Dent Mater 8:2-6, January, 1992

The effect of corrosive environment on the porcelainto-metal bond--A fracture mechanics investigation M. Herrmann ~, R. Rottenegger ~,J. Tinschert 2, R. Marx 2 IDepartment of ProstheticDentistry, Universityof Cologne, Cologne,Germany eDepartment of ProstheticDentistry, Technical Universityof Aachen, Aachen, Germany

Abstract. Microcracks, flaws, and voids inside a metal-porcelain restoration may cause the restoration to fracture in service. Such cracks result in the concentration of stresses. The dynamic nature of the stresses due to mastication promotes crack growth. In addition, corrosive components of the oral environment enhance the growth rate. In the present investigation, fracture mechanics has been used to analyze the in vitro resistance to fracture of the porcelain-fusedto-metal restoration. The risk of a clinical failure of porcelainfused-to-metal decreases with enlarged crack resistance (increasing work of fracture). The work of fracture represents an average of the energy for initiation and propagation of a crack through the interface separating porcelain and metal. This work also indicates a material's ability to stop a crack once it is moving. This study utilized the three-point bending test for the crack resistance measurement, and investigated one palladium and five base metal alloys. Corrosive components of the oral environment and the details of firing were of crucial importance for long-term bond stability. Alarge number of tests has been described to measure the bond strength between metal and porcelain (Anusavice et al., 1980; Lavine and Custer, 1966; Schwickerath, 1987; Vofi, 1969). These tests concentrate mostly on the shear strength of the interface. The interface, however, behaves in a brittle manner, like the porcelain body, and it is therefore more sensitive to tensile rather than to shear stresses. If the goal is to study the bond strength, methods provided by fracture mechanics should be utilized (Broek, 1974; Groll et al., 1983). Such methods reflect the clinical requirements, since they are a measure of the sensitivity of the porcelain-fused-to-metal restoration to tensile stresses. Fracture toughness is useful to assess the resistance to fracture of the stress-bearing parts of the restoration. Only recently was fracture mechanics also successfully applied to the durability of composites (Rasmussen, 1978; Morena et al., 1986; Mair and Vowles, 1989). Failures originate preferentially from cracks or flaws. Microcracking is an important part of the wear process. The periodically repeated stresses of mastication concentrate at the crack tip to open up the crack. Gradual growth of the crack occurs if the stress exceeds the strength of the bond. Unstable catastrophic growth follows, which may result in the destruction of the restoration. A crack may take some time for initiation, but, once formed, it represents a significant risk to the strength and durability of the restoration. Only the cracks and flaws which reach the threshold of severity cause fracture. For dental porcelain to be

strengthened, it is essential that a mechanism exist to prevent crack propagation under low tensile stresses. The rate at which a crack will increase in size depends on the environmental conditions which are present at its tip. The wet corrosion environment of the mouth can accelerate the rate of growth, since the crack corrosion at the crack front weakens the chemical bonds between ceramics and metal substrates (e.g., Si0 bonds). It is well-known that immersion, even into ordinary water, reduces the fracture toughness of porcelain (Garg and Frotman, 1980: Phillips, 1982). Obviously, in the mouth this effect will be strongly enhanced, since this environment is known to contain many corrosive components (Phillips, 1982). The stress intensity at the crack tip depends on two factors, the structure of the material and the geometry of the piece. In composites, the pinning of the crack front has been attributed to filler particles (Mair and Vowles, 1989). In porcelain, leucite crystals embedded into the glassy matrix may have similar properties. Smaller crystals are believed to be more effective than larger crystals (Broek, 1974). For a given width, the deeper the crack in relation to the specimen dimension, the greater the concentration of stress. Therefore, the crack velocity increases with the crack depth. If the structure is under tensile stress, the concentrated stress can easily exceed the strength of the porcelain body, and the depth of the crack increases. Finally, critical crack growth occurs rapidly when the stress intensity reaches its critical value, and an almost explosive fracture of the bond occurs. Therefore, for dental porcelain to be strengthened, it is essential for cracks and flaws to be avoided and a mechanism provided to pin the crack front and prevent crack propagation under low tensile stresses. Note that, under a compressive stress, the crack is not self-propagating, and the stress from outside is resisted more successfully. In metals, stresses can be relieved by plastic deformation, but, because porcelain is non-ductile, stress relief is not possible. Fracture mechanics classifies three modes of failure: Mode I (tensile mode fracture) crack opening is due to tensile stresses which are perpendicular to the crack propagation direction; mode II (shear mode fracture) crack opening by longitudinal shear stresses; and mode III crack opening by transverse shear stresses. The fracture experiment can be adapted to the goal of clinical investigations by selecting the most appropriate mode. This is generally the tensile mode, since, as has been mentioned, the interface between metal and porcelain is sensitive to primarily tensile stresses. Clinically, we may refer to a spallation, which starts at a cusp of the grinding surface or at the border of a crown, finally becoming separated due to the periodic load of mastication. Fortunately, mode I is convenient

