Manuscript ITSC 2003

8/6/2019 Manuscript ITSC 2003

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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

1. M.R. Dorfman, Thermal Spray Basics, AdvancedMaterials & Processes, 160, No 7 (2002), 47-50.

2. H. Kreye, A Comparison of HVOF Systems Behavior of Materials and Coating Properties,

Proceedings of the 4th

HVOF Colloquium, Erdin,

Bavaria, Germany, November 13-14, 1997, 13-21.3. J.G. Legoux, B. Arsenault, C. Moreau, V. Bouyer,

and L. Leblanc, Evaluation of Four High Velocity

Thermal Spray Guns Using WC-10Co-4Cr Cermets,

Proceedings of the International Thermal SprayConference, Montreal, Quebec, Canada, May 8-11,

2000, 479-486.

4. T.C Hanson, C.M. Hackett, and G.S. SettlesIndependent Control of HVOF Particle Velocity and

Temperature, J. of Thermal Spray Technol., 11, No

1 (2002), 75-85.

5. D.L. Gilmore, R.C. Dykhuizen, R.A. Neiser, T.JRoemer, and M.F. Smith, Particle Velocity andDeposition Efficiency in the Cold Spray Process, J

of Thermal Spray Technol., 4, No 8 (1999),.576-582.

6. T. Stoltenhoff, J. Voyer, and H. Kreye, ColdSpraying State of the Art and Applicability

Proceeding of the International Thermal SprayConference, Essen, Germany, March 4-6, 2002, 366-

374.

7. A.N. Papyrin, V.F. Kosarev, S.V. Klinkov, and A.PAlkimov, On the Interaction of High Speed Particles

with a Substrate under the Cold Spraying

Proceeding of the International Thermal Spray

Conference, Essen, Germany, March 4-6, 2002, 380-384.

8. J.A. Browning, The Magic Box? Spraytime, 7, No4, (2000), 19.

9. A.A. Verstak, and V.E. Baranovski, Dynamic AirDeposited Coatings for Power and Black LiquorRecovery Boilers, Corrosion99, NACE

International, Paper No 74 (1999).

Manuscript ITSC 2003

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