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Tribological and mechanical properties of HFCVD diamond-coated WC-Co substrates with different Cr interlayers
Surface & Coatings Technology 203 (2008) 704–708
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Tribological and mechanical properties of HFCVD diamond-coated WC-Co substrates with different Cr interlayers Chau-Chang Chou a,⁎, Jyh-Wei Lee b, Yen-I Chen a a b
Department of Mechanical & Mechatronic Engineering, National Taiwan Ocean University, No.2, Pei-Ning Rd., Keelung 20224, Taiwan, ROC Department of Mechanical Engineering, Tungnan University, No. 152, Sec.2, Pei-Shen Road, Shen-Ken, Taipei 222, Taiwan, ROC
a r t i c l e
i n f o
Available online 4 September 2008 Keywords: Chemented tungsten carbide Diamond Chromized interlayer PVD CVD
a b s t r a c t Chromium interlayers between WC-Co substrates and diamond coating by hot filament chemical vapor deposition (HFCVD) process were produced by two different techniques: using pack chromization and bipolar symmetry pulsed DC reactive magnetron sputtering processes. Both chromium surfaces were found partially transformed into carbide compound layers by energy dispersive X-ray spectrometer (EDS) and X-ray diffractometer (XRD). Daimler–Benz Rockwell-C indentation tests were conducted to evaluate the adhesion properties of diamond coatings with interlayers created by these two techniques. Delamination outside the indentation zones was also observed by a scanning electron microscopy (SEM). Wear resistance of both coatings were investigated by pin-on-disk wear tests. Rather low wear volumes revealed their excellent anti-wear behavior. Good diamond adhesion on chromized interlayers has been attributed to chromium carbide formation on pack chromized film surfaces as well as on PVD ones during the HFCVD process. It is concluded that the adhesion properties and tribological performance of CVD diamond coatings are significantly improved by the proposed two chromium interlayers. However, chromium interlayer produced by pack chromization is more beneficial to the adhesion of diamond coating than that, by PVD process. One reason is the former interlayer's role as a diffusion barrier of cobalt binder, the other is the low cost of the facilities for the pack chromization technique. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The cemented tungsten carbides are used widely in the cutting, drilling and molding tools. Diamond coating on these tools can improve their performance as well as their serving lives. Along with great wear resistance, the advantages include high surface hardness, low friction, high thermal conductivity and better corrosion protection [1]. The existence of cobalt binder in cemented diamond carbide suppresses diamond growth during high temperature deposition process and thus induced poor adhesion of diamond coating. Various approaches have been reported to reduce the catalyst effect of cobalt on the growth of diamond films. Removing the cobalt binder from the substrate by acid etching is usually conducted to solve this problem. Its drawback is the significant reduction of the substrate's mechanical properties and toughness. Besides, the large residue stress
⁎ Corresponding author. E-mail address: [email protected] (C.-C. Chou). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.08.055
will be raised due to the mismatch of the thermal expansion. Diffusion barriers seem to be a better way to prevent the Co catalyst effect but the adhesion of the diamond coating still needs to be improved. In this study, chromium interlayers between WCCo substrates and diamond surface coatings were produced by two techniques: pack chromization and PVD processes. Chromization processes can be implemented by pack [2,3], salt bath [4] , fluidized bed [5], and chemical vapor deposition methods [6]. Chromization is usually divided into two categories: soft chromization and hard chromization [7]. The former is used on the alloys with less than 0.1 wt.% carbon to improve their corrosion and high temperature oxidation properties [7–10]. While hard chromization process, on the alloys with carbon concentration higher than 0.3 wt.%, is applied to achieve improved mechanical properties, including higher hardness [5,7,11–14] and good adhesion properties [14]. One of the authors has conducted a systematic investigation on the microstructures and mechanical properties of steels with different carbon contents [15]. In these works it was found that layers of chromium carbide formed during chromization process are responsible for the improved mechanical properties. However, forming chromium carbide interlayer on tungsten carbides by hard chromization process
C.-C. Chou et al. / Surface & Coatings Technology 203 (2008) 704–708
705
2. Experimental procedure
Fig. 1. Surface morphology of WC-Co substrate deposited with PVD-Cr interlayer: (a) SEM image; (b) EDS of the cobalt particle on the surface.
