ICMCTF2012 Session B1-2: PVD Coatings and Technologies

Due to high deposition rate and ability to form hard and dense coating, arc ion plating (AIP) has been applied in many industrial applications. Meanwhile, emission of macro-particles (MPs) and delamination of the coating due to large compressive residual stress are considered as drawbacks of AIP process. Emission of MPs can be reduced by controlling the movement of arc spot during the discharge which can be manipulated by application of external magnetic field. The new arc cathode was developed through the magnetic field design of the cathode surface. The relationship between the vacuum arc plasma state and the property of the deposited various nitride coatings were investigated for conventional and the newly developed arc cathode. There were three advantages as vacuum arc cathode for new arc cathode. First, the number of MPs on the coating deposited by new cathode can be reduced resulting much improved surface morphology. Yamamoto et al. reported that the improvement of the surface morphology of the coating deposited by new cathode was correlated with the velocity of cathode spot on the surface of the target ^[1]. Second, Although deposition varies depending on the cathode material, we have observed general increase in deposition rate in case of the new cathode. Third, the residual compressive stress of the TiAl(50/50)N coating deposited by new cathode can be halfed compared to conventional cathode, so it is possible to deposit the thick coating on the sharp edge of the cutting tool by the new cathode. To investigate mechanism for these improvements by the new cathode, optical emission analysis of vacuum arc plasma with conventional and new cathode was conducted.

[1] K. Yamamoto et al., presented at ICMCTF 2010 at San Diego G6-7

A review will be given on the preparation of carbon and boron nitride based super-hard coatings by excimer laser deposition. In particular, the conditions under which tetrahedral amorphous carbon (ta-C), nano-crystalline diamond (n-D) and cubic boron nitride (c-BN) films form will be presented. The growth rates, microstructure and mechanical properties of those films will be discussed and compared to other deposition methods and the special advantages of the pulsed laser deposition method will be emphasized.

It will be shown that ta-C, n-D and c-BN films with high hardness in the range of 45 – 65 GPa can be deposited at high growth rates. As the films show high internal stresses due to the high energy of the film forming particles, which results in poor adherence, the preparation of micrometer thick films of those materials requires a suitable method of stress reduction during the deposition process. We developed a special pulsed laser annealing technique, which is applied alternating to the deposition process of thin sub-layers. It will be shown that stress-free ta-C films with 80 to 85 % sp³ bonds, 60 – 65 GPa hardness and thicknesses in the µm-range can be prepared by using a KrF-excimer laser of 248 nm wavelength for annealing. In the case of c-BN films, a F₂-laser of 157 nm wavelength has to be used for annealing, where the stresses could be reduced by some 50 %, so far.

The great advantage of the method in comparison to conventional thermal annealing is the short time it requires to completely remove stress (the process itself requires only a few µs) and that it is possible to go over directly from deposition to annealing and vice versa. The latter is of particular significance for the deposition of ta-C films as the substrate temperature during actual film growth must not exceed 90 °C.

In the second part of the talk, the hardness, adhesion, friction coefficient and wear rates of ta-C, n-D and c-BN films prepared by our method on WC-hard metal inserts and various types of steel in dependence of the deposition parameters will be presented and discussed.

Finally, some examples of coated components and tools will be shown and the industrial potentialities of the method will be discussed taking into account the commercially available lasers as well as the costs.

Increasing demands on tribological performance drive the development of innovative coating materials and deposition methods. Recent studies of reactively sputtered chromium carbide films show promising results with regards to coefficients of friction [1], and these coatings may be suitable for commercial purposes. However, transfer of a process to industrial conditions poses challenges, such as growing the films at significantly higher deposition rates. While this is known to influence the microstructure and phase content, which in turn determine the tribological performance, the implications for the Cr-C system are not fully understood.

The objective of the present study is to investigate chromium carbide films deposited by direct current magnetron sputtering (DCMS) and high power impulse magnetron sputtering (HiPIMS) in an industrial deposition system, at growth rates up to 500 nm/min. While DCMS offers a higher deposition rate, HiPIMS is known to result in well-adherent, dense, and smooth coatings. Microstructural changes are expected as well as modified tribological and mechanical properties owing to the high deposition rate.

Cr-C coatings were synthesized by sputtering of a Cr target in an Ar/C₂H₂ atmosphere. XRD shows an X-ray amorphous Cr-C phase, in contrast to the nanocrystalline structure usually reported [2-3]. For DC sputtered coatings, XPS reveals a C-C phase which increases according to the phase diagram for Cr-C. The coatings produced by HiPIMS feature a meta-stable, supersaturated Cr-C phase, with a higher C content than predicted for the thermodynamically stable phase Cr₃C₂. Coefficients of friction were obtained from dry-sliding experiments (typically 0.35 – 0.50 for 34 – 0 % C-C), and the values confirm a dependence on phase fractions of C-C and C-Cr. Regarding the mechanical properties, coatings deposited by DCMS have a hardness of 14±1 to 18±1 GPa (51 to 17 at.% C), while coatings deposited by HiPIMS have a hardness of 7±3 GPa to 14±4 GPa (59 to 26 at.% C). Unexpectedly, the coatings deposited by HiPIMS are softer than the ones deposited by DCMS, and this difference may be due to a different grain size or phase content. SEM cross-sections indicate deep Cr implantation in the substrate, which is known to improve adhesion. Further results will be presented for a wide compositional range, and compared to literature.

References

[1] Mitterer et al, Proc IME J J Eng Tribol, 223 (2009) 751-757

[2] Gassner et al, Tribol Lett, 27 (2007) 97-104

[3] Agarwal et al, Thin Solid Films, 169 (1989) 281-288

DLC (diamond like carbon) thin film depositions are carried out in a wide range of CVD- and PVD technologies. Main applications are: tribologically stressed machine components as well as wear protection of tools. As a matter of fact, it could be verified that the hydrogen free DLC-coatings result in advanced thin film properties such as higher hardness which is caused by a higher sp³-bonding content. Therefore evaporation of solid graphite by PVD-technologies has been of considerable advantages.

Especially the vacuum arc evaporation technique stands out for a very high degree of single and multiple ionised carbon atoms with increased energy necessary for condensation in the dense tetragonal amorphous diamond- like carbon film structure (ta-C). Beside low friction also super hard coatings can be deposited by means of that technology.

A commercial hard material coating system has been equipped with a laser arc module (LAM) for the deposition of hydrogen-free ta-C-films. For industrial applications it was necessary to increase deposition rates. Therefore arc pulse current and pulse frequency were varied form 800 A to 1.600 A and 150 Hz to 300 Hz, respectively.

The carbon plasma during deposition process were examined by optical emission spectroscopy (OES) and Langmuir probe and effect on plasma parameters of the thin film properties will be shown.

The influence of the deposition rate on thin film properties is caused by thermal effects. One limitation is a maximal deposition temperature of about 150 °C in order to avoid the graphite film structure which results in reduction of hardness finally.

The thin film depositions were carried out on samples of circular blanks and carbide drills. Hardness, adhesion and elastic force module are the first criteria for evaluating the quality of the deposited ta-C- films. As real application tests the ta-C coated drills were used to machine an abrasive AlSi-alloy with high Si-content.

cBN (cubic boron nitride) shows outstanding mechanical properties such as hardness and wear resistance. In contrast to diamond, cBN is compatible with ferrous materials, which makes it possible to be used as coating materials for machining of ferrous materials at high temperatures. However, amorphous BN and hBN layers inevitably formed before nucleation of cBN weaken the interface stability between cBN film and substrate. Furthermore high stress developed during deposition of cBN film deteriorates adhesive strength of cBN film. Recently, the adoption of compositionally gradient B-C-N buffer layer [1] and the addition of oxygen [2] or hydrogen [3] have been suggested as useful technique to improve interface (hBN) stability and reduce residual stress of cBN film, respectively. By further reduction of residual stress of cBN film with stable interface of hBN layer, the application of cBN as protective coating material is believed to be realized. In this study, therefore, we have investigated the effect of hydrogen addition on the residual stress of cBN film prepared with compositionally gradient B-C-N layer deposited by magnetron sputtering of B₄C target. The deposition was performed on Si (100) substrate under the chamber pressure of 0.27 Pa with substrate bias of -250V. After the deposition of B₄C layer, up to 5 sccm hydrogen was added to a gas mixture of argon and nitrogen flowing 25 sccm and 5 sccm, respectively during sample preparation. The compressive stress of cBN film was observed to be decreased from 9.2 GPa to 4.3 GPa, with increasing hydrogen flow up to 5 sccm. The cBN fraction in these films, however, remained to be about 65%, irrespective of the amount of hydrogen added. The stress reduction observed in cBN film deposited with the addition of hydrogen was discussed in terms of the relation between the penetration probabilities of hydrogen and argon ions into the film, which was main origin of compressive residual stress of the compositionally gradient B-C-N layer with hBN structure.

[1] K. Yamamoto, M. Keunecke and K. Bewilogua, Thin Solid Films 377-378 (2000) 331-339

[2] M. Lattemann, S. Ulrich and J. Ye, Thin Solid Films 515 (2006) 1058–1062

[3] H.-S. Kim, J.-K. Park, W.-S. Lee and Y.-J. Baik, Thin Solid Films 519 (2011) 7871–7874

This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea