Optimization of the active element and reinforcement proportion for improving joint strength of brazed Si3N4/42CrMo

Micron-TiN particles and Ti powder were introduced into the AgCu powder filler to design a particle-reinforced composite filler. This TiN particle (TiNp) modified brazing material was adopted to reliably braze Si3N4 ceramics onto 42CrMo steel, and the ratio of active element Ti to TiNp reinforcement was optimized for joint strength improvement. The reaction layer’s thickness (TiN+Ti5Si3) adjacent to the ceramic decreased by introducing TiNp. Nevertheless, the reaction layer’s thickness (TiC) at the 42CrMo side did not vary too much. TiNp showed excellent wettability with Ag-Cu-Ti braze, and there was no obvious reaction at the interface. Cu-Ti intermetallic compounds were formed and randomly distributed in the brazing seam. When the TiNp content did not exceed 8 vol.%, the joints’ mechanical properties first sharply decreased and then remained basically unchanged when Ti increased. Raising the TiNp content to 10 vol.%, the joint strength showed the changing trends of first increasing and then dropping by increasing Ti content. The bending strength reached the maximum value of 396 MPa when 8 vol.% TiNp and 4 wt.% Ti were adopted.


Introduction
An increasing interest has been focused on the Si 3 N 4 ceramic as a structural component because of its superior heat resistance and corrosion resistance [1].However, due to its inherent brittleness, large dimensions or complex shapes Si 3 N 4 ceramic is difficult to produce.Therefore, people often need a reliable Si 3 N 4 ceramic joining technology, especially with metal materials.Due to the excellent repeatability and perfect joint size or shape adaptability, active brazing is a recognized technique for connecting ceramics to metals [2].In order to join Si 3 N 4 itself or metals, the most commonly used is Ag-Cu-Ti, in which Ag and Cu elements are in eutectic proportion [3].However, due to the significant differences in mechanical properties between the two substrates, significant residual stresses typically generated during the brazing process [4].These stresses can cause defects and form characteristic crack contours similar to the dome-shaped, detaching the brazing substrates.Therefore, the residual stress relieving in metal-ceramic brazing seams is of great significance [5].
The thermal mismatch between the substrates and filler material can be reduced by adopting low thermal expansion coefficient (CTE) materials (such as carbon fiber, SiC, WC, or Mo).Then some of the residual stress between the filler material and the substrates can be relieved [6,7].But all reported reinforcing materials above exhibit strong interactions with Ti, which will complicate the microstructure and negatively influence the interaction between the substrates and solder.Therefore, the interaction between reinforcing particles and active elements should be considered carefully when designing composite fillers.
The reaction products between Si 3 N 4 and Ti are TiN and Si, so the reaction between active Ti elements and TiN is considered relatively debilitated.Moreover, the CTE of Ag-Cu-Ti is significantly higher than that of TiN [8], so adding TiNp to the alloy can reduce the CTE of filler.However, TiN is a nonstoichiometric compound with a composition ranging from TiN 0.37 to TiN 1.2 [9].Therefore, further research is still needed on the reaction mechanism between Ti and TiN.This paper designs a particle reinforced composite filler with different Ti and TiNp additions for the brazing between Si 3 N 4 ceramics and 42CrMo steel, to achieve the goal of obtaining the maximum joint strength.

Experimental procedures
Commercially polycrystalline Si 3 N 4 was used in this study, with a metal companion of 42CrMo steel.The raw ceramic materials were diced with dimensions of 3 mm×4 mm×18 mm using a diamond cutting machine.Wire electric discharge machining was performed on the 42CrMo steel, resulting in a size of 3×4×18 mm 3 sample.Ti particles (~50 µm) together with micron TiNp (~10 μm) were added to Ag-Cu eutectic powder filler (Ag-28Cu, wt.%), and then a planetary mill was used to grind the mixture in a vacuum for 2 hours to prepare the composite.The mass fraction of Ti varied between 4% and 10%.The TiNp content (volume fractions) in the brazing fillers were set to be 10%, 8%, and 5%, respectively.For each brazed joint, the total weight of the brazing powder was set to be the same (20 mg).
Before joining, the brazing surfaces (3.0×4.0 mm 2 ) of the Si 3 N 4 ceramic and 42CrMo steel were ground by SiC paper with a grit size from 200 to 1200.Then, in the final preparation step, a diamond suspension, whose average particle size is around 1 µm, was adopted for polishing.All polished samples were cleaned with alcohol ultrasound for 10 minutes and then dried with an electric hair dryer.
A composite paste was prepared by using a small quantity of hydroxyethyl cellulose in the filler.The paste was then placed between the brazing substrates.A vacuum furnace was adopted to braze the brazing assemblies.Firstly, the assemblies were heated up to 700°C at a heating speed of 10 degrees per minute and then reduced the heating speed to 5°C per minute until 900°C.Subsequently, the brazing pair was maintained at this brazing temperature for 5 minutes and then the temperature was lowered to 300°C at a slow rate (5°C/min).Then the joint in the furnace was naturally cooled to 20°C.The vacuum inside the furnace was maintained at (1.3-1.7)×10 -3 Pa during the joining process.
After brazing, epoxy resin was adopted to embed the joining samples.Then the embedded samples were processed perpendicular to the brazing seam.The samples were ground and polished until the surface finish reached 0.25 μm.The joint morphology was observed using scanning electron microscopy (abbreviated as SEM).The phases' chemical composition was analyzed by energy dispersive X-ray spectroscopy (abbreviated as EDS) installed into the SEM machine.The focused ion beam (abbreviated as FIB) technique was adopted to prepare a sample for transmission electron microscopic (abbreviated as TEM) examination.The phases' diffraction patterns in the seam were analyzed using TEM at 300 kV.Four-point bending strength with a speed of no more than 0.5 mm/min was applied to evaluate the joint performance.At least three brazing samples were used for the evaluation of joint performance under each joining condition.

Results
3.1 Microstructure of Si 3 N 4 /42CrMo joint 5 vol.%TiNp modified Ag-Cu-Ti4 (wt.%) filler was adopted to braze Si 3 N 4 and 42CrMo at 900°C for 5 min, and the microstructures were compared with the joint with no TiNp, which are presented in Figures 1(a-d).The composite filler shows good wettability with the matrix, and continuous and dense interfacial reaction layers were generated at both interfaces between the substrates and the composite.As shown in Figure 1  When TiNp reinforcements were not added in the filler, as shown in Figure 1 (a), the white Ag-rich phases and light gray Cu-rich phases in the brazing layer mainly present as the eutectic form.A continuous reaction layer near Si 3 N 4 and the second particle layer in its vicinity are formed at the interface of Si 3 N 4 /brazing, as shown in Figure 1 (b).
It is well known that the active Ti elements in the molten filler will diffuse toward the Si 3 N 4 ceramic during brazing and react with the ceramic to form an inner fine-grain TiN.Then the released Si will diffuse into the braze to form outer coarse-grain Ti 5 Si 3 .However, the granular Ti 5 Si 3 adjacent to the dense reaction layer disappears when TiNp modified composite is used, as indicated in Figure 1  (d).Besides, the thickness of the TiN+Ti 5 Si 3 layer brazed with TiNp modified filler (3.4 µm) is thinner than that without TiNp reinforcements (7.7 µm).Nevertheless, the thickness of the reaction layer close to 42CrMo does not vary too much.The difference in interfacial layers' thickness indicates that the introduction of TiNp limits the enrichment of Ti on both sides of the substrates.
The morphology of the brazing layer close to the filler/steel was analyzed by the TEM method.Figure 2(a) shows that the TiNp and Ag-Cu-Ti filler have good bonding, and no obvious reaction can be observed around the interface.For the interfacial reaction phases at 42CrMo/braze side, TiC is confirmed by the diffraction pattern analysis shown in Figure 2

Mechanical properties of Si 3 N 4 /42CrMo joint
The variability of joint strength of Si 3 N 4 /42CrMo joints brazed with different reinforcements and active elements is shown in Figure 3(a).When the reinforcement content is less than or equal to 8 vol.%, all the joint strengths decrease shapely when increasing the Ti content from 4 wt.% to 6 wt.%.For the 5 vol.%TiNp joints, the four-point bending strength stays around 65 MPa when the active element content increases from 6 wt.% to 10 wt.%.While the reinforcement content is fixed at 8 vol.%, the joint strength climes gradually from 25 MPa to 85 MPa when the active element content decreases from 10 wt.% to 6 wt.%.However, when the reinforcement content rises to 10 vol.%, the joint strength shows the changing trend of first dropping and then increasing when decreasing the Ti content.The bending strength reaches 221 MPa with 6 wt.% Ti for the 10 vol.%TiNp contained composite filler, which is 84.6% higher than that for the case with 4 wt.%Ti.
When we fix the active element content and investigate the effect of reinforcements content on the joint bending strength, it can be easily found from Figure 3(a) that the variation of reinforcements content has little effect on the joint strength when the Ti content is above 6 wt.%.When the content of the active element is 4 wt.%, the bending strength increases a little and then decreases sharply to 120 MPa.When the Ti content increases to 6 wt.%, the mechanical strength also increases a little firstly and then increases up to 221 MPa while raising the reinforcements content to 10 vol.%.The bending strength reached the maximum value of 396 MPa when 8 vol.%TiNp and 4 wt.%Ti were adopted.To investigate the fracture characteristics of brazed joints, further inspection was conducted on the fracture surface after the bending test.Two different forms of failure modes can be observed, which are presented in Figures 3(b-c).The figure displayed in Figure 3(b) indicates that the crack initiates from the Si 3 N 4 near the brazing seam and ultimately fractures at the ceramic far from the brazing joint, leaving sharp edges on the Si 3 N 4 .This kind of fracture mode is always corresponding to an excellent joint mechanical property.In the case of the fraction illustrated in Figure 3(c), a characteristic crack contours similar to dome shaped can be observed, indicating considerable residual stresses are generated in the joints [10].

Discussion
TiNp dispersion reduces the effective CTE of composite brazing, which is beneficial for Si 3 N 4 /42CrMo brazing joints.This phenomenon can be concluded by the mechanical test results of 6 wt.% Ti, showing the increasing trend of joint strength when TiNp content increases.However, when a higher content of TiNp is added, the ductile deformation of the brazing alloy decreases a lot.The experimental results demonstrate the deteriorated plastic deformation effect on the mechanical strength when the content of reinforcements rises from 8 to 10 vol.% when the Ti content is 4 wt.%.
In addition, an appropriate thickness of the interface reaction layer is the key point to acquire highperformance brazed joints.An excessively thick reaction layer can decrease the joint strength because of its inherent brittleness.An extremely thin reaction layer also reduces the joint strength because it cannot transmit sufficient load.An appropriate thickness of the interface reaction layer is crucial, and this can be achieved by adjusting the active element and reinforcement content in the composite filler.Moreover, Cu-Ti intermetallics will precipitate when a higher Ti content is used.The hardness and modulus of elasticity for Ag and Cu are lower than those of Cu-Ti.Because of the hardening effect, brazing alloys adjacent to Cu-Ti also have a higher hardness and modulus.The particle reinforcement of the brazing layer is beneficial to a certain extent for the Si 3 N 4 /42CrMo joints.However, the formation of intermetallics will consume Cu in the filler, and a large amount of intermetallics in the joints will also deteriorate the plastic deformation of the filler alloy.The four-point bending strength of the joint with a large number of intermetallics is relatively low, as indicated in Figure 3, further confirming that the formation of a large number of intermetallics in our brazing system is not conducive to improving joint performance.
In summary, the TiNp or Ti content has a significant impact effect in the CTE mismatch between joining materials, interface reaction layer thickness, the filler plastic deformation ability, and the Cu-Ti precipitation, ultimately affecting the mechanical property.The reasonable utilization of composite fillers with optimal TiNp and Ti content can obtain a reliable joint.

Conclusion
The Si 3 N 4 ceramic and 42CrMo steel were brazed successfully by designing a novel Ag-Cu-Ti+TiNp composite.The microstructure evolution of brazed joints was studied and the conclusions are as follows: (1) By introducing TiNp, the interfacial layers's thickness at the Si 3 N 4 ceramic side decreased, whereas the TiC reaction layer's thickness at the 42CrMo side remained stable.
(2) TiNp showed excellent wettability with Ag-Cu-Ti braze, and there was no obvious reaction at the interface.Intense plastic deformation occurred during the brazing process, as evidenced by massive dislocations found in the Ag or Cu matrix.Cu-Ti intermetallic compounds were formed and randomly distributed in the seam.
(3) When the TiNp content did not exceed 8 vol.%,The joints' mechanical properties first sharply decreased and then remained stable when Ti increased.Raising the TiNp content to 10 vol.%, the joint strength showed the changing trends of first increasing and then dropping by increasing Ti content.The bending strength reached the maximum value of 396 MPa when 8 vol.%TiNp and 4 wt.%Ti were adopted.
, the Si 3 N 4 /42CrMo joint can be separated as three typical regions: (I) TiN+Ti 5 Si 3 reaction layer near the Si 3 N 4 ; (II) brazing seam composed of solid solutions or TiNp additions; (III)reaction layer adjacent to filler/steel.
(b).Numerous dislocations are observed in the brazing zone, indicating strong plastic deformation during joining.Besides, some tiny particles are generated and randomly distributed in the braze, as illustrated in Figure2(a).Mainly Cu and Ti elements can be detected in these precipitates by the EDS analysis, as shown in Figure2(f), indicating that Cu-Ti intermetallics were presented during brazing.Further analysis of the HRTEM image in Figure2(g) confirms that one of the black reaction phases is Cu 4 Ti 3 .