Timothy Michael Gross

69.3: A Mechanics Framework for Ion‐Exchanged Cover Glass with a Deep Compression Layer

A cover glass that is capable of having a deep ion exchange compression layer enables one to tailor the glass to the intended device application. Key to this effort is having a glass mechanics framework that includes resistance to visible and strength-limiting contact damage as well as maintaining sufficient strength to survive localized glass flexing during contact events.

Competing Indentation Deformation Mechanisms in Glass Using Different Strengthening Methods

Chemical strengthening via ion exchange, thermal tempering, and lamination are proven techniques for strengthening of oxide glasses. For each of these techniques, the strengthening mechanism is conventionally ascribed to the linear superposition of the compressive stress profile on the glass surface. However, in this work we use molecular dynamics simulations to reveal the underlying indentation deformation mechanism beyond the simple linear superposition of compressive and indentation stresses. In particular, the plastic zone can be dramatically different from the commonly assumed hemispherical shape, which leads to a completely different stress field and resulting crack system. We show that the indentation-induced fracture is controlled by two competing mechanisms: the compressive stress itself and a potential reduction in free volume that can increase the driving force for crack formation. Chemical strengthening via ion exchange tends to escalate the competition between these two effects, while thermal tempering tends to reduce it. Lamination of glasses with differential thermal expansion falls in between. The crack system also depends on the indenter geometry and the loading stage, i.e., loading vs. after unloading. It is observed that combining thermal tempering or high free volume content with ion exchange or lamination can impart a relatively high compressive stress and reduce the driving force for crack formation. Therefore, such a combined approach might offer the best overall crack resistance for oxide glasses.

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

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.

Corning Incorporated: Designing a New Future with Glass and Optics

Corning Incorporated is a world leader in glass and ceramic products, and has been innovating in these materials since 1851. The company sells component-level technical products that are integrated into systems made by its customers. In most cases, those systems are significantly more efficient or in some instances fundamentally enabled by the performance of the Corning product. Corning calls its products “keystone components” for this reason. Keystone components often result from a combination of both material and process innovations, which tend to be difficult for other companies to duplicate. Developing keystone components requires patient investment in R&D (both materials and process) over long periods of time, and depends upon a culture of innovation and dedication to fundamental understanding. We highlight in this chapter three different keystone components developed by Corning in the past two decades—Corning® Gorilla® Glass for touch-enabled displays, Epic® sensors for drug discovery, and ClearCurve® optical fiber. In each case we provide an overview of Corning’s contributions to each field, describe the areas of technical challenge that still need to be addressed by the research community, and link those to the skills and capabilities that are needed to ensure further success in each.