Bond Strengthening and Grain Size Refinement in Superhard Metal Borides

通过调整材料的特性来不断创造和发展工具，已经成为人类社会发展的基石。高机械硬度是工业环境中用于加工和切削的材料非常理想的特性，因为它大大减少了磨损，从而减少了加工工具的周转率。工业标准的超硬材料是金刚石，这是目前已知的最硬的材料。然而，金刚石的问题在于，它不仅需要昂贵的高压、高温合成，而且在应用上也受到限制。这是因为它在空气中热不稳定，当用于切割含铁材料时，金刚石会分解形成碳化铁。这两者都导致了金刚石工具的高周转率，并且无法与普通含铁材料（如钢）一起使用。较便宜的替代品，如碳化钨（WC）的合成更容易，成本更低，但缺乏金刚石的极高硬度值，因此具有较高的周转率，效果较差。在这个项目中，由NSF材料研究部门的固态和材料化学项目和陶瓷项目支持，主要研究人员设计和制造了接近金刚石中所见的高硬度的超硬材料，同时复制了WC中发现的低成本，环境压力合成。这些由硼制成的超硬材料不仅降低了合成成本，而且提高了可以制造的工具的使用寿命，因此，降低了工业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使得等离子切割等高精度切割和成型工具的使用成为可能，这是目前金刚石等电绝缘材料无法使用的，这进一步降低了这些工具形成的成本和浪费。在这项研究之外，主要研究人员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，还有整个高等教育。从事这个项目的研究生也可以通过他们进行的研究以及他们与导师一起参与的指导和推广项目获得宝贵的技能。硬度是一种机械性能，由材料抵抗不可逆形状变化（即塑性变形）的能力来定义。给定材料的硬度取决于几种不同材料的特性，但它们总体上可以分为两类：内在键合效应和晶界效应。这两个因素对硬度的贡献并不相互排斥，因此可以单独优化和组合，以显着提高材料的硬度。该项目得到了NSF材料研究部固态与材料化学项目和陶瓷项目的支持，采用了所描述的双管齐下的超硬材料设计方法，并将过渡金属硼化物系统的合成与高压研究相结合，以获得有关体块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小元素掺杂到金属硼化物的硼位点是如何影响键合的。使用二硼化物和四硼化物体系，其中含有不同数量的碳，主要研究人员研究了不同的碳键合机制，以及它们对硬度的影响。此外，还探索了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制造已知的超硬金属硼化物（如ReB2， WB2和WB4）的纳米晶形式，通过最大化晶界的数量来进一步提高这些材料的硬度，从而阻止塑性变形。这些纳米晶体材料也允许新的分析技术，这是不可能的块状材料，如Rietveld织构分析。这两种硬化金属硼化物的方法可以结合起来产生纳米晶固溶体，从而受益于改进的键合效应和晶界效应。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，研究生和本科生的博士学习和本科研究机会的培训，以及有可能提高工业制造和加工工具质量的新型超硬材料的开发。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值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.