Wear 257 (2004) 41–46

Influence of spray parameters on the particle in-flight properties and theproperties of HVOF coating of WC-CoCr

Lidong Zhao∗, Matthias Maurer, Falko Fischer, Robert Dicks, Erich LugscheiderMaterials Science Institute, Aachen University of Technology, Juelicher Street 344a, D-52070 Aachen, Germany

Received 22 April 2003; received in revised form 31 July 2003; accepted 31 July 2003

Abstract

Tungsten carbide-cobalt based spray coatings are widely used in industry for applications requiring abrasion, sliding, fretting anderosion resistance. High velocity oxy-fuel (HVOF) flame spraying has been used for producing high quality carbide coatings. In this study,a WC-CoCr powder was sprayed using a HVOF process. The spray parameters were varied to study their influence on the particle in-flightproperties and the coating properties using on-line particle monitoring. The wear behavior of the coatings was evaluated both by rubberwheel tests and by pin-on-disk tests. It was found that the total gas flow rate and the powder feed rate could strongly influence the particlein-flight properties under the spray conditions in the study. By contrast, the spray distance had less influence than the above two parameters.In general, the higher the total gas flow rate, the lower the powder feed rate and the shorter the spray distance, the higher the particlevelocity and temperature, and the denser and harder the coating. The rubber wheel tests showed that the total gas flow rate was of largesignificance for the wear resistance of the coatings.© 2003 Elsevier B.V. All rights reserved.

Keywords: HVOF; WC-CoCr; Coatings; Hardness; Wear

1. Introduction

Tungsten carbide-cobalt based spray coatings are widelyused in industry for applications requiring abrasion, sliding,fretting and erosion resistance. The hard WC particles in thecoatings lead to high coating hardness and high wear resis-tance, while the metal binder (Co, Ni, or CoCr) supplies thenecessary coating toughness. The tungsten carbide cermetspowder can be sprayed using different spray processes suchas conventional flame spraying, plasma spraying and HVOFspraying process. The coating properties are influenced notonly by the properties of the used powders but also signifi-cantly by the used spray process and spray parameters[1–3].

High velocity oxy-fuel (HVOF) flame spraying has beenan industrially established process since the mid 1980s. Thisprocess has been widely used for producing high qualitycarbide cermets coatings due to its moderate process tem-peratures and high gas velocities.

In recent years, the enormous development in process di-agnostics enables on-line particle monitoring[4–6]. Manysystems for on-line particle monitoring are commerciallyavailable. One of these systems is the DPV-2000 system.

∗ Corresponding author. Tel.:+49-241-166020; fax:+49-241-1660217.E-mail address: zhao@msiww.rwth-aachen.de (L. Zhao).

The DPV-2000 system enables the on-line measurement ofsurface temperature, velocity and diameter of single parti-cles in a spray plume by the principle of two-wavelengthpyrometry. In this study, a WC-CoCr powder with a sizedistribution of−45 +11�m was sprayed using HVOF pro-cess. The spray parameters were varied to study their influ-ence on the particle in-flight properties using the DPV-2000system. The corresponding coatings were studied in termsof their microstructure, hardness and wear behavior.

2. Experimental procedure

In this study, a commercially available WC-CoCr spraypowder with a size distribution of−45 + 11�m (WOKA9010Cr) was used as the starting spray powder. Commer-cially available mild steel St 37 was used as the substrate.The HVOF spray experiments were carried out using thecommercially available Sulzer-Metco DJ 2600 spray sys-tem. To investigate the influence of spray conditions on theparticle in-flight properties and coating properties, the sprayparameters were varied. The spray parameters are shown inTable 1.

