Сraig. Dental Materials

572 Chapter I 8 CERAMICS

Problem 2

A porcelain crown fractured as a result of excessive porosity. What can be done to avoid porosity?

Solution a

The mix of porcelain should not be stirred too much because bubbles may be entrapped.

Solution b

Porcelain should be heated gradually when first fired to evaporate the water holding the particles together without creating steam. Rapid heating results in steam, which can create porosity.

Solution c

The vacuum level in the porcelain furnace was improperly set. Air-firing does not allow proper elimination of pores during sintering.

Problem 3

A patient showed considerable wear of enamel on the teeth in occlusion with porcelain crowns. Explain.

Solution

Dental porcelain is harder than tooth enamel and can wear it away. Porcelain fused to metal with metal lingual surfaces on anterior crowns results in less wear. However, because the porcelain crowns are already cemented, the use of a bite splint in the case of bruxism may be indicated.

Problem 4

A heat-pressed leucite-reinforced crown frac-

tured after firing of the veneering ceramic.

Explain.

Solution a

Improper core material was used. Heatpressed ceramic ingots are available either for the staining technique or for the veneering technique. These ingots have different coefficients of thermal expansion. If the veneering

ceramic was fired on a core material for the staining technique, the thermal expansion mismatch between the materials can lead to fracture or cracking upon cooling.

Solution b

Improper veneering ceramic was used; for example, an aluminous veneering ceramic with unmatched coefficient of thermal expansion.

Problem 5

A heat-pressed, all-ceramic crown pressed incompletely. Explain the possible causes of incomplete pressing.

Solution a

The pressing temperature was not reached because of an erroneous temperature calibration. Heat-pressing furnaces should be calibrated regularly. If the pressing temperature was not reached, the viscosity of the ceramic ingot was too high to allow complete pressing of the crown.

Solution b

The air-pressure in the heat-pressing furnace was too low. Ideal pressure should be at least 65 psi for proper pressing.

Solution c

Only one ceramic ingot was used when two were needed. Wax-patterns should be weighed before pressing to ensure that enough material is available for pressing.

Problem 6

The porcelain on a ceramic-metal crown fractured at the drying stage. Explain.

Solution a

The recommended drying time was improperly set or the heating rate was too high.

Solution b

The ceramic slurry was insufficiently condensed.

Problem 7

The surface of a ceramic-metal crown exhibits several dark inclusions. What are the possible causes of these defects?

Solution a

The inclusions are metallic inclusions created during framework grinding; They were not eliminated because of inadequate cleaning after grinding.

Solution b

Dirty instruments were used to mix the porcelain powder or dirty brushes to apply the ceramic.

Solution c

Metallic instruments were used at any time during porcelain preparation or application.

Anusavice KJ, Gray A, Shen C: Influence of initial flaw size on crack growth in air-tempered porcelain, J Dent Res 70:131, 1991.

Asaoka K, Nuwayama N, Tesk JA: Influence of tempering method on residual stress in dental porcelain, J Dent Res 71:1623, 1992.

Baran GR, O'Brien WJ, Tien TY: Colored emission of rare earth ions in a potassium feldspar glass, J Dent Res 56:1323, 1977.

Barreiro MM, Riesgo 0,Vicente EE: Phase identification in dental porcelains for ceramo-metallic restorations, Dent Muter 5:51, 1989

Brodbelt RHW, O'Brien WJ, Fan PL: Translucency of dental porcelain, J Dent Res 59:70, 1980.

Denry IL, Rosenstiel SF, Holloway JA et al: Enhanced chemical strengthening of feldspathic dental porcelain, J Dent Res 72:1429, 1993.

Dong JK, Luthy H, Wohlwend A et al: Heatpressed ceramics: technology and strength,

Int J Prosthodont 5:9, 1992.

Gray HS: The porcelain jacket crown, NZ Dent J 59:283, 1963.

Hodson JT: Some physical properties of three dental porcelains, J Prosthet Dent

9:235, 1959.

Johnston WM, O'Brien WJ: Color analysis of dental modifying porcelains, J Dent Res 60 (Spec Issue A):441, 1981.

Jones DW, Wilson HJ: Some properties of dental ceramics, J Oral Rehabil2:379, 1975.

Kelly JR, Giordano R, Pober R et al: Fracture surface analysis of dental ceramics: clinically failed restorations, Int J Prosthodont 3430, 1990.

Kelly JR, Nishimura I, Campbell SD: Ceramics in dentistry: historical roots and current perspectives, J Prosthet Dent 75:18, 1996.

