To obtain better performance from materials which are used in harsh environments, their mechanical attributes with respect to their microstructure must be well established. This can be accomplished using the XPM™ accelerated property mapping method from Bruker along with the xSol® High-Temperature Stage, which allows fast and dependable measurements at high temperatures.
XPM is a measurement technique which employs nanoindentation to gather a large number of quantitative values rapidly, at up to six a minute. The indentations are created in a gridstatistics pattern so that statistics are obtained on property distribution very quickly, and localized mechanical properties are mapped as to spatial distribution.
The importance of XPM measurements is their speed, which prevents or minimizes several crucial problems that could otherwise complicate nanoindentation testing at high temperatures, such as the occurrence of chemical reactions which may both blunt the probe and cause sample breakdown. On the other hand, by increasing the speed of measurements 500-fold, XPM maximizes their value. In addition, thermal drift creates minimal or insignificant effects at this speed of testing.
Figure 1. Optical image of 400 °C testing location (left) used to find a suitable testing location. SPM image post XPM testing (center) where the circular regions are SiC fibers embed within the SiC matrix. 400 indents were performed in both the fiber and matrix material. The resulting property map of hardness (right) shows a higher hardness in the matrix material and edge effects around the peripheral of the fibers.
In the current experiment, a Hysitron® TI 980 TriboIndenter® fitted with an xSol® High-Temperature Stage was used with the XPM to examine the mechanical properties of a silicon wafer as well as a fiber-matrix composite of silicon carbide at a temperature between 400 °C and 800 °C. For this purpose, a diamond Berkovich probe was selected.
The samples were placed in an atmosphere of 95% argon, 5% hydrogen throughout the test period. A tip area function was carried out both pre- and post-test on fused quartz, but there was no tip degradation to a measurable extent.
On the silicon sample, an indentation grid of 10 x 10 was created at each of the temperatures, using a peak load of 7 mn and indents spaced at 5μm. Each 100-indent grid was completed in around 30 seconds.
On the silicon carbide sample, each grid contained 20 x 20 indents at 750 nm spacing, and with a peak load of 4 mN, being created in about 100 seconds. The grid included both matrix and fibers during the same test.
To ensure proper location of the grid, both optical imaging and scanning probe microscopy (SPM) were utilized, as shown in Figure 1.
In Table 1, the results of measurement of the hardness and elastic modulus are given, both at 400 °C and 800 °C. At room temperature, silicon has a hardness quite comparable to that at 400 °C, but there was a steep decrease at 800 °C. Figure 2 shows post-XPM SPM imaging of silicon performed in situ at 800 °C.
In Figure 3 the results of testing of the composite fiber-matrix material of silicon carbide are shown in the form of a histogram of the reduced modulus and hardness of this material. The matrix increased in reduced modulus and hardness at both temperatures, but the hardness was reduced to a much greater extent as the temperature went up. At 800 °C the fiber and matrix were almost equal in hardness.
A double Gaussian curve was fitted to the histogram values to generate the hardness and the elastic modulus. The adjusted R-squared values lay between 0.91 and 0.98, and are seen in Table 1.
Table 1. Mechanical properties and standard deviations.
Figure 2. SPM image of silicon taken at 800 °C following XPM testing.
Figure 3. Histograms of silicon carbide fiber-matrix composite hardness and elastic modulus results obtained from XPM indentation testing at 400 °C and 800 °C. Fiber properties remain relatively constant over the temperature range, while the modulus and hardness decrease by approximately 18% and 40%, respectively.
Silicon increases markedly in hardness as the temperature rises, and this is due to a change from brittleness to ductility. In the case of silicon carbide, the mechanical properties of the matrix and fiber do not change together, as the matrix shows a much slower increase in hardness with rising temperature than the fiber, which therefore remains stronger at the higher temperature.
The Bruker xSol heating stage used in combination with XPM permits mechanical property assessment at high speed, so that the localized properties of any material can be fully characterized and adjusted to match the operating temperature at which it is designed to function.
This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.
For more information on this source, please visit Bruker Nano Surfaces.