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Glasses and amorphous materials

Which correctly describes how sheet glass is strengthened by air quenching (thermal tempering)?

  1. AQuenching leaves tension at the surface and compression in the interior, and the internal compression stops cracks from growing so the glass resists breaking
  2. BQuenching makes the surfaces solidify first, and the interior, cooling and shrinking later, pulls them into compression and goes into tension itself
  3. CQuenching crystallizes only the surface, forming a hard crystalline layer that prevents scratches and so resists breaking
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Answer: B. Quenching makes the surfaces solidify first, and the interior, cooling and shrinking later, pulls them into compression and goes into tension itself

When air is blown on both faces of a glass sheet heated to about 650 °C, the surfaces cool and solidify first. The interior is still soft at this stage, so the shrinkage of the surfaces is absorbed by flow in the interior. When the interior later cools and tries to shrink, it pulls in the already rigid surfaces, so at room temperature compressive stress in the surfaces and tensile stress inside remain in balance. Glass fracture starts in tension from surface flaws, so the glass does not break until the applied tension exceeds the surface compression, and its strength becomes about 3–5 times that of annealed glass. However, when it breaks, the strain energy stored inside shatters the whole sheet into small granules, and it cannot be cut after tempering. The statement that tension remains at the surface has it backward, and no crystallization takes place.

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More Glasses and amorphous materials questions

  1. In thermally tempered glass of thickness t = 10 mm, let y be the distance from the mid-plane; the stress follows the parabolic distribution σ(y) = σc(1 − 12y²/t²) (tension positive, σc is the tensile stress at the mid-plane). If the surface compressive stress is 120 MPa, which correctly pairs the mid-plane tensile stress σc with the depth from the surface to the end of the compressive layer?
  2. Which correctly describes chemical strengthening (strengthening by ion exchange), used for smartphone cover glass and similar products?
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