A study of CVD diamond deposition on cemented ...

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[10] have obtained adhesion improvement of diamond coatings on cemented carbide with high cobalt content using Nb, Cr and Ta inter- layers. According to ...
Diamond & Related Materials 63 (2016) 51–59

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Reprint of “A study of CVD diamond deposition on cemented carbide ball-end milling tools with high cobalt content using amorphous ceramic interlayers”☆ Yu-xiao Cui, Wei-song Wang, Bin Shen, Fang-hong Sun ⁎ State Key Laboratory of Mechanical System and Vibration, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

a r t i c l e

i n f o

Article history: Received 23 June 2015 Received in revised form 31 July 2015 Accepted 3 September 2015 Available online 6 February 2016 Keywords: CVD Diamond films WC–Co substrates Interlayer Adhesion

a b s t r a c t In this paper, CVD diamond coatings are deposited on cemented carbides with 10 wt.% Co using amorphous SiO2 and amorphous SiC interlayers. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Raman spectrum and X-ray diffraction (XRD) are carried out to characterize the microstructure and composition of as-deposited films. Moreover, the adhesion and cutting performance of asfabricated diamond coatings are studied. Indentation tests show that the amorphous ceramic interlayers can enhance the adhesion between diamond films and WC–Co substrates. The cutting tests against zirconia indicate that the tools with amorphous ceramic interlayered diamond coatings exhibit improved cutting performance. The amorphous ceramic interlayers can improve the adhesive strength and wear endurance of diamond coatings on WC–10 wt.% Co substrates, which provide a viable way for adherent diamond coatings on cemented carbide tools with high cobalt content. © 2015 Elsevier B.V. All rights reserved.

1. Introduction CVD diamond coatings on cemented carbide tools can increase the tool lifetime and cutting performance considerably due to their excellent physical and chemical properties, such as high hardness, low friction coefficient and wear resistance [1–3]. CVD diamond coated tools are very suitable for machining carbon fiber reinforced plastic (CFRP), ceramic, printed circuit board (PCB), metal matrix composite (MMC) and graphite [4–8]. However, the cobalt in cemented carbides can cause graphitization at the film/substrate interface, which can lead to the deterioration of nucleation and growth of CVD diamond. Cobalt removal by chemical etching results in adhesion improvement and is widely used in industry. Nevertheless, the low depth etching will lead to the diffusion of residual cobalt during CVD process, and high depth etching will cause a brittle Co-depleted layer at the interface of film/substrate [9–10]. Either case is detrimental to the adhesion between diamond coatings and WC–Co substrates. At present, the substrate materials of most diamond coated tools are cemented carbides with low cobalt content, typically in the 3%–6% range [11]. Since higher cobalt content can improve the toughness and ☆ A Publisher’s error resulted in this article appearing in the wrong issue. The article is reprinted here for the continuity of the Special Issue: 9th International Conference on New Diamond and New Carbons (NDNC) 2015. For citation purposes, please use the original publication details; Diamond and Related Materials, Volume 59, pp. 21–29. DOI of original article: http://dx.doi.org/10.1016/j.diamond.2015.09.002. ⁎ Corresponding author. E-mail address: [email protected] (F. Sun).

http://dx.doi.org/10.1016/j.diamond.2016.01.017 0925-9635/© 2015 Elsevier B.V. All rights reserved.

strength of cemented carbides, which is extensively used in the intermittent cutting process, well adherent diamond coatings on these cemented carbide tools with high cobalt content is of great importance to the improvement of cutting performance and tool lifetime in difficult-to-cut material machining [12]. To our knowledge, there are a few papers published that investigate the deposition of CVD diamond on the high-cobalt-content cemented carbides by employing either chemical etching [11,13–14] or interlayers [15–17]. Mallika et al. [11] have found that by using chemical etching strong adherent diamond coatings can be deposited on high cobalt cemented carbides. And Xu et al. [10] have obtained adhesion improvement of diamond coatings on cemented carbide with high cobalt content using Nb, Cr and Ta interlayers. According to their study, the specimens performed with both chemical etching and interlayers exhibit better adhesion than those performed with only chemical etching or interlayers. In conclusion, considerable success has been made in obtaining adherent diamond coatings on high-cobalt-content cemented carbides. However, there is little subsequent machining data to verify the effectiveness of as-fabricated diamond coatings. Amorphous ceramic is in good grace from wide aspects, considering yield strength, break strength, abrasive resistance, corrosion resistance and thermal characteristic [18–19], which makes it very a promising interlayer material. Endler et al. [20] have synthesized a-SiC, a-Si3N4 and aSiCxNy interlayers by CVD and performed diamond deposition on those amorphous interlayers. And our former investigations show that a-SiC interlayers can improve the adhesion, frictional behavior and cutting performance of diamond coatings on WC-6 wt.% Co substrates [21–22].

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It has been proved that polycrystalline diamond films can be deposited on SiO2 with reasonable nucleation rate [23]. Nevertheless, to our knowledge there is few publications to date that describe the application of a-SiO2 as interlayer material in CVD diamond deposition, which may have pronounced effects on the adhesion of diamond coatings. Therefore, it is necessary to study the influence of a-SiO2 interlayer on the adhesion of diamond coatings. Zirconia has a broad range of industrial applications. The excellent properties of zirconia, such as high fracture toughness, chemical resistance, low heat conductivity and good biocompatibility, make it very suitable for producing dental and orthopedic implants in medical field. Nevertheless, such ceramics are not easy to be manufactured due to the abrasive character of ceramic debris during machining. CVD diamond coated tools are very promising in machining abrasive materials. And yet there is very few data available insofar as zirconia is concerned as test medium [6]. In this paper, by the combination of amorphous ceramic interlayers and chemical etching, CVD diamond coatings with improved adhesion have been fabricated on WC-10 wt.% Co substrates. In particular, the amorphous ceramic interlayered diamond coatings are deposited on cemented carbide ball-end milling tools (Co 10 wt.%). The effectiveness of amorphous ceramic interlayers has been validated by milling zirconia ceramic. 2. Experimental The cemented carbide samples with 10 wt.% Co are used as substrates. A two-step chemical pretreatment is performed in prior to remove surface cobalt and roughen the substrate as well, which is reported elsewhere [21]. The WC–Co substrates are dipped in the Murakami's regent (10 g K3[Fe(CN)]6 + 10 g KOH + 100 ml H2O) in ultrasonic vessel for 15 min; then the surface binder phase was washed away in the acid (30 ml H2SO4 + 70 ml H2O2) for 1 min. Subsequently the amorphous ceramic interlayers are synthesized in a home-made CVD apparatus, with tetraethoxysilane (TEOS) and dimethyldiethoxysilane (DMDEOS) as the precursors of a-SiO2 and a-SiC respectively. After the fabrication of amorphous ceramic interlayers, as-pretreated WC–Co substrates are scratched with 3 μm diamond powder and cleaned in ultrasonic deionized water bath to enhance diamond nucleation prior to diamond deposition. Then CVD diamond coatings are deposited with acetone and hydrogen as reactant. The deposition parameters of interlayers and CVD diamond are given in Table 1. Table 2 gives the different deposition processing conditions in this study. In particular, conventional WC–Co substrates with lower Co content (Sample 2 and Sample 6) are also employed in this work, which are prepared using the same pretreatment and deposition process. The interlayer material (a-SiO2 and a-SiC) is investigated by transmission electron microscopy (TEM) and selected area electron diffraction (SAED) to study the microstructure and phase composition. The interlayer material is mixed with alcohol, and it is ground into sub-microscale regimes in a mortar and pestle. Subsequently, the turbid liquid with ground interlayer material is placed in ultrasonic vessel for 15 min to be well mixed. After that 50 μl of the turbid liquid is dropped onto carbon film, which is mounted on copper grid as support. The carbon film with turbid Table 1 Deposition parameters of interlayers and diamond coatings. Parameters

Pressure Gas flow Precursor source/H2 ratio Acetone/H2 ratio Filament-substrate distance Filament temperature Substrate temperature Negative bias current

a-SiO2 and a-SiC

12 Torr 100 sccm 0.5% – 15 mm 2200 °C 700 °C –

Diamond Nucleation

Growth

15 Torr 300 sccm – 1% 10 mm 2200 °C 800 °C 4A

30 Torr 300 sccm – 1% 10 mm 2200 °C 800 °C 4A

Table 2 List of different deposition processes for WC–Co cemented carbide substrates. Sample

Chemical etching

Interlayer

Co content

Diamond deposition

1 2 3 4 5 6 7 8

Yes Yes Yes Yes Yes Yes Yes Yes

– – a-SiO2 (40 min) a-SiC (30 min) – – a-SiO2 (40 min) a-SiC (30 min)

