Depositing highly adhesive optical thin films on acrylic substrates Tomoaki Takahashi,1 Toshinori Harada,1 Hiroshi Murotani,1,* and Shigeharu Matumoto2 1
Course of Electro Photo Optics, Graduate School of Engineering, Tokai University, 4-1-1 Kitakaname, Hiratuka, Kanagawa 259-1292, Japan 2
Shincron Co., Ltd., 4-3-5 Minato Mirai, Nishi-ku, Yokohama-City, Kanagawa, Japan *Corresponding author: [email protected]‐tokai.ac.jp
Received 4 September 2013; revised 25 October 2013; accepted 28 October 2013; posted 29 October 2013 (Doc. ID 196876); published 15 January 2014
Optical thin films are used to control the reflectance and transmittance of optical components. However, conventional deposition technologies applicable to organic (plastic) substrates typically result in weak adhesion. We overcame this problem by using vacuum deposition in combination with sputtering to directly deposit a SiO2 optical thin film onto an acrylic resin substrate. We observed neither yellowing nor deformation. The hardness of the film is 2H as measured by the pencil hardness test, indicating successful modulation of optical properties without sacrificing substrate hardness. © 2014 Optical Society of America OCIS codes: (310.0310) Thin films; (310.6870) Thin films, other properties. http://dx.doi.org/10.1364/AO.53.00A287
1. Introduction
Optical thin films are essential for controlling the reflectance and transmittance from optical devices. Such devices historically consisted of inorganic materials (e.g., optical glass and quartz crystal), a primary reason being their good adhesion to optical thin films. As a result of recent demands for reductions in size, weight, and cost, researchers now often use organic materials such as resins (plastics) for optical devices. Since acrylic substrates are particularly useful for their excellent transparency, it is important to overcome their substrate adhesion problem [1,2]. Acrylic substrate adhesive properties have been improved by coating an intermediate layer between the substrate and optical thin film, but the intermediate layer deteriorates the surface precision and leads to problems such as light scattering and image distortion.
1559-128X/14/04A287-04$15.00/0 © 2014 Optical Society of America
To overcome these problems, we used vacuum deposition and sputtering to yield sufficient adhesion between a SiO2 optical thin film and an acrylic resin substrate, without using an intermediate layer, via direct deposition. Using either method by itself is insufficient. Sputtering results in excellent adhesion, but plasma and high-energy particles induce substrate yellowing, substrate surface heating, and other problems [3–5]. Although vacuum deposition does not damage a substrate, film adhesion is problematic. We devised an approach that combines the advantages of vacuum deposition and sputtering, yet overcomes their individual limitations. 2. Experimental Method A. Film Deposition Conditions
As a study of the first layer, we directly deposited a film onto an acrylic substrate (Nitto Jushi Kogyo) by radio-frequency (r.f.) sputtering (Table 1) and evaluated the deposition in terms of adhesive properties. Figure 1 shows a schematic diagram of the combination of coating equipment used in this study. The 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Table 1.
Sputtering Deposition Conditions
Target material Inlet gas r.f. power (W) O2 flow rate (sccm)
3. Results and Discussion
Si Ar, O2 100, 200, 300 1, 2, 5, 10
A. Optical Characteristics of Films
1. Sputtering Deposition Figure 2 shows the spectral characteristics of our optical thin films deposited by sputtering deposition on a BK-7 substrate as a function of the O2 flow rate and the r.f. power. A film deposited at an r.f. power of 100 or 200 W and an O2 flow rate of 1 sccm (sccm denotes 100 90
Transmittance (%)
80
Before deposition O2 flow rate: 1 sccm O2 flow rate: 2 sccm O2 flow rate: 5 sccm O2 flow rate: 10 sccm
70 60 50 40 30 20 10
Fig. 1. Schematic diagram of combination coating equipment (sputtering and vacuum deposition).
0 200
400
600
800 1000 1200 1400 1600 1800 2000
Wavelength (nm)
Film Evaluation
We evaluated the adhesive properties of a SiO2 optical thin film directly deposited on an acrylic substrate, by using a cross-hatch test (ISO 9211-4 [6]). The rate of tape removal was the snap rate. We evaluated the hardness of films by using the pencil hardness test (JIS K 5600-5-4 [7]). The spectral characteristics were measured with a spectrophotometer (V-570, JASCO). In addition, we evaluated the stress characteristics of the film with a laser Fizeau interferometer (F601, Fujinon). Substrate effects hindered evaluation of the optical and mechanical characteristics of the SiO2 optical thin films deposited on acrylic substrates. Therefore, we evaluated these characteristics after deposition on BK-7 optical glass (Schott). Table 2.
Condition Condition Condition Condition Condition
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1 2 3 4 5
Sputtering and Vacuum Deposition Conditions
Sputtering r.f. Power: 200 (W) Ar∶O2 70∶10 (sccm) Time (s)
Vacuum Deposition Electron beam current: 120 (mA) Time (s)
120 240 480 720 960
68 60 52 45 38
APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014
90 80
Transmittance (%)
B.
100
70 60
Before deposition O2 flow rate: 1 sccm O2 flow rate: 2 sccm O2 flow rate: 5 sccm O2 flow rate: 10 sccm
50 40 30 20 10 0 200 400
600
800 1000 1200 1400 1600 1800 2000
Wavelength (nm)
100 90 80
Transmittance (%)
feature of this deposition equipment is that the sputtering and vacuum deposition are done in one chamber. We adjusted the compound ratio of the film by vacuum deposition and sputtering. The total thickness of the optical thin film was approximately 500 nm. Table 2 shows the film formation conditions.
70 60 50
Before deposition O2 flow rate: 1 sccm O2 flow rate: 2 sccm O2 flow rate: 5 sccm O2 flow rate: 10 sccm
40 30 20 10 0 200
400
600
800 1000 1200 1400 1600 1800 2000
Wavelength (nm) Fig. 2. Spectral transmission of thin films (sputtering deposition). r.f. power (a) 100 W. (b) 200 W. (c) 300 W.
100
Transmittance(%)
90
Before deposition Conditions 1 Conditions 2 Conditions 3 Conditions 4
80
70
60
50
400
600
800
1000
1200
1400
1600
1800
2000
Wavelength(nm) Fig. 3. Spectral transmission of thin films (sputtering and vacuum deposition).
cubic centimeters per minute at standard temperature and pressure) exhibited light absorption. A film deposited at an r.f. power of 300 W and an O2 flow rate of 1 or 2 sccm also exhibited light absorption. These results suggest that, in terms of optical properties, sputtering alone is insufficient for useful thin film deposition. 2. Sputtering and Vacuum Deposition Figure 3 shows the spectral characteristics of SiO2 films directly deposited by using sputtering and vacuum deposition on an acrylic substrate; we observed neither yellowing nor deformation. B.
Mechanical Characteristics of Films
1. Sputtering Deposition Figure 4 shows the cross-hatch test results for the SiO2 films deposited by modulating the sputtering deposition as a function of the O2 flow rate and the r.f. power. This shows that highly adhesive properties were obtained when the r.f. power was 200 W and the O2 flow rate was 10 sccm. Highly adhesive r.f. power: 100 W r.f. power: 200 W r.f. power: 300 W
ISO Classification
0 1
Fig. 5. Internal stress of SiO2 films deposited by using sputtering deposition.
Figure 5 shows the stress characteristics of our SiO2 films, and suggests that the internal stress corresponded to the compressive stress in all of the thin films and the compressive strength tended to decrease in accordance with increasing the O2 flow rate. The film deposited at 200 W shows low absorption and sufficient adhesion. The difference between deposition batches of the stress becomes small in the condition of O2 flow rate of 10 sccm. It is thought that this is because Si is oxidized enough in this condition and SiO2 is deposited. In these films, Figs. 4 and 5 show that, as the internal stress decreases, adhesion tends to increase. Table 3 shows the pencil hardness test results. The hardness of our optical thin films deposited by sputtering was the same as that of the acrylic substrate (2H). However, a film deposited at an r.f. power of 300 W and an O2 flow rate of 5 or 10 sccm exhibited decreasing hardness. 2. Sputtering and Vacuum Deposition Figure 6 shows the cross-hatch test results of SiO2 thin films deposited by using sputtering in combination with vacuum deposition. We obtained little delamination under either condition 1 or 2 and large delamination under conditions 3–5. In addition, the sputtering time necessary decreased and the substrate tended to be highly adhesive. This suggests that reducing sputtering time avoids damageinduced weak adhesion.
