IL29787A - Coating and chemically treating glass or ceramic articles for strengthening them - Google Patents

Description

COATING AND CHEMICALLY TREATING GLASS OR CERAMIC ARTICLES FOR STRENGTHENING THEM This invention relates to processes for strengthening vitreous, vitrocrystalline and ceramic bodies.

It is known that glass oan be tempered chemically by causing metal ions to enter exterior layers of the glass from a contacting medium under suitable conditions. Two types of chemical tempering process can be distinguished. In one type, an ion exchange between the glass and the contacting medium takes place at a temperature sufficiently high for stress relaxation to occur in the glass and the ions entering the glass are such as to confer a lower coefficient of thermal expansion on the external glass layers. In the other type of chemical tempering process, ions in exterior layers of the glass are replaced by larger ions and the ion exchange is effected while the exterior glass layers are at a temperature below the annealing point (corresponding to a viscosity of 1 2 poises) so that stress relaxation will not occur or will do so in a limited way only.

The tensile strength of glass can be very considerably increased by the performance of a chemical tempering process but the process does not increase the surface hardness of the glass to any material extent, if at all.

By means of the present invention, a glass body can be processed to produce a body of substantial tensile strength and a surface hardness substantially greater than that of the glass body. The invention can also be usefully applied starting with a body of vitrocrystalline or ceramic material.

According to the invention, compressive stresses are produced in surface layers of a vitreous, vitro- crystalline or ceramic .body by a chemical tempering treatment in which ion exchange takes place between surface layers of said body and/ a- liquid . or' gaseous medium, through a- coating layer adherent , to said ;bpdy -' and applied, thereto prior to the chemical tempering treatment, and said layer forms, or is modified, af ,r the start of the chemical tempering .treatment. to form a crystalline or partly crystalline coating with a surface hardness greater than that of the coated surface . The invention ,can be .applied' ith particular advantage for strengthening glasses of ordinary composition, i.e.,. glasses formed from easily available inexpensive constituents, e.g., silica, soda,, lime and feldspar.

The chemical tempering treatment performed, in carrying out. the invention is preferably a treatment ■ involving, the exchange of alkali metal ions, e.g., the substitu on of sodium ions in the body undergoing treatment by potassium ions or lithium io s-.

The coating present on the. body at the commencement of the chemical tempering process ma have the same physical and chemical. composition as the substance initiall applied to the body or the coating' initially' formed may . . be subjected to .' some modification in situ preparator to ^ the commencement of chemical tempering. For example, the body may be initially coated with an amorphous substance or subst nces' which can,then be converted to form a crystalline or crystall sable coating. The conversion may involve reaction' between ingredients of the initial coating or betv/een a substance or substances in the initial' coating and another- substance brought into . contact with the- coating. These possibilities are of potential importance' because there is in consequence a wide selection, of substance's from which an initial, thin layer adherent to the body can be formed. In general, adherent coatings can be formed more easily by amorphous than by crystalline substances. "'■:." '■■ ' Crystallisation in the coating, ma occur at least in part during the che ical treatment, e.g., during immersion of the ..body; in a chemical , tempering, bath such as a bath composed of a molten salt or salts. The bath may include an ingredient for promoting crystallisation in the coating. ■·''■ Thus, crystallisation in an aluminium-containing coating can be. promoted; by an agent selected from the; group: . Ti, Mn, Mg, Co, Oil, Ni, Zn, Cr.

Crystallisation in a. zirc0:nium-c0n'tainin coating can be promoted by an agent selected, from the group: Pe, Ca, Mg, Ti, and rare earths. For silicon-containing coatings, the crystallisat on agent can be a halogen and suitable agents for promoting crystallisation in titanium-containing coatings are: : Zn, Cd, Bi, ΤΊί, Al, Cr, the halogens, and water vapour. The said crystallisation-promoting agents, if added to a bath of medium in atomic' or molecular form may become transformed,, sometimes .. spontaneously, into simple or. compound ions by reaction with the medium, without loss of their activity.. .The. said agents can also be introduced in the form of compounds, e.g., sulphates, chlorides or oxides. In general, crystallisation tends to be promoted by the presence of lithium ions but their concentration should not exceed 2%. It is also possible to form: the, chemical composition of the final coating by reaction between one or more initially applied substances and an ingredient of the contacting medium, such as the bath of molten salt or salts, used for the chemical tempering treatment, and in such a process the crystallisation may also occur at least in part during the chemical tempering treatment or it may be brought about subsequently, depending on the composition of the coating. By way of example the body may. be initially coated with an element by a conventional coating technique such as evaporation in vacuo, cathodic sputtering or hydrolysis and the coated article can then be immersed in a bath of molten salt in which, while the ion exchange proceeds, the coating is converted to its final chemical composition and crystallisation takes place.

