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Strong glass
Journal of Non-Crystalline Solids 73 (1985) 233-246 North-Holland, Amsterdam
233
STRONG GLASS *
Robert G A R D O N Research Staff Ford Motor Company, Dearborn, Michigan, USA
Factors that govern the strength of bulk glass are reviewed, along with some processes by which that strength may be increased. Most involve compressive pre-stressing of surfaces, and some - notably thermal tempering and ion stuffing - are highly developed for certain applications. Strengths attained in the laboratory and in industrial practice are contrasted. More fundamentally, the usable strength of glass is limited by its brittleness, and the question is raised whether composites with glass-like properties can be made less brittle.
1. Introduction
"Gliack und Glas, wie leicht bricht das!" In the popular mind, glass is almost synonymous with fragility. Under the circumstances I can but hope that my luck will not have given out when I agreed to a title that seems to challenge this old G e r m a n saying. - Two considerations emboldened me to do so. One is that glass is really one of the strongest materials we have, with an intrinsic strength, measured in the laboratory on flawless glass, of about 10000 MPa (100000 k g / c m 2 or 1 500000 psi). It is only the glass encountered in everyday life that is so proverbially fragile, some with usable strengths 300 times lower. The second is that great progress has been made - since the days that proverbs were coined - both in understanding whence this discrepancy and in raising the useful strength of glass. Here I would mention some recent reviews on strengthening glass [1-3] that appeared in a volume co-edited by our guest of honor. I have leaned on them, and I would encourage the reader to do likewise, for they cover the subject in much greater depth than I propose to. Here I intend only to touch upon the salient factors affecting the strength of glass, mention a few recent papers, and engage in a little speculation to start a discussion of what the future may hold in this regard. To take stock, table 1 gives an overview of the range of glass strengths. * A speculation, affectionately dedicated to Norbert J. Kreidl on the occasion of his 80th birthday. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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Table 1 Range of glass strengths and surface compressive stresses Strength (MPa) minimum (1% ile)
Ref.
median (50% ile)
Bending strength of annealedfloat glass upon scoring with glass cutter Minimum reasonable design strength of large commercial sheet of small commercial sheet as manufactured
[231 15 30 40 40 100
45 100 200
Surface compression (MPa)
Surface compressionproduced by Thermal tempering, comml, 5 mm glass, air quench Thermal tempering, exptl, 5 mm glass, air quench Thermal tempering, exptl, 25 mm glass, liq. quench Ion stuffing, diffusive, comml. (15 min) Ion stuffing, diffusive, exptl. (16 hr) Ion stuffing, elect, assist, exptl. (2 min)
100 200 500 100 700 1000
[24] [14(b)] [16]
[251 [13(a)l
Average strength (MPa) Design strength of glass fibers for FGRP Tensile strength of pristine glass fibers Measured "ultimate tensile strength" of Soda lime glass at room temperature Soda lime glass at liq. nitrogen temp. Fused silica at liq. nitrogen temp.
2000 3000 3600 7600 14000
[261 [26] [27]
2. Factors affecting the strength of glass Interestingly, it is the very brittleness of glass that is also one source of its great inherent strength when in a flawless condition. Thus, the high inherent strength of pristine glass is determined primarily by two factors. The rigidity of the glassy network precludes local "yielding", so that glass responds elastically to stresses right up to the breaking stress; and this, in turn, reflects the high strength of the chemical bonds of the oxides that make up the glassy network. In compression glass is stronger yet. The same factors manifest themselves very differently in glass that is not flawless - that is all glass encountered in daily life. In the view generally accepted since Griffith's classic work on brittle fracture [4], a flaw - i.e. any crack or other discontinuity, however small - will act as a stress concentrator, such that stresses, particularly tensile stresses, in its vicinity will be orders of magnitude higher than the nominal stress corresponding to the applied load. In ductile materials such locally high stresses can be relieved by yielding, and failure thus delayed - or even avoided. Not so in brittle materials. Since their response remains elastic even to the highest stresses they can sustain, the
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geometry of flaws and their stress-concentrating effect remain unchanged. And, if the concentrated local stress at the tip of the critical flaw reaches the bond-strength of the material, the material will fail at a nominal tensile stress far below its intrinsic strength. In polycrystalline ceramics critical flaws may be located in the interior of the material or in the surface. In glasses they invariably occur in the surfce. The failure of glass thus always originates at a critical flaw in a region of the surface under tension. It follows that the strengthening of glass may i~e attempted along three broad lines of attack: (1) At the most fundamental level one might seek to raise the intrinsic strength of the glass or, more usefully, to reduce its brittleness. If past evidence is anything to go by, this is not a particularly promising approach; but neither should it be passed over altogether, and I will return to it later. (2) Somewhat more realistically, and accepting the brittle nature of glass, one might seek to increase it usable strength by preserving its flaw-free, pristine surfaces. Unfortunately, however, such surfaces are so vulnerable that anything applied to preserve them will itself damage them. (3) This brings us to the only approach widely used in practice to enhance the strength of bulk glass. In this, one accepts both the brittleness of glass and the inevitability of flaws in its surface, and one seeks to counteract their effect by compressively prestressing the surface. A secondary factor bearing on the strength of glass is the propensity of flaws to grow by "stress corrosion", a consequence of the increased reactivity of freshly formed surfaces, especially with moisture. "Static fatigue" results from the slow growth of sub-critical flaws until failure of the glass occurs under a load that it had previously sustained. Encapsulation in a moisture proof environment and low temperatures are thus also means to maintain higher strength. The data at the end of table 1 suggest that static fatigue comes into play even in the strongest (flaw-free?) glass.
3. Maintaining flaw-free surfaces? Even in the absence of visible, gross injuries to the surface, the strength of glass is likely to be impaired by minute flaws, which - for all that they are invisible - have a marked effect on strength. In an elegant paper, Ernsberger [5] rendered these small flaws visible, and he also showed how very vulnerable surfaces are to touch, to atmospheric contamination and to thermal nucleation of flaws in the form of microcrystals. Thus, while it may be possible to restore surfaces to a flawless condition -as, for example, by acid polishing - to keep them flawless is another matter. Indeed, anything that might be applied to a surface to prevent damage would also cause some. Even so, a relatively high level of strength can be preserved in glass fibers used for re-inforcing plastics by coating them immediately after they are drawn and before they come into contact with other fibers or any other solid. Such fibers, with useful strengths of the order of 2000
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MPa, represent the strongest glass currently in use. No comparable usable strength has been preserved in any massive body of glass. (cf. table 1) While this example illustrates a practical utilization of this approach to strengthening glass, the long term maintenance of a flaw free surface is as idle a dream as that of a ductile glass. 4. Learning to live with surface flaws This brings us to the approach most widely used in practice to enhance the strength of bulk glass. Inasmuch as the stress concentrating effect of flaws is critical only for tensile stresses, this approach consists of compressively prestressing the surface of the glass. Any applied load must then neutralize the compressive pre-load before potentially damaging tensile stresses can arise in the surface. The usable strength of the glass is thus increased by the amount of the compressive prestress. As we shall see, there may also be some additional benefits. Largely for historical reasons, processes that strengthen glass by this route are called "tempering".
