**5. Composites microstructure**

In present chapter author was focused on properties of "classical" particulate composites. It means materials containing the second phase particles randomly distributed into matrix. It means that the amount of additives must be lower than a percolation threshold. To assure that situation examples of composite materials contain 10 vol. % of tungsten phase with the same grain size distribution were manufactured.

Commercial powders were used as a starting materials (alumina – TM-DAR Taimicron, zirconia – Tosoh 3Y-TZ, tungsten carbide – Baildonit). Powders homogeneity was assured by 30 min. of attrition mixing of constituent powders in ethyl alcohol.

88 Tungsten Carbide – Processing and Applications

**4. Composite manufacturing** 

W2C) were present in the product.

[22, 34].

(see Table 3).

nowadays too expensive for wider application [23, 29, 30].

in the composite was relatively high (~30-40 vol. %).

same grain size distribution were manufactured.

**5. Composites microstructure** 

Preparation of dense particulate composite bodies with randomly distributed inclusions meets potential difficulties during composite powder preparation and during sintering. The most popular method of second phase dispersion in the matrix is simple mixing. This method is widely used for zirconia/WC system [26-28]. The mixing process utilizing intensive mills (atrittors, rotation-vibrational mills) in short time assures the proper tungsten carbide particles distribution within the matrix in the wide range of WC content 10 – 50 vol. %. More sophisticated methods as for instance decomposition of organic WC precursors are

In alumina/WC composite system the mechanical mixing is also the main preparation method of the composite powder. Anyway, there were some experiments [23] utilizing selfpropagating high temperature synthesis (SHS) process for *in-situ* synthesis of alumina/tungsten carbide composite powder. In this process both tungsten carbides (WC,

Sintering of composites with oxide matrices and dispersed WC particles is a typical example of sintering with "rigid inclusions" widely described in literature [31-33]. In fact, during this process, diffusional mechanisms of densifications appear only in the oxide matrix. The presence of carbide particles makes the sintering driving forces much weaker. This effect is as stronger as higher is tungsten carbide particles volume content. The high relative density demand for structural applications (> 97 % of theo.) can be assured using pressureless sintering method when WC content not exceed 20 vol. %. Additionally, sintering temperature in this case must be relatively high (1550C for zirconia and 1600C for alumina). It is not profitable for sinters microstructure because the grain growth phenomenon. The inert sintering atmosphere demanded for preservation of WC from oxidation at high temperatures causes some additional factor of stabilization in zirconia

Practically, the most often sintering method for both type of composites is hot-pressing technique. Application of this method allows to assure high densities (> 98 % of theo.) in relatively short time (30 – 60 min.). Such conditions limits the grain growth in the matrix

There were some investigations utilizing pulsed electric current sintering (PECS) for zirconia/WC composite densification [27]. These methods were profitable when WC content

In present chapter author was focused on properties of "classical" particulate composites. It means materials containing the second phase particles randomly distributed into matrix. It means that the amount of additives must be lower than a percolation threshold. To assure that situation examples of composite materials contain 10 vol. % of tungsten phase with the Materials for test were fabricated by hot-pressing technique (HP) due to guarantee the maximum of the densification of investigated samples. The sintering conditions were as follow: the maximum temperature - 1500°C (for zirconia and zirconia/WC composite) and 1650°C (for alumina and alumina/WC composite) with 1 hour soaking time and maximum applied pressure - 25 MPa.

A typical SEM microstructures of hot-pressed composites were showed in the Figs. 7 and 8. Table 2 collects data about the grain size of individual phases.


**Table 2.** The mean grain size of phases existing in sinters containing 10 vol. % of WC.

**Figure 7.** The typical SEM image of thermally etched ZrO2/WC composite microstructure.

**Figure 9.** TEM micrograph of Al2O3/WC composite. Dark grains – WC; bright ones – alumina.

**Figure 10.** TEM micrograph of ZrO2/WC composite.

**Figure 8.** The typical SEM image of thermally etched Al2O3/WC composite microstructure.

Measurements performed in the TEM revealed that oxide matrices and tungsten carbide grains close adhered and no discontinuities were observed (Figs. 9 and 10).

