Surface hardening of titanium by pulsed Nd:YAG laser irradiation at 1064- and 532-nm wavelengths in nitrogen atmosphere

The surface hardness of titanium modified by laser irradiation at different wavelengths in nitrogen atmosphere was investigated. Further, surface characteristics such as morphology, chemical state, and chemical composition in the depth direction were also studied. The size and depth of the craters observed in the laser-irradiated spots increased monotonically with an increase in the laser power. Furthermore, the crater formed by the 532-nm laser was deeper than that formed by the 1064-nm laser for the same laser power. Laser power beyond a certain threshold value was required to obtain a titanium nitride layer. When the laser power exceeds the threshold value, a titanium nitride layer of a few tens of nanometers in thickness was formed on the substrate, whereas a titanium oxide layer containing small amounts of nitrogen was formed when the laser power is below the threshold value. Thus, it was shown that laser irradiation using appropriate laser parameters can successfully harden a titanium substrate, and the actual hardness of the titanium nitride layer, measured by nanoindentation, was approximately five times that of an untreated titanium surface.     

Excimer laser surface treatment of aluminum alloy in nitrogen

The excimer laser nitriding process reported is developed to enhance the mechanical and chemical properties of aluminum alloys. An excimer laser beam is focused onto the alloy surface in a cell containing 1-bar nitrogen gas. A vapor plasma expands from the surface and a shock wave dissociates and ionizes nitrogen. It is assumed that nitrogen from plasma in contact with the surface penetrates to some depth. Thus it is necessary to work with a sufficient laser fluence to create the plasma, but this fluence must be limited to prevent laser-induced surface roughness. The nitrogen-concentration profiles are determined from Rutherford backscattering spectroscopy and scanning electron microscopy coupled to energy-dispersive X-ray analysis. Crystalline quality is evidenced by an X-ray diffraction technique. Transmission electron microscopy gives the in-depth microstructure. Fretting coefficient measurements exhibit a lowering for some experimental conditions. The polycrystalline nitride layer obtained is several micrometers thick and composed of a pure AlN (columnar microstructure) top layer (200–500 nm thick) standing on an AlN (grains) in alloy diffusion layer. From the heat conduction equation calculation it is shown that a 308-nm laser wavelength would be better to increase the nitride thickness, as it corresponds to a weaker reflectance R value for aluminum. Received: 17 October 2000 / Accepted: 19 October 2000 / Published online: 23 May 2001     

Laser diffusion nitriding of Ti-6Al-4V for improving hardness and wear resistance

Owing to poor tribological properties, titanium (Ti) alloys are usually surface-treated to enhance their surface properties. Laser surface nitriding, among others, is a common method employed to increase hardness and wear resistance for Ti alloys. Conventional laser nitriding involves surface melting of Ti alloys in a nitrogen atmosphere. This inevitably results in a roughened surface and post-treatment might be required. The present study aims at laser diffusion nitriding Ti alloys without surface melting via carefully selecting the laser processing parameters. The nitrided surface was characterized by X-ray diffractometry (XRD), optical microscopy (OM), scanning-electron microscopy (SEM), and profilometry. The nitride layer formed was about 1.62 μm upon repeated passes. The change in surface roughness resulting from laser diffusion nitriding was only minimal. Nanoindentation measurements revealed that the hardness of the nitride layer was around 11.3 GPa, being about 2.3 times that of Ti-6Al-4V. Ball-on-slab sliding wear test recorded a reduction in wear volume by about 8 times. The results of the present work thus demonstrate the feasibility of diffusion nitriding of Ti-6Al-4V by laser treatment for enhancing its surface properties and performance.     

Surface nitriding of Ti–6Al–4V alloy with a high power CO2 laser

Surface nitriding of a Ti–6Al–4V alloy by laser melting in a flow of nitrogen gas has been investigated, with the aim of increasing surface hardness and hence improving related properties such as wear and erosion resistance. The effect of the scanning speed, nitrogen dilution, and nitrogen flow rate on microstructure, microhardness, and cracking of the nitrided layers was studied. Optical, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction (XRD) were used to reveal the microstructure and to identify the phases formed. It is shown that smooth, deep, and crack-free nitride layers of a surface hardness ranging between 500 and 800 HV can be obtained by controlling the processing parameters. Cracks are present in the sample processed at slow scanning speed and high laser power. Dilution of the nitrogen gas with argon gas leads to a crack-free nitride layer at the expense of a reduction in surface hardness. Slow scanning speeds lead to the formation of a deep and hard surface layer, and increasing the nitrogen flow rate results in a rough surface with a slight increase in hardness.     

X-ray photoelectron spectroscopic study on surface reaction on titanium by laser irradiation in nitrogen atmosphere

The surface reaction on titanium due to pulsed Nd:YAG laser irradiation in a nitrogen atmosphere was investigated using X-ray photoelectron spectroscopy (XPS). The laser, with a wavelength of 532 nm (SHG mode), was irradiated on a titanium substrate in an atmosphere-controlled chamber, and then the substrate was transported to an XPS analysis chamber without exposure to air. This in situ XPS technique makes it possible to clearly observe the intrinsic surface reaction. The characteristics of the surface layer strongly depend on the nitrogen gas pressure. When the pressure is 133 kPa, an oxynitride and a stoichiometric titanium nitride form the topmost and lower surface layers on the titanium substrate, respectively. However, only a nonstoichiometric titanium oxide layer containing a small amount of nitrogen is formed when the pressure is lower than 13.3 kPa. Repetition of laser shots promotes the formation of the oxide layer, but the formation is completed within a few laser shots. After the initial structure is formed, the chemical state of the surface layer is less influenced by the repetition of laser shots.     

CO2 laser gas assisted nitriding of Ti-6Al-4V alloy

Laser gas assisted nitriding of Ti-6Al-4V alloy is carried out and nitride compounds formed and their concentration in the surface vicinity are examined. SEM, XRD and XPS are accommodated to examine the nitride layer characteristics. Microhardness across the nitride layer is measured. Temperature field and nitrogen distribution due to laser irradiation pulse is predicted. It is found that the nitride layer appears like golden color; however, it becomes dark gold color once the laser power irradiation is increased. The δ-TiN and ?-TiN are dominant phases in the surface vicinity. The needle like dendrite structure replace with the feathery like structure in the surface region due to high nitrogen concentration. No porous or microcracks are observed in the nitrided layer, except at high power irradiation, in this case, elongated cracks are observed in the surface region where the nitrogen concentration is considerably high.     

光纤激光气体氮化钛合金发射光谱及等离子体研究

激光气体氮化工艺可在钛合金表面快速生成氮化层,提高钛合金表面硬度和耐磨性,促进钛合金应用.采用光纤激光气体氮化Ti-6Al-4V合金,为了明确氮化过程光谱发射区是否形成等离子体,采用探针法检测了光谱发射区导电性;为了研究工艺参数对光谱特性、光谱发射区温度及等离子数量的影响,采用光谱仪采集了氮化过程发射光谱,并采用高速摄...     

Comparison of surface films formed on titanium by pulsed Nd:YAG laser irradiation at different powers and wavelengths in nitrogen atmosphere

The nitridation of titanium (Ti) caused by a Q-switched Nd:YAG laser under nitrogen gas atmosphere was investigated in situ using X-ray photoelectron spectroscopy (XPS). A laser having a wavelength of 1064 nm and 532 nm (SHG mode) was irradiated on a titanium substrate in an atmosphere-controlled chamber, and the substrate was then transported to an XPS analysis chamber without exposing it to air. The characteristics of the surface layer strongly depend on the laser power. When the power is relatively low, a titanium dioxide layer containing a small amount of nitrogen is formed on the substrate. Laser irradiation beyond a certain laser power is required to obtain a stoichiometric titanium nitride (TiN) layer. A TiN layer and an oxynitride layer with a TiO_xN_y-like structure are formed as the topmost and the lower surface layer, respectively, when the laser power exceeds this threshold value. The threshold laser power strongly depends on the wavelength of the laser, and this threshold value for the 532-nm laser is quite lower than that for the 1064-nm laser.     

