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Modified surface morphology in surface ablation of cobalt-cemented tungsten carbide with pulsed UV laser radiation
Applied Surface Science 172 (2001) 331±344
Modi®ed surface morphology in surface ablation of cobalt-cemented tungsten carbide with pulsed UV laser radiation Tiejun Lia,b,*, Qihong Loua, Jingxing Donga, Yunrong Weia, Jingru Liub a
Shanghai Institute of Optics and Fine Mechanics, The Chinese Academy of Sciences, PO Box 800-211, Shanghai 201800, China b Northwest Institute of Nuclear Technology, PO Box 69-13, Xi'an 710024, China Received 13 August 2000; accepted 28 October 2000
Abstract Surface ablation of cobalt-cemented tungsten carbide hardmetal has been carried out in this work using a 308 nm, 20 ns XeCl excimer laser. The in¯uence of ablation rate, surface roughness, surface micromorphology as well as surface phase structure on laser conditions including laser irradiance and pulse number have been investigated. The experimental results showed that the ablation rate and surface roughness were controlled by varying the number of pulses and laser irradiance. The microstructure and crystalline structure of irradiated surface layer varied greatly with different laser conditions. After 300 shots of laser irradiation at irradiance of 125 MW/cm2, the surface micromorphology characterizing a uniform framework pattern of ``hill±valleys''. With the increment of laser shots at laser irradiance of 125 MW/cm2, the microstructure of cemented tungsten carbide transformed from original polygon grains with the size of 3 mm to interlaced large and long grains after 300 shots of laser irradiation, and ®nally to gross grains with the size of 10 mm with clear grain boundaries after 700 shots. The crystalline structure of irradiated area has partly transformed from original WC to b-WC1ÿx , then to a-W2C and CW3, and ®nally to W crystal. At proper laser irradiance and pulse number, cobalt binder has been selectively removed from the surface layer of hardmetal. It has been demonstrated that surface ablation with pulsed UV laser should be a feasible way to selectively remove cobalt binder from surface layer of cemented tungsten carbide hardmetal. # 2001 Elsevier Science B.V. All rights reserved. PACS: 81.65.Cf; 81.05.Bx Keywords: Surface ablation; Tungsten carbide; Cobalt; Excimer laser
1. Introduction Surface treatments of metals and alloys by pulsed laser irradiation have been widely investigated and * Corresponding author. Present address: Shanghai Institute of Optics and Fine Mechanics, The Chinese Academy of Sciences, PO Box 800-211, Shanghai 201800, China. E-mail address: [email protected] (T. Li).
reported in the literatures. The surface ablation is one of the most phenomena induced by this kind of laser. The wavelength is ranging from visible to ultraviolet with the pulse duration from femtosecond to nanosecond. The metals and alloys commonly used are gold [1], silver [2], copper [3], nickel [4], titanium and Ti± Al alloy [5], Fe [6], iron [7], etc. However, to our knowledge, there are few reports on the ablation of cobalt-cemented tungsten carbide hardmetal with
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 8 8 1 - 3
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pulsed UV laser in the literatures up to now [8,9]. In this paper, we report results on ablation of cobaltcemented tungsten carbide hardmetal with pulsed UV laser. We have chosen this kind of hardmetal because it has been widely used as substrate for diamond coating (DC) tools. The widespread used and cheaper bulk material for tool inserts, the WC±Co hardmetal, is convenient and pro®table as a substrate for diamond ®lm coatings. Unfortunately, the Co-rich binder phase constitutes a severe obstacle for diamond deposition in chemical vapor deposition (and related) processes. Because of the catalytic effect for amorphous carbon or soot formation, the presence of cobalt actually results in a detrimental effect both on diamond nucleation and adhesion to substrate [10±13]. Therefore, it is necessary to remove cobalt binder from the surface of cemented carbide substrate when people seek for depositing diamond ®lm on the surface of hardmetal. In this paper, we intend to seek for the possibility of selective removal of cobalt binder from surface layer of cement tungsten carbide hardmetal by surface ablation with pulsed UV laser due to the difference of ablation behavior between cobalt and tungsten carbide. It is therefore interesting to investigate the behavior of cobalt-cemented tungsten carbide hardmetal under pulsed UV laser irradiation. In this work, surface ablation of cobalt-cemented tungsten carbide hardmetal has been studied with pulsed UV laser. The ablation rate, surface roughness, surface micromorphology as well as surface phase structure have been investigated with different laser conditions including laser irradiance and pulse number. The possibility of selective cobalt removal from surface layer of cemented tungsten carbide hardmetal by pulsed laser ablation was ®nally demonstrated. 2. Experimental Commercial cobalt-cemented carbide tool inserts (YG6, diamond brand) used for general machining application were used as ablation samples. It is composed of 94 wt.% tungsten carbide particles with the size of 3 mm and 6% cobalt binder. A repetitive XeCl pulsed excimer laser (model: SY-200, l 308 nm, t 20 ns, 450 mJ maximum for each pulse) was used as the light source to deliver irradiation on the surface of the hardmetal. After passing through an automatical
co-axis optical attenuator (Patent No. ZL96229370.9), the output laser pulse was focused on the surface of the hardmetal by a quartz lens f 150 mm with the spot size of 1 2 mm2 . The laser irradiance delivered on the surface of hardmetal therefore could easily be controlled to desired value by simply adjusting this continuous optical attenuator. The pulse energy was measured with a pyroelectric energy meter and the energy deviation among different pulses was found within 10%. In order to minimize the in¯uence of energy ¯uctuation of laser pulse on ablation rate, 100 pulses were accumulated for measurement of crater depth. The surface roughness as well as the depth of craters irradiated by laser were measured with Taylor± Hobson stylus pro®lometer (model: Talysurf-120L). The ablation rate was therefore determined by dividing the depth of the crater by the number of pulses. The uncertainty for the measured ablation rate is within 10%. The surface micromorphology of samples was photographed by Shimadzu EPMA-8705QH2 Electron Probe Microanalyzer. The surface phase structure was examined by Rigaku D/max-rA X-ray diffractometer. 3. Results Fig. 1 shows the dependence of ablation rate on laser irradiances with the spot area of 0.003 cm2. It is shown that the threshold irradiance for the ablation of cobalt-cemented tungsten carbide hardmetal is around 80 MW/cm2 (corresponding to 4.8 mJ for each laser pulse) under 308 nm laser irradiation. Obviously, the threshold irradiance for hardmetal is much higher than that for cobalt metal which is around 30 MW/cm2 [14, this volume]. As shown in Fig. 1, in low irradiances range, the ablation rate rises almost linearly with the irradiance up to around 200 MW/cm2, and then increases slowly with the irradiance up to about 450 MW/cm2. At even higher irradiance, the ablation rate approaches saturation. This behavior should be probably due to the vapor attenuation induced by laser. At low irradiances irradiation, the temperature of the target surface is not heated high enough to generate signi®cant evaporation of the target. As a result, the vapor is so dilute that the attenuation of laser by vapor can be neglected. The ablation rate increases almost linearly with the irradiance. According to BaÈuerle
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Fig. 1. Dependence of ablation rate on laser irradiance. The spot area is 0.003 cm2.
[15], for surface absorption, and laser irradiances around threshold irradiance at which the vapor is weak, the depth Dh can be estimated by Dh F=r DHv , where F is the laser irradiance, r the mass density of target and DHv the latent heat for evaporation. Here, this approximation has shown to be a good description to the behavior of the ablation at low irradiances. With further increase of laser irradiances, the vaporization becomes more signi®cant, and the absorption of the vapor grows considerably high and cannot be neglected any more. The attenuation of vapor therefore reduces the laser energy delivered onto the surface and slows down the increase of
ablation rate. At even higher irradiances, the particles in vapor are almost fully ionized at a very high temperature. Laser-induced plasma is then ignited and leads to the appearance of plasma shielding which separated the target surface from incoming laser [16± 18]. Little energy could reach on the target surface to cause a further heating of target surface. A saturation of the ablation rate therefore appears. As shown in Fig. 1, the irradiance threshold for plasma ignition is 450 MW/cm2. The dependence of the surface roughness as a function of laser irradiances and number of shots is shown in Fig. 2. The evaluation standard used in
Fig. 2. Dependence of surface roughness on laser irradiance.
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surface roughness measurements is Rz-DIN. As shown in Fig. 2, the surface roughness increases nonlinearly with the increment of laser irradiances. At low irradiances from 80 to 200 MW/cm2, there is a sharp rise in surface roughness. At medium irradiances from 200 to 500 MW/cm2, the rise of surface roughness somewhat slows down. At even high irradiances, the surface roughness returns to a sharp rise. Obviously, laser irradiance has a signi®cant in¯uence on the surface roughness. The in¯uence of laser pulse number on surface roughness is shown in Fig. 3. The corresponding laser irradiance is 125 MW/cm2. As shown in Fig. 3, with pulse number increasing from 0 to 100 shots, there is a sharp rise in surface roughness. With laser shots further increasing, the rise of surface roughness slows down gradually and ®nally approaches to a plateau of saturation. It can be easily seen that, roughness of irradiated surface is mainly controlled by initial several hundreds of laser shots. Those subsequent laser shots give only limited contribution to surface roughness. Fig. 4(a)±(d) shows the dependence of surface micromorphology on laser irradiances. The corresponding irradiances are 400, 200, 125 and 50 MW/ cm2, respectively. The pulse number for each case is kept at 100 shots. As shown in Fig. 4(a) and (b), at high irradiances, the surface of hardmetal exhibits irregular melting pattern. There are many round blowholes with the size of around 3±5 mm distributing dispersedly on
the surface of hardmetal, which is probably due to the escaping of gases originated from violent ablation process. Obviously, both cobalt and tungsten carbide have been ablated together. One cannot expect to selectively remove only cobalt binder from the surface of cemented hardmetal at these irradiances level. As shown in Fig. 4(c), the surface micromorphology at 125 MW/cm2 appears as a completely different picture other than those obtained at high irradiances. There are many interlaced ``mountain±valley'' solid structures distributed on the irradiated surface of hardmetal. Based on the fact that the melting and boiling point of tungsten carbide (3143 K [15] and 6273 K [19]) is much higher than that of cobalt (1768 K [15] and 3200 K [15]), tungsten carbide should be much more dif®cult to be ablated than cobalt. It is therefore reasonable that those prominent ``mountains'' and those hollow ``valleys'' should be probably corresponding to unablated tungsten carbide grains and evaporated cobalt binder, respectively. At this irradiance, tungsten carbide grains have been successfully exposed with the selective removal of cobalt binder in the meantime. Fig. 4(d) shows the surface micromorphology at 50 MW/cm2. There seems to be little change on the irradiated surface of hardmetal compared to as-received surface in Fig. 5(a). Therefore, it is impossible to remove cobalt binder at this low irradiance due to low temperature rise by laser pulses heating.
Fig. 3. Dependence of surface roughness on number of laser shots. The corresponding laser irradiance is 125 MW/cm2.
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Fig. 4. Surface morphology of hardmetal at different laser irradiances. The pulse number was set to be 100 at each case. The corresponding laser irradiance is: (a) 400 MW/cm2; (b) 200 MW/cm2; (c) 125 MW/cm2; (d) 50 MW/cm2.
The modi®ed surface micromorphology with different laser shots at 125 MW/cm2 are shown in Fig. 5(a)±(h). Fig. 5(a) shows the metallography of as-received sample (etched by 10% alkaline solution of potassium ferricyanide), which exhibits white polygon tungsten carbide grains with the size of around
3 mm and black cobalt binder. The corresponding pulse number to Fig. 5(b)±(h) is 0, 50,100, 200, 300, 500 and 700, respectively. Fig. 5(b) shows the surface micromorphology of as-received sample, which exhibits a smooth pattern with some grinding marks. After 50 shots of laser irradiation as shown in
336 T. Li et al. / Applied Surface Science 172 (2001) 331±344 Fig. 5. Surface morphology after different shots of laser irradiation. The laser irradiance applied here is 125 MW/cm2. (a) The metallography micrograph of as-received sample. The corresponding pulse number is: (b) 0; (c) 50; (d) 100; (e) 200; (f) 300; (g) 500; (h) 700.
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Fig. 5(c), there starts to appear some small hills and pits on the surface of hardmetal, but the spatial difference of these hills and valleys is still very small. The surface features look like a melt-induced structure with small pits, which are probably due to the melting and evaporation of cobalt by pulsed laser irradiation. Moreover, there are some round blowholes on the irradiated surface area, but the sizes are relatively small. These blowholes should probably result from the escaping of carbon element during the ablation process. Here, these small blowholes probably indicate that effects of carbon element escaping and dissolution of tungsten carbide due to the heating of pulsed laser irradiation are still not signi®cant. With pulse number further increasing from 100 to 200 shots as shown in Fig. 5(d) and (e), the surface micromorphology turn to exhibit a uniform framework pattern of hill±valleys, which are probably tungsten carbide hills and cobalt valleys due to the fact that tungsten carbide is much more dif®cult to be ablated than cobalt binder. The sharp corners of original tungsten carbide grains have disappeared, and the microstructure of the surface has transformed from original separated polygon carbide grains to interlaced large and long carbide grains with round-corner shape. In addition, there appears clearly the growing up of tungsten carbide from original 3 to 5±6 mm. After more shots of laser irradiation up to 300 as shown in Fig. 5(f), the hills turn to be more prominent and the
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valleys become much deeper, which indicate that a large amount of cobalt binder has been ablated out of the surface layer with tungsten carbide grains still standing on the surface. The spatial difference of hills and valleys has become signi®cant. Obviously, tungsten carbide grains have been clearly exposed on the irradiated surface of hardmetal. This result coincides well with the reliance of surface roughness on number of laser shots under 125 MW/cm2 pulsed laser irradiation as described above. Moreover, tungsten carbide grains have grown up to approximately by 8 mm in size, which are much bigger than the original ones with the size of 3 mm. However, things start to change when pulse number increases up to 500 shots. As shown in Fig. 5(g), the surface seems to be a result of completely remelting process. There are some small hills and pits distributing irregularly on the ¯at surface. In the meantime, there also appear a number of round blowholes with the size of 3 mm on the melted surface, which are comparatively larger than blowholes appearing in Fig. 5(c). These blowholes should probably indicate that a certain amount of carbon element has escaped from the irradiated surface layer. With pulse number increasing to 700 shots, the surface micromorphology as shown in Fig. 5(h) turn to be a picture characterizing uniform separated gross grains with the size of 10 mm just like ferrite. Those grains have less difference in height but have clear grain boundaries, which
Fig. 6. Electron probe spectrum at the surface of gross grain as shown in Fig. 5(h).
