Translated Abstract
Spark plasma sintering (SPS) is a material processing technique which shows several advantages, such as high heating rate, short holding time, low sintering temperature and so on. Nowadays, SPS technique was becoming an ordinary method for the preparation of ceramics. Solid reactions and grain growth are two fundamental issues during the processing of SPS. Therefore, this work is focused to these two aspects and the aim is to get the general principles and furtherly, to tailor the microstructures of ceramics effectively. Firstly, the reactions between titanium (Ti) and aluminum nitride (AlN) during SPS were investigated and the high-purity Ti2AlN ceramic was obtained. The results indicated that the SPS is beneficial to the solid reactions. After replacing Ti by TiH2 which shows smaller particle size, the single phase Ti2AlN was prepared and the mechanical properties were investigated. By tailoring the shape of the graphite mold, the Ti2AlN/TiN functionally graded material(FGM) was fabricated due to the thermal gradient and the decomposition of Ti2AlN. The Ti2AlN/TiN FGM shows obvious hardness variation across the whole sample. Secondly, the grain growth behavior of hydroxyapatite (HAp) nanopowders during SPS was studied. It was revealed that there was a kinetic window in which the grain growth was suppressed while beyond which the nanopowders can not totally densified or the rapid grain growth was activated.It was found that the reactions between Ti and AlN is a complicated dynamic process. As the diffusion of AlN into the Ti particles, several reaction products, such as TiN, Ti3Al, Ti3AlN, Ti2AlN and TiAl, were firstly identified and finally only the Ti2AlN phase containing a minor of TiN phase was obtained. It was also revealed that the reaction between TiN and TiAl plays a key role on the purity of final Ti2AlN phase in that the plastic flow of TiAl in the reaction compact on one hand increased the possibility of contacting with the TiN and on the other hand, the TiN was remained when the TiAl was squeezed out from the reaction compact. This is why there is still a minor of TiN in the Ti2AlN matrix when sintered at 1400 oC with a holding time of 5 min.When the TiH2 was used to react with AlN during SPS, the Ti2AlN single phase was achieved at 1200 oC by holding 5 min. It was revealed that the diffusion path of AlN towards to Ti particle was shortened when the TiH2 powder was used since it shows smaller particle size and thus, the synthesis temperature was decreased. The received single phase Ti2AlN ceramic shows good properties. The bending strength, fracture toughness and the hardness are 354 MPa, 9.3 MPa•m1/2 and 4.1 GP, respectively. The indentation behavior indicates that the radial cracks along the indentations were avoided by the delamination of Ti2AlN grains which gives Ti2AlN ceramic a good machinability. During SPS, a large temperature gradient (over 100 oC) inside the sintering compact can be reached when a designed graphite mould was adopted. By this way, the continuous microstructural evolution during the reactive process between TiH2 and AlN was observed. This offers an effect method to study the solid reactions between different powders especially then the reactions are complicated. Furthermore, the Ti2AlN/TiN FGM was prepared when the temperature is high enough. This is due to the different thermal decomposition of Ti2AlN across the whole sample. Finally, the obtained Ti2AlN/TiN FGM shows a continuous variation of hardness from 3.9 GPa on the Ti2AlN side to 17 GPa on the TiN side.Two different HAp nanopowders, spray dried (SD) and hydrothermal (HT), were consolidated by SPS and their grain growth behaviors were investigated. It is indicated that the densification starts at about 700 oC for both powders and along with the densification, the elongated nanopowders has changed into equiaxed grains. During this process, only minor grain coarsening was found. However, a rapid grain growth was observed when the sintering temperature reached a critical value. The temperature period from densification onset temperature to rapid grain growth temperature can be defined as a kinetic window. When the sintering temperature is in the kinetic window, the grain growth can be suppressed and the highly transparent HAp ceramics were fabricated at 925 oC for SD powder and at 775 oC for HT powder by optimizing the sintering parameters during SPS. The large different temperatures for preparing transparent HAp ceramics may attribute to the more packing defects and intrinsic defects introduced by SD granulated nanopowders. These defects were difficult to eliminate since they were inherited during SPS. Therefore, higher temperature is required.. On the other hand, the densification were retarded in the presence of large defects. Besides, the transparent HAp bulk ceramics with different grain sizes were obtained by controlling the sintering parameters during SPS and even in these tranparent HAp ceramics, the trace of defects are still present.The rapid grain growth has been occurred at 1000 oC for SD powder and at 950 oC for HT powder. At the same time, the small pores were found in the dense HAp ceramics and the transparency was deteriorated. It is revealed that the dehydroxyalation of HAp plays an important role on the rapid grain growth since the water vapor derived from dehydration may give a local hydrothermal environment in the dense HAp ceramic and therefore, the diffusion rate was accelerated. For HT powders, the release of carbonate groups activated the hydrothermal reactions and as a result, the interfacial energy of HAp grains were changed. This is why the anisotropic grain growth was present in the HT powders.
Translated Keyword
[Spark plasma sinteringSolid reactionGrain growthTi2AlNHydroxyapatite]
Corresponding authors email
