Tribological behaviors of nanostructured surface layer processed by means of surface mechanical attrition treatment
Y.N. Shi a, Z. Han b
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy
of Sciences, Shenyang, 110016, China
a yinongshi@imr.ac,
b zhonghan@imr.ac
Keywords:Surface mechanical attrition treatment (SMAT); Friction and wear; Nanocrystalline metallic materials
Abstract. Surface mechanical attrition treatment, an approach to fabricate nanostructured surface layer
on bulk metallic materials has been extensively investigated in the past few years with respect to grain refinement mechanism, friction and wear behavior and the subsequent chemical treatments. The present paper briefly overviews the friction and wear behaviors of the surface nanocrystalline layers generated by SMAT on Cu, steels and Mg alloy with emphasis on reciprocating sliding wear behaviors. The potential applications of the present approach are also prospected.
1. Introduction
窦唯个人资料Nanocrystalline (NC) metallic materials often possess appealing properties compared with their coarse-grained (CG) polycrystalline counterparts as they are structurally characterized by a large volume fraction of grain boundaries. Owing to the potential applications of NC materials, there has been considerable interest in the exploration of synthesis, structure characterization and properties of NC materials over the past couple of decades since Gleiter [1] proposed the concept of “NC materials”. After many years of intense research, one critical question that people may concern is the real application of these NC materials, as Koch [2] pointed out in a recent review paper on structural NC materials.
Wear, usually refers to the gradual damage that may result when two materials in contact move relati
ve to each other [3], is a quite common phenomenon that mainly concerns the surface structure and properties of materials in regard with material aspect. To promote the friction and wear properties of engineering materials has long been the objective that material scientists endeavor to attain. Surface modification of engineering materials is one of the routes to get to the objective by optimizing the properties of material surface. As more and more evidences of appealing properties of NC materials have been found, it is reasonable to expect that a NC layer on a bulk material may have enhanced properties compared with the bulk matrix. Surface nanocrystallization [4] aims at the generation of nanostructured surface layer to improve the overall properties and behaviors of the materials without changing bulk properties. As one of the methods of surface nanocrystallization, surface mechanical attrition treatment (SMAT) [5] has been found to be able to generate NC layer on pure metals [6-9] and alloys [10-13]. The idea of SMAT is to introduce a large number of defects and/or interface into the surface layer by plastic deformation, which is realized by the repeatedly impacting of flying balls to the material surface. The mechanical attrition of the balls will introduce plastic strain to the surface layer of the treated material. The accumulation of plastic strain and the
increasing of strain rate along the depth from the deep matrix to the top surface of the treated material will gradually refine the original coarse grains into ultra-fine grains (ufg) and/or nanometer-sized grains. The thickness of the nanostructured surface layer varies from 10 to about 100 µm [6-13] in different materials. These nanostructured surface layers have been found to have a much more enhanced hardness and strength than the matrix [6, 13-19].
It is the purpose of the present paper to review some recent results on friction and wear behaviors of SMAT metallic materials. Most of them will concentrate on dry sliding wear, with a few investigations concerning the oil-lubricated sliding and fretting wear. The possible wear mechanisms are addressed. The potential applications of SMAT are also prospected.
2. The grain refinement mechanism of SMAT
The principle of SMAT is to refine the grain size of the surface layer of a bulk material by severe plastic deformation (SPD), which is achieved by repeated impacts of the flying balls onto the surface of the treated material, as is illustrated in Fig.1. Steel spherical balls are placed in a chamber similar to that used in ball milling, the chamber is vibrated by a vibration generator, with a vibration frequency in the range of 50 Hz to 20 kHz. The size of the balls typically used is 8-10 mm in diamete
安吉丽娜 朱莉主演的电影r. Samples are fixed to the opposite of the generator in the chamber. The surface to be treated is impacted by a large number of resonated balls with a rather high striking speed within a short period of time, which will in turn generate a decreased gradient in strain and strain rate from the top treated surface to the deep matrix with the top surface having the highest value. As schematically illustrated in Fig.2, the corresponding grain size and the micro-strain in the layer will also have a gradient distribution. The grain size of the top surface can be refined into nanometer range, with an average grain size of a few nanometer to tens of nanometer (Table 1), which may vary with materials [6-13].
Fig. 1. Schematic illustration of the SMAT set-up.
Fig. 2. Strain and strain rate variation along depth from the very surface to the matrix of materials
subjected to SMAT.
Table.1 Wear resistance of the selected metal materials after SMAT
Material Average grain size of
the top surface layer
(nm)
好听网名大全Thickness of the
nanostructured
surface layer
(µm)
Hardness (GPa)
CG as-SMAT
Experimental condition
Dominate wear
mechanism
If SMAT
promotes
the wear
resistance
企划案范文Cu 10 25 0.7 1.3 Unlubricated reciprocal
sliding
Unlubricated fretting
Lubricated fretting
Abrasive and
oxidation
Adhesive and
oxidation
Adhesive
Yes
Yes
Yes
Low carbon steel 10~20 10 1.9 3.0 Unlubricated reciprocal
sliding
Abrasive Yes Fe 10 15 2.0 3.8 Unlubricated scratch Abrasive Yes
AZ91D Mg alloy 30±5 100 0.9 1.8 Unlubricated reciprocal
sliding
Abrasive and
mild oxidation
Yes
The mechanism of surface nanocrystallization by SMAT process has been systematically investigated in pure metals, i.e. Fe [6], Co [7], Ti [8] , Cu [9] and alloys, i.e. Al alloy [11]low carbon steel [12], AISI304 stainless steel [10], AZ91D Mg alloy [13], AISI52100 steel [20] etc. These investigations demonstrated that grain refinement mechanism depends mostly on the lattice structure and stacking fault energy (SFE) of the treated material, which normally affect the plastic deformation 10~100 µ
mechanism and dislocation activities. For materials with more dislocation slip systems and higher SFE (such as Fe), plastic deformation will be carried out by dislocation slip while for those with relati
vely fewer slip system and lower SFE (Mg alloy or Ti for example), plastic deformation tends to deform by mechanical twinning. Generally, dislocation activity and twining will intertwine each other during the deformation process and will generate various configurations of dislocation arrangement (e.g. dislocation walls, tangles, arrays, etc.) or twins to subdivide the original coarse grains into sub-grains or cells which will finally be divided into nanometer-scaled grains. Recent investigations also indicated that other factors, such as melting point [13], phase transformation [10], second phase particles [20] will also affect the grain refinement process.
