- Email: [email protected]
Characterization of nanostructured multiphase Ti–Al–B–N thin films with extremely small grain size
Surface and Coatings Technology 148 (2001) 206–215
Characterization of nanostructured multiphase Ti–Al–B–N thin films with extremely small grain size D.V. Shtanskya,*, K. Kanekob, Y. Ikuharac, E.A. Levashova a
SHS-Center, Moscow Steel and Alloys Institute, Leninsky pr. 4, 164, Moscow 119991, Russia b Japan Fine Ceramic Center, Atsuta, Nagoya 456-8587, Japan c Engineering Research Institute, School of Engineering, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan Received 4 April 2001; accepted in revised form 22 June 2001
Abstract The microstructure and chemical composition of nanostructured multiphase Ti–Al–B–N films deposited by DC magnetron sputtering of a multiphase composite Ti–Al–B–N target (five phases) in a gaseous mixture of argon and nitrogen were studied by means of Auger electron spectroscopy, X-ray diffraction, conventional and high-resolution transmission electron microscopy and electron energy-loss spectroscopy. The (TiqAl)yB ratio of films deposited at low nitrogen partial pressure replicates that of the target. At higher values of nitrogen partial pressure, the target is preferentially sputtered. The chemical composition of the films is not influenced significantly by either the substrate temperature or the bias voltage, whereas the phase composition is strongly affected by the PVD process parameters. Evidence for the formation of mixtures of nanocrystalline (Ti,Al)N, TiB2 and h-BN phases in an amorphous matrix is presented. Most of the Ti–Al–B–N films show a grain size in the range 0.6–4 nm. The grain size reduces further to 0.3–1 nm with decreasing N content (TiAl0.2 B0.7 N0.7 film). This is the smallest grain size determined from TEM investigations which has hitherto been observed, being only one–three unit cells in dimension. Evidence for the ordered structure of grain boundaries in the nanostructured TiAl0.3 B0.5 N1.9 film is obtained. The amorphous phase forms as individual grains rather than as a thin, intergranular amorphous layer of uniform thickness, even though only a limited number of grain boundaries were studied in detail. The applicability of the equilibrium phase diagram for the prediction of the phase composition in the Ti–Al–B–N films is also discussed. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Magnetron sputtering; Nanostructured thin films; Transmission electron microscopy (TEM); Electron energy loss spectroscopy (EELS); X-Ray diffraction; Electron diffraction
1. Introduction Multicomponent thin films of titanium-based borides and nitrides are expected to provide superior physical, chemical and engineering properties. The interpretation of these properties is rather complex because many factors affect film characteristics. In order to understand trends in the physical and mechanical behavior, a detailed characterization of the atomic structure, overall chemical composition, phase composition, grain size and morphology is needed. * Corresponding author. Tel.: q7-95-230-4535; fax: q7-95-2365298. E-mail address: [email protected] (D.V. Shtansky).
Recently, a new concept for the design of superhard nanocrystalline films has been developed w1–3x. These materials have been shown to be nanocomposites, consisting of either nanocrystallites in an amorphous matrix, or a mixture of two nanocrystalline phases. Such microstructures are believed to prevent grain boundary sliding and a softening of the film due to an inverse Hall–Petch effect. Nanocrystalline thin films consist of very small grains, typically less than 20 nm in diameter. The grain size, however, is further reduced to a few nm when the composition of the film becomes more complex. For instance, most of the Ti–Si–N w3–6x and Ti–Al–Si–N w6,7x thin films exhibit grain sizes in the range 3–8 nm. According to Shtansky et al. w8–10x, microstructural
0257-8972/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 3 4 1 - X
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
analysis of the Ti–Si–B–N and Ti–Si–C–N films revealed extremely small grain sizes that ranged from 1 to 4 nm. The TiN and TiB2 grain sizes of the Ti–B–N films were reported to range from 2 to 5 nm w11–13x, whereas Baker et al. w14x, using the Scherrer formula, calculated the crystallite size to be as small as 1.7 and 0.9 nm. Mollart et al. w15x suggested that the TiB1.7N1.8 films consisted of clusters of atoms with varying composition, representing neither a fully nanocrystalline nor a homogeneous amorphous state. A quasiamorphous arrangement of the TiB2 and TiB phases in the Ti–B–N films has been proposed by Mitterer et al. w16x. It has repeatedly been shown that the solubility limit of solid solutions of cubic and hexagonal Ti–B–N can be extended by the incorporation of boron and nitrogen atoms in the TiN and TiB2 lattices, respectively w8,13,16–18x. Thus, it is still unclear whether the phase composition of films deposited under non-equilibrium conditions would be the same as predicted from the equilibrium phase diagram as proposed by Gissler and co-workers w11,12,14,19x, who claimed that the phase composition of Ti–B–N films coincided well with that derived from the ternary phase diagram. In the case of nanostructured multiphase thin films, the structure cannot be unambiguously determined by only using the diffraction technique. It is well known that nanocrystalline multicomponent thin films show very broad diffraction peaks with low intensity, which are usually attributed to amorphous material, even though complementary microanalytical techniques had revealed the crystalline nature of fine-grained films. Recently, modern microanalytical techniques, such as Xray photoelectron spectroscopy (XPS) w14,15,19x, Raman spectroscopy w20x and extended X-ray absorption fine structure (EXAFS) measurement w14,15x, were employed to reveal the phase composition of the various multicomponent films. High-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS) are further powerful tools to reveal and identify the structure and phase composition of nanostructured multiphase films. The present work is part of a systematic study to deposit various multicomponent thin films by magnetron sputtering of composite targets w8–10x. Only limited data are available from the literature concerning the deposition of films by sputtering of multiphase targets. In the present study, Ti–Al–B–N films with crystallite sizes ranging from 0.3 to 8 nm have been examined by means of X-ray diffraction (XRD), Auger electron spectroscopy (AES), selected-area electron diffraction (SAED), EELS and HRTEM. 2. Experimental The composite target for PVD was manufactured by means of self-propagating high-temperature synthesis
207
Table 1 Target characteristics Phase composition
(wt.%)
TiB2 TiAl Ti2AlN Ti3Al Al3N4
51 30 13 3 3
Chemical composition
(at.%)
Ti Al B N O (TiqAl)yB (TiqAl)y(BqNqO)
Before sputtering 43.3 7.7 32.5 13.4 3.1 1.6 1.0
After sputtering 47 7.2 27.8 15.4 2.6 1.9 1.2
(SHS). The target was synthesized from an exothermic mixture containing 66.7 wt.% Ti, 20 wt.% Al, and 13.3 wt.% B using the combined SHS–load consolidation technology w21x. The phase composition of the target as determined by Auger electron spectroscopy is shown in Table 1. The major phases in the target were TiB2, TiAl and Ti2AlN; in addition, minor phases such as Ti3Al and Al3N4 phases were detected by X-ray diffraction. The synthesized target was subjected to magnetron sputtering in a gaseous mixture of argon and nitrogen. The diameter of the target was 12.5 cm and the distance from the target to the substrate was 8 cm. The total pressure was maintained at 0.7 Pa, and the nitrogen partial pressure was varied from 10 to 25% of the total pressure. Films were deposited on stainless steel and nickel substrates for 30 min. Prior to deposition, the substrates were mechanically polished and then sputtercleaned by ionic etching with argon at a bias voltage of y600 V for 2 min. The substrate temperature was kept at 7008C during sputter cleaning. The average film thickness was 2 mm. The applied bias (Ubias) was controlled from y500 to 0 V and the substrate temperature ranged from 100 to 4008C. Thin foils for plane-view TEM studies were prepared from 3-mm disks using a standard technique, involving mechanical grinding to a thickness of approximately 0.07 mm from the side of substrate, followed by mechanical dimpling and one-side ion-milling to perforation at a voltage of 3–4 kV. The structure of the films was examined in a Hitachi 9000NAR transmission electron microscope with a point resolution of 0.19 nm operating at 300 kV. The XRD spectra of films were obtained by a Geigerflex X-ray diffractometer. The composition of the films was analyzed by AES. For analytical electron microscopy (AEM), we used dedi-
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
208
Table 2 Sputtering conditions and film characteristics No.
