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Hafnium carbide hard coatings produced by pulsed laser ablation and deposition
Surface and Coatings Technology 151 – 152 (2002) 531–533
Hafnium carbide hard coatings produced by pulsed laser ablation and deposition R. Teghila,*, A. Santagataa, M. Zaccagninoa, S.M. Barinovb, V. Marottac, G. De Mariad a b
Dipartimento di Chimica, Universita` della Basilicata, via N. Sauro 85, 85100 Potenza, Italy High Tech Ceramics Research Centre, Russian Academy of Sciences, Moscow 119361, Russia c CNR Istituto Materiali Speciali, via S. Loja, Tito Scalo, Italy d Dipartimento di Chimica, Universita` ‘La Sapienza’, Ple A. Moro 5, 00185 Roma, Italy
Abstract In this paper the results of the deposition of thin films of hafnium carbide by pulsed laser ablation deposition are reported. The coatings characteristics, investigated by conventional techniques such as X-ray diffraction (XRD), scanning and transmission electron microscopy (SEMyTEM), and atomic force microscopy (AFM) are discussed in relation with the properties of the plasma produced in the interaction between the laser source (frequency doubled Nd:YAG laser) and the target. The plasma analysis, performed by emission spectroscopy, optical imaging and time of flight mass spectrometry, is also used to clarify the ablation mechanism and to compare it with those of the other carbides of the same group. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Hafnium carbide; Pulsed laser ablation; Thin film
1. Introduction Hafnium carbide films have found wide application in various tribological engineering devices owing to their excellent hardness and wear resistance. Furthermore, this carbide like the other carbides of the group four elements, shows a very high melting point, good resistance to corrosion and low thermal conductivity; ideal characteristics for use in high temperature applications. In previous works w1–3x we studied the pulsed ablation deposition of other two carbides of group four, TiC and ZrC, and the results evidenced a very different behaviour between the two systems. In particular, in the ablation of titanium carbide we found the presence of two different mechanisms: at low laser fluence (up to 3 Jycm2) a thermal mechanism with a non-congruent vaporisation; and at high laser fluence ()3 Jycm2) a congruent vaporisation probably related to a phase explosion mechanism w4x, or in any case, to a nonthermal mechanism. In the case of zirconium carbide * Corresponding author. Tel.: q39-971-202225; fax: q39-971202223. E-mail address: [email protected] (R. Teghil).
we found only one mechanism, corresponding to the phase explosion. In this work we extend our analysis to hafnium carbide, with the aim to give a deeper insight to the ablation and deposition mechanisms of group four carbides. 2. Experimental The experimental apparatus consists of a vacuum chamber equipped with quartz windows for laser beam inlet and in situ optical analyses, heatable substrate holder and rotating target support. The background pressure inside the chamber was 1.5=10y4 Pa. The laser source was a frequency doubled Nd-YAG laser (ls532 nm, ts10 ns, 10 Hz repetition rate) impinging on the target with an incidence angle of 458. The substrate (111 oriented silicon) was kept at distance of 2.5 cm from the target (hot pressed carbide tablets 99% pure from Cerac). The characterisation of the plasma has been carried out by emission spectroscopy (OMA-ICCD system, EG&G), optical imaging (ICCD camera, EG&G) and time of flight (TOF) mass spectrometry (LAMMA 500 from Leybold). The spatial and temporal resolution of the ICCD systems were 150 mm and 5 ns, respectively.
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 5 8 9 - 4
532
R. Teghil et al. / Surface and Coatings Technology 151 – 152 (2002) 531–533
Fig. 1. TOF mass spectrum of positive ions produced from the ablation of a HfC target.
A scanning electron microscope (SEM) Cambridge 100 with a 7 nm resolution was used for a morphological characterisation of the deposited films and the X-ray diffraction patterns have been recorded by a Siemens D5000 difractometer using Cu Ka1 radiation. The film hardness was measured using a Leica VMHT apparatus (Leica Ltd., Switzerland) equipped with a
Fig. 2. Overall light emission from HfC plume recorded by an ICCD camera, 600 ns after the laser shot. The contours in the figure represent the light intensity plotted at 10% steps. The grid step is 3 mm.
