Sintering kinetics and microstructure development of synthetic lunar highlands and mare regolith

Kinetics and microstructure development during pressureless sintering of two Lunar regolith simulants are reported and discussed. Dry-pressed cylindrical specimens of either a Highlands simulant or a Mare simulant were heated at 80 K min-1 to a prescribed sintering temperature and held for ten hours while dimensional changes were tracked in situ by optical dilatometry. The axial and volumetric strains and densification rates were calculated as a function of time and temperature. Possible phase transformations were evaluated by quantitative X-ray powder diffraction. The relative density increased with the logarithm of time at any temperature, and densification rates decreased continuously and sharply with increasing density. Sintered microstructures and compositional characteristics were observed by scanning electron microscopy and X-ray microprobe analysis of polished surfaces. The microstructures of both materials show evidence of heterogeneous densification, with large but isolated high-density domains surrounded by lower-density regions with interconnected porosity. No significant compositional differences were detected between the higher- and lower-density regions, meaning that the heterogeneous local densities are caused by gradients in particle size or packing that induce locally varying thermodynamic driving forces for densification. The apparent activation enthalpy for densification was measured as 445 kJ mol-1 and 440 kJ mol-1 for the Highlands and Mare simulants, respectively, values that were observed to be independent of temperature and sintered density to within the measurement uncertainty.