Granulation of Nano Alumina Powder for Improved Flowability by Spray Drying

Transcription

1 DOI /s TECHNICAL PAPER TP 2579 Granulation of Nano Alumina Powder for Improved Flowability by Spray Drying S. Sathish M. Geetha A. Udayakumar S. Senthil Kumar R. Asokamani Received: 9 April 2012 / Accepted: 24 July 2012 Ó Indian Institute of Metals 2012 Abstract This paper reports on the methodology adopted for obtaining near spherical agglomerated Al 2 O 3 powders which enables them to be coated by plasma spray technique on titanium alloys. An attempt was made on spray drying alumina slurries with polyethylene glycol as binder and sodium hexametaphosphate as a dispersant. The effect of ph on the stability and viscosity of the slurry was established and also the rheological behavior of the slurry was studied in detail. Uniform agglomerated particles were obtained by spray drying the slurry containing 22 vol% alumina, 6 wt% polyethylene glycol and 0.02 wt% sodium hexametaphosphate as a dispersant. Keywords Alumina Polyethylene glycol Sodium hexametaphosphate Spray drying 1 Introduction Alumina (Al 2 O 3 ) coatings having superior hardness, chemical stability and refractory character are commonly utilized to resist wear caused by friction and solid particle erosion [1, 2]. As a surface modification technique, atmospheric plasma spraying (APS) has been well-established to deposit various ceramic coatings [3 5]. Recent works on nanoceramic coating shows enhanced S. Sathish M. Geetha (&) R. Asokamani School of Mechanical and Building Sciences, VIT University, Vellore , India geethamanivasagam@vit.ac.in A. Udayakumar S. Senthil Kumar Materials Science Division, Council of Scientific and Industrial Research National Aerospace Laboratories, Banglore , India tribological behavior when compared to the conventional micron size powder coatings [6 9]. However, it is difficult to achieve good flowability using plasma spray gun. The sub-micron/nano sized ceramic powders (\1 lm) cannot be sprayed by particle injection into the plasma as it lacks the momentum necessary to penetrate into the plasma or to impinge on the surface while the plasma sweeps over the substrate. In order to fulfill plasma spray process requirements, it is essential to have suitable particle size, morphology and density. Hence to fulfill these requirements, it has been found that the powder should consist of dense agglomerated particles in the size ranging from 10 to 20 lm in diameter with bulk density of around 1.7 g/cc [10]. Hence, it becomes necessary to process the procured nano particles to achieve an appropriate size range. This is easily accomplished by spray drying the slurry consisting of appropriate concentration of solid loading of alumina particles, dispersant and the binder. Spray drying is a process by which a fluid feed material (slurry) is transformed into a dry powder by spraying the feed into a hot medium. Spray dried powder is known for its spherical morphology as well as free flowing nature which allows the ease with which the plasma spraying process can be effected. Betrand et al. [11] have extensively studied the influence of dispersant level, ph and binder addition on the slurry characteristics and observed that the flocculated slurry leads to solid granules while the dispersed suspension leads to the formation of hollow granules. Walker et al. [12] have shown the correlations between the aqueous alumina slurry formulation, slurry yield stress and granule characteristics and their studies revealed that high deflocculant level which corresponds to low slurry yield stress leading to the formation of hollow granules. Cao et al. [10] have obtained highly dense agglomerated powders with larger particle size and good

2 flowability using polyethyleneimine (PEI) as a binder and dispersant and also confirmed that the suspension preparation is the controlling factor for the properties of spray dried alumina powders. Most of these works were based on polyvinyl alcohol (PVA) binder and poly acrylic acid dispersant. But it is also evident from these works that PVA reduces the density of the spray dried powder. Hence, the objective of this work is to agglomerate Al 2 O 3 powders by spray drying using polyethylene glycol (PEG) binder and sodium hexametaphosphate dispersant. N 2 Inlet CR N SC Air Inlet HE 2 Experimental Procedure 2.1 Slurry Preparation C The present experiment used alumina powder having an average particle size of lm, procured from Alcoa (grade A16), polyethylene glycol 400 (Rolex make) and sodium hexametaphosphate (LOBA Chemie make). In order to obtain appropriate agglomeration, experiments were performed using various volume percentages of the solid particles (Al 2 O 3 ), binders (PEG) and dispersant (NaPO 3 ) 6. These ingredients were ball milled in aqueous medium for about 48 h. The ph values of the slurries were adjusted with standard analytical grade HCl or NaOH solutions so as to obtain stable slurry suitable for spray drying. The viscosity of the slurries was measured using Brookfield rotational viscometer (DV-III ULTRA, Brookfield, USA) at a shear rate of 51 s -1. Further, sedimentation experiments were conducted to study the effect of ph and solid loading. Relative settling height (RSH) was measured using the graduated cylindrical jars. The slurries of different ph were prepared and filled in different jars. Initial height of the slurry was noted and subsequent heights were measured with respect to time. 2.2 Spray Drying Spray drying was carried out with the help of co-current spray dryer (Basic Technology Pvt. Ltd; Kolkata, India). The spray drying apparatus consists of a peristaltic pump which feeds the slurry into the stream of hot gases allowing a rapid evaporation of the liquid and leaving behind the agglomerated alumina particles (Fig. 1). The hot air inlet and outlet temperatures were kept at around 300 and 100 C respectively. The spray dryer essentially consists of pneumatic spray gun with 700 lm nozzle diameter to atomize the slurry. The slurry feed rate was maintained around ml/min and spraying was performed with Nitrogen gas compressed at a pressure of 0.9 to 1.2 bar. Feed Slurry 2.3 Sintering P The agglomerated spray dried powder was collected from cyclone separator and sintered at high temperature. The dead sintering was carried out on the spray dried powders at a temperature of about 1,300 C for about 4 h to retain the spherical morphology and improve the handling strength. 2.4 Characterization Coarse N 2 Inlet - Nitrogen gas Inlet CR - Compressor N - Nozzle SC - Spray chamber C - Cyclonic filter HE - Electrical heater P - Pump Fig. 1 Schematic diagram for spray drying apparatus Fine The powder morphologies of the spray dried and sintered powders were observed using scanning electron microscope (JEOL JSM-360). Phase analysis was performed on these powders using X-ray diffractrometer (Philips 3121) with CuK a radiation. The current and voltage were set at 40 kv and 20 ma and the data were collected in the 2h ranges in a step scan mode with a step of 2 /min. Also the particle size of these powders was established by particle size analyzer (Sedigraph 5100, M/s Micromeritics, USA make). Bulk density of the powder is determined by measuring the weight of the volume of powder with a graduated

3 cylinder which is gently tapped until a constant volume is obtained. Flowability studies were carried out using hall flow meter as per ASTM B standard. Initially 50 g of Al 2 O 3 powder without moisture is weighed and allowed to flow inside the funnel. The time taken for the flow of 50 g of powder is noted down. This gives the flowability of the powder. 3 Results and Discussion High solid loading is an essential factor for preparing highly dense agglomerates. Due to the relatively higher surface tension, the solid loading in water is high. In this context, water is preferred over other solvents for slurry preparation. From the preliminary sedimentation experiments on alumina water system, it was understood that very high Al 2 O 3 content ([25 vol%) destabilizes the slurry due to flocculation. Hence, the Al 2 O 3 content in the slurry was maintained around 22 vol%. As the viscosity increases with binder content, it is crucial to maintain the binder content as minimum as possible. It was also evident from our initial trials that the slurry ceased to flow during spraying due to the high binder content (10 wt%). Accordingly the binder content was kept at 6 wt% with respect to Al 2 O 3 content for later experiments. The optimized parameters used for spray drying are given in Table 1. PEG was considered as binder for this study due to the fact that PVA has an adverse effect by reducing density and particle size of spray dried powder [13, 14]. Fig. 2 Effect of ph on RSH hexametaphosphate (dispersant) for settling studies. Figure 2 depicts relative sediment height (RSH) with respect to ph of the slurry. It was noticed that RSH remained almost stable at lower ph (\3.85) up to 10 h. The slurry with ph of 3.85 displayed higher RSH value due to flocculation. Similarly the slurries with higher ph ([5.24) behaved like viscoelastic colloidal gels and got dried over time. Accordingly it is preferable to maintain ph below Moreover, slurries with high RSH values are advantageous in achieving dense agglomerates which in turn 3.1 Effect of ph At lower solid loading, the suspension is less dense which make the particles to behave as if they are isolated. But in the case of highly loaded slurries, interaction between particles provokes the influence of ph on sedimentation behavior [15]. In addition, the binder also changes the state of dispersion of the suspension (stable or unstable) hence, it is essential to consider slurry with binder for settling studies. Consequently, 22 vol% Al 2 O 3 slurry was prepared with 6 wt% PEG (binder) and 0.02 wt% sodium Table 1 Optimized parameters for spray drying Slurry composition and spray drying parameters Alumina 22 vol% Polyethylene glycol 6 wt% Sodium hexametaphosphate 0.02 wt% Air entry temperature 300 C Air exit temperature 100 C Pressure of N 2 gas bar Fig. 3 Effect of ph on viscosity of the slurry

4 improves the density of the powder. Further it is evident from the Fig. 3 that the viscosity of the slurries was below 60 cp at the ph in the range of which indicated that the slurries were suitable for spray drying. As very low ph slurries are corrosive in nature, the ph was fixed in the range of for the spray drying experiments. 3.2 Rheological Behavior of the Slurry Rheograph (Fig. 4) shows the rheological behaviour of the Al 2 O 3 slurry whose ph was adjusted to 3.2. The experiment was conducted over the shear rate range of s -1 from the upward sweep followed by downward sweep. The viscosity of the slurry was around 21 cp at 51 s -1. The difference in the viscosities at any given shear rate gives the information about thixotropy which can be defined as the property of the slurry that are thick under normal conditions and flows over time when it is stressed. From the rheograph, it was evident that the thixotropy was negligible which means that slurry was sufficiently stable. Further, slurry displayed only a mild shear thinning behaviour. The magnitude of shear thinning i.e. reduction in viscosity with respect to increasing shear rate reveals information about inter-particle network formation [15]. High shear thinning behaviour means high inter-particle network which in turn offers more resistance to flow. Accordingly for the purpose of spray drying, it is preferable to have slurries with weak or no inter-particle network [16]. Hence the prepared Al 2 O 3 slurry was found to be suitable for spray drying. 3.3 Spray Drying of Slurry After the optimization of solid loading, binder content and ph of the slurry, the slurries were spray dried under controlled conditions. In order to achieve larger particle size, it is essential to maintain higher feed rate [10]. But it was found that there is limitation in maintaining higher feed rate as it will otherwise lead to chocking. However, this problem was effectively circumvented by maintaining sufficiently higher temperature during spraying (*300 C). Spray dried powders were collected from spraying chamber as well as from cyclone filter. The powders collected directly from chamber were coarse and free flowing in nature, whereas, the fine powders were obtained from cyclone filter. Accordingly the powders collected from the chambers were found to be more suitable for plasma spraying process. 3.4 Agglomerate Size and Morphology The prepared spray dried alumina powder was sintered at high temperature of about 1,300 C to study the effect of sintering on particle morphology. Sedigraph particle size distribution (Fig. 5) revealed the median particle size (d 50 ) of spray dried powder was around 4 lm. Further it showed a narrow unimodal distribution of particles i.e. the particles were narrowly distributed over the range of 3 7 lm. On the other hand in the case of sintered powder, it was confirmed that particle size (d 50 ) had further grown to 18 lm and about 80 % of particles were above 10 lm. Accordingly it was ensured that the sintered powder can be utilized for plasma spraying purpose. Scanning electron micrograph (Fig. 6) revealed that the initial particles were irregular and flaky in nature, whereas, the particles of spray dried powder were dense and exhibited spherical morphology. It was also confirmed that the spherical morphology was retained even after sintering at high temperature. Thus, it is evident that sintering increased the particle size without affecting the particle morphology. Further the absence of hollow granules ensured that the dispersant content in the slurry was maintained at an optimum level. The particle sizes of the spray dried and the sintered powders were found to be in the ranges of 3 20 and 8 30 lm, respectively, as obtained from the clemex vision image analysis software attached with optical microscope. Thus, the particle sizes which are obtained from the micrographs are well corroborated with that of the sedigraphs obtained. 3.5 Powder Density Fig. 4 Rheological behavior of the slurry The bulk density (g/cm 3 ) of as received, spray dried and sintered powders are found to be 1.2, 1.5 and 1.6, respectively. Thus, the bulk densities of the spray dried and sintered Al 2 O 3 powders are found to be higher than that of reported values, which are in the range of g/cm 3 [16]. The higher particle density is the ample evidence for

5 Fig. 5 Particle size distribution a spray dried and b sintered alumina powder the formation of the solid core particles which is essential for plasma spray process. The porous particle will float on the plasma flame surface or be evaporated by superheating, whereas, dense particle will diffuse to the center of the plasma flame and improve the coating efficiency. 3.6 Flowability The flowability of the as received, spray dried and sintered powders were found to be 5.14 s/50 g, 22.3 s/50 g and 28.3 s/ 50 g respectively. From the above results, it is evident that the spray died and sintered powders possess better flowability. This is mainly due to the high density and spherical morphology of the spray dried and sintered powders. Fig. 6 SEM Images of a starting powder b spray dried and c sintered alumina powder 3.7 X-ray Diffraction Studies XRD patterns of the as received, spray dried and sintered alumina powders revealed only single phase a-alumina structure (Fig. 7). It is evident from the XRD pattern that the peaks of the as received Al 2 O 3 powder were fairly

6 Viscosity of the optimized slurry displayed negligible thixotropy and only a mild shear thinning behavior. Particle size (d 50 ) of the obtained spray dried powder were around 4 lm which was further increased to 18 lm after sintering. Scanning electron micrographs revealed the spherical morphology of spray dried powder. Also it was confirmed that the morphology was retained even after sintering. Thus, the obtained powder had bulk density of about 1.6 g/ cm 3 as required for plasma spraying. Accordingly it was ensured that the prepared agglomerated powder can be suitable for plasma spray process. Acknowledgments The authors Sathish and Geetha Manivasagam gratefully acknowledge the Department of Science and Technology for the funding and the Director of NAL CSIR, Bangalore for permitting to utilize facilities. The authors also thank Mr. P. Kumara and Mr. B. Abdul Haleem of NAL for their help regarding the spray drying. Thanks are also extended to Dr. R. Ramachandra Rao, Ms. H. N. Roopa for their help in conducting Relative Sediment Height experiments. Fig. 7 XRD patterns of a starting Al 2 O 3 powder b spray dried powder c spray dried powder sintered at 1,300 C broadened indicating the nanocrystalline nature of the powder. On the other hand, the peaks of the sintered powders showed crystalline nature indicating that some grain growth has taken place during sintering. 4 Conclusions In order to coat nanoceramic powders using plasma spraying process, the powders are required to be agglomerated to the required size (10 20 lm). In this work, agglomerated alumina powder was produced by spray drying process by optimizing solid loading, ph and the concentration of binder. For this purpose, polyethylene glycol and sodium hexametaphosphate were used as binder and dispersant, respectively. The process parameters were optimized and found that slurry with 22 vol% Al 2 O 3,6 wt% PEG and 0.02 wt% sodium hexametaphosphate yielded the good flowable powder. The slurry with ph in the range of was found to be suitable for spraying. References 1. Hawthorne H M, Erickson L C, Ross D, and Tai H, Wear 11 (1997) Ramachandran K, Selvarajan V, Ananthapadmanabhan P V, and Sreekumar K P, Thin Solid Films 315 (1998) Westergard R, Erickson L C, Axen N, Hawthorne H M, and Hogmark S, Tribol Int 31 (1998) Li C-J, and Sun B, Thin Solid Films 450 (2004) Tao S, Yin Z, Zhou X, and Ding C, Tribol Int 43 (2010) Zeng Y, Lee S W, and Ding C X, Mater Lett 57 (2002) Jordan E H, Gell M, Sohn Y H, Goberman D, Shaw L, Jiang S, Wang M, Xiao T D, Wang Y, and Strutt P, Mater Sci Eng; A 301 (2001) Wang Y, Jiang S, Wang M, Wang S, Danny Xiao T, and Strutt P R, Wear 237 (2000) Kabacoff L T, The AMPTIAC Newsletter Spring 6 (2002) Cao X Q, Vassan R, Schwartz S, Jungen W, and Tietz F S, J Eur Ceram Soc 20 (2000) Betrand G, Filiatrec C, Mahdjous H, Foussy A, and Coddet C, J Eur Ceram Soc 23 (2003) Walker W J, Reed J S, and Verma S K, J Am Ceram Soc 82 (1997) Yama shita O, and Kishimoto Y, Powder Metall 41 (1998) Frey R G, and Halloran J W, J Am Ceram Soc 67 (1984) Mohanta K, and Bhargava P, J Am Ceram Soc 91 (2008) Mishra P K, Nayak B B, and Mishra B K, Powder Technol 196 (2009) 272.