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Preparation of CVD diamond coatings on gamma titanium aluminide using MPECVD with various interlayers
Vacuum 82 (2008) 1325–1331
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Preparation of CVD diamond coatings on gamma titanium aluminide using MPECVD with various interlayers Saleh B. Abu Suilik a, *, Masayuki Ohshima a, Toshimitsu Tetsui b, Kazuhiro Hasezaki a a b
Department of Materials Science, Shimane University, Nishikawatsu-cho 1060, Matsue, Shimane 690-8504, Japan Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
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Article history: Received 14 December 2007 Received in revised form 31 March 2008 Accepted 6 April 2008
CVD diamond coatings were deposited on to g-TiAl surfaces using a microwave plasma enhanced CVD to improve wear properties and the performance of g-TiAl. Diamond coatings were directly deposited on to g-TiAl substrates and deposited on to TiC, Ti5Si3, Al2O3 þ TiO2, and Si interlayers prepared on g-TiAl substrates. The diamond coatings deposited directly on g-TiAl suffered severe delamination and cracked. Those deposited on TiC and Ti5Si3 interlayers partially delaminated, whereas those deposited on Al2O3 þ TiO2 and Si interlayers adhered well to the underlying surfaces. The diamond films obtained were characterized using scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. Raman spectra showed that polycrystalline and nanocrystalline diamond films grew on g-TiAl. Residual internal stresses of the diamond coatings deposited on interlayered-g-TiAl were estimated experimentally from Raman spectra. The coatings prepared on Al2O3 þ TiO2/g-TiAl and Si/g-TiAl showed lower residual stresses. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Gamma titanium aluminide CVD diamond coatings MPECVD Residual stresses Interlayers
1. Introduction Gamma titanium aluminides (g-TiAl) are lightweight structural materials originally designed and developed for high-temperature applications in the aerospace and automotive industries, due to their promising high-temperature properties. g-TiAl compounds are being considered for other applications including automotive engine valves [1], other engine components [2], and biomedical applications [3]. Adoption of g-TiAl in such applications is highly regarded because of the lightweight, high specific strength, good biological compatibility [4], and excellent corrosion resistance. Exploiting g-TiAl successfully in such applications requires superior wear properties and reasonable ductility. However, titanium-based materials have poor wear properties [5,6] such as poor abrasive and fretting wear resistances [7,8] and high coefficients of friction [9]. In general, wear resistance of titanium-based materials can be improved by increasing tensile strength and/or hardness, and applying hard surface treatment [7]. Moreover, the ductility and toughness of g-TiAl alloys are considerably improved through a two-phase lamellar microstructure composed of parallel lamellas of g-TiAl and a2-Ti3Al [10]. In addition, alloying using various elements combined with heat treatment can improve ductility and other properties of the bulk g-TiAl, including microstructure control and alteration into lamellar microstructure. The latter is * Corresponding author. Tel./fax: þ81 852 32 6402. E-mail address: [email protected] (S.B. Abu Suilik). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.04.053
a desired microstructure for industrial applications [10,11]. Wear properties of g-TiAl have been improved by surface engineering to combat wear. The surface engineering includes plasma carburizing coating [12], nitriding–carburizing coating [13], plasma nitriding coating [9], pre-oxidation thermal treatment [3], and acetylene plasma coating [14]. These coating methods were aimed at improving the surface tribological properties of g-TiAl. However, surface engineering adopting CVD diamond coatings can also be expected to offer outstanding wear performance. CVD diamond coatings are important as wear-resistant protective coatings for anti-wear parts and as scratch- or corrosionresistant coatings due to their superhardness, chemical inertness, high thermal conductivity [15], and low coefficient of friction. To date, studies of CVD diamond coatings on g-TiAl are very rare. Almost all the research on CVD diamond on titanium-based materials has concentrated on titanium and titanium alloys [16–21]. Applying diamond films on titanium-based materials still faces major problems in adhesion of diamond coatings, due to large differences in coefficients of thermal expansion (CTE) [16–20]. The use of interlayers between a diamond film and its underlying substrate is an effective measure to improve adhesion, minimize internal stresses, and enhance deposition of CVD diamond films [22–24]. Many types of interlayers have been adopted, including carbides [24], silicon-containing materials [24], nitrides [23,25], and Si [26,27] interlayers. In addition to application of a suitable interlayer, adoption of a simple two-step deposition approach is expected to further enhance deposition of CVD diamond films. The
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two-step deposition method is a process where deposition is made at two different experimental regimes preferentially for better nucleation density, at the first step, and for better growth of CVD diamond films at the second step [17,28]. The objectives of this work are: (i) to prepare CVD diamond coatings on g-TiAl using microwave plasma enhanced chemical vapor deposition (MPECVD) adopting the two-step deposition method; (ii) to solve the adhesion problem of diamond coatings using various interlayers on g-TiAl; and (iii) to explore new interlayer materials suitable for application on g-TiAl. The emphasis of this study is on preparation of diamond coatings and their adhesion to the underlying g-TiAl. Accordingly, CVD diamond coatings were deposited directly on to g-TiAl, and deposited on to titanium carbide (TiC), titanium silicide (Ti5Si3), alumina þ titania (Al2O3 þ TiO2), and Si interlayers. These interlayer materials were selected due to their ability to form strong bond with the g-TiAl, to prevent outward diffusion of g-TiAl during diamond deposition, and to promote diamond nucleation and growth [29]. The diamond coatings and interlayers were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), optical microscopy, and Raman spectroscopy. Residual stresses of the coatings were evaluated based on Raman spectra of the diamond films. 2. Experimental details 2.1. Materials and CVD diamond deposition
