PATENT DOCUMENT

Publication Number: US-10760175-B2
Application Number: US-201615333070-A
Country: US
Kind Code: B2

Title: White anodic films with multiple layers

Abstract:
Anodic films that have a white color, and methods for forming the same, are described. According to some embodiments, the anodic films have multiple metal oxide layers. A first layer can provide scratch and chemical resistance and a second layer can provide a light diffusing pore structure that diffusely reflects incoming light and provides a white appearance to the anodic film. According to some embodiments, the anodic films also include a smoothed barrier layer that specularly reflects incoming light so as to brighten the appearance and enhance the white color of the anodic film. The resulting anodic films have an opaque white appearance not achievable using conventional techniques. The anodic films are well suited for providing cosmetically appealing coatings for consumer products, such as housings for electronic products.

Claims:
What is claimed is: 
     
       1. A method of forming an anodic substrate having a white appearance, the method comprising:
 forming a first anodic layer that overlays an aluminum alloy substrate by anodizing the aluminum alloy substrate in a first electrolyte, wherein the first anodic layer includes a first porous region defining a first plurality of pores having first diameters, the first porous region being defined by first pore walls that extend from openings in an external surface of the first anodic layer; and 
 forming a second anodic layer, overlaid by the first anodic layer, by anodizing the aluminum alloy substrate in a second electrolyte different than the first electrolyte, the second anodic layer including a second porous region defining a second plurality of pores having second diameters greater than the first diameters, wherein the second porous region shares the openings in the external surface, and is defined by second pore walls contiguous with and thicker than the first pore walls, the second pore walls including light-diffusing surface features. 
 
     
     
       2. The method of  claim 1 , further comprising:
 forming a barrier layer that is disposed between the aluminum alloy substrate and the second anodic layer. 
 
     
     
       3. The method of  claim 2 , wherein a thickness of the barrier layer is between 150 nm to 800 nm. 
     
     
       4. The method of  claim 1 , wherein the first pores have a columnar shape. 
     
     
       5. The method of  claim 1 , wherein the first electrolyte includes oxalic acid or sulfuric acid, and the second electrolyte includes phosphoric acid. 
     
     
       6. An anodized substrate having a white appearance for a consumer electronic product, the anodized substrate comprising:
 an aluminum alloy substrate; 
 a first anodic layer overlaying the aluminum alloy substrate and having an external surface, the first anodic layer including a first porous region defining a first plurality of pores having first diameters, wherein the pores of the first plurality of pores are defined by first pore walls and are accessible from openings in the external surface; and 
 a second anodic layer disposed between the aluminum alloy substrate and the first anodic layer, the second anodic layer including a second porous region defining a second plurality of pores having second diameters greater than the first diameters, wherein the second plurality of pores are accessible via the openings in the external surface, and are defined by second pore walls having light-diffusing surfaces, the second pore walls being thicker than the first pore walls. 
 
     
     
       7. The anodized substrate of  claim 6 , wherein the first and second anodic layers have a combined W 10  value of at least 75. 
     
     
       8. The anodized substrate of  claim 6 , further comprising:
 a barrier layer disposed between the aluminum alloy substrate and the second anodized layer, and the barrier layer has a thickness between 150 nanometers to 800 nanometers. 
 
     
     
       9. The anodized substrate of  claim 6 , wherein the light-diffusing surfaces are oriented other than orthogonal to the external surface of the first anodic layer. 
     
     
       10. The anodized substrate of  claim 6 , wherein the second pore walls are irregular. 
     
     
       11. The anodized substrate of  claim 8 , wherein the barrier layer has a profile variance of no greater than 6% of a thickness of the barrier layer. 
     
     
       12. The anodized substrate of  claim 6 , wherein the second porous region is overlaid by the first porous region. 
     
     
       13. The anodized substrate of  claim 6 , wherein the second porous region extends from the first porous region and towards the aluminum alloy substrate. 
     
     
       14. The anodized substrate of  claim 6 , wherein the light-diffusing surfaces diffusely reflect visible light incident upon the external surface, thereby imparting the white appearance to the anodized substrate. 
     
     
       15. An anodized part for an electronic device, the anodized part comprising:
 an aluminum alloy substrate; 
 a first anodic layer overlaying the aluminum alloy substrate and having a first porous region that extends from openings in an external surface of the first anodic layer, wherein the first porous region defines a first plurality of pores having first diameters and defined by first pore walls; and 
 a second anodic layer disposed between the aluminum alloy substrate and the first anodic layer, wherein the second anodic layer includes a second porous region defining a second plurality of pores having second diameters greater than the first diameters, the second plurality of pores sharing the openings in the external surface and being defined by second pore walls that have light-diffusing surface features and are thicker than the first pore walls. 
 
     
     
       16. The anodized part of  claim 15 , further comprising:
 a non-porous barrier layer disposed between the aluminum alloy substrate and the second anodic layer. 
 
     
     
       17. The anodized part of  claim 15 , wherein the first and second anodic layers have an L* value of 80 or higher, a b* value between −3 to +6, and an a* value between −3 to +3. 
     
     
       18. The anodized part of  claim 15 , wherein the first and second anodic layers have a W 10  value of at least 75. 
     
     
       19. The anodized substrate of  claim 8 , wherein a metal oxide particle is infused within at least one of the first or second porous regions. 
     
     
       20. The anodized substrate of  claim 6 , wherein the first diameters are between 30 nanometers to 100 nanometers.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application No. 62/249,079, filed Oct. 30, 2015, and entitled “ANODIZED FILMS WITH PIGMENT COLORING,” which is incorporated herein by reference in its entirety and for all purposes. 
     Any publications, patents, and patent applications referred to in the instant specification are herein incorporated by reference in their entireties. To the extent that the publications, patents, or patent applications incorporated by reference contradict the disclosure contained in the instant specification, the instant specification is intended to supersede and/or take precedence over any such contradictory material. 
    
