Injection molding of ceramic cutting tools for wood-based materials

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Journal of the European Ceramic Society 33 (2013) 3115–3122

Injection molding of ceramic cutting tools for wood-based materials F. Sommer ∗ , F. Kern, R. Gadow Institut für Fertigungstechnologie keramischer Bauteile, Universität Stuttgart (Institute for Manufacturing Technologies of Ceramic Components and Composites, University of Stuttgart), Allmandring 7b, D-70569 Stuttgart, Germany Received 4 March 2013; received in revised form 29 April 2013; accepted 5 May 2013 Available online 15 June 2013

Abstract In this study a mutable mold for ceramic cutting tools with inserts of different cutting angles and two different injection positions was designed. Three alumina-based ceramic feedstocks with different types and amount of second phases were developed. A mold filling study was carried out for both sprue positions in order to prove the molding behavior of the feedstock and the functionality of the mold. Debindering and sintering of molded green parts was arranged for each composition, respectively. Mechanical properties, microstructure and achieved cutting edge sharpness of produced tools were investigated. Results show that the mold design and injection molding process play a key role in order to manufacture cutting tools of best possible sharpness enabling a wood machining process. Feedstocks exhibit a good mold filling behavior resulting in comparatively sharp cutting edges of ≈10 ␮m after sintering. Mechanical properties show high potential for application of wood machining cutting tools. © 2013 Elsevier Ltd. All rights reserved. Keywords: CIM; Injection molding; Ceramic composites; Cutting tools; Wood machining

1. Introduction Cemented carbide tools for industrial wood cutting have become state of the art as they can provide a good ratio of hardness and fracture toughness.1 Small cutting angles and sharp cutting edges with micrometer sized radii can be realized, which are mandatory for the cutting of wood and wood-based materials.2 The wood cutting process itself is quite challenging as cutting speeds are normally ∼5 times faster compared to conventional metal cutting.3 Moreover wood as a workpiece is anisotropic and inhomogeneous because it often contains knots and silicates.4,5 Abrasive wear but also tannins in the wood, which chemically attack the binder phase of hard metals, frequently define the end of the lifetime for hard metal wood cutting tools.6–9 When the radius of a cutting edge has become too large, blunted tools cannot cut wood fibers anymore, resulting in bad surface qualities of the machined product. Wood machining technology of has dramatically improved during the last decades. Higher accuracy, feed rates and spindle

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Corresponding author. Tel.: +49 711 68568234; fax: +49 711 68568301. E-mail address: [email protected] (F. Sommer).

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rotations do not only have influence on process time but also surface qualities. In contrary, cutting tool materials were not studied such intensively. Spindle rotations for currently used hard metal tools are limited due to their high centrifugal forces and the clamping forces of the tool holder. The demands of improved wear behavior, the strong rise of the tungsten carbide price and limited rotation speeds of high specific density cemented carbides give rise to new investigations of cutting tool materials. Promising materials are oxide ceramics as they exhibit high hardness, excellent wear resistance and no sensitivity to chemical attack. Moreover densities are approximately only one quarter of the density of tungsten carbide and do thus not limit the rotation speeds. First studies on alumina based cutting ceramics for wood machining were done by Gogolewski et al.10 Chipping was the main problem but when grain sizes were reduced main failure mechanism was abrasive wear. Gel-casting and cold isostatic pressing of plates were chosen for defectfree processing of ceramic blanks. Tools were subsequently machined out of casted plates. Investigations on hot-pressed SiC particle reinforced Si3 N4 ceramics showed outperforming properties compared to standard tungsten carbide tools.11 However, also post-processing steps led to too high production costs for

