FIELD

[0001] This disclosure relates generally to aluminum films and aluminum plating methods. In particular, described are aluminum films that have hardening particles embedded therein and methods for forming the same.

BACKGROUND

[0002] 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.

[0003] 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

[0004] This paper describes various embodiments that relate to aluminum films that have hardening particles that increase the hardness of the aluminum film.

[0005] According to one embodiment, a method for forming a hardened aluminum layer on a substrate is described. The method includes exposing at least a portion of a surface of the substrate to a mixture of a number of aluminum ions and a number of hardening particles. The hardening particles are dispersed within the aluminum ions. An average size of the hardening particles is larger than an average size of the aluminum ions. The method also includes creating a flow of aluminum ions toward the substrate surface by applying an appropriate electric field to the mixture. The method further includes creating a flow of hardening particles toward the substrate surface in accordance with the flow of aluminum ions. The aluminum ions and the hardening particles form an aggregate of aluminum metal and hardening particles on the substrate surface, the aggregate having a hardness value greater than that of aluminum metal.

[0006] According to an additional embodiment, a part is described. The part includes a substrate. The part also includes a hardened aluminum layer disposed on the substrate. The hardened aluminum layer includes aluminum metal. The hardened aluminum layer also includes a supportive network having a number of hardening particles substantially uniformly dispersed within the aluminum metal. The supportive network adds a hardening quality to the aluminum metal such that the hardened aluminum layer is more resistant to denting compared to an aluminum metal layer without the supportive network.

[0007] According to a further embodiment, a method for plating aluminum on a surface of a substrate is described. The method includes exposing the substrate surface to an aluminum electrolytic bath having a number of hardening particles dispersed therein. The method also includes causing the aluminum to deposit onto the substrate surface by applying an electric field to the electrolytic bath. The hardening particles are co-deposited with the aluminum forming an aggregate layer on the substrate surface. The aggregate layer includes a network of hardening particles substantially uniformly dispersed within the aluminum.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] 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.

[0009] FIG. 1 shows an electrolytic cell configured for co-depositing aluminum with hardening particles on a cathode substrate.

[0010] FIGs. 2A-2C show cross section views of a part undergoing a plating process where hardening particles are co-deposited with aluminum.

[0011] FIG. 2D shows a close-up cross section view of an aluminum layer and aluminum oxide layer both having hardening particles embedded therein. [0012] FIG. 3 shows a flowchart indicating an aluminum plating process with hardening particles in accordance with the part shown in FIGs. 2A-2D.

DETAILED DESCRIPTION

[0013] 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.

[0014] This application relates to aluminum films and providing aluminum films on substrates using plating methods. 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 exterior portions of enclosures and casings for electronic devices.

[0015] Described herein are methods for providing aluminum layers having superior hardness. Methods involve providing hardening particles within the aluminum layers that increase the overall hardness of the aluminum layers. The hardening particles can be co-depositing with aluminum during a plating process.

[0016] These and other embodiments are discussed below with reference to FIGs. 1 - 3. 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.

[0017] As described above, it can be difficult to produce a pure or a nearly pure aluminum layer that has the satisfactory durability for many industrial applications. Pure or nearly pure aluminum typically has a hardness value of less than about 40 Hv, making it too soft for many applications. One method for improving the hardness of an aluminum layer is to convert a surface of the aluminum layer to an aluminum oxide layer. Aluminum oxide layers typically have hardness values of 300 Hv and over, and therefore can provide a protective hard coating over the softer aluminum. However, even with the protective oxide layer, a surface of a part can still be easily gouged and marred. This is because the relatively soft pure aluminum layer is situated between a relatively hard underlying substrate and a relatively hard aluminum oxide layer.

