Fracture phenomena and strength properties of chemically and physically strengthened glass

JOURNAL OF NON-CRYSTALLINESOLIDS 1 (1968) 49-68 © North-Holland Publishing Co., Amsterdam

F R A C T U R E P H E N O M E N A AND S T R E N G T H P R O P E R T I E S OF C H E M I C A L L Y AND P H Y S I C A L L Y S T R E N G T H E N E D GLASS I. GENERAL SURVEY OF STRENGTH AND FRACTURE BEHAVIOUR OF STRENGTHENED GLASS A. L. ZIJLSTRA and A. J. BURGGRAAF Development Centre of the Glass Division, Philips' Gloeilampenfabrieken, Eindhoven, The Netherlands

Received 28 June 1968 One of the methods most frequently used for reinforcing glass objects is to introduce compressive stresses at the surface. Such stresses can be produced physically, as in the thermal toughening process, or chemically, by applying a film of a lower coefficient of thermal expansion than the glass body itself, or by producing a compressive surface layer making use of the "ion stuffing" technique. The advantages and disadvantages of the different strengthening methods are discussed with relation to the observed strength values (measured either by quasi-static loading or by impact loading) and the fracture behaviour of reinforced glass objects.

1. Introduction In the last ten years there have been enormous developments in the strengthening of glass. In particular, chemical toughening methods based on ion exchange have shown that it is possible to give glass much higher values of mechanical strength than had hitherto been possible by the conventional thermal toughening methods. Encouraged by the spectacular successes achieved with the chemical strengthening methods, several investigators in recent years have made a closer study of the possibilities of further improving on the thermal toughening process. The strengthening achieved in this way is now of the same magnitude as obtained from ion-exchange methods, reaching values of more than 100 kg/mm 2. As with the ion-exchange methods, however, it remains questionable whether these improved thermal methods will prove to be a commercial proposition on a large scale within the not too distant future. Both strengthening systems have their advantages and disadvantages, and which one is used will depend to a large extent on the application of the strengthened glass. It seems likely that both techniques will find their own 49

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individual fields of application, as long as there remain distinct differences in the strength and fracture behaviour of glass objects, depending on the manner in which the reinforcement is effected.

2. Strength of unstrengthened glass It is nowadays generally assumed that the main reason for the relatively low strength of glass objects lies in the presence of numerous surface flaws which can act as stress concentrators when the object comes under load. The macroscopic stress at which the object breaks is only a small fraction of the microscopic stress occurring at the flaw tips, the latter stress constituting the direct cause of the fracture as soon as a critical breaking stress is reached. The stress-concentrating action depends closely on the form and dimensions of the surface flaws or cracks, and in normal glass objects may amount to at least a factor of 100 or more. This appears from the observed bending or tensile strengths, which are normally no more than 10 kg/mm 2, compared with the theoretical strength of glass, which is more than 1000 kg/mm 2. The latter value is approached by "virgin" glass fibres and is not so far removed from the strength of "virgin" or acid-etched solid glass (200 to 400 kg/mm2). For more details on the theoretical strength of glass and about the influence of surface flaws on the strength of glass, reference may be made to a number of review articles on the subject1-7). In this connection, however, we should not omit to mention two important practical aspects of the strength behaviour of glass objects, namely the influence of fatigue and the influence of the size of the loaded surface area. Because of these influences it is not possible to connect a particular type of surface damage directly with a particular value of the breaking strength of a glass object. 2.1. THE FATIGUEEFFECT The phenomenon of fatigue is generally ascribed to stress-induced corrosion. Under the influence more particularly of water vapour from the atmosphere the glass surface is attacked, but the rate of reaction in the case of a surface under tensile stress is greatest at the flaw tips. As a result, the centrating action continuously increases. The macroscopic stress remaining constant, the tensile stress at the flaw tip reaches at a given moment a critical value and fracture occurs. The dependence of the observed strength on the loading time (static fatigue) or as a function of the loading speed (dynamic fatigue) is partly governed by such parameters as the glass composition, the nature of the surface damage,

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temperature and relative humidity. In the case o f " u s u a l l y " damaged glass a loading time of one second under "normal" atmospheric conditions (20°C and 50~ relative humidity) can reduce the originally unfatigued strength by as much as 50~. The value of the strength without fatigue can be measured at loading times of 10 - 4 o r 10 - 5 second, and is often referred to as the impact strength. After very prolonged loading the strength may drop to about 20~ of the original value. 2.2. THE SIZE EFFECTIN FRACTURE A glass surface generally contains a large number of flaws or cracks of diverse shapes and dimensions, all of which therefore have different stressconcentrating actions. Under uniform surface loading it may be expected that the fracture will start at the most "dangerous" flaw. The larger the area under tensile stress, the greater the chance of increasingly dangerous flaws and the lower the observed strength. Where "usual" damage is concerned, the difference in the strength observed in a tensile test and a three-point bending test on identical rods may amount to a factor of two. In the case of strongly concentrated loads, as applied for example by pressing spherical objects against the glass surface, very high breaking strengths are observed, even with damaged glass. A glass plate that has a bending strength of 10 kg/mm 2 or less in a quasi-static bending test under normal room conditions, may under the same conditions have an indentation strength of more than 100 kg/mm 2 when a steel ball with a diameter of lmm is pressed into it. 2.3.