During spraying, a commercially available DPV-2000system (made by TECNAR, Canada) was used to monitor

0043-1648/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/j.wear.2003.07.002

42 L. Zhao et al. / Wear 257 (2004) 41–46

the particle in-flight properties. The measured volume bythe DPV-2000 system is with about 0.5 mm3 very small.For analysis of particle properties, a cross-section of theparticle jet was initially scanned with the DPV-2000 sys-tem. Then, the spot with the maximum particle flow ratedetermined by the scanning measurement was selected asthe spot for a point measurement. The maximum number ofdetected particles was 2000 for a point measurement. Themeasured values for particle velocity and temperature arethe average values of detected particles.

Metallographical investigation was carried out using op-tical microscopy and scanning electron microscopy (SEM).The microhardness of the coatings (Vickers scale) was mea-sured on the prepared cross-sections of the sprayed coatingsusing a micro hardness tester made by Buehler Ltd., USA.The load used for measurements was 300 g. The porositymeasurements were carried out on the cross-sections of thecoatings using a picture analysis system NIBAS made byNikon. The measured area for a measurement was 370�m×250�m. The porosity given here is an average of eight mea-surements.

The pin-on-disk tests and rubber wheel abrasion tests werecarried out to evaluate the wear performance of the coatings.Pins of 14 mm diameter were coated on the top surface. Forwear tests, a disk covered with 400-SiC grinding paper wasused. The pins were held onto the surface of the grindingpaper. The pressure on a pin amounted to 0.03 N/mm2. Thesliding speed was 0.36 m/s. The mass loss of the pins wasmeasured every 25 m. The grinding paper was changed every25 m. The total sliding distance amounted to 200 m. Therubber wheel abrasion tests were carried out using a methodvery similar to ASTM G65-00. The samples of 40 mm×60 mm× 20 mm were coated. The coating thickness wasabove 300�m. The pressure was 300 N. The wear distancewas 2500 m. The surface speed of the wheel was 5 m/s. Theused quartz sand had a size distribution of 0.6–0.85 mm.

3. Results and discussion

The particle in-flight properties measured by theDPV-2000 system are shown inTable 2. It can be seen thatunder the spray conditions given inTable 1, the particle

Table 1Spray parameters

Coating O2 (l/min) H2 (l/min) Carrier gas (N2) (l/min) Feed rate (g/min) Spray distance (mm)

1 231 609 20 40 2902 231 609 20 40 2503 175 545 20 40 2904 228 652 20 65 2705 202 578 20 65 2706 202 578 20 100 2707 202 578 20 30 2708 204 636 20 80 2509 176 504 20 65 270

Table 2Particle in-flight properties measured by the DPV-2000 system

Coating Temperature (◦C) Velocity (m/s)

1 2121 5122 2145 5213 1956 4654 2116 4965 2031 4716 2003 4517 2061 4998 2016 4749 1967 436

temperatures were high, lying between 1956 and 2145◦C.The high particle temperatures were attributed to the axialpowder injection of the DJ 2600 spray system. It can be seenthat about 10% variation in the particle temperature couldbe caused by varying the spray parameters. The parametersfor coating 2 resulted in the highest particle temperature,while the parameters for coating 3 led to the lowest parti-cle temperature. Besides the parameters for coating 3, theparameters for coating 9 led also to a temperature below2000◦C. By contrast, all other parameters led to a particletemperature above 2000◦C. Under the given spray condi-tions, the particle velocity varied between 521 and 436 m/s.The parameters for coating 2 resulted in the highest particlevelocity, while the parameters for coating 9 led to the low-est particle velocity. The variation in the particle velocity ofabout 20% occurred by varying the spray parameters. It canbe seen the particle velocity was much more sensitive tothe variation of spray parameters than the particle tempera-ture. Comparing the results inTable 2, it can be found thatthe total gas flow rate, the powder feed rate and the spraydistance all influenced the particle in-flight properties, butto different degrees. With increasing the total gas flow rate,both the particle velocity and the temperature increasedstrongly. Comparing coatings 4 and 9, it can be seen thatthe particle velocity increased from 436 to 496 m/s and theparticle temperature increased from 1967 to 2116◦C, whenonly increasing the total gas flow rate from 680 to 880 l/minwhile keeping other parameters unchanged. The particlevelocity and temperature increased by about 14 and 8%,respectively. When decreasing the powder feed rate, the

L. Zhao et al. / Wear 257 (2004) 41–46 43

particle velocity and temperature also increased. Comparingcoatings 6 and 7, it can be seen that the particle velocityincreased from 451 to 499 m/s and the particle temperatureincreased from 2003 to 2061◦C, when only decreasingthe powder feed rate from 100 to 30 g/min while keep-ing other parameters unchanged. The particle velocity andtemperature increased by about 11 and 3%, respectively.When decreasing the spray distance, the particle velocityand temperature also increased. Comparing coatings 1 and2, it can be seen that the particle velocity increased from512 to 521 m/s and the particle temperature from 2121 to2145◦C, when only decreasing the spray distance from 290to 250 mm while keeping other parameters unchanged. Theparticle velocity and temperature increased only by about 2and 1%, respectively. Therefore, it can be said that in theselected parameter field, the total gas flow rate was the mostsignificant influence factor for the particle in-flight proper-ties. The powder feed rate also showed a strong effect on theparticle in-flight properties. By contrast, the spray distanceshowed a slight influence on the particle in-flight properties.

The metallographical investigation showed that all coat-ings had a similar coating microstructure.Fig. 1 shows across-section of coating 4 in a low enlargement.Fig. 2shows a SEM-photograph of this coating with a high en-largement. It can be seen that the coating is very dense andhas a very good contact with the substrate, indicating a verygood bonding to the substrate. The WC particles distributeuniformly in the coating. The sharp irregular shapes of thesmall WC-particles indicate that these WC-particles werenot molten during spraying. Therefore, it can be assumedthat the metal binder was partly or fully melted, while themost of the WC-particles remained in the solid state duringspraying.

The porosity data of the coatings are shown inTable 3.The porosity of the coatings varied between 4.0 and 0.2%.Coating 4 was the most dense, while coating 3 was the mostporous.Fig. 3shows the relationship between the measuredparticle temperature and coating porosity. It can be seen thatin general, the porosity decreases with increasing the parti-

Fig. 1. Cross-section of coating 4.

Fig. 2. SEM backscattered image of coating 4.

cle temperature. It can also been seen that below a certainthreshold particle temperature, the porosity is high. Whereasabove 2000◦C, the temperature does not have a significanteffect on the porosity any more. The porosity is low.Fig. 4shows the relationship between the measured particle veloc-ity and coating porosity. A clear trend as shown inFig. 3can not be seen inFig. 4. However, it can be seen that whenthe particle velocity is above 470 m/s, the porosity is low.Comparing the data inTables 1–3, it can be seen that the to-tal gas flow rate is of large significance for producing densecoatings under the given spray conditions. When the totalgas flow rate was not less than 780 l/min, the measured par-ticle temperature was over the threshold value and the ve-locity was over 470 m/s so that the dense coatings could beproduced.

The microhardness data of the coatings are also shownin Table 3. The microhardness of the coatings lies between928 and 1330 HV0.3. The variation in the microhardness wassignificant. Coating 2 had the highest hardness, while coat-ing 9 had the lowest hardness. It can be seen that only coat-ings 3 and 9 had a microhardness of less than 1000 HV0.3,while other coatings had a microhardness near or above1100 HV0.3. The influence of the particle in-flight properties

Table 3Porosity and microhardness data and results of pin-on-disk and rubberwheel tests

Coating Porosity(%)

Hardness(HV0.3)

Pin-on-diskmass loss (mg)

Rubber-wheelmass loss (mg)

1 1.2 1106 12 3782 0.5 1330 11 3833 4.0 976 12 4284 0.2 1248 11 3645 1.2 1107 12 3806 1.1 1089 12 3867 1.2 1081 11 3648 1.5 1075 12 3839 3.3 928 13 417

44 L. Zhao et al. / Wear 257 (2004) 41–46

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

1900 1950 2000 2050 2100 2150 2200

Particle temperature (˚C)

Fig. 3. Relationship between the measured particle temperature and coating porosity.

on the coating hardness can be seen inFigs. 5–6. In general,the coating hardness increases with increasing the particletemperature and particle velocity. The total gas flow rate isof large significance for coating hardness. When the totalgas flow rate was not less than 780 l/min, coatings had highhardness near or above 1100 HV0.3.