    
     FIELD 
     The described embodiments relate to anodized films having a white appearance and methods forming the same. More specifically, methods involve techniques for forming multiple-layered anodic film structures that result in a white appearing anodic film having high durability and chemical resistivity. 
     BACKGROUND 
     Anodizing is an electrochemical process that thickens a naturally occurring protective oxide on a metal surface. An anodizing process involves converting part of a metal surface to an anodic film. Thus, an anodic film becomes an integral part of the metal surface. Due to its relative hardness, an anodic film can provide corrosion resistance and wear protection for an underlying metal. In addition, an anodic film can enhance a cosmetic appearance of a metal surface. For example, anodic films can have a porous microstructure that can be infused with dyes to impart a desired color to the anodic films. 
     Conventional methods for coloring anodic films, however, have not been able to achieve an anodic film with an opaque and saturated white appearance. In particular, the underling metal substrate can often be seen through the anodic film such that the anodized substrate appears to have a silver or metallic appearance. Thus, conventional techniques result in films that appear to be off-white or have a grey or silver hue. What is needed are improved methods for forming white appearing anodic films. 
     SUMMARY 
     This paper describes various embodiments that relate to white anodized films and methods for forming white anodized films. The colored anodized films can have an opaque white appearance. 
     According to one embodiment, a method of forming a white appearing metal oxide film is described. The method includes forming a first layer of the metal oxide film by anodizing a substrate in a first electrolyte. The method also includes forming a second layer of the metal oxide film by anodizing the substrate in a second electrolyte different than the first electrolyte. The second layer is more porous than the first layer and has pore wall surfaces that diffusely reflect visible light incident an exterior surface of the metal oxide film so as to impart the white appearance to the metal oxide film. 
     According to another embodiment, an anodized substrate having a white appearance is described. The anodized substrate has an anodic coating including a first metal oxide layer having an exterior surface corresponding to an exterior surface of the anodized substrate. The anodic coating also includes a second metal oxide layer adjacent the first metal oxide layer. The second metal oxide layer is more porous than the first metal oxide layer and has pore wall surfaces that diffusely reflect visible light incident an exterior surface of the anodic coating so as to impart a white appearance to the anodic coating. 
     According to a further embodiment, an enclosure for an electronic device is described. The enclosure includes an aluminum alloy substrate. The enclosure also includes an anodic coating having a white appearance disposed on the aluminum alloy substrate. The anodic coating has a first metal oxide layer, a second metal oxide layer adjacent the first metal oxide layer, and a barrier layer. The second metal oxide layer pore wall structure that diffusely reflects incident visible light. The barrier layer is positioned between the second metal oxide layer and the aluminum alloy substrate, wherein a thickness of the barrier layer is between about 150 nanometers and about 800 nanometers. 
     These and other embodiments will be described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements. 
         FIG. 1  shows perspective views of devices having metallic surfaces that can be protected using anodic oxide coatings described herein. 
         FIG. 2  shows a cross section view of an anodized part illustrating how an anodized part using a conventional anodizing process can have a translucent appearance. 
         FIGS. 3A-3E  show cross section views of an anodized part that having a multiple layered structure that provides a white appearance, in accordance with some embodiments. 
         FIG. 4  shows a flowchart indicating a process for forming a multiple layered anodic film having a white appearance, in accordance with some embodiments. 
         FIGS. 5A-5C  show SEM cross section images of different parts at various stages of forming a multiple layered anodic oxide coating, in accordance with some embodiments. 
         FIGS. 6A-6D and 7A-7D  show SEM cross section and top view images of a part indicating how a barrier layer smoothing process can affect a structure of and anodic film, in accordance with some embodiments. 
         FIGS. 8A-8D  show how a circularly polarizing filter can be used to determine whiteness of a part, including parts having multiple layered anodic films in accordance with some embodiments. 
         FIGS. 9A-9B, 10A-10B and 11A-11B  show SEM images of anodic film prior to and after barrier layer smoothing processes to illustrate the extent that a barrier layer smoothing process can smooth an interface surface of a barrier layer, in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims. 
     Described herein are processes for providing a white color to anodic films. In particular embodiments, the anodic films have multiple layers, where a first layer, which can correspond to an outer or external layer of the anodic film, has a relatively high density of metal oxide material, thereby providing a hardness and chemical resistivity to the anodic film. A second layer, beneath the first layer, can include a pore wall structure that diffusely reflects incoming visible light, thereby providing a white appearance to the anodic film. The pore wall structure of the second layer can include pore wall surfaces that are at non-orthogonal orientations with respect to the outer surface of the anodic film, thereby providing a structure for diffusely reflecting incident light. In some cases, the anodic films include a smoothed barrier layer that defines a flat interface surface between the barrier layer and an underlying substrate. The flat interface surface can specularly reflect incoming light, thereby increasing a brightness and enhancing the white appearance of the multiple layered anodic film. The barrier layer smoothing process can also flatten pore terminuses of the second layer, thereby providing additional flat surfaces for specularly reflecting incoming light. 
     Methods for forming the multiple layered anodic films can include performing a first anodizing process using a first electrolyte and a second anodizing process using a second electrolyte different than the first electrolyte. In some embodiments, the first electrolyte includes oxalic acid, which can form a dense and chemically resistant first layer. In some embodiments, the first electrolyte includes sulfuric acid, which can form a substantially colorless and cosmetically appealing anodic film. In some embodiments, the second electrolyte includes phosphoric acid, which can form an irregular pore structure that includes light diffusing pore walls. The second anodizing process can result in a more porous second layer than the first anodizing process. In embodiments where a barrier layer smoothing process is used, the anodic film can be exposed to a third anodizing process that is performed in a non-dissolution (i.e., non-pore forming) electrolyte. In particular embodiments, the non-pore forming electrolyte includes borax or boric acid. The multiple layered anodic film can be sealed using a sealing process so as to further increase its chemical resistance and the corrosion resistance. The resultant white appearing anodic film can have a hardness of at least 150 HV (Vickers Pyramid Number as measured using Vickers hardness test) in order to withstand abrasion forces that may occur during normal use of a consumer product (e.g., an electronic device as described above). The resultant white appearing anodic film can also be characterized as having an L* value of at least 80 (in some cases at least 85), a b* value between about −3 and about +6, and an a* value of between about −3 and about +3. In some embodiments, a suitable white color can be achieved without infusing a dye or pigment within the multiple layered anodic film. In some embodiments, a suitable white color is achieved by infusing a dye or pigment within the multiple layered anodic film. 
     The present paper makes reference to anodizing of aluminum and aluminum alloy substrates. It should be understood, however, that the methods described herein may be applicable to any of a number of other suitable anodizable metal substrates, such as suitable alloys of magnesium. As used herein, the terms anodized film, anodized coating, anodic oxide, anodic coating, anodic film, anodic layer, anodic coating, anodic oxide film, anodic oxide layer, anodic oxide coating, metal oxide film, metal oxide layer, metal oxide coating, oxide film, oxide layer, oxide coating etc. can be used interchangeably and can refer to suitable metal oxides, unless otherwise specified. 
     Methods described herein are well suited for providing cosmetically appealing surface finishes to consumer products. For example, the methods described herein can be used to form durable and cosmetically appealing finishes for housing for computers, portable electronic devices, wearable electronic devices, and electronic device accessories, such as those manufactured by Apple Inc., based in Cupertino, Calif. 
     These and other embodiments are discussed below with reference to  FIGS. 1-11B . However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these Figures is for explanatory purposes only and should not be construed as limiting. 
     The methods described herein can be used to form durable and cosmetically appealing coatings for metallic surfaces of consumer devices.  FIG. 1  shows consumer products that can be manufactured using methods described herein.  FIG. 1  includes portable phone  102 , tablet computer  104 , smart watch  106  and portable computer  108 , which can each include housings that are made of metal or have metal sections. Aluminum alloys, such as 5000 series, 6000 series or 7000 series aluminum alloys, can be a choice metal material due to their light weight and ability to anodize and form a protective anodic oxide coating that protects the metal surfaces from scratches. In some cases the anodic oxide coatings are colorized to impart a desired color to the metal housing or metal sections. 
     Conventional anodic oxide coloring techniques involve infusing dyes, such as organic dyes or metal-based colorants, within the pores of the anodic oxide. It can be difficult, however, to impart an opaque white appearance to anodic oxide coatings. This is, in part, because white colorants can be composed of relatively large particles that can be difficult to infuse within the nano-scale diameter pores of anodic oxide coatings. Conventional colorizing techniques often result in off-white or silver colored anodic oxide coatings. This is because the underlying metal substrate can still be observable through the anodic oxide such that the anodic oxide finish retains a metallic look. Described herein are improved techniques for providing opaque white anodic oxide finishes to metal substrate, such as those on housing of devices  102 ,  104 ,  106  and  108 . 
     In general, white is the color or appearance of an object if the material of the object diffusely reflects back most of visible light that strikes it. Non-dyed anodic oxide coatings may have a slight whitened or colored appearance, depending on the anodizing conditions and processing parameters for forming the anodic oxide coating. However, many types of non-dyed anodic oxide coating can be generally characterized as translucent in that the underlying metal substrate is typically clearly visible through the non-dyed anodic oxide coating. 
     To illustrate,  FIG. 2  shows a cross section view of a surface portion of anodized part  200 . Part  200  includes metal substrate  202  and metal oxide coating  204 . Metal oxide coating  204  is composed of a metal oxide material  203  and includes pores  206  formed during the anodizing process. In this way, pores  206  are defined by pore walls  205 , which are composed of metal oxide material  203 . The size of pores  206  can vary depending on the anodizing process conditions. For example, some type II anodizing processes, as defined by MIL-A-8625 industry standards, which involve anodizing in a sulfuric acid bath, can result in pores  206  having diameters of about 20 nanometers (nm) to about 30 nanometers. Metal oxide coating  204  is uncolorized in that pores  206  do not include dye or metal colorants. Thus, much of the visible light incident metal oxide coating  204  can pass through metal oxide coating  204 . For example, light ray  208  can enter outer surface  210  of metal oxide coating  204  and pass through metal oxide material  203  and pores  206 , reflect off of underlying metal substrate  202 , and reflect back out of metal oxide coating  204 . In this way, underlying metal substrate  202  can be visible through metal oxide coating  204 , thereby giving anodized part  200  a metallic look. 
