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Plasma enhanced magnetron sputter deposition of Ti–Si–C–N based nanocomposite coatings
Surface & Coatings Technology 203 (2008) 538–544
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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 e v i e r. c o m / l o c a t e / s u r f c o a t
Plasma enhanced magnetron sputter deposition of Ti–Si–C–N based nanocomposite coatings Ronghua Wei ⁎ Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238-5166, USA
A R T I C L E
I N F O
Available online 22 May 2008 Keywords: Nitrides Nanocomposite Thick coatings Erosion resistance Turbine blades
A B S T R A C T This paper reviews a Plasma Enhanced Magnetron Sputtering (PEMS) technology and a series of studies of Ti– Si–C–N nanocomposite coatings deposited using this technique. The PEMS technology is discussed briefly and compared with conventional magnetron sputter deposition. In PEMS, an electron source is used to generate plasma in addition to the magnetron plasma for both ion cleaning of the surfaces before deposition and enhanced ion bombardment during the deposition. The coatings thus obtained are very dense with nearly no columnar structure. Using this technology, many Ti–Si–C–N coatings have been prepared. These coatings are found to be nanocomposite coatings composed of nanocrystalline TiCN with grain sizes of 4.7– 100 nm in a matrix of amorphous SiCxNy. Their hardness ranges from 25 to 43 GPa and has a Si concentration up to about 3.5 at.%. They exhibit erosion resistance over 100 times better than the uncoated Ti–6Al–4V substrate. It has been observed that the toughness of the coating, or the resistance to crack formation and propagation, evaluated using H3/E⁎2, correlates well with the coating erosion resistance. The relationship among the processing parameters and the coating microstructure, nanohardness, adhesion, and erosion resistance are discussed. In addition, single and multilayered coatings have also been compared. These coatings are intended to be used against severe sand erosion for gas turbine compressor blades and vanes. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nanocomposite coatings, especially the Ti–Si–N based coatings, have been studied extensively due to their unique properties of extreme high hardness and high oxidation resistance. The early work may date back to 1992 when Li Shizhi et al. published their study of Ti– Si–N coatings using plasma enhanced chemical vapor deposition (PECVD) [1]. They observed that the films with a Si concentration near 10–15 at.% exhibited very high hardness — over 60 GPa, and concluded that the coatings were multiphased composites consisting of crystalline TiN and amorphous Si3N4. Later, Veprek et al. conducted extensive studies and observed that the hardness of some coatings could even be over 100 GPa [2–15]. They further explained that the superhardness of these coatings was the result of the nanocrystalline TiN in the amorphous phased Si3N4. The structure is now commonly written as nc-TiN/a-Si3N4. Musil et al. observed that a composite coating consisting of a hard ceramic phase such as MeN (metal nitride) and a soft metal also had high hardness [16–24]. Other groups have also studied the Ti–Si–N system and other ternary, quaternary, or even pentanary systems [25–41]. The high hardness of these nanocom-
⁎ Southwest Research Institute®, 6220 Culebra Road, San Antonio, Texas 78238-5166, USA. Tel.: +1 210 522 5204; fax: +1 210 522 6220. E-mail address: [email protected]. 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.05.019
posite coatings has already demonstrated superb properties for tribological applications such as in high speed, dry cutting and drilling [42–44] and on aero engine turbine blades against severe erosion [45–48]. PECVD is a commonly used process in preparing the nanocomposite coatings, [1–15,42,49–55]. On the other hand, various PVD processes have also been developed including ion beam assisted deposition [30], magnetron sputtering [16–27,29,31–35,37–41,43–48], cathodic arc deposition [28,36,58] and hybrid deposition using both magnetron sputtering and arc evaporation [56,57]. Recent reviews on various deposition technologies can be found in refs. [59,60]. Compared with PECVD, PVD has many advantages such as the low deposition temperature, which allows the processing of low temperature substrate materials. Besides, PVD is a commonly used technique for depositing TiN-based hard coatings for the cutting tool industry, which can easily adopt the Ti–Si–N coating process. Within the PVD family, magnetron sputtering is probably the most commonly used technique in the nanocomposite coating research, and more literature can be found in Refs. [61–67]. This paper discusses a particular technique – Plasma Enhanced Magnetron Sputtering (PEMS) and reviews the research and development of a special type of nanocomposite coating – Ti–Si–C–N. The principle and the advantage of the PEMS will be discussed. Furthermore, the microstructural properties and micro-nanomechanical properties will be presented. Finally, the tribological performance, particularly the erosion resistance, will also be reviewed.
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2. Plasma enhanced magnetron sputtering (PEMS) 2.1. Principle of PEMS The PEMS process is based on the magnetron sputtering process, but an electron source such as a hot filament, heated by an AC power supply, is introduced to generate electrons. With a DC power supply (discharge) applied between the filament and the chamber wall, the electrons are accelerated to the wall. Because of the presence of the gas (Ar) in the vacuum chamber, electron-neutral collisions occur, thereby resulting in ionization and hence generating plasma. Detailed descriptions of the PEMS process can be found in Refs. [45–48,68–70]. 2.2. Comparison of PEMS and conventional magnetron sputter deposition The major difference between PEMS and conventional magnetron sputtering (CMS) is the introduction of the filament-generated plasma (FGP), which can also be generated using other electron sources. There are a few advantages using the FGP. First, the FGP is produced in the large volume defined by the entire vacuum chamber; therefore a very high current density can be obtained, and up to 25 times the current density was measured. In addition, the FGP is independent of the magnetron-generated plasma (MGP). The high current density obtained from the FGP can be used during both the ion cleaning stage and the sputter deposition stage. During the ion cleaning, the magnetrons are not turned on and the MGP is not needed because the FGP is sufficient to clean the oxide from the sample surface. In CMS, magnetrons may have to be turned on to provide the system with plasma for the ion cleaning. When the magnetrons are on, the target materials are inevitably sputtered and can be deposited on the parts before they are even cleaned. Therefore, in the PEMS process surface cleanliness is guaranteed using the FGP alone. During the deposition stage the FGP remains for enhanced ion bombardment to densify the
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coating. The advantages of the PEMS process can be understood from Fig. 1, which shows Al films prepared using the CMS process (Fig. 1 a) and the PEMS process (Fig. 1 b) under identical processing parameters except the FGP. It is clear that high ion current density bombardment substantially densifies the film. Nearly identical results have also been observed on Cr films [48]. In comparison with the columnar structure produced by CMS, very dense Cr films have been obtained using the PEMS technique. The advantages of the PEMS process has been further demonstrated on cutting tools including end mills, gear shaper cutters and gear hobs. A reduction of two to five times in land wear, flank wear, and nose wear, compared with the tools coated using conventional PVD techniques, has been observed [68–70]. Because of the advantages of the PEMS technique over the CMS technique, we choose to use the PEMS for developing the Ti–Si–C–N coatings. 3. Ti–Si–C–N based Nanocomposite coatings prepared using the PEMS process The primary reasons for developing the Ti–Si–C–N based nanocomposite coatings over the Ti–Si–N coatings are the simplicity of the deposition process and the uniformity of the coating composition. In CMS, there may be three methods to deposit nc-TiN/a-Si3N4 nanocomposite coatings and each one has its own limitations. In the first method, Ti and Si are sputtered from separated targets. In this case, it is very difficult to obtain homogeneous coating composition in a large vacuum chamber. In addition, compared to Ti the sputtering rate of Si is usually low because it is a semiconductor. Besides, the rate control for Si is difficult especially in the nitrogen environment. In the second method, composite targets of Ti and Si can be used. This will overcome the coating composition uniformity issue, but many targets with various compositions of Ti and Si have to be made to achieve the desired composition. In addition, even if the ideal composition is selected, the cost of composite targets is very high, particularly for production. In the third method, Ti can be sputtered from Ti targets while Si is obtained from silane (SiH4) or silicon tetrachloride (SiCl4). This will resolve the uniformity issue and the composite target cost issue, but SiH4 is undesirable because it is a pyrophoric gas and requires special gas handling equipment for safe use and training for the operators. Even though it is commonly used in semiconductor industry or in research environments, it is rarely used for commercial tribological coatings. As for SiCl4, it is corrosive to the vacuum system. In our studies, Ti is sputtered from Ti targets, while Si comes from trimethylsilane ((CH3)3SiH or TMS) precursor. This method is similar to the last one described above, but the advantage of using TMS is significant. TMS is flammable but not pyrophoric and can be handled in a similar way as many other flammable gases such as acetylene (C2H2) and methane (CH4). It is not corrosive to the vacuum chamber and the pumps. However, TMS contains carbon and the coatings are Ti–Si–C–N based coatings. What is needed is to understand whether this kind of coating performs in a similar manner as the Ti–Si–N coatings. Therefore, the research has been focused on understanding the Ti–Si–C–N coatings produced using the PEMS technology by sputtering Ti in the environment of N2 and TMS. If the microstructure, hardness and tribological performance of these Ti–Si–C–N coatings are better than, or even only similar to, those of the Ti–Si–N coatings, it has been proven that the PEMS technology together with the use of the TMS to form the Ti–Si–C– N coatings is advantageous, particularly for large-scaled production. 4. Microstructure of the Ti–Si–C–N coatings
Fig. 1.