PIIID复合强化处理轴承钢表面TiN膜层的XPS表征

用等离子体浸没离子注入与沉积(PIIID)复合强化新技术在AISI52100轴承钢基体表面成功合成了硬而耐磨的氮化钛薄膜。膜层表面的化学组成和相结构分别用X射线衍射(XRD)和X射线光电子能谱(XPS)表征;膜层表面的原子力显微镜(AFM)形貌显示出TiN膜结晶完整,结构致密均匀。XRD测试结果表明,TiN在(200)晶面衍射峰最强,具有择优取向。Ti(2p)的XPS谱峰泰勒拟合分析揭示出,Ti(2p_1/2)峰和Ti_2p3/2峰均有双峰出现,表明氮化物中的Ti至少存在不同的化学状态;N(1s)的XPS谱峰在396.51, 397.22和399.01 eV附近出现了三个分峰,分别对应于TiNO_y,TiN和N—N键中的氮原子。结合O(1s)的XPS结果,证实膜层中除生成有稳定的TiN相外,还有少量钛的氧化物和未参与反应的单质氮。整个膜层是由TiN,TiO₂,Ti—O—N化合物和少量单质氮组成的复合体系。     

Surface nanostructural modifications of Ti implanted by N+ ions as a function of energy

The surface modification of titanium foil/sheet samples (0.5?mm thickness) implanted by nitrogen ions of different energy and fluence of 1?×?10^18?N^+?cm^?2 was studied using X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy and secondary-ion mass spectrometry (SIMS). XRD patterns showed the development of titanium nitride with different compositions in the implanted samples, and the presence of different titanium compositions such as titanium oxides was also observed. AFM images at 16 and 20?keV showed the formation of grains, which were attributed to the initial sputtering of grain boundaries. The morphology of the surface changed at 25?keV showing granular structure with an almost uniform background and lowest surface roughness relative to lower and higher implantation energies. A correlation was obtained between all results for XRD, SIMS and AFM except the titanium nitride maximum intensity at 25?keV N⁺ implantation. In order to achieve more detailed information about the role of N⁺ energy in this kind of work it is proposed that a further investigation is needed on both N⁺ energy and substrate temperature as well as some theoretical studies.     

Excimer laser crystallization of amorphous silicon on metallic substrate

An attempt has been made to achieve the crystallization of silicon thin film on metallic foils by long pulse duration excimer laser processing. Amorphous silicon thin films (100 nm) were deposited by radiofrequency magnetron sputtering on a commercial metallic alloy (N42-FeNi made of 41 % of Ni) coated by a tantalum nitride (TaN) layer. The TaN coating acts as a barrier layer, preventing the diffusion of metallic impurities in the silicon thin film during the laser annealing. An energy density threshold of 0.3 J?cm^?2, necessary for surface melting and crystallization of the amorphous silicon, was predicted by a numerical simulation of laser-induced phase transitions and witnessed by Raman analysis. Beyond this fluence, the melt depth increases with the intensification of energy density. A complete crystallization of the layer is achieved for an energy density of 0.9 J?cm^?2. Scanning electron microscopy unveils the nanostructuring of the silicon after laser irradiation, while cross-sectional transmission electron microscopy reveals the crystallites’ columnar growth.     

Laser/sol–gel synthesis: a novel method for depositing nanostructured TiN coatings in non-vacuum conditions

In this work results of experiments on the in situ production of titanium nitride by the reaction of titania sol–gel with a nitrogenous admixture under laser irradiation are reported. A diode laser beam at different powers and traverse speeds was applied to the mixture placed on EN43 mild steel and 316L stainless steel substrates. Composite coatings of titanium nitride and titanium oxide with a hardness of 17–21 GPa have been achieved by this new method. Surface morphology and microstructure of the deposited coatings and substrate surface layers were examined using optical microscopy, scanning electron microscopy, and field-emission gun scanning electron microscopy. Chemical composition was determined by energy-dispersive X-ray analysis. The phases were identified by X-ray diffraction. Results of microhardness and nanohardness at the top surface were evaluated. PACS 81.15.Fg; 81.20.Fw; 81.05.-t     