has never been studied in the literatures. On the other hand, Crbased films deposited by PVD deposition on copper substrates [16] and steel substrates [17–19] were studied. Investigation on these Cr-based interlayers between diamond film and WC-Co substrates was first reported by Polini et al. [20]. They found that chromium carbide formation on PVD film surfaces during the CVD process was also beneficial to diamond adhesion. The aim of this work is to study and compare the effect of Cr-base interlayers produced by pack chromization and PVD techniques on the microstructure, the adhesion, and the wear resistance of the diamond-coated WC-Co tools.
Cemented tungsten carbide samples, WC-6 wt.% Co, were used as substrates. Specimens with 15 × 15 × 1 mm3 dimensions were ground and polished up to 0.03 µm (Ra), washed in distilled water, ultrasonically degreased in acetone and rinsed in alcohol. A supersonic-assisted etching process by means of a 1:1 HNO3/H2O solution was optionally applied for 10 min to study the effect of cobalt element in the substrate's surface. Cr-based interlayers were prepared by two processes. They are bipolar symmetry pulsed DC reactive magnetron sputtering process, a PVD process, and pack chromization process, respectively. By the PVD plant, a 0.5 µm Cr thin film was deposited onto WC-Co samples. On the other hand, pack cementation for chromizing was conducted in a double steel can which was designed by Meier [2] at 950 °C for 4 h. The powder mixture for chromizing contained 30 wt.% ferrochromium powder (71 wt.% Cr, 0.03 wt.% C and balanced Fe), 2.5 wt.% ammonia chloride activator and 67.5 wt.% filler (alumina powder). Argon protection was employed to prevent oxidation of chromium. To enhance diamond's nucleation, specimens were seeded by sinking the WCCo substrates in a diamond acetone suspension and preprocessed by a supersonic vibrator for 60 min. The diamond powder with 4– 12 nm particle diameter was suspended in the acetone suspension. Diamonds were deposited in a stainless steel hot filament chemical vapor deposition (HFCVD) chamber. The gas phase, a mixture of methane and hydrogen with a CH4/H2 volume ratio fixed at 2.0%, was activated by three parallel tungsten filaments (0.8 mm in diameter and 8 mm apart) positioned 6 mm from the substrate. The filament temperature (2100 °C) was monitored by an infrared pyrometer. A thermal couple was also installed inside the holder to detect the substrate's temperature. Gas mixture's pressure was 50 torr (7.245 kpa) with flow rate 255 sccm. All samples with chromized interlayer were subject to 5 h CVD process. Scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) was adopted to examine the surface morphologies of specimens with chromium interlayers and their later diamondcoated surfaces. Phases of these specimens were identified by grazing incidence (1°) X-ray diffraction (XRD) using a Panalytical X' Pert Pro MPD diffractometer. Micro Vickers hardness test was implemented to measure the surface hardness of the original substrate and the interlayer-deposited specimens. The Daimler–Benz Rockwell-C (HRCDB) adhesion test [21] was used to access the adhesion of coatings. The load of 1471 N was applied to cause a coating damage adjacent to the boundary of indentation. Three indentations were conducted for each specimen. A pin-on-disk tribological test rig was used for the present investigation. The cemented tungsten carbide (WC + 6 wt.% Co) ball of 1/4″ (6.350 mm) diameter was adopted as a stationary pin. The
Fig. 2. Cobalt mapping of WC-Co substrate with chromized interlayer after diamond deposition: (a) SEM cross section image; (b) chromium mapping; (c) cobalt mapping.
