Materials Science and Engineering A323 (2002) 1 – 8
www.elsevier.com/locate/msea
Aluminum phosphate sealed alumina coating: characterization of
microstructure
Minnamari Vippola a,*, Samppa Ahmaniemi a, Jaakko Keränen a, Petri Vuoristo a,
Toivo Lepistö a, Tapio Mäntylä a, Eva Olsson b
a
Tampere Uni6ersity of Technology, Institute of Materials Science, P.O. Box 589, 33101 Tampere, Finland
b
Uppsala Uni6ersity, Analytical Materials Physics, P.O. Box 534, 75121 Uppsala, Sweden
Received 5 October 2000; received in revised form 11 January 2001
Abstract
The microstructure of aluminum phosphate sealed plasma-sprayed alumina coating was characterized by X-ray diffractometry,
scanning electron microscopy, and analytical transmission electron microscopy. Microstructural characterization was carried out
to identify the phases of the coating and to understand better the strengthening effect of aluminum phosphate sealant in the
coating. The main phases in the coating are metastable g-Al2O3 and stable a-Al2O3. The overall structure of the coating is lamellar
with columnar g-Al2O3 grains. The aluminum phosphate sealant shows good penetration into the coating to the depth of about
300 mm filling the structural defects such as pores, cracks and gaps between the lamellae. The sealant in the coating has the relative
composition of 26 at.% aluminum and 74 at.% phosphorus giving the molar ratio P:Al of 3, which refers to the metaphosphates
Al(PO3)3. There is also some crystalline aluminum phosphate in the coating, in the form of berlinite-type orthophosphate AlPO4,
owing to the reaction between the sealant and the alumina coating. Thus, the phosphate bonding in the alumina coating is based
both on chemical bonding resulting from the chemical reaction with the alumina coating and on adhesive binding resulting from
the formation of the condensed phosphates in the structural defects of the coating. © 2002 Elsevier Science B.V. All rights
reserved.
Keywords: Plasma-sprayed alumina coatings; Aluminum phosphate sealing; Microstructure of alumina coating; Microscopy of alumina coatings
1. Introduction
Plasma-sprayed oxides such as alumina are widely
used as corrosion- and wear-resistant coatings in industry. Plasma-sprayed coatings have a layered structure
formed when molten droplets of powder flatten and
solidify on the surface of the substrate. The lamellar
structure of the alumina coating always has some
porosity due to entrapped gas, shadowing of the previously solidified droplets, and cracking of the lamellae.
These pores and the incomplete bonding between lamellae decrease the strength, wear resistance, and corrosion
resistance of the coating [1 – 3]. One way to enhance
wear and corrosion resistance of alumina coatings is
through sealing treatment. The sealing is commonly
* Corresponding author. Tel.: + 358-3-3652350; Fax: +358-33652330.
E-mail address: [email protected] (M. Vippola).
achieved by impregnating the coating with inorganic or
organic solutions [4,5]. According to our earlier studies
[6–8], aluminum phosphate provides an effective sealing for alumina coating. It improves significantly the
abrasive and erosive wear resistance as well as the
corrosion resistance of the alumina coating [6–8]. There
have also been a few other studies on phosphate-based
sealing treatments for coatings [4,9 –13]. For instance,
in the late 1980s, orthophosphoric acid [9] and, recently, commercial phosphate solutions [4] have been
studied as a sealant for porous oxide coatings. Sealing
with aluminum phosphate originates from the field of
refractories and especially their binders. They are
formed from the reaction of orthophosphoric acid or
acidic aluminum phosphates with alumina [10,14 –17]
that results in the formation of crystalline or amorphous aluminum phosphates. The reaction yields different products depending on the composition of binder,
the reaction time, and the temperature. The phosphate
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M. Vippola et al. / Materials Science and Engineering A323 (2002) 1–8
bonding may occur in two ways: by chemical bonding
resulting from the chemical reaction, and by adhesive
binding resulting from the formation of crystalline or
amorphous condensed phosphates having appropriate
adhesive properties for binding surfaces [10,14–17].
Aluminum phosphates have also found other applications as a binder such as for metal– matrix composites
[18] and for coatings as already mentioned.