Femtosecond laser-induced periodic surface structures on steel and titanium alloy for tribological applications

Laser-induced periodic surface structures (LIPSS, ripples) were generated on stainless steel (100Cr6) and titanium alloy (Ti6Al4V) surfaces upon irradiation with multiple femtosecond laser pulses (pulse duration 30 fs, central wavelength 790 nm). The experimental conditions (laser fluence, spatial spot overlap) were optimized in a sample-scanning geometry for the processing of large surface areas (5 × 5 mm²) covered homogeneously by the nanostructures. The irradiated surface regions were subjected to white light interference microscopy and scanning electron microscopy revealing spatial periods around 600 nm. The tribological performance of the nanostructured surface was characterized by reciprocal sliding against a ball of hardened steel in paraffin oil and in commercial engine oil as lubricants, followed by subsequent inspection of the wear tracks. For specific conditions, on the titanium alloy a significant reduction of the friction coefficient by a factor of more than two was observed on the laser-irradiated (LIPSS-covered) surface when compared to the non-irradiated one, indicating the potential benefit of laser surface structuring for tribological applications.     

Laser controlled melting of pre-prepared inconel 718 alloy surface

Laser treatment of Inconel 718 alloy surface is carried out. The alloy surface is coated with a carbon layer containing 7% TiC particles prior to the laser treatment. The carbon coating provides increased absorption of the incident laser beam and holds TiC particles. The microstrutural and morphological changes in the laser treated region are examined using optical and scanning electron microscopes, energy dispersive spectroscopy, and X-ray diffraction. The microhardness of the surface is measured and the residual stress formed at the surface vicinity is determined from the XRD technique. It is found that partial dissolution of carbide particles takes place at the surface. The composition of fine grains at the surface vicinity, nitride compounds formed, and dissolution of Laves phase at the surface region enhances the hardness at the treated surface. In addition, laser treated surface is free from the micro-crack network and cavities.     

激光喷丸改善TC6钛合金组织和力学性能

为了使激光冲击强化技术能较好地应用于TC6钛合金的发动机叶片,对TC6钛合金进行试验研究。通过X射线衍射仪、透射电子显微镜等测试技术分析了不同参数下TC6钛合金的微观组织变化,用显微硬度计和残余应力测试仪分别表征表层硬度和残余应力变化,并测试材料冲击后的振动高周疲劳性能。试验结果表明:激光冲击材料后表面组织得到明显细化,随着冲击次数的增加,先后出现了高密度位错、位错胞、亚晶和纳米晶。性能方面,表面硬度在冲击一次即可提高19%,硬度影响深度达到700 m;与此同时表面残余应力最高达到-608.5 MPa,在500 m深度上仍具有-100 MPa左右的应力存在。经三次冲击后,标准疲劳试片的疲劳极限提高近20%。     

Electro-spark alloying using graphite electrode on titanium alloy surface for biomedical applications

In order to improve the biomedical properties of a titanium alloy surface, electro-spark surface alloying was carried out using a graphite electrode in air, in a nitrogen gas atmosphere and in silicone oil. The morphology and microstructure of the strengthened layers were analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The hardness distributions as a function of depth were measured by a micro-hardness tester. Corrosion resistance capacities of the modified layers were evaluated using potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS). In addition, wear resistance and corrosive wear properties in a simulated body fluid (SBF) were studied with a pin-on-disk tribometer. Alloyed layers, completely covering the substrate surface and about 40 μm thick mainly composed of the TiC phase and with strong metallurgical bonding and adhesion to the substrate, were obtained. This can markedly improve hardness and wear resistance of the surface layer of the substrate. In comparison to coatings prepared in air and nitrogen gas atmospheres, the coating produced in silicone oil media exhibits a denser and more perfect surface structure. The wear resistance in air and corrosive wear resistance in SBF solution is the best for the coating produced in silicone oil. For instance, the wear rate in air with a GCr15 steel ball counterpart is reduced by a factor of 29 compared with the original titanium alloy and the corrosive wear rate in SBF solution with a corundum ball can decrease by a factor of 13.8. Simultaneously, the effect of electron-spark surface alloying of the titanium alloy surface on biocompatibility and biological activity was also investigated. The electron-spark surface strengthened layer treated in silicone oil shows good biocompatibility and biological activity, and can help cell attachment to the substrate surface.     

