WO2023105536A1

WO2023105536A1 - A heat treatable solar control glass article comprising titanium nitride and niobium-based functional layers

Info

Publication number: WO2023105536A1
Application number: PCT/IN2022/051057
Authority: WO
Inventors: Priyesh DHANDHARIA; Vincent Reymond; Sacha ABADIE; Soumyadeep MISRA; Uditendu MUKHOPADHYAY
Original assignee: Saint-Gobain Glass France
Priority date: 2021-12-09
Filing date: 2022-12-07
Publication date: 2023-06-15

Abstract

A material comprising a transparent substrate deposited with a stack of thin layers on at least one of its surface to act on the solar and/or infrared radiation likely to strike said surface is disclosed. The stack of thin layers successively comprises, starting from the substrate not more than two functional layers Fl, F2 based on Nb/NbN and TiN, respectively and three dielectric coatings Ml, M2, M3 comprising at least one dielectric layer such that each of the metallic functional layer is sandwiched between two dielectric coatings. The proposed material has desirably low solar heat gain coefficient (SHGC) or solar factor (SF); desirable visible transmission; thermal stability and color matchability upon heat treatment such as tempering and desirable selectivity value.

Description

A HEAT TREATABLE SOLAR CONTROL GLASS ARTICLE COMPRISING TITANIUM NITRIDE AND NIOBIUM-BASED FUNCTIONAL LAYERS Technical Field The present invention relates, in general relates to a material comprising a transparent substrate, on the surface of which a stack of thin layers is deposited comprising two functional infrared (IR) reflecting and absorbing layers sandwiched between at least one dielectric layers making it possible to act on the solar and/or infrared radiation likely to strike said surface. More specifically, the present invention relates to a heat treatable solar control glass article that realizes one or more of: desirable glass side (RG) and/or coating side (RC) reflective color values; desirably low solar heat gain coefficient (SHGC) or solar factor (SF); desirable visible transmission; thermal stability and color matchability upon heat treatment such as tempering and desirable selectivity value. Background Solar factor (SF or g-value) and selectivity values are desired in some applications, particularly in hot and humid weather climates. Solar factor (SF), calculated in accordance with EN standard 410, relates to a ratio between the total energy entering a room or the like through a glazing and the incident solar energy. Thus, it will be appreciated that lower SF values are indicative of good solar protection against undesirable heating of rooms or the like protected by windows/glazings. A low SF value is indicative of a coated article (e.g., IG window unit) that is capable of keeping a room fairly cool in summer time during hot ambient conditions. Selectivity relates to the ratio of the light transmission to the solar factor, (TL/g). High selectivity values are often desirable, because this combines high or desirable visible transmission with a low SF value which is indicative of good infra-red blockage. When good selectivity (TL/g) is achieved, there is provided a higher ratio of visible transmission to solar factor (SF), which will be appreciated by those skilled in the art. In other words, good selectivity values are desired in combination with rather low SF values. This permits coated articles and/or IG window units, for example, to realize desirable visible transmission while at the same time blocking significant undesirable radiation (e.g., IR) from reaching a building interior or the like. While low SF and high selectivity values, are desirable for coated articles such as IG window units and/or monolithic windows, the achievement of such values may come at the expense of sacrificing coloration and/or reflectivity values. The absence of any substantial adverse effect upon heating the coating or its substrate, defines what is meant herein by the term "heat treatable". While in certain situations some characteristics may change somewhat during heat treatment, to be "heat treatable" as used herein means that the desired properties such as emissivity, sheet resistance, durability and corrosion resistance of the ultimate layer system and overall product must be achieved despite the fact that the coated glass has been subjected to one or more of the heat treatments (i.e. bending, tempering and/or heat strengthening). For most architectural purposes contemplated by this invention optimized heat treatability means that the glass and its layered coating remains substantially unchanged in at least its emissivity, sheet resistance, durability and with minimal color shift as between the pre-heat treated product and the final product after heat treatment. Solar control coatings are very well known in the art. For example, solar control coatings having a layer stack of glass/Si3N4/TiN/Si3N4/TiN/Si3N4 are known in the art. For example, U.S. Patent Document 2018/0186691 is incorporated herein by reference. While layer stacks of U.S. Patent Document 2018/0186691 provide reasonable solar control and are overall good coatings, they are lacking in certain respects. The coating side reflective a* values (a* under R_FY) in Examples 1 and 5 in Table 5 of US '691 are +4.5 and +4.8, respectively, and the coating side visible reflectance values (R_FY) in Examples 1 and 5 are 26.4% and 12.3%, respectively. Examples 1 in US '691 is undesirable because the coating side visible reflectance (R_FY) values are too high at less than 30%, and because the coating side reflective a* values are too positive at 4.5. And even when R_FY is reduced down to 12.3% in Example 5, this results in the coating side reflective a* color value in Example 5 still being too positive with a value of +4.8 resulting in red color. Thus, the coatings described in US '691 were not able to achieve a combination of acceptable visible