AVS2011 Session TR-ThA: Advanced Tribological Materials

The microstructure, nanomechanical and nanotribological properties of ZrB₂ thin films grown by DC magnetron sputtering have been studied as a function of Ar pressure, substrate bias, and substrate temperature. Films, ~ 500 nm thick, were deposited onto Si (001) and Al₂O₃ (0001) substrates from a compound target using an industrial chamber CC-800/9 from CemeCon operated at a fixed target-to-substrate distance of 7 cm.

X-ray diffraction patterns show that 0001-oriented films can be obtained on both substrates at a substrate bias of -80 V without any external heating. Transmission electron microscopy of samples grown at different conditions reveal the presence of an amorphous 100-300 nm thick layer close to the substrate, followed by the nucleation of ZrB₂(0001). The same oriented structure appears for samples grown up to 150 ^oC, but at higher temperatures this orientation is gradually degraded. At 500 ^oC, cross-sectional scanning electron microscopy shows a columnar microstructure with re-nucleation during the growth. For films grown at 100 °C, little impact on the texture is observed when the substrate bias is changed from floating to -200 V.

Nanomechanical and nanotribological properties measured with a Hysitron Triboindenter^TM TI 950 reveal that the films have high hardness and elastic recovery, and low friction. For films grown at low temperature, the hardness, reduced elastic modulus, and elastic recovery decrease from 25 to 19 GPa, 290 to 200 GPa, and 96 to 92%, respectively, when the amorphous interface increases from 100 to 300 nm. Nano-frictional tests were done in a load-controlled feedback mode using a force of 1 mN; a total of 40 reciprocating passes were performed for each test using a diamond 90º probe with a 1 µm tip radius. The friction tests reveal a friction coefficient µ in the range 0.10-0.13 for ZrB₂ samples grown at different conditions, in contrast of µ = 0.6 for a pure Zr film.

Deposition processes like the high power impulse magnetron sputtering (HIPIMS) indicate high metal ion ratios in the plasma, which result in increased structural and mechanical properties. The generally low deposition rate, compared to direct current magnetron sputtering (DCMS), narrows the industrial application range of this technology. The modulated pulse power (MPP) deposition technique on the other hand uses multiple complex pulsing steps to increase the metal ion ratio in the plasma without dramatically reducing the deposition rates as compared to DCMS.

Recently we showed that a multilayer architecture of CrN and TiN, deposited using the hybrid HIPIMS/DCMS deposition technique, results in coatings exhibiting friction coefficients in the range of diamond-like-carbon (DLC) coatings when tested at RT and ambient air conditions. Here we show results of MPP/DCMS deposited CrN/TiN multilayer coatings indicating comparable mechanical and tribological properties, hardness values around 25 GPa and coefficient of friction below 0.05. Furthermore, investigations on their dependence to the atmospheric conditions used during dry sliding as well as theoretical investigations of the layered structure using density function theory simulations were carried out.

Friction and wear mitigation is typically accomplished by introducing a shear accommodating layer (e.g., a thin film of liquid) between surfaces in sliding and/or rolling contacts. When the operating conditions are beyond the liquid realm, such as in extreme environments, attention turns to solid coatings. The focus of this talk is how contacting surfaces and subsurfaces change both structurally and chemically in order to accommodate interfacial shear for two multifunctional coating systems: nanocomposite MoS₂/Sb₂O₃/Au and Ni/TiC/graphite. It was determined that the coatings exhibit velocity accommodation modes (VAM) of interfacial sliding and intrafilm shear, as determined by advanced electron microscopy (3-D focused ion beam serial cross-sectioning, HAADF-STEM, and HRTEM) and spectroscopy (Raman, Auger and EDS wear maps) techniques.

In the case of amorphous-based MoS₂/Sb₂O₃/Au nanocomposite sputtered coatings, the main mechanism responsible for low friction and wear in both dry and humid environments is governed by the interfacial sliding between the wear track and the friction-induced transfer film on the counterface ball. In dry environments, the nanocomposite has the same low friction coefficient as that of pure MoS₂ (~0.007). But unlike pure MoS₂ coatings which wear through in air (50% RH), the composite coatings showed minimal amount of wear with wear factors of ~1.2-1.4 x 10^-7 mm³/Nm in both dry nitrogen and air. Cross-sectional TEM of wear surfaces revealed that frictional contact resulted in amorphous to crystalline transformation in MoS₂ with 2H-basal (0002) planes aligned parallel to the sliding direction. In air, the wear surface and subsurface regions exhibited islands of Au. The mating transfer films were also comprised of (0002)-orientated basal planes of MoS₂ resulting in predominantly self-mated ‘basal-on-basal’ interfacial sliding, and thus low friction and wear.

In the case of laser deposited Ni/TiC/graphite composite coatings, it was determined during sliding that a wear-induced tribochemical and structural change from microcrystalline graphite to amorphous carbon/nanocrystalline graphite hybrid layer resulted in decreased friction and wear. Other novel insights were determined from 3-D microstructural evolution during wear, such as a mechanically mixed layer developed consisting of predominately refined nanocrystalline Ni grains (~10 nm grain size) and disordered carbon below this hybrid layer. The formation of these low interfacial shear strength films and recrystallized zones were responsible for intrafilm shear VAM to achieve low friction coefficients (~0.09) and wear factors (~6.8 x 10^-7 mm³/Nm).

Friction at the nanoscale shows a strong and complex relationship to surface roughness and atomic disorder [1]. Recent research in superconductivity dependent friction [2-5], along with reports that quantum size effects [6] can influence diffusion (and thus friction) of adsorbed layers, has motivated our investigation. In particular, we have performed friction measurements of adsorbed nitrogen and helium films sliding on nanostructured lead films substrates that have been deposited on titanium, a substrate that lead does not wet. Varying the lead coverage results in a spectrum of percolated morphologies. We prepare these films on a quartz crystal microbalance (QCM) and probe their topologies by means of adsorption onto the surface [7].

Measurements have been recorded on nanoclustered lead films with coverages crossing the critical concentration for percolation. We study the substrate in the superconducting and normal states, which allows us to isolate and quantify the contribution of electronic and phononic dissipation to the total friction present [2]. Submonolayer adsorbate converages have allowed us to probe the edge effects of surface nanoclusters, while multilayer coverages have let us explore the strength and proximity effects of surface roughness. We compare our measurements to those reported by Pierno et al. on films of ordered Pb(111) terraces, where atomic step edges are present [3], and conclude that the variation in reported values of friction on nanostructured lead is due to phononic effects at the step edges.

Funding provided by NSF DMR.

[1] Y. Braiman et al., Physical Review E 59, R4737-40 (1999).

[2] M. Highland and J. Krim, Physical Review Letters 96, 1-4 (2006).

[3] M. Pierno et al., Physical Review Letters 105, 1-4 (2010).

[4] Q. Ding et al., Wear 265, 1136-1141 (2008).

[5] M. Kisiel et al., Nature Materials 10, 119-122 (2011).

[6] M. Özer et al., Physical Review B 72, 3-6 (2005).

[7] V. Panella and J. Krim, Physical Review E 49, 4179-4184 (1994).

United Protective Technologies (UPT) has developed a room temperature plasma assisted chemical vapor deposition (PACVD) coating process to build carbon based coatings. Versions of this coating are in use for infrared optical applications, galvanic corrosion barriers and tribological modifications to critical components. The process used to produce SP³EC^TM coatings is compatible with a wide range of materials including semiconductors, metals, polymers and composites. Deposition parameters for the SP³EC™ process can be controlled to produce a wide range of carbon based thin films, ranging from 120 nm diamond crystalline grains to low friction glassy amorphous carbon films. Layered “nano-composite” structures constructed of these films have been proven to improve the life; performance and reliability of components under high wear conditions and corrosion conditions. Additionally, the SP³EC™ process is non-toxic and environmentally friendly.