WO2022007985A1

WO2022007985A1 - Adapter of sample holder for evaluation of mechanical durability of thin films and method for evaluation of quality of mechanical durability of thin films via this adapter

Info

Publication number: WO2022007985A1
Application number: PCT/CZ2020/000039
Authority: WO
Inventors: Radim Čtvrtlík; Petr Boháč; Jan Tomáštík; Miroslav Hrabovský; Lubomír Jastrabík; Lukáš Václavek; Karel Cvrk; Miroslav Veselský; Petr Abrhám; Václav Koula
Original assignee: Univerzita Palackého v Olomouci
Priority date: 2020-07-10
Filing date: 2020-08-24
Publication date: 2022-01-13
Also published as: EP4042138A4; CZ2020405A3; EP4042138A1; CZ308746B6

Abstract

The invention is an adapter of sample holder for pursuance of method for evaluation of quality of mechanical durability of thin films which is formed by a hollow body (1), whose shell (101) is from below closed with a lid (8) which allows fixation of the adapter to a console of the holder of testing device and from above with a contact front face (2), on which is not only from the top fixed a tested sample (3) consisting of a substrate (31) and on it deposited thin film (32) but also there is from the bottom fixed a piezoelectric sensor (4) of acoustic emission which is equipped with a mechanical damper (5) and a preamplifier (6) of an electric signal which is through a connector (41) taken out from the body (1), where the essence of the invention is that the shell (101) and the contact front face (2) are designed as an assembly of two individual structural parts with different material composition, whereas the contact front face (2) is made of a transparent material which has mechanical stiffness and stability with the value of Young's modulus of elasticity higher than 40 GPa and at the place of its fixation there is an inner peripheral recess (103) in the shell (101), in which is placed LED illumination (7) which is connected through a coupler (71) with a source of electric energy which is placed outside of the body (1). Likewise, is the invention the method of evaluation of quality of mechanical durability of thin films deposited on transparent substrate with use of continuous monitoring and record of acoustic emission during low cycle repetitive scratch test with subcritical force by the help of the adapter.

Description

Adapter of sample holder for evaluation of mechanical durability of thin films and method for evaluation of quality of mechanical durability of thin films via this adapter

Art Domain

This invention concerns testing of physical-technical characteristic of solid materials by application of mechanical stress to nontransparent functional thin film of tested material deposited on transparent substrate through a wear test with use of „in situ" acoustic emission detection (AE) and optical detection of penetration of an abrasive body through a film/substrate interface. The body is the tip of nanotester with simple geometric shape whose tip rounding is typically in range of tenth of pm up to mm units, which mechanically acts on the surface of tested film.

Present Prior Art

Thin films with thickness in range from tents nm to tents pm improve or change surface characteristic of substrates i.e. functional and design elements of various components of machines, devices, tools, apparatuses or their parts which are in operation or production exposed to one time or repeated mechanical stresses. The substrates can be for example metals, alloys, semi-conductors, glasses, ceramics, polymers and possibly organic materials. Based on nature of deposition process and material of thin film, is substrate-thin film system in particular extent of operational conditions in metastable state, because modern deposition processes allow creation of materials with unique physical characteristic which are beyond reach of classical technologies.

Various, for example optical, protective or photoactive films are often in operation exposed to cyclic loading with specific subcritical load, which however causes mechanical damage of the film but not sooner than after several cycles, so called low cycle fatigue damage of the film. Typical for these cases is accumulation of microscopic damage during particular load cycles. There are patented various special testing devices for research of fatigue and wear of different components which are described for example in the files US3170321A, WO2016/102968 A1 or IN206754. Also, there are known devices for testing of materials for wear resistance, for example according to the files US9341555B2 or CN101344470B. Thin films are specific in the way, that according to the deposition process/method, they do not have to be always homogenous along the whole cross-section. Critical is also their adhesion to the substrate. Wear resistance is standardly tested on a Pin-on-disc device (see the patent US4966030A) or on various tribotesters, which are described for example in the DE10390125B4, US20080034837A1 or W02003060487A1 which combine fatigue and abrasion wear. However, they do not always, with regard to their specific use, offer relevant outcomes. That is why there is effort to develop devices which would more precisely simulate practical wear conditions, see for example the patent CN204679360U which describes a testing device for photovoltaic films on glass.

There are likewise known methods of use of acoustic emission for monitoring of processes of additive production (CN109477737) or structural integrity of components (CN207850994, CN108195943, US5170666, RU2684709). There also exist designs based on acoustic-optical scanning (JP2014126400, JP6086719, US9513260, JP3944578). Yet these methods or devices are dedicated to specific purposes and are not suitable for evaluation of spatially localized damage of thin films and coatings during regular nano/micro-tribo/mechanical tests.