reflectivity values (low reflection values) and reflective a* coloration values (achieving neutral color). Another example of solar control coatings having a layer stack of glass/Si3N4/NiCrN/Si3N4/TiN/Si3N4 are also known in the art. German patent document DE102014114330 is incorporated herein by reference. While layer stack of DE '330 optimizes the internal reflection to neutral color, this is achieved at the cost of external reflection % (RG) being too high and resulting in intensive colors (ranging from greenish-blue to greenish-yellow) as mentioned in paragraph [0033] of DE '330. The layer stack of DE '330 is further not heat treatable. Yet another example of solar control coatings having a layer stack of glass/Si3N4/NiCr or NiCrW/Si3N4/TiN/Si3N4 is known from U.S. Patent Document 2017/0197874 and is herein incorporated by reference. The layer stack of US ‘874 achieves neutral appearance in layer-side (RC) reflection and low reflection values. Yet the visible transmittance of the layer stack ranging between 0.1 to 2% results in a virtually opaque substrate with a glass side relatively neutral reflectance, thereby giving an absorbent black appearance. Thus it would be desirable according to example embodiments of the present invention for a solar control coating to be designed so as to have a combination of acceptable visible transmission (TL), desirable reflective coloration (e.g., desirable a* and b* reflective color values) in glass side (RG) and coating side (RC), low SF and high selectivity for a coated article in window applications. Coated articles according to certain example embodiments of this invention substantially reduce the intensive external colors while retaining a low visible reflectance in glass side (RG) and coating side (RC) and also maintain good mechanical, chemical and environmental durability and low emissivity properties. Most of the known solar control coatings comprise a layer stack having either a double TiN functional layers or have single TiN layer in combination with NiCr/ NiCrN functional layer. There is no prior known solar control coatings that comprise of a combination of TiN and Nb/NbN functional layers and as reasoned in US '691 niobium based layers such as Nb, NbN, NbZr or NbZrN have in general high emissivity and solar heat gain coefficient (SHGC) values and low selectivity. Hence skilled artisans preferred to employ solar control coatings comprising either a double TiN functional layers or have single TiN layer in combination with NiCr/ NiCrN functional layer. However, the inventors of the present invention have surprisingly found that for layer stacks comprising a combination of TiN and Nb/NbN sandwiched between dielectric layers, the TiN functional layer contributes to the solar control performance of the layer stack and the Nb/NbN functional layer can be used to control the reflection color, reflection value and other optical characteristics of the solar control coating and improve aesthetics of the resulting solar control substrate. Further, it was found that the introduction of a niobium based layer between the glass substrate and the TiN functional layer helped in maintaining the internal reflection color (RC) of the solar control coating similar to that of a solar control coating having a layer stack of Glass/Si3N4/TiN/Si3N4. Furthermore, it was found that the introduction of a niobium based layer between the glass substrate and the TiN functional layer provided means for controlling the optical characteristics of the solar control coated articles without significantly compromising the performance properties of the solar control articles. Most of the known and above referenced coating system in the prior art either achieve a lower internal reflection or a lower external reflection, often at the cost of increasing the other. Whereas the embodiments of the present invention disclose that the inventors were able to achieve a relatively low internal (RC) and external (RG) reflection values while achieving desirable external reflection colors ranging from neutral, green, bronze, blue etc., and non-intensive internal reflection colors. In addition to achieving desirable visible light transmittance, low SF and high selectivity. It is thus a purpose of this disclosure to help achieve all the said characteristics, detail of which will become apparent to the skilled artisan once given the following disclosure. Certain example embodiments of this invention relate to a solar control glass article comprising a layer stack of glass/Si3N4/Nb or NbN/Si3N4/TiN/Si3N4 having an internal reflection (RC) of less than 15% and an external reflection (RG) of less than 23%; neutral external reflection (RG) color; visible light transmission (T_L) ranging between 5% and 65%; selectivity of more than 0.7. Certain example embodiments of this invention also relate to a heat treatable solar control glass article. Summary of the Disclosure In one aspect of the present disclosure, a solar control glass article comprising a transparent substrate having a first surface deposited with a stack of thin layers comprising, n functional layers and n+1 dielectric layers, wherein n=2 such that each functional layer is sandwiched between 2 dielectric layers and optionally an overcoat layer, wherein the overcoat layer forms the outermost layer of the thin multilayer coating is disclosed. The solar control glass article is characterized in that the first functional layer (F1) comprises niobium-based material having a thickness range of 1 nm to 14 nm and the second functional layer (F2) comprises TiN having a thickness range of 20 nm to 50 nm, an internal reflection (RC) of less than 15% and an external reflection (RG) of less than 23%. The solar control glass article is heat treatable from the glass side with a ΔE*G of less than or equal to 3 and a has selectivity of more than 0.7. Other features and aspects of this disclosure will be apparent