J Am Ceram Soc 73 [7] 2111-13 (19901

Strengthening of Soda-lime Glass Rods by Heat Treatment Alone and Heat Treatment plus Silanes

Patrick Duly* and Stanley Byers Department of industry and Technology, Ball State University, Muncie, Indiana 47306

A study with soda-lime glass rods showed that modulus of rupture values can be increased from approximately 124 X lo6 Pa (18 ksi) to 276 X lo6 Pa (40 ksi) by heat treat- ment, Dipping the rods into diethyldichlorosilane prior to heating at 593°C (1100°F) or exposing hot glass to the silane vapor following heat treatment increased strength an addi- tional 30% over the average strength obtained by heating only at this temperature. Similar treatment with phenyl- trichlorosilane or diethyldichlorosilane did not give strengths statistically higher than those obtained by heating alone. [Key words: soda-lime, strengthening, heat treat- ment, silanes, glass.]

I. Introduction

ANUFACTURERS of glass products are continually seach- M ing for practical ways to increase the strength of glass and particularly to reduce those articles on the low end of the strength distribution curve. This study investigates the use of silanes plus heat-treating as a process for increasing glass strength.

11. Experimental Procedure

Soda-lime glass rods (3-mm diameter), with the chemical analysis shown in Table I, were cleaned by immersing for 20 min in a low-alkaline solution of deionized water, rinsed, then immersed in a 5% HCI solution for 20 min. All rods used in the study, including those called “as received,” were given this preliminary washing. The 12.7-cm (5.0-in.) rods were then exposed to varying heat treatment times and tempera- tures, as well as various silane surface treatments prior to heating. In some trials, silane was introduced while the rods were at heat-treating temperatures. The modulus of rupture was then measured on a universal testing machine: using a four-point bending jig with 10.2 cm (4.0 in.) between lower loading points, 5.1 cm (2.0 in.) between upper loading points, and a crosshead speed of 2.54 cm (1.0 in.) per minute. Rods were removed from the heating environment and allowed to air quench. No evidence of retained stress was observable under a polariscope.

111. Results

Figure 1 plots average modulus of rupture (MOR) of glass rods heated at temperatures between 565°C (1050°F) and 732”

T. Michalske-contributing editor

Manuscript No. 198059. Received October 17, 1989; approved December

*Member, American Ceramic Society. ‘Now with Overmeyer Mold Co., Winchester, IN. ‘Model 1022, Instron Corp., Canton, MA.

21, 1989.

(1350°F) for times ranging from 2 to 60 min; the starting point represents control tests.

Figure 2 compares strengths of rods with different heat treatments and heat treatment plus exposure to silanes, and it shows that heat treatment at 593°C (1100°F) for 1 h increased strength approximately 70% (from 124 MPa (18 ksi) to ap- proximately 207 MPa (30 ksi)). Diethyldichlorosilane plus heat-treating increased strength more than 100% from the ini- tial level (to approximately 276 MPa (40 ksi)), whether ap- plied by dipping prior to heating or as a vapor introduced onto hot rods after heating. The mean strength measured with the diethyldichlorosilane treatment was statistically higher than samples heat-treated only at the 99.5% confi- dence level. However, treatment with phenyltrichlorosilane or dimethyldichlorosilane did not show a statistically significant strength improvement. Also, rods which were dipped in silane, but not heat-treated, showed no strength increase.

IV. Discussion of Results

Roach and Coo er,’ Lawn, Jakus, and Gonzalez,* and Marshal and Lawn show that increased strength of aged, in- dented glass correlates with postindentation crack growth. These authors provide convincing arguments that it is stress relaxation near a flaw which causes strength to increase with aging. Roach and Cooper also show that annealed indenta- tions gave sharply increased strengths with higher annealing temperatures. Figure 1 shows similar results but over a higher and broader temperature range than the cited work. It is in- teresting that the strength increase at 565°C (1050°F) in this study (20%) is less than that found by Roach and Cooper at 400°C (750°F) (40%) when testing abraded rods. However, this study shows strength increases of more than 100% at ternpera- tures of 648°C (120°F).

