Superlubricity and tribochemistry of polyhydric alcohols

The anomalous low friction of diamondlike carbon coated surfaces lubricated by pure glycerol was observed at $80°\text{C}$. Steel surfaces were coated with an ultrahard 1 µm thick hydrogen-free tetrahedral coordinated carbon (ta-C) layer produced by physical vapor deposition. In the presence of glycerol, the friction coefficient is below 0.01 at steady state, corresponding to the so-called superlubricity regime (when sliding is then approaching pure rolling). This new mechanism of superlow friction is attributed to easy glide on triboformed OH-terminated surfaces. In addition to the formation of OH-terminated surfaces but at a lower temperature, we show here some evidence, by coupling experimental and computer simulations, that superlow friction of polyhydric alcohols could also be associated with triboinduced degradation of glycerol, producing a nanometer-thick film containing organic acids and water. Second, we show outstanding superlubricity of steel surfaces directly lubricated by a solution of myo-inositol (also called vitamin Bh) in glycerol at ambient temperature $\left(25°\text{C}\right)$. For the first time, under boundary lubrication at high contact pressure, friction of steel is below 0.01 in the absence of any long chain polar molecules. The mechanism is still unknown but could be associated with friction-induced dissociation of glycerol and interaction of waterlike species with steel surface.

Figure 1

. Chemical formula of some polyhydric alcohols: glycerol, pyrogallol, resorcinol, pentaerythritol, and myo-inositol (also called vitamin Bh). All these molecules are small and have different shapes such as spherical, crab shape or spider shape. They do not contain aliphatic chains like most of amphiphilic molecules used in traditional lubricants. In this study, we focus on glycerol and myo-inositol.

Figure 2

Steady-state friction coefficients values obtained with steel/steel, $a$-C:H/a-C:H, and ta-C/ta-C friction pairs materials, lubricated in three different environments. All tests were performed at a temperature of $80°\text{C}$. The friction tests in UHV were performed with the ECT tribometer. The friction tests in liquid pure glycerol and $\text{glycerol}+1\text{wt}\%$ myo-inositol were performed with a SRV machine at 270 MPa contact pressure and $0.3\text{m}{\text{s}}^{-1}$ sliding speed. Superlubricity is achieved for the first time with ta-C under boundary lubrication regime with glycerol. In these conditions, sliding is approaching pure rolling.

Figure 3

(Color) Formation of water molecules by molecular dynamics simulation (shown with small circles) during lubrication of OH-terminated ta-C/ta-C in presence of initially layered glycerol molecules. Contact pressure is 0.5 GPa and surfaces were slid for 4 ps with a sliding speed of 1 ps/nm

Figure 4

Evolution of peak intensities at mass 18 (${\text{H}}_2\text{O}$ and/or OD) and at mass 20 $\left({\text{D}}_2\text{O}\right)$ during a friction test performed with gaseous deuterated glycerol ${\text{C}}_3{\text{H}}_5{\left(\text{OD}\right)}_3$. The peak due to heavy water increases while peak due to water or DO decreases. This result strongly suggests the formation of water originating from the degradation of deuterated glycerol during friction test.

Figure 5

Evolution of friction coefficient vs the number of cycles for highly polished steel/steel friction pairs lubricated with glycerol at ambient temperature and for different contact pressures. The friction tests were performed with the pin-on-flat tribometer at a sliding speed of $3\text{mm}{\text{s}}^{-1}$ and with a track length of 2 mm. The friction reduction mechanism is pressure dependent for glycerol lubricated steel systems. A high contact pressure is necessary to activate the tribochemical reaction.

Figure 6

(Color) Wear track observed on the flat after a friction test of 100 cycles with pure glycerol. The contact was kept closed without releasing the load during 2 h. A mark was observed on the flat where the ball stayed static during the 2 h. (a) Optical image, (b) SEM image of the dashed area.

Figure 7

Superlubricity of steel/steel friction pair lubricated by a drop of a solution of 1 mass % myo-inositol in glycerol (at ambient temperature; $25°\text{C}$). The friction tests were performed with the reciprocating pin-on-flat tribometer with a sliding speed of $3\text{mm}{\text{s}}^{-1}$, a track length of 2 mm, and under a constant normal load of 10 N (corresponding to a maximum contact pressure of 800 MPa). In these conditions of high pressure and considering surfaces roughness of 30 nm, a glycerol viscosity of 934 mPa and a glycerol pressure viscosity coefficient of $5.4{\text{GPa}}^{-1}$. The calculated minimum film thickness is about 10 nm and the lambda ratio is inferior to unity. So the lubrication regime is boundary and EHL lubrication is excluded. After 400 cycles, friction tends to increase because inositol has been consumed by the tribochemical reaction. Heating at $80°\text{C}$ suppresses the superlow regime, probably by desorption of inositol molecules on the OH-terminated surface.

Figure 8

Steady-state friction coefficient values for different friction pairs vs glycerol partial pressure in the ECT tribometer. All friction experiments were carried at $80°\text{C}$. The sliding speed was 1 mm/s and the track length was 2 mm under a normal load of 1 N (corresponding to a maximum contact pressure of 300 MPa). After the induction period corresponding to dangling bonds formation, the glycerol molecules react with the carbon surface providing efficient lubrication. This set of experiments also suggests that previously formed OH-terminated surface (Si-OH) is important for the friction reduction of silicon by glycerol. Glycerol can hardly lubricate native and untreated silicon surfaces.