94 Prakt. Met. Sonderband 41 (2009)
view, the preparation and imaging of micro-size specimens (with cross sections of only a few hun- Step in order tı
dred micrometers) using metallographic methods must be conducted in a meticulous manner. A lit- in subsequent
erature based approach for conducting such an investigation is not size- specific but rather is mate- the direction «
rial-type-specific. Even for gold alloy materials the literature recommendations are generalized and cleaned in an
do not consider that the concentration and type of alloying elements can critically influence the out-
come of a prepared surface. The present study will therefore focus on a preparation approach which _Table 2. Final y
highlights the importance of time and sequential use of chemical etchants on the AuAgCu ternary Trial
alloy. 1
2 Experimental procedure
The samples investigated in the present study were produced by a microcasting method called in-
vestment casting [5]. This method is particularly suitable for casting small structures as it allows for
an excellent replication of mould patterns. The microcasting was performed in preheated ceramic TKCN is a stron
moulds made from a phosphate bonded investment using a vacuum pressure casting (VPC) machine
(Prestomat by Degussa, Germany). A mould temperature of Tr = 700 °C was used for all the speci- Table 3. Etchin
mens produced. The cast specimens were of tensile test sample geometry with a cross sectional di- dal
mension of 130 x 260 um”, length of 780 um at the gauge region, and a total length of 4 mm. The RA
cast micro specimens were freed from the mould, chemically cleaned to remove investment material
retained on the surface and then separated from the runner for analysis. Isothermal heat treatment
was done on a selected amount of the as-cast micro specimens at the three conditions: 400 °C for 15
min, 750 °C for 15 min (recommended solution treatment condition by material manufacturer — De-
gussa Dental GmbH — Germany) and 750 °C for 15 min + 400 °C for 15 min (Aging step). The heat
treated micro specimens were air cooled.
Metallographic investigations on the micro specimens were conducted using standard procedures.
The length-wise face of the micro specimen was prepared, i.e. the 130 um thickness direction was
embedded. Therefore grinding and polishing processes were all done by hand in order to ensure that
the extent to which the samples are thinned down are readily assessed. Two somewhat similar The prepared
preparation approaches were applied each yielding markedly different outcomes. In these two ap- MAT. Reveals
proaches, the embedding, cutting and initial polishing steps are similar (see Table 1) but different ference contra
final polishing (etch-polishing) and etching protocols were applied. focus making
Table 1. List of embedding, grinding and initial polishing conditions used for the preparation. Processing by thickness fle
hand at disc rotating speed; 200 rpm. _ tively. The ha
Step Treatment Medium Grade _ Lubricant Time | First the mice
[ Embedding Technorit 4071 - 15 min This ensures {
Rough grinding SiC-paper 320 Water 1 min face is flat an
Medium grinding SiC-paper 600 Water 30sec was used to of
+ Mild grinding SiC-paper 1000 Water 30sec
Fine grinding SiC-paper 4000 Water 10 — 30sec
Initial polishing Synthetic cloth Diamond 3um Water —based 2.— 5 min
The procedures used for the final steps have been listed in Tables 2 and 3 for the two preparation
trials. Special attention was given to rinsing of the sample surfaces with water after each grinding
