Reducing Wear of Steel Rolling Against Ti6Al4V Operating in Vacuum Timothy L. Krantz* Abstract This work was motivated by a qualification test of a mechanism for a space telescope. During the test  undesired wear debris was formed. In this project alterative materials and coatings were tested with intent to reduce wear and debris when steel has a misaligned rolling contact against Ti6Al4V. Testing was done using a vacuum roller rig mimicking the mechanism’s contact conditions. Ten configurations were tested. Most configurations resulted in significant debris. A sputtered 1-micrometer-thick nanocomposite molybdenum disulfide (MoS 2) film provided the best wear protection. The best configuration made use of  the MoS 2coating on both materials, and in preparing for sputtering the anodized Ti6Al4V working surface  was smoothed using an ultrasonic process.  Introduction This work was motivated by results of a qualification test of a mechanism to be used for the James Webb  Space Telescope. The mechanism is used to move a magnet for certain operations of the telescope’snear infrared spectrograph (NIRSpec) instrument. The motion of the magnet is guided by a set of preloaded steel rollers in contact with anodized Ti6Al4V flat surfaces. The qualification testing was accomplished in a cold vacuum chamber to match as closely as possible the extreme deep space environment. Post-test inspections of the qualification test article revealed some wear of the steel rollers  and mating Ti6Al4V surfaces, and some loose debris was found. The NASA Engineering Safety Center (NESC) investigated the potential risk of the wear and resulting debris to hinder the full capability of the  NIRSpec instrument. This article describes results of roller tests of ten material configurations with the  project intent to minimize wear debris.  Background – Qualification Test  Article and Test Results Following is a description of the qualification test article, test conditions, and results that motivated this  research [1]. The mechanism of interest is a translator assembly that features a set of 11 rollers to guide  the motion of a magnet (Figure 1). The translation of the magnet is used to control the positions of microshutters. The in-plane position is established by a set of four rollers contacting both sides of a guide rail  defining the direction of motion. The out-of-plane posit ion and motion is guided by a set of seven rollers.  Three rollers are in contact with a base plate defining one plane and four rollers are in contact with a cover defining a second parallel plane. A motor and linkages provide the motive force. The translator assembly moves in a straight line for approximately 200 mm on each stroke. The translation time in one  direction during testing was 10.5 seconds. The test duration was 96,000 forward and return motions. Roller preload was established by a shimming procedure to provide a nominal normal load of 38 N. The  roller profile was a circular crown with a flat feature superimposed on the center of the crown. Assuming line contact across the flat, the Hertz contact maximum pressure was 240 MPa. The rollers were machined from free machining 440F steel, annealed and passivated. Each roller was supported by a pair  of deep groove ball bearings. The fastener securing the roller bearings to the axle was a locking style bolt assembled with a small amount of axial play. The rollers were in contact with Ti6Al4V that had been  stress relieved and anodized. *NASA Glenn Research Center, Cleveland, OH /g51/g85/g82/g70/g72/g72/g71/g76/g81/g74/g86/g3/g82/g73/g3/g87/g75/g72/g3/g23/g21/g81/g71/g3/g36/g72/g85/g82/g86/g83/g68/g70/g72/g3/g48/g72/g70/g75/g68/g81/g76/g86/g80/g86/g3/g54/g92/g80/g83/g82/g86/g76/g88/g80/g15/g3/g49/g36/g54/g36/g3/g42/g82/g71/g71/g68/g85 /g71/g3/g54/g83/g68/g70/g72/g3/g41/g79/g76/g74/g75/g87/g3/g38/g72/g81/g87/g72/g85/g15/g3/g48/g68/g92/g3/g20/g23/g16/g20/g25/g15/g3/g21/g19/g20/g23/g3 87