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Activated Combustion HVAF Coatingsfor Protection against Wear and High Temperature Corrosion
A. Verstak, V. Baranovski
UniqueCoat Technologies, Ashland, Virginia USA
Abstract
Activated Combustion HVAF Spraying (AC-HVAF) involvesa jet of air-fuel combustion products to deposit coatings of
metallic and carbide powders. In the process, spray particles
are heated below their melting temperature while acceleratedto velocity typically 700-850 m/s, forming a coating upon
impact with a substrate. Extremely low oxygen content and
high density are distinguished features of the AC-HVAF
coatings, resulting in their excellent performance under condi-
tions of severe wear and corrosion. Besides new level of
coating quality, the AC-HVAF process demonstrates outstan-
ding technological efficiency and spray rates 5-10 times
exceeding those of the HVOF counterparts. The paper presentsresults on characterization of selected metallic and carbide
coatings and describes their applications.
Introduction
Activated Combustion High-Velocity Air-Fuel process (AC-
HVAF) is recently developed technology for deposition of
metallic and metal-carbide coatings of commercial powdered
materials. The specific of the process is that spray powder
particles are heated below their melting point, while accele-
rated to velocity well above 700 m/s to form dense andpractically non-oxidized deposits with minimal thermal
deterioration. Thus, this is a solid particle spray technology
where particle temperature remains an important factor ofcoating formation. The process can be described as warm
kinetic spraying, positioned in between family of HVOFprocesses and Cold Gas-Dynamic Spraying [1-8] (Fig.1).
The AC-HVAF gun combusts a mixture of compressed air and
fuel gas (propane, propane-butane, propylene or MAPP-gas)
in original combustion chamber, generating high velocity jet
of combustion products exhausting out of cascade-type nozzle.
The combustion process is activated by a hot wall of thechamber, containing high temperature catalyst. Such design
provides stable air-fuel combustion within very short chamber,
allowing axial injection of spray particles through it. Secon-
dary fuel and air are introduced in the cascade nozzle
providing secondary combustion along its walls. Passing
through the chamber and cascade nozzle, spray particles aregradually heated to targeted temperature and accelerated to
velocity approaching that of the gaseous jet. Impacting asubstrate, the powder particles form a coating.
Figure 1: Comparison of spray particle temperature Tp and
velocity Vp for thermal spray processes.
Combustion of gases within the combustion chamber is aprimary source of spray particle energy. Secondarycombustion is used for fine regulation of particle velocity and
temperature. The nozzle length, type of fuel and consumptionof gases for primary (in-chamber) and secondary (in-nozzle)combustion are major technological factors of the process.
Absence of spray material fusion and high impact velocities
are distinguished characteristics of the AC-HVAF coating
deposition process.
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Equipment
The AC-HVAF process is realized with Intelli-Jet spray sys-
tem, designed and manufactured by UniqueCoat Technologies
LLC, Ashland, VA (USA). The system includes two basic
models of spray gun, SB-250 and SB-500 (Fig.2), fullyautomated control console equipped with a touch-screen
operator interface, necessary peripheral equipment (powder
feeder, fuel gas vaporizer, etc.).
Figure 2: SB-500 and SB-250 guns of the Intelli-Jet (AC-
HVAF) Spray System.
The SB-500 gun generates an equivalent of about 500 kW of
energy, combusting up to 248 SLPM (8.8 SCFM) of propane
and 7.8 m3/min (280 SCFM) of air at 7 bar (100 PSI). The gun
is capable spraying WC-based powders with productivity up
to 30 kg/hr (65 lb/hr), Cr3C2-based powders up to 18 kg/hr (40
lb/hr) and metallic alloys up to 22 kg/hr (48 lb/hr).
Appearance of the jet (Fig.3) is rather unusual: at spraydistance, the gaseous jet diameter is about 20 mm while
powder jet diameter (spray pattern) is only 6-8 mm.
