Miniature Cvd-diamond Coring Drills for Robotic Sample Collection and Analysis

Introduction: Coring tools have been used effectively on the Moon, but to date no such tools have been used on any other extraterrestrial surface. The lunar experience includes both manual (Apollo) and robotic (Luna) systems. These coring systems were concerned primarily with acquiring samples from depth for return to Earth or for the creation of instrument emplacement holes (e.g., heat probes). Current designs for planetary drills differ from the lunar tools primarily in that they are integrated with robotic instrumentation for remote analysis, but the role of the drilling or coring system remains one of acquiring samples that must be extracted from the core barrel for analysis. Missing from current sample collection systems is a tool that can double as both a coring device and a sample holder. This dual utility can minimize the number of motions, the mass, and the power required for several classes of instruments in planetary surface exploration. To be effective, such a system must be durable and simple in operation. Hollow CVD diamond drills possess the hardness, excellent cutting properties, and heat resistance required for drilling into a wide variety of rocks and minerals. Because CVD diamond is also unreactive and transparent to infrared radiation and to X-rays of moderate to high energy, it can be used as a sample holder in various instruments for X-ray diffraction (XRD), Xray fluorescence (XRF), infrared spectroscopy, Raman spectroscopy, and thermal analysis. The specific application explored in this study is that of XRD. A small robotic instrument for combined XRD/XRF analysis [1] has been developed and has proved effective in the determination of mineralogy from XRD data on small (<1 mg) powdered samples of rocks and minerals. One of the obstacles to space applications of this instrument, however, is the need to acquire and insert rock powders robotically. Powdered samples are necessary to obtain accurate mineral identifications and quantitative mineral abundances by XRD. The low X-ray absorption coefficient of diamond permits transmission of X-rays with energies typically used in XRD (e.g., Cu Kα at 8.04 keV) with little attenuation. One of the standard sample mounting systems for powder XRD is based on glass capillaries in which small amounts of sample powder are held and rotated to present a large number of random crystal orientations to the X-ray beam. The capillaries used are made of silica glass with typical wall thicknesses of 10 μm and internal bores of 100-500 μm. A miniature, hollow CVD diamond drill can be as effective as a sample holder, exploiting the lower X-ray absorption coefficient of diamond to compensate for its thicker walls and taking advantage of the absence of amorphous X-ray scattering in crystalline diamond. Hollow CVD diamond drills were produced by the Instituto Nacional de Pesquisas Espaciais (INPE), the national space research institute of Brazil [2]. These drills were produced by chemical-vapor deposition on Fe wires in a methane-plasma furnace. Two types of drills were produced, the first consisting of thick-wall (230 μm) CVD diamond grown over smooth wires of 300 μm diameter. After deposition around the wire forms, these drills could be simply pulled off of the wire for use. The second type was formed by growth of CVD diamond over 475-μm diameter Fe wires in which spiral grooves had been cut. This type of drill has a raised spiral along the inner wall that assists in drawing rock powder into the hollow drill as it turns. For this type of drill the Fe form is removed by acid dissolution. Wall thickness of the spiral drills was reduced to 80-150 μm to improve X-ray transmission.