     It should be noted that metal oxide coating  204  includes a porous layer  201  (defined by thickness  212 ), which includes pores  206  and a barrier layer  209 , (defined by thickness  214 ), which corresponds to a generally non-porous portion of metal oxide coating  204  between metal substrate  202  and porous layer  201 . Both porous layer  201  and the barrier layer  209  include metal oxide material  203  from converting surface portions of metal substrate  202  to a corresponding metal oxide material  203 . Interface surface  216  of barrier layer  209 , defined on one side by metal substrate  202  and on another side by barrier layer  209 , has a shape that is partially defined by pore terminuses  218  of pores  206 . In particular, the curved shaped pore terminuses  218  can cause interface surface  216  to have a scalloped geometry or shape. In three dimensions, interface surface  216  can be characterized as having a series of curved, hemispherical, cup-like features. 
     One of the challenges associated with imparting a white appearance to metal oxide coating  204  is that many white colorants, such as titanium oxide particles, can be too big to fit within pores  206 . Thus, conventional methods can make it impossible to accomplish a visibly saturated, rich, highly opaque white color to metal oxide coating  204 . Even when some whitening is accomplished, significant amounts of incoming light can still pass through metal oxide coating  204  so as to give part  200  a silver hue as viewed from surface  210 . In addition, light incident metal oxide coating  204  can become trapped within metal oxide coating  204  due to the scalloped shaped interface surface  216 , thereby darkening the appearance of metal oxide coating  204  and preventing a brightness necessary for a providing white appearance. 
     The methods described herein involve forming a multiple layered anodic coating that can provide a saturated, opaque and bright white appearance.  FIGS. 3A-3E  illustrate cross section views of part  300  undergoing an anodizing process for forming a white appearing multiple layered coating, in accordance with some described embodiments. 
       FIG. 3A  shows part  300  after metal substrate  302  is anodized using a first anodizing process. Metal substrate  302  can be any suitable anodizable material, such as suitable aluminum and aluminum alloys. In some embodiments, metal substrate  302  is a 5000 series, 6000 series or 7000 series aluminum alloy. The first anodizing process converts a portion of metal substrate  302  to first metal oxide layer  304 . First metal oxide layer  304  is composed of metal oxide material  303 , the composition of which depends on the composition of metal substrate  302 . For example, an aluminum alloy metal substrate  302  can be converted to a corresponding aluminum oxide material  303 . First metal oxide layer  304  includes porous portion  301  (defined by thickness  312 ) and the barrier layer  309  (defined by thickness  314 ). The porous portion  301  includes pores  306 , which are formed during the anodizing process, and are formed within metal oxide material  303 . The barrier layer  309  is generally free of pores  306  and is situated between metal substrate  302  and porous portion  301 . The thickness of first metal oxide layer  304 , corresponding to thicknesses  312  of the porous portion plus thickness  314  of barrier layer  309 , can vary depending on the application. In some embodiments, barrier layer has a thickness  314  of about 100 nanometers or less. In some embodiments, the thickness of first metal oxide layer  304  is between about 3 micrometers and about 15 micrometers. 
     In some embodiments, first metal oxide layer  304  has a pore structure that provides high mechanical strength and chemical resistance to first metal oxide layer  304 . This can be accomplished by adjusting process conditions of the first anodizing process. For example, anodizing in a bath including oxalic acid can result in pores  306  that are generally wider than those formed in an electrolytic bath including sulfuric acid. For example, in some embodiments using oxalic acid-based anodizing results in pores  306  having diameters  320  between about 30 nanometers and about 100 nanometers, compared to sulfuric acid-based anodizing that can result in pores  306  having diameters  320  between about 10 nanometers and about 40 nanometers. 
     Although pores  306  are generally wider using oxalic acid anodizing, the density of pores  306  is less compared to the density of pores  306  using sulfuric acid anodizing. That is, the density of metal oxide material  303  and the width of pore walls  305  can be generally greater when oxalic acid anodizing compared to sulfuric acid anodizing. This greater relative density of metal oxide material  303  (using oxalic acid-based anodizing) can result in metal oxide layer  304  being harder and more chemically resistant than a sulfuric acid-based oxide film, which can be useful in applications where first oxide layer  304  corresponds to an exterior surface of a consumer product (e.g., devices of  FIG. 1 ). In some embodiments, good results were found when the electrolyte has a relatively low concentration of oxalic acid, such as about 10 g/l of oxalic acid or less—which is lower than conventional oxalic acid anodizing processes. 
     It should be noted that oxalic acid-based anodizing can, in some cases, cause first metal oxide layer  304  to have a yellow hue, sometimes associated with using an organic acid-based anodizing bath. Since this may serve against providing a white appearing anodic coating, it may be preferable to use a sulfuric acid-based anodizing process in some cases. However, in some embodiments, the oxalic acid-based anodizing can result in a sufficiently white an colorless anodic film. In some cases, such a yellow hue can be offset using barrier layer thickening techniques, which will be described below with reference to  FIG. 3C . 
       FIG. 3B  shows part  300  after a second anodizing process is performed, causing more of metal substrate  302  to be converted to second metal oxide layer  322 . Second metal oxide layer  322  grows beneath first metal oxide layer  304  and reforms barrier layer  309  (defined by thickness  327 ) adjacent metal substrate  302 . Thus, the thickness of the multiple layered anodic oxide coating can be defined by thickness  312  of first metal oxide layer  304 , thickness  333  of second metal oxide layer  322 , and thickness  327  of barrier layer  309 . 
     As shown, pores  323  within second metal oxide layer  322  are generally wider than pores  306  of first metal oxide layer  304 . In some embodiments, the diameter  324  of pores  323  are about 100 nanometer or more, in some embodiments between about 100 nm and about 300 nm. In addition, second metal oxide layer  322  has a pore walls  325  that are irregular, in that pore walls  325  have pore wall surfaces  326  are oriented non-orthogonally with respect to the outer surface  310 . This anodic pore structure can be accomplished, for example, by performing the second anodizing process in a bath including phosphoric acid. 
     The irregular pore structure of second metal oxide layer  322  can impart a white appearance to the anodic coating by diffusely reflecting incoming visible light. This is illustrated by first light ray  328  entering outer surface  310  of first metal oxide layer  304 , reflecting off of pore wall surfaces  326  of second metal oxide layer  322 , and exiting outer surface  310  at a first angle. Second light ray  329  enters outer surface  310  of first metal oxide layer  304 , reflects off of pore wall surfaces  326 , and exits outer surface  310  at a second angle different than the first angle. Third light ray  330  enters outer surface  310  of first metal oxide layer  304 , reflects off of pore wall surfaces  326 , and exits outer surface  310  at a third angle different than the first angle and the second angle. In this way, pore wall surfaces  326  within second metal oxide layer  322  can diffusely reflect visible light and impart a white appearance to the multiple layered anodic oxide coating of part  300 . In some embodiments, a good whitening results were found when the second anodizing process involves using an electrolytic bath having a relatively low concentration of phosphoric acid, such as about 17 g/l of phosphoric acid or less—which is much lower than conventional phosphoric acid anodizing processes. 
     It should be noted that first metal oxide layer  304 , which can generally have more mechanical strength and be more dense (i.e., have more volume percent of metal oxide material) than second metal oxide layer  322  can provide structural integrity to the anodic film, while underlying second metal oxide layer  322 , while generally more porous than first metal oxide layer  304 , can provide the porous structure suitable for providing a white appearance to the anodic film. 
     At  FIG. 3C , barrier layer  309  is optionally smoothed and thickened in order to enhance the whitening of the multiple layered anodic oxide coating. The smoothing of barrier layer  309  can smooth out interface surface  316  of barrier layer  309 , which previously had a scalloped geometry. This can cause incoming light that does not diffusely reflect off of pore wall surfaces  326  to specularly reflect of flat interface surface  316 . For instance, light ray  317  enters outer surface  310  of first metal oxide layer  304 , passes through first metal oxide layer  304  and second metal oxide layer  322 , reflects off of interface surface  316 , and exits outer surface  310  at a first angle. Light ray  319  enters outer surface  310  of first metal oxide layer  304 , passes through first metal oxide layer  304  and second metal oxide layer  322 , reflects off of interface surface  316 , and exits outer surface  310  at the same first angle as light ray  317 . Additionally or alternatively, the barrier layer smoothing process can flatten or smooth pore terminuses  318  of pores  323 , such that flattened pore terminuses  318  can also specularly reflect incoming light. In this way, the smooth (i.e., flat) interface surface  316  and/or pore terminuses  318  can cause light that does not diffusely reflect off of pore wall surfaces  326  to specularly reflect off interface surface  316  and/or pore terminuses  318 , resulting in brightening and enhancing the white appearance of the multiple layered anodic coating. That is, the specular reflectivity of flattened interface surface  316  increases the lightness of the whiteness caused by diffuse reflection off of pore walls  326  (see e.g., light ray  329 ) to produce a bright white appearance. In some embodiments, the barrier layer smoothing process is necessary in order to accomplish a particular level of lightness, which can be measured using, for example, L* values as defined by CIE 1976 L*a*b* color space model standards. 
     The barrier layer smoothing process can be accomplished by anodizing part  300  using a third anodizing process that promotes anodic film growth without substantially promoting anodic film dissolution, i.e., a non-pore-forming electrolyte. In some embodiments, the non-pore forming electrolyte contains one or more of Na 2 B 4 O 5 (OH) 4 .8H 2 O (sodium borate or borax), H 3 BO 3  (boric acid), C 4 H 6 O 6  (tartaric acid), (NH 4 ) 2 .5B 2 O 3 .8H 2 O (ammonium pentaborate octahydrate), (NH 4 ) 2 B 4 O 7 .4H 2 O (ammonium tetraborate tetrahydrate), C 6 H 10 O 4  (hexanedioic acid or adipic acid), C 6 H 16 N 2 O 4  (ammonium adipate), (NH 4 ) 2 C 4 H 4 O 6  (ammonium tartrate), C 6 H 8 O 7  (citric acid), C 4 H 4 O 4  (maleic acid), C 2 H 4 O 3  (glycolic acid), C 6 H 4 (COOH) 2  (phthalic acid), Na 2 CO 3  (sodium carbonate), [Six(OH) 4-2x ] n  (silicic acid), and H 3 NSO 3  (sulfamic acid). Suitable barrier layer smoothing processes are described in detail in U.S. provisional application No. 62/249,079, filed Oct. 30, 2015, which is incorporated herein by reference in its entirety. In some embodiments, the non-pore-forming electrolyte includes borax, boric acid or adipic acid. Below are listed some example process parameters used for a barrier layer smoothing process, in accordance with some embodiments. 
     Example 1 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Electrolyte composition 
                 Borax 
               
