US 6,014,872 AGrant
Methods of Making Insulating Glass Units With Neutral, High Performance, Durable Low-E Glass Coating Systems
Issue Date:2000-01-18
•3 Claims
•1 Drawing Sheets
Abstract
A low-E sputter-coated layer system for automotive and architectural purposes of the basic Si.sub.3 N.sub.4 /NiCr/Ag/NiCr/Si.sub.3 N.sub.4 type improved by either an undercoat of TiO.sub.2 or the use of stainless steel in the Si.sub.3 N.sub.4 layers, or both. By selection of appropriate thicknesses the layer coatings may be rendered heat treatable. I.G. units are an advantage product in which the coatings may be employed.
Metadata
Assignee
- Guardian Industries Corp.
Inventors
- Klaus W. Hartig
- Philip J. Lingle
- Steven L. Larson
Application Information
Application Number:US 031840&
Filing Date:1998-02-27
Priority Date:1995-11-02
Art Unit:171
Classifications
IPC:
C03B 2324
Field of Search:
6532.4;34;36;58;60.2;60.4;60.8;67
Patent Drawings (1 sheets)
Description
FIELD OF THE INVENTION
This invention relates to coating systems for glass substrates which exhibit very low emissivity values and are substantially neutral in color. More particularly, this invention relates to glass articles, such as insulating glass units (e.g. doors and windows) which are provided with these coating systems, and methods of making them.
BACKGROUND OF THE INVENTION
The importance of sputter-coated glass layer systems for achieving solar management properties in many types of glass articles, such as architectural windows and doors, is now well established in commerce. In addition, the importance of using such layer systems in insulating glass units (known as "IG" units in the art) is equally well established. Examples of this latter use include multipaned windows and doors made up of at least two panes of glass sealed at their peripheral edges to form an insulating chamber therebetween. Such chambers, after the edges are sealed, are then popularly made commercially by displacing the air in the chamber with another gas such as argon. However, in certain modern technology mostly under development (or in certain older, more expensive technology) a vacuum chamber is employed while the glass sheets are heated to a temperature whereby the glass edges are fused together.
Important to the acceptance of solar management glasses, including IG units, in the marketplace are the following characteristics which relate directly to the sputter-coated layer system employed:
1) the desired amount of visible transmittance coupled with an acceptable level of infrared radiation reflectance;
2) a non-mirror-like appearance (i.e. a low visible "reflectance" as defined below);
3) a substantially neutral visible reflected color when viewed from the glass side (i.e. a color falling within the range of from colorless to slightly blue);
4) resistance to weathering or other chemical attack, often referred to as "chemical durability" (this term is defined below); and
5) resistance to abrasion (often referred to as "mechanical durability", a term also defined below) during handling, particularly during the various steps necessary to produce an IG window or door from two or more sheets of glass, at least one of which has been pre-sputter-coated with the aforesaid layer system.
In addition to these physical characteristics, the coating system employed must be economical to produce. If it is not, the ultimate product, such as in an IG unit, may become so expensive as to inhibit demand.
It is well-known in the art that these desired characteristics often conflict when attempting to achieve them, and that, therefore, trade-offs often become necessary. For example, achievement of acceptable levels of transmittance or IR (infrared) reflection may have to be at the expense of durability (either chemical or mechanical, or both). In other trade-offs, undesirable colors and mirror-like windows (or doors) become unavoidable. In still further trade-offs, cost of production becomes a significant factor. Such problems create a need in the art for a new sputter-coated layer system which can achieve a better balance among these characteristics.
In U.S. Pat. No. 5,344,718 there are disclosed various excellent sputter-coated layer systems which achieve acceptably low values of emissivity (E), and thus are properly classified as a family of "Low-E" systems (i.e. a family of high IR reflectance coatings). In addition, such coating systems, as a family, generally exhibit durability characteristics which approach or equal those of pyrolytic coatings and thus are quite acceptable. Still further, these coatings, particularly in their preferred embodiments, exhibit high visible transmittance. At the same time they also exhibit a reasonably neutral color, ranging somewhat into the green side of blue which is, however, reasonably masked by the level of visible reflectance achieved to thereby appear substantially neutral. In addition, these visible reflectance characteristics are below 20% and thus also avoid an undesirable mirror-like appearance when viewed from either the inside or outside when used, for example, as a window or door.
The family of layer systems disclosed in U.S. Pat. No. 5,344,718 employs various layers of Si.sub.3 N.sub.4 and nickel or nichrome to sandwich one or more layers of IR reflecting metallic silver between them, in a selected order, thereby to achieve the desired end properties. The entire disclosure of this patent, including the "BACKGROUND" section thereof is incorporated herein by reference.
Generally speaking this prior patent ('718) achieves its unique results by the use of a system comprised of five or more layers wherein from the glass outwardly the system comprises:
a) an undercoat layer of Si.sub.3 N.sub.4 ;
b) a layer of nickel or nichrome;
c) a layer of silver;
d) a layer of nickel or nichrome; and
e) an overcoat layer of Si.sub.3 N.sub.4.
When the system consists essentially of these five (5) layers, the following thicknesses are generally employed:
When, in this prior patent ('718), more than five layers are employed, such as when two silver layers are employed, the system from the glass outwardly usually includes the following layers:
glass/Si.sub.3 N.sub.4 /Ni:Cr/Ag/Ni:Cr/Ag/Ni:Cr/Si.sub.3 N.sub.4, and the total thickness of the silver remains the same (e.g. 95 .ANG.-105 .ANG.) such that each layer of silver itself is only about 50 .ANG., to make up the total.
While such systems as disclosed in this prior '718 patent constitute a significant improvement over then existing prior art systems, particularly those discussed in the "BACKGROUND" section of that patent; nevertheless, there remained room for improvement in the characteristic of "emissivity". For example, in the systems of the '718 patent, normal emissivity (E.sub.n) was generally less than or equal to about 0.12 while hemispherical emissivity (E.sub.h) was generally less than about 0.16. However, in practice, the lower limits realistically or commercially achievable were generally, for E.sub.n about 0.09 and for E.sub.h were about 0.12. Achievable sheet resistances (R.sub.s) in this respect were generally from about 9-10 ohms/.sub.sq.
Inhibiting the achievement of better IR reflection (i.e. decreased "E" values) was the generally held belief that if the thickness of silver were increased to achieve higher IR reflectance (and thus lower "E" values) at least one or more of the following four detrimental effects would occur: (1) there would result a loss of durability; (2) the ultimate product would be too highly reflective, and thus become mirror-like; (3) the color would be driven to an unacceptably high purple or red/blue appearance; and/or (4) visible transmittance would become unacceptably low.
