Some of the intrinsic properties of various forms of Si3N4 and SiC are listed and limitations of such materials' availability are pointed out. The essential features/parameters to characterize a batch of powder are discussed including the standard techniques for such characterization. In process characterization, parameters in sintering, reaction sintering, and hot pressing processes are discussed including the factors responsible for strength limitations in ceramic bodies. Significant improvements in material properties can be achieved by reducing or eliminating the strength limiting factors with consistent powder and process characterization along with process control

Dutta, S.

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.r IRnYIr'`i IN GU TUAEINE:
^OWDER ANC PFCCESS CHAPAC TEDNATICN (NASA)
P HC AC2/NF 01
	 CSCL 21E
CERAMICS IN GAS TURBINE:
POWDER AND PROCESS CHARACTERIZATION
N78-17059
Onc las
3/07 04479
by Sunil Dutta
Lewis Research Center
Cleveland, Ohio 44135
TECHNICAL PAPER presented at the
Conference on Composites and Advanced Materials
sponsored by the American Ceramic Society
Cocoa Beach, Florida, "'unary 17-19, 1977
CEWIICS IN CAS TURBINES:
POWDF;R AND PROCESS C1iARACTERIZATION
by Suril Dutta
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
ABSTRACT
The role of powder and process characterization in producing high
quality silicon ritride and silicon carbide components, for gas turbine
applications, is described. Some of the intrinsic properties of
various form~ of Si 3N 4
 and SIC are listed and limitations of Such
materials' availability have been pointed out. The essential features/
Ln	 parameters to characterize a hatch of powder have been discussed in-
s
w
	 cluding the standard techniques for such characterization. In process
characterization, parau:9ters in sintering, reaction sintering, and
hot pressing processes are discussed including the factors responsible
for strength limitations in ceramic bodies. It is inevitable that
significant improvements in naterial properties can be achieved by
reducing or eliminating the strength limiting factors with consistent
powder and process characterization along with process control.
STAR Category 27
2INTRODUCTION
The high temperature ceramic gas turbine is a candidate power plant
because it offers potential solutions to some of the problems of energy,
air pollution, and materials resources. The high efficiency ceramic gas
turbine with its smaller volume and weight, has significant potential for
use in aircraft and automobile engines, and in electric power generating
systems. Advanced c prarAcs such as silicon nitride and silicon carLide
have been widely considered for gas turbine components. Their material
properties such as high strength, and good thermal shock, oxidation
resistance, and phase Stability make them well suited for gas turbine..
Because of their Ligh use temperatures these ceramics offer the gas tur-
bine important advantages including reduction In cooling air requirements
and in specific fuel consumption, and may permit the use of lower grade fuels.
The need for ceramics in gas turbines has been well recognized.
However, the lack of consistent methods of powder and process charac-
terization and the lack of consistent materials sources, in addition to the
problems of designing with brittle materials, inhibit their use. The
'Jbjective of this paper is to describe the material and process charac-
terization required for process control in thF, manufacture of gas turbine
components.
PROPFRTY REQUIRDEMS
The material requirements for high temperature gas turbines vary for
each component depending; on the exact nature of the operational environ-
ment (mechanical, thermal, chemical, etc). For example, Table 1 shows
the ceramic materials considered for components of a vehicular gas tur-
hine 1 , and gives the maximum operating temperature for each component.
3TABLE. 1
Ceramic Materials for Vehicular
Gas Turbine Components 
COMPONENT MAXIMUM OPERATING CERAMIC MATERIAI.
TLWERATURE'C (F)
Combustor 1648 (3000) Sic
First Stage Stator 1315 (2400) Si3N4
First Stage Rotor Shroud 1093 (2000) Si3N4
First Stage Rotor 1280 (2300) Si3N4, Sic
Second Stage Stator 1150 (2100) Si3N4
Second Stage Rotor 1093 (2000) Si3N4, Sic
Regenerator 982 (1800) LAS/r AS*
As noted in the introduction, the essential properties for gas turbine
components are high temperature mechanical strength, good oxidation,
thermal shock resistance, and phase stability.
Table 2 indicates some important properties of Some forme of
S1 3N 4 and Sic ceramics which are under investigation for gas turbine
components.
*I.?AS - Lithium Aluminum Silicate
MAS - Magnesium Aluminum Silicate
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5Among the properties discussed, thermal expansion and thermal con-
(iuctivity, along with the modulus of elasticity, control the thermal
stresses developed. Thus they determine the thermal shock exposure a
material of a given strength can tolerate. Of the nurerous ceramics
available, Si 3N4 and SiC have lower thermal expansion and higher thermal
conductivity, and consequently higher thermal shock resistance. In ad-
dition, these ceramics have low densities and possess excellent mechani-
cal strength, at room and high temperatures. There Fore, the selection of
Si 3N4 and SiC as candidates for gas turbines was made on the basis of
strength and resistance to thertral shock. However, these properties are
very sensitive to processing history such as powder preparation, fabrica-
tion cond3.tions, and fabrication techniques, which are responsible for
porosity, foreign inclusions, low melting phases, stoichioraetry etc., all
of which limit the strength. Therefore, further improvements in material
properties remain to be achieved by consistent powder and process control.
t:ATERIALS AVAILABILITY
Presently, materials costs and availability of silicon nitride and
silicon carbide are problem areas. Since no large and continuing mar-
ket exists for these materials, their present cost is high and their
availability is quite limited. Table 3 shows a list of manufacturers,
%,,I-.o have been engaged in synthesizing or manufacturing high purity,
micron and sub•-raicron Si 3N 4 and SiC powders.
6TABLE 3
Potential Sources of High Purity, Si 3N 4 and SiC Powders
Silicon Nitride (Si 3N 4 )	 Silicon Carbide (SiC)
Kawecki Berylco/Advanced Materials
	
