Study on stainless steel 316L coatings sprayed by high pressure HVOF
B. Sun and H. Fukanuma, Saitama/J
The conventional high-velocity oxy-fuel (HVOF) process has characteristics of high flame velocity and moderatetemperature, and is widely used to deposit cements, metals and alloys coatings such as WC-Co, nickel andstainless steel. In this paper, a high pressure HVOF system with combustion chamber pressure up to 3.0MPa, andwith characteristics of higher flame velocity and lower temperature was developed.
In-flight particle velocity was measured using the DPV-2000 system at combustion chamber pressures from 1.0 to
3.0MPa, and stainless steel 316L powder was deposited at a combustion chamber pressure of 3.0MPa. The
influence of spray conditions on the coating microstructure, deposition efficiency and micro-hardness were
investigated. It was shown that the combustion chamber pressure has significant influence on particle velocity.
Dense coatings composed of unmolten or partially molten particles could be deposited by varying the sprayparameters. In the experiment, deposition efficiency up to 90% was achieved at the optimized spray conditions.
1

Introduction

In the high velocity oxygen-fuel (HVOF) process,liquid or gaseous fuel and oxygen are injected into a
confined combustion chamber, then ignited andburned to produce heat and pressure. The velocity of
combustion products is accelerated by passing
through a convergent-divergent nozzle or aconvergent nozzle. The powder feedstocks are
injected into the combustion flame jet andsubsequently are heated and accelerated throughheat and momentum transfer from the flame jet.
Compared to other spray processes the HVOF
process has special features of higher particle velocity
and lower particle temperature, which give it asuperior ability in producing denser, more wearresistant and more corrosion- resistant coatings. It has
been widely used to deposit carbide-based cements,
metals and alloy coatings such as WC-Co, nickel andstainless steel materials [1, 2]. However, as a sprayprocess employed in atmosphere the ambient air
entrainment into the flame jet is inevitable whichusually causes in-flight particle oxidation [ 3 ]. The
oxidation may undesirably affect phase composition,
microstructure and performance of the depositedcoatings, especially for materials or applications
sensitive to oxidation [4, 5]. In addition, during the
HOVF spraying WC-Co cermet process, it iscommonly accompanied by the decarburization oftungsten carbide (WC), which results in the formationof dicarbide (W2C) and metallic tungsten (W) [6]. The
reduction of WC in the coating after the spraying will
degrade the wear resistance of the coatings [7, 8].
To improve the abovementioned shortcomings of the
HVOF process, A. Dolatabadi and Pershin designed aprotective shroud attaching to the end of the HVOF
nozzle, which can prevent ambient air from
penetrating the spray flame jet. The experimentresults showed that the protective shroud brings not
only a significant reduction in in-flight particle
oxidation but also a more uniform particle velocity andtemperature distribution [9 ]. In consideration of thecharacteristics of the cold spray process, recently
other approaches named warm spray or low
temperature oxy-fuel (LTOF) were reported [10, 11].
By injecting additional nitrogen gas or water into the

combustion chamber of the conventional HVOF torch,the flame jet temperature can be controlled at lower
range and thereby the particles can be heated to atemperature below the melting point but higher thanthat of cold spray. The warm spray or LTOF
processes covered the existing gap between theconventional HVOF and cold spraying, and provided
potential applications to produce coatings which are
difficult to be deposited by the cold spray process,such as WC-Co, titanium alloys and some superalloy
coatings [12, 13]. In the cold spray process the coatings are formed by
the high velocity impact of solid powders. Many
researchers have argued that only the spray particles
at a velocity higher than the critical velocity canachieve deposition, and higher particle velocity
enhances the deformation, adhesion and cohesion of
the particles [14, 15, 16]. As for the warm spray and
LTOF spray, the process operated in the temperature
range between HVOF spraying and cold spraying, thepowder materials remain unmolten during spraying. It
can be reasonably supposed that higher velocity
particles will form denser coatings with highperformance. However, there is no clear evidence
showing that higher particle velocity is obtained in thewarm spray or LTOF spray process.
In this paper, an approach aimed to accelerate thespray particle to higher velocity in partially molten or
un-molten state was proposed. A high pressure HVOF
system with combustion chamber pressure up to
3.0MPa, and with characteristics of higher flame
velocity and lower temperature was developed.
Stainless steel 316L particle was used as the spraymaterial. The influence of spray parameters on theparticle velocity, deposition efficiency, microstructure
and microhardness of the coating were investigated.
2

Experiment equipment and methods

2.1

High pressure HVOF equipment

In the one-dimensional two-phase flow (gas andparticles) the acceleration of the particle velocity canbe equated to the drag force on the particle:

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dVp
dt

Vp

dVp
dx

U
3
CD (Vg  Vp )Vf  Vp g
4
U p dp(Eq .1)

where, Vp: particle velocity, Vg :gas velocity, Cd: dragcoefficient, ǹp: particle density, ǹg : gas density
Therefore, for a given type of spray process and spray
powders, the particle velocity is determined by the gas
velocity, density and the acceleration distance in thebarrel. Higher gas velocity and density, as well as alonger barrel will contribute to accelerate the sprayparticle to higher velocity.
Considering the HVOF process as simplified onedimensional isentropic supersonic flow, gas velocity
within the convergent-divergent nozzle is a function of
only the stagnation gas temperature and the nozzle
geometry (Eq.2, Eq.3). Gas pressure does not affect
the gas velocity. However, examination of Eq.1, Eq.4,
Eq.5 and Eq.6 shows that the drag force on theparticle is linearly dependent on the stagnation gas
pressure. Therefore, gas pressure is a key factor in
determining the particle velocity.

k 1 2
M
2

T0
T

1

A
A*

1 ª§ 2 ·§ k 1 2 ·º2( k1)
M ¸»¸¨1«¨¹¼
M ¬©k  1¹©
2

U0
U§ k 2 ·k1¨1 M ¸© 2¹

(Eq. 2)k1

M

P0(Eq. 3)

1

Vg
kRTg

U0 RT0(Eq. 4)(Eq. 5)

O2 mole19.35
Fuel mole

2.2

Experiment materials and process

The spray material used in the experiment wascommercially available stainless steel 316L powder
with nominal size range of -44 +16 Pm (Micro-melt®
316L). The powder presents a spherical morphology(Fig. 1). A stainless steel 304 sheet with size of 50 x
50 x 3 mm was used as a substrate. The deposition
efficiency was measured by calculating the ratio
between the mass of sprayed powder and thedeposited mass onto the steel cylinders with adiameter of 260mm. Prior to spraying, the substrate
was sandblasted with #36 brown fused alumina grits.
In these studies, the stainless steel 316L particle
velocity was measured by the DPV-2000 system
under chamber pressures from 1.0 to 3.0MPa. The
influence of ǳ and spray distance on deposition
efficiency, microstructure and microhardness of thecoating were investigated. A comparison to the JP5000 system was also deduced.

(Eq.6)

where, k : ratio of specific heat, R: gas constant, Tg:
gas
local gas temperature, To: stagnation
temperature, U 0: stagnation gas density, U : local gas
density, Po: stagnation gas pressure, A*:critical area,
M: mach number
Based on the above analysis, a novel high pressure
HVOF torch with combustion chamber pressure up to
3.0 MPa was developed. This chamber pressure ishigher than the most commercial industry available
HVOF systems, which commonly operated below 1.0
MPa. Kerosene was used as fuel.
Particle temperature was controlled by lowering theflame jet temperature in this study. Different frominjecting additional nitrogen gas or water into the
combustion chamber, in this study, the flame jettemperature is controlled by adjusting the oxygen/fuel(O/F) ratio. Here we defined the normalized O/F ratio
as variableǳin Eq.7.

O

where, O2 mole is the mole quantity of the oxygen gas,
Fuel mole is the mole quantity of kerosene, and 19.35 is
the stoichiometric flow ratio of kerosene
Therefore, ǳ 㧪 1 corresponds to overstoichiometricconditions, indicating that excess oxygen gas flow
was injected into the chamber. Thermal expansion of
the unreacted free oxygen gas will exhaust reaction
heat energy which results a drop in the flame jettemperature. Thus, the flame jet temperature can becontrolled by varying ǳHigher ǳ results in lower
flame temperature and consequently lower particle
temperature.
Therefore, theoretically the particle velocity andtemperature can be controlled by varying the chamberpressure and ǳ.

(Eq. 7)

Fig. 1. Morphology of Micro-melt stainless steel 316L.
3

Experiment results

3.1

Particle Velocity

The influence of chamber pressure, ǳ and spraydistance on the particle velocity were shown in Fig. 2,
Fig. 3 and Fig. 4 respectively. The particle velocities
were measured at a stand-off distance of 380mm fromthe nozzle exit. Under high pressure HVOF

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increased from 100 to 200mm, and decreased withthe further increase of spray distance to 380 mm.
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conditions, the particle temperature can not bemeasured by the DPV-2000 system due to the low
particle radiation intensity.
The influence of combustion chamber pressure on theparticle velocity is shown in Fig. 2. The measurement
was conducted at ǳ = 2.0. With the increase ofchamber pressure from 1.0 to 2.0MPa, average
particle velocity increased greatly from 430 to 640m/s,
which is higher than that of the JP-5000 550m/s. With
the further increase of chamber pressure to 3.0MPa
the particle velocity increased slightly to 675 m/s. The
measurement results indicated that improvements toparticle velocity by higher chamber pressure areavailable.

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Fig. 4. Particle velocity as a function of spray distance(ǳ= 2.0, P = 3.0MPa)

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Fig. 2. Particle velocity distribution as a function ofchamber pressure (ǳ= 2.0 )The influence of ǳ on the particle velocity wasmeasured at a chamber pressure of 1.0MPa as shownin Fig. 3. Compared to the chamber pressure, ǳ
exerts no significant influence on the particle velocity.

Deposition efficiency

In the thermal spray process, the spray distanceshould be adjusted according to the type of sprayprocess, power, spray materials, substrate anddesired coating performance etc. Observation of thehigh pressure HVOF flame jet shows that the flame
temperature may turn out to be quite lower with theincrease of the ǳ . The flame jet may exert lessoxidation or distortion effect on the substrate ordeposited coatings. To investigate the effect of spraydistance, a wide spray distance range from 50 to 300
mm was adopted to deposit coatings at the chamberpressure of 3.0MPa. Fig. 5 shows the deposition
efficiency as a function of the spray distance at
different ǳ.
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Fig. 3. Particle velocity distribution as a function of ǳ
(P = 1.0MPa)
Fig. 4 shows the particle average velocity as afunction of spray distance at a chamber pressure of
3.0 Mpa and ǳ = 2.0. The particle velocity increased
from 705 to 730 m/s when the spray distanceFig. 5. Deposition efficiency (P = 3.0MPa)
At the short spray distance of 50 and 175mm, thedeposition efficiency increased with the ǳ andreached a maximum of 90% at the spray distance of
50mm. It is higher than that of the JP-5000 and isapproximate to the up-to-date cold spray process [17].

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However, the deposition efficiency presents a
decreasing trend as a function of ǳ at the spraydistance of 300 mm.
3.3

Microstructure and microhardness

The typical microstructure of JP-5000 and highpressure HVOF sprayed stainless steel 316L coatings
are shown in Fig. 6. The Ȝ increased from 1.0 to 3.0
under the chamber pressure of 3.0MPa at the spraydistance of 50mm. As an exception, the spraydistance was set at 300 mm when ǳ=1.0 to preventthe substrate from overheating due to the flame jet.

at the spray distance of 300mm in both ǳ=2.0 and
3.0. It is noticeable that compared to the Fig. 6b, theFig. 7a shows less oxidation in the coating. It can be
deduced that the oxidation appearing in Fig. 6boccurred after the particle impacted on the substrate
due to the short spray distance, as opposed to in-flightoxidation. Effective controlling of the substratetemperature during spraying may improve or eliminate
the oxidation in the coating.

(a) ǳ= 2.0, P = 3.0MPa (a) ǳ= 3.0, P = 3.0MPa
spray distance = 300mm spray distance = 300mm
Fig.7 Microstructure of the stainless steel 316L
coatings at different spray distances(a) ǳ1.0, P = 3.0MPa (b) ǳ= 2.0, P = 3.0MPa
spray distance = 300mm spray distance = 50mm

The microhardness of the coatings as a function of ǳ
is shown in Fig. 8. The coatings microhardness
decreased slightly with the ǳ and reduction in spraydistance was benefit to obtain higher coating
microhardness. The JP-5000 sprayed coating has a
microhardness of 445.
(c) ǳ= 3.0, P = 3.0MPa
spray distance = 50mm

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spray distance = 380mm

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Fig.6 Microstructure of the stainless steel 316L
coatings at different ǳ
At ǳ=1.0 (Fig.6a), the coating shows a near lamellar
structure indicate that the spray powders experienced
partial melting during spraying. The dark thin lamellae
between the particles are oxides. At ǳ = 2.0 (Fig.6b),
the particle deformation extent shows no significant
change compared toǳ = 1.0. However, the oxidation
inclusions between the interfaces of the particle
comparatively decreased. At ǳ= 3.0 (Fig.6c), except
for few fine pores at the interface of adjacent particles,
there are no visible oxidation inclusions in the coatingthat can be observed. It is noticeable that the particles
deformation extent at ǳ= 3.0 is much less than that ofǳ =1.0 or 2.0. The JP-5000 sprayed coating iscomposed of well deformed particles and it shows
typical lamellar structure (Fig.6d). Massive oxidation
inclusions indicate that the spray particles
experienced were well molten during spray process.
The coating microstructures sprayed at different spraydistances are shown in Fig. 7. Comparing the
microstructure of Fig.6b / Fig.7a and Fig.6c / Fig.7b
deposited at different spray distance, it can be foundthat visible pores appeared between particle interface

