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A GUİDE TO DRY COATİNG

A Guide to Dry Coating

Dry coating is a powerful tool to obtain desirable coating solutions. Tungsten disulfide, molybdenum disulfide, graphite, and PTFE are commonly combined with dry coating methods to achieve durable and self-lubricating coatings. This guide dives into the details of tungsten disulfide, molybdenum disulfide, graphite, and PTFE dry coatings, their application areas, weigh outs their advantage and disadvantages, and concludes which one is the best option.

What is Dry Coating?

Coating is without a doubt one of the most important integral of various technological systems. Coatings provide protection against a number of external threats such as corrosive chemicals, heat, and moisture; provide lubrication and resistance to extreme temperatures. The coating techniques are mainly categorized as wet and dry coating techniques. Wet (liquid) coatings are the most traditional coating techniques including brushing of liquid on to the surface or dipping or immersing the material into the liquid coating medium. On the other hand, the dry coating is a novel approach to coating technologies that has been used for the last 40 years. This coating technology involves the use of coating material in the powder form. This is why dry coating is often referred to as powder coating as well. Powder spraying and burnishing methods are the two of the well-known dry coating techniques while out of the two, burnishing is much less common. In the powder spraying method, the powdered material is applied electrostatically to the surface and often cured with heat or ultraviolet light.

Being a novel approach, the dry coating offers a wide variety of advantages thanks to the advanced technologies behind it. In comparison to the wet coating techniques, one of the most important advantages of dry coating is the reduced emission of volatile organic compounds (VOCs) which are harmful pollutants affecting the environment and human health. Liquid coating media contain solvents that have high amounts of VOCs while powder coatings have little to negligible amounts of VOCs. Powdered chemicals used in dry coating are much easier to clean in the case of an accident reducing the health and safety risks. Furthermore, the dry coating process is much more time and cost efficient, reduces ventilation requirements, and most importantly provides a more durable product. The advantages of dry coating over its alternatives can be summarized as;

  • Durability
  • Time and Cost Efficiency
  • Environmental Friendly
  • Ease of Use
  • Improved Health and Safety Conditions

The most common applications of this exciting coating technology are;

  • Coating of steel materials against heat damage, cold damage, and corrosion
  • Coating of metallic roofs and curtain walls
  • Coating of car parts under frequent stress or usage such as door handles, shock absorbers, coil springs, and frames (White, 2018)
  • Coating of solid lubricants such as WS2, MoS2, graphite, and PTFE
  • Coating of aircraft and spacecraft parts

Out of this plateau of applications, we will be focusing on the coating of solid lubricants and diving into their advantages and disadvantages as well as their applications.

 

Dry Coating with Tungsten Disulfide (WS2)

Tungsten disulfide (WS2) is a synthetic powder material with a layered structure. W-S-W units of tungsten disulfide are strongly bonded in a hexagonal arrangement. On the other hand, the W-S-W layers are weakly connected by the intramolecular of van der Waals forces (Chate, Sathe, & Hankare, 2013). WS2 is chemically well-defined and is a well-known solid lubricating material with a molecular weight of 248 g/mol, a density of 7.5 g/ml, and a contact angle of 93° in H2O (Brian A. Baumgarten, 2011). It is insoluble in water. The layered crystal structure of WS2 is demonstrated in Figure 1.

Figure 1. The layered structure of tungsten disulfide (Hussain, Sevilla, Rader, & Hussain, 2013)

