Wear, 66 (1981) 379 - 387 @ Elsevier Sequoia S.A., Lausanne -
379 Printed in The Netherlands
INFLUENCE OF THE PLASTIC DEFORMATION OF METALS DURING MIXED FRICTION ON THEIR CHEMICAL REACTION RATE
(Received June 6,198O)
Summary Mechanical effects such as friction may activate chemical reactions between solids and their surrounding media. Several factors can contribute to this effect: heat of friction, the removal of protective surface layers, an increase in the effective surface area, faster transport of reacting particles and production of lattice defects by plastic defo~ation. The effect of lattice defects on the rate of different chemical reactions was measured. It was shown that this effect can act toincrease the reaction rate by one to two orders of m~itude. The mechanical effect depends not only on the metal and the tribological stress but also on the reaction partner and the experimental conditions.
1, Introduction Friction and wear are markedly influenced by chemical reactions with the surrounding medium. Well-known examples are the reactions of metals with oxygen or water vapour resulting in tribo-oxidation and the formation of abrasive oxide [l -31. Vice versa, chemical reactions can be activated by the friction process, e.g. the reaction of the metal of hypoid gears with extreme pressure (EP) additives, producing protective surface layers [ 4 ] . On the one hand the additives are not corrosive; on the other hand the surface compounds must be formed as soon as the asperity peaks come into contact. The sudden increase in the reaction rate is usually attributed to the heat of friction, and junction temperatures have been calculated under this assumption [ 5 1. However, other effects may also contribute to the mechanical activation of chemical reactions: the removal of protective surface layers, e.g. metal oxides;an increase in the effective surface area; increased transport rates of reaction partners; the formation of lattice defects, i.e. areas of increased free energy by plastic deformation of metals, for example. Striking examples of shifting chemical equilibria by mechanical work on solids are not as well known in friction as in grinding experiments. For
example a fortyfold increase in the carbon dioxide pressure at 400 “C has been measured after grinding limestone . Copper oxide can be formed by grinding copper powder in a carbon dioxide atmosphere although the free energy of reaction is found to be positive at all temperatures when the free energy of annealed copper is used in the calculation [ 71. The kinetic activation of chemical reactions of solids after mechanical work was shown as early as 1885 by Osmond and Wirt [ 81 who found a marked increase in the dissolution rate of iron in dilute acids after rigorous cold working. The formation of Ni(CO), from its components was found to be significantly enhanced when the nickel plates were shaken with silicon carbide grains, the reaction rate remaining nearly constant down to temperatures of -83 “C [ 91. The interaction of corrosion and mechanical wear has been extensively investigated, e.g. the dissolution of stainless steel, chromium and nickel in sulphuric acid during scraping with an aluminium oxide plate [lo]. In most work the total effect of different parameters such as temperature, lattice defects and increase in surface area has been measured and attributed to the parameter which would be expected to be predominant under the experimental conditions used. The aim of this paper is to determine the isolated effect of lattice defects after eliminating all other factors. In two previous investigations of electrochemical reactions [ 11, 121 it has been shown that the lattice defects created by friction can increase the reaction rates by factors between 2.2 and 8.8. The lowest one was found for the anodic dissolution of cadmium in phosphoric acid, the highest one for the dissolution of nickel in sulphuric acid. When copper was dissolved in sulphuric acid it was found that there was little load effect. The effect of the reaction partner of the dissolving metal did not seem to be important for the dissolution of copper in sulphuric acid, phosphoric acid and ammonia [ 121. However, these results cannot be extrapolated to other reactions, the most interesting one being that of gear metals with EP additives. The experiments mentioned above were carried out using aqueous solutions, two of them producing a copper complex with water ligands and one an ammonium complex. Therefore reactions in non-aqueous media were investigated. Iron electrodes were used in most cases. The effect of the potential on the increase in the rate of dissolution produced by friction was also measured.
