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Surface morphology evolution and properties of silicon coating on silicon carbide ceramics by advanced plasma source ion plating
Surface & Coatings Technology 207 (2012) 204–210
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Surface morphology evolution and properties of silicon coating on silicon carbide ceramics by advanced plasma source ion plating G.L. Liu a,⁎, Z.R. Huang a, b, J.H. Wu c, X.J. Liu a a
Structural Ceramic Engineering Research Center, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China c Inorganic Coating Materials Research Center, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China b
a r t i c l e
i n f o
Article history: Received 16 March 2012 Accepted in revised form 20 June 2012 Available online 29 June 2012 Keywords: Silicon coating Plasma Surface morphology Residual stress Silicon carbide
a b s t r a c t Effects of substrate surface defects and deposition parameters on deposition of silicon coatings onto silicon carbide ceramics by plasma source ion plating for surface modification were studied in this paper. Substrate surface defects like holes and protuberances were not duplicated during the coating process. The main discontinuities on silicon coatings turned out to be notches which were probably generated by combined impact of excessive internal stress and persistent ion bombardment during the coating process. Continuous, homogenous and well-bonded 2 μm thick amorphous silicon coatings with smooth surface were obtained by improving plasma power, slowing down deposition rate and rising substrate temperature. The total reflectance of silicon coatings within 400–750 nm wavelength was also studied. The higher the residual compressive stress, the higher is the total reflectance. Rising substrate temperature would also significantly improve the total reflectance. The enhancement might be attributed to shortened Si\Si bonding under high compressive stress and higher compactness at higher substrate temperature, respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The use of surface coatings is an important alternative as a means of extending the performance of materials in a wide range of applications. Silicon carbide (SiC) has been recently applied in optical components for large space telescope [1–5] due to its high hardness/volume density ratio, high thermal conductivity and excellent wear-resistant ability. There is a demand for high surface smoothness of SiC material to get high imaging quality. Because of the high hardness of SiC, it is not easy to obtain high surface smoothness by traditionally mechanical polishing process [6]. Moreover, microstructure defects, like pores, steps at different phases and grain boundary damages are unavoidable under certain surfacing condition and present further difficulty in polishing this material. The application of modifying coatings like chemical vapor deposited (CVD) SiC coating [7] and physical vapor deposited (PVD) silicon (Si) coating [8] is therefore interesting to obtain mirror surface of SiC ceramic. Application of Si coating can reduce surface wearing resistance of SiC ceramic without changing mechanical properties of the bulk materials. Efficiency of surface finishing for large optical components can be greatly improved as well. Moreover, microstructure defects on polished ⁎ Corresponding author. Tel.: +86 21 52414231; fax: +86 21 52413903. E-mail address: [email protected] (G.L. Liu). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.06.069
surface of SiC ceramic mentioned above can be covered up by the coating process. Thus, mirror surface with high surface smoothness and low scattering can be obtained, meeting optical requirements. For optical applications, an amorphous structure without microstructure defects is needed owing to its absence of optical anisotropy and low surface scattering. Amorphous Si coating can be deposited by thermal evaporation [9], sputtering [10,11] and CVD methods [12,13]. The advanced plasma source ion plating (AP-IP) is a promising technique which combines the economic advantages of conventional evaporation with improved film properties of ion-assisted deposition [14]. In the ion plating process surface coverage is improved by sputtering and redeposition of the depositing material as well as some degree of backscattering of sputtered materials. The additional ion impact leads to the desorption of the weakly bonded fragments as well as to a densification of the film. The aim of this work is to study the possibility of obtaining continuous, homogenous and well bonded amorphous Si coatings on SiC ceramics by means of AP-IP. Characteristics like surface morphology, residual stress, bonding strength and optical reflectance of as deposited Si coatings were identified. Effects of substrate surface defects and deposition parameters on deposition of silicon coatings were studied. The evolution of surface morphology and contribution of residual stress and substrate temperature to the total reflectance of Si coatings within 400–750 nm wavelength were also discussed.
G.L. Liu et al. / Surface & Coatings Technology 207 (2012) 204–210
2. Experimental
205
Table 2 Coating conditions for Si deposition on SiC ceramic substrate.