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Fig. 7. X-ray diffraction spectra of surface after different shots of laser irradiation. The laser ¯uence is 125 MW/cm2. The corresponding pulse number is: (a) 0; (b) 50; (c) 100; (d) 200; (e) 300; (f) 700. ($) WC; (^) cobalt; () b-WC1ÿx ; (*) a-W2C; ( ) CW3; ( ) W.
separate them into individual grains. To make clear what these grains are, electron probe microanalyzer was used to identify these grains. A number of quantitative analyses have been carried on different grains, but the results are same with little difference. The energy spectrum is shown in Fig. 6. The result of the
analysis shows that the grain is composed of 93% tungsten and 6% cobalt. Note that because of the limitation of electron probe microanalyzer, carbon element cannot be analyzed. As a result, there are no peaks corresponding to carbon element. Combined with surface micromorphology and analyses results of
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Fig. 7 (Continued ).
electron probe, it is speculated that those grains should probably be tungsten carbide grains, but the surface of grains is probably covered with a layer of cobalt. That is to say that, an amount of cobalt metal has re-entered into irradiated surface layer of hardmetal after 700 shots of laser irradiation. The dependence of surface XRD spectra on different pulse number is shown in Fig. 7(a)±(f). The
corresponding pulse number is 0, 50, 100, 200, 300, and 700, respectively. The laser irradiance is set to be 125 MW/cm2 at each case. As shown in Fig. 7(a), XRD spectrum of as-received sample shows only tungsten carbide and cobalt sharp peaks. After 50 shots of laser irradiation shown in Fig. 7(b), the b-WC1ÿx phase is observed and the intensity of cobalt peaks start to decline, which indicates a fast evapora-
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Fig. 7 (Continued ).
tion of cobalt from the irradiated surface. With the increment of laser shots up to 200 as shown in Fig. 7(c) and (d), peaks corresponding to a-W2C and CW3 phase are observed, while the intensity of cobalt peaks continue to decline. At 300 shots as shown in Fig. 7(e), the intensity of cobalt peaks fall down to an unde-
tectable level, which probably indicates that there has been little cobalt binder left in surface layer after 300 shots of pulsed laser irradiation. In addition, sharp peaks corresponding to tungsten crystal start to appear in this spectrum. Based on these phase transformation described above, it is obvious that there is the escape
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of carbon element with the increment of laser shots, which leads to the formation of non-stoichiometric carbide phases with less content of carbon and tungsten crystal. This effect should be primarily owing to the remelting of tungsten carbide by pulsed laser irradiation, which results in the reaction of carbon in the tungsten carbide with oxygen in air to form volatile gases such as CO or CO2. Although there is the formation of non-stoichiometric carbide phases in surface layer due to the accumulation of laser shots, it can be easily seen from diffraction pattern that tungsten carbide remains to be the dominating phase in surface layer and most of cobalt binder in surface layer has been selectively removed. However, when pulse number further increases up to 700 shots, there appears a further phase transformation. As shown in Fig. 7(f), those peaks corresponding to tungsten crystal turn to be stronger and sharper, which indicates a further escaping of carbon element from irradiated surface layer. In the mean time there reappear the cobalt peaks, which indicates that a certain amount of cobalt has re-entered into surface layer of hardmetal. Combined with analysis result by electron probe as described above, it has been con®rmed that an amount of cobalt has re-entered into surface layer of hardmetal. Based on the experimental results described above, one should always apply proper laser irradiance and
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proper pulse number of laser shots when seeking to remove cobalt binder by surface ablation with pulsed UV laser. In this experiment, laser irradiance should be higher than 50 MW/cm2 but lower than 200 MW/ cm2 in order to selectively remove cobalt binder from the surface layer of hardmetal. At laser irradiance of 125 MW/cm2, the total number of laser shots in the ablation process should be limited around 300 shots for selective removal of cobalt binder. At laser irradiance of 125 MW/cm2 and 300 shots of laser irradiation, cobalt binder has been successfully removed from the surface layer of cobalt-cemented tungsten carbide hardmetal. 4. Discussion Since this kind of hardmetal is composed of tungsten carbide and cobalt binder, it is therefore essential to know the properties and ablation behavior of these two components ®rstly. As to cobalt, the melting and boiling point is 1768 K [15] and 3200 K [15], respectively. Based on the numerical calculation results as shown in Fig. 8 [14], the surface temperature rises to the boiling point of 3200 K at 90 MW/cm2, further reaches 5000 K at 175 MW/cm2, and then approaches saturation temperature of 5500 K at irradiances higher than 200 MW/cm2. (The numerical calculation
Fig. 8. Calculated surface temperature as a function of laser irradiance at 12 ns with and without vapor attenuation l 308 nm; t 20 ns [14].
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assumes only a single 308 nm laser pulse irradiation on the surface of cobalt, and the vapor attenuation effect during ablation process has been considered. The results will be published elsewhere.) As to tungsten carbide (WC), the melting and boiling point is 3143 K [15] and 6273 K [19], which is much higher than that of cobalt. In order to selectively remove cobalt binder from the surface of hardmetal and retain tungsten carbide as much as possible, one should necessarily apply proper laser irradiance on the surface of hardmetal. If laser irradiance is too low, one cannot expect selective removal of cobalt from the surface of hardmetal due to low heating temperature induced by pulsed laser irradiation. If high irradiance is applied, the surface temperature will be heated up to a rather high point, at which not only cobalt binder will be removed but also tungsten carbide will be also ablated together. Therefore, one should always apply proper irradiance on the surface of hardmetal when seeking to selectively remove cobalt binder from the surface of cobalt-cemented hardmetal by pulsed laser ablation. As have been described above in this experiment, at laser irradiance of 125 MW/cm2, cobalt binder has been selectively removed from the surface layer of cobalt-cemented tungsten carbide hardmetal. While at irradiance which is higher than 200 MW/ cm2 or lower than 50 MW/cm2, surface ablation of both cobalt and tungsten carbide or no surface abla-
tion has occurred instead of selective ablation of cobalt. The phase transformations in this ablation process could be understood through following processes. When initial laser pulses with the irradiance of 125 MW/cm2 irradiate the smooth surface of hardmetal, the surface layer will be heated to a rather high temperature of approximately 4500 K according to Fig. 8 [14]. This temperature is much higher than the boiling point of cobalt (3200 K) but much lower than the boiling temperature of tungsten carbide. As a result, cobalt binder will be fast evaporated from the surface layer with tungsten carbide grains still sticking on the surface. This effect therefore results in a large amount of hollow valleys on the surface due to the fast evaporation of cobalt binder. On the other hand, at this irradiance, the surface temperature rises sharply to the maximum temperature within 15 ns and then falls down quickly to approximately 2500 K within 40 ns as shown in Fig. 9 [14]. That is to say, this irradiation process is a fast heating and fast cooling process. Because the thermal conductivity of tungsten carbide K 0:29 W= cm K [15] is much lower than that of cobalt K 0:74 W= cm K [15], it is much more dif®cult for tungsten carbide to conduct heat than for cobalt metal. Moreover, because of the fast evaporation of cobalt binder from the surface in this heating process, a great
Fig. 9. Calculated temporal evolution of surface temperature with vapor attenuation during the process of pulsed laser irradiation of cobalt l 308 nm; t 20 ns [14].
