Acta Polytechnica doi:10.14311/APP.2020.27.0121 Acta Polytechnica 27(0):121–125, 2020 © Czech Technical University in Prague, 2020 available online at http://ojs.cvut.cz/ojs/index.php/app BENEFITS OF USE OF ACOUSTIC EMISSION IN SCRATCH TESTING Lukáš Václaveka,∗, Jan Tomáštíkb, Hana Chmelíčkováa, Radim Čtvrtlíkc a Joint Laboratory of Optics of Palacky University and Institute of Physics of the Czech Academy of Sciences, Faculty of Science, Palacky University, 17. listopadu 12, 77146 Olomouc, Czech Republic b Institute of Physics of the Czech Academy of Sciences, Joint Laboratory of Optics of Palacky University and Institute of Physics AS CR, Olomouc, Czech Republic, 17. listopadu 50a, 77207 Olomouc, Czech Republic c Regional Centre of Advanced Technologies and Materials, Joint Laboratory of Optics of Palacky University and Institute of Physics of the Czech Academy of Sciences, Faculty of Science, Palacky University, 17. listopadu 12, 77146 Olomouc, Czech Republic ∗ corresponding author: lukas.vaclavek@upol.cz Abstract. Scratch test is regularly used for assessment of cohesive and adhesive strength of thin films and coatings. By default, its evaluation is based on analysis of depth-load-time record and microscopic observation of residual scratch groove. The visual analysis of the residual groove provides the most detailed description of the final damage of the surface (crack patterns, extent of plastic deformation, delamination, etc.), but it may be a time demanding approach. Although the continuous recording of indenter penetration depth and applied load offers instantaneous information about the performance of the tested material, it may not provide sufficient description of the sample’s deformation behaviour. Therefore, other complementary techniques for description of the deformation response to scratch loading are desirable. Continuous recording of acoustic emissions (AE) generated during the test could be a possible solution. It is especially the ability of AE method to detect the very first and even subsurface failures of the material that is of the most importance and otherwise inaccessible by standardly used techniques. What is more, it is a non-destructive and real-time method. In principle, AE method can be beneficially employed for a wide range of materials explored via scratch test. The strength of the AE analysis of the nano/micro scratch test will be demonstrated on various types of materials including optical thin films, durable metal and hard ceramic films as well as bulk laser cladding. Selected phenomena and features of the use of AE during scratch testing will be presented, including effect of scratch load on character of AE records (burst vs. continuous) for TiO2 on glass, subsurface damage of SiC films on silicon and selective failure of hard carbide phase embedded in a metal matrix in case of laser re-melted layers. Keywords: Acoustic emission, laser cladding, scratch test, thin films. 1. Introduction 1.1. Scratch test The scratch test is a standard tribo-mechanical test for determination of cohesive-adhesive properties of coated materials. The test is based on lateral move- ment of the loaded tip across the sample surface, while different types of damage and/or wear can be simulated depending on the experimental setup (tip geometry, load, loading rate, environment, etc.). In most cases, a progressive loading with a chosen maxi- mum is used. Nevertheless, abrupt loading can also be employed for example in wear test, where con- stant sub-critical load is applied several times over the same track [1]. In fact, various scenarios could take place under the action of the generated pressure field based on the nature of the film/substrate proper- ties. Depending on the actual loading conditions, the material deforms beneath the moving indenter and various types of failures could occur including brittle film cracking, tensile cracking, ploughing, and film delamination according to mechanical properties and adhesion of the studied film-substrate system [2]. The scratch test applicability for evaluation of lay- ered materials durability is given by the reliable de- tection of the onset of their failures, either in films, substrate or film/substrate interface. Standard meth- ods of scratch test evaluation are based on accurate identification of the critical loads representing on- sets of those failures by visual observation of residual scratch groove using a microscope and/or analysis of load-displacement-time data. Specific failure modes including different crack types along with plastic de- formation are usually observable using modern micro- scopic techniques with