XPS analysis of the surface of leucite-reinforced feldspathic ceramics

dental materials Dental Materials 17 (2001) 1±6

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XPS analysis of the surface of leucite-reinforced feldspathic ceramics T. Hooshmand, R. Daw, R. van Noort*, R.D. Short Centre for Biomaterials and Tissue Engineering, University of Shef®eld, Shef®eld, UK Received 2 March 1999; revised 29 February 2000; accepted 20 March 2000

Abstract Objective: The bond of a silane-coupling agent to a ceramic surface is expected to be in¯uenced by the composition and chemical state of the ceramic surface. The purpose of this study was to determine the variation in the composition and the chemical states of the surfaces of a range of leucite-reinforced feldspathic (LRF) ceramics using X-ray photoelectron spectroscopy (XPS). Methods: Five LRF ceramic discs (IPS Empress,Optec HSP,VMK 68, Mirage, and a modi®ed Mirage) were produced and polished to a 1 mm ®nish. A further nine discs of the modi®ed Mirage were produced. The discs were stored for 48 h in a vacuum oven at 1108C to remove absorbed water. The surfaces of these discs were analysed by XPS. Survey scans at 308 take-off angle were taken and surface composition (in at%) was calculated from the narrow scans for Si 2p, O 1s, Al 2p, Mg 2s, K 2p, Na 1s, Ca 2p and N 1s. Results: Atomic concentration of elements (after exclusion of C) for the ®ve LRF ceramics were in the range: O, 45.0±51.6%; Si, 26.7± 35.6%; Al, 6.3±9.7%; Mg, 4.9±8.8%; K, 0.5±2.2%; N, 0.9±2.9% and less than 1% of Na and Ca. The shapes of the O 1s and Si 2p narrow scan core lines of the ®ve LRF ceramics were virtually identical. Signi®cance: All ®ve LRF ceramics were found to have a silica-rich surface layer due to a reduction in K and Na relative to the bulk composition. Both the composition and chemical states of the surfaces for the ®ve LRF ceramics were very similar. q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Leucite-reinforced; X-ray photoelectron spectroscopy; Surface analysis; Ceramic; Feldspathic

1. Introduction In a previously published study by Della Bona and van Noort [1], it was shown that the shear bond strength test commonly used to test the resin±ceramic bond has some limitations due to a predisposition towards cohesive fracture of the ceramic base. A rod-to-rod tensile bond strength test arrangement was proposed. This technique was used by Hoosmand et al. [2,3] to show that the silane bond to a leucite-reinforced feldspathic ceramic could be improved to such a degree that cohesive failure occurred within the luting resin rather than at the resin±ceramic interface. This was the case despite there being no contribution from micromechanical bonding by using a ceramic surface polished to a 1 mm ®nish. Therefore the use of hydro¯uoric acid (HF) to create a strong resin±ceramic bond may not be necessary. However, the experimental design for the bond strength tests was such that a modi®cation of the ceramic used was * Corresponding author. Department of Restorative Dentistry, School of Clinical of Dentistry, Claremont Crescent, Shef®eld S10 2TA, UK. Tel.: 144-114-271-7932; fax: 144-114-266-5326. E-mail address: r.vannoort@shef®eld.ac.uk (R. van Noort).

necessary in order to prevent ceramic fractures and ensure good bonding between the ceramic and the Ni/Cr alloy rods used. Consequently, the bond strength test technique cannot be applied to a wide range of leucite-reinforced ceramics as these will not bond adequately to the Ni/Cr alloy rods. Thus it is not clear if the results of any assessment of silane treatments using this rod-to-rod specimen design will be applicable to all leucite-reinforced feldspathic (LRF) ceramics. When a signi®cant contribution to the resin±ceramic bond is provided by surface roughening, such as HF etching, then this will depend on the microstructure of the etched ceramic surface, which will vary from one ceramic to another. However, if the ceramic surface is smooth, the bond is dependent only on the silane-coupling agent and this then becomes a function of the surface composition and chemistry of the ceramic and not its surface topography. Hence, if LRF ceramics exhibit very similar surface compositions and chemical states of silicon and oxygen, then it is reasonable to hypothesise that the silane bond will be the same for all LRF ceramics as these represent the binding sites for the silane-coupling agent to the ceramic surface. This would obviate the need to carry out extensive bond

