J. Dent. 1991; 19: 51-55
The strength
51
of dental ceramics*
P. F. Messer, V. Piddockt and C. H. Lloyd (Editor)+ School of Materials, University of Sheffield, University of Dundee, Scotland, UK
jTurner
Dental School, University
of Manchester
and *Dental
School,
ABSTRACT Considerable changes have taken place in the range of ceramic materials available for dental use. Although the appearance of dental porcelain is good and biocompatibility excellent, its mechanical properties are somewhat limited. As a consequence, a number of distinct developments have taken place primarily to achieve greater strength; other goals having been to improve accuracy and simplification of production procedure. Progress has been made, but at the present time newer ceramics must still be regarded as complementary to established porcelains. KEY WORDS: J. Dent. 1991; Correspondence UK.
Dental ceramics, Strength 19: 51-55
(Received 6 September
1990;
accepted 17 September 1990)
should be addressed to: Mr C. H. Lloyd, Dental School, University of Dundee, Dundee DDl 4HN.
The Dental Materials Group of the British Society for Dental Research (BSDR) has, for a number of years, organized workshops as part of the programme at the Society’s annual meeting. For the 1990 meeting (held in King’s College, London) the Group selected the subject of ‘Dental Ceramics’, a material category in which significant developments have taken place in recent years. Advances in dental ceramic technology have been stimulated by the need to improve the mechanical properties of conventional materials whilst maintaining the benefits of lasting aesthetics and biocompatibility, associated with dental ceramic restorative materials. Improvements in strength and fracture toughness have been achieved and enhanced material integrity has been demonstrated in the laboratory. Consequently at this workshop, what determines the strength of any ceramic was addressed (Dr Messer) before considering recent developments in specific dental ceramic systems (Dr Piddock).
UNDERSTANDING THE STRENGTH BEHAVIOUR OF CERAMICS Properties of a material can be considered to be of two types: the characteristic and the behavioural. Characteristic properties, constitutional or structural, describe the state of the material. The amounts, types and compositions *Based on the Proceedings of the British Society for Dental Research Dental Materials Group Workshop, 1990. @1991 Butterworth-Heinemann 0300-5712/91/010051-05
Ltd.
of the phases including pores are constitutional. The sizes and shapes of the phase regions together with their crystal structures are structural. The behaviour of a material depends upon its characteristic properties and upon the stimuli which are acting. To understand any behavioural property, an analysis of the property is required in terms of the characteristic properties. Tensile strength
and toughness
From fracture mechanics a relationship is obtained between the behavioural properties of strength (OF), fracture toughness (K,, = (2Eyi)‘a5), and the structural characteristic of flaw size (C). This is: oF = constant Klc C-O.’= constant (2Eyi)‘33 C-o.5 (1) where the constant depends upon the structural characteristics of the flaw and how the ceramic is loaded; E and ‘/i are Young’s modulus and the effective surface energy for fracture initiation respectively. Hence to understand strength behaviourK,, orE and yi need to be measured and analysed in terms of the characteristic properties. Flaw types, origin, severity
and observation
For a flaw to initiate fracture it must have a sharp cracklike feature associated with it. Flaws can occur on the
52
J. Dent. 1991;
19: No. 1
edges, surfaces and in the volume of a ceramic, and for the same size, their severity decreases in that order. In strong ceramics, the flaws are often of grain size dimensions, typically a few microns in size. Surface and edge flaws, which may be formed during surface grinding or from abrasion in service, might growby stress corrosion to initiate fracture. Cracks can also develop on cooling after tiring by separation at grain or inclusion boundaries as a result of thermal expansion mismatch or a phase transition involving a volume change. The origin of the failure can be identified by fractography (Rice, 1974) but often no identifiable defect is observed when the flaw, such as a grinding crack, is planar and parallel to the fracture surface. Flaws in ceramics of low and modest strength are correspondingly larger. They have been identified using fractography as: large pores which are often fissure shaped; pore clusters; large grains with cracks around their boundaries; large inclusions, which are either associated with circumferential or radial cracks. The size of the three-dimensional part of the flaw can often be measured on the fracture surface using scanning electron microscopy (SEM). Large pores can result from the burnout of organic impurites, but often in ceramics which have been sintered they develop from a non-uniform shrinkage during densification. Non-uniform shrinkage results from one or more of the characteristic properties in the green ceramic being non-uniform (Agbarakwe et al., 1989). The characteristic properties most commonly found to be non-uniform in the green state are porosity, composition, particle size and variable alignment of anisotropic particles. Model flaws can be employed in calculations of flaw severity using values for oF and K,c in equation (1) with an appropriate constant. The most frequently employed is an internal penny-shaped slit of radius C, the plane of which is perpendicular to the direction of tensile stress. For this case, equation (1) becomes 0
F
=
0.5 n 0e5K,c c-o’5
(2)
A spherical pore raises the stress only by a factor of two on its equator. Because these flaws are frequently found at fracture origins, it is necessary to consider them to be associated with sharp cracks to form a pore-crack combination. The constants have been evaluated for two model pore-crack flaws (Evans et al., 1979). Fracture toughness If fracture initiates from a flaw of known shape, dimensions, position and orientation in a test-piece whose geometry, size and method of loading are known, K,c can be determined. This requires the constant in equation (1) to be evaluated from the above information. When ceramics are being investigated, a sawn notch of narrow width usually forms a macroscopic flaw in test-pieces. Sharp cracks are assumed to emanate from the root of the
notch to make it equivalent to a sharp crack with a length taken to be equal to the notch depth. Pores, inclusions and grains in ceramics cause the Young’s modulus to vary with position and cause the applied stress adjacent to the crack tip to differ from the continuum case. Similarly any residual stresses will affect the crack tip stress. Consequently, the measured Kit has the effects of these departures from the continuum built in. Porosity reduces both E and yi, since it reduces the load bearing area, raises stress locally and reduces the area of the fracture surfaces formed. This leads to: Klc = Klc*
exp - kp
(3)
