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Surf. Interface Anal. 26, 1027È1034 (1998)
XPS Studies of Low-temperature Plasma-produced
Graded Oxide–Silicate–Silica Layers on Titanium
Michael Steveson, Pawittar S. Arora and Roger St. C. Smart*
Ian Wark Research Institute, University of South Australia, The Levels Campus, Mawson Lakes, South Australia 5095
A method has been developed to produce a silica coating bonded to an oxidized titanium substrate, graded in
composition and structure through the surface layer, providing the base for deposition of bioactive glasses and
hydroxyapatite. A low-temperature air/water/tetraethoxysilane plasma coating method is reported. Characterization of the graded surface layer using XPS and SEM is compared with previously reported studies of similar
layers on nickel substrates. Depth proÐling and the modiÐed Auger parameter were used to elucidate the structure
of the interphase region between the titanium and silica. A functionally-graded layer, from the titanium metal
through the oxide, orthosilicates, pyrosilicates, chain silicates and layer silicates to bulk silica, was demonstrated.
The thickness of the layer was in the range 30–50 nm, depending on the duration of oxidation and tetraethoxysilane deposition. ( 1998 John Wiley & Sons, Ltd.
KEYWORDS : XPS ; x-ray photoelectron spectroscopy ; Ti ; SiO ; Auger parameter ; depth proÐling
Owing to their strength, metals (e.g. CoCr alloy, stainless steel, titanium) are the materials of choice for stressbearing medical and dental implants. When an implant
is Ðtted, the way the gap between the implant and the
bone is Ðlled by the body is determined by the type of
material used. The body surrounds materials such as
stainless steel, referred to as a biotolerant material, with
a layer of Ðbrous tissue. This prevents direct boneÈ
implant contact and reduces the rigidity of the joint,
thus stainless-steel implants have been replaced in many
instances by titanium implants. The spaces between a
bioinert material, such as titanium, and the bone are
Ðlled by growing bone, producing a much more rigid
In the 1980s, bioactive materials, e.g. bioglass and
hydroxyapatite, were developed for implant use.1,2
Bone integrates with the surface of these compounds
resulting in the strongest interface possible.1
These ceramic or glass bioactive materials have lower
mechanical strength than the metals previously used for
implant manufacture and hence are generally applied as
surface coatings to the metals rather than as bulk
materials. With the development of high strength at the
bone/implant interface with modern bioactive materials,
there is increased need for correspondingly highstrength adhesion between the coating and the metal
implant. Currently, titanium implants are coated with
hydroxyapatite using plasma spraying. In this process, a
high-speed, high-temperature (15 000 ¡C) stream of partially melted hydroxyapatite particles is Ðred at the titanium surface, resulting in a relatively thick ([10 lm)
coating that is usually poorly bound to the titanium
* Correspondence to : R. St. C. Smart, Ian Wark Research Institute,
University of South Australia, The Levels Campus, Mawson Lakes,
South Australia 5095
CCC 0142È2421/98/131027È08 $17.50
( 1998 John Wiley & Sons, Ltd.
surface. It is brittle and often contains a variety of
unwanted compounds (e.g. CaO) produced by thermal
decomposition of the hydroxyapatite. One study found
that the load required for failure of the titanium/apatite
interface was 12 MPa, whereas the bone/apatite interface failed at 17 MPa.1 Hence, the advantages of the
new high-strength bioactive materials was not being
realized due to poor coating quality.
Other methods to coat titanium with bioactive
materials have been suggested, e.g. sol-gel-derived coatings,3 but bonding at the metal/ceramic interface
remains weak and hydrolysis can detach the layer. It is
evident that this is a research area requiring more development.
Arora and Smart4 reported a low-temperature
plasma reaction using a radio frequency source (100 W)
at \40 ¡C to produce compositionally and structurally
graded oxide/silicate/silica layers on nickel substrates.
The surface of the nickel was cleaned in an air or argon
plasma and, then air saturated with water vapour was
introduced into the plasma to oxidize the nickel surface.
