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Effect of TiAl-based interlayer on the surface morphology and adhesion of nanocrystalline diamond film deposited on WC–Co substrate by hot filament CVD
Surface & Coatings Technology 258 (2014) 108–113
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Effect of TiAl-based interlayer on the surface morphology and adhesion of nanocrystalline diamond film deposited on WC–Co substrate by hot filament CVD Jong-Keuk Park ⁎, Hak-Joo Lee, Wook-Seong Lee, Young-Joon Baik Electronic Materials Center, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Sungbuk-gu, 136-791, Seoul, Republic of Korea
a r t i c l e
i n f o
Article history: Received 19 May 2014 Accepted in revised form 25 September 2014 Available online 2 October 2014 Keywords: Adhesion Nanocrystalline diamond film Surface morphology Interlayer WC–Co insert
a b s t r a c t The effect of various interlayers of TiAl, TiAl/TiAlN, and TiAl/TiAlN/TiAl on the surface morphology and adhesive strength of nanocrystalline diamond (NCD) film on WC–Co substrate was investigated in this study. NCD film was deposited on WC–6 wt.% Co substrate by hot filament CVD (HFCVD) technique with H2–5 vol.% CH4 gas mixture. In contrast to the NCD film deposited on the TiAl terminated interlayers (TiAl and TiAl/TiAlN/TiAl) showing flat surface morphology, hump structure (like cauliflower) was observed for the NCD film deposited on the TiAlN terminated interlayer (TiAl/TiAlN). The surface morphology and adhesion behavior of NCD film deposited on WC–Co substrate with various TiAl(N)-based interlayers were closely related to the nucleation density of NCD and Co diffusion from the substrate. The seed density was observed to be higher when we adopted the TiAlterminated interlayer, whereas the Co diffusion from the WC–6 wt.% Co substrate was much more retarded by the incorporation of TiAlN into the interlayer. As a result, the adhesion improvement of NCD film was noticeable for the TiAl/TiAlN/TiAl-coated WC–Co substrate. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Diamond film in mechanical parts has attracted great attention due to its low friction coefficient as well as high hardness and wear resistance [1,2]. The problem in the application of diamond as a surface coating material on steel-based substrate and WC–Co cermet is the deteriorated adhesive strength originating from transition metal such as Fe, Co, and Ni in the substrate, which promotes the formation of sp2 graphitic species. The adhesion improvement of diamond film on the WC–Co substrate has been tried in many reports [3–12] such as in the removal of Co at the surface to be coated by thermal and chemical etching [3–6] and in the incorporation of various metal- and metal nitridebased interlayers [7–12], which block the diffusion of Co from the interior to the surface of the substrate to be coated. Another problem to be considered for the application of diamond film is its roughness, which influences the surface finish of the workpiece after machining process [6]. In addition, the roughness plays an important role in anti-wear low friction components like mechanical seals and bearings. The problem usually observed in microcrystalline diamond film due to its large and facetted grain has been overcome by the development of (ultra) nanocrystalline diamond (NCD) with a grain
⁎ Corresponding author. Tel.: +82 2 958 6763; fax: +82 2 958 5509. E-mail address: [email protected] (J.-K. Park).
http://dx.doi.org/10.1016/j.surfcoat.2014.09.053 0257-8972/© 2014 Elsevier B.V. All rights reserved.
size less than 10 nm. The NCD film was successfully applied to the mechanical part demanding lower friction coefficient with graphite [13]. For the deposition of NCD film on WC–Co substrate, however, the buffer layer which suppresses Co diffusion from the substrate should be carefully designed in terms of surface roughness as well as adhesive strength because hump structure (like cauliflower) usually observed in NCD film, which is influenced by the nucleation density of sp3 phase, is not favorable to reduce the surface roughness of diamond film. In various metal nitrides, TiAlN has been used as a main protective coating material due to its excellent wear and oxidation resistance [14]. In contrast to the Ti(N) interlayer which was reported ineffective in improving the adhesive strength of diamond film on WC–Co substrate [8,10], TiAl(N) needed to be checked as an effective interlayer because Al was reported to increase the nucleation density of diamond [8]. Furthermore, the elucidation of the role of TiAl(N) interlayer on the adhesion of diamond film on WC–Co has an important meaning in terms of practical as well as scientific point of views because TiAl-based nitride coatings have been successfully adopted to increase the lifetime and performance of many types of WC–Co tools. In this study, we investigated the role of TiAl(N) containing interlayer on the deposition behavior of NCD film on WC–Co substrate. Various interlayers of TiAl, TiAl/TiAlN, and TiAl/TiAlN/TiAl were designed, and the effect of the interlayer structure on the surface morphology and adhesion of NCD film grown on WC–Co substrate was checked.
