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Interpenetrating network ceramic-resin composite dental restorative materials
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Interpenetrating network ceramic-resin composite dental restorative materials M.V. Swain a,∗ , A. Coldea b,1 , A. Bilkhair c , P.C. Guess c a b c
Faculty of Dentistry, Health Science Centre, Kuwait University, Kuwait Vita Zahnfabrik, Bad Säckingen, Germany Prosthetic Department, Faculty of Dentistry, University of Freiburg, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. This paper investigates the structure and some properties of resin infiltrated
Received 17 June 2015
ceramic network structure materials suitable for CAD/CAM dental restorative applications.
Received in revised form
Methods. Initially the basis of interpenetrating network materials is defined along with
5 September 2015
placing them into a materials science perspective. This involves identifying potential advan-
Accepted 9 September 2015
tages of such structures beyond that of the individual materials or simple mixing of the
Available online xxx
components.
Keywords:
are summarized. These include the strength, fracture toughness, hardness and damage
Interpenetrating network material
tolerance, namely to pointed and blunt (spherical) indentation as well as to burr adjustment.
Results. Observations from a number of recently published papers on this class of materials
Resin infiltrated ceramic
In addition a summary of recent results of crowns subjected to simulated clinical conditions
Mechanical properties
using a chewing simulator are presented. These results are rationalized on the basis of existing theoretical considerations. Significance. The currently available ceramic-resin IPN material for clinical application is softer, exhibits comparable strength and fracture toughness but with substantial R-curve behavior, has lower E modulus and is more damage tolerant than existing glass-ceramic materials. Chewing simulation observations with crowns of this material indicate that it appears to be more resistant to sliding/impact induced cracking although its overall contact induced breakage load is modest. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The basis for the selection of dental materials for restorative purposes has been somewhat determined by whether the material is metallic, ceramic or polymer based. The nature of the interatomic bonding forces of each of these classes
∗
1
of materials dictated the mechanical response especially the elastic modulus. Requirements associated with the high contact loading and abrasive conditions on the occlusal surface, especially with posterior teeth, resulted in the development of stiffer and harder ceramic particle filled composites. However with all these classes of materials there was a distinct gap between the properties of the enamel and dentin they
Corresponding author. E-mail address: [email protected] (M.V. Swain). On maternity leave from.
http://dx.doi.org/10.1016/j.dental.2015.09.009 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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were being asked to replace. Interestingly the closest matching systems were gold and amalgam with that of enamel and some cements and highly filled composite resins with dentin. The current advent of greater emphasis on aesthetics has resulted in usage of materials with either much lower (polymer composite) or higher (crystalline ceramics such as zirconia or alumina) elastic properties than enamel. The question now arises as to how can the palette of existing dental materials for CAD/CAM based restorative dentistry be increased? The requirements for such materials not only include aesthetics, biocompatibility and longevity but also precision and rapid reliable millability to relative thin dimensions (<500 m). Such demands, which have become more acute during the past decade, have resulted in a limited number of novel materials and approaches. Included in the former are pre- or partially-cerammed glass-ceramics as well as partially sintered crystalline zirconia and alumina ceramics. These materials generally involve a time consuming heat processing step between milling and clinical utilization. In the past few years two alternate options have appeared on the market place. These include a very heavily particle filled resin cured at higher temperature/pressure and a ceramic based resin interpenetrating network (IPN) material, with the latter the basis of this paper. The former include Lava Ultimate from 3M and CerasmartTM from GC. What are IPN materials? These are multi-phase structures in which the constituent phases are mutually continuous and interconnected [1]. The three-dimensional interconnectivity of IPN materials differs from traditional composites, such as discrete fiber or particle reinforced and laminated composites. As can be appreciated from previous studies the purpose of developing synthetic IPNs is driven, amongst others, by the attempt to enhance or tailor the physical properties of the constituent phases, e.g. fracture toughness [2], fracture strength [3], contact and grinding damage tolerance [4] etc. Each constituent phase within IPNs contributes its own properties resulting in enhanced effective properties of the topologically interconnected microstructure [5]. In contrast with conventional composites in which only the matrix phase is continuous IPNs exhibit some physical properties that are different from and often superior. Feng et al. [6] proposed a unit cell model to estimate the effective properties of IPNs, since for example the elastic modulus, the strength or the fracture toughness of IPNs depend not only on the volume fractions, but also, on the spatial distribution of the constituent phases. According to Feng et al. [6] for IPNs the reinforcing phase is able to distribute stresses more effectively in all directions. The greatest benefit (according to [5]) of network structured materials with interpenetrating phases is in distributing an enhanced resistance to various breakdown phenomena [5]. Such as, a three-dimensional reinforcement phase (e.g. polymer) offers resistance to crack propagation by bridging cracks introduced to the lower strain to failure matrix material (e.g. ceramic). In aligned fibrous composites, in contrast to IPNs, cracks propagating parallel to the fibers cannot be deflected [5]. Therefore, to develop R-curve (resistance curve) behavior, a range of inhomogeneity on more than one scale, as typically occurs in biological materials with their multiple hierarchical structures, is required, which is the case with IPNs. Even though materials with interpenetrating networks are relatively
common for natural biomaterials, for instance bones in mammals and the trunks and limbs of plants, there are currently only few that are synthetically developed [5]. In dentistry a family of IPN all-ceramic systems has existed for more than 2 decades based upon the pioneering research of Sadoun and Vita Zahnfabrik dental company, namely the In-Ceram® ALUMINA group of materials. The latter materials consist of an open porous ceramic structure typically 70% dense consisting of alumina, alumina-zirconia or spinel that is infiltrated with a glass. These materials not only could be fabricated by slip casting (or milling from pre-fabricated porous blocks) to the specific shape but also there was no dimensional change upon subsequent sintering and infiltration with glass. The In-Ceram®ALUMINA system continues to have reliable clinical outcomes for applications as individual crowns in the posterior and short span bridges in the anterior region of the mouth [7]. In this paper the results of a number of recent specific studies of a commercially available material (Enamic, Vita Zahnfabrik) along with some experimental materials will be reported. In addition recent outcomes by Nguyen et al. [8] using very high pressure infiltration of sintered and unsintered ceramic blocks will be considered. Of particular importance with the latter is that sintering of the ceramic particles prior to high pressure infiltration had very limited influence upon the resultant mechanical properties. Specific emphasis will be placed upon the properties of these materials in comparison with a range of existing all-ceramic materials. Of particular importance will be to address, where possible, the issue of tolerance of these various materials to common clinical conditions namely contact and grinding induced damage.
2.
Materials and methods
2.1.
IPN based dental materials
2.1.1.
In-ceram family of all-ceramic materials
The all-ceramic In-ceram group of ceramics, developed by Sadoun more than 25 years ago and commercialized by Vita Zahnfabrik in 1989, were the first specific dental materials to utilize the IPN concept. They also overcame the major obstacle associated with the use of all-ceramic systems namely sintering induced shrinkage of ∼20% by the development of a technique that involved precision net shape forming. This consisted of utilising a combination of coarse and fine alumina particles that could be slip cast to approx. 70% density, which upon sintering to 1000–1200 ◦ C did not shrink despite the formation of necks between the individual particles. This was achieved by the presence of the coarse grains which prevented contraction and resulted in an open fine porous structure throughout the alumina body. Subsequent infiltration of this structure with glass at 950–1000 ◦ C, which completely wetted and under the influence of capillary forces, resulted in near completely dense structures. These materials found application as crowns in both anterior and posterior regions of the mouth and as short span anterior bridges and two unit posterior bridge structures. These materials may be considered the first successful all-ceramic system and reports on 18 year
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2.2. Mechanical property characterization of IPN ceramic-resin materials A comparative assessment of the mechanical properties of IPN based dental materials along with a number of existing conventional dental ceramic systems are listed in Table 1. The majority of this data was reported in the study by Coldea et al. [12]. It has been complimented with additional published data from the recent study by Nguyen et al. [8]. The basis for including the latter is that as mentioned above these authors investigated a similar ceramic framework to that studied by Coldea et al. [12] but also investigated the influence of pre-sintering a slip cast powder as well as conventional press-filtration and drying. Direct comparisons between the strengths and fracture toughness of these materials is not appropriate as the results by Nguyen et al., [8] were obtained using different span dimensions for strength testing and the notched prismless (NPL) technique rather than the single edge V-notched bend (SEVNB) test for toughness.
2.3.
Fig. 1 – Comparison of the microstructure of two IPN based dental systems: In-Ceram® ALUMINA (a), and Enamic resin infiltrated glass-ceramic (b).
clinical performance outcomes are available [9]. A typical example of the microstructure of an In-ceram® ALUMINA material is shown in Fig. 1a.
