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CVD diamond-coated steel inserts for thermoplastic mould tools—Characterization and preliminary performance evaluation
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1085–1091
journal homepage: www.elsevier.com/locate/jmatprotec
CVD diamond-coated steel inserts for thermoplastic mould tools—Characterization and preliminary performance evaluation ´ V.F. Neto ∗ , R. Vaz, M.S.A. Oliveira, J. Gracio Centre for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro, 3810-193 Aveiro, Portugal
a r t i c l e
i n f o
a b s t r a c t
Article history:
The increasing demand for quality on moulded polymer parts is stressing the research and
Received 11 June 2007
development on new and enhanced injection moulding materials and tools. The latter, is the
Received in revised form
objective of the study reported here focusing the research on moulds with considerable lower
6 March 2008
adhesion walls and simultaneously better mould heat extraction rates. Polycrystalline dia-
Accepted 14 March 2008
mond coatings enable the attainment of both characteristics above highlighted, inferring to the mould wall a higher thermal conductivity coefficient and one of the lowest friction coefficients. Recent developments in the diamond chemical vapour deposition (CVD) process,
Keywords:
combined with the usage of appropriated interlayer systems, open the possibility to apply
Diamond CVD coatings
polycrystalline diamond coatings onto steel mould inserts. Therefore, this study reports
Mould tools
results obtained on the properties of diamond thin films deposited on typical mould steel,
Steel substrates
and preliminary results on the interaction between the diamond/steel system and the melt
Interlayer
polymer.
Thermoplastic injection moulding
1.
Introduction
Polycrystalline diamond, in microcrystalline or nanocrystalline morphology, detains a number of extreme properties that point it as a technology suitable for exploitation in numerous industrial applications. In a review paper, May (2000) reported that diamond possesses an extreme mechanical hardness (ca. 90 GPa) and wear resistance, one of the highest bulk modulus (1.2 × 1012 N m−2 ), the lowest compressibility (8.3 × 10−13 m2 N−1 ), the highest room temperature thermal conductivity (2 × 103 W m−1 K−1 ), a very low thermal expansion coefficient at room temperature (1 × 10−6 K) and is very resistant to chemical corrosion. Most of these properties are attractive for cavities and mould tools, nevertheless coating an entire cavity with poly-
∗
Corresponding author. Tel.: +351 234 370 830; fax: +351 234 370 953. E-mail address: [email protected] (V.F. Neto). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.03.012
© 2008 Elsevier B.V. All rights reserved.
crystalline diamond is presently an utopia. Chemical vapour deposition (CVD) systems are considerably size limited due to the means of activating (thermal, electric discharge, or combustion flame) the gas phase carbon-containing precursor molecules. A second problem related to the usage of CVD on mould tools is concerned with the fact that the diamond cannot be directly coated onto ferrous substrates. Carbon, the precursor element of diamond, easily diffuses into the ferrous matrix, leaving behind no matter to start the diamond nucleation process. To bypass the latter, appropriate interlayers can be used. A suitable interlayer is the one that promotes a diffusion block from and to the substrate material, enhances the adhesion between the diamond coating and the mould, and does not affect the properties of the dia-
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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1085–1091
mond film or those of the mould tool, as pointed by Neto et al. (2008). A third constraint is the typical diamond deposition temperatures. The process temperature could be a limitation, since high temperatures may change the heat treatment induced in the steel material and alter its properties. Nevertheless, in the past years, CVD temperatures have been experiencing a significant reduction, a fact that has been pointed out by Petherbridge et al. (2001) and Dong et al. (2002), claiming depositions of good quality diamond at temperatures as low as 400 ◦ C, and not at the typical ∼900 ◦ C. With the expertise accomplished in the past recent years by diamond surface engineering researchers, it is now possible to coat small metal inserts for polymer moulding and evaluate the diamond coating performance. In this study, a small steel plate has been diamond coated, mechanically characterized and tested under service conditions, to produce high-density polyethylene (HDPE) components.
2.