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Fig.8 Coatings microhardness
4

Discussions

4.1

Particle velocity

The particle velocity measurement results showedthat combustion chamber pressure exerts significant
influence on the particle velocity (Fig. 2). The
stainless steel 316L particle velocity of 675 m/s at thechamber pressure of 3.0MPa is comparable or evenhigher than that of the cold spray process [16].
Compared to chamber pressure, the ǳ gives nosignificant influence on particle velocity at a chamber

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pressure of 1.0 MPa (Fig. 3). Theoretically, reducing
ǳ will improve the particle velocity due to the higher
flame jet velocity. However, the combustion products
flow also decreased meanwhile with the reducing of
ǳ , which results in lower gas density andconsequently
lower
particlevelocity.
The
independence of particle velocity to ǳ provided afeasible solution to independently control the particle
velocity and temperature by merely varying thechamber pressure and ǳ . This method is moreconvenient and economic compared to adjusting thepowder injection location [18].
Short spray distance leads to improved deposition
efficiency in the high pressure HVOF process asshown in Fig. 8. For example, 90%, 84% and 63% at
the spray distance 50, 175 and 300mm respectively
when ǳ = 3.0. However, examining the particle
velocity it can be found that the particle velocity at
spray distance of 50 mm is lower than that at 175mm.
Therefore, the low deposition efficiency at long spraydistance can be attributed to the cooling effect to thespray particle because the lowering of particle
temperature results in an increase in the criticalvelocity. At the same time, it can be found that whenthe ǳ = 3.0, it exerts more significant influence on thedeposition efficiency than ǳ= 1.0 and 2.0. Since thehigher ǳ  corresponds to the lower particle
temperature, this experimental result is in accordancewith the cold spray process, which is commonly
operated at the short spray distance less than 50mm.
The deposition efficiency at the spray distance of 50
and 175mm increased slightly with the  ǳ . The
reason is not clear but may be related to the particle
oxidation film destruction during impact.
4.2

ǳ= 2.0. Correspondingly, the coating microhardness
at ǳ= 2.0 is slightly higher than that of ǳ=3.0 (Fig.
8). Compared to the cold spray, high pressure HVOF
shows the ability to the heat particles to include a wide
range of melting states. As for the JP-5000 (Fig. 9d),the splat and coating surface morphology indicate thatthe spray particles experienced a completely molten
state during spraying.
Other than ǳ, spray distance also influences on the
microstructure and microhardness of the coatings.
Coatings deposited at short spray distance trends toobtainhighermicrohardness
anddensermicrostructure. These influences may be attributed tothe cooling and deceleration effect on the particles
due to the penetration of air into the flame jet [19].

(a) ǳ = 1.0 spray distance: 300 mm P = 3.0 MPa(b) ǳ = 2.0 spray distance: 50 mm P = 3.0 MPa

Microstructure and properties

High pressure HVOF and JP-5000 sprayed stainlesssteel 316L coatings present different microstructure asshown in Fig. 6. The particle velocity measurement
results and the coating microstructures reveal thatparticle velocity is the key factor in forming dense
coatings. Although the particle temperature can not bemeasured in this studies, it is helpful to understandthe particle melting state by observing the particle
splat and coating surface morphology, as shown in
Fig. 9. At ǳ = 1.0 (Fig. 9a), the unmolten splat shows
sufficient plastic deformation and the coating iscomposed of a mixture of partially molten and molten
particle. It should indicate that a few numbers ofmolten or partially molten splats could be observed in
the low magnification microphoto at ǳ = 1.0. We candeduce that quite a number of particles have reachedmolten or partially molten state during in-flight at ǳ=
1.0. The splat deformation degree decreased whenthe ǳ increased over 2.0 and the coatings werecompletely composed of unmolten particles as shownin Fig. 9b and Fig. 9c. Compared to ǳ = 3.0, thecoating presents a more dense microstructure when(c) ǳ = 3.0 spray distance: 50 mm P = 3.0 MPa(d) JP-5000
Fig. 9 Splat and coating surface morphology
5

Conclusions

A high pressure HVOF system with combustion
chamber pressure up to 3.0MPa, and withcharacteristics of higher flame velocity and lower

DVS 276temperature was developed. Chamber pressure
exerts significant influence on the particle velocity.
The stainless steel 316L powders average velocity
reaches 675m/s at the chamber pressure of 3.0MPa.
The ǳ is the key factor in determining the particle
melting state. The ǳ and the spray distance havesignificant influence on the microstructure, deposition
efficiency and microstructure of the coatings. Dense
coatings composed of unmolten or partially molten
particles can be deposited at different sprayparameters. This process can also be expected todeposit high-performance carbide-based cermet,
metal and other alloy coatings.
6

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