Quality Control Metrics to Assess MoS₂ Sputtered Films for Tribological Applications

Molybdenum disulfide (MoS₂) solid lubricants have historically been used as coatings in space applications because of their low friction coefficients (µ < 0.05) and wear rates (k< 1 × 10^–6 mm³/Nm) in inert and vacuum environments [1‐3]. Pure MoS₂ coatings are not commonly used in terrestrial applications due to high wear rates and oxidation when exposed to water and oxygen [4‐11]. To mitigate adverse interactions in terrestrial environments, dopants such as C, Sb₂O₃, Au, Ni, Ta, and Ti are commonly added to improve the tribological performance and environmental robustness [12‐20]. While reported mechanisms for performance improvement of composite coatings vary depending on additives, composites such as MoS₂/Sb₂O₃/Au have been shown to help facilitate the expression and retention of MoS₂ at the interface through agglomeration of Au nanoparticles [21]. Other composites such as MoS₂/C/Sb₂O₃ have been shown to exhibit extremely low shear strengths. The sliding interface of MoS₂/C/Sb₂O₃ changes during sliding depending on the environment, with the surface becoming carbon-rich in humid testing conditions and MoS₂-rich in dry, inert environments [22].

A common hypothesis for the improved environmental resilience of composite MoS₂ coatings involves densification and hardening imparted by dopants [23‐28]. Sputtered pure MoS₂ coatings have been shown to exhibit low densities (ρ ~ 3.3–4 g/cm³, theoretical = 4.8–5.06 g/cm³ [29, 30]), which varies depending on coating microstructure [31]. Buck [31] observed the density of sputter-deposited amorphous coatings (ρ ~ 3.3 g/cm³) was lower than that of crystalline coatings (ρ ~ 3.95 g/cm³ for basally oriented coatings) due to the presence of microscopic vacancies and a high degree of disorder. Lince et al. [32] found that a higher oxygen content in MoS_2-xO_x coatings led to an increase in film density due to a reduction in crystallite size. Interactions between oxygen molecules and MoS₂ increased the defect density of the coating, thereby forming a disordered microstructure [32]. Techniques such as ion-beam assisted deposition (IBAD) have been shown to improve both the density of pure MoS₂ coatings (ρ ~ 4.4 g/cm³) and wear resistance in humid and dry environments [29].

A prevailing hypothesis for enhanced wear resistance of doped-MoS₂ is that it is linked to improved coating density [23‐28], though there is little to no direct evidence or a well-developed fundamental understanding of the role of this relationship present in the literature. A major barrier to this understanding lies in the difficulty in depositing fully dense pure films, or even films with consistent density. Variability in coating morphology is one of the main challenges limiting the widespread commercial use of sputtered pure MoS₂ films in engineering applications.

From a research perspective, understanding temperature dependence, environmental resilience, or load-dependent friction behavior becomes even more challenging when coating microstructure varies from one deposition to the next. Relationships between detailed coating structure and deposition parameters such as substrate temperature [33, 34], argon partial pressure [31, 32], bias voltage [37], and target-substrate distance [8] have been studied for decades. While many parameters are controllable during deposition, there exist other variables and/or by-products that cannot. For instance, Buck [38] studied the effects of water vapor in the plasma on the microstructure and tribological behavior of MoS₂ coatings. By varying the partial pressure of water, he showed that increased water vapor produced low density, less crystalline coatings that exhibited poor wear resistance. A secondary source of water vapor was also found to be a result of substrate heating during deposition via desorption of water from surfaces. The study found this source of contamination also led to less wear resistant coatings. Interestingly, substrate heating is often changed to improve the performance of MoS₂ coatings, yet it can have a negative effect depending on the cleanliness of the system.

