A simple method to control nanotribology behaviors of monocrystalline silicon

Abstract

A simple method was proposed to control the nanotribology behaviors of monocrystalline silicon against SiO2 microsphere by adjusting relative humidity (RH). Experimental results indicated that adhesion work, friction coefficient, and nanowear of silicon against SiO2 microsphere significantly varied between 60% and 90% RH. Under 60% RH, adhesion work was 119 mN/m, and friction coefficient was about 0.53. However, adhesion work and friction coefficient decreased to ∼70 mN/m and ∼0.3 under 90% RH, respectively. An apparent wear track ∼13 nm deep formed on the silicon surface under 60% RH, whereas no obvious wear scar was observed on the silicon surface under 90% RH. Analysis indicated that such tribological behaviors were due to different water condensations on the silicon surface under 60% and 90% RH. Under 60% RH, the water that condensed on the surfaces of the silicon sample and SiO2 tip mainly consisted of ice-like water. As a result, adhesion work was enlarged by the breaking force of the ice-like water bridge in the contact area. Given that a ≡Si–O–Si≡ bonding bridge easily formed between the silicon surface and the SiO2 tip with the help of water condensation under 60% RH instead of 90% RH, the friction coefficient was large and the nanowear of the silicon sample was severe under 60% RH. These results may help elucidate the nanotribology behaviors of silicon and facilitate the tribological design of dynamic microelectromechanical systems working under humid conditions.

Effect of Humidity on Friction and Wear—A Critical Review

“…On the Si-H surface, as humidity increases, friction decreases monotonically, but wear increases initially, reaches a maximum value, and then eventually decreases [161,162,164]. In contrast, on the SiO x surface, both friction and wear first increase to a maximum value at 50% RH and then decrease below the detection limit of AFM at RH above 85% or in water (Figure 7a) [163,[165][166][167][168][169][170][171]. It is also noted that the effect of humidity on wear of SiO x -covered Si wafer varies depending on the sliding speed ( Figure 7b) [168].…”

An Investigation of the Wear on Silicon Surface at High Humidity

“…It is well known that the wear behaviors in micro/nano scale are affected by contact pressure [ 14 , 15 ]. Although the loading condition remained unchanged during the wear tests at 60% RH and 90% RH, the contact pressure for the pair of Si/SiO 2 might still change with the variation of humidity [ 7 , 9 , 11 ]. Based on the contact theory established by Derjaguin, Muller and Toporov (DMT) [ 8 , 14 , 15 , 16 ], the contact pressure P c in the present study can be estimated by Equation (1).…”

“…Since the silicon sample is covered by the native oxide layer (SiO x ), the key factor for the different wear of silicon samples is whether there is enough activation energy for the tribochemical wear of SiO x in the wear region. In the present study, the activation energy of the native oxide layer (SiO x ) refers to the bond energy of Si-O [ 7 , 11 , 28 ]. Therefore, the required energy E req to remove all the SiO x in the wear region should be calculated to verify the existence of the native oxide layer after the wear tests.…”

“…For this condition, the Si-O-Si bridges will easily generate between the interfaces of Si/SiO 2 pair, see Equation (4) [ 38 ]. ≡Si-OH + HO-Si≡ → ≡Si-O-Si≡+H 2 O With the help of shearing action, the native oxide layer will be gradually removed by the Si-O-Si bridges [ 7 , 11 ]. After that, hydrolysis reactions will take place on the surface of Si(100) substrate, see Equation (5) [ 7 , 11 , 38 ].…”

“…It has been reported that the wear on silicon surface changes with the variation of humidity in normal humid air (RH < 70%) [ 7 , 8 , 9 ]. However, the wear behaviors and mechanism of silicon at high humidity (RH > 70%) are far less understood [ 7 , 10 , 11 ].…”