Fig. 3. X-Ray diffraction spectrum of a HfC film deposited on Si (111) with a laser fluence of 6 Jycm2 and a substrate temperature of 400 8C.
standard Vickers pyramidal indenter (square-based diamond pyramid of face angle 1368). 3. Results and discussion The ablation rate, measured through the target weight loss, can give an indication about the ablation mechanism involved in the laser–material interaction. In the case of hafnium carbide a linear trend was found in the whole laser fluence range (0.5–22 Jycm2). This is the same behaviour found in the zirconium carbide ablation and we considered it an indication that also in this case, a single ablation mechanism takes place. If we consider the TOF mass spectrum of the positive ions produced from the ablation of a HfC target (Fig. 1), we can note the presence of carbide clusters with formula HfCq n , with n up to six, together with the metal peak. The peak corresponding to HfCq 2 does not show a particularly high intensity and this is another feature that makes this system similar to ZrC. In fact, the presence of a high dicarbide peak should indicate a thermal mechanism w2x because this cluster is one of the products of thermal equilibrium vaporisation w5x. This type of cluster is present in a large amount in the vapour phase produced from the low fluence ablation of TiC, but only in traces in the gas phase produced from the ablation of ZrC. In the negative ions spectrum only carbon clusters were found, in agreement with all other carbide systems. The similarity between HfC and ZrC is confirmed by the study of the dynamics of the plume expansion performed by an ICCD camera. The front velocity (3.2=106 cmys) and the cosine exponent (ns2.7) are similar to those of the other group four carbides, but the presence of a single emission maximum in the plume indicates a low ionisation degree, as found in ZrC but
R. Teghil et al. / Surface and Coatings Technology 151 – 152 (2002) 531–533
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with higher energy compared to neutral ones and could interfere with the formation of large islands w6x, that is the first step of columnar growth, favouring the layerby-layer growth. Not by chance TiC films show exactly a layered structure. The films adherence to the substrate was good and their hardness was comparable with that of bulk HfC (18–29 GPa at a load of 0.5 N) w7x. 4. Conclusions
Fig. 4. SEM microphotograph of the cross-section of the same film in Fig. 3.
not in TiC, where a double maximum was found at a laser fluence higher than 3 Jycm2 w3x. An image of the plume produced from the ablation of the HfC target is shown in Fig. 2. The deposition of the films was performed at different substrate temperatures, from 25 up to 800 8C. The films were always composed of crystalline HfC and the best results were obtained for a substrate temperature of 400 8C. In Fig. 3 the X-ray diffraction spectrum of a film deposited in these conditions is shown. From SEM analysis the deposits result are smooth and compact, with only few droplets on the surface. The cross-section of the deposited films, analysed by SEM and presented in Fig. 4, showed a columnar structure, confirming the analogies with the zirconium carbide w2x. In fact, in both systems the columnar structure could be connected with an island growth mechanism, related to the plasma ionisation degree. The role of charged particles in the deposition processes is not clear, but they certainly impinge on the substrate
In conclusion, the behaviour of HfC targets under laser irradiation is very similar to that of ZrC ones. The different behaviour in respect to TiC could be explained considering the differences in the physico-chemical properties of the three carbides. It is apparent from the thermal conductivity values w8x that for ZrC and HfC targets is easier to reach a very high local temperature at lower laser fluence in comparison to the TiC system. A very rapid increase in local temperature up to the critical temperature could give rise to a situation in which a phase explosion takes place. The different growth mechanism we found in the couple HfC–ZrC in respect to TiC has been explained in terms of plume ionisation degree, leading to columnar (HfC–ZrC) or layered (TiC) structures. References w1x L. D’Alessio, A.M. Salvi, R. Teghil, et al., Appl. Surf. Sci. 134 (1998) 53. w2x L. D’Alessio, A. Santagata, R. Teghil, et al., Appl. Surf. Sci. 168 (2000) 284. w3x R. Teghil, L. D’Alessio, M. Zaccagnino, D. Ferro, V. Marotta, G. De Maria, Appl. Surf. Sci. 173 (2001) 233. w4x R. Kelly, A. Miotello, Nucl. Instrum. Methods B 122 (1997) 374. w5x C.A. Stern, F.J. Kohl, High. Temp. Sci. 2 (1990) 274. w6x H. Sankur, T.I. Cheung, Appl. Phys. A 47 (1988) 271. w7x S.M. Barinov, D. Ferro, C. Bertuli, L. D’Alessio, submitted for publication in J. Mater. Sci. w8x A. Krajewski, L. D’ Alessio, G. De Maria, Cryst. Res. Technol. 33 (1998) 34.
Hafnium carbide hard coatings produced by pulsed laser ablation and deposition R. Teghila,*, A. Santagataa, M. Zaccagninoa, S.M. Barinovb, V. Marottac, G. De Mariad a b
Dipartimento di Chimica, Universita` della Basilicata, via N. Sauro 85, 85100 Potenza, Italy High Tech Ceramics Research Centre, Russian Academy of Sciences, Moscow 119361, Russia c CNR Istituto Materiali Speciali, via S. Loja, Tito Scalo, Italy d Dipartimento di Chimica, Universita` ‘La Sapienza’, Ple A. Moro 5, 00185 Roma, Italy
Abstract In this paper the results of the deposition of thin films of hafnium carbide by pulsed laser ablation deposition are reported. The coatings characteristics, investigated by conventional techniques such as X-ray diffraction (XRD), scanning and transmission electron microscopy (SEMyTEM), and atomic force microscopy (AFM) are discussed in relation with the properties of the plasma produced in the interaction between the laser source (frequency doubled Nd:YAG laser) and the target. The plasma analysis, performed by emission spectroscopy, optical imaging and time of flight mass spectrometry, is also used to clarify the ablation mechanism and to compare it with those of the other carbides of the same group. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Hafnium carbide; Pulsed laser ablation; Thin film
1. Introduction Hafnium carbide films have found wide application in various tribological engineering devices owing to their excellent hardness and wear resistance. Furthermore, this carbide like the other carbides of the group four elements, shows a very high melting point, good resistance to corrosion and low thermal conductivity; ideal characteristics for use in high temperature applications. In previous works w1–3x we studied the pulsed ablation deposition of other two carbides of group four, TiC and ZrC, and the results evidenced a very different behaviour between the two systems. In particular, in the ablation of titanium carbide we found the presence of two different mechanisms: at low laser fluence (up to 3 Jycm2) a thermal mechanism with a non-congruent vaporisation; and at high laser fluence ()3 Jycm2) a congruent vaporisation probably related to a phase explosion mechanism w4x, or in any case, to a nonthermal mechanism. In the case of zirconium carbide * Corresponding author. Tel.: q39-971-202225; fax: q39-971202223. E-mail address: [email protected] (R. Teghil).
we found only one mechanism, corresponding to the phase explosion. In this work we extend our analysis to hafnium carbide, with the aim to give a deeper insight to the ablation and deposition mechanisms of group four carbides. 2. Experimental The experimental apparatus consists of a vacuum chamber equipped with quartz windows for laser beam inlet and in situ optical analyses, heatable substrate holder and rotating target support. The background pressure inside the chamber was 1.5=10y4 Pa. The laser source was a frequency doubled Nd-YAG laser (ls532 nm, ts10 ns, 10 Hz repetition rate) impinging on the target with an incidence angle of 458. The substrate (111 oriented silicon) was kept at distance of 2.5 cm from the target (hot pressed carbide tablets 99% pure from Cerac). The characterisation of the plasma has been carried out by emission spectroscopy (OMA-ICCD system, EG&G), optical imaging (ICCD camera, EG&G) and time of flight (TOF) mass spectrometry (LAMMA 500 from Leybold). The spatial and temporal resolution of the ICCD systems were 150 mm and 5 ns, respectively.
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 5 8 9 - 4
532
R. Teghil et al. / Surface and Coatings Technology 151 – 152 (2002) 531–533
Fig. 1. TOF mass spectrum of positive ions produced from the ablation of a HfC target.
A scanning electron microscope (SEM) Cambridge 100 with a 7 nm resolution was used for a morphological characterisation of the deposited films and the X-ray diffraction patterns have been recorded by a Siemens D5000 difractometer using Cu Ka1 radiation. The film hardness was measured using a Leica VMHT apparatus (Leica Ltd., Switzerland) equipped with a
Fig. 2. Overall light emission from HfC plume recorded by an ICCD camera, 600 ns after the laser shot. The contours in the figure represent the light intensity plotted at 10% steps. The grid step is 3 mm.