sufficient resolution. However, there are film-substrate systems, where initial crack- ing is not detectable by microscopic techniques or depth-change data. The examples may be cracking at the film-substrate interface or substrate cracking 121 http://dx.doi.org/10.14311/APP.2020.27.0121 http://ojs.cvut.cz/ojs/index.php/app L. Václavek, J. Tomáštík, H. Chmelíčková, R. Čtvrtlík Acta Polytechnica Figure 1. Acoustic emission: a) AE holder implemented in the NanoTest system, b) burst AE envelope, c) continuous AE envelope d) AE hit waveform; b) and c) represents data from thin optical film on glass substrate. Sample Film Deposition techniques Substrate Spherical tip radius [µm] Max. Load [mN] Thickness [nm] SiC film SiC Magnetron sputtering Si wafer 5 300 and 500 2500 Cr film Cr Magnetron sputtering steel (CSN 16 720) 5 and 10 500 250 TiO2 film TiO2 Magnetron sputtering glass 5 100 and 300 285 Laser cladding Remelted iron based powder Laser cladding steel (CSN 12 050) 10 500 ∼6x106 Table 1. Overview of tested samples and experimental conditions used in this study. under the opaque film [3]. Therefore, new reliable methods are desirable. Acoustic emission can be an appropriate solution for such cases [4, 5]. 1.2. Acoustic emission At macro scale, acoustic emission (AE) is established as a standard non-destructive method in defectoscopy for the interfacial and cohesion failure detection, local- ization and evaluation of material defects and cracks of large-scaled objects [6, 7]. The energy levels re- leased during macromechanical tests are many times higher than those released from the highly localized process volumes in nanomechanical and nanotribologi- cal tests [8]. Therefore, AE sensors have to feature an optimized design that is vital to achieve sufficient sen- sitivity and reliability for applications at nano/micro scale. Besides, the standard AE approaches and proto- cols have to be also adopted for specifics of nano/micro mechanical tests. The AE method principle is detection of the elastic waves generated by a sudden release of energy from a localized source in the material [9, 10]. Energy release occurs when the material undergoes changes in its in- ternal structure, for example due to cracking or plastic deformation arising from temperature gradients or ex- ternal mechanical forces. Typically, the piezoelectric material-based sensors attached to the sample are used for detection of AE signals (Fig. 1a). These sen- sors are capable of detecting very small events with low AE signal amplitude. Traditionally, the acoustic 122 vol. 27/2020 Benefits of use of Acoustic Emission in scratch testing emission record is represented by the so-called AE envelope. It is a time-compressed signal that provides a quick overview about AE activity during the test and can be categorized into two types: i) burst emis- sions and ii) continuous emissions. A burst emission is a discrete packet of waves associated with a single event, whereas continuous emission is considered as a superposition of many small events, see Fig. 1b and 1c. The AE envelope mainly gives information about the onset of the first cracks or an apparent change in failure mode. However, more throughout analysis of individual AE events can also be performed based on evaluation of the high-frequency sampled AE signal with sub-microsecond resolution. The discrete AE events are then represented by so called “AE hits” and can be further analysed in terms of their energy, and time and frequency parameters, see Fig. 1d. In this paper, the high frequency AE monitoring was used during scratch testing of different thin films, coatings and bulk in order to demonstrate the benefits and features of AE technique. In the following sections the effect of scratch load on AE character for optical TiO2 films on glass, subsurface damage of SiC films on silicon and selective failure of hard carbide phase embedded in a metal matrix in case of laser re-melted layers are shown. 