Bond Strengthening and Grain Size Refinement in Superhard Metal Borides

Non-technical Summary

The continuous creation and development of tools through the tuning of materials’ properties has been a cornerstone of much of the development seen in human societies. High mechanical hardness is a very desirable property for materials used in industrial settings for machining and cutting as it dramatically reduces wear and therefore turnover rate of machining tools.

The industry standard superhard material is diamond, the hardest material currently known. The issue that arises with diamond, however, is that not only does it have an expensive high pressure, high temperate synthesis, but it is limited in its applications. This is because it is thermally unstable in air and when used to cut iron containing materials, diamond breaks down to form iron carbides.

These both result in a high turnover rate for diamond tools, and an inability to be used with common iron containing materials, like steel. Cheaper alternatives such as tungsten carbide (WC) have an easier, low-cost synthesis, but lack the extremely high hardness values of diamond and therefore have high turnover rates and are less effective. With this project, supported by the Solid State and Materials Chemistry program and the Ceramics program, both in NSF’s Division of Materials Research, the principal investigators design and create superhard materials that approach the high hardness seen in diamond, while replicating the low cost, ambient pressure synthesis found in WC.

These superhard materials made from boron not only lower the cost of synthesis, but they improve the lifetime of the tools that can be created and therefore, lower the amount of waste generated in industrial machining. Additionally, the metallic nature of these transition metal borides enables the use of high precision cutting and shaping instruments like plasma cutting, which is currently not usable with electrically insulating materials like diamond, which additionally reduces cost and waste in the formation of these tools.

Beyond this research, the principal investigators undertake educational outreach in the greater Los Angeles area. This includes developing lessons and experiments for K-12 schools and presenting them to teachers, along with speaking to students in grade school about not just science, but higher education as a whole. Graduate students who work on this project also gain valuable skills through both the research they conduct as well as through the mentorship and outreach programs they participate in alongside their mentors.

Technical Summary

Hardness is a mechanical property that is defined by a material’s ability to resist irreversible shape change, known as plastic deformation. The hardness of a given material is dependent on several different materials’ properties, but they can overall be grouped into two categories: intrinsic bonding effects and grain boundary effects. These two contributors to hardness are not mutually exclusive and therefore can be optimized separately and combined to dramatically improve the hardness of a material.

This project, with support from the Solid State and Materials Chemistry program and the Ceramics program, both in NSF’s Division of Materials Research, uses the described two-pronged approach towards superhard materials design and combines the synthesis of transition metal boride systems with high-pressure studies to obtain information about the internal deformation mechanisms of bulk and nanocrystalline materials. The research groups at UC Los Angeles study how small element doping into the boron sites of the metal borides affects the bonding.

Using systems of di- and tetra- borides with varying amounts of carbon in them, the principal investigators investigate the different carbon bonding regimes, and their impact on hardness. Additionally, synthetic routes for the formation of nanostructured metal borides are explored. The principal investigators utilize new synthetic routes to create nanocrystalline forms of known superhard metal borides such as ReB2, WB2 and WB4 to further increase the hardness of these materials by maximizing the number of grain boundaries which can impede plastic deformation.

These nanocrystalline materials also allow for new analytical techniques which are not possible for bulk materials such as Rietveld texture analysis. These two approaches to hardening metal borides can then be combined to create nanocrystalline solid solutions which benefit from both the improved bonding effects and grain boundary effects. The broader impacts of the project are multifaceted and include extensive outreach conducted by the principal investigators aimed at grade school children, the training of both graduate and undergraduate students in their Ph.D. studies and undergraduate research opportunities, respectively, and the development of novel superhard materials which have the potential to improve the quality of industrial manufacturing and machining tools.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.