Tribological Behavior of Thermal Spray Coatings, Deposited by ...

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A. Lanzutti at al., Tribology in Industry Vol. 35, No. 2 (2013) 113‐122strength of material. The carbides are mainlyproduced by Cr and Mo. The hardness of thematerial is about 180±20 HV 0,3 and the ferriticgrain size is about 45±15 µm.c)Fig. 2. SEM images and microstructuralcharacterization of thermal spray coatings: a) NiCr80/20, b) WC CoCr 18/4 and c) NiCr 80/20+ Cr 2 O 3 .Fig. 1. Microstructure of gr. 22 steel.In Fig. 2 are shown the SEM micrographsobtained for Thermal spray coatings and therelative data acquired by mechanicalcharacterization and image analysis. In Tab. 3the thermal spray coatings’ properties are listed.a)b)Table 3. Results of thermal spray coatings’characterization.Coating Thickness Porosity Hardness HV0,3[µm] vol.%NiCr 98±16 6.5 359±18WCCoCr 105±15 3.45 1027±21NiCr+Cr2O3 (38+187) ± 25 5.5+10.1 (341+1118) ±24As can be observed, the three types of thermalspray coatings present different thickness andporosity. The porosity, acquired by imageanalysis, is higher for the coatings deposited byAPS technique compared to the HVOF deposits.This difference could be related to both powderssize and impact velocity that is lower in APStechnique with respect to HVOF. Indeed, thedifference in kinetic energy of the moltenpowders, that is higher in HVOF technique, leadsto a different density on deposited coating. Thehardness acquired is associated to the materialdeposited and the values acquired are similar todata available in scientific literature for thermalspray coatings [1‐13].The SEM micrographs obtained on cross sectionof Ni/SiC composite coatings previously etched(acetic acid: nitric acid 1:1) are shown in Fig. 3.In Table 4 are listed the electrodepositedcoatings’ properties.Table 4. A result of Ni/SiC electrodepositscharacterization.Coating Thickness[µm]SiC wt.% HardnessHV0.3Ni 78±7 ‐ 172±7Ni/µSiC 73±8 0.8 247±8Ni/nSiC 75 ± 5 0.15 270 ±9116
A. Lanzutti at al., Tribology in Industry Vol. 35, No. 2 (2013) 113‐122a)b)micro‐particles leads to a microstructurecolumnar with a slight modification oforientation, caused probably by the deviation ofelectrical field in proximity of ceramic particlesthat are in non‐conductive material. On thecontrary, the addition of nanoparticles gives agrain refinement and a multi‐orientation ofcolumns. The SiC amount is higher in the microcompositecoatings compared to the nanocompositeone. The addition of particles leads toa noticeable microhardness increase due to boththe presence of the particles and the grainrefinement.All the analysed samples showed a surfaceRoughness Ra of about 0.5µm, obtained after thesurface’ grinding.b. Tribological characterizationAll the wear tracks obtained for both bare steel,thermal spray and composite electrodeposits atboth room temperature and 300 °C are shown inFigs. 4‐6.c)In Fig. 4 the top views of the wear tracksobtained for gr. 22 steel are shown, tested atboth room temperature and 300 °C.Room temperature 300°CFig. 3. SEM images and microstructuralcharacterization of electrodeposited coatings:a) pure Ni, b) Ni/µSiC and c) Ni/nSiC.The microstructure of the electrodeposits iscolumnar. In the case of the pure Ni the metalcolumns are oriented along the direction ofelectrical fields. The addition of SiC microparticlesleads to a slight modification of the Nicolumns orientation and size. On the other hand,the codeposition of SiC nano‐particles leads tothe formation of a fine grained deposit in whichthe Ni columns are not oriented. The addition ofFig. 4. SEM images of the wear tracks obtained for thebare steel at both RT and 300 °C.The steel is subjected to triboxidative wear atboth room temperature and 300 °C. At roomtemperature the oxide produced is adherent tometal substrate and very homogeneus. At 300 °Cthe oxide produced is concentrated on the weartrack’ sides. This phenomenon is related to theloss of mechanical properties of the substratesthat permits the countermaterial to destroy theoxide layer that is thus deposited on the sides ofthe wear track. At 300 °C are highlighted the117
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