materials

Article Development of a System for Additive Manufacturing of Matrix Composite Structures Using Laser Technology

Stefan Polenz 1,*, Willy Kunz 2 , Benjamin Braun 3, Andrea Franke 4, Elena López 1, Frank Brückner 1,5 and Christoph Leyens 1,6

1 Fraunhofer Institute for Material and Beam Technology (IWS), 01277 Dresden, Germany; [email protected] (E.L.); [email protected] (F.B.); [email protected] (C.L.) 2 Fraunhofer Institute for Ceramic Technologies and Systems (IKTS), 01277 Dresden, Germany; [email protected] 3 Space Structures GmbH, 12435 Berlin, Germany; [email protected] 4 AXIAL Ingenieurgesellschaft für Maschinenbau mbH, 01445 Radebeul, Germany; [email protected] 5 Product and Production Development, Department of Engineering Sciences and Mathematics, Luleå University of Technology, 97187 Lulea, Sweden 6 Institute of Materials Science, Technische Universität Dresden, 01069 Dresden, Germany * Correspondence: [email protected]

Abstract: Ceramic matrix composites (CMCs) are refractory ceramic materials with damage-tolerant behavior. Coming from the space industry, this class of materials is increasingly being used in other applications, such as automotive construction for high-performance brake discs, furnace technology,   heat for pipe systems and landing flaps on reusable rocket sections. In order to produce CMC faster and more cost-efficiently for the increasing demand, a new additive manufacturing Citation: Polenz, S.; Kunz, W.; Braun, process is being tested, which in the future should also be able to realize material joints and higher B.; Franke, A.; López, E.; Brückner, F.; component wall thicknesses than conventional processes. The main features of the process are as Leyens, C. Development of a System for Additive Manufacturing of follows. A ceramic fiber bundle is de-sized and infiltrated with ceramic suspension. The bundle Ceramic Matrix Composite Structures infiltrated with matrix material is dried and then applied to a body form. During application, the Using Laser Technology. Materials matrix material is melted by laser radiation without damaging the fiber material. For the initial 2021, 14, 3248. https://doi.org/ validation of the material system, samples are pressed and analyzed for their absorption properties 10.3390/ma14123248 using integrating sphere measurement. With the results, a suitable processing laser is selected, and initial melting tests of the matrix system are carried out. After the first validation of the process, a Academic Editor: A. test system is set up, and the first test specimens are produced to determine the material parameters. Javier Sanchez-Herencia Keywords: ceramic matrix composite; additive manufacturing; laser technology; wavelength depen- Received: 30 March 2021 dent absorption rate; integrating sphere measurement Accepted: 8 June 2021 Published: 12 June 2021

Publisher’s Note: MDPI stays neutral 1. Introduction with regard to jurisdictional claims in published maps and institutional affil- The conventional production of fiber-reinforced ceramic components mainly takes iations. place via infiltration (PIP), melt infiltration process (MI), chemical vapor infiltration (CVI) or processes [1–9]. Another method that is still being researched on a small laboratory scale is electrophoresis deposition [10]. In the following, these methods will be briefly described, and their advantages and disadvantages will be mentioned. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. In the case of the PIP, a fiber preform is produced as the starting framework, e.g., made This article is an open access article of fibers. This is infiltrated with a matrix polymer in a temperature range between ◦ distributed under the terms and 150 and 300 C, followed by the chemical crosslinking of the polymer. The resulting green conditions of the Creative Commons body is referred to as carbon fiber-reinforced polymer (CFRP). By means of pyrolysis under ◦ Attribution (CC BY) license (https:// inert conditions from 700 up to 1600 C, a ceramic matrix composite (CMC) arises from the creativecommons.org/licenses/by/ CFRP. Due to the volume shrinkage, which occurs during pyrolysis, the matrix is being 4.0/). pervaded by pores and cracks. By the repetition of the process, steps of pure filtration,

Materials 2021, 14, 3248. https://doi.org/10.3390/ma14123248 https://www.mdpi.com/journal/materials Materials 2021, 14, 3248 2 of 10

crosslinking and pyrolysis several times, the resulting can be reduced iteratively to a minimum. In the end, however, a low remaining porosity must be tolerated. The advantages and disadvantages of the PIP and other methods are summarized in Table1.

Table 1. Advantages and disadvantages of different production methods for fiber-reinforced ceramic components [1–11].

