Investigating the Cold Sintering Process of Bioactive Materials

The cold sintering process (CSP) is a non-conventional sintering technique that densifies ceramic materials using a transient liquid phase under applied pressure, typically at low temperatures (<300 °C). This processing window enables co-sintering of ceramic-polymer phases and offers unique advantages over conventional high-temperature sintering, which can compromise material integrity and degrade functional performances.
Bioactive materials play a pivotal role in clinical orthopedic applications because they stimulate the biological events that mediate bone repair and regeneration. This thesis investigates CSP of three distinct bioactive material systems: hydroxyapatite (HA), ternary bioglass (BG, SiO₂–CaO–P₂O₅), and hydroxyapatite-polylactide composite (HA-PLA). The key role of transient liquids, their chemistry, and the associated pressure-solution creep pathway in enabling low-temperature densification was studied. Phosphoric acid was found to be an effective transient liquid for densifying HA to ~90 % and BG to ~89 % under 360 MPa at 200 °C. In both cases, 2 M phosphoric acid drives pressure-solution creep via incongruent dissolution-reprecipitation-recrystallization pathways. In CSP of the HA-PLA composite, consolidation is governed by the HA-to-PLA ratio and the thermal-induced flowability of PLA, as assessed by comparing unplasticized and plasticized PLA.
Finally, the temperature-dependent dynamics of densification in HA, BG, and consolidation in HA-PLA composite were established through in-situ monitoring and in-situ Raman analysis in collaboration with the National Institute of Standards and Technology (NIST), USA. Overall, this thesis provides a fundamental understanding of transient liquid chemistry and the underlying sintering mechanism governing CSP of bioactive ceramics and composite systems, guiding future efforts to advance the technology readiness level (TRL) of CSP for the fabrication of synthetic grafts.