Barrier coatings on spacecraft materials
This application is a continuation of application Ser. No. 07/604,378, filed Oct. 29, 1990 now abandoned, which is a continuation of application Ser. No. 07/400,779, filed Aug. 30, 1989, now abandoned which is a continuation of application Ser. No. 07/126,895, filed Nov. 30, 1987 now abandoned. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The invention relates to a process for coating the surface of an organic polymer to protect it against erosion by oxygen atoms. More specifically, the invention relates to such a process which includes the step of depositing a thin protective barrier film on the surface. 
     2. Description of Prior Art 
     All matter on the earth&#39;s surface, both living and inanimate, is protected by the atmosphere. This is not the case in outer space, where the atmosphere&#39;s density decreases rapidly with distance above the earth&#39;s surface; this decreases its ability to attenuate such harmful effects as, for example, energetic radiation (e.g. ultraviolet, UV). The insufficiency of life-sustaining molecular oxygen gas (due to the partial vacuum) and bombardment by short wavelength (high energy) electromagnetic radiation and by charged particles (electrons and ions) render any form of life impossible even a short distance (tens or hundreds of kilometers) above the earth&#39;s surface. The effects of charged particles are further enhanced in the so-called magnetosphere, some hundreds to thousands of kilometers above the earth&#39;s surface, where the earth&#39;s magnetic field traps and concentrates charged particles. The well-known van Allen radiation belts are related to this effect, as are such visible phenomena as the Aurora Borealis. 
     The present invention is primarily concerned with man-made objects (satellites) in earth orbit, and more specifically with those in &#34;low earth orbit&#34; (LEO), namely at altitudes of about 150 to 700 km. At these altitudes the major atmospheric constituent is atomic oxygen, O, which occurs as a result of UV-induced dissociation of molecular oxygen, O 2 . Experience on shuttle flights has shown that even a short-term exposure to this environment can have harmful effects on spacecraft surfaces, particularly if these are composed of or covered with organic (polymeric) materials. Organic polymers are important to the design of present and future generations of spacecraft, as they are used in advanced composites for structural members, and for thermal protection in the form of films and coatings. For instance, at the altitude of the space shuttle (about 250 km), the atomic oxygen density (≃10 -19  atoms cm -3 ) is not very high, even though its concentration exceeds that of the next most abundant atmospheric constituent, molecular nitrogen, N 2 . However, due to the high orbital velocity (about 8 km/sec) the O atom flux is quite high, about 10 15  atoms/cm 2  sec. Furthermore, this high velocity corresponds to collisions with highly energetic (about 5.0 eV) atomic projectiles. The observed result has been that exposed polymer surfaces are eroded by the chemically highly reactive O atoms, and that they suffer appreciable roughening and loss of mass. This, in turn, leads to irreversible degradation of the physical characteristics for which the surface or structural member were designed. 
     Erosion of organic materials by atomic oxygen in space has been described by various authors, for example by D. G. Zimcik and C. R. Maag, &#34;Results of Apparent Atomic Oxygen Reactions with Spacecraft Materials during Shuttle Flight STS-41G&#34;, presented at AIAA &#34;Shuttle Environment and Operations II Conference&#34;, Houston, Tex., November 1985, paper No. 85/7020. 
     Additional damage to insulating spacecraft materials such as organic polymers may result from charge buildup due to impinging electrons and ions. As described, for example, by K. G. Balmain and W. Hirt in IEEE Transactions on Electrical Insulation, Vol. EI-18, pages 498 to 503, 1983, charging by energetic electrons in space can lead to surface discharge arcs or flashover, which can permanently damage the polymer&#39;s structure and integrity. Furthermore, electrical transients associated with these discharge events can perturb or destroy electronic circuitry aboard the space vehicle. 
     Several authors have suggested remedies to the problems described above. For example, B. A. Banks et al. (&#34;Ion Beam Sputter Deposited Thin Film Coatings for Protection of Spacecraft Polymers in Low Earth Orbit&#34; presented at the 23rd AIAA Aerospace Science Meeting, Reno, Nev., Jan. 14-17, 1985) J. S. Sovey et al (U.S. Pat. No. 4,664,980) describe the use of ion beam sputter deposited thin coatings as oxidation barriers. Their coatings are composed of metal oxides (from the group of Si, Al, In, Sn) and up to 20 percent of a molecularly mixed flexible polymer (such as polytetrafluoroethylene), the purpose of the latter--according to the authors--being to provide the required flexibility. It was found, in both ground-based laboratory and in-space tests, that these thin protective coatings can act as barriers against oxidative erosion of the underlying polymer. Other authors have also shown thin layers of pure Teflon and sputtered or vapor deposited indium-tin oxide (ITO) to possess similar protective properties against attack by atomic oxygen. 
     SUMMARY OF INVENTION 
     Banks et al. (loc. cit.) have summarized the properties required for protective barrier coatings as follows: 
     the barrier must, of course, itself be unaffected by atomic oxygen bombardment; 
     it must be thin, light-weight and strongly adherent; 
     it should be flexible, abrasion resistant, and allow adhesive bonding; 
     it should be u.v. tolerant but not alter the substrate&#39;s optical properties; 
     Finally, an additional desirable property (not listed by Banks et al.) is that surface conductivity should be high in order to prevent the buildup of harmful potential gradients which result from charging. 
     It is an object of the present invention to provide protective coatings to organic polymer structures where said coatings have the above-named characteristics and are applied by glow discharge deposition. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Films and coatings may be prepared by a variety of physical and chemical deposition techniques as described, for example, by R. F. Bunshah et al. in &#34;Deposition Technologies for Films and Coatings&#34;, Noyes Publications, Park Ridge, N.J., 1982. These include physical vapor deposition (PVD) by evaporation, sputtering, and ion plating; chemical vapor deposition (CVD), plasma enhanced (or-assisted) chemical vapor deposition (PECVD or PACVD), flame spraying or plasma spraying, deposition from aqueous solutions, and various other methods. Some of these methods are suitable for providing protective coatings on surfaces of organic polymers, the object of the present invention, particularly those which do not appreciably raise the surface temperature of the thermally sensitive polymer substrate. These include the above-mentioned PVD techniques, and methods involving low pressure plasma (also known as glow discharge). The latter category constitutes a particularly advantageous embodiment of the present invention. 
     The use of glow discharges for preparing thin solid films is well known (see, for example, J. Mort and F. Jansen &#34;Plasma Deposited Thin Films&#34;, CRC Press, 1986): The workpiece (or pieces) to be coated is placed into a reactor chamber which can be evacuated with the help of a suitable vacuum pump. Following this, a flow of the chosen reagent gas (or gas mixture) is introduced, and the dynamic pressure in the chamber is adjusted to an appropriate value, typically in the range from 10 milli Torr to a few, e.g. 2, Torr (1 Torr=133 Pa). A glow discharge is then established in the chamber by applying electrical energy to the reagent gas flow, which results in chemical reaction leading to the desired thin film deposit. The electrical means, for example parallel plate electrodes, used to apply the said electrical energy is well known to those skilled in the art, and needs no further description here. Generally, high frequency electrical energy is preferred, typically in the audio-, radio-, or microwave frequency ranges, and this too is well known to those persons skilled in the art. In this manner, various types of thin film materials can be prepared, namely: 
     a) Plasma polymers, 
     b) inorganic insulating films, 
     c) semiconducting and conductive materials. 
     In category (a), films may be derived from halocarbon (e.g. fluorocarbon), hydrocarbon, or organosilicon &#34;monomer&#34; compounds such as hexamethyldisiloxane (HMDSO). Typical category (b) materials are inorganic silicon compounds such as &#34;plasma silicon dioxide&#34;, P-SiO 2 , plasma silicon nitride (P-SiN), or plasma silicon oxynitride (P-SiON). These may be prepared by reacting silane (SiH 4 ) with an appropriate second gas (or mixture) such as N 2  O, NH 3 , N 2 , etc. Films in category (c) include amorphous, hydrogenated silicon (a-Si:H) and germanium, respectively prepared from SiH 4  and GeH 4  gas, metal films prepared from volatile organometallic compounds, and semiconducting metal oxide films such as tin oxide. The latter material may be prepared by co-reacting tetramethyltin [Sn (CH 3 ) 4  ] with oxygen in the glow discharge. Among the above-mentioned materials many can, in principle, provide suitable barriers against attack by O atoms, except those prepared from hydrocarbon reagents. 
     In the present work, a microwave plasma reactor of the type described by Weissfloch et al. in U.S. Pat. No. 3,814,983 was used to deposit various thin film materials onto organic polymers which are important to application in spacecraft, namely Kapton™ polyimide, polyethylene terephthalate (Mylar)™, and Epoxy resin. An important advantage of this electrodeless reactor type is the ability to coat large objects, for example of cylindrical geometry, which is not readily feasible with conventional, (e.g. capacitively-coupled) radio-frequency discharge systems. In a second plasma reactor of the same type, a plasma in a flow of pure O 2  gas was used to generate O atoms. By exposing coated and control samples to this latter plasma, a laboratory simulation of LEO environment was achieved. The erosion rate of the various samples was determined gravimetrically by measuring weight loss with an electronic microbalance. 
     Typical examples of treatment results obtained using the method of the present invention are shown in Tables I to III. 
     
