Revealing the mechanical properties of potassium dihydrogen phosphate crystals by nanoindentation
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C. Zhanga) and M. Liu Laboratory for Precision and Nano Processing Technologies, The University of New South Wales, New South Wales 2052, Australia
F.H. Zhang School of Mechatronics Engineering, Harbin Institute of Technology, People’s Republic of China
K. Mylvaganam and W.D. Liu Laboratory for Precision and Nano Processing Technologies, The University of New South Wales, New South Wales 2052, Australia (Received 1 September 2015; accepted 26 February 2016)
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.
I. INTRODUCTION
Potassium dihydrogen phosphate, KH2PO4 (KDP), and its deuterated analogue K(DXH1 X)PO4 (DKDP) are hydrogen bonded crystals.1,2 Owing to its remarkable photoelectric properties, KDP has been widely used for electro-optic switches and frequency conversion crystals in the large-aperture, high-power laser systems.3,4 In such a system, hundreds of large-aperture KDP components of the size of 420 420 10 mm are needed to meet the requirement of the laser performance. However, a KDP crystal is one of the most difficult-to-cut nonlinear optical materials, because it is soft, deliquescent, easy to fracture, and sensitive to temperature change. As such, it is extremely difficult to make a KDP element with damage-free subsurface and super smooth optical surfaces. Likewise, the laser-induced damage threshold (LIDT), which is the most important property for a KDP element to function in a high-power system, is far below what the intrinsic crystal should have provided.5–9 It has Contributing Editor: George M. Pharr a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.91 1056
J. Mater. Res., Vol. 31, No. 8, Apr 28, 2016
been suspected that the low LIDT is due to the poor surface integrity of a KDP element caused by a surface machining process, during which damages, such as dislocations, micro/nanocracks, and phase changes, could have been introduced to the KDP crystal. However, the mechanism behind it is not clear. To improve the laser damage resistance of a machined KDP element, research efforts have been taken to understand the mechanical properties of this type of complex materials subjected to mechanical loading. To date, however, the works are limited to the characterizat
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