Biomaterials 23 (2002) 3395 3403
Fluoroapatite glass-ceramic coatings on alumina: structural,
mechanical and biological characterisation
"
E. Vernea,*, M. Bosettib, C. Vitale Brovaronea, C. Moisescuc, F. Lupoa, S. Sprianoa,
M. Cannasb
a
Materials Science and Chemical Engineering Department, Polytechnic of Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy
b
Department of Medical Sciences, Human Anatomy, University of Eastern Piedmont, Novara, Italy
c
Otto Schott Institute, Friedrich-Schiller-University, Fraunhoferstraße 6, Jena, Germany
Received 27 July 2001; accepted 1 January 2002
Abstract
The aim of this work was to realise bioactive coatings on full density a-alumina substrates.
An SiO2 CaO-based glass (SC) and an SiO2 Al2O3 P2O5 K2O CaO F -based glass-ceramic (SAF) were used for this purpose.
Specifically, SAF is a fluoroapatite containing glass-ceramic and previous studies have shown that it is a highly bioactive
biomaterial. Furthermore, these fluoroapatite crystals possess a needle-shaped morphology which mimics that of hydroxylapatite
found in human hard tissues, particularly in teeth.
SAF is a very viscous glass-ceramic and for this reason an intermediate, less viscous, SC layer was interposed in direct contact
with alumina aiming to obtain a good coating adhesion.
Moreover, this intermediate layer strongly lowers the Al3+ diffusion and thus minimises both compositional changes in the SAF
outer layer and the risk of detrimental modifications of the nature of the crystalline phases.
A complete characterisation of the coated samples was performed by means of X-ray diffraction, optical and scanning
microscopy. Coating adhesion on alumina was tested by comparative shear tests while biocompatibility was investigated on
alumina, bulk SAF and on the realised coatings. For this purpose, cytotoxicity, adhesion and proliferation of human osteoblast-like
cells were cultured onto the three materials. Results showed that the interposition of the SC layer was successful in allowing a good
softening and spreading of the SAF outer layer and in avoiding the crystallisation of undesired crystalline phases maintaining the
good bioactive properties of the bulk one. In vitro results on the coatings showed osteoblast-like cell behaviour similar to bulk
fluoroapatite glass-ceramic and better respect to alumina substrates, being a promising index of bone material integration and of its
in vivo possible applications. r 2002 Elsevier Science Ltd. All rights reserved.
Keywords: Glass-ceramic; Coatings; Osteoblast-like cells; Biocompatibility
1. Introduction as the quality of mechanical fit between implant and
bone or the surface roughness of alumina, and can cause
Alumina is considered a reliable material for several problems as loosening and clinical failure [2].
biomedical applications due to its high wear resistance A way to improve alumina osteointegration is to
and fracture toughness. When implanted in vivo realise a coating on it using materials with a certain
alumina shows the formation of a non-adherent fibrous degree of biological activity.
capsule at the tissue interface, without exhibiting any Bioactive glasses and glass-ceramics are known to be
chemical bonding with bone, it is considered an almost the materials that promote new tissue formation and
inert material [1]. This non-adherent fibrous capsule can enhance interactions of cells with the biomaterial. Their
become quite thick depending on different factors, such essential requirement to bond with a living bone results
in the formation of a biologically active layer of
hydroxyapatite on their surface. The bioactivity me-
*Corresponding author. Tel.: [phone]; fax: [phone]-
chanism may differ in consequence of the glass
4699.
"
E-mail address: [e-mail] (E. Verne). composition and in the presence of different crystalline
0142-9612/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.
PII: S 0142- 9612( 02) 00040- 6
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3396 E. Verne et al. / Biomaterials 23 (2002) 3395 3403
Table 1
phases [3]. In the field of bioactive glass-ceramics, a
!
