Mechanical Properties and Performance of Engineering Ceramics and Composites VIII

Ceramic Engineering and Science Proceedings Volume 34, Issue 2 - Mechanical Properties and Performance of Engineering Ceramics and Composites VIII 

A collection of 21 papers from The American Ceramic Society’s 37th International Conference on Advanced Ceramics and Composites, held in Daytona Beach, Florida, January 27-February 1, 2013. This issue includes papers presented in Symposium 1 -
Mechanical Behavior and Performance of Ceramics and Composites.

• Language: English
• Publisher: Wiley
• Release date: Dec 2, 2013
• ISBN: 9781118807453

Book preview

Preface

This volume is a compilation of papers presented in the Mechanical Behavior and Performance of Ceramics & Composites symposium during the 37th International Conference & Exposition on Advanced Ceramics and Composites (ICACC) held January 27 to February 1, 2013, in Daytona Beach, Florida.

This long-standing symposium received presentations on a wide variety of topics thus providing the opportunity for researchers in different areas of related fields to interact. This volume emphasizes some practical aspects of real-world engineering applications of materials such as oxidation, fatigue, wear, nondestructive evaluation, and mechanical behavior as associated with systems ranging from rare earth aluminates to multilayer composite armor. Symposium topics included:

Composites: Fibers, Interfaces Modeling and Applications

Fracture Mechanics, Modeling, and Mechanical Testing

Nondestructive Evaluation

Processing-Microstructure-Properties Correlations

Tribological Properties of Ceramics and Composites

Significant time and effort is required to organize a symposium and publish a proceeding volume. We would like to extend our sincere thanks and appreciation to the symposium organizers, invited speakers, session chairs, presenters, manuscript reviewers, and conference attendees for their enthusiastic participation and contributions. Finally, credit also goes to the dedicated, tireless and courteous staff at The American Ceramic Society for making this symposium a huge success.

DILEEP SINGH

Argonne National Laboratory

JONATHAN SALEM

NASA Glenn Research Center

Introduction

This issue of the Ceramic Engineering and Science Proceedings (CESP) is one of nine issues that has been published based on manuscripts submitted and approved for the proceedings of the 37th International Conference on Advanced Ceramics and Composites (ICACC), held January 27–February 1, 2013 in Daytona Beach, Florida. ICACC is the most prominent international meeting in the area of advanced structural, functional, and nanoscopic ceramics, composites, and other emerging ceramic materials and technologies. This prestigious conference has been organized by The American Ceramic Society’s (ACerS) Engineering Ceramics Division (ECD) since 1977.

The 37th ICACC hosted more than 1,000 attendees from 40 countries and approximately 800 presentations. The topics ranged from ceramic nanomaterials to structural reliability of ceramic components which demonstrated the linkage between materials science developments at the atomic level and macro level structural applications. Papers addressed material, model, and component development and investigated the interrelations between the processing, properties, and microstructure of ceramic materials.

The conference was organized into the following 19 symposia and sessions:

The proceedings papers from this conference are published in the below nine issues of the 2013 CESP; Volume 34, Issues 2–10:

Mechanical Properties and Performance of Engineering Ceramics and Composites VIII, CESP Volume 34, Issue 2 (includes papers from Symposium 1)

Advanced Ceramic Coatings and Materials for Extreme Environments III, Volume 34, Issue 3 (includes papers from Symposia 2 and 11)

Advances in Solid Oxide Fuel Cells IX, CESP Volume 34, Issue 4 (includes papers from Symposium 3)

Advances in Ceramic Armor IX, CESP Volume 34, Issue 5 (includes papers from Symposium 4)

Advances in Bioceramics and Porous Ceramics VI, CESP Volume 34, Issue 6 (includes papers from Symposia 5 and 9)

Nanostructured Materials and Nanotechnology VII, CESP Volume 34, Issue 7 (includes papers from Symposium 7 and FS3)

Advanced Processing and Manufacturing Technologies for Structural and Multi functional Materials VII, CESP Volume 34, Issue 8 (includes papers from Symposium 8)

Ceramic Materials for Energy Applications III, CESP Volume 34, Issue 9 (includes papers from Symposia 6, 13, and FS4)

Developments in Strategic Materials and Computational Design IV, CESP Volume 34, Issue 10 (includes papers from Symposium 10 and 12 and from Focused Sessions 1 and 2)

The organization of the Daytona Beach meeting and the publication of these proceedings were possible thanks to the professional staff of ACerS and the tireless dedication of many ECD members. We would especially like to express our sincere thanks to the symposia organizers, session chairs, presenters and conference attendees, for their efforts and enthusiastic participation in the vibrant and cutting-edge conference.