4.1. Tempering processes Early examples of glass strengthened by precompression of its surfaces were the thermally tempered ware of de la Bastie and of Siemens and the clad glass of Schott, all of about 100 years ago. Thermal tempering came into its own as an important industrial process in the 1930s. Cladding, except for decoration and for optical fibers, appears to have died out. It was succeeded in the 1960s by a number of other, potentially less laborious and more versatile methods of achieving the same effect. The term "chemical tempering" is now applied to all, even though they involve several different physical and chemical phenomena. For all the differences between them, cladding, the various chemical tempering processes and thermal tempering have one feature in common: they all put the surface of glass into compression by creating a surface layer that has a larger specific volume (in the unstressed state) than the bulk of the glass. Details of chemistry apart, this will involve one of three physical mechanisms: (1) In Schott's cladding process, glass tubes were coated with a layer of a glass having a lower coefficient of expansion than the base glass. As this composite tube was cooled from its forming temperature, the surface layers tended to contract less than the bulk, so that they were put under compression. The same strengthening mechanism is involved in some chemical tempering processes that also produce surface layers having a lower coefficient of expansion than the base glass. These include surfacecrystallization [6], certain ion-exchange treatments at temperatures above the strain point of the glass [7], and partial leaching and reconsolidation of the surface layers of a phase-separated glass. [8].
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(2) Cladding of a different nature is obtained by yet another method of chemical tempering, called "ion stuffing" [9,10]. In this process, surface compression is obtained by producing surface layers having an intrinsically higher specific volume (now referred to unit mass of network forming oxides) than the base glass. This is done by an ion exchange - conducted below the strain-point of the glass - that replaces smaller cations in the glass surface with larger ones. This appears to be the only one of the chemical tempering processes to be used on a commercial scale. Many variations have been played on the theme of chemical tempering, and many more remain to be explored. Thus, for example, ion exchange can be aided by an applied electrical field [9], different mechanisms can perhaps be brought into play together, and, most importantly, a variety of materials is available for ingredients of the base glass and the ion exchange medium. (3) In thermal tempering the desired surface compression is obtained in a chemically homogeneous material as a permanent result of transient temperature gradients that accompany the cooling of glass: in the first stage of rapidly cooling glass, a large temperature difference is developed between surface layers and the interior. Since the interior is then relatively fluid and incapable of sustaining any stresses, the glass solidifies in a substantially stress-free condition. From that time until the glass reaches isothermal conditions at room temperature, the interior cools through a larger temperture interval than the surfaces, and - since the glass is by then increasingly solid throughout - this puts the surfaces under compression, the interior under tension. A second mechanism involved in thermal tempering merits mention here: In a rapidly cooled massive body of glass local cooling rates are position-dependent, as is therefore also the resulting structure (fictive temperature) of the glass. Differential-contraction-induced differences in specific volume are therefore accompanied by differences in specific volume related to a structural inhomogeneity of the glass. In a representative case, the latter account for about a quarter of the stresses produced [11]. However, while the structural inhomogeneity of the glass increases with severity of quenching, its contribution to temper stresses decreases; so that, in conventional tempering processes at least, structural effects play a progressively smaller role as the glass is more highly tempered.
4. 2. Characteristics of tempered glass The results of chemical and thermal tempering differ greatly: the former usually produces very thin compression layers (less than 100 microns thick), in which stress gradients are very steep. Compressive stresses in the surface can be very high, up to 1000 MPa. Thermal tempering, on the other hand, usually produces more modest surface compressions (of the order of only 100 MPa), but with compression layers that extend over 20% of the glass thickness on each side. The dicing, upon fracture, of thermally tempered glass - which is
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often another desired attribute of such glass - is brought about by tensile stresses in its interior that balance compressive stresses in its surface layers. As a consequence of the thinness of compression layers in chemically strengthened glass, the balancing tensile stresses in its interior are likely to be low - unless the glass itself is also very thin - so that chemically tempered glass tends to break into only a few large pieces, rather like annealed glass. The strength of tempered glass has usually been taken to be the sum of the strength of the base glass, as determined by its population of flaws, and the surface compressive stress. Conventional thermal tempering can thus raise the useful strength of ( - 5 mm thick) float glass from a minimum of about 30 MPa (cf. table 1) to 130 MPa, or - if tempering is done on less damaged glass - from perhaps 100 to 200 MPa. The dicing characteristics, which are determined by tempering stresses alone, will be the same in either case; and, except for gross accidents to its surface, the glass with the higher initial strength will remain stronger indefinitely (cf. fig. 1). These gains in strength, though not large when measured against the intrinsic strength of glass, are nonetheless significant. Coupled with the enhancement of the fracture pattern of thermally tempered glass, they have led to greatly expanded uses of glass: in vehicles and for security in buildings, to name but two. An advantage of thermally tempered glass that has only recently been documented is the protection against strength-impairing surface damage that its compressed surface layers afford it [12]. This fig. 1 shows that a given severity of abrasion reduces the strength of tempered glass less than that of annealed glass. The enhancement of strength due to tempering is, therefore, even greater after exposure of the glass to abrasion in service than before. Efforts to counter the effect of flaws by surface compression seem thus also to have brought about a partial accomplishment of the earlier objective of eliminating flaws! The recognition that the susceptibility of glass to damage is less after tempering than before places added value on tempering glass that has as good a surface as possible. It remains to be determined just how far it may be worthwhile to go in that direction, for the nucleation of flaws during the heating stage of tempering may well place an upper limit to the attainable surface quality of the product. Chemical strengthening can readily raise glass strengths by a larger margin; and the potential for improvements to 1000 MPa, or about one-tenth of the ultimate strength of glass, has been demonstrated in the laboratory [13(a)]. On the other hand, chemically tempered glass remains vulnerable to surface flaws deep enough to extend through its relatively thin compression layer. Since the growth of this layer by diffusion proceeds as the square root only of time, increasing its thickness can be rather time consuming and therefore impracticable economically. Electrically driven ion exchange is a more difficult process to engineer for most shapes of glass parts, but it has the advantages of proceeding much faster
R. Gardon /Strong glass
239
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even in the face of a mild abrasion that reduced the strength of etched but untempered glass by a factor of 10. In view of this, the writers called this glass "abrasion resistant" rather than "strong" - though, by any standard, it was that, too. Interestingly, the strength of 1000 MPa was obtained at a certain thickness of the compression layer only, and increasing this - as well as decreasing it - sharply reduced the strength. 4.3. M i x e d strengthening mechanisms
Several Russian investigators have worked with systems that combined thermal tempering - notably with reactive liquid quenching media - with acid etching and surface treatments aimed at altering what they called the "structure" of the surface [14]. They certainly produced the highest compressive surface stresses reported by anyone for thermal tempering, up to 500 MPa in 25 mm thick glass. They also reported breaking strengths in excess of 1000 MPa, but these seem of questionable significance since etching was the last step in their treatment, and etching will, for a short time at least, make any glass very strong. Finally, they also attributed some 100-300 MPa of the observed increase in strength to an unspecified "structural" change of their surfaces. This was later interpreted as, possibly, a low-expansion silica film derived from the interaction of the hot glass with the siloxane liquid used as a quenching medium [6]. If so, this would be an instance of combined chemical and thermal tempering. However, it is also possible that the 100-300 MPa in question was simply the result of a difference between how etching affected annealed and highly tempered glass, or how their surfaces fared between etching and testing. A re-evaluation of work along these lines might be in order now, in the light of new knowledge of chemical tempering and the degradation of freshly etched surfaces. In considering ways to enhance the strength of glass, it seems natural to think of combining various strengthening mechanisms. With due regard to the temperatures at which they must be conducted, chemical and thermal tempering could be combined - perhaps to yield the strength of the former and the abrasion resistance a n d / o r fracture pattern of the latter. It is also quite likely - in some cases unavoidable - that the mechanisms that dominate different processes do not come into play by themselves. Thus, for example, it may happen that a combination of surface layer and bulk glass - engineered for a difference in specific volumes, say - also exhibits differences in expansion coefficients or glass transition temperatures. With the overlap of the temperature ranges in which various tempering mechanisms can come into play, several may be involved in one process; and, in doing so, they may reinforce or counteract one another. - A clear example of a destructive interplay of various mechanisms occurs in diffusive ion stuffing. For this to proceed at a reasonable rate, it must be conducted at elevated temperatures, at which stress relaxation may come into play along with an acceleration of the ion exchange that is generating the desired stresses. The process must clearly be run at a temperature optimal for the results expected in a given time.