There also exist devices for example CZ305833B6, which are dedicated to AE detection during local mechanical testing of thin films. Generally, they are not suitable for complex and reliable evaluation of wear or gradual mechanical degradation of thin films. The adapter of the holder according to the CZ305833B6 formed from a one-side opened hollow body whose shell has cylindrical shape and whose inner space is from the side of a supporting face closed by a lid, which is modified for fixation to the holder of the nanotester and on the opposed side to the lid with a flat contact face which forms a monolithic unit with the shell and is produced from the same material, preferably from steel. Mechanical waves propagate, during the testing, through the whole monolith of the shell which is not optimal in terms of quality of obtained results due to attenuation of mechanical waves and in turn also due to worse transfer of elastic energy to piezoelement. Research of low cycle fatigue wear combined with abrasive wear is possible to do also on nanotesters with coupled microscopic head which also enables to carry out a repetitive scratch test. Experimental comparison of wear resistance of SiC and nano/micro-crystal diamond films tested by the repetitive scratch test at constant subcritical load was published in Taylor a., et al, Diam. Rel. Mater. 69 (2016) 13-18. Similar tribological tests of functional films were published in Tomastik J., et al., Sci. Rep. (2018) 8:10428, tests of functional films on transparent substrates in Simurka L, et al., Int. J. Appl. Glass Sci., (2018) 1-10; Simurka L, et al. (2018) https://doi.Org/10.10Q7/s11696-018- 0420 2

For evaluation of extent of resulting damage, i.e. wear process and extent of wear, it is necessary not only to identify onset of the damage (usually sub-surface) but also the extent of cumulative damage. In case of films with dominant plastic deformation during contact loading, especially the reliable identification of fatal cumulative damage is crucial, i.e. penetration of testing body (tip) up to the interface with the substrate. However, this is possible only post factum by chemical mapping of wear tracks (scratch scars) in vacuum (SEM-EDS-Scanning Electron Microscopy - Energy Dispersive Spectroscopy), XPS-X-ray Photoelectron Spectroscopy). This makes the process highly demanding in terms of time and experimental equipment, significantly complicates the test evaluation and in turn substantially increases costs. Other methods, for example a friction sensor attached to the tip or possibly monitoring the change of electric properties during penetration of the tip to substrate do not have to be sufficiently precise from the point of view of lateral resolution at nano/micro scale. Even detailed optical microscopy or AFM - Atomic Force Microscopy analysis of wear tracks does not have to reveal the point of scratching through the layer/thin film and thus clearly identify the fatal damage of the film. The experience shows that even continuous scanning of the penetration depth of the tip during scratch passes, when load is applied, does not have to provide relevant information concerning scratching through the film.

eventually between batch of scratch passes, d^^s not have to always provide relevant outcomes.

possible to carry out „in situ"

^{and the} sample has to be transferred into another measuring apparatus, eventually placed back afterwards into nanotester for continuation of the test. Nevertheless, even the most modern nanotesters do not enable identical fixation of the samples that in principle prevents of continuation of the repetitive wear tests performed on the nano/micro scale. This fact is a critical and principal obstacle for reliable use and evaluation of wear tests of functional thin films on transparent substrates in industry.

Because there has not been published other, safer designs/solutions and it does not provide even an adapter of sample holder described in the file CZ 305833 B6, it is always necessary to unfix the sample from the adapter, carry out the examination under the external microscope and then put the sample back again on the adapter and finally install the adapter into the mechanical testing device, i.e. nanotester. It is especially the precise re-fixing of the sample back on the holder that is the most crucial point, as it is nearly impossible to place it exactly in the same position, orientation and condition (possible contamination, water vapors adsorption and similar) as it was before detachment from the holder.

The aim of presented invention is to design an adapter for holder of samples for evaluation of mechanical durability of thin films which would feature not only sufficient mechanical stiffness and stability, but also AE sensitivity for use in microtesters as well as in nanotesters, and the method of evaluation of quality of mechanical durability of thin films by the help of this holder which, by the help of a light beam which penetrates the transparent substrate, allows revealing/evaluating of fatal cumulative wear in the wear track formed during the repetition scratch or other penetration test.

