FILM STACK COMPOSITION, RELATED METHOD, AND APPLICATIONS

Abstract

A multi-layer PVD film stack contains high index of refraction base material(s), followed by a semi-transparent low index of refraction cap layer. The base layer(s) provide the color range of the film. The thickness of the cap layer dictates the degree to which the film retains the color properties of the base material and the reflectivity of the cap material. The cap layer not only increases the reflectivity of the base material, but it also decreases the reflectivity lost when the PVD film is topcoat lacquered. The lacquering is advantageous in protecting the PVD film but it decreases the reflectivity of the high index of refraction materials (generally n value>1.9 at 632 nm). These materials are desirable due to their color properties but are too dark after lacquering for many commercial applications. We address this problem by utilizing a high refractive index metal, or metals, as an opaque first layer followed by a thin semi-transparent second layer of a low refractive index metal. Upon topcoat lacquering, the resulting coating retains most of the aesthetically pleasing properties of the base metal(s) but minimizes the darkening of the metal(s) after topcoat due to the low refractive index of the second layer, which is in contact with the lacquer.

Claims

1. A thin film stack composition for application onto a substrate, comprising: a first layer disposed immediately adjacent the substrate surface comprising one or more metals each having an index of refraction equal to or greater than 1.9, said first layer having a total thickness equal to or greater than 250 angstroms (A); a second layer disposed immediately adjacent the first layer comprising one or more metals each having an index of refraction equal to or less than 1.5, said second layer having a total thickness equal to or less than 200 A, wherein the second layer is semi-transparent over the visible spectrum; and a non-metallic topcoat layer disposed immediately adjacent the second layer, wherein the topcoat layer is substantially transparent over the visible spectrum, whereby darkening of the first layer by the topcoat layer is reduced or eliminated.

2. The thin film stack composition of claim 1, wherein the first layer is at least one of Co, Cr, Fe, Ge, Mo, Ni, Pb, Pd, Pt, Re, Rh, Si, Sn, Ti, V, W, Zn or alloys thereof.

3. The thin film stack composition of claim 1, wherein the second layer is at least one of Ag, Al, Au, Cu, In, Jr, Mg, Mo or alloys thereof.

4. The thin film stack composition of claim 1, wherein the first layer is Chromium and the second layer is Aluminum.

5. The thin film stack composition of claim 1, wherein the first layer is Titanium and the second layer is Aluminum.

6. The thin film stack composition of claim 1, wherein the first layer is Cobalt and the second layer is Aluminum.

7. The thin film stack composition of claim 1, wherein the topcoat layer is a lacquer or a paint.

8. The thin film stack composition of claim 1, wherein the metallic thin film stack composition is a PVD stack.

9. A method for creating a highly reflective, decorative, film/thin film coating on a substrate, comprising: providing a substrate to be decoratively coated; using PVD to deposit a first layer of one or more metals on the substrate, each metal having an index of refraction equal to or greater than 1.9 measured at 632 nm, at a total thickness equal to or greater than 250 A; using PVD to deposit a second layer of one or more metals onto the first layer, each metal having an index of refraction equal to or less than 1.5 measured at 632 nm, at a total thickness equal to or less than 200 A, wherein the second layer is semi-transparent over the visible spectrum; and applying a protective, non-metallic, substantially transparent topcoat layer on the second layer.

10. The method of claim 6, further comprising drying the topcoat layer.

11. The method of claim 6, wherein the topcoat layer is a lacquer or a paint.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0022] FIG. 1 schematically displays the substrate, organic smoothing layer, PVD stack, and topcoat lacquering according to an exemplary, non-limiting embodiment.

[0023] FIG. 2 is a graphical model showing reflectivity of an opaque chromium PVD film in air compared to the reflectivity of that same opaque chromium PVD with a 50 A layer of aluminum on top in air. It shows an increase of 20% in reflectivity by adding 50 A of aluminum.

[0024] FIG. 3a is a graphical model comparing the reflectivity in air of opaque chromium PVD v. opaque chromium PVD with a topcoat of lacquer. The lacquer topcoat results in a 13% decrease in reflectivity. FIG. 3b is a graphical model comparing the reflectivity in air of the same opaque chromium film with an additional 50 A thick layer of aluminum v. the same stack with a lacquer topcoat ranging in thickness from 5 um to 200 um. The lacquer topcoat decreases the reflectivity by 9%, which is an improvement over opaque chromium alone when adding the topcoat lacquering.

