Residual stress model for CaF 2
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Nanoindentation tests and finite element analysis that considers elastic-mesoplastic deformation for single crystals were used to investigate the mechanical properties of CaF2 under spherical indentation. The goal was to gain a better understanding of microfractures and crystalline anisotropy and their effect on the surface quality of CaF2 during manufacturing. In this analysis, indentations of the three main crystallographic planes (100), (110), and (111) were studied and compared to examine the effects of crystalline anisotropy on the load–displacement curves, surface profiles, contact radius, spherical hardness, stress distributions, and cleavage at two stages, namely at the maximum indentation load and after the load had been removed. Our model results were compared with experimental observation of surface microroughness, subsurface damage, and material removal rate in grinding of CaF2.
I. INTRODUCTION
Calcium fluoride (CaF2) is a cubic ionic single-crystal material with important properties for optical instrumental applications. These properties include wide transmission range, low refractive index, high permeability, and low birefringence. CaF2 is widely used in the manufacturing of mirrors, lenses, windows, and prisms for both ultraviolet and infrared applications. The optical performance of CaF2 is highly correlated to its surface quality. For instance, Stenzel et al. investigated laser damage behavior of CaF2 under various polishing steps.1 In contrast to conventional hard polish, advanced methods, such as ductile machining or chemical polishing, lead to a distinct increase in its damage threshold. Using optical interferometry and atomic force microscopy (AFM), Retherford et al. examined the effect of surface quality on transmission performance for the (111) surface of CaF2.2 Their results showed that improved surface quality and lower subsurface damage could lead to a greater increase in transmittance. Kukleva et al. measured the dependence of the coefficient of specular light reflection on the surface roughness for the (100), (110), and (111) planes of CaF2.3 Their calculations showed the specular reflection coefficient increased for smoother surfaces. To thoroughly exploit its optical characteristics, a great deal of effort has been devoted to investigating the mechanical properties of CaF2 during its surface finisha)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0348 2796
J. Mater. Res., Vol. 22, No. 10, Oct 2007
ing process to produce high-quality finished parts. For example, the finished surface characteristics and polishing parameters of CaF2 under different methods such as magnetorheological finishing (MRF), single-point diamond turning (SPDT), ultraprecision float polishing, and ultraprecision grinding have been examined and compared.4–7 It was found that microfracturing and crystallographic anisotropy are the main factors affecting surface preparation. Structural defects, such as dislocations, are usually generated during material removal.
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