NASA / TM--2002-211821

Foreign Object Damage Behavior of Two

Gas-Turbine Grade Silicon Nitrides by Steel

Ball Projectiles at Ambient Temperature

Sung R. Choi

Ohio Ae:rospace Institute, Brook Park, Ohio

J. Michael Pereira, Lesley A. Janosik, and Ramakrishna 'I: Bhatt

Glenn Research Ce:n-_er, Clevel.and, Ohio

August 2002
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NASA/TM--2002-211821

Foreign Object Damage Behavior of Two

Gas-Turbine Grade Silicon Nitrides by Steel

Ball Projectiles at Ambient Temperature

Sung R. Choi

Ohio Aerospace Institute, Brook Park, Olnio

J. Michael Pereira, Lesley A. Janosik, and Ramakrishna 'I: Bhatt

Glenn Research Center, CleveI.and, Ohio

National Aeronautics and

Spa ce Ad.minis tration

Glelm Research Center

August 2002
Acknowledgments

The authors are thankful to R. Pawlik for the experimental work during the course of this stud}: This work was

supported by Higher Operating Temperature Propulsion Components (HOTPC) program, NASA Glenn Research

Center, Cleveland, Ohio.

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ideas from a technical peer group.

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identification only. This usage does not constitute an official

endorsement, either expressed or implied, by the National

Aeronautics and Space Administration.

The Aerospace Propulsion and Power Program at

NASA Glenn Research Center sponsored this work.

NASA Center for Aerospace Informa{ion

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Available frorn

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Available electronically at bttp: //gltrs.grc.nasa.gov
FOREIGN OBJECT DAMAGE BEHAVIOR OF TWO GAS-TURBINE GRADE

SILICON NITRIDES BY STEEL BALL PROJECTILES AT AMBIENT

TEMPERATURE

Sung R. Choi*

Ohio Aerospace Institute

Brook Park, Ohio 44142

J. Michael Pereira, Lesley A. Janosik, and Ramakrishna T. Bhatt

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

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 well- or 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 qV- l .......

; --"_ql . . .

Pressure Projectdeveloclty _X I

readout Laser measurment ....1__.... •............. \

system _ rotating stage

Figure 1. Schematic of impact testing apparatus Clamps

Holder

Test

Specimen

2.

...... E ................

Details on

specimen holding

NASA/TM--2002-211821 3
500

450

400

Vel 350

oei 300

ty

(in/250

s) 200

150

100

50

0

J

y = 99.834Ln(x) -

.... R2 = 0.9929

0 100 200 300 400 500 600 700 800 900

Pressure (psi)

Figure 2. Velocity of steel ball projectiles with a diameter of 1.59 mm, as a function of He-pressure applied

inside a gun barrel. Each data point represents an average of four measurements with an error bar of +1.0

standard deviation.

Post-Impact Strength Testing

Strength testing for impacted specimens was performed in ambient-temperature air using a

SiC four-point flexure fixture with 20-mm inner and 40-mm outer spans in accordance with

ASTM C 1161 [17]. Each impact-tested specimen was placed in the fixture such that its impact

site was located in tension side within the inner span. An electromechanical test frame (Model

8562, Instron, Canton, MA) was used in displacement control with an actuator speed of 0.5

mm/min. Slow crack growth, which can occur during strength testing in air at ambient

temperature for some ceramics such as alumina, is not an issue for silicon nitrides as well as

silicon carbides. A limited fractographic analysis was performed after strength testing to

determine failure origin, flaw configuration and mode of fracture. 'As-received' flexural strength

was also determined for each material with a total of 20 test specimens using the same test fixture,

test frame, and test conditions that were utilized for the impacted test specimens.

Fracture Toughness and Indentation Strength Testing

Fracture toughness was determined at ambient temperature for AS800 and SN282 using the

single-edge-precracked beam (SEPB) method in accordance with ASTM C 1421 [20]. A total of

five flexure test specimens with nominal dimensions of 3 mm (width) by 4 mm (depth) mm by 45

mm (length) were used for each material. A sharp precrack was introduced on the 3mm x 4mm

plane located at the center of each test specimen. The average size of precracks, produced through

Vickers indentation (with an indentation load of 9.8N) and subsequent bridge compression, was

about 2 mm for both materials. Precracked specimens were fractured to determine fracture loads

using a four-point flexure test fixture with 20-mm inner and 40-mm outer spans at 0.5 mm/min in

an electromechanical test frame. Details regarding the SEPB test method in terms of relationship

between indent load, bridge spans, and precrack size can be found elsewhere [21].

Indentation strength testing was also conducted using flexure test specimens measuring 3 mm

(depth) by 4 mm (width) by 23 mm (length) in order to determine the material's strength response

to indentation damage by a Vickers indenter. This indentation-strength response has been used to

model the quasi-static, sharp-particle impact behavior of brittle materials [2,3]. Hence, the use of

indentation strength may enable one to at least qualitatively grasp an important material parameter

that has the most significant effect on resistance to sharp-particle impact damage. Note that

indentation strength data can also be used to determine R-curve behavior of brittle materials [22]

in conjunction with fracture toughness data obtained by the SEPB method. Four different

indentation loads of 29, 49, 98 and 196 N were applied on the 4-mm wide side at the center of

each AS800 and SN282 test specimen. Indentation strength was determined using a four-point

flexure fixture with 10-mm inner and 20-mm outer spans at a test rate of 0.5 mm/min with three

test specimens tested at each indentation load. The number of test specimens, three at each

NASA/TM--2002-211821 4
indentationload,wasstatisticallysufficientconsideringaverysmallstrengthscatter(typically

yieldingacoefficient ofvariationof<5%)forbothAS800andSN282siliconnitrides.

