**3. Graded structure and tribology design for optimal lubricating properties in laminated ceramic composites**

#### **3.1. Design and preparation of laminated-graded self-lubricating ceramic composites**

From the results above, it can be concluded that the layered structure design is a good strategy to enhance the mechanical properties of monolithic ceramics, which can efficiently improve the bending strength and work of fracture. Nevertheless, the friction and wear rate of these materials under dry sliding conditions are still high. To overcome this problem, the laminatedgraded structure self-lubricating ceramic composites were designed. Figure 7 shows the schematic of self-lubricating ceramic composites with laminated-graded structure, where d1=d2, the A layer is the Al2O3 or ZrO2-Al2O3, and the B layer is Al2O3-ZrO2 or ZrO2. The center area is composed of laminated composites that are similar to that of in the section 2, which provides high strength for the whole material. The content of SLs is graded, increased from center to two sides, and finally reaches a fixed value on the surface to ensure the excellent lubricating function of the materials. In this study, each couple of ZrO2(3Y)-Al2O3 and ZrO2(3Y) or Al2O3 and Al2O3-ZrO2 has the same SL content. The SL content of each couple *f*(x) is determined by the following equation [18]:

$$f\left(\mathbf{x}\right) = \left(\mathbf{x}/m\right)^{p} \times f\left(\mathbf{s}\right) \tag{1}$$

Where *x* is the number of the couple, *m* is the total number of the couples in the gradient area, *p* is the gradient exponent, and *f*(s) is the content of SLs in surface layers. Commercially available Al2O3, ZrO2, Y2O3, CuO, TiO2, and SLs (graphite+CaF2+BaSO4 and graphite+CaF2 in two kinds of laminated-graded structure self-lubricating ceramic composites, respectively) were used. Experimental details for preparation and characterization are described in these references [17, 18].

**Figure 7.** Schematic of the laminated-graded structure self-lubricating composites.

ZrO2(3Y)-Al2O3 layers is 15 %, the bending strength and work of fracture of the ZrO2(3Y)- Al2O3/ZrO2(3Y)-laminated composite reach to 968 MPa and 3751 J m–2, respectively (Fig. 6a

**Figure 6.** Mechanical properties of ZrO2(3Y)-Al2O3/ZrO2(3Y)-laminated composites and monolithic ceramic.

**in laminated ceramic composites**

66 Advances in Functionally Graded Materials and Structures

determined by the following equation [18]:

**3. Graded structure and tribology design for optimal lubricating properties**

From the results above, it can be concluded that the layered structure design is a good strategy to enhance the mechanical properties of monolithic ceramics, which can efficiently improve the bending strength and work of fracture. Nevertheless, the friction and wear rate of these materials under dry sliding conditions are still high. To overcome this problem, the laminatedgraded structure self-lubricating ceramic composites were designed. Figure 7 shows the schematic of self-lubricating ceramic composites with laminated-graded structure, where d1=d2, the A layer is the Al2O3 or ZrO2-Al2O3, and the B layer is Al2O3-ZrO2 or ZrO2. The center area is composed of laminated composites that are similar to that of in the section 2, which provides high strength for the whole material. The content of SLs is graded, increased from center to two sides, and finally reaches a fixed value on the surface to ensure the excellent lubricating function of the materials. In this study, each couple of ZrO2(3Y)-Al2O3 and ZrO2(3Y) or Al2O3 and Al2O3-ZrO2 has the same SL content. The SL content of each couple *f*(x) is

**3.1. Design and preparation of laminated-graded self-lubricating ceramic composites**

( ) = ´ ( ) () *<sup>p</sup>*

Where *x* is the number of the couple, *m* is the total number of the couples in the gradient area, *p* is the gradient exponent, and *f*(s) is the content of SLs in surface layers. Commercially

*f x xm f s* (1)

and b).

#### **3.2. The mechanical and tribological properties of traditional self-lubricating ceramic composites**

For comparison, the mechanical and tribological properties of traditional self-lubricating ceramic composites were first conducted. Figure 8 shows the microstructure of two kinds of traditional self-lubricating ceramic composites. It can be seen that there are lots of tiny pores in the sintered samples. There is no doubt that these defects will greatly degenerate the mechanical properties of the materials. The mechanical properties of two kinds of traditional self-lubricating ceramic composites (Al2O3-graphite and Al2O3-LaF3 composites) are given in Figure 9. It can be seen clearly that the bending strength and work of fracture decrease rapidly with the increase of the content of SLs. For the Al2O3-LaF3 composites, when the volume content of lubricants increase to 40%, the bending strength and work of fracture reduced to as low as 67 MPa and 44 J m–2, which were 6.3 and 2.9 times lower than those of monolithic Al2O3 ceramic. Therefore, the traditional self-lubricating ceramic composites exhibit poor mechanical properties mainly because of the lots of SLs that destroyed the continuity of ceramic matrix. The ceramic composites may exhibit good lubricating properties when proper amounts of lubricants were added [1,4]. Nevertheless, this kind of ceramics possesses poor anti-destructive and reliability, which is the key obstacle to its practical application. Therefore, as mentioned earlier, improving high-strength and high-toughness ceramic-matrix self-lubricating materials for practical applications is significant.

**Figure 8.** SEM micrographs of fracture surface of traditional Al2O3-graphite (a) and Al2O3-LaF3 (b) self-lubricating ce‐ ramic composites.

**Figure 9.** Mechanical properties of Al2O3-graphite (a) and Al2O3-LaF3 (b) self-lubricating composites.

#### **3.3. The performance of laminated-graded structure self-lubricating ceramic composites**

Compared to the traditional self-lubricating ceramic composites, laminated-graded structure self-lubricating ceramic composites exhibit excellent mechanical properties. Table 1 describes the bending strength of Al2O3-laminated-graded structure self-lubricating ceramic composites and of some monolithic self-lubricating ceramic composites. It can be seen from Table 1 that the bending strength of laminated-graded structure self-lubricating ceramic composites are much higher than any one of monolithic materials. The bending strength reached 348 MPa, which is approximately five times higher than that of the traditional monolithic Al2O3/SL and Al2O3-ZrO2(3Y)/SL ceramics, and which basically approached the properties of general monolithic Al2O3 and Al2O3-ZrO2(3Y) ceramics [17].

Additionally, the gradient exponent *p* has a remarkable influence on the mechanical properties of laminated-graded structure self-lubricating composites [18]. As shown in Figure 10, the bending strength of the ZrO2(3Y)-Al2O3/ZrO2(3Y)/SL FGM increased, with the increase of *p* up to 2.0, and then decreased rapidly when *p* exceeds 2.0. This phenomenon is caused by the residual stress between the adjacent layers in gradient area. The variation of *p* causes the change of content of SLs in gradient layers, and then the residual stress that is generated from the thermal mismatch because of the difference in thermal expansion coefficients between the adjacent graded layers (as shown in Figure 11) is influenced. This shows that a reasonable residual stress is essential to adjust the mechanical properties of these materials.


**Table 1.** Bending strength of laminated-graded structure self-lubricating ceramic composites [17].

earlier, improving high-strength and high-toughness ceramic-matrix self-lubricating materials

**Figure 8.** SEM micrographs of fracture surface of traditional Al2O3-graphite (a) and Al2O3-LaF3 (b) self-lubricating ce‐

**Figure 9.** Mechanical properties of Al2O3-graphite (a) and Al2O3-LaF3 (b) self-lubricating composites.

monolithic Al2O3 and Al2O3-ZrO2(3Y) ceramics [17].

**3.3. The performance of laminated-graded structure self-lubricating ceramic composites**

Compared to the traditional self-lubricating ceramic composites, laminated-graded structure self-lubricating ceramic composites exhibit excellent mechanical properties. Table 1 describes the bending strength of Al2O3-laminated-graded structure self-lubricating ceramic composites and of some monolithic self-lubricating ceramic composites. It can be seen from Table 1 that the bending strength of laminated-graded structure self-lubricating ceramic composites are much higher than any one of monolithic materials. The bending strength reached 348 MPa, which is approximately five times higher than that of the traditional monolithic Al2O3/SL and Al2O3-ZrO2(3Y)/SL ceramics, and which basically approached the properties of general

for practical applications is significant.

68 Advances in Functionally Graded Materials and Structures

ramic composites.

**Figure 10.** The bending strength of ZrO2(3Y)-Al2O3/ZrO2(3Y)/SL FGMs varies with the gradient exponent.

[18].

The laminated-graded structure ceramics not only showed excellent mechanical properties, it also maintained good tribological performance. As shown in Figure 12, in the temperature range of 25–800 °C, the friction coefficient of Al2O3 and ZrO2(3Y) laminated-graded structure composite was less than 0.55, which was approximately half of that of monolithic Al2O3 and ZrO2 ceramics. The decrease of friction coefficients were achieved by the presence of graphite, CaF2, and BaSO4, which have excellent lubricating property under different temperatures.

Figure 10. The bending strength of ZrO2(3Y)-Al2O3/ZrO2(3Y)/SL FGMs varies with the gradient exponent.

Figure 11. Variation of the difference value of coefficients of thermal expansion between the adjacent layers with the gradient exponent *p*

monolithic Al2O3 and ZrO2 ceramics. The decrease of friction coefficients were achieved by the presence of graphite, CaF2,

The laminated-graded structure ceramics not only showed excellent mechanical properties, it also maintained good tribological performance. As shown in Figure 12, in the temperature range of 25–800 °C, the friction coefficient of Al2O3

and ZrO2(3Y) laminated-graded structure composite was less than 0.55, which was approximately half of that of

**Figure 11.** Variation of the difference value of coefficients of thermal expansion between the adjacent layers with the gradient exponent *p* [18].

Graphite has a good lubricating property at room temperature to 300 °C, and CaF2 at 250 °C to 1000 °C. In addition, BaSO4 also possesses excellent self-lubricating performance over a broad temperature range. During the sliding process, these SLs form the self-lubricating film that is helpful to reduce direct contact between the ceramics and further improved the tribological properties of the materials [17,18].

**Figure 12.** The friction coefficients of two kinds of laminated-graded self-lubricating composites at room temperature to 800 °C.

In conclusion, laminated-graded structure self-lubricating ceramic composites realize the integration of mechanical and tribological properties. Their excellent mechanical and tribo‐ logical properties indicate that the laminated-graded structure self-lubricating ceramic composites have numerous high-technology applications and promising prospect as structural materials.
