Effect of pulsed laser parameters on in-situ TiC synthesis in laser surface treatment

Commercial titanium sheets pre-coated with 300-μm thick graphite layer were treated by employing a pulsed Nd:YAG laser in order to enhance surface properties such as wear and erosion resistance. Laser in-situ alloying method produced a composite layer by melting the titanium substrate and dissolution of graphite in the melt pool. Correlations between pulsed laser parameters, microstructure and microhardness of the synthesized composite coatings were investigated. Effects of pulse duration and overlapping factor on the microstructure and hardness of the alloyed layer were deduced from Vickers micro-indentation tests, XRD, SEM and metallographic analyses of cross sections of the generated layer. Results show that the composite cladding layer was constituted with TiC intermetallic phase between the titanium matrix in particle and dendrite forms. The dendritic morphology of composite layer was changed to cellular grain structure by increasing laser pulse duration and irradiated energy. High values of the measured hardness indicate that deposited titanium carbide increases in the conditions with more pulse duration and low process speed. This occurs due to more dissolution of carbon into liquid Ti by heat input increasing and positive influence of the Marangoni flow in the melted zone.     

Plasma transferred arc boriding of a low carbon steel: microstructure and wear properties

Borided coatings on AISI 1018 steel with different boron contents were produced using plasma transferred arc (PTA) melting. The thickness of the coatings ranged from 1 to 1.5 mm and their hardness from 400 to 1600 HV. Hypoeutectic or hypereutectic compositions consisting of primary ferrite or primary Fe₂B borides, respectively, and a eutectic constituent of -Fe+Fe₂B were obtained. The presence of FeB attested in coatings with the highest boron contents seems to be responsible for the intergranular cracks extending from the surface of the coatings to the substrate. Crack free coatings corresponding to the minimum quantity of eutectic and with a minor quantity of FeB were subjected to pin on disk wear testing and compared to the steel of the substrate. It was found that the wear rate of the borided coatings was about four orders of magnitude lower than the wear rate of the steel substrate. A transition from mild to severe wear was observed for the steel substrate material, but it was absent in the case of the borided coatings for the entire range of the applied loads examined. It is shown that the transition in the case of steel occurs when grooving and plastic deformation is replaced by intense cracking of the material above a critical load. In the case of the borided layer the dominant wear mechanism is delamination of the eutectic, however, the platelike borides are able to support the load and remain in the mild wear range for all the loads tested. Both borided and plain steel surfaces have the same friction coefficient after a short transition period, because both develop an oxide layer leading roughly to the same tribosystem with the alumina counterbody.     

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.     

Effect of the pulsed laser deposition conditions on the tribological properties of thin-film nanostructured coatings based on molybdenum diselenide and carbon

The structural state and tribological properties of gradient and composite antifriction coatings produced by pulsed laser codeposition from MoSe₂(Ni) and graphite targets are studied. The coatings are deposited onto steel substrates in vacuum and an inert gas, and an antidrop shield is used to prevent the deposition of micron-size particles from a laser jet onto the coating. The deposition of a laser jet from the graphite target and the application of a negative potential to the substrate ensure additional high-energy atom bombardment of growing coatings. Comparative tribological tests performed at a relative air humidity of ∼50% demonstrate that the “drop-free” deposition of a laser-induced atomic flux in the shield shadow significantly improves the antifriction properties of MoSe _x coatings, decreasing the friction coefficient from 0.07 to 0.04. The best tribological properties, which combine a low friction coefficient and high wear resistance, are detected in drop-free MoSe _x coatings additionally alloyed with carbon (up to ∼55 at %) and subjected to effective bombardment by high-energy atoms during growth. Under these conditions, a dense nanocomposite structure containing the self-lubricating MoSe₂ phase and an amorphous carbon phase with a rather high concentration of diamond bonds forms.     

