Microstructural observations

7.1.5. Microstructural observations

Low magnification SEM micrographs of the in-plane composite structure of Specimen 1025 containing purely submicron particles revealed large matrix shrinkage cracks in matrix rich regions (Figure 7.16). SEM micrographs of Specimen 999 containing 20% 1 micron particles revealed seemingly fewer and smaller shrinkage cracks (Figure 7.17), whilst Specimen 1015 containing 50% 1 micron particles and Specimen 1010 containing 80% 1 micron particles revealed little evidence of matrix shrinkage cracking (Figure 7.18 and Figure 7.19). Fibre tows were seemingly well infiltrated with matrix material, however large-scale pores of irregular size and distribution were evident for all specimens observed.

Figure 7.16. SEM micrograph of Specimen 1025 containing purely submicron alumina particles (0%) showing large shrinkage cracks in matrix rich regions.

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Figure 7.17. SEM micrograph of Specimen 999 containing 20% 1 micron alumina particles showing small shrinkage cracks in matrix rich regions.

Figure 7.18. SEM micrograph of Specimen 1015 containing 50% 1 micron alumina particles showing little evidence of shrinkage cracks in matrix rich regions.

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Figure 7.19. SEM micrograph of Specimen 1010 containing 80% 1 micron alumina particles showing little evidence of shrinkage cracks in matrix rich regions.

7.2. Discussion

Flexural strength values obtained for composites containing 20, 40 and 50% 1 micron particles were statistically significantly greater than composites containing purely submicron particles. Whilst values obtained for 30% 1 micron particles were not different from composites containing only submicron particles, the range of values was very large and encompassed the entire data set. It is therefore very difficult to draw any conclusions based on this data, and will be discussed in further detail below. For the purpose of discussion, composites containing 20, 40 and 50% 1 micron particles will be classified as ‘moderate amounts’ of 1 micron particles.

1 micron alumina particles were employed to reduce the number of shrinkage cracks and large-scale pores formed during particle densification and sintering. Hypothetically, submicron particles fill the inter-particle spaces between large particles, to achieve a high

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packing density and to create a non-shrinking network (Figure 7.20). This ultimately reduces the number of shrinkage cracks and pores, subsequently improving composite performance.

Figure 7.20. Microstructural design of an oxide-oxide CMC with a bimodal particle size distribution (After [42]).

The presence of a non-shrinking network was implied by calculation of linear contraction during sintering. Composites manufactured with 30 and 40% 1 micron particles revealed a significant decrease in linear contraction compared with composites containing only submicron particles. These findings suggests that moderate amounts of large particles within the matrix inhibited shrinkage during sintering, leading to less interparticle spacing and fewer shrinkage cracks, resulting in an improvement in flexural strength. Furthermore, microstructural analysis of composites containing 20% 1 micron particles revealed fewer and smaller shrinkage cracks compared with composites containing purely submicron particles, and composites containing 50% 1 micron particles revealed little evidence of shrinkage cracks. On the contrary, a significant decrease in short beam shear values obtained for composites containing 20, 40 and 50% 1 micron particles was evident. The short beam shear test is designed to induce shear stresses at the mid-plane resulting in a delamination failure, and is greatly affected by matrix strength and matrix adhesion with reinforcing fibres. As suggested previously, 1 micron particles provide a non-shrinking network that resists densification. Whilst this proved beneficial to flexural strength, incomplete densification results in both a weaker matrix and poorer fibre/matrix adhesion. Subsequently, composites

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containing a moderate amount of 1 micron particles were less able to withstand shear stresses at the mid-plane, resulting in premature delamination.

Composites containing 80% 1 micron particles demonstrated significantly poorer flexural and short beam shear strength compared with composites containing 0-50% 1 micron particles.

Microstructural observations revealed little evidence of shrinkage cracks in matrix rich regions, providing a more uniform structure. Nevertheless, it has already been suggested that the incorporation of less-sinterable 1 micron particles results in a less dense composite with poorer matrix strength. Calculation of composite density by mass/volume calculations revealed that composites containing 80% 1 micron particles were significantly less dense than composites containing only submicron particles. When a composite with an exceptionally weak matrix is loaded in three-point bend, it is possible that failure does not occur by tension at the top surface but by delamination at the mid-plane, resulting in premature failure. Further investigation would be required to confirm this suggestion.

Composites containing 30% 1 micron particles exhibited a large range in flexural strength data, and data was randomly distributed. Variability is an inherent characteristic of oxide composite materials and by the nature of three-point bend testing. Nevertheless, there is no reasonable explanation as to why flexural strength values obtained for composites containing 30% 1 micron particles should differ to any other data set. 37 specimens were tested in three-point bend which were diamond cut from three composite flat plates manufactured using the same processing technique. Bowles and Frimpong proposed that an increase in void content may lead to an increase in scatter [128], however it is apparent from Figure 7.21 that a large amount of variability occurred not only between plates, but also within plates, eliminating factors such as porosity, density or matrix volume fraction. Furthermore, frequency

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histograms illustrating the distribution of data revealed that flexural strength values obtained for composites containing 30% 1 micron particles were randomly distributed. Conversely, flexural strength values obtained for 0, 10, 20, 40, 50 and 80% 1 micron particles demonstrated a normal distribution, whereby the mean, median and mode values are approximately equal, and the data is approximately symmetrical. Normal distribution allows a large population of data to be described completely by two parameters, mean and standard deviation.

Figure 7.21. Flexural strength values for composites containing 30% 1 micron particles.

There were no observable differences in the stress-displacement response for specimens loaded in three-point bend manufactured with 30% 1 micron particles, despite a maximum strength value more than double that of minimum strength. Equally, there were no observable differences in the stress-displacement response for specimens loaded in three-point bend manufactured with 0-80% 1 micron particles. The response was typically linear elastic until peak load was achieved, after which load bearing capacity decreased significantly. A small number of specimens demonstrated a region of linear elasticity followed by non-linear

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behaviour prior to failure, revealing the presence of matrix micro-cracking prior to composite failure.

7.3. Concluding remarks

Incorporating a moderate amount of 1 micron alumina particles into slurry during manufacture improved composite flexural strength, yet proved detrimental to short beam shear strength. Composites containing 10% 1 micron particles were statistically not different from composites containing purely submicron particles in both flexural and short beam shear strength. The addition of 1 micron particles did not reveal any manufacturing advantages and will therefore not be pursued throughout the remainder of the project.

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Chapter 8 - The effect of sintering temperature on processing and