R. Gardon / Strong glass
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4.4. Where can we go from here?
As far as thermal tempering is concerned, I am less aware of demands for higher temper stresses than of demands to extend the benefits of tempering to thinner glass, more complex shapes and parts made of low-expansion glasses. This situation may, of course, simply be a manifestation of " m a r k e t pull" in one direction, with no "technology push" in the other. (The latter may be up to us!) In any case, the objectives in greater demand entail some of the same problems as would attempts to raise the level of temper, and we shall briefly address that issue. Relatively modest increases in thermally produced temper stresses perhaps by as much as a factor of two, depending on circumstances - can be achieved by processes that are " m o r e of the same", such as quenching the glass more severely. Raising the quenching rate is relatively easy: sublimating solids and fluidized beds and jets have been added to the old repertory of air jets and liquids. It is generally more difficult to meet such concomitant requirements as raising the initial temperature of the glass without risk to its form. Nevertheless, this sort of thing - with greater or lesser variations - is being done all the time. An interesting variant of thermal tempering is to conduct it with a time-dependent quenching rate. This was used in a process that enabled sheets of thin glass, from 3 m m down to 0.75 ram, to be tempered. Simply to employ more severe quenching than usual, using a liquid coolant, appears not to have been practicable. Instead, this process entailed quenching in two stages, first in air~ then in a liquid [15]. One can estimate the maximum strengthening effect attainable by thermal tempering by recalling that temper stresses are (roughly) determined by some average of temperature differences across the half-thickness of the glass during the critical phase of cooling. The highest value that this can usefully attain is the difference between the high end of the glass transition range and some reasonable coolant temperature. Correspondingly, the ultimately attainable surface compression in soda-lime glass might be about 350 MPa. It would be less in a low-expansion glass; and harder to approach in a thin sheet of glass than in a thick one. This explains why modest increases in temper stress above - 100 MPa in 5 m m glass are relatively easy to obtain. The highest compressive stress of 500 MPa in 25 m m glass, reported i/a ref. [14(b)], is another matter! To approach the limiting surface compression would - except perhaps in very thick glass - be more difficult than simply to quench the glass more severely, and I would look to some qualitative change in the cooling process. The two-stage quench of ref. [15] - though addressed to reducing the thickness of tempered glass rather than to raising temper stresses - may, among other things, also have been a step in that direction. It is a measure of its effectiveness that, while engineering advances in equipment have now made the tempering of 3 m m glass possible by more-or-less conventional techniques also,
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this process is - to my knowledge - still the only one for which it is claimed that it can temper sheets of 1 mm glass. Processes that take advantage of modulated quenching may be one possible route to take beyond what can readily be accomplished by conventional thermal tempering. To approach the limiting surface compression attainable yet more closely would be very difficult. Conceptually, it might be done by supplying heat to the interior of the glass, even as its surfaces are being rapidly cooled. While such a process is not out of question, it is not likely to be developed without some strong new motivation. With chemical tempering one can already achieve higher compressive stresses in the surface, so that the practical challenge, once again, is not to raise these further, but rather to obtain thicker compression layers in a reasonable time and to make the process useful for a greater variety of products. Electrically driven ion exchange may hold promise of high productivity, but more work is needed to establish both its capabilities and its practicality. Beyond that, materials, processes and techniques of execution are all appropriate subjects for further research and development. All seem to be receiving a great deal of attention, in the Japanese patent literature especially. For Japanese practice with chemically strengthened bottles, see ref. [16]. An interesting development is Asahi Glass Company's work [17] on ion-exchange strengthening float glass while still in the form of a continuous ribbon. The wide-spread availability of such glass - cuttable like annealed glass, but with a strength perhaps four times higher - could perhaps, slowly, change popular perceptions regarding the extreme fragility of glass.
5. Changing the intrinsic strength and brittleness of glass? Finally, I would like to touch on an approach to strengthening glass that may be both the most fundamental and - probably for good reasons - the least discussed. But then, if I cannot stick my neck out on an occasion such as this, when can I? The first step is a relatively safe one. Modification of the compositions of glasses is among the oldest pursuits of glass makers, and composition should clearly affect the intrinsic strength of glass, (cf. last two lines of table 1) To identify this effect one must be able to distinguish the intrinsic strength from that usually observed, or, at least, to account for the effect of flaws on the observed strength. Using techniques of fracture mechanics, Eagan and Swearengen [18] measured critical stress intensity factors of various compositions of alumino- and borosilicate glasses. Not only were differences between these two types of glass demonstrated, but, by choosing compositions that maximized the density of network forming bonds, the fracture toughness of either type could be raised. Taking the fracture toughness measured in these experiments as being roughly proportional to ultimate strength, we can infer that gains in strength over the base soda-silica glass ranged from 15 to 50%.
