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Glass surfaces then and now
Journal of Non-Crystalline Solids 49 (1982) 331-338 North-Holland Publishing Company
331
Part IV. Glass surfaces and interfaces
GLASS SURFACES THEN AND NOW N o r b e r t J. K R E I D L Department of Ceramic Engineering, University of Missouri-Rolla. Rolla, Missouri, USA and Department of Chemical Engineering, Universi(v of New Mexico, Albuquerque, New Mexico, USA
A reservoir of efficient instrumentation capable of detailed and quantitative exploration of glass surface features is now available in the face of the recognition of their crucial importance for both surface and bulk properties as well as performance. Yet it may be instructive to glance at what had been learned about glass surfaces by investigators before the advent of these intricate tools. Among such earlier findings are the recognition of the altered composition and structure of the surface layer, the extension of this disturbance to considerable depth, the effect of surface structure on adsorption, adhesion, wetting, finishing, etc. and on bulk properties. It was recognized that not only bulk composition, but the processing of the glass decisively influenced the composition, structure and behavior of the surface. And, in turn, certain compositional constituents, even in small concentrations, were found to be able to alter the composition and structure of the surface, thus affecting its function.
1. Introduction T h e s t u d y of glass surfaces a n d their i m p o r t a n t influence on b u l k b e h a v i o r is b o u n d to d o m i n a t e p r e s e n t e n d e a v o r s in glass science, engineering, development, a n d p r o d u c t i o n . This is due to b o t h a m o r e m a t u r e recognition of the role of solid surfaces a n d the i n t r o d u c t i o n of s o p h i s t i c a t e d i n s t r u m e n t a t i o n p e r m i t t i n g the q u a n t i t a t i v e analysis of surfaces. This does n o t m e a n that long b e f o r e the a p p e a r a n c e of expensive a n d efficient tools, a n u m b e r of glass scientists d i d n o t e x p l o r e a n d e x p l a i n surface behavior. N o r does this m e a n that their findings h a d n o t s t i m u l a t e d the skill of technologists in d e a l i n g with the p r o b l e m s clearly affected b y surface p h e n o m ena. It seems fitting at the start of a C o n f e r e n c e d e d i c a t e d to the p r e s e n t impressive w o r k c o n c e r n i n g glass surfaces to r e m i n d ourselves of these earlier achievements,
2. The nature of glass surfaces Before the a d v e n t of tools p e r m i t t i n g the q u a n t i t a t i v e d e s c r i p t i o n of surfaces a n d their changes u n d e r i m p o s e d conditions, the m o s t i m p o r t a n t p h e n o m e n a requiring, as well as supplying, i m p o r t a n t qualitative conclusions on glass surfaces were: wetting, friction, adhesion, corrosion, a n d polishing. 0022-3093/82/0000-0000/$02.75
© 1982 N o r t h - H o l l a n d
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N.J. Kreidl / Glass surfaces then and now
By about 1960, it had become clear [1-2] that the underlying principle of surfaces was the presence of strongly unbalanced electrical forces. In this respect, of course, glasses resemble crystalline solids. However, the nature of bonding in some cases allows the adjustment of potential unbalance by changes in the electronic orbitals and the position of atoms in the surface layer. Examples are polymerized hydrocarbons [1] and, potentially, even certain inorganic glasses. It is the unbalanced forces on fresh surfaces which explain their attraction for other substances. In fact, it is recognized that a true glass surface does not persist; foreign contaminants such as OH and organic groupings from the atmosphere will immediately form a layer which represents the practical surface of glasses under conventional conditions. On the other hand, freshly blown or radically cleaned glass surfaces will stick together when contacted immediately in order to restore electrical interatomic balance. The wetting by other liquids and gases thus becomes a specific chemical phenomenon governed by the specific unbalances as a result of broken bonds in a specific glass composition. From the above, the conclusion was drawn that surface tension is by no means a simple consequence of the relative attractive forces within the bulk of a substance, but is a measurement of the degree to which attractive forces on the surface remained unsatisfied when the surface was processed [1]. Subtle chemical reactions on the surface affecting surface tension have been known for a long time. For instance, a droplet running down an inclined plane shows a greater contact angle at the advanced edge, probably as it replaces contaminants [3-5]. Another striking example is that, in contrast to the usual observation that mercury does not wet glass, highly purified mercury does [6]. Contact angles of various liquids on various glasses were measured about 50 years ago [7]. For many organic liquids (e.g. glycerine, turpentine, acetic acid), a 0 ° contact angle was found for soda-lime glasses. These authors had used freshly drawn tubes, being fully aware of the effect of aging and conventional cleaning. The systematic studies of Moser [8] on water and other fluids suggested larger contact angles and thus must be taken with caution. The surface tension of freshly broken SiO a can be very high due to the presence of excess unbound oxygen and oxygen-deficient tetrahedra. The low polarizability of the constituent ions in SiO 2 and conventional glasses makes an adjustment difficult. Yet, processing can cause a different situation. Even elongating a glass sample to form a rod over a sufficient period of time may permit an adjustment that affects the symmetry of the constituent tetrahedra to a considerable depth [2]. The general propagation of surface unbalance in depth was discussed in great detail by Weyl [9]. Nonetheless, the high value of contact angle (35 ° ) for lead glasses stands out and is in agreement with the concept that the polarizability of the lead ion permits the adjustment of unbalanced bonds on the surface [2].
