Using Acoustic Emission for Identify Cohesive Failure of Screen Protectors

Handheld electronic devices are extremely common amongst consumers in today’s age of information. However, these portable multifunctional devices can be quite expensive. Screen protectors can be used to protect the fragile components, such as the glass interface. How effective are these screen protectors? Nanovea’s Mechanical Tester’s Micro Module with an acoustic emission attachment can be used to clearly identify critical loads at which the screen protector fails.

Importance of Acoustic Emissions for Scratch Testing

Acoustic emission is a powerful technique and detects changes such as fracturing, cracking and deformation occurring in the material. Combining this with scratch testing means that it can be used to find critical loads where cohesive failure, such as chipping, fractures, cracking or adhesive failures, such as delamination, occur. Acoustic emission can detect underlying changes that cannot be observed by a microscope as they occur under the surface.

Measurement Objective

Nanovea’s Mechanical Tester’s Micro Module was used to test two different commercial screen protectors. Three scratch tests per sample were conducted and the parameters are listed in Table 1. The acoustic emission, normal force, friction force, and true depth were closely monitored in-situ. The critical loads at which the samples failed are identified by acoustic emission and optical microscopy.

Picture of test setup (left) and scratch test parameters (right)

Figure 1. Picture of test setup (left) and scratch test parameters (right)

Test Conditions and Procedure

Table 1. Test Parameters for Scratch Testing on Screen Protectors

Test Parameters All Samples
Load type Progressive
Initial Load (N) 0.1
Final Load (N) 12
Loading rate (N/min) 12
Scratch Length (mm) 3
Scratching speed (mm/min) 3
Indenter geometry 120° Conical
Indenter material (tip) Diamond
Indenter tip radius (µm) 50

Results

Table 2. Summary of Results

Critical Load #1 Critical Load #2
Screen Protector A 2.918 ± 0.249 10.40 ± 0.78
Screen Protector B 5.404 ± 1.026 8.721 ± 0.633

Two critical loads were identified from scratch tests using both optical microscopy and acoustic emissions. Critical load #1 is identified by signs of radical cracking seen under the microscope and this is the point where consistent cracking at the surface of the glass is observed. Critical load #2 is the first change in acoustic emission and this is the point where the glass begins to consistently fracture.

It is very difficult to accurately identify critical loads from imaging techniques after the scratch test is complete because fracturing causes substantial damage to the surface of the sample. Fortunately, acoustic emission can clearly define the point at which critical loads are reached. Full results from the tests are shown below.

Screen Protector A

Table 3. Critical Loads from Scratch Testing on Screen Protector A

Critical Load #1 Critical Load #2
1 2.614 10.53
2 3.223 11.277
3 2.918 9.379
Average 2.918 10.40
Standard Deviation 0.249 0.78

Data graphed from scratch test on Screen Protector A – (A) Critical Load #1, (B) Critical Load #2

Figure 2. Data graphed from scratch test on Screen Protector A – (A) Critical Load #1, (B) Critical Load #2

Optical Microscopy of Critical Load #1 (Left) and Critical Load #2 (Right) for Screen Protector A. Taken with 100x and 50x magnification (0.442mm and 0.9001mm image width) respectively.

Figure 3. Optical Microscopy of Critical Load #1 (Left) and Critical Load #2 (Right) for Screen Protector A. Taken with 100x and 50x magnification (0.442 mm and 0.9001 mm image width) respectively.

Full length image of post scratch test for Screen Protector A – (A) Critical Load #1, (B) Critical Load #2

Figure 4. Full length image of post scratch test for Screen Protector A – (A) Critical Load #1, (B) Critical Load #2

Screen Protector B

Table 4. Critical Loads from Scratch Testing on Screen Protector B

Critical Load #1 Critical Load #2
1 5.647 8.539
2 6.521 9.571
3 4.044 8.054
Average 5.404 8.721
Standard Deviation 1.026 0.633

Data graph from scratch test on Screen Protector B – (A) Critical Load #1, (B) Critical Load #2

Figure 5. Data graph from scratch test on Screen Protector B – (A) Critical Load #1, (B) Critical Load #2

Optical microscopy of Critical Load #1 (Left) and Critical Load #2 (Right) for Screen Protector B. Both images were taken with 100x magnification (0.442 mm image width).

Figure 6. Optical microscopy of Critical Load #1 (Left) and Critical Load #2 (Right) for Screen Protector B. Both images were taken with 100x magnification (0.442 mm image width).

Full length image of post scratch test for Screen Protector B – (A) Critical Load #1, (B) Critical Load #2

Figure 7. Full length image of post scratch test for Screen Protector B – (A) Critical Load #1, (B) Critical Load #2

Conclusion

From the testing conducted, two critical loads – surface fracturing and cracking – were identified. Although Screen Protector A appears to damage earlier than Screen Protector B, it fractures at a higher load then Screen Protector B.

The critical load #1 were 2.918 ± 0.249 and 5.404 ± 1.026 for Screen Protectors A and B respectively. Critical load #2 were 10.40 ± 0.78 and 8.721 ± 0.633 for Screen Protectors A and B respectively. These values were identified accurately with the help of the acoustic emissions attachment on Nanovea’s Mechanical Tester’s Micro Module and by optical microscopy.

Theory of Scratch Testing

Principle

The scratch testing method is a quantitative test in which the critical loads at which failures appear in the samples are used in order to evaluate the relative adhesive or cohesive properties of a coating or the scratch resistance of a bulk material.

Scratches are made during the test with a sphero-conical stylus which is either drawn at a constant speed across the sample under a constant load or, more commonly, a progressive load with a fixed loading rate.

Different radii (the “sharpness” of the stylus) are available for sphero-conical styluses. Commonly, radii are from 20 up to 200 mm for micro/macro scratch tests, and from 1 to 20 mm for nano scratch tests.

Critical load is defined as the smallest load at which a recognizable failure occurs in a progressive load test. In a constant load test however, the critical load is the load at which a regular occurrence of such failure along the track is observed.

Comments on the Critical Load

The scratch test is a quantitative test and it has a high repeatability. The critical load is dependent on the mechanical strength (cohesion, adhesion) of a combined coating-substrate system as well as on several other parameters. Some of these are directly related to the test itself, whilst others are related to the coating-substrate system.

Testing parameters affecting critical load:

  • Scratching speed
  • Loading rate
  • Indenter material (and also indenter tip wear)
  • Indenter tip radius

Sample specific parameters affecting critical load:

  • Internal stresses in the material
  • Coating thickness
  • Coating hardness and roughness
  • Substrate hardness and roughness
  • Friction coefficient between surface and indenter

It is possible to obtain very repeatable data to quantifiably compare samples by keeping the parameters constant.

Means for Critical Load Determination

The most reliable method to detect surface damage is microscopic observation. With this technique it is possible to differentiate between adhesive failure at the interface of the coating-substrate system and cohesive failure within the coating.

Recording the friction force means that the force fluctuations along the scratch can be studied and correlated to the failures observed under the microscope. Often, a failure in the sample will cause a change (a change in slope or a step) in coefficient of friction. The frictional responses to failures are highly specific to the coating-substrate system in study.

Sometimes, the depth sensor recording can indicate where a failure occurs. A significant fall in the depth will typically indicate that the indenter has broken through one layer of a sample and down to the next.

Deformation of a sample surface can also be studied using depth recording. Performing pre- and post-scans of the scratch can show plastic and elastic deformation.

Nanovea.

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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