Preparation of a ceramic core with high strength using an inorganic precursor and the gel-casting method

    Preparation of a ceramic core with high strength using an inorganic precursor and the gel-casting method Geun-Ho Cho, Jing Li, Eun-Hee Kim, Yeon-Gil Jung PII: DOI: Reference:

S0257-8972(15)00567-8 doi: 10.1016/j.surfcoat.2015.09.062 SCT 20653

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

1 April 2015 4 September 2015 4 September 2015

Please cite this article as: Geun-Ho Cho, Jing Li, Eun-Hee Kim, Yeon-Gil Jung, Preparation of a ceramic core with high strength using an inorganic precursor and the gel-casting method, Surface & Coatings Technology (2015), doi: 10.1016/j.surfcoat.2015.09.062

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Preparation of a ceramic core with high strength using an

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inorganic precursor and the gel-casting method

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Geun-Ho Cho, Jing Li, Eun-Hee Kim, and Yeon-Gil Jung*

School of Materials Science and Engineering, Changwon National University,

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Changwon, Gyeongman 641-773, Republic of Korea

*Corresponding author. Tel: +82-55-213-3712, Fax: +82-55-262-6486

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E-mail: [email protected]

Abstract A new process for the preparation of a ceramic core with high formability and fracture strength is proposed. It involves the use of powders coated with inorganic binders, and gelcasting. Two types of powders were used to investigate the effect of the starting material on the mechanical properties (the green and sintering strength values) of the ceramic core, according to the components and particle sizes. Both the green and sintering strengths of the

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ACCEPTED MANUSCRIPT core samples prepared in Case II exhibited reasonable values, due to the long gelation time and the effective glassification of the inorganic precursor, respectively; a strength of about 10

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MPa was achieved after 1 h heat treatment at 1000 oC. Furthermore, the prepared core

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samples completely dissolved in 40 wt% NaOH solution. Specific experimental conditions were optimized for the application of precision casting, for example, the composition ratio (of

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the starting powders), the inorganic precursor, and the gel-casting method.

Keywords: Ceramic core; Inorganic precursor; Gel-casting; Fracture strength; Glass phase;

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Dissolubility.

1. Introduction Conventionally, a ceramic core used for hollow components in investment casting or precision casting is prepared by an injection molding method from a mixture of organic compounds (wax and surfactants) and starting powders based on zircon flour (ZrSiO4) and fused silica (SiO2) [1,2]. Polymer materials, such as paraffin wax and crystal wax, are added to form the green body of the core, and surfactants are used to maximize blending between

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ACCEPTED MANUSCRIPT the starting powders and the wax. However, polymer materials that have long chains of ultrahigh molecular weight do not mix easily with the powders due to the high viscosity,

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despite the addition of surfactants. This results in a decline in the formability of the core and

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easily induces collapse of the green body before heat treatment. In addition, the fracture strength after heat treatment arises only from the effect of sintering between particles. This is

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due to the volume shrinkage and shape deformation of the core. Although much research has

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focused on preparing the core with good mechanical properties and with no shape deformation [3–6], these studies were limited to considering variations in the composition

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ratio (of the starting powders) and heat treatment temperature, etc., within the investment casting process.

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In the present study, the gel-casting method and an inorganic precursor were used to fabricate a ceramic core with reasonable green strength and sintering strength; here, the

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fracture strength of the core after heat treatment is simply referred to as the sintering strength.

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The shape of the core created using the gel-casting process is fixed through the network structure of the polymer generated by the gelation of a prepolymer (a monomer and a dimer), which results in the enhancement of the green strength in the core. The inorganic precursor was used to increase the sintering strength of the core, due to the glass phase formed on the surfaces of the starting particles [7]. The prepolymer and inorganic precursor added with the liquid phase would be particularly effective in improving the mechanical properties. 2. Experimental procedure 2.1. Materials and methods Commercially available fused silica, zircon flour, and silica (of various particle sizes) were used as starting ceramic powders to prepare the core. The composition ratios of the starting powders are given in Table 1. Tetraethyl orthosilicate (TEOS; Sigma-Aldrich Korea, Yongin, Korea) and sodium methoxide (NaOMe, Sigma-Aldrich Korea, Yongin, Korea) were

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ACCEPTED MANUSCRIPT used as the SiO2 and Na2O precursors, respectively. The following materials (all from SigmaAldrich Korea, Yongin, Korea) were used as additives in the gel-casting process: monomer acid;

N,N-dimethyl-3-oxo-butanamide;

ethylene

carboxamide),

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(acrylic

dimer

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(bisacrylamide), initiator (homotaurine, ammonium sulfate, ammonium peroxydisulfate), and catalyst (N,N,N-trimethylethylenediamine and N,N,N,N-tetramethylethylenediamine).

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Formulations used to prepare the core samples under various conditions are given in

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Table 2. First, the starting powders were mixed with the inorganic precursor, and then the mixture was filtered. The precursor-coated powders were dried at 80 °C for 24 h. Thereafter,

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the precursor-coated powders and prepolymers (monomer and dimer) were blended in a suitable solvent for the homogeneous dispersion of the two-phase heterogeneous materials.

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After the addition of initiator and catalyst, the mixture became a gel. The gelation time was indirectly measured by varying the temperature of inner system during polymerization. The

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gelated core sample was dried at 25 C for 48 h. This formed the green body of the core. The

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green body was then heat-treated at 1000 C for 1 h. Details of the fabrication process for the preparation of the core are given in Fig. 1.

2.2. Characterization

The strength of the core samples before and after heat treatment was measured using a universal testing machine (Instron 5566; Instron Corporation, Norwood, MA, USA) in the four-point mode, at a rate of 0.5 mmmin–1. Five runs were performed to determine the standard deviation of the strength. The fracture morphology in the core samples after heat treatment at 1000 °C was observed using a scanning electron microscope (SEM; model JSM5610; JEOL, Tokyo, Japan). Elemental analysis of samples was carried out using an energy dispersive X-ray spectrometer (energy resolution 133 eV) (Oxford Instruments, Oxford, UK).

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ACCEPTED MANUSCRIPT Dissolubility of the core samples was checked using a 40 wt% NaOH solution at 50 C for 10

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h.

3. Results and discussion

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In this study, the gelation and heat treatment processes were used to enhance the green and sintering strengths. The prepolymer and inorganic precursor with the liquid phase

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were used for the homogeneous mixing of the heterogeneous materials selected for use. In the polymerization of prepolymers with two or more functionalities, the polymer chains grow

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into a network. As the polymerization proceeds, the viscosity of this system increases and gelation commences. Furthermore, the temperature in the system increases during

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polymerization because chain growth is an exothermic reaction. Gelation begins immediately at the starting point of the highest temperature. Therefore, a stronger gel is synthesized at

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longer gelation times. This, in turn, leads to improvement in the green strength of the core.

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Experimental details pertaining to the gelation time and temperature used for the starting powders of different composition ratios are shown in Fig. 2. In Case II (Fig. 2(b)), the gelation time was longer and the temperature was higher than in Case I (Fig. 2(a)). The gelation is directly related to the network structure and polymer density. As a result, the core prepared in Case II would probably have a higher green strength compared with the core prepared in Case I. The reaction mechanisms involving the two inorganic precursors used in this study, silicate and sodium methoxide, are as follows [8,9]: Sol–gel reaction: Si(OEt)4 + 4H2O  Si(OH)4 + 4EtOH

(1)

Si(OH)4  SiO2 + 2H2O

(2)

Hydrolysis reaction: 5

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(3)

Heat treatment: (4)

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SiO2 + 2NaOH  SiO2·Na2O + H2O

where Si(OEt)4, Si(OH)4, EtOH, SiO2, NaOMe, NaOH, and MeOH denote TEOS, silanol,

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ethyl alcohol, silica, sodium methoxide, sodium hydroxide, and methyl alcohol, respectively [10,11]. Sodium methoxide is hydrolyzed to form sodium hydroxide, and TEOS is converted

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to silica and ethyl alcohol. The SiO2 and NaOH synthesized via the above mechanisms are glassified to form sodium silicate (SiO2·Na2O) during heat treatment. Specifically, liquid-

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phase precursors are changed to solid-phase glass via the sol–gel reaction and heat treatment, subsequently leading to an improvement in the sintering strength of the core.

