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Microstructures and thermal damage mechanisms of sintered polycrystalline diamond compact annealing under ambient air and vacuum conditions
Int. Journal of Refractory Metals and Hard Materials 54 (2016) 138–147
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Microstructures and thermal damage mechanisms of sintered polycrystalline diamond compact annealing under ambient air and vacuum conditions Jiansheng Li a, Wen Yue a,b,⁎, Chengbiao Wang a,b a b
School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, PR China Key Laboratory on Deep Geo-drilling Technology of the Ministry of Land and Resources, China University of Geosciences (Beijing), Beijing 100083, PR China
a r t i c l e
i n f o
Article history: Received 5 May 2015 Received in revised form 16 July 2015 Accepted 19 July 2015 Available online 29 July 2015 Keywords: PDC Microstructures Thermal damage mechanisms Annealing Ambient air Vacuum
a b s t r a c t The microstructures and thermal damage mechanisms of sintered polycrystalline diamond compact (PDC) were studied in ambient air and vacuum at the temperature up to 1000 °C. The microstructures and compositions of the annealed PDC were characterized by white light interferometer, X-ray diffractometry (XRD), Raman spectroscopy and scanning electron microscopy (SEM). The results showed that no visible change in the morphologies of surface of PCD layers (PDC surfaces) was observed at 200 °C both in ambient air and vacuum. After annealing at 500 °C, numbers of spalling pits appeared on the PDC surface, and the stress-induced spall mechanism was the dominant thermal damage mechanism in ambient air and vacuum. With the temperature up to 800 °C, the annealed PDC surface in ambient air was seriously damaged with a mixed thermal damage mechanism such as graphitization, oxidation and stress-induced micro-cracks. Whereas, the thermal damage mechanism in vacuum was nearly the same as that at 500 °C. At 900 °C, only a dendritic phase of Co3O4 was contained on the annealed PDC surface due to extensive graphitization and oxidation in ambient air. When it comes to vacuum environment, many cracks were observed on the PDC surface and some fine diamond grains near the cracks spalled, which demonstrated that the thermal damage mechanisms consisted of stress-induced crack and spall mechanisms caused by the different thermal expansion coefficients between the diamond and Co phase. Compared with that at 900 °C, the degree of thermal damage reduced at 1000 °C in vacuum because of the diffusion of unevenly distributed Co. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Sintered polycrystalline diamond compact (PDC) is a kind of ultrahard materials that involved sintering a polycrystalline diamond (PCD) layer to a cemented carbide (WC–Co) substrate during a high pressure/high temperature (HP/HT) process [1,2]. PDC possesses the characteristics of diamond (high hardness, excellent wear resistance) as well as the advantages of WC–Co substrate (excellent impact toughness, good machinability). Due to these excellent properties, PDC has been widely used in cutting tools and drilling bits. Moreover, PDC is considered as an excellent material for space drilling for its outstanding physical and chemical properties under vacuum condition [3–6]. However, a high temperature, during the processes of cutting and drilling, will accelerate the formation of thermal damage such as graphitization and micro-cracks [7–9], which rapidly leads to failure of PDC tools. Cobalt, as a binder existing in the gaps among diamond grains as well as the interface between PCD layer and WC substrate [8,10], plays a very ⁎ Corresponding author at: School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, PR China. E-mail addresses: [email protected], [email protected] (W. Yue).
http://dx.doi.org/10.1016/j.ijrmhm.2015.07.024 0263-4368/© 2015 Elsevier Ltd. All rights reserved.
important role in graphitizing and initiating of micro-cracks of diamond at high temperature (above 600 °C) [11,12]. As numbers of previous experimental results showed, the microstructures and thermal damage mechanisms of PDC were affected by the temperature and environment. Mehan et al. [13] proposed that the thermal damage of PDC, with a pregnant period, was associated with graphitization in nitrogen. Micro-cracks initiated from the edge of diamond grains and eventually developed into transgranular cracks. Miess et al. [14] studied the stability of three kinds of PCDs (diamond grain sizes of about 4 μm, 10 μm and 30 μm, respectively) in oxygen, nitrogen and hydrogen at the temperature from 600 °C to 800 °C, and observed the second phase of WC–Co extruded out of all kinds of PCD as well as micro-cracks induced by thermal stress. Wang et al. [15] further studied the effect of distribution of binder Co on the cracks, and demonstrated that the various distributions of Co directly led to different kinds of cracks at 700 °C in nitrogen. The mechanisms of thermal damage of PCD, at the temperature from 800 °C to 900 °C in ambient air, were concluded by Wang et al. [16], which confirms that the four forms of thermal damage are micro-cracks, micro-wrinkles, ball-like bugles, and micro-holes. When PCD was heated above 800 °C, diamond grains were converted into CO2 and CO, and micro-cracks and micro-
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Fig. 1. SEM image of the original PDC surface: (a) secondary electron image and (b) back scattering image.
wrinkles occurred on the surface of diamond due to thermal stress. With the disappearance of diamond and increasing of Co3O4, ball-like bugles and micro-holes eventually appeared. Recently, Masina et al. [17] heated PDCs by laser in ambient air to explore the thermally induced defects. It presented that the diffusion of Co and W resulted in the formation of CoxOy and WxOy on PDC surface, but no graphitization of diamond was found. Up to now, researchers only have paid attention to the microstructures and thermal damage mechanisms of annealed PDC around the induced phase transition temperature (700–850 °C). The thermal damage below the induced phase transition temperature is associated with the progress of cutting and drilling with low speed. However, few researches on lower temperature (below 700 °C) and higher temperature (above 850 °C) have been reported. For better understanding of the thermal damage of cutting and drilling tools, it is vital to systematically study the microstructures and thermal damage mechanisms from room temperature (RT) to 1000 °C under ambient air condition. In addition, as a kind of potential material for space exploration, the microstructures and thermal damage of PDC will affect its service life in vacuum conditions. Some works have been performed to study the thermal stability of PDC with a nonmetal binder in vacuum
[18,19], whereas the microstructures and thermal damage mechanisms of PDC with Co binder have been rarely studied under vacuum condition. In this work, annealing experiments were performed in ambient air and vacuum to explore the mechanism of thermal damage of PDC samples so as to further realize the thermal failure mechanism of PDCs in cutting and drilling under ambient air and vacuum conditions. 2. Experimental details 2.1. Materials The commercially sintered PDC produced by Zhongnan Diamond Co., Ltd. was used. It consisted of a PCD layer and a substrate. In the PCD layer, the mean size of diamond grains was 25 μm and the content of the cobalt binder was about 5 wt.%. The substrate was cemented carbide (WC–16 wt.% Co). The commercial PDC samples were cut into 8 × 8 mm2 samples by wire-electrode cutting with a polished cross section. The SEM images of original PDC surface are shown in Fig. 1. Fig. 1a and b shows the secondary electron image and back scattering image of
Fig. 2. Cross section image of the PDC: (a) optical image, (b) SEM image of selected region A in (a), and panels (c) and (d) are the corresponding EDS mapping images of (b).