Kelly JR, Tesk JA, Sorensen JA: Failure of allceramic fixed partial dentures in vitro and in vivo: Analysis and modeling, J Dent Res 74:1253, 1995.

Kingery WD, Bowen HK, Uhlmann DR: Introduction to ceramics, (ed 2) New York, 1976,John Wiley & Sons.

Kulp PR, Lee PW, Fox JE: Impact test for dental porcelain, J Dent Res 40:1136, 1961.

Kurzeja R, O'Brien WJ: The fluorescence of porcelain containing cerium, J Dent Res 60 (Spec Issue A):435, 1981.

Leone EF, Fairhurst CW: Bond strength and mechanical properties of dental porcelain enamel, J Prosthet Dent 18:155, 1967.

Mackert JRJ, Rueggeberg FA, Lockwood PE

et al: Isothermal anneal effect on microcrack density around leucite particles in dental porcelains, J Dent Res 73:1221, 1994.

Mackert JR Jr, Twiggs SW, Evans-Williams AL: Isothermal anneal effect on leucite content in dental porcelains, J Dent Res

741259, 1995.

McLaren EA, Sorensen JA: High-strength alumina crowns and fixed partial dentures generated by copy-milling technology,

Quint Dent Techno1 l8:3l, 1995.

574 Chapter 18 CERAMICS

McLean JW: A higher strength porcelain for crown and bridge work, BY Dent J 119:268, 1765.

McLean JW: The alumina reinforced porcelain jacket crown, J Am Dent Assoc 75621, 1967.

McLean JW, Hughes TH: The reinforcement of dental porcelain with ceramic oxides, Br Dent J 117:251, 1765.

Meyer JM, O'Brien WJ, Yu R: Sintering of dental porcelain enamels, J Dent Res

55676, 1976.

Milleding P, ~ r t e n g r e nV, Karlsson S: Ceramic inlay systems: some clinical aspects, J Oral Rehabil22:571, 1775.

Mora GP, O'Brien WJ: Thermal shock resistance of core reinforced all-ceramic crown systems, J Biomed Mater Res 28:187, 1994.

Morena R, Lockwood PE, Fairhurst CW: Fracture toughness of commercial dental porcelains, Dent Mater 2:58-62, 1786.

O'Brien WJ: Ceramics, Dent Clin North Anz 27:851, 1785.

O'Brien WJ: Recent developments in materials and processes for ceramic crowns, J Anz Dent Assoc 110:547, 1785.

O'Brien WJ, Craig RG, editors: Proceedings of conference on recent developments i n dental ceramics, Ceramic Eng and Sci Proc, Columbus, Ohio, 1785, American Ceramic Society.

O'Brien WJ, Johnston WJ, Fanian F: Filtering effects of body porcelain on opaque color modifiers, J Dent Res, IADR Abstracts 61:330, 1982.

O'Brien WJ, Nelson D, Lorey RE: The assessment of chroma sensitivity to porcelain pigments, J Prosthet Dent 4963, 1783.

Pichi. PW, O'Brien WJ, Groh CL et al: Leucite content of selected dental porcelains,

J Biomed Mater Res 28:603, 1974.

Preston JD, Bergen SF: Color science and dental art, St Louis, 1780, Mosby.

Probster L, Diehl J: Slip-casting alumina ceramics for crown and bridge restorations, Quint Int 2325, 1772.

Rekow ED: A review of the developments in dental CAD/CAM systems, Curr Opin Dent 2:25, 1772.

Seghi R, Sorensen J: Relative flexural strength of six new ceramic materials. I n t J Prosthodont 8(3):237-246, 1795.

Sherrill CA, Jr, O'Brien WJ: The transverse strength of aluminous and feldspathic porcelains, J Dent Res 53:683, 1774.

Smith BB: Esthetic restoration of anterior teeth, with emphasis on rapid fabrication of fired porcelain units, J A m Acad Gold Foil Oper

6:6, 1763.

Smith BI3: Porcelain inlays for the general practitioner, Dent Clin North A m 171, Mar. 1967.

Thompson JY, Anusavice KJ, Naman A et al: Fracture surface characterization of clinically failed all-ceramic crowns, J Dent Res 73(12):1824-1832, 1994.

Thompson JY, Anusavice KJ: Effect of

surface etching on the flexure strength and fracture toughness of Dicor disks containing controlled flaws, J Dent Res

73:505, 1774.

Vaidyanathan TK, Vaidyanathan J, Prasad A: Properties of a new dental porcelain.

