The Effect of Dissolved Water on the Tribological Properties of Polyalkylene Glycol and Polyolester Oils

The effect of water dissolved in polyalkylene glycol and polyolester oils on the tribological behavior of two material contact pairs in three test environments is evaluated. The material contact pairs are M2 tool steel against 390 aluminum and M2 tool steel against gray cast iron. The three oils are a polyalkylene glycol (PAG) and two polyolester (PEl and PE2) oils. The test environments are R134a, air and argon. The tests are conducted in a specially designed high pressure tribometer which provides an accurate control of the test variables. The results indicate that the P AG oil performed better than the esters for both material contact pairs. The wear on the aluminum plates for the tests conducted with the P AG oil in all three environments is greatest at the lowest moisture content levels. From the stand point of friction and wear, it is beneficial to have a water content level of 5000 ppm or greater in the PAG oil when the plate material is 390 aluminum. The wear on the cast iron plates, when using a PAG oil as the lubricant showed a slight increase with water content in a R134a environment. This trend is opposite when air is the test environment. Both ester oils lubricated aluminum much better than the cast iron . The difference in the amount of wear can be as high as two orders of magnitude. This is probably due to the ability of the esters to form bidentate bonds with aluminum. Esters do not form such bonds with iron. The plate wear is greater for the PEl tests than for the PE2 tests for both material contact pairs. This is most likely due to the difference in the viscosity of the oils. In PE2 oil, water does not seem to affect the friction and wear of both aluminum/steel and cast iron/steel contacts when R134a is the test environment. On the contrary, for the aluminum/steel contacts, the water content significantly influences wear when argon or air is the test environment. For the cast iron/steel contacts, the wear is strongly influenced by the water content when the test is conducted in argon, but it is not influenced by the water content when the test is conducted in air.Air Conditioning and Refrigeration Center Project 0