IL32977A - Process and apparatus for strengthening chemically tempered bodies made of glass,vitrocrystalline material,ceramic or rock - Google Patents

Description

PROCESS AND APPARATUS FOR STRENGTHENING CHEMICALLY TEMPERED BODIES MADE OF GLASS , VITROCRYSTALLINE MATERIAL. CERAMIC OR ROCK imn «ΙΓ»3·>3Τ o* ifflg o^fflTa^ l»taa>- ignm ywrt The present invention relates to a process for treating chemically tempered bodies made of glas&, vitrocr stalline material,, ceramic or rock, and to apparatus for use in the performance of such process.

It is known that the tensile strength of an article made of glass, vitrocrystalline material, ceramic or rock is usually lower than that which the article should, theoretically possess having regard to its cross-sectional dimensions.

For instance, the actual tensile strength of a vitreous article of conventional composition, such as a soda-lime silicate, is approximately 6 kg/mm . The strength which the same article should possess according to theoretical calculation is several hundred times higher than that actual value.

It is known that glass oan be strengthened by causing substance to enter the glass from a contacting medium and appropriately controlling the temperature during and after such entry so that exterior layers of the glass are placed in a state of compressive stress or of increased compressive stress. Such a process of setting up or increasing surface compressive stresses is known as a chemical tempering process.

Usually, chemical tempering involves the replacement of ions in the glass by ions deriving from the contacting treatment medium. It is possible for example to set up compressive stresses by substituting ions in exterior layers of the glass by ions which confer a lower coefficient of thermal expansion on such exterior layers, at a temperature which is sufficiently high and maintained for a time sufficiently long for stress relaxation to occur, and subsequently allowing the glass to cool.

Alternatively, surface compressive stresses can be set up by substituting ions in exterior layers of the glass "by larger ions while the surface layers of the glass are at an elevated temperature which however is too low to allow substantial, or at least not a complete, stress relaxation, in the length of time for which such temperature is maintained, the glass being then cooled to normal temperature so that the induced stresses are "frozen* in. Furthermore it is possible to introduce surface compressive stresses by causing ions to enter into the glass through a surface thereof from a contacting medium under the influence of an electric field, without accompanying equal movement of ions from the glass into the contacting medium through that surface.

A chemical tempering process can be applied not only to a body of glass but also to a body of vit ocrys talline material. As applied to such a body, the tempering treatment gives the best results when the phase of the vit ocrys alline material which is susceptible to the greater or greatest amount of diffusion of substance from the contacting medium is well dispersed throughout the surface of the body. A chemical tempering process can also be applied to a body of ceramic or rock provided that there is sufficiently uniform distribution of a vitreous phase or phases at the surface of the material to permit surface compressive stresses to be set up or increased by tho introduction or exchange of substances as referred to.

From British Patent IOII638 it has been known to treat glass articles by means of an etching agent which is dissolved in a bath of molten salt so as to remove a thin and uniform layer of glass and obtain a faultless surface.

This treatment may be performed in a single stage wherein the bath of salt contains also cations adapted to be diffused in the glass in exchange for cations already present in the uppermost layers of glass or the treatment may be performed in two stages, in the first instance employing a bath of salt which contains an etching agent and does not contain exchangeable ions, whereupon in a second step the glass article is submitted to a treatment in a bath of dissolved salt wherein exchangeable ions are contained.

Possibly the etching agent may be accompanied by a catalyst and the operations may be performed at elevated tempera ures.

T e invention provides a simple process whereby a property or properties of a body of glass # vi rocrys talline material, ceramic or rock which has been chemically tempered can be modified.

According to the present invention, this modification is brought about by subjecting the chemically tempered body to an after treatment wherein a surface stratum is removed from the material under compressive stress.

Since the strengthening of a body achieved by a chemical tempering is due to external layers of the body being in a state of compressive stress, it was not to be expected that partial removal of such layers would be beneficial; on the contrary one would have expected such removal seriously to weaken the body.

