Tribological behavior of alumina-added apatite–wollastonite glass–ceramics in simulated body fluid

Tribological behavior of alumina-added apatite–wollastonite glass–ceramics in simulated body fluid

Materials Chemistry and Physics 124 (2010) 113–119 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.e...

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Materials Chemistry and Physics 124 (2010) 113–119

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Tribological behavior of alumina-added apatite–wollastonite glass–ceramics in simulated body fluid Jongee Park a,∗ , Sang-Hee You b , Dong-Woo Shin b , Abdullah Ozturk c a b c

Department of Materials Engineering, Atilim University, Incek Golbasi, Ankara 06836, Turkey Division of Advanced Materials Science and Engineering, Engineering Research Institute, Gyeongsang National University, Jinju, Gyeongnam 660-701, South Korea Department of Metallurgical and Materials Engineering, Middle East Technical University, Ankara 06531, Turkey

a r t i c l e

i n f o

Article history: Received 22 January 2010 Received in revised form 21 May 2010 Accepted 2 June 2010 Keywords: Dental ceramics Sintering Wear Simulated body fluid Tribological properties

a b s t r a c t Tribological properties of an alumina-added apatite–wollastonite glass–ceramic produced by controlled heat treatment of a glass in the system MgO–CaO–SiO2 –P2 O5 –Al2 O3 have been evaluated and compared with those of selected commercial dental ceramics, Duceragold and IPS Empress. Tribological tests were performed in dry condition and in simulated body fluid (SBF) using a pin-on-disk apparatus. The friction coefficient and specific wear rate of the tested materials were measured in dry and in artificial saliva (simulated body fluid: SBF) in order to elucidate the appropriateness of the alumina-added apatite–wollastonite (A–W) glass–ceramic for dental applications. Wear rate of the materials investigated varied from 0.96 × 10−4 mm3 N−1 m to 41.37 × 10−4 mm3 N−1 m depending on the bioenvironmental test conditions. The results of this study revealed that the alumina-added A–W glass–ceramic becomes more wear resistant as sintering temperature is increased and exhibits tribological properties similar to those of the commercial dental materials investigated. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Glass–ceramics containing apatite and wollastonite crystals (A–W glass–ceramics) have received great importance as biomaterials since their discovery in 1982 [1], especially in the repair and replacement of natural bone. Interest in these materials was developed due to their improved biological and mechanical properties [2–4]. A–W glass–ceramics bond to living bone in a short period, and maintain high mechanical properties such as toughness and strength for a long period in a body environment [3–6]. Because of these desirable properties A–W glass–ceramics have found special applications in clinic, either in powder form as bone filler or as bulk material for fabricating iliac crest prostheses, artificial vertebrae, inter-vertebra discs, spinous process spacers, etc. Park and Ozturk [7,8] recognized the potential of A–W glass–ceramics for use in dental materials, in view of the fact that A–W glass–ceramics possess similar properties to commercial dental materials currently used in crown and bridge work. However, A–W glass–ceramic releases calcium ions when in contact with living tissues and supersaturates the surrounding fluid with calcium ions which triggers a precipitation of calcium and phosphate as apatite on the glass–ceramic surface [5,6]. This effect is not desirable for materials to be used in dental restorations. Several investigations [3,9,10] have showed

∗ Corresponding author. Tel.: +90 312 586 8786; fax: +90 312 586 8091. E-mail address: [email protected] (J. Park). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.06.001

that non-bioactive high strength A–W glass–ceramics could be produced, without changing the crystalline phases, by adding small amount of Al2 O3 to the composition of the base glass. Ohtsuki et al. [9] reported that CaO–SiO2 glasses containing Al2 O3 do not form apatite crystals on their surfaces in simulated body fluid even after 30 days. The addition of Al2 O3 improves the chemical durability of the glass–ceramic and consequently decreases the rate of dissolution of it in aqueous condition [11]. Determination of the tribological properties of alumina-added non-bioactive A–W glass–ceramics is of great importance since the understanding of tribological performance of this material could extend its utilization in special dental applications such as veneers, inlays, onlays and crowns. Several studies [12–14,15] have concluded that wear could be controlled by altering the microstructure of the glass–ceramics. A clear understanding of the tribological behavior of A–W glass–ceramics is necessary for correlating the tribological properties with structural characteristics of this material system. This publication demonstrates that alumina-added A–W glass–ceramic could be produced with the tribological properties similar to commercially available dental porcelain materials. Hence, studies carried out on the tribological behavior of aluminaadded non-bioactive A–W glass–ceramics have both scientific and technological significances. Duceragold® (Degussa Dental GmbH, Germany) and IPS Empress® (Ivoclar Vivadent, Liechtenstein) are commercially available dental materials. Duceragold® is leucite-reinforced feldspathic ceramic with Young’s modulus of 69 GPa while IPS Empress®