Pittsburgh Plate Glass
Engineering
General Telephone b Electronics
	
General Electric Co.
Norton Co./Lucas Ltd.
	 Carborundum Co.
Indussa Corporation 	 Sonneborn Refractories b
Chemicals Corporation
Silicon nitride materials are in such an early stage of develop-
ment that they are undergoing constant changes in methods of manufac-
turing, processing, additives etc., thus resulting in different chemis-
tries in the final synthesized powder, from one batch to another.
Sinterable silicon carbide powder is less available than silicon
nitride powder. No high purity, sub-micron SiC powder is available com-
mercially. Although Pittsburgh Plate Glass was a source of B-SJ.0 powder,
this powder was withdrawn from the market. General Electric does not
manufacture b- S iC for sale. Carborundum has announced a fine grained
a-SiC powder, but it is not yet commercially available. In any case,
progress is hindered by a lack of powders having reproducible charac-
teristics.
POWDER CHAPkCTERIZATION
The essential features to characterize in a batch of powder either
in research and development programs or for quality control in produc-
tion are shown in Table 4.
7Table 4
Characterization Features of Silicon Nitride and Silicon Carbide Powder
X-ray Density
Chemical Composition/Srsichiometry
Trace Cation and Anion Impurities
Surface Area
Average Particle Size
Particle Shape
Particle Size Distribution
Phase Composition
Unreacted Phases (Si,C)
Silicon nitride and silicon carbide powders from different vendors
have wide compositional variations with respect to impurities, stoichi-
ometry, chemistry etc. They can also vary from batch to batch in ma-
trial prepared at different times. Therefore, a quantitative charac-
terization of these features (Table 4) is very important to understand
the cause and effect relationshi p s in material properties.
Some of the standard techniques of characterizing ceramic powders
are listed in Table 5.2
8TABLE 5(2)
Standard Techniques	 for Ceramic Powder Characterization (Si 3N 4 and SiC)
Technique	 Characterization Features
Wet Chemistry	 quantitative Elemental Analysis
X-ray Fluorescence	 Trace Elements
Emission Spectroscopy	 Cation Analysis 1-10 ppm
Mass Spectroscopy	 Cations b Anions below 10 ppm
Vacuum fusion	 Nitrogen
Neutron Activation	 Oxygen less than 0.3 percent
Non-dispersive X-ray	 (Elemental analysis) of particles
> 1 um size
X-ray Diffraction 	 Phase Identification, Crystal structure
Electron Diffraction	 Particles > 0.1 um size
Optical Microscopy	 Particles > 0.5 um size
Screening/Sieving	 Particles > 400 mesh
Sedimentation	 0.05 - 100 um size range
Electron Microscopy	 0.001 - 5 um size range
Of these techniques, wet chemical analysis, X-ray diffraction, op-
tical microscopy, electron microscopy, and screening/ sieving are most
routinely used while others are less commonly used due to inadequate
laboratory facilities and an excessive time and cost required. How-
ever, their use is increasing. For example, more and more researchers
are using the neutron activation technique for quantitative estimation
9of oxyhen 3 in silicon nitride, a. ininum nitride etc., instead of the
vacuum fusion technique.
Effort is underway to improve chemical analyses because of ano-
malies in analysis of the same starting powde:. To illustrate the
problems, Table 6 (4) shows chemical analyses of two silicon nitride
powders performed in different laboratories.
(4)
TABLE. 6
Wet Chemical Analysis Performed at Various Labo
SILICON NITRIDE POWDER (2-grades)
	