Graham and Moore4 heat-treated glass containers with ar- tificially damaged surfaces at temperatures of 648°C (1200°F) for more than 2 min and obtained an increase in breaking pressure of up to 50% but saw little increase when heated to 621°C (1150°F) for 4 min. Brockway and Walters’ showed that strength increases of more than 100% were obtained by heat- ing soda-lime glass rods with a laser (energy density from 1.0

9

Table I. Chemical Composition of Glass Rods Component Amount (wt%)

SiO? 72.700 13.870 10.900 0.520 0.440 0.130 0.048

Not detected Total 99.608

2111

2112

276 (40)

- - 249 (361 Y)

Y - 220 (32) '0

193 (28) x

165 (241

OI

O 138 ( 2 0 ) E

Communications of the American Ceramic Society

T

110 (16) L . 0 5 10 15 30 45 60

TIME (MIN.) SOAKED AT DESIGNATED TEMPERATURE

Fig. 1. Average strength of soda-lime glass rods with increasing temperatures and soak times.

to 100 J/cm), although there was considerable scatter between trials. Interestingly, that work shows up to 39% increase in bursting strength of pressurized glass containers when heated to 551°C (1050°F) and additional strengthening of -30% when laser heat-treated. However, confidence levels are not pro- vided. The above work is cited to corroborate the consistent strengthening trends obtained by heat-treating and to note that specific experimental conditions appear to influence the levels of strengthening obtained.

Sil'vestrovich and Boguslavskii6 report on the use of silicon-organic films to produce hydrophobic glass surfaces at low (200°C (392°F)) heat treatment temperatures and to in- crease bending strength when heat-treated to higher (650°C (1200°F)) temperatures. In that study, the contact angle of water droplets measured on rods heat-treated after being

1 2 9 0 ( 4 2 )

2 6 2 - - co Y 2 3 4

.- 2 0 7

a a. 1 7 9

- a 0

x

c CI Is1 5 1 5 2 L c,

0 = 565°C (1050'F) 0 = 593T (lIOO°F) A = 615T (1150'F) O = 648°C (12OOOF) X = 704'C (1300'F)

20 samples per point I = 95% Confidence Limits

I

4

26) t I 2 2

v) 1 2 4 ( 1 8

9 7 ( 1 4

6 9 ( 1 0

1 2 3

1 4 5 6

T

1 a

I = 95% Confidence Limits

Fig. 2. Strength vs heating cycle and/or silane treatments: (1) control-as-received glass rods (30 specimens); (2) heated at 537°C (1000"F), 1 h (30 specimens); (3) heated at 593°C (1100"F), 1 h (40 specimens); (4) rods dipped in diethyldichlorosilane, heated at 593°C (1100"F), 1 h (30 specimens); (5) rods dipped in di- ethyldichlorosilane, heated at 593°C (1100"F), 1 h (30 specimens); (6) rods dipped in phenyltrichlorosilane, heated at 593°C (1100"F), 1 h (30 specimens); (7) rods heated to 593°C (1100"F), 1 h, then treated with vapor of diethyldichlorosilane while hot (30 s eci mens); (8) rods dipped in dimethyldichlorsilane, heated at 993°C (1100"F), 1 h (30 specimens).

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dipped in silane was markedly higher than that of uncoated glass, indicating that a hydrophobic surface was present. Therefore, one way to account for the strength increase added by the diethyldichlorosilane is by the formation of a moisture barrier which reduces the availability of water to the glass surface, thus reducing stress corrosion crack growth. Mould and show that static fatigue can reduce the strength of glass by the amount observed, and Ritter" shows that glass rods coated with protective polymeric coatings (acrylic, silicone, and epoxy) increased the short-term bending strength of abraded glass rods by approximately 15%, 20%, and 35%, respectively, for a crosshead speed of 0.5 cm/min. This increase was attributed to the coatings limiting the availability of water to the glass surface, although the stress corrosion sensitivity was essentially unchanged.