               
                   
                 Electrolyte temperature (degrees Celsius) 
                 20-25 
               
               
                   
                 Electrolyte pH 
                 9.0-9.2 
               
               
                   
                 Maximum voltage range 
                 400 V-460 V 
               
               
                   
                   
               
            
           
         
       
     
     Example 2 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Electrolyte composition 
                 Boric acid 
               
               
                   
                 Electrolyte temperature (degrees Celsius) 
                 20-25 
               
               
                   
                 Electrolyte pH 
                 4.9-5   
               
               
                   
                 Maximum voltage 
                 400 V 
               
               
                   
                   
               
            
           
         
       
     
     Example 3 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Electrolyte composition 
                 Adipic acid 
               
               
                   
                 Electrolyte pH 
                  3.0-10.0 
               
               
                   
                 Maximum voltage range 
                 400 V-550 V 
               
               
                   
                   
               
            
           
         
       
     
     Barrier layer  309  can be smoothed to differing amounts, depending on a desired final smoothing outcome and process limitations. In some embodiments, the barrier layer smoothing process is performed until interface surface  316  achieves a profile variance of no more than about 30 nanometers, where the profile variance is defined as a distance d between an adjacent peak and valley of the interface surface  316  over a predefined distance across part  300 . In some embodiments, the profile variance is no more than about 6% of the thickness t of barrier layer  309 . Profile variance can be measured, for example, using a scanning electron microscope (SEM) cross section image of the part  300 . SEM cross section images of some samples are described below with reference to  FIGS. 9A-9B, 10A-10B and 11A-11B . 
     In addition to smoothing barrier layer  309 , the barrier layer thickening process can also thicken barrier layer  309  to thickness t. That is, thickness t is greater than thickness  314  (in  FIG. 3A ) prior to the barrier layer smoothing process. This aspect can be used to compensate for any discoloration of the anodic oxide coating. For example, as described above, anodizing in organic acids such as oxalic acid can cause first metal oxide layer  304  to have a yellow hue. To offset this yellowing, barrier layer  309  can be used to reflect light via thin film interference. For example, objects that reflect a yellow color will have a positive b* value and objects that reflect a blue color will have a negative b* value, according to CIE 1976 L*a*b* color space model measurements. Thus, thickness t of barrier layer can be tuned to create light interference effects that add a blue hue (negative b* value) to offset a yellow hue (positive b* value) of first barrier layer  304 . Likewise, thickness t of barrier layer can be tuned to create light interference effects that add a magenta hue (positive a* value) to offset a green hue (negative a* value) of first barrier layer  304 . In this way, a more color-neutral anodic coating conducive to a white appearance can be achieved. Some discussion as to use of barrier layers for thin film interference coloring are described U.S. provisional application No. 62/249,079, filed Oct. 30, 2015, and U.S. non-provisional application Ser. No. 14/312,502, each of which is incorporated herein its entirety. 
     Thus, the final thickness t of barrier layer  309  can be chosen so as to sufficiently smooth barrier layer  309  as well as to reflect a desired range of wavelengths of light by thin film interference. In some embodiments, thickness t of barrier layer  309  is at least about 200 nanometers. In some embodiments, thickness t is about 300 nanometers or more. In some embodiments, thickness t is about 400 nanometers or more. In some embodiments, thickness t about is between about 150 nanometers and 800 nanometers. 
       FIG. 3D  shows part  300  after an optional pigment infusing process is performed, which involves depositing particles  321  within the multiple layered anodic oxide coating. Particles  321  should have a white appearance or otherwise be highly optically reflective. In some embodiments, particles  321  are composed of one or more of a titanium oxide (e.g., TiO 2 ), an aluminum oxide (e.g., Al 2 O 3 ) and a zinc oxide (e.g., ZnO). Particles  321  can be infused using any suitable method. In some cases, part  300  is immersed in a solution that has particles  321  suspended therein. In some embodiments, the solution is an aqueous solution with a controlled pH conducive to promoting diffusion of particles  321  within pores  306 . Particles  321  thereby become infused within pores  306  and get trapped such that, when part  300  is removed from the solution, at least some of particles  321  remain within pores  306 . In some embodiments, particles  321  become infused within both second metal oxide layer  322  and first metal oxide layer  304 . 
     Particles  321  can diffusely reflect incoming visible light (e.g., light ray  332 ), thereby further enhancing the whiteness of the multiple layered anodic oxide coating. Thus, incoming light can diffusely reflect off of pore walls  326  of second metal oxide layer  322  (e.g., light ray  329 ), diffusely reflect off of particles  321  (e.g., light ray  332 ), and specularly reflect off of flattened interface surface  316 , resulting in a bright and white appearance. Note that the particle infusing process shown in  FIG. 3D  is optional. That is, in some embodiments, a white enough multiple layered anodic oxide coating is achieved without infusing particles  321 . In some embodiments, however, the addition of particles  321  may be beneficial to achieving adequate levels of whiteness. 
       FIG. 3E  shows part  300  after an optional pore sealing process is performed in order to enhance the chemical resistance and corrosion resistance of the anodic oxide coating. The sealing process can hydrate the metal oxide material  303  of at least top portions of pore walls  305  of first metal oxide layer  304 . In particular, the sealing process can convert metal oxide material  303  to its hydrated form  334 , thereby causing swelling of pore walls  305  and sealing of pores  306 . The chemical nature of hydrated metal oxide material  334  will depend on the composition of metal oxide material  303 . For example, aluminum oxide (Al 2 O 3 ) can be hydrated during the sealing process to form boehmite or other hydrated forms of aluminum oxide. The amount of hydration and sealing can vary depending on the sealing process conditions. In some embodiments, only a top portion of pores  306  of first metal oxide layer  304  is hydrated, while in some embodiments substantially the entire length of pores  306  of first metal oxide layer  304  is hydrated. In some cases, a portion of pores  323  of second metal oxide layer  304  are also hydrated. Any suitable pore sealing process can be used, including exposing part  300  to hot aqueous solution or steam. In some cases additives are added to the aqueous solution, such as nickel acetate or other commercial additives, such as Okuno Chemical H298 (manufactured by Okuno Chemical Industries Co., Ltd., based in Japan). 
     After sealing, the multiple layered anodic coating of part  300  can have superior hardness and scratch resistance and appear an opaque white color. The sealing of pores  306  may also help retain particles  321  within the multiple layered anodic coating (in embodiments that include particles  321 ). In some embodiments, the multiple layered anodic coating of part  300  is characterized as having a hardness value of at least 150 HV. In some embodiments, the multiple layered anodic coating of part  300  is characterized as having an L* value of 80 or higher, a b* value between about −3 and about +6 and an a* value of between about −3 and about +3. Note that in some embodiments, the barrier layer smoothing process can be necessary to achieve a certain level of lightness, related to the whiteness, of the multiple layered metal oxide film. For example, one multiple layered metal oxide coating sample was characterized as having an L* value of 74.16, a b* value of 1.75, and an a* value of 0.05, and visually appeared grey prior to performing the barrier layer smoothing process. After the barrier layer smoothing process, the multiple layered metal oxide coating sample was characterized as having an L* value of 84.30, a b* value of 1.85, and an a* value of −0.38, and visually appeared white. Thus, the barrier layer smoothing process can be used to increase the lightness (L*) and/or reduce discoloration (b* or a*) of the multiple layered anodic film. 
     In some cases, the whiteness of the anodic coating can be characterized using whiteness index (WI) ratings. One equation used for the measuring WI is the CIE standard illumination D65 formulae for whiteness W 10 :
 
 W   10   =Y   10 800( x   n,10   −x   10 )+1700( y   n,10   −y   10 )
 
