Scratch Resistance of Glass Introduction
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Scratch Resistance Problematic
The smartphones require a thin glass screen capable of being swiped multiple times and survive being stored with other damaging items such as keys. The glass manufacturers are investing in research that can lead to stronger and more scratch resistant materials to be used by smartphone manufacturers. The quality of a good chemically strengthened glass therefore resides in its ability to resist scratching by particles or items that will come in contact with its surface. The challenge of using a generally brittle material such as glass is that one must increase its elasticity and reduce its brittleness at the same time to achieve the functionality required for its applications. Although scratch testing is an obvious choice to test the scratch resistance of these products, this test can bring researchers much more information than just the scratch resistance. Therefore, scratch testing presents clear advantages for researchers and quality control engineers to improve and control these glass products.
Scratch Resistance Test – Principles of Testing
A scratch is created by dragging a diamond tip of known geometry atop the surface of the sample of interest as shown in Figure 1. As the tip is moved along the surface, the normal load applied to the tip is increased linearly, creating an increase in the severity of the contact. Following the scratch, images are taken of the entire scratch to analyze the different deformation and failures.
Figure 1: Scratch Testing Principle
Figure 2: Confocal Image of Scratch Failure
The normal loads at which failures happen are called critical loads1, 2. Critical loads are found using either imaging or a combination of imaging and signals (Acoustic emission for example). Multiple signals can be recorded during the scratch test, allowing the user to correlate behaviors and specific measures.
Scratch Test Conditions
|Load Application Profile||Linear increasing|
|Scratch Length||1 mm|
|Initial Load||50 mN|
|Final Load||2 N|
|Scratch Speed||2 mm/min|
|Stylus||Rockwell with 20 µm radius|
Table 1: Test conditions
Figure 3: Glass sample in the instrument
Figure 4: Plots of Penetration depth, friction force, acoustic emission, and critical loads (vertical lines) for glass A sample
In all cases, as the normal force increases, the glasses experience at least one type of failure which is identified as cracks on either side of the scratch groove. Two samples further experienced a complete failure of the material with complete fracture and chipping of the material. Figure 2 shows the resulting penetration depth and coefficient of friction for sample B as the normal scratch load is increased up to 2N.
In this case two types of failures are recorded:
Lc1: First cracks on the sides of the groove
Figure 5: Adhesion Failure at the interface
Chipping and complete failure of glass
Figure 6: Chipping of glass A (Lc2)
Confocal and Bright Field imaging of Failure
Figure 7: Confocal and Bright Field images of Glass A in the failures area
Scratch Testing Of Glass Conclusions
The three chemically strengthened glasses can be ranked from worst to best: A, B, and C. Although all glasses present a first crack failure observed at the edge of the groove, only A and B chip under 2 N maximum scratch force. Furthermore, all failures look similar on all three samples leading to the conclusion that similar stresses are causing Lc 1 and Lc 2 on all three samples.
Figure 8: Critical loads summary for the 3 glass samples
The scratch testing technique was used to simulate a “worst case” scenario for the wear and damage of chemically strengthened glass such as Gorilla® glass. The stresses generated during the scratch test provide information on the strength of the different glasses and allow for the ranking of the different treatments used to produce those samples. The combination of image and signal analysis provides the most advanced understanding of glass failure behaviors. The same instrument is also used to study wear on those same samples although those results were not reported here.
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