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amount of energy will be carried out of the surface, resulting in additional cooling of the surface. Therefore, unlike the fast evaporation of cobalt, tungsten carbide grains will be partly melted within this short heating time. Although this heating process is rather short in time, because temperature takes more effect on the growing up of tungsten carbide grains than heating time according to knowledge of powder metallurgy [20], this heating process induced by pulsed laser irradiation will subsequently result in the growing up of tungsten carbide grains. Due to the actions of diffusion and congregation, melted tungsten carbide will recrystallize on the surface of unmelted large tungsten carbide grains, resulting in further growing up of tungsten carbide grains and joining of adjacent grains. The growing up of tungsten carbide grains will subsequently lead to the formation of prominent hills framework as shown in micrograph above. With the accumulation of this heating process induced by pulsed laser irradiation, original small tungsten carbide grains will gradually grown-up into large and long carbide grains that stand out prominently on the surface of hardmetal looking just like undulating hills. However, when laser pulses increase up to 300 shots, tungsten carbide grains have been clearly exposed with a large amount of cobalt binder having been removed from the surface. The surface looks like an undulating hill±valleys area. With pulse number further increasing, because tungsten carbide hills have stood out prominently on the surface, most of laser energy will be received by tungsten carbide hills which results in subsequent remelting of tungsten carbide grains. Only a small part of laser energy could reach hollow cobalt valleys, i.e., hollow cobalt valleys have been shielded by prominent tungsten carbide hills. Thus, there will be no fast evaporation of cobalt from the surface, which will subsequently result in additional cooling of the surface. As a result, the surface temperature will be heated to rather high temperature. The remelting and the recrystallization process of tungsten carbide will be greatly accelerated, resulting in further growing up of tungsten carbide grains. With laser shots accumulating high enough, tungsten carbide grains will grow up to gross carbide grains. Because this heating process is rather a fast heating and fast cooling process, the crystal orientation of grown-up carbide grains differs greatly with
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each other in this recrystallization process, leading to clear grain boundaries between gross carbide grains. On the other hand, although there is only a small part of laser energy reaching cobalt binder valleys after 300 shots of laser irradiation, the irradiance is still high enough to melt and evaporate cobalt binder. However, this process is somewhat moderate. This effect results in the diffusion of melted cobalt binder towards the surface of hardmetal, which further leads to mixing with remelted tungsten carbide and forming a layer of cobalt on the surface of tungsten carbide grains. Nevertheless, it is not expected that cobalt binder re-enter into surface layer of hardmetal. Therefore, one should have to control the pulse number of laser shots, while seeking for selective removal of cobalt binder in surface ablation of cemented hardmetal. 5. Conclusion Based on the experimental results described above, surface ablation of cobalt-cemented tungsten carbide hardmetal has been carried out using a 308 nm, 20 ns XeCl excimer laser. The in¯uence of ablation rate, surface roughness, surface micromorphology as well as surface phase structure on laser conditions including laser irradiance and pulse number have been investigated. The experimental results showed that the ablation rate and surface roughness were controlled by varying the number of pulses and laser irradiance. The microstructure and crystalline structure of irradiated surface layer varied greatly with different laser conditions. After 300 shots of laser irradiation at irradiance of 125 MW/cm2, the surface micromorphology characterizing a uniform framework pattern of hill±valleys. With the increment of laser shots at laser irradiance of 125 MW/cm2, the microstructure of cemented tungsten carbide transformed from original polygon grains with the size of 3 mm to interlaced large and long grains after 300 shots of laser irradiation, and ®nally to gross grains with the size of 10 mm with clear grain boundaries after 700 shots. The crystalline structure of irradiated area has partly transformed from original WC to b-WC1ÿx , then to a-W2C and CW3, and ®nally to W crystal. At proper laser irradiance and pulse number, cobalt binder has been selectively removed from
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the surface layer of hardmetal. It has been demonstrated that surface ablation with pulsed UV laser should be a feasible way to selectively remove cobalt binder from surface layer of cemented tungsten carbide hardmetal. Moreover, this technique is expected to be used in any desired selective removal of one component from the surface layer of material, which is composed of two or more components with distinct properties between them. Acknowledgements The authors would like to appreciate Prof. Xiaolong Jiang for valuable discussion, and Mrs. Jianhua Gao for excellent works in sample analysis. This work was supported by National Advanced Technology Project of China and National Key Laboratory of Laser Technology of Huazhong University of Science and Technology. References [1] T.D. Bennett, C.P. Grigoropoulos, D.J. Krajnovich, J. Appl. Phys. 77 (1995) 849. [2] W. Svendsen, O. Ellegaard, J. Schou, Appl. Phys. A. 63 (1996) 247.
[3] R. Jordan, D. Cole, J.G. Lunney, K. Mackay, D. Givord, Appl. Surf. Sci. 86 (1995) 24. [4] S. Press, A. Demchuk, M. Stuke, Appl. Phys. A. 61 (1995) 33. [5] S. Tosto, A.D. Bartolomeo, P.D. Lazzaro, Appl. Phys. A. 63 (1996) 385. [6] S. FaÈhler, H.U. Krebs, Appl. Surf. Sci. 96 (1996) 61. [7] R. Jordan, J.G. Lunney, Appl. Surf. Sci. 127 (1998) 968. [8] J. Guo, B. Li, J. Zhao, J. Jia, Q. Li, W. Wu, M. Isao, I. Hidek, Trans. Nonferrous Met. 5 (1995) 142. [9] J. Jandeleit, A. Horn, R. Weichenhain, E.W. Kreutz, R. Poprawe, in: Proceedings of the 1997 Laser Materials Processing Conference, Proceedings V. 83 n Pt 1, Vol. A106, Laser Institute of America, Orlando, FL, 1997. [10] C.T. Kuo, T.Y. Yen, T.H. Huang, J. Mater. Res. 5 (1990) 2515. [11] M. Murakawa, S. Takeuchi, H. Miyazawa, Y. Hirose, Surf. Coat. Technol. 36 (1998) 303. [12] S. Soderberg, A. Gerendas, M. Sjostrand, Vacuum 41 (1990) 1317. [13] B. Lux, T. Haubner, P. Renard, Diamond Rel. Mater. 1 (1992) 1035. [14] T. Li, Q. Lou, J. Dong, Y. Wei, J. Liu, Appl. Surf. Sci. 172 (2001) 356±365. [15] D. BaÈuerle, Laser Processing and Chemistry, 2nd Edition, Springer, Berlin, 1996. [16] A.M. Prokhorov, V.I. Konov, I. Ursu, I.N. Mihailescu, Laser Heating of Metals, Hilger, Bristol, 1990. [17] C. Liu, High Power Laser and Particle Beams 2 (1990) 418. [18] J. Lu, X. Ni, A. He, Chinese J. Laser B 4 (1995) 434. [19] D.L. Zeng, Materials in Powder Metallurgy, Metallurgy Industry Press, Beijing, 1989. [20] G.D. Wang, Principles of Powder Metallurgy, Metallurgy Industry Press, Beijing, 1988.
