Fracture behavior in Amorphous and Ceramic Coatings on Aluminum Alloy Substrate
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punch (SP) testing technique developed at Ames Laboratory has been shown to be a useful tool for studying radiation embrittlement and mechanical property degradation of various structural alloys under aggressive thermal environments [3-5]. Because of the use of thin SP specimens and the biaxial stress state, it appears that the SP test has the ability to examine the mechanical properties of coated materials with the aid of acoustic emission (AE) techniques for monitoring coating cracking. The present paper summarizes recent results of Ni-P amorphous and overlay ceramic coatings on Al alloy substrates [6]. Coating cracking behavior during the deformation of SP specimens is examined by scanning electron microscopy (SEM) and AE methods. Complex fracture behavior is explained by residual stresses produced during coating processing and the formation of coating cracks. EXPERIMENTAL PROCEDURE The materials used in this study were 7075 type Al alloy substrates coated with a 10 ýI.m thick Ni-10 wt.%P (Ni-P) amorphous and overlay YSZ (0-1.0 pm thick). The Ni-P coating with the overlay YSZ coatings (0, 0.25, 0.5 and 1.0 ptm thick) are designated as NP, NPZ1, NPZ2, and NPZ3, respectively. The Ni-P and YSZ coatings were made using electroless plating and r.f. magnetron sputtering methods [6]. SP specimens with the dimension of 10 mm square and 0.5 mm thick were machined from a coated Al alloy disc. The coating was located on a side of the SP specimen. SP 853
Mat. Res. Soc. Symp. Proc. Vol. 356 ©1995 Materials Research Society
specimens of a uncoated Al alloy were made by machining off the coating. SP testing jigs consisted of a loading ball puncher, four clamping screws and lower and upper die holders [3,4]. The SP specimen was set in the specimen holder so that the coating surface could be subjected to tensile stresses. SP tests were carried out using a scFrew-driven Instron testing machine at room temperature with a cross head speed of 2 x 10f m/s. The morphology of coating cracking in fractured and load-interrupted SP specimens was examined by SEM. AE events were measured during the SP test using a Smart Acoustic Monitor of Physical Acoustic Co. An AE sensor with a wide flat frequency response (0.1-1.0 MHz) was attached to the puncher head. The AE data were stored intermittently every 3 seconds corresponding to the deflection increment of 6above x 10-themRMS through gain of 55 dB and a floating threshold which had been set at 3 dB signalthelevel.
RESULTS AND DISCUSSION As shown in Fig. 1, many surface cracks extensively propagated along both the radial and tangential directions in the coated SP specimens. The numbers of radial coating cracks and cracked segments (Nr and N.) were determined by SEM observation on intermittently 30
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Figure 1. Scanning electron micrograph of fractured SP specimens coated with Ni-P amorphous and ceramic coating (1.0 pim thick).
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