[0018] One method for providing a plated aluminum layer having increased hardness involves adding hardening particles to the electrolytic plating bath such that the hardening particles become co-deposited with aluminum onto the substrate. FIG. 1 shows electrolytic cell 100, which includes tank 102, power supply 106, cathode substrate 108, anode 110, and electrolytic bath 104. Anode 110 can contain any suitable aluminum-containing material. Cathode substrate 108 can include any suitable material, including suitable metal materials. In some embodiments, cathode substrate includes an aluminum-containing material. In some cases, cathode substrate includes a strike layer, such as copper and/or nickel, which will be described in detail below. Prior to electroplating, cathode substrate 108 can undergo any of a number of suitable pre-plating cleaning processes to remove trace amounts of impurities and oxide on its surface. Electrolytic bath 104 includes a mixture of hardening particles 112 dispersed within aluminum ions 111. In some embodiments, electrolytic cell 100 is in an inert environment. For example, electrolytic cell 100 can be placed in a closed system having inert gas such as nitrogen gas. The composition of electrolytic bath can depend upon the purity of aluminum being plated. In some embodiments where pure or nearly pure aluminum is plated, electrolytic bath 104 includes a non- aqueous electrolyte. In some embodiments, where an aluminum alloy is plated, electrolytic bath 104 includes an aqueous electrolyte.

[0019] During a plating procedure, power supply 106 applies a voltage across cathode substrate 108 and anode 110 causing positively charged aluminum ions 111 to migrate toward cathode substrate 108. Thus, applying a voltage creates a flow of aluminum ions 111 toward substrate 108. Hardening particles 112 dispersed within electrolytic bath 104 become entrained with the flow of aluminum ions 111 toward cathode substrate 108. In this way, an aggregate of aluminum metal and hardening particles 112 become deposited onto cathode substrate 108. Hardening particles 112 can be added to electrolytic bath 104 prior to or during the plating process. Hardening particles 112 can be made of material that is harder than aluminum. For example, hardening particles 112 can be made of carbides, nitrides, or mixtures thereof. In some embodiments, hardening particles 112 are substantially neutral in charge when placed in electrolytic bath 104. In some embodiments, hardening particles 112 have a net positive charge when placed in electrolytic bath 104. Note that although hardening particles 112 shown in FIG. 1 appear as round or spherical in shape, hardening particles 112 can have any suitable shape. For example, hardening particles 112 can have amorphous or angular shapes. The average size of hardening particles 112 can vary depending on application requirements. The average size of hardening particles 112 is generally larger than the average size of aluminum ions 111. In some applications, the size of hardening particles 112 depends on the thickness of the final plated aluminum layer. In some embodiments, the average width (or diameter, if hardening particles 112 are spherical) of hardening particles 112 is less than about one third of the final thickness of the plated aluminum layer. In some embodiments, the average width or diameter of hardening particles 112 is about 10 microns or smaller. In some embodiments, the average width or diameter of hardening particles 112 range in the nanometer scale. In some embodiments, hardening particles 112 have an average width or diameter greater than about one nanometer.

[0020] In some cases, hardening particles 112 have a tendency to settle due to the force of gravity. For example, hardening particles 112 can settle to the bottom of tank 102 once they are added to electrolytic bath 104. In such cases, it can be beneficial to provide a mechanism to keep hardening particles 112 from settling. In addition, it can be beneficial to keep hardening particles 112 evenly distributed within electrolytic bath 104 during the plating process such that hardening particles 112 are substantially uniformly distributed within the resultant aluminum layer. In some embodiments, hardening particles 112 are mechanically agitated within electrolytic bath 104 during the plating process. In some embodiments, agitating hardening particles 112 is achieved by using a bubbler that introduces bubbles of gas within electrolytic bath 104 that force movement of hardening particles 112 within bath 104. In some embodiments, the gas for the bubbler is an inert gas, such as nitrogen or argon. The placement of the gas outlet of the gas bubbler can be chosen for most efficient agitation of settling hardening particles 112. For example, the gas bubbler outlet can be positioned at the bottom of tank 102. In some cases, multiple outlets can be situated at multiple locations of the tank 102.

