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On effectiveness of various surface treatments on adhesion of HF-CVD diamond coating to tungsten carbide inserts
Diamond and Related Materials 11 (2002) 1660–1669
On effectiveness of various surface treatments on adhesion of HF-CVD diamond coating to tungsten carbide inserts B. Sahoo, A.K. Chattopadhyay* Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur 721 302, WB, India Received 12 July 2001; received in revised form 15 April 2002; accepted 23 April 2002
Abstract An investigation has been carried out to study the effectiveness of various surface treatments by HNO3 qH2 O (1:1), HNO3q HClqH2O (1:1:1) and HNO3q3HCl on adhesion of HF-CVD diamond coating on WCqCo substrate. HNO3 qH2 O could not reduce the surface cobalt to a very low value. Prolongation of etching time with this etchant could not reduce surface cobalt significantly. A strong etchant like HNO3 q3HCl could reduce surface cobalt of the tool insert substantially, but it also attacked WC grains and no substantial increase in surface roughness observed. Interestingly HNO3 qHClqH2 O (1:1:1) could bring down the surface cobalt content to the lowest level. In addition, surface roughness of the etched surface appeared to be highest under this treatment. The diamond coating has been found to show highest adhesion when deposited on HNO3qHClqH2O (1:1:1) treated WCqCo substrate. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Cemented carbide; Substrate treatment; Diamond film
1. Introduction Synthesis of diamond by low pressure has opened up new opportunities to use synthetic diamond in cutting tool applications. In many application it is advantageous to use diamond coated cutting tools wherein a thin film of diamond is directly deposited on a cemented carbide substrate by CVD and such coated tool is free from weakness shown by natural diamond crystal tool and PCD compacts w1x. Adequate properties of diamond coating like wear resistance, microhardness, edge coverage (covering face and flank), smoothness of the coating are essential for the diamond coated tool. However, good adhesion of the diamond coating with the substrate to stop flaking is the fundamental requirement to ensure consistent performance of a diamond coated insert. Numerous investigations have undoubtedly established the detrimental effect of Co present in cemented carbide insert substrate on nucleation and growth of diamond and ultimately on the adhesion of the coating with the substrate. Various methods like chemical etch*Corresponding author. Tel.: q91-3222-82914; fax: q91-322255303. E-mail address: [email protected] (A.K. Chattopadhyay).
ing, vacuum evaporation have been tried to reduce the content of surface cobalt on the substrate in order to improve adhesion of the coating with the substrate w2x. Another aspect of this etching technique is to achieve adequate surface roughness on the etched substrate surface which, on the other hand, can ensure good anchorage of the diamond film with the substrate. Cobalt removal by acid etching is an important surface treatment process. Cemented carbide substrates etched with HNO3qH2O (1:3) and seeded with 6 mm diamond particles aided nucleation and growth of diamond film w3x. Compared to unetched substrate WC–Co substrate etched with HNO3qH2O solution at room temperature and ultrasonic vibration achieved better film deposition, morphology and adhesion of diamond to the substrate w4,5x. Morphology, structure and adhesion of diamond film could also be improved by treatment with HClq H2O (1:1) boiling solution w6x. It has been further observed that HNO3q3HCl combination is more effective in removal of Co. However, etching WC grains with Murakami solution desirably increases the substrate surface roughness substantially but also causes increase in the surface Co content w7x. It can be realized that substantial surface roughness with low cobalt content is
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 2 . 0 0 1 3 7 - 1
B. Sahoo, A.K. Chattopadhyay / Diamond and Related Materials 11 (2002) 1660–1669
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Fig. 1. SEM micrograph of cutting edge and surface morphology of tools before and after various treatments, (a) cutting edge, (b) rake surface.
the most desirable surface for deposition of diamond film. It has been observed that mechanical interlocking effect caused by a rough surface can contribute signifi-
cantly to improved adhesion. Enhancement of surface roughness through cobalt etching is beneficial in that, the diamond ‘seed’ can well-retained in the cavity and
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Table 1 Summary of various substrate treatment and diamond deposition process Sl No.
Substrate: WC–6%Co tool inserts Geometry: SPGN120308 Substrate treatment conditions (etching reagents, time, condition)
Code for the treatment and residual cobalt %
Code used for the diamond coated tool
Deposition conditions (similar for all the tools)
1
HNO3qH2O (1:1) 15 min, with ultrasonic vibration HNO3qH2O (1:1) 30 min, with ultrasonic vibration HNO3qHClqH2O (1:1:1) 15 min, with ultrasonic vibration
treat1 (1.41%) treat2 (1.25%)
T1
treat3 (0.46%)
T3
Substrate: WC–6%Co tool inserts Geometry: SPGN120308 Filament substrate distance: 4.5 mm Filament temperature: 2000–2200 8C Substrate temperature: 740 8C (measured on substrate holder) Flow of gases: H2 (99.995% pure): 99.5 sccm CH4 (99.95% pure): 0.5 sccm Chamber pressure: 30 Torr Duration of deposition: 8 h Growth rate: (0.5–1.0 mmyh)
2
3
improve nucleation density and provide well anchored roots of subsequently grown diamond. The aim of the present work is to make a systematic study on effectiveness on various etching process to reduce cobalt content on substrate surface and, on the other hand, their effectiveness to enhance surface roughness and the resulting effect on coating morphology, coating substrate interface, interfacial bond.
T2
Under the influence of elastic–plastic stresses caused by indentation a lateral crack develops and propagate along the coating–substrate interface. The radius of the lateral crack is governed by the indentation load and geometry, existence of residual stresses and fracture resistance of the film–substrate interface. For good adhesion flaking occurs in the crater area only. If adhesion is poor, flaking occurs over a larger area.
2. Experimental procedure and conditions Cemented carbide turning inserts of geometry SPGN120308, ISO K10 grade with 6% cobalt supplied by Sandvik AB were used as the substrate. Substrates were treated with different etching reagents and etching time as summarised in Table 1 at room temperature under ultrasonic vibration. After etching selected samples were seeded with fresh diamond microcrystals of 0–2 mm grit size. Thin CVD diamond films were deposited on these seeded substrates by a hot-filament CVD process. Different deposition conditions are also mentioned in Table 1. The uncoated, etched and diamond coated surfaces of the carbide inserts were characterized under scanning electron microscope (SEM) for cutting edge, surface morphology and fractograph, EDAX for analyses of elements on the uncoated and etched substrate surface, X-ray diffraction for crystal structure and Micro-Raman spectroscopy for chemical quality evaluation of diamond. Roughness of uncoated, etched and diamond coated surface were also measured by Talysurf (Taylor–Hobson Surtronic 3P Talysurf). Adhesion test of the hard coatings on carbide tools by indentation method is well known w8x. Interfacial failure in coating–substrate occurs by brittle fracture.
Fig. 2. Surface roughness and cobalt content of WC–Co inserts before and after various treatments.
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Fig. 3. EDAX spectra of the surface of uncoated as received surface and substrate surface undergone various treatments.
In the present study, the indentation adhesion test was done with a standard Rockwell hardness tester having modified loading system to facilitate fractional loading from 20 to 100 kgf in steps of 10 kgf. The indented samples were seen under optical microscope for measurement of average crack diameters against the indentation loads and finally studied under SEM for nature and morphology of indentations at various indentation loads. 3. Results and discussion SEM micrograph of the as-received uncoated tool tip shows fine grinding marks in Fig. 1. High magnification SEM photo reveals smeared layer of cobalt in Fig. 1.
Smearing effect is less pronounced at the cutting edge. Similar non-homogeneous nature of as received surface with smeared cobalt has also been reported earlier w6x. Fig. 1 shows that WC–Co substrate with 6% Co, when treated with dilute nitric acid, HNO3qH2O (1:1) with ultrasonic vibration for 15 min under treatment ‘treat1’, the smeared layer of cobalt was partially removed exposing WC grains. Fig. 2 indicates that because of etching, surface cobalt has been reduced to 1.41%, whereas surface roughness has slightly increased. Removal of non-tungsten material i.e. cobalt from the surface, with similar treatment has also been reported w4,9x. However, no quantitative data on residual Co were presented. For further studying the effectiveness of such treatment, the etching time under the treatment ‘treat1’
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Fig. 4. SEM micrograph of cutting edge and rake face of inserts after diamond film deposition, (a) cutting edge; (b) rake surface.
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Fig. 6. Raman spectra of diamond film on WC–Co substrate surface.
50% diluted HNO3, surface cobalt could be reduced to 0.3–0.4% from a bulk content of 9–12%. The present investigation, in contrast, undoubtedly reveals that, 50% diluted HNO3 did not give good results. There was an increase of roughness of substrate surface when etching time was extended from 15 to 30 min as indicated in Fig. 2. However, etching depth was less than depth of grinding marks, which are clearly visible in Fig. 1. It appears from Fig. 2 that the substrate treated with a solution of HNO3qHClqH2O (1:1:1) for 15 min under treatment ‘treat3’, could bring down Co as low as 0.46% and surface roughness has gone up further. The SEM view of the etched surface clearly reveals the WC grains (Fig. 1). The grinding marks have also been completely removed as a result of strong etching. The variation in cobalt percentage on the untreated surface and after treatment ‘treat1’ and ‘treat3’ have also been indicated in EDAX spectra of Fig. 3. From the above observations, it is clear that Co has been effectively etched by HNO3qHClqH2O (1:1:1). It is well known that Co is readily dissolved in hydrochloric and nitric acid forming aqueous solution of cobalt chloride and cobaltous nitrate w10,11x. This has been
Fig. 5. X-ray diffraction spectra of uncoated diamond coated tungsten carbide cutting tool inserts.
was extended to 30 min in treatment ‘treat2’. This resulted in a reduction of residual cobalt content to 1.25% (Fig. 2). It has been observed that etching by HNO3 may not completely remove Co from the substrate surface w9x. Another investigation w5x claimed that with
Fig. 7. Surface roughness of diamond coated tools.
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Fig. 8. SEM fractograph of the cutting edge and coating thickness of diamond coated tools (a) coating at cutting edge covering flank and rake face; (b) coating at rake face.
clearly reflected in the present result that the combination of HCl and HNO3 has accelerated removal of Co and also increase of roughness on the substrate surface (Fig. 2). SEM micrograph of the surface of the diamond coated tool T1 (Table 1) in Fig. 4 shows good coverage of coating with well faceted crystals having cubic (1 0 0) and octahedral (1 1 1) structures, (1 1 1) being most common, which has been well evident from XRD diagram in Fig. 5. The same figure also shows XRD diagram of the uncoated substrate. A sharp peak at 1337.5ycm of the Raman spectra obtained on the coating of the tool T1 in Fig. 6 confirms the quality of the
coating to be good. The crystal size varies in the range 2–5 mm, fine crystals 2–3 mm are most common. But, outgrowths at some locations are also noticed. Due to presence of such outgrowths, the surface appears to be rough as observed from Fig. 7. SEM fractograph of the tool T1 in Fig. 8 shows diamond coating with a thickness approximately 4–4.5 mm. The coating has a columnar growth. No interfacial void is observed with the tool T1. SEM pictures of the rake face of the tool T2 coated on substrate with treatment ‘treat2’ having 1.25% surface cobalt have been shown in Fig. 4. The coating also shows good coverage with (1 0 0) and (1 1 1) faceted crystals. However, (1 1 1) texture of the coating is found
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Fig. 9. SEM micrograph of the indentation crack morphology for diamond coated tools under various indentation loads, (a) under 30 kgf load; (b) under 60 kgf load; (c) under 100 kgf load.
to be predominant. The XRD spectra of the tool was similar to that of tool T1 shown in Fig. 5. The crystal size varies in the range 2–3 mm, 2 mm crystals being most common. The surface is free from outgrowth and is smooth and regular. Compared to T1 tool, overall grain uniformity is much improved as a result of which average surface roughness has been found to be lower as is evident from Fig. 7. Fractograph of the tool T2 in Fig. 8 shows coating thickness 3.5 to 4 mm. In Fig. 8 SEM picture further reveals dense and crystalline nature of coating similar to that in tool T1. The coating substrate interface appears to be better interlocked due
to more anchoring sites. This might be the result of slightly lower surface cobalt content. Similar results of fine and smooth morphology with lower cobalt content has been reported in a previous investigation w5x. Having better results with lower cobalt content under ‘treat2’ compared to ‘treat1’, attempt was made to deposit diamond on substrates with still lower cobalt content obtained by the most effective treatment ‘treat3’. Tool T3 was coated under such substrate treatment. SEM morphology of the cutting edge of the tool T3 in Fig. 4 shows well-faceted crystals predominantly with (1 1 1) morphology. Rake face morphology reveals sim-
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Fig. 10. Crack diameter vs. indentation load for diamond coated tools.