Scanning Microscopy 3:1023-1033, 1787. Vines RF, Semmelman JO: Densification of den-

tal porcelain, J Dent Res 36950, 1957. Wagner WC, O'Brien WJ, Mora GP: Fracture surface analysis of a glaze-strengthened

magnesia core material, Int J Prosthodont 5:475-478, 1792.

Weinstein M, Katz S, Weinstein AB: Fused porcelain-to-metal teeth. US Patent No. 3,052,782, September 11, 1762.

Wozniak WT, Moore BK, Smith E: Fluorescence spectra of dental porcelain, J Dent Res 55(Spec Issue B):B186, 1776.

Yamada HN: Dentalporcelain, the state of the art-19 77, Los Angeles, 1777, University of Southern California.

High fusing temperature of the alloy. The fusing temperature must be substantially higher (>100•‹C) than the firing temperature of the ceramic and solders used to join segments of a bridge.

Low fusing temperature of the ceramic. The fusing temperature must be lower than ceramic used for all-ceramic restorations so no distortion of the coping takes place during fabrication.

The ceramic must wet the alloy readily when applied as a slurry in order to prevent voids forming at the interface. In general, the contact angle should be 60 degrees or less.

A good bond between the ceramic and metal is essential and is achieved by the interactions of the ceramic with metal oxides on the surface of metal (Fig. 19-2) and by the roughness of the metal coping. Compatible coefficients of thermal expansion of the ceramic and metal so the

Fig, 19-1 Cross section of a ceramic-metal crown showing a gold alloy or base metal coping, the opaque body (dentin), and enamel ceramic layers.

ceramic does not crack during fabrication. The system is designed so the value for the metal is slightly higher than for the ceramic, thus putting the ceramic in compression (where it is stronger) during cooling (Fig. 19-3).

Adequate stiffness and strength of the alloy core. This requirement is especially important for fixed bridges and posterior crowns. High stiffness in the alloy reduces stresses in the ceramic by reducing deflection and strain. High strength is essential in the interproximal regions in fixed bridges. High sag resistance is essential. The alloy copings are relatively thin; no distortion should occur during firing of the ceramic

578 Chapter 19 CERAMIC-METAL SYSTEMS

or the fit of the restoration will be compromised.

8 . An accurate casting of the metal coping is required even with the higher fusing temperature of the alloy.

9.Adequate design of the restoration is critical. The preparation should provide for adequate thickness of alloy (see 5 on p. 576) and also provide enough space for an adequate thickness of ceramic to yield an esthetic restoration. In some instances, a ceramic-metal restoration has an advantage over an all-ceramic restoration because

less tooth structure needs to be removed to provide adequate bulk for the all-ceramic restoration. However, in cases of small, lower, anterior teeth, an all-ceramic restoration has an advantage with respect to esthetics, because with a ceramic-metal restoration it is difficult to remove enough tooth structure to provide space for the coping and the esthetic ceramic layer. The geometry of the shoulder should be flat with a rounded angle or a chamfer to allow enough bulk of ceramic and avoid fracture in this area. If full ceramic coverage is not used ( e g , a metal occlusal) the position of the ceramic-metal joint should be located as far as possible from areas of contact with opposing teeth.

2 ld

CERAMIC-METAL BONDfNG'"."1";

The bond strength between the ceramic and metal is perhaps the most important requirement and thus will be given special attention. In general, the bond is a result of chemisorption by diffusion between the surface oxides on the alloy and in the ceramic. These oxides are formed during wetting of the alloy by the ceramic and firing of the ceramic. The most common mechanical failure of these restorations is ceramic debonding from the metal. Many factors control metal-ceramic adhesion: the formation of strong chemical bonding, mechanical interlocking between the two materials, and residual stresses. In addition, as noted earlier the ceramic must wet

and fuse to the surface to form a uniform interface with no voids. These factors are also important for ceramic coatings on metallic implants.

An interface between a metal and a ceramic with many strong chemical bonds between them, with the bonds acting as tags that hold the two materials together, would obviously lead to strong bonding. However, methods producing a ceramic-metal interface with strong chemical bonding have not been developed. But the formation of oxides on the surface of the metal have been proven to contribute to the formation of strong bonding. Noble metals, which are resistant to oxidizing, must have other, more easily oxidized elements added, such as indium and tin, to form surface oxides. When these more easily oxidized elements are added, bonding is improved. The common practice of "degassing" or preoxidizing the metal coping before ceramic application creates surface oxides that improve bonding.

