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Particle behavior of nanocrystalline 316-stainless steel during high velocity oxy-fuel thermal spray
NanoStmchmd
Pergamon PII SO9659773(99)00126-9
PARTICLE
BEHAVIOR HIGH
OF NANOCRYSTALLINE VELOCITY OXY-FUEL
Materials, Vol. 12, pp. 319-322, 1999 Elsevier Science Ltd 0 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 09659773/99/$-&front matter
31CSTAINLESS STEEL DURING THERMAL SPRAY
M.L. Lau, V.V. Gupta and E.J. Lavernia Department of Chemical and Biochemical Engineering and Materials Science University of California, Irvine, Irvine, CA 92697-2575
Abstract--The present paper investigates the particle behavior of nanocrystalline 316 stainless steel powders during high velocity oxy-fuel (HVOF) spray. The feedstock powders were synthesized by mechanical milling to produce flake-shaped agglomerates with an average grain size of less than 50 nm. The powders were then introduced into the HVOF spray to produce a nanocrystalline coating. A mathematical model is developed to study the variation of dtperent agglomerate sizes resulting from the mechanical milling process on the particle behavior during thermal spraying. A generalized Newtonian equation describing the momentum transfer was used, which incorporates a geometric ratio to account for the size and morphological variations of the micron-sized agglomerates with nano-grained structure. Particle velocity from the numerical calculations indicated that the velocity profile depends strongly on the thickness of the particles, which is ahnction of milling time and milling media, 01999 Acta Metallurgica Inc. INTRODUCTION Thermal spray is a coating process which has evolved to produce various metallic, non-metallic, and ceramic coatings (1). In recent year, thermal spraying using nanoctystalline feedstock powders has yielded coatings with higher hardness, strength, and corrosion resistance than those of the conventional counterparts (2-5). For instance, high velocity oxy-fuel spray of WCY12Co and WC/lSCo nanocomposite coatings shows a 28% increase in hardness while the toughness of the nanocomposite remains constant when compared to that of the conventional coatings (6). More recently, inert gas atomized 3 16-stainlesssteelpowders were mechanically milled in methanol and liquid nitrogen separately to produce nanocrystalline powders and the powders were thermally sprayed by HVOF (7). The resultant coatings exhibited significant increasein microhardness. For example, an 48% increasein microhardnesswas observed in the thermal sprayed coating when methanol milled stainlesssteel powders was used as the feedstock (7). The objective of the present paper is to investigate the particle behavior during the thermal spraying process. In particular, a mathematical model was used to predict the momentum behavior of the micron-sized agglomerateswith nanocrystalline grain structure during the HVOF spraying process. The generalized Newtonian momentum equation is changed to account for the variation of the flake-shapedagglomeratesproduced by mechanical milling.
319
FOURTH
INTERNATIONAL CONFERENCE
EXPERIMENTAL
ON NANOSTRUCTURED
MATERIALS
PROCEDURE
Pre-alloyed 316-stainless steel powders (17 wt.% Cr, 12 wt.% Ni, 2.5 wt.% MO, 1 wt.% Si, 0.1 wt.% C and balance Fe, Sulzer Metco (US) Inc.) with a nominal particle size of 45fll pm were chosen for the study. The powders were mechanical milled in either methanol or liquid nitrogen environment for 10 hrs. Subsequently the nanocrystalline powders was thermally sprayed using a Sulzer Metco DJ 2600 HVOF spray system on the 1020-stainless steel substrates. Hydrogen gas was used as fuel with a hypersonic gas velocity of approximately 2000 m/s and a pressure of 0.28 MPa was generated. The details associated with the HVOF experiment can be found elsewhere (7). The generalized equation describing the velocity profile of the impinging particles can be expressed by the Newtonian equation (8-10). Although the momentum transport between the hypersonic flame gas and the particles involves the drag force, force due to pressure gradients and added mass, Basset history term, and external potential forces, the drag force is dominant in the case of the HVOF thermal spraying (1).
VPPP
dVP -
dt
= A,
hag
Pg
1 Vg
- VP
1 bg
- VP )
VP is the volume of the particles (m3); p, and p, are densities of the particle and flame gas respectively (kg/m3); vp, vs are particle velocity and flame gas velocity (m/s); t is the time (s); AP is particle area (m*); and Ctig is the drag coefficient (unitless). Although the drag coefficient of non-spherical particles is difficult to predict due to the influence of particle orientation, the flow patterns are similar to those described for spheres (11). The drag coefficient for spherical particles, which is a function of the Reynolds number, is defined as C,,,, = (1 + 0.189Re0.6”2)
[2] at low Reynolds number range (21
The Reynolds number is expressed as Re = e
(1).
