1. Brazing Characteristics
Brazing ceramic to ceramic or ceramic to metal components can be quite challenging. Most brazing filler metals form spherical shapes on the ceramic surface, which leads to weak or no wetting.
Brazing filler metals that can wet ceramics tend to form various brittle compounds at the joint interface (such as carbides, silicides, and ternary or multicomponent compounds), which affect the mechanical properties of the joint.
Furthermore, due to significant differences in coefficients of thermal expansion between ceramics, metals, and brazing filler metals, residual stress may exist in the joint after cooling from the brazing temperature to room temperature, potentially leading to joint cracking.
The use of active brazing filler metals created by adding reactive metallic elements to conventional filler metals can improve wetting on ceramic surfaces. The use of low temperature and short duration brazing can reduce the impact of interface reactions.
Designing appropriate joint configurations and utilizing single or multiple metal layers as interlayers can help minimize thermal stress on the joint.
Brazing Filler Metals Brazing ceramics to metals is typically performed in a vacuum furnace, hydrogen atmosphere, or argon atmosphere. Brazing filler metals for vacuum sealing electronic devices have additional specific requirements in addition to the general characteristics.
For example, the filler metal should not contain elements that generate high vapor pressures to avoid problems such as dielectric leakage and cathodic poisoning in devices. It is generally specified that the vapor pressure of the filler metal during device operation should not exceed 10-3Pa, and the content of high vapor pressure impurities should be in the range of 0.002% to 0.005%.
The oxygen content of the filler metal (W(o)) should not exceed 0.001% to avoid the generation of water vapor during brazing in a hydrogen atmosphere, which could cause splashing of the molten brazing filler metal. Furthermore, the filler metal must be clean and must not have oxides on the surface.
When brazing ceramics after metallization, brazing alloy filler metals such as copper (Cu), nickel (Ni), silver-copper (Ag-Cu) and gold-copper (Au-Cu) can be used . When brazing ceramics directly to metals, brazing filler metals containing reactive elements such as titanium (Ti) and zirconium (Zr) should be selected.
Among binary brazing filler metals, Ti-Cu and Ti-Ni are commonly used and can be employed in the range of 1100°C. In ternary brazing filler metals, Ag-Cu-Ti (with (W)Ti content less than 5%) is often used for direct brazing of various ceramics to metals.
This ternary system can be used with Ag-Cu sheet, powder, or eutectic filler metal combined with Ti powder. Brazing filler metal B-Ti49Be2 exhibits similar corrosion resistance to stainless steel and has a lower vapor pressure, making it a preferable choice for vacuum sealed joints that require oxidation and leak prevention.
In the Ti-V-Cr system, the addition of Cr effectively reduces the melting temperature range, with the lowest melting temperature achieved at 30% W(V). Cr-free B-Ti47.5Ta5 brazing filler metal has been used for direct brazing of alumina and magnesia, allowing the joint to operate at an ambient temperature of 1000°C. Active brazing loads for direct bonding of ceramics to metals are shown in Table 14.
Table 14. Active Brazing Loads for Brazing Ceramics to Metals
| Composition of the welding material (mass fraction) (%)· |
Welding temperature (°C)· |
Applications and joint materials |
| B-Ag69Cu26Ti5 | 850~880 | Ceramics-Cu, Ti, Nb, etc. |
| B-Ag85Ti15 | 1000 | Ceramic oxide-Ni, Mo, etc. |
| B-Ag85Zr15 | 1050 | Ceramic oxide-Ni, Mo, etc. |
| B-Cu70Ti30 | 900~1000 | Ceramics-Cu, Ti, refractory metals, etc. |
| B-Ni83Fe17 | 1500~1675 | Ceramics-Ta (joint strength 140MPa) |
| B-Ti92Cu8 | 820~900 | ceramic metal |
| B-Ti75Cu25 | 900~950 | ceramic metal |
| B-Ti72Ni28 | 1140 | Ceramic-ceramic, ceramic-metal, ceramic-graphite |
| B-Ti50Cu50 | 980~1050 | ceramic metal |
| B-Ti49Cu49Be2 | 1000 | ceramic metal |
| B-Ti48Zr48Be4 | 1050 | ceramic metal |
| B-Ti68Ag28Be4 | 1040 | ceramic metal |
| B-Ti47.5Zr47.5Ta5 | 1650~2100 | Tantalum ceramic |
| B-Zr75Nb19Be6 | 1050 | ceramic metal |
| B-Zr56V28Ti16 | 1250 | ceramic metal |
2. Brazing Technology
Pre-metallized ceramics can be welded in high purity inert gas, hydrogen or vacuum environments. When directly brazing non-metallic ceramics, vacuum brazing is generally recommended.
(1) General Brazing Process for Ceramics and Metals
(1) The general brazing process for ceramics and metals can be divided into seven process steps: surface cleaning, paste application, ceramic surface metallization, nickel plating, brazing and post-brazing inspection.
Surface cleaning is carried out to remove oil stains, sweat marks and oxide films from the surface of the base material. Metal parts and brazing material must be degreased and then treated with acid or alkali to remove oxide films, followed by rinsing under running water and drying.
High-quality parts must undergo heat treatment at the appropriate temperature and duration in a vacuum oven or hydrogen oven (ion bombardment can also be used) to purify the surface of the part.
Cleaned parts should not come into contact with oily objects or bare hands and should immediately proceed to the next process or be placed in a dryer, avoiding prolonged exposure to air.
Ceramic parts should be cleaned with acetone using ultrasonic cleaning, rinsed under running water, and finally boiled twice in deionized water for 15 minutes each time.
Applying the paste is a crucial step in ceramic metallization. It consists of applying the paste to the ceramic surface to be metallized using a brush or paste applicator.
The coating thickness is generally between 30-60 micrometers, and the paste is typically composed of pure metal powder with a particle size of approximately 1-5 micrometers (sometimes with the addition of suitable metal oxides) and an organic binder.
The paste-applied ceramic parts are then placed in a hydrogen furnace and sintered at a temperature of 1300-1500°C for 30-60 minutes using wet hydrogen or cracked ammonia. For ceramics with hydrides applied, they must be heated to about 900°C to decompose the hydride and react with pure metal or residual titanium (or zirconium) on the ceramic surface to obtain a metallic coating.
In the case of the Mo-Mn metallization layer, to promote wetting with the brazing material, a 1.4-5 micrometer thick nickel layer is electroplated or coated with nickel powder. If the brazing temperature is below 1000°C, the nickel layer also needs to undergo pre-sintering in a hydrogen furnace at a temperature and time of 1000°C/15-20 minutes.
Treated ceramics are treated like metal parts and assembled with stainless steel, graphite or ceramic molds to form a whole. The brazing material is applied to the joint and the part must be kept clean throughout the operation, avoiding touching it with bare hands.
Brazing is carried out in an argon gas, hydrogen gas or vacuum furnace. The brazing temperature depends on the brazing material, and to avoid cracking the ceramic, the cooling rate should not be too fast. Furthermore, certain pressure may be applied during brazing (approximately 0.49-0.98 MPa).
After brazing, the welded parts must undergo surface quality inspection, in addition to thermal shock and mechanical performance tests. Sealing components used in vacuum devices must also be leak tested in accordance with relevant regulations.
(2) Direct Brazing
During direct brazing (active metal method), the ceramic and metal parts to be welded are first subjected to surface cleaning and then assembled.
To prevent cracks due to different coefficients of thermal expansion, a rotating buffer layer (one or multiple metal sheets) can be placed between the joints. Whenever possible, brazing material should be placed between the two workpieces or in the spaces filled with brazing material and then welded in a manner similar to conventional vacuum brazing.
When using Ag-Cu-Ti brazing material for direct brazing, the vacuum brazing method must be employed. Heating must begin when the degree of vacuum in the oven reaches 2.7×10-3Pa.
At this point, rapid heating can be applied; however, when the temperature approaches the melting point of the brazing material, it must be heated slowly to ensure even temperature distribution throughout all parts of the joint.
Once the brazing material has melted, the temperature must be raised quickly to the brazing temperature, with a waiting time of 3 to 5 minutes. During cooling, slow cooling must be carried out before reaching 700°C, while after 700°C natural cooling can be allowed.
For direct brazing with Ti-Cu active brazing material, the brazing material can take the form of Cu foil with Ti powder or Cu component with Ti foil, or Ti powder can be applied to the ceramic surface followed by adding of Cu foil.
All metal parts must be degassed in vacuum, the degassing temperature for oxygen-free copper is 750-800°C, and for Ti, Nb, Ta, etc., the degassing temperature should be 900°C for 15 minutes. The degree of vacuum at this stage must not be less than 6.7×10 -3 Pai.
During brazing, the components to be brazed must be assembled in an electrical device and heated in a vacuum oven at a temperature between 900-1120°C, with a holding time of 2 to 5 minutes. During the entire brazing process, the vacuum pressure should not be less than 6.7×10 -3 Pai.
The brazing process using the Ti-Ni method is similar to the Ti-Cu method, with the brazing temperature being 900±10°C.
(3) Oxide brazing method
The oxide brazing method uses oxide brazing material that forms a glassy phase when melted, allowing it to infiltrate the ceramic and wet the metal surface, thus achieving reliable connections. This method can be used to bond ceramic to ceramic or ceramic to metal.
The main components of oxide brazing materials are Al 2 Ó 3 CaO, BaO, MgO and the addition of B 2 Ó 3 Y2O3, Ta 2 Ó 3 etc. can produce brazing materials with various melting points and linear expansion coefficients.
Furthermore, fluorine brazing materials composed mainly of CaF 2 and NaF can also be used to join ceramics and metals, providing high-strength, heat-resistant joints.
