DE3006270C2 - Process for the production of sintered bodies from silver and predominantly silver-containing powders - Google Patents

Description

The invention relates to a method for producing sintered bodies with a high final density Degree of space filling> 0.9 from silver and predominantly silver-containing powders, in which the powders with given pressure pressed into a pressed body and then during a heat treatment in ^ Atmosphere to be sintered.

For many technical sintered materials, a high final density is required for reasons of high quality properties aimed at. In powder metallurgy processes, the density increase takes place in one press and one Sintering step. When pressing metal powder, the pressing density increases with increasing pressing pressure. At the Sintering generally leads to a further increase in density. However, there are sintering systems that operate at high Press presses show a decrease in density, d. that is, the compact is inflated during sintering. With these sintering systems densities during sintering can only be achieved when low compression pressures are used.

Up to now it was mostly worked with a small pressure. The sintered bodies were continued by pressing condensed. If you worked in the pressure area in which expansion already took place during sintering was, the sintered bodies could also be compacted by repressing at high pressure a subsequent thermal load, however, the density drops again.

It is the object of the invention to provide a method with which it is possible to produce sintered bodies from silver powder or predominantly silver-containing high-density powders with a degree of bulk Manufacture> 0.9 with the additional requirement that the gradient of the length change curve at the end of the sintering time is positive is or with a further heat treatment at the isothermal sintering temperature also a positive one Has gradients of change in length.

This object is achieved according to the invention in that the heat treatment of the pressed body in at least two successive temperature levels with increasing temperature characteristics up to Reaching the sintering end temperature is carried out and that at each temperature level during a predetermined Isothermally sintered for a period of time.

The level of the pressing pressure has a great influence on the sintering behavior, especially of pressed bodies made of silver powder or similar ductile metal powders. At a low press density, there is an increase in density achieved during sintering (reduction in volume and thus negative change in length). With increasing pressing pressures an increase in volume or puffing is obtained during sintering. In intermediate Pressure ranges are lower values of the sintering shrinkage with positive gradients of the change in length and a further decrease with increasing pressure the relative change in length with negative gradients at the end of the sintering time, with even higher gradients Press printing is not obtained by sintering compaction, but rather by puffing up. Here there are over the whole Sinter phase a negative gradient of the change in length.

If one chooses such a high pressure, at which a negative gradient of the change in length at the end of the sintering time again leads to a decrease in density, one can with the same pressing pressure when using the method according to the invention (step sintering) avoid the effect of the decrease in density at the end of the sintering time and thus achieve a higher sinter compaction than with direct sintering.

The invention is further explained with the aid of the drawing and exemplary embodiments.

It shows

F i g. 1 graphically shows the negative relative change in length during direct sintering of silver compacts,

Fig. 2 shows the negative relative change in length at Step sintering of silver compacts.

F i g. 1 shows the negative relative change in length during direct sintering of compacts made from electrolysis pure silver powder. The sintering time t in minutes is on the abscissa and the negative relative one on the ordinate

Change in length γ- removed in percent. Curve

11 shows the course of the relative change in length of the silver pressed body, which was pressed with a pressing pressure of 100 MPa and sintered at a temperature of 1123 K. At the end of the sintering period, there is a negative relative change in length (sintering shrinkage) of 11%.
When the compression pressure is increased from 100 MPa to 200 MPa, the lower measurement curve 12 is obtained. At the end of the sintering period, the higher compression pressure already shows a clear drop in the sintered density achieved due to expansion due to gas inclusions from a maximum of 6.5% to 5.5% .

In the graphic representation according to FIG. 2 is the sintering time in minutes on the abscissa and on the The ordinate plotted the negative relative change in length in percent. The curve 21 shows the relative change in length when sintering according to the invention (step sintering) of electrolysis pure silver powder with a Compression pressure of 200 MPa and then pressed in steps at the temperatures indicated in the curves was sintered. A linear sintering shrinkage of 7.9% is achieved, which is higher than that of the Direct sintering in FIG. 1 of 5.5%. It is particularly important that the curve according to FIG. 2 one shows positive gradient, while curve 12 in FIG. 1 has a negative gradient and thus a Shows the decrease in density.

The compacts can, for. B. from pure silver powder, from a powder of silver / metal compound, z. B. AgCdO 10 to 30, made of silver / metal, e.g. B. AgNi 10-40 or made of silver / metalloid, e.g. B. AgC 2 - 5 exist. she are particularly suitable as materials for electrical contact pieces.

1 sheet of drawings

Claims (1)

Patent claims: Process for the production of sintered bodies of high final density with a degree of space filling > 0.9 from silver and predominantly silver-containing powders, in which the powder is applied at a given pressure pressed into a compact and then during a heat treatment in a Nj atmosphere are sintered, characterized in that the heat treatment of the compact in at least two successive temperature levels with increasing temperature characteristics up to is carried out to reach the sintering end temperature, and that at each temperature level during is sintered isothermally for a predetermined period of time.