Ultraprecision Machining Characteristics of Poly-Crystalline Germanium

Abstract

Germanium is an excellent infrared optical material. On most occasions, singlecrystalline germanium is used as optical lens substrate because its homogeneous structure is beneficial for fabricating uniform optical surfaces. In this work, we attempt to use poly crystals as lens substrates instead of single crystals, which may lead to a significant reduction in production cost. We conducted ultraprecision cutting experiments on poly-crystalline germanium to examine the microscopic machinability. The crystal orientations of specific crystal grains were characterized, and the machining characteristics of these crystal grains including surface textures, cutting forces, and grain boundary steps were investigated under various machining conditions. It was possible to produce uniformly ductile-cut surfaces cross all crystal grains by using an extremely small undeformed chip thickness (∼80 nm) under negative tool rake angles (∼ −45• ). This work indicates the possibility of fabricating high-quality infrared optical components from poly-crystalline germanium.

Microstructure Effects on Cutting Forces and Flow Stress in Ultra-Precision Machining of Polycrystalline Brittle Materials

“…From the above results, it is evi dent that the workpiece material in grain A is easier to deform and softer than that in grain B. The difference in the springback between the two grains can be attributed to the existence of a grain boundary step on the machined surface as suggested by Yan et al [9]. Although the grains are machined at a constant unde formed chip thickness, the actual surface height on the machined surface is different due to the springback and thereby creating grain boundary steps.…”

“…The critical grain boundary shear stress is given by Blackenhagen et al [21] rb = 0.057 G (9) The value of ^Hp is computed as 3.0129 MPa ml/2 for M = 3. The variation of grain size causes a variation in the normal flow stress according to the Hall-Petch relation.…”

“…Yan et al [9] conducted ductile cutting experiments using a sharp nose diamond tool on polycrystalline germanium. The cutting is performed on three grains and the surface finish and forces are monitored.…”

Studies of the ductile mode of cutting brittle materials (A review)

“…The authors of [64,75,76] also consider the chip critical thickness as a necessary parameter to achieve the ductile mode of machining semiconductors, and this value should be lower than the cutting edge radius. In some papers [59,[70][71][72][73][74][75][76][77][78][79][80] the critical chip thickness (after exceeding which the brittle fracture starts), is specifically indicated. According to [48], the maximum chip thickness should not be above 0.5 μm in turning silicon in the <110> direction of the crystal lattice.…”

“…The effects of the tool cutting edge geo metric parameters, cutting depth, and chip thickness on the machined surface quality were studied experi mentally in [26,76,84] for silicon, in [80] for germanium, and in [75] for CaF 2 . Figure 5 gives the scheme of the surface fracture mechanism of silicon in the ductile mode cutting [84].…”