Synthesis and processing of MAX phases by Powder Injection Moulding and Additive Manufacturing

• Autores: Eduardo Tabares Lorenzo
• Directores de la Tesis: Sophia Alexandra Tsipas (dir. tes.), Antonia Jimenez Morales (codir. tes.)
• Lectura: En la Universidad Carlos III de Madrid ( España ) en 2022
• Idioma: inglés
• Tribunal Calificador de la Tesis: José Manuel Torralba Castelló (presid.), Javier Hidalgo García (secret.), Konstantina Lambrinou (voc.)
• Programa de doctorado: Programa de Doctorado en Ciencia e Ingeniería de Materiales por la Universidad Carlos III de Madrid
• Materias:
  • Física
    • Física del estado sólido
      • Materiales compuestos
• Enlaces
  • Tesis en acceso abierto en: e-Archivo
• Resumen

  • This doctoral thesis with title “Synthesis and processing of MAX phases by Powder Injection Moulding and Additive Manufacturing” has been carried out at the Group of Powder Technology (GTP) from the Universidad Carlos III de Madrid (UC3M), in the framework of the doctoral programme in Ciencia e Ingeniería de Materiales of the UC3M, under the supervision of Dr. Sophia A. Tsipas and Dr. Antonia Jiménez Morales. This thesis has been developed, mainly, at the UC3M and also, through a research stay of 4 month at RHP Technology GmbH located in Seibersdorf, Austria. The work at RHP was performed under the supervision of Dr. Erich Neubauer and Dr. Michael Kitzmantel.

    MAX phases are a family of ternary materials with a fixed stoichiometry and a general formula of Mn+1AXn, where M is a transition metal, A is generally an element of groups IIIA and IVA of the periodic table, X is either carbon or nitrogen and n a value between 1 and 3. Their nano-laminated structure gives these materials an unusual combination of (1) metallic properties, such as, good electrical and thermal conductivity, machinability, high damage tolerance; and (2) ceramic properties such as high rigidity, resistance to corrosion and oxidation and good mechanical properties at high temperatures. This unique combination of properties makes these materials very promising candidates for industrial applications with demanding conditions, which has prompted the study, design and development of these family of materials.

    The main goal of this thesis is to study and develop new processing routes for MAX phases, starting from the synthesis of MAX phases powders followed by scaling-up of the powder production and the processing of the material. These new processing routes are built on feedstock-based processing such as powder injection moulding (PIM) and additive manufacturing (AM) through composite extrusion modelling (CEM). With all this in consideration four main sections define this work: MAX phase synthesis, processing of MAX phases by conventional methods, powder injection moulding and composite extrusion modelling.

    MAX phase synthesis MAX phase synthesis process has been in constant study due to the difficulty of obtaining high purity or single-phase elements off these materials. Researchers are continuously proposing new synthesis routes or adjusting existing ones to develop high purity MAX phases, through the understanding of the synthesis mechanism. The production of MAX phase powders usually starts form the elemental powders that conform the specific phase that wants to be obtained. It is also common to combine those elemental powders with carbides or nitrides which also match with the MAX phase desired. Molar ratios of the starting materials are commonly designed depending on the stoichiometry of the MAX phase, with some adjustments taken into consideration the sublimation properties of the powders.

    As an example, the stability of MAX phases at high temperatures between 1100 and 1400 ºC, proposes these family of materials as a great candidate for numerous applications in aggressive high temperature applications. As structural high temperature materials, MAX phases can operate in heated elements in gas turbine engines. Materials used in the aerospace industry require strict performances at aggressive environments. Most importantly, component weight is a special factor that needs to be studied in order to reduce cost. In this sense, with good behaviour at high temperatures and low density, suggest MAX phases as a promising material for this applications. As an example, Ti3SiC2 has half the density of some of the alloys used in turbine engines and can withstand high temperatures under severe strength conditions.