2.1.2.
Enamic, ceramic based resin infiltrated technology
The concept of resin replacing glass for the infiltration of porous In-ceram structures had a long history before the successful development and commercial release of Enamic in 2012. The major difference between the infiltration of a glass versus a resin was that the curing shrinkages of the resin (∼5% [10]) were much greater than the differential shrinkage upon cooling of the glass-ceramic system («1%). The stresses generated by the substantial shrinkage of the curing resin and the rigid ceramic framework generally resulted in debonding between the resin and framework leading to greater opacity because of the interface gaps that developed. Judicious selection of resin, silanation enhanced bonding between the resin and ceramic and high pressure during the curing phase overcame the problems resulting in a dense aesthetically appealing material. A typical example of the microstructure of Enamic is shown in Fig. 1b. A study by Franco-Steier et al. [11] of resin infiltrated porous alumina ceramic frameworks explored the role of pressure on the strength and such IPN systems. Simultaneously Sadoun and students have investigated a range of modified IPN resin infiltrated ceramic networks [8].
Clinical simulation tests
In an attempt to simulate the response of various materials to the rigors experienced by a molar crown, tests are often conducted with crowns cemented to various teeth and subjected to multiple normal and sliding contact in a chewing simulator [13,14]. In the present study, conducted at the Uniklinik Freiburg, crowns of Enamic, Mark II and emaxCAD were investigated. The CAD/CAM (Cerec software Version 3.80, Cerec In-lab MC XL, Sirona, Bensheim, Germany) fabricated crowns were provided by Vita Zahnfabrik, and were cemented to resin based composite dies (Tetric EvoCeram A2, Ivoclar) with an adhesive cement (Variolink II, Ivoclar-Vivadent). Prior to cementation, all crown specimens (occlusal thickness 2 mm, axial wall thickness 1.5 mm) were etched with hydrofluoric acid (Vita Ceramics etch): Vita Enamic 60 s, IPS e.max CAD 20 s and Vita Mark II 60 s. Specimens were embedded in a selfpolymerizing resin (Technovit 4000, Heraeus Kulzer, Germany) and then stored in distilled water at 37 ◦ C for at least 24 h before loading. Mechanical cycling tests on 14 cemented crowns of each group were subjected to nominal mouth-motion fatigue using a fatigue simulator (Willytec, Germany) with a load of 198 N for 1.2 million cycles at 1.6 Hz. These tests were performed by loading then sliding a 6 mm diameter steatite sphere for 0.5 mm down the mesio-lingual cusp towards the central fossa. The samples were simultaneously exposed to thermal cycling from 5 to 55 ◦ C for 60 s intervals. The specimens were inspected to determine whether cracking or complete fracture occurred. Subsequently the specimens were loaded with a 6 mm diameter steel ball, placed on the cusps, to failure with a universal testing machine (Zwick Germany).
3.
Results and discussion
3.1.
Strength
The strength values of the resin infiltrated ceramic IPN materials lie clearly between that of a typical porcelain (VM9, Vita)
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Table 1 – Mechanical properties of the materials compared in this study. Material
Strength (SD) [MPa]
E modulus (SD) [GPa]
MarkII PICN1(Enamic) PICN2 ICAlumina VM9 YTZP emaxCAD
137.8 (12.4) 144.4 (9.61) 158.5 (7.14) 402.1 (34.54) 121.6 (11.61) 1358.5 (136.54) 344.1 (64.5)
57.2 (3.6) 31.7 (1.4) 26.5 (1.1) 211.8 (13.1) 57.1 (2.5) 184.2 (2.5) 79.7 (4.9)
Hardness (SD) [GPa] 6.24 (0.43) 2.41 (0.08) 1.71 (0.01) 11.76 (0.59) 6.29 (1.24) 13.91 (0.9) 6.02 (0.21)
and glass-ceramic materials including Vita Mark 2 and leucite reinforced systems (Empress 1, Ivoclar) with the lithiumdisilicate glass ceramics (Empress 2, Ivoclar) providing an upper boundary. A feature of the IPN strengths [12] was that the results were exceptionally consistent with a CV of <5% and a Weibull modulus of approximately 20. This was also the situation for one of the glass ceramic materials in question and is consistent with the observations by Quinn et al. [15] for this material. Similar consistent values for the strength were not measured by Nguyen et al. [8] for their experimental IPN materials. In fact they noted greater reliability with the nonsintered porous ceramic. In contrast Franco-Steier et al. [11] observed very high Weibull modulus values for the majority of the specimens they investigated using a bi-axial strength method. The latter authors also noted an increasing strength of their IPN materials with increasing pressure during polymerization and minimal influence of heating rate.
3.2.
Elastic modulus
The elastic modulus values of the IPN materials are much lower than most typical ceramic materials with values lying between enamel and dentin. As pointed out by Wegner and Gibson [16] prediction of the elastic properties for materials with such large differences in E modulus using classic analytical models is not straightforward. In addition a study by Knackstedt et al. [17], which summarises both theoretical, numerical (FEA) and experimental approaches to realistically model these systems relied heavily on high resolution microCT imaging. In the initial study by Coldea et al. [18] for a range of ceramic to resin volume fractions the values were found to fit a modified Halpin-Tsai approach however more recent theoretical and modelling studies have shown that at the volume fraction of porosity under consideration the precise model used is relative insensitive to the nature of the shape and form of the pores and infiltration material [17].
3.3.
Fracture toughness
The fracture toughness values listed in Table 1 show that the SEVNB fracture toughness depended upon the polymer content of the infiltrated ceramic network. The value varied √ between 1 and 1.5 MPa m for the 72 and 64% dense ceramic √ material using the SEVNB approach and 1.4 and 1.9 MPa m with the indentation strength in bending (ISB) approach. The latter values are again much lower than the NPL measured results of Nguyen et al. [8]. This difference may arise because the NPL method measures the toughness following stable
K1c (SEVNB) (SD) √ [MPa m]
K1c (IS) (SD) √ [MPa m]
1 (0.06) 1 (0.04) 1.51 (0.11) 3.73 (0.13) 0.82 (0.06) 4.94 (0.28) 2.37 (0.28)
1.25 (0.14) 1.41 (0.17) 1.89 (0.27) 3.64 (0.45) 0.96 (0.02) 4.97 (0.28) 2.27 (0.16)
Crack size at fracture [m] 13.1 (2.5) 12.1 (1.6) 22.5 (2.2) 21.6 (3.7) 11.3 (2.1) 3.3 (0.7) 13.0 (5.1)
Strain at fracture [%] 0.24 (0.02) 0.45 (0.03) 0.6 (0.03) 0.11 (0.01) 0.21 (0.02) 0.65 (0.06) 0.43 (0.08)
crack extension, in other words if the material has an R-curve then the results may approach the steady state value rather than for crack initiation which the SEVNB approach measures. A further indication that there may well be an R-curve associated with the resin IPN materials is the critical defect size estimated from the strength and fracture values and also listed in Table 1. An image of a crack extending from the corners of a Vickers hardness impression, see for instance Fig. 7 in Coldea et al. [19], shows that while radial cracks develop for the higher ceramic content system the polymer network appears to form bridges across the crack thereby enhancing crack resistance. The extent of the crack closure associated with such polymer bridges will depend upon a number of factors including; the quality of bonding with the ceramic matrix, the residual stress within the polymer, the extent of polymeric conversion of the mono(oligo)mer along with the yield stress and strain to failure of the bridging polymer. The higher pressure confinement of the polymer by Nguyen et al. [8] would have assisted with movement of additional polymer into the depths of the ceramic framework, which along with a hold period at a temperature just below the Tg of the polymer may have relaxed the importance of the residual curing stresses that would typical arise during polymerization. In the study by Franco-Steier et al., [11] these authors only investigated the fracture toughness of the alumina-resin material produced under the highest external pressure and found that √ although the value was relatively modest (2.3 MPa m) with the SEVNB method the material exhibited an extensive R√ curve with crack initiation commencing at 1.5 MPa m and after 2.2 mm of crack extension the crack resistance was √ greater than 4MPa m. It is very likely that similar extensive R-curve behavior exits for the feldspar/resin IPN systems that Coldea et al. [18] and Nguyen et al. [8] investigated. The basis for toughening ceramic materials was succinctly reviewed by Evans [20] where he clearly identified a number of mechanisms primarily associated with crack tip shielding, namely; phase transformation about the crack tip as with zirconia toughened materials, micro-cracking and plastic deformation about the extending crack and crack bridging developing behind the crack tip. The theoretical situation for the fracture toughness of IPN materials is contingent upon a detailed physical appreciation of the mechanisms responsible for the enhanced toughness. An SEM image of the crack tip about an extended but unloaded crack generated with a Vickers indenter (see Fig. 7 in Coldea et al. [19]) clearly shows that cracking predominantly appears to be through the ceramic matrix with additional fracture along the polymer-porcelain interface as well as extended polymer deformation ligaments
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that bridge the crack well behind the crack tip. This was also reported for the alumina-resin IPN system [11]. There have been a number of studies of the mechanics of metal–ceramic composites in the 80s and 90s (see [20–23]). A specific analysis of IPN structures based on the above concepts for metal-metal based systems was developed by Wegner and Gibson [21]. These authors argued that two approaches were appropriate to determine the fracture toughness of such materials, (i) the contribution of the bridging ligaments, and (ii) the contribution from the plastic zone that develops about the crack tip as the crack extends and is important for development of R-curve behavior. Additional mechanisms not considered here, as there appears to be limited evidence from the SEM images of extended cracks, may also include microcracking about the crack tip. Considering the contribution from bridging ligaments the relevant expression for the increased energy for crack extension (or J-Integral approach) [21] is given by
u∗
J = fb
(u) du
(1)
0
where fb is the volume fraction of ligament material, (u) is the stress resistance of the ligament material as a function of bridging displacement u, and u* is the displacement at failure. This reduces to J =
2fb2 ε∗ y dst 3(1 − fb )
(2)
where dst is the mean diameter of the polymer component and ε* is the failure strain. In a critical assessment of the contribution of ductile ligament bridging to the increase in toughness Sun and Yeomans [24] present the following expression for the integration of Eq. (1) J = fb y dst ,
(3)
where is a parameter that is dependent upon the interfacial bonding between the ductile ligaments and ceramic matrix. If the bonding is very strong then this term is low (<1) whereas for modest bonding it may increase to values of 4–5. The latter arises because the effective gauge factor of the bridging ligament is then much greater than the mean ductile ligament diameter. The consequence is that this term can vary from 30 to 200 J/m2 for the IPN materials depending upon the value of . For the plastic zone absorption developed behind the advancing crack tip the additional energy is given by J = y ε∗p h
(4)
where is a constant close to unity, ε∗p is the plastic strain at failure and h is the width of the plastic zone. The latter expression is determined from 1 h= 3
K1c y
2 (5)
where K1c is the critical stress intensity factor and y is the yield stress approximated by Hardness/3.
