21-Mat-A6 Materials Selection and Design for Materials Processing · Dec-12-Mtl-A6 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
Certain sodium borosilicate compositions ($\text{Na}_2\text{O}$–$\text{B}_2\text{O}_3$–$\text{SiO}_2$, the basis of the Vycor process) sit inside a metastable liquid–liquid immiscibility dome: on cooling into, or reheating within, this composition/temperature region, the homogeneous melt spontaneously splits — by spinodal decomposition or by nucleation-and-growth phase separation, depending on how deep into the dome the glass is taken — into two intimately mixed amorphous phases: one SILICA-RICH and chemically durable, the other alkali-borate-rich and chemically LEACHABLE (readily dissolved by hot mineral acid). Reheating the phase-separated glass in the immiscibility dome (typically ∼500–600 °C) coarsens this two-phase structure to a controlled, mutually interconnected (percolating, for compositions near the spinodal) length scale.
The part is then leached in hot dilute acid, dissolving out the alkali-borate-rich phase and leaving behind a porous, high-purity (∼96% $\text{SiO}_2$) silica-rich skeleton, which is finally consolidated (fired further, closing the residual porosity by viscous sintering) into a dense, ultra-high-silica glass with a much higher softening point and better chemical/thermal-shock resistance than the parent borosilicate composition. Phase separation is thus exploited DELIBERATELY here (rather than avoided, as it normally is for an ordinary transparent borosilicate glass, e.g. Pyrex-type labware, where a single, chemically uniform glass phase is wanted) to engineer a controlled, selectively leachable two-phase microstructure.
The annealed plate is reheated uniformly to well above its glass transition, close to (but below) its softening point so it does not sag, then rapidly and uniformly quenched on BOTH faces simultaneously (typically by air jets). The surface layers cool and become rigid first, while the still-hot interior has not yet contracted; as the interior subsequently cools and tries to shrink to its own smaller equilibrium volume, it is constrained by the already-rigid outer layers, which forces the interior into residual TENSION and, by overall force balance across the section, forces the surface layers into residual COMPRESSION.
Glass almost always fails by brittle fracture initiating at a pre-existing SURFACE flaw under an applied TENSILE stress. Because the surface now carries a built-in COMPRESSIVE residual stress, any externally applied tensile (bending) load must first overcome that compressive pre-stress before a net tensile stress ever develops at a surface flaw — raising the practical fracture (bending) strength typically by a factor of 4–6× relative to the same, annealed glass, even though the material's own intrinsic (flaw-tip) strength is unchanged. Borosilicate's comparatively LOW thermal expansion coefficient (versus soda-lime glass) is a complication here: achievable temper stress scales with the product of $E$, CTE and the temperature difference frozen in during the quench, so borosilicate needs a higher tempering temperature and a much more intense quench (higher air-jet heat-transfer coefficient) to reach a useful surface compression. Chemical (ion-exchange) strengthening is of limited help for borosilicate because its low alkali content leaves few Na$^+$ ions to exchange.
Devitrification is UNCONTROLLED partial crystallization of a glass — the opposite of the deliberately engineered, fine-grained crystallization of a glass-ceramic (Question 5) — typically arising as an unwanted side effect of holding, or cooling too slowly, in the glass's own crystallization-nose temperature range (e.g. during reforming, over-slow annealing, or prolonged elevated-temperature service). Crystals nucleate, often heterogeneously at surfaces, inclusions, or tool-contact points, and grow COARSELY and non-uniformly, frequently concentrated near one surface or interface rather than distributed uniformly through the volume.
When it is used deliberately. For some glasses devitrification is imposed on purpose as a heat treatment: the glass (often with a nucleating agent) is reheated through a nucleation range and then held in its crystal-growth range, as in the glass-ceramic cycle of Question 5(b), or a devitrifying sealing/solder glass is crystallized in place after it has flowed. Done this way it produces a fine, uniform crystalline body that is stronger, harder, stiffer and tougher than the parent glass (crystals deflect and arrest cracks, and bending strength can rise two- to three-fold), with a higher deformation temperature and, for low-expansion crystal phases, better thermal-shock resistance, though it loses transparency.
Effect when it is uncontrolled. Unplanned devitrification is almost always detrimental. The coarse crystals differ from the surrounding residual glass matrix in thermal expansion coefficient, generating internal residual stresses and microcracks at the crystal–glass interfaces on cooling; the crystals and their associated interfacial flaws act as large, uncontrolled Griffith flaws — far more severe strength-limiting defects than the sub-micron surface flaws that normally control as-formed glass strength — so devitrified glass is typically much weaker and more brittle than either the same composition fully amorphous, or a properly engineered fine-grained glass-ceramic of comparable crystallinity. Devitrified glass is also frequently visibly hazy or opaque (light scattering at the coarse crystal–glass interfaces), a visual tell distinct from a glass-ceramic's often translucent-to-opaque but mechanically sound appearance.
Photochromic glass darkens REVERSIBLY under UV/visible light exposure and fades back to clear once the light is removed (or on mild heating). The mechanism relies on a dispersed phase of ultra-fine (nanometre-scale) silver halide crystallites (typically AgCl or AgBr) precipitated within the glass during a controlled heat treatment. On light exposure, $\text{Ag}^+$ ions in the halide crystallites are photoreduced to metallic $\text{Ag}^0$ colloidal clusters, which absorb visible light and darken the glass — a photochemical reaction that is fully REVERSIBLE because the surrounding glass matrix confines the silver and halide species close enough together to recombine once the light is removed (unlike photographic film, where the halide reaction is chemically fixed and made permanent). Used in self-darkening (photochromic) ophthalmic lenses.
Photosensitive glass undergoes a PERMANENT, irreversible change on UV exposure (typically through a mask, i.e. photolithographically), followed by a SEPARATE, deliberate heat treatment step. The glass contains a small dopant of $\text{Ce}^{3+}$ together with a noble-metal ion (Au, Ag, or Cu). UV exposure photo-oxidizes $\text{Ce}^{3+}\to\text{Ce}^{4+}+e^-$; the released electron reduces the noble-metal ion to a neutral atom. The SEPARATE post-exposure heat treatment (typically ∼500–600 °C) then nucleates and grows these metal atoms into colloidal particles, which themselves act as heterogeneous nucleation sites for a crystalline phase (e.g. lithium metasilicate) — so that only the UV-EXPOSED regions crystallize (becoming opaque and, in some formulations, selectively etchable in dilute HF), while unexposed regions remain clear glass. Used to fabricate precisely, permanently patterned glass or glass-ceramic microstructures (the FotoForm/FotoCeram-type process), not a reversible light-transmission effect.
Net difference: photochromic behaviour is reversible, needs no separate heat-treatment step for each darken/fade cycle, and relies on silver-halide photoreduction/recombination; photosensitive behaviour is irreversible, requires a distinct post-exposure heat treatment to develop a permanent, spatially patterned crystallization/coloration, and is used for precision micro-patterning rather than variable light transmission.