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industrial scale cutting tools. A further approach was done by Strehler et al. Knive-shaped liquid phase Si3 N4 /SiC ceramics were uniaxially pressed and subsequently diamond-machined for the final shape.12,13 Cutting tools were solidified by pressureless sintering and post-hipping. Results showed that final post-sintering heat treatment led to a partial crystallization of the intergranular phase, which was indispensable for good edge integrity. Compared to hot-pressed composites,11 gas-pressure sintered samples displayed higher bending strength, lower hardness and toughness and larger grain sizes. Anyhow results showed twice as long lifecycles than standard tungsten carbide tools.14 Regarding the requirements for applicable ceramic cutting tools and industrial viability, ceramic injection molding (CIM) shows high potential as a processing technique. CIM offers the ability of economical mass production of three dimensional parts. Cost-intensive post-processing can frequently be avoided, because parts can be produced with tight dimensional tolerances.15–19 The aim of this study is to investigate if ceramic wood cutting tools can be manufactured by ceramic injection molding in sufficient quality. Tools must provide high hardness, fine microstructures and sharp cutting angles. In order to meet these requirements the use of sub-␮m powders seems indispensable. However, the preparation of injection molding feedstocks from fine initial powders and the processing is quite challenging and needs careful selection of all processing parameters involved. Ceramic cutting tools of three different alumina-based developed feedstocks were injection molded. Debindering and sintering of molded green parts was tailored for each composition, respectively. Mechanical properties, microstructure and achieved cutting edge sharpness of produced tools were investigated. Further investigations will focus on wear behavior of ceramic tools in wood machining but are not part of this study.

Table 2 Powder loadings of feedstocks developed. Feedstock

Powder loading [vol.%]

Al2 O3 ACY AS

60.5 60.5 59.3

USA, particle size d50 = 400 nm, SBET = 8 m2 /g). Powders used for the reinforcing dispersion were SiC C25 (Ceram GmbH, d50 = 400 nm, SBET = 25 m2 /g) and YCrO3 (synthesized acc.20 ). Sintering additives added to Al2 O3 –SiC composites were MgAl2 O4 (99%, S30CR, Baikowski, France, particle size d50 = 200 nm, SBET = 30 ± 5 m2 /g) and yttria (99.9%, Alfa Aesar, Karlsruhe, Germany, d50 = 3.39 ␮m, surface by mercury porosimety SHg = 9.16 m2 /g). An aqueous suspension of each composition was prepared and subsequently bead milled (Dispermat SL, VMA Getzmann, Germany) at 3000 rpm. The deagglomerated suspension was then blended with a polyethylene wax and polyethylene glycole based commercial binder (Licomont EK 583 G, eMBe Products, Germany). Mixing in a sigma-blade kneader (Hermann Linden Maschinenfabrik, Germany) was carried out at 140 ◦ C until the solvent was evaporated and a thermoplastic compound was formed. The amount of thermoplastic binder was chosen between 40.7 vol.% and 39.5 vol.% with regard to the nanopowder content of each composition (Table 2). Feedstocks were subsequently granulated and mixed in a twin screw extruder (d = 16 mm, l/d = 25, Thermo Fisher Scientific, Germany). The homogenizing step was carried out for two times at 140 ◦ C at 250 rpm to improve homogeneity by introducing high shear forces. After extrusion through a d = 3 mm die, the cold feedstocks were crushed for further processing. 2.2. Mold design

2. Experimental 2.1. Materials processing Three alumina-based feedstocks were developed from commercially available ceramic powders and thermoplastic binder. Composites recipes are shown in Table 1. Starting powders chosen were Al2 O3 APA0.5 (Ceralox, USA, particle size d50 = 300 nm, SBET = 8 m2 /g) and Al2 O3 SPA0.5 (Ceralox, Table 1 Powder compositions used for feedstocks. Feedstock

Matrix [vol.%]

Al2 O3

100 Al2 O3 (APA0.5)

ACY

97.9 Al2 O3 (APA0.5)

AS

95 Al2 O3 (SPA0.5)

Second phase

Dopants 1500 ppm MgAl2 O4

2.1 vol.% YCrO3 5 vol.% SiC (C25)

A mutable mold was designed for the manufacturing of ceramic cutting tools. Angles of blades are variable due to mold inserts of different angles. By changing the length of ejector pins, holes can be created in the blades, enabling a secure locking during milling by metal pins of the tool holder. Sprue position and geometry are changeable by assembling the sprue orientated mold part to an upper or lower position. Injection is thus either possible by a pin gate of d = 3.5 mm in central position or film gate from the end of the cutting blade (b × h = 11 × 0.6 mm). In order to assist complete filling the mold was prepared for optional vacuumizing. Fig. 1 shows a 3D image of a cutting tool with 55◦ cutting angle and optional pin or film gate.

