The Role of Materials and Surface Science in Quality Assurance

Abstract

As a manufacturer of large high-technology aerospace structures, Martin Marietta Corporation is dedicated to applying advanced analytical techniques such as scanning transmission electron microscopy (STEM), X-ray photoelectron microscopy (XPS) and specular reflectance fourier transform infra-red spectroscopy (SR-FTIR) to aid in quality assurance. The Corporate Research Center, Martin Marietta Laboratories, has taken an active role in this effort and in this paper we discuss examples of how these techniques have been applied to achieve improved quality control at the manufacturing level.

The use of adhesive bonding to join metal parts as well as polymeric composites has increased dramatically in the past several years. Recognizing that this joining technique can be used successfully only if careful attention is given to surface preparation and avoidance of surface contamination, we have demonstrated the value of sophisticated surface science equipment to assist in manufacturing control of these parameters. For example, it has been shown that the high resolution capabilities of the STEM, when used in the SEM mode, is invaluable for monitoring the effectiveness of surface preparation procedures such as the Forest Products Laboratories (FPL) etch and the phosphoric acid anodize (PAA) processes for treating Al prior to bonding. An extensive review of this work has appeared recently and should be referred to for more details¹. Briefly, it has been observed that the major function of these etching and anodization processes, besides removing rolling mill oils, etc., is to develop a microporous oxide on the Al surface which interlocks with the adhesive, forming a much stronger bond than if the surface were smooth. The importance of this microporous oxide cannot be overemphasized; we have shown, for example, that, all other things being equal, the strength of a bond made to a microporous oxide surface may be up to five times stronger than if the oxide is devoid of microporesity. Since important features of the microporous oxide are less than 200Å, we have found it necessary to use the high resolution capabilities of a STEM (30Å resolution in a SEM mode) in order to characterize the oxide structures. Currently, the STEM is used for in-process control of these surface preparation treatments and, in fact, has been of considerable value for quickly detecting variations in processing procedures that could have resulted in bonding problems had they not been caught at the surface preparation level.

Advanced Surface analytical techniques have also been very effective in avoiding problems with bonding composites-to-composites. Specifically, the use of FTIR and XPS has enabled us to identify certain types of contaminants that can severely degrade bond strength. The most important contaminants identified in these studies are mold release agents that are used to allow separation of a composite part from the tool upon which it is formed and cured. Using both specular reflectance FTIR and XPS, Matienzo et al^2,3 have shown that these release agents transfer to the composite surface and can cause serious problems if attempts are made to bond one cured composite part to another. In particular, it was shown that as little as 12 atomic percent Si (as silicone) contamination from mold release agents can cause significant reductions in lap shear strength; at the 22% Si level the strength is an order-of-magnitude less than uncontaminated controls. This work has led to the use of certain types of peel plies which, when stripped off the cured composite, leave behind an uncontaminated surface which yields excellent bond strengths².

The above are only a few examples of the role that surface science and materials science can play in assuring the integrity of high technology aerospace structures. It has been gratifying to witness, over the past ten years, the use of these sophisticated techniques and talents of the surface and materials scientists to assist the production and quality engineers perform a better job.