DOE/NNSA/DE-FG03-03NA00069 Annual Report 1
OAK-B135 This project was undertaken to enhance our understanding and control of the response of ductile materials to extreme loading conditions such as those involved in impact loading. The project was designed with a focus on the role of friction in high rate deformation and shock studies. The work involves collaboration of the tribology group in Materials Science and Engineering at OSU and two groups at LANL, an impact loading group in the dynamic experimentation division and a computer simulation group in the applied physics division. The two teams are investigating the same materials pairs: Cu/Cu, Al/stainless steel (SS) and Ta/Al. The LANL team is providing impacted specimens for characterization at OS U. The LANL team has designed and built a rotating barrel gas gun apparatus that allows measurement of frictional force at an impacted interface over time scales of 0 to 50 {micro}s. Impact pressures are 0-150 MPa and sliding speeds can be up to 50 m/s. The stainless steel barrel can rotate at rates from 0-5000 rpm. An impactor rod is driven at up to 12 m/s against a target rod of the specimen material. Initial tests have been with OFHC Cu/Cu annealed to relax strains from the machining process. The grains are equiaxed and have 40 {micro}m grain size. In the velocity range 0-6 m/s, the friction force increases with velocity for time scales of order 25 {micro}s. The LANL team has also performed molecular dynamics (MD) simulations of sliding for Cu/Cu and Al/Ta using embedded atom potentials as well as simpler systems using Lennard-Jones potentials (also used at OSU). The results show extensive plastic deformation and, in some cases, the formation of nanocrystals at the sliding interface. The dependence of friction force on sliding velocity, v, shows two regimes: a low speed regime in which friction force rises with v and a high speed regime in which it decreases. The experimental work at OSU has focused on three tasks: (1) designing and building an improved system for sliding tests at intermediate velocities, (2) developing appropriate pre-testing surface preparation and (3) developing post-test characterization techniques. The new pin/disk wear testing system can achieve sliding speeds up to 1 m/s in a range of environments and contact times as small as 0.1 s. Transmission Electron Microscopy (TEM) was done on cross-sections of the as-machined annular OFHC copper samples. This revealed substructures consistent with extensive subsurface. These features would complicate our efforts to study the changes produced by impact with sliding. The samples should have a minimal amount of subsurface deformation prior to testing, so the deformation due to sliding will not be obscured. Therefore, a study was conducted to find a test specimen preparation method that would minimize subsurface deformation. Three machining methods were analyzed: lathe turning, fly-cutting, and electrical discharge machining (EDM). Post-machining annealing at 275 C for one hour in a vacuum furnace was also performed to remove deformation remaining from the machining processes. Microhardness was measured as a function of the distance from the machined surface. This was a simple way to determine the extent of subsurface deformation. The results show that annealed fly-cut samples are best for our purposes. Similar tests on pure aluminum samples suggest that annealing of fly-cut samples at 200 C for an hour is sufficient to remove subsurface deformation. The material tested at OSU was characterized using optical microscopy, SEM and TEM. Wear tracks and wear debris were analyzed using SEM and energy dispersive spectroscopy (EDS). TEM samples were prepared using different techniques including dimpling, jet-polishing and chemical polishing. Innovative techniques involving a Focused Ion Beam (FIB) have also been explored. MD modeling at OSU has focused on simple amorphous materials. The results suggest that the flow of material close to the sliding interface is characterized by the formation of eddies, intimate mixing and ''diffusion-like'' growth of the mixed layer. When the sliding speed is sufficiently high, the strain rate allows vorticity to develop. A comparison of eddy sizes with nanocrystal sizes in actual tribomaterial suggests that vorticity is directly responsible for the formation of such nanocrystal material. It is suggested that the flow of material near the interface is similar to turbulent flow in fluids. The resulting eddies affects frictional energy dissipation and mechanical mixing. A mixing-layer model of turbulent flow is qualitatively consistent with velocity profiles and friction behavior revealed by MD simulations.
- Research Organization:
- The Ohio State University, Columbus, OH (US)
- Sponsoring Organization:
- USDOE National Nuclear Security Administration (NNSA) (US)
- DOE Contract Number:
- FG03-03NA00069
- OSTI ID:
- 822009
- Report Number(s):
- DOE/NA/00069-1; TRN: US200413%%23
- Resource Relation:
- Other Information: PBD: 13 Mar 2004
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ALUMINIUM
CHEMICAL POLISHING
COMPUTERIZED SIMULATION
CROSS SECTIONS
ENERGY LOSSES
GRAIN SIZE
ION BEAMS
LENNARD-JONES POTENTIAL
OPTICAL MICROSCOPY
STAINLESS STEELS
STRAIN RATE
TRANSMISSION ELECTRON MICROSCOPY
TURBULENT FLOW
VACUUM FURNACES
IMPACT LOADING
IMPACT WITH FRICTION
MATERIALS CHARACTERIZATION
MOLECULAR DYNAMICS SIMULATIONS