Table XI. Approximate Glass Compositions and Properties
Component SLS‡ (mol%) SAS† (mol%)
SiO2 72 66.2
Al2O3 0 10.8
MgO 6 5.6
CaO 9 0.5
Na2O 13 13.5
K2O 0 2.3
Other 0 1.0
Property SLS* SAS*
Elastic Modulus (GPa) 70 72
Poisson’s Ratio 0.22 0.22
Strain Point (°C) 490 576
Annealing Point (°C) 530 622
Softening Point (°C) 710 870
Stress-optical Coef. (TPa-1) 2.75^ 2.75^
‡ SLS = generic float SLS, example composition after Seward and Varshneya35
† SAS = Corning 0317, composition from Dumbaugh36
* SAS and SLS properties from Seward and Varshneya35
^ Stress-optical coefficient estimates after Varshneya12
Chemical strengthening was performed using a paste-based potassium nitrate source. Two compositions were utilized. For chemical strengthening performed at 350 °C and higher temperatures, a paste of weight percent composition 25% C&C ball clay (Spinks Clay Company), 25% potassium nitrate (technical grade), and 50% distilled water was used, prepared as a 1,300 g batch. Below 350 °C, a paste of weight percent composition 26% porcelain, 18% potassium nitrate (technical grade), 4% potassium hydroxide (technical grade), and 52% distilled water was used, prepared in a 120 g batch.
A 70/30 mole percent eutectic mixture of potassium nitrate and potassium hydroxide was utilized to attempt to depress the melting point of the salt mixture within the paste. These pastes were thoroughly mixed using a hand-held drill with stir attachment prior to application to the coupons. For each set, two coupons were fully masked on their 16 mm x 8 mm faces using Scotch™ tape with approximately 2 mm overhang, leaving the 16 mm x H edges exposed to the paste. One coupon was half-masked on the 16 mm x
8 mm faces, leaving half of those faces exposed to the paste. The final coupon had all faces exposed to the paste. Paste was applied by dip-coating the coupons into the freshly- mixed paste. After paste coating, the coupons were placed on aluminum foil and dried at 85 °C for one hour. After low-temperature drying, the paste had become sufficiently rigid to remain adhered to the glass and to maintain its shape (leather-hard), at which time the tape masks were carefully removed using tweezers to avoid disturbing the adherent paste. This masking process was highly effective for applying the paste to the intended surfaces. Paste-coated coupon sets were transferred to separate fine-wire stainless steel mesh setters for chemical strengthening. A muffle furnace was first pre-heated to the target exchange temperature and then setters with coated coupons were placed in the furnace. The furnace temperature was independently monitored using a probe thermocouple placed within 25 mm of the coated coupons. The chemical strengthening temperatures and times used for each glass are given in Table XII. For each chemical strengthening temperature, multiple setters were often placed within the furnace, and then removed individually when the target time had elapsed for each setter. When the chemical strengthening time was one hour or less, setters were independently chemically strengthened to prevent adverse effects from temperature fluctuations. After removal from the furnace, setters with coated coupons were allowed to cool to room temperature, after which coupons were rinsed with tap water to remove the paste and dried with paper towels. Samples were immediately placed in polyethylene bags and labeled after washing and drying.
Table XII. Chemical Strengthening Temperature and Time Parameters
Temperature (°C) Time (hours)
SLS SAS
250 - 239
300 - 72, 108, 144, 216, 288
350 - 4, 8, 16, 32, 64, 143
400 9, 16, 25, 37, 46, 66 1, 2.25, 4, 6.25, 9, 16 450 4, 9, 16, 25, 37, 50 1, 2.25, 4, 6.25, 9, 16 500 1, 2.25, 4, 6.25, 9 0.25, 1, 2.25, 4, 6.25
Figure 16 schematically depicts (A) the coating locations, (B) resulting swelling after chemical strengthening, and (C) locations of sampled line profiles that are further detailed below. Table XIII also provides an overview of the coupon purpose for each set.
