Analysis of Polycarbonate Lenses Using a Profilometer, Tribometer, and a Mechanical Tester

In a wide variety of optical applications, polycarbonate lenses are commonly used. Due to their high impact resistance, low weight, and relatively cheaper cost of high-volume production, they are more practical than traditional glass in many different applications [1].

A few of these applications necessitate conditions of safety (e.g. safety eyewear), complexity (e.g. Fresnel lens), or durability (e.g. traffic light lens). Further, these criteria are often difficult to satisfy without the use of plastics.

Thus, plastic lenses stand out in its field, due to its ability to cheaply meet many diverse requirements while maintaining sufficient optical qualities. Even polycarbonate lenses have certain limitations. For instance, one of the main concerns for consumers is the ease with which they can be scratched. Thus, to compensate for this weakness, extra processes can be carried out to apply an anti-scratch coating.

Nanovea examines some important properties of polycarbonate lenses by utilizing three of its proprietary metrology instruments: the Profilometer, Tribometer, and Mechanical Tester.

Importance of Testing Polycarbonate Lenses

To calculate the surface roughness and radius of curvature, the surface data of a lens is ideal, since these properties infl­uence the optical quality of the lens. While the radius of curvature affects the lens’ optical power, its surface roughness will have an impact on its ability to scatter light. It is also important to measure the thickness of the lens, which will in turn affect its effective focal length.

The more the defects on the surface of the lens, the more the quality of the lens will decrease. What’s more, material with high scratch resistance has a tendency to wear less over time and is thus less prone to defects caused by external sources. Scratch hardness is defined as the resistance of the sample to scratch defects, a property that can be used to determine the scratch hardness of the bulk material or effectiveness of a scratch-resistant coating. In addition, the process of adhesive scratch testing can be undertaken in order to determine the quality of adhesion between the coating and the lens.

Using tribology testing against various materials, it is possible to obtain the Coefficient of Friction (COF). A practical consideration would be to understand how other materials will behave when interacting with the lens, since polycarbonate lenses find use in many different applications.

In other words, friction can be minimized (or maximized) when selecting complementary materials. Thus, the use of wear testing demonstrates the durability of the sample under different conditions.

This study’s testing and results represent how the sample will perform in real life applications. Results from this study can be used to determine which type of material, process, or design is ideal for the user’s specific application. Using these highly accurate instruments, effective quality control testing can also be repeatedly conducted.

Measurement Objective

This case study conducts a general investigation on several important properties of a polycarbonate lens. The profilometer, tribometer, and mechanical tester demonstrates the following properties: surface roughness, radius of curvature, thickness, scratch hardness, COF against various materials, as well as wear rate.

Example of polycarbonate lens about to be tested on Nanovea Profilometer

Example of polycarbonate lens about to be tested on Nanovea Profilometer

Example of polycarbonate lens about to be tested on Nanovea Tribometer

Example of polycarbonate lens about to be tested on Nanovea Tribometer

Example of polycarbonate lens about to be tested on Nanovea Mechanical Tester

Example of polycarbonate lens about to be tested on Nanovea Mechanical Tester

Profilometry

Equipment Featured

NANOVEA HS2000

  • Advanced Automation
  • High Speed
  • Precision Flatness Measurement
  • Customizable Options
  • Rigid and Stable Structure
  • User-Friendly Design

NANOVEA PS50

  • 50 mm x 50 mm XY
  • Compact Benchtop
  • Ideal Upgrade From Stylus and Laser Technologies

Radius Of Curvature And Roughness

Measurement Parameters

Table 1. Test parameters for roughness and radius measurements on the lens

Test Parameter Value
Instrument Nanovea HS2000
Optical Sensor L1 Lens (200 μm Z-range)
Scan size (mm) 10 mm x 10 mm
Step size (µm) 5 μm x 5 μm
Scan time (h:m:s) 00:01:02

Figure 1 and 2 displays results of the pro­filometry measurements. The Figures below are 2D and 3D images of the lens’s true form.

Radius

The area scan on the lens ensures the capture of the radius of curvature at the apex of the curve. In an effort to observe the symmetry of the lens, radius of curvature was calculated from both the X- and Y-axis. For the front side, values of 142.1 and 135.5 mm were obtained, while these values were 137.0 mm and 139.2 mm were for the back side.

