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Scratch resistance analysis of coatings on glass and polycarbonate
Thin Solid Films 517 (2009) 3121–3125
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Scratch resistance analysis of coatings on glass and polycarbonate W. Boentoro ⁎, A. Pflug, B. Szyszka Fraunhofer Institute of Surface Engineering and Thin Films (IST), Braunschweig, Germany
a r t i c l e
i n f o
Available online 25 November 2008 Keywords: Scratch test Finite element Coating, Glass Polycarbonate
a b s t r a c t Optical coatings are nowadays broadly used in automotive and architectural glazings, displays, etc. When applied to the outside of laminated glass panels or on single glass sheets the coatings are subject to abrasive processes like cleaning and dusty or sandy circumstances. A high mechanical stability is therefore required under various environmental loads in life cycle. In this work, scratch tests have been carried out for antimony doped tin oxide film (SnO2:Sb) on glass and silicon oxide film (SiO2) on polycarbonate to analyze the impact of film thickness on the scratch resistance. In addition to that, the taber test is used for the analysis of SiO2 on polycarbonate. Simulations of the experimental set-up based on a finite element model are developed in order to investigate the elasto-plastic behaviour of the coating and substrate. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Optical coatings for outside application on glass and polycarbonate have achieved a strong interest in the last few years. Transparent and conductive oxides such as antimony doped tin oxide film (SnO2:Sb) are useful as low emissivity (low-e) coating to prevent moisture formation on the outer glass surface [1]. For the emerging field of polycarbonate (PC) glazing, scratch protecting films are necessary to achieve mechanical properties similar to glass on soft substrates. The exposed surface can degrade the function of the coating over the time, for instance due to abrasive processes. This may lead to the damage of the underneath layers and substrate and after all, to the premature fracture and failure [2]. The mechanical behaviour, such as elastic modulus, hardness and fracture toughness are responsible for the coating resistance of the overall system, which is usually a multilayer stack [3]. The goal of the work described here is to analyze the impact of the coating thickness on the scratch and wear characteristics of metal oxides on glass and polycarbonate. The systems glass / SnO2:Sb and polycarbonate/SiO2 were investigated for coating thickness in the range from 50 nm to 1600 nm for SnO2:Sb and from 500 nm to 4000 nm for SiO2. Scratch and taber tests were used for mechanical testing. An approach for the simulation of the scratch test using the finite element method is described. 2. Experimental details In this work, two types of samples system were investigated namely SnO2:Sb conductive film on glass substrate and SiO2 protecting coating on PC. The first system, the glass substrate was borosilicate
⁎ Corresponding author. Tel.: +49 531 2155 668. E-mail address: [email protected] (W. Boentoro). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.11.119
glass AF45 (1.1 mm) which was heated to 300 °C during the deposition process by reactive DC magnetron sputtering. The second system, material SiO2 deposited on PC unheated substrate (Makrolon AL2647) by AC magnetron sputtering. The set of samples is described in Table 1. The scratch tests were performed using commercial scratch equipment (CSEM Revetest Scratch Tester) with a Rockwell indenter (Radius 200 µm). The layout of the system is shown in Fig. 1. The sample was first adapted and cut to the suitable size of the scratch test table and then properly cleaned by blowing high-pressure nitrogen. The indenter tip was cleaned before and after every test by using ethanol to avoid the influence of the adhered coating rest. Scratch test load was increased linearly from 1 N to 20 N with sliding distance of 10 mm and velocity of 10 mm/min. The same scratch procedure was applied four times for each sample. Then, the scratches were observed under light microscopy and measured for comparing the visible scratches. The definition of the force where onset of damage occurred is based on the identification of the scratch in the light microscope image. The scratch test of the coating is also
Table 1 Overview on layer stacks analyzed Material
SnO2:Sb
SiO2
Substrate
Borosilicate glass Schott AF45 (1.1 mm)
Deposition process TS [°C]
Reactive DC magnetron sputtering 300
Polycarbonate Bayer Makrolon (AL2647) Schott AF45 (1.1 mm) Reactive AC magnetron sputtering
d [nm]
50, 200, 400, 800, 1600
Substrate temperature Film thickness
unheated 500, 1000, 2000, 4000
3122
W. Boentoro et al. / Thin Solid Films 517 (2009) 3121–3125
3. Results 3.1. Scratch test of SiO2 on polycarbonate
Fig. 1. Schematic of the scratch test of the experimental set-up.