Figure 3: Appearance of the AC-HVAF jet in spray process
The SB-250 gun consumes 65% of gases compared to the SB-
500. This smaller version of the gun operates at slightly higher
pressure, thus is hotter. It was specifically designed for
spraying carbides, as well as to operate in hand-held mode
Both guns work with single fuel gas source, such as propane
propane-butane, propylene or MAPP-gas. Start-up ignition is
provided by internal electric spark plug. Nitrogen is used as acarrier gas for powder. The guns are air-cooled. No other
gases or pilot flames are needed for the system operation.
The Intelli-Jet is intrinsically safe system. Indeed, since flame
propagation velocity in air-fuel gas mixtures is rather low(thousand-fold smaller than in oxygen-fuel mixtures), the
combustion is practically impossible outside of the
combustion chamber or the cascade nozzle. The danger of
flashback does not exist. Pressure in the combustion chamber
does not exceed 5 bar (71 PSI), what excludes safety problems
known for high-pressure equipment. Message alert or
automatic shutdown of the system is provided at abnormaoperating conditions.
Particle Temperature and Velocity
Particle surface temperature and velocity at spray distancewere measured with SprayWatch 2i optical equipment (Oseir
Ltd., Finland) for the SB-500 gun, operating with propane as a
fuel gas. Average data for alloy 625-type and WC-10Co-4Cr
powders are presented in Table 1, as well as in Fig. 4 and 5.
Table 1: Average particle velocity and particle surfacetemperature in a jet of the SB-500 gun, air-propane
combustion.
Powder
material
Particle
size, m
Particle
temperature,oC
Particle
velocity, m/s
Alloy 625 16-45 1180 810
WC-10Co-
4Cr
5-30 1285 775
Figure 4: Histogram of the WC-10Co-4Cr particle velocityVp in the SB-500 jet at spray distance 150 mm (6 inches);
air-propane combustion.
Vp, m/s
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Figure 5: Distribution of the WC-10Co-4Cr particle surface
temperature Tp across the SB-500 jet at spray distance 150
mm (6 inches); air-propane combustion.
Presented data revealed that the particle surface temperature
was 200-250oC lower than melting temperature of used
metallic alloys. This was a primary result of relatively low air-
propane combustion temperature. The particle velocity for
both powders was noticeably higher than that known for theHVOF processes. It is important to note that calculated
gaseous jet velocity was only about 900 m/s (i.e. lower than in
the HVOF spraying), proving high efficiency of the gun inaccelerating of spray powders.
Characterization of Coatings
Oxides and Porosity: Since spray particle is not fused and itsvelocity is very high, shortening the particle residence time in
the AC-HVAF jet, spray material oxidation is very limited in
the process. In metallic coatings, oxygen is present not in a
form of oxide scales but rather as dissolved gas. Thus,oxidation of material is not visible in coating micrographs
(Fig. 6). For instance, at standard spraying conditions total
oxygen content in the alloy 671 type (Ni-45Cr-1Ti) AC-
HVAF coating was only 0.20 wt.% (0.06 wt.% in powderstock). Due to high chromium content, this particular material
is prone to oxidation during thermal spraying. For comparison,
in different HVOF coatings of the same powder total oxygen
content varied from 0.95 to over 2.0 wt.%.
Usually, apparent metallographic porosity is hardly detectible
in the AC-HVAF coatings. Taking into account restrictions of
optical metallography, it would be correct to assume that suchcoatings as in Fig.6 might have porosity below 1.0%.
Figure 6: Micrographs of the alloy 625 type (a), alloy 671
type (b) and Cu-Ni-In (c) AC-HVAF coatings, x 100
Distance across the jet, mm
T ,oC
a)
c)
b)
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Bond Strength: Due to high-velocity impact of spray particles,
the AC-HVAF coatings provide high bond strength to metallic
substrates: over 65-75 MPa (10-12 KSI) for carbides and 45-
75 MPa (6.7- 12 KSI) for metals onto steel, cast iron and
superalloy substrates. The AC-HVAF metallic coatingswithstand impacts by a hummer or welding over without cra-
cking or delamination.
There are several characteristics of the AC-HVAF coatings,worth to emphasize regarding their mechanical properties:
a) Coating bond strength depends very little on their thickness,
indicating low level of residual stresses. For instance, bond
strength of the Ni-Si-B alloy coating to gray cast iron
remained 67-73 MPa (10-11 KSI) when the coating thickness
varied from 0.5 to 2.0 mm (20 to 60 mils).
b) Coatings reveal extremely high bond strength to Al-, Mg-
or Ti-based substrates known forming strong oxide scales,
which usually prevent good bonding of thermal spray
coatings. Stiff solid particles break oxides through, penetratinginto metal and forming strong bonds (Fig. 7). For instance, 4
mm (160 mils) thick austenitic stainless steel coating revealed
bond strength of 50 MPa (7.5 KSI) to aluminum (99%)
substrate.
Figure 7: Micrograph of the Stellite 12-type AC-HVAF
coating onto aluminum substrate (x 500), revealing deep
penetration of spray particles into aluminum surface.
Resistance to High Temperature Corrosion: Absence of oxide
scales and high density of the AC-HVAF coatings results in
their excellent performance in high temperature corrosionenvironment. In oxidizing (Fig.8) and sulfidizing (Fig.9)
environments the AC-HVAF coatings of alloy 625 and alloy
671substantially outperformed their counterparts sprayed with
electric arc and HVOF [9]. Beneficial factor was that the AC-HVAF coatings efficiently sintered and formed diffusion
zones with a substrate at elevated temperatures, then per-
forming as a solid metal. Such sintering and inter-diffusion
is restricted in other thermal spray coatings due to the oxide
scales are efficient barriers for diffusion.
5.638
5.279
3.982
0
1
2
3
4
5
6
ARC HVOF AC-HVAF
Weightgain,
mg/cm2
Figure 8: Weight gain of alloy 671 coatings sprayed with
electric arc, HVOF and AC-HVAF, after testing in N2-1%H2S-1%HCl gas at 400
oC during 1440 hours.
0
0.5
1
1.5
2
2.5
3
3.5
4
ARC HVOF AC-
HVAF
Stock
Weight
gain,
mg/cm2
alloy 671
alloy 625
Figure 9: Weight gain of alloy 671 coatings ands stoc
materials after testing in air at 700oC during 1000 hours.
Characteristics of Carbide Coatings: The AC-HVAF WC-based and Cr3C2-based coatings are very dense (Fig. 10, 11)
with little if any traces of oxidation or carbide therma
deterioration. The latter is a primary result of the spray particlelow temperature. Hardness of the WC-17Co, WC-12Co, WC
10Co-4Cr, WC-20Cr-7Ni, Cr3C2-25%(Ni-20Cr) and Cr3C220%(Ni-20Cr) coatings is similar or higher than for the HVOF
counterparts.
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Figure 10: Micrographs of the WC-10Co-4Cr AC-HVAFcoating, x 100 (a), X 500 (b).
Figure 11: SEM micrographs of the Cr3C2-25%(Ni-20Cr
AC-HVAF coating, x 1000Some specific properties of carbide coatings were found
during development of several commercial applications.
a) AC-HVAF carbide coatings allow achieving very highsurface quality when superfinishing. In particular, al
mentioned above coatings are routinely superfinished to
optical mirror range, i.e. better then Ra 0.012 micron (0.5inch). Figure 12 demonstrates 350 mm (14 inch) diameterroller with WC-10Co-4Cr coating (hardness 1250 HV300superfinished to surface roughness Ra 0.010 micron (Ra 0.4
inch). It was sprayed to thickness over 0.5 mm (20 milsproviding necessary resistance to impact and scratching by
tooling.
Figure 12: Appearance of the WC-10Co-4Cr AC-HVAFcoating superfinished to Ra 0.010 micron (0.4 inch).
b) The AC-HVAF coatings reveal outstanding crack resistance(fracture toughness). One of the methods for measuring of
fracture toughness coefficient K1C for brittle materials
involves indentation with Vickers pyramid and measuring of
induced crack length. Since K1C ~ (a/c)3/2
, when a>c (a is
diagonal of indentation, c is length of induced crack), the ratio
a/c can be used as crack resistance factor. In our tests the
crack resistance factor was found between 1 and 4 fordifferent HVOF coatings of WC-based and Cr3C2-based
materials (loading on pyramid was 300 g). Corresponding AC-
HVAF coatings revealed numbers in the range 100-200indicating dramatically improved crack resistance.
