Vickers Indentation Cracking of Ion-Exchanged Glasses: Quasi-Static vs. Dynamic Contact

Timothy M. Gross,James J. Price

doi:10.3389/fmats.2017.00004

Abstract

The indentation deformation and cracking responses of ion-exchanged glasses were measured using quasi-static and dynamic loading cycles. Two glass types were compared, a normal glass that deforms to a large extent by a shearing mechanism and a damage resistant glass that comparatively deforms with less shear and more densification. The quasi-static indentation cracking threshold for median/radial cracks for the ion-exchanged normal glass was determined to be 7 kilograms force (kgf), while the ion-exchanged damage resistant glass required loads exceeding 30 kgf. The increased cracking threshold of the damage resistant glass composition is attributed to the deformation mechanism, i.e. deformation with greater densification/less shear results in less subsurface damage and less residual stress. Both glass types were also subjected to dynamic indentation where the contact event time was more than 10,000 times shorter than the quasi-static condition. Under dynamic loading conditions, the cracking thresholds of the ion-exchanged normal and damage resistant glasses increased to greater than 50 kgf and greater than 150 kgf, respectively. The stress induced optical retardation was compared for quasi-static and dynamic indents made at sub-cracking threshold loads for both glasses. For indents made at the same sub-cracking threshold load in the normal glass, optical retardation mapping indicates less residual stress surrounding dynamic indents when compared to quasi-static indents. This suggests a rate dependence on the deformation mechanism in normal glasses with higher rates promoting densification in favor of shear. However, for damage resistant glass, the stress induced optical retardation is the same for indents made at both quasi-static and dynamic indentation rates.

Highlights

Vickers indentation is a valuable tool for comparing the sharp contact cracking resistance of glasses
The indentation deformation and cracking behavior of ionexchanged normal and damage-resistant glasses is highly dependent on the glass structure
An unexpected finding was that the formation of strength limiting flaws and simultaneous part failure occurs on the loading half cycle in dynamic Vickers indentation of ion-exchanged glass in a rigidly supported configuration

Summary

Introduction

Vickers indentation is a valuable tool for comparing the sharp contact cracking resistance of glasses. Failure occurs during drops onto irregular, hard surfaces that generate large median/radial cracks originating in the subsurface of a sharp contact impression that penetrate the depth of compressive layer (DOL) and enter the central tension (CT) region. Crack extension through the depth of layer typically occurs with minimal assistance from bend induced stresses since most devices utilize a design that keeps the Vickers Cracking: Quasi-Static vs Dynamic cover glass fairly rigid. The Vickers indentation cracking test is used to measure the resistance to the formation of large median/ radial cracks under controlled/repeatable testing conditions in a rigid configuration and closely replicates the failure mechanism seen in cover glass in the field (Price et al, 2009)

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Conclusion