Hydrodynamic effect on the superlubricity of phosphoric acid between ceramic and sapphire

Abstract

Abstract:In this work, a super-low friction coefficient of 0.003 was found between a silicon nitride ball and a sapphire plate lubricated by phosphoric acid solution. The wear mainly occurred in the running-in period and disappeared after superlubricity was achieved. The friction coefficient was effectively reduced from 0.3 to 0.003 at a constant speed of 0.076 m/s, accompanied by a 12-nm-thickness film. The lubrication regime was indicated to change from boundary lubrication in the running-in period to elastohydrodynamic lubrication in the superlubricity period, which is also supported by the results of the friction coefficient versus sliding speed. In addition, the experimental results showed good agreement with theoretical calculations based on the elastohydrodynamic lubrication theory, suggesting a significant hydrodynamic effect of phosphoric acid on superlubricity.

A review of recent advances in tribology

“…Liquid superlubricity has been proposed for more than 20 years [52−55]. Its mechanism can be summarized as hydration effect [56], chemical reaction layer [57], hydrodynamic effect [58,59], double electric layer interaction which will be more important in superlubricity [60,61], and the combination of multiple effects [62]. Superlubricity lubricants have been developed from pure water [63], brine [64], acids [65] to acid-alcohol system [66,67], alcohol system [68], bioliquids [69], oil-based system [70], and surfactants [71].…”

Macroscale Superlubricity Achieved With Various Liquid Molecules: A Review

“…Phosphoric acid as a lubricant was first found to achieve superlubricity at the Si 3 N 4 /glass interface in 2011 (Li et al, 2011). Thereafter, acid-based lubricants as a new series of lubricants were investigated by the authors in detail at various tribopairs, such as Si 3 N 4 /glass, Si 3 N 4 /SiO 2 , Si 3 N 4 /sapphire, and ruby/sapphire (Deng et al, 2014;. As depicted in Figure 4a, when a Si 3 N 4 ball slides on a glass disc under the lubrication of H 3 PO 4(aq) , the COF decreases from 0.45 to 0.004 after a wearing-in period of 600 s. The wearing-in period is vital to the achievement of superlubricity and can be divided into two stages according to the evolution of the COF with time.…”

Speed dependence of liquid superlubricity stability with H3PO4 solution

“…23 Moreover, the sliding of two surfaces with each other results in a hydrodynamic effect in the limited hydrated water layer, which also plays an important role in the lubrication state of the liquid system. 21 The hydrodynamic effect in the contact area signicantly weakens with the decrease in speed. The subdued lubricating lm can barely support the high contact pressure from the upper Si 3 N 4 ball and results in the direct solid-solid contact of tribopair surfaces, thereby failing to achieve superlubricity state.…”

“…19 Further studies showed that superlubricity mechanism was mainly attributed to the formation of a three-layer water-containing nanolm with hydrogen bond network (hydrated water layer) between two contacting surfaces aer the tribochemical reaction between acid solution and tribopair surfaces during the running-in process. [20][21][22] The special composite lubricating nanolm can effectively separate the tribopair surfaces and provide a hydrated water layer with ultra-low shear strength instead of a direct frictional contact. Meanwhile, the inuences of the types of acid, hydrogen ion concentration and volume of lubricant on the superlubricity behavior of Si 3 N 4 /glass pair have also been studied.…”