Effect of Ti₃AlC₂ content on properties of polycrystalline diamond

Objectives: In superhard materials, polycrystalline diamond (PCD) has become a hot topic in scientific research because it inherits the advantages such as high hardness, high wear resistance, and high thermal conductivity from diamond. The type and content of the binder have a great influence on the properties of polycrystalline diamond. Methods: The microstructure of polycrystalline diamond samples is analyzed by FEI INSPECT F50 scanning electron microscope (SEM) of FEI Company in the United States, and the bonding state between diamond and binder is observed. The distribution of each element in the polycrystalline diamond sintered body is tested and analyzed by an energy dispersive spectrometer attached to the scanning electron microscope. An A8 ADVANCE X-ray diffractometer (XRD, λ = 0.154 06 nm, Germany, scanning speed: 10°/ min, scanning range: 20°～90°) is used to analyze the phase of polycrystalline diamond samples and raw materials to determine their phase composition. The sample density is measured by Archimedes' principle. The Vickers hardness of the sample is measured using a FM-ARS900 microhardness tester (load: 9.81 N, holding time: 10s). The fracture toughness is calculated using the fracture toughness formula according to the crack length, the hardness, and the elastic modulus of the material. Results: It can be concluded from XRD analysis that PCD is mainly composed of diamond, TiC, and Al₄C₃, and that the Ti₃AlC₂ phase is not detected in any sample. Therefore, Ti₃AlC₂ is considered to have been completely decomposed. The Ti-C bond in Ti₃AlC₂ is a strong covalent bond, while the Ti-Al bond is a weak metallic bond. In addition, Ti₃AlC₂ is a metastable phase, while TiC is stable at high pressure and high temperature. Therefore, Ti₃AlC₂ is decomposed into TiC and Al-Ti alloy, and Al-Ti reacts with diamond to form TiC and Al₄C₃. The existence of TiC and Al₄C₃ diffraction peaks also confirms this finding. From the SEM analysis, it can be concluded that when the binder content is 10% and 15%, there are a small number of holes and cracks on the surface of PCD; when the binder content is 20%, the diamond is tightly wrapped by the binder, and there are no obvious holes between the particles. The crystal form is complete without any fragmentation, and the diamond is arranged closely and distributed evenly, with better combination and compactness, indicating that the synthesis process of the PCD sintered body is well controlled. When the binder content is 25% and 30%, the excess binder appears to aggregate and cause dispersed diamonds and more holes. Moreover, no Ti₃AlC₂ grains with a layered structure were found under these conditions, which was consistent with the results of XRD analysis. Under high temperature and high pressure, the increase in Ti₃AlC₂ content will decompose more Al-Ti alloy into the liquid phase, which enhances the flow and uniform distribution of diamond and the generated hard phases in the system. At the same time, the product of Ti₃AlC₂ decomposition reacts with diamond under high temperature and high pressure to form TiC and Al₄C₃ with strong covalent bonds, which improves the bonding state between diamond particles, thus improving the comprehensive mechanical properties of PCD. The relative density shows a trend of increasing first and then decreasing. When the amount of binder is too much, the bonding performance between diamond and binder becomes worse, and the wear ratio decreases. In the electron microscope image, it is observed that the aggregation and porosity of the binder appear in the PCD composites, which explains the deterioration of mechanical properties. With the increase of Ti₃AlC₂ content, the Vickers hardness and fracture toughness of PCD increase first and then decrease. When the mass fraction of Ti₃AlC₂ is 20%, the hardness of PCD reaches the maximum value of 54.0 GPa. The fracture toughness of PCD reaches the maximum value of 5.23 MPa·m^1/2 when the mass fraction of Ti₃AlC₂ is 25%. When the mass fraction of Ti₃AlC₂ is more than 20%, the high content of binder in the sintered body will lead to a decrease in the relative density of PCD, resulting in a decrease in the Vickers hardness of the sample. Conclusions: Polycrystalline diamond is prepared at 5.5 GPa, 1500 °C, and 6 minutes by using diamond with a particle size of 5 μm as raw material and Ti₃AlC₂ as the binder. The effect of Ti₃AlC₂ content on the structure and properties of PCD is studied. Ti₃AlC₂ completely decomposes and reacts with diamond under high temperature and high pressure. Diamond, TiC, and Al₄C₃ are the main components in the sintered body. The binder reacts with diamond to form TiC and Al₄C₃. Through the combination of TiC and Al₄C₃ as intermediates, the diamond particles are firmly bonded together to form a dense microstructure. An appropriate amount of binder makes the diamond and the bonding material evenly distributed, and the PCD sintered body becomes denser. When the mass fraction of Ti₃AlC₂ is 20%, the relative density, Vickers hardness, and wear ratio of PCD reach maximum values of 99.3%, 54.0 GPa and 5 733.3, respectively. When the mass fraction of Ti₃AlC₂ is 25%, the fracture toughness of PCD reaches the maximum value of 5.23 MPa·m^1/2. When the Ti₃AlC₂ content reaches 25% and 30%, the density, hardness, and wear ratio of the PCD samples decrease due to the excessive aggregation and pores of the binder and diamond in the sintered body.