Chemically Strengthened Glass Deep Compressive Layer Design
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Solution Overview
Problem
Chemically strengthened glasses used in electronic devices face challenges in survivability when subjected to tensile stresses and deep flaws, particularly from contact with hard/sharp surfaces, requiring improved compressive stress profiles to enhance durability.
Innovation Solution
The development of chemically strengthened glass articles with deep compressive surface layers, achieved through a two-step ion exchange process, where the first step involves immersing alkali aluminosilicate glass in a bath at temperatures above 400°C to create a compressive stress layer of at least 45 µm, followed by a second step in a different bath at 350°C to refine the stress profile, resulting in a compressive stress of 700 MPa to 1200 MPa at the surface and extending to a depth of 45 µm.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If chemically strengthened glass is used in electronic devices, then the glass provides structural integrity and surface protection, but it exhibits reduced survivability when subjected to tensile stresses and deep flaws from contact with hard/sharp surfaces
Solution Approach 1:
The patent applies parameter changes by modifying the compressive stress profile through a two-step ion exchange process. The first step creates a deep compressive layer (DOC ≥ 45 μm) with moderate compressive stress (150-400 MPa), while the second step adds a shallow highly compressed layer (depth 3-12 μm) with high compressive stress (400-1200 MPa). This dual-parameter approach (depth and stress magnitude) resolves the contradiction by providing both deep flaw resistance and surface tensile stress resistance.
Solution Approach 2:
The patent segments the compressive stress profile into two distinct portions: a first portion extending to depth da (3-12 μm) with steep slope (40-200 MPa/μm) providing surface protection, and a second portion extending to DOC (≥45 μm) with moderate slope (2-8 MPa/μm) providing deep flaw resistance. This segmentation allows each layer to specialize in resisting different types of damage mechanisms.
2Ease of manufacture
If a single linear compressive stress profile is used, then the manufacturing process is simpler, but the glass exhibits reduced resistance to both surface flaws and deep flaws
Solution Approach 1:
The patent segments the ion exchange process into two distinct steps with different bath compositions, temperatures, and durations. The first ion exchange step creates the deep compressive layer, while the second step creates the shallow highly compressed layer. This segmentation enables optimization of each layer's properties independently, achieving superior fracture resistance while maintaining manufacturing feasibility through standardized ion exchange equipment.
Solution Approach 2:
The patent employs periodic action by conducting ion exchange in two sequential batches rather than continuously. The first batch establishes the baseline deep compressive profile, and the second batch periodically adds the enhanced surface compression. This periodic approach allows precise control over each layer's characteristics and is readily implementable in existing production workflows.
3Strength
If the compressive stress layer extends deeper into the glass, then the glass resists deep flaws better, but the surface compressive stress magnitude decreases
Solution Approach 1:
The patent segments the compressive stress distribution into two functional zones: a deep zone (DOC ≥ 45 μm) with moderate compressive stress (150-400 MPa) that resists deep flaws, and a shallow zone (depth 3-12 μm) with high compressive stress (400-1200 MPa) that resists surface initiation of cracks. This segmentation resolves the inverse relationship between depth and stress magnitude by dedicating different depth ranges to different protective functions.
Solution Approach 2:
The patent applies local quality by tailoring the compressive stress characteristics to specific depth regions. The surface region receives high compressive stress with steep gradient for immediate flaw resistance, while the subsurface region receives moderate compressive stress extending deeper for crack propagation resistance. Each region's stress profile is locally optimized for its specific protective role.
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 glass articles exhibit a 60% survival rate in an inverted ball drop test from 100 cm and an equibiaxial flexural strength of at least 98 N (10 kgf), demonstrating improved resistance to fracture and durability.
Implementation Method 1
conducting a first ion exchange step by immersing an alkali aluminosilicate glass article in a first ion exchange bath at a temperature of greater than 400°C
Implementation Method 2
the first ion exchange step by immersing an alkali aluminosilicate glass article in a first ion exchange bath
Implementation Method 3
immersing an alkali aluminosilicate glass article in a first ion exchange bath at a temperature of greater than 400°C for a time sufficient
Data Source
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AI summary
Chemically strengthened glass articles having at least one deep compressive layer extending from a surface of the article to a depth of at least about 45 μm within the article are provided. In one embodiment, the compressive stress profile includes a single linear segment extending from the surface to the depth of compression DOC. Alternatively, the compressive stress profile includes two linear portions: the first portion extending from the surface to a relatively shallow depth and having a steep slope; and a second portion extending from the shallow depth to the depth of compression. The strengthened glass has a 60% survival rate when dropped from a height of 100 cm in an inverted ball drop test and an equibiaxial flexural strength of at least 10 kgf as determined by abraded ring on ring testing. Methods of achieving such stress profiles are also described.