The detail observation of Al2O3/WC and ZrO2/WC microstructures and chemical analyses performed as a line scan across the interphase boundaries (Figs.11 and 12) showed that there are differences in elements diffusion in investigated systems. The change of chemical composition near alumina/tungsten carbide boundary is sharp and distinct (Fig. 9). There is no evidence of Al diffusion into WC or W diffusion into Al2O3. In the case of zirconia/tungsten carbide boundary chemical composition is changing near the interphase boundary. It could be a slight confirmation of thermo-dynamically described tendency to creation of ZrC and W2C in this system.

Results of TEM investigations have shown specific crystallographic relationships between alumina and zirconia matrix and tungsten carbide phase [35]. Crystal correlations may partially explain significant improvement of mechanical properties of alumina- and zirconia-based composites comparing with a pure oxide matrices. However, apart from crystallographic factors, the properties of material under investigation may be affected by interfacial defects and interphase boundary structure.

**Figure 9.** TEM micrograph of Al2O3/WC composite. Dark grains – WC; bright ones – alumina.

**Figure 10.** TEM micrograph of ZrO2/WC composite.

90 Tungsten Carbide – Processing and Applications

**Figure 8.** The typical SEM image of thermally etched Al2O3/WC composite microstructure.

grains close adhered and no discontinuities were observed (Figs. 9 and 10).

creation of ZrC and W2C in this system.

interfacial defects and interphase boundary structure.

Measurements performed in the TEM revealed that oxide matrices and tungsten carbide

The detail observation of Al2O3/WC and ZrO2/WC microstructures and chemical analyses performed as a line scan across the interphase boundaries (Figs.11 and 12) showed that there are differences in elements diffusion in investigated systems. The change of chemical composition near alumina/tungsten carbide boundary is sharp and distinct (Fig. 9). There is no evidence of Al diffusion into WC or W diffusion into Al2O3. In the case of zirconia/tungsten carbide boundary chemical composition is changing near the interphase boundary. It could be a slight confirmation of thermo-dynamically described tendency to

Results of TEM investigations have shown specific crystallographic relationships between alumina and zirconia matrix and tungsten carbide phase [35]. Crystal correlations may partially explain significant improvement of mechanical properties of alumina- and zirconia-based composites comparing with a pure oxide matrices. However, apart from crystallographic factors, the properties of material under investigation may be affected by

**Figure 13.** TEM micrograph of Al2O3/WC composite. a – bright field (BF) image, b – selected area

**Figure 14.** TEM micrograph of Al2O3/WC composite. a – BF image, b –SEAD from WC grain, c - SEAD

electron diffraction (SEAD) from WC grain, c - SEAD from Al2O3 grain.

from Al2O3 grain.

**Figure 11.** High resolution TEM microstructure of Al2O3/WC and the line scan across alumina and tungsten carbide boundary.

**Figure 12.** High resolution TEM microstructure of ZrO2/WC and the line scan across zirconia and tungsten carbide boundary.

tungsten carbide boundary.

tungsten carbide boundary.

**Figure 11.** High resolution TEM microstructure of Al2O3/WC and the line scan across alumina and

**Figure 12.** High resolution TEM microstructure of ZrO2/WC and the line scan across zirconia and

**Figure 13.** TEM micrograph of Al2O3/WC composite. a – bright field (BF) image, b – selected area electron diffraction (SEAD) from WC grain, c - SEAD from Al2O3 grain.

**Figure 14.** TEM micrograph of Al2O3/WC composite. a – BF image, b –SEAD from WC grain, c - SEAD from Al2O3 grain.

Alumina and WC grains were indexed using the SAED "(Selected Area Electron Diffraction) and two characteristic crystal relationships between above phases were identified (Figs. 13 and 14):

$$(0\ 111)\,\text{WC}\,\parallel\,(1\ 105)\,\text{Al}\,\text{O}\,\text{s}\tag{7}$$

Tungsten Carbide as an Reinforcement in Structural Oxide-Matrix Composites 95

[ **1** 010] WC ‖ [010] tetragonal ZrO2 (12)

Furthermore, EBSD analysis made by Faryna at. all. [37, 38] proved statistically that crystallographic correlation in investigated composite systems are not an unique property,

The basic mechanical properties of investigated materials were collected in Table 3. Both composites were well densified but is worth to noticed that there is about 1 % of difference between Al2O3/WC and ZrO2/WC composites. Zirconia matrix and zirconia-basing material is almost fully dense. Alumina-basing composite has "only" 98.8 % of theoretical density and is 0.5 % worse densified than alumina matrix. This difference is not much but certainly

It is characteristic that Al2O3/WC material has lower bending strength than "pure" matrix material. Different effect is observed for ZrO2/WC composite. The mean value of the bending strength of ZrO2/WC is similar to that registered for zirconia matrix. But the highest strength value registered during tests was over 10% higher than that measured for zirconia matrix. This fact showed that there is a potential of strength improvement in this system.

It is not surprise that hardness of composites is higher than that measured for matrices. Spectacular is the increase of the fracture toughness. In both investigated composite systems

> Young's modulus, *E*, GPa

Alumina 99.3 ± 0.1 17.0 ± 1.2 379 ± 6 3.6 ± 0.3 600 ± 120 780 6

WC 98.8 ± 0.1 18.7 ± 0.8 394 ± 7 5.5 ± 0.7 450 ± 45 550 12 Zirconia s.s. 99.5 ± 0.1 14.0 ± 0.5 209 ± 5 5.0 ± 0.5 1150 ± 75 1250 18

WC 99.7 ± 0.1 17.0 ± 0.9 232 ± 6 8.0 ± 1.0 1100 ± 130 1380 7

Experiments of subcritical crack propagation performer using Double Torsion method (DT) [39- 41] showed that composites were much more resistant for this disadvantageous

Fracture toughness, *KIc*, MPam0,5

Bending strength – the mean value, *σ*, MPa

Bending strength – maximum value, *σmax*, MPa

Weibull parameter, *m*

*KIc* increased more than 50 % when compared with the suitable matrix.

Vickers hardness, *HV*, GPa

± denotes the confidence interval on the 0.95 confidence level (for ρ, *HV* and *KIc* measurements);

**Table 3.** Mechanical properties of the matrices and composites.

± denotes the standard deviation of the mean value of 40 results of measurements (for σ measurements).

but they are very often.

Material

Alumina/10vol.%

Zirconia/10vol.%

**6. Mechanical properties** 

influence observed bending strength test results.

Relative density, ρ*wzgl.,* %

$$\left[11\,23\right] \text{WC} \parallel \left[23\,11\right] \text{AlcO} \tag{8}$$

and

$$(0\ 111)\text{ WC} \parallel (1011)\text{ Als}\text{O}\tag{9}$$

$$\left[ \begin{array}{c} \begin{bmatrix} 2 \ 1 \ 10 \end{bmatrix} \text{WC} \ \right] \text{ [} \left[ \begin{matrix} 01 \ 11 \end{matrix} \right] \text{AlzO} \end{array} \tag{10}$$

These relationships were found on several sites investigated on the thin foil.

**Figure 15.** TEM micrograph of ZrO2/WC composite. A – BF image, B –SEAD from ZrO2 grain, C– SEAD from WC grain, D – SEAD from the grain boundary region.

Similarly, in ZrO2/WC system crystallographic relationships were identified (Fig. 15) [36]:

[0001] WC ‖ [001] tetragonal ZrO2 (11)

$$\left[\mathbf{1}\,010\right]\,\text{WC} \parallel \left[010\right]\,\text{tetragonal }\mathbf{ZrO}.\tag{12}$$

Furthermore, EBSD analysis made by Faryna at. all. [37, 38] proved statistically that crystallographic correlation in investigated composite systems are not an unique property, but they are very often.