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Much larger gains might be expected if the rigidity of the glassy network could somehow be reduced. This would likely lead to a material with an intrinsic strength lower than that of the original glass, but with a higher usable strength. Great progress along these lines was made in the last thirty years in the field of plastics, where the impact resistance of "rubber-modified" glassy polymers was raised by factors of 10 to 20 above that of the unmodified material. - To do the same for inorganic glasses would be something great to wish for Norbert's 100th birthday! This may be something of a pipedream, for - almost by definition - glass per se cannot be made ductile. So, if we want toughness, we must needs talk composites. Analogies with plastics abound. Thus fiber glass reinforced plastics now have their counterparts in SiC-fiber-reinforced glass [19]. Such materials are clearly beyond the scope of this paper; but I would like to pursue briefly an analogy with rubber-modified glassy plastics, which was prompted by a recent paper by Krstic et al. [20]. This reports the toughening of a glass by the inclusion of metallic particles. The materials in question were made by hot pressing mixtures of glass frits with various metal powders. Most of them were no stronger than the base glass. However, in the combination of a certain glass and oxide-covered aluminum particles - the oxide layer serving as a bonding agent - the particles acted as ductile "brakes" to the propagation of fractures though the glassy matrix, in a manner illustrated schematically by fig. 2. This approach may be contrasted with earlier attempts to strengthen glass with dispersions of rigid particles, intended to limit the size of flaws in the glassy matrix [21]. At high volume fractions ( - 45%) of small (15-20 micron) particles, and correspondingly small spacings between them, increases in strength up to about 70% were obtained. (cf. fig. 3). In contrast, in the work with ductile inclusions, particle loadings of only 5 to 20 vol% sufficed to raise the toughness (stress intensity factor) to levels 3 to 6 times that of the base glass. (cf. fig. 4). Breaking strengths (in units of stress) cannot be inferred from
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244
R. Gardon / S t r o n g glass
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R. Gardon / Strong glass
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6. C o n c l u s i o n
We have reviewed some factors that govern the strength of bulk glass and some processes by which that strength may be increased. Most involve compressive prestressing of surfaces, and some - notably thermal tempering and ion stuffing - are highly developed for certain applications. Work in the laboratory has already demonstrated greater strengths than are yet attainable industrially, and their utilization in practice presents a challenge comparable to that of further advances in the laboratory. I have also raised the truly long-range - if, indeed, not " f a r out" - question of whether composites with glass-like properties can be made less brittle. I hope that our discussion may help to define some of these problems more clearly and pinpoint opportunities more closely. - Advances may ensue from improved insights into the strength and fracture characteristics of glasses, or,
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R. Gardon /Strong glass
again, they may remain unforeseen and flow - as thermal tempering did a half century before Griffith identified the flaws named after him - from the inventive genius of practitioners of the glass maker's art.
References [1] F.M. Ernsberger, in: Glass: Science and Technology, Vol. 5 eds. D.R. Uhlmann and N.J. Kreidl (Academic Press, New York, 1980) p. 133. [2] R. Gardon, ibid., p. 145. [3] R.F. Bartholomew and H.M. Garfinkel, ibid., p. 217. [4] A.A. Griffith, Phil. Trans. Roy. Soc., Ser. A221 (1921) 163. [5] F.M. Ernsberger, in: Advances in Glass Technology, Proc. 6th Int. Congress on Glass, Washington, D.C., July 1962, Part 1 (Plenum, New York, 1962) p. 511. [6] S.D. Stookey, J.S. Olcott, H.M. Garfinkel and D.L. Rothermel, ibid., p. 397, [7] H.P. Hood and S.D. Stookey, US Patent 2 779 136 (1957). [8] M.G. Drexhage and P.K. Gupta, J. Am. Ceram. Soc. 63 (1980) 72. [9] N. Weber, US Patent 3 218.220 (1965). [10] S.S. Kistler, J. Am. Ceram. Soc. 45 (1962) 59. [11] O.S. Narayanaswamy, J. Am. Ceram. Soc. 61 (1978) 146. [12] (a) B.R. Lawn, D.B. Marshall and S.M. Wiederhorn, J. Am. Ceram. Soc. 62 (1979) 71; (b) H. Dannheim, H.J. Oel and W. Prechtl, (in German) Glastechn. Ber. 54 (1981) 312. [13] (a) M. Abou-el-leil and A.R. Cooper, Glass Techn. 21 (1980) 57; (b) M. Abou-el-leil and A.R. Cooper, J. Am. Ceram. Soc. 64 (1981) 141. [14] (a) F.F. Vitman, I.A. Boguslavskii and V.P. Pukh, Dokl. Akad. Nauk. SSSR 145 No. 1 (1962) 85 (Engl. transl.: Soviet Phys. - Doklady 7, No. 7, 1963) 650; (6) I.A. Boguslavskii, Steklo i Ker. 21 No. 10 (1964) 4 (Engl. transl.: Glass & Ceramics, USSR, 21 (1964) 562. [15] R. Melling, D.C. Wright and J. Pickup, US Patent 3 890 128 (1975). [16] H. Ono, Glass. Techn. 22 (1981) 173. [17] Asahi Glass Co., Japan. Patents 79 132 620 (1979) and 80 80 744 (1980) (cf. Chem. Abstrs. 92:133939 and 94: 6990k). [18] R.J. Eagan and J.C. Swearengen, J. Am. Ceram. Soc. 61 (1978) 27. [19] J.J. Brennan and K.M. Prewo, J. Mat. Sci. 17 (1982) 2371. [20] V.V. Krstic, P.S. Nicholson and R.G. Hoagland~ J. Am. Ceram. Soc. 64 (1981) 499. [21] D.P.H. Hasselman and R.M. Fulrath, J. Am. Ceram. Soc. 49 (1966) 68. [22] (a) N. Miyata and H. Jinno, J. Mat. Sci. 16 (1981) 2205; (b) A.G. Evans, Phil. Mag., Set. 8, 26 (1972) 1327. [23] H. Woelk and K. Elsenheimer, (in German) Glastechn. Ber. 52 (1979) 14. [24] R. Gardon, Paper No. 79 in: Proc. 7th Int. Congress on Glass, Brussels (1965). [25] M.E. Nordberg, E.L. Mochel, H.M. Garfinkel and J.S. Olcott, J. Am. Ceram. Soc. 47 (1964) 215. [26] F.M. Ernsberger, Phys. Chem. Glasses 10 (1969) 240. [27] B.A. Proctor, I. Whitney and J.W. Johnson, Proc. Roy. Soc., Ser. A297 (1967) 534.
233
STRONG GLASS *
Robert G A R D O N Research Staff Ford Motor Company, Dearborn, Michigan, USA
Factors that govern the strength of bulk glass are reviewed, along with some processes by which that strength may be increased. Most involve compressive pre-stressing of surfaces, and some - notably thermal tempering and ion stuffing - are highly developed for certain applications. Strengths attained in the laboratory and in industrial practice are contrasted. More fundamentally, the usable strength of glass is limited by its brittleness, and the question is raised whether composites with glass-like properties can be made less brittle.