Although alumina coatings are widely used in industry, there are few studies on their microstructure [1–
3,19–22]. An even more unknown area is the
microstructure of the aluminum phosphate sealed alumina coatings. Therefore, X-ray diffractometry (XRD),
scanning electron microscopy (SEM), and analytical
transmission electron microscopy (AEM) were used for
a microstructural characterization of the aluminum
phosphate sealed alumina coatings, to identify the phases
in the coating, and to characterize the aluminum phosphate sealant in order to better understand the sealing
and strengthening mechanisms.
2. Experimental procedure
The alumina coatings were produced with PlasmaTechnik A3000S plasma spray equipment by Sulzer
Metco AG (Switzerland). The particle size of the Amperit
spray powder Al2O3 740.1 from H.C. Starck GmbH
(Germany) ranged from 22.5 to 45 mm. The coating was
sprayed with the optimized parameters: current I, 610 A;
voltage U, 72 V; gas concentrations for Ar and H2, 41
and 14 slpm. The spray distance was 120 mm, and the
coating was sprayed to a thickness of about 800 mm. The
plasma-sprayed alumina coating was sealed with aluminum hydroxide (Al(OH)3) and orthophosphoric acid
(H3PO4) solution diluted with 20 wt.% deionized water.
The Al(OH)3:H3PO4 ratio was 1:4.2 by weight, giving the
molar ratio P/Al of about 3. The solution was permitted
to react at a temperature slightly above the room
temperature until the solution became clear. Then it was
spread onto a porous plasma-sprayed alumina coating
and allowed to impregnate into the coating at room
temperature under normal pressure for 12 h. Thereafter,
the impregnated coating was heat-treated for 2 h at
100°C, followed by 2 h at 200°C, and 2 h at the final
curing temperature of 400°C.
XRD of the coatings were performed with a Model
D500 diffractometer by Siemens (Germany) using copper
Ka radiation. The scan step of 2q in the measurement
was 0.02° with a step time of 1.2 s. Some of the XRD
analysis was carried out for sealed alumina coating
surfaces where a layer of the coating was removed by
grinding with dry SiC paper. The sealed alumina coating
was also examined with scanning electron microscope
Model XL30 (Philips, The Netherlands), equipped with
an energy dispersive spectrometer (EDS) (Model DX-4;
EDAX International, USA). Cross-sectional AEM samples of the coating were examined with analytical transmission electron microscopes (Models JEM 2010 and
JEM 2000FX II; JEOL, Japan) and with a Link
AN10000 energy dispersive spectrometer by Oxford
Instruments Microanalysis Group (UK), operated at 200
kV. Cross-sectional AEM samples were embedded into
the Ti grids, thinned by mechanical polishing with a
‘Dimple Grinder’ Model 656 (Gatan Inc., USA), and
followed by ion milling with the Precision Ion Polishing
System Model 691 (Gatan Inc., USA).
3. Results and discussion
3.1. XRD analysis
Spray powder of Al2O3 consists of stable a-Al2O3
having a hexagonal unit cell with a= b= 0.475 nm and
c= 1.299 nm (JCPDS File Card No. 10-173). The crystallographic structure of alumina changes during plasma
spraying [2]. Fig. 1(a) shows the XRD spectrum of the
as-sprayed alumina coating. The alumina phases are
indicated in the spectrum with symbols g and a. The
XRD spectrum shows that the coating consists mostly of
metastable g-Al2O3 having a face-centered cubic unit cell
with a= 0.790 nm (JCPDS File Card No. 10-425) and
some a-Al2O3.
The results of the XRD analysis of the as-sprayed
coating are in good agreement with the literature [2,19–
22]. The reason for the formation of metastable g-Al2O3
in the coating is the high cooling rate of the molten
particles and the easy nucleation of g-Al2O3 from the melt
[2,20,21]. The presence of a-Al2O3 is due either to
unmolten particles or to the transformation from the g
to the a phase, or both [21,22]. The transformation from
the g to the a phase can occur when the heat removal
is not sufficient for rapid cooling, such as in the middle
layers of the rather thick coating [22]. The SEM micrograph for the polished cross-section of the sealed
coating (Fig. 2(a)) shows that there are some round-like
particles, some of which are marked with arrows, in the
lamellar coating structure. These particles resemble unmelted or partially melted powder particles, indicating
that the presence of a-Al2O3 in the coating is due to
unmolten alumina.
The basic reaction route for alumina or aluminum
hydroxide with orthophosphoric acid is as follows [16].
First, the solution transforms to either acidic mono-aluminum phosphate (Al(H2PO4)3) or an amorphous phase.
Heating of the solution causes dehydration and the
formation of the metaphosphates Al(PO3)3. Alu-minum
metaphosphates (Al(PO3)3) are trivalent cation longchain polyphosphates, which have five different crystalline forms A, B, C, D, and E [23,24]. Other
M. Vippola et al. / Materials Science and Engineering A323 (2002) 1–8
reactions with the alumina produce phosphates with a
lower P/Al molar ratio, such as an orthophosphate
AlPO4. Aluminum orthophosphate (AlPO4) is a trivalent cation monophosphate having atomic arrangements similar to the silica structure. All the typical
polymorphs of silica have also been observed for this
compound: two quartz-like (berlinite), two cristobalitelike, and two tridymite-like polymorphs [23]. However,
the reaction route and the phases from the aluminum
phosphate reactions vary, for instance, depending on a
P/Al molar ratio of the solution, the reaction time and
the reaction temperature [10,16,17].
During the heat treatment, the excess of aluminum
phosphate formed a cake-like porous layer of crystallized sealant on the top of the coating. The porous layer
was removed and crushed into the powder for XRD
analysis (Fig. 3). In our earlier studies [25], one of the
main phases in the sealant was identified to be cyclohexaphosphate (Al2P6O18) having a monoclinic unit cell
with a=0.88 nm, b =1.56 nm, c = 0.63 nm, and i=
108°. Fig. 1(b) shows the XRD spectrum for the sealant
in which the cyclohexaphosphate peaks are marked C.
The average composition of the sealant was 26 at.%
aluminum and 74 at.% phosphorus, giving the P/Al
molar ratio of 3 [25]. The other phases in the sealant
were other polymorphs of metaphosphates (Al(PO3)3).
The metaphosphate polymorph B is known to correspond to cyclophosphates and more exactly to a cyclic
hexameric anion P6O18 [24]. Aluminum cyclohexaphosphate (Al2P6O18) is a trivalent cation cyclohexaphos-
3
phate and is isotypic with the corresponding chromium
compound [24]. The results of the phase analysis for the
sealant are in agreement with literature [10,16,17,23,24].
Due to the heat treatment, the aluminum phosphate
sealant solution has been dehydrated and has formed a
mixture of long-chain polyphosphates Al(PO3)3 including the cyclohexaphosphate Al2P6O18 that corresponds
to the B-type polymorph of Al(PO3)3.
Fig. 1(c),(d) show the XRD patterns of the aluminum
phosphate sealed alumina coating. The XRD analysis
was first carried out on the polished surface where the
cake-like porous sealant layer had been removed and
then after removing a 50 mm layer of the coating from
the surface (Fig. 3). Some small extra peaks marked
with B and X in the diffractograms of Fig. 1(c),(d)
indicate the presence of phosphate phases in the sealed
coating. It should be noted that it is difficult to identify
the peaks reliably on the basis of the XRD spectrum
because of the small number of the peaks and the high
background level due to the many strong alumina
peaks. The best fit to the peaks, labelled B, could be
achieved with the berlinite-type aluminum orthophosphate (AlPO4), having a hexagonal unit cell with a=
b=0.49 nm and c= 1.09 nm (JCPDS File Card No.
10-423). The berlinite phase is not the only phosphate
phase in the coating. There are other peaks, marked
with X, which seem to correspond to the peaks of the
metaphosphate Al(PO3)3 phases. There is probably also
amorphous aluminum phosphate in the coating, although the verification of this was impossible on the
Fig. 1. XRD patterns (a) for the as-sprayed alumina coating, (b) for the aluminum phosphate sealant, (c) for the polished surface of the sealed
alumina coating, (d) for the ground surface of the sealed alumina coating after removing a 50 mm layer, and (e) for the ground surface of the
sealed alumina coating after removing a 350 mm layer. The separate alumina phases are indicated with the symbols g and a, cyclohexaphosphate
with C, aluminum orthophosphate with B, and other aluminum phosphates with X.
