SPUTTERED TITANIUM OXYNITRIDE FILMS

Abstract: A titanium oxynitride film, method for its production, and coated articles comprising titanium oxynitride in combination with other metal-containing films are disclosed.

Description: The present invention relates generally to the art of sputtering metal-containing films on non-metallic substrates, and more particularly to the art of magnetic sputtering of multiple-layer metal-dielectric transparent films on glass. U.S. Patent No. 3,990,784 to Gelber discloses a coated architectural glass system comprising a transparent substrate and a multilayer coating comprising first and second metal layers with a dielectric layer between them, wherein the first and second metal layers have a thickness ratio so that the transmission of the coating can be changed independent of its reflection properties by varying the thickness of the metal layers while maintaining the ratio constant. The dielectric has a thickness such that the reflection from the coating is not strongly colored. U.S. Patent No. 4,022,947 to Grubb et al discloses a transparent panel capable of transmitting a desired portion of visible radiation while reflecting a large portion of incident solar radiation and a method of preparing same, by sputtering an iron, nickel and chromium alloy to obtain a transparent metal film, and reactively sputtering the same or a similar alloy in the presence of oxygen to form an oxide film. In one preferred embodiment, the metal film lies between the substrate and the metal oxide film. In another preferred embodiment, the metal oxide film lies between the substrate and the metal film. U.S. Patent No. 4,534,841 to Hartig, et al. discloses solar-control glazing produced by applying first an oxide layer having an optical thickness of 20 to 280 nanometers to a transparent substrate by cathodic evaporation and second a chromium nitride layer having a geometric thickness of 10 to 40 nanometers. An optical third dielectric layer may be applied to the second layer. The oxide layer is selected from oxides of tin, titanium, and aluminum. U.S. Patent No. 4,535,000 to Gordon discloses placing a thin film of metal nitride, e.g. titanium nitride, on a glass substrate by mixing a metal halide with a reducing gas like ammonia at 250 to 320°C and reacting the gases at the glass surface heated to 400° to 700°C to form the film on the glass. U.S. Patent No. 4,546,050 to Amberger et al discloses a glass sheet with a multilayer coating selected from the group consisting of copper, stainless steel, titanium dioxide; copper, titanium, titanium dioxide; and copper, titanium, titanium nitride. Architectural glass products with metallic and/or metal oxide films are growing in importance as energy demands for heating and cooling become increasingly expensive. Coated glass architectural products generally fall into two categories, solar energy control and high transmittance/low emissivity coated products. Solar energy control products are generally glass substrates, often tinted, coated with a low visible transmittance colored film which reduces solar energy transmittance through the windows into the building interior, thereby reducing air conditioning costs. These products are most effective in warm climates and are most often seen in commercial construction. In areas where heating costs are of greater concern, and particularly in residential construction, high transmittance/low emissivity coatings are desirable in order to allow high transmittance of visible light into the interior while reflecting infrared radiation to retain heat inside the building. High transmittance/low emissivity coatings are typically multiple layer films wherein an infrared reflecting metal such as silver, gold or copper is sandwiched between anti-reflective metal oxide layers such as bismuth, indium and/or tin oxides. Solar energy control films, on the other hand, are typically single layer films of one or more of the metals or oxides of metals such as cobalt, iron, chromium, nickel, copper, etc. Wet chemical methods for producing metallic films for solar energy control are well known from U.S. Patents 3,846,152; 4,091,172; 3,723,158 and 3,457,138. Pyrolytic methods for producing metal oxide films for solar energy control are well known from U.S. Patents 3,660,061; 3,658,568; 3,978,272 and 4,100,330. Sputtering technologies for producing high transmittance/low emissivity multiple layer coatings are disclosed in U.S. Patents No. 4,462,884 and No. 4,508,789. Sputtering techniques for producing solar control films are disclosed in U.S. Patents No. 4,512,863 and No. 4,594,137. Summary of the Invention The present invention provides a novel and superior dielectric film for use in a wide variety of multiple-layer architectural coatings on glass. The present invention involves sputtering a titanium cathode in an atmosphere comprising oxygen and nitrogen in order to deposit a coating comprising titanium oxynitride. The titanium oxynitride film of the present invention may be