Essence of the invention

Set goal is accomplished with invention, which is an adapter of sample holder that enables to perform a method for evaluation of quality of mechanical durability of thin films, which is formed by a hollow body whose shell is from below closed with a lid which enables fixation of an adapter to a console of the holder of testing device and from above with a contact front face on which is not only from the top fixed tested sample, which consists of substrate and on it deposited tested thin film, but also there is, from the bottom fixed a piezo sensor of acoustic emission which is equipped with a mechanical damper and a preamplifier of electric signal which is through a connector taken out from the body. The essence of the invention is that the shell and the contact face are designed as a set of two independent structural parts of different material composition whereas the contact front face is formed from a transparent material featuring a high mechanical stiffness and stability with Young’s modulus of elasticity value higher than 40 GPa and in the place of its anchoring there is an inner peripheral recess in the shell where a LED illumination is installed, which is connected through a coupler with source of electric energy, which is placed outside the body.

In preferable design is the contact front face formed either as one integral part or consists of firmly attached support plate and contact plate.

Also the essence of the invention is a method for evaluation of quality of mechanical durability of thin films deposited on transparent substrate during a low cycle repetitive scratch test with constant subcritical force by the help of an adapter, whose essence is, that after determination of subcritical force which consists in selection of size and geometry of testing tip with a well-defined actual shape and determination of testing force, which is selected in the way for elastic deformation to be predominant and after relief to remain only required low residual deformation, then is, with this way predetermined constant sub-critical force, carried out the repetitive scratch test on testing device over the same wear trajectory in one direction with simultaneous monitoring of acoustic emissions and actual position of the testing tip, whereas is, after a particular scratch test when is applied subcritical force or group of scratch tests, which is done without removal of tested sample from the testing device, carried out topographic scanning of the wear track with the same tip with topographic force, which does not cause any deformation of residual wear track, after whose evaluation together with AE signal is predicted place where is expected fatal cumulative wear of tested film and without removal of the tested sample from the device is, by illumination of this wear track, localized and/or confirmed the exact position of fatal damage of the film, whereas evaluation criterion is not only the number of cycles of the scratch test realized until the fatal cumulative damage of the thin film occurs but also the sequence number of scratch test when subsurface damage is detected by the help of AE record through exceeding a threshold value, and also the extent of cumulative damage. In preferable design, it is simultaneously with the mechano-acousto-optical exploring carried out also a continual detection of tangential/frictional force which acts during on-load passes, i.e. when load is applied during scratch testing, as well as continual scanning of changes of electric characteristic of contact area between the testing tip and tested thin film-substrate system.

With presented invention is reached new and higher effect because it is possible, by the help of a special acousto-optical adapter of the sample holder, after basic determination of subcritical force for testing of low cycle wear of tested non-transparent thin film deposited on transparent substrate, to carry out „in situ" the repetitive scratch test over the same trajectory at simultaneous AE monitoring and scanning of the wear track profile during and after one or more load passes with use of depth sensing indentation technique (DSI - Depth Sensing Indentation) of nanotester that allows to obtain a group of inspection depth profiles and corresponding AE records for purpose of prediction of place of fatal cumulative wear of the film, which is consequently evaluated and optically verified through illumination of the residual wear track. Evaluating criterion is the number of cycles of repetitive scratch test realized until fatal cumulative damage of the film occurs, the moment of initial subsurface adhesion-cohesion damage and the extent of scratching through of the film.

Higher effect of the sample holder itself is in implementation of illumination at preserving sufficient mechanical stiffness for use in modern nano/micro-testers and provision of high sensitivity of AE detection, whereas its sectional design based on separation of the steel shell from the transparent contact front face, which serves as transparent membrane, offers more suitable conditions for propagation and detection of weak mechanical vibrations. This is achieved by formation of a waveguide represented by the contact front face and also by elimination of one material interface between the metal and the material of the contact front face in comparison with an adapter CZ305833 B6 provided that it would be equipped with an external illumination and a transparent membrane. In case of use of glass as material for the contact front face, the mechanical wave passes through one material interface less and herewith is reached more efficient transfer of elastic energy from the sample to a piezo element, which is essential in case of wear test, where microscopic damages in the course of the test are expected, thus even lower energetic AE activity. Experimental evidence of higher effect of given adapter in comparison with the CZ 305833 B6 equipped with external illumination and transparent membrane is mentioned in Fig.5, which compares signals from three nano-indentation tests carried out on standard/calibration silicon monocrystal. From graphs, it is evident that submitted design of the adapter of the sample holder in comparison with the CZ 305833 B6 shows higher values of amplitudes of AE signal (Fig.5a) and higher energy of detected AE events (Fig.5b).