g-TiAl substrates with a chemical composition of Ti–46Al–7Nb– 0.31C at.% [30] were used in this study. The phase composition of the g-TiAl was identified by XRD pattern (Fig. 1a). g-phase TiAl was the main constituent of the substrates. Some a2-Ti3Al was also present. Microstructure of the as-received material was lamellar (Fig. 1b). Hardness of the g-TiAl was measured using a Vickers hardness tester at 200 g for 15 s. The hardness was approximately 377 HV. Specimens were cut from materials made by precision casting using the levitation casting (LEVICAST) method [31], with minimum level of impurities and defects. Impurities of the substrate material were less than 700 and 100 ppm for oxygen and nitrogen, respectively [32]. Subsequent hot isostatic pressing (HIP) was made to enhance homogeneity and defect elimination. The HIP was made at 1250 C, and 1.2 108 Pa for 2 h. This treatment also aimed at achieving lamellar microstructure and improving ductility and fracture toughness of the g-TiAl. Tabular specimens with dimensions of 10 10 2 mm3 were cut, ground using SiC emery paper down to 1500#, and polished using 1 mm Al2O3 powder and 1–2 mm diamond powder, and then ultrasonically cleaned using acetone, methanol or ethanol, and deionized water for 5 min in turn. Deposition of CVD diamond coatings was made in MPECVD system (2.45 GHz, 1 kW, and quartz chamber) with CH4–H2 precursors. Before insertion into the chamber, the substrates were ultrasonically agitated for 30 min in a diamond slurry (1.5 g of 1–2 mm diamond powder in 50 ml ethanol) to improve nucleation density of diamond. Ultrasonic cleaning of the pretreated substrates was made after the diamond pretreatment, using acetone, methanol, and deionized water for 3 min, respectively. Deposition of CVD diamond coatings was divided into two experimental sets. In the first, CVD diamond coatings were deposited directly on the g-TiAl. In the second set, CVD diamond coatings were deposited on interlayers (TiC, Ti5Si3, Al2O3 þ TiO2, and Si) which were previously prepared on g-TiAl substrates. Deposition of diamond coatings was made at the same experimental parameters for both sets. Experimental conditions used are shown in Table 1, applying the two-step deposition for better nucleation and growth of CVD diamond. The experimental parameters were selected based on a previous study [28]. Substrate temperature was measured using an optical pyrometer along with a conventional thermocouple placed beneath
Fig. 1. As-received g-TiAl substrate (a) XRD pattern and (b) bright field TEM image of the lamellar microstructure.
the substrate. Substrate temperatures given in this study are the readings of the optical pyrometer. Total gas flow rate was 75 sccm (standard cubic centimeters) for all the experiments, and substrate temperature was controlled by regulating the microwave power in the range of 270–350 W without additional heating. Various interlayers were prepared to improve adhesion and growth of the diamond coatings, and to alleviate the large difference in CTE of the diamond coatings and underlying surfaces. TiC, Ti5Si3, Al2O3 þ TiO2, and Si were prepared on g-TiAl substrates and the diamond coatings were then deposited on the interlayers. TiC and Ti5Si3 interlayers were prepared by solid carburizing and siliconizing using carbon (graphite) and silicon powders, respectively. g-TiAl substrates were inserted into a quartz ampoule containing graphite powder, and the ampoule was then put under vacuum and closed. Solid carburizing was made at 1000 C for 15 h. Similarly, siliconizing of g-TiAl substrates was made in a quartz ampoule containing Si powder under vacuum and heat treated at 1000 C for 15 h. Al2O3 þ TiO2 interlayer was formed by thermal pre-oxidation of the g-TiAl substrates at 850 C for 30 h in air. A polycrystalline Si interlayer (110 nm) was prepared on g-TiAl using a radio frequency Table 1 Experimental parameters used in deposition of CVD diamond on to g-TiAl Deposition condition
Nucleation
Growth
Substrate temperature ( C) CH4/H2 (Vol. %) Deposition time (min) Deposition pressure (Pa) Total mass flow rate (sccm) Plasma power (W)
590 4 60 980 75 270–310
600 2 60 950 75 270–320
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(RF) magnetron sputtering technique (power 200 W and growth speed 0.5 nm s1). Thicknesses of the prepared interlayers and diamond films were estimated from SEM and optical micrographs, and the sputtering parameters. Hardness of the diamond coatings was measured using a Vickers hardness tester at 50 g for 15 s. 2.2. Characterization The CVD diamond coatings and interlayers were observed using SEM (Hitachi S-3500H, 10–15 kV) and optical microscopy. Diamond quality was evaluated using Raman spectroscopy (Renishaw; system 10005 with Arþ laser line of 514.5 nm) using laser beam spot size of 0.7 mm at a power of 25 mW and measuring time of 10–60 s. The quality was assessed by studying the first-order Raman spectra. Raman peaks were analyzed using the Gaussian curve fitting technique. In addition, structures of the diamond and interlayers were identified using XRD (RIGAKU CuKa, RINT 2500 and RINT 2000). 3. Results and discussion 3.1. Direct deposition of CVD diamond on g-TiAl Diamond coatings deposited directly on g-TiAl surfaces are shown in Fig. 2. Although the coatings were continuous and had a very fine morphology, the coatings delaminated from the g-TiAl surfaces and were severely cracked. Crack propagation was observed at both low (Fig. 2a) and high magnification (Fig. 2b). Thickness of the delaminated diamond coating was 1–2 mm. Deposition of the diamond coatings was made at low substrate temperature (590–600 C), as this expected to minimize the thermal stresses in the deposited coatings [17]. However, delamination and crack propagation of the coating occurred due to the large contrast in CTE between the coating and the underlying g-TiAl. The CTEs of CVD diamond coating, g-TiAl, and interlayer materials are summarized in Table 2 (CTE information from Refs. [33–39]). Upon cooling to room temperature, the difference in CTE caused compression stresses in the diamond coating on g-TiAl, as the diamond CTE is very low [21]. This eventually leads to delamination and damage of the coating due to stress relaxation. Moreover, chemical reactions at the surface affect the interface properties, affecting adhesion strength of the diamond coatings [40]. Delamination of diamond coatings is a common problem faced during the deposition of diamond films on titanium and titanium alloys [18,20–22,40]. Raman spectrum of the diamond coating obtained through direct deposition on g-TiAl is shown in Fig. 3. Raman spectrum showed the diamond peak centered at 1334 cm1 and a broad peak centered at 1550 cm1, corresponding to disordered carbon. In addition, the Raman spectrum exhibited a relatively small peak at 1140 cm1 corresponding to nanocrystalline diamond component. Similar nanocrystalline diamond films were reported by Askari et al. [16]. The Raman spectrum was measured at a delaminated spot where the stress relaxation had already occurred (after cooling to RT, the diamond film split from the underlying g-TiAl substrate), and almost no residual stresses remained after the relaxation process. The diamond peak is centered at 1334 cm1, which refers to 2.16 GPa compressive stress as will be shown later. The thermal stresses of a diamond coating can be estimated theoretically from Eq. (1) [41]. Z TD Ed sthermal ¼ ðad aTiAl ÞdT (1) ð1 vd Þ TR Eq. (1) can be simplified to: sthermal ¼ ðEd =ð1 vd ÞÞ Da DT, where Ed ¼ Young’s modulus of diamond, vd ¼ Poisson’s ratio of diamond, ad and aTiAl are CTEs of diamond and g-TiAl, respectively. TR ¼ room temperature, and TD ¼ deposition (substrate) temperature.
Fig. 2. SEM micrographs of a diamond coating deposited directly on g-TiAl: (a) wide view, and (b) delamination and crack. Experimental conditions are given in Table 1.
According to Eq. (1) and taking Ed ¼ 1050 GPa, nd ¼ 0.07, and ad and aTiAl from Table 2, the diamond coating obtained in this section should have thermal stresses of 5.34 GPa (compression thermal stresses). Based on investigations made by Ager and Drory [41], the diamond coating obtained here should have accommodated this amount of thermal stress, but the coating failed to do so, causing delamination. This was probably due to weak sticking force of the coating and intrinsic internal stresses of the diamond films [19].
Table 2 Coefficients of thermal expansion of diamond, g-TiAl, and interlayer materials Material
Diamond [33] g-TiAl [34,35] TiC [36] Ti5Si3 [37] Al2O3 [38] TiO2 [39] Si [38]
CTE (106/K) 293 K
873 K
1 10
4 11.3 7.7 at RT to 1273 K 11 at 443–1343 K 6.7 at RT 2.1–2.8 at RT 2.8 at RT
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Fig. 3. Raman spectrum of the diamond coating deposited directly on g-TiAl.
3.2. Deposition of CVD diamond on to g-TiAl using interlayers 3.2.1. Formation of interlayers on to g-TiAl XRD surface patterns of the interlayers prepared on to g-TiAl are shown in Fig. 4. The phase compositions of the interlayers were confirmed as TiC, Ti5Si3, and Al2O3 þ TiO2. In addition, a Si interlayer was obtained on g-TiAl although Fig. 4c did not show Si pattern
most probably due to the small thickness (110 nm) of the Si interlayer. Thicknesses of the interlayers prepared were (Table 3) 4.4 mm (TiC), 5.2 mm (Ti5Si3), 1–2 mm (Al2O3 þ TiO2), and 110 nm (Si). Applying an interlayer between the diamond coating and the underlying surface is a common practice to improve bonding of diamond films to the titanium and titanium alloys [23,25]. Generally, interlayers should adhere well to the underlying surface, and should have desirable properties like low CTE to alleviate the large contrast in CTE between the coating and the substrate. The selected interlayers had good adhesion (diffusion bonding) to the g-TiAl. This was expected to improve adhesion of the diamond films. TiC and Ti5Si3 were selected based on the fact that TiC- and Sicontaining materials provide good diffusion bond with the g-TiAl, and promote nucleation and deposition of diamond [24]. Si interlayer is widely used to enhance adhesion of diamond and DLC coating with many substrate materials [26,27,42]. Likewise, Al2O3 þ TiO2 is a common product of the pre-oxidation process of g-TiAl at a controlled environment [3]. Likewise, Al2O3 þ TiO2 has low CTE (Table 2), and adhere well to the underlying g-TiAl. In this work, Al2O3 þ TiO2 is used for the first time as interlayer material between diamond coating and g-TiAl. 3.2.2. Deposition of CVD diamond on to interlayers/g-TiAl Morphology and coating coverage of CVD diamond coatings deposited on to g-TiAl with TiC, Ti5Si3, Al2O3 þ TiO2, and Si interlayers are shown in Fig. 5. Low-magnification SEM micrographs of the diamond coatings (Fig. 5a and b) showed partial delamination of diamond coatings deposited on TiC/g-TiAl and Ti5Si3/g-TiAl. The partial delamination is indicated by the white arrows in the SEM micrographs (Fig. 5a and b), whereas the lowercase letters (b and d) show adhered diamond coatings using TiC and Ti5Si3 interlayers. Moreover, diamond coatings deposited on to Al2O3 þ TiO2/g-TiAl and Si/g-TiAl adhered well to the underlying surfaces, with good coverage over large areas (Fig. 5c and d). No delamination was observed. The morphology of the grown diamond coatings was observed in higher-magnification SEM micrographs (Fig. 5a!–d!). The diamond coatings consisted of fine features with ball-like or cauliflower-like morphology. The spherical morphology prevailed because of the low deposition temperature (590–600 C) adopted in this study [28]. The average grain size of diamonds ranged from 0.66 to 2.0 mm. After deposition time of 2 h, the diamond grains almost coalesced, forming continuous coatings on interlayered-g-TiAl surfaces. The average thicknesses of the diamond coatings obtained on the interlayers are shown in Table 3. The Raman spectra of the diamond coatings obtained using interlayers are shown in Fig. 6. These spectra were measured at the adhered side of the coatings (indicated by lowercase letters b, d, f, and h in Fig. 5). All of the coatings showed diamond peaks near 1332 cm1 and broad peaks centered at 1450–1600 cm1, corresponding to the disordered carbon components coexisting in the diamond coatings. The Raman spectra of diamond coatings deposited on TiC and Ti5Si3 interlayers showed relatively weak diamond peaks compared to those grown on the Al2O3 þ TiO2 and Si interlayers. It is thought that Al2O3 þ TiO2 and Si interlayers enhanced nucleation and deposition of diamond because they
Table 3 Average thickness of the obtained diamond coatings and the interlayers
Fig. 4. XRD patterns of the interlayers prepared on g-TiAl.
