Makoto Chiba and Masahiro Seo
Keywords: Nano-scratching test, Friction coefficient, Single crystal<br>iron, Passive film
Abstract:
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Volume 6 Paper C016 Potential Dependence of Frictional Coefficient Evaluated by In-situ Nano-scratching for the Passive Iron Surface Makoto Chiba and Masahiro Seo Graduate School of Engineering, Hokkaido University, Kita-13Jo, Nishi8Chome, Kita-ku, Sapporo 060-8628, Japan, chiba@elechem1-mc.eng.hokudai.ac.jp Abstract In-situ (kept potentiostatically in the passive state) and ex-situ (in air after passivation) nano-scratching tests were performed to the iron single crystal (100) surfaces passivated at 0.0 V – 1.0 V (SHE) in pH 8.4 borate solution to evaluate the friction coefficient of the passive surface and its potential dependence. The friction coefficient obtained with in-situ nano-scratching for the passive iron (100) surface was significantly larger than that obtained with ex-situ nano- scratching. The friction coefficient obtained with ex-situ nanoscratching was almost independent of potential in the passive region. On the other hand, the friction coefficient obtained with in-situ nanoscratching increased with increasing potential in the passive region. These results were explained in terms of a series of mechano- electrochemical reaction such as the film rupture, active dissolution and repassivation taking place at the moving front of the indenter during in-situ nano-scratching. Keywords: Nano-scratching test, Friction coefficient, Single crystal iron, Passive film Introduction There have been many studies of passive films on metals. Most of studies reported so far were focused on the chemical properties [1- 13] such as composition, structure and thickness of passive film, or semiconductive properties, were while few studies there on mechanical properties of passive films on metals. Recently it reported has that been the chemical or electrochemical properties of passive metal surfaces correlate their mechanical properties to some extent [14-19]. If the correlation between chemical or electrochemical and mechanical properties is made clear, the mechano-electrochemical knowledge valuable for better understanding of the passivity breakdown and repassivation will be acquired. The recent development of nano-indentation or nano-scratching technique has made feasible to evaluate the mechanical properties of surface thin films on solids such as hardness, elastic modulus or friction coefficient [20-25]. In our previous studies [15,17-19], the first attempt of in-situ nano-indentation and in-situ nano-scratching in solution to the single crystal iron surfaces as kept at a constant potential in the passive region succeeded in getting the substantial information regarding the nano-mechano-electrochemical properties of the passive surfaces. In this study, in-situ (kept potentiostatically in the passive state) and ex-situ (in air after passivation) nano-scratching techniques were applied to the single crystal iron (100) surfaces passivated at various 2 potentials in pH 8.4 borate solution to evaluate the friction coefficient of the passive iron surface and its potential dependence. Experimental Single crystal iron (100) disk-plate with a diameter of 10 mm and a thickness of 1.4 mm were used for experiments. The iron specimen was mechanically polished and then finally electropolished with a constant current density of 32 mA cm-2 in a mixture of 70 % HClO4 and glacial CH3COOH (1 : 20) at 17 ˚C. The electrolyte solution employed for passivation was pH 8.4 borate solution. The passivation of the iron specimen was performed with a potentiostatic polarization at 0.0 V – 1.0 V (SHE) for 1 h in pH 8.4 borate solution, after cathodic reduction of an air-formed film at a constant current density of –30 µA cm-2 for 10 min. A small electrochemical cell made from Diflon was specially designed for in-situ nano-scratching test in solution. Platinum ring and wire were used as counter and reference electrodes, respectively. The transducer (Hysitron Co. Ltd., Triboscope) for nano-scratching was combined with AFM (Digital Instruments, Nanoscope IIIa) in which the electrochemical cell was set. Normal and lateral forces (FN and FL), and normal and lateral displacements (DN and D L) can be measured simultaneously by using this transducer. The iron specimen was potenstatically passivated at various potentials in the passive region for 1 h in pH 8.4 borate solution at 25 ˚C. The in-situ scratching performed nanoon was the specimen surface kept at the same passive potential in pH 8.4 borate solution after passivation for 1 h. The ex- 3 situ nano-scratching was also performed on the specimen taken out from the solution in air after passivation, washed with milli-Q filtered water and dried with a jet of nitrogen gas. A conical diamond indenter with a tip radius less than 1 µm and an included angle of 90 degree was employed for nano-scratching and attached to a tungsten rod for use in liquid. After nano-scratching test, the morphology of the scratched area can be observed with AFM by using the same tip used for nano-scratching. Figure 1 shows the outline of nano-scratching process. At first, the tip was indented to the sample surface by a normal force, FN, in the range of 100 – 1000 µN and then the tip was moved to the lateral direction under the constant normal force for 10 s at a scratching rate of 0.2 µm s-1 between a distance of 2 µm to