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Improvement in the corrosion resistance of magnesium ZK60SiC composite by excimer laser surface treatment
Scri~taMaterialia.Vol.38.No.2. DD.191-198.1998 EiskvierS&e Ltd Copyright0 1998ActaMetallurgica Inc. Printedin the USA.Allrightsreserved. 1359-6462/98 $19.00+ .oO
Pergrumon PI1 S1359-6462(97)00495-S
IMPROVEMENT IN THE CORROSION RESISTANCE OF MAGNESIUM ZK60/SiC COMPOSITE BY EXCIMER LASER SURFACE TREATMENT T.M. Yue, A.H. Wang and H.C. Man Department
of Manufacturing Engineering, The Hong Kong Polytechnic Hung Horn, Kowloon, Hong Kong
University,
(Received June 24, 1997) (Accepted October 13, 1997)
Introduction Recent advances in manufacturing technology have resulted in the development of magnesium metal matrix composites (Mg-MMCs) [ 1,2]. However, the problem of serious galvanic corrosion is one of the major concerns when using these materials. Therefore, protective measures are certainly required for magnesium-base composites before they can be adopted for use in some aggressive environments. In addition to the conventional corrosion protection methods such as anodizing or organic coating, novel surface treatment techniques such as ion implantation [3] and laser surface treatment [4-61 have been considered to be useful countermeasures for combating corrosion in magnesium alloys. Subramanian [4] and Wang [:5] have reported that laser cladding of Mg-Zr powders on magnesium and its alloys by using high power CO, lasers can significantly increase the corrosion resistance of the materials. However, in order to avoid serious oxidation of magnesium alloys, laser cladding is normally required to be carried out in an inert gas chamber. In contrast to CO, lasers, the extremely high pulse power and short pulse duration of excimer lasers have enabled the modification of material surfaces with minimal thermal effect. This could mean that some active materials such as magnesium, aluminum and titanium can be surface treated by excimer lasers in a much less demanding atmosphere. Moreover, with the low surface reflectivity for metals in the ultraviolet range, excimer lasers are considered to be an ideal source for surface modification of magnesium alloys. As was reported by Koutsomichalis [6] that the equilibrium potential of the excimer laser treated magnesium alloy AZ3 1B was some 20mV more noble than that of the untreated alloy. They also suggested that magnesium nitrides may be formed on the excimer treated surfaces; however, no direct evidence was given. In our previous study [7], it was found that using excimer laser surface treatment the number of cathodic sites on the surface of an Al/SIC composite was igreatly reduced. As a result, the corrosion resistance of the composite was significantly improved. Nonetheless, studies of the laser surface treatment of Mg/SiC composites using excimer lasers have not been reported. With this in view, a study was carried out to assess the effects of excimer laser surface treatment on the corrosion properties of Mg/SiC composites. In this paper, the emphasis is placed on the effects of different shielding gases on the resulting corrosion properties of the Mg-ZK60/SiC composite. 191
192
EXCIMER
Figure
LASER
I. Microstructure
SURFACE
TREATMENT
of the MgZK6WSiC
Exuerimental
Vol. 38. No. 2
composite
Procedures
The composite used in this study was a 17(vol%) SIC particulate reinforced ZK60 (Mg-6Zn-O.SZr wt.%) magnesium composite which was produced by using the powder metallurgy route and was in the extruded rod form. The average size of the SIC particle is about 3Frn (Fig.1). The specimens for corrosion testing were finished by grinding with 800-grit emery paper and subsequently cleaned with distilled water and alcohol prior to laser treatment. Laser surface treatment was conducted using a KrF excimer laser which was operated at a wavelength of 248nm. Pulse duration was fixed at 25ns. The beam delivery system was so designed that a circular aperture was irradiated by a homogeneous laser beam from the rear and imaged down to a 1.2 mm spot size on the surface of the specimen. Laser surface treatment was performed under three different atmospheric surroundings, namely: in air, in Ar and in N,. In the latter