Experimental details
A 50 mm × 50 mm × 5 mm steel (AISI P20) plate, supplied by MoldAveiro (Aveiro, Portugal) was diamond coated in a hot-filament CVD reactor, described by Ali et al. (2003). A commercial chromium nitride (CrN) interlayer was deposited onto the steel plate, in a Microcoat PVD-arc system, by Prirev (Vagos, Portugal). Prior to the diamond deposition the sample was subjected to a 3-h ultrasonic bath in 1/4 m polycrystalline diamond solution, followed by a 2-min cleaning with isopropanol. Diamond deposition parameters were optimized considering the nature of the interlayer and the steel substrate, as presented in Table 1 and Fig. 1. Following Ali et al. (2003), it was used CH4 modulations with respect to deposition time, as
Table 1 – Diamond deposition conditions Parameter
Value
Pressure (kPa) Substrate temperature (◦ C) Filament temperature (◦ C) Filament–substrate distance (mm) Hydrogen (H2 ) flow (sc cm)
3.9 800 2100 8 200
Fig. 1 – Time-modulation of the methane (CH4 ) gas related to H2 .
shown in Fig. 1, to enhance the nucleation density in the beginning of the deposition and to promote secondary nucleation during the film growth, resulting in denser and homogeneous coating. The substrate surface temperature was measured using a K-type thermocouple, which was located directly underneath and touching the bottom of the substrate during deposition. The filament temperature was measured by an IMPAC optical pyrometer. The coated steel plate was tested for polymer moulding, using a mould tool specially designed to accommodate the
Fig. 2 – (A) Mould cavity and (B) close-up of the diamond-coated insert plate.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1085–1091
Table 2 – Cycle injection moulding processing conditions Parameter Injection pressure (bar) Melt temperature (◦ C) Hold pressure (bar) Hold time (s) Cooling time (s)
Value 50 200 30 5 22
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After the injection moulding cycles were carried out, a series of indentation tests were performed in a Frank hardness tester Frankoskop 38180 with a 2.5-mm steel Brinell indenter, employing a range of loads between 153.2 N and 613 N. Vickers micro-hardness of the coated samples where measured using a Shimadzu micro-hardness tester HMV-2000, with a range of loads from 0.49 N to 1.96 N. Surface morphology of the diamond coating (before and after polymer injections moulding) and indentation zones were assessed by scanning electron microscopy (SEM), with a Hitachi S-4100 SEM system equipped with energy dispersive X-ray spectroscopy (EDS). A surface profiler (Hommelwerke, T1000, Japan) was used to measure the film surface roughness of the deposited diamond film. Stress state and quality of the diamond coatings were assessed by Raman spectroscopy. Raman spectra of the samples were obtained at room temperature, using an ISA JOBIN YVON-SPEX T6400 system with a 514.5-nm-Ar+ ion laser.
3.
Results and discussion
The deposited diamond film, as it can be seen in Fig. 2, presents a ∼30 mm diameter spot centred in a square substrate, where the film has a homogeneous growth. Outside this spot, the film looses its crystallinity and just amorphous carbon is present. This is probably due to the limitations of CVD equipment used which has a theoretical 50 mm × 50 mm deposition area. Inside the above-referred spot, the diamond film was analysed in a SEM before and after the injection routine cycle. As it can be depicted from Fig. 3(A), of the as-deposited diamond surface, the film presents a predominant (1 1 1) and (1 0 0) crystallite orientation, which is typical for the used deposition temperatures. For diamond growth, the three main surfaces for adsorption and growth are the triangular (1 1 1) surface, the square (1 0 0) surface, and the less well-defined (1 1 0) surface (May, 2000). This statement has also been confirmed by X-ray diffraction, not shown here. It can also be depicted from Fig. 3(A), from measurements, that the predominant crystal size is under 500 nm. The film thickness is
Fig. 3 – SEM image of the (A) as-deposited diamond film, of the (B) diamond film after the routine injection cycle, and of (C) a lower amplification of (B).
insert (Fig. 2) and mounted in an “Inautom EuroInj D65” injection moulding machine to perform a cycle of 250 HDPE sample plates. The injection moulding processing conditions are presented in Table 2.
Fig. 4 – HDPE injected plate.
