Bulk Metallic Glasses (BMGs) are promising materials for several applications owing to their high elastic limit and resistance to permanent deformation. However, BMGs have lower wear resistance than their crystalline counterparts during dry sliding. The formation of a composite material with crystalline phases dispersed in the BMG matrix through devitrification and partial crystallization at elevated temperatures has recently been proposed as an effective way to improve the wear resistance. However, our understanding of the origin of the improved wear behavior of annealed BMGs is still elusive. Here, a systematic evaluation of the effect of annealing temperature (from temperatures lower than the BMG glass transition temperature to temperatures higher than the BMG recrystallization temperature) on the friction and wear response of a Zr-based BMG, namely Vit105 (Zr52.5Cu17.9Ni14.6Al10Ti5), was performed. The results indicate that annealing Vit105 improves its wear resistance while also reducing the steady-state friction response when the annealing temperature is close to the glass transition temperature. Notably, the formation of a transfer film on the sapphire countersurface is highly dependent on the applied normal load and sliding time. Finally, the wear mechanism was found to be strongly dependent on the annealing temperature as a transition from a predominantly adhesive wear mechanism to an abrasive-dominated one was observed as the annealing temperature crossed the glass transition temperature. Altogether, the results of this work aid to our understanding of the tribological behavior of Zr-based BMGs in general, while also providing clues to strategies for the effective use of BMGs in tribological applications.
I. INTRODUCTION
Bulk Metallic Glasses (BMGs) are a class of materials characterized by a disordered (amorphous) structure. Because of the lack of crystalline defects (e.g., dislocations) able to mediate plastic deformations, BMGs generally exhibit higher strength and hardness than crystalline solids.1–3 Despite the resulting, widespread interest in BMGs for a variety of structural applications, their technological implementation has severely been limited by the room-temperature brittleness of BMGs as they catastrophically fail with minimal plastic deformations.2–4 Notably, overcoming the brittleness of BMGs has remained a challenge owing to the difficulties associated with establishing and tailoring structure–property relationships for BMGs.
Over the past decades, our understanding of the plasticity of BMGs below their glass transition temperature has progressively been advancing. It is now well accepted that plastic deformations are accommodated in BMGs through shear bands,5–7 i.e., local regions with low density arising from diffusive jumps and leading to cooperative shearing of clusters of atoms. As the introduction of regions with a higher free volume locally facilitates plastic events in BMGs, increasing the number of regions with lower density through, for example, cryogenic thermal cycling8 has been shown to enhance plastic deformations9 while, at the same time, reducing yield strength and thermal stability.10 In contrast, the densification of BMGs through the annihilation of loosely packed regions has been shown to increase the yield strength and thermal stability, while reducing ductility as demonstrated by the increase in the brittleness of BMGs upon low-temperature annealing.11–15
BMGs, being thermodynamically metastable at room temperature,16 can also undergo devitrification and partial crystallization when exposed to energetic inputs (e.g., mechanical and thermal activation), which induces the precipitation of secondary phase and strongly affects the resulting mechanical and chemical properties. For example, the partial devitrification of the amorphous structure of BMGs was reported to improve the corrosion resistance of these alloys through the formation of protective surface layers.13,14 Other studies, however, reported a reduction in the corrosion resistance of BMGs following the partial crystallization of the alloys at elevated temperatures.17,18 As another example, in the case of Zr-based BMGs, the formation of nanocrystalline phases within the amorphous matrix by isothermal annealing was shown to improve the mechanical properties (e.g., higher Young’s modulus and fracture strength) compared to fully disordered BMGs.16,19–23 Hofmann et al. prepared a Zr–Ti–Nb–Cu–Be BMG through arc melting and reinforced it by precipitating a dendritic phase via heat treatments. The results of tensile tests indicated an increase in ductility at room temperature compared to its monolithic counterpart.24 Similarly, improved tensile properties were measured for a Cu–Zr–Al bulk metallic glass through the precipitation of spherical B2–CuZr phase in the matrix.25 This crystalline phase underwent stress-induced phase transformation to accommodate plastic strain upon deformation and work-hardened the material. While these promising results led to a few recent attempts to employ BMGs in aerospace applications, where materials are exposed to harsh environments without access to routine maintenance during service,26,27 other studies provided evidence that long exposures to elevated temperatures can enhance the brittle behavior of glassy alloys.22
Even though the effect of devitrification and partial crystallization on the mechanical properties of BMGs has extensively been evaluated, our understanding of the influence of the precipitation of crystalline phases on the tribological behavior of glassy alloys is still elusive. Jin et al. reported an inverse relationship between the wear volume and microhardness in a structurally relaxed and partially crystallized Zr42Ti15.5Cu14.5Ni3.5Be24.5 BMG,28 which suggested that the wear resistance of these materials follows Archard’s equation for wear29 (note: this relationship was originally developed upon considering crystalline metals but has subsequently been shown to describe the wear behavior of ceramics and some BMGs30). However, other experiments provided evidence that BMGs do not follow Archard’s equation for wear and suggested that other factors, such as surface oxide and degree of crystallinity,31–33 contribute to determining the wear behavior of these materials.