where K,c * is the pore free value of K,c,p the volume of porosity and k a constant. Inclusions bonded to the matrix will also affect E and yi. The average E may be different to that of the matrix, and if the crack passes around the inclusions (thereby increasing the area of the fracture surface) yi will be raised. The inclusions and the matrix will probably have different thermal expansion behaviour or the former may exhibit a phase transition involving a volume change on cooling. These will give rise to residual stresses and possibly microcracking for the larger inclusions. Although the stresses developed are independent of the size of the inclusions, it is observed that microcracking depends upon their size. A similar observation is made for grains which undergo anisotropic contraction. These observations are explained by assuming that incipient cracks may be present whose severity tends to increase with the inclusion or the grain size. Coarse inclusions contracting more than the matrix are enveloped by circumferential cracks. This makes their effect upon Klc similar to that of porosity. Coarse inclusions which shrink less than the matrix cause radial cracks to form. If present in high concentrations, they may link up and cause K,c to have a low value. A similar problem can occur for coarse-grained ceramics. When little microcracking occurs on cooling because the inclusions or grains are sufficiently small, the formation of microcracks can be triggered by the stress field around a crack as it advances. This causes a process zone to form around the crack, which can cause the measured Klc to be increased significantly. The toughening effect can occur through microcrack shielding and through the effect of dilatancy, which could be more significant. The expansion (of the inclusions, grains or the matrix adjacent to the crack) behind the crack tip, tends to close up the crack. The more inclusions or grains involved and the larger the dilatancy associated with each, the greater will be the toughening increment. The width of the zone depends upon the inclusion or grain size. Very small inclusions or grains require higher stresses to cause microcracking. Hence, the width of the zone will decrease when the inclusion or grain size becomes very small and the measured Kit will be smaller.
Messer et al.: Strength
Strength
measurement
In fracture toughness tests which involve bending testpieces, such as the three- or four-point tests on bars, the Cring test and the various tests in which discs are flexed, the flaws which are likely to cause failure will be at the surface or (except for disc tests) at the edges of the test-pieces. This is because the applied stress decreases rapidly from the surface in these tests and edge and surface flaws are more severe than internal flaws of the same size. The straight-pull tensile test subjects the gauge length to a uniform applied stress provided bend stresses are eliminated. Failure should result from the most severe flaw in the test piece. However, it is easier to carry out the ring-bursting test. In this, more of the volume of the ring is subjected to stresses close to the maximum than occurs in bend tests, so that failure is commonly observed from internal flaws. Concluding ceramics
remarks on the strength
of
The strength test should be chosen to suit the purpose for which the data is required and the test-pieces should have the same characteristics as the ceramic components to be used in practice. In all cases, the first step in understanding the strength behaviour is to determine the fracture origin using fractography and, if possible, the flaw size. Second, KIc should be measured and oF, C and K,= tested for consistency using equation (1) for the appropriate flaw model. It should be remembered that the test-pieces might be macroscopically prestressed, so that oF is the sum of the residual and applied stresses at failure. Finally the testpieces should be adequately characterized so that the reasons for changes in strength from batch to batch or with preparative methods can be understood.
RECENT ADVANCES
IN DENTAL CERAMICS
In this survey ‘recent’ has been used as a restriction for advances made in the eighties. However, research on established materials still continues to produce necessary and interesting results. Omission of these established materials is a consequence of a need to concentrate upon newer materials and not because the former are becoming any less important. Only as the newer materials mature with the passage of time will we be able to judge whether they are replacements for, or complementary to, currently established materials. Improved
porcelain
A further improvement in tensile strength of alumina reinforced porcelain has been achieved by increasing the proportion of alumina crystals. A 33 per cent increase in flexural strength has been reported for Hi-Ceram core porcelain (Vita Zahnfabrik, Bad Sackingen, FRG),
of dental ceramics
53
compared with conventional alumina reinforced material (Oilo, 1988), and a similar increase in disc strength has been observed (Piddock, 1989). A significant improvement in fracture toughness has also been determined (Kvam, 1989). O’Brien (1985) has described a high expansion coefficient core porcelain reinforced with magnesia crystals which is compatible with metalloceramic porcelains. The strength is similar to that of alumina reinforced material, but can be increased by the application of a compressive surface glaze. In Inceram Core Ceramic (Vita Zahnfabrik) a different approach has been adopted to achieve higher strength, liquid glass infiltration of a porous crystal preform. High strength copings of this type are formed by condensing a slurry of fine alumina powder onto a special refractory die. The carved coping is dried slowly and the temperature is gradually raised to 1100“C over a period of 2 h, at which it is maintained for a further 2 h. The shrunken refractory is removed and a shade match glass slurry applied to the surface. Firing at 1100“C for 4 h results in densitication of the alumina preform by the low viscosity glass. The manufacturers claim that this new core material has a flexural strength three times greater than that of conventional core porcelain. Shrink-free
ceramic
coping
The Cerestore System (Johnson and Johnson, East Winsor, NJ, USA) was introduced as a means of improving the tit of all-ceramic restorations. The problems of sintering shrinkage and platinum foil matrix usage are eliminated by transfer moulding a plasticized tablet containing alumina, magnesia and a glass frit onto an epoxy die (Sozio and Riley, 1983). Prolonged firing up to a maximum temperature of 1515°C produces a reaction between the magnesia and alumina. Formation of magnesium aluminate spine1 is accompanied by a volume expansion which exactly compensates for the sintering shrinkage. Several investigators have examined sectioned Cerestore restorations (Chan et al., 1985; Davis, 1988; Scharer et al., 1988; Dickinson et al., 1989) and marginal discrepancies ranging from 1 urn to 313 pm have been measured. A number of processing variables have been shown to influence fitting accuracy (Campbell, 1986; Sato et al., 1986a, b). The strength and fracture toughness of the Cerestore core material has been compared with conventional alumina reinforced porcelain. Josephson et al. (1985) and Oilo (1988) demonstrated significantly higher strengths for Cerestore. However, Philp and Brukl(l984) found no statistically significant differences and Piddock et al. (1987) obtained greater disc strengths for conventional alumina reinforced porcelains. Superior fatigue strength has been calculated for Cerestore (Morena et al.. 1986) but lower fracture toughness measurements have been reported (Kvam, 1989). Further development of the coping material has led to the production of Alceram shrink-free crowns (Innotek
54
J. Dent. 1991;19:
No. 1
Dental Corp., Lakewood, CO, USA). The manufacturers claim that the new core material is 65-75 per cent stronger than Cerestore.
Castable
glass ceramics
The production of tetrasilicic-mica glass ceramic restorations has been described by Adair and Grossman (1984). Molten glass is cast to the required shape and is converted to a partially crystalline state by a prolonged heat treatment. The formation of mica-type crystals is claimed to enhance the strength and toughness of the ceramic, whilst a high degree of translucency is maintained. Shade matching is achieved by the application of tinted glazes, although coloured cements are an alternative. A value of 152 MPa has been quoted for the modulus of rupture of Dicer glass-ceramic (Dentsply International, York PN, USA) (Adair and Grossman, 1984). More recently the flexural strength has been reported as 240 MPa compared with a value of 115 MPa for alumina reinforced porcelain (Oilo, 1988). The fracture toughness of Dicer has been found to be similar to that of feldspathic porcelain with the ceramming treatment producing a small increase in Kit (Jones ,et al., 1988). However, Kvam (1989) has indicated that the fracture toughness of cerammed Dicer is inferior to that of conventional core porcelain. Marginal fit measurements giving discrepancies ranging from 10 urn to 88 urn have been reported (Davis, 1988; Scharer et al., 1988; Dickinson et al., 1989). Techniques similar to those used for the manufacture of Dicer crowns, are utilized in the production of Cera Pearl restorations (Kyocera, Tokyo, Japan). A calcium phosphate-based Cera Pearl glass is cast and then heat treated to induce crystallization of the oxyapatite within the amorphous matrix (Hobo and Iwata, 198Sa). The former is converted to hydroxyapatite when it comes into contact with moisture. Again, colour matching is achieved by superficial glazing. Formation of the crystalline phase is claimed to produce a three-fold increase in tensile strength (Hobo and Iwata, 1985b). Light refractive index, density, hardness, thermal expansion and thermal conductivity have all been found to be similar to natural enamel.
Developments
in bonded foil ciowns
The Renaissance crown, sometimes termed the Cera Platin or Ceplatec crown, uses a metal foil to support the overlying porcelains (Schossow, 1984). The swaged, gold coated foil is made rigid by soldering the folds with the melted outer gold layer. Poor marginal tit has been reported by Scharer et al. (1987) although improved fit can be obtained by modifying the technique. A study of the compressive strength of simulated crowns has indicated that the strength of the Renaissance restoration is inferior to conventionally produced porcelain jacket crowns (Brukl and Ocampo, 1987).
Ion exchange
strengthening
The recent introduction of an ion strengthening paste (Ceramicoat, GC Industrial Corp., Tokyo, Japan) offers the possibility of strengthening dental porcelains without the need for potentially hazardous molten salt baths described previously (Southan, 1970). Improvements in strength of up to 120 per cent have been obtained by immersion in molten potassium nitrate (Dunn et al., 1977). Such findings give some indication of the technique’s promise. Increases in strength of up to 44 per cent have been noted in a preliminary study in which the new paste was used (Piddock and Qualtrough, 1990).However, the degree of improvement is dependent on the composition of the porcelain treated.
DISCUSSION The discussion opened with an enquiry on how fracture stress, as it is measured in practice, relates to the strength of a crown. It is obvious that reproduction of conditions prevailing in viva is fundamental to obtain valid data during in vitro testing. There was general agreement that the crown surface should be replicated on the test-piece, although it was pointed out that this is known to present problems during specimen production and leads to compromises. All too often the environment in which the crown exists is oversimplified, and previously dismissed environmental factors might now be reconsidered. For example, what is the effect of an acidic luting cement? Should we consider the crown, cement and dentine as an entity and test accordingly? Though no one positively supported separation of the ceramic from the other components in the restoration, everyone acknowledged that this is what happens in practice. Leading on from this, a point of view was put. that in many respects tooth enamel has very limited mechanical properties yet survives well, as do thin laminate veneers (for which low fracture forces would be measured in the laboratory). Perhaps therefore the concept of veneering might be extended and a more conservative approach considered as an alternative to seeking higher and higher bulk strengths from ceramics. It is recognized that the quality and success of a ceramic restoration is dependent upon the combined skills of the dentist and the technician. Not surprisingly an opinion should be expressed, that often the measurement of strength of a porcelain is as much a measure of the technical skill used to make the test-piece as any inherent properties of the material. Inevitably, the subsequent discussion on the durability and aesthetics of ‘rival systems proved inconclusive given the differences in clinical preparation, the variable quality of technical skill in addition to any differences between materials. That fairly reflects the current position, for no material nor technique is universally acclaimed as the best. There has been a trend towards greater heterogeneity in dental porcelain containing crystalline dispersions as a
Messer et al.: Strength
consequence of compositional changes made to achieve greater strength. Unfortunately, the strength of ceramics is, in general, limited by the degree of heterogeneity. However, it may well be possible to produce greater strengths in these systems by optimizing dispersion parameters. Alternative approaches to the goal of high strength dental ceramics including the closure of pores by hot isostatic processing (‘hipping’) or CAD-CAM of fully dense monolithic stock have yet to be fully explored. Finally the group was reminded that market forces strongly influence the development and use of any new dental material. A UK dental ceramic market distinct from an arguably more progressive continental European market may be developing due to the domination of a restrictive and cost conscious National Health Service. This is probably not a uniquely British phenomenon. Any (socialized) state welfare scheme or (free enterprise) health insurance company which achieves a dominant market position in any country will inevitably draw up an approved list (whether to limit government expenditure, or improve profitability). This could be viewed as restrictive. Under such circumstances a niche may be created for a material tailored to that particular national market. Although dental materials have become internationalized increasingly, and are seen as such, local conditions still have an influence upon choice and differentiate national markets. Group members were left to reflect on the fact that ultimately it is those who pay for the treatment who will decide on whether the improvements brought about by research can be afforded, and consequently whether particular new ceramic systems are viable.
References Adair P. J. and Grossman D. G. (1984) The castable ceramic crown. Int. J. Periodont. Rest. Dent. 2, 33-45. Agbarakwe U. B., Banda J. S. and Messer P. F. (1989) Non uniformities and pore formation. Mat. Sci. Eng. A109, 9-16. Brukl C. E. and Ocampo R. R. (1987) Compressive strength of a new foil and porcelain fused to metal crowns. J. Prosthet. Dent. 57, 404-410. Campbell S. D. (1986) The effect of sintering temperature on the dimensional stability of a new ceramic coping. J. Prosthet. Dent. 55, 309-312. Chan C., Haraszthy G., Geis-Gerstorfer J. et al. (1985) The marginal>lit of Cerestore full-ceramic crowns-a preliminary report. Quintessence Int. 16, 399-402. Davis D. R. (1988) Comparison of fit of two types of allceramic crowns. J. Prosthet. Dent. 59, 12-16. Dickinson k J. G., Moore B. K, Harris R. K et al. (1989) A comparative study of the strength of aluminous porcelain and all-ceramic crowns. J. Prosthet, Dent. 61,297-304.
of dental ceramics
55
Dunn B., Levy M. N. and Reisbick M. H. (1977) Improving the fracture resistance of dental ceramic. J Dent. Res. 56, 1209-1213. Evans A G., Biswas D. R. and Fulrath R. M. (1979) Some effects of cavities on the fracture of ceramics: II. Spherical cavities. J. Am. Ceram. Sot. 62, 101-106. Hobo S. and Iwata T. (1985a) Castable apatite ceramics as a new biocompatible restorative material. II. Fabrication of the restoration. Quintessence Int. 16,207-216. Hobo S. and Iwata T. (1985b) Castable apatite ceramics as a new biocompatible restorative material. I. Theoretical considerations. Quintessence Int. 16, 135-141. Jones D. W., Rizkalla A. S., King H. W. et al. (1988) Fracture toughness and dynamic modulus of tetrasilicic-micaglassceramic. J. Can. Ceram. Sot. 57, 39-46. Josephson B. A., Schulman A., Dunn Z. A. et al. (1985) A compressive strength study of an all-ceramic crown. J. Prosthet. Dent. 53, 301-303. Kvam K. (1989) Fracture toughness determination of ceramic and resin based dental composite. J. Dent. Res. 68, 997. Morena R., Beaudreau G. M., Lockwood P. E. et al. (1986) Fatigue of dental ceramics in a simulated oral environment. J. Dent, Res. 65, 993-997. O’Brien W. J. (1985) Magnesia ceramic jacket crowns. Dent. Clin. North Am. 29, 719-723. Oilo G. (1988) Flexural strength and internal defects of some dental porcelains. Acta Odontol. Stand. 46, 313-322. Philp G. K. and Brukl C. E. (1984) Compressive strengths of conventional, twin foil, and all-ceramic crowns. J. Prosthet. Dent. 52, 215-220. Piddock V. (1989) Evaluation of a new high-strength aluminous porcelain. Clin. Mater. 4, 349-360. Piddock V. and Qualtrough A. J. E. (1990) Ion strengthening of dental porcelains. J Dent. Res. 69, 975. Piddock V., Marquis P. M. and Wilson H. J. (1987) The mechanical strength and microstructure of all-ceramic crowns. J. Dent. 15, 153-158. Rice R. W. (1974) Fracture topography of ceramics. Mater. Sci. Res. 7,439-472. Sato T., Wohlwend A and Scharer P. (1986a) “Shrink-free” ceramic crown system: factors influencing the core margin lit. Quintessence Dent. Technol. 10, 81-87. Sato T., Wohlwend A. and Scharer P. (1986b) Marginal tit in a “shrink-free” ceramic crown system. Int. J. Periodont. Res. Dent. 3, 9-21. Scharer P., Sato T. and Wohlwend A (1987) Marginal fit in the Cera-platin crown system. Quintessence Dent. Technol. 11, 11-25. Scharer P., Sato T. and Wohlwend A. (1988) A comparison of the marginal tit of three cast ceramic crown systems. .J Prosthet. Dent. 59, 534-542. Schossow D. (1984) The Kera-platinum technique porcelain crown: “Renaissance of the jacket crown”. Quintessence Dent. Technol. 8, 231-255. Southan D. E. (1970) Strengthening modern dental porcelain by ion exchange. Aust. Dent. J. 15, 507-510. Sozio R. B. and Riley E. J. (1983) The shrink-free ceramic crown. J. Prosthet. Dent. 49, 182-187.
