Abstract:
A method for forming a porous coating with nanosize pores on a substrate includes the steps of (a) forming a suspension of sinterable particles in a carrier fluid; (b) maintaining the suspension by agitating the carrier fluid; (c) applying a first coating of the suspension to the substrate; and (d) sintering the sinterable particles to the substrate. A thin layer of this nanoporous coating is deposited onto a substrate having micropores. The substrate provides strength and structural support while the properties of the nano powder layer controls flow and filtration aspects of the device. This composite has sufficient strength for handling and use in industrial processes. Since the nano powder layer is thin, the pressure drop across the layer is substantially less than conventional thicker nano powder structures.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This patent application relates to, and claims priority to, U.S. Provisional Patent Application Ser. No. 60/848,423 titled “Sinter Bonded Porous Metallic Coatings,” that was filed on Sep. 29, 2006. The subject matter of that provisional patent application is incorporated by reference in its entirety herein. 
     
     U.S. GOVERNMENT RIGHTS 
       [0002]    N. A. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    This invention relates to a method to form a porous metallic coating on a substrate. More particularly, a suspension of nanosize particles in a carrier fluid is deposited on the substrate and heated to evaporate the carrier fluid while sintering the particles to the substrate. 
         [0005]    2. Description of the Related Art 
         [0006]    There are numerous applications requiring a porous open cell structure including filtration and gas or liquid flow control. These structures are typically formed by compacting metallic or ceramic particles to form a green compact and then sintering to form a coherent porous structure. Particle size, compaction force, sintering time and sintering temperature all influence the pore size and the structure strength. When the pore size is relatively large, such as microsize (having an average diameter of one micron (μm) or greater), the structure thickness relative to pore size is modest for sufficient strength to be handled and utilized in industrial applications. When the pore size is relatively small, such as nanosize (having an average diameter of less than one micron), the structure thickness is much greater than pore size for sufficient strength to be handled and utilized in industrial applications. As a result, the structure has high resistance to passing a gas or liquid through the long length, small diameter pores and there is a high pressure drop across the filter. Note that for this application, the diameter is to be measured along the longest axis passing from one side of a particle to the other side and also passing through the particle center. 
         [0007]    A number of patents disclose methods for depositing a porous coating on a substrate. U.S. Pat. No. 6,544,472 discloses a method for depositing a porous surface on an orthopedic implant. Metallic particles are suspended in a carrier fluid. The carrier fluid may contain water, gelatin (as a binder) and optionally glycerin (as a viscosity enhancer). Evaporation of the water results in the metallic particles being suspended in a gelatinous binder. Heating converts the gelatin to carbon and sinters the metallic particles to the substrate. 
         [0008]    U.S. Pat. No. 6,652,804 discloses a method for the manufacture of a thin openly porous metallic film. Metal particles with an average particle diameter between one micron and 50 microns are suspended in a carrier fluid having as a primary component an alcohol, such as ethanol or isopropanol, and a binder. This suspension is applied to a substrate and heated to evaporate the alcohol component. A green film of microparticles suspended in the binder is then removed from the substrate and heated to a temperature effective to decompose the binder and sinter the metallic particles. 
         [0009]    U.S. Pat. No. 6,702,622 discloses a porous structure formed by mechanical attrition of metal or ceramic particles to nanosize and then combining the nanosized particles with a binder, such as a mixture of polyethylene and paraffin wax to form a green part. The green part is then heated to a temperature effective to decompose the binder and sinter the particles. 
         [0010]    U.S. Pat. Nos. 6,544,472; 6,652,804; and 6,709,622 are all incorporated by reference in their entireties herein. 
         [0011]    In addition to the thickness constraint discussed above, the inclusion of a binder and optional viscosity enhancer may further increase the pressure drop across a structure. During sintering, the binder and viscosity enhancer decompose, typically to carbon. This carbonatious residue may in whole or in part block a significant number of pores necessitating a high pressure drop across the structure to support adequate flow. 
         [0012]    There remains, therefore, a need for a method to deposit a thin nano powder layer on a substrate that does not suffer from the disadvantages of the prior art. 
       BRIEF SUMMARY OF THE INVENTION 
       [0013]    In accordance with an embodiment of the invention, there is provided a method for forming a porous coating on a substrate. This method includes the steps of (a) forming a suspension of sinterable particles in a carrier fluid; (b) maintaining the suspension by agitating the carrier fluid; (c) applying a first coating of the suspension to the substrate; and (d) sintering the sinterable particles to the substrate. It is a feature of certain embodiments of the invention that a thin coating of a nano powder material may be deposited onto a substrate having micropores. A first advantage of this feature is that the microporous substrate provides strength and structure support and the nano powder layer may be quite thin. As a result, a nanoporous material which has sufficient strength for handling and industrial processes is provided. Since the nano powder layer is thin, the pressure drop across the layer is substantially less than conventional thicker nano powder structures. 
         [0014]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  illustrates in flow chart representation a method for depositing a porous coating in accordance with an embodiment of the invention. 
           [0016]      FIG. 2  schematically illustrates a system for depositing the porous coating formed in accordance with an embodiment of the invention. 
           [0017]      FIG. 3  illustrates a porous tube suitable for gas flow regulation or filtration having a porous coating in accordance with an embodiment of the invention. 
           [0018]      FIG. 4  is a scanning electron micrograph of a surface of the porous coating formed in accordance with an embodiment of the invention. 
           [0019]      FIG. 5  is a scanning electron micrograph of a cross section of the porous coating of  FIG. 4 . 
           [0020]      FIG. 6  graphically illustrates the effect of successive layers of the porous coating of  FIG. 4  on the gas flux. 
           [0021]      FIG. 7  illustrates a fuel cell component having a porous coating in accordance with an embodiment of the invention. 
           [0022]      FIG. 8  illustrates a frit for use in a liquid chromatography column having a porous coating in accordance with an embodiment of the invention. 
           [0023]      FIG. 9  illustrates a catalytic surface suitable for an industrial catalytic converter having a porous coating in accordance with an embodiment of the invention. 
           [0024]      FIG. 10  illustrates an adhesively bonded composite having a porous coating effective to enhance adhesion in accordance with an embodiment of the invention. 
       
