Suspension plasma spraying of ceramics by using an ink jet printer S. Kuroda and P. Blazdell, Tsukuba/J Suspensions-Plasmaspritzen von Keramiken mit einem Tintenstrahldrucker Für den Einsatz in einem Tintenstrahldrucker wurde aus YPSZ-nanosortiertem Pulver und einem organischen Lö-sungsmittel keramische Tinte hergestellt. Der Druckkopf konnte einen gleichbleibenden Spritzstrahl mit einer Teil-chengröße von ungefähr 100 µm erzeugen. Dieser wurde in einen Gleichstromplasmastrahl eingebracht, der bei verschiedenen Lichtbogenströmen und Gaskombinationen arbeitete. Es stellte sich heraus, dass die einzelnen Tröpfchen im Plasmastrahl explodieren und in noch feinere Tröpfchen aufgeteilt werden, wenn sie auf die Substrat-oberfläche auftreffen. Durch die Erwärmung der Substratoberfläche war es möglich, regelmäßig geformte „Splats“ zu erzeugen, die viel kleiner waren als die, die durch herkömmliches Plasmapulverspritzen erreicht werden. Mikro-risse innerhalb dieser „Splats“ wurden unter dem SEM untersucht. Dabei stellte sich heraus, dass in den „Splats“, die kleiner waren als der kritische Durchmesser von 5 µm, keine Risse auftraten. 1. Introduction It is well known that certain physical and mechanical properties of materials exhibit significant improve-ments as their grain size is reduced. Attractive fea-tures include lower temperature sintering and im-provements in hardness, ductility and dielectric proper-ties. Conventional plasma spraying processes nor-mally use a 10–100 µm sized feed stock to form the coating. When finer particles are used as a feedstock, coatings with finer lamella and improved properties were obtained [1]. However, it is not possible to use very fine ceramic powders as they have a tendency to flow badly or to agglomerate heavily [2]. Hence, if the solid particles traditionally used as a feedstock are replaced by a stream of droplets containing stabilized submicron ceramic suspension, further reduction in the grain size can be obtained. This may lead to supe-rior properties. A previous study [3] showed that by using a direct continuous ink jet printer it is possible to produce a stream of monosized, equi-spaced ceramic drops, which could be injected into the conventionally pro-duced plasma flame. This technique significantly re-duced the size of the splats produced. Another advantage of using suspension as a feed-stock is that one can control the chemical composition of the deposit easily. This has been demonstrated in an attempt to fabricate solid electrolyte [4]. This paper investigates the effect of the plasma condi-tion as well as the substrate temperature upon the splats formed in an attempt to optimize the process for coating formation. 2. Experimental Procedure 2.1 Preparation of ceramic ink and substrate Ceramic ink containing 2% volume of 5 wt % Y2O3 - ZrO2 (HSY3, Daiichi Kigenso, Japan) in butly acetate and 2 wt% of a commercial oligomeric dispersant was used for this study. The zirconia has an average parti-cle size of 0.1 µm and a specific surface area of 7 m2/g. The ink was prepared as detailed by Teng et al. [5]. Details of the zirconia, butyl acetate and dispers-

ant (KD1) have already been given by Teng et al. [6]. Instead of using an ultrasonic bath for dispersion of the inks, however, it was found that an ultrasonic probe [USP 600, Shimadzu, Japan] produced a better dispersion. 2.2 Ink jet printer and plasma spray configuration A continuous jet printer was used [Domino Inkjet, Ja-pan], Fig.1. The ceramic ink is forced under pressure (up to 0.5Mpa) through a 50 µm diameter nozzle. As the fluid passes through the gun body, a pressure wave is superimposed by a piezoelectric drive rod at a frequency of 64 kHz. This regulates the break up of the stream into individual droplets of approximately 100 µm. For observation purposes the stream was back-illuminated by a light emitting diode (LED) strobed in synchronization with the jet excitation fre-quency. Spraying was performed onto stainless steel sub-strates which had been polished using 0.1 µm alu-minia grit. The substrates were heated to the required