Method Advantages Disadvantages

- similarity in design and - high cost of ceramic precursors manufacturing with conventional - very long process time CFRP standards and low tooling - multiple repetitions of the costs leads to cost-efficiency infiltration, crosslinking and - possibility of in situ joining of pyrolysis pre-hardened parts in the - high residual porosity and low

Polymer Infiltration Pyrolysis autoclaving process interlamellar shear strength

- processing time under 5 days - residual metal limits the application - dense parts (porosity under 2%) temperature - low cost - high process temperatures during - complex and near-net shapes may infiltration be fabricated - possible attack of fibers by metal Melt Infiltration - high electrical conductivity

- high flexibility in the variety of - very long process time shapes - only small wall thicknesses (3–5 - in large ovens, many parts of mm) processable different shapes can be infiltrated - closed residual porosity with pore simultaneously diameters up to 0.5 mm

Chemical Vapor Infiltration - only one infiltration step

- use of conventional sintering - only for oxide matrix materials technology - porosity values around 20% - inexpensive - low operating temperatures around - low processing temperatures at Sintering 1600 ◦C - 1000 to 1200 ◦C

- complex powder preparation and dispersion - cold process - limitation to thin component - without fiber damage thicknesses - complex process technology

Electrophoresis - porosity

- simple processing - high flexibility in the variety of - strength level unknown yet shapes - sensitive fiber bundle during - no size limitations processing - implementation of functional - elaborate fiber processing materials or components possible Laser-induced Fabrication of CMC

The MI process is similar to the PIP process but enables the production of CMCs with a higher . In this case, the PIP process is interrupted after the first pyrolysis. Instead of repeating the previous process steps, the remaining void is infiltrated with metal instead (for example, liquid infiltration (LSI)). This process adaption enables the reduction of porosity. The infiltrated metal closes the pores. In order to avoid the damage of the fibers by metal, the infiltration process must be carried out as quickly as possible. The third conventional method is the CVI. Again, the basis is a fiber braid stabilized near the final contour. The fiber preform is heated in an oven and then flushed by the process gas. The process gas reacts, and some of the reaction products deposit on the fibers Materials 2021, 14, 3248 3 of 10

and form the matrix of the CMC. The remaining reaction products are filtered out of the process chamber. Another variant for the production of oxidic CMC is sintering for matrix genera- tion. Here, the matrix material is produced from starting materials by high-temperature treatment. These matrix precursor materials have lower melting temperatures (1000 to 1200 ◦C) than the final matrix (about 1600 ◦C). This allows processing without damaging the oxidic fibers by too high temperatures during processing. The matrix material is milled to powders in the nanometer particle size range and then processed into suspensions. The slurries are introduced between the fibers and sintered at the sintering temperature with high volume shrinkage. Another method for matrix generation of CMC that is not yet conventionally used is electrophoresis. This involves dispersing electrically charged ceramic matrix particles in a liquid. The matrix particles are then deposited on oppositely charged fiber surfaces and between the fibers (electrodes) in a direct current electric field. The spaces between the fibers are filled with matrix material until the capillaries between the fibers are closed so far that the charged particles can no longer infiltrate. Here too, pore space remains open. In the experiments presented below, a CMC is to be produced for the first time using laser technology (Coherent Diamond J-3 10.6 400 W OEM laser, Coherent, Palo Alto, CA, USA) and a computer-controlled laydown mechanism (Axial Ingenieurgesellschaft GmbH, Dresden, Germany). For this purpose, samples of the powder materials SiO2 and Y2O3 are first pressed and then melted by the laser. The SiC fiber material mixed into these samples as particles are given special consideration in the subsequent ceramography. It should not melt under the determined process parameters in order to avoid mixing with the matrix material and thus guarantee the typical CMC properties. The Y-Si-O matrix system was selected because it forms yttrium in thermodynamic equilibrium, which have sufficient mechanical stability and excellent stability at both room and high temperatures [11–13]. Likewise, the coefficient of is close to that of SiC fibers, and there are no chemical interactions between the components [14,15].

2. Materials and Methods A new process based on additive manufacturing methods, as published in the patent specification DE102015205595B3 [16], gives the opportunity to overcome long processing times and the dependency on furnace technology. A ceramic fiber bundle is to be con- tinuously de-sized after unwinding from the fiber supply roll and then infiltrated with a ceramic slurry. Followed by a drying process, the infiltrated fiber bundle is positioned by a CNC-controlled depositing mechanism, and at the same time, the matrix material around the fibers is consolidated by means of laser treatment. Compared to conventional methods, the new additive process offers various advan- tages (see Table1). One of them is shaping. This is to take place via a quasi-continuous process step. As described in the patent, a fiber bundle is to be infiltrated with matrix material and then deposited. The aim is to melt the matrix powder during the depositing process and to compact it around the fibers without damaging them. Since the require- ments for the matrix starting materials are lower than for sintering, they are currently cheaper to procure. The new method should make it possible to integrate metallic fastening elements and sensor technology during the process through shorter, local energy input in the manufacturing process. Joining is addressed as a further possible application of the process. The process can theoretically be scaled well, as the area of the unit is regarded as the limit and can be adapted to the necessary conditions. The fiber material, SiC, was defined as the point for the material system. The following conditions for the matrix system and the beam source to be used are derived from this: • Comparatively high beam absorption of the matrix compared to the fiber material [17] • Meltability of the matrix material Materials 2021, 14, 3248 4 of 10