Durability, both mechanical and chemical, is an important factor to achieve in architectural glass generally whether used as a monolithic sheet or, for example, when used in an IG unit. As aforesaid, the handling, assembling and sealing of IG units places a premium on mechanical durability, while the need to edge seal the panes to create an insulating chamber therebetween creates the need for chemical durability due primarily to the nature of the sealant which inevitably contacts the coating. Aesthetically, both mirror-like and purple color qualities may eliminate the marketability of any product exhibiting these characteristics. Loss of visible transmittance while undesirable, does not become truly undesirable until, in a monolithic sheet, it drops below about 70% and in an IG unit it drops below about 63%. However, in certain uses, particularly where low shading coefficients (i.e. less than about 0.6) are desired, transmittance may actually be too high, even though emissivity is reasonably low. Generally speaking, where shading qualities are desired (i.e. to lower air conditioning costs), monolithic visible transmittance should be kept below 75% and preferably below 73%, while in a typical IG unit visible transmittance should be about 65% to 68%.
In partial confirmation of the above beliefs, is the rather complex layer system disclosed in U.S. Pat. No. 5,302,449 as well as its presumed commercial counterpart in IG unit form, known as Cardinal 171 sold by Cardinal IG Company. The layer system as taught in this patent varies the thicknesses and types of materials in the layer stack to achieve certain solar management qualities, as well as employing an overcoat of an oxide of zinc, tin, indium, bismuth, or oxides of their alloys including zinc stannate, to achieve abrasion resistance. In addition, the system employs one or two layers of gold, copper or silver to achieve its end results. When two layers of silver are used it is said that the first is between 100 .ANG.-150 .ANG. and preferably about 125 .ANG. in thickness while the second, based thereon, is to be between 125 .ANG.-175 .ANG.. When only one silver layer is employed, it is taught that its thickness is to be about 100 .ANG.-175 .ANG., and preferably 140 .ANG.. Nowhere does this patent disclose the use of nickel or nichrome, nor the use of silicon nitride as an element(s) in the stack arrangement.
In actual commercial practice, the aforesaid Cardinal IG units have been found to achieve quite acceptable solar management properties including acceptable color characteristics and relatively good non-mirror-like visible reflectance (an example is reported hereinbelow for comparison). However, this otherwise quite acceptable system has been found lacking in chemical durability, and, as defined herein, may be said to lack chemical durability since it fails the prescribed boil test. While the precise reason for this is not known, the simple conclusion is that, as has been indicative of the prior art, sacrifices had to be made in at least one desirable characteristic, in order to achieve desirable levels of the others. In addition, due to the nature of the stack and elements used, the system is quite expensive to produce principally due to the number and thickness of the layers required to achieve the desired result.
In the "BACKGROUND" section of the aforesaid '718 patent there is disclosed a further prior art architectural glass layer system which commercially has become known as Super-E III, a product of the Airco Corporation. This system, from the glass outwardly consists of the following layer stack:
Si.sub.3 N.sub.4 /Ni:Cr/Ag/Ni:Cr/Si.sub.3 N.sub.4
It has been found in practice that in this Super-E III system the Ni:Cr alloy is 80/20 by weight Ni/Cr, respectively (i.e. nichrome), the two nichrome layers are reported as being 7 .ANG. thick, the Ag layer is specified as being only about 70 .ANG. thick [except that it is stated that the silver may be about 100 .ANG. thick], and the Si.sub.3 N.sub.4 layers are relatively thicker (e.g. 320 .ANG. for the undercoat and about 450 .ANG. for the overcoat). In reality, because of its thinness (i.e. about 70 .ANG.), the silver (Ag) layer has been found, in practice, to actually be rather semi-continuous in nature.
While this coating achieved good "durability" (i.e. the coating was scratch resistant, wear resistant and chemically stable) and thus achieved an important measure of this characteristic as compared to pyrolytic coatings, for glass at about 3 mm thick, E.sub.h is only about 0.20-0.22, and E.sub.n is about 0.14-0.17. Both of these emissivity values are rather high. In addition, sheet resistance (R.sub.s) measures a relatively high 15.8 ohms/.sub.sq (the more acceptable value being about 10.5 or less). Thus, while both mechanical and chemical durability are found to be quite acceptable and its monolithic sheet visible transmittance was a rather high 76.+-.1%, and while these coatings also proved to be compatible with conventional sealants used in IG units, its ability to handle IR radiation was less than desirable. In addition, its rather high monolithic visible transmittance of 76.+-.1% made such a system rather undesirable when lower shading characteristics were required.
Airco has followed its Super-E III system with what it has designated as its Super-E IV system. This system includes as its layer stack from the glass outwardly the following:
This system is quite similar in performance to the Super-E III except that visible transmittance is higher (e.g. greater than 80%), emittance is lower (e.g. less than about 0.10) and shading coefficient is significantly higher (e.g. approx. 0.80). In addition, due to the use of TiO.sub.2 as an undercoat, the system is expensive to produce.
Another group of layer systems, somewhat similar perhaps to those of Super-E III and IV, is reported in U.S. Pat. Nos. 5,377,045 and 5,563,734. In the systems reported in these two patents, a single silver layer (for example) is sandwiched between two nichrome layers (wherein the Cr is nitrided in '734) which, in turn, are sandwiched between a lower layer of, for example, TiO.sub.2 or Si.sub.3 N.sub.4 alone or doped with Zr; and an outer layer of Si.sub.3 N.sub.4 or Si.sub.3 N.sub.4 doped with Zr. When TiO.sub.2 is used as the lower layer, Si.sub.3 N.sub.4 is not used between this lower TiO.sub.2 layer and the next above precoat nichrome layer. When Si.sub.3 N.sub.4 is used as the lower layer, TiO.sub.2 is not used beneath it.
The above-described layer system of this '045 patent, in practice, is known to generally be of a purple color, has been found not to be chemically durable as defined by the below-described boil test, is not heat treatable, and has a rather high emissivity. The nitriding of the Cr in the '734 patent virtually assures that the system will not be heat treatable. Moreover, according to both of these patents, special sputtering techniques are advised in order to reduce what is referred to as the "tintrinsic stress" in one of the dielectric layers in order to achieve mechanical and chemical desirability.
