13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 3 Adhesive and corrosive wear mechanisms After a short while vibration, the tribotester would stop owing to the overlarge friction. Figure 2 shows the worn morphologies of loading heads and restriction beams failed in nitrogen and ethanol vapor environments. On restriction beams only the feature of blunt peaks can be found and the bearing area is no more than 6% of the total nominal contact area. Hence these devices failed at the early stage of wear. The reason of blunt peaks can be inferred from the morphologies of the loading heads, where the worn area concentrated in a small region less than 5µm×5µm with more wear features. For samples in nitrogen atmosphere, scratches and pie-like wear debris distributed on the wear scars, while glass-like materials around bearing spots without scratches were found on samples working in ethanol vapor atmosphere. These glass-like materials can decompose in wet hydrofluoric acid vapor and is considered as the reaction products of silicon and ethanol in rubbing conditions. Therefore wear in nitrogen is attributed to adhesive type while in ethanol vapor it is corrosive type. Figure 2. Worn morphologies of driving shuttles (a, b) and loading heads (c, d) after device failure. (a, c) Dry nitrogen atmosphere. (b, d) Ethanol vapor environment. The scale bar is 1µm. The two wear mechanisms can be further convinced by the shuttle vibration degradation. Assuming each initial rough peak has a parabolic shape 2 0 h r r⋅ = and the average friction force ρ is proportional to the total bearing area A= 0 N A⋅ = / ) ( 0 N h r ⋅π . For adhesive wear, combining the classical Archard wear equation [10, 11] rA s V v t A h 1λ = Δ Δ = ⋅Δ ⋅Δ (1) (V is the wear volume, s is the sliding length, v is the sliding speed, and λ1 is the wear coefficient) and the simplified relationship between atomic contact area Ar and normal load Fn of Ar=λ2·Fn [12], the transient wear rate is inversely proportional to the wear depth as h F N v r A F v dt dh n n ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ⋅ ⋅ = = π λλ λλ 0 1 2 1 2 , (2) which induces t h∝ or t ∝ρ . And for corrosive wear in strong media, only the reacted materials
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