Preparation and Properties of Silicon Nitride Ceramics by Nitrided Pressureless Sintering (NPS) Process

Abstract

Silicon nitride ceramics were prepared by new nitrided pressureless sintering (NPS) process in this study. The microstructures, strengths and thermal properties of the NPS silicon nitride ceramics containing three types of Al2O3 and Y2O3 sintering additives were investigated. Additionally, we have investigated the effect of silicon metal contents changing with 0, 5, 10, 15 and 20 wt% in each composition. The silicon nitride was successfully densified using NPS process, particularly at the starting composition of 5 wt.% Al2O3, 5 wt.% Y2O3, and 5 wt.% Si addition. The maximum flexural strengths and relative densities of these specimens were 500 MPa and 98%, respectively. The flexural strength of sintered specimens after the thermal shock test between 30oC and 1300oC for 20,000 cycles was maintained with the original laboratory strength of 500MPa by low thermal expansion coefficient, 2.9 × 10-6/oC, and high thermal conductivity, 28 W/m⋅oC.

Properties of silicon nitride for aluminum melts prepared by nitrided pressureless sintering

“…However, it is noteworthy that the strength degradation is not significant relative to the inert laboratory strength of the NPS Si 3 N 4 before corrosion tests. As degassing pipe and rotor component is operated at high temperature, [8][9][10][11] hot corrosion study is currently investigated.…”

“…Degassing pipe and rotor are used to stir molten aluminum and bubble gases for removal of hydrogen, because detrimental impurities remain such as H 2 , Na or Ca component in the aluminum. 9,10 Degassing is conducted at high temperature, >700 • C in a refractory-lined box that contains 226.8-453.6 kg of aluminum at a time. The degassing pipe has an impeller at the end and it is rotated (200-600 rpm).…”

“…Therefore Si 3 N 4 degassing pipe and rotor blade are used in aluminum melts due to these superior thermal, chemical and mechanical properties. [9][10][11] However, one of the drawbacks is high cost in the preparation of Si 3 N 4 components. It is generally accepted that high quality of structural Si 3 N 4 ceramics can only be produced from submicron raw powders, under 1 m, with high purity (>99.9%).…”

“…12 To improve the mechanical properties of RBSN, NPS is replaceable new process because the mechanical properties of NPS silicon nitride can be much improved by post sintering at 1800-1820 • C. Moreover, the lower sintering temperature and almost no dimensional change in the 1st stage of sintering step have economical advantage. Even though several studies have been performed on the NPS Si 3 N 4 , [9][10][11] there is no systematic study on the properties of NPS Si 3 N 4 .…”

“…Therefore, in this study, we utilized NPS (nitrided pressureless sintering) one-step process, [9][10][11] which can make cost-effective Si 3 N 4 product using reaction-bonding process up to certain temperature, and obtain the dense sample with high mechanical properties by consequent pressureless sintering process at the final stage of heat treatment. In the present study, we investigate the mechanical properties such as four-point flexural strength, hardness, and fracture toughness.…”