Elastic and inelastic deformations of ethylene-passivated tenfold decagonal $\mathrm{Al}-\mathrm{Ni}-\mathrm{Co}$ quasicrystal surfaces

The adhesion and friction force properties between a tenfold $\mathrm{Al}\text{‐}\mathrm{Ni}\text{‐}\mathrm{Co}$ decagonal quasicrystal and a titanium nitride (TiN)-coated tip were investigated using an atomic force microscope in ultrahigh vacuum. To suppress the strong chemical adhesion found in the clean quasicrystal surfaces, the sample was exposed to ethylene that formed a protective passivating layer. We show that the deformation mechanism of the tip-substrate junction changes from elastic to inelastic at a threshold pressure of $3.8\text{to}4.0\mathrm{GPa}$. Images of the indentation marks left above the threshold pressure indicate the absence of new steps, and indicate that surface damage is not accompanied by formation of slippage planes or dislocations, as found in plastically deforming crystalline materials. This is consistent with the lack of translational periodicity of quasicrystals. The work of adhesion in the inelastic regime is five times larger than in the elastic one, plausibly as a result of the displacement of the passivating layer. In the elastic regime, the friction dependence on load is accurately described by the Derjaguin-Müller-Toporov (DMT) model, consistent with the high hardness of both the TiN tip and the quasicrystal sample. Above the threshold pressure, the friction versus load curve deviates from the DMT model, indicating that chemical bond formation and rupture contribute to the energy dissipation.

Figure 1

(a) A LEED pattern of the clean surface at an electron energy of $75\mathrm{eV}$. The inset is a schematic representation of the LEED pattern, showing two rings of tenfold spots and satellite spots in-between. (b) $100\mathrm{nm}\times 100\mathrm{nm}$ STM image of a tenfold decagonal $\mathrm{Al}-\mathrm{Ni}-\mathrm{Co}$ quasicrystal surface ($V_s=1.0\mathrm{V}$, $\mathrm{I}=0.1\mathrm{nA}$). Height profile in the inset along the line between A and B shows the terraces with average width of $50–100\mathrm{nm}$ separated by single steps $0.2\mathrm{nm}$ high.

Figure 2

(a) $500\mathrm{nm}\times 290\mathrm{nm}$ contact mode topographic and (b) friction images acquired with an applied load of $400\mathrm{nN}$. (c) $500\mathrm{nm}\times 200\mathrm{nm}$ topographic and (d) friction images at a load of $750\mathrm{nN}$. (e) Height profile through the broken line in (a). The step height and width of terraces is similar as that observed with STM although the resolution is lower. In (c) and (d), the images show the effect of stick-slip, producing the dark bands at the beginning of the scan (top), suggesting that at this higher load the chemical nature of the tip-sample contact has been changed.

Figure 3

(a) $0.95\mu \mathrm{m}\times 1.0\mu \mathrm{m}$ topographic AFM image and corresponding friction images (b), acquired at an applied load of $150\mathrm{nN}$. A central area, previously scanned with a load of $750\mathrm{nN}$ (corresponding to a pressure of $4.1\mathrm{GPa}$), is inelastically deformed, showing a trench with a depth of $1.0\mathrm{nm}$, and debris at the edge. Height (c) and friction (d) profiles across the trench show that the friction force inside the trench is $74\mathrm{nN}\pm 13\mathrm{nN}$, higher than that of outside $(14\mathrm{nN}\pm 4\mathrm{nN})$ by a factor of 5.

Figure 4

Plot of adhesion force as a function of applied load. In the elastic regime, up to an applied load of $600\mathrm{nN}$, the adhesion force is $13\mathrm{nN}$. It increased up to $70\mathrm{nN}$ in the inelastic regime. Two force-distance curves, corresponding to these two regimes are shown in insets.

Figure 5

Plot of the friction force as a function of applied load. The lines are DMT and JKR fittings using the constraint of an adhesion force $\left(L_c\right)$ of $13\mathrm{nN}$. The DMT curve fits very well with the experimental data in the elastic regime, consistent with contact of two hard materials, TiN and the passivated quasicrystal. Open circles represents the inelastic contribution to the friction force and energy dissipation, which is the difference between the experimental friction and the extrapolated DMT values.