The research group reports on the discovery of internal defects caused by heteroatom monolayers.

In collaboration with the Institute of Physics at the Chinese Academy of Sciences (IPCAS), researchers from Beijing University of Technology (BJUT) have discovered a new type of internal grain damage in the ceramic phase in TiC doped cemented tungsten carbides.

These structural defects were found to be caused by the ordered distribution of heteroatoms on the specific planes of tungsten (W) and carbon (C). Importantly, these newly identified faults show distinct characteristics that distinguish them from well-known systematic faults, such as phase boundaries, grain boundaries, double boundaries, shear faults and types.

This work is published in Advanced Powder Tools, and include atomic-scale details of the structure, composition and crystallography of a new type of planar defect. In addition, detailed calculations of the species were carried out to assess their strength and stability, and to shed light on their nature.

The research team, led by Professor Xiaoyan Song from BJUT, found that the presence of a titanium (Ti) monolayer on the base planes of WC was caused by the decomposition of (W,Ti)C._x complexions, which formed on the WC/Co interfaces by decay-wind method sintering of the powder mixture. A stable Ti monolayer can provide a nucleation site for WC crystal growth [0001]_WC instructions. This possibility was confirmed by model calculations.

“We also explored the possibility of creating similar defects in WC grains by doping V, Zr, Nb, Mo, and Hf through the process,” explained Song. “We found that the Ti-monolayer induced planar defects were more stable inside the WC grains, and they were also easier to form in the bonded carbides.”

This work shows the importance of planar faults and surface stability that prevent the long-distance movement of fault storage and intra-grain movement, and act as barriers to the propagation of transgranular cracks. Therefore, the risk of transgranular fracture, which is the main failure mechanism of covalent crystals in ceramics and ceramic matrix composites, can be significantly reduced.

“We demonstrated in detail that changing the density of planar defects improves mechanical performance, resulting in a unique balance between the strength and durability of materials,” Song added. “Our study paves the way for improved material technology by deliberately introducing and customizing heteroatomic monolayer-induced grain-internal planar defects.”

In addition, the methods described in this article, using cemented tungsten carbides as a representative, can be extended to other materials. By carefully selecting the dopants and controlling the sintering parameters, it is possible to control the density of planar defects to achieve the desired properties of various ceramic systems.