Changes in surface properties of nitrogen-implanted AISI316L stainless steel

The effect of nitrogen ion implantation with an energy of 125 keV and doses of 1 × 10¹⁷–1 × 10¹⁸ at/cm² on such tribological characteristics of AISI316L stainless steel as the friction coefficient, wear resistance, and microhardness was studied. The steel surface layer composition was studied by the methods of RBS, XRD, GXRD, SEM, and EDX. The friction coefficient and abrasion resistance of AISI316L stainless steel were measured in air, oxygen and argon atmospheres, and in vacuum. An increase in the abrasion resistance after implantation was detected, which was different for various media. The largest increase in the wear resistance was observed during testing in air. The largest decrease in the friction coefficient was observed for all implanted samples in argon atmosphere. Tribological tests resulted in an increase in nitrogen, carbon, and oxygen concentrations in worn sample fragments in comparison with their concentrations in surface layers immediately after implantation.     

Process mapping of laser surface modification of AISI 316L stainless steel for biomedical applications

A 1.5-kW CO₂ laser in pulsed mode at 3 kHz was used to investigate the effects of varied laser process parameters and resulting morphology of AISI 316L stainless steel. Irradiance and residence time were varied between 7.9 to 23.6 MW/cm² and 50 to 167 μs, respectively. A strong correlation between irradiance, residence time, depth of processing and roughness of processed steel was established. The high depth of altered microstructure and increased roughness were linked to higher levels of both irradiance and residence times. Energy fluence and surface temperature models were used to predict levels of melting occurring on the surface through the analysis of roughness and depth of the region processed. Microstructural images captured by the SEM revealed significant grain structure changes at higher irradiances, but due to increased residence times, limited to the laser in use, the hardness values were not improved.     

Cross-sectional transmission electron microscopy of ultra-fine wires of AISI 316L stainless steel

Starting with 190?µm diameter wire of 316L stainless steel, ultra-thin wire just 8?µm in diameter has been made and characterized. There was no intermediate heat treatment used in the process of drawing, and the amount of true stain was about 6.3. A specimen preparation method for the cross-sectional transmission electron microscopy (TEM) of ultra-fine wires of 316L stainless steel has been developed. The ultra-fine wire was sandwiched between silicon chips and the bonded assembly then sliced to produce longitudinal and transverse sections of the wire in a form suitable for further processing into electron transparent samples. TEM reveals that the heavily deformed wire consists of nanoscale fine elongated structures along the drawing direction. The diffraction patterns indicate that a substantial amount of austenite has transformed into martensite. The TEM dark field images show nanosized patches of martensite distributed among the debris of austenite along the drawing direction. The evidence strongly suggests that severe deformation leads to mechanical stabilization of austenite against the growth of martensite.     

New route to form micro-pores on 316L stainless steel surface

In order to seek an effective way for preventing restenosis after coronary stent implantation, a proposal of increasing the amount of loaded drug without changing the size of struts was given. Thereafter, a process of fabricating in-situ formed sub-micro-pores on 316L stainless steel (316L SS) was demonstrated. An aluminum thin film was deposited by magnetron sputtering on a 316L substrate. The aluminum film was then anodized in different acids (0.3 M oxalic and 10 vol.% sulfuric) by regulating direct current power supply. Through an appropriate chemical dissolution, the anodic alumina film was removed and the underlying porous 316L was obtained. The morphology of the porous 316L surface was examined by scanning electron microscope and the composition of the pores was investigated by energy dispersive X-ray analysis. The corrosion behavior of the porous 316L was evaluated by the polarization measurement. The results indicate that the shape and size of pores could be affected evidently by the acids used in anodization. The pores density is found to change with variation of the applied voltage in anodization. The corrosion current of the anodized specimens decrease and the corrosion voltage increase, compared with the untreated specimens.     

