Influence of feedstock preparation on ceramic injection molding and microstructural features of zirconia toughened alumina

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ScienceDirect Journal of the European Ceramic Society 34 (2014) 745–751

Influence of feedstock preparation on ceramic injection molding and microstructural features of zirconia toughened alumina Frank Sommer a,∗ , Hartmut Walcher b , Frank Kern a , Marko Maetzig b , Rainer Gadow a a

Universität Stuttgart, Institut für Fertigungstechnologie keramischer Bauteile (IFKB), D-70569 Stuttgart, Germany b Arburg GmbH&Co. KG, D-72286 Loßburg, Germany Received 18 February 2013; received in revised form 23 September 2013; accepted 27 September 2013 Available online 20 October 2013

Abstract Feedstocks for ceramic injection molding of ZTA containing 90 vol.% of sub-␮m alumina and 10 vol.% of zirconia nanopowder were prepared by different processing techniques. Feedstocks were prepared by mixing in a sigma-blade kneader and subsequent homogenizing by twin-screw extrusion or shear roll compaction. Two other feedstocks were previously bead milled and subsequently processed by the same procedure. Compounding technology strongly influences the injection molding behavior and microstructures of the final product. Despite higher energy input of the shear roll compactor, powder agglomerates cannot be completely avoided. Pre-milling is effective to disperse and deagglomerate ceramic powders. Injection pressures of feedstocks from pre-milled powders were about 200 bar lower compared to pressures needed for non-milled feedstocks. Present feedstock preparation methods are feasible to produce homogeneous feedstocks, which strongly influence microstructures. In order to produce high solid loaded sub-␮m/nm feedstocks, processing methods, pre-treatment and solid content have to be carefully chosen. © 2013 Elsevier Ltd. All rights reserved. Keywords: Feedstock; Mixing technique; Injection molding; CIM; Microstructure

1. Introduction Ceramic injection molding (CIM) offers the manufacturing of complex three dimensional parts with tight dimensional tolerances. Cost-intensive post-processing can frequently be avoided, which enables economical mass production of ceramic components. Multiple processing steps have to be controlled in order to manufacture fully functional products. Research on CIM in the last decades focused on the selection of initial powders with favorable morphologies1–3 and appropriate organic vehicles.4–6 The latter must permit sufficient flow behavior for defect-free molding and a smooth debindering behavior, in other words a controlled thermal decomposition of organic content.7,8 Mold design and injection parameters strongly influence properties of green bodies and consequently the final product. Disadvantageous parameters will lead to products with defects such as voids, cracks or sink marks, thus unrectifiable rejects. Due to

∗

Corresponding author at: IFKB, Universität Stuttgart, Allmandring 7b, D70569 Stuttgart, Germany. Tel.: +49 711 68568234; fax: +49 711 68568301. E-mail address: [email protected] (F. Sommer). 0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.09.020

high shear rates during injection molding powder-binder separation and particle orientation effects in the thermoplastic melt have been frequently reported.9,10 Anisotropic mechanical properties and shrinkage have to be taken into consideration for new components.11–13 Latest research focuses on mold filling simulations of solid loaded fluids, but challenges to describe the complete process still remain.14,15 Within the multi-stage process chain, the compounding of the powder-binder-mixture is the most critical factor since it defines the molding and debindering behavior.16 From a theoretical point of view feedstocks with high solid loadings providing high green densities are favored but an accompanying increase of viscosity limits the flowability of the feedstock. The threshold solid loading of 63.7 vol.% for monosized particles is under practical conditions never achieved because real powders never show perfect spherical morphologies.17,18 Feedstocks with a minimum of necessary binder but low viscosity are desired, which allow for complete mold filling. This becomes even more difficult for powders with small particle sizes since the large surface has to be wetted completely by the binder. For production in industrial scale a reproducible feedstock is essential for stable processing conditions and series manufacturing. The optimal feedstock is