2. Experimental In order to isolate the effect of mechanical activation and in particular to exclude effects due to frictional heat, electrochemical reactions were chosen. Their rate can be measured continuously via the current, and the temperature effect was excluded by separating the mating partners in the experimental device described in ref. 11. It has been shown that 0.3 s after separation the temperature rise was less than 2 “C. When suitable electrode-
electrolyte systems are used, protective surface layers can be excluded. Any increase in the surface area can be detected by measuring the double-layer capacity. The reactions took place in a cylindrical vessel, the interior of which was coated with Teflon. Three pins of diameter 10 mm were screwed symmetrically into the Teflon bottom and were electrically connected below as working electrodes. A platinum wire counterelectrode was inserted in the centre. A rotating disc inserted in a Teflon cylinder was kept in position above the pins by a bearing. A normal load pressed the vessel with the pins upwards against the disc and was applied by a dead weight via a lever. When interruption of frictional contact was required, the lever was released so that the vessel with the pins dropped through 4 mm. The pins and the disc were immersed in the electrolyte and air was excluded by bubbling nitrogen through it. The disc was made of alumina which was shown experimentally to give the same results as discs of the pin material. A Teflon tube attached to one pin was used as a Luggin capillary. The reference electrode was mercurous sulphate for aqueous electrolytes and mercurous acetate [ 131 for organic electrolytes. A constant deviation from chemical equilibrium, i.e. a constant potential, was maintained by a Jaissle potentiostat, except for organic electrolytes with lower conductivities when a Wenking potentiostat with compensation for the IR potential drop was used. The current before, during and after friction was recorded. When rapid current changes occurred after separating the mating surfaces, an oscilloscope was used. In these cases the current measured 0.3 s after separating the surfaces was assumed not to be influenced by frictional heat [ 111. The current density after friction was referred to the rubbed area. The pins were annealed before use. The double-layer capacity, which was assumed to be proportional to the “true” surface, was determined by galvanostatic transients. In most cases the capacity increased by 20 - 50% after the friction experiment, which is small compared with the effect of the lattice defects. Only in one case, when it was more than 100’S, was it used for calculating the current density. Pitting corrosion which resulted in a low frictional effect was observed in some experiments with iron pins, and the results of these were ignored. The temperature was 40 “C, the load was 157 N and the sliding speed was about 19 cm s-l. The front of the pins was rubbed for at least 20 min before measuring the current after friction. The electrolytes were prepared as far as possible from analytical grade chemicals and doubly distilled water. Almost water-free organic sulphuric acid solution was prepared using sulphuric acid containing 30% sulphur trioxide. The rest of the water was determined by Karl Fischer titration as small traces, the highest amount being the water content of sulphuric acid in ethanol (0.23% HsO) after the experiment which was partly due to esterification.
3. Results A characteristic example of a friction experiment is shown in Fig. 1 where the dissolution of copper in sulphuric acid-copper sulphate solution is shown before, during and after rubbing with an aluminium oxide disc. At the beginning of friction the current decreases because most of the pin is covered by the nonconducting disc. After separation the current steeply increases because of separation and activation and then decreases because the most disordered atoms of the lattice are dissolved before the less disordered ones. The results are different when the dissolution of nickel in sodium sulphate-sulphuric acid solution is considered (Fig. 2). Here the increase in reactivity is large enough to increase the anodic current during friction. The current transport in the partial split between the rubbed pin surface and the disc is due to the high conductivity and convection of the solution. The time lag of the current maximum after release is accounted for by the time required to form a catalytic surface compound [ 141. Similar observations have been made with iron within the first few seconds [ll] and can be explained in the same way [ 151. At the maximum the current density of nickel dissolution in the rubbed area is 88 times higher than before rubbing. The effect of the potential on the relative increase in current after friction is shown in Fig. 3 where the current density-potential diagrams for annealed copper in copper phosphate-phosphoric acid solution obtained before and after friction are compared. The diagrams show not only that the copper dissolution was accelerated but also the counter-reaction of copper deposition at the disturbed lattice. However, the relative increase in the copper deposition rate was lower and thus the open-circuit potential was shifted in the negative direction. At larger deviations from the equilibrium potential the relative rate increase became smaller as the accelerating effect of the overvoltage became dominant over the effect of the lattice defects.
Fig. 1. Dissolution of copper in aqueous sulphuric acid (1 N HzS04-1 during and after friction: overvoltage, +5 mV; load, 119 N. Fig. 2. Dissolution of nickel in sulphuric acid (0.5 M Na2S04, after friction: potential ELI, +19 mV; load, 119 N.
N CUSO~), before,
pH 1) before, during and
cd 4 2
O -2 -L
Fig. 3. Dissolution and deposition of copper in phosphoric acid (CcU+ = 0.1 M, pH 1) before (0) and after (X) friction. Fig. 4. Current density us. potential for iron in perchloric acid (0.5 M NaC104, pH 1) before (0) and after (X) friction.
A stronger effect of mechanical activation is observed for iron dissolution in an aqueous perchloric acid-sodium perchlorate solution (Fig. 4). The measurements were not extended to potentials at which the counter-reaction (hydrogen evolution) was accelerated by friction, but there is no doubt that the electrocatalytic activity was increased by lattice defects [ 16, 171 although the increase in the anodic reaction was larger. Mechanical activation experiments with reactions other than the anodic dissolution of metals to form hydrated metal ions were performed using iron, which is the most interesting metal from a technical viewpoint, and sulphuric acid dissolved in anhydrous organic solvents. Preliminary tests were carried out to select solvents in which the electrode was not covered by a surface layer, the resistance of which determined the rate of reaction. This was ensured by measurements with high currents and by measuring the ohmic IR drop with transients. Figure 5 shows the current density-potential diagram for iron in sulphuric acid dissolved in dimethyl sulphoxide. There is only a slight increase of the anodic current. Although the counter-reaction is more accelerated ai
22 5 I 2 0 i
-2 -4 -6 ~B -10
Fig. 5. Current density us. potential for iron in anhydrous sulphuric acid-dimethyl phoxide before (0) and after (X) friction.
very negative potentials, the increase in the rate of iron dissolution is dominant near the corrosion potential, as shown by the shift in the potential at which the current is zero, i.e. the anodic and cathodic reaction rates are equal. Similar small accelerations of iron dissolution were found in other solvents such as ethanol or NJVdimethylformamide. For NJVdimethylformamide the slight acceleration in the counter-reaction was more dominant near the corrosion potential which was displaced in the anodic direction.