Interlayer
Interlayer thickness
Diamond film thickness (mm)
TiC Ti5Si3 Al2O3 þ TiO2 Si
4.4 mm 5.2 mm 1–2 mm 110 nm
1.2 1.5 2.8 3.0
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Fig. 5. SEM micrographs of the diamond coatings deposited on various interlayers: (a and a!) TiC interlayer, (b and b!) Ti5Si3 interlayer, (c and c!) Al2O3 þ TiO2 interlayer, and (d and d!) Si interlayer.
provide suitable environment for nucleation and growth of diamond, while TiC and Ti5Si3 did not function properly. This point is supported by the fact that diamond coatings prepared on Al2O3 þ TiO2 and Si interlayers were thicker than those of TiC and
Ti5Si3 interlayers. The intensity of the diamond peaks in the Raman spectra is related to the portion, quality, and thickness of diamond components in the coatings [43,44], and improved peak intensity indicates a greater diamond component and quality. In addition,
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Fig. 6. Raman spectra of the diamond coatings deposited on g-TiAl over various interlayers.
Fig. 7. XRD patterns of diamond coatings deposited on (a) Si/g-TiAl and (b) Al2O3 þ TiO2/g-TiAl.
nanocrystalline diamonds were incorporated in the diamond film deposited on Si interlayer. The nanocrystalline diamond is evident in the Raman spectrum (Fig. 6, Si interlayer) at 1140 cm1 [16]. In addition, upward shift of the diamond peak position was observed in the diamond deposited on TiC and Ti5Si3 interlayers. The XRD patterns (Fig. 7) confirmed the structures of diamond films deposited on Al2O3 þ TiO2/g-TiAl and Si/g-TiAl. The main diamond peaks at 43.9 (111), 75.3 (220), and 91.5 (311) 2q were confirmed. Peaks from the Al2O3 þ TiO2 interlayer (Fig. 7b), and g-TiAl were also evident. The shift of the diamond peak from the unstressed peak of 1332 cm1 is a beneficial tool for estimating the residual stresses in the CVD diamond coatings [20,25,40]. The thermal stresses of diamond coatings were estimated using Eq. (1). In addition, biaxial residual stresses can be experimentally estimated from the shift of the Raman diamond peak position [41]. Eq. (2) was used to estimate biaxial residual stresses in a polycrystalline diamond film:
stresses of the diamond coatings deposited on interlayers are: diamond/TiC/g-TiAl (3.24 GPa), diamond/Ti5Si3/g-TiAl (17.28 GPa), diamond/Al2O3 þ TiO2/g-TiAl (2.16 GPa), and diamond/Si/g-TiAl (5.4 GPa). Summary of the estimated biaxial residual stresses of the diamond coatings deposited on interlayered-g-TiAl is shown in Table 4. Almost all the diamond coatings showed compression residual stresses, except for the diamond coating deposited on Al2O3 þ TiO2 interlayer, which had tension residual stresses. The difference between the calculated thermal stresses and the compression and tension internal stresses of the diamond coatings is explained in terms of intrinsic stresses of diamond coatings originating from the coalescence process and the growth of diamonds, which differ based on the interlayer applied on g-TiAl surfaces. The tension residual stresses in diamond coating deposited on Al2O3 þ TiO2/g-TiAl are thought to reflect the tension between diamond grains during growth process. A similar phenomena has been noted in many reports [45,46]. The Vickers hardness of the diamond coatings at 50 g was approximately 700–900 HV which is higher than the hardness of g-TiAl.
sestimat: ¼ 1:08ðv v0 Þ
(2)
where sestimat.: is the biaxial residual stress in a diamond film (GPa), v: Raman shift in a deposited diamond film (cm1), and v0: Raman shift of the unstressed diamond 1332 cm1. The thermal stresses in the diamond coatings should be 5.34 GPa (Eq. (1)). Likewise, the estimated biaxial residual internal Table 4 Summary of the estimated biaxial residual stresses of the diamond coatings deposited on interlayers 1 a
b
Interlayer
Dv (cm )
sestimat. (GPa)
Stress type
Delamination
TiC Ti5Si3 Al2O3 þ TiO2 Si
3 Upward 16 Upward 2 Downward 5 Upward
3.24 17.28 2.16 5.4
Compression Compression Tension Compression
Partial Partial No No
a b
Dv: Raman shift in diamond peak position. sestimat.: The estimated biaxial residual stresses.
4. Conclusions Experimental studies of CVD diamond coatings deposition on to
g-TiAl were made to develop adhered and wear-resistant coatings suitable for g-TiAl. The experimental results from CVD diamond coatings deposited directly and using TiC, Ti5Si3, Al2O3 þ TiO2, and Si interlayers show: 1. Diamond coatings were deposited directly on to g-TiAl. However, the coatings obtained delaminated from the underlying surfaces and cracked severely. 2. Diamond coatings were also deposited on TiC, Ti5Si3, Al2O3 þ TiO2, and Si interlayers applied to g-TiAl. The coatings partially delaminated from TiC/g-TiAl and Ti5Si3/g-TiAl, whereas adhesion improved considerably on Al2O3 þ TiO2/gTiAl and Si/g-TiAl.
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3. Al2O3 þ TiO2 and Si interlayers alleviated the large contrast in coefficients of thermal expansion between the diamond coatings and the underlying g-TiAl surfaces, leading to improved diamond growth and quality. 4. Residual stresses estimated based on Raman spectra showed high residual compression stresses when using Ti5Si3 interlayer. Diamond coatings deposited on TiC and Si interlayers showed relatively low residual compression stresses. Diamond coatings deposited on the Al2O3 þ TiO2 interlayer showed low-tension residual stresses. 5. Deposition of diamond coatings using Al2O3 þ TiO2 and Si interlayers improved adhesion of the coatings to the underlying g-TiAl. Applying CVD diamond coatings with strong adhesion will improve the wear properties and performance of g-TiAl.