Base-metal alloys contain elements, such as nickel, chromium, and beryllium, which form oxides easily during degassing, and care must be taken to avoid too thick an oxide layer. Manufacturers' specify conditions to form the optimal oxide and often indicate the color of the oxide. Oxides rich in NiO tend to be dark gray, whereas those rich in Cr,03 are greenish. These oxides dissolve in the ceramic during fixing and may discolor it or be visible through thin layers of ceramic near the gingival edge of the restoration.

The oxides are not completely dissolved during the fusion of the ceramic and thus the oxidealloy interface can be the site of mechanical failure. This situation is especially true with some alloys that form layers rich in Cr,O,, which does not adhere well to the alloy. These alloys typically require the application of a bonding agent to the alloy surface to modify the type of oxide formed.

Alloys containing Be normally form welladhering oxides. Be0 is a slow-growing oxide that does not delaminate from the surface of the alloy. Rare earth elements such as yttrium can be added to the alloy to improve adherence by forming oxides, tying the alloy to the oxide layer.

From both theoretical and practical standpoints, the roughness, or more generally the topography, of a ceramic-metal interface plays a large part in adhesion. The ceramic penetrating into a rough metal surface can mechanically interlock with the metal, like Velcro, improving adhesion. The increased area associated with a rougher interface also provides more room for chemical bonds to form. However, rough surfaces can reduce adhesion if the ceramic does not penetrate into the surface and voids are present at the interface;this may happen with improperly fired porcelain or metals that are poorly wetted by the porcelain. Sandblasting is often used to remove excess oxide and to roughen the surface of the metal coping to improve the bonding of the ceramic.

High residual stresses between the metal and ceramic can lead to failure. If the metal and ceramic have different thermal expansion coefficients, the two materials will contract at different rates during cooling and strong residual stresses will form across the interface. If these stresses are strong enough the ceramic on the restoration will crack or separate from the metal. Even if the stresses are less strong and do not cause immediate failure, they can still weaken the bond. To avoid these problems the ceramics and alloys are formulated to have closely matched thermal expansion coefficients. Most porcelains have coefficients of thermal expan-

between the metal and ceramic causes the metal to contract slightly more than does the ceramic during cooling after firing. This condition puts the ceramic under slight residual compression, which makes it less sensitive to applied tensile forces.

Wetting is important to the formation of good ceramic-metal bonding. During firing, the ceramic must wet and flow over the metal surface. The contact angle between the ceramic and metal is a measure of the wetting and, to some extent, the quality of the bond that forms. The wetting of the alloy surface by the fused ceramic indicates an interactionbetween surface atoms in

the metal with the ceramic. Low contact angles indicate good wetting. The contact angle of ceramic on a gold type of ceramic alloy is about 60 degrees. The surface of the noble alloys containing tin and indium after heating have these oxides present, and they diffuse into and interact with the ceramic, forming an adhesive bond. The oxide surface of an Au-Pt-Pd 98% noble alloy is shown at high magnification in Fig. 19-2.

EVALUATION OF CERAMIC-METAL BONDING

Many tests have been used to determine the bond strength between ceramics and metals; however, the ideal test currently does not exist. In addition, data obtained from different tests are often not comparable. One of the established bond-strength tests is the planar shear test. Other commonly used tests are the flexural tests. The flexural tests require layers of ceramic to be bonded to a strip or plate of metal. The coated metal plate is flexed in a controlled manner until the ceramic fractures off. In the 3-point flexure bend test, ceramic is fired to one side of a rectangular strip of metal. The metal-ceramic strip is supported by two knife edges, and the specimen is loaded in the center with the ceramic surface down until failure of the ceramic occurs.

An adequate bond occurs when the fracture stress is >25 MPa; however, with many metalceramic systems values of 40 to 60 MPa are common. In a variant of this test, opaque and body ceramics are applied and fired to a thickness of approximately 1 mm on a 20-mm x 5-mm x 0.5-mm alloy sheets. The specimen is then bent over a 1-cm-diameter rod (with the ceramic on the outside) and then straightened. The surface is viewed under low magnification and the percent of the surface with retained ceramic is reported. Tests based on tensile and torsional loading schemes have also been used.

A ceramic-metal bond may fail in any of three possible locations (Fig. 19-4). Knowing the location of the fracture provides considerable information. The highest strength metal-ceramic specimens will fracture in the ceramic when tested (see Fig. 19-4, 0;this is observed with some alloys that were properly prepared and had

Fig. 19-4 Diagram showing three observed types of bond failure in ceramic-metal systems:A, metalmetal oxide; B, metal oxide-metal oxide; and

C, ceramic-ceramic.Note: The dimensions of the layers are not to scale.

ceramic applied and fused. Testing these highstrength specimens using the push-through shear test shows the maximum strengths are approximately the same as the shear strength of the ceramic. Fractures through the oxide (see Fig. 19-4, B) and metal-metal oxide fracture (see Fig. 19-4, A) are commonly observed with poor bonding. Base-metal alloys commonly fracture through the oxide (Fig. 19-5) if an excessively thick oxide layer is present. Interfacial fracture is observed with metals that are resistant to forming surface oxides, such as pure gold or platinum.