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3. Hygroscopy and condensation It has been observed that lead glasses are much less hygroscopic than conventional soda-lime glasses. For instance, weight gain in mg per cm 3 glass in 4 h is found to be 35 for window glass, 5.6 for 45% PbO optical flint glass, and 4.9 for 5.1% PbO lead borate glass [1]. Even more convincing was the experiment by Sonders et al. [5], who measured the capillary rise of water in glass tubes the surfaces of which had been treated with various cations. After 120 h a blank showed a rise of 62 mm and the lead impregnated surface, 37 mm. Other cations of higher polarizability than Ca also showed some effect; for instance, Zn2+ :53 and Ni 2+ :48. Rayleigh [10] observed that passing a flame over a glass plate qualitatively changed the appearance of condensed water. The surface changed from white - caused by the scattering of droplets - to become dark but transparent when a continuous film wetted the plate. The flame was later found to cause the emission of ions, and use was made of this knowledge in cleaning glass by glow discharge before depositing optical films [2]. Silver is attracted preferentially to the flame track, thus making it visible and permanent [10]. Surface condensation of H 2 0 is affected by compositional changes, most easily obtained by base exchange at low temperature with silver. This exchange at low temperature and to a modest depth is invisible, but manifests itself in a different condensation pattern when the glass is breathed. This phenomenon has been developed as an "invisible trade mark". A more extensive study of condensation patterns included the detection of the Griffith flaws, so important in the assessment of mechanical strength [11].
4. De-aikalization While the reaction of the virgin glass surface with H 2 0 and ambient contaminants is of primary importance for the performance of a practical glass surface, there are other important reactions, some of which were discovered long ago. One which is of industrial importance at the present time is the incidental or deliberate reaction with furnace gases at annealing temperatures. The beneficial effect on durability was first studied and described by Keppeler [ 12,13], who patented a procedure in 1930. The underlying reaction was found to be: N a 2 0 + SO 2 + ½02 --, Na2SO 4
(1)
depleting Na from the glass. The oxidation of SO 2 needs a catalyst. Fortunately, water is a catalyst for both this reaction and for the migration of Na + to the glass surface. In the absence of SO2, an improvement of the surface can also be obtained by other processes of de-alkalization. Flame de-alkalization was first described by Williams et al. [14]. De-alkalization by base exchange on exposure to
334
N.J. Kreidl / Glass surfaces then and now
diluted acids, followed by heating to remove protons, has been considered in the case of optical glasses, Williams et al. [14] developed surface de-alkalization by exposure to dehydrated kaolin: Na ÷ (glass) + H + (clay) --, n + (glass) + S a + (clay), (500-600°C)
H ÷ (glass)
--,
HzO + de-alkalized glass surface.
(2)
(3)
Reaction (1) is much accelerated by water vapor.
5. Glass composition, surface enrichment and surface tension
5.1. Surface enrichment Components lowering surface tension were found to enrich in the surface [2,15]. This enrichment of the surface in a component lowering surface tension is the consequence of the requirement to minimize the total free energy of a multicomponent system [16,17]. The difference ( A X ) in concentration of a component will be [17]:
AX-
R1T ( 8 0 / 8 l n ' X ) r ,
where o is the surface energy; X is the mole fraction of the component; R is the gas constant; and T is the absolute temperature. In the idealized case of an ideal solution in a binary system and of a monolayer surface, Overbury et al. [18] obtained:
X~/X~ = x2b/x b exp(o I -- 02)a/RT, where: the X values are the concentrations of two components identified by indices 2 and l, in the surface and bulk, identified by superscripts s and b; o2, o t are the surface energies of X; and a -- d A / d n , in the chemical potential (pt) equation, # = OG/On - oSA/On, with A being the area and G being the energy. This approach only gives a description of the nature of the enrichment. For non-ideal solutions and multilayers, much more complicated relations have been worked out [18]. The relation of heat of vaporization or sublimation to surface-free energy has been postulated to be approximately 6 : 1 for a closed-packed metal with 12 nearest neighbors. This estimate is based on the simple assumption that in sublimation, six times more bonds have to be broken than in obtaining a new surface. In oxides, we have to deal with deeper interaction and relaxation into new equilibria at the surfaces involving surplus orbitals. This is why quantitative experimental evidence will be necessary to establish fundamentals for the case of glasses.