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Preparation of CVD diamond coatings on gamma titanium aluminide using MPECVD with various interlayers Saleh B. Abu Suilik a, *, Masayuki Ohshima a, Toshimitsu Tetsui b, Kazuhiro Hasezaki a a b
Department of Materials Science, Shimane University, Nishikawatsu-cho 1060, Matsue, Shimane 690-8504, Japan Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 December 2007 Received in revised form 31 March 2008 Accepted 6 April 2008
CVD diamond coatings were deposited on to g-TiAl surfaces using a microwave plasma enhanced CVD to improve wear properties and the performance of g-TiAl. Diamond coatings were directly deposited on to g-TiAl substrates and deposited on to TiC, Ti5Si3, Al2O3 þ TiO2, and Si interlayers prepared on g-TiAl substrates. The diamond coatings deposited directly on g-TiAl suffered severe delamination and cracked. Those deposited on TiC and Ti5Si3 interlayers partially delaminated, whereas those deposited on Al2O3 þ TiO2 and Si interlayers adhered well to the underlying surfaces. The diamond films obtained were characterized using scanning electron microscopy, Raman spectroscopy, and X-ray diffraction. Raman spectra showed that polycrystalline and nanocrystalline diamond films grew on g-TiAl. Residual internal stresses of the diamond coatings deposited on interlayered-g-TiAl were estimated experimentally from Raman spectra. The coatings prepared on Al2O3 þ TiO2/g-TiAl and Si/g-TiAl showed lower residual stresses. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Gamma titanium aluminide CVD diamond coatings MPECVD Residual stresses Interlayers
1. Introduction Gamma titanium aluminides (g-TiAl) are lightweight structural materials originally designed and developed for high-temperature applications in the aerospace and automotive industries, due to their promising high-temperature properties. g-TiAl compounds are being considered for other applications including automotive engine valves [1], other engine components [2], and biomedical applications [3]. Adoption of g-TiAl in such applications is highly regarded because of the lightweight, high specific strength, good biological compatibility [4], and excellent corrosion resistance. Exploiting g-TiAl successfully in such applications requires superior wear properties and reasonable ductility. However, titanium-based materials have poor wear properties [5,6] such as poor abrasive and fretting wear resistances [7,8] and high coefficients of friction [9]. In general, wear resistance of titanium-based materials can be improved by increasing tensile strength and/or hardness, and applying hard surface treatment [7]. Moreover, the ductility and toughness of g-TiAl alloys are considerably improved through a two-phase lamellar microstructure composed of parallel lamellas of g-TiAl and a2-Ti3Al [10]. In addition, alloying using various elements combined with heat treatment can improve ductility and other properties of the bulk g-TiAl, including microstructure control and alteration into lamellar microstructure. The latter is * Corresponding author. Tel./fax: þ81 852 32 6402. E-mail address: [email protected] (S.B. Abu Suilik). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.04.053
a desired microstructure for industrial applications [10,11]. Wear properties of g-TiAl have been improved by surface engineering to combat wear. The surface engineering includes plasma carburizing coating [12], nitriding–carburizing coating [13], plasma nitriding coating [9], pre-oxidation thermal treatment [3], and acetylene plasma coating [14]. These coating methods were aimed at improving the surface tribological properties of g-TiAl. However, surface engineering adopting CVD diamond coatings can also be expected to offer outstanding wear performance. CVD diamond coatings are important as wear-resistant protective coatings for anti-wear parts and as scratch- or corrosionresistant coatings due to their superhardness, chemical inertness, high thermal conductivity [15], and low coefficient of friction. To date, studies of CVD diamond coatings on g-TiAl are very rare. Almost all the research on CVD diamond on titanium-based materials has concentrated on titanium and titanium alloys [16–21]. Applying diamond films on titanium-based materials still faces major problems in adhesion of diamond coatings, due to large differences in coefficients of thermal expansion (CTE) [16–20]. The use of interlayers between a diamond film and its underlying substrate is an effective measure to improve adhesion, minimize internal stresses, and enhance deposition of CVD diamond films [22–24]. Many types of interlayers have been adopted, including carbides [24], silicon-containing materials [24], nitrides [23,25], and Si [26,27] interlayers. In addition to application of a suitable interlayer, adoption of a simple two-step deposition approach is expected to further enhance deposition of CVD diamond films. The
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two-step deposition method is a process where deposition is made at two different experimental regimes preferentially for better nucleation density, at the first step, and for better growth of CVD diamond films at the second step [17,28]. The objectives of this work are: (i) to prepare CVD diamond coatings on g-TiAl using microwave plasma enhanced chemical vapor deposition (MPECVD) adopting the two-step deposition method; (ii) to solve the adhesion problem of diamond coatings using various interlayers on g-TiAl; and (iii) to explore new interlayer materials suitable for application on g-TiAl. The emphasis of this study is on preparation of diamond coatings and their adhesion to the underlying g-TiAl. Accordingly, CVD diamond coatings were deposited directly on to g-TiAl, and deposited on to titanium carbide (TiC), titanium silicide (Ti5Si3), alumina þ titania (Al2O3 þ TiO2), and Si interlayers. These interlayer materials were selected due to their ability to form strong bond with the g-TiAl, to prevent outward diffusion of g-TiAl during diamond deposition, and to promote diamond nucleation and growth [29]. The diamond coatings and interlayers were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), optical microscopy, and Raman spectroscopy. Residual stresses of the coatings were evaluated based on Raman spectra of the diamond films. 2. Experimental details 2.1. Materials and CVD diamond deposition