The excellent lubricating properties of WS2 stem from the dynamic distribution of the weak binding forces between these layers of WS2. Tungsten disulfide has the lowest reported dynamic friction coefficient of 0.03 and a static friction coefficient of 0.08 in a solid material (Ilie & Tita, 2008). These low friction coefficient values make WS2 an ideal candidate for solid lubrication in various applications. Tungsten disulfide is commonly applied with dry coating techniques. The appearance of the resulting WS2 coating is silver-gray and smooth. WS2 coatings commonly have a thickness of approximately 0.5 microns and mirror all the characteristics of the substrate. Furthermore, WS2 coatings are non-toxic, non-corrosive, and inert. However, WS2 cannot inhibit the inherent corrosion on exceptionally low corrosion materials. These coatings are impermeable to most solvents, refined fuels, and chlorinated solvents. On the other hand, it is important to note that, WS2 is not resistant to fluorine gasses, sulfuric and hydrofluoric acids, and hot caustic alkaline solutions. Another important advantage of WS2 coatings is the ease of coating process which does not require any heat or curing process and binders. The coating process is performed at room temperature. The resulting WS2 dry coating is compatible with petrochemical oils, greases, synthetic oils, silicone lubricants, and hydraulic fluids. Furthermore, WS2 coatings have a wide operation range which is from -273-650°C at ambient conditions and -188-1316°C in vacuum environments. The high operating temperature of WS2 makes it compatible with bolts, shafts, exhaust parts, and many other industrial pieces. One of the most attractive properties of WS2 coatings is the ability to withstand high loads of over 300,000 psi. Due to these excellent properties, WS2 dry coatings provide;

Tungsten Disulfide (WS2) Micronpowder

  • Reduced mechanical lubrication and build-up problems
  • Improved performance and extended service life for machinery
  • Vacuum compatible lubrication
  • Wide temperature operating range
  • Easy release of parts
  • Elimination of galling, seizing, and cold welding
  • Reduction of pressure and mold wear

As it is with any material, there are of course drawbacks to WS2 dry coatings as well. WS2 is;

  • Vulnerable to fluorine gasses, sulfuric and hydrofluoric acids, and hot caustic alkaline solutions, as it was mentioned earlier.
  • Not a food grade material
  • Not highly resistant to abrasion due to low hardness values
  • High cost compared to other solid lubricants

 

Composites of WS2 with various materials are utilized to enhance the corrosion and abrasion resistance of WS2 coatings. The Vickers Hardness of pure WS2 coatings is approximately 300 VH which is significantly lower than that of steel which can go up to 900 VH depending on the degree of treatment. WS2 is often used in combination with chromium (Cr), Carbide (C), and Titanium (Ti).

For example, a study by F. Ilie et. al. has investigated the effect of Ti doping on the hardness and wear performance of WS2 coatings as well as the friction coefficient of the composite material. This study has interestingly suggested that Ti doping significantly improved the wear resistance of WS2 coating while also improving the friction coefficient of the pure WS2 material. The friction coefficient of the WS2/Ti has decreased to 0.03 from the value of 0.05 of pure WS2 coating. This improvement in the friction coefficient is attributed to the formation of TiO2 by the authors. Furthermore, the wear tracks on the testing samples coated with WS2/Ti are found to be much smoother than the samples coated with pure WS2 (Ilie & Tita, 2008).

 

Another study by F. Gustavsson et. al. focusing on WS2/C/Ti and WS2/C/Cr has demonstrated the positive effect of Ti and Cr as well as C on the hardness of WS2 coatings. Cosputtering of WS2 with lighter elements such as C or N provides better results in humid air and enhances the mechanical properties of the coating. This study also shows that incorporating Cr and Ti significantly improves the mechanical properties of WS2 whilst preserving the desirable friction coefficients. The hardness of WS2 is increased to ≈700 VH for WS2/C/Cr and to ≈1800 VH WS2/C/Ti indicating that while both options increase the mechanical properties of the coating Ti gives more significant results (Gustavsson, Bugnet, Polcar, Cavaleiro, & Jacobson, 2015).

https://nanografi.com/blog/a-review-case-on-dry-lubricants-ptfe-and-graphite/

Thanks to these desirable properties, WSis utilized as a solid lubricating material applied by dry coating techniques. Common application areas of WS2 dry coatings are;

  • Cryogenic pumps,
  • Electrical connectors,
  • Slide mechanisms,
  • High vacuum applications,
  • Cutting tools,
  • Seamer rolls,
  • Circuit breakers and switches,
  • Compressors and rheostats
  • Engine parts,
  • Pilot valves,
  • Pins and taps etc.

