Additive Manufacturing Of Polymer Derived Ceramics

Author : Shruti Gupta
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (142 download)

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Download or read book Additive Manufacturing of Polymer-Derived Ceramics written by Shruti Gupta and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: This dissertation describes the stereolithography-based additive manufacturing of ceramic matrix composites starting from preceramic polymers. Preceramic polymer route to produce high performance silicon carbide based ceramic matrix composites has been explored with various techniques to produce parts with high strengths. This route makes polymer processing amenable to ceramics. Ceramic matrix composites (CMCs), especially those based on SiC or SiOC, possess low weight and exhibit superior high temperature properties. It is possible to attain a higher degree of synthetic control over the microstructure and properties due to the tailorable polymer precursor chemistry and processing. Polymer-derived ceramics (PDCs) produced upon pyrolysis are mostly amorphous solids often containing a large amount of free carbon. However, the main issue with producing such ceramics is the inherent density increase from the preceramic polymer (1 g/cc) to 3.21 g/cc of the SiC ceramic. This results in a tremendous amount of shrinkage during the polymer to ceramic transformation causing cracks and pore formation in the ceramic matrix. The mechanical strength of PDCs suffers from these cracks, pores and microstructure inhomogeneities. Despite its benefits, the SLA process faces the challenge of producing PDCs with high strengths. The goal of this dissertation is to identify techniques to combat defects in SLA printed ceramics and increase their mechanical strength. This dissertation details exploratory work in identifying 3 main materials and processing pathways to predict and resolve such shrinkage-related microstructural defects in the PDCs. Firstly, if the shrinkage in the preceramic polymer matrix governs the pore and crack formation, it is vital to determine how the mechanical properties of these SLA printed green bodies affect the strength of the PDC formed upon pyrolysis. Therefore, the first study was based on comparing the mechanical properties of the printed green bodies of the preceramic resins to those of the pyrolyzed ceramics. Properties of the printed preceramic polymer such as the modulus and strain-at-break played an important role in determining the failure in the PDCs. These properties can be modulated by choosing an appropriate crosslinking chemistry Such comparative studies between these properties of the green bodies and the ceramic microstructure have not been conducted in the past. This work identifies strain-at-break as a new parameter to be considered to predict the amount of shrinkage sustained by the matrix without cracking. By eliminating cracks, higher strengths can be achieved. The compressive strengths of the lattices produced in this work were comparable to the highest reported values in the literature. Secondly, if inert fillers such as ceramic particles are added to the preceramic resin as second phase heterogeneities, then the overall density of the PDC will increase and the shrinkage will be reduced as shrinkage will not occur in the volume occupied by the inert fillers. In the next part of this dissertation, the effect of silicon nitride inert fillers in the PDC matrix on the strength of the composite PDCs was studied. It was observed that not only the amount of particle loading, but also the particle size affected the strengths of such ceramic composites. Besides, preliminary studies on polymer infiltration and pyrolysis (PIP) of ceramics produced with high solids loading were carried out. This process generated flexural strengths of 197 MPa, higher than other techniques reported in this work. A combination of SLA printing and PIP showed promise in future development of high strength industrially important parts such as gas turbine blades, splash plates and heat exchangers. Lastly, if reactive components (or active fillers) such as transition metals are present in the preceramic polymer system, upon pyrolysis, the reaction of such active fillers with the gaseous byproducts will cause an expansion in the volume of the PDC matrix during pyrolysis, thereby curbing the shrinkage. Importantly for the additive manufacture of ceramics, this is the first report on the incorporation of active fillers in vat photopolymerization techniques. Chapter 7 will describe iron as an active filler which when present in catalytic amounts in siloxane-based resins lowered the crystallization temperature of SiC to ~1200°C. This process will enable low temperature fabrication of more complex PDCs while also improving their strengths. The presence of crystalline domains of silicon carbide served as reinforcements to the PDC matrix. Preliminary work was also carried out on titanium as an active filler. In both cases, addition of active filler significantly increased the strengths of the PDCs.