0109-5641/01/$20.00 + 0.00 q 2001 Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII: S 0109-564 1(00)00032-4

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T. Hooshmand et al. / Dental Materials 17 (2001) 1±6

Table 1 Materials information Product name

Manufacturer

IPS Empress Optec HSP

Ivoclar-Vivadent, Liechtenstein Jeneric/Pentron., Wallingford, U.S.A Mirage Myron Int, Kansas City, U.S.A Modi®ed Mirage Myron Int, Kansas City, U.S.A VMK 68 Vita Zahnfabrik, Bad SaÈckingen, Germany

Batch No Type 484078 E2833H

Body A1 Body A2

J2715K J2715K 2678

Body D4 Body D4 Body B2

strength studies for all the LRF ceramics used in dentistry with respect to the effectiveness of the silane-coupling agent. The aim of this study was to determine the chemical composition and environmental characteristics of the surfaces of a number of LRF ceramics using X-ray photoelectron spectroscopy (XPS). Although XPS has been used to examine the surfaces of dental casting alloys [4], titanium implants [5], hydroxyapatite coatings [6] and ceramic orthodontic brackets [7], such work has not previously been undertaken for dental ceramics used in the construction of all-ceramic restorations. 2. Materials and methods Four types of leucite-reinforced feldspathic ceramic for all-ceramic resin-bonded restorations (IPS Empress, Optec HSP, Mirage, and a modi®ed Mirage) were used. In addition, a leucite-containing feldspathic ceramic for metal± ceramic restorations (VMK 68) was selected. The details are listed in Table 1. The modi®ed Mirage ceramic was prepared using a milling process. A suspension of 18 vol% of Mirage ceramic powder in distilled water, containing a few drops of sodium hexameta phosphate as a de¯oculant, was prepared.

Fig. 1. Survey scans of ®ve types of LRF ceramics. 1. IPS Empress, 2. Mirage, 3. Modi®ed Mirage, 4. Optec HSP, 5. VMK 68.

This suspension was mixed in a cylindrical pot 75 mm diameter and 140 mm long containing 20 zirconia balls, 12.5 mm in diameter. The pot was rotated on rollers at 75 rpm for 40 h to break down the coarser particles [8]. Optec HSP, Mirage, and modi®ed Mirage powders were condensed into discs using Mirage porcelain condenser liquid (Chameleon Dental Products, Kansas City, USA). VMK 68 ceramic powder was condensed with Vita modelling liquid (Vita Zahnfabrik, Bad SaÈckingen, Germany). One disc of each type of the ceramic was prepared. The discs were approximately 10 mm in diameter and 2 mm in thickness and were ®red in a vacuum oven according to the manufacturer's instructions. An IPS Empress ceramic disc was prepared using the lost-wax and hot-pressing technique. The ceramic discs were ground with 600-grit SiC abrasive and polished to a 1 mm ®nish with diamond compounds (Buehler Ltd, Coventry, UK) in order to ensure a high-quality signal from the surface for analysis. The samples were then cleaned ultrasonically in acetone for 15 min. The ®rst problem encountered was that water vapor contamination in the high-vacuum chamber of the spectrometer, arising from absorbed water in the bulk of the ceramic, prevented the creation of an adequate vacuum. This was overcome by placing the specimens in a vacuum oven at 1108C for 48 h and transferring them to a vacuum desiccator at room temperature on the day before analysis. Adsorption of water from the atmosphere during transportation from the oven to the spectrometer was found not to be a problem. The XPS analysis of the LRF ceramic discs was carried out on a VG CLAM 2 (Vacuum Generators, East Grinstead, UK) with the Mg Ka X-ray source run at 100 W, 10 kV and 10 mA. Survey scans with a range corresponding to binding energies of 0±1100 eV were used for quanti®cation at a pass energy of 100 eV. To obtain higher resolution scans (narrow scans) for determination of chemical states, a pass energy of 20 eV was selected. All samples were analysed at a 308 takeoff angle. The binding energies for all spectra were calibrated by assuming that the major component of the C 1s peak resulted from hydrocarbon contamination at a binding energy of 285.0 eV [9]. If this were not the case, one would be able to determine this from the peak shape. It was assumed that the chemical state of the silicon (Si 2p) may vary according to the number of oxygen atoms to which it is bonded. The Si 2p peak position could, therefore, vary between samples. Hence, the Si 2p peak was not used for charge correction and charge correction was performed using the hydrocarbon component of the C 1s core level. Atomic concentrations were calculated using the Scienta software program (Scienta Instruments, Upsala, Sweden) and relevant sensitivity factors for the elements [10]. Since the analysis of large number of specimens for statistical variations would impractical, 10 ceramic discs of only one material (modi®ed Mirage) were analysed in order to calculate the specimen to specimen reproducibility of the XPS analyses.

