Patent Publication Number: US-2019177858-A1

Title: Coating an alloy substrate

Description:
BACKGROUND 
     Metal alloys exhibit a wide variety of characteristics that make them suitable for different applications ranging from commercial and industrial materials to military and medical equipment. In general, the type of properties possessed by a metal alloy is determined by the constituents of the metal alloy. The properties possessed by a metal alloy, in turn determine use of the metal alloy for a given application. 
     The properties and characteristics of the metal alloys can be customized during manufacturing of the alloys, depending upon composition of the alloy, and process used for fabricating such alloys. The process of manufacturing metal alloys is generally a controlled process. The composition of the alloy as well as the process parameters are monitored to obtain a metal alloy having characteristics in accordance with an end application. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is provided with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and components. 
         FIG. 1  illustrates a sectional view of an die-casted metal alloy with an exterior coat deposition, in accordance with an implementation of the present subject matter; 
         FIG. 2  illustrates different stages of treating a surface of an alloy substrate for depositing an exterior coat, according to an implementation of the present subject matter; 
         FIG. 3  illustrates an example method for treating a surface of an alloy substrate, according to an implementation of the present subject matter; and 
         FIG. 4  illustrates an example method for depositing an exterior coat on an alloy substrate, according to an implementation of the present subject matter. 
     
    
    
     DETAILED DESCRIPTION 
     Generally, metal alloys, such as magnesium alloys, titanium alloys and aluminum alloys have high surface porosity with large number of pores and cavities on their surfaces. The pores and cavities make the surface uneven, hard and chemically unstable for coating with other materials. Such large number of pores and cavities may also provide reduced surface contact of an applied material with the surface of the metal alloys and prevent the applied material to have sufficient binding with the surface. 
     Further, the surface of the metal alloys is also reactive and tend to oxidize with gases of the atmosphere to form an unstable oxide layer on the surface. The unstable oxide layer reduces stability of the surface for adhering to an exterior coat, and makes the surface chemically resistant towards different materials. 
     Therefore, to overcome such issues, the surface of the metal alloys are generally subjected to multiple surface treatment processes, followed by deposition of a putty and the exterior coat. However, the multiple surface treatment processes make the overall process of treating the surface and applying the exterior coat complex, time and resource consuming. 
     In accordance with an implementation of the present subject matter, techniques for efficiently treating the surfaces alloy substrates are described. The techniques reduce porosity and reactivity of the surfaces of the metal alloys and make the surfaces stable for adhering to exterior coats. 
     In an example implementation of the present subject matter, an alloy substrate of a die-casted metal alloy is received. As would be understood, the die-casted metal alloy is obtained through casting or molding of molten metal alloy in a die or a mold, and then cooling and solidifying are performed to obtain the alloy substrate. The surface of the alloy substrate is porous having multiple pores on the surface. Thereafter, an electrically conductive layer is deposited on the surface of the alloy substrate. The electrically conductive layer is deposited on the surface to cover the surface entirely. In an example, the electrically conductive layer may be deposited through a spraying technique or may be deposited manually. The electrically conductive layer may be composed of metal particles and electrically conductive polymers. 
     After depositing the electrically conductive layer, the surface is subjected to a machining process that may include scrubbing and polishing of the surface. During machining, excess material of electrically conductive layer may be removed and the surface is made smooth by allowing the metal particles to fill the pores, thereby reducing porosity of the surface. After the metal alloys are machine processed, the surface of the alloy substrate is oxidized based on an oxidation process to form an oxidation layer on the surface. 
     The oxidation layer so formed provides stability to the surface for adhering to exterior coats and insulates the surface from the outside environment to prevent direct exposure of the surface to the environment and reduce reactivity of the surface and corrosion. After the surface is oxidized, an exterior coat including at least one coating layer is deposited on the surface of the alloy substrate. The exterior coat may provide a smooth and lustrous appearance to the alloy substrate. 
