Abstract:
Process for depositing an amorphous carbon layer on substrates, such as heat exchangers, and the coated substrates so formed, such as an aluminum fined heat exchanger.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to surface heat sinks (heat exchangers) having a plurality of fins that effectively increases the surface area thereby enhancing the heat transfer to the air.  
         BACKGROUND OF THE INVENTION  
         [0002]    The heat dissipated by convection from the fins on heat exchanger is strongly limited by the thermal barrier of any oxide layer formed on the surface of the fins. To overcome this limitation, it was suggested that the roughness of the surface of the fins be increased and/or increase the number of fins or length of the fins thereby enhancing the heat transfer of the air by turbulating the air. One problem with turbulating the air is that the pressure drop increase greatly outweighs the increase in heat transfer due to the laws of physics, and hence the limiting factor is fan power and noise under forced convection applications. Under natural convection applications turbulating the airflow may cause the airflow to stall.  
         OBJECTS OF THE INVENTION  
         [0003]    It is an object of the invention to provide a surface area coated with a thin amorphous carbon layer with the properties approaching those of diamonds.  
           [0004]    It is another object of the invention to provide an extended surface heat sink with a plurality of aluminum fins coated with a thin amorphous carbon layer with the properties approaching those of diamonds.  
           [0005]    It is another object of the invention to provide a surface heat sink with a thin amorphous carbon layer coating to be used in industrial applications such as computer chip cooling, electronic cabinet cooling, car radiators, air conditioning condensers, aircraft heat exchangers and the like.  
           [0006]    It is another object of the invention to provide a process for depositing a thin amorphous carbon layer (DLC) coatings on a wide range of substrates. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    The sole drawing is a schema of a plasma enhanced chemical vapour deposition system (PECVD) for use in applying a DLC coating on substrates. 
     
    
     SUMMARY OF THE INVENTION  
       [0008]    The invention in one aspect, relates to a heat exchanger comprising a heat sink having a portion of its surface a thin layer of an amorphous carbon having a hardness of at least 2000 Kg/mm 2 , a specific resistivity of at least 10 8  ohm cm, and a dielectric strength of at least 10 6  V/cm. Preferably the thin amorphous carbon layer has properties approaching those of diamonds such as a hardness between about 2000 to 9000 Kg/mm 2 , a specific resistivity between 10 8  to 10 13  ohm cm, and dielectric strength more than 10 6  V/cm. Another preferable property of DLC is a low friction of 0.02 in vacuum and less than 0.1 in air. The thickness of the DLC layer is preferably in the range of 1 to 5 μm. The DLC layer can be homogeneously deposited and adhere well to metals and semiconductors which form stable carbides. Virtually there is no adhesion on gold, copper, iron but intermediate layers (nickel or some other or material) can be used to solve the problem. DLC coatings adheres also on ceramic, glass, poly-carbonate, polyamides (nylon) and on all materials forming carbides (i.e. plastic) that can withstand 150° C. Preferably DLC coating can have a roughness, as coated, of 0.05 μm Ra. Basically roughness is the copy of that of the base material coated. Prior to coating, parts should be cleaned either ultrasonically or by immersion cleaning. Depends on the application, solvent or aqueous cleaner can be used.  
         [0009]    Preferably, the heat exchanger having extending fins, such as aluminum, copper, or steel with the more preferable being aluminum, and such fins having a DLC film deposited thereon. The best way to practice the invention is to remove the oxide layer on the fins and then apply the DLC coating on the surface of the fins.  
         [0010]    The heat transfer performance of any cooled extended surface heat exchanger that uses aluminum fins can be used for car radiators/air conditioning condensers, truck radiators, aircraft heat exchangers, etc. Since most of these types of heat exchanger use turbulators on the fins, a DLC film on the fins may replace the turbulators. This would have an effect of reducing noise as the increase in air pressure drop due to the DLC film being effectively zero. Heat sinks that rely on conduction only of heat to the aluminum fins, i.e. small chip or electronic device heat sinks, would also benefit from this invention, since this is based on heat exchanger design with extended surface heat exchangers in which the heat transfer from the fins to the air being the poorest heat transfer region or greatest resistance to heat transfer (unless working with very high viscosity fluids in pipes or air in pipes).  
         [0011]    The novel amorphous carbon (DLC) layer or coating is an PECVD coating and has the following properties;  
         [0012]    Adhesion. DLC generally adheres well to metals and semiconductors which form stable carbides. Metals including aluminum, titanium and molybdenum have been successfully coated as have the semiconductors, silicon and germanium. There is virtually no adhesion to copper, gold or other metal which do not form carbides, but intermediate layers may be introduced, such as aluminum, zinc, silicon, germanium and other materials that form carbides. Good adhering DLC films have also been deposited on silica, various glasses, titanium nitride and alumina. The bonding strengths of DLC to various plastics are similar to those of films chemically bonded by carbide formation. However, adhesion to plastic must be due to a different mechanism. Thicker DLC films (more than 6 μm) may have adhesion problem because of the high compressive stress of about 10 9  Pa. For example, films thicker than 2 μm on germanium may cause spalling problems while films on aluminum have a 50 μm thickness or larger without any problems because of the plastic flow in the substrate.  
