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ABSTRACT Machine parts like rolling bearings or gears are stressed during operation in a changing mechanical strain. This causes wear by impacts and wear by rolling which is marked by the so called surface ruin. The appearance of surface fatigue is based upon structural transformation, cracking and cracking-growth processes and ends with the separation of debris particles caused by the above mentioned permanent changing strain /1/. The final stage, which is equivalent to the component failure, is the so called 'pitting' on the technical surface, which is characteristically named "surface fatigue'. The impact tester is used for detailed research about failure mechanisms of thin films. Statements about the adherence of hard material coatings under dynamic compressive stress can be made using this test method, due to the possibility to simulate some effects of rolling strain. Therefore a hard metal ball strikes with a frequency of up to 50 Hz onto the surface. The altitude stress can be varied to get a detailed evaluation of fatigue strength under reversal strain. Selected hard material coatings were analized after testing with the described method applying an impact force of 300 N, 500 N and 700 N. In the framework of this

presentation MSIP (Magnetron-Sputter-lon-Plating) coatings on titanium- and chromium basis were used. The fatigue defects and the results of this study will be discussed depending on structure and morphology of thin films.

2. MATERIAL (TARGETS, SUBSTRATES AND REACTIVE GAS) targets: - Ti-target

TiN-coating

- TiAI-target

TiAIN- coating

- Cr-target

CrN- coating

/

TiCN- coating

I

CrCN- coating

substrate: - 10OCr6 (ball bearing steel)

reactive gas: - N2 - CH 4 - C2H2

nitrogen methane

ethine

39 Mat. Res. Soc. Symp. Proc. Vol. 505 0 1998 Materials Research Society

3. FATIGUE STRENGTH TESTS (IMPACT TEST) As already mentioned, the impact test was developed to simulate compressive stress on (coated) surfaces. One important advantage of this test method is, that the impulsive strain, which hard material films are exposed to in different ranges of applications, can be already simulated in the laboratory stage /2, 3/. So it is possible to get some specific information about the performance of components to a very early point of time. To evaluate the resistance of coatings against impulsive strain, the testing apparatus offers the possibility to load the samples by impacting a hard metal ball. The used hard metal ball has a diameter of 5 mm and is connected firmly to a force transducer and an impact mass. The impact is carried out centrically in a liner while the impact force and the number of impacts are registered and regulated by a computer. The generation of the impact is realized by an electromagnet, that stimulates into vibration the structural components, the permanent magnet, the force transducer and the hard alloy ball. The oscillation frequency and by that the impact force is adjusted by the coil current of the electromagnet. The frequency can amount up to 50 Hz, the impact force up t