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C.-C. Chou et al. / Surface & Coatings Technology 203 (2008) 704–708
Fig. 3. XRD patterns of a chromized interlayer-coated WC-Co substrate (a) before and (b) after diamond deposition.
coated specimens were attached to an as-hardened AISI 1045 steel, 30.0 mm × 5.0 mm in dimension, by an adhesive. A normal load of 4.9 N was applied. The sliding speed was 0.012 m/s with a wear track of 8 mm. The wear time was 30 min for each test. Worn specimens were ultrasonically cleaned in acetone for 2 min to remove wear debris after wear tests. The depth profile of each wear scar was measured with a contour measurement system (SE500-58D, Kosaka lab., Japan). 3. Results and discussion 3.1. Microstructures, phases and thickness of Cr-interlayers From the EDS test as illustrated in Fig. 1, small cobalt particles could be identified on the PVD-Cr surface. To avoid the catalytic effect of cobalt binder, the acid etching process as mentioned in the last section was applied before the PVD process. Network-like surface structures were observed for the specimen chromized at 950 °C for 4 h, which was the same as that on the chromized steel [22]. On these surfaces, no cobalt particle was found. The thickness of chromized interlayers is measurable from the Cr mapping in Fig. 2b of the cross section in Fig. 2a, which is about 8 µm and is consistent with the number from EDS results. After the high temperature process of diamond deposition it is worthy to notify that nearly no Co could be found in the chromized interlayer, as shown in the cobalt mapping of Fig. 2c. The chromized interlayer was observed being able to prevent cobalt's diffusion successfully. Structures of interlayer-coated WC-Co samples diamond deposition were identified by X-ray diffraction. For the specimen with PVD Cr interlayer, the XRD pattern identified that no cobalt existed on this pre-etching WC-Co specimen after the diamond deposition. The original chromium interlayer has also transformed into chromium carbide which is known as a good binder to enhance the adhesion of diamond coating. For the specimen with chromized interlayer as shown in Fig. 3a, chromium–iron nitride and chromium carbide presented with WC in the XRD pattern before diamond deposition. After the diamond deposition, as depicted in Fig. 3b, two diamond peaks appear instead of the WC
ones. This could further verify the capability of chromized layer as a diffusion barrier of cobalt. 3.2. Substrate roughness and hardness The comparison of the surface roughness and hardness of two interlayer-deposited WC-Co substrates are depicted in Table 1. The hardness reduction of substrate with PVD-sputtered Cr interlayer demonstrates that the acid etching process has induced the lower toughness of the material beneath the surface. On the contrary, substrate with chromized interlayer becomes tougher. However, the increase of the surface roughness of this chromized substrate could be some issue to the growth of the diamond film. It needs an intensive investigation in the later work. 3.3. The Daimler–Benz Rockwell-C (HRC-DB) adhesion test Adhesion properties of diamond-coated WC-Co specimens are evaluated by HRC-DB adhesion test. Microstructures of indentation craters on specimens after the HRC-DB adhesion tests are shown in Fig. 4. They are SEM images by backscattered electrons (BSE). For the specimen without any interlayer, large and deep lateral spalls circumferentially around the indentation crater are observed in Fig. 4a. The surface of the cemented WC-Co substrate can be found just nearby the crater. When the specimen was pre-coated with PVDsputtered Cr interlayer, lateral spalls still appear around the crater as shown in Fig. 4b. But the hard interlayer (chromium carbide) remains partially on the exposed surface. This interlayer is helpful to improve the adhesion of diamond coating on it even the original Cr
Table 1 Surface roughness and micro Vickers hardness of interlayer-deposited WC-Co substrate Status
Hardness (HV, kgf/mm2)
Surface roughness (Ra, μm)
Original PVD-sputtered Cr interlayer-deposited Chromized interlayer-deposited
938.533 ± 75.620 722.833 ± 10.651 1125.80 ± 119.10
0.027 0.022 0.294
C.-C. Chou et al. / Surface & Coatings Technology 203 (2008) 704–708
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interlayer is only 0.6 um thick. For the specimen with chromized interlayer, the spalls around the indentation crater are in a rather smaller scale compared with the PVD-sputtered one, as shown in
Fig. 5. A typical wear track of diamond-coated WC-Co substrate (with PVD-sputtered Cr interlayer).
