ICMCTF2012 Session E1-2: Friction Wear Lubrication Effects & Modeling

Thermal dewetting of metallic thin films is a relatively simple and effective method to create uniform nano-scale patterns on solid surfaces. Nano-patterns formed by this method have been utilized for various applications such as catalysts to grow carbon nanotubes, etch masks for fabrication of nano/micro patterns and friction reduction films. Generally, it has been known that reduction of real contact area is advantages to decrease adhesion and frictional force. In this regard, fabrication of nano-pattern is one of the ways to make the real contact area smaller than a flat and smooth film.

In this study, Ag thin film of about 20 nm in thickness was deposited on Si wafer by RF sputtering. Then, annealing was conducted in a vacuum chamber for 1 hour. Different annealing temperatures from 250 to 550°C were applied to assess the effects of temperature on the film morphology. As a result, circular shaped Ag nano-patterns were formed on the Si wafer by thermal dewetting mechanism. As expected, the morphology of the Ag nano-pattern varied with respect to the annealing temperature.

Experiments were carried out to assess the frictional behavior using the Ag nano-patterned specimens. Friction tests were performed using an atomic force microscope (AFM) with a modified cantilever. A micro-sized sphere was attached to the cantilever to increase the contact region compared with the normal sharp probe tip to investigate the effect of contact area between the nano-pattern and the counter surface. Both initial and steady state frictional forces were measured. It was found that the frictional force of nano-patterned specimens was significantly lower than the smooth Ag thin film. Moreover, there was an optimum annealing temperature that resulted in the lowest frictional force. The effect of contact area on the frictional force was also assessed based on the experimental results and contact analysis.

ACKNOWLEDGEMENT

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MEST) (No. 2011-0000409).

Molybdenum disulfide coatings, doped with metals such as Au or Ti, exhibit good tribological properties over a wide range of environmental conditions and contact pressures. For macroscopic contact sizes (order of 10 – 100 microns), solid lubrication, friction reduction and wear resistance are accomplished by the formation of thin, stable transfer films (order of 10 – 100 nm in thickness). Recent studies in the literature have proposed the use of MoS₂ coatings for micro-electromechanical systems (MEMS), such as gears and switches, and therefore increased the demand for investigating microscale contacts (order of 0.1 - 10 microns) on these coatings. The goal of this work was to provide a direct comparison between the tribological performance of a Ti-MoS₂ coating at the two length scales. A ‘real time’ study of the transfer film behavior and velocity accommodation modes (VAMs) at the macro-scale was conducted with an in situ tribometer, while on the micro-scale, transfer films were analyzed ex situ on the counterface by means of atomic force microscopy. Higher friction was observed with microtribology compared to macrotribology and was attributed to, in some cases, different velocity accommodation modes as compared to macroscopic scales. For dry sliding, the behavior of microscopic contacts on Ti-MoS₂ deviates only slightly from macroscopic results, showing higher limiting friction and microplowing. For humid sliding, microscopic contacts deviate significantly from macroscopic behaviour, showing plowing behaviour and absence of transfer films.

Transition metal dichalcogenides (TMDs) such as WS₂ are well-known for their layered structure and solid lubricant properties. However, beside low friction, a solid lubricant coating must also have a long wear life in order to perform well in a tribological situation. Thus, by adding carbon to the material the mechanical properties can be improved. However, when using a magnetron sputtering process, the resulting thin films are found to be sub-stoichiometric with respect to sulphur. This is due to a number of different effects; take-off angle, scattering, different sticking coefficients and energetic particle bombardment of the substrate.

In this work we have used a non-reactive magnetron sputtering process to see how these effects affect the resulting film stoichiometry, and hence the tribological properties. This was done by changing the process pressure, DC-RF power, the location of the substrate (in and off axes) and by adding carbon to the material. Also, a newly developed Monte Carlo computer model is presented which makes it possible to simulate and predict how these changes will affect the resulting film stoichiometry.

Carbon nitride has fascinat e properties such as high hardness and high current de nsity of field em ission and so on. In addition, if c-C3N4 or β- C3N4 structure can be sy nthesized, it is possibl e to obtain high ha rdness excee ding that of diam ond. Authors tried to obtain cry stal carbon nitride, cry st alline deposit s were obtained from a CH4-N2 reaction gas sy stem using mi crowave plasm a C VD . On the other hand, it has been reported that the friction coefficient of am orphous carbon nitride (CNx) was lower than 0.01 i n N2 atmo sphere. However, tribo logical characteristics of cry stalline carbon nitride were not cleared . So, investigation was carried out on the tribol ogical characteristics of cry stalline carbon nitrid e sy nthesized using MW- PCVD.

Carbon nitride was sy nthes ized using mi crowave plasma CVD. The m ixture of CH4-N2 gas was used a s a reaction gas. CH4 flow rate was varied from 1 to 3 SCCM, and N2 flow rate was fixed to 100 SCCM. Sy nthesis pressure was fixed to 4.0 kPa, and microwave po wer was fix ed to 20 0 W. Reaction time was fixed to 3h. Si was used as the substrate. Surfaces of the deposits were obser ve d using SEM. The deposits were esti mated by Ra ma n spectroscopy, AES, and XPS. Tribologi cal properties of th e deposits were estimated by using of a ball-on-dis k friction tester . Measurements are conducted by usi ng of load 0.1N, speed of sliding 6.2mm/s and counterpart materi als SU J2 or Si3N4 4.7mm in dia meter, respe cti vely. Wear properties of the deposits we re esti mated by using of a surface r oughness test er.

As a result of SEM observation, crystalline deposits similar to the rods of hexagonal were observed for all conditions. The particle size was increased with increasing of CH4 flow rate. From AES esti mation, the peaks of C, N, O, and Si were obser ved in AES sp ectra of eac h samp les. N2 content was increased wi th increasing of CH4 flow rate. From XPS measurem ent, C-N bond and Si3N4 were observed in X PS spectra of each sample s. The lowest coefficient about 0.51 against SUJ2 was obtained for deposits sy nthesized in CH4 flow rate 3 SCCM in the estimation of tribological properties. And the lo west co efficient is about 0.41 agains t Si3N4 for deposits sy nthesized in CH4 flow rate 1 SCCM. As a r esult of estim ation of wear dept h, wear depth was decr ea sed with increasing of CH4 flow rate.

As a conclusion, tri bologi cal characteristics of cry st alline carbon nitride sy nt hesized using MW-PCVD were depended on N2 content of the deposits.

This presentation will discuss how defect structure in atomic layer deposited ceramic coatings (transition metal oxides and in situ formed carbides) determines the thermal/oxidative and friction/wear properties in cellular solids, such as carbon-based composites and foams. Specifically, we will discuss (a) how interstitial carbide and oxide phases, such as ZrC and ZrO₂, provide thermal and oxidation resistance to carbon, and (b) how lubricious, nanocrystalline layered ceramics, such as high basal stacking fault density ZnO, and low crystallographic shear, oxygen deficient Magnéli phases, such as TiO_2-x, mitigate friction and wear. Two important questions will be addressed: (1) Can the coating systems be processed with thermodynamically and kinetically stable oxide and carbide phases and interfaces? (2) How will the defect structure (planar stacking faults and vacancies/interstitials) of these phases be able to accommodate interfacial shear while providing sufficient hardness and elastic modulus?

Robust solid lubricant coatings are needed to survive humid oxidizing environments in many applications. The addition of dopants to MoS₂ films has been previously shown to enhance the lubricant durability [Acta Mater., 58 Scharf et al.]. This study focuses on the synthesis of Au doped MoS₂ nano-composite thin films created by co-sputter deposition as a function of temperature. The ratio of the deposition flux was adjusted to control the composition to about 10% Au by weight. (Scanning) transmission electron microscopy (S)TEM revealed that the room temperature deposited nano-composite consisted of 2-4 nm size Au particles in a matrix of semi-crystalline MoS₂. With increasing growth temperatures, the nano-composite exhibited dramatic structural changes: the Au nanoparticles coarsened by diffusion-driven Ostwald ripening to 5-10 nm size and the MoS₂ basal planes encapsulated the Au nanoparticles thereby forming a novel solid Au core MoS₂ nano-onion like structures. In comparison, a room temperature deposited film was heated, post deposition, to 600°C inside a TEM. In situ TEM heating revealed that Au nanoparticles also coarsened, but unlike heating during film growth, the highly ordered basal planes with reactive edge sites did not encapsulate the Au. This suggests that MoS₂ surface diffusivity and energy during film growth is different than MoS₂ bulk diffusion where the thermal activation energy for diffusion is likely smaller in the latter. Friction and wear measurements were made using a ball-on-disk tribometer in air with 50% RH. The role of these novel structures on the tribological behavior of the films in humid environments will be discussed.

Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000

Gold-multiwalled carbon nanotube (MWCNT) composite films were elaborated by electrodeposition and co-deposition of CVD process. Tribological properties were investigated by a pin-on-disk type friction testing using an electronical system to mesured in same time the electrical conductivity of the composite coatings.

Friction coefficient of gold–MWCNT composite films decreased with increasing MWCNT content. Implantation of MWCNT on the surface, plating parameters, MWCNT concentration and the stirring rate of the bath were systematically investigated in order to found a relationship between incorporation rate V and friction coefficient and electrical conductivity. The Au-0.5 mass% MWCNT composite film showed the minimum friction coefficient value of 0.15. In this case, wear resistance of Au-MWCNT is tree time better.

Hydrogen-free tetrahedral amorphous carbon (ta-C) films are known to display super-low friction behaviour under mixed lubrication conditions in combination with specific polyalcohols. There is a huge interest to use this effect for technical application in order to reduce friction loss e.g. on engine components or gears and using environmentally friendly lubricants at the same time. Although there are some tribological investigation and first approaches to explain the phenomenon there is still a lack of comprehensive data and understanding of superlubricity behaviour of ta-C.

This study evaluated the wear resistance of FeB/Fe₂B layers applying the four-ball lubricant test. First, the boride layers were obtained at the surface of AISI 52100 steels by developing the powder-pack boriding method. The treatment was carried out at a temperature of 1223 K with 1 h of exposure. The boriding of AISI 52100 steels results in the formation of a superficial flat front growth of FeB/Fe₂B layers with a total layer thickness of 72 microns. In addition, the hardness at the surface of borided steels was 1900 HV.

The four-ball lubricant tests were performed in the borided samples (with a surface roughness of 0.043 microns) and unborided steels with a surface roughness of 0.025 microns considering dry and lubricant conditions. The wear resistance of borided and unborided steels in the dry condition was estimated by three different loads of 49, 98 and 147 N with a constant speed and time test of 1200 rpm and 150 s, respectively. For the case of the lubricated condition, the borided and non-borided steels were exposed to applied loads of 147 and 392 N according to the ASTM D4172 standard using commercial SAE 15W40 engine oil. For all cases, the friction coefficients of borided and unborided steels were monitoring by a full bridge load cell, and the temperature was sensed through a thermocouple in real time during the four-ball lubricant test. The wear scar diameter and the wear scar surfaces of the AISI 52100 borided and non-borided steels in both experimental conditions were measured using an optical microscope and scanning electron microscopy (SEM) to understand the wear mechanisms involved.