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Foreign Object Damage Behavior of Two Gas-Turbine Grade Silicon Nitrides by Steel Ball Projectiles at Ambient Temperature
| Content Provider | Semantic Scholar |
|---|---|
| Author | Sung Choi, Rino |
| Abstract | Foreign object damage (FOD) behavior of two commercial gas-turbine grade silicon nitrides, AS800 and SN282, was determined at ambient temperature through strength testing of flexure test specimens impacted by steel-ball projectiles with a diameter of 1.59 mm in a velocity range from 220 to 440 m/s. AS800 silicon nitride exhibited a greater FOD resistance than SN282, primarily due to its greater value of fracture toughness (Kic). Additionally, the FOD response of an equiaxed, fine-grained silicon nitride (NC132) was also investigated to provide further insight. The NC132 silicon nitride exhibited the lowest fracture toughness of the three materials tested, providing further evidence that Kic is a key material parameter affecting FOD resistance. The observed damage generated by projectile impact was typically in the forms of wellor illdeveloped ring or cone cracks with little presence of radial cracks. INTRODUCTION Ceramics, because of their brittle nature, are susceptible to localized surface damage/cracking when subjected to impact by foreign objects. It is also true that ceramic components may fail structurally even by soft particles when the kinetic energy of impacting objects exceeds certain limits. The latter case often has been found in aerospace engines in which combustion products, metallic particles or small foreign objects cause severe damage to blade/vane components, resulting in serious structural problems. Therefore, foreign object damage associated with particle impact needs to be considered when ceramic materials are designed for structural applications. In view of this importance, a considerable amount of work on impact damage of brittle materials by sharp particles as well as by "blunt" particles or by plates has been accumulated both experimentally and analytically [1 10]. The understanding of particle impact phenomena has been based on the concept of indentation fracture mechanics for sharp particle impact [2] and on Hertzian contact analysis for "blunt" or ball impact [1], leading to simplified quasi-static phenomenological models of strength degradation. This paper describes in detail the ambient-temperature, foreign object damage (FOD) behavior of two currently representative gas-turbine grade silicon nitrides, AS800 and SN282. Some of the results in this work were also reported previously [11]. Ceramic target specimens in the form of flexure bars were impacted at their centers with steel ballprojectiles with a diameter of 1.59 mm in a velocity range from 220 to 440 m/s. Post-impact strength was determined as a function of impact velocity to accurately evaluate the severity of impact damage. Fractography was performed before and after post-impact strength testing to determine impact morphologies and the nature of strength-controlling flaw configurations. A previously-proposed phenomenological model [1] was used to better understand the distinct difference in FOD behavior between the two silicon nitrides. To gain further insight, impact and post-impact strength testing was also conducted using an additional, equiaxed, fine-grained silicon nitride, NC 132. Data obtained using this third silicon nitride material was utilized to further pinpoint a key material parameter affecting the FOD resistance of structural silicon nitrides. *NASA Senior Resident Research Scientist at Glenn Research Center. NASA/TM--2002-211821 1 EXPERIMENTAL PROCEDURES Materials and Test Specimens Materials used in this work included two commercially available gas-pressure sintered silicon nitrides, AS800 (fabricated by Honeywell Engines, Phoenix, AZ, '99 vintage) and SN282 (fabricated by Kyocera, Vancouver, WA, '00 vintage). These two silicon nitrides are currently considered strong candidate materials for gas-turbine applications in view of their substantially improved elevated-temperature properties [12 14]. Both materials are toughened silicon nitrides, with microstructures tailored to achieve elongated grain structures. AS800 silicon nitride has been used at the NASA Glenn Research Center in life prediction programs [15,16]. The billets for each material were machined into flexure