Correlation between the in-flight conditions of HVOF sprayed alloy particles and the coating structure T. Fukushima, H. Yamada, J. Kawakita and S. Kuroda, Tsukuba/J Korrelation der In-Flight-Partikeleigenschaften und der Schichtstruktur beim HVOF-Spritzen Pulver aus 316L rostfreiem Stahl wurde mit einer HVOF-Pistole unter verschiedenen Spritzbedingungen verarbeitet. Dabei kam auch eine im Labor entwickelte Einrichtung zum Einsatz, die die Oxidation des Pulvermaterials während des Spritzvorganges verringern sollte. Die Geschwindigkeit der Spritzpartikel wurde mittels einer DPV-2000-Messeinrichtung bestimmt. Der geschmolzene Anteil der Spritzpartikel wurde mittels einer im Labor entwickelten Messmethode bestimmt. Dabei wurde ein Gel verwendet, auf das die geschmolzenen Spritzpartikel auftrafen und so ausgewertet werden konnten. Die Porengrößenverteilung innerhalb der Schicht wurde mit einem Quecksilber-Porosimeter bestimmt. Die Inertgas-Fusionsmethode diente zur Bestimmung des Sauerstoffgehaltes. Es wurde festgestellt, dass sich die Porosität der Schicht mit zunehmender Partikelgeschwindigkeit verringert. Ab einer Parti-kelgeschwindigkeit von über 750 m/s wurde eine Porosität unterhalb der Nachweisgrenze des Luftdurchlässigkeits-prüfers festgestellt. Eine Wechselwirkung zwischen der Porosität in der Schicht und dem geschmolzenen Anteil der Spritzpartikel konnte dagegen nicht eindeutig festgestellt werden. Durch Kombination von Gasabschirmung und optimierter Rohrlänge sowie durch optimierte Einsatzbedingungen war es möglich, sehr dichte Schichten mit gerin-gen Sauerstoffgehalten von weniger als 0,2 Gewichts-% zu erzielen. 1. Introduction High Velocity Oxy-Fuel (HVOF) spray is a relatively new thermal spray technique, which propels powder material by a supersonic flame jet to velocity over 500 m/s. HVOF sprayed coatings are generally dense with less oxidation of feedstock materials as compared to coatings formed by other atmospheric thermal spray processes such as plasma spray or wire arc spray. Hence, their use in industry has been increasing in the fields of wear and corrosion resistant coatings. Porosity in sprayed alloy coatings is critical if the coat-ing is used as a corrosion barrier in the as-sprayed condition [1]. Oxidation is also important since it tends to degrade the coatings’ performances as corrosion barrier or TBC bond coat [2, 3]. One of the ultimate goals of thermal spray technology is therefore to form a dense (zero porosity) and clean coating (low oxida-tion). In order to achieve such a goal, however, it is necessary to understand how the conditions of sprayed particles, substrate and the surrounding envi-ronment affect the coatings’ porosity and oxidation. In a previous report [4], we investigated the oxidation of HVOF sprayed 316L stainless steel coatings by using a nitrogen-gas shield attached to the substrate. It was revealed that under the standard spraying con-dition oxidation during flight is around 0.2 wt% and oxidation on the substrate is about 0.1wt%, resulting in the 0.3 wt% oxygen content of the coating. Control of oxidation by attaching a gas shroud to the HVOF gun has been attempted and oxygen content below 0.15 % was achieved while maintaining the deposition effi-ciency over 73 %. The porosity in the deposit, how-ever, increased to over 2.5 %, which was conjectured to be due to the cooling of sprayed particles within the gas shroud. Other researchers also reported that gas shrouding is effective for controlling oxidation in HVOF thermal spray [5, 6].

In this paper, by combining a longer barrel (200 mm) with the gas shroud, we further investigate the correla-tion between the state of HVOF sprayed particles, i.e. velocity and molten fraction, with the density and oxy-gen content of the resultant coatings of 316L stainless steel. Realization of dense and low-oxygen content coatings by optimizing the operation conditions will be demonstrated.

2. Experimental A high-pressure HVOF spray gun (JP5000, TAFA, Concord, NH, US) was used to spray 316L stainless steel powder onto SS400 (JIS) mild steel substrates.

