Development of a continuous microscratch technique in an atomic force microscope and its application to study scratch re
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A method to measure friction during scratching at linearly increasing loads in a commercial atomic force/friction force microscope (AFM/FFM) has been developed. The normal load was increased in small increments over the required range for the scratch using a software module while the friction signal was measured via a breakout box and data acquisition computer. Topography images of the scratch were obtained in situ with the AFM in tapping mode with minimal loss of damage event information. This technique was employed to study the scratch resistance of hard amorphous carbon coatings of thicknesses ranging from 20 nm down to 3.5 nm deposited by different commercially available deposition techniques on a silicon substrate. I. INTRODUCTION
Today, several applications including magnetic media storage and microelectromechanical systems (MEMS) require the use of ultrathin scratch and wear-resistant coatings that are less than 10-nm thick.1 Hence it is important to be able to evaluate the scratch resistance of such ultrathin coatings and to understand associated failure mechanisms. Scratch tests, in which a normal load is applied to a scratch tip and is gradually increased during scratching, have been widely used to measure adhesion and scratch resistance of coatings for some time.2 Such “continuous scratch” tests are used to identify the minimum normal load or critical load at which a failure event occurs (such as detachment of the coating or sudden increase in damage to the sample), which is used as a basis for comparing scratch resistance or adhesion of coatings.1– 4 Methods to determine critical load during scratching include monitoring acoustic emission5–9 (AE) and monitoring of tangential or friction force during scratching.6,8,10,11 In the latter method, the normal load at which a sharp increase in the friction force encountered by the tip occurs is usually considered to be the critical load. This method has been employed to study microscratch resistance and adhesion of thin coatings by several researchers using a commercial nanoindenter as well as other instruments.1 In such cases postscratch imaging is done with a separate instrument such as scanning electron or optical microscope, which is inconvenient and may also result in loss of damage event information during sample handling between the instruments. The atomic force microscope/friction force microscope (AFM/FFM) is another popular and effective tool to study micro/nanoscale tribological phenomena due to its ability to apply low loads (on the range of nano- to micronewtons) and high vertical and lateral resolutions. Commercial atomic force microscopes are extensively J. Mater. Res., Vol. 16, No. 2, Feb 2001
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used to study tribological and mechanical properties of materials, surface layers, and thin coatings. Scratching at constant loads can be performed using any commercial AFM/ FFM and has been commonly used to compare scratch resistance of coatings and materials.1,12 But the continuous scratch test (a
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