Ti6Al4V合金表面激光沉积复合涂层的组织和性能

为了增强Ti6Al4V钛合金的耐磨性,采用激光沉积制造方法在其表面上制备了以原位生成的TiC颗粒和直接添加的WC颗粒为增强相的耐磨涂层,观察了各涂层的微观组织,并测量了涂层的显微硬度和涂层在室温大气条件下的摩擦磨损性能。结果表明各涂层和基体呈现冶金结合,原位自生的TiC和部分熔化的WC颗粒均能够均匀弥散分布于基体上,由于增强相颗粒的弥散强化及激光沉积组织的细晶强化作用,基材的硬度和耐磨性均得到了提高。原位自生的TiC涂层比WC涂层硬度梯度分布平缓,但耐磨性稍差。     

Abrasive resistance of arc sprayed carbonitride alloying self-shielded coatings

Wear-resistant coatings were prepared on the surface of the Q235 low-carbon steel plate by HVAS with the carbonitride alloying self-shielded flux-cored wire. Detection and analysis on the microstructure and properties of the coatings were carried out by using scanning electron microscope, microhardness tester and wear tester. The forming, the wear resistance and its mechanism of the coatings were studied. The results show that the coatings have good forming, homogeneous microstructure and compact structure. The coatings have good hardness, the average microhardness value reaches 520 HV_0.1, and the highest value is up to about 560 HV_0.1. As a result, the coatings have good abrasive wear performance and adhesion strength.     

A TiB2 metal matrix composite coating enriched with nitrogen: Microstructure and wear properties

Metal matrix composites containing titanium nitrides or titanium borides raise great interest to researchers due to their high wear resistance and enhanced corrosion properties. In the present investigation composite coatings containing both titanium nitrides/carbonitrides and titanium diborides were produced on plain steel substrates using the plasma transferred arc (PTA) technique with argon-nitrogen mixtures in the plasma and shielding gas. The microstructure of the metal matrix composites (MMC) obtained was thoroughly studied and found to consist of primary titanium diboride particles surrounded by a eutectic matrix containing, apart from ferrite, both titanium diboride and titanium carbonitride particles. The wear behavior of the composite coatings was assessed by pin on disk experiments. The wear rate against both a tool steel counterbody and an alumina counterbody is of the order of 10⁻⁴ mm³/m. The friction coefficient for both the alloyed layer-tool steel system and the alloyed layer-alumina system increases up to sliding speed of 0.30 m/s and then decreases, when the sliding speed increases further. Specifically, the friction coefficients are varied between the values 0.5 and 0.65. The wear mechanism for the tribosystem alloyed layer-tool steel is characterized by plastic deformation and adherence of material coming from the alloyed layer to the surface of the ball, while for the tribosystem alloyed layer-alumina ball, severe plastic deformation and formation of oxide layer are observed.     

Self-lubricative coating grown by micro-plasma oxidation on aluminum alloys in the solution of aluminate-graphite

Aluminum trioxide ceramic coatings with high hardness were grown on surfaces of 2024 Aluminum alloys by micro-plasma oxidation in an aluminate electrolytic solution, which highly improve wear-resisting properties of 2024 Aluminum alloys. However, ceramic coating surfaces are porous and very coarse, which is disadvantageous to practical applications. In this paper, in order to increase the density of the pores and decrease the friction coefficient of the ceramic coatings, different concentrations (2-8 g/l) of graphite were added into the aluminate electrolytic solution. The thickness and hardness of the produced ceramic coatings were measured by HVS-100 micro-hardness tester and thickness tester. The friction coefficient of the coatings was studied by a frictionometer. The phase composition and surface morphology of the MPO films were evaluated through X-ray diffraction (XRD) and scanning electron microscope (SEM). The results show that the thickness of the ceramic coating is about 22 ± 1 μm, surfaces of the ceramic coatings are very uniform and that the coatings consist of mainly aluminum trioxides and a certain amount of graphite, which indicates graphite have entered the ceramic films during the micro-plasma oxidation process. Wear properties results show that the friction coefficient of the ceramic coatings decreased when graphite entered the ceramic films. When the concentration of graphite is 4 g/l, the wear properties of the coatings is the most excellent and the friction coefficient decreases to the lowest, that is 0.09.     

Microstructure performance and formation mechanism of laser alloying rare earth oxides modified nanocrystalline layer on TA7

Nanoscale particles (NP) were observed in a Ni60–Ag–Si₃N₄–Y₂O₃ laser alloying (LA) layer on a TA7 titanium alloy, NP usually locate on the grain boundaries, which are able to block the motion of dislocation in a certain extent. Such layer mainly consisted of γ-Ni, TiN, γ-(Fe, Ni), TiAg and lots of amorphous phases. The wear resistance of such layer with laser scanning speed 3 mm/s was better than that of a LA layer with 6 mm/s, which was mainly ascribed to an uniform microstructure and less defect of layer. The high laser scanning speed made the existing time of laser molten pool be shorter than before, favoring the formation of a fine microstructure. However, the defects, such as pores were produced in LA layer (higher scanning speed), decreasing the wear resistance.     

