Patent Publication Number: US-2007098993-A1

Title: Article with multilayer diamond-like carbon film

Description:
TECHNICAL FIELD  
      The present invention generally relates to diamond-like carbon films, and more particularly relates to an article with multilayer diamond-like carbon film having gradient composition.  
     BACKGROUND  
      A diamond-like carbon film was first deposited by Aisenberg et al. Since this initial investigation of depositing the diamond-like carbon film, a variety of different techniques about the diamond-like carbon film have been developed.  
      Diamond-like carbon is a mostly metastable amorphous material but can include a microcrystalline phase. Diamond-like carbon contains both sp 2  and Sp 3  hybridised carbon atoms. Diamond-like carbon includes amorphous carbon (a-C) and hydrogenated amorphous carbon (a−C:H) containing a significant Sp 3  bonding. The amorphous carbon containing 85% or more of Sp 3  bonding is called highly tetrahedral amorphous carbon (ta-C). The Sp 3  bonding provides valuable diamond-like properties such as mechanical hardness, low friction, optical transparency and chemical inertness onto a diamond-like carbon film. The diamond-like carbon film has some advantages, such as being capable of deposition at room temperature, deposition onto steel or plastic substrates and superior surface smoothness.  
      Because of excellent properties such as corrosion resistance and wear resistance, the diamond-like carbon film is a suitable protective film material for various articles such as molds, cutting tools and hard disks. However, diamond-like carbon film also shows several drawbacks. One of the most serious practical problems is their poor adhesion to a substrate. This difficulty is caused by the high compressive stresses present in the diamond-like carbon film and the high compressive residual stresses present between the diamond-like carbon film and the substrate. Due to this problem, commercial application of the diamond-like carbon film is restricted to a certain extent.  
      What is needed, therefore, is an article with the multilayer diamond-like carbon film having good corrosion resistance, good wear resistance and high adhesion to substrate.  
     SUMMARY  
      A preferred embodiment provides an article including a substrate and a multilayer diamond-like carbon film formed on the substrate. The multilayer diamond-like carbon film includes a number of diamond-like carbon layers stacked one on another. Each diamond-like carbon layer is comprised of carbon, hydrogen and a metal-containing component selected from a group consisting of chromium, titanium, chromium-titanium alloy and chromium nitride. A content of the metal-containing component in each diamond-like carbon layer gradually decreases with increasing distance away from the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Many aspects of 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 present embodiment. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views.  
       FIG. 1  is a schematic view of a multilayer diamond-like carbon film formed on a substrate according to a preferred embodiment;  
       FIG. 2  is similar to  FIG. 1 , but showing a multilayer diamond-like carbon film formed on a substrate, with an intermediate layer interposed therebetween;  
       FIG. 3  is a schematic view of a multi-target co-sputtering system for forming a multilayer diamond-like carbon film according to a preferred embodiment; and  
       FIG. 4  is a top view of multi-target co-sputtering targets and rotation substrates of the multi-target co-sputtering system shown in  FIG. 3 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
      Embodiments will now be described in detail below and with reference to the drawings.  
      Referring to  FIG. 1 , an article  40  including the multilayer diamond-like carbon film  10  and a substrate  20  according to a preferred embodiment is shown. The multilayer diamond-like carbon film  10  is formed on the substrate  20 . The article  40  can be a hard disk, a mold or a cutting tool. The substrate  20  can be magnetic media material or metal alloy such as sintered alloy, iron-based alloy, titanium-based alloy, aluminum-based alloy and copper-based alloy. The multilayer diamond-like carbon film  10  including a number of diamond-like carbon layers stacked one on another is shown. The diamond-like carbon layers have a gradient composition.  
      The multilayer diamond-like carbon film  10  is composed of n layers of diamond-like carbon layer, i.e. a first layer  11 , a second layer  12 , a third layer  13  . . . , a (n−2)th layer  14 , a (n−1)th layer  15 , and a nth layer  16  stacked one on top of the other in that order, wherein n is an integer, preferably in a range from 6 to 30. The first layer  11  is an innermost layer of the multilayer diamond-like carbon film  10  that is adapted to adhere to the substrate  20 . The second layer  12  is a formed on the first layer  11 , the third layer  13  is formed on the second layer  12 , similarly, the (n−2)th layer  14  is the third layer counting from the outermost layer of the diamond-like carbon film  10  and is formed on the (n−3)th layer, the (n−1)th layer  15  is formed on the (n−2) layer  14 , and a nth layer  16  is the outermost layer of the multilayer diamond-like carbon film  10 . The nth layer  16  is formed on the (n−1)th layer  15  and is distant from the substrate  20 .  