ilar features as is evident from XRD spectra in Fig. 5. The size of crystals on the tool surface appears to be relatively large. However, 3–4 mm crystals are most common. The coating is free from defects like outgrowths and pinholes. Due to large size crystals, the surface appears to be relatively rough as evident in Fig. 7. SEM fractograph of the tool T3 in Fig. 8 reveals that the thickness of the coating is 5–5.5 mm. SEM picture clearly shows that the coating has grown from the inner crests of the substrates with a good number of anchoring sites. The coating is dense at the substrate surface without any crack or discontinuity. However, some porous structures have been observed towards the coating surface. Among the tools developed, T3 has a comparatively thicker coating, which might have resulted due to very low Co content favouring nucleation and growth at the intermediate temperature 740 8C. The thicker film resulting in larger surface crystals contributed significantly to higher surface roughness. Nondiamond carbon phases are not observed from the SEM picture, which is further confirmed from the Raman spectra. SEM micrograph of the indentation crack morphology on tool T1 in Fig. 9 under 30 kgf load shows lateral flaking of the coating beyond the crater region. With increase of indentation load to 60 and 90 kgf, flaked area on the coating also grew more as can be seen from
SEM picture shown in the same figure. Comparing the SEM picture of the indentation crack morphology and ‘d vs P’ graphs for the tools T1 and T2 in Fig. 9 and Fig. 10, one can arrive at the conclusion that just by reducing the surface cobalt percentage from 1.41 to 1.25, adhesion of the coating to the substrate can be improved. Result of the indentation test for the tool T3 further confirmed that through reduction of surface cobalt, adhesion of diamond film with WC substrate could be enhanced remarkably. In case of T3 tool the amount of surface cobalt was just 0.46%. At 30 kgf load the coating underwent deformation without experiencing any crack in a localized region unlike that of tools T1 and T2. It is important to note that the very resistance to flaking of the coating during indentation has gradually improved with the reduction in surface cobalt content. This is quite evident when one looks at the SEM pictures in Fig. 9 and ‘d vs P’ graph for tools T1, T2 and T3. The much improved adhesion of tool T3 is clearly demonstrated in Fig. 10. The slope of the characteristic graph ‘d vs P’ is also found to be minimum for tool T3. 4. Conclusions (1) Dilute HNO3 (50% concentration) could remove partially surface cobalt from a WCqCo substrate when
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used under ultrasonic vibration for 15 min. The amount of surface cobalt was reduced to 1.41% from a nominal amount of approximately 6%. The etched surface of the substrate was found to be rougher than the unetched surface. (2) Prolongation of the etching time from 15 to 30 min with the same diluted HNO3 could reduce the surface cobalt only to 1.25%. The roughness of the etched surface was found to be slightly more than what was observed after etching for 15 min. (3) A solution of HNO3qHClqH2O (1:1:1) was found to be very effective etchant for removing cobalt from WCqCo substrate. It could bring down the surface cobalt to less than 0.5% when used under ultrasonic vibration just for 15 min. Roughness of the surface obtained after etching was found to be still higher. (4) The residual Co on the substrate surface was found to be approximately 0.6% when treated with aqua regia, HNO3q3HCl for 15 min also under ultrasonic vibration. However, roughness of the etched surface was found to be lower than what was obtained on the surface etched with the solution of HNO3qHClqH2O (1:1:1). (5) Film deposited on differently etched surface showed diamond predominantly with (1 1 1) morphology. Raman spectra also did not reveal existence of any non-diamond phase in the coating. Diamond coating deposited on HNO3qHClqH2O (1:1:1) solution etched surface showed larger crystals than that deposited on diluted HNO3 etched surface. Coating thickness was also greater in the former in comparison to what was deposited on the latter. (6) Etching with HNO3qHClqH2 O (1:1:1) solution not only reduces surface cobalt content substantially but also creates deep anchoring sites for ‘seed diamond’ on which diamond film forms and grows. (7) Adhesion test by indentation clearly shows improvement in the film substrate adhesion with reduced
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cobalt content on the substrate surface and in this respect a solution of HNO3qHClqH2O (1:1:1) has been found to be an effective etchant since it not only removes cobalt from the substrate surface efficiently without attacking WC grains but also increases the surface roughness of the etched surface significantly. References w1x D.A. Stephenson, J.S. Agapiou, Metal Cutting Theory and Practice, Marcel Dekker Inc. Publication, NY, 1997. w2x S. Hosomi, I. Yoshida, Diamond CVD researches as patent applied, Application of Diamond Films and Related Materials, Elsevier Science Publishers, 1991, pp. 15–24. w3x T.H. Huang, C.T. Kuo, T.S. Lin, Tribological behaviour of chemical vapour deposition diamond films on various cutting tools, Surf. Coat. Technol. 56 (1993) 105–108. w4x A.K. Mehlmann, S.F. Dirnfeld, Y. Avigal, Investigation of lowpressure diamond deposition on cemented carbides, Diamond Relat. Mater. 1 (1992) 600–604. w5x T.H. Huang, C.T. Kuo, C.S. Chang, C.T. Kao, H.Y. Wen, Tribological behaviours of the diamond-coated cemented carbide tools with various cobalt contents, Diamond Relat. Mater. 1 (1992) 594–599. w6x M. Alam, D.E. Peebles, D.R. Tallant, Diamond deposition onto WC–6%Co cutting tool material, Thin Solid Films 300 (1997) 164–170. w7x F. Deuerler, H. Van den Berg, R. Tbersky, A. Freundlieb, V. Buck, Pretreatment of substrate surface for improved adhesion of diamond films on hard metal cutting tools, Diamond and Related Materials 5 (1996) 1478–1489. w8x P.C. Jindal, D.T. Quinto, G.J. Wolfe, Adhesion measurement of chemically vapor deposited and physically vapor deposited hard coatings on WC–Co substrates, Thin Solid Films 154 (1987) 361–375. w9x S. Chatterjee, A.G. Edwards, A. Nichols, C.S. Feigerle, Analysis of surface preparation treatments for coating tungsten carbide substrates with diamond thin films, J. Mater. Sci. 32 (1997) 2827–2833. w10x S.R. Young, Cobalt, Reinhold Publishing Corporation, New York, 1948. w11x W. Betteridge, Cobalt and its Alloys, Wiley, 1982.