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M. Vippola et al. / Materials Science and Engineering A323 (2002) 1–8
Fig. 2. SEM micrographs of the polished cross-section of the sealed
coating. (a) The particles (marked with arrows) resemble the unmelted or partially melted alumina powder particles. (b) The topside
of the coating is less porous. The sealant penetration to a depth of
about 300 mm is seen.
basis of the measured XRD patterns. The number and
the intensity of the small extra peaks in the XRD
pattern of the polished surface (Fig. 1(c)) are higher
than those of the ground surface (Fig. 1(d)). Thus, there
is less crystalline aluminum phosphate deeper inside the
coating.
The results of XRD studies for the aluminum phosphate sealed coating can be explained on the basis of
the literature [10,14–17] indicating that, in addition to
the metaphosphates Al(PO3)3, the orthophosphates
AlPO4 can be formed due to the availability and further
reaction with alumina. Aluminum phosphate sealant
closer to alumina lamellae surfaces is sufficiently high in
aluminum content to form orthophosphates AlPO4 [17].
This also indicates that the sealing mechanism of aluminum phosphate in the alumina coating is not only
based on adhesive binding resulting from the formation
of crystalline or amorphous condensed phosphates in
the structural defects of the coating, but also on chemical bonding that could result from the chemical reaction of the sealant with the alumina coating. Anyway,
the reliable identification of the type and phases of the
aluminum phosphate in the coating and on the top of
the coating is difficult on the basis of XRD analysis.
There are many choices of phases and such factors as
the P/Al molar ratio of the solution, and the reaction
time and temperature, which affect the products of
aluminum phosphate reactions [10,16,17].
The penetration depth of the aluminum phosphate
sealant was investigated with a layer removal technique.
XRD analysis was repeated after removing 50 mm
layers from the surface down to a depth of 400 mm. The
average penetration depth of the copper Ka radiation
to the alumina is about 20–30 mm. Fig. 1(e) shows the
XRD pattern of the aluminum phosphate sealed alumina coating where a 350 mm layer has been removed
from the coating surface (Fig. 3). It was found that
Fig. 3. Schematical description for the aluminum phosphate sealed alumina coating after the sealing heat treatment. Regions for the XRD analysis
(b), (c), (d) and (e) are indicated.
M. Vippola et al. / Materials Science and Engineering A323 (2002) 1–8
5
Fig. 4. The structure of the fractured cross-section of the sealed
coating. The coating has a lamellar structure with some structural
defects such as pores and gaps between the lamellae.
aluminum phosphate sealant had penetrated into the
coating to a depth of about 300 mm. No traces of
aluminum phosphate phases were observed in the spectra at further depths. The reason for the incomplete
penetration of the sealant into the alumina coating can
be the reaction between the sealant and the coating,
which retards the penetration.
3.2. Microstructural studies
The microstructure of the coating was determined
from cross-sectional samples of the sealed coating, from
both the polished and fractured surface. Fig. 2(b) shows
the SEM micrograph of the polished surface of the
coating cross-section. The coating presents less porosity
in the topside of the coating, indicating that the sealant
has penetrated into the coating to a depth of about 300
mm, about the same depth as found in the XRD studies.
The aluminum phosphate penetration was also confirmed with the EDS analysis, which indicated phosphorus in the topside of the coating, but not below 300 mm.
In the coating (Fig. 2(a)), there are also some particles,
some of which are marked with arrows, resembling
unmelted or partially melted alumina powder particles.
These particles explain the presence of a-Al2O3 in the
coating. Fig. 4 shows the structure of the fracture
surface of the sealed coating cross-section. The alumina
coating has a lamellar structure. There are also some
pores and gaps between the lamellae even in the sealed
coating. Thus, the aluminum phosphate sealant has not
sealed the coating completely. The alumina coating has
always some closed porosity, which is impossible to
seal. Fig. 5 shows a polished cross-section of the sealed
coating studied with elemental X-ray mapping. The
elemental mapping of phosphorus (Fig. 5(c)) at the
depth of about 50 mm from the surface indicates good
penetration of aluminum phosphate sealant into the
Fig. 5. The polished cross-section of the sealed coating studied with
elemental X-ray mapping. (a) The region of the coating for elemental
mapping. (b) The selected area for elemental mapping. (c) The
phosphorus map for the selected area.
