Laminated Si 3 N 4 -Mo Composites Produced by Hot Pressing

In this paper, the physical, mechanical and friction behavior of Si 3 N 4 -Mo laminated composites were investigated. It can be found that the multilayer structure could improve the fracture toughness of laminated composite compared with pure Si 3 N 4 . Through the thermodynamic and kinetic calculations of chemical reactions, we conclude that the main product of the interface is Mo 5 Si 3 which caused poor physical and mechanical properties of Si 3 N 4 -Mo layered ceramics. The higher content of Mo was beneficial to hinder the diffusion of cracks and improved the fracture toughness of the material, but with the lower content of Mo, Si 3 N 4 -Mo laminated ceramic composite had better tribology properity . In addition, with the increase of the load, the tribology properity of Si 3 N 4 -Mo laminated ceramic composite decreased, and the friction mechanism of Si 3 N 4 -Mo/TC4 is a mixed wear of adhesive wear and abrasive wear.


Introduction
As one of the most important structural materials, silicon nitride (Si 3 N 4 ) based ceramic composites had been widely used in bearings, piston pumps, engine parts and high-speed cutting tools. However, this kind of composites showed low fracture toughness in application [1,2] . Therefore, a method to effectively toughen Si 3 N 4 based composites was greatly needed.
In the 1970s, researchers designed and prepared bionic structural ceramics by analysing and imitating the structure of nacre shell in nature, which provided a new idea for the toughening and strengthening of ceramic materials [3,4] . From then on, scholars tried to add solid lubricant like BN to ceramic materials as a sandwich. D Kovar [5] et al. prepared and studied the effect of the toughness of the weak interface layer on the mechanical properties of the Si 3 N 4 /BN laminated composite. The result showed that the composition of interface layer had little influence on the strength of layered materials, but the fracture work of materials decreased sharply with the increase of Si 3 N 4 content. Similarly, Q Zan [6] et al studied the Si 3 N 4 /BN laminated composite and found that the strength of layered materials decreased with the increase of the number of layers. Tan Jun [7] prepared and studied B 4 C/BN laminated ceramic composite by water-based tape casting and laminated hot-pressed sintering. It is found that the thickness ratio and the composition of the weak interfacial layer had an effect on the mechanical properties and microstructure of the layered ceramics. In the laminated ceramic composites, not only the dispersion toughening caused by adding second phase, but also the toughening mechanism such as crack deflection, bifurcation and delamination caused by the layered structure, which effectively improves the mechanical properties of the ceramic materials. In addition, many scholars added metal materials as interlayers to ceramic materials. K Zuo [8] et al.
found that the strength and toughness Al 2 O 3 -Ni/Ni laminated composite material was higher than that of Al 2 O 3 /Ni layered composite with the same structural parameters.
However, the mechanical properties of layered ceramic materials have always been the focus of attention, while the tribological properties and lubrication mechanism are rarely reported.
Our group has been devoted to the tribological research of Si 3 N 4 ceramic composites in various fields, and the developed Si 3 N 4 -hBN ceramic composites have certain practical functions in many fields [9][10][11][12][13][14][15][16][17] . However, in Si 3 N 4 -hBN ceramic composites, the binding capacity of hBN and ceramic matrix is poor, and excessive addition of hBN will cause the ceramic composites to lose their original high strength and high hardness. According to existing research findings [11] , the layered ceramic materials made by adding metal Mo as weak surface layer have good corrosion resistance and can achieve toughening effect of ceramic materials. Based on this, we add Mo interface layer to Si 3 N 4 ceramic composites to prepare Si 3 N 4 /Mo layered composites, and explore its physical and mechanical properties. We hope that this study would provide data supporting for the application of ceramic materials and polymer materials in marine engineering equipment,.

Experimental procedure 2.1 Raw materials
In this study, commercial α-silicon nitride (Si 3 N 4 : 99.9% pure with an average particle size of 1.5 μm from HeFei Aijia New Material Co., Ltd.) and molybdenum (Mo: 99.0% pure from Sinopharm Chemical Reagent Co., Ltd, China) were used.
Ytterbium oxide (Y 2 O 3 : an average particle size of 0.37 μm and purity of 99%) and aluminium oxide (Al 2 O 3 : an average particle size of 1.17 μm and purity of 99%) were used as sintering aids.

Fabrication procedure
The Si 3 N 4 powder with 4% Y 2 O 3 and 6% Al 2 O 3 powders was ball-milled using zirconia oxide balls for 5 h at 100 rpm in alcohol, after that, these mixed powders were constantly stirred and dried in a drying oven. Then, the dried powders of mixed-ceramics and Mo were weighted into some shares, the compositions of ceramics and Mo powders were shown in Table 1. The ceramics and Mo powders were stacked the layers in sequence in a stainless-steel die, and the green body of multilayer sample was pressed at a pressure of 30 MPa for 10 minutes. Next, the multilayer green bodies of 11Si 3 N 4 -Mo and 9Si 3 N 4 -Mo were hot-pressed at a pressure of 30 MPa and at 1800 ℃ for 30 minutes in a nitrogen atmosphere, respectively. The fabrication procedure of the laminated samples is shown in Fig. 1. In addition, the pure Si 3 N 4 was prepared as the contrastive sample by the same process of ball milling, drying and hot-pressing. with a span length of 16 mm and a crosshead speed of 0.5 mm/min. The indentation toughness was calculated from the lengths of radial cracks and indents diagonals using a formula given as follows [19] : where, a is the half length of indentation diagonal, c is the radial half-crack length and HV is Vickers hardness of materials.
Sintered specimens were grinded, polished to a range of 0.8~1μm by a diamond grinding tool and chemically etched in molten NaOH at 400°C for 2 min. The microstructures were then studied by using a VEGA II XMU scanning electron microscope.
The tribological test of the laminated samples was conducted using a pin-on-disc tribometer against an Φ 44 mm × 6 mm TC4 disc. Before the wear test, the sintered composites were carefully polished to obtain a surface roughness of 0.  Table 2). The worn surface of the composites was analyzed by scanning electron microscopy (SEM, FEI Company, Hillsboro, USA). Structure analysis of the laminated ceramics was conducted by X-ray diffraction (XRD, D/max2200PC, Rigaku, Japan).

Phase composition
The XRD analysis of the prepared Si 3 N 4 /Mo layered ceramic composite material is shown in Fig. 2. The results show that the Si 3 N 4 has also completed the transformation from α phase to β phase under this process, and no α-Si 3 N 4 has been detected. As shown in Fig. 3, Si 3 N 4 completely exist in the β phase in the form of rods. However, Mo 5 Si 3 phase that is the product of the reaction between Si 3 N 4 and Mo has been detected, and there is a molybdenum-silicon compound at the interface between the ceramic and the metal. The main phase formed is Mo 5 Si 3 . This substance has a large difference in lattice constant (a/c≈2), the coefficient of thermal expansion is anisotropy (α c /α a ≈2), therefore, Mo 5 Si 3 will have cracks during the growth of single crystals, which is also the reason for the large number of cracks found in the metal region in Figure 4. In addition, no Mo element was detected in the metal layer, indicating that Mo may remain small or have completely reacted. As shown in Fig. 5(d), there is indeed elemental Mo in the area near the center of the metal layer, and when the detection gradually extends from "D" to "C", the Si element suddenly appears, passing through the "Y" area, the relative ratio with Mo element reached a stable state of about 3:5, and substances in this area was Mo 5 Si 3 .
Therefore, when extending outward from the area near the center of the metal, the substance gradually changed from elemental Mo to Mo 5 Si 3 through the transition area "Y". As shown in Fig. 5(b), the dark ceramic area is completely in the form of Si 3 N 4 .
When the detection gradually extends from the "A" through the dark and light color boundary line (that is, the interface layer of the layered material), the element Si gradually decreases , while the Mo element began to appear and gradually increased.
After the transition of the "X" region, the relative ratio of Mo and Si elements reached a stable state of about 5:3, and substances in this area was Mo 5 Si 3 . Therefore, when the dark ceramic area extends toward the metal layer, the substance gradually changes from Si 3 N 4 to Mo 5 Si 3 through the transition area "X". According to some studies [13,[18][19][20] , Si 3 N 4 and Mo will undergo the following series of reactions at high temperature: 3  Therefore, it can be speculated that the substance present in the transition region "X" is mainly MoSi 2 , and the substance present in the transition region "X" is mainly Mo 3 Si. In summary, the material distribution from the interface of the Si 3 N 4 /Mo laminated ceramic composite layer to the center of the metal layer is Si 3 N 4 →MoSi 2 →Mo 5 Si 3 →Mo 3 Si→Mo.

Mechanical properties
It can be seen from Table 3 that all three materials maintain high hardness, which is due to the phase transformation strengthening of Si 3 N 4 through the high-temperature sintering process, but the appearance of Mo

Thermodynamic analysis of interface layer
As can be seen from the analysis before the text, the Si 3 N 4 /Mo laminated composites have chemical reactions between composite ceramic materials and metals during high temperature sintering, and the forming compounds, such as MoSi 2 , Mo 5 Si 3 and Mo 3 Si, extended into the central area at the layer interface. Due to the appearance of molybdenum silicides, resulting in the decrease in the bending strength and density of laminated composites. However, it's not clear about the formation of the material at the interface of the Si 3 N 4 /Mo layered composite layer and the conditions under which it occurs. Therefore, we analyzes the evolutionary mechanism of material formation at the interface of the layer according to the principle of material thermodynamics.
As can be seen from the above, at high temperatures, Si 3 N 4 ceramics react with Mo to form molybdenum silicides. As can be seen from the laws of thermodynamics, at high temperatures, the necessary condition for a chemical reaction to be spontaneous is that gibbs free energy was negative, therefore, we can judge whether these reactions can proceed spontaneously at the molding temperature according to this prerequisite. A mathematical expression known for gibbs free energy at a specific temperature T: Among them: ΔG is the difference in Gibbs free energy of the chemical reaction, ΔH is the difference in the enthalpy of the chemical reaction, ΔS is the difference in the entropy of the chemical reaction, is the reaction temperature (T = C+273.15, Kelvin), Cp is the molar heat capacity of the substance at 298K. According to thermodynamic principles, when the gibbs free energy change value of a chemical reaction under a certain condition is negative, the chemical reaction will proceed spontaneously under this condition. Table 4 lists the enthalpy and entropy of the products and reactants involved in the continuous chemical reaction that may occur at the interface of the layered material layer at 298K. According to formula (4), the reaction equations (1) and (2) at 2098K are calculated. And (3) Gibbs free energy, in order to determine the possibility of chemical reaction of the material at the process molding temperature. The Gibbs free energy calculation results of the chemical reaction formula of the Si3N4/Mo layered composite material can be seen in Table 5. From the calculation results in the table, it can be seen that the Gibbs free energy of each formula is negative. From this we can judge that the reaction formulas (1), (2) and (3) can proceed spontaneously at room temperature. This proves that at a sintering temperature of 1800°C, Si3N4 and Mo will undergo a chemical reaction to form MoSi2, Mo5Si3, and Mo3Si. And, because the Gibbs free energy of formula (2) is smaller, this reaction will occur more easily, which also proves that Mo5Si3 generates the largest proportion of intermetallic molybdenum-silicon compounds in the metal layer.  Fig. 7. When the load of the friction pair increases, the friction factor shows a certain upward trend. Moreover, in the same lubricating environment, the friction coefficient of the 9Si 3 N 4 -Mo/TC4 friction pair is slightly higher than that of the 11Si 3 N 4 -Mo/TC4 friction pair and Si 3 N 4 /TC4. On the other hand, from the wear of the Si 3 N 4 /Mo and TC4 friction pair in Fig. 8 , it can be seen that the wear rate of the layered material pin sample is in the order of 10 -5 mm 3 /(N·m), and the wear rate of the titanium alloy disc sample is in the order of 10 -3 mm 3 /(N·m). In addition, the signal wear rate and total wear rate of 11Si 3 N 4 -Mo/TC4 friction pair are lower than 9Si 3 N 4 -Mo/TC4 friction pair, indicating that the tribological properties of 11Si 3 N 4 -Mo/TC4 are slightly better than 9Si 3 N 4 -Mo/TC4.   Fig. 9, it can be found that as the load increases, the degree of substance adhesion gradually increases. Perform EDS energy spectrum analysis on the three zones: "A", "B" and "C" in Fig. 9(c), and the results are shown in Fig.10 Fig.   11. It can be found from the picture that there are a large number of trench ploughs on the wear surface of the TC4 disc, and as the load increases, the wear surface of the TC4 disc became more and more rough. Therefore, it can be concluded that the friction mechanism of Si 3 N 4 -Mo/TC4 is a mixed wear of adhesive wear and abrasive wear.

Conclusions
In the present study, by adding Mo as metal interlayers to Si 3 N 4 , Si 3 N 4 -Mo laminated ceramic composite with a layered structure was prepared via hot-pressing at