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at specified temperature, and after 30 min insulation, it was cooled down in ambient environment. 2.3. Analysis methods
Fig. 3. Optical photo of the high vacuum quartz tube.
the PDC surface, respectively. The inset figure of Fig. 1a is the threedimensional surface profile. As Fig. 1a exhibits some tiny holes are on the surface, which are attributed to the sintering process of PDC. The original surface roughness of PDC is 4.3 nm. Fig. 1b shows the distribution of the Co binder and the diamond grains. The Co binder is distributed at the bright region along the diamond grain boundaries and the dark region contains diamond. Fig. 2 is the cross-sectional image of PDC. Fig. 2a is the optical cross-sectional image of the original PDC, and it shows that the total thickness of the PDC sample is 3.7 mm with 3.2 mm of WC–Co substrate and 0.5 mm of PCD layer. Fig. 2b is the SEM image of the selected region A in Fig. 2a. Fig. 2c and d shows the corresponding EDS mapping images of Fig. 2b. Fig. 2c presents the Co concentrates at the gaps among the diamond grains as well as the interface between PCD layer and WC–Co substrate. The Co phase in PCD layer is due to the infiltration of the Co from the WC–Co substrate during sintering [8]. Fig. 2d shows that large amount of W appears in WC–Co substrate. However, the content of the W is negligible compared to that of Co showed in Fig. 2c. The previous references [8,17] have indicated that W, with very low quantity, comes from thermal diffusion and infiltration during sintering process.
2.2. Annealing tests The thermal annealing treatments were performed at room temperature (RT), 200 °C, 500 °C, 800 °C, 900 °C and 1000 °C in a muffle furnace (SX-8-10) for 30 min under ambient air condition. In order to achieve high vacuum environment for the vacuum annealing treatment, PDC samples were capsulated in a high vacuum quartz tube at ~ 1 × 10−5 Pa (Fig. 3). Then the tube was put into the muffle furnace
The optical microscope (Olympus BX51M) was used to record the surface and cross-section image of the annealed PDC samples. The three-dimensional surface topography of the annealed PDC surface was measured by white light interferometer (NanoMap-D). The X-ray diffraction (XRD) patterns were evaluated by an automated D/max2000 diffractometer to study the phase transformation. Raman experiments were performed with a LabRAM HR Evolution spectrometer using a 514.5 nm line of Ar+ laser as the excitation wavelength to investigate the graphitization. The SSX-550 field emission scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscope (EDS) was used to study the microstructure and chemical compositions of the annealed PDC samples. 3. Experimental results and discussion 3.1. Morphologies The morphologies of the annealed PDC surfaces are shown in Fig. 4. It exhibits that the morphologies at 200 °C and 500 °C in ambient air and vacuum are almost the same as that of original PDC surface, which indirectly imply that the ambient air has little or no influence on the visual change of PDC below 500 °C. After annealing at 800 °C in vacuum, the PDC sample still exhibits similar surface with the original PDC sample. However, visible variation can be obviously discovered in ambient air. The color of PDC surface becomes dull and tarnished (Fig. 4c). The WC–Co substrate stretches out on the edge of PDC surface attributed to the thermal expansion and oxidation. This phenomenon can be further presented evidently in Fig. 4d. It is noted that the PCD layer has shed off because of an excess of oxidization (Fig. 4d). While for the sample annealed at 1000 °C in ambient air, the surface cannot be photographed due to the extensive damage. However, the morphologies of samples annealed at 900 °C and 1000 °C in vacuum are quite different from those in ambient air. Fig. 4i and j shows that a large number of bucking deformations appear on the PDC surface, especially on the edge of the PDC surface. These surface variations will be further presented by the surface roughness measured by white light interferometer (Table 1). As Table 1 shows the surface roughness increases with a rise in temperature. Higher temperature has a significant effect on rougher surface. Fig. 5 shows the morphologies of cross section of the annealed PDC at 200 °C, 500 °C and 800 °C in ambient air and vacuum. It is obvious to find that the cross-sectional morphologies, annealed at 200 °C in ambient air as well as at 200 °C, 500 °C and 800 °C in vacuum, are almost
Fig. 4. Morphologies of the annealed PDC surfaces.
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Table 1 Surface roughness (Ra) of the annealed PDC surfaces at different temperatures in ambient air and vacuum. Temperature (°C)
RT
200
500
800
900
1000
Surface roughness in ambient air (nm) Surface roughness in vacuum (nm)
4.3 4.3
3.8 4.2
15.1 14.3
105.3 21.7
– 391.7
– 260.1
identical. After annealing at 500 °C in ambient air, the oxidization reaction of W and Co has occurred with the change of the color of WC–Co substrate (Fig. 5b), which has been mentioned in previous reference [17]. However, Fig. 5c presents that the macro-cracks appear at the interface between PCD layer and WC–Co substrate at the temperature up to 800 °C in ambient air. This can be attributed to the high stress caused by volume expansion of oxidation for WC–Co substrate. The WC–Co substrate after expanding may be evidently observed in Fig. 4c and d. An interesting result (Fig. 6) is that PCD layers annealed at 900 °C and 1000 °C both exfoliate from the WC–Co substrate due to the interface thermal stress, but the former at a lower temperature, with a coarser surface, is easier to exfoliate than the later sample (Table 1). This abnormal phenomenon may be attributed to the diffusion of the unevenly distributed Co binder.
have weak effects on the microstructures and thermal damage of PCD layer due to the small quantity of WC phase. Fig. 7a also presents that a new phase of Co3O4 has formed with the disappearance of Co binder at 800 °C and the intensity of Co3O4 peak weaken at 900 °C. The weaker Co3O4 peak indicates that the reaction of chemical decomposition has occurred. The reaction equations are as follows:
3.2. Composition and structure analysis
3Co þ 2O2 → Co3 O4
ð1Þ
2Co3 O4 → 6CoO þ O2 :
ð2Þ
3.2.1. XRD analysis As Fig. 7a shows diamond and Co binder exist on PDC surfaces at the low temperature (RT, 200 °C and 500 °C) in ambient air, but large amounts of cobalt oxide (Co3O4, CoO) together with small amount of amorphous carbon and graphite, as well as the negligible WC, are found at 800 °C and 900 °C. In order to identify the presence of WC in the PCD layer, the XRD curve of the RT sample has been amplified (shown in the inset of Fig. 7d). It shows that the intensity of the WC is far weaker than that of Co, which is consistent with the EDS result of Fig. 2c and d. Masina et al. [17] have reported that large amounts of WxOy phase caused by laser heating on the PDC surface can change the microstructure and weaken the mechanical properties. However, it is difficult to find the existence of WxOy phase on the PDC surface through the XRD result shown in Fig. 7. It brings into correspondence with that of reference [12]. A reasonable supposition is that W could
Fig. 6. Morphologies of cross section of the annealed PDC at 900 °C and 1000 °C in vacuum.
With the temperature increasing from 200 °C to 1000 °C, it is apparent to find that the intensity of diamond peak firstly becomes strong and then decreases in ambient air as well as in vacuum (Fig. 7a and c). The turning temperatures, with the strongest intensity of diamond peak, are 500 °C in ambient air and 800 °C in vacuum, respectively. A plausible explanation is that a proper annealing temperature can create more crystalline diamond and improve the parameter of atomic order [20, 21]. Fig. 7c shows that the intensities of diamond peaks at different temperatures have less variation in vacuum compared with those in ambient air, which suggests that the process of graphitization can be restrained in vacuum. In addition, Fig. 7c indicates that graphitization occurs at the temperature of 900 °C and exacerbates at 1000 °C. It can
Fig. 5. Morphologies of cross section of the annealed PDC at 200 °C, 500 °C and 800 °C in ambient air and vacuum.