1 2 3 4 5 6 7
Chemical composition (at.%)
PVD characteristics Tsub (8C)
Ubias (V)
N2yAr =100
O
Ti
Al
B
N
(TiqAl)y B
100 250 400 250 250 250 250
0 0 0 250 500 250 250
14 14 14 14 14 10 25
4.1 3.5 3.5 3.5 3 3 3
30.8 30 30.6 33.8 32 37.4 25.9
9.2 9.5 9.2 8.3 9 7.6 8.1
21.6 22 23.4 24.2 23 27.3 13.2
34.3 35 33.1 30.2 33 24.7 49.8
1.8 1.8 1.7 1.7 1.8 1.7 2.6
cated scanning transmission electron microscopes (Thermo, Vacuums-Generators HB 601 UX) operated at 100 kV, equipped with a high-resolution pole piece and a cold field emission gun. Energy resolution of better than 0.4 eV was attained in the full width at half-maximum (FWHM) of the zero loss with the spectrometer entrance aperture limited to approximately 12 mrad. A parallel electron energy-loss spectroscope (PEELS) equipped with photodiode array (Gatan 766) was used for the quantitative chemical analysis, and it had a typical sensitivity better than 0.1 monolayers of impurity segregation at internal interfaces w22x. Regions of appropriate specimen thickness determined by EELS (less than half the mean free path) were carefully selected for the EELS measurements. To measure the EELS edges, the beam current was kept at approximately 0.2 nA with a mean beam diameter of 0.3 nm (FWHM). The Auger analysis was performed on a LHS-10 SAM spectrometer. To remove the surface oxide, samples were etched prior to analysis by a 4-keV argon ion beam with a current of 0.1 mA. For quantification of films, the negative peak intensity ratio method was employed w19x. 3. Results and discussion 3.1. Sputtering of composite target To the best of our knowledge, this is the first attempt to deposit Ti–Al–B–N thin films by magnetron sputtering of a multiphase target (TiB2qTiAlqTi2 AlN, with a minor amount of Ti3Al and Al3N4 phases). Formerly, Ti–Al–B–N films were prepared from hot-pressed TiAlB8N10 w23x and TiAl0.68B3.67N2.44 w24x targets, and most recently, by simultaneously sputtering from TiAl and TiB2 targets in an AryN2 mixture w25x. It is frequently observed that ion bombardment of composite targets results in preferential sputtering. It has been shown that the target was impoverished either by metal atoms w18,26,27x or by metalloid atoms w28,29x during reactive magnetron sputtering. The elemental composition of the composite target used in the present study before and after sputtering is shown in Table 1. The
Grain size (nm)
Formula
– 0.6–3 – 1.5–4 0.9–3 0.3–1 2.5–8
TiAl0.3B0.7N1.1 TiAl0.3B0.8N1.2 TiAl0.3B0.8N1.1 TiAl0.2B0.7N0.9 TiAl0.3B0.7N TiAl0.2B0.7N0.7 TiAl0.3B0.5N1.9
presence of nitrogen is a result of the active interaction between titanium and nitrogen in the air during SHS. The chemical composition of the target during sputtering was fairly constant, with the titanium content increased slightly, while that of boron decreased. 3.2. Composition of the films The influence of the substrate temperature in the range 100–4008C, bias voltage of 0 to y600 V, and nitrogen partial pressure of 10–25%, on the chemical composition of the Ti–Al–B–N films was examined, all other parameters being kept constant. The parameters of the PVD processing, the chemical composition of the films and some other information are presented in Table 2. It is evident that the elemental composition of the films was moderately affected by both the substrate temperature and the bias voltage, supporting the results from w9x. The nitrogen content of the films depends substantially on the nitrogen partial pressure. As the nitrogen partial pressure was increased from 10 to 25%, the nitrogen content of the films increased from 25 to 49%. The as-deposited Ti–Al–B–N films contained 3– 4% oxygen. The oxygen content in the film was equal to that of the target. The (TiqAl)yB ratio of films 1– 6 was found to be close to that of the target. A further increase in the nitrogen partial pressure from 14 to 25% revealed an increase in the concentration of metal atoms in the film, in comparison with that of the target. Thus, the composite target was being sputtered preferentially under sputtering condition 7. To calculate the phase composition of Ti–Al–B–N films, let us consider the simplified Ti(Al)–B–N phase diagram, ignoring, for the sake of simplicity, that microstructure may be far away from thermodynamic equilibrium. Gissler and co-workers w11,12,14,19x showed that the phase composition of Ti–B–N films can be derived from the thermodynamic equilibrium phase diagram at 15008C by Novotny et al. w30x, and the experiments appear to be in agreement with their predictions. Wahlstrom et al. w31x reported that the structure of the
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
209
Fig. 1. A simplified phase diagram of the Ti–B–N system w30x with the film compositions examined in the present study indicated. n, film 1; d, film 2; q, films 3 and 5; m, film 4; s, film 6; and h, film 7.
Ti1yxAlxN films depends on the Al content, being of single phase NaCl type TiN structure for 0FxF0.52. Since the Al content in our films did not exceed 9.5%, it is reasonable to assume that aluminum replaces titanium atoms in the TiN lattice. The chemical compositions of the Ti–Al–B–N films, determined by AES, are marked on the Ti(Al)–B–N phase diagram presented in Fig. 1 and their phase compositions calculated are shown in Table 3. 3.3. Microstructure of the films 3.3.1. X-Ray diffraction Fig. 2 shows the typical XRD spectra of the TiAl0.3B0.7N and TiAl0.2B0.7N0.9 films. The position lines for the stoichiometric TiB2 and TiN phases are also shown in Fig. 2. Two broad peaks from the films can be distinguished with FWHM values of 8–98 (2u). The positions of the center of gravity of the first and second diffraction peaks differ slightly from one film to another, ranging from 0.256 to 0.251 and from 0.21 to 0.208 nm, respectively. There are several factors which may explain such peak broadening: overlapping of diffraction lines from several crystalline phases with very small
Fig. 2. X-Ray diffraction spectra of films 4 and 5 deposited on nickel substrates.
grain size; inhomogeneous distribution of grains with various compositions; or the presence of both crystalline and amorphous phases. For instance, Gissler w11x presented a computer-modeled diffraction spectrum for a TiN structure assuming a random orientation of crystallites with an average grain size of 2 nm. His calculation showed that the FWHM values of (111) and (200) peaks amounted to approximately 4.5 and 5.58 (2u), respectively. Thus, the line-broadening effects observed in the present spectra can be interpreted as a superposition of diffraction lines from the nanocrystalline TiN and TiB2 phases. A further peak broadening can be achieved if the film consisted of crystallites with different chemical compositions, resulting in various lattice parameters. It has been shown that face-centered cubic (f.c.c.) and hexagonal Ti–B–N phases were formed due to the incorporation of boron and nitrogen in the TiN and TiB2 lattice, respectively w8,9,13,16,32x. The composition of grains can also change from grain to grain,
Table 3 Phase composition of Ti–Al–B–N films predicted from Ti(Al)–B–N phase diagram and phases suggested by various experimental methods Film no.
1 2 3 4 5 6 7
Phase composition predicted from phase diagram (%)
XRD
TiB2
TiN
BN
Ti2N
N2
21 21 23 28.5 24 35.5 –
71 70 69 71 71 47.5 69.5
8 9 8 0.5 5 – 28
– – – – – 17 –
– – – – – – 2.5
TiN, TiB2, h-BN TiN, TiB2, h-BN
SAED
HRTEM
ELNES Ti-L23
B-K
h-BN
TiN
TiN
TiN
TiN TiN TiN, TiB2 TiN
TiN
TiN, TiB2
h-BN TiN
TiN, TiB2
TiB2
210
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
Fig. 3. Dark-field TEM micrographs (plain-view) of the as-deposited Ti–Al–B–N films formed from parts of the first two diffraction rings with corresponding SAED patterns inserted. (a) Film 2; (b) film 5; (c) film 4; and (d) film 7.
as was shown by Shtansky et al. w10x in Ti–Si–C–N films using lattice parameter measurements. The first peaks are much broader than the second peaks and show more extended tails on the low-angle side. This feature may be attributed to the spectral contribution of the (001) peak of TiB2 phase andyor the (002) peak of hexagonal boron nitride (h-BN). The position of the second peak and its relative intensity also suggests the superposition of the (111) peak from the cubic boron nitride (c-BN) phase, having a maximum at 0.2087 nm. Further microanalytical analysis is obviously required to establish the phase composition. Note that in the case of nanostructured multiphase thin films, estimation of the crystallite size from the peak width in XRD using the Debye–Scherrer formula, as performed by several authors w12,14,15x, is questionable, because several phases make contributions to the peak width. 3.3.2. Transmission electron microscopy (TEM) Most of the Ti–Al–B–N films were characterized by a very small grain size that ranged from 1 to 3 nm, as estimated from the DF TEM images (see Fig. 3 and Table 2). The grain size increased up to 5–8 nm when the nitrogen content of the films was increased (film 7). The SAED patterns presented in Fig. 3 identified these films as having a cubic structure (lattice type B1). The lattice parameters calculated on the basis of the planes (111) and (220) were correspondingly higher that that
of the (200) line. Anisotropy of the lattice parameter has been reported, and could originate from metalloid atoms (B, O) occupying tetrahedral sites in the cubic TiN lattice w8,33x. The TiAl0.2B0.7N0.7 grain size was estimated from the DF TEM image to be approximately 0.3–1 nm (Fig. 4a). This is the smallest grain size which has hitherto been reported, being only one–three unit cells in dimension. Strictly speaking, the determination of grain size in nanostructured material using DF images is not a perfect method, because a DF image of amorphous material also consists of a speckle of white spots against a dark background. The size of the speckle depends on defocus and varies within the range 0.2–1.5 nm. Note that the question of whether a material is really amorphous or is nanocrystalline is still debated, even for simpler material. For instance, Graczyk and Chaudhari w34x proposed a random network as the correct structure for amorphous Si, whereas Rudee and Howie w35x presented TEM evidence for the microcrystalline model, assuming a grain size of 1.4 nm. The SAED pattern presented in Fig. 4b provides further evidence of the nanocrystalline nature of the TiAl0.2B0.7N0.7 film. The SAED pattern exhibits two continuous diffraction rings, showing an essentially random orientation. The observed degree of ring broadening corresponds to scattering from those crystallographic planes which have d spacings within the range 0.289–
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
211
Fig. 5. Plane-view HRTEM image of film 4 showing a TiN crystallite with a grain size of 4 nm. The incident beam direction is w001x.
Fig. 4. (a) Dark-field TEM micrograph (plain-view) of film 6 formed from parts of the first two diffraction rings, showing very small speckles of white spots ranging from 0.3 to 0.8 nm against a dark background; and (b) SEAD pattern.
0.236 and 0.222–0.194 nm, respectively. This agrees well with XRD results. The line broadening and overlapping do not allow the phase composition of the film to be determined unambiguously. However, the positions of the centers of the diffraction rings and their broadening infers that the rings are formed by a superposition of diffraction lines (111)TiN, (100)TiB2, (200)TiN and (101)TiB2. 3.3.3. High-resolution transmission electron microscopy (HRTEM) HRTEM is a powerful tool for revealing the crystalline structure of an individual grain. In combination with other methods, it allows the phase composition of nanocrystalline film to be determined. Fig. 5 is a planeview HRTEM micrograph of TiAl0.2B0.7N0.9 film 4, showing a TiN crystallite with a grain size of 4 nm oriented along a N001M zone axis. Therefore, both sets of {100}TiN planes are visible. The image around the crystallite shows an amorphous-like structure, although such an image may also result from other effects, such as local strain, overlapping of the grains, or impurity segregation.
Fig. 6 shows a plane-view HRTEM image of a typical area in film 2 containing a number of grains. The areas showing lattice fringes alternate with those displaying an image of randomly scattered intensities, which is characteristic of a disordered structure. It is evident that this film is characterized by a very small grain size, ranging from 1.5 to 2 nm. It is also evident from the HRTEM results that the f.c.c. TiN phase was formed in TiAl0.3B0.8N1.2 film. Fig. 7a,b are HRTEM micrographs of cubic TiN crystallites, 1.5–2 nm in diameter, taken along the w001x and w110x directions, respectively. The angles of the lattice fringes and interplanar spacing are clear evidence for the presence of cubic crystallites embedded in an amorphous matrix. The TiAl0.2B0.7N0.7 film 6 exhibits extremely small grain sizes, less than 1 nm. The HRTEM micrograph of Fig. 8 shows small areas, 0.3-0.8 nm in size, with welldeveloped parallel fringes, as shown by arrows. Although some would assert that such an image indicated disorder structure, others would argue that it could have arisen from an extremely small nanocrystallites embedded in amorphous matrix. Note that the DF image and SAED pattern presented in Fig. 4 are further supportive of the formation of order-disorder structure.
Fig. 6. Plane-view HRTEM image of film 2 showing a number of crystallites with a grain size ranging from 1.5 to 2 nm.
212
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
Fig. 8. Plane-view HRTEM image of film 6 showing an ordered– disordered structure. Arrows indicate the location of certain ordered regions.
Fig. 7. Plane-view HRTEM images of film 2 showing TiN crystallites with a grain size of (a) 1.5 and (b) 2 nm. The incident beam direction is (a) w001x and (b) w110x.
In order to get atomic resolution a satisfactory, the foil thickness must be of the same order as, or less than, the grain size, since overlapping of randomly oriented crystallites with a dimension of a few angstroms makes the interpretation of the microstructure by HRTEM ambiguous. Even if the foil thickness is reduced to approximately 3–5 nm, the specimen is still much too thick to obtain high-contrast image. An example of an area exhibiting the well-developed parallel fringes is shown in Fig. 9. The observed d spacing was consistent with the value of 0.33 nm, which is characteristic of basal planes of the h-BN phase that are oriented edge on w36x. Fig. 9 also illustrates that this crystallite is quite small with spacing as little as three lattice fringes. Such small h-BN crystallites are frequently observed in the sp2bonded layer of BN films w37x. Taking into account all the information obtained from XRD, DF TEM, SAED and HRTEM it can be suggested that the TiAl0.2B0.7N0.7 film consisted of a mixture of nanocrystalline clusters with various symmetries and amorphous matrix. The atomic structure of a grain boundary is also of great importance for the physical properties of nanostructured materials. For instance, a theoretical concept
for designing nanocrystalline superhard material is based on the assumption that a thin amorphous layer around the nanocrystallites prevents the formation and multiplication of dislocations w38x. Therefore, it is often speculated that the nanocrystals would be completely surrounded by a thin amorphous layer. The atomic structure of grain boundaries was studied in film 7, where the grain size was relatively large, in order to obtain a clear image of the interface. The two grains in Fig. 10 were identified to be TiN nanocrystallites with diameters of approximately 6 nm. The right grain was oriented close to a w001x zone axis and the lattice fringes in both grains were from {100} planes. Fig. 10 also shows that there were well-developed growth ledges on
Fig. 9. Plane-view HRTEM image of film 6 showing an area, approximately 1=3 nm in size, with well-developed parallel fringes.
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
213
Fig. 10. Plane-view HRTEM image of film 7 showing a single grain boundary between two TiN crystallites of approximately 6 nm in size. The incident beam direction is w001x. Arrows indicate well-developed growth ledges.
both sides of the TiN crystallite, with the ledge height being equal to one unit lattice height of TiN in the w100x direction. No intergranular precipitation of glassy phase was observed. This is in keeping with the experimental data by Shtansky et al. w10x, who observed ordered grain boundaries in nanostructured Ti–Si–B–N films. 3.3.4. Electron energy-loss spectroscopy The energy-loss near-edge structure (ELNES) exhibits fine structure, which is specific to the type and arrangement of atoms in the near-neighbor shell. This structure is called a ‘coordination fingerprint’. A structural determination can be approximately made from a comparison of the spectrum with a ‘coordination fingerprint’ obtained from a reference material w39x. Fig. 11 shows the Ti-L23 ELNES from films 2, 4 and 6, in comparison with the ELNES from TiN and TiB2 reference films w8,32x. The Ti-L23 ELNES from TiN film exhibited two distinct major peaks, with peak shapes similar to those of TiB2 film, but they were correspondingly shifted towards higher energy by approximately 2.2 and 1.5 eV, respectively. Consequently, the peak separation was larger for TiB2 in comparison with TiN. The Ti-L23 spectrum for film 2 exhibited peak shapes and positions similar to those of TiN reference film. The only feature that can be evident is a small shift of the peaks towards lower energy, by approximately 0.2 and 0.4 eV, respectively, suggesting a slight change in the atomic arrangement of the TiN phase. This is probably due to the incorporation of aluminum and boron atoms in the cubic phase. The peak positions of film 4 and 6 were midway between TiN and TiB2 phases. It is reasonable to assume that the spectra observed are a linear combination of the TiN and TiB2 reference spectra, although the formation of a cubic phase (with dissolved aluminum and boron atoms) cannot completely be excluded.
Fig. 11. N-K and Ti-L23 ELNES from the Ti–Al–B–N films 2, 4 and 6 and the reference TiN and TiB2 films for comparison.
The B-K ELNES from the as-deposited films are plotted in Fig. 12, together with the reference spectra for boron and its’ compounds. Comparisons between the spectrum obtained from film 6 and that from the TiB2 reference film showed that they were similar. In the case of films 2 and 4, however, the presence of
Fig. 12. B-K ELNES from the Ti–Al–B–N films 2, 4 and 6 and the reference TiB2, h-BN, c-BN and B materials for comparison.
214
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
TiB2 phase can hardly be recognized. Film 2 had only one feature at 197.7 eV, which resembles the h-BN spectrum. There is no p–p* loss peak, suggesting that the film was in a poorly crystallized form w40x. Note that it also has a broad peak at approximately 206 eV, which does not correspond to any feature in the h-BN spectrum. As for film 4, the signalynoise is so poor that no serious comment can be made regarding its comparison with h-BN or c-BN phases. These results suggest that the major part of BN is present as an amorphous phase. 3.4. Equilibrium phase diagrams for the prediction of phase composition in multicomponent thin films: use and abuse It has been suggested that the ternary T–B–N phase diagram provides a good basis from which the phase composition for any particular film composition can be estimated w11,12,14,19x. It is not surprising, however, that the present results do not always support this interpretation. In contrast, the equilibrium phase diagram does not appear to be suitable for an interpretation of the phase composition of nanostructured Ti–Al–B–N films. In the present study, evidence for the (Ti,Al)N, TiB2 and h-BN phases were observed. The presence of h-BN crystallites in film 6 was suggested by HRTEM, but this phase was not predicted from the simplified Ti(Al)–B–N phase diagram. The formation of TiB2 phase in films 2 and 4 follows from the Ti(Al)–B–N phase diagram, but the B-K ELNES from these films did not reflect the structure of TiB2 phase. Thus, the formation of TiB2 phase in films 2 and 4 is questionable. The remarkable peaks shift of the Ti-L23 ELNES obtained from films 4 and 6 towards lower energy appeared to indicate that the titanium environment in these films was substantially different from that in the TiN phase. From these arguments, it may be concluded that the cubic phase contained aluminum and a certain amount of boron. Further research is necessary to clarify this point. Note that equilibrium phase diagrams often do not describe the thermodynamics of highly metastable states in thin films, because many additional factors, such as high interdiffusion rates w41x, high quenching rates w42x and energetic ion bombardment, may affect the phase composition. It is also suggested that the composition of grains may vary from one grain to another, similar to previous results. 3.5. Atomic structure of the grain boundaries in nanocrystalline materials The atomic structure of the grain boundaries in nanocrystalline materials has been the subject of extensive discussion during recent years w43–46x. The interest in this topic has been increasing because a significant
amount of the atoms in nanocrystallite materials may be located at the grain boundaries, raising the question as to whether a novel state of matter exists. According to the molecular dynamics simulations by Keblinski et al. w45,46x, the microstructure of nanocrystalline materials consists of ordered crystalline grains with glassy intergranular films of constant, uniform thickness between them. Thus, they have postulated that a nanocrystalline microstructure with random grain orientations contains only high-energy grain boundaries. In contrast to their conclusions, other experimental studies of nanocrystallite thin films have indicated that the grain boundaries are not disordered w47x. A detailed study of the grain boundaries in nanocrystalline Ti–Si–C–N films by Shtansky et al. w10x showed that the grain boundaries had both ordered and disordered regions. In the present paper, we present further evidence for the ordered structure of the grain boundaries. It is also inferred from HRTEM observations that an amorphous phase forms as individual grains rather than as intergranular layers. Note that the HRTEM technique appears to have a clear limit in its application for the characterization of nanocrystalline films with a grain size of less than 1 nm. Finally, it should be emphasized that this paper deals with an extremely complicated problem in characterization of nanostructured multicomponent thin films, and can therefore be expected to provide only a first approximation of the phase composition in the films. This viewpoint is also based on the results of previous works on nanostructured films w14–16,19,24,25x. 4. Summary A multiphase (five phases) composite target was used for deposition of dense, nano-structured, multiphase Ti– Al–B–N films with random orientation. The following results were obtained. 1. The (TiqAl)yB ratio of the films was close to that of the target when the nitrogen partial pressure was low. Thus, no preferential sputtering had occurred. The absence of this effect is an advantage for practical applications. 2. In the range of PVD process parameters examined, substrate temperature and bias voltage had a minor effect on the film microstructure and chemical composition, whereas nitrogen partial pressure affected the film composition and grain size significantly. Moreover, the phase composition was strongly affected by the PVD process parameters. 3. A combination of various microanalytical techniques has enabled the qualitative identification of the phase composition of Ti–Al–B–N films with an extremely small grain size. The structure of Ti–Al–B–N films consisted of various nanocrystalline mixtures of (Ti,Al)N, TiB2 and h-BN phases in an amorphous
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
matrix. The chemical composition of cubic (Ti,Al)N (with boron atoms dissolved) and hexagonal TiB2 phases appears to vary from one grain to another. It was shown that the equilibrium phase diagram does not always correctly predict the phase composition of Ti–Al–B–N films under the deposition conditions used in the present study. 4. Evidence for the ordered structure of grain boundaries in the nanostructured TiAl0.3B0.5N1.9 films was obtained. The amorphous phase formed as individual grains of several nm, rather than as a thin, intergranular amorphous layer of uniform thickness. 5. The TiAl0.2B0.7N0.7 film consisted of crystalline clusters, less than 1 nm in size, being only one–three unit cells in dimension. Further study is necessary to clarify the structure of grain boundaries in this film. References w1x S. Veprek, S. Reiprich, Thin Solid Films 268 (1995) 64. w2x S. Veprek, J. Vac. Sci. Technol. 17 (1999) 2401. w3x S. Veprek, P. Nesladek, ´ ´ A. Niederhofer, F. Glatz, M. Jılek, M. ˇ Sima, Surf. Coat. Technol. 108y109 (1998) 138. w4x X. Sun, J.S. Reid, E. Kolawa, M.-A. Nicolet, J. Appl. Phys. 81 (1997) 656. w5x F. Vaz, L. Rebouta, B. Almeida et al., Surf. Coat. Technol. 120 y121 (1999) 166. w6x A. Niederhofer, P. Nesladek, ´ ¨ H.-D. Mannling, K. Moto, S. ´ Veprek, M. Jılek, Surf. Coat. Technol. 120y121 (1999) 173. ˇ w7x P. Holubar, ´ M. Jılek, ´ M. Sima, Surf. Coat. Technol. 120y121 (1999) 184. w8x D.V. Shtansky, E.A. Levashov, A.N. Sheveiko, J.J. Moore, J. Mater. Synth. Proc. 6 (1) (1998) 61. w9x D.V. Shtansky, E.A. Levashov, A.N. Sheveiko, J.J. Moore, J. Mater. Synth. Proc. 7 (3) (1999) 187. w10x D.V. Shtansky, E.A. Levashov, A.N. Sheveiko, J.J. Moore, Metall. Mater. Trans. 30A (1999) 2439. w11x W. Gissler, Surf. Coat. Technol. 68y69 (1994) 556. w12x T.P. Mollart, M. Baker, J. Haupt, A. Hammer, W. Gissler, Surf. Coat. Technol. 74y75 (1995) 491. w13x P. Losbichler, C. Mitterer, P.N. Gibson, W. Gissler, F. Hofer, P. Warbichler, Surf. Coat. Technol. 94y95 (1997) 297. w14x M.A. Baker, T.P. Mollart, P.N. Gibson, W. Gissler, J. Vac. Sci. Technol. A 15 (1997) 284. w15x T.P. Mollart, P.N. Gibson, M.A. Baker, J. Phys. D: Appl. Phys. 30 (1997) 1827. w16x C. Mitterer, P. Losbichler, F. Hofer, P. Warbichler, P.N. Gibson, W. Gissler, Vacuum 50 (1998) 313. w17x M. Tamura, H. Kubo, Surf. Coat. Technol. 54y55 (1992) 255. w18x D.V. Shtansky, E.A. Levashov, V.I. Kosjanin, E.V. Shtanskaja, Proceedings of the European Conference on Advanced PM Materials, Birmingham, UK, 23–25 October 1995, 1995, p. 435.
215
w19x M.A. Baker, A. Steiner, J. Haupt, W. Gissler, J. Vac. Sci. Technol. 13 (1995) 1633. w20x C.P. Constable, J. Yarwood, W.-D. Munz, ¨ Surf. Coat. Technol. 116–119 (1999) 155. w21x J.J. Moore, A. Kunrath, R. Torres et al., Int. J. SHS 6 (1997) 277. w22x H. Mullejans, ¨ in: D. Kolar, D. Survorov (Eds.), First Slovene– German Seminar on Joint Projects in Material Science and Technology, Bilateral Seminars of the International Bureau, 20, ¨ Forschungzentrum, Julich, 1994, p. 9. w23x C. Mitterer, P. Rodhammer, ¨ ¨ F. Jeglitsch, J. Vac. Sci. H. Stori, Technol. A 7 (1989) 2646. w24x C. Rebholz, H. Ziegete, A. Leyland, A. Matthews, J. Vac. Sci. Technol. A 16 (1998) 2851. w25x C. Rebholz, J.M. Schneider, A.A. Voevodin et al., Surf. Coat. Technol. 113 (1999) 126. w26x J.E. Greene, Ion Beam-Assisted Film Growth, Elsevier, Amsterdam, 1988, chapter 5. w27x W. Palmer, M. Huttinger, W. Bench, Thin Solid Films 174 (1989) 147. w28x J.S. Reid, E. Kolawa, C.M. Garland et al., J. Appl. Phys. 79 (1996) 1109. w29x E. Kolawa, J.M. Molarius, C.N. Nieh, M.-A. Nicolet, J. Vac. Sci. Technol. A 6 (1990) 3006. w30x H. Novotny, F. Benesovsky, C. Brucl, O. Schob, Monatsh. Chem. 92 (1961) 403. w31x U. Wahlstrom, L. Hultman, J.-E. Sundgren, F. Adibi, I. Petrov, J.E. Green, Thin Solid Films 235 (1993) 62. w32x R.A. Andrievsky, G.V. Kalinnikov, N.P. Kobelev, Y.M. Soifer, D.V. Shtansky, Solid-State Phys. 39 (1997) 1859. w33x D.S. Rickerby, A.M. Jones, B.A. Bellamy, Surf. Coat. Technol. 37 (1985) 111. w34x J.F. Graczyk, P. Chaudhari, Phys. Status Solidi b 58 (1973) 163. w35x M.L. Rudee, A. Howie, Philos. Mag. 25 (1972) 1001. w36x D.V. Shtansky, O. Tsuda, Y. Ikuhara, T. Yoshida, Acta Mater. 48 (2000) 3745. w37x D.V. Shtansky, Y. Yamada-Takamura, T. Yoshida, Y. Ikuhara, Sci. Technol. Adv. Mater. 1 (2001) 219. w38x S. Veprek, M. Haussmann, S. Reiprich, L. Shizhi, J. Dian, Surf. Coat. Technol. 86y87 (1996) 394. w39x K. Kaneko, M. Yoshiya, I. Tanaka, S. Tsurekawa, Acta Mater. 47 (1999) 1281. w40x S.D. Berger, D.R. McKenzie, P.J. Martin, Philos. Mag. Lett. 57 (1988) 6. w41x D.G. Rickerby, Philos. Mag. B 68 (1993) 939. w42x H. Holleck, J. Vac. Sci. Technol. A 4 (1986) 2661. w43x S.R. Phillpot, D. Wolf, H. Gleiter, Scr. Metall. Mater. 33 (1995) 1245. w44x P. Keblinski, D. Wolf, S.R. Phillpot, H. Gleiter, Philos. Mag. Lett. 76 (1997) 143. w45x P. Keblinski, S.R. Phillpot, D. Wolf, H. Gleiter, Acta Mater. 45 (1997) 987. w46x P. Keblinski, D. Wolf, S.R. Phillpot, H. Gleiter, Scr. Mater. 41 (1999) 631. w47x G.J. Thomas, R.W. Siegel, J.A. Eastman, Scr. Metall. Mater. 24 (1990) 201.
Characterization of nanostructured multiphase Ti–Al–B–N thin films with extremely small grain size D.V. Shtanskya,*, K. Kanekob, Y. Ikuharac, E.A. Levashova a
SHS-Center, Moscow Steel and Alloys Institute, Leninsky pr. 4, 164, Moscow 119991, Russia b Japan Fine Ceramic Center, Atsuta, Nagoya 456-8587, Japan c Engineering Research Institute, School of Engineering, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan Received 4 April 2001; accepted in revised form 22 June 2001
Abstract The microstructure and chemical composition of nanostructured multiphase Ti–Al–B–N films deposited by DC magnetron sputtering of a multiphase composite Ti–Al–B–N target (five phases) in a gaseous mixture of argon and nitrogen were studied by means of Auger electron spectroscopy, X-ray diffraction, conventional and high-resolution transmission electron microscopy and electron energy-loss spectroscopy. The (TiqAl)yB ratio of films deposited at low nitrogen partial pressure replicates that of the target. At higher values of nitrogen partial pressure, the target is preferentially sputtered. The chemical composition of the films is not influenced significantly by either the substrate temperature or the bias voltage, whereas the phase composition is strongly affected by the PVD process parameters. Evidence for the formation of mixtures of nanocrystalline (Ti,Al)N, TiB2 and h-BN phases in an amorphous matrix is presented. Most of the Ti–Al–B–N films show a grain size in the range 0.6–4 nm. The grain size reduces further to 0.3–1 nm with decreasing N content (TiAl0.2 B0.7 N0.7 film). This is the smallest grain size determined from TEM investigations which has hitherto been observed, being only one–three unit cells in dimension. Evidence for the ordered structure of grain boundaries in the nanostructured TiAl0.3 B0.5 N1.9 film is obtained. The amorphous phase forms as individual grains rather than as a thin, intergranular amorphous layer of uniform thickness, even though only a limited number of grain boundaries were studied in detail. The applicability of the equilibrium phase diagram for the prediction of the phase composition in the Ti–Al–B–N films is also discussed. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Magnetron sputtering; Nanostructured thin films; Transmission electron microscopy (TEM); Electron energy loss spectroscopy (EELS); X-Ray diffraction; Electron diffraction
1. Introduction Multicomponent thin films of titanium-based borides and nitrides are expected to provide superior physical, chemical and engineering properties. The interpretation of these properties is rather complex because many factors affect film characteristics. In order to understand trends in the physical and mechanical behavior, a detailed characterization of the atomic structure, overall chemical composition, phase composition, grain size and morphology is needed. * Corresponding author. Tel.: q7-95-230-4535; fax: q7-95-2365298. E-mail address: [email protected] (D.V. Shtansky).
Recently, a new concept for the design of superhard nanocrystalline films has been developed w1–3x. These materials have been shown to be nanocomposites, consisting of either nanocrystallites in an amorphous matrix, or a mixture of two nanocrystalline phases. Such microstructures are believed to prevent grain boundary sliding and a softening of the film due to an inverse Hall–Petch effect. Nanocrystalline thin films consist of very small grains, typically less than 20 nm in diameter. The grain size, however, is further reduced to a few nm when the composition of the film becomes more complex. For instance, most of the Ti–Si–N w3–6x and Ti–Al–Si–N w6,7x thin films exhibit grain sizes in the range 3–8 nm. According to Shtansky et al. w8–10x, microstructural
0257-8972/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 3 4 1 - X
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
analysis of the Ti–Si–B–N and Ti–Si–C–N films revealed extremely small grain sizes that ranged from 1 to 4 nm. The TiN and TiB2 grain sizes of the Ti–B–N films were reported to range from 2 to 5 nm w11–13x, whereas Baker et al. w14x, using the Scherrer formula, calculated the crystallite size to be as small as 1.7 and 0.9 nm. Mollart et al. w15x suggested that the TiB1.7N1.8 films consisted of clusters of atoms with varying composition, representing neither a fully nanocrystalline nor a homogeneous amorphous state. A quasiamorphous arrangement of the TiB2 and TiB phases in the Ti–B–N films has been proposed by Mitterer et al. w16x. It has repeatedly been shown that the solubility limit of solid solutions of cubic and hexagonal Ti–B–N can be extended by the incorporation of boron and nitrogen atoms in the TiN and TiB2 lattices, respectively w8,13,16–18x. Thus, it is still unclear whether the phase composition of films deposited under non-equilibrium conditions would be the same as predicted from the equilibrium phase diagram as proposed by Gissler and co-workers w11,12,14,19x, who claimed that the phase composition of Ti–B–N films coincided well with that derived from the ternary phase diagram. In the case of nanostructured multiphase thin films, the structure cannot be unambiguously determined by only using the diffraction technique. It is well known that nanocrystalline multicomponent thin films show very broad diffraction peaks with low intensity, which are usually attributed to amorphous material, even though complementary microanalytical techniques had revealed the crystalline nature of fine-grained films. Recently, modern microanalytical techniques, such as Xray photoelectron spectroscopy (XPS) w14,15,19x, Raman spectroscopy w20x and extended X-ray absorption fine structure (EXAFS) measurement w14,15x, were employed to reveal the phase composition of the various multicomponent films. High-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS) are further powerful tools to reveal and identify the structure and phase composition of nanostructured multiphase films. The present work is part of a systematic study to deposit various multicomponent thin films by magnetron sputtering of composite targets w8–10x. Only limited data are available from the literature concerning the deposition of films by sputtering of multiphase targets. In the present study, Ti–Al–B–N films with crystallite sizes ranging from 0.3 to 8 nm have been examined by means of X-ray diffraction (XRD), Auger electron spectroscopy (AES), selected-area electron diffraction (SAED), EELS and HRTEM. 2. Experimental The composite target for PVD was manufactured by means of self-propagating high-temperature synthesis
207
Table 1 Target characteristics Phase composition
(wt.%)
TiB2 TiAl Ti2AlN Ti3Al Al3N4
51 30 13 3 3
Chemical composition
(at.%)
Ti Al B N O (TiqAl)yB (TiqAl)y(BqNqO)
Before sputtering 43.3 7.7 32.5 13.4 3.1 1.6 1.0
After sputtering 47 7.2 27.8 15.4 2.6 1.9 1.2
(SHS). The target was synthesized from an exothermic mixture containing 66.7 wt.% Ti, 20 wt.% Al, and 13.3 wt.% B using the combined SHS–load consolidation technology w21x. The phase composition of the target as determined by Auger electron spectroscopy is shown in Table 1. The major phases in the target were TiB2, TiAl and Ti2AlN; in addition, minor phases such as Ti3Al and Al3N4 phases were detected by X-ray diffraction. The synthesized target was subjected to magnetron sputtering in a gaseous mixture of argon and nitrogen. The diameter of the target was 12.5 cm and the distance from the target to the substrate was 8 cm. The total pressure was maintained at 0.7 Pa, and the nitrogen partial pressure was varied from 10 to 25% of the total pressure. Films were deposited on stainless steel and nickel substrates for 30 min. Prior to deposition, the substrates were mechanically polished and then sputtercleaned by ionic etching with argon at a bias voltage of y600 V for 2 min. The substrate temperature was kept at 7008C during sputter cleaning. The average film thickness was 2 mm. The applied bias (Ubias) was controlled from y500 to 0 V and the substrate temperature ranged from 100 to 4008C. Thin foils for plane-view TEM studies were prepared from 3-mm disks using a standard technique, involving mechanical grinding to a thickness of approximately 0.07 mm from the side of substrate, followed by mechanical dimpling and one-side ion-milling to perforation at a voltage of 3–4 kV. The structure of the films was examined in a Hitachi 9000NAR transmission electron microscope with a point resolution of 0.19 nm operating at 300 kV. The XRD spectra of films were obtained by a Geigerflex X-ray diffractometer. The composition of the films was analyzed by AES. For analytical electron microscopy (AEM), we used dedi-
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
208
Table 2 Sputtering conditions and film characteristics No.
1 2 3 4 5 6 7
Chemical composition (at.%)
PVD characteristics Tsub (8C)
Ubias (V)
N2yAr =100
O
Ti
Al
B
N
(TiqAl)y B
100 250 400 250 250 250 250
0 0 0 250 500 250 250
14 14 14 14 14 10 25
4.1 3.5 3.5 3.5 3 3 3
30.8 30 30.6 33.8 32 37.4 25.9
9.2 9.5 9.2 8.3 9 7.6 8.1
21.6 22 23.4 24.2 23 27.3 13.2
34.3 35 33.1 30.2 33 24.7 49.8
1.8 1.8 1.7 1.7 1.8 1.7 2.6
cated scanning transmission electron microscopes (Thermo, Vacuums-Generators HB 601 UX) operated at 100 kV, equipped with a high-resolution pole piece and a cold field emission gun. Energy resolution of better than 0.4 eV was attained in the full width at half-maximum (FWHM) of the zero loss with the spectrometer entrance aperture limited to approximately 12 mrad. A parallel electron energy-loss spectroscope (PEELS) equipped with photodiode array (Gatan 766) was used for the quantitative chemical analysis, and it had a typical sensitivity better than 0.1 monolayers of impurity segregation at internal interfaces w22x. Regions of appropriate specimen thickness determined by EELS (less than half the mean free path) were carefully selected for the EELS measurements. To measure the EELS edges, the beam current was kept at approximately 0.2 nA with a mean beam diameter of 0.3 nm (FWHM). The Auger analysis was performed on a LHS-10 SAM spectrometer. To remove the surface oxide, samples were etched prior to analysis by a 4-keV argon ion beam with a current of 0.1 mA. For quantification of films, the negative peak intensity ratio method was employed w19x. 3. Results and discussion 3.1. Sputtering of composite target To the best of our knowledge, this is the first attempt to deposit Ti–Al–B–N thin films by magnetron sputtering of a multiphase target (TiB2qTiAlqTi2 AlN, with a minor amount of Ti3Al and Al3N4 phases). Formerly, Ti–Al–B–N films were prepared from hot-pressed TiAlB8N10 w23x and TiAl0.68B3.67N2.44 w24x targets, and most recently, by simultaneously sputtering from TiAl and TiB2 targets in an AryN2 mixture w25x. It is frequently observed that ion bombardment of composite targets results in preferential sputtering. It has been shown that the target was impoverished either by metal atoms w18,26,27x or by metalloid atoms w28,29x during reactive magnetron sputtering. The elemental composition of the composite target used in the present study before and after sputtering is shown in Table 1. The
Grain size (nm)
Formula
– 0.6–3 – 1.5–4 0.9–3 0.3–1 2.5–8
TiAl0.3B0.7N1.1 TiAl0.3B0.8N1.2 TiAl0.3B0.8N1.1 TiAl0.2B0.7N0.9 TiAl0.3B0.7N TiAl0.2B0.7N0.7 TiAl0.3B0.5N1.9
presence of nitrogen is a result of the active interaction between titanium and nitrogen in the air during SHS. The chemical composition of the target during sputtering was fairly constant, with the titanium content increased slightly, while that of boron decreased. 3.2. Composition of the films The influence of the substrate temperature in the range 100–4008C, bias voltage of 0 to y600 V, and nitrogen partial pressure of 10–25%, on the chemical composition of the Ti–Al–B–N films was examined, all other parameters being kept constant. The parameters of the PVD processing, the chemical composition of the films and some other information are presented in Table 2. It is evident that the elemental composition of the films was moderately affected by both the substrate temperature and the bias voltage, supporting the results from w9x. The nitrogen content of the films depends substantially on the nitrogen partial pressure. As the nitrogen partial pressure was increased from 10 to 25%, the nitrogen content of the films increased from 25 to 49%. The as-deposited Ti–Al–B–N films contained 3– 4% oxygen. The oxygen content in the film was equal to that of the target. The (TiqAl)yB ratio of films 1– 6 was found to be close to that of the target. A further increase in the nitrogen partial pressure from 14 to 25% revealed an increase in the concentration of metal atoms in the film, in comparison with that of the target. Thus, the composite target was being sputtered preferentially under sputtering condition 7. To calculate the phase composition of Ti–Al–B–N films, let us consider the simplified Ti(Al)–B–N phase diagram, ignoring, for the sake of simplicity, that microstructure may be far away from thermodynamic equilibrium. Gissler and co-workers w11,12,14,19x showed that the phase composition of Ti–B–N films can be derived from the thermodynamic equilibrium phase diagram at 15008C by Novotny et al. w30x, and the experiments appear to be in agreement with their predictions. Wahlstrom et al. w31x reported that the structure of the
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
209
Fig. 1. A simplified phase diagram of the Ti–B–N system w30x with the film compositions examined in the present study indicated. n, film 1; d, film 2; q, films 3 and 5; m, film 4; s, film 6; and h, film 7.
Ti1yxAlxN films depends on the Al content, being of single phase NaCl type TiN structure for 0FxF0.52. Since the Al content in our films did not exceed 9.5%, it is reasonable to assume that aluminum replaces titanium atoms in the TiN lattice. The chemical compositions of the Ti–Al–B–N films, determined by AES, are marked on the Ti(Al)–B–N phase diagram presented in Fig. 1 and their phase compositions calculated are shown in Table 3. 3.3. Microstructure of the films 3.3.1. X-Ray diffraction Fig. 2 shows the typical XRD spectra of the TiAl0.3B0.7N and TiAl0.2B0.7N0.9 films. The position lines for the stoichiometric TiB2 and TiN phases are also shown in Fig. 2. Two broad peaks from the films can be distinguished with FWHM values of 8–98 (2u). The positions of the center of gravity of the first and second diffraction peaks differ slightly from one film to another, ranging from 0.256 to 0.251 and from 0.21 to 0.208 nm, respectively. There are several factors which may explain such peak broadening: overlapping of diffraction lines from several crystalline phases with very small
Fig. 2. X-Ray diffraction spectra of films 4 and 5 deposited on nickel substrates.
grain size; inhomogeneous distribution of grains with various compositions; or the presence of both crystalline and amorphous phases. For instance, Gissler w11x presented a computer-modeled diffraction spectrum for a TiN structure assuming a random orientation of crystallites with an average grain size of 2 nm. His calculation showed that the FWHM values of (111) and (200) peaks amounted to approximately 4.5 and 5.58 (2u), respectively. Thus, the line-broadening effects observed in the present spectra can be interpreted as a superposition of diffraction lines from the nanocrystalline TiN and TiB2 phases. A further peak broadening can be achieved if the film consisted of crystallites with different chemical compositions, resulting in various lattice parameters. It has been shown that face-centered cubic (f.c.c.) and hexagonal Ti–B–N phases were formed due to the incorporation of boron and nitrogen in the TiN and TiB2 lattice, respectively w8,9,13,16,32x. The composition of grains can also change from grain to grain,
Table 3 Phase composition of Ti–Al–B–N films predicted from Ti(Al)–B–N phase diagram and phases suggested by various experimental methods Film no.
1 2 3 4 5 6 7
Phase composition predicted from phase diagram (%)
XRD
TiB2
TiN
BN
Ti2N
N2
21 21 23 28.5 24 35.5 –
71 70 69 71 71 47.5 69.5
8 9 8 0.5 5 – 28
– – – – – 17 –
– – – – – – 2.5
TiN, TiB2, h-BN TiN, TiB2, h-BN
SAED
HRTEM
ELNES Ti-L23
B-K
h-BN
TiN
TiN
TiN
TiN TiN TiN, TiB2 TiN
TiN
TiN, TiB2
h-BN TiN
TiN, TiB2
TiB2
210
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
Fig. 3. Dark-field TEM micrographs (plain-view) of the as-deposited Ti–Al–B–N films formed from parts of the first two diffraction rings with corresponding SAED patterns inserted. (a) Film 2; (b) film 5; (c) film 4; and (d) film 7.
as was shown by Shtansky et al. w10x in Ti–Si–C–N films using lattice parameter measurements. The first peaks are much broader than the second peaks and show more extended tails on the low-angle side. This feature may be attributed to the spectral contribution of the (001) peak of TiB2 phase andyor the (002) peak of hexagonal boron nitride (h-BN). The position of the second peak and its relative intensity also suggests the superposition of the (111) peak from the cubic boron nitride (c-BN) phase, having a maximum at 0.2087 nm. Further microanalytical analysis is obviously required to establish the phase composition. Note that in the case of nanostructured multiphase thin films, estimation of the crystallite size from the peak width in XRD using the Debye–Scherrer formula, as performed by several authors w12,14,15x, is questionable, because several phases make contributions to the peak width. 3.3.2. Transmission electron microscopy (TEM) Most of the Ti–Al–B–N films were characterized by a very small grain size that ranged from 1 to 3 nm, as estimated from the DF TEM images (see Fig. 3 and Table 2). The grain size increased up to 5–8 nm when the nitrogen content of the films was increased (film 7). The SAED patterns presented in Fig. 3 identified these films as having a cubic structure (lattice type B1). The lattice parameters calculated on the basis of the planes (111) and (220) were correspondingly higher that that
of the (200) line. Anisotropy of the lattice parameter has been reported, and could originate from metalloid atoms (B, O) occupying tetrahedral sites in the cubic TiN lattice w8,33x. The TiAl0.2B0.7N0.7 grain size was estimated from the DF TEM image to be approximately 0.3–1 nm (Fig. 4a). This is the smallest grain size which has hitherto been reported, being only one–three unit cells in dimension. Strictly speaking, the determination of grain size in nanostructured material using DF images is not a perfect method, because a DF image of amorphous material also consists of a speckle of white spots against a dark background. The size of the speckle depends on defocus and varies within the range 0.2–1.5 nm. Note that the question of whether a material is really amorphous or is nanocrystalline is still debated, even for simpler material. For instance, Graczyk and Chaudhari w34x proposed a random network as the correct structure for amorphous Si, whereas Rudee and Howie w35x presented TEM evidence for the microcrystalline model, assuming a grain size of 1.4 nm. The SAED pattern presented in Fig. 4b provides further evidence of the nanocrystalline nature of the TiAl0.2B0.7N0.7 film. The SAED pattern exhibits two continuous diffraction rings, showing an essentially random orientation. The observed degree of ring broadening corresponds to scattering from those crystallographic planes which have d spacings within the range 0.289–
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
211
Fig. 5. Plane-view HRTEM image of film 4 showing a TiN crystallite with a grain size of 4 nm. The incident beam direction is w001x.
Fig. 4. (a) Dark-field TEM micrograph (plain-view) of film 6 formed from parts of the first two diffraction rings, showing very small speckles of white spots ranging from 0.3 to 0.8 nm against a dark background; and (b) SEAD pattern.
0.236 and 0.222–0.194 nm, respectively. This agrees well with XRD results. The line broadening and overlapping do not allow the phase composition of the film to be determined unambiguously. However, the positions of the centers of the diffraction rings and their broadening infers that the rings are formed by a superposition of diffraction lines (111)TiN, (100)TiB2, (200)TiN and (101)TiB2. 3.3.3. High-resolution transmission electron microscopy (HRTEM) HRTEM is a powerful tool for revealing the crystalline structure of an individual grain. In combination with other methods, it allows the phase composition of nanocrystalline film to be determined. Fig. 5 is a planeview HRTEM micrograph of TiAl0.2B0.7N0.9 film 4, showing a TiN crystallite with a grain size of 4 nm oriented along a N001M zone axis. Therefore, both sets of {100}TiN planes are visible. The image around the crystallite shows an amorphous-like structure, although such an image may also result from other effects, such as local strain, overlapping of the grains, or impurity segregation.
Fig. 6 shows a plane-view HRTEM image of a typical area in film 2 containing a number of grains. The areas showing lattice fringes alternate with those displaying an image of randomly scattered intensities, which is characteristic of a disordered structure. It is evident that this film is characterized by a very small grain size, ranging from 1.5 to 2 nm. It is also evident from the HRTEM results that the f.c.c. TiN phase was formed in TiAl0.3B0.8N1.2 film. Fig. 7a,b are HRTEM micrographs of cubic TiN crystallites, 1.5–2 nm in diameter, taken along the w001x and w110x directions, respectively. The angles of the lattice fringes and interplanar spacing are clear evidence for the presence of cubic crystallites embedded in an amorphous matrix. The TiAl0.2B0.7N0.7 film 6 exhibits extremely small grain sizes, less than 1 nm. The HRTEM micrograph of Fig. 8 shows small areas, 0.3-0.8 nm in size, with welldeveloped parallel fringes, as shown by arrows. Although some would assert that such an image indicated disorder structure, others would argue that it could have arisen from an extremely small nanocrystallites embedded in amorphous matrix. Note that the DF image and SAED pattern presented in Fig. 4 are further supportive of the formation of order-disorder structure.
Fig. 6. Plane-view HRTEM image of film 2 showing a number of crystallites with a grain size ranging from 1.5 to 2 nm.
212
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
Fig. 8. Plane-view HRTEM image of film 6 showing an ordered– disordered structure. Arrows indicate the location of certain ordered regions.
Fig. 7. Plane-view HRTEM images of film 2 showing TiN crystallites with a grain size of (a) 1.5 and (b) 2 nm. The incident beam direction is (a) w001x and (b) w110x.
In order to get atomic resolution a satisfactory, the foil thickness must be of the same order as, or less than, the grain size, since overlapping of randomly oriented crystallites with a dimension of a few angstroms makes the interpretation of the microstructure by HRTEM ambiguous. Even if the foil thickness is reduced to approximately 3–5 nm, the specimen is still much too thick to obtain high-contrast image. An example of an area exhibiting the well-developed parallel fringes is shown in Fig. 9. The observed d spacing was consistent with the value of 0.33 nm, which is characteristic of basal planes of the h-BN phase that are oriented edge on w36x. Fig. 9 also illustrates that this crystallite is quite small with spacing as little as three lattice fringes. Such small h-BN crystallites are frequently observed in the sp2bonded layer of BN films w37x. Taking into account all the information obtained from XRD, DF TEM, SAED and HRTEM it can be suggested that the TiAl0.2B0.7N0.7 film consisted of a mixture of nanocrystalline clusters with various symmetries and amorphous matrix. The atomic structure of a grain boundary is also of great importance for the physical properties of nanostructured materials. For instance, a theoretical concept
for designing nanocrystalline superhard material is based on the assumption that a thin amorphous layer around the nanocrystallites prevents the formation and multiplication of dislocations w38x. Therefore, it is often speculated that the nanocrystals would be completely surrounded by a thin amorphous layer. The atomic structure of grain boundaries was studied in film 7, where the grain size was relatively large, in order to obtain a clear image of the interface. The two grains in Fig. 10 were identified to be TiN nanocrystallites with diameters of approximately 6 nm. The right grain was oriented close to a w001x zone axis and the lattice fringes in both grains were from {100} planes. Fig. 10 also shows that there were well-developed growth ledges on
Fig. 9. Plane-view HRTEM image of film 6 showing an area, approximately 1=3 nm in size, with well-developed parallel fringes.
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
213
Fig. 10. Plane-view HRTEM image of film 7 showing a single grain boundary between two TiN crystallites of approximately 6 nm in size. The incident beam direction is w001x. Arrows indicate well-developed growth ledges.
both sides of the TiN crystallite, with the ledge height being equal to one unit lattice height of TiN in the w100x direction. No intergranular precipitation of glassy phase was observed. This is in keeping with the experimental data by Shtansky et al. w10x, who observed ordered grain boundaries in nanostructured Ti–Si–B–N films. 3.3.4. Electron energy-loss spectroscopy The energy-loss near-edge structure (ELNES) exhibits fine structure, which is specific to the type and arrangement of atoms in the near-neighbor shell. This structure is called a ‘coordination fingerprint’. A structural determination can be approximately made from a comparison of the spectrum with a ‘coordination fingerprint’ obtained from a reference material w39x. Fig. 11 shows the Ti-L23 ELNES from films 2, 4 and 6, in comparison with the ELNES from TiN and TiB2 reference films w8,32x. The Ti-L23 ELNES from TiN film exhibited two distinct major peaks, with peak shapes similar to those of TiB2 film, but they were correspondingly shifted towards higher energy by approximately 2.2 and 1.5 eV, respectively. Consequently, the peak separation was larger for TiB2 in comparison with TiN. The Ti-L23 spectrum for film 2 exhibited peak shapes and positions similar to those of TiN reference film. The only feature that can be evident is a small shift of the peaks towards lower energy, by approximately 0.2 and 0.4 eV, respectively, suggesting a slight change in the atomic arrangement of the TiN phase. This is probably due to the incorporation of aluminum and boron atoms in the cubic phase. The peak positions of film 4 and 6 were midway between TiN and TiB2 phases. It is reasonable to assume that the spectra observed are a linear combination of the TiN and TiB2 reference spectra, although the formation of a cubic phase (with dissolved aluminum and boron atoms) cannot completely be excluded.
Fig. 11. N-K and Ti-L23 ELNES from the Ti–Al–B–N films 2, 4 and 6 and the reference TiN and TiB2 films for comparison.
The B-K ELNES from the as-deposited films are plotted in Fig. 12, together with the reference spectra for boron and its’ compounds. Comparisons between the spectrum obtained from film 6 and that from the TiB2 reference film showed that they were similar. In the case of films 2 and 4, however, the presence of
Fig. 12. B-K ELNES from the Ti–Al–B–N films 2, 4 and 6 and the reference TiB2, h-BN, c-BN and B materials for comparison.