10 wt.% 6 wt.% 10 wt.% 10 wt.% 10 wt.% 6 wt.% 10 wt.% 10 wt.%

– – – – 4h 4h 4h 4h

liquid is dried under a lamp for 30 min. Then the carbon film is placed in the vacuum cavity and the microstructure of grinded interlayer material is detected by TEM. Field emission gun scanning electron microscopy (FEG-SEM) is used to investigate the morphologies of amorphous ceramic intermediate films and CVD diamond films. Energy dispersive X-ray spectroscopy (EDX) measurements are used to give an elemental analysis of the aSiO2 interlayer coated substrates and conventional chemical etched substrates. The quality and crystalline microstructure of as-synthesized diamond coatings are analyzed by Raman spectroscopy and X-ray diffraction (XRD). In order to evaluate the adhesive strength between diamond coatings and WC–Co substrates, Rockwell indentation tests are performed on as-synthesized diamond coatings, with a constant load of 100 kg. The indentation on each specimen is investigated by SEM respectively. To analyze the cutting performance of as-fabricated diamond coated tools, comparative milling tests are conducted for diamond coated tools with/without amorphous interlayers, with zirconia ceramic as the work piece. The details of zirconia ceramic are given in Table 3. The milling parameters are as follows: spindle speed, 8000 rpm; feed rate, 0.3 mm/rev, radial cutting width, 0.3 mm; cutting depth, 0.5 mm. No lubricants are used in the cutting test. The worn morphology of cutting edge and the flank wear values are investigated by optical microscope and SEM respectively.

3. Results and discussion The amorphous ceramic interlayers (a-SiO2 and a-SiC) are synthesized by pyrolysis of molecular precursors. In order to investigate the microstructure and phase composition of interlayer material, TEM analysis is applied. After sufficiently mechanical crushing, the interlayer material is grinded into sub-microscale regimes. As shown in Fig. 1a–b, the dark particles of a-SiO2 and a-SiC can be observed, with sizes between 100 and 200 nm. For both types of interlayer materials, the SAED patterns of sub-microscale regimes consist of only broad and dull halos, indicating the presence of amorphous structure. The SEM micrographs and EDX patterns of amorphous ceramic interlayers and chemical etched WC–Co substrates (Samples 1–4) are shown in Fig. 2 by SEM. It can be seen from Fig. 2a and c that after chemical etching the surface topography of Samples 1–2 is quite alike. The cobalt on cemented carbide substrate surface is washed away and the WC grains present a rough and porous Co-depleted layer. When amorphous ceramic interlayers are employed, as shown in Fig. 2e and g, homogeneous pasty-like materials, i. e. a-SiO2 and a-SiC, are covered on the rugged WC grains, and the surface topography is close in texture with ball-like Table 3 Properties of zirconia ceramic. Density (g/cm3)

6.03

Mean grain size (μm) Young's modulus (GPa) Fracture Toughness (Pam1/2) Hardness (kg/mm2)

0.4 222 10.7 ± 0.7 1180 ± 13

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Fig. 1. TEM images and SAED patterns of sub-microstructure of (a) a-SiO2 and (b) a-SiC material.

appearance. EDX microanalyses reveal that the main component elements of TEOS decomposition products (Fig. 2f) are O and Si. The component elements of DMDEOS decomposition products (Fig. 2h) are Si, C and O. The main composition of TEOS decomposition products is SiO2 [24]. The decomposition products of DMDEOS are called Si–O–C hybrid material and contain both Si–C and Si–O bonds [25]. As shown in Fig. 2h, the content of O is much lower than that of Si and C. The main composition of DMDEOS decomposition products is supposed to be SiC, which contains a small amount of O. Subsequently, diamond coatings are deposited on these substrates. The surface topography and fractography of as-fabricated diamond coatings is illustrated in Fig. 3. For all four samples, their morphologies are similar to each other. Pyramidal shaped (1 1 1) faceted crystallites can be observed, whose size reaches 4–5 μm. Fig. 3e–hshows the fractography of Samples 5–8. 4–5 μm-thick diamond coatings are synthesized on each sample, with obvious columnar structure. Besides, for Samples 7–8, it can be observed that amorphous mud-like interlayers exist between the diamond coatings and substrates. The thickness of a-SiO2 interlayer and a-SiC interlayer is 0.4 μm and 1 μm respectively. The Raman spectra of as-fabricated diamond coatings are shown in Fig. 4, using an Ar+ laser with an excitation wavelength of 632.8 nm. The intense band located at about 1332 cm−1 for each sample is attributed to polycrystalline diamond. Moreover, a broad G band (graphite phase) can be observed at around 1500 cm−1. All four spectra are similar. Fig. 5 gives the XRD patterns of as-fabricated diamond coatings (Samples 5–8). As can be observed in all four samples, except for the diffraction peaks attributable to tungsten carbide, intense (1 1 1) and (2 2 0) diamond peaks exist at 2θ ~ 43.9° and 75.3°. Moreover, when amorphous ceramic interlayers are employed, weak diffraction peaks of the interfacial cobalt-silicides are detected at 2θ ~ 29.3°, 34.3°, 46.6° and 80.8°, which can be ascribed to CoSi2 and CoSi respectively. This indicates that the surface residual cobalt on chemical etched cemented carbide substrates has reacted with the interlayer material in CVD process, which is in accord with former study [20–21,26–27]. These intermetallic compounds are expected to have no deleterious effects like cobalt [26]. Besides, it should be noted that the deposition of oxide films on WC–Co might cause the oxidation of WC–Co surface. However, as illustrated in Fig. 5c, no oxidized product (WO2, WO3, CoO, etc.) can be detected when a-SiO2 interlayer is employed. We conjecture that the thermal decomposition of TEOS droplets might take place near the hot filaments in the atmosphere of H2. Then a-SiO2 particles come into