RESULTS AND DISCUSSION

Post-Impact Strength

The two-parameter Weibull plots of 'as-received' flexure strengths of both AS800 and SN282

are shown in Figure 3, where lnln[1/(1-F)] was plotted as a function of ln(yf. The variables F and

9 are failure probability and flexure strength, respectively. Weibull modulus (m) and

characteristic strength ((Yo) were m 21 and (Yo 795 MPa for AS800, and m 11 and (Yo 623 MPa

for SN282, respectively. The mean strength was 775+45MPa for AS800 and 595+64 MPa for

SN282. In many cases failure origins were associated with surface-connected defects such as

machining flaws and pores, and with elongated grains.

The results of strength testing for impacted test specimens are shown in Figure 4, where postimpact strength was plotted as a function of impact velocity for both materials. "As-received"

flexure strength of both silicon nitrides was also plotted for comparison. Frequently, specimens

impacted at lower velocity were not fractured from impact sites and were not included in the postimpact strength data. The "zero" strength shown in the figure represents the specimens broken

upon impact at higher velocity, where the impact force (or impact energy) was sufficient to break

test specimens into two pieces, with failure originating from the impact site. This velocity is

called a "critical impact velocity" in this paper. It should be noted that for a given impact

condition and material the critical impact velocity depends on test specimen size and geometry, as

well as on the relative configuration of specimen support on impact test fixtures.

Strength degradation with respect to the "as-received" strength was evident with increasing

impact velocity for both materials. For a given impact velocity, however, corresponding postimpact strength was greater for AS800 than for SN282. Also, AS800 silicon nitride exhibited a

higher critical impact velocity (Vo) as compared to that of SN282 silicon nitride such that

Vc = 400 m/s for AS800

Vc = 300 m/s for SN282

250 400 1000 [MPa] l

_=

_=

,<

©

,<

1

o

-1

-2

-3

-4

5.5

RT FLEXURE

A

20/40 mm spans

SN282 Si3N4/

_e=¢7 =11 ,'_ 37

i i

6 6.5

O

_AS800 Si3N 4

m=21

O0=795 MPa

I

7 7.5

FLEXURAL STRENGTH, In fff [MPa]

Figure 3. Weibull distributions of 'as-received' flexure strengths of AS800 and SN282 silicon nitrides

determined at ambient temperature ("RT"). m and fro indicate Weibull modulus and characteristic strength,

respectively. The straight lines represent best-fit lines.

NASA/TM--2002-211821 5
1000

a00

Z 600

400

<

200

AsR

Flexure Specimens/RT

(failedfrom impact)

100 500

_ AS800 SisN 4

/ _'_ 0 0

SN282 SisN 4 vcAx_ I

200 300 400

IMPACT VELOCITY, V [m/s]

Figure 4. Results of post-impact strength as a function of impact velocity, determined for AS800 and SN282

silicon nitrides flexure specimens impacted at ambient temperature ("RT") by steel ball projectiles with a

diameter of 1.59mm. "AsR" indicates 'as-received' mean flexure strength of two materials with error bars of

+1.0 standard deviations. Vc (with arrows) indicates the "critical impact velocity."

Sharp strength degradation occurred in the region of impact velocities close to the critical impact

velocity of each material. The post-impact strength behavior of both materials was better

represented in a two-parameter Weibull scheme, which is depicted in Figure 5. The figure is for a

given impact velocity of 300 m/s using a somewhat large number (15 to 17) of test specimens.

For AS800 silicon nitride, nine of a total 17 test specimens failed from impact sites and the

remaining eight specimens failed from inherent material defects, resulting in a uni-modal strength

distribution (with a Weibull modulus of about 20, close to that of the as-received strength). By

contrast, for SN282 silicon nitride, all 15 test specimens fractured from impact sites; however, five

of these 15 specimens broke upon impact, thereby resulting in an overall bimodal strength

distribution. Note that the ("zero") strength for the specimens that failed upon impact for SN282

was taken to be about 10 MPa for graphical presentation.

Impact Morphology

The hardened steel ball projectiles, without exception, were flattened after impact, as a result

of considerable plastic deformation. In some cases, particularly at higher impact velocity, the

projectiles were subjected to both extreme heat (evidenced by burning marks) and cracking, as

shown in Figure 6. The degree of plastic deformation of the projectiles in terms of projectilediameter decrease was about 20 to 40% depending on velocity. The mechanical properties of a

projectile material, of course, are important factors affecting FOD behavior of a target material. A

preliminary study showed that annealed steel ball projectiles were more susceptible to plastic

deformation than the hardened steel ball projectiles, and thus caused less damage to the target

material.

Impact sites were characterized with three distinct regions of inner, intermediate and outer

zones, see Figure 7. The inner zone was typically on the top of cone cracks, the intermediate zone

was believed to be the outer boundary of lateral cracks or other damage, and the outer zone was of

the impression of flattened steel-ball projectiles. Some material transfer from the projectiles to the

target specimens was seen at the outer zone, due to an instantaneous 'cold welding' effect (by

metallic projectiles) at the contact area. In post-impact strength testing, failure of specimens

impacted at velocities below critical impact velocity commonly originated from the outer

boundary of lateral cracks (see Figure 7(b)). The failure path in this case was straight and cut

through the boundary of the intermediate damage zone. Well-developed cone cracks were

observed in specimens impacted at velocity close to and above the "critical impact velocity" in

which test specimens fractured into two pieces upon impact. The surface failure pattern associated

with this well-developed cone crack was clearly typified with a fracture path that was kinked at the

inner damage zone that was an upper part of the cone (Figure 7(c)).