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thus a homogeneous binder-solid mixture with high solid loading and minimum necessary binder content. Feedstocks should provide solids with a thin binder coating and no accumulations of the single components. The effect of powder treatment on rheology and injection molding of zirconia ceramics has already been documented by Trunec. Rheological behavior and thus mechanical properties were improved due to the favorable treatment effect.19 Feedstocks containing zirconia powder of higher surface area (15 m2 /g) were prepared by Walcher.20 The mixing capability of a sigma-kneader was compared to a shear roll compactor. Results show that the feedstock quality is directly linked to injection pressures needed and feedstock temperatures during molding. The aim of this study is to correlate different feedstock preparation methods to molding behavior and microstructural features of a nano composite ceramic. Thus zirconia toughened alumina feedstocks including 10 vol.% of nanosized zirconia and 90 vol.% of sub-␮m alumina powder were prepared. With respect to the high amount of nanopowder a very high solid loading of 57.5 vol.% was chosen in order to illustrate mixing capabilities with undergoing risk of defects from molding or debindering. Feedstocks were prepared from identical powder and binder batches with equivalent solid loadings and processing parameters. 2. Experimental procedure 2.1. Materials Two commercially available powders were used. ␣-Alumina (APA0.5, Sasol, North America) with a specific surface of 8 m2 /g and a particle size of d50 = 0.3 ␮m (manufacturer’ specification). Unstabilized zirconia (UEP, Daiichi, Japan) powder has a specific surface of 20–30 m2 /g and a particle size of 0.4–0.7 ␮m (manufacturer’ specification). A commercially available binder system Licomont EK 583 G (eMBe, Germany), on basis of polyethylene wax and polyethylene glycol, was used as binder system. 2.2. Feedstock preparation Zirconia toughened alumina feedstocks consisting of 10 vol.% of zirconia, 90 vol.% of alumina and a solid loading of 57.5 vol.% were prepared by four different routes as shown in Fig. 1. Ceramic powders and binder were mixed in a double Z-blade kneader (Hermann Linden, Germany) at 140 ◦ C for 100 min. After output by extrusion half of the granulated mixture was remelted and homogenized by a twin screw extruder (d = 16 mm, L/d = 25, Haake Rheomex, Thermofisher Scientific, Germany) at same temperature for two times. The diameter of the die was d = 3 mm. Feedstock 1 is following denominated as “FS1”. The other half was homogenized by a shear roll compactor (BSW 135-1000, Bellaform, Germany) with a gap width of 0.5 mm. Both shear rolls were heated by two heating zones each, one at feeding side and one at granulation side. Temperature of roll 1 was set to 96 ◦ C and 75 ◦ C, roll 2 was set

Fig. 1. Feedstock preparation routes (FS1–FS4).

to 103 ◦ C and 79 ◦ C. The feedstock was homogenized for two times at 106 rpm (“FS2”). Two other compounds were prepared by an additional pre-milling step. An aqueous suspension of Al2 O3 and 10 vol.% ZrO2 was bead milled with 3Y-TZP milling balls of d = 0.8 mm (Dispermat SL, VMA Getzmann, Germany). The suspension and the binder were subsequently mixed in the kneader for 100 min at 140 ◦ C until the complete water was evaporated. Following homogenizing steps were carried out by twin screw extruder (“FS3”) and shear roll compactor (“FS4”) as previously described with same processing parameters. Because set-ups cannot be compared directly, optimal parameters for best mixing and homogenization were chosen based on broad experience. 2.3. Injection molding Injection molding was carried out on a hydraulic injection molding machine (Allrounder 270S 400-70, Arburg, Germany) with 15 mm diameter screw. The machine was equipped with position regulated screw and CIM cylinder assembly. Test plates (36 mm × 28 mm × 3.5 mm) of FS1–FS4 were produced with equivalent parameters shown in Table 1. Pressure at switch-over point and temperatures in all zones were traced. The machine was running under serial production set-up. 2.4. Debindering and sintering Debindering was performed in a combined process. The water-soluble fraction was removed by extraction in deionized Table 1 Injection molding parameters of FS1–FS4. Parameter Injection velocity [cm3 /s] Packing pressure [bar] Temperature (Zone 1/2/3/4) [◦ C] Mold temperature [◦ C]