ASSESSMENT OF S T R E N G T H B E H A V I O U R IN P R A C T I C E

The practical assessment of the strength behaviour of glass objects on the basis of experience with simple laboratory tests is a speculative matter. For this reason many laboratory tests and quality tests have been adapted to existing practical situations. For fundamental research into strength behaviour, strength tests that can be interpreted in a not too complicated manner remain of course indispensable. It is therefore particularly regrettable that the numerous investigations into the strength behaviour of glass have been carried out by the most widely diverse methods, which makes a comparative analysis of the results virtually impossible. A limited number of standardized strength testing methods would be of great benefit to research in this field. It would also be necessary to standardize the preceding surface-damaging procedures coupled with these methods in order to obtain a defined surface condition. The maximum permissible loading of glass objects depends on the con-

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ditions of use. In the case of glass panes for windows, for example, it is the usual practice to reckon with a maximum permissible tensile stress of about 2 kg/mm 2 s). In constructions where the glass is under tensile stress for a very long time over a fairly large area the usual upper limit is put at no more than about 1 kg/mm 2; this is done, for example, in the case of (evacuated) television tubes, for many glass seals, etc. Allowance is of course made in this low permissible stress for a certain safety margin, but it is not excessively large, particularly not when account has to be taken of the considerable strength variations naturally present (standard deviations of 15 to 25~) and of the possibility of serious surface damage. The limiting strength (endurance limit) for soda-lime-silica glass with usual surface damage has been put at about 1.4 kg/mm 2 9). It is not likely that this figure will vary much for other glass compositions. If the measurement, calculation or prediction of static or quasi-static bending stresses in glass objects is already no simple matter in the ordinary course of events, the situation becomes even more complicated where the stresses are due to impact, i.e. to dropping, knocks, striking objects, etc. As a rule, the loaded surface area is small in these cases, so that locally very high contact stresses may be reached, giving rise to fracture. A fracture of this kind need not cause the object to break up, and might not even result in distinct cracks. Sometimes the damage remains limited to a small flaw, often scarcely visible to the naked eye, but which of course gives rise to a serious local weakening of the surface. The contact stresses can be calculated by means of the Hertzian equations10) provided the striking objects are spherical - which is seldom the case in practice - and provided the contact time is known. Impact causes not only local deformation but also a general deformation of the object, with the associated bending stresses. In the case of solid, thick-walled objects the bending is relatively slight and there is a considerable chance that the contact stress will be the primary cause of fracture. With relatively less thick-walled objects the kinetic energy of the striking object is largely converted into an elastic energy which deforms the object in such a way that the probability of fracture is primarily determined by the bending stress and to a much lesser extent by the contact stress. Fig. 1 gives a schematic survey of the breaking strengths observed on a soda-lime-silica glass in relation to some loading conditions. The surface condition is defined as "normal", that is to say the state as it exists some time after the usual treatment, where there are no serious flaws visible to the naked eye. Extra damage caused by sandblasting decreases the strength by an average of 30~, particularly where the loaded area is large, and scratching with very hard objects, such as diamond, causes a decrease of about 60~.

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kg/mm 2 300 l INDENTATION STRENGTH (STATIC) (INDENTER 0.5 m m in DIAMETER) lOOj 4 ~

INDENTATION STRENGTH (DYNAMIC) (INDENTER 10 rnm in DIAMETER)

605~ 40-

INDENTATION STRENGTH (STATIC) (INDENTER 10 rnm in DIAMETER)

20-

IMPACT BENDING STRENGTH ( 10-S sec.)

108-

SHORT-TERM STATIC BENDING STRENGTH

( 10 sec.)

64-

LONG-TERM STATIC BENDING STRENGTH ( 107 s e c . )

2-

Fig. 1. Schematic representation of breaking strengths of sheet glass in relation to the method of loading. 3. Possibilities of reinforcing glass Apart from a large number of publications on specific methods of strengthening, some useful review articles have appeared which give a more or less comprehensive picture of the possibilities of strengthening glass objects ll-la). It will be sufficient here, therefore, to mention briefly a few of the most wellknown strengthening methods, supplemented by various processes described in recent literature, and an attempt will be made to indicate the specific advantages and disadvantages of the individual methods. Methods designed to remove serious surface flaws, or to make them less dangerous, thereby strengthening the glass object, have been in common use for a very long time; examples are mechanical polishing, acid polishing, fire polishing, etc. In principle, very high strength values can be achieved if steps are taken to prevent the occurrence of serious surface flaws; for strong glass a procedure of this kind has found industrial application only in the manu-

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facture of fibre glass. The manufacture of more solid glass without surface flaws encounters serious practical difficulties. The combination of glass with other materials (composites) is another possible form of strengthening. Glass-ceramics may be mentioned as a practical realization of this, being a combination of glass phase with crystalline phases where the latter make a positive contribution to the strength behaviour by their high elasticity modulus and possibly also by the "stopping" of microcracks in the glass phase on the boundary of the crystals and by ductile flow of the crystals. Many glass-ceramic materials have a bending strength which is a factor of two or three higher than that of glass. Being primarily crystalline, however, glass-ceramics do not fall within the scope of this article. 3.1. THERMALTOUGHENING The most important method of strengthening glass is undoubtedly the introduction of a compression layer in the surface. The most familiar and widely used method is known as thermal or physical toughening or tempering, where the glass object is heated to a temperature not far below the softening point and then rapidly quenched. The temperature distribution (gradient) in the temperature range where the stress begins to build up in the core and the coefficient of thermal expansion of the material determine the relative difference in shrinkage to room temperature, and this shrinkage difference between the inner and outer zones indicates, after recovery of the temperature equilibrium, what the stress distribution will be. The principle of this method of strengthening glass was known as long ago as in the seventeenth century (the "Prince Rubert drop") but thermal toughening on a commercial scale has been known only for a few decades. A quantitative theory of the thermal toughening process is of fairly recent origin. Since its initiation by Bartenev a4) in 1949, a number of studies have appeared, from which more especially that by Gardon 15) gives an excellent insight into this subject. In the thermal toughening procedure the glass is cooled as a rule by forced convection using impinging air jets. It is a fairly simple matter to obtain a heat transfer coefficient of about 5 × 10-3 cal/cm 2 °C sec which, in a sheet of glass of 6 mm (¼") thick, gives a compressive stress at the surface of approx. 2500 m/~/cm or 8 to l0 kg/mm 2. Using cooling air it is more complicated to achieve heat transfer coefficients higher than this; more than l0 x l0 -3 cal/cm 2 °C sec is virtually unattainable, and, even if it is, the compressive stress built up in the surface layer of a 6 mm thick glass pane is no more than 15 kg/mm 2. As a rule the stress distribution is roughly parabolic, that is to say the

