ICMCTF2011 Session E5: Nano- and Microtribology

Wednesday, May 4, 2011 1:30 PM in Room California

Wednesday Afternoon

Start	Invited?	Item
1:30 PM		E5-1 Microstructural Tailoring of Metallic Multilayer Thin Films by Laser Interference Metallurgy for Enhanced Tribological Properties Carsten Gachot, Frank Thomas Muecklich (University of Saarland, Germany) Laser-Interference-Metallurgy is a rather new surface processing technology, allowing a quick as well as direct structuring of geometrically precise periodic and long range ordered microstructures on macroscopic areas. In this technique, a high power nanosecond laser pulse is split into several coherent sub beams which interfere on the surface of the sample. This technique facilitates various metallurgical processes such as melting, recrystallization, recovery and the formation of intermetallic phases for example on the lateral scale of the microstructure but also topography effects in metals, ceramics or polymers. With regard to topography effects, laser surface texturing is for many years an established method to reduce stiction in magnetic storage devices or to enhance the tribological properties by the production of micro dimples serving as lubricant reservoirs. In this research work, we will focus on the microstructural tailoring of metallic multilayer thin films i.e. TiAl multilayer films. The idea is to create lateral periodic intermetallic phase composites consisting of hard and soft regions and therefore providing an improved wear resistance. Many classical wear theories only emphasize the importance of hardness as the main factor influencing wear resistance. According to Archard´s equation, the volume loss per sliding distance is linearly proportional to the applied normal load F_N and reciprocal to hardness H. Recent studies revealed the relevance of the ratio of hardness to Young´s modulus (H/E), called the elastic strain to failure. Therefore, thin films being composed of hard hard intermetallic phases laterally arranged in a ductile matrix could exhibit superior properties. By controlling the size and distribution of the corresponding phases, it could be possible to make a balance between hardness and elastic modulus which is decisive with respect to tribological applications.
1:50 PM		E5-2 Nanotribological Properties of CrN Films Deposited in an Industrial Chamber by HIPIMS and DC Magnetron Sputtering Esteban Broitman, Grzegorz Greczynski, Lars Hultman (Linköping University, Sweden) The microstructure, composition, throwing power, mechanical, and nanotribological properties of CrN_x films grown by reactive high power pulsed magnetron sputtering (HIPIMS) and direct current magnetron sputtering (DCMS) have been studied as a function of the substrate inclination angle α, defined as the angle between the substrate and target normal directions. Films were deposited on Si(001) substrates using an industrial chamber CC-800/9 from CemeCon. Similar conditions for nitrogen pressure, DC bias, and discharge power were used for both, HIPIMS and DCMS depositions. For all angular positions 0 ≤ α ≤ 180^o, DCMS yield thicker films than HIPIMS. However, the last ones have a lower variation in thickness. XPS characterization shows a very low variation in stoichiometry with α. Mechanical and nanotribological properties measured with a Hysitron TI 950 TriboIndenter^TM reveal that HIPIMS films have higher hardness and lower wear rate than DCMS ones. For a load of 680 µN and a diamond Berkovich probe, HIPIMS films have a uniform hardness of 17 ± 1 GPa, while the hardness of DCMS films varies with α from 7 to 13 GPa. ScanningWear^TM tests using a 1 µm 90º conical probe with a force of 75 µN shows that HIPIMS films have an average wear depth of 6 ± 1 nm for all α values, while the wear depth for DCMS films varies from 12 to 28 nm. The superior throwing power and uniform mechanical and nanotribological properties for HIPIMS films can be correlated to the higher Cr ionization in the plasma, as determined by optical emission spectroscopy.
2:10 PM		E5-3 Nano-/Micro Scale Fretting and Reciprocating Wear of Thin Films and Si(100) Ben Beake (Micro Materials Ltd, UK); Tomasz Liskiewicz (University of Leeds, UK); Jim Smith (Micro Materials Ltd, UK) The accelerated nano/micro-wear capability in a commercial ultra-low drift nanomechanical test system (NanoTest, Micro Materials Ltd) has been used on a range of hard and soft thin films on Si and on uncoated Si(100) to optimise for enhanced durability and gain mechanistic understanding of the processes controlling the onset of nano-wear in these materials. The nano-wear test fills a key measurement gap in terms of pressures, forces, contact areas and sliding speed between AFM based wear testing and classical macrotribometers and hence more directly maps onto contact conditions in a range of coating applications including automotive engines and biomedical devices. In situ monitoring of nano-wear over periods of several hours enables tests to be usefully run for greater duration and at lower contact pressures than is typical in previous nanotribological tests. The influence of applied load and its rate of application on the behaviour of uncoated Si(100) in 10000 cycle nano-fretting wear tests with small spherical indenters has been investigated. Damage morphology is contrasted to that after other mechanical contact situations such as nanoindentation, nano-scratch or impact.
2:30 PM		E5-4 The Effect of Environmental and Contact Conditions on Micro-Tribology Experiments on Engineering Coatings Mark Gee, J.W. Nunn, L.P. Orkney (National Physical Laboratory, UK) Micro-tribology experiments were carried out with a novel micro-tribology test system to investigate the friction and wear performance of a selection of engineering coatings in model single asperity micro-tribology experiments. Single and multiple pass experiments were carried out at a range of test loads from 50 mN to 200 mN, and with a range of probes including diamond indenters with tip radii from 1 to 200 micrometres and a 2 mm steel ball. The tests were made in moist air, dry nitrogen and vacuum. The tests made under vacuum made use of the in-situ SEM capability of the micro-tribometer yielding a near realtime image record of the deformation that had taken place. The coatings that were examined included TiN, TiAlN, CrN, MOST, and various types of DLC coatings. The deformation that took place was evaluated with confocal optical microscopy and AFM. It was found that the friction and wear that was found was dependent on both the contact geometry and the test environment. These results are discussed with respect to observations of the scratches made with a high resolution SEM.