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is leucite-based pressable dental core ceramic with the elastic modulus of 65 GPa. They have similar fracture toughness of 1.1–1.2 MPa m−1/2 [16–18]. Both restoration materials have showed good clinical success without occurring secondary caries or high biologic risks [18–20]. The purpose of the present study was to produce a biologically inert A–W glass–ceramic by sintering glass powder compacts in the MgO–CaO–SiO2 –P2 O5 –Al2 O3 system at different sintering temperatures, and to determine its tribological properties in dry and artificial saliva conditions, in order to better understand the possibility of using these materials in dental applications. Properties of alumina-added A–W glass–ceramics were compared with the properties of selected commercial dental materials to evaluate the tribological similarities among these materials and to elucidate the appropriateness of alumina-added A–W glass–ceramics for dental applications. 2. Experimental procedures 2.1. Specimen preparation A batch consisting of a mixture composed of MgO, CaO, SiO2 , P2 O5 and Al2 O3 in weight ratio 3.6, 40.4, 33.2, 16.5, 6.3, respectively, was prepared from reagent grade powders of MgO, CaCO3 , SiO2 , CaHPO4 ·2H2 O and Al2 O3 . Batches of ∼15 g were melted in a platinum crucible at 1500 ◦ C for 1 h in an electric furnace. Melting took place under normal laboratory conditions without controlling the atmosphere. The melt was poured onto a stainless steel plate to obtain bulk glass. The glass was ground to the size of 250 mesh and mixed with isopropyl alcohol in the weight ratio of 1:1. The dried glass powder was uniaxially pressed into a disc of 40 mm in diameter and 7 mm in thickness at a load of 40 kN. The samples of alumina-added A–W glass–ceramics were prepared by sintering the disk-shaped glass compacts at four different temperatures: 780 ◦ C, 900 ◦ C, 1000 ◦ C and 1100 ◦ C. The heating rate up to the maximum temperature was 5 ◦ C min−1 for all of the samples. Each sample was held at the maximum temperature for 2 h. The commercial dental materials Duceragold® (Degussa Dental GmbH, Germany) and IPS Empress® (Ivoclar Vivadent, Liechtenstein) were obtained in powder form from the manufacturers. Five specimens of each were fabricated according to the instructions quoted by the manufacturers. The specimens for Duceragold were prepared in disk-shape with nominal dimensions of 14 mm× 2 mm (diameter and thickness) by firing at 800 ◦ C. The specimens for IPS Empress were prepared in the same nominal dimensions by heat-pressing at 1075 ◦ C. The opposite faces of the test specimens were polished to a mirror-like finish to assure surface parallelism and smoothness prior to the tribological tests. The specimens were polished by application of a series of abrasive papers and 0.3 ␮m alumina powder solution on a cloth. After polishing, the samples were cleaned ultrasonically in distilled water for 10 min using an ultrasonic bath.

Table 1 Chemicals and their quantity in SBF in 1 l of distilled water. Chemical

Quantity (g)

NaCl NaHCO3 KCl K2 HPO4 MgCl2 ·2H2 O CaCl2 ·2H2 O NaSO4 Tri(hydroxymethyl)aminomethane 1 M HCl

7.996 0.35 0.22 0.174 0.305 0.368 0.071 6.057 ∼40 ml

England) to determine the wear track depth and wear area. The cross-sectional area of the wear track was calculated by averaging the wear area of four points of maximum mutual distance (90◦ spacing) on the wear track of the disk following the wear test, from the profiles recorded at the four locations. Wear volume was then calculated by multiplying the cross-sectional area of the wear track by the circumference of the track. The morphologies of fracture surface and wear tracks of the samples of the tested materials were observed by scanning electron microscope, SEM (Jeol 6400, Japan).

