PATENT DOCUMENT

Publication Number: US-10842035-B1
Application Number: US-201916562164-A
Country: US
Kind Code: B1

Title: Nitrided titanium surfaces with a natural titanium color

Abstract:
This application relates to an enclosure for a portable electronic device. The enclosure includes a titanium substrate having interstitial nitrogen atoms, where the titanium substrate is characterized as having an a* value that is less than 1, a b* value that is less than 5, and an L* value that is more than 70.

Claims:
What is claimed is: 
     
       1. An enclosure for a portable electronic device, the enclosure comprising:
 a titanium substrate defining an external surface, the titanium substrate comprising at least 10 at % interstitial nitrogen atoms at or near the external surface, the titanium substrate having an a* value that is less than 1 and a b* value that is less than 5. 
 
     
     
       2. The enclosure of  claim 1 , wherein the titanium substrate has a surface hardness of 850 HV 0.1  or greater. 
     
     
       3. The enclosure of  claim 1 , wherein the titanium substrate has an L* value of 65 or more. 
     
     
       4. The enclosure of  claim 1 , wherein the color is characterized according to CIE L*a*b* color-opponent dimension values. 
     
     
       5. The enclosure of  claim 1 , wherein the titanium substrate has a surface hardness of 850 HV 0.1  or greater and a hardness of the titanium substrate decreases as a depth from the external surface increases. 
     
     
       6. The enclosure of  claim 1 , wherein the titanium substrate includes a chemically etched external surface. 
     
     
       7. The enclosure of  claim 1 , wherein the titanium substrate has a thickness of at least 10 μm or greater. 
     
     
       8. The enclosure of  claim 7 , further comprising:
 a metal oxide layer that overlays the titanium substrate. 
 
     
     
       9. The enclosure of  claim 8 , wherein the metal oxide layer is modified to exhibit a range of colors relative to a thin film interference effect. 
     
     
       10. An enclosure for a portable electronic device, the enclosure comprising:
 a titanium substrate; and 
 a titanium nitride layer overlaying the titanium substrate, the titanium nitride layer comprising at least 10 at % interstitial nitrogen atoms and having a thickness of less than 100 μm, the titanium nitride layer having an a* value less than 1 and a b* value less than 5. 
 
     
     
       11. The enclosure of  claim 10 , wherein the interstitial nitrogen atoms are uniformly dispersed throughout the titanium nitride layer. 
     
     
       12. The enclosure of  claim 10 , wherein the color is characterized according to CIE L*a*b* color-opponent dimension values. 
     
     
       13. The enclosure of  claim 10 , further comprising:
 an anodized layer that overlays the titanium nitride layer. 
 
     
     
       14. The enclosure of  claim 10 , wherein the titanium nitride layer has an external surface that is blasted, textured or polished.

Description:
FIELD 
     The described embodiments relate generally to a titanium part having an external surface hardened by nitriding. More particularly, the described embodiments relate to a method for forming a nitrided titanium part having a natural titanium color. 
     BACKGROUND 
     Enclosures for portable electronic devices may be formed from a variety of different materials. In certain instances, the enclosure may be formed from titanium. However, in the consumer electronics space, where there is a greater demand for wear resistance, titanium and its alloys thereof may be insufficient to provide sufficient hardness and strength to protect operational components carried by these enclosures. Although titanium and its alloy thereof are capable of undergoing a “case-hardening” process, the resulting titanium nitride is characterized as having a distinctive gold/brown color that many consumers find unattractive. 
     SUMMARY 
     This paper describes various embodiments that relate generally to a titanium part having an external surface hardened by nitriding. More particularly, the described embodiments relate to a method for forming a nitrided titanium part having a natural titanium color. 
     According to some embodiments, an enclosure for a portable electronic device is described. The enclosure includes a titanium substrate having interstitial nitrogen atoms, where the titanium substrate has a color characterized as having an a* value that is less than 1 and a b* value that is less than 5. 
     According to some embodiments, an enclosure for a portable electronic device is described. The enclosure includes a titanium substrate and a titanium nitride layer that overlays the titanium substrate, where the titanium nitride layer has a thickness less than 100 micrometers, and the titanium nitride layer has a color characterized as having an a* value less than 1 and a b* value less than 5. 
     According to some embodiments, a method for forming an enclosure for a portable electronic device, the enclosure including a nitrided titanium substrate, is described. The method includes forming an anodized nitrided part from the nitrided titanium substrate, wherein the anodized nitrided part includes an anodized layer overlaying the nitrided titanium substrate, and forming a nitrided part by removing the anodized layer of the anodized nitrided part, wherein the nitrided part has an a* value that is less than 1 and a b* value that is less than 5. 
     Other aspects and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the described embodiments. 
    
