On December 3, 2021, the research group of Professor Chen Yuan from the School of Mechanical and Electrical and Information Engineering of Shandong University published a paper entitled "nano-ceramicreplacing cobalt in cemented carbide as binder phase: is itfeasible?" in the Journal of Alloys and CompoundsThe study of the toughening mechanism of nano-oxide ceramics.
Original link:
About the study
WC-6Al2O3, WC-6ZRO2 and WC-6MGO nanocomposite cemented carbides were sintered at 1700 and 40MPa pressures and were nearly fully densified. XRD consists of the main WC and ceramic-bonded phase peaks, while no other phases are detected, indicating that the ceramic-bonded phases (Al2O3, Zro2, MGO) have good thermodynamic compatibility with WC. SEM shows that the nano-ceramic binders are uniformly distributed between the WC grain matrix, and this uniformly distributed ceramic bond is crucial to improve the mechanical properties of cemented carbide materials.
After sintering, the WC grains retain the initial grain size, and the second phase significantly inhibits the grain growth of the WC matrix by limiting the grain boundary mobility. Dislocations were observed in all three ceramic-bonded cemented carbide materials, and dislocations improved the fault tolerance of cemented carbide. It was found that some nano-Zro2 grains were distributed along the WC grain boundary, while more ZRO2 nano-grains were distributed inside the WC grains, forming the so-called intragranular nanostructure. Compared to the ceramic binding phase at the WC grain boundary, the ceramic inside the WC grain is much smaller.
In the process of high-temperature sintering cooling, due to the difference in thermal expansion coefficient, residual tensile stress occurs around the ceramic binder phase, which is conducive to crack deflection when the crack reaches the stress field. When the external load acts on the nano-ceramic bonded material, the difference in elastic modulus will cause the redistribution of the microstress, thereby improving the toughness of the material. The crack bridging phenomenon of the three types of ceramic-bonded cemented carbide is exhibited, which effectively reduces the crack propagation energy. Crack non-bifurcation is also found in cemented carbide, which greatly increases the energy consumption of main crack propagation and effectively delays crack propagation.
During the fracture process of WC-ZRO2 cemented carbide, when the external stress acts on the cemented carbide, the stress concentration near the crack tip is generated, which promotes the transformation of T-ZRO2 to monoclinic M-ZRO2. This transition significantly hinders crack propagation by enhancing stress relaxation near the crack tip. In addition, the volume expansion caused by the phase transformation compresses the surrounding matrix, which is conducive to crack closure. In addition, the phase transformation of the surface of the material will cause the generation of surface compressive stress, which greatly improves the toughness of the material.
Figure 1: TEM microscopy of ceramic-bonded cemented carbide** (A) dislocations in WC-6Al2O3 (B) dislocations in WC-6ZRO2 (C) dislocations in WC-6ZRO2 (D) Intragranular and intergranular structures of WC-6ZRO2.
Figure 2: Toughening mechanism of WC-6Al2O3.
Figure 3: Toughening mechanism of WC-6ZRO2.
Figure 4: Toughening mechanism of WC-6AMGO.
Figure 5: XRD spectra of the polished and fractured surfaces of the WC-6ZRO2 specimen.
Conclusions of the study
In summary, WC-6Al2O3, WC-6ZRO2 and WC-6MGO nanocomposite cemented carbides were developed by hot-pressing sintering technology, and compared with unbonded cemented carbides and traditional WC-CO cemented carbides, the hardness and fracture toughness of nano-ceramic cemented cemented carbides were enhanced at the same time compared with micro-ceramic cemented cemented carbides. This excellent hardness of nano-ceramic-bonded cemented cemented carbide is essential for high-speed machining applications and is expected to be a candidate material for high-speed machining tools.