3000 ppm MgAl2 O4 / 3000 ppm Y2 O3

2.3. Injection molding and heat treatment Injection molding was carried out on a hydraulic machine (Allrounder 270C 400-100, Arburg, Germany) with an 18 mm screw. The machine was equipped with position regulated screw and CIM cylinder assembly. Injection molding parameters were optimized for each feedstock. Green parts were debindered in a two-step process. First the water soluble binder part was

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Fig. 2. Particle size distributions of initial powders used. Fig. 1. 3D CAD image of a cutting tool showing sprue position and geometry.

dissolved in distilled water for five days at 30 ◦ C and further 5 days at 60 ◦ C in order to generate an open pore network. A further thermal debindering step in air up to 400 ◦ C was carried out to completely remove the residual binder. Components were presintered up to 600–800 ◦ C in the same run providing sufficient component stability for final sintering under appropriate atmospheres and sintering parameters. Heat treatment was adjusted to each feedstock dependingly on initial powders and composites developed. Final sintering was carried out in hydrogen or argon atmosphere (XVAC, Xerion, Germany). Table 3 shows heat treatment parameters of tools of the three different feedstocks. Final sintering temperatures were varied with 25 K increments. 2.4. Physical and microstructural characterization Densities were determined using the Archimedes principle. The theoretical densities were calculated from the composition of the specimen using the rule of mixture assuming 3.98 g/cm3 and 3.21 g/cm3 for the theoretical density of ␣-alumina, and ␣-silicon carbide, respectively. Sharpness of cutting edges was measured from polished cross sections (Leica MeF4, Zeiss Axiocam MRc, Germany). The microstructure of polished and thermally etched surfaces or fracture faces were analyzed by SEM (5000–10,000× magnification) in secondary electron mode at low acceleration voltage of 3 kV (Zeiss, DSM982 Gemini, Germany). Grain sizes were Table 3 Thermal treatment parameters of tools of four feedstocks. Feedstock

Final sintering temperature [◦ C]

Dwell [min]

Atmosphere

Al2 O3

1400–1475 (25 K increment; A–D) 1425–1475 (25 K increment; A–C) 1475 (D) 1725–1800 (25 K increment; A–D)

120

H2

90

H2

60 180

H2 Ar

ACY20

AS

measured by linear intercept method of SEM images ∼75 grains each, using Mendelson’s correction factor of 1.558.21 2.5. Mechanical characterization Sintered samples were lapped and polished (Struers Rotopol, Germany) to a 1 ␮m finish with diamond suspension (Buehler, UK). Microhardness HV0.1 was tested with a microindenter on at least 12 indents (Fischerscope, Germany). Indentation modulus Eind was calculated from the loading–unloading curve of the microhardness measurements according to the universal hardness method. HV10 hardness was measured with a Vickers indenter (Bareiss, Germany) on 5 indents. Indentation fracture toughness was determined by direct crack length measurements of five HV10 indents and calculating KIC,IND according to the model of Niihara (Median crack) and Evans.22,23 3. Results and discussion 3.1. Powder processing Fig. 2 shows laser granulometry measurements of the initial powders used. Silicon carbide and alumina powders show a monomodal particle size distribution with a D50 value of ≈300 nm. However the SPA0.5 alumina powder exhibits a broader size distribution compared to the APA0.5 alumina powder. Laser granulometry measurements confirm maximum D50 values of manufacturer’s data for all initial powders used. A TEM image of C25 silicon carbide powder is shown in Fig. 3 (100 kV, 0.2 nm/line, 120k× magnification). The image reveals an angular shaped powder morphology with a large number of nano-sized primary particles <100 nm. Morphology of APA0.5 alumina has been recently shown by the authors.20 SPA0.5 shows similar spherical particle shape as APA0.5, which is preferred for injection molding. 3.2. Injection molding A mold filling study was carried out in order to prove the filling behavior of the mold insert. Volume of injection was

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F. Sommer et al. / Journal of the European Ceramic Society 33 (2013) 3115–3122 Table 4 Basic injection molding parameters of feedstocks. Parameter [cm3 /s]

Injection velocity Packing pressure [bar] Barrel temperature (zone 1-2-3-4) [◦ C] Mold temperature [◦ C] Cooling time [s] Injection pressure (max.) [bar]

Fig. 3. TEM image of initial silicon carbide C25 powder.

Fig. 4. Filling study of pin gate cavity.

increased stepwise, while other parameters were kept constant. Fig. 4 shows a filling study of cutting tools by injection through the central positioned pin gate. A symmetrical filling with two flow fronts is clearly visible starting from the central position to the corner parts. Cracks orientated ±45◦ at the flow direction form during the first phase of form filling. These shear cracks are shifted along the edge until the mold is filled completely. A filling study though a film gate located at the front face is displayed in Fig. 5. A fountain like flow front with only limited jetting is visible. Although no packing pressure was applied during the

Fig. 5. Filling study of film gate cavity.

Al2 O3 /ACY

AS

20 300 150–145–145–140

20 700 145–145–145–140

62 28 867 ± 12/853 ± 7

62 25 1130 ± 6

filling studies the cavity was almost filled, no matter which sprue geometry or position was chosen. This implies an appropriate sprue and gate design in combination with good flow behavior of the feedstocks. For final manufacturing of the tools, packing pressure was applied in order to compensate the shrinkage of the feedstock during cooling to completely fill the cavity and create sharp cutting edges. By applying the identical injection speed of the Al2 O3 feedstock, resulting injection pressure showed a decrease of ≈20% for central pin gate injected compared to pin gate injected tools. Otherwise only limited packing pressures could be applied because higher packing pressures yielded in cracks along the sprue, in other words defect parts. For tools with very low packing pressures optical control revealed chunking or incompletely filled edges. Moreover cracks moving along the tool edge during filling were considered disadvantageous for final edge stability. Further experiments were only carried out with the film gate. Injection molding parameters were adjusted for each feedstock separately with respect to the optical control of cutting edges. Basic injection parameters of feedstocks just slightly vary among each other (Table 4). Further parameters such as metering time, melt cushion or dynamic pressure are not displayed. First investigations concentrated on speed of injection. In literature it is well documented that high injection speeds and thus high shear forces may cause powder-binder segregation, especially for sub␮m and nanopowders.24,25 However slow injection speeds often result in short shots as the feedstock will rapidly cool in the sprue and the cavity of the mold. Hence higher packing pressures must be applied for complete filling of the cavity, which can introduce residual stress in the component. These stresses may relax during the thermal treatment and result in defective components.26 In this study components produced with 10 cm3 /s revealed noncomplete edges. Even a high packing pressure of 700 bar could not develop sharp edges. Hence injection speed was increased up to 20 cm3 /s, whereupon edges where duly completed. A high packing pressure of 700 bar was only applied for the AS feedstock, where the quality of the edges increased. Because neither edge quality nor the recorded total volume of injection increased for all tools of other feedstocks a packing pressure of 300 bar was applied. Generally standard deviations of injection pressures recorded for all feedstocks are quite low indicating high feedstock qualities and good repeat accuracy of the injection molding machine. To provide injection speed of 20 cm3 /s injection

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Fig. 6. Relative density and grain sizes of produced Al2 O3 , ACY and AS cutting tools.

pressures resulted in 867 bar and 853 bar for the Al2 O3 and ACY feedstock, respectively. Much higher pressure was required for the AS feedstock, where only 5 vol.% of SiC second phase was present. Even though the adjusted nozzle temperature was not identical with ACY feedstocks, pressure required was much higher than expected. Fines of <100 nm and plate shape larger SiC grains in the initial powder (Fig. 3) seem to be the reason for the strong increase in injection pressures. Despite a 1.2 vol.% lower powder loading the large surface of silicon carbide particles and accompanying higher shear forces of particles show an enormous influence on the molding. 3.3. Physical and microstructural features A reduced amount of remaining pores, hence high relative densities have been shown beneficial for long life-time cutting tools.12 In contrast to processing techniques such as dry pressing or cold isostatic pressing where final densities closely reach theoretical densities, high relative densities from ceramic injection molding parts are quite challenging.27,28 The comparably low green density due to the high binder content mandatory for the thermoplastic molding is often the reason for incomplete densification. Hydrogen sintering has been documented to increase densification29 and was thus chosen as atmosphere for the plain alumina and ACY tools. Because nitrogen atmosphere supports the formation of nitrides during sintering, alumina silicon carbide tools where sintered in argon atmosphere. The sintering of alumina silicon carbide has been successfully shown by Smirnov et al.30 Fig. 6 gives the relative densities of three material systems developed at different heat treatments each. Densities measured are very promising for cutting tool application as densities minimal reached ≈96%. With increasing sintering temperature densities rise as expected. Note that ACY sample of heat treatment D is not in a row with increasing sintering temperatures. Heat treatment of 1475 ◦ C and 60 min dwell is similar to heat treatment B (sintering at 1450 ◦ C and 90 min). Density results are thus in good agreement. Very high sintering temperatures are required in order to densify alumia silicon carbide composites, even with very low second phase content. The sintering of alumina silicon carbide has been intensively studied by Ihle.

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The formation of CO, SiO, Al2 O and Al was detected at high sintering temperatures, whereas at sintering below 1850 ◦ C no change in compositions was detected by XRD.31–33 In order to reduce sintering temperatures and enhance densification behavior sintering additives as MgAl2 O4 and Y2 O3 were used. High relative densities >97.3% confirmed the successful liquid phase sintering strategy for AS tools. Density rises with temperature until 1775 ◦ C. The dropdown in density could be attributed to a possible formation of Al6 Si2 O13 , Al4 C3 or Al4 SiC4 at higher temperatures. Presuming a perfect mold filling and accordingly cutting tools of perfect sharpness, special attention has to be attributed to initial powders used. Grain sizes of sintered tools finally limit its accomplishable sharpness considering one grain as a final cutting edge. Therefore fine-grained microstructures are not only favored due to higher mechanical properties and wear behavior but also edge sharpness. Fig. 7 shows polished cross-sections of sintered cutting tools. Edge radii of ≈10–11 ␮m were achieved after sintering without further post-processing steps. Images show relatively rough flank spaces and some small pores in the central part. Microstructures of polished and thermally etched surfaces are displayed in Fig. 8. Surfaces reveal almost defect-free structures, which should be also the case for the outer part of injection molded tools (compare Fig. 7). Matrix grain sizes increased with increasing temperature as expected due to the driving force to reduce internal energy by reducing the grain boundary area. In literature it is well documented that the formation of YAG results in higher creep resistance.34–36 This phenomenon is linked to the fact that higher sintering temperatures must be applied in order to reach high densities. Accompanying grain growth is thus indispensable. Matrix grain sizes were measured based on the linear intercept method of at least 75 grains each. Alumina grains from plain APA resulted in grains of 904 nm size up to 1385 nm at highest sintering temperature. Alumina grains in ACY composite generally exhibit larger matrix grain sizes starting at 1284 nm at lowest up to 2406 nm for highest sintering temperature (Fig. 6). From images it can be concluded that YAG precipitates form clusters of 1–2 ␮m size and are not homogenously distributed at matrix triple points. Single YAG grains form YAG agglomerates are of 300–400 nm size. Grain sizes of AS composites are only roughly estimated from fracture faces. Alumina grains increased to ≈3 ␮m even for lowest sintering parameters applied, while small SiC grains remain at size of ≈100–600 nm size. Fracture of AS samples proceeds predominantly intergranular along grain boundaries. 3.4. Mechanical properties Fig. 9 shows Vickers hardness of plain Al2 O3 , ACY and AS cutting tools. Maximum hardness of 2087 HV10 and 1900 HV10 was measured for Al2 O3 and ACY tools, respectively. Increasing hardness can be attributed to the rise in density but just slow increase of grain sizes. With increasing sintering temperatures the strong grain growth seems to overcompensate the density effect. Hardness values consequently remain on similar level or smoothly drop as already stated by Rice et al.37 This effect