2 Herrmann et aL/Porcelain-to-metal bond--A fracture mechanics investigation

for use in the experimental test and yields data from which the bond strength can be easily calculated. Note that any kind of applied stress may be reduced to combinations of two basic types, compression and tension. Compressive stress is not usually of interest in studies of porcelain fracture, because to a certain extent it pushes cracks and flaws together. Fracture toughness may be characterized by the stress intensity factor K~¢ or, alternatively, by the rate of energy release G. The stress intensity factor relates the stress which is applied along a test piece to the resultant stress which acts at the crack tip to open up the crack. We will not discuss K~c further, because it is relevant only for homogeneous specimens. Instead, we consider the energy release rate G, which is appropriate for heterogenous specimens (compounds of metal and ceramics). G (unit J/m 2) is also known as the specific work of fracture. G is defined as the elastic energy of deflection per unit area stored in a specimen under an applied load. At the moment of crack propagation, this energy will be released. In accordance with conservation of energy, G is also equal to the work to separate the compound specimen into two pieces. Therefore, it is a measure for the crack resistance of the interface or, equivalently, for the bond strength. G may also be interpreted as the force which is needed to open a crack front of unit length (note that J/m2=N/m).

MATERIALS AND METHODS The present investigation concentrates upon opaque porcelain (VMK 68, Vita, Bad S~ickingen, Germany), since this layer of the veneer is next to the metal and is therefore responsible for bonding the porcelain to the prepared metal substrate. We investigated four Ni-Cr alloys [Ivotect P (73Ni-18Cr-4Mo) Ivoclar, Lichtenstein, FL; Wirocron (XNi-23Cr-9Fe) Ni concentration (X) is not known--this alloy is no longer sold; Wirolloy (63Ni-23Cr-9Fe) Bego, Bremen, FRG; Wiron 88 (64Ni-24Cr10Mo) Bego] and a Co-Cr alloy [Wirobond (64Co-31Cr-3Mo) Bego, Bremen, FRG] and a palladium alloy [Pors-on4 (58Pd30Ag-6Sn-4In) Degussa, Hanau, FRG]. For a careful fracture mechanical investigation of precious alloys, refer to Groll et al. (1983). We simulated the oral environment by using corrosive baths of two acidities (pH = 4.2 and 5.2). The compositions of the corrosive baths are given in Table 1. We determined the bond TABLE1: COMPOSITIONOF CORROSIONBATHS Solution pH = 5.2: Sodium Chloride 460 mg/L Potassium Chloride 400 mg/L

load F porcelain







~-stramgaugest ///

load -F/2


V Fig. 1. Three-point bending test with clip gauges for the compliance measurement. Refer to text. Distance between the two lower load application points ~ = 24 mm, width d = 3 ram, height h = 6 mm, starter crack length ao= 3 mm (typically). Since compliance means bending/load (b/F) as a function of the reduced crack length a/h, a calibration is needed to determine b = b(w) (refer to Fig. 2).