2. Experimental settings Diverse types of samples with different mechanical properties were explored using scratch test. Specifi- cally, a laser cladding on steel, magnetron sputtered Cr film on steel, SiC film on Si and TiO2 on glass substrate, see Table 1. In all cases, samples were fixed using a low temperature wax on the special AE holder installed in the NanoTest instrument (MicroMaterials, Wales), where all the nano/micro mechanical tests were performed, see Fig. 1a. Scratch tests were per- formed with a fully calibrated NanoTest instrument at room temperature with diamond spherical indenters. Tips with end radius of 5 and 10 µm were used in order to generate different stress fields. During the tests, the normal force was linearly increased up to pre-set maximum (see Table 1) over the scratch track of 450 µm. Acoustic emission was recorded during the test using the ZEDO system (Dakel, Czech Republic). This AE system consists of a dedicated sample holder with a piezoelectric sensor and an in-built preampli- fier, a recording unit and a controlling software. AE signals were obtained with 16 bits sampling resolu- tion, sampling frequency of 10 MHz and frequency bandwidth from 30 kHz to 2 MHz. The scratch test evaluation was performed using a combination of three independent approaches: post process evaluation of residual scratch groove using microscopic techniques, the depth-change record analysis, and the acoustic emission envelope signal analysis. Measured samples along with used experimental details are listed in the Table 1. Figure 2. Scratch test performed up to 300 mN on SiC thin film: a) AE envelope b) residual scratch groove. 3. Results and discussion 3.1. TiO2 thin film – burst vs. continuous AE AE envelope recorded during scratch testing on trans- parent TiO2 film on transparent substrate is shown in Fig. 1. The TiO2 film with thickness of 285 nm on glass was tested up to 100 and 300 mN. It can be clearly seen that with the increase of normal load the character of AE signal also changes. A scratch test of the sample with a maximum load of 100 mN leads to the burst AE, induced by film cracking and breakage, see Fig. 1b. Increase of maximum load to 300 mN changes the character of the AE envelope to a continuous one, as shown in Fig. 1c. This change is caused by scratching through the film directly to glass substrate. Cracking of the glass is then responsible for strong continuous AE signal. 3.2. Sub-surface cracking of SiC coating on Si Sub-surface damage is a typical effect that might lead to a substantial overestimation of the tribolog- ical/mechanical durability of the film/substrate sys- tem if not considered. Despite the possibilities of up-to-date microscopic approaches, they are limited to detection of surface damage. In other words, very limited options for detection of the sub-surface dam- age are available. The effect of sub-surface damage on scratch test evaluation and beneficial use of AE method can be presented for amorphous SiC film mag- netron sputtered on Si substrate. A series of scratch tests with a maximum load of 300 mN and 500 mN were performed, similarly to [5]. High resolution visual observation with the laser scanning confocal microscope combined with the anal- ysis of depth-load-time record does not reveal any damage in the film tested up to 300 mN (Fig. 2) and only faint cracking beyond 460 mN (Fig. 3) sug- gesting high durability of the films Nevertheless, AE signal clearly reveals extensive subsurface cracking taking place already from 270 mN. Subsurface crack- 123 L. Václavek, J. Tomáštík, H. Chmelíčková, R. Čtvrtlík Acta Polytechnica Figure 3. Scratch test performed up to 500 mN on SiC thin film: a) AE envelope b) residual scratch groove. ing was directly confirmed on similar samples using the SEM-FIB cross-sectioning [5]. When we compare these values to the visually identified critical load for the first surface distinguishable cracks, then we can conclude that microscopic analysis overestimates the critical load by 70 %. 3.3. Effect of indenter radius on AE signal for Cr thin films on steel In general, acoustic emission has been known to be a very effective method for brittle materials. Nev- ertheless, it can be used also for systems of metal films deposited on steel substrates. Figure 4 shows an AE envelope recorded during a scratch test on Cr film sputtered on steel substrate up to 500 mN. Since scratch tests on non-coated steel surface did not produce any detectable AE signal it can be con- cluded, that AE is produced solely by the Cr film cracking. Microscopic analysis of the residual scratch grooves also supports that i) Cr film damage perfectly corresponds to the AE envelope as can be seen from Fig. 4 and ii) no signs of cracking were observed on the uncoated steel substrate (steel CSN 16 720). Scratch test is a very sensitive method to the used experimental conditions [11, 12] and direct compari- son between various studies have to be made with care. It is apparent that use of indenter with a different radius substantially changes the stress field beneath the moving indenter [13, 14]. Figure 4 also compares AE envelopes for the Cr film on steel substrate tested at the same experimental conditions with 5 µm and 10 µm sphere. It can be clearly seen that distinguish- able AE signal undoubtedly above the