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Typical SEM morphologies and energy dispersive X-ray spectroscopy (EDS) results of core samples after heat treatment at 1000 °C are shown in Fig. 3, as a function of the

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composition ratio of the starting powders. The sintering strength of the core prepared with

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inorganic precursors arises from the sintering effect between the particles and the glass phase generated on the surfaces of the starting particles. However, in this study, heat treatment was conducted at only 1000 C, even though the temperature is a little low to give enough strength to the core. However, the inhomogeneous distribution of glass phase during quenching to room temperature was formed on the surfaces of particles due to the low viscosity of glass phase at above 1000 oC, resulting in the shape distortion and cracks in the ceramic core. Therefore, the glass phase should be homogeneously coated on the surfaces of the particles and the conversion efficiency to the glass phase should be sufficiently high. In Case II, the glass phase was formed uniformly on the surfaces of the particles and the inorganic precursor was mostly converted to the glass phase. In contrast, in Case I, the glass phase was not fully developed on the particles, perhaps because the fused silica, composed of starting powder in Case II, is not involved in the glassification reaction. Specifically, the 6

ACCEPTED MANUSCRIPT reaction between precursors (SiO2 and Na2O) and the reaction between fused silica in the starting powder and Na2O takes place simultaneously in Case II. In EDS analysis, the

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elements Na, Si, O, and Zr from the inorganic precursor and the starting powder were

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detected, irrespective of the composition ratios of the starting powders.

The green strength and sintering strength of core samples before and after heat

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treatment are shown in Fig. 4. All samples exhibited similar or higher green strength values

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compared with the values of the core prepared by the conventional injection molding method. The core samples prepared in Case II exhibited higher green strength than those prepared in

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Case I. This resulted from an increase in the polymer density and the network structure after the longer gelation time and higher temperature (referred to in Fig. 2). The core sample in

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Case II exhibited a sintering strength of about 10 MPa, which is very similar to that achieved with conventional investment casting, after heat treatment for 24 h at 1200 C. However,

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when using an inorganic precursor, the core realized a similar strength value at the relatively

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lower temperature of 1000 C and within a shorter time of 1 h. This is due to the successful formation of the glass phase on the surfaces of the starting particles. Furthermore, the prepared core samples completely dissolved in a 40 wt% NaOH solution (see Fig. 5). Consequently, the conditions and processes proposed in this study (composition, gel-casting process, and inorganic binder), such as those in Case II, can be used for preparing ceramic cores that have useful strength and dissolubility.

4. Conclusions The gel-casting method and an inorganic precursor have been introduced to prepare a ceramic core with good mechanical properties (green and sintering strengths) and without shape deformation. The green strength was closely related to the network structure of polymer generated by gelation of the prepolymer. The core sample prepared in Case II had

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ACCEPTED MANUSCRIPT higher green strength than that prepared in Case I, owing to the longer gelation time and higher gelation temperature. In Case II, the core sample showed a sintering strength of about

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10 MPa after heat treatment of only 1 h, due to the glass phase on the surfaces of particles.

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However, the core prepared in Case I had a lower strength of about 6 MPa, resulting from insufficient and partial glassification of the inorganic precursors. In addition, the core sample

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prepared by the gel-casting process and an inorganic binder was completely dissolved in a

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sodium hydroxide (NaOH) solution.

