Residual stress determination of silicon containing boron dopants in ceramic matrix composites

Abstract

Raman spectroscopy was utilized to investigate residual stresses found within a SiC/SiC ceramic matrix composite containing Hi-Nicalon ™ fibers, a slurry meltinfiltrated matrix of silicon carbide particles, and silicon matrix. Large gradients of electrically active boron are found throughout various regions within the crystalline lattice of the silicon matrix. The regions were identified by the varying degrees of asymmetry and peak width measured in the resonant Fano profile of the doped silicon. A methodology to determine the residual stress state of silicon exhibiting varying degrees of electrically active boron is presented by utilizing the changes in the Raman profile parameters. Previous works on similar SiC/SiC CMCs have attributed spatial gradients in the wavenumber to large fluctuations in stress. By applying the proposed methodology, we show that these observations are related to active boron that is segregated in various matrix areas. Utilizing this methodology, mean compressive stresses in various silicon regions were found to be approximately 300 MPa, with complementary tensile silicon carbide particle stresses of approximately 300 MPa.

Measuring the effects of heat treatment on SiC/SiC ceramic matrix composites using Raman spectroscopy

“…Multiple constituents were observed before and after heat treatment using the Raman system for ex situ measurements: silicon carbide particles (which are translucent to the 514.5 nm light and allow collection of spectra from the silicon below), silicon found on the surface of the matrix, and silicon between the individual fibers within the fiber tow (ie, intratow silicon). The effective penetration depth for SiC particles using the 514.5 nm is greater than 700 µm, much larger than the SiC particles investigated, while the effective penetration depth for silicon is approximately 0.67 µm . This essentially returns a surface measurement for silicon interrogated on the surface of the CMC; however, the silicon stress state is much more complex and three‐dimensional for silicon found underneath SiC particles.…”

“…Specific details of sample preparation and Raman spectroscopy measurements can be found in previous literature . Ex situ and in situ heat treatments were both performed; ex situ testing utilized one CMC sample 25 mm × 25 mm × 4 mm in size (4 mm in the transverse direction) and one CMC sample 4 mm × 3 mm × 4 mm in size.…”

“…This line shape is described by Equation 1 below, where is the abscissa, is the Fano wavenumber, is the asymmetry parameter, is the half width at half maximum (HWHM), and is a fitting constant. Further discussion on the utilization of these parameters is found in previous work …”

“…Raman spectroscopy provides a simple and inexpensive method to determine residual stresses on a microscale in silicon and silicon carbide, both common constituents in high‐performance CMCs . Employment of these methods to determine stresses in silicon, however, requires consideration of dopants (additives) found within the silicon lattice as well as known loading conditions . Boron itself is a common dopant in silicon melt‐infiltrated composites .…”

“…As the amount of electrically active boron increases within the silicon lattice, the Fano profile becomes increasingly asymmetric, the width increases, and the characteristic wavenumber decreases . Utilizing a least‐squares fitting routine, the Fano parameters can be obtained from the Raman spectra and are readily utilized to determine the hydrostatic stress state of boron‐doped silicon . Notably, the term “hydrostatic” is used throughout this paper not to imply an isotropic stress state, but to instead refer to the mean of the normal stress components of the stress tensor.…”