5
As is generally recognized, obtaining precise values for many of the terms in the above expressions is exceptionally difficult and approximate values are used to gain a sense of the magnitude of the toughening contributions. The fracture toughness of the porous ceramic components amounts to only a few J/m2 with the fully dense ceramic having a J or G value of ∼10 J/m2 and so for a porous material the corresponding toughness would be approximately proportional to the E modulus difference that is G of 2–4 J/m2 for the feldspathic frameworks The implication here is that the toughness of the IPN material is dominated by the toughening contribution from the bridging ligaments and associated plastic deformation zone about the crack tip. The width of the plastic zone h from the values for H and K1c in Table 1 lie in the range from 0.1 to 1 m for the PICN materials in Table 1 to approx. 3.5 m for the IPN systems by Nyugen et al. [8]. The latter are greater because of their higher toughness and lower hardness. The increased energy because of the plastic zones and assuming the yield stress of the resin was 100 MPa for the results of Coldea at al. [18] and 150 MPa for those of Nguyen et al. [8] are then 0.1 J/m2 for PICNM1, 6 J/m2 for PICNM2 and 43 J/m2 for a typical system from Nyugen et al. [8]. The higher yield stress for these materials is considered to arise because of the highly constrained small volume of resin, which was found to have a significant effect for metallic materials [25], and also the higher confining fabrication pressures used by Nguyen et al. [8]. If we now consider the bridging components from Eqs. (2) and (3) above we have major uncertainties regarding the factor . An alternative method to determine the contribution of the bridging ligaments was developed by Lalande et al. [26] based upon the shape of the crack opening displacement. The hardness values of the IPN materials are strongly dependent upon the polymer volume fraction. These range from 2 GPa for the higher 72% ceramic to 1 GPa for the 59% ceramic structure [18]. These values are considerable less than enamel and the lower values are comparable to dentin. In all instances the values are far less than existing ceramic dental restorative materials but greater than existing resin based composite systems.
3.4.
Damage tolerance of IPN ceramic-resin systems
A feature evident in the results reported in Table 1 is that the IPN materials all exhibited a higher strain to failure than most typical ceramic materials with Y-TZP being the possible exception. In addition the extent of the estimated defect size at failure was far greater than the microstructural features of the IPN materials. The typical approximate repeat unit cell or representative volume is only a few microns as is dictated by the size of the typical large particles. For the polymer IPN systems the defect size is far larger by a factor of at least 5 and for the materials from Nyugen et al. [8] approaches 10 because of the R-curve included value of the fracture toughness. In an attempt to assess the damage tolerance of these materials in relation to existing ceramic materials Coldea et al. [18,19] investigated the consequence of indentation with sharp [18] and spherical indenters [19] on the retained strength. While it is recognized that a Vickers diamond indenter is not the typical damage inducing event in the mouth it does appraise the resistance to particle impact or diamond
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Fig. 2 – Vickers indentation load to cause a 25 and 50% reduction in the retained strength of the ceramic material tested. Original results for the plots of retained strength versus indentation load are shown in Fig. 2 of [17].
burr abrasion. Whereas indentation with spheres is perhaps closer to more typical occlusal contact without the sliding aspect that typically occurs. The force-displacement curves with sharp and blunt indenters scale with the E modulus and hardness of the materials (see [19]). The clear difference in the load-deflection curves (see Fig. 5 in [19]) for the softer and lower modulus resin infiltrated ceramic IPN materials is that they display creep deformation during hold period at the maximum load.
Fig. 3 – Critical indentation load for the onset of strength reduction upon indentation with the two spheres for the materials listed in Table 1. The original date is shown in Fig. 4 in [18].
in a residual impression and also less well defined cone cracks. According to Auerbach’s law the indentation load for the onset of cone cracking scales with the critical stress intensity factor and the radius of the sphere (Pc = ˛Kic R) where ˛ is a constant. Again the exceptional materials are the resin infiltrated IPN materials because of their plasticity and also the presence of a substantial R-curve behavior.
3.4.3. 3.4.1.
Pointed indenter damage
The typical response for most of these materials upon indentation with a Vickers pyramid indenter was the development of a plastic impression and with four radial cracks emanating at the corners. The extent of the cracks scaled with the toughness and indentation load and subsequent strength testing with indentation impression in tension resulted in fracture initiating from these cracks. Observations of the strength as a function of Vickers indentation load for the materials listed in Table 1 are shown in Coldea et al. [18]. Rather than reproduce such results here we simply compare the indentations loads to cause a 25 and 50% reduction in the retained flexural strength, Fig. 2. These results clearly show that the two resin infiltrated IPN materials require much greater contact loads to induce such strength reductions. It is also interesting to note that the other IPN material, the all-ceramic In-Ceram® ALUMINA, also shows greater strength reduction tolerance. In this respect Mark II is also interesting as it consists of a mixture of feldspathic crystalline components with a glass matrix, almost an IPN in its own right.
3.4.2.
Spherical or blunt contact damage tolerance
Contact loading with spherical indenters shows that the materials behaved in a more elastic manner in that almost complete recovery occurred upon unloading. At heavier loads cone cracks were seen to develop about the impression with minimal indication of any plastic residual impression formation. Associated with the formation of a cone crack strength degradation occurred. The critical load for the onset of strength reduction is plotted in Fig. 3 for the two indenters used with the different materials. For the IPN materials, more plastic deformation and creep were observed resulting
Milling induced damage
A critical feature for materials suitable for CAD/CAM milling with diamond cutters is their ability to machine rapidly, without chipping and with minimal strength reduction. In a very recent study Coldea et al. [27] investigated the strength degradation of all the materials in Table 1 following a 2 s burr adjustment grinding in a transverse and longitudinal direction. The maximum force applied was 10 N and the 2.3 mm diameter burr cut the specimen dry at 20,000 rpm, which is typical of the motor driven grinding devices used by technicians. A special jig was constructed to maintain the grinding force for the 2 s of grinding, details of which are available in Ref. [27]. The measured 3 point bend strengths for the materials for 3 different burrs; coarse, medium and extrafine are available in the same publication. In Fig. 4 the % reduction in strength is plotted for the three burrs and in the two grinding directions. Of major significance was that the Y-TZP material suffered minimal to no strength degradation and has not been included whereas all the other materials showed a much higher strength reduction when ground in the transverse direction especially with coarser burrs. The results again clearly show that the IPN materials exhibited the least strength reduction. Transverse grinding is anticipated to generate more consequential flaws as the major grinding furrows are normal to the applied tensile stress during subsequent loading in flexure. Additional information that was measured during the milling induced study was the volume of substrate removed during the 2 s milling period for the different materials. There have been various approaches proposed for determining the rate of milling based upon the mechanical properties of the materials and also the mechanisms considered responsible for material removal with the sharp diamond grits of the burrs.
