Ch. Täschner

Deposition of hard crystalline Al2O3 coatings by bipolar pulsed d.c. PACVD

Abstract A bipolar pulsed d.c. PACVD technique was used to deposit crystalline Al2O3 coatings onto WC/Co cemented carbide cutting inserts and onto steel substrates using a gas mixture of AlCl3, H2, O2 and Ar. Different deposition conditions were tested, substrate temperatures (Ts) between 500 and 700°C and a gas mixture ratio O/AlCl3 in the range from 2 to 20. At Ts≥600°C transparent crystalline coatings of the γ-Al2O3 phase were obtained having the best crystallinity for O/Al ratio of 4 and 6. At a substrate temperature of 700°C and an O/AlCl3 ratio of 2, α-Al2O3 or a mixture of α- and γ-Al2O3 phases was formed. The microhardness HV[0.02] of the crystalline coatings was found to be in the range 19–23 GPa. The γ-Al2O3 layers appeared in most cases transparent with smooth surfaces. The wear properties of Al2O3-coated cutting inserts produced by the PACVD method were evaluated and compared with a thermal CVD κ-Al2O3 coating in a machining test in a ball bearing steel. From the test results it could be concluded that the PACVD Al2O3 coatings showed wear properties very similar to the thermal CVD κ-Al2O3 coatings.

Morphology controlled NH4V3O8 microcrystals by hydrothermal synthesis

Single crystalline ammonium trivanadate NH4V3O8 with variable morphologies, including shuttles, flowers, belts, and plates, was synthesized by the hydrothermal treatment of NH4VO3 with acetic acid. The crystals optimally grow under gentle conditions of 140 °C for 48 h. The resulting NH4V3O8 microcrystals were characterized by means of X-ray diffraction, scanning electron microscopy, infrared and Raman spectroscopy, static magnetization studies, and thermal analysis. The key factors to control the size and morphology of the crystals are the pH value and the vanadium concentration. A tentative microscopic growth mechanism is proposed and it is demonstrated how shape and morphology of the resulting microcrystalline material can be tuned by appropriate synthesis parameters.

Synthesis of aligned carbon nanotubes by DC plasma-enhanced hot filament CVD

Abstract Vertically aligned carbon nanotubes (CNTs) films have been prepared by a direct current plasma-enhanced hot filament chemical vapour deposition method. The growth of multi-walled carbon nanotubes on metal (Fe, Co, Ni) precoated silicon substrate are studied between 500 and 1000 °C using gas mixtures of methane, hydrogen and argon. The influence of substrate pre-treatment, substrate voltage and deposition temperature on the structure and the alignment of the CNTs are discussed. In dependence on thickness of the thin metal layers, thermal pre-treatment or hydrogen plasma etching and plasma parameters during the deposition process different carbon nanostructures, especially regarding tube diameter, morphology and arrangement could be proved. Transmission electron microscopy images of isolated tubes showed an imperfectly trained tubular structure of graphite shells, formed as ‘bamboo’-like shapes with partly crystalline metal droplets on the tip. Because the alignment of the tubes is induced by the discharge voltage and the electrical field, the characteristic plasma parameters as plasma potential, electron energy and electron density, measured by Langmuir probe investigations were discussed.

Deposition of hard crystalline Al2O3 coatings by pulsed d.c. PACVD

Abstract Two kinds of pulsed d.c. plasma assisted chemical vapour deposition (PACVD) processes (unipolar and bipolar excitation) were studied to deposit crystalline Al 2 O 3 coatings on steel or WC/Co cemented carbide inserts using aluminium trichloride (AlCl 3 ), nitrous oxide (N 2 O) or oxygen (O 2 ) and hydrogen/argon gas mixtures. At substrate temperatures as low as 650–700°C and a gas mixture ratio of 1 for N 2 O/AlCl 3 , coatings with different contents of α-Al 2 O 3 and γ-Al 2 O 3 besides amorphous alumina could be obtained under unipolar plasma excitation conditions. The ratio of the crystalline phases depends on plasma power density and substrate temperature, and has been determined qualitatively by X-ray diffraction. The dark grey or transparent coatings have a rough, less well crystallized surface in comparison to surfaces known from the thermal CVD process. The coating thickness differed significantly between the centre and the edges of the samples. The crystallite sizes were estimated to be 9 and 20 nm for the α-Al 2 O 3 and even smaller for γ-Al 2 O 3 containing coatings. By increasing the substrate temperature to 800°C, the content of α-Al 2 O 3 in the coatings was increased. Lower substrate temperatures or conditions with a deficiency of oxygen in the gas mixture lead to the formation of aluminium or aluminium nitride. Crystalline α-Al 2 O 3 or very smooth, transparent γ-Al 2 O 3 were obtained at 700°C dependent on the gas mixture ratio using O 2 /AlCl 3 as precursors and bipolar pulsed d.c. plasma excitation. A clear trend becomes apparent for the formation of γ-A 2 O 3 and amorphous alumina films if the substrate temperature is lowered and the O/AlCl 3 gas ratio is changed appropriately. The hardness of the smooth, transparent coatings are found to be in the range 20–24 GPa. The wear properties of some smooth, transparent γ-Al 2 O 3 PACVD coatings were evaluated in a machine cutting test using ball bearing steel and compared with thermal CVD alumina coatings. It could be concluded that the bipolar pulsed PACVD-Al 2 O 3 coatings showed very similar wear properties to thermal CVD coatings.

