GB2212800A

GB2212800A - Method of coating articles

Info

Publication number: GB2212800A
Application number: GB8823440A
Authority: GB
Inventors: Peter Finlay James; Harold Rawson; Tianhe Wang
Original assignee: University of Sheffield
Current assignee: University of Sheffield
Priority date: 1987-10-06
Filing date: 1988-10-06
Publication date: 1989-08-02
Also published as: GB8823440D0

Abstract

A method of coating an article with a glass comprises forming a melt of a glass that is silica-free or has a silica content less than 55 wt % and a low viscosity at the dipping temperature, preheating the article to a degree to suit the particular glass composition, dipping the article in the melt, maintaining the article within the melt for a predetermined period and withdrawing the article from the melt at a predetermined controlled rate. The method is used to strengthen articles, particularly glass articles, by applying a coating having a thermal expansion coefficient less than that of the article.

Description

METHOD OF COATING ARTICLES This invention relates to a method of coating articles, and is particularly concerned with the provision of a glass coating on an article formed from metal, glass or ceramic material.

It is already known that metal, glass or ceramic articles can be coated with a glass using a variety of methods, the best known of which is commonly used in the vitreous enamelling industry. Here, a suspension of powdered glass (vitreous enamel) in water or alcohol is applied to the article by spraying or dipping, and, after drying, the article is fired at a temperature to fuse the powder into a substantially continuous coating. The difficulty is that it cannot be totally guaranteed that all of the surface of the article is evenly coated with powder. It is also not uncommon that, during the creation of the coating, there is the formation of bubbles within the coating, the presence of which impare considerably the effectiveness of the coating.

The object of the present invention is to provide a method of coating an article with a glass, free from the disadvantages mentioned above.

According to the present invention, a method of coating an article with a glass comprises forming a melt of a glass composition which is silica-free or has a silica content less than 55 wt % (preferably less than 50 wt %), and which has a low viscosity at the dipping temperature, preheating the article to a degree to suit the particular glass composition, dipping the article in the melt, maintaining the article within the melt for a predetermined period, and withdrawing the article from the melt at a predetermind controlled rate. The thickness of the glass coating produced on the substrate article depends on a number of factors including the viscosity of the glass melt, the temperature of the preheated substrate article and the speed of the withdrawal of the article from the melt.In particular, the higher the substrate temperature the thinner will be the coating layer thickness after withdrawal from the melt.

With the article having been preheated, relatively high dipping temperatures of 1250"C can be used without risk of damage to the article, and it is preferable that at such temperatures the glass has a viscosity within the range 1 - 100 Pas, and preferably less than 10 Pas. As a result relatively short immersion periods can be employed e.g., less than 2 seconds, with the substantial guarantee that the article will be totally and evenly coated. For example, when coating a substrate article composed of commercial soda-lime-silica glass the article should be preheated to 6000C prior to dipping, immersed for less than 2 seconds in the melt'and withdrawn from the melt at a speed typically in the range 5 to 20 cm per second. The glass article should then be cooled as rapidly as possible-to the annealing temperature for the substrate glass.

By ensuring that the glass for the coating has a thermal expansion coefficient less than that of the article, any possibility of the coating crazing on the article is avoided and there is the additional advantage that, after cooling, the glass coating is under a compressive stress, and as a result generates considerably improved strength characteristics for the coated article in comparison with an uncoated article.

Alternatively a glass coating in compressive stress may be produced by dip-coating the article with a glass composition which is then heat-treated to produce a surface crystalline layer.

Examples of the invention are given below: EXAMPLE 1 Glass rods of a commercial soda-lime-silica composition were coated through the method described above, namely direct dipping in a glass having the following composition, ZnO 45.7 wt %, B203 38.3 wt %, A1203 1.3 wt %, CaO 7.3 wt %, P205 7.4 wt %. The temperature at which dipping was performed was about 12000C. The coated layers are very uniform and free of bubbles (at the interface between the substrate and coating layer). Thus coated samples are mechanically very strong, having a bending strengh of 419 MPa. The reason for the high strength obtained is the lower value of the thermal expansion coefficient (7 x 10-6 oC-l) for the coating glass in relation to the soda-lime-silica composition. This results in a compressive stress in the coating layer.

EXAMPLE 2 Commercial soda-lime-silica glass rods were successfully coated by the direct dipping method with a glass of the following composition, Li2O 12.0 wt %, A1203 22.0 wt %, SiO2 40.0 wt %, B203 16.0 wt %, ZnO 10.0 wt %. The coating was done by simply dipping the preheated soda-lime-silica glass rods into the glass melt at 12500C and removing them immediately. In this way the rods were coated uniformly. Next, the rods were heat-treated at 590"C for 3 hours to produce a uniform surface crystal layer. The samples thus obtained had an average bending strength of 377 MPa. Because the crystallised coating layers are mechanically very hard and have a much higher toughness, these samples had much greater resistance to surface damage which can reduce the strength greatly.Also, strengthening resulted from the low thermal expansion coefficient of the the crystallised coating layer, for a similar reason to that given for Example 1.

