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AlON nanomultilayers synthesized by reactive magnetron sputtering technology
Journal of Alloys and Compounds 485 (2009) 435–438
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Epitaxial growth and superhardness effect of TiN/AlON nanomultilayers synthesized by reactive magnetron sputtering technology Ming Kong, Xiaoyan Wu, Bilong Huang, Geyang Li ∗ State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dong Chuan Street, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 17 March 2009 Received in revised form 25 May 2009 Accepted 26 May 2009 Available online 2 June 2009 Keywords: Reactive sputtering Nanomultilayer Crystallization Epitaxial growth Superhardness effect
a b s t r a c t Ti and Al2 O3 targets were used to synthesize a series of TiN/AlON nanomultilayers in the gas mixture of Ar and N2 through reactive magnetron sputtering technology. The formation of AlON and the effect of AlON thickness on microstructures and mechanical properties of the multilayers were evaluated by means of energy dispersive spectrometry, X-ray diffractometry, high-resolution field transmission electron microscopy and nanoindentation. The results revealed that AlON was formed when Al2 O3 target was sputtered in the gas mixture of Ar and N2 resulting from O atoms in Al2 O3 being partially replaced by N atoms. Under normal deposition conditions, AlON was amorphous. However, as a component of nanomultilayers, AlON, when thickness being less than 0.6 nm, was forced to crystallize and grew epitaxially on TiN due to the template effect, and the resultant nanomultilayer yielded superhardness effect with a maximum hardness value of 40.5 GPa. When AlON’s thickness was increased up to 0.6 nm, its growth mode returned from crystalline back to amorphous, and the epitaxial structure was destroyed, resulting in the disappearance of superhardness effect. © 2009 Elsevier B.V. All rights reserved.
1. Introduction With the development of dry and high-speed cutting technology, cutter protective coatings are required to possess not only high hardness but also good oxidation resistance at high temperatures, which cannot be satisfied by coatings in existence. On one hand, although the commonly used nitride coatings such as TiN, TiCN and TiAlN are welcomed for their outstanding mechanical properties, they are not stable at temperatures higher than 800 ◦ C [1], much lower than 1000 ◦ C, a temperature that tips of cutting tools will reach at dry and high-speed cutting. On the other hand, oxide films such as Al2 O3 although perform well at high temperatures; they are low in hardness, resulting in failure when being applied as coatings individually. For instance, Al2 O3 layers as thick as 0.5 m were inserted into the nitride coatings in an attempt to solve oxidation problem of the coatings under high temperatures [2,3]. Unfortunately, the results were not satisfactory because the addition of oxides would bring down the overall hardness. Based on the superhardness effect [4] that has been found in some nitride nanomultilayers, Sproul [5] proposed an oxide/oxide nanomultilayer synthetic route, trying to combine both high temperature oxidation resistance and sufficient hardness. However, the expected superhardness effect was not observed in Al2 O3 /ZrO2 [6]
∗ Corresponding author. Tel.: +86 21 54742261; fax: +86 21 54742268. E-mail address: [email protected] (G. Li). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.133
or Y2 O3 /ZrO2 [7] multilayers prepared by them. The similar shear moduli of the two oxide components (Al2 O3 /ZrO2 ) or the formation of amorphous structure (Y2 O3 /ZrO2 ) were considered to be reasons for the loss of high hardness. Recently, superhard nitride/oxide nanomultilayers such as TiN/SiO2 [8] and TiN/Al2 O3 [9] were successfully synthesized. Investigations showed that when their thickness were reduced to be less than 1 nm, SiO2 and Al2 O3 layers, which normally exist in amorphous structure, were forced to crystallize due to the template effect of crystalline TiN layers and grew epitaxially with TiN, accompanied with achievements of superhardness. HV hardness as high as 44.5 GPa and 37.9 GPa were reported in these nanomultilayers, respectively. However, it is hard for this kind of coatings to be implanted into real industrial mass productions