Dry Coating with Molybdenum Disulfide (MoS2)

Molybdenum disulfide (MoS2) is a naturally occurring inorganic material known as the mineral Molybdenite. This material has a molecular weight of 160.08 g/mol, a density of 5.06 g/ml, and a water contact angle of 60°. Just like WS2, MoS2 shows a layered structure that contributes to its desirable properties. In this structure, Mo atoms form hexagonal planes with covalently bonded S atoms at both sides. The strong covalent bonds in the planes and weak Van de Waals bonds between planes result in easy shearing tangential to the planes. Much like tungsten disulfide the desirable lubricating properties of MoS2 are caused by this easy shearing. Figure 2 gives a representation of the planar structure of MoS2.

Figure 2. The planar structure of MoS(Vazirisereshk, Martini, Strubbe, & Baykara, 2019)

MoS2 is one of the most common dry lubricating materials on the market due to its desirable friction coefficient and availability. Dry coating of MoSoften referred to as Moly coatings, can be done through various techniques such as burnishing, spraying, and rubbing. The resulting coating has an appearance of bluish grey and a smooth surface. The friction coefficient of MoS2 coatings can be as low as 0.03 under vacuum. However, in humid environments, its friction coefficient is in the range of 0.06-0.15 (Gradt & Schneider, 2016). The working temperature range of dry MoS2 coatings also depends on the operational environment. The maximum operating temperature is reported as 1100°C under vacuum and 400°C under ambient conditions. At the other end of the spectrum, MoS2 can operate at cryogenic temperatures down to -185°C both under vacuum and ambient conditions. Hence, MoS2 dry lubricants are used as an excellent lubricating material under vacuum and cryogenic temperatures. Furthermore, MoS2 dry coatings can be used in high load applications up to 250,000 psi. MoS2 dry coatings offer;

  • Non-toxic, inert, non-corrosive coatings,
  • Wear resistance,
  • Long-lasting lubrication
  • Resistance to evaporation and aging,
  • Controlled film thickness and adjustable load bearing,
  • Prevention against galling, pick-up, and seizure,
  • Excellent adhesion,
  • Application on all ferrous and non-ferrous metals, man-made solids, and plastics.

There are of course some disadvantages of MoS2 dry coating despite it being one of the most preferred dry coating materials. These disadvantages are;

  • High sensitivity to humidity resulting in oxidation of the coating material. Under normal humid conditions, the friction coefficient can rise to 0.15-0.3 accompanied by wear.
  • Increasing friction coefficient with decreasing temperatures,
  • Not food grade material.

In dry coating applications, MoS2 is often doped with various materials in the hopes of decreasing its friction coefficient and providing resistivity to humid environments. The most common dopants in MoS2 dry coatings are titanium (Ti), gold (Au), nickel (Ni), and antimony trioxide (Sb2O3). Amongst these dopants, Ti is the most widely studied option. Ti dopants are desirable because of its homogenous distribution among the MoS2 structure.

A study by Hoa Li et. al. have investigated the tribological behavior of MoS2/Ti composite coatings and showed that Ti-doped molybdenum disulfide coatings provide improved oxidation resistance without compromising the friction coefficient across a range of different humidities. According to this study, the inclusion of Ti also improves the hardness of the coating which in turn provides better wear resistance (H. Li, Zhang, & Wang, 2016).

A comparative study on the effect of dopants on MoS2 dry coatings has focused on the Ni, Au, Fe, and Sb2O3. This study has shown that all dopants improve the friction coefficient and wear resistance of MoS2 coatings and concluded that the best option was Sb2O3/Au (Zabinski, Donley, Walck, Schneider, & McDevitt, 1995). Hard dopants like Ni, Cr, and Sb2Oaffect the performance of MoS2 coatings by increasing the hardness and the density. On the other hand, softer materials such as Au provides a thinner and more even coating of MoS2 leading to lower wear. In terms of oxidation resistance, the increase in the density of MoS2 reduces the number of active sites, passivates the edges, and sacrificially bonds with oxygen. Hence, the inclusion of dopants to the MoS2 structure reduces the detrimental effect of humidity and improves performance and storability in air. As a result, doping with Sb2O3/Au is reported to offer the best of both worlds to the MoS2 coatings. A study conducted by Matthew A. Hamilton et. al. have shown that Sb2O3/Au doped MoScoatings have an order of magnitude lower wear rate than Sb2O3-doped MoS2 at room temperature without compromising from the friction coefficient (Hamilton et al., 2008). The improvement in wear resistance is especially useful in space applications since the wear resistance is crucial. For example, the James Webb Space Telescope (JWST) which is planned for launch in 2021 and is the successor to the Hubble Space Telescope uses MoS2 coatings as a solid lubricant on the sensitive parts in a number of precision instruments including Near-Infrared Spectrograph (NIRSpec) and Mıd-Infrared Instrument (MIRI) (Krause et al., 2010). In addition to space applications, MoS2 coatings are utilized in a number of other areas.

