Sintering of Silicon Nitride Ceramics with Additive Mixtures Based on Yttria, Aluminium Nitride and Alumina

摘要

The objective of this work was to compare the sinterability and the mechanical properties of Sl_3N_4 ceramics with 14 vol./100 of additive mixtures based on yttrium oxide, Y_2O_3, and a mixed concentrate of yttrium and rare earth oxides, CTR_2O_3. The mixtures prepared were Y_2O_3/AlN (YAN), CTR_2O_3/AlN (CAN), Y_2O_3/SiO_2 (YS), CTR_2O_3/SiO_2 (CS), Y_2O_3/Al_2O_3 (YAl and CTR_2O_3/Al_2O_3 (CAl). The Si_3N_4 doped with the systems YS, CS, YAl and CAl were sintered in a first step at l800deg.C under 0.5 MPa of N_2 for 30 minutes and in a second step at l900deg.C under l0 MPa of N_2 during 30 minutes. The samples YA and CA were sintered at l900deg.C under 2 MPa of N_2 fOr 30 minutes and in a second step at l950deg.C and at l0 MPa of N_2 during 30 minutes. The samples were characterized by their relative densities, phase analysis, microstructure and mechanical properties. The relative densities obtained were the following: YS 97.07/100, CS 97.43/100, YAl 98.57/100, CAl 99.57/100, YA 93.30/100 and CA 93.50/100. The inferior densities of the materials YA and CA are due to the formation of the SiAION phase, thus reducing the amount of liquid during sintering. The X-ray diffiactograms presented the β- Si_3N_4 phase for all sample compositions studied, besides the α-SiAlON, Y_2O_3 and Y_2Si_3N_4O_3 phases for the YA and CA samples. In the other samples, no other phases besides the β-Si_3N_4 have been detected, indicating that the intergranular phase remained amorphous. The microstructures observed by SEM revealed fine grained and elongated β-Si_3N_4 grains with high aspect ratio in the samples with SiO_2 and Al_2O_3 addition. In the case of AlN addition, more equiaxed grains are observed, which are attributed to be the SiAlON phase. The best mechanical properties in terms of strength and fracture toughness were obtained by the samples YAl, 723 MPa and 8.39 MPam~1/2, respectively. The samples doped with AlN, YAN and CAN, exhibited superior hardness of l8l9 HV2 and l882 HV2, respectively.