In fact, following the surface removal, the body, e.g. a sheet of glass, is more durable. In general the body even shows greater resistance to breakage. The effect of the surface removal is therefore surprising.

The body treated in accordance with the invention may constitute a manufactured article. Alternatively the body may constitute part of an article, e.g., the body may be a coating on a substrate. In the latter case the compressive stresses due to chemical tempering may be confined to external layers at the exposed surface of the body.

The invention is particularly but by no means exclusively intended to be applied in the treatment of glass in sheet form. The invention can be applied to a sheet which has been chemically tempered over its whole area or to a sheet only a certain part or only certain parts of which has or have been chemically tempered, e.g. , the edges or marginal portions of the sheet where breakage can often commence. Even if the whole sheet has been chemically tempered, the treatment according to the invention whereby a surface stratum is removed from the material under compressive stress may be confined to a part or parts of the sheet, e.g., to edges or marginal portions thereof.

It has been found that it is advantageous for the ultimate strength of the body to remove a top stratum of between 0.1 and microns in thickness from the or each outer layer which is under compressive stress. Below this range the efficacy of the treatment is not so marked and above the said range, the increase in strength progressively diminishes as the thickness removed increases.

The removal of the surface stratum is preferably performed by dissolving the stratum with a fluorine-containing medium. Fluorine-containing media are recommended because they enable the treatment to be performed very quickly.

Particularly when treating glass, it is preferable to use a fluorine-containing acid medium. If the medium used has sufficient acidity, the optical properties of the surface are not impaired.

Preferably the medium has a pH value less than . Experiments show that transparent glass can be treated by such a medium without any loss in transparency, whereas if less acid media are used there is often a tendency for the glass surface to be rendered light-diffusing.

A very suitable treatment medium is an aqueous solution of hydrofluoric acid. Another suitable medium is ammonium bifluoride. As another example, a medium may be used which comprises a fluorine-containing compound, e.g., sodium fluoride (NaF) together with sulphuric acid. A particularly effective medium is one comprising both hydrofluoric acid and sulphuric acid. For instance, a satisfactory result has been obtained in a treatment period of only a few minutes by using an aqueous solution containing approximately 6% by volume of each of the said acids.

While preference is given to fluorine-containing media, the removal of the surface stratum can be achieved with other etching media. Thus, for example, a surface stratum can be removed from the compressively stressed exterior layers of a chemically tempered glass body by using a medium containing OH ions, e.g., a solution of sodium hydroxide. When using a basic medium it is preferable for the medium in contact with the body to be continuously renewed to avoid impairment of the optical properties of the surface.

Preferably, the temperature of the treatment medium is between 0°C and 80°C. The speed of removal of material from the surface of the body can be accurately controlled by appropriate choice of the temperature within this range.

The treatment can advantageously be applied by constantly renewing the treatment medium in contact with the surface of the body, by sprinkling or otherwise continuously supplying the latter with the medium and allowing the medium to stream along the surface to be treated. This method has the advantage of quickly renewing the medium polluted by the reaction with the surface of the body. The same trealanent can also be applied by means of a fluorinated compound in the form of a gas or a vapour.

An alternative way of performing the process is to dip the chemically tempered body in a bath of the treatment medium. Reaction products are likewise effectively removed from the surface being treated when proceeding in that way.

Advantageously, the treatment medium and the body are given a relative oscillatory movement of sonic or ultrasonic frequency. It has been found that this movement accelerates the beneficial action of the treatment medium; a given improvement in mechanical strength is thus obtained in a shorter time.

It is advantageous to follow the etching treatment whereby a surface stratum is removed, by a treatment with water or a basic medium. The advantage lies in the fact that by this after-treatment, the strength of the body can be still further improved. The after-treatment may be performed by exposing the body to a humid atmosphere, e.g. an atmosphere which is saturated or near saturation point, or by immersing the body in water or an aqueous solution of a base, e.g. sodium hydroxide, the after-treatment being continued for a sufficiently long time for the strengthening effect to take place. If a base is used the reaction products are preferably continuously removed if transparency has to be preserved. The required after-treatment period can easily be ascertained by tests and of course depends in any given case on the composition of the treatment medium and its temperature. When treating ordinary soda lime glass, good results have been obtained by an immersion in a bath consisting of a 20$ aqueous solution of sodium hydroxide at 30°C.

