Performance of Austenitic High-Nitrogen Steels under Gross Slip Fretting Corrosion in Bovine Serum

Abstract

Modular artificial hip joints are a clinical standard today. However, the release of wear products from the head-taper interface, which includes wear particles in the nm size range, as well as metal ions, have raised concerns. Depending on the loading of such taper joints, a wide variety of different mechanisms have been found by retrieval analyses. From these, this paper concentrates on analyzing the contribution of gross slip fretting corrosion at ultra-mild wear rates using a bovine calf serum solution (BCS) as the lubricant. The parameters were chosen based on biomechanical considerations, producing wear rates of some ng/m wear path. In parallel, the evolution of tribomaterial (third bodies) was analyzed as to its constituents and generation rates. It has already been shown earlier that, by an advantageous combination of wear mechanisms and submechanisms, certain constituents of the tribomaterial remain inside the contact area and act like extreme-pressure lubricant additives. For the known wear and corrosion resistance of austenitic high-nitrogen steels (AHNSs), which outperform CoCrMo alloys even under inflammatory conditions, we hypothesized that such steels will generate ultra-mild wear rates under gross slip fretting. While testing AHNSs against commercially available biomedical-grade materials of CoCrMo and TiAlV alloys, as well as zirconia-toughened alumina (ZTA) and against itself, it was found that AHNSs in combination with a Ti6Al4V alloy generated the smallest wear rate under gross slip fretting corrosion. This paper then discusses the wear behavior on the basis of ex situ analyses of the worn surfaces as to the acting wear mechanisms and submechanisms, as well as to the tribological reaction products.

Keywords: austenitic high-nitrogen steels; fretting corrosion; gross slip; ultra-mild wear; wear mechanisms.

Figure 1

Microstructures of the investigated austenitic high-nitrogen steels. (a) FeCN0.9; (b) FeCN0.6.

Figure 2

(a) Scheme of the fretting test rig. (b) Elements of the tribological system and loading parameters.

Figure 3

Mean frictional work of experiments with FeCN0.9 runs against (a,b) fluted Ti6Al4V and (c,d) fluted FeCN0.6.

Figure 4

OCP over the entire duration of the fretting experiments for different pin materials against FL-FeCN0.6.

Figure 5

Wear appearances on a fluted Ti6Al4V cylinder (a,b) after 40,000 cycles against FeCN0.6 (c,d). The samples have been sonicated in ethanol.

Figure 6

EDS scans of a contact ridge and grainy debris on fluted Ti6Al4V cylinder. The samples have been sonicated in ethanol.

Figure 7

Pile of grainy debris (areas and spectra 19 and 21) within a valley (areas and spectra 20 and 22) of a fluted Ti6Al4V cylinder after the fretting test against polished FeCN0.9. The samples have been sonicated in ethanol. Blank cells within this and all further EDS tables mean “not detected”.

Figure 8

EDS scans of a contact ridge and grainy debris on the FeCN0.6 pin. The samples have been sonicated in ethanol.

Figure 8

EDS scans of a contact ridge and grainy debris on the FeCN0.6 pin. The samples have been sonicated in ethanol.

Figure 9

Wear appearances on a fluted CoC0.06 cylinder (a,b) after 40,000 cycles against polished FeCN0.6 pins (c,d). The samples have been sonicated in ethanol.

Figure 10

EDS scans of a contact ridge and grainy debris on a fluted CoC0.06 cylinder. The samples have been sonicated in ethanol.

Figure 11

EDS scans of grainy debris between the wear grooves on the FeCN0.6 pin. The samples have been sonicated in ethanol.

Figure 12

Wear appearances on a fluted FeCN0.6 cylinder (a,b) after 40,000 cycles against CoC0.06 pins (c,d). The samples have been sonicated in ethanol.

Figure 13

EDS scans of a contact ridge and grainy debris on a fluted FeCN0.6 cylinder. The samples have been sonicated in ethanol.

Figure 13

EDS scans of a contact ridge and grainy debris on a fluted FeCN0.6 cylinder. The samples have been sonicated in ethanol.

Figure 14

EDS scans of grainy debris on a polished CoC0.06 pin. The samples have been sonicated in ethanol.

Figure 15

Wear appearances on a fluted FeCN0.6 cylinder (a,b) after 40,000 cycles against polished FeCN0.9 pins (c,d). The samples have been sonicated in ethanol.

Figure 16

EDS scans of the debris pushed out to both sides of the contact area of the fluted FeCN0.6 cylinder. The samples have been sonicated in ethanol.

Figure 17

EDS scans of grainy debris on a polished FeCN0.9 pin. The samples have been sonicated in ethanol.

Figure 17

EDS scans of grainy debris on a polished FeCN0.9 pin. The samples have been sonicated in ethanol.

Figure 18

Wear appearances on a fluted FeCN0.6 cylinder (a,b) after 40,000 cycles against ZTA (c,d). The samples have been sonicated in ethanol.

Figure 18

Wear appearances on a fluted FeCN0.6 cylinder (a,b) after 40,000 cycles against ZTA (c,d). The samples have been sonicated in ethanol.

Figure 19

EDS scans of a contact ridge and grainy debris on fluted FeCN0.6 cylinder. The samples have been sonicated in ethanol.

Figure 20

The graded structure of sliding interfaces according to reference [30].

Figure 21

Accumulated frictional work per wear path W_acc/l in mNm/m of the different combinations of materials. (a) W_acc/l of the fluted couples, this work; (b) W_acc/l of all couples tested [20,39,44].

Figure 22

Normalized gross MML generation rate g_MML in ng/m and the ΔOCP in V of the different combinations of materials (* ZTA/metal couples). (a) This work; (b) This and former work [20,39,44].

Figure 23

Normalized gross wear rate w_FC vs. the normalized gross MML generate rate g_MML of the different combinations of materials (* ZTA/metal couples). (a) This work; (b) This and former work [20,39,44].