**6. Mechanical properties**

94 Tungsten Carbide – Processing and Applications

and 14):

and

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

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

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

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

from WC grain, D – SEAD from the grain boundary region.

(0 111) WC ‖ (1 105) Al2O3 (7)

[11 23] WC ‖ [23 11] Al2O3 (8)

(0 111) WC ‖ (1011) Al2O3 (9)

[2 1 10] WC ‖ [01 11] Al2O3 (10)

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 influence observed bending strength test results.

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


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

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

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

Experiments of subcritical crack propagation performer using Double Torsion method (DT) [39- 41] showed that composites were much more resistant for this disadvantageous phenomenon. Such experiments were previously conducted for alumina and zirconia materials [42, 43] but not for composites containing tungsten carbide particles. Results of performed investigations (see Fig. 16) showed that the threshold value of *KI* coefficient in both composite systems significantly increased. In Al2O3/WC material the threshold *KI* value was ~4.0 MPam-0.5 (compared with 2.6 MPam-0.5 for alumina). In ZrO2/WC material the threshold *KI* value was ~4.4 MPam-0.5 (compared with 3.6 MPam-0.5 for zirconia). The most probably reason of such behaviour was the residual stresses state in dense sintered composite bodies. Distribution of these stresses around composite hindered breaking of atomic bonds on the tip of flaws presented in composites.

Tungsten Carbide as an Reinforcement in Structural Oxide-Matrix Composites 97

The type of Dry Sand Test based on ASTM test [46], which indicates wear susceptibility of material for wear during abrasive action of hard particles without any lubricant. The test

The Miller Test based on ASTM test [47] allows to determine of SAR (Slurry Abrasion Response) number during the wear in slurry. The test duration was 6 hours. In both test the

Material Miller Test, SAR number, Dry Sand Test, wear rate in mm3

The results showed difference in wear mechanisms between tests conducted in dry and wet environments for composites with two matrices: α-alumina and tetragonal zirconia. Figures 17 and 18 collects SEM images of worn surfaces after the Dry Sand Test and the Miller Test. The fundamental difference visible at these figures, especially for alumina phase, is the presence of intensive grain boundary etching during work in wet environment. It could leads even to the whole grain scouring from the sintered bodies. During the Dry Sand Test the dominant wear mechanism consist in grains fracture. The presence of hard WC particles

The worn surface profilographic analysis (see Table 5) allows to establish that second phase particle addition modifies alumina microstructure significantly. Alumina-basing composite surface after both tests were much more smooth, than the pure matrix surface. It proved that

Comparing the pure zirconia and zirconia-basing composite materials is visible that composite surface roughness is higher. Anyway, the wear rates for zirconia-basing

Wear properties of both (alumina or zirconia) composite types are distinctly different in

Conducted tests established that incorporation of second phase grains into alumina matrix influences wear properties changes in high scale. Changes observed for zirconia based

Results of performed wear tests suggest that investigated materials are predicted to work at different environments. The wear resistant parts for work at wet environments seems to be

Alumina 163.51 55.17 Alumina/10vol.% WC 9.57 9.57 Zirconia s.s. 9.45 15.71 Zirconia/10vol.% WC 7.41 11.46

duration was 2000 rotation of the wheel.

**Table 4.** Results of performed wear tests.

low value of test result means the better material behaviour.

Results of performed wear tests were collected in Table 4.

in both investigated oxide matrices limits the wear rates.

the dominant wear mechanisms were significantly limited.

composites were lower than for pure zirconia material.

composites are not so spectacular but still significant.

spite of wear environment.

**Figure 16.** The crack velocity VI vs. stress intensity factor KI. A, Z, A/WC, Z/WC – stand for alumina, zirconia, alumina/WC composite and Zirconia/WC composite, relatively.

### **7. Wear resistance**

One of the most important field of structural ceramic application is using them as a part of devices resistant for abrasive wear. From this point of view it is important how the material behaves at different working conditions (the range of loads) and environments (the presence of humidity). The oxide matrices are sensitive for water presence in environment. Even under low load rate it can cause the subcritical crack growth [42 - 45]. If the load are serious, degradation of the oxide matrices in the presence of water could be very significant.

The chapter presents the results of investigation on wear of alumina and zirconia-basing composites by hard abrasive particles in different environments. The results of two tests (The Dry Sand Test and the Miller Test in pulp) were compared. As an abrasive medium coarse silicon carbide grains were used in both cases.

The type of Dry Sand Test based on ASTM test [46], which indicates wear susceptibility of material for wear during abrasive action of hard particles without any lubricant. The test duration was 2000 rotation of the wheel.

The Miller Test based on ASTM test [47] allows to determine of SAR (Slurry Abrasion Response) number during the wear in slurry. The test duration was 6 hours. In both test the low value of test result means the better material behaviour.


Results of performed wear tests were collected in Table 4.

**Table 4.** Results of performed wear tests.

96 Tungsten Carbide – Processing and Applications

atomic bonds on the tip of flaws presented in composites.

phenomenon. Such experiments were previously conducted for alumina and zirconia materials [42, 43] but not for composites containing tungsten carbide particles. Results of performed investigations (see Fig. 16) showed that the threshold value of *KI* coefficient in both composite systems significantly increased. In Al2O3/WC material the threshold *KI* value was ~4.0 MPam-0.5 (compared with 2.6 MPam-0.5 for alumina). In ZrO2/WC material the threshold *KI* value was ~4.4 MPam-0.5 (compared with 3.6 MPam-0.5 for zirconia). The most probably reason of such behaviour was the residual stresses state in dense sintered composite bodies. Distribution of these stresses around composite hindered breaking of

**Figure 16.** The crack velocity VI vs. stress intensity factor KI. A, Z, A/WC, Z/WC – stand for alumina,

One of the most important field of structural ceramic application is using them as a part of devices resistant for abrasive wear. From this point of view it is important how the material behaves at different working conditions (the range of loads) and environments (the presence of humidity). The oxide matrices are sensitive for water presence in environment. Even under low load rate it can cause the subcritical crack growth [42 - 45]. If the load are serious,

The chapter presents the results of investigation on wear of alumina and zirconia-basing composites by hard abrasive particles in different environments. The results of two tests (The Dry Sand Test and the Miller Test in pulp) were compared. As an abrasive medium

degradation of the oxide matrices in the presence of water could be very significant.

zirconia, alumina/WC composite and Zirconia/WC composite, relatively.

coarse silicon carbide grains were used in both cases.

**7. Wear resistance** 

The results showed difference in wear mechanisms between tests conducted in dry and wet environments for composites with two matrices: α-alumina and tetragonal zirconia. Figures 17 and 18 collects SEM images of worn surfaces after the Dry Sand Test and the Miller Test. The fundamental difference visible at these figures, especially for alumina phase, is the presence of intensive grain boundary etching during work in wet environment. It could leads even to the whole grain scouring from the sintered bodies. During the Dry Sand Test the dominant wear mechanism consist in grains fracture. The presence of hard WC particles in both investigated oxide matrices limits the wear rates.

The worn surface profilographic analysis (see Table 5) allows to establish that second phase particle addition modifies alumina microstructure significantly. Alumina-basing composite surface after both tests were much more smooth, than the pure matrix surface. It proved that the dominant wear mechanisms were significantly limited.

Comparing the pure zirconia and zirconia-basing composite materials is visible that composite surface roughness is higher. Anyway, the wear rates for zirconia-basing composites were lower than for pure zirconia material.

Wear properties of both (alumina or zirconia) composite types are distinctly different in spite of wear environment.

Conducted tests established that incorporation of second phase grains into alumina matrix influences wear properties changes in high scale. Changes observed for zirconia based composites are not so spectacular but still significant.

Results of performed wear tests suggest that investigated materials are predicted to work at different environments. The wear resistant parts for work at wet environments seems to be

the best area of application for zirconia composites. Alumina based materials show the best properties during dry abrasion.

Tungsten Carbide as an Reinforcement in Structural Oxide-Matrix Composites 99

**Figure 18.** A typical microstructures of worn surfaces after Miller Test; alumina (left top), zirconia s. s.

Selected information about properties of composite materials basing on alumina or zirconia matrices containing dispersed tungsten carbide inclusions presented in this chapter indicated that these materials have potential to be widely used as a structural

Properties improvement in these composites is not only an effect of introducing an additional toughening mechanisms connected with crack path/inclusion interacting (crack deflection, crack branching, crack bridging), but also relatively strong interphase grain boundaries confirmed by the unique phenomenon of privileged crystallographic correlation

(right top), alumina/10vol.% WC (left bottom), zirconia/10vol.% WC (right bottom).

**8. Summary** 

of oxide and carbide phases.

material.

**Figure 17.** A typical microstructures of worn surfaces after Dry Sand Test; alumina (left top), zirconia s. s. (right top), alumina/10vol.% WC (left bottom), zirconia/10vol.% WC (right bottom).


± denotes of the standard deviation of 5 measurements

**Table 5.** Profilographic parameter Ra of material surfaces worn during wear tests.

**Figure 18.** A typical microstructures of worn surfaces after Miller Test; alumina (left top), zirconia s. s. (right top), alumina/10vol.% WC (left bottom), zirconia/10vol.% WC (right bottom).
