For the past 100 years, there have been a large number of wear testing instruments developed. There has been considerable work conducted for the development of standardized test procedures and preferred tester is normally specified by most industries. In case you are not sure on the starting point, start by contacting research industry associations to find out if an accepted test procedure exists. Other information sources include organizations that develop test standards, such as ASTM International and ISO.
In case a method does not exist or it is decided not to follow the industry standard, it is important to select a test that models the wear system that needs to be studied. Ideally the test will duplicate a wear situation seen in an intended application exactly for instance, a field test. However with wear complexity, an exact simulation is either not possible or practical and certain differences need to be accepted. This is because wear involves two or more bodies, one or more materials and is based on a broad range of influences. Hence test development is subject to trial and error and is dependent on the capability of the developer.
Features to Consider in Simulating a Wear System
The key elements involved in the simulation of a wear system include specimen preparation, apparatus design, test protocol and measurement. The main features that must be considered are described.
- Apparatus – The test apparatus must be robust, should be of a rugged design to provide repeatable and reproducible results. Parameters such as speed, speed, rigidity of apparatus construction, alignment, and supply of abrasive require adequate control to ensure stable wear conditions.
- Materials involved – The wear system structure includes the specimen and counter-body. One must 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 abrasive types 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 impact on the rate of wear.
- Shape – Particles that are blocky or angular in shape can cause up to ten times the wear rate as compared to rounded particles.
- Size - The particle size is critical, as smaller particles cause proportionally less wear than larger particles.
- Type – Popular abrasive particles include aluminium oxide and 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.
- Friability – This is how easily 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. In certain systems it may be required that the specimen and abradant to “wear-in”, thus establishing a uniform and stable contact geometry.
- Contact pressure (applied load) – With an accelerated test, the load may be more that what is actually in the field. This parameter normally involves the force that the abrading material is pushed against the specimen during the rubbing action.
- Sliding speed (sliding velocity) – This is the speed at which the abradant moves over the specimen. While it is desirable that the test be accelerated, 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 impact the frictional characteristics of a material. It is 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 tests involving metals, surface roughness, geometry of the specimens, microstructure, homogeneity, and hardness need to be measured. Similar controls are also essential for the counter-face and the wear-producing mediums.
- Environment – Many materials are sensitive to humidity and temperature variations and changing the test environment will influence results.
It is not common for an engineer concerned with product life and consistency 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.
This information has been sourced, reviewed and adapted from materials provided by Taber Industries.
For more information on this source, please visit Taber Industries.