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;
- 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;
- 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).
Thanks to these desirable properties, WS2 is 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.