REACTIVELY SPUTTERED CR NITRIDE COATINGS STUDIED USING THE ACOUSTIC-EMISSION SCRATCH TEST TECHNIQUE

This paper reports the application of the acoustic scratch test technique, developed for adhesion measurements, in a prenormative study related to the cyclic fatigue performance of motor valve springs. The scratch test technique has been considered reliable for adhesion measurements; however, few systematic studies exist, and studies of coatings on contoured shapes are rare. The present study can be considered as a novel mechanical testing methodology based on a comprehensive compilation of results obtained in a surface engineering project for the improvement of the fatigue properties of coated springs. The critical load for chromium nitride coatings, deposited using well-specified plasma conditions on steel substrates, has been measured by optical inspection and derived by studying horizontal force and acoustic emission as a function of load. An Alcatel 650 r.f/d.c. sputtering equipment, with two sputtering cathodes in a sandwich configuration and a computer-controlled reactive gas dosage system defining controllable sputter deposition points, was used. Thus it was possible to measure the sputter rates from both targets and to compare the properties of coatings deposited from lower (L) and upper (U) sputtering targets. To facilitate the establishment of correlations between the plasma conditions and coating characteristics, the PARADOX database system was used for sorting the data. The data presented show a need for more prenormative research in areas related to scratch testing, such as the initiation and propagation of cracks, in order to obtain more basic information on coating performance.