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Test Methods for Combined Thermal Cycling and Environment Exposure of Advanced Materials
Dr. R.D. Kane and Dr. M.S. Cayard
CLI International, Inc.
Houston, Texas USA
rdk@clihouston.com
Abstract
Test Methods for Combined Thermal Cycling and Environment Exposure of Advanced Materials
Dr. R.D. Kane and Dr. M.S. Cayard
CLI International, Inc.
Houston, Texas USA
rdk@clihouston.com
Abstract
This paper describes the development of test methods specifically designed to simulate combined thermal cycling and environment exposure. Materials evaluated in this program included several classes of refractory composite materials including those based on carbon, ceramic and glass matrices strengthened by various carbon and ceramic fibers. Combined thermal and environment profiles were developed to evaluate the resistance to environmental degradation. Evaluation parameters included mass change and coating integrity.
High specific strength refractory composites are the subject of recent investigations for their high potential serviceability for extreme conditions including exposure to very high temperatures up to 2000 C in some applications.1-5 However, there has been major concern regarding exposure to oxidative atmospheres. These concerns are typically enhanced for applications where environmental exposure is concomitant with thermal and pressure loads. Whereas most developmental studies evaluate resistance to oxidation and mechanical loads separately, these two conditions are most often encountered simultaneously thus promoting possible synergistic effects. Therefore, special methods are required to properly assess the serviceability of materials exposed to such conditions.
In this study, a specially designed apparatus was designed which employed hypothetical temperature, pressure and mechanical profiles versus time to simulate effects that could be encountered in actual service conditions. This apparatus was used in an effort to screen refractory composite materials and the behavior of surface coatings using mass changes and visual examination of the failure mode and distribution of deterioration.
No currently available standard test methods were identified that
cover the range and severity of the conditions needed for the
present study. Therefore, the first step was to prioritize the
requirements needed to be simulated in order to produce a reasonable
evaluation of performance. The requirements identified included
the following:
To provide both rapid heat-up and high temperature capabilities,
specially designed heaters were employed that utilized resistance
heating via molybdenum disilicide elements rated for 2800 F. Induction
heating was considered, but while producing rapid heating, it
could not be used since many of the refractory materials under
consideration were non-metallic and non-electrically conductive
in nature. An additional constraint was the time limitation required
for the materials evaluation effort. Rapid evaluation of available
equipment, necessary modifications and integration had to be made
to meet project milestones which eliminated the possibility for
the procurement of a highly specialized, long lead time test apparatus.
Custom designed and assembled thermal cycling units (TCU's) were
designed to conduct the environmental tests. The TCU was constructedon a vertical test frame with a fixed conditional tube and a mobile
cross member assembly
They employed two resistance heaters and a "cold" zone
above the conditioning tube to attain the various temperature
regions necessary for thermal cycling. This apparatus is also
shown schematically in Figure 2. The specimens were held in a
fixture attached to the cross member and were cycled between the
various temperature regions in the conditioning tube. The movement
of the cross member assembly was programmed to automate the thermal
cycling process. The outer ceramic tube and feed-throughs were
used to contain and maintain the test environment for the thermal
cycling tests.
The test temperature was monitored using a thermocouple whose tip was located just behind the specimens. The required temperature profiles were achieved by manipulation of a baffle thickness on the specimen holder and/or by varying the exterior tube insulation. Calibration runs were performed on dummy specimens to demonstrate that characteristic test profiles had been attained.
Materials and Specimens
Several types of refractory composite materials were evaluated
in this program. These included:
The data in this paper will be limited to that obtained for the coated carbon/carbon composites.
Carbon is very susceptible to environmental deterioration at high temperatures in both oxidizing and reducing environments. Therefore, emphasis was placed on the evaluation of coatings based on ceramics that would act as a barrier to the test environments. The specimens consisted of a multiply lay-up in the form of flat coupon. The specimen dimensions were nominally: 22 cm long x 3 cm wide x 3 to 4 mm thick. Typically six to nine specimens of each material were exposed and evaluated in each environment.