deposited in combination with an infrared reflective film such as silver to form a multilayer low emissivity film. The titanium oxynitride film of the present invention may also be deposited in combination with a metal alloy film such as stainless steel or Inconel to form a variety of colored multilayer coatings with relatively saturated colors. The titanium oxynitride film of the present invention may also be deposited in combination with both an infrared reflective film such as silver and a metal film which reduces the luminous reflectance, particularly a metal alloy film such as Inconel, to produce a multilayer coating which has a relatively saturated color and low emissivity. Brief Description of the Drawing Figure 1 illustrates the transmittance at 550 nanometers (nm) of a titanium oxynitride film on glass as a function of film thickness, measured in number of cathode passes, at various percentages of oxygen in nitrogen. Figure 2 illustrates the deposition rate of titanium oxynitride, in Angstroms per cathode pass, as a function of the percentage of oxygen in the atmosphere of the coating chamber. Figure 3 illustrates the absorption of a titanium oxynitride film about 600 Angstroms thick as a function of the percentage of oxygen in the atmosphere of the coating chamber. Figure 4 illustrates the transmittance at 550 nanometers of a titanium oxynitride film over an Inconel film as a function of film thickness at various cathode power levels. Detailed Description of the Preferred Embodiments A transparent, nonmetallic substrate, preferably glass, is coated by cathode sputtering, preferably magnetron sputtering, to provide a product comprising titanium oxynitride with desirable durability and aesthetic properties. In a conventional magnetron sputtering process, a substrate is placed within a coating chamber in facing relation with a cathode having a target surface of the material to be sputtered. Preferred substrates in accordance with the present invention include glass, ceramics and plastics which are not detrimentally affected by the operating conditions of the coating process. The cathode may be of any conventional design, preferably an elongated rectangular design, connected with a source of electrical potential, and preferably employed in combination with a magnetic field to enhance the sputtering process. At least one cathode target surface comprises titanium which is sputtered in a reactive atmosphere to form a titanium oxynitride film. The anode is preferably a symmetrically designed and positioned assembly as taught in U.S. Patent No. 4,478,702 by Gillery et al. The titanium oxynitride of the present invention is deposited by sputtering a titanium cathode in an atmosphere comprising oxygen and nitrogen. The composition of the atmosphere preferably ranges from 10 to 50 percent oxygen and from 90 to 50 percent nitrogen. An atmosphere comprising 10 to 25 percent oxygen and the balance nitrogen is particularly preferred. The figures show that the properties of the titanium oxynitride change gradually and continuously as the gas composition changes. In contrast, titanium sputtered in an oxygen/argon atmosphere exhibits an abrupt change from oxide to metal. The figures further show that it is possible to choose deposition conditions such that a titanium oxynitride film with desired transmittance and absorbance properties can be sputtered at a desired rate. Certain desired coating colors can be produced for architectural purposes by combining a colorless dielectric material with inner and outer colored metal layers, or by combining colored metal oxide with a reflective metal. In accordance with the present invention, desired coating colors are obtained by combining titanium oxynitride with a highly infrared reflective metal such as silver to produce intense colors with a high degree of saturation as well as low emissivity. If the luminous reflectance of such a coating is higher than desired, it can be reduced, without sacrificing color purity or emissivity, with an optional coating of a neutral metal such as alloys of nickel and iron, particularly Inconel and stainless steel. The present invention provides the capability of making a series of colored coatings with a minimum of layers and materials. The coating system of the present invention has relatively low reflection, high color saturation and monolithic durability. It is known that a color series can be made with first and second metal layers surrounding a layer of a transparent dielectric material; the color being varied by changing the thickness of the dielectric layer. However, no previously practiced dielectric has had the required properties of fast sputtering, high refractive index and good durability. The titanium oxynitride of the present invention has the above properties, as well as the ability, in combination with a suitable metal film, to produce intensely colored architectural coatings. For example, titanium oxynitride in combination with a nickel alloy can be used to make a