because all the modulation layers in these multilayers were deposited through directly sputtering ceramic TiN, SiO2 or Al2 O3 targets which are low in deposition rate. Very recently, many attempts have been made to increase the deposition rate of these oxide-composed nanomultilayers, and by using reactive sputtering method with metallic V, Al, Zr and ceramic SiO2 , Al2 O3 acting as target, VN/SiO2 [10], VN/AlON [11], AlN/SiO2 [12] and ZrN/AlON [13] were fabricated, respectively. Satisfactory results with both high hardness and high deposition rate were achieved in these systems. In contrast, Ti and Al2 O3 are two most commonly used coating materials and thus more manufacturing implantable. Therefore, in this paper, a reactive magnetron sputtering method by using Ti metallic and Al2 O3 ceramic target was employed to prepare TiN/AlON nanomultilayers
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and the resultant film’s microstructure and mechanical properties were studied. 2. Experimental An ANELVA SPC-350 multi-target magnetron sputtering unit was employed to deposit TiN/AlON nanomultilayers and TiN, AlON monolithic films. A pure Ti (99.99%) metal target whose diameter is 75 mm was controlled by a DC-controlled cathode and an Al2 O3 (99.9%) ceramic target of the same diameter was controlled by a RF cathode. Mirror polished stainless steel and silicon wafer substrates were ultrasonically cleaned in acetone and alcohol for at least 15 min before being mounted on the rotating substrate holder in vacuum chamber. The distance between the target and the substrates was kept constant at 50 mm. After a base pressure of 4.0 × 10−4 Pa was attained, high purity Ar gas (99.999%) and N2 gas (99.999%) were successively aerated into the chamber to act as sputter and reactive gas respectively. During the deposition, the total pressure of the gas mixture was 3.0 × 10−1 Pa with N2 partial pressure being 8.0 × 10−2 Pa. Prior to the deposition of nanomultilayers, a 150 nm thick TiN transition layer was first deposited to promote adhesion between the substrate and the film. Then the substrate holder began to rotate, which enables the substrates alternately stayed in front of the Ti and Al2 O3 targets to acquire the individual materials (TiN and AlON) for the purpose of forming multilayered structure. The accurate control of individual TiN and AlON layer thickness was realized through carefully adjusting time that the substrates were exposed to each target. By this means, a series of nanomultilayer samples with thickness of AlON (lAlON ) layer being varied from 0.3 to 1.5 nm while TiN layer (lTiN ) fixed at 3.2 nm were prepared. For hardness measurement purpose, the accumulated thickness for each multilayer film was controlled to be 2 um. Composition and microstructure of these films were characterized by Rigaku Dmax-rC X-ray diffractometer using Cu K␣ radiation (XRD), Philips CM200 FEG highresolution transmission electron microscope (HRTEM) and EDAX DX-4 X-ray energy dispersive spectrometer (EDS). The hardness of the film was measured by a Fisherscope H100VP nanoindenter with Vickers indenter tip and a maximum applied load of 20 mN. Each hardness value was an average of at least 10 measurements.
3. Results and discussion 3.1. Composition and structure of Al2 O3 monolithic films In the course of reactively synthesizing nanomultilayers at atmosphere of Ar and N2 gas mixture from sputtering Ti and Al2 O3 targets, not only Ti reacted with N2 to produce TiN film, but also O atoms in Al2 O3 were found to be partially replaced by N atoms, forming AlON compound. The content of N in AlON monolithic films as a function of N2 partial pressure in Ar/N2 gas mixture was examined by EDS and illustrated in Fig. 1. It turned out that the as-deposited films were found to contain a certain amount of N even at very low N2 partial pressure, and with the increase of N2 partial pressure, the N content in AlON also increased correspondingly. However, the speed that O being replaced by N was initially very high and then gradually slowed down, providing an eventual N content of less than 25% even as N2 partial pressure accounts for 50% of the total pressure. Based on this plot, a N2 partial pressure
Fig. 1. N content (atomic ratio, nN /nN+O ) in AlON films as a function of N2 partial pressure (pN2 /pN2 + pAr ) in gas mixture.