In this work, we systematically evaluated the effect of annealing temperature (from temperatures lower than the BMG glass transition temperature to temperatures higher than the BMG recrystallization temperature) on the friction and wear response of a Zr-based BMG, namely Vit105 (Zr52.5Cu17.9Ni14.6Al10Ti5). Based on the results of sliding experiments and material characterization, a correlation between wear behavior, friction response, and annealing temperature was established.
II. MATERIALS AND EXPERIMENTS
A. Isothermal annealing and material characterizations
The samples (dimensions: 15 × 15 × 1.5 mm3) used in this work were sectioned from a Vitreloy 105 (Vit105, Zr52.5Cu17.9Ni14.6Al10Ti5) metallic glass plate (105 × 30 × 1.5 mm3, supplied by Eutectix, USA) with a precision low-speed diamond saw (Allied High Tech Products Inc., USA) operating at 300 rpm.
To determine the glass transition temperature (Tg) of Vit105, differential scanning calorimetry (DSC) measurements were performed using a Q100 DSC (TA Instruments, USA) at two heating rates (i.e., 10 and 20 °C/min). Based on the DSC results, the annealing temperatures to be used in subsequent experiments with Vit105 were determined and could be categorized into three groups: (1) below Tg (250 and 350 °C), (2) between Tg and the crystallization temperature (410 °C), and (3) above the crystallization temperature (480 °C). Vit105 coupons were annealed at these temperatures for 12 h in a tube furnace (MTI Corporation, USA) connected to a mechanical roughing pump, which allowed a base pressure of ∼10−3 Torr to be achieved. Sacrificial Ti strips (met specifications of ASTM B265; supplied by McMaster-Carr, USA) were also included in the furnace tube during heat treatments to scavenge oxygen. At the end of the annealing process, each sample was allowed to cool down to room temperature inside the tube furnace for 12 h.
After being heat treated, the samples (together with a reference, non-annealed sample) were mechanically polished with emery paper (P250, P500, P1200, and P2400; Struers GmbH, Germany) and a series of diamond paste on polishing clothes (3 µm, 1 µm, and 250 nm; Struers GmbH, Germany). The root-mean-square roughness surface roughness (Rq) of the samples was less than 20 nm (measured with a VK-X1100 optical profilometer, Keyence, Japan).
To evaluate the formation of crystalline phases as a result of annealing, x-ray diffraction (XRD) measurements were carried out on all samples using a MiniFlex 600 diffractometer (Rigaku, Japan). XRD spectra were acquired with a monochromatic Cu Kα x-ray source (wavelength: 1.54 Å). The 2θ range was set between 20° and 80° with a step size of 0.04° and a scan speed of 2°/min.
The hardness and reduced elastic modulus of the samples were measured with a Hysitron TI 950 TriboIndenter (Bruker, USA) operated in a constant strain rate mode (0.5 s−1) and at a peak load of 12 mN. Sixteen indents were created on each sample for statistical purpose, and the Berkovich tip was calibrated using the Oliver–Pharr method34 before and after the experiment. The penetration depth was measured to be larger than 100 nm for all indents.
B. Tribological testing
Reciprocating ball-on-flat tribological experiments were conducted using a UMT-2 TriboLab (Bruker, USA) in open air (45%–50% relative humidity) without the presence of any liquid lubricants. Sapphire hemispheres (diameter: 2 mm, Swiss Jewel, USA) were used as a countersurface. Sliding tests were carried out as a function of applied normal load (2.5, 5, and 9.8 N, corresponding to an average Hertzian contact pressure of 1.12, 1.42, and 1.77 GPa). The sliding speed, stroke length, and number of cycles were 5 mm/s, 5 mm, and 1000, respectively. Three independent measurements were performed to check reproducibility. The root-mean-square roughness surface roughness (Rq) of the sapphire hemisphere was 76 ± 2 nm.