range of attractive colors with excellent durability. Using reflectance circle diagrams and computer calculations, it is determined that a two layer combination of metal and dielectric has an optimum thickness for both layers which gives the minimum reflectance and highest color saturation combination. The higher the refractive index of the dielectric, the lower the transmission of the coating at the optimum, and the higher the color saturation. Metals with low n and high k, where n and k are the real and complex parts of the complex refractive index, defining the propagation of electromagnetic radiation in the medium, tend to give the lowest transmission and highest saturation. If the thickness of the metal is increased in an attempt to lower the transmission, the reflectance is increased and a weak color results. Depositing a very thin metal layer prior to the deposition of the dielectric layer can decrease the reflectance and give a more saturated color. If the thickness of the primary metal layer is increased in combination with the deposition of a very thin metal layer, a low transmitting, low reflectance, highly coloured coating can be produced. If two primary metal layers are used, a low refractive index dielectric in combination with a low n, high k, metal gives the most attractive appearance. The calculations show that at 20 percent light transmission, adequate saturation can be obtained using a metal in combination with a dielectric with a refractive index of 2.3. For lower light transmission, a metal-dielectric-metal system is preferred. With the titanium oxynitride of the present invention, many metal or metal alloy films can be used to provide a multiple-layer coating with good properties. Preferred films include metals such as titanium, and metal alloys such as nickel alloys and iron alloys. A nickel alloy is preferred since it is highly chemical resistant, neutral in color and easy to deposit. A clean glass substrate is placed in a coating chamber which is evacuated, preferably to less than 1.33x10⁻² Pa (10⁻⁴ torr), more preferably less than 2.66x10⁻³ Pa (2x10⁻⁵ torr). A selected atmosphere of reactive gases, preferably nitrogen and oxygen, is established in the chamber to a pressure between 6.65x10⁻² and 10 Pa (5x10⁻⁴ and 10⁻² torr.) A cathode having a target surface of titanium is operated over the surface of the substrate to be coated. The target metal is sputtered, reacting with the atmosphere in the chamber to deposit a titanium oxynitride coating layer on the glass surface. After the initial layer of titanium oxynitride is deposited, the coating chamber is evacuated, and an inert atmosphere such as pure argon is established at a pressure between 6.65x10⁻² Pa and 10 Pa (5x10⁻⁴ and 10⁻² torr). A cathode having a target surface of metal or metal alloy is operated over the titanium oxynitride coated surface. The target is sputtered to deposit a metallic layer on the titanium oxynitride coated glass surface. A preferred metal is titanium. Preferred metal alloys include Inconel, a nickel alloy, and stainless steel, an iron alloy, preferably sputtered at a pressure of 4 to 6 millitorr in pure argon. In some preferred embodiments of the present invention, a metal film is deposited under, as well as over, the titanium oxynitride film. As in the case of a two layer film, the dominant wavelength of the reflected color from the uncoated surface depends almost totally on the thickness of the titanium oxynitride layer. The thickness of the top metal layer is varied until the transmission has about the required value, then the thickness of the underlying metal layer is varied until the desired reflection from the uncoated side of the article is attained. Final modification of the top metal film thickness may be required to obtain the optimum final transmission. Within the thickness range of interest, increasing the thickness of the top metal film decreases the transmission and increases the reflectance from the uncoated side of the coated article. Increasing the thickness of the bottom metal film decreases the transmission and decreases the reflectance from the uncoated side. In a preferred embodiment of the present invention, a multiple layer film is deposited by cathode sputtering to form a high transmittance, low emissivity coating. In addition to the titanium target, at least one other cathode target surface comprises a metal to be sputtered to form an infrared reflective metallic layer. A multiple layer coating having an infrared reflective metallic layer in combination with an anti-reflective titanium oxynitride layer is produced as follows. A clean glass substrate is placed in a coating chamber which is evacuated, preferably to less than 10⁻² Pa (10⁻⁴ torr), more preferably less than 2.66x10⁻³ Pa (2x10⁻⁵ torr). A selected atmosphere of reactive gases, preferably nitrogen and oxygen, is established in the chamber to a pressure between 6.65x10⁻²Pa and 10⁻³Pa (5x10⁻⁴ and 10⁻⁵ torr). A cathode having a target surface of titanium is