Nanocrystalline Surface Layer Research Articles

Tribological Behaviour of Pure Ti with a Nanocrystalline Surface Layer Under Different Loads

Surface mechanical attrition treatment (SMAT), a novel surface severe plastic deformation method, was carried out for titanium (Ti) to create a gradient-structured Ti (SMAT Ti). The tribological behaviour was studied under different loads and dry sliding conditions. The results showed that the deformation layer of SMAT Ti was about 160 μm. The friction and wear results showed that the wear resistance of SMAT Ti was enhanced compared to the coarse-grained (CG) counterpart. SMAT Ti showed abrasive wear under 1 and 5 N, and exhibited abrasive and adhesive wear under 2 N. While CG Ti showed abrasive and adhesive wear under 1–2 N, and exhibited abrasive wear under 5 N for the work hardening effects.

Wear Behaviour of Pure Ti with a Nanocrystalline Surface Layer

A nanocrystalline (NC) layer with the thickness of 30 µm was produced on pure titanium surface by surface mechanical attrition treatment (SMAT). Microstructure observation indicated that the grain size increases with depth from the treated surface. The friction coefficient decreases and the wear resistance increases with the SMAT sample as compared to its coarse-grained counterpart. The improvement of the wear properties could be attributed to the higher hardness of SMAT sample.

The study on vulcanization fouling behavior of nanocrystalline layer

Mold fouling can degrade the physical properties and reduce the aesthetic value of the molded product. A plethora of research efforts have tried addressing these drawbacks, however, an effective method to prevent the foregoing adverse effects of mold fouling is still a challenge. Here, the fouling performance of aluminum alloy with a nanocrystalline surface layer fabricated by High Pressure Shot Peening (HPSP) during vulcanization has been investigated. Results indicate that an HPSP‐treated sample shows excellent resistance of mold fouling with only 1/8 weight gain after 25 cycles, compared to its HPSP‐free counterpart. The improved release performance can be attributed to the compact passivation layer with a larger thickness and low‐energy surface obtained after HPSP treatment, as well as the strengthened surface with a lower friction coefficient. Copyright © 2011 John Wiley & Sons, Ltd.

Enhanced Saturation Magnetization of Nanocrystalline Cobalt Produced by Surface Mechanical Attrition Treatment

A nanocrystalline surface layer is produced in Co plate by surface mechanical attrition treatment (SMAT). The characterization of microstructure and composition indicates that elements of Fe, Cr diffuse from hardened steel balls into the surface layer during SMAT. The diffusion phenomenon results in the composition deviation in the surface layer, leading to higher value of saturation magnetization(Ms) for nanocrystalline Co surface layer in comparison with its coarse-grained counterpart.

Study on wear and friction resistance of nanocrystalline Fe nitrided at low temperature

A nanocrystalline surface layer was fabricated on a pure Fe plate by means of a surface mechanical attrition treatment (SMAT). The SMAT process is able to enhance gas nitriding kinetics so that a nanostructured nitride layer can be formed at low nitriding temperature. Nanoindentation measurements indicate that the hardness value of the nitrided SMAT sample is about 6.1 GPa at the top surface, and decreases gradually to about 2.0 GPa in the matrix. Nanoscratch tests show that the friction coefficient decreases and the wear resistance increases with the SMAT sample. The improvement in friction and wear properties may be attributed to the higher hardness in nanocrystalline nitride layer.

Microstructure and Performance of Surface Nanostructure 316L Stainless Steel Induced by Wire-Brushing Deformation

Nanostructured surface layer was synthesized on 316L stainless steel by means of high-speed rotation wire-brushing deformation (HRWD). The refined microstructure features were systematically characterized by optical microscopy (OM), X-ray diffraction (XRD) analysis, and scanning electron microscopy (SEM) observations. Furthermore, the microhardness was examined by microhardness tester. After HRWD treatment, obvious grain refinement was observed and a nanocrystalline surface layer was formed on 316L stainless steel. It was found that a gradient microstructure with grain size from nanoscale to microscale was obtained along the depth of its surface layer. The thickness of the nanocrystalline surface layer varies from a few to about 20μm depending upon the treatment duration and compressive stress. The microhardness of nanostructured surface layer was enhanced significantly, and along the depth from the top surface, the microhardness in the surface gradually decreased to that of the matrix. Besides, the grain refinement mechanism and behaviors were discussed.