The treatment whereby a surface stratum is removed from the compressively stressed exterior layers preferably takes place very soon after the chemical tempering, and preferably the process according to the invention is performed as part of a continuous process wherein bodies are successively chemically tempered (preferably by a process involving an exchange of alkali metal ions, and most preferably involving a substitution of alkali metal ions in the body by larger alkali metal ions) and then subjected to the surface removal treatment. If an after-treatment with water or a base is performed, that treatment may likewise proceed in immediate succession to the preceding one. In the treatment of glass, preference is given to continuous processes wherein a chemical tempering treatment is performed, involving an ex- change of alkali metal ions between the glass and a contacting medium, and wherein a surface stratum is removed from the external compressive-ly stressed layers within a few minutes of the termination of chemical tempering. By avoiding or reducing handling of the body between the tempering and the following treatment, the risk of the surface of the body being spoiled is likewise avoided or reduced.

After the treatment whereby a surface stratum is removed following chemical tempering, and after the subsequent treatment with water or a base (if such after treatment is applied) at least a portion of the treated surface of the body can be coated with one or more layers which modifies a surface property of the body, e.g. its surface hardness. The improvement of the tensile strength is considerably increased by the tempering and surface removing treatments but other properties, for example the surface hardness of the body are not necessarily improved by these treatments. It is therefore very advantageous to give protection by applying one or more layers which confer improved mechanical properties on the body, for instance, a hard layer or a layer with visco-elastic characteristics adequate to impart resilience to the surface in response to localised pressure. The improvement re-suiting from the application of the protecting layer or layers is greater than the improvement which would result from the application of the same layer or layers without previous tempering and surface removal treatments.

A protective layer or layers can be deposited by different methods, for instance, by evaporation in vacuo, by cathode sputtering or else by applying the coating substance as a liquid, e.g. by dipping the body in a solution containing an organometallic substance, and then giving the film thus deposited a heat treatment. It is advantageous to apply one or more coating layers comprising at least one of the following substances: i02, Si02, Al^, Cr20 , Fe^, Ζ >2, Ta2°5' V2°5' Th02, Y20^, Ce02, nO, SiC, TiN, TaC, ZrC, B2C, TiC, A1B, B^C, zircon, beryl, topaz, ZnCrO^, τΒ2· Layers formed from these compounds have considerable hardness. Advantageously, the tempering, surface removal and coating treatments are performed successively in a continuous process wherein contact (if any) between the body and other bodies after the surface removal and prior to the coating treatment is reduced to a minimum.

Preferably, the treatments are performed in an apparatus comprising means for spraying etching medium, a tank for collecting the etching medium after it has streamed along the body, and means for supporting the body to be treated in the zone of action of the spraying means.

Such apparatus enables the etching liquid in contact with the surface of the body to be continually renewed and also enables the solution to be collected after it has streamed over the body. Advantageously, the apparatus also comprises means for spraying or sprinkling the ion exchange treatment medium and a tank for collecting this medium after it has streamed over the body. Such an apparatus enables the etching and ion exchange treatments to be performed in a continuous process.

Advantageously, the or each collecting tank is connected to a recycling circuit v/hich recycles the liquid back to the sprinklers or sprayers for re-use. The apparatus may incorporate means for filtering and/or regenerating the media during recycling thereof.

The performance of a process according to the invention using a fluorine-containing etching medium can be detected by examination of the body itself. If an ion exchange has taken place involving replacement of ions in the body by other ions, these other ions will be in the heavier concentration in an external layer of the body, and in a surface stratum of such external layer the body will contain fluorine ions. In the case of a body which has been chemically tempered by an ion exchange between sodium ions in the glass and potassium ions in a contacting medium, the body, after the etching treatment, is characterised in that it contains sodium ions and, in an exterior layer of the body, potassium ions in a concentration greater than in an inner layer of the body (where the concentration of potassium ions may be nil) , and in that there are fluorine ions in a surface stratum of such exterior layer. The actual surface of such a body is hydrophobic. The invention includes a body having such characteristics.

