Abstract:
A component bonding preparation method employs plasma to prepare a component for bonding. A first plasma is used to hydroxylate a component surface. A second plasma is used to silanize the component surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This is a related application to U.S. patent application Ser. No. 12/255,203, filed on Oct. 21, 2008 and U.S. patent application Ser. No. 12/255,177, filed on Oct. 21, 2008. 
     
    
     STATEMENT OF GOVERNMENT INTEREST 
       [0002]    The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. N00019-02-C-3003 awarded by the United States Navy. 
     
    
     BACKGROUND 
       [0003]    The present invention relates to a method of component bonding preparation. More particularly, the present invention relates to a method of component bonding preparation where pre-bonding preparations are performed using plasmas. 
         [0004]    A typical process for preparing a component surface for bonding may include several steps. First, the component surface may be abraded with water and grit or sanded by hand. Abrading and sanding a delicate component, such as a fan inlet shroud fairing, can result in damage to the component. Components may also have geometries that are not conducive to hand sanding. An example of such a component is a fan inlet shroud fairing having a U-shaped bend at a leading edge and two trailing edges that extend from the bend. The trailing edges are rigid, yet fragile and may be separated by less than one inch (2.54 cm). The trailing edges may also contain fragile embedded electrical components. Hand sanding the interior surfaces of the trailing edges is extremely difficult to perform due to the geometry involved and presents a high risk of damage to fragile elements. Abrading and sanding by hand are also labor intensive processes. Second, once abraded, the component surface may be rinsed with water to remove debris from the abrading or sanding and dried. The water used for rinsing must be removed before continuing the bonding preparation. Removal is typically performed by placing the component in an oven for several hours or even up to one full day. Once dried, the component then needs to cool to room temperature. These drying and cooling steps take a significant amount of time. Third, a solvent may be applied to the component surface to further clean the surface, followed by a silane primer. The silane primer is typically applied by wiping or brushing the silane primer onto the component surface. Again, some components, such as fan inlet shroud fairings, have geometries that are not conducive to the application of a silane primer by a brush or cloth. Once the silane primer has been applied, the primed component is cured for a time in humidity conditions greater than fifty percent humidity. The component surface may be bonded following the curing of the primed component. Due to the instability of the silane primer when wiped or brushed onto the component surface, bonding typically must take place within eight hours of curing. 
         [0005]    Due to the difficulties that unique geometries and fragile components present to the typical bonding preparation process, an improved process for bonding preparation is desired. Additionally, a process that extends the shelf life of the applied primer past eight hours is also desired. 
       SUMMARY 
       [0006]    In a method according to the present invention, a component is placed inside a processing vessel. A surface of the component is hydroxylated with a plasma by introducing a hydroxylating agent into the processing vessel and applying electromagnetic radiation to excite the hydroxylating agent into a first plasma, which adds hydroxyl groups to the component surface. The component surface is silanized with a plasma by introducing a silane into the processing vessel and applying electromagnetic radiation to excite the silane into a second plasma, which is hydrolyzed by the hydroxyl groups on the component surface to bond a silane layer to the component surface. 
         [0007]    In a component bonding method, the method includes cleaning a component surface, plasma hydroxylating the component surface, plasma silanizing the component surface and bonding the plasma silanized component surface to a substrate. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a view showing the arrangement of a plasma processing apparatus. 
           [0009]      FIG. 2  is a schematic illustration of one embodiment of a component bonding preparation method using plasma. 
           [0010]      FIG. 3  is a schematic illustration of an additional embodiment of a component bonding preparation method using plasma. 