[0021] In some embodiments, agitation is accomplished by circulating electrolytic bath 104 using, for example, a mechanical pump. The circulating fluid of electrolytic bath 104 can cause movement of hardening particles 112 and keep hardening particles 112 from settling within electrolytic bath 104. In some embodiments, sound waves are passed through bath 104 (ultrasonics) to agitate hardening particles during plating. In some embodiments, agitation is accomplished by moving, such as vibrating or spinning, tank 102 during the plating process. In some embodiments, hardening particles 112 are added to electrolytic bath 104 incrementally over a period of time during which plating occurs. In some embodiments, hardening particles 112 are added in batches. In other embodiments, hardening particles 112 are added continuously during the plating process to allow a constant flow of hardening particles 112 to pass by cathode substrate 108. In some embodiments, hardening particles 112 are treated with a dispersant coating, such as a surfactant or polymer, prior to being place in bath 104. Once in bath 104, the dispersant coating can cause hardening particles 112 to repel each other and thereby suspend hardening particles 112 within electrolytic bath 104. In some embodiment, one or more of the agitating and coating methods described above are used to evenly distribute hardening particles 112 within electrolytic bath 104. In some embodiments, the cathode substrate is positioned, for example, at the bottom of tank 102 such that the force of gravity can assist in the co- depositing of hardening particles 112 with aluminum ions 111.

[0022] FIGs. 2A-2C show cross sections of part 200 undergoing a plating process where hardening particles are co-deposited with aluminum. At FIG. 2A, part 200 includes substrate 202 having optional strike layer 204 deposited thereon. Substrate 202 can be made of any suitable material that can be used in a plating process. In some embodiments, substrate 202 is a metal or metal alloy, such as aluminum or aluminum alloy. In general, strike layer 204 is a very thin (typically a few microns or less) metal layer that adheres well with substrate 202 and promotes adhesion of a subsequently plated aluminum layer. Typical strike layer 204 metals can include, but are not limited to, copper and nickel. In some embodiments strike layer 204 is not used. In some embodiments, an electroless plating process is used to form strike layer 204. In electroless plating there is no electrical bias so there is substantially no current density distribution across the part. Thus, strike layer 204 can grow at the same rate along the surface of substrate 202, creating a very evenly distributed strike layer 204. Thus, electroless plating can be referred to as a "self-leveling" process.

[0023] At FIG. 2B, aluminum layer 206 having hardening particles 210 distributed therein is deposited on strike layer 204. In embodiments where strike layer 204 is not used, aluminum layer 206 is deposited directly deposited onto substrate 202. In some embodiments, hardening particles 210 are substantially evenly distributed within aluminum layer 206. The even distribution can be accomplished, for example, using an agitation or dispersion technique described above with reference to FIG. 1. Aluminum layer 206 can be deposited using a plating procedure, such as one of the plating procedures described above, such that hardening particles 210 co-deposit with aluminum. Hardening particles 210 can increase the hardness of aluminum layer 206, thereby making aluminum layer 206 more resistant to gouging, scratching, or denting compared to an aluminum layer without hardening particles 210 or compared to pure or nearly pure aluminum. In this way, hardening particles 210 can form a supportive network within aluminum layer 206 that increases the hardness of aluminum layer 206. The hardness of aluminum layer 206 can be measured using any of a number of suitable techniques. For example, hardness can be measured using the Vicker's hardness test, whereby an indenting force required to deform aluminum layer 206 is measured. Aluminum layer 206 with hardening particles 210 is found to withstand a greater indenting force as measured using a Vicker's hardness test compared to an aluminum layer without hardening particles 210. In some embodiments, the hardness of aluminum layer 206 has a hardness value on the Vicker's scale of about 80 HV or greater. In some embodiments, aluminum layer 206 has a hardness value of about 100 HV or greater.

[0024] At FIG. 2C, a portion of aluminum layer 206 is optionally converted to aluminum oxide layer 208. Conversion to aluminum oxide layer 208 can be accomplished using any suitable method, such as an anodizing process. Aluminum oxide layer 208 has a porous structure with a number of pores 212 that run in a substantially vertically direction from the top region to the bottom region of aluminum oxide layer 208. As shown, aluminum layer 206 and aluminum oxide layer 208 have hardening particles 210 embedded within them.