J.-K. Park et al. / Surface & Coatings Technology 258 (2014) 108–113
1000
2. Experimental details
(111)1
800
Intensity (arbit. unit)
Disk-type WC–6 wt.% Co with 2.5 cm in diameter was used as a substrate after polishing it with 0.25 μm diamond slurry. The WC–Co substrate with a grain size of 1 μm was not etched chemically or thermally to remove Co at the surface before the deposition of interlayer. TiAl and TiAlN layers were deposited on the WC–Co substrate by D.C. magnetron sputtering of TiAl target which was 5.1 cm in diameter. After it was cleaned with acetone and ethanol, the WC–Co substrate was put into a chamber. The base pressure of the chamber was less than 1.3 × 10− 4 Pa, and the substrate temperature was raised to be 300 °C before deposition process. TiAl and TiAlN layers were deposited under a chamber pressure of 0.4 Pa with Ar and Ar–64 vol.% N2 reactive gases at a substrate bias of −75 V, respectively. The target power was set to 200 W for all the deposition conditions. TiAl, TiAl/TiAlN, and TiAl/TiAlN/TiAl interlayers were prepared on the WC–Co substrate by sequential deposition of TiAl and TiAlN layers. The total thickness of interlayers was controlled to be 500 nm irrespective of the structure. The ultrasonic seeding of the WC–Co substrates with various interlayers was performed in a methanol suspension of NCD particles of 5 nm in average size. Hot-filament chemical vapor deposition (HFCVD) system was used to deposit NCD film on WC–6 wt.% Co substrates. The mixture of CH4 and H2 was used as reactant gas. The chamber pressure supplied with 100 sccm of H2–5 vol.% CH4 gas was maintained at 103 Pa. For all the deposition experiments, the substrate temperature was fixed at 770 °C. The deposition time was 1 h–5 h. The growth rate of NCD film was measured as 0.75 μm/h under the deposition condition. The surface morphology and grain size of deposited NCD film were examined by scanning electron microscopy (SEM, Hitachi S-2400). The seed density on the TiAl- and TiAl/TiAlN-coated WC–6 wt.% Co substrate after ultrasonic seeding was characterized by high-resolution scanning electron microscopy (HR-SEM, FEI Nova 200 NanoSEM HRSEM). The in-depth analysis of chemical compositions for the NCDcoated WC–Co substrates with various interlayers was measured by Auger electron spectroscopy (AES, PHI-700 & LC-TOFMS LECO). The TiAl-based interlayers, as well as the phases formed during the deposition of NCD film, were analyzed by X-ray diffractometer (Rigaku, D/ MAX-2500) with a glazing incidence angle of 2°. The Rockwell indentation apparatus (Wilson Instrument Inc.) with diamond cone was used to access the adhesion of the deposited NCD film. The load was 588 N– 1470 N. The carbon-based phases at the delaminated regions after the indentation were analyzed by Raman spectroscopy (Renishaw Invia Micro Raman Spectroscopy) with Nd:YAG laser of 532 nm wavelength.
109
s
s
s : substrate 1 : TiAl 2 : Ti3Al
s
(002)2
s
3 : Ti3Al5
(401)3
s
4 : TiAlN
(002)1
600
(201)2 (200)4
(201)1
400
(441)3
(b) 200
(a) 0 20
30
40
50
60
70
2θ (º) Fig. 1. X-ray diffraction patterns obtained from the (a) TiAl and (b) TiAl/TiAlN layers deposited on the WC–6 wt.% Co substrates.
Fig. 2. During the reaction, WC in contact with TiAl was believed to be decarburized to W. As a Co-containing phase, aluminum cobalt titanium alloy (Ti32.49Co28Al59.51) [17], which was formed by the reaction of TiAl and diffused Co from the WC–Co substrate, was detected in all the NCDcoated specimens. With the formation of NCD during deposition, Ti from the TiAl alloy was observed to transform to TiC, as indicated in Fig. 2. In comparison to the NCD film with the TiAl terminated interlayer (Fig. 2(b) and (d)), the intensity of the Ti32.49Co28Al59.51 (440) peak, which was observed at 2θ~43o, was much lower for the NCD film with the TiAlN terminated interlayer (Fig. 2(c)). Fig. 3(a), (b), and (c) show the surface morphologies of Rockwell indentations for the NCD films deposited on WC–Co substrates for 1 h with TiAl, TiAl/TiAlN, and TiAl/TiAlN/TiAl interlayers, respectively. For comparison, the surface images of NCD films before the indentations were shown together. For the NCD film deposited on the TiAlterminated interlayers, the surface morphology was observed to be flat (Fig. 3(a) and (c)), whereas the hump structure (like cauliflower) was shown for the NCD film deposited on the TiAlN terminated interlayer (Fig. 3(b)). Although the Rockwell indentation was performed with the same load of 588 N, the indentation did not initiate a delamination around the indentation for the specimen prepared with the TiAl interlayer (Fig. 3(a)), while a severe delamination occurred for the NCD film deposited with the TiAl/TiAlN interlayer (Fig. 3(b)). The NCD film *:Aluminuim Cobalt Titanium alloy (Ti32.49Co28Al59.51)
s
3. Results and discussion
# : TiC, d : diamond, W: tungsten, S : substrate
(200)W
(110)w (111)d (200)#
(220)#
s
Intensity (arbit. unit)
Fig. 1 shows the XRD patterns of TiAl and TiAl/TiAlN layers deposited on the WC–6 wt.% Co substrate. XRD peaks of TiAl-based alloy such as TiAl, TI3Al, and Ti3Al5 in addition to TiAlN were observed, which indicates that the TixAly alloy and TiAlN layers have polycrystalline structures. It was reported [15] that the TiAl metastable structure, which was prepared by DC magnetron sputtering, was stabilized to an equilibrium structure of TiAl and Ti3Al by annealing in a hydrogenated argon atmosphere. In our study, the TiAl layer prepared by magnetron sputtering of TiAl target in an argon atmosphere at 300 °C was also composed of various TixAly alloys as shown in Fig. 1. Fig. 2 shows X-ray diffraction patterns obtained from the NCDcoated WC–Co substrates with various interlayers. Tungsten (W) and TiC, in addition to diamond, were clearly observed for the NCD-coated WC–Co substrate with TiAl(N)-based interlayers. The W is believed to come from the WC in the substrate. Considering the Gibbs free energy of formation of TiC and WC at 1000 K, which are − 173.115 kJ/mol and − 35.805 kJ/mol, respectively [16], the Ti in the TiAl interlayer in contact with the WC–Co substrate is carburized to TiC with C from NCD and/or WC under the NCD deposition condition as shown in
(222)* (400)*
s
(622)*
(511)* (422)*
(800)*
(440)*
(e)
s
s
(d) (c) (b) (a) 20
30
40
50
60
70
2θ (º) Fig. 2. X-ray diffraction patterns obtained from the (a) WC–6 wt.% Co substrate and the NCD-coated WC–Co substrates (b) with TiAl, (c) TiAl/TiAlN, (d) TiAl/TiAlN/TiAl (for 1 h), and (e) TiAl/TiAlN/TiAl (for 5 h) interlayers.
