Brazing of ceramics and metals

1. Brazeability

It is difficult to braze ceramic and ceramic, ceramic and metal components. Most of the solder forms a ball on the ceramic surface, with little or no wetting. The brazing filler metal that can wet ceramics is easy to form a variety of brittle compounds (such as carbides, silicides and ternary or multivariate compounds) at the joint interface during brazing. The existence of these compounds affects the mechanical properties of the joint. In addition, due to the large difference of thermal expansion coefficients among ceramic, metal and solder, there will be residual stress in the joint after the brazing temperature is cooled to room temperature, which may cause joint cracking.

The wettability of the solder on the ceramic surface can be improved by adding active metal elements to the common solder; Low temperature and short time brazing can reduce the effect of interface reaction; The thermal stress of the joint can be reduced by designing a suitable joint form and using a single or multi-layer metal as the intermediate layer.

2. Solder

Ceramic and metal are usually connected in vacuum furnace or hydrogen and argon furnace. In addition to general characteristics, brazing filler metals for vacuum electronic devices should also have some special requirements. For example, the solder should not contain elements that produce high vapor pressure, so as not to cause dielectric leakage and cathode poisoning of devices. It is generally specified that when the device is working, the vapor pressure of the solder shall not exceed 10-3pa, and the high vapor pressure impurities contained shall not exceed 0.002% ~ 0.005%; The w (o) of the solder shall not exceed 0.001%, so as to avoid water vapor generated during brazing in hydrogen, which may cause splashing of molten solder metal; In addition, the solder must be clean and free of surface oxides.

When brazing after ceramic metallization, copper, base, silver copper, gold copper and other alloy brazing filler metals can be used.

For direct brazing of ceramics and metals, brazing filler metals containing active elements Ti and Zr shall be selected. The binary filler metals are mainly Ti Cu and Ti Ni, which can be used at 1100 ℃. Among the ternary solder, Ag Cu Ti (W) (TI) is the most commonly used solder, which can be used for direct brazing of various ceramics and metals. The ternary filler metal can be used by foil, powder or Ag Cu eutectic filler metal with Ti powder. B-ti49be2 brazing filler metal has similar corrosion resistance to stainless steel and low vapor pressure. It can be preferentially selected in the vacuum sealing joints with oxidation and leakage resistance. In ti-v-cr solder, the melting temperature is the lowest (1620 ℃) when w (V) is 30%, and the addition of Cr can effectively reduce the melting temperature range. B-ti47.5ta5 solder without Cr has been used for direct brazing of alumina and magnesium oxide, and its joint can work at an ambient temperature of 1000 ℃. Table 14 shows the active flux for direct connection between ceramic and metal.

Table 14 active brazing filler metals for ceramic and metal brazing

2. Brazing technology

The pre metallized ceramics can be brazed in high-purity inert gas, hydrogen or vacuum environment. Vacuum brazing is generally used for direct brazing of ceramics without metallization.

(1) Universal brazing process the universal brazing process of ceramic and metal can be divided into seven processes: surface cleaning, paste coating, ceramic surface metallization, nickel plating, brazing and post weld inspection.

The purpose of surface cleaning is to remove oil stain, sweat stain and oxide film on the surface of base metal. The metal parts and solder shall be degreased first, then the oxide film shall be removed by acid or alkali washing, washed with flowing water and dried. Parts with high requirements shall be heat treated in vacuum furnace or hydrogen furnace (ion bombardment method can also be used) at appropriate temperature and time to purify the surface of parts. The cleaned parts shall not contact with greasy objects or bare hands. They shall be immediately put into the next process or into the dryer. They shall not be exposed to the air for a long time. Ceramic parts shall be cleaned with acetone and ultrasonic, washed with flowing water, and finally boiled twice with deionized water for 15min each time

Paste coating is an important process of ceramic metallization. During coating, it is applied to the ceramic surface to be metallized with a brush or paste coating machine. The coating thickness is generally 30 ~ 60mm. The paste is generally prepared from pure metal powder (sometimes appropriate metal oxide is added) with a particle size of about 1 ~ 5um and organic adhesive.

