W. Drawl

Silicon carbonitride, a new hard material and its relation to the confusion about ‘harder than diamond’ C3N4

Abstract The claim by Cohen (Science 261 (1993) 307) that powerful computational tools allow us to predict ‘properties of substances even before we have created them’ was made in conjunction with the claim of special properties for a hypothetical phase, C 3 N 4 . Among such properties was hardness, and it was asserted that, the covalent form of C 3 N 4 could be ‘harder than diamond.’ This assumption contradicted what chemists have known since 1816 in their experimentation with carbon nitrides. Never was there a single hint of the existence of a covalent, single bond C–N network. In the last decade some 400 papers have been written about this non-existent material of dubious significance (R.C. DeVries, Mater. Res. Innovat. 1 (1997) 161). No C 3 N 4 material has ever been made and the claims on both the chemical composition and crystal structure are clearly in error. The impact of such exaggerated claims on the scientific enterprise cannot be ignored. In contrast, we report herein on a related but real hard material, silicon carbonitride, with the α -Si 3 N 4 crystal structure modified by the introduction of carbon atoms. Synthesis of this Si–N–C crystalline material was possible by using a CH 4 /H 2 /N 2 microwave plasma etching of solid Si. Films on Si, SiC, Si 3 N 4 and diamond, as well as crystal agglomerates of a few mm 3 volume, have been prepared. This phase possesses a micro-hardness lower than cubic boron nitride and a band gap of 3.8 eV. The present experiments indicate that only 6 at.% of C have been incorporated into α -Si 3 N 4 . We might suggest that ‘first principles’ calculations be undertaken to explain the limited solubility of carbon in the α -Si 3 N 4 phase.

Interlayers for diamond-coated cutting tools

Abstract Boron-containing chemical vapor deposition (CVD) films are being explored for use as interlayers between WC-Co substrates and CVD diamond coatings to enhance the adherence between these materials. The wear resistance and hardness of diamond coatings on tough WC-Co substrates gives an ideal coated tool material for many machining applications. CVD diamond applied directly onto WC-Co exhibits very poor adherence. Factors contributing to this problem include the solubility and diffusivity of carbon in cobalt, resulting in limited diamond nucleation and precipitation of graphite, and the large thermal expansion mismatch between WC-Co and diamond. Boron-containing films act as diffusion barriers to carbon diffusion into the cobalt and enhance chemical bonding at the coating-substrate interface. Compositional grading of the interlayer maximizes its effectiveness as an aid to adherence. The experimental results leading to these observations are discussed in this paper.

In‐process ellipsometric monitoring of diamond film growth by microwave plasma enhanced chemical vapor deposition

In‐process monitoring of diamond film growth was performed with near‐infrared ellipsometry (1550 nm). The trajectories in the ellipsometric parameters (ψ,Δ) differ according to the method of substrate pretreatment and the CO/H2 gas ratio used in the microwave plasma‐enhanced chemical vapor deposition process. The nucleation density determined from ellipsometry shows qualitative agreement with that from scanning electron microscopy performed after deposition. The rate at which nuclei develop is also monitored, and the observed induction time is shorter for conditions leading to a higher nucleation density.

The effect of carbon ion implantation on the nucleation of diamond on Ti-6Al-4V alloy

Abstract The heterogeneous nucleation of diamond particles during the early stages of diamond film formation on non-diamond substrates is not well understood. In this work, we used ion implantation as a pretreatment process, to control the nucleation of diamond particles. The well known surgical alloy Ti-6Al-4V (“mirror” polished) was used as the substrate. Carbon ions at 30 keV energy were implanted at room temperature into masked regions on the samples, up to doses of 1×1016-7×117 ions cm-2. At the high doses, carbon concentrations up to 80 at.% were generated as shown by Rutherford backscattering spectrometry (RBS). After ion implantation, diamond depositions were conducted at MRL utilizing a Toshiba microwave chemical vapour deposition (CVD) system. Operating conditions were 12 kPa total system pressure, 1% methane in hydrogen at a total flow of 100 standard cm3 min-1, substrate temperature of 1000 °C, deposition time 5 h. The deposited films were studied by scanning electron microscopy, microfocus Raman scattering and RBS. The results indicate (a) a reduction in nucleation density up to 8 times with increasing ion dose, (b) near perfect diamond particles (Raman scattering) and (c) large internal stresses leading to partial flaking when the film becomes continuous. The role of a possible “carbided layer” formed by ion implantation is discussed.

