ZnO nanowires have received much interest owing to their particular structural and piezoelectric properties. For widespread application of ZnO nanowires in various nanotechnologies, the mechanical reliability of the nanowires should be assessed. In this paper, the damage characteristics of vertically grown ZnO nanowires due to contact sliding against a 2 mm diameter steel ball under relatively low loads were investigated. Frictional behavior and wear characteristics of the specimens were assessed. Furthermore, contact sliding tests were performed inside an SEM to monitor the progression of damage of the nanowires. It was found that the friction coefficient was about 0.35 under all loads while the damage characteristics of the nanowires were quite different for each load. The large diameter nanowires tended to fracture earlier than the small diameter nanowires. Wear tests performed inside the SEM confirmed the surface damage characteristics observed during the friction tests.
ZnO nanowires have been receiving much interest as a novel material due to its semiconducting and piezoelectric properties [
In this paper, the contact damage characteristics of ZnO nanowires grown vertically on a silicon substrate were investigated. The progression of nanowire damage was monitored by performing contact sliding experiments using a pin-on-reciprocator type tribotester as well as using a wear tester mounted inside a scanning electron microscope (SEM). The frictional behavior of the ZnO nanowires was also investigated by sliding the nanowires against a steel ball under loads ranging from 2 to 6 mN. The motivation was to identify the contact conditions under which ZnO nanowires get damaged and to understand the damage mechanism. The details of the experiments are described in the following sections.
ZnO nanowires were synthesized on (100) Si wafer with using the hydrothermal method. Prior to the coating processes, Si wafer went through an intensive cleaning process to effectively remove all the organic residues. Si wafer was rinsed with warm (>55°C) acetone and methanol and cleaned by RCA number 1 method using 27% ammonium hydroxide (NH4OH) and 30% hydrogen peroxide (H2O2). Si wafers were then dipped into hydrogen fluoride (HF) to remove the native silicon dioxide (SiO2) layer [
The specimen fabrication process consisted of two steps: (1) ZnO seed-layer coating and (2) growth of ZnO nanowire arrays. For ZnO seed layer coating, a solution of 20 mM EtOH and Zinc acetate dihydrate ((C2H3O2)2Zn·2H2O) were aged for more than 6 hours. The process of spin coating was repeated several times to attain adequate thickness. Si wafer with the seed layer was immersed in the solution of HPLC (high-performance liquid chromatography) grade deionized water, zinc nitrate hexahydrate (Zn(NO3)2), and hexamethylene-tetramine (HMT, C6H12N4) at 80°C condition for 6 hours [
The morphology of the ZnO nanowires coated by the above described method is shown in SEM images of Figure
SEM images of ZnO nanowires (a) evenly coated on Si wafer and (b, c) at higher magnification that shows a mixture of nanowires with two distinct diameters.
Contact sliding tests were performed using a pin-on-reciprocator type tribotester to investigate the frictional behavior of the ZnO nanowires. Frictional force was monitored in real time while reciprocating a 2 mm diameter steel ball in contact with the ZnO nanowire specimen surface. The tests were performed for 20 seconds with a linear speed of 4 mm/sec under the normal loads of 2, 4, and 6 mN. The friction tests were performed five times under the same condition. After each test, the steel ball was carefully cleaned with acetone, EtOH, and de-ionized water to minimize the effect of ball contamination. The wear of the steel ball was found to be insignificant after each test. Thus, the same ball could be used for all the friction tests after cleaning. The wear tracks formed on the ZnO nanowire specimen during the friction tests were observed using the SEM.
In order to assess the progression of ZnO nanowire damage due to contact sliding without interruption of the sliding motion wear tests were performed inside an SEM. A pin-on-disk type wear tester was mounted inside the SEM to observe the wear behavior of ZnO nanowires in real time. Figure
Schematic of the pin-on-disk type wear tester mounted inside the SEM.
The contact sliding tests were conducted using the ZnO nanowire specimens under various loads while obtaining the frictional force in real time. Figure
Frictional force of ZnO nanowires sliding against a 2 mm diameter steel ball with respect to number of sliding cycles for different applied loads.
Figure
Friction coefficient of ZnO nanowires sliding against a 2 mm diameter steel ball with respect to number of sliding cycles for different applied loads.
