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Dynamic fragmentation process and fragment microstructure evolution of alumina particles in a vacuum kinetic spraying system
Scripta Materialia xxx (2015) xxx–xxx
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Dynamic fragmentation process and fragment microstructure evolution of alumina particles in a vacuum kinetic spraying system Hyungkwon Park, Jinyoung Kim, Changhee Lee ⇑ Kinetic Spray Coating Laboratory (NRL), Division of Materials Science and Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, South Korea
a r t i c l e
i n f o
Article history: Received 2 April 2015 Revised 12 June 2015 Accepted 13 June 2015 Available online xxxx Keywords: Vacuum kinetic spraying (VKS) process Shock wave Dynamic fragmentation Fragment size Deposition mechanism
a b s t r a c t In a vacuum kinetic spraying (VKS) system, submicron ceramic particles impact the substrate and undergo dynamic fragmentation within nano-seconds, resulting in a nano grain film. In this report, a hypothesized mechanism for dynamic fragmentation generated during Al2O3 particle deposition is suggested and tested via Auto-dyn simulation linked to an analysis of distinct fragment microstructures and debris size calculations. Based on this data, shock-induced dynamic fragmentation, one of the crucial elements of clarifying the deposition mechanism, was investigated. Ó 2015 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.
Vacuum kinetic spraying (VKS) is an attractive ceramic film fabrication process for a wide variety of industrial fields due to various positive advantages, including (primarily) room temperature processing, high deposition rates, low processing costs, and simple operation methods as compared to similar film processes such as: sol–gel, sputtering, and CVD processes [1]. For these reasons, considerable research has been conducted toward the development of applications for VKS. However, a fundamental understanding of processes including the mechanism of deposition is largely lacking. This may be due to the fact that many process events occur simultaneously, making analysis very complicated. Various things, like the material response against a high strain rate caused by a shock wave, unexpected nanoscale material behavior, ceramic plasticity, nanosecond reaction time, and so on, occur during particle deposition of the VKS process. In this regard, investigations of the mechanisms of VKS have been conducted based on particle impact simulation [1,2] and/or microstructural observation [3,4]. Several studies [1,3] have suggested that plasticity and fragmentation are crucial elements of the deposition mechanism because flattened and nano-sized grains are always observed in the coating microstructure. Accordingly, shock-induced plasticity and fragmentation phenomena were suggested from simulation results based on postmortem microstructural evidence [5].
⇑ Corresponding author. E-mail address: [email protected] (C. Lee).
In addition to the previous results [5], shock-induced dynamic fragmentation is one focus of this study. The hypothesis of a dynamic fragmentation process was tested via Auto-dyn simulation, film microstructure observation, and fragment size calculation. Furthermore, dynamic fragmentation, one of the crucial keys for clarifying the deposition mechanism, is discussed in detail. A commercial a-Al2O3 powder with irregular morphology shown in Fig. 1(a) and soda lime glass were used as the feedstock material and substrate, respectively. The powder particles had an average diameter of 0.3 lm with a relatively wide size range, as confirmed in Fig. 1(b). Prior to deposition, the powder was dried for several hours to remove moisture and avoid particle agglomeration. The substrates were also cleaned via ultrasonication to eliminate surface contaminants. The powders were aerosolized via aerosol chamber vibration and transferred to the deposition chamber. The particles were accelerated to supersonic velocities using the pressure difference between the two chambers and a slit-type nozzle with an area of 5.0 0.4 mm2. Helium was used as a carrier gas and flow rate was regulated from 2 to 18 L/min to control particle velocity [1]. Two types of experiments were conducted. One experiment involved an individual particle test to analyze particle dynamic behavior. For the experiment, a small quantity (0.01 g) of powder was used with only one pass being performed. Traverse speed and standoff distance were fixed at 1 mm/s and 10 mm, respectively. A 10-pass coating was fabricated using approximately 80.0 g of powder, although spraying conditions were the same as in the individual particle test. Using the 10-pass coating specimen,
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Fig. 1. (a) FE-SEM micrograph of the powder morphology, (b) size distribution of Al2O3 particles, and (c) corresponding numerical modeling design of 0.3-lm-diameter Al2O3 in Auto-dyn simulation.
the elastic modulus was measured to estimate the atomic bonding state for each coating. Powder morphology and deposition fragment shapes were observed via field emission scanning electron microscopy (FE-SEM, SIGMA, Carl Zeiss). For measuring the elastic modulus, nanoindentation experiments were performed using a Nanoindenter XP (MTS, Oak Ridge, TN) with a Berkovich tip. The indenter displacement was set to 500 nm and the data was averaged over 20 measurements. A non-linear transient finite element method (FEM) was performed using an ANSYS Auto-dyn v. 13.0 commercial simulation tool to analyze the dynamic behavior of a single alumina particle. To simulate the process economically and effectively, the model was designed to be simple and symmetric, as shown in Fig. 1(c). The particle size was set to 0.3 lm in diameter, the value of which was the same as the average powder size. For analysis of the complicated processes that occur during supersonic particle impact, a simple spherical-shaped particle was deliberately designed. The smallest element was intentionally set to 5 nm, considering the smallest grain size observed in a prior report [1]. Two material combinations, an Al2O3 particle on a glass substrate and an Al2O3 particle on an Al2O3 substrate, represented the particle impact on a glass substrate and the deposited coating layer subsequently formed on the glass substrate, respectively. The Johnson–Holmquist 2 (JH2) model was chosen because it can be used to predict the dynamic impact behavior of a ceramic material. A detailed explanation of the model can be found in the literature [6]. The material properties of Al2O3 and glass were set based on values referenced in the software library. The only variable was the particle impact velocity, which was set between 100 and 700 m/s at 100 m/s intervals in consideration of the estimated value reported in a prior study [1]. Principal stress was monitored at the particle south pole (gauge 1), and the corresponding contact point of the substrate (gauge 9), as shown in Fig. 1(c). In this simulation, principal stress 1 and 2 indicated that stress occurred in the x- (axial stress) and y-axis (lateral stress) directions, respectively.
Fig. 2(a–c) show the material status change during particle impact with a 400 m/s velocity as time proceeds from 0.1 ns in Fig. 2(a), to 0.5 ns in Fig. 2(b), to 1.0 ns in Fig. 2(c) after the impact event. The pale red directional marks represent compressive force, indicating the opposite direction. As the force increased, the strain rate correspondingly increased. Compressive lateral pressures are generated by a momentary inertial force if the strain rate is sufficiently high [7–9]. This compressive pressure suppresses crack formation and instead induces plasticity in the ceramic material as a means of consuming the suddenly released energy [9,10]. With regard to the VKS process, although there were no factors confining the particle, high strain shock loading generated a momentary inertial confining force [5,9]. Thus, the particle was expected to exhibit plasticity at the start of the impact when there was strong confinement hindering crack formation. However, the force gradually decreased as the kinetic energy was consumed by plastic deformation, friction, and heat energy. Therefore, the pale red arrows in Fig. 2(b) became faint and extensive bulk failure was simultaneously abruptly generated, especially at the bottom of the particle. It is worth noting that the bulk fail rate was significantly reduced thereafter as confirmed by differences in the fraction of failures over similar time intervals, 0.4 ns between Fig. 2(a and b) and 0.5 ns between Fig. 2(b and c). In consideration of the fact that the kinetic energy was 50% consumed within 0.22 ns and took around 0.4–0.7 ns to expend 90% of the energy (depending on the substrate, i.e., glass or Al2O3) for all velocity cases, the impact reaction event may be completed prior to 1.0 ns of elapsed time in Fig. 2(c). Therefore, extensive plasticity and failure took place within a very short time scale: 0.55 ns on average. Another important observation was the upper component of the particle, which remained intact even 1.0 ns after impact. As mentioned previously, the unexpected plasticity and dynamic fragmentation of ceramic particles are closely related to changes in the confining pressure of the VKS system. Fig. 2(d) shows the change of the maximum principal stress 2 (P. stress 2) for different particle velocities. Although the P. stress 2 changes