Oxidation of AISI 304L stainless steel surface with atomic oxygen

Oxidation of stainless steel surface in oxygen atmosphere was investigated by Auger electron spectroscopy (AES) depth profiling. The samples made of AISI 304L stainless steel were exposed to highly non-equilibrium oxygen atmosphere at different temperatures between 300 and 800 K and for different periods between 5 and 600 s. The degree of dissociation of oxygen molecules was of the order of 10%. A thin oxide layer formed on the stainless steel surface consisted of the iron oxide. The thickness depended on the sample temperature. At room temperature it was 7 nm, and it remained the same up to 200 °C. With further increase of temperature, the thickness of the oxide layer increased and reached 40 nm at 450 °C. The thickness was independent of exposure time. The results were explained by two mechanisms of oxide growth. Up to 200 °C the oxidation was run by electro-migration, while at higher temperatures the thermal induced migration prevailed.     

Micro-hot-embossing of 316L stainless steel micro-structures

In this paper, the fabrication of an array of 316L stainless steel cylindrical micro-structures by hot-embossing is studied. The effects of various embossing parameters on the filling of the micro-cavities of silicon mold insert and the demolding of micro-structures are investigated. They include different silicon mold insert configurations, embossing pressure, embossing temperature, embossing time and the thickness of embossing substrate on which the micro-structures stand. Two types of silicon mold insert configurations are investigated. Hot-embossing study with the silicon mold insert attached on the upper mold plate shows that it is not feasible to achieve complete filling. Incomplete filling occurs and increasing the embossing pressure causes excessive thinning of the embossing substrate, so that micro-structures are torn off during demolding. Experiments show that the configuration that the silicon mold insert is positioned flush with lower mold surface improves complete filling. Using suitable embossing pressure and temperature, the larger substrate thickness and longer embossing time ensure the fabrication of defect-free fully filled micro-structures.     

Collagen immobilization on 316L stainless steel surface with cathodic deposition of calcium phosphate

Collagen fibril/(calcium phosphate and carbonate) composite coatings on 316L stainless steel were developed with a cathodic deposition technique. The response of SaOS-2 osteoblast-like cells to the collagen/calcium salt-coated 316L steel was investigated. The collagen fibrils were self-assembled on the 316L steel surface and immobilized by their partial incorporation into a calcium salt layer electrodeposited cathodically in Hanks’ solution. The amount of calcium salt depended on the applied cathodic potential. The mineralization of collagen fibrils was observed. The collagen coverage localized and the composition of calcium salts varied on the same specimen. Such non-uniform surfaces affected the cell response. The observed outlines of cell bodies and nuclei on the thin collagen coating were clearer than those on the thick collagen coating in most cases. The collagen coating did not significantly influence the mean viability of cells on the whole specimen surface. Interestingly, the alkaline phosphatase activity per cell on the collagen/calcium salt-coated specimens was higher than that on the as-received specimen. It was revealed that cathodic deposition is an effective technique to immobilize collagen fibrils on a 316L steel surface.     

Corrosion-wear monitoring of TiN coated AISI 316 stainless steel by electrochemical noise measurements

A modified electrochemical noise (EN) technique has been applied to monitor corrosion-wear of TiN coated AISI 316 stainless steel sliding against corundum in 0.5 M H₂SO₄. Experimental results show that the EN technique can sensitively detect potential and current variations during a corrosion-wear process. Corrosion-wear mechanisms of TiN coatings depend on their substrate properties. When the substrate is passive, such as AISI 316 stainless steel, the potential and current variations reflect the properties of coatings. Depassivation and repassivation alternately take place on the tribo-activated wear area during the steady-state phase.     

Electrophoretic deposition of PVA coated hydroxyapatite on 316L stainless steel

Polyvinyl alcohol (PVA) coated hydroxyapatite was deposited onto a 316L stainless steel substrate by electrophoretic deposition. Deposition was carried out in a methanol suspension at pH 5.5 using a graphite rod as an anode. Parameters such as PVA concentration, deposition voltage and time were optimized to achieve a homogeneous, crack-free adhesive coating. Techniques such as X-ray diffraction and Fourier transform infrared spectroscopy were used to study the phase composition of the coated materials and the stability of hydroxyapatite in the presence of PVA.     