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maximum compression at the glass surface amounts, in absolute terms, to about twice the value of the maximum tensile stress in the central zone. The thickness of the compression layers is about 21~ of the sheet thickness.

3.1.1. Strength behaviour of thermally toughened glass Since the effective strength of a glass object whose surface is under compressive stress is equal to the "intrinsic" strength plus the value of the compressive stress, a surface compression of e.g. 8 kg/mm z causes an appreciable strengthening by a factor of two to three, as found for example in static or quasi-static loading tests on fiat glass panes under normal room conditions. Under an impact load the strengthening will show less result, and in the case of indentation the strengthening will have hardly any effect at all. Thermal toughening is widely employed for flat glass, such as for example the windscreens of motor vehicles, window panes for houses, offices, shop windows, showcases, glass doors, panels and so on, Apart from flat glass plates, slightly bent plates as used for car windscreens can also be strengthened in the same way. Objects of more complicated shape and/or unequal wall-thickness are less easy to strengthen homogeneously, on the one hand because it is then more difficult to apply a uniform heat transfer coefficient during the forced cooling over the whole surface area, and on the other hand because the built-up stress is dependent on the local wall-thickness. Glass objects of not unduly complicated shape, such as pans, beakers and other thick glass objects for household use, can, however, be given a strengthening from 4 to 8 kg/mm 2. Other objects suitable for strengthening are lenses, caps and shades for fittings, high-tension insulators, etc. In many of the applications mentioned the strengthening is intended not only to provide better resistance to mechanical loading, but above all to make the object better able to withstand temperature fluctuations (i.e. to give it greater thermal endurance). The improvement achieved in this connection is almost comparable with that for a quasi-static mechanical load, that is to say a toughening of 8 kg/mm 2 can increase the temperature shock which the object can withstand upon cooling up to a factor of three. In addition to glasses of the soda-lime-silica type, which have a relatively high thermal expansion coefficient (approx. 90x 10-7/°C), glasses with a lower expansion coefficient can also be thermally toughened. In their case, however, the degree of temper is less for the same heat transfer coefficient during cooling, the toughening being roughly inversely proportional to the thermal expansion coefficient of the glass. Glasses of the borosilicate type (thermal expansion coefficient about 40 X I0-7/°C) can be strengthened to a limited extent; for example, borosili-

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cate glass 6 mm thick can be toughened up to about 4 kg/mm z upon cooling with a heat transfer coefficient of 5 x 10 -3 cal/cm 2 °C sec. This is turned to advantage more particularly for improving the thermal endurance of this type of glass which, thanks to its low expansion, is then greater than that of toughened soda-lime-silica glass. Cooking vessels and frying pans of welltoughened borosilicate glass can be used straightaway on an open gas flame and subsequently rapidly cooled in cold water, without any danger of fracture. 3.1.2. Fracturebehaviour of thermally toughenedglass The fracture of a thermally toughened glass object is characterized, as is well known, by the occurrence of a large number of small, light fragments, each being more or less rectangular in shape and therefore less likely to cause serious cuts. Compared with non-toughened glass, which fractures into large, pointed and very sharp splinters which can be extremely dangerous, this constitutes a great advantage. In some applications it is an advantage that carries much more weight than the increased strength. In many countries, indeed, safety glass as used in motor vehicles is required to comply with certain standards, which lay down a minimum number of fragments per unit surface. The number of fragments per unit surface, called "fragment density", is greater the higher the degree of toughening. This may be qualitatively explained by assuming that a large part of the stored elastic energy is converted into surface energy of the newly formed surfaces. In a recent investigation by Akeyoshi and Kanai 16) the fragment density is determined as a function of the maximum central tensile stress (varying from 3 to 9 kg/mm 2) for various plate thicknesses (1.8 to 8.2 mm). In a very recent study Barsom17), on the basis of the experimentally determined force needed for splitting a fracture - the crack extension f o r c e established a relation between the maximum tensile stress in the central zone and the quotient of fragment weight and plate thickness. For plate thicknesses of 1/8 to 3/8 inch and tensile stress values from 5 to 10 kg/mm 2, the theory describes the experimentally found values very well. The requirements to be met by safety glass differ from one application and from one country to another, but usually a fragment density - i.e. the number of fragments per 25 cm z (square of 50 × 50 mm) - of at least 60 is required for plates 4 to 12 mm thick. To meet this requirement a maximum central tensile stress ~rcTof about 4½ kg/mm z is needed in a plate 6 mm thick. In some cases the permissible fragment density is 40 (corresponding to a Crcr of approx. 4 kg/mm 2) oi" even 10 (acT = approx. 3 kg/mmZ), whereas in other cases a minimum of 180 is laid down (acT=approx. 6 kg/mm2).