E1167 Verne
good feature is the presence of hydroxyapatite or
fluoroapatite crystalline phases [4,5]. The use of SiO2 CaO Al2O3 P2O5 K2O F
bioactive glasses and glass-ceramics is generally not just
SC 53.7 46.3    
limited to load-bearing implants: dentistry, maxillofacial
SAF 26.2 16.4 17.9 19.6 10.5 8.3
surgery and small bone replacements. Yet, the use of the
latter material as coatings on high-strength ceramic or
metallic substrates, could widen their application fields
by combining the mechanical properties of the substrate
with the bioactivity of the coated surface [6 12]. In fact, SC glass was prepared by melting the raw products
the final goal is to combine the bioactive properties of (SiO2, CaCO3) in a platinum crucible at 16001C for 1 h
the coating with the strength of this inert bioceramic and and by pouring the melt on a pre-heated stainless steel
to propose this biomaterial for middle ear surgery, plate. The SAF glass-ceramic was prepared by melting
fingers prostheses, intervertebral discs and dental the starting products (SiO2, Na2CO3, K2CO3, CaCO3,
implants. The main difficulties involved in coating Al(PO3)3, Na3AlF6, K3AlF6 and AlOOH H2O) in an
alumina substrates are connected with its low thermal alumina crucible at 15001C, followed by grounding and
expansion coefficient (8 8.5 10 6/1C) and its high remelting in a platinum crucible at 15501C. The melt
reactivity with glasses and glass-ceramics at high was then poured on a carbon mould. The as-poured
temperatures involved in the coating preparation. SAF was further thermally treated in order to stimulate
In this work, double-layer coatings on alumina the crystallisation of a fluoroapatite phase having a
substrates were developed and the obtained material needle-shaped morphology, with crystals about 5 10 mm
was characterised by studying the nature of its crystal- in length [4]. The fluoroapatite phase is essential to
line phases and its in vitro biocompatibility using human impart bioactive properties to SAF and its peculiar
osteoblast-like cells. Previous studies [7,8] have shown morphology was engineered in order to mimic the shape
that at high temperatures, Al3+ extensively diffuses of hydroxylapatite in human hard tissues. The thermal
from the substrate towards the coating surface and other expansion coefficient of SAF was determined based on
researchers assessed that very low Al3+ content can 5 5 25 mm3 bars by dilatometry (Netzsch, Model
completely inhibit the bioactivity mechanism [2]. Aiming 402 E, Exton, PA) whereas the thermal expansion
to avoid contamination of the glasses by Al3+ diffusion, coefficient of SC was known from literature [6].
an intermediate SC glass layer was interposed between After pouring, both SC and SAF were ball-milled and
alumina substrates and the fluoroapatite-based glass- sieved up to a grain size o100 mm.
ceramic outer layer (SAF) as this procedure proved to
be effective in a previous work [7,8]. This intermediate 2.2. Coating preparation
layer was also planned to increase the coating adhesion
to the substrate as preliminary tests showed that SAF is Full-density medical-grade a-alumina (sintered by FN
too viscous to provide a satisfactory adhesion to S.p.A Italy) was used as substrate. Specimens with a
alumina. The choice of the two coating materials was 1cm2 surface were ultrasonically cleaned in acetone for
also made considering their thermal expansion coeffi- 10 min. The substrates were coated first by an SC glassy
cients that were similar to that of the alumina and thus layer and second by of SAF. The reactivity between the
would not induce crack formation due to residual substrate and the SAF glass-ceramic was extensively
thermal stresses. Furthermore, as the inorganic compo- investigated as any compositional change may lead to
nent of human teeth is fluoroapatite, a fluoroapatite- the formation of additional crystalline phases which
based glass-ceramic (SAF) was used aiming to propose may inhibit the bioactivity of the material [6]. To
the realised biomaterials even for dental implants. achieve a good control on the reactivity of alumina
towards the applied layers, the time and temperature-
processing conditions were carefully calibrated. A series
of preliminary experiments was performed depositing an
2. Materials and methods SAF layer in direct contact with alumina, but the
coatings showed a high tendency to delaminate due to
2.1. SC and SAF preparation and characterisation the low wettability of SAF on alumina connected with
its very high viscosity. Furthermore, these SAF coatings
Two different materials were used as coatings: a glass extensively reacted with the substrate and this reaction
belonging to the system SiO2 CaO (SC) and a glass- modified their composition leading to the crystallisation
ceramic belonging to the system SiO2 Al2O3 P2O5K2O of undesired, non-bioactive crystalline phases [6]. For
CaO F (SAF). The exact compositions of these two this reason, we decided to use an intermediate SC layer,
coating materials are reported in Table 1 [4,7,8]. which in a previous work showed a very good adhesion
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E. Verne et al. / Biomaterials 23 (2002) 3395 3403 3397
to alumina [8]. In this way, we planned to obtain an coatings. The specimens were glued to stainless steel
outer SAF layer well adherent to the SC one and thus to grips with Araldite AV 119 (Ciba-Geigy) and cured for
the alumina substrate. 40 min at 1201C. The mechanical shear tests on the
Specifically, this approach involved two thermal joined samples were performed with a compression
treatments that were singularly optimised in order to machine (SINTECD/10). Ten specimens were used for
accommodate the different thermal and viscous proper- this test.