On effectiveness of various surface treatments on adhesion of HF-CVD diamond coating to tungsten carbide inserts B. Sahoo, A.K. Chattopadhyay* Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur 721 302, WB, India Received 12 July 2001; received in revised form 15 April 2002; accepted 23 April 2002
Abstract An investigation has been carried out to study the effectiveness of various surface treatments by HNO3 qH2 O (1:1), HNO3q HClqH2O (1:1:1) and HNO3q3HCl on adhesion of HF-CVD diamond coating on WCqCo substrate. HNO3 qH2 O could not reduce the surface cobalt to a very low value. Prolongation of etching time with this etchant could not reduce surface cobalt significantly. A strong etchant like HNO3 q3HCl could reduce surface cobalt of the tool insert substantially, but it also attacked WC grains and no substantial increase in surface roughness observed. Interestingly HNO3 qHClqH2 O (1:1:1) could bring down the surface cobalt content to the lowest level. In addition, surface roughness of the etched surface appeared to be highest under this treatment. The diamond coating has been found to show highest adhesion when deposited on HNO3qHClqH2O (1:1:1) treated WCqCo substrate. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Cemented carbide; Substrate treatment; Diamond film
1. Introduction Synthesis of diamond by low pressure has opened up new opportunities to use synthetic diamond in cutting tool applications. In many application it is advantageous to use diamond coated cutting tools wherein a thin film of diamond is directly deposited on a cemented carbide substrate by CVD and such coated tool is free from weakness shown by natural diamond crystal tool and PCD compacts w1x. Adequate properties of diamond coating like wear resistance, microhardness, edge coverage (covering face and flank), smoothness of the coating are essential for the diamond coated tool. However, good adhesion of the diamond coating with the substrate to stop flaking is the fundamental requirement to ensure consistent performance of a diamond coated insert. Numerous investigations have undoubtedly established the detrimental effect of Co present in cemented carbide insert substrate on nucleation and growth of diamond and ultimately on the adhesion of the coating with the substrate. Various methods like chemical etch*Corresponding author. Tel.: q91-3222-82914; fax: q91-322255303. E-mail address: [email protected] (A.K. Chattopadhyay).
ing, vacuum evaporation have been tried to reduce the content of surface cobalt on the substrate in order to improve adhesion of the coating with the substrate w2x. Another aspect of this etching technique is to achieve adequate surface roughness on the etched substrate surface which, on the other hand, can ensure good anchorage of the diamond film with the substrate. Cobalt removal by acid etching is an important surface treatment process. Cemented carbide substrates etched with HNO3qH2O (1:3) and seeded with 6 mm diamond particles aided nucleation and growth of diamond film w3x. Compared to unetched substrate WC–Co substrate etched with HNO3qH2O solution at room temperature and ultrasonic vibration achieved better film deposition, morphology and adhesion of diamond to the substrate w4,5x. Morphology, structure and adhesion of diamond film could also be improved by treatment with HClq H2O (1:1) boiling solution w6x. It has been further observed that HNO3q3HCl combination is more effective in removal of Co. However, etching WC grains with Murakami solution desirably increases the substrate surface roughness substantially but also causes increase in the surface Co content w7x. It can be realized that substantial surface roughness with low cobalt content is
0925-9635/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 2 . 0 0 1 3 7 - 1
B. Sahoo, A.K. Chattopadhyay / Diamond and Related Materials 11 (2002) 1660–1669
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Fig. 1. SEM micrograph of cutting edge and surface morphology of tools before and after various treatments, (a) cutting edge, (b) rake surface.
the most desirable surface for deposition of diamond film. It has been observed that mechanical interlocking effect caused by a rough surface can contribute signifi-
cantly to improved adhesion. Enhancement of surface roughness through cobalt etching is beneficial in that, the diamond ‘seed’ can well-retained in the cavity and
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Table 1 Summary of various substrate treatment and diamond deposition process Sl No.
Substrate: WC–6%Co tool inserts Geometry: SPGN120308 Substrate treatment conditions (etching reagents, time, condition)
Code for the treatment and residual cobalt %
Code used for the diamond coated tool
Deposition conditions (similar for all the tools)
1
HNO3qH2O (1:1) 15 min, with ultrasonic vibration HNO3qH2O (1:1) 30 min, with ultrasonic vibration HNO3qHClqH2O (1:1:1) 15 min, with ultrasonic vibration
treat1 (1.41%) treat2 (1.25%)
T1
treat3 (0.46%)
T3
Substrate: WC–6%Co tool inserts Geometry: SPGN120308 Filament substrate distance: 4.5 mm Filament temperature: 2000–2200 8C Substrate temperature: 740 8C (measured on substrate holder) Flow of gases: H2 (99.995% pure): 99.5 sccm CH4 (99.95% pure): 0.5 sccm Chamber pressure: 30 Torr Duration of deposition: 8 h Growth rate: (0.5–1.0 mmyh)
2
3
improve nucleation density and provide well anchored roots of subsequently grown diamond. The aim of the present work is to make a systematic study on effectiveness on various etching process to reduce cobalt content on substrate surface and, on the other hand, their effectiveness to enhance surface roughness and the resulting effect on coating morphology, coating substrate interface, interfacial bond.