Fig. 4c. The exposed surfaces under these spalls are identified the chromized interlayer and no WC-Co substrate is found in these areas. The adhesion strength is hereby much better than the PVD-sputtered one. 3.4. The wear property of diamond-coated WC-Co specimens A typical wear scar morphologies of diamond-coated WC-Co specimens with chromium interlayers is illustrated in Fig. 5. Very rough surface morphologies are found in their wear tracks, these “bumps” are material transferred from the upper specimens during the sliding contact. They are identified by EDS analysis as the material from the balls of cemented tungsten carbide. The depth profiles of wear scars of diamond-coated WC-Co specimens with chromized interlayer and PVD-sputtered Cr interlayer after pin-ondisk wear test are plotted in Fig. 6a and b, respectively. Plateaux of transferred material spread over both wear tracks, especially on the surface of the one with chromized interlayer. But, for the surface of diamond-coated WC-Co specimen with PVD-sputtered Cr interlayer, a deep groove across the whole surface deposition system indicates the weakness of its substrate to bear the alternating loading condition. 4. Conclusions
Fig. 4. Surface morphologies of indention craters of diamond-coated WC-Co specimens (a) without chromium interlayer, (b) with PVD-sputtered Cr interlayer, and (c) with chromized interlayer after the Diamler–Benz Rockwell-C adhesion tests.
1. A PVD-sputtered Cr interlayer was deposited on the cemented tungsten carbide substrate before diamond deposition. This layer was later transformed into chromium carbide during the high temperature process of diamond deposition. It is helpful to improve the adhesion of diamond coating. However, since Cr interlayer cannot prevent cobalt binder's diffusion under the high temperature condition, a supersonic-assisted acid etching treatment of the substrate is applied to avoid this situation. The drawback of this treatment is the brittle behavior of the substrate which reduces the wear resistance and adhesion of diamond coating. 2. Hard chromizing surface treatments on WC— 6.0 wt.% Co at 950 °C during 4 h has been carried out. A chromized layer with chromium–iron nitride at the surface and chromium carbides underneath was formed. This layer not only can improve the adhesion of diamond coating but also protect the diffusion of cobalt binder from
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Fig. 6. Depth profiles of wear scar of diamond-coated WC-Co substrate (a) with chromized interlayer and (b) with PVD-sputtered Cr interlayer.
the WC substrate. This technique is beneficial than the PVD-sputtered one due to its excellent anti-wear capability and low cost of the facilities.
[11] [12] [13] [14]
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Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Tribological and mechanical properties of HFCVD diamond-coated WC-Co substrates with different Cr interlayers Chau-Chang Chou a,⁎, Jyh-Wei Lee b, Yen-I Chen a a b
Department of Mechanical & Mechatronic Engineering, National Taiwan Ocean University, No.2, Pei-Ning Rd., Keelung 20224, Taiwan, ROC Department of Mechanical Engineering, Tungnan University, No. 152, Sec.2, Pei-Shen Road, Shen-Ken, Taipei 222, Taiwan, ROC
a r t i c l e
i n f o
Available online 4 September 2008 Keywords: Chemented tungsten carbide Diamond Chromized interlayer PVD CVD
a b s t r a c t Chromium interlayers between WC-Co substrates and diamond coating by hot filament chemical vapor deposition (HFCVD) process were produced by two different techniques: using pack chromization and bipolar symmetry pulsed DC reactive magnetron sputtering processes. Both chromium surfaces were found partially transformed into carbide compound layers by energy dispersive X-ray spectrometer (EDS) and X-ray diffractometer (XRD). Daimler–Benz Rockwell-C indentation tests were conducted to evaluate the adhesion properties of diamond coatings with interlayers created by these two techniques. Delamination outside the indentation zones was also observed by a scanning electron microscopy (SEM). Wear resistance of both coatings were investigated by pin-on-disk wear tests. Rather low wear volumes revealed their excellent anti-wear behavior. Good diamond adhesion on chromized interlayers has been attributed to chromium carbide formation on pack chromized film surfaces as well as on PVD ones during the HFCVD process. It is concluded that the adhesion properties and tribological performance of CVD diamond coatings are significantly improved by the proposed two chromium interlayers. However, chromium interlayer produced by pack chromization is more beneficial to the adhesion of diamond coating than that, by PVD process. One reason is the former interlayer's role as a diffusion barrier of cobalt binder, the other is the low cost of the facilities for the pack chromization technique. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The cemented tungsten carbides are used widely in the cutting, drilling and molding tools. Diamond coating on these tools can improve their performance as well as their serving lives. Along with great wear resistance, the advantages include high surface hardness, low friction, high thermal conductivity and better corrosion protection [1]. The existence of cobalt binder in cemented diamond carbide suppresses diamond growth during high temperature deposition process and thus induced poor adhesion of diamond coating. Various approaches have been reported to reduce the catalyst effect of cobalt on the growth of diamond films. Removing the cobalt binder from the substrate by acid etching is usually conducted to solve this problem. Its drawback is the significant reduction of the substrate's mechanical properties and toughness. Besides, the large residue stress
⁎ Corresponding author. E-mail address: [email protected] (C.-C. Chou). 0257-8972/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.08.055
will be raised due to the mismatch of the thermal expansion. Diffusion barriers seem to be a better way to prevent the Co catalyst effect but the adhesion of the diamond coating still needs to be improved. In this study, chromium interlayers between WCCo substrates and diamond surface coatings were produced by two techniques: pack chromization and PVD processes. Chromization processes can be implemented by pack [2,3], salt bath [4] , fluidized bed [5], and chemical vapor deposition methods [6]. Chromization is usually divided into two categories: soft chromization and hard chromization [7]. The former is used on the alloys with less than 0.1 wt.% carbon to improve their corrosion and high temperature oxidation properties [7–10]. While hard chromization process, on the alloys with carbon concentration higher than 0.3 wt.%, is applied to achieve improved mechanical properties, including higher hardness [5,7,11–14] and good adhesion properties [14]. One of the authors has conducted a systematic investigation on the microstructures and mechanical properties of steels with different carbon contents [15]. In these works it was found that layers of chromium carbide formed during chromization process are responsible for the improved mechanical properties. However, forming chromium carbide interlayer on tungsten carbides by hard chromization process
C.-C. Chou et al. / Surface & Coatings Technology 203 (2008) 704–708
705
2. Experimental procedure
Fig. 1. Surface morphology of WC-Co substrate deposited with PVD-Cr interlayer: (a) SEM image; (b) EDS of the cobalt particle on the surface.
has never been studied in the literatures. On the other hand, Crbased films deposited by PVD deposition on copper substrates [16] and steel substrates [17–19] were studied. Investigation on these Cr-based interlayers between diamond film and WC-Co substrates was first reported by Polini et al. [20]. They found that chromium carbide formation on PVD film surfaces during the CVD process was also beneficial to diamond adhesion. The aim of this work is to study and compare the effect of Cr-base interlayers produced by pack chromization and PVD techniques on the microstructure, the adhesion, and the wear resistance of the diamond-coated WC-Co tools.
Cemented tungsten carbide samples, WC-6 wt.% Co, were used as substrates. Specimens with 15 × 15 × 1 mm3 dimensions were ground and polished up to 0.03 µm (Ra), washed in distilled water, ultrasonically degreased in acetone and rinsed in alcohol. A supersonic-assisted etching process by means of a 1:1 HNO3/H2O solution was optionally applied for 10 min to study the effect of cobalt element in the substrate's surface. Cr-based interlayers were prepared by two processes. They are bipolar symmetry pulsed DC reactive magnetron sputtering process, a PVD process, and pack chromization process, respectively. By the PVD plant, a 0.5 µm Cr thin film was deposited onto WC-Co samples. On the other hand, pack cementation for chromizing was conducted in a double steel can which was designed by Meier [2] at 950 °C for 4 h. The powder mixture for chromizing contained 30 wt.% ferrochromium powder (71 wt.% Cr, 0.03 wt.% C and balanced Fe), 2.5 wt.% ammonia chloride activator and 67.5 wt.