Patent Publication Number: US-6218344-B1

Title: Antiwear additives for spacecraft lubricants

Description:
RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to improved lubricants for spacecraft. 
     The use of satellites for communication and navigation is ever increasing in both military and commercial applications. The high costs of building and launching satellites are driving the need to extend the useful life of satellites from the current 5 to 8 years to at least 15 years. 
     Spacecraft utilize many moving assemblies. The current tribological requirements of such assemblies are usually satisfied by a variety of lubricants and materials. To date, spacecraft lifetime is limited primarily by the failure of systems such as power supplies, electronics, thermal systems, optical systems and positioning systems. Technological advances in these systems are making them more reliable. As spacecraft life expectancy increases, more spacecraft failures will be attributed to tribological limitations if corresponding advances in tribology do not occur. 
     Lubrication demands on satellite platforms generally fall into three categories: high speed, low speed and mixed speed. Some manufacturers of mechanisms on satellite platforms prefer liquid lubricants while others prefer grease lubricants. Low speed satellite mechanisms operate below the speeds required to produce an elastohydrodynamic lubrication (EHL) film, and thus have metal to metal contact. Such metal to metal contact leads to high wear and eventual mechanism failure. High speed mechanisms operate at speeds where the EHL film is maintained throughout the life of the bearing system. Although the presence of the EHL film minimizes wear, there is still intermittent asperity contact at full speed and high wear during start-up. Mixed speed mechanisms operate at times at high speed and at other times low speed, and are exposed to both EHL and boundary lubrication. 
     Two factors are critical in maintaining good lubrication in liquid/grease lubricated systems over an extended time, in an extremely high vacuum. First, the lubricant base oil must remain in place, without volatilizing or creeping into other areas, and it must not change in other ways, such as becoming thicker or changing chemically. Second, additives in the lubricant must not evaporate or be consumed, thus leaving the base oil to carry the load with no additive-produced film. New, improved base fluids for satellite applications are much less volatile than previously and currently used mineral oils; these new base fluids, including, but not necessarily limited to narrow molecular weight range polyalphaolefins (PAO), multiply alkylated cyclopentanes (MAC) and silahydrocarbons (SiHC), are gradually being inserted into satellite applications. 
     Additives also need to have low volatility. Hydrocarbon base lubricants are readily enhanced with a wide variety of additive chemical classes. In atmospheric pressure applications, commercial additives are a mature technology because hydrocarbon base oils have a very large industrial market. One problem for high vacuum applications is that commercial additives are often supplied in a carrier fluid, such as a mineral oil or an ester oil, which is more volatile than the additive and therefore undesirable for satellite applications. Further, most commercial additives are not made especially for vacuum operation, so the choice is limited. Commercial additive producers have little incentive to make less volatile additives for the satellite lubricant market because of its extremely small volume. 
     U.S. Pat. No. 5,196,130, issued Mar. 23, 1993 to L. J. Gschwender and C. E. Snyder, Jr, discloses a lubricity additive, tris(4-chlorophenoxyphenyl)phosphate, for high-temperature gas turbine engine oils. We have now found that this additive is also useful for satellite lubrication applications. 
     Accordingly, it is an object of the present invention to provide lubricants for satellite applications. 
     Other objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
     DESCRIPTION OF THE INVENTION 
     In accordance with the present invention there are provided lubricants for satellite applications which consist essentially of a base fluid having low volatility and a minor amount, i.e., about 0.1 to 10 weight percent, of a chlorinated tris(phenoxyphenyl)phosphate. If long-term storage on earth, prior to use, is anticipated, a minor amount of an antioxidant may be added to the lubricants. In one aspect of the invention, there is provided a lubricating oil consisting essentially of a base fluid having low volatility and about 0.1 to 3.0 weight percent of a chlorinated tris(phenoxyphenyl)phosphate. In another aspect of the invention, there is provided a grease composition consisting essentially of a base fluid having low volatility, a suitable thickener and about 0.1 to 9.9 weight percent of a chlorinated tris(phenoxyphenyl)phosphate. 
     The base fluids, as noted previously, includes narrow molecular weight range polyalphaolefins (PAO), multiply alkylated cyclopentanes (MAC) and silahydrocarbons (SiHC). Commercially available polyalphaolefins may contain low molecular weight components which can be removed by vacuum distillation. For example, SHF-82, available commercially from Mobil Chemical Company, contains approximately 10% C 30 , 35% C 40 , and 55% C 50+ . Distillation of this stock at 0.7 Pa (0.005 torr), 240° to 250° C., provides a “bottom cut” with a composition of about 6% C 40 , balance C 50+ . 
    