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3.1.3. Advantages and disadvantages of thermal toughening Using a gaseous cooling medium, such as compressed air, the thermal toughening procedure is relatively simple provided the shape of the glass object is such that the heat transfer coefficient is uniform over the whole surface, or at least over the part of the surface to be strengthened. The formation of a homogeneous stress calls for an object with little variation in wall-thickness. Since the initial temperature has to be high, there is a risk of the object suffering viscous deformation. To minimize this risk the object must be heated as quickly as possible to the desired initial temperature of the toughening process. The toughening itself takes up very little time; the stress formation begins after two or three seconds and the important temperature range is traversed after about fifteen seconds. The whole process need take no longer than about five minutes. The strengthened object shows a distinct improvement in mechanical strength under static and quasi-static load over a relatively large surface area, but no considerable improvement under static or impact loading on a concentrated surface area. Nevertheless, the thermally toughened glass offers reasonable resistance to blows, knocks, etc. because any damage that occurs, such as cracking or checking, need not lead to the destruction of the object as long as the damage remains confined to the compressive stress zone. The compression layer is relatively thick and therefore offers fairly high resistance to bruising. If the damage penetrates to the central tensile stress zone, the object breaks up into a large number of fragments. This means that it is not possible to subject a thermally toughened object to subsequent operations such as sawing, cutting or drilling. The damage which this causes in a tensile stress zone of approx. 3 kg/mm z or more - the minimum value present in the central zone of a thermally toughened object - usually causes immediate and total destruction. 3.1.4. New developments in thermal toughening In recent years there have been some new developments in thermal toughening, especially aimed at higher heat transfer coefficients with a view to obtaining a greater degree of temper or to giving a reasonable degree of strengthening to thin-walled glass objects. Akeyoshi and Kanai 16) describe the thermal toughening of thin plates of window glass by means of a solid contact method with watercooled metal plates separated from the glass to be toughened by glass fibre cloth. Compressive stresses up to about l 5 kg/mm 2 were produced in this way in 2 mm

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thin glass plates. The toughening can be controlled by varying the contact pressure. Similar processes have been described in various patentslS-21). Liquids have been used as a cooling medium for some considerable time. A frequent drawback of the mineral oil often used in the past was that the high viscosity of the fluid hampered the removal of heat. Wartenberg22) describes a method of thermal toughening in organic fluids, heated to a little below their boiling point, in which glasses with a low coefficient of thermal expansion (about 30 x 10-7/°C) have to be treated in a liquid containing OH groups and possessing a heat of evaporation of less than 200 cal/g, whereas for glasses with a high expansion coefficient (about 100 × 10-7/°C) a heat of evaporation of no more than 150 cal/g is permitted. After immersion of the hot glass in the cooling liquid a gas film is formed whose life depends on the liquid used and which prevents initially too rapid cooling, involving the formation of tensile stress and the risk of fracture. For moderate toughening (the formation of coarse fragments upon fracture) the use of e.g. carbon tetrachloride (CC14) is recommended, for greater toughening (fine fragmentation upon fracture) propanol (C3HTOH) or pentanol (CsHllOH). In Russia a marked preference has been shown for some time for silicones as the cooling medium. Apart from being a very suitable heat-transferring fluid, resulting in relatively high degrees of toughening, silicone oil is also said to have other beneficial effects on the glass surface, improving the strength behaviour2a). Thin window glass (3 mm thick) can be strengthened in this way to 30 kg/mm 2. A poly-organosiloxane liquid 24) has proved to be a good cooling medium. The Russian investigations have shown that the degree of strengthening in thermal toughening is not only attributable to the magnitude of the compressive stress at the surface but also to a structural change at the glass surface. This structural contribution is noticeable only after a thin surface layer has been etched away (e.g. with HF), when it appears that the contribution of the etched strength of a thermally toughened glass is much greater than that of an annealed glass etched in the same way. Particularly in the case of thin-walled glass this extra contribution from the structure can be very considerable, thanks to the rapid cooling. The combined effect of thermal toughening and acid etching is sometimes referred to as "thermophysical hardening" 25, 26). Although the combination of thermal toughening and etching can give very high strength values, the method does not seem to be particularly successful in practical applications since the increase of strength gained by etching is destroyed by subsequent damage, so that only the contribution from the compressive stress remains. The search therefore continues for media that possess a high cooling capacity. Simply raising the cooling capacity of the immersion fluid may cause serious

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difficulties because there would then be a risk of glass objects breaking during the toughening process. The cooling capacity of various eligible cooling liquids, however, is highly temperature-dependent. Use can be made of this property by choosing a cooling medium which is matched to the type of glass under treatment ~7). The use of organic liquids, however, also has disadvantages, as for example non-uniform cooling, deformation of the surface, the control of the bath temperature, etc. These drawbacks are said to be less serious in a recently introduced toughening method 28,29) using molten metals, which moreover results in an even higher degree of toughening. Very thin-walled window glass (1.3 mm) quenched in molten Wood's metal or molten tin can be given a bending strength of up to 70 kg/mm 2 or more. It seems more likely, however, that more solid glass objects would run a serious risk of breaking if quenched in a bath of molten metal. 3.2.