operated, preferably at a power level of 5 to 10 kilowatts, over the surface of the substrate to be coated. The target metal is sputtered, reacting with the atmosphere in the chamber to deposit a titanium oxynitride coating layer on the glass surface. After the initial layer of titanium oxynitride is deposited, the coating chamber is evaluated and an inert atmosphere such as pure argon is established at a pressure between 6.65x10⁻²Pa and 10 Pa (5x10⁻⁴ and 10⁻² torr). A cathode having a target surface of silver metal is operated over the titanium oxynitride coated surface. The target metal is sputtered and deposits a uniform, highly infrared reflective, conductive metallic layer on the titanium oxynitride coated glass surface. A second layer of titanium oxynitride is deposited on the silver layer under essentially the same conditions used to deposit the first titanium oxynitride layer. The present invention will be further understood from the descriptions of specific examples which follow. EXAMPLE I A titanium cathode target measuring(5 by 17 inches) 12.7 by 43.2 centimeters is powered at 10 kilowatts in a vacuum chamber containing an atmosphere of 23 percent oxygen and 77 percent nitrogen at a pressure of 4 millitorr. The cathode is stationary while a glass substrate passes under the sputtering target surface at a rate of(120 inches) about 3 meters per minute. Four passes deposit a film comprising titanium oxynitride on the glass surface to a luminous transmittance of 75.7 percent. EXAMPLE II A glass substrate is coated with a first layer comprising titanium oxynitride as in Example I. The titanium oxynitride coated surface is then coated with a uniform layer of silver by sputtering a silver cathode target powered at 0.27 kilowatts in an argon atmosphere at a pressure of 5.32x10⁻¹Pa (4 millitorr)to a final luminous transmittance of 68 percent. To protect the silver from oxidation, a very thin protective coating of titanium is deposited in one pass of the titanium cathode powered at 0.03 kilowatts in argon at 5.32x10⁻¹Pa (4 millitorr)to a final luminous transmittance of 67.5 percent. EXAMPLE III A glass substrate is coated with titanium oxynitride and silver as in the previous examples. After depositing the thin, protective layer of titanium, a second layer of titanium oxynitride is deposited to a final luminous transmittance of 82.1 percent, thereby producing a high transmittance, low emissivity coated article. EXAMPLE IV A titanium cathode target measuring(5 by 17 inches) about 12.7 by 43.2 centimeters is powered at 10 kilowatts at 645 volts in a vacuum chamber containing an atmosphere comprising 23 percent oxygen and 77 percent nitrogen at a pressure of 5.32x10⁻¹Pa (4 millitorr). A glass substrate passes the cathode once at a speed of (108 inches) about 2.74 meters per minute and is coated with titanium oxynitride. The chamber is evaluated and an atmosphere of pure argon is introduced at a pressure of 5.32x10⁻¹Pa (4 millitorr). A silver cathode is powered at 441 volts at 2.5 amps to sputter a silver film over the titanium oxynitride coated surface in one pass at(120 inches) about 3.05 meters per minute. To protect the silver film from oxidation, a very thin layer of nickel alloy is deposited over the silver. A target of Inconel 625, which comprises 18.6 percent chromium, 3 percent iron, 4 percent columbium, 9 percent molybdenum and the balance nickel, is powered at 1 amp at 352 volts. The nickel alloy is sputtered in pure argon at 4 millitorr while the substrate passes at(120 inches) about 3.05 meters per minute. The coated article has a luminous transmittance of 21.3 percent and reflectance from the uncoated side of 54.6 percent. The color coordinates from the uncoated surface are x = .3516 and y = .3805. The observed color is pale yellow. EXAMPLE V A titanium oxynitride film in combination with a silver film provides a sufficiently reflective and a sufficiently saturated yellow colored film to simulate the appearance of a gold film. A titanium cathode powered at 10 kilowatts at 640 volts is sputtered as in Example IV except that the atmosphere at 5.32x10⁻¹ Pa (4 millitorr)pressure comprises less oxygen. One pass at(120 inches) about 3.05 meters per minute with a slightly oxygen-deficient atmosphere produces a titanium oxynitride film which is somewhat more absorbing than the oxynitride film of Example IV. A silver cathode powered at 441 volts at 2.4 amps is sputtered in pure argon at 4 millitorr to deposit a silver film over the titanium oxynitride coated surface in one pass at (120 inches) about 3.05 meters per minute. To protect the silver film from oxidation, a very thin film of nickel alloy as in Example IV is sputtered in argon at 5.32x10⁻¹ Pa (4 millitorr) in one pass at (120 inches) 3.05 meters per minute by a cathode target of Inconel 625 metal powered at 356 volts at one amp. The coated article has approximately the same luminous transmittance as the article of Example IV, but the reflectance from the uncoated surface is 40.2 percent and the