Investigation on fabrication, interface formation, and properties of hot-processed titanium-doped copper/diamond composites

Abstract

Efficient thermal management is crucial for next-generation high-power electronic devices, driving the need for high-performance heat sink materials. Copper/diamond composites have emerged as promising candidates due to their exceptional thermal conductivity (TC) and tailorable coefficient of thermal expansion (CTE). However, their performance is limited by weak interfacial bonding, caused by the poor chemical affinity between copper and diamond and severe phonon scattering due to mismatched lattice vibrations. To address these challenges, interface engineering strategies such as pre-coating diamond particles or pre-alloying the copper matrix with carbide-forming elements (e.g., Ti, W, Zr, Cr, B) have been extensively explored. Among these, titanium has shown promise through the formation of a stable titanium carbide (TiC) layer, which significantly enhances interfacial bonding and composite performance. This thesis investigates the thermophysical and mechanical performance of titanium-doped copper/diamond composites fabricated via hot-processing techniques, hot-forging and hot-pressing, with focuses on optimizing fabrication parameters, elucidating interface formation mechanisms, and establishing the relationship between the interfacial microstructure and composites performance. Ti-coated diamond/Cu composites were successfully fabricated via hot-forging, and the effects of forging temperature, diamond volume fraction, and deformation degree were systematically evaluated. The study highlights the crucial role of hot-forging temperature in optimizing TiC interface formation and enhancing interfacial bonding. 800°C is the optimal hot forging temperature to facilitate the dynamic formation of TiC interface layer and ensure a high interface coverage rate on the diamond particles. Additionally, the volume fraction of diamond affects the interface microstructure and TC of the hot-forged composites by affecting the heating behaviour of the composites during induction heating. The relatively large deformation degree can effectively contribute to the densification and interfacial bonding of the composites. 800 °C hot-forged composite with 45 vol% diamond and 80% deformation degree exhibited excellent TC of 529 W/mK, along with a high tensile strength of 241 MPa, attributed to the synergy of uniform distribution of diamond in the matrix, a rough interface, and strong interfacial bonding between the copper and the diamond. The fabrication of copper/diamond composites via hot-pressing, was further investigated employing two interfacial engineering strategies: titanium carbide formation through diamond surface metallization and copper matrix alloying with titanium additives. The results show that diamond metallization shows a better effectiveness in enhancing both mechanical and thermophysical performance than matrix alloying. Notably, Cu/50 vol% Ti-coated diamond composite fabricated at 540 MPa achieved an exceptional TC of 565 W/mK and a tensile strength of about 147 MPa. For the copper-Ti/diamond composite containing 0.5 wt% Ti processed at 1050 °C, it exhibited a comparable TC (554 W/mK), but a relatively low tensile strength (93 MPa). The optimal processing pressures for maximizing TC and mechanical strength in hot-pressed Cu/Ti-coated diamond composites differ due to interface microstructure: continuous interface enhances mechanical strength through strong interfacial bonding, whereas jagged interface arranged in parallel on the diamond surface minimizes interfacial thermal resistance. A comparative study between the two processing routes revealed that hot-forging outperforms hot-pressing at high diamond content (55 vol%) due to greater matrix deformation and more uniform diamond distribution, whereas hot-pressing becomes advantageous at lower diamond fractions (45-50 vol%), where applied pressure provides effective densification and interfacial microstructure becomes the dominant factor governing composite performance. A detailed investigation into interface formation mechanisms was conducted on 540 MPa hot-pressed composites with 50 vol% Ti-coated diamond. The results revealed that the distinct atomic configuration of individual diamond facets govern the nucleation and growth behaviour of TiC at the interface, giving rise to different interfacial microstructures: a continuous Cu/TiC/diamond structure on diamond-{100} facets, and a multilayered Cu/Cu (Ti)/TiC/diamond structure on diamond-{111} facets. Beyond the formation of TiC interlayer on both facets, the semi-coherent orientation relationships (ORs) were identified between TiC and both diamond-{111} facet and copper: (11 ̅1 ̅) TiC // (11 ̅1 ̅) Diamond and [1 ̅1 ̅0] TiC // [1 ̅1 ̅0] Diamond, (111) TiC// (1 ̅11) Cu, [01 ̅1] TiC// [211] Cu. These ensure a high degree of atomic ordering at the interface, enhancing phonon transmission across the interface. Concurrently, Cu segregation at TiC grain boundaries refines grains and strengthens interfacial bonding, further reducing thermal resistance. Furthermore, sub-stoichiometric TiCx present on both facets introduces a mixed metallic-covalent bonding character, facilitating electron-mediated heat transport and promoting nanotwin formation, which migrates phonon scattering by enhancing dislocation mobility. Overall, this work demonstrates that the thermophysical and mechanical performance of copper/diamond composite is governed by the synergistic interplay between processing parameters and interfacial microstructure. The facet-dependent atomic configuration of diamond surface dictate distinct interfacial microstructures, while processing parameters such as temperature and pressure provide effective levers for controlling TiC stoichiometry, interlayer continuity, and interfacial bonding strength. Together, these factors determine the balance between thermal transport efficiency and mechanical strength, establishing a rational framework for interface engineering in high-performance Cu/diamond composites.