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Interface bonding between particle and substrate during HVOF spraying
Applied Surface Science 317 (2014) 908–913
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Interface bonding between particle and substrate during HVOF spraying Ce Sun a,b , Lei Guo a,b , Guanxiong Lu a,b , Yanbing Lv a,b , Fuxing Ye a,b,∗ a b
School of Materials Science and Engineering, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China Tianjin Key Laboratory of Advanced Joining Technology, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 15 July 2014 Received in revised form 27 August 2014 Accepted 29 August 2014 Available online 6 September 2014 Keywords: Deposition behavior Energy distribution Impact model HVOF
a b s t r a c t The impact processes of Ni particles at initial temperature of 900 K on Al, Cu and Steel substrates were numerically analyzed by using ANSYS/LS-DYNA. Initial kinetic energy of the particle dissipated to particle and substrate simultaneously, the proportion of which was defined as energy distribution coefficient (K). The K values for Ni/Al, Ni/Cu and Ni/steel combinations were approximated to 4, 0.4 and 0.1, respectively. Individual Ni60 particles were deposited experimentally onto 6061–T6 aluminum alloy, copper and 304 stainless steel by High Velocity Oxy-fuel (HVOF) spraying. The contact between Ni particles and three substrates was not perfect. The bonding ratio, which is the effective contact area divided by total area, for Ni/Cu combination is 55.41%, larger than those for Ni/Al (40.78%) and Ni/steel (32.70%) combinations, indicating that moderate K value is beneficial for interface bonding between particle and substrate. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Thermal spray technology, as one of the most effective surface modification techniques, is extensively used to produce coatings to protect the mechanical components from corrosion, erosion and heat. Since the spraying coating is formed by the gradual accumulation of individual splats, the coating properties are significantly affected by particles deposition behaviors. To understand the bonding mechanism of particle and substrate, many studies have been conducted for the particle impact process based on experimental, theoretical and numerical methods [1–6]. During coating spraying, the particles may be in molten, semi-molten or solid states, which mainly depend on the spray technique used. The cold spraying usually generates solid particles, the plasma spraying produces molten droplets due to high temperature, and semi-molten particles are generated during High Velocity Oxy-fuel (HVOF) spraying due to relative low temperature and high velocity. For plasma spraying, almost all of the droplet kinetic energy dissipates to splat, thus substrate has little effect on the particle morphology [7]. However, for cold spraying and HVOF spraying, the initial kinetic energy also
∗ Corresponding author at: School of Materials Science and Engineering, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China. Tel.: +86 22 2740 6261; fax: +86 22 2740 7022. E-mail addresses: [email protected], [email protected] (F. Ye). http://dx.doi.org/10.1016/j.apsusc.2014.08.196 0169-4332/© 2014 Elsevier B.V. All rights reserved.
dissipated to substrate, so the substrate properties play an important role in particle deposition behaviors. For cold spraying, energy conversion and distribution occur during particle impacted on the substrate, which are affected by the properties of particle and substrate. Bae et al. [8] pointed out that effective combination between particle and substrate could only be formed when the elastic strain energy (rebound energy) was less than the plastic deformation energy (adhesion energy). Wu et al. [9] studied the energy conversion in cold spraying by numerical simulation method, and found that the ratio of energy distributed in solid particle and substrate was about 0.4 ∼ 0.6 when the particle impact with a critical velocity. During HVOF spraying, there also exists conversion and distribution of initial kinetic energy. Trompetter et al. [7] found that the conversion of particle kinetic energy during HVOF spraying was strongly depended on the substrate hardness. However, the report on the conversion and distribution of initial kinetic energy during the impact process of particle on the substrate is limited, and the bond mechanism of semi-molten particle and substrate is still unclear. In this study, the deposition behaviors of semi-molten Ni particle on Al, Cu and Steel substrates were numerically analyzed by using ANSYS/LS-DYNA software, and individual Ni60 particles were deposited experimentally onto 6061–T6 aluminum alloy, copper and 304 stainless steel by HVOF spraying. Bonding features were characterized with respect to the flattening ratio and compression ratio of particles, as well as the bonding ratio between particles and
C. Sun et al. / Applied Surface Science 317 (2014) 908–913
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Table 1 Parameters of Johnson–Cook model and EOS used in study [5,13,15]. Material
Density(g/cm3 )
Young’s modulus (GPa)
Melting point Tm (K)
A (MPa)
B (MPa)
n
C
m
Al Cu Ni Steel
2.70 8.93 8.90 7.89
68.9 124 207 217.5
916 1356 1726 2370
148 90 169 793
345 292 648 510
0.183 0.31 0.33 0.26
0.001 0.025 0.006 0.014
0.895 1.09 1.44 1.03
Table 2 Operation parameters of HVOF spraying.
Oxygen Propane Nitrogen
Pressure (MPa)
Flow rate (m3 /h)
0.7 0.7 0.6
15 1.4 1.2
The materials elastic strain and plastic deformation behaviors were described by a linear Mie–Gruneisen equation of state (EOS) and a Johnson–Cook plasticity model, respectively, which could be used to solve the strain hardening, strain rate hardening, and thermal softening questions. The Von Mises stress () of material was expressed as follows [11,12]: = (A + Bεn )(1 + Cln(¯ε∗ ))(1 − (T ∗ )m ) p
ε¯ ∗ =
ε¯ p ε¯ 0
(1)
(2)
is the equivalent flow stress, A, B, n, C and m are the constants related to the materials. εp and ε¯ p are the equivalent plastic strain (PEEQ) and strain rate, respectively, and ε¯ 0 is the reference strain rate. T* is the non-dimensional temperature defined as follow:
Fig. 1. Finite-element model.
substrates. Attempt is made to understand the deposition behaviors of particle and substrate by combining simulation and experiment. 2. Numerical modeling The process of high-speed Ni particle impacted on substrate was simulated by using a commercial explicit finite element analysis program LS-DYNA. Arbitrary Lagrangian Eulerian (ALE) formulation based on the mass, momentum and energy conservation equations was selected to avoid the excessive distortion of elements. Computational zone was simplified as a 2D axisymmetric model because of axisymmetric characteristic of the actual collision process as shown in Fig. 1. The nominal meshing sizes for the particle and interfacial contact region were 0.2 m for more accurate computations. The substrate length and width were taken to be five times larger than the particle diameter (40 m) to exclude the influences of boundary. In this simulation, the symmetry boundary A–B–C was constrained in X displacement and the bottom boundary C–D in X and Y displacements, and the other boundaries were set as free. An automatic 2D single interface contact algorithm available in LS-DYNA was implemented for the particle and substrate interaction. Because the high-speed impact process was mainly dominated by the inertial force, other volume forces, such as gravity, were neglected. The particle/substrate interaction was assumed to be an adiabatic process due to relative shorter thermal diffusivity distance compared with the actual dimension [10].
T∗ =
⎧ 0 ⎪ ⎪ ⎨
T < Trefer (T − Trefer )
(T −T ) ⎪ ⎪ ⎩ melt refer 1
Trefer ≤ T ≤ Tmelt
(3)
Tmelt < T
Tmelt and Trefer are the melting temperature and reference temperature, respectively. The parameters of Johnson–Cook model and Mie–Gruneisen EOS used in the simulation were shown in Table 1.
3. Experimental procedures 6061–T6 aluminum alloy, copper and 304 stainless steel were selected as substrates materials for High Velocity Oxy-fuel (HVOF) spraying. The commercially available Ni60 powders with a distribution size of 10 ∼ 70 m were used as spraying material, the chemical composition of which is 17.5Cr-4.05B-4.5Si-1.0C-13.7FeNi (wt.%). Substrates were polished and cleaned with acetone, and then the Ni60 powders were sprayed on the three substrates by HVOF spraying (TJ-9000). The spraying parameters were listed in Table 2. In order to collect individual particles, a stainless steel sieve with 0.5 mm holes was fixed between the torch and substrate, which could favor individual particles collected and reduce the influence of flame on the substrates. The morphologies of splats were characterized with Scanning electron microscopy (FE-SEM S4800, Hitachi).
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C. Sun et al. / Applied Surface Science 317 (2014) 908–913
Fig. 2. Simulation result of Ni particle impacted on Cu substrate at a speed of 500 m/s.
4. Results and discussion 4.1. Numerical modeling the interface bonding between particle and substrate The impact processes of particles on substrates were simulated by LS-DYNA. In order to characterize the semi-molten state, the particle initial temperature was 900 K and speed was selected in the range of 250 ∼ 700 m/s. For the sake of brevity, only simulation result of Ni particle impact on Cu substrate at a speed of 500 m/s was presented, as shown in Fig. 2. Both particle and substrate exhibit morderate plastic deformation, and adiabatic shear instability jet (ASI) can be found, which is the result of severe plastic deformation in the interfacial region. It is worthwhile to note that the maximum effective plastic deformation (Max-PEEQ) is concentrated at the surrounding of the contact interface, rather than the center point of impacting direction. Before particle impact on the substrate, the total energy of the system is equal to the initial kinetic energy of the particle. During the impact process, the initial kinetic energy manily converts to plastic dissipation energy (EPL ), elastic strain energy (EEL ), viscous dissipation energy (EV ) and frictional dissipation energy (EF ). Due to energy conservation, the follow relationship can be obtained: EK = EPL + EEL + EV + EF
Fig. 3. The energy evolution after Ni particle impacted the Cu substrate at a speed of 500 m/s.
