A) Ring-Cone
3. EXPERIMENTAL PROCEDURES
3.3 Sample Preparation
3.3.1 Grinding
The nine optical glasses came in bulk annealed form and were cut into smaller rectangular pieces about 30 mm L x 25 mm W x 4 mm H using a large diamond saw*. Approximately four samples were cut for each composition. Grinding and polishing of the samples was then carried out to achieve a flat, near-optical-quality surface with few scratches and pits. No cutting was necessary for the vitreous silica specimen. The float and Pyrex™
borosilicate samples were cut from sheets of glass 3.2 and 3.5 mm thick, respectively, and no grinding was necessary. All specimens, except for the vitreous silica (Suprasil 312), were mounted on circular aluminum metal blocks using thermoplastic glue, in order to facilitate grinding and polishing.
The vitreous silica specimen was large enough to grind and polish without using metal blocks. A Buehler grinder† was used for grinding. Samples were ground on both sides using SiC paper with either distilled water or polishing oil. First 320-grit, followed by 400-grit, then 600-grit SiC paper was used.
The grit numbers correspond to average SiC grain sizes of approximately 34 µm, 22 µm, and 14 µm, respectively. Samples NS1 and S3 were ground with polishing oil to reduce water interaction with the glass, while the rest of the samples, including the vitreous silica specimen, were ground in distilled
*Discotom-5 Cutting Saw, Struers Corporation, USA
†Buehler Ecomet 3 Variable Speed Grinder/Polisher, Buehler Corporation, USA
water. Each pad was saturated with either the oil or distilled water prior to grinding. Samples were ground five minutes per side. Each sample was rinsed with either water, for the water-ground specimens, or reagent alcohol (90% ethanol, 5% methanol, 5% isopropanol) for the oil-ground specimens, between steps to remove residue. Polishing of the specimens took place next.
3.3.2 Polishing
The two oil-ground specimens were polished using diamond pastes* of 15 µm, 6 µm, 3 µm, and 1 µm, in that order. Polishing oil‡ was put on a pad**
secured to a polishing wheel spun by a motor, along with some diamond paste. The samples were moved in a circular direction opposite the rotation of the wheel and polished for approximately five minutes per side. Additional polishing oil was squirted onto the pad about every thirty seconds. Between polishing steps, all samples were placed in a beaker containing the reagent alcohol, which was then placed in an ultrasound bath for a couple of minutes to remove residue. Samples were then gently dried with laboratory towels.
New pads were used for each diamond paste size. The last step consisted of polishing with a cerium oxide polishing compoundon a new pad, followed by cleaning. All surfaces were subsequently checked with a light microscope to ensure good quality.
*METLAB Diamond Compound, METLAB Corporation, USA
‡Polishing Oil, Buehler Corporation, USA
**METLAB PAN W Polishing Pads, METLAB Corporation, USA
Cerium Oxide Polishing Compound, Buehler Corporation, USA
When necessary, samples were re-polished with the cerium oxide compound to remove any obvious imperfections. The samples were then removed from the aluminum blocks by heating the blocks on a hot-plate until the glue softened, at which point the samples were removed. The samples were then placed, one at a time, in a beaker containing acetone and ultrasonicated for approximately five minutes. This was necessary in order to remove the glue from the samples. The samples were then wiped with reagent alcohol to remove any acetone residue, and then stored in a drying oven held at ~100 °C to help keep away water.
The water-ground specimens were polished using diamond sprays, along with the polishing pads. The sprays came in diamond sizes of 15 µm, 6 µm, and 1 µm. Samples were polished starting with the largest diamond size first. More spray was added every thirty seconds, and samples were polished approximately five minutes per side, and cleaned with the reagent alcohol and ultrasound polishing steps. After polishing with the 6-µm spray, the specimens were polished using the 3-µm diamond paste and a glycol-based diamond thinner fluid. Polishing with the cerium oxide compound was the last polishing step, followed by specimen cleaning and storage, as before.
The float-glass specimens were cleaned with the reagent alcohol and ultrasound, while one side of the 7740 Pyrex™ borosilicate specimens was polished with the 1-µm diamond spray, followed by polishing with the cerium
oxide compound. This was done to remove any surface layers that may have been present.
For glasses NS1 and S3, only the specimens used for the in-situ crack initiation studies were oil polished; the rest of these specimens were polished via the water route described above for all other tests. The toxicity, smell, and greasiness of the oil were some of the reasons why it was not used to prepare all the specimens.
3.3.3 Annealing
The polished glass specimens, except the vitreous silica specimen, were annealed in an electric muffle furnace§ at 10 K above their respective glass transition (Tg) or annealing (TA) temperatures to remove any residual stresses. Values of Tg were supplied by the optical glass manufacturer, who used a 2.2 K/minute heating rate. The glass transition temperature of the float glass was determined in a previous study53 and the annealing temperature of the 7740 Pyrex™ borosilicate glass was found in the literature. Two specimens of the same glass were placed inside the annealing furnace on a piece of platinum foil. The temperature was increased at 10 K/minute until the desired temperature of 10 K above Tg or TA was reached, at which point the specimen was soaked at this temperature for one hour.
After one hour, the temperature was decreased at a rate of≈1 K/minute to
§Thermolyne Model 1300, Thermolyne Corporation, USA
room temperature (≈22 °C). The specimens were then removed and stored in the drying oven. Table X lists the glass transition and annealing temperatures.