Presentation on theme: "Recent Advances in Improving Strength of Glass"— Presentation transcript:
1Recent Advances in Improving Strength of Glass Suresh T. GulatiResearch Fellow & ConsultantCORNING Incorporated
2Chronology ... G. Galilei (1638): C. A. Coulomb (~1770): C. E. Inglis (1913):A. A. Griffith (1920):G. R. Irvin (1957):S. M. Wiederhorn (1970):… (and many others)observation of size-dependencein fatigue of ships(µ )1/2tm - *µsm = S0:shear stress tm causes fracture at internal friction µ, normal stress sm and intergranular cohesion S0quantification of stress concentration at elliptical defects in glass plates: A=s(1+2a/b); abrelation of strain energy to surface energy and critical stress to defect size:c2 2E/(a) c << E/10extension of Griffith’s equation by considering plastic work in total fracture energy G: G = 2adefinition of the stress intensity factor K and Kc: r 1/2 f() = KIexperimental description of crack speed regimes, environmental fatigue and stress corrosion in glasses and other materials...
3Chronology σ = 10-2Σσini O. Schott, A. Winkelmann, et al. G. Gehlhoff, Z. tech. Phys. 6 (1925) , et al.
4What do we mean by Strengthening? High Surface Strength?High Edge Strength ?Resistance to Surface Damage/Abrasion?Improvement in Short Term Strength?Improvement in Long Term Strength?All Surfaces in Compression?How Deep a Compression Layer?How High the Internal Tension?
5Basic Principles of Strengthening Minimize flaw severity by modifying surfaces- grinding & polishing- fire polishing- acid etchingProtect modified surfaces from further damage- coating
6Basic Principles of Strengthening Introduce beneficial stresses in surfaces- thermal tempering- chemical tempering- high temperature lamination- lamination plus tempering- differential densification
13Strengthening by Post-Processing AnnealedStrengthSurface CompressionFinal StrengthHigh TempLamination200 MPa140 MPa lam’n+200 MPa temper540 MPaClass 100 cleanFloat Process +Coating> 300 MPa
14Thermal Tempering Ideal for float glass, i.e. high CTE glasses Ideal for deep compression layerSimple, clean and easy to implement in productionRequires good surface quality including edgesProof testing prior to tempering may prove beneficial
15Thermal TemperingTemper level may be improved by increasing max. temperature and/or cooling rateTwo levels of tempering:a) heat strengtheningb) fully temperedSee overhead presentation
16Higher Quench Rates during Thermal Tempering Increase heat transfer rate by usinga) moist air orb) liquid medium like oil orc) organic fluids ord) salt bathHeat transfer rate can be increased from to 0.02 cal /cm2 oC sec.High quench rates will increase temporary tensile stress on surfaces and edges causing premature cracking, hence surface and edge defects should be minimized prior to tempering
17Challenges in Tempering Obtaining good temperEliminating breakage during temperingControlling final shape of article
18Tempering Steps Heating the glass Sag bending or press bending Air quenching or chillingInspecting
19Heating Step Uniform heat is critical with little or no gradients Max. temperature > annealing temperatureToo high a temperature causes distortionToo low a temperature causes breakage during quenching
20Quenching StepRapid quenching from 650+°C to 500-°C will give good temperTemper level improves with cooling rate and the square of glass thicknessNonuniform cooling results in distortion and regional stresses (visible under polarized light)Breakage during quenching indicates either too low a temperature or defects on surfaces and edgesPurposely induced differential regional stress helps control break pattern and minimize spleen formation, e.g. by nonlinear positioning of air nozzlesMax. surface tension (temporary tension) occurs a few seconds (2 to 4 secs.) after start of quenching
21Inspection Step Inspect shape for distortion Inspect for breakage and originedge break?surface break?before quenching?after quenching?Inspect for parabolic stress pattern through the thickness; use polarized light