NASA/TM--2002-211821 6
_, 2

1

o

-1

<

0 -2

-3

< -4

20 55 150 400 1100 [MPa]

V = 300 m/s

Failed from imp_,_J

SN282 S_N 4

 a lme Upon .7"

Failed from impact

i i i i i

3 4 5 6 7

POST-IMPACT STRENGTH, In of [MPa]

ailed not

m impact

AS800 S[3N 4

Figure 5. Weibull strength distributions for AS800 and SN282 silicon nitrides flexure specimens impacted at

300 m/s by 1.59-mm-diameter steel ball projectiles. The strength for the SN282 specimens completely

fractured upon impact was taken to be about 10 MPa for presentation purpose. The open symbols for AS800

indicate the specimens not failed from impact sites, whereas the closed symbols represent the specimens

failed from impact sites.

()

iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii_iiiii_i_i_L_:_

iiiiiiiiiii!i!i!!%iiiiiiiiiiiiii__

iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii ..........

()

Figure 6. A typical example of a steel ball projectile with a diameter of 1.59 mm subjected to an impact

velocity of 350 m/s for AS800 silicon nitride, showing severe plastic deformation (flattened), heating and

cracking: (a) side view; (b) top view from the arrow. Note the impression marks in (b), imprinted from the

machining marks of an AS800 target specimen.

NASA/TM--2002-211821 7
(a) (b2) (c2)

Figure 7. Typical examples of the impact sites, fracture surfaces and fracture paths for both AS800 and

SN282 silicon nitrides subjected to steel ball projectiles with a diameter of 1.59 mm: (a) Overall impact site

showing three distinct regions ("A": inner zone, "B": intermediate zone and "C": outer zone) for AS800 at

an impact velocity of V 350 m/s; (b) Top fracture path (bl) and fracture surface (b2) of an AS800 specimen

failed (in strength testing) from the outer boundary of the intermediate damage zone; strength 650 MPa; (c)

A SN282 specimen failed upon impact at V 350 m/s showing kinked fracture path (cl) at the cone and

fracture surface (c2) showing a well-developed cone crack.

Failed in

strength test Impact site

Fragment

Figure 8. An AS800 test specimen broken at a distance of about 5 mm from the right end upon impact at a

velocity of 300 m/s. The impact site is visible. The specimen was strength-tested, but failure did not

originate from the impact site.

NASA/TM--2002-211821 8
Thetensileprincipalstress, accordingtotheHertziancontactanalysis, occursjustoutsidethe

contactareabetweentwocontactingbodiesinwhichaconecrackinitiatesandthenpropagates

throughthelocusofmaximumtensilestress[5,9].Incasesofimpactswithhardprojectilesvs.

hardtargetmaterials(uchasceramicballsvs.ceramictargetmaterials), reasonableagreement

betweenthecalculatedcontactarea(radius)andtheuppersize(radius)of a conehasbeen

demonstrated [4].ThecontactareacanbeestimatedbasedontheHertziancontacttheorytogether

withtheprincipleofconservation ofimpactenergyasfollows[1,4,5,8]:

a = a(klE)l/5pl/5R V 2/5 (1)

where a is the contact area, cz is a constant (=1.3), k = [(1-v2)+(1-v'2)E/E '] with v being Poisson's

ratio and the primes denoting variables associated with the projectile, E is the elastic modulus of

target material, p is the density of the projectile, R is the radius of the projectile, and V is the

impact velocity. The calculated contact area based on Equation (1) was significantly smaller

(approximately one-fifth) than the upper cone sizes observed in the current effort. Significant

plastic deformation of steel ball projectiles is believed to be a major cause of such a discrepancy

between the calculated and the observed contact size. Evidently the 'soft' steel balls generated

much less contact area compared to that generated with 'hard' ceramic balls. Hence, the impact

events in this work would be characterized as 'plastic (in projectile)-elastic (in target material)'

impact rather than 'elastic-elastic' impact, which would be the case for ceramic balls vs. ceramic

target. Of course, plastic deformation may possibly occur even in ceramic projectiles and target

materials upon Hertzian impact. In the current effort, any plastic deformation potentially

occurring in the ceramic target materials was assumed negligible as compared with the degree of

plastic deformation of the steel ball projectiles.

Finally it was found that in some rare cases (particularly at 300 m/s) some specimens failed

upon impact at a distance of about 5-8 mm from one end of the test specimen, as shown in

Figure 8. This is presumably a result of stress wave interaction (dynamic effect) upon impact,

similar in principle to the situation of Hopkinson bar experiments. Although this was observed as

a rare case in the current experimental setting, such dynamic effects must be considered in

instances where real components are subjected to high impact energy.

Fracture Toughness, Indentation Strength and R-Curve

A summary of fracture toughness for AS800 and SN282 silicon nitrides determined by the

SEPB method is shown in Table 2. The table also includes the value previously evaluated for the

conventional fine-grained NC132 silicon nitride [15]. Fracture toughness was Kic = 8.1+0.3

MPa_/m and Kic = 5.5_+0.2 MPa_/m, respectively, for AS800 and SN282 silicon nitrides. Both

AS800 and SN282 silicon nitrides are known as in-situ toughened materials tailored to achieve

elongated grain structures. Consequently, the tailored microstructure of these materials results in

increased fracture toughness, as compared to conventional equiaxed, fine-grained silicon nitrides.

