Research on Solid Lubricants for Use within Gas-Cooled Power Reactors

The gas-cooled power reactors operated by the C.E.G.B. are refuelled on-load. Thus there are a number of devices which operate in the coolant, and solid lubricants offer an attractive solution to the lubrication problems encountered. These problems are discussed, and the characteristics of suitable lubricants are elucidated. The main requirement is that of long lifetime at constant (but not necessarily extremely low) friction.

Present solutions to some of the problems are described, and their limitations are pointed out. A brief review of some possible solid lubricants is given, with an indication of the temperature ranges within which they are usable.

Results of friction and endurance tests on sprayed films of unbonded molybdenum disulphide, tungsten disulphide and graphite are presented for tests at 200 and 400°C. At 400°C tungsten disulphide was the most promising lubricant, whereas molybdenum disulphide was best at 200°C. In general, lifetimes of sprayed films showed a wide scatter.

Preliminary experiments on some hot-pressed composite materials containing a metallic (nickel) matrix and a lubricant powder (tungsten diselenide or calcium fluoride) are described. None of the formulations tested was fully satisfactory at 100°C or 400°C when both wear rate and friction were considered. However, all proved superior to plain nickel when rubbed against stainless steel.

The important factors in the method of application of solid lubricants are not understood. It is clear that much more basic research is needed, and the directions this might take are discussed.