A significant improvement in the prior art is disclosed in our co-pending application Ser. No. 08/356,515 filed Dec. 15, 1994, now U.S. Pat. No. 5,514,476, and entitled LOW-E GLASS COATING SYSTEM AND INSULATING GLASS UNITS MADE THEREFROM. The unique layer systems disclosed therein are comprised of a silver layer sandwiched between two nichrome layers which, in turn, are sandwiched between a lower and outer layer of Si.sub.3 N.sub.4. By appropriate adjustment of layer thicknesses, the coating systems of that invention achieved advantageously low emissivities (i.e. E.sub.n <0.07, E.sub.h <0.075, R.sub.s <5.5 ohms/.sub.sq.). In addition, the reflectance characteristics made them quite acceptable (i.e. no mirrorlike appearance) for use in insulating glass units ("IG's"). The transmittance characteristics, as well, were in the appropriate range and the problem of undesirable purple color of past prior art systems was eliminated.
While these layer systems were quite advantageous, it has been found that as one seeks to achieve even lower emissivity values (i.e. reduced infrared transmission, which is a principal object of many layer systems used in architectural and automotive glass), principally through the attempt to thicken the silver layer (the primary IR-reflecting layer), visible transmittance, color, and reflectance characteristics are adversely affected. For example, thickening of the silver layer has been found to severely decrease visible transmittance to below the acceptable 70% level. In addition, the glass side appearance of an article (e.g. IG unit) so coated with too thick a silver layer can often become highly purple and mirrorlike. Thus while the system described above achieved properties theretofore unachieved in combination by the prior art, there was still a need for improvement, if at all possible.
In addition to the layer systems described above, other coatings containing silver and/or Ni:Cr as layers for infrared reflectance and other light management purposes have been reported in the patent and scientific literature. See, for example, the Fabry-Perot filters and other prior art coatings and techniques disclosed in U.S. Pat. Nos. 3,682,528 and 4,799,745 (and the prior art discussed and/or cited therein). See also the dielectric, metal sandwiches created in numerous patents including, for example, U.S. Pat. Nos. 4,179,181; 3,698,946; 3,978,273; 3,901,997; and 3,889,026 just to name a few. While such other coatings have been known or reported, it is believed that prior to our invention, none of these prior art disclosures teach or have achieved the ability to employ the highly productive sputter-coating process and, at the same time, achieve an architectural glass which not only approaches or equals the durability of pyrolytic coatings, but which also achieves excellent solar management qualities as well.
In view of the above, it is apparent that there exists a need in the art for a sputter-coated layer system which optimizes rather than unduly sacrifices on a perceived priority basis, the above-described characteristics for coated glass sheets generally, and IG units more particularly, in an economical way. It is a purpose of this invention to fulfill this and other needs in the art which will become more apparent to the skilled artisan once given the following disclosure.
SUMMARY OF THE INVENTION
This invention achieves its superior results by using as a starting point the very advantageous layer system as just described above [i.e. as disclosed in U.S. Pat. No. 5,514,476 which for simplicity may be illustrated as: glass/Si.sub.3 N.sub.4 /NiCr/Ag/NiCr/Si.sub.3 N.sub.4 ] and changing this layer system in one of three basic, alternative ways:
1. admixing stainless steel with the layers of Si.sub.3 N.sub.4, or
2. employing an undercoat of TiO2 between the glass substrate and the first Si.sub.3 N.sub.4 layer; or
3. a combination of 1) and 2) above.
In those instances, in this respect, when the NiCr layers comprise a substantial amount of nitrided Cr (i.e. CrN.sub.x) the systems are generally not "heat treatable". However, by employing a requisite range of thicknesses of the layers and forming the NiCr layers so as to be substantially free of any nitrided Cr, the layer systems so formed are found to be "heat treatable" as that term is defined herein below.
In all instances, however, whether heat treatable or not, excellent emissivity values are achieved, and equally unexpectedly, the visible reflectance and color appearance of the article when viewed from the glass side remains non-mirrorlike and is substantially neutral (i.e. falls within the color range of truly neutral to only slightly blue), while the layer system itself remains chemically and mechanically durable despite at times an increase in silver layer thickness. As such, especially when heat treatable, the layer systems of this invention find particular utility as layer systems in insulating glass units such as IG doors and windows, especially when such units are manufactured using heat sealing techniques.
In one aspect of this invention then there is provided in a sputter-coated glass article comprised of a glass substrate having on a planar surface thereof, from the glass outwardly, a layer system including:
a first layer of Si.sub.3 N.sub.4 ;
a first layer of nickel or nichrome;
a layer of silver;
a second layer of nickel or nichrome; and
a second layer of Si.sub.3 N.sub.4 ;
the improvement selected from the group consisting of: (a) at least one of said Si.sub.3 N.sub.4 layers including from about 0.5% -15% by weight stainless steel, (b) an undercoat layer of TiO.sub.2, and (c) a combination thereof, and wherein
when said glass substrate has a thickness of about 2 mm-6 mm, said unheat-treated coated glass substrate has a normal emissivity (E.sub.n)of about 0.06 or less, a hemispherical emissivity (E.sub.h)of about 0.08 or less, a sheet resistance (R.sub.s) of about 7.0 ohms/sq. or less and having a substantially neutral visible reflected color when viewed from the glass side.
Given the above alternatives, then, in a particular embodiment of this invention there is provided a sputter-coated glass article comprised of a glass substrate having on one of its planar surfaces, from the glass outwardly, a layer system including:
a) a layer comprised of Si.sub.3 N.sub.4 and stainless steel, wherein the stainless steel is in an amount of about 0.5%-15% by weight of said layer;
b) a layer of nickel or nichrome;
c) a layer of silver;
d) a layer of nickel or nichrome; and
e) a layer comprised of Si.sub.3 N.sub.4 and stainless steel,
wherein the stainless steel is in an amount of about 0.5%-15% by weight of said layer, wherein
when the glass substrate has a thickness of about 2 mm-6 mm, the coated glass substrate has a normal emissivity (E.sub.n) of about 0.06 or less, a hemispherical emissivity (E.sub.h)of about 0.07 or less, a sheet resistance (R.sub.s) of about 5.0 ohms/.sub.sq. or less and has a substantially neutral visible reflected color when viewed from the glass side.
In yet another embodiment of this invention there is provided a sputter-coated glass article comprised of a glass substrate having on one of its planar surfaces, from the glass outwardly, a layer system including:
a) an undercoat layer of TiO.sub.2 ;
b) a layer comprised of Si.sub.3 N.sub.4 ;
c) a layer of nickel or nichrome;
d) a layer of silver;
e) a layer of nickel or nichrome; and
f) a layer comprised of Si.sub.3 N.sub.4 ; wherein when the glass substrate has a thickness of about 2 mm-6 mm, the coated glass substrate has a normal emissivity (E.sub.n) of about 0.04 or less, a hemispherical emissivity (E.sub.h) of about 0.05 or less, a sheet resistance (Rs) of about 5.0 ohms/sq. or less and has a substantially neutral visible reflected color when viewed from that glass side.
In certain embodiments of this invention, stainless steel, as aforesaid, may be combined in either or both of the Si.sub.3 N.sub.4 layers in an amount of about 0.5%-15% by weight of said layer.
In certain embodiments of this invention the layer system consists essentially of the above-described six (6) or five (5) layers. In both such embodiments, moreover, the relative thicknesses of the layers may be adjusted so that in further preferred embodiments the layer system is "heat treatable" as this term is defined below.
In certain further embodiments of this invention an aforesaid single sheet of glass of about 2 mm-6 mm having a layer system as contemplated by this invention on one of its planar surfaces, has the following reflectance and color coordinate characteristics:
a) when viewed from the GLASS SIDE
R.sub.G Y is about 8 to 18
a.sub.h is about -3 to +3
b.sub.h is about 0 to -15 and
b) when viewed from the FILM SIDE
R.sub.F Y is about 4 to 15
a.sub.h is about 0 to +8
b.sub.h is about -5 to -20
wherein RY is reflectance and a.sub.h, b.sub.h are the color coordinates as measured in Hunter units, Ill. C, 10.degree. observer.
In certain further embodiments of this invention a sheet of glass, provided on one of its surfaces with a layering system as above-described, is employed with at least one other sheet of glass such that each is substantially parallel to the other, but spaced therefrom and sealed at their peripheral edges to form an insulating chamber therebetween thereby to form an insulating glass unit useful as a window, door or wall wherein the layer system is located on surface 24 as shown in FIG. 2 such that the reflectance and color coordinate characteristics when viewed from outside are:
R.sub.G Y, about 10 to 20
a.sub.h, about -2 to +2
b.sub.h, about 0 to -10
and when viewed from inside are:
R.sub.F Y, about 8 to 18
a.sub.h, about 0 to +4
b.sub.h, about 0 to -10
and the visible transmittance is at least about 61%. When the coating system is located on surface 26, the reflectance and color coordinates are reversed from that above, but the transmittance remains the same.
The term "outside" as used herein means as observed by a viewer from outside the dwelling in which the coated glass sheet (i.e. IG unit) is employed. The term "inside" as used herein means the opposite of "outside", i.e. the side as observed by a viewer from inside the dwelling in which the unit is placed (e.g. from within a room of a house or office building looking toward the "outside").
As stated above, it is further contemplated by this invention that certain of the layer systems that are within the scope of this invention have the characteristic of being heat treatable. As used herein, the term "heat treatable" means that a layer system is capable of being subjected to at least one of the following conventional heating processes and not have its desired end-characteristics adversely affected thereby. The conventional heating processes contemplated are tempering, bending, heat strengthening or the heat sealing step(s) employed to seal two or more glass sheets together when forming an IG unit. In this latter instance, this may or may not include heating the glass stack sufficiently high to slump the edges of the glass sheets themselves.
By having the characteristic of being heat treatable, a particular coating system of this invention may be chosen for a particular end use. For example, if the layer system is to be used in an automotive bent and/or tempered windshield, it will be one chosen so as to be capable of going through these processes. If useful in architectural windows requiring the same appearance in both untempered and tempered panes, as another example, then the coating is so chosen to achieve this result by being heat treatable via the tempering process. of course, to be "heat treatable" the coating need only be capable of going through at least one, but not all, of the above-listed heat treatments.
In this respect, certain coatings contemplated may or may not be capable of withstanding tempering, hardening or bending, but are still considered "heat treatable" if they can withstand the heat employed to seal an IG unit during its fabrication (either with or without glass slumping), particularly when the method of forming the IG unit includes the step of evacuating the air from (i.e. degassing) the insulating chamber during sealing and leaving the chamber in vacuum or refilling it with an inert gas such as argon. Thus in certain embodiments of this invention there is fulfilled yet another need in the art by providing:
in a method of making an insulating glass unit comprised of at least two sheets of glass sealed at their peripheral edges to each other thereby to define at least one insulating chamber therebetween, the method including the steps of spacing the sheets apart, heating the sheets at an elevated temperature, and sealing the peripheral edges of the sheets to each other at or above the elevated temperature, the improvement comprising using on a planar surface of at least one of the glass sheets, a sputter-coated, heat treatable layer system according to this invention so located that the layer system is within the insulating chamber of the IG unit formed by sealing the glass sheet edges to one another.
This invention will now be described with respect to certain embodiments thereof, along with reference to the accompanying illustrations, wherein:
IN THE DRAWINGS
FIG. 1 is a partial side sectional view of an embodiment of a layer system according to this invention.
FIG. 1A is a partial side sectional view of another embodiment of a layer system according to this invention.
FIG. 2 is a partial cross-sectional view of an IG unit as contemplated by this invention;
FIG. 3 is a partial schematic perspective view of a house employing as a window, door and wall an IG unit such as illustrated in FIG. 2; and
FIG. 4 is a partial cross-sectional schematized view of an embodiment of an IG unit in its pre-fabrication stage prior to evacuation and sealing as contemplated by this invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
Certain terms are prevalently used in the glass-coating art, particularly when defining the properties and solar management characteristics of coated glass used in the architectural field. Such terms are used herein in accordance with their well-known meaning. For example, as used herein:
Intensity of visible wavelength light, "reflectance" is defined by its percentage and is reported as R.sub.X Y (i.e. the Y value cited below in ASTM 308-85), wherein "X" is either "G" for glass side or "F" for film side. "Glass side" (e.g. "G") means, as viewed from the side of the glass substrate opposite that on which the coating resides, while "film side" (i.e. "F") means, as viewed from the side of the glass substrate on which the coating resides. When reported for an IG unit the subscript "G" denotes "outside" and "F" denotes "inside" (i.e. from "outside" the dwelling, or from "inside" the dwelling, as the case may be).
Color characteristics are measured on the "a" and "b" coordinates. These coordinates are indicated herein by the subscript "h" to signify the conventional use of the Hunter method (or units) Ill. C, 10.degree. observer, according to ASTM D-2244-93 "Standard Test Method for Calculation of Color Differences From Instrumentally Measured Color Coordinates" Sept. 15,1993 as augmented by ASTM E-308-85, Annual Book of ASTM Standards, Vol. 06.01 "Standard Method for Computing the Colors of Objects by Using the CIE System".
The terms "emissivity" and "transmittance" are well understood in the art and are used herein according to their well-known meaning. Thus, for example, the term "transmittance" herein means solar transmittance, which is made up of visible light transmittance, infrared energy transmittance, and ultraviolet light transmittance. Total solar energy transmittance is then usually characterized as a weighted average of these other values. With respect to these transmittances, visible transmittance, as reported herein, is characterized by the standard Illuminant C technique at 380-720 nm; infrared is 800-2100 nm; ultraviolet is 300-400 nm; and total solar is 300-2100 nm. For purposes of emissivity, however, a particular infrared range (i.e. 2,500-40,000 nm) is employed, as discussed below.
Visible transmittance can be measured using known, conventional techniques. For example, by using a spectrophotometer, such as a Beckman 5240 (Beckman Sci. Inst. Corp.), a spectral curve of transmission is obtained. Visible transmission is then calculated using the aforesaid ASTM 308/2244-93 methodology. A lesser number of wavelength points may be employed than prescribed, if desired. Another technique for measuring visible transmittance is to employ a spectrometer such as a commercially available Spectragard spectrophotometer manufactured by Pacific Scientific Corporation. This device measures and reports visible transmittance directly. As reported and measured herein, visible transmittance (i.e. the Y value in the CIE tristimulus values, ASTM E-308-85) uses the Ill. C., 10.degree. observer.
"Emissivity" (E) is a measure, or characteristic of both absorption and reflectance of light at given wavelengths. It is usually represented by the formula:
For architectural purposes, emissivity values become quite important in the so-called "mid-range", sometimes also called the "far range" of the infrared spectrum, i.e. about 2,500-40,000 nm., for example, as specified by the WINDOW 4.1 program, LBL-35298 (1994) by Lawrence Berkeley Laboratories, as referenced below. The term "emissivity" as used herein, is thus used to refer to emissivity values measured in this infrared range as specified by the 1991 Proposed ASTM Standard for measuring infrared energy to calculate emittance, as proposed by the Primary Glass Manufacturers' Council and entitled "Test Method for Measuring and Calculating Emittance of Architectural Flat Glass Products Using Radiometric Measurements". This Standard, and its provisions, are incorporated herein by reference. In this Standard, emissivity is reported as hemispherical emissivity (E.sub.h) and normal emissivity (E.sub.n).
The actual accumulation of data for measurement of such emissivity values is conventional and may be done by using, for example, a Beckman Model 4260 spectrophotometer with "VW" attachment (Beckman Scientific Inst. Corp.). This spectrophotometer measures reflectance versus wavelength, and from this, emissivity is calculated using the aforesaid 1991 Proposed ASTM Standard which has been incorporated herein by reference.
Another term employed herein is "sheet resistances". Sheet resistance (R.sub.s) is a well-known term in the art and is used herein in accordance with its well-known meaning. Generally speaking, this term refers to the resistance in ohms for any square of a layer system on a glass substrate to an electric current passed through the layer system. Sheet resistance is an indication of how well the layer is reflecting infrared energy, and is thus often used along with emissivity as a measure of this characteristic. "Sheet resistance" is conveniently measured by using a 4-point probe ohmmeter, such as a dispensable 4-point resistivity probe with a Magnetron Instruments Corp. head, Model M-800 produced by Signatone Corp. of Santa Clara, Calif.
"Chemical durability" or "chemically durable" is used herein synonymously with the term of art "chemical resistance" or "chemical stability". Chemical durability is determined by boiling a 2".times.5" sample of a coated glass substrate in about 500 cc of 5% HCl for one hour (i.e. at about 220.degree. F.). The sample is deemed to pass this test (and thus the layer system is "chemically durable" or is deemed to have "chemical durability") if the sample's layer system shows no pinholes greater than about 0.003" in diameter after this one hour boil.
"Mechanical durability" or "mechanically durable" as used herein is defined by one of two tests. The first test uses a Pacific Scientific Abrasion Tester (or equivalent) wherein a 2".times.4".times.1" nylon brush is cyclically passed over the layer system in 500 cycles employing 150 gm of weight, applied to a 6".times.17" sample. In the other, alternative test, a conventional Taber abrader (or equivalent) is used to subject a 4".times.4" sample to 300 revolutions of two C.S. 10F abrasion wheels each having attached a 500 gm weight. In either test, if no substantial, noticeable scratches appear when viewed with the naked eye under visible light, the test is deemed passed, and the article is said to be mechanically durable.
Thicknesses of the various layers in the systems reported are measured by, and thus the term, "thickness" as used herein is defined by alternative techniques. In one technique, known optical curves, or, in the alternative, the use of a conventional needle ellipsometer (i.e. profilometer) is employed. In another and particularly advantageous technique, an "n & k" analyzer is used (n & k Technology, Inc., Santa Clara, Calif.). This technique is believed to be generally described in U.S. Pat. No. 4,905,170, along with the ability to determine the "n" (i.e. refractive index) and "k" (i.e. the coefficient of extinction) values of the film under investigation. The disclosure of this patent is incorporated herein by reference. Such procedures and techniques are well-known to the skilled artisan and thus need no further explanation, except to note that thicknesses reported and used herein are reported in angstrom units.
Turning now to FIGS. 1 and 1A, there is shown a partial cross-sectional sketch of two embodiments of this invention. As can be seen, there is employed a conventional glass substrate 1 used in the architectural art. Such glass is preferably made by the conventional "float" process and thus is referred to as "float glass". The usual thickness thereof may be from about 2 mm-6 mm. The composition of the glass is not critical, and may vary widely. Typically, the glass employed is one of the soda-lime-silica family of glass well-known in the glass art.
The process and apparatus used to form the various layers on glass substrate 1 may be a conventional multi-chamber (multi-target) sputter-coating system such as is produced by Airco, Inc. In this respect, the preferred sputter-coating process for use herein is the same as that disclosed in U.S. Pat. No. 5,344,718. It is to be noted here that it is an aspect of this invention that its unique results are achieved through the use of conventional sputter-coating techniques without the need for special processes to relieve intrinsic stresses as reported in U.S. Pat. No. 5,377,045 or 5,563,734.
