Wüest M. 51 Wykes M. 82 Yamaguchi M. 17 Ybarra G. 129 Yubero F ...

JUNE 28 WEDNESDAY MORNING
WS-18-WeM-INV.12 THE EFFECT OF MECHANICAL AND FRICTIONAL PHE-
NOMENA ON THE QUALITY OF VACUUM IN ADVANCED TECHNOLOGIES. R.A.
Nevshupa. Department of Vacuum Mechanics and Tribology (MT-11), Bauman Moscow State
Technical University, 2-Baumanskaia 5, Moscow 105005, Russia. J.L. de Segovia, E. Roman. Department
of Surface Physics and Engineering, Institute of Material Science of Madrid, C/ Sor Juana
Ines de la Cruz 3, Cantoblanco, Madrid 28049, Spain
By the last quarter of the twentieth century, as the result of several progresses in materials, coatings,
degassing procedure, and pumping technologies, attaining of extremely high vacuum below 10 -8 Pa
had became almost routine task in many technologies ranging from electronic and optoelectronic industries
to large particle accelerators and physical experimental systems. These progresses allowed
to control precisely desorption of gases from stationary surfaces faced to vacuum and to obtain specific
desorption rate from these surfaces below 10 -8 Pa·m 3·m -2·s -1 . In this circumstances secondary
desorption phenomena come to the foreground to provide quality of the total and partial pressures in
technological systems. Tribodesorption, i.e. desorption of gas stimulated by mutual friction, indentation
and other types of mechanical action, is one of the most important among these desorption phenomena
due to high desorption yield and abundance of moving parts in modern ultrahigh vacuum
technological equipment. Specific desorption rate during sliding of stainless steel, other metals and
tribological coatings can be as high as 10 -2 - 10 -3 Pa·m 3·m -2·s -1 . Although contact area, which molecules
are desorbed from, is usually very small, instant pressure increase due to translation of moving
parts inside vacuum system can be of the order of 10 -6 - 10 -7 Pa that is critical for many technological
processes. Moreover, friction is a source of hydrocarbons, carbon oxides, water vapours and other
contaminating substances, which might harm sensitive production.
Tribodesorption is a complex phenomenon based on three main physical sources: desorption of the
topmost adsorbed gas layers, emission of gas molecules from the shallow subsurface zones and tribochemical
reactions. In high and ultrahigh vacuum the late two sources are dominating. The composition
of desorbed gas depends on many factors including kind of material, content of the gas in
the material, presence of adsorbed phases and so on. Time behaviour of tribodesorption has maximum
at the beginning of friction of fresh surface and slowly decreases during friction. However, tribodesorption
rate can be restored after resting time during 12 or more hours or after moderate heating
of the rubbed surface. These facts point the diffusion of gas atoms and molecules in the material
bulk as a precursor of tribodesorption.
In addition, tribodesorption is strongly correlated with any damage of the surface, i.e. plastic deformation,
fracture, cracking, wear and so on. This feature is very promising for developing of reliable
tribological coatings for vacuum applications.
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