Temperature dependence of friction under cryogenic conditions in vacuum
Summary (1 min read)
1. Introduction
- Many thermodynamic and transport properties of solids depend dramatically on the temperature.
- In both metals and semiconductors, the electrical resistivity can have a strong temperature dependence, particularly in the cryogenic regime.
- The video frames are analyzed to yield the acceleration and force on the sliding block, from which the sliding friction coefficient can be determined.
- Contrary to their initial expectations, their results for the friction coefficients show very little temperature dependence in the range 4–200 K.
2. Experimental methods
- Two of the faces of the copper block slider have three 3.175 mm (1/8 inch) diameter balls made from bearing steel attached so that approximately half of the ball is exposed; these hemispherical contact points support the load and define the tribological contact points.
- Most of their measurements were done with steel balls, but one experimental run was done with sapphire balls (purchased from Small Parts).
- Once the radiation shields of the cryostat reach their final temperatures of approximately 4 and 77 K, respectively, the region surrounding the tribometer is effectively cryopumped by the shields, and the pressure presumably drops to very low values.
- To avoid the stick-slip motion characteristic of sliding near the critical angle, sliding friction measurements were performed by clamping the block at the end of the track using the solenoid, inclining the track at an angle approximately 10 higher than the critical angle, and then releasing the block.
- The camera was carefully mounted in the horizontal position to within ±0.1 using a digital level.
3. Friction as a function of temperature
- Friction measurements as described above were performed using steel balls on three types of tracks: stainless steel, polished sapphire, and PTFE coated steel shown in figures 5, 6 and 8, respectively.
- The authors found that the run-in history of the samples strongly affected the friction.
- The only substantial temperature dependence evolves after considerable wear, when the samples show a reproducible and reversible increase from lkG0.4 to lkG0.8 in the range 300K<T<450K.
- The Hertzian contact stress is approximately 276 MPa.
- Both steel and sapphire balls on sapphire produced a visible track in the sapphire as well as numerous fine white particles which were presumably sapphire.
4. Conclusion
- There are many theoretical reasons to expect a strong temperature dependence of the friction coefficient, particularly in the cryogenic regime.
- Wear was found to be an important factor.
- In these materials, the authors believe that the effects of wear overwhelm any possible temperature dependent friction mechanisms.
- The authors results show that PTFE coatings have many practical advantages for cryogenic engineering.
- The low temperature dynamic friction of steel on PTFE was approximately 0.18, which was less than one half that of the other materials the authors tested.
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Citations
84 citations
Cites methods from "Temperature dependence of friction ..."
...In separate experiments, employing a cryostat with a tilting slider-block tribometer using steel pins on PTFE, steel pins on sapphire, and sapphire on sapphire, no discernable temperature dependence from 4 to 300 K was observed, although wear was apparent in these systems [5]....
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82 citations
Cites result from "Temperature dependence of friction ..."
...In contrast, previous studies with unfilled PTFE have found no clear dependence of wear on vacuum conditions, though the coefficient of friction of this material shows some dependence at elevated sliding speeds [4, 21–24]....
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71 citations
60 citations
Cites background from "Temperature dependence of friction ..."
...Burton et al. [ 17 ] found inconclusive trends during tilted sled experiments with sapphire, PTFE, and other materials over a range of 4‐400 K under dry sliding in vacuum but notes that ‘‘…we believe that the effects of wear overwhelm any possible temperature...
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47 citations
References
5,834 citations
"Temperature dependence of friction ..." refers background in this paper
...More macroscopic theories [2] of frictional phenomena use models based on effects such as local melting of asperities, viscoelastic effects, plastic flow, and tribochemistry [3]....
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291 citations
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115 citations
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