    
    
       [0025]    Like reference numbers and designations in the various drawings indicated like elements. 
       DETAILED DESCRIPTION 
       [0026]    For purposes of this application, a “binder” is a carrier fluid component that remains after the carrier fluid is transformed from a liquid, such as by evaporation. A “viscosity enhancer” is a liquid that when added to the carrier fluid increases the viscosity of the carrier fluid beyond that of a primary component of the carrier fluid. A “suspension” is a mixture of a powder in a solvent. A “substrate” is a device or a part of a device to which the porous metallic coatings of the invention are applied. The substrate is typically porous, but may be solid in certain embodiments. A “nano powder coating” is the porous coating applied to the substrate from a powder having an average particle size of less than 10 microns. 
         [0027]    As illustrated in flowchart representation in  FIG. 1 , the sinterable particles used to form a porous coating in accordance with the invention are suspended  10  in a carrier fluid. The sinterable particles are typically nanosize and have an average maximum diameter sufficiently small to remain in solution in the carrier fluid in the presence of agitation without requiring an addition of a binder or viscosity enhancer. The sinterable particles preferably have an average maximum diameter of from 10 nanometers to 10 microns and more preferably have an average maximum diameter of from 10 nanometers to less than one micron. The sinterable particles are preferably metal or metal alloy powders but may also be other materials such as metal oxides and ceramics as long as such powders are capable of sinter bonding to each other and/or to a substrate. Preferred materials for the sinterable particles include nickel, cobalt, iron, copper, aluminum, palladium, titanium, platinum, silver, gold and their alloys and oxides. One particularly suitable alloy is Hastelloy C276 that has a nominal composition by weight of 15.5% chromium, 2.5% cobalt, 16.0% molybdenum, 3.7% tungsten, 15.0% iron, 1.0% manganese, 1.0% silicon, 0.03% carbon, 2.0% copper and the balance nickel. 
         [0028]    The sinterable particles may be a mixture of materials. For example, a platinum powder may be mixed with 316L stainless steel, zinc, silver and tin powders to promote better adhesion of the coating at lower temperatures. Lower temperatures better retain the nano structure during the sintering process. The mixed coatings may be deposited from suspension containing the mixture of powders and deposited simultaneously on to a substrate. Other benefits of applying a mixture of materials include mechanically alloying the coating, dilute and isolated particle distributions, enhanced bonding to the substrate at lower temperatures and controlled Thermal Coefficients of Expansion (TCE). Under the rule of mixtures, when 50% of component A and 50% of component B are combined and sintered, the coating would have a TCE that is the average of the respective TCE&#39;s of A and B. More than two components and other ratios of components may also be utilized and the TCE of the mixture calculated. 
         [0029]    The carrier fluid is a liquid that evaporates essentially completely without a residue remaining dispersed in the sinterable particles. As such, the carrier fluid is substantially free of binders and viscosity enhancers. “Substantially free” means there is insufficient binder to form a compact without sintering and is nominally less than 0.05%, by volume. Preferred carrier fluids are alcohols. A most preferred alcohol for the carrier fluid is isopropanol (also referred to as isopropyl alcohol). 
         [0030]    The suspension is formed in an inert atmosphere to prevent oxidation of the particles and because nanosized metallic particles are sometimes pyrophoric and may spontaneously ignite when exposed to air. The coating may be a mixture of different powders in which case these powders are first mixed in an inert atmosphere, such as argon. Once the powders are mixed, a carrier fluid is added to form the suspension. Nominally, equal volumes of carrier fluid and sinterable particles are utilized. However, other volume fractions may be used, dependant primarily on the method of deposition. While Brownian motion will cause the nanosized sinterable particles to remain in suspension for an extended period of time, agitation  12  is utilized to extend the period of suspension consistency. The agitation  12  may be by any effective means to maintain carrier fluid motion such as an impeller or ultrasonic vibration. 
