Evaluation of Friction Characteristics and Low Friction Mechanism of Tungsten  Disulfide for Space Solid Lubricant at Elevated Temperature in a Vacuum  Ayaka Takahashi*, Josaphat Tetuko S S* and Keizo Hashimoto** Abstract  A development for a synthetic aperture radar for exploration of another planet is planned. A synthetic  aperture radar satellite needs a deployment system for a large antenna for both telecommunication and  observation of the planet. The spacecraft will be required to withstand harsh environments without  maintenance during the mission. One option is to use a solid lubricant in the sliding portions. Tungsten  disulfide (WS 2) for use as a solid lubricant at elevated temperatures in space has been examined. In order  to simulate the friction during deployment, measurements for the friction coefficient at room temperature  and high temperature in a vacuum using three types of WS 2 coating films on a metal disk have been  conducted. The first WS 2 bonded film was compared to molybdenum disulfide (MoS 2) bonded film with the  same coating condition and using the same equipment. MoS 2 has been mainly utilized in current spacecraft  and it has similar crystal structure as that of WS 2. The wear life of WS 2 bonded film shows longer life than  MoS 2 bonded film at 500°C under vacuum. Sputtered films and shot peening films were also examined.  The friction coefficient of WS 2 shot peening films are extremely low at room temperature and high  temperatures. Furthermore, an effort was made to confirm the low friction mechanism from the result of  analysis after the friction tests. The friction mechanism of these materials is attributed to weakly bonding of  the (0002) plane. It was shown that a specimen having strong texture on the (0002) plane has a low friction  coefficient.   Introduction  Spacecraft mechanisms cannot be maintained or inspected after launch. The extreme environments of  interplanetary space present a more challenging wear environment, introducing failure modes not seen in  terrestrial environments. Therefore, any satellite components subjected to sliding wear must be sufficiently  lubricated to survive mission requirements. Recent exploration programs subject spacecraft to more  extreme environments than seen in the earth orbital environment. One option to provide lubrication for  interplanetary missions is solid lubrication. [1, 2].   There is evidence in the literature by NASA in 1969 [3] that WS 2 has demonstrated a great potential for a  high-temperature solid lubricant for space. MoS 2 has been utilized successfully as a solid lubricant for space  equipment. Recently, nanoscale multilayers of MoS 2/WS 2 [4] and nanocomposite films [5-8] have been studied  for their tribological properties using X-ray diffraction (XRD) and Transmission Electron Microscopy.  Although the friction coefficient of WS 2 was very low at elevated temperatures, studies of WS 2 have not  been as extensive as that of MoS 2 which has the same crystal structure. The crystallographic structure of  WS 2 is shown in Figure 1. The friction characteristics of WS 2 are attributed to the crystal structure, which is  composed of hexagonal close-packed tungsten atomic layers sandwiched between two hexagonal closepacked sulfur atomic layers. Therefore, two adjacent sulfur layers held by Van der Waals force are weakly  bonded and slide easily on the (0002) sulfur basal planes.   To simulate the deployment and driving friction, pin-on-disk reciprocating tribometer and rotary tribometer  experiments have been conducted under vacuum at ambient and elevated temperatures with WS 2 on the  metal disk.  *Center for Environmental Remote Sensing National University Corporation, Chiba University, Japan ** Department of Aerospace Engineering, Teikyo University, Japan Proceedings of the 44th Aerospace Mechanisms Symposium, NASA Glenn Research Center, May 16-18, 2018  NASA/CP—2018-219887 113