Formation of titanium carbide layer by laser alloying with a light-transmitting resin

The weight reduction of mechanical components is becoming increasingly important, especially in the transportation industry, as fuel efficiency continues to improve. Titanium and titanium alloys are recognized for their outstanding potential as lightweight materials with high specific strength. Yet they also have poor tribological properties that preclude their use for sliding parts. Improved tribological properties of titanium would expand the application of titanium into different fields.Laser alloying is an effective process for improving surface properties such as wear resistance. The process has numerous advantages over conventional surface modification techniques. Many researchers have reported the usefulness of laser alloying as a technique to improve the wear resistance of titanium. The process has an important flaw, however, as defects such as cracks or voids tend to appear in the laser-alloyed zone.Our group performed a novel laser-alloying process using a light-transmitting resin as a source for the carbon element. We laser alloyed a surface layer of pure titanium pre-coated with polymethyl methacrylate (PMMA) and investigated the microstructure and wear properties. A laser-alloyed zone was formed by a reaction between the molten titanium and thermal decomposition products of PMMA at the interface between the substrate and PMMA. The cracks could be eliminated from the laser-alloyed zone by optimizing the laser alloying conditions. The surface of the laser-alloyed zone was covered with a titanium carbide layer and exhibited a superior sliding property and wear resistance against WC-Co.     

Mössbauer spectroscopy of laser surfaces treated metallic materials

Laser treated surfaces of the Fe₈₃Si₁₇ alloy and of the coatings prepared on low-carbon steel by laser surface alloying with Ni and Al were investigated by means of Mossbauer spectroscopy. The short range order in the surface layer after irradiation by neodymium laser pulses was found to be similar to that before irradiation. The high quenching rate of a single melt pool after single pulse action seems to be masked by annealing due to the heat produced by successive pulses covering the whole surface. A detailed phase analysis of the coatings prepared by laser surface alloying was done. Seven different phases were found in dependence on chemical composition of alloy coatings and on traverse speed, i.e. the speed of relative motion of sample and the continuous CO₂-laser beam.     

磁场辅助激光熔覆制备Ni60CuMoW复合涂层

采用磁场辅助激光熔覆技术,在Q235钢表面制备了Ni60CuMoW复合涂层,借助SEM,EDS 和XRD 等表征手段对涂层进行了微观组织和物相分析,利用维氏硬度计测试了复合涂层截面的显微硬度分布,通过摩擦磨损实验和电化学测试系统研究了复合涂层的磨损性能和耐腐蚀性能。研究结果表明:涂层主要由-Ni,Cu）固溶体、硅化物和硼化物组成,Cr3Si晶粒细化且均匀致密;磁场辅助作用下,激光熔覆涂层平均显微硬度达到913HV0.5,为无磁场辅助涂层的1.5 倍,磨损失重仅为无磁场涂层的36%,自腐蚀电位上升了100 mV,腐蚀电流密度降低了70%,耐磨耐蚀性能得到了显著改善。     

Fretting wear behaviour of microarc oxidation coatings formed on titanium alloy against steel in unlubrication and oil lubrication

Ceramic coatings were formed on Ti6Al4V alloy surface by microarc oxidation (MAO) in a Na₂SiO₃ system solution. Unlubricated, smear oil and oil bath lubricated fretting tests were performed on MAO coatings against 52100 steel on a fretting wear tester. Microstructural investigation of the worn surfaces was performed and the wear mechanisms were studied. The results show that the coatings are mainly composed of rutile and a small amount of anatase TiO₂, both in nano grain structure. Friction coefficient of microarc oxidation coatings under oil bath lubrication was significantly reduced, favorable stable at 0.15, which indicates that the coatings with oil lubricated lowered the shear and adhesive stresses between contact surfaces, consequently alleviating the possibility of initiation and propagation of cracks in the inner layer of the coating or titanium alloy substrate.     

Structure and properties of the bronze laser alloyed with titanium

The paper describes the microstructure and properties (microhardness and wear resistance) of the bronze laser alloyed with titanium. The laser alloying was done using a pulsed Nd:YAG laser with a generated beam energy of 25-35 J. A very fine microstructure was formed under such rapid solidification conditions like laser treatment. The high chemical homogeneity and fine structure of the melted zone were attributed to high cooling rates due to the short interaction time with Nd:YAG pulsed laser radiation and relatively small volume of the melted material. The structure obtained in the surface layer after laser alloying permits to get a high level of hardness and an improved wear resistance.     

