The effect of CNTs on the sintering and grain growth of ceramics

The effect of carbon nanotubes (CNTs) on the sintering behaviour and grain growth of ceramics was studied using Yttria partially stabilised Zirconia and Boron Carbide as model systems. The CNTs simultaneously enhance densification and retard grain growth. The sintering activation energy was calculated using isothermal conditions produced by Spark Plasma Sintering (SPS) with a rapid heating rate for the monolithic materials as well as ceramic CNT composites (0.5 and 2 vol%). These compositions look at the effect both below and above the electrical percolation threshold respectively. A significant reduction of the activation energy for the ceramic 2vol% CNT composites is attributed to the formation of a percolating network of CNTs providing a lower energy diffusion pathway. The sintering mechanism was found to be grain boundary diffusion for all samples suggesting that the presence of CNTs does not change the sintering mechanism but does lower the activation energy for the rate limiting step in the sintering process. The reduction in grain growth is attributed to the poor wetability of the CNTs acting as a physical barrier to prevent grain growth.