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J.-K. Park et al. / Surface & Coatings Technology 258 (2014) 108–113
(a)
NCD pore
pore
TiAl/TiAlN interlayer 200 μm
1 μm
500 nm Fig. 4. Cross sectional image of the NCD film deposited on the WC–6 wt.% Co substrate with the TiAl/TiAlN interlayer for 1 h.
(b)
delamination 200 μm
1 μm
(c)
film and TiAlN-terminated interlayer clearly show that the lower heterogeneous nucleation density of NCD on TiAlN induces the hump structure (like cauliflower) shown in Fig. 3(b). Although the Cocontaining phase (Ti32.49Co28Al59.51) was observed for the NCD-coated specimens with TiAl-terminated interlayers, the NCD film was not delaminated by 1 hour-deposition, which was due to the lower Co contamination of TiAl-terminated interlayers by shorter deposition time. Fig. 5 shows the HR-SEM images of the seed densities on TiAl- and TiAl/TiAlN-coated WC–6 wt.% Co substrates after ultrasonic seeding. From the HR-SEM images of the dispersed nanodiamond seed particles, the seed densities on the TiAl- and TiAl/TiAlN-coated WC–Co substrates were characterized to be 2.6 × 108 and 1.1 × 108 nuclei/cm2, respectively. Lee et al. have reported [18,19] that the seed density on substrate after ultrasonic seeding with diamond slurry was closely related to the relative values of the respective zeta potentials of the substrates and
(a) 500 nm
200 μm
1 μm
Fig. 3. Surface morphologies of the Rockwell indentations on the NCD films deposited on the WC–6 wt.% Co substrates for 1 h with (a) TiAl, (b) TiAl/TiAlN, and (c) TiAl/TiAlN/TiAl interlayers. The indentation loads for (a), (b), and (c) were 588 N, 588 N, and 1470 N, respectively. The surface images of the NCD films before indentation were shown together.
deposited with the TiAl/TiAlN/TiAl interlayer was not delaminated even at an indentation load of 1470 N. It is well-known that the Co phase at the surface of WC–Co favors the catalytic formation of graphitic carbon under the diamond deposition environment. The Co phase can be diffused out through the interlayer, which causes the poor adhesion between NCD film and WC–Co substrate [5]. In our investigation, the Co-containing phase was detected to be an aluminum cobalt titanium alloy (Ti32.49Co28Al59.51) as clearly observed in X-ray diffraction patterns (Fig. 2). As shown in Fig. 3, however, the delamination of NCD film was observed only for the specimen prepared with the TiAlN-terminated interlayer, which shows a lower peak intensity of the Ti32.49Co28Al59.51 phase. This implies that the delamination observed in Fig. 3(b) was not due to the Co-containing phase formed at the early stage of NCD deposition. On the contrary, the lower nucleation density of NCD on TiAlN was believed to induce the hump structure (like cauliflower) as shown in Fig. 3(b), causing delamination of the NCD film even with a lower indentation load of 588 N. Fig. 4 shows the cross sectional image of the NCD film deposited on the TiAl/TiAlN interlayer. The pores observed at the interface between NCD
2 μm
(b) 500 nm
2 μm
Fig. 5. Dispersion of nanodiamond seed particles by ultrasonic treatment on the (a) TiAland (b) TiAl/TiAlN-coated WC–6 wt.% Co substrates. The seeds are indicated by white arrows.
the seed powders, which indicates the coulombic nature of the seeding process. In this respect, the difference in diamond seed density observed in the TiAl- and TiAl/TiAlN-coated WC–6 wt.% Co substrates was believed to related to the surface bonding of TiAl metal and nitrogen. The lower seed density observed in the TiAl/TiAlN-coated WC–Co substrate (Fig. 5(b)) was believed to induce the lower heterogeneous nucleation site and generate pores at the interface between NCD film and interlayer, which are believed to intensify stress in the NCD film under a lower indentation load (588 N) and cause the NCD film to delaminate, as reported previously in diamond-coated WC–Co [20]. Fig. 6(a) and (b) show the surface morphologies of Rockwell indentations for the NCD film deposited on WC–Co substrates for 5 h with TiAl and TiAl/TiAlN/TiAl interlayers, respectively. In the NCD-coated WC–Co specimen with the TiAl interlayer (Fig. 6(a)) a delamination of the NCD film was clearly observed at a load of 588 N. With the TiAl/TiAlN/TiAl interlayer (Fig. 6(b)), however, NCD film was not observed to be delaminated from the substrate even at a load of 1470 N although through-thickness cracks were formed. This clearly shows that the TiAlN, sandwiched by TiAl, effectively suppresses Co diffusion from the WC–Co substrate while the nucleation density of NCD films was increased by TiAl-terminated layer, which improved the adhesion of NCD film on the WC–Co substrate. The delaminated surface regions shown in Figs. 3(b) and 6(a) after the Rockwell indentations were checked by the Raman spectroscopy. Fig. 7(a) and (b) show the Raman spectra obtained at the delaminated regions of NCD-coated WC–6 wt.% Co substrates with (a) TiAl/TiAlN and (b) TiAl interlayers. For the NCD-coated WC–Co specimen with TiAl/TiAlN interlayer, a broad peak, which is believed to come from
(a)
delamination
200 μm
(b)
200 μm Fig. 6. Surface morphologies of the Rockwell indentations on the NCD film deposited on the WC–6 wt.% Co substrate for 5 h with (a) TiAl and (b) TiAl/TiAlN/TiAl interlayers. The indentation loads for (a) and (b) were 588 N and 1470 N, respectively.