1. Introduction
"Gliack und Glas, wie leicht bricht das!" In the popular mind, glass is almost synonymous with fragility. Under the circumstances I can but hope that my luck will not have given out when I agreed to a title that seems to challenge this old G e r m a n saying. - Two considerations emboldened me to do so. One is that glass is really one of the strongest materials we have, with an intrinsic strength, measured in the laboratory on flawless glass, of about 10000 MPa (100000 k g / c m 2 or 1 500000 psi). It is only the glass encountered in everyday life that is so proverbially fragile, some with usable strengths 300 times lower. The second is that great progress has been made - since the days that proverbs were coined - both in understanding whence this discrepancy and in raising the useful strength of glass. Here I would mention some recent reviews on strengthening glass [1-3] that appeared in a volume co-edited by our guest of honor. I have leaned on them, and I would encourage the reader to do likewise, for they cover the subject in much greater depth than I propose to. Here I intend only to touch upon the salient factors affecting the strength of glass, mention a few recent papers, and engage in a little speculation to start a discussion of what the future may hold in this regard. To take stock, table 1 gives an overview of the range of glass strengths. * A speculation, affectionately dedicated to Norbert J. Kreidl on the occasion of his 80th birthday. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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R. Gardon /Strong glass
Table 1 Range of glass strengths and surface compressive stresses Strength (MPa) minimum (1% ile)
Ref.
median (50% ile)
Bending strength of annealedfloat glass upon scoring with glass cutter Minimum reasonable design strength of large commercial sheet of small commercial sheet as manufactured
[231 15 30 40 40 100
45 100 200
Surface compression (MPa)
Surface compressionproduced by Thermal tempering, comml, 5 mm glass, air quench Thermal tempering, exptl, 5 mm glass, air quench Thermal tempering, exptl, 25 mm glass, liq. quench Ion stuffing, diffusive, comml. (15 min) Ion stuffing, diffusive, exptl. (16 hr) Ion stuffing, elect, assist, exptl. (2 min)
100 200 500 100 700 1000
[24] [14(b)] [16]
[251 [13(a)l
Average strength (MPa) Design strength of glass fibers for FGRP Tensile strength of pristine glass fibers Measured "ultimate tensile strength" of Soda lime glass at room temperature Soda lime glass at liq. nitrogen temp. Fused silica at liq. nitrogen temp.
2000 3000 3600 7600 14000
[261 [26] [27]
2. Factors affecting the strength of glass Interestingly, it is the very brittleness of glass that is also one source of its great inherent strength when in a flawless condition. Thus, the high inherent strength of pristine glass is determined primarily by two factors. The rigidity of the glassy network precludes local "yielding", so that glass responds elastically to stresses right up to the breaking stress; and this, in turn, reflects the high strength of the chemical bonds of the oxides that make up the glassy network. In compression glass is stronger yet. The same factors manifest themselves very differently in glass that is not flawless - that is all glass encountered in daily life. In the view generally accepted since Griffith's classic work on brittle fracture [4], a flaw - i.e. any crack or other discontinuity, however small - will act as a stress concentrator, such that stresses, particularly tensile stresses, in its vicinity will be orders of magnitude higher than the nominal stress corresponding to the applied load. In ductile materials such locally high stresses can be relieved by yielding, and failure thus delayed - or even avoided. Not so in brittle materials. Since their response remains elastic even to the highest stresses they can sustain, the
R. Gardon /Strong glass
235
geometry of flaws and their stress-concentrating effect remain unchanged. And, if the concentrated local stress at the tip of the critical flaw reaches the bond-strength of the material, the material will fail at a nominal tensile stress far below its intrinsic strength. In polycrystalline ceramics critical flaws may be located in the interior of the material or in the surface. In glasses they invariably occur in the surfce. The failure of glass thus always originates at a critical flaw in a region of the surface under tension. It follows that the strengthening of glass may i~e attempted along three broad lines of attack: (1) At the most fundamental level one might seek to raise the intrinsic strength of the glass or, more usefully, to reduce its brittleness. If past evidence is anything to go by, this is not a particularly promising approach; but neither should it be passed over altogether, and I will return to it later. (2) Somewhat more realistically, and accepting the brittle nature of glass, one might seek to increase it usable strength by preserving its flaw-free, pristine surfaces. Unfortunately, however, such surfaces are so vulnerable that anything applied to preserve them will itself damage them. (3) This brings us to the only approach widely used in practice to enhance the strength of bulk glass. In this, one accepts both the brittleness of glass and the inevitability of flaws in its surface, and one seeks to counteract their effect by compressively prestressing the surface. A secondary factor bearing on the strength of glass is the propensity of flaws to grow by "stress corrosion", a consequence of the increased reactivity of freshly formed surfaces, especially with moisture. "Static fatigue" results from the slow growth of sub-critical flaws until failure of the glass occurs under a load that it had previously sustained. Encapsulation in a moisture proof environment and low temperatures are thus also means to maintain higher strength. The data at the end of table 1 suggest that static fatigue comes into play even in the strongest (flaw-free?) glass.
3. Maintaining flaw-free surfaces? Even in the absence of visible, gross injuries to the surface, the strength of glass is likely to be impaired by minute flaws, which - for all that they are invisible - have a marked effect on strength. In an elegant paper, Ernsberger [5] rendered these small flaws visible, and he also showed how very vulnerable surfaces are to touch, to atmospheric contamination and to thermal nucleation of flaws in the form of microcrystals. Thus, while it may be possible to restore surfaces to a flawless condition -as, for example, by acid polishing - to keep them flawless is another matter. Indeed, anything that might be applied to a surface to prevent damage would also cause some. Even so, a relatively high level of strength can be preserved in glass fibers used for re-inforcing plastics by coating them immediately after they are drawn and before they come into contact with other fibers or any other solid. Such fibers, with useful strengths of the order of 2000
236
R. Gardon / Strong glass
MPa, represent the strongest glass currently in use. No comparable usable strength has been preserved in any massive body of glass. (cf. table 1) While this example illustrates a practical utilization of this approach to strengthening glass, the long term maintenance of a flaw free surface is as idle a dream as that of a ductile glass. 4. Learning to live with surface flaws This brings us to the approach most widely used in practice to enhance the strength of bulk glass. Inasmuch as the stress concentrating effect of flaws is critical only for tensile stresses, this approach consists of compressively prestressing the surface of the glass. Any applied load must then neutralize the compressive pre-load before potentially damaging tensile stresses can arise in the surface. The usable strength of the glass is thus increased by the amount of the compressive prestress. As we shall see, there may also be some additional benefits. Largely for historical reasons, processes that strengthen glass by this route are called "tempering".