    
     
       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  illustrates perspective views of various portable electronic devices having enclosures that may be processed using the techniques described herein, in accordance with some embodiments. 
         FIGS. 2A-2E  illustrate cross-sectional views of a process for forming a nitrided titanium part, in accordance with some embodiments. 
         FIG. 3  illustrates a magnified cross-sectional view of a nitrided titanium substrate, in accordance with some embodiments. 
         FIG. 4  illustrates a magnified cross-sectional view of a nitrided titanium part, in accordance with some embodiments. 
         FIG. 5A  illustrates an exemplary graph depicting hardness as a function of depth for a nitrided titanium substrate, in accordance with some embodiments. 
         FIG. 5B  illustrates an exemplary graph depicting hardness as a function of depth for a nitrided titanium part, in accordance with some embodiments. 
         FIG. 6  illustrates a method for forming a nitrided titanium part, in accordance with some embodiments. 
         FIG. 7  illustrates an exemplary graph depicting hardness as a function of depth for exemplary nitrided titanium parts processed according to various techniques, in accordance with some embodiments. 
         FIG. 8  illustrates a method for forming a nitrided titanium part, in accordance with some embodiments. 
     
    
    
     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. 
     In the following detailed description, references are made to the accompanying drawings, which form a part of the description and in which are shown, by way of illustration, specific embodiments in accordance with the described embodiments. Although these embodiments are described in sufficient detail to enable one skilled in the art to practice the described embodiments, it is understood that these examples are not limiting; such that other embodiments may be used, and changes may be made without departing from the spirit and scope of the described embodiments. 
     Enclosures for portable electronic devices may be formed from a variety of different materials. In certain instances, the enclosure may be formed from titanium, which is attractive over stainless steel and aluminum due to titanium&#39;s high strength-to-weight ratio. Additionally, titanium is highly corrosion-resistant. However, in the consumer electronics space, where there is a greater demand for wear resistance, titanium and its alloys thereof may be insufficient to provide sufficient hardness and strength to protect operational components carried by these enclosures. Although titanium and its alloy thereof may undergo a “case-hardening” process via exposure to a nitriding process, the resulting titanium nitride is characterized as having a distinctive gold/brown color that many consumers find unattractive. Yet, the increased hardness of titanium and its alloys thereof imparted by the nitriding process is very desirable for manufacturers in the consumer electronics space. 
     Accordingly, the embodiments described herein set forth techniques for forming a nitrided titanium part, and subsequently restoring the color of the nitrided titanium part from its gold or brown color to a natural titanium color. In particular, the techniques involve nitriding a titanium or titanium alloy substrate in a controlled nitriding process. The controlled nitriding process includes controlling parameters that include the temperature, duration, and/or the atmospheric pressure. Controlling these parameters can be used to reduce the amount of effort, time, and expense that is required for subsequent steps for forming the nitrided titanium part having the natural titanium color. 
     Subsequent to the nitriding process, the titanium nitride layer of the nitrided titanium substrate that is responsible for the gold/brown color is chemically stripped away by subjecting the nitrided titanium substrate to a succession of anodization and chemical etching processes. Notably, the titanium nitride layer cannot be chemically stripped away absent the anodization process. Although the titanium nitride layer has been stripped away, the remaining portion of the nitrided titanium substrate still exhibits elevated hardness (e.g., &gt;850 Hv) due to diffusion of nitrogen atoms that penetrated deep into the metal matrix of the titanium or titanium alloy substrate. 
     Notably, it is surprising and unexpected that the titanium nitride layer has such a shallow thickness, and that the gold/brown color can be removed by etching less than 1 μm of the nitrided titanium material from the external surface of the nitrided titanium substrate. 
     As used herein, the terms anodic film, anodized film, anodic layer, anodized layer, anodic oxide coating, anodic layer, anodic oxidized layer, metal oxide layer, oxide film, oxidized layer, and oxide layer can be used interchangeably where appropriate. In one example, an anodized layer can result from an electrochemical anodization process of titanium or a titanium alloy. In another example, metal oxide layers can result from a non-electrolytic oxidation process. It should be noted that the processes for forming an anodized layer and a metal oxide layer may be different. As used herein, the terms segment, region, and section can also be used interchangeably where appropriate. 
     In some examples, the color of the nitrided titanium part may be characterized according to CIE L*a*b* color-opponent dimension values. The L* color opponent dimension value is one variable in an L*a*b* color space. In general, L* corresponds to an amount of lightness. L*=0 represents the darkest black while L*=100 represents white in general, a* indicates amounts of red color and green color in a sample. A negative a* value indicates a green color, while a positive a* value indicates a red color. Accordingly, samples having a positive a* value will indicate that more red than green is present. In general, b* indicates amounts of blue color and yellow color in a sample. A negative b* value indicates a blue color, while a positive b* value indicates yellow color. Accordingly, samples having a positive b* value will indicate more yellow than blue is present. 
     According to some embodiments, a non-nitrided titanium substrate or titanium alloy substrate may have natural titanium color, which is defined as an L*a*b color space value of an L* value greater than 70, an a* value less than 1, and a b* value less than 5. As described herein, the modified nitrided part may have a color that is within ˜1 ΔE94 of the natural titanium color. 
     According to some embodiments, an enclosure for a portable electronic device is described. The enclosure includes a titanium substrate having interstitial nitrogen atoms, where the titanium substrate has a color characterized as having an a* value that is less than 1 and a b* value that is less than 5. 
     These and other embodiments are discussed below with reference to  FIGS. 1-8 ; 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. 
       FIG. 1  illustrates various portable electronic devices that can be processed using the techniques as described herein. The techniques as described herein can be used to process surfaces of enclosures of the portable electronic devices. In some examples, the enclosures can include at least one of a metal, a metal alloy, a polymer, glass, ceramics, or a thermoplastic. In particular, metallic enclosures can include titanium or a titanium alloy (e.g., Ti6Al4V). 
       FIG. 1  illustrates exemplary portable electronic devices including a smartphone  102 , a tablet computer  104 , a smartwatch  106 , and a portable computer  108  that include enclosures that may be processed using the techniques as described herein. These exemplary portable electronic devices may be capable of using personally identifiable information that is associated with one or more users. It is well understood that the use of personally identifiable information should follow privacy policies and practices that are generally recognized as meeting or exceeding industry or governmental requirements for maintaining the privacy of users. In particular, personally identifiable information data should be managed and handled so as to minimize risks of unintentional or unauthorized access or use, and the nature of authorized use should be clearly indicated to users. 
     Surface(s) of the portable electronic devices  102 ,  104 ,  106 ,  108  described herein may assume any number of desired surface geometries and surface finishes. In some examples, the enclosures may have a three-dimensional structure having a height, width, and depth, and any type of geometry. In particular, the enclosure is characterized as rectangular, polygonal, circular, beveled edges, angular edges, elliptical, etc. 
       FIGS. 2A-2D  illustrate cross-sectional views of a process for forming a nitrided titanium part having a natural titanium color, in accordance with some embodiments. In some embodiments, a metal part  200 , that is processed according to the techniques described herein, has a near net shape of a final part, such as the enclosures of the portable electronic devices  102 ,  104 ,  106 , and  108 . 
       FIG. 2A  illustrates a cross-sectional view of a metal part  200  prior to undergoing a process for forming a doped metal oxide coating. In some examples, the metal part  200  includes a metal substrate  204 , where the metal substrate  204  is formed of titanium or a titanium alloy (e.g., Ti6Al4V). In some examples, Ti6Al4V has a Vickers hardness of 320 Hv 0.1 . Notably, the hardness of the metal substrate  204  may be insufficient to provide sufficient hardness and strength for protecting operational components that are carried by the respective enclosures of the portable electronic devices  102 ,  104 ,  106 , and  108 . However, the metal substrate  204  may have a natural titanium color that consumers may consider attractive. 
       FIG. 2B  illustrates a cross-sectional view of a nitrided titanium substrate  210  subsequent to a nitriding process, in accordance with some embodiments. During the nitriding process, the metal part  200  may be exposed to a heat treating process that involves exposing the metal substrate  204  to a nitrogen-rich atmosphere at a temperature between 850° C. to 1100° C. The metal substrate  204  is annealed in the nitrogen-rich atmosphere having a partial pressure of N2 of ˜625 Torr (0.82 atm). In some embodiments, the nitriding process is a controlled process. 
     The nitriding process causes nitrogen atoms to diffuse into the metal substrate  204 . As a result of the nitriding process, the nitrided titanium substrate  210  includes a titanium nitride layer  214 . The titanium nitride layer  214  is a thin, ceramic coating (e.g., ˜less than 1 μm) that provides a hardened protective coating or “case hardening.” However, the titanium nitride layer  214  has a distinctive gold/brown color due to a significant amount of nitrogen atoms that diffuse into the metal substrate  204 . Notably, consumers may consider this distinctive gold/brown color to be aesthetically unattractive. In some examples, this gold/brown color is characterized as having an a* value greater than 1 and a b* value greater than 5. In some examples, interstitial nitrogen atoms  218  penetrate beyond the external surface  216  of the nitrided titanium substrate  210  up to a depth (D h ) of several 10 s of μms. The titanium nitride layer  214  is characterized as having a thickness of less than 1 μm due to the interstitial nitrogen atoms  218  penetrating to a depth (D n ) from the external surface  216 . 
     As illustrated in  FIG. 2B , the nitrided titanium substrate  210  includes the titanium nitride layer  214  that overlays the hardened titanium region  212  and the metal substrate  204 . The titanium nitride layer  214  includes nitride compounds  219  that form within the metal matrix of the metal substrate  204 . In some embodiments, the titanium nitride layer  214  includes an amount of the nitride compounds  219  that is sufficient to impart the titanium nitride layer  214  with a gold/brown color. 
     Relative to the hardened titanium region  212 , the titanium nitride layer  214  has an elevated concentration of the interstitial nitrogen atoms  218  that causes the titanium nitride layer  214  to have a gold/brown color. Indeed, there is a corresponding relationship between hardness and concentration of the interstitial nitrogen atoms  218 . The interstitial nitrogen atoms  218  diffuse into the metal substrate  204  such as to form a hardened titanium region  212 . As defined herein, the term “interstitial” refers to the interstitial nitrogen atoms  218  occupying interstitial sites of the titanium matrix of the metal substrate  204 . It should be noted that the interstitial nitrogen atoms  218  may not completely penetrate the entire thickness of the metal substrate  204 . Thus, some interstitial sites of the titanium matrix are not occupied by the nitrogen atoms. 