2:50 PM		E5-6 Nanomechanics of Thin Films: a Cross Sectional Approach Carlos Alberto Botero Vega, Emilio Jimenez-Piqué (Universitat Politècnica de Catalunya, Spain); Tushar Kulkarni (Boston university); Luis Miguel Llanes Pitarch (Universitat Politècnica de Catalunya, Spain); Vinod Sarin (Boston University) Nanoindentation has become one of the most widely used techniques for measuring mechanical properties of thin films. Conventionally, nanoscratch and nanoindentation tests are performed on the surface of the films to evaluate its mechanical integrity, elastic modulus and hardness. However, in complex systems such as compositionally graded thin films, small spatial variations in mechanical properties are difficult to distinguish using this approach. In this work, polished cross sections of functionally graded CVD mullite coatings on silicon carbide substrates have been evaluated. To assess the intrinsic mechanical properties and their spatial variation, nanoindentation tests have been carried on mullite coatings with constant and graded Al:Si ratios. Additionally, transverse nanosctratch tests to evaluate the adhesive and cohesive resistance of the coatings and the structural integrity of the system were preformed. Different damage morphologies were identified at the interface and inside the coating by using complementary characterization techniques. In the case of functionally graded coatings a gradual increase in the hardness and elastic modulus with increasing distance from the substrate/coating interface was observed. Nanoscratch on the cross sections allowed the determination of the critical loads for adhesive and cohesive damage. Graded coatings exhibited the best combination of properties for structural applications.
3:10 PM		E5-7 Sub-Micro-Pillar Compression Tests on Nanocrystalline Nickel Tribofilms Corbett Battaile, Somuri Prasad, Joseph Michael, Brad Boyce (Sandia National Laboratories) Frictional contact on metal surfaces can produce complex micro- and nano-structural changes underneath the wear surface. Our previous work has shown that the formation of thin, nanocrystalline material in the subsurface region can result in substantial reduction of the coefficient of friction between a hard sphere and a single crystal of nickel. A direct measurement of the mechanical properties of this tribofilm would be very useful in explaining the friction transitions associated with this phenomenon, but such an endeavor is complicated by the fact that the wear surfaces are rough, and the tribofilm is usually less than 200 nm thick, making quantitative nanoindentation difficult. In order to circumvent these obstacles, wear tracks were created on a {110}-oriented single crystal of nickel in both <110> and <211> directions, by unidirectional sliding of a Si3N4 sphere in a dry nitrogen environment. A focused ion beam was used to mill sub-micro-pillars into both the worn and unworn nickel surfaces. These pillars were cylindrical in shape, with a slight taper, and approximately 250 nm in diameter and 500 nm high. The mechanical properties of the pillars were directly measured by performing compression tests in a nanoindenter with a flat 12 µm diameter platen. The worn material was found to be approximately two to four times softer than the unworn crystal. In addition, the unworn material exhibited significant work hardening, whereas the worn material showed virtually none. Scanning electron microscopy showed that deformation in the worn material was localized near the tops of the pillars, whereas deformation in the unworn material occurred throughout the lengths of the pillars. In this presentation, we will outline the tribological tests used to create the worn material; describe the technique for creating the sub-micro-pillars; and discuss the implications of the results for both the grain size dependent strength, and the tribological properties, of the nanocrystalline tribofilms.
3:30 PM		E5-8 Nanoscale Mechanical Imaging of Multilayered Films for Flexible Display Using Contact Resonance Force Microscopy Jun-Hee Hahn (Korea Research Institute of Standards and Science, Korea); Dae-Hyun Kim, Hyo-Sok Ahn (Seoul National University of Science and Technlogy, Korea) Contact resonance force microscopy (CRFM) has been used to evaluate the elastic properties of multilayered films for flexible display: LiF, Alq3 and aluminum. With CRFM, quantitative images of the spatial distribution in nanoscale elastic properties were acquired. The average values of plane strain modulus for each layers were 62.1 ~ 65.4 GPa, 11.5 ~ 13.0 GPa and 74.8 ~ 76.3 GPa for the LiF, Alq3 and aluminum, respectively. Obtained values showed good agreement ( ~ 15 % difference) with values determined by nanoindentation. These results provide insight into using CRFM methods to attain reliable, accurate measurements of elastic properties on the nanoscale.
3:50 PM	Invited	E5-9 Effect of Surface Coating Topography on the Tribological Properties of Nanoparticle Films Mustafa Akbulut (Texas A&M University) In the tribology research community, efforts are underway to develop improved lubricants and lubricant additives to improve energy efficiency. With the advent of nanotechnology, research into lubricants and lubricant additives has experienced a paradigm shift. New nanomaterials and nanoparticles are currently under investigation as lubricants or lubricant additives because of their unusual properties as alternatives to traditional materials. Most of these investigations focus on the nanoparticle film friction on one type of surface rather than investigating nanoparticle friction in relation to surface roughness and structure. By contrast, it has been shown that even a 1-2 nm roughness can significantly affect adhesion and friction in the case of dry lubrication. However, to date, there is still no general theory for the interactions of rough and structured surface coatings across either nanoparticle films or any systemic experiments that identify the main trends. Therefore, it is increasingly necessary to correlate adhesion and friction of nanoparticle films to surface roughness morphology and compliance. This presentation will highlight the current state of art on these issues and describe our recent results on effects of surface coating roughness, structure, and mechanical properties on the adhesion and friction interactions across nanoparticle films.