Transcript Strengthening of Glass

Recent Advances in Improving
Strength of Glass
Suresh T. Gulati
Research Fellow & Consultant
CORNING Incorporated
Chronology
G. Galilei (1638):
C. A. Coulomb (~1770):
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
C. E. Inglis (1913):
quantification of stress concentration at elliptical
defects in glass plates: sA=s(1+2a/b); ab
A. A. Griffith (1920):
relation of strain energy to surface energy and
critical stress to defect size:
sc2  2E/(a)  sc << E/10
G. R. Irvin (1957):
extension of Griffith’s equation by considering
plastic work in total fracture energy G: G = s2a
definition of the stress intensity factor K and Kc:
r 1/2 s f() = KI
S. M. Wiederhorn (1970):
… (and many others)
experimental description of crack speed
regimes, environmental fatigue and stress
corrosion in glasses and other materials
...
Chronology
σ=
10-2Σσini
O. Schott, A. Winkelmann, et al.
G. Gehlhoff, Z. tech. Phys. 6 (1925) 544-554, et al.
What do we mean by
Strengthening?
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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?
Basic Principles of
Strengthening
• Minimize flaw severity by modifying surfaces
- grinding & polishing
- fire polishing
- acid etching
• Protect modified surfaces from further damage
- coating
Basic Principles of
Strengthening
•
Introduce beneficial stresses in surfaces
- thermal tempering
- chemical tempering
- high temperature lamination
- lamination plus tempering
- differential densification
Strengthening by Post-Processing
PostProcess
Annealed
Strength
Surface
Compressi
on
0
Final
Strength
None
70 MPa
Thermal
70 MPa
Tempering
100 MPa
170 MPa
Chemical 70 MPa
Tempering
550 MPa
620 MPa
70 MPa
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)
Various Approaches
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Thermal Tempering
Chemical Tempering
High Temperature Lamination
Coating
Acid Etching
Low Temperature Lamination
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
Strength of Glass
• Strength is extrinsic property
• Toughness is intrinsic property
(sc)
(KIc)
 KIc = Ysc ac0.5
 Y = flaw tip geometry factor = 1.2
 ac = critical flaw depth
 sc = failure stress = strength of glass
Strengthening by Post-Processing
PostProcess
Annealed
Strength
Surface
Compressi
on
0
Final
Strength
None
70 MPa
Thermal
70 MPa
Tempering
100 MPa
170 MPa
Chemical 70 MPa
Tempering
550 MPa
620 MPa
70 MPa
Strengthening by Post-Processing
Post
Process
Final
Annealed Surface
Compressi Strength
Strength
High Temp
Lamination
200 MPa
on
Class 100 clean > 300 MPa
Float Process +
Coating
140 MPa
lam’n
+
200 MPa
temper
0
540 MPa
> 300 MPa
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
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
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
Challenges in Tempering
• Obtaining good temper
• Eliminating breakage during tempering
• Controlling final shape of article
Tempering Steps
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Heating the glass
Sag bending or press bending
Air quenching or chilling
Inspecting
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
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
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
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
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
Estimate of Temper Level
   2

s s  Compressio n 
  
1   3

E   1

Center
s c Tension 



1   3

Surface
  10  5  106 psi
  0.22
   170 10 7 in / in / C
Sigm as  153   14000psi
Sigm ac  77   7000psi
   90 C
Estimate of Cooling Rate
t2R
 
8k
k  thermal diffusivity  0.0013 in2 / sec
  100 t 2 R
ΔT (°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
120
0.118
86
120
0.090
148
Estimate of Temporary Tension
E    t 2 R 
st 


1     8k 
1.77 10 6 17 10 6 2

t R
0.55  8  0.0013
 5260 t 2 R
0.150”
0.118”
0.090”
R
35°C/sec
57°C/sec
99°C/sec
st
4140 psi
4175 psi
4220 psi
ΔT
80°C
80°C
80°C
0.150”
0.118”
0.090”
44°C/sec
72°C/sec
124°C/sec
5210 psi
5260 psi
5260 psi
100°C
100°C
100°C
0.150”
0.118”
0.090”
53°C/sec
86°C/sec
148°C/sec
6270 psi
6300 psi
6300 psi
120°C
120°C
120°C
t
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
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
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
Schematic of Ion Exchange
Strength vs. Treatment Time
Strength Distribution before and
after Ion Exchange
Strength Distribution vs. Ion Exchange Treatment
Time
Effect of Surface Abrasion on Strength of Ion
Exchanged Glass
Applications of Chemical
Tempering
•Ophthalmic lenses
•Aircraft windows
•Lightweight containers
•Centrifuge tubes
•Automotive backlite
•Photocopier transparencies
•Cell phone cover glass
•Touch pads
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
Science of Chemical Tempering
Stress Relaxation
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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
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
Innovations in Ion Exchange
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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
Question
• Could atomic mechanisms helping open
network doorways for enhanced diffusion
also lead to accelerated stress relaxation?
• Most likely, YES !
Summary of Chemical Tempering
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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
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.
Strengthening by Lamination
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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
Strengthening by Post-Processing
PostProcess
Annealed
Strength
Surface
Compressi
on
0
Final
Strength
None
70 MPa
Thermal
70 MPa
Tempering
100 MPa
170 MPa
Chemical 70 MPa
Tempering
550 MPa
620 MPa
70 MPa
Strengthening by Post-Processing
Post
Process
Final
Annealed Surface
Compressi Strength
Strength
High Temp
Lamination
200 MPa
on
Class 100 clean > 300 MPa
Float Process +
Coating
140 MPa
lam’n
+
200 MPa
temper
0
540 MPa
> 300 MPa
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)
PPAD plot: 11/27/2006
impact of edge & treatment conditions
Failure Probability, %
99.5
98
95
90
80
60
40
20
10
5
2
1
1100
1000
900
800
700
600
500
400
300
200
100
Strength, MPa
Data Summary
ground edge: (23 Specs)
Mean=150; Stdev=19.3; m=8.82; S0=158
ground + acid: (10 Specs)
Mean=533; Stdev=250; m=2.80; S0=588
polished edge: (25 Specs)
Mean=189; Stdev=19.6; m=11.9; S0=197
polished + ion exch.: (24 Specs) Mean=291; Stdev=21.2; m=15.9; S0=300
ground + acid: Ranks: 1 to 6; (6 Specs) m=5.96; S0=440
ground + acid: Ranks: 6 to 10; (5 Specs) m=1.46; S0=540
WeiPPAD 5.010
(01/29/03) (XOnProb) (MedianEst)