Enhanced mechanical properties and in vitro cell response of surface mechanical attrition treated pure titanium

Surface mechanical attrition treatment (SMAT) was used to fabricate nanocrystalline surface layers on the commercial purity titanium. X-ray diffraction and transmission electron microscopy results indicate that the top layer contained nanograins. Enhanced strength and microhardness were achieved due to the surface nanostructure. Cell culture tests have shown a greater adhered cell density and more extensively spreading morphologies of Saos-2 cells on the SMAT substrates compared to those on the as-received Ti counterparts. Enhanced cell viability and cell cycle were also achieved on the SMAT Ti substrates. These could be attributed to the nanostructure grains with the increased surface hydrophilicity and roughness on the SMAT Ti.

Microstructure and Evaluation of Wire-brushed Mg Sheets

Wire-brushing was applied to surfaces of AZ31B Mg alloy sheets, and the microstructure, mechanical properties and corrosion resistance were investigated. A nanocrystalline surface layer, which consists of equiaxed nano-crystals, can be obtained by the wire-brushing. The mean grain size in the nanocrystalline layer depends on the condition, and the smallest mean grain size was 68.2nm. The grain size depends on the feed speed rather than pressing load. The proof stress and tensile strength slightly increased after the wire-brushing. Corrosion resistance of the Mg sheet significantly increased after the wire-brushing.

The Effect of Initial Microstructure on Surface Nanocrystallization of Quenched and Tempered Steel

Nanocrystalline surface layer was fabricated on a quenched and tempered Cr-Si alloy steel by using Surface mechanical treatment. The microstructure features of various sections in the surface layer were characterized by using transmission electron microscopy (TEM). By analyzing the microstructural characteristic at different depths in the treated surface, the effect of the initial microstructures on grain refinement process of quenched and tempered steel was investigated. Experimental evidence showed the initial subgrains with small angle boundary and lower dislocation density were firstly developed into Lamellar-type dislocation cells (DCs) with dense dislocation walls (DDWs). Some initial subboundaries were moved to DDWs by dislocation activities. The width of lamellar-type DCs was 2-3 times of that of initial lathy subgrains. The size of the DCs and subgrains formed in the interim of refinement process was not uniformity. On the top surface the cementite granules were decomposed or fragmented to hyperfine particles, and the size of the grains tended to uniformity. Experimental analysis indicated the configuration of microstructure was affect by the initial microstructure in the initial stage and the interim of the grain refinement process. Surface nanocrystallization of Cr-Si steel can be attributed to dislocation activities.

High-temperature surface alloying of nanocrystalline nickel produced by surface mechanical attrition treatment

A nanocrystalline surface layer is synthesized in Ni by surface mechanical attrition treatment (SMAT). The characterization of microstructure and composition indicates that foreign elements from hardened balls can permeate and diffuse into the surface layer during SMAT, which can further diffuse and homogenize in the surface layer when annealing at high temperature, leading to formation of new alloy. Foreign elements permeating into the surface layer during SMAT followed by diffusing and homogenizing at high temperature provide an alternative effective technology for surface modification.

