Understanding the effect of processing conditions on the properties of cold sintered ceramics

Sintering is an essential step in ceramic processing which transforms powder compacts into useful dense solids with the requisite strength and functionality. Conventional sintering methods involve a thermal treatment at temperatures above 1000°C, raising environmental concerns and technological challenges associated with limited materials integration and microstructural control. Recent advances in sintering research have led to the development of the cold sintering process (CSP), which enables densification of various ceramics at low temperatures, typically below 350°C, by utilizing external pressure and chemical pathways. However, the current state of the art still lacks a fundamental understanding of this chemo-mechanical densification mechanism and its effect on the mechanical properties of cold sintered parts. This thesis investigates the densification mechanism during cold sintering through modifications of the liquid phase chemistry. The aim is to understand and improve the structural integrity of cold sintered ceramics by taking ZnO as a model system. Furthermore, the generation of processing defects by CSP is thoroughly investigated by ultrasonic nondestructive testing, biaxial bending and fractography. Densification and microstructural evolution in cold sintering are found to be chemically driven phenomena, governed by the dissolution¿precipitation mechanism of pressure solution creep. An aqueous solution of formic acid is proposed as an effective transient phase, achieving high densification (~97%) and a ~40% increase in strength compared to literature values. Densification gradients and defects, such as cracks and delamination, are found to be associated with inhomogeneous pressure application and inadequate control of liquid phase evaporation. Their elimination has been successfully demonstrated, resulting in structurally sound cold sintered ZnO ceramics with twice the strength (~120 MPa) of previously reported values. A mechanical comparative analysis with conventionally sintered ZnO has revealed that the lower fracture resistance of cold sintered samples is attributed to a reduced fracture toughness due to their nanometric grain size. This thesis also explores manufacturing challenges with CSP and demonstrates the feasibility of its scaling up, representing a first step toward application development. The insights gained in this thesis on the mechanical reliability of cold sintered materials provide important implications for the further development of the CSP towards establishing its potential as a viable alternative sintering technique.