Friction and Wear Mechanisms of Nanocrystalline Nickel in Ambient and Inert Atmospheres
- PDF / 854,899 Bytes
- 11 Pages / 593.972 x 792 pts Page_size
- 11 Downloads / 153 Views
nc) metallic materials are normally characterized by high strength and hardness but limited ductility.[1–4] These properties are generally attributed to the large volume fraction of grain boundaries inhibiting the mobility of dislocations.[5,6,7] For example, when the grain size of Ni was reduced from 15 lm to 12 nm, its hardness was reported to increase from 0.9 to 6.9 GPa.[8,9] The increase in strength and hardness that accompanies grain refinement is of interest from the tribological point of view, in particular for the design of new materials and surfaces with improved wear resistance. Farhat et al.[10] tested magnetron-sputtered nc Al using a pin-on-disc tribometer. In ambient air, an 80 pct reduction in the wear rate and a 60 pct reduction in the peak coefficient of friction (COF) value MEHDI SHAFIEI, Ph.D Candidate, and AHMET T. ALPAS, Professor, are with the Department of Mechanical, Automotive and Materials Engineering, University of Windsor, Windsor, ON N9B 3P4, Canada. Contact e-mail: shafi[email protected] Manuscript submitted March 21, 2006. Article published online June 29, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
of the 15-nm grain size Al were measured compared to an Al with a grain size of 1 mm. In the grain size (d) range of 15 to 100 nm, where Hall–Petch type strengthening[11,12] was observed, the wear rate (W) of the nc Al was found to be proportional to the applied load (P) consistent with the Archard’s law[13] and obeyed the following equation:[10] P W ¼ W0 þ K ½1 H0 þ kd0:5 On the other hand, for electrodeposited nc Ni subjected to abrasion tests, it was reported that the breakdown in Hall–Petch hardening at d = 12 nm was accompanied by a change in the wear mechanism.[9] Schuh et al.[9] also compared the scratch resistance of Ni samples with grain sizes of 12 nm and 15 lm. No change in the COF was observed during the nanoscratch tests performed using a Berkovich-type indenter, and a COF of 0.25 was measured for both samples. Jeong et al.[14] investigated the abrasive wear resistance of electrodeposited nc Ni coatings in ambient air and found that a 44 pct increase in the wear resistance VOLUME 38A, JULY 2007—1621
occurred for Ni with d = 13 nm compared to Ni with d = 90 lm. On the contrary, Mishra et al.[15] reported COF values of 0.16 and 0.29 for electrodeposited nc Ni coatings with d = 8 and 10 nm, respectively, and a COF of 0.62 for a mc Ni with d = 61 lm. There is clearly a need for further experimental work to characterize micromechanisms of wear in nc metallic materials, and to rationalize microscopic processes leading to generation of wear debris or surface damage. In examining the wear behavior of nc materials, attention should be given to the role of testing atmosphere as the wear rates depend on the environmental conditions to which a material is subjected. The nc materials have a larger density of grain boundaries that act as preferential nucleation sites for oxides.[16] Grain boundaries also provide high diffusion paths for oxygen, and thus, the role of surface oxidation d
Data Loading...