Controlled Phase 85/15*	 High
Element
	
AM.E (U.K.) NASA-Lewis
	
AMF.(U.K.)	 KBI
wt.%	 wt/"	 Wt%	 wt%
ratories
Purity *
NASA-Lewis
wt%
Oxygen 1.3 2.29 0.55 0.55 1.8
Fe 0.5 0.78 0.63 0.40 0.27
Ca 0.2 0.40 0.25 0.02 0.05
Al 0.5 1.17 0.50 0.10 0.09
Ti -- 0.036 0.042 -- 0.019
Mn __ 0.015 --
C 0.2 0.16 0.15 -- 0.55
Not analyzed
*Advanced Materials Engineering, U.K.
10
For examplt, in controlled phase 85/15 grade powder, the oxygen content
analyzed at AME (Advanced Materials Engingeering Laboratory) was 1.3 wt
percent, while Sanders at NASA-Lewis, obtained a higher value of 2.29 wt
percent. Similarly, the aluminum content analyzed by AMF, to be 0.5 per-
cent as compared to a higher value of 1.17 percent. reported by Sanders.
In high purity grade S1 3N 4 powder, the mayor variations were found in
the analysis of oxygen, aluminum, calcium and carbon contents. A1-
thcugh the variation in results could be due to errors in the specific
experimental technique, a l:cl of accepted methods of analyses raises
questions about correctness of the data, since there are no suitable ways
to cross check results from one experimental technique to other among
labo.atories.
Characterization of ceramic powders such as si_icon nitride and
silica:, carbide is an extremely time consuming and expensive task, and
varies between one user to another. The manufacturers of such powders
need to carry out extensive characterization on the powder to maintain
quality. Several methods should be employed to cross check results, be-
cause the reliance upon one technique can lead to erroneous inter-
pretations. With S1 3N 4 and SiC powders, an importan'. parameter is re-
sidual oxygen, in the synthesized powder. Al • , surface siIfra (Si02)
and the increase in 510 2
 content during storage, should be determined
quantitatively to understand the basic nature of the starting raw ma-
terials.
It is anticipated that when all the above features are determined
accurately and correlated with the material properties, a suitable speci-
fication can be developed for a powder, with reproducible characteristics.
11
PROCESS CHARACTERIZATION
The major role of process characterization is to identify the
source of the strength limiting factors in ceramics.
They are listed in Table 7.
Table 7
b:rengtn Limiting ractors in u.ramics
Porosity
Inclusions
Agglomerates
Cooling Cracks
Unreacted Phabe(s)
Phase Transformation
Large Grain Size
Surface Defects
For example, inclusions, agglomerates etc, are usually intro-
duced during powder handling and processing; while porosity, coarse
grains and pores etc, result during densification. All of these de-
fects can be raA omly distributed throughout the material and cause
wide scatter in strength, resulting in a low Weibull "m" value. There-
fore, in order to have maximum reliability, and to produce material of
consistent quality, careful characterization of each processing step is
essential. For example, during milling, characterization of milling
media, milling contaminants, particle dis*_ribticions etc, to determine
the quality of nixing, and the nature and level of impurities introduced
during the milling operation are essential.
l"
Similarly, during cold forming such as injection molding processes,
parameters of the injected mix such as viscosity of organic binder,
particle distribution, particle shape, etc, should be charactetized in
order to control the molding behavior (flow characteristics) of a par-
ticular Lomposition and the resulting quality of the molded product.
.Aintering, reaction sintering, and hot pressing are the major hot
forming processes. The parameters that need careful characterization in
these processes are shown in Table B. In all three processes, LLarac-
terizati.on of various parameters are crucial for prucess control, to pro-