4. Discussion The measurements demonstrate that lattice defects created by friction can cause a marked increase in the reactivity of metals. The acceleration factor decreases with increasing distance from the corrosion or equilibrium potential, and hence there is an increased Tafel slope in a logarithmic plot if there is any linear part of the curve after friction. The increase in reaction rate due to plastic deformation cannot be measured completely even with the method used here. The effect begins to decrease immediately after rubbing ceases; however, correct measurement is only possible after the temperature decrease, the electrochemical double layer is rearranged and in some cases catalytic surface compounds are formed. In this period the most activated areas of the lattice may already have formed reaction products. Even the process of deformation in addition to the unavoidable temperature increase may activate chemical reactions. For example Frisch and Roper  have interpreted tribochemical reactions by the migration rate of dislocation lines. Therefore the acceleration rates reported here are only minimum values. Some energy considerations can be added to the kinetic measurements, although conclusions extrapolated from thermodynamics to kinetics and vice versa have to be treated with care. The increase in the free enthalpy of the reactions cannot be measured by the shift in open-circuit potential after friction nor by any shift in chemical equilibria, e.g. after shaking nickel plates with grains of a hard material in a carbon monoxide atmosphere. A more negative potential is obtained by measuring the potential of a copper electrode in a solution of copper ions after friction, but this is not the equilibrium potential of the disturbed surface as is sometimes presumed. Forward and backward reactions are not reversible in this case. The most disturbed lattice areas dissolve preferentially. However, the deposition does not form similarly disturbed crystallites. The measured potential is a mixed one at which two reactions occur at the same rate: the dissolution of copper atoms of increased energy, and the deposition of copper at suitable sites, which are even more numerous on a disturbed surface, as can be seen by the increase in the deposition rate in Fig. 3. The mixed potential is much more positive than an imaginary equilibrium potential at which dissolved and deposited copper are equally disturbed. Therefore the real increase in the
free enthalpy is much higher than that calculated from the potential shift after friction. Vice versa, it appears impossible to estimate the expected reaction rate increase from data regarding the excess energy stored in the disturbed lattice. In friction experiments it is not the bulk excess energy which is relevant but the energy per gram atom stored in the outermost surface which, corresponding to the effective deformation, decreases rapidly with depth [ 191. The energy stored in friction tracks on thin aluminium foils has been estimated using electron microscopy in order to determine dislocation densities [ 20,21 J , The energy of a step dislocation line can be estimated from r223 E=
In 2 i ro )
where G is the shearing modulus of elasticity, b is the Burgers vector, L is the length of the dislocation line and or is Poisson’s ratio. The radii rl and r. are assumed to be half the distance to the next dislocation line and about half the Burgers vector. However, the results obtained with thin aluminium foils gave such small amounts of energy that they could not cause a measurable increase in the reaction rate. Higher values are obtained for the stored energy if the maximum data for dislocation densities are used, which in plastically deformed pure metals range up to 101’ crne2 [ 231. An extremely high dislocation density (5 X 1012 cm-s) has been found in a friction track on steel by X-ray diffraction line broadening and is due to the prevention of dislocation inflation by the chromium and v~adium coning [ 241. If this value of the dislocation density is used and the amount of energy due to the interaction of dislocations is assumed to be the same as the energy stored within the dislocation lines [ 251, a stored elastic energy of about 1300 J (g atom)-’ is obtained. Sometimes it is proposed that the increase in the reaction rate can be calculated from the Arrhenius equation by assuming that the pre~xponenti~ factor is constant and the activation energy is decreased by the total amount of stored energy. For highly deformed‘steel this calculation would predict a rate increase of about 63%. To obtain agreement with experiment, we would have to postulate stored energies which have never been measured. Moreover, the assumption is con~dic~d by other results: the reaction rate is increased by a factor which is not constant as would be expected from the Arrhenius equation but is very dependent on the potential; measurements on the same metal after the same tribological stress gave very different increases in dissolution rate for different reactions, e.g. formation of different solvates of the iron ion. In addition, the conditions for using the Arrhenius equation are not fulfilled as the stored energy is not distributed over the surface in thermal equ~ib~um but is concentrated at the ends of dislocation lines. These end points are nuclei for the dissolution; a screw dislocation in particular offers an immediate step for dissolution with a lower activation energy, producing an etch pit. However, for the reasons given earlier the rate increase cannot be
explained by the increased number of dislocation lines alone but depends on the ~dividu~ d~olution kinetics of each reaction.
Acknowledgments The author is indebted to Dr. P. Studt for helpful discussions, to them. Ing. R. Kriiger for assistance during the initial experiments and to the Deutsche Fo~chungsgemeinschaft for financial support.
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