Intensity (arbit. unit)
J.-K. Park et al. / Surface & Coatings Technology 258 (2014) 108–113
111
2D
(a)
D+G 2D'
(b)
2000
2500
3000
3500
4000
Raman shift (cm-1) Fig. 7. Raman spectra obtained at the delaminated surface regions of NCD-coated WC– 6 wt.% Co substrates with (a) TiAl/TiAlN and (b) TiAl interlayers.
carbon-based phases such as graphite [21,22] and sp2-, sp3-bonded hydro-carbons [23], was observed in the region between 2500 and 3100 cm−1. The broad peak was observed in all the NCD-coated WC– Co specimens that we have prepared, irrespective of the adhesion of NCD film coated. When the TiAl interlayer was adopted, however, clear peaks at 2697, 2940, and 3245 cm− 1 which correspond to the 2D, D + G and 2D′ peaks as reported in the carbon nanowalls [21] were observed. The peak observed at the 2697 cm−1 was also reported in carbon nanotube grown by CVD using a Co-containing catalyst [22]. The carbon-based phase detected at the delaminated region indicates that the delamination took place between interlayer and NCD film. Furthermore, the clear difference in the Raman spectra observed at the delaminated regions of the NCD-coated WC–Co specimens with TiAl/ TiAlN and TiAl interlayers shows that the delamination of the NCD film was caused by different reasons, lower seed density and graphite formation at the interface between NCD film and interlayer due to the Co diffusion, respectively, as mentioned previously. The Co diffusion and phase formation of Ti32.49Co28Al59.51 during the deposition of NCD on WC–Co with TiAl-terminated interlayers were analyzed in depth with the elemental depth profile measured by AES, which was shown in Fig. 8. For the NCD-coated WC–Co specimen prepared with the TiAl interlayer, the Co content over 27 at.% was observed to diffuse into the TiAl layer as shown in Fig. 8(a). However, the diffused Co contents in the TiAl layers in contact with NCD and WC–Co substrate were measured to be much lower (~13 at.%) when NCD film was deposited on the WC–Co substrate with the TiAl/TiAlN/TiAl interlayer. When the deposition time was increased to 5 h, the Co content in the TiAl layer in contact with WC–Co substrate was increased to 19 at.%. For the specimen, however, the Co content in the TiAl layer in contact with NCD was found unchanged (~ 13 at.%), which implied that the TiAlN layer sandwiched by upper and lower TiAl layers had an effect on the suppression of Co diffusion from WC–Co substrate during the deposition of the NCD film. Furthermore, the Ti and Al in TiAlN layer could barely react with Co to form a Ti32.49Co28Al59.51 alloy phase, as indicated by the lower Co content (~4 at.%,) in the TiAlN layer even after the deposition of NCD for 5 h. Although the carbon content in the TiAlN layer was increased to 15 at.% in the NCD coated specimen prepared with the TiAl/ TiAlN/TiAl interlayer for 5 h, still higher nitrogen content (~40 at.%) in the TiAlN layer represented that TiAlN was stable and not to be
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J.-K. Park et al. / Surface & Coatings Technology 258 (2014) 108–113
Atomic concentration (%)
100
(a)
80
C
60
40
W Ti
20
Co Al O
0 0
10
20
30
40
50
Sputter time (min)
Atomic concentration (%)
100
(b)
80
of Co, which promotes graphite (sp2) formation. As reported by Li et al. [8], the Al thin film coated on the WC–Co substrate dissolved into, or reacted with the binder phase Co during CVD process, greatly decreases the graphitization tendency during diamond deposition as in the case of Al–Fe alloy substrate. In our study, the Co binder is reacted with the TiAl interlayer and alloyed into the aluminum cobalt titanium alloy (Ti32.49Co28Al59.51) as shown in the XRD patterns of Fig. 2. The Co content in the aluminum cobalt titanium alloy, which seems comparable to, or even larger than the WC–Co substrate, is not a critical factor to be considered for the efficacy of TiAl-based interlayers if the Co is alloyed to the phase which decreases the graphitization tendency. Although the TiAlN layer itself is stable under the deposition condition of the NCD film, the two maximum Co content peaks observed at the upper and lower TiAl layers in the NCD-coated WC–Co substrate prepared with TiAl/TiAlN/TiAl interlayer imply that Co can diffuse through the grain boundary of polycrystalline TiAlN layer and form a Ti32.49Co28Al59.51 alloy phase with the TiAl-terminated layer. In this respect, the amorphous phase of metal nitride with no grain boundary can be suggested as a more effective diffusion barrier of Co from the WC–Co substrate in the deposition of the NCD film. 4. Conclusion
C
60
N Ti
40
W
Al
20
Co O 0 0
10
20
30
40
50
The effect of various TiAl-based interlayers on the surface morphology and adhesive strength of nanocrystalline diamond (NCD) film on WC–Co substrate was investigated. The incorporated TiAlN interlayer between TiAl layers improved the adhesion of the NCD film noticeably. Although the TiAlN layer was effective in suppressing Co diffusion, the TiAlN-terminated interlayer was observed to generate the surface of the NCD film hump structure (like cauliflower) due to the lower nucleation density of diamond. By the adoption of the sandwiched TiAlN interlayer by TiAl (TiAl/TiAlN/TiAl), a smooth surface, as well as high adhesive strength of the NCD film on the WC–Co substrates, was obtained.
Sputter time (min) Acknowledgment
Atomic concentration (%)
100
(c)
This work was supported by a grant from the Fundamental R&D Program for Core Technology of Materials (2MR1690), funded by the Ministry of Trade, Industry & Energy, Republic of Korea.