Table 2. Fracture toughness of AS 800, SN282 and NC 132 silicon nitrides determined in

four- 3oint flexure at ambient temperature

No. of test Fracture

Material specimens toughness, Kic Method [20]

(MPa_/m)

AS800 Si3N4 5 8.1+0.3 SEPB

SN282 Si3N4 5 5.5+0.2 SEPB

NC132 Si3N 4 [15] 5 4.6+0.4 SEPB

NASA/TM--2002-211821 9
Indentstrengthasafunctionof indentloaddeterminedforbothsiliconnitrides(AS800and

SN282)isshowninFigure9. Foragivenindentload,indentstrengthwasgreaterforAS800than

forSN282.Basedonindentationfracturemechanics, generalizedindentstrength(Gf) can be

expressed as a function of indent load (P) as follows [22]:

2fl 1

O'if = 0 p3+2_ (2)

_. 700

600

500

400

300

Z

_a

200

Z

_a

Z 100

_o

SN

slope=-0.25 --

SN282 SN

i i i i i i I i

20 30 40 5060 80100 200 300

INDENT LOAD, P [N]

Figure 9. Indent strength as a function of indent load determined for AS800 and SN282 silicon nitrides. The

solid lines represent the best-fit lines in the log (indent strength) vs. log (indent load) relation. Error bars

indicate +1.0 standard deviations.

10

¢)

Z 8

<

=)

= _"_ 4

¢)

< 2

I I

_=o.13

/__=0.10

_- _=o

I

ASS00

SN282

NC132

0 I I I I

0 500 1000 1500 2000 2500

CRACK SIZE, c [gm]

Figure 10. Fracture resistance (R-curve) as a function of crack size for AS800 and SN282 silicon nitrides.

The fine-grained NC 132 silicon nitride data [15] is also included for comparison.

NASA/TM 2002-211821 10
where 0 is a parameter associated with elastic modulus, hardness, indent crack geometry and

fracture resistance. The parameter fl represents the degree of fracture resistance ("R-curve") as

defined in [22]

K R = tcc ¢ (3)

where KR is fracture resistance, c is the crack size, and _: is a parameter. Equation 2 is a common

expression of rising R-curve behavior for ceramic materials. When 13=0, the case reduces to a flat

R-curve such that _j=OP -1/3 and Ka=K=-KIc. R-curve behavior of a brittle material can be

estimated based on Equation (3) by determining the parameter 13from (the slope of) the indent

strength vs. indent load data (such as Figure 9) and then using this value to determine the

parameter _; using a known value of fracture toughness (obtained by, for example, SEPB)

evaluated with the particular precrack size (c) used. The resulting R-curve behavior thus

estimated for AS800 and SN282 is shown in Figure 10. The previously determined R-curve of

NC132 silicon nitride [15] is also included in the figure. AS800 exhibits not only greater fracture

toughness but also stronger R-curve behavior than SN282, while the conventional, equiaxed finegrained NC132 silicon nitride typically exhibits a flat R-curve and low fracture toughness.

AnalFtical Considerations on Strength Degradation

A phenomenological model of strength degradation due to ball impact was proposed

previously by Wiederhorn and Lawn [1], based on assumptions that the impact event was elastic

and quasi-static and that strength degradation was attributed to a formation of cone cracks. Also,

another important assumption was that the "effective" size of strength controlling flaws was

proportional to the base radius of the cone [1,5,9]. The latter simplification was based on the fact

that the stress intensity factor solution of a cone crack was not available and that the geometry of a

cone crack system varied with projectile, target materials and impact conditions (velocity), etc.

With those assumptions, strength degradation was modeled using Hertzian contact analysis, the

principle of energy conservation, and indentation fracture relations. The model, despite several

assumptions, was in good agreement with experimental data determined for glass impacted by

steel or tungsten carbide spherical projectiles [1]. The resulting strength degradation as a function

of impact velocity is expressed as follows [1]:

_---_-2/15 1/5_ 2/3T74/3 V 2/5

(_f : _[K I L) p K IVkIC (4)

where • is a constant. Equation (4) can also be expressed in terms of impact kinetic energy (Uzd

of a projectile using

to yield

1

U_ = =my2 (5)

2

CYZ= • ,(k/E) 2/15R 1/15Kic4/3 UK1/5 (6)

r 1/5 where m is the mass of a projectile and • =(2_/3) _. For a given target material and a given 2/5

material and geometry of a projectile, the post-impact strength depends on [impact velocity] or 1/5 [impact kinetic energy] as seen from Equation (4) or (6). The post-impact strength data shown

in Figure 4 were plotted as a function of impact kinetic energy in Figure 11 based on Equation (6).

It is noted from the figure that a discrepancy in slope between the prediction (=-1/5) and the

experimental data was significant for both AS800 and SN282 silicon nitrides. This discrepancy

might have arisen from several factors, including: (1) Significant plastic deformation of a

projectile upon impact might invalidate the model's assumption of idealized elastic impact;

NASA/TM--2002-211821 11
(2) Formation of well-developed cone cracks might not have always occurred (as seen from

fractography); sometimes radial or lateral cracks or ill-defined damage might have controlled

2000

1000 AsR

988 -

600

500

400 SN282 SN

E- 300

200

[...