[0025] FIG. 4 is a plot of the reflectivity of chromium topped with aluminum and lacquer, according to an exemplary, non-limiting embodiment. Reflectivity, that is, brightness, increases with increasing aluminum thickness. Reflectivity is measured at 532 nm (left) and 635 nm (right).

[0026] FIG. 5 is a plot of the L*a*b* color values (widely accepted as a method based on the complementary relationships between yellow, blue, green, and red, whereby humans perceive color) of each of five (four and a reference) chromium-based samples against the LAB color space background. Addition of aluminum pushes B values down, towards the blue/purple area of the plot and away from the yellow/red area. L*a*b* values for samples with aluminum and lacquer (samples 2-4) are closer to those of bare chromium than those with only chromium and lacquer (sample 1). Background color is provided as a guideline. While L*a*b* values are objective, the appearance of colors depends on monitor settings, individual vision, and other factors and should not be taken literally.

[0027] FIG. 6 is an illustration of color changes across the chromium-based samples with application of aluminum and lacquer shown in FIG. 5, according to an exemplary, non-limiting embodiment. Colors are generated from measured L*a*b* values, which are objective. The subjective appearance of these colors will depend on computer monitor settings, individual vision, and other factors and should not be taken literally.

[0028] FIG. 7 is a plot of the reflectivity of five (four and a reference) samples of titanium topped with aluminum and lacquer, according to an exemplary, non-limiting embodiment. Reflectivity, that is, brightness, increases with increasing aluminum thickness. Reflectivity is measured at 532 nm (left) and 635 nm (right).

[0029] FIG. 8 is a plot of the L*a*b* color values of each of the titanium-based samples against the L*a*b* color space background, according to an exemplary, non-limiting embodiment. Addition of aluminum pushes A and B values down, towards the blue/purple area of the plot and away from the yellow/red area. L*a*b* values for samples with aluminum and lacquer (2-4) are bluer than even those of bare titanium and significantly bluer than those with only titanium and lacquer (sample 1). Background color is provided as a guideline. While L*a*b* values are objective, the appearance of colors depends on monitor settings, individual vision, and other factors and should not be taken literally.

[0030] FIG. 9 is an illustration of color changes across the titanium-based samples of FIG. 8 with application of aluminum and lacquer, according to an exemplary, non-limiting embodiment. Colors are generated from measured L*a*b* values, which are objective. The subjective appearance of these colors will depend on computer monitor settings, individual vision, and other factors and should not be taken literally.

[0031] FIG. 10 is a plot of the reflectivity of five (four and a reference) samples of cobalt topped with aluminum and lacquer, according to an exemplary, non-limiting embodiment. Reflectivity, that is, brightness, increases with increasing aluminum thickness. Reflectivity is measured at 532 nm (left) and 635 nm (right).

[0032] FIG. 11 is a plot of the L*a*b* color values of each of the cobalt-based samples of FIG. 10 against the L*a*b* color space background, according to an exemplary, non-limiting embodiment. Addition of aluminum pushes A and B values down, towards the blue/purple area of the plot and away from the yellow/red area. L*a*b* values for samples with aluminum and lacquer (2-4) are bluer than even those of bare titanium and significantly bluer than those with only titanium and lacquer (sample 1). Background color is provided as a guideline. While L*a*b* values are objective, the appearance of colors depends on monitor settings, individual vision, and other factors and should not be taken literally.

[0033] FIG. 12 is an illustration of color changes across the cobalt-based samples of FIG. 10 with application of aluminum and lacquer, according to an exemplary, non-limiting embodiment. Colors are generated from measured L*a*b* values, which are objective. The subjective appearance of these colors will depend on computer monitor settings, individual vision, and other factors and should not be taken literally.

DETAILED DESCRIPTION OF EXEMPLARY, NON-LIMITING EMBODIMENTS

[0034] Aspects and embodiments disclosed herein and claimed address the problem of darkening or reduced reflection of a highly reflective metallic base layer on a substrate due to the application of a protective non-metallic topcoat, typically lacquer.

