Pressureless sintering of silicon nitride/boron nitride laminate composites

Silicon nitride (Si3N4) and boron nitride (BN) are materials commonly used in the production of layered ceramics [1–3] and their derivatives [4]. In composites made from these two materials, the Si3N4 provides strength and the BN deflects cracks that intersect its basal planes at right angles [5]. The process of crack deflection greatly increases the length of the crack path through the sample, resulting in greater energy absorption as compared to the relatively straight crack path often observed in a monolithic ceramic. Composites made from these materials are densified by hot-pressing because of the reluctance of BN to sinter [2, 4]; For example, h-BN similar to that used in this study was hot-pressed for 1.5 hrs at 1740 ◦C under 25 MPa with no apparent sintering observed between BN particles [6]. It is the strong covalent bonds on the basal planes that make for an unreactive, non-wetting compound which severely hampers the densification of BN. Previous research on Si3N4/BN composites has shown that during the hot-pressing step, the liquid phase formed in the Si3N4 cells is forced into the BN cell boundaries [5]. This glassy phase, which has a composition that is a function of the sintering aids used and the silica on the surface of the silicon nitride, acts as a “glue” to hold the BN platelets together. It is proposed that if the sintering aids are added in the appropriate proportion directly to the BN powder [7], as well as to the Si3N4 [8–15], laminate composites may be densified by pressureless sintering. α-silicon nitride (UBE Industries E-10, ∼0.5 μm diameter) was ball milled with 4 wt% Al2O3 (Alcoa A16SG, ∼0.4 μm in diameter) and 4 wt% Y2O3 (Alfa Aesar REacton, ∼10 μm in diameter) to create the silicon nitride layers (presently identified as Si3N4-4A4Y). From prior work [16], the Si3N4-4A4Y composition investigated for the Si3N4 was determined to sinter to 92.3% and be composed of entirely β-Si3N4. The BN layer was made with h-BN (Advanced Ceramics HCP) to which 34.3 wt% each of alumina and yttria powders, 17.1 wt% α-Si3N4 and 2 wt% SiO2 (Alfa Aesar REacton, <45 μm) were also added. Prior work showed this composition to sinter to 66% of its theoretical density [16]. This composition of BN powders will be referred to as 88BN in the rest of the text. The powders that were used to form the Si3N4-4A4Y and 88BN layers were ball-milled in methanol for 1 hr using zirconia milling media prior to assembling the layered ceramic. The compact was fabricated by adding alternating layers of 2 g of Si3N4-4A4Y powder, followed by a 0.9 g layer of 88BN powder, to a steel die with inner dimensions of 76 mm × 38 mm × 6 mm. Note that a total of 15 layers were stacked, with the top and bottom layers composed of Si3N4-4A4Y. In addition, the 88BN powder intentionally did not completely cover the 76 mm × 38 mm area; rather, the perimeter was kept free of the 88BN powder so that adjacent layers of Si3N4-4A4Y would touch. The thickness of the perimeter was ∼3 mm. It is thought that sintering of the Si3N4-4A4Y powder would provide an additional compaction force to the 88BN layers. After all the layers were added, the composite was pressed in a laboratory press at ∼7 MPa. The compact was removed from the die and packed in a powder bed consisting of 60 wt% α-Si3N4 and 40 wt% BN, and sintered in a BN crucible at 1800 ◦C for 3 hrs in flowing nitrogen gas in a Centorr Testorr furnace. In addition to the layered ceramic, individual compacts of Si3N4-4A4Y powder and the 88BN powder were pressed and sintered similarly to the layered ceramic. Table I presents the properties of these monolithic ceramics [17]; these properties were used to predict the expected mechanical properties of the layered ceramic. The average density of three 15-layered test specimens was ∼2.1 g/cm3, approximately 66% of its theoretical density of ∼3.4 g/cm3. The theoretical density (TD) was calculated based on the composite having ∼65 vol% Si3N4-4A4Y (TD ∼3.26 g/cm3) and ∼35 vol% 88BN (TD ∼3.66 g/cm3), as determined from point-count analysis. X-ray diffraction of the faces parallel to the pressing direction, using Cu Kα radiation, revealed only β-Si3N4 and h-BN peaks. Standard 4 pt. bend mechanical test specimens [18], 3 mm × 4 mm × 50 mm, were machined from the Si3N4-4A4Y/88BN layered composite. The as-sintered surface was not polished prior to testing due to minor surface curvature. A fully articulated SiC 4 pt-bend flexural fixture with inner and outer spans of 20 and 40 mm, respectively, was used to fracture the speci-