The goal of this PhD thesis was to develop dense aluminium compound-cubic boron nitride
composites with a high cBN content. To achieve this goal, two different strategies were used:
infiltration of cBN preforms and hot pressing of cBN-Al mixtures. The particle size of the cBN
and the amount of aluminium were systematically varied and the influence of these parameters
on densification and selected properties was evaluated. A basic understanding of the product
that was formed over certain temperatures and times provides information that can be used in
optimizing the infiltration and hot pressing of cBN with Al. For this reason, the reaction kinetics
between Al and cBN was initially investigated.
The reaction kinetics of the chemical interactions between Al and cBN was investigated in detail
using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The reaction was
studied using samples containing 50 volume percent of Al and 50 volume percent of cBN (12 μm)
hot pressed at 800 oC. The prepared samples were allowed to react isothermally at temperatures
between 1 000 oC and 1 400 oC under nitrogen (N2) and argon (Ar) atmospheres. It was found
that the degree and rate of the reactions increased with increasing temperature in both N2 and
Ar atmospheres. The degree of aluminium nitride (AlN) formation was considerably higher
under N2 than under Ar. The difference in the formation of AlN between the two atmospheres
was attributed to the reaction of N2 gas with the sample due to the open porosity. The infiltration of partially hexagonalized cBN matrix with molten Al was studied. The samples
were found to have a density higher than 97% of the theoretical density. It was found that the
amount of soft hBN phase present in the sample (due to hexagonalization) increases at
temperatures higher than 1 300 oC, resulting in a hardness of the final material of Hv10 = 6.5 ±
4.8 GPa. Because of thie poor hardness this route was abandoned.
Cubic boron nitride powder (12 μm) without hexagonal boron nitride (hBN) was also infiltrated
with aluminium. The infiltrated samples were found to exhibit a density higher than 96% of the
theoretical density. The Al reacted with cBN and no hBN was observed at temperatures below
1 400 oC. The resulting product showed a Vickers hardness of Hv10 = 14.4 ± 1.6 GPa compared
with Hv10 = 6.5 ± 4.8 GPa obtained with the partially hexagonalized cBN matrix. Infiltration of
3 μm cubic boron nitride increased the Vickers hardness to 22.0 ± 0.6 GPa. However, this
infiltration was not very reproducible. Al-cBN cermets were hot pressed at temperatures between 800 oC and 1 750 oC and at a
pressure of 50 MPa in vacuum. The effect of the particle size of the starting powders, as well as
the effect of the starting compositions and temperature, was investigated. The materials could
only densify up to 80 – 92% of the theoretical density. After hot pressing at 800 oC, only Al and
cBN could be observed by XRD, whereas higher hot-pressing temperatures resulted in the
formation of AlN and AlB2 which retard the densification. The microstructure of the hot-pressed
materials was studied using SEM. It was observed that there are oxide layers at the interface
between the Al and cBN phases. The presence of these oxide layers prevented the Al from
spreading, thereby preventing full densification.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/5876 |
| Date | 09 December 2008 |
| Creators | Sithebe, Humphrey Samkelo Lungisani |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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