    
     The following example illustrates the invention: 
     EXAMPLE 
     The following fluids were tested to determine their suitability for use as lubricants for satellite applications: 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                 Designation 
                 Type 
                 Source 
                 Tradename 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 PAO-1 
                 polyalphaolefin 
                 Nye Lubricants, Inc. 
                 New Bedford, 
                 Nye Synthetic 
               
               
                   
                   
                   
                 MA 
                 Oil 179 
               
               
                 PAO-2* 
                 polyalphaolefin 
                 Mobil Chemical 
                 Edison, NJ 
                 SHF-82 
               
               
                   
                   
                 Company 
               
               
                 MAC 
                 multiply alkylated 
                 Nye Lubricants, Inc. 
                 New Bedford, 
                 Pennzane 2000 
               
               
                   
                 cyclopentane 
                   
                 MA 
               
               
                 SiHC-1 
                 silahydrocarbon 
               
               
                 SiHC-2 
                 silahydrocarbon 
               
               
                 SiHC-3 
                 silahydrocarbon 
               
               
                   
               
               
                 (distilled as noted previously  
               
            
           
         
       
     
     The fluids designated SiHC-1, -2 and -3 were synthesized in-house according to the procedure in Chen et al, U.S. patent application Ser. No. 09/385,397, filed Aug. 30, 1999. Briefly, the procedure comprises reacting an alkyl silane having the formula H—SiR 1   3  with a compound having at least one vinyl group of the formula R n —Si—(CH═CH 2 )( 4− n), wherein R and R 1  are alkyl groups having 1 to 18 carbon atoms, and n is an integer having a value of 0 to 3, in the presence of a transition metal salt or transition metal complex catalyst. SiHC-1 has the formula CH 3 Si(CH 2 CH 2 Si—(n-C 10 H 21 ) 3 ) 3 , SiHC-2 has the formula CH 3 Si(CH 2 CH 2 Si—(n-C 8 H 17 ) 3 ) 3 , formula CH 3 Si(CH 2 CH 2 Si—(n-C 6 H 13 ) 3 ) 3 . 
     Viscosity, viscosity index and thermographic data for these fluids are shown in Table I, below: 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE I 
               
             
            
               
                   
                   
               
               
                   
                   
                 MAC 
                   
               
               
                   
                 PAO 
                 Penn- 
                 Silahydrocarbon 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Fluid type 
                 PAO-1 
                 PAO-2 
                 zane 
                 SiHC-1 
                 SiHC-2 
                 SiHC-3 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Viscosity, cSt 
                   
                   
                   
                   
                   
                   
               
               
                 100° C. 
                 14.58 
                 12.33 
                 14.4 
                 15.2 
                 12.17 
                 9.98 
               
               
                 40° C. 
                 104 
                 93.5 
                 106 
                 94.4 
                 71.22 
                 56.5 
               
               
                 −17.8° C. 
                 4860 
                 5030 
                 5158 
                 3051 
                 2059 
                 1514 
               
               
                 −40° C. 
                 * 
                 * 
                 77870 
                 34910 
                 20780 
                 14870 
               
               
                 −54° C. 
                 * 
                 * 
                 * 
                 * 
                 157300 
                 110790 
               
               
                 Visc. Index 
                 145 
                 126 
                 139 
                 170 
                 169 
                 165 
               
               
                 TGA T ½ , ° C. 
                 240 
                 265 
                 286 
                 350 
                 304 
                 257 
               
               
                 TGA T 0 , ° C. 
                 150 
                 235 
                 280 
                 336 
                 288 
                 246 
               
               
                 T ½  − T 0   
                 90 
                 30 
                 6 
                 14 
                 16 
                 11 
               
               
                   
               
               
                 *No Flow  
               
            
           
         
       
     
     In contrast, two commercial base fluids, Coray 100 and Vac-Kote, had T ½  of about 170° C. and 215° C., respectively; Fomblin Z, a fluid now used in spacecraft, has a T ½  of about 390° C. 
     Four-ball wear tests of formulations of these base fluids with chlorinated alkylated tris(phenoxyphenyl)phosphate were conducted in accordance with ASTM D4172 with the exception that a fitted plastic cage was placed around the apparatus and dry nitrogen was purged through the chamber for at least 15 minutes before and during the test. The calculated initial stress in this test is 4312 MPa (494,811 psi). Average wear scar data, in mm, are shown in Table II, below: 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 PAO 
                 MAC 
                 SiHC 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Base Fluid 
                 1.8 
                 2.7 
                 1.6 
               
               
                   
                 With Additive, % 
                 1.1, 1% 
                 2.2, 0.1% 
                 0.8, 1% 
               
               
                   
                   
                   
                 0.55, 0.25% 
               
               
                   
                   
               
            
           
         
       
     
     Having thus described exemplary embodiments of the present invention, it should be noted by those skilled in the art that the disclosures herein are exemplary only and that alternatives, adaptations and modifications may be made within the scope of the present invention.