Compared to other SPD approaches currently employed in grain refinement of bulk metallic materials, SMAT has a much higher strain rate at the top surface of the sample which is estimated to be about 102-3/s. This has been approved to be quite crucial in the generation of NC grains in the top surface layer of the treated materials [9]. A recent finite element modeling result [21] indicated that the size of the spherical balls and the impact velocity are also very important for SMAT to get an NC layer with a considerable thickness.
3. Friction and wear behaviors of the surface nanostructured layers
The surface nanostructured layer generated by SMAT generally has a much higher hardness compa
red with the deep matrix of the sample (CG), as listed in Table 1. Investigations have shown that the hardness of the top surface layer can be as much as two times higher than that of the CG matrix [6, 13, 17, 22, 23]. It was also demonstrated that the increased hardness was attributed to the grain refinement rather than the residual stress in the surface [5, 17]. Figure 3 presents a typical curve of the hardness variation along depth from the top surface to matrix taken from AZ91D Mg alloys subjected to SMAT [13]. It is shown that the hardness of the top surface is 1.8 GPa, and the values decrease down to the matrix of 0.9 GPa. Detections in other materials, e. g. Fe [6], Cu [17], Al alloy [22] and stainless steel [23] also demonstrated a similar variation trend. According to the Archard’s law [24]: V/L = KW/H, (1)where V is the wear volume; L the sliding distances; W the normal loads;
H the hardness of the softer material in the two contacting pair; and K is the wear coefficient, which is valid to both the adhesive and abrasive wears, the wear volume per unit sliding distance(wear rate) is inversely proportional to the hardness of the worn material, which means materials with higher hardness may have a better wear resistance than those with lower hardness. It is reasonable to expect that material surfaces subjected to SMAT may have an enhanced wear resistance than the matrix, although the actual situation might be quite complicated under different wear configurations. The hardness of a material is one of the most important mechanical properties in wear and has been 到贵州旅游攻略
widely employed as a criterion to determine the abrasive wear resistance [25]. For some annealed pure metals and steels, the abrasive wear resistance was found to be linearly proportional to their hardness. In contrast, although there was a considerable increase in hardness by mechanical work hardening for some pure metals and heat-treated steels, their abrasive wear resistance remained relatively unchanged with increasing hardness and, in some cases, even decreased. Recently, investigations have been carried out on the friction wear behaviors of pure copper, low carbon steel, stainless steel, Mg alloy after SMAT. Some of these are presented here to have an overview.康熙来了关颖
3.1 Friction and wear behavior of SMAT Cu
It would be easy to start with Cu, a widely used pure metal with an fcc structure and an SFE of 78 mJ/m2. Work on SMAT of Cu [9] showed that different processes of SMAT would result in a surface layer with nano-sized grains or nano-sized twins. Exploration of friction and wear behavior of the SMAT Cu involves investigations on the surface layer with nano-sized grains carried out under dry sliding [17] and fretting [26] condition, as well as oil-lubricated [18, 27] fretting. Also involved is the exploration on the surface layer with nano-sized twins [28] conducted under dry sliding wear condition.
Fig. 3. Variations of hardness and grain (cell) size along the depth of SMAT AZ91D Mg alloy.
3.1.1 Dry sliding wear [17, 28]
Fig. 4(a) gives a TEM bright field image of the surface layer of Cu after 30 minutes SMAT, the grain size distribution is also presented in Fig.4 (b). The average grain size of the surface layer is about 10 nm and the thickness of the nanostructured surface layer is around 25 µm and that of the deformed layer is about 800 µm. The hardness of the top nanostructured surface layer is 1.3 GPa, 80% higher than that of the CG Cu. The coefficient of friction (COF) and the wear rate of the SMAT and CG Cu sample were detected under dry sliding condition with variation of the applied load from 1 to 11 N and the sliding velocity from 0.01 to 0.1 m/s.
As illustrated in Fig. 5, the COF of both the SMAT and CG Cu all decreaseed with the increasing of the applied load, the SMAT Cu exhibited lower COF than that of the CG Cu within the whole applied load. The wear rate of the SMAT Cu was much lower than that of the CG sample, as shown in Fig. 6. When the applied load was 1-3 N, the wear rate of the SMAT Cu was approximately one-fourth of that of the CG Cu. When the applied load was 11 N, The average wear rate of the CG Cu was 4 times higher than that of the SMAT Cu. These results clearly evidenced that SMAT can pronouncedly enhance the wear resistance of conventional CG Cu within the applied load in the experiments provided the NC surface layer after SMAT is not worn off during the sliding process. 0
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6 d (nm)H a r d n e s s (G P a )Distance from the top surface (µm)
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