PROTECTIVE OVERCOAT FOR SOLAR-SHIELDING FILMS

Abstract: Transparent glass sheets (10) comprising thereon titanium-nitride as the principal solar screening layer (24) for reducing transmission of near infrared and, thereover, a layer of tin oxide (26) of about 300 to 800 angstroms thick. The tin oxide layer serves to protect the titanium nitride from oxidation during high-­ temperature processing procedures, increases the abrasion-­ resistance, and, at roughly the midpoint of the 300 to 800 angstrom range, is useful in reducing the colour and visible reflectivity properties of some titanium nitride films. In practice, some of the tin oxide (22), that is not required for physical protection of the TiN or visual effective, can be positioned between the nitride and the glass substrate.

Description: This invention relates to the improvement of infra red radiation barrier films, particularly those used in shielding windows from excessive heat gain during warm weather. It has been proposed in US-A-3,885,855 to produce solar control films by reactive sputtering of the nitrides, carbides or borides of the metals titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, or tungsten. While effective optical properties were known for some of these materials, any large-scale production of architectural glass by the vacuum electrical method of reactive sputtering would be rather slow and expensive. Control of the solar energy coming into windows is important in maintaining comfortable indoor conditions in warm climates. Solar control has been achieved commonly by adding light-responsive colorant material to the glass. Tinting of glass in this way has disadvantages in production, however, since a long time may be required to change shades. More recently, reflecting and absorbing films have been applied to clear glass, to achieve solar control. Reflection of unwanted radiation is more efficient than absorption, since reflection eliminates the radiation completely, whereas part of the absorbed heat is eventually carried into the building. In US-A-4,535,000 there is disclosed a rapid process for chemical vapour deposition of transparent solar screens of titanium nitride while avoiding haze and imperfections on the sheet being coated. Such problems could have been caused by premature, powder-forming condensation of reactants in reactor conduits. There are a large number of other procedures and products characterized by the deposition of titanium nitride on substrates. Most of these relate to wear-resistant layers of the nitride which, because of their thickness, render the film opaque. All questions of light transmission or optical quality of such materials are not applicable to such materials and procedures. The improvements in titanium nitride deposition procedures did not avoid some inherent problems in achieving broad acceptance of TiN as a glass-coating material in such markets as architectural glass. These problems included some susceptibility to mechanical and chemical attack and some limitations with respect to visual characteristics of TiN film. The invention described below arose out of work directed to the solving of these problems. According to the present invention there is provided a process for making a sheet glass product which comprises the steps of forming a sheet of transparent glass on a glass production line coating the glass sheet by chemical vapour deposition with a barrier film of titanium nitride forming over said titanium nitride barrier film an exterior film or a protective oxide selected from oxides of tin, silicon and aluminium and mixtures thereof and subjecting subsequently the glass sheet to annealing, tempering or bending at a temperature in excess of 800°F (426.5°C), said titanium nitride being oxidation resistant during said annealing, tempering or bending. By means of the invention there can be provided an oxidation-resistant protective layer over a thin, transparent infrared radiation barrier film of titanium nitride, which enables the coated sheet glass product to be subjected to annealing, tempering, bending or other processes at temperatures higher than 800°F (426.5°C). The protective oxide film can also impart excellent abrasion resistance to the barrier layer, which in turn has superior light-attenuating characteristics, particularly reduced visible reflectivity and reduced colour, when used as a solar shield on architectural windows, to reduce the solar heat gain during hot weather. The protective exterior film is preferably of tin oxide. Silicon oxide and aluminium oxide also have good protective properties. However, tin oxide, particularly fluorine-doped tin oxide, is believed to have the most favourable combination of mechanical, optical, and infrared emissivity properties to complement solar-shield applications. Titanium nitride is used to form the barrier layer because of the relative ease with which gaseous precursors to the coating can be formed and handled on processing equipment. The thinner tin oxide films, i.e. those of about 50 nm thick, are preferable for use on high-light-transmission glass sheets, e.g., those over a 30 nm thick TiN film. An example would be primary vision areas of automotive glass which would have a visible light transmission of about 70%. In such an instance, total solar transmission would be about 50%. In lower-transmission applications (e.g., an automotive sun roof or architectural glass), a titanium nitride thickness of 60 to 100 nm again in combination with a 50 nm overcoat film of tin oxide, would provide a highly suitable combination for obtaining a 15% visible transmission and 8% transmission of total solar transmission. Among preferred products of the invention are transparent glass sheets comprising thereon titanium-nitride as the principal solar screening layer for reducing transmission of near infrared and, thereover, a layer of tin oxide of 30 to 80 nm thick. The tin oxide layers serves to protect the titanium nitride from oxidation during high temperature processing procedures, increases the abrasion-resistance, and, at 50 nm, or roughly the midpoint of the 30 to 80 nm range, is useful in reducing the color and reflectivity properties of the titanium nitride films. In practice, some of the tin oxide, that is not required for physical protection of the TiN or visual effect, can be positioned between the nitride and the glass substrate. The performance of these compound solar-control coatings is only slightly dependent on the glass substrate on which they are placed. However, more exact values of optimum thickness can be determined for an individual application of the invention after one selects a particular substrate and the desired parameters for emissivity and solar transmission values. In one highly advantageous application of the invention, the titanium-nitride based coating is sandwiched between two protective oxide coatings, e.g., two tin-oxide coatings. In such systems, the titanium nitride film serves the principal function of reducing near-infrared (wavelengths of from about 0.7 to 2 µm) transmission. The outer layer of tin oxide serves to increase the abrasion resistance and serves as an oxidation-protective layer during manufacture, after the coating operation itself, in subsequent, high-temperature fabrication steps such as annealing, tempering or bending processes. The inner layer of tin oxide, that layer positioned between the glass substrate and the titanium nitride, promotes a desirably uniform nucleation of the titanium nitride during the deposition of TiN and serves to insulate the TiN from any undesirable chemical interaction with the glass substrate. Both tin oxide layers serve to increase the ratio of transmission of visible light to the transmission of near infrared light because of their high refractive index. The tin oxide films are also particularly effective in reducing emissivity (reflecting far-infrared) when they have been fluorine-doped according to manufacturing procedures known in the art. In some advantageous products, the tin oxide films total about 100 nm, in thickness. For example, each tin oxide film may be about 50 nm thick. Thicker TiN films, those with otherwise highly desirable shading characteristics for some architectural use, have, taken alone on a glass substrate, reflection characteristics which are aesthetically undesirable. Thus, TiN films of about 175 nm, in thickness have an almost mirror-like bluish reflection color from the coated side. This is particularly true when the coating is formed according to the process described by Gordon in U.S. Patent 4,535,000, i.e., by reacting ammonia and titanium chloride. When a tin oxide film of about 57.5 nm is placed on top of the TiN, the reflected color becomes virtually undetectable to most observers. Indeed, the highly reflective nature of the transparent sheet also drops markedly, roughly approximating the reflectance of untreated architectural glass. Such reflection-diminished film will generally have a visible transmission of 5-6% with a total solar transmission that is even lower. While the products of the process of the invention are highly advantageous in use because of their optical and solar-shielding properties, and the superior abrasion resistance of tin oxide relative to TiN, the use of a tin oxide coating is particularly important in processing procedures. Thus, for example, transparent glass products prepared according to the invention can be annealed at typical glass-plate annealing temperatures of about 1100°F (593°C) for an hour without any significant, or even detectable, oxidation of the solar-control film. Similarly, tempering at temperatures of about 1150°F (621°C) for five minutes results in no deterioration of the product. Similar testing was carried out by measuring the contact resistance between the tin oxide film, through the TiN layer, to a silicon substrate. Any oxidation of the TiN would have resulted in a large increase in the electrical contact resistance measured during the test. Another important process aspect of the invention is the conservation of titanium nitride which would otherwise be lost to oxidation during the process of coating hot glass with TiN according to such high temperature procedures as are illustrated by the process disclosed in US-A- 4,535,000. In such a procedure, and assuming the titanium nitride film would take about 20 seconds to move 10 feet (3.05m) from the exit of the tin float bath to the tin oxide over coating, then 10 to 15 nm of the top surface of titanium oxide could be oxidized to titanium oxide. Such a loss of TiN can often be tolerated or