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Fig. 6 – Brinell hardness of the steel substrate, of the steel substrate-coated with CrN and the steel substrated with CrN interlayer and diamond coated. Fig. 5 – EDS of the diamond film.
estimated to be within 1.5–2.0 m and the measured average roughness is 0.14 m. After this preliminary analyses of the as-deposited diamond film, the coated plate was inserted in a mould tool as shown in Fig. 2 and mounted in the injection moulding machine. HDPE test samples, presented in Fig. 4, were obtained using the conditions presented in Table 2. The first injected samples presented some small dark spots, as the result of the pealing of the amorphous carbon. After 10 cycles, the injected objects were perfectly clear of any coating contamination. After the 250 injection cycles, the insert was again removed from the mould tool and re-analysed to evaluate any wear
and degradation of the polycrystalline diamond coating. Parts production can be catalogued as short, average, long or as big productions, if the number of objects being processed are from 1 to 30 000, 30 0000 to 250 000, more than 250 000 and more than 500 000, respectively (Manual do Projectista, 2003). Although in this work a reduced number of moulding cycles were carried out, being this a preliminary work of an investigation into the application of diamond coatings to moulds inserts used in plastic injection moulding, which may enhance wear resistance of the inserts and improve the moulding release, it is important to assess the characteristics of the diamond films after this number of injections. It can be depicted from Fig. 3(B), that no relevant morphological changes were observed, a fact considered an evidence
Fig. 7 – SEM images of Brinell hardness indentations, using a 2.5-mm steel sphere and loads of 613 N, 490 N, 306.5 N and 153.2 N.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1085–1091
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Fig. 8 – SEM images of Brinell hardness indentations boundary zone, using a 2.5-mm steel sphere and loads of 613 N, 490 N, 306.5 N and 153.2 N.
for the good mutual adhesion of the steel–interlayer–diamond system, at least for this number of routine cycles. The image shows some darker spots that may be small amounts of plastic attached to the diamond film. Fig. 3(C) shows a lower amplification SEM image of the diamond-coated plate after the routine injection cycles, where it can be seen that the film is quite homogeneous. Also, from Fig. 5, it can be seen that on the EDS spectra of the coated plates, before and after injection, the ele-
ments exhibited are carbon, probably from the diamond and enhanced form HDPE residues on the after injection spectrum, chromium from the interlayer, and iron from the substrate matrix. The EDS spectra of the coated plates, before and after injection, are very similar, indicating that no significant modification occurred during the injection cycles. The measured Brinell hardness for the coated system was determined to be 226 HB, presenting a decrease of 27.9% in
Fig. 9 – Raman spectrum of the diamond coating before and after Brinell indentation at 613 N (in the centre, in the middle and in the boundary of the indentation).
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hardness when compared with the steel–CrN system (289 HB). The steel substrate coated with CrN also presents a decrease of 8.3% of Brinell hardness compared with the bare steel substrate (313 HB). These reductions on the hardness of the coated systems are due to steel and CrN phase transformations occurred when the CrN and diamond films are formed. Diamond growth, as showed in Table 1, was performed at a substrate temperature of 800 ◦ C, above the ␣ → ␥ transformation temperature. The PVD-arc CrN coating, although performed at low temperature, involves high kinetic energy (material leaving the cathode at a velocity of around 10 km/s) (Fig. 6). The Brinell hardness indentations serve also to qualitatively analyse the coatings adhesion. Fig. 7 shows the effect of the 2.5 mm steel ball indenter with applied loads of 613 N, 490 N, 306.5 N and 153.2 N. It can be observed that no significant delamination occurred, even at the maximum tested
load. The coated film deformed to the shape of the Brinell indenter tip. The deformation of the coating to the shape of the indenter and in a synchronised form with the more plastic steel and CrN system, without slating, is an indication of the good adhesion of the diamond film (Fig. 8). Concentric cracks around the indentation spots can be observed in the 613 N, 490 N and 306.5 N indentations, more noteworthy in the highest load and decreasing with the load decrease. The cracks may be due to the external stress imposed onto the film during indentation loading that may force the film to crack in order to dissipate stress and/or energy. Raman spectroscopy was used to assess the diamond Raman quality (ratio between the intensity of the diamond species and the intensity of non-diamond species), proposed by Kulisch et al. (1996) and to estimate the residual stresses of the diamond film following Ager and Drory (1993) work.
