221 Degradation of Sputter -Deposited Nanocomposite   MoS 2 Coatings for NIRCam during Storage  in Air  

Jeffrey  R. Lince,* Stuart H. Loewenthal ** and Charles S. Clark**  

    Abstract  

The Focus and Alignment Mechanism (FAM) in NIRCam  on the J ames Webb Space Telescope will be  lubricated with nanocomposite sputter -deposited MoS 2 coatings . To respond to concerns regarding how  exposure to humid air during storage would affect performance, a study was conducted involving real -time  (i.e., unaccelerated) storage of coatings in air at 59% RH  for over two years . Pin-on-disk friction testing  showed that the endurance of the coatings dropped significantly during storage, but still provided  acceptabl e margin for FAM. The endurance results correlated well with X -ray Photoelectron Spectroscopy  results that measured the conversion of MoS 2 to nonlubricating MoO 3 during storage.  

NIRCam Background  

Nanocomposite sputter -deposited MoS 2 coatings are being used to lubricate sliding surfaces within the  Focus and Alignment Mechanism (FAM) in the Near Infrared Camera (NIRCam); NASA chose NIRCam as  one of four science instruments for the James Webb Space Telescope (JWST) (See Figure 1).1 The three  FAM s are linear actuators  required to provide micron level  positioning in tip, tilt and piston to the Pickoff  Mirror that reflects the starlight  into the rest of the NIRC am instrument. The FAM,  along with the other  NIRCam mechanisms such as the filter wheel assembly and pupil imaging assembly , are required to  operate at approximately 37K for near infrared imaging during the 5- year JWST mission .2    The FAM consist s of a fine pitch lead screw driven by 3 -phase stepper motor through a high- ratio Harmonic  Drive.  It was required to pass a cryogenic 2X life test of 200,000 revolutions with repeatable position  accuracy of less than 4 microns and with less  than a 30% increase in threshold motor  current. These  demanding requirements and the need to op erate at cryogenic temperatures put a high premium on  selecting durable solid lubricant  films.  Sputter -deposited nanocomposite MoS 2-based coatings  manufactured by Hohman Plating, Inc. were chosen for this application.  

Because of the high precision nature of the FAM, the plan was to store the flight article in either a vacuum  or a dry nitrogen purged environment during ground processing after delivery. This represented a conservative position due to the known sensitivity of the MoS 2 lubricant to humidity. However, providing  such an environment proved to be impractical after integration with the next higher assembly , where it will  be stored in air with 30 to 60% RH . Although non- reversible degradation of MoS 2 in the coatings to oxides  is expected in poorly controlled storage environments, the specific amount of degradation over long periods of time has not been quantified.    Sputter -deposited MoS 2-based coatings are increasingly being used in spacecraft applications where solid  lubricant formulations based on MoS 2 powders are precluded.3 Sputter -deposited coatings differ from such  coatings in that they are thin (i.e., ~0.5– 1 μm thick), have smaller crystallite sizes, higher crystalline defect  levels, and do not use bonding agents. In their pure form, sputtered coatings exhibit significant sensitivity  to oxidation in humid atmospheres. However, i mprovements in wear life have been achieved by cosputtering MoS 2 with vari ous species4-7 to form nanocomposites  (as are used in the FAM) , or forming  multilayer coatings by alternating deposition of a metal with MoS 2.8 These mixed coatings should exhibit                                                    * The Aerospace Corporation, El Segundo, CA   ** Lockheed Martin Space Systems Company , Sunnyvale, CA  

Proceedings of the 43rd Aerospace Mechanisms Sym posium, NASA Ames Research Center, May 4-6, 2016