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Fig. 7. Cross section images and edge radii of produced Al2 O3 , ACY and AS cutting tools.

becomes even stronger for AS tools as hardness dramatically drops for higher sintering temperatures. Highest hardness of 2013 HV10 was measured for the lowest sintering temperature. Wear and fracture behavior are of major interest as they finally define the life of a cutting tool. Toughness was measured keeping in mind that values from toughness indentation methods have been criticized in literature lately.38 On the other hand measured toughness values cannot be directly linked to real working conditions. Fracture behavior of tools should thus also be examined in further tests of real application. Maximum fracture toughness √ measured for final √ tools in this study√were 2.8 (3.7) MPa m, 3 (4.3) MPa m and 3.7 (4.1) MPa m based on Evans formula for ACY, Al2 O3 and AS respectively. The fracture toughness of ceramics has been

Fig. 8. SEM microstructures of produced Al2 O3 , ACY and AS cutting tools.

Fig. 9. HV 10 hardness and fracture toughness (Evans formula) of produced Al2 O3 , ACY and AS cutting tools.

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also discussed in detail by Ponton and Rawlings. 19 most frequently used toughness equations were compared with each other based on different ceramic materials.39 Considering single edge notched beam tests as real toughness, values calculated from the Evans formula seems most appropriate as concluded for non-transformable alumina ceramics by Ponton.40 Because Niihara formula is often used for toughness calculations in literature, Niihara toughness values are quoted in brackets for comparison. 4. Summary and conclusion Three different alumina-based feedstocks were successfully developed from commercially available initial sub-␮m powders. All ceramic feedstocks provided good mold filling behavior. The drawback of pin gate in this study is the lower holding pressure, which can be applied without crack formation. Green parts reveal chunking edges after molding. Injection molding with film gate is thus favored due to sharper cutting edges achieved. Injection molding parameters were optimized for each feedstock. Injection pressures were 853–1130 bar in order to retain injection speed of 20 cm3 /s, which showed best mold filling obligatory for sharp cutting edges. Cutting tools were subsequently debindered and sintered. Sintering atmosphere and parameters were chosen for material systems each. Cutting edge radii of ≈10–11 ␮m were measured after sintering without postmachining processing. High relative densities of 96% up to 98.7% were achieved and confirmed by SEM microstructures. Toughness values were just of moderate level but have to be examined in real cutting operations. The very high hardness values of up to 2087 HV10 show a promising prospect for future ceramic wood cutting tools. Results of this study have shown the successful production of ceramic wood cutting tools by injection molding. Further investigations in real wood machining must further verify the potential application of produced tools. 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. References 1. Schneider SJ. Engineered materials handbook – ceramics and glasses, vol. 4. ASM International; 1992. p. 808–10. 2. Prakash LJ. Application of fine grained tungsten carbide based cemented carbides. Int J Refract Met Hard Mater 1995;13:257–64. 3. Costes JP, Larricq P. Towards high speed in wood milling. Ann For Sci 2002;59:857–65. 4. Fengel D, Wegener G. Wood – chemistry, ultrastructure, reactions. Berlin: Walter de Gruyter; 1984. p. 34–5. 5. Stewart HA. High temperatures wear tools when wood machining. In: Mod. Woodworking.; 1993. p. 1–4. 6. Darmawan W, Rahayu IS, Tanaka C, Marchal R. Chemical and mechanical wearing of high speed steel and tungsten carbide tools by tropical woods. J Trop For Sci 2006;18(4):255–60.

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