     where Y is the Y tristimulus value (relative luminance), (x,y) is the chromaticity coordinate in the CIE 1931 color space, (x n ,y n ) is the chromaticity coordinate of the perfect diffuser (reference white), and the subscript ten (10) indicates the CIE 1964 standard observer. 
     In general, the higher the W 10  value, the greater the whiteness. In some embodiments, the multiple layered anodic coating of part  300  has a W 10  value of at least 75. It should be noted that in some embodiments these whiteness index values can be achieved without the use colorants (e.g., dyes, pigments or metal colorant) within the anodic oxide coating. In other embodiments, the anodic coating should include a colorant, such as pigment particles described above with reference to  FIGS. 3D and 3E , in order to achieve these whiteness index values. 
     Thickness  312  of first metal oxide layer  304 , thickness  333  of second metal oxide layer  322 , and thickness t of barrier layer  309  can vary depending on desired mechanical or color properties of the multiple layered anodic coating. In particular embodiments, thickness  312  of first metal oxide layer  304  is between about 3 micrometers and about 15 micrometers, thickness  333  of second metal oxide layer  322  is between about 2 micrometers and about 15 micrometers, and thickness t of barrier layer  309  is at least about 200 nanometers (in some embodiments up to about 800 nanometers). In some embodiments, a final thickness of the multiple layered anodic coating of part  300  (including thickness  312 , thickness  333  and thickness t) is between about 5 micrometers and about 30 micrometers. 
     It should be noted that thickness t of barrier layer  309  is dependent, in part, on the voltage used during the barrier layer smoothing process, with higher voltages associated with a thicker barrier layer  309 . If the voltage used in the barrier layer smoothing process is too high, this could cause first metal oxide layer  304  and/or second metal oxide layer  322  to breakdown. Thus, the voltage should be kept sufficiently low to prevent such breakdown. This means that a maximum thickness t of barrier layer  309  is limited. In some embodiments, thickness t is grown to a maximum of about 800 nanometers. As described above, however, thickness t should be large enough to be associated with sufficient flattening of interface surface  316 . This means that in some embodiments, thickness t should range between about 150 nanometers and about 800 nanometers. In some embodiments, thickness t of the barrier layer is at least about 6% of a total thickness of the anodic coating (t+333+312). 
       FIG. 4  shows flowchart  400 , which indicates a process for forming a multiple layered anodic coating having a white appearance, in accordance with some embodiments. At  402 , a substrate undergoes an optional surface pretreatment. In some embodiments, the surface pretreatment involves polishing a surface of the substrate to a mirror polish reflection. In some embodiments, the substrate surface is polished until the surface achieve a gloss value of 1500 gloss units or greater, as measured at 20 degree reflection. In a particular embodiment, the gloss value is about 1650 gloss units as measured at 20 degree reflection. The level of flatness/smoothness of the substrate surface prior to anodizing can be important in some embodiments in order to help achieve a sufficiently smooth barrier layer after a barrier layer smoothing process is performed (see  FIG. 3C ). Other surface pretreatment processes can include degreasing and de-smutting (e.g., exposure to a nitric acid solution for 1-3 minutes). Care should be taken, however, to assure the degreasing and de-smutting do not significantly damage the mirror polished surface of the substrate. The substrate can be composed of any suitable anodizable material, such as a suitable aluminum alloy. 
     At  404 , a first layer of a metal oxide film is formed using a first anodizing process. In some cases, the first anodizing process involves using a first electrolyte that includes oxalic acid or sulfuric acid. In some embodiments, the first electrolyte has an oxalic acid concentration of between about 5 g/l and about 60 g/l. In some embodiments, the oxalic acid concentration is about 10 g/l or less—which is lower than conventional oxalic acid anodizing. In some embodiments, the temperature of the electrolyte during anodizing is between about 20 degrees C. and about 40 degrees C., using an anodizing voltage of between about 40 volts and about 100 volts, using an anodizing current density of between about 1 A/dm 2  and about 4 A/dm 2 . The anodizing time period will vary depending on a desired thickness of the first metal oxide layer. In some embodiments, the first anodizing time period is between about 1 minute and 5 minutes. 
     At  406 , a second layer of the metal oxide film is formed using a second anodizing process. The second layer can be structurally different than the first layer in that the second layer can have more pore wall surfaces that diffusely reflect visible light incident an exterior surface of the metal oxide film compared to the first layer. For example, the first layer of the metal oxide film can have pore walls that are substantially orthogonal to the exterior surface of the anodic coating, whereas pore wall surfaces of the second layer of the metal oxide film can be oriented non-orthogonally with respect to the exterior surface such that light can reflect off the pore wall surfaces (see  FIGS. 3A-3D ). 
     In some embodiments, the second electrolyte includes phosphoric acid in a concentration of between about 15 g/l and about 250 g/l. In some embodiments, the phosphoric acid concentration is about 17 g/l or less—which is lower than conventional phosphoric acid anodizing. In some embodiments, the temperature of the second electrolyte during anodizing is between about 5 degrees C. and about 70 degrees C., using an anodizing voltage of between about 70 volts and about 150 volts, using an anodizing current density of between about 0.5 A/dm 2  and about 5 A/dm 2 . In some embodiments, the electrolyte temperature is maintained at about 60 degrees C. or higher during the anodizing, which is higher than conventional voltages used in phosphoric acid anodizing. The anodizing time period will vary depending on a desired thickness of the second metal oxide layer. In some embodiments, the second anodizing time period is between about 25 minute and 50 minutes. 
     At  408 , the barrier layer of the multiple layered metal oxide film is smoothed using a third anodizing process, which can be referred to as a barrier layer smoothing process. The third anodizing process can be performed in a non-pore forming electrolyte such that the additional metal oxide material is non-porous, effectively thickening the substantially non-porous barrier layer. In some embodiments, the non-pore forming electrolyte contains one or more of Na 2 B 4 O 5 (OH) 4 .8H 2 O (sodium borate or borax), H 3 BO 3  (boric acid), C 4 H 6 O 6  (tartaric acid), (NH 4 ) 2 .5B 2 O 3 .8H 2 O (Ammonium pentaborate octahydrate), (NH 4 ) 2 B 4 O 7 .4H 2 O (ammonium tetraborate tetrahydrate), C 6 H 10 O 4  (hexanedioic acid), C 6 H 16 N 2 O 4  (ammonium adipate), (NH 4 ) 2 C 4 H 4 O 6  (ammonium tartrate), C 6 H 8 O 7  (citric acid), C 4 H 4 O 4  (maleic acid), C 2 H 4 O 3  (glycolic acid), C 6 H 4 (COOH) 2  (phthalic acid), Na 2 CO 3  (sodium carbonate), [Six(OH) 4-2x ] n  (silicic acid), and H 3 NSO 3  (sulfamic acid). 
     In particular embodiments, the third anodizing process involves anodizing in an electrolyte having borax in a concentration of between about 10 g/l and 20 g/l (at a pH between about 9 and 9.2) held at an anodizing temperature of between about 20 degrees C. and 30 degrees C. In another embodiment, an electrolyte having boric acid in a concentration of between about 10 g/l and 20 g/l (at a pH of about 6) held at an anodizing temperature of between about 20 degrees C. and 30 degrees C. was used. The voltage of the anodizing process can vary depending, in part, on a desired interference coloring provided by the barrier layer. In some embodiments, a voltage of between about 200 volts and about 550 volts, with low current density, is used. In a particular embodiment, a DC voltage is applied and increased at a rate of about 1 volt/second until a voltage of between about 300 volts and about 500 volts is achieved, which is maintained for about 5 minutes. 
