Patent Publication Number: US-2019169763-A1

Title: Methods for improving adhesion of aluminum films

Description:
FIELD 
     This disclosure relates generally to aluminum films and methods for depositing aluminum films. In particular, described are various methods for improving adhesion of deposited aluminum films. 
     BACKGROUND 
     Electroplating is a process widely used in industry to provide a metal coating having a desirable physical quality on a part. For example, electroplated coatings can provide abrasion and wear resistance, corrosion protection and aesthetic qualities to the surfaces of parts. Electroplated coating may also be used to build up thickness on undersized parts. 
     Aluminum substrates, in particular, can be difficult to plate since aluminum surfaces rapidly acquire an oxide layer when exposed to air or water, and thus tend to inhibit good adhesion of an electrodeposited film. In addition, since aluminum is one of the more anodic metals, there is a tendency to form unsatisfactory immersion deposits during exposure to a plating solution, which can cause discontinuous plating or breakdown of the plating process. Furthermore, if plating an aluminum film, plating methods usually involve the plating of pure aluminum metal onto the substrate. Although pure aluminum has an ordered microstructure and good cosmetic properties, it is relatively soft and easily scratched. Therefore, there are significant challenges to plating aluminum in industrial applications where durability is a desirable characteristic of a plated film. 
     SUMMARY 
     This paper describes various embodiments that relate to aluminum films with improved adhesion. 
     According to one embodiment, a method for forming a protective coating on a surface of an aluminum substrate is described. The method includes forming an adhesion-promoting layer on a surface of the aluminum substrate. The adhesion-promoting layer has a number of cavities having side walls oriented substantially normal to the surface of the aluminum substrate. The adhesion-promoting layer is chemically compatible with a subsequent anodizing process. The method also includes depositing an aluminum layer on the adhesion-promoting layer. The aluminum layer has a number of anchor portions disposed within corresponding cavities of the adhesion-promoting layer. The anchor portions engage with the side walls of the adhesion-promoting layer resisting a shearing force applied to the aluminum layer securing the aluminum layer to the adhesion-promoting layer. 
     According to an additional embodiment, a method for forming an aluminum layer on a substrate is described. The method includes forming an aluminum oxide adhesion layer on the substrate. The aluminum oxide adhesion layer has a number of pores defined by a plurality of corresponding pore walls. The method also includes, during the forming, controlling an average pore size of the aluminum oxide adhesion layer by simultaneously allowing growth of the pore walls and dissolving the pore walls such that the average pore size is sufficiently large to allow aluminum material to form therein during a subsequent aluminum layer depositing process. The method also includes depositing the aluminum layer on the aluminum oxide adhesion layer. During the depositing, anchoring portions of the aluminum layer are formed within at least a portion of corresponding pores. The anchor portions engage with the pore walls resisting a shearing force applied to the aluminum layer securing the aluminum layer to the aluminum oxide layer. 
     According to a further embodiment, a composite coating for an aluminum substrate is described. The composite coating includes a first aluminum oxide layer disposed on the aluminum substrate. The first aluminum oxide layer has a first hardness. The composite coating also includes a second aluminum oxide layer disposed on the first aluminum oxide layer. The second aluminum oxide layer being more optically transparent than the first aluminum oxide layer. The first aluminum oxide layer is integrally bounded to the second oxide layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The described embodiments and the advantages thereof may best be understood by reference to the following description taken in conjunction with the accompanying drawings. These drawings in no way limit any changes in form and detail that may be made to the described embodiments by one skilled in the art without departing from the spirit and scope of the described embodiments. 
         FIGS. 1A-1C  show cross-section views of a part undergoing a pretreatment involving an anodizing process to improve adhesion of a deposited aluminum layer. 
         FIGS. 2A and 2B  show cross-section scanning electron microscope (SEM) views of a part that includes an aluminum oxide adhesion layer formed using a phosphoric acid anodizing process. 
         FIGS. 3A-3C  show cross-section views of a part undergoing a pretreatment involving a zincating process to improve adhesion of a deposited aluminum layer. 
         FIGS. 4A-4C  show cross-section views of a part undergoing a pretreatment involving a surface roughening process to improve adhesion of a deposited aluminum layer. 
         FIGS. 5A-5C  shows cross-section views of a part undergoing aluminum depositing and anodizing processes where a portion of substrate is anodized. 