Laser and plasma nitriding of titanium using CW-CO2 laser in the atmosphere

Surface nitriding of the titanium by the mixing technology with laser and plasma (LPN) in atmosphere has been investigated. Comparing with the technique of laser nitriding, we could obtain the titanium nitride at relatively low laser power density and the oxidation was prevented without the chamber. The synthesized layers comprised of titanium nitrides were about 178 μm depth. The effect of the laser power density, scanning velocity, and plasma flow rate on the components consisting of the material of the nitrided layer was studied. X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to reveal the components consisting of the material of the nitrided layer.     

Analysis of the interface between a pulsed laser deposited calcium phosphate coating and a titanium alloy substrate

Calcium phosphate coatings deposited on titanium alloy are intended to add a bioactive surface to medical implants. This work presents the characterisation of the interface between Ti-6Al-4V and a crystalline calcium phosphate coating obtained by pulsed laser deposition, with a KrF excimer laser, at 575 °C and under a 45 Pa water-vapour atmosphere. The coating–substrate system was studied by secondary-ion mass spectrometry, scanning electron microscopy, X-ray diffractometry, Raman spectroscopy and X-ray photoelectron spectroscopy. The results show that the deposition process promotes the interdiffusion of substrate elements into the coating and coating elements into the substrate oxide layer. Thus, a graded layer of mixed calcium phosphate and amorphous titanium oxide is formed. For the substrate, a hydroxyapatite coating acts more as a barrier for oxygen incoming from a gas than as an oxygen source during deposition. Moreover, oxygen diffusion into the substrate occurs. Thus, the content of oxygen of this oxide layer diminishes with depth. When the oxygen concentration is low enough it is incorporated in solid solution in the titanium alloy . PACS 81.15.Fg; 68.55.-a; 87.68.+z     

Thermal stability of titanium nitride coatings prepared by the mixing technology with laser and plasma

Titanium samples were treated by the mixing technology with laser and plasma (LPN) using different laser power densities. These nitrided samples were then annealed at 473 K, 673 K, 873 K, and 1073 K for 2 h in vacuum, respectively. The samples before and after annealing were characterized at room temperature and compared in terms of microstructure. X-ray diffraction and cross-sectional optical microscopy studies showed that the layer structure of the titanium nitride coating is preserved after annealing at 1073 K when the coating is formed using a laser power density of 8.0 × 10⁵ W/cm². Therefore, titanium nitride coatings produced by LPN demonstrate excellent thermal stability and are potential candidates for high temperature tribological applications.     

Surface improvement and biocompatibility of TiAl24Nb10 intermetallic alloy using rf plasma nitriding

The present work describes the surface improvement and biocompatibility of TiAl₂₄Nb₁₀ intermetallic alloy using rf plasma nitriding. The nitriding process was carried out at different plasma power from 400 W to 650 W where the other plasma conditions were fixed. Grazing incidence X-ray diffractometry (GIXRD), Auger electron spectroscopy (AES), tribometer and a nanohardness tester were employed to characterize the nitrided layer. Further potentiodynamic polarization method was used to describe the corrosion behavior of the un-nitrided and nitrided alloy. It has been found that the Vickers hardness (HV) and corrosion resistance values of the nitrided layers increase with increasing plasma power while the wear rates of the nitrided layers reduce by two orders of magnitude as compared to those of the un-nitrided layer. This improvement in surface properties of the intermetallic alloy is due to formation of a thin modified layer which is composed of titanium nitride in the alloy surface. Moreover, all modified layers were tested for their sustainability as a biocompatible material. Concerning the application area of biocompatibility, the present treated alloy show good surface properties especially for the nitrided alloy at low plasma power of 400 W.