2.1. Substrate materials
Deposition parameters
In this study, SiC ceramics with high density and excellent mechanical properties were prepared by solid state sintering at high temperature of 2100 °C for 2 h at argon atmosphere. Sintered SiC ceramics used as base materials (20 mm in diameter and 3 mm in thickness) for Si coating were finished by grinding and polishing on UNIPOL 802 precision polishing machine. Diamond suspensions with 5 μm, 3 μm and 1 μm abrasive grain sizes were used sequentially in polishing process. After polishing, the samples were ultrasonically cleaned in acetone for 10 min twice, rinsed with alcohol and then distilled water, and finally dried under high pressure air prior to coating experiment. Surfaces with rms (root mean square) surface roughness of 2.2 nm and diffuse reflectance below 0.7% within 400–750 nm wavelength were obtained for coating technique. Physical properties of SiC ceramics and surface quality after polishing in this study were shown in Table 1. 2.2. Coating procedure Si particles were evaporated from an electron beam gun. The deposition rate was monitored and controlled by a quartz microbalance system. A Leybold advanced plasma source (APS) [14,15] was applied to generate the ion beam. The working gas was argon in a continuous flow with a constant pressure of 1.6 × 10 −2 Pa in the cavity. Prior to the deposition, the polished SiC ceramic substrates were sputtered by Ar ions for 180 s. Thickness of Si coatings was dominated to be 2 μm by adjusting the deposition time using a quartz microbalance system. Coating conditions selected for this experiment are shown in Table 2. In order to optimize deposition parameters of AP-IP process for Si coating on SiC ceramic efficiently. Several coating conditions with variations of plasma power, deposition rate and substrate temperature were selected primarily. In these conditions, the self bias of −110 V and −130 V was varying with the given plasma power of 4 kW and 6 kW, respectively. The deposition rate was given to be 0.4 nm·s−1 and 0.6 nm·s−1. Once preferable values of the plasma power and the deposition rate were determined from above, the substrate temperature was ranged from 100 °C to 200 °C. 2.3. Characterization techniques The microstructure and morphology were characterized by scanning electron microscopy (SEM, Model JSM-6700F and JEOL-JXA8100) equipped with an energy dispersive X-ray spectroscopy (EDS). X-ray diffraction analyses (Rigaku D/max 2550 V Kα(Cu) radiation) were carried out to identify phase composition. The rms surface roughness was calculated from atomic force microscopy (AFM, Model SPI3800N&SPA300HV) images within 10 × 10 μm 2 area. The bonding strength of the coating was tested on an adhesive power testing
Naming of coating conditions 6P4D-15a 4P4D-15 6P6D-15 4P6D-15 6P4D-10 6P4D-20
Plasma power (kW) Self bias (V) Deposition rate (nm·s−1) Substrate temperature (°C) Pressure (×10−2 Pa)
6
4
6
4
6
−130 0.4
−110 0.4
−130 0.6
−110 0.6
−130 0.4
−130 0.4
150
150
150
150
100
200
1.6
1.6
1.6
1.6
1.6
Relative density (%)
instrument (BF-2) by scratching method, in which critical load was determined by sudden jump of the sound signal intensity. The residual stress was calculated with a formula derived from the following equation [16]: 2
σ¼
Es t s 6 1−ν 2s t f
1 1 − R2 R1
Flexural strength (MPa)
496
Fracture toughness (MPa·m1/2) Young's modulus (GPa) Poisson's ratio Thermal expansion coefficient (10−6 K−1) (at room temperature)
5.0 404 0.165 2.34
Rms surface roughness (nm) Diffuse reflectance (%, 400–750 nm)
ð1Þ
where σ is the residual stress of the coating, Es the Young's modulus of the substrate, νs the Poisson's ratio of the substrate, ts and tf the thickness of the substrate and the coating respectively, R1 and R2 the radii of curvature before and after deposition respectively. R1 and R2 were measured by means of an optical interferometer (ZYGO). Optical reflectance was determined at a room temperature with a Cary500 UV–VIS–NIR spectrophotometer. 3. Results 3.1. Phase composition of substrate and coating Fig. 1 shows the XRD patterns of the Si coating and the SiC substrate. The analysis of Bragg peak intensities extracted from XRD patterns of the SiC substrate clearly evidences polycrystalline silicon carbide and a small amount of graphite (Residual Carbon, C). No distinct diffraction peak from the silicon coating could be found, as shown in Fig. 1. The XRD patterns of the coatings under other deposition conditions in this study present to be similar with this and are not shown here. It indicates that Si coatings deposited by AP-IP process in this study are amorphous.
Surface quality after polishing 99.4
1.6
a Name 6P4D-15 represents the coating parameters of plasma power, deposition rate and substrate temperature is 6 kW, 0.4 nm·s−1and 150 °C, respectively.
Table 1 Physical properties of SiC ceramics and surface quality after polishing. Physical properties
6
2.2 b0.7
Fig. 1. XRD patterns of the Si coating and the SiC substrate.
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Fig. 2. Back-scattered electron SEM image of SiC substrate after corrosion.
For microstructure observation, the polished surface of SiC ceramic was eroded by melting alkaline moderately. As shown in Fig. 2, graphite as an additive almost remains at the boundary of SiC grains after high temperature sintering according to SEM and EDS results. The mean grain size of SiC is about 5 μm while the graphite phase is about 2 μm. It can be considered that the phase composition of the SiC substrate has no obvious effect on that of the Si coating. 3.2. Surface topography of substrate and silicon coating The polished surface of SiC ceramics as substrates for coating is shown in Fig. 3, from which we can see surface defects like holes and protuberances. The holes were demonstrated to be the result of grain pulling-out [17]. The protuberances were identified to be residual carbon and caused by different hardness of SiC grains and graphite [18]. Fig. 4 shows secondary electron SEM images of silicon coatings deposited on SiC under different coating conditions listed in Table 2. Fig. 5 shows the high resolution SEM image of the Si coating. It can be seen that the as deposited Si coating is with homogenous microstructure. The surface of silicon coatings does not duplicate the surface topography of SiC substrate during the coating process, as shown in Fig. 4. The main discontinuation on silicon coatings turned out to be notches with sizes ranging from half to 2 μm. The depth of notches is measured to be about 50 nm, as shown in Fig. 6, which is less than the total 2 μm thickness of the coating. It means that the SiC substrate can be fully covered up and the effect of substrate surface defects on the surface morphology of the Si coating can be overlooked.