From each set of coupons, coupons #1 and #2 were used for surface profile measurement of edge swelling resulting from 16 mm x H edge exchange. Surface profile measurements were made using a white light profilometer (Zygo Corporation model NV5000 5032). Prior to each measurement session, a system error profile of a certified silicon carbide reference flat (approximately 5.9 nm peak-to-valley error over a 25 mm aperture) was measured and used for all subsequent measurements to subtract any aberrations introduced by the profilometer optics. A step height standard (VLSI Standards Incorporated, 1.8 μm step) was also measured and the average of 15 measurements was used to adjust profilometer calibration constants that were susceptible to drift with varying ambient temperature and humidity. Note, drift from this type of error was often less than 3 nm. With use of the reference profile and calibration to the step height standard, the profiler generated height measurement accuracy of
±11 nm, precision of ±6 nm, and resolution of 1 nm or better. The lateral resolution was similar to that of a standard optical microscope, on the order of 5 μm for the 20x objective with 1x zoom (1.13 μm pixel resolution). Profiles of a 0.30 mm x 8.75 mm region (i.e. the 8 mm coupon dimension was exceeded to ensure the edge was captured) between the mid-points of the 16 mm edges of the 16 mm x 8 mm face of the coupon were stitched from multiple measurements using the profilometer software, where individual profiles had spatial dimensions of 360 μm x 270 μm with profile overlap of approximately 13% percent for stitch alignment. From the measured profile, a line profile of 25 μm width was extracted, drawn to be perpendicular to the 16 mm edge and drawn to avoid regions of obvious edge chip-out. Height values at each position along the profile were an average of the height values of the 25 pixels across the line width.
Line profiles were leveled and translated to a common reference position by a procedure detailed in Appendix C.
Figure 16. Coupon selective-surface chemical strengthening, swelling, and measurement depictions: (A) paste-coated regions indicated by shaded blocks, (B) dimensional swelling after chemical strengthening where solid blue coloration indicates chemically strengthened surface, (C) arrow and eye indicating the profile or measurement line. Note, diffusion is in the z-direction and swelling and chemical diffusion gradients are not drawn to scale.
Table XIII. Summary of Purpose for Coupons in Each Set
Coupon ID Purpose Measurement Location
1 & 2 Edge swelling by white light profilometer
16 mm x 8 mm Exterior surface,
Line connecting midpoints of 16 mm edges
3 Step height by white light profilometer
16 mm x 8 mm Exterior surface,
Entire surface
4
Stress-birefringence by polarized light microscopy with compensator &
Chemical diffusion by electron microprobe analysis
8 mm x H Interior surface,
Line connecting the midpoints of the 8 mm edges
Coupon #3 from each set was used for surface profile measurement of step height.
The white light profilometer was again used, with initialization and calibration equivalent to that described above. The 5x objective with 0.4x zoom (11.3 μm pixel resolution) was utilized to assemble full-surface profiles of the 16 mm x 8 mm surfaces. Afterward, five lines of several millimeters in length by 0.28 mm (25 pixels) width were drawn across the step-containing mid-section of the full-surface profiles. Height at each position along the profile was the average height of the 25 pixels across the 0.28 mm width. Each line profile was leveled by linear line subtraction, and then the step height was taken as the difference of the height peak and valley points near the step region. The mean step height was established from the step heights of the five lines.
