Design and selection: materials to manage friction and wear 239

11.5 Design and selection: materials to manage friction and wear 239

Burst velocity

a) 100

Pressure limited

a) 100

Full hydrodynamic

P (MP

by yield

lubrication

(MP 10 P 10

1 Boundary ing pressure

1 Velocity limited

Safe use

by overheating

area

ing pressure

lubrication

Bear 0.1 Bear 0.1 Dry sliding

Sliding velocity v (m/s)

Sliding velocity v (m/s)

(a)

(b)

Figure 11.11 (a) A P–v curve for a bearing material. (b) P–v envelopes for various lubricated

and unlubricated sliding, ultimately limited by the disintegration of the bearing under centrifugal force at very high velocities.

to suppress vibration. Intimate details are trade secrets, but here is the idea. The brake pads on your bike are made up of a synthetic rubber with particles of a cheap silicate to reduce pad wear and give wet friction. Those on your car have

a phenolic matrix with particles of silicates, silicon carbide (sandpaper grit) to control friction and graphite or MoS 2 as a lubricant. Those on a military jet, a 747 or an F1 car are carbon or ceramic—here high temperature is the problem; replacement every 10 days is acceptable if they stand the heat (try calculating how much aluminum you could melt with the energy dissipated in bringing a 200-tonne 747 to rest from 200 kph).

Waging war on wear

As explained earlier, the rate of wear increases with the bearing pressure P (equation (11.4)). It also increases with sliding velocity v because the faster the sliding, the more work is done against friction and it all turns into heat. A given bearing material has an acceptable P–v envelope within which it is usable; ven- ture outside it and seizure or catastrophic wear awaits you (Figure 11.11(a)). To extend the envelope upwards, we select materials with higher strength; to extend it laterally, we choose materials with higher thermal conductivity and melting point—or lower friction.

Lubrication, of course, reduces friction. Figure 11.11(b) shows that this does not increase the admissible bearing pressure—that is a static property of the material—but it does increase the permissible sliding velocity. Boundary lubri- cation expands the envelope a little; hydrodynamic lubrication expands it much more—but note its drop-off at low velocities because there is not enough sliding speed to build up the pressure profiles of Figure 11.9.

At very high rotational velocities, approached in ultra-centrifuges, gyro- scopic navigation systems and super-flywheels for energy storage, there is an

240 Chapter 11 Rub, slither and seize: friction and wear

ultimate cut-off. It is the speed at which centrifugal forces exceed the strength of the material and the whole system disintegrates.

Hard materials have low wear rates but they tend to be brittle. Soft ones are tougher but they wear rapidly. Wear is a surface property, toughness a property of the bulk. So the way to fight wear is to take a tough material and give it a hard surface coating, using techniques of surface treatment. High-carbon steels are surface hardened by rapidly heating with a flame, an electron beam or a laser beam and then quenching the surface. Carburizing, nitriding and boriding give wear resistance to components like crankshafts or ball races by diffusing carbon, nitrogen or boron atoms into a thin surface layer, where they react to give particles of carbides, nitrides or borides. Hard, corrosion-resistant layers of alloys rich in tungsten, cobalt or chromium can be sprayed onto surfaces, but a refinishing process is then necessary to restore dimensional precision.

Hard ceramic coatings of alumina (Al 2 O 3 ) or of tungsten or titanium carbide (WC, TiC) can be applied by plasma spraying to give both wear resistance and resistance to chemical attack, and chemical and physical vapor deposition meth- ods allow surface coatings of other metals and ceramics, including diamond- like carbon.

Records for all these treatments (and more) are contained in the CES soft- ware under the heading ‘Surface treatment’. The software allows selection of coatings to meet given design requirements, of which wear resistance is one of the more important. We return to these in Chapter 18 on selecting processes.