The ceramics used for porcelain-fused-to- metal restorations must fulfill five requirements:

(1) they must simulate the appearance of natural teeth, (2) they must fuse at relatively low temperatures, (3) they must have thermal expansion coefficients compatible with the metals used for ceramic-metal bonding, (4) they must withstand

Fig. 19-5 Bond failure through the metal oxide layer of a nickel-based ceramic-metal crown.

(From O'BrienWJ, in Yamada H, editor: Dental porcelain: the state of the art-1977, Los Angeles, 1977, University of Southern California.)

the oral environment, and (5) they must not unduly abrade opposing teeth. The ceramic is carefully formulated to achieve these requirements. These ceramics are composed of crystalline phases in an amorphous and glassy (vitreous) matrix. They comprise primarily SiO,, A1,03, Na,O, and K,O (Table 19-1). Opacifiers (TiO,, ZrO,, SnO,) and various heat-stable pigments are also added to the ceramic. Because of their composition, they can be considered a type of glass. To match the appearance of tooth structures, small amounts of fluorescing pigments such as rare earth oxides (CeO,) are added. The nature of ceramics, with their glassy matrix and crystalline phases, produces a translucency much like that of teeth, whereas pigments and opacifiers control the color and translucency of the restoration. The ceramic is supplied as a fine powder.

In developing ceramics for ceramic-metal bonding, a major breakthrough was formulating products that had sufficiently high thermal expansion coefficients to match those of dental alloys. This higher expansion was made possible

by the addition of potassium oxide and the formation of a high-expansion phase called leucite (KA1Si2O6).This phase increased the thermal expansion of the porcelain so it could match that of dental alloys.

These materials have other qualities that make them well suited for ceramic-metal restorations. They fuse at lower temperatures than do many other ceramic materials, lessening the potential of distorting the metal coping. Sodium and potassium oxides in the glassy matrix are responsible for lowering the fusing temperatures to the range 930" to 980" C; low-fusing ceramics have hydroxyl groups and more Na20 to lower fusing temperatures to as low as 660" C. These ceramics do not corrode, and are also resistant to the fluids present in the oral environment. They can, however, be abrasive to opposing teeth because of their hardness; this becomes a significant problem if the porcelain surface is rough due to improper processing or becomes rough in the oral environment. Newer products have been shown to be less abrasive to natural teeth. These new ceramics are also strong in compression, which permits their use on the occlusal surfaces of the restorations. The ceramics used to bond to metals have tensile strengths of 35 MPa, com-

pressive strengths of 860 MPa, shear strengths of 120 MPa, and flexural strengths of 60 MPa.

Chronologically the alloys developed for ceramic-metal restorations were Au-Pt-Pd, Ni-Ci, Co-Cr, Au-Pd-Ag, Pd-Ag, Au-Pd, Pd-Cu, and Ti.

COMPOSITION AND PROPERTIES OF NOBLE METAL ALLOYS

The composition ranges and color of five types of noble alloys for ceramic-metal restorations are listed in Table 19-2.The properties of these alloys are given in Table 19-3.

Au-Pt-Pd Types These alloys contain a very high noble metal content, mainly gold with platinum and palladium to increase the melting range. The high-noble content provides good corrosion resistance. Indium, tin, and iron are present and form oxides to produce a ceramicmetal bond. Rhenium is added as a grain refiner. Hardening of Au-Pt-Pd type alloys results from solid solution hardening and the formation of an FePt, precipitate. Optimum heat treatment for hardening is 30 minutes at 550" C, but practically the hardening occurs during firing of the ceramic. During the casting of these alloys some of these base elements are lost; it is therefore recommended that 50%new alloy be used with a spme button if it is used to make a second casting. The new alloy will provide enough of the base elements so adequate oxides and hardening result.

From Table 19-3 it is seen that these alloys have high stiffness (elastic modulus), strength, and hardness and reasonable elongation; however, they have somewhat low sag resistance. The alloys are very costly because of their high noble-metal content and high density; they are sold on a weight basis but used on a volume basis. The casting temperature is reasonably high, and although reasonably easy to solder, care must be taken because the soldering temperature is only about 50 " C below the melting