In various tests it has been found that there is a tendency for the ion exchange to proceed more rapidly through a coating which is non-crystalline and from this point of view there is an advantage in carrying out the invention so that at least some of the crystallisation is deferred until after completion of the ion exchange treatment.

The effect of carrying out the present invention is that the properties of the vitreous or other body are improved more than can be predicted from the known effects of chemical tempering on the one hand and the provision of a relatively hard surface coating on the other. It is surprising that the tensile strength is more improved by carrying out the ion-exchange through the coating. ion donating medium or of the ions into the coating so that compressive stresses are set up in the coating during cooling. This appears probable e.g., when the ions donated to the glass or other body are provided by molten salts such as KC1 or LINO.,, and molecules of the salt diffuse into the coating and particularly if crystallisation in the coating then ensues. However the reason for the improvements are not wholl understood. It is possible, depending on the composition of the coating layer and the ion-donating medium, not only that the composition of this layer will undergo some chemical modification as a result of the diffusion of ions, atoms or molecules into the coating from the contacting medium and/or as a result of the diffusion of ions into the coating from the coated body, but that a substance or substances entering the coating will participate in crystal formation in the layer, and this also has beneficial effects. In some processes it has been observed that as a result of the simultaneous penetration of ions from the coated body into the coating on the one hand and the penetration of ions from the medium into the coated body and. the coating on the other hand, the coated body and the coating became modified with formation of similar lattices of a vitreous nature in the one case and crystalline nature in the other and that in consequence the adherence of the coating to the body is considerably improved. In some processes ion exchange occurs between the medium and the coating as well as between the medium and the coated body, so In order to achieve a high degree of surface hardness it is useful to form a coating comprising at least one crystalline phase formed by (corundum), SiC, Si02, ZrB2, TIN, TaC, ZrC, TiC, AIB, ZrO≥, B C, iOg, zircon (F.0H)2, a205, Y205, Ce02.

The ion exchange can be promoted by an electric field. This may be generated by a direct or alternating voltage and may extend through the body undergoing treatment between two quantities of molten salt in contact with opposite sides of the coated body. It suffices to use voltages of the order of several tens of volts.

Alternatively or in addition, the ion exchange can be promoted by means of a substance which weakens the diffusion barrier, e.g., by countering the diffusion-inhibiting effects of the ions which diffuse into the bath of molten salt or other medium from the body undergoing treatment. Substances which can be used for weakening the diffusion barrier are, e .g. ,· halogens, compounds pro-viding alkaline earth metal ions, particularly calcium or magnesium ions, and substances which form complexes with ions migrating from the body, e.g., substances in the groups : Pe, Mi, NH^+, CN~.

It is to be understood that the invention includes processes in which the ion exchange which takes place through a coating layer, affects only a part of the total surface area of the body. For example, in the case of a sheet of glass, the invention includes a process in which only the edge surfaces of the sheet or only the edge surfaces and contiguous marginal portions of the major surfaces are coated and subjected to the chemical tempering treatment.

The invention includes a vitreous, vitrocrystalline or ceramic body which has at least one chemically tempered surface or surface portion covered by a coating with a greater surface hardness than the coated body. In particular the invention includes such a body in which there exists in a zone comprising the surface layers of the body and the contacting bottom superficial layers of the coating, a continuous variation in the contents of alkali metal ions originating from the glass and of ions derived from a substance other than that used for forming the coated body or the coating.

The following are examples of processes and materials according to the invention : Example I A sheet of soda-lime glass of ordinary composition, formed from a batch containing as its main constituents, 72.4 % of Si02, 13.7 % of Na20, 12.0 of CaO and 1.9 of Alp0. (the percentage being by weight), and having a thickness of 25 mm was introduced into a chamber and metallised in vacuo at a vacuum of 10"^ mm Hg. In the chamber the sheet of glass was coated on both surfaces with a layer of 3508 of by cathode sputtering using an alumina-coated electrode. The applied voltage The sheet of glass thus treated was kept immersed for 40 hours in a bath of potassium nitrate at a temperature of 490°C. .

During this phase of the treatment, potassium ions diffused through the A120.-. layer and replaced sodium ions of the glass. The latter diffused towards the bath of potassium nitrate. In a comparative test, 15$ by weight of CaClg was introduced into the nitrate bath and this was found to counter the tendency for sodium ions in the bath to block the diffusion of the potassium ions.