The main application areas of MoS2 dry coatings are;

  • Aerospace/space applications,
  • Cryogenic pumps,
  • Low moisture applications,
  • Vacuum applications,
  • Compressors,
  • Engine and drive train parts,
  • O ring seals,
  • Medical and dental equipment,
  • Refrigeration and liquid nitrogen pumps.

Dry Coating with Graphite

Graphite has attracted considerable attention since its discovery. It is the most well-known layered structure whose fame has exceeded the science community. Each layer of graphite consists of carbon atoms arranged in a hexagonal crystal structure. Unlike previous materials mentioned in this text, graphite is a mono-atom material. However, similar to WS2 and MoS2 has strong bonds between the carbon atoms at the same layer and weak bonds between the layers facilitating the easy shear movement. Graphite has a high melting point, 3600°C for pristine graphite, and a density of 2.09-2.23 g/cm3 which is lower than diamond. This naturally occurring material is insoluble in water and organic solvents making it a solvent resistant material. The lubricating properties of graphite are strongly dependant on the moisture content of the environment. Graphite’s lubricating mechanism involves both the utilization of interlayer shear and water intercalation. In addition to these mechanisms, previous studies have also shown that graphite flakes have indicated the formation of graphite scrolls at the tribological interface (Spreadborough, 1962). These scrolls are favorable for decreasing the surface energy and reducing friction in the sliding interfaces.This is why water vapor is crucial for obtaining low friction coefficients with graphite. It is reported that the friction coefficient of graphite is around 0.5-0.6 in dry environments and around 0.1-0.2 in humid environments (Berman, Erdemir, & Sumant, 2014). Because of these desirable lubricating properties and its abundance, graphite has been used as a solid lubricant for more than 40 years. However, the dry coating of graphite is a new technology offering even distribution of graphite, better performance, and control over the coating properties. Graphite lubricants applied with dry coating techniques commonly have a thickness of 0.2-5 μm and an appearance of opaque black. The operating temperature range of graphite dry coatings is reported as -35 – 600°C. It is also important to note that the properties of graphite dry coatings are considerably related to the substrate material which affects the friction coefficient and the wear resistance of graphite dry coatings. For example, a study conducted by Yongxin Wang et. al. has shown the differences between Ti, Al, brass, and GCr15 steel. According to this study and later works on the matter, graphite coatings show the lowest friction coefficient and wear when it is coupled with Ti under humid conditions. On the other hand, the highest friction coefficient in both dry and humid environments was observed for brass as 0.47 and 0.14 respectively. In terms of wear rates, graphite coatings coupled with aluminum have shown the highest wear rates in both dry and humid environments (Y. Wang, Wang, Li, Chen, & Xue, 2013). However the differences, most of the studies on the graphite dry coatings suggest that they are suitable to dry lubricants, especially under humid conditions.

Expanded Graphite

The advantages of graphite dry coating can be listed as;

  • The low friction coefficient and high thermal stability
  • Good lubrication in high humidity,
  • Protection against fretting corrosion,
  • Abundance and low-cost of the material,
  • Easy application without the need for curing steps,
  • Can be sprayed onto the hot surfaces,
  • Chemically inert so it can resist environmental changes,
  • Prevents galling and binding,
  • The application can be topped up.

