The micro-wear technique and its application to ultrathin film systems
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A micro-wear testing technique has been developed by incorporating a piezoelectric pusher into an existing microindenter system. The pusher and its associated servo-control circuitry were designed to generate a precise reciprocating horizontal motion at the indenter tip for implementing a microscaled wear test. The information acquired from the test includes the wear loading curve, i.e., the normal applied load versus wear penetration depth, the friction force, and in turn, the apparent wear friction coefficient. Measuring the electrical resistance across the coating thickness is also possible if an electrical conducting indenter is utilized. Furthermore, in conjunction with the surface characterization tools, the wear morphology revealed useful information regarding the coating failure mechanism(s) and shed some light toward understanding coating tribology. The tester design concepts, operating procedure, data acquisition, and analysis will be examined. Experimental results on ultrathin carbon coatings with various thicknesses will be employed to illustrate the capabilities of the micro-wear tester.
I. INTRODUCTION
B. Tangential loadcell
By incorporating a PZT pusher, tangential force gauge, and electrical resistance measuring circuitry into a microindenter platform,1"* a new micromechanical testing technique has been developed to perform microscaled wear testing on thin coating systems. With the capability of continuously monitoring applied normal load, tangential force as well as simultaneous electrical resistance during the entire course of the wear process, this micro-wear technique provides powerful methodology to not only characterize the mechanical durability but also promote understanding of tribological issues of protective coating systems.
A PZT force sensor (© in Fig. 2), embedded between the PZT pusher and indenter translation carriage (® in Fig. 2), was used to monitor the tangential force applied on the indenter. With a load sensitivity of 0.57 gmf/V, the tangential loadcell is capable of detecting the minute force induced by the friction between the indenter tip and coating surface. The extraction of the interfacial friction force from the tangential force measurement will be discussed in more detail later. Two patterned leaf springs not only served as the suspension system for the indenter carriage, but also guided the indenter reciprocating movements in this newly developed tester. A properly configured leaf-spring suspension provides not only sufficient compliance along the wear direction but also enough rigidity otherwise to maintain good stability for the indenter motion.
II. APPARATUS A. PZT driver for reciprocating motion A PZT pusher5 was employed as a driver to provide a horizontal reciprocating motion for the indenter (® in Fig. 1). By adopting closed loop servo-control (Fig. 2) and using the position probe (© in Fig. 1) output as a feedback signal, the indenter achieved a nearly perfect oscillation motion with frequency ranging from 0 to 100 Hz. However, due to the current restrictio
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