         FIG. 6  shows a flowchart indicating a high-level process involving substrate pretreatment to improve adhesion of a deposited aluminum layer. 
         FIG. 7  shows a plating rack assembly suitable for plating a number of parts. 
     
    
    
     DETAILED DESCRIPTION 
     Representative applications of methods and apparatus according to the present application are described in this section. These examples are being provided solely to add context and aid in the understanding of the described embodiments. It will thus be apparent to one skilled in the art that the described embodiments may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order to avoid unnecessarily obscuring the described embodiments. Other applications are possible, such that the following examples should not be taken as limiting. 
     This application relates to aluminum films and providing aluminum films on substrates. As used herein, the terms “film” and “layer” are used interchangeably. Unless otherwise described, as used herein, “aluminum” and “aluminum layer” can refer to any suitable aluminum-containing material, including pure aluminum, aluminum alloys or aluminum mixtures. As used herein, “pure” or “nearly pure” aluminum generally refers to aluminum having a higher percentage of aluminum metal compared to aluminum alloys or other aluminum mixtures. The aluminum films are well suited for providing both protective and attractive layers to consumer products. For example, methods described herein can be used for providing protective and cosmetically appealing coatings for enclosures and casings for electronic devices. 
     Described herein are methods for improving adhesion of deposited aluminum layers on a substrate. Methods described herein can be used to improve the adhesion of an aluminum layer to a substrate without the use of a strike layer. Methods involve substrate pretreatments prior to depositing of an aluminum layer. The pretreatments providing a three-dimensional surface having gaps or cavities on the substrate that can act as anchoring regions for securing the aluminum layer to the substrate. In some embodiments, methods involve providing a thin aluminum oxide adhesion layer on the substrate prior to depositing aluminum. In some embodiments, methods involve providing a zincating layer on the substrate prior to depositing aluminum. In some embodiments, methods involve roughening the surface of the substrate prior to depositing aluminum. Some embodiments involve a combination of two or more substrate pretreatments. 
     These and other embodiments are discussed below with reference to  FIGS. 1-7 . 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. 
     As described above, it can be difficult to deposit onto aluminum substrates since aluminum substrates quickly acquire a natural oxide layer when exposed to air or water. The natural oxide layer can inhibit the adhesion of many metal materials, such as aluminum, to the surface of the aluminum substrate. Conventional methods for providing better adhesion include forming a thin layer of copper or nickel plating, referred to as a strike or strike layer. The strike layer generally has good adhesion to the aluminum substrate and also to the subsequently deposited aluminum layer. However, use of a strike layer can have has several disadvantages. For example, a strike layer can make the part more susceptible to galvanic corrosion during a plating process. In particular, if external coating layers are scratched exposing the strike layer next to the plated aluminum layer (and possibly the aluminum substrate), the exposure of dissimilar materials can create a galvanic cell on the part. This can increase the risk of corrosion in the plated and anodized part later on. 
     There can also be manufacturing challenges to using a strike layer. In some manufacturing processes, an entire aluminum layer is converted to aluminum oxide using an anodizing process. During the anodizing process, the strike layer can be exposed in localized areas creating a varied current density distribution across the part. Locally thinner areas of the aluminum layer can become anodized through sooner, resulting in an anodized layer having a varying thickness across the part. Furthermore, materials from the strike layer can contaminate the anodizing bath and create defects in the resultant aluminum oxide. To avoid exposing the strike layer during these processes, a buffer layer of plated aluminum can be positioned between the substrate and the aluminum oxide layer. However, the buffer layer can add thickness to the overall aluminum and aluminum oxide stack. 
     To avoid reaching the strike layer, a buffer layer of aluminum can be left between the strike layer and the remainder of the aluminum layer. Since the thickness of the aluminum layer can be variable across a part (due to variations in current density), the thickness of the buffer layer is generally dictated by the minimum thickness across the part. One of the disadvantages of using a buffer layer is that it can add an undesired extra thickness to the aluminum and aluminum oxide stack. In addition, if during an anodizing process the aluminum oxide layer grows too close to or beyond the thickness of the aluminum layer, the anodizing solution can contact and react with the strike layer. The reaction products can contaminate the anodizing solution and cause defects in the resultant aluminum oxide layer. For at least these reasons, it can be beneficial in certain applications to avoid the use of a strike layer. However, it can be difficult to plate aluminum directly onto substrates since aluminum generally does not adhere well to substrates during electroplating, especially when plating pure or nearly pure aluminum. In addition, if the substrate is also made of aluminum, the aluminum substrate has a strong affinity to form a natural oxide layer on its surface making it difficult to plate thereon. 