On the other hand, there some disadvantages to graphite coatings such as;

  • Limited use under vacuum or dry conditions,
  • Defects in the structure might cause oxidation and lower the actual working temperature down to 600°C.

Graphite can be applied as a pure dry coating or as a composite with other materials. The composites graphite coatings are utilized either to improve the lubricating properties of durable coatings or the lubricating properties of graphite under vacuum and dry conditions. An example of graphite composite dry coatings utilizes the graphite/tantalum composite. A comprehensive study was conducted on the lubricating properties of this composite material by Zuoping Wang et. al. This study has shown that the inclusion of Ta into graphite dry coatings result in improved friction coefficient and wear behavior without affecting the binding abilities of the coating. The friction coefficient lowers down to 0.01 which is approximately ten folds better than pure graphite coatings. Furthermore, this study suggests that Graphite/Ta composite coatings show better performance under dry conditions (Z. Wang, Feng, & Shen, 2018).

Another composite material that is used in combination with graphite dry coating is chromium. According to a study conducted by S. K. Field et. al., Graphite/Cr coatings provide superior wear resistance to pure graphite coatings and reduced friction coefficients. The friction coefficient of these composite graphite coatings is as low as 0.05 and shows no change under cyclic loading. In addition, after 10,000 cycles no visible cracks or wear were observed on the composite coating suggesting a great wear resistance. Furthermore, the wear rate of composite coatings has also shown improvement (Field, Jarratt, & Teer, 2004).

The graphite dry coatings have a wide range of application areas whether applied as a composite or as a pure coating. These application areas can be listed as;

  • Engine components and machinery,
  • Medical applications such as artificial hip joints and medical tools,
  • Drilling equipment,
  • Baking lines,
  • Small intricate components that are sensitive to dust and gunk,
  • Electronics,
  • Textile machines.

Dry Coating with PTFE

Polytetrafluoroethylene (PTFE) is an important polymer-based engineering material. This material is an organic polymer consisting of a carbon backbone and two fluorine atoms bonded to each carbon atom. Most of the desirable properties of PTFE stem from these fluorine atoms which do not easily react with any other material, not even other fluorine atoms. Fluorine is mostly the reason for the lubricating nature of PTFE coatings since they create surface energy as low as 18 dynes/g. PTFE does not require humidity to perform as a lubricant and doesn’t have a layered structure. However, macromolecules of PTFE slip easily along each other similar to lamellar structures. Pure PTFE coatings have a friction coefficient range of 0.2-0.15. Despite the low friction coefficients, pure PTFE shows very high wear rates. Thanks to the fluorine atoms in its structure, PTFE is also a highly inert material having a great chemical resistance and stability. Furthermore, PTFE shows electrical inertness. The density of PTFE is around 2.1-2.3 g/cm3 while its melting point is approximately 327°C. PTFE coating is one of the most commonly used self-lubricating coatings. You might not have heard about PTFE but surely you’ve heard about Withford Xylan™ or better yet Chemours Teflon™ which are essentially the in-house versions of PTFE dry coatings. Because of these well-known grades of PTFE, it is commonly associated with the term non-stick”. The appearance of PTFE coatings is whitish or gray with a smooth surface. However, they can be reformulated to achieve various colors. The operating temperature range of PTFE coatings is often reported as -180 – 250°C and the optimal coating thickness is reported as 5-10 μm (lubricants, 2014).

 

The advantages of PTFE dry coatings can be listed as;

  • Excellent chemical resistance,
  • Good resistance to light, UV, and weathering,
  • Low friction coefficients,
  • Food and medical grade material,
  • Good resistance to high and low temperatures,
  • Excellent hydrophobicity,
  • Provides self-cleaning properties to surfaces due to high hydrophobicity,
  • Low dielectric constant (2.0).

On the other hand, the disadvantages of PTFE dry coatings are;

  • Low wear resistance due to the relative softness of PTFE,
  • Sensitivity to creep and abrasion,
  • Poor heat dissipation due to the low thermal conductivity which causes premature failure of the coating,
  • Use is limited to low-speed sliding application,
  • Low-load carrying capacity,
  • Low resistance to radiation,
  • The difficulty of coating around the joining,
  • Corrosive and prone to toxic fumes.