    The first objective of this PhD thesis was the synthesis of high purity MAX phases for the later processing by PIM and AM. For this purpose, several molar ratios and initial powders were studied varying the heat treatment temperatures and the phases present after the pressureless sintering process were analysed. All samples were characterised by XRD and SEM to control the phase evolution of the Ti3SiC2 synthesis. In addition, the synthesis mechanism was studied through thermodynamic calculations and a reaction mechanism for the selected powder mixtures is proposed. Finally, a specific mixture was selected analysing the variation of the heat treatment time and particle size of the initial powders to study their effect on the Ti3SiC2 synthesis, obtaining high purity powders. From this part of the work, powders of Ti3SiC2 with a purity of 94 vol.% were synthesised using Ti/SiC/C as initial powders with a molar ratio of 3:1.5:0.5, adjusting the heat treatment at 1300 ºC for 6 h under vacuum conditions. Regarding Cr2AlC, the synthesis was performed starting from elemental powders of Cr, Al and C with a molar ratio at 2:1.2:1 with a purity of 98% after a heat treatment at 1300 ºC for 6 h under protective argon atmosphere. In addition, Ti2AlC/Ti3AlC2 composite MAX phase was also synthesised starting from Ti, Al and TiC with a molar ratio of 1:1:0.75 after a heat treatment at 1400 ºC for 4 h under protective argon atmosphere. It can be stated that, the synthesis procedure that is followed for MAX phases is influenced by the starting powders and the initial molar ratios. In essence, the starting material should be carefully selected depending on the synthesis route that is going to be followed. The synthesis methods developed were simple, reproducible and scalable and produced high purity MAX phase powder with controlled powder size distribution. Furthermore, thermodynamic calculations were performed to establish the synthesis mechanism of the MAX phases. Albeit they are theoretical calculations, the proposed synthesis reactions give step-by step description of how the synthesis is achieved and explain the different secondary phases found in the final products. Nevertheless, the grain size of the materials used for the synthesis have an effect on the final synthesis and the initial composition should also be carefully selected.

    Ti3SiC2 and Cr2AlC MAX phases were selected for the study of new processing routes and, for this purpose, the scalability of the powder production was successfully achieved obtaining purities of 92% and 96%, respectively. The scale-up process of the powder production was accomplished with the compaction of the powders by cold isostatic pressing (CIP). Although final purity of the powders was reduced by a 2% for both MAX phases, the amount of material obtained after this synthesis methods was increased by a 5000%.

    Conventional MAX phase processing After the study of the synthesis and powder production of MAX phases Ti3SiC2 and Cr2AlC and in order to validate the properties of the synthesised powders, this part of the work intends to analyse different processing techniques: uniaxial pressing and sintering, cold isostatic pressing and sintering and hot-pressing. After the optimisation of the sample production densification of the powders was analysed, all while maintaining high purity levels of MAX phase. Mechanical properties of the samples showed a close dependence to the porosity values. Elastic modulus of the Ti3SiC2 samples processed displayed lower values (83 kN/mm2) than those reported in the literature, due to the low densifications obtained. On the other hand, Cr2AlC sample exhibited similar elastic modulus to those in the literature, due to the high densification attained in these samples (225 kN/mm2). In addition, cyclic compressive strength showed no hysteresis, with no noticeable difference between the cold isostatic press and sintered and hot-pressed samples. Furthermore, the wear behaviour of the samples was studied by reciprocating sliding, analysing the wear tracks and wear rate of the samples, in order to establish a wear mechanism. The self-lubricating effect characteristic of MAX phases was observed for all samples analysed. From these tests, a dependence on the porosity was also observed. Cold isostatic pressed and sintered Ti3SiC2 and Cr2AlC exhibited different wear properties that were not only affected by the difference in the porosity of the samples but on the different wear mechanisms of each materials. Although both materials presented a combination of abrasive and adhesive wear, the abrasive effect was more noticeable for Ti3SiC2. On the other hand, hot-pressed samples exhibited a similar wear rate for all loads studied (5 and 10 N). This effect seems to be a combination of the lubricating effect of the material and the small grain size obtained after the process, resulting in a lower amount of generated debris during the tests. The wear mechanism of the MAX phases analysed can be distinguished by a quasi-plastic deformation combining abrasive and adhesive mechanism for Ti3SiC2 and a predominant adhesive behaviour for Cr2AlC.