     In order to improve the adhesion of an aluminum layer on a substrate, methods described herein involve pretreating the substrate prior to an aluminum deposition process. The pretreatments avoid the use of a strike layer and, therefore, do not include some of the downsides of using a strike layer. The pretreatments involve creating a three-dimensional adhesion-promoting surface on the substrate. When an aluminum layer is deposited on the adhesion-promoting surface, portions of the aluminum can become deposited within the gaps or cavities of the three-dimensional adhesion-promoting surface. These portions of the aluminum layer can anchor the aluminum layer to the substrate surface and provide better adhesion of the aluminum layer to the substrate. The adhesion-promoting layer can be substantially free of non-aluminum metal agents, such as agents containing copper and/or nickel, and therefore chemically compatible with a subsequent anodizing process. In some embodiments, creating the adhesion-promoting surface involves forming an aluminum oxide adhesion layer on the substrate surface, which is described below with reference to  FIGS. 1A-1C and 2A-2B . In other embodiments, creating the adhesion-promoting surface involves forming a zincating layer on the substrate surface, which is described below with reference to  FIGS. 3A-3C . In other embodiments, creating the adhesion-promoting surface involves roughening the substrate surface, which is described below with reference to  FIGS. 4A-4C . It should be understood that these embodiments are presented as suitable examples and are not meant to limit the types and scope of possible methods of providing adhesion-promoting surfaces. 
     One way of forming an adhesion-promoting surface on a substrate is by forming a thin aluminum oxide layer that has adhesion-promoting properties on the substrate.  FIGS. 1A-1C  show cross-section views of part  100  undergoing an aluminum deposition process involving formation of an aluminum oxide adhesion layer in accordance with some embodiments. At  FIG. 1A , a thin portion of aluminum substrate  102  is converted to aluminum oxide adhesion layer  104 . Prior to forming aluminum oxide adhesion layer  104 , the top surface of aluminum substrate  102  can be treated with any suitable finishing technique. For example, aluminum substrate  102  can be polished to have mirror shine. In other embodiments, aluminum substrate  102  is textured to have a textured or roughened surface. Since forming aluminum oxide adhesion layer  104  is a conversion process whereby a portion of aluminum substrate  102  is consumed, aluminum oxide adhesion layer  104  is integral to and well adhered to aluminum substrate  120 . Aluminum oxide adhesion layer  104  should be thick enough to create a good anchoring surface and thin enough so that the surface of substrate  102  remains conductive for a subsequent electroplating process. In some embodiments, aluminum oxide adhesion layer  104  has a thickness of less than about 5 micrometers. In some embodiment, aluminum oxide adhesion layer  104  has a thickness of about 3 micrometers or less. 
     Aluminum oxide adhesion layer  104  can be formed using an anodizing process that includes the use of an acidic electrolytic solution. In some embodiments, the electrolytic solution includes phosphoric acid, oxalic acid, or a combination of phosphoric acid and oxalic acid. The phosphoric and/or oxalic acid can promote the formation of pores  105  having a larger average diameter compared to the average diameter of standard anodic pores. In addition, aluminum oxide adhesion layer  104  generally has lower pore density compared to standard anodized aluminum oxide layers. It is believed that the phosphoric and/or oxalic acid tends to dissolve portions of the pore walls of pores  105  as pores  105  are being grown, thereby creating the larger diameter pores  105  and lower pore density. That is, anodizing in phosphoric acid and/or oxalic acid conditions allow for simultaneous growth and dissolving of the pore walls. In some embodiments, pores  105  have a diameter of about 100 nm or greater. In some embodiments, the acidic electrolytic solution contains chromic acid and/or sulfuric acid. In some embodiments, the acidic electrolytic solution contains a mixture of two or more of phosphoric acid, oxalic acid, chromic acid, and sulfuric acid. In some embodiments, aluminum oxide adhesion layer  104  is exposed to an inert atmosphere prior to a subsequent aluminum plating process in order to activate aluminum oxide layer  104  and promote better adhesion with the plated aluminum. The inert atmosphere can include exposing part  100  to a non-oxidative atmosphere such as a nitrogen or argon environment. 