Table 1: Chemical composition of the spray powder and the substrate.

(mass%) SUS316L (TAFA1236F), Powder size:25-53µm Fe bal, Cr 16.8, Ni 10.8, Mo 2.05, N 0.131, O 0.026 Low carbon steel (SS400), Plate: 50x100x5mm Fe bal, C 0.11, Si 0.22, Mn 0.5, P 0.017, S 0.016

Table 2: List of spray conditions including gas shroud attachment. Ox (STD) Ne Re Fuel flow rate (l/min) 0.33 0.41 0.44 Oxygen flow rate (l/min) 860 670 600 Combustion pressure (MPa)

0.65 0.59 0.57

Barrel length (mm) 100, 200 Torch standoff (mm) 380 Torch traverse velocity (m/s)

0.7

Powder feed rate (g/min) 60 N2-1 flow rate (l/min) 1500, 2500 N2-2 flow rate (l/min) 450

The chemical composition and size of these powder and substrate are listed in Table 1 and the spraying conditions are given in Table 2. A schematic illustration of HVOF spraying with a gas shroud is shown, Fig.1 (a). A mixture of fuel (kero-sene) and oxygen is ignited in the combustion cham-ber and the resultant high-temperature gas flows through the converging-diverging nozzle and the bar-rel. The barrel is essentially a water-cooled straight copper tube, whose length is chosen depending on the nature of the spray powder. 100, 150 and 200 mm length barrels are available from the manufacturer and we compared the 100 and 200 mm long barrels in the present study. The gas shroud attachment already reported is used to inject an inert gas at the exit of the barrel in order to control oxidation of sprayed particles during flight, Fig.1 (b) [4]. It consists of a double-walled metal tube, which is water-cooled and possesses two inlet ports to introduce nitrogen gas. One port (N2 gas 1) is located at the entrance of the shroud and the other port (N2 gas 2) is located at the exit of the shroud. The length of the shroud is approximately 200 mm and the inner diameter at the entrance is 20 mm and 30 mm at the exit. In this study, the flow rate of the upper stream nitrogen was set at two levels of 1,500 and 2,500 sl/min whereas at the exit it was fixed at 450 sl/min. These values have been chosen based on some pre-liminary experiments showing that larger upstream flow rate is beneficial for suppressing in-flight oxida-tion. The location of a substrate with respect to the powder feed position is kept constant in the present study, regardless of the barrel length and use of the gas shroud. The spray distance 380 mm listed in Ta-ble 1 is the distance from the exit of the 100 mm bar-rel. In the case of 200 mm barrel with the gas shroud, therefore, the distance between the exit of the gas shroud and the substrate is only 80 mm. The flying velocity of HVOF sprayed particles was measured by an in-flight particles diagnostic instru-ment (DPV2000, TECNAR, Quebec, Canada) and the

molten fraction of sprayed particles was measured by the gel-capture method developed in the laboratory [7]. In this technique, sprayed particles were captured by an agar gel target located at the substrate position. Since molten particles or molten fraction of semi-molten particles were capture in the surface layer of the target whereas the solid particles and solid portion of semi-molten particles penetrate deeper into the gel, it is possible to separate these two and determine the weight ratio with respect each other. Since the two-color pyrometry is not reliable for HVOF sprayed alloy powders due to the change in the surface color caused by oxidation, this technique has been very useful for quantifying the degree of melting of sprayed particles. Pore size distribution within the deposits was meas-ured by a mercury intrusion (Autopore II 9220, Micro-meritics, USA)and the oxygen content was determined by the inert-gas fusion method. 3. Results and Discussion 3.1 Effects of barrel length and gas shroud on

