Abstract:
A method for surface treating aluminum or aluminum alloy, the method comprising the following steps of: providing a substrate made of aluminum or aluminum alloy; forming a TiON coating on the substrate by magnetron sputtering, using aluminum as a target, and nitrogen and oxygen as reactive gases; and forming a chromium oxynitride coating on the TiON coating by magnetron sputtering, using chromium as a target, and nitrogen and oxygen as reactive gases.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to co-pending U.S. patent applications (Attorney Docket No. US35132, US35134), each entitled “PROCESS FOR SURFACE TREATING ALUMINUM OR ALUMINUM ALLOY AND ARTICLE MADE WITH SAME”, by Chang et al. These applications have the same assignee as the present application. The above-identified applications are incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The disclosure generally relates to processes for surface treating aluminum or aluminum alloy and articles made of aluminum or aluminum alloy treated by the process. 
         [0004]    2. Description of Related Art 
         [0005]    Due to having many good properties such as light weight and quick heat dissipation, aluminum and aluminum alloy are widely used in manufacturing components (such as housings) of electronic devices. However, aluminum and aluminum alloy have a lower erosion resistance. 
         [0006]    Therefore, there is room for improvement within the art. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Many aspects of the embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the exemplary process for surface treating aluminum or aluminum alloy and articles made of aluminum or aluminum alloy treated by the process. Moreover, in the drawings like reference numerals designate corresponding parts throughout the several views. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like elements of an embodiment. 
           [0008]      FIG. 1  is a cross-sectional view of an exemplary article treated by the present process. 
           [0009]      FIG. 2  is a schematic view of a magnetron sputtering machine for processing the article in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    An exemplary process for surface treating aluminum or aluminum alloy may include the following steps. 
         [0011]    Referring to  FIG. 1 , a substrate  20  is provided. The substrate  20  is made of aluminum or aluminum alloy. 
         [0012]    The substrate  20  is pretreated. For example, the substrate  20  is ultrasonically cleaned with a solution (e.g., alcohol or Acetone) in an ultrasonic cleaner, to remove impurities such as grease or dirt from the substrate  20 . Then, the substrate is dried. 
         [0013]    An aluminum oxynitride (AlON) coating  30  and a chromium oxynitride (CrON) coating  40  are formed on the substrate  20  by magnetron sputtering. The AlON coating  30  is directly formed on a surface  201  of the substrate  20 . The CrON coating  40  is directly formed on the AlON coating  30 . The AlON coating  30  has close physical properties (such as thermal expansion coefficient) to the substrate  20 , thus the AlON coating  30  can improve the bonding of the substrate  20  and the CrON coating  40 . The magnetron sputtering for depositing the AlON coating  30  and the CrON coating  40  may be performed by the following steps. 
         [0014]    First, the AlON coating  30  is directly deposited on the substrate  20  by magnetron sputtering. The substrate  20  is held by a rotating bracket  14  in a vacuum chamber  12  of a magnetron sputtering machine  1  as shown in  FIG. 2 . The vacuum chamber  2  maintains an internal pressure of about 6×10 −3  Pa to about 8×10 −3  Pa. The temperature in the vacuum chamber  2  is maintained at a temperature of about 100° C. to about 150° C. The speed of the rotating bracket  4  is in a range from about 0.5 revolutions per minute (rpm) to about 1 rpm. Argon, oxygen, and nitrogen are simultaneously supplied into the vacuum chamber  12 , with the argon as a sputtering gas, and the oxygen and nitrogen as reactive gases. The flux of the argon is from about 150 Standard Cubic Centimeters per Minute (sccm) to about 300 sccm. The flux of the oxygen is in a range from about 30 to about 60 sccm, and the flux of the nitrogen is in a range from about 15 to about 40 sccm. A bias voltage is applied to the substrate  20  in a range from about −100 volts (V) to about −300V. At least one aluminum target  5  is evaporated at a power of about 6 kW to about 12 kW for about 0.5 hours to about 1 hour, to deposit the AlON coating  30  on the substrate  20 . The power may be a medium-frequency AC power. 
         [0015]    Then the CrON coating  40  is directly formed on the AlON coating  30  by magnetron sputtering. This step may be carried out in the magnetron sputtering machine  1 . The aluminum targets  5  are switched off. The flux of the oxygen is adjusted in a range of about 40 sccm to about 100 sccm and the flux of the nitrogen is adjusted in a range of about 30 sccm to about 60 sccm. At least one chromium target  6  is evaporated at a power of about 8 kW to about 10 kW for about 0.5 h to about 2 h, depositing the CrON coating  40  on the AlON coating  30  with the remaining conditions maintained same with depositing the AlON coating  30 . 
