Microstructure, Mechanical and Tribological Properties of Si3N4/Mo-Laminated Composites

(1) Background: the applications of ceramic materials in a friction pair and a moving pair are limited, just because of their poor toughness and unsatisfactory tribological characteristics. In view of this, Mo as a soft metal layer was added into a Si3N4 matrix to improve its toughness and tribological characteristics. (2) Methods: The microstructure and metal/ceramic transition layer were examined using X-ray diffraction, scanning electron microscope, electron dispersive X-ray spectroscopy, and Vickers hardness. Bending strength and fracture toughness were also measured. Tribological characteristics were obtained on the pin-on-disc wear tester. (3) Results: It can be found that the multilayer structure could improve the fracture toughness of laminated composite compared with single-phase Si3N4, but the bending strength was significantly reduced. Through microstructure observation, the transition layer of Si3N4/Mo-laminated composite was revealed as follows: Si3N4→MoSi2→Mo5Si3→Mo3Si→Mo. Moreover, the addition of the Mo interface to silicon nitride ceramic could not significantly improve the tribological properties of Si3N4 ceramic against titanium alloy in seawater, and the friction coefficients and wear rates of the sliding pairs increased with the increase in load. (4) Conclusions: The process failed to simultaneously improve the comprehensive mechanical properties and tribological performance of Si3N4 ceramic by adding Mo as the soft interfacial layer. However, the utilization of metal interfacial layers to enhance the toughness of ceramics was further recognized and has potential significance for the optimization of ceramic formulation.


Introduction
Ceramic materials have high hardness, high strength, good chemical stability and excellent corrosion/wear resistance, and they have been applied in the manufacturing industry, building trades and even in the medical domain [1][2][3][4][5]. At present, ceramic materials have been used in various industrial fields, such as brake pads, radome, engines and cutting tools [6][7][8][9]. However, further applications of monolithic ceramic materials are limited due to their low resistance to fracture. Laminated composites are one of the main ways for improving the brittleness of ceramics, and are being paid more and more attention from a biomimetic point of view.
Since the 1970s, scholars have discovered that ceramic materials can be improved by bionic structural design, e.g., laminated nacre shell [10][11][12][13][14]. Zuo prepared Al 2 O 3 /Nilaminated composites via hot-press sintering at 25 MPa under an argon atmosphere at 1400 • C for 1 h, and found that the ceramic/metal composites exhibited a higher fracture toughness of 16.10 MPa·m 1/2 and a higher strength of 417.41 MPa than those of monolithic Al 2 O 3 [15]. Laminated Ti/Al 2 O 3 composite was fabricated via tap-casting and hot-press powder are shown in Figure 1. In addition, Y2O3 powder (with a purity of 99%, average particle size: 0.37 μm, Aijia New Material Science & Technology Ltd., Hefei, China) and Al2O3 powder (with a purity of 99.9%, average particle size: 1.17 μm, Aijia New Material Science & Technology Ltd., Hefei, China) were also adopted as sintering aids.
(a) (b) Aiming to realize the uniform distribution of Mo layer, the silicon nitride matrix composite with two-layer thickness ratio (11:1 and 9:1) of Si3N4/Mo was designed under the conditions of a certain sample thickness according to the relevant reference [34]. The laminated structure of Si3N4/Mo composite was firstly designed as shown in Figure 2, and the design parameters (e.g., layer number, thickness ratio and Mo mass ratio) is shown in Table 1.  Then, the Si3N4 powders with 4% Y2O3 and 6% Al2O3 powders were ball-milled using zirconia oxide balls for 5 h at 100 rpm in alcohol, and then the mixed powders were constantly stirred and dried in a drying oven. Subsequently, the dried powders of mixedceramics and Mo powder were weighted according to the relevant design parameters. Next, the ceramic and Mo powders were successively stacked in layers in a stainless-steel Aiming to realize the uniform distribution of Mo layer, the silicon nitride matrix composite with two-layer thickness ratio (11:1 and 9:1) of Si 3 N 4 /Mo was designed under the conditions of a certain sample thickness according to the relevant reference [34]. The laminated structure of Si 3 N 4 /Mo composite was firstly designed as shown in Figure 2, and the design parameters (e.g., layer number, thickness ratio and Mo mass ratio) is shown in Table 1.
Materials 2022, 15, x FOR PEER REVIEW 3 of 16 powder are shown in Figure 1. In addition, Y2O3 powder (with a purity of 99%, average particle size: 0.37 μm, Aijia New Material Science & Technology Ltd., Hefei, China) and Al2O3 powder (with a purity of 99.9%, average particle size: 1.17 μm, Aijia New Material Science & Technology Ltd., Hefei, China) were also adopted as sintering aids.
(a) (b) Aiming to realize the uniform distribution of Mo layer, the silicon nitride matrix composite with two-layer thickness ratio (11:1 and 9:1) of Si3N4/Mo was designed under the conditions of a certain sample thickness according to the relevant reference [34]. The laminated structure of Si3N4/Mo composite was firstly designed as shown in Figure 2, and the design parameters (e.g., layer number, thickness ratio and Mo mass ratio) is shown in Table 1.  Then, the Si3N4 powders with 4% Y2O3 and 6% Al2O3 powders were ball-milled using zirconia oxide balls for 5 h at 100 rpm in alcohol, and then the mixed powders were constantly stirred and dried in a drying oven. Subsequently, the dried powders of mixedceramics and Mo powder were weighted according to the relevant design parameters. Next, the ceramic and Mo powders were successively stacked in layers in a stainless-steel  Then, the Si 3 N 4 powders with 4% Y 2 O 3 and 6% Al 2 O 3 powders were ball-milled using zirconia oxide balls for 5 h at 100 rpm in alcohol, and then the mixed powders were constantly stirred and dried in a drying oven. Subsequently, the dried powders of mixedceramics and Mo powder were weighted according to the relevant design parameters. Next, the ceramic and Mo powders were successively stacked in layers in a stainless-steel mold, and the slab of multilayer sample was cold pressed for 10 min at a pressure of  30 MPa. Finally, the multilayer slab was hot-pressed sintered for 30 min at a pressure of 30 MPa and a temperature of 1800 • C in a nitrogen atmosphere. In this way, a Si 3 N 4 /Mo composite disc with a size of Φ 45 mm × 6 mm was prepared, and a pure Si 3 N 4 disc was also prepared as reference for comparison with the mechanical properties of layered composite. Additionally, then, the test piece with a size of 35 mm × 3 mm × 4 mm was cut from the disc sample for its physical and mechanical properties, and the test piece with a size of 10 mm × 5 mm × 5 mm was also cut for tribological properties as shown in Figure 2.

Test Procedure
To obtain the physical and mechanical properties of a laminated ceramic composite, the density and porosity of the ceramic composite were measured according to the Archimedes methods, the bending strength of composite was determined by a three-point bending test with a span length of 30 mm and a crosshead speed of 0.5 mm/min. The Vickers hardness was measured on polished surface with a load of 10 N for 15 s, and each sample has at least 10 Vickers indentations on its surface. The indentation toughness is calculated by the redial crack length and the indentation diagonal length.
To obtain the friction coefficient and wear rate of the laminated ceramic composite, the tribological test of composite sliding against TC4 in artificial seawater was conducted with a pin-on-disc tribometer. In this test tribometer, an upper pin contacts a stationary disc. The pin specimen (11SM and 9SM in Table 1) with a filleted square end was used to form flat contacts; the disc, as the mating materials, was machined from TC4, in a size of 44 mm in diameter and 5 mm in thickness. The TC4 disc was finished by grinding to achieve a surface roughness (Ra) of about 0.1 µm, and the laminated composite was carefully polished to a surface roughness of Ra 0.1-0.3 µm. The pin and disc samples were both ultrasonically cleaned in fresh alcohol. The discs were fixed, and the composite pin was rotated at a speed of 500 r/min (0.836 m/s) and normal loads of 10 N (0.4 MPa), 20 N (0.8 MPa) and 30 N (1.2 MPa). Meanwhile, the total sliding time was set as 20 min. Additionally, the liquid medium artificial seawater prepared according to Standard ASTM D 1141-98 (as shown in Table 2). The initial running-in period was not accounted for the calculation of friction coefficient (f ) and wear rate (w). The friction coefficient is directly determined by the tester. Additionally, the wear rate is defined by w = ∆m/(ρPL), where ∆m represents the mass wear volume assessed by weight loss using a microbalance (accuracy = 0.1 mg), P is the normal load, L is the sliding distance, and ρ is the density. Friction coefficients and wear rates were obtained from the average of the values taken from three runs. The composite samples were deeply etched in a solution of NaOH for 2 min, and the microstructure of ceramic composite was observed by scanning electron microscope (SEM). Additionally, the phase composition of composite was analysed by X-ray diffract meter (XRD). The morphological analysis and chemical characterization of the wear surfaces were made by SEM/EDS. In this case, the toughening mechanism and wear mechanism of Si 3 N 4 /Mo composite was revealed in this study, as shown in Figure 3.

Results and Discussion
In this study, one new laminated material-the Si3N4/Mo composite-was developed to improve the mechanical and tribological properties of silicon nitride ceramic. Due to the dependence of material performance on the microstructure of materials, the phase composition and microstructure was firstly analyzed in this section. Then, the mechanical properties of the composite material were also analyzed, and the underling toughening mechanism was also discussed. Meanwhile, the tribological performance was tested, and the wear mechanism is analyzed in depth in the folowing subsection.

Phase Composition and Microstructure
It is well known that α-phase silicon nitride starting powder changed into β-phase silicon nitride bulk during the sintering process. In this study, we must reveal the influence of Mo powder on Si3N4 phase transformation and the existence form of Mo powder in the ceramic composite. The phase composition of Si3N4/Mo-laminated composite was analyzed by XRD, and the result is shown in Figure 4. It can be seen that the laminated composite is composed of β-Si3N4 and Mo5Si3 phases were detected on the surface of 9SM composite. Obviously, α-Si3N4 was completely transited to β-Si3N4 during the sintering process. The other main phase Mo5Si3 is one molybdenum-silicon compound with a certain brittleness, and this compound should be a reaction product between Si3N4 and Mo during the fabrication process. No Mo was detected on the composite surface, and the Mo layer on the surface should react with silicon nitride to some compounds (e.g., Mo5Si3).

Results and Discussion
In this study, one new laminated material-the Si 3 N 4 /Mo composite-was developed to improve the mechanical and tribological properties of silicon nitride ceramic. Due to the dependence of material performance on the microstructure of materials, the phase composition and microstructure was firstly analyzed in this section. Then, the mechanical properties of the composite material were also analyzed, and the underling toughening mechanism was also discussed. Meanwhile, the tribological performance was tested, and the wear mechanism is analyzed in depth in the folowing subsection.

Phase Composition and Microstructure
It is well known that α-phase silicon nitride starting powder changed into β-phase silicon nitride bulk during the sintering process. In this study, we must reveal the influence of Mo powder on Si 3 N 4 phase transformation and the existence form of Mo powder in the ceramic composite. The phase composition of Si 3 N 4 /Mo-laminated composite was analyzed by XRD, and the result is shown in Figure 4. It can be seen that the laminated composite is composed of β-Si 3 N 4 and Mo 5 Si 3 phases were detected on the surface of 9SM composite. Obviously, α-Si 3 N 4 was completely transited to β-Si 3 N 4 during the sintering process. The other main phase Mo 5 Si 3 is one molybdenum-silicon compound with a certain brittleness, and this compound should be a reaction product between Si 3 N 4 and Mo during the fabrication process. No Mo was detected on the composite surface, and the Mo layer on the surface should react with silicon nitride to some compounds (e.g., Mo 5 Si 3 ). Figure 5 shows the micromorphology of metal layer and ceramic matrix for 9SMlaminated composites. Figure 5a shows the microstructure of the metal layer, and it can be seen that some cracks appear in the metal layer, and the characteristics of brittle phase are very obvious. Figure 5b shows the microstructure of silicon nitride matrix, and it can be clearly seen that the ceramic matrix is mainly composed of columnar crystal, and some special compounds (indicated by the red arrows) are distributed sporadically on the matrix. Materials 2022, 15, x FOR PEER REVIEW 6 of 16  Figure 5 shows the micromorphology of metal layer and ceramic matrix for 9SMlaminated composites. Figure 5a shows the microstructure of the metal layer, and it can be seen that some cracks appear in the metal layer, and the characteristics of brittle phase are very obvious. Figure 5b shows the microstructure of silicon nitride matrix, and it can be clearly seen that the ceramic matrix is mainly composed of columnar crystal, and some special compounds (indicated by the red arrows) are distributed sporadically on the matrix.  Figure 6 gives the enlarged morphologies of interface between ceramic matrix and metal layer, and the corresponding EDS analysis results. Figure 6a shows the enlarged morphology o the interface area near the ceramic matrix ("A" area), and it can be seen that there is a transition area between ceramic matrix ("A" area) and the interface area ("B" area). Figure 6b shows the corresponding EDS analysis result from "A" to "B" area, and it can be found that Mo element gradually appears and increases to a certain value. After the transition of the "X" region, the relative ratio of Mo and Si elements reached a stable state of about 5:3. Combined with the XRD result, it can be confirmed that Mo5Si3 formed in the interface between the ceramic matrix and the Mo layer. Figure 6c shows the enlarged morphology of the interface area near the metal layer, and the corresponding EDS analysis result is shown in Figure 6d. Figure 6d presents that along the path from the interface area ("C" area) to metal layer ("D" area), the Si element gradually decreases, while the Mo element gradually increases. After passing through the "Y" area, the relative ratio with Mo element reached a stable state of about 3:5. As discussed above, it can be   Figure 5 shows the micromorphology of metal layer and ceramic matrix for 9SMlaminated composites. Figure 5a shows the microstructure of the metal layer, and it can be seen that some cracks appear in the metal layer, and the characteristics of brittle phase are very obvious. Figure 5b shows the microstructure of silicon nitride matrix, and it can be clearly seen that the ceramic matrix is mainly composed of columnar crystal, and some special compounds (indicated by the red arrows) are distributed sporadically on the matrix.  Figure 6 gives the enlarged morphologies of interface between ceramic matrix and metal layer, and the corresponding EDS analysis results. Figure 6a shows the enlarged morphology o the interface area near the ceramic matrix ("A" area), and it can be seen that there is a transition area between ceramic matrix ("A" area) and the interface area ("B" area). Figure 6b shows the corresponding EDS analysis result from "A" to "B" area, and it can be found that Mo element gradually appears and increases to a certain value. After the transition of the "X" region, the relative ratio of Mo and Si elements reached a stable state of about 5:3. Combined with the XRD result, it can be confirmed that Mo5Si3 formed in the interface between the ceramic matrix and the Mo layer. Figure 6c shows the enlarged morphology of the interface area near the metal layer, and the corresponding EDS analysis result is shown in Figure 6d. Figure 6d presents that along the path from the interface area ("C" area) to metal layer ("D" area), the Si element gradually decreases, while the Mo element gradually increases. After passing through the "Y" area, the relative ratio with Mo element reached a stable state of about 3:5. As discussed above, it can be  Figure 6 gives the enlarged morphologies of interface between ceramic matrix and metal layer, and the corresponding EDS analysis results. Figure 6a shows the enlarged morphology o the interface area near the ceramic matrix ("A" area), and it can be seen that there is a transition area between ceramic matrix ("A" area) and the interface area ("B" area). Figure 6b shows the corresponding EDS analysis result from "A" to "B" area, and it can be found that Mo element gradually appears and increases to a certain value. After the transition of the "X" region, the relative ratio of Mo and Si elements reached a stable state of about 5:3. Combined with the XRD result, it can be confirmed that Mo 5 Si 3 formed in the interface between the ceramic matrix and the Mo layer. Figure 6c shows the enlarged morphology of the interface area near the metal layer, and the corresponding EDS analysis result is shown in Figure 6d. Figure 6d presents that along the path from the interface area ("C" area) to metal layer ("D" area), the Si element gradually decreases, while the Mo element gradually increases. After passing through the "Y" area, the relative ratio with Mo element reached a stable state of about 3:5. As discussed above, it can be concluded that one of the interface compounds is Mo 5 Si 3 , and there are still other Si-Mo compounds in the interface layer. According to the relevant studies [35], the reaction of Si3N4 and Mo can take place high temperatures, as follows. 3  Mo Si 48Mo 3Mo Si + → From the chemical equations above, MoSi2, Mo3Si and Mo5Si are all the chemic products of Si3N4 with Mo. Combined with EDS analysis results, it can be inferred th substance in region "X" (in Figure 6b) should be MoSi2, and the substance in region " (in Figure 6d) should be Mo3Si. Therefore, the material distribution from the ceram matrix to the Mo layer is: Si3N4→MoSi2→Mo5Si3→Mo3Si→Mo; namely, the transition lay between ceramic matrix and metal layer is Si3N4→MoSi2→Mo5Si3→Mo3Si→Mo.
From the discussion above, Si3N4 ceramics reacted with Mo to form molybdenu silicide. It is well known that the thermodynamic condition for these reactions is that t corresponding Gibbs free energy must be negative. Additionally, the mathematic expression for the Gibbs free energy at a given temperature T can be described below.
( ) where ΔG is the difference in Gibbs free energy of the chemical reaction, ΔH is t difference in the enthalpy of the chemical reaction, ΔS is the difference in the entropy According to the relevant studies [35], the reaction of Si 3 N 4 and Mo can take place at high temperatures, as follows.
From the chemical equations above, MoSi 2 , Mo 3 Si and Mo 5 Si are all the chemical products of Si 3 N 4 with Mo. Combined with EDS analysis results, it can be inferred that substance in region "X" (in Figure 6b) should be MoSi 2 , and the substance in region "Y" (in Figure 6d) should be Mo 3 Si. Therefore, the material distribution from the ceramic matrix to the Mo layer is: Si 3 N 4 →MoSi 2 →Mo 5 Si 3 →Mo 3 Si→Mo; namely, the transition layer between ceramic matrix and metal layer is Si 3 N 4 →MoSi 2 →Mo 5 Si 3 →Mo 3 Si→Mo.
From the discussion above, Si 3 N 4 ceramics reacted with Mo to form molybdenum silicide. It is well known that the thermodynamic condition for these reactions is that the corresponding Gibbs free energy must be negative. Additionally, the mathematical expression for the Gibbs free energy at a given temperature T can be described below.
where ∆G is the difference in Gibbs free energy of the chemical reaction, ∆H is the difference in the enthalpy of the chemical reaction, ∆S is the difference in the entropy of the chemical reaction, T is the reaction temperature (T = C + 273.15, Kelvin), and C p is the molar heat capacity of the substance at 298 K. Table 3 lists the relevant enthalpy and entropy of the products and reactants. According to Formula (4), the Gibbs free energies of reaction Equations (1)-(3) are calculated to determine the possibility of the spontaneous occurrence for the chemical reactions.  (2) is the lowest, the drive for this reaction to happen should be highest. Because of this, Mo 5 Si 3 occupied the largest proportion in the transition layer. Overall, it can be verified that for Si 3 N 4 /Mo-laminated composite, one transition layer (Si 3 N 4 →MoSi 2 →Mo 5 Si 3 →Mo 3 Si→Mo) formed between β-phase Si 3 N 4 matrix and Mo metal layer.

Mechanical and Tribological Properties
Based on the above, it can be confirmed that one chemical transition layer formed between Si 3 N 4 and Mo. Meanwhile, the Mo-Si compounds such as Mo 5 Si 3 present brittle characteristics (as shown in Figure 5a). Such a microstructure should affect the mechanical and tribological properties of the laminated composite. Table 5 gives the mechanical properties of Si 3 N 4 /Mo-laminated composites. Compared with single-phase Si 3 N 4 , the composite presented lower bending strength and higher toughness. The ceramic matrix in the composite presented a similar hardness compared with single-phase Si 3 N 4 . It is obvious that the Si 3 N 4 /Mo-laminated composites do exhibit good toughness, but the strength is very poor. The similar hardness of the ceramic matrix to the single Si 3 N 4 would be attributed to the completed phase transformation of Si 3 N 4 during the same sintering process (as shown in Figure 5). Meanwhile, Mo 5 Si 3 , as a reaction product, is a brittle substance between the ceramic matrix and the metal layer. This substance has a large difference in lattice constant (a/c ≈ 2), and the coefficient of thermal expansion is anisotropy (α c /α a ≈ 2). Therefore, cracks would appear during the growth of a Mo 5 Si 3 single crystal, and this is the reason for  Figure 5. Consequently, the ability of the laminated composite to resist cracking is reduced. Thus, the bending strength of laminated composite was obviously lower than that of the single-phase ceramic.
On the other hand, due to the difference of thermal expansion coefficient between the interface layer and the matrix layer, tensile stress and compressive stress were generated at the interface between the ceramic matrix and metal layer. In this case, when the crack propagated in the laminated composite, the existence of residual stress caused the crack to deflect at the interface of the laminated composite, thus consuming the fracture energy and improving the fracture toughness of the laminated composite. Figure 7 shows the tendency of a zigzag crack propagation of 9SM and 11SM composites. Therefore, the fracture toughness of laminated composite is higher than that of single-phase ceramic. From Figure 7, it can be also seen that the crack deflects as it passes through the metal layer in 9SM composite, while some cracks directly penetrate the metal layer and the ceramic matrix in 11SM composite. Combined with Table 1, it can be concluded that, when the content of Mo in the laminated composite is higher, it is beneficial to prevent the diffusion of cracks and improve the fracture toughness of the material.
The similar hardness of the ceramic matrix to the single Si3N4 would be attributed to the completed phase transformation of Si3N4 during the same sintering process (as shown in Figure 5). Meanwhile, Mo5Si3, as a reaction product, is a brittle substance between the ceramic matrix and the metal layer. This substance has a large difference in lattice constant (a/c ≈ 2), and the coefficient of thermal expansion is anisotropy (αc/αa ≈ 2). Therefore, cracks would appear during the growth of a Mo5Si3 single crystal, and this is the reason for the large number of cracks observed in the metal layer as shown in Figure 5. Consequently, the ability of the laminated composite to resist cracking is reduced. Thus, the bending strength of laminated composite was obviously lower than that of the singlephase ceramic.
On the other hand, due to the difference of thermal expansion coefficient between the interface layer and the matrix layer, tensile stress and compressive stress were generated at the interface between the ceramic matrix and metal layer. In this case, when the crack propagated in the laminated composite, the existence of residual stress caused the crack to deflect at the interface of the laminated composite, thus consuming the fracture energy and improving the fracture toughness of the laminated composite. Figure  7 shows the tendency of a zigzag crack propagation of 9SM and 11SM composites. Therefore, the fracture toughness of laminated composite is higher than that of singlephase ceramic. From Figure 7, it can be also seen that the crack deflects as it passes through the metal layer in 9SM composite, while some cracks directly penetrate the metal layer and the ceramic matrix in 11SM composite. Combined with Table 1, it can be concluded that, when the content of Mo in the laminated composite is higher, it is beneficial to prevent the diffusion of cracks and improve the fracture toughness of the material. The tribological characteristics of the laminated composite were also carried out, and the friction coefficients and wear rates of Si3N4/Mo-laminated composite sliding against TC4 pairs at different load in seawater are shown in Figure 8. From this figure, it can be seen that the friction coefficients of the sliding pairs increase with the increase in load (from 10 N to 30 N) in Figure 8a. In general, the friction coefficient of laminated composite/TC4 is at a range from 0.3 to 0.5, and the friction coefficient of 9SM/TC4 pair is highest. From Figure 8b,c, the wear rates of pin and disc both increase with the increase in load. Additionally, the wear rates of laminated composite/TC4 pair are higher than the single ceramic/TC4 pair. Combined with the results of friction coefficient and wear rate, it can be seen that adding Mo as the interfacial layer to the Si3N4 ceramic matrix did not improve its tribological properties. The tribological characteristics of the laminated composite were also carried out, and the friction coefficients and wear rates of Si 3 N 4 /Mo-laminated composite sliding against TC4 pairs at different load in seawater are shown in Figure 8. From this figure, it can be seen that the friction coefficients of the sliding pairs increase with the increase in load (from 10 N to 30 N) in Figure 8a. In general, the friction coefficient of laminated composite/TC4 is at a range from 0.3 to 0.5, and the friction coefficient of 9SM/TC4 pair is highest. From Figure 8b,c, the wear rates of pin and disc both increase with the increase in load. Additionally, the wear rates of laminated composite/TC4 pair are higher than the single ceramic/TC4 pair. Combined with the results of friction coefficient and wear rate, it can be seen that adding Mo as the interfacial layer to the Si 3 N 4 ceramic matrix did not improve its tribological properties.  Figure 9 shows the morphologies of the worn surfaces of 11SM pins in seawater. From the figures, it can be seen that the metal transfer layer gradually appears on the worn surface of the laminated composite with the increase in load (Figure 9a-c). At loads of 20 N and 30 N, the worn surface becomes significantly coarser (Figure 9b,c). The EDS analysis results of the worn surface of 11SM pin at a load of 30 N is shown in Figure 10. It can be seen that the ceramic matrix region is mainly composed of two different regions ("A" and "B" zone in Figure 9c). The region "A" is mainly composed of Si elements, while the region "B" is mainly composed of metallic elements such as Ti, Mo and V ( Figure  10a,b). Obviously, some metal transfer layers formed on the wear surface of composite pin. The wear surface of metal interface layer ("C" zone in Figure 9c) is mainly composed of Ti, V and other elements as shown in Figure 10c. It is interesting that the Mo element is not detected on the metal interface layer, which may be also attributed to the formation of metal transferred layer from TC4 disc. From the analysis results above, it can be found that even under the cooling and lubrication of seawater, obvious adhesion wear still appeared when the laminated composite ceramic was matched with the titanium alloy.  Figure 9 shows the morphologies of the worn surfaces of 11SM pins in seawater. From the figures, it can be seen that the metal transfer layer gradually appears on the worn surface of the laminated composite with the increase in load (Figure 9a-c). At loads of 20 N and 30 N, the worn surface becomes significantly coarser (Figure 9b,c). The EDS analysis results of the worn surface of 11SM pin at a load of 30 N is shown in Figure 10. It can be seen that the ceramic matrix region is mainly composed of two different regions ("A" and "B" zone in Figure 9c). The region "A" is mainly composed of Si elements, while the region "B" is mainly composed of metallic elements such as Ti, Mo and V (Figure 10a,b). Obviously, some metal transfer layers formed on the wear surface of composite pin. The wear surface of metal interface layer ("C" zone in Figure 9c) is mainly composed of Ti, V and other elements as shown in Figure 10c. It is interesting that the Mo element is not detected on the metal interface layer, which may be also attributed to the formation of metal transferred layer from TC4 disc. From the analysis results above, it can be found that even under the cooling and lubrication of seawater, obvious adhesion wear still appeared when the laminated composite ceramic was matched with the titanium alloy.  The worn surfaces of the TC4 disc against the 11SM pin are shown in Figure 11. In the figure, obvious furrows can be observed on the wear surfaces of the TC4 disc. Meanwhile, when the load is 20 N and 30 N, the worn surface of TC4 disk appears the characteristics of repeated adhesion. Obviously, when the Si3N4/Mo composite slid against the TC4, the surface of the titanium alloy disc was ploughed by the micro-bulge on the surface of composite pin. Due to the incorporation of the Mo layer, more serious adhesion wear occurred for the composite/TC4 pair compared with Si3N4/TC4. Therefore, the friction coefficient and wear rate of the Si3N4/Mo against the TC4 were both higher than those of the Si3N4 against the TC4. The worn surfaces of the TC4 disc against the 11SM pin are shown in Figure 11. In the figure, obvious furrows can be observed on the wear surfaces of the TC4 disc. Meanwhile, when the load is 20 N and 30 N, the worn surface of TC4 disk appears the characteristics of repeated adhesion. Obviously, when the Si 3 N 4 /Mo composite slid against the TC4, the surface of the titanium alloy disc was ploughed by the micro-bulge on the surface of composite pin. Due to the incorporation of the Mo layer, more serious adhesion wear occurred for the composite/TC4 pair compared with Si 3 N 4 /TC4. Therefore, the friction coefficient and wear rate of the Si 3 N 4 /Mo against the TC4 were both higher than those of the Si 3 N 4 against the TC4. Based on the discussion above, when the Mo was added into the Si3N4 matrix as an interface layer, the fracture toughness of the ceramics was slightly enhanced to 11.2 MPam 1/2 , but the strength was reduced to 330 MPa. K. Balazsi [36] prepared a layered silicon nitride-zirconia composite with MLG with a fracture toughness of 4.6 MPam 1/2 , and a bending strength of 264 MPa. Sun Mengyong [37] also indicated that a SiC/BN composite presented a higher fracture toughness of 8.5 MPam 1/2 and a lower bending strength of 300 MPa. Compared with the experimental data, the mechanical properties of the Si3N4/Mo-laminated composite are better, especially for the high fracture toughness.
However, even in seawater, the tribological properties of Si3N4 against the titanium alloy could not be effectively improved. On the contrary, the incorporation of the Mo interface layer played a role in the degradation.
It can be seen from the research work in this paper that the Si3N4/Mo composite materials have lower strength and poor tribological properties, so they are not suitable as a friction pair material. However, the composite materials also have higher toughness and moderate hardness, and the addition of the metal Mo should improve the thermal conductivity of the silicon nitride material; therefore, these composite materials can be considered for use in a radome and other non-stressed parts. Based on the discussion above, when the Mo was added into the Si 3 N 4 matrix as an interface layer, the fracture toughness of the ceramics was slightly enhanced to 11.2 MPa·m 1/2 , but the strength was reduced to 330 MPa. K. Balazsi [36] prepared a layered silicon nitridezirconia composite with MLG with a fracture toughness of 4.6 MPa·m 1/2 , and a bending strength of 264 MPa. Sun Mengyong [37] also indicated that a SiC/BN composite presented a higher fracture toughness of 8.5 MPa·m 1/2 and a lower bending strength of 300 MPa. Compared with the experimental data, the mechanical properties of the Si 3 N 4 /Mo-laminated composite are better, especially for the high fracture toughness.
However, even in seawater, the tribological properties of Si 3 N 4 against the titanium alloy could not be effectively improved. On the contrary, the incorporation of the Mo interface layer played a role in the degradation.
It can be seen from the research work in this paper that the Si 3 N 4 /Mo composite materials have lower strength and poor tribological properties, so they are not suitable as a friction pair material. However, the composite materials also have higher toughness and moderate hardness, and the addition of the metal Mo should improve the thermal conductivity of the silicon nitride material; therefore, these composite materials can be considered for use in a radome and other non-stressed parts.

Conclusions
In this study, Mo was added into a silicon nitride matrix as a metal interface layer, and then the Si 3 N 4 /Mo-laminated ceramic composite was successfully fabricated via hot-press sintering. The mechanical and tribological properties were investigated in this study, and the following conclusions were obtained.
(1) The transition layer from the ceramic matrix to Mo interface layer was Si 3 N 4 →MoSi 2 → Mo 5 Si 3 →Mo 3 Si→Mo. Meanwhile, the compounds including MoSi 2 , Mo 5 Si 3 and Mo 3 Si were all the products of the reaction between the ceramic matrix and the metal interface layer. The transition layer was mainly composed of brittle phase Mo 5 Si 3 , which had a negative effect on the mechanical properties of laminated composite. (2) The ductility of the metal Mo layer and the residual stress between the ceramic matrix and the metal layer resulted in crack deflection and branching, as well as a higher fracture toughness for the laminated ceramic composite. (3) The incorporation of the Mo interface layer to the silicon nitride matrix degraded the tribological properties of the Si 3 N 4 ceramic sliding against the TC4 in seawater. Meanwhile, with the increase in load, the friction coefficients and wear rates both also increased.
In general, the addition of the Mo metal as an interface layer to the Si 3 N 4 ceramic matrix was intended to improve the mechanical and tribological properties of ceramics, which failed. However, this research also suggests that adding a metal as an interface layer to a ceramic matrix does indeed toughen the ceramic material. Hence, the authors of this paper indicate that the composite can be considered for use in a radome and other non-stressed parts.