At the end of tribological experiments, the wear tracks were analyzed by optical profilometry to determine the specific wear rate and morphology, whereas the sapphire hemispherical countersurfaces were examined with a VEGA3 scanning electron microscope (Tescan, Czech Republic) operated at an acceleration voltage of 10 kV for imaging and 20 kV for mapping the distribution of transfer film/wear debris with energy dispersive x-ray spectroscopy (EDS). All countersurfaces were sputter-coated with Au before EDS analyses to avoid charging.
III. RESULTS AND DISCUSSION
A. Thermal properties and phase transformation
Figure 1(a) displays the DSC curves for as-received Vit 105 acquired at two different heating rates (i.e., 10 and 20 °C/min). The glass transition temperature was found at 406.3 and 405.5 °C upon performing measurements at a heating rate of 10 and 20 °C/min, respectively. Above Tg, two exothermic peaks were detected and indicate a two-step crystallization for this material, in agreement with previous reports in the literature.35,36 The structural evolution of Vit105 was characterized by x-ray diffraction (XRD) [Fig. 1(b)]. Compared to the XRD spectrum of as-received Vit105, which only exhibited a broad envelope at 35°–40°, the spectra collected on samples annealed at 250 and 350 °C for 12 h did not exhibit any well-defined, narrow diffraction peaks, thus indicating that no crystalline phases were formed within the amorphous matrix upon annealing at temperatures below Tg. However, the XRD spectra collected on specimens annealed at 410 °C (∼4 °C above from Tg) and 480 °C (above the crystallization temperature) for 12 h were characterized by intense diffraction signals, which provided evidence for the partial crystallization of the material. Notably, in agreement with Kündig et al.,36 these diffraction peaks could be assigned to Zr2Ni—an intermetallic compound with a tetragonal crystal structure. The absence of any diffraction feature that could be attributed to NiTi2-type cubic phases, which would form upon annealing in the high-oxygen-content Vit105 samples,37 confirms the effectiveness of the Ti strip in scavenging oxygen during the annealing process.
(a) DSC curves of the as-received Vit105 sample, where the glass transition temperature Tg is highlighted by an arrow. The blue curve is shifted for clarity. (b) XRD patterns of as-received Vit 105 (i.e., reference) as well as Vit105 annealed at different temperatures. The diffraction peaks assigned to the Zr2Ni crystalline phase are marked with stars. The patterns are shifted for clarity.
(a) DSC curves of the as-received Vit105 sample, where the glass transition temperature Tg is highlighted by an arrow. The blue curve is shifted for clarity. (b) XRD patterns of as-received Vit 105 (i.e., reference) as well as Vit105 annealed at different temperatures. The diffraction peaks assigned to the Zr2Ni crystalline phase are marked with stars. The patterns are shifted for clarity.
B. Nanoindentation
The mechanical properties of Vit105 samples before and after annealing were evaluated by nanoindentation. Figure 2(a) shows representative load–displacement curves acquired on Vit105 samples at a constant strain rate (0.5 s−1), while the computed hardness and reduced elastic modulus values are reported in Fig. 2(b). The loading curves collected on as-polished Vit105 (i.e., not annealed, reference) exhibited serrated features (see Fig. S.1 of the supplementary material). These features, which have also been observed in indentation experiments carried out on a number of BMGs under various loading conditions at room temperature,6,38–40 are attributed to the formation of shear bands upon loading that mediate plastic deformations in amorphous solids.6,41 The hardness and reduced elastic modulus were found to be 6.9 ± 0.2 GPa and 97.7 ± 1.4 GPa, respectively, which agree well with the values reported for Zr-based BMGs in the literature.42–44 While similar serrated features appeared in the loading curves collected on Vit105 annealed at 250 °C (note: no statistically significant variations in reduced elastic modulus and hardness were observed upon annealing Vit105 at 250 °C), they were not detected in indentation data acquired on Vit105 samples annealed at 350 °C. Since annealing at 350 °C did not induce the precipitation of any crystalline phases as indicated by XRD measurements, the absence of serrated features in loading curves acquired on Vit105 annealed at 350 °C can be attributed to the annealing-induced reduction in free volume in the glass, which increases the activation barrier for the motion of shear bands12,28 and results in an increase in hardness [Fig. 2(b)]. While indentation experiments performed on Vit105 specimens annealed at higher temperatures (i.e., 410 and 480 °C) did not exhibit any serration and, thus, suggest a densification of the amorphous matrix, the partial crystallization of the glass suggested by XRD analyses can contribute to the progressive increase in reduced elastic modulus and hardness with annealing temperature. These results are in agreement with previous studies evaluating the impact of the precipitation of crystalline phases on the mechanical properties of BMGs.19–22
(a) Representative load–displacement curves collected on Vit105 before (reference) and after annealing at different temperatures. (b) Hardness and reduced elastic modulus computed from indentation experiments.
(a) Representative load–displacement curves collected on Vit105 before (reference) and after annealing at different temperatures. (b) Hardness and reduced elastic modulus computed from indentation experiments.
C. Friction response
Load-dependent reciprocating ball-on-flat experiments were performed in open air and without the presence of any liquid lubricants to evaluate the effect of annealing on the tribological response of Vit105. Figure 3 displays the evolution of the coefficient of friction (CoF) as a function of sliding cycles measured in experiments carried out at different applied normal loads. A short running-in (duration: ∼20 cycles) was always detected upon sliding a sapphire countersurface on the specimens (as-prepared and annealed Vit105) in agreement with the results of previous published studies evaluating the tribological response of BMG systems.45–50 Notably, a decrease in the fluctuations of the coefficient of friction (quantified as the standard deviation over one sliding cycle) was observed upon increasing the annealing temperature of Vit105 samples above the glass transition temperature. This was especially detected in the case of the experiments performed at an applied normal load of 2.5 N (see Fig. S.2 of the supplementary material). Furthermore, stochastic, sharp variations in friction coefficient were measured in experiments carried out at 5 and 9.8 N normal load with annealed samples (above 350 °C in the case of tests performed at 9.8 N and at 410 and 480 °C in the case of tests performed at 5 N). These sudden changes in friction upon sliding could be attributed to the continuous formation and removal of the transfer film on the countersurface, as previously reported in the literature.47,51
Evolutions of the coefficient of friction as a function of cycles for experiments carried out at 2.5 N (a), 5 N (b), and 9.8 N (c) of Vit105 samples before (reference) and after annealing at 250, 350, 410, and 480 °C.
Evolutions of the coefficient of friction as a function of cycles for experiments carried out at 2.5 N (a), 5 N (b), and 9.8 N (c) of Vit105 samples before (reference) and after annealing at 250, 350, 410, and 480 °C.
The steady-state CoF, together with the corresponding standard deviation, was computed by averaging the CoF from the 200th to the 850th cycle, ensuring that the initial running-in period was excluded, and a stable sliding regime was reached. Figure 4 displays the variation in the steady-state CoF with the temperature at which the samples were annealed. First, the steady-state CoF values for the as-prepared (reference) sample are in good agreement with those reported by Jones et al. for a sapphire pin sliding against Vitreloy 1b (i.e., a Zr-based BMG with similar composition to the one used in the present work) in open air.52 Second, the steady-state CoF decreases with annealing temperature in tribological tests performed at 2.5 N applied normal load. However, a completely different evolution of the steady-state CoF with annealing temperature was observed in tests carried out at applied normal loads of 5 and 9.8 N: while a slight increase in steady-state CoF was found to occur upon increasing the annealing temperature up to 350 °C, a significant decrease in the steady-state CoF was observed upon annealing the samples near the glass transition temperature (i.e., 410 °C), above which the steady-state CoF slightly increased.
Steady-state coefficient of friction as a function of annealing temperature for sliding experiments carried out at 2.5 N (a), 5 N (b), and 9.8 N (c) applied normal loads. The steady-state coefficient of friction for as-prepared Vit105 (reference) is also displayed.
Steady-state coefficient of friction as a function of annealing temperature for sliding experiments carried out at 2.5 N (a), 5 N (b), and 9.8 N (c) applied normal loads. The steady-state coefficient of friction for as-prepared Vit105 (reference) is also displayed.