CEMS study of stainless steel films deposited by pulsed laser ablation of AISI316

Thin stainless steel films deposited on SiO₂/Si wafer were prepared by a pulsed laser ablation of austenite stainless steel (AISI316), and characterized by conversion electron Mössbauer spectrometry (CEMS) using a He gas proportional counter. As-deposited films were composed of a magnetic phase. When deposited films were heated in air at various temperatures, the hyperfine field of the magnetic phase increased. Hematite was produced on the surface, and the magnetic orientation changed from parallel to in-plane at random with the increase of heating temperatures. The metallic iron and magnetite were produced at 400°C in dry Ar + 5%H₂ atmosphere. When the film was heated in wet Ar + 5%H₂ atmosphere at 600°C, maghemite was produced on the surface, and austenite phase was produced in the inner film.     

Microstructure evolution occurring in the modified surface of 316L stainless steel under high current pulsed electron beam treatment

High current pulsed electron beam (HCPEB) surface treatment of 316L stainless steel (SS) was carried out with a wide spectrum of treating parameters. Microstructure changes occurring in the modified surface were characterized with microscopy, X-ray diffractometry and electron backscatter diffractometry (EBSD) techniques. The evolution regularities of surface craters and microstructure refinement, as well as preferred orientation of (1 1 1) crystal planes were discussed on considering the coupled temperature-stress fields formed in surface layers after an absorption of HCPEB energy.     

316L不锈钢粉末光纤激光选区熔化特性

采用自行研制的光纤激光选区熔化快速成型设备,研究了选区激光熔化316L不锈钢粉末工艺参数、能量输入与样件致密度、表面形貌之间的关系以及微观组织特征。结果表明：扫描速度对成型效果影响最为显著;样件致密度随着激光能量密度提高有逐渐增大的趋势;能量密度作为选区激光熔化工艺的技术指标具有可操作性;表面形貌由激光功率与扫描速度比值所决定。深入探讨了能量输入、熔化凝固行为、激光功率与扫描速度比值与样件致密度、表面形貌的关系。结果表明：选区激光熔化凝固组织层内、层间熔合处为弧形,且为冶金结合,金相组织主要由柱状晶与等轴晶组成,层内靠近熔合线周围是柱状晶,而层间靠近熔合线附近主要是细小等轴晶,晶粒直径为1 μm左右。     

316L不锈钢粉末光纤激光选区熔化特性

采用自行研制的光纤激光选区熔化快速成型设备,研究了选区激光熔化316L不锈钢粉末工艺参数、能量输入与样件致密度、表面形貌之间的关系以及微观组织特征。结果表明:扫描速度对成型效果影响最为显著;样件致密度随着激光能量密度提高有逐渐增大的趋势;能量密度作为选区激光熔化工艺的技术指标具有可操作性;表面形貌由激光功率与扫描速度比值所决定。深入探讨了能量输入、熔化凝固行为、激光功率与扫描速度比值与样件致密度、表面形貌的关系。结果表明:选区激光熔化凝固组织层内、层间熔合处为弧形,且为冶金结合,金相组织主要由柱状晶与等轴晶组成,层内靠近熔合线周围是柱状晶,而层间靠近熔合线附近主要是细小等轴晶,晶粒直径为1μm左右。     

316L不锈钢在静态液态锂中的腐蚀行为研究

利用脉冲激光诱导击穿光谱仪(LIBS)结合SEM、EDX和维氏硬度(HV)等分析手段,分析了316L不锈钢在350℃的液态锂中静态腐蚀500h后的腐蚀特性。微观形貌分析表明：样品存在较明显的晶间腐蚀,其腐蚀层厚度约为2.7μm。硬度分析表明：腐蚀后表面硬度值约HV234.72,约是腐蚀前硬度值的0.78倍。LIBS组分分析表明：经液态锂腐蚀后的样品表面Ni、Cr等元素含量相对减少;在被腐蚀区域Li元素含量随激光击打深度的增加而减少,而Ni、Cr等元素含量与之近似呈成正比递增;母材区Li元素的渗透深度约3.5μm,约是焊缝区的0.6倍。     

Low-strain fatigue in AISI 316L steel surface grains: a three-dimensional discrete dislocation dynamics modelling of the early cycles I. Dislocation microstructures and mechanical behaviour

The early stages of the formation of dislocation microstructures in low-strain fatigue are analysed, using three-dimensional discrete dislocation dynamics modelling. Simulations under various conditions of loading amplitude and grain size have been performed. Both the dislocation microstructures and the associated mechanical behaviour are accurately reproduced in single-slip as well as in double-slip loading conditions. The microstructures thus obtained are analysed quantitatively, in terms of number of slip bands per grain, band thickness and band spacing. The simulations show the crucial role of cross-slip both for the initial spreading of strain inside the grain and for the subsequent strain localization in the form of slip bands. A complete and detailed scheme for the persistent slip band formation is proposed, from the observation of the numerical dislocation arrangements.     

