Films using Nanoindentation
MSC507: Practical Electron Microscopy
MSC507: Practical Electron Microscopy
Background and Introduction
|Experimental Procedure||Results and Discussion||
Conclusions and Acknowledgements
BACKGROUND AND INTRODUCTION
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.
3. Plasma Cleaning
Plasma cleaning involves the removal of impurities and contaminants from surfaces through the use of an energetic plasma created from gases. The plasma is created by using high frequency voltages (typically kHz to >MHz) to ionize the low pressure gas (typically around 100-200 mTorr), although atmospheric pressure plasmas are now also common. If the gas used is oxygen, the plasma is an effective, economical, environmentally safe method for critical cleaning. A cleaning action is carried out by the oxygen species created in the plasma (O2+, O2−, O3, O, O+, O−, ionized ozone, metastable excited oxygen, and free electrons). These species react with organic contaminants to form H2O, CO, CO2, and lower molecular weight hydrocarbons. These compounds have relatively high vapour pressures and are evacuated from the chamber during processing. The resulting surface is ultra-clean.
4. Differential Interference Contrast Microscopy
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.
1. Sample Preparation
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.
Fig. 1: (a) Schematic load distribution on thesample; (b) Showing actual load distribution on the MLD sample
Fig.2: Showing the load distribution on the Niobia sample
2. Initial Imaging
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.
Fig. 3 : Optical imaging was done to instantly check for the quality of indentations (MLD sample)
Fig. 4 : Contamination Square seen in the centre
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.
3. FIB milling
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.
3. Final Imaging
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.
Results and Discussion
1. Initial Observations
The first specimen to be worked on was the multilayer specimen. As shown in Fig. 2a indents in a 3x3 matrix were obtained for increasing load. Figures 5 through 10 show the cracking values reported for the indents of varying loads.
As seen in the Fig. 5 through 8, we see very little marks arising out of the indentation for the lighter loads. As the loads increase, the features we are looking for get clearer. Unfortunately, increasing the load also increases the possibility of the plastic deformation passing from the film and entering the substrate. When this happens, the data is no longer valid. Hence, a right balance has to be struck between the resolution of the data and its efficacy. The least load to produce well resolved images were obtained for a load of 8mN.
Fig. 5: Indentation Measurements for indent #1 on MLD
2. FIB Milling
Images of the milled region are shown below in Fig. 11. The first 3 images are milled regions in the multilayer film. The fourth image is that of the milled region in the Niobia film. As we can see, the difficulty here was in observing the internal walls of the milled region due to obstruction created by the geometry. Images of higher magnification showed certain discrepancies in the film, but we couldn’t ascertain if it was due to the indentation or due to manufacturing defects, or possibly milling artifacts. Two such images are shown in Fig. 12 and Fig. 13.
3. SE2 vs. In-Lens
One would expect the SE2 (Everhart-Thornley) detector to work better at long working distance, and the inlens to work better at shorter working distances. But as seen in the following images, that is not always so. As shown in fig.14 and 15, we can see the comparison between the in-lens image and theSE2 image for the same spot. In case of Fig. 14, we hardly see any difference in the two. But for Fig. 15, we see an appreciable difference. This is simply explained by the fact that if the sample is tilted away from the detector, then the image seen is not as bright but if it is tilted towards the sample, then the beam is much brighter. Also, the in-lens detectors did a far better job looking after the surface morphology, while the SE2 provided a far better contrast.
4. Cube Corner Tip
It was seen that the indents in the niobia specimen were not of a good quality. The indented images lacked symmetry. Given the quality of the indents, it was not sure if there was an error in the orientation of the film, or if there was any defect in the indenter itself, like the indenter being chipped. Further investigation was needed in this direction before a conclusion could be arrived at.
4. Localized Plastic Deformation
As expected from theoretical knowledge, we can see in Fig. 15 that the indentation produces a downward stress field in the specimen. However, due to the geometry of the specimen, a component of this is passed into the sideward direction following the Poisson’s Ratio. As a result, we see a spherical region around the indent, whose stress concentration is very high. This region is colored in red. This region shown ductile behavior and shows a plastic deformation. The region beneath it, however, shows brittle behavior. Hence, as seen in the orange-red interface, due to the difference in the ductility of the material, we can see the fracture cracks propagating through the material, with the origin at the interface.
4. Fracture Toughness Calculation
Fracture toughness of the material can be calculated by using the formula:
Where, Kc is th fracture toughness, E is the modulus of elasticity, H is the hardness, P is the applied stress and c is the crack length, as recorded from the above image Fig. 9.
The objective behind this exercise was to try to find the fracture toughness of optical oxide thin films on glass substrate, while learning to use the electron microscope and its ancillary equipment. Different stages of thin Film deformation upon loading could be observed.
But clearly, the work has created more questions than it has solved. Further investigation is needed to understand the deformations resulting from this process. Deeper indentations than the ones done for this process are required so that we can understand a more about the effects of the indentation inside the material. Also, in this case, the indent geometry has to be verified. Also, the calculation of fracture toughness by above principle has to be verified by other processes, including Finite Element Analysis.
I sincerely thank Brian McIntyre for being a great source of ideas, criticism, and good humor, and for being an integral part of my problem solving thought process. I also thank Karan Mehrotra for providing me with the inspiration and the specimens for this work.
1. A. C. Fischer-Cripps, Ibis Handbook of nanoindentation
2. D. S. Harding, W. C. Oliver and G. M. Pharr; Cracking during nanoindentation and its use in measuring fracture toughness; Thin Films: Stresses and Mechanical Properties V, Fall 1994.
Rohit G.Puranik, April 27, 2012