ever chemical analysis of structural changes due to thermal cycling is lacking. A study conducted by Saravanan et.al [14] analysed multilayer polymer and MoS 2 coating that was annealed to 500 °C, concluding that the heat treatment improved the wear durability of the film by forming hard MoO 3 and soft carbon-like material from the polymer layer. However, the effect of heat treatment in isolated MoS 2 and Ti doped MoS 2 is still unclear. It has been previously determined that annealing up to 450 °C introduces defects in MoS 2 , where S atoms are lost due to the breakage of S-Mo-S bonds [15]. This has been confirmed by Raman analysis, where the E 12g and A 1g peak shifts are analysed, which are the in-plane and out-of-plane vibrations respectively. This has also been supported by Wu et al. [16], where decomposition of MoS 2 was observed at temperatures over 400 °C. On the contrary, some studies have shown MoS 2 degradation takes place at temperatures as low as 260 °C [17], which has been conformed with decreased Raman intensity. Aus Wissenschaft und Forschung 50 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0006 Introduction Molybdenum disulfide (MoS 2 ) is widely used as a lubricant in space applications such as satellites and spacecraft [1]. For this reason, its tribological properties have been extensively analysed in vacuum environment [2,3] and air [4,5]. Due to the deteriorating tribological properties in air, MoS 2 has been doped with Ti to increase density of the coating [6] and therefore minimising oxygen substitution. MoS 2 / Ti coatings have shown reduced sensitivity to air, where the coefficient of friction (CoF) is significantly reduced in air, in comparison to pure MoS 2 [5,7-9]. Apart from metal doping, thermal treatments, such as annealing have been applied to MoS 2 for various reasons, including optimisation of mechanical properties, such as desorption of contaminants [10] and releasing of stress [11]. However, mechanical systems for satellites, are exposed to wide range of temperatures during testing, launch and travel, ranging from -80 °C to 250 °C in air and vacuum [12], therefore exposing MoS 2 based coatings to thermal cycling. For this reason, chemical analysis of structural changes is required in order to gain full understanding of wear and failure mechanisms. It has already been established that the coefficient of friction (CoF) in MoS 2 decreases with increasing temperature, up to 120 °C [13]. Some studies have also analysed the effect of thermal cycling, where the temperature alternates between -150 °C and 250 °C [12]. How- Impact of thermal cycles on tribological properties and oxidation of MoS 2 coatings Kristine Brittain, Ardian Morina, Liuquan Yang, Anne Neville* Eingereicht: 22.9.2021 Nach Begutachtung angenommen: 21.2.2022 Dieser Beitrag wurde im Rahmen der 62. Tribologie-Fachtagung 2021 der Gesellschaft für Tribologie (GfT) eingereicht. MoS 2 based solid lubricants are extensively used in space applications, where the materials are exposed to large fluctuations of temperatures. Storage takes place in ambient conditions and so does testing, to test the mechanical systems in the harshest environments. In this work, the coatings were exposed to 40 °C, 75 °C and 250 °C for 1 hour and cooled in air to determine the effect of exposure to increased temperature. It was confirmed that MoS 2 / Ti is thermally stable and not significantly affected by heat treatments. Pure MoS 2 observes increased strain in the structure after exposure to 250 °C, which is due to oxygen substitution that takes place in S vacancies, followed by oxide formation. Although coefficient of friction is not affected by heat treatments, wear is reduced due to increase in hardness. Keywords Molybdenum disulfide, solid lubrication, surface analysis, wear mechanisms, oxidation, friction Abstract * Kristine Brittain - Corresponding author Orcid-ID: https: / / orcid.org/ 0000-0003-2440-3720 School of Mechanical Engineering University of Leeds, Leeds, LS2 9JT, United Kingdom Professor Ardian Morina Institute of Functional Surfaces, University of Leeds Leeds, LS2 9JT, UK Dr Liuquan Yang Institute of Functional Surfaces, University of Leeds Leeds, LS2 9JT, UK Professor Anne Neville Institute of Functional Surfaces, University of Leeds Leeds, LS2 9JT, UK Oxide formation has been observed upon exposure to temperatures above 260 °C for long period of time, where the presence of molybdenum trioxide (MoO 3 ) was confirmed [16]. Defects can be detrimental to MoS 2 as the charge transfer in S vacancies are much higher than in defect free structure. Therefore, contaminants