The invention will now be illustrated by a number of examples which are non-limiting. In all these Examples the thickness of the surface stratum removed was determined by measuring the loss of weight of material as a result of the etching treatment.

Example I A sheet of vitrocrystalline material produced from the following composition by weight: SiO, A1203 Na≥0 CaO TiO„ was treated for ≥h hours at 520 C in a bath of potassium nitrate.

The resulting breakage stress was 80 kg/mm - about four times higher than that before the tempering treatment.

A sheet having the same composition was given an analagous treatment and was then immersed at 20°C in a bath having the following composition: 8*8 litres of water 0.6 litre H S0. 0. litre H ( 70^) The treatment lasted for 3 minutes. The modulus of rupture of the sheet, determined by flexure, was 110 kg/mm . A stratum 2 microns in thickness had been removed from the surface of the sheet on each side thereof by the treatment* By lengthening the acid treatment to minutes, surface layers 1 microns in thickness were removed and the 2 rupture modulus was increased only to 94 kg/mm .

In a comparative test, a comparable rupture modulus to that possessed by the sheet treated with acid for 3 minutes as above described was obtained by treating a similar sheet with the acid for one minute at 20°C, accompanied by mechanical vibrations of a frequency of 45 cycles/sec and a power of 2 watt/cm . In this case, however, only 0.9 microns were removed from the surface.

Example II A sheet of soda-lime glass formed from the following main constituents: was given an ion exchange treatment. Na ions were substituted by Li ions in a bath containing 2% LiNO^ and 8$ NaNO^ at a temperature of 580°C. The treatment lasted for 20 minutes. The glass was removed from the bath, cooled and dried. The glass was then placed in an aqueous solution containing 10% by volume of hydrofluoric acid, kept at 50°C. After a treatment lasting 30 seconds, the glass was removed from the acid bath, washed with distilled water and dried in isopro-pylic alcohol vapour. A thickness of 3 microns had been removed from each surface by the acid treatment. The modulus of rupture was 2 kg/mm . Before treatment in the acid bath, the modulus was only 15 2 kg/mm . The treated sheet was somewhat light-diffusing.

Example III A sheet of soda-lime glass was given an ion exchange treatment in a bath of KNO^. As a result of the tempering, surface layers of the glass 25 microns in thickness were in a state of compression and the tensile strength of the glass was 100 kg/mm . The glass was given a treatment in the acid bath used in Example I; the treatment was stopped after 2 minutes. A surface stratum 2 microns in thickness was removed from each side of the sheet by this treatment. The 2 resulting glass had a tensile strength of 130 kg/mm .

A layer of ZrO^ was then deposited on a sample of the treated glass by immersing it in an alcoholic solution of the corresponding acetate and then heat treating the coating film thus formed, at 300°C, The tensile strength had slightly diminished, but the surface hardness was better than before the coating step.

Example IV Fig. 1 shows diagrammatically an apparatus enabling the ion exchange and acid treatments to be applied in a continuous process.

A sheet of glass 17 was disposed vertically on a support 18 which was advanced by a mechanism (not shown), e.g. a chain or carriage.