two cases, the surface of the specimen was gently flushed with 99% pure Ar and 99% pure N, respectively at a flow rate of 40 Wmin. Pulse frequency and energy were fixed at 1OOHz and 45mJ respectively. The laser scanning velocity was set at 2mm/s, and a 25% overlap condition was used. Before and after the laser surface treatment, the surface chemistry of the specimen was analyzed by XPS. Anodic polarization tests were carried out in a 3.5wt% NaCl solution which was prepared using analytical grade reagents. The initial pH value of the solution was 5.7. The specimen was driven from polarization a E,,, of -250mV to - 1.OV at a scanning rate of 1.OmVs- ’ to produce potentiodynamic plots. All potentials were measured with reference to a standard calomel electrode (SCE). For each of the three different laser treated conditions, as well as for the untreated condition, three corrosion tests were performed. The corrosion rate (CR) of the specimens was calculated according to Faraday’s Law, and can be expressed as [S]: CR=K+w
(1)
where CR is given in mpy (10 p3 in/year); K is a constant and is equal to 0.1288; p is the density of the tested material, i,,,., is the open circuit current, EW is the material equivalent weight and is considered
Vol. 38, No. 2
EXCIMER LASER SURFACE TREATMENT
193
to be dimensionless. In the calculation of the corrosion rate of the Mg/SiC composite using equation (l), p and EW were taken to be 1.97g/cm3 and 12.15 respectively [8], whilst i,,, was measured from the potentiodynambc anodic polarization curves.
Results and Discussion Analysis
of laser treated surfaces
Figures 2(a-c) show the excimer laser treated surfaces of the Mg-composite produced under various gas environments. Disregarding the types of gases used, all the Sic particles on the surface are virtually covered by a laser-modified layer; however, networks of microcracks are also present. The results of the XPS analysis of the various laser treated surfaces are presented in Figs.3 and 4. Figs,3(a-d) compare the Mg peak of the untreated sample and the three different laser treated samples, i.e. in air, in Ar and in NZ. The results show that a relatively well defined single peak profile of Mg was obtained for the untreated and the Ar and N,-treated samples, whilst an expanded profile was obtained for the air-treated samples (Fig.3b). On the other hand, only on the N,-treated samples was a clear N, peak detected (Fig.4d). During laser surface treatment, interactions between the laser beam, the gas(es) of the surrounding and the composite material are expected to occur. When the sample was laser treated in air, the 0, and the N, bonds cculd be broken by the emitted photons of the KrF excimer laser [9] as the binding energy of these two gases is only 5.0eV and 1OeV respectively. As a result, clouds of 02- and N’- were generated, and at the same time the magnesium alloy matrix was melted. Accordingly, a laser gas alloying phenomenon would have occurred as the melted magnesium alloy reacted with ions of 02and/or N3 ~. However, a comparison of the Gibbs free energy for the formation of magnesium oxide and magnesium nitride indicates that MgO tends to form instead of Mg,N, because the former has a lower Gibbs free energy at temperatures below the boiling point of magnesium. This has been confirmed by the XPS result that no distinct nitrogen peak was detected (Fig. 4b) on the surface of the air-treated samples. A dark color was observed in the air-treated specimen and is believed to be due to the formation of some MgO on the specimen surface. When Ar was used as the shielding gas, the noble gas was not easily ionized by the laser, and the molten magnesium alloy was well protected by the inert gas. As a result, no fsignificant oxidization of the surface was observed, and a golden color was observed in the specimen. Again, no distinct nitrogen peak was obtained for the Ar-treated samples (Fig. 4~). Now, when N, was used, ions of nitrogen could react with the molten magnesium to form Mg,N,. The XPS analysis of the N-treated samples shows a nitrogen content of as high as 10 atomic %. This result thus strongly suggests that magnesium nitride has been formed. The presence of Mg,N, gives a gray color to the specimen.
