Journal of Materials Processing Technology 229 (2016) 785–794 Contents lists available at ScienceDirect Journal of Materials Processing Technology jo ur nal ho me page: www.elsevier.com/locate/jmatprotec Evaluation of grinding-induced subsurface damage in optical glass BK7 Hao Nan Li ∗ , Tian Biao Yu, Li Da Zhu, Wan Shan Wang School of Mechanical Engineering and Automation, Northeastern University,Shenyang, PR China a r t i c l e i n f o Article history: Received 22 February 2015 Received in revised form 2 November 2015 Accepted 5 November 2015 Available online 10 November 2015 Keywords: Subsurface damage Optical glass BK7 Grinding a b s t r a c t Optical glass BK7 is widely used in optical industries but the grinding process of it with aggressive machin- ing parameters (e.g., fast infeed rate, big depth of cut) easily leads to subsurface damage (SSD). Many methods have been proposed to evaluate SSD but they unavoidably changed or even destroyed the ground BK7 surfaces, required expensive equipment and operator skills, or needed tedious preparation works. In this paper, an explicit relation between SSD and R z in the BK7 grinding process is established based on the analytical calculation of the classic brittle material crack system and the grinding kinematics. To validate the relation, the SSD values evaluated by the proposed relation are compared with the values obtained in both the experiments and other previous studies. The results indicate that the proposed relation can nondestructively and conveniently evaluate grinding-induced SSD, in situ and ex situ, by using a simple handheld profilometer. Furthermore, the proposed relation is transformed into another two forms, relating SSD with grinding process parameters (such as undeformed chip thickness, wheel and workpiece speed and wheel depth of cut). The two transformed relations indicate that the proposed relation can not only evaluate SSD after the grinding process but also optimize process parameters to control SSD during the grinding process. The study of this paper is expected to be meaningful in the SSD measurement and design, manufacture and application of optical glass BK7. © 2015 Elsevier B.V. All rights reserved. 1. Introduction As a type of high-quality optical materials, optical glass BK7 enjoys high linear optical transmission, good wear resistance and colorless appearance, and is often used as a standard of comparison for other glass materials (Bach and Neuroth, 1998). It accounts for more than 5% of the total industry production of glass in Europe (1.685 million tons) and brings about 7 trillion Euro output value in 2014 according to the Glass Alliance Europe (2015). Compared with the lapping process, the grinding process has been considered as a promising “rough” machining technology for BK7 manufacturing because of its high efficiency and low cost (Malkin and Hwang, 1996). However, the grinding process of BK7 with aggressive machining parameters (e.g., fast infeed rate, big depth of cut) easily leads to serious subsurface damage (SSD) due to the brittle and hard nature of BK7. Serious SSD is characterized by a layer below the ground surface containing appreciable micro- crack nucleation and extending of the order of 10–100 m into BK7 substances (Hed and Edwards, 1987; Malkin and Guo, 2008). This ∗ Corresponding author. E-mail addresses: lhnlwfb@163.com, Haonan.Li@nottingham.ac.uk (H.N. Li). SSD influences the service life, optical performances and mechani- cal properties of optical components (Camp et al., 1998). Thus, it is meaningful to evaluate grinding-induced SSD for optical glass BK7. The evaluation of SSD is tricky as SSD occurs beneath the ground surfaces (Li et al., 2015). Therefore, great efforts were made to this issue and the proposed methods can be divided into the destructive methods and the nondestructive methods. The destructive methods employed various additional process- ing operations to the ground specimens to expose SSD and make SSD visible and measurable. Esmaeilzare et al. (2014) proposed the angle polishing method and the experiments showed that this method was efficient for examining the distribution of micro- cracks on the ground surface. However, it was time-consuming for SSD measurements because the specimens need to be first angle cut and then carefully angle polished. Moreover, the measured SSD val- ues on the polished cross section need to be converted to obtain the realistic values according to geometrical relationships. Aida et al. (2014) explored SSD of GaN substrate by using cathodolumines- cence method in which electrons impacting on a material caused the emission of photons which had wavelengths in the visible spec- trum and were used to measure SSD depths. The predicted results showed that this method was effective and accurate but only appli- cable to luminescent materials. Instead of mechanical polishing, http://dx.doi.org/10.1016/j.jmatprotec.2015.11.003 0924-0136/© 2015 Elsevier B.V. All rights reserved.