     At  FIG. 1B , aluminum layer  106  is deposited on aluminum oxide adhesion layer  104 . Aluminum layer  106  can be deposited using any suitable process including a plating process, such as an electroplating process. In some embodiments, aluminum layer  106  is deposited on aluminum oxide adhesion layer  104  very soon after formation of aluminum oxide adhesion layer  104  to avoid exposure of aluminum oxide adhesion layer  104  to moisture or air. In some embodiments, aluminum oxide adhesion layer  104  is placed in a moisture free atmosphere, such as a nitrogen or argon environment. As shown, pores  105  are sufficiently large such that anchoring portions  107  of aluminum layer  106  can form within pores  105 . Pore walls  105  are substantially normal to the top surface of substrate  102  such that when a shearing force  109  is applied to aluminum layer  106 , anchoring portions  107  can engage with pore walls  105  and secure aluminum layer  106  to aluminum oxide adhesion layer  104 . The adhesion strength of aluminum layer  106  to aluminum oxide adhesion layer  104  can be measured by a standard pull test. In some embodiments, the adhesion of aluminum layer  106  to aluminum oxide adhesion layer  104 , as measured by pull testing, ranges from about 70 MPa and about 85 MPa. 
     At  FIG. 1C , a portion of aluminum layer  106  is optionally converted to aluminum oxide layer  108 . As shown, aluminum oxide layer  108  has pores  112  that are generally smaller in diameter compared to pores  105  of aluminum oxide adhesion layer  104 . Aluminum oxide layer  108  can be formed using any suitable process, including conventional anodizing processes. Note that part  100  does not include a strike layer, thereby eliminating the occurrence of defects caused by anodizing interfering materials in a strike layer. That is, there is no chance of reaching a defect-causing material during an anodizing process. Thus, aluminum layer  106  can be thinner than a corresponding aluminum layer using a strike layer, thereby reducing the stack thickness of aluminum oxide adhesion layer  104 , aluminum layer  106 , and aluminum oxide layer  108 . In some embodiments, substantially all of aluminum layer  106  is converted to an aluminum oxide layer. In some embodiments, substantially all of aluminum layer  106  and a portion of aluminum substrate  102  are converted to an aluminum oxide layer. These embodiments are achievable because there is no intervening strike layer. That is, aluminum oxide adhesion layer  104  is compatible with an anodizing process. 
       FIGS. 2A and 2B  shows cross-section SEM (scanning electron microscope) images of an actual aluminum part  200  illustrating the features described above with reference to  FIGS. 1A-1C .  FIG. 2B  is an inset showing a close-up view of a portion of the SEM image of  FIG. 2A . Part  200  includes aluminum substrate  202 , aluminum oxide adhesion layer  204 , aluminum layer  206 , and aluminum oxide layer  208 . Aluminum oxide adhesion layer  204  was formed using an anodizing solution containing phosphoric acid. As clearly shown in the inset  FIG. 2B , portions of aluminum layer  206  deposit within the pores of aluminum oxide adhesion layer  204 , mechanically interlocking aluminum layer  206  and aluminum oxide adhesion layer  204  together. The pores of aluminum oxide adhesion layer  204  are substantially normal to the top surface of substrate  202  such that when a shearing force is applied to aluminum layer  206 , the deposited portions of aluminum layer  206  can engage with the pore walls of the pores and secure aluminum layer  206  to aluminum oxide adhesion layer  204 . 
     Another way forming an adhesion-promoting surface on a substrate is by forming a zincating layer on the substrate.  FIGS. 3A-3C  show cross-section views of part  300  undergoing an aluminum deposition process involving formation of a zincating layer in accordance with some embodiments. At  FIG. 3A , zincating layer  304  is deposited onto aluminum substrate  302 . Zincating layer  304  is generally a thin conductive crystalline layer that can be formed by exposing aluminum substrate  302  to a zincate solution. The zincate solution contains tetrahydroxozincating ion (Zn(OH) 4   2 ), which can remove a natural oxide layer that forms on aluminum substrate  302 . Once formed, the zincating layer  304  can prevent re-oxidation of aluminum substrate  302 . In some embodiments, a cyanide multi-metal based zincatc solution is used. Zincating layer  304  is generally chemically computable with a subsequent anodizing process. In some embodiments, zincating layer  304  is very thin, in some cases less than about 0.5 micrometers thick. In some embodiments, zincating layer  304  is exposed to an inert atmosphere prior to a subsequent aluminum plating process in order to promote better adhesion with the plated aluminum. 