N.J. Kreidl / Glass surfaces then and now
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5.2. Effect of some components The surface tension of conventional oxide glasses was found to be increased by A1203, MgO, and CaO [19,20] and to be decreased by K 2 0 [20], B203 [21], As203 [2,22], V205 [2,22], Cr203 [22], MoO 3 [22], PbO [23-28], F [29], I [30], SO 3 [31] and OH [32]. Assuming additivity within limited ranges, the relative effects of some oxides were shown as factors per 1% oxide in dyn c m - t ; e.g. according to Lyon [20]: AI203 MgO SiO2 N a 2 0 K 2 0 5.98 5.77 3.25 1.27 0 or to Dietzel [33] (900°C): A1203 PbO 6.2 1.2 " The strong effect of PbO is of particular interest; moreover, much PbO can be accommodated in oxide glasses. The data in table I show the influence of PbO concentration according to various authors (dyn c m - i). The tendency, at high PbO content, for an increase in surface tension with temperature will be discussed later. The very large decrease in surface tension by very small concentrations (0.01%) of (AsO4) 3-, (MOO4) 2- , (VO4) 2 and ( C r O 4 ) 2 - is explained by surface enrichment, invariably in the higher oxidation species [15,22]. Around 1950 this concept was used to control bifocal glass interfaces by MoO 3 additions much below 1%. In the case of boron, the strong effect is attributed to enrichment at the surface in the (BO3) 3- group [21]. Accordingly, the higher the alkali content, known to promote (BO4)5- groupings, the less the effect of boron in lowering surface tension [34]. In the case of SO 3, 1% decreases the surface tension of a conventional soda-lime glass from about 0.3 to about 0.25 N m - i [35-40]. At low tempera-
Table 1 The influence of PbO content according to various authors PbO (%) 35.6 40 44.6 56 70.7 89 89
800°C
900°C
209
211
160
164
1000°C
1200°C
1400°C
Ref.
234 220 213 202 184 142 140
236
235
[27] [28] [23-28] 1271 [23-25] [27] [28]
208 154 increase
336
N.J. Kreidl / Glass surfaces then and now
tures, immiscibility contributes to this effect. The phenomenon is important in melting practice because of improved wetting of residual sand grains by the initial flux, rich in sulfate. In silicate glasses, fluorine was found to decrease surface tension (at 800°C) and to enrich in the surface [29], but if NaF substitutes for NaPO 3 in phosphate glasses, surface tension increases [41]. Iodine (also bromine) in very small quantities decreases surface tension as it enriches in the surface [30]. These authors applied the phenomenon to the fining of optical glasses in Pt crucibles attributing the effect to the decreased wetting at the interface allowing gas bubbles to escape. It has been understood that the OH content, particularly by absorption, plays a large role [42]. At 21 mbar H20 vapor pressure, at 550°C, the surface tension dropped to 205 × 1 0 - 3 N m -I compared with 315 in vacuum. The decrease in adsorption at higher temperatures becomes manifest by a decrease in the influence of the atmosphere [31].
6. Effect of temperature on surface tension
Increased temperature will generally decrease surface tension; i.e. do/dT< 0. The decrease is of the order of 4 to 10 × 10-3N m -~, but when highly active components, such as PbO, B303 and MoO 3 have decreased surface tension by enrichment and orientation, increased temperature disturbs this ordering and do/dT> 0 [28,42,43].
7. Friction
The friction coefficient of clean glass was found to be exceptionally high [44], somewhat more so at low loads [45]. To obtain consistent data, a well-defined cleaning procedure (glow discharge) was soon recognized to be necessary. Friction against metals is particularly high if softness allows the metal to cling a n d / o r if oxide layers adhere to the clean glass surface [1]. This high friction enables the marking of glasses by metal [46]. It is not surprising that it became clear at an early date [44] that liquids and monomolecular films would greatly affect friction depending on their chemical reactivity with the glass. Nor is it surprising that the effect was different for different solids in contact with glass. Of great practical importance was, and still is, the treatment of glass surfaces with organics forming films several monolayers thick, e.g. silicone. This process is derived from the accidental discovery that glass surfaces in contact with methylchlorosilane vapors (during the synthesis of methylchlorosilanes) became water repellent. On this basis, the water-proofing of glass was patented [47]. The recognition of damage by friction as the chief cause of fracture in bottles led to the adoption of siliconing in the manufacturing process.
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8. Adhesion Similarly, adhesion has for a long time been understood to depend on the initiation of specific chemical reactions, such as the presence or absence of oxide films of metals in contact with glass. An early example of such studies is the adhesion of indium to glass [48]. The discussion in even modest detail of the adhesion of glass to metals, or polymers, would require an entire proceedings. In glass-polymer composites, a chief consideration of early workers was the low alkali content of the glass [49] and the use of coupling agents [50].
9. Polishing Finally, credit should be given to the early classical work on polished glass surfaces. Careful experimentation and unbiased interpretation led Beilby [51] to the conclusion that the polishing process is not just a refined abrasion, but involves some flow. This was elaborated upon in detail by French [52] who coined the term "Beilby Layer." A host of subsequent studies led to controversy over flow initiated just by heat and pressure, versus flow assisted by chemical reaction. The effect of a chemical reaction between the very small crystals of the polishing agent with surface anions and cations of the glass was propounded particularly by Kaller [53]. He and the chief proponents of a thermal-plastic mechanism [54,55] agreed later that the chemical mechanism has importance, though it is limited to the last stage of polishing. The situation has been treated extensively, and with a sense of history and much insight by Holland [ i ].