strength as a function of immersion time (T = 37°C), with the acidity as a parameter. In addition, we investigated the influence of the firing procedure [normal fusion run (program F) and retarded fusion run (Program S)] as is described in Table 2 (porcelain furnace "Multimat MCII", DeTrey/Dentsply, Dreieich, Germany). We utilized the three-point bending test to measure the energy release rate (Fig. 1). For the non-precious alloys, the specimen consisted of two wings prepared from each type of metal. The opaque porcelain was fused between the inner faces of these wings so that they were aligned. For the palladium alloy, the wings are machined out of brass. The porcelain was fused between the opposite faces of two cast plates of the palladium alloy. These small plates were about 1 mm thick. This triple-layered sandwich was then glued by a cyano-acrylic adhesive between the two brass wings. We initiated a starter crack with the help of a 20 pro-thick platinum foil glued to the lower one-third of one of the metal facings (Fig. 1). During firing, the foil did not stick to either the porcelain or the metal. In an earlier stage of the present investigation, we experimented with chevron-notch test pieces. We abandoned these experiments in favor of the platinum foil as a spacer, since it was difficult to locate the notch precisely in the interface between metal and porcelain. Moreover, while the notch was being cut, there was the high risk of initiating

Calcium Chloride-2-hydrate795 mg/L


Sodium hydrogen-phosphate-l-hydrate 690 mg/L

Program S

Program F

Uric Acid 1000 mg/L

Preheating 600°C, 3 min

same as S

Drying 3 min

same as S

Lactic acid 10.1 g

Fusion 960°C, 3 min (2 min vac)

same as S

Sodium Chloride 5.84g

Annealing 830°C, 16 min

Aqua dest. ad 1000g

Cooling 19 min

Sodium lactate as a buffer solution

(Vacuum 50 hPa, heating rate 50°C/minute)

Solution pH = 4.2:

Dental Materials~January 1992 3

bending b/opening w


dC/d~ (l/N)

C (v/N)



6 ~





" ////


3 2 0





0 0.2




reduced crack length a/h



Fig. 2. Bending/opening (b/w) as a function of reduced crack length a/h. Note that, by a simple geometric argument, b/w = 6 holds for a = O. Open diamonds: measured points (the smooth line is meant as a guide to the eye).

additional cracks into the specimen, so that the defect area would be indefinite increased. Average starter crack lengths have been, typically, & = a/h = 0.5, where a is the distance from crack tip to base of the spe°cimen (shown in Fig. 1) and 'h' is the specimen thickness. This starter crack channels the stress to the interface between porcelain and metal. The use of a platinum foil instead of a chevron-notch as a tool to initiate a starter crack could result in a somewhat higher bond strength, since the bottom of the crack initiated by a spacer should be considerably smoother than one which is due to a notch. Note that the surface tension of the molten porcelain during fusion equalizes all unevenness. The energy release rate is calculated by Eq. 1: G = Fc~ . d C 2d d~


provided the specimen behaves elastically under three-point load. Elastic behavior has been checked on the occasion of the calibration experiment to determine the compliance, as will be described below. F is the peak load at fracture. C(~) is the compliance (bending/load, b/F; unit pro/N) of the specimen as a function of the reduced crack length ~ = a/h. dC(~)/d~ is the corresponding differential quotient at the actual crack length ~o. According to Eq. 1, the bond strength is related to the square of the peak load at fracture as well as to the elastic energy








reduced crack length a/h

Fig. 3. Left-hand scale: Compliance C as a function of reduced crack length a/h. The open diamonds are measured points; the smooth line corresponds to the function kl*exp(K2(a/h)"), with kl = 0.082, k~= 4.358, and k3= 0.55. Right-hand scale: dC/d,~as a function of reduced crack length a/h [the dashed line corresponds to the function klk2k3(a/h)" al*exp(K2a/h)k3)).