D. Analysis of the countersurface and wear tracks
The wear tracks were analyzed by optical profilometry. Figure 5 displays optical profilometry images of wear tracks generated on as-prepared (reference) Vit105 and Vit105 samples annealed at different temperatures. The morphology of the wear tracks indicated that, independently of the applied normal load, the wear mechanism for as-prepared Vit105 is abrasive. This finding agrees well with previous studies that found the wear behavior of various BMG systems to be dominated by abrasive wear when sliding against rigid countersurfaces in open air.46,48,53–55 While abrasive wear was also observed to occur on all annealed samples used for experiments carried out at an applied normal load of 9.8 N, a change in wear mechanism to adhesive–abrasive wear could be detected in the case of specimens annealed above 350 °C (410 °C) and employed in tests performed at 2.5 N (5 N) applied normal load. This variation in wear mechanism, which has not been reported so far in the case of BMGs, will be discussed in more detail in Sec. III E.
Optical profilometry images of the wear tracks created on Vit105 samples (before and after annealing) at different applied normal loads.
Optical profilometry images of the wear tracks created on Vit105 samples (before and after annealing) at different applied normal loads.
Optical profilometry also allowed for the quantification of the specific wear rate, i.e., the volume of material mechanically removed from the sample surface normalized by the applied normal load and sliding distance.29 Figure 6 shows the effect of the temperature at which Vit105 was annealed on the specific wear rate measured at the end of sliding tests carried out at different applied normal loads. A statistically significant decrease in wear rate was measured upon increasing the annealing temperature of Vit105, indicating that devitrified and partially crystallized specimens have a higher wear resistance than fully glassy Vit105. This finding agrees well with the work by Zhou et al., which provided evidence for a decrease in wear volume upon crystallizing a Zr61Ti2Cu25Al12 BMG through annealing.56
Specific wear rate as a function of annealing temperature for sliding experiments carried out at 2.5 N (a), 5 N (b), and 9.8 N (c) applied normal loads. The specific wear rates for as-prepared Vit105 (reference) are also displayed.
Specific wear rate as a function of annealing temperature for sliding experiments carried out at 2.5 N (a), 5 N (b), and 9.8 N (c) applied normal loads. The specific wear rates for as-prepared Vit105 (reference) are also displayed.
The correlation between the wear resistance of the specimens and their nanomechanical properties indicated a linear decrease in specific wear rate with surface hardness (measured via nanoindentation) (Fig. 7), which suggests that the wear behavior of the samples evaluated in the present study follows Archard’s wear model.
Specific wear rate as a function of hardness (measured by nanoindentation, see Fig. 2) for sliding experiments carried out at 2.5, 5, and 9.8 N applied normal loads.
Specific wear rate as a function of hardness (measured by nanoindentation, see Fig. 2) for sliding experiments carried out at 2.5, 5, and 9.8 N applied normal loads.
To gain insights into the phenomena occurring at the sliding interface and dictating the variation in wear mechanism, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) measurements were performed on the sapphire countersurfaces (note: while SEM/EDS analyses were performed to evaluate the distribution of transfer film on the countersurfaces, surface-analytical measurements by, for example, XPS are required to determine the composition and element chemical state in the near-surface region of the transfer film). Figure S.3 of the supplementary material displays secondary electron (SE) SEM micrographs of the sapphire spheres used in tribological experiments carried out on Vit105 samples (both as-prepared and annealed) at different applied normal loads. The representative SEM/EDS micrographs of the countersurface employed for tests performed at 2.5 N applied normal load are shown in Fig. 8(a) and provide evidence for the formation of a Zr-rich transfer layer from the Vit105 surface. The fragments of the transfer layers are also present on the Vit105 surface after breaking the contact, as shown in Fig. 5. While no transfer film was found within the contact area on the countersurfaces employed in experiments carried out at 9.8 N applied normal load, the patches of Zr-rich material could be located outside the contact region and attributed to the removal of the transfer film from the contact upon sliding. This difference in the distribution of transfer film on the sapphire countersurface provides clues for the different evolution of the coefficient of friction during sliding experiments (Fig. 3): while the formation of a stable transfer film on the sapphire sphere does not result in any sudden variations in the coefficient of friction during sliding tests at 2.5 N normal load, the continuous generation and removal of this layer in the tests performed at 5 and 9.8 N on annealed samples (above 350 °C in the case of tests performed at 9.8 N and at 410 and 480 °C in the case of tests performed at 5 N) can be ascribed to be the origin of the measured stochastic variations in the coefficient of friction. Notably, the thickness of the transfer film patches on the sapphire countersurfaces was always several micrometers (see Fig. S.5 of the supplementary material for a representative profilometry height map of the sapphire countersurface used in tribological tests performed at 2.5 N on Vit105 annealed at 480 °C).