When tetraethoxysilane (TEOS, (C H O) Si) vapour
2 5 4 as deterwas introduced, SiO radicals were produced,
mined by mass spectroscopy of the plasma gas, which
reacted with the oxidized surface to produce silicon
oxides. As the reaction proceeded, orthosilicate (SiO )
structures were formed, then pyrosilicates, followed by
layer silicates, chain silicates and bulk silica. Surface
analysis by XPS, with silicate structures elucidated from
modiÐed Auger parameter data, was used to characterize the surface layers as the reaction proceeded. The
integrity of the layer was tested by dissolution testing in
pH 2 nitric acid. The silica coating reduced the nickel
dissolution rate by more than two orders of magnitude
over [30 days, demonstrating the continuity and
coherence of the layer.
In this paper we will report the use of a similar
method to coat titanium with silica, producing a coherent, continuous coating and strong metalÈceramic
bonding. The transition from titanium, through the
Received 1 May 1998
Accepted 14 August 1998
oxide and silicates to silica is characterized by XPS and
SEM, as previously.4 The silica-coated titanium can be
used as a substrate for hydroxyapatite, bioactive glass
or ceramic coating.
Titanium sheet (99.99%) and tetraethoxysilane (or
tetraethylorthosilicate) (98%) were supplied by Aldrich
Chemical Co. All water introduced into the plasma was
puriÐed in a Milli-Q system. Before use, the titanium
sheet was cut into D1.5 cm square pieces that were
cleaned following the method described by Wilson,5 i.e.
washing in 5 wt.% KOH solution followed by a water
rinse, then a warm 1.6 wt.% HF/33 wt.% HNO solution for 2È3 min followed by another water rinse. 3
The plasma reaction method has been described previously.4 The plasma was generated in a Harrick
Plasma Cleaner/Sterilizer (PDC-32G) run at 100 W, frequency 13.5 MHz and D2 Torr pressure, as measured
by an Edwards High Vacuum Model 8/1 pressure
gauge. The sample was placed on a Petri dish to
support it in the centre of the plasma. In the Ðrst step of
the reaction, air is introduced to the chamber to clean
impurities (particularly hydrocarbons) o† the metal
surface. In the second step, water vapour was introduced into the plasma by bubbling the incoming air
through water. In the third step, silicon species were
added to the plasma by adding TEOS to the water with
air bubbling through it. A typical treatment was 10 min
of air plasma, 20 min of water plasma and 10 min of
TEOS/water plasma.
The treated titanium surfaces were analysed by x-ray
photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). A Physical Electronics PHI
5600 ESCA system with a magnesium (hl \ 1253.6 eV)
x-ray source operating at 300 W (15 kV, 20 mA) was
used for XPS surveys (pass energy 99.3 eV) and elemental regions (pass energy 58.7 eV). The bremsstrahlung
radiation of the source was used for Auger electron
spectroscopy of the D1600 eV Si KLL transition. The
vacuum in the analysis chamber was maintained at
\10~8 Torr during analysis. All spectra were corrected
for charging e†ects using the hydrocarbon contamination peak (uncharged) at 284.8 eV. Depth proÐles
were obtained by etching the surface with an argon ion
gun, with the etching rate calibrated using a 100 nm
tantalum oxide layer on a tantalum substrate. At 3 kV,
the etch rate was determined to be 1.2 nm min~1.
The inelastic mean free path (IMFP) can be calculated from the Seah and Dench relationship for inorganic compounds.6 For the O 1s XPS signal, using the
bulk density of rutile TiO , the IMFP is calculated to
be 0.43 nm. As 95% of the2 analysed photoelectrons are
emitted from a depth less than or equal to 3 ] IMFP,
this depth is taken to represent the analysis depth. For
the angle-resolves XPS measurements, the analysis
depth used was 3IMFPsinh. The values of these analysis
depths are summarized in Table 1. For more general
XPS measurements, an analyser angle of 45¡ was used.