Besides improved coating performance, this fact has very
practical results in creating of new markets for thermal spray
In particular, improved coating crack resistance results in
possibility to apply very thick layers of carbides (Fig.13). The
AC-HVAF technology is relatively insensitive to surface
temperature during coating application, this way improving
coating quality consistency, etc.
a)
b)
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Figure 13: The 12.5 mm (0.5 inch) thick WC-12Co AC-
HVAF coating onto steel coupons.
Resistance to Elevated Temperature Erosion: Number of tests
was performed for estimation of elevated temperature erosion
resistance of AC-HVAF coatings, using a blast nozzle type
erosion tester. Erodent was the bed ash retrieved from CFBboiler (average hardness 780 HV100, average particle size 0.3
mm). Test conditions: erodent particle velocity 60 m/s,
sample temperature 300oC, impact angle 30 degrees, test time
5 hours, ash total load 375 grams. Table 2 presents some
principle results on different coatings thickness loss in
comparison with carbon steel (boiler tubing material).
Table 2: Coating thickness loss during elevated temperature
erosion testing.
No. Spray material Spray
method
Coating
thickness loss
during test,
m
1 Cr3C2-25%Ni-Cr HVOF 29
2 Cr3C2-25%Ni-Cr AC-HVAF 29
3 WC-10Co-4Cr HVOF 19
4 WC-10Co-4Cr AC-HVAF 11
Ref. AISI 1018 carbon
steel
185
According to presented results, erosion resistance of chrome
carbide based AC-HVAF coating was similar to HVOF
counterpart, while tungsten carbide AC-HVAF coating
outperformed HVOF coating of similar material.
Applications
In spite of the AC-HVAF technology is rather new process,
the coatings have already found their use in industry. To name
few, the following applications have proven reliable
performance of the AC-HVAF coatings:
a) Power generation:
- corrosion resistant coatings on furnace waterwall ofpulverized coal and waste-to-energy boilers;
- erosion-corrosion resistant coatings on waterwall ofcirculating fluidized bed combustion boilers;
- erosion-corrosion resistant coatings on coal impellers ofcircular burners;
- corrosion and high-temperature wear resistant coatingson critical components of gas turbines.
b) Steelworks
- thick wear resistant coatings on process rolls;- wear and corrosion resistant coating on sink roll in zinc
galvanizing;
- wear resistant coatings on hearth rolls of annealingfurnace;
- erosion-corrosion resistant coatings onto a hood ooxygen blowing furnace.
c) Pulp and paper- wear resistant coatings on dryer cans and calender rolls
of paper machines;
- corrosion resistant coatings on furnace waterwall andfloor of black liquor recovery boilers.
d) Food processing- erosion resistant coating on impellers of centrifuga
blowers.
e) Film making
- wear resistant and functional coatings on process rolls.
f) Textile
- wear resistant coatings on aluminum clutch hubs ofwiring machines;
- friction coatings on housing and hubs of brakes.
g) Hard chrome alternative coatings in general machinebuilding, printing, plastic extrusion and hydraulic components
Currently the AC-HVAF coatings are being tested to certify
for applications in aircraft industry and land gas turbines.
Summary
1. Activated Combustion HVAF is a new high-velocity spray
technology, depositing metallic and cemented carbide coatings
of heated but not fused powder particles.
2. For utilization of the AC-HVAF technology, Intelli-Jet
spray system is developed and commercialized. Equipment
characteristics include extremely high spray rates, reliability
and intrinsic safety.
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3. The AC-HVAF coatings revealed extremely low level of
oxidation, high density, reliable mechanical properties,
improved corrosion and erosion resistance.
4. The AC-HVAF coatings demonstrated outstanding
performance and found commercial applications in powergeneration, steelworks, pulp and paper, food processing, film-
making and other industries.
References
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