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Si3N4 multilayer coatings
Surface & Coatings Technology 205 (2011) 3588–3595
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Structure and mechanical properties of TiAlSiN/Si3N4 multilayer coatings K. Zhang a, L.S. Wang a, G.H. Yue a, Y.Z Chen a, D.L. Peng a,⁎, Z.B. Qi b, Z.C. Wang b a b
Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, China College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
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Article history: Received 14 July 2010 Accepted in revised form 20 December 2010 Available online 29 December 2010 Keywords: Reactive magnetron co-sputtering TiAlSiN/Si3N4 multilayer Mechanical properties
a b s t r a c t TiAlSiN/Si3N4 multilayer coatings which have different separate layer thicknesses of TiAlSiN or Si3N4 were deposited onto glass sheets, single-crystal silicon wafers and polished WC–Co substrates by reactive magnetron co-sputtering. The morphology, crystalline structure and thickness of the as-prepared multilayer coatings were characterized by TEM, SEM, XRD and film thickness measuring instrument. The mechanical properties of the coatings were evaluated by a nanoindenter. The effects of monolayer thickness on the microstructure and properties of TiAlSiN/Si3N4 multilayer coatings were explored. The coatings showed the highest hardness when the thickness of Si3N4 and TiAlSiN monolayers was 0.33 nm and 5.8 nm, respectively. The oxidation characteristics of the coatings were studied at temperatures ranging from 700 °C to 900 °C for oxidation time up to 20 h in air. It was found that the coatings displayed good oxidation resistance. © 2010 Elsevier B.V. All rights reserved.
1. Introduction TiN hard coatings deposited by Physical Vapour Deposition (PVD) have been paid great attention to their technological applications [1]. In the last two decades, multi-element systems have received more attention to further improving the performance [2]. The microstructure, composition, mechanical properties, wear and cutting performance of quaternary TiAlSiN have been studied recently [3–6]. It was reported that the microstructure and mechanical properties of TiAlSiN depended on the Si contents [7,8]. Besides the new multi-component coatings, great efforts have been made to form multilayer structure with periods in nanometer range and these represent a new class of engineering materials [9–11]. The multilayer coatings, made of alternation of two individual layers with different crystalline structures or, alternatively, crystalline or amorphous combinations, appear as promising coatings and have recently attracted increasing interests, especially for nc-MenN/ a-Si3N4 (Me = W, V, (TiAl), …) coatings. The origin of superhardness in nc-MenN/a-Si3N4 nanocomposites has been reported [12,13]. In recent years, the developments of TiAlSiN/Si3N4 multilayer coatings have shown improved mechanical properties and oxidation resistance as compared to the single layer coatings [14,15]. The TiAlSiN/ Si3N4 coatings with improved mechanical properties been synthesized by different techniques such as arc ion plating, magnetron sputtering and enhanced chemical vapor deposition [16–18]. The properties of TiAlSiN/Si3N4 coatings were found strongly dependent on the thickness of Si3N4 [19,20]. It is well known that Si3N4 show high temperature stability and low friction coefficient. TiAlSiN/Si3N4 multilayer coatings
⁎ Corresponding author. Tel./fax: +86 592 2180155. E-mail address: [email protected] (D.L. Peng). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.12.035
are expected to combine the properties of constituent materials and improve the comprehensive properties compared to the individual single layer film. In this study, the TiAlSiN/Si3N4 coatings were deposited with different monolayer thicknesses, and their microstructure and mechanical properties were investigated. 2. Experimental The TiAlSiN/Si3N4 coatings were deposited on glass sheets, singlecrystal Si wafers and WC–Co substrates with dimensions of 12.7 × 12.7 mm2 by magnetron co-sputtering. All the substrates were cleaned by an ultrasonic agitator with acetone and alcohol for 15 min respectively, and then dried with argon. A high purity Si (99.99%) target with a diameter of 3 in. and 4 mm in thickness was connected to RF source sputter gun, which was installed with an angle of 30° to the horizontal plane. The Ti50Al50 (99.99%) target with a diameter of 5 in. and 4 mm in thickness was connected to DC source sputter gun, which was installed perpendicular to the horizontal plane. The substrates were placed on a rotating holder, with a rotational speed of 25 rpm during deposition, which paralleled to the Ti50Al50 target surface with a vertical distance of 9 cm to the target. The chamber was evacuated to a base pressure lower than 9.0× 10−4 Pa. Before deposition, pre-sputtering was performed for 20 min to clean the target surface. Then, the TiAlSiN/Si3N4 coatings were fabricated in a flowing N2 + Ar gas mixture with N2/(N2 + Ar) proportion keeping at 25% and the working pressure was kept at 0.6 Pa. All the depositions were conducted using a power of 100 W for the Ti50Al50 alloy target and Si target. The substrate was heated to 300 °C by a graphite heater furnace and was kept at 300 °C by a temperature control meter (SR93SHIMADEN) during deposition. The thickness of the monolayer was controlled by the open time of baffle and calculated by multiplying the
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Fig. 4. XRD patterns of TiAlSiN/Si3N4 coatings prepared with different TiAlSiN thicknesses. Fig. 1. Cross-sectional HAADF image of TiAlSiN/Si3N4 coatings with calculated TiAlSiN thickness of 5.8 nm and Si3N4 thickness of 1.2 nm.
Fig. 2. HRTEM image of TiAlSiN/Si3N4 coatings with calculated TiAlSiN thickness of 5.8 nm and Si3N4 thickness of 1.2 nm.
