CAPITULO II: MARCO TEÓRICO
2.4. DIFERENCIAS ENTRE EL INVIERTE.PE Y EL SNIP
TEM imaging and analysis is required to determine the structure of the interface. Fig 5.18. shows a TEM micrograph of UDM4. the smooth line being indicative of interface formation by reaction with the fibre as stated in Chapter 2. ( Note : UDM4 contains NL-201 fibres ). Using light element analysis the constitution of the interface can be investigated as presented in Fig 5.18. for UDM4. The presence of a carbon enriched interfacial layer is indicated by the light element traces, even though other elemental species (especially oxygen ) exist in this layer. For the fibre analysis some matrix elements are present, representing diffusion of magnesium and aluminium into the fibre and this can be seen by the darker layer extending into the fibre ~ 110- I80nm. As was the case in Fig 5.9. for CF2, ion beam damage of the matrix has occurred and can be seen in Fig 5.18.
For composites processed with a matrix from the commercially sourced CDM powder, as with the BGMC powder, similar results are attained, with the
presence of the carbon interfacial layer being detected in all cases. This can be seen in Fig 5.19. which shows a micrograph and analysis results for CUDM7 ( Note : CUDM7 contains NL-607 fibres ). The analysis shown in this figure, as for Fig 5.18.. shows that carbon enrichment occurs at the interface. It can also be seen that the diffusion of matrix elements into the fibre is lessened in this case. However, the analysed area is further into the fibre than for UDM4. As for UDM4, other elemental species are present at the interface. As was the case for the TEM micrograph of CF2, Fig 5.9., a band can be observed on the micrograph presented in Fig 5.19. around the interface region. This is an artefact of the printing process and does not represent a real specimen effect. Upon thermal aging treatment to 700°C the effect upon the interface is as indicated in Fig 5.20. Again on this micrograph a band is present around the interface region. This, as in the above case, is an artefact of the printing process. From the analysis results it can be seen that the carbon enrichment is no longer present at the interface. The interfacial structure is different form the non-aged specimens and analysis shows that silicon and oxygen are the major elements present in areas such as that shown, with smaller amounts of magnesium and aluminium. Hence, for this case, a silica bridge has formed across this part of the interface, although the bridging is not complete for this aging temperature. Further examination of the sample shows areas with carbon rich interfacial regions as is observed in the non-aged composite ( Fig 5.19. ). For this composite aged at !000°C a silica plug is formed at the interface near the edges of the composite, and SEM x-ray linescans indicate no carbon at the interface with only silicon and oxygen present in major amounts with small residual amounts of magnesium and aluminium.
These results presented above relate directly to the lhr crystallisation isotherm. However, similar results were obtained for the 2 and 3hr isotherm composites and hence they are believed to be generic. The longer processing times can not only affect the degree of crystallisation of the matrix, but can change the interfacial development also. For the 3hr isotherm analysis indicates a carbon rich interfacial layer, which has a thickness ~35-7()nm, but the interface in some areas shows evidence of bubble formation, as will be discussed at the end of this chapter.
Figure 5.19. TEM micrograph and light element analysis results for CUDM7.
Figure 5.20. TEM micrograph and light element analysis results for CUDM7 after aging at 700°C for lOOhrs.
of the interface. The remainder of this section concentrates on the micromechanical response of the interface. The micromechanical properties of the debond energy 2 f and
the shear sliding resistance T, were measured as described in Chapter 3., Section 6., with an example of an indented fibre shown in Fig 5.21. This figure illustrates how accurately the indenter tip can be positioned on the fibre surface. Normally the fibre would not show any radial cracks and the test results are not used after cracking has occurred. The results of this evaluation are presented in Table 5.1., and are based on an average of ten indents per sample. For the sample aged at 1000°C it should be noted that indentations taken within a few fibre diameters from the sample edge produced very different results with 2 r and T much higher than those for the middle of the sample
as presented in the table. This indicates that oxygen ingress into the sample causes a marked effect on the interfacial properties to within a few fibre diameters of the edge of the sample.
The table shows the marked difference between the NL-607 and NL-201 fibre types within the composite and this may be responsible for the differences in the mechanical response between these composites ( see Chapter 6. ). This difference can
■Sample designation and stale MAS / NL - 201 as fabricated MAS / NL - 607 as fabricated aged 450°C aged 700°C aged 1000°C CUDM18 unidirectional CUDM19 unidirectional Micromechanical Properties. 2 r (Jnv2) t (MPa). 30.4±7.2 239±150 12.4±5.4 48± 15 13.6±4.4 108±54 35.6±29.2 2481120 14.2±8.4 4217 2.6±1.6 4519 8.2±3.8 70116
Table 5.1. Micromechanical property measurements for composites in this study. be further illustrated by the indenter load versus tip displacement plots shown in Fig 5.22. Results for the thermal aging experiments show the differences in the interfacial response due to exposure in air. and are shown graphically in Fig 5.23. This illustrates the rise in micromechanical properties at intermediate temperatures for composites fabricated with NL-607 fibre and the 3hr isothemi.
L o a d (N) MAS/NL-201. L o a d ( N ) MAS/NL-607.
Figure 5.22. Load versus tip displacement plots for the NL-201 and NL-607 fibres. I 19
Energy (Jm:). 7 0 - , In t c rf a c ia l D c b o n d E n e r g y ( 2 T ) .
1 ... '
1 0 0 2 0 0 3 0 0 4(X) 5(X ) 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 Temperature (°C). Stress (MPa). 4 0 0-, 3 5 0 - 300^ 2 5 0- 200- 1 5 0 - 100- 5 0 0 In t c rfa c ia l S h e a r S lid in g S t r e s s (t). I 0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 8 0 0 9 0 0 1 0 0 0 1 1 0 0 Temperature (l'C).Figure 5.23. Graphical plot of the interfacial micromechanical properties 2T and T,
compared to thermal aging temperature.