Effects of Testing Temperatures and Thermal Treatments on Flexural Strength of Si₃N₄‐MoSi₂‐BN Composites

Abstract

bridge S360) and energy dispersive spectroscopy (EDS, INCA Energy 300, Oxford Instruments, UK) on polished surfaces. For this purpose, the specimens were polished with diamond paste down to 0.25 lm.Vickers microhardness (HV1.0) was measured with a load of 9.81 N, using a Zwick 3212 tester. Young's modulus (E) was measured by the resonance frequency method on 28 × 8 × 0.8 mm 3 specimens using a Hewlett Packard gainphase analyzer. Fracture toughness (J Ic ) was evaluated using the chevronnotched beam (CNB) in flexure. The test bars, 25 × 2 × 2.5 mm 3 (length × width × thickness, respectively), were notched with a 0.08 mm diamond saw; the chevron-notch tip depth and average side length were about 0.12 and 0.80 of the bar thickness, respectively. The flexural tests were performed on a semiarticulated silicon carbide four-point jig with a lower span of 20 mm and an upper span of 10 mm on a universal screw-type testing machine (Instron mod. 6025). The specimens were deformed with a crosshead speed of 0.05 mm/min. The "slice model" equation of Munz et al. [25] was used to calculate J Ic . On the same machine and with the same flexural jig, the flexural strength (r), up to 1300°C in air, was measured on chamfered bars 25 × 2.5 × 2 mm 3 (length × width × thickness, respectively), using a crosshead speed of 0.5 mm/min. For the high-temperature tests, a soaking time of 18 min was set to reach thermal equilibrium. Five specimens were used for each temperature point.The electrical resistivity measurements were carried out by a four-probe DC method at room temperature, inducing a current in bar specimens of 2 × 2.5 × 25 mm 3 . The current and the voltage reading were detected at the same time in two different digital high-resolution multimeters. The resistivity values were determined from the electrical resistance measurement, taking into account the distance between the test leads and the cross-sectional area of the samples.[Verborgen]Si 3 N 4 -based ceramics are among the most important materials for structural applications due to a unique combination of properties such as strength, hardness and thermal stability. However after sintering, these materials are very difficult to be machined in complex shapes because of their brittleness and high hardness. Hexagonal BN (h-BN) can be used to improve machinability by the self-lubricating effects due to its plate-like structure. [1][2][3][4] To obtain high density Si 3 N 4 -BN composites, pressure-assisted techniques are COMMUNICATIONS ADVANCED ENGINEERING MATERIALS 2006, 8, No. 10