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Fig. 4 – Strength reduction (% of initial strength) following a 2 s adjustment with three different grades of diamond burrs for the materials listed in Table 1. Data from which the current figure is obtained originally published in [26].
Jahanmir et al. [28] reviewed this topic very thoroughly and present a number of equations for the volume of material lost per unit distance of indenter sliding. Considering first that the hardness of the materials governs the material removal then the relationship is simply; V=
a H
materials, one leucite and one lithium disilicate containing, showed greatest antagonist wear while another glass ceramic (Mark II) had far less antagonist wear but more self-wear. Another feature of the two former materials was the extensive fretting related cracking that occurred adjacent to the edge of
(6)
where H is the hardness and a is a constant related to the shape of the cutting tip. The fit of this relationship with the data for the coarse burr is shown in Fig. 5a. If on the other hand chipping about the scratch generated is the major mechanism then the analysis of Evans and Marshall [29] as to the extent of lateral cracking either side of the scratch is the determinant of the volume removed. The expression from [29] is given by V=
g(E/H)4/5 1/2
(Kc H5/8 )
(7)
where g is a constant. The results when compared with this relationship are shown in Fig. 5b. In the case where grain dislodgement is the considered model then Jahanmir et al. [28] propose the following relationship V = y(E4/5 /KH9/5 )
(8)
where y is a constant. The results compared with this relationship are shown in Fig. 5b. In all cases there is reasonably good agreement with the observations.
3.5.
Wear response
The wear behavior of a range of dental restorative materials, including one of the IPN materials, was recently evaluated by Mörmann et al. [30]. These authors investigated not only repetitive contact loading between an enamel antagonist against each material subjected to thermocycling but also simulated tooth brushing induced wear of the materials. They found that polished zirconia showed the least wear in both tests and caused minimal enamel wear. In contrast two glass ceramic
Fig. 5 – Volume of material removed in the 2 s burr adjustment procedure with the coarse bur for the various materials listed in Table 1. In (a) the volume loss is related simply to the inverse of the materials hardness as in Eq. (6), while in (b) the volume is related to the expressions in Eqs. (7) and (8). Reasonable correlation of the volume removed exists with all three expressions.
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Fig. 6 – Optical images of typical cracks that formed on the (a) Mark II and (b) e.max CAD crowns during chewing simulator fatigue tests after 1.2 million loading cycles with a 6 mm diameter steatite sphere under a max load of 200 N.
the contact area. This was less evident in the Mark II material. The presence of fretting induced cracks, which are also a form of damage, would potentially reduce the mechanical strength similar to the indentation results mentioned above. For the IPN material (Enamic) they investigated this showed the least antagonist wear but more self-wear than the Mark II. Again the resultant surface finish the authors observed was very similar to the originally prepared surface. The tooth-brushing wear studies showed minimal differences between the changes of surface gloss for all the glass ceramic materials but the softer IPN (Enamic) material along with the composite resin based materials all showed significant reductions in surface gloss commensurate with their lower hardness values.
3.6.
Clinical simulation response
The outcome of the clinical simulation tests was that none of the IPN (Enamic) crowns failed, while 6 IPS e.max CAD had minor cracking and 12 Vita Mark II restorations revealed significant crack failures resulting in simulated 5 year survival rates of 100%, 57,14%, 14.28%, respectively. Bulk fractures as well as superficial cohesive fractures were not observed. Most of the Vita Mark II crowns (n = 12) that failed did so in the range of 900.000–1.2 M cycles, while all minor failures in the IPS e.max CAD group were only discovered after fatigue testing during inspection with a binocular microscope. Typical examples of the cracking are shown in Fig. 6. These cracks tended to be either radial cracks from the margin edges or partial Hertzian cone cracks within the sliding contact area. Subsequently all samples were loaded to fracture with a ball resting on the fossa area with minimal difference in the strength of the cracked versus uncracked specimens. Further details on the test outcomes will be published in due course. The incidence of cracking during fatigue testing for the two ceramic materials Mark II and emaxCAD appears to scale with the fracture toughness of these materials. Whereas in the case of the Enamic material the lower E modulus and hardness along with the substantial R-Curve behavior appears to have imparted substantially improved resistance to crack initiation and growth. The latter is in agreement with the results shown
in Fig. 3 where the load for the onset of strength degradation was much higher for the IPN (Enamic). As will be reported on in due course the wear volume generated on the Enamic crown was greater than for the other two ceramics.
4.
Conclusions
This paper has focused on the properties of resin infiltrated IPN based CAD/CAM materials suitable for dental restorative purposes and compared them with an array of existing ceramic materials. It was pointed out that these materials have lower hardness and elastic modulus but comparable or higher fracture toughness than many existing porcelain and glass-ceramics currently used for the same purpose. The results indicate that IPN materials have substantial Rcurve behavior which imparts greater tolerance to contact and grinding induced damage. This combination of low hardness and modulus coupled with higher toughness and R-curve behavior enables rapid milling with minimal edge chipping consequences as well as greater ability to cope with severe occlusal contact fatigue loading. Considerable further work is needed to better appreciate the failure criteria for such materials as well as fatigue damage and wear response. It is hoped this paper will stimulate substantive further research on this class of dental materials.
Acknowledgements The authors are grateful to the support of Prof J Strub, Prof J Fischer and Dr N Thiel during the course of this work.
references
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[2] O’Brien DJ, Parquette B. Polymer toughness transfer in a transparent interpenetrating glass-polymer composite. Composites Science and Technology 2012;73:57–63. [3] Chen H, Dong X, Zeng T, Zhou Z, Yang H. The mechanical and electric properties of infiltrated PZT/polymer composites. Ceramics International 2007;33:1369–74. [4] Ashby MF. Hybrids to fill holes in material property space. Philosophical Magazine 2005;85:3235–57. [5] Clarke DR. Interpenetrating phase composites. Journal of the American Ceramic Society 1992;75:739–59. [6] Feng XQ, Mai YW, Qin QH. A micromechanical model for interpenetrating multiphase composites. Computational Materials Science 2003;28:486–93. [7] Selz CF, Strub JR, Vach K, Guess PC. Long-term performance of posterior InCeram Alumina crowns cemented with different luting agents: a prospective, randomized clinical split-mouth study over 5 years. Clinical Oral Investigations 2014;18:1695–703. [8] Nguyen JF, Ruse D, Phan AC, Sadoun MJ. High-temperature-pressure polymerized resin-infiltrated ceramic networks. Journal of Dental Research 2014;93:62–7. [9] Rinke S, Tsigaras A, Huels A, Roediger M. An 18-year retrospective evaluation of glass-infiltrated alumina crowns. Quintessence International 2011;42:625–33. [10] Chen Q, Zhao Y, Wu W, Xu T, Fong H. Fabrication and evaluation of Bis-GMA/TEGDMA dental resins/composites containing halloysite nanotubes. Dental Materials 2012;28(10):1071–9. [11] Franco Steier VF, Koplin C, Kailer A. Influence of pressure-assisted polymerization on the microstructure and strength of polymer infiltrated ceramics. Journal of Materials Science 2013;48:3239–47. [12] Coldea A, Swain MV, Thiel N. Mechanical properties of polymer-infiltrated-ceramic-network materials. Dental Materials 2013;29:419–26. [13] Coelho PG, Silva NR, Bonfante EA, Guess PC, Rekow ED, Thompson VP. Fatigue testing of two porcelain-zirconia all-ceramic crown systems. Dental Materials 2009;25(9):1122–7. [14] Guess PC, Bonfante EA, Silva NR, Coelho PG, Thompson VP. Effect of core design and veneering technique on damage and reliability of Y-TZP-supported crowns. Dental Materials 2013;29:307–16. [15] Quinn GD, Hoffman K, Quinn JB. Strength and fracture origins of a feldspathic porcelain. Dental Materials 2012;28:502–11. [16] Wegner LD, Gibson LJ. The mechanical behavior of interpenetrating phase composites I Modelling. International Journal of Mechanical Sciences 2000;42: 925–42.
