Used for roughness quality control during production. The heart of a ballpoint pen is the ball of between 0.4 and 1.6 mm in diameter, made of stainless steel or tungsten carbide which is used as the writing tip. From a cartridge the ink flows through tiny grooves to the tip where a rolling ball applies the ink to the paper.
The image shows a tungsten carbide ball's surface with a diameter of 0.5 mm. To ensure the ink is uniformly transferred onto the paper a precisely defined surface structure of the ball is needed. The precision of the diameter and roundness must be extremely exact and the surface must have a defined roughness. For optimum ink flow the roughness must be between 2 and 10 nm.
The AFM provides accurate roughness measurements down below 1nm on small balls. The measurement was done in "Static Force" operating mode.