Adsorption fatigue

Below is a brief account of the mechanical aspect contribution into corrosive wear. Failure of materials is treated today as a discrete process of jerky local elementary acts repeated periodically in time [15]. Since metal friction has a pulse behavior and metals wear by the fatigue mechanism [16], so friction and wear in corrosion-active media should be treated as the interrelation of the process with adsorption and corrosive fatigue. Adsorption fatigue usually paves the way for corrosion processes in the materials and precedes stronger effects in the media where the rubbing bodies operate [17]. 

Results from our fatigue tests have shown that, in the one-to two-month accelerated fatigue tests, blood is a less severe environment than saline or air. This may be due to adsorption of the blood components into the microcracks, resulting in reduced stress concentrations. More work is anticipated to study the effect of blood on the fatigue life of elastomers. 

Electron emission occurs when plastic deformation, abrasion, or fatigue cracking disturbs a material surface. Triboelectrons are emitted from freshly formed surface. The emission reaches a maximum immediately after mechanical initiation. When mechanical initiation is stopped, the emission decays with time. Strong emission has been observed for both metals and metal oxides. There is a strong evidence that the existence of oxides is necessary. The exoelectron emission occurs from a clean, stain-free metallic surface upon adsorption of oxygen (Ferrante 1977). 

The service performance of rubber products can be improved by the addition of fine particle size carbon blacks or silicas. The most important effects are improvements in wear resistance of tire treads and in sidewall resistance to tearing and fatigue cracking. This reinforcement varies with the particle size, surface nature, state of agglomeration and amount of the reinforcing agent and the nature of the elastomer. Carbon blacks normally are effective only with hydrocarbon rubbers. It seems likely that the reinforcement phenomenon relies on the physical adsorption of polymer chains on the solid surface and the ability of the elastomer molecules to slip over the filler surface without actual desorption or creation of voids. 

When one talks about reversibility of the Rehbinder effect, the presence of a thermodynamically stable interface between mutually saturated solid phase and the liquid, as well as complete disappearance of these effects upon the removal of the medium (e.g. by evaporation) are implied. These features emphasize principal difference between the Rehbinder effect and corrosive action of the medium. At the same time, one has to realize that it is not possible to draw here a distinct border line. The term disintegration covers a broad range of processes from idealized cases of purely mechanical breaking to destruction by corrosion or dissolution. The Rehbinder effect, i.e. the lowering of strength due to adsorption and chemisorption, stress-caused corrosion, and corrosion fatigue, occupies some intermediate place between these extremes. All these phenomena represent a certain degree of combination between the mechanical work performed by external forces and chemical (physico-chemical) interaction with the medium. 

This idea is very similar to the hypothesis explaining the fatigue strength of silica glass under load [106], There is an opinion that the adsorption of moisture from the environment results in the hydrolysis reaction ... 

The reasons that determine the properties modification by tear and fatigue can be the following 1) the appearance of free macroradicals under the deformation conditions 2) the activation of some reactions with medium components 3) chains reorientation and packing in supramolecular formations 4) the appearance of adsorption and superficial-active properties of the mechano-destruc-tion products and 5) the ordering and plastic properties modification of the contact surfaces by friction with the polymer [595]. 

In the discussion of the reversibility of the Rehbinder effect, it was implied that there is a thermodynamically stable interface present between the mutually saturated solid phase and liquid medium and that the effect vanishes when the liquid medium is ranoved, for example, by evaporation. These two peculiarities make the Rehbinder effect principally different from the corrosion caused by the action of aggressive media. At the same time, one must realize that complete segregation is not possible various processes can cover a fairly broad spectrum from idealized cases involving purely mechanical failure to purely corrosive processes (or dissolution). The Rehbinder effect, which involves the adsorption-induced lowering of strength, stress-facilitated corrosion, and corrosive fatigue, often occupies intermediate positions in these series. In this type of phenomenon, the action of external forces and the action of chemically active media both contribute to the net result in certain proportions. 

In the case of intensive repetitive actions, the facilitation of plastic deformation in the surface layer may at some point result in the opposite effect, namely, an additional strength increase due to the accelerated accumulation of distortions in the metal structure. Direct observations by electron microscopy, conducted by Kostetskiy et al., indicated a significant increase in the dislocation density in the surface layer. Under the appropriate conditions (temperature, stress, velocity, etc.), such a peculiar sample training may be used in the improvement of the structure and the mechanical properties of the surface layer. However, this already corresponds to the adsorption-induced fatigue region, studied in detail by Karpenko et al. These studies showed that at a certain level of stress the adsorption-caused acceleration of defect accumulation within the surface layer may lead to the premature development of cracks and partial failure after a certain number of cycles (cyclic fatigue). 

---