*2.3.1.3. Effect of the test medium*

The wear resistance of a Zr61Ti2Cu25Al12 (ZT1) BMG sliding against Si<sup>3</sup> N4 balls in dry (air), and lubricated states (deionized water, and simulated physiological media) in a ball-on-flat contact configuration was investigated by Wang et al. [64]. It has been shown that the wear resistance of ZT1 BMG in air and deionized water was superior to that of Ti6Al4V alloy but inferior to that of 316 L stainless steel and CoCrMo alloy when similar conditions were used. However, in simulated physiological media, such as phosphate buffered solution (PBS), and DMEM + FBS (Dulbecco's modified Eagle medium with 10 vol.% fetal bovine serum), the Zr-based BMG exhibited lower wear resistance than that of Ti6Al4V, 316 L stainless steel, and CoCrMo. This was likely attributed to its moderate pitting corrosion resistance, and its poor depassivation-repassivation kinetics, as induced by passive film breakdown in the solution containing chloride ions during the tribo-corrosion conditions. The presence of proteins in the test solution had a significant effect on the rapid decrease in the pitting resistance of the Zr-based BMG, which caused greater and severe wear damage. Therefore, improvement in the pitting resistance of the Zr-based BMG in physiological media is necessary if it concerns its possible use as a biomedical implant. In addition, screening of a good material as a counterpart to coupling with Zr-based BMG is an additional key factor in ensuring better wear resistance (e.g., by lowering its wear rate). In dry conditions, abrasive wear was predominant mechanism in the case of ZT1 BMG. Under lubricated sliding conditions with the presence of deionized water, the wear damage caused by abrasion can be mitigated. In a simulated physiological environment with the presence of chloride ions, synergistic effects of abrasive and corrosive wear more likely control the wear process. For the four metals studied, the wear resistance had no distinct correlation with the hardness, while the material with high Young's modulus had better wear resistance.

In another investigation conducted by Hua et al. [65], the tribological behavior of a Zr53Al16Co23.25Ag7.75 BMG in air and phosphate buffer saline (PBS) solution was evaluated using ball-on-disk reciprocating sliding contacts. A biomedical alloy Ti-6Al-4V was used for comparison purposes. The wear resistance of the Zr-Al-Co-Ag BMG sliding in air has been shown to be superior to that of Ti-6Al-4V alloy. Under such dry friction conditions, the wear damage on Zr-based BMG was governed by combined oxidational and adhesive wear processes, whereas the wear mechanisms on the Ti-6Al-4V alloy was mainly driven by conjoint processes of abrasion and adhesion damage. Similarly, under lubricated contact conditions, both oxidational and abrasive wear were the main wear mechanisms of the Ti-6Al-4V alloy in PBS. The Zr-based BMG exhibited lower wear resistance under lubricated sliding conditions (i.e., PBS) over dry friction, but higher than that of Ti-6Al-4V alloy. That low wear resistance was likely attributed to the low pitting corrosion resistance of Zr-Al-Co-Ag BMG in the medium containing chloride ions, and to the synergistic effects of abrasive and corrosive wear during the tribocorrosion tests. The pitting resistance in PBS solution of Zr-Al-Co-Ag BMG was lower than that of Ti-6Al-4V alloy, and was considered a key factor in the tribocorrosion behavior of the Zr-based BMG under the conditions considered.

### *2.3.2. Fe-based BMGs*

Zr41Ti14Cu12.5Ni10Be22.5 amorphous alloys using a pin-on-disk tribometer at room temperature under relatively heavy loads. The μ depended on the counterbody material. The highest value of μ was measured for AISI 5120 steel (0.30) under a normal load of 150 N, whereas the lowest value (0.15) was recorded for the Zr-based BMG disk for a normal applied load of 100 N. The wear weight loss of the crystalline material disk was lower than that of the pins. The opposite result was obtained when the Zr-based BMG was used as the counterbody material. For the three different counterbody materials, the crystalline disks showed obvious piled-up material without severe peeling-off. In addition, microcracks were found on the AISI 5120 steel disk worn surface, whereas the plastic flow has become one of the dominant wear mechanisms on the AISI 52100 steel disk. However, rough detached surface damage and wave-like patterns accompanied by pronounced delamination have been found on the BMG pins when amorphous disks were used as counterpart. Differences in wear mechanisms have been observed between the crystalline materials and the amorphous disks. The wear mechanisms of Zr-based BMG pins included grooves, micro-cracks, peeling-off and vicious

The tribological behavior of a Zr52.5Cu17.9Ni14.6Al10Ti5 (at.%) BMG sliding against yttriastabilized zirconia counterparts was investigated using pin-on-disk in two distinct environments, namely air and argon [62]. It was found that the wear of the Zr-based BMG was reduced by more than 45% due to the removal of oxygen from the test environment at two different loads, namely, 16 and 23 N. The surface wear pins were examined using X-ray diffractometry, differential scanning calorimetry, scanning electron microscopy and optical surface profilometry. A number of abrasive particles and grooves were observed on the worn surface of the pin tested in air, while a relatively smooth worn surface was found for specimens tested in argon. In ambient air, an abrasive wear mechanism dominates the degradation mechanism of

