Mechanical Characterization of Diamond-Like Carbon (DLC) Coated Polycarbonates
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MECHANICAL CHARACT!ERIZATION OF
DIAINXD-LIKE CARBON (DLC) COATED POLYCARBCNATES
ALEX J. HSIEH, PAUL HUANG, SHANKAR K. VENKATARAMAN , AND DAVID L. KOHLSTEDT ,VS Army Research Laboratory, Watertown, MA 02172-0001 Dept. of Chemical Engineering and Materials Science, U. of Minnesota, Minneapolis, MN 55455 ABSTRACT Evaluations of DLC coated polycarbonates have been carried out via and continuous high speed projectile impact, three point bend, microscratch tests. Radial and circumferential microcracks appeared on the back surfaces and near the regions of impact as a result of the high Despite the impact, the adhesion of the DLC speed impact stresses. coating in the area between the cracks remained, except in the immediate In the three point bend tests, transverse cracks vicinity of impact. developed and propagated in the DLC coating when the applied strain No delamination occurred in reached the fracture limit of the coating. the continuous microscratch tests; instead, cracking of the coating was Microcracking was the dominant observed in DLC coated polycarbonates. mode of failure which was consistent in these three diversed mechanical tests. Results of the SEM studies further confirmed microcracking prior to delamination as observed in the high speed impact tested coupons. INTRODUCTION Polycarbonates have a variety of military and civilian applications goggles for including helicopter canopies and aircrew windshields, soldiers and chemical laboratory personnel, and face protection for police and riot control personnel. Unfortunately, polycarbonates suffer from sensitivity to scratching as well as to liquid exposure with a loss of the desirable optical and physical properties. Diamond-like carbon (DLC) coatings have been demonstrated to provide polycarbonates with significant enhancement in the resistance to abrasion and to chemical attack. In addition, DLC coatings maintained the high Despite the performance speed impact strength of polycarbonate. improvements and recent advancements in thin film fabrication techniques, an understanding of the micromechanical properties of interfaces lags far behind. The objective of this work was to assess the adhesion strength of DLC continuous coated polycarbonates subjected to high speed impact, microscratch, and three point bend tests. The mode of failure of the DLC among these mechanical tests was compared. MATERIALS Injection molded polycarbonate coupons of 2.3 mm thickness coated The DLC with DLC were obtained from Diamonex, Inc. (Allentown, PA). coatings were deposited by direct ion beam deposition; two 11 cm ion beam sources were operated on a mixture of methane and argon gas for the Uniformity of the deposition thickness was coating deposition. approximately ± 1.5%. Typical coupons with 0.1 pm DLC appeared optically clear with a slight tan/brownish tint. Mat. Res. Soc. Symp. Proc. Vol. 308. @1993 Materials Research Society
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EXPERI1ENTAL HIGH SPEED IMPACT TEST Impact testing (1) was carried out using a fragment simulator projectile weighing 1.10 grams. Projectiles were
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