Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 129
alternative suggestion is that the H atoms dissolve preferentially in regions
of high triaxial stress (such as exist at the tip of cracks), where they locally
lower the interatomic cohesion of the lattice.
There does not appear to be a single origin of hydrogen cracking: different
micromechanisms of failure have been proposed for various materials, and
further research is required before a full understanding is achieved.
3.4.4 Wear of metals and alloys
Wear occurs when two solid surfaces slide over each other, either with or
without a lubricant and empirical sliding wear tests have been devised to
simulate practical situations and to provide design data on wear rates. A
common test rig employs a pin pressed against a rotating disc or ring, the pin
being the specimen and the rotating surface the counterface. Several test rig
methods are the subject of national standards, and contact stresses, thermal
conditions, sliding speeds and chemical environment are all significant
parameters in any wear test.
Wear is assessed by measuring the change in specimen dimensions during
the course of the test, expressing the data as the amount of material removed
per distance slid as a function of the normal load. Figure 3.36 shows, on
logarithmic axes, the wear rate of a leaded α/β brass pin sliding against a
hard stellite ring as a function of load, and the pattern of behaviour typifies
that in many metallic systems in both lubricated and unlubricated conditions.
The electrical contact resistance between the pin and the ring is also plotted,
allowing the extent of metallic contact to be estimated.
There are seen to be two regimes of wear: mild wear at low loads and a
transition to severe wear at higher loads. In each regime, the data can be
expressed in terms of the Archard wear equation:
Q = KW/H [3.15]
where Q is the wear rate per unit sliding distance, W the normal load, H the
indentation hardness of the softer surface and K a dimensionless constant
known as the wear coefficient. The quantity K/H is given the symbol k and
called the dimensional wear coefficient; it represents the volume of material
removed by wear per unit distance slid.
In Fig. 3.36, the region of mild wear corresponds to the sliding surfaces
being separated by oxide films, with only occasional direct metallic contact,
hence the relatively high contact resistance. The debris formed consists of
mixed oxides of copper, zinc and iron originating from the slider and the
ring, and wear rates of both are similar. In the regime of severe wear, the
wear rate of the harder counterface is negligible and the wear debris is
metallic brass. There is extensive metallic contact, as shown by the low
contact resistance.