The Relationship between Resistivity and Boron Doping Concentration of Single and Polycrystalline Diamond

Boron Doped Diamond for Real-Time Wireless Cutting Temperature Monitoring of Diamond Coated Carbide Tools

Among the unique opportunities and developments that are currently being triggered by the fourth industrial revolution, developments in cutting tools have been following the trend of an ever more holistic control of manufacturing processes. Sustainable manufacturing is at the forefront of tools development, encompassing environmental, economic, and technological goals. The integrated use of sensors, data processing, and smart algorithms for fast optimization or real time adjustment of cutting processes can lead to a significant impact on productivity and energy uptake, as well as less usage of cutting fluids. Diamond is the material of choice for machining of non-ferrous alloys, composites, and ultrahard materials. While the extreme hardness, thermal conductivity, and wear resistance of CVD diamond coatings are well-known, these also exhibit highly auspicious sensing properties through doping with boron and other elements. The present study focuses on the thermal response of boron-doped diamond (BDD) coatings. BDD coatings have been shown to have a negative temperature coefficient (NTC). Several approaches have been adopted for monitoring cutting temperature, including thin film thermocouples and infrared thermography. Although these are good solutions, they can be costly and become impractical for certain finishing cutting operations, tool geometries such as rotary tools, as well as during material removal in intricate spaces. In the scope of this study, diamond/WC-Co substrates were coated with BDD by hot filament chemical vapor deposition (HFCVD). Scanning electron microscopy, Raman spectroscopy, and the van der Pauw method were used for morphological, structural, and electrical characterization, respectively. The thermal response of the thin diamond thermistors was characterized in the temperature interval of 20-400 °C. Compared to state-of-the-art temperature monitoring solutions, this is a one-step approach that improves the wear properties and heat dissipation of carbide tools while providing real-time and in-situ temperature monitoring.

Raman imaging and Kelvin probe microscopy for the examination of the heterogeneity of doping in polycrystalline boron-doped diamond electrodes

The issue of the heterogeneity of boron doping in microcrystalline diamond films was addressed by four different methods: micro-Raman spectroscopy and Raman imaging, Kelvin probe force microscopy, conducting atomic force microscopy, and scanning electrochemical microscopy. The samples were commercially available films from Windsor Scientific, with an average boron concentration of about 5 x 10(20) cm(-3). In agreement with previous works, all of the methods showed that the boron uptake was nonuniform across the surface of the electrode. Two different types of regions were evidenced, with metallic or semiconducting properties that were characterized with different types of Raman spectra. The line shape of these spectra was strongly dependent on the excitation wavelength. Local variations in electroactivity were evidenced by the SECM curves, which are related to the electronic properties of the individual grains, which, in turn, are governed by the boron content of the individual crystallites. In this study, two different micro-Raman imaging techniques were used that reveal the grain structure of the films: the images constructed from the diamond line intensity perfectly reproduced the optical image obtained by illuminating the sample in reflection. The method also allows detection of the presence of nondiamond carbon, especially in the metallic parts of the samples. Other spectral features (intensity of the boron-related broad lines, as well as the frequency and width of the diamond line) were used to construct images. In every case, the grain structure of the film was revealed, as well as twinning within individual crystallites. All approaches revealed that no enhanced doping or boron depletion occurred at the grain boundaries.

Impact of Grain-Dependent Boron Uptake on the Electrochemical and Electrical Properties of Polycrystalline Boron Doped Diamond Electrodes

A combination of high-resolution electrical and electrochemical imaging techniques, in conjunction with cathodoluminescence (CL), is used to investigate the electrochemical behavior of oxygen-terminated highly doped polycrystalline boron doped diamond (BDD). The BDD has a dopant density approximately 5 x 10(20) atoms cm(-3), grain size ca. 5-40 microm, and thickness 500 microm. CL imaging demonstrates that boron uptake is nonuniform across the surface of BDD, and conducting atomic force microscopy (C-AFM) highlights how this impacts on the local conductivity. While C-AFM shows no evidence for enhanced grain boundary conductivity, two characteristic conductivity domains are found with resistances of ca. 100 kOmega and ca. 50 MOmega. With the use of scanning electrochemical microscopy (SECM), local heterogeneities are also observed in the electroactivity of the BDD surface, consistent with the two different types of conducting regions. Local currents of the magnitude expected for metal-like behavior are observed in some regions, suggesting degenerative doping of the grains (supported by CL studies). In other regions, slower electron transfer is apparent. However, even for the reduction of Ru(NH(3))(6)(3+), which occurs at potentials far negative of the flat-band potential for oxygen-terminated BDD, all areas of the surface show some electroactivity. This study highlights that the spatially heterogeneous conductivity and corresponding electroactivity of BDD are readily resolved using a combination of C-AFM, SECM, and CL.