The strength
51
of dental ceramics*
P. F. Messer, V. Piddockt and C. H. Lloyd (Editor)+ School of Materials, University of Sheffield, University of Dundee, Scotland, UK
jTurner
Dental School, University
of Manchester
and *Dental
School,
ABSTRACT Considerable changes have taken place in the range of ceramic materials available for dental use. Although the appearance of dental porcelain is good and biocompatibility excellent, its mechanical properties are somewhat limited. As a consequence, a number of distinct developments have taken place primarily to achieve greater strength; other goals having been to improve accuracy and simplification of production procedure. Progress has been made, but at the present time newer ceramics must still be regarded as complementary to established porcelains. KEY WORDS: J. Dent. 1991; Correspondence UK.
Dental ceramics, Strength 19: 51-55
(Received 6 September
1990;
accepted 17 September 1990)
should be addressed to: Mr C. H. Lloyd, Dental School, University of Dundee, Dundee DDl 4HN.
The Dental Materials Group of the British Society for Dental Research (BSDR) has, for a number of years, organized workshops as part of the programme at the Society’s annual meeting. For the 1990 meeting (held in King’s College, London) the Group selected the subject of ‘Dental Ceramics’, a material category in which significant developments have taken place in recent years. Advances in dental ceramic technology have been stimulated by the need to improve the mechanical properties of conventional materials whilst maintaining the benefits of lasting aesthetics and biocompatibility, associated with dental ceramic restorative materials. Improvements in strength and fracture toughness have been achieved and enhanced material integrity has been demonstrated in the laboratory. Consequently at this workshop, what determines the strength of any ceramic was addressed (Dr Messer) before considering recent developments in specific dental ceramic systems (Dr Piddock).
UNDERSTANDING THE STRENGTH BEHAVIOUR OF CERAMICS Properties of a material can be considered to be of two types: the characteristic and the behavioural. Characteristic properties, constitutional or structural, describe the state of the material. The amounts, types and compositions *Based on the Proceedings of the British Society for Dental Research Dental Materials Group Workshop, 1990. @1991 Butterworth-Heinemann 0300-5712/91/010051-05
Ltd.
of the phases including pores are constitutional. The sizes and shapes of the phase regions together with their crystal structures are structural. The behaviour of a material depends upon its characteristic properties and upon the stimuli which are acting. To understand any behavioural property, an analysis of the property is required in terms of the characteristic properties. Tensile strength
and toughness
From fracture mechanics a relationship is obtained between the behavioural properties of strength (OF), fracture toughness (K,, = (2Eyi)‘a5), and the structural characteristic of flaw size (C). This is: oF = constant Klc C-O.’= constant (2Eyi)‘33 C-o.5 (1) where the constant depends upon the structural characteristics of the flaw and how the ceramic is loaded; E and ‘/i are Young’s modulus and the effective surface energy for fracture initiation respectively. Hence to understand strength behaviourK,, orE and yi need to be measured and analysed in terms of the characteristic properties. Flaw types, origin, severity
and observation
For a flaw to initiate fracture it must have a sharp cracklike feature associated with it. Flaws can occur on the
52
J. Dent. 1991;
19: No. 1
edges, surfaces and in the volume of a ceramic, and for the same size, their severity decreases in that order. In strong ceramics, the flaws are often of grain size dimensions, typically a few microns in size. Surface and edge flaws, which may be formed during surface grinding or from abrasion in service, might growby stress corrosion to initiate fracture. Cracks can also develop on cooling after tiring by separation at grain or inclusion boundaries as a result of thermal expansion mismatch or a phase transition involving a volume change. The origin of the failure can be identified by fractography (Rice, 1974) but often no identifiable defect is observed when the flaw, such as a grinding crack, is planar and parallel to the fracture surface. Flaws in ceramics of low and modest strength are correspondingly larger. They have been identified using fractography as: large pores which are often fissure shaped; pore clusters; large grains with cracks around their boundaries; large inclusions, which are either associated with circumferential or radial cracks. The size of the three-dimensional part of the flaw can often be measured on the fracture surface using scanning electron microscopy (SEM). Large pores can result from the burnout of organic impurites, but often in ceramics which have been sintered they develop from a non-uniform shrinkage during densification. Non-uniform shrinkage results from one or more of the characteristic properties in the green ceramic being non-uniform (Agbarakwe et al., 1989). The characteristic properties most commonly found to be non-uniform in the green state are porosity, composition, particle size and variable alignment of anisotropic particles. Model flaws can be employed in calculations of flaw severity using values for oF and K,c in equation (1) with an appropriate constant. The most frequently employed is an internal penny-shaped slit of radius C, the plane of which is perpendicular to the direction of tensile stress. For this case, equation (1) becomes 0