2 3
Table 3.
Pencil Hardness Test Results of SiO2 Films Deposited by Sputtering Deposition
4
Pencil Hardness Test Results O2 Flow Rate (sccm)
5 0
Weakly adhesive
2
4
6
8
10
O2 flow rate (sccm)
Fig. 4. Cross-hatch test results of SiO2 films deposited by using sputtering deposition.
r.f. Power (W) 100 200 300
1
2
5
10
2H 2H 2H
2H 2H 2H
2H 2H H
2H 2H H
1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Highly adhesive
Table 4. Pencil Hardness Test Results of SiO2 Films Deposited by Sputtering and Vacuum Deposition
0
Condition 1 2 3 4 5
ISO Classification
1
2
2H 2H 2H 2H 2H
3
6H or more, further validating our conclusion that substrate hardness was retained after thin film deposition.
4
5
Condition1
Condition2
Condition3
Condition4
Condition5
4. Conclusion
Weakly adhesive Fig. 6. Cross-hatch test results of SiO2 films deposited by sputtering and vacuum deposition.
100
Compressive internal stress (MPa)
Pencil Hardness Test Results
95 90 85 80 75 70 65 60 55 50 Condition1
Condition2
Condition3
Condition4
Condition5
Fig. 7. Internal stress test results of SiO2 films deposited by sputtering and vacuum deposition.
We obtained useful adhesion when depositing, via sputtering, SiO2 thin films on a BK-7 substrate by optimizing the O2 flow rate and r.f. power. High adhesion tended to occur when we deposited the thin films, via a combination of sputtering and vacuum deposition, directly on an acrylic substrate. The hardness of the resulting thin films was the same as that of the acrylic substrate. Our deposition method will be useful for controlling the reflectance and transmittance of optical thin films deposited on acrylic substrates without sacrificing substrate hardness. We acknowledge Mr. Seino, Mr. Kaite, and Mr. Abe of Fine Crystal Co., Ltd. for preparing the thin film samples. We thank Mr. Miyamoto and Mr. Haraki of the Joint Technology Management Office at the Future Science and Technology Joint Research Center of Tokai University for assistance with the measurements. References
Figure 7 shows the stress characteristics of our thin films. The internal stress corresponded to the compressive stress in all of our thin films and the compressive strength tended to increase in accordance with increasing sputtering time. In summary, Figs. 6 and 7 show that as internal stress decreases, adhesion tends to increase. We used the sputtering condition (r.f. power 200 W and O2 flow rate 10 sccm) for the first layer in sputtering and vacuum deposition. Because this sputtering condition showed stable low stress and high adhesive. At this time, the films with low stress showed a tendency toward high adhesion. Table 4 shows the pencil hardness test results. The hardness of our thin films deposited by sputtering and vacuum deposition is the same as that of the acrylic substrate (2H). The hardness of the thin films prepared by sputtering and vacuum deposition on a BK-7 substrate is
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APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014
1. J. Allen and A. Tregunna, “Antireflection coatings for plastic optics,” J. Phys. D 21, S92–S95 (1988). 2. P. Munzert, C. Praefke, U. Schulz, and N. Kaiser, “Adhesion of vacuum deposited optical coatings on PMMA and polycarbonate,” J. Adhes. Sci. Technol. 26, 2269–2276 (2012). 3. U. Schulz, P. Munzert, and N. Kaiser, “Surface modification of PMMA by DC glow discharge and microwave plasma treatment for the improvement of coating adhesion,” Surf. Coat. Technol. 142–144, 507–511 (2001). 4. J. E. Klembeg-Sapieha, L. Martinu, N. L. S. Yamasaki, and C. W. Lantman, “Tailoring the adhesion of optical films on polymethyl-methacrylate by plasma-induced surface stabilization,” Thin Solid Films 476, 101–107 (2005). 5. Ph. Duchatelard, G. Baud, J. P. Besse, and M. Jacquet, “Alumina coatings on PMMA: optimization of adherence,” Thin Solid Films 250, 142–150 (1994). 6. International Organization of Standardization, “Optics and photonics—Optical coatings—Part 4: specific test methods,” ISO 9211-4 (2012). 7. “Testing method for paints—Part 5: mechanical property of film—Section 4: scratch hardness (Pencil method),” JIS K 5600-5-4 (ISO/DIN 15184) (1999).
