All metals and materials in contact with each other during work require lubrication.
Sandy, dusty and contaminated environments, using wet lubricants like oil and grease are detrimental to the system
Dust and sand mixed with lubricant turns in to mud which can harden and remove lubricating effect from the system.
Solid lubricant coatings are inert, resistant to moisture and corrosion.
Coating with solid lubricant are protecting the metals from corrosion and reduce the friction at the same time.
These solid lubricant are stable at temperatures where oils will decompose and will not leak out of the gear boxes.