such as O 2 and H 2 O bind to the defect sites more strongly [11]. It has been suggested that water and oxygen molecules only physisorb onto MoS 2 coating. Density functional theory (DFT) calculations have shown that chemisorption is not possible due to high energy barrier, however the physisorbed molecules deplete MoS 2 of electrons [11,17], where charge transfer takes place from MoS 2 to O 2 or H 2 O. Previous research has confirmed that E 12g mode decreases and A 1g increases with increasing defect density formed through heat treatment such as annealing [15]. However, the reason of peak shifts and linewidth changes are not fully understood. Lack of research exists linking thermal cycling to tribological properties. It is unclear whether some of the desorbed contaminants adsorb again during cooling period, hindering low friction and wear and whether increased oxidation is induced due to sulfur loss. Also, how these structural changes affect friction and wear is also unclear. In this study, MoS 2 and MoS 2 / Ti coatings were thermally treated before friction test at room temperature (RT). All the coatings were treated and tested in air, as satellite materials testing takes place under worstcase conditions to ensure material durability, as per ECSS-E-ST-33-01C [18]. The impact of exposure to temperature fluctuations has not been widely researched, therefore irreversible structural changes are unclear and will be examined further. Methodology MoS 2 and MoS 2 / Ti (also known as MoST) was deposited by Teer Coatings Ltd (Droitwich, UK), using DC closed-field unbalanced magnetron-sputter ion plating (CFUBMS). AISI 440C substrates were polished, sonicated in acetone for 5 minutes and rinsed with isopropanol alcohol (IPA) to remove any contaminants. The substrates were stored in vacuum desiccator, until used for deposition. Detailed deposition procedure is provided elsewhere [19]. Each coating was heated in oven in air for 1 hour and cooled in air before stored in a vacuum desiccator. Three temperatures were selected to analyse the extent of chemical changes in the structure with increasing temperature. Coatings were heated to 40 °C, 75 °C and 250 °C before cooled and analysed. Dry contact pin-on-plate tribometer (Biceri, UK) was used, with 40 mm radius EN31 steel pins as a counter body. The plate was 30mm diameter and 6mm thick, made of AISI 440 C steel, coated with 1µm thick MoS 2 coating. Each test was repeated at least three times and the operating conditions for friction tests are listed in Table 1. Table 1: Operating conditions Operational parameter Value Speed (m/ s) 0.02 Contact pressure (MPa) 530 Stroke length (mm) 10 Cycles 1800 Temperature (°C) 25 Humidity (RH%) 40 - 50 Any chemical changes in the structure were monitored using Raman by Renishaw InVia spectrometer (UK). 488 nm Ar ion laser [20] with 0.5 mW power was used with laser step size of 2 µm. A mapping area of 24 × 20 µm 2 was obtained. The unit cell structure was analysed using X-ray diffraction (XRD) by Rigaku SmartLab (Tokyo, Japan) with Cu Kα radiation (λ = 1.54186 Å) equipped with D/ tex Ultra 250 silicon strip detector. 2 mm incident beam mask was used to minimise the irradiated area within wear scars. The crystallite size in c direction (along (002) plane), was calculated using Debye Scherrer equation [21]. Wear analysis was conducted using NPFlex, non-contact white light interferometer by Bruker (USA) using vertical resolution of < 0.15 nm. Three areas of each wear scar were scanned, ensuring repeatability. The data was analysed using Vison64 software and specific wear rate was calculated using Archard’s wear equation [22]. The quantification of oxide coverage was conducted by optical microscope by Leica DM 6000 M (Leica Microsystems, Wetzlar, Germany), and Raman, where different areas were analysed, with dark regions identified as oxide containing MoS 2 . Optical microscope was used to obtain images of the wear scars, followed by image analysis software (ImageJ), quantifying the dark regions within the wear scar by counting pixels. Wear debris outside of the wear scar were excluded for quantification. Energy dispersive X-Ray spectroscopy (EDX) was used to obtain elemental analysis in the middle of wear scar using scanning electron microscopy (SEM) by Carl Zeiss Evo MA15-SEM coupled with AZtecEnergy EDX system using CZ STEM detector. Aus Wissenschaft und Forschung 51 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0006 high CoF in MoS 2 , and high wear rate in untreated MoS 2 , the wear rate in heat treated MoS 2 decreases significantly. This indicates that the heat treatment