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Recent Advances in Improving Strength of Glass
Suresh T. GulatiResearch Fellow & Consultant
CORNING Incorporated
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Chronology 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-dependence in fatigue of ships
(µ2 + 1)1/2m - *µm = S0: shear stress m causes fracture at internal friction µ, normal stress m and intergranular cohesion S0
quantification of stress concentration at elliptical defects in glass plates: A=(1+2a/b); ab
relation of strain energy to surface energy and critical stress to defect size: c
2 2E/(a) c << E/10
extension 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() = KI
experimental description of crack speed regimes, environmental fatigue and stress corrosion in glasses and other materials
...
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Chronology
O. Schott, A. Winkelmann, et al.
G. Gehlhoff, Z. tech. Phys. 6 (1925) 544-554, et al.
σ = 10-2Σσini
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What 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?
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Basic Principles of Strengthening
• Minimize flaw severity by modifying surfaces- grinding & polishing- fire polishing- acid etching
• Protect modified surfaces from further damage- coating
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Basic Principles of Strengthening
• Introduce beneficial stresses in surfaces- thermal tempering- chemical tempering- high temperature lamination- lamination plus tempering- differential densification
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Strengthening by Post-Processing
Post-Process
Annealed Strength
Surface Compression
Final Strength
None 70 MPa 0 70 MPa
Thermal Tempering
70 MPa 100 MPa 170 MPa
Chemical Tempering
70 MPa 550 MPa 620 MPa
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Glass Quality Requirements• Glass batch free of contamination.e.g. NiS• Center Strength > 25 MPa (chemtemper)
> 50 MPa (thermal temp)
> 120 MPa ( lam’n & temper )
> 300 MPa ( Class 100 clean Float Process)
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Various Approaches
• Thermal Tempering• Chemical Tempering• High Temperature Lamination• Coating• Acid Etching• Low Temperature Lamination
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Defects in Glass
• Bulk defects in interior due to inhomogeneities from batch or mfg process
• Surface defects due to handling, scoring or contact with dissimilar materials
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Strength of Glass
• Strength is extrinsic property (c)
• Toughness is intrinsic property (KIc)
KIc = Yc ac0.5
Y = flaw tip geometry factor = 1.2 ac = critical flaw depth c = failure stress = strength of glass
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Strengthening by Post-Processing
Post-Process
Annealed Strength
Surface Compression
Final Strength
None 70 MPa 0 70 MPa
Thermal Tempering
70 MPa 100 MPa 170 MPa
Chemical Tempering
70 MPa 550 MPa 620 MPa
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Strengthening by Post-Processing
Post Process
AnnealedStrength
Surface Compression
Final Strength
High TempLamination
200 MPa 140 MPa lam’n+200 MPa temper
540 MPa
Class 100 cleanFloat Process +Coating
> 300 MPa 0 > 300 MPa
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Thermal Tempering
• Ideal for float glass, i.e. high CTE glasses• Ideal for deep compression layer• Simple, clean and easy to implement in
production• Requires good surface quality including
edges• Proof testing prior to tempering may prove
beneficial
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Thermal Tempering
• Temper level may be improved by increasing max. temperature and/or cooling rate
• Two levels of tempering:a) heat strengtheningb) fully tempered
• See overhead presentation
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Higher Quench Rates during Thermal Tempering
• Increase heat transfer rate by using a) moist air ororb) liquid medium like oil orc) organic fluids ord) salt bath
• Heat transfer rate can be increased from 0.005 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
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Challenges in Tempering
• Obtaining good temper• Eliminating breakage during tempering• Controlling final shape of article
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Tempering Steps
• Heating the glass• Sag bending or press bending• Air quenching or chilling• Inspecting
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Heating Step
• Uniform heat is critical with little or no gradients
• Max. temperature > annealing temperature• Too high a temperature causes distortion• Too low a temperature causes breakage
during quenching
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Quenching Step• Rapid quenching from 650+°C to 500-°C will give good
temper• Temper level improves with cooling rate and the square
of glass thickness• Nonuniform 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 edges• Purposely induced differential regional stress helps
control break pattern and minimize spleen formation, e.g. by nonlinear positioning of air nozzles
• Max. surface tension (temporary tension) occurs a few seconds (2 to 4 secs.) after start of quenching
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Inspection Step
• Inspect shape for distortion• Inspect for breakage and origin
– edge break?– surface break?– before quenching?– after quenching?