CHEMICAL STRENGTHENING METHODS

Whereas physical toughening is based on the different shrinkage of the outside and inside of the glass in the elastic range as a result of the application of temperature gradients, chemical toughening is based on the introduction of a compressive stress in the surface by applying to the surface a layer of a composition different from the underlying bulk glass. This can be done by two basically different techniques: (a) The application, at temperatures above the strain point, of a surface layer with a lower coefficient of thermal expansion than the bulk glass. (b) The introduction of larger (alkali) ions to replace smaller ones at temperatures below the strain point. The latter process is sometimes referred to as "crowding" or "ion stuffing". The first principle (a) can again be subdivided into the following three important methods: (a. I) Glass having a high thermal expansion is coated with glass having a low expansion coefficient. Schott's "compound glass" (1891) 3°) may be seen as a first example of this. A particular limitation of this method is the lack of appropriate lowmelting enamels with a sufficiently low thermal expansion. This is much less of a drawback when the material to be strengthened is not glass but a ceramic or glass ceramic, which have a much higher softening point than most glasses. Certain types of glass ceramic for household utensils are nowadays strengthened in this way, bringing the quasi-static bending strength to more than 25 kg/mm 2 12). The compressive stress built up is about 5 kg/mm 2 at a difference of expansion of 10× 10-7/°C. It is thus possible in principle to achieve fairly high strengthening with this coating,

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but an excessive difference in thermal expansion leads to very sharp stress transitions from the compression layer to the bulk glass, with the danger that coating will peel off. In many cases, moreover, the enamel coating causes undesirable optical errors, etc. This technique has therefore found only limited application for the strengthening of glass. (a. 2) The application of a surface layer of low thermal expansion by means of an ion-exchange process was introduced about ten years ago by Hood and Stookey 31). A soda-lime-silica glass treated in a lithium-containing bath can be strengthened from 15 to 20 kg/mm 2 by the exchange Li + for Na + at high temperatures. The process has not found commercial application. It has also proved possible to treat glass of the window-glass type at temperatures above the strain point in cupro-halide vapour, the copper ions being exchanged for sodium ions from the glass in such a way that no reduction takes place to metallic copper. Strengthening up to 50 kg/mm 2 is said to be possible in this way with treatment times from 15 to 60 mina2). The thickness of the compression layer varies from 40 to 500 p. Ion exchange can also take place at very high temperatures, for example during the hot working of the glass. Use can be made at the same time of electric fields3a). The strengthening of flat glass is described by a process in which an electric potential is applied between an electrode in the glass bath and the atmosphere above it. The atmosphere above the glass bath contains ionized particles, for example of LiC1. As a result of the electric field, Li + diffuses in the glass immediately before and during the drawing of the glass, while the Na + ions of the window glass penetrate deeper into the glass. Compressive stresses from 5 to 6 kg/mm 2 have been achieved in this way. It is possible to reach 7 to 8 kg/mm 2 by the use of magnesium salt. This type of ion exchange might be used not only for strengthening purposes but also for other refinements of the glass surface, to improve chemical resistance or optical properties. (a. 3) A layer with a low thermal expansion coefficient can also be formed by surface crystallization. The crystalline phase consists usually of flspodumene or/~-eucriptite, which has a very low or even negative thermal expansion. This technique was described by Olcott and Stookey a4) in 1962. Suitable glasses of the lithium-alumino-silicate type can be strengthened in this manner to about 70 kg/mm 2. Provided the surface layers are not too thick (e.g. no more than 100 #) the crystalline phase is not very disturbing because the birefringence is weak and the effective refractive index is well matched to that of the glass. In another ion exchange process a soda-alumino-silica glas containing TiO2 is subjected in a salt bath to the exchange Li + for Na + resulting in simultaneous crystallization as).

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Cornelissen et al. 36) describe a strengthening process by surface crystallization of a Na20-Li20-A1203-SiO 2 glass in which, by the simultaneous exchange of Li + for Na + and diffusion of Ag ions from the salt bath, an intermediate layer of lithium-metasilicate with a high expansion coefficient is formed between the bulk glass and the surface layer of fl-eucriptite. Surface crystallization processes for strengthening glass objects do not find much practical application, partly because the treatment has to take place at temperatures above the strain point, thus involving the risk of viscous deformation, and because fairly long treatment times are needed in order to obtain a sufficiently thick crystallized layer, which may moreover have unwanted properties. An abrupt transition from crystalline phase to glass phase is associated with sharp stress transitions, which can again lead to peeling. (b) The technique mentioned under (b) was first described by Kistler aT) in 1962. This investigator treated a soda-lime-silica glass in a KNO3 melt at temperatures below the strain point (e.g. 350 °C) and found after some considerable time a substantial compressive stress up to 90 kg/mm 2. The compression zone was limited, however, to a very thin layer, and therefore, because of the surface flaws penetrating through this layer, there was scarcely any observable strengthening. The ion exchange K + for Na + reported by Kistler was used by Nordberg et al aS) for strengthening alkali-aluminosilicate glass; values up to 70 kg/mm 2 were obtained as a result of the buildup within a fairly short time of a compression layer sufficiently thick to withstand "usual" damage without loss of strength. Among the chemical strengthening methods this ion stuffing technique is the most important method of reinforcing glass in a not too complicated way, at temperatures at which there is no risk of viscous deformation and within a reasonable time. It produces a quasi-static bending strength three to six times greater than that of glass strengthened by conventional thermal toughening. It is therefore not surprising that the greater part of the literature since 1962 on chemical methods of strengthening glass has been devoted to the ion stuffing technique. Originally, a great deal of research was aimed at finding the most favourable combination of ion exchange treatment and type of glass, with a view to building up the highest possible compressive stress in the shortest possible time. Burggraaf39, 4o) has shown that the glass obtained by ion exchange has a considerably greater density than glass of the same composition obtained by a normal melting procedure. As a result the observed stresses are lower than might be expected, for example only about 4 0 ~ of the calculated value in the case of K + for Na + exchange. The best results have been achieved with alkali-alumino-silicate glasses. The diffusion rates in these glasses are relatively high, the Na + for Li +

