Fracture Properties of Yttria-Stabilized Zirconia (YSZ) Thin-Film Modified Constructs

Dental restoration technologies allow for patients to return function, integrity, and aesthetics to damaged or missing teeth. Recent studies on the link between oral and general health, has led to a shift in focus towards oral-health-related quality of life. Within the United States alone, an ageing population has driven demand for health systems and technology. Similarly, increasing global life expectancy and economic development has led to a strong focus on personal heath/wellness. With recent studies indicating links between oral and general health, much attention has been spent on improvements to dental materials and technologies. All-ceramic materials, such as porcelain and partially-stabilized zirconia (PSZ), have made strong headways into recent use as dental restorations, where aesthetic properties are highly coveted. Particularly, recent attention has been given to dental ceramics due to the relative ease in which their appearance can be matched to natural teeth. However when compared to their metal and metal-ceramic hybrid counterparts, ceramic restorations are disadvantaged by their reduced capacity to resist crack propagation. As a result, strengthening properties are of particular interest in all-ceramic materials where common preparation techniques involve surface roughening by abrasion or etching to increase the bonding surface area of a construct. However, these roughening techniques may also impart surface flaws, resulting in premature failure of a restoration. Under tension, surface flaws may act as potential stress-concentration and crack-initiation sites, allowing cracks to propagate more easily, thereby limiting the strength of ceramics. The corrosion and wear resistance properties of ceramics, such as partially-stabilized zirconia (PSZ), along with the unique microstructures achievable by thin-film deposition, have made ceramic thin-films particularly appealing for use as protective coatings. Furthermore, it has been shown that the application of thin-films to a dental ceramic can result in increased flexural strength. Recent investigations have shown an improvement in fracture strength in dental ceramics through sputter deposition of yttria-stabilized zirconia (YSZ) thin-films. It is hypothesized that films with differing microstructure and compressive film stress will further enhance a construct's fracture strength. Here, feldspathic porcelain substrates were cut from commercially available porcelain (ProCAD, Ivoclar-Vivadent) blocks and wet-polished through 1200-grit using SiC abrasive. Utilizing radio frequency (r.f.) magnetron sputtering, yttria-stabilized zirconia films (2-3im) were deposited with varying deposition parameters. Film and flaw properties were characterized by optical microscopy, scanning electron microscopy (SEM), x-ray diffraction (XRD), and wafer bow analysis. Flexural strength was determined by ring-on-ring biaxial flexure, and three-point bending. Fracture toughness values were calculated from flaw size and fracture strength. Fractography and Weibull analysis was then conducted to supplement fracture behavior observations and to develop a mechanistic understanding of observed trends. By depositing PSZ films of varying microstructures and film stresses, this study aims to show a further increase in mechanical properties in PSZ-altered constructs. Data show improvements in fracture strength of up to 55% over unmodified specimens. XRD analysis shows that films deposited with higher substrate bias displayed a high %monoclinic volume fraction (19%) compared to non-biased deposited films (87%), and resulted in increased film stresses and modified YSZ microstructures. SEM analysis shows critical flaw sizes of 63+1im leading to fracture toughness improvements of 55% over unmodified samples. Data supports surface modification of dental ceramics with YSZ thin film coatings to improve fracture strength. Increase in construct strength was attributed to increase in compressive film stresses and modified YSZ microstructures. It is believed that this surface modification may lead to significant improvements and overall reliability of all-ceramic dental restorations. This work also aims to propose a simplistic model of strengthening mechanisms to assist in predicting fracture behavior improvements.