Fig. 10 – SEM images of Vickers micro-hardness indentations, using load 0.49 N, 0.98 N and 1.96 N for 10 s and 5 s.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1085–1091
Raman quality was determined to be around 51%, where 100% would be pure diamond. According to Ager and Drory (1993), an experimental estimation of the residual stresses () can be achieved from the Raman spectrum shift (), by averaging the stress shift relation over all crystallite orientations: = −1.08s , = −0.384d ,
for singlet phonon for doublet phonon
(1) (2)
However, as it can be seen in Fig. 9, the large peak width of the diamond peak, does not allow the observation of the singlet and doublet phonons, so according to Ralchenko et al. (1995), it can be consider the peak measured to the centre point between singlet and doublet, 0.5(s + d ) and find: = −0.567
(3)
To verify the changes on the coatings residual stress, Raman spectroscopy was performed before and after the indentation tests for different loads. As it can be observed in Fig. 9, the as-deposited film has a spectrum shift of ∼14 cm−1 , which corresponds to ∼7.9 GPa of compressive stress, not very different from the one determined by Ralchenko et al. (1995) on a diamond film growth on steel, at similar conditions as the ones presented in this work. The Raman spectra acquired in the 613 N indentation area, present a slightly superior compressive stress, decreasing from the boundary of the indentation ( ∼21 cm−1 / ∼11.9 GPa) to its centre ( ∼18 cm−1 / ∼10.2 GPa). Complimentary to the Brinell hardness measurements, Vickers micro-hardness indentations, using 0.49 N (50 g), 0.98 N (100 g) and 1.96 N (200 g) and a holding time of 10 s and 5 s, were performed. The measured micro-hardness stabilised at 483 HV (±10%) for the set of measurements performed at 0.98 and 1.96 N for both the holding times. The 0.49 N load, held for 10 s, presented 270 HV. For the 5 s holding time, at the above-referred load, it was not possible to measure the hardness value accurately. Fig. 10 shows the SEM images of the micro-indentations zones. All indented spots present surface modifications, in what seems to occur a reduction of the adhesion of the diamond film to the substrate.
4.
Conclusions and further work
Polycrystalline diamond was deposited in mould steel, with the usage of a commercial CrN interlayer film and appropriated deposition conditions. The work presented in this paper demonstrates the possibility to use CVD polycrystalline diamond to enhance plastic injection moulding, although much work remains to be done.
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Increased number of routine cycles must be performed in order to correlate some statistical degradation of the diamondcoated insert. Also different diamond films must be tested and incorporated in the statistical degradation investigation. It is important to observe the influence on the results of different film morphologies (different crystal size, crystal orientation, etc.), different film thicknesses, and different roughness coatings.
Acknowledgments V.F. Neto thanks the Universidade de Aveiro for the PhD funding support. The authors are grateful to FCT, Portugal, for funding the project: POCTI/EME/60816/2004. The authors would like to acknowledge Mr. Victor Marques, MoldAveiro (Aveiro, Portugal), for supplying the substrate material, Eng. Luis Godinho and Eng. Rui Pimenta, from Prirev (Vagos, Portugal), for producing the CrN interlayers and also ˜ Teles for the helpful Eng.a Tatiana Zhiltsova and Eng. Joao collaboration operating the injection-moulding machine.
references
Ager, J.W., Drory, M.D., 1993. Quantitative measurement of residual biaxial stress by Raman spectroscopy in diamond grown on a Ti alloy by chemical vapour deposition. Phys. Rev. B 48, 2601–2607. ´ Ali, N., Neto, V.F., Gracio, J., 2003. Promoting secondary nucleation using methane modulations during diamond CVD to produce smoother, harder and better quality films. J. Mater. Res. 18 (2), 296–304. Dong, L., Ma, B., Dong, G., 2002. Diamond deposition at low temperature by using CH4 /H2 gas mixture. Diam. Relat. Mater. 11, 1697–1702. Kulisch, W., Ackermann, L., Sobisch, B., 1996. On the mechanisms of bias enhanced nucleation of diamond. Phys. Status Solidi A 154, 155–174. Manual do Projectista, 2003. CENTIMFE–Technological Center for the Mouldmaking, Special Tooling and Plastic Industries, Marinha Grande, Portugal. May, P.W., 2000. Diamond thin films: a 21st century material. Philo. Trans. Roy. Soc. Lond. A 358, 473–495. ´ Neto, V.F., Shokuhfar, T., Oliveira, M.S.A., Gracio, J., Ali, N., 2008. Polycrystalline diamond coatings on steel substrates. Int. J. Nanomanuf. 2 (1/2), 99–115. Petherbridge, J.R., May, P.W., Pearce, S.R.J., Rosser, K.N., Ashfold, M.N.R., 2001. Low temperature diamond growth using CO2 /CH4 plasmas: molecular beam mass spectrometry and computer simulation investigations. J. Appl. Phys. 89 (2), 1484–1492. Ralchenko, V.G., Smolin, A.A., Pereverzev, V.G., Obraztsova, E.D., Korotoushenko, K.G., Konov, V.I., Lakhotkin, Y.V., Loubnin, E.N., 1995. Diamond deposition on steel with CVD tungsten intermediate layer. Diam. Relat. Mater. 4, 754–758.
journal homepage: www.elsevier.com/locate/jmatprotec
CVD diamond-coated steel inserts for thermoplastic mould tools—Characterization and preliminary performance evaluation ´ V.F. Neto ∗ , R. Vaz, M.S.A. Oliveira, J. Gracio Centre for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro, 3810-193 Aveiro, Portugal
a r t i c l e
i n f o
a b s t r a c t
Article history:
The increasing demand for quality on moulded polymer parts is stressing the research and
Received 11 June 2007
development on new and enhanced injection moulding materials and tools. The latter, is the
Received in revised form
objective of the study reported here focusing the research on moulds with considerable lower
6 March 2008
adhesion walls and simultaneously better mould heat extraction rates. Polycrystalline dia-
Accepted 14 March 2008
mond coatings enable the attainment of both characteristics above highlighted, inferring to the mould wall a higher thermal conductivity coefficient and one of the lowest friction coefficients. Recent developments in the diamond chemical vapour deposition (CVD) process,
Keywords:
combined with the usage of appropriated interlayer systems, open the possibility to apply
Diamond CVD coatings
polycrystalline diamond coatings onto steel mould inserts. Therefore, this study reports
Mould tools
results obtained on the properties of diamond thin films deposited on typical mould steel,
Steel substrates
and preliminary results on the interaction between the diamond/steel system and the melt
Interlayer
polymer.
Thermoplastic injection moulding
1.
Introduction
Polycrystalline diamond, in microcrystalline or nanocrystalline morphology, detains a number of extreme properties that point it as a technology suitable for exploitation in numerous industrial applications. In a review paper, May (2000) reported that diamond possesses an extreme mechanical hardness (ca. 90 GPa) and wear resistance, one of the highest bulk modulus (1.2 × 1012 N m−2 ), the lowest compressibility (8.3 × 10−13 m2 N−1 ), the highest room temperature thermal conductivity (2 × 103 W m−1 K−1 ), a very low thermal expansion coefficient at room temperature (1 × 10−6 K) and is very resistant to chemical corrosion. Most of these properties are attractive for cavities and mould tools, nevertheless coating an entire cavity with poly-
∗
Corresponding author. Tel.: +351 234 370 830; fax: +351 234 370 953. E-mail address: [email protected] (V.F. Neto). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.03.012
© 2008 Elsevier B.V. All rights reserved.
crystalline diamond is presently an utopia. Chemical vapour deposition (CVD) systems are considerably size limited due to the means of activating (thermal, electric discharge, or combustion flame) the gas phase carbon-containing precursor molecules. A second problem related to the usage of CVD on mould tools is concerned with the fact that the diamond cannot be directly coated onto ferrous substrates. Carbon, the precursor element of diamond, easily diffuses into the ferrous matrix, leaving behind no matter to start the diamond nucleation process. To bypass the latter, appropriate interlayers can be used. A suitable interlayer is the one that promotes a diffusion block from and to the substrate material, enhances the adhesion between the diamond coating and the mould, and does not affect the properties of the dia-
1086
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1085–1091
mond film or those of the mould tool, as pointed by Neto et al. (2008). A third constraint is the typical diamond deposition temperatures. The process temperature could be a limitation, since high temperatures may change the heat treatment induced in the steel material and alter its properties. Nevertheless, in the past years, CVD temperatures have been experiencing a significant reduction, a fact that has been pointed out by Petherbridge et al. (2001) and Dong et al. (2002), claiming depositions of good quality diamond at temperatures as low as 400 ◦ C, and not at the typical ∼900 ◦ C. With the expertise accomplished in the past recent years by diamond surface engineering researchers, it is now possible to coat small metal inserts for polymer moulding and evaluate the diamond coating performance. In this study, a small steel plate has been diamond coated, mechanically characterized and tested under service conditions, to produce high-density polyethylene (HDPE) components.
