Robert S. Polvani

Chemical Aspects of Tool Wear in Single Point Diamond Turning

Abstract A hypothesis is proposed that ascribes chemical wear of diamond tools to the presence of unpaired d electrons in the sample being machined. This hypothesis is used to explain a range of results for metals, alloys, and other materials including “electroless” nickel. The hypothesis is further tested by experiments presented here on the machining of a range of high purity elements. The implications for diamond turnability of other materials are discussed.

Detection of subsurface damage: studies in sapphire

A variety of nondestructive characterization techniques has been used to detect and measure subsurface damage in single- crystal sapphire to develop methods suitable to inspect high performance optics for sub-surface damage. These techniques include polarized light microscopy, x-ray diffraction topography, transmission electron microscopy (TEM) and Raman spectroscopy. TEM examination shows that for ground surfaces damage can extend up to 6 - 7 micrometers into the bulk and includes cracks, twins and dislocations, while under polished surfaces only dislocations are seen. X-ray diffraction topography can image defects such as long-range strain, dislocations, residual surface scratches (not visible optically) and low-angle grain boundaries (lineage). Polarized light is also sensitive to strain and provides a relatively easy method for detecting defects such as cracks and lineage. Of all of the techniques Raman spectroscopy offers the best potential for quantifying strain in terms of stress.

Form-Profiling of Optics Using the Geometry Measuring Machine and the M-48 CMM at NIST

We are developing an instrument, the Geometry Measuring Machine (GEMM), to measure the profile errors of aspheric and free form optical surfaces, with measurement uncertainties near 1 nm. Using GEMM, an optical profile is reconstructed from local curvatures of a surface, which are measured at points on the optic’s surface. We will describe a prototype version of GEMM, its repeatability with time, a measurements registry practice, and the calibration practice needed to make nanometer resolution comparisons with other instruments. Over three months, the repeatability of GEMM is 3 nm rms, and is based on the constancy of the measured profile of an elliptical mirror with a radius of curvature of about 83 m. As a demonstration of GEMM’s capabilities for curvature measurement, profiles of that same mirror were measured with GEMM and the NIST Moore M-48 coordinate measuring machine. Although the methods are far different, two reconstructed profiles differ by 22 nm peak-to-valley, or 6 nm rms. This comparability clearly demonstrates that with appropriate calibration, our prototype of the GEMM can measure complex-shaped optics.

X-ray Topography Study of Surface Damage in Single-Crystal Sapphire

X-ray diffraction topography was used to investigate the relationship between subsurface damage, near-surface microstructure, and fracture strength in a series of sapphire modulus of rupture (MOR) bars which had been fabricated to proof test fabrication processes. The strength of the bars was determined by failure in four point bending. The tensile surface of the bars was also examined using optical microscopy and non-contacting surface profilometry. Both show that the bars have good surface finish, with a typical RMS roughness of 0.7 nm. No correlation was found between RMS surface finish and fracture strength. Although the bars appeared to be indistinguishable, topographs taken prior to fracture testing revealed that they are of two distinct types. Type 1 has an oriented microstructure consisting of a pattern of linear features running the length of the bars. Type 2 was typical of well-polished sapphire, containing individual dislocations and occasional damage from handling. We attribute the Type 1 microstructure to fabrication damage that was not removed by subsequent processing and/or polishing. Fracture strength data showed that the Type I (damaged) bars had strengths less than 70 % of the bars without damage. Topography is sensitive to near-surface damage that can be correlated to fracture strength. Neither low magnification optical microscopy nor conventional surface finish statistics could be correlated to strength.

Sapphire statistical characterization and risk reduction program

The Sapphire Statistical Characterization and Risk Reduction Program tested 1400 4-point flexure bars with different crystal orientations at different temperatures to establish a mechanical strength database for engineering design. Sapphire coupons were selected to represent surfaces on two different missile windows and a missile dome. Sapphire was obtained from the same suppliers used for the windows or dome and, as much as possible, coupons were fabricated in the same manner as the corresponding part of the window or dome. For one missile window, sapphire from one fabricator was 50% stronger than sapphire made to the same specifications from the same blanks by another fabricator. In laser thermal shock tests, sapphire performed better than predicted from flexure tests. Of several nondestructive methods evaluated for their ability to identify mechanically weak specimens, only x-ray topography was correlated with strength for a limited set of specimens.

Metrology for micro-optics at NIST with μXCALIBIR

Microlenses play a key role in photonic devices for the applications such as communications, medical imaging, wavefront detection, machine vision and others. Microlenses can be either refractive or diffractive with apertures ranging from few tens of µm to about a mm. Lenses can be made as discrete lenses but for may applications lens arrays are fabricated. Figure metrology for micro-lenses is likely to pose unique challenges. When compared to “macro-” optical elements, the aperture of microlenses is no longer very much larger than the wavelength of source light. Diffraction effects can thus no longer be neglected. For example, the focus shift may lead to a substantial systematic error in a radius-of-curvature measurement of a microlens.

Progress with the XCALIBIR interferometer at NIST

In response to the increasing need for the characterization of precision optics, the National Institute of Standards and Technology has initiated a program for the development of SI traceable surface and wavefront metrology. A key instrument in this program is the “eXtremely accurate CALIBration InterferometeR” (XCALIBIR). XCALIBIR is a multi-configuration phase-measuring precision interferometer for the measurement of flat, spherical, and aspheric surfaces. We describe the interferometer and the calibration of the 300 mm aperture Fizeau configuration and the spherical Fizeau configuration. An uncertainty of 0.2 nm rms was achieved in a 3-flat calibration of three 300 mm diameter optical flats. Additional recent measurement results will be presented that illustrate the capabilities of the XCALIBIR interferometer.

Some Observations on Tool Wear in Single Point Diamond Turning of Stainless Steel

Single point diamond turning (SPDT) is a cost effective technique for production of certain types of optical, opto-electronic, and mechanical elements. SPDT is often considered a “new” technology, even though single crystal natural diamonds have been used by engravers for millennia and as lathe tools for centuries1. The technology which, today, is commonly referred to as single point diamond turning (SPDT) has roots in parallel developments in Europe and the USA starting in the 1940s.

Characterization of Subsurface Damage in Glass and Metal Optics

Even unusual care in polishing leads to optics with somewhat damaged working faces. The question is to what degree? the physical nature of this damage depeids on the ductility or plasticity of the constniction material. Glassy materials (glass brittle metals and semiconductors) fracture while ductile materials work harden. Beryffium suffers from both modes. To physically model damage we oversimplify and picture a surface layer with two features a depth and an intensity of the IvjIk property changes. A statistical alternative to intensity is the visible flaw density. We use a variety of analytical mathods but mainly x-ray diffraction metallography and instrumented microindentation and frequently use them in combination. Our problem is to develop a general hasis and means of characterizing subsurface damage. To illustrate the diversity difficulty and our progress with the problem we will report experiences with a variety of examples. The talk will consider damage in ductile regimeN ground classically polished fused silica BK-7 pyrex optics esuperu polished BK-i precision ground silicon nitride and aluminaceramics and precision CBN turned beryllium.