M. Vippola et al. / Materials Science and Engineering A323 (2002) 1–8
6
Fig. 6. (a) AEM image from the sealed coating cross-section. Coating lamellae consist of columnar g-Al2O3 grains, marked A, extending through
the lamella thickness. Between the lamellae, there is aluminum phosphate sealant, which has filled the gaps between the lamellae. P in the sealant
indicates the area for electron diffraction, and arrows indicate other sealant areas. (b) Electron diffraction pattern [011] from the columnar g-Al2O3
grain marked A. (c) Electron diffraction pattern from the aluminum phosphate sealant between the lamellae, indicated with P.
coating following the structural defects, such as pores,
cracks, and gaps between the lamellae.
3.3. AEM studies
The AEM studies were carried out for the cross-sec-
tional coating samples. Fig. 6(a) shows a cross-section
of the sealed alumina coating. As can be seen, the
coating lamellae consist of columnar grains extending
through the lamella thickness. The columnar grains are
approximately equiaxed on the plane parallel to lamellae. Between the lamellae, there can also be seen alu-
M. Vippola et al. / Materials Science and Engineering A323 (2002) 1–8
minum phosphate sealant. The sealant has filled the
gaps and cracks between the lamellae, and such areas
are indicated with arrows in Fig. 6(a). Fig. 6(b) presents
the [011] electron diffraction pattern from the columnar
grain marked with A in Fig. 6(a). The electron diffraction pattern from the grain ensures that it is g-Al2O3
with a face-centered cubic unit cell. Fig. 6(c) presents
7
the electron diffraction pattern from the aluminum
phosphate sealant, marked P in Fig. 6(a). The ring
pattern indicates the amorphous or nanocrystalline
structure of the sealant. The relative composition of the
sealant in the coating is of about 26 at.% aluminum and
74 at.% phosphorus, giving the molar ratio P/Al of
about 3, which refers to metaphosphates Al(PO3)3. Fig.
Fig. 7. (a) AEM image from the aluminum phosphate sealant between the lamellae indicated with the marker. The solid black line from the sealant
to the coating indicates the path for the EDS point analysis. The numbers 1 – 6 next to the line indicate analysis points. (b) The distribution of
aluminum and phosphorus along the point analysis line.
8
M. Vippola et al. / Materials Science and Engineering A323 (2002) 1–8
7(a) shows a closer image of the amorphous aluminum
phosphate. The solid black line from the sealant to the
coating indicates the path along which the EDS point
analysis was performed, and numbers 1– 6 next to the
line indicate separate measurements. Fig. 7(b) shows
the distribution of aluminum and phosphorus along the
line shown in Fig. 7(a). The average composition of the
sealant in the coating is similar to the sealant on the top
of the coating. However, the XRD measurements indicated the crystalline aluminum phosphate phase AlPO4
with a molar ratio P/Al of 1 in the coating. This
suggests that the aluminum phosphate sealant forms,
due to the reaction with the alumina coating, phosphates with a lower P/Al molar ratio into the reaction
layer between the sealant and the coating lamellae.
There, the Al concentration is sufficiently high for
AlPO4 formation. Therefore, more detailed studies of
the areas between the sealant and the lamellae are
needed.
4. Conclusions
The results of the XRD, microstructural, and AEM
studies show that the coating structure is a lamellar
structure with the columnar g-Al2O3 grains extending
through the lamella thickness with a face-centered cubic
unit cell. There is also some a-Al2O3 in the coating due
to the unmelted or partially melted alumina particles.
During the heat treatment, the excess of aluminum
phosphate sealant dehydrates and forms a porous cakelike layer of crystallized sealant onto the top of the
coating. The crystallized sealant is the mixture of longchain polyphosphates Al(PO3)3 including the cyclohexaphosphate Al2P6O18 that corresponds to the B-type
polymorph of Al(PO3)3. In the coating, the aluminum
phosphate sealant shows good penetration into the
coating to a depth of about 300 mm following structural
defects such as pores, gaps, and cracks between the
lamellae. Aluminum phosphate sealant in the coating
has the relative composition of 26 at.% aluminum and
74 at.% phosphorus, giving the molar ratio P/Al of 3,
which refers to a long-chain polyphosphate Al(PO3)3.