     As illustrated in  FIG. 2B , the interstitial nitrogen atoms  218  penetrate the external surface  216  to a depth of several 10 s of μms. This region of the nitrided titanium substrate  210  having the interstitial nitrogen atoms  218  is referred to as the hardened titanium region  212 . The volume of the titanium matrix that is occupied by the interstitial nitrogen atoms  218  as a result of the nitriding process is significantly greater than the extent of protection provided by the thin layer of a pure titanium nitride. In contrast to the metal substrate  204 , the nitrided titanium substrate  210  has a Vickers hardness of about 925 Hv 0.1 . In some examples, the increased hardness of the nitrided titanium substrate  210  may be attributed to the hardness of the titanium nitride layer  214 , where the titanium nitride layer  214  has a Vickers hardness between about 1800 Hv 0.1  to 2100 Hv 0.1 . 
     Although the nitrided titanium substrate  210  has an ideal hardness value that is suitable for protecting operational components for use with portable electronic devices  102 ,  104 ,  106 , and  108 , the color of the titanium nitride layer  214  may be considered unattractive. However, it is nearly impossible to chemically strip the titanium nitride layer  214  from the remaining portion (e.g., hardened titanium region  212 ). For example, exposing the nitrided titanium substrate  210  to an acid etching process without an intermediate anodizing step would have no effect as the gold/brown color would remain even after prolonged exposure to the chemical stripping process. Accordingly, the gold/brown color of the nitrided titanium substrate  210  is preferably corrected using an intermediate anodizing process as described herein. Notably, the remaining portion of the hardened titanium region  212  does not include a sufficient amount of interstitial nitrogen atoms  218  to cause the gold/brown color. 
       FIG. 2C  illustrates a cross-sectional view of an oxidized nitride substrate  220  subsequent to an anodizing process, in accordance with some embodiments. During the anodizing process, the nitrided titanium substrate  210  is exposed to an electrolytic anodizing solution that includes sulfuric acid (H 2 SO 4 ). In some examples, the anodizing process involves exposing the nitrided titanium substrate  210  an anodizing voltage between about 40 V-60 V for a duration of between 45 seconds-90 seconds. The anodizing process described herein involves penetrates the entire thickness of the nitrided titanium substrate  210 . Accordingly, the anodizing process involves anodizing the nitrided titanium material of the nitrided titanium substrate  210  to form a metal oxide layer  222 . In some embodiments, the metal oxide layer  222  may also be referred to as an anodized layer. 
     As illustrated in  FIG. 2C , subsequent to the anodizing process, the oxidized nitride substrate  220  includes the metal oxide layer  222 . The metal oxide layer  222  may include TiO x  or TiO 2 . The metal oxide layer  222  may have a thickness (D O1 ) of about 100 nm. In contrast to the titanium nitride layer  214 , the metal oxide layer  222  is colorless. According to some embodiments, the nitrided titanium substrate  210  may be anodized to various thicknesses, as described with reference to application Ser. No. 16/182,473, entitled “DURABLE COSMETIC FINISHES FOR TITANIUM SURFACES,” which is incorporated herein by reference in its entirety for all purposes. 
       FIG. 2C  illustrates that the oxidized nitride substrate  220  still includes the hardened titanium region  212 . The hardened titanium region  212  corresponds to the interstitial nitrogen atoms  218  penetrating to a depth (D h ) of several 10 s of μms from an external surface  224  of the oxidized nitride substrate  220 . The oxidized nitride substrate  220  may be characterized as having a hardness of 900 Hv 0.1 . In other words, the anodizing process does not sacrifice the hardness of the oxidized nitride substrate  220 . 
       FIG. 2D  illustrates a cross-sectional view of a modified nitrided part  230  subsequent to a chemical stripping process, in accordance with some embodiments. Notably, whereas prior to the anodizing process, the titanium nitride layer  214  could not be chemically stripped from the remaining portion (e.g., hardened titanium region  212 ), the chemical solution can be used to strip the metal oxide layer  222  from the remaining portion. According to some examples, the chemical stripping process involves exposing the oxidized nitride substrate  220  to a concentrated phosphoric acid solution (H 3 PO 4 ) at a temperature (e.g., 85° C.) sufficient to etch the metal oxide layer  222 . 
       FIG. 2D  illustrates that the metal oxide layer  222  has been chemically stripped off such as to reveal the natural titanium color while maintaining the hardness conferred by the nitriding process. In particular, the modified nitrided part  230  may include an elevated concentration of nitrogen (e.g., &gt;10 at %). 
     For example, the modified nitrided part  230  has a Vickers hardness of 850 Hv 0.1  or greater and a natural titanium color defined as an L* value greater than 70, an a* value less than 1, and a b* value less than 5. In some embodiments, only the absolute minimum amount of the titanium nitride layer  214  is removed such as to correct the color. Accordingly, in some examples, a majority of and/or an entirety of the titanium nitride layer  214  is removed. In some embodiments, the chemical stripping process restores the color of the modified nitrided part  230  to within ˜1 ΔE94 of the color of the metal part  200 . 
       FIG. 2D  illustrates that the modified nitrided part  230  still includes the hardened titanium region  212 . In some examples, the hardened titanium region  212  is referred to as a layer that overlays a non-nitrided region of the metal substrate  204 . The hardened titanium region  212  corresponds to the interstitial nitrogen atoms  218  penetrating to a depth (D h ) of several 10 s of μms from an external surface  232  of the modified nitrided part  230 . The chemical stripping process removes approximately less than 1 μm of an upper region of the hardened titanium region  212  (e.g., the titanium nitride layer  214 ). In some examples, the hardened titanium region  212  has a thickness less than 100 μm. In some examples, the hardened titanium region  212  has a thickness less than 50 μm. In some examples, the hardened titanium region  212  has a thickness less than 20 μm. In some examples, the thickness of the hardened titanium region  212  does not exceed an amount that causes the hardened titanium region  212  to have an a* value greater than 1 and a b* value greater than 5. 
     Alternatively,  FIG. 2E  illustrates a cross-sectional view of a modified nitrided oxidized part  240 , in accordance with some embodiments. In particular, the modified nitrided oxidized part  240  may exhibit a range of colors based upon a thin film interference effect. For example, only a portion of the metal oxide layer  222  is chemically stripped from the oxidized nitrided substrate  220 . As a result, the modified nitrided oxidized part  240  may still include a thin layer of the modified metal oxide layer  242 ; thereby, exhibiting a color based on the thin film interference effect. In some examples, the modified metal oxide layer  242  is less than 100 nm in thickness. In some embodiments, the modified metal oxide layer  242  of the modified nitrided oxidized part  240  may have a thickness D O2  where (D O2 )&lt;(D O1 ). 
     The thin film interference effect is dependent upon a refractive index of the modified metal oxide layer  242 . In particular, the color of the anodized layer is a function of the thickness of the modified metal oxide layer  242 . As noted above, only an amount of the modified metal oxide layer  242  is removed as is necessary to correct for the gold/brown color. In some instances, it may be desirable for the modified nitrided part  230  to exhibit a color via the thin film interference effect. It should be noted that any one of the techniques described herein for forming the modified nitrided part  230  may also be directed towards forming the modified nitrided oxidized part  240 . 
     As described herein, the nitriding process for converting the metal part  200  to the modified nitrided part  230  and/or the modified nitrided oxidized part  240  involves a controlled process, such as carefully controlling the appropriate nitriding temperature. The controlled nitriding process described herein involves exposing the metal substrate  204  to the nitrogen gas and increasing the temperature to a target temperature between 850° C. to 1100° C. Establishing the appropriate target temperature is critical for achieving the appropriate hardness depth profile and/or color profile for the modified nitrided part  230 . In particular, the nitriding process involves tailoring the color profile and/or the hardness profile of the modified nitrided part  230  and the modified nitrided oxidized part  240 . For example, if the hardness profile of the nitrided titanium substrate  210  is very shallow, then subsequent to performing the anodizing process, then the hardness profile of the modified nitrided part  230  will be very shallow and may be unsuitable for sufficiently protecting the portable electronic devices  102 ,  104 ,  106 , and  108 . Additionally, if the gold/brown color profile of the nitrided titanium substrate  210  is too deep, then it may be very difficult to correct for this color. In other words, correctly tailoring the color profile and/or the hardness profile reduces the amount of expense, effort, and time required during the anodizing and chemical stripping processes. 
       FIG. 3  illustrates a magnified cross-sectional view of the nitrided titanium substrate  210 , in accordance with some embodiments. As illustrated in  FIG. 3 , the nitrided titanium substrate  300  includes a hardened titanium region  212  that encompasses a titanium nitride layer  214 . The hardened titanium region  212  overlays the metal substrate  204 . During the nitriding process, nitrogen atoms react with the metal matrix  320  of the metal substrate  204 , thereby causing nitride compounds  318  (e.g., TiN) to form within the metal matrix  320  of the metal substrate  204 . The titanium nitride layer  214  includes an elevated concentration of the nitride compounds  318  relative to the remaining portion of the hardened titanium region  212 . As a result, the titanium nitride layer  214  is characterized as having a gold/brown color. In some embodiments, the titanium nitride layer  214  includes a TiN compound material  316  that forms along an upper region (e.g., depth &lt;1 μm) of the nitrided titanium substrate  300 . In some embodiments, the TiN compound material  316  includes a dense amount of the nitride compounds  318  such as to impart the gold/brown color. 
     As illustrated in  FIG. 3 , the nitrogen atoms diffuse into the metal matrix  320 , where a concentration of the nitrogen atoms  218  within the metal matrix  320  decreases as the depth from the external surface  330  increases. Accordingly, there is a greater concentration of nitrogen atoms  318  along the upper region (e.g., thickness &gt;50 μm) than the lower region of the nitrided titanium substrate  300 . 
       FIG. 4  illustrates a magnified cross-sectional view of the modified nitrided part  230 , in accordance with some embodiments. As described herein, the modified nitrided part  400  does not include the titanium nitride layer  214  nor the metal oxide layer  222 . As a result, the modified nitrided part  400  has a reduced amount of interstitial nitrogen atoms  218  relative to the nitrided titanium substrate  300 . Nevertheless, the modified nitrided part  400  has an elevated concentration of interstitial nitrogen atoms  218  (e.g., &gt;10 at %), which is more than sufficient to impart the modified nitrided part  400  with a Vickers hardness of at least 850 Hv 0.1  or greater. In some embodiments, the modified nitrided part  400  has a Vickers hardness of greater than 800 Hv 0.1 . 
       FIGS. 5A-5B  illustrate exemplary hardness depth profiles of the nitrided titanium substrate  210  and the modified nitrided part  230 , respectively.  FIG. 5A  illustrates an exemplary hardness depth profile as a function of depth from the external surface  216  of the nitrided titanium substrate  210 . According to some examples, the nitrided titanium substrate  210  has a thickness of about 100 μm. As described herein, the nitrided titanium substrate  210  has a titanium nitride layer  214  that extends up to 1 μm from the external surface  216 . The titanium nitride layer  214  has the greatest concentration of the interstitial nitrogen atoms  218 , thereby imparting the titanium nitride layer  214  with the gold/brown color. 
     As depicted by the hardness depth profile, at a depth of less than 1 μm from the external surface  216 , the nitrided titanium substrate  210  has a hardness greater than 1000 Hv 0.1 . As depicted by the hardness depth profile, at a depth of 1 μm from the external surface  216 , the nitrided titanium substrate  210  has a hardness of about 1000 Hv 0.1 . As depicted by the hardness depth profile, at a depth of 50 μm from the external surface  216 , the nitrided titanium substrate  210  has a hardness of about 500 Hv 0.1 . 
       FIG. 5B  illustrates an exemplary hardness depth profile as a function of depth from the external surface  232  of the modified nitrided part  230 . According to some examples, the modified nitrided part  230  has a thickness of about 100 μm. The modified nitride part  230  is formed by anodizing the nitrided titanium substrate  210 , and then subsequently stripping the metal oxide layer  222  from the oxidized nitrided substrate  220 . The anodizing and stripping process removes less than 1 μm of the nitrided titanium matrix material from the oxidized nitrided substrate  220 . According to some examples, the nitrided titanium matrix material that is removed corresponds to the titanium nitride layer  214 . As a result, the modified nitride part  230  has a natural titanium color. 
     As depicted by the hardness depth profile, at a depth of 1 μm from the external surface  232  which may correspond to the thickness of the titanium nitride layer  214  that was removed, the modified nitride part  230  has a hardness of about 850 Hv 0.1  or greater. As depicted by the hardness depth profile, “case hardening” of the modified nitrided part  230  extends up to a depth of 100 μm from the external surface  232 . 
       FIG. 6  illustrates a method  600  for forming nitrided titanium part, in accordance with some embodiments. In some embodiments, the method  800  may be implemented in conjunction with a closed feedback loop that is implemented by an optical detection system and/or a controlled electrolytic anodization system. 
     As illustrated in  FIG. 6 , the method  600  begins at step  602 , which involves forming a metal substrate—e.g., the metal substrate  202 . The metal substrate includes titanium or a titanium alloy. In some examples, the metal substrate has a Vickers hardness of about 320 Hv 0.1 , which may be insufficient to protecting operational components carried by the portable electronic devices  102 ,  104 ,  106 , and  108 . Notably, the metal substrate is described as having a natural titanium color, which is defined as an L*a*b color space value of an L* value greater than 65 together with an a* value less than 1 and a b* value less than 5. 
     At step  604 , the metal substrate  204  is exposed to a nitriding process. The nitriding process includes heating the metal substrate  204  in a nitrogen-rich atmosphere at a temperature between 850° C. to 1100° C. In some embodiments, the nitriding process is a controlled process in order to carefully monitor and control the hardness profile and color profile of the resulting nitrided titanium substrate  210 . 
     Subsequent to the nitriding process, nitrogen atoms diffuse into the metal substrate  204  to form a hardened titanium region  212 , which also encompasses the titanium nitride layer  214 . While the titanium nitride layer  214  provides an elevated amount of hardness and strength, the titanium nitride layer  214  imparts the nitrided titanium substrate  210  with an unattractive gold/brown color. 
     At step  606 , the nitrided titanium substrate  210  is exposed to an anodizing process to form the oxidized nitrided substrate  220 . Notably, the titanium nitride layer  214  cannot be chemically stripped from the remaining portion of the hardened titanium region  212 , where the remaining portion does not include a sufficient amount of interstitial nitrogen atoms  218  to cause the gold/brown color. Accordingly, the entire thickness of the nitrided titanium substrate  210  may be anodized to form a metal oxide layer  222 . In some examples, the metal oxide layer  222  has a thickness of about 100 nm. In some examples, the metal oxide layer  222  has a thickness less than 200 nm. 
     At step  608 , at least a portion of the metal oxide layer  222  is removed from the oxidized nitrided substrate  220  as is necessary to revert the color of the modified nitride part  230  to a natural titanium color. In some examples, the portion may constitute a majority and/or an entirety of the metal oxide layer  222 . 
     In some embodiments, an optical detection system may be utilized to monitor the color of the modified nitride part  230 . In some embodiments, the optical detection system may determine whether the color of the modified nitride part  230  satisfies predetermined value and/or range. For example, the optical detection system may determine at least one of whether the L* value of the modified nitride part  230  satisfies a predetermined L* value, the a* value of the modified nitride part  230  satisfies a predetermined a* value or the b* value of the modified nitride part  230  satisfies a predetermined b* value. In another example, the optical detection system may determine whether the color of the modified nitride part  230  is within ˜1 ΔE94 of the color of the metal substrate  202 . If the optical detection system determines that the modified nitride part  230  has a color that does not satisfy any one of the predetermined L*, a* or b* values, then additional material (e.g., the nitrided titanium matrix) may be removed to satisfy any one of the predetermined L*, a* or b* values. 
     Alternatively, at step  610 , if the optical detection system determines that the modified nitride part  230  has a color that satisfies any one of the predetermined L*, a* or b* values, then the external surface  232  of the modified nitride part  230  may be subjected to process, such as a polishing, texturizing or blasting process. 
       FIG. 7  illustrates an exemplary hardness profile for various nitrided titanium substrates subsequent to a nitriding process, in accordance with various examples. In particular, the nitriding process results in a range of gold and brown colors for various nitrided titanium substrates—e.g., the nitrided titanium substrate  210 . Furthermore, the micro-hardness rating for each of the nitrided titanium substrates varies according to the depth from an external surface. As illustrated in  FIG. 7 , six different nitrided titanium substrates were formed as a result of utilizing different nitriding parameters (e.g., partial pressure, temperature, and duration). 
     The nitrided titanium substrate processed under Trial 1 exhibited a light yellow, shallow hardness depth. Trial 1 includes subjecting the nitrided titanium substrate to a duration of 30 minutes and a partial pressure between 0.5-5.0 Torr. 
     The nitrided titanium substrate processed under Trial 2 exhibited a dark yellow, shallow hardness depth. Trial 2 includes subjecting the nitrided titanium substrate to a duration of 30 minutes and a partial pressure between 500-700 Torr. 
     The nitrided titanium substrate processed under Trial 3 exhibited a high hardness, deeper hardness depth. Trial 3 includes subjecting the nitrided titanium substrate to a duration of 120 minutes and a partial pressure between 0.5-5.0 Torr. 
     The nitrided titanium substrate processed under Trial 4 exhibited a deeper hardness depth. Trial 4 includes subjecting the nitrided titanium substrate to a duration of 120 minutes and a partial pressure between 500-700 Torr. 
     The nitrided titanium substrate processed under Trial 5 exhibited a deeper hardness depth than the sample processed under Trial 4. Trial 5 includes subjecting the nitrided titanium substrate to a duration of 120 minutes and a partial pressure between 500-700 Torr, but a temperature greater than the sample processed under Trial 4. 
     The nitrided titanium substrate processed under Trial 6 exhibited a deeper hardness depth than the samples processed under Trials 4 and 5. Trial 6 includes subjecting the nitrided titanium substrate to a duration of 120 minutes and a partial pressure between 500-700 Torr, but a temperature greater than the samples processed under Trials 4 and 5. 
     Accordingly, the hardness depth profile illustrated in  FIG. 7  depicts that modifying any one of the partial pressure, temperature, and/or duration of the nitriding process can significantly alter the hardness depth profile and/or color profile for the nitrided titanium substrate  210 . Thus, it may be necessary to control the nitriding parameters in order to form a nitrided titanium substrate having a shallow depth of color (i.e., gold/brown color does not exceed 1 μm depth) and a deep hardness depth (e.g., hardness of ˜850 Hv at ˜100 nm depth). Beneficially, controlling the nitriding parameters can reduce the amount of work required during the anodizing and/or chemical stripping processes to achieve a modified nitrided part  230  having a deep hardness and a natural titanium color. 
       FIG. 8  illustrates a method  800  for forming nitrided titanium part, in accordance with some embodiments. In contrast to the method  600 , the method  800  involves performing the nitriding, oxidizing, and stripping processes in a controlled atmosphere. The method  800  may be implemented in conjunction with a closed feedback loop that is implemented by an optical detection system and/or a controlled electrolytic anodization system. 
     The method  800  begins at step  802 , which involves exposing the metal substrate  202  to a nitriding process (e.g., N 2  at an elevated temperature) to form the nitrided titanium substrate  210 . 
     At step  804 , while the nitrided titanium substrate  210  remains exposed to the elevated temperature, the N 2  is replaced with O 2  during a non-electrolytic oxidation process such as to form the oxidized nitrided substrate  220 . The oxidized nitrided substrate  220  includes the metal oxide layer  222 . 
     At step  806 , at least a portion of the metal oxide layer  222  is removed from a remaining portion of the oxidized nitrided substrate  220  while the oxidized nitrided substrate  220  remains exposed to the elevated temperature in an argon-rich atmosphere. In particular, the metal oxide layer  222  can be dissolved by merely reducing the partial pressure of oxygen present. In other words, an etching solution (e.g., phosphoric acid) is not required to remove the metal oxide layer  222 . The portion of the metal oxide layer  222  that is removed is based upon an amount necessary to revert the color of the modified nitride part  230  to a natural titanium color. In some examples, the portion may constitute a majority and/or an entirety of the metal oxide layer  222 . 
     In some embodiments, an optical detection system may be utilized to monitor the color of the modified nitride part  230 . In some embodiments, the optical detection system may determine whether the color of the modified nitride part  230  satisfies predetermined value and/or range. For example, the optical detection system may determine at least one of whether the L* value of the modified nitride part  230  satisfies a predetermined L* value, the a* value of the modified nitride part  230  satisfies a predetermined a* value or the b* value of the modified nitride part  230  satisfies a predetermined b* value. In another example, the optical detection system may determine whether the color of the modified nitride part  230  is within ˜1 ΔE94 of the color of the metal substrate  202 . If the optical detection system determines that the modified nitride part  230  has a color that does not satisfy any one of the predetermined L*, a* or b* values, then additional material (e.g., the nitrided titanium matrix) may be removed to satisfy any one of the predetermined L*, a* or b* values. 
     Alternatively, at step  808 , if the optical detection system determines that the modified nitride part  230  has a color that satisfies any one of the predetermined L*, a* or b* values, then the external surface  232  of the modified nitride part  230  may be subjected to process, such as a polishing, texturizing or blasting process. 
     Any ranges cited herein are inclusive. The terms “substantially”, “generally,” and “about” used herein are used to describe and account for small fluctuations. For example, they can refer to less than or equal to ±5%, such as less than or equal to ±2%, such as less than or equal to ±1%, such as less than or equal to ±0.5%. 
     The various aspects, embodiments, implementations or features of the described embodiments can be used separately or in any combination. Various aspects of the described embodiments can be implemented by software, hardware or a combination of hardware and software. The described embodiments can also be embodied as computer readable code on a non-transitory computer readable medium. The non-transitory computer readable medium is any data storage device that can store data which can thereafter be read by a computer system. Examples of the non-transitory computer readable medium include read-only memory, random-access memory, CD-ROMs, HDDs, DVDs, magnetic tape, and optical data storage devices. The non-transitory computer readable medium can also be distributed over network-coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. 
     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.

Metadata:
Filing Date: 20190905
Publication Date: 20201117
Grant Date: 20201117
Priority Date: 20190905
Inventors: CURRAN, JAMES A.
FEINBERG, ZECHARIAH D.
MINTZ, TODD S.
Assignee: APPLE INC
CPC Classifications: [{"code": "H05K5/10", "inventive": true, "first": false, "tree": "[]"}, {"code": "C23C8/24", "inventive": true, "first": true, "tree": "[]"}, {"code": "C25D11/26", "inventive": true, "first": false, "tree": "[]"}, {"code": "C23C28/04", "inventive": true, "first": false, "tree": "[]"}, {"code": "C23C8/80", "inventive": true, "first": false, "tree": "[]"}, {"code": "C23C8/42", "inventive": true, "first": false, "tree": "[]"}, {"code": "C23C8/24", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01R13/506", "inventive": true, "first": false, "tree": "[]"}, {"code": "C25D11/26", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05K5/04", "inventive": true, "first": true, "tree": "[]"}, {"code": "C23C8/24", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05K5/04", "inventive": true, "first": true, "tree": "[]"}, {"code": "C25D11/26", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01R13/506", "inventive": true, "first": false, "tree": "[]"}, {"code": "H05K5/0004", "inventive": true, "first": false, "tree": "[]"}, {"code": "C23C8/24", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 73264201