Potentiodynandc
anodic polarization
curves
Figure 5 displays some typical potentiodynamic anodic polarization curves of the various excimer laser treated samples as well as the untreated sample. Apparently, the curves of the laser-treated specimens were shifted to ithe left of the curve of the untreated specimen, which indicates that the corrosion current (i,,,) of the laser-treated specimens is smaller than that of the untreated specimen. The results of corrosion rate of the untreated sample as well as the various laser treated samples are presented in Figure 6. These results show that by applying excimer laser surface treatment, the corrosion rate of the Mg/SiC compo:site was reduced significantly, and the increase in corrosion resistance influenced by the
194
EXCIMER
Figure 2. Surface morphology
LASER
SURFACE
of the laser-treated
TREATMENT
surfaces:
Vol. 38, No. 2
(a) in air, (b) in Ar, (c) in N,.
type of shielding gas is of the order of N, > Ar > air. Comparing the untreated and the N,-laser treated samples a reduction by a factor of 50 in the average corrosion rate was recorded. Figs. 7(a-d) show the appearance of the various corroded specimens which have been driven from E corr -250mV and interrupted at E,,,, +230mV. These figures clearly show that the untreated sample has been badly corroded, whilst a much cleaner surface was obtained for the laser treated samples. The improved corrosion resistance of the laser treated samples is not considered due to the covering up of the SIC particles by the laser-modified layer, as Hihara [IO] and Nunez [ 1 l] have shown that there was no enhanced attack at or near the magnesium matrix/SiC interface due either to galvanic corrosion or
195
EXCIMER LASER SURFACE TREATMENT
Vol. 38, No. 2
XPS sp big 2p/2 -
XPS Sp Mg 2p/l -
6’0 5’5 Blndlng Energy / eV
BindIng Energy / e\l
5‘0
(b) XPS Sp Mg ?p/l -
XPS Sp Mg 2~11 -
JS Sp Mg
T
g
200
2 Binding Energy / eV
60 55 50 Binding Energy / eY
cc> Figure 3. XPS analysis showing the magnesium air, (c) treated in Ar, (d) treated in N,.
45
Cd) 2p spectrum of the (a) untreated
sample, and the laser-treated
samples: (b) in
preferential corrosion of interfacial phases. They have discounted that galvanic corrosion associated with coupling of magnesium and Sic is a major problem, for the electrical conductivity of SIC is relatively low. The main corrosion cause in magnesium based-sic composites is still due to localized galvanic corrosion originating at the coarse eutectic phase and iron contamination. Furthermore, Nunez [l l] has found that the corrosion penetration rate of Mg-SiC composites can be reduced by heat treatment. This improvement is due to the partial dissolution of the eutectic bands and a more uniform distribution of the eutectic phase. In the present study, it was found that although using a lower pulse energy than 45mJ can still produce a laser-modified layer covering the SIC particles, the resulting corrosion resistance was found to be inferior. It is believed that only at high enough laser energies can the eutectic phase and the intermetallics be dissolved. And the improvement in corrosion resistance brought about by excimer laser treatment is believed to be primarily a result of microstructural refinement, particularly of the eutectic phase, due to rapid solidification, although the possibility of the formation of an amorphous surface structure can not be ruled out. Finally, the improvement in corrosion resistance obtained for the N,-treated specimens is considered to be due to the formation of Mg,N,, which is very stable in an aggressive environment [ 121.
EXCIMER
196
LASER
SURFACE
TREATMENT
XPS Sp N Is/l -
410 405 400 Binding Energy / eV
Vol. 38, No. 2
XPS Sp N Is/l -
395
390 Binding Energy i' eV
XPS Sp N Is/l -
XPS Sp N Is/l xl
s i 3800
k =3700 2 5 0 3600 0 .
I..‘.‘,‘..!~,“.t..“r
405 400 410 Binding Energy / eV
395
39(
,h3500 .r 2 b 3400 2 410 405 400 Binding Energy / eV
cc> Figure 4. XPS analysis showing the nitrogen (c) in Ar, (d) in N,.
395
390
Cd)
Is spectrum of the (a) untreated sample, and the laser-treated
samples: (b) in air,
Conclusions The corrosion resistance of the Mg-ZK60/SiC composite was improved significantly after excimer laser surface treatment. Amongst the three different atmospheric conditions used, i.e., air-treated, Ar-treated and N,-treated, the N,-treated samples had the lowest corrosion rate, which on average is some 50 times less than that of the untreated samples. The superior corrosion resistance of the laser-treated samples over that of the untreated samples is believed due to a refinement of the surface microstructure. Furthermore, the enhanced corrosion resistance obtained for the N,-treated samples is considered to be owing to the formation of magnesium nitrides in the laser-modified layer. Acknowledgements This research was supported by the Research Grants Council (RGC) under project no. PolyU133196E. The assistance offered by Professor W.M. Kwok, Department of Chemistry of The Chinese University of Hong Kong, in carrying out the XPS analysis is highly appreciated.
