Ni-based superalloys are often used to fabricate turbine airfoils (Figure 1), which require high mechanical strength, corrosion resistance and creep deformation resistance. However, these three properties make sample preparation a difficult process. The use of a high-speed planar grinder for planar grinding helps to gain more successful material removal and the high daily volume requirements. This article presents solutions for preparing airfoil specimens made of Ni-based superalloys.
Figure 1. Turbine airfoil
Using a precision cutter is the best practice to section airfoils to minimize deformation. Sectioning was performed in the direction of the sample bulk so the coating was compressed, to reduce the odds of the coating breaking off the substrate. An abrasive saw with a superalloy wheel can be used when the airfoil is too big for a precision saw. The IsoMet™ 4000 or 5000 was used to section the airfoil with the following consumable; IsoCut™ Precision Blade CBN HC
The SimpliMet XPS1 mounting press, was used to mount the airfoil in the EpoMet G mounting media.
Grinding and Polishing
Grinding of the airfoils specimens’ plane was achieved using the PlanarMet 300. Initial grinding with a 120 [P120] grit alumina stone results in a high removal of material in a reduced time period of operation, compared to using conventional SiC abrasive papers. The reduction ranges from 10 minutes to a little more than one minute. The parameters used are listed in Table 3.
Table 1. Grinding and Polishing Parameters
||10 lbs [45 N] (six samples)
||Aluminum oxide wheel, 120grit [P120]
Polishing was carried out using the Ecomet/AutoMet 300 with a 12 in [305 mm] platen (Figure 2). The samples were polished in three steps in a central force mode (Table 4 and Figure 3).
Figure 2. Surface finish after PlanarMet 300 at 5X.
Figure 3. Surface finish after MasterMet™ colloidal silica polishing on ChemoMet™ at 20X.
Table 2. 4-Step Method for Ni-based SuperAlloys using the PlanarMet™ 300 and EcoMet™/AutoMet™ 300
||Load – lbs [N]/Specimen
||Platen speed [rpm]
||Head speed [rpm]
|Alumina Grinding Stone
||120 [P120] grit
||9 µm MetaDiTM Supreme Diamond*
||3 µm MetaDi Supreme Diamond*
||0.02- 0.06 µm MasterMetTM Colloidal Silica
= Specimen Holder
*Plus MetaDi Fluid Extender as desired
Imaging and Analysis
A Nikon Eclipse MA200 inverted microscope provided a 3.1 MP uEye digital microscopy camera was utilized to inspect the prepared specimens. The inspection was achieved with bright field illumination (BF). Figures 4 and 5 show BF micrographs of the airfoil etched with molybdic acid reagent.
Figure 4. Airfoil etched with Molybdic Acid Reagent at 5X.
Figure 5. Airfoil etched with Molybdic Acid Reagent at 50X.
A clear depiction of Ni-based superalloys can be gained in a rapid and quick manner by employing PlanarMet 300 Planar Grinder and advanced preparation techniques. The total processing time is shortened by 41% in comparison with conventional methods that use several steps of SiC grinding paper.
This information has been sourced, reviewed and adapted from materials provided by Buehler.
For more information on this source, please visit Buehler.