F
=
0.5 n 0e5K,c c-o’5
(2)
A spherical pore raises the stress only by a factor of two on its equator. Because these flaws are frequently found at fracture origins, it is necessary to consider them to be associated with sharp cracks to form a pore-crack combination. The constants have been evaluated for two model pore-crack flaws (Evans et al., 1979). Fracture toughness If fracture initiates from a flaw of known shape, dimensions, position and orientation in a test-piece whose geometry, size and method of loading are known, K,c can be determined. This requires the constant in equation (1) to be evaluated from the above information. When ceramics are being investigated, a sawn notch of narrow width usually forms a macroscopic flaw in test-pieces. Sharp cracks are assumed to emanate from the root of the
notch to make it equivalent to a sharp crack with a length taken to be equal to the notch depth. Pores, inclusions and grains in ceramics cause the Young’s modulus to vary with position and cause the applied stress adjacent to the crack tip to differ from the continuum case. Similarly any residual stresses will affect the crack tip stress. Consequently, the measured Kit has the effects of these departures from the continuum built in. Porosity reduces both E and yi, since it reduces the load bearing area, raises stress locally and reduces the area of the fracture surfaces formed. This leads to: Klc = Klc*
exp - kp
(3)
where K,c * is the pore free value of K,c,p the volume of porosity and k a constant. Inclusions bonded to the matrix will also affect E and yi. The average E may be different to that of the matrix, and if the crack passes around the inclusions (thereby increasing the area of the fracture surface) yi will be raised. The inclusions and the matrix will probably have different thermal expansion behaviour or the former may exhibit a phase transition involving a volume change on cooling. These will give rise to residual stresses and possibly microcracking for the larger inclusions. Although the stresses developed are independent of the size of the inclusions, it is observed that microcracking depends upon their size. A similar observation is made for grains which undergo anisotropic contraction. These observations are explained by assuming that incipient cracks may be present whose severity tends to increase with the inclusion or the grain size. Coarse inclusions contracting more than the matrix are enveloped by circumferential cracks. This makes their effect upon Klc similar to that of porosity. Coarse inclusions which shrink less than the matrix cause radial cracks to form. If present in high concentrations, they may link up and cause K,c to have a low value. A similar problem can occur for coarse-grained ceramics. When little microcracking occurs on cooling because the inclusions or grains are sufficiently small, the formation of microcracks can be triggered by the stress field around a crack as it advances. This causes a process zone to form around the crack, which can cause the measured Klc to be increased significantly. The toughening effect can occur through microcrack shielding and through the effect of dilatancy, which could be more significant. The expansion (of the inclusions, grains or the matrix adjacent to the crack) behind the crack tip, tends to close up the crack. The more inclusions or grains involved and the larger the dilatancy associated with each, the greater will be the toughening increment. The width of the zone depends upon the inclusion or grain size. Very small inclusions or grains require higher stresses to cause microcracking. Hence, the width of the zone will decrease when the inclusion or grain size becomes very small and the measured Kit will be smaller.
Messer et al.: Strength
Strength
measurement
In fracture toughness tests which involve bending testpieces, such as the three- or four-point tests on bars, the Cring test and the various tests in which discs are flexed, the flaws which are likely to cause failure will be at the surface or (except for disc tests) at the edges of the test-pieces. This is because the applied stress decreases rapidly from the surface in these tests and edge and surface flaws are more severe than internal flaws of the same size. The straight-pull tensile test subjects the gauge length to a uniform applied stress provided bend stresses are eliminated. Failure should result from the most severe flaw in the test piece. However, it is easier to carry out the ring-bursting test. In this, more of the volume of the ring is subjected to stresses close to the maximum than occurs in bend tests, so that failure is commonly observed from internal flaws. Concluding ceramics
remarks on the strength
of
The strength test should be chosen to suit the purpose for which the data is required and the test-pieces should have the same characteristics as the ceramic components to be used in practice. In all cases, the first step in understanding the strength behaviour is to determine the fracture origin using fractography and, if possible, the flaw size. Second, KIc should be measured and oF, C and K,= tested for consistency using equation (1) for the appropriate flaw model. It should be remembered that the test-pieces might be macroscopically prestressed, so that oF is the sum of the residual and applied stresses at failure. Finally the testpieces should be adequately characterized so that the reasons for changes in strength from batch to batch or with preparative methods can be understood.
