Recent Advances in Improving Strength of Glass

Suresh T. GulatiResearch Fellow & Consultant

CORNING Incorporated

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

...

Chronology

O. Schott, A. Winkelmann, et al.

G. Gehlhoff, Z. tech. Phys. 6 (1925) 544-554, et al.

σ = 10-2Σσini

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?

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

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

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

• 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 (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

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

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

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 strengtheningb) fully tempered

• See overhead presentation

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

Challenges in Tempering

• Obtaining good temper• Eliminating breakage during tempering• Controlling final shape of article

Tempering Steps

• 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 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

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

31

1 ECenter

Tensionc

32

1 Surface

nCompressios

psi610510 22.0

Cinin //10170 7

psiSigmac 700077

psiSigmas 14000153

C 90

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

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

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 oni) bath concentrationii) treatment timeiii) 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• 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 (20m)

• Bath composition is sensitive to contamination• Accessibility to flaws may be different on tin vs. air

side

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

Question

• Could atomic mechanisms helping open network doorways for enhanced diffusion also lead to accelerated stress relaxation?

• Most likely, YES !

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

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.

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

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

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

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)