2.
Experimental details
A 50 mm × 50 mm × 5 mm steel (AISI P20) plate, supplied by MoldAveiro (Aveiro, Portugal) was diamond coated in a hot-filament CVD reactor, described by Ali et al. (2003). A commercial chromium nitride (CrN) interlayer was deposited onto the steel plate, in a Microcoat PVD-arc system, by Prirev (Vagos, Portugal). Prior to the diamond deposition the sample was subjected to a 3-h ultrasonic bath in 1/4 m polycrystalline diamond solution, followed by a 2-min cleaning with isopropanol. Diamond deposition parameters were optimized considering the nature of the interlayer and the steel substrate, as presented in Table 1 and Fig. 1. Following Ali et al. (2003), it was used CH4 modulations with respect to deposition time, as
Table 1 – Diamond deposition conditions Parameter
Value
Pressure (kPa) Substrate temperature (◦ C) Filament temperature (◦ C) Filament–substrate distance (mm) Hydrogen (H2 ) flow (sc cm)
3.9 800 2100 8 200
Fig. 1 – Time-modulation of the methane (CH4 ) gas related to H2 .
shown in Fig. 1, to enhance the nucleation density in the beginning of the deposition and to promote secondary nucleation during the film growth, resulting in denser and homogeneous coating. The substrate surface temperature was measured using a K-type thermocouple, which was located directly underneath and touching the bottom of the substrate during deposition. The filament temperature was measured by an IMPAC optical pyrometer. The coated steel plate was tested for polymer moulding, using a mould tool specially designed to accommodate the
Fig. 2 – (A) Mould cavity and (B) close-up of the diamond-coated insert plate.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1085–1091
Table 2 – Cycle injection moulding processing conditions Parameter Injection pressure (bar) Melt temperature (◦ C) Hold pressure (bar) Hold time (s) Cooling time (s)
Value 50 200 30 5 22
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After the injection moulding cycles were carried out, a series of indentation tests were performed in a Frank hardness tester Frankoskop 38180 with a 2.5-mm steel Brinell indenter, employing a range of loads between 153.2 N and 613 N. Vickers micro-hardness of the coated samples where measured using a Shimadzu micro-hardness tester HMV-2000, with a range of loads from 0.49 N to 1.96 N. Surface morphology of the diamond coating (before and after polymer injections moulding) and indentation zones were assessed by scanning electron microscopy (SEM), with a Hitachi S-4100 SEM system equipped with energy dispersive X-ray spectroscopy (EDS). A surface profiler (Hommelwerke, T1000, Japan) was used to measure the film surface roughness of the deposited diamond film. Stress state and quality of the diamond coatings were assessed by Raman spectroscopy. Raman spectra of the samples were obtained at room temperature, using an ISA JOBIN YVON-SPEX T6400 system with a 514.5-nm-Ar+ ion laser.
3.
Results and discussion
The deposited diamond film, as it can be seen in Fig. 2, presents a ∼30 mm diameter spot centred in a square substrate, where the film has a homogeneous growth. Outside this spot, the film looses its crystallinity and just amorphous carbon is present. This is probably due to the limitations of CVD equipment used which has a theoretical 50 mm × 50 mm deposition area. Inside the above-referred spot, the diamond film was analysed in a SEM before and after the injection routine cycle. As it can be depicted from Fig. 3(A), of the as-deposited diamond surface, the film presents a predominant (1 1 1) and (1 0 0) crystallite orientation, which is typical for the used deposition temperatures. For diamond growth, the three main surfaces for adsorption and growth are the triangular (1 1 1) surface, the square (1 0 0) surface, and the less well-defined (1 1 0) surface (May, 2000). This statement has also been confirmed by X-ray diffraction, not shown here. It can also be depicted from Fig. 3(A), from measurements, that the predominant crystal size is under 500 nm. The film thickness is
Fig. 3 – SEM image of the (A) as-deposited diamond film, of the (B) diamond film after the routine injection cycle, and of (C) a lower amplification of (B).
