Solid Lubrication at High-Temperatures-A Review

Abstract

Understanding the complex nature of wear behavior of materials at high-temperature is of fundamental importance for several engineering applications, including metal processing (cutting, forming, forging), internal combustion engines, etc. At high temperatures (up to 1000 °C), the material removal is majorly governed by the changes in surface reactivity and wear mechanisms. The use of lubricants to minimize friction, wear and flash temperature to prevent seizing is a common approach in engine tribology. However, the degradation of conventional liquid-based lubricants at temperatures beyond 300 °C, in addition to its harmful effects on human and environmental health, is deeply concerning. Solid lubricants are a group of compounds exploiting the benefit of wear diminishing mechanisms over a wide range of operating temperatures. The materials incorporated with solid lubricants are herein called 'self-lubricating' materials. Moreover, the possibility to omit the use of conventional liquid-based lubricants is perceived. The objective of the present paper is to review the current state-of-the-art in solid-lubricating materials operating under dry wear conditions. By opening with a brief summary of the understanding of solid lubrication at a high temperature, the article initially describes the recent developments in the field. The mechanisms of formation and the nature of tribo-films (or layers) during high-temperature wear are discussed in detail. The trends and ways of further development of the solid-lubricating materials and their future evolutions are identified.

Keywords: friction; glaze layer; high temperature; self-lubrication; smart materials; solid lubricants; tribology; wear.

Figure 1

Number of published articles on (a) high-temperature wear and self-lubrication (1980–2020); and (b) high-temperature self-lubrication based on respective solid lubricants (percentage, 2000–2021), as recovered from Scopus database.

Figure 2

(a) Classification of HT solid-lubricants based on their chemical composition; and (b) a scheme showing the mechanism of their friction reduction.

Figure 3

Diffusion mechanism of soft metal-based solid lubrication during HT tribological operations.

Figure 4

HT sliding results of Ag-based composites and coatings as found in recent literature (a) CoF; (b) Relative wear rate (in relation to their corresponding RT wear). Values are labeled as per Ag concentration, matrix material, and fabrication method [10,13,14,15,16,17,18,19,20,21].

Figure 5

Crystal structures demonstrating the weak Vander walls forces present in the inter-lamellar layers/planers resulting in an easy slip between them (a) MoS₂ (or WS₂); (b) a single layer of graphene and graphite as a heap of multiple graphene layers; and (c) hexagonal boron nitride (h-BN) [17].

Figure 6

HT sliding results of MoS₂-based composites and coatings as found in recent literature (a) CoF; (b) Relative wear rate (in relation to their corresponding RT wear). Values are labeled as per MoS₂ concentration, matrix material, and fabrication method [47,48,49,50,51,52,53].

Figure 7

HT sliding results of hBN-based composites and coatings as found in recent literature (a) CoF; (b) Relative wear rate (in relation to their corresponding RT wear). Values are labeled as per hBN concentration, matrix material, and fabrication method [80,81,82,83,84,85,86].

Figure 8

HT sliding results of WS₂-based composites and coatings as found in recent literature (a) CoF; (b) Relative wear rate (in relation to their corresponding RT wear). Values are labeled as per WS₂ concentration, matrix material, and fabrication method [10,87,88,90,91,92].

Figure 9

HT sliding results of Fluorides-based composites and coatings as found in recent literature (a) CoF; (b) Relative wear rate (in relation to their corresponding RT wear). Values are labeled as per Fluorides concentration, matrix material, and fabrication method [10,33,97,98,99]. Synergic effect of more than one solid lubricant is labeled with S.

Figure 10

Schematic showing synergism of solid lubricants, i.e., soft metal/laminar solids and fluorides to broaden the range of lubricating temperature.

Figure 11

A collection of images from ref. [4] showing the mechanism of wear reduction at HT due to the formation of thick, homogeneous tribolayer. Tribolayer consisted of TiO₂ and boric acid.

Figure 12

Contemplation of oxides forming Magnéli phase based on their de-cohesion energy (G) and elastic constant (C₄₄) as a function of the distance between the cleaved layers [109].

Figure 13

HT sliding results of oxide-based lubrication and coatings as found in recent literature (a) CoF; (b) Relative wear rate (in relation to their corresponding RT wear). Values are labelled as per oxide formation (in situ), matrix material, and fabrication method [4,13,118,120,123,124,125,126,127].

Figure 14

A graphical representation of effective temperature ranges for solid-lubricating materials (solid lubricants).

Figure 15

An approximate range of CoF under effective temperature ranges for widely used solid-lubricating materials (solid lubricants).

Figure 16

(a) Evolution of an HT solid-lubricating material in terms of CoF and wear rate; (b) features of a ‘smart’ solid-lubricating material [3].