The results of the rubber wheel abrasion tests are alsoshown inTable 3. Fig. 7shows the relationship between themass loss and coating hardness. It can be seen that except

0

0,5

1

1,5

2

2,5

3

3,5

4

4,5

5

430 440 450 460 470 480 490 500 510 520 530

Particle velocity (m/s)

Fig. 4. Relationship between the measured particle velocity and coating porosity.

for the two coatings with lowest microhardness and highestporosity, the coatings had similar mass losses. Coatings 3and 9 had a mass loss of 428 and 417 mg, respectively. Themass loss of other coatings lay between 386 and 364 mg. Itcan be seen that the total gas flow rate is also of large sig-nificance for the wear resistance. When the total gas flowrate was not less than 780 l/min, a relatively constant wearresistance of the coating was realized. Comparing coatings6 and 7, it can be seen that the influence of the powder feed

L. Zhao et al. / Wear 257 (2004) 41–46 45

800

900

1000

1100

1200

1300

1400

1900 1950 2000 2050 2100 2150 2200

Particle temperature (˚C)

Fig. 5. Relationship between the measured particle temperature and coating hardness.

rate on the wear resistance is not as significant as the totalgas flow rate. About 6% mass loss reduction was realizedwhen reducing the feed rate by 70%. However, because thedecrease in the powder feed rate means the increase in spraytime and consumption of spray gases and thus, the produc-tion costs. Therefore, it can be seen that a control of thecoating hardness and wear resistance by the total gas flowrate is much more reasonable than by the powder feed rate.

800

900

1000

1100

1200

1300

1400

430 440 450 460 470 480 490 500 510 520 530

Particle velocity (m/s)

Fig. 6. Relationship between the measured particle velocity and coating hardness.

The results of the pin-on-disk tests are also shown inTable 3. It can be seen the mass losses of all coatings aftera sliding distance of 200 m were very low and similar. It laybetween 11 and 13 mg. It can be seen that it was difficult toevaluate the difference of the coatings under the given testconditions, because all coatings were very wear resistant inthe given circumstances. A correlation between the coatinghardness and the mass loss could not be found, either.

46 L. Zhao et al. / Wear 257 (2004) 41–46

300

320

340

360

380

400

420

440

800 900 1000 1100 1200 1300 1400

Hardness (HV0.3)

Fig. 7. Relationship between the mass loss and coating hardness.

4. Conclusions

In this study, the influence of spray parameters on theparticle in-flight properties and coating properties duringHVOF-spraying of WC-CoCr powder was investigated us-ing on-line particle monitoring. It was found that the sprayparameters such as the total gas flow rate, the powder feedrate and the spray distance influenced the particle prop-erties and the coating properties to different degrees. Ingeneral, the higher the total gas flow rate, the lower pow-der feed rate and the shorter the spray distance, the higherthe particle velocity and temperature. It was found that theparticle velocity was more sensitive to the spray parame-ters than the particle temperature. In general, the coatinghardness increased with increasing the particle temperatureand velocity and the coating porosity decreased. Underthe experimental conditions, the total gas flow rate showedmore influence than the powder feed rate, which again hadmore influence than the spray distance. The rubber wheeltests showed that the total gas flow rate was of large signif-icance for the wear resistance of the coatings. A control of

wear resistance by the total gas flow rate should be morereasonable than by the powder feed rate.

References

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