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Fabrication of Short-Fibre Reinforced SiCN by Injection Moulding of Pre-Ceramic Polymers

A. Müller-Köhn, M. Ahlhelm, A. Neubrand, H. Klemm, T. Moritz, A. Michaelis

Pre-ceramic polymers were used as organic vehicle in this study forming a ceramic matrix after pyrolysis. Carbon short-fibres were chosen as fibre materials. Because of inadequate processing properties of the used pre-ceramic polymer, blending with thermoplastic waxes and process additives was necessary. An amorphous Si–C–N network was obtained as the matrix of the short-fibre composite after pyrolysis of the polysilazane precursor. In this study, the influence of different fibre types, fractions and lengths on com- pounding and moulding has been analysed. Torque measurements were used for characterising the flow ability and the thermal stability of the feedstock. Simulation of injection moulding processes was used as tool for predicting the form filling of test moulds as well as for supporting future development and moulds construction. In fibre-reinforced parts, the fibre distribution and alignment affect the mechanical proper- ties. With injection moulding simulation, the fibre alignment can be predicted and arranged. This offers the chance for smart comprehensive design of parts and moulds to match the mechanical requirements. The resulting composite parts have been analysed regarding their microstructure and mechanical properties.

1 Introduction of a ceramic powder as filler and a ther- visualized and analysed for strength or Producing advanced ceramics by injection moplastic binder system acting as organic damage tolerance improvement. In previous moulding allowed the application in areas ve­hicle. So far, in industrial application, the publications, it was shown that a PIM simu- of high cost pressure. In powder injection thermoplastic polymers are completely re- lation, in this case with Moldex3D® (Sim- moulding, the so-called feedstock consists moved after shaping. The polymer content paTec Simulation & Technology Consulting in the feedstock reaches 35–50 vol.-%. GmbH), is a valuable tool for predicting the Problems, arising with the binder removal, flow behaviour of a ceramic powder-filled A. Müller-Köhn, A. Ahlhelm, H. Klemm, were found to cause defects during binder polymer melt and its effects on the final cer­ T. Moritz, A. Michaelis burnout or high shrinkage during further am­ic part. Fraunhofer Institute for Ceramic thermal treatment [1]. In contrast to estab- After pyrolyzing, a highly porous, amorph­ Technologies and Systems (IKTS) lished powder injection moulding, the appli- ous ceramic matrix is formed, which could 01277 Dresden cation of pre-ceramic polymers allows using provide a basis for Ceramic Matrix Com- Germany the organic vehicle as ceramic matrix after posites (CMCs). Previous work used pre- pyrolysis. In this way, high solids loading, ceramic polymers with ceramic powders in A. Neubrand almost zero shrinkage and reduced porosity order to achieve Si–C, Si–C–N or Si–O– Fraunhofer Institute for Mechanics of shall be reached. C–N ceramics [2–5]. The ceramic compos- Materials (IWM) The application of pre-ceramic polymers in ites showed for example low density, high 79108 Freiburg injection moulding requires special proper- strength at high temperatures, high and Germany ties of the polymer. The pre-ceramic poly- stable friction coefficient and relatively high mers have to be meltable with a certain damage tolerance in comparison with other Corresponding author: A. Müller-Köhn freezing point, they shall show a low viscos- ceramics [6]. For injection moulding, only E-mail: ity and limited thermal cross-linking. These short fibres are suitable for processing of [email protected] aspects demand compromises in ceramic CMCs analogical to manufacturing of fibre- yield of the polymer. reinforced plastics [7]. Carbon short-fibres Keywords: CMC, injection moulding, Injection moulding process simulation can were also used in warm-pressing of CMCs carbon fibre, fibre reinforced provide accurate and detailed predictions in the LPI-route for producing brake discs or of 3D-fibre orientation, which can be also pads for automotive application [8].

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Tab. 1 Precursor properties Tab. 2 Fibre properties

Trade Name Melting Temperature Density Ceramic Yield Length Fibre Type Finish [°C] [g/cm³] [%] [mm] Water-based Toho Tenax® HT C124 6 ML 33 S66 75 1,01 73 (removed by washing) SGL Sigrafil C 0,5 00B none 0,5 Ceraset HTT1800 N/A 1,02 86,3 Toho Tenax® HT M100 none 0,1

In comparison to these materials different stability were conducted at production tem- Tab. 3 Powder properties properties are expected since fibre spread- perature (150 °C). As input data for process Powder Type Trade Name d50 ing in injection moulded parts will be more simulation, temperature conductibility (DIN [µm] dominant than the pressed fibre bundles. EN 821), specific heat capacity (DIN 51007) SiC Sika Unikiln FCP07 3,0 Nevertheless, advanced properties are ex- and thermal expansion coefficient (DIN pected like high and stable friction coeffi­ 51045) were determined, thermal conduc- cient, excellent thermal shock resistance tivity was calculated (Tab. 4). Tab. 4 Simulation data input and damage-tolerant behaviour [6, 8, 9]. The rheological behaviour of the prepared Temperature conductibility 0,61 mm2/s feedstocks was characterised by high-pres- (DIN EN 821) 2 Experimental sure capillary rheometry (RH10, Malvern Specific heat capacity 0,911 J/(g ⋅ K) Instruments/DE) at 130, 140 and 150 °C, (DIN 51007) 2.1 Materials respectively, with a 1,5 mm diameter die. Thermal expansion 2,94 ⋅ 10–5 /K Two different types of polysilazane were The apparent shear rate was increased coefficient (DIN 51045) −1 used as pre-ceramic polymers. For shaping stepwise in the range 50–5000 s . The Thermal conductivity 1,0 W/(m ⋅ K) the meltable ML33 S66 (Clariant/DE) and compound showed typical shear thinning Density 1,41 g/cm³ for infiltration Ceraset HTT1800 resin (KDT behaviour without any flow anomalies. The Inc./US) were chosen. The properties of the specific volume in correlation of pressure meltable precursor concerning viscosity and and temperature (pVT) was determined Moldex3D® (Simpatec/DE, CoreTec Inc.TW) wetting behaviour for the filler material according to ISO 17744 under isobar cool- uses an Improved Anisotropic Rotary Dif- were not adequate. For this reason, appli- ing (6 K/min) in a pressure range between fusion (iARD) technique in contrast to the cation of thermoplastic waxes as plasticizer 200–1600 bar from 40–170 °C by PVT100 conventional Folgar-Tucker-model-based and lubricants were necessary. pVT-measuring-system (SWO Polymertech- techniques. The obtained material charac- Characteristic data of pre-ceramic polymers nik GmbH/DE). teristics were fitted into the Moldex3D® are summarised in Tab. 1. Ceramic yield was Porosity of pyrolyzed samples was deter- (Release 10) material models. The meshing determined after pyrolysis in argon atmos- mined by high-pressure mercury porosim- of different test geometries was done by phere at 1000 °C and 1 K/min heating rate. etry (Micromeritics AutoPore IV 9500). Moldex 3D Mesh which bases on the Rhi- Three different commercial types of carbon Mechanical testing was performed by noceros Modeling Tools for Designers v. 4.0 short-fibres with diameter of 7 µm, 100 µm the 4-point-bending test (in dependence (Robert McNeel & Associates). and 500 µm as well as 6 mm in length with on DIN EN 843-3, however with testing or without water-soluble coating were used geom­etry as fired), Young’s modulus was 2.4 Sample preparation in this study. The influence of different fibre measured by the sonic resonance method. After heating at 100 °C for removing re- fractions and lengths was analysed. Before For testing, the sample geometry was not sidual moisture, the fibres and powders compounding, the water-soluble coating modified. The SEVNB-method was used were compounded with the polymers in a had been removed by washing. The differ- for measuring fracture toughness [10]. The double Z-blade kneader (LKII, Linden/DE) ent type of fibres are summarised in Tab. 2, ­ratio of the notch depth a to the specimen at 150 °C for 1–2 h. The compound was properties of used powders are shown in height W was 0,22 (1,5 mm notch depth for granulated on a cutting mill (Pulverisette Tab. 3. 7 mm specimen height). 19, Fritsch/DE). The fibre content varied between 17–25 vol.-%, smaller fibre con- 2.2 Characterisation methods 2.3 Injection moulding simulation tent in feedstock number 3 was selected Feedstock development was started in a The injection moulding of fibre-reinforced because of reduced fibre degradation and torque rheometer (Plastograph, Brabender/ composites struggles with fibre-induced better flowability. The compound compos­ DE) equipped with a W50 measuring mixer anisotropic mechanical properties strongly itions are described in Tab. 5. with a chamber volume of 55 cm³. This as- depending on the fibre alignment. In a re- Shaping was carried out on an injection sembly allowed compounding of small ma- sult, the moulded products may have high moulding machine (Allrounder 370C, Ar- terial amounts under practice-oriented con- internal stress. The flow-induced fibre orien- burg/DE) at a feedstock temperature of ditions. Measurements of the cross-linking tation and anisotropic shrinkages in injec- 150 °C and a mould temperature of 40 °C. behaviour, wetting properties and mixture tion moulding are complex 3D-behaviours. The injection unit was equipped with hard- cfi/Ber. DKG 97 (2020) No. 7-8 E 47 PROCESS ENGINEERING

Tab. 5 Feedstock composition careful heating up to 950 °C within 270 h. Material Polymer Solid Fibre Fibre Powder Powder Reinfiltration process was executed under Feedstock Type Content Loading Type Content Type Content vacuum with Ceraset HTT1800. Curing 25 vol-% 25 vol.-% and following pyrolysis were also done in 1 C –SiCN 29,1 mass-% 50 vol.-% 0,1 mm SiC f 25,5 mass-% 45,4 mass-% oxygen-free argon flow of 5 l/h and heating 25 vol.-% 25 vol.-% up to 950 °C within 72 h. 2 C –SiCN 29,1 mass-% 50 vol.-% 0,5 mm SiC f 25,5 mass-% 45,4 mass-% 3 Results and discussion 17 vol.-% 33 vol.-% 3 C –SiCN 28,3 mass-% 50 vol.-% 6 mm SiC f 15,3 mass-% 56,4 mass-% 3.1 Compound development For processing by conventional injection Tab. 6 Injection moulding parameters moulding machines, a thermoplastic be- Nozzle Tool Injection Cooling Injection Cycles haviours of the compound is needed. Fur- Temperature Temperature Flow Rate Time Pressure Time thermore, a requirement for mould filling 150 °C 40 °C 9 cm³/s 30 s 560 bar 41 s is a good flowability and for ejection a fast freezing as well as good warm strength. Because of noncompliance of pure polysila- zane polymer material, it is necessary to (i) blend the used precursor with thermoplas- tic polymers to ensure shaping, and (ii) with additives for filler wetting and dispersion. In a measuring mixer, blends of precur- sor with process additives were tested for 80 min under different shear conditions (Fig. 2). The rotation speed of the blades in the mixer was set from 10–150 rpm. As exemplary filler system, a composition of 25 vol.-% 100 µm carbon fibres and 25 vol.-% SiC was applied. For appointing processing temperatures, it is necessary not to excess a cross-linking temperature of Fig. 1 Green dimensions of sample geometry 180 °C. Separations were detected by increasing torque during the testing procedure. These separations in pure polysilazane and add­ itional plasticizer compounds were caused by the high aspect ratio of the fibres and gravity was observed due to slight differ- ences in densities of polymers and filler. Such separations, detected by increasing torque over testing time indicate a poor fibre-polymer-bonding which can be regu- lated by wetting and lubrication additives.

3.2 Compound preparation In compounding of fibres in polymers, dif- ferent challenges were met. The biggest problem was the plasticization of the fibre- polymer-mixture because of the low bulk- Fig. 2 Compound stability test on a measuring mixer at processing temperature (150 °C) density. This fact resulted in long periods for mixing which induced to cross-linking of the precursor. The initial length of used metal screw and cylinder for wear reduc- moulding parameters are summarised in fibres was 100 µm, 500 µm and 6 mm. tion. Specimen rectangular bars measuring Tab. 6. ­After processing, fibre length was measured 7 mm × 6 mm × 70 mm were produced in Pyrolysis of moulded parts was carried out by optical microscopy and by FESEM in- a quad-cavity mould (Fig. 1). The injection in an oxygen-free argon flow of 5 l/h and by vestigations. During processing, the longer

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fibres were cut in lengths of 250–350 µm. The reason for this strong shortening was the appearance of high shear-forces while compounding which is necessary for plas- ticization and homogenization. An alterna- tive compounding route could be the appli- cation of specific injection units which are equipped directly in line with an upstream compounding extruder for endless fibre feeding.

3.3 Simulation results Simulation of a simple bar geometry results in typical fibre orientation profile which is well known from injection moulded fibre- reinforced plastics (Fig. 3). Central areas are randomized in their orientation while outer areas are strongly aligned in flow dir­ ection (Fig. 4). The reason of this distribu- tion is the shear and velocity profile during mould filling. In presence of obstacles, for example the mould cores for creating the through-holes (Fig. 5), there is a high orien- Fig. 3 Fibre orientation in bars after complete mould filling, orientation grade from low tation around that barrier geometries. After (blue) to high (red) passing the first hindrance, a diagonal shift of flow profile occurs because of the asym- metrical position of the inserts, resulting in different velocity profile. In the broader partial stream, areas of high randomization are developed. In the area of coalescence, a weld line is generated with highly oriented fibres. At these locations, injection moulding defects like entrapped air and cold shuts are ex- pected. Such regions are weak points in further processing or applications where cracks may arise.

3.4 Sample characterisation In the mould filling period, non-optimised feedstock compositions have shown in- stable flowing or even faults like jetting because of bad filler connection and high viscosity of compounds. An optimised poly- mer blend composition provided stable flow front and consistent processing parameters. Fig. 4 Cross section in detail: highly aligned surface layer and randomized core, Inconsistent flowing causes defect for- asymmetry caused by gate positioning mation during thermal treatments. Fig. 6 shows injection faults and optimised feed- alignment during injection and mould fill- with 6 mm fibre initial length because of stock flowing. ing determines the mechanical behaviour. highest solid loading and highest percent- With increasing fibre content, flowability Pyrolyzed specimens of applied feedstock age of silicon carbide powder. Properties of and mould filling properties of feedstocks compositions show externally no cracks and specimen after pyrolysis and infiltration are had been changed to the worse. High in- only 1–2 % linear shrinkage. In the 4-point- summarised in Tab. 7. jection pressures and distinctive defect bending-test, a strength level of 49,4– Reinfiltration resulted in lower porosity and formation after pyrolysis were the results. 60 MPa was reached after reinfiltration. also in higher bending strength as well as The typical fibre distribution caused by Highest strength obtained compositions better stiffness. In bending test, no typical cfi/Ber. DKG 97 (2020) No. 7-8 E 49 PROCESS ENGINEERING

force-deflection graphs were detected. Ma- terial compositions with high fibre amount show more processing faults which lead to more sample defects. In this point of view, differences in mechanical properties between several material compositions re- sult from these defects and porosity. Fibre loading itself is mainly not the key fact for mechanical properties, but defect formation and controlling. Fracture toughness was investigated for non-infiltrated specimen only. The values increased from 0,68–1,62 MPa√m corres­ ponding to increasing fibre initial length. Also crack deflection rised to the same de- gree (Fig. 7).

3.5 Microstructure Amorphous SiCN is located between the short fibres and the powder. Because of the application of pre-ceramic polymers, ceramic matrix was formed in first pyroly- Fig. 5 Simulation results of bars with through-holes sis (Fig. 8). On this route, the CMC-semi- finished products obtain even higher green densities than conventional ceramic injec- tion moulded or dry pressed parts. Fibre shortening is significant for long ini- tial length, which depends directly on the volume content of fibres. Higher contents have to suffer quite higher shortening than compositions with lower contents. For feed- stock composition with 6 mm initial fibre length, the volume content decreased to only 17 vol.-%. Tab. 8 shows results of the FESEM investigations of the fibre length dis- tribution. The specimens show typical fibre orientation profile which is well known from injection moulded fibre-reinforced plastics. Central areas were randomized in their orien­ta­tion while outer areas were strongly aligned in flow direction. The occurrence of Fig. 6 Injection faults and optimised flowing in the mould filling period for different randomized areas depends on the fibre con- feedstock compositions (17–40 vol.-% fibre content, 6 mm initial fibre length) tent. High amounts of fibres result in very strict divided orientation areas. Between these differently orientated fibre areas, high Tab. 7 Resulting properties of pyrolized specimen strains appeared in thermal treatment be- Young’s Porosity Density 4-Point-Bending-Strength cause of the different shrinkage behaviour. Feedstock Modulus [%] [g/cm³] [MPa] On these boundary layers, defects could be [GPa] developed. Fig. 9 shows randomized fibre 25 vol.-% 0,1 mm 24,4 1,81 33,8 24 orientation areas and resulting defects after 25 vol.-% 0,1 mm inf. 13,8 1,99 49,4 42 pyrolysis. 25 vol.-% 0,5 mm 27,0 1,71 37,8 28 4 Conclusions 25 vol.-% 0,5 mm inf. 20,6 1,85 59,8 41 In this study, the feasibility of injection 17 vol.-% 6 mm 18,9 1,94 49,7 48 moulding of carbon short-fibre filled pre- 17 vol.-% 6 mm inf. 10,7 2,17 60,0 59 ceramic polymers was verified. Polymeric

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Fig. 7 Increasing crack deflection in the SEVNB test

Fig. 8 Microstructure after pyrolysis: after first pyrolysis (l.), and after two times infiltration and pyrolysis (r.) precursor incorporating carbon short-fibres Tab. 8 Results of FESEM investigation of fibre length and ceramic powders up to 50 vol.-% Average Initial Fibre Length Range of Shortened Fibres loading blended with technical thermo- Composition [mm] [mm] plastic polymers can be compounded and (Manufacturer Information) (Measured) processed by standard injection moulding 25 vol.-% 0,1 mm 0,1 0,085–0,125 machines. 25 vol.-% 0,5 mm 0,5 0,1–1,175 The fibre orientation can be predicted by 17 vol.-% 6 mm 6 0,175–0,350 numerical calculations using commercial with Moldex3D® mould filling simulation tool. The knowledge about fibre distribu- tion and constructive manipulation of them is important for technical realisation of complex-shaped components and optimisa- tion of tool designs. By smart design, weak areas can be avoided or shifted deliberately to lower loaded regions. The process simu- lation data can also be the starting point for further numerical analysis. Software like DIGIMAT or even ANSYS may Fig. 9 Randomized fibre orientation areas and resulting defects after pyrolysis be used as tool for transforming injection moulding simulation data to input data for mechanical simulation. In this way, an im- ened by processing down from 100 µm to shortening and thermal cross-linking would proved part design can by developed ­under 350 µm depending on the initial length. The be restricted. contribution of manufacturing process spe- reason for this strong shortening effect is Mechanical properties of tested specimen cialties. the appearance of high shear forces in the were disappointing in comparison with the Further pyrolysis of moulded parts was car- compounding and injection step. By ap- warm-pressed short-fibre reinforced sam- ried out. The carbon short-fibres were short- plication of a double screw extruder, fibre ples [6]. In injection moulded parts, carbon cfi/Ber. DKG 97 (2020) No. 7-8 E 51 PROCESS ENGINEERING / INDEX TO ADVERTISERS

short-fibres are dispersed in the matrix and References [6] Krenkel, W.: Ceramic matrix composites. Fiber are shortened during the intensive com- [1] German, R.M.; Bose A.: Injection molding of reinforced ceramics and their applications. pounding and shaping process. Because of metals and ceramics. Princeton, Metal Powder Weinheim 2008 this, shortening the benefit of effective fibre Industries Federation, 1997 [7] Herzog, A.; et al.: Novel application of ceramic length for toughening or damage tolerant [2] Kroke, E.; et al.: Silazane derived ceramics and precursors for the fabrication of composites. J. effects is limited. Highly porous material related materials. Mater. Science and Engin. of the Europ. Ceram. Soc. 25 (2005) 187–192 compositions are an excellent starting point 26 (2000) 97–199 [8] Krenkel, W.; Heidenreich, B.; Renz, R.: C/C–SiC for infiltration, nitridation or siliconizing pro- [3] Zhang, T.; Evans, J.; Woodthorpe, J.: Injection composites for advanced friction systems. Adv. cesses. By infiltration, mechanical properties moulding of silicon carbide using an organic Engin. Mater. 81 (2002) [4] 427–436 could be significantly improved. Longer -fi vehicle based on a preceramic polymer. J. [9] Herzog, A.; Wötting, G.; Vogt, U.F.: Short-fibre- bres introduce better fracture toughening of the Europ. Ceram. Soc. 15 (1995) 729– reinforced reaction-bonded silicon nitride effects by crack deflection. Defect forma- 734 (RBSN) by precursor route: Processing and tion determines in general the mechanical [4] Gonon, M.; et al.: Manufacture of monolithic properties. J. of the Europ. Ceram. Soc. 27 properties and there is a further challenge ceramic bodies from polysilazane precursor. (2007) 3561–3572 in processing for technical application. University of Limerick, 1994 [10] Choi, S.R.; Gyekenyesi, J.P.: Assessments of [5] Walter, S.; et al.: Injection moulding of poly­ fracture toughness of monolithic ceramics: Acknowledgement siloxane/filler mixtures for oxycarbyde ceramic SEPB versus SEVNB methods. NASA/Techical Financial support of the Fraunhofer Society/ composites. J. of the Europ. Ceram. Soc. 16 Memorandum 2006-214090, NASA, Glenn DE is gratefully acknowledged. (1996) 387–393 Research Center, 2006

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