Thomas Industrial Library

12.4 ABRASIVE WEAR

Abrasion occurs in two modes, referred to as the two-body and three-body abrasive wear processes.[7] Two-body abrasion refers to a 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. Three-body abrasion refers to the introduction of hard particles between two sliding surfaces, at least one of which is softer than the particles . The hard particles abrade material from one or both surfaces. Lapping and polishing are in this category. Abrasion is then a material removal process in which the affected surfaces lose mass at some controlled or uncontrolled rate .

UNCONTROLLED ABRASION Machine 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 should continue.

CONTROLLED ABRASION In addition to designing systems to avoid abrasion, engineers also design them to create controlled abrasive wear. Controlled abrasion is widely used in manufacturing processes. Two-body grinding 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.

Abrasion-Resistant Materials

Some 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.

COATINGS 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 are: 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 Metal Finishers at http:// www.namf.org/.