Characterization of thermally sprayed micro- and nanocrystalline cylinder wall coatings by means of a cavitation test

Abstract

The future development of motor engine design and technology governs the increasingly demanding requirements on materials in terms of friction and wear properties. In order to reduce emissions and achieve economical and environmentally sound solutions, thermal-spray processes became an interesting alternative to manufacture wear resistant and low-friction cylinder surfaces. Three thermal-spray processes are of major interest: twin wire arc system, high-velocity oxygen fuel and plasma transferred wire arc. The energy to heat the feedstock and the kinetic energy to accelerate the molten particles to the substrate differ within these processes consequently resulting in differing lamellar microstructures of the coatings. Hitherto, low-alloyed carbon steels with 0.1–0.8 wt% carbon have been successfully used for cylinder coatings. The motivation to alleviate friction losses is currently based on the material development of a new iron-base alloy with chromium and boron. This should solidify into a nanocrystalline or even amorphous matrix of high wear resistance. Prior to engine tests these coatings are characterized in laboratory by means of bench tests for adhesive tensile strength, tribological properties, and fatigue of the composite. Thus cavitation tests should reveal the tribological stability of these coatings and their ability to resist high-frequency cyclic impact stresses. The objective in this investigation is to describe the cavitation behaviour and the influence of voids like pores or oxides.