from the following description and the accompanying drawings. Brief Description of the Drawings Embodiments are illustrated by way of example and are not limited to those shown in the accompanying figures. FIG. 1 illustrates a stack of thin layers deposited on a transparent glass substrate, according to one embodiment of the present disclosure; and FIG. 2 illustrates a stack of thin layers deposited on a transparent glass substrate, according to one other embodiment of the present disclosure; Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the invention. Detailed Description Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or similar parts. Embodiments disclosed herein are related to material having a desirable visible light transmittance, low SF, high selectivity while maintaining a relatively low internal (RC) and external (RG) reflection values and achieving neutral external reflection color and non-intensive internal reflection colors. FIG. 1 illustrates a structure of a stack of thin layer having two functional layers F1, F2 deposited on a transparent substrate 10. Each of the functional layers 50, 100 is positioned between dielectric coatings 20 (M1), 40 (M2), 80 (M3) such that: the first functional layer 50, starting from the substrate, is positioned between the dielectric coatings 20, 40 and the second functional layer 100 is positioned between the dielectric coatings 40, 80. The dielectric coatings 20, 40 and 80 each comprise at least one dielectric layer. The stack of thin layers may further optionally comprise at least one overcoat layer 1000 (not represented) in contact with the dielectric coating 80 (M3). Achieving desired optical characteristics while maintaining the performance characteristics becomes challenging for a 5-layered solar control coating having a layer stack of glass/Si3N4/NiCr/Si3N4/NiCr/Si3N4 or glass/Si3N4/TiN/Si3N4/TiN/Si3N4. The inventors of the present invention have surprisingly found that the introduction of a niobium-based functional layer as an intermediate layer between the glass substrate and the second functional layer made of TiN in a solar control coating provides the best opportunity to control the optical characteristics of the solar control coating without negatively impacting the solar control performance of the solar control coated glass article. It was further found by the inventors of the present invention that, the thickness of this intermediate niobium-based functional layer significantly controls the internal reflection (RC) and external reflection (RG) values of the solar control coated glass article. Furthermore, it is specifically elucidated by the inventors of the present invention that, the first functional layer made of Nb or NbN controls the reflection levels and the overall aesthetics of the solar control coated glass article, while the second functional layer made of TiN controls the solar control performance of the solar control coated glass article. While most of the known solar control coating as highlighted in the earlier section optimize either the internal or external reflection of the coated glass article, the teachings of the present invention demonstrate that desired reflection % and color values in reflection can be achieved by the introduction of the intermediate niobium-based functional layer in close proximity to the glass substrate. Hence the solar control coated article as taught by the teachings of the present invention achieve superior internal and external aesthetics while also achieving improved solar control properties. Within the meaning of the present invention, the label “first”, “second” for the functional layers and “first”, “second”, “third” for the dielectric coatings are defined starting from the substrate bearing the stack and with reference to the layers or coatings having the same function. For example, the functional layer closest to the substrate is the first functional layer, the one farthest from the substrate is the second functional layer. Likewise, the dielectric coating closest to the substrate is the first dielectric coating, the next one moving away from the substrate is the second dielectric coating etc. Thicknesses stated in the present document with no other specifications are physical, real or geometric thicknesses referred to as T and are expressed in nanometers (and not optical thicknesses). The thickness of the dielectric coatings is represented as T. T1, T2 and T3 according to the specific dielectric coating they refer to. The inventors of the present invention have worked with two functional layer materials for F1 and F2: Nb or NbN and TiN. Thus the thin multilayer coating proposed by the present invention comprise in multiple embodiments: Glass | Dielectric Coating (M1) | Nb or NbN (F1) | Dielectric coating (M2) | TiN (F2) | Dielectric coating (M3). Teachings of the present invention in the following sections will establish how the introduction of the intermediate niobium-based functional layer in close proximity to the glass substrate controls the internal or external reflection of the coated glass article to achieve desired optical and solar control performance characteristics that is not achievable by any of the previously described known solar control coatings. The following disclosures and examples will demonstrate how the above stated factor influence the internal reflection (RC), external reflection (RG) light transmission (TL), solar factor (SF) and selectivity. According to one embodiment of the present invention, thickness of F1 comprising materials Nb or NbN preferably ranges between 1 nm and 14 nm and thickness of F2 comprising TiN preferably ranges between 20 nm and 50 nm. These thickness ranges for F1 and F2 are best suited for obtaining internal reflection (RC) of less than 15% and an external reflection (RG) of less than 23%. According to one other embodiment of the present invention, F1 comprising materials