MEDICINSKI GLASNIK

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236 Medicinski Glasnik, Volumen 6, Number 2, August 2009 INTRODUCTION Ceramics are being used increasingly as a restorative material in a variety of dental restorations, including metal-ceramic crowns, all-ceramic restorations, and fixed partial dentures, mainly as a result of their excellent aesthetic properties, durability, biocompatibility and resistance to wear (1). Ceramic for dental reconstructive work are multiphase silicate ceramics, glass ceramics or monophased glasses with varying compositions (2,3). Structure composed of ceramic layers on a metal frame combined the strength of a metal substrate (dental alloy) with aesthetic of a ceramic. Currently, these ceramic fused to metal appliances are widespread in use in prosthodontics. Because of their inherently brittle nature susceptibility to their failure was identified at localized areas of high stress concentration on the ceramic surface, metalceramic interface or within the microstructure (4). In any laminate composite system the strength of the interfacial bond between the individual laminates is a major factor in determining the overall resistance of the system to deformation and failure (5,6). A strong interface should provide sufficient stress transfer between the individual laminates to allow the applied loads to be transferred and accommodated. Conversely, a weak interface will frequently result in failure by a process of delaminating under an applied load possibly arising from crack initiation and propagation within and along the layer (7). These bilayered composites have attracted considerable attention from laboratory researchers seeking to understand the failure characteristics of ceramic fused to metal systems. Alterations to the interfacial region between bilayered structures are of considerable interest and authors have reported the effects of variations in the interfacial surface roughness on the mechanical properties of metal-ceramics specimens (8). By improving a final surface treatment of metal substructure could be significantly improved functional durability of metal-ceramic appliances. Oxidation heat treatment of the metal is used to remove the entrapped gas, eliminate surface contaminants, and form the metal oxide layer. An alloy is deliberately given an oxidation treatment prior to ceramic application, or whether it oxidizes during the portion of the firing cycle before flow of the ceramics begins, the fusing ceramics comes into immediate contact with oxide rather than with metal surface (9,10). Different opinions exist as to how this oxide interacts with ceramic during the firing cycle. It is widely believed, that the fusing ceramic dissolves away the oxide originally formed and produces an interaction zone responsible for the formation of a bond (11). King rejected the oxide layer theories extend at that time (Kauzt 1936.) which postulated that a layer of oxide adherent to the metal is wetted by the ceramics and becomes the transition zone between the metal and glassy matrices (12). Pask (13) otherwise suggest a direct chemical bonding between the ceramic and metal. According to Mackert (11) the chromium-containing alloys all contain oxygen-active elements: beryl, aluminium, vanadium, titanium, and/or yttrium. Boron oxide makes these alloys self-fluxing during melting and gives them unique melting and casting behaviour. The addition of aluminium to these alloys adversely affects this behaviour because of its tendency to produce slag (14). Because of the close correspondence between oxide adherence and ceramic bonding, it can only be concluded that the adherence of the oxide plays a dominant role in ceramic bonding (11). The aim of this study was to investigate the influence of different cast surface finishing process on metal-ceramics bond strength. Table 1. Procedures of metal surface treatment for each group of samples Specimen Metals surface treatment 1 Sand blasting with 110 μm Al2O3 particles 2 3 4 Sand blasting with 110 μm Al2O3 particles Oxidation Sand blasting with 110 μm Al2O3 particles Sand blasting with 250 μm Al2O3 particles Oxidation Sand blasting with 250 μm Al2O3 particles Sand blasting with 110 μm Al2O3 particles Extended oxidation Sand blasting with 110 μm Al2O3 particles Sand blasting with 110 μm Al2O3 particles Oxidation 5 Sand blasting with 110 μm Al2O3 particles Bonding agent 6 Etching in acid mixture
MATERIALS AND METHODS Six groups of same three metal cast plates, 25×3×0.5 mm have been produced according to the manufacturer’s instructions. The used alloy (Wirobond C, BEGO, Bremen, Germany) Mehulić et al Surface finishing and bond strength (weight percentage: Cr 26%, Mo 6 %, W 5 %, Si 1 %, Fe 0.5 %, Ce 0.5 %, C 0.02 %, and the rest Co) belongs to the group of cobalt-chrome alloys free of the contents of beryllium and nickel. Thus produced samples are cleaned and handled in same direction, and the surfaces to which ceram- Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Sample 6 Figure 1. Specimens’ surface of the sample prepared and recorded by a scanning electronic microscope (SEM) with the secondary electron detector (SE) (Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Croatia, 2008., with permission) 237
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MEDICINSKI GLASNIK

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