Modi®ed surface morphology in surface ablation of cobalt-cemented tungsten carbide with pulsed UV laser radiation Tiejun Lia,b,*, Qihong Loua, Jingxing Donga, Yunrong Weia, Jingru Liub a
Shanghai Institute of Optics and Fine Mechanics, The Chinese Academy of Sciences, PO Box 800-211, Shanghai 201800, China b Northwest Institute of Nuclear Technology, PO Box 69-13, Xi'an 710024, China Received 13 August 2000; accepted 28 October 2000
Abstract Surface ablation of cobalt-cemented tungsten carbide hardmetal has been carried out in this work using a 308 nm, 20 ns XeCl excimer laser. The in¯uence of ablation rate, surface roughness, surface micromorphology as well as surface phase structure on laser conditions including laser irradiance and pulse number have been investigated. The experimental results showed that the ablation rate and surface roughness were controlled by varying the number of pulses and laser irradiance. The microstructure and crystalline structure of irradiated surface layer varied greatly with different laser conditions. After 300 shots of laser irradiation at irradiance of 125 MW/cm2, the surface micromorphology characterizing a uniform framework pattern of ``hill±valleys''. With the increment of laser shots at laser irradiance of 125 MW/cm2, the microstructure of cemented tungsten carbide transformed from original polygon grains with the size of 3 mm to interlaced large and long grains after 300 shots of laser irradiation, and ®nally to gross grains with the size of 10 mm with clear grain boundaries after 700 shots. The crystalline structure of irradiated area has partly transformed from original WC to b-WC1ÿx , then to a-W2C and CW3, and ®nally to W crystal. At proper laser irradiance and pulse number, cobalt binder has been selectively removed from the surface layer of hardmetal. It has been demonstrated that surface ablation with pulsed UV laser should be a feasible way to selectively remove cobalt binder from surface layer of cemented tungsten carbide hardmetal. # 2001 Elsevier Science B.V. All rights reserved. PACS: 81.65.Cf; 81.05.Bx Keywords: Surface ablation; Tungsten carbide; Cobalt; Excimer laser
1. Introduction Surface treatments of metals and alloys by pulsed laser irradiation have been widely investigated and * Corresponding author. Present address: Shanghai Institute of Optics and Fine Mechanics, The Chinese Academy of Sciences, PO Box 800-211, Shanghai 201800, China. E-mail address: [email protected] (T. Li).
reported in the literatures. The surface ablation is one of the most phenomena induced by this kind of laser. The wavelength is ranging from visible to ultraviolet with the pulse duration from femtosecond to nanosecond. The metals and alloys commonly used are gold [1], silver [2], copper [3], nickel [4], titanium and Ti± Al alloy [5], Fe [6], iron [7], etc. However, to our knowledge, there are few reports on the ablation of cobalt-cemented tungsten carbide hardmetal with
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 8 8 1 - 3
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pulsed UV laser in the literatures up to now [8,9]. In this paper, we report results on ablation of cobaltcemented tungsten carbide hardmetal with pulsed UV laser. We have chosen this kind of hardmetal because it has been widely used as substrate for diamond coating (DC) tools. The widespread used and cheaper bulk material for tool inserts, the WC±Co hardmetal, is convenient and pro®table as a substrate for diamond ®lm coatings. Unfortunately, the Co-rich binder phase constitutes a severe obstacle for diamond deposition in chemical vapor deposition (and related) processes. Because of the catalytic effect for amorphous carbon or soot formation, the presence of cobalt actually results in a detrimental effect both on diamond nucleation and adhesion to substrate [10±13]. Therefore, it is necessary to remove cobalt binder from the surface of cemented carbide substrate when people seek for depositing diamond ®lm on the surface of hardmetal. In this paper, we intend to seek for the possibility of selective removal of cobalt binder from surface layer of cement tungsten carbide hardmetal by surface ablation with pulsed UV laser due to the difference of ablation behavior between cobalt and tungsten carbide. It is therefore interesting to investigate the behavior of cobalt-cemented tungsten carbide hardmetal under pulsed UV laser irradiation. In this work, surface ablation of cobalt-cemented tungsten carbide hardmetal has been studied with pulsed UV laser. The ablation rate, surface roughness, surface micromorphology as well as surface phase structure have been investigated with different laser conditions including laser irradiance and pulse number. The possibility of selective cobalt removal from surface layer of cemented tungsten carbide hardmetal by pulsed laser ablation was ®nally demonstrated. 2. Experimental Commercial cobalt-cemented carbide tool inserts (YG6, diamond brand) used for general machining application were used as ablation samples. It is composed of 94 wt.% tungsten carbide particles with the size of 3 mm and 6% cobalt binder. A repetitive XeCl pulsed excimer laser (model: SY-200, l 308 nm, t 20 ns, 450 mJ maximum for each pulse) was used as the light source to deliver irradiation on the surface of the hardmetal. After passing through an automatical
co-axis optical attenuator (Patent No. ZL96229370.9), the output laser pulse was focused on the surface of the hardmetal by a quartz lens f 150 mm with the spot size of 1 2 mm2 . The laser irradiance delivered on the surface of hardmetal therefore could easily be controlled to desired value by simply adjusting this continuous optical attenuator. The pulse energy was measured with a pyroelectric energy meter and the energy deviation among different pulses was found within 10%. In order to minimize the in¯uence of energy ¯uctuation of laser pulse on ablation rate, 100 pulses were accumulated for measurement of crater depth. The surface roughness as well as the depth of craters irradiated by laser were measured with Taylor± Hobson stylus pro®lometer (model: Talysurf-120L). The ablation rate was therefore determined by dividing the depth of the crater by the number of pulses. The uncertainty for the measured ablation rate is within 10%. The surface micromorphology of samples was photographed by Shimadzu EPMA-8705QH2 Electron Probe Microanalyzer. The surface phase structure was examined by Rigaku D/max-rA X-ray diffractometer. 3. Results Fig. 1 shows the dependence of ablation rate on laser irradiances with the spot area of 0.003 cm2. It is shown that the threshold irradiance for the ablation of cobalt-cemented tungsten carbide hardmetal is around 80 MW/cm2 (corresponding to 4.8 mJ for each laser pulse) under 308 nm laser irradiation. Obviously, the threshold irradiance for hardmetal is much higher than that for cobalt metal which is around 30 MW/cm2 [14, this volume]. As shown in Fig. 1, in low irradiances range, the ablation rate rises almost linearly with the irradiance up to around 200 MW/cm2, and then increases slowly with the irradiance up to about 450 MW/cm2. At even higher irradiance, the ablation rate approaches saturation. This behavior should be probably due to the vapor attenuation induced by laser. At low irradiances irradiation, the temperature of the target surface is not heated high enough to generate signi®cant evaporation of the target. As a result, the vapor is so dilute that the attenuation of laser by vapor can be neglected. The ablation rate increases almost linearly with the irradiance. According to BaÈuerle
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Fig. 1. Dependence of ablation rate on laser irradiance. The spot area is 0.003 cm2.
[15], for surface absorption, and laser irradiances around threshold irradiance at which the vapor is weak, the depth Dh can be estimated by Dh F=r DHv , where F is the laser irradiance, r the mass density of target and DHv the latent heat for evaporation. Here, this approximation has shown to be a good description to the behavior of the ablation at low irradiances. With further increase of laser irradiances, the vaporization becomes more signi®cant, and the absorption of the vapor grows considerably high and cannot be neglected any more. The attenuation of vapor therefore reduces the laser energy delivered onto the surface and slows down the increase of
ablation rate. At even higher irradiances, the particles in vapor are almost fully ionized at a very high temperature. Laser-induced plasma is then ignited and leads to the appearance of plasma shielding which separated the target surface from incoming laser [16± 18]. Little energy could reach on the target surface to cause a further heating of target surface. A saturation of the ablation rate therefore appears. As shown in Fig. 1, the irradiance threshold for plasma ignition is 450 MW/cm2. The dependence of the surface roughness as a function of laser irradiances and number of shots is shown in Fig. 2. The evaluation standard used in
Fig. 2. Dependence of surface roughness on laser irradiance.
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surface roughness measurements is Rz-DIN. As shown in Fig. 2, the surface roughness increases nonlinearly with the increment of laser irradiances. At low irradiances from 80 to 200 MW/cm2, there is a sharp rise in surface roughness. At medium irradiances from 200 to 500 MW/cm2, the rise of surface roughness somewhat slows down. At even high irradiances, the surface roughness returns to a sharp rise. Obviously, laser irradiance has a signi®cant in¯uence on the surface roughness. The in¯uence of laser pulse number on surface roughness is shown in Fig. 3. The corresponding laser irradiance is 125 MW/cm2. As shown in Fig. 3, with pulse number increasing from 0 to 100 shots, there is a sharp rise in surface roughness. With laser shots further increasing, the rise of surface roughness slows down gradually and ®nally approaches to a plateau of saturation. It can be easily seen that, roughness of irradiated surface is mainly controlled by initial several hundreds of laser shots. Those subsequent laser shots give only limited contribution to surface roughness. Fig. 4(a)±(d) shows the dependence of surface micromorphology on laser irradiances. The corresponding irradiances are 400, 200, 125 and 50 MW/ cm2, respectively. The pulse number for each case is kept at 100 shots. As shown in Fig. 4(a) and (b), at high irradiances, the surface of hardmetal exhibits irregular melting pattern. There are many round blowholes with the size of around 3±5 mm distributing dispersedly on
the surface of hardmetal, which is probably due to the escaping of gases originated from violent ablation process. Obviously, both cobalt and tungsten carbide have been ablated together. One cannot expect to selectively remove only cobalt binder from the surface of cemented hardmetal at these irradiances level. As shown in Fig. 4(c), the surface micromorphology at 125 MW/cm2 appears as a completely different picture other than those obtained at high irradiances. There are many interlaced ``mountain±valley'' solid structures distributed on the irradiated surface of hardmetal. Based on the fact that the melting and boiling point of tungsten carbide (3143 K [15] and 6273 K [19]) is much higher than that of cobalt (1768 K [15] and 3200 K [15]), tungsten carbide should be much more dif®cult to be ablated than cobalt. It is therefore reasonable that those prominent ``mountains'' and those hollow ``valleys'' should be probably corresponding to unablated tungsten carbide grains and evaporated cobalt binder, respectively. At this irradiance, tungsten carbide grains have been successfully exposed with the selective removal of cobalt binder in the meantime. Fig. 4(d) shows the surface micromorphology at 50 MW/cm2. There seems to be little change on the irradiated surface of hardmetal compared to as-received surface in Fig. 5(a). Therefore, it is impossible to remove cobalt binder at this low irradiance due to low temperature rise by laser pulses heating.
Fig. 3. Dependence of surface roughness on number of laser shots. The corresponding laser irradiance is 125 MW/cm2.
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Fig. 4. Surface morphology of hardmetal at different laser irradiances. The pulse number was set to be 100 at each case. The corresponding laser irradiance is: (a) 400 MW/cm2; (b) 200 MW/cm2; (c) 125 MW/cm2; (d) 50 MW/cm2.
The modi®ed surface micromorphology with different laser shots at 125 MW/cm2 are shown in Fig. 5(a)±(h). Fig. 5(a) shows the metallography of as-received sample (etched by 10% alkaline solution of potassium ferricyanide), which exhibits white polygon tungsten carbide grains with the size of around
3 mm and black cobalt binder. The corresponding pulse number to Fig. 5(b)±(h) is 0, 50,100, 200, 300, 500 and 700, respectively. Fig. 5(b) shows the surface micromorphology of as-received sample, which exhibits a smooth pattern with some grinding marks. After 50 shots of laser irradiation as shown in
336 T. Li et al. / Applied Surface Science 172 (2001) 331±344 Fig. 5. Surface morphology after different shots of laser irradiation. The laser irradiance applied here is 125 MW/cm2. (a) The metallography micrograph of as-received sample. The corresponding pulse number is: (b) 0; (c) 50; (d) 100; (e) 200; (f) 300; (g) 500; (h) 700.
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Fig. 5(c), there starts to appear some small hills and pits on the surface of hardmetal, but the spatial difference of these hills and valleys is still very small. The surface features look like a melt-induced structure with small pits, which are probably due to the melting and evaporation of cobalt by pulsed laser irradiation. Moreover, there are some round blowholes on the irradiated surface area, but the sizes are relatively small. These blowholes should probably result from the escaping of carbon element during the ablation process. Here, these small blowholes probably indicate that effects of carbon element escaping and dissolution of tungsten carbide due to the heating of pulsed laser irradiation are still not signi®cant. With pulse number further increasing from 100 to 200 shots as shown in Fig. 5(d) and (e), the surface micromorphology turn to exhibit a uniform framework pattern of hill±valleys, which are probably tungsten carbide hills and cobalt valleys due to the fact that tungsten carbide is much more dif®cult to be ablated than cobalt binder. The sharp corners of original tungsten carbide grains have disappeared, and the microstructure of the surface has transformed from original separated polygon carbide grains to interlaced large and long carbide grains with round-corner shape. In addition, there appears clearly the growing up of tungsten carbide from original 3 to 5±6 mm. After more shots of laser irradiation up to 300 as shown in Fig. 5(f), the hills turn to be more prominent and the
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valleys become much deeper, which indicate that a large amount of cobalt binder has been ablated out of the surface layer with tungsten carbide grains still standing on the surface. The spatial difference of hills and valleys has become signi®cant. Obviously, tungsten carbide grains have been clearly exposed on the irradiated surface of hardmetal. This result coincides well with the reliance of surface roughness on number of laser shots under 125 MW/cm2 pulsed laser irradiation as described above. Moreover, tungsten carbide grains have grown up to approximately by 8 mm in size, which are much bigger than the original ones with the size of 3 mm. However, things start to change when pulse number increases up to 500 shots. As shown in Fig. 5(g), the surface seems to be a result of completely remelting process. There are some small hills and pits distributing irregularly on the ¯at surface. In the meantime, there also appear a number of round blowholes with the size of 3 mm on the melted surface, which are comparatively larger than blowholes appearing in Fig. 5(c). These blowholes should probably indicate that a certain amount of carbon element has escaped from the irradiated surface layer. With pulse number increasing to 700 shots, the surface micromorphology as shown in Fig. 5(h) turn to be a picture characterizing uniform separated gross grains with the size of 10 mm just like ferrite. Those grains have less difference in height but have clear grain boundaries, which
Fig. 6. Electron probe spectrum at the surface of gross grain as shown in Fig. 5(h).
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Fig. 7. X-ray diffraction spectra of surface after different shots of laser irradiation. The laser ¯uence is 125 MW/cm2. The corresponding pulse number is: (a) 0; (b) 50; (c) 100; (d) 200; (e) 300; (f) 700. ($) WC; (^) cobalt; () b-WC1ÿx ; (*) a-W2C; ( ) CW3; ( ) W.
separate them into individual grains. To make clear what these grains are, electron probe microanalyzer was used to identify these grains. A number of quantitative analyses have been carried on different grains, but the results are same with little difference. The energy spectrum is shown in Fig. 6. The result of the
analysis shows that the grain is composed of 93% tungsten and 6% cobalt. Note that because of the limitation of electron probe microanalyzer, carbon element cannot be analyzed. As a result, there are no peaks corresponding to carbon element. Combined with surface micromorphology and analyses results of
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Fig. 7 (Continued ).
electron probe, it is speculated that those grains should probably be tungsten carbide grains, but the surface of grains is probably covered with a layer of cobalt. That is to say that, an amount of cobalt metal has re-entered into irradiated surface layer of hardmetal after 700 shots of laser irradiation. The dependence of surface XRD spectra on different pulse number is shown in Fig. 7(a)±(f). The
corresponding pulse number is 0, 50, 100, 200, 300, and 700, respectively. The laser irradiance is set to be 125 MW/cm2 at each case. As shown in Fig. 7(a), XRD spectrum of as-received sample shows only tungsten carbide and cobalt sharp peaks. After 50 shots of laser irradiation shown in Fig. 7(b), the b-WC1ÿx phase is observed and the intensity of cobalt peaks start to decline, which indicates a fast evapora-
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Fig. 7 (Continued ).
tion of cobalt from the irradiated surface. With the increment of laser shots up to 200 as shown in Fig. 7(c) and (d), peaks corresponding to a-W2C and CW3 phase are observed, while the intensity of cobalt peaks continue to decline. At 300 shots as shown in Fig. 7(e), the intensity of cobalt peaks fall down to an unde-
tectable level, which probably indicates that there has been little cobalt binder left in surface layer after 300 shots of pulsed laser irradiation. In addition, sharp peaks corresponding to tungsten crystal start to appear in this spectrum. Based on these phase transformation described above, it is obvious that there is the escape
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of carbon element with the increment of laser shots, which leads to the formation of non-stoichiometric carbide phases with less content of carbon and tungsten crystal. This effect should be primarily owing to the remelting of tungsten carbide by pulsed laser irradiation, which results in the reaction of carbon in the tungsten carbide with oxygen in air to form volatile gases such as CO or CO2. Although there is the formation of non-stoichiometric carbide phases in surface layer due to the accumulation of laser shots, it can be easily seen from diffraction pattern that tungsten carbide remains to be the dominating phase in surface layer and most of cobalt binder in surface layer has been selectively removed. However, when pulse number further increases up to 700 shots, there appears a further phase transformation. As shown in Fig. 7(f), those peaks corresponding to tungsten crystal turn to be stronger and sharper, which indicates a further escaping of carbon element from irradiated surface layer. In the mean time there reappear the cobalt peaks, which indicates that a certain amount of cobalt has re-entered into surface layer of hardmetal. Combined with analysis result by electron probe as described above, it has been con®rmed that an amount of cobalt has re-entered into surface layer of hardmetal. Based on the experimental results described above, one should always apply proper laser irradiance and
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proper pulse number of laser shots when seeking to remove cobalt binder by surface ablation with pulsed UV laser. In this experiment, laser irradiance should be higher than 50 MW/cm2 but lower than 200 MW/ cm2 in order to selectively remove cobalt binder from the surface layer of hardmetal. At laser irradiance of 125 MW/cm2, the total number of laser shots in the ablation process should be limited around 300 shots for selective removal of cobalt binder. At laser irradiance of 125 MW/cm2 and 300 shots of laser irradiation, cobalt binder has been successfully removed from the surface layer of cobalt-cemented tungsten carbide hardmetal. 4. Discussion Since this kind of hardmetal is composed of tungsten carbide and cobalt binder, it is therefore essential to know the properties and ablation behavior of these two components ®rstly. As to cobalt, the melting and boiling point is 1768 K [15] and 3200 K [15], respectively. Based on the numerical calculation results as shown in Fig. 8 [14], the surface temperature rises to the boiling point of 3200 K at 90 MW/cm2, further reaches 5000 K at 175 MW/cm2, and then approaches saturation temperature of 5500 K at irradiances higher than 200 MW/cm2. (The numerical calculation
Fig. 8. Calculated surface temperature as a function of laser irradiance at 12 ns with and without vapor attenuation l 308 nm; t 20 ns [14].
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assumes only a single 308 nm laser pulse irradiation on the surface of cobalt, and the vapor attenuation effect during ablation process has been considered. The results will be published elsewhere.) As to tungsten carbide (WC), the melting and boiling point is 3143 K [15] and 6273 K [19], which is much higher than that of cobalt. In order to selectively remove cobalt binder from the surface of hardmetal and retain tungsten carbide as much as possible, one should necessarily apply proper laser irradiance on the surface of hardmetal. If laser irradiance is too low, one cannot expect selective removal of cobalt from the surface of hardmetal due to low heating temperature induced by pulsed laser irradiation. If high irradiance is applied, the surface temperature will be heated up to a rather high point, at which not only cobalt binder will be removed but also tungsten carbide will be also ablated together. Therefore, one should always apply proper irradiance on the surface of hardmetal when seeking to selectively remove cobalt binder from the surface of cobalt-cemented hardmetal by pulsed laser ablation. As have been described above in this experiment, at laser irradiance of 125 MW/cm2, cobalt binder has been selectively removed from the surface layer of cobalt-cemented tungsten carbide hardmetal. While at irradiance which is higher than 200 MW/ cm2 or lower than 50 MW/cm2, surface ablation of both cobalt and tungsten carbide or no surface abla-
tion has occurred instead of selective ablation of cobalt. The phase transformations in this ablation process could be understood through following processes. When initial laser pulses with the irradiance of 125 MW/cm2 irradiate the smooth surface of hardmetal, the surface layer will be heated to a rather high temperature of approximately 4500 K according to Fig. 8 [14]. This temperature is much higher than the boiling point of cobalt (3200 K) but much lower than the boiling temperature of tungsten carbide. As a result, cobalt binder will be fast evaporated from the surface layer with tungsten carbide grains still sticking on the surface. This effect therefore results in a large amount of hollow valleys on the surface due to the fast evaporation of cobalt binder. On the other hand, at this irradiance, the surface temperature rises sharply to the maximum temperature within 15 ns and then falls down quickly to approximately 2500 K within 40 ns as shown in Fig. 9 [14]. That is to say, this irradiation process is a fast heating and fast cooling process. Because the thermal conductivity of tungsten carbide K 0:29 W= cm K [15] is much lower than that of cobalt K 0:74 W= cm K [15], it is much more dif®cult for tungsten carbide to conduct heat than for cobalt metal. Moreover, because of the fast evaporation of cobalt binder from the surface in this heating process, a great
Fig. 9. Calculated temporal evolution of surface temperature with vapor attenuation during the process of pulsed laser irradiation of cobalt l 308 nm; t 20 ns [14].