                                           TABLE I                                 
__________________________________________________________________________
Kapton Polyamide Substrate                                                
              Coating conditions.sup.(a)                                  
                                  Thick-                                  
Sample                                                                    
    Sample description            ness                                    
                                      Weight loss.sup.(b)                 
No. (coating type)                                                        
              T.sub.s (°C.)                                        
                  F.sub.SiH.sbsb.4 (sccm)                                 
                         Gas/Flow (sccm)                                  
                                  (μm)                                 
                                      (mg/cm.sup.2 · h)          
__________________________________________________________________________
1   control (untreated)                                                   
              --  --     --       --  4.60                                
2   a-Si:H    330  5     Ar/30    0.5 0.05                                
3   &#34;         250  5     Ar/45    1.0 0.01                                
4   P-SiN     250 25     NH.sub.3 /25                                     
                                  0.5 0.00                                
5   P-SiO.sub.2                                                           
               25 25     N.sub.2 O/25                                     
                                  0.5 0.10                                
6   P-SiON     25 25     NH.sub.3 /25;N.sub.2 O/25                        
                                  0.5 0.09                                
7   PP HMDSO   25 --     HMDSO/45 0.5 0.28                                
__________________________________________________________________________
 .sup.(a) pressure: 100 m Torr; microwave power: 100 W                    
 .sup.(b) O.sub.2 flow: 200 sccm; pressure: 100 m Torr; microwave power:  
 300 W                                                                    
 
