Influence of Y2O3 sintering aids on performance of c-BN-reinforced AlN/BN composite ceramics

Abstract

AlN/h-BN composite ceramics exhibit considerable anisotropy, which significantly constrains their application in areas such as semiconductor packaging and light-emitting diodes. The introduction of c-BN can significantly enhance mechanical properties and effectively reduce the anisotropy. However, this results in lower thermal conductivity, which is important issue that needs to be addressed. In this study, hot-press sintering was employed to fabricate AlN-10 wt% h-BN-15 wt% c-BN-x wt.% Y2O3 (AB15CxY, with x = 0, 1, 2, 3, 4) composite ceramics. Effects of Y2O3 content on microstructure, anisotropy, thermal conductivity, and flexural strength of AlN/BN composite ceramics were systematically investigated. Results indicate that during high-temperature sintering, c-BN undergoes phase transformation into "onion-like'' h-BN, and Y2O3 reacts with Al2O3 film on the surface of AlN powder to form Y3Al5O12 (YAG) phase. YAG phase promotes sintering densification as well as directional arrangement of lamellar h-BN, endowing composite ceramics with relative density of 99.6 % (sample AB15C1Y) with increased degree of anisotropy (the index of orientation preference is |IOP| = 68–93). Thermal conductivity of AlN/BN composite ceramics is directly correlated with lattice oxygen content, and as Y2O3 content increases, lattice oxygen content decreases monotonically. Lattice oxygen content of sample AB15C3Y is as low as 0.35 wt%, and the maximum values of K∥ and K⊥ are 100 and 59 W‧m−1‧K−1, respectively, which represent increases of 64.8 % and 51.3 % compared with those of AB15C0Y ceramic, respectively. With the increase in Y2O3 content, AlN grains become gradually larger, and the proportion of YAG phase, which is characterized by brittle grain boundaries, increases, leading to gradual reduction in the flexural strength of AlN/BN composite ceramics.