Tribocorrosion is an irreversible transformation of a material resulting from simultaneous physicochemical and mechanical surface interactions taking place in a tribological contact. Tribocorrosion involves numerous synergy effects between mechanical and chemical or electrochemical phenomena including corrosion accelerated occurs in many types of contacts such as sliding contacts, rolling contacts, fluid or particle impingement, as well as erosion and abrasion phenomena in fluids due to suspended particles. Technical systems of sliding contact in aqueous media are found for example in pumps, mining equipment, medical implants, metal grinding. Fretting corrosion is a particular case of tribocorrosion of solid-solid contacts resulting from low amplitude oscillations in a corrosive environment.
(Edited By: S. Virtanen, P. Schmuki, G.S. Frankel, Critical Factors in Localized Corrosion IV, p. 618)
Elastomer Friction:
Elastomers are usually formulated to have high friction coefficients against a wide variety of counterfaces but the friction coefficients can be very low in some cases. Table 4 shows friction coefficients for some common elastomers against hardened stainless steel; the coefficients are quite high. On the other hand, in water, the coefficient of polyurethane against most other solids is less than 0.2. The slippery nature of this elastomer became apparent when these materials came into vogue in the 70’s for floor finishes. They produced a very abrasion resistant surface, but when wet they were a safety hazard. Their use as tires and floor toppings has almost ceased but they are now used for seals and wear parts in pumps where their slipperiness and abrasion resistance have a synergistic effect.
The coefficient of friction of many elastomers against other solids is often 1 or more, but the use environment may significantly alter the friction characteristics.
(K. C. Ludema,Raymond George Bayer,ASTM Committee G-2 on Erosion and Wear,Tribological modeling for mechanical designers, p.113)
Ultrasonic Transducer:
The mechanical construction of a typical ultrasonic transducer used in contact testing is shown in Fig. 1.7. A very thin (app. 100µm thick) piezoelectric crystal is plated on both faces; it is attached through a small electrical network contained in the transducer housing to the external BNC or microdot of the transducer. Since the crystal is very fragile, a ceramic wear plate protects the front face of the crystal, as shown. The back face of the crystal is attached to a layer of epoxy loaded with tungsten particles. This backing acts as a highly attenuating medium that controls the shape and duration of the pulse.
There are actually two types of contact transducers. They are distinguished by the types of motion generated in the crystal when excited by a voltage pulse and the corresponding types of motion subsequently present in the ultrasound beam launched from the transducer into the part. Figure 1.8(a) shows a contact P-wave transducer with the crystal excited in a mode that causes its thickness to expand and contract normal to the surface, thereby producing a wave with similar motions that is called a P-(pressure) wave. Figure 1.8(b) in contrast shows S-wave transducer with the crystal excited in a shearing type of motion, thereby producing an S-wave (shear) wave.
(Lester W. Schmerr, Fundamentals of ultrasonic nondestructive evaluation: a modeling approach, p.6)
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