Course of Electro Photo Optics, Graduate School of Engineering, Tokai University, 4-1-1 Kitakaname, Hiratuka, Kanagawa 259-1292, Japan 2
Shincron Co., Ltd., 4-3-5 Minato Mirai, Nishi-ku, Yokohama-City, Kanagawa, Japan *Corresponding author: [email protected]‐tokai.ac.jp
Received 4 September 2013; revised 25 October 2013; accepted 28 October 2013; posted 29 October 2013 (Doc. ID 196876); published 15 January 2014
Optical thin films are used to control the reflectance and transmittance of optical components. However, conventional deposition technologies applicable to organic (plastic) substrates typically result in weak adhesion. We overcame this problem by using vacuum deposition in combination with sputtering to directly deposit a SiO2 optical thin film onto an acrylic resin substrate. We observed neither yellowing nor deformation. The hardness of the film is 2H as measured by the pencil hardness test, indicating successful modulation of optical properties without sacrificing substrate hardness. © 2014 Optical Society of America OCIS codes: (310.0310) Thin films; (310.6870) Thin films, other properties. http://dx.doi.org/10.1364/AO.53.00A287
1. Introduction
Optical thin films are essential for controlling the reflectance and transmittance from optical devices. Such devices historically consisted of inorganic materials (e.g., optical glass and quartz crystal), a primary reason being their good adhesion to optical thin films. As a result of recent demands for reductions in size, weight, and cost, researchers now often use organic materials such as resins (plastics) for optical devices. Since acrylic substrates are particularly useful for their excellent transparency, it is important to overcome their substrate adhesion problem [1,2]. Acrylic substrate adhesive properties have been improved by coating an intermediate layer between the substrate and optical thin film, but the intermediate layer deteriorates the surface precision and leads to problems such as light scattering and image distortion.
1559-128X/14/04A287-04$15.00/0 © 2014 Optical Society of America
To overcome these problems, we used vacuum deposition and sputtering to yield sufficient adhesion between a SiO2 optical thin film and an acrylic resin substrate, without using an intermediate layer, via direct deposition. Using either method by itself is insufficient. Sputtering results in excellent adhesion, but plasma and high-energy particles induce substrate yellowing, substrate surface heating, and other problems [3–5]. Although vacuum deposition does not damage a substrate, film adhesion is problematic. We devised an approach that combines the advantages of vacuum deposition and sputtering, yet overcomes their individual limitations. 2. Experimental Method A. Film Deposition Conditions
As a study of the first layer, we directly deposited a film onto an acrylic substrate (Nitto Jushi Kogyo) by radio-frequency (r.f.) sputtering (Table 1) and evaluated the deposition in terms of adhesive properties. Figure 1 shows a schematic diagram of the combination of coating equipment used in this study. The 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
A287
Table 1.
Sputtering Deposition Conditions
Target material Inlet gas r.f. power (W) O2 flow rate (sccm)
3. Results and Discussion
Si Ar, O2 100, 200, 300 1, 2, 5, 10
A. Optical Characteristics of Films
1. Sputtering Deposition Figure 2 shows the spectral characteristics of our optical thin films deposited by sputtering deposition on a BK-7 substrate as a function of the O2 flow rate and the r.f. power. A film deposited at an r.f. power of 100 or 200 W and an O2 flow rate of 1 sccm (sccm denotes 100 90
Transmittance (%)
80
Before deposition O2 flow rate: 1 sccm O2 flow rate: 2 sccm O2 flow rate: 5 sccm O2 flow rate: 10 sccm
70 60 50 40 30 20 10
Fig. 1. Schematic diagram of combination coating equipment (sputtering and vacuum deposition).
0 200
400
600
800 1000 1200 1400 1600 1800 2000
Wavelength (nm)
Film Evaluation
We evaluated the adhesive properties of a SiO2 optical thin film directly deposited on an acrylic substrate, by using a cross-hatch test (ISO 9211-4 [6]). The rate of tape removal was the snap rate. We evaluated the hardness of films by using the pencil hardness test (JIS K 5600-5-4 [7]). The spectral characteristics were measured with a spectrophotometer (V-570, JASCO). In addition, we evaluated the stress characteristics of the film with a laser Fizeau interferometer (F601, Fujinon). Substrate effects hindered evaluation of the optical and mechanical characteristics of the SiO2 optical thin films deposited on acrylic substrates. Therefore, we evaluated these characteristics after deposition on BK-7 optical glass (Schott). Table 2.
Condition Condition Condition Condition Condition
A288
1 2 3 4 5
Sputtering and Vacuum Deposition Conditions
Sputtering r.f. Power: 200 (W) Ar∶O2 70∶10 (sccm) Time (s)
Vacuum Deposition Electron beam current: 120 (mA) Time (s)
120 240 480 720 960
68 60 52 45 38
APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014
90 80
Transmittance (%)
B.
100
70 60
Before deposition O2 flow rate: 1 sccm O2 flow rate: 2 sccm O2 flow rate: 5 sccm O2 flow rate: 10 sccm
50 40 30 20 10 0 200 400
600
800 1000 1200 1400 1600 1800 2000
Wavelength (nm)
100 90 80
Transmittance (%)
feature of this deposition equipment is that the sputtering and vacuum deposition are done in one chamber. We adjusted the compound ratio of the film by vacuum deposition and sputtering. The total thickness of the optical thin film was approximately 500 nm. Table 2 shows the film formation conditions.
70 60 50
Before deposition O2 flow rate: 1 sccm O2 flow rate: 2 sccm O2 flow rate: 5 sccm O2 flow rate: 10 sccm
40 30 20 10 0 200
400
600
800 1000 1200 1400 1600 1800 2000
Wavelength (nm) Fig. 2. Spectral transmission of thin films (sputtering deposition). r.f. power (a) 100 W. (b) 200 W. (c) 300 W.
100
Transmittance(%)
90
Before deposition Conditions 1 Conditions 2 Conditions 3 Conditions 4
80
70
60
50
400
600
800
1000
1200
1400
1600
1800
2000
Wavelength(nm) Fig. 3. Spectral transmission of thin films (sputtering and vacuum deposition).
cubic centimeters per minute at standard temperature and pressure) exhibited light absorption. A film deposited at an r.f. power of 300 W and an O2 flow rate of 1 or 2 sccm also exhibited light absorption. These results suggest that, in terms of optical properties, sputtering alone is insufficient for useful thin film deposition. 2. Sputtering and Vacuum Deposition Figure 3 shows the spectral characteristics of SiO2 films directly deposited by using sputtering and vacuum deposition on an acrylic substrate; we observed neither yellowing nor deformation. B.
Mechanical Characteristics of Films
1. Sputtering Deposition Figure 4 shows the cross-hatch test results for the SiO2 films deposited by modulating the sputtering deposition as a function of the O2 flow rate and the r.f. power. This shows that highly adhesive properties were obtained when the r.f. power was 200 W and the O2 flow rate was 10 sccm. Highly adhesive r.f. power: 100 W r.f. power: 200 W r.f. power: 300 W
ISO Classification
0 1
Fig. 5. Internal stress of SiO2 films deposited by using sputtering deposition.
Figure 5 shows the stress characteristics of our SiO2 films, and suggests that the internal stress corresponded to the compressive stress in all of the thin films and the compressive strength tended to decrease in accordance with increasing the O2 flow rate. The film deposited at 200 W shows low absorption and sufficient adhesion. The difference between deposition batches of the stress becomes small in the condition of O2 flow rate of 10 sccm. It is thought that this is because Si is oxidized enough in this condition and SiO2 is deposited. In these films, Figs. 4 and 5 show that, as the internal stress decreases, adhesion tends to increase. Table 3 shows the pencil hardness test results. The hardness of our optical thin films deposited by sputtering was the same as that of the acrylic substrate (2H). However, a film deposited at an r.f. power of 300 W and an O2 flow rate of 5 or 10 sccm exhibited decreasing hardness. 2. Sputtering and Vacuum Deposition Figure 6 shows the cross-hatch test results of SiO2 thin films deposited by using sputtering in combination with vacuum deposition. We obtained little delamination under either condition 1 or 2 and large delamination under conditions 3–5. In addition, the sputtering time necessary decreased and the substrate tended to be highly adhesive. This suggests that reducing sputtering time avoids damageinduced weak adhesion.
2 3
Table 3.