         [0016]      FIG. 1  shows a cross-section of a portion of an exemplary article  10  made of aluminum or aluminum alloy processed by the surface treating as described above. The article  10  may be housings for electronic devices, such as mobile phones. The article  10  includes the substrate  20  made of aluminum or aluminum alloy, the AlON coating  30  formed on the substrate  20 , and the CrON coating  40  formed on the AlON coating  30 . In the AlON coating  30 , the atomic percentage of Al is about 40% to about 65%; the atomic percentage of O is about 25% to about 50%; the atomic percentage of N is about 10% to about 20%. In the CrON coating  40 , the atomic percentage of Cr is about 50% to about 70%; the atomic percentage of O is about 20% to about 45%; the atomic percentage of N is about 5% to about 10%. The thickness of the AlON coating  30  may be about 0.4 μm to about 0.8 μm. The thickness of the CrON coating  40  may be about 0.5 μm to about 2.0 μm. The CrON coating  40  formed by this exemplary method comprises crystal grains having an average particle diameter of about 4 nm to about 7 nm. Crystal grains having an average diameter of about 4 nm to about 7 nm have smaller spaces between crystal grains than in materials have larger average particle diameters. Thus, the CrON coating  40  has improved compact density and the article  10  coated with the CrON coating  40  has improved erosion resistance since it becomes harder for contaminants to enter the spaces between the crystal grains. 
       EXAMPLES 
       [0017]    Experimental examples of the present disclosure are described as follows. 
       Example 1 
       [0018]    A sample of aluminum alloy substrate was ultrasonically cleaned for about 30 minutes and then was placed into the vacuum chamber  2  of the magnetron sputtering machine  1 . The vacuum chamber  2  was evacuated to maintain an internal pressure of about 8×10 −3  Pa and was heated to maintain a temperature of about 120° C. The speed of the rotating bracket  4  was about 0.5 rpm. Argon, oxygen, and nitrogen were simultaneously fed into the vacuum chamber  2 . The flux of the argon was about 150 sccm. The flux of the oxygen was about 20 sccm, and the flux of the nitrogen was about 15 sccm. The bias voltage applied to the substrate was about −200V. Aluminum targets were evaporated at a power of about 8 kw with the duty cycle of about 50% for about 0.5 h, depositing a AlON coating on the substrate. Then the aluminum targets were switched off. The flux of the oxygen was adjusted to 40 sccm. The flux of the nitrogen was adjusted to 30 sccm. chromium targets were evaporated at a power of about 8 kW with the duty cycle of about 50% for about 1 h, depositing a CrON coating on the AlON coating with the remaining parameters were unchanged. 
       Example 2 
       [0019]    Unlike the example 1, in the example 2, the flux of the oxygen was about 30 sccm, and the flux of the nitrogen was about 20 sccm during sputtering the AlON coating. The flux of the oxygen was about 80 sccm, and the flux of the nitrogen was about 50 sccm during sputtering the CrON coating. Except the above difference, the remaining experiment conditions of example 2 were same with example 1. An article of aluminum alloy coated with an AlON coating and a CrON coating was obtained according to example 2. 
         [0020]    The samples processed in example 1 and 2 have similar microcosmic configuration and surface topography, so have similar erosion resistance. 
       Comparison Example 
       [0021]    A sample of aluminum alloy substrate was processed by magnetron sputtering in the magnetron sputtering machine  1 . Unlike the example 1, the target material was chromium and the reactive gas was nitrogen in the comparison example. The flux of the nitrogen was about 50 sccm. The bias voltage applied to the substrate was about −200V. The chromium targets were evaporated at a power of about 8 kW with the duty cycle of about 50% for about 0.5 h. Except for the above differences, the remaining experiment conditions of the comparison example were same as example 1. A chromium nitride (CrN) coating was deposited on the aluminum alloy substrate. 
       Results of the Above Examples 
       [0022]    An neutral salt spray test was implemented to the samples coated with the AlON coating and the CrON coating and the sample coated with CrN coating. The test conditions included 5% NaCl (similar to salt-fog chloride levels), that was neutral at 35° C. to simulate condensing gases with moisture and salt. The test was an accelerated corrosion test for assessing coating performance. Obvious erosion was observed with the sample coated with CrN coating after about 4 h. However, after about 72 h, erosion began to be observed the samples coated with the AlON coating and the CrON coating. 
         [0023]    It is to be understood, however, that even through numerous characteristics and advantages of the exemplary disclosure have been set forth in the foregoing description, together with details of the system and function of the disclosure, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrangement of parts within the principles of the disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.