3. Results and discussion The X-ray diffraction patterns shown in Fig. 1 reveal that crystals precipitated during the sintering of the base glass at different sintering temperatures. The XRD patterns suggest that the glass–ceramics are composed of glass and crystal phases, and that the content of the glassy phase decreases and that of crystals increases with increasing sintering temperature. The sample sintered at 780 ◦ C, designated as A780, contained only apatite (Ca10 (PO4 )6 (OH)2 : JCPDS #9-432) crystals. The samples sintered at 900 ◦ C and 1000 ◦ C, designated as AW900 and AW1000, respectively, were composed of apatite and ␤-wollastonite (CaO·SiO2 : JCPDS #10-489) crystals. In the XRD pattern of the sample sintered at 1100 ◦ C, designated as AW1100, crystals of whitlockite (3CaO·P2 O5 : JCPDS #9-169) in addition to apatite and ␤-wollastonite were detected. AW1100 showed the maximum (3 2 0) peak intensity of the wollastonite crystal, implying that more and more wollastonite precipitated as the sintering temperature increased. However, the (2 1 1) peak intensity of apatite crystal reached its maximum in AW1000 and then decreased in AW1100, since some of the apatite crystals were converted to whitlockite

2.2. Testing and analyses The crystalline phases precipitated from the parent glass in the sintering process were identified using an X-ray diffractometer, XRD, (Rigaku Geigerflex-DMAK/B). Scans were run from 20◦ to 50◦ 2 at a speed of 2◦ min−1 with 0.02◦ increment using Cu-K␣ radiation on the sample surface. Microhardness measurements were conducted using a Knoop Hardness tester (Dukson tester, USA). At least five measurements were performed at different locations of the flat surface of the specimens through application of 500 gf for 15 s. The bulk density of sintered samples was determined by the Archimedean method. The averages and the standard deviation of the measurements were calculated. The tribological tests were performed in both dry and artificial saliva (simulated body fluid: SBF) using a pin-on-disc tribometer (CSM Instruments, Switzerland) at a load of 10 N, rotating speed of 0.25 cm s−1 , sliding distance of 50 m. The wear track diameter was 1 cm and data acquisition frequency was 1 Hz. The tribometer measures the tangential force between the two contacting surfaces and, using the TriboX2.0 software (CSM Instruments, Switzerland), calculates the coefficient of friction as the ratio of the tangential force to the load. The SBF was prepared according to the instructions given by Kokubo et al. [2,5]. It was buffered at a pH of 7.25 with tri(hydroxymethyl)aminomethane and HCl solution. The chemicals used and their quantities in SBF are given in Table 1. All specimens were immersed in SBF for 12 h before the wear test started, and clamped and supported on the sample stage located at the bottom of the wear cell filled with SBF. The level of SBF in the wear cell was maintained such that the specimen remained immersed in the fluid during the entire duration of the test. A 5 mm diameter high purity zirconia ball was chosen as the antagonist since it has recently achieved wide spread use in dentistry as a core and for fixed partial dentures [21]. After each individual tribological test, the surface profile of the specimen was measured by using a stylus profilometer (Surtronic 3+, Taylor Hobson Precision Ltd.,

Fig. 1. XRD patterns of the alumina-added A–W glass–ceramics obtained after sintering at temperatures of (a) 780 ◦ C, (b) 900 ◦ C, (c) 1000 ◦ C and (d) 1100 ◦ C.

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Fig. 2. The fracture morphology of the glass–ceramics sintered at (a) 780 ◦ C, (b) 900 ◦ C, (c) 1000 ◦ C and (d) 1100 ◦ C.

crystals during sintering at 1100 ◦ C. The findings are in good agreement with those reported in the literature [2,5,22]. Fig. 2 illustrates the SEM images taken from the fracture surface of the alumina-added A–W glass–ceramics sintered at different temperatures. Significant changes in fracture morphology occur with increasing sintering temperature. The sample sintered at 780 ◦ C (Fig. 2(a)) has large pores, but the sample sintered at 1100◦ C has a dense and well-developed grain morphology in Fig. 2(d). Values of Knoop hardness (KH), bulk density, wear rate, and mean friction coefficient for the materials investigated are presented in Table 2. KH values of the materials investigated varied from 304(8) HV for A780 to 505(6) HV for AW1100. The bulk density also varied from 1.93(0.06) g cm−3 for A780 to 2.67(0.03) g cm−3 for AW1100. The values in the parentheses next to the average hardness indicate the ±standard deviation of the data from the averages. The glass–ceramic becomes denser and harder when the sintering temperature is increased. This is attributed to the precipitation of more and more wollastonite and whitlockite crystals with

increasing sintering temperature. Wollastonite crystal has a surface crystallization mechanism and it grows in column shape from surface to interior [7]. The existence of wollastonite crystals results in an increase in the mechanical properties, due to the whiskerlike shape of grains which act as a filler reinforcing matrix. Kokubo et al. [2,5] reported that the presence of wollastonite crystals can effectively increase the fracture surface energy of the crystallized product, even when they are present as fine dendrites rather than long fibres. KH for the glass–ceramics containing apatite and wollastonite crystals was not significantly different from the values for Duceragold and IPS Empress. Wear rate of the materials investigated varied from 0.96 × 10−4 mm3 N−1 m (for AW1100 in SBF) to 41.37 × 10−4 mm3 N−1 m. (for A780 in dry) depending on the tribological test environment. The materials had lower wear rates in SBF than in the dry condition due to the lubricating action of SBF which removed the wear debris in the form of glass–ceramic particles or blocks formed on the wear surface during testing.

