H. Leiste

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.

Investigation and characterisation of silicon nitride and silicon carbide thin films

Abstract Thin films of silicon nitride (Si3N4) and silicon carbide (SiC) have been deposited by radio frequency (r.f.) magnetron sputtering of stoichiometric targets in non-reactive argon and in the case of Si3N4 additionally in reactive nitrogen–argon atmospheres. The influence of the sputtering atmosphere, the substrate temperature and the substrate bias on the composition and on the mechanical and microstructural properties of the thin films was investigated. FTIR and Raman spectroscopy was used to identify the chemical bonding configuration and to control the chemical composition. Raman investigation showed a change in the bonding configuration from amorphous silicon carbide to a crystalline structure and the incorporation of nitrogen in silicon nitride thin films with increasing substrate bias.

Microstructure and properties of multilayer coatings with covalent bonded hard materials

Abstract Single layer thin films of, preferably, covalent hard coatings SiC, Si 3 N 4 have been deposited by non-reactive r.f. and reactive d.c. magnetron sputtering. By sequential deposition they are combined with metallic hard coatings (TiC, TiN) to multilayer systems with an individual layer thickness of 1–500 nm of alternating covalent and metallic hard material layers on cemented carbide or quarz substrates. The reactive deposition was carried out at 400°C in an atmosphere of N 2 or CH 4 containing Ar by elementary targets (Si, Ti). The non-reactive deposited films were grown by the use of stoichiometric targets (SiC, Si 3 N 4 , TiN, TiC) between room temperature and 550°C. The constitution of the single layer and multilayer thin films was studied by XRD, EMPA and HRTEM in dependence on the deposition parameters (substrate temperature, reactive gas flow) and related to the mechanical properties (hardness, adhesion). SiC films deposited by non-reactive sputtering in a stochiometric composition at temperatures between 200 and 550°C show an amorphous structure in XRD-studies. The microhardness was measured to 2600HV0.05 and the critical loads of failure in the scratch test below 30 N for hardmetal/SiC composites. The composition of SiC x thin films deposited by reactive magnetron sputtering has been varied up to x ≤1.5. The deposition rate of 0.17 nm/s is weakly dependent on the CH 4 gas flow. The films show a hardness up to 3500HV0.05 and critical loads of failure in the scratch test of 70 N. Using XRD studies in Bragg–Brentano geometry, the (102) line of hexagonal SiC has been found at a substrate temperature of 400°C.

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.

Sensitivity optimization of injection-molded photonic crystal slabs for biosensing applications

For label-free assays employing photonic crystal slabs (PCSs), the sensitivity is one of the most important properties influencing the detection limit. We investigate the bulk sensitivity and the surface sensitivity of 24 different PCSs fabricated by injection molding of PMMA and subsequent sputtering of a Ta2O5 high-index layer. The duty cycle of the linear grating is varied in steps of 0.1 between 0.2 and 0.7. Four different Ta2O5 layer thicknesses (89 nm, 99 nm, 189 nm, 301 nm) are deposited. Both bulk and surface sensitivity are optimal for a Ta2O5 layer thickness of 99 nm. The maximum bulk sensitivity of 138 nm/RIU is achieved for a duty cycle of 0.7, while the maximum surface sensitivity of 47 nm/RIU is obtained for a duty cycle of 0.5. Good agreement between experimental results and finite-difference time-domain (FDTD) simulations is observed. The PCSs sensitivity is linked to the mode intensity distribution.

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.

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.

Composition-dependent structure of polycrystalline magnetron-sputtered V–Al–C–N hard coatings studied by XRD, XPS, XANES and EXAFS

V–Al–C–N hard coatings with high carbon content were deposited by reactive radio-frequency magnetron sputtering from a segmented sputter target. The composition-dependent coexisting phases were analysed using the complementary methods of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine-structure spectroscopy (EXAFS).