The effect of the load on the wear behavior of Zr55Cu30Ni5Al10 BMG under linear reciprocating sliding conditions was investigated by Tao et al. [63]. An increase in the normal load results in a decrease of the μ of the Zr-based BMG (from 0.348 down to 0.226), and an increase of the wear volume loss. The wear mechanism evolves during friction, combining different processes such as welding, adhesive, and abrasive wear. At low loads, adhesive wear governs

and lubricated states (deionized water, and simulated physiological media) in a ball-on-flat contact configuration was investigated by Wang et al. [64]. It has been shown that the wear resistance of ZT1 BMG in air and deionized water was superior to that of Ti6Al4V alloy but inferior to that of 316 L stainless steel and CoCrMo alloy when similar conditions were used. However, in simulated physiological media, such as phosphate buffered solution (PBS), and DMEM + FBS (Dulbecco's modified Eagle medium with 10 vol.% fetal bovine serum), the Zr-based BMG exhibited lower wear resistance than that of Ti6Al4V, 316 L stainless steel, and

N4

balls in dry (air),

the BMG pin, whereas, in argon, an adhesive wear controls the wear process.

The wear resistance of a Zr61Ti2Cu25Al12 (ZT1) BMG sliding against Si<sup>3</sup>

the degradation wear mechanism of the Zr-based BMG.

*2.3.1.3. Effect of the test medium*

flow when they slid against crystalline materials.

*2.3.1.2. Effect of the loading conditions*

126 Metallic Glasses - Properties and Processing

The sliding wear behavior of a range of rapidly solidified alloy samples based on the composition of Fe68Cr18Mo2B12 (prepared by planar-flow casting) against a cobalt-bonded tungsten carbide counterface was evaluated using a modified crossed-cylinder wear testing rig [66]. A range of microstructures was examined, and the effect on the wear performance was evaluated. Alloys in ribbon forms were investigated in the as-cast amorphous state and after devitrification at various temperatures, and with the bulk-devitrified sample prepared by hot extrusion of the crushed ribbon. Low-temperature crystallization of the ribbon produced a high volume fraction of metastable Fe<sup>3</sup> B in a ferritic matrix, whereas high-temperature crystallization produced M<sup>2</sup> B and Mo-rich borides, still in a ferrite matrix. The wear results showed that the alloy having the stable M2 B and Mo-rich borides had the best wear performance, whereas that of the amorphous material had the highest wear rate, and the alloy with Fe<sup>3</sup> B precipitates exhibited an intermediate wear behavior. The wear volume was systematically higher for the amorphous alloy than for the corresponding devitrified material. This was believed to be the result of the continuous crystallization of the glassy ribbon on the worn surface. Crystallization produced a microstructure of deformed ferrite containing a uniform dispersion of very fine boride precipitates. The combination of extensive microcracks and the inability of the precipitates to support the load led to an increase in the wear rate.

wear rates. Two different tribocorrosion mechanisms were observed in the ZrCuAlNi BMG alloy. The amorphous structure suffered from wear-accelerated corrosion generating larger wear rates due to galvanic coupling effects. The crystalline structure did not suffer from wear accelerated corrosion (no galvanic coupling effects due to lack of passivity) and thus lower wear rates were measured. The amorphous BMG material generated a large amount of wear debris during tribocorrosion tests, which act as very abrasive particles, thereby entailing the wear of the counterpart material (alumina). This was due to the formation of grooves and the growth/breakdown sequences of an amorphous oxide film and the high availability of Zr in the material. Crystalline BMG material generated low amount of wear debris, which could be attributed to recrystallization (nano-crystallization), and the presence of a new intermetallic

A Tribo-Electrochemical Investigation of Degradation Processes in Metallic Glasses

http://dx.doi.org/10.5772/intechopen.79387

129

Cu), promoting an improvement of its mechanical properties under tribocorrosion

In another comparable study [71], using a reciprocating ball-on-disk tribometer equipped with an electrochemical cell, the degradation mechanisms due to mechanical wear and tribocorrosion of Zr-based BMGs, with nominal compositions of Zr55Cu30Ni5Al10 and Zr65Cu18Ni7Al10 used as potential load-bearing implant materials, were examined. The Zr65-BMG with a higher Zr content showed an increase in plasticity but a reduction in its wear resistance during sliding was noticed. Both BMGs experienced abrasive wear mechanisms after dry sliding wear test conditions (under applied load of 2 N). The worn surface had a higher hardness and no wear-induced recrystallization was confirmed *via* FIB cross-section analysis. Therefore, the hardening mechanism was due to the free-volume annihilation under suppressed shear deformation. The more passive nature of the Zr65-BMG had consequently a negative influence on its tribocorrosion resistance since wear-accelerated corrosion speededup its governing degradation mechanism. It has been shown that a galvanic coupling established between the depassivated wear track and the surrounding passive area was behind the main degradation mechanism of passive Zr65-BMG subjected to the tribocorrosion environment. In order to apply the BMGs for load-bearing biomedical devices, it is required to find out a balance between their wear-controlled mechanisms, namely wear-accelerated corrosion

phase (Zr<sup>2</sup>

conditions.

and static corrosion resistance.

Address all correspondence to: a.berradja@gmail.com

[1] Kramer J. The amorphous state of metals. Zeitschrift für Physik. 1937;**106**:675-691

nickel or cobalt. J. Research of the National Bureau of Standards. 1950;**44**:109

[2] Brenner A, Couch DE, Williams EK. Electrodeposition of alloys of phosphorus with

MTM Department, K.U. Leuven, Leuven, Belgium

**Author details**

Abdenacer Berradja

**References**