There are numerous studies and formulations of PTFE coatings that focus on eliminating the drawbacks of PTFE coatings. Most of these studies especially deal with the high wear rate of the material. The wear rate of pure PTFE coatings changes depending on several different parameters however published results are between 4.8-11.2×10-4 mm3/Nm.For this purpose, several different fillers including plastics, graphite, molybdenum disulfide, fiber glass, bronze, dental silicate, silicon, titanium dioxide, silver, copper, tungsten, and molybdenum were suggested to be included in the PTFE coatings.

A comprehensive study has investigated the friction coefficient and wear behavior of PTFE coatings with different filler materials including carbon, graphite, glass fibers, MoS2, and poly-phenyleneterephthalamide (PPDT) fibers. The results of this study show that all of these filler materials has a positive effect on the wear resistance of PTFE dry coatings and little to no effect on the friction coefficient. The highest wear resistance was found for composites containing 18% carbon + 7% graphite, 20% glass fibers + 5% MoS2 and 10% PPDT fibers. Furthermore, the PPDT fibers were found to be the most effective fillers in interrupting large-scale fragmentation. The study also indicates a relationship between the wear behavior of the coating and the thermal stability, thermal conductivity, and the characteristics of filler materials (Khedkar, Negulescu, & Meletis, 2002).

Significant improvement in the wear resistance is also observed when alumina nanoparticles are used as the filler material in the PTFE coatings. The study on the subject has resulted that at filler concentrations of 20 wt% the wear resistance of the PTFE coatings improved 600 times. The minimum wear rate of alumina filled PTFE at this concentration was reported as 1.2×10-6 mm3/Nm which is considerably lower than the unfilled PTFE dry coatings. Meanwhile, the friction coefficient of the PTFE coating has shown a slight increase however it remains on the 0.15-.02 range (Sawyer, Freudenberg, Bhimaraj, & Schadler, 2003).

Studies on the composite PTFE coatings are endless and in general, suggest that fillers provide better wear resistance to PTFE dry coatings. As another example of successful PTFE fillers, ZnO has reported to improve the wear resistance of PTFE dry coatings to 13×10-6 mm3/Nm (F. Li, Hu, Li, & Zhao, 2001). Bronze filled PTFE coatings show higher thermal and electrical conductivity hence can be applied for extreme temperature applications. Carbon fiber and carbon fillings reduce creep and increase the hardness of PTFE dry coatings improving the wear resistance. Studies on the composite PTFE coatings are endless and in general, suggest that fillers provide better wear resistance to PTFE dry coatings.

The desirable properties of PTFE dry coatings are utilized as a dry lubricant in several different areas including;

  • Aerospace and automotive parts,
  • Packing machines,
  • Roller and chutes in the transport ranges,
  • Separation of casting resin applications,
  • Medical industry (as coatings that inhibit bacteria),
  • Chemical industry (on vessels, impellers, heat exchangers, etc.)
  • Cookware (non-stick pans, pots, etc.)

In addition to its traditional dry lubrication applications, PTFE coatings are also utilized as water and dirt repellent coatings on tents and camping equipment, fabrics, carpets, umbrellas, hot hair tools, fast food, and condiment containers.

Which Material is Better?

Tungsten disulfide, molybdenum disulfide, graphite, and PTFE are all excellent materials for dry coating. Their outstanding lubrication properties make them very valuable in various application areas where liquid lubrication is just not an option or the dirt and mold attracted by liquid coatings pose great problems. But which material is better out of the four? To answer this question we need to compare these dry coatings on several different points. The first point that comes to mind is the friction coefficient of these materials since it is the main indicator of a good dry lubricant. Graphite and PTFE dry coatings show higher friction coefficients in comparison to molybdenum disulfide and tungsten disulfide. Furthermore, graphite is not suitable for vacuum applications due to high friction coefficients under vacuum conditions. Even though molybdenum disulfide and tungsten disulfide has comparable friction coefficients under vacuum conditions, the friction coefficient of molybdenum disulfide under ambient conditions is significantly higher than the tungsten disulfide. The tribological behavior of molybdenum disulfide considerably gets affected by the water vapor in the air and thus molybdenum disulfide dry coatings are prone to failure in the case of environmental changes. As a result with regards to the friction coefficient, tungsten disulfide provides better results under a wider range of conditions.