Fig. 2. Material status change of Al2O3 particles with a 400 m/s impact velocity at (a) 0.1, (b) 0.5, and (c) 1.0 ns, and (d) principal stress 2 graph with different velocities [A and B in Fig. 2(b) indicate specific points in plasticity and the fractured region, respectively]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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every moment during impact and reverts to 0 when fracture is complete, the observed maximum values indicate the largest stress recorded at each gauge and velocity. In the graph, the triangle and inverted triangle indicate measurements at gauges 9 and 1, respectively. Thus, the P. stress 2 of an alumina particle in the case of both substrates and the contact point of the alumina substrate was marked, except for the contact point of the glass substrate. As explained in the white box in Fig. 1(c), P. stress 2 indicates the stress in the direction perpendicular to particle impact, with the value always being negative, indicative of compressive stress. As a result, stress was directed toward the inside of the particle, indicating a confining pressure. This can be confirmed by regions A and B in Fig. 2(b), in which the P. stress 2 values were around 0.3 GPa at A and 0 GPa at B, respectively. Thus, the plasticity region completely failed following the disappearance of the confining pressure. The confining pressure became increasingly negative as the velocity increased. Moreover, the value for impact on the Al2O3 substrate was larger than was measured on the glass substrate. This suggests that the confining pressure increased with increasing substrate hardness. Fragment microstructure evolution was observed via individual particle testing. Fig. 3 presents the FE-SEM micrographs of debris morphology. Although not all fragments had the same shape as those observed in Fig. 3(a–d), the morphology seen in the images demonstrates particular phenomenological features generated during shock-induced dynamic fragmentation. Given the flattened fragment shape caused by plasticity and the particle size range in Fig. 1(b), the debris size is reasonable. Based on the fragmentation patterns observed in prior studies [5,11], the debris morphology was formed via shear fracture resulting from shock loading [7,8]. In Fig. 3(a), it appears that localized plasticity was generated at the bottom of the particle, in agreement with the simulation results. Fragments were also barely attached to the part at 2 L/min. It was postulated that the debris exhibited incomplete fragmentation due to a lack of kinetic impact energy. Additionally, the fragmentation pattern was quite similar to the surface morphology of a typical vacuum kinetic sprayed film [3,4]. Thus, the debris appeared in the middle of dynamic fragmentation. Conversely, it seems that the debris in Fig. 3(b) resulted from complete dynamic fragmentation, although some fragments were not perfectly detached (marked by white arrows). However, several small fragments that detached from the large debris were distinctly observed (marked by yellow dashed circles). Fig. 3(c) reveals fragments stuck in an anchoring layer. The debris appears as if it were undergoing plasticity due to its partial fragmentation. Moreover, the presence of an anchoring layer was also confirmed by the simulation results in Fig. 2(c) and served an important role in successful particle deposition [3,12]. The fragment in Fig. 3(d) appears to be breaking away from the anchoring layer, consisting of crystallites around 100 nm in size. Based on an analysis of the experimentally observed results in conjunction with the corresponding simulation results, a hypothesis describing the dynamic fragmentation process in a VKS system
3
can be proposed as follows. At the initial particle impact stage, the particle underwent localized plasticity aided by confining pressures when the impact velocity is sufficiently high, as shown in Fig. 4(a). Shock-induced plasticity was prevalent in the bottom portion of the particle, which was stuck by a strong confining inertial force in conjunction with the supporting substrate and pressure from the upper part of the particle in Fig. 4(b). At this point in time, components of the particle that were not confined, i.e., the upper and sides of the particle, began to shatter. However, the confining force weakened, and thus the region exhibiting plasticity also began to experience bulk failure as illustrated in Fig. 4(c). Finally, the region was completely dynamically fragmented in Fig. 4(d). Some parts of the particle that already failed might travel with the gas stream flowing onto the substrate. Consequently, a variety of microstructural features resulting from dynamic fragmentation can be observed in Fig. 3(a–c). As the front of the shock wave propagates, ceramic material suffers from elastic and/or plastic deformation accompanied by fragmentation [8,9]. Additionally, the damage level gradually progresses in the order of sudden elastic shock, failure ramp, deformation shock, to the Hugoniot state as the shock wave became stronger [7,8]. Furthermore, development can be accelerated even at low shock loading levels depending on microstructural conditions such as: grain size, interphases (grain boundary, triple line junction, quadratic node), crack size and number, porosity, and more [8–10]. Moreover, it was previously observed that dynamic strength data corresponded to resolved shear stress ranging from approximately G/25 to G/40, where G was the shear modulus of the ceramic [8]. If the G of Al2O3 was 150 GPa, the dynamic strength was in the range of 3.75–6.0 GPa. Furthermore, considering the impact pressure was measured to be between 3 and 11 GPa according to a prior report [5], the ceramic particle or previously deposited layer could be sufficiently beyond the failure ramp state in the VKS process. Consequently, the debris pattern, e.g., the fragments in Fig. 3(a–d), could be effectively observed via the individual particle test. Additionally, fragment sizes could be estimated from an equation related to the size of the fracture zone debris based on energy balance arguments [7,8]:
