The bioactivity of hydroxyapatite (HA) and the strength and stiffness of carbon nanotubes (CNTs) suggest that composites of these two materials may be mechanically-superior as a bone replacement material. This work is the first direct comparison of spark plasma sintering (SPS) and pressureless sintering (PLS) to prepare dense composites of HA and CNTs. Previous studies of HA-CNT composites have typically used one or the other of these techniques, but since the exact powder properties, including phase purity, particle size distribution, and CNT morphology, can affect sinterability, it is difficult to compare the results of different authors. It is thus necessary to use the same powder to make a direct comparison of the two sintering techniques. The sintering conditions for HA-CNT composites are crucial. The high temperatures necessary for sintering can oxidize CNTs, while the presence of water vapour or high pressure is necessary to retain HA’s phase purity and hydroxylation. SPS offers the advantages of fast processing times (minutes) and lower sintering temperatures, but requires expensive equipment and limited geometry. PLS, on the other hand, involves cheap, readily-available equipment, but requires much longer processing times (hours) and higher sintering temperatures. The atmosphere must also be controlled carefully to balance CNT retention and preservation of HA phase purity and hydroxylation. HA-CNT powders (CNT loadings between 0 - 5 wt.%) were prepared by co-precipitation of HA in the presence of multi-walled CNTs. The material was then dried, ground, and pressed into tablets. SPS tablets were sintered at 1050°C for 3 min. at 70 MPa. PLS tablets were sintered in an atmosphere of carbon monoxide and hydrogen (optimized in a previous study for density and CNT retention) at 1200°C for 1 hr. Powder from the same batch was used for both techniques. The sintered material was characterized for phase purity (XRD), hydroxylation (FTIR), % porosity, CNT retention, microhardness, microstructure (SEM), and HA-CNT interface (TEM). Tablets produced by both techniques were found to be phase pure and hydroxylated. The SPS tablets were more dense (97-99% vs. 74-94%, depending on CNT loading) and had far superior CNT retention (99+% vs. up to 50%). In correlation with density, the microhardness of the SPS materials was up to 500% greater compared with their PLS counterparts. SEM and TEM analysis showed a much smaller grain size for SPS materials and better coating of the CNTs with HA. PLS can offer an affordable method to prepare phase-pure HA-CNT composites but at the expense of density and mechanical properties. SPS, on the other hand, produces fully dense materials with no CNT loss, enhanced coating of CNTs with HA, and increased microhardness.
|Publication status||Published - Apr 2011|
|Event||2011 MRS Spring Meeting & Exhibit - San Francisco, USA|
Duration: 1 Apr 2011 → …
|Conference||2011 MRS Spring Meeting & Exhibit|
|Period||1/04/11 → …|