insert (Fig. 2) and mounted in an “Inautom EuroInj D65” injection moulding machine to perform a cycle of 250 HDPE sample plates. The injection moulding processing conditions are presented in Table 2.
Fig. 4 – HDPE injected plate.
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Fig. 6 – Brinell hardness of the steel substrate, of the steel substrate-coated with CrN and the steel substrated with CrN interlayer and diamond coated. Fig. 5 – EDS of the diamond film.
estimated to be within 1.5–2.0 m and the measured average roughness is 0.14 m. After this preliminary analyses of the as-deposited diamond film, the coated plate was inserted in a mould tool as shown in Fig. 2 and mounted in the injection moulding machine. HDPE test samples, presented in Fig. 4, were obtained using the conditions presented in Table 2. The first injected samples presented some small dark spots, as the result of the pealing of the amorphous carbon. After 10 cycles, the injected objects were perfectly clear of any coating contamination. After the 250 injection cycles, the insert was again removed from the mould tool and re-analysed to evaluate any wear
and degradation of the polycrystalline diamond coating. Parts production can be catalogued as short, average, long or as big productions, if the number of objects being processed are from 1 to 30 000, 30 0000 to 250 000, more than 250 000 and more than 500 000, respectively (Manual do Projectista, 2003). Although in this work a reduced number of moulding cycles were carried out, being this a preliminary work of an investigation into the application of diamond coatings to moulds inserts used in plastic injection moulding, which may enhance wear resistance of the inserts and improve the moulding release, it is important to assess the characteristics of the diamond films after this number of injections. It can be depicted from Fig. 3(B), that no relevant morphological changes were observed, a fact considered an evidence
Fig. 7 – SEM images of Brinell hardness indentations, using a 2.5-mm steel sphere and loads of 613 N, 490 N, 306.5 N and 153.2 N.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1085–1091
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Fig. 8 – SEM images of Brinell hardness indentations boundary zone, using a 2.5-mm steel sphere and loads of 613 N, 490 N, 306.5 N and 153.2 N.
for the good mutual adhesion of the steel–interlayer–diamond system, at least for this number of routine cycles. The image shows some darker spots that may be small amounts of plastic attached to the diamond film. Fig. 3(C) shows a lower amplification SEM image of the diamond-coated plate after the routine injection cycles, where it can be seen that the film is quite homogeneous. Also, from Fig. 5, it can be seen that on the EDS spectra of the coated plates, before and after injection, the ele-
ments exhibited are carbon, probably from the diamond and enhanced form HDPE residues on the after injection spectrum, chromium from the interlayer, and iron from the substrate matrix. The EDS spectra of the coated plates, before and after injection, are very similar, indicating that no significant modification occurred during the injection cycles. The measured Brinell hardness for the coated system was determined to be 226 HB, presenting a decrease of 27.9% in
Fig. 9 – Raman spectrum of the diamond coating before and after Brinell indentation at 613 N (in the centre, in the middle and in the boundary of the indentation).
