1

Introduction

Base metal alloys, in particular cobalt-base alloys, are used in dentistry in a wide range of indications. The advantages of these alloys are their high stiffness, low specific density, low thermal conductivity, as well as their superior biocompatibility and corrosion resistance in the oral cavity . Due to their good biocompatibility, cobalt-base alloys are also used in medical fields other than dentistry, for example as material for stents or endoprostheses . Aside from their mechanical and biological characteristics, financial considerations also play a decisive role in the application of these base metal alloys in dentistry, as the prices of traditional nobel metal alloys have increased considerably in recent years .

For the manufacture of crowns and FDPs, cobalt-base alloys are commonly used as core material. The color of the restorations can be improved by subsequent veneering of these cores with glass ceramics. Various investigations have shown that the bond strength between core material and veneer ceramic is in the same range for cobalt-base alloys as for traditionally used nobel metal alloys . Adhesion between alloy and veneer ceramic is determined by several factors, including contraction forces due to differences in the thermal expansion coefficients of the components as well as macro- and micro-mechanical retention or chemical bonds . The strongest bonds are those in the oxygen bridges between oxides in the alloys and silicon atoms in the ceramics . These bonds are due to adherence oxides which develop on the alloys’ surface within the initial firing cycle of the veneering process . Various elements then diffuse across the contact zone between alloy and ceramic . With cobalt-base alloys, predominantly chromium oxides migrate into the ceramic layer and form chemical bonds . In opposite to nobel metal alloys, which must contain a fraction of easily oxidized elements to form adherence oxides, this procedure is not needed for base metal alloys, as they already contain high levels of elements which can form a superficial adherent oxide layer. However, the uncontrolled formation of an excessively thick oxide layer should also be avoided with base metal alloys, as this results in poor adherence with possible chipping and delamination of the veneering ceramic ( Fig. 1 ) . In addition to bonding mechanisms, the wettability of the alloy surface by the veneering ceramic has a crucial influence on the bond strength between these components . Only if the alloy surface is adequately wetted, it can be ensured that the bonding mechanisms described can take effect.

Delamination and stress cracks occurring in the ceramic layer during the veneering process of a cobalt-base alloy framework.

The use of an oxide dissolving primer is a recently introduced strategy to improve the wettability of base metal alloy surfaces, as well as to avoid an excessively thick oxide layer within the veneering process. However, to the authors’ best knowledge up to now there are no publications available reporting on the effect of this kind of primer. Therefore, the aim of the present study was to investigate the influence of such an oxide dissolving primer on the adhesion between two different cobalt-base alloys and a veneer ceramic. The effects of primer application were evaluated – both quantitatively in a fracture-mechanics approach and qualitatively by means of scanning electron microscopy. It was hypothesized that an oxide dissolving primer would improve the wettability of the alloy surfaces and adhesion between alloy and ceramic layers.

2

Materials and methods

This study investigated the influence of an oxide dissolving primer (HeraCeram NP-Primer, Heraeus Kulzer GmbH, Hanau, Germany) on the adhesion of a veneer ceramic (HeraCeram, Heraeus Kulzer GmbH, Hanau, Germany) to two different cobalt-base alloys ([BL] Bärlight, Selection GmbH, Hollern, Germany; [CP] Cara Process, Heraeus Kulzer GmbH, Hanau, Germany). According to the manufacturer’s data the composition of the NP-Primer is as follows (weight %): 79.0% SiO ₂ , 3.0% Al ₂ O ₃ , 1.5% K ₂ O, 4.5% Na ₂ O, 1.5% Li ₂ O, 9.5% B ₂ O ₃ , <1.0% CaO. Detailed information about the alloys used is listed in Table 1 . Bilayer alloy/veneer specimens were manufactured for a four-point bending test. The strain energy release rate during stable crack growth at the bonding interface was estimated as a parameter for the bond strength of the bilayer composites .

40 rectangular plates (25 mm length × 8 mm width × 1.25 mm thickness) were cast of the BL alloy, while 40 specimens with identical dimensions were processed of the CP alloy using a CAD/CAM (Computer Aided Design/Computer Aided Manufacturing) technique. All specimens were ground with silicon carbide paper to achieve flat and smooth surfaces. One of the 25 mm × 8 mm sides was sandblasted using 110 μm alumina at a pressure of 3 bar, followed by steam cleaning of the surfaces. Prior to veneering on half of the specimens of each alloy ( n = 20), the NP-Primer was applied onto the sandblasted surface and fired according to the manufacturers’ recommendations ( Table 2 ). The corresponding paste opaquer of the veneering system was then applied to all alloy plates. Afterwards, specimens were placed into a separable steel mold, in which 1.75 mm clearance was available for the veneer layer (our own construction, Hannover Medical School, Hannover, Germany). The dentin veneering ceramic powder was mixed with its corresponding mixing liquid, filled into the mold and condensed. Excess mixing liquid was removed by applying tissue paper onto the surface of the veneer. After the mold had been removed, the specimens were transferred to a firing tray, placed in the ceramic furnace (Multimat MC II, De Trey Dentsply, Konstanz, Germany) and sintered with the first dentin firing cycle ( Table 2 ). Due to sintering shrinkage, the alloy plates were not entirely covered with the veneer layer after the first sintering process. Therefore, the specimens were returned to the mold, a second layer of the veneering material was applied and the sintering was repeated in the second dentin firing cycle ( Table 2 ). All veneering steps were performed by one experienced dental technician for all test groups.