References [l] L. Holland, The properties of glass surfaces (J. Wiley, New York, 1964). [2] W. Weyl and E. Marboe, The constitution of glass, Vol. l (lnterscience, New York, 1964); E. Marboe, The constitution of glass Vol. 2 (Interscience, New York, 1967). [3] N. Adam, The physics and chemistry of surfaces, Third Ed. (Oxford University Press, London, 1941) quoting Raley (1890). [4] R. Shuttleworth and G. Bailey, Disc. Faraday Soc. 3 (1948) I I. [5] L. Sonders, D. Enright and W. Weyl, J. Appl. Phys. 21 0950) 338. [6] L. Briggs, J. Chem. Phys. 24 (1953) 488. [7] F. Bartell and E. Merril, J. Phys. Chem. 36 (1932) 1178. [8] F. Moser, The Glass Industry, (April, 1956): Am. Soc. Test Mat. Bull. 169 (1950) 62. [9] D. Day, ed., Glass surfaces (North-Holland, Amsterdam, 1975) p. 1. [10] Lord Rayleigh, Nature 86 (191 l) 416. [I l] W. Levengood, J. Am. Cer. Soc. 38 0955) 178. [12] G. Keppeler, Glastech. Ber. 5 0927) 97. [13] G. Keppeler, Sprechsaal 61 (1928) 300. [14] H. Williams and W. Weyl, Glass Ind. 26 (1945) 275; 324. [15] C. Amberg, PhD Dissertation, Department of Ceramics, Pennsylvania State University (1948).
338 [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
N.J. Kreidl / Glass surfaces then and now
H. Defray, Surface tension and adsorption (J. Wiley, New York, 1966). R. Swalin, Thermodynamics of solids, 2nd Ed. (J. Wiley, New York, 1972). S. Overbury, P. Bertrand and G. Somorjai, Chem. Rev. 75 (1975) 547. A. Dietzel, Sprechsaal 27 (1942) 82. K. Lyon, J. Am. Cer. Soc. 27 (1944) 186. H. Scholze, Glas (Springer, Berlin, 1964) p. 261. C. Amberg, J. Am. Cer. Soc. 29 (1946) 81. A. Appen, Zh. Fiz, Khim. 26 (1952) 1339. A. Appen, Silikattech 5 (1956) 11. A. Appen, Adv. Glass Tech., Vol II, eds., Matson and Rindone (Plenum, New York, 1963). M. Hino, T. Ejima and M. Kameda, J. Jap. Inst. Met. 31 (1967) 113; tabulated in O. Mazurin, M. Streltsina, A. Shvaiko-Shavaikovshaya, Svoistva Stokl; Stekloobrazuyushtshkikh Rasplavov IV (Nauka Leningrad, 1980) p. 190. [27] L. Shartsis, S. Spinner and A. Smock, J. Am. Cer. Soc. 31 (1948) 23. [28] H. Ito, T. Yana Gose and Y. Suginohara, J. Mining Metall. Inst. Jap. 77 (1961) 895. [29] V. Budov, Uy. Kruchinin and F. Solinov, Glass and Ceramics 22 (1965) 660. [30] H. Hafner, G. Brewster and R. Weidel, US 2 702 749 (Feb. 22, 1955) abstr: Cer. Abst. (July, 1955) p. 122b). [31 ] A. Dietzel and E. Wegner, Mitteilungen Verb. deutscher Emailfachleute 2 (1954) 13. [32] G. Tamman and H. Rabe, Anorg. Allg. Chem. 162 (1927) 17. [33] A. Dietzel, Kolloid Z. 100 (1942) 368. [34] L. Shartsis and W. Capps, J. Am. Cer. Soc. 35 (1952) 169. [35] H. Jebsen-Marwedel, Kolloid Z. 137 (1954) 118. [36] H. Jebsen-Marwedel, Glastech, Ber. 29 (1956) 223. [37] H. Jebsen-Marwedel, Kolloid Z. 150 (1957) 137. [38] H. Jebsen-Marwedel, Glastech. Ber. 31 (1958) 431. [39] H. Jebsen-Marwedel, Naturwiss 45 (1958) 260. [40[ H. Jebsen-Marwedel, CR. Symp. Fusion du Verre Brussel (1958). [41] D. Williams, B. Bradbury and W. Maddocks, J. Soc. Glass Tech. 43 (1959) 308. [42] N. Parikh, J. Am. Cer. Soc. 41 (1958) 18. [43] H. Scholze, Glas Springer, Berlin (1977). [44] W. Hardy and J. Hardy, Phil. Mag. 38 (1919) 32. [45] R. Southwick, Preston Lab. Rep. no. 57-077 (1957). [46] L. Gehring and J. Turnbull, Bull. Am. Cer. Soc. 19 (1940) 290. [47] W. Patnode, US 2 306 222 (1940). [48] K. Rose, Sci. Am. 170 (I 944) 154. [49] J. Brossy, J. Case, A. Houze and A. Lasday, Soc. Plast. Ind., 12th Annual Meeting (1957) Sec. 16A. [50] J. Bjorksten, and L. Yaeger, Modem Plastic (1952) 29 124. [51] G. Beilby, Proc. Roy. Soc. A72 (1903) 218; 226. [52[ J. French, Opt. Sci. Instr. Maker 62 (1921) 253. [53] A. Kaller, Silikattechnik, 7 (1956) 380; Jenaer Jahrbuch, VEB., Carl Zeiss, Jena p. 181. [54] E. Brueche, A. Kaller and H. Poppa, Silikattech. 10 (1959) 213. [55] H. Poppa, J. Soc. Glass. Tech. 40 (1956) 513T.