Environments
For evaluation purposes, specimens were cycled in two nominal ranges of temperature: (1) High temperature (1400 C - 500 C) and (2) Low temperature (800 C - 450 C). Thermal conditioning environments utilized were (1) air, (2) reduced partial pressure air (less than 10 torr) and (3) gas mixtures involving argon as an inert gas with hydrogen and oxygen as reactive gases. Selected cycles were also conducted with periodic exposure to room temperature at both high (100%) and low (40%) relative humidity.
Evaluation
The initial and final dimensions, mass and physical appearance of the specimens were recorded. Mass changes were plotted versus exposure time to determine the rate of degradation. Visual indications of change were very important since it helped to characterize the mode and distribution of attack. Common surface discontinuities observed included cracking, spalling, pinholes, bubbles, bumps, and discoloration.
Thermal Cycling Profiles
Figures 3 and 4 show the typical thermal cycling profiles obtained. The first profile shows a rapid increase in temperature to approximately 1300 C from near ambient temperature (<250 C).. This is then followed by a slow cool down cycle with a hold at about 600 C. The second profile involved two rapid heating cycles. They had an intermediate rapid cool down and hold periods. Following the second heating cycle the specimen was rapidly cooled to near ambient conditions.
Figure 3
Figure 4Coated Carbon/Carbon Composites (CCC)
These materials were characterized by loss of coating continuity followed by substrate deterioration. Typically, the coatings blistered,
developed cracks and then spalled (See Figure 5). In some cases,
a glassy sublimate was observed around the cracked areas of the
coating as shown in Figure 6. Following spalling large areas of
the substrate were exposed leading to massive deterioration (Figure
7). Spalling was typically the synergistic effect resulting from
combined thermal cycling and environmental exposure. The thermal
stresses and coating cracks induced by the temperature cycling
provided a path for the environment to the substrate.
Figure 5
Figure 6
Figure 7Subsequent temperature cycling accelerated coating deterioration and substrate attack. In most cases, the mismatch between the thermal expansion coefficients of the coating and the substrate constitutes the driving force for this behavior.
Figure 8 shows a typical response of several materials to the
high temperature cycle in an oxidizing environment. This shows
a decrease in specimen mass with number of cycles. The change
in mass ranged from 4 percent to over 20 percent. The effect of
the catastrophic failure and spalling of the coating system can
be seen in the bottom curve. In this case, the coating exhibited
failure after two cycles which results in a mass loss from about
2 percent after two cycles to 13 percent after the third cycle.
By comparison, the upper curves were typical of materials that
had relatively protective coating which minimized mass loss to
between 2 and 7 percent
In some cases, the coatings developed blisters and pinholes during
manufacturing or during initial exposures presumably from outgassing
(Figure 9). These pinholes can provide a path for diffusion of
the environment to the substrate if they were continuous defects
resulting in mass loss of the substrate eventhough the coating
does not disbond or spall. However, in many cases, this type of
degradation was not found to be as severe as for cases where catastrophic
coating failure was observed. However, in some cases, the build-up
of gas pressure between the coating and substrate could also promote
accelerated degradation.
Figure 9
In reducing atmospheres (low partial pressure oxygen and argon/hydrogen
mixtures) produced generally less attack than did the oxidative
exposures conducted in air. Blistering of the coatings was less
obvious under these conditions. Cracking of the coating was the
predominant degradation mode. In these cases, it appeared that
the temperature cycling (i.e. thermal expansion/contraction mismatch)
was the primary limitation in coating performance more so than
environmental degradation.
The addition of intermittent humidity exposures did not produce significant effects on the mode of oxidative attack. The only significant change was an increase in mass after each humidity exposure presumably by the absorption of moisture into the specimen. In the case of the reducing environments, the attack following the humidity exposure appeared to be lessened.
Based on the investigation presented herein the following conclusions
were made: