DEVELOPMENT AND APPLICATION OF A DRY ULTRAMICROTOMY TECHNIQUE FOR THEPREPARATION OF GALVANNEAL SHEET COATINGS

The formability of galvanneal steel sheets used in the automotive indu stry is influenced by the presence and distribution of brittle and dif ficult to distinguish Zn-Fe intermetallics in the coating. Characteriz ation of these intermetallics requires a high spatial resolution techn ique such as analytical transmission electron microscopy (ATEM). Sampl e preparation by ion milling is impossible due to iron redeposition, a nd traditional ultramicrotomy using water affects the coating chemistr y. A technique based on dry ultramicrotomy has therefore been develope d. To optimize the technique, different parameters (knife angle, cutti ng medium, thickness setting on the ultramicrotome, cutting speed) hav e been investigated for the preparation of galvanneal coatings and pur e Al sections. Results show that dry cutting does not affect the coati ng chemistry but shortens the life of the knife. Knife quality (cleanl iness, sharpness and absence of defects) is a major factor to obtain g ood dry sections. The best results for the more ductile pure Al are ob tained with a 35 degrees knife whilst for the harder galvanneal coatin g it is recommended to use a 55 degrees knife. These results suggest t hat the sectioning mechanism for the harder material involves more a c leavage-fracture mechanism whilst a greater amount of shear is involve d when sectioning relatively ductile Al. The optimum parameters for se ctioning galvanneal coatings are established and results obtained by p arallel electron energy loss spectrum imaging and energy dispersive X- ray spectrometry in the TEM are given. This study shows that with a go od control of all the sectioning parameters it is possible to obtain g ood sections repeatedly and rapidly. (C) 1995 Wiley-Liss, Inc.