What do brittle materials tend to fail prematurely?
- Due to stress intensification at crack tips causing the materials to fail more dependent on surface or internal flaw conditions than the chemical makeup of the solid- Substantiated by the Griffith-Irwin theory, which states the stress at failure is directly proportional to the 1/sqrt(half-ellipse) of the size of the largest microscopic flaw
How do ductile solids fail that make them tougher?
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Why are crack tips not the limiting factor in ductile solids?
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What is the method for cutting glass? Why does it work?
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What is the theoretical fracture strength of glass? Why is this typically not achieved? What is one way higher fracture strengths CAN be achieved?
- Theoretical fracture strength: 9800 MPa-- However after normal handling, goes down to 50 MPa; due to microscopic surface flaws that get introduced- Theoretical strengths can be achieved with Fiberglass: 3300 MPa-- Due to containerless formation of a flaw-free fiber, coated with a polymer to protect against mutual abrasion
What is the cause of "static fatigue" in soda-lime glasses?
- By "stress corrosion"; reducing the structural integrity of the glass by applying a static load to the glass (esp.) in water-- Glass network is stretched open, facilitating the flow of water into the crack tip, this leaches out alkali (replacing it with hydronium ion) further converting Silica to Silicon Hydroxide. -- Causes crack to slowly grow until it is of a size which causes the entire glass to fail
What are two ways to strengthen glass? What is the goal of both methods?
- Flame polishing-- Can increase strengths up to 1400 MPa- Etching with HF-- Long exposure would totally dissolve the glass-- Brief exposure attacks sharp corners preferentially; similar smoothing as flame polishing-- Can increase strengths up to 1700-3400 MPa- Goal of both = reduce number of cracks in glass reducing stress concentration in the glass thus increasing its strength - Baseline strength for normally handled glass = 690 MPa
What is involved in the tempering process?
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What is the strengthening mechanism of tempering?
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Benefits of tempered glass?
- Strength of tempered glass is 5x that of annealed glass- Referred to as safety glass because during fracture large granules are formed rather than large shards, safer in the window glass of cars
How does Ion Exchange Strengthening of glass work?
- Glass containing Na+ is immersed in Potassium Nitrate solution at a temperature below Tg: ~460C for 6 hours-- The process is driven by the entropy of mixing, some of the Na+ in the glass network is replaced by larger K+
Why does Ion Exchange strengthen the glass?
- The "stuffing" of K+ puts the glass surface in compression- Corning Gorilla Glass and Schott Xensation are ion-exchange sodium aluminosilicate glasses--- Glass thickness 0.4-2mm compressive layer: 50 µm
What are laminated glasses?
Un-tempered glass panels pressure joined to plastic layers
What are uses of laminated glasses?
- Automotive windshields: upon fracture, glass fragments adhere to the plastic, and do not propel into the passenger compartment- "Bullet-proof (resistant)": Type of laminated glass, over 6cm thicc
What causes a phase separation of Sodium borosilicate glasses?
- A compositional range of liquid immiscibility which results in phase-separated glasses (see 2f-2)-- With appropriate heat treatment the B2O3-Na2O-Rich regions become a continuous phase (see 2f-3)-- This phase can be leeched (etched) out with acid (eg Sulfuric acid at 90C)-- Resulting porous glass can have a very narrow pore size ranges
What are applications for phase separated glasses? Why?
- Component for gas separation -- Gases of smaller size can pass through, but larger ones cannot- Used as a desiccator: Water adsorbed onto vast surface area-- Porous glass provides a tremendous amount of surface area: "Thirsty glass"- Used for catalysis-- Acceleration of gaseous reactions through interaction with a surface
How is vycor glass manufactured? What is its main benefit?
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What is main benefit of using beta quartz as a glass-ceramic material? And beta eucriptite?
- Beta Quartz has a negative, near-zero CTE-- see 2g-1- Beta eucriptite has a near-zero CTE-- Both have excellent thermal shock resistance
What is the main composition used to mimic the behavior of beta quartz? Why does this occur?
- Compositional range of 1 Lithia: 1 Alumina: 6 Silica; which crystallize to beta-eucriptite solid solution (ss)-- beta-eucriptite closely emulates the beta-quartz structure-- This forms because AlO4 (5-) tetrahedra form with charge compensation provided by the Li+
How is beta-eucriptite glass made? (hint: Titania)
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What is the advantage of the use of nucleating agents in beta-eucriptite manufacture?
- Since the original phase-separation was uniformly dispersed, crystals quickly grow to uniform impingement-- Results in very fine grain size (~50 nm) and no porosity
With increasing heat treatment temperature on beta-eucriptite glass, what new phase forms? What's is chemical composition? What is good about this phase?
- At 1000C beta-spodumene (ss) forms beta-spodumene (ss): LiAl(Si2O6)-- also has a very low CTE
How do the optical properties of beta-eucriptite and beta-spodumene differ? How does this drive applications?
- beta-eucriptite (ss) forms very small crystals of low birefringence-- transparent; applications: (Visions) cookware, range tops- beta-spodumene (ss) forms larger, more birefringent crystals-- Opaque white; applications: Corningware cookware
What are other applications of beta-eucriptite and beta-spodumene ceramic glasses?
- Precision optics; fabricated using spin casting. Annealed for 8 months to form Zerodur® low-expansion glass-ceramic- High strength glass; Chemchor® process: Sodia/Potassia-Alumina-Silica-(titania) ion exchanged with molten lithium sulfate-- thin ion exchanged layers form beta-eucriptite and/or beta-spodumene ceramic-- Low CTE applies compressive stresses on glass interiors-- Thin to preserve transparency; almost 2x as strong as tempered glass
How are machinable glass ceramics formed?
- Through the devitrification of phases of a mica composition-- Can be easily cleaved, can be drilled lathed etc -- see 2g-8
What is meant by the chemical machining of glass ceramics?
- A photographic process to define locations to devitrify which are more soluble to an enchant -- See 2g-9 for the process of the selective etching of Lithium metasilicate