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Fig. 7. XRD patterns of annealed PDC surfaces in ambient air and vacuum: (a) XRD patterns of different temperatures in ambient air, (b) amplified curves of (a) at 800 °C and 900 °C, (c) XRD patterns of different temperatures in vacuum, (d) amplified curves of (c) at 900 °C and 1000 °C, and the inset of (d) is the amplified curve at RT.
Fig. 8. Raman spectra of diamond grains at position A (the edge of diamond grains) and position B (the center of diamond grains) after annealing in ambient air and vacuum: (a) and (b) are the Raman spectra of diamond grains at position A annealed in ambient air and vacuum, respectively, (c) is Raman spectra of annealed diamond grains (air 800 °C and vac. 900 °C) at positions A and B, and (d) is the optical photo of PDC surface.
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be clearly observed that no cobalt oxide is formed, which confirms that the degree of high vacuum (~10−5 Pa) in annealing test is reliable. 3.2.2. Raman analysis Raman spectra, obtained for all annealed PDC surfaces at position A (the edge of diamond grains) in ambient air and vacuum, are showed in Fig. 8a and b. Fig. 8a presents that only a sharp diamond peak
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(~1332 cm−1) appears at the temperature of RT, 200 °C and 500 °C. It infers that the original PDC surface is composed of diamond grains with high purity [22] and possesses good graphitization resistance at least 500 °C in ambient air. With the PDC surfaces heated to 800 °C, a broad peak (G mode) centered at ~1580 cm−1 appears. The G peak demonstrates that the diamond grains start to graphitize. For a higher temperature of 900 °C, the Raman spectra (Fig. 8a) show that the
Fig. 9. SEM images of annealed PDC surfaces in ambient air and vacuum: (a) air 200 °C, (b) vac. 200 °C, (c) air 500 °C, (d) vac. 500 °C, (e) air 800 °C, (f) vac. 800 °C, (g) air 900 °C, (h) vac. 900 °C, (i) air 1000 °C, and (j) vac. 1000 °C.
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Fig. 9 (continued).
diamond grains have disappeared but Co3O4 remained with four peaks (~480, ~522, ~619 and ~693 cm−1), which is consistent with the result of the XRD pattern (Fig. 7a). As Fig. 8b presents the graphitization temperature in vacuum is higher than that in ambient air. With the temperature increasing from 900 °C to 1000 °C, it can be clearly observed that the diamond peak begins to broaden and the G peak starts to strengthen accounting for the increase of the degree for graphitization. In order to confirm whether the edge of diamond graphitize at first, Raman experiments are further performed to obtain the Raman spectra of position A and position B (the center of diamond grains) at the turned temperature in ambient air and vacuum, respectively (Fig. 8c). It presents that the weak G peak only appears at position A at turned temperature. Thus, it confirms that the graphitization initiates at the edge of diamond grains for PDC surface. 3.2.3. SEM and EDS analysis As shown in Fig. 9, the images of annealed PDC surfaces strongly depend on the annealing temperature, especially at high temperature. Fig. 9a and b exhibits no visible variation for PDC surface is observed in ambient air and vacuum compared to that of original sample (Fig. 1a), which indicates that the physical microstructure of PDC surface does not change at 200 °C. Moreover, the three-dimensional surface profiles showed in the inset and the roughness values presented in Table 1 also support this result. With the temperature up to 500 °C, many spalling pits appear on the PDC surface as shown in Fig. 9c and d. A reasonable explanation is associated with exfoliation of fine
intergranular-diamond particles, which is demonstrated by the further magnified SEM image (Fig. 10a). The selected regions from I to III clearly show the spalling progress. Fig. 10b indicates that the spalling pits maintain high amounts of Co and it is difficult to find the peaks of W. The previous work [6] indicates that the Co phase can aggregate the formation of spalling pits. Therefore, the formation of spalling pits can be essentially described as the following progress. As Fig. 10a shows some fine intergranular-diamond particles among original defects are on the PDC surface. It is confirmed in previous references [23,24] that the diamond materials with defects have a very weak thermal resistance. Thus, the fine intergranular-diamond particles are easy to exfoliate when heated. In addition, the thermal stress, caused by the difference between the thermal expansion coefficients of the diamond and Co phase, leads to formation of cracks. Finally, fine intergranulardiamond particles exfoliate and spalling pits appear. Fig. 9e and f shows the images of annealed PDC surfaces at 800 °C in ambient air and vacuum. The huge variation of the micrographs indicates that the PDC surface is seriously damaged in ambient air. As references [25–28] showed oxygen can reduce the graphitization temperature and accelerate the oxidization of diamond. Huge numbers of oxidized holes have appeared on the PDC surface (Fig. 9e). What's more, the Co phase is easy to react with oxygen at 800 °C to form Co3O4. Co3O4 phase is extruded out of the diamond grain boundaries (Fig. 9e), which can promote the surface roughness of PDC surface. Fig. 11a exhibits that the micro-crack induced by the extruded Co3O4 appears on the PDC surface, and Fig. 11b further confirms that the
Fig. 10. SEM image and EDS of the annealed PDC surface at 500 °C in ambient air: (a) the enlarge SEM image and (b) EDS surface chemical composition marked in points A and B of (a).
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Fig. 11. SEM image and EDS of the annealed PDC surface at 800 °C in ambient air: (a) the enlarge SEM image and (b) EDS surface chemical composition marked in point C of (a).
extruded phase is composed of Co and O. However, there is nearly no distinction between Fig. 9d and f. Taking into consideration the X-ray diffraction curves of 500 °C and 800 °C in vacuum, it is confirmed that no chemical reaction has occurred on PDC surface. Hence, the thermal damage mechanisms of 500 °C and 800 °C in vacuum are identical. With the temperature increasing up to 900 °C, the micrographs both in ambient air and vacuum have greatly changed. It is difficult to find the diamond grains but not the dendritic structure in Fig. 9g. Fig. 12a and Table 2 show that the dendritic structure contains large amounts of Co and O and small amounts of W. EDS results further confirm that the dendritic phase is Co3O4. W may come from the substrate because of thermal diffusion and excessive oxidation of the PCD layer. Many cracks shown in Fig. 9h initiate near the holes among diamond grains (Crack A) and then develop into transgranular cracks (Crack B). It is noticed that the cracks can induce the spalls of diamond and further increase the number of the spalling pits. The roughness of PDC surface increases rapidly at 900 °C in vacuum. To further explore the factors for the initiation of cracks, the elemental analysis of the selected regions E and F was performed, and the result shows that region E contains Co (Fig. 12b). Hence it can conclude that defects and thermal stress lead to the initiation of cracks together. At 1000 °C the micrograph of annealed PDC surface in ambient air is not performed due to the extensive damage (Fig. 9i). However, comparing Fig. 9j with h, a common thermal damage mechanism is observed at 900 °C and 1000 °C in vacuum. A noticeable phenomenon is the reduction of thermal damage at a higher temperature (1000 °C), which is indirectly presented by the value of surface roughness in Table 1. The detailed explanation may be attributed to the thermal stress caused by the unevenly distributed Co binder in the interface between the PCD layer and WC–Co substrate.