100

0.1

Flexure Specimens/RT

(Failed from impact)

1

Vo

AS800 SN

I I I I I I I

0.2 0.3 0.40.5 0.7 1 2

IMPACT KINETIC ENERGY, U k [J]

Figure 11. Post-impact strength as a function of impact kinetic energy determined for AS800 and SN282

silicon nitrides. The slope (=-1/5) indicates a theoretical value based on Equation (4). "AsR" indicates "asreceived" flexure strength for both materials. For clarity of graphical representation, mean strength was used

with error bars of +1.0 standard deviations.

strength; (3) Quasi-static assumption of the model, which would have been operative for soft

glasses, might not have been applicable to harder silicon nitrides.

A KeF Material Parameter Affecting FOD Resistance

For most silicon nitrides, elastic modulus (E), hardness, density and Poisson's ratio (v) are

quite similar. Hence for a given projectile, impact velocity, and given target specimen geometry,

the post-impact strength, according to Equation (4) or (6), depends on the fracture toughness of a

target material with a relation of _yf_ [Kic] 4/3. This leads to a simple expression of the post-impact

strength ratio between AS800 and SN282 silicon nitrides as follows:*

_ f /AS800 KIC/AS800 4/3

- [--] (7)

Gf/SN282 KIC/SN282

Use of this relation together with the values of fracture toughness (see Table 2) determined for

both AS800 and SN282 yielded that the post-impact strength of AS800 was 1.68 times greater

than that of SN282. The actual strength ratio at 200 m/s and 300 m/s was found to be 1.20 and

1.52, respectively, resulting in reasonable agreement, particularly at 300 m/s. Despite an

appreciable discrepancy in the post-impact strength vs. impact kinetic energy relation between the

prediction and the experimental data, as seen in Figure 11, Equation (4) or (6) provides important

insight regarding AS800's greater resistance to FOD as compared to that of SN282. It is therefore

reasonable to conclude that a silicon nitride with greater fracture toughness can possess greater

FOD resistance than another silicon nitride with lower fracture toughness.

In order to better understand (and thus to further confirm) this major conclusion, additional

FOD testing was conducted using a conventional, equiaxed, fine-grained silicon nitride (NC132)

for target/strength test specimens, as described previously in the Experimental Procedure. The

results of post-impact strength testing for NC132 silicon nitride are presented in Figure 12,

This relation, based on indentation fracture mechanics [23], is valid for indent strength for a given indent load. The ratio

of indent strengfla between AS800 and SN282 silicon nitrides shown in Figure 9 was in the range of 1.36 to 1.52, resulting

in some discrepancy with flae flaeoretical prediction (=1.68) but overall in reasonable agreement.

NASA/TM 2002-211821 12
"_1000

800

Z 600

400

<

[..

O

200

i i i i

Flexure Specimens/RT

ASR (failed from impact)

a00

0 100 200 300 400 500

IMPACT VELOCITY, v [m/s]

Figure 12. Results of post-impact strength as a function of impact velocity for NC132 silicon nitride,

presented together with AS800 and SN282 silicon nitride data for comparison. Mean strength was used for

graphical clarity with error bars of +1.0 standard deviations. Vo indicates critical impact velocity.

_a 600

;_ 500

400

3OO

-<

200

100

-<

[.. 0

2

I I I

Flexure Bar Specimens

(4mmx3mmx45mm)

AS800

_General trend

i I i I = I i

4 6 8

FRACTURE TOUGHNESS, Kic [MPam v:]

1.32

0.74

0.44

10

i

i

Figure 13. General trend in critical impact velocity as a function of fracture toughness for two in-situ

toughened AS800 and SN282 silicon nitrides and one conventional, equiaxed fine-grained NC132 silicon

nitride.

NASA/TM--2002-211821 13
degradation of NC132 with respect to "as-received" strength was significant even at lower impact

velocities (< 220 m/s), resulting in the lowest critical impact velocity of Vo 230 m/s among the

three silicon nitrides tested. Figure 13 shows a summary of critical impact velocity as a function

of fracture toughness for the three silicon nitrides tested. The general trend manifest from the

figure is that critical impact velocity increases with increasing fracture toughness. This indicates

undoubtedly that fracture toughness is a key material parameter affecting FOD resistance in

silicon nitrides. This is also understandable if one considers that fracture toughness is a measure

of resistance to crack initiation-and-propagation. The NC132 with the lowest value of fracture

toughness, Kic 4.6 MPa_/m, exhibited the lowest FOD resistance; whereas, the AS800 with the

greatest value of fracture toughness, Kic 8.1 MPa_/m, showed the greatest FOD resistance. In

other words, the in-situ toughened silicon nitride, tailored with an elongated grain structure to

achieve more fracture resistance (and rising R-curve behavior also) via crack deflection and/or

bridging, additionally possesses much better FOD resistance than the conventional, equiaxed, finegrained silicon nitride which has no such features as deflection and/or bridging, and typically

exhibits a flat R-curve (see Figure 10).

Comparison of FOD Behavior between 'Blunt" (Ball) Proiectile and Sharp-Particle Impact

A comparison of post-impact strength of silicon nitrides subjected to a single impact event

with steel balls (this study) and sharp SiC-particles (grit # 16 and 46) [3,24] is shown in Figure 14.