      Each diamond-like carbon layer has a different composition. Each diamond-like carbon layer is composed of carbon, hydrogen and a metal-containing component. The metal-containing component is selected from a group consisting of chromium, titanium, chromium-titanium alloy and chromium nitride. The metal-containing component in each diamond-like carbon layer gradually decreases in content from the first layer  11  to the nth layer  16 . For example, if an mth layer is any one of the diamond-like carbon layers of the multilayer diamond-like carbon film  10 , a composition of the mth layer can be represented by a formula of a−C:H:(n−m+1)X, wherein m is an integer in a range from 1 to n, C represents a carbon component, H represents a hydrogen component, and X represents a metal-containing component. Therefore, the composition of each diamond-like carbon layer can be represented by a concrete formula. For example, the first layer  11  is represented by a−C:H:nX, the second layer  12  is represented by a formula of a−C:H:(n−1)X, the third layer  13  is represented by a−C:H:(n−2)X . . . , the (n−1)th layer  15  is represented by a−C:H:3X, the (n−1)th layer  15  is represented by a−C:H:2X, and the nth layer  16  is represented by a−C:H:X. A content of the metal-containing component in each diamond-like carbon layer is in a range from 0.2% to 1.0%.  
      Accordingly, the first layer  111  has greatest atomic percentage of the metal-containing component and the nth layer  16  has least atomic percentage of metal-containing component. The metal-containing component can enhance strength of the diamond-like carbon layer, whilst the corrosion resistance and wear resistance of the diamond-like carbon layer are weakened. Therefore, the properties of each diamond-like carbon layer depend on the atomic percentage of the metal-containing component thereof.  
      The first layer  11  is the innermost layer of the multilayer diamond-like carbon film  10  that is adapted to contact with the substrate  20 . The substrate  20  is usually made of a metal material, thus, an increased content of the metal-containing component in the first layer  111  of multilayer diamond-like carbon film  10  facilitates an adhesion to the substrate  20 . In other words, the multilayer diamond-like carbon film  10  adheres relatively easily to the substrate  20 .  
      The nth layer  16  is the outermost layer of multilayer diamond-like carbon film  10 . That the nth layer  16  has the least atomic percentage of the metal-containing component enhances its properties, such as hardness, corrosion resistance and wear resistance, smoothness, etc.  
      The atomic percentage of the metal-containing component in each layer of the multilayer diamond-like carbon film  10  gradually decreases from the first layer  111  to the nth layer  16 . For example, the atomic percentage of the metal-containing component in the first layer  111  is 1%. The atomic percentage of the metal-containing component in the nth layer  16  is 0.2%. The atomic percentage of the metal-containing component in other layers is gradually reduced from 1% to 0.2%.  
      A thickness of each diamond-like carbon layer of the multilayer diamond-like carbon film  10  is in a range from 0.1 nanometers to 30 nanometers. A thickness of the multilayer diamond-like carbon film  10  is in a range from 0.6 to 900 nanometers.  
      In one example, the substrate  20  is magnetic media material of a hard disk, the each diamond-like carbon layer thickness is in a range from 0.2 nanometers to 0.5 nanometer and the thickness of the multilayer diamond-like carbon film  10  is a range from 1.2 nanometers to 15 nanometers. Preferably, a thickness of the multilayer diamond-like carbon film  10  is a range from 1.5 nanometers to 3 nanometers. The thinner multilayer diamond-like carbon film  10  gives better magnetic recording performance.  
      In another example, the substrate  20  is metal alloy of a mold or a cutting tool, the each diamond-like carbon layer thickness is in a range from 1 nanometer to 30 nanometers and a thickness of the multilayer diamond-like carbon film  10  is in a range from 6 nanometers to 900 nanometers. Preferably, a thickness of the multilayer diamond-like carbon film  10  is a range from 30 nanometers to 450 nanometers.  
      The multilayer diamond-like carbon film  10  may have good performance of corrosion resistance, adhesion, and wear resistance by optimizing the gradient composition and multilayer structure. The multilayer diamond-like carbon film  10  with good corrosion resistance, good wear resistance and high adhesion to the substrate  20  can be served as thin protective film on articles  40  such as molds, cutting tools and hard disks.  
      Referring to  FIG. 2 , the multilayer diamond-like carbon film  10  formed on the substrate  20  with an intermediate layer  30  sandwiched therebetween is shown.  