     At  FIG. 3B , aluminum layer  306  is deposited on zincating layer  304 . As shown by the inset view, zincating layer  304  has a three-dimensional crystalline structure that includes cavities surrounded by walls  305 . When aluminum layer  306  is deposited onto zincating layer  304 , anchoring portions  307  become deposited within the cavities of zincating layer  304 . Walls  305  can be substantially normal to the top surface of substrate  302  such that when a shearing force  309  is applied to aluminum layer  306 , anchoring portions  307  can engage with walls  305  and secure aluminum layer  306  to substrate  302 . In some embodiments, the adhesion of aluminum layer  306  to zincating layer  304 , as measured by pull testing, ranges from about 30 MPa and about 65 MPa. 
     At  FIG. 3C , a portion of aluminum layer  306  is optionally converted to aluminum oxide layer  308 , which has anodic pores  312  formed therein. Aluminum oxide layer  308  can be formed using any suitable process, including a conventional anodizing process. Zincating layer  304  allows for the elimination of a strike layer within part  300 , thereby eliminating the occurrence of defects caused by anodizing interfering materials in a strike layer. Thus, in some embodiments, substantially all of aluminum layer  306  is converted to an aluminum oxide layer. In some embodiments, substantially all of aluminum layer  306  and a portion of aluminum substrate  302  are converted to an aluminum oxide layer. 
     An additional way of forming an adhesion-promoting surface on a substrate is by creating a textured or roughened surface on the substrate.  FIGS. 4A-4C  show a cross-section view of part  400  undergoing an aluminum deposition process involving substrate surface roughening in accordance with some embodiments. At  FIG. 4A , top surface of aluminum substrate  402  is textured to have a rough surface  404  having a series of peaks and valleys. Any suitable surface texturing or roughing process can be used. In some embodiments, a blasting procedure is used, whereby a blasting media is impinged on top surface of aluminum substrate  402 . In some embodiments, a laser texturing procedure is used, whereby a continuous or pulsed laser is scanned across the top surface of aluminum substrate  402  creating random or organized patterns of pits. In some embodiments, an acid etching procedure is used, whereby an acidic solution etches and creates a roughened top surface of aluminum substrate  402 . In some embodiments, aluminum substrate  402  is exposed to an inert atmosphere prior to a subsequent aluminum plating process in order to promote better adhesion with the plated aluminum. 
     At  FIG. 4B , aluminum layer  406  is deposited on aluminum substrate  402 . As shown, anchoring portions  407  are formed within the valleys of roughened surface  404 . Walls  405  of the valleys of roughened surface  404  are substantially normal to the top surface of substrate  402  such that when a shearing force  1309  is applied to aluminum layer  406 , anchoring portions  407  engage walls  405  securing aluminum layer  406  to substrate  402 . Note that each of the valleys of rough surface  404  are generally larger in width compared to the width of each pore  105  of aluminum oxide adhesion layer  104  described above with reference to  FIGS. 1A-1C . In some embodiments, the adhesion of aluminum layer  306  to substrate  402 , as measured by pull testing, is about 29 MPa. 
     At  FIG. 4C , a portion of aluminum layer  406  is optionally converted to aluminum oxide layer  408 , which has pores  412  formed therein. Aluminum oxide layer  408  can be formed using any suitable process, including a conventional anodizing process. In some embodiments, substantially all of aluminum layer  406  is converted to an aluminum oxide layer. Roughened surface allows for the elimination of a strike layer within part  400 , thereby eliminating the occurrence of defects caused by anodizing interfering materials in a strike layer. Thus, in some embodiments, substantially all of aluminum layer  406  is converted to an aluminum oxide layer. In some embodiments, substantially all of aluminum layer  406  and a portion of aluminum substrate  402  are converted to an aluminum oxide layer. 