Effects of surface nanocrystallization pretreatment on low-temperature ion sulfurization behavior of 1Cr18Ni9Ti stainless steel

A nanocrystalline surface layer of 10 μm thickness was fabricated on 1Cr18Ni9Ti stainless steel by means of supersonic fine particles bombarding (SFPB). The followed low-temperature ion sulfurizing was carried out on the original and the SFPBed (SFPB treated) surface, respectively, forming sulfide layers with certain thickness. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were employed to analyze the phase constituents and grain size of the nanocrystallized surface layer. The surface morphologies and compositions of the sulfide layers were examined by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). X-ray photoelectron spectroscope (XPS) was used to detect the valence states of the sulfide layers. Elemental distribution with depth was measured by augur energy spectroscopy (AES). The results show that the microstructure of the surface layer is refined to nano-grains with the grain size about 30 nm and random crystallographic orientations by SFPB treatment. The surface nanocrystallization pretreatment can significantly improve the thickness, density, and the FeS content ratio of the sulfide layers. The analysis indicates that, the enhancement in efficiency of the ion sulfurization treatment by SFPB surface nanocrystallization treatment is mainly attributed to the high-density crystal defects and the increase of surface chemical activity.

Friction and wear behaviors of nanocrystalline surface layer of medium carbon steel

A nanocrystalline (nc) surface layer, which is about 30 μm thick, was fabricated on a medium carbon steel plate using high-energy shot peening (HESP). The tribological behavior of the nanocrystalline surface layer was investigated under the unlubricated slide using a ring-on-disc tester. The observation of the surface topography shows that the nanocrystalline surface layer may reduce the effect of fatigue wear and improve the friction and wear properties of medium carbon steel. Experimental results show that the friction coefficient and wear weight loss decrease and the wear resistance increases with the surface nanocrystallized samples under lower loads.

Thermal Stability in Nanocrystalline Fe-30 Wt Pct Ni Alloy Induced by Surface Mechanical Attrition Treatment

By means of surface mechanical attrition treatment (SMAT), a nanocrystalline surface layer is produced in Fe-30 wt pct Ni alloy, accompanying the formation of the strain-induced martensite. The thermal stability of nanocrystalline martensite and parent phase austenite in Fe-30 wt pct Ni alloy is studied by X-ray diffraction (XRD) and transmission electron microscope (TEM). The grain growth kinetics parameters, time exponent, n, and activation energy, Q, for both martensite and austenite, are determined, respectively. The TEM observations indicate that abnormal grain growth occurs during annealing at high temperatures.

Ultrasonic Surface Mechanical Attrition of Commercially Pure Ti to Induce Nanocrystalline Surface Layer

In the domain of incremental nanotechnology, surface mechanical attrition treatment is a technique which can transform superficial structure of a material to nanocrystalline without changing the chemical composition. This study is a part of the development and implementation of the technique by using ultrasonic vibrations. The material used is pure titanium in rolled and annealed condition. The nanocrystalline structure is characterized using X-ray diffraction (XRD), and transmission electron microscopy (TEM). The measured grain size is in the order of 5~60 nm. A correlation in the results of XRD and TEM is also discussed.

Effect of Combined Shot Peening Process for Surface Nanocrystallization Method on Fatigue Strength of TC4

In order to enhance the fatigue limit of titanium (TC4), the combined shot peening were employed. Firstly, nano-crystallization in surface layer of TC4 were carried out by high-energy shot peening, and then the surface repair shot peening with small balls were introduced for repairing the surface damage by high-energy shot peening, and reducing the surface roughness, thus to enhance the quality of nanocrystalline surface layer of TC4. The fatigue tests were performed using the specimens treated by the combined shot peening. In addition, the influence factors of fatigue limit, the behavior of fatigue crack initiation and propagation etc. are discussed. The results showed that the fatigue limit of TC4 specimen treated by high-energy shot peening is increased by 13%, while the fatigue limit of TC4 specimen treated by the compound shot peening process is increased by 34% (165MPa). The main reason of fatigue improvement is considered to be that the combined shot peening process may effectively repair the surface damage, and also there is the certain relationship between the fatigue limit improvement and the surface roughness reducing.