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Part IV. Glass surfaces and interfaces
GLASS SURFACES THEN AND NOW N o r b e r t J. K R E I D L Department of Ceramic Engineering, University of Missouri-Rolla. Rolla, Missouri, USA and Department of Chemical Engineering, Universi(v of New Mexico, Albuquerque, New Mexico, USA
A reservoir of efficient instrumentation capable of detailed and quantitative exploration of glass surface features is now available in the face of the recognition of their crucial importance for both surface and bulk properties as well as performance. Yet it may be instructive to glance at what had been learned about glass surfaces by investigators before the advent of these intricate tools. Among such earlier findings are the recognition of the altered composition and structure of the surface layer, the extension of this disturbance to considerable depth, the effect of surface structure on adsorption, adhesion, wetting, finishing, etc. and on bulk properties. It was recognized that not only bulk composition, but the processing of the glass decisively influenced the composition, structure and behavior of the surface. And, in turn, certain compositional constituents, even in small concentrations, were found to be able to alter the composition and structure of the surface, thus affecting its function.
1. Introduction T h e s t u d y of glass surfaces a n d their i m p o r t a n t influence on b u l k b e h a v i o r is b o u n d to d o m i n a t e p r e s e n t e n d e a v o r s in glass science, engineering, development, a n d p r o d u c t i o n . This is due to b o t h a m o r e m a t u r e recognition of the role of solid surfaces a n d the i n t r o d u c t i o n of s o p h i s t i c a t e d i n s t r u m e n t a t i o n p e r m i t t i n g the q u a n t i t a t i v e analysis of surfaces. This does n o t m e a n that long b e f o r e the a p p e a r a n c e of expensive a n d efficient tools, a n u m b e r of glass scientists d i d n o t e x p l o r e a n d e x p l a i n surface behavior. N o r does this m e a n that their findings h a d n o t s t i m u l a t e d the skill of technologists in d e a l i n g with the p r o b l e m s clearly affected b y surface p h e n o m ena. It seems fitting at the start of a C o n f e r e n c e d e d i c a t e d to the p r e s e n t impressive w o r k c o n c e r n i n g glass surfaces to r e m i n d ourselves of these earlier achievements,
2. The nature of glass surfaces Before the a d v e n t of tools p e r m i t t i n g the q u a n t i t a t i v e d e s c r i p t i o n of surfaces a n d their changes u n d e r i m p o s e d conditions, the m o s t i m p o r t a n t p h e n o m e n a requiring, as well as supplying, i m p o r t a n t qualitative conclusions on glass surfaces were: wetting, friction, adhesion, corrosion, a n d polishing. 0022-3093/82/0000-0000/$02.75
© 1982 N o r t h - H o l l a n d
332
N.J. Kreidl / Glass surfaces then and now
By about 1960, it had become clear [1-2] that the underlying principle of surfaces was the presence of strongly unbalanced electrical forces. In this respect, of course, glasses resemble crystalline solids. However, the nature of bonding in some cases allows the adjustment of potential unbalance by changes in the electronic orbitals and the position of atoms in the surface layer. Examples are polymerized hydrocarbons [1] and, potentially, even certain inorganic glasses. It is the unbalanced forces on fresh surfaces which explain their attraction for other substances. In fact, it is recognized that a true glass surface does not persist; foreign contaminants such as OH and organic groupings from the atmosphere will immediately form a layer which represents the practical surface of glasses under conventional conditions. On the other hand, freshly blown or radically cleaned glass surfaces will stick together when contacted immediately in order to restore electrical interatomic balance. The wetting by other liquids and gases thus becomes a specific chemical phenomenon governed by the specific unbalances as a result of broken bonds in a specific glass composition. From the above, the conclusion was drawn that surface tension is by no means a simple consequence of the relative attractive forces within the bulk of a substance, but is a measurement of the degree to which attractive forces on the surface remained unsatisfied when the surface was processed [1]. Subtle chemical reactions on the surface affecting surface tension have been known for a long time. For instance, a droplet running down an inclined plane shows a greater contact angle at the advanced edge, probably as it replaces contaminants [3-5]. Another striking example is that, in contrast to the usual observation that mercury does not wet glass, highly purified mercury does [6]. Contact angles of various liquids on various glasses were measured about 50 years ago [7]. For many organic liquids (e.g. glycerine, turpentine, acetic acid), a 0 ° contact angle was found for soda-lime glasses. These authors had used freshly drawn tubes, being fully aware of the effect of aging and conventional cleaning. The systematic studies of Moser [8] on water and other fluids suggested larger contact angles and thus must be taken with caution. The surface tension of freshly broken SiO a can be very high due to the presence of excess unbound oxygen and oxygen-deficient tetrahedra. The low polarizability of the constituent ions in SiO 2 and conventional glasses makes an adjustment difficult. Yet, processing can cause a different situation. Even elongating a glass sample to form a rod over a sufficient period of time may permit an adjustment that affects the symmetry of the constituent tetrahedra to a considerable depth [2]. The general propagation of surface unbalance in depth was discussed in great detail by Weyl [9]. Nonetheless, the high value of contact angle (35 ° ) for lead glasses stands out and is in agreement with the concept that the polarizability of the lead ion permits the adjustment of unbalanced bonds on the surface [2].