214
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
TiB2 phase can hardly be recognized. Film 2 had only one feature at 197.7 eV, which resembles the h-BN spectrum. There is no p–p* loss peak, suggesting that the film was in a poorly crystallized form w40x. Note that it also has a broad peak at approximately 206 eV, which does not correspond to any feature in the h-BN spectrum. As for film 4, the signalynoise is so poor that no serious comment can be made regarding its comparison with h-BN or c-BN phases. These results suggest that the major part of BN is present as an amorphous phase. 3.4. Equilibrium phase diagrams for the prediction of phase composition in multicomponent thin films: use and abuse It has been suggested that the ternary T–B–N phase diagram provides a good basis from which the phase composition for any particular film composition can be estimated w11,12,14,19x. It is not surprising, however, that the present results do not always support this interpretation. In contrast, the equilibrium phase diagram does not appear to be suitable for an interpretation of the phase composition of nanostructured Ti–Al–B–N films. In the present study, evidence for the (Ti,Al)N, TiB2 and h-BN phases were observed. The presence of h-BN crystallites in film 6 was suggested by HRTEM, but this phase was not predicted from the simplified Ti(Al)–B–N phase diagram. The formation of TiB2 phase in films 2 and 4 follows from the Ti(Al)–B–N phase diagram, but the B-K ELNES from these films did not reflect the structure of TiB2 phase. Thus, the formation of TiB2 phase in films 2 and 4 is questionable. The remarkable peaks shift of the Ti-L23 ELNES obtained from films 4 and 6 towards lower energy appeared to indicate that the titanium environment in these films was substantially different from that in the TiN phase. From these arguments, it may be concluded that the cubic phase contained aluminum and a certain amount of boron. Further research is necessary to clarify this point. Note that equilibrium phase diagrams often do not describe the thermodynamics of highly metastable states in thin films, because many additional factors, such as high interdiffusion rates w41x, high quenching rates w42x and energetic ion bombardment, may affect the phase composition. It is also suggested that the composition of grains may vary from one grain to another, similar to previous results. 3.5. Atomic structure of the grain boundaries in nanocrystalline materials The atomic structure of the grain boundaries in nanocrystalline materials has been the subject of extensive discussion during recent years w43–46x. The interest in this topic has been increasing because a significant
amount of the atoms in nanocrystallite materials may be located at the grain boundaries, raising the question as to whether a novel state of matter exists. According to the molecular dynamics simulations by Keblinski et al. w45,46x, the microstructure of nanocrystalline materials consists of ordered crystalline grains with glassy intergranular films of constant, uniform thickness between them. Thus, they have postulated that a nanocrystalline microstructure with random grain orientations contains only high-energy grain boundaries. In contrast to their conclusions, other experimental studies of nanocrystallite thin films have indicated that the grain boundaries are not disordered w47x. A detailed study of the grain boundaries in nanocrystalline Ti–Si–C–N films by Shtansky et al. w10x showed that the grain boundaries had both ordered and disordered regions. In the present paper, we present further evidence for the ordered structure of the grain boundaries. It is also inferred from HRTEM observations that an amorphous phase forms as individual grains rather than as intergranular layers. Note that the HRTEM technique appears to have a clear limit in its application for the characterization of nanocrystalline films with a grain size of less than 1 nm. Finally, it should be emphasized that this paper deals with an extremely complicated problem in characterization of nanostructured multicomponent thin films, and can therefore be expected to provide only a first approximation of the phase composition in the films. This viewpoint is also based on the results of previous works on nanostructured films w14–16,19,24,25x. 4. Summary A multiphase (five phases) composite target was used for deposition of dense, nano-structured, multiphase Ti– Al–B–N films with random orientation. The following results were obtained. 1. The (TiqAl)yB ratio of the films was close to that of the target when the nitrogen partial pressure was low. Thus, no preferential sputtering had occurred. The absence of this effect is an advantage for practical applications. 2. In the range of PVD process parameters examined, substrate temperature and bias voltage had a minor effect on the film microstructure and chemical composition, whereas nitrogen partial pressure affected the film composition and grain size significantly. Moreover, the phase composition was strongly affected by the PVD process parameters. 3. A combination of various microanalytical techniques has enabled the qualitative identification of the phase composition of Ti–Al–B–N films with an extremely small grain size. The structure of Ti–Al–B–N films consisted of various nanocrystalline mixtures of (Ti,Al)N, TiB2 and h-BN phases in an amorphous
D.V. Shtansky et al. / Surface and Coatings Technology 148 (2001) 206–215
matrix. The chemical composition of cubic (Ti,Al)N (with boron atoms dissolved) and hexagonal TiB2 phases appears to vary from one grain to another. It was shown that the equilibrium phase diagram does not always correctly predict the phase composition of Ti–Al–B–N films under the deposition conditions used in the present study. 4. Evidence for the ordered structure of grain boundaries in the nanostructured TiAl0.3B0.5N1.9 films was obtained. The amorphous phase formed as individual grains of several nm, rather than as a thin, intergranular amorphous layer of uniform thickness. 5. The TiAl0.2B0.7N0.7 film consisted of crystalline clusters, less than 1 nm in size, being only one–three unit cells in dimension. Further study is necessary to clarify the structure of grain boundaries in this film. References w1x S. Veprek, S. Reiprich, Thin Solid Films 268 (1995) 64. w2x S. Veprek, J. Vac. Sci. Technol. 17 (1999) 2401. w3x S. Veprek, P. Nesladek, ´ ´ A. Niederhofer, F. Glatz, M. Jılek, M. ˇ Sima, Surf. Coat. Technol. 108y109 (1998) 138. w4x X. Sun, J.S. Reid, E. Kolawa, M.-A. Nicolet, J. Appl. Phys. 81 (1997) 656. w5x F. Vaz, L. Rebouta, B. Almeida et al., Surf. Coat. Technol. 120 y121 (1999) 166. w6x A. Niederhofer, P. Nesladek, ´ ¨ H.-D. Mannling, K. Moto, S. ´ Veprek, M. Jılek, Surf. Coat. Technol. 120y121 (1999) 173. ˇ w7x P. Holubar, ´ M. Jılek, ´ M. Sima, Surf. Coat. Technol. 120y121 (1999) 184. w8x D.V. Shtansky, E.A. Levashov, A.N. Sheveiko, J.J. Moore, J. Mater. Synth. Proc. 6 (1) (1998) 61. w9x D.V. Shtansky, E.A. Levashov, A.N. Sheveiko, J.J. Moore, J. Mater. Synth. Proc. 7 (3) (1999) 187. w10x D.V. Shtansky, E.A. Levashov, A.N. Sheveiko, J.J. Moore, Metall. Mater. Trans. 30A (1999) 2439. w11x W. Gissler, Surf. Coat. Technol. 68y69 (1994) 556. w12x T.P. Mollart, M. Baker, J. Haupt, A. Hammer, W. Gissler, Surf. Coat. Technol. 74y75 (1995) 491. w13x P. Losbichler, C. Mitterer, P.N. Gibson, W. Gissler, F. Hofer, P. Warbichler, Surf. Coat. Technol. 94y95 (1997) 297. w14x M.A. Baker, T.P. Mollart, P.N. Gibson, W. Gissler, J. Vac. Sci. Technol. A 15 (1997) 284. w15x T.P. Mollart, P.N. Gibson, M.A. Baker, J. Phys. D: Appl. Phys. 30 (1997) 1827. w16x C. Mitterer, P. Losbichler, F. Hofer, P. Warbichler, P.N. Gibson, W. Gissler, Vacuum 50 (1998) 313. w17x M. Tamura, H. Kubo, Surf. Coat. Technol. 54y55 (1992) 255. w18x D.V. Shtansky, E.A. Levashov, V.I. Kosjanin, E.V. Shtanskaja, Proceedings of the European Conference on Advanced PM Materials, Birmingham, UK, 23–25 October 1995, 1995, p. 435.
215
w19x M.A. Baker, A. Steiner, J. Haupt, W. Gissler, J. Vac. Sci. Technol. 13 (1995) 1633. w20x C.P. Constable, J. Yarwood, W.-D. Munz, ¨ Surf. Coat. Technol. 116–119 (1999) 155. w21x J.J. Moore, A. Kunrath, R. Torres et al., Int. J. SHS 6 (1997) 277. w22x H. Mullejans, ¨ in: D. Kolar, D. Survorov (Eds.), First Slovene– German Seminar on Joint Projects in Material Science and Technology, Bilateral Seminars of the International Bureau, 20, ¨ Forschungzentrum, Julich, 1994, p. 9. w23x C. Mitterer, P. Rodhammer, ¨ ¨ F. Jeglitsch, J. Vac. Sci. H. Stori, Technol. A 7 (1989) 2646. w24x C. Rebholz, H. Ziegete, A. Leyland, A. Matthews, J. Vac. Sci. Technol. A 16 (1998) 2851. w25x C. Rebholz, J.M. Schneider, A.A. Voevodin et al., Surf. Coat. Technol. 113 (1999) 126. w26x J.E. Greene, Ion Beam-Assisted Film Growth, Elsevier, Amsterdam, 1988, chapter 5. w27x W. Palmer, M. Huttinger, W. Bench, Thin Solid Films 174 (1989) 147. w28x J.S. Reid, E. Kolawa, C.M. Garland et al., J. Appl. Phys. 79 (1996) 1109. w29x E. Kolawa, J.M. Molarius, C.N. Nieh, M.-A. Nicolet, J. Vac. Sci. Technol. A 6 (1990) 3006. w30x H. Novotny, F. Benesovsky, C. Brucl, O. Schob, Monatsh. Chem. 92 (1961) 403. w31x U. Wahlstrom, L. Hultman, J.-E. Sundgren, F. Adibi, I. Petrov, J.E. Green, Thin Solid Films 235 (1993) 62. w32x R.A. Andrievsky, G.V. Kalinnikov, N.P. Kobelev, Y.M. Soifer, D.V. Shtansky, Solid-State Phys. 39 (1997) 1859. w33x D.S. Rickerby, A.M. Jones, B.A. Bellamy, Surf. Coat. Technol. 37 (1985) 111. w34x J.F. Graczyk, P. Chaudhari, Phys. Status Solidi b 58 (1973) 163. w35x M.L. Rudee, A. Howie, Philos. Mag. 25 (1972) 1001. w36x D.V. Shtansky, O. Tsuda, Y. Ikuhara, T. Yoshida, Acta Mater. 48 (2000) 3745. w37x D.V. Shtansky, Y. Yamada-Takamura, T. Yoshida, Y. Ikuhara, Sci. Technol. Adv. Mater. 1 (2001) 219. w38x S. Veprek, M. Haussmann, S. Reiprich, L. Shizhi, J. Dian, Surf. Coat. Technol. 86y87 (1996) 394. w39x K. Kaneko, M. Yoshiya, I. Tanaka, S. Tsurekawa, Acta Mater. 47 (1999) 1281. w40x S.D. Berger, D.R. McKenzie, P.J. Martin, Philos. Mag. Lett. 57 (1988) 6. w41x D.G. Rickerby, Philos. Mag. B 68 (1993) 939. w42x H. Holleck, J. Vac. Sci. Technol. A 4 (1986) 2661. w43x S.R. Phillpot, D. Wolf, H. Gleiter, Scr. Metall. Mater. 33 (1995) 1245. w44x P. Keblinski, D. Wolf, S.R. Phillpot, H. Gleiter, Philos. Mag. Lett. 76 (1997) 143. w45x P. Keblinski, S.R. Phillpot, D. Wolf, H. Gleiter, Acta Mater. 45 (1997) 987. w46x P. Keblinski, D. Wolf, S.R. Phillpot, H. Gleiter, Scr. Mater. 41 (1999) 631. w47x G.J. Thomas, R.W. Siegel, J.A. Eastman, Scr. Metall. Mater. 24 (1990) 201.