Vol. 38. No. 2
EXCIMER LASER SURFACE TREATMENT
-1.050
-
-1.150
-
1-
-1.250
-
-1.350
-
-1.450
-
-1.550
-
197
untreated in air
2-
treated
34-
treated in Ar treated in N2
2 !! r w
-1.650
_IIIIII111111111111 -9 -6
-7
-6
-5 Wi,
Figure 5. Potentiodynamic
anodic polarization
-4
-3
-2
-1
AkmA2)
curves of the untreated
and the laser-treated
samples.
al
5
600
E ‘Z 2
400
5 200
7
untreated Figure 6. The corrosion
rate (CR) calculated
air
0 10”
Ar
for the untreated
N2 and the laser-treated
samples.
EXCIMER
198
LASER
SURFACE
Vol. 38, No. 2
TREATMENT
WI Figure 7. Surface morphology
of the corroded
surfaces:
(a) untreated,
(b) treated in air, (c) treated in Ar. (d) treated in N,.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
T. E. Wilks, Lightweight magnesium composites for automotive applications, Proceedings of metal matrix composites, Detroit, 49 (1994). A. Luo, Processing, microstructure, and mechanical behavior of cast magnesium metal matrix composites, Metall. Mater. Trans. A. 26A, 2445 (1995). I. Nakatsugawa, R. Martin, and E. J. Knystautas, Improving corrosion resistance of AZ9lD Mg-alloy by nitrogen ion implantation, Corrosion Science. 52, 921 (1996). R. Subramanian, S. Sircar, and J. Mazumder, Laser cladding of zironium on magnesium for improved corrosion properties, J. Mater. Sci. 26, 951 (1991). A. A. Wang, S. Sircar, J. Mazumder, Laser cladding of Mg-Al alloys, J. Mater. Sci. 28, 5113 (1993). A. Koutsomichalis, L. Saettas, and H. Badekas, Laser treatment of magnesium, J. Mater. Sci. 29, 6543 (1994). X. M. Zhang, H. C. Man, and T. M. Yue, Corrosion properties of excimer laser surface treated AI-Sic metal matrix composite, Scripta Materialia. 35, 1095 (1996). ASTM Standards, Gl02-89 Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements. E. Schubert and H. W. Bergmann, Modification of metallic surface by means of excimer lasers: fundamentals and applications, Laser in Engineering. 2, 115 (1993). L. H. Hihara and P. K. Kondepudi, The galvanic corrosion of SIC monofilamenttZE41 Mg metal-matrix composites in 0.5 M NaNO,, Corrosion Science. 34, 1761 (1993). C. A. Nunez-Lopez, P. Skeldon, G. E. Thompson, P. Lyon, H. Karimzadeh, and T. E. Wilks, The corrosion behaviour of Mg-alloy ZC7l/SiC, metal matrix composite, Corrosion Science. 37, 689 (1995). W. A. Ferrando, Review of corrosion and corrosion control of magnesium alloys and composite, J. Mater. Eng. 11, 310 (1989).
