**2.1. ZrO2/WC system**

Zirconia for structural applications is used in the form of solid solutions of yttria, magnesia, calcia or rare earth metals in ZrO2 [18-20]. Using data from [21] one can calculate that free enthalpy of mixing of zirconia and any stabilizing element is significantly lower than the error of determination of free enthalpy for chemical reactions in ZrO2-WC (or ZrO2-WC-C) systems. It allows to calculate, with a reasonable approximation, the possibility of reactions proceeding using thermo-dynamical data for zirconia only.

Potential chemical reactions taken into account were as follow:

$$\text{ZrCr} + \text{WC} = \text{ZrC} + \text{WOr} \tag{1}$$

Tungsten Carbide as an Reinforcement in Structural Oxide-Matrix Composites 83

Reaction (2) can proceed when the partial pressure of CO in the system is lower than 0.95

These calculations were verified experimentally [22]. The presence of ZrC and W2C was determined in sinters containing WC inclusions sintered at different temperatures (Fig. 1.).

If sintering process is conducted using hot-pressing (HP) technique, composite powder could be in contact with carbon from the press die or stamps. It suggest that the third

ZrO2 + 3C = ZrC + 2CO (3)

Reaction (3) can proceed when the partial pressure of CO in the system is lower than 0.054 atm at 1400C, 0.20 atm at 1500C, 0.65 atm at 1700C and 1.89 atm at 1700C. These data suggest that is possible to produce ZrC precipitates even in pure zirconia sinters when one can assure right value of CO pressure [17]. Conducting of sintering process at relatively low

**Figure 2.** Result of XRD analysis of 3Y-ZrO2/10vol.% of WC composite hot-pressed at 1500C. T: stand

Potential chemical reactions in alumina – tungsten carbide systems taken into account:

2Al2O3 + 3WC = Al4C3 + 3WO2 (4)

2Al2O3 + 6WC = Al4C3 + 3W2C + 3CO2 (5)

atm at 1400C, 5.20 atm at 1500C, 23.8 atm at 1700C and 93.0 atm at 1700C.

reaction should be also taken into account:

for tetragonal phase of the zirconia solid solution.

**2.2. Al2O3/WC system** 

temperature (1400C ) allows to avoid ZrC appearance (Fig. 2).

$$\text{ZrO} + 6\text{WC} = \text{ZrC} + 3\text{WAC} + \text{CO} \tag{2}$$

Calculations showed that reaction (1) cannot proceed in the range of potential sintering temperatures (1400 - 1700C) because of fact that standard free enthalpy (ΔGr 0) of that reaction is much higher than zero.

**Figure 1.** Results of XRD analyses of 3Y-ZrO2/10vol.% of WC composite pressureless sintered at different temperatures. T: stand for tetragonal phase of the zirconia solid solution, sintering temperatures indicated on the left side of plots.

Reaction (2) can proceed when the partial pressure of CO in the system is lower than 0.95 atm at 1400C, 5.20 atm at 1500C, 23.8 atm at 1700C and 93.0 atm at 1700C.

These calculations were verified experimentally [22]. The presence of ZrC and W2C was determined in sinters containing WC inclusions sintered at different temperatures (Fig. 1.).

If sintering process is conducted using hot-pressing (HP) technique, composite powder could be in contact with carbon from the press die or stamps. It suggest that the third reaction should be also taken into account:

$$\text{ZrO} + \text{\textdegree 3C} = \text{ZrC} + \text{2CO} \tag{3}$$

Reaction (3) can proceed when the partial pressure of CO in the system is lower than 0.054 atm at 1400C, 0.20 atm at 1500C, 0.65 atm at 1700C and 1.89 atm at 1700C. These data suggest that is possible to produce ZrC precipitates even in pure zirconia sinters when one can assure right value of CO pressure [17]. Conducting of sintering process at relatively low temperature (1400C ) allows to avoid ZrC appearance (Fig. 2).

**Figure 2.** Result of XRD analysis of 3Y-ZrO2/10vol.% of WC composite hot-pressed at 1500C. T: stand for tetragonal phase of the zirconia solid solution.

#### **2.2. Al2O3/WC system**

82 Tungsten Carbide – Processing and Applications

**2. Thermodynamical aspect** 

reaction is much higher than zero.

temperatures indicated on the left side of plots.

**2.1. ZrO2/WC system** 

properties measurements and abrasive wear tests.

proceeding using thermo-dynamical data for zirconia only.

Potential chemical reactions taken into account were as follow:

fracture in composites. These observations were put together with the results of mechanical

Zirconia for structural applications is used in the form of solid solutions of yttria, magnesia, calcia or rare earth metals in ZrO2 [18-20]. Using data from [21] one can calculate that free enthalpy of mixing of zirconia and any stabilizing element is significantly lower than the error of determination of free enthalpy for chemical reactions in ZrO2-WC (or ZrO2-WC-C) systems. It allows to calculate, with a reasonable approximation, the possibility of reactions

ZrO2 + WC = ZrC + WO2 (1)

ZrO2 + 6WC = ZrC + 3W2C + CO (2)

Calculations showed that reaction (1) cannot proceed in the range of potential sintering

0) of that

temperatures (1400 - 1700C) because of fact that standard free enthalpy (ΔGr

**Figure 1.** Results of XRD analyses of 3Y-ZrO2/10vol.% of WC composite pressureless sintered at different temperatures. T: stand for tetragonal phase of the zirconia solid solution, sintering

Potential chemical reactions in alumina – tungsten carbide systems taken into account:

$$2\text{AlAlCl} + 3\text{WC} = \text{AlCl} + 3\text{WCl} \tag{4}$$

$$2\text{AlAlO} + 6\text{WC} = \text{Al} \cdot \text{C} + 3\text{WAC} + 3\text{CO} \tag{5}$$

$$2\text{AlxO} + \text{9WC} = \text{AlxC} + \text{9W} + \text{6CO} \tag{6}$$

Tungsten Carbide as an Reinforcement in Structural Oxide-Matrix Composites 85

The results of FEM simulations were visualized at Figures 3 and 4. They present the distribution of principal maximal stresses around in the hypothetical composite microstructure. Calculations were made for the same schematic microstructure. Matrix was assumed as zirconia or alumina, respectively. The inclusion phase was WC. Calculations

**Figure 3.** The principal maximal stresses calculated for ZrO2/WC composite. Dark blue color represents the maximal values of compressive stresses, brown color represents the maximal values of tensile

Generally, the maximum value of principal maximal stresses in the zirconia matrix is about 30 % higher than in the alumina one. The tensile stress level near the interphase boundary in the zirconia matrix materials exceeds 1000 MPa all around the inclusion grain (Fig. 3). In the

This fact influences the path of crack in the investigated materials. In zirconia-based composites crack goes along the interphase boundary (Fig. 5). The crack course in composites with alumina matrix is different. It usually goes near the inclusion grains, but it is deflected before it reaches the interphase boundary (Fig. 6). This means that the crack goes

alumina based materials maximum stress values in this area are much lower (Fig. 4).

stresses. At this Figure WC inclusions are generally in blue color.

through alumina grains.

were made for Al2O3/WC and ZrO2/WC composites.

Calculations showed that reactions (4 - 6) cannot proceed in the range of potential sintering temperatures (1400 - 1700C) because of fact that standard free enthalpy (ΔGr 0) of that reaction is much higher than zero. That results were also confirm by Niyomwas [23], who stated that Al2O3/WC system is thermodynamically stable up to 2000C.