Fig. 3. XRD patterns of TiAlSiN/Si3N4 coatings prepared with different Si3N4 thicknesses.
deposition rate by the sputtering time. To determine the deposition rate, monolayers TiAlSiN and Si3N4 were firstly deposited for 1 h respectively and then the thickness of the monolayers TiAlSiN and Si3N4 was measured by a surface profiler (Dektak-III), and the deposition rate thus can be calculated. As a result, monolayer thickness in multilayer coatings can be calculated by multiplying the deposition rate by the sputtering time. Unless being pointed out, the thickness values mentioned in the following text are all calculated values. X-ray diffraction (XRD) with Cu Kα radiation (40 kV, 30 mA) was used to analyze the crystal structure and phases of the coatings. For morphological observation, a scanning electron microscope operating at 20 kV accelerating voltages was used. Transmission electron microscopy (TEM) and scanning TEM (STEM) were performed using a Tecnai F30 microscope operating at an accelerating voltage of 200 kV for the in-depth characterizations on the film structure. The hardness measurements of the coatings were performed by using a nanoindenter (CSM instruments) that employed a Berkovich diamond indenter. Each sample was made of three indentations and the value of the hardness reported here is the average of the three measured values and the maximum indentation depth was restricted to onetenth thickness of the coatings to minimize the effect of substrate on the hardness measurement. Adhesion was measured by the microscratch test using a Rockwell diamond stylus. The loading speed was 138 N/min and the maximum load was 70 N. The critical load was evaluated by spallation of the coating and determined by optical
Fig. 5. XRD patterns of TiAlSiN/Si3N4 coatings prepared with different total thicknesses.
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microscopy after each scratch test. Three scratches were performed on each coating and an average value was calculated. Sliding wear tests against Al2O3 ball of 6 mm diameter were undertaken in ambient
air at room temperature on a ball-on-disc high-temperature tribometer (CSM Instruments SA, Switzerland). The test was carried under the following conditions: a normal load of 1 N, wear track of
Fig. 6. Surface morphological and cross-sectional SEM images of TiAlSiN/Si3N4 coatings deposited with various Si3N4 thicknesses: (a) 0 nm, (b) 0.33 nm, (c) 0.50 nm, and (d) 1.2 nm.
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4 mm diameter, rotation speed of 0.05 m/s and sliding cycle number of 1000.
3. Results and discussion 3.1. Structural characterization Fig. 1 is the cross-sectional high-angle annular dark-field (HAADF) image of TiAlSiN/Si3N4 coatings obtained in STEM mode. The HAADF technique collects electrons scattered at high angles to form the image. The intensity in the HAADF image roughly depends on Z2 (Z represents atomic number), so elements with a larger Z will be bright in the image. From this image, a multilayered structure is evident, i.e. the brighter area corresponds to the TiAlSiN layer (has a higher average Z) while the darker layer is Si3N4. Such a layered structured was also confirmed by energy dispersive X-ray (EDX) analyses. The thicknesses of TiAlSiN and Si3N4 layers are measured to be 1.2 nm and 5.8 nm, respectively. Each alternating thin layer is about 7 nm, which is consistent with the thickness calculated by deposition rate and the sputtering time. Fig. 2 is the bright field high-resolution TEM (HRTEM) image of TiAlSiN/Si3N4 coatings. Nanocrystals with a dimension of a couple of nanometers can be detected in the TiAlSiN/Si3N4 coatings. On the other hand nanocrystals will develop in the TiAlSiN/Si3N4 multilayer coating when the Si3N4 thickness is 1.2 nm. Fig. 3 shows the typical XRD patterns of the as-deposited multilayer coatings with different Si3N4 layer thicknesses and a fixed thickness (5.8 nm) of the TiAlSiN layers. The coatings exhibit a cubic structure of TiN, single fcc phase, with preferred (111) orientation. The (111) reflection of multilayer coating with Si3N4 layer thickness of 0.12 nm shows a sharp increase compared to the monolayer TiAlSiN coating. This may result from the very thin thickness of the separate Si3N4 layer and the preparation process of the multilayer film. The separate layer thickness of 0.12 nm is too thin to interrupt the TiAlSiN layer growing. However, for the case of thicker separate Si3N4 layer (N0.33 nm), the growth of the TiAlSiN layer was interrupted, and thus, this led to a TiAlSiN grain refinement in multilayer coatings and the intensities of (111) reflections decreased gradually. From the TEM results, it can be found that the coating was constituted with many nanocrystals when the Si3N4 thickness is 1.2 nm. The presence of nanocrystals leads to broadened or even amorphous-like XRD patterns. Compared to the monolayer TiAlSiN coating, cubic-(111) peak of TiAlSiN/Si3N4 broadened as the Si3N4 layer thickness increased, which could come from TiAlSiN grain refinement in the coatings. On the other hand, the growth of TiAlSiN grains in the multilayer structure should be suppressed and smaller grains would develop, which resulted in a broadening of peaks. In addition, with the Si3N4 layer thickness increasing, the diffraction peak has a small shift towards lower angles. This shift can be attributed to residual stresses in the coatings [21]. And in the process of alternate growth of TiAlSiN and Si3N4, the residual stresses result in the increase in the interplanar spacing of TiAlN (111). The as-deposited coatings with different thicknesses of TiAlSiN layer but fixed thickness (0.33 nm) of Si3N4 layer are shown in Fig. 4. The coatings also formed a cubic structure of TiN with preferred (111) orientation. With the TiAlSiN layer thickness increasing, the intensity of (111) peak of TiAlSiN/Si3N4 multilayer coatings shows an increasing tendency, indicating a larger grain size or a better crystlallinity with a large TiAlSiN thickness. This trend could also be related to a decrease in the amorphous phase (Si3N4 layers) in the coatings. The XRD results of the TiAlSiN/Si3N4 coatings with different total thicknesses but fixed thicknesses of TiAlSiN (5.8 nm) and Si3N4 (0.33 nm) layers are shown in Fig. 5. All the deposited coatings except the coating with thickness of 200 nm indicated a cubic single phase with preferred (111) orientation, and the intensity increased rapidly with the increasing coating thickness.
Fig. 7. XPS spectra of Si2p taken from the outermost surface: (a) as-deposited TiAlSiN, (b) as-deposited TiAlSiN/Si3N4 with TiAlSiN thicknesses of 5.8 nm and Si3N4 thicknesses of 0.33 nm.