         [0031]    A substrate is then coated  14  with the suspension by any suitable means such as spraying, rolling, dipping, use of a doctor blade, or other method by which a thin, uniform coating thickness of about five microns maybe deposited. As described below, sequence of coating and sintering may be repeated multiple times to achieve a desired total coating thickness. The substrate may be porous or non-porous and may have either a rough or a smooth surface finish. The substrate is formed from a material to which the sinterable particles may be sinter bonded. 
         [0032]    One preferred substrate is a porous metal having a thickness on the order of 0.1 inch and pores with an average diameter on the order of 5 μm. This substrate has sufficient strength to be handled and to withstand the rigors of an industrial process. At least one side of this substrate is coated with nanoporous particles by the method of the invention to a thickness effective to continuously coat the surface. This composite structure is effective for filtration and gas or liquid flow control on the nanoscale while having the strength and durability of a much coarser structure. 
         [0033]    One method to deposit porous coatings of the inventions utilizes the spray system  16  schematically illustrated in  FIG. 2 . A suspension  18  of sinterable particles in a carrier fluid is retained within a pressure cup  20 . An impeller  22  driven by a motor  24  or other means maintains the suspension  18  by agitation. Recirculating pump  26  draws the suspension  18  from the pressure cup  20  to a spray head  28  and returns nondeposited suspension back to pressure cup  20  in the direction generally indicated by arrows  30 . The system  16  is pressurized from an external high pressure source  32  such as air pressurized to 40 psi. A positive pressure of about 1 psi is maintained in pressure cup  20 . Depressing trigger  34  deposits a fine spray of suspension on a substrate (not shown). 
         [0034]    Referring back to  FIG. 1 , following coating  14 , the coated substrate is heated  36  for a time and temperature effective to evaporate the carrier fluid and sinter  36  the sinterable particles to the substrate. To prevent oxidation, sintering is typically in a neutral or reducing atmosphere or under a partial vacuum. While the sintering temperature is dependent on the composition of the substrate and sinterable particles, for iron alloy or nickel alloy components, a temperature from about 1,200° F. to about 1,800° F., and preferably from about 1,400° F. to about 1,600° F. for a time from about 45 minutes to 4 hours, and preferably from about 1 hour to 2 hours. 
         [0035]    Shrinkage during the sintering process may be detected if the coating step  14  deposits a suspension layer greater than about 10 microns. Preferably, the maximum coating thickness deposited during one coating cycle is on the order of five microns. If a coating thicker than 5-10 microns is desired, multiple coating cycles may be used by repeating  38  the coating and sintering steps. For smooth substrates, complete coverage can usually be achieved with a single coating and sintering cycle. When the substrate is rough and/or porous, multiple coating cycles are typically required to achieve complete coverage. When coating a Media Grade 2 porous substrate, typically three coating cycles are required to achieve complete coverage. For a Media Grade 1 substrate, two coating cycles are usually sufficient, while for a Media Grade greater than 2, several coating cycles may be required for complete coverage. A Media Grade 1 substrate is characterized by a nominal mean flow pore size of 1 μm and a Media Grade 2 substrate is characterized by a nominal mean flow pore size of 2 μm. Larger pore size substrates, such as Media Grade 40 or Media Grade 100 may also be coated with the coatings described herein. 
