Study of the Influence of Sintering Atmosphere and Mechanical Activation on the Synthesis of Bulk Ti2AlN MAX Phase Obtained by Spark Plasma Sintering.

The influence of the mechanical activation process and sintering atmosphere on the microstructure and mechanical properties of bulk Ti2AlN has been investigated. The mixture of Ti and AlN powders was prepared in a 1:2 molar ratio, and a part of this powder mixture was subjected to a mechanical activation process under an argon atmosphere for 10 h using agate jars and balls as milling media. Then, the sintering and production of the Ti2AlN MAX phase were carried out by Spark Plasma Sintering under 30 MPa with vacuum or nitrogen atmospheres and at 1200 °C for 10 min. The crystal structure and microstructure of consolidated samples were characterized by X-ray Diffraction, Scanning Electron Microscopy, and Energy Dispersive X-ray Spectroscopy. The X-ray diffraction patterns were fitted using the Rietveld refinement for phase quantification and determined their most critical microstructural parameters. It was determined that by using nitrogen as a sintering atmosphere, Ti4AlN3 MAX phase and TiN were increased at the expense of the Ti2AlN. In the samples prepared from the activated powders, secondary phases like Ti5Si3 and Al2O3 were formed. However, the higher densification level presented in the sample produced by using both nitrogen atmosphere and MAP powder mixture is remarkable. Moreover, the high-purity Ti2AlN zone of the MAX-1200 presented a hardness of 4.3 GPa, and the rest of the samples exhibited slightly smaller hardness values (4.1, 4.0, and 4.2 GPa, respectively) which are matched with the higher porosity observed on the SEM images.

Synthesis and Characterization of a Nearly Single Bulk Ti2AlN MAX Phase Obtained from Ti/AlN Powder Mixture through Spark Plasma Sintering.

MAX phases are an advanced class of ceramics based on ternary carbides or nitrides that combine some of the ceramic and metallic properties, which make them potential candidate materials for many engineering applications under severe conditions. The present work reports the successful synthesis of nearly single bulk Ti2AlN MAX phase (>98% purity) through solid-state reaction and from a Ti and AlN powder mixture in a molar ratio of 2:1 as starting materials. The mixture of Ti and AlN powders was subjected to reactive spark plasma sintering (SPS) under 30 MPa at 1200 °C and 1300 °C for 10 min in a vacuum atmosphere. It was found that the massive formation of Al2O3 particles at the grain boundaries during sintering inhibits the development of the Ti2AlN MAX phase in the outer zone of the samples. The effect of sintering temperature on the microstructure and mechanical properties of the Ti2AlN MAX phase was investigated and discussed.

Electrochemically Exfoliated Graphene-Like Nanosheets for Use in Ceramic Nanocomposites.

In this work, the synthesis of graphene-like nanosheets (GNS) by an electrochemical exfoliation method, their microstructural characterization and their performance as fillers in a ceramic matrix composite have been assessed. To fabricate the composites, 3 mol % yttria tetragonal zirconia (3YTZP) powders with 1 vol % GNS were processed by planetary ball milling in tert-butanol to enhance the GNS distribution throughout the matrix, and densified by spark plasma sintering (SPS). According to a thorough Raman analysis and SEM observations, the electrochemically exfoliated GNS possessed less than 10 graphene layers and a lateral size lower than 1 µm. However, they contained amorphous carbon and vacancy-like defects. In contrast the GNS in the sintered composite exhibited enhanced quality with a lower number of defects, and they were wavy, semi-transparent and with very low thickness. The obtained nanocomposite was fully dense with a homogeneous distribution of GNS into the matrix. The Vickers hardness of the nanocomposite showed similar values to those of a monolithic 3YTZP ceramic sintered in the same conditions, and to the reported ones for a 3YTZP composite with the same content of commercial graphene nanosheets.

Critical Influence of the Processing Route on the Mechanical Properties of Zirconia Composites with Graphene Nanoplatelets.

Graphene-based nanostructures, used as potential reinforcement in ceramic composites, have a great tendency to agglomerate. This requires the use of homogenization techniques during the powder processing, posing the need to evaluate how these techniques affect the microstructure and the mechanical properties of the resulting composites. The influence of the processing route on the properties of 3YTZP (3 mol % yttria tetragonal zirconia polycrystals) ceramic composites with 10 vol % cost-effective GNP (graphene nanoplatelets) has been addressed. Four different powder processing routines combining ultrasonic powder agitation (UA) and planetary ball milling (PBM) in wet and dry media have been used and all the composites were densified by spark plasma sintering (SPS). The mechanical properties at room temperature in the macroscale have been assessed by Vickers indentations, four-point bending tests and the impulse-echo technique, while instrumented indentation was used to measure the hardness and Young's modulus at the nanoscale. The application of dry-PBM enhances greatly the mechanical and electrical isotropy of the composites, slightly increases the hardness and lowers the elastic modulus, independently of the application of UA. The combination of UA and dry-PBM enhances the flexure strength by 50%, which is desirable for structural applications.

Aqueous colloidal processing of single-wall carbon nanotubes and their composites with ceramics.

A unique combination of acid treatment, aqueous colloidal processing, and spark-plasma sintering (SPS) has been used to fabricate high-density Al2O3 /single-wall carbon nanotube (SWNT) composites with well-distributed SWNTs and other carbon nanostructures ('nano-onions', diamond) at Al2O3 grain boundaries. This approach could be used to obtain well-controlled microstructures of ceramic/SWNT composites for tailored mechanical, electrical, and thermal properties. In addition, the colloidal approach for dispersing SWNTs presented here could be used for the controlled manipulation of SWNTs.