being, fall on WC–Co substrate and pile up. The CVD process of a-SiO2 could be performed above WC–Co substrate rather than on WC–Co substrate surface. So the oxidation of WC–Co substrate is avoided. In order to evaluate the adhesive strength of film/substrate system, Rockwell indentation tests are conducted on as-fabricated diamond coatings with and without amorphous ceramic interlayers. The SEM characterization is shown in Fig. 6. As presented in Fig. 6a, the 100 kg load causes crack propagation around the indentation contact region of conventional diamond coating deposited on WC–10 wt.% Co substrate. Similarly, the diamond coating deposited on WC–6 wt.% Co substrate with no interlayer is also flaked and chipped by indentation, as shown in Fig. 6b. Nevertheless, for the amorphous ceramic interlayered diamond coatings (Fig. 6c–d), the indentation regions are circle in shape and no cracks extend from the indentations for the diamond coatings. It should be noted that for cemented carbide substrates, higher cobalt content means higher toughness and plasticity, which can enhance the indentation deformation and increase the delamination between films and substrates. It can be concluded by comparison among Samples 5, 7 and 8 that the adhesive strength of diamond coatings deposited on WC–10 wt.% Co substrates has been remarkably enhanced by the amorphous ceramic interlayers. The reasons for the adhesion enhancement are as follows. On one hand, as shown in Fig. 5, during CVD process the amorphous ceramic interlayers can react with the residual cobalt on the surface and the diffused cobalt from the bulk. As mentioned before, the reaction products are expected to have no deleterious effects like cobalt [26]. And thus the graphitization caused by cobalt can be remarkably reduced. On the other hand, as shown in Fig. 2, the interlayer material essentially acts as amorphous binder to fill up the voids of the porous zone on substrate surface after chemical etching, which can improve the interfacial contact between diamond and substrate and, consequently, the adhesive strength. In order to investigate the cutting performance of as-fabricated diamond coated ball-end milling tools, milling tests are performed with zirconia ceramic as the work piece. In particular, aiming at studying the effect of cobalt content, except for the substrates with 10 wt.% cobalt, conventional diamond coating is deposited on cemented carbide tool with 6 wt.% cobalt as well. Fig. 7 presents the flank wear of tools observed by optical microscope. As exhibited in Fig. 7, the flank wear of all samples exhibits very close value for the first 20 min. It shows a slow increase to 0.018 mm for Samples 5–8. Afterwards, however, the flank wear of Sample 5, i.e., conventional diamond coated tool with 10 wt.% Co, rises sharply, which is about 0.1 mm when milling for 30 min. Obvious flaking-off of diamond coatings can be seen. The milling tests for

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Fig. 2. Surface topography (a, c, e, g) and EDX patterns (b, d, f, h) of (a, b) Sample 1 (c, d) Sample 2 (e, f) Sample 3 and (g, h) Sample 4.