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References
60
C Ti
40
W
N Al
20
Co
O
0 0
10
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Sputter time (min) Fig. 8. Compositional depth profiles of NCD-coated WC–Co substrates with (a) TiAl, (b) TiAl/TiAlN/TiAl interlayers for 1 h and (c) TiAl/TiAlN/TiAl interlayer for 5 h. The profiles were measured by AES.
transformed fully to carbide phase such as TiC under the deposition condition of the NCD film. When the TiAl-based interlayer was employed the Co concentration at the diamond/substrate interface seems comparable to, or even larger than that of bulk WC–Co as shown in Fig. 8. However, the important thing to be considered is not the amount of Co, but the catalytic effect
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Effect of TiAl-based interlayer on the surface morphology and adhesion of nanocrystalline diamond film deposited on WC–Co substrate by hot filament CVD Jong-Keuk Park ⁎, Hak-Joo Lee, Wook-Seong Lee, Young-Joon Baik Electronic Materials Center, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Sungbuk-gu, 136-791, Seoul, Republic of Korea
a r t i c l e
i n f o
Article history: Received 19 May 2014 Accepted in revised form 25 September 2014 Available online 2 October 2014 Keywords: Adhesion Nanocrystalline diamond film Surface morphology Interlayer WC–Co insert
a b s t r a c t The effect of various interlayers of TiAl, TiAl/TiAlN, and TiAl/TiAlN/TiAl on the surface morphology and adhesive strength of nanocrystalline diamond (NCD) film on WC–Co substrate was investigated in this study. NCD film was deposited on WC–6 wt.% Co substrate by hot filament CVD (HFCVD) technique with H2–5 vol.% CH4 gas mixture. In contrast to the NCD film deposited on the TiAl terminated interlayers (TiAl and TiAl/TiAlN/TiAl) showing flat surface morphology, hump structure (like cauliflower) was observed for the NCD film deposited on the TiAlN terminated interlayer (TiAl/TiAlN). The surface morphology and adhesion behavior of NCD film deposited on WC–Co substrate with various TiAl(N)-based interlayers were closely related to the nucleation density of NCD and Co diffusion from the substrate. The seed density was observed to be higher when we adopted the TiAlterminated interlayer, whereas the Co diffusion from the WC–6 wt.% Co substrate was much more retarded by the incorporation of TiAlN into the interlayer. As a result, the adhesion improvement of NCD film was noticeable for the TiAl/TiAlN/TiAl-coated WC–Co substrate. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Diamond film in mechanical parts has attracted great attention due to its low friction coefficient as well as high hardness and wear resistance [1,2]. The problem in the application of diamond as a surface coating material on steel-based substrate and WC–Co cermet is the deteriorated adhesive strength originating from transition metal such as Fe, Co, and Ni in the substrate, which promotes the formation of sp2 graphitic species. The adhesion improvement of diamond film on the WC–Co substrate has been tried in many reports [3–12] such as in the removal of Co at the surface to be coated by thermal and chemical etching [3–6] and in the incorporation of various metal- and metal nitridebased interlayers [7–12], which block the diffusion of Co from the interior to the surface of the substrate to be coated. Another problem to be considered for the application of diamond film is its roughness, which influences the surface finish of the workpiece after machining process [6]. In addition, the roughness plays an important role in anti-wear low friction components like mechanical seals and bearings. The problem usually observed in microcrystalline diamond film due to its large and facetted grain has been overcome by the development of (ultra) nanocrystalline diamond (NCD) with a grain
⁎ Corresponding author. Tel.: +82 2 958 6763; fax: +82 2 958 5509. E-mail address: [email protected] (J.-K. Park).
http://dx.doi.org/10.1016/j.surfcoat.2014.09.053 0257-8972/© 2014 Elsevier B.V. All rights reserved.
size less than 10 nm. The NCD film was successfully applied to the mechanical part demanding lower friction coefficient with graphite [13]. For the deposition of NCD film on WC–Co substrate, however, the buffer layer which suppresses Co diffusion from the substrate should be carefully designed in terms of surface roughness as well as adhesive strength because hump structure (like cauliflower) usually observed in NCD film, which is influenced by the nucleation density of sp3 phase, is not favorable to reduce the surface roughness of diamond film. In various metal nitrides, TiAlN has been used as a main protective coating material due to its excellent wear and oxidation resistance [14]. In contrast to the Ti(N) interlayer which was reported ineffective in improving the adhesive strength of diamond film on WC–Co substrate [8,10], TiAl(N) needed to be checked as an effective interlayer because Al was reported to increase the nucleation density of diamond [8]. Furthermore, the elucidation of the role of TiAl(N) interlayer on the adhesion of diamond film on WC–Co has an important meaning in terms of practical as well as scientific point of views because TiAl-based nitride coatings have been successfully adopted to increase the lifetime and performance of many types of WC–Co tools. In this study, we investigated the role of TiAl(N) containing interlayer on the deposition behavior of NCD film on WC–Co substrate. Various interlayers of TiAl, TiAl/TiAlN, and TiAl/TiAlN/TiAl were designed, and the effect of the interlayer structure on the surface morphology and adhesion of NCD film grown on WC–Co substrate was checked.
