. I Methods of Applying Solid Lubricants
There are several methods for applying solid lubricants. Some of them are
(A) Powdered solids: The oldest and simplest method of application of solid lubricants.
(i) Burnishing: Burnishing is a rubbing process used to apply a thin film of dry powdered solid lubricant such as graphite, MoS2, etc., to a metallic surface. This process produces a highly polished surface that is effective where lubrication requirements and wear-life are not stringent, where clearance requirements must be maintained and where the wear debris from the lubricant must be minimized. Surface roughness of the metal substrate and particle size of the powder are critical to ensure good application.
(ii) Hand rubbing: Hand rubbing is a procedure for loosely applying a thin coating of solid lubricant.
(iii) Dusting: Powder is applied without any attempt to evenly spread the lubricant. This method results in a loose and uneven application that is usually unsatisfactory.
(iv) Tumbling: Parts to be lubricated are tumbled in a powdered lubricant. Although adhesion is not very good; the method is satisfactory for noncritical parts such as small threaded fasteners and rivets.
(v) Dispersions: Dispersions are mixtures of solid lubricant in grease or fluid lubricants. The most common solids used are graphite, MoS2, PTFE, and Teflon. The grease or fluid provides normal lubrication while the solid lubricant increases lubricity and provides extreme pressure protection. Addition of MoS2 to lubricating oils can increase load-carrying capacity, reduce wear, and increase life in roller bearings, and has also been found to reduce wear and friction in automotive applications. However, caution must be exercised when using these solids with greases and lubricating fluids. Grease and oil may prevent good adhesion of the solid to the protected surface. Detergent additives in some oils inhibit the wear-reducing ability of MoS2 and graphite, and some anti-wear additives may actually increase wear. Solid lubricants also affect the oxidation stability of oils and greases.
Consequently, the concentration of oxidation inhibitors required must be carefully examined and controlled. Aerosol sprays are frequently used to apply solid lubricant in a volatile carrier or in an air-drying organic resin. However, this method should be limited to short-term uses or to light or moderate-duty applications where thick films are not necessary.
(B) Bonded coatings: Bonded coatings provide greater film thickness and increased wear life and are the most reliable and durable method for applying solid lubricants. Under carefully controlled conditions, coatings consisting of a solid lubricant and binding resin agent are applied to the material to be protected by spraying, dipping, or brushing.