Considerable strength degradation occurs in the case of sharp particle impact even at much lower

impact kinetic energy, showing that the severity of impact damage was far greater in "sharp"

particle impact than in "blunt" (steel-ball) projectile impact. The sharp particle impact typically

produced radial cracks emanating from the impact sites (similar to the Vickers indent cracks that

originate from the comers of an impression site), thereby resulting in significant strength

degradation. It should be noted that the fracture toughness of AS440 silicon nitride was greater

(about 30%) than that of GN10 silicon nitride; hence, as expected, the post-impact strength

observed for sharp particle impact was greater for AS440 than for GN10. Figure 15 shows typical

morphologies generated by three different types of damages: steel-ball projectile impact (this

study), sharp SiC-particle impact [24], and Vickers indent [24]. Radial cracks, originating from

the impact site and causing appreciable strength degradation, were typified in sharp-particle

impact. Cone and lateral cracks, resulting in relatively insignificant strength degradation, were

exemplified in steel-ball projectile impact. Well-developed radial/median crack systems are

generally characterized in most silicon nitrides in response to Vickers indent. As a consequence,

for a given (qnasistatic) impact load, strength degradation is expected to be much greater in either

sharp-particle impact or static indentation than in ball projectile impact. Of course, the degree of

strength degradation for sharp particle impact also depends on additional features such as the

geometry and material of particles and target specimens.

NASA/TM--2002-211821 14
"_ 1000

900

__, 800

i__ 700

600

[.- 500

Z 400

[-- 300

E-,

< 200

0

-ASR

if<

Ball projectiles ""_

_$800 Slx

[This study]

SN282 SlX

_ AS440 SN

Sharp Si_ GN10 SN Vc

[disks; Refs 3,24]

I I I I I

10-4 10-3 10-2 10-1 10 0

IMPACT KINETIC ENERGY, U k [J]

100

10-5 101

Figure 14. A comparison of post-impact strength as a function of impact kinetic energy between "sharp" SiC

particle impact [3,24] and "blunt" steel-ball projectile impact (this study). "ASR" indicates "as-received"

flexure strength for both materials. Error bars represent +1.0 standard deviations.

(a) (b) (c)

Figure 15. Three different types of damages generated in machined surfaces of silicon nitrides at ambient

temperature by: (a) steel-ball projectile impact (this study), (b) sharp SiC-particle impact [24], and (c) a

Vickers indenter [24].

NASA/TM--2002-211821 15
Effect of Protective Laver: Role of Protective Coating

It is speculated that for a given projectile and target material, FOD resistance can be improved

by employing a certain protective coating. Essentially, this coating would reduce the severity of

impact damage by alleviating the magnitude of Hertzian contact stresses. With this in mind,

additional FOD testing for both AS800 and SN282 silicon nitrides was conducted at impact

velocities greater than the critical impact velocities for each of the two materials. The prospective

impact site on each target specimen was covered with 0.059mm-thin copper shim (tape), i.e., a

protective layer. Three specimens of each silicon nitride were tested, yielding the results shown in

Figure 16. The average post-impact strength of AS800 with the protective layer at an impact

velocity of 440 m/s was 580 MPa. The addition of a protective layer provided a remarkable

enhancement in FOD resistance, compared to the AS800 specimens tested previously (with no

protective layer) that resulted in 'zero' post-impact strength. The same effect was observed for

SN282, where an average post-impact strength of 365 MPa was achieved with the protective layer

at an impact velocity of 400 m/s, which is significantly greater than the critical impact velocity of

300 m/s observed previously in the case with no protective layer. This indicates that the thin

copper shim did indeed act as a protective layer, reducing the severity of impact damage by

reducing Hertzian contact stress. However, it should be noted that the protective layer just blew

up upon impact, see Figure 17, thus making the layer ineffective against multiple or repeated

impact events. These results indicate that the addition of a protective layer on the base silicon

nitride increases the critical impact velocity significantly and that adhesion between the protective

layer and the substrate is crucial to enhance the life of the protective layer. It is also implied that

greater fracture toughness of a protective layer will yield better FOD resistance as well as

increased life of the underlying silicon nitride material. Protective coatings have been an

important issue in demanding aeroengine environments. Application of thermal barrier coatings

(TBCs) and/or environmental barrier coatings (EBCs) has been shown to provide significant

performance enhancements. In addition to TBC/EBC, the application of protective coatings to

improve FOD resistance--'impact barrier coating' (IBC) or 'impact resistance coating'

(IRC)--must be considered, explored, and developed as an important necessity in aeroengine

design and applications.

1000

_" 800

[-

600

[..

400

L_

<

[-

©

2OO

0

0 500

Flexure Specimens/RT

iSR _ (Failed from impact)

• _ _ Cu shim

[ [ [ [ [ i _ __1

100 200 300 400

IMPACT VELOCITY, v [m/s]

Figure 16. Results of impact testing for AS800 and SN282 silicon nitrides with test specimens being covered

with 0.059-mm copper protective layer (shim). Impact velocity used was 400 and 440 m/s, respectively, for

SN282 and AS800 silicon nitrides. The results of the regular (no protective coating) FOD tests are also

shown for comparison.

NASA/TM--2002-211821 16
Impact center

Copper shim I

Figure 17. A typical appearance of the impact site of 0.059-mm thick copper shim and SN282 test specimen

subjected to an impact velocity of 400 m/s. Note a blew-up mode of copper shim upon impact. The postimpact strength of this specimen was determined as 327 MPa.