induces chemical changes in MoS 2 structure, however MoS 2 / Ti is unaffected by heat treatment. Therefore, the chemical changes must be analysed to confirm the source of oxide formation or oxide removal. The wear of the pin was also analysed, as shown in Figure 3, where untreated and 250 °C heat treated MoS 2 has the highest wear. Chemical changes in the structure were analysed by monitoring E 12g and A 1g modes. Figure 4 shows the peak shift changes in heat treated (a) MoS 2 and (b) MoS 2 / Ti before friction test, with dotted lines as the average peak Aus Wissenschaft und Forschung 52 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0006 Results Friction curves of MoS 2 show very small deviations between different heat treatment temperatures. Figure 1 shows one friction curve per each condition, however at least three repeats have been conducted. Figure 1(a) shows small increase in CoF when heat treated at 40 °C, however the average steady state CoF does not differ significantly from untreated MoS 2 . The CoF in MoS 2 / Ti coatings does not differ between heat treatments, as shown by Figure 1(b). The average steady state CoF, which is the average of at least three repeats, is obtained between 1200-1800 cycles, and is summarised in Figure 2(a) and wear in Figure 2(b). As expected, MoS 2 / Ti has lower CoF in treated and untreated samples. Despite Figure 1: Coeffcient of friction in untreated and heat treated (a) MoS 2 and (b) MoS 2 / Ti coatings Figure 2: (a) Steady state average CoF and (b) wear rate in MoS 2 and MoS 2 / Ti coating position in untreated coatings and the grey area show standard deviation of the peak positions across the mapping area. The only significant structural change is induced after MoS 2 is exposed to 250 °C, where both E 12g and A 1g modes increase, however, this is not observed in MoS 2 / Ti. The increase in both Raman modes is consistent with compressive strain [23]. The Raman analysis is applied to both coatings after friction test in the middle a wear scar, as shown in Figure 5. This was done to determine the effect of heat exposure on tribological properties and whether the structural changes induced by heat, impact MoS 2 after wear. Large increase in peak distance is seen in MoS 2 , with largest difference at treatment temperature of 250 °C. The increase in A 1g mode and decrease in E 12g mode indicate defect formation , which is due to Mo-S Aus Wissenschaft und Forschung 53 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0006 Figure 4: Raman peak shifts in untested coatings after heat treatment in (a) MoS 2 and (b) MoS 2 / Ti, the dotted lines are peak positions in untreated coatings with the grey area as standard deviation Figure 5: Raman peak shifts in the middle of wear scar in (a) MoS 2 and (b) MoS 2 / Ti, the dotted lines are peak positions in untreated coatings after RT friction test, with the grey area as standard deviation Figure 3: Wear rate in pins in MoS 2 and MoS 2 / Ti due to overlapping S and Mo signals, however it has been previously shown that EDX can be used to confirm loss of S [26]. Figure 6(b) shows small amounts of oxygen after wear, indicating removal of oxides, particularly in MoS 2 / Ti. Unlike MoS 2 , Raman analysis does not show significant defect formation. In worn MoS 2 / Ti, only in-plane (E 12g ) is affected, observing increase in all heat treated coatings. The impact of heat treatment and oxide formation during wear is analysed by optical microscope, where the dark Aus Wissenschaft und Forschung 54 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0006 bond breakage [24], where as a result, S atoms are lost, leaving vacancies, or defects in the structure. EDX analysis in Figure 6(a) show estimated ratio between S and Mo in worn coating, with and without heat treatment. In a defect free structure, the ratio between S: Mo is 2: 1, however in untreated worn MoS 2 and MoS 2 / Ti, the ratio is decreased to 1.6-1.7, as shown in Figure 6(a). The ratio between S and Mo has increased to 1.8-1.9 in worn heat treated MoS 2 and MoS 2 / Ti. As increase in S generally takes place after sulfurization, it is possible that Mo has been removed instead [25], after the formation of MoO 3 , increasing the ratio between S: Mo. It is important to note that EDX is semi-quantitative technique Figure 6: EDX analysis of (a) S: Mo ratio and (b) oxygen content in MoS 2 and MoS 2 / Ti after wear in untreated and heat treated coatings at 250 °C Figure 7: Optical microscope image of heat treated MoS 2 at 250 °C regions have been identified as oxide containing areas by Raman. This is illustrated in Figure 7, where the