Full text of DOI:10.1557/JMR.2002.0154

Microstructural evolution in nanocrystalline Ni coatings
M.L. Lau and J. He
Department of Chemical Engineering and Materials Science, University of California, Irvine,
Irvine, California 92697-2575
A.J. Melmed and T.A. Lusby
Materials Science and Engineering Department, Johns Hopkins University,
Baltimore, Maryland 21218-2689
R. Schweinfest and M. Ru¨hle
Max-Planck-Institute fu¨r Metallforschung, Seestrasse 92, D-70174 Stuttgart, Germany
E.J. Lavernia
Department of Chemical Engineering and Materials Science, University of California, Irvine,
Irvine, California 92697-2575
(Received 22 January 2001; accepted 12 February 2002)
Gas-atomized Ni powders were mechanically milled under liquid nitrogen and
subsequently sprayed using a high-velocity oxygen-fuel thermal spraying system. The
resultant coatings were evaluated using various analytical methods, including scanning
electron microscopy (SEM), transmission electron microscopy, and atom probe field
ion microscopy (APFIM). The results indicated that the majority of the grains in
nanocrystalline Ni coating were equiaxed with an average grain size of 50 ± 23 nm.
SEM elemental dot mapping analysis suggested that oxide particles were distributed
along the prior droplet boundaries and interpass boundaries in the coatings. The
APFIM analysis indicated that the distribution of oxygen was very inhomogeneous in
the nanocrystalline coating samples studied. Mostly small NiO precipitates with a size
range of 1–7 nm were found throughout the grains, and the average size of these
precipitates was approximately 4 nm. However, large oxide precipitates with a size up
to 55 nm were also observed. In addition, increased amounts of oxygen were found at
the grain boundaries and at structural defects.
aluminum and dilute aluminum alloys.14 The alloyed
I. INTRODUCTION
powders synthesized by this technique were strengthened
by aluminum oxynitride particles (2–10 nm in diameter
with a mean spacing of 50–100 nm).14 Whittenberger et
al. employed cryomilling to disperse Y2O3 in prealloyed
NiAl matrix and reported that the AIN dispersoids
formed during cryomilling provided more strengthening
in NiAl than the Y2O3 particles alone. In related studies,
cryomilled WC–Co particles remained less than 100 nm
after undergoing thermal exposure at 1000 °C for 4 h.16
A relatively high creep resistance was reported in an
ultrafine-grained Al–Mg alloy prepared by cryomilling
and subsequently hot extruded.17 TEM analysis, follow-
ing creep at 300 °C to approximately 0.2% strain for
1141 h, revealed the grain size was unchanged from its
as-extruded size, indicating significant thermal stability
of this material at relatively high fractions of the melting
temperature.
Significant advancements have been made in the
synthesis of nanocrystalline powders with controllable
chemical homogeneity. Several methods have been
scaled up for commercial use.1,2 As a result, the ap-
plication of nanocrystalline materials has diversified
into various fields. For instance, thermal spraying of na-
noscale materials has generated coatings with properties
reportedly superior to those of the micron-sized counter-
parts.3–11 In a recent study, nanocrystalline Ni powders
prepared by mechanical milling in methanol for 10 h
were sprayed by high-velocity oxygen-fuel (HVOF)
spraying to produce nanocrystalline coatings.12 In this
study, the coating exhibited a 20% increase in micro-
hardness despite a higher volume fraction of porosity as
compared to the coatings produced using conventional
inert-gas-atomized powders.12 Amongst the various tech-
niques available, cryomilling offers the advantage of
changing the deformation kinetics of the powders by
minimizing heat generation and favoring fracturing
over welding of the particles.13 The technique was first
employed by Luton et al. in the mechanical alloying of
To provide insight into the evolution of microstructure
in nanocrystalline materials during thermal spraying,
nanocrystalline coatings were synthesized by HVOF
thermal spraying using the cryomilled Ni powders
as feedstock powders. The powder temperatures were
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M.L. Lau et al.: Microstructural evolution in nanocrystalline Ni coatings
ured, using a two-color In-flight Particle Pyrometer
the powders were heated and propelled towards the sub-
strate where they were deposited as coatings with aver-
age thickness of 250 ␮m. The gas flow rates were
standard parameters provided for spraying Ni alloys by
the operation manual of the facility and are listed in
Table I.