4.1. Tempering processes Early examples of glass strengthened by precompression of its surfaces were the thermally tempered ware of de la Bastie and of Siemens and the clad glass of Schott, all of about 100 years ago. Thermal tempering came into its own as an important industrial process in the 1930s. Cladding, except for decoration and for optical fibers, appears to have died out. It was succeeded in the 1960s by a number of other, potentially less laborious and more versatile methods of achieving the same effect. The term "chemical tempering" is now applied to all, even though they involve several different physical and chemical phenomena. For all the differences between them, cladding, the various chemical tempering processes and thermal tempering have one feature in common: they all put the surface of glass into compression by creating a surface layer that has a larger specific volume (in the unstressed state) than the bulk of the glass. Details of chemistry apart, this will involve one of three physical mechanisms: (1) In Schott's cladding process, glass tubes were coated with a layer of a glass having a lower coefficient of expansion than the base glass. As this composite tube was cooled from its forming temperature, the surface layers tended to contract less than the bulk, so that they were put under compression. The same strengthening mechanism is involved in some chemical tempering processes that also produce surface layers having a lower coefficient of expansion than the base glass. These include surfacecrystallization [6], certain ion-exchange treatments at temperatures above the strain point of the glass [7], and partial leaching and reconsolidation of the surface layers of a phase-separated glass. [8].
R. Gardon /Strong glass
237
(2) Cladding of a different nature is obtained by yet another method of chemical tempering, called "ion stuffing" [9,10]. In this process, surface compression is obtained by producing surface layers having an intrinsically higher specific volume (now referred to unit mass of network forming oxides) than the base glass. This is done by an ion exchange - conducted below the strain-point of the glass - that replaces smaller cations in the glass surface with larger ones. This appears to be the only one of the chemical tempering processes to be used on a commercial scale. Many variations have been played on the theme of chemical tempering, and many more remain to be explored. Thus, for example, ion exchange can be aided by an applied electrical field [9], different mechanisms can perhaps be brought into play together, and, most importantly, a variety of materials is available for ingredients of the base glass and the ion exchange medium. (3) In thermal tempering the desired surface compression is obtained in a chemically homogeneous material as a permanent result of transient temperature gradients that accompany the cooling of glass: in the first stage of rapidly cooling glass, a large temperature difference is developed between surface layers and the interior. Since the interior is then relatively fluid and incapable of sustaining any stresses, the glass solidifies in a substantially stress-free condition. From that time until the glass reaches isothermal conditions at room temperature, the interior cools through a larger temperture interval than the surfaces, and - since the glass is by then increasingly solid throughout - this puts the surfaces under compression, the interior under tension. A second mechanism involved in thermal tempering merits mention here: In a rapidly cooled massive body of glass local cooling rates are position-dependent, as is therefore also the resulting structure (fictive temperature) of the glass. Differential-contraction-induced differences in specific volume are therefore accompanied by differences in specific volume related to a structural inhomogeneity of the glass. In a representative case, the latter account for about a quarter of the stresses produced [11]. However, while the structural inhomogeneity of the glass increases with severity of quenching, its contribution to temper stresses decreases; so that, in conventional tempering processes at least, structural effects play a progressively smaller role as the glass is more highly tempered.
4. 2. Characteristics of tempered glass The results of chemical and thermal tempering differ greatly: the former usually produces very thin compression layers (less than 100 microns thick), in which stress gradients are very steep. Compressive stresses in the surface can be very high, up to 1000 MPa. Thermal tempering, on the other hand, usually produces more modest surface compressions (of the order of only 100 MPa), but with compression layers that extend over 20% of the glass thickness on each side. The dicing, upon fracture, of thermally tempered glass - which is
238
R. Gardon / Strong glass
often another desired attribute of such glass - is brought about by tensile stresses in its interior that balance compressive stresses in its surface layers. As a consequence of the thinness of compression layers in chemically strengthened glass, the balancing tensile stresses in its interior are likely to be low - unless the glass itself is also very thin - so that chemically tempered glass tends to break into only a few large pieces, rather like annealed glass. The strength of tempered glass has usually been taken to be the sum of the strength of the base glass, as determined by its population of flaws, and the surface compressive stress. Conventional thermal tempering can thus raise the useful strength of ( - 5 mm thick) float glass from a minimum of about 30 MPa (cf. table 1) to 130 MPa, or - if tempering is done on less damaged glass - from perhaps 100 to 200 MPa. The dicing characteristics, which are determined by tempering stresses alone, will be the same in either case; and, except for gross accidents to its surface, the glass with the higher initial strength will remain stronger indefinitely (cf. fig. 1). These gains in strength, though not large when measured against the intrinsic strength of glass, are nonetheless significant. Coupled with the enhancement of the fracture pattern of thermally tempered glass, they have led to greatly expanded uses of glass: in vehicles and for security in buildings, to name but two. An advantage of thermally tempered glass that has only recently been documented is the protection against strength-impairing surface damage that its compressed surface layers afford it [12]. This fig. 1 shows that a given severity of abrasion reduces the strength of tempered glass less than that of annealed glass. The enhancement of strength due to tempering is, therefore, even greater after exposure of the glass to abrasion in service than before. Efforts to counter the effect of flaws by surface compression seem thus also to have brought about a partial accomplishment of the earlier objective of eliminating flaws! The recognition that the susceptibility of glass to damage is less after tempering than before places added value on tempering glass that has as good a surface as possible. It remains to be determined just how far it may be worthwhile to go in that direction, for the nucleation of flaws during the heating stage of tempering may well place an upper limit to the attainable surface quality of the product. Chemical strengthening can readily raise glass strengths by a larger margin; and the potential for improvements to 1000 MPa, or about one-tenth of the ultimate strength of glass, has been demonstrated in the laboratory [13(a)]. On the other hand, chemically tempered glass remains vulnerable to surface flaws deep enough to extend through its relatively thin compression layer. Since the growth of this layer by diffusion proceeds as the square root only of time, increasing its thickness can be rather time consuming and therefore impracticable economically. Electrically driven ion exchange is a more difficult process to engineer for most shapes of glass parts, but it has the advantages of proceeding much faster
R. Gardon /Strong glass
239
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240
R. Gardon / Strong glass
even in the face of a mild abrasion that reduced the strength of etched but untempered glass by a factor of 10. In view of this, the writers called this glass "abrasion resistant" rather than "strong" - though, by any standard, it was that, too. Interestingly, the strength of 1000 MPa was obtained at a certain thickness of the compression layer only, and increasing this - as well as decreasing it - sharply reduced the strength. 4.3. M i x e d strengthening mechanisms
Several Russian investigators have worked with systems that combined thermal tempering - notably with reactive liquid quenching media - with acid etching and surface treatments aimed at altering what they called the "structure" of the surface [14]. They certainly produced the highest compressive surface stresses reported by anyone for thermal tempering, up to 500 MPa in 25 mm thick glass. They also reported breaking strengths in excess of 1000 MPa, but these seem of questionable significance since etching was the last step in their treatment, and etching will, for a short time at least, make any glass very strong. Finally, they also attributed some 100-300 MPa of the observed increase in strength to an unspecified "structural" change of their surfaces. This was later interpreted as, possibly, a low-expansion silica film derived from the interaction of the hot glass with the siloxane liquid used as a quenching medium [6]. If so, this would be an instance of combined chemical and thermal tempering. However, it is also possible that the 100-300 MPa in question was simply the result of a difference between how etching affected annealed and highly tempered glass, or how their surfaces fared between etching and testing. A re-evaluation of work along these lines might be in order now, in the light of new knowledge of chemical tempering and the degradation of freshly etched surfaces. In considering ways to enhance the strength of glass, it seems natural to think of combining various strengthening mechanisms. With due regard to the temperatures at which they must be conducted, chemical and thermal tempering could be combined - perhaps to yield the strength of the former and the abrasion resistance a n d / o r fracture pattern of the latter. It is also quite likely - in some cases unavoidable - that the mechanisms that dominate different processes do not come into play by themselves. Thus, for example, it may happen that a combination of surface layer and bulk glass - engineered for a difference in specific volumes, say - also exhibits differences in expansion coefficients or glass transition temperatures. With the overlap of the temperature ranges in which various tempering mechanisms can come into play, several may be involved in one process; and, in doing so, they may reinforce or counteract one another. - A clear example of a destructive interplay of various mechanisms occurs in diffusive ion stuffing. For this to proceed at a reasonable rate, it must be conducted at elevated temperatures, at which stress relaxation may come into play along with an acceleration of the ion exchange that is generating the desired stresses. The process must clearly be run at a temperature optimal for the results expected in a given time.
