Chemically Strengthened Glass Surface Roughness Control
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Solution Overview
Problem
Existing chemically strengthened glass used in foldable devices is inadequate in crack strength during pen drop tests, particularly for extremely thin sheets, due to surface roughness issues such as core roughness depth and mean summit curvature.
Innovation Solution
A chemically strengthened glass with a thickness of 0.20 mm or smaller, featuring a core roughness depth of 0.90 nm or smaller and a mean summit curvature of 13.0×10−4/nm or smaller, achieved through ion exchange treatment and polishing using colloidal silica and cerium oxide as abrasive grains, to enhance crack resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If the glass sheet is thinned to achieve light weight, then the weight is reduced, but the crack strength and bending strength deteriorate
Solution Approach 1:
The patent applies chemical strengthening treatment to change the physical and chemical parameters of the glass surface. By immersing the thin glass sheet in a molten salt bath containing potassium nitrate and other salts at elevated temperatures (400-500°C), ion exchange occurs where smaller sodium ions in the glass are replaced by larger potassium ions from the salt bath. This creates a compressive stress layer at the surface with a depth of 5-20 μm and compressive stress of 300-1500 MPa, significantly enhancing crack strength and bending strength while maintaining the thin profile for light weight
2Strength
If chemical strengthening treatment is applied to enhance strength, then crack strength is improved, but surface roughness increases
Solution Approach 1:
The patent performs preliminary polishing of the glass sheet surface before applying the chemical strengthening treatment. The glass sheet is polished to achieve a surface roughness of Ra 0.3 μm or less, core roughness depth Sk 0.8 μm or less, and mean summit curvature Ssc 12.0×10^-4/nm or less. This preliminary action ensures that when the glass undergoes chemical strengthening and subsequent cooling, the surface maintains high flatness and low roughness, preventing defect formation while still achieving the desired compressive stress layer for enhanced crack strength
Solution Approach 2:
The patent carefully controls the parameters of the chemical strengthening process to balance strength enhancement with surface quality. By optimizing the salt bath composition (potassium nitrate, sodium carbonate, potassium carbonate, etc.), temperature (400-500°C), and treatment time (0.5-4 hours), the patent achieves a compressive stress layer depth of 5-20 μm while maintaining surface roughness within acceptable limits. The controlled ion exchange process strengthens the glass without excessive surface degradation
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution significantly increases crack strength in pen drop tests, making the glass more reliable for foldable devices by reducing surface stress concentrations and improving surface flatness.
Implementation Method 1
performing an ion exchange treatment of replacing the alkali metal ions existing in a surface of the glass sheet with alkali metal ions of another kind having larger ion radius than ion radius of the alkali metal ions in the surface of the glass sheet
Data Source
AI summary
The present invention relates to a chemically strengthened glass including alkali metal ions, having a thickness of 0.20 mm or smaller, and having a pair of major surfaces that are opposed to each other and have been subjected to a chemically strengthening treatment, in which at least one of the pair of major surfaces has a core roughness depth Sk of 0.90 nm or smaller and a mean summit curvature Ssc of 13.0×10−4/nm or smaller.