Superhard ceramic composites: Influence of interfaces on properties of low pressure sintered composites

• Applicant: Dr.-Ing. Anne-Kathrin Wolfrum
• Subject Area: Glass, Ceramics and Derived Composites ; Synthesis and Properties of Functional Materials ; Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 429418801

High-performance wear-resistant materials are indispensable in many areas of modern technology, from manufacturing tools to industrial components. Two of the hardest known materials, diamond and cubic boron nitride (cBN), offer exceptional potential in this regard. However, their application is limited by specific manufacturing requirements: when sintered under normal pressure, both are metastable and convert to the less hard hexagonal phases. Conventional high-pressure synthesis prevents this, but is expensive and limits the possible shapes and sizes of the products. Previous research has addressed these challenges and shown that particle coatings are indispensable: It was found that uncoated cBN and diamond convert to hBN and graphite during sintering under normal pressure, especially in the presence of a transient liquid phase. This resulted in porous, mechanically weak interfaces. Particle coatings therefore proved to be an important prerequisite for the production of wear-resistant composites. These coatings can protect against unwanted phase transformations and form a strong chemical and mechanical bond with the ceramic matrix. In previous work, TiN and SiC were identified as potential coatings. TiN showed limited stability under the conditions investigated, while SiC remained chemically and structurally stable on both cBN and diamond in Al₂O₃ and Si₃N₄ matrices. Therefore, the proposed project aims to establish a framework for the microstructural properties of coated cBN/diamond-reinforced Al₂O₃ and Si₃N₄ composites. The goal is to achieve dense materials with high toughness and wear resistance and a particle content of at least 20 vol.%. The project focuses on the question of whether the limited protection provided by TiN can be improved through an optimized coating design, whether the results also apply to 2 µm particles, which are required for crack-free Al₂O₃ sintered composites, and how liquid phases and grain boundary phases can be adapted to improve compaction and crack resistance. Furthermore, it will be investigated whether the solid integration of SiC-coated particles leads to higher wear resistance across all particle sizes and fractions and whether reactive sintering additives stabilize the coatings. Therefore, newly developed dense TiN, TiC(1−x)Nx, and SiC coatings in combination with spark plasma sintering (SPS) are used in the project. In addition to mechanical and tribological tests, artifact-free preparations and microstructural characterizations (FESEM, EBSD, XRD, micro-Raman, TEM) are performed to quantify interface properties, residual stresses, and wear behavior.

• DFG Programme: Research Grants