           [0011]      FIG. 4  is a bar graph illustrating relative bond strengths of bonds prepared with conventional silanes and plasma silanes. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    Plasma processing has been used for a variety of purposes. Plasmas have previously been used to clean articles and prepare surfaces for bonding. The present invention further advances plasma capabilities by providing a method of silanizing component surfaces using plasma and extending the stability and shelf life of silanized component surfaces. 
         [0013]      FIG. 1  illustrates one embodiment of a plasma processing apparatus  10  capable of preparing a component for bonding according to the present invention. Plasma processing apparatus  10  includes processing vessel  12 , power unit  14 , inlet  16  and exhaust line  18 . Processing vessel  12  includes an interior configured to accommodate component (target object) C, such as a fan inlet shroud fairing, and to process component C with a plasma. Power unit  14  supplies electromagnetic radiation R into processing vessel  12  and generates plasma P by the application of electromagnetic radiation R to a gas or silane in processing vessel  12 . Inlet  16  allows gases, vapors and silanes to enter processing vessel  12 . When electromagnetic radiation R is applied to the introduced gases or silanes, plasmas are formed. Exhaust line  18  allows for vacuum evacuation of processing vessel  12 . Plasma processing apparatus  10  optionally includes shelf  20 , component support  22  and silane deposition monitoring system  24 . Component support  22  and silane deposition monitoring system  24  are described in further detail in U.S. patent application Ser. No. 12/255,203, filed on Oct. 21, 2008 and U.S. patent application Ser. No. 12/255,177, filed on Oct. 21, 2008, respectively. The operation of plasma processing apparatuses to generate plasmas are known in the art. However, the present invention provides for plasma silanization of component C while extending the stability and shelf life of the resulting silanized component prior to bonding. 
         [0014]      FIG. 2  illustrates one embodiment of component bonding preparation method  30 . Component bonding preparation method  30  includes placing component C in plasma processing apparatus  10  (step  32 ), cleaning a surface of component C with a plasma (step  36 ), hydroxylating the surface of component C with a plasma (step  38 ), silanizing the surface of component C with a plasma (step  40 ) and bonding the plasma silanized surface of component C to a substrate (step  44 ). Steps  36 ,  38  and  40  are all capable of being performed within processing vessel  12  of plasma processing apparatus  10 . 
         [0015]    Component bonding preparation method  30  will work on surfaces made of various materials. Generally, any surface with organic functional groups can be prepared according to component bonding preparation method  30 . Component C surfaces capable of bonding preparation include polymers, metals and organic matrix composites. Suitable polymers include polyamides, polyimides, thermoset materials and combinations thereof. Suitable metals include titanium, beryllium, magnesium, magnesium alloys and combinations thereof. Additionally, a metal having alkaline functional groups at its surface is also suitable. “Organic matrix composites” refers to composite materials having one or more functional groups containing carbon atoms. Suitable organic matrix composites include carbon composites, thermoplastic composites and fiber-reinforced plastics. 
         [0016]    Beginning with step  32 , component C is placed inside processing vessel  12  of plasma processing apparatus  10 . Component C is typically placed within processing vessel  12  on shelf  20  or component support  22 . Plasmas are used during cleaning step  36 , hydroxylating step  38  and silanizing step  40 . During these steps, plasma will interact with all exposed surfaces of component C. 
         [0017]    In step  36 , the surface of component C is cleaned using a plasma. Cleaning the component surface refers to removing contaminants and weak boundary layers from the surface of component C. For components having a surface sufficiently clean or already cleaned by other means, cleaning step  36  becomes optional. Cleaning step  36  includes drawing a vacuum in processing vessel  12 , introducing gas into processing vessel  12 , applying electromagnetic radiation within processing vessel  12  and evacuating processing vessel  12 . A vacuum is drawn on processing vessel  12  via exhaust line  18 , which is connected to a vacuum pump (not shown) configured to create a vacuum in processing vessel  12 . Once the vacuum is applied, gas is introduced into processing vessel  12  via inlet  16 . Suitable gases for cleaning step  36  include argon, oxygen, tetrafluoromethane, hydrogen and combinations thereof. Once the gas is introduced and the pressure within processing vessel  12  stabilizes, power unit  14  delivers electromagnetic radiation R to the interior of processing vessel  12 . The application of electromagnetic radiation R to the gas introduced into processing vessel  12  results in excitation of the gas into a plasma state (plasma P). Power unit  14  delivers electromagnetic radiation R to processing vessel  12  to maintain plasma P for a predetermined time. During this time plasma P removes contaminants and weak boundary layers from the surface of component C. Once the predetermined time for electromagnetic radiation R delivery has expired, processing gas present in processing vessel  12  is evacuated via exhaust line  18 . 