      The intermediate layer  30  is formed on the substrate  20  and serves as a functional layer. The material of the intermediate layer  30  is selected according to the material used for the substrate  20 . For example, when the substrate  20  is magnetic media material of hard disks, the intermediate layer  30  can be a magnetic layer such as cobalt-chromium-tantalum (CoCrTa) alloy and cobalt-chromium-platinum-tantalum (CoCrPtTa) alloy. If the substrate  20  is metal alloy of molds or cutting tools, the intermediate layer  30  is a mirror-polished layer with composition such as ferrum-chromium-carbon-molybdenum-silicon-vanadium (FeCrCMoSiV) alloy. The intermediate layer  30  is bonded to the multilayer diamond-like carbon film  10  and the substrate  20  via metallic bonding. Thus the multilayer diamond-like carbon film  10  has higher adhesion to the substrate  20 .  
      The multilayer diamond-like carbon film  10  can be deposited using a co-sputtering process. Referring to  FIG. 3  and  FIG. 4 , a multi-target co-sputtering system  100  for forming a multilayer diamond-like carbon film  10  according to the preferred embodiment is shown. The multi-target co-sputtering system  100  includes an ion source  110  and a rotating stage  130 . The ion source  110  is used to disassociate and ionize a sputter gas to generate reactive plasma. The rotating stage  130  has a rotational axis A and three targets. The rotating stage  130  can turn around the rotational axis A. The first target  132  and the second target  134  are carbon targets, and the third target  136  is a metal-containing material target. The metal-containing material can be selected from a group consisting of chromium, titanium, chromium-titanium alloy and chromium nitride. Each target has a ring  140  with lots of diffusion holes  142  around an exterior edge thereof. The diffusion holes  142  can let the sputter gas to go surrounding each target to form reactive plasma for sputtering deposition the multilayer diamond-like carbon film  10  on the substrate  20 .  
      During the co-sputtering process, the multilayer diamond-like carbon film  10  is deposited on the substrate  20  in a vacuum environment. The sputter gas is introduced into the multi-target co-sputtering system  100  to maintain a sputter pressure in a range from 0.6 torrs to 5 torrs. The sputter gas for the first target  132  and the second target  133  can be selected from a group consisting of a mixture of argon and methane (with a percentage by volume of methane in a range from 5% to 20%), a mixture of argon and hydrogen (with a percentage by volume of hydrogen in a range from 5% to 20%), a mixture of argon and ethane (with a percentage by volume of ethane in a range from 5% to 20%), a mixture of krypton and methane (with a percentage by volume of methane in a range from 5% to 20%), a mixture of krypton and hydrogen (with a percentage by volume of hydrogen in a range from 5% to 20%), and the mixture of krypton and ethane (with a percentage by volume of ethane in a range from 5% to 20%). The sputter gas surrounds the first target  132  and the second target  134 . The ion source  110  energizes the sputter gas to form reactive plasma for sputtering deposition of carbon and hydrogen. The sputter gas for the third target  136  can be selected from a group consisting of argon, and a mixture of argon and nitrogen (with a percentage by volume of nitrogen in a range from 3% to 15%). The sputter gas also surrounds the third target  136  to form reactive plasma for sputtering deposition on metal.  
      Multi-target co-sputtering process allows a multilayer structure to form by adjusting the deposition parameters, such as sputter pressure, sputter temperature, substrate bias, and the sputter gas ratio. Each diamond-like carbon layer of multilayer diamond-like carbon film  10  can be deposited by controlling the deposition parameters. After depositing n layers, the multilayer diamond-like carbon film  10  with gradient composition and multilayer structure is formed on the substrate  20 . The multilayer diamond-like carbon film  10  with gradient composition and multilayer structure has better adhesion, better corrosion resistance and wear resistance, higher hardness and smoothness and longer lifetime.  
      Additionally, before forming the multilayer diamond-like carbon film  10 , the intermediate layer  30  is formed on the substrate  20  in order to further enhance the adhesion. The intermediate layer  30  can be formed using a method such as direct current (DC) magnetron sputtering, alternating current (AC) magnetron sputtering, and radio frequency (RF) magnetron sputtering.  
      While certain embodiments have been described and exemplified above, various other embodiments will be apparent to those skilled in the art from the foregoing disclosure. The present invention is not limited to the particular embodiments described and exemplified but is capable of considerable variation and modification without departure from the scope of the appended claims.