Turning first to the layer system shown in FIG. 1, five layers (a)-(e) are employed. Here no TiO.sub.2 undercoat is employed and the Si.sub.3 N.sub.4 layers have stainless steel admixed therewith. From the glass outwardly the layers and a preferred range of thicknesses, measured by the "n & k" technique above, for this particular embodiment are:
In particularly preferred embodiments, the thicknesses of the layers are:
In the five (5) layer embodiments represented by FIG. 1, particularly preferred thicknesses of the layers are about:
In sputter-coating the layers (a) and (e), silicon (Si) targets are preferably employed which have admixed with the Si, the desired amount of stainless steel (e.g. #316) to achieve the desired end amount in the film layer. By conducting the sputtering in nitrogen, Si.sub.3 N.sub.4 is formed and at least some of the chromium in the stainless steel forms chromium nitride. Optionally, aluminum may also be employed as a dopant in small amounts (e.g. 6% by wt.) for the purpose of keeping the target conductive. However, stainless steel also serves this purpose, and thus Al is not required to achieve the desired level of conductivity.
In this respect, it is stated herein that, in general, the amount of stainless steel generally contemplated for use in each layer is about 0.5%-15% by weight of the film. Since the sputter process generally involves sputtering the Si, and stainless steel (and Al if optionally present) at approximately the same rate, the amount of each ingredient employed (if reasonably uniformly disbursed) in the target itself may be assumed to be with reasonable accuracy for the purposes of this invention, the resultant amount in the layer after sputter-coating (as confirmed by analysis). Thus when stating herein that a layer includes a certain weight percent of stainless steel, it generally means that it was about that amount that was employed in the target.
Reference is now made to FIG. 1A where a six (6) layer embodiment of this invention is illustrated. Here the five top layers (a')-(e') are of the same ingredients as their counterpart layers (a)-(e) in FIG. 1 or the Si.sub.3 N.sub.4 layers may be formed free of any stainless steel (i.e. one or both of the Si.sub.3 N.sub.4 layers a' and e' need not be provided with stainless steel and the unique results of this invention may still be achieved). The sixth layer added, of course, is an undercoat layer UC of TiO.sub.2 such that the thicknesses of these layers are preferably about:
In certain preferred embodiments the layers have about the following approximate thicknesses:
If NiCr targets are used, for example, and no stainless steel is employed in forming layers a' and e', the sputter process of layers b', c' and d' is generally conducted in argon to prevent nitriding. For non-heat treatable embodiments where NiCr is employed, the Cr may be partially or substantially completely nitrided if desired (e.g. by conducting the sputter process in N.sub.2). As described below, if stainless steel is employed, heat treatability is still attainable despite some nitriding of Cr in the layers of the system, except when the higher temperatures associated, for example, with tempering, are used (e.g. about 1050.degree. F./3 min. or above).
In the six (6) layer embodiments of FIG. 1A, in this respect, particularly preferred thicknesses are about:
As stated above, the unique use of either stainless steel in the Si.sub.3 N.sub.4 layers and/or the use of an undercoat of TiO.sub.2 has proven to give rise to a family of layer systems (as represented by the examples of FIGS. 1 and 1A) which, if provided on the planar surface of a monolithic sheet of glass (e.g. "float glass") having a thickness of about 2 mm-6 mm, produces a glass article having a normal emissivity (E.sub.n) of about 0.06 or less, a hemispherical emissivity (E.sub.h)of about 0.08 or less, a sheet resistance (R.sub.s) of about 7.0 ohms/.sub.sq. or less and exhibits a substantially neutral visible reflected color (i.e. from neutral to slightly blue) when viewed from the glass side. By choosing the appropriate thicknesses, the visible transmittance is at least about 70%, and the monolithic sheet may be rendered heat treatable as defined above.
A typical range of reflectance and color coordinates for the embodiments of this invention using the thicknesses of monolithic glass sheet and layers as described above, are:
GLASS SIDE
R.sub.G Y, about 8 to 18
a.sub.h, about -3 to +3
b.sub.h, about 0 to -15
FILM SIDE
R.sub.F Y, about 4 to 15
a.sub.h, about 0 to +8
b.sub.h, about -5 to -20
In preferred embodiments such characteristics are:
GLASS SIDE
R.sub.G Y, about 9 to 15
a.sub.h, about -1 to +3
b.sub.h, about -2 to -10
FILM SIDE
R.sub.F Y, about 4 to 10
a.sub.h, about +3 to +7
b.sub.h, about -10 to -20
In the embodiments of FIG. 1 when the particularly preferred thicknesses above set forth for this five (5) layer embodiment are employed, the reflectance and color coordinates are:
GLASS SIDE
R.sub.G Y, about 10.2
a.sub.h, about 0.4
b.sub.h, about -4.7
FILM SIDE
R.sub.F Y, about 4.6
a.sub.h, about 6.5
b.sub.h, about -15.8
In the embodiments of FIG. 1A when the particularly preferred thicknesses, above set forth for this six (6) layer embodiment are employed, and stainless steel is employed in the Si.sub.3 N.sub.4 layers, the reflectance and color coordinates are:
GLASS SIDE
R.sub.G Y, about 11.0
a.sub.h, about 2.3
b.sub.h, about -8.8
FILM SIDE
R.sub.F Y, about 6.0
a.sub.h, about 5.4
b.sub.h, about -17.5
Such a monolithic glass sheet so produced from clear float glass, has in fact been found to have a visible transmittance of about 76%, while the visible transmittance of the above-described particularly preferred five (5) layer system is about 71%. In both instances the layer system has been found to be heat treatable, mechanically durable, and chemically durable.
While the amount of stainless steel may be varied to meet individual requirements, it has been found preferable in sputter-coating either the five layer or six layer embodiments of FIG. 1-1A when stainless steel is, in fact, employed to employ a silicon (Si) target having about 6% by weight stainless steel, and thus by the above-described assumption creating a layer with about 6% by weight stainless steel. It is also preferred in the practice of this invention when sputter-coating the two sandwiching nucleating layers b,d or b'd', to conduct that sputtering in a nitrogen environment if nichrome (e.g. 80/20, Ni/Cr by wt.) is employed, thereby to convert at least a portion of the chromium in the nichrome to a nitride (unless, as described above, the higher temperature heat treatable processes, e.g. tempering, are to be used). Similarly, of course, at least a portion of the chromium in the stainless steel will form a nitride when sputtering the Si/SS target in a nitrogen environment to form Si.sub.3 N.sub.4 (i.e. silicon nitride).
It is a surprising finding of this invention that when stainless steel is employed in the above-referenced layer system, and thicknesses are adjusted as above set forth, not only does the nitriding of the Cr in the stainless steel not prevent the attainment of heat treatability (as opposed to the nitriding of the Cr in the NiCr layers which usually prevents heat treatability at the aforesaid higher temperature processes), but it also aids in achieving certain enhancements in solar management properties as well, such as: reduced visible reflectance and improved reflected color.