In a first compartment 19» the sheet of glass was heated to the working temperature of the ion exchange treatment - i.e., ^O°C - by means of radiant elements (not shown). In the same compartment 19» sprayers or sprinkler pipes 20, 21 differently orientated and formed with holes 6, 7 are connected to a conduit 8 for feeding molten potassium nitrate. The glass 17 was therefore sprayed over all of its surfaces by jets or streams 9 of the ion exchange salt. The treatment was continued for 2h hours. The salt used was collected at the bottom of the compartment 19 in a gutter 10 which took the salt to a regenerator and a recycler (not shown). After this treatment, the glass was kept in an adjacent compartment 11 for one hour at a temperature of h °C to allow the salt to drain completely away. The glass was then cooled in the same compartment by means of coolers 12. When the glass reached a temperature of 50°C, the partition 13 was raised and the glass 17 was conveyed into a compartment Ik constructed similarly to the compartment 19. In the compartment l the acid medium treatment was performed by means of spray or sprinkler pipes 15, l6. The acid mixture consisting of hydrofluoric acid and sulphuric acid was sprayed on to the surfaces of the glass for one minute. The glass was then cooled and was thus ready for coating by a protective layer if required. The acid treatment removed a stratum 2.5 microns in thickness from each side of the glass sheet. By the acid treatment the tensile strength of the glass was increased from 90 to 120 kg/mm . In a comparative test in which a similar sheet was subjected to a similar treatment, but in a discontinuous way, the sheet surface being handled after the chemical tempering and before the acid treatment, the tensile strength was increased only to 105 kg/mm .

Example V A piece of ceramic formed from the following composition: AlgO^ : 07 Na20 : 0.

CaO, Mgp; FegO , Ti02 : .$ o was immersed in a bath of KNO^ at 50 C for 48 hours. After the chemical tempering, the piece was treated with an aqueous solution of sulphuric acid and hydrofluoric acid whereby a surface stratum 12 microns in thickness was removed from the exterior layer in a state of compressive stress. The piece thus acquired a tensile strength twice that which it had before the acid trealment.

Example VI A sheet of soda-lime glass was treated for 40 hours in a bath of KNO^ containing 0.2$ of K^CO^. The bath was maintained at 450°C. The 2 tensile strength of the sheet after that treatment was 90 kg/mm .

Potassium ions had penetrated up to 30 microns into the sheet. In a comparative test, a sheet of the same glass was subjected to the same chemical tempering treatment and was then immersed for one hour in a 2C^o aqueous solution of aOH at 30°C. The tensile strength of the 2 glass after that treatment was 110 kg/mm . A surface stratum 1.5 microns in thickness was removed from the surface of the glass sheet, on each side thereof, by this treatment.

Example VII A piece of nepheline rock ( a^KAl^Si^O^) was subjected to an ion exchange treatment in a bath of KO^ at 500°C. After 30 hours the sample was withdrawn from the bath, cooled and then immersed for three minutes in an acid medium containing ^¾ by volume of HP. The acid " treatment removed a surface stratum from the compressively stressed surface layer of the rock and the strength of the rock was considerably increased by such treatment.

Example VIII Sheets of soda-lime glass were tempered by immersion for twenty hours in a bath of KNO^ at 70°C followed by cooling of the sheets. The tensile strength of the tempered sheets was 90 kg/mm .

The chemically tempered sheets were sprinkled for 2 minutes with an aqueous solution cx^ntaining £¾ HP and &/o E^SO^ the temperature of the solution being 20°C. This treatment removed from each side of each sheet a surface stratum 1.5 microns in thickness. Following 2 this treatment the sheets had a tensile strength of 115 kg/mm .

The sheets were then immersed for minutes in boiling water.

This final treatment increased the tensile strength of the sheets to 135 kg/mm .

Clearly, the invention is not limited to the embodiments which have been described and illustrated by way of example, and modifications could be made thereto without exceeding the scope of the invention.

Claims (30)

WHAT IS CLAIMED IS:

1. A process for ijig' a body of glass, vitrocrystalline material, ceramic or rock, which body has been chemically tempered whereby material in an external part of such body is under compressive stress, characterised in that the chemically tempered body is subjected to an after-treatment wherein a surface stratum is removed from the material which is under compressive stress.

2. A process according to claim 1, characterised in that said body constitutes a manufactured article.

3. A process according to claim 1 or 2 , characterised in that said removed stratum is from 0.1 to 20 microns in thickness.

4. A process according to any preceding claim, characterised in that said stratum is removed by means of a medium containing fluorine.