3.2. Surface topography characterization Fig. 6 shows the surface morphological (left) and cross-sectional (right) SEM images of the TiAlSiN/Si3N4 coatings prepared at different thicknesses of Si3N4 layer. It can be seen from Fig. 6, the TiAlSiN monolayer shows significantly columnar grains and the direction of the grain growth is perpendicular to the membrane surface. Owing to the introduction of Si3N4 into the coating system, the fracture morphology of TiAlSiN is changed from a columnar microstructure to a fine-grained structure. During the deposition, the growth of TiAlSiN crystals was blocked periodically by the covering Si3N4 layer, and thus the grain growth could be suppressed. When the thickness of the Si3N4 layer is thin, the introduced Si3N4 layer does not block the growth of TiAlN columnar grain, and thus the structure of columnar grain does not change significantly. However, when the thickness of Si3N4 layer is more than 1.2 nm, the Si3N4 would completely block the growth of columnar grain and the nanocrystals would develop. The result is consistent with that of TEM and XRD. From these SEM images, we can see that the grain size of TiAlSiN/Si3N4 coatings is smaller than that of TiAlSiN monolayer. The small grain size indicates that the grains have been refined. In addition, the surface of the coating was dense, which could be caused by the introduction of thin Si3N4 layer. The crystal growth of
Fig. 8. XRD patterns of TiAlSiN/Si3N4 multilayer before and after oxidation at 800 °C for 20 h.
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TiAlSiN was blocked by the Si3N4 layer, so the grain size of TiAlSiN decreased. The change of the microstructure from a columnar to a refinement structure would have an effect on the mechanical properties. Thus, the TiAlSiN/Si3N4 multilayer is expected to display better properties than both constituent materials. Fig. 7 shows the XPS spectra of (a) TiAlSiN monolayer coating and (b) TiAlSiN/Si3N4 multilayer coatings with TiAlSiN thicknesses of 5.8 nm and Si3N4 thicknesses of 0.33 nm. It can be seen that similar XPS spectra were obtained about the binding energy with TiAlSiN/ Si3N4 multilayer coatings and TiAlSiN monolayer coating. The Si2p peak was centered at 101.6 eV, indicating that the Si was in the form of Si3N4 [22].
3.3. Oxidation resistance Fig. 8 shows the XRD of TiAlSiN/Si3N4 multilayer coating before and after oxidation at 800 °C for 20 hours. The thickness of TiAlSiN and Si3N4 is 5.8 nm and 0.33 nm, respectively. Little difference can be found for the multilayer coating before and after oxidation at 800 °C. Oxide phases were not clearly detected by XRD since the oxides might be in a nanocrystalline state. The chemical bonding of the TiAlSiN/Si3N4 multilayer coating after oxidation at 900 °C for 20 h was investigated by XPS. The thickness of TiAlSiN and Si3N4 is 5.8 nm and 0.33 nm, respectively. For the sample oxidized at 900 °C, Ar+ ion was adopted for sputter etching 100 nm to
Fig. 9. XPS spectra of Ti2p, Al2p, Si2p, N1s and O1s on the TiAlSiN/Si3N4 coatings oxidized at 900 °C for 20 h before (a) and after (b) sputtering in XPS measurements.
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remove the top layers. Fig. 9 shows the typical XPS spectra of Ti2p, Si2p, Al2p and N1s of the TiAlSiN/Si3N4 multilayer coatings before and after sputter etching. Based on this figure, the binding energies of the Al2p shift from 74.2 eV to 73.1 eV before and after sputter etching. The peak at 74.2 eV can be substantially attributed to the formation of Al2O3, and the peak locating at 73.1 eV might be due to the presence of Al in the form of amorphous Si3N4 layers in the coatings [23]. With respect to Ti2p, the spectrum shows two main peaks centered at 457.9 eV and 463.6 eV before sputter etching. After sputter etching,
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Ti2p also shows main peaks centered at 457.9 and 463.6 eV, and a weak peak at 454.8 eV, which can be reasonably attributed to the oxide phases and TiN, respectively [24]. In addition, no peak corresponding to Ti–N bond (454.8 eV) was observed before etching, indicating that the surface of the coating was completely oxidized. The Si2p shows a broad and quite weak peak after sputter etching. However, this weak peak was not detected before sputter etching. The reason for this absence of the Si spectrum may be attributed to the fact that the silicon ions are rarely mobile [23], as a result the Ti and Al ions
Fig. 10. The chemical bonding of the TiAlSiN/Si3N4 coating with TiAlSiN thicknesses of 5.8 nm and Si3N4 thicknesses of 0.33 nm after oxidation from 700 °C to 900 °C for 20 h.
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flowed more freely and diffused outward to cover it. Similar to Si2p, a weak peak occurred in the N1s region after sputter etching, and this peak was not detected before sputter etching, owing to the escape of nitrogen during oxidation. Fig. 10 shows the chemical bonding of the TiAlSiN/Si3N4 coating after oxidation from 700 °C to 900 °C for 20 h. The thickness of TiAlSiN and Si3N4 is 5.8 nm and 0.33 nm, respectively. The intensity of the spectrum derived from the oxidized surface of the multilayer coatings after oxidation from 700 °C to 900 °C for 20 h increased in the following order: Si2p b Al2 b Ti2p b O1s. This indicates that the oxides of Ti and Al were primary components of the outermost surface layer. The growth of SiO2 is dominated by the inward diffusion of oxygen, because the silicon in silicon oxides is relatively immobile due to the higher bonding energy of Si4+–O2− (465 kJ · mol− 1)[25–27]. In the case of TiAlSiN/Si3N4 multilayer coatings, Ti and Al ions flowed more freely and diffused outward. There was little difference of intensity for O1s after oxidation at 700 °C and 800 °C for 20 h though a slight increase was observed, and both Ti2p and Al2p show the same trend. In contrary, the intensities of O1s, Ti2p and Al2p significantly increased after oxidizing at 900 °C for 20 h, indicating that the oxidation degree increases dramatically as the temperature increases. For the N1s and Si2p peaks, with the oxidation temperature increasing, the intensity decreased gradually, especially from 800 °C to 900 °C. This is due to the escape of nitrogen ions during oxidation at high temperature. Therefore, the oxidation resistance of TiAlSiN/Si3N4 multilayer coatings can be attributed to the comparatively high activation energies of TiAlN and Si3N4 and the different values of Gibbs free energy for the oxide formation (ΔG = 378.2, 212.6 and 204.75 kcal/mol for Al2O3, TiO2 and SiO2, respectively) [28]. In addition, the amorphous Si3N4 phase boundary around the TiAlSiN crystallites suppresses diffusion of oxygen and therefore reduces the oxidation rate of the TiAlSiN/ Si3N4 nanocomposite coatings. The TiAlSiN/Si3N4 coating exhibited good oxidation resistance. However, during the process of oxidation, the decrease of nitrogen from the coating prevented the formation of oxides with good oxidation resistance, which would undermine its oxidation resistance. 3.4. Mechanical characterization Fig. 11 shows the mechanical properties of the synthesized multilayer coatings. Hardness of the TiAlSiN/Si3N4 multilayer coatings with different Si3N4 thicknesses, detected by nanoindenter, is presented in Fig. 11 (a), and that of different TiAlSiN thicknesses is illustrated in Fig. 11 (b). Fig. 11 (a) exhibited a maximum hardness of 36.3 GPa for the TiAlSiN/Si3N4 multilayer coatings with the Si3N4 thickness of 0.33 nm, while the hardness of monolayer TiAlSiN coating is 20.8 GPa. The hardness of the multilayer coatings decreased significantly with further increase in the Si3N4 thickness. When the Si3N4 thickness is 1.2 nm, the hardness of the coatings decreased to about 17.2 GPa, lower than that of monolayer TiAlSiN coating. The hardness of TiAlSiN/Si3N4 multilayer coatings with different TiAlSiN thicknesses displayed similar thickness dependence tendency to that of variation in Si3N4 thickness. The multilayer coatings exhibited a maximum hardness when the thickness of TiAlSiN was about 5.8 nm. However, with the TiAlSiN thickness further increasing, the hardness decreased to 24.2 GPa, still larger than that of monolayer TiAlSiN coating. The hardness enhancement can be attributed to the Koehler effect, which is based on the hindering of the dislocation movement [29]. When the two single layers with small thickness in the multilayer have different shear moduli, the dislocation blocking would occur. Different dislocation line energies required an additional stress to move the dislocation from the single layer with lower shear modulus into the layer with higher shear modulus [30]. The decrease of the hardness with Si3N4 thickness increasing can be attributed to the critical thickness of Si3N4 which becomes unstable and forms defects in the coatings with thicker thickness [31]. For thicker Si3N4 layers, defect formation and structural transformation would occur,
Fig. 11. Hardness dependence on monolayer thickness of (a) Si3N4 and (b) TiAlSiN.
which result in a decrease of hardness. For the thicker layer of TiAlSiN, movement or multiplication of dislocation will appear in the TiAlSiN layer firstly, so the binding effect of dislocation in the multilayer would become weak, resulting in a reduction in hardness. Scratch tests with the loads increasing continuously were conducted to investigate the adhesion of the multilayer coatings and the results are summarized in Fig. 12. TiAlSiN/Si3N4 multilayer coatings both with thickness variation of monolayer Si3N4 and TiAlSiN
Fig. 12. Critical load dependence on monolayer thickness of (a) Si3N4 and (b) TiAlSiN.
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good oxidation resistance. The mechanical properties of TiAlSiN/ Si3N4 are directly bound up with the Si3N4 monolayer thickness and the TiAlSiN monolayer thickness also has an effect on these TiAlSiN/ Si3N4 coatings. The coatings show the maximum hardness of 36.3 GPa when the thickness of Si3N4 and TiAlSiN layer is 0.33 nm and 5.8 nm, respectively.
Acknowledgements This work was partially supported by the National Key Technology R&D Program of China (2007BAE05B04), and by the National Natural Science Foundation of China (No. 50825101 and No. 50971108).
References
Fig. 13. Friction coefficient of TiAlSiN multilayer coatings with various S3N4 thicknesses against an Al2O3 ball.
show high critical loads larger than 40 N, indicating good adhesion between the coatings and substrates. For the multilayer coatings with different Si3N4 monolayer thicknesses, the largest critical load is 50.5 N when the thickness of monolayer Si3N4 is 0.33 nm. For the multilayer coatings with different TiAlSiN monolayer thicknesses, the load increases gradually with the thickness increase of TiAlSiN monolayer. A higher critical load for the TiAlSiN/Si3N4 multilayer coatings can be connected with the breaking of the columnar growth of the Si3N4 layer and the limitation of the crack propagation only to the single layer [32]. The relatively soft Si3N4 layers absorb more energy than the harder TiAlSiN, which leads to a crack formation at deeper indentation. In addition, critical load is closely related to the residual stress in the coatings, and reduces as the residual stress increases [33]. The friction property of the coatings was investigated by a ball-on-disk test at room temperature and the Al2O3 ball was used as the counterpart. The result is shown in Fig. 13. The TiAlSiN/ Si3N4 multilayer coatings show the highest friction coefficient of about 0.93. With the Si3N4 monolayer thickness increasing, the friction coefficient decreases, ranging from 0.93 to 0.68. This can be attributed to the formation of self-lubricant tribo-layers, which would be more activated with increasing silicon content. 4. Conclusion In this study, monolayer TiAlSiN and multilayer TiAlSiN/Si3N4 coatings were synthesized by a reactive magnetron co-sputtering system. Both TiAlSiN and TiAlSiN/Si3N4 with a small Si3N4 monolayer thickness exhibit B1-NaCl structure with preferred (111) orientation. The Si3N4 monolayer with thicker thickness would block the columnar grain growth of TiAlSiN to form nanocrystals in the multilayer coatings. TiAlSiN/Si3N4 multilayer coatings displayed
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Structure and mechanical properties of TiAlSiN/Si3N4 multilayer coatings K. Zhang a, L.S. Wang a, G.H. Yue a, Y.Z Chen a, D.L. Peng a,⁎, Z.B. Qi b, Z.C. Wang b a b
Department of Materials Science and Engineering, College of Materials, Xiamen University, Xiamen 361005, China College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
a r t i c l e
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Article history: Received 14 July 2010 Accepted in revised form 20 December 2010 Available online 29 December 2010 Keywords: Reactive magnetron co-sputtering TiAlSiN/Si3N4 multilayer Mechanical properties
a b s t r a c t TiAlSiN/Si3N4 multilayer coatings which have different separate layer thicknesses of TiAlSiN or Si3N4 were deposited onto glass sheets, single-crystal silicon wafers and polished WC–Co substrates by reactive magnetron co-sputtering. The morphology, crystalline structure and thickness of the as-prepared multilayer coatings were characterized by TEM, SEM, XRD and film thickness measuring instrument. The mechanical properties of the coatings were evaluated by a nanoindenter. The effects of monolayer thickness on the microstructure and properties of TiAlSiN/Si3N4 multilayer coatings were explored. The coatings showed the highest hardness when the thickness of Si3N4 and TiAlSiN monolayers was 0.33 nm and 5.8 nm, respectively. The oxidation characteristics of the coatings were studied at temperatures ranging from 700 °C to 900 °C for oxidation time up to 20 h in air. It was found that the coatings displayed good oxidation resistance. © 2010 Elsevier B.V. All rights reserved.
1. Introduction TiN hard coatings deposited by Physical Vapour Deposition (PVD) have been paid great attention to their technological applications [1]. In the last two decades, multi-element systems have received more attention to further improving the performance [2]. The microstructure, composition, mechanical properties, wear and cutting performance of quaternary TiAlSiN have been studied recently [3–6]. It was reported that the microstructure and mechanical properties of TiAlSiN depended on the Si contents [7,8]. Besides the new multi-component coatings, great efforts have been made to form multilayer structure with periods in nanometer range and these represent a new class of engineering materials [9–11]. The multilayer coatings, made of alternation of two individual layers with different crystalline structures or, alternatively, crystalline or amorphous combinations, appear as promising coatings and have recently attracted increasing interests, especially for nc-MenN/ a-Si3N4 (Me = W, V, (TiAl), …) coatings. The origin of superhardness in nc-MenN/a-Si3N4 nanocomposites has been reported [12,13]. In recent years, the developments of TiAlSiN/Si3N4 multilayer coatings have shown improved mechanical properties and oxidation resistance as compared to the single layer coatings [14,15]. The TiAlSiN/ Si3N4 coatings with improved mechanical properties been synthesized by different techniques such as arc ion plating, magnetron sputtering and enhanced chemical vapor deposition [16–18]. The properties of TiAlSiN/Si3N4 coatings were found strongly dependent on the thickness of Si3N4 [19,20]. It is well known that Si3N4 show high temperature stability and low friction coefficient. TiAlSiN/Si3N4 multilayer coatings
⁎ Corresponding author. Tel./fax: +86 592 2180155. E-mail address: [email protected] (D.L. Peng). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.12.035
are expected to combine the properties of constituent materials and improve the comprehensive properties compared to the individual single layer film. In this study, the TiAlSiN/Si3N4 coatings were deposited with different monolayer thicknesses, and their microstructure and mechanical properties were investigated. 2. Experimental The TiAlSiN/Si3N4 coatings were deposited on glass sheets, singlecrystal Si wafers and WC–Co substrates with dimensions of 12.7 × 12.7 mm2 by magnetron co-sputtering. All the substrates were cleaned by an ultrasonic agitator with acetone and alcohol for 15 min respectively, and then dried with argon. A high purity Si (99.99%) target with a diameter of 3 in. and 4 mm in thickness was connected to RF source sputter gun, which was installed with an angle of 30° to the horizontal plane. The Ti50Al50 (99.99%) target with a diameter of 5 in. and 4 mm in thickness was connected to DC source sputter gun, which was installed perpendicular to the horizontal plane. The substrates were placed on a rotating holder, with a rotational speed of 25 rpm during deposition, which paralleled to the Ti50Al50 target surface with a vertical distance of 9 cm to the target. The chamber was evacuated to a base pressure lower than 9.0× 10−4 Pa. Before deposition, pre-sputtering was performed for 20 min to clean the target surface. Then, the TiAlSiN/Si3N4 coatings were fabricated in a flowing N2 + Ar gas mixture with N2/(N2 + Ar) proportion keeping at 25% and the working pressure was kept at 0.6 Pa. All the depositions were conducted using a power of 100 W for the Ti50Al50 alloy target and Si target. The substrate was heated to 300 °C by a graphite heater furnace and was kept at 300 °C by a temperature control meter (SR93SHIMADEN) during deposition. The thickness of the monolayer was controlled by the open time of baffle and calculated by multiplying the
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Fig. 4. XRD patterns of TiAlSiN/Si3N4 coatings prepared with different TiAlSiN thicknesses. Fig. 1. Cross-sectional HAADF image of TiAlSiN/Si3N4 coatings with calculated TiAlSiN thickness of 5.8 nm and Si3N4 thickness of 1.2 nm.
Fig. 2. HRTEM image of TiAlSiN/Si3N4 coatings with calculated TiAlSiN thickness of 5.8 nm and Si3N4 thickness of 1.2 nm.
Fig. 3. XRD patterns of TiAlSiN/Si3N4 coatings prepared with different Si3N4 thicknesses.
deposition rate by the sputtering time. To determine the deposition rate, monolayers TiAlSiN and Si3N4 were firstly deposited for 1 h respectively and then the thickness of the monolayers TiAlSiN and Si3N4 was measured by a surface profiler (Dektak-III), and the deposition rate thus can be calculated. As a result, monolayer thickness in multilayer coatings can be calculated by multiplying the deposition rate by the sputtering time. Unless being pointed out, the thickness values mentioned in the following text are all calculated values. X-ray diffraction (XRD) with Cu Kα radiation (40 kV, 30 mA) was used to analyze the crystal structure and phases of the coatings. For morphological observation, a scanning electron microscope operating at 20 kV accelerating voltages was used. Transmission electron microscopy (TEM) and scanning TEM (STEM) were performed using a Tecnai F30 microscope operating at an accelerating voltage of 200 kV for the in-depth characterizations on the film structure. The hardness measurements of the coatings were performed by using a nanoindenter (CSM instruments) that employed a Berkovich diamond indenter. Each sample was made of three indentations and the value of the hardness reported here is the average of the three measured values and the maximum indentation depth was restricted to onetenth thickness of the coatings to minimize the effect of substrate on the hardness measurement. Adhesion was measured by the microscratch test using a Rockwell diamond stylus. The loading speed was 138 N/min and the maximum load was 70 N. The critical load was evaluated by spallation of the coating and determined by optical
Fig. 5. XRD patterns of TiAlSiN/Si3N4 coatings prepared with different total thicknesses.
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microscopy after each scratch test. Three scratches were performed on each coating and an average value was calculated. Sliding wear tests against Al2O3 ball of 6 mm diameter were undertaken in ambient
air at room temperature on a ball-on-disc high-temperature tribometer (CSM Instruments SA, Switzerland). The test was carried under the following conditions: a normal load of 1 N, wear track of
Fig. 6. Surface morphological and cross-sectional SEM images of TiAlSiN/Si3N4 coatings deposited with various Si3N4 thicknesses: (a) 0 nm, (b) 0.33 nm, (c) 0.50 nm, and (d) 1.2 nm.
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4 mm diameter, rotation speed of 0.05 m/s and sliding cycle number of 1000.
3. Results and discussion 3.1. Structural characterization Fig. 1 is the cross-sectional high-angle annular dark-field (HAADF) image of TiAlSiN/Si3N4 coatings obtained in STEM mode. The HAADF technique collects electrons scattered at high angles to form the image. The intensity in the HAADF image roughly depends on Z2 (Z represents atomic number), so elements with a larger Z will be bright in the image. From this image, a multilayered structure is evident, i.e. the brighter area corresponds to the TiAlSiN layer (has a higher average Z) while the darker layer is Si3N4. Such a layered structured was also confirmed by energy dispersive X-ray (EDX) analyses. The thicknesses of TiAlSiN and Si3N4 layers are measured to be 1.2 nm and 5.8 nm, respectively. Each alternating thin layer is about 7 nm, which is consistent with the thickness calculated by deposition rate and the sputtering time. Fig. 2 is the bright field high-resolution TEM (HRTEM) image of TiAlSiN/Si3N4 coatings. Nanocrystals with a dimension of a couple of nanometers can be detected in the TiAlSiN/Si3N4 coatings. On the other hand nanocrystals will develop in the TiAlSiN/Si3N4 multilayer coating when the Si3N4 thickness is 1.2 nm. Fig. 3 shows the typical XRD patterns of the as-deposited multilayer coatings with different Si3N4 layer thicknesses and a fixed thickness (5.8 nm) of the TiAlSiN layers. The coatings exhibit a cubic structure of TiN, single fcc phase, with preferred (111) orientation. The (111) reflection of multilayer coating with Si3N4 layer thickness of 0.12 nm shows a sharp increase compared to the monolayer TiAlSiN coating. This may result from the very thin thickness of the separate Si3N4 layer and the preparation process of the multilayer film. The separate layer thickness of 0.12 nm is too thin to interrupt the TiAlSiN layer growing. However, for the case of thicker separate Si3N4 layer (N0.33 nm), the growth of the TiAlSiN layer was interrupted, and thus, this led to a TiAlSiN grain refinement in multilayer coatings and the intensities of (111) reflections decreased gradually. From the TEM results, it can be found that the coating was constituted with many nanocrystals when the Si3N4 thickness is 1.2 nm. The presence of nanocrystals leads to broadened or even amorphous-like XRD patterns. Compared to the monolayer TiAlSiN coating, cubic-(111) peak of TiAlSiN/Si3N4 broadened as the Si3N4 layer thickness increased, which could come from TiAlSiN grain refinement in the coatings. On the other hand, the growth of TiAlSiN grains in the multilayer structure should be suppressed and smaller grains would develop, which resulted in a broadening of peaks. In addition, with the Si3N4 layer thickness increasing, the diffraction peak has a small shift towards lower angles. This shift can be attributed to residual stresses in the coatings [21]. And in the process of alternate growth of TiAlSiN and Si3N4, the residual stresses result in the increase in the interplanar spacing of TiAlN (111). The as-deposited coatings with different thicknesses of TiAlSiN layer but fixed thickness (0.33 nm) of Si3N4 layer are shown in Fig. 4. The coatings also formed a cubic structure of TiN with preferred (111) orientation. With the TiAlSiN layer thickness increasing, the intensity of (111) peak of TiAlSiN/Si3N4 multilayer coatings shows an increasing tendency, indicating a larger grain size or a better crystlallinity with a large TiAlSiN thickness. This trend could also be related to a decrease in the amorphous phase (Si3N4 layers) in the coatings. The XRD results of the TiAlSiN/Si3N4 coatings with different total thicknesses but fixed thicknesses of TiAlSiN (5.8 nm) and Si3N4 (0.33 nm) layers are shown in Fig. 5. All the deposited coatings except the coating with thickness of 200 nm indicated a cubic single phase with preferred (111) orientation, and the intensity increased rapidly with the increasing coating thickness.
Fig. 7. XPS spectra of Si2p taken from the outermost surface: (a) as-deposited TiAlSiN, (b) as-deposited TiAlSiN/Si3N4 with TiAlSiN thicknesses of 5.8 nm and Si3N4 thicknesses of 0.33 nm.
3.2. Surface topography characterization Fig. 6 shows the surface morphological (left) and cross-sectional (right) SEM images of the TiAlSiN/Si3N4 coatings prepared at different thicknesses of Si3N4 layer. It can be seen from Fig. 6, the TiAlSiN monolayer shows significantly columnar grains and the direction of the grain growth is perpendicular to the membrane surface. Owing to the introduction of Si3N4 into the coating system, the fracture morphology of TiAlSiN is changed from a columnar microstructure to a fine-grained structure. During the deposition, the growth of TiAlSiN crystals was blocked periodically by the covering Si3N4 layer, and thus the grain growth could be suppressed. When the thickness of the Si3N4 layer is thin, the introduced Si3N4 layer does not block the growth of TiAlN columnar grain, and thus the structure of columnar grain does not change significantly. However, when the thickness of Si3N4 layer is more than 1.2 nm, the Si3N4 would completely block the growth of columnar grain and the nanocrystals would develop. The result is consistent with that of TEM and XRD. From these SEM images, we can see that the grain size of TiAlSiN/Si3N4 coatings is smaller than that of TiAlSiN monolayer. The small grain size indicates that the grains have been refined. In addition, the surface of the coating was dense, which could be caused by the introduction of thin Si3N4 layer. The crystal growth of
Fig. 8. XRD patterns of TiAlSiN/Si3N4 multilayer before and after oxidation at 800 °C for 20 h.
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TiAlSiN was blocked by the Si3N4 layer, so the grain size of TiAlSiN decreased. The change of the microstructure from a columnar to a refinement structure would have an effect on the mechanical properties. Thus, the TiAlSiN/Si3N4 multilayer is expected to display better properties than both constituent materials. Fig. 7 shows the XPS spectra of (a) TiAlSiN monolayer coating and (b) TiAlSiN/Si3N4 multilayer coatings with TiAlSiN thicknesses of 5.8 nm and Si3N4 thicknesses of 0.33 nm. It can be seen that similar XPS spectra were obtained about the binding energy with TiAlSiN/ Si3N4 multilayer coatings and TiAlSiN monolayer coating. The Si2p peak was centered at 101.6 eV, indicating that the Si was in the form of Si3N4 [22].
3.3. Oxidation resistance Fig. 8 shows the XRD of TiAlSiN/Si3N4 multilayer coating before and after oxidation at 800 °C for 20 hours. The thickness of TiAlSiN and Si3N4 is 5.8 nm and 0.33 nm, respectively. Little difference can be found for the multilayer coating before and after oxidation at 800 °C. Oxide phases were not clearly detected by XRD since the oxides might be in a nanocrystalline state. The chemical bonding of the TiAlSiN/Si3N4 multilayer coating after oxidation at 900 °C for 20 h was investigated by XPS. The thickness of TiAlSiN and Si3N4 is 5.8 nm and 0.33 nm, respectively. For the sample oxidized at 900 °C, Ar+ ion was adopted for sputter etching 100 nm to
Fig. 9. XPS spectra of Ti2p, Al2p, Si2p, N1s and O1s on the TiAlSiN/Si3N4 coatings oxidized at 900 °C for 20 h before (a) and after (b) sputtering in XPS measurements.
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remove the top layers. Fig. 9 shows the typical XPS spectra of Ti2p, Si2p, Al2p and N1s of the TiAlSiN/Si3N4 multilayer coatings before and after sputter etching. Based on this figure, the binding energies of the Al2p shift from 74.2 eV to 73.1 eV before and after sputter etching. The peak at 74.2 eV can be substantially attributed to the formation of Al2O3, and the peak locating at 73.1 eV might be due to the presence of Al in the form of amorphous Si3N4 layers in the coatings [23]. With respect to Ti2p, the spectrum shows two main peaks centered at 457.9 eV and 463.6 eV before sputter etching. After sputter etching,
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Ti2p also shows main peaks centered at 457.9 and 463.6 eV, and a weak peak at 454.8 eV, which can be reasonably attributed to the oxide phases and TiN, respectively [24]. In addition, no peak corresponding to Ti–N bond (454.8 eV) was observed before etching, indicating that the surface of the coating was completely oxidized. The Si2p shows a broad and quite weak peak after sputter etching. However, this weak peak was not detected before sputter etching. The reason for this absence of the Si spectrum may be attributed to the fact that the silicon ions are rarely mobile [23], as a result the Ti and Al ions
Fig. 10. The chemical bonding of the TiAlSiN/Si3N4 coating with TiAlSiN thicknesses of 5.8 nm and Si3N4 thicknesses of 0.33 nm after oxidation from 700 °C to 900 °C for 20 h.