[3], where d, is the characteristic particle
dimension (m) and rls is the dynamic viscosity of the gas (Ns/m*).
RESULTS
AND DISCUSSION
Mechanical milling of the as-received stainless steel powders produced flake-shaped agglomerates with the size between 27-253 urn for the methanol milled powders and 28-85 urn for the cryomilled powders. The BET surface areas of the as-received and methanol milled and cryomilled powders, determined by a particle surface area analyzer (Coulter SA3 loo), are shown in TABLE 1. The process of mechanical milling increases the specific surface areas and hence the chemical reactivity of the powders. The velocity profile of the stainless steel agglomerates and thickness is shown in Figure 1. The area is expressed as the shape of two faces of an ellipse to account for the geometry of the irregular flakes, ,4d = 2nr 2 n [4]. In equation [4], r is the radius of the particle (m) and n is the aspect ratio (unitless). Furthermore, the volume, Vd is then expressed
FOURTH INTERNATIONAL CONFERENCE
ON NANOSTRUCTURED
MATERIALS
321
as V, = Adc [5], in which c is the thickness of the particle (m) measured by scanning electron microscopy. TABLE 1 Surface Areas of the As-received 3 16-stainless Steel, Methanol Milled and Cryomilled powders 1 Surface area (ml/g) As-received 3 16-stainless steel powders I 0.043 Cryomilled ( 10 hrs.) powders 0.698 Methanol milled (10 hrs.) powders 9.539 The calculated velocity profile shown in Figure 1 yields the variations in the particle behavior of methanol milled and cryomilled agglomerates. The particle size used in the present mathematical model corresponds to the cumulative weight fraction (die, d5s, and d9s) of the particle size distribution, determined by a Microtrac Standard Range Particle Analyzer, of the nanocrystalline powders feedstock produced by the mechanical milling process. The particle size distribution for the methanol milled powders indicates a wide size distribution, from 27 to 253 pm. The effect of the wide particle size distribution can be observed in the velocity profiles of the methanol milled powders. The particle velocity increases steeply initially, with respect to the axial distance from the powder injection to the exit of the gun barrel (approximately 70 mm). Then the particle velocity decreases due to the rapid mixing between the fuel gas and the surrounding atmosphere. As the particle thickness increases, the momentum transport decreases due to the increasing drag. According the Figure 1, the methanol milled steel agglomerates with the particle size of 27 pm attains a maximum velocity at approximately 1850 m/s and slowly decelerates to 1275 m/s before reaching the substrate. The velocity profile of methanol milled agglomerates behaves differently than that of cryomilled agglomerates. The surface areas between the methanol and cryomilled agglomerates contributed to the observed differences in the velocity profile. For instance, although the particle size corresponding to the 10 cumulative weight percent between methanol milled (d,,,=27 pm) and cryomilled (d10=28 pm) particles differs negligibly, the thickness of the corresponding cryomilled particles is approximately 75% larger than that of the methanol particles. In effect, the momentum transport of the methanol milled agglomerates is more efficient than that of the cryomilled agglomerates. Ongoing studies are continued to determine the energy transport of the methanol milled and cryomilled agglomerates.
CONCLUSIONS One-dimensional Newtonian equation is employed to predict the momentum transport between the nanocrystalline 316-stainless steel agglomerates and the flame gas during high velocity oxygen-fuel thermal spraying. A geometric ratio is used to account for the size and morphological variations of the micron-sized agglomerates with nano-grained structure. The calculated particle velocity indicated that the velocity profile depends strongly on the thickness of the particles, which is a function of milling time and milling media. The momentum transport of methanol milled particles is more efficient than that of cryomilled particles due to the difference in particle morphology.
322
FOURTH
INTERNATIONAL
&INFERENCE
ON NANOSTRUCTURED
Methanolmill dl0=27 Methanol til. d50=89 Methanol mll, d90=253 Ci-yomill, d10=28 mu, Cryomi d50=50 mx CryomQ d90=85 mu,
z -.i x x --b
MATERIALS
um, 3.0487 m m. 5.887 m um, 8.56B7 m 1.23E-6 m 2.87B6 m 4.51 E6 m
500 0
40
80
120
160
200
240
Distance along the gun barrel (mm)
Figure 1. Velocity profile of methanol milled and cryomilled 3 16-stainlesssteel agglomerates and thicknessalong the gun barrel during HVOF spraying. ACKNOWLEDGMENTS The authors would like to acknowledge the financial support by the OffIce of Naval Research under grant No.N00014-94-1-0017 and No.N00014-98-1-0569 for the present research. The authors also thank Coulter Corp. (M. Reyes, Miami, Florida) for the donation of the SA3100 surface particle analyzer. REFERENCES 1. Pawlowski, L., The Science and Engineering of Thermal Spray Coatings, John Wiley 8~ Sons, England, 1995. 2. Stewart, D.A., Dent, A.H., Harris, S.J., Horlock, A.J., McCartney, D.G. and Shipway, P.H., Journal of Thermal Spray Tech., 1998, in press. 3. Lau, M.L., Jiang, H.G. and Lavemia, E.J., J. ThermalSpray Tech., 1998, in press. 4. Kear, B.H., Skandan, G. and Sadangi, R., High pressure synthesis of nanophase WC/Co/diamond Powders: implications for thermal spraying, presented in Thermal Spray Processing of Nanoscale Materials, 1997, Davos, Switzerland. 5. Tellkamp, V., Lau, M., Fabel, A. and Lavemia, E.J., NanoStructured Mat/s., 1996,!, 489. 6. Jia, K. and Fisher, T.E., J. Thermal Spray Tech., 1997, in press. 7. Lau, M.L., Jiang, H.G. and Lavemia, E.J., Thermal Spraying: Meeting the Challenges of the 2lst Century, Coddet, C., ed. 1998, Nice, France, 379. 8. Liang, X., Lavemia, E.J., Wolfenstine, J. and Sickinger, A., J. ThermalSpra?, Tech.,1995, $2.52. 9. Pfender, E. and Lee, Y.C., Plasma Chem. and Plasma Proc., 198&z, 21 I. 10. Tawfik, H.H. and Zimmerman, F., J. ThermalSpray Tech., 1997, fj, 345. I 1. Clift, R., Grace, J.R. and Weber, M.E., Bubbles. Drops, and Particles, Academic Press, Inc., New York, 1978, p. 142.
Pergamon PII SO9659773(99)00126-9
PARTICLE
BEHAVIOR HIGH
OF NANOCRYSTALLINE VELOCITY OXY-FUEL
Materials, Vol. 12, pp. 319-322, 1999 Elsevier Science Ltd 0 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved 09659773/99/$-&front matter
31CSTAINLESS STEEL DURING THERMAL SPRAY
M.L. Lau, V.V. Gupta and E.J. Lavernia Department of Chemical and Biochemical Engineering and Materials Science University of California, Irvine, Irvine, CA 92697-2575
Abstract--The present paper investigates the particle behavior of nanocrystalline 316 stainless steel powders during high velocity oxy-fuel (HVOF) spray. The feedstock powders were synthesized by mechanical milling to produce flake-shaped agglomerates with an average grain size of less than 50 nm. The powders were then introduced into the HVOF spray to produce a nanocrystalline coating. A mathematical model is developed to study the variation of dtperent agglomerate sizes resulting from the mechanical milling process on the particle behavior during thermal spraying. A generalized Newtonian equation describing the momentum transfer was used, which incorporates a geometric ratio to account for the size and morphological variations of the micron-sized agglomerates with nano-grained structure. Particle velocity from the numerical calculations indicated that the velocity profile depends strongly on the thickness of the particles, which is ahnction of milling time and milling media, 01999 Acta Metallurgica Inc. INTRODUCTION Thermal spray is a coating process which has evolved to produce various metallic, non-metallic, and ceramic coatings (1). In recent year, thermal spraying using nanoctystalline feedstock powders has yielded coatings with higher hardness, strength, and corrosion resistance than those of the conventional counterparts (2-5). For instance, high velocity oxy-fuel spray of WCY12Co and WC/lSCo nanocomposite coatings shows a 28% increase in hardness while the toughness of the nanocomposite remains constant when compared to that of the conventional coatings (6). More recently, inert gas atomized 3 16-stainlesssteelpowders were mechanically milled in methanol and liquid nitrogen separately to produce nanocrystalline powders and the powders were thermally sprayed by HVOF (7). The resultant coatings exhibited significant increasein microhardness. For example, an 48% increasein microhardnesswas observed in the thermal sprayed coating when methanol milled stainlesssteel powders was used as the feedstock (7). The objective of the present paper is to investigate the particle behavior during the thermal spraying process. In particular, a mathematical model was used to predict the momentum behavior of the micron-sized agglomerateswith nanocrystalline grain structure during the HVOF spraying process. The generalized Newtonian momentum equation is changed to account for the variation of the flake-shapedagglomeratesproduced by mechanical milling.
319
FOURTH
INTERNATIONAL CONFERENCE
EXPERIMENTAL
ON NANOSTRUCTURED
MATERIALS
PROCEDURE
Pre-alloyed 316-stainless steel powders (17 wt.% Cr, 12 wt.% Ni, 2.5 wt.% MO, 1 wt.% Si, 0.1 wt.% C and balance Fe, Sulzer Metco (US) Inc.) with a nominal particle size of 45fll pm were chosen for the study. The powders were mechanical milled in either methanol or liquid nitrogen environment for 10 hrs. Subsequently the nanocrystalline powders was thermally sprayed using a Sulzer Metco DJ 2600 HVOF spray system on the 1020-stainless steel substrates. Hydrogen gas was used as fuel with a hypersonic gas velocity of approximately 2000 m/s and a pressure of 0.28 MPa was generated. The details associated with the HVOF experiment can be found elsewhere (7). The generalized equation describing the velocity profile of the impinging particles can be expressed by the Newtonian equation (8-10). Although the momentum transport between the hypersonic flame gas and the particles involves the drag force, force due to pressure gradients and added mass, Basset history term, and external potential forces, the drag force is dominant in the case of the HVOF thermal spraying (1).
VPPP
dVP -
dt
= A,
hag
Pg
1 Vg
- VP
1 bg
- VP )
VP is the volume of the particles (m3); p, and p, are densities of the particle and flame gas respectively (kg/m3); vp, vs are particle velocity and flame gas velocity (m/s); t is the time (s); AP is particle area (m*); and Ctig is the drag coefficient (unitless). Although the drag coefficient of non-spherical particles is difficult to predict due to the influence of particle orientation, the flow patterns are similar to those described for spheres (11). The drag coefficient for spherical particles, which is a function of the Reynolds number, is defined as C,,,, = (1 + 0.189Re0.6”2)
[2] at low Reynolds number range (21
The Reynolds number is expressed as Re = e
(1).
[3], where d, is the characteristic particle
dimension (m) and rls is the dynamic viscosity of the gas (Ns/m*).
RESULTS
AND DISCUSSION
Mechanical milling of the as-received stainless steel powders produced flake-shaped agglomerates with the size between 27-253 urn for the methanol milled powders and 28-85 urn for the cryomilled powders. The BET surface areas of the as-received and methanol milled and cryomilled powders, determined by a particle surface area analyzer (Coulter SA3 loo), are shown in TABLE 1. The process of mechanical milling increases the specific surface areas and hence the chemical reactivity of the powders. The velocity profile of the stainless steel agglomerates and thickness is shown in Figure 1. The area is expressed as the shape of two faces of an ellipse to account for the geometry of the irregular flakes, ,4d = 2nr 2 n [4]. In equation [4], r is the radius of the particle (m) and n is the aspect ratio (unitless). Furthermore, the volume, Vd is then expressed
FOURTH INTERNATIONAL CONFERENCE
ON NANOSTRUCTURED
MATERIALS
321
as V, = Adc [5], in which c is the thickness of the particle (m) measured by scanning electron microscopy. TABLE 1 Surface Areas of the As-received 3 16-stainless Steel, Methanol Milled and Cryomilled powders 1 Surface area (ml/g) As-received 3 16-stainless steel powders I 0.043 Cryomilled ( 10 hrs.) powders 0.698 Methanol milled (10 hrs.) powders 9.539 The calculated velocity profile shown in Figure 1 yields the variations in the particle behavior of methanol milled and cryomilled agglomerates. The particle size used in the present mathematical model corresponds to the cumulative weight fraction (die, d5s, and d9s) of the particle size distribution, determined by a Microtrac Standard Range Particle Analyzer, of the nanocrystalline powders feedstock produced by the mechanical milling process. The particle size distribution for the methanol milled powders indicates a wide size distribution, from 27 to 253 pm. The effect of the wide particle size distribution can be observed in the velocity profiles of the methanol milled powders. The particle velocity increases steeply initially, with respect to the axial distance from the powder injection to the exit of the gun barrel (approximately 70 mm). Then the particle velocity decreases due to the rapid mixing between the fuel gas and the surrounding atmosphere. As the particle thickness increases, the momentum transport decreases due to the increasing drag. According the Figure 1, the methanol milled steel agglomerates with the particle size of 27 pm attains a maximum velocity at approximately 1850 m/s and slowly decelerates to 1275 m/s before reaching the substrate. The velocity profile of methanol milled agglomerates behaves differently than that of cryomilled agglomerates. The surface areas between the methanol and cryomilled agglomerates contributed to the observed differences in the velocity profile. For instance, although the particle size corresponding to the 10 cumulative weight percent between methanol milled (d,,,=27 pm) and cryomilled (d10=28 pm) particles differs negligibly, the thickness of the corresponding cryomilled particles is approximately 75% larger than that of the methanol particles. In effect, the momentum transport of the methanol milled agglomerates is more efficient than that of the cryomilled agglomerates. Ongoing studies are continued to determine the energy transport of the methanol milled and cryomilled agglomerates.
CONCLUSIONS One-dimensional Newtonian equation is employed to predict the momentum transport between the nanocrystalline 316-stainless steel agglomerates and the flame gas during high velocity oxygen-fuel thermal spraying. A geometric ratio is used to account for the size and morphological variations of the micron-sized agglomerates with nano-grained structure. The calculated particle velocity indicated that the velocity profile depends strongly on the thickness of the particles, which is a function of milling time and milling media. The momentum transport of methanol milled particles is more efficient than that of cryomilled particles due to the difference in particle morphology.
322
FOURTH
INTERNATIONAL
&INFERENCE
ON NANOSTRUCTURED
Methanolmill dl0=27 Methanol til. d50=89 Methanol mll, d90=253 Ci-yomill, d10=28 mu, Cryomi d50=50 mx CryomQ d90=85 mu,
z -.i x x --b
MATERIALS
um, 3.0487 m m. 5.887 m um, 8.56B7 m 1.23E-6 m 2.87B6 m 4.51 E6 m
500 0
40
80
120
160
200
240
Distance along the gun barrel (mm)
Figure 1. Velocity profile of methanol milled and cryomilled 3 16-stainlesssteel agglomerates and thicknessalong the gun barrel during HVOF spraying. ACKNOWLEDGMENTS The authors would like to acknowledge the financial support by the OffIce of Naval Research under grant No.N00014-94-1-0017 and No.N00014-98-1-0569 for the present research. The authors also thank Coulter Corp. (M. Reyes, Miami, Florida) for the donation of the SA3100 surface particle analyzer. REFERENCES 1. Pawlowski, L., The Science and Engineering of Thermal Spray Coatings, John Wiley 8~ Sons, England, 1995. 2. Stewart, D.A., Dent, A.H., Harris, S.J., Horlock, A.J., McCartney, D.G. and Shipway, P.H., Journal of Thermal Spray Tech., 1998, in press. 3. Lau, M.L., Jiang, H.G. and Lavemia, E.J., J. ThermalSpray Tech., 1998, in press. 4. Kear, B.H., Skandan, G. and Sadangi, R., High pressure synthesis of nanophase WC/Co/diamond Powders: implications for thermal spraying, presented in Thermal Spray Processing of Nanoscale Materials, 1997, Davos, Switzerland. 5. Tellkamp, V., Lau, M., Fabel, A. and Lavemia, E.J., NanoStructured Mat/s., 1996,!, 489. 6. Jia, K. and Fisher, T.E., J. Thermal Spray Tech., 1997, in press. 7. Lau, M.L., Jiang, H.G. and Lavemia, E.J., Thermal Spraying: Meeting the Challenges of the 2lst Century, Coddet, C., ed. 1998, Nice, France, 379. 8. Liang, X., Lavemia, E.J., Wolfenstine, J. and Sickinger, A., J. ThermalSpra?, Tech.,1995, $2.52. 9. Pfender, E. and Lee, Y.C., Plasma Chem. and Plasma Proc., 198&z, 21 I. 10. Tawfik, H.H. and Zimmerman, F., J. ThermalSpray Tech., 1997, fj, 345. I 1. Clift, R., Grace, J.R. and Weber, M.E., Bubbles. Drops, and Particles, Academic Press, Inc., New York, 1978, p. 142.