Jeffrey R. Piascik

Adhesion/cementation to zirconia and other non-silicate ceramics: Where are we now?

Non-silicate ceramics, especially zirconia, have become a topic of great interest in the field of prosthetic and implant dentistry. A clinical problem with use of zirconia-based components is the difficulty in achieving suitable adhesion with intended synthetic substrates or natural tissues. Traditional adhesive techniques used with silica-based ceramics do not work effectively with zirconia. Currently, several technologies are being utilized clinically to address this problem, and other approaches are under investigation. Most focus on surface modification of the inert surfaces of high strength ceramics. The ability to chemically functionalize the surface of zirconia appears to be critical in achieving adhesive bonding. This review will focus on currently available approaches as well as new advanced technologies to address this problem.

Development of a novel surface modification for improved bonding to zirconia.

OBJECTIVE This report presents a novel pretreatment technique, whereby the zirconia surface is converted to a more reactive zirconium oxyfluoride, enabling improved chemical bonding to other dental substrates via conventional silanation approaches. METHODS The study leverages a novel gas-phase fluorination process that creates a thin oxyfluoride conversion layer on the surface of zirconia, making it more reactive for conventional adhesive bonding techniques. Zirconia specimens, polished and roughened, were pretreated and composite cylinders bonded using conventional adhesive techniques. All specimens were subjected to a force at a crosshead speed of 0.5mm/min in an electro-mechanical testing device. Single-factor analysis of variance (ANOVA) at a 5% confidence level was performed for the bonding strength data. Optical microscopy and scanning electron microscopy (SEM) were used to evaluate and quantify failure surfaces. RESULTS Shear bond strengths were analyzed using single-factor ANOVA (p<0.05). Mechanical testing results revealed that fluorinated zirconia specimens (both rough and polished) displayed the highest shear bond strengths as compared to other commercially available treatments. X-ray photoelectron spectroscopy analysis helped determine that this novel pretreatment created a more reactive, 2-4nm thick oxyfluoride conversion layer with approximate stoichiometry, ZrO(3)F(4). CONCLUSION Simple shear bond mechanical tests demonstrated that a fluorination pre-treatment is a viable method to chemically modify zirconia to produce a reactive surface for adhesive bonding.

On-chip electron-impact ion source using carbon nanotube field emitters

A lateral on-chip electron-impact ion source utilizing a carbon nanotube field emission electron source was fabricated and characterized. The device consists of a cathode with aligned carbon nanotubes, a control grid, and an ion collector electrode. The electron-impact ionization of He, Ar, and Xe was studied as a function of field emission current and pressure. The ion current was linear with respect to gas pressure from 10−4to10−1Torr. The device can operate as a vacuum ion gauge with a sensitivity of approximately 1Torr−1. Ion currents in excess of 1μA were generated.

Long-term microtensile bond strength of surface modified zirconia:

OBJECTIVE To compare long-term microtensile bond strength of zirconia, surface-modified via a novel treatment, to current surface conditioning methods for zirconia, when resin bonded to dental composite. METHODS Two ProCAD (porcelain) and 10 sintered ZirCAD (ZrO(2)) blocks (18 mm × 14 mm × 12 mm) were obtained from manufacturers. Twelve Herculite XRV composite blocks were fabricated (18 mm × 14 mm × 12 mm). Bonding surface of blocks was polished through 1200-grit SiC and air-abraded (50 μm alumina, 0.28MPa, 20s). Blocks were then separated into six groups: (1) porcelain (control), HF-etched/silane-treated, (2) ZrO(2), tribochemical-coated/silane-treated, (3) ZrO(2), primer-treated, (4) ZrO(2), modified via novel 3.2 nm silica layer/silane-treated, (5) ZrO(2), modified via novel 5.8nm silica layer/silane-treated, and (6) ZrO(2), modified via novel 30.4 nm silica layer/silane-treated. Blocks were bonded to composite using Clearfil Esthetic cement. Blocks were stored in distilled water (37°C, 24h), then cut into microtensile bars (n=8/gp), then bond strengths were measured using a universal testing machine at 0, 1, 3, and 6 months. All groups were statistically analyzed (ANOVA, Tukeys, p<0.05). RESULTS At 6 months (aging), all silica seed layer specimens displayed microtensile bond strength similar to CoJet specimens but less than that of silane-modified dental porcelain. CONCLUSION The deposition of a silica layer on zirconia resulted in similar or superior long-term resin bond strength when compared to traditional silanation and bonding techniques for zirconia but lower than that for silane-treated dental porcelain.

Enhanced bonding between YSZ surfaces using a gas-phase fluorination pretreatment.

The present investigation focuses on the surface modification, via gas-phase fluorination process, of yttria- stabilized zirconia (YSZ) to increase its wettability and chemical bonding directly to acrylate-based resin cements. YSZ plates and cylinders, as-received and roughened, were pretreated in a fluorine containing plasma and bonded with a commercially available resin cement for simple shear bond adhesion testing. No organo-silane coupling agent was used to enhance bonding between the two substrates. Shear bond tests revealed that bond strength increased with fluorination time. Furthermore, the pretreated, as received (nonroughened) specimen group displayed relatively high bond strengths suggesting surface reactivity and direct chemical bonding with the resin cement. X-ray photoelectron spectroscopy analysis revealed the surface conversion layer to be a mixture of phases; zirconium oxyfluoride, zirconium fluoride, and yttrium fluoride. It is hypothesized that these fluoride and oxyfluoride phases have the potential to increase surface hydroxylation, enabling direct covalent bonding between YSZ and resin cement. It is believed that this surface treatment has broad reaching impact when using high-strength ceramics in a multitude of bioapplications.

