Superhard Metals

技术综述过渡金属硼化物因其将高硬度和导电性与直接的常压合成相结合的物理性能而引起人们越来越多的兴趣。如果设计得当，超硬金属硼化物可以改进刀具和耐磨涂层。二硼化Re(ReB2)是这种新的、不断增长的超硬金属硼化物的一个很好的例子。在固态和材料化学计划的支持下，将探索通过使ReB2与其他过渡金属固溶体来激活位错钉扎机制来改善ReB2性能的可能性，这可能会产生更坚硬的材料。通过用相对便宜的过渡金属钨取代Re，可以合成价格较低的过渡金属硼化物。通过用W完全取代Re并增加硼浓度，可以得到另一种有趣的硼化物--四硼化钨(WB4)，它的硬度似乎与ReB2相当。在表征了其物理性质，包括微纳压痕、常压和高压X射线衍射以及多晶和单晶WB4的共振超声光谱后，将使用中子衍射重新检查其结构。这一强大的工具应该能够确定硼原子在单胞中的确切位置，这是目前尚不清楚的。这应该可以更好地了解WB4的体积弹性系数、高硬度和其他特性。将WB4与其他过渡金属元素如Ta、Mo、Cr和Mn进行固溶体，以潜在地增加硬度，并提供更好的防腐蚀性、热稳定性和断裂韧性。该项目的一个长期目标是探索将这些超硬金属硼化物用于刀具的可行性。通过利用其导电性，可以使用电火花加工(EDM)来切割所需的形状。刀具刀片将由电火花加工制成，并将进行耐磨性等性能测试。将开发涂层并进行摩擦试验。通过吸引本科生参与研究，并为高中生和教师提供材料及其应用的讲座和演示，该项目将产生更广泛的影响。非技术总结寻找新的超硬金属具有巨大的科学和实践兴趣。从科学上讲，我们将了解如何以及为什么可以将以延展性著称的材料转变为超不可压缩、超硬的化合物。实际上，这些金属材料，无论多么坚硬，都可以使用一种称为电火花加工的现成工艺来切割成精确的形状。这将使人们能够探索它们可能用于铣削、锯切和钻孔的可能性。电火花加工将被用来将这些新材料转化为工具，以测试它们切割黑色金属的能力。还将开发和测试具有低摩擦表面的抗划痕涂层。这些项目的更广泛影响将通过吸引本科生参与研究并为高中生和教师提供材料及其应用的讲座和演示来实现。

TECHNICAL SUMMARYTransition metal borides have attracted increasing interest due to their physical properties that combine high hardness and electrical conductivity with straightforward ambient pressure synthesis. If designed properly, superhard metallic borides could lead to improved cutting tools and wear-resistant coatings. Rhenium diboride (ReB2) is a good example of this new and growing class of superhard, metallic borides. With support from the Solid State and Materials Chemistry Program, the possibility of improving on the properties of ReB2 by making solid solutions of ReB2 with other transition metals to activate dislocation-pinning mechanisms that could create harder materials will be explored. Less expensive transition metal borides will be synthesized by replacing rhenium with the relatively inexpensive transition metal tungsten. By completely replacing Re with W and increasing the boron concentration, another interesting boride, tungsten tetraboride (WB4), can be made which appears to have comparable hardness to ReB2. After characterizing its physical properties including micro- and nano-indentation, ambient and high pressure X-ray diffraction and resonant ultrasound spectroscopy of both polycrystalline and single crystalline WB4, its structure will be reexamined using neutron diffraction. This powerful tool should enable the determination of the exact positions of the boron atoms in the unit cell, something that is currently not known. This should provide better insight into the bulk modulus, high hardness and other properties of WB4. Making solid solutions of WB4 with other transition metal elements such as Ta, Mo, Cr and Mn, will be carried out in order to potentially increase hardness and impart improved corrosion protection, thermal stability and fracture toughness. A long-term goal of this project is to explore the feasibility of using these superhard, metallic borides for cutting tools. By taking advantage of their electrical conductivity, desired shapes will be cut using electric discharge machining (EDM). Cutting tool inserts will be made by EDM, and properties such as wear resistance will be tested. Coatings will be developed and friction tests carried out. Broader impact of this project will be achieved by engaging undergraduates in the research and providing high school students and teachers with lectures and demonstrations of materials and their applications.NON-TECHNICAL SUMMARYThe search for new superhard metals holds both great scientific and practical interest. Scientifically, an understanding of how and why materials known for their malleability can be turned into ultra-incompressible, superhard compounds will be gained. Practically, these metallic materials, no matter how hard, can be cut into precise shapes using a readily available process known as electric discharge machining. This will enable the exploration of their possible use for milling, sawing and drilling. Electric discharge machining will be used to turn these new materials into tools that will be tested for their ability to cut ferrous metals. Scratch resistant coatings with low friction surfaces will also be developed and tested. Broader impact of these projects will be achieved by engaging undergraduates in the research and providing high school students and teachers with lectures and demonstrations of materials and their applications.