Nascent surfaces

Figure 8.10 is the variation of the potential and the current of the pyrite electrode with pyrite as the grinding media at different pressure at natural pH in the presence of xanthate. The results show that the corrosion potential decreases with the increase of mechanical pressure. The range of potential change is in 170- 190 mV. This change may arise from the formation of nascent surface... 

The newly formed (nascent) surface (Saint-Pierre 1964)... 

The mechanochemical reactions of aluminum were investigated under two reaction conditions, namely, during and after the milling. The active source may be different in the two reactions. However, high temperature, high pressure, and nascent surface appeared not to be active factors in this case, because preactivated aluminum was observed to react with butyl bromide even after the termination of milling. 

As the rubbing continues, the polyphosphate layer comes into closer contact with water in oil and is hydrolyzed to give a short-chain polyphosphate, e.g., Zn2P207 (Fuller et al., 1998). During the mechanically activated processes (friction coefficient, p > 0.4), a nascent surface is generated, and in the presence of oxygen, an iron oxide is formed. The acid - base reaction between polyphosphate glasses (hard Lewis bases) and the oxides (hard Lewis acids) is... 

The primaiy role of sulfur species in the tribochemical processes is passivation by sulfur species on nascent surfaces caused by a severe wear process (extreme pressure conditions) Fe2+ + S2" - FeS, the heat of formation AHf (FeS) = -1.04 eV. The sulfide can prevent adhesion and also the attack of oxygen species, AHf (FeO) = - 2.82 eV (Mori, 1995). There is enough sulfur in engine oil to initiate acid-base reactions in the formation of zinc sulfide (Martin, 1999). Organic sulfur species other than in thiophosphate form can react with the ZnO produced by the phosphate reactions according to the following ZnO + S2 - ZnS + O2. The zinc sulfide can also be directly produced if the polyphosphate contains sulfur atoms in the polymer chain (thiophosphate), for example ... 

The effect of the particular functional group on adsorption, however, is not always the same as seen on oxide surfaces. For example, carboxylic acids adsorb strongly on metal oxide surfaces, and the heat of adsorption of stearic acid is higher than that of the corresponding ester, methyl stearate (Hironaka et al., 1978). For adsorption on the nascent surface, however, propionic acid is a poor adsorbate, and the adsorption activity of propionic acid is lower than that of methyl propionate. Although propyl amine adsorbs easily on metal oxide surfaces, the adsorption activity of propyl amine is low on the fresh steel surfaces. Also, the heat of adsorption of organic sulfides on iron oxide is less than that of esters (Forbes et al., 1970b), but the results of adsorption activity on the nascent surface were the opposite. It is thus noteworthy that the chemical nature of the nascent surface of steel is often opposite to that of oxide-covered metal surfaces, with respect to adsorption. 

The adsorption of organic compounds on nascent surfaces can be considered as an acid-base reaction. According to the hard-soft acid and bases HSAB principle (Ho, 1977), polar compounds such as carboxylic acid and amine (with lone pair electrons on oxygen or nitrogen) are classified as "hard bases". A hard base reacts more easily with a hard acid than with a soft acid. Metals are classified as soft acids which react much more easily with soft bases than hard bases. The results in Table 5.1 can be explained with this concept. The soft bases (benzene, 1-hexene, diethyl disulfide) react easily with the nascent surface as a soft acid. On the other hand, the hard bases such as propionic acid, stearic acid, propyl amine and trimethyl phosphate exhibit a very low activity (Fischer et al., 1997a and 1997b Mori and Imazumi, 1988). 

When surfaces of tribological systems are involved in the mechanical activity of rubbing, direct reactions of surface adsorbed films with solid surfaces take place. The mechanically activated clean surface (nascent surface) of the metals and alloys is extremely reactive. Tribofilm formation is caused by the interaction between the metal (M, substrate) nascent surface under high energy and chemisorbed molecules of additive (adsorbate) (Buckley, 1981). 

Boundary lubrication - Nascent surfaces — Tribofilm formation... 

Nascent surface Explain the difference in the concept of liquid lubrication mechanism in (a) hydrodynamic, (b) elastohydrodynamic and (c) boundary lubrication. Which of the following characterize (a), (b), and (c) lubrication regime continuous fluid film, negligible deformation, complete separation of the surfaces, elastic and plastic deformation, no wear takes place, no contact between the sliding surfaces, involving surface topography, physical and chemical adsorption, catalysis and reaction kinetics, and tribochemical film formation ... 

Mechanism. Basically, fretting is a form of adhesive or abrasive wear, where the normal load causes adhesion between asperities and oscillatory movement causes ruptures, resulting in wear debris. Most commonly, fretting is combined with corrosion, in which case the wear mode is known as fretting corrosion. For example, in the case of steel particles, the freshly worn nascent surfaces oxidize (corrode) to FejO, and the... 

Erosion damage by solid particle impact or cavitation bubble collapse to an oxide or passive film will reveal the underlying nascent surface inducing a higher activity (higher corrosion... 

G. Steinberg (Memorex Corp.) On the subject of temperature-tIme superposition, one must define the system (as in all cases). When adhesion can occur, e.g. in thermoplastic systems, the frictional force will increase wide sliding and load. More and more adhesion (fusion) will take place as more and more "new" (nascent)surface is exposed, and "friction" wi11 increase. This results in stick-slip. Such systems can also "weld" when in contact after sliding stops adhesion. Rupture then occurs on restarting sliding. Please comment. 

J. J. Bikerman As long as the temperature of the slider-support interface is moderate, there is no difference between thermosetting and thermoplastic polymers. No fusion will be expected as long as this temperature is below the softening range of the polymer material. Usually, the "nascent" surface becomes covered with adsorbed gases before it comes in contact with another solid when a nail is driven into a polymer plate, a layer of air is still present between the two. The "stick-slip" kind of motion depends above all on the inertia of the measuring instrument. 

As mentioned above, tribological behavior in boundary lubrication is strongly affected by surface reactions as well as by chemisorption of additives on contacting surfaces. The molecular structure of the additives is not the only parameter responsible for any particular tribochemical behavior. During the friction process, contamination layers on the outer surface are removed mechanically, leading to the fomiation of fresh, nascent surfaces characterized by... 

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