1. Recent Advances in Improving Strength of Glass
Suresh T. Gulati
Research Fellow & Consultant
CORNING Incorporated

2. 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
(µ )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: A=s(1+2a/b); ab
relation of strain energy to surface energy and critical stress to defect size:
c2  2E/(a)  c << E/10
extension of Griffith’s 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
...

3. Chronology σ = 10-2Σσini O. Schott, A. Winkelmann, et al.
G. Gehlhoff, Z. tech. Phys. 6 (1925) , et al.

4. 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?

5. Basic Principles of Strengthening
Minimize flaw severity by modifying surfaces
- grinding & polishing
- fire polishing
- acid etching
Protect modified surfaces from further damage
- coating

6. Basic Principles of Strengthening
Introduce beneficial stresses in surfaces
- thermal tempering
- chemical tempering
- high temperature lamination
- lamination plus tempering
- differential densification

7. Strengthening by Post-Processing
Annealed Strength
Surface Compression
Final Strength
None
70 MPa
Thermal Tempering
100 MPa
170 MPa
Chemical Tempering
550 MPa
620 MPa

8. 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 clean Float Process)

9. Various Approaches Thermal Tempering Chemical Tempering
High Temperature Lamination
Coating
Acid Etching
Low Temperature Lamination

10. 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

11. 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

12. Strengthening by Post-Processing
Annealed Strength
Surface Compression
Final Strength
None
70 MPa
Thermal Tempering
100 MPa
170 MPa
Chemical Tempering
550 MPa
620 MPa

13. Strengthening by Post-Processing
Annealed
Strength
Surface Compression
Final Strength
High Temp
Lamination
200 MPa
140 MPa lam’n +
200 MPa temper
540 MPa
Class 100 clean
Float Process +
Coating
> 300 MPa

14. 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

15. 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

16. 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 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

17. Challenges in Tempering
Obtaining good temper
Eliminating breakage during tempering
Controlling final shape of article

18. Tempering Steps Heating the glass Sag bending or press bending
Air quenching or chilling
Inspecting

19. 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

20. 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

21. 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

22. Fully Tempered Glass σs~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

23. 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

24. Estimate of Temper Level

25. Estimate of Cooling Rate
ΔT (°C) t(in.) R(°C/sec)

26. Estimate of Temporary Tension
t R st ΔT
0.150” 35°C/sec psi 80°C
0.118” 57°C/sec psi 80°C
0.090” 99°C/sec psi 80°C
0.150” 44°C/sec psi 100°C
0.118” 72°C/sec psi 100°C
0.090” 124°C/sec psi 100°C
0.150” 53°C/sec psi 120°C
0.118” 86°C/sec psi 120°C
0.090” 148°C/sec psi 120°C

27. 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

28. 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

29. 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 on
i) bath concentration
ii) treatment time
iii) diffusion vs. stress relaxation kinetics

30. Schematic of Ion Exchange

31. Strength vs. Treatment Time

32. Strength Distribution before and after Ion Exchange

33. Strength Distribution vs. Ion Exchange Treatment Time

34. Effect of Surface Abrasion on Strength of Ion Exchanged Glass

35. Applications of Chemical Tempering
Ophthalmic lenses
Aircraft windows
Lightweight containers
Centrifuge tubes
Automotive backlite
Photocopier transparencies
Cell phone cover glass
Touch pads

36. 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

37. 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

40. 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

41. 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

42. Question
Could atomic mechanisms helping open network doorways for enhanced diffusion also lead to accelerated stress relaxation?
Most likely, YES !

43. 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

44. 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 , 1993.

45. 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

55. Strengthening by Post-Processing
Annealed Strength
Surface Compression
Final Strength
None
70 MPa
Thermal Tempering
100 MPa
170 MPa
Chemical Tempering
550 MPa
620 MPa

56. Strengthening by Post-Processing
Annealed
Strength
Surface Compression
Final Strength
High Temp
Lamination
200 MPa
140 MPa lam’n +
200 MPa temper
540 MPa
Class 100 clean
Float Process +
Coating
> 300 MPa

57. 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 clean Float Process)