Frictional and Adhesive Properties of Diamond-like Carbon/ Silicon Nitride Nanocontacts Erin E. Flater 1,2,3 , Jeffrey R. VanLangendon 1,2,3 , Erik H. Wilson 1,4,5 , Kumar Sriharan 1,4,5 , Robert W. Carpick 1,2,3,6,7 1 Department of Engineering Physics, 2 Engineering Mechanics Program, 3 Mechanics and Materials Research Program, 4 Nulear Engineering and Engineering Physics Program, 5 Center for Plasma Aided Manufacturing, 6 Rheology Research Center, 7 Materials Science Program University of Wisconsin-Madison, 1500 Engineering Drive, Madison, WI 53706, USA ABSTRACT Diamond-like carbon (DLC) is a unique material that exhibits both low friction and high hardness. This material is being considered as a coating for a wide range of applications, including micromachines, where friction and adhesion play a critical role in performance. In the present study, we seek to understand more fully the fundamental relations that govern the tribology of DLC at the nanoscale. In particular, we wish to understand the way in which humidity tends to reduce the superior frictional properties of DLC. Coatings of DLC were deposited on silicon flats using the plasma source ion deposition process. These coatings are studied using atomic force microscopy, where a nanoscale tip is placed in contact with a sample to measure relative adhesive and frictional forces. Dependence of friction and adhesion on relative humidity, load, and sliding history for a DLC/ silicon nitride interface are discussed. INTRODUCTION Friction is a ubiquitous physical phenomenon that is not well understood on a fundamental level. Most surfaces are rough on small scales, although macroscopically they may appear smooth. Surfaces that are apparently in complete contact are in fact only in contact at raised points, or asperities, which complicate surface interactions during sliding. The frictional behavior of a single asperity should be studied in order to obtain a clearer understanding of the most basic processes involved. Furthermore, understanding friction and wear on the nanoscale is especially important for the development of devices that work on the micro- or nano-scale, where surface forces dominate [1]. Diamond-like carbon (DLC), an amorphous solid composed of sp 2 - and sp 3 -bonded carbon, is a useful coating material for many applications. In particular it holds promise for application in micro-electro-mechanical systems (MEMS), since DLC exhibits low friction and high hardness [2] . The frictional properties of DLC depend strongly on environmental humidity, whereby the superior low friction behavior wanes at higher humidity. The mechanisms that govern the relationship between friction Figure 1: A correlation average from the tip calibration sample, which gives an approximation of the tip shape. and humidity for this material are not well understood. This paper presents a characterization of DLC film behavior in a humidity-controlled environment. EXPERIMENTAL PROCEDURE DLC films were deposited on silicon wafers using the non- line-of sight plasma source ion deposition process, developed at the University of Wisconsin-Madison [3]. Atomic force microscopy (AFM) was performed on the DLC to characterize its tribological properties. In AFM a cantilever with a nanoscale tip is brought into contact with a surface. Deflections of the tip and cantilever are measured using a laser beam reflected off the back of the cantilever. The bending and twisting of the cantilever as it moves across a surface provides a measure of normal and friction forces that act between the tip and the sample with sub-nanometer and sub-nanoNewton precision. A Nanoscope IV AFM (Digital Instruments, Santa Barbara, California) with a silicon nitride AFM cantilever was used for this study. The cantilever used was of rectangular geometry, with a manufacturer’s spring constant of 0.05 N/m for cantilever bending. Batch processing of AFM cantilevers by chemical vapor deposition causes considerable variation in the cantilever spring constants. Thus, this nominal value allows for the calculation of tip- surface forces only within an order of magnitude.