(4)
Since EV and EF are much lower than EPL and EEL , they can be neglected in the simulation [8], and Eq. (4) can be written as follows: EK ≈ EPL + EEL = EP + Esub
(5)
EP and Esub are the energy dissipated to the particle and substrate, respectively. Fig. 3 shows the energy evolution after Ni particle impacted on Cu substrate at a speed of 500 m/s. At the initial stage of collision (0 < t < 35 ns), EK decreases rapidly, and EP and Esub increase. Fig. 4 shows the evolution of particle acceleration. Particle acceleration first increases rapidly, at about 35 ns, the particle acceleration reaches to the maximum value. High-speed impact leads to high strain and interfacial temperature rising. Large plastic deformation causes strain hardening, while the increase of interfacial temperature results in thermal softening. At the initial stage of particle impact, strain hardening is dominant, thus the resistance force applied on the particle increases, leading to the increase in particle acceleration. At about 35 ns, strain hardening and thermal softening reach balance, the resistance force applied on the particle reaches to the largest value, leading to the maximum particle acceleration. As the process developing, thermal softening becomes dominant, leading to the formation of adiabatic shear instability jet (as shown in Fig. 2). At about 100 ns, particle acceleration diminishes to zero,
Fig. 4. The evolution of particle acceleration with time.
indicating that initial kinetic energy completely converts into EP and Esub . Fig. 5 shows the energy conversion of Ni/Al, Ni/Cu and Ni/steel at different particle velocity. With the increase of the particle velocity, Ek , Ep and Esub all monotonically increase for each case. The change of energy for Ni/Al combination is shown in Fig. 5a, Esub is very close to EK , indicating that the initial kinetic energy mainly dissipates to substrate, which would lead to susbtrate plastic deformation. For the case of Ni/Cu combination, the initial kinetic energy redistributed to particle is still much lower than that redistributed to substrate (shown in Fig. 5b), because most of intial kinetic energy of particle is dissipated into plastic deformation of the relative counterpart. For Ni/Al and Ni/Cu cases, much higher energy dissipated by substrate is achieved. Fig. 5c shows the Ni/steel case, when the particle velocity is below 400 m/s, EP is very close to EK , as the velocity increases, Esub increases apparently, which could be attributed
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Fig. 6. Plot of relationship between energy distribution coefficient K and particle velocity for Ni/Al, Ni/Cu and Ni/steel.
by particle plastic deformation due to adquate energy storage capacity of particle at low velocity. On the other hand, energy is not enough to make the particle effectively combined with the substrate due to relatively low particle velocity. When the impact velocity exceeds a critical velocity, effective combination occurs, and the K value almost keeps constant. According to the simulation results in the literature [8], the critical velocity values for different cases are mainly in the range of 450 ∼ 800 m/s. When particle speed exceeds critical velocity, the highest K value is obtained for Ni/steel case (K ∼ 4), Ni/Cu combination reveals moderate K value (K ∼ 0.4), and the K value for Ni/Al case is lowest (K ∼ 0.1). Since the redistributed energy mainly converts to plastic work and heat, K could be used to characterize the deformation degree of particle and substrate. It is known that the formation of adiabatic shear instability (ASI) in the interfacial region can be considered as good adhersion between particle and substrate. Therefore, one can obtain excellent particle/substrate bonding by optimizing K value. 4.2. Interface bonding between particle and substrate
Fig. 5. The energy conversion of (a) Ni/Al, (b) Ni/Cu and (c) Ni/steel at different particle velocity.
to the relative lower energy storage capability for particle due to the limitation of its volume. Energy distribution coefficient K was introduced to represent distribution proportion of initial kinetic energy EK between the particle and substrate, K can be defined as follow: K=
EP Esub
(6)
K can be used to measure the relative deformation degree between the particle and substrate. High K value suggests large proportion of particle dissipation energy, leading to severe deformation of particle, while low K value means severe deformation of substrate. Fig. 6 shows energy distribution coefficient K for Ni/Al, Ni/Cu and Ni/steel combinations. The K value first decreases with increasing particle velocity to 400 m/s. The decline tendency of K could be attributed to the follow two factors: on the one hand, more energy could be dissipated
Fig. 7 shows cross-sectional morphologies of Ni particles deposited on Al, Cu and 304 Stainless steel substrates. When Ni particle was deposited on Al substrate, Ni particle exhibits slight deformation while intensive plastic deformation could be found on the Al substrate, as shown in Fig. 7a. Fig. 7b shows the crosssectional morphology of Ni particles deposited on Cu substrate, both Ni particle and Cu substrate exhibit moderate deformation behaviors. For Ni/steel case, dramatic plastic deformation can be found in Ni particle, whereas no obvious deformation can be observed on steel substrate, as shown in Fig. 7c. Fig. 8 shows the flattening ratio and compression ratio of Ni particles deposited on Al, Cu and Steel substrates. The flattening ratio Rf and Rc can be calculated from the cross-sectional SEM images according to the follow equations [14]: Rf =
Dp dp
(7)
Rc =
dp − Hp × 100% dp
(8)
where Dp is the maximum spreading diameter of the splat perpendicular to impact direction, Hp is the maximum height of the splat in the impact direction, dp is the original particle diameter, which could be estimated form diameter Dp and Hp by assuming that particle volume keeps constant during impacting process.
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Fig. 9. Bonding ratio for the Ni/Al, Ni/Cu and Ni/steel cases.
Fig. 7. Cross-sectional images of individual Ni particle deposited onto (a) Al (b) Cu and (c) steel.
