H.-J. Scheibe

Interlayers for diamond deposition on tool materials

The direct deposition of diamond on such tool materials as hard metals and steels is difficult because graphitization occurs and adhesion is poor. The following hard coatings have been investigated concerning their suitability as interlayers for diamond growth: TiN, TiC, Si3N4, SiC, SiCxNy, (Ti, Si)Nx and pulsed arc deposited a-C (laser-arc). The diamond deposition was performed by hot-filament CVD. A sufficient diamond nucleation density was only reached by ultrasonic pretreatment with diamond powder. The nucleation density further depends on interlayer materials and substrate temperature. The determined nucleation densities were 105–108 cm−2 for the titanium- and silicon-containing interlayers. The ultrasonic-pretreated a-C layers had a nucleation density of about 1010 cm−2 compared with 4–6 × 107 cm−2 for untreated samples. No dependence of the nucleation density on a-C layer thickness was found. Raman spectroscopic results show that diamond films with low non-diamond quantity grew on interlayers of TiC, SiC and SiCxNy. In addition stress and adhesion were investigated. The poorest adhesive strength resulted for diamond on TiN and a-C. Silicon-containing interlayers such as Si3N4 and SiC showed good adhesion with critical loads up to 22 N measured by scratch test.

Deposition of superhard amorphous carbon films by pulsed vacuum arc deposition

The deposition of superhard amorphous carbon films demands high energies of all impinging particles for film formation by subplantation instead of condensation. Such conditions may be realized by vacuum arc discharge. By using pulsed arc currents and laser controlled ignition (Laser-Arc) the usual problems in arc ablation of carbon targets have been overcome. In this way, smooth and very hard films with hardness in the superhard range between 40 and 80 GPa have been prepared with high deposition rates comparable to conventional industrial arc deposition. Due to the high ion energies, low deposition temperatures are possible without reducing the adherence. They are even necessary for the formation of the diamond bonds by avoiding relaxation towards a graphitic structure. Hence, materials besides metals such as steels or aluminum, temperature-sensitive materials such as polymers have been successfully coated with these hard layers.

Non-destructive characterization of mechanical and structural properties of amorphous diamond-like carbon films

Abstract Amorphous diamond-like carbon (DLC) films show promising properties for wear protection applications. Pulsed laser deposition (PLD), laser-induced pulsed vacuum arc deposition (laser-arc) and mass-selected ion beam deposition (MSIBD) are techniques which enable deposition of DLC films with the desired properties by providing sufficiently high carbon energy. Two non-destructive methods were used to determine the properties of films deposited by these three deposition techniques: (1) ellipsometry, which gives information about the micro-structure through the optical properties; and (2) laser-acoustic analysis enabling the measurement of the Youngs modulus of thin films down to less than 100 nm film thickness. These methods were used to investigate the effect of the carbon ion energy and the substrate temperature during deposition on the film quality. A clear correlation between the Youngs modulus and the optical parameters was found. All three deposition techniques are characterized by a critical substrate temperature above which sp 2 rich films are deposited. The PLD and MSIBD systems have a higher ion energy and a lower deposition rate than the laser-arc system. This is the reason for the higher critical substrate temperature below which sp 3 rich films are formed for PLD and MSIBD compared with laser-arc.

Optical and photoemission studies of DLC films prepared with a systematic variation of the sp3:sp2 composition

Abstract The optical and photoemission properties of hydrogen-free DLC films prepared under conditions which result in different types of carbon bonding (previously determined from electron energy loss spectroscopy) are reported. The films were prepared using a mass selected ion beam deposition system covering the C+ energy range 10 eV–2 keV, mostly on Si held at room temperature. Previous measurements have shown that the sp3 content of these films varied from 0 to >80%. The optical constants (n, k, absorption coefficient (α)) of these films were measured by ellipsometry. The energy gaps were derived from Tauc plots (Eg) and from α = 104 cm−1 values (E04). The energy gaps were found to vary with the sp3 content from E

A laser-acoustic method for testing and classifying hard surface layers

Abstract The laser-acoustic method is accepted to be an interesting method of testing thin films. It is based on measuring the dispersion of surface acoustic waves which are generated by short laser pulses. A reliable test equipment was developed that allows a user-friendly operation. The method is non-destructive, the test takes little time and special sample preparation is not required. It is mainly applied to measure the Young’s modulus of thin films with thickness down to less than 50 nm. Recent studies showed these results to correlate with important microstructural and mechanical properties of hard and superhard films. The laser-acoustic technique was improved to test multilayer films consisting of two components. The approach of an effective medium of transversal symmetry is used to describe the elastic behavior of multilayer films. It enables the elastic anisotropy of the multilayer film to be evaluated. Applications are presented, performed at multilayers of diamond-like carbon and aluminum deposited by laser-arc on steel and silicon. The films consisted of four and twenty single layers, respectively. The Young’s modulus of the diamond-like carbon in the multilayer was determined with the laser-acoustic technique. The results reveal the reproducibility of the deposition technique and demonstrate the potential of the laser-acoustic technique to test multilayer films. The laser-acoustic method is shown to be sensitive to machining layers. The effect of grinding and polishing steel surfaces was studied. Studies were performed to compare the results of the laser-acoustic technique with those of membrane deflection and micro-indentation. TiN, CrN and TiCN films (thickness: 0.8–2.3 μm) were tested with laser-acoustics and micro-indentation, polysilicon films (thickness: 0.46 μm) with laser-acoustics and the membrane deflection technique.

Biocompatibility and mechanical properties of diamond-like coatings on cobalt-chromium-molybdenum steel and titanium-aluminum-vanadium biomedical alloys

Diamond-like carbon (DLC) films are favored for wear components because of diamond-like hardness, low friction, low wear, and high corrosion resistance (Schultz et al., Mat-wiss u Werkstofftech 2004;35:924-928; Lappalainen et al., J Biomed Mater Res B Appl Biomater 2003;66B:410-413; Tiainen, Diam Relat Mater 2001;10:153-160). Several studies have demonstrated their inertness, nontoxicity, and the biocompatibility, which has led to interest among manufacturers of surgical implants (Allen et al., J Biomed Mater Res B Appl Biomater 2001;58:319-328; Uzumaki et al., Diam Relat Mater 2006;15:982-988; Hauert, Diam Relat Mater 2003;12:583-589; Grill, Diam Relat Mater 2003;12:166-170). In this study, hydrogen-free amorphous, tetrahedrally bonded DLC films (ta-C) were deposited at low temperatures by physical vapor deposition on medical grade Co28Cr6Mo steel and the titanium alloy Ti6Al4V (Scheibe et al., Surf Coat Tech 1996;85:209-214). The mechanical performance of the ta-C was characterized by measuring its surface roughness, contact angle, adhesion, and wear behavior, whereas the biocompatibility was assessed by osteoblast (OB) attachment and cell viability via Live/Dead assay. There was no statistical difference found in the wettability as measured by contact angle measurements for the ta-C coated and the uncoated samples of either Co28Cr6Mo or Ti6Al4V. Rockwell C indentation and dynamic scratch testing on 2-10 μm thick ta-C films on Co28Cr6Mo substrates showed excellent adhesion with HF1 grade and up to 48 N for the critical load L(C2) during scratch testing. The ta-C coating reduced the wear from 3.5 × 10(-5) mm(3)/Nm for an uncoated control sample (uncoated Co28Cr6Mo against uncoated stainless steel) to 1.1 × 10(-7) mm(3)/Nm (coated Co28Cr6Mo against uncoated stainless steel) in reciprocating pin-on-disk testing. The lowest wear factor of 3.9 × 10(-10) mm(3)/Nm was measured using a ta-C coated steel ball running against a ta-C coated and polished Co28Cr6Mo disk. Students t-test found that the ta-C coating had no statistically significant (p < 0.05) effect on OB attachment, when compared with the uncoated control samples. There was no significant difference (p < 0.05) in the Live/Dead assay results in cell death between the ta-C coated Co28Cr6Mo and Ti6Al4V samples and the uncoated controls. Therefore, these ta-C coatings show improved wear and corrosion (Dorner-Reisel et al., Diam Relat Mater 2003;11:823-827; Affato et al., J Biomed Mater Res B Appl Biomater 2000;53:221-226; Dorner-Reisel et al., Surf Coat Tech 2004;177-178:830-837; Kim et al., Diam Relat Mater 2004;14:35-41) performance and excellent in vitro cyto-compatibility, when compared with currently used uncoated Co28Cr6Mo and Ti6Al4V implant materials.

A model for particle growth in arc deposited armophous carbon films

Abstract Arc evaporation processes are connected with the emission of small particles which are incorporated in the growing film on the substrate. SEM, AFM and optical microscopy indicate a size increase of such objects during film growth. Because of their cone-like shape we call them cone-like nodules. Using conventional measurement methods that can determine the structure, it is hard to explain the behaviour and properties of those nodules, as they do not provide clear information about their nature. Square resolved Raman measurements show a mixture of graphitic and diamond-like structures, that allows to conclude that the nodule consists of a graphitic particle originated at the cathode, covered by an amorphous carbon film. We propose a new model that describes the growth of cone-like nodules in the homogeneous carbon film very well. It is based on the dependence of the DLC film properties on the incidence angle of ions. The cone-like shape and the growth behaviour at perpendicular and grazed ion incidence was simulated using a simple computer program. A comparison of the simulated data and the SEM-images show a good qualitative and quantitative agreement.

DLC and metallic nanometer multilayers deposited by laser-arc

Abstract The method of laser-induced vacuum arc (laser-arc) combines the good controllability of pulsed laser deposition with the high efficiency of a vacuum arc technique. One advantage of this technique is the essential reduction of droplets allowing the deposition of high-quality amorphous carbon films. These hydrogen-free films with very high hardness up to the superhard range exhibit excellent wear resistance and low friction. In the present paper, another advantage of the laser-arc is demonstrated, i.e. the possibility of depositing multilayer coatings down to the nanometer level of each individual layer thickness with high efficiency and high accuracy. These possibilities open new ways to overcome the principal problem of hard PVD coatings, i.e. the high internal stress which restricts the film thickness. Multilayer systems of Al–C and Ti–C with systematic variations of single layer thickness and thickness relationship were analysed by electron microscopy and Auger electron spectroscopy. The Youngs moduli were measured by the non-destructive ultrasonic surface wave method (US–SAW). The alternating hard and ductile layers allowed a remarkable relaxation of the internal stresses. Furthermore, the growth of the particle induced defects (droplets) could be strongly reduced.

Elastic modulus as a measure of diamond likeness and hardness of amorphous carbon films

Abstract The elastic modulus directly reflects the bonding strength and structure of a solid state. Usually this factor can only be modified slightly for a given material, but in amorphous carbon films large variations in Youngs modulus (from less than 100 GPa up to more than 500 GPa) are found. This can be attributed mainly to the varying fraction of sp3 and sp2 bonds. Even for hard amorphous carbon films of diamond-like nature, the modulus achieves only 30%–50% of the value of crystalline diamond. Youngs modulus was measured by a method based on the determination of the frequency-dependent propagation velocity of ultrasonic surface waves and its mathematical evaluation. The potential of this non-destructive method has been demonstrated for amorphous carbon films down to a thickness of 100 nm. In addition to reference films deposited by various methods, films prepared by pulsed arc processes were investigated systematically. The influence of the technological key parameters (ion flux, substrate temperature and hydrogen content) has been stressed. The changes in elastic modulus reflect directly the diamond likeness of the carbon-carbon network. Hence, Youngs modulus represents a suitable parameter for the quantitative characterization and classification of amorphous carbon materials. Apart from certain contributions by compressive internal stresses, the hardness can be estimated to be about one-tenth of the elastic modulus.

Elastic modulus of diamond-like carbon films prepared by pulsed vacuum arc

Abstract Amorphous carbon films have been prepared by special pulsed vacuum arc deposition methods allowing high currents up to 1 kA and more. The Youngs modulus of these films has been determined with respect to various technological parameters. For these measurements a method based on the propagation of ultrasonic surface waves has been applied which has been specially designed for the investigation of thin films below one micrometer. Large changes of the elastic modulus, depending on the technology, have been observed, contrasting with the common understanding of invariability of the elastic behaviour. Reflecting the large structural variations possible in amorphous carbon, the elastic modulus represents a suitable parameter for characterizing the carbon—carbon network. Furthermore, the elastic modulus of amorphous carbon films may be used for a first estimation of film hardness because of the strong correlation of these two quantities.