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A detailed study of the scratch test was undertaken to determine its sensitivity, reproducibility, and soundness as a means of measuring the adhesion of aluminum films on fused quartz substrates. The complete-removal failure criterion was found to be quite subjective and often ambiguous, finally leading to the threshold adhesion failure (TAF) concept. Using the TAF approach, it was found that the scratch test is sensitive enough to discriminate between failure and nonfailure within 0.1 g of stylus loading at both low- and high-load levels. Also, the reproducibility of the mean TAF loads is excellent. In addition to the expected TAF load dependence on stylus tip radius of curvature and film thickness, it was also found that any one stylus has its own scribing characteristics which are reflected in the TAF load. The mean TAF loads are, therefore, relative measurements.
The occurrence of crescent fractures in the substrate at high-stylus loads puts an upper bound on acceptable testing loads. TAF loads are almost always well below the crescent fracture load level, but critical loads determined by the complete removal criterion (being so much higher) may have doubtful meaningfulness.
It appears that the TAF approach is measuring the lower limits, while the critical load approach is measuring the upper limits of film adherence. In either case, one obtains an estimate of the quality of film adherence, but not an unequivocal measurement of the adhesive forces.
adhesion, scratch tests, thin films, measurements, failure
Physicist, U.S. Naval Weapons Engineering Support ActivityNaval Research Laboratory, WashingtonWashington, D.C.D.C.
Cherryfield Stoneware, Cherryfield, Me.