ICMCTF2006 Session E3-3: Fracture, Adhesion, Friction and Wear

Strength, friction, and wear are dominant factors in the performance and reliability of materials and devices fabricated using microsystem technologies. While adequate for some applications, as-fabricated strength and wear properties severely restrict use of these devices in many dynamic applications. Applying coatings and films is one method to enhance device performance and reliability. Atomic Layer Deposition (ALD) is a method ideally suited for applying these films as it creates conformal, stress free, and well-adhered films. Of particular interest are ALD tungsten and alumina films where bulk crystalline counterparts exhibit high hardness and good frictional resistance. We have therefore begun a study of ALD tungsten and ALD tungsten and aluminum oxide nanolaminate fracture properties. The tungsten films were deposited to thicknesses ranging from several monolayers to 200 nm on mirror-polished silicon. Nanolaminates were then created from sequential deposition of tungsten and aluminum oxide films in four, eight, and sixteen bi-layer systems to form 100 nm thick films on silicon substrates. Following deposition, the fracture resistance of each film system was measured using nanoscratch techniques at loads characteristic of microsystem operation. The single layer ALD tungsten films exhibited a pronounced susceptibility to channel cracking and delamination while the nanolaminate films exhibited a decrease in fracture susceptibility that varied with bilayer thickness. In this presentation, we will use mechanics based fracture models to show that fracture resistance scales with film structure and bilayer thickness providing a means to tailor wear resistant film performance.

This work was supported by Sandia National Laboratories, a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

The thermomechanical properties of C-fiber reinforced Cu-C Metal Matrix Composites which are potential materials for high performance heat sinks depend crucially on wetting and adhesion between the chemically immiscible elements Cu and C. This work presents two approaches to adhesion manipulation: (i) the activation of the C-surface by a treatment in Nitrogen (N₂) Radio Frequency (RF) plasma and (ii) the deposition of a Mo-interlayer on the C-surface. Both approaches significantly increase the adhesion of 4 µm thick Cu-coatings deposited by magnetron sputtering immediately after pre treatment. Heat treatment (30 min, 800°C, High Vacuum furnace) leads to a drastic loss in adhesion for the plasma treated samples while the samples containing the Mo-interlayer retain excellent adhesion values. Thermal cycling experiments (RT - 500°C) with X-Ray Diffraction (XRD) measurements of the tensions in the Cu-coating show comparable results: The Cu-coating on the plasma treated sample delaminates after one cycle. The sample with the Mo-interlayer can go through several cycles and is able to sustain thermally induced tensions. The difference in the response of the two sample types to post deposition thermal treatment can be tracked back to the de-wetting behavior of Cu on the different substrates. Void formation is observed at the Cu-C interface in the case of plasma treatment but not for samples with a Mo-interlayer.

This work is supported by the Austrian Science Fund (FWF) under grant Nr. P-14534.

Adhesion is one of the fundamental parameters to characterize mechanical strength of thin films on substrates. Quantitative evaluation of adhesive strength of copper thin film on flexible polyimide film, which is used as a flexible print circuit, is tried in this research by using a new technique. Conventional evaluation methods, ex. peel test, have already been developed, and they are simple and easy to perform. However, extraction of information purely on the strength of interface is not so simple because the effect of plastic deformation can be hardly removed from the data obtained with conventional methodssuper[ 1,2].

Kamiya et al.^[3] recently developed a new technique to evaluate the toughness of interface, where interface crack was effectively extended by a local load with a minimum amount of plastic deformation in the film and substrate. This technique has already been successfully applied for the case of hard coatings, and its applicability is newly examined in the present study with a combination of softer film and substrate. The specimens were made of the polyimide film, which was diced like a block on the copper film, and loaded parallel to the film surface with a diamond needle until it was scratched off. Interface crack extension process was observed in detail by letting the paint penetrate into the crack during the experiment. The same process was also numerically simulated under the condition of constant energy release rate and compared with the results of experiment. The interface fracture energy of the specimen was determined so that it gives the same simulation result as obtained in the experiment, which was 110 J/m².

^[1] A. K. Moidu, A. N. Sinclair, J. K. Spelt, J. Test. Eval., 26(1998), 247-254.

^[2] J. Y. Song, J. Yu, Acta Mater., 50(2002), 3985-3994.

^[3] S. Kamiya, H. Nagasawa, K. Yamanobe, M. Saka, Thin Solid Films, 473(2005), 123-131.

Evaluation of mechanical (adhesion to the substrates, hardness and elastic modulus) and tribological (scratch and wear resistance) properties of nano-coatings is important for a high-tech industry.

The present work and its novel technology cover a wide range of thin films on various ceramic and plastic substrates, for example, flat-panel displays, magnetic disks and heads, optical disks and lenses, etc. The following test procedures were used for the comprehensive evaluation of hard-coated metal samples:

- Nano-indentation tests for nano-hardness and elastic modulus evaluation,

- Reciprocating wear tests for evaluation of thin-film friction and durability,

- Scratch-hardness tests under constant load for scratch-resistance and micro-hardness,

- Scratch-adhesion tests under increasing load for thin-film adhesion and toughness.

All the tests were performed on the same tester, re-configured for each type of test within a minute by installing corresponding replaceable modules. The Universal Nano & Micro Tester with multiple sensors (indentation and wear depth, normal and lateral forces, contact electrical resistance, acoustics, temperature) and integrated both atomic force and optical microscopes has been utilized to characterize mechanical properties of thin films, with in-situ monitoring their changes during micro and nano indentation, scratching, reciprocating and other tests.

Clear differences between ductile and brittle films, as well as between different deposition technologies, have been observed. For many films there was an excellent correlation of their nano-hardness with wear and scratch resistance, for some films there was a lower degree of correlation due to either different wear mechanisms or poor adhesion. The multi-sensing technology with in-situ digital optical microscopy and periodic AFM imaging helped discover and understand the differences.

Numerous studies have shown that the displacement amplitude has a critical impact on the fretting damages, it may promote cracking under small partial slip amplitude and wear under larger gross slip condition. Macroscopic fatigue stressing is classically superimposed to the component promoting a catastrophic propagation of the cracks initially activated by the fretting contact. Hence an optimized surface treatment is requested to display high wear resistance but also good performances against crack nucleation and crack propagation. Unfortunately these different aspects are usually studied separately.

The objective of this work is to shown how combined Fretting Wear (i.e only contact loadings are imposed) and Fretting Fatigue (i.e the contact loadings combined with fatigue stressing) test can provide a global overview of the surface treatment performance against fretting loadings. It is shown that these aspects can be quantified through a single and coherent methodology. Considering a representative pressure field a Fretting Wear analysis was conducted to identify the friction law, next the wear kinetics under gross slip conditions and the contact crack nucleation behaviour under partial slip. Applying similar partial slip fretting conditions, a fretting fatigue analysis has been performed to confirm the crack nucleation and propagation response as well as quantify the crack arrest conditions.

Transposed to a low carbon alloy (30NiCrMo), a duplex surface treatment (shot peening + HVOF WC-Co coating) has been investigated. It is shown and explained how and why this combined surface treatment displays very good compromise against wear and cracking fretting damages.