After the 40 hour immersion period, the sheet of glass was removed from the nitrate bath and cooled in air to ambient temperature. The tensile strength of the" sheet was found to be 1J0 kg/mm and its ohs surface hardness was 9. The Mohs surface hardness of the glass substrate was 5. .

Example 2 A layer of metallic aluminium was deposited on the surface of a sheet of glass of the same composition as the glass used in example I, b evaporation in vacuo. The metallic aluminium was carried by an electrically heated tungsten filament. The sheet of glass coated with the aluminium layer was then heated for one hour at a temperature of 4 0°C to transform the aluminium layer into The resulting layer had an amorphous structure.

The glass thus treated was then immersed for 45 hours in a potassium nitrate bath as used in example I but containing a chromium salt in sufficient concentration to provide 5$ potassium ions diffused into the glass through the layer and replaced sodium ions which diffused towards the bath. Also in the nitrate bath, the layer of amorphous acquired a crystalline structure so that its surface hardness increased. The Mohs surface hardness of the glass substrate was 5.5 and that of the corundum coating was 9.

Other crystallisation-promoting agents which could be used in place of chromium, and with comparable results, are : titanium, manganese, magnesium, cobalt, copper, nickel and zinc. The concentrations of such elements in the bath should preferably not exceed.10$ by weigth.

Example 3 ■ A sheet of borosilicate glass of the following composition : Si02:72.9$J a20:9.8. ; K20:0.1#j Ca0:0.4#; Mg0:0.2$j :10. # (percentages by weigth), was introduced into a chamber in which the pressure was of the order of 10~-½m Hg.. In the chamber, a 75 ni layer of silicon monoxide was deposited on the surfaces of the glass by thermal evaporation of silicon monoxide contained in a molybdenum crucible. The sheet thus treated was then heated in air to a temperature close to its deformation temperature to transform the monoxide into a partially amorphous and partially crystalline silicon dioxide coating.

The sheet of glass was then immersed for 35 hours in a bath of molten cesium nitrate at a temperature of 520°C, to cause cesium ions to diffuse in the glass in substitution for sodium ions. To assist the diffusion, an During the treatment in the bath, the crystallisation of the silica progressed only slowly, and did not hinder the diffusion of cesium ions into the coated glass.

After 19 hours treatment in the nitrate bath, 5$ by weigth of barium chloride was added to the bath. The chlorine ions thus introduced into the bath had two effects during the remaining period of treatment in the bath. On the one hand, the chlorine ions promoted the crystallisation of the Si02 layer; on the other hand, the chlorine ions increased the rate of diffusion of the cesium ions.

On conclusion of the period of treatment, the sheet of coated glass was removed from the bath and cooled in air. The tensile strength of the product was 110 kg/mm , and it had a Mohs surface hardness of 7· The Mobs surface hardness of the substrate was 5.2. It stood up very satisfactorily to severe abrasion tests and resisted chemical attack by HC1, .

Example A glass container, was coated with a layer of ZrB≥ and chemically toughened in a bath of potassium nitrate.

The glass was a soda-lime glass of similar composition to the glass used in example I.

The inside and outside surfaces of the container were coated by evaporation in vacuo with a layer containing both zirconium and boron. The vaporisation in vacuo of the two substances was performed simultaneously. The coated zirconium was vaporised from a/tungsten filament. The boron was va orised from a coatin on an electricall two substances was continued until a layer 10008 in thickness was obtained.

The container thus treated ;was then immersed for 25 hours in a bath of potassium nitrate kept at a temperature of 70°C. by weigth of titanium ions and 0.5$ by weigth of lithium ions (based on the weigth of potassium ions in ■each case) were added to the bath. These elements promoted the crystallisation of ZrBg. Potassium ions were substituted for sodium ions in the glass. The ZrBg was formed and crystallised while the ion exchange proceeded.

The container thus treated had a tensile strength of 102 kg/mm*" and a high degree of resistance to abrasion.

Its phs surface hardness was 7.5 whereas the Mohs surface hardness of the substrate was 5· 5· Iron, calcium, magnesium and rare earths could be used in similar concentrations, in place of titanium ions for promoting crystallisation.

Hard layers of Sic, TiN, TaC, ZrC, TiC, AIB, B^C, corundum and/or zircone, can likewise be formed by evaporation in vacuo of their constituent elements followed by combination of the elements in situ to form a crystalline coating in the chemical toughening bath, and the formation of such coatings can also be promoted by the agents above referred to.

Example 5 A vitrocrystalline body was dipped in a solution formed by adding to a solution of 200 cc of titanium lsopropyl in 200 cc of water, 1 litre of ethyl alcohol solution and bore a thin surface coating of the solution.

The coating was dried at 150°C for ten minutes and fired for 10 minutes at 450°C. The dried coating was composed of titanium and iron oxides and was amorphous.

The body thus coated was then toughened chemically in a potassium nitrate chemical tempering bath containing 6% by weigth of zinc ions to promote crystallisation of the oxide coating. During the tempering treatment s the coating organised. The final Mohs surface hardness of the body was 7.6 whereas that of the substrate was 7.0.

Water vapour, halogens, cadmium, bismuth, nickel, aluminium can be used instead of zinc as the crystallisation agent. It is preferable to use such agents in a concentration not exceeding 10$ by weigth. Crystallisation can likewise be promoted by the presence of lithium ions in the tempering bath, e.g., in a proportion of 0.5$ by weigth based on the weigth of the other alkali metal ions in the bath.

A similar wet coating process can be used for forming coatings composed of SiOg or a mixture of TiOg and S102.

Example 6 A sheet of soda-lime glass similar to that used in example I was coated with a titanium layer on each of its surfaces. The thickness of the layer deposited was 250 ¾. The layer was deposited by evaporation in vacuo.

The coated sheet was then immersed in a bath of potassium nitrate at a temperature of 450°C and containing an agent for promoting crystallisation. The nitrate bath chemical tempering of the coated sheet by ion exchange . between the sheet and the bath, sodium ions in the glass being replaced by potassium ions. The Mohs surface hardness of the coated sheet was 8.6 whereas that of the glass substrate was 5·5· The process according to the invention can be applied not only to ordinary glasses, in respect of. hich its use is particularly advantageous, but also to special glasses or ceramics, more particularly vitroceramics.

Example 7 A sheet of soda-lime glass of the composition specified in example I was coated by simultaneous cathode sputtering processes respectively using electrodes .coated with alumina and boron, with a coating layer 5000 ¾ in thickness composed of 90$. by weigth of and 10$ by weigth of- boron. The sheet was then treated in the same way as the coated sheet in example I by. immersion in a" ' potassium nitrate bath, . through which however nitrogen was bubbled. At the end of the treatment it was found that sodium ions in the coated surface layers of the glass had become replaced by potassium ions and that a "conversion of boron to boron nitride had', taken place in the coating.

Moreover, some nitrogen ions had penetrated into the glass sheet to a depth of 10 microns and had become converted to form aluminium nitride. The Mohs surface hardness of the sheet was 9 and its tensile strength was found to be 2 '.··■ .'''■'· 1 5 kg/mm .

Claims (5)

1. A process for strengthening a vitreous, vitro-crystalline or ceramic body wherein compressive stresses are produced in surface layers of the body b a chemical tempering treatment in which ion exchange .takes place between surface layers of said body and a liquid or gaseous medium through a coating layer adherent to said body and applied thereto prior to the chemical tempering treatment, and wherein said layer forms, or is modified after the start of the chemical tempering treatment to form a crystalline- . or partly crystalline coating with a surface hardness greater than that of the coated surface.

2. A process accordin to claim 1 wherein the coating layer through which the ion exchange commences, is noncrystalline and wherein the modification. of the coating to a crystalline or partly crystalline form takes place wholly or mainly after the completion of the ion. exchange treatment.

3. A process according to claim 1 or 2 wherein the coating layer through which the ion exchange commences is a layer resulting from the reaction in situ. of constituents applied separately to said body. 4-. A process according to any preceding claim wherein the chemical composition of the said coating in its final form results at least in part from a reaction between a constituent or constituents present in the coating at the commencement of the ion exchange treatment and another substance or substances, during such ion exchange treatment. crystallisation takes place in the layer, promoted by a halogen. 1

4. A process according. to any of claims 1 to 9, wherein the coatin is a titanium-containing coating and crystallisation takes place in the layer, promoted by an agent selected from the group : Zn, Cd, Bi, Ni, Al, Cr, the halogens and water vapour. 1

5. A process according to any preceding claim wherein the final coating comprises at least one crystalline phase formed by (corundum), SiC, SiOg, ZrB2, TiN, TaC, ZrC, TiC, AIB, B^C, i02, ZrOg, zircon (ZrSiO^), beryl CeOg. ein the body is composed of a soda-lime glass. 17. A process according to. any preceding claim wherein the ion exchange >is an 'exchange2-¾>ί· 'alkali metal ions. 18. A process of strengthening a vitreous, vitro-crystalline or ceramic body, substantially according to any of the examples herein. 19. A vitreous, vitrocrystalline or ceramic body which has been strengthened by a process according to any preceding claim. 20. A vitreous, vitrocrystalline or ceramic body which has at least one chemically tempered surface or surface portion covered by a coating with a greater surface hardness than the