As referenced above, FIG. 2 illustrates, somewhat schematically, a typical IG unit in accordance with this invention. In order to differentiate the "inside" of the IG unit labelled "In" from its "outside", labelled "Out", the sun 9 is schematically presented. As can be seen such an IG unit is made up of "outside" glass pane 11 and "inside" glass pane 13. These two glass panes (e.g. 2 mm-6 mm thick) are sealed at their peripheral edges by a conventional sealant 15 and desiccant strip 17. The panes are then retained in a conventional window or door retaining frame 19 (shown in partial schematic form). By sealing the peripheral edges of the glass sheets and replacing the air in chamber 20 with a gas such as argon, a typical, high insulating value IG is formed. Chamber 20 in this respect is typically about 1/2" in width.
By employing the layer systems of this invention, as above-described, as layer system 22 on wall (i.e. inner planar surface) 24 of outside glass sheet 11 within the chamber 20, as illustrated, or alternatively on wall (i.e. inner planar surface) 26 of the inside glass sheet 13 within chamber 20 (not shown), a particularly unique, non-mirror-like IG unit is formed which is substantially neutral in color when viewed from either the inside or outside of the dwelling in which the IG unit is installed. In this respect, it is understood, of course, that FIG. 2 illustrates only one embodiment of an IG unit in which the unique layer systems of this invention may be employed. In fact, the layer systems of this invention are available for use in a wide variety of IG units generally, including those having more than two panes of glass. Generally speaking, however, the IG units of the subject invention, when the layer system is located on the wall of either glass sheet within an insulating chamber of the IG unit, will typically have the following range of characteristics:
In this respect, in certain embodiments typical reflectance and color coordinates will fall within the following range when viewed from the outside or inside:
and the visible transmittance is at least 61% and preferably at least 63%.
In addition to the above characteristics, in certain preferred embodiments where the aforesaid particularly preferred five layer system represented by FIG. 1 (using the particularly preferred thickness listed above), the following performance characteristics are achieved if such a system is used in an IG unit with a 1/2" wide chamber 20 filled with argon as those characteristics are computed according to the software program known as "WINDOW 4.1" from Lawrence Berkeley Laboratories of Berkeley, Calif.; and, in addition, using a Hitachi spectrophotometer to get input data for: (1) visible and solar transmittance; (2) solar reflection, film side and glass side; and (3) a Beckman infrared spectrophotometer to measure emittance. The WINDOW 4.1 program, 1988-1994 is a copyrighted program of the Regents of the University of California entitled "Fenestration Production Thermal Analysis
In like manner using the same technique of measurement, the following performance characteristics are achieved in the aforesaid system when the particularly preferred six layer system represented by FIG. 1A (using the particularly preferred thicknesses listed above) is the layer system on either Wall 24 or Wall 26 (FIG. 2) as noted below:
In both these embodiments, the monolithic glass sheet may be subjected to both the boil test to determine chemical durability and the aforesaid Pacific Scientific Abrasion tester to determine mechanical durability. Both tests are passed for both embodiments.
SPECIFIC EXAMPLES USING STAINLESS STEEL
An Airco ILS-1600 research coater was employed to form the layer stacks of both FIG. 1 and FIG. 1A. This coater has the capability of employing either three or four targets (in the case of four targets, two at least must be by c-mag, e.g. Si and Ti). Here, for the embodiment of FIG. 1A, cathode #1 is titanium; cathode #2 is silicon with 5% Al and 6% #316 stainless steel admixed; cathode #3 is silver; and cathode #4 is nichrome (80/20 by weight Ni/Cr). As stated, cathode #1 and #2 may be in c-mag form. For the embodiment of FIG. 1, the titanium cathode is eliminated and the other three remain the same.
The two layer stacks are formed on a monolithic, glass sheet of soda-lime-silica float glass having a thickness of 0.087 inches. The following coater settings are employed:
The thicknesses, measured by the "n & k" technique above-described, are:
The optical and electrical properties of each system are as follows:
These two layer systems, as above described, are each formed into an IG unit of the type shown in FIG. 2 (coating on Wall 24, 1/2" argon insulating chamber or vacuum chamber). Its optical, thermal and electrical properties (by applying the above-described WINDOW 4.1 technique) are:
The effect of the stainless steel (and/or a TiO.sub.2 undercoat) on the optical and electrical properties is demonstrated by a comparison of the above five layer system characteristics (with 6% by wt. #316 stainless steel) and the later presented TiO.sub.2 undercoated six layer examples of this invention, with a duplicated five layer system of substantially the same thicknesses employing the same three targets, but in the duplicated system employing a Si target doped with 5% Al without any stainless steel admixed therein or an undercoat of TiO.sub.2. Here, the coater settings for the duplicated, non-stainless steel, non-TiO.sub.2 undercoated layer system are:
The optical and electrical properties of this duplicated monolithic five layer system (on the same glass substrate) without stainless steel are:
As can be seen (and as shown further below), the use of stainless steel and/or a TiO.sub.2 undercoat creates a significant improvement in the characteristics of the glass article, particularly in the lowering of the E values, which, in the case of the six layer system combined TiO.sub.2 /stainless steel aspects of this invention, are even lower.
By way of further comparison, and in contrast to the characteristics of the above two embodiments of this invention are the following characteristics by applying the above-described WINDOW 4.1 technique (1/2" argon chamber) to the aforesaid prior art commercial IG product "Cardinal-171".
It is to be pointed out, in this respect, that this Cardinal-171 IG product has achieved significant commercial acceptability in the marketplace. Its drawbacks are its lack of chemical and mechanical durability and unheat-treatability. Its precise layer stack system is not known. However, it is believed to be consistent with that as taught in the aforesaid U.S. Pat. No. 5,302,449.
EXAMPLES (NO STAINLESS STEEL/TIO.sub.2 UNDERCOAT)
A zoned Airco Corporation sputter coater as generally illustrated in FIG. 2 of U.S. Pat. No. 5,376,455 (and as generally described in U.S. Pat. Nos. 4,356,073 and 4,422,916) was employed to form two non-stainless steel containing layer systems on a clear float glass. The layer systems formed are illustrated in FIG. 1A. However, Example A is non-heat treatable, while Example B is heat treatable. Both have an undercoat of substantially stoichiometric TiO.sub.2.