Using the PEMS process, numerous Ti–Si–C–N coatings have been studied. In this review paper, we will highlight the characterization results from a series of studies [45–48,71] conducted using various techniques including SEM, EDAX, XRD and erosion test. Shown in Fig. 2 are the SEM morphological and cross-sectional images of a Ti–Si–C–N coating. In general, the coatings are dense and
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R. Wei / Surface & Coatings Technology 203 (2008) 538–544
both Si and C concentrations increase, as expected, but the N concentration decreases. On the other hand, when N2 flow rate increases, the N concentration in the coating increases as expected, while the concentration for both Si and C barely changes. The typical concentration for N is from 30–40% while for Ti it is from 50–40%. For the best performed coatings in erosion test, the Si concentration is from 1–3.5%. The detailed discuss on the processing parameters and the composition of the Ti–Si–C–N coatings can be found from Refs. [48,71]. Based on the EDAX analysis, there is an access amount of SiCN that cannot be observed and accounted for in the crystalline phases in the XRD analysis. Therefore, as in the study of Ti–Si–N films where there is an amorphous phase of Si3N4 (or a-Si3N4), it is concluded that the Ti– Si–C–N coatings has an amorphous phase of SiCxNy (or a-SiCxNy). 5. Micro and nanomechanical properties of the Ti–Si–C–N coatings
Fig. 2.
Microhardness measurement using Vicker's indenter at 50–100 g loads shows that the hardness of the Ti–Si–C–N coatings is from 2500 to 4300 kgf/mm2, depending on the deposition conditions (ion energy, ion current density, N2 flow rate and TMS flow rate). The microhardness data for selected samples are also shown in Table 1. The nanohardness and the modulus of elasticity have been measured for the Ti–Si–C–N coatings. From the measurement, the reduced modulus (E⁎ = E / (1 − ν2) where ν is the Poisson ratio = 0.3), plastic deformation energy (Wp), elastic deformation energy (We) and elastic recovery (Re, elastic deformation/total deformation) can be obtained. Most importantly, the value H3 / E⁎2, which represents the toughness of a coating or resistance to crack formation and propagation, can also be calculated. The detailed discussion on these parameters can be found in Refs. [20,72,73]. The data for selected
featureless. The microstructure is determined by the processing parameters including the flow rate of N2, the flow rate of TMS, the ion current density, and the ion energy. In general, the higher the values for these four processing parameters, the better the microstructure of the coating (dense without the columnar structure). Coating thickness also influences the microstructure. Thicker coatings tend to be rougher with more inclusions and defects. However, extremely thick coatings have been obtained and the microstructure looks reasonably well. Shown in Fig. 3 are the morphological (Fig. 3 a) and cross-sectional (Fig. 3 b) SEM images of a Ti–Si–C–N coating over 500 µm thick. Detailed relationships between the microstructure and the processing parameters can be found from Refs. [48,71]. Shown in Fig. 4 a is the typical XRD data for most Ti–Si–C–N coatings. As can be seen, only TiCN can be recognized, which can be further indentified as TiC0.3N0.7. It is noted that the peak locations for TiCN and TiN are very close at low angles, but a careful examination at high angles will enable us to separate TiCN from TiN. For a few coatings, in addition to the TiCN phase, other peaks can be observed including TiN with the rare appearance of Ti4N3, as shown in Fig. 4 b. The average gain size can be estimated using the predominant peak. Most good performing coatings (in erosion and microstructure) have the (200) preferred orientation as shown in Fig. 4 a and this peak was used to estimate the grain size. For all coatings we have studied, the grain size varies from 4–100 nm. Some selected samples with various measurement results are listed in Table 1, in which the samples with the grain size up to 30.2 nm are listed. For the coatings that perform well in erosion tests, which will be discussed in a later section, the grain size is about 4–7 nm. The composition of the coatings, measured using EDAX, shows a typical Si concentration of 0.5–3.5 at.%. The Si concentration for selected samples is shown in Table 1. Typically, the C concentration (not shown) is about 10–20%. With the increase of the TMS flow rate,
Fig. 3.
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Fig. 4.
samples are listed in Table 1. Typically the nanohardness of these coatings is from 15–35 GPa, while H3 / E⁎2 is from 0.05 to 0.25 GPa. The value H3 / E⁎2 has been found to correlate well with the erosion resistance, which will be discussed in a later section. 6. Coating adhesion ranking and microstructural ranking Rockwell C scale indentation is a simple method that has been widely accepted in evaluating coating adhesion [74–76]. Together
with the SEM observation of the cracks, the coating adhesion can be ranked from “1” for the best (showing nearly no cracks anywhere around the indentation) to “6” for the worst (showing the coating complete spallation). Using a similar method, the microstructure of the coatings can also be ranked, with “1” being the best (dense, few inclusions, less columnar structure, etc), and “4” being the worst (loose, many inclusions, clear columnar structure, etc) [48,71]. The rankings for adhesion and microstructure for selected samples are shown in Table 1. Using these values, coatings can be selected
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Table 1 Sample number
Erosion rate ave (x10−4 mm3/g)
XRD peaks
Grain sizes⁎ (nm)
Micro hardness (HV)
Nano hardness (GPa)
H3 / E⁎2 (GPa)
Si (at.%)
Micro-structure rank
Rc adhesion rank
Ti–6AI–4V SN1 SN2 SN3 SN5 SN6 SN7 SN8
312.80 218.04 118.11 103.36 61.26 17.05 9.79 8.51
– TiN TiCN TiCN TiN TiCN TiCN TiCN, TiN, SiC
– 30.2 11.8 7.1 10.2 5.5 7.2 4.8
– 2620.5 4110.0 2274.5 4014.8 2923.3 3234.9 2806.2
– – – 21.8 18.7 32.6 25.5 29.4
– – – 0.068 0.064 0.206 0.157 0.216
– 0.73 0.55 1.90 2.24 1.74 0.94 3.50
– 4 4 2 2 2 1 2
– 2 1 1 1 1 4 2
⁎Grain size was estimated using the predominant peak for each coating and the Jade 3.1 software by Material Data, Inc.
according to their hardness, adhesion, microstructure and erosion resistance for a specific application. 7. Erosion resistance of the Ti–Si–C–N coatings The intended application of the Ti–Si–C–N coatings is for gas turbine compressor blades where severe erosion occurs. The erosion properties of the coatings have been evaluated using a sand erosion tester at two incident angles of 30° and 90° using 50 µm Al2O3 powder. The incident particle velocity was about 14 m/s. The erosion rate (volume loss of the coating/gram of erodent consumed) for selected nanocomposite coated samples (SN1-SN9) together with the uncoated Ti–6Al–4V is shown in Table 1 and also plotted in Fig. 5. It should be mentioned that for a few samples, the erosion rate at 90° was basically zero. The micro-balance accuracy (0.01 mg) was used for their total mass loss, which corresponds to an erosion rate of 1.7 × 10− 4 mm3/g. From Fig. 5, it is noted that for uncoated Ti–6Al–4V the erosion rate is higher at the low incident angle of 30° than that at high angle of 90°. In contrast, for the Ti–Si–C–N coated samples, SN1, SN2, and SN4, the erosion rate is lower at the low incident angle of 30°. These trends are consistent with the theoretical predictions and experimental observations that the erosion rate of ductile materials peaks at about 15–30°, while the erosion rate of brittle materials peaks at 90° [77–81]. As for the best coatings (SN5, SN6 and SN7), the trend is reversed and the erosion rate at 90° is basically zero. It is clear that the best coatings reduce the erosion rate by 100–200 times over the uncoated substrate material Ti–6Al–4V.
the toughness (H3 / E⁎2 ~ 0.2), the better the erosion resistance of the coating. In contrast, for the poor performing coatings the toughness is low (H3 / E⁎2 ~ 0.06). The effect of Si content listed in Column 8 on the erosion resistance cannot be recognized clearly, although a high Si concentration seems to result in either low erosion or high erosion [71]. As for the micronanostructure (Column 9), the best performing coatings are all ranked high, either “1” or “2”, implying that these coatings are very dense. In contrast, the worst performing coatings are ranked very low, mostly “4”, implying that these coatings are porous and full of defects. However, the Rc adhesion ranking is nearly reversed, as shown in the last column of the table. For the best performing coatings, the adhesion is generally not as good as the worst performing coatings, which are ranked either “1” or “2”. This is possibly due to the higher internal stress. In general, dense coatings are more susceptible to indentation cracking than the porous coatings. Considering the erosion resistance, adhesion and microstructure, SN6 and SN8 are the best compromise and generally used in our depositions of components. 8.2. Multilayer coatings Multilayer coatings are commonly used for thick coatings to reduce the internal stress. It is believed the multilayered structure can reduce erosion by absorbing the solid particle impact energy. In order to further optimize the coatings, a comparison study was carried out in which a single layered coating and two multilayered coatings were
8. Discussion 8.1. Single layer coatings The measurement methods for selected samples listed in Table 1 have been discussed above. Now we shall discuss the meanings of the results. Listed in Column 2 is the average erosion rate, ranked from the highest to the lowest, based on which all the data are arranged. The erosion data can be divided into three groups from the highest erosion rate (SN1) to medium erosion rate (SN2-SN4) and to the lowest erosion rate (SN5-SN7). Listed in Column 3 are the XRD major peaks (mainly TiCN and TiN) as discussed previously. The coating grain size (Column 4), estimated from 4.8–30 nm seems to have a strong effect on the erosion resistance. For the best performing coatings, the grain size is about 4.7–7 nm. When the grain size is too big (20–30 nm) the coating loses its erosion resistance. For a few coatings the average grain size is over 100 nm (not shown in this paper), the erosion rate is even worse. The microhardness (Column 5) seems to have no effect on the coating performance, as is evident that a few samples show the “super” hardness (over 4000) but high erosion rate. As for the nanohardness (Column 6), it should be noted for samples SN1 and SN2, their nancohardness was not available due to their loose structures. For the best performing coatings, the nanohardness is from 25 to 33 GPa. However, it is clear the toughness (H3 / E⁎2) listed in Column 7 has a strong effect on the erosion rate. In general, the higher
Fig. 5.
R. Wei / Surface & Coatings Technology 203 (2008) 538–544
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and propagation. The erosion resistance can be 100 times higher than the uncoated Ti–6Al–4V. The microstructure, adhesion and erosion resistance, all determined by the processing parameters, have been correlated and ranked for the selection of a coating for specific applications. In some cases, compromise has to be made in selecting the “best” coatings. In addition, single layered coatings outperform multilayered coatings in erosion, but multilayered coatings are generally more resistant to Rc indentation cracking and spallation. The Ti–Si–C–N coatings outperform TiN coatings. Although the PEMS technology that utilizes heavy ion bombardment and TMS precursor to form the Ti–Si–C–N coatings seems to be advantageous for commercial production of hard coatings, large scale tests using actual blades have yet to be conducted to prove the process. Some droplets are sometimes still observed in the coatings and need to be eliminated since they can become the site for fatigue debt. In addition to the erosion applications, these Ti–Si–C–N coatings may also be used for cutting applications and a comprehensive study is needed. Fig. 6.
deposited and erosion tested. The coating parameters (N2 flow rate, TMS flow rate, ion energy and ion current density) remained constant, but in depositing the soft layer (Ti) for the multilayer coatings, both N2 and TMS were turned off. For the two multilayer coatings, both had the hard/soft/hard structure, but the hard layer for sample Multi1 was twice as thick as sample Multi2, while the soft coating Ti thickness remained the same. To compare the erosion resistance, all samples were deposited for about the same duration; therefore, Multi1 had fewer layers but the hard layers were thicker. Finally, the uppermost layer on the top was always the hard layer for both Multi1 and Multi2, as for the conventional multilayer coatings. Shown in Fig. 6 is the erosion rate obtained in the same way as discussed previously. For comparison purposes, the erosion rate for uncoated Ti–6Al–4V is also given. It is noted that the data for SN7 and SN8 are the same as in Table 1 and Fig. 5, while SN7-R and SN8-R represent the repeated test data (new samples deposited using the same parameters as SN7 and SN8), while Multi1 and Multi2 are the corresponding multilayered coatings. In order to compare with the Ti–Si–C–N nanocomposite coatings, one set of samples was coated with only TiN (no TMS) and the data for the single layer and multilayers are also shown in Fig. 6. From the data, first we note that the repeat data are close to the original data, implying the reproducibility of the process. Second, the single layered coatings show lower erosion than the multilayer coatings, while all Multi1 coatings (fewer number of hard layers, but thicker for each one) show lower erosion than all Multi2 coatings (more number of hard layers but thinner for each one). This may be understood in the following. The total thickness for all coatings is about the same, but the single-layered coating has the thickest hard layer, followed by the Multi1 and then Multi2. Therefore, the hard coating is responsible for the high erosion resistance. This result contradicts the conventional belief that a multilayer coating can absorb the impact energy and hence reduce the erosion. It should be mentioned here that multilayered coatings indeed show better adhesion in the Rc indentation tests. Finally, all Ti–Si–C–N coatings shown in this figure outperform the TiN coatings. 9. Conclusion and outlook In this paper, we have discussed the PEMS technique and reviewed the Ti–Si–C–N nanocomposite coatings produced using this technique. The Ti–Si–C–N coatings are nanocomposite coatings with the nanocrystalline TiCN and/or TiN (grain size from 4.7–100 nm) in the amorphous matrix of SiCxNy. The total Si content is less than 3.5 at.%. The hardness can be over 40 GPa. However, the erosion resistance barely correlates with the hardness, but it is strongly determined by the value of H3 / E⁎2, the toughness or resistance to crack formation
Acknowledgment The author wishes to thank Mr. Christopher Rincon and Mr. Edward Langa for the deposition of the coatings, Mr. James Spencer and Mr. Byron Chapa for the SEM and XRD characterizations, and Dr. Qi Yang, National Research Council, Canada, for the EDS and nanoindentation analyses. The internal research funding from Southwest Research Institute made the work possible. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
[30]
Li Shizhi, S. Yulong, P. Hongrui, Plasma Chem. Plasma Process. 12 (1992) 287. S. Veprek, S. Reiprich, Li Shizhi, Appl. Phys. Lett. 66 (20) (1995) 2640. S. Veprek, S. Reiprich, Thin Solid Films 268 (1–2) (November 1 1995) 64. S. Veprek, Surf. Coat. Technol. 97 (1–3) (December 1997) 15. S. Veprek, P. Nesladek, A. Niederhofer, F. Glatz, M. Jilek, M. Sima, Surf. Coat. Technol. 108–109 (1998) 138. S. Veprek, Thin Solid Films 317 (1998) 449. S. Veprek, P. Nesládek, A. Niederhofer, F. Glatz, Nanostruc. Mater. 10 (5) (July 1998) 679. S. Veprek, J. Vac. Sci. Technol. A17 (5) (1999) 2401. A. Niederhofer, P. Nesládek, H.-D. Männling, K. Moto, S. Veprek, M. Jílek, Surf. Coat. Technol. 120–121 (November 1999) 173. S. Veprek, A. Niederhofer, K. Moto, T. Bolom, H.-D. Mannling, P. Nesladek, G. Dollinger, A. Bergmaier, Surf. Coat. Technol. 133–134 (2000) 152. S. Veprek, A.S. Argon, Surf. Coat. Technol. 146–147 (September-October 2001) 175. S. Veprek, M. Jilek, Vacuum 67 (3–4) (September 26 2002) 443. S. Ma, J. Procházka, P. Karvánková, Q. Ma, X. Niu, X. Wang, D. Ma, K. Xu, S. Veprek, Surf. Coat. Technol. 194 (1) (April 20 2005) 143. S. Veprek, M.G.J. Veprek-Heijman, Surf. Coat. Technol. 201 (13) (March 26 2007) 6064. A.C Fischer-Cripps, P. Karvánková, S. Vepřek, Surf. Coat. Technol. 200 (18–19) (May 8 2006) 5645. J. Musil, P. Zeman, H. Hrubý, P.H. Mayrhofer, Surf. Coat. Technol. 120–121 (November 1999) 179. J. Musil, I. Leipner, M. Kolega, Surf. Coat. Technol. 115 (1) (June 15 1999) 32. P. Zeman, R. Cerstvy, P.H. Mayrhofer, C. Mitterer, J. Musil, Mater. Sci. Eng. A Struct. Mater. Prop. Microstruct. Process. 289 (1–2) (September 30 2000) 189. J. Musil, H. Poláková, Surf. Coat. Technol. 127 (1) (May 1 2000) 99. J. Musil, Surf. Coat. Technol. 125 (2000) 322. F. Regent, J. Musil, Surf. Coat. Technol. 142–144 (July 2001) 146. J. Musil, H. Hruby, P. Zeman, H. Zeman, R. Cerstvy, P.H. Mayrhofer, C. Mitterer, Surf. Coat. Technol. 142–144 (July 2001) 603. J. Musil, P. Karvánková, J. Kasl, Surf. Coat. Technol. 139 (1) (May 1 2001) 101. J. Soldán, J. Musil, Vacuum 81 (4) (November 6 2006) 531. Soo Hyun Kim, Ji Woong Jang, Sung Soo Kang, Kwang Ho Kim, J. Mater Process. Technol. 130–131 (December 20 2002) 283. D. Mercs, P. Briois, V. Demange, S. Lamy, C. Coddet, Surf. Coat. Technol. 201 (16–17) (May 21 2007) 6970. Harish C. Barshilia, B. Deepthi, A.S. Arun Prabhu, K.S. Rajam, Surf. Coat. Technol. 201 (1–2) (September 12 2006) 329. A. Winkelmann, J.M. Cairney, M.J. Hoffman, P.J Martin, A. Bendavid, Surf. Coat. Technol. 200 (14–15) (April 10 2006) 4213. Jyh-Wei Lee and Yue-Chyuan Chang, “Study on the microstructures and mechanical properties of pulsed DC reactive magnetron sputtered Cr–Si–N nanocomposite coatings,” Surface and Coatings Technology, In Press, Accepted Manuscript, 24 May 2007. Ping Zhang, Zhihai Cai, Wanquan Xiong, Surf. Coat. Technol. 201 (15) (April 23 2007) 6819.
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[31] Harish C. Barshilia, B. Deepthi, K.S. Rajam, Vacuum 81 (4) (November 6 2006) 479. [32] David Rafaja, Christina Wüstefeld, Milan Dopita, Milan Růžička, Volker Klemm, Gerhard Schreiber, Dietrich Heger and Michal Šíma, “Internal structure of clusters of partially coherent nanocrystallites in Cr–Al–N and Cr–Al–Si–N coatings,” Surf. Coat. Technol., In Press, Corrected Proof, Available online 11 April 2007. [33] J. Musil, H. Zeman, J. Kasl, Thin Solid Films 413 (1–2) (June 24 2002) 121. [34] T. Fu, Z.F. Zhou, K.Y. Li, Y.G. Shen, Surf. Coat. Technol. 200 (7) (December 21 2005) 2525. [35] E. Ribeiro, A. Malczyk, S. Carvalho, L. Rebouta, J.V. Fernandes, E. Alves, A.S. Miranda, Surf. Coat. Technol. 151–152 (March 1 2002) 515. [36] M.G. Faga, G. Gautier, R. Calzavarini, M. Perucca, E. Aimo Boot, F. Cartasegna and L. Settineri, “AlSiTiN nanocomposite coatings developed via Arc Cathodic PVD: Evaluation of wear resistance via tribological analysis and high speed machining operations,” Wear, In Press, Corrected Proof, Available online 23 May 2007. [37] Harish C. Barshilia, B. Deepthi and K.S. Rajam, “Deposition and characterization of CrN/Si3N4 and CrAlN/Si3N4 nanocomposite coatings prepared using reactive DC unbalanced magnetron sputtering,” Surf. Coat. Technol., In Press, Corrected Proof, Available online 11 April 2007. [38] J. Musil, R. Daniel, Surf. Coat. Technol. 166 (2–3) (March 24 2003) 243. [39] S. Carvalho, L. Rebouta, E. Ribeiro, F. Vaz, M.F. Denannot, J. Pacaud, J.P. Rivière, F. Paumier, R.J. Gaboriaud, E. Alves, Surf. Coat. Technol. 177–178 (January 30 2004) 369. [40] Ph.V. Kiryukhantsev-Korneev, D.V. Shtansky, M.I. Petrzhik, E.A. Levashov, B.N. Mavrin, Surf. Coat. Technol. 201 (13) (March 26 2007) 6143. [41] P.J. Martin, A. Bendavid, J.M. Cairney, M. Hoffman, Surf. Coat. Technol. 200 (7) (December 21 2005) 2228. [42] J.L. He, C.F. Chen, M.H. Hon, Wear 181–183 (1995) 189. [43] S. Carvalho, E. Ribeiro, L. Rebouta, C. Tavares, J.P. Mendonça, A. Caetano Monteiro, N.J.M. Carvalho, J.Th.M. De Hosson, A. Cavaleiro, Surf. Coat. Technol. 177–178 (January 30 2004) 459. [44] C.H. Zhang, X.C. Lu, H. Wang, J.B. Luo, Y.G. Shen, K.Y. Li, Appl. Surf. Sci. 252 (18) (July 15 2006) 6141. [45] R. Wei, E. Langa, C. Rincon, J. Arps, Surf. Coat. Technol. 201 (2006) 4453. [46] R. Wei, E. Langa, C. Rincon, J. Arps, in: Mark J. Jackson (Ed.), Surface Engineering: Proceedings of the 5th International Surface Engineering Congress (May 15–17, 2006, Seattle, Washington, USA), vol. 44073–0002, ASM International, Materials Park, Ohio, 2006, p. 78. [47] R. Wei, E. Langa, J. Arps, Q. Yang, L. Zhao, Plasma Process Polym. 4 (S1) (April 2007) S693. [48] R. Wei, “Plasma Enhanced Magnetron Sputter (PEMS) Deposition of Thick Nanocomposite Coatings for Erosion Protection,” in “Nanocomposite Coatings and Nanocomposite Material,” editors W. Ahmed and Prof. N. Ali, Trans Tech Publisher, in press. [49] D.-H. Huo, K.-W. Huang, Thin Solid Films 394 (2001) 72. [50] D.-H. Huo, K.-W. Huang, Thin Solid Films 394 (2001) 81. [51] D.-H. Huo, W.-Chieh Liao, Thin Solid Films 419 (2002) 11. [52] D. Ma, S. Ma, H. Dong, K. Xu, T. Bell, Thin Solid Films 494 (2006) 438.
[53] Y. Guo, S. Ma, K. Xu, Surf. Coat. Technol. 201 (9–11) (February 26 2007) 5240. [54] P. Jedrzejowski, J.E. Klemberg-Sapieha, L. Martinu, Surf. Coat. Technol. 188–189 (November-December 2004) 371. [55] P. Jedrzejowski, J.E. Klemberg-Sapieha, L. Martinu, Thin Solid Films 466 (1–2) (November 1 2004) 189. [56] S.R. Choi, I-.W. Park, J.H. Park, K.H. Kim, Surf. Coat. Technol. 179 (2004) 89. [57] Jun-Ha Jeon, Sung Ryong Choi, Won Sub Chung, Kwang Ho Kim, Surf. Coat. Technol. 188–189 (November-December 2004) 415. [58] A. Bendavid, P.J. Martin, E.W. Preston, J. Cairney, Z.H. Xie, M. Hoffman, Surf. Coat. Technol. 201 (7) (December 20 2006) 4139. [59] Sam Zhang, Deen Sun, Yongqing Fu, Hejun Du, Surf. Coat. Technol. 167 (2–3) (April 22 2003) 113. [60] D. Levchuk, Surf. Coat. Technol. 201 (2007) 6071. [61] J. Musil, Surf. Coat. Technol. 100–101 (March 1998) 280. [62] Z.G. Li, M. Mori, S. Miyake, M. Kumagai, H. Saito, Y. Muramatsu, Surf. Coat. Technol. 193 (1–3) (April 1 2005) 345. [63] H.C. Barshilia, B. Deepthi, A.S. Arun Prabhu, K.S. Rajam, Surf. Coat. Technol. 201 (1–2) (September 12 2006) 329. [64] G.S. Kim, B.S. Kim, S.Y. Lee, J.H. Hahn, Thin Solid Films 506–507 (May 26 2006) 128. [65] Sam Zhang, Deen Sun, Yongqing Fu, Hejun Du, Qing Zhang, Diamond Relat. Mater. 13 (10) (October 2004) 1777. [66] M. Nose, Y. Deguchi, T. Mae, E. Honbo, T. Nagae, K. Nogi, Surf. Coat. Technol. 174– 175 (2003) 261. [67] L. Rebouta, C.J. Tavares, R. Aimo, Z. Wang, K. Pischow, E. Elves, T.C. Rojas, J.A. Odriozola, Surf. Coat. Technol. 133–134 (2000) 234. [68] J.N. Matossian, R. Wei, J. Vajo, G. Hunt, M. Gardos, G. Chambers, L. Soucy, D. Oliver, L. Jay, C.M. Tylor, G. Alderson, R. Komanduri, A. Perry, Surf. Coat. Technol. 108–109 (1998) 496. [69] S.V. Fortuna, Y.P. Sharkeev, A.P. Perry, J.N. Matossian, A. Shuleopov, Thin Solid Films 377–378 (2000) 512. [70] R. Wei, J.J. Vajo, J.N. Matossian, M.N. Gardos, Surf. Coat. Technol. 158–159 (2002) 465. [71] R. Wei, “Plasma Enhanced Magnetron Sputtering Deposition of Superhard, Nanocomposite Coatings,” in Plasma Surface Engineering and its Practical Applications, ed. by Ronghua Wei, in press, Research Signpost Publisher. [72] J. Musil, M. Jirout, Surf. Coat. Technol. 201 (9–11) (February 26 2007) 5148. [73] J. Musil, F. Kunc, H. Zeman, H. Polakova, Surf. Coat. Technol. 154 (2002) 304. [74] P.C. Jindal, Dennis T. Quinto, George J. Wolfe, Thin Solid Films 154 (1987) 361. [75] W.-D. Munz, et al., J. Vac. Sci. Technol. A 11 (5) (Sep/Oct 1993) 2583. [76] W. Heinke, A. Leyland, A. Matthews, G. Berg, C. Friedrich, E. Broszeit, Thin Solid Films 270 (1995) 431. [77] W. Tabakoff, Surf. Coat. Technol. 39–40 (1989) 97. [78] R.J.K. Wood, Mater. Des. 20 (1999) 179. [79] V. Shanov, W. Tabakoff, J.A. Gunaraj, Surf. Coat. Technol. 94–95 (1997) 64. [80] R.V. Hillery, J. Vac. Sci. Technol. A4 (6) (1986) 2624. [81] K. Haugen, O. Kvernvold, A. Ronold, R. Sandberg, Wear 186–187 (1995) 179.
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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 e v i e r. c o m / l o c a t e / s u r f c o a t
Plasma enhanced magnetron sputter deposition of Ti–Si–C–N based nanocomposite coatings Ronghua Wei ⁎ Southwest Research Institute, 6220 Culebra Road, San Antonio, Texas 78238-5166, USA
A R T I C L E
I N F O
Available online 22 May 2008 Keywords: Nitrides Nanocomposite Thick coatings Erosion resistance Turbine blades
A B S T R A C T This paper reviews a Plasma Enhanced Magnetron Sputtering (PEMS) technology and a series of studies of Ti– Si–C–N nanocomposite coatings deposited using this technique. The PEMS technology is discussed briefly and compared with conventional magnetron sputter deposition. In PEMS, an electron source is used to generate plasma in addition to the magnetron plasma for both ion cleaning of the surfaces before deposition and enhanced ion bombardment during the deposition. The coatings thus obtained are very dense with nearly no columnar structure. Using this technology, many Ti–Si–C–N coatings have been prepared. These coatings are found to be nanocomposite coatings composed of nanocrystalline TiCN with grain sizes of 4.7– 100 nm in a matrix of amorphous SiCxNy. Their hardness ranges from 25 to 43 GPa and has a Si concentration up to about 3.5 at.%. They exhibit erosion resistance over 100 times better than the uncoated Ti–6Al–4V substrate. It has been observed that the toughness of the coating, or the resistance to crack formation and propagation, evaluated using H3/E⁎2, correlates well with the coating erosion resistance. The relationship among the processing parameters and the coating microstructure, nanohardness, adhesion, and erosion resistance are discussed. In addition, single and multilayered coatings have also been compared. These coatings are intended to be used against severe sand erosion for gas turbine compressor blades and vanes. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Nanocomposite coatings, especially the Ti–Si–N based coatings, have been studied extensively due to their unique properties of extreme high hardness and high oxidation resistance. The early work may date back to 1992 when Li Shizhi et al. published their study of Ti– Si–N coatings using plasma enhanced chemical vapor deposition (PECVD) [1]. They observed that the films with a Si concentration near 10–15 at.% exhibited very high hardness — over 60 GPa, and concluded that the coatings were multiphased composites consisting of crystalline TiN and amorphous Si3N4. Later, Veprek et al. conducted extensive studies and observed that the hardness of some coatings could even be over 100 GPa [2–15]. They further explained that the superhardness of these coatings was the result of the nanocrystalline TiN in the amorphous phased Si3N4. The structure is now commonly written as nc-TiN/a-Si3N4. Musil et al. observed that a composite coating consisting of a hard ceramic phase such as MeN (metal nitride) and a soft metal also had high hardness [16–24]. Other groups have also studied the Ti–Si–N system and other ternary, quaternary, or even pentanary systems [25–41]. The high hardness of these nanocom-
⁎ Southwest Research Institute®, 6220 Culebra Road, San Antonio, Texas 78238-5166, USA. Tel.: +1 210 522 5204; fax: +1 210 522 6220. E-mail address: [email protected]. 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.05.019
posite coatings has already demonstrated superb properties for tribological applications such as in high speed, dry cutting and drilling [42–44] and on aero engine turbine blades against severe erosion [45–48]. PECVD is a commonly used process in preparing the nanocomposite coatings, [1–15,42,49–55]. On the other hand, various PVD processes have also been developed including ion beam assisted deposition [30], magnetron sputtering [16–27,29,31–35,37–41,43–48], cathodic arc deposition [28,36,58] and hybrid deposition using both magnetron sputtering and arc evaporation [56,57]. Recent reviews on various deposition technologies can be found in refs. [59,60]. Compared with PECVD, PVD has many advantages such as the low deposition temperature, which allows the processing of low temperature substrate materials. Besides, PVD is a commonly used technique for depositing TiN-based hard coatings for the cutting tool industry, which can easily adopt the Ti–Si–N coating process. Within the PVD family, magnetron sputtering is probably the most commonly used technique in the nanocomposite coating research, and more literature can be found in Refs. [61–67]. This paper discusses a particular technique – Plasma Enhanced Magnetron Sputtering (PEMS) and reviews the research and development of a special type of nanocomposite coating – Ti–Si–C–N. The principle and the advantage of the PEMS will be discussed. Furthermore, the microstructural properties and micro-nanomechanical properties will be presented. Finally, the tribological performance, particularly the erosion resistance, will also be reviewed.
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2. Plasma enhanced magnetron sputtering (PEMS) 2.1. Principle of PEMS The PEMS process is based on the magnetron sputtering process, but an electron source such as a hot filament, heated by an AC power supply, is introduced to generate electrons. With a DC power supply (discharge) applied between the filament and the chamber wall, the electrons are accelerated to the wall. Because of the presence of the gas (Ar) in the vacuum chamber, electron-neutral collisions occur, thereby resulting in ionization and hence generating plasma. Detailed descriptions of the PEMS process can be found in Refs. [45–48,68–70]. 2.2. Comparison of PEMS and conventional magnetron sputter deposition The major difference between PEMS and conventional magnetron sputtering (CMS) is the introduction of the filament-generated plasma (FGP), which can also be generated using other electron sources. There are a few advantages using the FGP. First, the FGP is produced in the large volume defined by the entire vacuum chamber; therefore a very high current density can be obtained, and up to 25 times the current density was measured. In addition, the FGP is independent of the magnetron-generated plasma (MGP). The high current density obtained from the FGP can be used during both the ion cleaning stage and the sputter deposition stage. During the ion cleaning, the magnetrons are not turned on and the MGP is not needed because the FGP is sufficient to clean the oxide from the sample surface. In CMS, magnetrons may have to be turned on to provide the system with plasma for the ion cleaning. When the magnetrons are on, the target materials are inevitably sputtered and can be deposited on the parts before they are even cleaned. Therefore, in the PEMS process surface cleanliness is guaranteed using the FGP alone. During the deposition stage the FGP remains for enhanced ion bombardment to densify the
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coating. The advantages of the PEMS process can be understood from Fig. 1, which shows Al films prepared using the CMS process (Fig. 1 a) and the PEMS process (Fig. 1 b) under identical processing parameters except the FGP. It is clear that high ion current density bombardment substantially densifies the film. Nearly identical results have also been observed on Cr films [48]. In comparison with the columnar structure produced by CMS, very dense Cr films have been obtained using the PEMS technique. The advantages of the PEMS process has been further demonstrated on cutting tools including end mills, gear shaper cutters and gear hobs. A reduction of two to five times in land wear, flank wear, and nose wear, compared with the tools coated using conventional PVD techniques, has been observed [68–70]. Because of the advantages of the PEMS technique over the CMS technique, we choose to use the PEMS for developing the Ti–Si–C–N coatings. 3. Ti–Si–C–N based Nanocomposite coatings prepared using the PEMS process The primary reasons for developing the Ti–Si–C–N based nanocomposite coatings over the Ti–Si–N coatings are the simplicity of the deposition process and the uniformity of the coating composition. In CMS, there may be three methods to deposit nc-TiN/a-Si3N4 nanocomposite coatings and each one has its own limitations. In the first method, Ti and Si are sputtered from separated targets. In this case, it is very difficult to obtain homogeneous coating composition in a large vacuum chamber. In addition, compared to Ti the sputtering rate of Si is usually low because it is a semiconductor. Besides, the rate control for Si is difficult especially in the nitrogen environment. In the second method, composite targets of Ti and Si can be used. This will overcome the coating composition uniformity issue, but many targets with various compositions of Ti and Si have to be made to achieve the desired composition. In addition, even if the ideal composition is selected, the cost of composite targets is very high, particularly for production. In the third method, Ti can be sputtered from Ti targets while Si is obtained from silane (SiH4) or silicon tetrachloride (SiCl4). This will resolve the uniformity issue and the composite target cost issue, but SiH4 is undesirable because it is a pyrophoric gas and requires special gas handling equipment for safe use and training for the operators. Even though it is commonly used in semiconductor industry or in research environments, it is rarely used for commercial tribological coatings. As for SiCl4, it is corrosive to the vacuum system. In our studies, Ti is sputtered from Ti targets, while Si comes from trimethylsilane ((CH3)3SiH or TMS) precursor. This method is similar to the last one described above, but the advantage of using TMS is significant. TMS is flammable but not pyrophoric and can be handled in a similar way as many other flammable gases such as acetylene (C2H2) and methane (CH4). It is not corrosive to the vacuum chamber and the pumps. However, TMS contains carbon and the coatings are Ti–Si–C–N based coatings. What is needed is to understand whether this kind of coating performs in a similar manner as the Ti–Si–N coatings. Therefore, the research has been focused on understanding the Ti–Si–C–N coatings produced using the PEMS technology by sputtering Ti in the environment of N2 and TMS. If the microstructure, hardness and tribological performance of these Ti–Si–C–N coatings are better than, or even only similar to, those of the Ti–Si–N coatings, it has been proven that the PEMS technology together with the use of the TMS to form the Ti–Si–C– N coatings is advantageous, particularly for large-scaled production. 4. Microstructure of the Ti–Si–C–N coatings
Fig. 1.
Using the PEMS process, numerous Ti–Si–C–N coatings have been studied. In this review paper, we will highlight the characterization results from a series of studies [45–48,71] conducted using various techniques including SEM, EDAX, XRD and erosion test. Shown in Fig. 2 are the SEM morphological and cross-sectional images of a Ti–Si–C–N coating. In general, the coatings are dense and
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both Si and C concentrations increase, as expected, but the N concentration decreases. On the other hand, when N2 flow rate increases, the N concentration in the coating increases as expected, while the concentration for both Si and C barely changes. The typical concentration for N is from 30–40% while for Ti it is from 50–40%. For the best performed coatings in erosion test, the Si concentration is from 1–3.5%. The detailed discuss on the processing parameters and the composition of the Ti–Si–C–N coatings can be found from Refs. [48,71]. Based on the EDAX analysis, there is an access amount of SiCN that cannot be observed and accounted for in the crystalline phases in the XRD analysis. Therefore, as in the study of Ti–Si–N films where there is an amorphous phase of Si3N4 (or a-Si3N4), it is concluded that the Ti– Si–C–N coatings has an amorphous phase of SiCxNy (or a-SiCxNy). 5. Micro and nanomechanical properties of the Ti–Si–C–N coatings
Fig. 2.
Microhardness measurement using Vicker's indenter at 50–100 g loads shows that the hardness of the Ti–Si–C–N coatings is from 2500 to 4300 kgf/mm2, depending on the deposition conditions (ion energy, ion current density, N2 flow rate and TMS flow rate). The microhardness data for selected samples are also shown in Table 1. The nanohardness and the modulus of elasticity have been measured for the Ti–Si–C–N coatings. From the measurement, the reduced modulus (E⁎ = E / (1 − ν2) where ν is the Poisson ratio = 0.3), plastic deformation energy (Wp), elastic deformation energy (We) and elastic recovery (Re, elastic deformation/total deformation) can be obtained. Most importantly, the value H3 / E⁎2, which represents the toughness of a coating or resistance to crack formation and propagation, can also be calculated. The detailed discussion on these parameters can be found in Refs. [20,72,73]. The data for selected
featureless. The microstructure is determined by the processing parameters including the flow rate of N2, the flow rate of TMS, the ion current density, and the ion energy. In general, the higher the values for these four processing parameters, the better the microstructure of the coating (dense without the columnar structure). Coating thickness also influences the microstructure. Thicker coatings tend to be rougher with more inclusions and defects. However, extremely thick coatings have been obtained and the microstructure looks reasonably well. Shown in Fig. 3 are the morphological (Fig. 3 a) and cross-sectional (Fig. 3 b) SEM images of a Ti–Si–C–N coating over 500 µm thick. Detailed relationships between the microstructure and the processing parameters can be found from Refs. [48,71]. Shown in Fig. 4 a is the typical XRD data for most Ti–Si–C–N coatings. As can be seen, only TiCN can be recognized, which can be further indentified as TiC0.3N0.7. It is noted that the peak locations for TiCN and TiN are very close at low angles, but a careful examination at high angles will enable us to separate TiCN from TiN. For a few coatings, in addition to the TiCN phase, other peaks can be observed including TiN with the rare appearance of Ti4N3, as shown in Fig. 4 b. The average gain size can be estimated using the predominant peak. Most good performing coatings (in erosion and microstructure) have the (200) preferred orientation as shown in Fig. 4 a and this peak was used to estimate the grain size. For all coatings we have studied, the grain size varies from 4–100 nm. Some selected samples with various measurement results are listed in Table 1, in which the samples with the grain size up to 30.2 nm are listed. For the coatings that perform well in erosion tests, which will be discussed in a later section, the grain size is about 4–7 nm. The composition of the coatings, measured using EDAX, shows a typical Si concentration of 0.5–3.5 at.%. The Si concentration for selected samples is shown in Table 1. Typically, the C concentration (not shown) is about 10–20%. With the increase of the TMS flow rate,
Fig. 3.
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Fig. 4.
samples are listed in Table 1. Typically the nanohardness of these coatings is from 15–35 GPa, while H3 / E⁎2 is from 0.05 to 0.25 GPa. The value H3 / E⁎2 has been found to correlate well with the erosion resistance, which will be discussed in a later section. 6. Coating adhesion ranking and microstructural ranking Rockwell C scale indentation is a simple method that has been widely accepted in evaluating coating adhesion [74–76]. Together
with the SEM observation of the cracks, the coating adhesion can be ranked from “1” for the best (showing nearly no cracks anywhere around the indentation) to “6” for the worst (showing the coating complete spallation). Using a similar method, the microstructure of the coatings can also be ranked, with “1” being the best (dense, few inclusions, less columnar structure, etc), and “4” being the worst (loose, many inclusions, clear columnar structure, etc) [48,71]. The rankings for adhesion and microstructure for selected samples are shown in Table 1. Using these values, coatings can be selected
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Table 1 Sample number
Erosion rate ave (x10−4 mm3/g)
XRD peaks
Grain sizes⁎ (nm)
Micro hardness (HV)
Nano hardness (GPa)
H3 / E⁎2 (GPa)
Si (at.%)
Micro-structure rank
Rc adhesion rank
Ti–6AI–4V SN1 SN2 SN3 SN5 SN6 SN7 SN8
312.80 218.04 118.11 103.36 61.26 17.05 9.79 8.51
– TiN TiCN TiCN TiN TiCN TiCN TiCN, TiN, SiC
– 30.2 11.8 7.1 10.2 5.5 7.2 4.8
– 2620.5 4110.0 2274.5 4014.8 2923.3 3234.9 2806.2
– – – 21.8 18.7 32.6 25.5 29.4
– – – 0.068 0.064 0.206 0.157 0.216
– 0.73 0.55 1.90 2.24 1.74 0.94 3.50
– 4 4 2 2 2 1 2
– 2 1 1 1 1 4 2
⁎Grain size was estimated using the predominant peak for each coating and the Jade 3.1 software by Material Data, Inc.
according to their hardness, adhesion, microstructure and erosion resistance for a specific application. 7. Erosion resistance of the Ti–Si–C–N coatings The intended application of the Ti–Si–C–N coatings is for gas turbine compressor blades where severe erosion occurs. The erosion properties of the coatings have been evaluated using a sand erosion tester at two incident angles of 30° and 90° using 50 µm Al2O3 powder. The incident particle velocity was about 14 m/s. The erosion rate (volume loss of the coating/gram of erodent consumed) for selected nanocomposite coated samples (SN1-SN9) together with the uncoated Ti–6Al–4V is shown in Table 1 and also plotted in Fig. 5. It should be mentioned that for a few samples, the erosion rate at 90° was basically zero. The micro-balance accuracy (0.01 mg) was used for their total mass loss, which corresponds to an erosion rate of 1.7 × 10− 4 mm3/g. From Fig. 5, it is noted that for uncoated Ti–6Al–4V the erosion rate is higher at the low incident angle of 30° than that at high angle of 90°. In contrast, for the Ti–Si–C–N coated samples, SN1, SN2, and SN4, the erosion rate is lower at the low incident angle of 30°. These trends are consistent with the theoretical predictions and experimental observations that the erosion rate of ductile materials peaks at about 15–30°, while the erosion rate of brittle materials peaks at 90° [77–81]. As for the best coatings (SN5, SN6 and SN7), the trend is reversed and the erosion rate at 90° is basically zero. It is clear that the best coatings reduce the erosion rate by 100–200 times over the uncoated substrate material Ti–6Al–4V.
the toughness (H3 / E⁎2 ~ 0.2), the better the erosion resistance of the coating. In contrast, for the poor performing coatings the toughness is low (H3 / E⁎2 ~ 0.06). The effect of Si content listed in Column 8 on the erosion resistance cannot be recognized clearly, although a high Si concentration seems to result in either low erosion or high erosion [71]. As for the micronanostructure (Column 9), the best performing coatings are all ranked high, either “1” or “2”, implying that these coatings are very dense. In contrast, the worst performing coatings are ranked very low, mostly “4”, implying that these coatings are porous and full of defects. However, the Rc adhesion ranking is nearly reversed, as shown in the last column of the table. For the best performing coatings, the adhesion is generally not as good as the worst performing coatings, which are ranked either “1” or “2”. This is possibly due to the higher internal stress. In general, dense coatings are more susceptible to indentation cracking than the porous coatings. Considering the erosion resistance, adhesion and microstructure, SN6 and SN8 are the best compromise and generally used in our depositions of components. 8.2. Multilayer coatings Multilayer coatings are commonly used for thick coatings to reduce the internal stress. It is believed the multilayered structure can reduce erosion by absorbing the solid particle impact energy. In order to further optimize the coatings, a comparison study was carried out in which a single layered coating and two multilayered coatings were
8. Discussion 8.1. Single layer coatings The measurement methods for selected samples listed in Table 1 have been discussed above. Now we shall discuss the meanings of the results. Listed in Column 2 is the average erosion rate, ranked from the highest to the lowest, based on which all the data are arranged. The erosion data can be divided into three groups from the highest erosion rate (SN1) to medium erosion rate (SN2-SN4) and to the lowest erosion rate (SN5-SN7). Listed in Column 3 are the XRD major peaks (mainly TiCN and TiN) as discussed previously. The coating grain size (Column 4), estimated from 4.8–30 nm seems to have a strong effect on the erosion resistance. For the best performing coatings, the grain size is about 4.7–7 nm. When the grain size is too big (20–30 nm) the coating loses its erosion resistance. For a few coatings the average grain size is over 100 nm (not shown in this paper), the erosion rate is even worse. The microhardness (Column 5) seems to have no effect on the coating performance, as is evident that a few samples show the “super” hardness (over 4000) but high erosion rate. As for the nanohardness (Column 6), it should be noted for samples SN1 and SN2, their nancohardness was not available due to their loose structures. For the best performing coatings, the nanohardness is from 25 to 33 GPa. However, it is clear the toughness (H3 / E⁎2) listed in Column 7 has a strong effect on the erosion rate. In general, the higher
Fig. 5.