The scratch test on the uncoated polycarbonate shows that the “soft” substrate starts to deform plastically at the initial load (1 N) as showed in Fig. 2 and then followed by continuous smooth scratch formation. On the 0.5 µm thickness sample, the coating starts to fail under higher loading condition around 3 N. The observed first failure is the irregular darker form in the middle of the scratch track where the coating spalling took place. Not far from that point, the crack starts to form a transverse crack in the scratch direction, which starts from the one side the other side of the crack (Fig. 3). The load that induced the first crack and the complete transverse crack will be used for referencing the critical load. The dependence of the critical load on coating thickness showed in Fig. 4 reveals an increase of
Fig. 2. Scratch test on the uncoated polycarbonate.
Fig. 3. Scratch test on the 500 nm silicon oxide on polycarbonate.
compared with the uncoated glass substrate. The preliminary scratch tests on the uncoated glass statically show that the scratch initiated at critical load about 10 N. The formation of transverse cracks, i.e. spalling, as observed with light microscopy, is defined as criterion of a critical load.
the critical load from ∼3 to ∼5 N when the thickness is increased from 0.5 µm to 2 µm. The critical load for first failure on 4 µm film thickness remains constant. However, the critical load for the onset of transverse cracking increases from ∼5 to ∼10 N. For comparing the result of the scratch test with a common standardized test, we performed tests using
Fig. 4. Dependence of critical loads on film thickness for scratch testing of PC/SiO2 samples for first failure and transverse cracking.
Fig. 5. Dependence of Haze value after the Taber Abraser test (CS10F wheel, 500 g load per wheel) on film thickness for PC/SiO2.
W. Boentoro et al. / Thin Solid Films 517 (2009) 3121–3125
3123
Table 2 The material properties that are used in the simulation Material
E-Modul
Poisson ratio
[GPa] Glass SnO2:Sb
Fig. 6. Dependence of critical load on film thickness for scratch testing of Glass/SnO2:Sb.
the Taber Abraser. The samples haze values were statistically measured by a hazemeter at four different locations. The dependence of the haze level on film thickness: as prepared, after 100 and 1000 revolutions is depicted in Fig. 5. A strong decrease of haze is observed when the film thickness is raised. However, even for 4 µm thickness the haze is in the order of 12% after 1000 revolutions. 3.2. Scratch test of SnO2:Sb on glass The dependence of the critical load on film thickness showed in Fig. 6 reveals that the uncoated glass has higher scratch resistance than the coated glass up to 800 nm film thickness. The coating thickness of 1.6 µm protects the substrate to from the visible failure to ∼ 16 N. For higher film thickness, a strong increase of the critical load is observed.
72 139
Yield stress [MPa]
0.21 0.36
50 1112
Fig. 7 depicts the typical scratch pattern for the different loads and film thicknesses from the scratch test. Fig. 7 at the left part (First Failure) shows the onset of crack formation, where the first failures were visible on the samples. In the middle part: the failure at load of 13.6 N has been chosen to show the influence of the coating thickness visually on the samples at the same loading. On the uncoated glass sample (a), the scratch track was formed by adjacent the circular patterns. The tracks of the indenter on the coated glass with 50 nm coating thickness (b) were formed by intersections of the circular pattern. The failure formations on the thicker coating (c) and (d) have the similar pattern with (b), however a denser circular pattern with the increase of the coating thickness is observed. The tracks on the 800 nm coating thickness (e) and 1600 nm (f) were dominated by dense semi-circular failures, which are obviously fade out with the increase of the coating thickness. The Fig. 7 in the right part, shows the failure at the end of the scratch test (20 N) where the relative less failure were observed at the coated samples of 800 nm and 1600 nm. 3.3. Simulation A 3D model of an indenter scratching on the coated substrate was developed using finite element method ABAQUS Standard 6.7. The indenter was modeled as a rigid ball with radius of 200 µm. The coating with thickness of 1600 nm on glass substrate was assumed to
Fig. 7. Light microscope images of the scratch pattern. Sample (a) is uncoated glass, compared with the coating thickness of (b) 50 nm, (c) 200 nm, (d) 400 nm, (e) 800 nm, (f) 1600 nm. For comparing the formation of the failure related to the coating thickness, the failure at the similar loading (13.6 N) were performed.
