T. J. Lee

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.

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...

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.

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.

Preparation an study of laser plasma diamond

Abstract Films of diamond-like material can be deposited with a laser plasma source of carbon ions in an ultrahigh vacuum environment without involving hydrogen in the growth mechanism. These films are distinguished by transparency at visible wavelengths which is a result of a high percentage of sp3 bonds. They resemble materials first quenched from ion beams at very slow deposition rates. In our method an Nd:YAG laser was focused on a graphite feedstock in an ultrahigh vacuum chamber at intensities in excess of 5 × 1011 W cm-2. A high current discharge confined to the path of the laser-ignited plasma provided further heat and aided processing of the ion flux. At a laser repetition rate of 10 Hz, a deposition rate of 0.5 μm h-1 over a 100 cm2 area was attainable with no measurable substrate heating. The substrates required no special preparation or seeding and materials including silicon, fused silica, glass, gold, copper, germanium, InP, ZnS, and polycarbonate and polyimide plastics were readily coated. Complex shapes could be accommodated and spheres of 440C stainless steel were covered successfully. Over 1000 samples were prepared to a variety of specifications with thickness reaching 5 μm and hardness exceeding 37 GPa.

Mechanical and adhesion properties of amorphic diamond films

Abstract Films of amorphic diamond can be deposited with a laser plasma source of carbon ions in an ultrahigh vacuum environment without involving hydrogen in the growth mechanism. This technique produces films that adhere more readily to materials for which there are important applications such as protective coatings. In this study mechanical properties of amorphic diamond films were examined. The hardness and Youngs modulus were obtained using a nanoindentation technique. Analyses of these films on silicon and ZnS by Rutherford backscattering spectrometry (RBS) show significant interfacial layers. The adhesion properties were also studied under harsh environmental conditions.

Progress in the characterization of nanophase diamond films uniquely produced by a laser plasma discharge source

Abstract Films of nanophase diamond can be deposited at room temperature with a laser plasma discharge source of multiply charged carbon ions, without the use of any catalyst in the growth mechanism. The beam from a pulsed Nd:YAG laser is focused on graphite at intensities in excess of 1011W cm-2 and the resulting plasma ejects carbon ions carrying energies of about 1 keV through a discharge space to the substrates to be coated. The high energies of condesation produce interfacial layers between the film and substrate materials, which provide levels of adhesion which can support the protection of substrates subjected to harsh environmental conditions. In this paper, recent advances in the characterization of nanophase diamond films are given. Emphasis has been placed on studies of the bonding and properties realized in one example; i.e. the deposition of nanophase diamond on stainless steel. Measurements with Rutherford backscattering spectrometry showed that the diamond coatings deposited from laser plasmas were bonded to the stainless steel substrates through interfacial layers with significant thickness. The resistance to wear was estimated with a modified sandblaster and it was shown that a coating of only 2 μm of nanophase diamond can increase the lifetime of the sample by a factor of better than 36. The results of other mechanical measurements, such as those obtained by friction tests, are also given.