Fig. 1. The jet printer head.

temperature using a hand held gas burner. The tem-perature was recorded using a thermocouple attached just behind the front face. The stream produced was delivered into the hottest part of the plasma jet through a water-cooled copper pipe, Fig.2. This ensured that the turbulence induced in the stream of droplets by the plasma jet was kept to a minimum. A DC plasma torch (SG100, Praxair, USA.) was used to generate the plasma jet. Table 1 lists the operation conditions of the plasma torch. In a previous study, this set of conditions adding hydrogen as a secondary plasma gas was found to be highly effective to heat the ink droplets, remove the solvent and melt the ceramic particles [3]. Behavior of ceramic ink drops injected into a plasma

jet is schematically illustrated, Fig.3. When the plasma power is low, the surface layer of an ink drop is disin-tegrated due to rapid heating and the resultant tiny droplets are deprived of the solvent. Then, the ceramic particles in these fine droplets are molten but solidifies before they arrive at the substrate. Hence, a large splat of original drop surrounded by ceramic spheres is observed on a substrate. As the plasma power is raised, disintegration of the original drop further pro-ceeds toward the core. This results in a mixture of ceramic splats which were molten at the impact onto the substrate and ceramic spheres which were solidi-fied before the imact on the substrate. With pure Ar as the plasma gas, appearance of ceramic splats re-quires an arc current over 750A. Increasing the arc current over 1000A produced predominantly splats but addition of hydrogen is more effective for removing the solvent and provide ceramic droplets with sufficient heat to retain them in the molten state till they impact on the substrate. 3. Results SEM photos of the samples produced with various substrate preheating temperatures are shown, Fig.4. Using a substrate which had not been pre-heated showed an almost total lack of splats. The few splats which were produced were poorly formed and the central area of splats often seemed have dropped off possibly due to poor adhesion to the substrate. How-ever, there is no indication of the original submicron ceramic material suggesting that the thermal energy supplied by the plasma was sufficient to remove the solvent from these small drops and melt the ceramic

Fig. 2. Experimental configuration.

Fig.3. Behavior of ceramic ink drops during plasma.

Table 1. List of operating conditions for the plas-ma torch.

Primary gas, Flow rate

Ar, 45 l/min

Secondary gas, Flow rate

H2,

10 l/min Arc current 600A Arc voltage 57V

powder. Preheating the substrate to 373K produced a similar microstructure to that of the cold substrate. The splats produced in this case were slightly more circular than the previous sample but were still poorly formed. Preheating to 483K increased the circularity of the splats. For this sample a number of small ceramic spheres are visible on the surface. Preheating the substrate to 573K produced a marked improvement in the splat circularity but some degree of splashing was

visible on the surface which indicates that the sub-strate temperature was insufficient to allow the molten ceramic to spread on the surface before it fully cooled. Increasing the preheat temperature to 673K noticeably reduced the surface splashing. Increasing the preheat temperature to 733K produced predominantly circular splats. Our previous study showed that a droplet disin-tegrates in the plasma due to boiling of the solvent and explosion into a large number of smaller droplets. The

no preheating 373K

483K 573K

673K 733K Fig. 4. Effects of substrate temperature on the splat morphology.

