Understanding the Atomic Scale Mechanisms that Control the Attainment of Ultralow Friction and Wear in Carbon-Based Materials

Abstract

REPORT DOCUMENTATION PAGE Form Approved OMB No. 0704-0188The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services, Directorate (0704-0188). Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION. REPORT DATE (DD-MM-YYYY)16 SPONSOR/MONITOR'S ACRONYM(S)AFRL/AFOSR/IOA(AOARD) SPONSOR/MONITOR'S REPORT NUMBER(S)14IOA087_144071 12. DISTRIBUTION/AVAILABILITY STATEMENT Distribution Code A: Approved for public release, distribution is unlimited. SUPPLEMENTARY NOTES ABSTRACTThe wear behavior of ultrananocrystalline diamond (UNCD) was characterized at the nanoscale to understand the fundamental mechanisms controlling the initial stages of wear. To enable this, novel nanoscale sliding experiments were conducted in situ in a transmission electron microscope (TEM). UNCD is part of a class of materials with tremendous potential for tribological applications due to their low wear and low friction behavior. However, the scientific understanding of the initial wear process is lacking, and this prevents broader application of these materials. The experiments revealed that: (1) the wear follows a gradual, atomic-level removal mechanism, as opposed to fracture or plastic deformation; (2) the rate of wear (rate of material removal) in this nanoscale, single asperity contact is orders of magnitude larger than that measured at the macroscale; and (3) nevertheless, the wear follows the macroscale trend of an initially larger wear rate, followed by stabilization at a lower value. Strikingly, no amorphization during sliding was observed, in contrast with previous experimental and computational results for diamond. To better understand the contact stresses that drive the wear, a careful, long-range investigation of contact behavior was also initiated. Since adhesion is crucial in determining contact stresses, an experimental method for the nanoscale measurement of length and strength of adhesive interactions between asperities of arbitrary shape was successfully implemented. The wear behavior of ultrananocrystalline diamond (UNCD) was characterized at the nanoscale to understand the fundamental mechanisms controlling the initial stages of wear. To enable this, novel nanoscale sliding experiments were conducted in situ in a transmission electron microscope (TEM). UNCD is part of a class of materials with tremendous potential for tribological applications due to their low wear and low frict...