Samples 6–8 are subsequently performed for another 20 min and no severe rise of flank wear can be observed by optical microscope. Fig. 8 gives the worn morphology of flank wear of Samples 5–8 by SEM. It can be seen in Fig. 8a that for Sample 5, after milling for 30 min, large area of diamond is peeled off, which causes the sharp increase of flank wear. Sample 6 exhibits better wear resistance and longer lifetime than Sample 5. However, after milling for 50 min, small

pieces of diamond coatings begin to detach from the cutting edge for Sample 6, as shown in Fig. 8b. For the amorphous ceramic interlayered tools, the milling duration is also 50 min, and there are only craters on the worn cutting edge and no coating delamination is observed (see Fig. 8c–d). Fig. 9 shows the flank wear time evolution of Samples 5–8. The amorphous ceramic interlayered tools exhibit lowest flank wear and longest lifetime. This indicates that by using the amorphous ceramic

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Fig. 3. SEM images of (a, b, c, d) surface topography and (e, f, g, h) fractograph of (a, e) Sample 5 (b, f) Sample 6 (c, g) Sample 7 and (d, h) Sample 8.

interlayers, the cutting performance of diamond coated tools with 10 wt.% Co is significantly improved, which exhibit even better cutting performance than the diamond coated tool with 6 wt.% Co. As illustrated in milling tests, the main failures of tools are coating delamination and abrasive wear, which can be ascribed to the cutting heat concentration on the cutting edge and continuous impact of zirconia particles. The adhesion between diamond coatings and WC–Co substrates is of great importance to the cutting performance of diamond coated cutting tools. Sample 6 exhibits much longer lifetime than Sample 5 in this study. It can be concluded that the Co content of WC–Co substrate has great influence on the adhesion of diamond coatings.

The increase of Co content is detrimental to the adhesion between diamond coatings and substrates, even though chemical etching of Co removal is performed in prior. Moreover, according to the milling tests of Samples 7–8, the amorphous ceramic interlayers (a-SiO2 and a-SiC) can effectively improve the cutting performance of diamond coated tools with high Co content (10 wt.%). 4. Conclusions The CVD diamond coatings are deposited on cemented carbide milling tools with 10 wt.% Co using a-SiO 2 and a-SiC interlayers.

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Fig. 4. Raman spectra of diamond coatings: (a) Sample 5, (b) Sample 6, (c) Sample 7, (d) Sample 8.

The amorphous ceramic interlayers can reduce the catalytic effect of binder phase and improve the interfacial contact between diamond and substrates, thus enhancing the adhesion between diamond

coatings and WC–Co substrates. The milling tests against zirconia ceramic indicate that the increase of Co content is detrimental to the adhesive strength of diamond coatings on WC–Co substrates. For

Fig. 5. XRD patterns of (a) Sample 5, (b) Sample 6, (c) Sample 7, (d) Sample 8.

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Fig. 6. SEM micrographs of delaminated area on (a) Sample 5, (b) Sample 6, (c) Sample 7, (d) Sample 8.

Co-cemented tungsten carbide mills with 10 wt.% binder phase (WC–10 wt.% Co), the amorphous ceramic interlayers can improve the tool life by enhancing the adhesion of CVD diamond coatings.

In this study, by using the amorphous ceramic interlayers, the diamond coated tools with 10 wt.% Co exhibit even better cutting performance than that with 6 wt.% Co.

Fig. 7. Images of flank wear after milling (magnification ×30).

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Fig. 8. SEM images of worn surface of (a) Sample 5 after milling for 30 min and (b) Sample 6, (c) Sample 7, (d) Sample 8 for 50 min. The selected images with larger magnification are shown on the right.

Prime novelty statement Well adherent diamond coatings on these cemented carbide tools with high cobalt content are of great importance to the improvement of cutting performance and tool lifetime in difficult-to-cut material machining. In this paper, two types of amorphous ceramic interlayers (a-SiO2 and a-SiC) are employed to improve the adhesive strength between CVD diamond coatings and WC–Co ball-end milling tools with high cobalt content (10 wt.%). Indentation test shows that the amorphous ceramic interlayers have greatly enhanced the adhesion between diamond films and WC–Co substrates. Moreover, the milling test against zirconia ceramic indicates

that the tools with amorphous ceramic interlayered diamond coatings exhibit improved cutting performance. The amorphous ceramic interlayers can improve the adhesive strength and wear endurance of diamond coatings on WC–10 wt.% Co substrates, which provide a viable way for adherent diamond coatings on cemented carbide tools with high cobalt content. Acknowledgments This research is sponsored by the Research Fund for the Doctoral Program of Higher Education of China (NO. 20130073110036).

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Fig. 9. The measured flank wear values for as-fabricated diamond coated milling tools.

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