Conducting Wear / Abrasion Tests

How to select a wear tester

Over the past 100 years, there have been hundreds of wear testing instruments developed. Fortunately, there has been significant work conducted to develop standardized test procedures and many industries specify a preferred tester that is used. If you are unsure where to start, begin by contacting research industry associations to determine if an accepted abrasion test procedure exists. Other sources of information include organizations that develop test standards, such as ASTM International and ISO.

If a method does not exist or you decide not to follow the industry standard, you need to select a test that models the wear system that you wish to study. Ideally, the test would exactly duplicate a wear situation seen in an intended application, such as a field test. However, given the complexity of wear, an exact simulation is generally neither practical nor possible and some differences will have to be accepted. This is because wear involves two or more bodies, one or more materials, and is dependent on a wide range of influences. Therefore, test development is subject to trial and error and is dependent on the capability of the developer.

The primary elements involved in simulating a wear system include apparatus design, specimen preparation, test protocol, and measurement. The following describe the important features that should be considered:

  • Motion – see Types of Wear
  • Apparatus – The test apparatus should be of a rugged design to provide repeatable and reproducible results. Parameters such as load, speed, rigidity of apparatus construction, alignment, and supply of abrasive require adequate control to ensure stable wear conditions.
  • Materials involved – The structure of the wear system includes the specimen and counter-body (usually an abradant of some sort). Be aware that a material can wear differently when exposed to different situations, or may be influenced by the wear of the other contacting body.
  • Abradant(wear agent) – Popular types of abrasives include textiles, sandpaper, and engineered abrasives. Although abrasive particles may not be the primary cause of actual wear, they are often used to accelerate the test. Abrasive particles, regardless if they are embedded in a binder material or are loose have a strong influence on the rate of wear.
    1. Shape – Particles that are angular or “blocky” in shape can cause up to ten times the wear rate as compared to rounded particles.
    2. Size - The size of the particle is critical, as smaller particles cause proportionally less wear than larger particles.
    3. Type – Popular abrasive particles include silicon carbide and aluminum oxide. With sandpaper, silicon carbide creates a thinner scratch pattern due to being a sharper grain than aluminum oxide, and will typically cut faster. Both types are available as an open coat or closed coat sandpaper. (Open-coat has gaps and open spaces between the grit particles that help prevent clogging. Closed-coat is better for abrading materials such as metal finishes but clogs easily.)
    4. Friability – How easy the abradant breaks down and fragments under localized heat and pressure, creating new sharp edges.
  • Contact Geometry – This includes the shape of the abrading head or abradant, and contact between it and the specimen. Some systems may require the specimen and abradant to “wear-in”, thus establishing a uniform and stable contact geometry. Although, point-contact eliminates many alignment problems associated with other contact geometries, stress levels may change as wear progresses, requiring more complex data analysis and comparison techniques.
  • Contact pressure (applied load) – With an accelerated test, the load may exceed what is actually seen in the field. This parameter usually involves the force that the abrading material is pushed against the specimen during the rubbing action.
  • Sliding speed (sliding velocity) – How fast the abradant moves over the specimen. While acceleration in a test is desirable, if the speed is too fast for the material (abradant), the precision of the test may be compromised by introducing different phenomena.
  • State of lubrication – Lubrication will affect the frictional characteristics of a material. Usually involved with metals, some plastics formulations also include a lubricant.
  • Specimen Preparation – Specimen preparation and the details of test control vary with the test and materials involved. For example, surface roughness, geometry of the specimens, microstructure, homogeneity, and hardness must be controlled for tests involving metals. Similar controls are also necessary for the counter-face and the wear-producing mediums.
  • Environment – Many materials are sensitive to changes in temperature and humidity, and changing the test environment will influence results.

It is not uncommon for an engineer concerned with reliability and product life to require precise simulation of the wear system. In contrast, a material developer looking to rank the wear resistance of materials may accept a convenient test that does not exactly replicate intended use. In either case, a well thought out wear test can provide valid test data without exactly replicating the application.

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