obtain a lateral force, FL, and a normal displacement, DN, respectively, as a function of lateral displacement, DL. The friction coefficient, µ’, is defined with dividing a lateral force, FL, by a normal force, FN, [26] as represented by Eq. (1). µ’ = FL / FN (1) Results and Discussion Figure 2 shows the potentiodynamic anodic polarization curve of the iron (100) surface measured with a potential sweep rate of 100 mV min-1 in pH 8.4 borate solution. The vertical arrows in Fig. 2 represent the potentials at which the nano-scratching tests were performed. According to the ellipsometrical results by Sato et al. [2], the thickness of passive film formed potentiostatically on a polycrystalline iron surface for 1 h in pH 8.4 borate solution increases linearly with film formation potential and is in the range of 2 – 5 nm as shown in a dashed line of Fig. 2. Figure 3 shows the normal displacement, DN, and friction coefficient, µ’, as a function of lateral displacement, DL, obtained with in-situ (kept at 0.25 V) nano-scratching at a normal force of FN = 500 µN for the iron (100) surface passivated at 0.25 V (SHE) for 1 h in pH 8.4 borate solution. The normal displacement, DN, increases with increasing lateral displacement, DL, passing through a maximum, and 4 attains a steady state. On the other hand, the friction coefficient, µ’, increases monotonously with increasing lateral displacement, DL, and attains a steady state. In the steady state region where the friction coefficient is independent of the lateral displacement, the friction coefficient is insensitive to the change in the normal displacement. steady Here, state friction value coefficient the of was employed for discussion. Figure 4 shows the AFM image height-profile scratched area and the of the for the passive iron (100) surface after in-situ nano- scratching test. The scratching and the groove produced by the resultant protrusion can be observed clearly from the AFM image. From the height-profile along the dashed line in the AFM image of Fig. 4, it is seen that the depth of the groove, DG, is 50 – 100 nm, which is less than the normal displacement, DN, of 70 – 120 nm in Fig. 3 a). This difference between DG and DN may be ascribed to the contribution of elastic deformation to DN, since DN contains both contributions of elastic and plastic deformation, while DG after the scratching contains only the contribution of plastic deformation. Furthermore, as seen from the comparison between Fig. 3 a) and Fig. 4 b), the lateral changes of DG are less than the changes of DN with DL, which may be caused by the size effect of the tip because the tip radius (< 1 µm) is not sufficiently small to measure exactly the bottom depth of the groove. 5 Figure 5 shows the friction coefficient, µ’, as a function of normal force, FN, obtained with in-situ and ex-situ nano-scratching tests to the iron (100) surfaces passivated at 0.25 V and 0.75 V (SHE) for 1 h in pH 8.4 borate solution. Here, these friction coefficients were obtained by averaging the values measured 10-20 times at each normal force and their standard deviations were 25%. A single crystal may exhibit non-isotropic deformation behavior which would provide the dependence of friction coefficient on scratching direction. In the present nano-scratching tests, the scratching direction was not specified. The effects of scratching direction on friction coefficient, even they are present, might be masked in the standard deviation of 25 %. The friction coefficients obtained with in-situ nano-scratching are independent of normal force within the standard deviation of 25 %, while the friction with ex-situ nano- coefficients, µ’, obtained scratching increase with increasing normal force, FN. It should be noted that the friction coefficients of the iron (100) surfaces obtained with in-situ nano-scratching test are larger than those obtained with ex-situ nano- scratching test. Moreover, the friction coefficient of the iron passivated (100) at surface 0.75 V obtained by in-situ nano- scratching test is larger than that of the iron (100) surface passivated at 0.25 V. In contrast, there is no significant difference between the friction coefficient of the iron (100) surface passivated 0.25 V and 0.75 V obtained with ex-situ nano-scratching test. 6 In Figs. 6 and 7, the friction coefficients, µ’, obtained with in-situ and ex-situ nano-scratching tests under the normal forces, FN = 100 µN and 1000 µN are plotted as a function of potential for the iron (100) surface passivated at various potentials, E. The friction coefficients obtained with in-situ nano-scratching test increase with increasing potential, while those obtained with ex-situ nano- scratching have no significant potential dependence. The potential dependence (0.24 V-1) of the friction coefficient obtained with in-situ nano-scratching under FN = 100 µN is larger than that (0.18 V-1) under FN = 1000 µN. The normal displacements, DN, under FN = 100 µN and 1000 µN are 30 nm and 150 nm, respectively, irrespective of in-situ and ex-situ nano-scratching, which are much larger than the thickness (2 – 5 nm) of passive films as seen from the dashed line in Fig. 2. This means that the thickness of passive film does not significantly affect the friction coefficient, which is consistent with the no significant potential dependence of the friction coefficients obtained with ex-situ nano-scratching. In contrast, it seems difficult to explain the significant potential