A Noncontacting Scanning Photoelectron Emission Technique for Bonding Surface Cleanliness Inspection
Molecular contamination of bonding surfaces can drastically effect the bond strength. This in turn effects the structural integrity that can be achieved. The presence of thin contaminant films on bonding surfaces can result from inadequate or incomplete cleaning methods, from oxide growth during the time between cleaning and bonding, or from failure to properly protect cleaned surfaces from oxide growth during the time between clearing and bonding, or from failure to properly protect cleaned surfaces from oils, greases, fingerprints, release agents, or deposition of facility airborne molecules generated by adjacent manufacturing or processing operation.
Required cleanliness levels for desired bond performance can be determined by testing to correlate bond strength with contaminant type and quantity, thereby establishing the degree of contamination that can be tolerated based on the bond strength that is needed. Once the minimum acceptable contaminant level is defined, a method is needed to quantitatively measure the contaminant level on the bonding surface prior to bonding to verify that the surface meets the established cleanliness requirements.
This paper describes a unique photoelectron emission technique for the nondestructive inspection of various bonding surfaces, both metallic and non-metallic, to provide quantitative data on residual contaminant levels.
The performance of many critical components for the Space Shuttle and other flight hardware depends on the quality of bonding achieved during fabrication of the component. An example of a major Shuttle element where quality bonding is crucial to performance, reliability, and safety is the Solid Rocket Motor (SRM). Inadequate bonding of the rubber insulation to the case could result in exposure of the D6AC steel case to the hot gas from the burning propellant and result in burn-thru which could be disastrous. Also, low strength bonds between the various nozzle parts could significantly affect SRM flight success.
Because of the high reliance placed on bonded parts and the wide variability in strength that is often observed, a comprehensive program was initiated by the Materials and Processes Laboratory of the Marshall Space Flight Center (MSFC) to investigate ways of improving bonding process control during the manufacture of critical SRM hardware in order to improve overall bond quality and reduce within-part and part-to-part variability in bonding strength. The process control parameters that affect bonding integrity include adhesive variability, storage, mixing, pot life, contamination of the bonding surface, surface preparation, adhesive application, and curing. Thus, all of these were included in the program. In addition, since bonding process control must be an inherent part of the overall manufacturing process, it has to be specific with respect to the sensitivity of the adhesives, bonding surfaces, and subsequent bonds to the environments (moisture, thermal, contamination) encountered during the manufacturing flow. For this reason, the program was implemented as a joint endeavor between MSFC and the Wasatch Division of Morton Thiokol, manufacturer of the Solid Rocket Motor.
Of all of the above parameters that affect bonding, contamination is probably the most insidious and least understood. The presence of thin molecular films on bonding surfaces can drastically affect the strength of some bonding systems. These films can result from inadequate or incomplete cleaning, oxide growth during the time between cleaning (e.g., grit blasting) and bonding or from failure to properly protect cleaned surfaces from oils, greases, fingerprints, release agents, or deposition of airborne molecular species generated by adjacent manufacturing or processing operations. These films may or may not be uniformly deposited on a large area bonding surface which can lead to variation in bond quality across that surface. The thickness and chemistry of the film, its interaction with the adhesive, and the adherents, and the subsequent response to the curing process can all affect the degree and level of bonding achieved. Often these contaminants are invisible making detection and quantitative measurement difficult and expensive. Thus, in order to eliminate or minimize contamination as a threat to bond integrity, strict contamination control of bonding surfaces is required. To assure the proper degree of control, required cleanliness levels must first be determined. Second, a methodology must be established for the uniform cleaning of the surfaces to the established levels and maintaining them at these levels from the completion of cleaning to the initiation of bonding. Third, pre-bonding inspection of the surfaces is required to verify that they have been properly controlled.
Previously, the contamination control of bonding surfaces was severely restricted due to the lack of a fast, cost effective method for quantitatively measuring contaminant levels on hardware bonding surfaces after cleaning and prior to bonding to verfy compliance with established cleanliness requirements. Since such a method was needed for the SRM bonding improvement program, a development effort was initiated by MSFC which resulted in the use of OSEE for this purpose. The successful application of this technique depends on (1) a knowledge of the fundamental performance capabilities and limitations of the photoelectron emission contamination sensor, (2) calibration of the sensor output as a function of contaminant level on the specific surface to be inspected, and (3) the effect of contaminant level on bond quality.