[0035] The solution involves the deposition of a higher refractive index metal, or metals, as an opaque first layer having a total thickness equal to or greater than 250 A on a [typically] non-metallic substrate followed by the deposition of a ‘very thin’ (≤200 A), semi-transparent second layer of a lower refractive index metal. Upon topcoat lacquering, the resulting coating retains most of the aesthetically pleasing properties of the base metal(s), but minimizes the darkening of the metal(s) after topcoat, due to the low refractive index of the second layer, which is in contact with the lacquer.

[0036] As used herein, a list of comparatively higher refractive index metals includes Co, Cr, Fe, Ge, Mo, Ni, Pb, Pd, Pt, Re, Rh, Si, Sn, Ti, V, W, Zn or alloys thereof. These metals have indices n≥1.9 @ 632 nm; a list of comparatively lower refractive index metals includes Ag, Al, Au, Cu, In, Ir, Mg, Mo or alloys thereof, having n≤1.5 @ 632 nm; and a ‘very thin’ layer (as that term is used herein) has a thickness≤200 A.

[0037] In the embodiments disclosed and claimed herein, a high index of refraction metal used as a base or first layer on the substrate will be chosen for its inherent color and optical properties. A low index of refraction metal used as the second, very thin layer covering the first, base layer will be chosen to increase the reflectivity of the high index film layer and decrease the amount of reflectivity lost from topcoat lacquering.

[0038] In an aspect, a metal or plastic opaque substrate is mounted to a holding fixture for processing in a physical vapor deposition (PVD) machine with two or more metallic sources available for producing a multilayer film of at least one high index of refraction metal and one low index of refraction metal. The substrate may be basecoat lacquered with an organic, in order to smooth the surface to provide a specular reflection.

[0039] Once sufficient vacuum is achieved, the high index of refraction metal chosen for its color and optical properties is taken from bulk form to vapor form in order to transfer atoms or molecules of the base metal to the substrate. Once the atom or molecule contacts the substrate, it will condense and begin to form the first layer of the PVD film on the part. This will occur for enough time as required to build a film of 250 A or thicker on the surface of the substrate.

[0040] It is possible that the first layer of PVD film referenced above be a combination of two or more additional high index of refraction metals in order to tune the color and other film properties, such as adhesion or stress. These layers combined will be 250 A or thicker. Additional metals used in the high index of refraction layer require additional sources of material inside the PVD system. Depending on the PVD technique used, this may mean additional evaporant boats, banks of filaments, sputtering cathodes, etc.

[0041] Upon completion of the high index of refraction layer, a very thin layer (≤200 A) of low index of refraction metal is deposited. This layer has two purposes: firstly, it increases the reflectivity of the whole PVD film in comparison to the base high index of refraction layer. As an example, an opaque chromium PVD film in air will be 55% reflective at 632 nm (see FIG. 2). With 50 A of aluminum added to the chromium layer, reflectivity is increased to 75% in air (see FIG. 2). Secondly, it decreases the drop in reflectivity of the whole film stack when topcoat lacquered in comparison to the amount of reflectivity lost when just the high index of refraction metal is topcoat lacquered. As an example, an opaque chromium PVD film will lose 13% of reflectivity at 632 nm when comparing the film with topcoat lacquering and without. Adding 50 A of aluminum to the opaque chromium film and comparing before and after topcoat lacquering reflectivity again, the loss is only 9% at 632 nm.

[0042] The low index of refraction layer has a maximum limit of 200 A in thickness. Using the above example of aluminum on chromium, as this layer approaches 200 A, the whole PVD film begins to take on the color and optical properties of an opaque aluminum film and defeats the purpose of using the high index of refraction metals for their colors. The ability to tune the film for color is negligible beyond 200 A thickness for almost all low index of refraction metals.

High Index of Refraction Metal Selection Examples for Color/Optical Properties and Suitability of PVD Techniques

[0043] Chromium (n=3.1; k=3.3 @ 632 nm) applied via PVD is not subject to REACH restriction as it does not utilize Chrome VI. It is a natural choice to replace chrome plating due to its blue hues, but requires lacquering to enable the very thin PVD film to withstand environmental effects similar to the traditional very thick chrome plating. Chromium in bulk form, as a deposition material, sublimes at high temperatures to vapor and can be deposited via any of the previously mentioned PVD techniques, with the exception of thermal evaporation, which will be problematic due to high melting temperature.

