Wear 263 (2007) 1545–1550 Case study Tribological properties of commercial optical disks estimated from nanoindentation and scratch techniques J. Rodr´ ıguez a,∗ , A. Rico a , V. Soria b a Departamento de Ciencia e Ingenier´ ıa de Materiales, Universidad Rey Juan Carlos, 28935 M´ ostoles, Spain b ICMUV-Departamento de Quimica Fisica, Universitat de Val` encia, 46100 Burjassot, Valencia, Spain Received 31 July 2006; received in revised form 20 December 2006; accepted 7 January 2007 Available online 23 May 2007 Abstract The structure of optical disks is a complex superposition of several layers with different objectives. The most external layer is usually designed with a protective purpose. When the scratch of the substrate occurs, the optical properties of the device decrease inducing a deficiency in the storage or access of data. Nowadays, the latest commercial optical disks exhibit protective coatings made of polymeric materials (copolymers, and polymeric matrix composite materials). The efficiency of these layers depends on a combination of several mechanical properties like hardness and Young’s modulus. In this work, a compositional and mechanical study was carried out on four commercial optical disks. Infrared (IR) analyses were performed to determine the composition of the external polymeric layers. Nanoindentation tests were done at different maximum loads to determine mechanical properties like hardness, H, and Young’s modulus, E, and their load dependence. The scratch resistance and the friction coefficients were determined from nanoscratch tests. Atomic force microscopy was used to estimate the scratch track. The H/E ratio seems to be one of the key factors to explain the wear resistance of the tested materials. © 2007 Elsevier B.V. All rights reserved. Keywords: Optical disks; Scratch resistance; Nanoidentation; Polymers 1. Introduction Storage of data is one of the most important needs nowa- days. Several solutions have been developed in recent decades to optimize the devices designed for this aim. Optical discs seem to solve two essential characteristics related to storage of data: portability and high storage capacity. Their main drawback is the extreme sensitivity to mechanical damage. When the sub- strate appears scratched, the optical properties are decreased and, consequently, a deficiency in the storage performance is induced [1]. The structure of a typical optical disk is a complex superpo- sition of several layers with different objectives [2]. Over the recording layer, usually made of metallic alloys, a cover layer is coated with a protective function. It is traditionally made of polycarbonate, because this polymer is transparent to the laser beam. To enhance the protective role, the latest optical disks ∗ Corresponding author. E-mail address: jesus.rodriguez.perez@urjc.es (J. Rodr´ ıguez). have begun to incorporate protective coatings made of new advanced materials (copolymers, and polymeric matrix com- posites). These coatings maintain the transparency to the laser beam and increase the scratch resistance due to their superior mechanical properties (hardness, stiffness). Usual thickness of the cover layer is around 100 m, and the hard protective coat- ings exhibit a wide range of thickness typically from 2 to 50 m [3,4]. In this study, experiments focused on the scratch resistance of several protective coatings used in commercial optical disks were performed. The aim is to establish correlations between mechanical properties and scratch resistance of the polymeric coatings. Due to the low thicknesses of the layers involved, nanoindentation technique was used to determine properties such as hardness and Young’s modulus [1,5–7]. The scratch resistance was also evaluated in this small scale. Finally, the confidentiality of the fabrication details impedes to know how the analysed disks are exactly manufactured. To identify the coating type, composition and morphological structure infrared (IR) analysis and environmental scanning electron microscopy (ESEM) were used. 0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.01.124