ICMCTF1999 Session BP: BP Posters

Thin conductive wear resistant metal/ceramic coatings were deposited onto polysulfone S2010 substrates using a triple-cathode vacuum arc plasma source connected to a magnetized plasma duct. Various combinations of two-layer coatings having an inner metallic conductive layer of Ti, Zr, Cu, Ag or Al attached to the plastic, and an external wear resistant layer of TiN or ZrN, were studied and evaluated. The deposition parameters (arc current, magnetic field strengths, deposition time) were optimized (1) to obtain the required deposition rate and coating thickness, and (2) to obtain good adhesion of the coating to the substrate at low temperature.

It was shown that bi-layer Ti/TiN and Ti/ZrN coatings had good adhesion to the substrate and low wear rate, but their electrical resistivity was high (3-6.6 Ω/square). Cu, Ag and Al metal sub-layers had a superior adhesion to the polymer substrate and low electrical resistivity (less then 0.1 Ω/square). However, the deposition of TiN layer on Ag caused immediate coating delamination from the substrate. Similar behavior was observed also with the Ag/Ti/TiN combination. In the case of Ag/ZrN coatings, which initially possessed good adhesion to the substrate, cracking and delamination of ZrN at the Ag/ZrN interface was observed after some time (1-7 days), while the Ag layer remained adherent to the substrate. In the cases of Cu/TiN and Cu/ZrN coatings, the nitride layers had poor adhesion to the Cu layer. The best results were obtained for Al/ZrN coatings, which had low resistivity (0.08-0.11 Ω/square) with good adhesion of the Al layer to the substrate and of the nitride layer to Al.

The authors have developed an electron-beam (EB) evaporation technique for the deposition of complex-alloy hard ceramic coatings, based on modifications to the well-known Ti-N system by addition of Al and B to the evaporant source material [1,2]. The coatings exhibit hardnesses in excess of 40GPa [1] and can significantly out-perform commercial TiN coatings in drilling tests [2]; however the friction coefficient against steels is still quite high, at around 0.8 [1].

There are a wide range of phase composition possibilities within the Ti-Al-B-N system, which can include nitrides, diborides, boron-nitrides (both hexagonal and cubic) titanium-aluminide inter-metallics and TiN/TiB₂'nanocomposite' phases. These offer a number of routes to adjust the hardness, friction and adhesion/toughness properties of TiAlBN coatings for applications other than machining, if the appropriate degree of process control can be achieved. Although significant amounts of h-BN can be incorporated into these coatings, this does not appear to lower the friction coefficient as much as might be expected [1]. This work therefore examines the use of Plasma-Immersion Ion-Implantation (PI3) as a means to introduce a high carbon content to the surface/near-surface of Ti-Al-B-N coatings and provide improved frictional characteristics.

The PI³modified coatings were produced by depositing a plasma-assisted EBPVD TiAlBN coating onto a series of tool-steel test coupons, then using a weakly-ionised low-voltage argon-methane plasma (supported by hot-filament electron emission), onto which high voltage pulses were superimposed, to PI³treat the coated coupons for different times over a range of voltages between 5 and 15kV. The friction and wear behaviour of the resulting carbon-implantation modified coatings was then assessed using both pin-on-disc and reciprocating-sliding wear tests. It is demonstrated that the PI³method provides a flexible technique for modifying the tribological characteristics of PVD hard coatings, which can be integrated into the coating process.

1.'Structure, mechanical and tribological properties of TiBN and TiAlN multiphase thin films produced by electron-beam evaporation'

C. Rebholz, H. Ziegele, A. Leyland & A. Matthews In press - J. Vac. Sci. Technol. A.

2.'Deposition and characterisation of TiAlBN coatings produced by direct electron-beamco-evaporation of Ti and a Ti-Al-B-N alloy from a twin crucible source'

C. Rebholz, J. Housden, N. Watson, A. Leyland, C. Charitidis, S. Logothetidis & A. Matthews In press - Thin Solid Films

Even though high strengths have been obtained in most ceramic coatings, it has also been shown that some ceramic coatings undergo extensive plastic deformation during indentation, scratch, and wear tests. It is essential to understand the plastic flow and related friction and wear behaviour. Reciprocating multipass wear tests have been carried out on unbalanced magnetron sputtered TiN coatings. Deformation and failure mechanisms were identified based on high resolution microstructural examination through cross-sectional fracture surfaces.

Sputtered TiN coatings exhibit columnar or nanocrystalline characteristics, the strain energy and residual stress can be easily dissipated or relaxed by the plastic flow of the coatings. If the interface adherence between coating and substrate is strong enough, the process of plastic flow, plastic extension, microflaking, and thinning will continue until the total penetration of the coatings.