Nb or NbN is metallic or partially or fully nitride. In additional optional embodiments of the present invention, F1 comprising materials Nb or NbN, further comprise materials such as zirconium or Molybdenum, wherein the materials comprised in F1 NbZr, NbMo or NbZrMo. According to one other embodiment of the present invention, the functional layers F1 and/or F2 can be optionally sandwiched between barrier layers. In such embodiments, barrier layers are comprised of material selected from Titanium, NiCr, SiAl or absorbing SiAlNx or Niobium. In specific embodiments where the F2 is sandwiched by barrier layers, the barrier layer is made of Niobium. Thickness of such barrier layer range from 1 nm and 5 nm. According to optional embodiments, functional layers F1 and/or F2 can be optionally oxidized either completely or partially. According to multiple embodiments of the present invention, thickness of dielectric coating 20 (M1) preferably ranges between 5 nm and 70 nm; thickness of dielectric coating 40 (M2) preferably ranges between 5 nm and 60 nm; and thickness of dielectric coating 80 (M3) preferably ranges between 5 nm and 60 nm, inclusive of all said values mentioned for M1, M2 and M3. The three dielectric coatings 20, 40, 80 comprise at least one dielectric layer based on a material selected from silicon nitride, aluminum nitride, oxynitrides of silicon and aluminum, silicon aluminium nitride, zinc oxide, tin and zinc oxide, tin oxide, titanium oxide, silicon oxide, aluminum oxide or titanium and tin oxide. According to a preferred embodiment of the present invention, the dielectric coatings 20, 40, 80 is made of silicon nitride. According to a most preferred embodiment of the present invention, the dielectric coatings 20, 40, 80 is made of silicon nitride doped with aluminum. Dielectric coating 20 (M1), 40 (M2) and 80 (M3) have a barrier function according to the present invention and should be understood layers made of a material capable of forming a barrier to the diffusion of sodium, oxygen and/or water at high temperature, originating from either the transparent substrate or the ambient atmosphere towards the functional layer. The constituent materials of the dielectric layer having a barrier function thus must not undergo chemical or structural modification at high temperature which would result in a modification to their optical properties. The layer or layers having a barrier function are preferably also selected from a material capable of forming a barrier to the constituent material of the functional layer. The dielectric layers having a barrier function thus allow the stack to be subjected, without excessively significant optical change, to heat treatments of the annealing, tempering or bending type. According to an optional embodiment, the stack of thin layers may further comprise at least one overcoat layer 1000 in contact with the dielectric coating 80 (M3). The overcoat layer 1000 comprises titanium zirconium nitride or oxynitride, titanium zirconium hafnium nitride or oxynitride, zirconium oxide or titanium oxide or their combinations thereof. According to a preferred optional embodiment, the overcoat layer 1000 comprises titanium zirconium oxide. Typically, the configuration of the stack of thin layers is designed such that the external reflection (RG) is higher and the internal reflection (RC) is lower. This is because the high external reflection (RG) provides privacy during the day time and the low internal reflection (RC) provides a clear view of the external environment in addition to providing visual comfort. Alternatively, certain other architectural applications could desire low external reflection (RG) for improving the depth of the reflection color. Nevertheless, in view of visual comfort for people placed inside and outside the building it is critical that the solar control coated glass article achieve less reflection % internally on the coating side (RC) and externally on the glass side (RG) and as well obtain aesthetically pleasing and pleasant reflection and transmission colors and not result in any intensive colors in external reflection and internal reflection. According to a one embodiment of the present invention, a solar control glass constructed having the below configuration of the stack of thin layers: Glass | Dielectric Coating (M1) | Nb or NbN (F1) | Dielectric coating (M2) | TiN (F2) | Dielectric coating (M3), exhibits the following optical characteristics: - internal reflection (RC) less than external reflection (RG); - internal reflection (RC) less than 15%; - external reflection (RG) less than 23%; - ratio of internal reflection (RC) to external reflection (RG) not exceeding 5; - visible light transmission (TL) ranging between 5% and 65%. The transparent substrates according to the present invention are preferably made of an inorganic rigid material, such as glass, or an organic material based on polymers (or made of polymer). The substrate is preferably a sheet of glass or of glass-ceramic. The substrate is preferably transparent, colorless (it is then a clear or extra-clear glass) or colored, for example colored blue, grey, green or bronze. The glass is preferably of soda-lime-silica type, but it may also be made of glass of borosilicate or alumino-borosilicate type. The substrate advantageously has at least one dimension greater than or equal to 1 m, or even 2 m and even 3 m. The thickness of the substrate generally varies between 0.5 mm and 19 mm, preferably between 0.7 and 9 mm, in particular between 2 and 12 mm, or even between 4 and 10 mm. The substrate may be flat or curved, or even flexible. The material, that is to say the substrate coated with the stack, may undergo a high-temperature heat treatment such as an annealing, for example a flash annealing such as a laser or flame annealing and/or a tempering. The temperature of the heat treatment is greater than 500° C, preferably greater