• Physical and chemical compatibility with SiC (coefficient of linear thermal expansion, chemical interactions, oxidation).

As can be seen from sources [13,14], yttrium disilicate (Y2Si2O7) seems to be suitable as a matrix material. However, the absorption values are not described. Based on other Materials 2021, 14, x 4 of 10 oxides [17], however, it seems reasonable to expect that equally high absorption values can be achieved here for CO2 lasers (λ = 10.6 µm). The higher absorption of the matrix materials should ensure that they melt before the fiber material with the lower absorption As can be seen from sources [13,14], yttrium disilicate (Y2Si2O7) seems to be suitable value is affected.as a matrix material. However, the absorption values are not described. Based on other To confirmoxides the assumption,[17], however, theit seems absorption reasonable values to expect of the that mentioned equally high oxidic absorption base mate- values rials (Y2O3, Gradecan be C, achieved Höganäs here AB, for Höganäs, CO2 lasers Sweden; (λ = 10.6 SiOμm).2, TheZandosil higher 30, absorption Heraeus, of Hanau,the matrix Germany) werematerials analyzed should by ensure integrating that they sphere melt before measurements the material and with compared the lower with absorption the literature valuesvalue of is SiC. affected. The Varian Cary 5000 UV-VIS-NIR spectrometer (Agilent Tech- nologies Inc., SantaTo Clara,confirm CA, the assumption, USA) was usedthe absorption to measure values the of absorbance the mentioned values oxidic over base ama- wavelength rangeterials of (Y 0.32O3, toGrade 15 µ C,m. Höganäs To analyze AB, Höganäs, the absorption Sweden; values, SiO2, Zandosil samples 30, Heraeus, of powdery Hanau, Germany) were analyzed by integrating sphere measurements and compared with the matrix material with different compositions were pressed. The method was as follows: literature values of SiC. The Varian Cary 5000 UV-VIS-NIR spectrometer (Agilent Tech- • Attritionnologies Inc., in isopropyl Santa Clara, alcohol CA, USA) for 4 was h used to measure the absorbance values over a • Uniaxialwavelength Pressing to range 16 × of16 0.3× to4 15 mm μm.3; To analyze the absorption values, samples of powdery • Cold isostaticmatrix pressing material with at 250 different MPa compositions were pressed. The method was as follows: • Debinding• atAttrition 600 ◦C formilling 1 h in isopropyl air. alcohol for 4 h • Table2 gives anUniaxial overview Pressing of theto 16 sample x 16 x 4 compositions mm³ tested. • Cold isostatic pressing at 250 MPa • Debinding at 600 °C for 1 h in air. Table 2. Composition of the pressed samples. Table 2 gives an overview of the sample compositions tested.

Sample SiO2 (%) Y2O3 (%) Table 2. Composition of the pressed samples. 1 100 0 2Sample 0SiO2 (%) 100 Y2O3 (%) 31 66.6100 33.3 0 2 0 100 3 66.6 33.3 Based on the analyzed oxidic materials, a matrix system will be selected. The corre- sponding diagramBased on forthe theanalyzed matrix oxidic material materials, system a matrix consisting system ofwill SiO be2 selected.and Y2O The3 can corre- be found in sourcesponding [18 ].phase Following diagram the for material the matrix and material laser system selection, consisting initial of parameter SiO2 and Y tests2O3 can are carried outbe forfound melting in source the [18]. pressed Following samples the material under and variation laser selection, of the laserinitial powerparameter (PL tests) between 10 andare 100carried W, laserout for feed melting rate (v)the betweenpressed samples 200 and under 16,000 variation mm/min of the and laser laser power spot (PL) diameter (d) ofbetween 1 mm. 10 The and energy 100 W, densitylaser feed (E) rate was (v) consideredbetween 200 asand a 16,000 comparative mm/min value. and laser The spot samples werediameter evaluated (d) of by 1 meansmm. The of energy ceramographic density (E) was cross-sections, considered as electron a comparative microscope value. The and X-ray diffraction.samples were The bestevaluated parameter by means sets of form ceramogr the startingaphic cross-sections, point for the electron first tests microscope with and X-ray diffraction. The best parameter sets form the starting point for the first tests the new additive manufacturing process described in Figure1. with the new additive manufacturing process described in Figure 1.

FigureFigure 1.1. CAD model model of of the the machine machine for additive for additive manufacturing manufacturing of CMC of using CMC laser using technology. laser technology. Materials 2021, 14, x 5 of 10

Materials 2021, 14, 3248x 5 5of of 10

3. Results 3.3. Results ResultsUsing the pressed samples prepared as described in Section 2, the integrating sphere measurement was carried out to determine the absorption values of different composi- UsingUsing the the pressed pressed samples prepared as descri describedbed in Section 22,, thethe integratingintegrating spheresphere tions of SiO2 and Y2O3. The results are shown in Figure 2. measurementmeasurement waswas carriedcarried outout to to determine determine the th absorptione absorption values values of differentof different compositions composi- tions of SiO2 and Y2O3. The results are shown in Figure 2. of SiO2 and Y2O3. The results are shown in Figure2.

Figure 2. Absorption rate (A) of selected ceramic materials by different wavelengths (λ) using integrating sphere meas- Figureurements. 2. Absorption rate rate (A) (A) of of selected selected ceramic cerami materialsc materials by different by different wavelengths wavelengths (λ) using (λ) integratingusing integrating sphere measurements. sphere meas- urements. FromFrom thethe results results of of the the integrating integrating sphere sphere measurements, measurements, it can it becan seen be seen that thethat selected the se- oxidelectedFrom ceramic oxide the ceramic matrixresults materialsmatrixof the integratingmaterials SiO2 and SiO sphere Y2 Oand3 show measurements,Y2O3 significantly show significantly it higher can be absorptionhigher seen that absorption the values se- lected(aboutvalues oxide 90%)(about atceramic 90%) a comparatively at matrix a comparatively materials high laser SiO high wavelength2 and laser Y2 wavelengthO3 show of 10.6 significantlyµ mof than10.6 μ them higher SiCthan fiber the absorption materialSiC fiber valueswithmaterial 66% (about with [17 ].90%) 66% Due at[17]. toa comparatively theDue proven to the proven difference high laserdifference in wavelength absorption in absorption of values, 10.6 μvalues,m the than following the the following SiC fiber tests materialaretests carried are withcarried out 66%with out [17]. with a mixtureDue a mixtureto the of proven SiO of 2SiOand di2 andfference Y2O Y32Oin3 inin a absorption stoichiometrica stoichiometric values, ratio ratio the of offollowing 2:1. 2:1. TheThe testscompositioncomposition are carried isis derivedderivedout with fromfrom a mixture thethe eutecticeutectic of SiO produced2pr andoduced Y2O3 ininin thisthisa stoichiometric ratioratio andand thethe ratio associatedassociated of 2:1. The lowlow ◦ compositionmeltingmelting temperaturetemperature is derived (about(about from 17751775 the eutectic°CC[ [18].18]. produced in this ratio and the associated low meltingToTo investigatetemperatureinvestigate thethe (about effecteffect 1775 ofof the the°C molten[18].molten bathbath onon thethe fiberfiber material,material, particulateparticulate siliconsilicon carbidecarbideTo investigate (SiC;(SiC; F1200,F1200, the ESK-SICESK-SIC effect of GmbH,GmbH, the molten Frechen,Frechen, bath Germany)Germany) on the fiber withwith material, aa volumevolume particulate fractionfraction ofsiliconof 10%10% carbidewaswas addedadded (SiC; asas F1200, aa substitutesubstitute ESK-SIC toto theGmbH,the matrixmatrix Frechen, system.system. Germany) TheThe powderpowder with mixtures mixturesa volume werewere fraction thenthen of mixedmixed 10% waswithwith added organicorganic as binders,binders, a substitute groundground to the andand matrix processedprocessed system. intointo The granules.granules. powder ThisThis mixtures waswas formedformed were intointothen cuboidscuboids mixed measuringmeasuring 1616 mmmm ×x 16 mm ×x 44 mm mm by by cold isosta isostatictic pressing (see FigureFigure3 3).). The The organic organic with organic binders, ground and processed into granules.◦ This was formed into cuboids measuringmaterialmaterial waswas 16 thenthenmm deboundxdebound 16 mm atxat 4 temperaturestemperatures mm by cold aboveisostaabovetic 500500 pressing °C.C. (see Figure 3). The organic material was then debound at temperatures above 500 °C.

(a) (b) (a) (b) FigureFigure 3.3. PressedPressed testtest specimensspecimens ofof YY22O3-SiO-SiO22 withwith 10 10 vol.% vol.% SiC, SiC, ( (aa)) before before and and ( (bb)) after after first first tests. tests. Figure 3. Pressed test specimens of Y2O3-SiO2 with 10 vol.% SiC, (a) before and (b) after first tests. The pressed samples were partially melted in the first experiments using a laser µ (Diamond J-3 10.6 400 W OEM Laser, Coherent, Palo Alto, CA, USA) of 10.6 m wavelength. The investigation of the single-track tests showed a different reaction of the material to the laser parameter variation of laser power, feed rate and spot diameter. Figure4 shows a cross- section of the track tests with λ = 10.6 µm for the material composite. The sample shows a homogeneous melt pool with a dense structure that has not solidified in thermodynamic equilibrium due to the extremely high cooling rate. Materials 2021, 14, x 6 of 10

The pressed samples were partially melted in the first experiments using a laser (Di- amond J-3 10.6 400 W OEM Laser, Coherent, Palo Alto, CA, USA) of 10.6 μm wavelength. The investigation of the single-track tests showed a different reaction of the material to the laser parameter variation of laser power, feed rate and spot diameter. Figure 4 shows Materials 2021, 14, 3248 a cross-section of the track tests with λ = 10.6 μm for the material composite. The sample6 of 10 shows a homogeneous melt pool with a dense structure that has not solidified in thermo- dynamic equilibrium due to the extremely high cooling rate.

FigureFigure 4.4. Cross-sectionsCross-sections ofof YY22O33-SiO2 processedprocessed with with λλ = 10.6 10.6 μµm.m.

AA cross-sectioncross-section withwith higherhigher enlargementenlargement ofof aa presspress specimenspecimen withwith 1010 vol.%vol.% SiCSiC isis shownshown inin FigureFigure5 5.. According According to to the the phase phase diagram diagram [ [18],18], itit waswas toto bebe expectedexpected thatthat mainlymainly yttriumyttrium disilicatedisilicate withwith embeddedembedded SiCSiC wouldwould form.form.

Figure 5. Cross-section of Y O -SiO sample with 10 vol.% SiC melted with λ = 10.6 µm. Figure 5. Cross-section of Y22O33-SiO2 sample with 10 vol.% SiC melted with λ = 10.6 μm. Measuring the composition by EDS (Energy dispersive spectroscopy) (Figure6), four Measuring the composition by EDS (Energy dispersive spectroscopy) (Figure 6), four different phases could be identified by the stoichiometric ratio of the elements: yttrium different phases could be identified by the stoichiometric ratio of the elements: yttrium disilicate, yttrium monosilicate, silica and silicon . This non-equilibrium state is disilicate, yttrium monosilicate, silica and . This non-equilibrium state is not in accordance with the phase diagram but is caused by the rapid cooling of the melt. not in accordance with the phase diagram but is caused by the rapid cooling of the melt. However, the evaporation of Si-O species during processing leads to a partial reduction in However, the evaporation of Si-O species during processing leads to a partial reduction the yttrium disilicate with the formation of more yttrium monosilicate compared to the in the yttrium disilicate with the formation of more yttrium monosilicate compared to the residual amount of silica. A phase of yttrium disilicate with silicon oxide is deposited on residual amount of silica. A phase of yttrium disilicate with silicon oxide is deposited on theresidual edge ofamount the yttrium of silica. disilicate A phase crystals of yttrium by segregation disilicate with of the silicon yttrium oxide monosilicate. is deposited on the edge of the yttrium disilicate crystals by segregation of the yttrium monosilicate. the edgeThe cross-sectionsof the yttrium ofdisilicate the surface crystals melt by tests segregation (Figure7) of show the yttrium similar lasermonosilicate. penetration depths of around 140 µm from top to bottom, with reduced laser power, lower feed rate and the same energy density. Viewed from the surface, the samples show no macroscopic defects (, cracking, color change), even when repeatedly processing the same surface with the same parameters. This is essential for the planned welding process, as the layer-by-layer structure requires repeated laser exposure of a surface. Materials 2021, 14, x 7 of 10

Materials 2021, 14, x 7 of 10 Materials 2021, 14, 3248 7 of 10

Figure 6. EDS (Energy dispersive spectroscopy) measured composition of melted Y2O3-SiO2 sam- ple with 10 vol.% SiC.

The cross-sections of the surface melt tests (Figure 7) show similar laser penetration depths of around 140 μm from top to bottom, with reduced laser power, lower feed rate and the same energy density. Viewed from the surface, the samples show no macroscopic defects (delamination, cracking, color change), even when repeatedly processing the same Figure 6. EDS (Energy dispersive spectroscopy) measured composition of melted Y2O3-SiO2 sam- Figuresurface 6. withEDS (Energythe same dispersive parameters spectroscopy). This is essential measured for composition the planned ofmelted welding Y2O process,3-SiO2 sample as the ple with 10 vol.% SiC. withlayer-by-layer 10 vol.% SiC. structure requires repeated laser exposure of a surface. The cross-sections of the surface melt tests (Figure 7) show similar laser penetration depths of around 140 μm from top to bottom, with reduced laser power, lower feed rate and the same energy density. Viewed from the surface, the samples show no macroscopic defects (delamination, cracking, color change), even when repeatedly processing the same

surface with the same parameters. This is essential for the planned welding process,Reduced P as the layer-by-layer structure requires repeated laser exposure of a surface. L and v, same E Reduced P L and v, same E

FigureFigure 7.7. Cross-sectionsCross-sections of surface melt tests fr fromom top top to to bottom, bottom, reduced reduced laser laser power power (P (PL)L from) from 40 40 to to10 10W, W, lower lower feed feed rate rate (v) (v) from from 3200 3200 to 800 to 800 mm/min, mm/min, same same energy energy density density (E) at (E) 0.5 at J/mm². 0.5 J/mm 2.

TheThe characterizationcharacterization of of the the microstructure microstructure formation formation of of the the melting melting regions regions in in Figure Figure7 was7 was carried carried out out by by means means of cross-sectionsof cross-sections and an theird their analyses analyses in the in fieldthe field emission emission scanning scan- Materials 2021, 14,electron ningx electron microscope microscope (FESEM: (FESEM: NVision40 NVision40 or Ultra55, or Ultra55, Carl Zeiss Carl AG,Zeiss Oberkochen, AG, Oberkochen, Germany) Ger-8 of 10

(Figuremany) 8(Figure) and X-ray 8) and diffraction X-ray diffraction (XRD: D8, (XRD: Bruker D8, AXS, Bruker Karlsruhe, AXS, Karlsruhe, Germany). Germany). Figure 7. Cross-sections of surface melt tests from top to bottom, reduced laser power (PL) from 40 to 10 W, lower feed rate (v) from 3200 to 800 mm/min, same energy density (E) at 0.5 J/mm².

The characterization of the microstructure formation of the melting regions in Figure 7 was carried out by means of cross-sections and their analyses in the field emission scan- ning electron microscope (FESEM: NVision40 or Ultra55, Carl Zeiss AG, Oberkochen, Ger- many) (Figure 8) and X-ray diffraction (XRD: D8, Bruker AXS, Karlsruhe, Germany).

Figure 8. StructuralFigure 8. variationsStructural from variations left to right—Concentration from left to right—Concentration of Si-O and increased of Si-Oformation and increasedof yttrium disilicate formation (grey) of due to reduced energy input and lower feed rate at the same energy density. yttrium disilicate (grey) due to reduced energy input and lower feed rate at the same energy density. Depending on the power parameters of the laser, the chemical composition of the sam- ple changes. Higher energy inputs lead to vaporization of Si-O and thus to a reduction of the yttrium disilicate with the formation of yttrium monosilicate. It is a benefit for the pro- cess to reduce the laser power and the feed rate at the same energy density. As the tests (Figure 8) show, this resulted in a higher amount of the disilicate phase in the cross-sections. Based on the results of the melting experiments on pressed powder tablets, the next step was to test the CMC production by laser technology on fiber composites consisting of Y2O3-SiO2 (1–2 mol) and the fiber material (Tyranno SA3, Ube Ind., Tokyo, Japan), as can be seen in the detailed images in Figure 9, the first samples were set up with the newly devel- oped system.

(a) (b) (c) Figure 9. Details of the test set-up from left to right: (a) fiber bundle reservoir; (b) of the fiber bundle in the slurry with subsequent furnace drying; (c) fiber guide system.

Following the functional tests of the system, the first samples were processed by la- ser, as can be seen in Figure 10.

Figure 10. First Y2O3-SiO2 sample with SiC fiber, melted with λ = 10.6 μm, produced using the new system: without melting the matrix (left), with melted matrix (middle and right).