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W. Boentoro et al. / Thin Solid Films 517 (2009) 3121–3125
Fig. 8. Schematic illustration of the three-dimensional finite element model of the scratch test.
be ideally smooth, without defects and homogenous. The constitute behavior of the coating is modeled for the simplicity with an elastic and perfectly plastic material law. The friction coefficient of 0.01 was assumed to be constant and based on the result of the scratch test. The material properties have been set according to the literature [3], and are listed in Table 2. To simulate scratch test experiment, the finite element geometry dimensions were chosen to approximate the real life experiment as close as possible. Preloading, sample and tip geometry and load applied in experiments have been modeled properly. The sliding distance 10 mm, substrate thickness 1.1 mm and coating thickness of 1600 nm are the basic of the model. However, compared to the test samples, the length of the model has been reduced to decrease computational time. Due to the plane of symmetry of the problem, the width of the original mesh can be reduced by half. The scratch test simulation was divided into three steps. First, the coating-substrate was pre loaded to 1 N (displacement controlled of 2.88 µm). Second, the indenter tip will slide 100 µm and at the same time indented to 28.8 µm within the step and not time dependent of the material deformation response. Last step is the unloading step. The figures are mapped according to the stress topography which represents a certain value of stress level on each color. The observation direction is related to the Fig. 8.
Fig. 9. The model SnO2:Sb (1.6 µm) on glass preloaded by indenter (shown only the half section), The stress unit is N/µm2 (1 MPa = 106 N/µm2).
Fig. 10. Simulation result of the first step: 1 N preloading showed the concentration tension stress in x-direction (scratch direction) occurs on the coating around the indenter contact area.
The simulation result of the 1 N pre loading showed in Fig. 9. In order to have a better observation point of view, the indenter tip is not shown as a whole, but only as a curve to identify the front side of the indenter (Fig. 10). The concentrations of tension stress (800 MPa) in 1-direction (scratch direction) obviously occur on the coating around the indenter contact area. Compression stress about 200 MPa showed on the substrate directly under the indenter. The common coating failure as the indenter moved to the right direction is shown in the Fig. 11. The simulation of the sliding indenter revealed that the maximal compression stress concentration presents in front of the indenter (Fig. 12) which lead to ploughing. The maximal tensile stress appears under the indenter and behind the sliding indenter that may initiate the failure. The secondary maximal tensile stress in the scratch boundary, may lead to form the angular crack that visible at the early phase of the scratch test as also showed in Fig. 11. 4. Discussion Our results show a significant impact of the film thickness on the critical load in scratch testing for both, coatings on glass and on polycarbonate. For PC, we observe an increase of the critical load for first failure up to ∼5 N at 2 µm thicknesses while for the increase from 2 to 4 µm thicknesses, only the critical load for the transverse cracking can be raised to 10 N. For the bare glass, the critical load for first failure is ∼ 10 N. This shows that even for 4 µm SiO2 on PC the scratch resistance of glass has not been achieved. The corresponding results for the abrasive test show a decrease of the haze level when the thickness is raised. The criteria for the automotive application of ΔH b 3% at 1000 revolution has not been met [4]. However, optimized multilayer stacks were described [5,6] being capable of fulfilling these demands. For the systems PC/SiO2, a similar analysis of the thickness dependence of the critical load in scratch testing has been performed by other authors [7], where the performance of PECVD SiO2 on PC has been investigated. They also reported that critical load increases linearly with the increase of the film thickness. A good agreement with their result has been also shown by the similar order of magnitude of critical load for first failure. For the coatings on glass, we observe weak performance compared to the bared glass when thin films were deposited. Films up to 400 nm thickness reveal inferior critical load for the first failure. For thicker films, however, the critical load could be raised up to 16 N. Therefore,
W. Boentoro et al. / Thin Solid Films 517 (2009) 3121–3125
3125
Fig. 11. The common coating failure as the indenter moved (to the right direction in the figure).