compensated for. But, it introduces a wholly unnecessary variable into the process. The tin oxide is of particular value in avoiding the above-described loss of nitride during the coating operation. Such conversion of nitride to oxide also introduces another variable into the product and may result in color variations in product as it is subjected to post-coating annealing and tempering. The tin oxide offers particular value in protecting the newly-formed TiN from oxidation during annealing and tempering operations at temperatures exceeding 800°F (426.5°C) and, typically between 1000 and 1200°F (538-649°C). Consequently, the overcoating of the nitride with tin oxide is advantageously carried out in the non-oxidizing atmosphere of the tin bath structure in which the glass sheet to be coated is formed. This procedure allows the required amount of nitride to be used more efficiently as a solar shield and to be deposited more rapidly because no significant allowance need be made for oxidation. In the accompanying drawings, Figures 1 and 2 illustrate schematically specific embodiments of the invention. Figure 1 illustrates a transparent solar-shield product 8 suitable for use as architectural glass and comprising a soda lime glass substrate 10, a coating 12 of about 150 nm TiN, and a coating 14 of 40 nm of fluorine-doped tin oxide. Figure 2 illustrates another solar-shield product 20. The glass 10 is coated with 50 nm of fluorine-doped tin oxide (layer 22), 70 nm of TiN (layer 24) and 40 nm of tin oxide (layer 26). The coatings are achieved using reactant systems known in the art, i.e., those described in US-A-4,535,000 and 4,265,974 for TiN and SnO₂ respectively. The titanium nitride deposited on the glass of Figures 1 and 2 is deposited according to the process described in US-A-4,535,000. The tin oxide, is a fluorine-doped oxide and deposited according to the process described in US-A-4,146,657. The structure according to Figure 1 was modified by replacing the tin oxide with silicon oxide, in one instance, and aluminum oxide in another instance. Both of these oxides were good protective coatings.

Claims: A process for making a sheet glass product which comprises the steps of forming a sheet (10) of transparent glass on a glass production line coating the glass sheet by chemical vapour deposition with a barrier film (12, 24) of titanium nitride forming over said titanium nitride barrier film an exterior film (14, 26) of a protective oxide selected from oxides of tin, silicon and aluminium and mixtures thereof and subjecting subsequently the glass sheet to annealing, tempering or bending at a temperature in excess of 800°F (426.5°C), said titanium nitride being oxidation resistant during said annealing, tempering or bending. A method according to Claim 1 wherein said titanium nitride layer (12, 24) has a thickness of 30 to 170 nm. A method according to Claim 2 wherein said titanium nitride layer (12, 24) is formed with a thickness of 60 to 100 nm. A method according to any preceding Claim wherein the exterior film (14, 26) comprises tin oxide. A method according to Claim 4 wherein the tin oxide film is doped with fluorine. A method according to Claim 4 or Claim 5 wherein the exterior film of tin oxide is formed with a thickness of 30 to 80 nm. A method according to Claim 6 wherein the thickness of the tin oxide layer is about 50 nm and the thickness of the titanium nitride layer is at least 300 nm. A method according to any one of Claims 4 to 7 wherein the resulting exterior film of tin oxide has an abrasion resistance greater than that of the underlying infrared radiation barrier film. A method according to any one of Claims 4 to 8 which includes: forming a second film (22) of tin oxide between the glass substrate (10) and the titanium nitride barrier film (24). A method according to Claim 9 wherein the exterior film (26) of tin oxide and the second film (22) of tin oxide are each about 50 nm thick and wherein said titanium nitride film (24) is at least 300 nm thick. A method according to any preceding Claim wherein the titanium nitride layer (12, 24) is formed with a thickness sufficient to exhibit a coloured reflection characteristic when uncoated and wherein said exterior film (14, 26) comprises tin oxide of a thickness effective to reduce substantially said coloured reflection characteristics in said sheet product. A method according to Claim 11 wherein said nitride film is about 170 nm thick and wherein said exterior film of tin oxide is at least 50 nm thick. A method according to any preceding Claim wherein the titanium nitride layer is formed by reacting ammonia with titanium chloride. A method according to any preceding Claim wherein the deposition of the titanium nitride barrier film (12, 24) is carried out in a reducing atmosphere in a tin float bath enclosure of the production line. A method according to Claim 14 wherein the overcoating of the barrier film with the oxide (14, 26) is also carried out in the reducing atmosphere of the tin float bath enclosure. A process according to any preceding Claim wherein said annealing or tempering step is carried out at a temperature in excess of 1000°F (538°C).

Inventors: GORDON, ROY GERALD
Applicants: GORDON, ROY GERALD
Application year: 1987
Main classification: C03C17
IPC codes: G02B1, C03C17