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and propagation. The erosion resistance can be 100 times higher than the uncoated Ti–6Al–4V. The microstructure, adhesion and erosion resistance, all determined by the processing parameters, have been correlated and ranked for the selection of a coating for specific applications. In some cases, compromise has to be made in selecting the “best” coatings. In addition, single layered coatings outperform multilayered coatings in erosion, but multilayered coatings are generally more resistant to Rc indentation cracking and spallation. The Ti–Si–C–N coatings outperform TiN coatings. Although the PEMS technology that utilizes heavy ion bombardment and TMS precursor to form the Ti–Si–C–N coatings seems to be advantageous for commercial production of hard coatings, large scale tests using actual blades have yet to be conducted to prove the process. Some droplets are sometimes still observed in the coatings and need to be eliminated since they can become the site for fatigue debt. In addition to the erosion applications, these Ti–Si–C–N coatings may also be used for cutting applications and a comprehensive study is needed. Fig. 6.
deposited and erosion tested. The coating parameters (N2 flow rate, TMS flow rate, ion energy and ion current density) remained constant, but in depositing the soft layer (Ti) for the multilayer coatings, both N2 and TMS were turned off. For the two multilayer coatings, both had the hard/soft/hard structure, but the hard layer for sample Multi1 was twice as thick as sample Multi2, while the soft coating Ti thickness remained the same. To compare the erosion resistance, all samples were deposited for about the same duration; therefore, Multi1 had fewer layers but the hard layers were thicker. Finally, the uppermost layer on the top was always the hard layer for both Multi1 and Multi2, as for the conventional multilayer coatings. Shown in Fig. 6 is the erosion rate obtained in the same way as discussed previously. For comparison purposes, the erosion rate for uncoated Ti–6Al–4V is also given. It is noted that the data for SN7 and SN8 are the same as in Table 1 and Fig. 5, while SN7-R and SN8-R represent the repeated test data (new samples deposited using the same parameters as SN7 and SN8), while Multi1 and Multi2 are the corresponding multilayered coatings. In order to compare with the Ti–Si–C–N nanocomposite coatings, one set of samples was coated with only TiN (no TMS) and the data for the single layer and multilayers are also shown in Fig. 6. From the data, first we note that the repeat data are close to the original data, implying the reproducibility of the process. Second, the single layered coatings show lower erosion than the multilayer coatings, while all Multi1 coatings (fewer number of hard layers, but thicker for each one) show lower erosion than all Multi2 coatings (more number of hard layers but thinner for each one). This may be understood in the following. The total thickness for all coatings is about the same, but the single-layered coating has the thickest hard layer, followed by the Multi1 and then Multi2. Therefore, the hard coating is responsible for the high erosion resistance. This result contradicts the conventional belief that a multilayer coating can absorb the impact energy and hence reduce the erosion. It should be mentioned here that multilayered coatings indeed show better adhesion in the Rc indentation tests. Finally, all Ti–Si–C–N coatings shown in this figure outperform the TiN coatings. 9. Conclusion and outlook In this paper, we have discussed the PEMS technique and reviewed the Ti–Si–C–N nanocomposite coatings produced using this technique. The Ti–Si–C–N coatings are nanocomposite coatings with the nanocrystalline TiCN and/or TiN (grain size from 4.7–100 nm) in the amorphous matrix of SiCxNy. The total Si content is less than 3.5 at.%. The hardness can be over 40 GPa. However, the erosion resistance barely correlates with the hardness, but it is strongly determined by the value of H3 / E⁎2, the toughness or resistance to crack formation
Acknowledgment The author wishes to thank Mr. Christopher Rincon and Mr. Edward Langa for the deposition of the coatings, Mr. James Spencer and Mr. Byron Chapa for the SEM and XRD characterizations, and Dr. Qi Yang, National Research Council, Canada, for the EDS and nanoindentation analyses. The internal research funding from Southwest Research Institute made the work possible. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]
[30]
Li Shizhi, S. Yulong, P. Hongrui, Plasma Chem. Plasma Process. 12 (1992) 287. S. Veprek, S. Reiprich, Li Shizhi, Appl. Phys. Lett. 66 (20) (1995) 2640. S. Veprek, S. Reiprich, Thin Solid Films 268 (1–2) (November 1 1995) 64. S. Veprek, Surf. Coat. Technol. 97 (1–3) (December 1997) 15. S. Veprek, P. Nesladek, A. Niederhofer, F. Glatz, M. Jilek, M. Sima, Surf. Coat. Technol. 108–109 (1998) 138. S. Veprek, Thin Solid Films 317 (1998) 449. S. Veprek, P. Nesládek, A. Niederhofer, F. Glatz, Nanostruc. Mater. 10 (5) (July 1998) 679. S. Veprek, J. Vac. Sci. Technol. A17 (5) (1999) 2401. A. Niederhofer, P. Nesládek, H.-D. Männling, K. Moto, S. Veprek, M. Jílek, Surf. Coat. Technol. 120–121 (November 1999) 173. S. Veprek, A. Niederhofer, K. Moto, T. Bolom, H.-D. Mannling, P. Nesladek, G. Dollinger, A. Bergmaier, Surf. Coat. Technol. 133–134 (2000) 152. S. Veprek, A.S. Argon, Surf. Coat. Technol. 146–147 (September-October 2001) 175. S. Veprek, M. Jilek, Vacuum 67 (3–4) (September 26 2002) 443. S. Ma, J. Procházka, P. Karvánková, Q. Ma, X. Niu, X. Wang, D. Ma, K. Xu, S. Veprek, Surf. Coat. Technol. 194 (1) (April 20 2005) 143. S. Veprek, M.G.J. Veprek-Heijman, Surf. Coat. Technol. 201 (13) (March 26 2007) 6064. A.C Fischer-Cripps, P. Karvánková, S. Vepřek, Surf. Coat. Technol. 200 (18–19) (May 8 2006) 5645. J. Musil, P. Zeman, H. Hrubý, P.H. Mayrhofer, Surf. Coat. Technol. 120–121 (November 1999) 179. J. Musil, I. Leipner, M. Kolega, Surf. Coat. Technol. 115 (1) (June 15 1999) 32. P. Zeman, R. Cerstvy, P.H. Mayrhofer, C. Mitterer, J. Musil, Mater. Sci. Eng. A Struct. Mater. Prop. Microstruct. Process. 289 (1–2) (September 30 2000) 189. J. Musil, H. Poláková, Surf. Coat. Technol. 127 (1) (May 1 2000) 99. J. Musil, Surf. Coat. Technol. 125 (2000) 322. F. Regent, J. Musil, Surf. Coat. Technol. 142–144 (July 2001) 146. J. Musil, H. Hruby, P. Zeman, H. Zeman, R. Cerstvy, P.H. Mayrhofer, C. Mitterer, Surf. Coat. Technol. 142–144 (July 2001) 603. J. Musil, P. Karvánková, J. Kasl, Surf. Coat. Technol. 139 (1) (May 1 2001) 101. J. Soldán, J. Musil, Vacuum 81 (4) (November 6 2006) 531. Soo Hyun Kim, Ji Woong Jang, Sung Soo Kang, Kwang Ho Kim, J. Mater Process. Technol. 130–131 (December 20 2002) 283. D. Mercs, P. Briois, V. Demange, S. Lamy, C. Coddet, Surf. Coat. Technol. 201 (16–17) (May 21 2007) 6970. Harish C. Barshilia, B. Deepthi, A.S. Arun Prabhu, K.S. Rajam, Surf. Coat. Technol. 201 (1–2) (September 12 2006) 329. A. Winkelmann, J.M. Cairney, M.J. Hoffman, P.J Martin, A. Bendavid, Surf. Coat. Technol. 200 (14–15) (April 10 2006) 4213. Jyh-Wei Lee and Yue-Chyuan Chang, “Study on the microstructures and mechanical properties of pulsed DC reactive magnetron sputtered Cr–Si–N nanocomposite coatings,” Surface and Coatings Technology, In Press, Accepted Manuscript, 24 May 2007. Ping Zhang, Zhihai Cai, Wanquan Xiong, Surf. Coat. Technol. 201 (15) (April 23 2007) 6819.
