Phenomenology

Mechanically stimulated gas emission (MSGE) is a complex phenomenon embracing various physical and chemical processes occurring on the surface and in the bulk of a solid under applied mechanical stress and resulting in emission of gases. MSGE is a part of a more general phenomenon of Mechanically Stimulated Neutral Emission (MSNE). The specific characteristics of MSGE as compared with MSNE is that the emitted neutral particles are limited to gas molecules. MSGE is opposite to Mechanically Stimulated Gas Absorption that usually occurs under fretting

Fretting

Fretting refers to wear and sometimes corrosion damage at the asperities of contact surfaces. This damage is induced under load and in the presence of repeated relative surface motion, as induced for example by vibration...

 corrosion

Corrosion

Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

 of metals, exposure to gases at high pressures, etc.

There are three main sources of MSGE:

I. Gas molecules adsorbed on the surface of a solid

IIa. Gases dissolved in the material bulk

IIb. Gases occluded or trapped in micro- and nanovoids, discontinuties and on defects in the material bulk

III. Gases generated as a result of mechanical activation of chemical reactions.

Generally, for producing MSGE, the mechanical action on the solid can be of any type including tension, compression, torsion, shearing, rubbing, fretting, rolling, indentation, etc.

In previous studies carried out by various groups it was found that MSGE is associated mainly with plastic deformation, fracture, wear and other irreversible modifications of a solid. Under elastic deformation MSGE is almost negligible and only was observed just below elastic limit due to possible microplastic deformation.

In accordance to the main sources, the emitted gases usually contain hydrogen

Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 (source type IIa), argon

Argon

Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...

 (for coatings obtained using PVD

Physical vapor deposition

Physical vapor deposition is a variety of vacuum deposition and is a general term used to describe any of a variety of methods to deposit thin films by the condensation of a vaporized form of the desired film material onto various workpiece surfaces...

 in Ar plasma - source type IIb), methane

Methane

Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 (source type III), water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 (source type I and/or III), carbon mono-

Carbon monoxide

Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

 and dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 (source type I/III).

The knowledge on the mechanisms of MSGE is still vague. On the basis of the experimental findings it was speculated that the following processes can be related with MSGE:

1. Transport of gas atoms by moving dislocations
2. Gas diffusion in the bulk driven by gradient of mechanical stress
3. Phase transformation induced by deformation
4. Removal of oxide and other surface layers, which prevent exit of dissolved atoms on the surface
5. Extension of free surface

Thermal effect seems to be irrelevant to the gas emission under light load conditions.

Terminology

Emerging character of this interdisciplinar branch of science is reflected by lack of established terminology. There are different terms and definitions used by different authors depending on the main approach used (chemical, physical, mechanical, vacuum science, etc.), specific gas emission mechanism (desorption, emanation, emission, etc.) and type of mechanical activation (friction, traction, etc.):

Mechanically stimulated outgassing (MSO)

Tribodesorption

Triboemission,

Fractoemission

Atomic and Molecular emission

Outgassing stimulated by friction

Outgassing stimulated by deformation

Desorption (tribodesorption, fractodesorption, etc.) reffers to release of gases dissolved in the bulk and adsorbed on the surface. Therefore, desortpion is only one of the contributing processes to MSGE. Outgassing is a technical term usually utilized in vacuum science. Thus, term "gas emission" embraces various processes, reflects the physical nature of this complex phenomenon and is preferrable for use in scientific publications.

Experimental observations

Due to low emission rate experiments should be performed in ultrahigh vacuum (UHV

Ultra high vacuum

Ultra-high vacuum is the vacuum regime characterised by pressures lower than about 10−7 pascal or 100 nanopascals . UHV requires the use of unusual materials in construction and by heating the entire system to 180°C for several hours to remove water and other trace gases which adsorb on the...

). In some studies the materials were previously doped with tritium

Tritium

Tritium is a radioactive isotope of hydrogen. The nucleus of tritium contains one proton and two neutrons, whereas the nucleus of protium contains one proton and no neutrons...

. MSGE rate then was measured by radioactivity outcome from the material under applied mechanical stress.