Replication Experiments in Microgravity Liquid Phase Sintering
- PDF / 4,028,814 Bytes
- 14 Pages / 593.972 x 792 pts Page_size
- 37 Downloads / 210 Views
I.
INTRODUCTION
DURING liquid phase sintering a component consists of solid, liquid, and vapor phases and behaves as a viscous body that responds over time to both capillary and gravity forces. The measured viscosity during liquid phase sintering depends on the composition, but tends to range from 109 to 1011 Pa s.[1–4] Usually the wetting liquid supplies a capillary force on the solid grains to densify the structure, resulting in component shrinkage. Gravity results in slightly anisotropic shrinkage, sometimes where the vertical dimension shrinks and the perpendicular dimension expands. Capillary forces act to cause uniform shrinkage, but substrate friction resists shrinkage. The combination of viscous flow, gravity, shrinkage, and substrate friction leads to loss of precision during Earth sintering to produce ‘‘elephant foot’’ geometries.[5] Computer simulations include these factors to explain distortion, densification, and final geometry.[6–11] On a microscopic scale, sintering on Earth induces pore buoyancy and grain settling effects that are most evident with high liquid contents.[5,12,13] Gravity segregates the solid grain structure to increase grain bonding and reduce diffusion distances, resulting in a larger grain
RANDALL M. GERMAN, Professor, is with the Mechanical Engineering Department, San Diego State University, 5500 Campanile Dr., San Diego, CA 92182. Contact e-mail: [email protected] JOHN L. JOHNSON, Vice President, is with Elmet Technologies LLC, 1560 Lisbon St., Lewiston, ME 04240. Manuscript submitted July 21, 2015. Article published online February 10, 2016 2286—VOLUME 47A, MAY 2016
size and dense skeletal structure that resists distortion.[6,14–16] Accordingly, gravity effects on sintering depend on the component height, amount of liquid, grain size, constituent densities, initial porosity, and dihedral angle, leading to differences in solid-liquid ratio along the vertical dimension with concomitant changes in grain size and other parameters, including the rate of grain growth.[5,15,17] Without gravity the initial conjecture was that sintering would be more predictable since grain compression, solid–liquid separation, pore buoyancy, and substrate friction would be eliminated. Sintering is a relatively slow process so testing this theory required access to long-term microgravity conditions. Counter to expectations, experimental results from microgravity sintering show that seemingly subtle effects lead to large changes that are less predictable.[18] Removing gravity simply allows small system variations to dominate the sintering trajectory. The secondary factors, such as impurity reactions, are always present, but are small in impact compared with gravity. With the removal of gravity, these secondary factors emerge to dominate sintering behavior, leading to grain agglomeration, radial liquid– solid separation, and generation of massive pores.[19–25] Because of limited microgravity furnace availability, experimental data are often without replication. This hinders critical evaluation of the compute
Data Loading...