Pergrumon PI1 S1359-6462(97)00495-S
IMPROVEMENT IN THE CORROSION RESISTANCE OF MAGNESIUM ZK60/SiC COMPOSITE BY EXCIMER LASER SURFACE TREATMENT T.M. Yue, A.H. Wang and H.C. Man Department
of Manufacturing Engineering, The Hong Kong Polytechnic Hung Horn, Kowloon, Hong Kong
University,
(Received June 24, 1997) (Accepted October 13, 1997)
Introduction Recent advances in manufacturing technology have resulted in the development of magnesium metal matrix composites (Mg-MMCs) [ 1,2]. However, the problem of serious galvanic corrosion is one of the major concerns when using these materials. Therefore, protective measures are certainly required for magnesium-base composites before they can be adopted for use in some aggressive environments. In addition to the conventional corrosion protection methods such as anodizing or organic coating, novel surface treatment techniques such as ion implantation [3] and laser surface treatment [4-61 have been considered to be useful countermeasures for combating corrosion in magnesium alloys. Subramanian [4] and Wang [:5] have reported that laser cladding of Mg-Zr powders on magnesium and its alloys by using high power CO, lasers can significantly increase the corrosion resistance of the materials. However, in order to avoid serious oxidation of magnesium alloys, laser cladding is normally required to be carried out in an inert gas chamber. In contrast to CO, lasers, the extremely high pulse power and short pulse duration of excimer lasers have enabled the modification of material surfaces with minimal thermal effect. This could mean that some active materials such as magnesium, aluminum and titanium can be surface treated by excimer lasers in a much less demanding atmosphere. Moreover, with the low surface reflectivity for metals in the ultraviolet range, excimer lasers are considered to be an ideal source for surface modification of magnesium alloys. As was reported by Koutsomichalis [6] that the equilibrium potential of the excimer laser treated magnesium alloy AZ3 1B was some 20mV more noble than that of the untreated alloy. They also suggested that magnesium nitrides may be formed on the excimer treated surfaces; however, no direct evidence was given. In our previous study [7], it was found that using excimer laser surface treatment the number of cathodic sites on the surface of an Al/SIC composite was igreatly reduced. As a result, the corrosion resistance of the composite was significantly improved. Nonetheless, studies of the laser surface treatment of Mg/SiC composites using excimer lasers have not been reported. With this in view, a study was carried out to assess the effects of excimer laser surface treatment on the corrosion properties of Mg/SiC composites. In this paper, the emphasis is placed on the effects of different shielding gases on the resulting corrosion properties of the Mg-ZK60/SiC composite. 191
192
EXCIMER
Figure
LASER
I. Microstructure
SURFACE
TREATMENT
of the MgZK6WSiC
Exuerimental
Vol. 38. No. 2
composite
Procedures
The composite used in this study was a 17(vol%) SIC particulate reinforced ZK60 (Mg-6Zn-O.SZr wt.%) magnesium composite which was produced by using the powder metallurgy route and was in the extruded rod form. The average size of the SIC particle is about 3Frn (Fig.1). The specimens for corrosion testing were finished by grinding with 800-grit emery paper and subsequently cleaned with distilled water and alcohol prior to laser treatment. Laser surface treatment was conducted using a KrF excimer laser which was operated at a wavelength of 248nm. Pulse duration was fixed at 25ns. The beam delivery system was so designed that a circular aperture was irradiated by a homogeneous laser beam from the rear and imaged down to a 1.2 mm spot size on the surface of the specimen. Laser surface treatment was performed under three different atmospheric surroundings, namely: in air, in Ar and in N,. In the latter two cases, the surface of the specimen was gently flushed with 99% pure Ar and 99% pure N, respectively at a flow rate of 40 Wmin. Pulse frequency and energy were fixed at 1OOHz and 45mJ respectively. The laser scanning velocity was set at 2mm/s, and a 25% overlap condition was used. Before and after the laser surface treatment, the surface chemistry of the specimen was analyzed by XPS. Anodic polarization tests were carried out in a 3.5wt% NaCl solution which was prepared using analytical grade reagents. The initial pH value of the solution was 5.7. The specimen was driven from polarization a E,,, of -250mV to - 1.OV at a scanning rate of 1.OmVs- ’ to produce potentiodynamic plots. All potentials were measured with reference to a standard calomel electrode (SCE). For each of the three different laser treated conditions, as well as for the untreated condition, three corrosion tests were performed. The corrosion rate (CR) of the specimens was calculated according to Faraday’s Law, and can be expressed as [S]: CR=K+w
(1)
where CR is given in mpy (10 p3 in/year); K is a constant and is equal to 0.1288; p is the density of the tested material, i,,,., is the open circuit current, EW is the material equivalent weight and is considered
Vol. 38, No. 2
EXCIMER LASER SURFACE TREATMENT
193
to be dimensionless. In the calculation of the corrosion rate of the Mg/SiC composite using equation (l), p and EW were taken to be 1.97g/cm3 and 12.15 respectively [8], whilst i,,, was measured from the potentiodynambc anodic polarization curves.
Results and Discussion Analysis
of laser treated surfaces
Figures 2(a-c) show the excimer laser treated surfaces of the Mg-composite produced under various gas environments. Disregarding the types of gases used, all the Sic particles on the surface are virtually covered by a laser-modified layer; however, networks of microcracks are also present. The results of the XPS analysis of the various laser treated surfaces are presented in Figs.3 and 4. Figs,3(a-d) compare the Mg peak of the untreated sample and the three different laser treated samples, i.e. in air, in Ar and in NZ. The results show that a relatively well defined single peak profile of Mg was obtained for the untreated and the Ar and N,-treated samples, whilst an expanded profile was obtained for the air-treated samples (Fig.3b). On the other hand, only on the N,-treated samples was a clear N, peak detected (Fig.4d). During laser surface treatment, interactions between the laser beam, the gas(es) of the surrounding and the composite material are expected to occur. When the sample was laser treated in air, the 0, and the N, bonds cculd be broken by the emitted photons of the KrF excimer laser [9] as the binding energy of these two gases is only 5.0eV and 1OeV respectively. As a result, clouds of 02- and N’- were generated, and at the same time the magnesium alloy matrix was melted. Accordingly, a laser gas alloying phenomenon would have occurred as the melted magnesium alloy reacted with ions of 02and/or N3 ~. However, a comparison of the Gibbs free energy for the formation of magnesium oxide and magnesium nitride indicates that MgO tends to form instead of Mg,N, because the former has a lower Gibbs free energy at temperatures below the boiling point of magnesium. This has been confirmed by the XPS result that no distinct nitrogen peak was detected (Fig. 4b) on the surface of the air-treated samples. A dark color was observed in the air-treated specimen and is believed to be due to the formation of some MgO on the specimen surface. When Ar was used as the shielding gas, the noble gas was not easily ionized by the laser, and the molten magnesium alloy was well protected by the inert gas. As a result, no fsignificant oxidization of the surface was observed, and a golden color was observed in the specimen. Again, no distinct nitrogen peak was obtained for the Ar-treated samples (Fig. 4~). Now, when N, was used, ions of nitrogen could react with the molten magnesium to form Mg,N,. The XPS analysis of the N-treated samples shows a nitrogen content of as high as 10 atomic %. This result thus strongly suggests that magnesium nitride has been formed. The presence of Mg,N, gives a gray color to the specimen.
Potentiodynandc
anodic polarization
curves
Figure 5 displays some typical potentiodynamic anodic polarization curves of the various excimer laser treated samples as well as the untreated sample. Apparently, the curves of the laser-treated specimens were shifted to ithe left of the curve of the untreated specimen, which indicates that the corrosion current (i,,,) of the laser-treated specimens is smaller than that of the untreated specimen. The results of corrosion rate of the untreated sample as well as the various laser treated samples are presented in Figure 6. These results show that by applying excimer laser surface treatment, the corrosion rate of the Mg/SiC compo:site was reduced significantly, and the increase in corrosion resistance influenced by the
194
EXCIMER
Figure 2. Surface morphology
LASER
SURFACE
of the laser-treated
TREATMENT
surfaces:
Vol. 38, No. 2
(a) in air, (b) in Ar, (c) in N,.
type of shielding gas is of the order of N, > Ar > air. Comparing the untreated and the N,-laser treated samples a reduction by a factor of 50 in the average corrosion rate was recorded. Figs. 7(a-d) show the appearance of the various corroded specimens which have been driven from E corr -250mV and interrupted at E,,,, +230mV. These figures clearly show that the untreated sample has been badly corroded, whilst a much cleaner surface was obtained for the laser treated samples. The improved corrosion resistance of the laser treated samples is not considered due to the covering up of the SIC particles by the laser-modified layer, as Hihara [IO] and Nunez [ 1 l] have shown that there was no enhanced attack at or near the magnesium matrix/SiC interface due either to galvanic corrosion or
195
EXCIMER LASER SURFACE TREATMENT
Vol. 38, No. 2
XPS sp big 2p/2 -
XPS Sp Mg 2p/l -
6’0 5’5 Blndlng Energy / eV
BindIng Energy / e\l
5‘0
(b) XPS Sp Mg ?p/l -
XPS Sp Mg 2~11 -
JS Sp Mg
T
g
200
2 Binding Energy / eV
60 55 50 Binding Energy / eY
cc> Figure 3. XPS analysis showing the magnesium air, (c) treated in Ar, (d) treated in N,.
45
Cd) 2p spectrum of the (a) untreated
sample, and the laser-treated
samples: (b) in
preferential corrosion of interfacial phases. They have discounted that galvanic corrosion associated with coupling of magnesium and Sic is a major problem, for the electrical conductivity of SIC is relatively low. The main corrosion cause in magnesium based-sic composites is still due to localized galvanic corrosion originating at the coarse eutectic phase and iron contamination. Furthermore, Nunez [l l] has found that the corrosion penetration rate of Mg-SiC composites can be reduced by heat treatment. This improvement is due to the partial dissolution of the eutectic bands and a more uniform distribution of the eutectic phase. In the present study, it was found that although using a lower pulse energy than 45mJ can still produce a laser-modified layer covering the SIC particles, the resulting corrosion resistance was found to be inferior. It is believed that only at high enough laser energies can the eutectic phase and the intermetallics be dissolved. And the improvement in corrosion resistance brought about by excimer laser treatment is believed to be primarily a result of microstructural refinement, particularly of the eutectic phase, due to rapid solidification, although the possibility of the formation of an amorphous surface structure can not be ruled out. Finally, the improvement in corrosion resistance obtained for the N,-treated specimens is considered to be due to the formation of Mg,N,, which is very stable in an aggressive environment [ 121.
EXCIMER
196
LASER
SURFACE
TREATMENT
XPS Sp N Is/l -
410 405 400 Binding Energy / eV
Vol. 38, No. 2
XPS Sp N Is/l -
395
390 Binding Energy i' eV
XPS Sp N Is/l -
XPS Sp N Is/l xl
s i 3800
k =3700 2 5 0 3600 0 .
I..‘.‘,‘..!~,“.t..“r
405 400 410 Binding Energy / eV
395
39(
,h3500 .r 2 b 3400 2 410 405 400 Binding Energy / eV
cc> Figure 4. XPS analysis showing the nitrogen (c) in Ar, (d) in N,.
395
390
Cd)
Is spectrum of the (a) untreated sample, and the laser-treated
samples: (b) in air,
Conclusions The corrosion resistance of the Mg-ZK60/SiC composite was improved significantly after excimer laser surface treatment. Amongst the three different atmospheric conditions used, i.e., air-treated, Ar-treated and N,-treated, the N,-treated samples had the lowest corrosion rate, which on average is some 50 times less than that of the untreated samples. The superior corrosion resistance of the laser-treated samples over that of the untreated samples is believed due to a refinement of the surface microstructure. Furthermore, the enhanced corrosion resistance obtained for the N,-treated samples is considered to be owing to the formation of magnesium nitrides in the laser-modified layer. Acknowledgements This research was supported by the Research Grants Council (RGC) under project no. PolyU133196E. The assistance offered by Professor W.M. Kwok, Department of Chemistry of The Chinese University of Hong Kong, in carrying out the XPS analysis is highly appreciated.
Vol. 38. No. 2
EXCIMER LASER SURFACE TREATMENT
-1.050
-
-1.150
-
1-
-1.250
-
-1.350
-
-1.450
-
-1.550
-
197
untreated in air
2-
treated
34-
treated in Ar treated in N2
2 !! r w
-1.650
_IIIIII111111111111 -9 -6
-7
-6
-5 Wi,
Figure 5. Potentiodynamic
anodic polarization
-4
-3
-2
-1
AkmA2)
curves of the untreated
and the laser-treated
samples.
al
5
600
E ‘Z 2
400
5 200
7
untreated Figure 6. The corrosion
rate (CR) calculated
air
0 10”
Ar
for the untreated
N2 and the laser-treated
samples.
EXCIMER
198
LASER
SURFACE
Vol. 38, No. 2
TREATMENT
WI Figure 7. Surface morphology
of the corroded
surfaces:
(a) untreated,
(b) treated in air, (c) treated in Ar. (d) treated in N,.
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
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