     Thus, the described techniques of depositing the electrically conductive layer and performing the oxidation efficiently reduce porosity and reactivity of the surface and enhance binding of an applied exterior coat with the surface. Further, the techniques provide a time and resource efficient mechanism of treating the surface and making the surface stable for coating. 
     The above described techniques are further described with reference to  FIGS. 1 to 4 . It should be noted that the description and figures merely illustrate the principles of the present subject matter along with examples described herein and, should not be construed as a limitation to the present subject matter. It is thus understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present subject matter. Moreover, all statements herein reciting principles, aspects, and examples of the present subject matter, as well as specific examples thereof, are intended to encompass equivalents thereof. 
       FIG. 1  illustrates a sectional view of a die-casted metal alloy  100  with an exterior coat deposition, according to an implementation of the present subject matter. The die-casted metal alloy  100 , possesses a uniform surface devoid of pores and is obtained on subjecting an alloy substrate  102  to a surface treatment process, as per techniques described in herein. The description of the surface treatment process is provided in details with reference to subsequent figures. 
     Referring to  FIG. 1 , the alloy substrate  102  is composed of a metal alloy, for instance, a magnesium alloy, a titanium alloy, or an aluminum alloy. The alloy substrate  102  may be fabricated through techniques of manufacturing alloys, such as molding or die-casting of the metal alloy using a die wherein a molten metal alloy is poured and then allowed to cool to take the shape of the die. During manufacturing of the alloy substrate various imperfections, such as gas and air entrapments may occur that result in formation of pores and cavities on the surface of the alloy substrate  102 . 
     The die-casted metal alloy  100  has an electrically conductive layer  104  deposited over the alloy substrate  102 . The electrically conductive layer  104  is composed of single or multiple layers of metal particles and electrically conductive polymers. In one example, the metal particles include one of aluminum particles, magnesium particles, and titanium particles and the electrically conductive polymers can be one of polylactene, polyphenylenevinylene, polythienylenevinylene, polythiophene, poly-3-alkylthiopene, polypyrrole, polyaniline, polyphenylene, polyphenylene sulfide and polyfuran, and poly-3,4-ethylenedioxythiopene polystyrene sulfonate (PEDOT). The electrically conductive layer  104  reduces porosity of the surface by allowing the metal particles of the electrically conductive layer  104  to fill the pores and cavities of the surface of the alloy substrate  102 . 
     In an example implementation, the die-casted metal alloy  100  includes an oxidation layer  106 . In an example, the oxidation layer  106  is a dense ceramic protective layer that provides hardness and stability to the surface for binding with exterior coats and insulates the surface from the outside environment, thereby reducing reactivity of the surface. In an example, the oxidation layer  106  may be a magnesium oxide (MgO) layer with a thickness of about 3-15 micro meter (μm). 
     Further, the die-casted metal alloy  100  includes an exterior coat  108  disposed on the oxidation layer  106 . The exterior coat  108  may have a coating layer, such as a paint coat having several paint layers coated on the die-casted metal alloy  100  to provide a color and a texture to the surface. In an example, the coating layer may be a metallic coat composed of metallic powders. Such metallic coats provide a metallic luster to the surface of the alloy substrate  102 . Further, the exterior coat  108  makes the alloy substrate  102  water resistant, smooth, and soft and imparts an anti-bacterial, anti-smudge and anti-fingerprint characteristics to the alloy substrate  102 . The die-casted metal alloy  100  with the exterior coat having such characteristics may be used for applications in electronic devices, such as making back covers and housings for laptops, notebooks, and smartphones. 
     The details of various stages of treating the surface of the die-casted metal alloy  100  for deposition of the exterior coat  108  have been explained in conjunction with description of  FIG. 2 . 
       FIG. 2  illustrates various stages  200  of treating the surface of a metal alloy, implemented by various units, according to an example implementation of the present subject matter. For sake of explanation, each stage of the surface treatment process and coat deposition has been described with reference to an alloy substrate  202 . A unit, in context of the present description, can be an apparatus, a machine or a combination of apparatuses or machines for performing an operation at a stage. At different stages, different units interact with the alloy substrate  202  to add the different layers deposited over the surface of the alloy substrate  202  to create the die-casted metal alloy, such as the above-described die-casted metal alloy  100 . 
     In an example implementation, the alloy substrate  202  is obtained for surface treatment and exterior coat deposition. The alloy substrate  202  is then subjected to application of an electrically conductive layer  206  by an applying unit  204 . In an example, the applying unit  204  may be a spraying apparatus or multiple spraying apparatuses for spraying electrically conductive material on the surface of the alloy substrate  202  to form the electrically conductive layer  206  on the alloy substrate  202 . In another example, the electrically conductive layer  206  may be deposited manually. In manual deposition, an amount of fluid electrically conductive material is poured on the alloy substrate  202  to cover the surface of the alloy substrate  202  and the conductive material is spread over the surface using a spreading apparatus, such as a plastic knife. The electrically conductive layer  206  is alike the electrically conductive layer  104  described earlier and is composed of metal particles and conductive polymers. In an example, the electrically conductive layer  206  allows formation of a ceramic oxide layer over the surface of the alloy substrate  202 . The ceramic oxide layer is composed of the metal particles that are filled in the pores and cavities of the surface of the alloy substrate  202 . 
     The alloy substrate  202  with the electrically conductive layer  206  is then subjected to a machining process implemented by a machining unit  208 . The machining unit  208  may include machining tools and a polishing equipment for removal of excess electrically conductive material from the surface and scrub or brush the remaining conductive material to polish the surface. In an example, polishing is performed by rotating the polishing equipment on the surface of the electrically conductive layer  206 . The polishing of the surface allows the metal particles of the electrically conductive layer  206  to fill the pores and cavities of the surface to smoothen the surface and reduce porosity of the alloy substrate  202 . After completing the process of machining, a machined alloy substrate  210  is obtained. 
     In an example implementation, an oxidation process is performed on the machined alloy substrate  210  by an oxidation unit  212 . In an example, the oxidation process may be a Micro Arc Oxidation (MAO) and the oxidation unit  212  may include electrodes, a container with an electrolyte, and transducers for performing the MAO of the alloy substrate  202 , and form an oxidation layer  214  on the surface of the machined alloy substrate  210 . In an example, the MAO may be performed at a voltage of about 150-550 Volts (V) at a temperature of about 10-45 degree Celsius (° C.) for a duration of about 2-10 minutes. 
     Further, different chemicals may be used during the MAO, for instance, as electrolyte or for aqueous solutions used for oxidation. The chemicals includes sodium silicate, metal phosphate, potassium fluoride, potassium hydroxide or sodium hydroxide, fluorozirconate, sodium hexametaphosphate, sodium fluoride, ferric ammonium oxalate, phosphoric acid salt, graphite powder, silicon dioxide powder, aluminum oxide powder, metal powder and polyethylene oxide alkylphenolic ether. The chemicals may be used along with water with the composition of about 0.05-15 percent (%) of the amount of water at a pH value of about 8-13. The MAO enhances adhesion of the surface of the alloy substrate  202  with exterior coats applied on the alloy substrate  202  and prevents surface peeling issues. 
     After performing the oxidation, an exterior coat  216  is applied on the machined alloy substrate  210  by a coating unit  218 . For example, the coating unit  218  may be a paint apparatus or a spraying apparatus for spraying exterior coat material on the alloy substrate  202 . The exterior coat  216  may include at least one coating layer. The coating layer may be a paint coat having a paint layer or several paint layers. In an example, the paint layers may be of different colors and appearances. In an example, the paint layers include a single layer of an Ultraviolet (UV) coat or a Polyurethane (PU) coat. In another example, the paint layers include a base coat along with either the UV coat or the PU coat. 
     In another example implementation, the coating layer of the exterior coat  216  may include a metallic coat, such as a metallic UV coat or a metallic PU coat. The metallic coat is deposited over the surface to provide a metallic lustre to the alloy substrate  102 . In an example, the metallic UV coat is deposited at a temperature of about 50-55° C., with 600-1000 Millijoules (mj) UV exposure for a duration of about 10-12 minutes. The metallic UV coat is composed of pearl, metal powders, dyes and color pigments. In another example, the metallic UV coat may be composed of resins, such as polyurethane, polycarbonate, urethane acrylates, polyacrylate, polystyrene, polyetheretherketone, polyacryletheretherketone, polyesters, fluoropolymers, and a mixture of the resins. The thickness of the metallic UV coat is about 10-25 μm. 
     The metallic PU coat may be composed of polyurethane or urethane acrylates. In an example, the thickness of the metallic PU coat is about 5-20 μm. In an example implementation, the metallic PU coat may be deposited in two layers, a base coat (not shown in the figure) and a top coat (not shown in the figure). In an example, the base coat is deposited at a temperature of about 80-150° C. for a duration of about 20-40 minutes. The base coat may be composed of barium sulfate, talc, dyes, metal powders and color pigments. The base coat may also include base coat resin, such as polyurethane and acrylic-polyurethane and may contain aluminum particles for metallic luster. Thereafter, the top coat may be deposited at a temperature of about 80-140° C. for about 20-40 minutes. 
     In an example, the coating layer of the exterior coat  216  may include a primer and a powder coat (not shown in the figure) deposited on the oxidation layer  214  prior to depositing the metallic UV coat. In an example, the primer and the powder coat may be deposited to enhance adhesion and durability of exterior coats on sharp edges of the surface. The primer may be composed of resin such as epoxy, acrylic-epoxy hybrids, acrylics, polyurethane and acrylic-polyurethane. Further, the primer may also contain fillers from a group of carbon black, titanium dioxide, clay, mica, talc, barium sulphate, calcium carbonate, synthetic pigments, metallic powders, aluminium oxide, CNT, graphene, graphite, organic and inorganic powders. 
     The powder coat is composed of high ratio fillers such as talc, clay, graphene and high aspect ratio pigments. Further, the powder coat may include epoxy, poly (vinyl chloride), polyamides, polyesters, polyurethanes, acrylics, polyphenylene ether. The primer resin may be subjected to a temperature of about 80-160° C. for about 20-40 minutes. In an example, the powder coat and the primer may contain fillers from carbon black, titanium dioxide, clay, mica, talc, barium sulphate, calcium carbonate, synthetic pigments, metallic powders, aluminium oxide, CNT, graphene, graphite, organic and inorganic powders. 
     After depositing the exterior coat  216  by the coating unit  218 , a die-casted metal alloy  220  having the substrate alloy  202  with the exterior coat  216  as the top most layer is received. The described techniques provide for a surface treatment process including depositing an electrically conductive layer and an oxidation layer to reduce porosity and reactivity of the alloy substrate  102 . 
       FIG. 3  and  FIG. 4  illustrate methods  300  and  400  for treating surface of an alloy substrate for deposition of exterior coats. The order in which the methods  300  and  400  is described is not intended to be construed as a limitation, and any number of the described method blocks may be combined in any order to implement the methods  300  and  400 , or an alternative method. 
       FIG. 3  illustrates the method  300  for treating surface of an alloy substrate, according to an example implementation of the present subject matter. The surface of the alloy substrate generally has large number of pores and cavities that makes the surface uneven and resistant for coating with different materials. Further, the surface is also reactive with gases of the atmosphere and is unstable for holding exterior coats applied on the surface. Therefore, to reduce porosity and reactivity of the surface and make the surface stable for adhesion with exterior coats, a surface treatment process is performed. The exterior coat may be a single layer or multiple layers of paint material or a metallic coat deposited on the alloy substrate for providing smooth and lustrous appearance to the alloy substrate. 
     At block  302 , an electrically conductive layer is applied on the alloy substrate. The electrically conductive layer is composed of metal particles and electrically conductive polymers. The electrically conductive layer is applied such that the metal particles fill the multiple pores on the surface of the alloy substrate. 
     Thereafter at block  304 , the alloy substrate is subjected to an oxidation process to form an oxidation layer over the surface. The oxidation layer provides for adhesion of the surface with an exterior coat. The alloy substrate  102  with the exterior coat is a die-casted metal alloy, such as the above-described die-casted metal alloy  100  and  220  with characteristics of a surface quality that may be used in various applications, for example, making housing and back covers of electronic devices. 
       FIG. 4  illustrates the method  400  for treating surface of an alloy substrate and deposit an exterior coat, according to another example implementation of the present subject matter. 
     At block  402 , an alloy substrate is received. The alloy substrate is a die-casted metal alloy and has a surface which is porous with multiple pores and cavities. As would be understood, the alloy substrate may be obtained after molding or die-casting of the metal alloy in a die or a mold and the pores and the cavities may be due to imperfections, such as gas and air entrapment during molding or die-casting of the metal alloy. At block  404 , an electrically conductive layer is deposited on the surface of the alloy substrate. The electrically conductive layer is composed of metal particles and electrically conductive polymers. 
     In an example, the metal particles can be one of aluminum particles, magnesium particles, and titanium particles and the electrically conductive polymers can be one of polylactene, polyphenylenevinylene, polythienylenevinylene, polythiophene, poly-3-alkylthiopene, polypyrrole, polyaniline, polyphenylene, polyphenylene sulfide and polyfuran, and poly-3,4-ethylenedioxythiopene polystyrene sulfonate (PEDOT). In an example implementation, the electrically conductive layer  104  may be deposited over the alloy substrate  102 . 
     After depositing the electrically conductive layer, the surface of the alloy substrate is subjected to a machining process to smoothen the surface by allowing the metal particles to fill pores of the alloy substrate, at block  406 . Therefore, the machining process reduces porosity of the surface of the alloy substrate  102 . In an example, the machining process may include scrubbing, brushing or polishing of the surface to remove excess electrically conductive material from the surface of the alloy substrate. 
     Thereafter, at block  408 , an oxidation process is performed onto the surface to oxidize the surface and form an oxidation layer on the surface. The oxidation layer provides stability to the alloy substrate to adhere to exterior coats and reduces reactivity of the alloy substrate. The oxidation layer also provides hardness to the alloy substrate. In an example implementation, the oxidation process is the MAO process and the oxidation layer  106  is formed over the surface of the alloy substrate  102 . 
     At block  410 , the exterior coat is deposited on the surface of the alloy substrate. The exterior coat may include a single coating layer or multiple coating layers. In an example, the exterior coat may be a paint coat with a color and a smooth texture. The exterior coat may be deposited to provide a lustrous appearance to the alloy substrate with anti-bacterial, anti-fingerprint, anti-smudge and water resistant characteristics. 
     In another example implementation, the exterior coat may be a metallic coat composed of metallic powders, pearl, dyes, and color pigments. In an example, the metallic coat may include a metallic Ultraviolet (UV) coat or a metallic PU (Polyurethane) coat. The metallic coat provides a metallic luster appearance to the alloy substrate  102 . The alloy substrate coated with the metallic coat may then be used for application in electronic devices as back cover for laptop, notebook and smartphones. 
     The die-casted metal alloy obtained by the described method of surface treatment, may be used for various purposes where a metal alloy sheet with non-porous surface having high adherence to exterior coating is desired. For example, the die-casted metal alloy may be used to manufacture back covers for electronic devices, such as laptops and smart phones. 
     Therefore, the described techniques efficiently reduce porosity and reactivity of the metal alloys to enhance binding between an exterior coat and the surface of the metal alloys. Further, the described techniques provide a time and resource efficient mechanism of treating the surface of the metal alloys. 
     Although implementations of present subject matter have been described in language specific to structural features and/or methods, it is to be understood that the present subject matter is not necessarily limited to the specific features or methods described. Rather, the specific features and methods are disclosed and explained in the context of a few implementations for the present subject matter.