Information on the chemical nature of the siliconÈ
oxygen bonds in the surface coating was determined
using the modiÐed Auger parameter. The Auger paramSurf. Interface Anal. 26, 1027È1034 (1998)
Table 1. Average analysis depths for O1s corresponding to XPS analysis angles of
between 10 and 90Ä
Analysis Angle
IMFP (nm)
3 Ã IMFP (nm)
eter, introduced by Wagner,7,8 is now commonly
expressed in the modiÐed form
[email protected] \ KE ] BE
where KE is the kinetic energy of the most intense
Auger electron and BE is the binding energy of the
main photoelectron from the same element in the same
XPS spectrum. The modiÐed Auger parameter has the
advantage that surface charging is removed by the
equal and opposite shifts in each of the two contributing energies. More importantly, it can be shown that
the modiÐed Auger parameter is a measure of the
energy changes due to extra-atomic relaxation and is
therefore often more sensitive to the stoichiometry and
strength of bonding to the next nearest neighbours of
the emitting atom than the XPS BE shifts.9,10 Hence,
the modiÐed Auger parameter for silicon in the silicateÈ
silica coatings provides information about the changes
in siliconÈoxygen bonding as the layer is formed by
plasma reaction.
Scanning electron microscopy was performed using a
high-resolution, Ðeld emission CAMSCAN CS44FE
microscope operating at 20 kV. The samples were not
coated before examination in the microscope.
Initial oxidized titanium surface
Although the characteristics of titanium surfaces
exposed to air are already well documented, our
analyses of these surfaces are brieÑy summarized here
for comparison with the changes produced by air
plasma cleaning described in the next section.
Electron microscopy of the initial, naturally oxidized
titanium surface shows that this oxide layer has many
impurities and irregularities, as indicated by the pits,
scratches and di†erentiated regions in Fig. 1.
Chemically-cleaned titanium can form nonstoichiometric oxides and sub-oxides, TiO , where x is
Table 2. Titanium 2p 3/2 XPS binding energy
for the naturally oxidised titanium
surface compared to literature values
Ti 2p 3/2 Peak (eV)
Ti O
2 3
Oxidised Surface
( 1998 John Wiley & Sons, Ltd.
Figure 1. The naturally oxidized titanium surface showing an
irregular surface and pitting.
\2, on reoxidation.11 X-ray photoelectron spectroscopy analysis of the natural oxide layer revealed only
the dioxide (Table 2). There was no shoulder on the Ti
2p 458.5 eV peak to indicate suboxides. An overlayer
of [email protected]
organic carbon contamination was also indicated by
the XPS spectrum. Surface atomic concentrations calculated from XPS peak areas give 2È10% titanium,
15È30% oxygen and 60È80% carbon, varying from
sample to sample depending on their history of exposure to the environment. The hydrocarbon layer contains some oxygen, as evidenced by ether (e.g. 287 eV)
and carbonyl or carbonate (e.g. near 288.5 eV) oxidized
hydrocarbon peaks on the high-binding-energy side of
the main C 1s peak and by the presence of three peaks
in the surface oxygen spectrum, the Ðrst relating to the
oxide in TiO (BE \ 529.7 eV), the second to OH
2 eV) and the third oxygen as CxO on
species (D531.5
carbonate (BE \ 533 eV) (Fig. 2). These species have
been found previously on untreated titanium implant
Figure 2. Comparison of the O 1s XPS peak of the naturally oxidized titanium at different analysis angles.
( 1998 John Wiley & Sons, Ltd.
materials.12 The relative intensity of the oxygen peaks
changes as the analysing depth is increased by angle
resolution, with the OH and CxO oxygens being more
prominent in the more surface-speciÐc spectra (i.e. 10¡).
With the instrument set to record from the deepest
level, i.e. 90¡, the organic oxygen peak is seen only as a
shoulder on the side of the much larger oxide peak (Fig.
2). The depth of analysis at each angle is recorded in
Table 1.
Even at 90¡, the depth of analysis is less than the
oxide layer thickness ; hence, an ion-etched depth proÐle
was used to determine the oxide thickness. Figure 3
shows an ion-etched depth proÐle of the natural oxide
layer. As expected, the carbon signal from surface contamination drops to a constant level after the Ðrst 30 s.
The etch rate of 1.2 nm min~1 gave a contamination
layer thickness of \0.6 nm, in reasonable agreement
with the angle-resolved XPS results. The surface oxygen
concentration reaches a plateau at D20 at.% at a
depth of D15 nm. This depth corresponds to the stabilization of the Ti signal, predominantly as the metal,
at D65 at.%. It should be noted that the carbon and
oxygen atomic concentrations stabilize at around 10%
and 20%, respectively, rather than going to zero. There
are a number of reasons for this observation. The
surface of the titanium used is rough (Fig. 1) and some
faces of the surface undulations are shadowed from the
angled ion beam. The pressure in the instrument during
measurement (i.e. 10~8È10~9 Torr) was too high to
maintain a clean surface, i.e. reaction with residual H O
and CO is likely. The titanium may have oxidized down
grain boundaries or the oxide may be of uneven thickness on di†erent facets. The 3 kV ion beam may also
have knocked some of the surface carbon and oxygen
further into the titanium.