     At  410 , a white pigment is optionally infused within the metal oxide film. Any suitable white coloring agent can be used. In some embodiments, the white pigment includes particles composed of a titanium oxide (e.g., TiO 2 ), an aluminum oxide (e.g., Al 2 O 3 ), a zinc oxide (e.g., ZnO), or any suitable combination thereof. In some embodiments, the white pigment is infused by exposing the metal oxide film to an aqueous solution having white pigment particles suspended therein such that the pigment particles deposit into and get trapped within the anodic pores of at least the second layer. 
     At  412 , the multiple layered metal oxide film is optionally sealed to seal at least top portions of the pores of the first layer. This can increase the mechanical strength and corrosion resistance of the multiple layered metal oxide film. In some embodiments, a target hardness of the multiple layered metal oxide film is at least about 150 HV, suitable for use in housing for electronic devices. In addition, the sealing process can retain the white pigment particles (if used) within the anodic pores of the metal oxide film. 
       FIGS. 5A-5C  show SEM cross section images of different parts at various stages of forming a multiple layered anodic oxide coating, in accordance with some embodiments.  FIG. 5A  shows part  500  after a first anodizing process converts a portion of substrate  502  to first metal oxide layer  504 . Substrate  502  is composed of an aluminum alloy and the anodizing process involved using a sulfuric acid-based bath. The resulting first metal oxide layer  504  has pores  506  and barrier layer  509 . Pores  506  have diameters of about 40 nm to about 50 nm. First oxide layer  504  has a thickness  508  of about 14.5 micrometers and barrier layer  509  has a thickness  510  between about 50 nm and about 70 nm. 
       FIG. 5B  shows part  530  after two anodizing processes have been performed. Like part  500 , a first anodizing process using a sulfuric acid-based electrolyte is used to form first metal oxide layer  504 , which as a thickness  508  of about 4.7 micrometers. In addition, a second anodizing process is performed, whereby another portion of substrate  502  is converted to second metal oxide layer  512 . The second anodizing process involved using a phosphoric acid-based bath and has a thickness  511  of about 6.7 micrometers. Barrier layer  509  is grown to a thickness  516  of about 150 nm. 
     As shown, pores  514  within second metal oxide layer  512  are generally wider than pores  506  of first metal oxide layer  504 . In particular, the diameters of pores  514  are about 100 nanometers or more, compared to pores  506  having diameters between about 40 nm and about 50 nm. Likewise, the pore walls between pores  514  of second metal oxide layer  512  are generally wider (thicker) than the pore walls between pores  506  of first metal oxide layer  504 . In addition, second metal oxide layer  512  has irregular pore walls with surfaces that are oriented non-orthogonally with respect to the outer surface  518 . 
       FIG. 5C  shows part  540  after three anodizing processes have been performed. Like parts  500  and  530 , a first anodizing process using an sulfuric acid-based electrolyte is used to form first metal oxide layer  504  (in this case having a thickness  508  of about 5.1 micrometers) and a second anodizing process using an phosphoric acid-based electrolyte is used to form second metal oxide layer  512  (in this case having a thickness  508  of about 4.5 micrometers). In addition, a barrier layer smoothing and thickening anodizing process is performed, where barrier layer  509  is smoothed and thickened to thickness  520  of about 550 nm. The barrier layer process involved exposing part  540  to a non-pore-forming anodizing process using a non-dissolution electrolyte (e.g., borax, boric acid, etc.) The resulting multilayered anodic coating (first oxide layer  504 +second oxide layer  512 +smoothed barrier layer  509 ) has a white appearance as viewed from outer surface  518 . 
       FIGS. 6A-6D and 7A-7D  show SEM cross section and top view images of a part indicating how a barrier layer smoothing process can affect a structure of and anodic film, in accordance with some embodiments. The part includes an anodic film  602 , formed using a phosphoric acid-based anodizing process, and barrier layer  604 .  FIGS. 6A-6D  show images of the part before a barrier layer smoothing and thickening process, and  FIGS. 7A-7D  show images of the part after a barrier layer smoothing and thickening process is performed. The barrier layer was smoothed using borax-based barrier layer smoothing process. 
       FIGS. 6A and 6B  show cross section images of the part at different magnifications, with  FIG. 6B  at a higher magnification. As shown in  FIG. 6B , barrier layer  604  prior to the barrier layer smoothing process has an uneven and inconsistent boundary.  FIG. 7B  shows that barrier layer  604 , after the barrier layer smoothing process is performed, has a much more even boundary that is more conducive to producing a white appearance. The barrier layer smoothing process also involves thickening barrier layer  604  (i.e., from about 200 nm thick to about 800 nm thick). Whiteness measurements of the part prior to the barrier layer smoothing process is characterized as having a whiteness value W 10  of 64.7, and after the barrier layer smoothing process having whiteness value W 10  of 70.48. This data indicates that the barrier layer smoothing process can significantly increase the whiteness of an anodic film. 
       FIGS. 6C and 6D  show top views of the anodic film at different magnifications, with  FIG. 6C  at a higher magnification.  FIGS. 7C and 7D  show top views of the anodic film after the barrier layer smoothing process. As shown, the barrier layer smoothing process did not significantly change the pore structure of anodic film  602 . In particular, the pore diameters were between about 200 nm and about 260 nm before and after the barrier layer smoothing process. This data indicates that the integrity of anodic film  602  is not significantly affected by the barrier layer smoothing and thickening process. 
     It can be difficult to determine a level of whiteness of a part based on L*a*b* color space values alone since bright metallic surfaces can have similar L*a*b* measurements as white surfaces.  FIGS. 8A-8D  show how a circularly polarizing filter can be used to determine whiteness of a part, including parts having multiple layered anodic films, in accordance with some embodiments. 
       FIG. 8A  shows a top view of part  800 , which is composed of an aluminum alloy substrate and which is anodized using a type II anodizing process. The type II anodizing process results in providing an anodic coating that is relatively transparent such that the silver metal appearance of the aluminum alloy substrate is highly visible through the anodic coating. First filter  802  and second filter  804 , which are both circularly polarized filters of the same type, are positioned on top of part  800 . First filter  802  has a first orientation (e.g., left circularly polarized) with respect to part  800 , and second filter  804  has a second orientation (e.g., right circularly polarized) with respect to part  800 . First filter  802  is oriented such that the bright silver appearance of part  800  is minimized barely visible through first filter  802  (i.e., has a dark appearance). Second filter  804  is oriented such that the bright silver appearance of part  800  is maximized and clearly visible through second filter  804 . L* measurements, which correspond to an amount of lightness, are taken of part  800  through first filter  802  and second filter  804 . The difference between the L* values are then quantified as ΔL* (L/R), where L corresponds to left circularly polarized and R corresponds to right circularly polarized. This ΔL* (L/R) can be used to distinguish between a white surface and a light reflection off of a metallic surface (e.g., a silver colored metal surface of an aluminum alloy). 
     To illustrate, Table 1 below summarizes some color space value measurements for part  800  and Table 2 summarizes the same color space value measurements for a white piece of paper. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Type II anodized aluminum alloy 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 W 10   
                 73 
               