Plasma-assisted deposition of hard material layers from organometallic precursors

Abstract Using amido-substituted titanium or zirconium compounds as starting substances, the deposition temperature for hard material layers, e.g. Ti(C,N) and Zr(C,N), can be decreased below the temperature of the usual plasma-assisted chemical vapour deposition process. In the temperature range from 550 to 850 K “hard-material-like” coatings were obtained, the deposition rate of which amounted to 2–3 μm h -1 . The layers have been characterized regarding composition, morphology and microstructure. The C and N contents of the coatings are dependent on temperature, gas phase composition and plasma power. Transmission electron microscopy investigations of “TiN” layers revealed a small grain size in the initial range followed by a typical columnar growth. Fragments or crack products of the organometallic precursors inserted in the layers influence the hard material properties unfavourably. The experiments are compared with thermally activated deposition and discussed taking into consideration thermodynamical calculations for the Ti-N-C-H system (with regard to organometallic starting compounds).

Deposition of TiN, TiC and Ti1-xAlxN coatings by pulsed d.c. plasma enhanced chemical vapour deposition methods

Abstract Unipolar and bipolar pulsed d.c. plasma enhanced chemical vapour deposition techniques have been used under various process conditions to deposit hard crystalline (Ti,Al)N, TiC and TiN coatings on steel and WC/Co hard metal substrates at temperatures between 500 and 700°C. As process gases mixtures of titanium tetrachloride (TiCl4), aluminium trichloride (AlCl3) and nitrogen or methane/hydrogen/argon were applied. The golden, violet grey or dark grey coloured TiN, (TiAl)N, TiC coatings deposited by varying the substrate temperature, plasma power density, excitation mode, and titanium to aluminium ratio in the gas phase were investigated with respect to their composition and structure. At substrate temperatures up to 700°C and gas mixture ratios of Ti/Al=0.3–0.5 using the unipolar pulsed d.c. method cubic (Ti,Al)N coatings with different aluminium and titanium content could be deposited. At constant gas phase ratios and plasma parameters an increased substrate temperature resulted in an increased titanium content. For the bipolar d.c. excitation mode higher aluminium trichloride concentration had to be used in order to obtain the same composition as for the unipolar case. Besides the cubic (Ti,Al)N phase, hexagonal AlN was found in samples prepared at 700°C and 4 mbar by XRD measurements. The crystallite size of the (Ti,Al)N coatings deposited by unipolar and bipolar activation were estimated to be approximately 10 nm. The hardness HV[0.02] was found to be in the range of 25–30 GPa for Ti1−xAlxN, up to 32 GPa for TiN, and up to 40 GPa for TiC. Coating thickness and element composition were determined by glow discharge optical emission spectroscopy depth profile analysis.

High temperature ferromagnetism of Li-doped vanadium oxide nanotubes

The nature of a puzzling high-temperature ferromagnetism of doped mixed-valent vanadium oxide nanotubes reported earlier by Krusin-Elbaum et al., Nature, 431 (2004) 672, has been addressed by static magnetization, muon spin relaxation, nuclear magnetic and electron spin resonance spectroscopy techniques. A precise control of the charge doping was achieved by electrochemical Li intercalation. We find that it provides excess electrons, thereby increasing the number of interacting magnetic vanadium sites, and, at a certain doping level, yields a ferromagnetic-like response persisting up to room temperature. Thus we confirm the surprising previous results on the samples prepared by a completely different intercalation method. Moreover our spectroscopic data provide first ample evidence for the bulk nature of the effect. In particular, they enable a conclusion that the Li nucleates superparamagnetic nanosize spin clusters around the intercalation site which are responsible for the unusual high-temperature ferromagnetism of vanadium oxide nanotubes.

Structure and electronic properties of Li-doped vanadium oxide nanotubes

The influence of Li-doping on the mixed-valent vanadium oxide nanotubes has been investigated using electron energy loss spectroscopy. In particular, the electron diffraction profiles and the vanadium L excitation edges have been studied. We observe that the structure of the vanadium oxide nanotubes is stable against electron transfer upon Li-doping. Excitations at the vanadium L edges show features which are associated with a reduction of the vanadium valency.

Plasma enhanced deposition of titanium aluminium composite films using organometallic aluminium precursors

Abstract Thin films of aluminium composites with or without additional titanium have been deposited by plasma enhanced CVD (pulsed d.c. discharge) at a deposition temperature of 770 K. (500 °C) using various organometallic aluminium starting compounds. The composition and the structure of the layers are determined by gas phase composition and plasma power density. Results concerning microhardness, adherence and coating structure are reported. A1N, AlON, and (Ti,A1)(0,N) coatings could be successfully prepared under the described conditions, but we have failed in depositing crystalline aluminium oxide layers.

Deposition of hard material coatings using an organometallic precursor

Abstract The deposition of titanium or zirconium carbonitride films is possible using organometallic precursors as starting material. In the temperature range from 550 to 850 K in combination with a d.c. plasma-assisted chemical vapour deposition process, golden-bronze or grey coatings were obtained depending on the gas-phase composition. The layers have been characterized regarding composition, morphology and microstructure and show differences in some properties (microhardness and structure) from plasma-assisted samples, using TiCl 4 -N 2 -H 2 -Ar as starting compounds. The C, N and O contents respectively are dependent on temperature, gas phase composition and plasma power. Scanning electron microscopy and transmission electron microscopy investigations revealed a small grain size (5–8 nm) in the initial stage followed by a typical columnar growth. By secondary-electron mass spectrometry investigations, fragments or crack products of the organometallic starting materials can be detected and they influence the hard material properties unfavourably. Thermodynamic calculations for the Ti-N-C-H system with regard to organometallic starting material were made and discussed in comparison with experiments.