From comparison among Fig. 4a, b, c and d, notches on Si coatings can be reduced by increasing the plasma power and self bias and deceasing the deposition rate. Si coatings deposited at 6 kW plasma power (Fig. 4a, c) present less notches on the surface than that deposited at 4 kW (Fig. 4b, d). Si coatings deposited at 0.4 nm·s−1 deposition rate (Fig. 4a, b) present less notches on the surface than that deposited at 0.6 nm·s−1 (Fig. 4c, d). The surface of Si coating deposited at 6 kW and 0.4 nm·s−1 in Fig. 4a presents least notches. It can be concluded that the deposition rate has a more powerful effect on reducing notches than the plasma power or self bias. The effect of substrate temperature on the surface morphology of the Si coating was also investigated. Si coating deposited at 100 °C presents more notches on the surface than at 150 °C, as shown in Fig. 7a and b. Nevertheless, Si coating deposited at 200 °C exhibits smooth surface without notches, as shown in Fig. 7c. It indicates that increasing the substrate temperature can effectively reduce defects on the coating surface and lead to smooth surface. 3.3. Residual stress, bonding strength and optical reflectance The residual stresses of as deposited Si coatings are compressive with values of −352 MPa, −455 MPa, −553 MPa, −636 MPa, −475 MPa and −538 MPa under deposition conditions of 6P4D-15, 4P4D-15, 6P6D-15, 4P6D-15, 6P4D-10 and 6P4D-20 in sequence. The residual stress has slight variation with deposition parameters in plasma enhanced ion plating process. Fig. 8 shows the cross-section SEM images of Si layers with low and high residual stress. Uniform section presents when the residual stress is much lower, as shown in Fig. 8 (b). There are several ways to investigate the bonding strength of the coating. Measuring the critical load in scratch test is one of them. The critical load of the Si coating deposited on SiC substrate in this study is about 34.5 N, as shown in Fig. 9. A more visual way is to observe the boundary between the coating and substrate. There are no distinct boundaries between the Si coating and SiC substrate, as shown in Fig. 8. It means that the two materials are chemical bond rather than mechanical bond. It indicates that the as deposited Si coatings exhibit quite good bonding strength with the SiC ceramic substrates necessarily for application without peeling. Fig. 10 shows the optical reflectance spectra of Si coatings within 400–750 nm wavelength. The total reflectance in Fig. 10 is the sum of the specular reflectance and the diffuse reflectance. It can be seen that the plasma power and deposition rate have slight affect on the total reflectance of Si coatings. However, the total reflectance increases significantly when the substrate temperature increases from 100 °C to 200 °C, as shown in Fig. 11. It can be considered that the effect of substrate temperature on the total reflectance is more significant than that of the residual stress. From above all, the optimized deposition parameters can be determined. The preferable values of plasma power, self bias, deposition rate and substrate temperature are 6 kW, − 130 V, 0.4 nm·s −1 and 200 °C, respectively. 4. Discussion 4.1. Surface morphology evolution of Si coating
Fig. 3. Surface topography of SiC substrate after polishing tested by AFM.
As mentioned above, the Si coating does not duplicate the surface topography of the substrate with surface defects like holes and protuberances caused by grain pulling-out and different hardness between two phases, respectively. What result in the notches on Si coatings? Experiment results show that notches can be avoided by optimizing deposition parameters as mentioned above. Deposition parameters are the main impact factors on the formation of the notches rather than surface defects of the SiC ceramic substrate.
G.L. Liu et al. / Surface & Coatings Technology 207 (2012) 204–210
207
Fig. 4. Secondary electron SEM images of Si coatings: (a) 6P4D-15; (b) 4P4D-15; (c) 6P6D-15; (d) 4P6D-15.
It is worth noting that the notches on coating surfaces mentioned above decrease as the residual stress decreases. There may be some correlation between the surface quality and the residual stress. The notches on the coating surface might be caused by the inertia ion bombardment under excessive internal stress. The high resolution image of the notches on Si coatings is shown in Fig. 12. There are steps and layers at the outskirt of the notch as well as islands at the bottom. That means notches were formed and partially filled during the coating process. In deposition process, ion plating takes place because of a negative self-biasing potential on the substrates. Because the substrate holder is negatively charged relative to the plasma, the ions from the plasma sheet are accelerated to the substrate and bombarding the growing film while the electrons are reflected. Increasing the plasma power and the self-bias can endow Si ions with much more momentum when condensate on the substrate. Rearrangement takes place during the formation process of the coating as well. It can be enhanced both by slowing down the deposition rate and increasing the substrate temperature, leading to relaxation of the internal stress and smooth surface.
Fig. 5. High resolution SEM image of Si coating deposited by AP-IP method.
4.2. Contribution of residual stress and substrate temperature to total reflectance The statement that residual stresses in PVD-coatings contain the thermal and condensation (structural) components is widespread and convincing [19–21]. The thermal component of the residual
Fig. 6. Surface topography of Si coating tested by AFM.
