PEO layers obtained from mixed aluminate–phosphate baths on Ti–6Al–4V: Dry sliding behaviour and influence of a PTFE topcoat

The present study focuses on the influence of PEO treatments (performed in the AC regime using mixed aluminate–phosphate baths) on the tribological behaviour of Ti–6Al–4V, dry sliding against EN100Cr6 steel. Three different sets of coatings were produced: (set 1) in an aluminate-rich electrolyte; (set 2) in a phosphate-rich electrolyte; and (set 3) identical to set 2, but with the addition of a spray-deposited PTFE topcoat. The topography, microstructure, phase constitution and surface microhardness of each set were characterized and dry sliding tests were carried out in the load range 5–120N using a block-on-ring tribometer. For each set, a transition load which corresponds to the end of coating life has been identified. The highest transition loads were observed for coatings produced using the aluminate-rich electrolyte (set 1). Intermediate values of transition load were measured for duplex PTFE/PEO coatings (set 3). Single-layer PEO coatings produced in the phosphate-rich electrolyte (set 2) showed the lowest values of transition load, due to the low coating thickness and poor uniformity. The deposition of the PTFE topcoat proved to be beneficial in terms of both friction and wear resistance, particularly in an intermediate (30–50 N) load range.