               
                   
                 L* 
                 92 
               
               
                   
                 a* 
                 0.6 
               
               
                   
                 b* 
                 1.8 
               
               
                   
                 ΔL* (L/R) 
                 34.3 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 White paper 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 W 10   
                 81.0 
               
               
                   
                 L* 
                 92.8 
               
               
                   
                 a* 
                 0.2 
               
               
                   
                 b* 
                 0.3 
               
               
                   
                 ΔL* (L/R) 
                 1.1 
               
               
                   
                   
               
            
           
         
       
     
     W 10  corresponds to a CIE standard illumination based on tristimulus value Y and chromaticity coordinate (x,y), as described above. In accordance with CIE D65 color space standards, L* corresponds to an amount of lightness, a* represents an amount of green or red/magenta, and b* represents an amounts of blue or yellow. Negative a* values indicate a green color while positive a* values indicate a red or magenta color. Negative b* values indicate a blue color and positive b* values indicate a yellow color. ΔL* corresponds to an amount of change of L* of the first filter compared to the second filter. W 10 , L*, a* and b* measurements are taken directly at the surfaces of the anodized part  800  and white piece of paper. The ΔL* value is based on measurements are taken through first filter  802  and second filter  804 . 
     Tables 1 and 2 indicate that the type II anodized aluminum substrate has similar W 10 , L*, a* and b* values as the white piece of paper. In fact, the W 10  value, which is an indicator of whiteness, for the visibly silver anodized part  800  is greater than the W 10  value of the white piece of paper. This is because the anodized part  800  has a high specular reflectance (i.e., high shine), which is associated with high lightness measurements. Thus, although W 10 , L*, a* and b* values can be an indication of how colorless and bright a part is, these values may not fully indicate a level of whiteness of a part. In contrast, the ΔL* for the anodized part  800  is much higher than that of the white piece of paper. In particular, the ΔL* value for the white piece of paper is relatively low (i.e., 1.1), whereas the ΔL* value of the bright silver anodized part  800  is much higher (i.e., 34.3). That is, a small ΔL* value is associated with a white color. 
       FIG. 8B  shows a top view of part  810 , which is composed of an aluminum alloy substrate (same type of aluminum alloy as the substrate of part  800 ) having been treated with multiple anodizing processes to form a white multilayered anodic film. In particular, part  810  includes a first layer formed by anodizing the substrate in an oxalic acid bath, a second layer formed by anodizing the substrate in a phosphoric acid bath, and a barrier layer that was smoothed and thickened using a barrier layer smoothing process. Circularly polarized filters  812  and  814  are positioned on top of part  810  at opposing orientations, as described above. Table 3 below summarized whiteness measurements of part  810 . 
     
       
         
           
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Multilayered anodic film on aluminum alloy 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 W 10   
                 84.2 
               
               
                   
                 L* 
                 90.8 
               
               
                   
                 a* 
                 2.5 
               
               
                   
                 b* 
                 1.3 
               
               
                   
                 ΔL* (L/R) 
                 8.9 
               
               
                   
                   
               
            
           
         
       
     
     Table 3 indicates that part  810  has a higher W 10  value than the silver appearing type II anodized part  800 . In addition, part  810  is characterized has having a much lower ΔL* value than the ΔL* value of part  810 . That is, part  810  measures less change in an amount of lightness L*, as measured through opposite-oriented polarized filters, compared to part  800 . This indicates that less of the lightness L* of part  810  is due to the specularly reflective underlying aluminum alloy substrate than the lightness L* of part  800 . In fact, to a human eye, part  800  has a silver appearance while part  810  has a distinctively white appearance. 
       FIG. 8C  shows a top view of part  820 , which is composed of an aluminum alloy substrate (same type of aluminum alloy as the substrate of parts  800  and  810 ) having been treated with a different multiple anodizing processes than part  810 . In particular, part  820  includes a first layer formed by anodizing the substrate in an oxalic acid bath, a second layer formed by anodizing the substrate in a phosphoric acid bath, a barrier layer that was smoothed and thickened using a barrier layer smoothing process, and white pigment (i.e., TiO 2 ). Circularly polarized filters  822  and  824  are positioned on top of part  820  at opposing orientations, as described above. Table 4 below summarized whiteness measurements of part  820 . 
     