4. Discussion 4.1. Chemical thermal damage mechanisms Sintered PDC surface is composed of fine diamond grains and Co binder. Diamond is a metastable phase of carbon at RT and atmospheric pressure. Under the condition of 200 °C and 500 °C, the reaction, converting from diamond to non-diamond (graphite, amorphous carbon, lonsdaleite), occurs with a normally undetectable rate, which is indirectly confirmed by X-ray diffraction pattern (Fig. 7). At a higher temperature of 800 °C, diamond will be converted quickly to graphite and amorphous carbon. However, some experimental results show that the transitional metal Co can catalyze the conversion from diamond to graphite [9,19,29,30]. It accounts for that PDC with Co binder is much easier to be graphitized than a single diamond. Moreover, the chemical thermal damage mechanisms are closely related to environmental ambience. In vacuum, the graphitization is the main form of chemical thermal damage at 900 °C and 1000 °C because of lack of reactive gases. But in ambient air, the oxygen will play an important role on thermal damage of PDC surface. The reaction between the Co phase and oxygen will occurr to create Co3O4 above 300 °C, which will promote the surface roughness and induce the initiation of micro-cracks. Diamond and graphite have low inoxidizability in ambient air. Previous researches have noted that the C atoms of diamond and graphite will be reacted with oxygen atoms to create CO and CO2 at high temperature [16,27, 28]. Thus, the surface of diamond will be etched and many oxide holes will remain (Fig. 9e). To sum up, the graphitization and oxidation mechanisms are the dominant chemical thermal damage mechanisms for PDC surface above 800 °C in ambient air. Whereas, the chemical thermal damage mechanism is only graphitization mechanism in vacuum.
Fig. 12. SEM images of annealed PDC surface at the temperature of 900 °C in ambient air and vacuum, respectively: (a) the enlarge SEM image in ambient air and (b) the enlarge SEM image in vacuum.
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substrate with a less deflection at higher temperature (1000 °C) in vacuum.
Table 2 EDS quantification (wt.%/at.%) of the selected regions (D, E and F in Fig. 12).
C Co O W
D
E
F
19.57/43.42 43.86/19.84 20.68/34.44 15.89/2.30
71.64/92.54 28.36/7.46 0/0 0/0
100/100 0/0 0/0 0/0
4.2. Physical thermal damage mechanisms As the PDC surfaces are heated, thermal stress will appear at the interface of diamond grains and Co binder due to the difference between the thermal expansion coefficients of the diamond (3.2 × 10−6/K) and Co (14.4 × 10− 6/K) both in ambient air and vacuum [31]. As Fig. 1a shows the origin PDC surface has some tiny holes caused by HP/HT sintering process. In addition, diamond grains near those holes are flaky. When the thermal stress reaches a critical value these diamond grains will exfoliate from the PDC surface and spalling pits will appear on the PDC surface (Fig. 9c, d and f). As the temperature is increased to 900 °C and 1000 °C, the thermal stress will be released by way of macro-cracks forming in vacuum (Fig. 9h and j). Thus the spall mechanism is the main physical thermal damage mechanism at 500 °C both in ambient air and vacuum, while the spall mechanism as well as the macro-crack mechanism compose the physical thermal damage mechanisms above 800 °C in vacuum. 4.3. Exfoliated mechanism of PCD layer For the composite material of PDC, the interfacial strength is a significant parameter to evaluate the mechanical property. As the PDC is heated, the thermal stress will appear in the interface, which is caused by the difference between the thermal expansion coefficients of the PCD layer and WC–Co substrate. According to the results of previous research, the thermal stress is linearly related to temperature [32]. If the interfacial strength of PDC is higher than the thermal stress, slight variation will appear (Fig. 5a, b, d, e and f). However, the PCD layer will be prone to warping and exfoliation when the thermal stress exceeds the critical value of interfacial strength. The degree of bending deflection depends on the value of thermal stress. It can be apparently obtained from Figs. 5 and 6 that the critical temperature in vacuum is about 900 °C. An abnormal result, observed in Fig. 6, is that the exfoliated PCD layer has warped seriously at 900 °C. But there is nearly no deflection for the exfoliated PCD layer at a higher temperature (1000 °C) in vacuum. The cross-sectional SEM images of origin PDC sample (Fig. 2b) have been performed to give a plausible explanation for this result. It presents that the Co concentrates at the gaps among the diamond grains as well as the interface between PCD layer and WC–Co substrate. When the PDC sample is heated, the uneven thermal stress will appear at the interface due to the uneven distribution of Co. Then the deflection and exfoliation of PCD layer will initiate at this Co-rich position. Thermal diffusion theory can account for this abnormal result based on the uneven distribution of Co and thermal stress. The thermal diffusion coefficient (D) will be used to evaluate diffusion rate, which can be described in the following equation: D ¼ D0 expð−Q =RTÞ
ð3Þ
where D0 is the diffusion constant, Q is the activation energy of diffusion, R is a constant with a value of 8.314 J/mol·K, and T is the thermodynamic temperature. Eq. (3) shows that D is exponentially related to −(1/T). With the temperature increasing from 900 °C to 1000 °C, Co will remarkably diffuse. The degree of the uneven distribution of Co will decline, which can lead to an even distribution of thermal stress in the interface. As a result, the PCD layer has exfoliated from the WC–Co
5. Conclusions In this work, the PDC samples were annealed at the temperature from 200 to 1000 °C in ambient air and vacuum. The microstructures and thermal damage mechanisms of annealed PDC are concluded as follows: (1) At the temperature of 500 °C, numbers of spalling pits appear on the PDC surface and the stress-induced spall mechanism is the dominant thermal damage mechanism for annealed PDC surfaces both in ambient air and vacuum. (2) At 800 °C, the annealed PDC surface in ambient air, compared with that in vacuum, is damaged seriously with the microstructural characteristics of micro-cracks, oxide holes and extruded Co3O4. It confirms that oxygen can accelerate process of thermal damage. The thermal damage mechanism is a mixed-mechanism including graphitization, oxidation and stress-induced microcrack mechanisms. Whereas, the thermal damage mechanism in vacuum is still the stress-induced spall mechanism. (3) The annealed PDC surface, at 900 °C in ambient air, only contains dendritic phase of Co3O4 because of extensive graphitization and oxidation. In vacuum, many cracks appear on the PDC surface and some fine diamond grains near the cracks spall. It demonstrates that the thermal damage mechanism consists of stressinduced crack and spall mechanisms caused by the different thermal expansion coefficients between the diamond and Co binder. (4) The thermal damage degree of the annealed PDC surface at 1000 °C in vacuum is lighter than that at 900 °C. The reason might be that Co has better thermal diffusion ability at a higher temperature of 1000 °C.
