Use of optical profilometry in the ASTM D4172 standard

Among the wear quantification techniques applied to point contacts, the most typical one consists of measuring the longitudinal and transversal wear scar diameters of the worn surface. However, this method does not provide information about the distribution of the wear along the worn surfaces. As a result, the wear prediction using wear scar diameters could be misrepresentative because of the irregular shape of the worn surfaces. This drawback should be considered especially when the scope of the test method is the determination of the relative wear preventive properties of lubricants in sliding contacts. This paper compares two hydraulic lubricants from different manufacturers using a four-ball tribometer under the ASTM D4172 standard. Wear was measured by the wear scar diameter and wear volume with an optical 3D profilometer. Experimental results reveal that the wear volume measurement is a more sensitive method than the wear scar diameter one. Even more, the use of the wear scar diameter could lead to contradictions when performing a comparison of very similar results. For this reason, this paper suggests the use of the wear volume measurement in the standard.

• The measurement of wear volume is more effective than the wear scar diameter, mainly when the wear surface has displaced or transferred material and/or irregular shape.
• The differences of wear volume among tests with similar testing times are larger than the differences in wear scar diameter.
• Similar wear scar diameters can correspond to different wear volumes.