Table 2 Knoop hardness, bulk density, wear rate and friction coefficient dry and in SBF. Material

A780 AW900 AW1000 AW1100 Duceragold IPS Empress

Knoop hardness (HV )

304 (8) 420 (15) 478 (8) 505 (6) 490 (10) 428 (7)

Bulk density (g cm−3 )

1.93 (0.06) 2.21 (0.04) 2.63 (0.04) 2.67 (0.03) 2.54 (0.06) 2.45 (0.03)

Wear rate (×10−4 m3 N−1 m)

Mean friction coefficient ()

Dry

In SBF

Dry

In SBF

41.37 11.16 9.66 1.68 2.26 2.89

2.74 2.17 1.55 0.96 1.33 1.41

0.93 0.89 0.87 0.72 0.88 0.82

0.85 0.82 0.76 0.62 0.78 0.75

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However, in the dry condition, glass–ceramic particles removed from the surface became part of the abrasive system, established a 3-body abrasion wear process and contributed to an increase in roughness rather than smoothing of the surface [23,24]. As seen in Table 2, the wear rate of the alumina-added A–W glass–ceramics decreased with increasing sintering temperature in both dry and SBF conditions. Wear rate for A780 was 41.37 × 10−4 mm3 N−1 m and 2.74 × 10−4 mm3 N−1 m in dry and in SBF conditions, respectively, but decreased to 1.68 × 10−4 mm3 N−1 m and 0.96 × 10−4 mm3 N−1 m for AW1100 in dry and in SBF conditions, respectively. The decrease in wear rate with increasing sintering temperature is attributed to the fact that the glass–ceramic becomes denser and harder as sintering temperature is increased due to the changes in the type and the proportion of crystals precipitated. As the glass–ceramic gets harder, it becomes more wear resistant. The results of the wear tests are in accord with the results of the hardness tests, implying an inverse relation between the hardness and wear rate of the alumina-added A–W glass–ceramics. This publication is the first in the open literature reporting the results of wear rate data for the alumina-added A–W glass–ceramics. Therefore the values of wear rate obtained in this study could not be compared directly. However, when a comparison is made between the wear rate of the alumina-added A–W glass–ceramic studied and that of the commercial dental materials, it is seen that, wear rate for AW1100 was lower than the values for Duceragold and IPS Empress both in the dry condition and in SBF. Though wear rate for the glass–ceramics sintered at lower temperatures was considerably higher than the values for Duceragold and IPS Empress in dry condition, the difference was less in SBF. Seghi et al. [25] stated that ideally, a dental restoration should wear at approximately the same rate as the enamel it replaces, and should not increase the wear rate of an opposing enamel surface. Therefore, AW1000 would be an alternative choice for materials used in restorative dentistry in the regions where direct contact with natural dentition occurs since it meets the requirements of the dental restoration. Since, AW1100 is harder, it could increase the wear rate of an opposing enamel surface, and therefore should be used as a core material. Mean friction coefficient () of the materials investigated ranged from 0.62 (for AW1100 in SBF) to 0.93 (for A780 in dry) depending on the tribological test conditions. The materials had relatively lower  in SBF than in the dry condition since SBF played a role of lubricant between the ball and surface of the material during the test. In general, materials comprising higher hardness exhibited relatively lower  than those having lower hardness, implying an inverse relation between  and hardness of the alumina-added A–W glass–ceramics. Variation in  with sliding distance in the dry condition and in SBF for the alumina-added A–W glass–ceramics is shown in Figs. 3 and 4, respectively. Variation in  with sliding distance in the dry condition and in SBF for the commercial dental materials is shown in Fig. 5. These representative curves illustrate that  of the materials studied is minimal at the beginning of the testing due to the point-contact between the ball and the sample. After the planar surface begins to break down,  increases rapidly because of higher contact area and the formation of wear particles which cause a 3-body abrasion wear process [23]. After a sliding distance ranging from 1 m to approximately 30 m depending on the sintering temperature,  reached a steady state level for the alumina-added A–W glass–ceramics as seen in Figs. 3 and 4. The steady state level increased with increasing sintering temperature for the aluminaadded A–W glass–ceramics. As for the commercial dental materials investigated, the sliding distances beginning the steady state level were about 2 m in the dry condition and 5 m in SBF as seen in Fig. 5.