Profile extraction in the X-axis (left) and Y-axis (right) of front side of polycarbonate lens

Figure 5. Profile extraction in the X-axis (left) and Y-axis (right) of front side of polycarbonate lens

Profile extraction in the X-axis (left) and Y-axis (right) of back side of polycarbonate lens

Figure 6. Profile extraction in the X-axis (left) and Y-axis (right) of back side of polycarbonate lens

Roughness

The form of the sample must be removed in order to obtain roughness data. To achieve this, a Gaussian filter with a nesting index of 0.25 mm was applied to obtain roughness height parameters. An Sa value of 26.76 nm was obtained for the front side of the polycarbonate lens, which was in contrast to 18.16 nm for the back side of the plastic lens. Their respective Sq values were 37.77 nm and 36.02. The results showed that these values are very low, making them ideal to minimize scattering of light upon the interaction of light with the lens’s surface.

Thickness

Measurement Parameters

Table 2. Test parameters for roughness and radius measurements on the lens

Test Parameter Value
Instrument Nanovea PS50
Optical Sensor PS5 (10000 μm Z-range)
Scan size (mm) 5 mm x 5 mm
Step size (µm) 10 μm x 10 μm
Scan time (h:m:s) 00:36:32

The thickness of the polycarbonate lens was obtained by using Nanovea’s point sensor system. This measurement was effective due to a focal point at each surface. Further, it is possible to have multiple focal points due to the axial chromatism technique. Using the sample’s index of refraction, the diff­erence in refraction between air and the sample is corrected. Thus, Figure 7 displays the two surfaces, top and bottom.

False-color view of top surface (left) and bottom surface (right)

Figure 7. False-color view of top surface (left) and bottom surface (right)

False-color view (left) and height parameters (right) for thickness of polycarbonate

Figure 8. False-color view (left) and height parameters (right) for thickness of polycarbonate

Due to the fact that the composition of this sample was unknown, this was set to a value of common plastic: polycarbonate – 1.58. Further, thickness was obtained by subtracting the two surfaces scanned. The mean thickness of the sample, scanned near the apex of the curvature, was found to be approximately 2.611 mm.

Mechanical Testing

Equipment Featured

NANOVEA PB1000

  • Fully Upgradeable
  • Nano to Macro Range without the need for exchange
  • Spacious Platform with Adjustable Height Clearance
  • Robust and Low Cost of Ownership
  • Fully Upgradeable

Scratch Hardness

Measurement Parameters

Table 3. Parameters used for scratch hardness testing on polycarbonate lens

Test Parameter Value
Load type Constant
Final Load (N) 15
Scratch Length (mm) 5
Scratching speed (mm/min) 18
Indenter geometry 120° cone
Indenter material (tip) Diamond
Indenter tip radius (μm) 200

In accordance with ASTM-G171, the scratch test was conducted. In order to minimize error caused by the curvature of the lens, scratches were created at the apex of the lens. Thus, a scratch hardness of 420.59 ± 8.69MPa was obtained. As was anticipated, the scratch hardness value is quite low. This is because of the nature of plastics. In contrast, the scratch hardness testing previously conducted on aluminum, copper, and steel in the past were 0.84, 0.52, and 3.20GPa respectively [2].

Despite the fact that testing conditions differed, the scratch resistance of the polycarbonate lens appears to be in the same magnitude as a soft, scratch-prone metal like copper.

Friction graph obtained from the scratch test

Figure 9. Friction graph obtained from the scratch test

Scratch hardness measurement conducted under an optical microscope. The blue dotted lines are positioned at the edge of the scratch to obtain scratch width.

Figure 10. Scratch hardness measurement conducted under an optical microscope. The blue dotted lines are positioned at the edge of the scratch to obtain scratch width.

Table 4. Results from scratch hardness test

. Measurement 1 (MPa) Measurement 2 (MPa) Measurement 3 (MPa)
Scratch 1 432.19 418.89 412.52
Scratch 2 431.51 416.25 413.4
Scratch 3 431.71 421.55 409.8

Scratch Imaging With Optical Profilometry

Using the profilometry instrument, the polycarbonate lens was profiled in order to closely inspect the outcome of the scratch test. Figure 13 shows that a great deal of material was found to be surrounding the area where the scratch took place. Further, it can also be seen that the volume of material around the scratch (Peak) and the volume lost (Hole) are approximately the same.

Thus, this study showed that the soft plastic seems to have been easily displaced during the scratch. This implied that the material has a low scratch resistance. The mean depth of the scratch ended up being 7.864 ± 0.2652 µm into the surface. As per Figure 14, this was obtained by extracting a series of profi­le across the scratched area and averaging the maximum valley depth (Pv) of each profile.

False-color view of a scratch made on the lens

Figure 11. False-color view of a scratch made on the lens

3-D view of scratch made on the lens

Figure 12. 3-D view of scratch made on the lens

Volume of a hole/peak analysis on the scratch created

Figure 13. Volume of a hole/peak analysis on the scratch created

Extracted series of profiles (left) and their primary profile parameters (right). Red line indicates the mean profile.

Figure 14. Extracted series of profiles (left) and their primary profile parameters (right). Red line indicates the mean profile.

Tribology

Equipment Featured

NANOVEA T50

  • Durable and Open Platform
  • High Micro Accuracy
  • Wide Range of Environmental Conditions
  • Longest Industry Warranty

Coefficient Of Friction

Measurement Parameters

Table 5. Parameters used for coefficient of friction testing on polycarbonate lens

Test Parameter
Load (N) 0.5
Test Duration (min) 5
Speed (rpm) 10
Radius (mm) 0.0-5.0
Total Distance (m) 0.78
Pin Geometry Ball
Pin Material Rubber, PTFE, ZrO2, Al3O2, SS440C
Pin Diameter (mm) 6

Table 6. Pin-On-Disk material combinations

Combination Disk Material Pin Material
1 Polycarbonate Rubber
2 Polycarbonate PTFE
3 Polycarbonate ZrO2
4 Polycarbonate Al3O2
5 Polycarbonate SS440C

Coefficient Of Friction

In an effort to ensure that the pins would pass over an unworn region throughout all tests, a Pin-On-Disk Spiral Test was performed. The fi­rst fi­ve revolutions were cropped from the graphs, to remove data when the radius was near 0 (i.e. minimal tangential movement). When analyzing the COF data, the curvature of the lens must be kept in consideration. Thus, the test results rank the following material from highest COF to lowest COF: Rubber, Al2O3, ZrO2, PTFE, SS440C. All tests were performed with a small normal force, in order to minimize the effects of wear on the sample.

COF graphs of 1) Rubber, 2) PTFE, 3) ZrO2, 4) Al2O3, 5) SS440C

Figure 15. COF graphs of 1) Rubber, 2) PTFE, 3) ZrO2, 4) Al2O3, 5) SS440C

Table 7. Results of COF testing on Plastic Lens

Pin Material Max COF Min COF Average COF
Rubber 0.947 0.277 0.734
PTFE 0.210 0.027 0.089
ZrO2 0.193 0.043 0.106
Al3O2 0.215 0.051 0.120
SS440C 0.175 0.022 0.072

Linear Wear

Measurement Parameters

Table 8. Parameters used for linear wear testing on polycarbonate lens

Test Parameter Value
Load (N) 20
Test Duration (min) 20
Speed (rpm) 100
Amplitude (mm) 10
Total Distance (m) 40
Ball Material ZrO2
Ball Diameter (mm) 6

Finally, to minimize eff­ects from curvature, the linear test was conducted near the apex of the lens. It is possible to observe two stages of wear by analyzing the COF graph. Between 0 and 200 revolutions, the two surfaces adapted to the surface of the sample. However, after 200 revolutions, significant wear began to kick in. Thus, three-body abrasion wear was created when loose particles created from the wear test are now rampant along the surface of the worn area.

Finally, profiling the wear track, analytically removing the curvature from the lens, and a volume of a hole study was conducted to accurately calculate wear rate and the volume loss (Figure 18). In total, a volume of 577,479,379 µm3 was lost. The zirconium oxide wore an average of 61.69 ± 6.830 µm into the plastic lens.

Friction graph from linear wear testing

Figure 16. Friction graph from linear wear testing

Extracted series of profiles (left) and their primary profile parameters (right). Red line indicated the mean profile.

Figure 19. Extracted series of profiles (left) and their primary profile parameters (right). Red line indicated the mean profile.

Table 9. Linear wear testing results

Max COF Min COF Average COF Volume Loss (µm3) Wear Rate x 10-5 (mm3/Nm)
0.459 0.037 0.336 577479379 72.185

Conclusion

Nanovea’s metrology instruments were used to investigate critical properties of polycarbonate lens. For material selection and quality control processes, the ability to accurately measure and quantify properties of materials is of high importance.

The different types of tests conducted show that Nanovea can target a wide variety of specific applications with its precision instruments. With Nanovea’s temperature, humidity, lubrication, and corrosion modules, diverse environmental conditions can also be easily applied.

References

[1] Kogler, Kent. "Selection of plastics for optical applications.” Advanced materials and processes technology (1999).

[2] Li, Duanjie. "SCRATCH HARDNESS MEASUREMENT USING MECHANICAL TESTER." (2014).

This information has been sourced, reviewed and adapted from materials provided by Nanovea.

For more information on this source, please visit Nanovea.

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