g-TiAl substrates with a chemical composition of Ti–46Al–7Nb– 0.31C at.% [30] were used in this study. The phase composition of the g-TiAl was identified by XRD pattern (Fig. 1a). g-phase TiAl was the main constituent of the substrates. Some a2-Ti3Al was also present. Microstructure of the as-received material was lamellar (Fig. 1b). Hardness of the g-TiAl was measured using a Vickers hardness tester at 200 g for 15 s. The hardness was approximately 377 HV. Specimens were cut from materials made by precision casting using the levitation casting (LEVICAST) method [31], with minimum level of impurities and defects. Impurities of the substrate material were less than 700 and 100 ppm for oxygen and nitrogen, respectively [32]. Subsequent hot isostatic pressing (HIP) was made to enhance homogeneity and defect elimination. The HIP was made at 1250 C, and 1.2 108 Pa for 2 h. This treatment also aimed at achieving lamellar microstructure and improving ductility and fracture toughness of the g-TiAl. Tabular specimens with dimensions of 10 10 2 mm3 were cut, ground using SiC emery paper down to 1500#, and polished using 1 mm Al2O3 powder and 1–2 mm diamond powder, and then ultrasonically cleaned using acetone, methanol or ethanol, and deionized water for 5 min in turn. Deposition of CVD diamond coatings was made in MPECVD system (2.45 GHz, 1 kW, and quartz chamber) with CH4–H2 precursors. Before insertion into the chamber, the substrates were ultrasonically agitated for 30 min in a diamond slurry (1.5 g of 1–2 mm diamond powder in 50 ml ethanol) to improve nucleation density of diamond. Ultrasonic cleaning of the pretreated substrates was made after the diamond pretreatment, using acetone, methanol, and deionized water for 3 min, respectively. Deposition of CVD diamond coatings was divided into two experimental sets. In the first, CVD diamond coatings were deposited directly on the g-TiAl. In the second set, CVD diamond coatings were deposited on interlayers (TiC, Ti5Si3, Al2O3 þ TiO2, and Si) which were previously prepared on g-TiAl substrates. Deposition of diamond coatings was made at the same experimental parameters for both sets. Experimental conditions used are shown in Table 1, applying the two-step deposition for better nucleation and growth of CVD diamond. The experimental parameters were selected based on a previous study [28]. Substrate temperature was measured using an optical pyrometer along with a conventional thermocouple placed beneath
Fig. 1. As-received g-TiAl substrate (a) XRD pattern and (b) bright field TEM image of the lamellar microstructure.
the substrate. Substrate temperatures given in this study are the readings of the optical pyrometer. Total gas flow rate was 75 sccm (standard cubic centimeters) for all the experiments, and substrate temperature was controlled by regulating the microwave power in the range of 270–350 W without additional heating. Various interlayers were prepared to improve adhesion and growth of the diamond coatings, and to alleviate the large difference in CTE of the diamond coatings and underlying surfaces. TiC, Ti5Si3, Al2O3 þ TiO2, and Si were prepared on g-TiAl substrates and the diamond coatings were then deposited on the interlayers. TiC and Ti5Si3 interlayers were prepared by solid carburizing and siliconizing using carbon (graphite) and silicon powders, respectively. g-TiAl substrates were inserted into a quartz ampoule containing graphite powder, and the ampoule was then put under vacuum and closed. Solid carburizing was made at 1000 C for 15 h. Similarly, siliconizing of g-TiAl substrates was made in a quartz ampoule containing Si powder under vacuum and heat treated at 1000 C for 15 h. Al2O3 þ TiO2 interlayer was formed by thermal pre-oxidation of the g-TiAl substrates at 850 C for 30 h in air. A polycrystalline Si interlayer (110 nm) was prepared on g-TiAl using a radio frequency Table 1 Experimental parameters used in deposition of CVD diamond on to g-TiAl Deposition condition
Nucleation
Growth
Substrate temperature ( C) CH4/H2 (Vol. %) Deposition time (min) Deposition pressure (Pa) Total mass flow rate (sccm) Plasma power (W)
590 4 60 980 75 270–310
600 2 60 950 75 270–320
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(RF) magnetron sputtering technique (power 200 W and growth speed 0.5 nm s1). Thicknesses of the prepared interlayers and diamond films were estimated from SEM and optical micrographs, and the sputtering parameters. Hardness of the diamond coatings was measured using a Vickers hardness tester at 50 g for 15 s. 2.2. Characterization The CVD diamond coatings and interlayers were observed using SEM (Hitachi S-3500H, 10–15 kV) and optical microscopy. Diamond quality was evaluated using Raman spectroscopy (Renishaw; system 10005 with Arþ laser line of 514.5 nm) using laser beam spot size of 0.7 mm at a power of 25 mW and measuring time of 10–60 s. The quality was assessed by studying the first-order Raman spectra. Raman peaks were analyzed using the Gaussian curve fitting technique. In addition, structures of the diamond and interlayers were identified using XRD (RIGAKU CuKa, RINT 2500 and RINT 2000). 3. Results and discussion 3.1. Direct deposition of CVD diamond on g-TiAl Diamond coatings deposited directly on g-TiAl surfaces are shown in Fig. 2. Although the coatings were continuous and had a very fine morphology, the coatings delaminated from the g-TiAl surfaces and were severely cracked. Crack propagation was observed at both low (Fig. 2a) and high magnification (Fig. 2b). Thickness of the delaminated diamond coating was 1–2 mm. Deposition of the diamond coatings was made at low substrate temperature (590–600 C), as this expected to minimize the thermal stresses in the deposited coatings [17]. However, delamination and crack propagation of the coating occurred due to the large contrast in CTE between the coating and the underlying g-TiAl. The CTEs of CVD diamond coating, g-TiAl, and interlayer materials are summarized in Table 2 (CTE information from Refs. [33–39]). Upon cooling to room temperature, the difference in CTE caused compression stresses in the diamond coating on g-TiAl, as the diamond CTE is very low [21]. This eventually leads to delamination and damage of the coating due to stress relaxation. Moreover, chemical reactions at the surface affect the interface properties, affecting adhesion strength of the diamond coatings [40]. Delamination of diamond coatings is a common problem faced during the deposition of diamond films on titanium and titanium alloys [18,20–22,40]. Raman spectrum of the diamond coating obtained through direct deposition on g-TiAl is shown in Fig. 3. Raman spectrum showed the diamond peak centered at 1334 cm1 and a broad peak centered at 1550 cm1, corresponding to disordered carbon. In addition, the Raman spectrum exhibited a relatively small peak at 1140 cm1 corresponding to nanocrystalline diamond component. Similar nanocrystalline diamond films were reported by Askari et al. [16]. The Raman spectrum was measured at a delaminated spot where the stress relaxation had already occurred (after cooling to RT, the diamond film split from the underlying g-TiAl substrate), and almost no residual stresses remained after the relaxation process. The diamond peak is centered at 1334 cm1, which refers to 2.16 GPa compressive stress as will be shown later. The thermal stresses of a diamond coating can be estimated theoretically from Eq. (1) [41]. Z TD Ed sthermal ¼ ðad aTiAl ÞdT (1) ð1 vd Þ TR Eq. (1) can be simplified to: sthermal ¼ ðEd =ð1 vd ÞÞ Da DT, where Ed ¼ Young’s modulus of diamond, vd ¼ Poisson’s ratio of diamond, ad and aTiAl are CTEs of diamond and g-TiAl, respectively. TR ¼ room temperature, and TD ¼ deposition (substrate) temperature.