55mm, 493K 45mm, 803K

Fig. 6. SEM photos of the coating surface sprayed onto substrates with different spray distance and prehea-ting temperature.

diameter of each splat in the photo at 733K was measured and its frequency distribution is shown, Fig.5. The majority of splats are below 5 µm and nu-merous particles below 1 µm were observed. Since there is little sign of splashing of droplets on the sub-strate in this sample, this size distribution should re-flect the disintegration of the original droplet in the plasma jet. Another important point to notice is than larger splats are segmented by microcracks into pieces of several microns whereas splats smaller that several microns are mostly intact. This fact seems to indicate that there exists a critical splat size below which splats do not crack. Cracking is believed to be due to the tensile

stress which develops within a splat during cooling after solidification because the thermal contraction of ceramic is constrained by the underlying substrate [7]. In conventional powder plasma spray, zirconia splats are mostly cracked because their size is above this criterion. How this size depends on the ceramic mate-rial as well as the substrate is a subject of great inter-est. Preliminary runs to form coatings were carried out. To investigate the effect of spray distance on coating formation a coating was deposited by over spraying 40 passes of a substrate at a linear scan velocity of 100 mm/s. SEM photos of the surface of the coatings ob-tained are shown, Fig.6. The top surface of a coating produced at a substrate distance of 55mm (recorded pre-heat temperature 493K) consisted of a number of discrete splats which had not been densified by the plasma. Conversely, using a substrate distance of 45mm (recorded pre-heat temperature 803K) pro-duced a much denser surface where considerable evidence of sintering and densification was visible. Such short spray distance insured that the majority of ceramic droplets are still molten when they hit the substrate and the temperature of the substrate to be sufficiently high throughout the spraying period.

4. Discussion In plasma spraying, a molten particle strikes the sub-strate at high velocity. Its kinetic energy is transformed into viscous flow, thermal and surface energy on im-pact and the droplet spreads, cools and solidifies. This process has been a subject of intensive study recently by theoretical analysis as well as experimental investi-gation. A numerical study which takes account of the spreading as well as the heat transfer and solidifica-tion during this process showed that under normal plasma spraying conditions (typically a molten droplet with 100 µm diameter impinging onto the substrate at a velocity of 100m/s), spreading will be essentially completed before the onset of solidification [8]. Transition of splat morphology with respect to the sub-

0 5 10 150

20

40

60

230

240

250

260

270

no cracking cracked

Num

ber o

f spl

ats

Splat diameter (µm)

Fig. 5. Size distribution of splats produced on a substrate preheated to 733K.

strate temperature has been studied systematically under conventional plasma spray conditions by using Ni powder sieved to a narrow size range around 45 µm [9]. It was found that the shape of splat changes from a star-shape with a small core at the center and spreading fingers around it to a circular disk-shape as the substrate temperature is raised. Also, the lateral dimension of the star-shaped splats is much larger than the disk-shaped ones. Since this transition occurs within a narrow temperature range less than 100K, transition temperature Tt was defined at which the ratio of the two types of splats is 1:1. It was conjec-tured that below Tt , solidification starts within the droplet in contact with the substrate at an early stage of spreading and the solidified portion acts as a barrier for the molten metal to spread smoothly, resulting in jetting away from the center to form disintegrated fin-gers. In the present study, circular disk splats were formed at a substrate temperature above 673K. Below this temperature, most splats were irregularly shaped. The spreading behavior of liquid droplet can be character-ized by two parameters, i.e., Reynolds number Re and Weber number We.

Re=ρvD/η, We= ρv2D/γ

where v is the velocity, D the diameter of the liquid droplet and η is the viscosity, ρ the density, and γ the surface tension of the liquid. It is apparent that assum-ing the droplet velocity to be the same, reduction in the droplet size results in reduction of these two parame-ters. Even though we have not measured the droplet velocity for the present study, it is unlikely that the impinging velocity to exceed 300 m/s, a measured flow velocity of the plasma gas itself at the present substrate position [10]. However, as observed, size of the droplets in the current experiment is an order of magnitude smaller than conventional powders. This implies that the driving force for spreading (kinetic energy) is much smaller in comparison with the vis-cosity and surface tension. In other word, the present experiment is analogous in terms of fluid flow to con-ventional sized powder impinging at a significantly low velocity. Therefore, it is more likely that solidification interferes with spreading. At a low substrate temperature large thermal gradients will be set up between the substrate-splat interface and the top surface of the splat. This will result in rapid cooling of the splat which will prevent the molten mate-rial from flowing and spreading into the disk shaped splat. As the substrate temperature is increased the temperature gradient between the splat-substrate interface and the top surface of the splat decreases and the wetting of the substrate by the molten droplet is improved. This will allow a droplet to remain molten for a longer time which facilitates more comprehensive flow over the surface of the substrate. This picture agrees with the fact that the degree of spreading in-creases with the raising substrate temperature in the present study. The effects of substrate temperature are also ex-pected to be prevalent in coatings. As is seen in Fig.6 increasing the substrate temperature causes the

splats to flow much more comprehensively and to loose their individuality. This may be more prevalent when using grit blasted substrates as the spats pro-duced using a low substrate temperature may not be able to flow comprehensively into the valleys of the grit blasted topography. This will severely affect the final coating properties such as adhesion to the substrate. 5. Conclusions A stream of mono-sized ceramic suspension has been successfully fed into a plasma jet and the operating condition of plasma as well as the substrate tempera-ture to obtain regularly shaped splats were established. Deposition onto a low temperature substrate produced poorly shaped splats. Increasing the substrate tem-perature above 673K produced circular splats and at 733K splats attained almost complete regularity. Size distribution study of the circular splats confirmed disin-tegration of original droplets within the plasma and revealed the existence of critical size for crack forma-tion within splats. Coatings produced at a low sub-strate temperature showed a poorly densified surface. Coatings produced using a higher substrate tempera-ture resulted in denser, more coherent structure. 6. Acknowledgements We acknowledge Mr. T. Kanno for his skilful design and manufacturing of a crucial component for the ex-periment and Mr. T. Awane for his assistance in SEM. 7. Literature [1] Notomi, A. and N. Hisatome: Application of plasma spraying to

solid oxide fuel cell production. Pure and Appl. Chem., 68 (1996), pp.1101/ 1106.

[2] Lange, F.F., B.I. Davis and I.A. Aksay: Processing-related fracture origins 3. Differential sintering of ZrO2 agglomerates in Al2O3/ZrO2 composite. J. Am. Ceram. Soc., 66 (1983), pp.407/408.

[3] Blazdell, P and S. Kuroda: Plasma spraying of submicron ceramic suspensions using a continuous ink jet printer. Sur-face and Coat. Technol., 123(2000), pp.239/246.

[4] Gitzhofer, F., M-E Bonneau and M.I. Boulos: Double doped ceria electrolyte synthesized by solution plasma spraying with induction plasma technology. Thermal Spray 2001: New Sur-faces for a New Millennium, 2001, C.C. Berndt et al. Ed., ASM Int., pp. 61/68.

[5] Teng, W.D. and M.J. Edirisinghe: Development of ceramic inks for direct continuous jet printing. J. Am. Ceram. Soc, 81(1998), pp.1033/1036.

[6] Teng, W.D., M.J. Edirisinghe and J.R.G. Evans: Optimization of dispersion and viscosity of a ceramic let printing ink. J. Am. Ceram. Soc, 80 (1997), pp.486/494.

[7] Kuroda, S. and T. W. Clyne: The quenching stress in thermally sprayed coatings. Thin Solid Films, 200(1991), pp.49/66.

[8] Trapaga, G., E.F. Matthys, Valencia and J. Szekely: Fluid-flow, heat-transfer, and solidification of molten-metal droplets im-pinging on substrates – Comparison of numerical and experi-mental results. Met. Trans. B, 23B(1992), pp.701/718.

[9] Fukumoto, M. and Y. Huang: Flattening mechanism in thermal sprayed nickel particle impinging on flat substrate surface. J. Thermal Spray Technol., 8(1999), pp.427/432.

[10] Vardelle, M., A. Vardelle, P. Fauchais and M.I. Boulos: Plasma-particle momentum and heat transfer: modelling and meas-urements. AICh EJ, 29(1983), pp.236/243.