Tribological and corrosive characteristics of electrochemical coatings based on cobalt and iron superalloys

Abstract

UDC 621.762 The tribological and corrosive characteristics of binary and ternary alloys electrodeposited from CoW, FeW, CoMoP, and CoWP citrate solutions are studied. The tungsten content of CoW alloys reaches 31 at.% and of FeW alloys 34 at.%. The introduction of phosphorus into the alloys reduces the content of tungsten from 26.7 to 19.7 at.% (at 4.5 at.% P). The molybdenum content of CoMoP alloys is 0.7-1.0 at.% at 5 to 8 at.% P. The electrolytic tungsten alloys are nanocrystalline and subgrains are 4 to 7 nm in size. The nanohardness of tungsten-rich alloys (∼13 GPa) is comparable with that of electrolytic chromium coatings. The wear resistance of the deposited alloys is quite high and is commensurable with that of hard coatings such as TiB 2 , TiN, and TiAlN. The tribooxidation of FeW alloys in dry friction is revealed. The corrosion resistance of the coatings is similar to that for electrolytic chromium.

Evaluation of Corrosion and Tribological Behavior of Electrodeposited Tungsten Alloys

“…Thus, it was depicted that the corrosion current density decreases from 4.610 -5 (for thin coatings) to 1.4 10 -5 A cm -2 (for 20 µm), meanwhile the corrosion potential for both samples was similar -0.60±0.05 V [48]. It was not determined a clear correlation between the tungsten content in the deposit and corrosion current density in NaCl + Na 2 SO 4 solution neither for Co-W nor Fe-W alloys [13,48] (Fig. 4).…”

“…However, for Co-W case the corrosion current density was higher than that in neutral single NaCl solution and this change might be caused by different dissolution mechanism in the presence of Cl -and SO 4 2-ions. An addition of organic additives (saccharin, neonol, OP-10 emulgator, neoinogenic SAS) to the electrolyte prior the electroplating of alloys did not reveal any favourable effect on examined corrosion resistance of Co-W alloys in different neutral solutions, in fact in all cases the prepared coatings demonstrated relatively lower corrosion resistance in neutral NaCl + Na 2 SO 4 solution [13,49]. Significantly poorer anticorrosive protection was noticed for Co-W alloy coatings in alkaline (6 M KOH) media, when the corrosion current density value increased up to 0.84×10 -3 A cm -2 and corrosion potential decreased up to -1.03 V at 24 at.% of W [49].…”

“…Meanwhile for electrodeposited Co-W and Fe-W alloy coatings the crystallographic structure is changing from polycrystalline to nanocrystalline at tungsten content above 20 at.% [4,11]. During the structural transformations the crystallite sizes are reduced from 60 nm to 7 nm for Co-W, from 41 nm to 3-4 nm for Fe-W and Ni-W alloys [13]. From practical point of view, alloys having nanocrystalline or amorphous structures have gained more interest since they usually possess better corrosion resistance and tribological properties (e. g. wear resistance).…”

“…Nevertheless, Co-W alloy electrodeposits in some works has been characterized by the lower rate of wear due to their relatively high hardness combined with the formation of the stable hexagonal close-packed (hcp) structure that is resistant to the high loads [30]. Meanwhile the fretting of Fe-W alloys at dry friction is accompanied by triboxidation during which iron oxides are accumulated leading to increased wear loss [13]. …”

AFM SURFACE ANALYSIS OF Fe-Co-Mo ELECTROLYTIC COATINGS

“…One of the common surface protection methods is electroplating, wherein coatings by binary and ternary alloys of iron subgroup metals with refractory components are of great practical interest [1,2]. Such coatings are distinguished due to combination therein functional properties that exceed the corresponding parameters for alloying metals [3].…”