Chapter IV: Aluminum Nitride/Boron Nitride Composites
4.3 Results & Discussion
4.3.4 Self-healing in AlN/BN Composites
4.3.4.2 Bend Strength of AlN/BN Composites
It was desired to probe how successful Al18B4O33 whiskers could be at bridging and healing cracks on the surface of the composite. Previous studies on ceramic matrix composites containing SiC and h-BN have shown that self-healing, also referred to as self-sealing or crack-healing, can be achieved when such composites are exposed to oxidizing
atmospheres [2,48,49]. Under these conditions, h-BN can oxidize into liquid-phase B2O3, also termed the healing agent, which can travel into pre-existing cracks within the composite. Once in contact with the crack walls (which are made of SiC or SiO2), the liquid oxide can react to form borosilicate. Upon cooling to room temperature, the volume expansion of the glass allows the oxidation product to seal the cracks and thus reinforce the material. Similar healing effects have also been seen in SiC/B4C [50] and SiC/B3C2
[51] composites, as well as in ZrO2/MoSi2 through the formation of ZrSiO4 as the load bearing material [52].
Figure 4.10. Images of healed AlN/BN beams exposed to dry air at 1250 °C for 48h after flexural test: surface (a) and cross-sectioned view (b). c) Indentations before treatment (yellow), after dry air heat-treatment (blue), and after dry argon heat-treatment (orange),
showing both secondary electron (top) and backscattered electron (bottom) micrographs to better illustrate the absence of cracks from the indents after heat-treatment. d) XRD
spectra and e) surface micrograph of beam exposed to dry argon at 1250 °C for 48 h, showing AlN (○), BN (◌), and Al18B4O33 (●).
The role of Al18B4O33 on the mechanical strength of AlN/BN composites was thus investigated through a serious of 4-point bend flexural tests. A summary of the measured flexural strengths can be found in Table 4.3. In this experiment, the strength of as-received beams was compared to heat-treated beams, which were exposed to either dry air or dry argon. Cracks were induced on the surface of these beams by introducing three indentations along the center of the beam (created with loads of 196 N), an example of which is illustrated in Figure 4.10a. The bend strength of the as-received bars was measured to be 223 ± 29 MPa, with failure originating from random locations in every specimen. The flexural strength of the indented AlN/BN was measured to be 139 ± 23 MPa, with failure occurring repeatedly through the indentations.
Table 4.3. Summary of measured flexural strength of AlN/BN composites.
Description of AlN/BN Beams Flexural Strength (MPa) Failure Origin
As-received 223 ± 29 Random
Indented 139 ± 23 Indentation
Indented + exposed to dry air (1250 °C for 48 h)
83 ± 24 Random
Indented + exposed to dry argon (1250 °C for 48 h)
194 ± 27 Indentation
Indentations were similarly introduced in beams subjected to heat-treatment, prior to being treated. Figure 4.10b confirms that bars exposed to dry air at 1250 °C for 48 h developed whiskers on the surface of the beam. Given the results from Figure 4.5, this temperature and holding time was selected to maximize the whisker strength, considering that hollow crystals did not develop under these conditions. When measuring the flexural strength of these indented and heat-treated beams, the origin of failure was found to be random, derived from the porous nature of the interlocking whiskers, seen in Figure 4.10b. We observed that the cracks produced by the three indentations were indiscernible from the rest of the oxide layer (seen in Figure 4.10c) and were not the origin of failure. The porosity of this oxide layer meant that the flexural strength of the oxidized beams was measured to be 83 ± 24 MPa, much lower than the previous two results.
Indented AlN/BN bars were also exposed to dry argon at 1250 °C for 48h, to develop an understanding of the effects that heating in an inert atmosphere could have on the strength of the indented composite. The flexural strength was measured to be 194 ± 27 MPa, with failure occurring repeatedly through the indentations even though Figure 4.10c shows the absence of cracks emerging from the indents after heat-treatment. The flexural strength of AlN/BN after heat-treatment in argon was significantly higher than it was before the treatment. This result is indicative of either residual stress relief of the indentation cracks or self-healing through oxide growth, and thus prompted further chemical and microstructural investigation. X-ray diffraction seen in Figure 4.10d suggested that small quantities of Al18B4O33 developed on the surface of the beams after heat-treatment in dry argon, despite the gas only containing 2 ppm of O2 and H2O combined. This result highlights the effectiveness of O2 and H2O in oxidizing h-BN and AlN, which might have been further promoted by B2O3 vapors in the system and the Al2O3 crucibles holding the samples. Scanning electron micrographs in Figure 4.10e show crystals of Al18B4O33
forming on the surface of the beam, where the widths of the grains remain below 2 µm. It is thought that the improved flexural strength of AlN/BN heat-treated in dry argon originates from the development of small Al18B4O33 crystals on the crack walls, which delayed the extension of the crack through friction between the walls and improved the strength of the fractured composite. Further, residual stress relief from the indentation stress field should also be considered as an additional factor. The increased strength of the composite could have arisen from annealing effects during dry argon exposure. Lawn demonstrates a more than doubling of the indentation strength through residual stress relief [53]. The occurrence of Al18B4O33 in the system highly suggests that oxide growth is the principle mechanism for the healing of the indentation cracks, however more work needs to be done to better decouple the two factors and establish to what extent the residual stress relief has influenced the improved strength of the composite.