Fig. 2. SEM micrographs of a diamond coating deposited directly on g-TiAl: (a) wide view, and (b) delamination and crack. Experimental conditions are given in Table 1.
According to Eq. (1) and taking Ed ¼ 1050 GPa, nd ¼ 0.07, and ad and aTiAl from Table 2, the diamond coating obtained in this section should have thermal stresses of 5.34 GPa (compression thermal stresses). Based on investigations made by Ager and Drory [41], the diamond coating obtained here should have accommodated this amount of thermal stress, but the coating failed to do so, causing delamination. This was probably due to weak sticking force of the coating and intrinsic internal stresses of the diamond films [19].
Table 2 Coefficients of thermal expansion of diamond, g-TiAl, and interlayer materials Material
Diamond [33] g-TiAl [34,35] TiC [36] Ti5Si3 [37] Al2O3 [38] TiO2 [39] Si [38]
CTE (106/K) 293 K
873 K
1 10
4 11.3 7.7 at RT to 1273 K 11 at 443–1343 K 6.7 at RT 2.1–2.8 at RT 2.8 at RT
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Fig. 3. Raman spectrum of the diamond coating deposited directly on g-TiAl.
3.2. Deposition of CVD diamond on to g-TiAl using interlayers 3.2.1. Formation of interlayers on to g-TiAl XRD surface patterns of the interlayers prepared on to g-TiAl are shown in Fig. 4. The phase compositions of the interlayers were confirmed as TiC, Ti5Si3, and Al2O3 þ TiO2. In addition, a Si interlayer was obtained on g-TiAl although Fig. 4c did not show Si pattern
most probably due to the small thickness (110 nm) of the Si interlayer. Thicknesses of the interlayers prepared were (Table 3) 4.4 mm (TiC), 5.2 mm (Ti5Si3), 1–2 mm (Al2O3 þ TiO2), and 110 nm (Si). Applying an interlayer between the diamond coating and the underlying surface is a common practice to improve bonding of diamond films to the titanium and titanium alloys [23,25]. Generally, interlayers should adhere well to the underlying surface, and should have desirable properties like low CTE to alleviate the large contrast in CTE between the coating and the substrate. The selected interlayers had good adhesion (diffusion bonding) to the g-TiAl. This was expected to improve adhesion of the diamond films. TiC and Ti5Si3 were selected based on the fact that TiC- and Sicontaining materials provide good diffusion bond with the g-TiAl, and promote nucleation and deposition of diamond [24]. Si interlayer is widely used to enhance adhesion of diamond and DLC coating with many substrate materials [26,27,42]. Likewise, Al2O3 þ TiO2 is a common product of the pre-oxidation process of g-TiAl at a controlled environment [3]. Likewise, Al2O3 þ TiO2 has low CTE (Table 2), and adhere well to the underlying g-TiAl. In this work, Al2O3 þ TiO2 is used for the first time as interlayer material between diamond coating and g-TiAl. 3.2.2. Deposition of CVD diamond on to interlayers/g-TiAl Morphology and coating coverage of CVD diamond coatings deposited on to g-TiAl with TiC, Ti5Si3, Al2O3 þ TiO2, and Si interlayers are shown in Fig. 5. Low-magnification SEM micrographs of the diamond coatings (Fig. 5a and b) showed partial delamination of diamond coatings deposited on TiC/g-TiAl and Ti5Si3/g-TiAl. The partial delamination is indicated by the white arrows in the SEM micrographs (Fig. 5a and b), whereas the lowercase letters (b and d) show adhered diamond coatings using TiC and Ti5Si3 interlayers. Moreover, diamond coatings deposited on to Al2O3 þ TiO2/g-TiAl and Si/g-TiAl adhered well to the underlying surfaces, with good coverage over large areas (Fig. 5c and d). No delamination was observed. The morphology of the grown diamond coatings was observed in higher-magnification SEM micrographs (Fig. 5a!–d!). The diamond coatings consisted of fine features with ball-like or cauliflower-like morphology. The spherical morphology prevailed because of the low deposition temperature (590–600 C) adopted in this study [28]. The average grain size of diamonds ranged from 0.66 to 2.0 mm. After deposition time of 2 h, the diamond grains almost coalesced, forming continuous coatings on interlayered-g-TiAl surfaces. The average thicknesses of the diamond coatings obtained on the interlayers are shown in Table 3. The Raman spectra of the diamond coatings obtained using interlayers are shown in Fig. 6. These spectra were measured at the adhered side of the coatings (indicated by lowercase letters b, d, f, and h in Fig. 5). All of the coatings showed diamond peaks near 1332 cm1 and broad peaks centered at 1450–1600 cm1, corresponding to the disordered carbon components coexisting in the diamond coatings. The Raman spectra of diamond coatings deposited on TiC and Ti5Si3 interlayers showed relatively weak diamond peaks compared to those grown on the Al2O3 þ TiO2 and Si interlayers. It is thought that Al2O3 þ TiO2 and Si interlayers enhanced nucleation and deposition of diamond because they
Table 3 Average thickness of the obtained diamond coatings and the interlayers
Fig. 4. XRD patterns of the interlayers prepared on g-TiAl.