deformation of the surface. The stress concentrations may lead to the initiation of the possible failure generation. 5. Conclusion and outlook The analysis of the scratch resistance, based on the critical criteria of the failure form, has been investigated. The experiment shows that the thickness has a strong influence on the scratch resistance of the coatings for both systems, SiO2 on PC and SnO2:Sb on glass. In addition, to understand the phenomena during the scratch test, a three-dimensional finite element model has been developed. The simulation is necessary in order to give the representation of the origin and development of the failure during the scratch test. For the future work, the simulation model will be further enhanced with respect to more specific of the friction dependency and material elastic-plastic properties that will be obtained from nano-indentation measurements. Fig. 12. Stress distribution of the scratch test mode after ∼3 N sliding ““that showed the ploughing in front of the indenter and tensile stress behind the indenter.
these coatings may serve as a durable coating for outside low-e application. Ultra hard DLC coatings on glass, however, reveal a critical load for first failure up to ∼50 N [8]. Our work on the computer simulation of the scratch test using the finite element simulation shows that the pattern of the stress distribution corresponds to the experimentally observed failure mode. The simulation describes the stress situation as the diamond tip sliding over the coating surface, and also the elastic and plastic
References [1] I. Hamberg, J.S.E.M. Svensson, T.S. Eriksson, C.G. Granqvist, P. Arrenius, F. Norin, Appl. Optics 26 (1987) 2131. [2] E. Ando, S. Suzuki, N. Aomine, M. Miyazaki, M. Tada, Vacuum 59 (2000) 792. [3] E.G. Berasategui, S.J. Bull, T.F. Page, Thin Solid Films 447–448 (2004) 26. [4] Deutsches Institut für Normung e. V., Testing of glass and plastics, abrasion test; method using abrasion wheels and measurement of scattered light, DIN 52347, 1987-12. [5] K. Schmauder, K.D. Nauenburg, K. Kruse, G. Ickes, Thin Solid Films 502 (2006) 270. [6] D. Katsamberis, K. Browall, C. Iacovangelo, M. Neumann, H. Morgner, Progr. Org. Coat. 34 (1998) 130. [7] D. Rats, V. Hajek, L. Martinu, Thin Solid Films 340 (1999) 33. [8] V.S. Veerasamy, H.A. Luten, R.H. Petrmichl, S.V. Thomsen, Thin Solid Films 442 (2003) 1.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Scratch resistance analysis of coatings on glass and polycarbonate W. Boentoro ⁎, A. Pflug, B. Szyszka Fraunhofer Institute of Surface Engineering and Thin Films (IST), Braunschweig, Germany
a r t i c l e
i n f o
Available online 25 November 2008 Keywords: Scratch test Finite element Coating, Glass Polycarbonate
a b s t r a c t Optical coatings are nowadays broadly used in automotive and architectural glazings, displays, etc. When applied to the outside of laminated glass panels or on single glass sheets the coatings are subject to abrasive processes like cleaning and dusty or sandy circumstances. A high mechanical stability is therefore required under various environmental loads in life cycle. In this work, scratch tests have been carried out for antimony doped tin oxide film (SnO2:Sb) on glass and silicon oxide film (SiO2) on polycarbonate to analyze the impact of film thickness on the scratch resistance. In addition to that, the taber test is used for the analysis of SiO2 on polycarbonate. Simulations of the experimental set-up based on a finite element model are developed in order to investigate the elasto-plastic behaviour of the coating and substrate. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Optical coatings for outside application on glass and polycarbonate have achieved a strong interest in the last few years. Transparent and conductive oxides such as antimony doped tin oxide film (SnO2:Sb) are useful as low emissivity (low-e) coating to prevent moisture formation on the outer glass surface [1]. For the emerging field of polycarbonate (PC) glazing, scratch protecting films are necessary to achieve mechanical properties similar to glass on soft substrates. The exposed surface can degrade the function of the coating over the time, for instance due to abrasive processes. This may lead to the damage of the underneath layers and substrate and after all, to the premature fracture and failure [2]. The mechanical behaviour, such as elastic modulus, hardness and fracture toughness are responsible for the coating resistance of the overall system, which is usually a multilayer stack [3]. The goal of the work described here is to analyze the impact of the coating thickness on the scratch and wear characteristics of metal oxides on glass and polycarbonate. The systems glass / SnO2:Sb and polycarbonate/SiO2 were investigated for coating thickness in the range from 50 nm to 1600 nm for SnO2:Sb and from 500 nm to 4000 nm for SiO2. Scratch and taber tests were used for mechanical testing. An approach for the simulation of the scratch test using the finite element method is described. 