test specimens measuring 3 mm by 4 mm by 45 ram, respectively, in depth, width and length in accordance with a test standard ASTM C1161 (size "B') [17]. All AS800 test specimens were annealed prior to testing at 1200 °C in air for 2h to eliminate or minimize damage and/or residual stresses presumably associated with machining. All SN282 test specimens were annealed by the manufacturer prior to testing with proprietary annealing condition. A conventional, hot-pressed, equiaxed, fine-grained silicon nitride, NC132 (fabricated by Norton Advanced Ceramics, Northboro, MA, '90 vintage) [15], was also used for comparison. The dimensions and machining condition of flexure test specimens of NC132 silicon nitride were the same as those of AS800 and SN282. Test specimens were all annealed with the same annealing conditions applied to AS800. The basic mechanical and physical properties of the three silicon nitrides as well as of the steel-ball projectile material (SAE 52100 bearing steel) are shown in Table 1. Foreign Obiect Damage Testing Foreign object damage (FOD) testing was carried out at ambient temperature using the experimental apparatus shown in Figure 1. A detailed description of the apparatus can be found elsewhere [11]. Hardened (HRC_>60) chrome steel-balls with a diameter of 1.59 mm were inserted into a 300 ram-long gun barrel with an inner diameter of 1.59 ram. A He-gas cylinder and relief valves were used to pressurize the reservoir to a specific level depending on the prescribed impact velocity. Upon reaching a specific level of pressure, a solenoid valve was instantaneously opened accelerating a steel-ball projectile through the gun barrel to impact a target specimen that was firmly supported on a metallic specimen holder. Each target specimen was aligned such that the projectile impacted at the center (4 mm-wide side) of the test specimen with a normal incidence angle. For a given pressure, the velocity of each steel projectile was determined using two pairs of laser transmitter and receiver, in which the two transmitters were aimed at the respective receivers through two holes in the gun barrel. The distance between the two holes was 25 ram, with the front hole located about 70 mm away from the front end of the gun barrel. The time traveled by a projectile between the two holes was measured with a digital storage oscilloscope connected to the two pairs of laser transmitter and receiver. The velocity was then calculated based on the distance-time relationship. It was found that velocity increased with increasing pressure, rising sharply at lower pressure but moderately at higher pressure as shown in Figure 2. The range of impact velocity applied in this work was from 220 to 440 m/s. Typically 7 to 15 test specimens were impacted at each chosen velocity for a given material. In particular, to determine the statistical post-impact strength behavior of AS800 and SN282, a total of 15 to 17 test specimens were tested at an impact velocity of 300 m/s. Impact morphologies were selectively examined optically right after impact testing but prior to post-impact strength testing. NASA/TM--2002-211821 2 Table 1. Basic mechanical and physical properties of AS800, SN282 and NC132 silicon nitrides and steelball projectile material at ambient temperature Material Elastic modulus _ E (GPa) Poisson's ratio _ V Density e p (g/cm 3) 3.27 Hardness _ H (GPa) AS800 Si3N 4 309 0.27 13.6+1.4 SN282 Si3N 4 304 0.28 3.32 15.3+0.2 NC132 Si3N 4 [15] 315 0.30 3.20 14.0+0.2 Chrome steel (for ball 200* 0.30* 7.80* HRC>60* projectiles) (SAE 52100) Notes: 1. By the impulse excitation technique, ASTM C 1259 [18] 2. By mass/volume method 3. By Vickers microhardness indentation, ASTM C 1327 [19] * From the manufacturer's data; HRC Hardness in Rockwell C scale. Oscilloscope "Shoot" switch 2000PSI r-He tank _]" ..... ] Test l . -"x Furnace Pressure Solenoid i Projectile Spec,me% transducer valve \ i insertion Gun I \ -I _,,i \ /--barrel ] _'a i_ Pressure_ ' I ,,<> l--1 '_ i r-r qVl ....... ; --"_ql . . . Pressure Projectdeveloclty _X I readout Laser measurment ....1__.... •............. \ system _ rotating stage Figure 1. Schematic of impact testing apparatus Clamps Holder |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020082954.pdf |
| Alternate Webpage(s) | https://permanent.access.gpo.gov/lps51542/20020082954.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
