Wear Resistance of Screen Protector Tempered Glass

Characterize glass surface – scratch & wear scratch resistance, roughness, thickness and more using SMT-5000 indentation and scratch tester.

 

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Wear Resistance Of Screens Introduction

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Wear Resistance Test Problematic

Screen protectors can be swiped multiple times with dust on it and can be stored with many other items that can damage or wear the screen.They typically serve as a sacrificial layer to protect the smartphone but are still expected to last. With all these products claiming similar resistance, this study wanted to test a few of these products from different brands to evaluate their wear resistance and see if they were equivalent as claimed.
Key Damage on Screen Protector

Figure 1: Key damage on screen protector

To represent the damage made to these screen protectors, this study focused on the two most common damaging items: sand particle and keys. Sand particles can be represented by an asperity having a radius varying from 10 to 100 microns. For the key, this study used the confocal microscope to measure the radii present on 3 different teeth of a key.

Wear Resistance Methodology

Asperity characterization:

The first step was the characterization of the key asperity to pick the right equivalent radius. This was achieved using the Rtec-Instruments Lambda head profilometer to image 3 different teeth of a key and measuring the radii at the different edges (Figure 2).

Wear Measurement:

To simulate the different asperities in contact with the screen protectors, diamond tips of different radii are dragged repeatedly along the surface of the samples to create a wear track following ASTM G133. A constant normal force is applied to the tip to simulate the force of an asperity sliding on the screen protector surface. The full wear track is imaged as constant interval to observe its evolution and quantify the wear as a function of time. The test is stopped when complete chipping failure along the wear track is observed. Multiple signals can be recorded during the scratch test, allowing the user to correlate behaviors and specific measures.

Key Tooth profilometry with Rtec Lambda Profilometer
Figure 2: Key Tooth Profilometry

Wear Resistance Testing Conditions

The Lambda head profilometer was used to image the key teeth. A confocal image using a 50X magnification objective was obtained and further analyzed to determine the average asperity radius to be used in the wear test.

The Indentation Scratch Tester (SMT-5000) was used to create wear tracks (Figure 3) on three different tempered glass screen protectors. A simple reciprocating linear wear test was performed on the different samples. Two different diamonds were used to simulate sand (20 microns radius) and a key (100 microns radius). The normal forces on the diamonds were also chosen to mimic the real-life situations.

A Confocal image of the full track was taken after every 300 cycles. Finally, the wear volume was measured after 1500 cycles to compare the different samples.

The test parameters are summarized in Table 1

Load Application Profile Constant Load
Track Length 1 mm
Normal Load 2 & 7 N
Linear Reciprocating Frequency 5 Hz
Stylus Diamond spheres 20 µm & 100 µm radius

Table 1: Test Conditions

Screen protector with diamond stylus in the SMT-5000

Figure 3: Screen protector with diamond stylus in the SMT-5000

Test Results

Asperity Radius Characterization

Three teeth of a key were imaged to be representative of different angles and radii. Each tooth was cross sectioned in two orthogonal directions as shown on Figure 4. The average of all radii measured on the key was calculated to be 102.7 microns, yielding the use of a 100 microns radius diamond stylus to perform the wear test.

3D image of a key tooth and the radii measurements on its cross profiles
Figure 4: 3D images of a key tooth and the radii measurements on its cross profiles

Wear Study of Wear Resistance Samples

The linear reciprocating tests lead to three phases of wear on the samples. The first stage is the creation of a groove in the material this occurs within the first few hundred cycles. The next stage is the apparition of Hertzian cracks either in the wear track or at one of the ends of the track. Finally, the cracks propagate and leads to complete failure of the glass by chipping.

The number of cycles necessary to observe this complete failure of the screen protector was recorded for each sample and compared to quantify the resistance of each of the products. 

The results are shown in Table 3. 

Wear Volume (µm3)
20 µm stylus 100 µm stylus
A 2789 5116
B 5674 8992
C 4851 6497
Table 2: Wear track volume for the different samples
Wear track on sample A after 300 cycles
Figure 5: Wear track on Sample A after 300 cycles
Volume Wear
Figure 6: Volume Wear

After 1000 cycles, the continuous application of the load in the wear groove starts to create Hertzian cracks in the tempered glass as shown in both confocal and bright field images. The dark field image confirms the presence of crack as indicated on Figure 7. 

Starting with the first cracks, some glass particles enter the sliding contact and accelerate the degradation of the groove.  Although the apparition of first cracks would incite the user to change its screen protector, this study pushed the linear reciprocating test to the point of chipping the tempered glass to compare the ultimate limit on each sample. Figure 8 shows such failure in the middle of the wear track.

Hertzian cracks on Sample B
Figure 7: Hertzian cracks on sample B
(confocal image on top and bright field at the bottom)

Starting with the first cracks, some glass particles enter the sliding contact and accelerate the degradation of the groove. 

Although the apparition of first cracks would incite the user to change its screen protector, this study pushed the linear reciprocating test to the point of chipping the tempered glass to compare the ultimate limit on each sample. Figure 8 shows such failure in the middle of the wear track. 

The number of cycles necessary to observe this complete failure of the screen protector was recorded for each sample and compared to quantify the resistance of each of the products. 

The results are shown in Table 3. 

Figure 9 summarizes this study illustrating both the wear volume and cycle to complete failure for each sample.

# of cycles required for complete failure
20 µm stylus 100 µm stylus
A 3900 4500
B 1800 2100
C 3300 3900
Table 3: Cycles to complete failure of samples
Chipping failure of the tempered glass on the wear track for sample C
Figure 8: Chipping failure of the tempered glass on the wear track for sample C
wear volumes and cycles to failure for each screen protector
N
Conclusions

Previous application notes focused on the scratch resistance of glasses (screen cover) for the smartphone applications (See Rtec Instruments application notes on Scratch Resistance of Glass). The same instruments can bring a deeper understanding of glasses by providing a full tribology study as demonstrated above. The ASTM G133 or other relevant standards can be run on the Rtec-Instruments to further the understanding of such glasses and their protection ability for the smartphone they are supposed to protect. 

The automatic image acquisition at different intervals provides a unique visualization of the wear track and different failures. The confocal images allow for volume and area calculations therefore simplifying the tribology analysis of such study. 

Although those three different screen protectors claimed similar performances, this study showed a clear difference in wear resistance and fracture resistance for these samples. 

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