The XRD studies indicated a small amount of crystalline aluminum phosphate, berlinite-type orthophosphate AlPO4, in the coating. The orthophosphate
AlPO4 has formed in addition to the metaphosphates
Al(PO3)3 due to the availability and further reaction
with the alumina coating. This indicates that the sealing
mechanism of aluminum phosphate in the alumina
coating is not only based on adhesive binding resulting
from the formation of the condensed phosphates, but
also on chemical bonding resulting from the chemical
reaction with the alumina coating. It should be noted
that both the a-Al2O3 and the aluminum orthophos-
phate AlPO4 have only been detected by the XRD
measurements. Therefore, the more reliable determination of the phases in the aluminum phosphate sealed
alumina coating requires more profound studies by
AEM.
Acknowledgements
The authors thank the Academy of Finland for
financing this work.
References
[1] S. Safai, H. Herman, Thin Solid Films 45 (1977) 295 –307.
[2] R. McPherson, Thin Solid Films 83 (1981) 297 – 310.
[3] R. McPherson, B.V. Shafer, Thin Solid Films 97 (1982) 201 –
204.
[4] P. Chraska, V. Brozek, B.J. Kolman, J. Ilavsky, K. Neufuss, J.
Dubsky, K. Volenik, in: C. Coddet (Ed.), Thermal Spray; Meeting the Challenges of the 21st Century, ASM International,
Materials Park, OH, 1998, pp. 1299 – 1304.
[5] J. Knuuttila, P. Sorsa, T. Mäntylä, J. Thermal Spray Tech. 8
(1999) 249 – 257.
[6] K. Niemi, P. Sorsa, P. Vuoristo, T. Mäntylä, in: C.C. Berndt, S.
Sampath (Eds.), Thermal Spray Industrial Applications, ASM
International, Materials Park, OH, 1994, pp. 533 – 536.
[7] E. Leivo, M. Vippola, P. Sorsa, P. Vuoristo, T. Mäntylä, J.
Thermal Spray Tech. 6 (1997) 205 – 210.
[8] S. Ahmaniemi, P. Vuoristo, T. Mäntylä, in: J. Meneve, K.
Vercammen (Eds.), Proceedings of the 2nd COST 516 Tribology
Symposium, Flemish Institute for Technological Research,
Antwerpen, 1999, pp. 185 – 192.
[9] A. Borisova, A. Tkachenko, Sov. Powder Metall. Met. Ceram.
26 (1987) 888 – 892.
[10] J. Morris, P. Perkins, A. Rose, W. Smith, Chem. Soc. Rev. 6
(1977) 173 – 194.
[11] A. Frolov, M. Trofimov, E. Verenkova, Chem. Abstr. 68 (1968)
52889.
[12] W. Koenig, G. Cope, Chem. Abstr. 68 (1968) 52889v.
[13] N. Silina, R. Sagitullina, Chem. Abstr. 71 (1969) 128162n.
[14] W. Kingery, J. Am. Ceram. Soc. 33 (1950) 239 – 241.
[15] M. O’Hara, J. Duga, H. Sheets, Ceram. Bull. 51 (1972) 590 –595.
[16] J. Reed, Introduction to the Principles of Ceramic Processing,
Wiley, New York, 1988, pp. 166 – 168.
[17] F. Gonzalez, J. Halloran, Ceram. Bull. 59 (1980) 727 –731, 738.
[18] J.-M. Chiou, D. Chung, J. Mater. Sci. 28 (1993) 1435 –1487.
[19] D. Fargeot, P. Lortholary, A. Dauger, TEM study of plasma
sprayed alumina coatings, in: P. Vincenzini (Ed.), Ceramic Powders, Elsevier, Amsterdam, 1983, pp. 977 – 985.
[20] R. McPherson, J. Mater. Sci. 15 (1980) 3141 – 3149.
[21] R. McPherson, Surf. Coat. Technol. 39/40 (1989) 173 –181.
[22] J.M. Guilemany, J. Nutting, M.J. Dougan, J. Thermal Spray
Tech. 6 (1997) 425 – 429.
[23] M.-T. Averbuch-Pouchot, A. Durif, Topics in Phosphate Chemistry, World Scientific Publishing, Singapore, 1996, pp. 82, 196,
284.
[24] A. Durif, Crystal Chemistry of Condensed Phosphates, Plenum
Press, New York, 1995, pp. 146, 288.
[25] M. Vippola, J. Keränen, X. Zou, S. Hovmöller, T. Lepistö, T.
Mäntylä, J. Am. Ceram. Soc. 83 (2000) 1834 – 1836.