     In some embodiments, one or more of the above-described pretreatment techniques can be used in combination. For example, an aluminum substrate can be treated with a surface roughening process, followed by formation of an aluminum oxide adhesion layer, and followed by deposition of an aluminum layer. In another embodiment, an aluminum substrate is treated with a surface roughening process, followed by formation of a zincating layer, and followed by deposition of an aluminum layer. In some embodiments, combining multiple pretreatment techniques can improve the adhesion of an aluminum layer to a substrate. 
     As described above, one of the advantages of the absence of a strike layer is that more portions of the aluminum layer, and possibly the substrate itself, can be converted to aluminum oxide without creating strike layer material defects. This allows for more flexibility during a subsequent anodizing process and the more possible variations in forming aluminum oxide layers on a substrate.  FIGS. 5A-5C  shows cross-section views of part  500  undergoing aluminum depositing and anodizing processes where a portion of substrate is anodized. 
     At  FIG. 5A , adhesion-promoting surface  504  is formed on aluminum substrate  502 . Adhesion-promoting surface  504  is any suitable surface that has a three-dimensional quality that allows for formation of anchoring portions during a subsequently aluminum depositing process. As described above, adhesion-promoting surface  504  can correspond to a surface of an aluminum oxide adhesion layer, a surface of a zincating layer, or a roughed surface of substrate  502 . Substrate  502  can be made of any suitable anodizable metal or metal alloy. 
     At  FIG. 5B , aluminum layer  506  is deposited on adhesion-promoting layer. Aluminum layer  506  can have the same or different composition as substrate  502 . In one embodiment, aluminum layer  506  and substrate  502  are both made of the substantially the same aluminum alloy. In some embodiments, aluminum layer  506  is made of substantially pure aluminum and substrate  502  is made of an aluminum alloy. Embodiments where aluminum layer  506  is substantially pure aluminum may be preferable in applications where it is desirable to have an optically brighter top layer for part  500 . Substrate  502  can be made of aluminum alloy since aluminum alloy is generally harder than pure aluminum and can provide good structural support for part  500 . 
     At  FIG. 5C , substantially all of aluminum layer  506  is converted to first oxide layer  508  and a portion of substrate  502  is converted to second oxide layer  510 , forming a composite coating for part  500 . Since first oxide layer  508  and second oxide layer  510  are formed during a single anodizing process, first oxide layer  508  can be integrally bonded with second oxide layer  510 . First oxide layer  508  and second oxide layer  510  are separated by interface  514 . Pores  512  formed during the anodizing process can be formed within first oxide layer  508 , transcend interface  514  and continue to within second oxide layer  510 . In embodiments where aluminum layer  506  and substrate  502  have different compositions, first oxide layer  508  and second oxide layer  510  can have different physical qualities and/or appearances. For example, an aluminum oxide layer resulting from conversion of a pure aluminum layer can be more optically transparent or translucent compared to an aluminum oxide layer resulting from conversion of an aluminum alloy layer. That is, aluminum oxide obtained from aluminum alloys can appear more yellow and have more of a hazy or matt quality. First oxide layer  508  and second oxide layer  510  can also have different hardness qualities. In one embodiment, second oxide layer  508  is harder than first oxide layer  510 . 
       FIG. 6  shows flowchart  600  indicating a high-level process involving substrate pretreatment and aluminum depositing, in accordance with described embodiments. At  602 , a surface of a substrate is pretreated forming an adherence-promoting surface. The substrate can be made of an anodizable material such as aluminum or alloys thereof. The pretreatments can include providing a three-dimensional surface that has gaps or cavities. Examples of pretreatments include one or more of forming an aluminum oxide adherence layer, forming a zincating layer and providing a roughen substrate surface. In some embodiments, the adhesion-promoting layer has a thickness of less than about 3 micrometers. At  604 , the adherence-promoting surface is optionally activated by exposing the adherence-promoting surface to an inert atmosphere. Suitable inert atmospheres can include exposure to nitrogen and/or argon gas. 
     At  606 , an aluminum layer is deposited onto the adherence-promoting surface of the substrate. In some embodiments, the aluminum layer is deposited using a plating process, such as an electroplating process. The aluminum layer can have substantially the same or different composition as the substrate. In one embodiment, the substrate is made of an aluminum alloy and the aluminum layer is made of substantially pure aluminum. The aluminum layer can be deposited to any suitable thickness. In some embodiments, the aluminum layer is deposited to a thickness ranging from about 1 micrometer and about 10 micrometers. In some embodiments, the aluminum layer is deposited to a thickness ranging from about 2 micrometers and about 4 micrometers. 