dependence of the friction coefficient obtained with in-situ nano-scratching without taking into account a series of mechano-electrochemical reaction which would take place at the moving front of the indenter tip during in-situ nano- scratching as discussed later. It is generally accepted that the lateral force, FL, consists of the adhesion term, Fa, and the ploughing term, Fp [26]. FL = Fa + Fp (2) The adhesion term, Fa, is given by Fa = sA (3), where A is the horizontal cross sectional area of the conical indenter tip in contact with the sample surface (see Fig. 1) and s is the shear strength at the scratched interface. Hardness of material surface, H, is defined with dividing FN by A [20]. H = FN / A (4) 7 By substituting Eq. (4) into Eq. (3), the adhesion term, Fa, is represented as follows. Fa = s (FN / H ) (5) The contribution of the adhesion term to the friction coefficient, µ’a, therefore, is given by µ’a = s / H (6). The ploughing term, Fp, is represented by Fp = A’ pf (7), where A’ is the vertical cross sectional area of the conical indenter tip in contact with the sample surface (see Fig. 1) and pf is the plastic flow pressure of materials against nano-scratching. The proportional relation holds between A and A’ from the geometry of a conical indenter. If the conical indenter has an ideal geometry (ø = 45˚), the proportional constant is k = A’ / A = (cot ø) / π = 0.318 [26]. The ploughing term, Fp, therefore, can be replaced by Fp = k A pf (8). By substituting Eq. (4) into Eq. (8), the ploughing term can be represented as follows. Fp = (k pf / H) FN (9) The contribution of the ploughing term to the friction coefficient, µ’p, is eventually given by µ’p = k pf / H (10). The net friction coefficient, µ’, containing both contributions of adhesion and ploughing terms is eventually represented by µ’ = µ’a + µ’p = s / H + k pf / H (11). For a single crystal iron, the value of s is less than 100 MPa at room temperature [27] and the value of H obtained with nano- indentation is about 3 GPa [17], from which µ’a is estimated to be less 8 than 0.033. Assuming that pf is equivalent to H [26], µ’p = 0.318 is estimated from Eq. (10). The net value of µ’ is 0.35, which is close to the experimental values obtained with ex-situ nano-scratching in the range of FN < 300 µN for the passive iron (100) surface. The experimental value, however, deviates upward from the estimated value with increasing FN in the range of FN > 300 µN. The dependence of µ’ on FN suggests that pf is not equivalent to H for the passive iron surface. It has been considered so that plastic the far [26] flow pressure, pf, is peculiar to materials, independent i.e., of the normal force, FN. It is deduced from the present results that the plastic flow pressure, pf, against nano-scratching does not only depend on materials themselves but also the normal force, FN. Furthermore, the experimental values of µ’ obtained with in-situ nano-scratching are significantly larger than those obtained with ex-situ nano-scratching, which could not be explained without consideration taking a series into of mechano-electrochemical reaction (such as the film rupture, active dissolution from the rupture sites and the repassivation) taking place at the moving front of the indenter tip during in-situ nano-scratching. Since the normal displacement, DN, is significantly larger than the thickness of passive film, the 9 passive film is ruptured at the moving front of the indenter tip during nano-scratching. In case of ex-situ nano-scratching, the rupture sites of passive film would be repaired by air-oxidation. On the other hand, in case of in-situ nano-scratching, active dissolution from the rupture sites and followed by the repassivation would take place. The repassivation may be promoted by a potential difference between the iron substrate at the rupture sites and solution. The repassivation rate, therefore, increases with increasing film formation potential. The pile up of the passive film due to repassivation would provide the high impedance against the movement of the indenter tip to increase the friction coefficient. The significant potential dependence of the friction coefficient obtained with in-situ nano-scratching may be explained by the increase in repassivation rate due to the increase in the potential difference between the iron substrate and solution. No significant potential dependence of the friction coefficient obtained with ex-situ nano-scratching may be explained by the constant repassivation rate due to air oxidation. At present, there is no appropriate explanation for the potential dependence of friction coefficient obtained with in- situ nano-scratching except for the assumption that a series of mechano-electrochemical reaction such as film rupture, active dissolution from the rupture sites and the repassivation takes place at the moving front of the indenter tip. Conclusions The following conclusions were drawn from the in-situ and ex-situ nano-scratching tests for the iron (100) surface passivated at 0.0 V – 1.0 V (SHE) for 1 h pH 8.4 borate solution. 1) The friction coefficient obtained with in-situ nano-scratching for the passive iron (100) surface was always larger than that of obtained with ex-situ nano-scratching. 2) The friction coefficient of the passive iron (100) surface obtained with in-situ nano-scratching increased with increasing the passive film formation potential. 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