Evaluation of crystallinity and film stress in yttria-stabilized zirconia thin films

Yttria (3 mol %)-stabilized zirconia (YSZ) thin films were deposited using radio frequency (rf) magnetron sputtering. The YSZ thin films were deposited over a range of temperatures (22–300 °C), pressures (5–25 mTorr), and gas compositions (Ar∕O ratio). Initial studies characterized a select set of properties in relation to deposition parameters including: refractive index, structure, and film stress. X-ray diffraction (XRD) showed that the films are comprised of mainly monoclinic and tetragonal crystal phases. The film refractive index determined by prism coupling, depends strongly on deposition conditions and ranged from 1.959 to 2.223. Wafer bow measurements indicate that the sputtered YSZ films can have initial stress ranging from 86 MPa tensile to 192 MPa compressive, depending on the deposition parameters. Exposure to ambient conditions (25  °C, 75% relative humidity) led to large increase (∼100MPa) in the compressive stress of the films. Environmental aging suggests the change in compressive stress ...

Stress evolution as a function of substrate bias in rf magnetron sputtered yttria-stabilized zirconia films

An increase in compressive stress was observed in rf magnetron sputtered yttria-stabilized zirconia thin films upon exposure to ambient conditions (25°C and 75% relative humidity). This increase was attributed to absorption of water molecules into intergranular pores. It was shown that increasing substrate bias power disrupted columnar grain growth and reduced the percent change in compressive stress when exposed to ambient environments. Transmission electron microscopy confirmed a reduction in intergranular porosity for substrate bias depositions but an increase in lateral defects. These defects are hypothesized to be stress-induced microcracks caused by a tetragonal to monoclinic phase transformation.

High voltage microelectromechanical systems platform for fully integrated, on-chip, vacuum electronic devices

We demonstrate a fully integrated, on-chip, vacuum microtriode capable of handling voltages up to 800V. The ability to operate at such high voltages is achieved by the addition of a 10μm thick silicon dioxide layer to the device. The device is fabricated using microelectromechanical systems fabrication principles and utilizes carbon nanotubes as field emitters. A dc amplification factor of 600 was obtained. This is the highest value reported for carbon nanotube-enabled microtriode devices. The high voltage capability of these microscale devices will enable their use in a wider variety of applications.

Fracture toughness improvements of dental ceramic through use of yttria-stabilized zirconia (YSZ) thin-film coatings

OBJECTIVES The aim of this study was to evaluate strengthening mechanisms of yttria-stabilized zirconia (YSZ) thin film coatings as a viable method for improving fracture toughness of all-ceramic dental restorations. METHODS Bars (2mm×2mm×15mm, n=12) were cut from porcelain (ProCAD, Ivoclar-Vivadent) blocks and wet-polished through 1200-grit using SiC abrasive. A Vickers indenter was used to induce flaws with controlled size and geometry. Depositions were performed via radio frequency magnetron sputtering (5mT, 25°C, 30:1 Ar/O2 gas ratio) with varying powers of substrate bias. Film and flaw properties were characterized by optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD). Flexural strength was determined by three-point bending. Fracture toughness values were calculated from flaw size and fracture strength. RESULTS Data show improvements in fracture strength of up to 57% 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 67±1μm leading to fracture toughness improvements of 55% over unmodified specimens. SIGNIFICANCE Data support surface modification of dental ceramics with YSZ thin film coatings to improve fracture toughness. Increase in construct strength was attributed to increase in compressive film stresses and modified YSZ thin film microstructures. It is believed that this surface modification may lead to significant improvements and overall reliability of all-ceramic dental restorations.

Zirconia–parylene multilayer thin films for enhanced fracture resistance of dental ceramics

Abstract Recent research has shown that the application of specific thin films can enhance the material properties of a laminate construct. In this study, the effect of different mono/multilayered films on the strength of a ceramic specimen is demonstrated. It is well established that cracks can initiate and/or propagate from the internal surfaces of all-ceramic dental restorations. Modifying that surface by thin-film deposition might help increase clinical longevity and applicability. Specimens were divided into the following groups according to different surface treatments received: uncoated (control group), 10 μm yttria-stabilized zirconia (YSZ) thin film, 10 μm parylene thin film, 9.75 μm YSZ + 0.25 μm parylene film, and a multilayered film (five layers of 1.25 μm YSZ + 0.75 μm parylene). Depositions were performed using a radio-frequency magnetron sputter system (working pressure 15 mT, 150 °C, 30:1 Ar/O2 gas ratio) to produce the YSZ layers, and a vapour deposition process was used to produce the parylene layers. Flexural strength measurements were carried out by three-point bending (span = 10 mm) in a servo-electric material testing system in deioinized (DI) water (37 °C). The results showed that the strength of the specimen significantly increased with the deposition of all types of coating, showing the greatest increase with the multilayered film (∼32 per cent). It is hypothesized that a multilayer thin film (brittle/ductile) can promote crack deflection, causing strength enhancement of the brittle construct.