The oxide depth measured in this proÐle appears to
be in the range 10È15 nm.
Figure 3. X-ray photoelectron spectroscopy ion-etched depth
profile of the naturally oxidized titanium surface. Sputter rate was
1.2 nm minÉ1 ; scale is atomic concentration (%).
Surf. Interface Anal. 26, 1027È1034 (1998)
Plasma-modiÐed titanium surfaces
Air plasma cleaning. Figure 4 shows a titanium surface
after 10 min in an air plasma. The pits seen in the
surface before plasma treatment (Fig. 1) have been
The air plasma has also reduced the surface organic
carbon layer. The carbon concentration, measured at
45¡, is 60È80 at.% for the natural oxide. After plasma
cleaning, the amount of carbon contamination varies
between samples, due to their di†erent exposure times
to the atmosphere before XPS analysis, but is generally
30È40 at.%. Also indicative of the removal of organic
carbon contamination is that the 0 ls peak at 45¡
becomes a single peak at 529.7 eV with no OH or CxO
shoulder on the high-energy side found before plasma
cleaning (Fig. 2).
This is evidence that the plasma actually removes
almost all of the surface contamination but the titanium
samples are recontaminated when they are reoxidized
on exposure to air during transferral between the
plasma chamber and the XPS. A series of samples were
prepared in which contamination was minimized by
backÐlling the plasma chamber with argon and transferring the sample from plasma to XPS as fast as possible (\30 s) in an argon-Ðlled vessel. These samples
had only 7È8 at.% surface carbon and, signiÐcantly,
only 16È17 at.% oxygen. The titanium surface has been
cleaned extensively and is likely to be highly reactive on
re-exposure to air. Hence, the hydrocarbon contamination can be reduced signiÐcantly, supporting the proposition that contamination in the plasma chamber is
very low. The surface of titanium on exposure to air
is pock-marked with irregularities. The air plasma
removed these markings to leave a smooth, even
surface. As the response of tissue to a metal implant
surface is inÑuenced by the chemical and structural
properties of the surface,12 e.g. hydrocarbon contamination and microscopic structure (e.g. pits), these variations may lead to an equally variable boneÈimplant
bond strength. Thus, if the sample surface is not
removed before plasma coatings are applied, the air
cleaning should lead to better implant performance.
Subsequent plasma treatments (as described below) are
then applied to this cleaned, reactive surface.
Figure 4. Naturally oxidized titanium surface after 10 min of air
plasma cleaning, showing no pitting.
Surf. Interface Anal. 26, 1027È1034 (1998)
Air/water vapour plasma oxidation. The aim of air/water
vapour plasma oxidation was to increase controllably
the depth of the oxide layer. Figure 5 shows a depth
proÐle of the titanium surface layers after 10 min in an
air plasma and then 20 min in air/water vapour plasma.
The depth of the oxide layer was D30 nm, after which
both the oxygen and titanium concentrations reach
stable values. This indicates a 67È100% increase in the
depth of the oxide due to this plasma reaction.
The SEM images of the air/water vapour plasmaoxidized titanium surface are closely similar to Fig. 4.
vapour plasma reactions. The
introduction of tetraethoxysilane (TEOS) vapour into
the plasma, along with air and water vapour, results in
a silica reaction with the metal surface.4 The modiÐed
Auger parameter in XPS provides a means of elucidating the structure of the silica surface coating produced on titanium. The data can be displayed on a silicon
chemical state plot as in Fig. 6. This plot shows the Si
2p binding energy on the x-axis, the Si KLL Auger
kinetic energy on the y-axis and, hence, the modiÐed
Auger parameter on diagonals. Figure 6 is a silicon
chemical state plot with a series of siliconÈoxygencontaining standards analysed by Wagner et al.,13 with
the energies adjusted to a C ls standard of 284.8 eV
rather than the original 284.6 eV. It shows that the different SiwO bond types from orthosilicates (SiO )
through Si O species to SiO can be determined from
this XPS data.