2. Experimental details In this work, two types of samples system were investigated namely SnO2:Sb conductive film on glass substrate and SiO2 protecting coating on PC. The first system, the glass substrate was borosilicate
⁎ Corresponding author. Tel.: +49 531 2155 668. E-mail address: [email protected] (W. Boentoro). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.11.119
glass AF45 (1.1 mm) which was heated to 300 °C during the deposition process by reactive DC magnetron sputtering. The second system, material SiO2 deposited on PC unheated substrate (Makrolon AL2647) by AC magnetron sputtering. The set of samples is described in Table 1. The scratch tests were performed using commercial scratch equipment (CSEM Revetest Scratch Tester) with a Rockwell indenter (Radius 200 µm). The layout of the system is shown in Fig. 1. The sample was first adapted and cut to the suitable size of the scratch test table and then properly cleaned by blowing high-pressure nitrogen. The indenter tip was cleaned before and after every test by using ethanol to avoid the influence of the adhered coating rest. Scratch test load was increased linearly from 1 N to 20 N with sliding distance of 10 mm and velocity of 10 mm/min. The same scratch procedure was applied four times for each sample. Then, the scratches were observed under light microscopy and measured for comparing the visible scratches. The definition of the force where onset of damage occurred is based on the identification of the scratch in the light microscope image. The scratch test of the coating is also
Table 1 Overview on layer stacks analyzed Material
SnO2:Sb
SiO2
Substrate
Borosilicate glass Schott AF45 (1.1 mm)
Deposition process TS [°C]
Reactive DC magnetron sputtering 300
Polycarbonate Bayer Makrolon (AL2647) Schott AF45 (1.1 mm) Reactive AC magnetron sputtering
d [nm]
50, 200, 400, 800, 1600
Substrate temperature Film thickness
unheated 500, 1000, 2000, 4000
3122
W. Boentoro et al. / Thin Solid Films 517 (2009) 3121–3125
3. Results 3.1. Scratch test of SiO2 on polycarbonate
Fig. 1. Schematic of the scratch test of the experimental set-up.
The scratch test on the uncoated polycarbonate shows that the “soft” substrate starts to deform plastically at the initial load (1 N) as showed in Fig. 2 and then followed by continuous smooth scratch formation. On the 0.5 µm thickness sample, the coating starts to fail under higher loading condition around 3 N. The observed first failure is the irregular darker form in the middle of the scratch track where the coating spalling took place. Not far from that point, the crack starts to form a transverse crack in the scratch direction, which starts from the one side the other side of the crack (Fig. 3). The load that induced the first crack and the complete transverse crack will be used for referencing the critical load. The dependence of the critical load on coating thickness showed in Fig. 4 reveals an increase of
Fig. 2. Scratch test on the uncoated polycarbonate.
Fig. 3. Scratch test on the 500 nm silicon oxide on polycarbonate.
compared with the uncoated glass substrate. The preliminary scratch tests on the uncoated glass statically show that the scratch initiated at critical load about 10 N. The formation of transverse cracks, i.e. spalling, as observed with light microscopy, is defined as criterion of a critical load.
the critical load from ∼3 to ∼5 N when the thickness is increased from 0.5 µm to 2 µm. The critical load for first failure on 4 µm film thickness remains constant. However, the critical load for the onset of transverse cracking increases from ∼5 to ∼10 N. For comparing the result of the scratch test with a common standardized test, we performed tests using
Fig. 4. Dependence of critical loads on film thickness for scratch testing of PC/SiO2 samples for first failure and transverse cracking.
Fig. 5. Dependence of Haze value after the Taber Abraser test (CS10F wheel, 500 g load per wheel) on film thickness for PC/SiO2.