[0044] Cobalt (n=2.8; k=4.3 @ 632 nm) is another transition metal that provides a lustrous, silvery color. It is more reflective than chromium and gives an even neutral color across the visible spectrum. Cobalt can be deposited with any of the PVD process techniques but is advantageously suited for sputtering or electron beam evaporation.

[0045] Nickel (n=1.9; k=3.7 @ 632 nm) as a base PVD layer gives a higher reflection than chromium but displays a red/yellow hue. Nickel presents unique challenges for PVD processing. It can be thermally evaporated but electron beam evaporation is advantageous. Additionally, using high strength magnetrons, nickel can effectively be sputtered.

[0046] Titanium (n=2.2; k=2.9 @ 632 nm) is silvery gray in color and displays a yellow hue. It is less reflective than chromium. Titanium can effectively be deposited with any of the PVD processes.

[0047] Stainless Steel (n=2.8; k=3.8 @ 632 nm) is less lustrous and less reflective than chromium. It is gray in color and gives a pleasing, muted tone. A sputtering deposition technique is advantageous for Stainless Steel.

Low Index of Refraction Metal Selection Examples for Reflectivity Properties and Suitability of PVD Techniques

[0048] Indium (n=1.1; k=2.3 @ 550 nm) is a silvery white low index of refraction post transition metal. It can effectively be deposited with any of the referenced PVD processes.

[0049] Silver (n=0.1; k=4.0 @ 632 nm) is a precious metal that readily avails itself to any of the PVD techniques. It is bright and lustrous with a very high natural reflectivity in the visible wavelength spectrum.

[0050] Aluminum (n=1.4; k=7.6 @ 632 nm) is a metal that is relatively cheap and abundant. It is highly reflective, although less than silver. It is silvery gray with a slight yellow hue. It is ideally suited for thermal evaporation or sputtering but can advantageously be deposited with any of the commercially available PVD techniques. Aluminum, although bright after top coating, is not a desirable base PVD film due to its yellow hue and inability to match the blue hues of traditional chrome plating. It is, however, an advantageous choice for a semi-transparent, second layer coating as light is either transmitted or reflected with low absorption.

Topcoat Lacquer Application

[0051] PVD films can be topcoat lacquered with many off the shelf commercial solutions. These can be air dried at room temperature, thermally cured, UV cured, or some combination thereof. Red Spot Paint and Varnish Company and Momentive are examples of companies who make formulations specifically for adhesion to, and protection of, PVD films for many different applications.

[0052] These paint lines, often robotic, take PVD coated parts from the PVD machine, perform an ionized air de-stat operation, paint the part through spray or dip coating, prepare the part for curing in an elevated temperature environment and then cure at a further elevated temperature with time or through UV exposure.

Reflectivity and Color Evaluation

[0053] An integrating sphere and appropriate wavelength laser can be utilized to measure the reflectivity of the coating stack to include the topcoat paint. One can measure the strength of the laser by focusing the laser directly in the center of the integrating sphere and using a detector and power meter. The laser than can be moved parallel to the opening of the integrating sphere and the PVD coated, and painted part, can be held at a 45-degree angle reflecting the laser into the integrating sphere. By measuring the laser power output using the same detector and power meter, a simple calculation of the measured power reflecting off the film to the measured baseline power of the laser will give the percent reflectivity of the PVD film stack with topcoat paint.

[0054] Color can be measured using a spectrophotometer to define the L*a*b* values to see the degree to which the thickness of the top layer of PVD affects the color layer, and the degree to which the topcoat lacquer affects the entire PVD film stack. Spectrophotometers measure reflectance and transmittance through a material as a function of wavelength (R. Berns, Billmeyer and Saltzman's Principles of Color Technology, Third Edition, 88, (2000)). They are useful instruments as they can separate out gloss levels and closely match how the human eye determines color. They are a way to clearly define color values of the multi-layer PVD film as well as the entire film stack with topcoat lacquer.

Exemplary Embodiment: Aluminum on Chromium

[0055] With reference to FIGS. 4-6, four samples were prepared as four respective thin film stacks, each first comprising of a sputtered layer of chromium, with a thickness of >50 nm. Aluminum was then sputtered atop the chromium to thicknesses as denoted below.

