**2. Design, fabrication, and properties of laminated ceramic composites**

#### **2.1. Design and fabrication of laminated composites**

tribological areas. Generally, incorporating solid lubricants (SLs) in ceramic matrixes solves the friction problems, which can reach a positive effect. Moreover, compound lubricants can exhibit excellent self-lubricating abilities in a wide range of temperatures because the lubri‐ cants can promote the formation of well-covered lubricating films on the surfaces of ceramics that can work effectively under different temperature [1-3]. Unfortunately, subsequent studies have shown that these composites are homogenous in terms of mechanical and tribological properties. Thus, the strength of ceramics and the lubrication of SLs cannot be fully utilized. Because the continuity of ceramic phases is destroyed by the layered structural SL phase, the mechanical property of this type of material is reduced [4,5]. In these situations, it is necessary

62 Advances in Functionally Graded Materials and Structures

to develop a high-strength and high-toughness self-lubricating ceramic composites.

Lamination is one of the new strategies being used to enhance the mechanical properties of ceramics. The ideas of laminated composites inspired from natural biomaterials, such as shells and teeth, are made of layered architectures combining materials with different properties. During the past decade, there are large amounts of layered ceramic composites that have been fabricated and studied [6-8]. These kinds of materials have non-catastrophic fracture behavior and damage tolerance, which exhibit much higher fracture toughness and work of fracture in them than in monolithic ceramics. Moreover, the unique configurations of the layered material allow design flexibility. Therefore, the combination of the laminated design of ceramic materials and self-lubricating ceramic composites with excellent lubricating property is a promising way to achieve the integration of mechanical and tribological properties [9-12].

For laminated self-lubricating ceramic composites, interfacial residual stress between the adjacent layers may have an important effect on their mechanical properties. Any modification or change of the interfacial structure and composition will be a determining factor in the strength of the interfacial bond and will eventually affect the toughness, strength, and fracture behavior of laminated composites [13]. Therefore, a reasonable residual stress between the adjacent layers is essential to improve the mechanical properties. Previous studies have shown that the graded design of the materials is an effective method to eliminate the interface stress of dissimilar material system [14-16]. This design concept of functionally graded materials (FGMs) was first raised by Japanese scientists in 1987 as reported in reference [14]. That is, components with different properties or structures disperse by a gradient change along with one direction instead of a homogeneous manner. Thus, the composite can exhibit different properties that are mutually exclusive at the same time, and the gradient change can eliminate the interface between components. This new-style and non-uniform composite realized the integration of structure and function, making it to have a wider prospect of application in

Based on the above background, the authors prepared high-performance structural/lubricat‐ ing-functional integration ceramic composites using the design of graded laminated structure [4,17,18]. This design is conductive to the combination of mechanical and tribological properties while retaining all the advantages of these materials. The aim of this chapter is to illustrate the design, fabrication, and properties of alumina and zirconia self-lubricating composites with laminated-graded structure and to provide guidance forthe optimum design

extreme conditions.

of these materials.

Figure 1 illustrates the schematic and the design concept of laminated composites. The thickness of the A layer and B layer are d1 and d2, respectively, where the A layer is the Al2O3 or ZrO2-Al2O3 and the B layer is Al2O3-ZrO2 or ZrO2. Commercially available Al2O3, ZrO2, Y2O3, CuO, and TiO2 were used in this study. The material was manufactured using the following steps [17-20]: (1) ball-milling of powder, (2) sequential stacking of layers in steel mold, and (3) hot-pressing in graphite mold. Hot-pressing was performed at 1350-1400 °C and 25 MPa using graphite die in an argon atmosphere for 100-120 minutes. Monolithic Al2O3 and ZrO2 with sintering aids were also sintered at same condition as comparisons. The micro‐ structures of the composites were observed using scanning electron microscopy (JSM-5600LV). The sintered specimens were sliced into test bars for bending strength and work of fracture.

**Figure 1.** Schematic of laminated composite structure.

An example of the microstructure of the ZrO2(3Y)-Al2O3/ZrO2(3Y)-laminated composites is shown in Figure 2, where the dark layer is the ZrO2(3Y)-Al2O3 layer and the light layer is the ZrO2(3Y) layer. The multilayer structure with a relatively straight interface can be observed without clear delamination. It can also be seen from Figure 2 that the ZrO2(3Y)-Al2O3 layer and ZrO2(3Y) layer have the same thickness of approximately 160 μm.

#### **2.2. The mechanical properties of laminated composites**

The geometric parameters of the layered structure are the key factors for the optimal design of laminated composites. These parameters mainly include the layer numbers and thickness ratio of the two layers. The mechanical properties of Al2O3/Al2O3-10wt.%ZrO2(3Y)-laminated composites with different layer numbers are shown in Figure 3 [19,20]. As shown in Figure 3, a relatively large number of layers are likely to improve the mechanical properties of the materials. When the number of layers is 41, the bending strength and work of fracture of materials reach the maximum value. The relationship between the mechanical properties and

**Figure 2.** SEM photograph of profile of laminated composites.

layer thickness ratio is displayed in Figure 4 [19,20]. One can see that the layer thickness ratio also has an enormous effect on the mechanical properties of laminated composites. The bending strength and work of fracture of all of the laminated materials are higher than that of the monolithic materials and decrease with the increase of the layer thickness ratio. When the layer thickness ratio is 1:1 and the thickness of each layer is 80 μm, the bending strength and work of fracture of the Al2O3/Al2O3-ZrO2(3Y) laminated composites could reach to 740 MPa and 3892 J m–2, respectively [19,20].

**Figure 3.** Effect of the layer numbers on the bending strength and work of fracture.

In addition, the compositions of the two layers also have significant effects on the mechanical properties of the laminate composites. The bending strength and work of fracture of Al2O3/ Al2O3-ZrO2(3Y)-laminated composites with different content of ZrO2(3Y) in Al2O3-ZrO2(3Y)

High-performance Self-lubricating Ceramic Composites with Laminated-graded Structure http://dx.doi.org/10.5772/62538 65

**Figure 4.** Effect of the thickness ratio on the bending strength and work of fracture.

layer thickness ratio is displayed in Figure 4 [19,20]. One can see that the layer thickness ratio also has an enormous effect on the mechanical properties of laminated composites. The bending strength and work of fracture of all of the laminated materials are higher than that of the monolithic materials and decrease with the increase of the layer thickness ratio. When the layer thickness ratio is 1:1 and the thickness of each layer is 80 μm, the bending strength and work of fracture of the Al2O3/Al2O3-ZrO2(3Y) laminated composites could reach to 740 MPa

and 3892 J m–2, respectively [19,20].

**Figure 2.** SEM photograph of profile of laminated composites.

64 Advances in Functionally Graded Materials and Structures

**Figure 3.** Effect of the layer numbers on the bending strength and work of fracture.

In addition, the compositions of the two layers also have significant effects on the mechanical properties of the laminate composites. The bending strength and work of fracture of Al2O3/ Al2O3-ZrO2(3Y)-laminated composites with different content of ZrO2(3Y) in Al2O3-ZrO2(3Y)

**Figure 5.** Relationship between mechanical properties of Al2O3/Al2O3-ZrO2(3Y)-laminated composites and content of ZrO2(3Y) in the Al2O3-ZrO2(3Y) layers.

layers are shown in Figure 5 [19,20]. As can be seen from the figure, with the increase of the content of ZrO2(3Y), first, the bending strength and work of fracture of the material increase and then they decrease gradually. When the mass content of ZrO2(3Y) is 10%, both bending strength and work of fracture reach the optimal value. This is mainly because the variation of content of ZrO2(3Y) in Al2O3-ZrO2(3Y) layers causes significant changes in the residual stresses between adjacent layers and the contribution of phase transformation toughening to the crack propagation energy of the materials, thus realizing the optimization of the materials [19,20]. The same design principles used for designing Al2O3/Al2O3-ZrO2(3Y)-laminated composites apply to designing ZrO2(3Y)-Al2O3/ZrO2(3Y) material. When the mass content of Al2O3 in 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 and b).

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