# Why can solid lubricants operate only at lower temperature in oxidizing environment but higher temperature in reducing environment?

The two main dry lubricants are graphite and molybdenum disulfide. They offer lubrication at temperatures higher than liquid and oil-based lubricants operate. Dry lubricants are often used in applications such as locks or dry lubricated bearings. Such materials can operate up to 350 °C (662 °F) in oxidizing environments and even higher in reducing / non-oxidizing environments (molybdenum disulfide up to 1100 °C, 2012 °F).

Source: Wikipedia contributors, "Dry lubricant," Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Dry_lubricant&oldid=870516892 (accessed July 24, 2019).

Why can solid lubricants operate only at lower temperature in oxidizing environment than in reducing environment?

• Likely they can get oxidised and at a certain point even burn. .. – Alchimista Jul 24 at 9:32
• Ever use a charcoal grill? Of what element are charcoal and graphite made? – DrMoishe Pippik Jul 25 at 4:29

Graphite is elemental carbon, and carbon is unstable in the presence of oxygen because they react to form carbon dioxide. The rate of this reaction: $$\ce{C + O2 = CO2}$$ is very slow and has a high activation energy, so a piece of graphite can sit in oxygen at room temperature for millions of years without anything happening to it. Once temperature increases, the reaction becomes much more rapid. From my own experience, a fist sized chunk of graphite can survive for several hours when heated to above 1000 °C in atmospheric oxygen.
Molybdenum sulfide is composed of two things that are likewise not stable in Earth's surface atmosphere: molybdenum and sulfur. It reacts by: $$\ce{2MoS2 + 7O2 → 2MoO3 + 4SO2}$$. As with graphite, this reaction is strongly temperature dependent.