22Fully Tempered Glass σs~14000 psi σs~7000 psi Measure particle size, weight and distribution when center-punchedSpontaneous breakage-NiS stone in tension zone?Verify by cooling glass to -40°C-Propagation of surface defect by externalstressing
23Heat-Strengthened Glass 3500 < σs < 10,000 psi5500 < σs < 9,700 psiFragment size < annealed glassbut > tempered glassHS glass used in place of annealed for higher strength, e.g. laminated side windows
25Estimate of Cooling Rate ΔT (°C) t(in.) R(°C/sec)
26Estimate of Temporary Tension t R st ΔT0.150” 35°C/sec psi 80°C0.118” 57°C/sec psi 80°C0.090” 99°C/sec psi 80°C0.150” 44°C/sec psi 100°C0.118” 72°C/sec psi 100°C0.090” 124°C/sec psi 100°C0.150” 53°C/sec psi 120°C0.118” 86°C/sec psi 120°C0.090” 148°C/sec psi 120°C
27Chemical Tempering Ideal for non-flat and complex shapes Ideal for thin glassesIdeal for high surface compressive stress (500 MPa)Exchange of large alkali ions for small alkali ions, hence “ion exchange process”Ion exchange temperature < Strain PointNo optical or physical distortion of product
28Limitations of Chem-tempering Depth of compression layer < 0.05 mmGlasses with low alkali content do not chem-temper efficientlyChem-treatment time can be long; 2 to 24 hoursHigher cost than thermal tempering
29Ion Exchange ProcessTreat glass article in molten salt bath, i.e. KNO3Exchange K+ ion for Na+ ion at T < S.P.Magnitude and depth of compression layerdepend oni) bath concentrationii) treatment timeiii) diffusion vs. stress relaxation kinetics
32Strength Distribution before and after Ion Exchange
33Strength Distribution vs. Ion Exchange Treatment Time
34Effect of Surface Abrasion on Strength of Ion Exchanged Glass
35Applications of Chemical Tempering Ophthalmic lensesAircraft windowsLightweight containersCentrifuge tubesAutomotive backlitePhotocopier transparenciesCell phone cover glassTouch pads
36Science of Chemical Tempering Diffusion KineticsExchange of ions on one to one basisInterdiffusion coeff. approximated by error functionInfluence of generated stressStress GenerationOne-dimensional difference between molar volumes of equimolar alkali glasses as function of local compositionLinear network dilatation coeff. similar to linear coeff. ofthermal expansion
37Science of Chemical Tempering Stress RelaxationViscous flowLow temperature network adjustmentCharacterization by stress measurementCharacterization by strength measurementStrength measurement must include abrasion specsProposed ASTM standard based on surface compression and depth of compression layerUniform biaxial strengthening
40Practical Aspects of Ion Exchange Only alkali containing glasses can be strengthenedSoda-lime-silica glass may have high surface compression but depth of compression is low (20mm)Bath composition is sensitive to contaminationAccessibility to flaws may be different on tin vs. air side
41Innovations in Ion Exchange Sonic assistMicrowave assistElectric field assistDiffusion rates are enhanced by above assistsSome conccerns over localized microwave absorption due to microwave field gradients
42QuestionCould atomic mechanisms helping open network doorways for enhanced diffusion also lead to accelerated stress relaxation?Most likely, YES !
43Summary of Chemical Tempering Slow and glass selective processProcess control is criticalExpensive processConsumer education on strength issues is importantNew glass products being chemically strengthened and soldNew innovations are needed to reduce cost without compromising effectiveness
44Reference“Technology of Ion Exchange Strengthening of Glass: A Review”by A.K.Varshneya & W.C.LaCoursein Ceramic Transaction, Vol. 29, The American Ceramic Society, pp , 1993.
45Strengthening by Lamination Definition of laminated glassLamination processResidual stressesDepth of compression layerImprovement in surface strengthThermal tempering of laminated glassStored energy and frangibility