Pencil Hardness Test Results of SiO2 Films Deposited by Sputtering Deposition
4
Pencil Hardness Test Results O2 Flow Rate (sccm)
5 0
Weakly adhesive
2
4
6
8
10
O2 flow rate (sccm)
Fig. 4. Cross-hatch test results of SiO2 films deposited by using sputtering deposition.
r.f. Power (W) 100 200 300
1
2
5
10
2H 2H 2H
2H 2H 2H
2H 2H H
2H 2H H
1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Highly adhesive
Table 4. Pencil Hardness Test Results of SiO2 Films Deposited by Sputtering and Vacuum Deposition
0
Condition 1 2 3 4 5
ISO Classification
1
2
2H 2H 2H 2H 2H
3
6H or more, further validating our conclusion that substrate hardness was retained after thin film deposition.
4
5
Condition1
Condition2
Condition3
Condition4
Condition5
4. Conclusion
Weakly adhesive Fig. 6. Cross-hatch test results of SiO2 films deposited by sputtering and vacuum deposition.
100
Compressive internal stress (MPa)
Pencil Hardness Test Results
95 90 85 80 75 70 65 60 55 50 Condition1
Condition2
Condition3
Condition4
Condition5
Fig. 7. Internal stress test results of SiO2 films deposited by sputtering and vacuum deposition.
We obtained useful adhesion when depositing, via sputtering, SiO2 thin films on a BK-7 substrate by optimizing the O2 flow rate and r.f. power. High adhesion tended to occur when we deposited the thin films, via a combination of sputtering and vacuum deposition, directly on an acrylic substrate. The hardness of the resulting thin films was the same as that of the acrylic substrate. Our deposition method will be useful for controlling the reflectance and transmittance of optical thin films deposited on acrylic substrates without sacrificing substrate hardness. We acknowledge Mr. Seino, Mr. Kaite, and Mr. Abe of Fine Crystal Co., Ltd. for preparing the thin film samples. We thank Mr. Miyamoto and Mr. Haraki of the Joint Technology Management Office at the Future Science and Technology Joint Research Center of Tokai University for assistance with the measurements. References
Figure 7 shows the stress characteristics of our thin films. The internal stress corresponded to the compressive stress in all of our thin films and the compressive strength tended to increase in accordance with increasing sputtering time. In summary, Figs. 6 and 7 show that as internal stress decreases, adhesion tends to increase. We used the sputtering condition (r.f. power 200 W and O2 flow rate 10 sccm) for the first layer in sputtering and vacuum deposition. Because this sputtering condition showed stable low stress and high adhesive. At this time, the films with low stress showed a tendency toward high adhesion. Table 4 shows the pencil hardness test results. The hardness of our thin films deposited by sputtering and vacuum deposition is the same as that of the acrylic substrate (2H). The hardness of the thin films prepared by sputtering and vacuum deposition on a BK-7 substrate is
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APPLIED OPTICS / Vol. 53, No. 4 / 1 February 2014
1. J. Allen and A. Tregunna, “Antireflection coatings for plastic optics,” J. Phys. D 21, S92–S95 (1988). 2. P. Munzert, C. Praefke, U. Schulz, and N. Kaiser, “Adhesion of vacuum deposited optical coatings on PMMA and polycarbonate,” J. Adhes. Sci. Technol. 26, 2269–2276 (2012). 3. U. Schulz, P. Munzert, and N. Kaiser, “Surface modification of PMMA by DC glow discharge and microwave plasma treatment for the improvement of coating adhesion,” Surf. Coat. Technol. 142–144, 507–511 (2001). 4. J. E. Klembeg-Sapieha, L. Martinu, N. L. S. Yamasaki, and C. W. Lantman, “Tailoring the adhesion of optical films on polymethyl-methacrylate by plasma-induced surface stabilization,” Thin Solid Films 476, 101–107 (2005). 5. Ph. Duchatelard, G. Baud, J. P. Besse, and M. Jacquet, “Alumina coatings on PMMA: optimization of adherence,” Thin Solid Films 250, 142–150 (1994). 6. International Organization of Standardization, “Optics and photonics—Optical coatings—Part 4: specific test methods,” ISO 9211-4 (2012). 7. “Testing method for paints—Part 5: mechanical property of film—Section 4: scratch hardness (Pencil method),” JIS K 5600-5-4 (ISO/DIN 15184) (1999).