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[17] Knackstedt MA, Arns CA, Saadatfar M, Sender TJ, et al. Elastic and transport properties of cellular solids derived from 3D tomographic images. Proceedings of The Royal Society A: Mathematical Physical and Engineering Sciences 2006;462:2833–62. [18] Coldea A, Swain MV, Thiel N. In-vitro strength degradation of dental ceramics and novel PICN material by sharp indentation. Journal of the Mechanical Behavior of Biomedical Materials 2013;26:34–42. [19] Coldea A, Swain MV, Thiel N. Hertzian contact response and damage tolerance of dental ceramics. Journal of the Mechanical Behavior of Biomedical Materials 2014;34:124–33. [20] Evans AG. Perspective on the development of high toughness ceramics. Journal of the American Ceramic Society 1990;73:187–206. [21] Wegner LD, Gibson LJ. The mechanical behavior of interpenetrating phase composites III Fracture toughness. International Journal of Mechanical Sciences 2001;43:1771–91. [22] Wegner LD, Gibson LJ. The mechanical behavior of interpenetrating phase composites II. A case study of a three dimensionally printed material. International Journal of Mechanical Sciences 2000;42:943–64. [23] Wegner LD, Gibson LJ. The mechanical behavior of interpenetrating phase composites III: Resin impregnated porous stainless steel. International Journal of Mechanical Sciences 2001;43:1061–72. [24] Sun X, Yeomans JA. Ductile phase toughened brittle materials. Journal Materials Science and Technology 1996;12:124–34. [25] Bannister M, Ashby MF. The deformation and fracture of constrained metal sheets. Acta Materialia 1991;39:2575–82. [26] Lalande J, Scheppokat S, Janssen R, Claussen N. Toughening of alumina/zirconia ceramic composites with silver particles. Journal of the European Ceramic Society 2002;22(13):2165–71. [27] Coldea A, Fischer J, Swain MV, Thiel N. Damage tolerance of indirect restorative materials (including PICN) after simulated bur adjustments. Dental Materials 2015;31:684–94. [28] Jahanmir S, Xu H, Ives KL. Mechanisms of material removal. In: Jahanmir S, Ramula M, Koshy P, editors. Machining of ceramics and composites. NY: Marcel Decker; 1999. p. 11–83. [29] Evans AG, Marshall DB. Wear mechanisms in ceramics. In: Fundamentals of friction and wear of materials. Edt. Rigney D A (ASM); 1980. p. 439–52. [30] Mörmann W, Stawarczyk B, Ender A, Sener B, Attin T, Mehl A. Wear characteristics of current aesthetic dental restorative CAD/CAM materials: two-body wear, gloss retention, roughness and martens hardness. Journal of the Mechanical Behavior of Biomedical Materials 2013;20:113–25.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Interpenetrating network ceramic-resin composite dental restorative materials M.V. Swain a,∗ , A. Coldea b,1 , A. Bilkhair c , P.C. Guess c a b c
Faculty of Dentistry, Health Science Centre, Kuwait University, Kuwait Vita Zahnfabrik, Bad Säckingen, Germany Prosthetic Department, Faculty of Dentistry, University of Freiburg, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. This paper investigates the structure and some properties of resin infiltrated
Received 17 June 2015
ceramic network structure materials suitable for CAD/CAM dental restorative applications.
Received in revised form
Methods. Initially the basis of interpenetrating network materials is defined along with
5 September 2015
placing them into a materials science perspective. This involves identifying potential advan-
Accepted 9 September 2015
tages of such structures beyond that of the individual materials or simple mixing of the
Available online xxx
components.
Keywords:
are summarized. These include the strength, fracture toughness, hardness and damage
Interpenetrating network material
tolerance, namely to pointed and blunt (spherical) indentation as well as to burr adjustment.
Results. Observations from a number of recently published papers on this class of materials
Resin infiltrated ceramic
In addition a summary of recent results of crowns subjected to simulated clinical conditions
Mechanical properties
using a chewing simulator are presented. These results are rationalized on the basis of existing theoretical considerations. Significance. The currently available ceramic-resin IPN material for clinical application is softer, exhibits comparable strength and fracture toughness but with substantial R-curve behavior, has lower E modulus and is more damage tolerant than existing glass-ceramic materials. Chewing simulation observations with crowns of this material indicate that it appears to be more resistant to sliding/impact induced cracking although its overall contact induced breakage load is modest. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The basis for the selection of dental materials for restorative purposes has been somewhat determined by whether the material is metallic, ceramic or polymer based. The nature of the interatomic bonding forces of each of these classes
∗
1
of materials dictated the mechanical response especially the elastic modulus. Requirements associated with the high contact loading and abrasive conditions on the occlusal surface, especially with posterior teeth, resulted in the development of stiffer and harder ceramic particle filled composites. However with all these classes of materials there was a distinct gap between the properties of the enamel and dentin they
Corresponding author. E-mail address: [email protected] (M.V. Swain). On maternity leave from.
http://dx.doi.org/10.1016/j.dental.2015.09.009 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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were being asked to replace. Interestingly the closest matching systems were gold and amalgam with that of enamel and some cements and highly filled composite resins with dentin. The current advent of greater emphasis on aesthetics has resulted in usage of materials with either much lower (polymer composite) or higher (crystalline ceramics such as zirconia or alumina) elastic properties than enamel. The question now arises as to how can the palette of existing dental materials for CAD/CAM based restorative dentistry be increased? The requirements for such materials not only include aesthetics, biocompatibility and longevity but also precision and rapid reliable millability to relative thin dimensions (<500 m). Such demands, which have become more acute during the past decade, have resulted in a limited number of novel materials and approaches. Included in the former are pre- or partially-cerammed glass-ceramics as well as partially sintered crystalline zirconia and alumina ceramics. These materials generally involve a time consuming heat processing step between milling and clinical utilization. In the past few years two alternate options have appeared on the market place. These include a very heavily particle filled resin cured at higher temperature/pressure and a ceramic based resin interpenetrating network (IPN) material, with the latter the basis of this paper. The former include Lava Ultimate from 3M and CerasmartTM from GC. What are IPN materials? These are multi-phase structures in which the constituent phases are mutually continuous and interconnected [1]. The three-dimensional interconnectivity of IPN materials differs from traditional composites, such as discrete fiber or particle reinforced and laminated composites. As can be appreciated from previous studies the purpose of developing synthetic IPNs is driven, amongst others, by the attempt to enhance or tailor the physical properties of the constituent phases, e.g. fracture toughness [2], fracture strength [3], contact and grinding damage tolerance [4] etc. Each constituent phase within IPNs contributes its own properties resulting in enhanced effective properties of the topologically interconnected microstructure [5]. In contrast with conventional composites in which only the matrix phase is continuous IPNs exhibit some physical properties that are different from and often superior. Feng et al. [6] proposed a unit cell model to estimate the effective properties of IPNs, since for example the elastic modulus, the strength or the fracture toughness of IPNs depend not only on the volume fractions, but also, on the spatial distribution of the constituent phases. According to Feng et al. [6] for IPNs the reinforcing phase is able to distribute stresses more effectively in all directions. The greatest benefit (according to [5]) of network structured materials with interpenetrating phases is in distributing an enhanced resistance to various breakdown phenomena [5]. Such as, a three-dimensional reinforcement phase (e.g. polymer) offers resistance to crack propagation by bridging cracks introduced to the lower strain to failure matrix material (e.g. ceramic). In aligned fibrous composites, in contrast to IPNs, cracks propagating parallel to the fibers cannot be deflected [5]. Therefore, to develop R-curve (resistance curve) behavior, a range of inhomogeneity on more than one scale, as typically occurs in biological materials with their multiple hierarchical structures, is required, which is the case with IPNs. Even though materials with interpenetrating networks are relatively
common for natural biomaterials, for instance bones in mammals and the trunks and limbs of plants, there are currently only few that are synthetically developed [5]. In dentistry a family of IPN all-ceramic systems has existed for more than 2 decades based upon the pioneering research of Sadoun and Vita Zahnfabrik dental company, namely the In-Ceram® ALUMINA group of materials. The latter materials consist of an open porous ceramic structure typically 70% dense consisting of alumina, alumina-zirconia or spinel that is infiltrated with a glass. These materials not only could be fabricated by slip casting (or milling from pre-fabricated porous blocks) to the specific shape but also there was no dimensional change upon subsequent sintering and infiltration with glass. The In-Ceram®ALUMINA system continues to have reliable clinical outcomes for applications as individual crowns in the posterior and short span bridges in the anterior region of the mouth [7]. In this paper the results of a number of recent specific studies of a commercially available material (Enamic, Vita Zahnfabrik) along with some experimental materials will be reported. In addition recent outcomes by Nguyen et al. [8] using very high pressure infiltration of sintered and unsintered ceramic blocks will be considered. Of particular importance with the latter is that sintering of the ceramic particles prior to high pressure infiltration had very limited influence upon the resultant mechanical properties. Specific emphasis will be placed upon the properties of these materials in comparison with a range of existing all-ceramic materials. Of particular importance will be to address, where possible, the issue of tolerance of these various materials to common clinical conditions namely contact and grinding induced damage.
