Measurement of microscale residual stresses in multi-phase ceramic composites using Raman spectroscopy

doi:10.1016/j.actamat.2017.03.015

Acta Materialia

Volume 129, 1 May 2017, Pages 482-491

https://doi.org/10.1016/j.actamat.2017.03.015 Get rights and content

Abstract

A methodology is described for characterizing the spatial distribution of thermal mismatch stresses at grain level in B₄C-SiC-Si ceramic composites using Raman spectroscopy. Unlike traditional methods to detect residual stresses (e.g., X-ray diffraction) which provide average values of stress over the entire specimen surface, Raman peak-shift analysis provides residual stress distributions within the microstructure at high spatial resolution. While classical formulation predicts uniform compressive stress within a Si-phase surrounded by the ceramic matrix, the Raman measurements revealed non-uniform residual stress distributions in Si when the particle size was larger than 5 microns. For large irregular shaped particles, the two methods coincide only along the interface between the particle and matrix, but vary drastically both in magnitude and nature in the interior of the particle where large tensile stresses have been measured. The presence of anomalous tensile stress in the interior of the minor Si-phase results in defect generation and structural disorder which has been confirmed by TEM analysis. Raman spectroscopic mapping was also used to compute an average macroscale residual stress value for a given material composition allowing links to be drawn between processing, microstructure and properties. The average residual stress within the microstructure was found to correlate well with the estimates based on volume fraction of the constituents and material properties.

Graphical abstract

Introduction

Structural ceramics, such as boron carbide (B₄C) and silicon carbide (SiC), offer exceptional mechanical properties and have applications ranging from lightweight armor to wear-resistant tooling [1], [2], [3]. These ceramics are commonly produced by hot pressing or pressureless sintering. Recently, reaction bonding has become a promising alternative because it offers rapid fabrication times, reduced processing temperatures, and can produce complex, near net-shape parts [4], e.g., a one-piece ceramic helicopter seat [5], [6]. In this method, a porous ceramic (B₄C and/or SiC) powder preform is placed in a vacuum furnace along with Si lumps. As the temperature is raised above the melting point of silicon (1410 °C), the molten silicon (Si) infiltrates the ceramic preform [7] and reacts with a carbon (C) source (free carbon or residual carbon in B₄C) to form SiC. The reaction-formed SiC and residual (unreacted) Si bond the microstructure together. Due to the complex microstructure of reaction bonded ceramics, an in-depth understanding of microstructure-property relationships is crucial for their effective use in intended applications.

An important consideration in these multiphase ceramic composites processed at high temperatures is residual misfit stress that evolves due to coefficient of thermal expansion mismatch and in some cases, even defect generation [8]. Patent literature [9] has revealed that residual compressive stresses in the Si phase can be leveraged as a “toughening” mechanism. Because Si is the weakest mechanical constituent in the composite microstructure it is of utmost interest in this study [10], [11].

The current work is motivated by three considerations: (i) Limited work has been reported on thermal mismatch stresses in reaction bonded materials, especially at microstructural level; (ii) A method for evaluating the spatial distribution of stress across different phases of varying size and shape is not readily available. Such an effort is critically important to understand the failure behavior of multi-phase composites. Traditional techniques to measure residual stresses, such as X-ray diffraction, yield only global average values and cannot provide stress distributions at the micron-scale with high spatial resolution. Lastly, (iii) classical formulation for calculation of residual stress in a small region (or a particle) surrounded by semi-infinite matrix assumes a perfect spherical geometry and uniform residual stress within the particle. In reality, these particles occur in various shapes and sizes, and the residual stress within the particles cannot be assumed constant. In this manuscript, a methodology is described to quantify the magnitude and spatial distribution of residual stress at micron-scale spatial resolution using micro-Raman spectroscopy on the Si phase of the reaction bonded B₄C and SiC ceramics. Transmission electron microscopy (TEM) was then used to identify structural disorder resulting from these stresses.

Section snippets

Materials and microstructure

Three reaction bonded ceramics (see Table 1 ), with a naming convention based on starting powder composition, were examined: (i) B₄C powder (BC ceramic), (ii) B₄C-SiC powders (BSC ceramic), and (iii) SiC powder (SC ceramic). Two types of BC ceramics were assessed: one contained only B₄C particles with no preform additives (BC-1) and the other contained 10 wt% of diamond particles (BC-D) in the preform as an additional carbon source to reduce the residual Si content [12]. Three BSC ceramics with

Theoretical estimation of residual stress

If the microstructure is assumed to be an assembly of uniformly sized, elastic, spherical Si particles (p) surrounded by a semi-infinite ceramic (B₄C or SiC) matrix (m), the magnitude of residual stress ( $σ$ ) in Si regions can be approximated using the following classical equation [25], $σ_{m - p} = - \frac{Δ α \cdot Δ T}{[\frac{0.5 (1 + v_{m}) + f_{p} (1 - 2 v_{m})}{E_{m} (1 - f_{p})} + \frac{1 - 2 v_{p}}{E_{p}}]}$ where the misfit stress developed in the particle is a function of the difference ( $Δ T$ ) between processing and ambient temperatures, elastic modulus (E) and Poisson's

Residual stress

The vast differences in thermo-mechanical properties of B₄C, SiC, and Si, shown in Table 2, cause thermal residual stress in the microstructure. Because residual Si is the weakest mechanical phase in the composite, it was of utmost interest in this analysis. Shown in Fig. 2 are Raman spectra for a silicon standard (reference material), raw Si used for infiltration (prior to reaction bonding) in its bulk and powder form, and selected scans of residual Si in a reaction bonded ceramic with

Conclusions

Raman spectroscopy has been effectively used in this investigation to assess residual stress and the level of defectiveness developed in processed multi-phase ceramics. The method provides a convenient route for characterizing local microscale stress heterogeneity at high spatial resolution. It was determined that Si regions smaller than 5 microns in diameter are in a nearly uniform state of compression as predicted by analytical equations. However, the classical formulation breaks down when

Acknowledgements

Financial support for Phillip Jannotti from the National Defense Science and Engineering Graduate (NDSEG) Fellowship program was provided with Government support awarded by DOD, AirForce Office of Scientific Research, NDSEG, 32 CFR 168a. The authors sincerely acknowledge the support from the Department of Army, US Army RDECOM contract no. W91CRB-10-D-0001-0006.

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Full length articleMeasurement of microscale residual stresses in multi-phase ceramic composites using Raman spectroscopy