(Idaho Falls, ID) to monitor the powder behavior during
flight. The microstructural analysis of the resultant coat-
ings was performed using scanning electron microscopy
(SEM), transmission electron microscopy (TEM), and
atom probe field ion microscopy (APFIM).
The temperature profile of the conventional Ni and
the sieved cryomilled Ni powders was experimentally
measured using a two-color In-flight Particle Pyrometer
IPP-2010 with a remote iris sensor head (Inflight Ltd.
Co., Idaho Falls, ID). A schematic diagram of the ex-
perimental apparatus is shown in Fig. 1. The sensor was
set perpendicular to the spray gun. Measurements were
performed at 10-s intervals at various distances from
the nozzle of the sprayed gun until the distance corre-
sponding to the location of the substrate (0.23 m) was
reached.
SEM observations were made using a Philips XL30
(Mahwah, NJ) a field emission gun scanning electron
microscope equipped with an energy dispersive spectros-
copy (EDS) detector. The TEM studies were performed
with a JEOL 2000FX operated at 200kV. Details of the
sample preparation procedure for the milled powders and
coatings are described in detail elsewhere.12 To prepare
the coatings for APFIM analysis, the coating was me-
chanically sectioned into rods with a square cross
section. The rods were tapered to sharp points by electro-
polishing in solutions of HCl in glycerin. Initial exami-
nation of the internal structure of the coating was
performed by field ion microscopy (FIM) to search for
atomic and nanoscale features of interest, such as small
scale precipitates, structural defects, phase boundaries,
and grain boundaries for compositional analysis. Achiev-
ing a smooth pointed surface was much more difficult
than for ordinary bulk Ni and Ni alloys, due to structural
and compositional inhomogeneities resulting from the
thermal spraying process. Therefore, most of the FIM
imaging was done by H-FIM or Ar-FIM to minimize the
field stress, rather than the conventionally preferred Ne-
FIM. Compositional characterization was done in a con-
ventional, electrically pulsed energy-compensated
APFIM with no image gas present.
II. EXPERIMENTAL
Inert-gas-atomized Ni (0.14 wt.% Co, 0.01 wt.%, Fe,
0.42 wt.% O, 0.013 wt.% N, and 99.8 wt.% Ni) powders
[Sulzer Metco (US) Inc., Westbury, NY] with a nominal
particle size of 45–11 ␮m were selected for the present
Study. The atomized powders were mechanically milled
in a modified Union Process 01-ST (Akron, OH) attritor
mill with a grinding tank capacity of 0.0057 m3. Stainless
steel balls 0.635 cm in diameter were used with a
powder-to-ball mass ratio of 1:20. The mill was operated
at 180 rpm with a shaft clearance of 0.635 cm from the
bottom of the grinding tank to minimize powder accu-
mulation. Liquid nitrogen was continuously introduced
in the attritor tank to ensure complete immersion of the
powders. The temperature was maintained between 83
and 98 K. The powders were removed from the tank of
the attritor and separated from the balls by sieving. The
cryomilled powders used for thermal spraying were
sorted into three different particle sizes (d < 38 ␮m,
38 ␮m < d < 45 ␮m, d < 45 ␮m) using standard sieving
techniques (Metal Powder Industries Federation Stan-
dard 05), with a set of calibrated meshes according to the
United States Standard Sieve Series. A Microtrac Stan-
dard Range Particle Analyzer (Microtrac Inc., Clearwa-
ter, FL) was used to determine the particle size
distribution of the milled powders. On the basis of
ASTM E1019 and ASTM E1097 standards, chemical
analysis of the powders and coatings was carried out by
Luvak Inc., a professional chemical analysis company
located in Boylston, Massachusetts.
Thermal spraying of the nanocrystalline powders was
performed with a Sulzer Metco DJ 2600 HVOF spray
system and 1020 carbon steel substrates. Hydrogen and
nitrogen were used as fuel and carrier gas, respectively.
Before spraying, the surface of the substrate was grit
blasted with abrasive Al2O3 particles to create a super-
ficial roughness, which improves the bond strength be-
tween the coating and the substrate. In the DJ 2600
(Westbury, NY) spraying gun the powders were injected
axially into the jet gas by a fluidized bed. After injection,
TABLE I. Spraying parameters for the Ni powders.
Pressure (MPa)
Gas flow (m3/h)
O2
H2
N2
Air
O2
H2
N2
Air
FIG. 1. Schematic flow diagram illustrating the acquisition of the
temperature profile of the Ni particles during thermal spraying.
1.17
0.96
1.034
0.69
13.8
41.0
0.792
22.3
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M.L. Lau et al.: Microstructural evolution in nanocrystalline Ni coatings
powder particles and the milling temperature.21 When
III. RESULTS AND DISCUSSION
A. Powder characteristics
the particle size reaches the limit of comminution, the
fracturing process ends. The surface area of the Ni pow-
der agglomerates after milling is significantly increased,
partly because of the flat agglomerate morphologies with
high aspect ratios, and most likely because the fractured
surface morphologies due to sandwichlike microstructure
formed during milling.21 Differences in the strain for
fracture among the layers in a sandwichlike microstruc-
ture lead to a relatively rough fracture surface (free sur-
face of the powder agglomerate) with a high density of
ridges and cervices. The rate of structural refinement is a
function of the mechanical energy input and the rate of
work hardening of the material.13 As a result, milling in
liquid nitrogen leads to a decrease of the grain size, as
shown in Table II.
In related studies, several mechanisms of grain refine-
ment during mechanical milling were proposed.19,22–25
Benjamin, the developer of mechanical milling/alloying,
and his colleagues stated that the process involves re-
peated welding, fracturing and re-welding of powder par-
ticles under a highly energetic ball charge.19 On the basis
of a study on microstructures of ball-milled Ru and
Mechanical milling of the as-received spherical pow-
ders produced irregular, flake-shaped agglomerates. Sec-
ondary electron images illustrate the morphological
differences between the as-received powders [Fig. 2(a)]
and the cryomilled (10 h) Ni powders [Fig. 2(b)]. The
surface areas of conventional Ni powders and cryomilled
(10 h) powders determined by a particle surface analyzer
(Model SA 3100, Coulter Corp., Miami, FL) were 0.043
and 0.285 m2/g, respectively. Table II lists particle sizes
obtained from the Microtrac Standard Range Particle
Analyzer, as well as thickness measured by SEM imag-
ing, and average grain size and aspect ratio by TEM
dark-field imaging (the aspect ratio is calculated by
measuring maximum and minimum values of grain
diameter on microscopy micrographs). For comparison,
relevant data from Ni powders milled in methanol envi-
ronment are also included.12
Chemical analyses of the cryomilled (10 h) Ni pow-
ders are shown in Table III. The oxygen content of cryo-
milled powders is mostly attributed to the pickup during
the transfer from the attritor to the glove box. The chemi-
cal analyses of Ni powders milled in methanol for the
same milling time are also included in Table III for com-
parative purposes. The percentage of carbon and nitrogen
in the milled powders is significant, and the amount of
nitrogen in the cryomilled powders is nearly 10 times
higher than that of the powder milled in methanol. The
carbon and nitrogen are likely to originate from methanol
or liquid nitrogen as milling media.
TABLE II. Characteristic powder properties of mechanically milled
Ni for 10 h for different milling conditions.
Milling
Thickness Agglomerate size Grain size Aspect
environment
(␮m)
(␮m)
(nm)
ratio
methanol
liquid nitrogen
4 ± 2
10 ± 3
77
84
44 ± 25
19 ± 11
1.48
1.47
Mechanical milling has been shown to be an effective
method for producing fine nanocrystalline metallic pow-
ders.18 The formation of irregular Ni agglomerates by
cryomilling is attributed to the continuous cold welding
and fracturing of the powder particles caused by the col-
lision between the powders and the milling media.19,20
The extent of welding and fracturing often depends on
the deformation behavior (both shear and friction) of the
TABLE III. Chemical analysis of milled (10 h) Ni powders.
Element Cryomilled powders (wt%) Methanol milled powders (wt%)
O
N
C
Fe
Ni
0.37
0.28
Not detected
0.085
0.44
0.036
0.24
0.078
99.206
99.265
FIG. 2. SEM analysis illustrating the morphological differences between (a) the as-received powders and (b) the Ni powders cryomilled for 10 h.
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M.L. Lau et al.: Microstructural evolution in nanocrystalline Ni coatings
AlRu, the formation of a nanocrystalline structure was
Ni powder increased with increasing distance. This
measured result is in general agreement with previous
studies on nanostructured WC–12% Co system.10
thought to evolve from the development of dislocation
cell structures within shear bands.22 Plastic deforma-
tion leads to the formation of dislocation cells within
shear bands; then dislocation cells transform into low-
angle grain boundaries and finally form nanocrystalline
grains surrounded by high-angle grain boundaries via
grain rotation.22–24 During intermediate stages of cryo-
milling of Inconel 625 alloy, grains in the powder were
deformed into elongated grains with a high density of
deformation faults, and then fractured by cycling impact
loading in random direction. The fractured fragments
from the elongated coarse grains formed nanoscale
grains. The evolution of the elongated grains from de-
velopment to disappearance suggested that repeated
strain fatigue fracturing caused by cycling impact loading in
random directions and cold welding were responsible for
the formation of a nanocrystalline microstructure.25
The measured difference in powder temperature be-
tween the conventional Ni and the cryomilled Ni pow-
ders can be rationalized by the difference in their
geometry (see Fig. 2). Due to the difference in morphol-
ogy and surface roughness, the surface area of the cryo-
milled Ni is more than 6.5 times the surface area of the
conventional Ni powders as determined by the surface
area analysis. Accordingly, the heat transfer between the
flame gas and the cryomilled Ni powders is more effi-
cient than that of conventional powders. Therefore, at the
exit of the gun, the temperatures of the cryomilled Ni
powders were nearly as high as the flame temperature of
approximately 2100 K. The temperature of the conven-
tional Ni powder at the exit of the gun was 1320 K, and
was thus lower than the flame temperature and the melt-
ing temperature of pure Ni (1726 K). Therefore, the un-
molten conventional Ni powder was heated up further by
the gas flame causing the temperature increases with in-
creasing flight distance. The temperatures (between 2050
and 2200 K) of the cryomilled Ni powders were initially
near the temperature of gas flame at the exit of the gun
(2100 K). Thus the temperatures of the powders de-
creased with the temperature of flame gas for increasing
distance from the exit of the gun.
B. Powder temperature during flight
The cryomilled powders used for thermal spraying
were sorted into 3 different particle sizes (d < 38 ␮m,
38 ␮m < d < 45 ␮m, d > 45 ␮m). A comparison of the
temperature profile obtained by the pyrometric measure-
ments between the cryomilled powder particles sieved
prior to thermal spraying and the conventional Ni pow-
ders (45–11 ␮m) is shown in Fig. 3. Insufficient data
were collected during thermal spraying of the cryomilled
powders with particle sizes greater than 45 ␮m due to
poor flowability of the powders, which led to severe
clogging of the fluidized powder feeder.
On the basis of the results in Fig. 3, it can be seen that
the temperatures of cryomilled Ni powders were higher
than those of conventional Ni powder. However, the tem-
perature of the cryomilled powders decreased with in-
creasing distance, while the temperatures of conventional
C. Coating microstructure
The results of SEM analysis performed on the cross
sections of various sprayed coatings generated using con-
ventional Ni powders (hereafter referred to as conven-
tional coatings) and cryomilled powders (nanocrystalline
coatings) are shown in Figs. 4 and 5. Elemental dot map-
ping was also performed on the coatings to determine the
distribution of oxides, as shown in Figs. 4(b) and 5(b).
Chemical analyses performed on the conventional and
nanocrystalline coatings, shown in Table IV, indicate a
slightly higher oxygen content in the nanocrystalline
coating; this is consistent with the elemental dot map
image in Fig. 5(b). The results showed that the oxides are
distributed along prior droplet boundaries (i.e., between
individual particles) and inter-pass boundaries (i.e.,
boundaries that develop as the HVOF scans the surface
of the target) in coatings, suggesting that oxidation may
occur in-flight during thermal spraying or/and during
particle impingement.
Transmission electron microscopy analysis of the
coatings, shown in Fig. 6(a), revealed that the majority of
grains observed were equiaxed with an average grain size
of 50 ± 23 nm and an aspect ratio of 1.42. In addition,
regions of large elongated grains with a grain size of
130–200 nm were also observed, as shown in Fig. 6(b).
The elongated grains observed in the cross section of the
coating suggested that the powders melted during
FIG. 3. Temperature profile of conventional Ni and sieved cryo-
milled Ni powders obtained by two-color pyrometric measurements.
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M.L. Lau et al.: Microstructural evolution in nanocrystalline Ni coatings
FIG. 4. Cross sections of a conventional Ni coating: (a) backscattered SEM image, (b) oxygen distribution as indicated in red by SEM x-ray dot
mapping.
FIG. 5. Cross sections of a nanocrystalline Ni coating: (a) backscattered SEM image, (b) oxygen distribution as indicated in red by SEM x-ray
dot mapping.
TABLE IV. Chemical composition of conventional and nanocrystal-
line Ni coatings.
was determined at sites located at or near grain boun-
daries, at structural defects occasionally found, and at
locations distant from grain boundaries.27 Many small-
Conventional Ni
coating (wt%)
Nanocrystalline Ni
coating (wt%)
sized (about 1–7 nm) NiO precipitates were found during
compositional depth profiling at or within a few tens of
nanometers from grain boundaries, and larger NiO pre-
cipitates (up to about 55 nm) also were encountered at
grain boundaries, along with the presence of the small
precipitates.
Element
O
N
Ni
1.52
Not detected
98.3
1.77
0.06
98.17
NiO precipitates, identified by atom probe, were con-
sistently found to image with dark contrast by FIM, and
the amount of such precipitates varied along a given
grain boundary, among grain boundaries found in any
given type of coating, and among grain boundaries found
in the differently processed coatings. Some examples are
shown in the FIM micrographs of Figs. 7–9. A relatively
uncontaminated section of a grain boundary in a
nanocrystalline Ni coating, which might have originated
from the inside of a particle, is indicated in Fig. 7. Fig-
ure 8 shows a large NiO precipitate located at a grain
boundary triple point in nanocrystalline Ni specimen.
There are several grains visible in this micrograph taken
in the APFIM, and the probe hole (illuminated to clearly
mark its position) can be seen entirely within the large
oxide precipitate, which is outlined between three
grains. The thick oxide-decorated boundary continues
the thermal spraying process and subsequently solidified
after impact, presumably by heterogeneous nucleation.
The solidification front should be perpendicular to the
substrate surface. Therefore, columns should roughly
grow opposite to the deposition direction. The formation
of columnar grains can also be attributed to the increased
temperature of the residual liquid due to the liberation
of the latent heat of fusion (recalescence effect).26 In
addition, the diffraction patterns of NiO were also iden-
tified by TEM.
D. Oxide phases in the coatings
The thermal sprayed samples were characterized by
FIM to investigate internal microstructural features of the
coatings such as nanoscale precipitates, structural de-
fects, phase boundaries, and grain boundaries. Composition
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M.L. Lau et al.: Microstructural evolution in nanocrystalline Ni coatings
to the lower left of the micrograph. Grain boundaries
be seen from Fig. 10. Also, the imposition determined
from a given depth profile clearly was affected by the
number of precipitates encountered. Precipitates larger
than the probe diameter were found to consist of almost
entirely NiO.
The inhomogeneity in oxide distribution discussed
above precludes any attempt to quantitatively specify
grain boundary compositions. The APFIM compositional
results are summarized in Table V, where the average
compositions found for nanocrystalline Ni coatings
and for conventional coatings at various numbers of ran-
dom sites at grain boundary sites and away from grain
containing relatively large numbers of small NiO disper-
soids in a conventional Ni specimen are seen in Fig. 9,
where many of the oxide dispersoids are circled in white
for ease of locating the grain. It is note that grain sizes in
any specific FIM specimen used were, due to its small
volume, strictly fortuitous and FIM was not used to de-
termine grain size in the present work. The effective
lateral probe diameter for composition measurements in
the present study was 1–3 nm. This meant that the com-
position fluctuations found in depth profiling were de-
pendent on the sizes of precipitates encountered,27 as can
FIG. 6. Bright-field TEM image showing cross sections of nanocrystalline Ni coating: (a) equiaxed grain structure and (b) large grain structure.
FIG. 8. Ar-FIM image of a nanocrystalline Ni coating at about 85 K
FIG. 7. H-FIM image of nanocrystalline coating at about 40 K with a
grain boundary indicated by a white line.
with probe hole (illuminated) positioned on a large NiO precipitate
between three grains (boundaries marked with white lines).
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M.L. Lau et al.: Microstructural evolution in nanocrystalline Ni coatings
boundaries (grain interiors) are given. The only signifi-
cant compositional difference between the grain bound-
aries and the grain interiors was the amount of oxide.
Also, there was significantly more oxide found at grain
boundaries in the nanocrystalline coatings compared to
the conventional coatings, although the grain interiors
were quite similar in composition (the standard deviation
of the measured values that reflect repeatability was less
than 0.3 wt% for oxygen). Note that due to the large
degree of compositional inhomogeneity in these
nanograined materials, the compositions determined by
APFIM cannot be directly compared to compositions de-
termined by more macroscopic methods.27
Compositional analysis by APFIM showed differences
in oxygen content in the grain boundaries and in the Ni
matrix between the conventional coating and coatings
sprayed with the cryomilled powders. Increased levels of
O and larger NiO precipitates at the grain boundary were
also observed in the nanocrystalline coatings, indicating
that the location of the interstitial atoms depended on the
milling environment and conditions. The distribution of
impurities, such as O, observed at or near the grain
boundaries suggested that these impurities could have
been introduced at the surface of the molten particles.
Alternatively, the oxides would also be introduced be-
tween each splat layer during spraying as each splat layer
was exposed to air before the stacking of another layer.
The N content of the nanocrystalline coating was higher
than that of the conventional coating, as observed in
Table IV. The increase in N content is mostly attributed
to the milling environment during powder synthesis.
Oxidation in thermal sprayed coatings can affect the
microstructure, the composition, and the phase distribu-
tion of the coating.28 The process occurs rapidly in the
presence of oxygen in the surrounding air or/and excess
oxygen during HVOF spraying.29 The extent of oxida-
tion can ultimately influence many physical properties
such as wear and erosion resistance of a coating. For
instance, oxidation of coatings is often a serious problem
for corrosion-resistant coatings and thermal barrier coat-
ings. However, in some applications of mild steel wire
sprayed coatings on cylinder walls of aluminum engines,
the metal oxides formed were observed to improve the
wear resistance of the coating.28
FIG. 9. Ne-FIM image of conventional Ni coating at about 40 K.
