| 其他摘要 | Increasing interests have been focused on nanocystalline (NC) materials with the anticipation that their properties will be different from, and often superior to those of conventional coarse-grained materials. During the past decade, their tribological behaviors have also attracted attention from scientists due to their potential industrial application. However, up to now, a clear scenery of tribological properties and mechanisms of NC materials is still lacking. Because of difficulties in obtaining “ideal” (i.e., flaw-free, contamination-free and with a sufficiently large sample size) NC pure metal samples by using the present preparation techniques, most investigations on the friction and wear behaviors of nanostructured materials have focused on dual-phase alloys and composites. It is a good choice to fabricate a sample suitable for tribological research by the generation of a nanostructured surface layer on bulk materials, for in many cases, the failures such as wear, erosion and fatigue take place on the surface of engineering materials. Research on friction and wear behaviors of NC pure metals can eliminate some extrinsic effects, such as solid solution hardening and precipitation hardening, helpful for understanding the grain refinement effect on tribological properties of NC materials. Therefore, systematic studies on tribological behaviors of NC pure metals become more and more crucial.
In this work, a nanostructured surface layer with equiaxed nanocrystallites or nano-twins has been successfully achieved by means of surface mechanical attrition treatment (SMAT). The average structure size and micro-hardness at the top surface layer is about 10 nm and 1.3 GPa, respectively. The average structure size gradually increases while micro-hardness decreases with the depth. Friction and wear tests under different conditions were performed by using SRVⅢ oscillating tester. The morphologies and composition of the worn surfaces and debris were investigated by using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) to discover involved wear mechanisms. Optical microscopy (OM), X-ray diffraction (XRD), transmission electron microscopy (TEM) were used to characterize the subsurface microstructure of pure copper induced by friction deformation. The main conclusions of this work are as follows:
1. Under dry condition, the wear volume of the nanostructure surface layer of Cu is only half to one-sixth of that of the coarse-grained Cu under the loads and speeds applied in our test, correlated with the strengthening of grain boundaries or twin boundaries. The sliding wear resistance of the nano-twinned surface of Cu sample is inferior to the NC surface layer of Cu (NC Cu) sample, and this difference increases with an increasing load, which may be attributed to their different hardness gradient and wear mechanisms. For three kinds of samples, both friction coefficients and wear rates show slight fluctuation under frequencies lower than 40 Hz, when the frequency exceeds 40 Hz, there exists a steep increase of the friction coefficients and wear rates for all the samples, due to the removal of the surface oxide layer.
2. Under dry fretting condition, tribological behaviors of the NC Cu are improved obviously, and it exhibits a reduction of the friction coefficient by 0.1-0.2, wear volume by 50-70% compared with coarse-grained Cu. Under a frequency of 20 Hz, when load exceeds a critical value (30 N for the NC Cu and 20 N for the as-annealed Cu), a continuous oxide layer can form on Cu/WC-Co tribopair, leading to a sharp increase of the friction coefficient and wear volume. Under a load of 20 N, for both samples, there are two sharp increases of the friction coefficient and wear volume at 50 Hz and 175 Hz respectively, the former is correlated with the formation of a continuous oxide layer, while the latter corresponds to direct metallic contact and softening of the material caused by high flash temperature.
3. Under lubricated fretting condition, the NC Cu sample exhibits a much improved load-bearing ability and an increased friction coefficient relative to their coarse-grained form. Its wear volume is one order of magnitude lower than that of the as-annealed Cu samples. When fretting against a WC-Co ball, the steady-state friction coefficient of the NC Cu sample shows a different trend with that of the as-annealed Cu sample. The former increases with an increasing frequency, while the latter increases with an increasing fretting frequency up to 100 Hz and thereafter decreases. The wear tracks of the NC Cu are characterized by ploughing marks in the center zone and pile-up of the wear debris at edges zone, while a lots of slip bands caused by plastic deformation can be found on the wear tracks of the as-annealed Cu. A discontinuous metal transfer layer can be found on the WC-Co ball surface only after fretting against the NC Cu sample.
4. Under lubricated fretting condition, a continuous transfer layer consisting of Cu is formed on the AISI 52100 steel ball after fretting against the NC Cu sample, while no material transfer occurs for the as-annealed Cu sample. Experimental parameters, especially slip amplitude and fretting frequency and hardness of Cu samples are important factors influencing the transfer behavior of the NC Cu, while the applied load have trial effect on it. The transfer layer in between the NC Cu/steel ball pair contributes slightly to the increased friction coefficient, while partly account for the enhanced fretting wear resistance of the NC Cu relative to the as-annealed Cu. The increased friction coefficient for the NC Cu can be attributed to its higher hardness in comparison with the as-annealed Cu.
5. Because of severe plastic deformation, the grain size in the surface layer of the pure coarse-grained copper subjected to friction has been refined into the nanometer level. Owing to the gradient variation of microstructures, the surface layer can be subdivided into four sections along depth from the top most surface, i.e., nanostructured regime, submicro-sized regime and matrix with plastic deformation evidences. The average transverse and longitudinal grain size are about 20 nm and 37 nm in the top layer respectively, and with the depth increasing, the size of grains or cells increases，while microstrain decreases.
Keywords: surface mechanical attrition treatment (SMAT); Cu; nanocrystalline; nano-twin; friction and wear; oxide layer; transfer layer; plastic deformation. |
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