Description of images in enclosed drawings

Particular example of design of acousto-optical adapter of sample holder for evaluation of quality of mechanical durability of thin films according to the invention and particular phases of use of this method of evaluation are illustrated in enclosed drawings, where

Fig.1 is schematic cross section of basic design of the adapter of the sample holder,

Fig.2 is schematic cross section of an alternative design of the adapter of the sample holder,

Fig.3 is AE record of individual scratch passes of abrasive body, i.e. a tip of nanotester in a wear track. In the first record, there is also displayed a synchronization signal on right perpendicular axis,

Fig.4 is principal scheme of the low cycle wear test, where T represents topographic pass at nearly zero normal load force and S represents on-load pass performed at constant subcritical normal force whereas the force is applied abruptly in order to maximize the portion of the test under the constant load. Number of cycles states number of load tests.

Fig.5 is comparison of prototype of the subjected adapter and the CZ 305833 B6, a1) and a2) AE amplitudes measured during three calibration tests, b) Cumulative AE energy for individual tests. Calibration nano-indentation test was carried out with Berkovich pyramidal indenter at maximal force of 500 mN, loading and unloading took 20 s, creep’s period was 10 s, temperature 24 °C.

The drawings which illustrate the adapter of the sample holder according to the invention and express examples of evaluation of quality of thin film do not in any case and anyhow limit the extent of the protection which is mentioned in the definition, they only clarify the essence of the invention. Examples of design of the invention

The adapter according to the illustration in Fig.1 is formed by a cylindrically shaped hollow body 1, whose shell 101 is made preferably of stainless steel, that ensures increased reliability and longtime stability of parameters of detection of acoustic emission. From above is the inner space of the body 1 closed with an inserted contact front face 2 made of transparent material which has mechanical stiffness and stability with value of Young’s modulus of elasticity higher than 40 GPa. On outside wall of the contact front face 2 is fixed, preferably glued, tested sample 3 which consists of a substrate 31 and on it deposited tested thin film 32 of the material. On the bottom wall of the contact front face 2, there is fixed a piezoelectric sensor 4 of acoustic emission (AE) for transformation of AE signal to electric signal. The AE sensor 4 is equipped with an mechanical damper 5 and a preamplifier 6 of electric that is through an output connector 41 taken out from the shell 101 into a non-illustrated control, record and evaluation unit of the device for controlling the scratch test or AE system. The shell 101 of the body 1 is in the area of fixation of the contact front face 2 with the tested sample 3 equipped with a peripheral recess 103, in which is placed LED illumination 7, which is connected through a coupler 71 with non-illustrated source of electric energy which is placed outside of the body 1 The LED Illumination 7 is formed by set of two, four or more LED diodes which are evenly placed around the perimeter of the recess 103. The body 1 is from below closed with a lid 8 which is equipped with a screw coupling 8 for possibility of fixation of the adapter to console of the holder of testing device.

This special acousto-optical adapter of the sample holder ensures that the condition, position and orientation of the sample 3 stay unchanged with respect to the track of the testing tip for whole time of duration of low cycle wear test, it means also after topography scan of surface of the wear track, its illumination and possibly further continuation of the test. Described design is not the only possible design of the adapter but the shell 101 does not have to be, for certain types of tests, cylindrical but can also be square or with another cross section. Also the contact front face 2 does not have to be formed as one structural piece but it can be sectional and be formed by a firmly attached support plate 22 and a contact plate 21, as is it illustrated in Fig.2, and can be made of another material than glass for example transparent plastic of monocrystal with given strength parameters. The process of testing of low cycle wear with repetitive scratch test consists in the first phase of selection of geometry and size, i.e. diameter in case of a spherical tip, and determination of subcritical force for testing, which is determined from regular scratch test or from indentation test of tested film or films via nanotester, for example in the way for elastic deformation during the deformation process to be predominant and after unloading to remain only low residual depth. There is also possible to use another method for determination of subcritical force.

The repetition test itself is then carried out in the way that at first is carried out the topography of the surface of the film in a given direction and required length with topographic normal force that does not promote any deformation of the sample, then follows scratch pass with linear steep increase of normal force to constant pre-determined subcritical level with the same tip whereas the tip moves over the identical trajectory as in the case of initial topographic pass. Then is carried out another topographic pass, again over the same trajectory. The low cycle wear test then continues with repeating scratch and topographic passes over the identical trajectory. The procedure is schematically illustrated in Fig. 4 according to which the test continues until the occurrence of fatal cumulative damage of the film. Topographic pass of the wear track can be repeated after each scratch test (on-load pass) or after a series of repeated scratch tests. Likewise, it is possible to apply normal force suddenly during on-load passes.