Fig. 3. Variation in the friction coefficient with sliding distance in the dry condition for the alumina-added A–W glass–ceramics.

Fig. 4. Variation in the friction coefficient with sliding distance in SBF for the alumina-added A–W glass–ceramics.

Fig. 5. Variation in the friction coefficient with sliding distance in the dry condition and in SBF for the commercial dental materials investigated.

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Fig. 6. SEM micrographs of the wear track for alumina-added A–W glass–ceramics. The tracks were obtained after wear tests (a) dry (b) in SBF.

A material with lower friction coefficient is preferred in a dental restoration, because the friction can cause lateral forces on the teeth, which, if excessive, are destructive to the supporting tissues. From that point of view, AW1000 and AW1100, whose  is more or less the same as  of the commercial dental materials in SBF, would be an appropriate material for dental applications. SEM micrographs of the wear tracks obtained after wear tests on the alumina-added A–W glass–ceramics sintered at different temperatures are shown in Fig. 6. Differences in the appearance of wear tracks are noticed, owing to the microstructural dissimilarities among the materials and the bioenvironmental test conditions. Although microcracks, wear debris and deep ploughing grooves are observed on the wear tracks, the width of the wear tracks decreases from A780 to AW1100. The images confirm the tribological findings, which show that the glass–ceramic becomes more wear resistant as the sintering temperature is increased. Significant differences were observed on the wear tracks obtained in the dry condition and in SBF. Wear tracks were rather clear and visible in the dry condition but become imperceptible in

SBF. Wear tracks of the commercial dental materials also showed similar features. In the dry conditions, the wear tracks were coarse and harsh, while in SBF they were light and pale as seen in Fig. 7. The grooves and signs of material loss are clearly seen in all of the materials studied. The SEM micrographs of the wear tracks suggest that different failure modes occur at the wear surfaces during wear test. Wear debris in the form of particles or blocks observed on the wear tracks imply that abrasive and adhesive wear mechanisms have occurred during wear test. The propagation of the fissures on the surface and the loss of material reveal that the dominant wear mechanism is abrasion in A780. However, in the samples sintered at higher temperature the primary wear mechanisms involved was adhesion. The deep ploughing grooves, ridges and chips observed on the wear tracks are the features of abrasive and adhesive wear [25–27]. As seen in the micrographs in Fig. 7, one of the wear mechanisms involved in the commercial dental materials was fatigue, caused by fissure nucleation on the subsurface and its propagation as a result of the repeated cycles.

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Fig. 7. SEM micrographs of the wear track for commercial dental materials. The tracks were obtained after wear tests (a) dry (b) in SBF.

sintering temperature. Wear profiles obtained in the dry condition illustrate relatively deeper and wider plough than those obtained in SBF, for all of the materials studied. 4. Conclusions An investigation of the tribological properties of a glass–ceramic produced in the MgO–CaO–SiO2 –P2 O5 –Al2 O3 system has led to the following conclusions:

Fig. 8. Wear track profiles for alumina-added A–W glass–ceramics. The profiles were obtained after wear tests (a) dry (b) in SBF.

More ploughed marks and debris were observed on the wear track of the materials having relatively higher values of wear rate, friction coefficient and lower values of hardness. The wear track profiles shown in Figs. 8 and 9 support these findings. Wear track profiles shown in Fig. 8 reveal that the depth and width of the wear track as well as the wear area decreased steadily with increasing

1. Tribological properties and microhardness of alumina-added A–W glass–ceramics depend strongly on the crystals precipitated and microstructure developed during sintering. 2. The tribological test environment has a profound effect on the wear rate and mean friction coefficient of alumina-added A–W glass–ceramics. Wear rate and mean friction coefficient obtained in SBF are lower than those obtained in the dry condition. 3. Strong correlation exists between mean friction coefficient, wear rate and microhardness of alumina-added A–W glass–ceramics. 4. The alumina-added A–W glass–ceramics investigated show similar tribological properties and microhardness to selected commercial dental materials when they are sintered at temperature 1000 ◦ C or greater. These glass–ceramics are expected to play a role in dental applications in the near future. Acknowledgements This work was in part supported by the Scientific and Technical Research Council of Turkey, TUBITAK, project number: 104M400. The authors are grateful to TUBITAK. References

Fig. 9. Wear track profiles for commercial dental materials. The profiles were obtained after wear tests (a) dry (b) in SBF.

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