Recent Advances in Improving Strength of Glass

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Recent Advances in Improving Strength of Glass

• Suresh T. Gulati
• Research 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)1/2tm - µsm S0 shear
stress tm causes fracture at internal friction µ,
normal stress sm and intergranular cohesion
S0 quantification of stress concentration at
elliptical defects in glass plates ?As(12a/b)
a?b relation of strain energy to surface energy
and critical stress to defect size ?c2 ?
2?E/(a?) ? ?c ltlt E/10 extension of Griffiths
equation by considering plastic work in total
fracture energy G G ?2a? definition 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
s 10-2Ssini
O. Schott, A. Winkelmann, et al. G. Gehlhoff, Z.
tech. Phys. 6 (1925) 544-554, et al.
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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 gt 25 MPa (chemtemper)
• gt 50 MPa (thermal temp)
• gt 120 MPa ( lamn temper )
• gt 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 (sc)
• Toughness is intrinsic property (KIc)
• KIc Ysc ac0.5
• Y flaw tip geometry factor 1.2
• ac critical flaw depth
• sc 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 Annealed Strength Surface Compression Final Strength
High Temp Lamination 200 MPa 140 MPa lamn 200 MPa temper 540 MPa
Class 100 clean Float Process Coating gt 300 MPa 0 gt 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 strengthening
• b) fully tempered
• See overhead presentation

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Higher Quench Rates during Thermal Tempering

• Increase heat transfer rate by using
• a) moist air or
• b) liquid medium like oil or
• c) organic fluids or
• d) 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 gt 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 650C 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 Glass

• ss14000 psi
• ss7000 psi
• Measure particle size, weight and distribution
  when center-punched
• Spontaneous breakage
• -NiS stone in tension zone?
• Verify by cooling glass to -40C
• -Propagation of surface defect by external
• stressing

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Heat-Strengthened Glass

• 3500 lt ss lt 10,000 psi
• 5500 lt ss lt 9,700 psi
• Fragment size lt annealed glass
• but gt 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
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Estimate of Cooling Rate
?T (C) t(in.) R(C/sec) 80 0.150 35 80
0.118 57 80 0.090 99 100 0.150 44 100
0.118 72 100 0.090 124 120 0.150 53 1
20 0.118 86 120 0.090 148
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Estimate of Temporary Tension

• t R st ?T
• 0.150 35C/sec 4140 psi 80C
• 0.118 57C/sec 4175 psi 80C
• 0.090 99C/sec 4220 psi 80C
• 0.150 44C/sec 5210 psi 100C
• 0.118 72C/sec 5260 psi 100C
• 0.090 124C/sec 5260 psi 100C
• 0.150 53C/sec 6270 psi 120C
• 0.118 86C/sec 6300 psi 120C
• 0.090 148C/sec 6300 psi 120C

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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 lt Strain Point
• No optical or physical distortion of product

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Limitations of Chem-tempering

• Depth of compression layer lt 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 lt S.P.
• Magnitude and depth of compression layer
• depend on
• i) bath concentration
• ii) treatment time
• iii) 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
  (20mm)
• 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.LaCourse
• in 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 Annealed Strength Surface Compression Final Strength
High Temp Lamination 200 MPa 140 MPa lamn 200 MPa temper 540 MPa
Class 100 clean Float Process Coating gt 300 MPa 0 gt 300 MPa
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Glass Quality Requirements

• Glass batch free of contamination.e.g. NiS
• Center Strength gt 25 MPa (chemtemper)
• gt 50 MPa (thermal temp)
• gt 120 MPa ( lamn temper )
• gt 300 MPa ( Class 100 clean Float
  Process)

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