Excess veneering material on the edges of the specimens was ground off and the veneering surface of the specimens was ground flat in a rotary polishing machine (Variable Speed Grinder/Polisher Power Pro 4000, Buehler LTD, Lake Bluff, USA) with a 45 μm diamond abrasive wheel, until a total specimen thickness of 2.5 mm (1.25 mm alloy + 1.25 mm veneer layer) was attained.

The specimens were notched using the single-edge V-notched-beam (SEVNB) method, as described in ISO 23146 . A 0.2 mm deep pre-notch was created across the width in the middle of the veneer ceramic side, using a 0.2 mm thick rotary diamond cutting blade (IsoMet 4000, Buehler LTD, Lake Bluff, USA) and lubricant (Isocut Plus cutting fluid, Buehler LTD, Lake Bluff, USA). In a second step, the notch was manually expanded with a razor blade and 6 μm diamond paste until the root of the notch was V-shaped, with a total depth of 0.5 mm. Then all specimens were inspected by light microscopy (M3Z, Wild, Heerbrugg, Switzerland) to check that the notches had defined geometry and to ensure that no irregular flaws occurred.

The testing procedure was performed in a four-point bending jig, according to DIN EN 843 , with the specimens mounted in a universal testing machine (Z2010, Zwick, Ulm, Germany) ( Fig. 2 ). The distance between the inner loading rollers was 10 mm and between the outer support rollers 20 mm. The specimens were placed in the jig, with the veneer ceramic side on the outer support rollers, and continuously loaded on the side of the alloy substructures at a crosshead speed of 0.01 mm/min. Data for crosshead displacement and load were collected and crack growth in the specimens was visually inspected. The testing procedure was stopped after the crack had reached the inner loading rollers and no further crack growth could be observed.

Schematic illustration of the four-point bending configuration.

During stable crack growth along the bonding interface, the load/crosshead displacement curves exhibited a plateau region, in which the load was almost constant ( Fig. 3 ). The strain energy release rate ( G , J/m ² ) was calculated from the average loading data, recorded at the plateau region of the curves, employing the following equation :

where F is the mean load to induce stable crack growth, l is the distance between the inner and outer rollers, b and h are width and the total thickness of the specimen and ν _A and E _A are Poisson’s ratio and elastic modulus of the alloy substructures, respectively. The parameter η includes all geometrical parameters of the specimens and is calculated from:

with:

where h _A and h _V are the thickness of the alloy substructure and the veneer and ν _V and E _V are Poisson’s ratio and the elastic modulus of the veneer, respectively. The values for ν and E used in this calculation to characterize the tested alloys are listed in Table 1 . For the veneer ceramic an elastic modulus (E) of 85 GPa (given by the manufacturer) and a Poisson’s ratio of 0.20 were assumed.

Typical load/crosshead displacement curve of the four-point bending test configuration.

Statistical analysis was performed using SPSS for Windows, version 19.0 (IBM, Ehringen, Germany). The normal distribution of data and homogeneity of variance were checked using the Kolmogorov–Smirnov and Levene tests, respectively. Comparison of the strain energy release rate ( G -value) between several alloy/veneer combinations in consideration of a primer application was performed using two-factor one-way analysis of variance (ANOVA). Differences between groups were checked for significance with Bonferroni multiple comparison. The level of significance was set at 0.05.

After bond strength testing, the path of the cracks at the adhesion interface and the fracture surface of the tested specimens were finally analysed by optical and scanning electron microscopy (SEM, Leo 1455VP, Carl Zeiss, Jena, Germany).

2

Materials and methods

This study investigated the influence of an oxide dissolving primer (HeraCeram NP-Primer, Heraeus Kulzer GmbH, Hanau, Germany) on the adhesion of a veneer ceramic (HeraCeram, Heraeus Kulzer GmbH, Hanau, Germany) to two different cobalt-base alloys ([BL] Bärlight, Selection GmbH, Hollern, Germany; [CP] Cara Process, Heraeus Kulzer GmbH, Hanau, Germany). According to the manufacturer’s data the composition of the NP-Primer is as follows (weight %): 79.0% SiO ₂ , 3.0% Al ₂ O ₃ , 1.5% K ₂ O, 4.5% Na ₂ O, 1.5% Li ₂ O, 9.5% B ₂ O ₃ , <1.0% CaO. Detailed information about the alloys used is listed in Table 1 . Bilayer alloy/veneer specimens were manufactured for a four-point bending test. The strain energy release rate during stable crack growth at the bonding interface was estimated as a parameter for the bond strength of the bilayer composites .

Table 1

Material characteristics of tested alloys. Data for alloying elements and elastic modulus are given by the manufacturers. Data for Poisson’s ratios of cobalt-base alloys is given in .

	Material characteristics of alloys
	Alloying elements (%)	Elastic modulus (GPa)	Poisson’s ratio
BL (Bärlight)	Co/54.5, Cr/25, Ga/6, Mo/4.5, In/3.5, Pt/2.0, Au/1.5, others/<1	185	0.33
CP (Cara Process)	Co/61, Cr/28, W/8.5, Si/1.6, others/<0.5	190	0.33

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