Other Considerations

Designing aeroengine components to withstand FOD events is a complex task. Consideration

of many factors is required, both in the generation of FOD data as well as in actual component

design efforts. A sample of these numerous factors includes the following:

Eftbct of projectile material/geometry

Effect of test specimen material/geometry: monolithic vs. composites approach

Effect of test-specimen support and component attachment

Effect of temperature/environment

Appropriate protective coatings

Geometrical design of components to enhance FOD resistance

FOD/Reliability/Life prediction codes

Not only must each of these factors be scrutinized individually, but the effects of interactions

between multiple factors also must be accommodated. Notwithstanding the immense challenges

this poses, a strategy for mitigating FOD damage must be developed and employed in order to

achieve the most desirable performance of components in service. Hence, some of these factors

are immediate subjects of study and the related work is under way, such as in the tasks reported in

this manuscript. Others are long-term efforts and are pursued continually in the quest for

improving the efficiency and reliability of aeroengine components.

CONCLUSIONS

Based on the results of extensive FOD testing at ambient temperature for two in-situ

toughened, gas-turbine grade silicon nitrides (AS800 and SN282) and one conventional silicon

nitride (NC132), the following conclusions were made:

(1) The overall resistance--estimated by post-impact strength - to foreign object damage

(FOD) by steel-ball projectiles with a diameter of 1.59 mm was found to be greater for

AS800 silicon nitride than for SN282 silicon nitride in an impact velocity range of 220 to

440 m/s.

NASA/TM--2002-211821 17
(2) Thecriticalimpactvelocity,inwhichflexuretestspecimens fractureduponimpact,was

about400m/sand300m/s,respectively, forAS800andSN282siliconnitrides.The

criticalimpactvelocityof theconventional equiaxedfine-grainedNC132siliconnitride

wasabout230m/s.

(3) ThedifferenceinFODresistancebtweenAS800andSN282siliconnitrideswasfound

tobeduetodifferences infracturetoughness, asalsosupportedbyadditional FODdata

onNC132withlowfracturetoughness. Thecriticalimpactvelocity,in general, was

linearlydependent onfracturetoughness, leadingtoaconclusionthatfracturetoughness

isakey material parameter affecting FOD resistance of silicon nitrides.

(4) The impact damage by 'blunt' steel-ball projectiles was found to be far more

insignificant for a given impact kinetic energy than that by 'sharp' SiC particles.

(5) The addition of a protective layer of 0.059-mm copper shim significantly improved the

FOD resistance of both AS800 and SN282 silicon nitrides by considerably reducing the

severity of impact damage via alleviating Hertzian contact stresses. This gives an insight

into some practical design philosophy regarding FOD protective coatings in actual

aeroengine components.

REFERENCES

1. S.M. Wiederhorn and B.R. Lawn, "Strength Degradation of Glass Resulting from Impact

with Spheres," J. Am. Ceram. Soc., 6019-10] 451-458 (1977).

2. S.M. Wiederhorn and B.R. Lawn, "Strength Degradation of Glass Impact with Sharp

Particles: I, Annealed Surfaces," J. Am. Ceram. Soc., 6211-2] 66-70 (1979).

3. J.E. Ritter, S.R. Choi, K. Jakus, P.J. Whalen, and R.G. Rateick, "Effect of Microstructure

on the Erosion and Impact Damage of Sintered Silicon Nitride," J. Mater. Sci., 26 5543-

5546 (1991).

4. Y. Akimune, Y. Katano, and K. Matoba, "SphericaMmpact Damage and Strength

Degradation in Silicon Nitrides for Automobile Turbocharger Rotors," J. Am. Ceram.

Soc., 72[8] 1422-1428 (1989).

5. C.G. Knight, M.V. Swain, and M.M. Chaudhri, "Impact of Small Steel Spheres on Glass

Surfaces," J. Mater. Sci., 12 1573-1586 (1977).

6. A.M. Rajendran and J.L. Kroupa, "Impact Design Model for Ceramic Materials," J. Appl.

Phys, 66[8] 3560-3565 (1989).

7. L.M. Taylor, E.P. Chen, and J.S. Kuszmaul, "Microcrack-Induced Damage Accumulation

in Brittle Rock Under Dynamic Loading," Comp. Meth. Appl. Mech. Eng., 55, 301-320

(1986).

8. R. Mouginot and D. Maugis, "Fracture Indentation beneath Flat and Spherical Punches,"

J. Mater. Sci., 20 4354-4376 (1985).

9. A.G. Evans and T.R. Wilshaw, "Dynamic Solid Particle Damage in Brittle Materials: An

Appraisal," J. Mater. Sci., 12 97-116 (1977).

10. B.M. Liaw, A.S. Kobayashi, and A.G. Emery, "Theoretical Model of Impact Damage in

Structural Ceramics," J. Am. Ceram. Soc., 67 544-548 (1984).

11. S.R. Choi, J.M. Pereira, L.A. Janosik, and R.T. Bhatt, "Foreign Object Damage of Two

Gas-Turbine Grade Silicon Nitrides at Ambient Temperature," (in print) Ceram. Eng. Sci.

Proc., 23[3,4] (2002).

12. T. Ohji, "Long Term Tensile Creep Behavior of Highly Heat-Resistant Silicon Nitride for

Ceramic Gas Turbines," Ceram. Eng. Sci. Proc., 22[3] 159-166 (2001).

13. F. Lofaj, S.M. Wiederhorn, and P.R. Jemian, "Tensile Creep in the Next Generation

Silicon Nitride," Ceram. Eng. Sci. Proc., 22[3] 167-174 (2001).

14. H.T. Lin, S.B. Waters, K.L. More, J. Wimmer, and C.W. Li, "Evaluation of Creep

Property of AS800 Silicon Nitride from As-Processed Surface Regions," Ceram. Eng.