ACerS and the ECD invite you to attend the 38th International Conference on Advanced Ceramics and Composites (http://www.ceramics.org/daytona2014) January 26–31, 2014 in Daytona Beach, Florida.

To purchase additional CESP issues as well as other ceramic publications, visit the ACerS-Wiley Publications home page at www.wiley.com/go/ceramics.

SOSHU KIRIHARA, Osaka University, Japan

SUJANTO WIDJAJA, Corning Incorporated, USA

Volume Editors

August 2013

Characterization and Modeling of Ceramic Matrix Composites

ACOUSTIC EMISSION AND ELECTRICAL RESISTIVITY MONITORING OF SIC/SIC COMPOSITE CYCLIC BEHAVIOR

Christopher R. Baker and Gregory N. Morscher

Mechanical Engineering Department, The University of Akron, Akron, OH

1. Abstract

Successful implementation of SiC/SiC composites in rotating components demands an excellent understanding of their cyclic stress and fatigue behavior. Two fiber-reinforced melt-infiltrated composites were tested under several cyclic stress conditions. In situ acoustic emission and electrical resistance was used to monitor damage accumulation and understand time and stress dependant degradation of mechanical properties. Post test microscopy and fractography was then used to observe microstructural anomalies which suggest possible damage progression mechanisms. Electrical resistance measurements were in good agreement with decreasing interfacial shear stress and show potential as a monitoring technique for fatigue related damage.

2. Introduction

Ceramic matrix composites (CMCs) are being investigated for potential use in the hot section of aerospace and power generating turbines. Their exceptional thermal stability at elevated temperatures (~1200°C) makes them uniquely able to allow current hot-section temperatures to be raised. However, should CMCs ever be able to be implemented in rotating components, an excellent understanding of fatigue performance and damage accumulation is necessary. This is made more difficult in the presence of damage (e.g. matrix cracking, fiber pull-out, etc.). The mechanics of loading, unloading and reloading after damage have initiated have been examined in the literature¹,². Similarly, cyclic loading effects on the interphase have been investigated and are an important part in the mechanical behavior of a damaged CMC³.

Non-destructive techniques have been investigated to better understand the damage initiation, progression and intensity in many CMC systems³–⁵. These include acoustic emission (AE), electrical resistance (ER), flash thermography and acousto-ultrasonics. ER techniques are particularly novel and a cohesive mechanical-electrical model has not yet been developed, although a few have been suggested. The models suggested typically rely on factors such as pristine material resistivity, number of cracks per unit distance, stress, etc.

In this work, two different materials were loaded cyclically in different loading parameters. With the aim of understanding the physical nature of the damage accumulation through non-destructive health-monitoring techniques, each test was monitored using Modal Acoustic Emission and ER. Comparison of the pristine mechanical behavior and cyclically loaded material through these measurements, one may be better able to understand the true nature of the damage accumulation in CMCs.

3. Experimental

Two balanced 0°/90° 5-harness-satin liquid silicon infiltrated composites were tested in varying cycling conditions. All composites are slurry cast SiC/SiC composites each with a different fiber type: Sylramic-iBN* and Tyranno ZMI**. Fiber volume fractions were determined by dividing the cross-sectional area of the composite by the fiber area (determined by multiplying fiber area, number of fibers per tow, number of plies and tows per ply.) All composites were manufactured by Goodrich*** and machined into dog-bone coupons. Mechanical, physical and geometric properties are listed in Table I. Coupons were loaded/unloaded at a rate of 6kN/min. The ZMI reinforced composite underwent 110 total cycles and the Sylramic-iBN composite was loaded/unloaded 250 cycles.

Table I. List of physical and mechanical properties of each composite

Strain was measured using an 1" (25.4mm) extensometer with ±2% travel on an MTS self-aligning system with hydraulic pressure grips. ER was measured using an Agilent**** 34420A digital multimeter over the gage length of 25.4 mm. AE signals were acquired using a Digital Wave***** Fracture Wave Detector. Wide-band sensors (50kHz-2MHz) were used and a threshold crossing technique was used to determine lengthwise location of events, similar to a technique used by Morscher⁶. Only events occurring in the gage section (middle 25.4 mm) were used in the analysis. Figure 1 shows the typical layout for a coupon prior to testing.

Figure 1. Experimental layout of a CMC prior to testing

4. Results and Discussion

Stress-strain behavior is shown in Figure 2 for each of the composites. The composites were loaded for several cycles under a particular stress, then loaded to a higher stress for several more cycles. This pattern continued, as shown in Figure 2. Note that none of the composites were loaded to failure. The higher strains accumulated in the ZMI composite are due to the lower fiber modulus compared to the Sylramic-iBN. Interestingly, strain seems to accumulate both at the zero stress condition and the fully loaded condition more with each cycle. This is consistent with both samples, and the interphase degradation is likely the reason for such behavior.

Figure 2. Stress Strain curves for both composites.

4.1. Acoustic Emission and Damage Accumulation

Acoustic emission has been shown to be effective at quantifying matrix cracking in various CMCs⁷. In particular, the cumulative AE energy (summation of the energies of each successive event) has been shown to have a direct correlation with stress-dependent crack density because high energy events, which dominate the cumulative energy curve, correspond to large matrix cracks. The acoustic and electrical behavior for each composite is shown in Figure 3. Figure 3a and 3b show that the most of the acoustic energy, i.e., matrix crack formation, occurs during the first cycle loading at each stress level. For the ZMI composites, high energy events occurred on the second and third cycles at the lowest stress condition (Figure 3). However, after the composite reached the crack saturation stress (~ 140 MPa), no further significant high energy events were detected. For the Sylramic-iBN composite (Figure 3b), high energy events did not occur after the first stress cycle.

Figure 3a. ZMI composite AE and ER behavior

Figure 3b. Sylramic-iBN composite AE and ER behavior

4.2. Interfacial Shear Stress Degradation

Rouby and Reynaud³ proposed an exponential decay of the interfacial shear stress, τ, as a function of cycles. This is bounded by a lower limiting value, τ∞. It takes the following form:

(1) equation

Where, for each composite system, τ0, t and τ∞ are unique parameters. In this work, τ was calculated for each cycle by combining equations (3) and (7) in Curtin² and is given by:

(2) equation

Where:

(3) equation

and r is the fiber radius, σt, is the total stress (applied and residual), ρc is the crack density. Fiber volume fraction used in the equation is only the fiber volume fraction in the loading direction (i.e. half the total fiber volume fraction.) Crack density was found using the technique of Morscher⁷, wherein a fraction of total accumulated AE energy is normalized by total cumulative AE energy, then multiplied by the saturation crack density, giving a stress-dependent crack density. Saturation crack densities were found from post-test microscopy of similar materials. Matrix modulus was calculated using a rule of mixtures approach, that is:

(4) equation

The values of τ are shown in Figure 4. Curve fitting parameters adopted from the model in (1) are also shown. Note that τ values for the first 50 cycles of the Sylramic-iBN coupon resulted in no cracking, so the chart considers the first cycle at 141 MPa to be the first cycle. the power-law fits the data reasonably well.

Figure 4. ISS cyclic degradation

4.3. Electrical Resistance

After the first cycle at a particular stress, the acoustic activity is negligible, although the ER continues to increase. Looking again at Figure 2, we can see that strain continues to accumulate. This suggests that what controls increasing ER is what causes increasing strain accumulation, which is most likely the continued wear of the interphase, responsible for the decrease in ISS shown in Figure 3. Also, cycling in the elastic region of the Sylramic-iBN composite yielded no significant change in resistivity, indicating that ER is more sensitive to damage than due to a stress response of the material.

Figure 5 shows conductivity (the reciprocal of the resistivity) and the decrease in ISS due to cyclic loading for the ZMI and Sylramic-iBN composites under the highest stress condition (ZMI) and intermediate stress condition (Sylramic-iBN). The two properties decrease very similarly with increasing number of cycles. Also, in the initial cycles of each composite, there is a recovery, to some degree, of the electrical resistance to a lower value upon unloading (see inset in Figure 3a). As cycling continues, this ER recovery becomes smaller and eventually becomes completely negligible. Since the matrix is generally much more conductive than the fiber, interphase or CVI SiC (due to the presence of continuous Si), the resistivity of the composite largely relies on the ability of the matrix to conduct and transfer electric current. As cycling progresses, conductivity of the composite decreases, and is likely associated with the increasing difficulty of current transfer to the matrix in a matrix segment between matrix cracks. A possible mechanism is the cracks not fully closing on unloading, but this needs further investigation.

Figure 5. Conductivity-ISS relationship; ZMI (top) and Sylramic-iBN (bottom)

Observing the inlay of Figure 3a, the ER consistently recovers some conductivity upon unloading, and increases to new maximum value each cycle on loading. Because there is very little new acoustic activity after the first cycle, we can deduce that the increase in resistivity isn’t due to new matrix crack formation, but rather an increase in fiber-matrix contact resistance, which is, in turn, associated with reduced ISS, shown in Figure 5. Presumably, interfacial wear reduces fiber-to-matrix contact and less amount of current carried by the matrix.

After the composite was fully cycled, monotonic tensile loading to failure was performed on the Sylramic-iBN to compare fracture surfaces to a non-cycled monotonic test of the same material. Post-test micrographs are shown in Figure 6. It can be easily seen that the degree of fiber pull-out is more significant in the post-cycled composite than the monotonic to-failure composite. The larger scale if fiber pull-out indicates that there is, in fact, a degradation of the ISS due to cyclic loading.

Figure 6. Figure showing fracture surface of cycled Sylramic-iBN composite (left) and non-cycled Sylramic-iBN Composite (right)

5. Conclusion

It has been shown that there is a reduction of the interfacial shear stress due to cyclic loading, indicated by accumulated strain in the composite, fracture surface comparison and increased electrical resistivity. A power-law model of the decreasing ISS is appropriate for these composites, and may be appropriate for the ER accumulation due to cycles. Furthermore, there appears to be a direct correlation between τ and the electrical conductivity of the composite. Future modeling of damage in composites should contain changing interphase condition as a function of loading cycles, whether hysteretic or constant load.

6. References

1. Pryce, a. W., & Smith, P. a. (1993). Matrix cracking in unidirectional ceramic matrix composites under quasi-static and cyclic loading. Acta Metallurgica et Materialia, 41(4), 1269–1281. doi:10.1016/0956-7151(93)90178-U

2. Curtin, W. A., Ann, B. K., & Takeda, N. (1998). Modeling Brittle and Tough Stress-Strain Behavior in Unidirectional Ceramic Matrix Composites, 46(10), 3409–3420.

3. Rouby, D., & Reynaud, P. (1993). Modification During Load Cycling in Ceramic-Matrix Fiber Composites, 48, 109–118.

4. Smith, C., Morscher, G., & Xia, Z. (2008). Monitoring damage accumulation in ceramic matrix composites using electrical resistivity. Scripta Materialia, 59(4), 463–466. doi:10.1016/j.scriptamat.2008.04.033

5. Morscher, G. N., & Baker, C. (2012). GT2012-69167 Use of Electrical Resistivity and Acoustic Emission to Assess Impact Damage States in Two SiC-Based CMCs. ASME Turbo Expo 2012, 1–6.

6. Morscher, G. N. (1999). Modal acoustic emission of damage accumulation in a woven SiC / SiC composite. Composites Science and Technology, 59, 687–697.

7. Morscher, G. N. (2004). Stress-dependent matrix cracking in 2D woven SiC-fiber reinforced melt-infiltrated SiC matrix composites. Composites Science and Technology, 64(9), 1311–1319. doi:10.1016/j.compscitech.2003.10.022

*Dow Corning, Inc., Midland MI

**Ube Industries, Japan

***Brecksville, OH

****Santa Clara, CA

*****Centennial, CO

CHARACTERIZATION OF SIC/SICN CERAMIC MATRIX COMPOSITES WITH MONAZITE FIBER COATING

Enrico Klatt¹, Klemens Kelm², Martin Frieß¹, Dietmar Koch¹, and Heinz Voggenreiter¹

¹Institute of Structure and Design, German Aerospace Center (DLR), Stuttgart, Germany

²Materials Research Institute, German Aerospace Center (DLR), Cologne, Germany

ABSTRACT

Non-oxide ceramic matrix composites (CMCs) based on silicon carbide fibers (Tyranno SA3; UBE Industries Ltd., Japan) were manufactured via PIP-process using a polysilazane precursor. Prior to PIP-process, SiC-fabrics (two fabric types: PSA-S17I16PX and PSA-SI 7F08PX) were coated with oxidation resistant monazite (LaPO4) which was derived from rhabdophane solution. Two different monazite coating processes were used: Foulard coating (FC) and dip coating (DC). Several iterations have been performed to reach the final coating thickness of ~100 nm and ~300 nm for FC and DC, respectively. Evolution of monazite coating for FC/DC was determined by calculating theoretical coating thickness and scanning electron microscopy (SEM) and differences will be discussed. After PIP-process, CMC materials were mechanically (3pt.-bending test) and microstructurally (SEM, TEM) characterized before and after exposure to air (T = 1100°C, 20 hr). Before exposure, CMC materials with homogeneous fiber coating exhibit moderate strength (~150–170 MPa) and damage tolerant behavior with significant fiber-pullout. After exposure, strength degradation of only 16%, less damage tolerant behavior and less fiber-pullout could be observed. Reasons for degradation might be silica formation which was observed on fibers and matrix as well as aluminum (incorporated in Tyranno SA3-fiber as a sintering aid) which diffused into fiber coating. Possible oxidation mechanisms will be presented and discussed.

INTRODUCTION

Non-oxide CMCs like SiC/SiC materials exhibit very high specific strength and excellent high temperature resistance. Nevertheless, one of the outstanding disadvantages is the intrinsic lack of oxidation resistance. The application of oxidation resistant fiber coatings