Fig. 3. X-Ray diffraction spectrum of a HfC film deposited on Si (111) with a laser fluence of 6 Jycm2 and a substrate temperature of 400 8C.
standard Vickers pyramidal indenter (square-based diamond pyramid of face angle 1368). 3. Results and discussion The ablation rate, measured through the target weight loss, can give an indication about the ablation mechanism involved in the laser–material interaction. In the case of hafnium carbide a linear trend was found in the whole laser fluence range (0.5–22 Jycm2). This is the same behaviour found in the zirconium carbide ablation and we considered it an indication that also in this case, a single ablation mechanism takes place. If we consider the TOF mass spectrum of the positive ions produced from the ablation of a HfC target (Fig. 1), we can note the presence of carbide clusters with formula HfCq n , with n up to six, together with the metal peak. The peak corresponding to HfCq 2 does not show a particularly high intensity and this is another feature that makes this system similar to ZrC. In fact, the presence of a high dicarbide peak should indicate a thermal mechanism w2x because this cluster is one of the products of thermal equilibrium vaporisation w5x. This type of cluster is present in a large amount in the vapour phase produced from the low fluence ablation of TiC, but only in traces in the gas phase produced from the ablation of ZrC. In the negative ions spectrum only carbon clusters were found, in agreement with all other carbide systems. The similarity between HfC and ZrC is confirmed by the study of the dynamics of the plume expansion performed by an ICCD camera. The front velocity (3.2=106 cmys) and the cosine exponent (ns2.7) are similar to those of the other group four carbides, but the presence of a single emission maximum in the plume indicates a low ionisation degree, as found in ZrC but
R. Teghil et al. / Surface and Coatings Technology 151 – 152 (2002) 531–533
533
with higher energy compared to neutral ones and could interfere with the formation of large islands w6x, that is the first step of columnar growth, favouring the layerby-layer growth. Not by chance TiC films show exactly a layered structure. The films adherence to the substrate was good and their hardness was comparable with that of bulk HfC (18–29 GPa at a load of 0.5 N) w7x. 4. Conclusions
Fig. 4. SEM microphotograph of the cross-section of the same film in Fig. 3.
not in TiC, where a double maximum was found at a laser fluence higher than 3 Jycm2 w3x. An image of the plume produced from the ablation of the HfC target is shown in Fig. 2. The deposition of the films was performed at different substrate temperatures, from 25 up to 800 8C. The films were always composed of crystalline HfC and the best results were obtained for a substrate temperature of 400 8C. In Fig. 3 the X-ray diffraction spectrum of a film deposited in these conditions is shown. From SEM analysis the deposits result are smooth and compact, with only few droplets on the surface. The cross-section of the deposited films, analysed by SEM and presented in Fig. 4, showed a columnar structure, confirming the analogies with the zirconium carbide w2x. In fact, in both systems the columnar structure could be connected with an island growth mechanism, related to the plasma ionisation degree. The role of charged particles in the deposition processes is not clear, but they certainly impinge on the substrate
In conclusion, the behaviour of HfC targets under laser irradiation is very similar to that of ZrC ones. The different behaviour in respect to TiC could be explained considering the differences in the physico-chemical properties of the three carbides. It is apparent from the thermal conductivity values w8x that for ZrC and HfC targets is easier to reach a very high local temperature at lower laser fluence in comparison to the TiC system. A very rapid increase in local temperature up to the critical temperature could give rise to a situation in which a phase explosion takes place. The different growth mechanism we found in the couple HfC–ZrC in respect to TiC has been explained in terms of plume ionisation degree, leading to columnar (HfC–ZrC) or layered (TiC) structures. References w1x L. D’Alessio, A.M. Salvi, R. Teghil, et al., Appl. Surf. Sci. 134 (1998) 53. w2x L. D’Alessio, A. Santagata, R. Teghil, et al., Appl. Surf. Sci. 168 (2000) 284. w3x R. Teghil, L. D’Alessio, M. Zaccagnino, D. Ferro, V. Marotta, G. De Maria, Appl. Surf. Sci. 173 (2001) 233. w4x R. Kelly, A. Miotello, Nucl. Instrum. Methods B 122 (1997) 374. w5x C.A. Stern, F.J. Kohl, High. Temp. Sci. 2 (1990) 274. w6x H. Sankur, T.I. Cheung, Appl. Phys. A 47 (1988) 271. w7x S.M. Barinov, D. Ferro, C. Bertuli, L. D’Alessio, submitted for publication in J. Mater. Sci. w8x A. Krajewski, L. D’ Alessio, G. De Maria, Cryst. Res. Technol. 33 (1998) 34.