Acknowledgments The authors would like to thank the National Natural Science Foundation of China (51375466), the International Science & Technology Cooperation Project of China (2011DFR50060) and the Fundamental Research Funds for the Central Universities (2652015072, 2652015074) for the financial support. References [1] H.P. Bovenkerk, F.P. Bundy, H.T. Hall, H.M. Strong, & R.H. Wentorf, Preparation of diamond, Nature 184 (1959) 1094–1098. [2] R.H. Wentorf, R.C. Devries, & F.P. Bundy, Sintered superhard materials, Science 208 (1980) 873–880. [3] Y.H. Zhao, W. Yue, F. Lin, & Wu.Z.Y. Wang Cb, Friction and wear behaviors of polycrystalline diamond under vacuum conditions, Int. J. Refract. Met. Hard Mater. 50 (2015) 43–52. [4] K. Zacny, D. Currie, G. Paulsen, T. Szwarc, & P. Chu, Planet. Space Sci. 71 (2012) 131–141. [5] I.A. Crawford, M. Anand, C.S. Cockell, H. Falcke, D.A. Green, R. Jaumann, et al., Planet. Space Sci. 74 (2012) 3–14. [6] X. Yu, J. Ai, L. Yang, & C.B. Wang, Exploring tribological behavior of diamond film by hot-filament chemical vapour deposition on tungsten carbide for lunar exploration, Vacuum 100 (2014) 41–45. [7] J. Qian, C. Pantea, J. Huang, T.W. Zerda, & Y. Zhao, Graphitization of diamond powders of different sizes at high pressure–high temperature, Carbon 42 (2004) 2691–2697. [8] M. Yahiaoui, L. Gerbaud, J-Y Paris, J. Denape, & A. Dourfaye, A study on PDC drill bits quality, Wear (2013) 298–299 (32–41). [9] J.N. Boland, & X.S. Li, Microstructural characterisation and wear behaviour of diamond composite materials, Materials 3 (2010) 1390–1419. [10] M. Akaishi, H. Kanda, Y. Sato, N. Setaka, T. Ohsawa, & O. Fukunaga, Sintering behavior of the diamond–cobalt system at high temperature and pressure, J. Mater. Sci. 17 (1982) 193–198.
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Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Microstructures and thermal damage mechanisms of sintered polycrystalline diamond compact annealing under ambient air and vacuum conditions Jiansheng Li a, Wen Yue a,b,⁎, Chengbiao Wang a,b a b
School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, PR China Key Laboratory on Deep Geo-drilling Technology of the Ministry of Land and Resources, China University of Geosciences (Beijing), Beijing 100083, PR China
a r t i c l e
i n f o
Article history: Received 5 May 2015 Received in revised form 16 July 2015 Accepted 19 July 2015 Available online 29 July 2015 Keywords: PDC Microstructures Thermal damage mechanisms Annealing Ambient air Vacuum
a b s t r a c t The microstructures and thermal damage mechanisms of sintered polycrystalline diamond compact (PDC) were studied in ambient air and vacuum at the temperature up to 1000 °C. The microstructures and compositions of the annealed PDC were characterized by white light interferometer, X-ray diffractometry (XRD), Raman spectroscopy and scanning electron microscopy (SEM). The results showed that no visible change in the morphologies of surface of PCD layers (PDC surfaces) was observed at 200 °C both in ambient air and vacuum. After annealing at 500 °C, numbers of spalling pits appeared on the PDC surface, and the stress-induced spall mechanism was the dominant thermal damage mechanism in ambient air and vacuum. With the temperature up to 800 °C, the annealed PDC surface in ambient air was seriously damaged with a mixed thermal damage mechanism such as graphitization, oxidation and stress-induced micro-cracks. Whereas, the thermal damage mechanism in vacuum was nearly the same as that at 500 °C. At 900 °C, only a dendritic phase of Co3O4 was contained on the annealed PDC surface due to extensive graphitization and oxidation in ambient air. When it comes to vacuum environment, many cracks were observed on the PDC surface and some fine diamond grains near the cracks spalled, which demonstrated that the thermal damage mechanisms consisted of stress-induced crack and spall mechanisms caused by the different thermal expansion coefficients between the diamond and Co phase. Compared with that at 900 °C, the degree of thermal damage reduced at 1000 °C in vacuum because of the diffusion of unevenly distributed Co. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Sintered polycrystalline diamond compact (PDC) is a kind of ultrahard materials that involved sintering a polycrystalline diamond (PCD) layer to a cemented carbide (WC–Co) substrate during a high pressure/high temperature (HP/HT) process [1,2]. PDC possesses the characteristics of diamond (high hardness, excellent wear resistance) as well as the advantages of WC–Co substrate (excellent impact toughness, good machinability). Due to these excellent properties, PDC has been widely used in cutting tools and drilling bits. Moreover, PDC is considered as an excellent material for space drilling for its outstanding physical and chemical properties under vacuum condition [3–6]. However, a high temperature, during the processes of cutting and drilling, will accelerate the formation of thermal damage such as graphitization and micro-cracks [7–9], which rapidly leads to failure of PDC tools. Cobalt, as a binder existing in the gaps among diamond grains as well as the interface between PCD layer and WC substrate [8,10], plays a very ⁎ Corresponding author at: School of Engineering and Technology, China University of Geosciences (Beijing), Beijing 100083, PR China. E-mail addresses: [email protected], [email protected] (W. Yue).
http://dx.doi.org/10.1016/j.ijrmhm.2015.07.024 0263-4368/© 2015 Elsevier Ltd. All rights reserved.
important role in graphitizing and initiating of micro-cracks of diamond at high temperature (above 600 °C) [11,12]. As numbers of previous experimental results showed, the microstructures and thermal damage mechanisms of PDC were affected by the temperature and environment. Mehan et al. [13] proposed that the thermal damage of PDC, with a pregnant period, was associated with graphitization in nitrogen. Micro-cracks initiated from the edge of diamond grains and eventually developed into transgranular cracks. Miess et al. [14] studied the stability of three kinds of PCDs (diamond grain sizes of about 4 μm, 10 μm and 30 μm, respectively) in oxygen, nitrogen and hydrogen at the temperature from 600 °C to 800 °C, and observed the second phase of WC–Co extruded out of all kinds of PCD as well as micro-cracks induced by thermal stress. Wang et al. [15] further studied the effect of distribution of binder Co on the cracks, and demonstrated that the various distributions of Co directly led to different kinds of cracks at 700 °C in nitrogen. The mechanisms of thermal damage of PCD, at the temperature from 800 °C to 900 °C in ambient air, were concluded by Wang et al. [16], which confirms that the four forms of thermal damage are micro-cracks, micro-wrinkles, ball-like bugles, and micro-holes. When PCD was heated above 800 °C, diamond grains were converted into CO2 and CO, and micro-cracks and micro-
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Fig. 1. SEM image of the original PDC surface: (a) secondary electron image and (b) back scattering image.
wrinkles occurred on the surface of diamond due to thermal stress. With the disappearance of diamond and increasing of Co3O4, ball-like bugles and micro-holes eventually appeared. Recently, Masina et al. [17] heated PDCs by laser in ambient air to explore the thermally induced defects. It presented that the diffusion of Co and W resulted in the formation of CoxOy and WxOy on PDC surface, but no graphitization of diamond was found. Up to now, researchers only have paid attention to the microstructures and thermal damage mechanisms of annealed PDC around the induced phase transition temperature (700–850 °C). The thermal damage below the induced phase transition temperature is associated with the progress of cutting and drilling with low speed. However, few researches on lower temperature (below 700 °C) and higher temperature (above 850 °C) have been reported. For better understanding of the thermal damage of cutting and drilling tools, it is vital to systematically study the microstructures and thermal damage mechanisms from room temperature (RT) to 1000 °C under ambient air condition. In addition, as a kind of potential material for space exploration, the microstructures and thermal damage of PDC will affect its service life in vacuum conditions. Some works have been performed to study the thermal stability of PDC with a nonmetal binder in vacuum
[18,19], whereas the microstructures and thermal damage mechanisms of PDC with Co binder have been rarely studied under vacuum condition. In this work, annealing experiments were performed in ambient air and vacuum to explore the mechanism of thermal damage of PDC samples so as to further realize the thermal failure mechanism of PDCs in cutting and drilling under ambient air and vacuum conditions. 2. Experimental details 2.1. Materials The commercially sintered PDC produced by Zhongnan Diamond Co., Ltd. was used. It consisted of a PCD layer and a substrate. In the PCD layer, the mean size of diamond grains was 25 μm and the content of the cobalt binder was about 5 wt.%. The substrate was cemented carbide (WC–16 wt.% Co). The commercial PDC samples were cut into 8 × 8 mm2 samples by wire-electrode cutting with a polished cross section. The SEM images of original PDC surface are shown in Fig. 1. Fig. 1a and b shows the secondary electron image and back scattering image of
Fig. 2. Cross section image of the PDC: (a) optical image, (b) SEM image of selected region A in (a), and panels (c) and (d) are the corresponding EDS mapping images of (b).
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at specified temperature, and after 30 min insulation, it was cooled down in ambient environment. 2.3. Analysis methods
Fig. 3. Optical photo of the high vacuum quartz tube.
the PDC surface, respectively. The inset figure of Fig. 1a is the threedimensional surface profile. As Fig. 1a exhibits some tiny holes are on the surface, which are attributed to the sintering process of PDC. The original surface roughness of PDC is 4.3 nm. Fig. 1b shows the distribution of the Co binder and the diamond grains. The Co binder is distributed at the bright region along the diamond grain boundaries and the dark region contains diamond. Fig. 2 is the cross-sectional image of PDC. Fig. 2a is the optical cross-sectional image of the original PDC, and it shows that the total thickness of the PDC sample is 3.7 mm with 3.2 mm of WC–Co substrate and 0.5 mm of PCD layer. Fig. 2b is the SEM image of the selected region A in Fig. 2a. Fig. 2c and d shows the corresponding EDS mapping images of Fig. 2b. Fig. 2c presents the Co concentrates at the gaps among the diamond grains as well as the interface between PCD layer and WC–Co substrate. The Co phase in PCD layer is due to the infiltration of the Co from the WC–Co substrate during sintering [8]. Fig. 2d shows that large amount of W appears in WC–Co substrate. However, the content of the W is negligible compared to that of Co showed in Fig. 2c. The previous references [8,17] have indicated that W, with very low quantity, comes from thermal diffusion and infiltration during sintering process.
2.2. Annealing tests The thermal annealing treatments were performed at room temperature (RT), 200 °C, 500 °C, 800 °C, 900 °C and 1000 °C in a muffle furnace (SX-8-10) for 30 min under ambient air condition. In order to achieve high vacuum environment for the vacuum annealing treatment, PDC samples were capsulated in a high vacuum quartz tube at ~ 1 × 10−5 Pa (Fig. 3). Then the tube was put into the muffle furnace
The optical microscope (Olympus BX51M) was used to record the surface and cross-section image of the annealed PDC samples. The three-dimensional surface topography of the annealed PDC surface was measured by white light interferometer (NanoMap-D). The X-ray diffraction (XRD) patterns were evaluated by an automated D/max2000 diffractometer to study the phase transformation. Raman experiments were performed with a LabRAM HR Evolution spectrometer using a 514.5 nm line of Ar+ laser as the excitation wavelength to investigate the graphitization. The SSX-550 field emission scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectroscope (EDS) was used to study the microstructure and chemical compositions of the annealed PDC samples. 3. Experimental results and discussion 3.1. Morphologies The morphologies of the annealed PDC surfaces are shown in Fig. 4. It exhibits that the morphologies at 200 °C and 500 °C in ambient air and vacuum are almost the same as that of original PDC surface, which indirectly imply that the ambient air has little or no influence on the visual change of PDC below 500 °C. After annealing at 800 °C in vacuum, the PDC sample still exhibits similar surface with the original PDC sample. However, visible variation can be obviously discovered in ambient air. The color of PDC surface becomes dull and tarnished (Fig. 4c). The WC–Co substrate stretches out on the edge of PDC surface attributed to the thermal expansion and oxidation. This phenomenon can be further presented evidently in Fig. 4d. It is noted that the PCD layer has shed off because of an excess of oxidization (Fig. 4d). While for the sample annealed at 1000 °C in ambient air, the surface cannot be photographed due to the extensive damage. However, the morphologies of samples annealed at 900 °C and 1000 °C in vacuum are quite different from those in ambient air. Fig. 4i and j shows that a large number of bucking deformations appear on the PDC surface, especially on the edge of the PDC surface. These surface variations will be further presented by the surface roughness measured by white light interferometer (Table 1). As Table 1 shows the surface roughness increases with a rise in temperature. Higher temperature has a significant effect on rougher surface. Fig. 5 shows the morphologies of cross section of the annealed PDC at 200 °C, 500 °C and 800 °C in ambient air and vacuum. It is obvious to find that the cross-sectional morphologies, annealed at 200 °C in ambient air as well as at 200 °C, 500 °C and 800 °C in vacuum, are almost
Fig. 4. Morphologies of the annealed PDC surfaces.
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Table 1 Surface roughness (Ra) of the annealed PDC surfaces at different temperatures in ambient air and vacuum. Temperature (°C)
RT
200
500
800
900
1000
Surface roughness in ambient air (nm) Surface roughness in vacuum (nm)
4.3 4.3
3.8 4.2
15.1 14.3
105.3 21.7
– 391.7
– 260.1
identical. After annealing at 500 °C in ambient air, the oxidization reaction of W and Co has occurred with the change of the color of WC–Co substrate (Fig. 5b), which has been mentioned in previous reference [17]. However, Fig. 5c presents that the macro-cracks appear at the interface between PCD layer and WC–Co substrate at the temperature up to 800 °C in ambient air. This can be attributed to the high stress caused by volume expansion of oxidation for WC–Co substrate. The WC–Co substrate after expanding may be evidently observed in Fig. 4c and d. An interesting result (Fig. 6) is that PCD layers annealed at 900 °C and 1000 °C both exfoliate from the WC–Co substrate due to the interface thermal stress, but the former at a lower temperature, with a coarser surface, is easier to exfoliate than the later sample (Table 1). This abnormal phenomenon may be attributed to the diffusion of the unevenly distributed Co binder.
have weak effects on the microstructures and thermal damage of PCD layer due to the small quantity of WC phase. Fig. 7a also presents that a new phase of Co3O4 has formed with the disappearance of Co binder at 800 °C and the intensity of Co3O4 peak weaken at 900 °C. The weaker Co3O4 peak indicates that the reaction of chemical decomposition has occurred. The reaction equations are as follows:
3.2. Composition and structure analysis
3Co þ 2O2 → Co3 O4
ð1Þ
2Co3 O4 → 6CoO þ O2 :
ð2Þ
3.2.1. XRD analysis As Fig. 7a shows diamond and Co binder exist on PDC surfaces at the low temperature (RT, 200 °C and 500 °C) in ambient air, but large amounts of cobalt oxide (Co3O4, CoO) together with small amount of amorphous carbon and graphite, as well as the negligible WC, are found at 800 °C and 900 °C. In order to identify the presence of WC in the PCD layer, the XRD curve of the RT sample has been amplified (shown in the inset of Fig. 7d). It shows that the intensity of the WC is far weaker than that of Co, which is consistent with the EDS result of Fig. 2c and d. Masina et al. [17] have reported that large amounts of WxOy phase caused by laser heating on the PDC surface can change the microstructure and weaken the mechanical properties. However, it is difficult to find the existence of WxOy phase on the PDC surface through the XRD result shown in Fig. 7. It brings into correspondence with that of reference [12]. A reasonable supposition is that W could
Fig. 6. Morphologies of cross section of the annealed PDC at 900 °C and 1000 °C in vacuum.
With the temperature increasing from 200 °C to 1000 °C, it is apparent to find that the intensity of diamond peak firstly becomes strong and then decreases in ambient air as well as in vacuum (Fig. 7a and c). The turning temperatures, with the strongest intensity of diamond peak, are 500 °C in ambient air and 800 °C in vacuum, respectively. A plausible explanation is that a proper annealing temperature can create more crystalline diamond and improve the parameter of atomic order [20, 21]. Fig. 7c shows that the intensities of diamond peaks at different temperatures have less variation in vacuum compared with those in ambient air, which suggests that the process of graphitization can be restrained in vacuum. In addition, Fig. 7c indicates that graphitization occurs at the temperature of 900 °C and exacerbates at 1000 °C. It can
Fig. 5. Morphologies of cross section of the annealed PDC at 200 °C, 500 °C and 800 °C in ambient air and vacuum.
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Fig. 7. XRD patterns of annealed PDC surfaces in ambient air and vacuum: (a) XRD patterns of different temperatures in ambient air, (b) amplified curves of (a) at 800 °C and 900 °C, (c) XRD patterns of different temperatures in vacuum, (d) amplified curves of (c) at 900 °C and 1000 °C, and the inset of (d) is the amplified curve at RT.
Fig. 8. Raman spectra of diamond grains at position A (the edge of diamond grains) and position B (the center of diamond grains) after annealing in ambient air and vacuum: (a) and (b) are the Raman spectra of diamond grains at position A annealed in ambient air and vacuum, respectively, (c) is Raman spectra of annealed diamond grains (air 800 °C and vac. 900 °C) at positions A and B, and (d) is the optical photo of PDC surface.
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be clearly observed that no cobalt oxide is formed, which confirms that the degree of high vacuum (~10−5 Pa) in annealing test is reliable. 3.2.2. Raman analysis Raman spectra, obtained for all annealed PDC surfaces at position A (the edge of diamond grains) in ambient air and vacuum, are showed in Fig. 8a and b. Fig. 8a presents that only a sharp diamond peak
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(~1332 cm−1) appears at the temperature of RT, 200 °C and 500 °C. It infers that the original PDC surface is composed of diamond grains with high purity [22] and possesses good graphitization resistance at least 500 °C in ambient air. With the PDC surfaces heated to 800 °C, a broad peak (G mode) centered at ~1580 cm−1 appears. The G peak demonstrates that the diamond grains start to graphitize. For a higher temperature of 900 °C, the Raman spectra (Fig. 8a) show that the
Fig. 9. SEM images of annealed PDC surfaces in ambient air and vacuum: (a) air 200 °C, (b) vac. 200 °C, (c) air 500 °C, (d) vac. 500 °C, (e) air 800 °C, (f) vac. 800 °C, (g) air 900 °C, (h) vac. 900 °C, (i) air 1000 °C, and (j) vac. 1000 °C.
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Fig. 9 (continued).
diamond grains have disappeared but Co3O4 remained with four peaks (~480, ~522, ~619 and ~693 cm−1), which is consistent with the result of the XRD pattern (Fig. 7a). As Fig. 8b presents the graphitization temperature in vacuum is higher than that in ambient air. With the temperature increasing from 900 °C to 1000 °C, it can be clearly observed that the diamond peak begins to broaden and the G peak starts to strengthen accounting for the increase of the degree for graphitization. In order to confirm whether the edge of diamond graphitize at first, Raman experiments are further performed to obtain the Raman spectra of position A and position B (the center of diamond grains) at the turned temperature in ambient air and vacuum, respectively (Fig. 8c). It presents that the weak G peak only appears at position A at turned temperature. Thus, it confirms that the graphitization initiates at the edge of diamond grains for PDC surface. 3.2.3. SEM and EDS analysis As shown in Fig. 9, the images of annealed PDC surfaces strongly depend on the annealing temperature, especially at high temperature. Fig. 9a and b exhibits no visible variation for PDC surface is observed in ambient air and vacuum compared to that of original sample (Fig. 1a), which indicates that the physical microstructure of PDC surface does not change at 200 °C. Moreover, the three-dimensional surface profiles showed in the inset and the roughness values presented in Table 1 also support this result. With the temperature up to 500 °C, many spalling pits appear on the PDC surface as shown in Fig. 9c and d. A reasonable explanation is associated with exfoliation of fine
intergranular-diamond particles, which is demonstrated by the further magnified SEM image (Fig. 10a). The selected regions from I to III clearly show the spalling progress. Fig. 10b indicates that the spalling pits maintain high amounts of Co and it is difficult to find the peaks of W. The previous work [6] indicates that the Co phase can aggregate the formation of spalling pits. Therefore, the formation of spalling pits can be essentially described as the following progress. As Fig. 10a shows some fine intergranular-diamond particles among original defects are on the PDC surface. It is confirmed in previous references [23,24] that the diamond materials with defects have a very weak thermal resistance. Thus, the fine intergranular-diamond particles are easy to exfoliate when heated. In addition, the thermal stress, caused by the difference between the thermal expansion coefficients of the diamond and Co phase, leads to formation of cracks. Finally, fine intergranulardiamond particles exfoliate and spalling pits appear. Fig. 9e and f shows the images of annealed PDC surfaces at 800 °C in ambient air and vacuum. The huge variation of the micrographs indicates that the PDC surface is seriously damaged in ambient air. As references [25–28] showed oxygen can reduce the graphitization temperature and accelerate the oxidization of diamond. Huge numbers of oxidized holes have appeared on the PDC surface (Fig. 9e). What's more, the Co phase is easy to react with oxygen at 800 °C to form Co3O4. Co3O4 phase is extruded out of the diamond grain boundaries (Fig. 9e), which can promote the surface roughness of PDC surface. Fig. 11a exhibits that the micro-crack induced by the extruded Co3O4 appears on the PDC surface, and Fig. 11b further confirms that the
Fig. 10. SEM image and EDS of the annealed PDC surface at 500 °C in ambient air: (a) the enlarge SEM image and (b) EDS surface chemical composition marked in points A and B of (a).
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Fig. 11. SEM image and EDS of the annealed PDC surface at 800 °C in ambient air: (a) the enlarge SEM image and (b) EDS surface chemical composition marked in point C of (a).
extruded phase is composed of Co and O. However, there is nearly no distinction between Fig. 9d and f. Taking into consideration the X-ray diffraction curves of 500 °C and 800 °C in vacuum, it is confirmed that no chemical reaction has occurred on PDC surface. Hence, the thermal damage mechanisms of 500 °C and 800 °C in vacuum are identical. With the temperature increasing up to 900 °C, the micrographs both in ambient air and vacuum have greatly changed. It is difficult to find the diamond grains but not the dendritic structure in Fig. 9g. Fig. 12a and Table 2 show that the dendritic structure contains large amounts of Co and O and small amounts of W. EDS results further confirm that the dendritic phase is Co3O4. W may come from the substrate because of thermal diffusion and excessive oxidation of the PCD layer. Many cracks shown in Fig. 9h initiate near the holes among diamond grains (Crack A) and then develop into transgranular cracks (Crack B). It is noticed that the cracks can induce the spalls of diamond and further increase the number of the spalling pits. The roughness of PDC surface increases rapidly at 900 °C in vacuum. To further explore the factors for the initiation of cracks, the elemental analysis of the selected regions E and F was performed, and the result shows that region E contains Co (Fig. 12b). Hence it can conclude that defects and thermal stress lead to the initiation of cracks together. At 1000 °C the micrograph of annealed PDC surface in ambient air is not performed due to the extensive damage (Fig. 9i). However, comparing Fig. 9j with h, a common thermal damage mechanism is observed at 900 °C and 1000 °C in vacuum. A noticeable phenomenon is the reduction of thermal damage at a higher temperature (1000 °C), which is indirectly presented by the value of surface roughness in Table 1. The detailed explanation may be attributed to the thermal stress caused by the unevenly distributed Co binder in the interface between the PCD layer and WC–Co substrate.