d¼
2Ec P2C
ð1Þ
where d indicates the characteristic debris size, E is the elastic modulus, c is the cohesive surface energy, and PC is the effective stress determining the elastic energy within a supporting particle. The elastic modulus and cohesive surface energy of aluminum oxide are E = 400 GPa and c = 5 J/m2, respectively. Additionally, PC could be substituted for the local confining pressure [7,8]. As a result, based on the P. stress 2 values in Fig. 2(d), the fragment size might be between 21 and 346 nm with mean size of 111 nm. This result is in agreement with the size shown in Fig. 3. The size of previously deposited fragments could be gradually reduced with successive particle impacts, generating consistent fragmentation until the
Fig. 3. FE-SEM micrographs of debris morphology: (a) localized plasticity at the bottom and fragmentation at the top at 2 L/min, (b) complete dynamic fragmentation at 6 L/ min, (c) stuck fragments in the anchoring layer at 14 L/min and (d) a fragment broken away from the layer at 18 L/min. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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In this report, a hypothesis related to the mechanism of dynamic fragmentation generated during particle deposition was suggested and tested via single particle impact simulations linked to microstructural evidence. Additionally, the nano-sized fragments usually generated in a VKS coating layer were calculated on the basis of the material behavior suffering shock loading. Fragmentation phenomena were investigated as a crucial key toward clarifying the mechanism of deposition. This work was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MEST) (NRF-2014R1A2A2A05007633).
Appendix A. Supplementary data
Fig. 4. Schematic illustration of the hypothesized dynamic fragmentation process: (a) material impact and localized plasticity, (b) shock-induced plasticity with high confining pressures, (c) limitation of plasticity due to a decrease in pressure, followed by (d) dynamic fragmentation.
fragments could not be continually fractured due to the high surface energy, c, caused by smaller size fragments [13], and/or the lattices collapsing into an amorphous phase [5,14]. Furthermore, this process could be accelerated with a drastic decrease in the elastic modulus at the nanoscale [14,15]. This point was confirmed via a nanoindentation experiment on a 10 pass coating prepared in this study. The elastic modulus was measured to be approximately 137 ± 16 GPa, the value of which was unexpectedly lower than the referenced value of 400 GPa for Al2O3. The low elastic modulus was caused by first increasing the interphase area in the nano-sized grain [16] and secondly from shock-induced lattice damage [11]. Considering that the elastic modulus indicates the bonding strength among the atoms against a separation force, this is understandable. Thus, acceleration of fragmentation is possible. Size should likely be determined from a competition between the desire to release the stored energy via crack generation/growth (fracture) and new surface generation to make it unstable in a fragment, based on the energy consideration of Griffith’s theory [17]. In consideration of the fact that there was no grain or particle size term in Eq. (1), it could be expected that similarly sized Al2O3 fragments would be produced if a larger particle were to impact the surface with the same force or confining pressure.
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.scriptamat.2015. 06.020. References [1] J. Akedo, J. Therm. Spray Technol. 17 (2008) 181–198. [2] D.-M. Chun, S.-H. Ahn, Acta Mater. 59 (2011) 2693–2703. [3] D.W. Lee, H.J. Kim, Y.H. Kim, Y.H. Yun, S.M. Nam, J. Am. Ceram. Soc. 94 (2011) 3131–3138. [4] F. Cao, H. Park, J. Heo, J. Kwon, C. Lee, J. Therm. Spray Technol. 22 (2013) 1109– 1119. [5] H. Park, J. Kwon, I. Lee, C. Lee, Scr. Mater. 100 (2015) 44–47. [6] D.S. Cronin, K. Bui, C. Kaufmann, G. McIntosh, T. Berstad, D. Cronin, in: 4th European LS-Dyna Users Conference, 2003, pp. 47–60. [7] D. Grady, J. Wise, in: Sandia National Labs., Albuquerque, NM (United States), 1993. [8] D. Grady, Mech. Mater. 29 (1998) 181–203. [9] J. Lankford, Int. J. Appl. Ceram. Technol. 1 (2004) 205–210. [10] G. Kanel, S. Bless, A. Rajendran, in: DTIC Document, 2000. [11] A.K. Mukhopadhyay, K.D. Joshi, A. Dey, R. Chakraborty, A. Rav, A.K. Mandal, J. Ghosh, S. Bysakh, S.K. Biswas, S.C. Gupta, Mater. Sci. Eng. A 527 (2010) 6478– 6483. [12] D.-W. Lee, H.-J. Kim, Y.-N. Kim, M.-S. Jeon, S.-M. Nam, Surf. Coat. Technol. 209 (2012) 160–168. [13] K. Nanda, A. Maisels, F. Kruis, H. Fissan, S. Stappert, Phys. Rev. Lett. 91 (2003) 106102. [14] M.A. Meyers, A. Mishra, D.J. Benson, Prog. Mater Sci. 51 (2006) 427–556. [15] H.S. Kim, M.B. Bush, Nanostruct. Mater. 11 (1999) 361–367. [16] M.K. Tripp, C. Stampfer, D.C. Miller, T. Helbling, C.F. Herrmann, C. Hierold, K. Gall, S.M. George, V.M. Bright, Sens. Actuators A 130 (2006) 419–429. [17] J.B. Wachtman, W.R. Cannon, M.J. Matthewson, Mechanical Properties of Ceramics, John Wiley & Sons, 2009.
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Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Dynamic fragmentation process and fragment microstructure evolution of alumina particles in a vacuum kinetic spraying system Hyungkwon Park, Jinyoung Kim, Changhee Lee ⇑ Kinetic Spray Coating Laboratory (NRL), Division of Materials Science and Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, South Korea
a r t i c l e
i n f o
Article history: Received 2 April 2015 Revised 12 June 2015 Accepted 13 June 2015 Available online xxxx Keywords: Vacuum kinetic spraying (VKS) process Shock wave Dynamic fragmentation Fragment size Deposition mechanism
a b s t r a c t In a vacuum kinetic spraying (VKS) system, submicron ceramic particles impact the substrate and undergo dynamic fragmentation within nano-seconds, resulting in a nano grain film. In this report, a hypothesized mechanism for dynamic fragmentation generated during Al2O3 particle deposition is suggested and tested via Auto-dyn simulation linked to an analysis of distinct fragment microstructures and debris size calculations. Based on this data, shock-induced dynamic fragmentation, one of the crucial elements of clarifying the deposition mechanism, was investigated. Ó 2015 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.
Vacuum kinetic spraying (VKS) is an attractive ceramic film fabrication process for a wide variety of industrial fields due to various positive advantages, including (primarily) room temperature processing, high deposition rates, low processing costs, and simple operation methods as compared to similar film processes such as: sol–gel, sputtering, and CVD processes [1]. For these reasons, considerable research has been conducted toward the development of applications for VKS. However, a fundamental understanding of processes including the mechanism of deposition is largely lacking. This may be due to the fact that many process events occur simultaneously, making analysis very complicated. Various things, like the material response against a high strain rate caused by a shock wave, unexpected nanoscale material behavior, ceramic plasticity, nanosecond reaction time, and so on, occur during particle deposition of the VKS process. In this regard, investigations of the mechanisms of VKS have been conducted based on particle impact simulation [1,2] and/or microstructural observation [3,4]. Several studies [1,3] have suggested that plasticity and fragmentation are crucial elements of the deposition mechanism because flattened and nano-sized grains are always observed in the coating microstructure. Accordingly, shock-induced plasticity and fragmentation phenomena were suggested from simulation results based on postmortem microstructural evidence [5].
⇑ Corresponding author. E-mail address: [email protected] (C. Lee).
In addition to the previous results [5], shock-induced dynamic fragmentation is one focus of this study. The hypothesis of a dynamic fragmentation process was tested via Auto-dyn simulation, film microstructure observation, and fragment size calculation. Furthermore, dynamic fragmentation, one of the crucial keys for clarifying the deposition mechanism, is discussed in detail. A commercial a-Al2O3 powder with irregular morphology shown in Fig. 1(a) and soda lime glass were used as the feedstock material and substrate, respectively. The powder particles had an average diameter of 0.3 lm with a relatively wide size range, as confirmed in Fig. 1(b). Prior to deposition, the powder was dried for several hours to remove moisture and avoid particle agglomeration. The substrates were also cleaned via ultrasonication to eliminate surface contaminants. The powders were aerosolized via aerosol chamber vibration and transferred to the deposition chamber. The particles were accelerated to supersonic velocities using the pressure difference between the two chambers and a slit-type nozzle with an area of 5.0 0.4 mm2. Helium was used as a carrier gas and flow rate was regulated from 2 to 18 L/min to control particle velocity [1]. Two types of experiments were conducted. One experiment involved an individual particle test to analyze particle dynamic behavior. For the experiment, a small quantity (0.01 g) of powder was used with only one pass being performed. Traverse speed and standoff distance were fixed at 1 mm/s and 10 mm, respectively. A 10-pass coating was fabricated using approximately 80.0 g of powder, although spraying conditions were the same as in the individual particle test. Using the 10-pass coating specimen,
http://dx.doi.org/10.1016/j.scriptamat.2015.06.020 1359-6462/Ó 2015 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.
Please cite this article in press as: H. Park et al., Scripta Mater. (2015), http://dx.doi.org/10.1016/j.scriptamat.2015.06.020
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Fig. 1. (a) FE-SEM micrograph of the powder morphology, (b) size distribution of Al2O3 particles, and (c) corresponding numerical modeling design of 0.3-lm-diameter Al2O3 in Auto-dyn simulation.
the elastic modulus was measured to estimate the atomic bonding state for each coating. Powder morphology and deposition fragment shapes were observed via field emission scanning electron microscopy (FE-SEM, SIGMA, Carl Zeiss). For measuring the elastic modulus, nanoindentation experiments were performed using a Nanoindenter XP (MTS, Oak Ridge, TN) with a Berkovich tip. The indenter displacement was set to 500 nm and the data was averaged over 20 measurements. A non-linear transient finite element method (FEM) was performed using an ANSYS Auto-dyn v. 13.0 commercial simulation tool to analyze the dynamic behavior of a single alumina particle. To simulate the process economically and effectively, the model was designed to be simple and symmetric, as shown in Fig. 1(c). The particle size was set to 0.3 lm in diameter, the value of which was the same as the average powder size. For analysis of the complicated processes that occur during supersonic particle impact, a simple spherical-shaped particle was deliberately designed. The smallest element was intentionally set to 5 nm, considering the smallest grain size observed in a prior report [1]. Two material combinations, an Al2O3 particle on a glass substrate and an Al2O3 particle on an Al2O3 substrate, represented the particle impact on a glass substrate and the deposited coating layer subsequently formed on the glass substrate, respectively. The Johnson–Holmquist 2 (JH2) model was chosen because it can be used to predict the dynamic impact behavior of a ceramic material. A detailed explanation of the model can be found in the literature [6]. The material properties of Al2O3 and glass were set based on values referenced in the software library. The only variable was the particle impact velocity, which was set between 100 and 700 m/s at 100 m/s intervals in consideration of the estimated value reported in a prior study [1]. Principal stress was monitored at the particle south pole (gauge 1), and the corresponding contact point of the substrate (gauge 9), as shown in Fig. 1(c). In this simulation, principal stress 1 and 2 indicated that stress occurred in the x- (axial stress) and y-axis (lateral stress) directions, respectively.
Fig. 2(a–c) show the material status change during particle impact with a 400 m/s velocity as time proceeds from 0.1 ns in Fig. 2(a), to 0.5 ns in Fig. 2(b), to 1.0 ns in Fig. 2(c) after the impact event. The pale red directional marks represent compressive force, indicating the opposite direction. As the force increased, the strain rate correspondingly increased. Compressive lateral pressures are generated by a momentary inertial force if the strain rate is sufficiently high [7–9]. This compressive pressure suppresses crack formation and instead induces plasticity in the ceramic material as a means of consuming the suddenly released energy [9,10]. With regard to the VKS process, although there were no factors confining the particle, high strain shock loading generated a momentary inertial confining force [5,9]. Thus, the particle was expected to exhibit plasticity at the start of the impact when there was strong confinement hindering crack formation. However, the force gradually decreased as the kinetic energy was consumed by plastic deformation, friction, and heat energy. Therefore, the pale red arrows in Fig. 2(b) became faint and extensive bulk failure was simultaneously abruptly generated, especially at the bottom of the particle. It is worth noting that the bulk fail rate was significantly reduced thereafter as confirmed by differences in the fraction of failures over similar time intervals, 0.4 ns between Fig. 2(a and b) and 0.5 ns between Fig. 2(b and c). In consideration of the fact that the kinetic energy was 50% consumed within 0.22 ns and took around 0.4–0.7 ns to expend 90% of the energy (depending on the substrate, i.e., glass or Al2O3) for all velocity cases, the impact reaction event may be completed prior to 1.0 ns of elapsed time in Fig. 2(c). Therefore, extensive plasticity and failure took place within a very short time scale: 0.55 ns on average. Another important observation was the upper component of the particle, which remained intact even 1.0 ns after impact. As mentioned previously, the unexpected plasticity and dynamic fragmentation of ceramic particles are closely related to changes in the confining pressure of the VKS system. Fig. 2(d) shows the change of the maximum principal stress 2 (P. stress 2) for different particle velocities. Although the P. stress 2 changes
Fig. 2. Material status change of Al2O3 particles with a 400 m/s impact velocity at (a) 0.1, (b) 0.5, and (c) 1.0 ns, and (d) principal stress 2 graph with different velocities [A and B in Fig. 2(b) indicate specific points in plasticity and the fractured region, respectively]. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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H. Park et al. / Scripta Materialia xxx (2015) xxx–xxx
every moment during impact and reverts to 0 when fracture is complete, the observed maximum values indicate the largest stress recorded at each gauge and velocity. In the graph, the triangle and inverted triangle indicate measurements at gauges 9 and 1, respectively. Thus, the P. stress 2 of an alumina particle in the case of both substrates and the contact point of the alumina substrate was marked, except for the contact point of the glass substrate. As explained in the white box in Fig. 1(c), P. stress 2 indicates the stress in the direction perpendicular to particle impact, with the value always being negative, indicative of compressive stress. As a result, stress was directed toward the inside of the particle, indicating a confining pressure. This can be confirmed by regions A and B in Fig. 2(b), in which the P. stress 2 values were around 0.3 GPa at A and 0 GPa at B, respectively. Thus, the plasticity region completely failed following the disappearance of the confining pressure. The confining pressure became increasingly negative as the velocity increased. Moreover, the value for impact on the Al2O3 substrate was larger than was measured on the glass substrate. This suggests that the confining pressure increased with increasing substrate hardness. Fragment microstructure evolution was observed via individual particle testing. Fig. 3 presents the FE-SEM micrographs of debris morphology. Although not all fragments had the same shape as those observed in Fig. 3(a–d), the morphology seen in the images demonstrates particular phenomenological features generated during shock-induced dynamic fragmentation. Given the flattened fragment shape caused by plasticity and the particle size range in Fig. 1(b), the debris size is reasonable. Based on the fragmentation patterns observed in prior studies [5,11], the debris morphology was formed via shear fracture resulting from shock loading [7,8]. In Fig. 3(a), it appears that localized plasticity was generated at the bottom of the particle, in agreement with the simulation results. Fragments were also barely attached to the part at 2 L/min. It was postulated that the debris exhibited incomplete fragmentation due to a lack of kinetic impact energy. Additionally, the fragmentation pattern was quite similar to the surface morphology of a typical vacuum kinetic sprayed film [3,4]. Thus, the debris appeared in the middle of dynamic fragmentation. Conversely, it seems that the debris in Fig. 3(b) resulted from complete dynamic fragmentation, although some fragments were not perfectly detached (marked by white arrows). However, several small fragments that detached from the large debris were distinctly observed (marked by yellow dashed circles). Fig. 3(c) reveals fragments stuck in an anchoring layer. The debris appears as if it were undergoing plasticity due to its partial fragmentation. Moreover, the presence of an anchoring layer was also confirmed by the simulation results in Fig. 2(c) and served an important role in successful particle deposition [3,12]. The fragment in Fig. 3(d) appears to be breaking away from the anchoring layer, consisting of crystallites around 100 nm in size. Based on an analysis of the experimentally observed results in conjunction with the corresponding simulation results, a hypothesis describing the dynamic fragmentation process in a VKS system
3
can be proposed as follows. At the initial particle impact stage, the particle underwent localized plasticity aided by confining pressures when the impact velocity is sufficiently high, as shown in Fig. 4(a). Shock-induced plasticity was prevalent in the bottom portion of the particle, which was stuck by a strong confining inertial force in conjunction with the supporting substrate and pressure from the upper part of the particle in Fig. 4(b). At this point in time, components of the particle that were not confined, i.e., the upper and sides of the particle, began to shatter. However, the confining force weakened, and thus the region exhibiting plasticity also began to experience bulk failure as illustrated in Fig. 4(c). Finally, the region was completely dynamically fragmented in Fig. 4(d). Some parts of the particle that already failed might travel with the gas stream flowing onto the substrate. Consequently, a variety of microstructural features resulting from dynamic fragmentation can be observed in Fig. 3(a–c). As the front of the shock wave propagates, ceramic material suffers from elastic and/or plastic deformation accompanied by fragmentation [8,9]. Additionally, the damage level gradually progresses in