The flattening ratio of Ni particle for Ni/Al combination is 1.23, lower than those of Ni particles for Ni/Cu case (1.46) and Ni/steel case (1.62). The compression ratio values of Ni particles for Ni/Al, Ni/Cu and Ni/steel are 14.80%, 35.93% and 48.19%, respectively. The
flattening ratio and compression ratio are related to the energy dissipated to the particle (EP ). Large EP would lead to large flattening ratio and compression ratio. Therefore, EP values increase in the order of Ni/Al case, Ni/Cu case and Ni/steel case, which are well agree with LS-DYNA simulation results. It can be seen from Fig. 7 that the contact between Ni particles and substrates is not perfect, which is the result of particle rebounding and solidification shrinkage. Therefore, effective contact area is introduced, and the bonding ratio is obtained from effective contact area divided by total area. Fig. 9 shows the bonding ratio for the three cases. The bonding ratio values of Ni/Al case, Ni/Cu case and Ni/steel case are 40.78%, 55.41% and 32.70%, respectively. It is known that large bonding ratio is beneficial to the interface bonding between particle and substrate. Therefore, Ni particle has the strongest adhesion to the Cu substrate due to the largest bond ratio for Ni/Cu case. According to simulation results, Ni/Cu combination has moderate energy distribution coefficient K, while the K value for Ni/Al case is too low and K value for Ni/steel is too high. Combining simulation and experiment results, a relationship between interface bonding strength and K could be found. Moderate K value means relatively good interface adhesion between particle and substrate, and too large or too low K value is detrimental to the interface bonding. Therefore, one can qualitatively evaluate the interface bonding strength between particles and substrates just form numerical simulation, if the K value is too large or too low, it indicates that the interface bonding must be poor, and if the K value is moderate, the interface bonding maybe good. The results could provide guidance on improving the interface bonding by optimizing K value during coating sparying. 5. Conclusions The impact processes of Ni particles on Al, Cu and Steel substrates at initial temperature of 900 K were numerically analyzed by using ANSYS/LS-DYNA, and individual Ni60 particles were deposited experimentally onto 6061–T6 aluminum alloy, copper and 304 stainless steel by HVOF spraying, bonding features of particles and substrates were characterized with respect to the flattening ratio, compression ratio and bonding ratio. Some conclusions can be drawn as follows:
Fig. 8. Flattening ratio and compression ratio of Ni particles deposited on Al, Cu and steel substrates.
(1) For Ni/Al, Ni/Cu and Ni/steel combinations, as particle velocity increasing, EK , EP and Esub all monotonically increased, K value decreased firstly and then almost kept constant. When the particle velocity over 400 m/s, the K values for Ni/Al, Ni/Cu and Ni/steel combinations were approximated to 4, 0.4 and 0.1, respectively.
C. Sun et al. / Applied Surface Science 317 (2014) 908–913
(2) The contact between Ni particles and the three substrates was not perfect. The flattening ratio and compression ratio of the particle impacted on steel substrate was the largest, while those of the particles impacted on Al substrate was the lowest. The bonding ratio for Ni/Cu combination was 55.41%, larger than those for Ni/Al (40.78%) and Ni/steel (32.70%) combinations. (3) Moderate K value was beneficial for interface bonding between particle and substrate, one could obtain enhanced interface bonding by optimizing K value during coating spraying. Acknowledgments This research was financially supported by the National Natural Science Foundation of China (No.51375332), the Natural Science Foundation of Tianjin (No.12JCYBJ12300) and the Specialized Research Fund for the Doctoral Program of Higher Education (No.20120032110031). The authors would like to thank the financial support. References [1] P.C. King, C. Busch, T.K. Sherri, M. Jahedi, Interface melding in cold spray titanium particle impact, Surf. Coat. Technol. 239 (2014) 191–199. [2] L. Guo, H.B. Guo, Evaluation of stress distribution and failure mechanism in lanthanum-titanium-aluminum oxides thermal barrier coatings, Ceram. Int. 39 (2013) 5103–5111. [3] P.C. King, S.H. Zahiri, M. Jahedi, Focused ion beam micro-dissection of coldsprayed particles, Acta Mater. 56 (2008) 5617–5626.
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[4] D.R. Yan, J.N. He, B.R. Tian, Y.C. Dong, The corrosion behavior of plasma sprayed Fe2 Al5 coating in molten Zn, Surf. Coat. Technol. 201 (2006) 2662–2666. [5] W.Y. Li, C. Zhang, C.J. Li, H. Liao, Modeling aspects of high velocity impact of particles in cold spraying by explicit finite element analysis, J. Therm. Spray Technol. 18 (2009) 921–933. [6] K. Kim, M. Watanabe, S. Kuroda, Bonding mechanisms of thermally softened metallic powder particle and substrates impacted at high velocity, Surf. Coat. Technol. 204 (2010) 2175–2180. [7] W. Trompetter, M. Hyland, D. McGrouther, P. Munroe, Effect of substrate hardness on splat morphology in high-velocity thermal spray coatings, J. Therm. Spray Technol. 15 (2006) 663–669. [8] G. Bae, Y. Xiong, S. Kumar, K. Kang, C. Lee, General aspects of interface bonding in kinetic sprayed coatings, Acta Mater. 56 (2008) 4858–4868. [9] X.K. Wu, X.L. Zhou, J.G. Wang, J.S. Zhang, Numerical investigation on energy balance and deposition behavior during cold spraying, Rare Met. Mater. Eng. 46 (2010) 385–389. [10] M. Yu, W.Y. Li, F.F. Wang, X.K. Suo, H.L. Liao, Effect of particle and substrate preheating on particle deformation behavior in cold spraying, Surf. Coat. Technol. 220 (2013) 174–178. [11] G.R. Johnson, W.H. Cook, Proc of 7th Int Symp Ball, Hague, The Netherlands, 1983, p. 541. [12] S. Kamnis, S. Gu, T.J. Lu, C. Chen, Numerical modeling the bonding mechanism of HVOF sprayed particles, Comput. Mater. Sci. 46 (2009) 1038–1043. [13] S. Yin, X. Wang, W.Y. Li, Effect of substrate hardness on the deformation behavior of subsequently incident particles in cold spraying, Appl. Surf. Sci. 257 (2011) 7560–7565. [14] W.Y. Li., H. Liao, C.J. Li, On high velocity impact of micro-sized metallic particles in cold spraying, Appl. Surf. Sci. 253 (2006) 2852–2862. [15] W.Y. Li, H. Liao, C.J. Li, H.S. Bang, C. Coddet, Numerical simulation of deformation behavior of Al particles impacting on Al substrate and effect of surface oxide films on interfacial bonding in cold spraying, Appl. Surf. Sci. 253 (2007) 5084–5091.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Interface bonding between particle and substrate during HVOF spraying Ce Sun a,b , Lei Guo a,b , Guanxiong Lu a,b , Yanbing Lv a,b , Fuxing Ye a,b,∗ a b
School of Materials Science and Engineering, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China Tianjin Key Laboratory of Advanced Joining Technology, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China
a r t i c l e
i n f o
Article history: Received 15 July 2014 Received in revised form 27 August 2014 Accepted 29 August 2014 Available online 6 September 2014 Keywords: Deposition behavior Energy distribution Impact model HVOF
a b s t r a c t The impact processes of Ni particles at initial temperature of 900 K on Al, Cu and Steel substrates were numerically analyzed by using ANSYS/LS-DYNA. Initial kinetic energy of the particle dissipated to particle and substrate simultaneously, the proportion of which was defined as energy distribution coefficient (K). The K values for Ni/Al, Ni/Cu and Ni/steel combinations were approximated to 4, 0.4 and 0.1, respectively. Individual Ni60 particles were deposited experimentally onto 6061–T6 aluminum alloy, copper and 304 stainless steel by High Velocity Oxy-fuel (HVOF) spraying. The contact between Ni particles and three substrates was not perfect. The bonding ratio, which is the effective contact area divided by total area, for Ni/Cu combination is 55.41%, larger than those for Ni/Al (40.78%) and Ni/steel (32.70%) combinations, indicating that moderate K value is beneficial for interface bonding between particle and substrate. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Thermal spray technology, as one of the most effective surface modification techniques, is extensively used to produce coatings to protect the mechanical components from corrosion, erosion and heat. Since the spraying coating is formed by the gradual accumulation of individual splats, the coating properties are significantly affected by particles deposition behaviors. To understand the bonding mechanism of particle and substrate, many studies have been conducted for the particle impact process based on experimental, theoretical and numerical methods [1–6]. During coating spraying, the particles may be in molten, semi-molten or solid states, which mainly depend on the spray technique used. The cold spraying usually generates solid particles, the plasma spraying produces molten droplets due to high temperature, and semi-molten particles are generated during High Velocity Oxy-fuel (HVOF) spraying due to relative low temperature and high velocity. For plasma spraying, almost all of the droplet kinetic energy dissipates to splat, thus substrate has little effect on the particle morphology [7]. However, for cold spraying and HVOF spraying, the initial kinetic energy also
∗ Corresponding author at: School of Materials Science and Engineering, Tianjin University, No. 92, Weijin Road, Tianjin 300072, China. Tel.: +86 22 2740 6261; fax: +86 22 2740 7022. E-mail addresses: [email protected], [email protected] (F. Ye). http://dx.doi.org/10.1016/j.apsusc.2014.08.196 0169-4332/© 2014 Elsevier B.V. All rights reserved.
dissipated to substrate, so the substrate properties play an important role in particle deposition behaviors. For cold spraying, energy conversion and distribution occur during particle impacted on the substrate, which are affected by the properties of particle and substrate. Bae et al. [8] pointed out that effective combination between particle and substrate could only be formed when the elastic strain energy (rebound energy) was less than the plastic deformation energy (adhesion energy). Wu et al. [9] studied the energy conversion in cold spraying by numerical simulation method, and found that the ratio of energy distributed in solid particle and substrate was about 0.4 ∼ 0.6 when the particle impact with a critical velocity. During HVOF spraying, there also exists conversion and distribution of initial kinetic energy. Trompetter et al. [7] found that the conversion of particle kinetic energy during HVOF spraying was strongly depended on the substrate hardness. However, the report on the conversion and distribution of initial kinetic energy during the impact process of particle on the substrate is limited, and the bond mechanism of semi-molten particle and substrate is still unclear. In this study, the deposition behaviors of semi-molten Ni particle on Al, Cu and Steel substrates were numerically analyzed by using ANSYS/LS-DYNA software, and individual Ni60 particles were deposited experimentally onto 6061–T6 aluminum alloy, copper and 304 stainless steel by HVOF spraying. Bonding features were characterized with respect to the flattening ratio and compression ratio of particles, as well as the bonding ratio between particles and
C. Sun et al. / Applied Surface Science 317 (2014) 908–913
909
Table 1 Parameters of Johnson–Cook model and EOS used in study [5,13,15]. Material
Density(g/cm3 )
Young’s modulus (GPa)
Melting point Tm (K)
A (MPa)
B (MPa)
n
C
m
Al Cu Ni Steel
2.70 8.93 8.90 7.89
68.9 124 207 217.5
916 1356 1726 2370
148 90 169 793
345 292 648 510
0.183 0.31 0.33 0.26
0.001 0.025 0.006 0.014
0.895 1.09 1.44 1.03
Table 2 Operation parameters of HVOF spraying.
Oxygen Propane Nitrogen
Pressure (MPa)
Flow rate (m3 /h)
0.7 0.7 0.6
15 1.4 1.2
The materials elastic strain and plastic deformation behaviors were described by a linear Mie–Gruneisen equation of state (EOS) and a Johnson–Cook plasticity model, respectively, which could be used to solve the strain hardening, strain rate hardening, and thermal softening questions. The Von Mises stress () of material was expressed as follows [11,12]: = (A + Bεn )(1 + Cln(¯ε∗ ))(1 − (T ∗ )m ) p
ε¯ ∗ =
ε¯ p ε¯ 0
(1)
(2)
is the equivalent flow stress, A, B, n, C and m are the constants related to the materials. εp and ε¯ p are the equivalent plastic strain (PEEQ) and strain rate, respectively, and ε¯ 0 is the reference strain rate. T* is the non-dimensional temperature defined as follow:
Fig. 1. Finite-element model.
substrates. Attempt is made to understand the deposition behaviors of particle and substrate by combining simulation and experiment. 2. Numerical modeling The process of high-speed Ni particle impacted on substrate was simulated by using a commercial explicit finite element analysis program LS-DYNA. Arbitrary Lagrangian Eulerian (ALE) formulation based on the mass, momentum and energy conservation equations was selected to avoid the excessive distortion of elements. Computational zone was simplified as a 2D axisymmetric model because of axisymmetric characteristic of the actual collision process as shown in Fig. 1. The nominal meshing sizes for the particle and interfacial contact region were 0.2 m for more accurate computations. The substrate length and width were taken to be five times larger than the particle diameter (40 m) to exclude the influences of boundary. In this simulation, the symmetry boundary A–B–C was constrained in X displacement and the bottom boundary C–D in X and Y displacements, and the other boundaries were set as free. An automatic 2D single interface contact algorithm available in LS-DYNA was implemented for the particle and substrate interaction. Because the high-speed impact process was mainly dominated by the inertial force, other volume forces, such as gravity, were neglected. The particle/substrate interaction was assumed to be an adiabatic process due to relative shorter thermal diffusivity distance compared with the actual dimension [10].
T∗ =
⎧ 0 ⎪ ⎪ ⎨
T < Trefer (T − Trefer )
(T −T ) ⎪ ⎪ ⎩ melt refer 1
Trefer ≤ T ≤ Tmelt
(3)
Tmelt < T
Tmelt and Trefer are the melting temperature and reference temperature, respectively. The parameters of Johnson–Cook model and Mie–Gruneisen EOS used in the simulation were shown in Table 1.
3. Experimental procedures 6061–T6 aluminum alloy, copper and 304 stainless steel were selected as substrates materials for High Velocity Oxy-fuel (HVOF) spraying. The commercially available Ni60 powders with a distribution size of 10 ∼ 70 m were used as spraying material, the chemical composition of which is 17.5Cr-4.05B-4.5Si-1.0C-13.7FeNi (wt.%). Substrates were polished and cleaned with acetone, and then the Ni60 powders were sprayed on the three substrates by HVOF spraying (TJ-9000). The spraying parameters were listed in Table 2. In order to collect individual particles, a stainless steel sieve with 0.5 mm holes was fixed between the torch and substrate, which could favor individual particles collected and reduce the influence of flame on the substrates. The morphologies of splats were characterized with Scanning electron microscopy (FE-SEM S4800, Hitachi).
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C. Sun et al. / Applied Surface Science 317 (2014) 908–913
Fig. 2. Simulation result of Ni particle impacted on Cu substrate at a speed of 500 m/s.