% filler (alumina powder). Argon protection was employed to prevent oxidation of chromium. To enhance diamond's nucleation, specimens were seeded by sinking the WCCo substrates in a diamond acetone suspension and preprocessed by a supersonic vibrator for 60 min. The diamond powder with 4– 12 nm particle diameter was suspended in the acetone suspension. Diamonds were deposited in a stainless steel hot filament chemical vapor deposition (HFCVD) chamber. The gas phase, a mixture of methane and hydrogen with a CH4/H2 volume ratio fixed at 2.0%, was activated by three parallel tungsten filaments (0.8 mm in diameter and 8 mm apart) positioned 6 mm from the substrate. The filament temperature (2100 °C) was monitored by an infrared pyrometer. A thermal couple was also installed inside the holder to detect the substrate's temperature. Gas mixture's pressure was 50 torr (7.245 kpa) with flow rate 255 sccm. All samples with chromized interlayer were subject to 5 h CVD process. Scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS) was adopted to examine the surface morphologies of specimens with chromium interlayers and their later diamondcoated surfaces. Phases of these specimens were identified by grazing incidence (1°) X-ray diffraction (XRD) using a Panalytical X' Pert Pro MPD diffractometer. Micro Vickers hardness test was implemented to measure the surface hardness of the original substrate and the interlayer-deposited specimens. The Daimler–Benz Rockwell-C (HRCDB) adhesion test [21] was used to access the adhesion of coatings. The load of 1471 N was applied to cause a coating damage adjacent to the boundary of indentation. Three indentations were conducted for each specimen. A pin-on-disk tribological test rig was used for the present investigation. The cemented tungsten carbide (WC + 6 wt.% Co) ball of 1/4″ (6.350 mm) diameter was adopted as a stationary pin. The
Fig. 2. Cobalt mapping of WC-Co substrate with chromized interlayer after diamond deposition: (a) SEM cross section image; (b) chromium mapping; (c) cobalt mapping.
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C.-C. Chou et al. / Surface & Coatings Technology 203 (2008) 704–708
Fig. 3. XRD patterns of a chromized interlayer-coated WC-Co substrate (a) before and (b) after diamond deposition.
coated specimens were attached to an as-hardened AISI 1045 steel, 30.0 mm × 5.0 mm in dimension, by an adhesive. A normal load of 4.9 N was applied. The sliding speed was 0.012 m/s with a wear track of 8 mm. The wear time was 30 min for each test. Worn specimens were ultrasonically cleaned in acetone for 2 min to remove wear debris after wear tests. The depth profile of each wear scar was measured with a contour measurement system (SE500-58D, Kosaka lab., Japan). 3. Results and discussion 3.1. Microstructures, phases and thickness of Cr-interlayers From the EDS test as illustrated in Fig. 1, small cobalt particles could be identified on the PVD-Cr surface. To avoid the catalytic effect of cobalt binder, the acid etching process as mentioned in the last section was applied before the PVD process. Network-like surface structures were observed for the specimen chromized at 950 °C for 4 h, which was the same as that on the chromized steel [22]. On these surfaces, no cobalt particle was found. The thickness of chromized interlayers is measurable from the Cr mapping in Fig. 2b of the cross section in Fig. 2a, which is about 8 µm and is consistent with the number from EDS results. After the high temperature process of diamond deposition it is worthy to notify that nearly no Co could be found in the chromized interlayer, as shown in the cobalt mapping of Fig. 2c. The chromized interlayer was observed being able to prevent cobalt's diffusion successfully. Structures of interlayer-coated WC-Co samples diamond deposition were identified by X-ray diffraction. For the specimen with PVD Cr interlayer, the XRD pattern identified that no cobalt existed on this pre-etching WC-Co specimen after the diamond deposition. The original chromium interlayer has also transformed into chromium carbide which is known as a good binder to enhance the adhesion of diamond coating. For the specimen with chromized interlayer as shown in Fig. 3a, chromium–iron nitride and chromium carbide presented with WC in the XRD pattern before diamond deposition. After the diamond deposition, as depicted in Fig. 3b, two diamond peaks appear instead of the WC
