Summary of Glass Surface Strengthening Techniques

Glass surfaces often have numerous micro-cracks and other defects, making them prone to breaking during practical applications. To enhance their durability, glass surfaces typically undergo various strengthening treatments to improve their impact resistance. There are several methods for glass surface strengthening, including tempering (both physical and chemical) and polishing (both physical and chemical). Below, we will explore the processes of glass tempering and surface polishing.

1. Tempering

Principle: Glass is a brittle material, inherently more suited to withstand compression than tension. The failure of glass usually occurs due to its low tensile strength. When glass is subjected to a load or impact that causes it to break, it is the excessive tensile stress on a specific area of the surface that leads to cracking. By inducing compressive stress on the surface beforehand, the tensile stress that would otherwise cause the glass to break can be partially or completely neutralized, thereby improving the glass's strength and impact resistance.

Categories: Physical Tempering, Chemical Tempering

Physical Tempering

Definition: Physical tempering involves treating ordinary flat or float glass under specific process conditions using quenching or air quenching methods (overall heating + medium cooling).

Process: Also known as quench-tempered glass, this method heats ordinary flat glass in a furnace to near its softening temperature (around 600°C). The internal stresses are relieved through self-deformation. The glass is then removed from the furnace and rapidly and uniformly cooled to room temperature by blowing high-pressure cold air onto both sides using multiple nozzles, resulting in tempered glass.

Characteristics: This glass is in a state where the interior is under tension and the exterior is under compression. If a localized area is damaged, the release of stress causes the glass to shatter into countless small fragments. These fragments have no sharp edges, making them less likely to cause injury.

Types: Gas Medium Tempering, Liquid Medium Tempering, Particle Tempering, Fog Tempering

Production Process Flow:

• Raw Glass Cutting → Pre-treatment (Cutting, Edge Breaking, Edge Grinding, Drilling, Washing, Drying) → Screen Printing → Tempering → Attachment Installation → Final Inspection and Packaging

Production Equipment:

• Tempering Furnace
• Air Grid
• Bending Press

Chemical Tempering – Ion Exchange Method

Definition: Chemical tempering involves altering the surface composition of ordinary flat or float glass through ion exchange, creating a compressive stress layer on the surface (surface ion exchange).

Process: Silicate glass containing alkali metal ions is immersed in molten lithium (Li⁺) salt. The surface Na⁺ or K⁺ ions are exchanged with Li⁺ ions, forming a Li⁺ ion exchange layer on the surface. Since the expansion coefficient of Li⁺ is smaller than that of Na⁺ or K⁺ ions, the outer layer shrinks less than the inner layer during cooling. Once cooled to room temperature, the glass remains in a state of tensile stress inside and compressive stress outside, similar to physically tempered glass.

Types: High-Temperature Ion Exchange, Low-Temperature Ion Exchange

Process Flow:

• Raw Glass Inspection → Cutting → Edge Grinding → Cleaning and Drying → Low-Temperature Preheating → High-Temperature Preheating → Ion Exchange → High-Temperature Cooling → Medium-Temperature Cooling → Low-Temperature Cooling → Cleaning and Drying → Inspection → Packaging

Advantages:

1. The strength of chemically tempered glass is comparable to that of physically tempered glass, with good thermal stability, low processing temperatures, minimal deformation, and suitability for various thicknesses and geometric shapes. The equipment required is simple, and the product is easily realizable.
2. The strength is several times higher than ordinary glass, with bending strength 3 to 5 times greater and impact resistance 5 to 10 times greater.

Disadvantages: Compared to physically tempered glass:

1. Longer production cycles (exchange times can be tens of hours).
2. Lower efficiency and higher production costs (molten salts cannot be recycled and require high purity).
3. Fragments are similar to ordinary glass, posing safety risks.
4. Performance instability (poor chemical stability).
5. Mechanical strength and impact resistance tend to diminish over time (also known as relaxation), with strength decaying rapidly over time.

Applications: Chemically tempered glass is widely used for various thicknesses of flat glass, thin-walled glass, and bottle-shaped glass products, as well as for fire-resistant glass. It is particularly suitable for strengthening ultra-thin, small, or complex-shaped glass products, as ion exchange treatment does not cause noticeable optical distortion.

Case Study: Corning Gorilla Glass

Related Companies:

• Corning
• Asahi Glass
• Jingbao
• Schott
• Lens Technology
• Bern Optics

2. Surface Polishing

Principle: Glass polishing involves the removal of surface imperfections such as lines, scratches, and other defects through chemical or physical methods, thereby enhancing the glass's transparency and refractive index. The core issues in polishing technology are precision and efficiency.

Categories: Physical Polishing, Chemical Polishing

Physical Polishing

Definition: Physical polishing smooths the surface of the object using mechanical methods.

Process Flow and Considerations:

1. During polishing, polishing powder is applied to the glass surface. The polishing process, like grinding, uses a grinding machine, so all the rules and corrections applicable to grinding also apply to polishing.
2. After grinding, the workpiece must be rinsed with warm water to effectively remove any adhered diamond powder. The workpiece is then prepared for polishing in a flat polishing machine, where the polishing pad must match the shape of the glass surface to ensure uniform friction and a bright mirror finish.
3. During polishing, avoid using too much polishing powder, as this can cause the polishing agent to slide rather than polish.
4. Polishing agents can be automatically dispensed by a machine, with the quantity determined by the operator based on inspection, as the degree of surface polishing can vary depending on the type of polishing agent used.

Chemical Polishing

Definition: Chemical polishing uses chemical solutions, typically hydrofluoric acid, to polish the glass surface. There are two main techniques: immersion bath method and single-sided immersion method.

Technique 1: Immersion Bath Method

• The entire glass piece is immersed in the chemical bath for 2–3 hours, then cleaned thoroughly.
• This method is used for polishing both sides of the glass simultaneously.

Technique 2: Single-Sided Immersion Method

• A protective film is applied to the non-polished side of the glass, then the glass is placed flat, and the chemical solution is poured onto the surface to a depth of no less than 3 mm, ensuring uniformity for 2–3 hours, followed by thorough cleaning.