2026-11-03 テキサスA&M大学
米国テキサスA&M大学とライス大学の研究チームは、超高速衝突によってダイヤモンド複合材料の一部がグラファイトへ瞬時に相転移し、衝撃エネルギーを吸収する現象を明らかにした。研究では、微細なダイヤモンド粒子に立方晶窒化ホウ素とコバルトを加え、放電プラズマ焼結法によって高硬度の複合材料を作製した。これに対し、二段式軽ガス銃を用いて、音速の約7.5～8.45倍で飛翔するアルミニウム弾を衝突させた。その結果、衝撃から数マイクロ秒以内にダイヤモンドの一部がグラファイトへ変化することを確認した。この相転移では炭素原子の結晶構造が変化し、その過程で衝突エネルギーの一部が吸収・散逸される。また、分子動力学シミュレーションによって、ダイヤモンドとグラファイトの界面構造や相転移の過程を解析した。本成果は、極超音速飛行体や宇宙機、航空機などが受ける極端な衝撃に耐える次世代防護材料の設計に貢献する。
Technical illustration of the 45-foot-long, two-stage light gas gun housed at Texas A&M University’s Hypervelocity Impact Laboratory. Credit: Texas A&M Engineering Experiment Station
＜関連情報＞
- https://stories.tamu.edu/news/2026/11/03/forging-diamonds-firing-hypersonic-projectiles-forming-graphite/
- https://www.sciencedirect.com/science/article/abs/pii/S1369702126003238
極超音速衝撃下でのダイヤモンドからグラファイトへの相転移 Diamond-to-graphite transformation under hypersonic impact
Abhijit Biswas,Aniket Mote,Rajib Sahu, Marcelo Lopes Pereira JuniorShuo Yang,Sudaice Kazibwe,Jishnu Murukeshan,Raphael Benjamim de Oliveira,Guilherme da Silva Lopes Fabris,Shreyasi Chattopadhyay, Gelu Costin,Jianhua Li, Robert Vajtai,Ching-Wu Chu,Lizhong Lang,Yu Zou, Liangzi Deng, Tobin Filleter, Douglas Soares Galvão, Christian Kübel…Pulickel M. Ajayan
Materials Today Available online: 6 August 2026
DOI:https://doi.org/10.1016/j.mattod.2026.103477
Highlights
- We stabilized bulk composites containing micron-size diamond particles via spark-plasma sintering (SPS), by using cubic boron nitride (cBN) as a matrix and cobalt as an additive.
- We conducted impact testing on these composites at hypersonic speed (near-hypervelocity, >Mach 8) using metal projectiles.
- The composite survives from smaller (∅1 mm) projectiles impact, however, fractures upon impacting with a larger (∅4 mm) projectile.
- Diamond undergoes a near-complete phase transformation to graphite due to impact, supported by the atomistic molecular dynamics simulations.
Abstract
Consolidation of diamond particles into high-strength bulk materials and composites is extremely challenging as it requires high sintering temperatures, at which diamond transforms into graphite, making it impossible to retain the sintered diamond phase. Here we report a synthetic approach based on spark plasma sintering that allows the stabilization of micron-scale diamond grains in composites, using cubic boron nitride (cBN) as the matrix and cobalt (Co) particles as a stabilizer, resulting in a hard-to-machine, tough material. We performed hypersonic speed impact tests on the composites using metal projectiles of different sizes. The composite withstands impact from ∅1 mm metal projectiles (speed Mach 7.5) but breaks apart during the impact from a larger ∅4 mm metal projectile (speed Mach 8.45). Interestingly, during this microsecond-scale impact and fracture event, embedded diamond particles in the composite undergo near-complete phase transformation to graphite. In-depth micro-structural characterizations of the fractured composite, supported by the molecular dynamics simulations reveal details on the impact-induced phase transformation and the creation of diamond-graphite interfaces, suggesting that the energy absorption is primarily enabled via phase change of diamond. Our findings provide a pathway to stabilize the diamond phase and a new understanding of phase transformation of diamond under extreme conditions.