W. Boentoro et al. / Thin Solid Films 517 (2009) 3121–3125
3123
Table 2 The material properties that are used in the simulation Material
E-Modul
Poisson ratio
[GPa] Glass SnO2:Sb
Fig. 6. Dependence of critical load on film thickness for scratch testing of Glass/SnO2:Sb.
the Taber Abraser. The samples haze values were statistically measured by a hazemeter at four different locations. The dependence of the haze level on film thickness: as prepared, after 100 and 1000 revolutions is depicted in Fig. 5. A strong decrease of haze is observed when the film thickness is raised. However, even for 4 µm thickness the haze is in the order of 12% after 1000 revolutions. 3.2. Scratch test of SnO2:Sb on glass The dependence of the critical load on film thickness showed in Fig. 6 reveals that the uncoated glass has higher scratch resistance than the coated glass up to 800 nm film thickness. The coating thickness of 1.6 µm protects the substrate to from the visible failure to ∼ 16 N. For higher film thickness, a strong increase of the critical load is observed.
72 139
Yield stress [MPa]
0.21 0.36
50 1112
Fig. 7 depicts the typical scratch pattern for the different loads and film thicknesses from the scratch test. Fig. 7 at the left part (First Failure) shows the onset of crack formation, where the first failures were visible on the samples. In the middle part: the failure at load of 13.6 N has been chosen to show the influence of the coating thickness visually on the samples at the same loading. On the uncoated glass sample (a), the scratch track was formed by adjacent the circular patterns. The tracks of the indenter on the coated glass with 50 nm coating thickness (b) were formed by intersections of the circular pattern. The failure formations on the thicker coating (c) and (d) have the similar pattern with (b), however a denser circular pattern with the increase of the coating thickness is observed. The tracks on the 800 nm coating thickness (e) and 1600 nm (f) were dominated by dense semi-circular failures, which are obviously fade out with the increase of the coating thickness. The Fig. 7 in the right part, shows the failure at the end of the scratch test (20 N) where the relative less failure were observed at the coated samples of 800 nm and 1600 nm. 3.3. Simulation A 3D model of an indenter scratching on the coated substrate was developed using finite element method ABAQUS Standard 6.7. The indenter was modeled as a rigid ball with radius of 200 µm. The coating with thickness of 1600 nm on glass substrate was assumed to
Fig. 7. Light microscope images of the scratch pattern. Sample (a) is uncoated glass, compared with the coating thickness of (b) 50 nm, (c) 200 nm, (d) 400 nm, (e) 800 nm, (f) 1600 nm. For comparing the formation of the failure related to the coating thickness, the failure at the similar loading (13.6 N) were performed.
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W. Boentoro et al. / Thin Solid Films 517 (2009) 3121–3125
Fig. 8. Schematic illustration of the three-dimensional finite element model of the scratch test.
be ideally smooth, without defects and homogenous. The constitute behavior of the coating is modeled for the simplicity with an elastic and perfectly plastic material law. The friction coefficient of 0.01 was assumed to be constant and based on the result of the scratch test. The material properties have been set according to the literature [3], and are listed in Table 2. To simulate scratch test experiment, the finite element geometry dimensions were chosen to approximate the real life experiment as close as possible. Preloading, sample and tip geometry and load applied in experiments have been modeled properly. The sliding distance 10 mm, substrate thickness 1.1 mm and coating thickness of 1600 nm are the basic of the model. However, compared to the test samples, the length of the model has been reduced to decrease computational time. Due to the plane of symmetry of the problem, the width of the original mesh can be reduced by half. The scratch test simulation was divided into three steps. First, the coating-substrate was pre loaded to 1 N (displacement controlled of 2.88 µm). Second, the indenter tip will slide 100 µm and at the same time indented to 28.8 µm within the step and not time dependent of the material deformation response. Last step is the unloading step. The figures are mapped according to the stress topography which represents a certain value of stress level on each color. The observation direction is related to the Fig. 8.
Fig. 9. The model SnO2:Sb (1.6 µm) on glass preloaded by indenter (shown only the half section), The stress unit is N/µm2 (1 MPa = 106 N/µm2).
Fig. 10. Simulation result of the first step: 1 N preloading showed the concentration tension stress in x-direction (scratch direction) occurs on the coating around the indenter contact area.