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exchange taking place even faster than the K + for Na + exchange. The highest degree of strengthening is obtained when the ratio MezO/A120 3 is close to unity, i.e. up to 100 kg/mm z for the K ÷ for Na ÷ exchange. In addition to glass containing A120 3, types of glass containing ZrO2 can also be reinforced quickly and substantially by this method41). Alkali-zinc-alumino-silicate glasses have also been reported 42) as being particularly suitable for ion exchanges such as Na + for Li ÷ or K ÷ for Li + or K ÷ for Na ÷. The exchange process is accelerated when the basic glass already contains the various types of alkali ions 41). The presence of a few per cent of P205 in the glass is also said 4z) to have an accelerating effect, preventing an attack of the glass surface and offering better chemical durability. The durability of glass subjected to an ion exchange K + for Na ÷ or K + for Li ÷ can also be improved by a subsequent treatment resulting in a very thin second layer (1 to 2 microns thick) where K + is replaced by e.g. H ÷ 44). The Ag ÷ for Li + exchange is faster than Na + for Li + in Li20-Na20A1203-SiO 2 glass. The Ag ÷ for Na ÷ exchange can also be applied to a sodalime-silica glass45). In this case, however, there is a risk of discolouration. There have been proposals 46, 47) to accelerate ion exchange processes by the use of electric fields. It is claimed that this would be particularly useful for building up within a reasonable time a sufficiently thick compression layer, e.g. with a K + for Na + exchange, in types of glass that are difficult to reinforce, e.g. soda-lime-silica glass. In recent years there have been many descriptions in the (patent) literature of strengthening methods in which different processes are combined, for example multiple ion exchanges or combinations of ion exchange and thermal toughening or of ion exchange and acid-etching. The aim here is to combine a number of favourable properties from each method so as to couple a high degree of strengthening with good resistance to specific types of damage and/or favourable fracture behaviour, better thermal endurance, and so on. Roy, Stacey and Webster 4s, 49), for example, describe a treatment of glass in a bath containing an alkali-borofluoride such as NaBF 4 or KBF 4 as etchant, dissolved in molten KNO3 with some AgNO3 as catalyst,for, to avoid discolouration, some Cu/O or Cu2C125°). A conventional etchant like H F is not suitable for the purpose, since a glass etched with H F [ a t temperatures above 150°C loses much of its strength due to destruction~of the glass surface. Etching with anhydrous etchants gives a glass surface better able to withstand the high temperature at which the ion exchange takes place, particularly when the surface is first baked out in vacuo51). It has proved possible in this way to give soda-lime-silica glass, in a slightly

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damaged state, strength values from 50 to 90 kg/mm 2 after 15 to 60 minutes treatment at temperatures between 350 ° and 450 °C. The procedure described above can also be combined with thermal toughening52), by quenching the glass from temperatures just below the softening point in the molten salt bath with the etchant. Given suitable dimensions, the object will then combine the favourable fracture behaviour of thermally toughened glass with the high strength of glass treated by the etching and ion exchange processes. Other patents 5~,5~) describe also the combination of thermal tempering and ion exchange. One of the results claimed is that delayed breakage due to the penetration of bruise checks into the tensile stress zone is much less likely to occur, because the thermal tempering gives a thicker compression layer which offers better protection against bruising. The impact strength (determined with a ball impact test) is claimed 55) to be substantially improved by means of two successive ion exchange treatments on alkali-alumino-silicate glasses. In the first treatment Li + from the glass is replaced in a relatively thick layer (200 to 300/t) by Na + ions from the salt melt, producing in this layer a relatively low compressive stress. In the second treatment, possibly carried out at the same time as the first, the Li + ions still present, together with the Na + ions introduced, are replaced by K + ions in a relatively thin layer (10 to 20 p), which then acquires a high compressive stress. The second treatment is said to have no effect on the static breaking strength. Certain types of glass, in particular soda-lime-silica glass, can be strengthened 56) by a double or even treble exchange while remaining readily workable (capable of being cut without splintering). The stress profile typical of this process is said to be W-shaped, successive layers being produced with a relatively high compressive stress at the surface (e.g. 20 kg/mm2), an intermediate layer with a relatively high tensile stress (e.g. 5 kg/mm2), both of roughly the same thickness (about 40 to 50 p), followed by a normal tensile-stress zone in the core of the glass with a low tensile stress level. Another example 57) of a double ion exchange treatment concerns the strengthening of a sodium-alumino-silicate glass, beginning with an Li + for Na + exchange above the strain point and followed by an Na + for Li + exchange below the strain point. The presence of P205 in the glass is recommended to prevent the occurrence of surface attack during the Li + exchange. The advantage of such a treatment is said to be that the strengthening (approx. 40 kg/mm 2) is not quickly lost when the glass is heated, as it is in the case of glass given a single ion exchange below the strain point. It is claimed that glass treated in this way can be kept at temperatures of up to 500 °C for long periods without any significant loss of strength.