A low coefficient of friction (?=0.06~0.08) was obtained when the wear is performed using diamond as the slider, even the occurrence of obvious plastic flow in the TiN coatings and substrate. In contrast, the coefficient of friction or energy loss is quite high (?=0.45~0.6) when the TiN coatings contact with WC/Co materials, even without the occurrence of obvious plastic flow. It is suggested that strong adhesion or interfacial bonds may be formed between the TiN coatings and WC/Co materials during the sliding process. The deformation loss is believed more important in relation to the wear or surface fatigue than in relation to friction.

Over the past few years, tetrahedral amorphous carbon (ta-C) films grown by Filtered Cathodic Vacuum Arc (FCVA) technique have attracted considerable interests owning to their unique combination of properties. Among the important properties of these films are high hardness, low coefficient of friction and optical transparency. The unusual combination of these properties has stimulated studies on various applications such as optical coatings, high endurance and low friction coatings. However, it is well known that typical films with diamondlike properties have very high residual compressive stresses after growth (12~14 GPa), which result in the delamination of thick films from their substrates. Therefore the large compressive stress has been one of the obstacles to the application of ta-C coatings. In this paper, the stress relief of ta-C films by post deposition thermal annealing is investigated.

The details of the FCVA deposition system have been published elsewhere. The system incorporates the off-plane double-bend (OPDB) filter to effectively remove all macro-particles. Tetrahedral amorphous carbon films were deposited at 100 eV ion energy (maximum sp³content) with the substrate held at room temperature. The film thickness was typically 70 nm. Two sets of experiments to anneal the films in low vacuum with argon (~10^-2 torr) at different temperatures ranging from 500 to 800 degree C were carried out. The samples in the two sets were kept at each annealing temperature for 2 minutes (Set A) and 30 minutes (Set B) respectively and allowed to cool down back to room temperature for other measurements. A Tencor P-10 surface profilometer was used to determine the stress of each film in all the sets. By using the Radius of Curvature Technique, the relative change in stress before and after heat treatment for each film was computed. To obtain qualitative information about the changes in the structure of the carbon films, Raman spectra were measured at room temperature on a Renishaw system using the 514.5 nm lines of Ar⁺ laser as excitation source. Typically, the spectra were acquired in the backscattering geometry over the range of 400 cm^-1 to 4000 cm^-1. Low input power was used in order to avoid annealing effects under laser irradiation.

The relative change in compressive stress as a function of heat treatment temperature for films subjected to different annealing time is plotted. In general, for all the films, the stress decreases (by 60% to 95%) with increasing heat treatment temperature. However for the same heat treatment temperature, the stress decreases much more prominently for films subjected to 2 minutes annealing time. A ta-C film subjected to 600 degree C heat treatment for 2 minutes annealing time has a stress reduction of 85% as compared to 70% for 30 minutes annealing time.

The Raman spectra of the ta-C films generally exhibit a broad intensity distribution at around 1560 cm^-1, which is characteristic of hydrogen-free amorphous diamond-like carbon films. In order to analyze the spectra quantitatively, the Raman spectra were fit to the two Gaussian peaks using PeakFit, a least-squares computer program. It can be seen for increasing temperature, the D peak becomes more noticeable and the G peak shifts toward 1585 cm^-1, which is also the wavenumber for the main peak of crystalline graphite. It is interesting to note that the integrated intensity ratio (I_D/I_G) obtained from the fitting results registered a smaller increase for films subjected to 2 minutes annealing. The I_D/I_G ratio for the ta-C film subjected to 600 degree C heat treatment for the 2 minutes annealing time is 0.35 as compared to 0.5 for the 30 minutes annealing time. The intensity ratio result correlates very well with the G peak position and its linewidth. An upward frequency shift of the G band and the narrowing of the G band with increasing heat treatment temperature were also observed. These changes in the Raman spectrum suggest graphitization of the films. It has been reported that the position of the G-band and its linewidth in the Raman spectrum for amorphous carbon films are correlated to at least two film properties - the sp³/sp² ratio and the film stress where a lower sp³ content and /or lower stress leads to a lower G linewidth. The relatively smaller change in the intensity ratio and the larger reduction in G linewidth observed for the 2 minutes annealed ta-C films suggests that higher stress relief of ta-C films can best be achieved by a shorter thermal annealing without sacrificing much degradation in its diamond-like properties.

In view of the high stress relief in the annealed films, subsequent deposition and annealing steps can be carried out to deposit thicker films. Initial results show that several layers can be deposited and yet high diamond-like quality films can be obtained.