ones. This could further verify the capability of chromized layer as a diffusion barrier of cobalt. 3.2. Substrate roughness and hardness The comparison of the surface roughness and hardness of two interlayer-deposited WC-Co substrates are depicted in Table 1. The hardness reduction of substrate with PVD-sputtered Cr interlayer demonstrates that the acid etching process has induced the lower toughness of the material beneath the surface. On the contrary, substrate with chromized interlayer becomes tougher. However, the increase of the surface roughness of this chromized substrate could be some issue to the growth of the diamond film. It needs an intensive investigation in the later work. 3.3. The Daimler–Benz Rockwell-C (HRC-DB) adhesion test Adhesion properties of diamond-coated WC-Co specimens are evaluated by HRC-DB adhesion test. Microstructures of indentation craters on specimens after the HRC-DB adhesion tests are shown in Fig. 4. They are SEM images by backscattered electrons (BSE). For the specimen without any interlayer, large and deep lateral spalls circumferentially around the indentation crater are observed in Fig. 4a. The surface of the cemented WC-Co substrate can be found just nearby the crater. When the specimen was pre-coated with PVDsputtered Cr interlayer, lateral spalls still appear around the crater as shown in Fig. 4b. But the hard interlayer (chromium carbide) remains partially on the exposed surface. This interlayer is helpful to improve the adhesion of diamond coating on it even the original Cr
Table 1 Surface roughness and micro Vickers hardness of interlayer-deposited WC-Co substrate Status
Hardness (HV, kgf/mm2)
Surface roughness (Ra, μm)
Original PVD-sputtered Cr interlayer-deposited Chromized interlayer-deposited
938.533 ± 75.620 722.833 ± 10.651 1125.80 ± 119.10
0.027 0.022 0.294
C.-C. Chou et al. / Surface & Coatings Technology 203 (2008) 704–708
707
interlayer is only 0.6 um thick. For the specimen with chromized interlayer, the spalls around the indentation crater are in a rather smaller scale compared with the PVD-sputtered one, as shown in
Fig. 5. A typical wear track of diamond-coated WC-Co substrate (with PVD-sputtered Cr interlayer).
Fig. 4c. The exposed surfaces under these spalls are identified the chromized interlayer and no WC-Co substrate is found in these areas. The adhesion strength is hereby much better than the PVD-sputtered one. 3.4. The wear property of diamond-coated WC-Co specimens A typical wear scar morphologies of diamond-coated WC-Co specimens with chromium interlayers is illustrated in Fig. 5. Very rough surface morphologies are found in their wear tracks, these “bumps” are material transferred from the upper specimens during the sliding contact. They are identified by EDS analysis as the material from the balls of cemented tungsten carbide. The depth profiles of wear scars of diamond-coated WC-Co specimens with chromized interlayer and PVD-sputtered Cr interlayer after pin-ondisk wear test are plotted in Fig. 6a and b, respectively. Plateaux of transferred material spread over both wear tracks, especially on the surface of the one with chromized interlayer. But, for the surface of diamond-coated WC-Co specimen with PVD-sputtered Cr interlayer, a deep groove across the whole surface deposition system indicates the weakness of its substrate to bear the alternating loading condition. 4. Conclusions
Fig. 4. Surface morphologies of indention craters of diamond-coated WC-Co specimens (a) without chromium interlayer, (b) with PVD-sputtered Cr interlayer, and (c) with chromized interlayer after the Diamler–Benz Rockwell-C adhesion tests.
1. A PVD-sputtered Cr interlayer was deposited on the cemented tungsten carbide substrate before diamond deposition. This layer was later transformed into chromium carbide during the high temperature process of diamond deposition. It is helpful to improve the adhesion of diamond coating. However, since Cr interlayer cannot prevent cobalt binder's diffusion under the high temperature condition, a supersonic-assisted acid etching treatment of the substrate is applied to avoid this situation. The drawback of this treatment is the brittle behavior of the substrate which reduces the wear resistance and adhesion of diamond coating. 2. Hard chromizing surface treatments on WC— 6.0 wt.% Co at 950 °C during 4 h has been carried out. A chromized layer with chromium–iron nitride at the surface and chromium carbides underneath was formed. This layer not only can improve the adhesion of diamond coating but also protect the diffusion of cobalt binder from
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Fig. 6. Depth profiles of wear scar of diamond-coated WC-Co substrate (a) with chromized interlayer and (b) with PVD-sputtered Cr interlayer.
the WC substrate. This technique is beneficial than the PVD-sputtered one due to its excellent anti-wear capability and low cost of the facilities.
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