Study of Isothermal Oxidation of Aluminizing and Chromizing Coating on Tungsten Electrical Contacts used in Automotive Disk Type Electromechanical Horns.

Electrical contacts are used in electromechanical horns to make and break the circuit. Electrical contacts are generally made of tungsten refractory material due to its high melting point, high hardness, and hence good abrasion and wear resistance among all refractory metals. However, the oxidation resistance of tungsten is poor, and thus tungsten contacts used in electrical appliances undergo severe high-temperature oxidation during making and breaking electric circuit. The tungsten oxide (WO3) layer, being not very conductive with respect to thickness, can cause electrical connectivity loss and subsequently cause horn failure. In other circumstances, if oxides grow thicker, they can peel off easily due to the frequent impact during make-and-break operations, leading to a high wear rate. Thus, to improve the life of horn, aluminizing and chromizing coating treatments on tungsten contact have been developed throughes halide activated pack cementation process. The behaviour of the developed coatings under isothermal oxidation at 1000 °C for different time intervals from 2 to 100 h was studied. To evaluate the rate of oxidation, the weight gain versus time of bare and coated tungsten contacts was analysed. Macroscopic morphological analysis indicates isothermal oxidation at 1000 °C resulted into severe oxidation of bare W within 10 h and aluminized W within 25 h. On the other hand, chromized W showed negligible oxidation even after 100 h at 1000 °C. The mass gain of chromized tungsten contacts after 100 h of isothermal oxidation was 2.2 mg/cm2, which was drastically lower than the aluminized (276.9 mg/cm2) and bare (270.02 mg/cm2) tungsten contacts. The parabolic oxidation rate constant (kp in units g2 cm−4 s−1) for chromized tungsten contact was obtained as 4.7 × 10−11, which is quite low compared to aluminized and bare tungsten of 6.3 × 10−7 and 1.0 × 10−6, respectively. This suggests that the chromizing coating effectively improved the oxidation resistance of tungsten contact. [ABSTRACT FROM AUTHOR]