Wear Testing and Wear Measurement

Why wear testing?
Wear test is carried out to predict the wear performance and to investigate the wear mechanism.
Two specific reasons are as follows:
– From a material point of view, the test is performed to evaluate the wear property of a material so as to determine whether the material is adequate for a specific wear application.
– From a surface engineering point of view, wear test is carried out to evaluate the potential of using a certain surface engineering technology to reduce wear for a specific application, and to investigate the effect of treatment conditions (processing parameters) on the wear performance, so that optimised surface treatment conditions can be realised.
Three levels of wear testing
Wear test is performed in three levels, namely,
(1) laboratory test,
(2) component simulation test,
(3) in-service test.
We may use an example to describe the difference among each type of test. A new surface engineering (SE) technology has been developed, which could be potentially used to improve the wear resistance of parts for a metal-on-metal hip joint (for human body). Perhaps the ideal and logical sequence of wear testing in this example will be as follows:
Stage 1 Laboratory test with small samples are initially carried out under testing conditions simulated insofar as conveniently possible to determine whether the surface engineering technology warrants further consideration, and if so, to find out under what treatment conditions, the highest wear resistance improvement can be achieved.
Stage 2 In the next stage, the optimised surface treatment condition will be used to treat some real joint parts, e.g. femoral head or cup, which will then be tested in a hip-joint simulator, with the testing conditions being controlled as close as possible to those for a real human joint, for example, lubricated with body liquid, temperature around 37°C, and moving like a human walking or running.
Stage 3 Only when the surface treated parts have survived the simulation tests, and indeed show considerably improved wear resistance yet without losing other properties (e.g. corrosion and bio-compatibility), can the in-service tests be carried out. The surface treated joint parts will be implanted in a human body and tested (monitored) for a pro-longed period of time if there is no immediate side effect after implantation.
As we can see, simulation test and in-service test produce more reliable results, however, laboratory test is cheap, safe and quick and thus serve a most useful function. In the following sections, we will introduce the methods for laboratory wear test. The working principles of three widely used laboratory wear testers will be given, followed by an introduction of wear measurement. An example of laboratory wear testing and measurement can be found in a short case study.