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In the opinion of the present authors, the most important trend in the literature quoted here is the use of multiple ion exchanges to obtain special diffusion profiles that give the glass specific, required properties. In addition, the use of etchant salt baths also seems to offer interesting prospects, particularly for strengthening ordinary soda-lime-silica glasses. It is difficult to avoid the impression that the special effects obtained by various methods reported in the literature are of a marginal nature. 3.2.1. Strength behaviour of chemically strengthened glass. Chemically strengthened glass can be given breaking strengths ranging from e.g. 50 kg/mm z to as high as 100 kg/mm 2. With conventional thermal toughening methods such values are not possible, and can only be achieved in certain cases by quenching in liquids. As in the case of thermally toughened glass, here too the observed strength is roughly equal to the sum of the intrinsic strength and the magnitude of the compressive stress in the surface. Nevertheless, there is an important difference, which is that the compression layer in thermally toughened glass is relatively thick, whereas in chemically toughened glass it is thin, varying from about 50 microns to usually no more than about 300 microns. The flaws "naturally" present in the damaged glass surface penetrate 5 to 10 # into the glass, which means that the compressive stress in the outer surface skin does not contribute to the strength. If the damage is more severe, as may be caused by scratches, etc., the resultant cracks may penetrate even deeper into the interior. In order for chemically strengthened glass to be reasonably resistant to such damage, it has been established that the total thickness of the compression layer should be at least about 50 # if sufficient benefit is to be derived from the compressive stress. A compression layer thicker than this is preferable, for example up to 100/t. Toughening values of 50 kg/mm 2 and higher result in a very substantial improvement of the static and quasi-static strength under loads over a relatively large surface area; the improvement may be a factor of 10 or more. This puts chemically strengthened glass into the category of very strong materials, comparable for instance with steel. Although further developments, and particularly applications research, are still in progress, chemically strengthened glass has already found various applications, especially where it has hitherto not proved possible to apply thermal toughening with any success, e.g. in very thin-walled objects or objects of complicated shape. Chemically strengthened glass has not yet, however, come into such wide use as might be supposed. One important reason for this is undoubtedly its limited resistance to the high stresses produced by a load concentrated on a

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small area. Although the chemically strengthened glass surface is slightly better resistant to concentrated loads than thermally toughened glass, in the sense that in the event of dropping, knocking, bruising etc., the stress required for a Hertzian fracture is appreciably higher, e.g. by a factor of two, once fracture does occur the risk that the damage will penetrate through the thin compression layer into the tensile stress zone is considerable. In spite of the high compressive stress, the thin compression layer offers only very limited protection against bruising. Therefore it would be necessary, while maintaining the present brittle fracture nature of glass, to introduce strengthening in the form of a surface compressive stress as high as 500 to 1000 kg/mm 2, which has not yet proved possible. 3.2.2. Fracture behaviour of chemically strengthened glass Another important reason why chemically strengthened glass still finds only limited application, is its totally different fracture behaviour as compared with that of thermally toughened glass. The thin compression layer in the chemically strengthened glass is compensated by a thick tensile stress zone, in which the average value of the tensile stress is low. If, for example, a 6 mm thick glass plate has compression layers on both sides of 75 p with a maximum compressive stress at the surface of 60 kg/mm 2 and an average compressive stress over the total layer of 30 kg/mm 1, the average value of the tensile stress in the central zone of 5.85 mm is about 0.8 kg/mm z. In the event of breakage this results in a small number of fragments which are fairly heavy, and moreover sharper, more pointed and therefore much more dangerous than the fragments of thermally toughened glass. Although there are various means of influencing the average value of the tensile stress in the central zone, there is not very much latitude in this respect and limits are set to the thinness of the glass. The average tensile stress in the central zone therefore fails in many cases to reach the value required in safety codes in order to comply with the specified minimum fragment density. In chemical strengthening, however, there are more possibilities of influencing the shape of the stress profile than in thermal toughening, and these can be utilized for improving the fracture behaviour. It has admittedly not proved possible to improve the fragment density for a given tensile stress in the central zone, but by choosing a suitable stress profile the nature of the fracture can be modified in such a way that the resultant fragments are far less pointed, acquire blunt edges and are therefore much less dangerous. Consequently, chemically strengthened glass in which the central tensile stresses have relatively low values can be used without running a serious

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risk of cut wounds in the event of fracture. This subject is dealt with in more detail in Part II. 3.2.3. Advantages and disadvantages of chemical strengthening methods The high degrees of strengthening achieved by ion stuffing seemed at first to make this method particularly suitable for quality improvements on a wide scale. Because the treatment is at temperatures below the strain point, there is no risk of viscous deformation. There are few restrictions with regard to the wall thickness of the object. These very favourable aspects as compared with the thermal toughening method are offset, however, by a number of drawbacks. These include the use of often aggressive salt melts, the problem of maintaining the activity of the salt baths, the fact that the exchange treatment is quite a lot longer than the thermal toughening process, the necessity of cleaning the object after the ion exchange treatment, the introduction of a surface layer of different composition with the risk of different properties, as for example a less favourable chemical durability, and above all the need to use special and expensive types of glass, preferably containing lithium, in order to achieve optimum results. Chemically strengthened glass objects remain susceptible to bruise damage but in such an event, unlike the case with thermally toughened glass, the result need not be the total destruction of the object provided the tensile stress at the central zone does not exceed a value of 1.5 to 2 kg/mm2; as a rule, however, this will not be the case. The low tensile stress level of less than about 1 kg/mm 2 often present in chemically toughened glass permits a subsequent operation such as cutting, sawing, drilling, etc., without the risk of fracture. The relatively large, dangerous fragments upon fracture, however, may often be a serious drawback or impermissible. Both chemically and thermally toughened glass find limited application at higher temperatures. Viscous stress relaxation rules out the use of thermally toughened window glass above about 300 °C for periods longer than about 100 hours without a pronounced loss of strength 58). In the case of chemically strengthened glass, moreover, ionic redistribution takes place, resulting in a marked decrease of strength at temperatures from about 200 °C 59.6o), unless a special double ion exchange treatment is employed~7).

References 1) O. L. Anderson, Fracture, Proceedings o f a Conference on the Atomic Mechanisms o f Fracture, Swampscott, Massachusetts, April 12-16, 1959 (Technology Press and Wiley, New York, 1959) p. 331. 2) R. J. Charles, Progress in Ceramic Science, Ed. J. E. Burke, Vol. 1 (Pergamon Press, New York, 1961) p. 1.