color coordinates are x = .3833 and y = .4093. The observed color is gold, a more saturated color than that of Example IV. This film survives thermal testing without developing haze. EXAMPLE VI A multiple layer coating of titanium oxynitride and nickel alloy is deposited on a glass substrate under the following conditions. A clean glass substrate is maintained in a vacuum chamber in an atmosphere of 15 percent oxygen and 85 percent nitrogen at a pressure of 7.98x10⁻¹ Pa (6 millitorr). With a titanium cathode powered at 6.7 kilowatts and a line speed of(120 inches) about 3 meters per minute, eight passes are required to produce a titanium oxynitride coating at a thickness having first order blue color. The titanium oxynitride coated glass surface is then passed under a nickel alloy target in pure argon. The nickel alloy in this example is Inconel 625, which comprises 18.6 percent chromium, 3 percent iron, 4 percent columbium, 9 percent molybdenum and the balance nickel. A layer of nickel alloy is sputtered to a sufficient thickness to reduce the transmittance to 22 percent. The chromaticity coordinates of this coating are x = .3198 and y = .2863 in reflectance from the uncoated glass surface. The observed color is purplish-pink and the luminous reflectance is 5.65 percent from the uncoated glass surface. EXAMPLE VII Using the layer system titanium oxynitride-Inconel as in Example VI, a coating with about 20 percent luminous transmittance and an attractive blue color is produced under the conditions given in Table I. Color control of the two layer coating is simple. The thickness of the titanium oxynitride controls the hue. If it is too green, the layer is too thick. If it is too red, the layer is too thin. The thickness of the titanium oxynitride also affects the transmission (or reflectance) since reddish-blue coatings generally have higher transmission than greenish-blue coatings. However, once the hue is fixed, the transmission (or reflectance) can be adjusted by changing the thickness of the Inconel layer. As would be expected, increasing the thickness decreases the transmittance and increases the reflectance. This change has an insignificant effect on the dominant wavelength of the hue. The effects of layer thickness changes, expressed as percentages of the coating thickness produced by conditions in Table I, on the color of five two-layer coatings of this example are presented in Table II. EXAMPLE VIII A glass substrate is sputter coated as in the previous examples with the first layer of Inconel 625 to luminous transmittance of 60 percent. A titanium oxynitride film is sputtered over the nickel alloy as in the previous examples. A second nickel alloy film is deposited to a final luminous transmittance of 22 percent. The chromaticity coordinates of the coating are x = .2644 and y = .2340 from the glass surface. The observed color is violet and the luminous reflectance is 8.9 percent from the uncoated glass surface. EXAMPLE IX A series of three-layer coatings is prepared by varying the thicknesses of the individual titanium oxynitride and Inconel layers. The results for these examples are presented in Table IV, wherein the thicknesses are expressed as percentages of thicknesses obtained using the conditions stated in Table III. EXAMPLE X A titanium oxynitride film is deposited on a glass surface as in Example VI. A stainless steel film is deposited over the titanium oxynitride. The chromaticity coordinates of this coating are x = .2466 and y = .2680 from the glass surface. The observed color is greenish-blue and the luminous reflectance is 18.5 percent from the uncoated glass surface. EXAMPLE XI A titanium oxynitride film is deposited in 8 passes on a glass surface as in the previous examples. A titanium metal film is deposited by sputtering a titanium cathode in argon. The chromaticity coordinates of the coating are x = .3317 and y = .3037 from the glass surface. The observed color is purplish-pink and the luminous reflectance is 5.17 percent from the uncoated glass surface. EXAMPLE XII A titanium oxynitride film is deposited in 9 passes on a glass surface as in Example XI. A titanium metal film is deposited by sputtering a titanium cathode in argon. The chromaticity coordinates of the coating are x = .2402 and y = .2265 from the glass surface. The observed color is purplish-blue and the luminous reflectance is 5.32 percent from the uncoated glass surface. The above examples are offered to illustrate the advantages of the present invention. The coatings in Tables II and III are not attacked in 24 hours by cold 20 percent hydrochloride acid or cold 30 percent nitric acid. In the five hour(275°F) 135°C heat test, there is a small transmittance change and a small reflected color change. This would be consistent with a growth of protective oxide on the Inconel surface, a process which should be self-limiting. In the Cleveland condensing humidity test at(150°F) about 66°C , no change was observed in the coatings in four months. The coatings are not