The simulation result of the 1 N pre loading showed in Fig. 9. In order to have a better observation point of view, the indenter tip is not shown as a whole, but only as a curve to identify the front side of the indenter (Fig. 10). The concentrations of tension stress (800 MPa) in 1-direction (scratch direction) obviously occur on the coating around the indenter contact area. Compression stress about 200 MPa showed on the substrate directly under the indenter. The common coating failure as the indenter moved to the right direction is shown in the Fig. 11. The simulation of the sliding indenter revealed that the maximal compression stress concentration presents in front of the indenter (Fig. 12) which lead to ploughing. The maximal tensile stress appears under the indenter and behind the sliding indenter that may initiate the failure. The secondary maximal tensile stress in the scratch boundary, may lead to form the angular crack that visible at the early phase of the scratch test as also showed in Fig. 11. 4. Discussion Our results show a significant impact of the film thickness on the critical load in scratch testing for both, coatings on glass and on polycarbonate. For PC, we observe an increase of the critical load for first failure up to ∼5 N at 2 µm thicknesses while for the increase from 2 to 4 µm thicknesses, only the critical load for the transverse cracking can be raised to 10 N. For the bare glass, the critical load for first failure is ∼ 10 N. This shows that even for 4 µm SiO2 on PC the scratch resistance of glass has not been achieved. The corresponding results for the abrasive test show a decrease of the haze level when the thickness is raised. The criteria for the automotive application of ΔH b 3% at 1000 revolution has not been met [4]. However, optimized multilayer stacks were described [5,6] being capable of fulfilling these demands. For the systems PC/SiO2, a similar analysis of the thickness dependence of the critical load in scratch testing has been performed by other authors [7], where the performance of PECVD SiO2 on PC has been investigated. They also reported that critical load increases linearly with the increase of the film thickness. A good agreement with their result has been also shown by the similar order of magnitude of critical load for first failure. For the coatings on glass, we observe weak performance compared to the bared glass when thin films were deposited. Films up to 400 nm thickness reveal inferior critical load for the first failure. For thicker films, however, the critical load could be raised up to 16 N. Therefore,
W. Boentoro et al. / Thin Solid Films 517 (2009) 3121–3125
3125
Fig. 11. The common coating failure as the indenter moved (to the right direction in the figure).
deformation of the surface. The stress concentrations may lead to the initiation of the possible failure generation. 5. Conclusion and outlook The analysis of the scratch resistance, based on the critical criteria of the failure form, has been investigated. The experiment shows that the thickness has a strong influence on the scratch resistance of the coatings for both systems, SiO2 on PC and SnO2:Sb on glass. In addition, to understand the phenomena during the scratch test, a three-dimensional finite element model has been developed. The simulation is necessary in order to give the representation of the origin and development of the failure during the scratch test. For the future work, the simulation model will be further enhanced with respect to more specific of the friction dependency and material elastic-plastic properties that will be obtained from nano-indentation measurements. Fig. 12. Stress distribution of the scratch test mode after ∼3 N sliding ““that showed the ploughing in front of the indenter and tensile stress behind the indenter.
these coatings may serve as a durable coating for outside low-e application. Ultra hard DLC coatings on glass, however, reveal a critical load for first failure up to ∼50 N [8]. Our work on the computer simulation of the scratch test using the finite element simulation shows that the pattern of the stress distribution corresponds to the experimentally observed failure mode. The simulation describes the stress situation as the diamond tip sliding over the coating surface, and also the elastic and plastic
References [1] I. Hamberg, J.S.E.M. Svensson, T.S. Eriksson, C.G. Granqvist, P. Arrenius, F. Norin, Appl. Optics 26 (1987) 2131. [2] E. Ando, S. Suzuki, N. Aomine, M. Miyazaki, M. Tada, Vacuum 59 (2000) 792. [3] E.G. Berasategui, S.J. Bull, T.F. Page, Thin Solid Films 447–448 (2004) 26. [4] Deutsches Institut für Normung e. V., Testing of glass and plastics, abrasion test; method using abrasion wheels and measurement of scattered light, DIN 52347, 1987-12. [5] K. Schmauder, K.D. Nauenburg, K. Kruse, G. Ickes, Thin Solid Films 502 (2006) 270. [6] D. Katsamberis, K. Browall, C. Iacovangelo, M. Neumann, H. Morgner, Progr. Org. Coat. 34 (1998) 130. [7] D. Rats, V. Hajek, L. Martinu, Thin Solid Films 340 (1999) 33. [8] V.S. Veerasamy, H.A. Luten, R.H. Petrmichl, S.V. Thomsen, Thin Solid Films 442 (2003) 1.