Crystallisation behaviour and high-temperature stability of stone wool fibres

The influences of the chemical composition and the heat-treatment conditions on both the crystallisation behaviour and the high-temperature stability (HTS) of three types of amorphous stone wool fibres of iron-bearing alumino-silicate composition are studied by performing thermal, structural, and compositional analyses. The fibres heat-treated under oxidising conditions exhibit outstanding HTS, i.e., the fibres are able to maintain their original geometric shape in a high-temperature environment. This is attributed to the formation of a nano-crystalline layer on the surface of the fibres as a consequence of the oxidation of Fe 2+ to Fe 3+. The layer remains after the bulk crystallisation process has been completed. The thickness of the layer increases with increasing initial content of Fe 2+ and Mg 2+ in the fibres. The nanolayer influences the bulk crystallisation in the manner that it lowers the crystallisation onset temperature, the activation energy, and the Avrami parameter. Such influence benefits the enhancement of the HTS. In contrast, the fibres heat-treated under reducing and inert conditions exhibit poor HTS. This is attributed to the early viscous heating of the fibres because of lack of the nano-crystalline surface layer. The HTS does not depend on the identity of the bulk crystalline phases.

A direct comparison in the fatigue resistance enhanced by surface severe plastic deformation and shot peening in a C-2000 superalloy

In this study, we present the first evidence and modeling efforts showing that surface severe plastic deformation (S2PD) can be more effective in producing metallic components with superior fatigue properties than shot peening (SP). With the aid of a wide battery of characterization techniques (i.e., X-ray diffractometry, optical microscopy, scanning electron microscopy, transmission electron microscopy, and 3-dimensional non-contact optical profilometry), micro- and nano-hardness testing, and finite element modeling, we have identified the underlying mechanism for the fatigue improvement. It is shown that the enhancement in the fatigue limit is derived from a nanocrystalline surface layer, a work-hardened surface region, and residual compressive stresses at the surface, all of which are introduced by S2PD and more substantial than that introduced by SP.

Experimental study of the microstructure and stress state of shot peened and surface mechanical attrition treated nickel alloys

The efficacies of shot peening (SP) and surface mechanical attrition treatment (SMAT) in inducing multiple changes in the microstructure and stress state of the workpiece are compared experimentally, for the first time, using depth-profile X-ray diffractometry. SMAT generates a thicker nanocrystalline surface layer, a deeper plastically deformed zone and a thicker surface region with larger residual compressive stresses than SP. These results can have important implications for improving the fatigue life and wear resistance of metals in the future.

Nanocrystallization process and mechanism in a nickel alloy subjected to surface severe plastic deformation

Bulk components made of a Ni-base C-2000 alloy with a face-centered cubic crystal structure and a very low stacking fault energy have been severely plastically deformed at the surface region to attain a grain size gradient ranging from nanocrystalline at the surface to coarse grained in the bulk. The evolution of microstructural characteristics has been studied as a function of the processing time employing a variety of analytical techniques, including extensive conventional and high-resolution transmission electron microscopy analyses. The thickness of the nanocrystalline surface layer is found to increase with the processing time. Deformation twinning is ubiquitous and occurs at the earliest stage of deformation and the deepest region of the material where plastic deformation has taken place in the surface severe plastic deformation process. A grain-refinement mechanism led by deformation twins and complemented by dislocation activity has been put forth to explain the nanocrystallization of the coarse-grained material employed in this investigation.

Microstructural characterization of nanocrystalline nickel produced by surface mechanical attrition treatment

By means of surface mechanical attrition treatment (SMAT), nanocrystalline surface layers are produced in pure Ni plates. The average crystallite size, root mean square (r.m.s.) microstrain, dislocation density, and stored elastic energy are determined by X-ray diffraction (XRD) line profile analysis. The average crystallite size obtained by XRD is compared with the grain size observed from transmission electron microscopy (TEM) image. The high-resolution TEM (HRTEM) micrograph confirms the presence of high density of dislocations obtained by XRD, and reveals that most of dislocations distribute at the subgrain boundaries with few inside the subgrains.