Secondary electron scanning electron microscopy (SEM) micrographs with the corresponding electron energy dispersive (EDS) maps of hemispherical sapphire countersurfaces used in sliding experiments carried out at 2.5 N (a) and 9.8 N (b) on Vit105 annealed at 350 °C.
Secondary electron scanning electron microscopy (SEM) micrographs with the corresponding electron energy dispersive (EDS) maps of hemispherical sapphire countersurfaces used in sliding experiments carried out at 2.5 N (a) and 9.8 N (b) on Vit105 annealed at 350 °C.
Altogether, these results indicate that the wear behavior of the samples evaluated in the present work is predominantly adhesive when sliding at a low applied normal load (in this case, 2.5 N). Under these contact conditions, a transfer film is formed and promotes interfacial sliding as the velocity accommodation mode.57 In the case of experiments carried out at higher loads (i.e., 5 and 9.8 N) on annealed specimens, the velocity is accommodated through bulk deformation, the fracture of third body particles, and surface slip. This conclusion is corroborated by the analysis of the morphology of the countersurfaces (Fig. S.4 of the supplementary material): while in the case of the formation of stable transfer films on the sapphire sphere, significant plastic deformations of the transferred material could be observed (Fig. S.4a of the supplementary material), the worn regions generated under conditions leading to the formation of no transfer films are characterized by a highly faceted fracture surface (intergranular fracture) (Fig. S.4b of the supplementary material), which is commonly observed in crystalline materials showing limited plastic deformation.58
While these experiments shed light on the interfacial phenomena controlling the variation in wear mechanisms of BMGs upon annealing these materials, they do not provide any insights into potential changes in wear behavior during the sliding process. As such, tribological experiments as a function of number of sliding cycles were performed.
E. Time-dependent wear behavior
To evaluate transitions in wear behavior upon sliding at an applied normal load of 9.8 N over time, tribological experiments were carried out while varying the number of sliding cycles (i.e., 250, 500, and 750 cycles) on the Vit105 samples annealed at 350 °C. Figure 9(a) shows the evolution of the coefficient of friction as a function of sliding cycles, while Fig. 9(b) displays the computed steady-state coefficient of friction as well as the specific wear rate (note: the range of sliding cycles used for computing the steady-state CoF was varied for the different experiments and was 200–700 for the tests carried out up to 750 cycles, 200–450 for the tests carried out up to 500 cycles, and 100–200 for the tests carried out up to 250 cycles). While no statistically significant variation in friction response was observed upon varying the number of sliding cycles, a slight increase in specific wear rate was detected upon increasing the sliding distance.
(a) Evolution of the coefficient of friction (CoF) as a function of sliding cycles for Vit105 samples annealed at 350 °C (applied normal load of 9.8 N). (b) Steady-state CoF and the specific wear rate as a function of the total number of sliding cycles.
(a) Evolution of the coefficient of friction (CoF) as a function of sliding cycles for Vit105 samples annealed at 350 °C (applied normal load of 9.8 N). (b) Steady-state CoF and the specific wear rate as a function of the total number of sliding cycles.
Despite these minute changes in wear response, the SEM and optical profilometry micrographs acquired on the Vit105 surfaces as well as sapphire countersurfaces indicated notable changes in wear mechanism upon increasing the number of sliding cycles, as displayed in Fig. 10. In the case of the experiments stopped at the 250th cycle, a transfer film was found to be present on the countersurface. However, the transfer film was progressively removed from the sapphire sphere as the number of sliding cycles increased, which resulted in the gradual transition from a predominantly adhesive wear behavior in the early stage of sliding to an abrasive-dominated wear behavior as sliding progressed, where the detached transfer film induces third-body abrasion at the contact.53 While the presence of a transfer film at the contact has been reported to significantly reduce friction in other material pairs,59 this effect was not observed in this work.
SEM micrographs of sapphire countersurfaces and optical profilometry images of Vit105 substrates (annealed at 350 °C) used in sliding experiments carried out at the applied normal load of 9.8 N for different numbers of cycles.
SEM micrographs of sapphire countersurfaces and optical profilometry images of Vit105 substrates (annealed at 350 °C) used in sliding experiments carried out at the applied normal load of 9.8 N for different numbers of cycles.