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利用脉冲激光诱导击穿光谱仪(LIBS)结合SEM、EDX和维氏硬度(HV)等分析手段,分析了316L不锈钢在350℃的液态锂中静态腐蚀500h后的腐蚀特性.微观形貌分析表明:样品存在较明显的晶间腐蚀,其腐蚀层厚度约为2.7μm.硬度分析表明:腐蚀后表面硬度值约HV234.72,约是腐蚀前硬度值的0.78倍.LIBS组分分析表明:经液态锂腐蚀后的样品表面Ni、Cr等元素含量相对减少;在被腐蚀区域Li元素含量随激光击打深度的增加而减少,而Ni、Cr等元素含量与之近似呈成正比递增;母材区Li元素的渗透深度约3.5μm,约是焊缝区的0.6倍.     

液态金属锂对于316L不锈钢焊接件的腐蚀行为研究

通过失重法结合SEM、金相分析等手段,研究了含焊缝的316L 不锈钢试样在350°C、0.15MPa 液态锂中500h 的静态腐蚀,并采用LIBS 结合台阶仪表征液态锂的渗入深度。结果表明,锂对含焊缝的316L 不锈钢样式的平均腐蚀速率为99.8mg·m^-2·h^-1;腐蚀深度为6.4μm;渗入深度在316L 不锈钢焊缝处为8.6μm,在母材处为4.8μm。在焊缝处锂对材料有明显的晶间腐蚀。     

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通过失重法结合SEM、金相分析等手段,研究了含焊缝的316L不锈钢试样在350°C、0.15MPa液态锂中500h的静态腐蚀,并采用LIBS结合台阶仪表征液态锂的渗入深度。结果表明,锂对含焊缝的316L不锈钢样式的平均腐蚀速率为99.8mg·m?2·h?1;腐蚀深度为6.4μm;渗入深度在316L不锈钢焊缝处为8.6μm,在母材处为4.8μm。在焊缝处锂对材料有明显的晶间腐蚀。     

Laser irradiation of AISI 316L stainless steel coated with Si3N4 and Ti

AISI 316L stainless steel was laser surface treated with different compositions of Si₃N₄ and Ti under various laser-processing parameters to improve its surface hardness through reinforcement of Ti-based silicides. The laser-treated regions exhibited improved surface hardness (250–1000 HV), variations in the surface morphology (smooth and bowl like) and presence of cracks and pores depending upon the Si₃N₄–Ti composition and laser-processing parameters. The study shows that when the Si₃N₄–Ti composition is 75–25 wt% and laser parameters are 1.5 kW laser power and 1.0 m min⁻¹ scan speed, a laser-treated region with high hardness of about 800 HV and smooth surface morphology as well as free from pores and cracks is observed. The X-ray diffractometer (XRD) and Energy-Dispersive Spectroscopy (EDS) analyses show that the laser surface-treated region has reinforced phase of Ti₅Si₃ and retained austenitic structure. The reinforced phase gives rise to very high hardness (or wear resistance) and also a corrosion resistance.     

One way of surface alloying treatment on iron surface based on surface mechanical attrition treatment and heat treatment

A method of surface alloying treatment has been developed: Ni powders were welded into the surface of iron plates by Surface Mechanical Attrition Treatment (SMAT), followed by annealing at certain temperature for 30 min. A Ni-Fe alloy layer with thickness about 100 μm in the sample surface was fabricated on pure iron plate. Scanning electron microscope (SEM), glow discharge spectrum (GDS), and X-ray diffraction (XRD) methods were used to analyze the microstructure, the composition and the phases of the alloy layer. Studies on the interface microstructure indicated that there was significant atomic diffusion and formation of multilayer of intermetallic compound and solid solution in SMAT process. Subsequent annealing accelerates the alloying process. The corrosion test shows the sample by SMAT treated with Ni powders exhibit the best corrosion resistance.