N.J. Kreidl / Glass surfaces then and now
333
3. Hygroscopy and condensation It has been observed that lead glasses are much less hygroscopic than conventional soda-lime glasses. For instance, weight gain in mg per cm 3 glass in 4 h is found to be 35 for window glass, 5.6 for 45% PbO optical flint glass, and 4.9 for 5.1% PbO lead borate glass [1]. Even more convincing was the experiment by Sonders et al. [5], who measured the capillary rise of water in glass tubes the surfaces of which had been treated with various cations. After 120 h a blank showed a rise of 62 mm and the lead impregnated surface, 37 mm. Other cations of higher polarizability than Ca also showed some effect; for instance, Zn2+ :53 and Ni 2+ :48. Rayleigh [10] observed that passing a flame over a glass plate qualitatively changed the appearance of condensed water. The surface changed from white - caused by the scattering of droplets - to become dark but transparent when a continuous film wetted the plate. The flame was later found to cause the emission of ions, and use was made of this knowledge in cleaning glass by glow discharge before depositing optical films [2]. Silver is attracted preferentially to the flame track, thus making it visible and permanent [10]. Surface condensation of H 2 0 is affected by compositional changes, most easily obtained by base exchange at low temperature with silver. This exchange at low temperature and to a modest depth is invisible, but manifests itself in a different condensation pattern when the glass is breathed. This phenomenon has been developed as an "invisible trade mark". A more extensive study of condensation patterns included the detection of the Griffith flaws, so important in the assessment of mechanical strength [11].
4. De-aikalization While the reaction of the virgin glass surface with H 2 0 and ambient contaminants is of primary importance for the performance of a practical glass surface, there are other important reactions, some of which were discovered long ago. One which is of industrial importance at the present time is the incidental or deliberate reaction with furnace gases at annealing temperatures. The beneficial effect on durability was first studied and described by Keppeler [ 12,13], who patented a procedure in 1930. The underlying reaction was found to be: N a 2 0 + SO 2 + ½02 --, Na2SO 4
(1)
depleting Na from the glass. The oxidation of SO 2 needs a catalyst. Fortunately, water is a catalyst for both this reaction and for the migration of Na + to the glass surface. In the absence of SO2, an improvement of the surface can also be obtained by other processes of de-alkalization. Flame de-alkalization was first described by Williams et al. [14]. De-alkalization by base exchange on exposure to
334
N.J. Kreidl / Glass surfaces then and now
diluted acids, followed by heating to remove protons, has been considered in the case of optical glasses, Williams et al. [14] developed surface de-alkalization by exposure to dehydrated kaolin: Na ÷ (glass) + H + (clay) --, n + (glass) + S a + (clay), (500-600°C)
H ÷ (glass)
--,
HzO + de-alkalized glass surface.
(2)
(3)
Reaction (1) is much accelerated by water vapor.
5. Glass composition, surface enrichment and surface tension
5.1. Surface enrichment Components lowering surface tension were found to enrich in the surface [2,15]. This enrichment of the surface in a component lowering surface tension is the consequence of the requirement to minimize the total free energy of a multicomponent system [16,17]. The difference ( A X ) in concentration of a component will be [17]:
AX-
R1T ( 8 0 / 8 l n ' X ) r ,
where o is the surface energy; X is the mole fraction of the component; R is the gas constant; and T is the absolute temperature. In the idealized case of an ideal solution in a binary system and of a monolayer surface, Overbury et al. [18] obtained:
X~/X~ = x2b/x b exp(o I -- 02)a/RT, where: the X values are the concentrations of two components identified by indices 2 and l, in the surface and bulk, identified by superscripts s and b; o2, o t are the surface energies of X; and a -- d A / d n , in the chemical potential (pt) equation, # = OG/On - oSA/On, with A being the area and G being the energy. This approach only gives a description of the nature of the enrichment. For non-ideal solutions and multilayers, much more complicated relations have been worked out [18]. The relation of heat of vaporization or sublimation to surface-free energy has been postulated to be approximately 6 : 1 for a closed-packed metal with 12 nearest neighbors. This estimate is based on the simple assumption that in sublimation, six times more bonds have to be broken than in obtaining a new surface. In oxides, we have to deal with deeper interaction and relaxation into new equilibria at the surfaces involving surplus orbitals. This is why quantitative experimental evidence will be necessary to establish fundamentals for the case of glasses.
