*2.2.1 Modified Brazilian test*

For the Flattened Brazilian Disc, the specimen would be separated into two halves. In fact, the crack initiation point is at the center of the specimen surface, the resulting crack would propagate in a plane normal to the loading direction. The tensile stress meets the maximum tensile strength criterion.

The FBD is more favorable to measure the tensile strength σ<sup>t</sup> [19].

For FB test, the form of specimens is a cylindrical with a thickness of 5 mm, a diameter of 30 mm and the width of the flat 2b of 5 mm (2α = 20°) (**Figure 1**).

Using the Griffith strength criterion, for a loading angle 2 α = 20°, the tensile strength (σt) was determined by this equation [20]:

$$
\sigma\_t = 0.9644 \frac{2P\_c}{\pi Dt} \tag{1}
$$

**Figure 1.** *Flattened Brazilian disc.*

Where D is the specimen diameter, Pc is the tensile load, t is the specimen thickness.

#### *2.2.2 Hardness measurements*

In order to exclude the effect of Titania, the samples are evaluated mechanically by NanoIndenter (INNOVASTTEST). The nanoindentation experiments were performed using the instrumented indentation techniques. The indented area was measured by optical microscopy for hardness calculation.

#### *2.2.3 Bending test*

The mechanical properties of the samples were assessed using Semi Circular Bending tests to determine the flexural strength σ<sup>f</sup> and the fracture toughness KIC. The samples were positioned on the loading platform by 3-point compressive loading, at an uniform loading speed of 0.075 mm/min. The SCB specimen diameter is equal to 30 mm and 5 mm for thickness (**Figure 2a**).

The flexural strength σ<sup>f</sup> is given according to Refs. [21–23]:

$$
\sigma\_f = \frac{P\_{mac}}{\pi R t} Q \tag{2}
$$

In which Pmax is the maximum load, t is the specimen thickness and R is the specimen radius. Q is the corresponding components of the dimensionless stress tensor, for the isotropic case Q = 5.132 [24].

In terms of fracture toughness KIC. The same specimen dimension was used by adding a crack of 4 mm in the semi disc, as shown in (**Figure 2b**). The crack-lengthto diameter ratio S/D was 0.13.

Using the SCB specimen with straight crack, the fracture toughness KIC was calculated with the following formula [21]:

$$K\_{IC} = \frac{P\_{\text{max}}\sqrt{\pi a}}{2Rt} Y\_I(a/R, \text{S}/R) \tag{3}$$

*Effect of Titania Addition on Mechanical Properties and Wear Behavior… DOI: http://dx.doi.org/10.5772/intechopen.99253*

**Figure 2.** *Semi-circular bending Disc: (a) uncracked SCB specimen and (b) cracked SCB specimen.*

Where a is the crack length, Pmax is the maximum load, D is the cylindrical block diameter and YI is the geometry factor. The latter is a function of the ratio of the crack length (a) over the semi-disc radius (R) and the ratio of the half-distance between the two bottom supports (S) over the semi-disc radius (R) (**Figure 2b**). The geometry factor YI is expressed as follows [21]:

$$Y\_I(a/R, S/R) = \frac{S}{R} \left( 2.91 + 54.39 \frac{a}{R} + 391.4 \left( \frac{a}{R} \right)^2 + 1210.6 \left( \frac{a}{R} \right)^3 - 1650 \left( \frac{a}{R} \right)^4 + 875 \left( \frac{a}{R} \right)^5 \right) \tag{4}$$

### *2.2.4 Compression test*

For the compressive test, D = 9 mm and l = 18 mm where D is the diameter and t is the length of the cylindrical specimen (**Figure 3**), as specified in ASTM standards [24, 25]. During the compressive test, the samples are positioned between compressive plates parallel and then compressed at a loading rate of 1 mm/min.

**Figure 3.** *Specimen for compressive test.*

#### **Figure 4.**

*Schematic representation of sliding (pion-on-disk configuration) test, showing experimental sliding parameters for Alumina- 10 wt.% TCP-TiO2 samples (pressureless sintering at 1600°C for 1h) against ZrO2 ball. The Sliding distance length is 377 m.*

In terms of compressive properties, the compressive strength σ<sup>c</sup> is given as follows:

$$
\sigma\_c = \frac{4P\_{\text{max}}}{\pi D^2} \tag{5}
$$

Where D is the cylindrical block diameter and Pmax is the maximum load.

#### **2.3 Tribological tests**

In order to evaluate the tribological properties of the Alumina-10 wt.% TCP-TiO2 composite, sliding wear tests were carried out against sintered composites under 9 N normal load [26, 27]. Wear tests were performed using a rotating pionon-disk tribometer, **Figure 4**, that was developed in the LGME lab [28]. A Zirconia ball of 10 mm diameter and 11 GPa hardness is fixed using a suitable device used to solicit our samples in sliding. All specimens are polished cylindrical discs of 30 mm diameter and 5 mm thickness. The parameters set for the sliding tests are a sliding velocity of 0.1 m/s. All the tests were achieved out without lubricated environment for the normal test duration of 3600 s, at room temperature (20°C) and with a relative humidity of 35 � 5%. For each test, the friction coefficient and the wear rate are measured.

The coefficient of friction is determined from the tangential effort and the normal force by the following formula (6) [29]:

$$\text{COF} = \frac{\text{Ft}}{\text{Fn}} \tag{6}$$

Where COF is the coefficient of friction (dimensionless), *Fn* is the normal applied force (N) and *Ft* is the tangential effort (N).

The wear rate of all compositions is deduced from the measurement of mass loss. It is determined using this following equation (Eq. (7)) [30]:

*Effect of Titania Addition on Mechanical Properties and Wear Behavior… DOI: http://dx.doi.org/10.5772/intechopen.99253*

$$W\_r = \frac{(M\_0 - M\_t)}{M\_0} \ast 100\tag{7}$$

in which *Wr* is the wear rate %ð Þ, *M*<sup>0</sup> is the initial mass and *Mt* the mass of the specimen after sliding test.

In order to control the wear of Zirconia ball, the ball was weighed before and after each sliding test.

The wear volume of all compositions is determined from the measurement of worn surfaces of tested samples. The wear tracks were determined using 2D-profilometer.

To determine the wear volume, a profilogram (2D-profiles) is drawn in the perpendicular direction of the wear scars over a length of 8 mm [31, 32] with which we can determine the area ð Þ *S*� of the wear track (**Figure 5**). The wear volume is determined using by the following formula (8):

$$W\_v = 2\,\text{\pi r\text{\\$}\_t\text{S}\_-} \tag{8}$$

Where is the wear volume (mm<sup>3</sup> ), *S*�is the area of the wear track and *rt* is the radius of the wear track.

#### **2.4 Physico-chemical characterization**

The characterization of the samples after the tests is carried out using several techniques.

X-ray powder diffraction analysis (DIFFRAC SUITE, Brucker, Germany) was conducted in order to analyze the phase transformation in the different structures of each composites before and after the sintering process. The Xray radiance was created by using CuKα radiation (λ = 1.5406 Å) in the 2θ range 5–60° at a current of 40 mA, a voltage of 40 kv, and a scanning rate of 1.2°/min. The identification phase was identified out by comparing the experimental XRD-patterns with the standard files assembled by ICDD (the International Center for Diffraction Data).

Scanning electron microscopy (JEOL JSM-5400) was used to observe the surfaces of the fractured samples after the sintering process. It was equally used

**Figure 5.** *Example of profilogram.*

for the assessment of wear mechanisms, the microstructure of the sliding zone of the tested sample.