EXAMPLE 3 Commercial soda-lime-silica rods were coated by direct dipping in a glass melt of the following composition (in weight percent) ZnO 60 B203 40 This glass has a thermal expansion coefficient of 4.8 x 10-6 oc-l The dipping temperature was approximately 11500C.

The coated layer was approximately 200 microns thick.

The measured bending strength for the coated rods was 548 MPa on average. The coated rods exhibited considerable resistance to surface damage such as by heavy abrasion and mechanical handling. In addition, when caused to fracture by an impact of suitable force, the presence of the coating caused the rod to shatter into a considerable number of small pieces, an important safety factor in many commercial applications of coated articles.

EXAMPLE 4 Commercial soda-lime-silica rods were coated by direct dipping in a glass melt having following composition (in weight percent).

ZnO 57.0 B203 28.5 A1203 5.0 v205 9.5 This glass has a thermal expansion coefficient of about 5.0 x 10-6 cc-l. The coating was carried out at 11400C.

The average bending strength for the coated glass rods was 368 MPa.

EXAMPLE 5 Commercial soda-lime-silica glass rods were coated by direct dipping in a glass having the following composition, Li2O 13 wt %, A1203 21 wt %, SiO2 43 wt %, B203 10 wt %, ZnO 10 wt %, V2O5 3 wt %. This composition was prepared as a homogeneous glass by melting at 13000C.

The glass transformation temperature and thermal expansion coefficient for this composition were about 4570C and 9.0 x 10-6 oC-1. Due to its relatively low viscosity (10 Pascal seconds at 13000C in comparison with 10 Pascal seconds at 1500"C for the base glass) coating by the direct dipping method can be performed at temperatures between 1200 - 13000C.

Commercial soda-lime-silica rods coated with the above composition (at 12500C) were subsequently heattreated at 5800C for an hour. During the heat-treatment, a complete surface crystal layer consisting mainly of the p-Eucryptyte phase was developed. Upon cooling to room temperature a compressive layer was formed on the coated rod surfaces. The rods thus obtained have an average strength of 400 MPa. Also the experiments show that these rods have strong resistance to surface damage. Such damage drastically reduces the strength of ordinary uncoated glasses.

EXAMPLE 6 A glass of the following composition was used to coat commercial soda-lime-silica glass rods by direct dipping at temperatures between 1200"C and 1300 C.

Li2O 14.0 wt %, A120, 21.0 wt %, SiO2 43.0 wt %, B203 12.0 wt %, ZnO 10 wt %. The glass transformation temperature and thermal expansion coefficient for this composition are respectively 4410C and 9.01 x 10-6 oc-1.

This glass can be easily melted at 13000C. The commercial soda-lime-silica glass rods coated with this composition at 12500C and subsequently heat-treated at 5800C for 50 minutes had an average bending strength of 366 MPa.

The glass rods thus obtained have a crystallised glass layer completely covering their surfaces. The crystallised layer consists mainly of the low expansion phase P-Eucryptite which ensured that it was in a state of compression. Also the layer was mechanically hard so that the resistance to surface damage was high.

In the two stage method of producing strengthened coated articles which is described in Examples 2, 5 and 6, the coatings may be glass compositions comprising, by weight, LiO2 9% to 15%, A1203 18% to 25%, SiOz 40% to 55% and Bs0, 5% to 20%, Na20 and/or K2O 0% to 4%, ZnO 0% to 15%, P205 and/or TiO2 and/or V205 0% to 10%.

By the use of the present invention, particularly in the preferred methods described in the foregoing Examples, coated articles of substantially increased strength are obtained.

Claims

1. A method of coating an article with a glass comprises forming a melt of a glass which is silicafree or has a silica content less than 55 % and which has a low viscosity at the dipping temperature, preheating the article to a degree to suit the particular glass composition, dipping the article in the melt, maintaining the article within the melt for a predetermined period and withdrawing the article from the melt at a predetermined controlled rate.

2. A method according to Claim 1 wherein the glass melt has a silica content less than 50 wt %.

3. A method according to Claim 1 or Claim 2 wherein the glass melt has a viscosity of less than 10 Pas.

4. A method according to any one of the preceding Claims wherein the article is maintained within the melt for a period less than 2 seconds.

5. A method according to any one of the preceding Claims wherein the article is withdrawn from the melt at a speed in the range of 5 to 20 cm per second.

6. A method of coating an article with a glass substantially as hereinbefore described in any one of the Examples.

7. Articles coated with a glass by a method according to any one of the preceding Claims.

8. For use in a method according to Claim 1 a glass composition comprising by weight, LiO2 9% to 15%, A1203 18% to 25%, SiO2 40% to 55% and B203 5% to 20%, Na2O and/or K2O 0% to 4%, ZnO 0% to 15%, P205 and/or TiO2 and/or V205 0% to 10%.

9. A method of strengthening an article by coating the article using a method according to Claim 1, the glass melt being a glass having a thermal expansion coefficient less than that of the article being coated.

10. A method of strengthening an article using a method according to Claim 1, the glass melt being formed of a glass composition which is capable of being crystallised in subsequent heat-treatment to form a coating having a lower coefficient of thermal expansion than the article which has been coated.