         [0018]    In step  38 , the surface of component C is hydroxylated. Hydroxylation refers to the addition of hydroxyl groups (—OH) onto the surface of component C. Hydroxylating step  38  includes introducing a hydroxylating agent (gas or vapor) into processing vessel  12 , applying electromagnetic radiation within processing vessel  12  and evacuating processing vessel  12 . During hydroxylating step  38 , processing vessel  12  remains under vacuum conditions. As with cleaning step  36 , the hydroxylating agent is introduced into processing vessel  12  via inlet  16  during hydroxylating step  38 . Suitable hydroxylating agents for hydroxylating step  38  include water vapor, hydrogen peroxide, methanol and combinations thereof. When hydrogen peroxide or methanol are used in hydroxylating step  38  they are introduced to processing vessel  12  as vapors. Once the hydroxylating agent is introduced and pressure within processing vessel  12  stabilizes, power unit  14  delivers electromagnetic radiation R to the interior of processing vessel  12 . The application of electromagnetic radiation R to the hydroxylating agent introduced into processing vessel  12  results in excitation of the hydroxylating agent into a plasma state (plasma P). Power unit  14  delivers electromagnetic radiation R to processing vessel  12  to maintain plasma P for a predetermined time. During this time plasma P introduces hydroxyl groups onto the surface of component C. Once the predetermined time for electromagnetic radiation R delivery has expired, processing gas or vapor present in processing vessel  12  is evacuated via exhaust line  18 . 
         [0019]    In step  40 , the surface of component C is silanized using a plasma. Silanization refers to the addition of a silane layer through self-assembly to the surface of component C. Silanes are a class of chemical compounds containing silicon and hydrogen. Silanes are commonly used to enhance adhesion between organic resins and inorganic substrates. Silanes generally improve the strength and integrity of a bond between components. A silane is often applied to bonding surfaces of aircraft components, such as fan inlet shroud fairings, prior to bonding the component to a frame or other component. 
         [0020]    Different types of silanes are used to improve the bonding properties of components, whether they are for aircraft or other commercial uses. The general formula of silanes used to enhance bonding is R n SiX (4-n) . These silanes typically contain a hydrolysable group (X), such as chlorine (Cl), a methoxy group (—OCH 3 ) or an ethoxy group (—OCH 2 CH 3 ), and a non-hydrolysable group (R). The non-hydrolysable group (R) is designed to provide reactive surfaces to the adhesive. The type of silane chosen for bonding preparation depends on the adhesive used for bonding. For example, vinyl silanes are typically chosen when the bonding adhesive is a silicone because vinyl silanes are compatible with the chemistry of the silicone adhesive. Suitable vinyl silanes include vinyltrimethylsilane, vinyltrimethylethoxysilane, vinyldimethylethoxysilane and vinyltrimethoxypropylsilane. Similarly, amino silanes are typically chosen when the bonding adhesive is bismaleimide because the amino silanes are chemically compatible with bismaleimide. Suitable amino silanes include 3-aminopropylethoxy silane, 3-aminopropyltriethoxysiland and 3-aminopropyltrimethoxysilane. Other silanes can also be used for different adhesives such as epoxies and urethanes. 
