Tribological Properties of Fire-Resistant, Nonflammable, and Petroleum-Based Hydraulic Fluids

Petroleum-based hydraulic fluids qualified under MIL-H-6083 are highly flammable. As a result, a fire-resistant polyalphaolefin (PAO)-based fluid, qualified under MIL-H-46170 and developed in the 1970s, and more recently, a nonflammable chlorotrifluoroethylene (CTFE) hydraulic fluid, were developed as potential replacements. However, the chemical and physical properties of both fluids are significantly different from those of conventional fluids. This paper outlines a laboratory study to define the interrelated parameters of degradation and wear of contacts lubricated with various hydraulic fluids, with particular reference to CTFE. The results rank the likely wear, corrosiveness, and oxidation properties of CTFE in comparison to currently used silicone-, PAO-, and petroleum-based fluids. In general, the antiwear characteristics of the CTFE hydraulic fluid were found to be similar to those of petroleum-based fluids but marginally inferior to those of PAO-based fluids. However, CTFE produced severe corrosion of brass at temperatures above approximately 135°C. As a result, it is believed that operation at very high temperatures is likely to cause unacceptable material removal from copper-based metals. Several surface treatment processes were identified to minimize potential side effects, evident under more severe operating conditions with CTFE.