       
         
           
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Multilayered anodic film with pigment on aluminum alloy 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 W 10   
                 83.5 
               
               
                   
                 L* 
                 88.9 
               
               
                   
                 a* 
                 1.4 
               
               
                   
                 b* 
                 2.0 
               
               
                   
                 ΔL* (L/R) 
                 4.2 
               
               
                   
                   
               
            
           
         
       
     
     Table 4 indicates that part  820 , like part  810 , has a higher W 10  value and much lower ΔL* value than the silver appearing type II anodized part  800 . Part  820  also appears to a human eye to have a distinctively white appearance. In this embodiment, the addition of TiO 2  pigment to the multilayered anodic film is shown to increase the W 10  value and decrease the ΔL* value compared to part  810  having a multilayered anodic film without pigment. 
       FIG. 8D  shows a top view of part  830 , which is composed of an aluminum alloy substrate (same type of aluminum alloy as the substrate of parts  800 ,  810  and  820 ) having been treated with a similar multiple anodizing processes as part  820 . In particular, part  830  includes a first layer formed by anodizing the substrate in a sulfuric acid bath, a second layer formed by anodizing the substrate in a phosphoric acid bath, a barrier layer that was smoothed and thickened using a barrier layer smoothing process, and TiO 2  pigment. Circularly polarized filters  832  and  834  are positioned on top of part  830  at opposing orientations, as described above. Table 5 below summarized whiteness measurements of part  830 . 
     
       
         
           
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Multilayered anodic film with pigment on aluminum alloy 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 W 10   
                 75 
               
               
                   
                 L* 
                 83 
               
               
                   
                 a* 
                 1.0 
               
               
                   
                 b* 
                 1.0 
               
               
                   
                 ΔL* (L/R) 
                 3.6 
               
               
                   
                   
               
            
           
         
       
     
     Table 5 indicates that part  830 , like parts  810  and  820 , has much lower ΔL* value than the silver appearing type II anodized part  800 . Part  830  also has an even lower ΔL* value than that of part  820 , which also has a multilayered anodic film with TiO 2  pigment. It is noted that part  830  has a lower W 10  value than parts  810  and  820  that also have multilayered anodic films, even though part  830  appears to have a distinctively white appearance. This indicates that, in some embodiments, ΔL* values may be as important than W 10  values in determining a whiteness of an anodic film. In any cases, the parts having the multilayered anodic films (parts  810 ,  820  and  830 ) each have a higher W 10  value than that of a single layered anodic film (part  800 ). 
     The data of Tables 1-5 and  FIGS. 8A-8D  indicate that, in some embodiments, multilayered anodic films formed using the methods described herein can be characterized as having L* values of at least 80, a b* value between about −3 and about +6, and an a* value of between about −3 and about +3. In some embodiments, the multilayered anodic films are characterized as having W 10  values of at least about 70 and a ΔL* value of no greater than about 10. It should be noted that L*, b*, a*, W 10 , ΔL* values can vary while still appearing white (e.g., not silver) to a human eye, and that process parameters and can used to adjust different structural properties of a multilayered anodic film in order to achieve a particular white appearance and hardness value. For example, the thicknesses of the first and second anodic film layers can be adjusted, as can the smoothness and thickness of the barrier layer and the amount and type of pigment used (if a pigment is used). 
       FIGS. 9A-9B, 10A-10B and 11A-11B  show SEM images of anodic film prior to and after barrier layer smoothing processes to illustrate the extent that a barrier layer smoothing process can smooth an interface surface of a barrier layer, in accordance with some embodiments. 
       FIGS. 9A-9B  show SEM cross section images of part  900  prior to a barrier layer smoothing process, with  FIG. 9B  showing a higher magnification. Part  900  includes substrate  902 , which is composed of an aluminum alloy, and anodic film  904 , which was formed using a sulfuric acid-based anodizing process (using a voltage of around 20 V) and which as pores  906 . Barrier layer  906  defines interface surface  908  between barrier layer  906  and anodic film  904 . As shown, interface surface  910  has a scalloped structure in accordance with the terminuses (bottoms) of pores  908 . Lines  912   a  and  912   b  demarcate the depth of the pore terminuses, which can be defined as thickness measurement of the curved bottom portions of pores  908 . The depth of pore  908  terminuses is found to be around 12 nm. 
     It should be noted that the chemistry of electrolytic bath and the voltage used during the anodizing process for forming anodic film  904  also has a relationship with the depth of pores  908 . For example, sulfuric acid-based anodizing generally results in pores that are smaller in diameter than pores formed form a phosphoric acid-based anodizing process. Also, higher voltages generally result in a less smooth interface surface. For example, a phosphoric acid-based anodizing process generally results in pores that are larger in diameter and that results in a barrier layer having less smooth interface than that of a sulfuric acid-based anodizing process (see  FIGS. 11A and 11B ). 
       FIGS. 10A and 10B  show SEM cross section images of part  900  after a barrier layer smoothing process. The barrier layer smoothing process involved applying a voltage of about 70 V to part  90  while immersed in a borax-base electrolyte. As shown, interface surface  910  is significantly smoothed and no longer has a scalloped structure. That is, the terminuses of pores  908  are flattened. This smoothing can also be characterized by a difference in the depth of the terminuses of pores  908 , as demarcated by lines  1002   a  and  1002   b . In particular, the depth of the terminuses of pores  908  is decreased to about 7 nm. According to some embodiments, the depth of the pore terminus is less than about 10 nm. 
     As described above, the chemistry of electrolytic bath and the voltage used during the anodizing process for forming an anodic film (prior to the barrier layer smoothing process) can affect the smoothness of an interface surface between the barrier layer and the porous portion of the anodic film. To illustrate,  FIGS. 11A and 11B  show top view and cross section SEM images, respectively, of part  1100 . Part  1100  includes substrate  1102 , anodic film  1104  and barrier layer  1106 . Anodic film  1104  was formed using a phosphoric acid-based anodizing process using a voltage of about 100 V. The resulting anodic film  1104  has pores  1108  with diameters around 50 nm and about 100 nm having a pore terminus depth  1110  of about 40 nm and about 76 nm. 
     The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.

Metadata:
Filing Date: 20161024
Publication Date: 20200901
Grant Date: 20200901
Priority Date: 20151030
Inventors: TATEBE, MASASHIGE
OSHIMA, TAKAHIRO
AKANA, JODY R.
NAKAGISHI, YUTAKA
KATAYAMA, JUNICHI
HARA, KENJI
ITO, YASUHIRO
HONGOU, Ayumi
Assignee: APPLE INC
CPC Classifications: [{"code": "C25D11/10", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25F3/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/16", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/24", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/06", "inventive": false, "first": false, "tree": "[]"}, {"code": "C25D11/22", "inventive": true, "first": true, "tree": "[]"}, {"code": "G06F1/1633", "inventive": false, "first": false, "tree": "[]"}, {"code": "C25F3/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05K5/04", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/06", "inventive": false, "first": false, "tree": "[]"}, {"code": "Y10T428/131", "inventive": false, "first": false, "tree": "[]"}, {"code": "C25D11/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/22", "inventive": true, "first": true, "tree": "[]"}, {"code": "H05K5/0243", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/08", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/10", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/08", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/22", "inventive": true, "first": true, "tree": "[]"}, {"code": "C25D11/06", "inventive": false, "first": false, "tree": "[]"}, {"code": "H05K5/0243", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/16", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05K5/04", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25F3/20", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/24", "inventive": true, "first": false, "tree": "[]"}, {"code": "G06F1/1633", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 58456159