There is a large global market in sourcing aftermarket parts to be used in the ongoing maintenance of aircraft. In some instances, airline and independent aircraft maintenance centers require verification that the sourced parts comply with the specifications of the original equipment vendor. In other cases, a local supplier may be the preferred source of these parts or cost is a critical factor for a part providing the same performance in the application as the OEM part.
An aftermarket aircraft parts supply company needed to identify a dark gray bushing. While ANALYZE cannot determine whether the part is functional in the designated application, ANALYZE can identify the composition of the part and, in certain cases, provide the source of the material.
Many part identifications can be done rapidly using a minimum number of analyses. For the identification of organic polymers, Fourier transform infrared spectroscopy (FTIR) provides an excellent starting point. As identification of unknown materials often proceed in a sequential manner, the information obtained from the initial FTIR analysis is used to design the subsequent analytical approach. In this particular case, visual inspection, FTIR, elemental analysis and specific gravity were employed.
In IR spectra, deflections (“IR absorbance bands”) from the baseline can be assigned to specific groupings of atoms; e.g., C-H, C=O. In many cases, identification of a material can be made based on the presence of a number of these bands. The availability of reference spectra of known compounds increases the probability of making a positive identification. The IR spectrum of the unknown material taken from the bushing is compared to a reference IR spectrum of DuPont Vespel™ SP-1 Polyimide resin in the following figure:
The excellent match of the two spectral ‘signatures’ suggests that Vespel™ polyimide has been used as the bushing resin.
At the time that this study was performed, Vespel™ was sold by DuPont in five product grades. According to the technical data sheet, two of products contained either 10 wt-% Teflon™, fluorocarbon or 15 wt-% molybdenum sulfide. While PFTE Teflon has strong and characteristic absorbance bands in its IR spectrum, the presence of polyimide as the major component causes interferences which reduces the utility of IR for detecting lower levels of fluorocarbon. Bulk elemental analyses for fluorine (marker for fluorocarbon) and sulfur/molybdenum (markers for moly sulfide) were done to verify or eliminate the presence of these resin grades. Inexpensive carbon, hydrogen and nitrogen analyses were done to confirm the presence of the nitrogen containing polyimide.
Based on the absence of elemental fluorine, sulfur and molybdenum, there was no fluorocarbon nor MoS2 present in the polyimide bushing material.
Of the remaining three Vespel™ grades, two contain graphite. Since the bushing was dark gray and the unfilled SP-1 resin grade has a yellow-brown coloration, the choice of resin narrowed to the two graphite containing materials: Vespel™ SP-21 (15 wt-% graphite) and Vespel™ SP-22 (40 wt-% graphite).
The density of the part was determined by accurately weighing the material in air and in a liquid of known density. The specific gravity measured for the unknown bushing is 1.42-1.44. Vespel™ SP-21 DF has a specific gravity of 1.42 and provided the best match for the experimentally determined value of the bushing.
This assignment could have been verified by determining the CHN elemental analysis for the reference Vespel™ SP-1 sample and determining the graphite loading from the additional carbon content of the bushing sample.
Conclusion and Benefit to the Customer
Within a few days, ANALYZE was able to perform a cost effective series of several analyses to determine, not only the generic identification of the polymer, but a specific product grade and source that the aftermarket parts supplier could employ in the fabrication of the replacement part.