     At  608 , at least a portion of the aluminum layer of the aluminum layer and the substrate is optionally converted to an oxide layer. In some embodiments, an anodizing process is used to form the oxide layer. In some embodiments, only a portion of the aluminum layer is converted to an aluminum oxide layer. The absence of a strike layer makes it possible to allow the anodizing process to convert a relatively larger percentage of the aluminum layer without concern as to strike layer material related defects. Thus, in some embodiments, substantially the entire aluminum layer, including portions proximate the substrate, is converted to aluminum oxide. In some embodiments, substantially the entire aluminum layer is converted to an aluminum oxide layer and a portion of the substrate is converted to an oxide layer. Anodizing process conditions can be chosen such that the aluminum oxide layer is durable and cosmetically appealing. In general, an aluminum oxide layer converted from a substantially pure aluminum layer can have a relatively transparent or translucent visual quality compared to an aluminum oxide layer converted from an aluminum alloy. 
     In a production environment, a number of parts can be plating in a single plating bath. The parts can be situated on a rack assembly, such as rack assembly  700  shown in  FIG. 7 . Rack assembly  700  is configured to support parts  702   a - 7021  during a plating process and, in some embodiments, during processes prior to or subsequent to a plating process. For example, rack assembly  700  can be used to support parts  702   a - 7021  during forming of an adhesion-promoting surface, during exposure to an inert atmosphere, during a plating process and/or during a post-plating anodizing process. This way, parts  702   a - 7021  can be transferred together as a unit from process station to process station without removing parts  702   a - 7021  from rack assembly  700 . 
     Rack assembly  700  can be placed within a plating bath during a plating process with bottom portion  711  oriented toward a bottom of the plating cell and top portion  713  oriented toward a top of the plating cell. Rack assembly  700  includes rack frame  704 , drainage bars  706 , and cut outs  710 . Parts  702   a - 7021  can be positioned within cut outs  710  such that each of parts  702   a - 7021  is separated a distance  712  from an edge of rack frame  704 . In addition, outward surfaces of parts  702   a - 7021  and outward surfaces of rack frame  704  are along the same plane. Distance  712  should be small enough such that, during a plating process, parts  702   a - 7021  and rack frame  704  approximate a single flat surface. The proximity of parts  702   a - 7021  to rack frame  704  and the positioning of parts  702   a - 7021  along the same plane as rack frame  704  can promote even current density and plating along edges, corners, and flat surfaces of parts  702   a - 7021 . In some embodiments, drainage bars  706  are added to rack assembly  700 . Drainage bars  706  are connected with and extend outward from rack frame  704  along a different plane as parts  702   a - 7021  and rack frame  704 . Drainage bars  706  can be positioned at an angle relative to rack frame  704  to promote good drainage of chemicals during the plating process. Drainage bars  706  can include connector portions  708  that connect with and fix parts  702   a - 7021  to drainage bars  706 . In some embodiments, connector portions  708  are secured to parts  702   a - 7021  using fasteners such as screws. It should be noted that rack assembly  700  illustrates a particular embodiment and that the shape and arrangement of rack frame  704 , drainage bars  706  and parts  702   a - 7021  can vary in other embodiments. 
     It should be noted that in processing aluminum alloy substrates for coating with a high purity aluminum, the rack material should be chemically compatible with various processing steps that may be employed. In some embodiments, the adhesion improvement processing (e.g., phosphoric anodizing) requires that substantially all surfaces presented for processing uniformly evolve a tenacious dielectric oxide layer for the process to proceed correctly. A subsequent processing step (inert atmosphere activation) can also benefit from having only aluminum surfaces exposed. Bare titanium can work successfully for adhesion but potentially cause cosmetic defects. Use of aluminum coated titanium racks avoids these defects. 
     Racks made entirely of an aluminum alloy may be successfully employed for the adhesion improvement step, the inert activation step and also for any cosmetic finish anodization after the high purity aluminum coating process, without changing the rack. In some embodiments, a titanium rack may also be employed for all these processing steps if it is first coated with aluminum. The utility of the titanium rack is that it will not be substantially degraded by normal post processing cleaning and restoration treatments. A rack made entirely of aluminum could potentially be consumed and destroyed by some number of complete processing cycles. 
     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 specific embodiments are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the described 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.