Surface reaction layer

Auger electron spectroscopy (AES) is particularly suited for surface analysis (depth 0.5-1 nm). AES depth profile analysis was employed to determine the thickness and composition of surface reaction layers formed under test conditions in the Reichert wear apparatus in the presence of four different ZDDPs additives at different applied loads (Schumacher et al., 1980). Using elemental sensitivity factors the concentration of the four elements (S, P, O, C) was determined at three locations corresponding to a depth of 1.8, 4.3, and 17 nm. No significant correlation between wear behavior and carbon or oxygen content of the reaction layer was observed. A steady state sulfur concentration is reached after a very short friction path. Contrary to the behavior of sulfur, phosphorus concentration in the presence of ZDDPs increases steadily with friction path, and no plateau value is reached. 

Like growth reactions, corrosion at surfaces involves a change in surface composition however, it is typically associated with a deleterious change. Corrosion reactions can be separated into two classes those involving the removal of material from the solid and those involving the formation of a surface reaction layer. In the first type of reaction, gas-or solution-phase species react with the surface to produce volatile or soluble products as in the etching of silicon by hydrofluoric acid. The second type of reaction involves the reaction of surface atoms to form a new compound which remains on the surface. The oxidation of many metals is considered a corrosion reaction of this type. 

Equations (92) and (93) assume fr (the surface reaction layer transmissivity) is a constant parameter. The steady-state sensitivity parameters, k, r, v, 5, Vg, and 5g... 

Stability relatiMshipa and the sequence of formation of surface reaction layers can he predicted from potential-pH diagrams. The Cu-O-S-HjO system will be used as an example- Figure 93-6 represents the Cu-O-S-H.O system at S = 10". Stable regions for Cu2S and CuS indicate that sulfur films will not form adjacent to CujS since the reaction... 

Chemical etching or ion milling can be used to mitigate this problem. Near surface reaction layers, if they form on the scale of a few tens of nanometers, cannot be seen using this technique, because these are commonly sputtered away during the time the beam stabilizes. 

Based on our previous studies of the dissolution and crystallization kinetics of potassium inorganic compounds based on linear nonequilibrium thermodynamics (Ji et al, 2010 Liu et al, 2009 Lu et al, 2011), we proposed to assume that the kinetic process of CO2 absorption by ILs comprised two steps surface reaction and diffusion, as shown in Fig. 17. Figure 17 demonstrates that when CO2 in the vapor phase and the ILs were in contact, the chemical reaction of CO2 with ILs occurred for the chemical absorption process of CO2 by ILs in the first step, which was named as the surface reaction layer, while for the physical mass transport process of CO2 by ILs in the first step, CO2 in the vapor phase would be transported into the IL phase, which was also named as the assumed surface reaction layer. As for the surface reaction layer, the driving force of the surface reaction was the chemical potential gradient of CO2 between CO2 at the vapor—Hquid interface and gas CO2. After that, in the second step, CO2 in the IL phase would... 

Quasi-equilibrium exposure regime. After tire 7 x 7 stmcture has been removed, quasi-equilibrium between etching and growtli of tire reaction layer is established. The reaction layer is about one monolayer tliick, and contains primarily SiF. Defects fonn near tire surface, partly from tire large reaction exotliennicity. 

Transition to steady-state etching. The surface becomes sufficiently disordered to dismpt tlie quasi-equilibrium, and tlie reaction layer becomes a tree stmcture of fluorosilyl chain stmctures tenninated by SiF., groups. 

Multilayers of Diphosphates. One way to find surface reactions that may lead to the formation of SAMs is to look for reactions that result in an insoluble salt. This is the case for phosphate monolayers, based on their highly insoluble salts with tetravalent transition metal ions. In these salts, the phosphates form layer stmctures, one OH group sticking to either side. Thus, replacing the OH with an alkyl chain to form the alkyl phosphonic acid was expected to result in a bilayer stmcture with alkyl chains extending from both sides of the metal phosphate sheet (335). When zirconium (TV) is used the distance between next neighbor alkyl chains is - 0.53 nm, which forces either chain disorder or chain tilt so that VDW attractive interactions can be reestablished. 

To produce wear-resistant or hardened surfaces, thin layers of borides can be prepared on metal surfaces by reaction and diffusion (see Metal SURFACE treatments). Boride powders can be formed iato monolithic shapes by cold pressing and sintering, or by hot pressiag. 

The thermodynamic phase stability diagrams appear to be preferred by corrosion scientists and technologists for the evaluation of gas-metal systems where the chemical composition of the gaseous phase consisting of a single gas or mixture of gases has a critical influence on the formation of surface reaction products which, in turn, may either stifle or accelerate the rate of corrosion. Also, they are used to analyse or predict the reason for the sequence of formation of the phases in a multi-layered surface reaction product on a metal or alloy. 

Phase Stability Diagrams and the Sequence of Phases in Surface Interaction Layers (Reaction Paths)... 

The micrographs in Fig. 7.88 show clearly how from a knowledge of the AG -concentration diagrams it is possible to select the exact reaction conditions for the production of tailor-made outermost surface phase layers of the most desired composition and thus of the optimum physical and chemical properties for a given system. In addition it shows that according to thermodynamics, there can be predictable differences in the composition of the same outermost phase layer prepared at the same conditions of temperature but under slightly different vapour pressures. 

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