T2
Under the influence of elastic–plastic stresses caused by indentation a lateral crack develops and propagate along the coating–substrate interface. The radius of the lateral crack is governed by the indentation load and geometry, existence of residual stresses and fracture resistance of the film–substrate interface. For good adhesion flaking occurs in the crater area only. If adhesion is poor, flaking occurs over a larger area.
2. Experimental procedure and conditions Cemented carbide turning inserts of geometry SPGN120308, ISO K10 grade with 6% cobalt supplied by Sandvik AB were used as the substrate. Substrates were treated with different etching reagents and etching time as summarised in Table 1 at room temperature under ultrasonic vibration. After etching selected samples were seeded with fresh diamond microcrystals of 0–2 mm grit size. Thin CVD diamond films were deposited on these seeded substrates by a hot-filament CVD process. Different deposition conditions are also mentioned in Table 1. The uncoated, etched and diamond coated surfaces of the carbide inserts were characterized under scanning electron microscope (SEM) for cutting edge, surface morphology and fractograph, EDAX for analyses of elements on the uncoated and etched substrate surface, X-ray diffraction for crystal structure and Micro-Raman spectroscopy for chemical quality evaluation of diamond. Roughness of uncoated, etched and diamond coated surface were also measured by Talysurf (Taylor–Hobson Surtronic 3P Talysurf). Adhesion test of the hard coatings on carbide tools by indentation method is well known w8x. Interfacial failure in coating–substrate occurs by brittle fracture.
Fig. 2. Surface roughness and cobalt content of WC–Co inserts before and after various treatments.
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Fig. 3. EDAX spectra of the surface of uncoated as received surface and substrate surface undergone various treatments.
In the present study, the indentation adhesion test was done with a standard Rockwell hardness tester having modified loading system to facilitate fractional loading from 20 to 100 kgf in steps of 10 kgf. The indented samples were seen under optical microscope for measurement of average crack diameters against the indentation loads and finally studied under SEM for nature and morphology of indentations at various indentation loads. 3. Results and discussion SEM micrograph of the as-received uncoated tool tip shows fine grinding marks in Fig. 1. High magnification SEM photo reveals smeared layer of cobalt in Fig. 1.
Smearing effect is less pronounced at the cutting edge. Similar non-homogeneous nature of as received surface with smeared cobalt has also been reported earlier w6x. Fig. 1 shows that WC–Co substrate with 6% Co, when treated with dilute nitric acid, HNO3qH2O (1:1) with ultrasonic vibration for 15 min under treatment ‘treat1’, the smeared layer of cobalt was partially removed exposing WC grains. Fig. 2 indicates that because of etching, surface cobalt has been reduced to 1.41%, whereas surface roughness has slightly increased. Removal of non-tungsten material i.e. cobalt from the surface, with similar treatment has also been reported w4,9x. However, no quantitative data on residual Co were presented. For further studying the effectiveness of such treatment, the etching time under the treatment ‘treat1’
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Fig. 4. SEM micrograph of cutting edge and rake face of inserts after diamond film deposition, (a) cutting edge; (b) rake surface.
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Fig. 6. Raman spectra of diamond film on WC–Co substrate surface.
50% diluted HNO3, surface cobalt could be reduced to 0.3–0.4% from a bulk content of 9–12%. The present investigation, in contrast, undoubtedly reveals that, 50% diluted HNO3 did not give good results. There was an increase of roughness of substrate surface when etching time was extended from 15 to 30 min as indicated in Fig. 2. However, etching depth was less than depth of grinding marks, which are clearly visible in Fig. 1. It appears from Fig. 2 that the substrate treated with a solution of HNO3qHClqH2O (1:1:1) for 15 min under treatment ‘treat3’, could bring down Co as low as 0.46% and surface roughness has gone up further. The SEM view of the etched surface clearly reveals the WC grains (Fig. 1). The grinding marks have also been completely removed as a result of strong etching. The variation in cobalt percentage on the untreated surface and after treatment ‘treat1’ and ‘treat3’ have also been indicated in EDAX spectra of Fig. 3. From the above observations, it is clear that Co has been effectively etched by HNO3qHClqH2O (1:1:1). It is well known that Co is readily dissolved in hydrochloric and nitric acid forming aqueous solution of cobalt chloride and cobaltous nitrate w10,11x. This has been
Fig. 5. X-ray diffraction spectra of uncoated diamond coated tungsten carbide cutting tool inserts.
was extended to 30 min in treatment ‘treat2’. This resulted in a reduction of residual cobalt content to 1.25% (Fig. 2). It has been observed that etching by HNO3 may not completely remove Co from the substrate surface w9x. Another investigation w5x claimed that with
Fig. 7. Surface roughness of diamond coated tools.