The damage of ZnO nanowires due to the frictional interaction was assessed by observing the wear tracks using the SEM. Figure
SEM image of the wear track on the ZnO nanowire specimen after the friction test under (a) 2 mN, (b) 4 mN, and (c) 6 mN applied load after 1–20 cycles of contact sliding. (
Under the 4 mN applied load, the damage of ZnO nanowires with large diameter occurred within the first cycle. The number of fractured ZnO nanowires with large diameter was slightly increased after 3 cycles. After 5 cycles, large diameter ZnO nanowires were damaged more severely. Small diameter ZnO nanowires also started to fracture into smaller pieces as the number of cycles increased. After 10 cycles under 4 mN applied load, the fractured ZnO nanowires were compressed to form a compacted surface morphology due to repetitive and relatively high contact stresses. After 20 cycles, it could be found that the extent of compressed fractured ZnO nanowires increased significantly (Figure
It was also interesting to observe the damage behavior of ZnO nanowires with the number of sliding cycles. Figure
SEM image of ZnO nanowire fractured in (a) parallel to the (0001) plane after 3 cycles under 6 mN applied load and (b) both parallel and nonparallel to the (0001) plane after 10 cycles under 6 mN applied load. Dotted lines show the fractured plane of the ZnO nanowires. (0001) is the perpendicular plane to the
In order to investigate the progression of damage of ZnO nanowires during repeated contact sliding, wear tests were performed inside the SEM. Figure
SEM image of the wear test under the load of (a) 2 mN, (b) 4 mN, and (c) 6 mN performed inside the SEM. Inset of triangle, arrow, and dotted regions show the large diameter, small diameter and compressed region of fractured ZnO nanowires, respectively.
For the 2 mN wear test, the amount of fractured nanowires was minimal after the 5th rotation while some fracture of the large diameter nanowires could be found after the 10th rotation as shown in Figure
From the results of the contact sliding tests the mechanism of contact damage of ZnO nanowires was postulated as illustrated in Figure
Illustration of ZnO nanowire damage process: (a) undamaged ZnO nanowires, (b) wear debris of fractured fragments of large diameter ZnO nanowires, (c) wear debris from fractured fragments of small diameter ZnO nanowires, and (d) compact surface formation due to compression of wear debris through repetitive and high contact stresses.
Unlike most sliding systems where the surface wears gradually, the damage of ZnO nanowires due to repeated contact sliding occurred mostly by fracture. Thus, the wear debris created were in the shape of fragmented portions of the nanowires. As mechanical stress was imparted on the nannowires due to contact sliding some of the nanowires experienced fracture along the (0001) plane of the ZnO crystal. Fracture planes other than (0001) were also found. The extent of damage increased with increasing applied load and number of sliding cycles. At the lowest load of 2 mN used in this work nanowire damage was not evident during the first several cycles of sliding. On the other hand, damage of the nanowires by fracture occurred within the first sliding cycle at loads higher than 4 mN.
An interesting point to note in the behavior of nanowire damage was that the large diameter nanowires tended to fail more readily than the small diameter ones. This outcome was attributed to two factors. The first factor is regarding the length of the nanowires. Since the large diameter nanowires were slightly longer than the small diameter nanowires, they could have been fractured first while protecting the small diameter nanowires from being damaged. However, considering the small percentage of nanowires with longer length, this effect was not considered to be significant. The second factor is regarding the stiffness of the nanowires. As mentioned before, the aspect ratio of large diameter nanowires was much smaller than the small diameter nanowires. Thus, high stiffness due to low aspect ratio of the large diameter nanowires could have resulted in early failure. Due to the high stiffness, the large diameter nanowires were not prone to elastic deflection that was needed to dissipate the frictional energy without being plastically deformed. In the case of the small diameter nanowires dissipation of the frictional energy was possible through accumulation of the elastic energy within the nanowire which was then released once the contact event was over. Nevertheless, even the small diameter nanowires got damaged by fracture when the load was sufficiently high or the number of contact cycles increased.
In this paper, the contact damage characteristics and frictional behavior of ZnO nanowires were investigated. The progression of damage of ZnO nanowires was investigated through analysis of the wear tracks after each cycle as well as by wear tests performed inside an SEM. Based on the experimental results, the following conclusions may be drawn. The friction coefficient was about 0.35 for the applied loads of 2, 4, and 6 mN throughout the entire sliding cycles. Large diameter nanowires tended to fracture more readily than small diameter nanowires during frictional interaction due to higher stiffness. Gradually, compressed fragments of fractured nanowires covered the surface of vertically grown ZnO nanowires which resulted in the formation of a continuous compact surface. The damage characteristics of ZnO nanowires could be classified into three steps: fracture of ZnO nanowires with large diameter, fracture of ZnO nanowires with large and small diameter, and compression of the fractured nanowires to form a compact surface.
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) no. 2011-0000409.