noise level was detected only for the test made with the indenter with smaller radius of 5 µm. 3.4. Laser cladding Although, scratch test has been used primarily for adhesion/cohesion testing of thin films, it may pro- vide useful information on mechanical behaviour of thick coatings deposited by laser cladding technology. In fact, these coatings with thickness up to several Figure 4. Cr thin film: a) AE envelope b) correspond- ing residual scratch groove made during the scratch test up to 500 mN with a sphere of radius of 5 µm, and c) AE envelope for the test performed up to 500 mN with a sphere of radius of 10 µm. Figure 5. Laser cladding sample: a) AE envelope graph and b) surface image with marked areas of cracks (bright spots represent carbide inclusions) millimeters, prepared by laser melting of micrometer size metal powder, may be considered as bulk. Here we report on a scratch test performed on a coating deposited from an iron-based mixture with 4.1 % of C, 32.0 % of Cr, 0.5 % of Si and 0.8 % of Mn on carbon steel substrate (steel CSN 12 050) using a fiber-coupled diode laser type LDM 3000-100 (wave- length 900-1070 nm). Although the depth changes records did not exhibit any dramatic fluctuations orig- inating from structural changes, a distinct AE signal was detected during the test. Subsequent microscopic observation of the residual groove revealed well defined microstructure with carbide inclusions. Comparison of the AE signal and microscopic images clearly points 124 vol. 27/2020 Benefits of use of Acoustic Emission in scratch testing out extensive cracking inside the carbide inclusions (see Fig. 5). This means that AE method is able to reflect microstructural features of the investigated material. 4. Conclusions Detection of acoustic emissions generated during the nano/micro-mechanical/tribological tests provides valuable information about deformation and failure mechanisms in the tested materials. It was demon- strated that application of the acoustic emission de- tection system during nano/micro scratch test proved to be very beneficial as it brings a capability to de- tect events previously undetectable by the standard scratch test evaluation approaches. Acoustic emis- sion can provide information about deformation and crack initiation in subsurface areas or film-substrate interfaces, which is undetectable neither by micro- scopic evaluation of residual scratch groove or from depth-change record data. The acoustic emission sig- nal, recorded simultaneously during the scratch test, can be clearly assigned to a certain point of resid- ual scratch groove, hence to the initiation of specific failure mode. Such analysis might reveal valuable information about deformation processes and their dynamics in both bulk as well as layered materials. Described AE method can be easily adapted to other local deformation or destructive methods like nanoin- dentation, pillar compression or cantilever bending tests. Acknowledgements The authors gratefully acknowledge the support by the Project TH03020245 of the Technology Agency of the Czech Republic and the Operational Pro- gramme Research, Development and Education, Projects Nos. CZ.02.1.01/0.0/0.0/17_049/0008422 and CZ.02.1.01/0.0/0.0/16_019/0000754 of the Min- istry of Education, Youth and Sports of the Czech Re- public. This work was also supported by the project IGA_PrF_2019_008 of Palacky University. References [1] L. Šimurka, R. Čtvrtlík, J. Tomaštík, et al. Mechanical and optical properties of SiO2 thin films deposited on glass. Chemical Papers 72(9):2143–2151, 2018. doi:10.1007/s11696-018-0420-z. [2] S. Bull. 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Proceedings of 30th Danubia Adria Symposium on Advances in Experimental Mechanics 2013. 125 https://doi.org/10.1007/s11696-018-0420-z https://doi.org/10.1016/0257-8972(91)90188-3 https://doi.org/10.3390/coatings8050196 https://doi.org/10.1007/s11837-019-03700-8 https://doi.org/10.1038/s41598-018-28704-3 https://doi.org/10.1007/978-3-540-69972-9 https://doi.org/10.5772/50270 https://doi.org/10.12989/sem.2015.54.6.1075 https://doi.org/10.1557/jmr.1998.0148 https://doi.org/10.1557/jmr.2003.0109 https://doi.org/10.1116/1.572845 https://doi.org/10.1016/j.msea.2005.09.121 https://doi.org/10.1016/S0257-8972(00)01097-5 Acta Polytechnica 27(0):122–126, 2020 1 Introduction 1.1 Scratch test 1.2 Acoustic emission 2 Experimental settings 3 Results and discussion 3.1 TiO2 thin film – burst vs. continuous AE 3.2 Sub-surface cracking of SiC coating on Si 3.3 Effect of indenter radius on AE signal for Cr thin films on steel 3.4 Laser cladding 4 Conclusions Acknowledgements References