Solid lubricants for applications at elevated temperatures

Abstract

Relative motion between mating surfaces at elevated temperatures often causes substantial material degradation due to friction and wear. Conventionally, solid lubricants have been used to reduce wear damage and friction drag under extreme conditions where liquid lubricants do not function properly. The recent trend towards higher operating temperatures in advanced power generating systems, i.e. turbomachinery, gas turbines, and hot adiabatic diesel engines, has imposed severe limitations on the currently available solid lubricants. The unusually aggressive conditions in these systems phased out most conventional solid lubricants and gave impetus to the search for more efficient materials. This paper discusses the lubricating characteristics of four different groups of materials known to provide lubricity under elevated temperature conditions. These groups are polymers, laminar solids, metal fluorides and metal oxides. Polymer lubricants are efficient lubricants within the range from room temperature to about 300 ~ Laminar solids extend that range to about 450 ~ Graphite, also a laminar solid, is an exception since it can offer excellent lubricity beyond 450 ~ in the form of gaseous oxidation products. Stable fluorides and metal oxides are useful lubricants between 500 and 1000 ~ though their performance is rather poor at lower temperatures.

Microtribology and Friction-Induced Material Transfer in Layered MoS2 Nanoparticles Sprayed on a Steel Surface

“…The weight decrease of 2 lm particles starts at around 400°C and is completed by 600°C. The change represents its thermal decomposition and the *20% weight loss in this temperature range is consistent with a conversion of MoS 2 to MoO 3 and gaseous SO 2 as reported previously [2,45,46]. This oxidation of MoS 2 by heating in air is believed to be dependent not only on temperature, but also on the particle size and availability of air [40].…”

The Lubricity of Oxides Revised Based on a Polarisability Approach

“…Data on the coefficient of friction of such oxides were collected from the published articles [1,2,4,23,24]. They prepared 50 atomic percentage compounds by mixing the appropriate quantities of the constituting simple oxides.…”

“…7 Variation of the coefficient of friction with the interaction parameter in binary/mixed oxides at T/T m = 0.7-0.8. Values for the coefficient of friction were taken from [2,4,23,24] In oxides, the mechanism of friction at different temperatures can be explained by considering the number of vacancies, N, at equilibrium given by [27] N ¼ c expðÀQ=kTÞ ð 7Þ…”

“…The coefficient of friction of these oxides were collected from literature [1][2][3][4][21][22][23][24]. Unfortunately, we have not found any reliable data of oxides like Sb 2 O 3 , SrO, and Cs 2 O-MoO 3 .…”

Tribological Properties of Spark-Plasma-Sintered Al2O3-SrSO4 Self-Lubricating Nanocomposites at Elevated Temperatures

“…Although there were few compounds which can work effectively as solid lubricants in air at temperatures ranging from room temperature to elevated temperature, some soft noble metals (Au, Ag), inorganic fluorides (CaF 2 , BaF 2 , BaF 2 /CaF 2 eutectic) and some metal oxides (MoO 3 , Cr 2 O 3 ) have been explored as high-temperature solid lubricants [1]. A series of self-lubricating composites based on these solid lubricants were developed by plasma spraying and powder metallurgy methods [2][3][4][5].…”