         [0021]    Silanizing step  40  includes introducing a silane or silane mixture into processing vessel  12 , applying electromagnetic radiation within processing vessel  12  and evacuating processing vessel  12 . During silanizing step  40 , processing vessel  12  remains under vacuum conditions. A silane or silane mixture is introduced into processing vessel  12  via inlet  16  during silanizing step  40 . Suitable silanes for silanizing step  40  include amino silanes and vinyl silanes, as described above, and other silanes depending upon the adhesive that will be used for bonding component C to a substrate following silanization. Once the silane or silane mixture is introduced and pressure within processing vessel  12  stabilizes, power unit  14  delivers electromagnetic radiation R to the interior of processing vessel  12 . The application of electromagnetic radiation R to the silane or silane mixture introduced into processing vessel  12  results in excitation of the silane or silane mixture into a plasma state (plasma P). Power unit  14  delivers electromagnetic radiation R to processing vessel  12  to maintain plasma P for a predetermined time, during which plasma P deposits a layer of silane molecules onto the surface of component C. Once the predetermined time for electromagnetic radiation R delivery has expired, residual silane present in processing vessel  12  is evacuated via exhaust line  18 . 
         [0022]    Silane deposition or grafting in silanizing step  40  is promoted by the presence of the hydroxyl groups on the surface of component C. Following completion of hydroxylating step  38 , the surface of component C contains hydroxyl groups. These hydroxyl groups attack and displace the hydrolysable group (X) of the silane plasma to form covalent silicon-oxygen (—Si—O—Si—) bonds on the surface of component C. Silicon-oxygen bonds are sufficiently stable for molecules of the silane to self-assemble into a layer on the surface of component C. Following silanization, a layer of silane is attached to the exposed surfaces of component C. The silane layer is arranged such that the covalent silicon-oxygen bonds are proximal with the surface of component C and the non-hydrolysable groups are distal to the surface of component C and free to interact with adhesive later-applied during bonding step  44 . 
         [0023]    The amount of electromagnetic radiation (power density) applied to processing vessel  12  during silanizing step  40  is significantly lower than the amount applied in cleaning step  36  and hydroxylating step  38 . The power density applied during silanizing step  40  is typically about ten percent to about thirty percent of the power density applied during cleaning step  36  and hydroxylating step  38 . In exemplary embodiments, the power density applied during silanizing step  40  is between about fifteen percent and twenty-five percent of the power density applied during cleaning step  36  and hydroxylating step  38 . Under the influence of plasma, covalent bonds (including silicon-oxygen bonds) are subject to fragmentation. Particularly active groups, vinyl groups, for example, are especially vulnerable to fragmentation. The combination of hydroxylating step  38  and the reduced power density applied during silanizing step  40  allows the silane to covalently bond with the surface of component C without destroying the other functionalities of the silane (e.g., the non-hydrolysable groups). The power density applied to processing vessel  12  during plasma silanizing step  40  is just sufficient to promote hydrolysis of the silane to the hydroxylated surface of component C while preventing or reducing undesired fragmentation. The presence of the hydroxyl groups on the surfaces of component C provides a preferred reaction path for silane hydrolysis and creates a generally ordered and unfragmented layer of silane molecules covalently bonded to the surface of component C. 
         [0024]    Plasma silanizing step  40  can be performed with any reactive silane having hydrolysable groups such as, but not limited to, methoxy, dimethoxy, trimethoxy, ethoxy, diethoxy, triethoxy, chloro-, dichloro- and trichloro-groups. The non-hydrolysable groups of the silane include, but are not limited to, vinyl, amine, alcohol, glycidyl, thio, butenyl, allyl, alkyl and perfluoroalkyl. The thickness of the generally ordered and unfragmented silane layer incorporated on the surfaces of component C will vary depending on the non-hydrolysable group(s) of the silane used in plasma silanizing step  40 . However, most silane layers prepared according to the present invention will typically have a thickness less than about 100 nm. 
         [0025]    Following silanizing step  40 , plasma silanized component C is removed from processing vessel  12  and plasma processing apparatus  10 . Silanized component C is then stored until needed for bonding. In an exemplary embodiment, plasma silanized component C is stable for about thirty days. Thirty days after the completion of plasma silanizing step  40 , the silane layer incorporated onto the exposed surfaces of component C remains suitable for enhancing the bonding of component C to a substrate. In step  44 , the surface of component C is bonded to a substrate. An adhesive is applied to either the substrate or the surface of component C and the substrate and the surface are positioned together as desired for bonding. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Silane Layer 
                 Relative Bond 
                   