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Fig. 8. Fracture cross-section images of Si coatings: (a) 6P4D-15, −352 MPa; (b) 4P6D-15, −636 MPa.
temperature during the deposition, and T0 is the temperature of the steady state after the deposition of the coating. The stress state of ion-plasma coating is determined to a large extent by structural (condensation) stresses, unlike, for example, the stress state of coatings prepared by thermal evaporation, in which the development of stresses is governed predominantly by the difference between the thermal expansion coefficients of the film and substrate materials [23]. In ion-plasma process, the development of structural compressive stresses results from the ion/atom bombardment directly during the deposition [23].
Fig. 7. SEM images of Si coatings deposited at different substrate temperatures: (a) 6P4D-10; (b) 6P4D-15; (c) 6P4D-20.
stresses is caused by the difference in the thermal expansion coefficients for the substrate and coating materials during the cooling from deposition to room temperature, the substrate and coating attain different values of strain, which results in the occurrence of residual stresses. The structural stresses are the consequence of the process of the increasing of the coating layer and are caused by strong imperfections of the condensate [22]. The contribution of the thermal factor can be calculated using the following relationship:
σT ¼
E ðα −α s ÞðT s −T 0 Þ 1ν c
ð2Þ
where αc is the thermal expansion coefficient of the coating, αs is the thermal expansion coefficient of the substrate, Ts is the substrate
Fig. 9. Bonding strength between Si coating and SiC substrate tested by scratching method.
G.L. Liu et al. / Surface & Coatings Technology 207 (2012) 204–210
209
Fig. 12. The morphology of notches on surface of Si coating.
Fig. 10. Total reflectance of Si coatings deposited with different parameters within 400–750 nm wavelength range: (a) 6P4D-15, −352 MPa; (b) 4P4D-15, −455 MPa; (c) 6P6D-15, −553 MPa; (d) 4P6D-15, −636MPa.
In the present work, tensile stress about 22 MPa arises in Si and SiC due to difference in their thermal expansion coefficients (αSi = 2.6 × 10 −6 K −1 [24], αSiC = 2.34 × 10 −6 K −1 for solid state sintered SiC ceramics) at 200 °C deposition temperature, which can be overlooked compared to the residual stress level of 538 MPa. It is interesting that the total optical reflectance spectrum slightly increased with the increasing residual stress caused by reducing plasma power and accelerating deposition rate, as shown in Fig. 10. Rising the substrate temperature could also greatly improve the total reflectance as shown in Fig. 11. The contribution of the substrate temperature to the total reflectance is much greater than that of the residual stress. It was reported that density is the major factor of the total reflectance and higher density leads to higher total reflectance [25]. In the ion-plasma, Si particles are ionized and deposited on the surface with momentum to some extent. Thus, Si coating is formed through ion combination rather than the accumulation of molecules. It is considered that the Si\Si bonding is more severely shortened under higher compressive stress. It contributes to higher compactness of the amorphous network leading to higher total reflectance. The compactness of Si coating is also considered to be greatly enhanced by rising the substrate temperature during the deposition process. That is why the total
reflectance significantly increases with the increase of the substrate temperature. It must be noted that stress relaxation takes place in the coating process and benefits from the rearrangement of the condensate. The rearrangement can be enhanced both by reducing the deposition rate and rising the substrate temperature, leading to low compressive stress. However, improving the substrate temperature could also intensify kinetic energies of bombarding ions, leading to high compressive stress. 5. Conclusion Continuous, homogenous and well-boned 2 μm thick amorphous Si coatings with compressive stress have been deposited on SiC ceramics by plasma source ion plating. The preferable coating parameters for plasma power, deposition rate and substrate temperature were 6 kW, 0.4 nm·s−1 and 200 °C, respectively. It was found that the composition and surface morphology of the Si coating were mainly affected by deposition parameters rather than substrate surface defects. Improving the plasma power, slowing down the deposition rate and rising the substrate temperature will efficiently reduce surface discontinuities, which present in the form of notches and probably result from ion bombarding under excessive stress. The total reflectance of silicon coatings within 400–750 nm wavelength was also studied. The higher the residual compressive stress, the higher is the total reflectance. The increase might be attributed to shortened Si\Si bonding under high compressive stress. Rising substrate temperature would also significantly improve the total reflectance, since the Si coatings would possess higher compactness at higher substrate temperature. Acknowledgments This work was sponsored by the fund from the National Natural Science of China no. 51102266. The help of the Inorganic Coating Materials Research Center by providing Leybold advanced plasma source ion plating facilities is highly appreciated. References
Fig. 11. Total reflectance of Si coatings deposited at different substrate temperatures within 400–750 nm wavelength range.
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[8] Y.M. Zhang, Y.F. Zhou, J.C. Han, J.H. Zhang, W. Yao, Y.Y. Han, Adv. Opt. Manuf. Technol. 6149 (2006) C1492. [9] H.N. Yang, Y.P. Zhao, G.C. Wang, T.M. Lu, Phys. Rev. Lett. 76 (1996) 3774. [10] V. Kulikovsky, V. Vorlicek, P. Bohac, M. Stranyanek, R. Ctvrtlik, A. Kurdyumov, L. Jastrabik, Surf. Coat.Technol. 202 (2008) 1738. [11] H. Takahashi, H. Nagata, H. Kataoka, H. Takai, J. Mater. Res. 10 (1995) 2736. [12] Y.Q. Fu, J.K. Luo, S.B. Milne, A.J. Flewitt, W.I. Milne, Mater. Sci. Eng. B-Solid State Mater. Adv. Technol. 124 (2005) 132. [13] S.R. Jadkar, J.V. Sali, A.M. Funde, N.A. Bakr, P.B. Vidyasagar, R.R. Hawaldar, D.P. Amalnerkar, Sol. Energy Mater. Sol. Cells 91 (2007) 714. [14] H.K. Pulker, Surf. Coat.Technol. 112 (1999) 250. [15] S. Pongratz, A. Zoller, J. Vac. Sci. Technol. A 10 (1992) 1897.
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Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Surface morphology evolution and properties of silicon coating on silicon carbide ceramics by advanced plasma source ion plating G.L. Liu a,⁎, Z.R. Huang a, b, J.H. Wu c, X.J. Liu a a
Structural Ceramic Engineering Research Center, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China c Inorganic Coating Materials Research Center, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China b
a r t i c l e
i n f o
Article history: Received 16 March 2012 Accepted in revised form 20 June 2012 Available online 29 June 2012 Keywords: Silicon coating Plasma Surface morphology Residual stress Silicon carbide
a b s t r a c t Effects of substrate surface defects and deposition parameters on deposition of silicon coatings onto silicon carbide ceramics by plasma source ion plating for surface modification were studied in this paper. Substrate surface defects like holes and protuberances were not duplicated during the coating process. The main discontinuities on silicon coatings turned out to be notches which were probably generated by combined impact of excessive internal stress and persistent ion bombardment during the coating process. Continuous, homogenous and well-bonded 2 μm thick amorphous silicon coatings with smooth surface were obtained by improving plasma power, slowing down deposition rate and rising substrate temperature. The total reflectance of silicon coatings within 400–750 nm wavelength was also studied. The higher the residual compressive stress, the higher is the total reflectance. Rising substrate temperature would also significantly improve the total reflectance. The enhancement might be attributed to shortened Si\Si bonding under high compressive stress and higher compactness at higher substrate temperature, respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The use of surface coatings is an important alternative as a means of extending the performance of materials in a wide range of applications. Silicon carbide (SiC) has been recently applied in optical components for large space telescope [1–5] due to its high hardness/volume density ratio, high thermal conductivity and excellent wear-resistant ability. There is a demand for high surface smoothness of SiC material to get high imaging quality. Because of the high hardness of SiC, it is not easy to obtain high surface smoothness by traditionally mechanical polishing process [6]. Moreover, microstructure defects, like pores, steps at different phases and grain boundary damages are unavoidable under certain surfacing condition and present further difficulty in polishing this material. The application of modifying coatings like chemical vapor deposited (CVD) SiC coating [7] and physical vapor deposited (PVD) silicon (Si) coating [8] is therefore interesting to obtain mirror surface of SiC ceramic. Application of Si coating can reduce surface wearing resistance of SiC ceramic without changing mechanical properties of the bulk materials. Efficiency of surface finishing for large optical components can be greatly improved as well. Moreover, microstructure defects on polished ⁎ Corresponding author. Tel.: +86 21 52414231; fax: +86 21 52413903. E-mail address: [email protected] (G.L. Liu). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.06.069
surface of SiC ceramic mentioned above can be covered up by the coating process. Thus, mirror surface with high surface smoothness and low scattering can be obtained, meeting optical requirements. For optical applications, an amorphous structure without microstructure defects is needed owing to its absence of optical anisotropy and low surface scattering. Amorphous Si coating can be deposited by thermal evaporation [9], sputtering [10,11] and CVD methods [12,13]. The advanced plasma source ion plating (AP-IP) is a promising technique which combines the economic advantages of conventional evaporation with improved film properties of ion-assisted deposition [14]. In the ion plating process surface coverage is improved by sputtering and redeposition of the depositing material as well as some degree of backscattering of sputtered materials. The additional ion impact leads to the desorption of the weakly bonded fragments as well as to a densification of the film. The aim of this work is to study the possibility of obtaining continuous, homogenous and well bonded amorphous Si coatings on SiC ceramics by means of AP-IP. Characteristics like surface morphology, residual stress, bonding strength and optical reflectance of as deposited Si coatings were identified. Effects of substrate surface defects and deposition parameters on deposition of silicon coatings were studied. The evolution of surface morphology and contribution of residual stress and substrate temperature to the total reflectance of Si coatings within 400–750 nm wavelength were also discussed.
G.L. Liu et al. / Surface & Coatings Technology 207 (2012) 204–210
2. Experimental
205
Table 2 Coating conditions for Si deposition on SiC ceramic substrate.