than 550° C, and better still greater than 600° C. The substrate coated with the stack may therefore be tempered. The heat treated solar control glass article has a superior color matchability with a ΔE* of less than 3.0 for external reflection and transmission, according to a preferred embodiment. The invention also relates to a glazing comprising a material according to the invention. Conventionally, the faces of a glazing are denoted starting from the outside of the building and by numbering the faces of the substrates from the outside towards the inside of the passenger compartment or room that it equips. This means that the incident solar light passes through the faces in the increasing order of their number. The stack is preferably positioned in the glazing so that the incident light coming from outside passes through the first dielectric coating before passing through the first functional layer F1. The stack is not deposited on the face of the substrate that defines the external wall of the glazing but on the inner face of this substrate. The stack is therefore advantageously positioned on face 2, face 1 of the glazing being the outermost face of the glazing, as is customary. The material may be intended for applications that require the substrate coated with the stack to have undergone a heat treatment at a high temperature such as a tempering or an annealing. The glazing of the invention may be in the form of monolithic, laminated or multiple glazing, in particular double glazing or triple glazing. In the case of a monolithic glazing, the stack is preferably deposited on face 2, that is to say that it is on the substrate that defines the external wall of the glazing and more specifically on the inner face of this substrate. A monolithic glazing comprises 2 faces; face 1 is on the outside of the building and therefore constitutes the external wall of the glazing, face 2 is on the inside of the building and therefore constitutes the internal wall of the glazing. A multiple glazing comprises at least two substrates kept at a distance so as to delimit a cavity filled by an insulating gas (e.g., dry air, Ar, Kr or their mixture). The materials according to the invention are very particularly suitable when they are used in double glazings with enhanced thermal insulation (ETI). A double glazing comprises 4 faces; face 1 is outside of the building and therefore constitutes the external wall of the glazing, face 4 is inside the building and therefore constitutes the internal wall of the glazing, faces 2 and 3 being on the inside of the double glazing. The stack may be on face 2, 3 or 4 of the glazing. In the same way, a triple glazing comprises 6 faces; face 1 is outside of the building (external wall of the glazing), face 6 is inside the building (internal wall of the glazing) and faces 2 to 5 are on the inside of the triple glazing. A laminated glazing comprises at least one structure of first substrate/sheet(s)/second substrate type. The stack of thin layers is positioned on at least one of the faces of one of the substrates. The stack may be on the face of the second substrate not in contact with the, preferably polymer, sheet. This embodiment is advantageous when the laminated glazing is assembled as double glazing with a third substrate. The glazing of the invention has colors in transmission in the L*a*b* color measurement system: a*T between -5 and +2, preferably between -4 and +1; and b*T between -9 and +11, preferably between -6 and +10. The glazing of the invention has colors in internal reflection (RC) in the L*a*b* color measurement system: This along with a low internal reflection (RC) value of less than 15% aid in visual comfort for people facing the interior of the building. The glazing of the invention has colors in external reflection (RG) in the L*a*b* color measurement system: This along with a low external reflection (RG) value of less than 23% aid in visual comfort for people facing the exterior of the building. Furthermore, these visual appearance remains virtually unchanged irrespective of the angle of incidence with which the glazing is observed (normal incidence and under an angle). This means that an observer does not have the impression of a significant lack of uniformity in color or in appearance. According to advantageous embodiments, the glazing of the invention has, in particular, the following performances: a solar factor less than or equal to 0.55, preferably less than or equal to 0.50, and/or a high selectivity, in order of increasing preference, of at least 0.7, of at least 0.8, and/or a low emissivity, in particular of less than 0.4. Preferably, the stack is deposited by magnetron sputtering. According to this advantageous embodiment, all the layers of the stack are deposited by magnetron sputtering. The invention also relates to the process for obtaining a material according to the invention, wherein the layers of the stack are deposited by magnetron sputtering. Examples Example 1 Preparation of the Substrates: Stack of thin layers Stack of thin layers, defined below, are deposited on substrates made of clear soda-lime glass with a thickness of 6 mm. In the example of the invention: The first functional layer is NbN; the second functional layer is TiN; and the dielectric layers are based on silicon nitride, doped with aluminum (Si₃N₄:Al). Table 1 lists the materials and thicknesses in nanometers for each layer or coating that forms the stacks as a function of their position with respect to the substrate bearing the stack (final line at the bottom of the table). The “Ref.” numbers correspond to the references from FIG.1, for samples 1 and 2. As for the comparative samples, comparative sample 1 is constructed from the prior art reference U.S. Patent Document 2018/0186691; and comparative sample 2 is constructed from the prior art reference German patent: DE102014114330. Table 1: Stack of thin layers

