HfC-MoSi 2 COMPOSITES

10.3.1 Introduction

As belonging to the class of Ultra-High-Temperature-Ceramics, HfC owns high hardness, high electrical conductivity and chemical stability.^11-14 HfC is a potential candidate material for aerospace applications, because of the high melting point and low self diffusion coefficient. However, despite all the potentialities, so far it has not been developed on industrial scale due to the low sinterability and poor fracture toughness.

The high melting point is the main reason for the low sinterability which does not allow HfC to be densified unless very high temperature and pressure are applied.

The literature on monolithic hafnium carbide is very scanty. Recent studies report on HfC-based coatings obtained through CVD method.¹¹ Other contributions are focused on hot pressed HfCx compounds, sintered at 2500°C, with mean grain size ∼40-60 μm.^12,13 The control of microstructure, i.e. grain coarsening and porosity, seems to be the main issue for this compound. In order to reduce the final mean grain size and improve the sinterability, nanosized HfC powders have been recently produced by carbothermal reduction synthesis.¹⁴ At the moment, however, very few results on the mechanical properties of HfC-based materials are reported in literature.

In this section, the results on microstructure and densification mechanisms of HfC-based materials are presented. These compounds were pressureless sintered using MoSi2 as sintering aid, in amounts of 5, 10 and 20 vol%.

10.3.2 Sintering behaviour

The densification data for HfC-based composites are presented in Tab. 10.II. Compared to the monolithic material (Fig. 10.1b), the composites showed an improved sinterability, with final densities in the range 96-98%. During sintering, significant weight loss occurred and it increased almost linearly with increasing the additive content, as reported in Tab. 10.II.

10.3.3 Microstructure

The X-ray diffraction patterns of the dense materials (Fig.10.8) showed the presence of the starting HfC and MoSi2 phases and traces of a HfMoSi phase, especially in the HC20 specimen.

Fig. 10.8: X-ray diffraction pattern of HC20 sample.

The fracture surfaces, Fig. 10.9a,c,e, are mainly intergranular for HC5 and HC10 and partially transgranular for HC20. An interesting feature of these materials is the increase of HfC grain size after sintering, in comparison with the as-received powder.

The mean grain size of the composites was about 3-4 μm, with large grains up to 7-8 μm in size. Apparently, the HC20 composite had a more homogenous microstructure, compared to HC5 and HC10. Furthermore, the mean grain size slightly decreased with increasing MoSi2 content (Table 10.II). The polished surfaces, shown in Fig. 10.9b,d,f, reveal bright squared HfC grains dispersed in the dark MoSi2 phase, which fills the space left by the matrix grains. In these microstructures, HfC grains retained a rounded shape and reduced size, around 1 μm, in areas where the MoSi2 phase was more abundant. In contrast, large faceted HfC grains up to 10 μm grew in areas where the MoSi2 phase was scarce. This feature indicates that MoSi2 could act as a grain growth inhibitor for HfC if the dispersion of the secondary phase is improved.

0

Fig: 10.9: Fracture and polished surfaces for a) and b) HC5, c) and d) HC10, e) and f) HC20.

A characteristic example of the microstructure is shown in Fig. 10.10.

Analogously to ZrC-system, mixed phases based on Hf-Mo-Si-C were detected by EDS analyses, among hafnium carbide grains (Fig. 10.11). The formation of these phases suggests a mutual solubility between the two main phases and their shape, with very low dihedral angles, indicates crystallization from a liquid phase.

a

c

e

b

d

f

Fig. 10.10: SEM micrograph of HC20 sample: dark phase is MoSi2, bright phase is HfC and the darker grey phase is Hf-Mo-Si.

Fig. 9.11: High magnification image of HC20 sample showing the reaction phase Hf-Mo-Si and the relative EDS spectrum.

A detailed microstructural characterization by TEM did not reveal any dislocations in HfC grains, but strain contrasts were often noticed. TEM-EDX analyses confirmed the formation of a (Hf,Mo)xSiy phase at the triple point junctions (see an example in Fig.

10.12). This aspect reveals a partial substitution of Hf into Mo sites. The interfaces of this mixed (Hf,Mo)xSiy phase are concave towards HfC and convex towards MoSi2, suggesting a wetting tendency only toward the silicide. As a matter of fact, the HfC-HfC grain boundaries were clean with plane interfaces. The size of the crystalline phase at a triple pocket varies between 150 and 200 nm. Two configurations of the triple point junction were observed: in one case at least one of the surrounding grains was MoSi2, whilst in many other cases all the three adjacent grains were hafnium carbide.

C

Hf Si

Hf Hf

Mo Mo Mo

2 μm

KeV

HfC

MoSi2

The composition of the mixed phase was calculated by EDS analyses on several triple points and different stoichiometries were detected. Most of the triple points had an average composition of Mo=32, Hf=32 and Si=36 at%. If we assume that the transition metal atoms substitute Mo within the unit cell, as reported by Sakida et al.,¹⁵ then the ratio metal/metalloid would be approximately 64/36. In addition, if we consider the phases in the system Hf-Si with a close ratio, then the possible phases are Hf2Si (66.7/33.3) and Hf5Si3 (62.5/37.5). In order to identify these mixed phase, several electron diffraction patterns were taken from one single grain with different tilting angles. The analysis of the d-values calculated from the diffraction patterns, indicated that the (Hf,Mo)xSiy phase had a structure not referable to the d-values of the known Mo-Si or Hf-Si phases. To experimentally obtain the cell constants, the parameters obtained from the diffraction patterns were checked with respect to tilting angles. The resulting unit cell has a hexagonal structure with a=7.4 Å and c=5.2 Å. Both structure and lattice constants are very close to the hexagonal Hf5Si3 with the space group n°

P63/mcm and lattice parameters of a=7.890 Å, c= 5.558 Å.

Fig. 10.12: Triple point junction in HC20 material, the EDS spectrum of the mixed Hf-Mo-Si phase and the corresponding diffraction patterns collected at different tilting angles.