George Levi

Aluminium-alumina interface morphology and thermodynamics from dewetting experiments

Abstract This work presents the morphology of micron-sized metallic drops (Al) equilibrated on ceramic substrates (sapphire), with emphasis on the interfacial region. The samples were made by controlled dewetting of a thin Al film on the basal surface of sapphire substrates, at temperatures above the melting point of Al. Morphological inspections showed that evaporation of the sapphire substrate atoms, which diffuse at the interface, has an important role in the evolution of the Al-sapphire interface morphology. The absolute value of the sapphire basal surface energy was extracted (1279 ± 78 mJ/m 2 at 900 °C). The values of the thermodynamic parameters determined from the interface morphology agree well with those from experiments made under identical conditions but using millimetre-sized drops. This confirms the validity of Young’s equation for large sessile drops in systems undergoing limited morphological changes (ridging) at the interface.

Oxygen induced interfacial phenomena during wetting of alumina by liquid aluminium

Abstract Sessile drop experiments of liquid Al on sapphire (α-Al2O3) were conducted under a low pressure (10−3 Torr) controlled Ar atmosphere as a function of oxygen partial pressure, temperature and/or time. Microstructural investigations of the samples from the experiments indicated that two different dominant processes occur at the liquid Al-sapphire interface: epitaxial growth of new α-Al2O3 layers on the sapphire substrate at temperatures below ≈1100°C and dissolution of the sapphire substrate at temperatures above ≈1100°C. Possible mechanisms by which oxygen affects wetting and adhesion of liquid Al on sapphire are examined. The non-wetting to wetting transition in this system may be explained by formation of an oxygen-rich interphase at the liquid Al-sapphire interface. It is concluded that dissolution of sapphire under Al oxidation conditions is capillary driven.

Structure refinement of titanium carbonitride (TiCN)

Rietveld structure refinement was conducted on X-ray powder diffraction data of TiCN. The existing structural model for TiCN, based on a cubic lattice with random occupation of sites by Ti, C and N, was found to be incorrect. The TiCN structure is better described based on the TiN structure (NaCl type) with substitution of N by C. Rietveld refinement showed that only a low concentration of vacancies exists in the nonmetallic sublattice.

Segregation of aluminium at nickel-sapphire interfaces

Sessile drop experiments of pure liquid Ni on the basal surface of pure sapphire were conducted under controlled atmosphere and temperature. This system has been traditionally considered as non-reactive, based on thermodynamic assessments. However, the results of this study demonstrate that a capillary driven interaction exists between the pure liquid Ni and the sapphire, which causes the dissolution of the sapphire substrate mainly at the triple junction. Oxygen and Al resulting from the dissolution process diffuse into Ni and segregate at its interfaces with the atmosphere and the sapphire (probably as AlxOy clusters), which reduces the interface energy. It is considered that this reduction is beneficial for the adhesion of both liquid and solid Ni on sapphire. The amount of Al introduced into the drop, and hence the segregation of Al that affects the interface energy (and adhesion), are related to the size of the sessile drop.

Free Surface and Interface Thermodynamics of Liquid Nickel in Contact with Alumina

Sessile drop experiments of Ni and Ni(2at.%Al) were conducted under controlled working conditions, at 1500°C, P(O2) ≤ 10−9 Torr. It is shown that Al and oxygen atoms engaged in the capillary driven mass transport at the interface have a significant impact on the surface/interface thermodynamics. The surface energy of liquid Ni determined from experiments in which Ni comes into contact with Al2O3 is significantly lower than that of high purity Ni, due to the segregation of Al. The free energy of segregation of Al to the free surface of Ni (Δ GS) was found to range from −164 to −152 kJ/mol, indicating a relatively strong tendency for segregation of Al to the free surface of Ni(Al). It is proposed that an Al(O)-rich liquid layer forms adjacent to the Ni-Al2O3 interface, which improves interfacial adhesion. In the Ni(Al)-Al2O3 system, an increase in the Al content of the alloy leads to the improvement of both wetting and adhesion of the alloy on the ceramic, correlating with the improvement in the interface strength after solidification.

Interfacial phenomena and microstructure evolution during solidification of binary and ternary Al–Mg–Si alloys cast with titanium carbonitride

Abstract In this work the microstructural evolution of Al–Mg 2 Si alloys cast with titanium carbonitride (TiCN) nano-powder was studied, based on Al(Mg)–TiCN and Mg(Si)–TiCN model systems. From the investigation of the model systems, it is concluded that the presence of TiCN leads to the formation of a Mg-rich (or pure Mg) bulk liquid layer in the alloy at the melt–TiCN interface, before solidification begins. The driving force for the formation of the interfacial Mg-rich layer is the reduction of the total interface energy. It is postulated that this layer forms due to the interaction between Mg segregated at the alloy–TiCN interface with Mg-rich Al–Mg-clusters and Si-rich Mg–Si-clusters from the molten alloys. It is concluded that a Mg-rich bulk layer also forms at the Al(Mg,Si)–TiCN interface in cast Al–Mg 2 Si alloys, which prevents nucleation of Mg 2 Si on the TiCN particles.