4. Discussion 4.1. Chemical thermal damage mechanisms Sintered PDC surface is composed of fine diamond grains and Co binder. Diamond is a metastable phase of carbon at RT and atmospheric pressure. Under the condition of 200 °C and 500 °C, the reaction, converting from diamond to non-diamond (graphite, amorphous carbon, lonsdaleite), occurs with a normally undetectable rate, which is indirectly confirmed by X-ray diffraction pattern (Fig. 7). At a higher temperature of 800 °C, diamond will be converted quickly to graphite and amorphous carbon. However, some experimental results show that the transitional metal Co can catalyze the conversion from diamond to graphite [9,19,29,30]. It accounts for that PDC with Co binder is much easier to be graphitized than a single diamond. Moreover, the chemical thermal damage mechanisms are closely related to environmental ambience. In vacuum, the graphitization is the main form of chemical thermal damage at 900 °C and 1000 °C because of lack of reactive gases. But in ambient air, the oxygen will play an important role on thermal damage of PDC surface. The reaction between the Co phase and oxygen will occurr to create Co3O4 above 300 °C, which will promote the surface roughness and induce the initiation of micro-cracks. Diamond and graphite have low inoxidizability in ambient air. Previous researches have noted that the C atoms of diamond and graphite will be reacted with oxygen atoms to create CO and CO2 at high temperature [16,27, 28]. Thus, the surface of diamond will be etched and many oxide holes will remain (Fig. 9e). To sum up, the graphitization and oxidation mechanisms are the dominant chemical thermal damage mechanisms for PDC surface above 800 °C in ambient air. Whereas, the chemical thermal damage mechanism is only graphitization mechanism in vacuum.
Fig. 12. SEM images of annealed PDC surface at the temperature of 900 °C in ambient air and vacuum, respectively: (a) the enlarge SEM image in ambient air and (b) the enlarge SEM image in vacuum.
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substrate with a less deflection at higher temperature (1000 °C) in vacuum.
Table 2 EDS quantification (wt.%/at.%) of the selected regions (D, E and F in Fig. 12).
C Co O W
D
E
F
19.57/43.42 43.86/19.84 20.68/34.44 15.89/2.30
71.64/92.54 28.36/7.46 0/0 0/0
100/100 0/0 0/0 0/0
4.2. Physical thermal damage mechanisms As the PDC surfaces are heated, thermal stress will appear at the interface of diamond grains and Co binder due to the difference between the thermal expansion coefficients of the diamond (3.2 × 10−6/K) and Co (14.4 × 10− 6/K) both in ambient air and vacuum [31]. As Fig. 1a shows the origin PDC surface has some tiny holes caused by HP/HT sintering process. In addition, diamond grains near those holes are flaky. When the thermal stress reaches a critical value these diamond grains will exfoliate from the PDC surface and spalling pits will appear on the PDC surface (Fig. 9c, d and f). As the temperature is increased to 900 °C and 1000 °C, the thermal stress will be released by way of macro-cracks forming in vacuum (Fig. 9h and j). Thus the spall mechanism is the main physical thermal damage mechanism at 500 °C both in ambient air and vacuum, while the spall mechanism as well as the macro-crack mechanism compose the physical thermal damage mechanisms above 800 °C in vacuum. 4.3. Exfoliated mechanism of PCD layer For the composite material of PDC, the interfacial strength is a significant parameter to evaluate the mechanical property. As the PDC is heated, the thermal stress will appear in the interface, which is caused by the difference between the thermal expansion coefficients of the PCD layer and WC–Co substrate. According to the results of previous research, the thermal stress is linearly related to temperature [32]. If the interfacial strength of PDC is higher than the thermal stress, slight variation will appear (Fig. 5a, b, d, e and f). However, the PCD layer will be prone to warping and exfoliation when the thermal stress exceeds the critical value of interfacial strength. The degree of bending deflection depends on the value of thermal stress. It can be apparently obtained from Figs. 5 and 6 that the critical temperature in vacuum is about 900 °C. An abnormal result, observed in Fig. 6, is that the exfoliated PCD layer has warped seriously at 900 °C. But there is nearly no deflection for the exfoliated PCD layer at a higher temperature (1000 °C) in vacuum. The cross-sectional SEM images of origin PDC sample (Fig. 2b) have been performed to give a plausible explanation for this result. It presents that the Co concentrates at the gaps among the diamond grains as well as the interface between PCD layer and WC–Co substrate. When the PDC sample is heated, the uneven thermal stress will appear at the interface due to the uneven distribution of Co. Then the deflection and exfoliation of PCD layer will initiate at this Co-rich position. Thermal diffusion theory can account for this abnormal result based on the uneven distribution of Co and thermal stress. The thermal diffusion coefficient (D) will be used to evaluate diffusion rate, which can be described in the following equation: D ¼ D0 expð−Q =RTÞ
ð3Þ
where D0 is the diffusion constant, Q is the activation energy of diffusion, R is a constant with a value of 8.314 J/mol·K, and T is the thermodynamic temperature. Eq. (3) shows that D is exponentially related to −(1/T). With the temperature increasing from 900 °C to 1000 °C, Co will remarkably diffuse. The degree of the uneven distribution of Co will decline, which can lead to an even distribution of thermal stress in the interface. As a result, the PCD layer has exfoliated from the WC–Co
5. Conclusions In this work, the PDC samples were annealed at the temperature from 200 to 1000 °C in ambient air and vacuum. The microstructures and thermal damage mechanisms of annealed PDC are concluded as follows: (1) At the temperature of 500 °C, numbers of spalling pits appear on the PDC surface and the stress-induced spall mechanism is the dominant thermal damage mechanism for annealed PDC surfaces both in ambient air and vacuum. (2) At 800 °C, the annealed PDC surface in ambient air, compared with that in vacuum, is damaged seriously with the microstructural characteristics of micro-cracks, oxide holes and extruded Co3O4. It confirms that oxygen can accelerate process of thermal damage. The thermal damage mechanism is a mixed-mechanism including graphitization, oxidation and stress-induced microcrack mechanisms. Whereas, the thermal damage mechanism in vacuum is still the stress-induced spall mechanism. (3) The annealed PDC surface, at 900 °C in ambient air, only contains dendritic phase of Co3O4 because of extensive graphitization and oxidation. In vacuum, many cracks appear on the PDC surface and some fine diamond grains near the cracks spall. It demonstrates that the thermal damage mechanism consists of stressinduced crack and spall mechanisms caused by the different thermal expansion coefficients between the diamond and Co binder. (4) The thermal damage degree of the annealed PDC surface at 1000 °C in vacuum is lighter than that at 900 °C. The reason might be that Co has better thermal diffusion ability at a higher temperature of 1000 °C.
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