544
R. Wei / Surface & Coatings Technology 203 (2008) 538–544
[31] Harish C. Barshilia, B. Deepthi, K.S. Rajam, Vacuum 81 (4) (November 6 2006) 479. [32] David Rafaja, Christina Wüstefeld, Milan Dopita, Milan Růžička, Volker Klemm, Gerhard Schreiber, Dietrich Heger and Michal Šíma, “Internal structure of clusters of partially coherent nanocrystallites in Cr–Al–N and Cr–Al–Si–N coatings,” Surf. Coat. Technol., In Press, Corrected Proof, Available online 11 April 2007. [33] J. Musil, H. Zeman, J. Kasl, Thin Solid Films 413 (1–2) (June 24 2002) 121. [34] T. Fu, Z.F. Zhou, K.Y. Li, Y.G. Shen, Surf. Coat. Technol. 200 (7) (December 21 2005) 2525. [35] E. Ribeiro, A. Malczyk, S. Carvalho, L. Rebouta, J.V. Fernandes, E. Alves, A.S. Miranda, Surf. Coat. Technol. 151–152 (March 1 2002) 515. [36] M.G. Faga, G. Gautier, R. Calzavarini, M. Perucca, E. Aimo Boot, F. Cartasegna and L. Settineri, “AlSiTiN nanocomposite coatings developed via Arc Cathodic PVD: Evaluation of wear resistance via tribological analysis and high speed machining operations,” Wear, In Press, Corrected Proof, Available online 23 May 2007. [37] Harish C. Barshilia, B. Deepthi and K.S. Rajam, “Deposition and characterization of CrN/Si3N4 and CrAlN/Si3N4 nanocomposite coatings prepared using reactive DC unbalanced magnetron sputtering,” Surf. Coat. Technol., In Press, Corrected Proof, Available online 11 April 2007. [38] J. Musil, R. Daniel, Surf. Coat. Technol. 166 (2–3) (March 24 2003) 243. [39] S. Carvalho, L. Rebouta, E. Ribeiro, F. Vaz, M.F. Denannot, J. Pacaud, J.P. Rivière, F. Paumier, R.J. Gaboriaud, E. Alves, Surf. Coat. Technol. 177–178 (January 30 2004) 369. [40] Ph.V. Kiryukhantsev-Korneev, D.V. Shtansky, M.I. Petrzhik, E.A. Levashov, B.N. Mavrin, Surf. Coat. Technol. 201 (13) (March 26 2007) 6143. [41] P.J. Martin, A. Bendavid, J.M. Cairney, M. Hoffman, Surf. Coat. Technol. 200 (7) (December 21 2005) 2228. [42] J.L. He, C.F. Chen, M.H. Hon, Wear 181–183 (1995) 189. [43] S. Carvalho, E. Ribeiro, L. Rebouta, C. Tavares, J.P. Mendonça, A. Caetano Monteiro, N.J.M. Carvalho, J.Th.M. De Hosson, A. Cavaleiro, Surf. Coat. Technol. 177–178 (January 30 2004) 459. [44] C.H. Zhang, X.C. Lu, H. Wang, J.B. Luo, Y.G. Shen, K.Y. Li, Appl. Surf. Sci. 252 (18) (July 15 2006) 6141. [45] R. Wei, E. Langa, C. Rincon, J. Arps, Surf. Coat. Technol. 201 (2006) 4453. [46] R. Wei, E. Langa, C. Rincon, J. Arps, in: Mark J. Jackson (Ed.), Surface Engineering: Proceedings of the 5th International Surface Engineering Congress (May 15–17, 2006, Seattle, Washington, USA), vol. 44073–0002, ASM International, Materials Park, Ohio, 2006, p. 78. [47] R. Wei, E. Langa, J. Arps, Q. Yang, L. Zhao, Plasma Process Polym. 4 (S1) (April 2007) S693. [48] R. Wei, “Plasma Enhanced Magnetron Sputter (PEMS) Deposition of Thick Nanocomposite Coatings for Erosion Protection,” in “Nanocomposite Coatings and Nanocomposite Material,” editors W. Ahmed and Prof. N. Ali, Trans Tech Publisher, in press. [49] D.-H. Huo, K.-W. Huang, Thin Solid Films 394 (2001) 72. [50] D.-H. Huo, K.-W. Huang, Thin Solid Films 394 (2001) 81. [51] D.-H. Huo, W.-Chieh Liao, Thin Solid Films 419 (2002) 11. [52] D. Ma, S. Ma, H. Dong, K. Xu, T. Bell, Thin Solid Films 494 (2006) 438.
[53] Y. Guo, S. Ma, K. Xu, Surf. Coat. Technol. 201 (9–11) (February 26 2007) 5240. [54] P. Jedrzejowski, J.E. Klemberg-Sapieha, L. Martinu, Surf. Coat. Technol. 188–189 (November-December 2004) 371. [55] P. Jedrzejowski, J.E. Klemberg-Sapieha, L. Martinu, Thin Solid Films 466 (1–2) (November 1 2004) 189. [56] S.R. Choi, I-.W. Park, J.H. Park, K.H. Kim, Surf. Coat. Technol. 179 (2004) 89. [57] Jun-Ha Jeon, Sung Ryong Choi, Won Sub Chung, Kwang Ho Kim, Surf. Coat. Technol. 188–189 (November-December 2004) 415. [58] A. Bendavid, P.J. Martin, E.W. Preston, J. Cairney, Z.H. Xie, M. Hoffman, Surf. Coat. Technol. 201 (7) (December 20 2006) 4139. [59] Sam Zhang, Deen Sun, Yongqing Fu, Hejun Du, Surf. Coat. Technol. 167 (2–3) (April 22 2003) 113. [60] D. Levchuk, Surf. Coat. Technol. 201 (2007) 6071. [61] J. Musil, Surf. Coat. Technol. 100–101 (March 1998) 280. [62] Z.G. Li, M. Mori, S. Miyake, M. Kumagai, H. Saito, Y. Muramatsu, Surf. Coat. Technol. 193 (1–3) (April 1 2005) 345. [63] H.C. Barshilia, B. Deepthi, A.S. Arun Prabhu, K.S. Rajam, Surf. Coat. Technol. 201 (1–2) (September 12 2006) 329. [64] G.S. Kim, B.S. Kim, S.Y. Lee, J.H. Hahn, Thin Solid Films 506–507 (May 26 2006) 128. [65] Sam Zhang, Deen Sun, Yongqing Fu, Hejun Du, Qing Zhang, Diamond Relat. Mater. 13 (10) (October 2004) 1777. [66] M. Nose, Y. Deguchi, T. Mae, E. Honbo, T. Nagae, K. Nogi, Surf. Coat. Technol. 174– 175 (2003) 261. [67] L. Rebouta, C.J. Tavares, R. Aimo, Z. Wang, K. Pischow, E. Elves, T.C. Rojas, J.A. Odriozola, Surf. Coat. Technol. 133–134 (2000) 234. [68] J.N. Matossian, R. Wei, J. Vajo, G. Hunt, M. Gardos, G. Chambers, L. Soucy, D. Oliver, L. Jay, C.M. Tylor, G. Alderson, R. Komanduri, A. Perry, Surf. Coat. Technol. 108–109 (1998) 496. [69] S.V. Fortuna, Y.P. Sharkeev, A.P. Perry, J.N. Matossian, A. Shuleopov, Thin Solid Films 377–378 (2000) 512. [70] R. Wei, J.J. Vajo, J.N. Matossian, M.N. Gardos, Surf. Coat. Technol. 158–159 (2002) 465. [71] R. Wei, “Plasma Enhanced Magnetron Sputtering Deposition of Superhard, Nanocomposite Coatings,” in Plasma Surface Engineering and its Practical Applications, ed. by Ronghua Wei, in press, Research Signpost Publisher. [72] J. Musil, M. Jirout, Surf. Coat. Technol. 201 (9–11) (February 26 2007) 5148. [73] J. Musil, F. Kunc, H. Zeman, H. Polakova, Surf. Coat. Technol. 154 (2002) 304. [74] P.C. Jindal, Dennis T. Quinto, George J. Wolfe, Thin Solid Films 154 (1987) 361. [75] W.-D. Munz, et al., J. Vac. Sci. Technol. A 11 (5) (Sep/Oct 1993) 2583. [76] W. Heinke, A. Leyland, A. Matthews, G. Berg, C. Friedrich, E. Broszeit, Thin Solid Films 270 (1995) 431. [77] W. Tabakoff, Surf. Coat. Technol. 39–40 (1989) 97. [78] R.J.K. Wood, Mater. Des. 20 (1999) 179. [79] V. Shanov, W. Tabakoff, J.A. Gunaraj, Surf. Coat. Technol. 94–95 (1997) 64. [80] R.V. Hillery, J. Vac. Sci. Technol. A4 (6) (1986) 2624. [81] K. Haugen, O. Kvernvold, A. Ronold, R. Sandberg, Wear 186–187 (1995) 179.