Temperature Dependence of Nucleation Density of Chemical Vapor Deposition Diamond

The temperature dependence of the nucleation density of Chemical Vapor Deposition diamond was carefully investigated by ellipsometric monitoring. Substrates were pre-treated by rubbing with diamond powder and then by wiping off the residual powder until visually clean. Nucleation density was greater than 1010/cm2 in some ranges of temperature. It was also observed that nucleation density increased suddenly to 860°C and then gradually decreased in the range of higher temperatures. The results were explained by a change in the adsorption state of the precursor, from a physical to a chemical.

Electron emission from disordered tetrahedral carbon

Electron field-emission tests have been performed on films grown by a modified microwave plasma assisted chemical vapor deposition diamond process. This modification includes the addition of N2 and O2 during the growth stage. Characterization of these films shows the presence of a disordered tetrahedral carbon structure. Raman spectroscopy indicates a disturbance in the cubic symmetry of the lattice and x-ray diffraction indicates a disordered tetrahedral structure. Field-emission testing indicate that current densities of 0.5 mA/cm2 can be obtained for applied fields of 5–8 V/μm. The results are explained in terms of a change in the band structure and the formation of electronic states in the band gap.

Precipitation of diamond from metallic liquids below 1 atm

We present evidence of the unique role of metallic liquids of composition MexCyHz (Me = Au, Ag, Cu, etc.) made by reacting mixtures of a metal and carbon in a microwave-stimulated H plasma at pressures near 100 Torr in diamond synthesis. Typical cubo-octahedral diamond crystals can be seen precipitating from such liquids in the temperature range 800–950 °C.

Field decrystallization and structural modifications of highly doped silicon in a 2.45-GHz microwave single-mode cavity

Highly doped n -type silicon powder responds aggressively to a 2.45-GHz microwave E-field, whereas it remains unperturbed in the H-field. In the E-field, after about 30 s of treatment, the silicon powder attained submelting temperatures and thus coagulated to a bulk solid piece. X-ray diffraction analysis of the surface and the cross section of this solid material failed to show any detectable peaks, ascertaining the fact that the material had decrystallized. The Raman spectra of the material had broad and shallow peaks quite different from the thin, sharp lines exhibited by Si wafer. It appears that the E-field treatment has considerably distorted the lattice structure creating lattice strains throughout the sample. These lattice strains were relieved by grinding (recrystallized).

Characterization and electron field emission from diamond coatings deposited by multiple laser process

Abstract A new diamond deposition process utilizing a plasma and a variety of interactions from a multiple laser system has been demonstrated, with WC/Co substrates. The process is conducted in open air and does not involve hydrogen. Structural characterization of the diamond coatings, which have exceptional adhesion to cutting tool inserts, indicates a cubic diamond structure. Tungsten and cobalt atoms are incorporated into the film and a layer depleted in cobalt exists at the diamond-WC/Co interface. Electron field emission current densities, useful for flat panel displays of 6 mA/cm2 at an applied voltage of 3000 V for a film-anode distance of 20 μm has been measured.

Infrared measurements of CVD diamond films

The growth surfaces of CVD diamond films are usually rough with polycrystalline crystallographic habits which presents a severe problem if CVD diamond films are to be used in infrared optics. Several methods are described in this paper in an effort to solve this problem. A polishing process was used to reduce the surface roughness by polishing the rough growth surface with a heated cast iron scaife. For polished films, near 70% transmittance was obtained over the whole range of 600-4000 cm-1, while the transmittance for non-polished films were much lower and varied strongly with the wavenumber. Absorptions believed due to carbon-hydrogen stretching bands and a silicon carbide phase were observed in the transmission spectra of polished diamond films.