Mechanical Reliability of Glass :

The Transparent Road Surface Layer

Carlo G Pantano

Department of Materials Science and Engineering

Materials Research Institute

The Pennsylvania State University

University Park, PA 16802

Materials Research Institute strengths in materials and nanotechnology

Apple Store, 5th Ave., NYC Glasgow, Scotland

Train Station/Strasbourg, France

the former “Sears Tower” Chicago, IL

New applications will benefit from stronger glass…..

GLASS

Strength of Glass

!2 iMILLIONpsd

E

o

th

but typically, σfracture = 5-20 ksi

What controls the strength?

1. Surface MICROcracks - intrinsic - extrinsic 2. Fatigue 3. Residual Stress

controlled - thermal tempering - chemical tempering +

-

uncontrolled due to forming (anneal them out)

fundamental issues in glass strength

• mechanical damage/flaws at the glass surfaces

• fatigue (stress corrosion/slow crack growth)

• low fracture toughness of the glass

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THE practical issue in glass strength: minimizing surface flaws

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failure strength versus flaw size: f = [2 gf E /p c*] 1/2

Figure 6.7. (a) Surface damage produced by light abrasion of one glass rod on another

Figure 5.15. Horse-shoe shaped cracks in the surface of glass produced by a small ball, sliding from left to right.

Failure strain measurements may provide information about ‘less brittle’ glasses

Setsuro Ito, 2002

IMI-14

GLASS

H20

• intrinsic strength and damage tolerance of the glass (E, γ, K1C, H)

• condition of the original surface (flaws, moisture, roughness)

• properties of the coating (E, H, friction, diffusion, residual stress)

• interfaces (weak vs strong)

Coatings for Strength

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● Scratch resistant

● Low maintenance

● Highly transparent

● Chemically inert

DiamondGuard® Glass that’s nearly diamond-like -- tough and beautiful. DiamondGuard is a family of permanent protective coatings that provides glass with exceptional scratch resistance. In fact, glass with one of our DiamondGuard coatings is proven to be over 10 times more scratch resistant than tempered and chemically strengthened glass.

Developed by Guardian’s Science and Technology Center using a patented process of diamond-like carbon deposition on glass, these coatings are not only renowned for their toughness, but their versatility and beauty as well. DiamondGuard is available on clear glass and sold in thicknesses ranging from 1.7 mm to 12 mm.

Hertzian Indentation

Quasi-plastic Damage Zone

First Ring Crack Concentric Ring Cracks

Radial Cracks

Cone Crack Median Cracks

Green and Pantano

Sample Reduced Modulus (GPa)

DLC Coated Glass 78.0 ± 10.

Uncoated Glass 70.6 ± 8.6

Reduced modulus values from Hertzian contact

Commercial DiamondGuard - diamond-like carbon on float

Spalling of coating

Sample Characteristic

Strength (GPa)

Mean Failure

Stress (GPa)

Median Failure

Stress (GPa)

Median Failure

Load (N)

DLC Coated Glass 3.35 3.23 1.49 558

Uncoated Glass 1.33 1.29 1.29 357

Sample Weibull Modulus (m)

DLC Coated Glass 14.1

Uncoated Glass 15.4

Surface Treatments for Glass Strength

• compressive surface residual stress

- Thermal(Physical) Tempering

- Chemical Tempering

>>>> perhaps best applied in combination with a coating…..

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Figure 72. Distribution of stresses in thermally toughened and chemically strengthened glass.

Figure 3. Processing and equipment used to temper glass. (a) Plot of surface and midplane glass temperatures versus time to show narrow tempering range. (b) Cross-section of heating portion of a tempering furnace. (c) Cross-section of quenching portion of a tempering furnace.

Annealed, Tempered, and Laminated Glass Fracture Patterns

CHEMICAL/ION EXCHANGE STRENGTHENING/TEMPERING

Corning® Gorilla® Glass

Strength comparison for: annealed,

heat strengthened, thermally tempered and

chemically strengthened glasses.

Laminated Safety Glass

Textured Glass

nano-structured thin films

nano-composite coatings

PRINTING ON GLASS: LEDS, solar cells, sensors….

Summary • Glasses are damage intolerant.

• Coated, tempered and laminated panels will be required.

• Surface textures need to be developed to optimize both friction and resistance to contact damage.

• Glass road surfaces are possible in selected areas, especially if developed in cooperation with future all-electric vehicles.