    
     
                                           TABLE II                                
__________________________________________________________________________
Epoxy Resin Substrate                                                     
              Coating conditions.sup.(a)                                  
                              Thick-                                      
Sample                                                                    
    Sample description        ness                                        
                                  Weight loss.sup.(b)                     
No. (coating type)                                                        
              F.sub.SiH.sbsb.4 (sccm)                                     
                     Gas/Flow (sccm)                                      
                              (μm)                                     
                                  (mg/cm.sup.2 · h)              
__________________________________________________________________________
 8  control (untreated)                                                   
              --     --       --  5.06                                    
 9  a-Si:H    10     Ar/45    0.5 0.04                                    
10  P-SiN     25     NH.sub.3 /25                                         
                              0.5 0.03                                    
11  P-SiON    25     NH.sub.3 24;N.sub.2 O/25                             
                              0.5 0.18                                    
12  P-SiO.sub.2                                                           
              25     N.sub.2 O/25                                         
                              0.5 0.09                                    
__________________________________________________________________________
 .sup.(a) T.sub.s = 25° C.; other conditions as in Table I         
 .sup.(b) as in Table I                                                   
 
    
     
                                           TABLE III                               
__________________________________________________________________________
Mylar Substrate                                                           
              Coating conditions.sup.(a)                                  
                              Thick-                                      
Sample                                                                    
    Sample description        ness                                        
                                  Weight loss.sup.(b)                     
No. (coating type)                                                        
              F.sub.SiH.sbsb.4 (sccm)                                     
                     Gas/Flow (sccm)                                      
                              (μm)                                     
                                  (mg/cm.sup.2 · h)              
__________________________________________________________________________
13  control (untreated)                                                   
              --     --       --  4.80                                    
14  a-Si:H    10     Ar/45    0.5 0.15                                    
15  P-SiN     15     NH.sub.3 /45                                         
                              0.5 0.00                                    
16  P-SiON    25     NH.sub.3 /25;N.sub.2 O/25                            
                              0.5 0.03                                    
17  P-SiO.sub.2                                                           
              25     N.sub.2 O/25                                         
                              0.5 0.11                                    
__________________________________________________________________________
 .sup.(a) as in Table II                                                  
 .sup.(b) as in Tables I and II                                           
 