Solar Control and Optical Properties Table 2 lists the main optical characteristics measured when the glazings are part of a monolithic glazing of 6 mm glass. For these monolithic glazings: T_L indicates: the light transmission in the visible region in %, measured according to the illuminant D65 Obs 2; a*T and b*T indicate the a* and b* colors in transmission in the L*a*b* system measured according to the illuminant D65 Obs 2 and measured perpendicularly to the glazing; RG indicates: the light reflection in the visible region in %, measured according to the illuminant D65 Obs 2 on the glass side of the glazing; a*RG and b*RG indicate the a* and b* colors in reflection in the L*a*b* system measured according to the illuminant D65 Obs 2 on the glass side of the glazing and thus measured at 8 deg from the glazing normal incidence; RC indicates: the light reflection in the visible region in %, measured according to the illuminant D65 Obs 2 on the coating side of the glazing; a*RC and b*RC indicate the a* and b* colors in reflection in the L*a*b* system measured according to the illuminant D65 Obs 2 on the coating side of the glazing and thus measured at 8 deg from the glazing normal incidence. The main optical characteristics of the comparative samples 1 & 2 were constructed from the patent information present in the corresponding prior art references. Table 2: Optical & Solar Control Properties

*SHGC (NFRC - 2001) From Tables 1 & 2, for samples 1 and 2 constructed according to the present invention, it can be vividly seen that the thickness variation is the only change between the stack configuration of sample 1 and 2. Comparison of the optical characteristics of the samples 1 & 2 prepared as per the teachings of the present invention and those of the comparative examples, it is evident that the comparative sample 1 & 2 have high internal and/or external reflection values and the external reflection colors are intensive as can be understood from the a*RG & b*RG values. On the contrary the external reflection color of samples 1 and 2 are neutral. The material comprised in F1 of the thin layer of stack taught in the present invention is significant in determining the optical characteristics of the resultant solar control coated glass article. Similarly, samples 1 and 2 achieve a less intensive external reflection color as compared to the comparative samples. This can be particularly attributed to the introduction of the intermediate functional layer based on niobium-based material. The dielectric properties of Nb or NbN disrupts the optical path which in turn impacts the external reflection without significantly impacting the internal reflection. Example 2 In order to demonstrate the role of the intermediate functional layer based on NbN, sample 1 was from the stack of thin layers, according to the present invention demonstrated in Table 2 was compared with comparative samples 3 & 4 constructed from prior art inventions. Table 4 lists the main optical characteristics measured when the glazings are part of monolithic structure, the stack being positioned on face 2 (face 1 of the monolithic glazing being the outermost face of the glazing, as is customary). Table 3: Stack of thin layers