Online continual monitoring of AE generated during deformation of the film in the course of individual passes allows determination of the moment when cohesion/adhesion failures in the film under the tip or in its close vicinity occur, i.e. formation of crack which generates pulse (discrete) AE event. AE detection allows continuous and real-time identification/monitoring of damage of the film, see Fig. 3. This is characterized by amplitude and length of AE signal (first event ~ 12s during the 4th. on-load pass). During each successive pass, the deformation followed by AE events is getting bigger till complete damage of the film occurs.

Control illumination of the wear track after each (n-th) topographic/scratch pass is allowed by the special acousto-optical adapter of the sample holder, which uses AE detection for continuous monitoring of emission events in the course of individual scratch passes. The adapter ensures stability of conditions and configuration of the sample as well as the wear track on its surface with regard to position and orientation of the movement of the testing tip. The advantage of majority of nanotesters is coupled microscope under which it is possible to illuminate the wear track and localize/monitor the extent of fatal cumulative wear.

When the first calibration test is carried out, during which AE response of tested system of thin film and used transparent substrate is mapped and the AE signal coming from the substrate is distinguished, then it is possible to define a threshold after whose exceeding it is possible to automatically terminate the wear test. This way it is possible, without the presence of operator, to automatically terminate the test of the sample. Consequently, it is possible to automatically carry out test of another sample, which is fixed on the same adapter of the sample holder. This fact has tremendous significance in case of industrial testing of more samples, for example for quality inspection. Subsequently, it is possible to verify fatal cumulative damage through illumination of the wear track.

Industrial utility

The method of testing of low cycle wear of thin films and coatings with use of the adapter according to the invention can be used everywhere where functional films and coatings in real operation are produced and used, for instance reflection films for laser mirrors, photovoltaic films, films for conversion of CO to CO2 etc.

Claims

P A T E N T CLAIMS

1. An adapter of sample holder that enables to perform a method for evaluation of the quality of mechanical durability of thin films, which is formed by a hollow body (1), whose shell (101) is from below closed with a lid (8) which enables fixation of the adapter to a console of the holder of testing device and from above with a contact front face (2), on which is not only from the top fixed a tested sample (3) consisting of a substrate (31) and on it deposited thin film (32), but also there is from the bottom fixed a piezoelectric sensor (4) of acoustic emission which is equipped with a mechanical damper (5) and a preamplifier (6) of an electric signal which is through a connector (41) taken out from the body (1), wherein, the shell (101) and the contact front face (2) are designed as an assembly of two individual structural parts with different material composition, whereas the contact front face (2) is made of a transparent material featuring mechanical stiffness and stability with the value of Young’s modulus of elasticity higher than 40 GPa and at the place of its fixation there is an inner peripheral recess (103) in the shell (101), in which is placed LED illumination (7), which is connected through a coupler (71) with a source of electric energy which is placed outside of the body (1).

2. An adapter of sample holder according to the claim 1 , wherein, the contact front face (2) is formed either as a solid part or it consists of firmly attached support plate (22) and a contact plate (21).

3. The method for evaluation of quality of mechanical durability of thin films deposited on the transparent substrate during low cycle repetitive scratch test with constant subcritical force by the help of the adapter according to the claim 1 , wherein, after determination of subcritical force consisting of a selection of size and geometry of testing tip with a well-defined actual shape and determination of subcritical force selected in the way that when it acts there comes primarily to elastic deformation and after release remains only required low residual depth of the scratch is with this way determined constant subcritical force carried out a repetitive scratch test on the testing device over the same trajectory in one and the same direction with simultaneous monitoring and recording of acoustic emission and actual position of the testing tip, whereas is after a particular scratch test/pass or groups of scratch passes when the subcritical force is applied, without extraction of the tested sample from the testing device, carried out a topographic scanning of the wear track with the same tip with topographic force, which does not cause any deformation of residual wear track, after its evaluation together with AE signal evaluation is predicted place of expected fatal cumulative wear of the tested film and without removal of the adapter from the testing device is by the help of illumination of this track localized and/or confirmed the place of fatal damage of the film, whereas the evaluation criteria are not only the number of cycles of the scratch test, i.e. scratch passes realized until fatal cumulative damage of the thin film occurs, but also the sequence number of the scratch pass when the subsurface damage is detected by the help of AE record of exceeding a threshold value, and also the extent of cumulative damage.

4. The method for evaluation of quality of mechanical durability of thin films deposited on the transparent substrate with use of continuous monitoring and record of acoustic emission during low cycle repetitive scratch test with constant subcritical force by the help of the adapter according to the claim 2, wherein, that simultaneously with mechano-acousto-optical scanning is carried out not only continuous detection of tangential/frictional force acting during the scratch passes but also continuous scanning of change of electric characteristic of contact area between the testing tip and the tested thin film-substrate system.