TABLE-US-00001 Sample Number Cr Layer Thickness Al Layer Thickness 1 >50 nm  0 nm 2 >50 nm  ~5 nm 3 >50 nm ~10 nm 4 >50 nm ~15 nm

[0056] Each of the samples 1-4 was then sprayed with a layer of protective lacquer and UV cured. FIG. 4 shows the reflectivity of chromium topped with aluminum and lacquer, measured at 532 nm (left) and 635 nm (right). FIG. 5 illustrates the color of each chromium-based samples in LAB space. Background color is provided as a guideline. As shown in FIG. 5, the addition of aluminum yields samples (2-4) that are brighter, i.e., more reflective, than bare chromium (1). FIG. 6 is an illustration of color changes across chromium-based samples with application of aluminum. Colors are generated from measured LAB values, which are objective. The subjective appearance of these colors will depend on computer monitor settings, individual vision, and other factors and should not be taken literally. While direct visualization of color depends on monitor settings, individual vision characteristics, as well as other factors, changes in color across the range of samples produced in this embodiment are dramatic and can be observed by the naked eye. Visually perceptible changes indicate the suitability of this approach to decorative applications.

[0057] As demonstrated, samples of chromium with thin, <20 nm, layers of aluminum on top of them are much more reflective following lacquering. Gains in reflectivity are as much as 25 percentage points. Simultaneously the natural chromium color can be significantly retained, despite addition of aluminum.

Exemplary Embodiment. Aluminum on Titanium

[0058] With reference to FIGS. 7-9, four samples were prepared as four respective thin film stacks, each first comprising of a sputtered layer of titanium, with a thickness of >50 nm. Aluminum was then sputtered atop the titanium with thicknesses as denoted below.

TABLE-US-00002 Sample Number Ti Layer Thickness Al Layer Thickness 1 >50 nm  0 nm 2 >50 nm  ~5 nm 3 >50 nm ~10 nm 4 >50 nm ~15 nm

[0059] Each of the samples 1-4 was then spray-coated with a layer of protective lacquer and UV-cured. FIG. 7 shows the reflectivity of titanium topped with aluminum and lacquer, measured at 532 nm (left) and 635 nm (right). FIG. 8 illustrates the color of each titanium-based samples in LAB space. Background color is provided as a guideline. As shown in FIG. 8, the addition of aluminum yields samples (2-4) that are brighter, i.e., more reflective, than bare titanium (1). FIG. 9 is an illustration of color changes across titanium-based samples with application of aluminum. Colors are generated from measured LAB values, which are objective. The subjective appearance of these colors will depend on computer monitor settings, individual vision, and other factors and should not be taken literally.

Exemplary Embodiment: Aluminum on Cobalt

[0060] With reference to FIGS. 10-12, four samples were prepared as four thin film stacks, each first comprising of a sputtered layer of cobalt, with a thickness of >50 nm. Aluminum was then sputtered atop the cobalt with thicknesses as denoted below.

TABLE-US-00003 Sample Number Co Layer Thickness Al Layer Thickness 1 >50 nm  0 nm 2 >50 nm  ~5 nm 3 >50 nm ~10 nm 4 >50 nm ~15 nm

[0061] Each of the samples 1-4 was then spray-coated with a layer of protective lacquer and UV-cured. FIG. 10 shows the reflectivity of cobalt topped with aluminum and lacquer, measured at 532 nm (left) and 635 nm (right). FIG. 11 illustrates the color each cobalt-based samples in LAB space. Background color is provided as a guideline. As shown in FIG. 11, the addition of aluminum yields samples (2-4) that are brighter, i.e., more reflective, than bare cobalt (1). FIG. 12 is an illustration of color changes across cobalt-based samples with application of aluminum. Colors are generated from measured LAB values, which are objective. The subjective appearance of these colors will depend on computer monitor settings, individual vision, and other factors and should not be taken literally.

[0062] While various disclosed embodiments have been described above, it should be understood that they have been presented by way of example only and not as a limitation. Numerous changes to the disclosed embodiments can be made in accordance with the specification herein without departing from the spirit or scope of this specification. Thus the breadth and scope of this specification should not be limited by any of the above-described embodiments; rather, the scope of this specification should be defined in accordance with the appended claims and their equivalents.