Interlayer
Interlayer thickness
Diamond film thickness (mm)
TiC Ti5Si3 Al2O3 þ TiO2 Si
4.4 mm 5.2 mm 1–2 mm 110 nm
1.2 1.5 2.8 3.0
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Fig. 5. SEM micrographs of the diamond coatings deposited on various interlayers: (a and a!) TiC interlayer, (b and b!) Ti5Si3 interlayer, (c and c!) Al2O3 þ TiO2 interlayer, and (d and d!) Si interlayer.
provide suitable environment for nucleation and growth of diamond, while TiC and Ti5Si3 did not function properly. This point is supported by the fact that diamond coatings prepared on Al2O3 þ TiO2 and Si interlayers were thicker than those of TiC and
Ti5Si3 interlayers. The intensity of the diamond peaks in the Raman spectra is related to the portion, quality, and thickness of diamond components in the coatings [43,44], and improved peak intensity indicates a greater diamond component and quality. In addition,
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Fig. 6. Raman spectra of the diamond coatings deposited on g-TiAl over various interlayers.
Fig. 7. XRD patterns of diamond coatings deposited on (a) Si/g-TiAl and (b) Al2O3 þ TiO2/g-TiAl.
nanocrystalline diamonds were incorporated in the diamond film deposited on Si interlayer. The nanocrystalline diamond is evident in the Raman spectrum (Fig. 6, Si interlayer) at 1140 cm1 [16]. In addition, upward shift of the diamond peak position was observed in the diamond deposited on TiC and Ti5Si3 interlayers. The XRD patterns (Fig. 7) confirmed the structures of diamond films deposited on Al2O3 þ TiO2/g-TiAl and Si/g-TiAl. The main diamond peaks at 43.9 (111), 75.3 (220), and 91.5 (311) 2q were confirmed. Peaks from the Al2O3 þ TiO2 interlayer (Fig. 7b), and g-TiAl were also evident. The shift of the diamond peak from the unstressed peak of 1332 cm1 is a beneficial tool for estimating the residual stresses in the CVD diamond coatings [20,25,40]. The thermal stresses of diamond coatings were estimated using Eq. (1). In addition, biaxial residual stresses can be experimentally estimated from the shift of the Raman diamond peak position [41]. Eq. (2) was used to estimate biaxial residual stresses in a polycrystalline diamond film:
stresses of the diamond coatings deposited on interlayers are: diamond/TiC/g-TiAl (3.24 GPa), diamond/Ti5Si3/g-TiAl (17.28 GPa), diamond/Al2O3 þ TiO2/g-TiAl (2.16 GPa), and diamond/Si/g-TiAl (5.4 GPa). Summary of the estimated biaxial residual stresses of the diamond coatings deposited on interlayered-g-TiAl is shown in Table 4. Almost all the diamond coatings showed compression residual stresses, except for the diamond coating deposited on Al2O3 þ TiO2 interlayer, which had tension residual stresses. The difference between the calculated thermal stresses and the compression and tension internal stresses of the diamond coatings is explained in terms of intrinsic stresses of diamond coatings originating from the coalescence process and the growth of diamonds, which differ based on the interlayer applied on g-TiAl surfaces. The tension residual stresses in diamond coating deposited on Al2O3 þ TiO2/g-TiAl are thought to reflect the tension between diamond grains during growth process. A similar phenomena has been noted in many reports [45,46]. The Vickers hardness of the diamond coatings at 50 g was approximately 700–900 HV which is higher than the hardness of g-TiAl.
sestimat: ¼ 1:08ðv v0 Þ
(2)
where sestimat.: is the biaxial residual stress in a diamond film (GPa), v: Raman shift in a deposited diamond film (cm1), and v0: Raman shift of the unstressed diamond 1332 cm1. The thermal stresses in the diamond coatings should be 5.34 GPa (Eq. (1)). Likewise, the estimated biaxial residual internal Table 4 Summary of the estimated biaxial residual stresses of the diamond coatings deposited on interlayers 1 a
b
Interlayer
Dv (cm )
sestimat. (GPa)
Stress type
Delamination
TiC Ti5Si3 Al2O3 þ TiO2 Si
3 Upward 16 Upward 2 Downward 5 Upward
3.24 17.28 2.16 5.4
Compression Compression Tension Compression
Partial Partial No No
a b
Dv: Raman shift in diamond peak position. sestimat.: The estimated biaxial residual stresses.
4. Conclusions Experimental studies of CVD diamond coatings deposition on to
g-TiAl were made to develop adhered and wear-resistant coatings suitable for g-TiAl. The experimental results from CVD diamond coatings deposited directly and using TiC, Ti5Si3, Al2O3 þ TiO2, and Si interlayers show: 1. Diamond coatings were deposited directly on to g-TiAl. However, the coatings obtained delaminated from the underlying surfaces and cracked severely. 2. Diamond coatings were also deposited on TiC, Ti5Si3, Al2O3 þ TiO2, and Si interlayers applied to g-TiAl. The coatings partially delaminated from TiC/g-TiAl and Ti5Si3/g-TiAl, whereas adhesion improved considerably on Al2O3 þ TiO2/gTiAl and Si/g-TiAl.
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3. Al2O3 þ TiO2 and Si interlayers alleviated the large contrast in coefficients of thermal expansion between the diamond coatings and the underlying g-TiAl surfaces, leading to improved diamond growth and quality. 4. Residual stresses estimated based on Raman spectra showed high residual compression stresses when using Ti5Si3 interlayer. Diamond coatings deposited on TiC and Si interlayers showed relatively low residual compression stresses. Diamond coatings deposited on the Al2O3 þ TiO2 interlayer showed low-tension residual stresses. 5. Deposition of diamond coatings using Al2O3 þ TiO2 and Si interlayers improved adhesion of the coatings to the underlying g-TiAl. Applying CVD diamond coatings with strong adhesion will improve the wear properties and performance of g-TiAl.
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