Overall, the results of the present work indicated that the tribological response of a class of Zr-based BMGs can be tuned by varying the annealing temperature. Annealing at temperatures below the glass transition temperature (i.e., 250 and 350 °C) induces structural relaxation, which reduces the total amount of free volume present in the system and increases the hardness by suppressing the ability to accommodate the plastic strain through shear banding.22,60,61 While these structural changes induce small variations in the friction coefficient as a function of load, they result in an enhancement of the wear resistance. Notably, annealing slightly above the glass transition temperature induces the partial crystallization of the amorphous matrix with the precipitation of a tetragonal phase (i.e., Zr2Ni) having reduced plasticity due to the lack of slip systems. The formation of a brittle crystalline phase within the amorphous matrix reduces friction and the specific wear rate, while also tending to change the wear mechanism from adhesive to abrasive.62,63 Increasing the annealing temperature above the glass transition temperature enhances the devitrification of the glassy matrix. The resulting composite material, consisting of an amorphous matrix reinforced with brittle inclusions, undergoes brittle fracture upon sliding owing to its limited capability to accommodate plastic strain, as indicated by SEM analysis. The results of this work do not only provide guidance for tailoring BMG-based materials for tribological applications through the generation of crystalline precipitates by annealing but also call for further studies evaluating the kinetics of formation of precipitates and their effect on the resulting mechanical and tribological properties.
Finally, it is worth noting, though, that the present work was carried out with a specific BMG (i.e., Vit105) and a countersurface material (i.e., sapphire). Thus, the findings presented herein might not be directly applicable to other tribological systems in which one of the mating surfaces is made of BMG or upon varying the chemical composition of the sliding countersurface.
IV. CONCLUSIONS
In this work, the effect of annealing temperature (from temperatures lower than the BMG glass transition temperature to temperatures higher than the BMG recrystallization temperature) on the friction and wear response of a Zr-based BMG, namely Vit105 (Zr52.5Cu17.9Ni14.6Al10Ti5), was systematically evaluated. The results of tribological experiments provided evidence that annealing Vit105 reduces the steady-state friction response when the annealing temperature is close to the glass transition temperature while progressively increasing the wear resistance owing to the increase in hardness induced by the densification of the amorphous matrix (below the glass transition temperature) together with the precipitation of a brittle crystalline phase (Zr2Ni) above the glass transition temperature. Furthermore, the wear mechanism was found to be dependent on the annealing temperature as a transition from a predominantly adhesive wear mechanism to an abrasive-dominated one was observed as the annealing temperature crossed the glass transition temperature. Altogether, the outcomes of this work demonstrate that the friction response and wear resistance of BMG can be tuned by varying the annealing temperature, thus aiding in the effective use of BMGs in tribological applications.
SUPPLEMENTARY MATERIAL
The supplementary material encompasses the following: a representative loading curve acquired on as-prepared Vit105; evolution of the standard deviation of the coefficient of friction (CoF) as a function of sliding cycles; secondary electron scanning electron microscopy (SEM) micrographs of sapphire countersurfaces used in sliding tests performed at different applied normal loads; SEM micrographs of the contact region on sapphire countersurfaces used in sliding tests performed at 2.5 and 9.8 N; and representative profilometry images of the transfer film within the wear track generated on Vit105 and the sapphire countersurface.
ACKNOWLEDGMENTS
The material is based upon work supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multi-mission laboratory managed and operated by the National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under Contract No. DE-NA0003525. F.M. acknowledges the support from the Welch Foundation (Grant No. F-2151-20230405) and the Walker Department of Mechanical Engineering and the Texas Materials Institute at the University of Texas at Austin. The authors thank Dr. B. D. Freeman and his student, J. J. Rosenthal, from the McKetta Department of Chemical Engineering at the University of Texas at Austin for their kind assistance with the DSC measurements.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
Hsu-Ming Lien: Data curation (lead); Formal analysis (lead); Investigation (lead); Writing – original draft (lead); Writing – review & editing (supporting). Nicolás Molina: Data curation (supporting); Investigation (supporting); Writing – review & editing (supporting). Aisha Lizaola: Data curation (supporting); Investigation (supporting); Writing – review & editing (supporting). Michael Chandross: Data curation (supporting); Investigation (supporting); Writing – review & editing (supporting). Filippo Mangolini: Conceptualization (lead); Formal analysis (supporting); Funding acquisition (lead); Investigation (equal); Writing – original draft (equal); Writing – review & editing (lead).
DATA AVAILABILITY
The data that support the findings of this study are available upon request from the corresponding author.