Testing Cell Phone Screen Protectors for Scratch Resistance

Even though phone screens are created to be resistant to scratching and shattering, they can still be damaged. Daily phone usage leads to wear and tear, e.g. gathering cracks and scratches. Screen protectors are an affordable damage prevention item which are frequently purchased to increase a screen's durability, as repairing these screens can be costly.

We are able to clearly establish critical loads at which screen protectors exhibit failure because of scratch testing by using the Nanovea PB1000 Mechanical Tester’s Macro Module paired with the acoustic emissions (AE) sensor, to generate a comparative study between two varieties of screen protectors.

The Importance of Testing Screen Protectors

Two typical varieties of screen protector materials are tempered glass and TPU (thermoplastic polyurethane). Tempered glass is considered the best of the two, as it supplies superior scratch and impact protection. Yet, it is also the most costly. On the other hand, TPU screen protectors cost less and are a popular choice for consumers who favor plastic screen protectors.

As screen protectors are usually made of materials with brittle properties, and are designed to absorb impacts and scratches, in-situ AE detection combined with controlled scratch testing is an optimal test setup for establishing the loads at which cohesive failures (e.g. chipping, cracking, and fracture) and/or adhesive failures (e.g. spallation and delamination) happen.

Measurement Objectives

During this study, using Nanovea’s PB1000 Mechanical Tester’s Macro Module, three scratch tests were carried out on two different commercial screen protectors. By utilizing an optical microscope and acoustic emissions sensor, the critical loads where each screen protector exhibited failure(s) could be identified.

Screen Protector sample on Nanovea PB1000 Mechanical Tester

Screen Protector sample on Nanovea PB1000 Mechanical Tester

Measurement Parameters

The Nanovea PB1000 Mechanical Tester was employed in order to test two screen protectors applied onto a phone screen and clamped down to a friction sensor table. The test parameters for all of the scratches are shown below in Table 1.

Table 1. Test parameters used for scratch testing.

Test Parameters
Load Type Progressive
Initial Load 0.1 N
Final Load 12 N
Sliding Speed 3.025 mm/min
Sliding Distance 3 mm
Indenter Geometry Rockwell (120° cone)
Indenter Material (tip) Diamond
Indenter Tip Radius 50 µm
Atmosphere Air
Temperature 24 °C (room temperature)

Image of TPU and tempered glass screen protectors on cell phone.

Figure 1. Image of TPU and tempered glass screen protectors on cell phone.

Table 2. Summary of critical loads for each sample.

Type of Screen Protector Critical Load #1 (N) Critical Load #2 (N)
TPU n/a 2.004 ± 0.063
Tempered Glass 3.608 ± 0.281 7.44 ± 0.995

Results and Discussion

As the screen protectors were created from different materials, they each showed differing kinds of failures. For the TPU screen protector just one critical failure was seen, yet the tempered glass screen protector showed two. The results for each sample can be seen below in Table 2.

Critical load #1 is determined as the load at which the screen protectors began to exhibit indications of cohesive failure under the microscope. Critical load #2 is determined by the first peak change observed in the acoustic emissions graph data. Critical load #2 corresponds with the location along the scratch where the protector began to visibly peel on the phone screen on the TPU screen protector.

Once Critical load #2 was surpassed for the remainder of the scratch tests, a scratch developed on the surface of the phone screen. Critical load #1 corresponds to the place where radial fractures started to appear for the Tempered Glass screen protector. Critical load #2 occurs near the end of the scratch at higher loads.

No damage was done to the phone screen even though the acoustic emission is a larger magnitude than the TPU screen protector. Critical load #2 corresponded to a large change in depth In both instances, showing that the indenter had pierced through the screen protector.

TPU Screen Protector

Table 3. Critical loads from scratch testing on TPU screen protector.

TPU Screen Protector
Scratch Critical Load #2 (N)
1 2.033
2 2.047
3 1.931
Average 2.003
Standard Deviation 0.052

Friction, Normal force, AE, and Depth vs Scratch length - TPU Screen Protector (B) Critical Load #2.

Figure 2. Friction, Normal force, AE, and Depth vs Scratch length - TPU Screen Protector
(B) Critical Load #2.

Optical microscopy of Critical Load #2 for TPU Screen Protector. Image was taken with 5x magnication (image width 0.8934 mm).

Figure 3. Optical microscopy of Critical Load #2 for TPU Screen Protector.
Image was taken with 5x magnication (image width 0.8934 mm).

Full length image of post scratch test for TPU Screen Protector. Image was taken after scratch test was performed.

Figure 4. Full length image of post scratch test for TPU Screen Protector.
Image was taken after scratch test was performed.

Tempered Glass Sample

Table 4. Critical loads from scratch testing on tempered glass screen protector.

Tempered Glass Screen Protector
Scratch Critical Load #1 (N) Critical Load #2 (N)
1 3.923 7.366
2 3.382 6.483
3 3.519 8.468
Average 3.653 6.925
Standard Deviation 0.383 0.624

Friction, Normal force, AE, and Depth vs Scratch length - Tempered Glass Screen Protector (A) Critical Load #1 (B) Critical Load #2

Figure 5. Friction, Normal force, AE, and Depth vs Scratch length - Tempered Glass Screen Protector
(A) Critical Load #1
(B) Critical Load #2

Optical microscope image of Critical Load #1 location (left) and Critical Load #2 location (right) - Tempered Glass Screen Protector Image was taken with 5x magnfication (image width: 0.8934 mm).

Figure 6. Optical microscope image of Critical Load #1 location (left) and Critical Load #2 location (right) - Tempered Glass Screen Protector
Image was taken with 5x magnfication (image width: 0.8934 mm).

Optical microscope image of Critical Load #1 location (left) and Critical Load #2 location (right) - Tempered Glass Screen Protector Image was taken after scratch test was performed.

Figure 7. Optical microscope image of Critical Load #1 location (left) and Critical Load #2 location (right) - Tempered Glass Screen Protector
Image was taken after scratch test was performed.

Conclusion

The Nanovea PB1000 Mechanical Tester’s capability to carry out repeatable and controlled scratch tests and use acoustic emission detection simultaneously, in order to accurately identify the loads at which cohesive and adhesive failure happen in screen protectors created from tempered glass and TPU is showcased in this article.

The experimental data shown supports the initial assumption for scratch prevention on phone screens Tempered Glass performs the best. The Nanovea Mechanical Tester provides repeatable and accurate indentation, scratch and wear measurement capabilities using ASTM and ISO compliant Micro and Nano modules.

The Mechanical Tester is a complete system, making it the perfect solution for establishing the complete scope of mechanical properties of thin or thick, soft or hard coatings, substrates and films.

Reference

1. “PET, TPU, or Tempered Glass – all you need to know to choose a screen protector.” phoneArena, 15 Jul. 2014, https://www.- phonearena.com/news/PET-TPU-or-Tempered-Glass--all-you-need-to-know-to-choose-a-screen-protector_id58204.

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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