the combustion flame and the conditions of sprayed powder

The appearance of combustion flames with the two barrels with and without gas shroud at the Ne condi-tion is shown, Fig.2. It is evident that the flame is elon-gated with the longer barrel and the gas shroud. The degree of elongation is less with the high flow rate 2500 sl/min of nitrogen, however. Molten fraction of sprayed powder at the substrate position was measured by the gel technique under various spraying conditions with and without the gas shroud, Fig.3 (a). Use of the longer barrel resulted in a higher molten fraction; the highest value over 60% at the Ne condition. Obviously this is due to the fact that a powder particle experiences a longer heating period within the barrel. As soon as the flame sprouts out of the barrel, cold air is vigorously mixed with the flame jet due to a strong vortex action [8]. The lowest molten fraction of approximately 5% was obtained at the standard spraying condition (Ox) specified by the manufacturer. Attaching the gas shroud and increas-ing the nitrogen flow rate lowered the molten fraction significantly. The velocity data are shown only for the Ne condition, Fig.3 (b). It is evident that the longer barrel generates particles whose average velocity is greater than the particles with the 100 mm barrel by approximately 100 m/s. Addition of the gas shroud with the nitrogen flow rate of 2500 sl/min has an effect to further accelerate sprayed particles by about 20 m/s. These are due to the extended acceleration region within the barrel and shroud. When the flow rate of nitrogen is not high enough, however, slight deceleration was observed with the 200 mm barrel. 3.2 Substrate temperature during spraying If the substrate temperature becomes too high during spraying, oxidation on the substrate becomes signifi-cant. Substrate temperature was measured by a ther-

(a)

(b)

Fig. 1. Schematic of (a) HVOF spraying with a gas shroud and (b) the detail of the nitrogen gas shroud attachment used to control the oxidation of HVOF sprayed particles during flight.

mocouple spot-welded to a substrate, which was heated by the HVOF gun without feeding powder but traveled exactly the same trajectory as in the coating fabrication, Fig.4. The plotted temperatures were re-corded at the steady state. Effect of nitrogen gas flow in reducing the substrate temperature is more pro-nounced with fuel-rich flames. This is because com-bustion is almost complete within the spray gun when the mixture is oxygen rich, whereas it is incomplete with fuel-rich mixture.

Fig. 2. Appearance of HVOF flames with various spray conditions and gas shroud.

Fig. 3. Effects of gas shroud and shroud gas flow rate on (a) the molten fraction and (b) the average velocity of sprayed particles at the substrate position.

Fig. 4. Effects of spray conditions and gas shroud flow rate on the steady state substrate temperature during HVOF spraying.

(a)

(b)

So, further combustion adds up heat when the com-bustion product exits to the ambient air, whereas no oxygen is added to it within the shroud and a large amount of cold nitrogen is mixed instead. With the cooling effect of the high nitrogen flow rate at 2500 sl/min, the substrate temperature could be controlled below 700K despite of the short distance between the shroud end and the substrate. 3.3 Characterization of splats and coatings

Effects of the barrel length and the shroud gas flow rate on (a) the splat morphology and (b) the coating cross section under the neutral flame condition (Ne) are shown, Fig.5. With the 100 mm barrel, the number of splashed splats clearly decreased with the gas

shroud. This is consistent with the significant reduction of the molten fraction from 42 % to 13% as shown in Fig.3 (a). With the 200 mm barrel, the molten fraction remained relatively high (over 40%) and hence the ratio of splashed splats did not decrease as much as with the 100 mm barrel. The cross sectional photos of the coatings obtained reveal that coatings with the shroud looks much more homogeneous and contain less oxides. The dark region is most significant with the 200 mm barrel without the shroud. The density of the coatings, however, cannot be evaluated from these photos. Coatings fabricated with the two types of barrel and the gas shroud are plotted in terms of the oxygen con-tent obtained by the inert gas fusion method and the total porosity as measured by the mercury porosime-

Fig. 5 (a). Effects of gas shroud and shroud gas flow rate on the morphology of splats

ter, Fig.6. As reported previously, use of the gas shroud resulted in significant increase in porosity with the 100 mm barrel. With the 200 mm barrel, however,

it was found that control of oxidation (0.2%) as well as zero porosity could be simultaneously achieved under the Ne and Re conditions with the shroud gas flow rate of 2500 sl/min. Such coatings are expected perform well for various applications such as a corrosion bar-rier in the as-sprayed form, for instance. 3.4 Discussion In order to examine the reason why coatings could be densified with the combination of the 200 mm barrel and the gas shroud, total porosity data were correlated with the average velocity of the sprayed particles and with the molten fraction, Fig.7 and Fig.8 respectively. For this purpose some additional experiments under the conditions not given in Table 2 were carried out to explore lower velocity regime with the 200 mm barrel with higher molten fraction. The results indicate that the velocity of the particles is the predominant factor for eliminating porosity. More specifically, average velocity over 750 m/s seems to be a critical condition for the powder used. From Fig.8 it appears that the

Fig. 5 (b). Effects of gas shroud and shroud gas flow rate on cross sections of sprayed coatings under the Ne flame condition.