the order of sudden elastic shock, failure ramp, deformation shock, to the Hugoniot state as the shock wave became stronger [7,8]. Furthermore, development can be accelerated even at low shock loading levels depending on microstructural conditions such as: grain size, interphases (grain boundary, triple line junction, quadratic node), crack size and number, porosity, and more [8–10]. Moreover, it was previously observed that dynamic strength data corresponded to resolved shear stress ranging from approximately G/25 to G/40, where G was the shear modulus of the ceramic [8]. If the G of Al2O3 was 150 GPa, the dynamic strength was in the range of 3.75–6.0 GPa. Furthermore, considering the impact pressure was measured to be between 3 and 11 GPa according to a prior report [5], the ceramic particle or previously deposited layer could be sufficiently beyond the failure ramp state in the VKS process. Consequently, the debris pattern, e.g., the fragments in Fig. 3(a–d), could be effectively observed via the individual particle test. Additionally, fragment sizes could be estimated from an equation related to the size of the fracture zone debris based on energy balance arguments [7,8]:
d¼
2Ec P2C
ð1Þ
where d indicates the characteristic debris size, E is the elastic modulus, c is the cohesive surface energy, and PC is the effective stress determining the elastic energy within a supporting particle. The elastic modulus and cohesive surface energy of aluminum oxide are E = 400 GPa and c = 5 J/m2, respectively. Additionally, PC could be substituted for the local confining pressure [7,8]. As a result, based on the P. stress 2 values in Fig. 2(d), the fragment size might be between 21 and 346 nm with mean size of 111 nm. This result is in agreement with the size shown in Fig. 3. The size of previously deposited fragments could be gradually reduced with successive particle impacts, generating consistent fragmentation until the
Fig. 3. FE-SEM micrographs of debris morphology: (a) localized plasticity at the bottom and fragmentation at the top at 2 L/min, (b) complete dynamic fragmentation at 6 L/ min, (c) stuck fragments in the anchoring layer at 14 L/min and (d) a fragment broken away from the layer at 18 L/min. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: H. Park et al., Scripta Mater. (2015), http://dx.doi.org/10.1016/j.scriptamat.2015.06.020
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H. Park et al. / Scripta Materialia xxx (2015) xxx–xxx
In this report, a hypothesis related to the mechanism of dynamic fragmentation generated during particle deposition was suggested and tested via single particle impact simulations linked to microstructural evidence. Additionally, the nano-sized fragments usually generated in a VKS coating layer were calculated on the basis of the material behavior suffering shock loading. Fragmentation phenomena were investigated as a crucial key toward clarifying the mechanism of deposition. This work was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MEST) (NRF-2014R1A2A2A05007633).
Appendix A. Supplementary data
Fig. 4. Schematic illustration of the hypothesized dynamic fragmentation process: (a) material impact and localized plasticity, (b) shock-induced plasticity with high confining pressures, (c) limitation of plasticity due to a decrease in pressure, followed by (d) dynamic fragmentation.
fragments could not be continually fractured due to the high surface energy, c, caused by smaller size fragments [13], and/or the lattices collapsing into an amorphous phase [5,14]. Furthermore, this process could be accelerated with a drastic decrease in the elastic modulus at the nanoscale [14,15]. This point was confirmed via a nanoindentation experiment on a 10 pass coating prepared in this study. The elastic modulus was measured to be approximately 137 ± 16 GPa, the value of which was unexpectedly lower than the referenced value of 400 GPa for Al2O3. The low elastic modulus was caused by first increasing the interphase area in the nano-sized grain [16] and secondly from shock-induced lattice damage [11]. Considering that the elastic modulus indicates the bonding strength among the atoms against a separation force, this is understandable. Thus, acceleration of fragmentation is possible. Size should likely be determined from a competition between the desire to release the stored energy via crack generation/growth (fracture) and new surface generation to make it unstable in a fragment, based on the energy consideration of Griffith’s theory [17]. In consideration of the fact that there was no grain or particle size term in Eq. (1), it could be expected that similarly sized Al2O3 fragments would be produced if a larger particle were to impact the surface with the same force or confining pressure.
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Please cite this article in press as: H. Park et al., Scripta Mater. (2015), http://dx.doi.org/10.1016/j.scriptamat.2015.06.020