J.-K. Park et al. / Surface & Coatings Technology 258 (2014) 108–113
1000
2. Experimental details
(111)1
800
Intensity (arbit. unit)
Disk-type WC–6 wt.% Co with 2.5 cm in diameter was used as a substrate after polishing it with 0.25 μm diamond slurry. The WC–Co substrate with a grain size of 1 μm was not etched chemically or thermally to remove Co at the surface before the deposition of interlayer. TiAl and TiAlN layers were deposited on the WC–Co substrate by D.C. magnetron sputtering of TiAl target which was 5.1 cm in diameter. After it was cleaned with acetone and ethanol, the WC–Co substrate was put into a chamber. The base pressure of the chamber was less than 1.3 × 10− 4 Pa, and the substrate temperature was raised to be 300 °C before deposition process. TiAl and TiAlN layers were deposited under a chamber pressure of 0.4 Pa with Ar and Ar–64 vol.% N2 reactive gases at a substrate bias of −75 V, respectively. The target power was set to 200 W for all the deposition conditions. TiAl, TiAl/TiAlN, and TiAl/TiAlN/TiAl interlayers were prepared on the WC–Co substrate by sequential deposition of TiAl and TiAlN layers. The total thickness of interlayers was controlled to be 500 nm irrespective of the structure. The ultrasonic seeding of the WC–Co substrates with various interlayers was performed in a methanol suspension of NCD particles of 5 nm in average size. Hot-filament chemical vapor deposition (HFCVD) system was used to deposit NCD film on WC–6 wt.% Co substrates. The mixture of CH4 and H2 was used as reactant gas. The chamber pressure supplied with 100 sccm of H2–5 vol.% CH4 gas was maintained at 103 Pa. For all the deposition experiments, the substrate temperature was fixed at 770 °C. The deposition time was 1 h–5 h. The growth rate of NCD film was measured as 0.75 μm/h under the deposition condition. The surface morphology and grain size of deposited NCD film were examined by scanning electron microscopy (SEM, Hitachi S-2400). The seed density on the TiAl- and TiAl/TiAlN-coated WC–6 wt.% Co substrate after ultrasonic seeding was characterized by high-resolution scanning electron microscopy (HR-SEM, FEI Nova 200 NanoSEM HRSEM). The in-depth analysis of chemical compositions for the NCDcoated WC–Co substrates with various interlayers was measured by Auger electron spectroscopy (AES, PHI-700 & LC-TOFMS LECO). The TiAl-based interlayers, as well as the phases formed during the deposition of NCD film, were analyzed by X-ray diffractometer (Rigaku, D/ MAX-2500) with a glazing incidence angle of 2°. The Rockwell indentation apparatus (Wilson Instrument Inc.) with diamond cone was used to access the adhesion of the deposited NCD film. The load was 588 N– 1470 N. The carbon-based phases at the delaminated regions after the indentation were analyzed by Raman spectroscopy (Renishaw Invia Micro Raman Spectroscopy) with Nd:YAG laser of 532 nm wavelength.
109
s
s
s : substrate 1 : TiAl 2 : Ti3Al
s
(002)2
s
3 : Ti3Al5
(401)3
s
4 : TiAlN
(002)1
600
(201)2 (200)4
(201)1
400
(441)3
(b) 200
(a) 0 20
30
40
50
60
70
2θ (º) Fig. 1. X-ray diffraction patterns obtained from the (a) TiAl and (b) TiAl/TiAlN layers deposited on the WC–6 wt.% Co substrates.
Fig. 2. During the reaction, WC in contact with TiAl was believed to be decarburized to W. As a Co-containing phase, aluminum cobalt titanium alloy (Ti32.49Co28Al59.51) [17], which was formed by the reaction of TiAl and diffused Co from the WC–Co substrate, was detected in all the NCDcoated specimens. With the formation of NCD during deposition, Ti from the TiAl alloy was observed to transform to TiC, as indicated in Fig. 2. In comparison to the NCD film with the TiAl terminated interlayer (Fig. 2(b) and (d)), the intensity of the Ti32.49Co28Al59.51 (440) peak, which was observed at 2θ~43o, was much lower for the NCD film with the TiAlN terminated interlayer (Fig. 2(c)). Fig. 3(a), (b), and (c) show the surface morphologies of Rockwell indentations for the NCD films deposited on WC–Co substrates for 1 h with TiAl, TiAl/TiAlN, and TiAl/TiAlN/TiAl interlayers, respectively. For comparison, the surface images of NCD films before the indentations were shown together. For the NCD film deposited on the TiAlterminated interlayers, the surface morphology was observed to be flat (Fig. 3(a) and (c)), whereas the hump structure (like cauliflower) was shown for the NCD film deposited on the TiAlN terminated interlayer (Fig. 3(b)). Although the Rockwell indentation was performed with the same load of 588 N, the indentation did not initiate a delamination around the indentation for the specimen prepared with the TiAl interlayer (Fig. 3(a)), while a severe delamination occurred for the NCD film deposited with the TiAl/TiAlN interlayer (Fig. 3(b)). The NCD film *:Aluminuim Cobalt Titanium alloy (Ti32.49Co28Al59.51)
s
3. Results and discussion
# : TiC, d : diamond, W: tungsten, S : substrate
(200)W
(110)w (111)d (200)#
(220)#
s
Intensity (arbit. unit)
Fig. 1 shows the XRD patterns of TiAl and TiAl/TiAlN layers deposited on the WC–6 wt.% Co substrate. XRD peaks of TiAl-based alloy such as TiAl, TI3Al, and Ti3Al5 in addition to TiAlN were observed, which indicates that the TixAly alloy and TiAlN layers have polycrystalline structures. It was reported [15] that the TiAl metastable structure, which was prepared by DC magnetron sputtering, was stabilized to an equilibrium structure of TiAl and Ti3Al by annealing in a hydrogenated argon atmosphere. In our study, the TiAl layer prepared by magnetron sputtering of TiAl target in an argon atmosphere at 300 °C was also composed of various TixAly alloys as shown in Fig. 1. Fig. 2 shows X-ray diffraction patterns obtained from the NCDcoated WC–Co substrates with various interlayers. Tungsten (W) and TiC, in addition to diamond, were clearly observed for the NCD-coated WC–Co substrate with TiAl(N)-based interlayers. The W is believed to come from the WC in the substrate. Considering the Gibbs free energy of formation of TiC and WC at 1000 K, which are − 173.115 kJ/mol and − 35.805 kJ/mol, respectively [16], the Ti in the TiAl interlayer in contact with the WC–Co substrate is carburized to TiC with C from NCD and/or WC under the NCD deposition condition as shown in
(222)* (400)*
s
(622)*
(511)* (422)*
(800)*
(440)*
(e)
s
s
(d) (c) (b) (a) 20
30
40
50
60
70
2θ (º) Fig. 2. X-ray diffraction patterns obtained from the (a) WC–6 wt.% Co substrate and the NCD-coated WC–Co substrates (b) with TiAl, (c) TiAl/TiAlN, (d) TiAl/TiAlN/TiAl (for 1 h), and (e) TiAl/TiAlN/TiAl (for 5 h) interlayers.
