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Wear 263 (2007) 1545–1550

Case study

Tribological properties of commercial optical disks estimated fromnanoindentation and scratch techniques

J. Rodrıguez a,∗, A. Rico a, V. Soria b

a Departamento de Ciencia e Ingenierıa de Materiales, Universidad Rey Juan Carlos, 28935 Mostoles, Spainb ICMUV-Departamento de Quimica Fisica, Universitat de Valencia, 46100 Burjassot, Valencia, Spain

Received 31 July 2006; received in revised form 20 December 2006; accepted 7 January 2007Available online 23 May 2007

bstract

The structure of optical disks is a complex superposition of several layers with different objectives. The most external layer is usually designedith a protective purpose. When the scratch of the substrate occurs, the optical properties of the device decrease inducing a deficiency in the

torage or access of data. Nowadays, the latest commercial optical disks exhibit protective coatings made of polymeric materials (copolymers, andolymeric matrix composite materials). The efficiency of these layers depends on a combination of several mechanical properties like hardnessnd 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

etermine the composition of the external polymeric layers. Nanoindentation tests were done at different maximum loads to determine mechanicalroperties like hardness, H, and Young’s modulus, E, and their load dependence. The scratch resistance and the friction coefficients were determinedrom 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 explainhe wear resistance of the tested materials.

2007 Elsevier B.V. All rights reserved.

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eywords: Optical disks; Scratch resistance; Nanoidentation; Polymers

. Introduction

Storage of data is one of the most important needs nowa-ays. Several solutions have been developed in recent decadeso optimize the devices designed for this aim. Optical discs seemo solve two essential characteristics related to storage of data:ortability and high storage capacity. Their main drawback ishe extreme sensitivity to mechanical damage. When the sub-trate appears scratched, the optical properties are decreasednd, consequently, a deficiency in the storage performance isnduced [1].

The structure of a typical optical disk is a complex superpo-ition of several layers with different objectives [2]. Over theecording layer, usually made of metallic alloys, a cover layer

s coated with a protective function. It is traditionally made ofolycarbonate, because this polymer is transparent to the laseream. To enhance the protective role, the latest optical disks

∗ Corresponding author.E-mail address: jesus.rodriguez.perez@urjc.es (J. Rodrıguez).

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043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.wear.2007.01.124

ave begun to incorporate protective coatings made of newdvanced materials (copolymers, and polymeric matrix com-osites). These coatings maintain the transparency to the laseream and increase the scratch resistance due to their superiorechanical properties (hardness, stiffness). Usual thickness of

he cover layer is around 100 �m, and the hard protective coat-ngs exhibit a wide range of thickness typically from 2 to 50 �m3,4].

In this study, experiments focused on the scratch resistancef several protective coatings used in commercial optical disksere performed. The aim is to establish correlations betweenechanical properties and scratch resistance of the polymeric

oatings. Due to the low thicknesses of the layers involved,anoindentation technique was used to determine propertiesuch as hardness and Young’s modulus [1,5–7]. The scratchesistance was also evaluated in this small scale. Finally, theonfidentiality of the fabrication details impedes to know how

he analysed disks are exactly manufactured. To identify theoating type, composition and morphological structure infraredIR) analysis and environmental scanning electron microscopyESEM) were used.

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. Materials

Four optical disks, which can be found in the market,ere studied in this work. They exhibit a multilayer structure.lthough all of them have a polycarbonate substrate, three of

he disks present protective layers made of advanced materialss was introduced in the previous section. A single polycarbon-te disk was used as reference material in the comparison toore specific protective coatings.

. Experimental techniques

.1. IR analysis

To determine the composition of the target layers, severalpecimens were prepared to make an IR analysis. The IR spec-ra were recorded by a NEXUS spectrometer, (ThermoNicolet

orp., Madison, WI, USA). In all cases, at least 64 scans with anccuracy of 2 cm−1 were signal-averaged. The frequency scaleas internally calibrated with a He–Ne reference to an accuracyf 0.2 cm−1 and externally with polystyrene. The optical disks

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Fig. 1. Infrared spectra

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ere placed on a horizontal holder for beam exposure, accordingo the alternative total reflection (ATR) spectroscopy technique,nd data processed with OMNIC software.