Another important aspect of dry coating is the operating temperature range of coating materials. The wider the temperature range, the richer the application areas of the coating. Furthermore, a wider temperature also indicates thermal resistance. PTFE has the narrowest temperature range out of the four and is not suitable for high-temperature applications over 250°C. Graphite is not suitable for low-temperature applications with a temperature limit of -35°C. Even though molybdenum disulfide and tungsten disulfide both have wide operating temperature ranges, tungsten disulfide can withstand much more extreme temperatures both under ambient and vacuum conditions.

Lastly, we need to assess the wear resistance of dry coatings. Out of all four options, molybdenum disulfide has the highest wear resistance while PTFE has the lowest wear resistance properties. Tungsten disulfide and graphite is located in between these options. However, there are several different composite options of each material that facilitate the wear resistance of dry coatings. The important properties of all four dry coating materials are summarized in the table below. As it can be deducted from the table and explained in detail above, tungsten disulfide is decided as the better dry coating material. It is no surprise that recently more and more research has been done on the tungsten disulfide dry coatings when we make a comparison with the other options on the market.

Table 1. Comparison of key properties of the dry coating materials

Friction Coefficient Operating Temperature Range Wear Resistance
Tungsten Disulfide 0.03-0.08 -273-650°C under ambient conditions-188-1316° under vacuum Medium
Molybdenum Disulfide 0.03 under vacuum0.06-0.15 under ambient conditions -185-1100 °C under vacuum-185-400 °C under ambient conditions High
Graphite 0.1-0.2 -35-600°C Medium
PTFE 0.15-0.2 -180 – 250°C Low

Conclusion

This work provides a quick guideline to dry coating with tungsten disulfide, molybdenum disulfide, graphite, and PTFE. These materials are all important dry lubricants that have been the center of studies. The dry coating is a new approach that uses the coating material in a powder form. The well-known dry coating methods are powder spraying and burnishing methods. Both of these offer advantages over traditional wet coating methods. These advantages include the reduced emission of VOCs, time and cost efficiency, reduced ventilation requirements, and durability. When combined with dry coating methods, these materials offer numerous advantages such as excellent lubricating properties without the dirt and inconvenience of wet lubricants, the ability to coat hard to reach places, ease of use, durability, time and cost efficiency, resistance to extreme temperatures, and wear resistance. As with any material, each of these dry coating options has its advantages and disadvantages. PTFE is a great material to use under mild conditions where wear resistance and extreme temperature are not of concern and chemical stability or hydrophobicity is important. Graphite is a great option under humid conditions since its lubricating mechanism performs the best in the presence of water vapor. On the other hand, graphite is not preferred for vacuum applications for the same reason. When it comes to heavy-duty applications such as aerospace applications, high-speed cutting, and sliding, high vacuum applications, or cryogenic pumps; tungsten disulfide and molybdenum disulfide are the two contestants for the job. While molybdenum disulfide has better wear resistance, it does not perform well under ambient conditions since humidity in air interferes with its lubricating mechanism and increases the friction coefficient of the coating. Hence, molybdenum disulfide dry coatings are ideal for high vacuum applications such as coating of spacecraft parts, etc. On the other hand, tungsten disulfide has the lowest friction coefficient both under vacuum and ambient conditions. It is also superior in terms of operating temperature range under both conditions and can withstand temperatures up to 1316°C under vacuum. The downside to tungsten disulfide dry coatings is their relatively lower wear resistance. However, studies have resulted in various options to remedy this problem by suggesting different composites of tungsten disulfide. The use of composites is also utilized for the coatings of molybdenum disulfide, graphite, and PTFE of course. Studies suggest that the downsides of these materials can be improved with the use of composite materials. Nevertheless, after a thorough comparison, tungsten disulfide dry coatings come out on top and prove to be the better option amongst these four dry coatings.

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