Abrasion tests on human enamel under wet and dry conditions

Wear behaviour of the cross sectional area of human enamel at a micrometer scale was studied with a nanoindenter transducer mounted on an AFM stage. Wear was applied by scanning a diamond indenter tip over a certain area with a load higher than typical for surface imaging. A conical tip with a radius of similar to 400 nm was used. Single lines of linearly incremented load were also scratched. The rounded tip mainly compresses the near surface material, and under certain conditions, fractures and removes material from the sample. A threshold load of 400 mu N for removing material was determined after scanning the sample with 50, 100 and 200 mu N without observing any spatted material, but deformed areas. The area of the worn surfaces was 2 x 2 mu m(2) since otherwise the tip crossed more than one mineral rod. Crossing a rod leads to a different condition under the tip due to the change in fibril orientation between the rods. For the 'rounded' tip the abrasion rate was larger under atmospheric conditions compared to the immersed in Hank's balanced salt solution (HBSS), when depth profiles were measured by Atomic Force Microscopy (AFM) imaging. However, similar results for both conditions were obtained when analysing the displacement data got from the transducer.