From an applied perspective, sputtered MoS₂ films are a preferred solid lubricant coating in inert environments, such as vacuum and space applications. However, despite the best efforts by coatings developers, batch-to-batch variations in film properties, which originate from difficulties in precisely controlling mutually influencing deposition parameters, pose significant challenges for hardware engineers owing to the resulting variability in functional behaviors, including tribological performance. Typically, aerospace hardware manufacturers provide witness coupons for each coating batch and subject them to in-house testing designed to qualify the batch. In recent years, several high-performance MoS₂ based composites have fallen out of favor with hardware engineers due to the inability to reliably achieve the same caliber of tribological performance they once did. While the motivation for more wear resistant, lower friction, and environmentally agnostic materials always remains, there is an applied need to develop metrics to quantify what properties make universally “good” MoS₂ coatings and encourage an understanding of what variables during or prior to deposition may be at play to disrupt this. This information will be invaluable in ensuring the quality and consistency of MoS₂ films, as well as metrics that can enable future materials discovery and optimization of film composition and structure for a range of applications.

Given that even experienced commercial suppliers of sputtered MoS₂ coatings can produce films with varying structure and performance due to uncontrolled deposition parameters, a method is needed to efficiently inspect coatings for critical attributes that will insure adequate tribological performance in the intended application. The purpose of this work is to show that density and hardness can be used as quality control metrics to insure the tribological performance of pure sputtered MoS₂ coatings.

Both the LD-2 and HD-1 coatings were manufactured in the same deposition chamber with nominally the same (controllable) deposition parameters, and by the same technician, albeit on different days. Additionally, the LD-1 coating was manufactured in a different chamber but with similar deposition parameters as LD-1 and HD-1. One of the most striking and quantifiable differences between the coating batches is the density. HD-1 (ρ = 4.5 g/cm³) has a density close to that of bulk MoS₂ (ρ = 4.8–5.06 g/cm³), exceeding the average density of IBAD coatings (ρ = 4.4 g/cm³) [29]. Both LD-1 (ρ = 3.55 g/cm³) and LD-2 (ρ = 3.04 g/cm³) have measured density values that are well below HD-1, likely due to the formation of the voids observed in the TEM (Fig. 1).

Differences in density and void formation could be due to variations in crystallite orientation and degree of crystallinity, as indicated by XRD. Buck observed that crystalline pure MoS₂ coatings (ρ ~ 3.8–3.95 g/cm³) are denser than amorphous coatings (ρ ~ 3.3 g/cm³), and that increased basal-orientation improves density due to decreased porosity [31]. Though our results show that LD-1, which is edge-oriented (as indicated by the (10 \(\overline{1 }\) 0) peak), is denser than the basally oriented LD-2 coating, both coatings have low peak intensities corresponding to their preferential orientations. Orientation and crystallinity can influence the friction and wear behavior of MoS₂, with highly crystalline, basally oriented coatings having lower initial friction coefficients and faster run-in to steady-state friction over amorphous microstructures [39, 40]. Although nanocrystal-amorphous coatings have been reported to have lower wear rates than nanocrystalline coatings [41], in this study it is challenging to decouple the individual effects of orientation and crystallinity from density on the tribological behavior. The impact of orientation on density and void formation is supported by the growth kinetics of MoS₂ during deposition. Low density, porous coatings are a result of the formation of edge-oriented MoS₂ crystallites providing reactive edge-sites for new deposits leading to a high vertical growth rate and decreased horizontal growth rate. As larger, vertically oriented lamellae form, they cause a shadowing effect, blocking incoming deposits resulting in the formation of voids [42, 43]. Void formation can greatly impact hardness. This phenomenon is widely studied in other material systems such as ceramics [44‐49] where it has been observed that hardness increases as porosity decreases.

Pure MoS₂ coatings deposited by PVD are typically sub-stoichiometric with a deficiency in sulfur [35, 50] and can have high levels of oxygen in the bulk (> 10%) [32, 51]. Oxygen substituted into the crystal lattice of MoS₂ forms MoS_2-xO_x by substituting sulfur and results in a peak shift of the (10 \(\overline{1 }\) 0) due to a reduced lattice constant [32]. Though the coefficients of friction observed for oxygen rich films are not as low as pure MoS₂ coatings, films containing high amounts of oxygen have been shown to have increased density from a reduced crystallite size, thereby producing lower wear rates than that of pure MoS₂ coatings [32]. Addition of oxygen, which can be viewed as a dopant, means that “pure” is a misnomer for MoS₂ films without dopants because of the added benefits oxygen can impose. In this work, no statistically significant difference in oxygen content in the coatings under investigation was detected. Notably, the oxygen content was very low and close to the accuracy of the analytical tool. While we do not believe that oxygen is contributing to the improved densification of HD-1, it is not unlikely that sources of contamination during the deposition process could introduce unwanted oxygen or water. Sources such as adsorbed water on the deposition chamber walls due to exposure to lab air during sample changing or transfer and a contaminated sputtering target could be key uncontrolled factors that influence the coatings density and tribological behavior.

