Michael Stüber

Microstructure and properties of low friction TiCC nanocomposite coatings deposited by magnetron sputtering

The objective of nanocomposite coatings combining hard and lubricant phases is the development of advanced multi-functional protective thin films showing abrasion resistance, and simultaneously, low friction. Up to now, no clear relation between constitution, microstructural properties and performance of such nanocomposite coatings based on dry lubricants like carbon or MoS2 has been evaluated. Deposition techniques, constitution, properties and performance of magnetron-sputtered nanocomposite coatings in the TiCC system are presented. The Vickers hardness could be optimized to values of polycrystalline TiC thin films, and at the same time, low friction coefficients against steel, similar to diamond-like amorphous carbon, could be realized. The mechanical properties and the tribological behavior of these thin films are related to the chemical composition and the microstructure of these advanced materials, characterized by electron microprobe analysis, Auger electron spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and high resolution transmission electron microscopy.

Graded layer design for stress-reduced and strongly adherent superhard amorphous carbon films

Diamond-like carbon thin films for tribological applications were deposited by d.c.-magnetron sputtering of a graphite target in a pure argon atmosphere or in a reactive hydrogen or methane atmosphere at pressures between 0.1 and 1 Pa in a graded constitution to improve adhesion and reduce residual stress. The temperature of the metallic, carbon- and ceramic-like substrates was below 100°C. The mechanical, thermal, electronic and optical properties of the carbon thin films show a significant dependence on the ion energy. Below 220 eV, strongly adherent black conductive films with hardness values up to 2000 HV0.05 were obtained. Hard and superhard diamond-like carbon thin films were deposited in an energy range between 220 and 370 eV with hardness values up to 4000 HV0.05. They are insulating, optically transparent and show a high degree of hardness combined with high compressive stress in the order of 4 GPa as well as a low adhesion, which means that the critical loads of failure are below 10 N. Above 370 eV, weak black conductive films with a poor adhesion were deposited. A new concept has been realized, which allows the conservation of the positive properties of superhard films, such as high hardness, sp3 content as well as a low friction coefficient (0.08–0.17 against 100Cr6 and Al2O3) with a simultaneously decreasing stress and increasing adhesion. First, a thin TiC interface layer was deposited, and then, the ion energy was gradually increased during the deposition of the carbon layer. Critical loads of failure in a scratch test of up to 50 N were reached when applying this concept. These amorphous carbon films have shown excellent tribological properties, especially low friction coefficients and low wear under dry sliding wear conditions against 100Cr6 and Al2O3. X-ray diffraction and TEM examinations confirmed a fully amorphous structure of hard carbon films. Substrate temperatures above 200°C result in the deposition of nanocrystalline graphite-like carbon films shown by Raman spectroscopy.

Influence of low energy ion implantation on mechanical properties of magnetron sputtered metastable (Cr,Al)N thin films

Metastable, nanocrystalline, ternary chromium aluminum nitride thin films have been deposited by reactive unbalanced magnetron sputtering of a chromium aluminum nitride target in a pure nitrogen atmosphere. The film constitution has been examined by X-ray microanalysis, X-ray reflectivity, X-ray diffraction, transmission electron microscopy and high-resolution electron microscopy. The mechanical properties such as Vickers hardness, elastic modulus and internal stress have been determined as a function of ion energy of bombarding particles during film growth. It was possible to show that the dependence of these properties on ion energy can be described by two physical mechanisms, both subsurface nitrogen ion implantation and nitrogen ion bombardment induced relaxation processes, whereas chemical composition is not affected in the case of our reactive deposition conditions.

Mechanical properties and performance of magnetron-sputtered graded diamond-like carbon films with and without metal additions

Abstract A specific gradient in the constitution and properties of the growing film can be used to improve the mechanical properties and performance of magnetron-sputtered carbon films up to a thickness of 10 μm. The films were deposited on polished WC hard metal inserts and on silicon substrates. Additional argon ion bombardment of the growing film, caused by the substrate bias voltage applied during deposition, could be applied to raise the film density and hardness. The metal additions to carbon films were achieved in a physical vapor deposition process by adding titanium carbide to graphite targets. In a series of basic experiments, the substrate bias voltage for the deposition of homogeneous pure carbon and titanium added films was varied between 0 and −800 V. Afterwards, both pure carbon films and carbon films with metal additions were deposited with a gradient based on the effect of increasing the substrate bias voltage. The resultant residual stress of these graded films grown at various substrate bias voltages could be reduced in comparison to conventionally sputtered films. The mechanical properties and performance, such as Vickers hardness, critical load of failure, friction coefficient and the residual stress of the pure carbon films and the films with added metal, deposited with and without this gradient, have been compared.

Characterisation of silicon carbide and silicon nitride thin films and Si3N4/SiC multilayers

Abstract Silicon carbide (SiC), silicon nitride (Si3N4−δ) monolayers as well as Si3N4/SiC multilayers have been deposited on silicon substrates and cemented carbide inserts by r.f. magnetron sputtering technique. The used target materials were high purity silicon carbide (99.5%) and silicon nitride (99.9% ex. MgO binder). The deposition parameters (like substrate temperature and gas composition) have been varied to optimise the properties of the coatings. The substrate temperature was varied from 20 °C up to 700 °C. The Ar/N2 gas composition for deposition of silicon nitride thin films was varied over a wide range. Both the SiC and Si3N4−δ monolayers and the SiC/Si3N4 multilayers have been characterised by Fourier transform infrared spectroscopy (FT-IR), Auger electron spectroscopy (AES) and X-ray diffraction (XRD). The mechanical properties such as Vickers hardness, residual stress and film adhesion have been investigated. SiC and Si3N4 monolayers with the best properties were chosen to design Si3N4/SiC multilayers. The number of monolayers in the multilayer system with a constant total layer thickness was varied in order to investigate the influence of the interface on film constitution and film properties.

The constitution and properties of cubic boron nitride thin films: a comparative study on the influence of bombarding ion energy

Abstract Boron nitride thin films were deposited by unbalanced radio frequency magnetron sputtering of a hot-pressed hexagonal boron nitride target in a mixed argon/nitrogen atmosphere of 0.2 Pa. The intensity of ion bombardment during deposition was varied by a direct current substrate bias between 0 and −500 V. Thin films containing more than 85% of the sp 3 -bonded cubic phase (c-BN), as confirmed by infrared spectroscopy, were produced in a wide voltage range from −200 to −400 V. The high c-BN content can be maintained even at an appreciably reduced bias of −80 V when a nucleation layer is formed in advance. In that case a decrease of compressive stress as well as an alteration of surface structure of the deposited c-BN films was observed. An analysis in light of electron energy loss spectroscopy indicates moreover the existence of an sp 2 -coordinated surface layer with an ion-energy-dependent thickness. The results are discussed within the frame of the subplantation model.

Influence of the process gas, gas pressure, r.f. power and geometrical arrangement on the magnetron plasma parameters for various thin film materials of the systems Ti=N and B=C=N

Abstract A retarding field analyzer was used to determine the ion energy, plasma potential, and ion current density of the magnetron plasma. The influence of the particle flux parameters of gas composition, gas pressure, r.f. power, and target geometry was investigated systematically. It is demonstrated by the results that both the plasma potential and the ion current density are hardly affected by the target material, as has been shown for TiN, C, BN, and BCN. At the same r.f. power input, targets of various diameters have various power densities. The plasma potential of a large target of 150 mm diameter is slightly lower than that of a target with a diameter of 76 mm. With increasing r.f. target power the resulting plasma potential will also remain constant, whereas the ion current density will be increased according to theoretical predictions. With an increasing working gas pressure, the plasma potential and ion current density decrease, which is in agreement with the theoretical calculations for both pure argon plasma and an argon/nitrogen gas mixture.

Synthesis and characterization of cubic boron nitride films — investigations of growth and annealing processes

Various attempts have been undertaken to reduce the compressive stress of c-BN thin films. An obvious decrease in stress was realized by post-deposition annealing up to 900 °C, as well as by film deposition with reduced bias after c-BN nucleation. A combined deposition/in-situ annealing (at 700 °C) multi-cycle process promotes improved film adhesion, and when implemented together with reduced ion impact during deposition, leads to additional stress relaxation for c-BN films.

Stress reduction in boron carbonitride films by ion energy-modulated multilayers

A multilayer concept for boron carbonitride films has been developed with optimization by stepwise graduation of the substrate bias. For this purpose, multilayers with a varying number of layers and with different single layer thicknesses were deposited by reactive magnetron sputtering in combination with ion bombardment. The energy of the ions was varied systematically. Film deposition was carried out in an argon/nitrogen gas mixture at a gas pressure of 0.35 Pa and a constant r.f. power of 400 W. During deposition, the substrates were cooled to a temperature below 100°C. The films produced were investigated by means of AES, XRD and TEM. The film properties and the film/substrate composite were characterized using different mechanical test methods. In particular, the film stress was determined by the bending of thin silicon substrates (0.18 mm). According to the AES measurements, the carbon concentration of the films amounted to about 22.5 at.%, with lower values being measured at the higher ion energies. The boron/nitrogen ratio was constant at 1.4 in all films. Graded multilayers were found to have stresses that were far below the theoretical values obtained by mere superpositioning of the stresses of deposited monolayers. This is attributed to the influence of the interface in the layer composite. A 2 μm thick multilayer consisting of seven single layers has a hardness of 25 GPa, a residual stress of 1.67 GPa (instead of 5.3 GPa) and a good adhesion with critical loads of failure of 44 N, as determined in a scratch test.

Monitoring the thin film formation during sputter deposition of vanadium carbide

The theoretical description and the experimental realisation of in situ X-ray reflectivity measurements during thin film deposition of polycrystalline vanadium carbide coatings are presented.