Revealing the mechanical properties of potassium dihydrogen phosphate crystals by nanoindentation

Potassium dihydrogen phosphate (KDP) is an important nonlinear optical crystal material for light frequency converters and Pockels photoelectric switches in laser systems. However, KDP is apt to fracture, is deliquescent, and can suffer from microstructural changes under a temperature variation. As such, KDP has been one of the most difficult-to-handle materials, but its properties have not been well understood. This paper aims to explore the mechanical properties of KDP crystals in detail with the aid of the nanoindentation technique using a Berkovich diamond indenter. It was found that the mechanical properties of KDP can be easily altered by machining-induced subsurface damage. It was also discovered that a KDP crystal is a visco-elasto-plastic material during micro/nanoscale deformation, although it is very brittle macroscopically.