stored in the specimen at the moment of fracture. The latter is closely related to the specimen's overall elastic properties and is characterized by its compliance. The compliance represents a correction term which makes allowance for the stress intensity due to the geometry and the elasticity of the specimen. The energy release rate and the bond strength are known when the peak load at fracture and the compliance are known. The peak load must be measured in a fracture experiment for each individual specimen; the compliance, however, is common to all specimens of equal shape and equal elastic properties. The calibration of the specimen's compliance C(~) was performed as follows: First, the crack opening, w, was measured as a function of load for several crack lengths "a" (Fig. 1). Second, we measured the relation between bending b and crack opening w (Fig. 2). Fig. 3 (left-hand scale) shows the compliance C = b/F as a function of reduced crack length a/h and Fig. 3 (right-hand scale) dC/d- (derivative of the compliance) as a function of a/h. The measurement of w is provided by a clip gauge. It consists of two springs with four strain gauges attached to the springs. During calibration, the clip gauge was mounted onto the bottom side of the specimen (refer to Fig. 1).














Ivotect P


I Od


m .......


Wiroo'o~ WlrOl~ I~rsic~ Time l Od

~7~7/J 3 0 d









Irxxnersion Time ~


Fig. 4. Bond strength in N/m of porcelain to five NEM alloys (program F). Initial values and values after immersion into corrosion bath (for composition, refer to Table 1). (a) pH = 4/2; (b) pH = 5.2.

4 Herrmann et al./Porcelain-to-metal bond~A fracture mechanics investigation



30 15



20 10 15

10 5 Pors-on4DH 4.2


] Od

...... u

Pors-on4,pH 5.2 Immersion Time 30d



Fig. 5. Bond strength in N/m of porcelain to a Pd alloy (program F). Initial values and values after immersion in corrosion bath (for composition, refer to Table 1). (a) pH = 4.2; (b) pH = 5.2.

The samples were loaded in a tensile testing machine at a cross-head speed of 50 ~m min -1. The bond strength was calculated on the basis of an average of at least five specimens.

RESULTS Figs. 4, 5, and 6 show the results of the present investigation. Figs. 4 and 5 represent the results for the specimens which have been immersed in corrosion baths. Fig. 6 shows the effects of both firing cycles and a corrosive environment. The bond strengths of two Ni-Cr alloys (Ivotect P, and Wirolloy) were weakened at a rate of a least 40% after as short a period as 10 days of immersion (Fig. 4). The corrosion bath (acidity pH = 4.2) reduced the bond strength of the alloy Wirocron to virtually zero after 10 days of immersion (Fig. 4a). The Ni-Cr alloy (Wiron 88) and the CoCr alloy (Wirobond) showed somewhat more stable behavior. Thirty and 90 days immersion resulted in a further decrease of the bond strength, as was anticipated (Fig. 4a). At the close of the investigation (longest period of immersion was 90 days), the majority of specimens showed a bond strength which was virtually zero. The bond strength of the Pd alloy (Pors-on 4) revealed much more resistant behavior (Fig. 5). In general, the lower pH corresponded to the greater decrease in bond strength. Fig. 6 demonstrates the considerable enhancement of bond strength after being annealed at 830°C, then cooled at a very low rate (Table 2, program S). The increase in strength persisted even under the condition of corrosive environment.

DISCUSSION The results revealed that the presently applied fracture experiment provided a sensitive test of the durability of the ceramometallic bond in a corrosive environment. It was evident that a corrosive environment and the modalities of firing (temperature, annealing) are crucial to the long-term stability of the bond. A significant factor in the weakening of porcelain is the effect of moisture contamination. Thus, the water in saliva may play a vital part in the fatigue. The process has been described as a replacement of the alkali ions in the glassy phase by hydrogen ions, which attract water molecules into the spaces originally occupied by the alkali. This water could act as a type of network modifier in weakening the porcelain. The water may interfere with the silicon-oxygen bond by making avail-

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The effect of corrosive environment on the porcelain-to-metal bond--a fracture mechanics investigation.

Microcracks, flaws, and voids inside a metal-porcelain restoration may cause the restoration to fracture in service. Such cracks result in the concent...
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