• Inspect for parabolic stress pattern through the thickness; use polarized light
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Fully Tempered Glassσs~14000 psi
σs~7000 psi• Measure particle size, weight and
distribution when center-punched• Spontaneous breakage
-NiS stone in tension zone? Verify by cooling glass to -40°C-Propagation of surface defect by external stressing
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Heat-Strengthened Glass
• 3500 < σs < 10,000 psi
• 5500 < σs < 9,700 psi• Fragment size < annealed glass
but > tempered glass• HS glass used in place of annealed for
higher strength, e.g. laminated side windows
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Estimate of Temper Level
31
1 ECenter
Tensionc
32
1 Surface
nCompressios
psi610510 22.0
Cinin //10170 7
psiSigmac 700077
psiSigmas 14000153
C 90
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Estimate of Cooling RatekRt
8
2
sec/0013.0 2inydiffusivitthermalk
Rt 2100ΔT (°C) t(in.) R(°C/sec)80 0.150 3580 0.118 5780 0.090 99100 0.150 44100 0.118 72100 0.090 124120 0.150 53120 0.118 86120 0.090 148
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Estimate of Temporary Tension
t R t ΔT0.150” 35°C/sec 4140 psi 80°C0.118” 57°C/sec 4175 psi 80°C0.090” 99°C/sec 4220 psi 80°C
0.150” 44°C/sec 5210 psi 100°C0.118” 72°C/sec 5260 psi 100°C0.090” 124°C/sec 5260 psi 100°C
0.150” 53°C/sec 6270 psi 120°C0.118” 86°C/sec 6300 psi 120°C0.090” 148°C/sec 6300 psi 120°C
kRtE
t 81
2
Rt 266
0013.0855.010171077.1
Rt 25260
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Chemical Tempering
• Ideal for non-flat and complex shapes• Ideal for thin glasses• Ideal for high surface compressive stress
(500 MPa)• Exchange of large alkali ions for small
alkali ions, hence “ion exchange process”• Ion exchange temperature < Strain Point• No optical or physical distortion of product
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Limitations of Chem-tempering
• Depth of compression layer < 0.05 mm• Glasses with low alkali content do not
chem-temper efficiently• Chem-treatment time can be long; 2 to 24
hours• Higher cost than thermal tempering
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Ion Exchange Process
• Treat glass article in molten salt bath, i.e. KNO3
• Exchange K+ ion for Na+ ion at T < S.P.• Magnitude and depth of compression layer
depend oni) bath concentrationii) treatment timeiii) diffusion vs. stress relaxation kinetics
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Schematic of Ion Exchange
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Strength vs. Treatment Time
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Strength Distribution before and after Ion Exchange
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Strength Distribution vs. Ion Exchange Treatment Time
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Effect of Surface Abrasion on Strength of Ion Exchanged Glass
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Applications of Chemical Tempering
•Ophthalmic lenses•Aircraft windows•Lightweight containers•Centrifuge tubes•Automotive backlite•Photocopier transparencies•Cell phone cover glass•Touch pads
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Science of Chemical Tempering
Diffusion Kinetics• Exchange of ions on one to one basis• Interdiffusion coeff. approximated by error function• Influence of generated stress
Stress Generation• One-dimensional difference between molar volumes of
equimolar alkali glasses as function of local composition• Linear network dilatation coeff. similar to linear coeff. of
thermal expansion
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Science of Chemical Tempering
Stress Relaxation• Viscous flow• Low temperature network adjustment• Characterization by stress measurement• Characterization by strength measurement• Strength measurement must include abrasion specs• Proposed ASTM standard based on surface
compression and depth of compression layer• Uniform biaxial strengthening
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Practical Aspects of Ion Exchange
• Only alkali containing glasses can be strengthened• Soda-lime-silica glass may have high surface
compression but depth of compression is low (20m)
• Bath composition is sensitive to contamination• Accessibility to flaws may be different on tin vs. air
side
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Innovations in Ion Exchange
• Sonic assist• Microwave assist• Electric field assist• Diffusion rates are enhanced by above
assists• Some conccerns over localized microwave
absorption due to microwave field gradients
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Question
• Could atomic mechanisms helping open network doorways for enhanced diffusion also lead to accelerated stress relaxation?
• Most likely, YES !
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Summary of Chemical Tempering
• Slow and glass selective process• Process control is critical• Expensive process• Consumer education on strength issues is important• New glass products being chemically strengthened
and sold• New innovations are needed to reduce cost without
compromising effectiveness
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Reference
• “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.365-378, 1993.
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Strengthening by Lamination
• Definition of laminated glass• Lamination process• Residual stresses • Depth of compression layer• Improvement in surface strength• Thermal tempering of laminated glass• Stored energy and frangibility
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Strengthening by Post-Processing
Post-Process
Annealed Strength
Surface Compression
Final Strength
None 70 MPa 0 70 MPa
Thermal Tempering
70 MPa 100 MPa 170 MPa
Chemical Tempering
70 MPa 550 MPa 620 MPa
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Strengthening by Post-Processing
Post Process
AnnealedStrength
Surface Compression
Final Strength
High TempLamination
200 MPa 140 MPa lam’n+200 MPa temper
540 MPa
Class 100 cleanFloat Process +Coating
> 300 MPa 0 > 300 MPa
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Glass Quality Requirements• Glass batch free of contamination.e.g. NiS• Center Strength > 25 MPa (chemtemper)
> 50 MPa (thermal temp)
> 120 MPa ( lam’n & temper )
> 300 MPa ( Class 100 clean Float Process)
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