13 500 160-155-145-140 60

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water at 30 ◦ C for seven days. The residual binder was removed by burn-out in air up to 400 ◦ C with a heating rate of 10 ◦ C/h and subsequent pre-sintering at 800 ◦ C with a heating rate of 300 ◦ C/h and 1 h dwell. Sintering was carried out in air at 1575 ◦ C (HTK 16/17, Thermconcept, Dr. Fischer, Germany) at a heating rate of 2 K/min and 2 h dwell. 2.5. Physical and microstructural characterization The porositiy distribution of the pre-sintered samples was monitored by mercury intrusion porosimetry (Pascal 140/440, Porotec, Germany). Sintered ZTA plates were lapped and polished to a 1 ␮m finish with diamond suspension (Struers Rotopol, Germany). The microstructural SEM characterization (1000–10,000× magnification) was carried out from polished and thermally etched surfaces in secondary electron mode at low acceleration voltage of 3 kV (Zeiss, DSM982 Gemini, Germany). Fracture images were taken from broken green parts (JEOL, JCM-5000 Neoscope). The other part of the broken sample was subsequently debindered and pre-sintered as described. Zirconia nanopowder was characterized by TEM (Jeol 1230, Japan).

Fig. 3. Particle size distribution of APA0.5 alumina.

3. Results 3.1. Materials A SEM picture and laser granulometry measurement of starting alumina powder is shown in Figs. 2 and 3. The powder has a uniform and almost spherical morphology. Particle size distribution shows a monomodal distribution with a D50 value of 0.29 ␮m. Neither a coarse powder fraction nor agglomeration is visible. TEM (100 kV, 0.2 nm/line, 250 k× magnification) of zirconia powder reveals crystallite sizes of 10–30 nm (Fig. 4). Laser granulometry shows D10 , D50 and D90 of 0.16 ␮m, 0.33 ␮m and 0.86 ␮m respectively and confirms manufacturer’ data. However, ∼5 vol.% of the initial powder is agglomerated, indicated by peaks in the 2 ␮m and 20 ␮m region (Fig. 5).

Fig. 2. SEM image of APA0.5 alumina.

Fig. 4. TEM image of UEP zirconia.

3.2. Injection molding Injection molding of each feedstock was started in semiautomatic mode for 20 test shots. Then the machine was run

Fig. 5. Particle size distribution of UEP zirconia.

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molded samples is displayed. Results are based on four single porosimetry measurements of FS1–4 each. Samples of porosimetry measurements were cut out of molded samples of same size and shape at identical positions. Pore volumes were normalized to cumulative pore volumes in order to compare porosity distributions. In SEM images of fracture faces studied at lower magnification large pores can be detected, which were formed during the mixing or molding process. FS1 and FS2 generally show broader porosity distributions (0.0316–0.076 ␮m) whereas in FS3 and FS4 very narrow distributions are visible (0.0316–0.0562 ␮m). Main peaks of pore radii are ∼0.042 ␮m for FS3 and FS4, respectively with similar amount of relative pore volume. FS2 showed the same size of the main peak but just limited relative pore volume. Main pore radius is shifted to ∼0.0562 ␮m for FS1. Fig. 6. Maximum injection pressures at switching point of FS1–FS4.

in automatic mode. Pressure at swich-over point of each injection cycle was recorded for 20 shots. Comparison of maximum pressure trends at switching point of FS1–4 is illustrated in Fig. 6. Average pressures of non-milled FS1 and FS2 are 790 bar and 725 bar to maintain speed of 13 cm3 /s. Pre-milled FS3 and FS4 generally need lowest injection pressures of <600 bar, respectively. Pressure deviations of non-milled feedstocks were ∼50 bar, whereas pre-milled feedstocks just showed deviations of ∼25 bar. Feedstocks prepared by milled powders only showed small maximum injection pressure differences in the range of the standard deviation. On the contrary maximum injection pressure of shear roll compacted feedstock FS2 was about 65 bar lower than twin screw extruded feedststock FS1, when non conditioned initial powders were used. 3.3. Porosimetry The porosity distributions of pre-sintered samples are shown in Fig. 7. The typical pore radii range of debindered injection

Fig. 7. Porosimetry distribution of pre-sintered samples FS1–FS4.

3.4. Microstructural characterization Fig. 8 shows SEM images of polished and thermally etched samples. Samples of FS1 and FS2 showed microscopic pores and presumably grain pull-outs due to the polishing process. Microstructures of FS3 only showed a few number of pores or rather grain pull-outs. Best microstructure without defects was seen for sample of FS4. At magnification of 1000× large alumina agglomerates of 15–25 ␮m size are visible in non-milled FS1 and FS2. These agglomerates are mostly formed by ∼15 single alumina grains or of higher number. Microstructure of pre-milled FS3 and FS4 shows a homogeneous distribution of zirconia particles, which primarily settle at triple points of the alumina matrix. Neither alumina nor zirconia agglomeration is visible. Microstructures of broken green parts, and parts, which were subsequently debindered are displayed in Figs. 9 and 10. Border zones of samples are free of defects whereas flow lines due to the injection and binder clusters are clearly visible in the center of the samples. Size of defects and distinctive flow lines are heavily reduced for sample of FS4 (Fig. 10). 4. Discussion From a theoretical point of view, a perfect feedstock consists of a high solid load with a minimal binder content providing high green density and sufficient mold filling. Therefore the mixing technique is decisive to distribute the two components of the ceramic powder and to homogeneously mix powders and binder. Inhomogeneities such as binder clusters, voids or powder agglomerations within the feedstock will be conveyed along the process chain to the final microstructure of the product. By applying high mixing energy in the compound, binder pockets can be reduced leading to ceramic particles coated with a thin binder film. When this state is achieved, the quality of the feedstock will be directly linked to the maximum injection pressure needed to maintain a distinct injection speed. In a homogeneous feedstock particles can slip on a thin binder film resulting in low injection pressures, whereas void nucleation and particleparticle friction requires energy and lead to higher injection pressures.

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Fig. 8. SEM images of polished and thermally etched surfaces (a-FS1, b-FS2, c-FS3, d-FS4).

Results of pressures at switch-over point show, that homogenization by a shear roll compactor (FS2) introduces higher energy in the feedstock than a twin screw extruder (FS1) as injection pressure of feedstocks in this study is about 65 bar lower. For pre-milled feedstocks no pressure differences were seen comparing twin-screw extruded and shear roll compacted feedstocks. Besides pressures recorded, standard deviations of injection pressures for pre-milled feedstocks amounted to just

half the values observed for non-milled feedstocks indicating feedstocks of higher homogeneity. In order to verify the “indirect viscosity measurements” carried out on the injection molding machine high pressure capillary rheometry was carried out (not shown). Some information confirming the said results can be deduced from the pressure drop measurements over the nozzle, which showed less pressure fluctuation in case of the pre-milled feedstocks.

Fig. 9. SEM images of green fracture faces (a-FS1, c-FS2) and pre-sintered fracture faces (b-FS1, d-FS2).

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Fig. 10. SEM images of green fracture faces (a-FS3, c-FS4) and pre-sintered fracture faces (b-FS3, d-FS4).

Capillary rheology measurements can provide a more general information on flow behavior without assuming geometrical boundary conditions. The measurements on the injection molding machine represent an objective criterion for the processability and reproducibility of the feedstock for a given mold and sprue geometry and thus for the manufacturing of a specific component. Porosity measurements of non-milled feedstocks FS1 and FS2 generally show broad pore size distributions. Agglomerates from initial powders or formed during processing cannot be completely destroyed by high energy mixing techniques resulting in larger pore radii and thus larger pore channels. Because pre-milling is quite effective to break agglomerates and homogeneously disperse zirconia in the alumina matrix, pore radius distributions of FS3 and FS4 are sharper and shifted toward smaller sizes. Generally shear roll compaction introduces slightly higher energy resulting in smaller pore radii but when powders are pre-milled this effect is leveled out. Smallest pore channels thus pore radii are measured for pre-milled FS3 and FS4. A maximum relative pore volume of ∼40% is both visible for a pore radius 0.042 ␮m. Results are in good agreement with microstructure of polished and etched samples. Large alumina but no zirconia agglomerates are visible in non-milled microstructures of FS1 and FS2. Because measurements of the initial alumina powder showed a homogeneous monomodal particle size distribution agglomerates are most likely formed during processing. A reason can be the incomplete distribution of alumina and zirconia particles resulting in alumina agglomeration during sintering.

As no zirconia agglomerates are seen in the microstructures, initial zirconia particles are only softly agglomerated and were destroyed during the homogenization process. Neither alumina nor zirconia agglomerates were seen in microstructures of FS3 and FS4. Pre-milling thus provides powder deagglomeration but moreover a good powder dispersion, which cannot be achieved by high energy homogenization methods only. Microstructure is strongly affected by the feedstock mixing techniques applied and by the injection process. Fracture images clearly reveal flow lines formed during the injection molding process in locations, where high shear stresses occur. Defects are most likely formed due to a powder-binder segregation already reported in literature. Moreover it is known that the bonding of water-soluble and water-insoluble component of the binder can be improved by several additives, resulting also in better injection behavior not part of this study. From images of fracture faces it can be concluded that flow lines and pores can be reduced by better homogenization of feedstocks. In this study pre-milled and shear roll compacted FS4 showed best characteristics. Debindering behavior will most probably also be affected by the procedure of feedstock manufacturing. Especially if as in the case of FS3 and FS4 very homogenously distributed powder binder mixtures are produced, volatiles originating from decomposed binder are forced to escape through extremely narrow capillaries, which in case of inappropriate time-temperature schedule may lead to defects by bloating or microcracking. Feedstocks of ultrafine powders are thus probably only applicable for components of limited wall thickness.

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5. Conclusions

References

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• Feedstock processing technique has strong influence of homogeneity of the feedstocks and directly influences injection pressures needed. Injection pressure differences of 200 bar were recorded between different compounds. • Shear roll compaction introduces higher energy in the compound than twin screw extrusion as seen from lower injection pressures and smaller pore radii. • Pre-milling is indispensable in order to produce high quality feedstocks. Homogeneously distributed 2-phase ceramics without agglomerates were obtained. High energy homogenization alone cannot substitute the milling step. • When feedstocks were pre-milled, lowest injection pressures needed were recorded. No difference of high energy mixing is seen from injection pressures with pre-milled feedstocks. • Improved powder-binder mixing of feedstocks is leads to less defects in the injection molding process. For high solid loaded homogeneous sub-␮m/nanopowder feedstocks shear roll compaction showed best characteristics. • A sharp distribution of small pore channels was detected for pre-milled and high energy mixed feedstocks. Special attention should be attributed to the debindering step, when parts from high solid loaded nanopowder feedstocks of higher thickness are produced. Present processing techniques clearly influence the feedstock quality such as powder dispersion and powder-binder homogeneity, hence the molding process. As a consequence processing techniques strongly affect the final microstructure and thus properties of parts. Acknowledgements The authors would like to thank the AIF (Industriegemeinschaft industrielle Forschung) for financing this study under grant no. 16611N/2. H.F. El-Maghraby (NRC, Egypt) is acknowledged for TEM images.