Tribological performance and microstructural evolution of α-brass alloys as a function of zinc concentration

Abstract

Tailoring a material's properties for low friction and little wear in a strategic fashion is a long-standing goal of materials tribology. Plastic deformation plays a major role when metals are employed in a sliding contact; therefore, the effects of stacking fault energy and mode of dislocation glide need to be elucidated. Here, we investigated how a decrease in the stacking fault energy affects friction, wear, and the ensuing sub-surface microstructure evolution. Brass samples with increasing zinc concentrations of 5, 15, and 36 wt% were tested in non-lubricated sphere-on-plate contacts with a reciprocating linear tribometer against Si 3 N 4 spheres. Increasing the sliding distance from 0.5 (single trace) to 5,000 reciprocating cycles covered different stages in the lifetime of a sliding contact. Comparing the results among the three alloys revealed a profound effect of the zinc concentration on the tribological behavior. CuZn15 and CuZn36 showed similar friction and wear results, whereas CuZn5 had a roughly 60% higher friction coefficient (COF) than the other two alloys. CuZn15 and CuZn36 had a much smaller wear rate than CuZn5. Wavy dislocation motion in CuZn5 and CuZn15 allowed for dislocation self-organization into a horizontal line about 150 nm beneath the contact after a single trace of the sphere. This feature was absent in CuZn36 where owing to planar dislocation slip band-like features under a 45° angle to the surface were identified. These results hold the promise to help guide the future development of alloys tailored for specific tribological applications.

Dislocation-mediated and twinning-induced plasticity of CoCrFeMnNi in varying tribological loading scenarios

“…The SAGB identified in Fig. 4a has been described as a dislocation trace line (DTL) in earlier publications [4,5,13]. The DTL in CoCrFeMnNi will be further discussed in the subsection 'Dislocation trace line and crystal rotation'.…”

“…In essence, a tribological system features two forces controlling the microstructural evolution: first, the normal force, which can be easily adjusted and second, the friction force, which is a function of the tribological system itself. Further parameters determining the deformation layer are cycle number [4,13,14], sliding velocity [15,16], temperature [16] and ambient environment [17].…”

“…It is known from uniaxial tests that the initial grain orientation is decisive for the occurrence of deformation twinning [23,24]. In contrast to high SFE materials [25][26][27], there is only little data available on medium SFE materials [7,13,28]. For example, nanoscratch tests in austenitic steel have demonstrated the influence of the initial crystal direction parallel to the normal direction on the formation of slip traces next to the wear track, pile-up and the wear track depth [28].…”

Subsurface Microstructural Evolution during Scratch Testing on Bcc Iron

“…This contrast was then referred to as the dislocation trace line (DTL). The occurrence of the DTL, which was also found in arrays of copper alloys and nickel [27,28], was explained through the motion of dislocations in the complex stress field associated with the moving sphere. It was recently observed that this dislocation self-organization is also the location where a localized simple shear of the subsurface material occurs in the sliding direction [24].…”

On the origin of plasticity-induced microstructure change under sliding contacts

“…This work is further motivated by the evidence that ductile crystalline materials often exhibit deformed layers (termed as "tribo-layers" or "third body" [19]) arising from localized plasticity when subjecting to contact and tribological loads [20][21][22][23][24]. Simulations have been performed to understand the interaction between the contact and microstructural changes [13,[25][26][27].…”