SYNTHESIS AND CHARACTERIZATION OF MOLYBDENUM-DISULFIDE FORMED FROM AMMONIUM TETRATHIOMOLYBDATE

An investigation has been carried out into the possibility of in situ formation of MoS2 within porous anodic films on aluminium, to improve subsequent tribological behaviour, by re-anodizing in thiomolybdate el ectrolyte. Acidification of thiomolybdate was employed to simulate the conditions for formation of the sulphide at the anodic film/electroly te interface, followed by appropriate vacuum heat treatments to study possible temperature effects on the sulphide due to either friction or Joule heating during anodizing. The products of both acidification an d heat treatment, characterized by X-ray powder diffraction and scanni ng electron microscopy, were compared with those formed by direct ther mal decomposition of ammonium tetrathiomolybdate crystals. The precipi tate formed by acidification was mainly amorphous molybdenum trisulphi de (MoS3), which on heat treatment at 450 and 850 degrees C yielded 3R -MoS2. 3R-MoS2 also formed by the thermal decomposition of thiomolybda te crystals. Thermogravimetric and differential thermal analyses showe d that the decomposition of MoS3 to MoS2 occurred in the range 220-370 degrees C and revealed the sequence of reaction steps. The findings s uggest that mainly amorphous MoS3 is formed as a consequence of change s in the pH of the film/electrolyte interface during re-anodizing but the product is relatively easily transformed to crystalline MoS2 on mo derate heating which may occur during wear processes.