| Superhard material is widely used in machining, wear-resistant coating, abrasive, nationalindustry such as space vehicle, fission reactor, high technology, high storage material,high-temperature semiconductor, oil production and high precision machining et al. due to itshigh hardness, high strength, high heat conductivity and excellent resistance to wear et al. Overthe past decades, the first generation superhard material only concentrated on diamond inindustry application. However, the diamond is not effective for cutting ferrous metals or steelbecause a chemical reaction occurs that produces iron carbide decreasing, the hardness ofdiamond rapidly. Although the second generation superhard material (c-BN, C3N4and BC4) cancompensate the shortage, its preparation is very expensive. Therefore, it is very necessary tostudy and develop novel superhard material.In recent years, the transition metal borides (TMBs) have attracted attention as the potentialsuperhard material owe to their high bulk modulus, high shear modulus, high hardness, excellentultra-incompressibility, good thermal stability and a degree of metallic behavior et al. However,numerous papers reported that these TMBs are not superhard materials. Therefore, it is of greatinterest and significance to search for novel TM superhard material and to study the superhardmechanism.As mentioned above, in this dissertation, we will present first-principle approach andarc-melting prepared method to investigate the structural stability, elastic properties, hardness,electronic structure and fracture mechanism of Ru-based borides. We will try to search for thenew Ru-based superhard materials and study the relationship between hardness and bondcharacteristic and fracture mechanism et al. The main contents and results of this thesis aresummarized as follows:In Chapter1, we firstly give a brief introduction to the research history of superhardmaterial. We analyze and discuss the merit and shortage of the first generation superhard material and second generation superhard material. Following, experimental and theoreticalcalculation studies on the third generation superhard material are reviewed. Also, the progress inthe investigation on the Ru-based borides is summarized. Finally, the purpose main contents andsignificance of this work is given.In Chapter2, we firstly introduce the basis theory of the first-principle approach based onthe density functional theory. Then, we introduce the Boron rule, bulk modulus, shear modulus,Young′s modulus and Poisson ratio of cubic structure, orthorhombic structure and hexagonalstructure. In this thesis, we use the Voigt-Reuss-Hill approach for calculating elastic properties.Finally, we introduce the hard theoretical model of superhard materials in recent years.In Chapter3, for the orthorhombic RuB2structure as the basis structure, we investigate thestructural stability, elastic modulus, hardness and electronic structure of RuB2-x(0≤x≤2) usingthe first-principle approach. The calculated results show that the poor-boron region is morestable than that of rich-boron region. The Debye temperature is calculated to be in a sequence ofRuB2>Ru2B3>RuB, which is consistent with the structural stable of RuB2-x.The calculated bulk modulus, shear modulus and Young′s modulus of Ru2B3are349GPa,202GPa and508GPa, respectively, which are bigger than that of RuB2and RuB. The intrinsichardness of Ru2B3is about of49.2GPa. Therefore, we predict that Ru2B3is a potentialsuperhard material. Its high hardness originates from the bond characteristic and electronicstructure. Its atomic arrangement can be viewed as the alternatively stacked Ru and B layersalong the b-direction. Moreover, the B layer is composed of two sub-boundary B layers.Therefore, the load plane is composed of B-B and Ru-B covalent bonds. This is a main reasonwhy this Ru2B3has high elastic modulus and hardness. Finally, we find that this Ru-basedborides exhibit a degree of metallic behavior.In Chapter4, the structural stability, elastic modulus, hardness and electronic structure ofCrB4-type borides are investigated by first-principle approach. The calculated results show thatthe tetraborides have high bulk modulus, high shear modulus and hardness. In addition, thecalculated elastic constant C22is bigger than that of C11and C33, indicating that it has highresistance to deformation along the b-axis. Moreover, the calculated B/G ratio of CrB4, ReB4,RuB4and OsB4is smaller than1.75, and the intrinsic hardness of CrB4and ReB4is about69.6GPa and41.6GPa, which means that they are potential superhard materials. The high hardness is derived from that the B-B bond cage states are composed of12B atoms. This B-B bond cagecan improve the resistance to the deformation and enhance its hardness.In Chapter5, the single RuB1.1is prepared by using arc-melting method and its structure isstudied by X-ray diffraction and Rietveld refinement. The Vickers hardness is measured byVickers micro-hardness and the fracture feature is characterized by scanning electronmicroscopy (SEM). Its elastic modulus including bulk modulus, shear modulus and B/G ratio isinvestigated by first-principles. The refined lattice parameters are: a=b=2.860and c=2.846. The first-principles calculations show that the bulk modulus, shear modulus and Young′smodulus of RuB1.1is346GPa,203GPa and509GPa, respectively. The calculated B/G ratio ofRuB1.1is smaller than1.75, implying that it exhibits a brittle behavior.The calculated intrinsic hardness of RuB1.1is33.2GPa, which is three times bigger thanthe experimental result (10.6GPa). We suggest that the origin of low hardness is derived fromthe specific bonding state, bonding orientation and fracture mechanism. The first-principlescalculations show that there is an isosceles triangle bonding state including B-B covalent bondsand Ru-B covalent bonds, in which the Ru-B bonds are on two sides, while the B-B covalentbond is on the base. Note that the Ru-B covalent bond is along the a-c plane. Moreover, theorientation of Ru-B covalent bonds is just on the load plane. Under load stress, therefore, theRu-B covalent bond breakages along this direction. This result is also demonstrated by thefracture surface. The SEM images show that twinning fracture follows the typical fracturemechanism with a lamellar structure.In conclusion, the structural stability and mechanical properties of Ru-based borides aresystematically investigated by first-principles. The calculated results show that the Ru2B3, CrB4and ReB4are potential superhard materials. Furthermore, the relationship between hardness andbond characteristic and fracture mechanism for RuB1.1is studied in this dissertation. Theseinvestigations will be helpful for preparing Ru-based superhard materials. |
PDF Full Text Request