In Example A (non-heat treatable), the line speed was 170 in/min. and the formation of the two nichrome layers (80/20 by wt. Ni:Cr) was conducted in a nitrogen/argon atmosphere in the same coater zone as silver so as to create a substantial amount of CrN.sub.x (i.e. nitrided chromium). In Example B (heat treatable), on the other hand, the line speed was 194 in/min. and the formation of the two nichrome layers (80/20 by wt. Ni:Cr) was conducted in the same coater zone as silver, but here in a substantially 100% argon atmosphere so as to prevent any significant amount of chromium nitride from being formed. The parameters of operation were as follows.
EXAMPLE A
EXAMPLE B
The coating so formed and with reference to FIG. 1A, had the following layer thicknesses:
The optical and thermal characteristics were determined for the monolithic sheet (Ill. C, 10.degree. observer, Hunter) and for an I.G. unit made of two such sheets of glass having 1/2" argon filled space between them (the latter's characteristics being determined by the Window 4.1 technique as described above). The layer system in the I.G. unit with reference to FIG. 2, was layer 22 on inner planar surface 24 of the I.B. unit. The characteristics are:
EXAMPLE A (NON-HEAT TREATABLE)
EXAMPLE B (HEAT TREATABLE)
As can be seen, by comparing the results of this invention with those of the above Cardinal-171 already commercially accepted product, the subject invention has achieved a high level of competitiveness using a significantly different and less expensive layering system. For example, while the Cardinal product achieves a bit higher visible transmittance than certain embodiments of the subject invention (73% vs. 70%), nevertheless this 70% value is not only well within acceptable levels but, when lower shading coefficients as explained above, are desirable (e.g. to reduce air conditioning costs), this 70% is more desirable commercially than the 73%. Of particular importance, moreover, is this invention's achievement of superior chemical and mechanical durability, as well as heat treatability (as demonstrated above). Both products have very low emissivities and substantially equal and excellent U values.
With respect to the above-referenced IG performance characteristics not previously defined herein, such as U.sub.Winter, R value etc., those terms are well understood in the art and are used here in accordance with their accepted meaning. For example, the "U" value is a measure of the insulating property of the IG system. U.sub.Winter and U.sub.Summer are determined according to NFRC 100-91 (1991), a standard subsumed in the WINDOW 4.1 software. "Shading coefficient" ("S.C.") is determined in accordance with NFRC 200-93 (1993), by first determining "Solar heat gain coefficient" and dividing by 0.87. "Relative heat gain" is determined by this same NFRC 200-93 procedure. "T.sub.solar " means total solar energy transmittance, a known combination of UV, visible and IR transmittance. "R.sub.solar " similarly, means total solar reflectance, a known combination of UV, visible, and IR reflectance.
FIG. 3 is a partial schematic view of a typical family dwelling 28 having various portals in which the subject invention may be employed. For example, window 30 may employ either a monolithic sheet of glass having a layer stack of this invention thereon, or employ as a "storm window" an IG unit of this invention such as is illustrated in FIG. 2. Similarly, sliding wall panel 32 or non-sliding wall panel 34 as well as front door panel 36 may be so constructed by employing this invention, either as a monolithic glass sheet or as an IG unit.
FIG. 4 is a schematic illustration of a typical two-pane IG unit stack prior to sealing. In certain embodiments of this invention, as stated above, the layer systems are heat treatable. In this Figure, the prestack employs two typical, clear float glass sheets 31 and 33 separated a selected distance (e.g. 0.1 mm) by glass beads 35. Lower sheet 33, of slightly larger size than upper sheet 31, has a layer system 37 according to this invention sputter-coated onto its inner planar surface (optionally, the inner planar surface of sheet 31 may be used for the layer coating). A conventional sealant 39 (e.g. a low melting ceramic) is then provided in the peripheral area 41 defined by centering the smaller sheet 31 on the larger sheet 33.
In conventional fashion, sufficient heat (e.g. approx. 500.degree. C.) is applied to flow sealant 39 to thereby create an insulating chamber 43. During this process a vacuum is applied to remove as much air and water vapor as is economically feasible, and optionally to either leave a vacuum or replace the air and water vapor with an inert gas such as argon. In an alternative technique the edges of the glass are flame sealed, rather than using a sealant. In either instance, heat must be applied to create the seal and drive off the water vapor. Thus the heat treatable embodiments of this invention find unique applicability in IG units of the type shown in FIG. 4 wherein the layer system must be able to withstand the heat employed during sealing without adversely affecting its desired characteristics.
In yet another alternative, a vacuum process is not employed and a spacing of 1/2" for the chamber is effected by various known, conventional techniques. In such a process the insulating chamber is usually filled with argon so as to displace the air and any water vapor (i.e. humidity or moisture) that might be present.
Once given the above disclosure many other features, modifications and improvements will become apparent to the skilled artisan. Such other features, modifications and improvements are therefore considered to be a part of this invention, the scope of which is to be determined by the following claims:
Claims
We claim:
1. In a method of making an insulating glass unit comprised of at least two sheets of glass sealed at their peripheral edges to each other thereby to define at least one insulating chamber therebetween, said method including the steps of spacing said glass sheets apart, heating said sheets at an elevated temperature, and sealing the peripheral edges of said sheets to each other at or above said elevated temperature, the improvement comprising using as at least one of said glass sheets a coated glass substrate having on a surface thereof a layer system including the following layers extending outwardly therefrom: a first layer of Si.sub.3 N.sub.4 ; a first layer of nickel or nichrome; a layer of silver; a second layer of nickel or nichrome; and a second layer of Si.sub.3 N.sub.4 ; wherein at least one of said Si.sub.3 N.sub.4 layers includes from about 0.5-15% by weight stainless steel, and wherein when said glass substrate has a thickness of about 2 mm-6 mm said coated glass substrate has a normal emissivity (E.sub.n) of about 0.06 or less, a hemispherical emissivity (E.sub.h) of about 0.08 or less, a sheet resistance (R.sub.s) of about 7.0 ohms/sq. or less and having a substantially neutral visible reflected color when viewed from the glass side.
2. In the method of claim 1 wherein said glass sheet having said layer system thereon is heat treatable.
3. In the method of claim 1 wherein said layer system on said at least one glass sheet consists essentially of the following layers having about the following thicknesses: Layer Thickness .ANG. ______________________________________ TiO.sub.2 100-400 Si.sub.3 N.sub.4 20-150 NiCr 7-100 Ag 75-250 NiCr 7-50 Si.sub.3 N.sub.4 50-600 ______________________________________
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