.2. Environmental scanning electron microscopy (SEM)

Samples were cross-sectioned using a micro-cutter Struersccutom-5 and prepared with conventional metallographic

echniques. Specimens were observed by means of an environ-ental scanning electron microscope (ESEM) Philips XL-30 to

valuate the internal layered structure of optical disks. Layerhicknesses were also measured.

.3. Nanoindentation

Nanoindentation tests with a Berkovich tip were carried out

n a MTS XP nanoindenter to determine the Young’s modulusnd hardness following the Oliver and Pharr methodology [8].he tests were performed at different peak loads: 1, 5, 10 and0 mN.

of the samples.

J. Rodrıguez et al. / Wear 263 (2007) 1545–1550 1547

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oremarkable: sample 4 (polyacrylate + SiO2) is clearly harder (itexhibits the lowest penetration) and stiffer (it exhibits the high-est slope in the unload branch at maximum load) than the otherthree samples. The values of the Young’s modulus and hardness

Fig. 2. ESEM images showing the layered

.4. Nanoscratch

Scratch tests were performed by applying a constant load todiamond Berkovich tip during a scratch length of 30 �m (the

ip displacement was perpendicular to one of its edges). Appliedoads ranged from 5 to 900 �N. TRIBOSCOPE® (Nanomechan-cal Test Instrument Hysitron Inc.) was employed to make thecratch on the layer surface with a scratch velocity of 0.2 �m s−1.oefficients of friction were also recorded during the scratch

est. The final scratch track width was evaluated taking 3-Dmages on the polymeric surfaces by using an AFM microscopehich scans at 0.1 Hz in 512 lines per area.

. Results

.1. Composition of the external layers

IR spectra allowed the identification of the main compositionf the protective layers. As was expected, the external layer ofample 1 is polycarbonate. Sample 2 is coated with an acrylicoating, sample 3 with a Tris(2-hydroxyethyl)isocyanurate tri-crylate layer, and sample 4 is coated with a composite materialomposed by polyacrylate + SiO2, as derived from peaks at 1050nd 550 cm−1, respectively (see Fig. 1).

.2. Layers morphology

In Fig. 2, ESEM images from the external layers of the disksre presented. The average thickness of the coatings ranges from500 �m in sample 1 (polycarbonate) to 8 �m for the sam-le 2 (acrylic coating). Sample 4 (polyacrylate + SiO2) exhibits

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ture of the optical disks used in this work.

n intermediate value of 15 �m, approximately, and sample(Tris(2-hydroxyethyl)isocyanurate triacrylate)shows a 30 �m

hickness external layer.

.3. Nanoindentation test

In Fig. 3, an example of the force–depth of penetration curvesbtained from nanoindentation is shown. Several aspects are

ig. 3. Nanoindentation load–displacement curves using 1 mN as indentationeak–load.

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Fig. 4. Mechanical properties of the optical discs, H

re plotted against the depth of penetration in Fig. 4. There isstrong dependence on the mechanical properties with the sizef the indentation due to the substrate influence in the nanoin-entation tests carried out at high loads. Interestingly, sample(polyacrilate + SiO2) is the hardest and stiffest at lower pen-

trations, but when the depth of penetration is increased itsechanical properties decrease up to the smallest values. The

ther three samples also present size dependence; nevertheless,he tendency for these samples is smoother than sample 4. Its necessary to point out that Young’s Modulus and hardnessalues evaluated from nanoindentation are usually higher than

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ig. 5. AFM images showing 3-D scratch track profiles carried out using a scratch loamples [9].

ess and Young’s modulus vs. depth of penetration.

hose measured by other mechanical techniques [9]. These dif-erences may be due to plasticity dominated phenomena likeile-up [10,11].