2.1. Substrate materials
Deposition parameters
In this study, SiC ceramics with high density and excellent mechanical properties were prepared by solid state sintering at high temperature of 2100 °C for 2 h at argon atmosphere. Sintered SiC ceramics used as base materials (20 mm in diameter and 3 mm in thickness) for Si coating were finished by grinding and polishing on UNIPOL 802 precision polishing machine. Diamond suspensions with 5 μm, 3 μm and 1 μm abrasive grain sizes were used sequentially in polishing process. After polishing, the samples were ultrasonically cleaned in acetone for 10 min twice, rinsed with alcohol and then distilled water, and finally dried under high pressure air prior to coating experiment. Surfaces with rms (root mean square) surface roughness of 2.2 nm and diffuse reflectance below 0.7% within 400–750 nm wavelength were obtained for coating technique. Physical properties of SiC ceramics and surface quality after polishing in this study were shown in Table 1. 2.2. Coating procedure Si particles were evaporated from an electron beam gun. The deposition rate was monitored and controlled by a quartz microbalance system. A Leybold advanced plasma source (APS) [14,15] was applied to generate the ion beam. The working gas was argon in a continuous flow with a constant pressure of 1.6 × 10 −2 Pa in the cavity. Prior to the deposition, the polished SiC ceramic substrates were sputtered by Ar ions for 180 s. Thickness of Si coatings was dominated to be 2 μm by adjusting the deposition time using a quartz microbalance system. Coating conditions selected for this experiment are shown in Table 2. In order to optimize deposition parameters of AP-IP process for Si coating on SiC ceramic efficiently. Several coating conditions with variations of plasma power, deposition rate and substrate temperature were selected primarily. In these conditions, the self bias of −110 V and −130 V was varying with the given plasma power of 4 kW and 6 kW, respectively. The deposition rate was given to be 0.4 nm·s−1 and 0.6 nm·s−1. Once preferable values of the plasma power and the deposition rate were determined from above, the substrate temperature was ranged from 100 °C to 200 °C. 2.3. Characterization techniques The microstructure and morphology were characterized by scanning electron microscopy (SEM, Model JSM-6700F and JEOL-JXA8100) equipped with an energy dispersive X-ray spectroscopy (EDS). X-ray diffraction analyses (Rigaku D/max 2550 V Kα(Cu) radiation) were carried out to identify phase composition. The rms surface roughness was calculated from atomic force microscopy (AFM, Model SPI3800N&SPA300HV) images within 10 × 10 μm 2 area. The bonding strength of the coating was tested on an adhesive power testing
Naming of coating conditions 6P4D-15a 4P4D-15 6P6D-15 4P6D-15 6P4D-10 6P4D-20
Plasma power (kW) Self bias (V) Deposition rate (nm·s−1) Substrate temperature (°C) Pressure (×10−2 Pa)
6
4
6
4
6
−130 0.4
−110 0.4
−130 0.6
−110 0.6
−130 0.4
−130 0.4
150
150
150
150
100
200
1.6
1.6
1.6
1.6
1.6
Relative density (%)
instrument (BF-2) by scratching method, in which critical load was determined by sudden jump of the sound signal intensity. The residual stress was calculated with a formula derived from the following equation [16]: 2
σ¼
Es t s 6 1−ν 2s t f
1 1 − R2 R1
Flexural strength (MPa)
496
Fracture toughness (MPa·m1/2) Young's modulus (GPa) Poisson's ratio Thermal expansion coefficient (10−6 K−1) (at room temperature)
5.0 404 0.165 2.34
Rms surface roughness (nm) Diffuse reflectance (%, 400–750 nm)
ð1Þ
where σ is the residual stress of the coating, Es the Young's modulus of the substrate, νs the Poisson's ratio of the substrate, ts and tf the thickness of the substrate and the coating respectively, R1 and R2 the radii of curvature before and after deposition respectively. R1 and R2 were measured by means of an optical interferometer (ZYGO). Optical reflectance was determined at a room temperature with a Cary500 UV–VIS–NIR spectrophotometer. 3. Results 3.1. Phase composition of substrate and coating Fig. 1 shows the XRD patterns of the Si coating and the SiC substrate. The analysis of Bragg peak intensities extracted from XRD patterns of the SiC substrate clearly evidences polycrystalline silicon carbide and a small amount of graphite (Residual Carbon, C). No distinct diffraction peak from the silicon coating could be found, as shown in Fig. 1. The XRD patterns of the coatings under other deposition conditions in this study present to be similar with this and are not shown here. It indicates that Si coatings deposited by AP-IP process in this study are amorphous.
Surface quality after polishing 99.4
1.6
a Name 6P4D-15 represents the coating parameters of plasma power, deposition rate and substrate temperature is 6 kW, 0.4 nm·s−1and 150 °C, respectively.
Table 1 Physical properties of SiC ceramics and surface quality after polishing. Physical properties
6
2.2 b0.7
Fig. 1. XRD patterns of the Si coating and the SiC substrate.
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Fig. 2. Back-scattered electron SEM image of SiC substrate after corrosion.