Table 4: Optical & Solar Control Properties

It can be seen from Table 4 that the comparative samples 3 & 4 achieve similar visible transmission as that of sample 1. However, in sample 1 due to the presence of the intermediate functional layer made of NbN, the external reflection (RG) properties are drastically modified. Nevertheless, the internal reflection (RC) value and transmission color remain unaltered. This decrease in external reflection (RG) is desirable to remove the discomfort for the people stationed outside the building. Further the external reflection color is similar for sample 1 and comparative sample 4. However, such an external optical characteristic is achieved in comparative sample 4 only at the cost of compromising the internal optical characteristics as is evident from the table. As a result of which comparative sample 4 is more reflective than the sample proposed by the present invention. Sample 1 and comparative samples 3 & 4 were heat treated (tempered) at 630°C for about 6 minutes and the color matchability post tempering was assessed. The results tabulated in table 5 demonstrates that the optical properties of sample 1 does not influence the tempering shift in the sample and the results are similar. The color shift of sample 1 in the glass side is very low. Table 5: Calorimetric Values

Solar Control Performance: Emissivity, solar factor and selectivity values of sample 1 and comparative sample 1 & 3 are provided in table 6. Table 6: Performance Properties

*SHGC It can be understood from Table 6 that the selectivity of sample 1 is superior to comparative sample 1 and almost equal to that of comparative sample 3. Hence it is evident that the introduction of the intermediate functional layer based on NbN has not impacted the solar control properties of the stack of thin layers known from the prior art but has rather enhanced the controllability of external and internal optics of the solar control glass article. Example 2 Preparation of the Substrates: Stack of thin layers Stack of thin layers, defined below, are deposited on substrates made of clear soda-lime glass with a thickness of 6 mm. In the example of the invention: The first functional layer is Nb; the second functional layer is either TiN; and the dielectric layers are based on silicon nitride, doped with aluminum (Si₃N₄:Al). Table 7 lists the materials and thicknesses in nanometers for each layer or coating that forms the stacks as a function of their position with respect to the substrate bearing the stack (final line at the bottom of the table). The “Ref.” numbers correspond to the references from FIG.1, for samples 5 – 7. Table 7: Stack of thin layers

Si3N4:Al 80 52.4 52.7 44.6

Si3N4:Al 40 58.8 44 23.8 Nb 50 1.1 1.5 2.8

Glass 10 6 mm 6 mm 6 mm Solar Control and Optical Properties Table 8 lists the main optical characteristics measured when the glazings are part of monolithic structure, the stack being positioned on face 2 (face 1 of the monolithic glazing being the outermost face of the glazing, as is customary). Table 8: Optical & Solar Control Properties