T. Hooshmand et al. / Dental Materials 17 (2001) 1±6

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Si 2p

COUNTS

IPS Empre Modified Mi

Optec HSP Mirage

VMK 68 102 Binding energy\eV

102

BINDING ENERGY [Ev]

Fig. 4. C 1s Narrow scan spectra. Fig. 2. Si 2p Narrow scan spectra.

3. Results Survey scans of the ®ve leucite-reinforced feldspathic ceramic discs by XPS analysis are shown in Fig. 1. The most intense photoelectron peak for each element was used for quanti®cation; aluminium (Al 2p), magnesium (Mg 2s), silicon (Si 2p), carbon (C 1s), potassium (K 2p), calcium (Ca 2p), nitrogen (N 1s), oxygen (O 1s), and sodium (Na 1s). Results for the narrow scan spectra of Si 2p and O 1s are presented in Figs. 2 and 3, respectively, showing similar core lines for the ®ve LRF ceramics. The C 1s narrow scan spectra (Fig. 4) show slight differences suggesting the presence of other functionalities besides hydrocarbons, especially apparent in the narrow scan of VMK 68. An analysis of the full-width-at-half-maximum (FWHM) of the Si 2p binding energies, calculated from the narrow scans, is presented in Table 2.

O 1s

COUNTS

IPS Empress

Modified Mira Optec HSP Mirage

VMK 68 531 531Binding energy\eV

BINDING ENERGY [Ev]

Fig. 3. O 1s Narrow scan spectra.

The surface composition (in atomic %) of the major elements present for the ®ve LRF ceramic discs are shown in Table 3. The percentage of the elements was calculated both prior to and after the removal of C. Atomic concentration of elements (after subtraction of carbon) for the ®ve LRF ceramics were: O, 45.0±51.6%; Si, 26.7±35.6%; Al, 6.3±9.7%; Mg, 4.9±8.8%; K, 0.5±2.2%; N, 0.9±2.9% and less than 1% of Na and Ca. Table 4 shows the mean concentration in atomic % and the standard deviation for the major elements on the surface of the modi®ed Mirage ceramic discs …n ˆ 10†: The small standard deviation and the narrow range indicate that there is a very small variation in the surface composition of the ceramic discs from one specimen to the next. 4. Discussion Silane-coupling agents are used to create a chemical bond between LRF ceramics and resin. The interaction between the silane and the ceramic is governed by surface phenomena and therefore is in¯uenced by the composition and chemical environment of the ceramic surface. A silanecoupling agent of general formula R±Si(OR 0 )3 contains both inorganic and organic reactive groups [11]. The alkoxy groups (R 0 ) are reactive with Si±OH groups on the ceramic surface where they react to form siloxane bonds (±Si±O± Si±). Therefore, the most important elements on the ceramic surface are silicon (Si) and oxygen (O). Table 2 Binding energy and FWHM of Si 2p core levels Sample

Position (eV)

FWHM (eV)

Mirage Modi®ed Mirage IPS Empress Optec HSP VMK 68

102.60 102.63 102.58 102.57 102.64

2.02 2.09 2.04 2.10 2.18

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T. Hooshmand et al. / Dental Materials 17 (2001) 1±6

Table 3 Surface composition in atomic percent of the ®ve LRF ceramics. The ®gures in parentheses correspond with the composition prior to the removal of the C 1s contribution Ceramic

%O

%Si

%Al

%Mg

%K

%Na

%N

%Ca

%C

Mirage

45.0 (29.5) 51.6 (32.1) 50.8 (36.0) 45.9 (27.7) 50.8 (29.5)

35.6 (23.3) 30.0 (19.7) 30.2 (21.4) 33.7 (20.3) 26.7 (15.5)

6.3 (4.1) 8.2 (4.8) 9.7 (6.9) 7.3 (4.4) 7.6 (4.4)

8.8 (5.8) 6.9 (3.1) 5.8 (4.1) 7.5 (4.5) 8.8 (5.1)

0.9 (0.6) 2.1 (1.0) 1.5 (1.1) 2.2 (1.3) 0.5 (0.3)

0.1 (0.1) 0.5 (0.3) 0.4 (0.3) 0.2 (0.1) 0.3 (0.2)

1.8 (1.2) 1.7 (1.1) 0.9 (0.6) 1.1 (0.7) 2.9 (1.7)

,0.2 ,0.1 0.7 (0.5) 0.5 (0.4) ,0.2 ,0.1 0.6 (0.4)

0 (34.4) 0 (37.5) 0 (29.1) 0 (39.7) 0 (41.8)

Modi®ed Mirage IPS Empress Optec HSP VMK 68

In the present study, results from survey scans for the ®ve LRF ceramics (Fig. 1) show them to have a similar elemental composition. The narrow scans of Si 2p and O 1s (Figs. 2 and 3) show that these elements are bound in a similar chemical environment. It is likely therefore that the surfaces of the ®ve ceramics are presenting the same chemical environment to the silane-coupling agent. Small differences in the chemical state of narrow scan spectra of C 1s Fig. 4 may be due to different levels of surface contamination. However, this is unlikely to result in a different interaction with the silane-coupling agent compared with the other ceramics, since the binding sites for the silane-coupling agent are primarily with the Si±OH groups on the ceramic surface. As shown in Table 2, the binding energies obtained for the Si 2p core level are very close for all samples, suggesting that in each sample the bonding state of the silicon is very similar. The positions of the peak centroids are consistent with Si bonded to two oxygens, representing either ±O± Si±O± or ±O±Si±OH binding states [12]. However, the FWHM of the Si 2p core levels are slightly greater than 2. Therefore, in each sample, there are also probably silicon atoms bonded to 1 or 3 oxygen atoms, although this will only be a small proportion, otherwise much larger FWHM values would be expected. A corresponding analysis for O 1s is not possible since the binding energy for O 1s is much less affected by the number of attached silicons. That is, silicon has a much smaller dynamic range of binding energies. The chemical compositions present on the surfaces of all LRF ceramic discs are virtually indistinguishable from one another (Table 3). The repeat experiments on the modi®ed Mirage ceramic discs (Table 4) shows that there is a very small variation from one specimen to the next and thus justi®es the use of only one specimen per ceramic. It should be noted that the quanti®cation of XPS is accurate to ^5% with an adequate signal-to-noise ratio. Oxygen (O 1s), silicon (Si 2p), aluminium (Al 2p) and magnesium (Mg 2s) were found to be the most dominant elements on the LRF ceramic surfaces with similar concentrations. The results indicate that there is no carbon bonded to the silica, which

contraindicates possible surface contamination from embedded SiC from the polishing process. However, the high surface concentration of carbon may be a consequence of the polishing process using diamond lapping compounds, which could result in diamond particles becoming embedded on the surface of the ceramic. Carbon contamination is dif®cult to remove from the surface and exposure in a plasma or soxhlet extraction may have been bene®cial. However, these cleaning procedures may also have an affect on the surface and this deserves further investigation. Based on the above observations, it is reasonable to speculate that the improvement in bond strength using a modi®ed method of silane treatment on a 1 mm polished modi®ed Mirage LRF ceramic as proposed by Hooshmand et al. [2,3] can also be applied to other LRF ceramics. This contrasts with the study by Eliades et al. [7] of ceramic bracket bases, which revealed wide variations in bracket composition and structure. In that case, the choice of bracket base could signi®cantly in¯uence the binding of the silane-coupling agent to the bracket. It should be noted that the surfaces of the ceramics investigated are composed of two phases, the glass matrix and leucite crystals, and the XPS used was only able to determine the average surface composition. The amount of leucite crystal varies from 19.3 wt% for Vita VMK68 to 50.6 wt% for Optec HSP [13]. Multiple ®ring, isothermal heat treatments, and cooling rate have been shown to be capable of altering the leucite content of dental ceramics [14,15]. Thus, there are local compositional and distribution variations of leucite and glass phases in the different ceramics. Table 5 compares the composition (in at%) of the major elements in the bulk of Mirage ceramic [16], the surface of Mirage ceramic (XPS data from this study), and the leucite crystals based on its chemical formula (K2O´Al2O3´4SiO2). Comparing the bulk composition of the Mirage with that of its surface, there is a tendency towards a silica-rich surface due to a considerable loss of K and Na, such that the silica content of the surface approaches 80%. Possibly, this is due to the surface being relatively disorganised compared with the bulk and as a consequence the K and Na in the glass network structure

T. Hooshmand et al. / Dental Materials 17 (2001) 1±6

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Table 4 Mean values of the concentration of each of the major elements in atomic percent and standard deviations for modi®ed Mirage …n ˆ 10†

Table 5 Comparison of the chemical composition (at%) of the bulk and surface of Mirage ceramic and leucite crystals

Elements

Mean (SD)

Range

Element

51.6 (2.9) 30 (2.1) 8.2 (1) 6.9 (1.2) 2.1 (0.7) 0.5 (0.1)

47.5±55 27.1±34.2 6.5±9.5 5±8.6 0.9±2.8 0.2±0.6

Mirage surface Composition

Leucite crystals

O 1s Si 2p Al 2p Mg 2s K 2p Na 1s

Mirage bulk Composition

Si O Al K Na

29.6 44.1 7.8 9.6 3.8

35.6 45.0 6.3 0.9 0.1

25.6 43.8 12.5 18.1 -

are readily dissolved and replaced by hydroxyl groups at the surface [17]. Therefore, one might also reasonably expect the surface of the leucite crystals to lose a substantial amount of K and Na and form a silica-rich surface layer in excess of the 70% bulk composition. Hence the surface composition of the LRF ceramic may not be that dissimilar from pure leucite. In addition, the chemical states of the silicon for the LRF ceramics tested are virtually the same, as indicated by the narrow scans of Si 2p, despite considerable differences in their leucite content. This can only be the case if the chemical composition and chemical state of the surface of the leucite crystals is very similar to that of the glass. From this, it can be inferred that the bonding of a silane-coupling agent will be unaffected by differences in leucite content of the LRF ceramics. However, this hypothesis will need to be tested. Obviously, if another crystalline phase such as alumina were present, then the local variation in surface composition might be considerable and one would expect this to have a signi®cant in¯uence on the silane bond. For example, an alumina reinforced core ceramic will have distinctive regions of pure alumina on its surface. Since silane-coupling agents do not bond well to alumina, the bond strength of resin composite to the ceramic will be affected, as noted by Kern and Thompson [18]. Thus, the proposition that silane bonding to leucite containing feldspathic glass-ceramics will be very similar, cannot be extrapolated to feldspathic glasses containing other non-silicate phases. 5. Conclusion This study is believed to be the ®rst report of the analysis of the surfaces of dental ceramics for all-ceramic restorations using X-ray photoelectron spectroscopy. It is concluded that the surface composition and chemical states of the ®ve LRF ceramics examined are virtually indistinguishable. These results support the hypothesis that the results of bond strength tests for one LRF ceramic, which was so designed as to focus on the silane contribution only, will be applicable to many other LRF ceramics. Further work is required to obtain more information about the surface composition and chemical states of other types of dental ceramics.

Acknowledgements We are grateful to the Laboratory of Surface and Interface Analysis, Department of Engineering Materials, University of Shef®eld, for providing access to the XPS facilities.

References [1] Della Bona A, van Noort R. Shear versus tensile bond strength of resin composite bonded to ceramic. J Dent Res 1995;74:1591±6. [2] Hooshmand T, van Noort R, Keshvad A, Shareef MY. Effect of silane application on ceramic/resin bond. J Dent Res 1997;76:69 (Abstr. No. 446). [3] Hooshmand T, van Noort R, Keshvad A. Bond durability of silane treated ceramic surfaces to resin. J Dent Res 1998;77:235 (Abstr. No. 1033). [4] Wataha JC, Malcolm CT. Effect of alloy surface composition on release of elements from dental casting alloys. J Oral Rehabil 1996;23:583±9. [5] Ameen AP, Short RD, Johns R, Schwach G. The surface analysis of implant materials. 1. The surface composition of a titanium dental implant material. Clin Oral Implants Res 1993;4:144±50. [6] Li X, Weng J, Tong W, Zuo C, Zhang X, Wang P, Liu Z. Characterization of hydroxyapatite ®lm with mixed interface by Ar 1 ion beam enhanced deposition. Biomaterials 1997;18:1487±93. [7] Eliades T, Lekka M, Eliades G, Brantley WA. Surface characterization of ceramic brackets: a multitechnique approach. Am J Orthod Dentofacial Orthop 1994;105:10±18. [8] Shareef MY, van Noort R, Messer PF. The effect of microstructural features on the biaxial ¯exural strength of leucite reinforced glassceramics. J Mater Sci: Mater Med 1994;5:113±8. [9] Briggs D, Seah MP. Practical surface analysis: vol. 1 Auger and X-ray photoelectron spectroscopy. 2nd ed.. Chichester: Wiley, 1990. [10] Ward RJ, Wood BJ. A comparison of experimental and theoretically derived sensitivity factors for XPS. Surf Interface Anal 1992;18:679± 84. [11] Plueddemann EP. Silane coupling agents. New York: Plenum Press, 1991. [12] Alexander MR, Short RD, Jones FR, Stollenwerk M, Zabold J, Michaeli W. An X-ray photoelectron spectroscopic investigation into the chemical structure of deposits formed from hexamethyldisiloxane/oxygen plasmas. J Mater Sci 1996;31:1879±85. [13] Schmid M, Fischer J, Salk M, Strub J. The microstructure of leucitereinforced glass ceramics. Schweiz Monatsschr Zahnmed 1992;102:1046±53. [14] Mackert JR, Evans AL. Quantitative x-ray diffraction determination of leucite thermal instability in dental porcelain. J Am Ceram Soc 1991;74:450±3.

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T. Hooshmand et al. / Dental Materials 17 (2001) 1±6

[15] Mackert JR, Evans AL. Effect of cooling rate on leucite fraction in dental porcelains. J Dent Res 1991;70:137±9. [16] Shareef MY, Piddock V, Messer PF, Van Noort R. Characterization and microstructural analysis of some dental porcelains. J Dent Res 1992;71:567 (Abstr. No. 414). [16] Doremus RM. Chemical durability of glass. In: Tomozawa M,

Doremus RM, editors. Treatise on material science and technology, vol. 17. 1979. [18] Kern M, Thompson VP. Bonding to glass in®ltrated alumina ceramic: adhesive methods and their durability. J Prosthet Dent 1995;73:240±9.