               
               
                 Age (Day) 
                 Shear Strength (%) 
                 % RSD 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 100 
                 8 
               
               
                 2 
                 94 
                 11 
               
               
                 3 
                 95 
                 4 
               
               
                 7 
                 94 
                 15 
               
               
                 9 
                 94 
                 5 
               
               
                 15 
                 95 
                 6 
               
               
                 22 
                 104 
                 7 
               
               
                 30 
                 106 
                 5 
               
               
                   
               
             
          
         
       
     
         [0026]    Table 1 indicates the relative shear strengths of bonds prepared on silane layers of different age. The shear strength of a bond prepared on Day 1 was used as a baseline value (100%) for comparison with the shear strengths of bonds prepared on aged silane layers. Table 1 indicates that the shear strengths of bonds prepared on silane layers aged up to about thirty days are nearly equivalent or exceed the shear strength of a bond prepared on Day 1. Thus, after about thirty days, the silane layer formed in plasma silanizing step  40  remains suitable for facilitating strong component bonding. 
         [0027]      FIG. 3  illustrates component bonding preparation method  30 A. Component bonding preparation method  30 A is component bonding preparation method  30  of  FIG. 2  with added optional steps. Component bonding preparation method  30 A also includes activating the component surface (step  34 ) and purging processing vessel  12  (step  42 ). 
         [0028]    In step  34 , the surface of component C is activated with oxygen. Activation refers to the addition of oxygen radicals to the surface of component C. Oxygen radicals are added to the surface of component C to make the surface more reactive. Activation step  34  follows the same process as cleaning step  32 . However, in activation step  34 , suitable gases include oxygen alone or mixtures of oxygen and argon or tetrafluoromethane. When oxygen or mixtures including oxygen are used in cleaning step  32 , the addition of oxygen radicals to the surface of component C may occur contemporaneously with the removal of contaminants and weak boundary layers. 
         [0029]    In step  42 , processing vessel  12  is purged of plasma by-products prior to the removal of component C. A high flow rate of gas is introduced into processing vessel  12  and maintained without the addition of electromagnetic radiation. Argon is a suitable gas for purging processing vessel  12 . Following the purge, processing vessel  12  is vented to the atmosphere and component C is removed from plasma processing apparatus  10 . 
       Example 
       [0030]    An example of one embodiment of the method of the present invention is described herein. Composite plaques were prepared for bonding using a plasma processing apparatus. The plasma processing apparatus included a processing vessel having a volume of approximately 1900 liters. 
         [0031]    Cleaning. The composite plaques were placed inside the processing vessel. A vacuum was drawn on the processing vessel to generate a base pressure of 30 millitorr (mT). Once the base pressure was reached, oxygen was introduced into the processing vessel at a flow rate of 1750 standard cubic centimeters per minute (sccm), providing a processing vessel pressure of 135 mT. The pressure inside the processing vessel was allowed to stabilize for approximately ten seconds. After stabilization, 2000 watts (W) of 13.56 MHz radio frequency (RF) power was applied providing a power density of approximately 0.10 W/cm 2  to excite the oxygen into a plasma state. The plasma was maintained for five minutes. After five minutes, oxygen in the processing vessel was evacuated and the processing vessel was allowed to stabilize at the base pressure of 30 mT. 
         [0032]    Hydroxylation. Argon was introduced into the processing vessel at a rate of 150 sccm. Ten seconds after the introduction of the argon, methanol (as methanol vapor) was added to the processing vessel at a rate of 45 mL/hr. The pressure inside the processing vessel was allowed to stabilize for approximately ten seconds. After stabilization, 3000 W of RF power was applied to excite the gas and vapor into a plasma state. The plasma was maintained for two minutes. The gas and vapor in the processing vessel was then evacuated and the processing vessel was allowed to stabilize at the base pressure of 30 mT. 
         [0033]    Silanization. Argon was introduced to the processing vessel at a rate of 150 sccm. After pressure stabilization, vinyltrimethoxypropylsilane (VTMS) was introduced to the processing vessel at a rate of 45 mL/hr. After stabilization, 500 W of RF power was applied to the process gas mixture to establish a plasma. The plasma was maintained for two minutes. The argon and silane in the processing vessel was then evacuated and the processing vessel was allowed to stabilize at the base pressure of 30 mT. 
         [0034]    Vessel purge. The processing vessel was purged of residual plasma by-products. A high flow rate of argon (1000 sccm) was introduced to the processing vessel and maintained without RF power for two minutes. The processing vessel was then vented to the atmosphere and the composite plaques were removed. 
         [0035]      FIG. 4  is a bar graph illustrating relative bond strengths of bonds made according to the present invention compared to a conventionally prepared bond that does not use a plasma silanized surface. Bar  100  on the far right indicates the average shear strength of a conventional bond preparation using SP-270, a silicone primer available from NuSil Technology (Carpinteria, Calif.). The average shear strength of the conventional bond is used as a baseline (100%) for comparison with the average shear strengths of bonds prepared according to the present invention. Bar  200  on the far left indicates the relative average shear strength of a bond prepared following vinyl silanization using a plasma (described in the Example above). The average shear strength of a bond prepared using plasma silanization with a vinyl silane (bar  200 ) is about 30% greater than the average shear strength of a bond prepared using the conventional (non-plasma) process (bar  100 ). Bars  220  and  240  indicate the relative average shear strengths of bonds prepared using butenyl and allyl plasma silanes, respectively. The average shear strengths of bonds prepared using plasma silanization with butenyl and allyl silanes (bars  220  and  240 , respectively) are more than 20% greater than the average shear strength of a bond prepared using the conventional process (bar  100 ). 
         [0036]    In summary, the present invention provides methods for component bonding preparation. The methods allow for the preparation of a component for bonding (cleaning and silanization) within a plasma processing apparatus. Methods of the present invention reduce the time required for preparation of the component prior to bonding when compared to other methods. The methods also allow for the preparation of components having geometries that present challenges for conventional cleaning and silane application. Additionally, the methods provide for extended stability and shelf life for silanized components. 
         [0037]    While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims