ICMCTF2011 Session E3-2: Tribology of Nanostructured and Amorphous Films

A lot of components in technical applications, for example bucket tappets or piston pins, are both mechanical and tribological highly loaded. To enhance their tribological properties they often get coated with amorphous carbon by PVD or PACVD. Although it is known, that surface treatment may strongly influence the fatigue properties of steels, there are only a few investigations about the influence of amorphous carbon coatings up to now. Besides that, there is a lack of methods for dimensioning such coated parts, not least because there do not exist adequate stress limits for coatings. Motivated by the described situation this article presents results of fatigue tests of 21MnCr5 specimens similar to ASTM 606 and coated with an a-C:H:W coating under zero-to-tension cyclic loading. It is discussed, how strong the coating influences the fatigue properties of the substrate and if it is possible to identify fatigue limits of coatings under defined stress conditions using conventional fatigue tests on resonant testing machines. First results from a set of fatigue tests indicates, that the amorphous carbon coating does not influence the static strength, in particular the tensile strength, but decreases the fatigue limit with increasing cycle number. This effect is not understood up to now, although there are some presumptions, which should be discussed and compared to other works, which studies the influence of different hard coatings on the fatigue limits of different substrates, too. Furthermore, in this article a method is suggested, which allows in a quite simple way the detection of fatigue limits of the coating using microsections of the fatigue specimens and their investigation by light microscopy. Compared to other known methods, for example cyclic impact tests, the suggested method provides fatigue limits under quite well defined stress conditions.

The 3D FEM model was developed for calculating the stress and strain distributions. The simulation model was applied to the scratch test contact conditions, when the spherical diamond tip was moving with increased load on a coated surface. The crack propagation during the empirical scratch testing was detected. It was observed that the first crack appeared for the both coatings as an angular crack representing the maximum stress experienced on the scratch groove edge. When combining the simulated stress values with empirical observation of coating fracture patterns the coating fracture toughness was determined. A major effect of the coating elastic modulus on the stress and fracture behaviour of the coatings was observed.

In the tribological testing the both coatings had a low friction performance. In the tribological testing with continuously increasing load, the critical load for coating delamination was higher for the a-C:H coating. This is in accordance with the results of FE modelling of stress accumulation and the fracture toughness evaluation of the coatings. The influence of coating characteristics on the performance properties in real applications will be discussed.

To achieve HDD areal densities of 1 Tb/sq.inch it is required that the magnetic spacing between the head sensor and the disk media is reduced to 6 nm. Especially, the overcoat (OC) film that protects the disk has to be as thin as 2.5 nm. Within this spacing budget the OC has to retain excellent tribological properties, lubricant compatibility, and anticorrosion protection of the Co-containing media.

TiSiN [1] and SiN_x [2] ultra-thin films are prospective materials for high density HDD disk OC. SiN_x films are denser than conventional diamond like carbon OC and can act as barriers that prevent migration and corrosion of Co. Unfortunately, SiN_x OC are unstable under high humidity and temperature. Water hydrolysis leads to SiO_x growth on the surface of SiNx OC resulting in HDD failures such as head crashes and irreversible disk damages.

Here, we show that adding Ti into SiN_x induces the formation of a TiO_x surface protection layer that allows reducing SiO_x growth and Co corrosion while concomitantly decreasing the OC surface energy. We present an extensive spectroscopy and microscopy study (XPS, XRR, FTIR, AFM, SEM, TEM, OSA) of 2.5 nm thick TiSiN OC, with focus on their composition and nanostructure, stability against hydrolysis, and anticorrosion protection.

OC were deposited by reactive magnetron sputtering on top of magnetic media on glass disks using different Ti_xSi_y targets to vary their content ratio R=Ti/(Ti+Si). These films were found to be different in nature than previously reported nanocomposite TiSiN [3]. They are completely amorphous with mass densities increasing with R (0 to 1) from 3.14 to 4.34 g/cc. These values are comprised between those of stoichiometric Si₃N₄ (3.2 g/cc) and TiN (5.4 g/cc). Moreover, angle-resolved XPS and FTIR showed that TiSiN OC are actually oxynitrides terminated with a native surface layer of TiO_x.

Overcoat stability against hydrolysis was studied in function of films Ti content. It was found that 4 at. % of Ti is enough to reduce SiO_x formation by half, and that higher Ti contents can block SiO_x surface growth. To reduce surface chemical reactivity and contaminant attraction it is desirable that disk OC have low surface energy. Surface energy of disks lubed with 1.2 nm of ZTMD, a perfluorinated lubricant, was measured by using the droplet contact angle method. TiSiN OC have much lower surface energy (16-19 mN/m) than SiN_x OC (31 mN/m). For comparison, we measured 17 mN/m for both TiN_x and TiO_x OC, which is consistent with TiSiN OC being capped with TiO_x.

[1] Q.Dai et al., US Pat. 6586070 B1 7/2003

[2] B.K.Yen et al., Vac. Sci. Technol. A 21(6), 1895, 2003

[3] S.Veprek et al., Surf. Coat. Technol. 202, 5063, 2008

Nanocrystallite carbon film has drawn much interests since it has not only high hardness and good wear performances comparable to those of diamond but also wonderful conductivity close to that of graphite. It has a brightly future as a new coating material in modern industry. Among the methods which are introduced to prepare carbon film, Electron Cyclotron Resonance (ECR) method has some exclusive advantages such as high energy efficiency, flexible working condition and simple facility. Therefore, ECR method is a very promising way to produce nanocrystallite carbon film in large scale.

In this study, we have prepared amorphous carbon films with different content of sp2 bonded nanocrystallite by ECR sputtering method. By controlling ECR plasma condition, either electron sheath or ion sheath was formed, and the film surface would be etched by either electron or ion during the film growth process, namely electron etching mode and ion etching mode. Carbon films with different nanocrystallite content were prepared under different etching mode and etching energy. The film's binding configurations and nano-structures were characterized by XPS spectrums and TEM pictures respectively. Friction coefficients and wear lives of the films were tested with a Pin-on-Disk tribometer, and super-low-friction phenomena were observed in certain films' wear tests.

Diamond-like carbon (DLC) coatings are particularly suited for applications that require minimum adhesion, low coefficient of frication (COF) and good wear properties against aluminum alloys. These properties however deteriorate rapidly at elevated temperatures. In this study, friction and wear behaviour of W doped DLC (W-DLC) was studied as a function of testing temperature to investigate the coating’s surface and subsurface damage at temperatures up to 400ºC. Pin-on-disk tests of 319 Al ran against W-DLC showed that the lowest COF was observed at 25ºC (0.20). While at 100ºC, 200ºC and 300ºC, high average steady state COF values of 0.59 were recorded and as the temperature reached 400 ºC, the COF reduced again to 0.18. Cross sections of the wear tracks prepared by focused ion beam (FIB) milling showed that at 100ºC the coating was spalled and aluminum was adhered to the exposed substrate and hence resulting in the observed high COF. However, at 400ºC, FIB cross sections of the wear track showed no evidence of coating failure or aluminum adhesion. Additional analyses illustrated that at 25ºC and 400ºC, a transfer layer was formed on aluminum contact surface, which prevented aluminum/DLC interactions, and reduced the friction by eliminating aluminum adhesion. Further chemical investigations of the transfer layer using micro Raman, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) were conducted to elucidate details of the mechanisms responsible for the low COF at elevated temperatures.

A series of WC/C nanostructured films with carbon contents ranging from 30 to 70 at.% was deposited on M2 steel substrates by magnetron sputtering of WC and graphite targets in argon. Depending on the amorphous carbon (a-C) incorporated in the coatings, nanocrystalline coatings (formed mainly by WC_1-x and W₂C phases) or nanocomposites (WC_1-x/a-C) were obtained with tunable mechanical and tribological properties. Ultrahardness values of 35-40 GPa were measured for the nanocrystalline samples whilst values between 16 to 23 GPa were obtained in the nanocomposite ones depending on the a-C content. The tribological properties were studied using a pin-on-disk tester versus steel (100Cr6) balls at 5N of applied load in dry sliding conditions. Three different zones were identified according to the observed tribological behaviour: I (μ>0.8; adhesive wear); II (μ:0.3-0.6; abrasive wear) and III (μ~0.2; self-lubricated). The wear tracks and the ball scars were observed by scanning electron microscopy (SEM) and Raman spectroscopy in order to elucidate the tribochemical reactions appearing at the contact and to determine the wear mechanism present in each type. A correlation among mechanical properties, crystal phases, a-C content and wear modes could be established for the series of WC/C coatings.