peak at 920 cm -1 confirms the presence of Mo-O bonds in MoS 2 [27]. The quantified oxide coverage is shown in Figure 9, where MoS 2 / Ti has larger area coverage of oxides, in comparison to MoS 2 . Both coatings observe increase in oxides at treatment temperature of 75 °C. However, MoS 2 maintains high oxide coverage at 250 °C, whereas MoS 2 / Ti shows a significant reduction in oxides after it has been exposed to 250 °C. Some changes have been observed in MoS 2 crystallinity, where outside the wear scar of 250 °C heat treated coating show slight increase in crystallinity, where peak at 33.5° emerges as shown in Figure 9(a). However, some crystallinity might have been lost after wear, as shown in Figure 9(b). This was analysed further, where the distance between MoS 2 layers is analysed using XRD. Overall it has been observed that the layer distance increase slightly after wear in all the heat treated and untreated MoS 2 coatings, as shown in Figure 10(a). However, the distance increases slightly after the untested coating has been exposed to 250 °C. The crystallite size was also monitored and summarised in Figure 10(b). As expected, the smallest crystalline coverage is observed in untreated samples. Aus Wissenschaft und Forschung 55 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0006 Figure 9: XRD of MoS 2 without heat treatment and after 75 °C and 250 °C heat treatment on (a) blank coating and (b) middle of a wear scar Figure 8: Oxide coverage across MoS 2 and MoS 2 / Ti wear scars Small temperature increase does not impact the structure of MoS 2 significantly, however, the exposure to 250 °C improves some crystallinity, which has been shown by XRD. This was also confirmed by Raman mapping of full width at half maximum (FWHM) of A 1g mode in Figure 11, Aus Wissenschaft und Forschung 56 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0006 Discussion The heat treatment has a minimal impact on CoF in both, MoS 2 and MoS 2 / Ti coatings and therefore understandably, lack of research exists analysing these effects. Figure 10: (a) The distance between interlayer and (b) crystallite size with increasing heat treatment temperature outside and within the wear scar using XRD Figure 11: Mapped area of FWHM of A 1g peak in MoS 2 outside wear scar in (a) untreated, (b) heat treated at 250 °C coating, within the wear scar in (c) untreated and (d) heat treated at 250 °C where the average FWHM decreases from 12.5 cm -1 in untreated MoS 2 (Figure 11(a)), to 11.7 cm -1 (Figure 11(b)). Furthermore, after wear, the crystallinity is lost, forming amorphous tribofilm over the wear scar. The increase in crystallinity as well as the increase in strain, as seen by Raman analysis, can be induced by oxygen substitutions [28]. The strain in untested samples exposed to 250 °C is confirmed by XRD, where the distance between the layers is increased, in comparison to untreated MoS 2 . During wear, the substituted oxygen contributes to oxide formation, as confirmed by the increase in oxide coverage. These reactions can confirm that friction reduces when recrystallization and water desorption takes place, however, as seen in Figure 11(d), excessive oxygen substitution leads to oxide formation and increased friction. For this reason, no significant changes in CoF were seen. Clear reduction in wear is seen in MoS 2 after heat treatment. It has been previously suggested that decrease in wear is due to formation of oxides, where the abrasive MoO 3 particles increase overall hardness of the coating [14]. Similar correlation was observed in this study, as shown in Figure 12, where in most heat treated MoS 2 coatings the wear rate decreases with increasing oxide coverage. The opposite is observed in untreated coating, where the highest wear rate is seen and the least amount of oxides are present. It is possible that in untreated coatings, the oxide layer has been removed upon friction. On the contrary, the analysis of the counterbody shows that the wear of the pin does not decrease after 250 °C heat treatment, indicating less efficient transfer film formation has occurred due to increased oxide coverage. This was also confirmed by optical microscope images, where larger area of transfer film is seen in 75 °C heat treated MoS 2 coating counterbody (Figure 13(a)), in comparison to 250 °C heat treated MoS 2 counterbody, where the transfer film is only seen in the middle of the contacting areas and large surrounding areas are depleted of transfer film. Titanium doped MoS 2 consists of densely packed and amorphous structure, yielding it to be more stable in air in comparison to pure MoS 2 , which already has been established [29,30]. No significant chemical changes in the structure after heat treatment were observed, which is as expected. Nonetheless, to confirm the chemically stable structure, maps of FWHM of A 1g mode have been obtained in untreated and treated coatings, as shown in Figure 14. It was observed that the exposure to heat only introduces some distortion, which is not affected by wear. Unlike in MoS 2 , the wear rate in MoS 2 / Ti is independent on oxide coverage as no correlation exists between both trends. Pure MoS 2 is more susceptible to loss of sulfur than MoS 2 / Ti, whereas MoS 2 / Ti becomes strained upon friction, which has been confirmed by Raman analysis as well as previous research by Colas et.al [31]. It has also been previously established that MoS 2 / Ti requires contaminants (H 2 O or O 2 ) in order to reduce the abrasive properties of Ti, by forming a phase consisting of Ti and O as well as MoS 2 and O [31]. Therefore, exposure to high temperatures leads to formation of new phases and increased strain. This has been confirmed by Raman Aus Wissenschaft und Forschung 57 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0006 Figure 13: Wear on pin after (a) 75 °C and (b) 250 °C heat treatment in MoS 2 coating Figure 12: Wear rate with oxide coverage in MoS 2 coatings ing to formation of MoO 3 . No change in CoF were seen as the friction reducing recrystallization and water desorption was balanced out by friction increasing oxide formation. • The removal of oxides during wear, removes Mo, forming defects in the structure. • Wear was reduced in pure MoS 2 as the oxides increase hardness of the coating. Oxides also hinder adhesion of MoS 2 , leading to removal of transfer film from the counterbody. • Contaminants are beneficial in MoS 2 / Ti as the formation of phases consisting of Ti and O reduce the abrasive properties of Ti, maintaining low friction and wear. Therefore, transfer film formation was not hindered, and wear was unaffected by heat treatments. Analytical tools such as Raman and SEM-EDX have been used to monitoring the structural changes and oxide formation. However, for future investigations, the exact S: Mo ratio can be quantified using XPS (X-ray photoelectron spectroscopy) in order to monitor S and oxide removal. In addition, the layer structure in MoS 2 and MoS 2 / Ti can be analysed using transmission electron microscope (TEM) to confirm any friction induced structural distortion or crystallisation and the formation of a new phase in MoS 2 / Ti. Aus Wissenschaft und Forschung 58 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 DOI 10.24053/ TuS-2022-0006 maps of FWHM in Figure 14, where the structure is preserved after heat treated wear in comparison to untreated wear. Pure MoS 2 , on the other hand, loses S and Mo atoms during exposure to high temperatures and during the cooling period, contaminants such as oxygen, binds to the vacant sites, restoring the structure and therefore recrystallization is observed by Raman and XRD. However, unlike in MoS 2 / Ti, where new phases are formed in the existing structure, in MoS 2 , the S and Mo replacement by O leads to reduced repulsion between S atoms, causing slight increase between the layers. This was confirmed by XRD, as well as the slight reduction in CoF in comparison to untreated MoS 2 [32]. Conclusion Although it has been established that Ti doped MoS 2 is more stable than pure MoS 2 , the effects of exposure to increased temperature in air were examined and linked to tribological properties in both coatings. The following can be concluded: • Oxidation occurs after MoS 2 has been exposed to temperatures as low as 75 °C. • High temperature heat exposure in pure MoS 2 leads to oxygen substitution induced recrystallisation. Upon friction, this induces excess oxygen substitution lead- Figure 14: Mapped area of FWHM of A 1g peak in MoS 2 / Ti outside wear scar in (a) untreated, (b) heat treated at 250 °C coating, within the wear scar in (c) untreated and (d) heat treated at 250 °C Aus Wissenschaft und Forschung 59 Tribologie + Schmierungstechnik · 69. Jahrgang · 1/ 2022 Acknowledgements We wish to acknowledge the support of the Henry Royce Institute through the Royce PhD Equipment Access Scheme enabling access to Thin Film XRD facilities at Royce@Manchester; EPSRC Grant Number EP/ R00661X/ 1. We would also like to thank Teer Coatings Ltd for providing MoS 2 and MoS 2 / Ti coatings. This work was funded by EPSRC (Engineering and Physical Sciences Research Council) grant number EP/ L01629X/ 1. 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