In the summer of 2014, Chevrolet NASCAR engine producer Earnhardt Childress Racing (ECR) witnessed various instances of damaged pistons present during the engine assembly process. Deep scratches on the piston skirts were found subsequent to rotating assembly measurements where the pistons were coupled to their respective engine block cylinders.
The engineers found a drop in the race engine performance and reliability due to a cleaning process of individual components and assemblies during engine production. Foreign object debris on the micro-meter scale caused deep scratches on the piston skirts, damaging several sets of NASCAR Sprint Cup Series pistons. The scratching formed small sharp burrs on the machined land surfaces and scored via the proprietary skirt coating into the aluminum substrate, making the pistons unusable.
ZEISS SteREO Discovery.V20 with a PlanApo S 0.63x objective at magnifications up to 95x was used to perform the initial investigation on the damaged pistons. The use of SteREO Discovery.V20 stereo microscope enabled engineers to identify various regions of interest (ROIs) on the piston skirts.
ROIs were determined within some of the larger scratch areas and where there were clear terminations of the scratches within the piston length. A macro image of the vertical skirt scratches is presented in Figure 1, indicating that the scratches appeared during the piston fit-up process, wherein the piston performs an axial movement in the respective cylinder bore.
Figure 1. Overview image of engine piston skirt damage
After fixturing the piston in a large vice clamp and positioning it on the motorized light microscope stage, the calibration of the part coordinates was done inside AxioVision software. Three relatively planar machined features on the piston were employed, serving as the fiducial markers for calibration.
This was followed by imaging of the ROIs as normal and saving them as .zvi format files, which save the parameters, including the coordinates and magnifications for Shuttle and Find, a unique correlative microscopy solution from ZEISS.
The ZEISS EVO MA25 scanning electron microscope (SEM) equipped with Bruker X Flash 6/30 energy dispersive x-ray (EDX) detectors and secondary electron, high definition backscatter electron (HDBSE) allowed further analysis of those ROIs non-destructively. The stage was moved into the SEM, where the SEM stage was calibrated using the AxioVision software to the three reference features.
The ROIs saved as .zvi image files were rapidly recalled after placing the piston in the SEM for the subsequent analysis of the regions, which were initially determined with the stereo microscope.
After positioning the electron gun at the ROIs, HDBSE imaging helped in determining embedded foreign particles, thanks to sensitivity to compositional variations. Atomic light elements appear dark and heavier elements light (Figure 2).
Figure 2. HDBSE image of an embedded particle, measuring 98µm x 44µm
The composition of the foreign particles as well as the surrounding substrate was determined using the SEM EDX detector for comparison purposes (Figures 3 and 4). With the elemental mapping capability, the EDX detector further analyzed another particle and surrounding area to corroborate the presence of aluminum oxide (Figures 5 and 6).
Figure 3. EDX spectra of particle showing Al and O peaks
Figure 4. EDX spectra of aluminum alloy substrate for comparison
Figure 5. SEM HDBSE image of a sharp multi-faceted Al2O3 particle at the score mark termination (yellow outlined area). (Dark outlying area is carbon rich skirt coating).
Figure 6. EDX mapping of the particle and surrounding area, with Al & O fitting the particle profile.
The following are the advantages of the solution used:
- Shuttle and Find between the light and electron microscopes provided faster part navigation to ROIs and bridged the gap of macro and micro analyses carried out
- The large chamber EVO MA25 needed no part sectioning and saving time and in other cases, good components
- Overlay of the light microscope and SEM images was also done, offering another unique analysis perspective
- Calibration was done on part features acting as fiducial markers versus those that are normally on correlative sample holders owing to part size
- EDX qualitative and quantitative analysis determined the composition of the foreign object, thereby facilitating in source identification
- Multi-particle composition analyses enabled faster root source determinations
The use of Shuttle and Find for correlative microscopy reduced time compared to conventional manual SEM navigation to ROIs predetermined using SteREO Discovery. V20. This time reduction led to the implementation of corrective actions in the production environment quickly.
Moreover, the use of the EVO MA25 class SEM and its large chamber that can handle over an 8" part height and 11" part diameter enabled the analysis of the entire 4"+ diameter piston without sectioning, saving roughly 4560min needed for the ROI analysis involving sectioning.
At relatively low magnifications, several embedded particles 30-100µm in size were rapidly identified by the SEM, using both secondary electron and then HDBSE imaging. HDBSE imaging distinguished the elemental variations of the particles, piston coating and aluminum substrate, which were then subsequently qualified and measured to determine their composition using EDX.
This data was used to further investigate the engine assembly process. What the engineers found was that the source of the contamination was the Scotch-Brite pads consisting of Al2O3 abrasive employed in engine block cleaning. The elimination of the use of Scotch-Brite pads has resulted in zero re-occurrences, thereby saving considerable time and money.
This information has been sourced, reviewed and adapted from materials provided by Carl Zeiss Microscopy GmbH.
For more information on this source, please visit Carl Zeiss Microscopy GmbH.