         [0013]    Hardness, friction and wear. DLC coating is a hard material and has a high cohesive energy, short bond length and a high degree of covalent bonding. DLC hardness is comparable to the conventional PVD and CVD typical materials such as TiN, SiC and Boron Carbide. Hardness can be up to 9000 Kg/mm 2  or higher. Coefficient of sliding friction of DLC against a steel ball is in the range of 0.005-0.02 under vacuum condition rising to 0.2 under 100% humidity. Dry nitrogen atmosphere is better in friction than dry oxygen. DLC coatings normally shows a high resistance to wear. The typical properties of the DLC layer, are hardness, low friction, generally chemical inertness, hydrophobic and the amorphous structure contribute to amplify the wear resistance of a hard material.  
         [0014]    Electrical properties. DLC is an insulator having a specific resistivity between 10 7  and 10 13  ohm cm. The value decrease when temperature increase. DLC has a high dielectric strength that is generally in the range of more than 10 6  V/cm.  
         [0015]    Optical properties. A DLC film 1 μm thick on glass appears dark brown when viewed in transmission. The color comes from a high absorption at the blue end of the spectrum and a low one in the red.  
         [0016]    DLC coatings can be used on heat sinks but this use is limited to enhancement of solid/solid heat transfer, that is, transfer between the base of the heat sink and en electronic component. Preferably, the oxide layer of an aluminum fin can be replaced with a highly thermally conductive layer (DLC) so as to reduce the overall resistance to heat transfer, that is the aluminum fins (sheets) will be able to give up heat to the air more effectively.  
         [0017]    Other applications for the DLC coatings are as follows:  
         [0018]    Textile: reeds, shuttles, healds, needles for mechanical looms, thread guides (ceramic, chromium plated, steel) to improve friction, and for chromium replacement in all the textile operations;  
         [0019]    Chemical: small pump pistons, rings, gear pumps parts, bearings&#39; 
         [0020]    Plastic and glass: moulding dies;  
         [0021]    Automotive: clutch pistons, clutch rings, parts for chromium replacement;  
         [0022]    Valves: ball valves for dry lubrication on soft seated valves;  
         [0023]    Surgical instruments, manufacturing machinery and window/glass coatings.  
         [0024]    The invention also relates to a process for depositing a thin amorphous carbon layer onto a substrate wherein said layer having a hardness of at least 2000 Kg/mm 2 , a specific resistivity of at least 10 8  ohm cm, and a dielectric strength of at least 10 6  V/cm; comprising the steps of preparing a plasma vapour deposition chamber; pumping into the chamber a carbon containing gas at a pressure range between about 10 −2  and 10 −5  mbar (preferably between 10 −3  and 10 −4  mbar); supplying a voltage (RF) of at least 1 kV to self sustain a glow discharge as the gas is ignited; depositing an amorphous carbon from the gas onto a substrate in the chamber and removing the coated substrate from the chamber.  
       DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    The sole drawing is coating system for DLC coatings. Specifically, the coating system for DLC thin layer deposition is a Plasma Enhanced Chemical Vapour Deposition (PECVD) that involve the dissociation of hydrocarbon gases (or extremely volatile hydrocarbon solvent). The drawing shows a general schema of the PECVD. The system is composed by a vacuum chamber  2  with its vacuum pumping system  4 , a multiple gas manifold  6  and a RF power generator  8 . When a gas is introduced in the preferred pressure range 10 −3 -10 −4  mbar and a RF voltage of one to several thousands is applied, a self sustained glow discharge is ignited. With a DC supply, the smaller electrode is normally connected to the negative terminal  10 . Between this electrode  10  and the plasma  12 , a dark space forms, across which there is the main potential drop, the plasma assuming a potential close to that of the anode. The negative electrode is therefor bombarded with high energy ions. A DC supply generally cannot be used if an insulating layer forms on the cathode (as in the case of DLC). With RF the smaller electrode charges negatively because the different mobility of ions and electrons. For a net zero current to flow during the cycle, the smaller electrode becomes negatively charged on average during cycle and is again bombarded by energetic ions. With a carrier gas containing carbon, hydrogen or some halogens is ionized, solid films are deposited on the electrode and this is the PECVD process. The growing films are bombarded with ions, probably giving rise to the metastable, mostly amorphous structures of PECVD layers. The layers will be dense and hard because weakly bonded atoms will be resputtered. With this technique, hard amorphous hydrogenated carbon films are obtained from hydrocarbon gases. Several process parameters are to be considered: type of gas, gas pressure, gas mixture ratio, substrate temperature, electrode and substrate geometry and electric field frequency. The most important parameters to control are the self bias potential, gas pressure and gas mixture ratio. The preferred procedure for producing hard DLC layers consist of first introducing argon at a pressure of 10 −2 -10 −3  mbar with a RF voltage of about 1 kV to clean components. Argon is then replaced by a hydrocarbon solvent (highly volatile at low pressure) at a pressure of 10 −1 -10 −2  mbar and later mixed with another hydrocarbon gas as discussed above.  
         [0026]    Those skilled in the art will recognize that changes may be made without departing in the scope or spirit from the present invention as more particularly defined in the claims below. For example, if the process requires a temperature exceeding 200° C., then a PVD procedure can be used.