4. Results and discussion 4.1. Numerical modeling the interface bonding between particle and substrate The impact processes of particles on substrates were simulated by LS-DYNA. In order to characterize the semi-molten state, the particle initial temperature was 900 K and speed was selected in the range of 250 ∼ 700 m/s. For the sake of brevity, only simulation result of Ni particle impact on Cu substrate at a speed of 500 m/s was presented, as shown in Fig. 2. Both particle and substrate exhibit morderate plastic deformation, and adiabatic shear instability jet (ASI) can be found, which is the result of severe plastic deformation in the interfacial region. It is worthwhile to note that the maximum effective plastic deformation (Max-PEEQ) is concentrated at the surrounding of the contact interface, rather than the center point of impacting direction. Before particle impact on the substrate, the total energy of the system is equal to the initial kinetic energy of the particle. During the impact process, the initial kinetic energy manily converts to plastic dissipation energy (EPL ), elastic strain energy (EEL ), viscous dissipation energy (EV ) and frictional dissipation energy (EF ). Due to energy conservation, the follow relationship can be obtained: EK = EPL + EEL + EV + EF
Fig. 3. The energy evolution after Ni particle impacted the Cu substrate at a speed of 500 m/s.
(4)
Since EV and EF are much lower than EPL and EEL , they can be neglected in the simulation [8], and Eq. (4) can be written as follows: EK ≈ EPL + EEL = EP + Esub
(5)
EP and Esub are the energy dissipated to the particle and substrate, respectively. Fig. 3 shows the energy evolution after Ni particle impacted on Cu substrate at a speed of 500 m/s. At the initial stage of collision (0 < t < 35 ns), EK decreases rapidly, and EP and Esub increase. Fig. 4 shows the evolution of particle acceleration. Particle acceleration first increases rapidly, at about 35 ns, the particle acceleration reaches to the maximum value. High-speed impact leads to high strain and interfacial temperature rising. Large plastic deformation causes strain hardening, while the increase of interfacial temperature results in thermal softening. At the initial stage of particle impact, strain hardening is dominant, thus the resistance force applied on the particle increases, leading to the increase in particle acceleration. At about 35 ns, strain hardening and thermal softening reach balance, the resistance force applied on the particle reaches to the largest value, leading to the maximum particle acceleration. As the process developing, thermal softening becomes dominant, leading to the formation of adiabatic shear instability jet (as shown in Fig. 2). At about 100 ns, particle acceleration diminishes to zero,
Fig. 4. The evolution of particle acceleration with time.
indicating that initial kinetic energy completely converts into EP and Esub . Fig. 5 shows the energy conversion of Ni/Al, Ni/Cu and Ni/steel at different particle velocity. With the increase of the particle velocity, Ek , Ep and Esub all monotonically increase for each case. The change of energy for Ni/Al combination is shown in Fig. 5a, Esub is very close to EK , indicating that the initial kinetic energy mainly dissipates to substrate, which would lead to susbtrate plastic deformation. For the case of Ni/Cu combination, the initial kinetic energy redistributed to particle is still much lower than that redistributed to substrate (shown in Fig. 5b), because most of intial kinetic energy of particle is dissipated into plastic deformation of the relative counterpart. For Ni/Al and Ni/Cu cases, much higher energy dissipated by substrate is achieved. Fig. 5c shows the Ni/steel case, when the particle velocity is below 400 m/s, EP is very close to EK , as the velocity increases, Esub increases apparently, which could be attributed
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Fig. 6. Plot of relationship between energy distribution coefficient K and particle velocity for Ni/Al, Ni/Cu and Ni/steel.
by particle plastic deformation due to adquate energy storage capacity of particle at low velocity. On the other hand, energy is not enough to make the particle effectively combined with the substrate due to relatively low particle velocity. When the impact velocity exceeds a critical velocity, effective combination occurs, and the K value almost keeps constant. According to the simulation results in the literature [8], the critical velocity values for different cases are mainly in the range of 450 ∼ 800 m/s. When particle speed exceeds critical velocity, the highest K value is obtained for Ni/steel case (K ∼ 4), Ni/Cu combination reveals moderate K value (K ∼ 0.4), and the K value for Ni/Al case is lowest (K ∼ 0.1). Since the redistributed energy mainly converts to plastic work and heat, K could be used to characterize the deformation degree of particle and substrate. It is known that the formation of adiabatic shear instability (ASI) in the interfacial region can be considered as good adhersion between particle and substrate. Therefore, one can obtain excellent particle/substrate bonding by optimizing K value. 4.2. Interface bonding between particle and substrate
Fig. 5. The energy conversion of (a) Ni/Al, (b) Ni/Cu and (c) Ni/steel at different particle velocity.
to the relative lower energy storage capability for particle due to the limitation of its volume. Energy distribution coefficient K was introduced to represent distribution proportion of initial kinetic energy EK between the particle and substrate, K can be defined as follow: K=
EP Esub
(6)
K can be used to measure the relative deformation degree between the particle and substrate. High K value suggests large proportion of particle dissipation energy, leading to severe deformation of particle, while low K value means severe deformation of substrate. Fig. 6 shows energy distribution coefficient K for Ni/Al, Ni/Cu and Ni/steel combinations. The K value first decreases with increasing particle velocity to 400 m/s. The decline tendency of K could be attributed to the follow two factors: on the one hand, more energy could be dissipated
Fig. 7 shows cross-sectional morphologies of Ni particles deposited on Al, Cu and 304 Stainless steel substrates. When Ni particle was deposited on Al substrate, Ni particle exhibits slight deformation while intensive plastic deformation could be found on the Al substrate, as shown in Fig. 7a. Fig. 7b shows the crosssectional morphology of Ni particles deposited on Cu substrate, both Ni particle and Cu substrate exhibit moderate deformation behaviors. For Ni/steel case, dramatic plastic deformation can be found in Ni particle, whereas no obvious deformation can be observed on steel substrate, as shown in Fig. 7c. Fig. 8 shows the flattening ratio and compression ratio of Ni particles deposited on Al, Cu and Steel substrates. The flattening ratio Rf and Rc can be calculated from the cross-sectional SEM images according to the follow equations [14]: Rf =
Dp dp
(7)
Rc =
dp − Hp × 100% dp
(8)
where Dp is the maximum spreading diameter of the splat perpendicular to impact direction, Hp is the maximum height of the splat in the impact direction, dp is the original particle diameter, which could be estimated form diameter Dp and Hp by assuming that particle volume keeps constant during impacting process.