    
     For all three untreated substrate materials the weight loss is comparable, 4.8±0.2 mg/cm 2 .h; all of the investigated coatings are seen to reduce the weight loss very substantially, in some instances (Nos. 4, 15) to an immeasurably small value. Thinner coatings (0.3 μm) have also been investigated and they yield comparable results. The coatings are flexible, strongly adherent and transparent, and therefore satisfy important requirements mentioned earlier. Regarding the last of these, electrical surface conductivity, the following additional tests have been conducted: Samples were exposed to corona discharge using a charging voltage of -4000 V; the decay rate of the surface potential was then measured using a noncontacting electrostatic voltmeter (Monroe Instruments Corp. &#34;Isoprobe&#34;). Some typical results of these tests are shown in Table IV: 
     
                                           TABLE IV                                
__________________________________________________________________________
Decay rate of negative surface potential                                  
                Negative surface potential (in volts)                     
Sample          at the indicated time                                     
No.  Description                                                          
                0 s 30 s                                                  
                       5 min (*)                                          
                              2 h                                         
                                 4 days                                   
__________________________________________________________________________
1    Kapton control                                                       
                2560                                                      
                    2560                                                  
                       2480   2080                                        
                                 820                                      
8    Epoxy control                                                        
                2500                                                      
                    2480                                                  
                       2400   1890                                        
                                 0                                        
13   Mylar control                                                        
                2460                                                      
                    2460                                                  
                       2440   2200                                        
                                 1700                                     
2    Kapton/a-Si:H                                                        
                1800                                                      
                     900                                                  
                         0    -- --                                       
4    Kapton/P-SiN                                                         
                2400                                                      
                    2340                                                  
                       1810 (380)                                         
                               990                                        
                                 0                                        
5    Kapton/P-SiO.sub.2                                                   
                 400                                                      
                     220                                                  
                        70      0                                         
                                 --                                       
6    Kapton/P-SiON                                                        
                 990                                                      
                     910                                                  
                        610    550                                        
                                 0                                        
7    Kapton/PP HMDSO                                                      
                2560                                                      
                    2560                                                  
                       2480 (330)                                         
                              2000                                        
                                 1250                                     
9    Epoxy/a-Si:H                                                         
                2000                                                      
                    1250                                                  
                        780 (120)                                         
                               90                                         
                                 0                                        
16   Mylar/P-SiON                                                         
                 450                                                      
                     330                                                  
                        80 (60)                                           
                                0                                         
                                 --                                       
17   Mylar/P-SiO.sub.2                                                    
                 750                                                      
                     720                                                  
                        510     0                                         
                                 --                                       
__________________________________________________________________________
 (*) Following 20 minutes of exposure to O.sub.2 plasma, and using same   
 charging and charge decay measuring procedure.                           
 
    
     Whereas the three control samples retain surface charge for lengthy periods (they become &#34;electrets&#34;), coated samples are seen to dissipate their surface charge quite rapidly. Not surprisingly, this decay is most rapid in the case of the semiconducting film a-Si:H, but even for the insulating materials such as P-SiO 2  and P-SiON it is far more rapid than for the untreated control samples. Surprisingly, the charge decay rate of coated samples is even more rapid, in practically all cases investigated, following exposure to oxygen plasma. For example, for samples Nos. 4, 7, 9, and 16 in Table IV, the surface potential had dropped to 380, 330, 120 and 0 volts, respectively, after 5 minutes following 20 minutes of O 2  plasma treatment and renewed corona charging. From this we infer that in space these coatings should be even more effective in preventing charge buildup than in the laboratory. 
     Although specific embodiments have been described, this was for the purpose of illustrating, but not limiting, the invention. Various modifications, which will come readily to the mind of one skilled in the art, are within the scope of the invention as defined in the appended claims.