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Fig. 8. SEM fractograph of the cutting edge and coating thickness of diamond coated tools (a) coating at cutting edge covering flank and rake face; (b) coating at rake face.
clearly reflected in the present result that the combination of HCl and HNO3 has accelerated removal of Co and also increase of roughness on the substrate surface (Fig. 2). SEM micrograph of the surface of the diamond coated tool T1 (Table 1) in Fig. 4 shows good coverage of coating with well faceted crystals having cubic (1 0 0) and octahedral (1 1 1) structures, (1 1 1) being most common, which has been well evident from XRD diagram in Fig. 5. The same figure also shows XRD diagram of the uncoated substrate. A sharp peak at 1337.5ycm of the Raman spectra obtained on the coating of the tool T1 in Fig. 6 confirms the quality of the
coating to be good. The crystal size varies in the range 2–5 mm, fine crystals 2–3 mm are most common. But, outgrowths at some locations are also noticed. Due to presence of such outgrowths, the surface appears to be rough as observed from Fig. 7. SEM fractograph of the tool T1 in Fig. 8 shows diamond coating with a thickness approximately 4–4.5 mm. The coating has a columnar growth. No interfacial void is observed with the tool T1. SEM pictures of the rake face of the tool T2 coated on substrate with treatment ‘treat2’ having 1.25% surface cobalt have been shown in Fig. 4. The coating also shows good coverage with (1 0 0) and (1 1 1) faceted crystals. However, (1 1 1) texture of the coating is found
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Fig. 9. SEM micrograph of the indentation crack morphology for diamond coated tools under various indentation loads, (a) under 30 kgf load; (b) under 60 kgf load; (c) under 100 kgf load.
to be predominant. The XRD spectra of the tool was similar to that of tool T1 shown in Fig. 5. The crystal size varies in the range 2–3 mm, 2 mm crystals being most common. The surface is free from outgrowth and is smooth and regular. Compared to T1 tool, overall grain uniformity is much improved as a result of which average surface roughness has been found to be lower as is evident from Fig. 7. Fractograph of the tool T2 in Fig. 8 shows coating thickness 3.5 to 4 mm. In Fig. 8 SEM picture further reveals dense and crystalline nature of coating similar to that in tool T1. The coating substrate interface appears to be better interlocked due
to more anchoring sites. This might be the result of slightly lower surface cobalt content. Similar results of fine and smooth morphology with lower cobalt content has been reported in a previous investigation w5x. Having better results with lower cobalt content under ‘treat2’ compared to ‘treat1’, attempt was made to deposit diamond on substrates with still lower cobalt content obtained by the most effective treatment ‘treat3’. Tool T3 was coated under such substrate treatment. SEM morphology of the cutting edge of the tool T3 in Fig. 4 shows well-faceted crystals predominantly with (1 1 1) morphology. Rake face morphology reveals sim-
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Fig. 10. Crack diameter vs. indentation load for diamond coated tools.
ilar features as is evident from XRD spectra in Fig. 5. The size of crystals on the tool surface appears to be relatively large. However, 3–4 mm crystals are most common. The coating is free from defects like outgrowths and pinholes. Due to large size crystals, the surface appears to be relatively rough as evident in Fig. 7. SEM fractograph of the tool T3 in Fig. 8 reveals that the thickness of the coating is 5–5.5 mm. SEM picture clearly shows that the coating has grown from the inner crests of the substrates with a good number of anchoring sites. The coating is dense at the substrate surface without any crack or discontinuity. However, some porous structures have been observed towards the coating surface. Among the tools developed, T3 has a comparatively thicker coating, which might have resulted due to very low Co content favouring nucleation and growth at the intermediate temperature 740 8C. The thicker film resulting in larger surface crystals contributed significantly to higher surface roughness. Nondiamond carbon phases are not observed from the SEM picture, which is further confirmed from the Raman spectra. SEM micrograph of the indentation crack morphology on tool T1 in Fig. 9 under 30 kgf load shows lateral flaking of the coating beyond the crater region. With increase of indentation load to 60 and 90 kgf, flaked area on the coating also grew more as can be seen from