    Powder Injection Moulding Powder injection moulding (PIM) is a powder metallurgy processing method for the production of small parts with complex shapes which starts from the raw material in powder form. The process is based on the mixing of the powders with a polymeric agent, called binder system, that will be responsible for giving the necessary viscosity to the powders to be injected. This powder-binder mixture is often referred as feedstock. Once the feedstocks are prepared, they are heated up in a barrel and pressure is applied to inject them into a mould with the desired final shape of the component to be produced. Powders and polymeric binder selection for the feedstock production are a critical step on the injection process since the adequacy of the initial materials will determine the viability of the process. In order to ensure the feasibility of the injection process, initial materials should comply certain requirements.

    In this sense, two multicomponent binders were selected for this study, firstly, an environmentally-friendly binder consisting of a combination of a sustainable polymer (polyethylene glycol, PEG) and a biopolymer (cellulose acetate butyrate, CAB), and secondly, a binder composed of the same sustainable polymer (PEG) and polypropylene (PP). MAX phase feedstocks were produced and rheologically characterised. From torque rheology, capillary rheology and homogeneity characterisation, the solid loadings selected as optimal were: 52 vol.% for Ti3SiC2/PEG-CAB, 51 vol% for Cr2AlC/PEG-CAB, 49 vol.% for Ti3SiC2/PEG-PP and 51 vol.% for Cr2AlC/PEG-PP. A special behaviour was observed for the MAX phase feedstocks, when the optimal solid loading was surpassed, feedstocks showed a decrease in the viscosity, this behaviour has not been previously observed and was attributed to the lubricating nature of MAX phases. However, these solid loading were not appropriate for PIM. All feedstocks exhibited a pseudoplastic behaviour with viscosities below those recommended for PIM (1000 s-1).

    The binder removal was performed in a two-step debinding process. First, PEG was removed by solvent debinding in distilled water at 60 ºC for 5 h. Up to a 91-95 wt.% of the PEG was removed in this process avoiding internal defects in the samples and creating the open porosity needed for the later backbone removal. These steps allow the use of a green solvent, compared to the typically organic solvents used for solvent debinding, having a lower environmental impact of the process. CAB was removed by thermal debinding maintaining the structural integrity of the samples and removing a 99 wt.% of the remaining binders. After sintering, porous injected samples were obtained without the use of space-holders with a tailored porosity while controlling the amount of solid loading in the mixtures.

    Composite extrusion modelling In the last years Additive Manufacturing (AM) has raised as go to technology for the production of prototypes and as a new processing route to develop near-net-shape parts. In the materials community, AM has become a “hot topic” for the designing of a wide range of materials. From structural purposes of commonly used stainless steels to functional applications of new materials, AM postulates as a promising route for the production of new designs and new materials never before used in the industry.

    One of the main reason of the thriving inception of Additive Manufacturing in the materials processing is the amount of technologies that encompasses allowing to select the ones more suited for a certain material. Besides polymer 3D printing, metals and ceramics materials have been widely studied due to the possibility of using this technology to obtain final part without posterior machining processes for a big number of applications with a freedom of design that distinguishes AM from the rest of processing technologies.

    Not all materials are suited for the AM process and do not comply with this specific requirements for initial materials. In summary, Powder bed fusion techniques are prone to use spherical powders for the correct deposition of the powder bed and the densification of the samples. Ceramics cannot be produced in a filament form for Direct Energy Deposition systems, and not all metals are successfully sintered by laser or electron beams. Powder morphology is a huge drawback for powder bed base technologies in the indirect AM, since the correct powder wettability of the binders depends on that morphology. Furthermore, not all polymers are suited for the filament production and the flexible properties of a feedstock filament is limited for a few number of polymers.