ties of SC and SAF. In fact, we needed to combine a
good softening of the coating materials while reducing, 2.5. Cell culture
as much as possible, the interactions between the
substrate and the coating and thus avoiding any Osteoblast-like cells were obtained by enzymatic
undesired modifications of the coating composition isolation from adult human bone trabecular fragments
and structure. obtained at surgery [18]. Briefly, particles 3 5 mm in
The coatings were prepared by controlled deposition diameter were plated in 90 mm tissue culture dishes with
of a suspension of SC or SAF powders in ethanol that 0.2 0.6 g of bone/dish after treatment with bacterial
was poured in a beaker containing the alumina collagenase (Sigma, Milano, Italy). The bone explants
substrates. The quantity of the dispersed powder were cultured in 10 ml ISCOVE S supplemented with
for the deposition process was chosen in order to obtain 20% FBS, 50 U/ml penicillin, 15 mcg/ml streptomycin
the desired coating thickness. The experimental route and 2 mm glutamine (all from Sigma) at 371C in 95%
involved the deposition of a first SC layer on air/5% CO2. Outgrowths of cells from the bone
the substrate, a drying stage to drive-off the ethanol fragments appeared within one week and formed a
and then a thermal treatment at 14501C for 20 min. This confluent monolayer at 3 4 weeks. The isolated bone
thermal treatment was chosen to allow the softening of cells have been characterised including osteoblast
SC and thus a good wetting of the alumina substrate. morphology, alkaline phosphatase expression and hor-
After these steps, the coating was cooled down to room mone responsiveness (PTH, 1,25(OH)2D3).
temperature and an SAF layer was deposited on top of it With the described isolation procedure we repeatedly
by gravity decantation of an SAF powder suspension. obtained an osteoblast-like cell population with 90 95%
After a drying stage, the second layer was thermally viability and 80 90% purity within sixth trypsin
treated at temperatures between 13501C and 14001C for treatment.
10 min. Finally, an annealing stage at 6001C for 2 h was For hystological observations, a cell density of
carried out to release residual thermal stresses. The 2 104 cells/cm2 had been chosen for short time
proposed experimental route was also effective for the incubation period (6 h of cell adhesion) and
maintenance of the SAF properties in the realised 1 104 cells/cm2 one for long term proliferation study
coating as will be deeply reported in a subsequent work. (4 days of cell proliferation). The choice of these cell
densities was based on pilot studies. To promote
2.3. Morphology, composition and structure attachment, the cells were allowed to adhere to the
disks at 371C for 1 h prior to the addition of the total
On the realised coating, a complete characterisation culture medium. To evidence only the effect of the
was performed in order to verify its correspondence with materials on the cell morphology and biochemistry,
the SAF bulk properties. The coating structure was osteoblast-like cells were cultured in ISCOVE s medium
investigated by X-ray diffraction (X Pert Philips dif- supplemented with 5% hot inactivated foetal bovine
fractometer) using the Bragg Brentano camera geometry serum and 2 mm glutamine. Cells cultured in the same
and the CuKa incident radiation. The morphology and conditions on Thermanoxs slides (Nunc, Milano, Italy)
composition of the coatings were assessed by scanning have been used as control.
electron microscopy (SEM Philips 525 M) and composi-
tional analysis by energy dispersion spectrometry (EDS, 2.6. Cell behaviour
Philips-EDAX 9100). For comparative purposes, bulk
SAF glass-ceramic specimens were also characterised. Since the chosen materials were not suitable for
normal transmitted light microscopy, we observed the
2.4. Mechanical characterisation cell morphology and the distribution with fluorescence
microscopy (FM) and scanning electron microscopy
The adhesion between the coating and alumina was (SEM). After 6 h (adhesion test) or 4 days (proliferation
determined by means of comparative shear tests using a test), the biomaterials were rinsed in PBS and then fixed
  single-lap  specimen configuration as described in for 20 min at 601C and stained for 5 min in a 0.025%
literature [13 17]. Sandwiches of alumina (5 15 mm2 acridine orange solution (a nucleic acid staining Fluka,
surface) joined by an SAF/SC/SAF layer were prepared Milano, Italy) [19] for FM observation. For each
using the same time and temperature schedule of the biomaterial, the cell morphology and number were
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3398 E. Verne et al. / Biomaterials 23 (2002) 3395 3403
evaluated using a fluorescent microscope connected to a
GIPS image processing software with an ITI PCV board
image analyser [20]. Osteoblast-like cell number was
referred to a surface area of 0.167 mm2 and was
evaluated at 250 microscope magnification.
