Formation And Properties Of ⅣB Transition Metal Nitride And Nanocomposite Materials

Nano-science and technologies have been developed in 1980s. As a newadvanced technology, nano-technology grows up rapidly today. It researchproper phenomena and function of nanomaterials with grain size at 0.1 100nm. Nanocomposite are usually classed as reinforced and functionnanocomposite. The reinforced nanocomposite are usually prepared bydropping particles or fibers with nanometer scale in matrix and then sinteringunder vacuum or by precipitating nanophase from matrix. The decrease ofgrain size will improve nanocomposite mechanical properties dramatically;meanwhile, large number of grain boundary obvious reduce the secondphase capacity which may distribute at grain boundary, so the negativeinfluence caused by boundary materials can be minimized;furthermore,small grain size is useful to improve fracture toughness of nanomaterials,grain boundary sliding is in favor of improving the plasticity of materials. So,the intensity, toughness and super plasticity of nanocomposite will greatlyimproved. The design and manufacture of advanced materials for applicationsat high stress and high temperature is one of the most challenging tasks ofmodern engineering. Refractory materials such as borides, nitrides,carbides , and silicides are natural candidates for these demandingapplications due to their exceptional hardness and stability at very hightemperatures.Transition metals of group ⅣB andⅤB boride (TiB2,ZrB2,HfB2) andnitride (TiN,AlN,BN,ZrN) ceramics have received increasing attention inrecent years because of their unique chemical and physical properties. Amongsuch materials, titanium nitride (TiN) is one of particular interest, due to itshigh hardness, good chemical and thermal stability, excellent corrosionresistance, and relatively high electrical conductivity. Another refractorymaterial diboride (TiB2) also possesses attractive properties of high meltingpoint, high hardness, good electrical conductivity, and excellent wear andcorrosion resistance, which is superior to that of TiC and TiN. In addition,TiB2 also demonstrates good ballistic performance. Therefore, the additionof TiB2 to form TiN-TiB2 composites has the potential of improving thehardness and toughness of TiN. Composites of TiN-TiB2 have been useful inthe applications such as cutting tools, electrodes, and wear resistancematerials.Different production routes generally prepare titanium nitride anddiboride. For example, TiN is often produced by the chemical or physicalvapor deposition (CVD or PVD) method, while TiB2 is usually sintered byhot isostatic pressing (HIP) or hot pressing (HP). Therefore, it is difficult toformulate composite featuring both ceramics by conventional approaches,combustion synthesis or self-propagating high-temperature synthesis (SHS)has been employed to produce a variety of advanced materials, includingborides, carbides, nitrides, silicides, carbonitrides, intermetallics,etc..In recent years, h-BN solid was used to prepare -TiNx or its composites.It was reported that -TiNx /TiB2 composite was prepared by hot pressurereaction between TiH2 and h-BN at high temperature of 1600 C andoself-propagating reaction between Ti and h-BN at high temperature of 1200oC.Theses facts indicate that formation of the -TiNx and TiB2 are related toexperimental conditions used. Laws and mechanisms of formation of -TiNxand TiB2 by solid reaction between Ti and BN is still problem needinvestigate.In the present experiment, evolution and products of reaction between Tiand BN are investigated by means of mechanical milling or isothermalannealing techniques, and mechanisms of formation of -TiNx and TiB2 arediscussed based on thermodynamics and kinetics.Nanocrystalline -TiNx was prepared by mechanical milling of the mixture ofTi and BN or annealing of the mixture milled for 70 h. The -TiNx is formedby diffuse reaction between Ti and a-BN and phase transition but notself-propagating reaction during milling process in the present experiment.Firstly Ti reacts with N in the a-BN to form amorphous Ti-N alloy, thencrystallizes into the -TiNx driven by local pressure and local temperatureinduced by collisions between balls or ball and vial.Formation of the -TiNx in annealing process is attributed to two processes.Below 600 C, a small amount of the -TiNx is formed by crystallization ofothe amorphous Ti-N in the 70h-milled mixture. When annealed above 600 C,oTi (N) solid solutions with N content higher than solubility limit decomposeinto Ti (N) solid solution with the solubility limit and the -TiNx. Most of the-TiNx is formed by decomposition of the Ti (N) in the annealing process. NoTiB2 was observed to form in both milling and annealing processes. That isdue to that heat of formation of TiN is more negative than that of TiB2 inthermodynamics and solubility limit is zero for B in Ti but is not for N in Ti inkinetics.Defect-induced Raman spectra were observed in the 150 -350 and450–700 cml regions and assigned to be correlated with powder composition.1Three Raman peaks, attributed to transverse acoustic (TA), longitudinalacoustic (LA) , and transverse optical (TO) modes of the m-TiNxrespectively, were found to shift gradually to lower frequency withincreasing milling time, and reach a minimum at the milling time of 150 h,and then shift gradually to high frequency. The change of lattice constant thatcaused by the N/Ti ratio is the main reason for the evolution in the Ramanspectra of -TiNx nanocrystalline with increasing milling time. The internalstress and grain size play a minor role in Raman spectra of-TiNxnanocrystalline in present study.Planetary milling and subsequent high-pressure high temperature treatmentfabricated TiN/TiB2 composite materials. The sample that milled for 70 hourswas transformed into TiN/TiB2 composite materials after subsequenthigh-pressure high temperature treatment (5GPa and 1300℃).Pressure has important influence on solid-state reaction. At normalpressure, annealing Ti and a-BN only forms -TiNx;while TiB2 are obtainedat 5 Gpa and 1300℃. This may attribute to high pressure promote B reactwith Ti,the activation energy between B and Ti reduced under high pressure.It was found that increase of pressure and temperature may reduce holes incomposite and increase density of materials, the grain size of materials touniformity. Increase the temperature and heat preservation may augment thegrain size.TiN/TiB2 nanocomposite have good mechanical, electrical and chemicalproperties. Its HV hardness is 7-8Gpa, electrical resistance coefficient is 10-4Ω?cm. the oxidation temperature of this composite is about 900 ℃.ZrN and ZrB2 are all belong to covalent crystals. Their diffuse coefficient athigh temperature are very low, it is very difficult to synthesis thesecomposites by conventional approaches. It is a promising method to synthesisZrN/ZrB2 composites by high pressure-high temperature technology ,furthermore,the raw materials of ZrN and ZrB2 are expensive,and it is alsoan economical route to synthesis ZrN/ZrB2 composites by high pressure-hightemperature technology.In this study, ZrN, ZrB2 and ZrN/ZrB2 composites were also fabricatedby mechanical milling, vacuum heat treatment and solid-state reaction underhigh-pressure high temperature. The formation mechanism and properties ofcomposites were researched.The -ZrNx is formed by diffuse reaction between Zr and a-BN. Firstly Zrreacts with N in the a-BN to form Zr(N) solid solution alloy, then Zr(N)decompose into -ZrNx driven by local pressure and local temperature inducedby collisions between balls or ball and vial. After milling Zr and BN (molratio 3:2) mixtures for 60 h, only found -ZrNx formed, no ZrB2 have beenformed during the whole milling process. Even annealing the mixtures at1050℃, 10-2pa for 2 h, there is still no ZrB2 formed. This is caused bylower formation enthalpy of ZrN compare with that of ZrB2, otherwise, Natoms are easier then B solute into Zr lattice to form Zr(N) solid solution atnormal pressure, so N atoms diffused and reacted with Zr prior to thereaction with B.After annealing amorphous BN and nanocrystalline Zr mixtures at highpressure (5Gpa) and high temperature (1200℃) for 15 min, not only ZrN,but also ZrB2 were all formed. This result shows that high pressure promotesB react with Zr atoms.