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amount of energy will be carried out of the surface, resulting in additional cooling of the surface. Therefore, unlike the fast evaporation of cobalt, tungsten carbide grains will be partly melted within this short heating time. Although this heating process is rather short in time, because temperature takes more effect on the growing up of tungsten carbide grains than heating time according to knowledge of powder metallurgy [20], this heating process induced by pulsed laser irradiation will subsequently result in the growing up of tungsten carbide grains. Due to the actions of diffusion and congregation, melted tungsten carbide will recrystallize on the surface of unmelted large tungsten carbide grains, resulting in further growing up of tungsten carbide grains and joining of adjacent grains. The growing up of tungsten carbide grains will subsequently lead to the formation of prominent hills framework as shown in micrograph above. With the accumulation of this heating process induced by pulsed laser irradiation, original small tungsten carbide grains will gradually grown-up into large and long carbide grains that stand out prominently on the surface of hardmetal looking just like undulating hills. However, when laser pulses increase up to 300 shots, tungsten carbide grains have been clearly exposed with a large amount of cobalt binder having been removed from the surface. The surface looks like an undulating hill±valleys area. With pulse number further increasing, because tungsten carbide hills have stood out prominently on the surface, most of laser energy will be received by tungsten carbide hills which results in subsequent remelting of tungsten carbide grains. Only a small part of laser energy could reach hollow cobalt valleys, i.e., hollow cobalt valleys have been shielded by prominent tungsten carbide hills. Thus, there will be no fast evaporation of cobalt from the surface, which will subsequently result in additional cooling of the surface. As a result, the surface temperature will be heated to rather high temperature. The remelting and the recrystallization process of tungsten carbide will be greatly accelerated, resulting in further growing up of tungsten carbide grains. With laser shots accumulating high enough, tungsten carbide grains will grow up to gross carbide grains. Because this heating process is rather a fast heating and fast cooling process, the crystal orientation of grown-up carbide grains differs greatly with
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each other in this recrystallization process, leading to clear grain boundaries between gross carbide grains. On the other hand, although there is only a small part of laser energy reaching cobalt binder valleys after 300 shots of laser irradiation, the irradiance is still high enough to melt and evaporate cobalt binder. However, this process is somewhat moderate. This effect results in the diffusion of melted cobalt binder towards the surface of hardmetal, which further leads to mixing with remelted tungsten carbide and forming a layer of cobalt on the surface of tungsten carbide grains. Nevertheless, it is not expected that cobalt binder re-enter into surface layer of hardmetal. Therefore, one should have to control the pulse number of laser shots, while seeking for selective removal of cobalt binder in surface ablation of cemented hardmetal. 5. Conclusion Based on the experimental results described above, surface ablation of cobalt-cemented tungsten carbide hardmetal has been carried out using a 308 nm, 20 ns XeCl excimer laser. The in¯uence of ablation rate, surface roughness, surface micromorphology as well as surface phase structure on laser conditions including laser irradiance and pulse number have been investigated. The experimental results showed that the ablation rate and surface roughness were controlled by varying the number of pulses and laser irradiance. The microstructure and crystalline structure of irradiated surface layer varied greatly with different laser conditions. After 300 shots of laser irradiation at irradiance of 125 MW/cm2, the surface micromorphology characterizing a uniform framework pattern of hill±valleys. With the increment of laser shots at laser irradiance of 125 MW/cm2, the microstructure of cemented tungsten carbide transformed from original polygon grains with the size of 3 mm to interlaced large and long grains after 300 shots of laser irradiation, and ®nally to gross grains with the size of 10 mm with clear grain boundaries after 700 shots. The crystalline structure of irradiated area has partly transformed from original WC to b-WC1ÿx , then to a-W2C and CW3, and ®nally to W crystal. At proper laser irradiance and pulse number, cobalt binder has been selectively removed from
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the surface layer of hardmetal. It has been demonstrated that surface ablation with pulsed UV laser should be a feasible way to selectively remove cobalt binder from surface layer of cemented tungsten carbide hardmetal. Moreover, this technique is expected to be used in any desired selective removal of one component from the surface layer of material, which is composed of two or more components with distinct properties between them. Acknowledgements The authors would like to appreciate Prof. Xiaolong Jiang for valuable discussion, and Mrs. Jianhua Gao for excellent works in sample analysis. This work was supported by National Advanced Technology Project of China and National Key Laboratory of Laser Technology of Huazhong University of Science and Technology. References [1] T.D. Bennett, C.P. Grigoropoulos, D.J. Krajnovich, J. Appl. Phys. 77 (1995) 849. [2] W. Svendsen, O. Ellegaard, J. Schou, Appl. Phys. A. 63 (1996) 247.
[3] R. Jordan, D. Cole, J.G. Lunney, K. Mackay, D. Givord, Appl. Surf. Sci. 86 (1995) 24. [4] S. Press, A. Demchuk, M. Stuke, Appl. Phys. A. 61 (1995) 33. [5] S. Tosto, A.D. Bartolomeo, P.D. Lazzaro, Appl. Phys. A. 63 (1996) 385. [6] S. FaÈhler, H.U. Krebs, Appl. Surf. Sci. 96 (1996) 61. [7] R. Jordan, J.G. Lunney, Appl. Surf. Sci. 127 (1998) 968. [8] J. Guo, B. Li, J. Zhao, J. Jia, Q. Li, W. Wu, M. Isao, I. Hidek, Trans. Nonferrous Met. 5 (1995) 142. [9] J. Jandeleit, A. Horn, R. Weichenhain, E.W. Kreutz, R. Poprawe, in: Proceedings of the 1997 Laser Materials Processing Conference, Proceedings V. 83 n Pt 1, Vol. A106, Laser Institute of America, Orlando, FL, 1997. [10] C.T. Kuo, T.Y. Yen, T.H. Huang, J. Mater. Res. 5 (1990) 2515. [11] M. Murakawa, S. Takeuchi, H. Miyazawa, Y. Hirose, Surf. Coat. Technol. 36 (1998) 303. [12] S. Soderberg, A. Gerendas, M. Sjostrand, Vacuum 41 (1990) 1317. [13] B. Lux, T. Haubner, P. Renard, Diamond Rel. Mater. 1 (1992) 1035. [14] T. Li, Q. Lou, J. Dong, Y. Wei, J. Liu, Appl. Surf. Sci. 172 (2001) 356±365. [15] D. BaÈuerle, Laser Processing and Chemistry, 2nd Edition, Springer, Berlin, 1996. [16] A.M. Prokhorov, V.I. Konov, I. Ursu, I.N. Mihailescu, Laser Heating of Metals, Hilger, Bristol, 1990. [17] C. Liu, High Power Laser and Particle Beams 2 (1990) 418. [18] J. Lu, X. Ni, A. He, Chinese J. Laser B 4 (1995) 434. [19] D.L. Zeng, Materials in Powder Metallurgy, Metallurgy Industry Press, Beijing, 1989. [20] G.D. Wang, Principles of Powder Metallurgy, Metallurgy Industry Press, Beijing, 1988.