duco	 dua,ity products. For example, time, temperature, and atmos-
phere are common to all three processes. In reaction sintering of sili-
con, the relation of temperature and pressure of the nitriding cycle
must be controlled to achieve high density products with homogeneous
microstructures and uniform porosity distribution. Similarly, in hot
pressing, control of parameters, e.g. die material, die liner, spacer
material along with the hot pressing pressure are essential. The pres-
sure may be applied at various stages of sintering, which could affect
the density, grain size, microstructures and strength. Fig. 1 sh,,ws
three different temperature-pressure profiles. Fig. IA indicates that
a small pre-sure is being applied during the early stage of hot pressing
until the compact reaches the selected sintering temperature, where the
maximum pressure is applied. On the other hand, in Fig. IB, maximum
pressure is applied at an intermediate stage before maximum temperature
is achieved, while in Fig. 1C, both pressure and temperature are increased
simultaneously such that the compact reaches maximum hot pressing pressure
at maximum sintering temperature.
X X
X X
X
X X
X
X
X
X X
X X
X X
X
X
13
TABLE 8
Characterization Parameters in Sintering, Reaction Sintering
and Hot Pressing Processes
Reaction Sintering	 Hot r ► essingFeatures	 Sintering
Sintering Aids X
Binder	 (vol %) X
Binder Removal X
Green Density X
Die Material
Die Liner
Spacer
Sintering Temperature X
Sintering Time X
Sintering Atmosphere X
Gas Pressure X
Hot Pressing Pressure
Dutta and Rubin (5) observed the effect of these temperature-
pressure profiles on density, grain growth and microstructures in hot
pressed alumina. For example, profile 1C yielded comparatively the
highest density at a given intermediate temperature, compared to
1(A) b 1(B), although there was very little difference in density
14
values at the very final stage of sintering. Also, profile 1C pro-
duced the most uniform microstructures with no exaggerated grain
growth, while profile IA, resulted in maximum exaggerated grain
growth. However, it is difficult to anticipate which one of these
temperature-pressure profiles would yield best results during hot
pressing of Si 3N 4 and SiC although it is noteworthy that profile 1C
is more conventional in commercial practice.
CONCLTMTNr REMARKS
Powder and process characterization are critically important to
the production of high performance Si 3N 4 and SiC ceramics for gas
turbine applications. It is absolutely essential to correlate pow-
der and process characterization data with material property data,
to develop specifications for material with reproducible characteris-
tics, in order to make the use of Si 3N 4 and SiC ceramics nrectical
for gas turbines.
REFERENCES
1. A. F. McLean; pp. 9-35 in Ceramics for High Performance Appli-
cations. Fdited by J. J. Burke, A. L. Corum, and R. N. Katz.
Brook Dill Pub. Co., Chestnut l'i.11, Yass., 1974.
2. R. N. Katz; pp. 35-49 in Treatise on Materials Science & Technol-
ogy, Vol 9. Edited by F. F. Y. Wang, Academic Press, Few York,
1976.
3. Priest, H., Army Materials & Mechanics Research Center, Watertown,
::ass. Priviate Communication.
4. Sanders, W. A., NASA-Lewis Research Center, Cleveland, CH. Private
Communication.
5. S. K. Dutta and J. A. Rubin, "Effect of Processing Variables on the
Properties of Hot-Pressed Ceramic Armor," Am. Ceram. Sec. Bull.,
51, [4] 421 (1972). (Abstract of Paper 2-SI-72 presented at Society
Symposuim I, Ceramic i.rmor. Watertown, '!ass., May 9, 1972.)
BT
Q-
FIGURE 1, TIME-TEMPERATURE-PRESSURE PROFILE IN HCT PRESSING OF CERAI".1CS


Ceramics in gas turbine: Powder and process characterization

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