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hardness when compared with the steel–CrN system (289 HB). The steel substrate coated with CrN also presents a decrease of 8.3% of Brinell hardness compared with the bare steel substrate (313 HB). These reductions on the hardness of the coated systems are due to steel and CrN phase transformations occurred when the CrN and diamond films are formed. Diamond growth, as showed in Table 1, was performed at a substrate temperature of 800 ◦ C, above the ␣ → ␥ transformation temperature. The PVD-arc CrN coating, although performed at low temperature, involves high kinetic energy (material leaving the cathode at a velocity of around 10 km/s) (Fig. 6). The Brinell hardness indentations serve also to qualitatively analyse the coatings adhesion. Fig. 7 shows the effect of the 2.5 mm steel ball indenter with applied loads of 613 N, 490 N, 306.5 N and 153.2 N. It can be observed that no significant delamination occurred, even at the maximum tested
load. The coated film deformed to the shape of the Brinell indenter tip. The deformation of the coating to the shape of the indenter and in a synchronised form with the more plastic steel and CrN system, without slating, is an indication of the good adhesion of the diamond film (Fig. 8). Concentric cracks around the indentation spots can be observed in the 613 N, 490 N and 306.5 N indentations, more noteworthy in the highest load and decreasing with the load decrease. The cracks may be due to the external stress imposed onto the film during indentation loading that may force the film to crack in order to dissipate stress and/or energy. Raman spectroscopy was used to assess the diamond Raman quality (ratio between the intensity of the diamond species and the intensity of non-diamond species), proposed by Kulisch et al. (1996) and to estimate the residual stresses of the diamond film following Ager and Drory (1993) work.
Fig. 10 – SEM images of Vickers micro-hardness indentations, using load 0.49 N, 0.98 N and 1.96 N for 10 s and 5 s.
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 1085–1091
Raman quality was determined to be around 51%, where 100% would be pure diamond. According to Ager and Drory (1993), an experimental estimation of the residual stresses () can be achieved from the Raman spectrum shift (), by averaging the stress shift relation over all crystallite orientations: = −1.08s , = −0.384d ,
for singlet phonon for doublet phonon
(1) (2)
However, as it can be seen in Fig. 9, the large peak width of the diamond peak, does not allow the observation of the singlet and doublet phonons, so according to Ralchenko et al. (1995), it can be consider the peak measured to the centre point between singlet and doublet, 0.5(s + d ) and find: = −0.567
(3)
To verify the changes on the coatings residual stress, Raman spectroscopy was performed before and after the indentation tests for different loads. As it can be observed in Fig. 9, the as-deposited film has a spectrum shift of ∼14 cm−1 , which corresponds to ∼7.9 GPa of compressive stress, not very different from the one determined by Ralchenko et al. (1995) on a diamond film growth on steel, at similar conditions as the ones presented in this work. The Raman spectra acquired in the 613 N indentation area, present a slightly superior compressive stress, decreasing from the boundary of the indentation ( ∼21 cm−1 / ∼11.9 GPa) to its centre ( ∼18 cm−1 / ∼10.2 GPa). Complimentary to the Brinell hardness measurements, Vickers micro-hardness indentations, using 0.49 N (50 g), 0.98 N (100 g) and 1.96 N (200 g) and a holding time of 10 s and 5 s, were performed. The measured micro-hardness stabilised at 483 HV (±10%) for the set of measurements performed at 0.98 and 1.96 N for both the holding times. The 0.49 N load, held for 10 s, presented 270 HV. For the 5 s holding time, at the above-referred load, it was not possible to measure the hardness value accurately. Fig. 10 shows the SEM images of the micro-indentations zones. All indented spots present surface modifications, in what seems to occur a reduction of the adhesion of the diamond film to the substrate.
4.
Conclusions and further work
Polycrystalline diamond was deposited in mould steel, with the usage of a commercial CrN interlayer film and appropriated deposition conditions. The work presented in this paper demonstrates the possibility to use CVD polycrystalline diamond to enhance plastic injection moulding, although much work remains to be done.
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Increased number of routine cycles must be performed in order to correlate some statistical degradation of the diamondcoated insert. Also different diamond films must be tested and incorporated in the statistical degradation investigation. It is important to observe the influence on the results of different film morphologies (different crystal size, crystal orientation, etc.), different film thicknesses, and different roughness coatings.
Acknowledgments V.F. Neto thanks the Universidade de Aveiro for the PhD funding support. The authors are grateful to FCT, Portugal, for funding the project: POCTI/EME/60816/2004. The authors would like to acknowledge Mr. Victor Marques, MoldAveiro (Aveiro, Portugal), for supplying the substrate material, Eng. Luis Godinho and Eng. Rui Pimenta, from Prirev (Vagos, Portugal), for producing the CrN interlayers and also ˜ Teles for the helpful Eng.a Tatiana Zhiltsova and Eng. Joao collaboration operating the injection-moulding machine.
references
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