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(a)
NCD pore
pore
TiAl/TiAlN interlayer 200 μm
1 μm
500 nm Fig. 4. Cross sectional image of the NCD film deposited on the WC–6 wt.% Co substrate with the TiAl/TiAlN interlayer for 1 h.
(b)
delamination 200 μm
1 μm
(c)
film and TiAlN-terminated interlayer clearly show that the lower heterogeneous nucleation density of NCD on TiAlN induces the hump structure (like cauliflower) shown in Fig. 3(b). Although the Cocontaining phase (Ti32.49Co28Al59.51) was observed for the NCD-coated specimens with TiAl-terminated interlayers, the NCD film was not delaminated by 1 hour-deposition, which was due to the lower Co contamination of TiAl-terminated interlayers by shorter deposition time. Fig. 5 shows the HR-SEM images of the seed densities on TiAl- and TiAl/TiAlN-coated WC–6 wt.% Co substrates after ultrasonic seeding. From the HR-SEM images of the dispersed nanodiamond seed particles, the seed densities on the TiAl- and TiAl/TiAlN-coated WC–Co substrates were characterized to be 2.6 × 108 and 1.1 × 108 nuclei/cm2, respectively. Lee et al. have reported [18,19] that the seed density on substrate after ultrasonic seeding with diamond slurry was closely related to the relative values of the respective zeta potentials of the substrates and
(a) 500 nm
200 μm
1 μm
Fig. 3. Surface morphologies of the Rockwell indentations on the NCD films deposited on the WC–6 wt.% Co substrates for 1 h with (a) TiAl, (b) TiAl/TiAlN, and (c) TiAl/TiAlN/TiAl interlayers. The indentation loads for (a), (b), and (c) were 588 N, 588 N, and 1470 N, respectively. The surface images of the NCD films before indentation were shown together.
deposited with the TiAl/TiAlN/TiAl interlayer was not delaminated even at an indentation load of 1470 N. It is well-known that the Co phase at the surface of WC–Co favors the catalytic formation of graphitic carbon under the diamond deposition environment. The Co phase can be diffused out through the interlayer, which causes the poor adhesion between NCD film and WC–Co substrate [5]. In our investigation, the Co-containing phase was detected to be an aluminum cobalt titanium alloy (Ti32.49Co28Al59.51) as clearly observed in X-ray diffraction patterns (Fig. 2). As shown in Fig. 3, however, the delamination of NCD film was observed only for the specimen prepared with the TiAlN-terminated interlayer, which shows a lower peak intensity of the Ti32.49Co28Al59.51 phase. This implies that the delamination observed in Fig. 3(b) was not due to the Co-containing phase formed at the early stage of NCD deposition. On the contrary, the lower nucleation density of NCD on TiAlN was believed to induce the hump structure (like cauliflower) as shown in Fig. 3(b), causing delamination of the NCD film even with a lower indentation load of 588 N. Fig. 4 shows the cross sectional image of the NCD film deposited on the TiAl/TiAlN interlayer. The pores observed at the interface between NCD
2 μm
(b) 500 nm
2 μm
Fig. 5. Dispersion of nanodiamond seed particles by ultrasonic treatment on the (a) TiAland (b) TiAl/TiAlN-coated WC–6 wt.% Co substrates. The seeds are indicated by white arrows.
the seed powders, which indicates the coulombic nature of the seeding process. In this respect, the difference in diamond seed density observed in the TiAl- and TiAl/TiAlN-coated WC–6 wt.% Co substrates was believed to related to the surface bonding of TiAl metal and nitrogen. The lower seed density observed in the TiAl/TiAlN-coated WC–Co substrate (Fig. 5(b)) was believed to induce the lower heterogeneous nucleation site and generate pores at the interface between NCD film and interlayer, which are believed to intensify stress in the NCD film under a lower indentation load (588 N) and cause the NCD film to delaminate, as reported previously in diamond-coated WC–Co [20]. Fig. 6(a) and (b) show the surface morphologies of Rockwell indentations for the NCD film deposited on WC–Co substrates for 5 h with TiAl and TiAl/TiAlN/TiAl interlayers, respectively. In the NCD-coated WC–Co specimen with the TiAl interlayer (Fig. 6(a)) a delamination of the NCD film was clearly observed at a load of 588 N. With the TiAl/TiAlN/TiAl interlayer (Fig. 6(b)), however, NCD film was not observed to be delaminated from the substrate even at a load of 1470 N although through-thickness cracks were formed. This clearly shows that the TiAlN, sandwiched by TiAl, effectively suppresses Co diffusion from the WC–Co substrate while the nucleation density of NCD films was increased by TiAl-terminated layer, which improved the adhesion of NCD film on the WC–Co substrate. The delaminated surface regions shown in Figs. 3(b) and 6(a) after the Rockwell indentations were checked by the Raman spectroscopy. Fig. 7(a) and (b) show the Raman spectra obtained at the delaminated regions of NCD-coated WC–6 wt.% Co substrates with (a) TiAl/TiAlN and (b) TiAl interlayers. For the NCD-coated WC–Co specimen with TiAl/TiAlN interlayer, a broad peak, which is believed to come from
(a)
delamination
200 μm
(b)
200 μm Fig. 6. Surface morphologies of the Rockwell indentations on the NCD film deposited on the WC–6 wt.% Co substrate for 5 h with (a) TiAl and (b) TiAl/TiAlN/TiAl interlayers. The indentation loads for (a) and (b) were 588 N and 1470 N, respectively.
