Study On Synthesis And Properties Of Ternary Nitride Ti₂AlN

A family of layered ternary compounds, as an important series of ceramics, hasgripped the physicists and material scientists' attention. These ternary compoundswith the general formula M_n+1AX_n (n=1 to 3), where M is an early transition metal, Ais an A-group (mostlyâ…¢A andâ…£A)element, and X is either C and/or N, arestructurally related. In these compounds, near-close-packed layersof transition-metalcarbide and/or nitride layers are interleaved with layers of a pure A-group element. Inthe M₂AX phases, every third layer is an A-group element layer. Ti₂AlN and Ti₂AlCare the representative substance. Ti₂AlN crystallizes as a hexagonal structure with aspace group of P6₃/mmc. Similarly to the other layered ternaries, Ti₂AlN combinesunusual properties of both metals and ceramics. Like metals, it is a good thermal andelectrical conductor, and has a low Vicker hardness, high Young's modulus and shearmodulus, and it is machinable. Like ceramics, it is elastically stiff; exhibits excellenthigh temperature mechanical properties. It is resistant to thermal shock and unusuallydamage tolerant, and exhibits excellent corrosion resistance. As far as I know pure bulkTi₂AlN is very hard to be fabricated in most case, for that these are many compoundsin the Ti-Al-N system such as Ti₂AlN, Ti₃Al₂N₂, Ti₃AlN₂ and Ti₄AlN₃ ect. Until now,Ti₂AlN, all reported, which is also less than Ti₂AlC, is synthesized by HIP(hightemperature insostatic pressing), which spends more time and money than other MAXmaterials. So we will find a new cheaper path to fabricate high purity, bulk, denseTi₂AlN, and research lots of properties of Ti₂AlNThe high purity bulk Ti₂AlN has been sintered by hot pressing with the startmolar ratio n(TiN):n(Ti):n(Al)=1.0:1.0:1.0 at 1300 with a pressure of 30 MPa inargon atmosphere. And denser Ti₂AlN at same ratio has been sintered by SPS.Results indicated that the reaction of Ti-Al-TiN system could be divided into twostages. Before 900℃, Ti reacts with Al to form TiAl intermetallics; above 900℃,TiAl reacts with TiN to produce dense Ti₂AlN composites. In the holding stage ofTi-Al-TiN system ternary Ti₂AlN develops to layered polycrystal and compositespyknosis at the meanwhile.Physical properties of Ti₂AlN with the crystal dimension of 10～15μm have been also carefully investigated. The measured flexural strength, fracture toughness andVickers hardness were 300MPa, 4.5MPa.m^1/2 and 4.0GPa. The high toughness andlow hardness make in machinable. Ti₂AlN is a good electrical and thermal conductor.The electrical conductivity at room temperature are 3.2×10⁶S·m^-1. The electricalresistivity increased linearly with the increasing of temperature, indicating thatTi₂AlN is a metallic conductor. The corrosion rate of Ti₂AlN in commom acidds anddilute NaOH is similar to that of Ti₃SiC₂.The oxidation behavior of Ti₃AlC₂ with the density 4.22g/cm³ was alsoinvestigated by means of TGA, XRD, SEM and EDS in the work. The isothermaloxidation behavior at 800～1000℃for 20h in air followed a parabolic rate law aswell as the cyclic oxidation behavior at 800～1000℃for 30 cycles. The defects inmatrix increased the kinetics of Ti₂AlN. The calculated activation energy was614.93kJ/mol. It had been demonstrated that Ti₂AlN had excellent oxidationresistance due to the dense continuous oxide scale consisted ofα-Al₂O₃ and TiO₂.Generally, the oxide scale was grown by the inward diffusion of O^2- and the outwarddiffusion of Ti⁴⁺ and Al³⁺. The rapid diffusion out of Ti⁴⁺ and Al³⁺, more often thannot, results in the formation of a "crack", gap, or void. Once formed the layer of porescan be, though not always, an effective barrier to further diffusion Ti⁴⁺ and/or Al³⁺ions outward, but does not hinder the continued inward diffusion of oxygen. Thecontinuous Al₂O₃ layer was attributed to the former oxidized behavior.