N.J. Kreidl / Glass surfaces then and now
335
5.2. Effect of some components The surface tension of conventional oxide glasses was found to be increased by A1203, MgO, and CaO [19,20] and to be decreased by K 2 0 [20], B203 [21], As203 [2,22], V205 [2,22], Cr203 [22], MoO 3 [22], PbO [23-28], F [29], I [30], SO 3 [31] and OH [32]. Assuming additivity within limited ranges, the relative effects of some oxides were shown as factors per 1% oxide in dyn c m - t ; e.g. according to Lyon [20]: AI203 MgO SiO2 N a 2 0 K 2 0 5.98 5.77 3.25 1.27 0 or to Dietzel [33] (900°C): A1203 PbO 6.2 1.2 " The strong effect of PbO is of particular interest; moreover, much PbO can be accommodated in oxide glasses. The data in table I show the influence of PbO concentration according to various authors (dyn c m - i). The tendency, at high PbO content, for an increase in surface tension with temperature will be discussed later. The very large decrease in surface tension by very small concentrations (0.01%) of (AsO4) 3-, (MOO4) 2- , (VO4) 2 and ( C r O 4 ) 2 - is explained by surface enrichment, invariably in the higher oxidation species [15,22]. Around 1950 this concept was used to control bifocal glass interfaces by MoO 3 additions much below 1%. In the case of boron, the strong effect is attributed to enrichment at the surface in the (BO3) 3- group [21]. Accordingly, the higher the alkali content, known to promote (BO4)5- groupings, the less the effect of boron in lowering surface tension [34]. In the case of SO 3, 1% decreases the surface tension of a conventional soda-lime glass from about 0.3 to about 0.25 N m - i [35-40]. At low tempera-
Table 1 The influence of PbO content according to various authors PbO (%) 35.6 40 44.6 56 70.7 89 89
800°C
900°C
209
211
160
164
1000°C
1200°C
1400°C
Ref.
234 220 213 202 184 142 140
236
235
[27] [28] [23-28] 1271 [23-25] [27] [28]
208 154 increase
336
N.J. Kreidl / Glass surfaces then and now
tures, immiscibility contributes to this effect. The phenomenon is important in melting practice because of improved wetting of residual sand grains by the initial flux, rich in sulfate. In silicate glasses, fluorine was found to decrease surface tension (at 800°C) and to enrich in the surface [29], but if NaF substitutes for NaPO 3 in phosphate glasses, surface tension increases [41]. Iodine (also bromine) in very small quantities decreases surface tension as it enriches in the surface [30]. These authors applied the phenomenon to the fining of optical glasses in Pt crucibles attributing the effect to the decreased wetting at the interface allowing gas bubbles to escape. It has been understood that the OH content, particularly by absorption, plays a large role [42]. At 21 mbar H20 vapor pressure, at 550°C, the surface tension dropped to 205 × 1 0 - 3 N m -I compared with 315 in vacuum. The decrease in adsorption at higher temperatures becomes manifest by a decrease in the influence of the atmosphere [31].
6. Effect of temperature on surface tension
Increased temperature will generally decrease surface tension; i.e. do/dT< 0. The decrease is of the order of 4 to 10 × 10-3N m -~, but when highly active components, such as PbO, B303 and MoO 3 have decreased surface tension by enrichment and orientation, increased temperature disturbs this ordering and do/dT> 0 [28,42,43].
7. Friction
The friction coefficient of clean glass was found to be exceptionally high [44], somewhat more so at low loads [45]. To obtain consistent data, a well-defined cleaning procedure (glow discharge) was soon recognized to be necessary. Friction against metals is particularly high if softness allows the metal to cling a n d / o r if oxide layers adhere to the clean glass surface [1]. This high friction enables the marking of glasses by metal [46]. It is not surprising that it became clear at an early date [44] that liquids and monomolecular films would greatly affect friction depending on their chemical reactivity with the glass. Nor is it surprising that the effect was different for different solids in contact with glass. Of great practical importance was, and still is, the treatment of glass surfaces with organics forming films several monolayers thick, e.g. silicone. This process is derived from the accidental discovery that glass surfaces in contact with methylchlorosilane vapors (during the synthesis of methylchlorosilanes) became water repellent. On this basis, the water-proofing of glass was patented [47]. The recognition of damage by friction as the chief cause of fracture in bottles led to the adoption of siliconing in the manufacturing process.
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8. Adhesion Similarly, adhesion has for a long time been understood to depend on the initiation of specific chemical reactions, such as the presence or absence of oxide films of metals in contact with glass. An early example of such studies is the adhesion of indium to glass [48]. The discussion in even modest detail of the adhesion of glass to metals, or polymers, would require an entire proceedings. In glass-polymer composites, a chief consideration of early workers was the low alkali content of the glass [49] and the use of coupling agents [50].
9. Polishing Finally, credit should be given to the early classical work on polished glass surfaces. Careful experimentation and unbiased interpretation led Beilby [51] to the conclusion that the polishing process is not just a refined abrasion, but involves some flow. This was elaborated upon in detail by French [52] who coined the term "Beilby Layer." A host of subsequent studies led to controversy over flow initiated just by heat and pressure, versus flow assisted by chemical reaction. The effect of a chemical reaction between the very small crystals of the polishing agent with surface anions and cations of the glass was propounded particularly by Kaller [53]. He and the chief proponents of a thermal-plastic mechanism [54,55] agreed later that the chemical mechanism has importance, though it is limited to the last stage of polishing. The situation has been treated extensively, and with a sense of history and much insight by Holland [ i ].
