Electron Stimulated Desorption (ESD) coupled with mass spectrometry was used to examine the chemical composition of the adsorbed layers and the surface chemical groups. The sample was bombarded with electrons using an electron gun positioned at a 60° angle to the surface normal. Unless otherwise mentioned, the electron energy used was 600 eV. The ions, which were generated on the surface due to electron bombardment, released from the sample and were detected using a quadrupole mass spectrometer, situated in front of the sample. More details about the setup and process of the ESD experimental system can be found elsewhere. Thermal Gravimetric analysis of thin films was used to analyse the gases emitted during progressive heating under both Ar and dry air flows using a mass spectrometer. The rate of linear heating was set at 5, 10, and 20 °C/min. The emitted gases were analysed in the range of 1-90 a.m.u. The Rutherford Backscattering Spectrometry (RBS) and Elastic Recoil Detection Analysis (ERDA) techniques, available at the Centre of Microanalysis of Materials of Autonomous University of Madrid, were employed to determine the elemental composition of the sample surfaces. Its 5MV linear tandem accelerator facility provides the ion beams to carry out the characterization with these techniques. In this study, a collimated He+ beam with energies of 3.035 or 4.260 MeV extracted from the accelerator was used, while choice of these energies’ values justified under resonant conditions for oxygen and carbon respectively. For RBS analysis, the backscattered ions were detected at an angle of 170° with respect to the direction of the incident ion beam. In the case of ERDA, the ions were directed to the surface at an incident angle of 75° with respect to the surface normal. Recoiled particles were collected at 30°. To filter out heavier ions, a 19 µm thick mylar film was placed in front of the detector to obtain the hydrogen depth profile. The total ion dose in each measurement was set to 15 µC with a particle flux of 5.5x1012 cm-2 s-1 and a probe size of 1.5x1.5 mm2. The spectra were taken with the samples at random orientations. For energy-to-depth conversion, common SRIM (Stopping and Range of Ions in Matter) energy loss data were used, along with reference samples of MgH2 Er-doped TiO2 coatings. The RBS-ERDA spectra were fitted using SIMNRA simulation software. The gases emitted during mechanical activation of the materials were analysed using an original UHV experimental system equipped with a quadrupole mass spectrometer (Hiden HALO), a reciprocating motion UHV-grade friction cell, and a dynamic gas expansion system. Such a configuration allows accurate quantification of minute emission rates down to 1 pmol s-1. The samples were rubbed under UHV using alumina spheres, 3 mm in diameter. The rubbing conditions, unless otherwise stated, were as follows: the normal load of 0.44 N, the frequency of reciprocating motion of 1 s-1, and the mean rubbing velocity of 0.18 m s-1. The experimental system is schematically shown in Figure 1 and described in detail elsewhere. Before the tests, the alumina spheres were thoroughly degreased consecutively in acetone and isopropanol ultrasonic baths. After drying, they were submerged in a hot Piranha solution to remove carbon and metal residues, rinsed with ultrapure water, and dried in an N2 stream. The differential mass spectra (DMS) were derived by subtracting the mean steady background mass spectra from the mean mass spectra recorded during the application of the mechanical stimulus. Only statistically significant changes (α=0.05) in DMS were analysed among the channels within the 1 – 100 a.m.u. range. To ensure comparability, the mass spectra were normalized by dividing by the total ion current in each spectrum. The tentative identification of ion species was based on reference cracking patterns from the NIST Webbook (NIST Webbook). The gas composition was determined through a backward stepwise regression method, in which we utilized reference mass spectra of various potential gas precursors. These spectra were fitted in various combinations to the experimental DMS with the aim of identifying the combination that included the fewest precursors and achieved a high R2adj value. Behavioural analysis (BA) was employed to develop better understanding of the mechanisms of underlying tribochemical processes. BA allows to explore the short- and long-term trends of highly dynamic emission time series, to establish the degree of correlation between the mass spectrometer signals, and to trace them back to the possible emission sources in the mechanically affected bulk material and/or on the mechanically affected surfaces. Chemical changes in the Mechanically Affected Zones (MAZ) were studied using vibrational spectroscopy (Raman and FTIR). All the results were benchmarked against the spectra obtained from neighbouring pristine surfaces. Raman spectra were measured using a 532 nm laser in air. Infrared micro-reflectance spectra were obtained using a micro-FTIR spectrometer.

FeS2 thin films were obtained by sulfuration of Fe coatings, which were deposited by thermal evaporation of iron powder (Goodfellow, 99.99%) on soda lime glass substrates under high vacuum. The initial thickness of the Fe coatings was 300±20 nm as measured using quartz crystal microbalance. The Fe coating was transferred into a glass ampoule, which contained a small amount of sulfur powder (Merck, 99.99%) placed at one end. The ampoule was evacuated down to 10-5 mbar and sealed. Then, sulfur was sublimated by heating to 300ºC for 20 h, while sulfur vapour at a pressure of about 0.065 bar reacted with the Fe film. After sulfuration, the coatings were kept in the same sealed ampoules at room temperature until they were characterized. Natural pyrite was used to contrast the results obtained for artificial FeS2 thin film. A sheet of iron pyrite was cut from a native crystal proceeding from Peru mines and polished. X-ray diffraction analysis showed a typical cubic crystal structure of the mineral sample. The iron coatings' crystal structure was analyzed both before and after sulfuration using grazing-angle X-ray diffraction (XRD). This was accomplished by employing Cu Kα radiation and maintaining a fixed incidence angle of 1.7°. To determine the mean crystallite size, the Scherrer formalism was applied to the main diffraction band (200). Film thickness measurements were conducted at the film edge utilizing a stylus profilometer, achieving an accuracy of 10 nm. The mechanical properties of the FeS2 coatings were investigated through nanoindentation (G200, KLA Corp.), utilizing a Berkovich diamond tip in dynamic contact mode. The maximum indentation depth was 100 nm constrained to remain below 10% of the total coating thickness. The loading cycle was carried out at a constant indentation strain rate of 0.1 s-1 and a small oscillating force was superimposed to this loading ramp (75 Hz of frequency, amplitude of 2 nm). Continuous measurement of the contact stiffness was achieved on the basis of the phase lag between the sinusoidal force and the penetration produced. X-ray Photoemission Spectrometry (XPS) was used to obtain information on chemical state of various elements under ultra-high vacuum (UHV) with a pressure below 10-8 Pa. Mg Kα radiation with an energy of 1253.6 eV was employed. To eliminate any airborne adsorbed contaminants from the sample surface, ensuring a pristine surface for subsequent XPS analysis, the samples underwent Ar+ ion sputtering with an energy of 1 keV and an incident angle of 60° with respect to the sample normal. The sputtering depth was around 1.3 Å. It should be noted that Ar+ ions for sputtering can potentially alter the chemical oxidation state of Fe and/or S and/or change the surface composition due to preferential sputtering. No additional treatment was performed. High-resolution XPS analysis of Fe 2p, S 2p, O 1s, and C 1s was conducted through the fitting process employing the minimum possible number of components compatible with the expected chemistry. For instance, both the Fe 2p3/2 and Fe 2 p1/2 spin-orbit peaks were fitted to ensure the coherence of the procedure, while assuming a Shirley background. For the sake of simplicity, only the Fe 2p3/2 bands are discussed here. The S 2p peak was fitted employing a S 2p3/2 S 2p1/2 doublet, considering the theoretical spin-orbit coupling ratio of 1:2. A fixed separation of 1.2 eV between the S 2p3/2 and S 2p1/2 was maintained based on literature for data processing.

Datasets of mass-spectrometry signals were obtained in the experiments with non-thermal tribochemical decomposition of synthetic thin-film iron sulphide and mineral iron pyrite. Tribochemical reactions were studied on a micrometre scale using localized rubbing under ultrahigh vacuum. Mechanically Stimulated Gas Emission Mass-Spectrometry (MSGE-MS) including the Dynamic gas expansion method was used to determine the kinetic parameters of gas emission and the composition of the emitted gases. The study was complemented by structural, morphological, tribological, mechanical and surface analyses. It was found that carbon-containing gases were dominating. The sulfur-containing gases comprised H2S, COS and CS2. The latter two were unexpected. The emission of these gases was traced back to solid-state chemical reactions kinetically controlled by the precursor concentrations and driven through non-thermal mechanisms, which we tentatively assigned to formation of sulfur radicals.

1. Dataset of mass-spectrometry time series of mechanically stimulated gas emission from sodium alanate (NaAlH4) pellets under vacuum.-- 2. Dataset of Thermal Programmed Desorption – Mass-Spectrometry (TPD-MS) analysis of sodium alanate.-- 3. Dataset of X-ray diffraction of sodium alanate.-- 4. Dataset of micro-FTIR spectra of pristine and mechanically activated surfaces of pellets of sodium alanate.-- 5. Dataset of Raman spectra measured on the surfaces of pellets of sodium alanate.

This study was co-funded by Spanish Ministry for Science and Innovation (grants PID2019-111063RB-I00, PID2020-112770RB-C22, PID2020-117573GB-I00, RTI2018-099794-B-I00, and TED2021-129950B-I00) and funding from Madrid Community (project S2018/NMT-4291 TEC2SPACE), Ministry of Science and Innovation of Spain (project CSIC13-4E-1794) and EU (FEDER, FSE).

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