RECENT ADVANCES
IN DENTAL CERAMICS
In this survey ‘recent’ has been used as a restriction for advances made in the eighties. However, research on established materials still continues to produce necessary and interesting results. Omission of these established materials is a consequence of a need to concentrate upon newer materials and not because the former are becoming any less important. Only as the newer materials mature with the passage of time will we be able to judge whether they are replacements for, or complementary to, currently established materials. Improved
porcelain
A further improvement in tensile strength of alumina reinforced porcelain has been achieved by increasing the proportion of alumina crystals. A 33 per cent increase in flexural strength has been reported for Hi-Ceram core porcelain (Vita Zahnfabrik, Bad Sackingen, FRG),
of dental ceramics
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compared with conventional alumina reinforced material (Oilo, 1988), and a similar increase in disc strength has been observed (Piddock, 1989). A significant improvement in fracture toughness has also been determined (Kvam, 1989). O’Brien (1985) has described a high expansion coefficient core porcelain reinforced with magnesia crystals which is compatible with metalloceramic porcelains. The strength is similar to that of alumina reinforced material, but can be increased by the application of a compressive surface glaze. In Inceram Core Ceramic (Vita Zahnfabrik) a different approach has been adopted to achieve higher strength, liquid glass infiltration of a porous crystal preform. High strength copings of this type are formed by condensing a slurry of fine alumina powder onto a special refractory die. The carved coping is dried slowly and the temperature is gradually raised to 1100“C over a period of 2 h, at which it is maintained for a further 2 h. The shrunken refractory is removed and a shade match glass slurry applied to the surface. Firing at 1100“C for 4 h results in densitication of the alumina preform by the low viscosity glass. The manufacturers claim that this new core material has a flexural strength three times greater than that of conventional core porcelain. Shrink-free
ceramic
coping
The Cerestore System (Johnson and Johnson, East Winsor, NJ, USA) was introduced as a means of improving the tit of all-ceramic restorations. The problems of sintering shrinkage and platinum foil matrix usage are eliminated by transfer moulding a plasticized tablet containing alumina, magnesia and a glass frit onto an epoxy die (Sozio and Riley, 1983). Prolonged firing up to a maximum temperature of 1515°C produces a reaction between the magnesia and alumina. Formation of magnesium aluminate spine1 is accompanied by a volume expansion which exactly compensates for the sintering shrinkage. Several investigators have examined sectioned Cerestore restorations (Chan et al., 1985; Davis, 1988; Scharer et al., 1988; Dickinson et al., 1989) and marginal discrepancies ranging from 1 urn to 313 pm have been measured. A number of processing variables have been shown to influence fitting accuracy (Campbell, 1986; Sato et al., 1986a, b). The strength and fracture toughness of the Cerestore core material has been compared with conventional alumina reinforced porcelain. Josephson et al. (1985) and Oilo (1988) demonstrated significantly higher strengths for Cerestore. However, Philp and Brukl(l984) found no statistically significant differences and Piddock et al. (1987) obtained greater disc strengths for conventional alumina reinforced porcelains. Superior fatigue strength has been calculated for Cerestore (Morena et al.. 1986) but lower fracture toughness measurements have been reported (Kvam, 1989). Further development of the coping material has led to the production of Alceram shrink-free crowns (Innotek
54
J. Dent. 1991;19:
No. 1
Dental Corp., Lakewood, CO, USA). The manufacturers claim that the new core material is 65-75 per cent stronger than Cerestore.
Castable
glass ceramics
The production of tetrasilicic-mica glass ceramic restorations has been described by Adair and Grossman (1984). Molten glass is cast to the required shape and is converted to a partially crystalline state by a prolonged heat treatment. The formation of mica-type crystals is claimed to enhance the strength and toughness of the ceramic, whilst a high degree of translucency is maintained. Shade matching is achieved by the application of tinted glazes, although coloured cements are an alternative. A value of 152 MPa has been quoted for the modulus of rupture of Dicer glass-ceramic (Dentsply International, York PN, USA) (Adair and Grossman, 1984). More recently the flexural strength has been reported as 240 MPa compared with a value of 115 MPa for alumina reinforced porcelain (Oilo, 1988). The fracture toughness of Dicer has been found to be similar to that of feldspathic porcelain with the ceramming treatment producing a small increase in Kit (Jones ,et al., 1988). However, Kvam (1989) has indicated that the fracture toughness of cerammed Dicer is inferior to that of conventional core porcelain. Marginal fit measurements giving discrepancies ranging from 10 urn to 88 urn have been reported (Davis, 1988; Scharer et al., 1988; Dickinson et al., 1989). Techniques similar to those used for the manufacture of Dicer crowns, are utilized in the production of Cera Pearl restorations (Kyocera, Tokyo, Japan). A calcium phosphate-based Cera Pearl glass is cast and then heat treated to induce crystallization of the oxyapatite within the amorphous matrix (Hobo and Iwata, 198Sa). The former is converted to hydroxyapatite when it comes into contact with moisture. Again, colour matching is achieved by superficial glazing. Formation of the crystalline phase is claimed to produce a three-fold increase in tensile strength (Hobo and Iwata, 1985b). Light refractive index, density, hardness, thermal expansion and thermal conductivity have all been found to be similar to natural enamel.
Developments
in bonded foil ciowns
The Renaissance crown, sometimes termed the Cera Platin or Ceplatec crown, uses a metal foil to support the overlying porcelains (Schossow, 1984). The swaged, gold coated foil is made rigid by soldering the folds with the melted outer gold layer. Poor marginal tit has been reported by Scharer et al. (1987) although improved fit can be obtained by modifying the technique. A study of the compressive strength of simulated crowns has indicated that the strength of the Renaissance restoration is inferior to conventionally produced porcelain jacket crowns (Brukl and Ocampo, 1987).
Ion exchange
strengthening
The recent introduction of an ion strengthening paste (Ceramicoat, GC Industrial Corp., Tokyo, Japan) offers the possibility of strengthening dental porcelains without the need for potentially hazardous molten salt baths described previously (Southan, 1970). Improvements in strength of up to 120 per cent have been obtained by immersion in molten potassium nitrate (Dunn et al., 1977). Such findings give some indication of the technique’s promise. Increases in strength of up to 44 per cent have been noted in a preliminary study in which the new paste was used (Piddock and Qualtrough, 1990).However, the degree of improvement is dependent on the composition of the porcelain treated.