Fig. 6. Summary of nitrogen gas shrouding in terms of total porosity and oxygen content. Ox, Ne, and Re correspond to different fuel-oxygen ratios shown in Table 2.

molten fraction of the particles is not so relevant but it is likely that particles need to be heated close to its melting point to enhance deformation at the impact onto the already deposited coating. Despite of the cooling effect on sprayed particles of the gas shroud, measured deposition efficiency re-mained over 73 % for all the conditions given in Table 2.

4. Conclusions Correlation between the state of sprayed particles and the properties of deposited coatings has been experi-mentally investigated. 316L stainless steel powder was sprayed by an HVOF apparatus with a barrel of different length and a gas shroud attachment was used to control oxidation. With such combination, it was possible to accelerate powder particles to a very high velocity over 750 m/s while avoiding excessive oxidation during flight and on the substrate. Results obtained are summarized as the followings.

1) With the 200 mm barrel, spray particles’ velocity was higher than that with the 100 mm barrel by about 100 m/s.

2) Molten fraction of particles was also significantly higher with the 200 mm barrel.

3) By attaching a gas shroud to the HVOF gun, oxi-dation could be controlled below 0.2 wt% for both the barrels. But zero porosity was only realized with the longer barrel.

4) With the 316L stainless steel powder tested, ve-locity over 750 m/s seems to a necessary condi-tion to achieve zero porosity, whereas the molten fraction did not show clear correlation with the po-rosity.

5. Acknowledgements The research reported here has been funded by the Frontier Research Project for the Structural Materials in the 21st Century (STX21). We deeply appreciate the continued support and encouragement from Dr. T. Kodama (NIMS). 6. Literature [1] Kuroda, S., T. Fukushima, M. Sasaki and T. Ko-

dama: Microstructure and corrosion resistance of HVOF sprayed 316L stainless steel and Ni base alloy coatings. Proc. ITSC 2000, Thermal Spray: Surface Engineering via Applied Research, C.C. Berndt, Ed., ASM International, pp.455/462.

[2] Moskowitz, L.N.: Application of HVOF thermal spraying to solve corrosion problems in the petro-leum industry. Proc. ITSC'92, May 1992, Or-lando, pp.611/618.

[3] Miller, R.A.: Thermal barrier coatings for aircraft engines: History and directions. J. Thermal Spray Technol. 6(1997), pp.35/42.

[4] Fukushima, T. and S. Kuroda: Oxidation of HVOF sprayed alloy coatings and its control by a gas shroud. Proc. ITSC 2001, New Surfaces for a New Millennium, C.C. Berndt, Ed., ASM Inter-national, pp.527/532.

[5] Pershin, V., J. Mostaghimi, S. Chandra, T. Coyle: A gas shroud nozzle for HVOF spray deposition. Proc. 15th ITSC, May 1998, Nice, pp.1305/1308.

[6]. Hacket, C.M. and G.S. Settles: Research on HVOF gas shrouding for coating oxidation control. Proc. 8th NTSC, September 1995, Houston, pp.21/29.

[7] Yamada, H., S. Kuroda, T. Fukushima and H. Yumoto: Capture and evaluat ion of HVOF thermal sprayed part ic les by a gel tar-get. Proc. ITSC 2001, New Surfaces for a New Millennium, C.C. Berndt, Ed., ASM International, pp.797/804.

[8] Hacket, C.M. and G.S. Settles: Turbulent mixing of the HVOF thermal spray and coating oxidation. Thermal spray, Proc. 7th NTSC, June 1994, Bos-ton, pp.307/312.

Fig. 7. Correlation between the total porosity of HVOF sprayed 316L stainless steel coatings and the average velocity of sprayed particles.

Fig. 8. Dependence of the total porosity of HVOF sprayed 316L stainless steel coatings on the molten fraction of sprayed particles