Bond Strengthening and Grain Size Refinement in Superhard Metal Borides

非技术概述通过调整材料的性能不断地创造和发展工具，一直是人类社会许多发展的基石。高机械硬度是工业设备中用于加工和切割的材料的一个非常理想的特性，因为它显著降低了磨损，从而降低了加工工具的周转率。行业标准的超硬材料是钻石，这是目前已知的最硬的材料。然而，钻石带来的问题是，它不仅具有昂贵的高压、高温合成，而且其应用也受到限制。这是因为它在空气中热不稳定，当被用来切割含铁材料时，钻石会分解形成碳化铁。这两种情况都导致金刚石工具的周转率很高，并且无法与常见的含铁材料(如钢)一起使用。碳化钨(WC)等成本较低的替代品具有更容易、低成本的合成，但缺乏钻石的极高硬度值，因此周转率高，效率较低。在NSF材料研究部固态和材料化学计划和陶瓷计划的支持下，主要研究人员设计和创造了接近钻石中所见的高硬度的超硬材料，同时复制了WC中的低成本、常压合成。这些由硼制成的超硬材料不仅降低了合成成本，而且还延长了可以制造的刀具的寿命，从而减少了工业机械加工中产生的废物量。此外，这些过渡金属硼化物的金属性质允许使用高精度的切割和成形工具，如等离子切割，这目前不适用于钻石等电绝缘材料，从而进一步降低了这些工具的形成成本和浪费。除了这项研究之外，主要调查人员还在大洛杉矶地区进行教育推广。这包括为K-12学校开发课程和实验，并将其呈现给教师，同时向小学学生讲述不仅是科学，而且是整个高等教育。参与这个项目的研究生通过他们进行的研究以及他们与导师一起参与的导师和外展项目，也获得了宝贵的技能。技术概述硬度是一种机械性能，由材料抵抗不可逆形状变化的能力定义，称为塑性变形。一种材料的硬度取决于几种不同材料的性质，但总体上可以分为两类：本征结合效应和晶界效应。这两个影响硬度的因素并不是相互排斥的，因此可以单独优化并组合在一起，以显著提高材料的硬度。该项目由NSF材料研究部的固态和材料化学计划和陶瓷计划支持，采用上述双管齐下的方法进行超硬材料设计，并将过渡金属硼化物系统的合成与高压研究相结合，以获得关于块体和纳米晶体材料内部变形机制的信息。加州大学洛杉矶分校的研究小组研究了金属硼化物中硼位置的小元素掺杂如何影响成键。使用含不同量碳的二硼化物和四硼化物体系，主要研究人员调查了不同的碳键制度，以及它们对硬度的影响。此外，还探索了形成纳米结构金属硼化物的合成路线。主要研究人员利用新的合成路线来创造已知的超硬金属硼化物的纳米晶形式，如ReB2、WB2和WB4，以通过最大限度地增加可阻止塑性变形的晶界数量来进一步增加这些材料的硬度。这些纳米晶体材料还允许采用新的分析技术，这是Rietveld织构分析等块状材料所不可能实现的。然后，这两种硬化金属硼化物的方法可以结合在一起，创建纳米晶固溶体，这两种方法都受益于改进的结合效应和晶界效应。该项目的广泛影响是多方面的，包括主要研究人员针对小学生进行的广泛推广，分别对研究生和本科生进行博士学习和本科生研究机会的培训，以及开发有潜力提高工业制造和机械加工工具质量的新型超硬材料。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical SummaryThe 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 SummaryHardness 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.