DISCUSSION The discussion opened with an enquiry on how fracture stress, as it is measured in practice, relates to the strength of a crown. It is obvious that reproduction of conditions prevailing in viva is fundamental to obtain valid data during in vitro testing. There was general agreement that the crown surface should be replicated on the test-piece, although it was pointed out that this is known to present problems during specimen production and leads to compromises. All too often the environment in which the crown exists is oversimplified, and previously dismissed environmental factors might now be reconsidered. For example, what is the effect of an acidic luting cement? Should we consider the crown, cement and dentine as an entity and test accordingly? Though no one positively supported separation of the ceramic from the other components in the restoration, everyone acknowledged that this is what happens in practice. Leading on from this, a point of view was put. that in many respects tooth enamel has very limited mechanical properties yet survives well, as do thin laminate veneers (for which low fracture forces would be measured in the laboratory). Perhaps therefore the concept of veneering might be extended and a more conservative approach considered as an alternative to seeking higher and higher bulk strengths from ceramics. It is recognized that the quality and success of a ceramic restoration is dependent upon the combined skills of the dentist and the technician. Not surprisingly an opinion should be expressed, that often the measurement of strength of a porcelain is as much a measure of the technical skill used to make the test-piece as any inherent properties of the material. Inevitably, the subsequent discussion on the durability and aesthetics of ‘rival systems proved inconclusive given the differences in clinical preparation, the variable quality of technical skill in addition to any differences between materials. That fairly reflects the current position, for no material nor technique is universally acclaimed as the best. There has been a trend towards greater heterogeneity in dental porcelain containing crystalline dispersions as a
Messer et al.: Strength
consequence of compositional changes made to achieve greater strength. Unfortunately, the strength of ceramics is, in general, limited by the degree of heterogeneity. However, it may well be possible to produce greater strengths in these systems by optimizing dispersion parameters. Alternative approaches to the goal of high strength dental ceramics including the closure of pores by hot isostatic processing (‘hipping’) or CAD-CAM of fully dense monolithic stock have yet to be fully explored. Finally the group was reminded that market forces strongly influence the development and use of any new dental material. A UK dental ceramic market distinct from an arguably more progressive continental European market may be developing due to the domination of a restrictive and cost conscious National Health Service. This is probably not a uniquely British phenomenon. Any (socialized) state welfare scheme or (free enterprise) health insurance company which achieves a dominant market position in any country will inevitably draw up an approved list (whether to limit government expenditure, or improve profitability). This could be viewed as restrictive. Under such circumstances a niche may be created for a material tailored to that particular national market. Although dental materials have become internationalized increasingly, and are seen as such, local conditions still have an influence upon choice and differentiate national markets. Group members were left to reflect on the fact that ultimately it is those who pay for the treatment who will decide on whether the improvements brought about by research can be afforded, and consequently whether particular new ceramic systems are viable.
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Dunn B., Levy M. N. and Reisbick M. H. (1977) Improving the fracture resistance of dental ceramic. J Dent. Res. 56, 1209-1213. Evans A G., Biswas D. R. and Fulrath R. M. (1979) Some effects of cavities on the fracture of ceramics: II. Spherical cavities. J. Am. Ceram. Sot. 62, 101-106. Hobo S. and Iwata T. (1985a) Castable apatite ceramics as a new biocompatible restorative material. II. Fabrication of the restoration. Quintessence Int. 16,207-216. Hobo S. and Iwata T. (1985b) Castable apatite ceramics as a new biocompatible restorative material. I. Theoretical considerations. Quintessence Int. 16, 135-141. Jones D. W., Rizkalla A. S., King H. W. et al. (1988) Fracture toughness and dynamic modulus of tetrasilicic-micaglassceramic. J. Can. Ceram. Sot. 57, 39-46. Josephson B. A., Schulman A., Dunn Z. A. et al. (1985) A compressive strength study of an all-ceramic crown. J. Prosthet. Dent. 53, 301-303. Kvam K. (1989) Fracture toughness determination of ceramic and resin based dental composite. J. Dent. Res. 68, 997. Morena R., Beaudreau G. M., Lockwood P. E. et al. (1986) Fatigue of dental ceramics in a simulated oral environment. J. Dent, Res. 65, 993-997. O’Brien W. J. (1985) Magnesia ceramic jacket crowns. Dent. Clin. North Am. 29, 719-723. Oilo G. (1988) Flexural strength and internal defects of some dental porcelains. Acta Odontol. Stand. 46, 313-322. Philp G. K. and Brukl C. E. (1984) Compressive strengths of conventional, twin foil, and all-ceramic crowns. J. Prosthet. Dent. 52, 215-220. Piddock V. (1989) Evaluation of a new high-strength aluminous porcelain. Clin. Mater. 4, 349-360. Piddock V. and Qualtrough A. J. E. (1990) Ion strengthening of dental porcelains. J Dent. Res. 69, 975. Piddock V., Marquis P. M. and Wilson H. J. (1987) The mechanical strength and microstructure of all-ceramic crowns. J. Dent. 15, 153-158. Rice R. W. (1974) Fracture topography of ceramics. Mater. Sci. Res. 7,439-472. Sato T., Wohlwend A and Scharer P. (1986a) “Shrink-free” ceramic crown system: factors influencing the core margin lit. Quintessence Dent. Technol. 10, 81-87. Sato T., Wohlwend A. and Scharer P. (1986b) Marginal tit in a “shrink-free” ceramic crown system. Int. J. Periodont. Res. Dent. 3, 9-21. Scharer P., Sato T. and Wohlwend A (1987) Marginal fit in the Cera-platin crown system. Quintessence Dent. Technol. 11, 11-25. Scharer P., Sato T. and Wohlwend A. (1988) A comparison of the marginal tit of three cast ceramic crown systems. .J Prosthet. Dent. 59, 534-542. Schossow D. (1984) The Kera-platinum technique porcelain crown: “Renaissance of the jacket crown”. Quintessence Dent. Technol. 8, 231-255. Southan D. E. (1970) Strengthening modern dental porcelain by ion exchange. Aust. Dent. J. 15, 507-510. Sozio R. B. and Riley E. J. (1983) The shrink-free ceramic crown. J. Prosthet. Dent. 49, 182-187.