5. » A process according to claim 4, characterised in that said medium has a pH less than ,

6. A process according to claim * or 5 » characterised in that said medium contains hydrofluoric acid in aqueous solution.

7. A process according to any of claims to 6 , characterised in that said medium contains sulphuric acid.

8. A process according to any of claims to 7 > characterised in that said stratum is removed by a medium at a temperature between 0° and 80°C.

9. A process according to any of claims 4 to 8 , characterised in that said after treatment also comprises, in a stage following the dissolving and removal of said stratum, a treatment which an aqueous medium.

10. A process according to any of claims h to 9» characterised in that said after-treatment comprises in a stage following the removal of said stratum, a treatment of the body with an alkaline medium.

11. A process according to claim 10, characterised in that said alkaline medium contains NaOH.

12. A process according to any of claims 1 to 9j characterised in that said after-treatment also comprises, in a stage following the removal of said stratum, an immersion of the body in water at such a temperature and for such a time that the strength of the body increases.

13. A process according to any of claims 1 to 9» characterised in that said after-treatment also comprises, in a stage following the removal of said stratum, an exposure of the body to a humid atmosphere at such a temperature and for such a time that the strength of the body increases.

14. 1 . A process according to any preceding claim, characterised in that said stratum is removed by means of a medium which is continuously supplied to and streams along said body.

15. A process according to any of claims 1 to 13, characterised in that said stratum is removed by immersing said body in a liquid medium which dissolves such stratum.

16. A process according to any preceding claim, characterised in that said stratum is removed by means of a medium between which and said body a relative oscillatory movement of sonic or ultrasonic frequency is produced.

17. A process according to any preceding claim, characterised in that said body has been subjected to a chemical tempering treatment involving an ion exchange between ions in said body and larger ions from a contacting substance.

18. A process according to claim 17, characterised in that said exchanged ions are alkali metal ions.

19. · A process according to any preceding claim characterised in that said stratum is removed immediately following the chemical tempering of said body.

20. A process according to any preceding claim characterised in that said after-treatment also comprises, in a stage following the removal of said stratum and following the subsequent treatment (if applied) with water or a base, a coating treatment stage wherein at least a portion of the previously treated surface of the body is coated with at least one layer which imparts a modified surface property to said body.

21. A process according to claim 20, characterised in that said treated surface is coated with a layer comprising at least one of the following substances: Ti02, Si02, Al^, Cr^, Fe^, Zr02, Ta205, V≥0 , ThOg, Y^, Ce02, ZnO, SiC, TiN, TaC, ZrC, B2C, TiC, A1B, B^C, zircon, beryl, topaz, ZnCrO^, τΒ2·

22. A process according to any preceding claim, characterised in that the chemical tempering and the one or more stages of the after-treatment are performed successively in a continuous process.

23. A process according to any preceding claim, characterised in that the body is a sheet of glass.

24. An apparatus suitable for use in performing a process according to any preceding claim, characterised in that the apparatus comprises means for spraying etching medium, a tank for collecting etching medium after it has streamed along a body being treated, and means for supporting a body to be treated in the zone of action of said spraying means.

25. An apparatus according to claim 24, characterised in that it also comprises means for spraying an ion exchange treatment medium, and a tanlc for collecting such medium after it has streamed along a body being treated.

26. An apparatus according to claim 21+ or 25, characterised in that the or each tank is connected to a recycling circuit which supplies the corresponding spraying means.

27. A process for strengthening a body of glass, vitrocrystalline material, ceramic or rock, substantially as hereinbefore described.

28. An apparatus for use in strengthening a body of glass, vitrocrystalline material, ceramic or rock, substantially as hereinbefore described.

29. A body of glass, vitrocrystalline material, ceramic or rock, which has been subjected to a process according to any of claims 1 to 22.

30. A body characterised in that it contains sodium ions, and, near the surface, contains potassium ions in concentration greater than in the centre of the body, and in that in the surface it also contains fluorine ions. COHEN ZEDEK & SP!SBACH P. O. Box 1169, Tel-Aviv