After the same incubation times, samples for SEM
analysis were fixed in 4% paraformaldehyde+2.5%
glutaraldehyde in 0.1 m cacodylate buffer, pH 7.4
(Karnowsky s solution at 4%), dehydrated in ethanol
(50 100%) and then in hexamethyldisilazane (all from
Sigma) [21,22]. The samples were then mounted on
appropriate stubs with colloidal silver, sputter-coated
with approximately 20 nm of gold palladium and
directly observed in a Philips 525M SEM at 10 kV.
2.7. Statistics
Fig. 2. Polished cross-sections of an SC/SAF-coated specimen.
Statistical analysis of the data was carried out using a
Dell computer equipped with SPSS for Windows soft-
ware. Duncan test was performed to compare adhesion
and proliferation results for the four materials tested; p
value was obtained from the ANOVA table. The
conventional 0.05 level was considered to reflect
statistical significance.
3. Results
3.1. Morphology, composition and structure
Both the SC and SAF powders are characterised by a
thermal expansion coefficient (9 10 6/1C) similar to
the alumina one (8.5 10 6/1C) and thus are suitable
materials to coat alumina substrates. The realised
coatings consisted of a bi-layer structure: an amorphous
SC layer in direct contact with the ceramic substrate and
an outer SAF glass-ceramic layer on top, as reported in
Fig. 1.
Fig. 2 shows the polished cross-sections of an SC/
SAF-coated specimen: about 400 450 mm thick coatings
SAF
Fig. 3. EDS results of mean analyses performed on the bulk SAF (a)
50µm
and on the surface of the outer SAF coating (b).
SC
400µm
were obtained. The SC intermediate layer (about 300
350 mm thick) shows a very good adhesion to the
alumina substrate without residual porosity. The outer
ALUMINA
SAF layer (about 50 mm thick) shows a continuous
interface with SC and very less porosity. Globally, the
whole coating is well adherent to the ceramic substrate
Fig. 1. Schematic representation of the coating structure. and shows a high degree of homogeneity.
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E. Verne et al. / Biomaterials 23 (2002) 3395 3403 3399
The EDS analyses made on bulk SAF and on the 3.2. Mechanical tests
outer part of the coatings on alumina, revealed that the
SAF glass-ceramic maintained the same starting com- From a mechanical point of view, the adhesion test
position after the thermal treatment involved with the showed the shear strength between 20 and 30 MPa70.3.
coating process. For this purpose, in Fig. 3a and b the In some cases, the mechanical test involved the failure of
EDS results of semi-quantitative analyses performed on the glue between the substrate and the metallic grips
the bulk SAF glass-ceramic and on the surface of the without breaking the coating/substrate interface. This
outer SAF coating are reported. latter behaviour clearly indicated that the interface
Fig. 4 reports an SEM micrograph of the coating strength of those samples was at least of the magnitude
surface, showing its glass-ceramic nature that was also that caused the glue failure. In a few cases, the failure
confirmed by the XRD results. Furthermore, the XRD occurred at lower loads, but the surface fracture
analysis performed on the coating surface showed that observation revealed the presence of residual porosity,
the crystalline phase detected corresponded to the bulk which induced an earlier and undesired crack propaga-
SAF one. For this purpose, in Fig. 5 the XRD patterns tion.
of the realised coating compared to the bulk SAF are
reported: in both cases, the only crystalline phase
detectable corresponds to fluoroapatite. The mainte- 3.3. Cell results
nance of the crystallisation of a fluoroapatite phase in
the SAF outer layer was obtained with an optimised Both in FM Acridine-orange-stained cells and at
thermal treatment at 13501C for 10 min. SEM analysis (Fig. 6), osteoblast-like cells attached for
6 h onto glass-ceramic SAF (Fig. 6c and d) and on the
coating (Fig. 6d and h) evidenced good cell adhesion
similar to Thermanoxs coverslips (Fig. 6a and e). This
latter material was used as control material because of
its good cell attachment and growth properties [23].
Bulk alumina (Fig. 6b and f) evidenced on its surface
less spread cells. Cell behaviour on coating, being better
than bulk alumina and analogous to bulk SAF
evidenced no Al3+ diffusion from the alumina sub-
strates during the coating procedure. The same beha-
viour was observed for cell proliferation (Fig. 7). In
particular, cells on alumina (Fig. 7b and f) were more
rounded and less spread than cells on Thermanoxs
coverslips control (Fig. 7a and e) and on SAF (Fig. 7c
and g), where cells appeared more closely in contact with
the biomaterial and were well flattened. Cells on the
coating (Fig. 7d and h) appeared similar to cells on bulk
SAF and a surface with plenty of fluoroapatite crystals
Fig. 4. SEM micrograph of the coating surface. was observed.
SAF coating on
alumina
SAF bulk
15 20 25 30 35 40 45 50 55
2 Theta
Fig. 5. XRD patterns of the coating surface (a) and compared to the bulk SAF (b).
Intensity [a.u.]
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3400 E. Verne et al. / Biomaterials 23 (2002) 3395 3403
Fig. 6. Morphology of osteoblast-like cells adhering on cell culture dish, alumina bulk SAF and the coating surface. Human osteoblast-like cells in
medium containing 5% hot inactivated FBS were attached for 6 h on Thermanoxs dish (a, e), on alumina (b, f), on bulk SAF (c, g) and on the
coating surface (d, h). Samples were examined by fluorescence microscopy at 400 magnification (a d) or scanning electron microscopy at 3000
magnification (e h).
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E. Verne et al. / Biomaterials 23 (2002) 3395 3403 3401
Fig. 7. Morphology of osteoblast-like cells proliferating on cell culture dish, alumina, bulk SAF and the coating. Thermanoxs dish (a, e), alumina
(b, f), bulk SAF (c, g) and coating surface (d, h) were incubated for 4 days with human osteoblast-like cells in a medium containing 5% hot
inactivated FBS. Samples were examined by fluorescence microscopy at 400 magnification (a d) or scanning electron microscopy at 1500
magnification (e h).
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3402 E. Verne et al. / Biomaterials 23 (2002) 3395 3403
chosen and the time and temperature schedule was
*
350
*
optimised in order to ascertain a very good contact
6hours 4days
between the substrate and the coating as well as the
300
absence of remarkable chemical reactions at the coating
interface. The firing temperature of the first layer was
250
high enough to allow the alumina wetting by the SC
glass and thus to realise a strong interface with the
200
ceramic substrate. The thermal treatment of the second
layer, the SAF one, was also lower than the previous one
150
but still high enough to involve a good adhesion and
100 infiltration between the two applied layers (SC and
SAF). The obtained results showed that the SC layer
50 provided a good surface on which the SAF glass-
ceramic could adhere without delamination during the
0 softening involved in the second thermal treatment.
Furthermore, both the temperatures and the times of the
two thermal treatments were sufficiently low and brief to
avoid an extensive Al3+ diffusion into the coating. This
bi-layer approach, involving two different coating
Fig. 8. Osteoblast-like cell quantification on the three studied sub- materials and thus two different thermal treatments,
strates. Cell number was the mean of 40 measurements on three
was therefore very effective to control both the
experiments (n ź 120) and was expressed as a cell percentage with
wettability of the substrate and the interface reactivity.
respect to loaded cells, *po0:05:
In fact, in this way, a well adherent outer SAF layer with
no compositional and structural differences in compar-
Cell count evidenced no statistical differences between ison with the bulk one was successfully obtained.
osteoblast-like cell number at 6 h adhesion on the three The shear test is an effective method to study the
different materials tested compared to Thermanoxs fracture energy of the coatings. This is a comparative
control cell culture dishes (Fig. 8). method which allows to determine the interfacial shear
At four days proliferation, osteoblast-like cell number strength of different kinds of coatings deposited on
was significantly higher on glass-ceramic materials (bulk substrates of various nature (metallic, ceramic, compo-
SAF and the coating) than on alumina. This latter site). The results obtained for this kind of coatings are
observation clearly indicated that   SAF glass-ceramic comparable with those of the most diffused bioactive
surfaces  induce better cell behaviour and higher cell coatings mechanically tested in the same conditions
proliferation than alumina. Furthermore, no differences [9,10,16] (i.e hydroxyapatite, glasses and composites
between bulk SAF and the coating were observed deposited on Ti6Al4V by vacuum plasma spray, or
confirming the absence of an extensive Al3+ diffusion glass, glass-ceramic and composite coatings deposited
from the alumina substrates during the coating proce- by conventional firing on zirconia or alumina sub-
dure. strates).
In vitro tests highlighted significant differences in 4
days proliferation of human primary osteoblast-like
4. Discussion and conclusions cells grown onto bulk SAF compared to cells grown
onto bulk alumina and thus suggested a direct influence
The thermal expansion coefficients of SC and SAF of the substrate on cellular turnover. One of the main
were very similar to each other and were also similar to regulators of proliferative rate in anchorage-dependent
that of alumina, which avoided the formation of cracks cells is the shape. Cells in a rounded configuration such
in the coating or at the interface between the coating and as on alumina (Figs. 6f and 7f) divide at a lower rate
the substrate due to residual thermal stresses. This than those flattened and well spread on bulk SAF.
feature was also assured by the annealing step at the end Consequently, cells which attach themselves to the
of the coating preparation. biomaterials but spread little will show significatively
Aiming to avoid the modification of the SAF lower proliferative rates than those in biomaterials
composition and structure, and then to affect its good which allow greater spreading (Fig. 8). A comparison
bioactivity [4,5], it was also necessary to control between cells grown onto bulk SAF and onto the
eventual detrimental reaction between the substrate coating highlighted no numerical and morphological
and the coating. In the meantime, it was also very differences between them confirming that there were no
important to obtain a continuous interface between the significant compositional and structural differences
coating and the substrate. Thus, a bi-layer structure was between bulk SAF and the coated cell contact surfaces.
Ctr
Coating
Alumina
Bulk SAF
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E. Verne et al. / Biomaterials 23 (2002) 3395 3403 3403
"
In conclusion, the application of sieved powders on [8] Verne E, Vitale Brovarone C, Ravaglioli A, Krajewski A.
Multilayered bioactive coatings on Al2O3 substrates. In: Biocera-
alumina substrates via controlled deposition proved to
mics. Singapore: World scientific, 1999. p. 491 4.
be low cost, effective way to realise adherent coatings of
!
[9] Ferraris M, Verne E, Appendino P, Moisescu C, Krajewski A,
the desired thickness. The interposition of an SC layer
Ravaglioli A, Piancastelli A. Coatings on zirconia for medical
was successful in allowing a good softening and
applications. Biomaterials 2000;21:765 73.
!
spreading of SAF and in avoiding the crystallisation of [10] Verne E, Ferraris M, Jana C, Paracchini L. BioveritsI base glass/
Ti particlulate biocomposite:   in situ  vacuum plasma spray
undesired crystalline phases. The coatings showed a
depositino. J Eur Ceram Soc 2000;20:473 9.
limited reactivity towards alumina since they did not
[11] Jiang G, Shi D. Coating of hydroxyapatite on highly porous
change their starting composition during the thermal
Al2O3 substrate for bone substitute. J Biomed Mater Res (Appl
treatment involved in their preparation. Thus, the
Biomater) 1998;43:77 81.
proposed approach led to the achievement of an outer [12] Kimihiro Y, Eiko Y, Xufeng D, Masaharu N, Takao U,
Motohide M. Electroforetic coating of multilayered apatite
SAF glass-ceramic coating maintaining the good biolo-
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gical properties of the bulk one.
1998;43:46 53.
Osteoblast-like cells result from the coatings, showing
[13] Ferraris M, Salvo M, Isola C, Appendino Montorsi M, Kohyama
osteoblast character analogous to bulk SAF and are
A. Glass-ceramic joining and coating of SiC/SiC for fusion
better when compared to bulk alumina one, which is a applications. J Nucl Mater 1998;(258 263):1546 50.
[14] Isola C, Salvo M, Ferraris M, Appendino Montorsi M. Joining of
promising index of bone material integration and of its
surface modified carbon/carbon composites using a barium
possible in vivo applications.
aluminum boro-silicate glass. J Eur Ceram Soc 1998;18:1017 24.
[15] Lin J, Kato H. Interfacial structure and strength of silicon silicon
diffusion bond. Mater Sci Technol 1995;11:1035 40.
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Italy) for the alumina substrates.
[17] Bushby RS, Scott VD. Liquid phase bonding of aluminium and
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