For MoS₂, relationships between coating porosity, hardness, and wear are not well established. Seynstahl et al. [52] varied the sample substrate rotation during deposition and observed that compact pure MoS₂ coatings with little to no porosity were harder (H = 5.69 GPa) and more wear resistant (k ~ 1 × 10^–7 mm³/Nm) than softer (H ~ 0.06–0.25 GPa), porous films (k ~ 5 × 10^–6—2 × 10 ⁻⁵ mm³/Nm). Although the authors did not directly measure density, we observe a similar trend with HD-1 exhibiting a higher hardness (H = 4.4 GPa) and lower wear rate (k = 5.74 × 10^–8 mm³/Nm) in dry N₂ than both LD-1 (H = 1.6 GPa, k = 7.98 × 10^–7 mm³/Nm) and LD-2 (H = 2 GPa, k = 1.59 × 10^–6 mm³/Nm). While density is a major factor contributing to the increased hardness of HD-1, the discrepancy between hardness and density for LD-1 and LD-2 could be due to differences in coating orientation. Though both LD-1 and LD-2 have weak peak intensities indicating nanocrystalline/amorphous microstructures, there is weak preferential vertical orientation of LD-1 (i.e., basal planes parallel to the indentation axis), compared to the more basally oriented LD-2 (basal planes perpendicular to the indentation axis), allowing for deformation to occur between low shear strength basal planes as the coating is deformed. For vertically oriented coatings, the indenter tip can advance further into the coating by pushing the columns apart resulting in a lower measured hardness [44]. The high hardness and density of HD-1 produced a greater wear resistance than that of widely established composite films such as MoS₂/Sb₂O₃/Au (k ~ 1 × 10^–7 mm³/Nm [21]) in dry N₂ environments. Improvements in the wear rates imparted by density and hardness are also observed in humid environments (Fig. 5), suggesting that wear performance measured in humid air could be a metric for a quality MoS₂ coatings, which for practical flight hardware, could be a useful metric if inert environments are unavailable or impractical to use.

Density as a driving factor for low wear MoS₂ coatings, and hardness as an indicator of coating density, provides a useful metric for the qualification of MoS₂ coatings to be used in practical applications. The low measured hardness of LD-1 and LD-2 would, for instance, indicate to an engineer that the batch of coatings will not meet specifications and should not be used. By using hardness as a metric, timely tribological testing of coating batches or costly characterization techniques such as RBS can be avoided, and only performed on batches such as HD-1 which meet an adequate hardness threshold. Additional useful metrics such as crystallinity and orientation, measured by XRD, would help provide a fast and accurate indication of a quality film.

Uncontrolled variation in mechanical properties of sputter deposited pure MoS₂ has limited their use in engineering applications. In this work, relationships between the mechanical properties and density of pure MoS₂ coatings were developed to establish deterministic parameters for high-quality coatings. Three batches of pure MoS₂ coatings were deposited using similar parameters, yet the wear rates of the coatings varied by ~ 100 × in dry N₂. Density of the coatings showed that each coating batch varied in density with the highest density, lowest wear coating (k = 5.74 × 10^–8 mm³/Nm, ρ = 4.5 g/cm³) having little to no void formation and achieving a density close to bulk MoS₂. The other coatings both had low density (ρ = 3.04 and 3.55 g/cm³) and significant void formation throughout the coatings. Nanoindentation showed distinct hardness and modulus differences between dense (H = 4.4 GPa, E = 83.7 GPa) and porous (H = 1.6–2 GPa, E = 50.4–67.8 GPa) coatings, correlating to changes in density and coating orientation. These results suggest that density is one of the dominant factors contributing to low wear rates for MoS₂ coatings and that the hardness of a coating is a key indicator of high-performance coatings. Furthermore, using hardness as a key metric to determine coating batch quality could help in reducing time consuming tribological testing or complex and expensive characterization.