of about 8.0 × 10−2 Pa (27% of the total pressure) was adopted to prepare nanomultilyers which yielded AlON an atomic ratio of N and (N + O) (nN /nN+O ) of about 0.22. XRD analysis revealed, on the other hand, that all the AlON monolithic films deposited under different N2 partial pressures were amorphous, in well agree with Xiao et al.’s results [14,15] who deposited AlON films by using CVD method. In addition, Xiao et al. reported that by using N atoms to partially take the place of O atoms, superior mechanical properties as compared to Al2 O3 and increased oxidation resistance temperature as high as 1200 ◦ C could be achieved as well. 3.2. Microstructure of nanomultilayers Fig. 2 provides several HRTEM images taken from TiN (3.2 nm)/AlON (0.6 nm) nanomultilayer. For the low-magnification image (Fig. 2a), a columnar crystal structure was observed and the column diameter was in the range of 30–50 nm. The middlemagnification image shown in Fig. 2b further revealed that in these columnar crystals, well-defined periodic TiN (bright contrast) and AlON (dark contrast) structure existed. A further zoom in on these regions provides high-magnification lattice image (Fig. 2c), which revealed that the lattice fringes were, in fact, growing continuously across several TiN and AlON layers. Selected area electron diffraction (SAED) pattern at the top left corner of Fig. 2c was indexed as only one set of FCC polycrystalline structure corresponding to TiN, and no new phase was identified. Clearly, AlON formed a kind of pseudomorphic structure which is identical to that of TiN. From the above HRTEM analysis, it can be concluded that the columnar crystals formed in nanomultilayers are distinctive from traditional columnar crystals in that they were characterized of continuous in structure while periodically changed in composition. The clear low-angle XRD superlattice peaks shown in Fig. 3 also indicated the existence of compositional periodic structure in these nanomultilayers when AlON thickness was over 0.4 nm. From /2 position of the superlattice peaks, the modulation periods were calculated in terms of a modified form of Bragg’s law [16] and the results agreed very well with HRTEM observation. On the other hand, because AlON layer is extremely thin or intermixture occurring, no obvious low-angle reflection was observed for multilayer with AlON = 0.3 nm and 0.4 nm. Nevertheless, its nominal thickness could be calculated by combining the EDS and HRTEM analysis, since TiN thickness was constant in these nanomultilayers while AlON thickness was in proportion to the Al/Ti atomic ratio. High-angle XRD patterns shown in Fig. 4 indicated that the as-deposited TiN monolithic film presented orientation growth preferred in (1 1 1)TiN crystalline plane. In nanomultilayers, when lAlON = 0.4 and 0.6 nm, a considerable increase in the intensity of (1 1 1)TiN diffraction peak was observed, showing that these nanomultilayers had superior crystallization to that of TiN monolithic film along (1 1 1) preferred orientation. This phenomenon results from a template effect accompanied with the growing of nanomultilayers [17] where AlON layer was forced to crystallize and grew epitaxially with TiN layers along (1 1 1)TiN preferred orientation. On the other hand, no new diffraction peak corresponding to AlON arises, indicating that AlON has crystallized into a kind of pseudomorphic structure which is identical to that of NaCl-typed fcc TiN crystals, in well agree with the HRTEM results shown in Fig. 2c. With a further increase in lAlON (≥1.2 nm), however, the intensity of (1 1 1)TiN peak decreased rapidly and left only a broaden and diffuse peak when AlON was increased to 1.5 nm. This can be explained by the fact that the growth mode of AlON returned back to amorphous when its thickness was increased, and hence blocked the epitaxial growth of columnar grains with strong (1 1 1)TiN orientation, leaving the structure of the present nanomultilayers to be an alternate stacking of nanocrystalline TiN and amorphous AlON.
M. Kong et al. / Journal of Alloys and Compounds 485 (2009) 435–438
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Fig. 2. Cross-sectional HRTEM images of TiN (3.2 nm)/AlON (0.6 nm) nanomultilayer (a. low-magnification; b. middle-magnification; c. high-magnification and selected area electron diffraction pattern).
Such microstructure evolution of the nanomultilayers can be understood by combining both thermodynamic and kinetic factors. In the thermodynamic aspect, coherent interface helps to reduce the total energy of the multilayer system, so AlON initially deposited on heterogeneous TiN crystal layer has a tendency of forming crystal and coherently growing with TiN layer. On the other hand, the formation of the above thermodynamically stable microstructure should also be cooperated kinetically by sufficient migration capability of the AlON adatoms on the growing surface. The higher migration ability the adatoms have, the easier that they overcome energy barrier and migrate from metastable positions (lower potential energy) to stable positions (lowest potential energy) will be. The mobility of adatoms is related not only to substrate temperature, residual energy when reaching growing surface, but also to the properties of the growing surface. If adatoms are deposited on het-
erogeneous materials, a higher mobility is usually expected, as the case in this study, the AlON atoms were deposited on TiN surface at the initial stage of each modulation period and had a high mobility to conquer kinetics threshold for crystallization, and therefore crystallized and epitaxially grew with TiN. However, after the stacking of several AlON monolayers, successively deposited AlON would have to grow on its own surface. As a result of atomic bond’s change, the dynamic conditions for AlON crystallization to be occurred were no longer assumed and it quickly altered back to amorphous growth mode, and newly deposited TiN on amorphous AlON will have to renucleate into nanocrystalline structure, bringing in a change in the multilayer’s structure from continuous epitaxial columnar crystals to alternate amorphous and nanocrystals. 3.3. Mechanical properties of nanomultilayers Hardness test of these samples gave an indication that the superhardness effect in nanomultilayers was closely related to the crystallization of AlON layer. As shown in Fig. 5, the hardness of
Fig. 3. Low-angle XRD patterns of nanomultilayers with different AlON thicknesses.
Fig. 4. XRD patterns of TiN monolithic film and TiN/AlON nanomultilayers with different AlON thicknesses (lTiN = 3.2 nm).
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Our research also indicated that it is feasible to synthesize nitride/oxide nanomultilayer through high-deposition-rate reactive sputtering method. Although the deposition rate is relatively low for Al2 O3 ceramic target, the fact that Al2 O3 layer only counts for a very small part of the nanomultilayers (only ∼0.6 nm thick) and the deposition rate of the whole nanomultilayer is predominately determined by the deposition rate of TiN layer provides a reasonable overall deposition rate. The value of this research not only lies in that it extends the current material combinations for nanomultilayers possessing high hardness, but also in that it provides a way to fabricate nanomultilayer coatings that can be industrialized. 4. Conclusions
Fig. 5. Dependence of hardness of nanomultilayers on AlON layer thickness, high HV corresponding to AlON’s crystallization.
TiN and AlON monolithic films are 28.0 GPa and 13.0 GPa, respectively. With the initial insertion of AlON layers, the hardness of nanomultilayers was found significantly increased and a maximum value of 40.8 GPa was achieved at lAlON = 0.6 nm. With the obtained high hardness, the TiN/AlON nanomultilayers demonstrates a high potential of being applied as hard protective coatings. However, such good mechanical properties will degrade if further increase AlON’s thickness, and the hardness quickly dropped down again to a value of less than 28 GPa when lAlON reached 1.5 nm. Based on the above microstructure analysis and hardness test results, the fact that the mechanical properties of TiN/AlON nanomultilayers have a close relationship with film’s microstructure, especially the crystallization state of AlON, is clear. At small thickness, AlON layer crystallized due to the template effect of TiN crystalline layer and formed coherent interface with TiN, resulting in a significant increase in hardness. With a further increase in thickness, AlON layer became amorphous, blocked the coherent growth and degraded the high hardness of nanomultilayers accordingly. With regard to the superhardness effect in nanomultilayers, several theories have been proposed to explain its mechanisms, including dislocation blocking by layer interfaces [18], strain effects at layer interfaces [19] and Hall–Petch strengthening [20], etc. Although emphasizing particularly on different points, they have some common prerequisites for multilayers to achieve superhardness: firstly, the two constituent layers should have different shear moduli, and therefore different dislocation line energies. Secondly, the individual layer thicknesses must be thin enough so that dislocation generation and movement cannot occur within the individual layers. Last but most importantly, the two constituent layers should form coherent interfaces. Microstructure analyses revealed that TiN/AlON nanomultilayers with lAlON less than 1 nm met all the above requirements and hence showed high hardness. Further increasing lAlON leads to the destruction of coherent interfaces due to the formation of amorphous AlON layers, and therefore the degradation of the nanomultilayers’ hardness.
In summary, the nitrogen content of AlON layers deposited by reactively sputtering the Al2 O3 target in a mixed atmosphere of Ar and N2 is directly dependent on nitrogen partial pressure, and the nitrogen content of AlON in the researched TiN/AlON nanomultilayers is lower than one-quarter of the total nonmetal contents. Affected by the template effect of TiN crystal layers, AlON layer is forced to crystallize when its thickness is less than 0.6 nm. Crystallized AlON forms a kind of pseudomorphic structure identical to that of TiN and grows epitaxially with TiN layers. Correspondingly, film’s hardness is significantly enhanced to a maximum value of 40.8 GPa. A further increase in AlON layer thickness leads to amorphous growth of AlON and the decline of film’s hardness. The expectation of greatly enhanced deposition rate along with its superior mechanical properties provides this kind of nanomultilayers very high promise of being applied in real industrial mass productions. Acknowledgements The authors gratefully acknowledge financial support from the National Natural Foundation of China, under Grant No. U0774001. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
S. PalDey, S.C. Deevi, Mater. Sci. Eng. A 342 (2003) 58–79. J.N. Lindstrom, R.T. Johannesson, J. Electrochem. Soc. 123 (1976) 555–559. A.A. Layyous, D.M. Freinkel, R. Israel, Surf. Coat. Technol. 56 (1992) 89–95. U. Helmersson, S. Todorova, S.A. Barnett, J. Appl. Phys. 62 (1987) 481–484. W.D. Sproul, Science 273 (1996) 889–892. W.D. Sproul, Surf. Coat. Technol. 86–87 (1996) 170–176. P. Yashar, S.A. Barnett, L. Hultman, W.D. Sproul, J. Mater. Res. 14 (1999) 3614–3622. L. Wei, F.H. Mei, N. Shao, M. Kong, G.Y. Li, J.G. Li, Appl. Phys. Lett. 86 (2005) 021919. L. Wei, M. Kong, Y.S. Dong, G.Y. Li, J. Appl. Phys. 98 (2005) 074302. J.L. Yue, W.J. Zhao, G.Y. Li, W. Gao, Mater. Lett. 62 (2008) 1621–1623. J.L. Yue, Y. Liu, W.J. Zhao, G.Y. Li, Scripta Mater. 55 (2006) 895–898. W.J. Zhao, M. Kong, Y. Wu, G.Y. Li, J. Appl. Phys. 103 (2008) 043506. Y.S. Dong, J.L. Yue, Y. Liu, G.Y. Li, J. Appl. Phys. D: Appl. Phys. 39 (2006) 4838–4842. X.D. Wang, F.M. Wang, W.C. Li, Mater. Sci. Eng. A 342 (2003) 245–250. W.D. Xiao, X. Jiang, Cryst. Growth 264 (2004) 165–171. C. Kim, S.B. Qadri, M.R. Scanlon, R.C. Cammarata, Thin Solid Films 240 (1994) 52–55. A. Madan, I.W. Kim, S.C. Cheng, P. Yashar, V.P. Dravid, S.A. Barnett, Phys. Rev. Lett. 78 (1997) 1743–1746. J.S. Koehler, Phys. Rev. B 2 (1970) 547–551. M. Kato, T. Mori, L.H. Schwartz, Acta. Metall. 28 (1980) 285–290. P.M. Anderson, C. Li, Nanostruct. Mater. 5 (1995) 349–362.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Epitaxial growth and superhardness effect of TiN/AlON nanomultilayers synthesized by reactive magnetron sputtering technology Ming Kong, Xiaoyan Wu, Bilong Huang, Geyang Li ∗ State Key Lab of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dong Chuan Street, Shanghai 200240, China
a r t i c l e
i n f o
Article history: Received 17 March 2009 Received in revised form 25 May 2009 Accepted 26 May 2009 Available online 2 June 2009 Keywords: Reactive sputtering Nanomultilayer Crystallization Epitaxial growth Superhardness effect
a b s t r a c t Ti and Al2 O3 targets were used to synthesize a series of TiN/AlON nanomultilayers in the gas mixture of Ar and N2 through reactive magnetron sputtering technology. The formation of AlON and the effect of AlON thickness on microstructures and mechanical properties of the multilayers were evaluated by means of energy dispersive spectrometry, X-ray diffractometry, high-resolution field transmission electron microscopy and nanoindentation. The results revealed that AlON was formed when Al2 O3 target was sputtered in the gas mixture of Ar and N2 resulting from O atoms in Al2 O3 being partially replaced by N atoms. Under normal deposition conditions, AlON was amorphous. However, as a component of nanomultilayers, AlON, when thickness being less than 0.6 nm, was forced to crystallize and grew epitaxially on TiN due to the template effect, and the resultant nanomultilayer yielded superhardness effect with a maximum hardness value of 40.5 GPa. When AlON’s thickness was increased up to 0.6 nm, its growth mode returned from crystalline back to amorphous, and the epitaxial structure was destroyed, resulting in the disappearance of superhardness effect. © 2009 Elsevier B.V. All rights reserved.
1. Introduction With the development of dry and high-speed cutting technology, cutter protective coatings are required to possess not only high hardness but also good oxidation resistance at high temperatures, which cannot be satisfied by coatings in existence. On one hand, although the commonly used nitride coatings such as TiN, TiCN and TiAlN are welcomed for their outstanding mechanical properties, they are not stable at temperatures higher than 800 ◦ C [1], much lower than 1000 ◦ C, a temperature that tips of cutting tools will reach at dry and high-speed cutting. On the other hand, oxide films such as Al2 O3 although perform well at high temperatures; they are low in hardness, resulting in failure when being applied as coatings individually. For instance, Al2 O3 layers as thick as 0.5 m were inserted into the nitride coatings in an attempt to solve oxidation problem of the coatings under high temperatures [2,3]. Unfortunately, the results were not satisfactory because the addition of oxides would bring down the overall hardness. Based on the superhardness effect [4] that has been found in some nitride nanomultilayers, Sproul [5] proposed an oxide/oxide nanomultilayer synthetic route, trying to combine both high temperature oxidation resistance and sufficient hardness. However, the expected superhardness effect was not observed in Al2 O3 /ZrO2 [6]
∗ Corresponding author. Tel.: +86 21 54742261; fax: +86 21 54742268. E-mail address: [email protected] (G. Li). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.05.133
or Y2 O3 /ZrO2 [7] multilayers prepared by them. The similar shear moduli of the two oxide components (Al2 O3 /ZrO2 ) or the formation of amorphous structure (Y2 O3 /ZrO2 ) were considered to be reasons for the loss of high hardness. Recently, superhard nitride/oxide nanomultilayers such as TiN/SiO2 [8] and TiN/Al2 O3 [9] were successfully synthesized. Investigations showed that when their thickness were reduced to be less than 1 nm, SiO2 and Al2 O3 layers, which normally exist in amorphous structure, were forced to crystallize due to the template effect of crystalline TiN layers and grew epitaxially with TiN, accompanied with achievements of superhardness. HV hardness as high as 44.5 GPa and 37.9 GPa were reported in these nanomultilayers, respectively. However, it is hard for this kind of coatings to be implanted into real industrial mass productions because all the modulation layers in these multilayers were deposited through directly sputtering ceramic TiN, SiO2 or Al2 O3 targets which are low in deposition rate. Very recently, many attempts have been made to increase the deposition rate of these oxide-composed nanomultilayers, and by using reactive sputtering method with metallic V, Al, Zr and ceramic SiO2 , Al2 O3 acting as target, VN/SiO2 [10], VN/AlON [11], AlN/SiO2 [12] and ZrN/AlON [13] were fabricated, respectively. Satisfactory results with both high hardness and high deposition rate were achieved in these systems. In contrast, Ti and Al2 O3 are two most commonly used coating materials and thus more manufacturing implantable. Therefore, in this paper, a reactive magnetron sputtering method by using Ti metallic and Al2 O3 ceramic target was employed to prepare TiN/AlON nanomultilayers
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M. Kong et al. / Journal of Alloys and Compounds 485 (2009) 435–438
and the resultant film’s microstructure and mechanical properties were studied. 2. Experimental An ANELVA SPC-350 multi-target magnetron sputtering unit was employed to deposit TiN/AlON nanomultilayers and TiN, AlON monolithic films. A pure Ti (99.99%) metal target whose diameter is 75 mm was controlled by a DC-controlled cathode and an Al2 O3 (99.9%) ceramic target of the same diameter was controlled by a RF cathode. Mirror polished stainless steel and silicon wafer substrates were ultrasonically cleaned in acetone and alcohol for at least 15 min before being mounted on the rotating substrate holder in vacuum chamber. The distance between the target and the substrates was kept constant at 50 mm. After a base pressure of 4.0 × 10−4 Pa was attained, high purity Ar gas (99.999%) and N2 gas (99.999%) were successively aerated into the chamber to act as sputter and reactive gas respectively. During the deposition, the total pressure of the gas mixture was 3.0 × 10−1 Pa with N2 partial pressure being 8.0 × 10−2 Pa. Prior to the deposition of nanomultilayers, a 150 nm thick TiN transition layer was first deposited to promote adhesion between the substrate and the film. Then the substrate holder began to rotate, which enables the substrates alternately stayed in front of the Ti and Al2 O3 targets to acquire the individual materials (TiN and AlON) for the purpose of forming multilayered structure. The accurate control of individual TiN and AlON layer thickness was realized through carefully adjusting time that the substrates were exposed to each target. By this means, a series of nanomultilayer samples with thickness of AlON (lAlON ) layer being varied from 0.3 to 1.5 nm while TiN layer (lTiN ) fixed at 3.2 nm were prepared. For hardness measurement purpose, the accumulated thickness for each multilayer film was controlled to be 2 um. Composition and microstructure of these films were characterized by Rigaku Dmax-rC X-ray diffractometer using Cu K␣ radiation (XRD), Philips CM200 FEG highresolution transmission electron microscope (HRTEM) and EDAX DX-4 X-ray energy dispersive spectrometer (EDS). The hardness of the film was measured by a Fisherscope H100VP nanoindenter with Vickers indenter tip and a maximum applied load of 20 mN. Each hardness value was an average of at least 10 measurements.
3. Results and discussion 3.1. Composition and structure of Al2 O3 monolithic films In the course of reactively synthesizing nanomultilayers at atmosphere of Ar and N2 gas mixture from sputtering Ti and Al2 O3 targets, not only Ti reacted with N2 to produce TiN film, but also O atoms in Al2 O3 were found to be partially replaced by N atoms, forming AlON compound. The content of N in AlON monolithic films as a function of N2 partial pressure in Ar/N2 gas mixture was examined by EDS and illustrated in Fig. 1. It turned out that the as-deposited films were found to contain a certain amount of N even at very low N2 partial pressure, and with the increase of N2 partial pressure, the N content in AlON also increased correspondingly. However, the speed that O being replaced by N was initially very high and then gradually slowed down, providing an eventual N content of less than 25% even as N2 partial pressure accounts for 50% of the total pressure. Based on this plot, a N2 partial pressure
Fig. 1. N content (atomic ratio, nN /nN+O ) in AlON films as a function of N2 partial pressure (pN2 /pN2 + pAr ) in gas mixture.
of about 8.0 × 10−2 Pa (27% of the total pressure) was adopted to prepare nanomultilyers which yielded AlON an atomic ratio of N and (N + O) (nN /nN+O ) of about 0.22. XRD analysis revealed, on the other hand, that all the AlON monolithic films deposited under different N2 partial pressures were amorphous, in well agree with Xiao et al.’s results [14,15] who deposited AlON films by using CVD method. In addition, Xiao et al. reported that by using N atoms to partially take the place of O atoms, superior mechanical properties as compared to Al2 O3 and increased oxidation resistance temperature as high as 1200 ◦ C could be achieved as well. 3.2. Microstructure of nanomultilayers Fig. 2 provides several HRTEM images taken from TiN (3.2 nm)/AlON (0.6 nm) nanomultilayer. For the low-magnification image (Fig. 2a), a columnar crystal structure was observed and the column diameter was in the range of 30–50 nm. The middlemagnification image shown in Fig. 2b further revealed that in these columnar crystals, well-defined periodic TiN (bright contrast) and AlON (dark contrast) structure existed. A further zoom in on these regions provides high-magnification lattice image (Fig. 2c), which revealed that the lattice fringes were, in fact, growing continuously across several TiN and AlON layers. Selected area electron diffraction (SAED) pattern at the top left corner of Fig. 2c was indexed as only one set of FCC polycrystalline structure corresponding to TiN, and no new phase was identified. Clearly, AlON formed a kind of pseudomorphic structure which is identical to that of TiN. From the above HRTEM analysis, it can be concluded that the columnar crystals formed in nanomultilayers are distinctive from traditional columnar crystals in that they were characterized of continuous in structure while periodically changed in composition. The clear low-angle XRD superlattice peaks shown in Fig. 3 also indicated the existence of compositional periodic structure in these nanomultilayers when AlON thickness was over 0.4 nm. From /2 position of the superlattice peaks, the modulation periods were calculated in terms of a modified form of Bragg’s law [16] and the results agreed very well with HRTEM observation. On the other hand, because AlON layer is extremely thin or intermixture occurring, no obvious low-angle reflection was observed for multilayer with AlON = 0.3 nm and 0.4 nm. Nevertheless, its nominal thickness could be calculated by combining the EDS and HRTEM analysis, since TiN thickness was constant in these nanomultilayers while AlON thickness was in proportion to the Al/Ti atomic ratio. High-angle XRD patterns shown in Fig. 4 indicated that the as-deposited TiN monolithic film presented orientation growth preferred in (1 1 1)TiN crystalline plane. In nanomultilayers, when lAlON = 0.4 and 0.6 nm, a considerable increase in the intensity of (1 1 1)TiN diffraction peak was observed, showing that these nanomultilayers had superior crystallization to that of TiN monolithic film along (1 1 1) preferred orientation. This phenomenon results from a template effect accompanied with the growing of nanomultilayers [17] where AlON layer was forced to crystallize and grew epitaxially with TiN layers along (1 1 1)TiN preferred orientation. On the other hand, no new diffraction peak corresponding to AlON arises, indicating that AlON has crystallized into a kind of pseudomorphic structure which is identical to that of NaCl-typed fcc TiN crystals, in well agree with the HRTEM results shown in Fig. 2c. With a further increase in lAlON (≥1.2 nm), however, the intensity of (1 1 1)TiN peak decreased rapidly and left only a broaden and diffuse peak when AlON was increased to 1.5 nm. This can be explained by the fact that the growth mode of AlON returned back to amorphous when its thickness was increased, and hence blocked the epitaxial growth of columnar grains with strong (1 1 1)TiN orientation, leaving the structure of the present nanomultilayers to be an alternate stacking of nanocrystalline TiN and amorphous AlON.
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Fig. 2. Cross-sectional HRTEM images of TiN (3.2 nm)/AlON (0.6 nm) nanomultilayer (a. low-magnification; b. middle-magnification; c. high-magnification and selected area electron diffraction pattern).
Such microstructure evolution of the nanomultilayers can be understood by combining both thermodynamic and kinetic factors. In the thermodynamic aspect, coherent interface helps to reduce the total energy of the multilayer system, so AlON initially deposited on heterogeneous TiN crystal layer has a tendency of forming crystal and coherently growing with TiN layer. On the other hand, the formation of the above thermodynamically stable microstructure should also be cooperated kinetically by sufficient migration capability of the AlON adatoms on the growing surface. The higher migration ability the adatoms have, the easier that they overcome energy barrier and migrate from metastable positions (lower potential energy) to stable positions (lowest potential energy) will be. The mobility of adatoms is related not only to substrate temperature, residual energy when reaching growing surface, but also to the properties of the growing surface. If adatoms are deposited on het-
erogeneous materials, a higher mobility is usually expected, as the case in this study, the AlON atoms were deposited on TiN surface at the initial stage of each modulation period and had a high mobility to conquer kinetics threshold for crystallization, and therefore crystallized and epitaxially grew with TiN. However, after the stacking of several AlON monolayers, successively deposited AlON would have to grow on its own surface. As a result of atomic bond’s change, the dynamic conditions for AlON crystallization to be occurred were no longer assumed and it quickly altered back to amorphous growth mode, and newly deposited TiN on amorphous AlON will have to renucleate into nanocrystalline structure, bringing in a change in the multilayer’s structure from continuous epitaxial columnar crystals to alternate amorphous and nanocrystals. 3.3. Mechanical properties of nanomultilayers Hardness test of these samples gave an indication that the superhardness effect in nanomultilayers was closely related to the crystallization of AlON layer. As shown in Fig. 5, the hardness of
Fig. 3. Low-angle XRD patterns of nanomultilayers with different AlON thicknesses.
Fig. 4. XRD patterns of TiN monolithic film and TiN/AlON nanomultilayers with different AlON thicknesses (lTiN = 3.2 nm).
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Our research also indicated that it is feasible to synthesize nitride/oxide nanomultilayer through high-deposition-rate reactive sputtering method. Although the deposition rate is relatively low for Al2 O3 ceramic target, the fact that Al2 O3 layer only counts for a very small part of the nanomultilayers (only ∼0.6 nm thick) and the deposition rate of the whole nanomultilayer is predominately determined by the deposition rate of TiN layer provides a reasonable overall deposition rate. The value of this research not only lies in that it extends the current material combinations for nanomultilayers possessing high hardness, but also in that it provides a way to fabricate nanomultilayer coatings that can be industrialized. 4. Conclusions
Fig. 5. Dependence of hardness of nanomultilayers on AlON layer thickness, high HV corresponding to AlON’s crystallization.
TiN and AlON monolithic films are 28.0 GPa and 13.0 GPa, respectively. With the initial insertion of AlON layers, the hardness of nanomultilayers was found significantly increased and a maximum value of 40.8 GPa was achieved at lAlON = 0.6 nm. With the obtained high hardness, the TiN/AlON nanomultilayers demonstrates a high potential of being applied as hard protective coatings. However, such good mechanical properties will degrade if further increase AlON’s thickness, and the hardness quickly dropped down again to a value of less than 28 GPa when lAlON reached 1.5 nm. Based on the above microstructure analysis and hardness test results, the fact that the mechanical properties of TiN/AlON nanomultilayers have a close relationship with film’s microstructure, especially the crystallization state of AlON, is clear. At small thickness, AlON layer crystallized due to the template effect of TiN crystalline layer and formed coherent interface with TiN, resulting in a significant increase in hardness. With a further increase in thickness, AlON layer became amorphous, blocked the coherent growth and degraded the high hardness of nanomultilayers accordingly. With regard to the superhardness effect in nanomultilayers, several theories have been proposed to explain its mechanisms, including dislocation blocking by layer interfaces [18], strain effects at layer interfaces [19] and Hall–Petch strengthening [20], etc. Although emphasizing particularly on different points, they have some common prerequisites for multilayers to achieve superhardness: firstly, the two constituent layers should have different shear moduli, and therefore different dislocation line energies. Secondly, the individual layer thicknesses must be thin enough so that dislocation generation and movement cannot occur within the individual layers. Last but most importantly, the two constituent layers should form coherent interfaces. Microstructure analyses revealed that TiN/AlON nanomultilayers with lAlON less than 1 nm met all the above requirements and hence showed high hardness. Further increasing lAlON leads to the destruction of coherent interfaces due to the formation of amorphous AlON layers, and therefore the degradation of the nanomultilayers’ hardness.
In summary, the nitrogen content of AlON layers deposited by reactively sputtering the Al2 O3 target in a mixed atmosphere of Ar and N2 is directly dependent on nitrogen partial pressure, and the nitrogen content of AlON in the researched TiN/AlON nanomultilayers is lower than one-quarter of the total nonmetal contents. Affected by the template effect of TiN crystal layers, AlON layer is forced to crystallize when its thickness is less than 0.6 nm. Crystallized AlON forms a kind of pseudomorphic structure identical to that of TiN and grows epitaxially with TiN layers. Correspondingly, film’s hardness is significantly enhanced to a maximum value of 40.8 GPa. A further increase in AlON layer thickness leads to amorphous growth of AlON and the decline of film’s hardness. The expectation of greatly enhanced deposition rate along with its superior mechanical properties provides this kind of nanomultilayers very high promise of being applied in real industrial mass productions. Acknowledgements The authors gratefully acknowledge financial support from the National Natural Foundation of China, under Grant No. U0774001. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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