Various TEOS plasma reaction times (up to 12 min)
were used to prepare a series of samples with surface
silicon concentrations of between 2.2 and 28 at.%.
Allowing for some hydrocarbon contamination,
25È30% silicon is the practical maximum for SiO . In
the samples that contain this amount of silicon,2 the
Air/H O/tetraethoxysilane
Figure 5. X-ray photoelectron spectroscopy ion-etched depth
profile of water vapour plasma-treated titanium. The spike in the
spectrum at 4.5 nm was caused by an electrical fault whilst recording the Ti spectrum.
( 1998 John Wiley & Sons, Ltd.
Figure 6. Silicon chemical state plot showing the distribution of a number of silicon–oxygen species. The silicon–oxygen stoichiometry is
shown in parentheses. (Adapted from Wagner et al .13).
plasma reaction was stopped when the silicon content
had just reached this level rather than being allowed to
continue to deposit as bulk silica. Figure 7 shows the
XPS silicon atomic concentrations after increasing reaction time superimposed on the chemical state plot. The
chemical state plot gives two di†erent kinds of information. As indicated by the plot of the siliconÈoxygen standards,13 SiwO bonds in di†erent structures are
distributed along the Auger parameter diagonals, i.e.
compounds with the same modiÐed Auger parameter
Figure 7. Silicon chemical state plot showing spread of silicon–oxygen structures with varying surface silicon concentrations (shown next
to data points). The TEOS plasma reaction times ranged from 0 to 12 min.
( 1998 John Wiley & Sons, Ltd.
Surf. Interface Anal. 26, 1027È1034 (1998)
can have di†erent structure. It is evident from Fig. 7
that there is a change in the structure of the siliconÈ
oxide bonds with increased silicon deposition. As the
silicon concentration rises, there is a shift from
orthosilicate-like structure to silica-like structure via the
intermediate pyro-, chain and layer silicates. The second
type of information on a chemical state plot is the stoichiometry of the silicon oxide. A series of papers by
Alfonsetti and colleagues14h16 have derived a parameter, b, that can be related siliconÈoxygen stoichiometry.
The di†erence between the modiÐed Auger parameter
for elemental silicon and that of the silicon oxide SiO ,
the b parameter, is related to the stoichiometric ratio, x,
in SiO via the following equations from refs 14È16,
b \ 1716.0 [ [email protected]
b \ 2.25x
indicating that the higher the value of [email protected] is, the fewer
oxygens per silicon. The [email protected] parameters for quartz (i.e.
1711.5 eV) and pure Si (i.e. 1716.0 eV) were used to calibrate the calculated b values as in our previous paper.4
The points in Fig. 7 lie in a line of lower slope than
y \ x (constant Auger parameter), indicating a change
in SiwO stoichiometry from SiO
at 2.8% Si to
SiO at 26% Si. These results may 1.4
seem contradictory
1.8 structural information in Fig. 7, which indicates
to the
that at 2.8% silicon the oxide is orthosilicate-like, i.e.
x \ 4, but this is not the case because both the low
silicon/oxygen ratio and the orthosilicate structure can
be explained by the mechanism of the plasma reaction.