References [l] L. Holland, The properties of glass surfaces (J. Wiley, New York, 1964). [2] W. Weyl and E. Marboe, The constitution of glass, Vol. l (lnterscience, New York, 1964); E. Marboe, The constitution of glass Vol. 2 (Interscience, New York, 1967). [3] N. Adam, The physics and chemistry of surfaces, Third Ed. (Oxford University Press, London, 1941) quoting Raley (1890). [4] R. Shuttleworth and G. Bailey, Disc. Faraday Soc. 3 (1948) I I. [5] L. Sonders, D. Enright and W. Weyl, J. Appl. Phys. 21 0950) 338. [6] L. Briggs, J. Chem. Phys. 24 (1953) 488. [7] F. Bartell and E. Merril, J. Phys. Chem. 36 (1932) 1178. [8] F. Moser, The Glass Industry, (April, 1956): Am. Soc. Test Mat. Bull. 169 (1950) 62. [9] D. Day, ed., Glass surfaces (North-Holland, Amsterdam, 1975) p. 1. [10] Lord Rayleigh, Nature 86 (191 l) 416. [I l] W. Levengood, J. Am. Cer. Soc. 38 0955) 178. [12] G. Keppeler, Glastech. Ber. 5 0927) 97. [13] G. Keppeler, Sprechsaal 61 (1928) 300. [14] H. Williams and W. Weyl, Glass Ind. 26 (1945) 275; 324. [15] C. Amberg, PhD Dissertation, Department of Ceramics, Pennsylvania State University (1948).
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H. Defray, Surface tension and adsorption (J. Wiley, New York, 1966). R. Swalin, Thermodynamics of solids, 2nd Ed. (J. Wiley, New York, 1972). S. Overbury, P. Bertrand and G. Somorjai, Chem. Rev. 75 (1975) 547. A. Dietzel, Sprechsaal 27 (1942) 82. K. Lyon, J. Am. Cer. Soc. 27 (1944) 186. H. Scholze, Glas (Springer, Berlin, 1964) p. 261. C. Amberg, J. Am. Cer. Soc. 29 (1946) 81. A. Appen, Zh. Fiz, Khim. 26 (1952) 1339. A. Appen, Silikattech 5 (1956) 11. A. Appen, Adv. Glass Tech., Vol II, eds., Matson and Rindone (Plenum, New York, 1963). M. Hino, T. Ejima and M. Kameda, J. Jap. Inst. Met. 31 (1967) 113; tabulated in O. Mazurin, M. Streltsina, A. Shvaiko-Shavaikovshaya, Svoistva Stokl; Stekloobrazuyushtshkikh Rasplavov IV (Nauka Leningrad, 1980) p. 190. [27] L. Shartsis, S. Spinner and A. Smock, J. Am. Cer. Soc. 31 (1948) 23. [28] H. Ito, T. Yana Gose and Y. Suginohara, J. Mining Metall. Inst. Jap. 77 (1961) 895. [29] V. Budov, Uy. Kruchinin and F. Solinov, Glass and Ceramics 22 (1965) 660. [30] H. Hafner, G. Brewster and R. Weidel, US 2 702 749 (Feb. 22, 1955) abstr: Cer. Abst. (July, 1955) p. 122b). [31 ] A. Dietzel and E. Wegner, Mitteilungen Verb. deutscher Emailfachleute 2 (1954) 13. [32] G. Tamman and H. Rabe, Anorg. Allg. Chem. 162 (1927) 17. [33] A. Dietzel, Kolloid Z. 100 (1942) 368. [34] L. Shartsis and W. Capps, J. Am. Cer. Soc. 35 (1952) 169. [35] H. Jebsen-Marwedel, Kolloid Z. 137 (1954) 118. [36] H. Jebsen-Marwedel, Glastech, Ber. 29 (1956) 223. [37] H. Jebsen-Marwedel, Kolloid Z. 150 (1957) 137. [38] H. Jebsen-Marwedel, Glastech. Ber. 31 (1958) 431. [39] H. Jebsen-Marwedel, Naturwiss 45 (1958) 260. [40[ H. Jebsen-Marwedel, CR. Symp. Fusion du Verre Brussel (1958). [41] D. Williams, B. Bradbury and W. Maddocks, J. Soc. Glass Tech. 43 (1959) 308. [42] N. Parikh, J. Am. Cer. Soc. 41 (1958) 18. [43] H. Scholze, Glas Springer, Berlin (1977). [44] W. Hardy and J. Hardy, Phil. Mag. 38 (1919) 32. [45] R. Southwick, Preston Lab. Rep. no. 57-077 (1957). [46] L. Gehring and J. Turnbull, Bull. Am. Cer. Soc. 19 (1940) 290. [47] W. Patnode, US 2 306 222 (1940). [48] K. Rose, Sci. Am. 170 (I 944) 154. [49] J. Brossy, J. Case, A. Houze and A. Lasday, Soc. Plast. Ind., 12th Annual Meeting (1957) Sec. 16A. [50] J. Bjorksten, and L. Yaeger, Modem Plastic (1952) 29 124. [51] G. Beilby, Proc. Roy. Soc. A72 (1903) 218; 226. [52[ J. French, Opt. Sci. Instr. Maker 62 (1921) 253. [53] A. Kaller, Silikattechnik, 7 (1956) 380; Jenaer Jahrbuch, VEB., Carl Zeiss, Jena p. 181. [54] E. Brueche, A. Kaller and H. Poppa, Silikattech. 10 (1959) 213. [55] H. Poppa, J. Soc. Glass. Tech. 40 (1956) 513T.