Spray-driedceramicpowders ...

1 Chemical Engineering Science 60 (2005) 95 102. Spray-dried ceramic powders: A quantitative correlation between slurry characteristics and shapes of the granules G. Bertranda, , P. Roya , C. Filiatreb , C. Coddeta a LERMPS UTBM, Site de S venans, 90010 Belfort cedex, France b LCMI UFC, 16 route de Gray, 25030 Besan on cedex, France Received 12 September 2003; received in revised form 9 December 2003; accepted 14 April 2004. Available online 11 September 2004. Abstract The characteristics and consequently the properties of ceramic coatings performed using plasma spraying means depend not only on the operating conditions but also on the powder feedstock. Oxide powders are commonly prepared in one stage or in a combination of stages of fusing, crushing, agglomerating and sintering.

2 The spray drying process (which corresponds to the agglomerating route) is a technique in which small constituent particles dispersed in water (which is called slurry) are sprayed in hot air and granulated thanks to a binder (latex or PVA). Spray drying is carried out for a variety of reasons, two major motivations being the production of composite and shape controlled powders for thermal spray applications. The aim of the present work was to establish a correlation between the slurry formulation and the characteristics of the spray dried granules for two oxide ceramics: Al2 O3 and Y2 O3 ZrO2 . Detailed studies on the dispersion and stability of the slurries were performed using sedimentation tests, electrophoretic mobility measurements as well as adsorption isotherms experiments.

3 Then, a drying experimental test based on the drying of a suspended droplet was developed to assess the drying mechanisms, identify the correlation between the slurry characteristics and the morphology of the dried granules and nally to predict the shapes of the spray-dried powders. It was shown that there is a qualitative relation between the sedimentation behaviour (as measured by the sediment ratio) and the granule shape (solid or hollow) and a quantitative relation between the thickness of the shell (for hollow granule) and the state of dispersion and the nature of the binder used. Finally, several powder batches were prepared in the spray-dryer which con rm the reliability of the drying simulation test.

4 2004 Elsevier Ltd. All rights reserved. Keywords: Spray drying; Ceramic powder; Hollow sphere 1. Introduction insulating coating, (Schulz et al., 2003). Plasma spraying is a complex process during which solid particles are in- Plasma sprayed oxide ceramics based on alumina, tita- jected into a plasma jet created either by a arc or a nia, zirconia, and so on are currently used as surface coat- eld, the material feedstock is molten and accelerated ing materials to improve resistance to corrosion, heat and in the high temperature region of the plasma, and then the wear for metal components and thus extend their service molten droplets spread onto the substrate to form a cohe- life (Normand et al.)

5 , 2000; Abdel-Samad et al., 2000; Wang sive coating. Among the numerous parameters that in uence et al., 2000; Jordan et al., 2001; Cao et al., 2004). For exam- the coating quality, the powder morphology has a great im- ple, 6 8 wt% yttria stabilized zirconia which is commonly portance (Allen et al., 2001; Friis et al., 2001). Two kinds deposited as top coating in thermal barrier coatings in the of ceramic powders are generally used: fused and crushed aeronautical industries has proved to be the most effective or agglomerated (spray-dried) and sintered. The latter ones have numerous advantages, and some of them are to present Corresponding author. Tel.: +333-8458-3240; fax: +333-8458-3286. spherical morphologies and homogeneous composition of E-mail address: (G.

6 Bertrand). different materials. 0009-2509/$ - see front matter 2004 Elsevier Ltd. All rights reserved. 96 G. Bertrand et al. / Chemical Engineering Science 60 (2005) 95 102. Ceramic spray drying which is the most versatile pow- dispersion, to nd out a quantitative relation between slurry der processing method consists in spraying a water-based behaviour and granule characteristics using a drying exper- suspension (called slurry) of the materials to agglomer- imental test and nally to show that the shape of the spray- ate into a stream of heated air. The rapid heat and mass dried ceramic granules can be predicted by slurry studies. transfer which occurs during drying combined with the This work was performed with two ceramic oxides, alumina presence of various slurry compounds result in dried and zirconia, and discussion will emphasize differences or granules having a large variety of shapes; from uni- similarities between both materials.

7 Form solid spheres which are regarded as ideal gran- ules for most ceramic systems to elongated, pancake, 2. Materials and experimental procedures donut-shaped, needle-like or hollow granules (Masters, 1985; Walton and Mumford, 1999). In the case of plasma An -alumina (P152SB, Aluminium Pechiney, France). spraying process, the powders requirements are free ow- and a 8 wt% yttria partially stabilized zirconia powder ing and narrow distribution size (Chuankian et al., 1984; (Saint-Gobain Ceramic Materials, France) were used in the Wigren et al., 1996). Thus the main characteristic of the present study. The physical characteristics of the different spray-dried powder is to have an external shape that is spher- powders are reported in Table 1.

8 Ical. It is well known that within such a shape two kinds of To achieve well-stabilised ceramic slurries, a commercial granule morphologies can be achieved by spray drying either polyelectrolyte an anionic dispersant the ammonium salt solid or hollow , and for the same hollow granules at least of polyacrylic acid (PAANH4 ), with an average molecular three types are identi ed: thin-walled fractured, thin-walled weight of 7000 8000 (Coatex , France) was added. dimple and thick-walled spherical (Lukasiewicz, 1989; A very strict mixing procedure was carried out. A spe- Minoshima et al., 2001; Liang et al., 2001; Charlesworth ci c amount of dispersant was rst added to distilled water and Marshall, 1960).

9 Previous studies have focused on the and stirred for 5 min. Then the oxide powder is gradually effect of the spray drying process conditions, such as the at- added and the suspension was stirred again for an additional omizer design, the drying temperature, etc. but also the feed 15 min. At that time the pH was adjusted with diluted solu- characteristics such as the initial solid content or the viscos- tions of HCl (1N) or NaOH ( ). Until equilibrium was ity of the slurry on the morphology of the granules (Walton reached, the suspension was continuously stirred (at least and Mumford, 1999; Lukasiewicz, 1989; Crosby and during 30 min). Finally a binder, either latex based on a Marshall, 1958).

10 At the same time, several works have dealt copolymer of styrene and acrylic ester (Rhodia, France) or with modelling the formation of the granule, with an empha- a hydro soluble polyvinyl alcohol PVA (Rhodia, France). sis on the drying stage (Minoshima et al., 2001; Liang et al., was added. 2001; Scherer, 1990). But, very few authors have discussed In order to produce spray dried ceramic powders, the on the importance of the slurry formulation on the character slurry must contain high solid content (from 15 vol% for of the ceramic granules and on the drying process while zirconia slurry to 30 vol% for alumina slurry) and be sta- noticing that it is a non-negligible factor. Recently, Takahashi ble during the process.