[0025] FIG. 2D shows a close-up cross section of aluminum layer 206 and aluminum oxide layer 208 of part 200. As shown, hardening particles 210 are embedded within aluminum layer 206 as well as within aluminum oxide layer 208. During the anodizing process pores 212 can grow around hardening particles 210. In this way, hardening particles 210 can be situated within aluminum oxide layer 208 but outside of pores 212. In some embodiments, the average width or diameter of hardening particles 210 are within scale of the diameter of pores 212. This so that hardening particles 210 do not substantially disrupt the pore structure of aluminum oxide layer 208 and/or affect the cosmetic quality of aluminum oxide layer 208. In some embodiments, hardening particles 210 have an average width or diameter of about 10 nanometers or greater. In one embodiment, hardening particles 210 have an average width or diameter in the order of about 100 to 200 nanometers. As described above, in other embodiments, hardening particles 210 can have an average diameter greater than about 200 nanometers. In some embodiments, the upper limit of the size of hardening particles 210 is about one third of the thickness of aluminum layer 206. In some cases hardening particles 210 can give aluminum layer 206 and/or aluminum oxide layer 208 a particular cosmetic characteristic, such as a hazy or matt appearing quality. Since aluminum layer 206 and aluminum oxide layer 208 have hardening particles 210 embedded therein, the hardness and resistance to scratching, gouging, or denting of each of aluminum layer 206 and aluminum oxide layer 208 is increased. That is, part 200 is more resistant to scratching, gouging, or denting.

[0026] FIG. 3 shows flowchart 300 indicating an aluminum plating process with hardening particles in accordance with part 200 of FIGs. 2A-2D. At 302, an optional strike layer is formed onto a substrate. The substrate can be made of any suitable material used in a plating process, such as a metal or metal alloy. In some embodiments, the substrate is comprised of a stainless steel alloy. In some embodiments, the substrate is comprised of aluminum or aluminum alloy. In general, a strike layer is a very thin (typically a few microns or less thick) metal layer that adheres well with the metal substrate. In some embodiments, the strike layer comprises copper and/or nickel.

[0027] At 304, aluminum is plated and hardening particles are co-deposited onto the strike layer or directly onto the substrate. In some applications, the plated aluminum is pure or nearly pure aluminum. Any of a number of suitable plating processes can be used, such as those described above with reference to FIGs. 1-2. In some embodiments, the hardening particles are mixed in an aluminum electroplating bath. The hardening particles can be made of a material that is harder than the aluminum being plated. In some embodiments, the hardening particles are made of carbides, nitrides, or a mixture thereof. In some embodiments, the hardening particles do not substantially negatively impact the electroplating process.

[0028] The hardening particles can be mixed and agitated in the electrolyte solution during the plating process. The concentration of hardening particles in the electroplating solution can vary depending, in part, upon the desired concentration of hardening particles in the plated metal. In some embodiments, concentrations of hardening particles ranged from about 0.5g to lOg per liter of electrolyte. The resultant particle-containing plated aluminum layer will preferably have a greater hardness than the plated aluminum alone. In some embodiments, the particle- containing plated aluminum has a hardness value on the Vickers scale of about 80 Hv or greater. This is compared to a hardness value ranging from about 20 Hv to 30 Hv for the same plating process without including the hardening particles. The aluminum layer can be deposited to any suitable thickness, depending in part on application requirements. In some embodiments, the aluminum layer is deposited to a thickness ranging from about 1 micron to about 10 microns. In some embodiments, the aluminum layer is deposited to thickness ranging from about 2 microns to about 5 microns.

[0029] At 306, at least a portion of the aluminum layer is optionally converted to an aluminum oxide layer. The aluminum oxide layer can add additional strengthening and durability to the surface of a part. In some embodiments, the conversion is accomplished using an anodizing process. The anodizing process can involve the use of an acidic anodizing bath such as an H ₂ SO ₄ solution. As the plated aluminum becomes converted to aluminum oxide, the hardening particles from the plated aluminum can become embedded within the aluminum oxide layer. In some embodiments, the hardening particles become embedded between the anodic pores of the aluminum oxide layer. The thickness of the aluminum oxide layer can depend, in part, on the thickness of the aluminum layer and on application requirements. In some embodiments, about half of the aluminum layer is converted to aluminum oxide. In some embodiments, the aluminum oxide layer ranges between about 2 microns to about 20 microns in thickness. In some embodiments, an aluminum oxide layer having a thickness of between about 8 microns to about 12 microns provides sufficient durability while providing good cosmetic quality.

[0030] 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.