Recent Advances in Improving Strength of Glass

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 (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: 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: r1/2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) 544-554, 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

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 100 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

13. Strengthening by Post-Processing

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 airor 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 /cm2oC 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) 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

26. Estimate of Temporary Tension tRstΔT 0.150” 35°C/sec 4140 psi 80°C 0.118” 57°C/sec 4175 psi 80°C 0.090” 99°C/sec 4220 psi 80°C 0.150” 44°C/sec 5210 psi 100°C 0.118” 72°C/sec 5260 psi 100°C 0.090” 124°C/sec 5260 psi 100°C 0.150” 53°C/sec 6270 psi 120°C 0.118” 86°C/sec 6300 psi 120°C 0.090” 148°C/sec 6300 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.365-378, 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