For microstructure observation, the polished surface of SiC ceramic was eroded by melting alkaline moderately. As shown in Fig. 2, graphite as an additive almost remains at the boundary of SiC grains after high temperature sintering according to SEM and EDS results. The mean grain size of SiC is about 5 μm while the graphite phase is about 2 μm. It can be considered that the phase composition of the SiC substrate has no obvious effect on that of the Si coating. 3.2. Surface topography of substrate and silicon coating The polished surface of SiC ceramics as substrates for coating is shown in Fig. 3, from which we can see surface defects like holes and protuberances. The holes were demonstrated to be the result of grain pulling-out [17]. The protuberances were identified to be residual carbon and caused by different hardness of SiC grains and graphite [18]. Fig. 4 shows secondary electron SEM images of silicon coatings deposited on SiC under different coating conditions listed in Table 2. Fig. 5 shows the high resolution SEM image of the Si coating. It can be seen that the as deposited Si coating is with homogenous microstructure. The surface of silicon coatings does not duplicate the surface topography of SiC substrate during the coating process, as shown in Fig. 4. The main discontinuation on silicon coatings turned out to be notches with sizes ranging from half to 2 μm. The depth of notches is measured to be about 50 nm, as shown in Fig. 6, which is less than the total 2 μm thickness of the coating. It means that the SiC substrate can be fully covered up and the effect of substrate surface defects on the surface morphology of the Si coating can be overlooked.
From comparison among Fig. 4a, b, c and d, notches on Si coatings can be reduced by increasing the plasma power and self bias and deceasing the deposition rate. Si coatings deposited at 6 kW plasma power (Fig. 4a, c) present less notches on the surface than that deposited at 4 kW (Fig. 4b, d). Si coatings deposited at 0.4 nm·s−1 deposition rate (Fig. 4a, b) present less notches on the surface than that deposited at 0.6 nm·s−1 (Fig. 4c, d). The surface of Si coating deposited at 6 kW and 0.4 nm·s−1 in Fig. 4a presents least notches. It can be concluded that the deposition rate has a more powerful effect on reducing notches than the plasma power or self bias. The effect of substrate temperature on the surface morphology of the Si coating was also investigated. Si coating deposited at 100 °C presents more notches on the surface than at 150 °C, as shown in Fig. 7a and b. Nevertheless, Si coating deposited at 200 °C exhibits smooth surface without notches, as shown in Fig. 7c. It indicates that increasing the substrate temperature can effectively reduce defects on the coating surface and lead to smooth surface. 3.3. Residual stress, bonding strength and optical reflectance The residual stresses of as deposited Si coatings are compressive with values of −352 MPa, −455 MPa, −553 MPa, −636 MPa, −475 MPa and −538 MPa under deposition conditions of 6P4D-15, 4P4D-15, 6P6D-15, 4P6D-15, 6P4D-10 and 6P4D-20 in sequence. The residual stress has slight variation with deposition parameters in plasma enhanced ion plating process. Fig. 8 shows the cross-section SEM images of Si layers with low and high residual stress. Uniform section presents when the residual stress is much lower, as shown in Fig. 8 (b). There are several ways to investigate the bonding strength of the coating. Measuring the critical load in scratch test is one of them. The critical load of the Si coating deposited on SiC substrate in this study is about 34.5 N, as shown in Fig. 9. A more visual way is to observe the boundary between the coating and substrate. There are no distinct boundaries between the Si coating and SiC substrate, as shown in Fig. 8. It means that the two materials are chemical bond rather than mechanical bond. It indicates that the as deposited Si coatings exhibit quite good bonding strength with the SiC ceramic substrates necessarily for application without peeling. Fig. 10 shows the optical reflectance spectra of Si coatings within 400–750 nm wavelength. The total reflectance in Fig. 10 is the sum of the specular reflectance and the diffuse reflectance. It can be seen that the plasma power and deposition rate have slight affect on the total reflectance of Si coatings. However, the total reflectance increases significantly when the substrate temperature increases from 100 °C to 200 °C, as shown in Fig. 11. It can be considered that the effect of substrate temperature on the total reflectance is more significant than that of the residual stress. From above all, the optimized deposition parameters can be determined. The preferable values of plasma power, self bias, deposition rate and substrate temperature are 6 kW, − 130 V, 0.4 nm·s −1 and 200 °C, respectively. 4. Discussion 4.1. Surface morphology evolution of Si coating
Fig. 3. Surface topography of SiC substrate after polishing tested by AFM.
As mentioned above, the Si coating does not duplicate the surface topography of the substrate with surface defects like holes and protuberances caused by grain pulling-out and different hardness between two phases, respectively. What result in the notches on Si coatings? Experiment results show that notches can be avoided by optimizing deposition parameters as mentioned above. Deposition parameters are the main impact factors on the formation of the notches rather than surface defects of the SiC ceramic substrate.
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Fig. 4. Secondary electron SEM images of Si coatings: (a) 6P4D-15; (b) 4P4D-15; (c) 6P6D-15; (d) 4P6D-15.
It is worth noting that the notches on coating surfaces mentioned above decrease as the residual stress decreases. There may be some correlation between the surface quality and the residual stress. The notches on the coating surface might be caused by the inertia ion bombardment under excessive internal stress. The high resolution image of the notches on Si coatings is shown in Fig. 12. There are steps and layers at the outskirt of the notch as well as islands at the bottom. That means notches were formed and partially filled during the coating process. In deposition process, ion plating takes place because of a negative self-biasing potential on the substrates. Because the substrate holder is negatively charged relative to the plasma, the ions from the plasma sheet are accelerated to the substrate and bombarding the growing film while the electrons are reflected. Increasing the plasma power and the self-bias can endow Si ions with much more momentum when condensate on the substrate. Rearrangement takes place during the formation process of the coating as well. It can be enhanced both by slowing down the deposition rate and increasing the substrate temperature, leading to relaxation of the internal stress and smooth surface.
Fig. 5. High resolution SEM image of Si coating deposited by AP-IP method.
4.2. Contribution of residual stress and substrate temperature to total reflectance The statement that residual stresses in PVD-coatings contain the thermal and condensation (structural) components is widespread and convincing [19–21]. The thermal component of the residual
Fig. 6. Surface topography of Si coating tested by AFM.
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Fig. 8. Fracture cross-section images of Si coatings: (a) 6P4D-15, −352 MPa; (b) 4P6D-15, −636 MPa.
temperature during the deposition, and T0 is the temperature of the steady state after the deposition of the coating. The stress state of ion-plasma coating is determined to a large extent by structural (condensation) stresses, unlike, for example, the stress state of coatings prepared by thermal evaporation, in which the development of stresses is governed predominantly by the difference between the thermal expansion coefficients of the film and substrate materials [23]. In ion-plasma process, the development of structural compressive stresses results from the ion/atom bombardment directly during the deposition [23].
Fig. 7. SEM images of Si coatings deposited at different substrate temperatures: (a) 6P4D-10; (b) 6P4D-15; (c) 6P4D-20.
stresses is caused by the difference in the thermal expansion coefficients for the substrate and coating materials during the cooling from deposition to room temperature, the substrate and coating attain different values of strain, which results in the occurrence of residual stresses. The structural stresses are the consequence of the process of the increasing of the coating layer and are caused by strong imperfections of the condensate [22]. The contribution of the thermal factor can be calculated using the following relationship:
σT ¼
E ðα −α s ÞðT s −T 0 Þ 1ν c
ð2Þ
where αc is the thermal expansion coefficient of the coating, αs is the thermal expansion coefficient of the substrate, Ts is the substrate
Fig. 9. Bonding strength between Si coating and SiC substrate tested by scratching method.
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Fig. 12. The morphology of notches on surface of Si coating.
Fig. 10. Total reflectance of Si coatings deposited with different parameters within 400–750 nm wavelength range: (a) 6P4D-15, −352 MPa; (b) 4P4D-15, −455 MPa; (c) 6P6D-15, −553 MPa; (d) 4P6D-15, −636MPa.
In the present work, tensile stress about 22 MPa arises in Si and SiC due to difference in their thermal expansion coefficients (αSi = 2.6 × 10 −6 K −1 [24], αSiC = 2.34 × 10 −6 K −1 for solid state sintered SiC ceramics) at 200 °C deposition temperature, which can be overlooked compared to the residual stress level of 538 MPa. It is interesting that the total optical reflectance spectrum slightly increased with the increasing residual stress caused by reducing plasma power and accelerating deposition rate, as shown in Fig. 10. Rising the substrate temperature could also greatly improve the total reflectance as shown in Fig. 11. The contribution of the substrate temperature to the total reflectance is much greater than that of the residual stress. It was reported that density is the major factor of the total reflectance and higher density leads to higher total reflectance [25]. In the ion-plasma, Si particles are ionized and deposited on the surface with momentum to some extent. Thus, Si coating is formed through ion combination rather than the accumulation of molecules. It is considered that the Si\Si bonding is more severely shortened under higher compressive stress. It contributes to higher compactness of the amorphous network leading to higher total reflectance. The compactness of Si coating is also considered to be greatly enhanced by rising the substrate temperature during the deposition process. That is why the total
reflectance significantly increases with the increase of the substrate temperature. It must be noted that stress relaxation takes place in the coating process and benefits from the rearrangement of the condensate. The rearrangement can be enhanced both by reducing the deposition rate and rising the substrate temperature, leading to low compressive stress. However, improving the substrate temperature could also intensify kinetic energies of bombarding ions, leading to high compressive stress. 5. Conclusion Continuous, homogenous and well-boned 2 μm thick amorphous Si coatings with compressive stress have been deposited on SiC ceramics by plasma source ion plating. The preferable coating parameters for plasma power, deposition rate and substrate temperature were 6 kW, 0.4 nm·s−1 and 200 °C, respectively. It was found that the composition and surface morphology of the Si coating were mainly affected by deposition parameters rather than substrate surface defects. Improving the plasma power, slowing down the deposition rate and rising the substrate temperature will efficiently reduce surface discontinuities, which present in the form of notches and probably result from ion bombarding under excessive stress. The total reflectance of silicon coatings within 400–750 nm wavelength was also studied. The higher the residual compressive stress, the higher is the total reflectance. The increase might be attributed to shortened Si\Si bonding under high compressive stress. Rising substrate temperature would also significantly improve the total reflectance, since the Si coatings would possess higher compactness at higher substrate temperature. Acknowledgments This work was sponsored by the fund from the National Natural Science of China no. 51102266. The help of the Inorganic Coating Materials Research Center by providing Leybold advanced plasma source ion plating facilities is highly appreciated. References
Fig. 11. Total reflectance of Si coatings deposited at different substrate temperatures within 400–750 nm wavelength range.
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