Advantages of Using Optical Profilometry in the ASTM D4172 Standard
Authors: Hernandez Battez, A., Torres, A., Garcia-Atance, G., Viesca, J.L., Gonzalez, R. and Hadfield, M.
Conference: 2010 STLE Annual Meeting & Exhibition
Dates: 16-20 May 2010Abstract:
There are many types of wear test methods with different purposes: to evaluate a material´s response to a specific type of wear, such as solid-particle erosion, sliding wear or two-body abrasive wear; to simulate a particular field application in order to screen materials, surface treatments, or lubricants. Standard wear tests methods -like any type of wear test- have both strengths and limitations .
Different wear quantification techniques are used in studies on wear. Historically, weighing the sample before and after the test has been the dominant wear quantification technique. This method has a limitation since in situations of mild wear, such as the wear of most tribological components involving lubricated sliding or rolling contact, the mass loss is often very small in relation to the total mass of the worn component. Usually, a precision balance has a resolution of 10-6 of the maximum load (e.g., 0.1 mg resolution at 100 g maximum load), which puts a limit to the minimum load possible to quantify in relation to the total weight of the component . Further, the mass of components may vary due to the adsorption of water (polymeric materials) and other contaminants. The component may also gain mass by material transfer from the counter surface. These variations often obscure the mass loss due to wear. Finally, weighing gives no information on the distribution of wear over the component. A second approach to wear quantification is based on dimensional measurements, either continuous measurements during wear or by determining the surface shape after wear. Continuous measurements are normally one-dimensional, exemplified by the measurement of the pin length reduction in a pin-on-disk test, using a high resolution displacement transducer. Shape determinations have been performed using mechanical and optical profilometers.
Now, it is possible to measure the volume of topographical features in the sub μm3 range (the mass of 1 μm3 steel ≈ 10-11 g!) by the introduction of high resolution instruments such as the atomic force microscope (AFM) and optical interferometry. Another benefit of this kind of instruments is that the topographical data is used directly to render images of the surface, including shading and height coded colours. The ASTM D4172 standard  covers a procedure for making a preliminary evaluation of the antiwear properties of fluid lubricants in sliding contact by means of the Four-Ball Wear Test Machine. This test method can be used to determine the relative wear preventive properties of lubricating fluids in sliding contact under the prescribed test conditions. The antiwear properties of the lubricant fluids are measured through the wear scar diameters (dimensional measurements) on the three lower balls using a microscope. This measurement method gives no information about the distribution of the wear over the worn surface and data can be corrupted by displaced or transferred material and/or irregular shape of the wear surfaces. This paper highlights how the ASTM D4172 can be improved by using the optical profilometry in order to measure wear volume instead of wear scar diameter measurements.
Preferred by: Antolin Hernandez Battez