Understanding the friction and wear of KDP crystals by nanoscratching

Potassium dihydrogen phosphate (KDP) is an important nonlinear optical crystal material, and has been widely used in large-aperture high-power laser systems. This paper aims to explore the friction and wear mechanisms of KDP by nanoscratching using Berkovich and conical indentation tips. Two scratching loading modes, the 'Constant' and the 'Continuous', were applied. During the experiment, the moisture and temperature in the testing chamber were controlled. The study found that the adhesive force plays an important role when the scratching load is low. The lateral force or the friction coefficient fluctuates due to the non-uniform deformation of the KDP in front of the indentation tip. During a constant loading scratching, the frequency and amplitude of the fluctuation vary with the level of the scratching load and the type of the indenter, while the velocity effect seems to be trivial. The slip in the KDP crystal, which is very much affected by contact stress, could be a main mechanism of plastic deformation. The Berkovich tip produces more obvious wavy groove walls, while the conical tip brings about more ploughing deformation along the wear track. The strain rate in the range investigated in this study seems to have a little effect on the micro-plastic deformation.