Microstructural optimization of active element and reinforcement proportion of Si3N4/42CrMo joints

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. The ratio of active element Ti and TiNp reinforcement was optimized according to the joints’ microstructuret. The reaction layer’s thickness close to the Si3N4 increased by decreasing the TiNp content or increasing the Ti content according to the microstructural observation. In addition, higher Ti content would result in more Cu-Ti intermetallics precipitation in the joint and Ti2N could be detected instead of TiNp when 10 wt.% of Ti was used.


Introduction
A reliable technology for joining Si 3 N 4 ceramic to metal materials is frequently required.Due to the excellent repeatability and perfect joint size or shape adaptability, active brazing is a recognized method for connecting ceramics to metals [1].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 [2].However, the physical properties are greatly different between metals and ceramics, significant residual stresses are typically generated during cooling [3].These stresses can cause defects and form characteristic crack contours similar to dome-shaped, detaching the brazing substrates.Therefore, residual stress relieving in ceramic-metal joints is of great significance.
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 [4].The interaction between reinforcements and Ti also attracted extensive attention in these years [5].One of the most common reinforcing materials is SiC particle [6].Other reinforcing materials also show strong interaction with Ti, such as BN, Si 3 N 4 , or TiB 2 [7], which will complicate the microstructure and negatively influence the reaction between substrates and brazing alloy.So the interaction between the 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 interaction between active Ti elements and TiN is considered to be debilitated.Moreover, the CTE of Ag-Cu-Ti alloy is significantly higher than that of TiN [8], so adding TiNp can achieve the goal of reducing the CTE of the filler.Thus, current research adopts TiNp as the reinforcing particle.This study designs a particlereinforced composite filler with different TiNp and Ti additions to join Si 3 N 4 ceramics and 42CrMo steel.The Si 3 N 4 /42CrMo brazing microstructure was characterized to elucidate the interfacial evolution.

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 4 mm×3 mm×18 mm.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 used a planetary mill 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 binder in the filler.The paste was then placed between the brazing substrates.A vacuum furnace was adopted to place the brazing assemblies.Firstly, the temperature was heated up to 700°C with a rate of 10 degrees per minute and then reduced the heating speed to 5°C per minute until 900°C.Subsequently, the brazing pair at this brazing temperature for 5 minutes and then down to 300°C at a slow rate (5°C/min).Finally, the assemblies was naturally cooled to 20°C.The vacuum inside the furnace was remained around 1.5×10 -3 Pa during joining process.
After brazing, epoxy resin was adopted to embed the joining samples.Then the embedded samples were ground and polished until the surface finish reached 0.25 μm.The morphology was characterized by using SEM, whose abbreviation was scanning electron microscopy.The phases chemistry was analyzed by EDS equipped with the SEM machine, whose abbreviation was energy dispersive X-ray spectroscopy.By employing grazing incidence XRD, whose abbreviation was X-ray diffraction, the phases were determined by XRD analysis.

Results
When 8 vol.%TiNp is adopted in the composite, the microstructure variation of the Si 3 N 4 /42CrMo with different Ti is shown in Figure 1.Similar interface structures are observed, which are independent of the changes in Ti content.However, microstructure observations reveal significant differences in the reaction layer's thickness close to the side of ceramie.The reaction layer's thickness at the side of ceramic (I) increases when increasing the Ti content, which is shown in Figure 1 (a-d), whereas the thickness close to the steel side does not change too much.The quantity of TiNp is relatively small when a high Ti content (10 wt.%) filler is used, compared with the other joints of lower Ti content.In addition, as shown in Figure 1 (c-d), lots of tiny phases appear when brazed with higher titanium filler.Cu and Ti elements are mainly detected by the EDS analysis, indicating that these intermetallic compounds are Cu-Ti compounds.The higher magnification SEM images around TiNp are indicated in Figure 2. It is worth noting that tiny Cu-Ti intermetallics can be hardly recognized when less than 6 wt.% Ti is used, as shown in Figure 2 (a-b).By increasing the Ti content to 8 wt.%, a large number of tiny Cu-Ti intermetallics appear.When the Ti content reaches 10 wt.%, more Cu Ti intermetallics appear, as indicated in Figure 2 (d).The fractured surface with 10 wt.% Ti was ground until the brazing alloy was exposed completely, as shown in Figure 3(a).The whole brazing surface is composed of fine grey phases, which is more obvious when referring to the magnified back-scattered micrograph on the top right of Figure 3(a).When the TiNp rises to 10 vol.%, the brazed Si 3 N 4 /42CrMo microstructure with different Ti contents is also investigated.As the microstructures for the 10 vol.%TiNp contained different Ti composite fillers show a similar changing trend with those in Figure 1, we do not put the SEM morphologies of the integral joints with 10 vol.%TiNp here.The backscattered micrographs around the TiNp with 10 vol.%TiNp and different Ti contents are displayed in Figure 4.A great number of Cu-Ti compounds can only be observed when the Ti content is up to 10 vol.%.When compared with the backscattered micrographs in Figure 2, we can easily find that, for the same Ti content, the amount of Cu-Ti intermetallics decreases significantly when TiNp increases from 8 vol.% to 10 vol.%.Therefore, we can further confirm that there are some interactions between the Ti and TiNp.

Discussion
Due to the higher titanium's chemical potential in Ag-Cu-Ti alloys compared to nitrides, Ti will diffuse and accumulate near TiNp when TiNp is added to the molten alloy [9].The consumption of Ti MATMA-2023 Journal of Physics: Conference Series 2691 (2024) 012077 by TiNp will decrease the Ti content that diffuses towards the substrates (42CrMo or Si 3 N 4 ), so it can be easily understood that the addition of TiNp in the filler will decrease the interfacial reaction layers' thickness close to the substrates.However, the equilibrium between the melt and TiNp is obtained by a low Ti content at the brazing temperature [9].The Gibbs free energy of the interfacial reaction (formation of Ti 5 Si 3 , TiN, and TiC) is much lower than that of intermetallic compounds [9], so most Ti only diffuses towards the substrate (Si 3 N 4 ceramic and 42CrMo) and reacts with them.Therefore, only a little Ti remains in the brazed joint.When the Ti content is below 6 wt.%, as illustrated in Figure 2 (a-b) and Figure 4 (a-b), the residual Ti in the brazing seam is not enough to form a large number of Cu-Ti intermetallics since that Ti can be also dissolved in the filler based on the phase diagram of Ag-Cu-Ti [10].So it is difficult to detect intermetallics in the brazed joints.Higher Ti content (8 wt.%) will result in a higher residual Ti content in the brazing seam, which can thermodynamically react with Cu to form more intermetallics.So a noticeable amount of intermetallics can be observed in the joints with higher Ti content (≥8 wt.%), as indicated in Figure 2 (c-d) and Figure 4 (d).

Conclusion
The Si 3 N 4 ceramic were brazed flawlessly to 42CrMo steel by designing a novel Ag-Cu-Ti+TiNp composite.The microstructure evolution of brazed joints was systematically studied.The conclusions are as follows: (1) By increasing the Ti content, the interfacial layer's thickness adjacent to Si 3 N 4 ceramic climbed, whereas the interfacial layer's thickness close to the steel did not vary too much.
(2) The higher the Ti content was, the more intermetallics precipitated.For the same Ti content, the amount of Cu-Ti intermetallics decreased by increasing TiNp content.
(3) The TiNp additions were hardly detected and Ti 2 N could be identified instead of TiN when high Ti content (10 wt.%) was used.
.1088/1742-6596/2691/1/012077 4 Additionally, the TiNp additions are hardly detected in the exposed brazing surface.The presence of various Cu Ti intermetallics is confirmed by the XRD results in Figure3 (b).Moreover, instead of TiN, Ti 2 N is identified in the XRD analysis results, indicating that most of the TiNp additions have reacted with Ti to form Ti 2 N when higher Ti content is applied in the brazed joints.