         [0036]    Once a coating of a desired thickness has been applied and sintered, either in one or multiple cycles, the coated surface may be finished  40  by secondary operations to cause an exterior portion of the coating to be mechanically deformed. Secondary operations include pressing, rolling, or burnishing to achieve a desired surface finish and/or finer pore size control. 
         [0037]    While the method of the invention deposits a nano power coating from a suspension having a carrier fluid that is substantially free of a binder, it is within the scope of the invention to deposit the nano powder coating and then apply a binder as a top coat over the applied coating prior to sintering. 
         [0038]    The invention described herein may be better understood by the examples that follow. 
       EXAMPLES 
     Example 1 
       [0039]    Filtration is generally performed using either cross flow or dead ended methods. In cross flow applications, only a portion of the filtrate is filtered in each pass while in dead ended applications, 100% of the fluid being filtered passes through the filter media. A process tube  42  illustrated in  FIG. 3  is useful for cross flow filtration and control of gas or liquid flow. The process tube  42  has a porous tubular substrate  44  with relatively large pores on the order of 5 μm. A porous coating  46  having a total coating thickness of about 25 microns and pores on the order of 50 nanometers (nm) in diameter covers the tubular substrate  44 . A process gas or liquid  48  flows into the process tube  42 . The filtered media  50  is sufficiently small to pass through the micropores of the porous coating  46  and exit through a wall of the process tube  42  while the waste stream  52  exits from an outlet side of the process tube. The process tube  42  depicted in  FIG. 3  may also be used for dead ended filtration by plugging exit end  53  of the tube, thereby forcing all of the fluid to pass through the tubular porous substrate  44  and the applied porous coating  46 . 
         [0040]    The process tube  42  was made with a tubular substrate formed from each one of 316L SS (stainless steel with a nominal composition by weight of 16-18 percent chromium, 10%-14% nickel, 2.0-3.0% molybdenum, less than 0.03% carbon and the balance iron, equally suitable is 316 SS, same composition without the restrictive limitation on carbon content), Inconel 625 (having a nominal composition by weight of 20% chromium, 3.5% niobium, and the balance nickel), and Hastelloy C276. The tubular substrate had pore sizes consistent with Media Grade 2. A slurry of Hastelloy C276 nanopowder and isopropyl alcohol was sprayed on the exterior wall of the tubular substrate to a thickness of between about 5-10 microns. The coating was sintered to the substrate by sintering at 1,475° F. for 60 minutes in a vacuum furnace. The process was repeated two additional times to achieve a total coating thickness of about 25 microns. 
         [0041]      FIG. 4  is a scanning electron micrograph of the nanoporous surface at a magnification of 40,000× illustrating the sintered nanoparticles and fine pores. The nanoparticles have an average diameter of about 100 nm and the nanopores have an average pore diameter of about 50 nm.  FIG. 5  is a scanning electron microscope at a magnification of 1,000× showing in cross-section the tubular substrate  44  and porous coating  46 . 
         [0042]    The performance of the process tube  42  was measured by determining the flux of nitrogen gas passing through the tube. The flux was measured at room temperature (nominally 22° C.) with a 3 psi pressure drop across the tube wall. The flux units are SLM/in 2  where SLM is standard liters per minute and in 2  is square inches. Table 1 and  FIG. 6  illustrate the flux values for the process tube with from 0 to 3 nano powder coating layers. The average flux on a Media Grade 2 substrate with a total coating thickness of about 25 microns and average pore size of about 50 nm was 6.69 SLM/in 2 . This compares extremely favorably with a conventional Media Grade 0.5 (nominal mean flow pore size of 0.5 μm) process tube that has a flux of 1.87 SLM/in 2  at 3 psi. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Flux at 3 psi (SLM/in 2 ) 
               
             
          
           
               
                 Coating 
                 Sample Number 
                   
               
             
          
           
               
                 Layers 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 Average 
               
               
                   
               
             
          
           
               
                 0 
                 15.23 
                 15.48 
                 17.09 
                 17.28 
                 17.67 
                 15.57 
                 16.39 
               
               
                 1 
                 9.34 
                 8.84 
                 14.38 
                 11.70 
                 10.17 
                 11.86 
                 11.05 
               
               
                 2 
                   
                   
                 9.07 
                 8.25 
                 8.06 
                 7.93 
                 8.33 
               
               
                 3 
                   
                   
                   
                   
                 6.81 
                 6.56 
                 6.69 
               
               
                   
               
             
          
         
       
     
       Example 2 
       [0043]      FIG. 7  illustrates in cross-sectional representation a membrane  54  useful in the production of hydrogen for fuel cell applications. A microporous substrate  56  is coated with a nanocoating  58  of palladium or platinum or their alloys. The substrate pore size is on the order of from 1 to 40 microns and more preferably from 1 to 10 microns. The coating include pores with diameters of from about 50 nm to 10 microns. Subsequent layers may be deposited onto the nanocoating such as by plating or layered deposition to generate an active surface for hydrogen generation. 
       Example 3 
       [0044]      FIG. 8  illustrates a particle retention barrier  60  effective to stop aluminum oxide beads from passing through a liquid chromatography column. The particle retention barrier  60  includes a microporous frit  62  that is typically formed from stainless steel, Hastelloy or titanium powders. Frit  62  has a diameter on the order of 0.082 inch (Media Grade 0.5 to 2). A nano powder layer  64 , usually of the same composition as the frit, coats one side of the frit  62 . The barrier  60  is formed by micropipetting or spraying a suspension of nano powder onto the surface and then vacuum sintering. 
       Example 4 
       [0045]      FIG. 9  illustrates a component  66  for improved catalytic performance. A nano powder layer  68  of platinum or other catalytic material coats a surface of a metal or ceramic support  70  for use in a catalytic converter, for industrial applications and/or automotive uses. 
       Example 5 
       [0046]      FIG. 10  illustrates a nano powder coating  72  applied to a surface of a substrate  74  to increase the surface area and provide locking pores for a polymer adhesive  76  thereby dramatically increasing the strength of the adhesive bond. 
       Example 6 
       [0047]    An example of creating a dilute distribution of isolated particles in a coating would be to create a 1:100 mixture of platinum particles in a stainless steel powder and then depositing this mixture onto a stainless steel substrate and sinter bonding. In this example, which would apply to a catalyst coating for fuel cell applications, one ends up with isolated platinum particles in a stainless steel surface. Here the stainless steel powder in the coating becomes indistinguishable from the substrate and the dilute platinum particles from the original coating are distributed over the surface of the substrate. 
       Example 7 
       [0048]    An example of bonding stainless steel to a substrate at lower temperatures would be to mix a lower temperature melting powder like tin with stainless steel 316 L SS powder that has a much higher melting temperature, coating the substrate with this mixture, and then follow up with sintering. The lower temperature component (tin) would diffuse at much lower temperatures than the stainless steel thus causing sintering and bonding at lower temperatures. 
       Example 8 
       [0049]    A sterilizing filter, useful to remove microbes such as bacteria and viruses from a liquid or gas medium requires a pore size of under 0.2 micron. This filter may be made by the process described herein. 
       Example 9 
       [0050]    A high efficiency filter for removing impurities from a gas or liquid medium utilizes depth filtration processes. An example of this would be to apply a relatively thick coating on the order of 200 microns on to a supporting substrate that utilizes the depth filtration technique to capture the very fine particulate/microbes for this kind of filtration. To build up this thickness, several thinner layers would be applied and sintered as described in the application to minimize shrinkage cracks during the sintering process. 
         [0051]    One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.