R. Gardon / Strong glass
241
4.4. Where can we go from here?
As far as thermal tempering is concerned, I am less aware of demands for higher temper stresses than of demands to extend the benefits of tempering to thinner glass, more complex shapes and parts made of low-expansion glasses. This situation may, of course, simply be a manifestation of " m a r k e t pull" in one direction, with no "technology push" in the other. (The latter may be up to us!) In any case, the objectives in greater demand entail some of the same problems as would attempts to raise the level of temper, and we shall briefly address that issue. Relatively modest increases in thermally produced temper stresses perhaps by as much as a factor of two, depending on circumstances - can be achieved by processes that are " m o r e of the same", such as quenching the glass more severely. Raising the quenching rate is relatively easy: sublimating solids and fluidized beds and jets have been added to the old repertory of air jets and liquids. It is generally more difficult to meet such concomitant requirements as raising the initial temperature of the glass without risk to its form. Nevertheless, this sort of thing - with greater or lesser variations - is being done all the time. An interesting variant of thermal tempering is to conduct it with a time-dependent quenching rate. This was used in a process that enabled sheets of thin glass, from 3 m m down to 0.75 ram, to be tempered. Simply to employ more severe quenching than usual, using a liquid coolant, appears not to have been practicable. Instead, this process entailed quenching in two stages, first in air~ then in a liquid [15]. One can estimate the maximum strengthening effect attainable by thermal tempering by recalling that temper stresses are (roughly) determined by some average of temperature differences across the half-thickness of the glass during the critical phase of cooling. The highest value that this can usefully attain is the difference between the high end of the glass transition range and some reasonable coolant temperature. Correspondingly, the ultimately attainable surface compression in soda-lime glass might be about 350 MPa. It would be less in a low-expansion glass; and harder to approach in a thin sheet of glass than in a thick one. This explains why modest increases in temper stress above - 100 MPa in 5 m m glass are relatively easy to obtain. The highest compressive stress of 500 MPa in 25 m m glass, reported i/a ref. [14(b)], is another matter! To approach the limiting surface compression would - except perhaps in very thick glass - be more difficult than simply to quench the glass more severely, and I would look to some qualitative change in the cooling process. The two-stage quench of ref. [15] - though addressed to reducing the thickness of tempered glass rather than to raising temper stresses - may, among other things, also have been a step in that direction. It is a measure of its effectiveness that, while engineering advances in equipment have now made the tempering of 3 m m glass possible by more-or-less conventional techniques also,
242
R. Gardon / Strong glass
this process is - to my knowledge - still the only one for which it is claimed that it can temper sheets of 1 mm glass. Processes that take advantage of modulated quenching may be one possible route to take beyond what can readily be accomplished by conventional thermal tempering. To approach the limiting surface compression attainable yet more closely would be very difficult. Conceptually, it might be done by supplying heat to the interior of the glass, even as its surfaces are being rapidly cooled. While such a process is not out of question, it is not likely to be developed without some strong new motivation. With chemical tempering one can already achieve higher compressive stresses in the surface, so that the practical challenge, once again, is not to raise these further, but rather to obtain thicker compression layers in a reasonable time and to make the process useful for a greater variety of products. Electrically driven ion exchange may hold promise of high productivity, but more work is needed to establish both its capabilities and its practicality. Beyond that, materials, processes and techniques of execution are all appropriate subjects for further research and development. All seem to be receiving a great deal of attention, in the Japanese patent literature especially. For Japanese practice with chemically strengthened bottles, see ref. [16]. An interesting development is Asahi Glass Company's work [17] on ion-exchange strengthening float glass while still in the form of a continuous ribbon. The wide-spread availability of such glass - cuttable like annealed glass, but with a strength perhaps four times higher - could perhaps, slowly, change popular perceptions regarding the extreme fragility of glass.
5. Changing the intrinsic strength and brittleness of glass? Finally, I would like to touch on an approach to strengthening glass that may be both the most fundamental and - probably for good reasons - the least discussed. But then, if I cannot stick my neck out on an occasion such as this, when can I? The first step is a relatively safe one. Modification of the compositions of glasses is among the oldest pursuits of glass makers, and composition should clearly affect the intrinsic strength of glass, (cf. last two lines of table 1) To identify this effect one must be able to distinguish the intrinsic strength from that usually observed, or, at least, to account for the effect of flaws on the observed strength. Using techniques of fracture mechanics, Eagan and Swearengen [18] measured critical stress intensity factors of various compositions of alumino- and borosilicate glasses. Not only were differences between these two types of glass demonstrated, but, by choosing compositions that maximized the density of network forming bonds, the fracture toughness of either type could be raised. Taking the fracture toughness measured in these experiments as being roughly proportional to ultimate strength, we can infer that gains in strength over the base soda-silica glass ranged from 15 to 50%.
R. Gardon /Strong glass
243
Much larger gains might be expected if the rigidity of the glassy network could somehow be reduced. This would likely lead to a material with an intrinsic strength lower than that of the original glass, but with a higher usable strength. Great progress along these lines was made in the last thirty years in the field of plastics, where the impact resistance of "rubber-modified" glassy polymers was raised by factors of 10 to 20 above that of the unmodified material. - To do the same for inorganic glasses would be something great to wish for Norbert's 100th birthday! This may be something of a pipedream, for - almost by definition - glass per se cannot be made ductile. So, if we want toughness, we must needs talk composites. Analogies with plastics abound. Thus fiber glass reinforced plastics now have their counterparts in SiC-fiber-reinforced glass [19]. Such materials are clearly beyond the scope of this paper; but I would like to pursue briefly an analogy with rubber-modified glassy plastics, which was prompted by a recent paper by Krstic et al. [20]. This reports the toughening of a glass by the inclusion of metallic particles. The materials in question were made by hot pressing mixtures of glass frits with various metal powders. Most of them were no stronger than the base glass. However, in the combination of a certain glass and oxide-covered aluminum particles - the oxide layer serving as a bonding agent - the particles acted as ductile "brakes" to the propagation of fractures though the glassy matrix, in a manner illustrated schematically by fig. 2. This approach may be contrasted with earlier attempts to strengthen glass with dispersions of rigid particles, intended to limit the size of flaws in the glassy matrix [21]. At high volume fractions ( - 45%) of small (15-20 micron) particles, and correspondingly small spacings between them, increases in strength up to about 70% were obtained. (cf. fig. 3). In contrast, in the work with ductile inclusions, particle loadings of only 5 to 20 vol% sufficed to raise the toughness (stress intensity factor) to levels 3 to 6 times that of the base glass. (cf. fig. 4). Breaking strengths (in units of stress) cannot be inferred from
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5 I0 15 20 V o l u m e F r o c t i o n of A I , %
I 25
Fig. 4. Fracture toughness versus volume fraction of AI particles in M-glass (Krstic et al. [20]). composites, but the latter can be viewed, macroscopically at least, as one material. It is also likely to be "simpler" to make. Thus, if our new material had enhanced strength along with desired glassy attributes, such as transparency, and if it could be made by the usual methods of forming glass possibly with phase-separation thrown in - I think it could fairly be called a dispersion-strengthened glass, a "stronger glass". In a parallel case, glassceramics have remained a product of the glass industry, while fiber-reinforced composites - whether glass fibers in plastics or ceramic fibers in a glass matrix - have been co-opted by others.
6. C o n c l u s i o n
We have reviewed some factors that govern the strength of bulk glass and some processes by which that strength may be increased. Most involve compressive prestressing of surfaces, and some - notably thermal tempering and ion stuffing - are highly developed for certain applications. Work in the laboratory has already demonstrated greater strengths than are yet attainable industrially, and their utilization in practice presents a challenge comparable to that of further advances in the laboratory. I have also raised the truly long-range - if, indeed, not " f a r out" - question of whether composites with glass-like properties can be made less brittle. I hope that our discussion may help to define some of these problems more clearly and pinpoint opportunities more closely. - Advances may ensue from improved insights into the strength and fracture characteristics of glasses, or,
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again, they may remain unforeseen and flow - as thermal tempering did a half century before Griffith identified the flaws named after him - from the inventive genius of practitioners of the glass maker's art.
References [1] F.M. Ernsberger, in: Glass: Science and Technology, Vol. 5 eds. D.R. Uhlmann and N.J. Kreidl (Academic Press, New York, 1980) p. 133. [2] R. Gardon, ibid., p. 145. [3] R.F. Bartholomew and H.M. Garfinkel, ibid., p. 217. [4] A.A. Griffith, Phil. Trans. Roy. Soc., Ser. A221 (1921) 163. [5] F.M. Ernsberger, in: Advances in Glass Technology, Proc. 6th Int. Congress on Glass, Washington, D.C., July 1962, Part 1 (Plenum, New York, 1962) p. 511. [6] S.D. Stookey, J.S. Olcott, H.M. Garfinkel and D.L. Rothermel, ibid., p. 397, [7] H.P. Hood and S.D. Stookey, US Patent 2 779 136 (1957). [8] M.G. Drexhage and P.K. Gupta, J. Am. Ceram. Soc. 63 (1980) 72. [9] N. Weber, US Patent 3 218.220 (1965). [10] S.S. Kistler, J. Am. Ceram. Soc. 45 (1962) 59. [11] O.S. Narayanaswamy, J. Am. Ceram. Soc. 61 (1978) 146. [12] (a) B.R. Lawn, D.B. Marshall and S.M. Wiederhorn, J. Am. Ceram. Soc. 62 (1979) 71; (b) H. Dannheim, H.J. Oel and W. Prechtl, (in German) Glastechn. Ber. 54 (1981) 312. [13] (a) M. Abou-el-leil and A.R. Cooper, Glass Techn. 21 (1980) 57; (b) M. Abou-el-leil and A.R. Cooper, J. Am. Ceram. Soc. 64 (1981) 141. [14] (a) F.F. Vitman, I.A. Boguslavskii and V.P. Pukh, Dokl. Akad. Nauk. SSSR 145 No. 1 (1962) 85 (Engl. transl.: Soviet Phys. - Doklady 7, No. 7, 1963) 650; (6) I.A. Boguslavskii, Steklo i Ker. 21 No. 10 (1964) 4 (Engl. transl.: Glass & Ceramics, USSR, 21 (1964) 562. [15] R. Melling, D.C. Wright and J. Pickup, US Patent 3 890 128 (1975). [16] H. Ono, Glass. Techn. 22 (1981) 173. [17] Asahi Glass Co., Japan. Patents 79 132 620 (1979) and 80 80 744 (1980) (cf. Chem. Abstrs. 92:133939 and 94: 6990k). [18] R.J. Eagan and J.C. Swearengen, J. Am. Ceram. Soc. 61 (1978) 27. [19] J.J. Brennan and K.M. Prewo, J. Mat. Sci. 17 (1982) 2371. [20] V.V. Krstic, P.S. Nicholson and R.G. Hoagland~ J. Am. Ceram. Soc. 64 (1981) 499. [21] D.P.H. Hasselman and R.M. Fulrath, J. Am. Ceram. Soc. 49 (1966) 68. [22] (a) N. Miyata and H. Jinno, J. Mat. Sci. 16 (1981) 2205; (b) A.G. Evans, Phil. Mag., Set. 8, 26 (1972) 1327. [23] H. Woelk and K. Elsenheimer, (in German) Glastechn. Ber. 52 (1979) 14. [24] R. Gardon, Paper No. 79 in: Proc. 7th Int. Congress on Glass, Brussels (1965). [25] M.E. Nordberg, E.L. Mochel, H.M. Garfinkel and J.S. Olcott, J. Am. Ceram. Soc. 47 (1964) 215. [26] F.M. Ernsberger, Phys. Chem. Glasses 10 (1969) 240. [27] B.A. Proctor, I. Whitney and J.W. Johnson, Proc. Roy. Soc., Ser. A297 (1967) 534.