Material science. Жаратылыстану факультеттерінің студенттеріне арналған әдістемелік құрал. Шолпан Гумарова

which serve to periodically switch on and off electric circuits operate under very arduous conditions.

      The materials of break contacts intended to open high-current, high-voltage circuits must be reliable enough to exclude burning of contacting surfaces or their sticking as the arc appears at the break, and to provide for low contact resistance when the contacts close the circuit.

      Break contacts are made from various alloys and powder-metallurgy composition (cermets) apart from pure high-melting metals. Most popular is the Ag-CdO material containing 12 to 20 % cadmium oxide by mass. It is obtained by heating a silver-cadmium alloy in the oxidizing atmosphere. The break contacts designed for service in high-powder devices are from Ag-Co, Ni-Cr-W-Mo-Ta, Cu-W-Mo, and Au-W-Mo compositions.

      The materials used for sliding contacts must be highly resistant to abrasion. These are cold-drawn (hard) copper, beryllium bronze, and the materials of the Ag-CdO group.

      Solders and fluxes

      Solders are special alloys used to obtain a mechanically strong (sometimes tight) joint or a permanent electrical contact of low resistance. Prior to soldering, the metal portions to be joined and the solder must be heated. Since the solder has a much lower melting point than the metals to be joined, it melts, while the metals remain hard. At the place where the molten solder comes in contact with the hard metal, complex physicochemical processes take place. The molten solder wets the metal, spreads over it and fills in the gaps between the metal parts. In this process, the solder diffuses into the base metal which dissolves in the solder and thus forms an intermediate layer (seam) that after hardening fastens the parts together into an integral assembly.