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Fig. 9. Bonding ratio for the Ni/Al, Ni/Cu and Ni/steel cases.
Fig. 7. Cross-sectional images of individual Ni particle deposited onto (a) Al (b) Cu and (c) steel.
The flattening ratio of Ni particle for Ni/Al combination is 1.23, lower than those of Ni particles for Ni/Cu case (1.46) and Ni/steel case (1.62). The compression ratio values of Ni particles for Ni/Al, Ni/Cu and Ni/steel are 14.80%, 35.93% and 48.19%, respectively. The
flattening ratio and compression ratio are related to the energy dissipated to the particle (EP ). Large EP would lead to large flattening ratio and compression ratio. Therefore, EP values increase in the order of Ni/Al case, Ni/Cu case and Ni/steel case, which are well agree with LS-DYNA simulation results. It can be seen from Fig. 7 that the contact between Ni particles and substrates is not perfect, which is the result of particle rebounding and solidification shrinkage. Therefore, effective contact area is introduced, and the bonding ratio is obtained from effective contact area divided by total area. Fig. 9 shows the bonding ratio for the three cases. The bonding ratio values of Ni/Al case, Ni/Cu case and Ni/steel case are 40.78%, 55.41% and 32.70%, respectively. It is known that large bonding ratio is beneficial to the interface bonding between particle and substrate. Therefore, Ni particle has the strongest adhesion to the Cu substrate due to the largest bond ratio for Ni/Cu case. According to simulation results, Ni/Cu combination has moderate energy distribution coefficient K, while the K value for Ni/Al case is too low and K value for Ni/steel is too high. Combining simulation and experiment results, a relationship between interface bonding strength and K could be found. Moderate K value means relatively good interface adhesion between particle and substrate, and too large or too low K value is detrimental to the interface bonding. Therefore, one can qualitatively evaluate the interface bonding strength between particles and substrates just form numerical simulation, if the K value is too large or too low, it indicates that the interface bonding must be poor, and if the K value is moderate, the interface bonding maybe good. The results could provide guidance on improving the interface bonding by optimizing K value during coating sparying. 5. Conclusions The impact processes of Ni particles on Al, Cu and Steel substrates at initial temperature of 900 K were numerically analyzed by using ANSYS/LS-DYNA, and individual Ni60 particles were deposited experimentally onto 6061–T6 aluminum alloy, copper and 304 stainless steel by HVOF spraying, bonding features of particles and substrates were characterized with respect to the flattening ratio, compression ratio and bonding ratio. Some conclusions can be drawn as follows:
Fig. 8. Flattening ratio and compression ratio of Ni particles deposited on Al, Cu and steel substrates.
(1) For Ni/Al, Ni/Cu and Ni/steel combinations, as particle velocity increasing, EK , EP and Esub all monotonically increased, K value decreased firstly and then almost kept constant. When the particle velocity over 400 m/s, the K values for Ni/Al, Ni/Cu and Ni/steel combinations were approximated to 4, 0.4 and 0.1, respectively.
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(2) The contact between Ni particles and the three substrates was not perfect. The flattening ratio and compression ratio of the particle impacted on steel substrate was the largest, while those of the particles impacted on Al substrate was the lowest. The bonding ratio for Ni/Cu combination was 55.41%, larger than those for Ni/Al (40.78%) and Ni/steel (32.70%) combinations. (3) Moderate K value was beneficial for interface bonding between particle and substrate, one could obtain enhanced interface bonding by optimizing K value during coating spraying. Acknowledgments This research was financially supported by the National Natural Science Foundation of China (No.51375332), the Natural Science Foundation of Tianjin (No.12JCYBJ12300) and the Specialized Research Fund for the Doctoral Program of Higher Education (No.20120032110031). The authors would like to thank the financial support. References [1] P.C. King, C. Busch, T.K. Sherri, M. Jahedi, Interface melding in cold spray titanium particle impact, Surf. Coat. Technol. 239 (2014) 191–199. [2] L. Guo, H.B. Guo, Evaluation of stress distribution and failure mechanism in lanthanum-titanium-aluminum oxides thermal barrier coatings, Ceram. Int. 39 (2013) 5103–5111. [3] P.C. King, S.H. Zahiri, M. Jahedi, Focused ion beam micro-dissection of coldsprayed particles, Acta Mater. 56 (2008) 5617–5626.
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