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Micro-Nano Tribology

Fatigue at Nanoscale: An Integrated Stiffness and Depth Sensing Approach to Investigate the Mechanisms of Failure in Diamondlike Carbon Film

[+] Author and Article Information
R. Ahmed1 n2

School of EPS,  Heriot-Watt University, Edinburgh, EH14 4AS, UK;College of Engineering, Alfaisal University, P.O. Box 50927, Riyadh, 11533, Saudi ArabiaR.Ahmed@hw.ac.uk

Y. Q. Fu

School of EPS,  Heriot-Watt University, Edinburgh, EH14 4AS, UK;Thin Film Centre, University of West of Scotland, Paisley, PA1 2BE, UK

N. H. Faisal

College of Engineering,  Alfaisal University, P.O. Box 50927, Riyadh, 11533, Saudi Arabia

1

Corresponding author.

2

Present address: Ahmed, Department of Mechanical Engineering, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, EH14 4AS, UK.

J. Tribol 134(1), 012001 (Feb 24, 2012) (8 pages) doi:10.1115/1.4005774 History: Received June 29, 2011; Revised January 02, 2012; Published February 21, 2012; Online February 24, 2012

Nanoscale impact fatigue tests were conducted to comprehend the relative fatigue performance and failure modes of 100 nm thick diamondlike carbon (DLC) film deposited on a 4 in. diameter Si (100) wafer of 500 μm thickness. The nanofatigue tests were performed using a calibrated TriboIndenter equipped with Berkovich indenter in the load range of 300–1000 μN. Each test was conducted for a total of 999 fatigue cycles (a low cycle fatigue test). Contact depth in this load range varied from 10 to 30 nm. An integrated contact stiffness and depth sensing approach was adapted to understand the mechanisms of fatigue failure. The contact depth and stiffness data indicated some peculiar characteristics, which provided some insights into the mechanisms of cohesive and adhesive failure in thin films. Based on the contact stiffness and depth data, and surface observations of failed DLC films using atomic force microscope and scanning probe microscopy, a five-stage failure mechanism is proposed. The failure of films starts from cohesive failure via cracks perpendicular to the film/substrate interface, resulting in a decrease in contact depth with number of fatigue cycles and no appreciable change in contact stiffness. This is followed by film delamination at the film/substrate interface and release of elastic stored energy (residual stress) resulting in an increase in contact stiffness. Finally, as the film breaks apart the contact stiffness decreases with a corresponding increase in contact depth.

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Copyright © 2012 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Dynamic model of indentation system (adapted from Ref. [25]); (I: indenter, F: film, and S: substrate)

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Figure 2

Fatigue at nanoscale: (a) 300 μN, (b) 500 μN, (c) 750 μN, and (d) 1000 μN loads (magnified view of data (enclosed oval windows))

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Figure 3

Indentation (or impact) cycle to adhesive failure (Nfa ) versus contact loads (lines between data points are polynomial fit of order 3, the error bars indicate the standard deviation of the data, x and y represents x-axis and y-axis data respectively, R represents correlation coefficient)

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Figure 4

SPM surface topography images of failures at (a) 750 μN and (b) 1000 μN loads. Corresponding force-gradient images are shown in (c) and (d). Circles indicate the periphery of adhesive failure at the film/substrate interface.

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Figure 5

Atomic force microscopy (AFM) images of failures at (a) 5000 μN and (b) 10 000 μN loads. Circles indicate the periphery of adhesive failure at the film/substrate interface.

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Figure 6

(i) Schematic of five stage failure mechanism in thin film-substrate system: (a) stage 1 showing increase in indentation depth (sinking-in), (b) stage 2 showing vertical cracks (similar to median vent) due to columnar structure and tensile stress at the edge of contact, cohesive failure starts, (c) stage 3 showing vertical cracks reach the interface leading to debonding at the film/substrate interface, (d) stage 4 showing debonding continues at the interface, and (e) stage 5 showing significant film failure. (ii) Schematic relationship between contact stiffness, contact depth, and the number of impact fatigue cycles identifying five stages of film failure[(S: substrate, F: film).

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Figure 7

(a) AFM surface topography of DLC film buckling using conical indenter, and (b) corresponding topography of buckled film at X-X

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Figure 8

Schematic of cantilever beam in contact with indenter (showing half space) (S: substrate, F: film)

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