Arora and Smart4 used mass spectroscopy of the
plasma to deduce the mechanism of the reaction on
nickel oxide. When TEOS was added to the plasma, the
m/e signals at 22 and 44 (singly and doubly charged
SiO) increased markedly. The peaks corresponding to
O, OH, H O, SiH and SiOH increased less dramatically. The 2silicon atom from SiO apparently occupies
the tetrahedral interstice in the surface oxide structure,
which in their case is nickel oxide but in this case is
titanium dioxide. The resulting structure has silicon
atoms connected to four oxygen atoms by orthosilicatelike bonds, hence the position of the 2.8% Si point on
Fig. 7, but the oxygen atoms are bonded to titanium
atoms as well, reducing the siliconÏs “shareÏ of the four
oxygens and explaining the apparent silicon/oxygen
ratio of 1.4. It was concluded that SiO was the species
responsible for the initial formation of SiO4~ in the
nickel oxide.4
Ion etching depth proÐles were performed on some of
the samples to determine the depth of the siliconcontaining layer. Figures 8È11 show that samples with
surface silicon concentrations of 6.1, 13, 18 and 26 at.%
had silicon concentrations down to depths of D0.8, 2.5,
6 and 18 nm, respectively. The results of these depth
proÐles, combined with the chemical state plot (Fig. 6),
indicate that as the TEOS plasma reaction proceeds, the
silicon-containing layer becomes thicker and more
silica-like, i.e. a compositionally graded structure exists
in the layer, as suggested by Arora and Smart.4
Further proof of this structure is found in the modiÐed Auger parameter depth proÐle, using ion beam
etching, through a completely coated (Si \ 28 at.%)
surface. Figure 12 shows that there is a systematic shift
of both SiwO structure and stoichiometry SiO as the
Surf. Interface Anal. 26, 1027È1034 (1998)
Figure 8. X-ray photoelectron spectroscopy ion-etched depth
profile of TEOS plasma-treated titanium with 6.1 at.% surface
ion etch continues through the layer. The nature of the
SiwO bonds on the outer surface are SiO -like, moving
through the layer silicates, the chain silicates,
the pyrosilicates and Ðnally the orthosilicates close to the titanium oxide (as the Si signal disappears).
It should be noted that as the ion etching cleaned the
hydrocarbon o† the surface within the Ðrst minute, no
charge correction of the points against the C ls peak at
Figure 9. X-ray photoelectron spectroscopy ion-etched depth
profile of TEOS plasma-treated titanium with 13 at.% surface
( 1998 John Wiley & Sons, Ltd.
Figure 10. X-ray photoelectron spectroscopy ion-etched depth
profile of TEOS plasma-treated titanium with 18 at.% surface
284.8 eV was performed. This does not reduce the relevance of these results because the modiÐed Auger
parameter is self-correcting for charge. If the surface
becomes positively charged due to the loss of electrons,
the binding energy would increase and the kinetic
energy will decrease equally.
The 30È50 mm silicate/silica layers are completely
deformable (to repeated 180¡ bending) without evidence
Figure 11. X-ray photoelectron spectroscopy ion-etched depth
profile of TEOS plasma-treated titanium with 26 at.% surface
of disbonding or cracking at the interface. Mechanical
tests on the hardness, tensile strength and frictional
forces are in progress.
plasma coating method has been shown to produce
Figure 12. Silicon chemical state plot showing the change in silicon–oxygen stoichiometry and structure within a 27.7 at.% surface silicon
coating. Produced from an XPS ion-etched depth profile. Sputter rate was 1.2 nm minÉ1 ; sputter times (in min) are shown next to the data
( 1998 John Wiley & Sons, Ltd.
Surf. Interface Anal. 26, 1027È1034 (1998)
compositionally and structurally graded layers from the
titanium metal through the oxide, orthosilicate, pyrosilicates, chain silicates and layer silicates to bulk silica, as
was found previously for nickel substrates. The initial
surface of the titanium metal in air has a thin layer of
hydrocarbon contamination (D0.6 nm) overlying a
layer of TiO 10È15 nm thick. The low-temperature air
plasma removed the bulk of the hydrocarbon contamination and substantially smoothed the surface by
annealing pits and scratches. An air/water vapour
plasma treatment increased the depth of the oxide layer
to 25È30 nm. The addition of tetraethoxysilane vapour
to the air/water plasma resulted in a surface reaction
producing the silicate structures with increasing SiwO
bonding as the reaction proceeded. The compositional
and structural gradation of the layer has been demonstrated in XPS depth proÐling and by information from
the modiÐed Auger parameter measurements from the
layer during deposition and in ion beam etching after
deposition to an overlayer of bulk SiO .
These bonded layers provide the basis for deposition
of overlayers of oxides, glasses and hydroxyapatite
using solÈgel formulations.1h3 These depositions have
been accomplished successfully and the results will be
reported in a separate publication.
The authors would like to thank Dr Bill Skinner for helpful advice
and Mr Len Green for his assistance on the scanning electron microscope. Funding for this research was provided by the Australian
Research Council through a large grant and a Senior Research Fellowship (R.St.C.S.).
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( 1998 John Wiley & Sons, Ltd.
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