The influence of plastik working MMC coatings on the surfach roughness reduction and strain hardening
Tomasz Dyl, Robert Starosta, Robert Skoblik
Quarterly No. 2, 2009 pages 144-148
DOI:
keywords: composite MMC coatings, plastic working of coatings, dispersion phase
abstract The paper presents the effect of plastic working on the selected properties of composite coatings on the nickel base of different contents dispersed phase of Al2O3. Coatings of Ni-Al-Al2O3 were deposited on the steel specimens using the subsonic process of flame spraying. The specimens were then subjected to both cold plastic working by rolling and also pressing by the hydraulic press. Plastic working is an alternative to machining, as the method of finishing of coats made by flame spraying. The paper presents the findings of the research into the possibility of using plastic working (rolling and pressing) to obtain the selected properties of the flame sprayed Ni-Al alloy coatings and Ni-Al-Al2O3 composite coatings. The alloy coatings had a single-phase structure, namely boundary solution , of the maximum 10% aluminium solubility in the crystal lattice of nickel, whereas in the composite coatings the volume content of non-metallic material Al2O3 was 15 and 30%. Flame spraying is widely used to apply coatings in the manufacture or reconditioning of a variety of machine parts (drive shaft journals in marine shafting, crank shaft main journals, pump rotors and casings, valve inserts and internal combustion engine cylinders, ship propellers and turbine rotor journals). Ceramic-and-metallic coatings are applied mainly to increase durability of machine parts. The paper aims for the preliminary analysis of the possibility for replacement of the traditional finishing methods, through machining, of the investigated coatings by surface plastic working. Steel specimens (C45) with the applied coating were subjected to cold rolling to obtain the following values of actual draft: h = 0.06; 0.12 and cold pressing h = 0.12. Flame-sprayed coatings were characterised by a considerable value of parameter Ra. The surface roughness obtained by flame spraying is not satisfactory in respect of the service qualities as large values of the order of Ra = 13.3 um are obtained. Therefore the coatings should be subjected to finishing in order to achieve the required surface quality. After plastic working, neither spalling nor exfoliation was observed on the actual surface or on the cross-section of the coatings. Qualitative assessment of the coatings’ adhesion using the so called ‘rapid temperature changes’ method did not show any adverse effect of the coating plastic strain on the tested property. Plastic working also contributed to the improvement in the stereometric structure (i.e. roughness) of the coatings. Another effect of plastic working was considerable reduction in the roughness average value. Profile measurements showed that the minimum values of Ra were obtained for rolled surfaces with the maximum values of relative strain, whereas the maximum values for pressed specimens.