Multiscale deformation processes during cold sintering of nanovaterite compacts

Abstract

Cold sintering is a promising route towards the manufacturing of dense ceramics at mild processing conditions, but our poor understanding of the process has prevented the wider spread of this attractive densification approach. Using nanovaterite powders with well-defined multiscale morphology, we perform in-situ X-Ray tomography on compacts subjected to controlled mechanical load and quantify the multiscale deformation processes responsible for the water-assisted cold sintering of this powder with the help of instrumented indentation experiments at the micro- and nano-scale. Our results reveal the crucial effect of water in promoting the macroscopic densification process and highlight the dominant role of the nanoparticle network inside agglomerates in controlling the cold sintering of compacts at high mechanical loads. By providing new insights into the deformation processes responsible for the densification effect, this study can potentially guide the discovery of novel chemical compositions and particle morphologies that can be more easily densified through room-temperature cold sintering with water.

Introduction

Cold sintering of powder compacts is a well-known geological process leading to rock formation in nature [1,2] and has recently been explored for the fabrication of advanced ceramics for electronic and structural applications [3], [4], [5], [6]. In geology, the densification of granular materials through pressure solution creep is an important mechanism underlying the formation and transformations of sedimentary rocks [2,7]. Pressure solution creep is a cold sintering process that enables the transport of atoms from inter-particle contacts to pore walls in a similar fashion to the conventional densification and sintering of ceramics at high temperatures. This mechanism is expected to dominate the creep deformation of geological calcite for pressures up to 500 MPa [8]. As far as ceramics are concerned, cold sintering offers an attractive sustainable pathway for the fabrication of stiff and strong functional materials in aqueous environment at much lower temperature compared to standard sintering conditions [3]. Given its relevance to such a broad spectrum of materials and research fields, cold sintering processes have been intensively studied by the geology and materials science communities in recent years [3,5,6,9].

The densification of powder compacts through cold sintering has been demonstrated for a wide range of chemical compositions and processing conditions [10]. While low-temperature dissolution and reprecipitation processes have long been known to enable the pressure-assisted densification of sedimentary rocks [11], the possibility to cold sinter salts, phosphates, carbonates and oxide ceramics featuring much lower solubility in water has only recently been evidenced by materials scientists [[3], [4], [5], [6], [7],[9], [10], [11], [12]]. Early works exploring this attractive processing route focused on the cold sintering of hydroxyapatite and lithium molybdate powders under temperatures and pressures in the range of 20–250 °C and 50–130 MPa [5,12]. Other studies revealed that similar pressures and temperatures can be used to densify ZnO powder compacts through hydrothermal processes [4,9]. More recent research has shown that hydrothermal conditions can be extended to cold sinter an impressive range of chemical compositions, including various simple and complex oxides, carbonates and salts [6]. Inspired by the densification of carbonates in geological formations, we have demonstrated that the cold sintering of nanovaterite powder is possible even at room temperature by compaction of the powder with water under pressures in the range 10–800 MPa [3]. Despite these several successful demonstrations of cold sintered materials, the microstructural deformation processes underlying the densification of such powders remain poorly understood. In particular, the morphology of the initial powders is expected to play an important role on the room-temperature sintering of nanovaterite with water, but its effect on the deformation processes within the compact under pressure has not yet been systematically studied.

Three-dimensional X-Ray tomography is a powerful in-situ experimental technique to assess the deformation processes within materials subjected to mechanical loading [13]. This has allowed for the space and time-resolved quantification of the porosity of metal powder compacts used in powder metallurgy [14,15], the mapping of strains and defects developed during the mechanical deformation of metals [16], and the analysis of matter transport mechanisms during high-temperature sintering of glass and oxide-based composites [17, 18]. Synchrotron based tomography has also been used to observe particle morphology changes during pressure solution creep of 100 µm sodium chloride particles [19]. Besides in-situ tomography, instrumented micro- and nano-indentation has also been extensively employed to quantify the mechanical response and deformation processes in consolidated or pressed materials [20]. By measuring the creep response of specimens tested with different indenter sizes, this technique provides a means to probe deformation processes at different length scales [21]. The combined use of such imaging and mechanical characterization tools should therefore be a powerful approach to study cold sintering phenomenon in powder compacts.

Here, we apply in-situ X-Ray tomography and instrumented mechanical indentation to investigate the deformation processes at play during the room-temperature cold sintering of nanovaterite under pressure with water. Because the investigated vaterite powder displays a multiscale morphology comprising agglomerates of interconnected nanoparticles, special attention is given to the quantification of the deformation mechanisms operating at different length scales. To this end, the displacement of vaterite agglomerates during uniaxial pressing is first observed through spatially- and time-resolved imaging using a custom-made mechanical testing setup positioned in a synchrotron beamline. Digital volume correlation is then applied on the reconstructed volumes to map local deformation of the compacts during stress relaxation experiments. This is later complemented with micro- and nano-indentation analysis to understand the role of the multiscale powder morphology on the cold sintering process. On the basis of these experiments, we finally propose a model to describe the deformation processes responsible for the water-assisted densification of nanovaterite powder under compression.