Laser Shockwave Sintering of Micro and Nanoscale Powders of Yttria-Stabilized Zirconia
Roa Baerga, Victor (2010) Laser Shockwave Sintering of Micro and Nanoscale Powders of Yttria-Stabilized Zirconia. Masters thesis, Iowa State University.
Full text available as:
Mechanical (non-thermal) sintering behavior of yttria stabilized zirconia powder compact was investigated as a function of particle size. Cubic crystalline form of ZrO2 containing 8 mole% Y2O3 powders of the nominal size 16 µm and 45 nm were acquired, cold pressed in a die to make green compacts and then subjected to a novel laser shock peening (LSP) process for densification. Results indicated that micropowder compacts (MC) excessively cracked and chipped while nanopowder compact (NC) and mixtures of micro/nanopowder compacts (MNC) underwent sintering without crack formation. However there was evidence of stress-assisted material removal and surface disruption in NC samples, making their surfaces much rougher than those of MC samples. In addition, MC exhibited a much higher increase in hardness over NC and MNC. A 64 % increase in hardness was obtained in MC compared to a 44 % increase in NC. Furthermore there was grain coarsening effect in NC and MNC as compared to MC. Overall an improved densification was obtained in NC and MNC samples. Analytical model results indicated residual stresses occurring deeper in the material for MC compared to NC and validated the higher hardness obtained in the MC samples. The NC samples were subjected to higher plastic strain compared to the MC samples that led to more plastic deformation in the NC samples. The plastically affected depths for the samples indicate the penetration of the LSP process reached over 25% of the 1 mm thick compacts. Lowering the repetition rate from 5 to 3 Hz increases the pulse pressure, plastically affected depth and surface plastic strain by 4% for both MC and NC, while the surface residual stresses increased by 85% (MC) and 104% (NC). We hypothesize that laser shock waves create a compressive stress pattern that causes plastic deformation of the particles and a large increase in the concentration of vacancies on the surface of the particles; this highly defective surface coupled with a high surface mobility of nanoparticles is responsible for the mass transport. In addition the strong bonding between nanoparticles and possible stress-assisted phase transition to monoclinic phase at the grain boundaries reduce significantly the pores thus resulting in a high density part. This hypothesis agrees with the results that were seen in the analytical model. Thermal conductivity measurements indicate that the MNC have approximately half the thermal conductivity of the MC. The reduction in thermal conductivity in MNC is attributed to the presence of nanoparticles at the pores that limit the mean free paths of phonons and photons.
Archive Staff Only: edit this record