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3) F. M. Ernsberger, Progress in Ceramic Science, J. E. Burke, Vol. 3 (The MacMillan Co., New York, 1963) p. 57. 4) W. B. Hillig, Modern Aspects of the Vitreous State, Ed. J. D. Mackenzie, Vol. 2 (Butterworths, Washington, 1962) p. 152. 5) C. J. Phillips, American Scientist 53 (1965) 20. 6) B. Sugarman, J. Materials Science 2 (1967) 275. 7) A. L. Zijlstra, General Survey prepared for the Symposium on the Mechanical Strength and Ways of Improving it, Florence, Sept. 25-29, 1961 (Union Scientifique Continentale du Verre, Charleroi, 1962). 8) G. L'Eglise and J. P. Collet, Verres et R6fract. 21 (1967) 295. 9) R. E. Mould, Glastechn. Ber. Sonderband 32K (1959) 111/18. 10) H. Hertz, Ges. Werke, Vol. 1 (Leipzig, 1895) p. 155. I1) F. M. Ernsberger, Glass Industry 47 (1966) 422, 481,542. 12) S. D. Stookey, High Strength Materials, Ed. V. F. Zackay (J. Wiley, London, 1965) p. 669. 13) B. Sugarman, Advances in Materials, Proceedings of a Symposium organised by the Institution of Chemical Engineering, Manchester, April 6-9, 1964 (Pergamon Press, London, 1966) p. 203. 14) G. M. Bartenev, J. Tech. Phys. (USSR) 19 (1949) 1423. 15) R. Gardon, Proceedings of the Seventh International Congress on Glass, Bruxelles, June 28-July 3, 1965 (International Commission on Glass) Paper No. 79. 16) K. Akeyoshi and E. Kanai, Proceedings of the Seventh International Congress on Glass, Bruxelles, June 28-July 3, 1965 (International Commission on Glass) Paper No. 80. 17) J. M. Barsom, J. Am. Ceram. Soc. 51 (1968) 75. 18) Japanese Patent, No. 315,298. 19) U.S.A. Patent, No. 1,895,548. 20) French Patent, No. 1,459,549. 21) Dutch Patent, No. 123,042. 22) German Patent, No. 1,182,782. 23) Russian Patent, No. 132,374. 24) S. I. Silvestrovich and I. A. Boguslavskii, Glass and Ceramics 17 (1960) 7. 25) F. F. Vitman, I. A. Boguslavskii and V. P. Pukh, Soviet Physics, Solid State 4 (1963) 1582. 26) I. A. Boguslavskii, Glass and Ceramics 21 (1964) 561. 27) I. A. Boguslavskii and O. I. Pukhlik, Glass and Ceramics 24 (1967) 1. 28) 1. I. Kitaigorodskii, S. I. Silvestrovich and V. M. Firsov, Soviet Physics - Doklady 9 (1965) 819. 29) S. I. Silvestrovich and V. M. Firsov, Glass and Ceramics 23 (1966) 6. 30) German Patent, No. 61,573. 31) U.S.A. Patent, No. 2,779,136. 32) Patent applied for in The Netherlands (application No. 6,704,745). 33) Patent applied for in The Netherlands (application No. 6,606,088). 34) J. S. Olcott and S. D. Stookey, Advances in Glass Technology, Technical Papers of the Sixth International Congress on Glass, Washington, July 8-14, 1962 (Plenum Press, New York, 1962) p. 400. 35) H. M. Garfinkel, D. L. Rothermel and S. D. Stookey, Advances in Glass Technology, Technical Papers of the Sixth International Congress on Glass, Washington, July 8-14, 1962 (Plenum Press, New York, 1962) p. 404. 36) J. Cornelissen, G. Piesslinger and A. M. M. de Rijk, Proceedings of the Symposium on the Surface of Glass and its Modern Treatments, Luxemburg, June 6-9, 1967 (Union Scientifique Continentale du Verre, Charleroi, 1967) p. 145, 37) S. S. Kistler, J. Am. Ceram. Soc. 45 (1962) 59. 38) M. E. Nordberg, E. L. Mochel, H. M. Garfinkel and J. S. Olcott, J. Am. Ceram. Soc, 47 (1964) 215.

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39) A. J. Burggraaf, Ph.D. Thesis, Technical University of Eindhoven, 1965 (Published in Philips Res. Rep., Suppl. 3 (1966)). 40) A. J. Burggraaf, Phys. Chem. Glasses 7 (1966) 169. 41) French Patent, No. 1,480,567. 42) British Patent, No. 1,018,890. 43) British Patent, No. 1,071,351. 44) British Patent, No. 1,048,580. 45) French Patent, No. 1,472,668. 46) French Patent, No. 1,437,672. 47) U.S.A. Patent. No. 3,218,220. 48) N. H. Ray, M. H. Stacey, and S. J. Webster, Phys. Chem. Glasses 8 (1967) 30. 49) British Patent No. 1,011,638. 50) British Patent No. 1,014,247. 51) British Patent No. 1,082,064. 52) British Patent No. 1,055,126. 53) British Patent No. 1,026,770. 54) British Patent No. 1,012,367. 55) British Patent No. 1,076,894. 56) British Patent No. 1,076,603. 57) British Patent No. 1,096,356. 58) M. J. Kerper and T. G. Scuderi, Bull. Am. Ceram. Soc. 42 (1963) 735. 59) M. J. Kerper and T. G. Scuderi, J. Am. Ceram. Soc. 49 (1966) 613. 60) H. M. Gartinkel, Proceedings of the Symposium on the Surface of Glass and its Modern Treatments, Luxemburg, June 6-9, 1967 (Union Scientifique Continentale du Verre, Charleroi, 1967) p. 166.