The Mechanical Properties Of Tungsten Borides And Nitrides Prepared At High Pressure And High Temperature

Superhard materials?Hv?40 GPa?were widely used in industry and military field due to their excellent physicochemical properties,such as wear resistance,high temperature resistant and chemical stability.Diamond and cubic boron nitride are two well known traditional superhard materials.However,they can't meet the demands of industrial production because diamond is unstable under high temperature and disable to cut iron and cubic boron is hard to be synthesized.To design and synthesize multi-functional superhard material or multi-functional hard material is an urgent assignment imposed on material scientists.The property of hardness is related to elastic and plastic deformation under indention test.So,materials with high hardness always have high ability to resist elastic and plastic deformation.In transition-metal borides?TMBs?and transition-metal nitrides?TMNs?,transition metals?TM?usually possess high bulk modulus because of greater repulsive forces within the crystal originating from higher concentration of valence electrons.Light elements,boron?B?and nitrogen?N?,can form strong covalent bonds which limit the motion of dislocations and thereby increase the shear modulus.So,TMBs and TMNs are two kinds of potential superhard materials.However,none of them demonstrate an asymptotic hardness larger than 40 GPa which is generally accepted as the threshold of superhard materials.Much of TMBs have layered structure with TM layers and B layers stacking alternately.The B layers would form strong covalent bonds.So,the complex chemical bonds of TM-B are very important to resist the layer slip caused by shear stress.The TM-B bond was predicted as strong covalent bond which plays an important role in forming superhard materials.However,none of the as-synthesized transition-metal light element shows the properties of superhard.Therefore,re-exploring the bond type of these transition-metal light element compounds is of great significance to understand the mechanism of hardness.To fabricate high nitrogen phase TMNs is hard in comparison with fabricating transition metal boride.It is very difficult to prepare the stoichiometric TMNs compounds primarily because the crystal lattice is thermodynamically unfavorable during the incorporation of nitrogen into tungsten at atmosphere pressure.The obtained products always present in the form of thin films or powder and posses a poor crystalline meanwhile due to the decomposition problems are often encountered in these methods.Up to now,there are few reports about the hardness mechanism of TMNs,and most of them are using theoretical simulation.The hardness values predicted by theoretical calculation can't be confirmed because it is difficult to synthesize high quality TMNs.Thus,to find a new method to prepare the bulk TMNs with stoichiometric is helpful to explore the mechanical properties for widely application.To study the hardness mechanism,WB₂?c-WN and ?–WN compounds were selected in this work.We explored the fabrication and mechanical properties of three compounds using both experimental and theory calculation method.The obtained results are as follow:1.We successfully synthesized high quality bulk WB₂ and measured its hardness.The asymptotic hardness of WB₂ is 25.6 GPa which clarifies that WB₂ is not a superhard material once more.2.Large electrons drifting from tungsten atoms toward the boron atoms were observed from the analysis of XPS.The large electrons drifting give rise to an ionic contribution to the bonding between W-B.We further conformed that the character of W-B bond should be more ionic based on the results obtained from Electron Localization Function?ELF?,density of states?DOS?,topological analysis of the static electron density and Mulliken population.WB₂ has layered structure with W layers and B layers stacking alternately.In B layers,the chemical bond between two boron atoms would form strong covalent bonds.So,the complex chemical bonds of TM-B are very important to resist the layer slip caused by shear stress.So,we proposed that the lack of W-B covalent bonds is the reason for the low hardness of WB₂.To enhance the strength of W-B bond is the key factor to improve the hardness of TMBs and TMNs.3.c-WN has been successfully synthesized at high pressure and high temperature via the decomposition of W3N4.The effects of pressure and temperature on the synthesis product were discussed in detail.It is concluded that high temperature can promote decomposing and high pressure played an important role in suppressing the as-synthesized samples further decomposing and restricting the movement of atoms.So,nitrogen-rich nitrides are preferred to be stable at a relatively low temperature.The Vickers hardness value of c-WN is 29 GPa under an applied load of 0.49 N.This value is so far the hardest Transition-metal nitrides with the sodium chloride structure.The compressible ability of c-WN is determined by in situ XRD at high pressure in a diamond anvil cell.The results suggested that c-WN was stable up to 29.6 GPa.This result suggests that c-WN is a mechanical stable phase material but not a mechanical unstable phase material which is predicted by theory calculation.In TMNs,most of TMNs always crystallize into sodium chloride structures which were considered unfavorable to resist shear strain,because the TM–N bonds in cubic nitrides are linearly distributed without involving three–dimensional?3D?network.So,TMNs with sodium chloride structures always exhibit relatively low indentation hardness,typically in the range of 10–20 GPa even under a low load.It is believed that the hardness is always related to the density and the distribution mode of chemical bond.In this work,we demonstrated that the hardness is not only determined by the conventional microstructural features but also determined by the nature of the bonding.4.We provided an effective route for synthesizing high-quality bulk TMNs and we first synthesized bulk phase pure ?–WN using this method.Lattice constants of ?–WN are a= 2.90114?3?? and c = 2.82709?3?? obtained from Rietveld refinements.We first obtained the hardness of ?–WN.The hardness of ?–WN is 13.8 GPa.W-N ionic bonds are mainly chemical bond in ?–WN based on the analysis of ELF,DOS and Mulliken population.The absence of covalent bonding between tungsten atoms and nitrogen atoms makes the structure,not 3D framework,easy to deform under shear stress and leads to the low hardness of ?–WN.