2.
Materials and methods
2.1.
IPN based dental materials
2.1.1.
In-ceram family of all-ceramic materials
The all-ceramic In-ceram group of ceramics, developed by Sadoun more than 25 years ago and commercialized by Vita Zahnfabrik in 1989, were the first specific dental materials to utilize the IPN concept. They also overcame the major obstacle associated with the use of all-ceramic systems namely sintering induced shrinkage of ∼20% by the development of a technique that involved precision net shape forming. This consisted of utilising a combination of coarse and fine alumina particles that could be slip cast to approx. 70% density, which upon sintering to 1000–1200 ◦ C did not shrink despite the formation of necks between the individual particles. This was achieved by the presence of the coarse grains which prevented contraction and resulted in an open fine porous structure throughout the alumina body. Subsequent infiltration of this structure with glass at 950–1000 ◦ C, which completely wetted and under the influence of capillary forces, resulted in near completely dense structures. These materials found application as crowns in both anterior and posterior regions of the mouth and as short span anterior bridges and two unit posterior bridge structures. These materials may be considered the first successful all-ceramic system and reports on 18 year
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2.2. Mechanical property characterization of IPN ceramic-resin materials A comparative assessment of the mechanical properties of IPN based dental materials along with a number of existing conventional dental ceramic systems are listed in Table 1. The majority of this data was reported in the study by Coldea et al. [12]. It has been complimented with additional published data from the recent study by Nguyen et al. [8]. The basis for including the latter is that as mentioned above these authors investigated a similar ceramic framework to that studied by Coldea et al. [12] but also investigated the influence of pre-sintering a slip cast powder as well as conventional press-filtration and drying. Direct comparisons between the strengths and fracture toughness of these materials is not appropriate as the results by Nguyen et al., [8] were obtained using different span dimensions for strength testing and the notched prismless (NPL) technique rather than the single edge V-notched bend (SEVNB) test for toughness.
2.3.
Fig. 1 – Comparison of the microstructure of two IPN based dental systems: In-Ceram® ALUMINA (a), and Enamic resin infiltrated glass-ceramic (b).
clinical performance outcomes are available [9]. A typical example of the microstructure of an In-ceram® ALUMINA material is shown in Fig. 1a.
2.1.2.
Enamic, ceramic based resin infiltrated technology
The concept of resin replacing glass for the infiltration of porous In-ceram structures had a long history before the successful development and commercial release of Enamic in 2012. The major difference between the infiltration of a glass versus a resin was that the curing shrinkages of the resin (∼5% [10]) were much greater than the differential shrinkage upon cooling of the glass-ceramic system («1%). The stresses generated by the substantial shrinkage of the curing resin and the rigid ceramic framework generally resulted in debonding between the resin and framework leading to greater opacity because of the interface gaps that developed. Judicious selection of resin, silanation enhanced bonding between the resin and ceramic and high pressure during the curing phase overcame the problems resulting in a dense aesthetically appealing material. A typical example of the microstructure of Enamic is shown in Fig. 1b. A study by Franco-Steier et al. [11] of resin infiltrated porous alumina ceramic frameworks explored the role of pressure on the strength and such IPN systems. Simultaneously Sadoun and students have investigated a range of modified IPN resin infiltrated ceramic networks [8].
Clinical simulation tests
In an attempt to simulate the response of various materials to the rigors experienced by a molar crown, tests are often conducted with crowns cemented to various teeth and subjected to multiple normal and sliding contact in a chewing simulator [13,14]. In the present study, conducted at the Uniklinik Freiburg, crowns of Enamic, Mark II and emaxCAD were investigated. The CAD/CAM (Cerec software Version 3.80, Cerec In-lab MC XL, Sirona, Bensheim, Germany) fabricated crowns were provided by Vita Zahnfabrik, and were cemented to resin based composite dies (Tetric EvoCeram A2, Ivoclar) with an adhesive cement (Variolink II, Ivoclar-Vivadent). Prior to cementation, all crown specimens (occlusal thickness 2 mm, axial wall thickness 1.5 mm) were etched with hydrofluoric acid (Vita Ceramics etch): Vita Enamic 60 s, IPS e.max CAD 20 s and Vita Mark II 60 s. Specimens were embedded in a selfpolymerizing resin (Technovit 4000, Heraeus Kulzer, Germany) and then stored in distilled water at 37 ◦ C for at least 24 h before loading. Mechanical cycling tests on 14 cemented crowns of each group were subjected to nominal mouth-motion fatigue using a fatigue simulator (Willytec, Germany) with a load of 198 N for 1.2 million cycles at 1.6 Hz. These tests were performed by loading then sliding a 6 mm diameter steatite sphere for 0.5 mm down the mesio-lingual cusp towards the central fossa. The samples were simultaneously exposed to thermal cycling from 5 to 55 ◦ C for 60 s intervals. The specimens were inspected to determine whether cracking or complete fracture occurred. Subsequently the specimens were loaded with a 6 mm diameter steel ball, placed on the cusps, to failure with a universal testing machine (Zwick Germany).
3.
Results and discussion
3.1.
Strength
The strength values of the resin infiltrated ceramic IPN materials lie clearly between that of a typical porcelain (VM9, Vita)
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Table 1 – Mechanical properties of the materials compared in this study. Material
Strength (SD) [MPa]
E modulus (SD) [GPa]
MarkII PICN1(Enamic) PICN2 ICAlumina VM9 YTZP emaxCAD
137.8 (12.4) 144.4 (9.61) 158.5 (7.14) 402.1 (34.54) 121.6 (11.61) 1358.5 (136.54) 344.1 (64.5)
57.2 (3.6) 31.7 (1.4) 26.5 (1.1) 211.8 (13.1) 57.1 (2.5) 184.2 (2.5) 79.7 (4.9)
Hardness (SD) [GPa] 6.24 (0.43) 2.41 (0.08) 1.71 (0.01) 11.76 (0.59) 6.29 (1.24) 13.91 (0.9) 6.02 (0.21)
and glass-ceramic materials including Vita Mark 2 and leucite reinforced systems (Empress 1, Ivoclar) with the lithiumdisilicate glass ceramics (Empress 2, Ivoclar) providing an upper boundary. A feature of the IPN strengths [12] was that the results were exceptionally consistent with a CV of <5% and a Weibull modulus of approximately 20. This was also the situation for one of the glass ceramic materials in question and is consistent with the observations by Quinn et al. [15] for this material. Similar consistent values for the strength were not measured by Nguyen et al. [8] for their experimental IPN materials. In fact they noted greater reliability with the nonsintered porous ceramic. In contrast Franco-Steier et al. [11] observed very high Weibull modulus values for the majority of the specimens they investigated using a bi-axial strength method. The latter authors also noted an increasing strength of their IPN materials with increasing pressure during polymerization and minimal influence of heating rate.
3.2.
Elastic modulus
The elastic modulus values of the IPN materials are much lower than most typical ceramic materials with values lying between enamel and dentin. As pointed out by Wegner and Gibson [16] prediction of the elastic properties for materials with such large differences in E modulus using classic analytical models is not straightforward. In addition a study by Knackstedt et al. [17], which summarises both theoretical, numerical (FEA) and experimental approaches to realistically model these systems relied heavily on high resolution microCT imaging. In the initial study by Coldea et al. [18] for a range of ceramic to resin volume fractions the values were found to fit a modified Halpin-Tsai approach however more recent theoretical and modelling studies have shown that at the volume fraction of porosity under consideration the precise model used is relative insensitive to the nature of the shape and form of the pores and infiltration material [17].
3.3.
