Physical tempering and chemical tempering are two common glass strengthening techniques, differing in principle and application focus.
We will detail their distinctions across four aspects: principle, process, performance characteristics, and application.
Physical Tempering: Rapid cooling creates compressive stress on the glass surface, with tensile stress at its core. Similar to “quenching,” it offers high strength but shatters into small particles upon severe impact.
Chemical Tempering: Ion exchange forms a compressive stress layer on the glass surface. This “gentle” process prevents spontaneous breakage, delivers exceptional surface flatness, and provides even greater strength. However, it may produce sharp fragments like ordinary glass when impacted.

Detailed Comparison Table

Characteristics	Physical Tempering	Chemical Tempering
Basic Principles	Thermodynamic Principle: Glass is heated near its softening point, then rapidly and uniformly cooled. The surface solidifies quickly while the interior cools slowly, creating strong compressive stress on the solidified surface during contraction	Chemical Principle: In a high-temperature molten salt bath, smaller ions on the glass surface (e.g., Na⁺) exchange with larger ions in the molten salt (e.g., K⁺). Larger ions occupy more space, forming a compressive stress layer on the glass surface.
Processing Steps	Heating → Rapid cooling (air quenching)	Immersion in molten potassium nitrate salt → Ion exchange → Cleaning
Stress Layer Depth	Deep, typically reaching 1/6 to 1/4 of the glass thickness (e.g., over 1mm for 6mm glass)	Shallow, typically tens to hundreds of micrometers (µm)
Surface Flatness	May exhibit slight deformation (e.g., waviness, bowing), unsuitable for ultra-thin or extremely flat applications	Excellent, with minimal alteration to glass flatness and optical properties, ideal for ultra-thin and precision glass
Strength	3-5 times stronger than ordinary glass	5-10 times stronger than ordinary glass, or even higher
Breakage Pattern	Safe breakage: Shatters into countless small particles (honeycomb-like), minimizing risk of severe cuts	Unsafe breakage: Fragments resemble ordinary glass, producing large sharp shards
Risk of Spontaneous Breakage	Yes. If the glass substrate contains impurities such as nickel sulfide, prolonged use may cause stress imbalance due to phase transitions, leading to spontaneous breakage	No. The chemical process is stable, eliminating the risk of spontaneous breakage
Processing Restrictions	All cutting, drilling, and other processing must be completed before tempering. Once tempered, the glass cannot be further processed	Minor cutting or processing is possible after tempering, but it will compromise the edge compressive stress layer
Product Thickness	Typically used for thicker glass (3mm and above)	Particularly suitable for ultra-thin glass (0.1mm - 2mm)
Cost and Energy Consumption	Relatively low cost with high production efficiency	Relatively high, with extended production cycles (hours to dozens of hours) and significant energy consumption
Primary Applications	Architectural doors/windows, curtain walls, shower enclosures, furniture, appliance panels, automotive side/rear windows	Electronic device covers (e.g., mobile phones, tablets, watch screens), aerospace glass, optical components, thin eyeglass lenses, etc

In-Depth Analysis
1. Physical Tempering
The core principle is “stress balance”: imagine it as a “sandwich.” The glass surface forms a hard compressive stress layer, while the core is a tensile stress layer. When external forces attempt to break the glass, they must first overcome the powerful compressive stress at the surface before reaching the relatively vulnerable tensile stress zone inside. This significantly enhances the glass's overall strength.
Drawback: “Spontaneous Breakage” Due to the internal tensile stress, if glass raw materials contain impurities (such as nickel sulfide inclusions), these impurities may undergo volume expansion over time. This expansion can instantly disrupt the delicate stress balance, causing the glass to shatter unexpectedly—a phenomenon known as “spontaneous breakage.”
Non-reworkable: Since the entire glass structure exists under high stress, any grinding of edges or cutting of surfaces causes the overall stress to release, shattering the entire piece.

2. Chemically Tempered Glass
The core principle is “ion exchange”: this process functions more like an “infiltration-based” reinforcement of the glass surface. It enhances strength by forming a dense, compressive stress layer on the surface, filled with large ions. Though this layer is shallow, it is exceptionally hard.
Advantages lie in precision and ultra-thin processing: Since it avoids severe thermal shock, it prevents deformation. It can treat ultra-thin glass as thin as 0.1mm while maintaining exceptional flatness—critical for touchscreen sensitivity and display quality.
Disadvantage: “Unsafe breakage.” Since the compressive stress layer is shallow, once a sharp object penetrates this “armor,” cracks rapidly propagate into the unreinforced interior, causing the glass to shatter like ordinary glass into sharp fragments. Therefore, chemically toughened glass typically requires pairing with an explosion-proof film (e.g., on smartphone screens).

Summary and Selection
For applications requiring high strength and safety (cut resistance) without extreme flatness demands—such as architectural doors and windows, shower enclosures—physical tempering offers an economical and efficient solution.
For precision applications demanding ultra-high strength, exceptional flatness, ultra-thin profiles, and zero risk of spontaneous breakage—such as electronic device screens and optical components—chemical tempering is the sole or optimal choice.

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