Grain boundaries are indicated by white lines, and oxide precipitates
are indicated by white circles. The square labeled D indicates the
location of a detector artifact.
TABLE V. Compositional analysis of conventional and nanocrystal-
line Ni coating by atom probe field ion microscopy.
Conventional coating (wt.%)
Nanocrystalline coating (wt.%)
Matrix
GBa
Matrix
GB (Ni–Ni)
Ni
O
N
99.3
0.7
4 sites: trace
98.9
1.1
2 sites: trace
99.4
0.6
3 sites: trace
98.3
1.7
5 sties: trace
FIG. 10. AP-FIM compositional depth profiles (atomic percent versus
depth in nanometers) for oxygen: (a) at a random site away from a
grain boundary, (b) a precipitate at a grain boundary, and (c) into a
grain boundary.
aGB ס grain boundary
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M.L. Lau et al.: Microstructural evolution in nanocrystalline Ni coatings
Extensive studies have been performed to determine
oxides were distributed along prior droplet boundaries
and inter-passing boundaries of the coatings. TEM dif-
fraction analysis indicated the oxide was NiO.
the dominant oxidation mechanism in metals. The for-
mation of an oxide layer comprises the complex proc-
esses of adsorption of oxygen molecules on the metal
surface, dissociation of the oxygen molecules, formation
of oxide nuclei, and finally the growth of nuclei to a
complete oxide layer.30 Molten particles can oxidize dur-
ing in-flight and after impingement onto the substrate
surface.29 The molten particles that populate the center of
the flame gas will oxidize rapidly by the surrounding
entrained oxygen in the atmosphere. The rate of oxida-
tion depends on the individual rates of the mass transfer
rate of oxygen from the gas phase to the particle surface,
the chemical rate of oxidation of the droplet, and the
diffusion of oxygen from the reacting surface of the par-
ticle to the interior.31 In the case of in-flight oxidation,
the level of oxide formation increases with increasing
spraying distance. However, if the rate of oxidation de-
creases with increasing spraying distance, the dominant
oxidation occurs at the substrate surface, where the splats
have a larger surface to volume ratio than the drop-
lets.28–32 The extent of oxidation depends on the splat
temperature and the time of exposure of the molten
splat to the surrounding environment.12 The presence of
oxides on the particle surface would thereby reduce the
heat transfer from the gas to the particle, resulting in low
thermal diffusivity in the coating and generating growth
stresses within the oxide.29 Furthermore, oxidation reac-
tions are highly exothermic and increase the transfer of
thermal energy.29
(3) According to the field ion microscopy analysis,
the distribution of oxygen was very inhomogeneous in
the nanocrystalline coating sample. Mostly small NiO
precipitates with a size range of 1–7 nm were found
throughout the grains, and the average size of these pre-
cipitates was approximately 4 nm. However, large oxide
precipitates with a size up to 55 nm were also observed.
In addition, increased amounts of oxygen were found at
the grain boundaries and at structural defects.
ACKNOWLEDGMENTS
The authors would like to acknowledge the financial
support by the Office of Naval Research under Grant
Nos. N00014-94-1-0017, N00014-98-1-0569, and
N00014-01-1-0882. The authors also are grateful for sup-
port by the Office of Naval Research under Grant No.
N00014-97-1-0559, to conduct the present APFIM work.
The authors thank Mr. M. Ice and Mr. R. Rodriguez for
the help of the diagnostic experiments and Ms. M. Sycha
for the preparation of the TEM samples.
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IV. SUMMARY
Nanocrystalline Ni powders produced by cryomilling
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follows:
(1) Thermal imaging results showed that the tempera-
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the conventional Ni powders for a given set of experi-
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