Fracture toughness
The fracture toughness values listed in Table 1 show that the SEVNB fracture toughness depended upon the polymer content of the infiltrated ceramic network. The value varied √ between 1 and 1.5 MPa m for the 72 and 64% dense ceramic √ material using the SEVNB approach and 1.4 and 1.9 MPa m with the indentation strength in bending (ISB) approach. The latter values are again much lower than the NPL measured results of Nguyen et al. [8]. This difference may arise because the NPL method measures the toughness following stable
K1c (SEVNB) (SD) √ [MPa m]
K1c (IS) (SD) √ [MPa m]
1 (0.06) 1 (0.04) 1.51 (0.11) 3.73 (0.13) 0.82 (0.06) 4.94 (0.28) 2.37 (0.28)
1.25 (0.14) 1.41 (0.17) 1.89 (0.27) 3.64 (0.45) 0.96 (0.02) 4.97 (0.28) 2.27 (0.16)
Crack size at fracture [m] 13.1 (2.5) 12.1 (1.6) 22.5 (2.2) 21.6 (3.7) 11.3 (2.1) 3.3 (0.7) 13.0 (5.1)
Strain at fracture [%] 0.24 (0.02) 0.45 (0.03) 0.6 (0.03) 0.11 (0.01) 0.21 (0.02) 0.65 (0.06) 0.43 (0.08)
crack extension, in other words if the material has an R-curve then the results may approach the steady state value rather than for crack initiation which the SEVNB approach measures. A further indication that there may well be an R-curve associated with the resin IPN materials is the critical defect size estimated from the strength and fracture values and also listed in Table 1. An image of a crack extending from the corners of a Vickers hardness impression, see for instance Fig. 7 in Coldea et al. [19], shows that while radial cracks develop for the higher ceramic content system the polymer network appears to form bridges across the crack thereby enhancing crack resistance. The extent of the crack closure associated with such polymer bridges will depend upon a number of factors including; the quality of bonding with the ceramic matrix, the residual stress within the polymer, the extent of polymeric conversion of the mono(oligo)mer along with the yield stress and strain to failure of the bridging polymer. The higher pressure confinement of the polymer by Nguyen et al. [8] would have assisted with movement of additional polymer into the depths of the ceramic framework, which along with a hold period at a temperature just below the Tg of the polymer may have relaxed the importance of the residual curing stresses that would typical arise during polymerization. In the study by Franco-Steier et al., [11] these authors only investigated the fracture toughness of the alumina-resin material produced under the highest external pressure and found that √ although the value was relatively modest (2.3 MPa m) with the SEVNB method the material exhibited an extensive R√ curve with crack initiation commencing at 1.5 MPa m and after 2.2 mm of crack extension the crack resistance was √ greater than 4MPa m. It is very likely that similar extensive R-curve behavior exits for the feldspar/resin IPN systems that Coldea et al. [18] and Nguyen et al. [8] investigated. The basis for toughening ceramic materials was succinctly reviewed by Evans [20] where he clearly identified a number of mechanisms primarily associated with crack tip shielding, namely; phase transformation about the crack tip as with zirconia toughened materials, micro-cracking and plastic deformation about the extending crack and crack bridging developing behind the crack tip. The theoretical situation for the fracture toughness of IPN materials is contingent upon a detailed physical appreciation of the mechanisms responsible for the enhanced toughness. An SEM image of the crack tip about an extended but unloaded crack generated with a Vickers indenter (see Fig. 7 in Coldea et al. [19]) clearly shows that cracking predominantly appears to be through the ceramic matrix with additional fracture along the polymer-porcelain interface as well as extended polymer deformation ligaments
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that bridge the crack well behind the crack tip. This was also reported for the alumina-resin IPN system [11]. There have been a number of studies of the mechanics of metal–ceramic composites in the 80s and 90s (see [20–23]). A specific analysis of IPN structures based on the above concepts for metal-metal based systems was developed by Wegner and Gibson [21]. These authors argued that two approaches were appropriate to determine the fracture toughness of such materials, (i) the contribution of the bridging ligaments, and (ii) the contribution from the plastic zone that develops about the crack tip as the crack extends and is important for development of R-curve behavior. Additional mechanisms not considered here, as there appears to be limited evidence from the SEM images of extended cracks, may also include microcracking about the crack tip. Considering the contribution from bridging ligaments the relevant expression for the increased energy for crack extension (or J-Integral approach) [21] is given by
u∗
J = fb
(u) du
(1)
0
where fb is the volume fraction of ligament material, (u) is the stress resistance of the ligament material as a function of bridging displacement u, and u* is the displacement at failure. This reduces to J =
2fb2 ε∗ y dst 3(1 − fb )
(2)
where dst is the mean diameter of the polymer component and ε* is the failure strain. In a critical assessment of the contribution of ductile ligament bridging to the increase in toughness Sun and Yeomans [24] present the following expression for the integration of Eq. (1) J = fb y dst ,
(3)
where is a parameter that is dependent upon the interfacial bonding between the ductile ligaments and ceramic matrix. If the bonding is very strong then this term is low (<1) whereas for modest bonding it may increase to values of 4–5. The latter arises because the effective gauge factor of the bridging ligament is then much greater than the mean ductile ligament diameter. The consequence is that this term can vary from 30 to 200 J/m2 for the IPN materials depending upon the value of . For the plastic zone absorption developed behind the advancing crack tip the additional energy is given by J = y ε∗p h
(4)
where is a constant close to unity, ε∗p is the plastic strain at failure and h is the width of the plastic zone. The latter expression is determined from 1 h= 3
K1c y
2 (5)
where K1c is the critical stress intensity factor and y is the yield stress approximated by Hardness/3.
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As is generally recognized, obtaining precise values for many of the terms in the above expressions is exceptionally difficult and approximate values are used to gain a sense of the magnitude of the toughening contributions. The fracture toughness of the porous ceramic components amounts to only a few J/m2 with the fully dense ceramic having a J or G value of ∼10 J/m2 and so for a porous material the corresponding toughness would be approximately proportional to the E modulus difference that is G of 2–4 J/m2 for the feldspathic frameworks The implication here is that the toughness of the IPN material is dominated by the toughening contribution from the bridging ligaments and associated plastic deformation zone about the crack tip. The width of the plastic zone h from the values for H and K1c in Table 1 lie in the range from 0.1 to 1 m for the PICN materials in Table 1 to approx. 3.5 m for the IPN systems by Nyugen et al. [8]. The latter are greater because of their higher toughness and lower hardness. The increased energy because of the plastic zones and assuming the yield stress of the resin was 100 MPa for the results of Coldea at al. [18] and 150 MPa for those of Nguyen et al. [8] are then 0.1 J/m2 for PICNM1, 6 J/m2 for PICNM2 and 43 J/m2 for a typical system from Nyugen et al. [8]. The higher yield stress for these materials is considered to arise because of the highly constrained small volume of resin, which was found to have a significant effect for metallic materials [25], and also the higher confining fabrication pressures used by Nguyen et al. [8]. If we now consider the bridging components from Eqs. (2) and (3) above we have major uncertainties regarding the factor . An alternative method to determine the contribution of the bridging ligaments was developed by Lalande et al. [26] based upon the shape of the crack opening displacement. The hardness values of the IPN materials are strongly dependent upon the polymer volume fraction. These range from 2 GPa for the higher 72% ceramic to 1 GPa for the 59% ceramic structure [18]. These values are considerable less than enamel and the lower values are comparable to dentin. In all instances the values are far less than existing ceramic dental restorative materials but greater than existing resin based composite systems.
3.4.
Damage tolerance of IPN ceramic-resin systems
A feature evident in the results reported in Table 1 is that the IPN materials all exhibited a higher strain to failure than most typical ceramic materials with Y-TZP being the possible exception. In addition the extent of the estimated defect size at failure was far greater than the microstructural features of the IPN materials. The typical approximate repeat unit cell or representative volume is only a few microns as is dictated by the size of the typical large particles. For the polymer IPN systems the defect size is far larger by a factor of at least 5 and for the materials from Nyugen et al. [8] approaches 10 because of the R-curve included value of the fracture toughness. In an attempt to assess the damage tolerance of these materials in relation to existing ceramic materials Coldea et al. [18,19] investigated the consequence of indentation with sharp [18] and spherical indenters [19] on the retained strength. While it is recognized that a Vickers diamond indenter is not the typical damage inducing event in the mouth it does appraise the resistance to particle impact or diamond
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Fig. 2 – Vickers indentation load to cause a 25 and 50% reduction in the retained strength of the ceramic material tested. Original results for the plots of retained strength versus indentation load are shown in Fig. 2 of [17].
burr abrasion. Whereas indentation with spheres is perhaps closer to more typical occlusal contact without the sliding aspect that typically occurs. The force-displacement curves with sharp and blunt indenters scale with the E modulus and hardness of the materials (see [19]). The clear difference in the load-deflection curves (see Fig. 5 in [19]) for the softer and lower modulus resin infiltrated ceramic IPN materials is that they display creep deformation during hold period at the maximum load.
Fig. 3 – Critical indentation load for the onset of strength reduction upon indentation with the two spheres for the materials listed in Table 1. The original date is shown in Fig. 4 in [18].
in a residual impression and also less well defined cone cracks. According to Auerbach’s law the indentation load for the onset of cone cracking scales with the critical stress intensity factor and the radius of the sphere (Pc = ˛Kic R) where ˛ is a constant. Again the exceptional materials are the resin infiltrated IPN materials because of their plasticity and also the presence of a substantial R-curve behavior.
3.4.3. 3.4.1.
Pointed indenter damage
The typical response for most of these materials upon indentation with a Vickers pyramid indenter was the development of a plastic impression and with four radial cracks emanating at the corners. The extent of the cracks scaled with the toughness and indentation load and subsequent strength testing with indentation impression in tension resulted in fracture initiating from these cracks. Observations of the strength as a function of Vickers indentation load for the materials listed in Table 1 are shown in Coldea et al. [18]. Rather than reproduce such results here we simply compare the indentations loads to cause a 25 and 50% reduction in the retained flexural strength, Fig. 2. These results clearly show that the two resin infiltrated IPN materials require much greater contact loads to induce such strength reductions. It is also interesting to note that the other IPN material, the all-ceramic In-Ceram® ALUMINA, also shows greater strength reduction tolerance. In this respect Mark II is also interesting as it consists of a mixture of feldspathic crystalline components with a glass matrix, almost an IPN in its own right.
3.4.2.
Spherical or blunt contact damage tolerance
Contact loading with spherical indenters shows that the materials behaved in a more elastic manner in that almost complete recovery occurred upon unloading. At heavier loads cone cracks were seen to develop about the impression with minimal indication of any plastic residual impression formation. Associated with the formation of a cone crack strength degradation occurred. The critical load for the onset of strength reduction is plotted in Fig. 3 for the two indenters used with the different materials. For the IPN materials, more plastic deformation and creep were observed resulting
Milling induced damage
A critical feature for materials suitable for CAD/CAM milling with diamond cutters is their ability to machine rapidly, without chipping and with minimal strength reduction. In a very recent study Coldea et al. [27] investigated the strength degradation of all the materials in Table 1 following a 2 s burr adjustment grinding in a transverse and longitudinal direction. The maximum force applied was 10 N and the 2.3 mm diameter burr cut the specimen dry at 20,000 rpm, which is typical of the motor driven grinding devices used by technicians. A special jig was constructed to maintain the grinding force for the 2 s of grinding, details of which are available in Ref. [27]. The measured 3 point bend strengths for the materials for 3 different burrs; coarse, medium and extrafine are available in the same publication. In Fig. 4 the % reduction in strength is plotted for the three burrs and in the two grinding directions. Of major significance was that the Y-TZP material suffered minimal to no strength degradation and has not been included whereas all the other materials showed a much higher strength reduction when ground in the transverse direction especially with coarser burrs. The results again clearly show that the IPN materials exhibited the least strength reduction. Transverse grinding is anticipated to generate more consequential flaws as the major grinding furrows are normal to the applied tensile stress during subsequent loading in flexure. Additional information that was measured during the milling induced study was the volume of substrate removed during the 2 s milling period for the different materials. There have been various approaches proposed for determining the rate of milling based upon the mechanical properties of the materials and also the mechanisms considered responsible for material removal with the sharp diamond grits of the burrs.
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Fig. 4 – Strength reduction (% of initial strength) following a 2 s adjustment with three different grades of diamond burrs for the materials listed in Table 1. Data from which the current figure is obtained originally published in [26].
Jahanmir et al. [28] reviewed this topic very thoroughly and present a number of equations for the volume of material lost per unit distance of indenter sliding. Considering first that the hardness of the materials governs the material removal then the relationship is simply; V=
a H
materials, one leucite and one lithium disilicate containing, showed greatest antagonist wear while another glass ceramic (Mark II) had far less antagonist wear but more self-wear. Another feature of the two former materials was the extensive fretting related cracking that occurred adjacent to the edge of
(6)
where H is the hardness and a is a constant related to the shape of the cutting tip. The fit of this relationship with the data for the coarse burr is shown in Fig. 5a. If on the other hand chipping about the scratch generated is the major mechanism then the analysis of Evans and Marshall [29] as to the extent of lateral cracking either side of the scratch is the determinant of the volume removed. The expression from [29] is given by V=
g(E/H)4/5 1/2
(Kc H5/8 )
(7)
where g is a constant. The results when compared with this relationship are shown in Fig. 5b. In the case where grain dislodgement is the considered model then Jahanmir et al. [28] propose the following relationship V = y(E4/5 /KH9/5 )
(8)
where y is a constant. The results compared with this relationship are shown in Fig. 5b. In all cases there is reasonably good agreement with the observations.
3.5.
Wear response
The wear behavior of a range of dental restorative materials, including one of the IPN materials, was recently evaluated by Mörmann et al. [30]. These authors investigated not only repetitive contact loading between an enamel antagonist against each material subjected to thermocycling but also simulated tooth brushing induced wear of the materials. They found that polished zirconia showed the least wear in both tests and caused minimal enamel wear. In contrast two glass ceramic
Fig. 5 – Volume of material removed in the 2 s burr adjustment procedure with the coarse bur for the various materials listed in Table 1. In (a) the volume loss is related simply to the inverse of the materials hardness as in Eq. (6), while in (b) the volume is related to the expressions in Eqs. (7) and (8). Reasonable correlation of the volume removed exists with all three expressions.
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Fig. 6 – Optical images of typical cracks that formed on the (a) Mark II and (b) e.max CAD crowns during chewing simulator fatigue tests after 1.2 million loading cycles with a 6 mm diameter steatite sphere under a max load of 200 N.
the contact area. This was less evident in the Mark II material. The presence of fretting induced cracks, which are also a form of damage, would potentially reduce the mechanical strength similar to the indentation results mentioned above. For the IPN material (Enamic) they investigated this showed the least antagonist wear but more self-wear than the Mark II. Again the resultant surface finish the authors observed was very similar to the originally prepared surface. The tooth-brushing wear studies showed minimal differences between the changes of surface gloss for all the glass ceramic materials but the softer IPN (Enamic) material along with the composite resin based materials all showed significant reductions in surface gloss commensurate with their lower hardness values.
3.6.
Clinical simulation response
The outcome of the clinical simulation tests was that none of the IPN (Enamic) crowns failed, while 6 IPS e.max CAD had minor cracking and 12 Vita Mark II restorations revealed significant crack failures resulting in simulated 5 year survival rates of 100%, 57,14%, 14.28%, respectively. Bulk fractures as well as superficial cohesive fractures were not observed. Most of the Vita Mark II crowns (n = 12) that failed did so in the range of 900.000–1.2 M cycles, while all minor failures in the IPS e.max CAD group were only discovered after fatigue testing during inspection with a binocular microscope. Typical examples of the cracking are shown in Fig. 6. These cracks tended to be either radial cracks from the margin edges or partial Hertzian cone cracks within the sliding contact area. Subsequently all samples were loaded to fracture with a ball resting on the fossa area with minimal difference in the strength of the cracked versus uncracked specimens. Further details on the test outcomes will be published in due course. The incidence of cracking during fatigue testing for the two ceramic materials Mark II and emaxCAD appears to scale with the fracture toughness of these materials. Whereas in the case of the Enamic material the lower E modulus and hardness along with the substantial R-Curve behavior appears to have imparted substantially improved resistance to crack initiation and growth. The latter is in agreement with the results shown
in Fig. 3 where the load for the onset of strength degradation was much higher for the IPN (Enamic). As will be reported on in due course the wear volume generated on the Enamic crown was greater than for the other two ceramics.
4.
Conclusions
This paper has focused on the properties of resin infiltrated IPN based CAD/CAM materials suitable for dental restorative purposes and compared them with an array of existing ceramic materials. It was pointed out that these materials have lower hardness and elastic modulus but comparable or higher fracture toughness than many existing porcelain and glass-ceramics currently used for the same purpose. The results indicate that IPN materials have substantial Rcurve behavior which imparts greater tolerance to contact and grinding induced damage. This combination of low hardness and modulus coupled with higher toughness and R-curve behavior enables rapid milling with minimal edge chipping consequences as well as greater ability to cope with severe occlusal contact fatigue loading. Considerable further work is needed to better appreciate the failure criteria for such materials as well as fatigue damage and wear response. It is hoped this paper will stimulate substantive further research on this class of dental materials.
Acknowledgements The authors are grateful to the support of Prof J Strub, Prof J Fischer and Dr N Thiel during the course of this work.
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Please cite this article in press as: Swain MV, et al. Interpenetrating network ceramic-resin composite dental restorative materials. Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.09.009