Solid Lubricants as Friction Modifi ers 185
Some type of substrate preparation for the load-bearing surface may be required to facilitate
the application of the solid lubricant. This is usually necessary for metal deformation processes
so that the fi lm thickness, fi lm uniformity, and durability of the applied lubricant on a billet will
be robust enough to lubricate. Typical treatments of the surface include phosphating, peening, and
shot blasting, which are especially useful for powder tumbling applications. With water-based
dispersions, heating the substrate to some elevated temperature is often necessary to activate the
bonding agents. Substrate heating serves a dual purpose: it facilitates the evaporation of the water
carrier, and it also initiates the physical/chemical bonding of the fi lm onto the substrate.
6.4 APPLICATIONS
Two major lubrication applications are considered here: metal wear protection lubrication and
lubrication for plastic deformation of metal. The former concerns applications such as constant
sliding or reciprocating motion, for example, gear, chain, or journal lubrication. The latter concerns
applications where metal is under plastic fl ow, such as metal-forming or metal-cutting applications.
6.4.1 WEAR PROTECTION AND GENERAL LUBRICATION
Wear protection and general lubrication applications are meant to include processes requiring hydro-
dynamic lubrication, elastohydrodynamic lubrication, and boundary lubrication. Examples of such
applications include chain lubrication, gear lubrication, and engine oil treatments. In essence, any
application where repetitive sliding or rolling contact occurs between two surfaces can be consid-
ered under the umbrella of wear protection lubrication. The intention is for the lubricant to reduce
the coeffi cient of friction and protect against wear (see Figure 6.10). The benefi ts include savings
in power consumption and service life of the component and effi ciency gains due to the increased
uptime resulting from proper lubrication.
Solid lubricants are useful and required for applications and conditions when conventional
liquid lubricants are inadequate. These conditions include the following:
- High operating temperatures that eliminate or reduce the functionality of the liquid lubricant
- Contact pressure of suffi cient magnitude that breaches the integrity of the liquid lubricant
%CHANNEL %CHANNEL
20.0
18.0
16.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.018.0
16.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
0.100 1.000 10.00 100.0^100020.0Size (μm)FIGURE 6.9 Coarse graphite particle size distribution.