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Acknowledgment

This work was supported by the Power Generation & Electricity Delivery

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(20142020103400) of the Korea Institute of Energy Technology Evaluation and Planning

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(KETEP) grant funded by the Ministry of Trade, Industry and Energy of Korea.

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ACCEPTED MANUSCRIPT References [1] S. Parrnaik, D.B. Karunakar, P.K. Jha, Developments in investment casting process – A

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review, Journal of Materials Processing Technology 212(2012) 2332–2348.

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[2] Y. Dong, K. Bu, Y. Dou, D. Zhang, Determination of interfacial heat-transfer coefficient during investment-casting process of single-crystal blades, Journal of Materials Processing

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Technology 211(2011) 2123–2131.

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[3] Massimiliano Guazzato, Mohammad Albakry, Simon P. Ringer, Michael V. Swain, Strength, fracture toughness and microstructure of a selection of all-ceramic materials. Part I.

[4]

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Pressable and alumina glass-infiltrated ceramics, Dental Materials 20(2004) 441–448. Y. Qin, W. Pan, Effect of silica sol on the properties of alumina-based ceramic core

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composites, Materials Science and Engineering A 508(2009) 71–75. [5] I.C. Huseby, M.P. Borom, C.D. Greskovich, High temperature characterization of silica-

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base core for superalloys, American Ceramic Bulletin 58(1979) 448–452.

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[6] G.R. Frank, K.A. Canfield, T.R. Wright. U. S. Patent no. 4837187 (1989). [7] R.J. Keller, R.S. Haaland, J.A. Faison, U. S. Patent no. 6578623 (2003). [8] M. Barsoum, Fundamentals of Ceramics, McGraw-Hill, Seoul, 1997. [9] W.D. Callister, Materials Science and Engineering: An Introduction, Wiley, New York, 1997. [10] S. Ege, Organic Chemistry, D. C. Heath and Company, Toronto, 1994. [11] N. Sasaki, A revolutionary inorganic core and mold making process, Foundry Management & Technology, Feb. 19, 2009.

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ACCEPTED MANUSCRIPT Table 1. Composition ratios of starting powders used in Case I and II

Case II (wt%)

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Case I (wt%)

Zircon flour 2

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Fused silica

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Silica powder (2 m*)

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Silica powder (45 m*)

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Silica powder (149 m*)

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Silica powder (100–200 m*)

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Zircon flour 1

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Starting powder

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* Numbers in parentheses indicate the average particle size of the added silica.

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ACCEPTED MANUSCRIPT Table 2. Formulations and conditions used to prepare core samples using the gel–casting

Inorganic precursor

Starting powder

Additive

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Run number Run (a-1)

Run (b-1)

TEOS: 38 g NaOMe: 56 g

60 vol%

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Run (a-2)

Monomer: 3.3 wt% Dimer: 0.7 wt% Dispersant: 1.1 wt% Solvent: 40 vol% Initiator: 0.15 g Catalyst: 0.15 g

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Case I

Case II

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Run (b-2)

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method and an inorganic precursor

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40 vol%

Condition Before heat treatment After heat treatment Before heat treatment After heat treatment

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and an inorganic precursor.

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Fig. 2. Gelation time and temperature with polymerization in the ceramic core with different starting powders: (a) Case I and (b) Case II.

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Fig. 3. SEM morphologies and EDS results of core samples heat-treated at 1000 C: (a) Case

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I and (b) Case II. Each number indicates low and high magnification, respectively. Fig. 4. Green strength and sintering strength of core samples: (a) Case I and (b) Case II.

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Fig. 5. Dissolubility of core samples prepared using Run (b-2): (a) untreated sample and (b)

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treated sample (40 wt% NaOH solution).

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Research Highlights:

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 The gel-casting method was used to fabricate the core with reasonable formability.  The glassification efficiency of the inorganic binder affected the strength.

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 Glass phases were homogeneously formed on the surfaces of starting particles.

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 The prepared core was completely dissolved in a NaOH solution.

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