Sci. Proc., 22[3] 175-182 (2001).

NASA/TM--2002-211821 18
15.S.R.ChoiandJ.P.Gyekenyesi, (a)"Elevated-Temperature "Ultra"-Fast FractureStrength

ofAdvancedCeramics: AnApproachtoElevated-Temperature "Inert"Strength," ASME

J. Eng. Gas Turbines & Powers, 121 18-24 (1999); (b) '°Ultra'-Fast Strength of

Advanced Structural Ceramics at Elevated Temperatures" pp. 27-46 in Fracture

Mechanics of Ceramics, Vol. 13, Edited by R.C. Bradt, D. Munz, M. Sakai, V.Ya.

Shevchenko, and K.W. White, Kluwer Academic/Plenum Publishers, New York, NY

(2002).

16. S.R. Choi and J.P. Gyekenyesi, "Slow Crack Growth Analysis of Advanced Structural

Ceramics under Combined Loading Conditions: Damage Assessment in Life Prediction

Testing," ASME J. Eng. Gas Turbines & Power, 123 277-287 (2001).

17. ASTM C 1161, "Test Method for Flexural Strength of Advanced Ceramics at Ambient

Temperatures," Annual Book of ASTM Standards, Vol. 15.01, ASTM, West

Conshohocken, PA (2002).

18. ASTM C 1259, "Test Method for Dynamic Young's Modulus, Shear Modulus, and

Poisson's Ratio for Advanced Ceramics by Impulse Excitation of Vibration," Annual

Book of ASTM Standards, Vol. 15.01, ASTM, West Conshohocken, PA (2002).

19. ASTM C 1327, "Test method for Vickers Indentation Hardness of Advanced Ceramics,"

Annual Book of ASTM Standards, Vol. 15.01, ASTM, West Conshohocken, PA (2002).

20. ASTM C 1421, "Test Methods for Determination of Fracture Toughness of Advanced

Ceramics at Ambient Temperature," Annual Book of ASTM Standards, Vol. 15.01,

ASTM, West Conshohocken, PA (2001).

21. S.R. Choi, A. Chulya, and J.A. Salem, "Analysis of Precracking Parameters for Ceramic

Single-Edge-Precracked-Beam Specimens, pp. 73-88 in Fracture Mechanics of

Ceramics, Vol. 10, Edited by R. C. Bradt, D. P. H. Hasselman, D. Munz, M. Sakai, and

V.Ya. Shevchenko," Plenum Press, New York, NY (1992).

22. R.F. Krause, "Rising Fracture Toughness from the Bending Strength of Indented

Alumina Beams," J. Am. Ceram. Soc., 7115] 338-343 (1988).

23. P. Chantikul, G.R. Anstis, B.R. Lawn, and D.B. Marshall," A Critical Evaluation of

Indentation Techniques for Measuring Fracture Toughness: II. Strength Method J. Am.

Ceram. Soc., 64[9] 539-543 (1981).

24. S.R. Choi, J.E. Ritter and K. Jakus, "Erosion and Impact Behavior of Various Advanced

Ceramics at Ambient and Elevated Temperatures," unpublished work, University of

Massachusetts, Amherst, MA (1988).

NASA/TM--2002-211821 19
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August 2002 Technicai Memorandum

4, TITLE AND SUBTITLE 5, FUNDING NUMBERS

Foreign Object Damage Behavior of Two Gas-Turbine Grade Silicon Nitrides by

Steel Ball Projectiles at Ambient Temperature

6. AUTHOR(S) WU-708-31-13-00

Sung R. Choi, J. Michael Pereira, I,esley A. Janosik, and Ramai<rishna T. Bhatt

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION

REPORT NUMBER

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field E----13513

Cleveland, Ohio 44135 - 3191

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AGENCY REPORT NUMBER

National Aeronautics and Space Administration

Washington, DiE 20546 - 0001 NASA TM------2002- 211821

11. SUPPLEMENTARY NOTES

Sung R. Choi, Ohio Aerospace Institute, Brook Park, Ohio, and NASA Senior Resident Research Scientist at Glenn

Research Center; J. Michael Pereira, Lesley A. Janosik, and Ramakrishna T. Bhatt NASA Glenn Research Center.

Responsible person, Sung R. Choi, organization code 5920, 216-433-8366.

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Subject Categories: 07 and 39 Distribution: Nonstandard

Available electronically at bttp://glt:.-s._rc.nasa._ov

"[his publication is avNlable from the NASA Center for AeroSpace Inl%rmadon, 301-621-0390.

12b. DISTRNBUTION CODE

13. ABSTRACT (Maximum 200 words)

Foreign object damage (FOB) behavior of two col_m_erciai 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 witl:

a diameter of 1.59 mm in a velocity range from 220 to 440 rtds. AS800 silicon nitride exhibited a greater FOD resistance

than SN282, primarily due to its greater value of fracture toughness (K[c). Additionaily, the FOD response of an equ]axed,

fine-grained silicon nitride (NC 132) was also investigated to prov] de further insight. The NCI 32 sill con nitride exhib]ted

the lowest fracture toughness of the three materials tested, providing further evidence that K[c is a :key material parameter

Nfecting FOD resistance. The observed damage generated by projectile impact was typicaily in the forms of well- or

ill-developed ring or cone cracks with little presence of radiai cracks.

14. SUBJECT TERMS

Foreign object damage; FOD; Turbine-grade silicon nitrides; Impact testing; Strength

testing; Mechanicai testing fbr ceramics; Strength degradation due to FOD

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