12.4 ABRASIVE WEAR
occurs in two modes, referred to as the
abrasive wear processes.
hard, rough material
sliding against a softer one
The hard surface digs into and removes material from the softer one. An example
is a file used to contour a metal part.
introduction of hard
particles between two sliding surfaces, at least one of which is softer than
. The hard
particles abrade material from one or both surfaces. Lapping and polishing are
in this category.
a material removal process in which the affected surfaces lose
mass at some controlled or uncontrolled rate
parts that operate in clean environments can be designed to minimize or
eliminate abrasive wear through proper selection of materials and finishes.
Smooth, hard materials will tend to not abrade soft ones in two-body contact.
Smooth finishes minimize abrasion at the outset and, unless hard particulate
contaminants are later introduced to the interface in service, that situation
addition to designing systems to avoid abrasion, engineers also design them to
controlled abrasive wear. Controlled abrasion is widely used in
manufacturing processes. Two-body
is perhaps the most common
example, in which abrasive media such as silicon carbide (Carborundum) are
forced against the part under high sliding velocities to remove material and
control size and finish. Cam surfaces are ground in this manner.
engineering materials are better suited to abrasive-wear applications than
others, based largely on their hardness. However, with hardness usually comes
brittleness, and thus their resistance to impact or fatigue loads can be less
than optimum. Table 12-1 shows the hardness of some materials that are suitable
for abrasive-wear applications.
Some ceramic materials can be
plasma-sprayed onto metal substrates to provide a hard facing that also has
high corrosion and chemical resistance. Some of these plasma-sprayed coatings
are quite rough upon application (like severely orangepeeled paint) and thus
must be diamond ground to obtain a finish suitable for a sliding joint. Some of
these coatings are also very brittle and may chip from the substrate if overstressed
either mechanically or thermally. Other hard coatings are available for steel
that take on, or even improve, the surface finish of the base material. Most of
these are proprietary formulas that are kept as trade secrets. Some trade names
Armaloy, Diamond Black,
Dicronite, Titancote, TitaniumNitriding
, and others.*
For more information on coatings, contact:
The American Electroplaters
and Surface Finishers Society
at http:// www.aesf.org/ or
The National Asociation of
Copyright 2004, Industrial
Press, Inc., New York, NY