affected by abrading with a pencil eraser, nor by the cycling bristle brush test, used to evaluate the coatings for internal monolithic glazing. However, rubbing with wet or dry pumice shows that the coating is not as hard as coatings comprising titanium nitride. The titanium oxynitride/metal alloy combination of layers can produce a few attractive products. However, the metal/titanium oxynitride/metal system can produce a much broader range of reflectance colors and transmissions using only two materials. Titanium oxynitride is transparent, chemically resistant, has a high-index of refraction and is as fast to deposit as the oxides of tin and zinc, which have inferior properties. The concentration of oxygen in nitrogen is not as critical to the process as might be thought unless the deposition rate is pushed to its absolute maximum. This relieves the complication that in-machine monitors are only reliable in the transmission mode which cannot distinguish a decrease in transmission from an increase in film thickness from a decrease in transmission due to an increase in absorption. Thus, color control for the two-layer coating should not be difficult. Color control is slightly more complicated from the three layer coating, which, for instance, if too reflective, can be made less so either by making the top metal layer thinner or the bottom metal layer thicker. The above examples are offered to illustrate the present invention. Various sputtering conditions may be employed, the ratio of oxygen and nitrogen may be varied and the titanium oxynitride film of the present invention may be employed at various thicknesses and configurations with other metal-containing films to provide a wide array of reflective colors. The scope of the present invention is defined by the following claims.

Claims: 1. A method of making a coated article by placing a substrate in a coating chamber, evacuating said chamber and depositing a coating onto a surface of a substrate, characterized by a. providing an atmosphere comprising oxygen and nitrogen in said chamber; b. placing a titanium cathode in said chamber facing a surface of said substrate; c. sputtering said titanium cathode in said atmosphere comprising oxygen and nitrogen in said chamber thereby depositing a titanium oxynitride coating on said surface of said substrate. 2. A method according to claim 1, wherein said substrate is glass. 3. A method according to claim 1, wherein said atmosphere comprises from 10 to 50 percent oxygen and 50 to 90 percent nitrogen. 4. A method according to claim 1, wherein a second layer of a highly infrared reflective metal film is deposited on a surface of said substrate. 5. A method according to claims 1 and 4, wherein as a further step a third layer is deposited on said substrate comprising titanium oxynitride. 6. A method according to claims 1, 4 and 5, wherein a first layer of titanium oxynitride is deposited on said substrate, a layer of a highly infrared reflective metal film is deposited over the titanium oxynitride, and a second titanium oxynitride layer is deposited over the highly infrared reflective metal film. 7. A method according to claim 4 wherein as a further step a third layer is deposited on said substrate comprising a second metallic film. 8. A method according to claim 7, wherein said second metallic film comprises a metal alloy selected from the group of nickel alloys and iron alloys, stainless steel and Inconel. 9. A method according to claims 4 and 6, wherein said highly infrared reflective metal film is selected from the group consisting of copper, silver or gold. 10. A method according to claim 1, wherein a second layer of a metallic film is deposited in an inert atmosphere on a surface of said substrate. 11. A method according to claims 1 and 10, wherein a third layer of a metallic film is deposited on said substrate. 12. A method according to claims 1, 10 and 11, wherein said titanium oxynitride film is deposited between said first and second metallic films. 13. A method according to claims 10, 11 and 12, wherein said metal is selected from the group consisting of nickel alloys, iron alloys, titanium and mixtures thereof. 14. A method according to claim 13, wherein said metal alloy is selected from the group consisting of stainless steel and Inconel. 15. A method according to claims 1 to 14, wherein said layers are deposited by sputtering. 16. An article of manufacture prepared by the method of claims 1 to 15. 17. Use of a coated article as a structural element according to claims 1 to 6, 9 and 15 for the reflectance of solar energy. 18. Use of a coated article as a structural element according to claims 1 to 9 and 15 for the reflectance of solar energy with reduced total luminous reflectance of the coated article. 19. Use of a coated article according to claims 1 to 3 and 10 to 15 as a colored architectural product.

Inventors: GILLERY, FRANK HOWARD
Applicants: PPG INDUSTRIES, INC.
Application year: 1988
Main classification: C23C16
IPC codes: C23C4, C23C14