    Composite Extrusion Modelling (CEM), also described as screw-based AM, stands as an alternative processing route to solve some of the problematics by conventional AM techniques and broaden the use of new materials. This technology combines the fundamental aspects of Powder Injection Moulding and Fused Deposition Modelling.

    With CEM, pelletised feedstocks can be used for the obtention of 3D parts. As for FDM the material is extruded, in this case by a screw, into the heated zone to give the feedstocks the necessary viscosity. Feedstocks are then extruded through a nozzle to be deposited into the bed in a layer-by-layer basis.

    In this work, Ti3SiC2 and Cr2AlC MAX phases feedstocks using a PEG/CAB as a binder were pelletised and printed through composite extrusion modelling (CEM). Several printing parameters were adjusted for the successful printing of the MAX phases, i.e.: extrusion temperature, bed temperature, layer height, layer thickness, extrusion speed and printing speed, amongst others. This last two parameters had a great effect on the quality of the final samples. Analysing the relative density of the printed parts under different conditions, it was possible to see that by increasing the extrusion speed the sample’s relative density increased due to the increase in the amount of material that was extruded. Once a certain speed is surpassed, the quality of the sample is lost by the excess of material being deposited. In the case of the printing speed, the quality of the samples improved with the reduction of the speed, achieving increasingly less quality improvement when further reducing the speed. This suggests a compromise between the improvement on the quality, related to the relative density, and the printing time of the parts. Best conditions were selected at an extrusion temperature of 210 ºC, printing speed of 5 mm/s and an extrusion speed of 550 steps per unit for Ti3SiC2 and a temperature of 230 ºC, a printing speed of 5 mm/s and an extrusion speed of 650 steps per unit for Cr2AlC.

    Furthermore, for the debinding process, temperature was set at 500 ºC and a heating and cooling rate of 0,5 ºC/min. With this modification the amount of polymer reduced after the process was of 99%, avoiding the appearance of cracks and the warping of the sample due to internal stresses generated during printing. Brown parts were immersed in zirconia balls during the sintering steep to control the warping of the samples. Sintering conditions were optimised at 1300 ºC for 6 hours with heating and cooling rates of 5 ºC/min. Samples exhibited a final relative density of 90% and 93% for Ti3SiC2 and Cr2AlC, respectively. Moreover, the volumetric shrinkage obtained for the samples was 10% for Ti3SiC2 and 21% for Cr2AlC, densifications relatively high considering the porosity of injected samples. This confirms the correct deposition of the layers achieved and optimum selection of the printing parameters, that resulted in control of the contraction generated by the internal stresses produced during the 3D printing process. All this, while controlling any cross contamination of the MAX phase powders due to a reaction with remaining binder and avoiding the decomposition of this materials.

    To summarise, in this work, MAX phase powders have been synthesised, scaling-up the process maintaining high purity levels of the powders. The powders’ quality was validated by conventional MAX phase processing analysing the mechanical properties and wear behaviour of the consolidated porous samples. Two routes were explored for the processing of near-net-shaped samples, powder injection moulding and additive manufacturing, using sustainable polymeric binders. Powder characterisation was performed to study the suitability for the feedstocks production and, in addition, an optimal solid loading was selected for each feedstock analysing and studying the rheological behaviour. Moreover, injected MAX phase samples were produced with tailored porosity without the use of space-holders. Lastly, these produced binders were processed by composite extrusion modelling to obtain 3D printed MAX phase samples. This process allowed the possibility of printing powders not commonly suited for additive manufacturing process and staring from feedstock pellet raw material. 3D printed MAX phases exhibited a good geometrical quality and relatively high densification. All this establishes porous and dense MAX phases, from synthesis to processing, by PIM or CEM routes, for applications such as, catalytic devices, filters or as high temperature heat exchangers.