From each set, coupon #4 was used for stress-birefringence thin slice fabrication and electron microprobe mount preparation. A slow-speed metallographic saw with low- concentration diamond-bonded blade lubricated with a kerosene reservoir was used to cut slices parallel with the 8 mm x H plane. The end of the coupon from the first cut was discarded. Several slices of 0.9 mm were extracted for stress-birefringence slice preparation. The remaining portion of the coupon was used for electron microprobe mount preparation. Slices for stress-birefringence measurement were mounted at the center of a 50 mm x 38 mm x 3 mm glass back-plane using thermoplastic cement with
10 mm x 10 mm x 1 mm microscope slide feet at each of the four corners of the back- plane. The back-plane arrangement allowed for improved thickness uniformity across the stress-birefringence slices. A final slice width (dimension W in Figure 15) of 150 μm to 350 μm with visually polished surfaces was obtained by grinding with 600 grit silicon carbide metallographic abrasive and water against a brass wheel, followed by grinding with 1000 grit silicon carbide metallographic abrasive and water against a float glass plate, and finally polishing with cerium oxide and water on a polishing cloth, affixed to a polishing wheel. Stress-birefringence slices were placed against 1 mm microscope slides with a 0.17 mm cover slip and mineral oil immersion fluid. Birefringence as a function of position from the edge was measured using a polarized light microscope (Olympus BX43) equipped with a Berek compensator. A 40x objective and 2.5 μm step size was used for stress profiles of less than 25 μm case depth, or 20x objective and 5 μm step size was used for stress profiles of 25 μm and greater case depth. Two compensation measurements were made at each location and their average was used in subsequent calculations. Care was taken to avoid regions of edge chip-out. The profile was measured near the mid-section of dimension L (Figure 15) to ensure the desired boundary conditions were maintained.37 Birefringence was converted to retardation using the manufacturer-provided lookup table for the Berek compensator. Retardation was then converted to biaxial stress assuming a biaxial plane strain condition, using the following equation:35
z
Wz
z yy
xx
1 (15)
where δ is the retardation, ν is Poisson’s ratio, β is the stress-optical coefficient, and W is the optical viewing path length (equivalent to slice dimension W in Figure 15). Refer to Table XI for values for ν and β. Stress error was established by propagating error for the following values: birefringence compensation ±0.2°, Poisson’s ratio ±0.01, stress-optical coefficient ±0.03 TPa-1, and optical viewing path length ±2 μm.
With the remaining portion of coupon #4, the interior cut surface (8 mm x H) was mounted facing outward in air-setting epoxy (Fulton Metallurgical Products Quickmount Resin and Hardener). Often several coupons were placed in one mount. Care was taken
to clearly label and track each coupon. After allowing the resin to setup for at least 10 hours, mounts were ground and polished using an autopolisher (Buehler model Automet 2 paired with Buehler model Ecomet 3). The preparation schedule is given in Table XIV. After grinding and polishing, mounts were dried at 80 °C for 8 hours, and then were carefully cleaned with methanol, followed by distilled water and dried with Kimwipes™. Mounts were either carbon coated under hard vacuum (approximately 30 nm carbon deposited) or sputter coated with gold-palladium under rough vacuum (approximately 10-15 nm gold-palladium deposited). The prepared mounts were used for the determination of potassium, sodium, and silicon chemical profiles by electron probe microanalysis (EPMA). An electron microprobe analyzer (JEOL model JXA-8200) was used to monitor counts from the following characteristic lines: potassium (Kα-1), sodium (Kα-1), and silicon (Kα-1). Other pertinent measurement parameters were: beam spot size 2 μm, beam accelerating potential 15 keV, beam current 10 nA, dwell time 3 seconds, and step size 2 μm. Parameters were chosen, in part, to prevent electron beam-induced alkali migration.19 A mean counts versus position profile was established from the mean of 10 rows, where each row was positioned perpendicular to the 8 mm edge and extended a minimum of 10 μm into the mount and a minimum of 10 μm past the potassium diffusion depth.
For the SAS series, stress profiles established from stress-birefringence measurements were input into elastic finite element simulations utilizing the thermoelastic framework of the finite element software package.38 Simulations utilized a plane strain boundary condition along the 16 mm coupon dimension. In-plane dimensions were one-quarter of the 8 mm x H cross-section. Materials properties of Young’s modulus and Poisson’s ratio were those given in Table XI. A suitable combination of coefficient of thermal expansion and temperature change were selected to insert the linear (not biaxial) elastic strain profile into the simulation.
Table XIV. Grinding and Polishing Schedule for EPMA Samples Grinding /
Polishing Surface
Lubricant
Force Per Sample
(lbs)
Platen Speed (rpm)
Minimum Time (minutes)
Platen Rotation Direction 240 grit SiC
paper Water 3 320 12 Counter-
clockwise 400 grit SiC
paper Water 3 320 12 Counter-
clockwise 600 grit SiC
paper Water 4 360 12 Counter-
clockwise 6 μm
polishing cloth
6 μm diamond suspension
4 280 16 Clockwise
1 μm polishing
cloth
1 μm diamond suspension
5 280 10 Clockwise