Farzin Davanloo

Amorphic diamond films produced by a laser plasma source

Recently, attention has been focused upon laser plasma sources of thin‐film diamond. These depend upon laser‐ignited discharges in which intense pulsed currents flow through the small volume of carbon plasma ablated from graphite feedstock by a focused laser beam. The materials produced in this way generally resemble the hard amorphic films deposited by ion beams. This paper reports a detailed characterization of these films which we call amorphic diamond. The combination of an optical band gap of 1.0 eV with a grain size of 100–200 A places this material far outside the range of possibilities available to the model of graphitic islands. A structure of very fine grained diamond would more readily explain the hardness of 13 GPa determined in the absence of any measurable fraction of hydrogen. Such amorphic diamond films have been grown uniformly on 100‐cm2 areas at ambient room temperatures with no seeding or abrasion of the substrate.

Laser plasma source of amorphic diamond

Amorphic diamond films characterized by a high percentage of sp3 bonds have been prepared in an UHV environment with a laser plasma source of carbon ions. Peak power densities in excess of 1011 W/cm2 were found necessary to produce films at growth rates of 0.5 μm/h over areas of 20 cm2 having optical quality sufficient to show bright interference colors.

MICROSTRUCTURE OF AMORPHIC DIAMOND FILMS

It has been previously reported that layers of amorphic diamond can be grown in a UHV environment free from hydrogen with a laser plasma source. Some advantages are offered by this technique which produces films that adhere more readily to materials for which there are important applications. Theory has recently suggested a structure for amorphic diamond that comprises nodules of carbon atoms linked by sp3 bonds in a matrix of other polytypes and the purpose of this article is to communicate strong evidence in support of that hypothesis. Extensive examinations of a variety of films with a scanning tunneling microscope show a clearly prevalent structure composed of dense nodules. Grain size is about 1000 A and the diamond character is attested by the agreement of morphology, high density, optical properties, soft x‐ray spectroscopy, hardness, and lack of appreciable hydrogen. Measurements agree in supporting a fraction of about 75% diamond contents. The principal conclusion is that this material prepared w...

Adhesion and mechanical properties of amorphic diamond films prepared by a laser plasma discharge source

Amorphic diamond films can be grown in an ultrahigh vacuum environment free from hydrogen with a laser plasma discharge source. This technique produces films that adhere more readily to materials for which there are important applications as protective coatings. In this work adhesion and mechanical properties of amorphic diamond films have been examined. A beam bending method has been used to measure the internal stress and a relatively low value of compressive stress was found. The dependence of stress on the laser intensities at the graphite ablation target has been studied. Analyses of these films on silicon, SiO2, ZnS, and TiAl6V4 by Rutherford backscattering spectrometry show significant interfacial layers with compositions of SiC, C0.5SiO2, C2.5ZnS, and C0.62Ti0.35Al0.05V0.02, respectively. Adhesion properties on ZnS and other substrates have also been examined for harsh environments. The mechanical properties of hardness, Young’s modulus, and stiffness have been obtained with a nanoindentation tech...

The bonding of protective films of amorphic diamond to titanium

Films of amorphic diamond can be deposited from laser plasma ions without the use of catalysts such as hydrogen or fluorine. Prepared without columnar patterns of growth, the layers of this material have been reported to have ‘‘bulk’’ values of mechanical properties that have suggested their usage as protective coatings for metals. Described here is a study of the bonding and properties realized in one such example, the deposition of amorphic diamond on titanium. Measurements with Rutherford backscattering spectrometry and transmission electron microscopy showed that the diamond coatings deposited from laser plasmas were chemically bonded to Ti substrates in 100–200‐A‐thick interfacial layers containing some crystalline precipitates of TiC. Resistance to wear was estimated with a modified sand blaster and in all cases the coating was worn away without any rupture or deterioration of the bonding layer. Such wear was greatly reduced and lifetimes of the coated samples were increased by a factor of better th...

High-power, repetitive-stacked Blumlein pulsers commutated by a single switching element

The stacked Blumlein pulse generators developed at the University of Texas at Dallas consist of several triaxial Blumleins stacked in series at one end. The lines are charged in parallel and synchronously commutated with a single switching element at the other end, In this may, relatively low charging voltages are multiplied to give a higher desired voltage across an arbitrary load. Extensive characterization of the stacked Blumlein pulsers indicates that these devices are capable of producing high-power pulses with rise times and repetition rates in the range of 0.3-50 ns and 1-300 Hz, respectively, using a conventional thyratron, spark gap, or photoconductive switch. This paper presents the progress in the development and use of these novel pulsers. Recent adaptation of the design has enabled the stacked Blumlein to produce 50-70 MW nanosecond pulses with risetimes on the order of 200-300 ps into nominally matched loads. The device has a compact line geometry and is commutated by a single photoconductive switch triggered by a low power laser diode.

Microstructural analyses of amorphic diamond, i‐C, and amorphous carbon

Recent experiments have identified the microstructure of amorphic diamond with a model of packed nodules of amorphous diamond expected theoretically. However, this success has left in doubt the relationship of amorphic diamond to other noncrystalline forms of carbon. This work reports the comparative examinations of the microstructures of samples of amorphic diamond, i‐C, and amorphous carbon. Four distinct morphologies were found that correlated closely with the energy densities used in preparing the different materials.

Fluorescence of high-pressure argon excited by an energetic flash X-ray source

The fluorescence from high-pressure argon plasmas (1-30 bar) has been excited with an intense flash X-ray device. For the first time a spectrum from 110 to 700 nm has been recorded under such conditions. The spectra obtained in the UV-VUV were very similar to those recorded with alpha -particle excitation at equivalent pressures. The dominant features were the so-called second and third continua of argon. The visible fluorescence was found to be negligible in comparison with the VUV continua.

Protective films of nanophase diamond deposited directly on zinc sulfide infrared optics

Nanophase diamond films can be grown at room temperature with a laser plasma discharge source without the use of any catalyst. This technique produces films that adhere readily to materials for which there are important applications as protective coatings. Described here is a study of the bonding and properties realized with the direct deposition of nanophase diamond on the II-VI compound of zinc sulfide. It was shown that adhesion and mechanical properties of the films can be correlated with the amounts of defects and impurities in the zinc sulfide substrates. In all cases significant interfacial layers caused by the deep penetration of carbon atoms into the substrates were observed. Resistance to wear was estimated with a modified sand blaster, and results indicated that only 1 mm coating of nanophase diamond can increase lifetimes of the zinc sulfide samples by a factor better than 5. Protection afforded by the nanophase diamond under harsh environmental conditions of rain impacts was also described.

Mass flow in laser-plasma deposition of carbon under oblique angles of incidence

The angular distribution of the mass flow in carbon laser plasmas, generated from graphite targets at laser power densities around 1011 W/cm2 and 1064 nm, was studied. Under oblique angles of incidence the mass flow is not perpendicular to the target surface but rather symmetrical around the bisecting angle between the laser beam and the surface normal. For all angles, however, a cos4ϑ-pattern is observed. Compared to normal incidence the mass flow is weaker by about a factor of 2 to 3 for 30° and 50° angle of incidence. The dependence of film quality on deposition angle with regard to the symmetry axis of the plume is demonstrated.