.4. Scratch test

Fig. 5 shows an example of the AFM images captured after

he scratch tests. The scratch profile is used to determine thecratch track widths. They are plotted versus the scratch loadn Fig. 6. This is the parameter used as a measurement of theample wear resistance [12]. The scratch track width for the

ad of 900 �N. Note the pile-up effect in the polycarbonate and acrylic coated

J. Rodrıguez et al. / Wear 263 (2007) 1545–1550 1549

Fig. 6. Scratch track width measured from 3-D scratch track profiles vs. scratchload.

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- Nanoindentation techniques provide an appropriate methodol-

ig. 7. Average coefficient of friction measured from scratch tests vs. scratchoad.

wo-layer poliacrylate + SiO2 sample is the lowest for the wholeoad range considered. In spite of its lower hardness, the tri-crylate sample exhibits similar track width values than theolyacrylate + SiO2 sample. It is also remarkable that the scratchesistance of the acrylic coating is higher than that of the poly-arbonate although no significant differences in hardness arebserved.

In Fig. 7, the average coefficient of friction measured duringcratch tests is presented as a function of the scratch load. Asn the curve of the scratch width, results obtained can be orga-ized in two groups. Polycarbonate and acrylic coating exhibitriction coefficients higher than 0.55. Additionally, a consider-ble load dependence is observed on the polycarbonate disks,eading to friction coefficients over 0.7 at the maximum load of00 �N. On the other hand, the more advanced coatings (triacry-aye and polyacrylate + SiO2) present load independent values

n the range analyzed. Both coatings have friction coefficientlose to 0.4–0.45.

Fig. 8. H/E ratio vs. depth of penetration.

. Discussion

In a first approximation to the scratch resistance of a mate-ial, it seems logical to assume that hardness is the key factor.evertheless, a more in depth approach usually points out a

ombination of properties. In fact, in Fig. 4, the variation of theardness with the depth of penetration makes difficult to predicthe wear resistance of the polymeric samples regarding only theardness variable. In addition to that, coefficient of friction isnsufficient to describe the tribological behaviour of the sam-le; for example, the sample 3 exhibits the lowest values but itsear resistance is smaller than that of the sample 4, as it can be

ppreciated in Fig. 6.One of the combinations of variables which usually describe

ore appropriately the tribological performance of a polymers the ratio hardness/stiffness [13]. The values of H/E corre-ponding to the samples tested in this study are also included inig. 8 plotted against the depth of penetration. In spite of the sizeependence, which makes difficult a direct correlation betweenhe wear resistance and the H/E ratio of the tested materials, thisarameter can be used to order the samples in the same way asn Fig. 6, the higher the H/E ratio the lower the scratch trackidth.Although, some preliminary conclusions can be derived from

he experiments performed in this case study, further researchs needed to enlighten more complex aspects. How relevant arehe nanoindentation and nanoscratch results to the actual serviceituation? Are the presented results very much affected by theeometry of the tips used? Are similar results expected frompherical indentation?

The answers to these questions and those related to the phys-cal mechanism of wear are under investigation.

. Conclusions

ogy to determine relevant mechanical properties of protectivecoatings used in the manufacturing of optical disks, because

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tests at penetration depths quite lower than thicknesses oflayers can be performed.Some degree of correlation between mechanical prop-erties measured by nanoindentation techniques and thescratch resistance can be established. In fact, the ratio H/Eseems to be useful to order materials, at least qualita-tively.Protective coatings made of polyacrylate + SiO2 and Tris(2-hydroxyethyl)isocyanurate triacrylate exhibit higher scratchresistance and lower friction coefficient than those corre-sponding to conventional disks.

cknowledgements

The authors thank ELM-Digitalia S.L. (Valencia, Spain)or providing optical discs samples and J. Abati for drawingis attention to hard-coating in optical discs. Authors are alsondebted to Comunidad de Madrid for the financial support to. Rico through grant order No. 5963-2005.

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