SEM picture shown in the same figure. Comparing the SEM picture of the indentation crack morphology and ‘d vs P’ graphs for the tools T1 and T2 in Fig. 9 and Fig. 10, one can arrive at the conclusion that just by reducing the surface cobalt percentage from 1.41 to 1.25, adhesion of the coating to the substrate can be improved. Result of the indentation test for the tool T3 further confirmed that through reduction of surface cobalt, adhesion of diamond film with WC substrate could be enhanced remarkably. In case of T3 tool the amount of surface cobalt was just 0.46%. At 30 kgf load the coating underwent deformation without experiencing any crack in a localized region unlike that of tools T1 and T2. It is important to note that the very resistance to flaking of the coating during indentation has gradually improved with the reduction in surface cobalt content. This is quite evident when one looks at the SEM pictures in Fig. 9 and ‘d vs P’ graph for tools T1, T2 and T3. The much improved adhesion of tool T3 is clearly demonstrated in Fig. 10. The slope of the characteristic graph ‘d vs P’ is also found to be minimum for tool T3. 4. Conclusions (1) Dilute HNO3 (50% concentration) could remove partially surface cobalt from a WCqCo substrate when
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used under ultrasonic vibration for 15 min. The amount of surface cobalt was reduced to 1.41% from a nominal amount of approximately 6%. The etched surface of the substrate was found to be rougher than the unetched surface. (2) Prolongation of the etching time from 15 to 30 min with the same diluted HNO3 could reduce the surface cobalt only to 1.25%. The roughness of the etched surface was found to be slightly more than what was observed after etching for 15 min. (3) A solution of HNO3qHClqH2O (1:1:1) was found to be very effective etchant for removing cobalt from WCqCo substrate. It could bring down the surface cobalt to less than 0.5% when used under ultrasonic vibration just for 15 min. Roughness of the surface obtained after etching was found to be still higher. (4) The residual Co on the substrate surface was found to be approximately 0.6% when treated with aqua regia, HNO3q3HCl for 15 min also under ultrasonic vibration. However, roughness of the etched surface was found to be lower than what was obtained on the surface etched with the solution of HNO3qHClqH2O (1:1:1). (5) Film deposited on differently etched surface showed diamond predominantly with (1 1 1) morphology. Raman spectra also did not reveal existence of any non-diamond phase in the coating. Diamond coating deposited on HNO3qHClqH2O (1:1:1) solution etched surface showed larger crystals than that deposited on diluted HNO3 etched surface. Coating thickness was also greater in the former in comparison to what was deposited on the latter. (6) Etching with HNO3qHClqH2 O (1:1:1) solution not only reduces surface cobalt content substantially but also creates deep anchoring sites for ‘seed diamond’ on which diamond film forms and grows. (7) Adhesion test by indentation clearly shows improvement in the film substrate adhesion with reduced
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cobalt content on the substrate surface and in this respect a solution of HNO3qHClqH2O (1:1:1) has been found to be an effective etchant since it not only removes cobalt from the substrate surface efficiently without attacking WC grains but also increases the surface roughness of the etched surface significantly. References w1x D.A. Stephenson, J.S. Agapiou, Metal Cutting Theory and Practice, Marcel Dekker Inc. Publication, NY, 1997. w2x S. Hosomi, I. Yoshida, Diamond CVD researches as patent applied, Application of Diamond Films and Related Materials, Elsevier Science Publishers, 1991, pp. 15–24. w3x T.H. Huang, C.T. Kuo, T.S. Lin, Tribological behaviour of chemical vapour deposition diamond films on various cutting tools, Surf. Coat. Technol. 56 (1993) 105–108. w4x A.K. Mehlmann, S.F. Dirnfeld, Y. Avigal, Investigation of lowpressure diamond deposition on cemented carbides, Diamond Relat. Mater. 1 (1992) 600–604. w5x T.H. Huang, C.T. Kuo, C.S. Chang, C.T. Kao, H.Y. Wen, Tribological behaviours of the diamond-coated cemented carbide tools with various cobalt contents, Diamond Relat. Mater. 1 (1992) 594–599. w6x M. Alam, D.E. Peebles, D.R. Tallant, Diamond deposition onto WC–6%Co cutting tool material, Thin Solid Films 300 (1997) 164–170. w7x F. Deuerler, H. Van den Berg, R. Tbersky, A. Freundlieb, V. Buck, Pretreatment of substrate surface for improved adhesion of diamond films on hard metal cutting tools, Diamond and Related Materials 5 (1996) 1478–1489. w8x P.C. Jindal, D.T. Quinto, G.J. Wolfe, Adhesion measurement of chemically vapor deposited and physically vapor deposited hard coatings on WC–Co substrates, Thin Solid Films 154 (1987) 361–375. w9x S. Chatterjee, A.G. Edwards, A. Nichols, C.S. Feigerle, Analysis of surface preparation treatments for coating tungsten carbide substrates with diamond thin films, J. Mater. Sci. 32 (1997) 2827–2833. w10x S.R. Young, Cobalt, Reinhold Publishing Corporation, New York, 1948. w11x W. Betteridge, Cobalt and its Alloys, Wiley, 1982.