The aim of this study was to functionalize the surface of yttria partially stabilized tetragonal zirconia ceramics (Y-TZP) with a nano-structured alumina coating to improve resin bonding.
Materials and methods
A total of 120 densely sintered disc-shaped specimens (15.5 ± 0.03 mm in diameter and 2.6 ± 0.03 mm thick) were produced from biomedical-grade TZ-3YB-E zirconia powder (Tosoh, Tokyo, Japan), randomly divided into three groups of 40 and subjected to the following surface treatments: AS – as-sintered; APA – airborne-particle abraded; POL – polished. Half of the discs in each group received an alumina coating that was fabricated by exploiting the hydrolysis of aluminium nitride (AlN) powder (groups AS-C, APA-C, POL-C). The coating was characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The shear-bond strength of the self-etching composite resin (RelyX Unicem, 3M ESPE, USA) was then studied for the coated and uncoated surfaces of the as-sintered, polished and airborne-particle abraded specimens before and after thermocycling (TC).
The SEM/TEM analyses revealed that the application of an alumina coating to Y-TZP ceramics created a highly retentive surface for resin penetration. The coating showed good surface coverage and a uniform thickness of 240 nm. The resin-bond strength to the groups AS-C, APA-C, POL-C was significantly higher than to the groups AS, APA and POL, both before and after TC ( p ≤ 0.05). During TC all the specimens in the POL and AS groups debonded spontaneously. In contrast, the TC did not affect the bond strength of the AS-C, POL-C and APA-C groups.
A non-invasive method has been developed that significantly improves resin-bond strength to Y-TZP ceramics. After surface functionalization the bond survives thermocycling without reduction in strength. The method is relatively simple and has the potential to become an effective conditioning method for zirconia ceramics.
Zirconia frameworks are produced mainly by CAD-CAM machining and sintering of the yttria partially stabilized tetragonal zirconia ceramics (Y-TZP), while prefabricated elements, including posts and cores, orthodontic brackets and implant abutments, are manufactured by the conventional ceramic-forming techniques, such as dry pressing and low-pressure injection moulding, followed by sintering . However, regardless of the manufacturing process, the intaglio surface needs pretreatment, since ceramic bonding relies on mechanical interlocking due to the micro-retentions created on the surface and the chemical bonding. Although superior in terms of mechanical, aesthetic and biological properties, bonding to this material still remains a challenge. In silica-based ceramics, a reliable bond can be achieved with hydrofluoric acid (HF) etching followed by silanization . In contrast, chemically stable, silica-free Y-TZP ceramics are acid etch resistant, and bonding protocols successfully used in silica-based ceramics cannot be implemented. Different mechanical and chemical conditioning methods have therefore been proposed to enhance the resin–ceramic bonding. Among these, airborne-particle abrasion has been commonly used to increase the surface roughness , thereby creating micro-retentions.
A more effective pretreatment is tribochemical silica-coating, in which the surface is first cleaned and roughened with alumina and then abraded with silica-modified alumina particles . Here, the blasting pressure is an important process parameter that determines the silica embedding rate . The silicated surface is finally silanized to enhance resin bonding. Silica-coating therefore combines micro-mechanical retention, produced by airborne-particle abrasion, with chemical bonding, and the process is well documented . However, Matinlinna et al. reported, that the silica content on the Y-TZP surface after silica-coating was too low for effective silanization . Further, air-particle abrasion methods in general have recently been questioned, since impact-induced cracks can be introduced in the surface of Y-TZP ceramics, reducing the strength of the material .
As there is no clinical consensus on the appropriate bonding procedures, considerable efforts are being made in order to develop alternative chemical and mechanical surface modifications of Y-TZP ceramics. New silane formulations , hydroxylation of zirconia surfaces prior to silanization and vapor-phase deposition have been reported to enhance the chemical bonding. Efforts have also been made to develop a mechanical pretreatment that would omit abrasion. Thus, for example, Derand et al. applied a low-fusing porcelain pearl layer to increase the roughness, while Phark et al. modified the Y-TZP surface with a slurry, containing zirconia powder, that was subsequently sintered. Aboushelib et al. proposed the so-called selective infiltration etching, which is based on inter-grain nano-porosities, created during thermal pre-stressing of the surface grains using a specific temperature regime. However, the clinical performance of these alternative surface-preparation techniques remains to be tested.
In this study, a new, non-invasive process to promote resin bonding is proposed. It relies on a functionalization of the Y-TZP’s surface, by applying a nano-structured alumina coating with a high surface area and good wetting ability. In this way a micro-mechanical interlocking can be achieved. Various surface coatings, including calcium phosphates, glass composites and hydroxyapatite, have been reported on Y-TZP surfaces . They were designed for biological reasons, mainly to promote cell ingrowths and osseointegration of implanted ceramics. These coatings were obtained by various physical, chemical and electrochemical methods and their thickness was between 30 and 300 μm. “In situ” formation of boehmite, using aluminium nitride (AlN) powder hydrolysis in an aqueous suspension to prepare a discontinuous nano-structured Al 2 O 3 coating, aimed at enhancing resin bonding, has however not previously been reported.
To fabricate the adhesive coating on ceramic substrates, we precipitated aluminium hydroxides, that originate from the hydrolysis of AlN powder in a diluted aqueous suspension. In previous work the preparation of the coating, based on exploiting the AlN powder hydrolysis, was described in detail . In brief, the AlN powder vigorously reacts with hot water; during the initial stages of hydrolysis, γ AlOOH (boehmite) and ammonia are formed following reaction (1) :
This reaction is exothermic and is accompanied by a rapid increase in the concentration of dissolved aluminium (poly)cations in the suspension, which favor the heterogeneous nucleation of lamellar boehmite on the substrate that is immersed in such a suspension. During a subsequent heat treatment, the precipitated boehmite will first thermally decompose forming a transitional alumina that later undergoes a series of polymorphic phase transformations following the sequence (2) :
γ AlOOH 300 ° − 500 ° γ A l 2 O 3 700 ° − 800 ° δ A l 2 O 3 900 ° − 1000 ° A l 2 O 3 1000 ° − 1100 ° α A l 2 O 3