NGK Spark Plug Co., Ltd. has been developing various silicon nitride materials, and the technology for fabricating components for ceramic gas turbines (CGT) using theses materials. We are supplying silicon nitride material components for the project to develop 300 kW class CGT for co-generation in Japan. EC-152 was developed for components that require high strength at high temperature, such as turbine blades and turbine nozzles. In order to adapt the increasing of the turbine inlet temperature (TIT) up to 1350 °C in accordance with the project goals, we developed two silicon nitride materials with further improved properties: ST-1 and ST-2. ST-1 has a higher strength than EC-152 and is suitable for first stage turbine blades and power turbine blades. ST-2 has higher oxidation resistance than EC-152 and is suitable for power turbine nozzles. On applying these silicon nitride ceramics to CGT engine, we evaluated various properties of silicon nitride materials considering the environment in CGT engine. Particle impact testing is one of those evaluations. Materials used in CGT engine are exposed in high speed gas flow, and impact damage of these materials is considered to be a concern. We tested ST-1 in the particle impact test. In this test, we observed fracture modes, and estimated the critical impact velocity. This paper summarizes the development of silicon nitride components, and the result of evaluations of these silicon nitride materials.

A new high quality turbine system using monolithic silicon-nitride ceramic is under development. In this study particle impact tests of the silicon-nitride have been tried at room and elevated temperatures with and without tensile load, which simulates centrifugal force of blade rotation. In the experiment 1 mm diameter particle is impacted at velocities up to 900 m s−1. In this paper, critical velocities for bending fracture and Hertzian cracks are examined. Moreover, strength degradation at elevated temperature and spall fracture of the blade are discussed. The main results are: 1) The bending fracture mode critical impact velocity for soft particles is higher than that for hard particles. 2)The impact parameter ϕ for initiation of Hertzian cracks ranges 1.08×10−5 – 1.56×10−5 for the materials tested. 3)Strength degradation at elevated temperature was clearly observed. 4) In the impact tests on blades spall fracture, which was caused by interaction of stress waves, appeared.

No abstract

In developing ceramic gas turbines (CGT), evaluation of impact strength of ceramic materials is one of the most important subjects. For the purpose of evaluating impact strength of ceramics, particle impact tests of ceramics are carried out at room temperature by using electrothermal (ET) gun and micro-sabot system. Power turbine nozzles and circular disk specimens made of three kinds of Si3N4 are impacted by samarium - cobalt (Sm - Co) magnet flyer particles in vacuum. Weight of particles is fixed to 4mg and particle velocities range from 300 to 900 m/sec. Thickness of specimens are 1.0, 1.5 and 2.0 mm. In this study critical impact velocity, Vc, is defined as threshold velocity which causes visible cracks on the rear surface of the impact side. The cracks seem to be brought by flexural fracture mode. It is found that Vc is related with bending strength, σ, and thickness, t, of the target material as : Vc = C · σ5/6 · t5/3. It is found that impact results with and without rear cracks are well categorized based on this empirical relationship.

No abstract