173

6

Selection and Application

of Solid Lubricants as

Friction Modifi ers

Gino Mariani

CONTENTS

6.1 Introduction ........................................................................................................................... 173
6.2 Solid Lubricant Properties .................................................................................................... 175
6.2.1 Graphite ..................................................................................................................... 175
6.2.1.1 Sources of Graphite ..................................................................................... 177
6.2.1.2 Lubrication .................................................................................................. 178
6.2.2 Molybdenum Disulfi de .............................................................................................. 179
6.2.3 Boron Nitride ............................................................................................................180
6.2.4 Polytetrafl uoroethylene ............................................................................................. 181
6.3 Preparation for Lubricant Application .................................................................................. 182
6.4 Applications .......................................................................................................................... 185
6.4.1 Wear Protection and General Lubrication ................................................................. 185
6.4.2 Lubrication for Plastic Deformation of Metals ......................................................... 189
References ...................................................................................................................................... 194

6.1 INTRODUCTION

Solid lubricants are considered to be any solid material that reduces friction and mechanical
interactions between surfaces in relative motion against the action of a load. Solid lubricants offer
alternatives to the lubricant formulator for situations where traditional liquid additives fall short
on performance. An example is a high-temperature lubrication condition in which oxidation and
decomposition of the liquid lubricant will certainly occur, resulting in lubrication failure. Another
example is for situations that generate high loads and contact stresses on bearing points of mating
surfaces, producing a squeeze-out of the liquid lubricant and a resulting lubricant starvation
(see Fig u re 6.1).
Solid lubricants, used as a dry fi lm or as an additive in a liquid, provide enhanced lubrication
for many different types of applications. Typical hot-temperature applications include oven chain
lubrication and metal deformation processes such as hot forging. Solid lubricants are also helpful
for ambient-temperature applications such as drawing and stamping of sheet metal or bar stock.
Solid lubricants are effectively used in antiseize compounds and threading compounds, which pro-
vide a sealing function and a friction reduction effect for threaded pipe assembly [1]. Applications
involving low sliding speeds and high contact loads, such as for gear lubrication, also benefi t from
solid lubricants. The solid lubricant effectively provides the required wear protection and load-
bearing performance necessary from gear oil, especially capable when used with lower-viscosity
base oils.