Intensity (arbit. unit)
J.-K. Park et al. / Surface & Coatings Technology 258 (2014) 108–113
111
2D
(a)
D+G 2D'
(b)
2000
2500
3000
3500
4000
Raman shift (cm-1) Fig. 7. Raman spectra obtained at the delaminated surface regions of NCD-coated WC– 6 wt.% Co substrates with (a) TiAl/TiAlN and (b) TiAl interlayers.
carbon-based phases such as graphite [21,22] and sp2-, sp3-bonded hydro-carbons [23], was observed in the region between 2500 and 3100 cm−1. The broad peak was observed in all the NCD-coated WC– Co specimens that we have prepared, irrespective of the adhesion of NCD film coated. When the TiAl interlayer was adopted, however, clear peaks at 2697, 2940, and 3245 cm− 1 which correspond to the 2D, D + G and 2D′ peaks as reported in the carbon nanowalls [21] were observed. The peak observed at the 2697 cm−1 was also reported in carbon nanotube grown by CVD using a Co-containing catalyst [22]. The carbon-based phase detected at the delaminated region indicates that the delamination took place between interlayer and NCD film. Furthermore, the clear difference in the Raman spectra observed at the delaminated regions of the NCD-coated WC–Co specimens with TiAl/ TiAlN and TiAl interlayers shows that the delamination of the NCD film was caused by different reasons, lower seed density and graphite formation at the interface between NCD film and interlayer due to the Co diffusion, respectively, as mentioned previously. The Co diffusion and phase formation of Ti32.49Co28Al59.51 during the deposition of NCD on WC–Co with TiAl-terminated interlayers were analyzed in depth with the elemental depth profile measured by AES, which was shown in Fig. 8. For the NCD-coated WC–Co specimen prepared with the TiAl interlayer, the Co content over 27 at.% was observed to diffuse into the TiAl layer as shown in Fig. 8(a). However, the diffused Co contents in the TiAl layers in contact with NCD and WC–Co substrate were measured to be much lower (~13 at.%) when NCD film was deposited on the WC–Co substrate with the TiAl/TiAlN/TiAl interlayer. When the deposition time was increased to 5 h, the Co content in the TiAl layer in contact with WC–Co substrate was increased to 19 at.%. For the specimen, however, the Co content in the TiAl layer in contact with NCD was found unchanged (~ 13 at.%), which implied that the TiAlN layer sandwiched by upper and lower TiAl layers had an effect on the suppression of Co diffusion from WC–Co substrate during the deposition of the NCD film. Furthermore, the Ti and Al in TiAlN layer could barely react with Co to form a Ti32.49Co28Al59.51 alloy phase, as indicated by the lower Co content (~4 at.%,) in the TiAlN layer even after the deposition of NCD for 5 h. Although the carbon content in the TiAlN layer was increased to 15 at.% in the NCD coated specimen prepared with the TiAl/ TiAlN/TiAl interlayer for 5 h, still higher nitrogen content (~40 at.%) in the TiAlN layer represented that TiAlN was stable and not to be
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Atomic concentration (%)
100
(a)
80
C
60
40
W Ti
20
Co Al O
0 0
10
20
30
40
50
Sputter time (min)
Atomic concentration (%)
100
(b)
80
of Co, which promotes graphite (sp2) formation. As reported by Li et al. [8], the Al thin film coated on the WC–Co substrate dissolved into, or reacted with the binder phase Co during CVD process, greatly decreases the graphitization tendency during diamond deposition as in the case of Al–Fe alloy substrate. In our study, the Co binder is reacted with the TiAl interlayer and alloyed into the aluminum cobalt titanium alloy (Ti32.49Co28Al59.51) as shown in the XRD patterns of Fig. 2. The Co content in the aluminum cobalt titanium alloy, which seems comparable to, or even larger than the WC–Co substrate, is not a critical factor to be considered for the efficacy of TiAl-based interlayers if the Co is alloyed to the phase which decreases the graphitization tendency. Although the TiAlN layer itself is stable under the deposition condition of the NCD film, the two maximum Co content peaks observed at the upper and lower TiAl layers in the NCD-coated WC–Co substrate prepared with TiAl/TiAlN/TiAl interlayer imply that Co can diffuse through the grain boundary of polycrystalline TiAlN layer and form a Ti32.49Co28Al59.51 alloy phase with the TiAl-terminated layer. In this respect, the amorphous phase of metal nitride with no grain boundary can be suggested as a more effective diffusion barrier of Co from the WC–Co substrate in the deposition of the NCD film. 4. Conclusion
C
60
N Ti
40
W
Al
20
Co O 0 0
10
20
30
40
50
The effect of various TiAl-based interlayers on the surface morphology and adhesive strength of nanocrystalline diamond (NCD) film on WC–Co substrate was investigated. The incorporated TiAlN interlayer between TiAl layers improved the adhesion of the NCD film noticeably. Although the TiAlN layer was effective in suppressing Co diffusion, the TiAlN-terminated interlayer was observed to generate the surface of the NCD film hump structure (like cauliflower) due to the lower nucleation density of diamond. By the adoption of the sandwiched TiAlN interlayer by TiAl (TiAl/TiAlN/TiAl), a smooth surface, as well as high adhesive strength of the NCD film on the WC–Co substrates, was obtained.
Sputter time (min) Acknowledgment
Atomic concentration (%)
100
(c)
This work was supported by a grant from the Fundamental R&D Program for Core Technology of Materials (2MR1690), funded by the Ministry of Trade, Industry & Energy, Republic of Korea.
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References
60
C Ti
40
W
N Al
20
Co
O
0 0
10
20
30
40
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Sputter time (min) Fig. 8. Compositional depth profiles of NCD-coated WC–Co substrates with (a) TiAl, (b) TiAl/TiAlN/TiAl interlayers for 1 h and (c) TiAl/TiAlN/TiAl interlayer for 5 h. The profiles were measured by AES.
transformed fully to carbide phase such as TiC under the deposition condition of the NCD film. When the TiAl-based interlayer was employed the Co concentration at the diamond/substrate interface seems comparable to, or even larger than that of bulk WC–Co as shown in Fig. 8. However, the important thing to be considered is not the amount of Co, but the catalytic effect
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