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flowed more freely and diffused outward to cover it. Similar to Si2p, a weak peak occurred in the N1s region after sputter etching, and this peak was not detected before sputter etching, owing to the escape of nitrogen during oxidation. Fig. 10 shows the chemical bonding of the TiAlSiN/Si3N4 coating after oxidation from 700 °C to 900 °C for 20 h. The thickness of TiAlSiN and Si3N4 is 5.8 nm and 0.33 nm, respectively. The intensity of the spectrum derived from the oxidized surface of the multilayer coatings after oxidation from 700 °C to 900 °C for 20 h increased in the following order: Si2p b Al2 b Ti2p b O1s. This indicates that the oxides of Ti and Al were primary components of the outermost surface layer. The growth of SiO2 is dominated by the inward diffusion of oxygen, because the silicon in silicon oxides is relatively immobile due to the higher bonding energy of Si4+–O2− (465 kJ · mol− 1)[25–27]. In the case of TiAlSiN/Si3N4 multilayer coatings, Ti and Al ions flowed more freely and diffused outward. There was little difference of intensity for O1s after oxidation at 700 °C and 800 °C for 20 h though a slight increase was observed, and both Ti2p and Al2p show the same trend. In contrary, the intensities of O1s, Ti2p and Al2p significantly increased after oxidizing at 900 °C for 20 h, indicating that the oxidation degree increases dramatically as the temperature increases. For the N1s and Si2p peaks, with the oxidation temperature increasing, the intensity decreased gradually, especially from 800 °C to 900 °C. This is due to the escape of nitrogen ions during oxidation at high temperature. Therefore, the oxidation resistance of TiAlSiN/Si3N4 multilayer coatings can be attributed to the comparatively high activation energies of TiAlN and Si3N4 and the different values of Gibbs free energy for the oxide formation (ΔG = 378.2, 212.6 and 204.75 kcal/mol for Al2O3, TiO2 and SiO2, respectively) [28]. In addition, the amorphous Si3N4 phase boundary around the TiAlSiN crystallites suppresses diffusion of oxygen and therefore reduces the oxidation rate of the TiAlSiN/ Si3N4 nanocomposite coatings. The TiAlSiN/Si3N4 coating exhibited good oxidation resistance. However, during the process of oxidation, the decrease of nitrogen from the coating prevented the formation of oxides with good oxidation resistance, which would undermine its oxidation resistance. 3.4. Mechanical characterization Fig. 11 shows the mechanical properties of the synthesized multilayer coatings. Hardness of the TiAlSiN/Si3N4 multilayer coatings with different Si3N4 thicknesses, detected by nanoindenter, is presented in Fig. 11 (a), and that of different TiAlSiN thicknesses is illustrated in Fig. 11 (b). Fig. 11 (a) exhibited a maximum hardness of 36.3 GPa for the TiAlSiN/Si3N4 multilayer coatings with the Si3N4 thickness of 0.33 nm, while the hardness of monolayer TiAlSiN coating is 20.8 GPa. The hardness of the multilayer coatings decreased significantly with further increase in the Si3N4 thickness. When the Si3N4 thickness is 1.2 nm, the hardness of the coatings decreased to about 17.2 GPa, lower than that of monolayer TiAlSiN coating. The hardness of TiAlSiN/Si3N4 multilayer coatings with different TiAlSiN thicknesses displayed similar thickness dependence tendency to that of variation in Si3N4 thickness. The multilayer coatings exhibited a maximum hardness when the thickness of TiAlSiN was about 5.8 nm. However, with the TiAlSiN thickness further increasing, the hardness decreased to 24.2 GPa, still larger than that of monolayer TiAlSiN coating. The hardness enhancement can be attributed to the Koehler effect, which is based on the hindering of the dislocation movement [29]. When the two single layers with small thickness in the multilayer have different shear moduli, the dislocation blocking would occur. Different dislocation line energies required an additional stress to move the dislocation from the single layer with lower shear modulus into the layer with higher shear modulus [30]. The decrease of the hardness with Si3N4 thickness increasing can be attributed to the critical thickness of Si3N4 which becomes unstable and forms defects in the coatings with thicker thickness [31]. For thicker Si3N4 layers, defect formation and structural transformation would occur,
Fig. 11. Hardness dependence on monolayer thickness of (a) Si3N4 and (b) TiAlSiN.
which result in a decrease of hardness. For the thicker layer of TiAlSiN, movement or multiplication of dislocation will appear in the TiAlSiN layer firstly, so the binding effect of dislocation in the multilayer would become weak, resulting in a reduction in hardness. Scratch tests with the loads increasing continuously were conducted to investigate the adhesion of the multilayer coatings and the results are summarized in Fig. 12. TiAlSiN/Si3N4 multilayer coatings both with thickness variation of monolayer Si3N4 and TiAlSiN
Fig. 12. Critical load dependence on monolayer thickness of (a) Si3N4 and (b) TiAlSiN.
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good oxidation resistance. The mechanical properties of TiAlSiN/ Si3N4 are directly bound up with the Si3N4 monolayer thickness and the TiAlSiN monolayer thickness also has an effect on these TiAlSiN/ Si3N4 coatings. The coatings show the maximum hardness of 36.3 GPa when the thickness of Si3N4 and TiAlSiN layer is 0.33 nm and 5.8 nm, respectively.
Acknowledgements This work was partially supported by the National Key Technology R&D Program of China (2007BAE05B04), and by the National Natural Science Foundation of China (No. 50825101 and No. 50971108).
References
Fig. 13. Friction coefficient of TiAlSiN multilayer coatings with various S3N4 thicknesses against an Al2O3 ball.
show high critical loads larger than 40 N, indicating good adhesion between the coatings and substrates. For the multilayer coatings with different Si3N4 monolayer thicknesses, the largest critical load is 50.5 N when the thickness of monolayer Si3N4 is 0.33 nm. For the multilayer coatings with different TiAlSiN monolayer thicknesses, the load increases gradually with the thickness increase of TiAlSiN monolayer. A higher critical load for the TiAlSiN/Si3N4 multilayer coatings can be connected with the breaking of the columnar growth of the Si3N4 layer and the limitation of the crack propagation only to the single layer [32]. The relatively soft Si3N4 layers absorb more energy than the harder TiAlSiN, which leads to a crack formation at deeper indentation. In addition, critical load is closely related to the residual stress in the coatings, and reduces as the residual stress increases [33]. The friction property of the coatings was investigated by a ball-on-disk test at room temperature and the Al2O3 ball was used as the counterpart. The result is shown in Fig. 13. The TiAlSiN/ Si3N4 multilayer coatings show the highest friction coefficient of about 0.93. With the Si3N4 monolayer thickness increasing, the friction coefficient decreases, ranging from 0.93 to 0.68. This can be attributed to the formation of self-lubricant tribo-layers, which would be more activated with increasing silicon content. 4. Conclusion In this study, monolayer TiAlSiN and multilayer TiAlSiN/Si3N4 coatings were synthesized by a reactive magnetron co-sputtering system. Both TiAlSiN and TiAlSiN/Si3N4 with a small Si3N4 monolayer thickness exhibit B1-NaCl structure with preferred (111) orientation. The Si3N4 monolayer with thicker thickness would block the columnar grain growth of TiAlSiN to form nanocrystals in the multilayer coatings. TiAlSiN/Si3N4 multilayer coatings displayed
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