Vacuum Tribology

The main difficulty in designing of an ultrahigh vacuum (UHV) tribometer combined with
tribophysical and tribochemical characterization techniques is to find the critical compromise between
the scientific requirements and technical or technological limitations from different subsystems and
components. The principal conflicts, their possible solutions and the recommended tribometer
configurations are analysed. The developed methodological principles were applied for designing and
construction of two UHV experimental tribological systems: TriDes-2 and Ca3UHV. The advances in the
design and development of the vacuum system as well as the UHV force sensor and sample holder are
presented and discussed.

Force measuring is used in various surface characterization techniques such as indentation,
scratch tests, tribological analysis, determination of gas content, etc. The main
problems related with force measurement under extreme conditions have been analysed.
A strategy that should be followed to solve these problems has been discussed and several
examples of successive solutions that recently were developed by the authors are
presented. The need to carry out the characterization under extreme conditions poses
serious problems for the designers of the measuring systems that may include the
incompatibility of the sensors with the test conditions, undesirable interactions with other
components, stability, precision and uncertainty issues, the measurement range, etc.
Resolving these problems must be based on a global approach in which the characterization
system is considered as a whole, while the designer must analyse and solve the
possible conflicts between the subsystems. The way how an appropriate force measuring
system can be selected is described. The proposed method is illustrated by an example
in which an indirect force measurement using optical fibre displacement sensor was used.
Another example describes measuring system developed for vacuum high-temperature
nanoindentation. At high temperature, proper heat management based on noncontact
heating and laminar flow cooling system is mandatory to avoid experimental data
being affected by external noise and thermal drift.

The main difficulty in designing of an ultrahigh vacuum (UHV) tribometer combined with tribophysical and tribochemical characterization techniques is to find the critical compromise between the scientific requirements and technical or technological limitations from different subsystems and components. The principal conflicts, their possible solutions and the recommended tribometer configurations are analysed. The developed methodological principles were applied for designing and construction of two UHV experimental tribological systems: TriDes-2 and Ca3UHV. The advances in the design and development of the vacuum system as well as the UHV force sensor and sample holder are presented and discussed.

Analysis of Mechanically Stimulated Gas Emission Spectrometry data combined with characterization of chemical composition, structure and mechanical properties of various hydrogenated amorphous carbon coatings has allowed shedding light on the possible reactions of tribochemical degradation. These reactions are conceivably driven by the mechanical deformation and stress rather than by frictional heating. A close correlation between the increase in proportion of sp3 CHx groups in the coating and the rate of CH4 emission was observed.  Emission of methane is suggested to involve abstraction of methyl radical followed by recombination with hydrogen atom. Generation of ethane has to be due to recombination of two abstracted methyl radicals rather than an ethyl radical with hydrogen atom. The rate of H2 emission steeply and non-linearly increased with the hydrogen content in the coating. Hydrogen generation can involve both liberation of occluded H2 and re-combinative reaction.