Although static fatigue is a possible explanation, other fac- tors may also be influencing the observed results. Roach et al. have studied the interfacial layers observed within cracks in mica and silicate glasses, highlighting the potential importance of surface chemistry in producing crack bridging deposits, which are regarded as restraining tractions at the crack walls behind the tip.

West et al." cite the synthesis of permethylpolysilane from dimethyldichlorosilane and sodium by the reaction

Me2SiCI2 3 (MezSi), + NaCl (1) In the case of heat-treated glass rods, sodium is known to

form films on the surface of glass,13 and such films, or bumps, were observed during this study.

According to West, the polysilane is pyrolyzed above 400°C to produce a polymeric carbosilane by the reaction

H I

(Me2Si), 3 -(--Si-CH2j,, (55% yield) (2) I

Me When prepared as in Eq. (2), the carbosilane is reported to

cross-link by surface oxidation in air. Perhaps, something analogous to Eq. (2) can occur when glass rods which have been dipped in silanes are heat-treated, even though the re- action is occurring in air. This suggestion is supported by the growth of whiskers on the silane-treated glass during heat- treating. However, the question of why only diethyldichloro- silane produced higher strengths during these experiments remains unanswered.

Further work is needed to better define the roll of stress corrosion, to optimize the methods of silane application, and to evaluate heat treatment temperatures and cycles.

July 1990 Communications of the American Ceramic Society 2113

V. Conclusion

The increased strength measured following the application of silane plus heat treatment could be of interest for a wide range of glass applications.

References ‘D.H. Roach and A .R . Cooper, “Effect of Contact Residual Stress

Relaxation on Fracture Strength of Indented Soda-Lime Glass,” J. Am. Cerum. Soc., 68 [ll] 632-36 (1985).

2B. R. Lawn, K. Jakus, and A. C. Gonzalez, “Sharp vs Blunt Crack Hy- pothesis in the Strength of Glass: A Critical Study Using Indentation Flaws,” J. Am. Ceram. Soc., 68 [I] 25-34 (1985).

3D. B. Marshall and B. R. Lawn, “Surface Flaws in Glass”; p. 221 in Frac- ture in Ceramic Materials. Edited by A. G. Evans. Noyes Publication, Park Ridge, NJ, 1984.

‘P.W. L. Graham and T.W. Moore, Jr., “Method of Strengthening Glass Articles,” U.S. Pat. No. 4231778, November 4, 1980.

5M. C. Brockway and C.T. Walters, “Laser Treatment Method for Im-

parting Increased Mechanical Strength to Glass Objects,” U.S. Pat. No. 4338114, July 6, 1982.

6s. 1. Sil’vestrovich and I. A. Boguslavskii, “The Use of Silica-Organic Compounds for Improving the Properties of Glass,” Sfekb Kerum., 1 , 7012 (1980).

’R. E. Mould, “Strength and Static Fatigue of Abraded Glass Under Con- trolled Ambient Conditions: 111, Aging of Fresh Abrasions,” J. Am. Ceram. Soc., 43 [3] 160-67 (1960).

* S . M. Wiederhorn and L. H. Bolz, “Stress Corrosion and Static Fatigue of Glass,”J. Am. Ceram. Soc., 53 [lo] 543 (1970).

9B. R. Lawn, D. B. Marshall, and T. P. Dabbs, “Fatigue Strength of Glass: A Controlled Flaw Study”; p. 316 in Fracture in Ceramic Materials. Edited by A. G . Evans. Noyes Publication, Park Ridge, NJ, 1984.

1°J. E. Ritter, “Stress Corrosion Susceptibility of Polymeric-Coated Soda- Lime Glass,”J. Am. Ceram. Soc., 56 [7] 402 (1973).

”D. H. Roach, S . Lathahal, and B. Lawn, “Interface1 Layers in Brittle Cracks,”J. Am. Ceram. Soc., 71 [2] 97-105 (1988).

I2R. West, D. D. Lawrence, P.O. Djurovich, H. Yu, and R. Sinclair, Am. Cerum. Soc. Bull., 62 [8] 899-903 (1983).

”R. M. Tichan, “Initial Stages of the Weathering Process on a Soda-Lime Glass Surface,” Glass Technol., 7 [l] (1966).