The pasted ceramic parts are sent to a hydrogen furnace and sintered with wet hydrogen or cracked ammonia at 1300 ~ 1500 ℃ for 30 ~ 60min. For the ceramic parts coated with hydrides, they shall be heated to about 900 ℃ to decompose the hydrides and react with pure metal or titanium (or zirconium) remaining on the ceramic surface to obtain a metal coating on the ceramic surface.

For the Mo Mn metallized layer, in order to make it wet with the solder, a nickel layer of 1.4 ~ 5um must be electroplated or coated with a layer of nickel powder. If the brazing temperature is lower than 1000 ℃, the nickel layer needs to be pre sintered in a hydrogen furnace. The sintering temperature and time are 1000 ℃ /15 ~ 20min.

The treated ceramics are metal parts, which shall be assembled into a whole with stainless steel or graphite and ceramic molds. Solder shall be installed at the joints, and the workpiece shall be kept clean throughout the operation, and shall not be touched by bare hands.

Brazing shall be carried out in an argon, hydrogen or vacuum furnace. The brazing temperature depends on the brazing filler metal. In order to prevent cracking of ceramic parts, the cooling rate shall not be too fast. In addition, brazing can also apply a certain pressure (about 0.49 ~ 0.98mpa).

In addition to the surface quality inspection, the brazed weldments shall also be subject to thermal shock and mechanical property inspection. The sealing parts for vacuum devices must also be subject to leakage test according to relevant regulations.

(2) Direct brazing when brazing directly (active metal method), first clean the surface of the ceramic and metal weldments, and then assemble them. In order to avoid cracks caused by different thermal expansion coefficients of component materials, the buffer layer (one or more layers of metal sheets) can be rotated between weldments. The brazing filler metal shall be clamped between two weldments or placed at the position where the gap is filled with brazing filler metal as far as possible, and then brazing shall be carried out like ordinary vacuum brazing.

If Ag Cu Ti solder is used for direct brazing, vacuum brazing method shall be adopted. When the vacuum degree in the furnace reaches 2.7 × Start heating at 10-3pa, and the temperature can rise rapidly at this time; When the temperature is close to the melting point of the solder, the temperature should be raised slowly to make the temperature of all parts of the weldment tend to be the same; When the solder is melted, the temperature shall be rapidly raised to the brazing temperature, and the holding time shall be 3 ~ 5min; During cooling, it shall be cooled slowly before 700 ℃, and it can be cooled naturally with the furnace after 700 ℃.

When Ti Cu active solder is directly brazed, the form of solder can be Cu foil plus Ti powder or Cu parts plus Ti foil, or the ceramic surface can be coated with Ti powder plus Cu foil. Before brazing, all metal parts shall be degassed by vacuum. The degassing temperature of oxygen free copper shall be 750 ~ 800 ℃, and Ti, Nb, Ta, etc. shall be degassed at 900 ℃ for 15min. At this time, the vacuum degree shall not be less than 6.7 × 10-3Pa。 During brazing, assemble the components to be welded in the fixture, heat them in the vacuum furnace to 900 ~ 1120 ℃, and the holding time is 2 ~ 5min. During the whole brazing process, the vacuum degree shall not be less than 6.7 × 10-3Pa。

The brazing process of Ti Ni method is similar to that of Ti Cu method, and the brazing temperature is 900 ± 10 ℃.

(3) Oxide brazing method oxide brazing method is a method to realize reliable connection by using the glass phase formed by the melting of oxide solder to infiltrate into ceramics and wet the metal surface. It can connect ceramics with ceramics and ceramics with metals. Oxide brazing filler metals are mainly composed of Al2O3, Cao, Bao and MgO. By adding B2O3, Y2O3 and ta2o3, brazing filler metals with various melting points and linear expansion coefficients can be obtained. In addition, fluoride brazing filler metals with CaF2 and NaF as the main components can also be used to connect ceramics and metals to obtain joints with high strength and high heat resistance.