Proceedings of SerbiaTrib '13

COF [-] COF [-]Fig.9: COF graphs at both room temperature and 300°Cfor the Ni/SiC composite coatings: a) Room temperaturetest, b) 300°C.At high temperature the COF graphs are verysimilar and this behaviour could be related to thechange of contact, compared with roomtemperature test, that is between Ni oxide and theWC sphere, instead of Ni slightly oxidized andWC.4. CONCLUSIONSTime [s]Time [s]In this work different type of coatings have beenanalysed deposited either by thermal spraytechniques or by electrodeposition. The coatingsdeposited by thermal spray are: NiCr (APS), WCCoCr (HVOF) and NiCr+Cr 2 O 3 (APS) . Theelectrodeposits are Ni/SiC coatings with nano- ormicro-sized particles embedded in metal matrix.The analysed coatings showed differentmicrostructure that depends on both depositedmaterial and deposition technique.Regarding the wear properties, The steelsubstrate showed the worst wear resistance at bothroom temperature and 300°C. This behaviour isrelated to the low mechanical properties of thissteel, that are decreasing as the temperatureincreases.All the tested metal matrix coatings underwenttriboxidation, that was increased at hightemperature test. The triboxidation behaviourdepends on metal oxidation resistance. The ceramica)b)coating was subjected to an intensive materialdetachment, caused mainly by the highinterconnected porosity of the thermal sprayedcoating. The detachment increased in function oftemperature because the ceramic oxide changedphase under the hertzian loads. For all the metalmatrix coatings was present a third body abrasioncaused mainly by both oxide descaling and ceramicreinforcement detachment from the metal matrix.Observing the wear rates, the WC CoCr coatingshowed the highest wear resistance at both roomtemperature and 300°C. This behaviour is related tothe microstructure of the deposit: the reinforcingparticles (WC) give high hardness also at hightemperature and the metal matrix (CoCr) increasesthe toughness of the coating and acts as binder forthe reinforcing particles. The electrodepositsNi/nSiC showed a wear behaviour that iscomparable with the WC CoCr one. For the nanocompositeelectrodeposits the synergy of both grainrefinement and nano-particles embedding leads toan increase of hardness at both room temperatureand 300°C. This effect probably enhances the wearresistance of the Ni metal matrix that is subjected tohertzian loads.The COF values are strongly dependent on thematerial analysed but it was observed, for thermalspray coating, similar COF values between theroom temperature test and the 300°C tests. Theceramic coating showed the lowest COF values athigh temperature caused mainly by the productionof brittle CrO 2 phase. The electrodeposits showedsome differences in the COF values between thehigh temperature tests and the room temperaturetests caused mainly by the change of hertziancontact from Ni slightly oxidized, at roomtemperature, to Ni strongly oxidized, at 300°C. Atroom temperature is visible a different in COFvalue between pure metal and composite coatings.This effect is related to the different mechanicalproperties of the coating and the possibleinteraction of reinforcing particles with thecountermaterial in the hertzian contact/motion.REFERENCES[1] N.F. Ak, C. Tekmen, I. Ozdemir, H.S. Soykan, E.Celik, NiCr coatings on stainless steel by HVOFtechnique, Surface and coatings technology, Vol.173-174, pp 1070-1073, 2003.[2] B.S. Sidhu, D. Puri, S. Prakash, Mechanical andmetallurgical properties of plasma sprayed andlaser remelted Ni-20Cr and stellite-6 coatings,Journal of Materials Processing Technology, Vol.159, pp 347-355, 2005.[3] H. Singh, D. Puri, S. Prakash, Some studies on hotcorrosion performance of plasma sprayed coatings13 th International Conference on Tribology – Serbiatrib’13 53