Materials 2021, 14, x 8 of 10 Materials 2021, 14, x 9 of 12

Materials 2021, 14, 3248 8 of 10 Figure 8.8. StructuralStructural variations variations from from left left to to right right—Concentration—Concentration of of Si-O Si-O and and increased increased formation formation of ofyttrium yttrium disilicate disilicate (grey) (grey) due toto reducedreduced energyenergy input input and and lower lower f eedfeed rate rate at at the the same same energy energy density. density.

DependingDependingDepending on on thethe the power power parametersparameters parameters ofof ofthe the the la laser,ser, laser, the the the chemical chemical chemical composition composition composition of ofthe ofthe sam- the samplesampleple changes. changes. changes. Higher Higher Higher energy energy energy inputs inputs inputs lead lead lead to to va to vaporizationporization vaporization of of Si-O of Si-O Si-O and and and thus thus thus to to to aa reductionreduction a reduction of ofofthe the yttrium yttrium disilicate disilicate disilicate with with with the the the formation formation formation of ofyttr of yttrium yttriumium monosilicate. monosilicate. monosilicate. It Itis Itisa isbenefita abenefit benefit for for the for the pro- the processcess to to toreduce reduce reduce the the the laser laser laser power power power and and and the the the feedfeed feed rate rate atat at thethe the same same same energy energy energy density. density. As As the thethe tests teststests (Figure(Figure(Figure 88) 8)) show, show, this this resulted resulted in in in a a ahigher higher higher amou amount amountnt of of of the the the disilicate disilicate disilicate phase phase phase in in inthe the the cross-sections. cross-sections.cross-sections. BasedBasedBased on onon the thethe results resultsresults of ofof the thethe melting meltingmelting experiments experimentsexperiments on onon pressed pressedpressed powder powderpowder tablets, tablets,tablets, the thethe next nextnext stepstepstep was waswas to toto test test the the CMC CMC production production by by by laser laser laser technology technology technology on on onfiber fiber fiber composites composites composites consisting consisting consisting of of YofY2O2O Y32-SiO3-SiOO3-SiO2 2(1 (1–2–2 (1–2mol) mol) mol)and and the andthe fiber fiber the material fiber material material (Tyranno (Tyranno (Tyranno SA3, SA3, Ube SA3,Ube Ind., Ind., Ube Tokyo, Tokyo, Ind., Japan), Tokyo, Japan), as Japan), ascan can be as be seencanseen be inin seen the detailed indetailed the detailed imagesimages images inin Figure Figure in Figure9, 9, the th 9firste, thefirst samples firstsamples samples were were set were setup set withup up with the with newlythe the newly newly devel- developeddevelopedoped system. system. system.

((aa)) ((bb)) ((c)) FigureFigureFigure 9. 9.9. Details Details of ofof the thethe test testtest set-up set-upset-up from fromfrom left leftleft to toto right: right:right: (a ()aa fiber)) fiberfiber bundle bundlebundle reservoir; reservoir;reservoir; (b ()(b bcoating)) coatingcoating of oftheof thethe fiber fiberfiber bundle in the slurry with subsequent furnace drying; (c) fiber guide system. bundlebundle inin thethe slurryslurry withwith subsequentsubsequent furnacefurnace drying;drying; ((cc)) fiberfiber guide guide system. system. Following the functional tests of the system, the first samples were processed by FollowingFollowing thethe functionalfunctional tests tests of of the the system, system the, the first first samples samples were were processed processed by by laser, la- laser, as can be seen in Figure 10. asser, can as becan seen be seen in Figure in Figure 10. 10.

Figure 10. First Y2O3-SiO2 sample with SiC fiber, melted with λ = 10.6 μm, produced using the new 2 3 2 system:FigureFigure 10.without10. FirstFirst YmeltingY2O3-SiO the2 samplesample matrix with with(left), SiC SiC with fiber, fiber, melted melted matrix with (middle λ == 10.6 and µμm,m, right). producedproduced usingusing thethe newnew system:system: withoutwithout meltingmelting thethe matrixmatrix (left),(left), withwith meltedmelted matrixmatrix (middle(middle andand right).right).

As can be seen in the cross-sections in Figure 11, it was confirmed that the fibers were not damaged by the laser, as expected. Damage only occurred in isolated cases where minimal matrix material was present. The transition areas between fiber and matrix correspond to the expectations of a CMC. In the images, the fibers appear to be embedded in the matrix. This is indicated by the grain structures of the matrix, which do not continue in the fiber, and a step in the cross-sections between fiber and matrix. Materials 2021, 14, x 10 of 12

As can be seen in the cross-sections in Figure 11, it was confirmed that the were not damaged by the laser, as expected. Damage only occurred in isolated cases where minimal matrix material was present. The transition areas between fiber and matrix correspond to the expectations of a CMC. In the images, the fibers appear to be embedded Materials 2021, 14, 3248 9 of 10 in the matrix. This is indicated by the grain structures of the matrix, which do not continue in the fiber, and a step in the cross-sections between fiber and matrix.

(a) (b)

(c) (d)

FigureFigure 11. Cross-section 11. Cross-section Y2O3-SiO 2Ysample2O3-SiO with2 sample SiC fiber, with melted SiC fiber, with λmelted= 10.6 µ withm, overview λ = 10.6 (a μ),m, higher overview magnification (a), of the transitionhigher (bmagnification), cross-section of along the thetransition fibers (c), ( areab), cross-section with embedded along fibers the in matrixfibers material(c), area ( dwith). embedded fibers in matrix material (d). 4. Conclusions 4. Conclusions In summary, it can be said that in the experiments with the press specimens, the use of the wavelength of 10.6 µm made it possible to melt the matrix systems. The tests In summary, it can be said that in the experiments with the press specimens, the use of with SiC-fiber material showed that a homogeneous infiltration of the fiber bundles with the wavelength ofmatrix 10.6 μ materialm made during it possible the winding to melt processthe matrix is possible. systems. However, The tests the with amount SiC- of matrix fiber material showedmaterial that between a homogeneous the fibers of theinfiltration fiber bundle of the still fiber has to bundles be increased. with Aftermatrix the welding material during theprocess, winding there process is still insufficientis possible. matrix However, between the theamount fibers. of Due matrix to the material high wavelength between the fibersof of the the CO fiber2 lasers bundle (λ = 10.6still µhasm), to the be laser increased. power is After predominantly the welding absorbed process, close to the there is still insufficientsurface matrix (100–200 betweenµm). Thus, the thefibers. melting Due process to the high takes wavelength place close to of the the surface. CO2 In order lasers (λ = 10.6 μm),to increase the laser the power penetration is predominantly depth, the laser absorbed parameters close are to varied the surface in further (100 experiments– 200 μm). Thus, the(feed melting rate, laserprocess power, takes laser place relative close movement).to the surface. The In fiber order material to increase (Tyranno the SA3, Ube penetration depth,Industries, the laser Tokyo,parameters Japan) are used varied was notin further damaged experiments by the laser (feed during rate, processing. laser If the laser power was too high (over 100 W), mixing with the matrix material occurred on the power, laser relative movement). The fiber material (Tyranno SA3, Ube Industries, Tokyo, fiber surface. Japan) used was not damagedAs an outlook by the for thelaser processing during processing. technique of If CMC the usinglaser laserpower technology was too described high (over 100 W),in mixing the paper, with the the following matrix canmaterial be noted. occurred The proportion on the fiber of infiltrated surface. matrix powder must As an outlookbe for increased, the processing by changing techniqu the bindere of CMC system, using or by laser increasing technology the penetration described depth of the in the paper, the followinglaser in the can melting be noted. area under The proportion controlled energy of infiltrated density matrix by varying powder the laser must power, feed be increased, by changingrate and scanning the binder strategy. system, To or prevent by increasing damage the to the penetration fibers, the depth laser power of the should be laser in the meltingkept area under under 50 Wcontrolled and the feed energy rate overdensity 4000 by mm/min. varying Thethe laser scanning power, strategy feed should be rate and scanningadapted strategy. so that To theprevent process damage zone is scannedto the fibers, several the times. laser For power example, should oscillating, be circular or rectangular scanning patterns of the laser beam could be used. kept under 50 W and the feed rate over 4000 mm/min. The scanning strategy should be adapted so that the process zone is scanned several times. For example, oscillating, circular or rectangular scanning patterns of the laser beam could be used. Materials 2021, 14, 3248 10 of 10

Author Contributions: Conceptualization, S.P. and W.K.; methodology, S.P., W.K. and A.F.; formal analysis, B.B.; investigation, S.P. and W.K.; resources, E.L., A.F., C.L. and F.B.; writing—original draft preparation, S.P.; writing—review and editing, W.K., B.B., A.F. and E.L.; supervision, F.B. and C.L.; project administration, E.L. and C.L.; funding acquisition, B.B., W.K and E.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by Bundesministerium für Bildung und Forschung (BMBF), grant number 03ZZ0222C. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: All the data is available within the manuscript. Acknowledgments: Based on these fundamental results, the project LasCer was recently funded within the framework of Agent3D from the Bundesministerium für Bildung und Forschung (BMBF). The authors would like to thank the BMBF for the funding. Conflicts of Interest: The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. The authors declare no conflict of interest.

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