Samples 5 – 7 illustrate the varied external reflection colors that can be achieved using teaching of the present invention. Sample 5 demonstrated a green external reflection color while samples 6 and 7 demonstrated blue and bronze reflection color, respectively. These samples further evidence that such external reflection colors are achieved while retaining the desired low internal and external reflection values and desired visible light transmission. Industrial Applicability The solar control glass article described in the present disclosure finds application as a glazed element in building. In this application case, the glazing may form a monolithic glazing with the coating side of the glass arranged facing the closed space inside the building. The glazing may also form a laminated glazing whose stack of layers may be in contact with the thermoplastic adhesive material connecting the substrates, in general PVB. The glazing may also be part of an insulation glazing window. The glazing of the present disclosure can also be annealed, strengthened, toughened and/or tempered. The tempered glazing can also be used in building wall cladding panel of curtain walling for interior applications. Further can also be used as a side window, rear window or sunroof for an automobile or other vehicle. Note that not all of the activities described above in the general description or the examples are required, that a portion of a specific activity may not be required, and that one or more further activities may be performed in addition to those described. Still further, the order in which activities are listed is not necessarily the order in which they are performed. Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims. The specification and illustrations of the embodiments described herein are intended to provide a general understanding of the structure of the various embodiments. The specification and illustrations are not intended to serve as an exhaustive and comprehensive description of all of the elements and features of apparatus and systems that use the structures or methods described herein. Certain features, that are for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features that are, for brevity, described in the context of a single embodiment, may also be provided separately or in a sub combination. Further, reference to values stated in ranges includes each and every value within that range. Many other embodiments may be apparent to skilled artisans only after reading this specification. Other embodiments may be used and derived from the disclosure, such that a structural substitution, logical substitution, or another change may be made without departing from the scope of the disclosure. Accordingly, the disclosure is to be regarded as illustrative rather than restrictive. The description in combination with the figures is provided to assist in understanding the teachings disclosed herein, is provided to assist in describing the teachings, and should not be interpreted as a limitation on the scope or applicability of the teachings. However, other teachings can certainly be used in this application. As used herein, the terms "comprises," "comprising," "includes," "including," "has," "having" or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a method, article, or apparatus that comprises a list of features is not necessarily limited only to those features but may include other features not expressly listed or inherent to such method, article, or apparatus. Further, unless expressly stated to the contrary, "or" refers to an inclusive-or and not to an exclusive-or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). Also, the use of "a" or "an" is employed to describe elements and components described herein. This is done merely for convenience and to give a general sense of the scope of the invention. This description should be read to include one or at least one and the singular also includes the plural, or vice versa, unless it is clear that it is meant otherwise. For example, when a single item is described herein, more than one item may be used in place of a single item. Similarly, where more than one item is described herein, a single item may be substituted for that more than one item. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The materials, methods, and examples are illustrative only and not intended to be limiting. To the extent that certain details regarding specific materials and processing acts are not described, such details may include conventional approaches, which may be found in reference books and other sources within the manufacturing arts. While aspects of the present disclosure have been particularly shown and described with reference to the embodiments above, it will be understood by those skilled in the art that various additional embodiments may be contemplated by the modification of the disclosed machines, systems and methods without departing from the spirit and scope of what is disclosed. Such embodiments should be understood to fall within the scope of the present disclosure as determined based upon the claims and any equivalents thereof. List of Elements TITLE: A HEAT TREATABLE SOLAR CONTROL GLASS ARTICLE COMPRISING TITANIUM NITRIDE AND NIOBIUM-BASED FUNCTIONAL LAYERS 10 Glass Substrate 20 First Dielectric Layer M1 40 Second Dielectric Layer M2 50 First Functional Layer F1 80 Third Dielectric Layer M3 100 Second Functional Layer F2 1000 Overcoat Layer

Claims

Claims 1) A solar control glass article comprising a transparent substrate having a first surface provided with a thin multilayer coating comprising: n functional layers and n+1 dielectric layers, wherein n=2 such that each functional layer is sandwiched between 2 dielectric layers and optionally an overcoat layer, wherein the overcoat layer forms the outermost layer of the thin multilayer coating, characterized in that: the first functional layer comprises niobium-based material having a thickness range of 1 nm to 14 nm; and the second functional layer comprises TiN having a thickness range of 20 nm to 60 nm, wherein said solar control glass article has an internal reflection (RC) of less than 15% and an external reflection (RG) of less than 23%, characterized in that the solar control glass article is heat treatable and a selectivity of more than 0.7 2) The solar control glass article as claimed in claim 1, wherein the first functional layer comprised of niobium-based material is metallic or partially or fully nitrided. 3) The solar control glass article as claimed in claim 1, wherein the dielectric layer comprises a material selected from the group consisting of silicon nitride, aluminum nitride, silicon aluminium nitride, oxynitrides of silicon and aluminum, zinc oxide, tin and zinc oxide, tin oxide, titanium oxide, silicon oxide, aluminum oxide or titanium and tin oxide. 4) The solar control glass article as claimed in claim 1, wherein the dielectric layer comprises silicon nitride. 5) The solar control glass article as claimed in claim 4, wherein the dielectric layer comprising silicon nitride is doped with aluminum. 6) The solar control glass article as claimed in claim 1, wherein the physical thickness of the first dielectric layer ranges between 5 nm to 70 nm. 7) The solar control glass article as claimed in claim 1, wherein the physical thickness of the second dielectric layer ranges between 5 nm to 60 nm. 8) The solar control glass article as claimed in claim 1, wherein the physical thickness of the third dielectric layer ranges between 5 nm to 60 nm. 9) The solar control glass article as claimed in claim 1 has a visible light transmission (T_L) ranging between 5% and 65%. 10) The solar control glass article as claimed in claim 1, wherein one or both functional layers are partially or completely oxidized. 11) The solar control glass article as claimed in claim 11, wherein one or both functional layers comprise oxygen not more than 5 wt%. 12) The solar control glass article as claimed in claim 1, wherein the overcoat comprises titanium zirconium nitride or oxynitride, titanium zirconium hafnium nitride or oxynitride, zirconium oxide or titanium oxide or their combinations thereof. 13) The solar control glass article as claimed in any of the preceding claims is a monolithic window. 14) The solar control glass article as claimed in any of the preceding claims is part of an insulation glazing window.