Electrical discharge machining of titanium alloy (Ti-6Al-4V)

In this study, the electrical discharge machining (EDM) of titanium alloy (Ti-6Al-4V) with different electrode materials namely, graphite, electrolytic copper and aluminium and process parameters such as, pulse current and pulse duration were performed to explore the influence of EDM parameters on various aspects of the surface integrity of Ti6Al4V. Scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectrograph (EDS) and hardness analysis were performed. The experimental results reveal that the value of material removal rate, surface roughness, electrode wear and average white layer thickness are tendency of increase with increasing current density and pulse duration. However, extremely long-pulse durations such as 200 μs led to decrease MRR and surface roughness. Furthermore, the surface hardness is increasing due to the Ti₂₄C₁₅ carbides formed on the surface and obvious cracks are always evident in re-solidified layer when machining copper electrode. The surface crack densities and critical crack lines were determined for the tested material. The graphite electrode is beneficial on material removal rate, electrode wear and surface crack density but relatively poorer surface finish.     

Principles of the alloying of steel in a beam of relativistic electrons

It is shown that it is possible to significantly strengthen steel by alloying a surface layer melted by the energy from a beam of relativistic electrons. A study was made of the effect of different treatment parameters (accelerating voltage, beam current, specimen velocity and temperature, etc.) on the structure, depth, hardness, and wear resistance of the alloyed layer. Several types of alloying mixtures were developed based on carbides of tungsten, chromium, and boron, and including special additions and modifiers. The proportions of the components was optimized. The technology of alloying in a beam of relativistic electrons is compared with vacuum electron-beam alloying. Heat treatment is used to additionally improve the structure of the layers.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 115–125, March, 1996.     

Wear resistance of reactive plasma sprayed and laser remelted TiB2-TiC0.3N0.7 based composite coatings against medium carbon steel

Wear resistance of reactive plasma sprayed TiB₂-TiC_0.3N_0.7 based composite coatings and the as-sprayed coating with laser surface treatment was investigated using plate-on-plate tests. Wear tests were performed at different normal loads and sliding speeds under dry sliding conditions in air. The surface morphologies of counterparts against as-sprayed and laser remelted coatings were investigated. The microstructure and chemical composition of wear debris and coatings were studied using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), respectively. The results show that the wear resistance of the laser remelted coating is improved significantly due to their increased microhardness and reduced flaws. The primary wear mechanism of the remelted coating is oxidation wear and its minor wear mechanisms are grain abrasion and fatigue failure during the course of wear test. In contrast, the primary wear mechanism of the as-sprayed coating is grain abrasion at the low sliding speed (370 rpm) and fatigue failure at the high sliding speed (549 rpm). The oxidation wear mechanism is a minor contributor for the as-sprayed coating.     

二硼化钛的高温高压制备及其物性

以化学计量配比的Ti,B元素为原料,在高温高压条件下成功制备出颗粒均匀、致密性大于99％的二硼化钛(TiB2)体材料.物性测试结果表明:TiB2的维氏硬度高达39.6 GPa(接近超硬材料的40 GPa);并呈现出金属导电特性,电阻率在10-8 Ω.m的数量级(接近TiB2单晶样品值).TiB2的高硬度与金属特性,可能与该方法制备的TiB2体材料中均匀的细小晶粒尺寸有关.该方法为制备功能陶瓷材料提供了新的思路.     

Microstructure and properties of TiB2–TiB reinforced titanium matrix composite coating by laser cladding

TiB₂ particle and TiB short fiber reinforced titanium matrix composite coatings were prepared utilizing in situ synthesized technique by laser cladding on the surface of Ti6Al4V alloy. Through the experiment, it was found that the surface of the single-track coatings appeared in the depression, but it can be improved by laser track overlapping. With the increase of laser power density, the amount of TiB short fiber was increased, and the distribution of TiB₂ and TiB became more uniform from the top to bottom. The micro-hardness of TiB₂/TiB coating showed a gradient decreasing trend, and the average micro-hardness of the coatings was two-fold higher than that of the substrate. Due to the strengthening effect of TiB₂ particle and TiB short fiber, the wear volume loss of the center of the coating was approximately 30% less than that of the Ti–6Al–4V substrate, and the wear mechanism of the coating was mild fatigue particle detachment.     

Microstructure and wear resistance of composite layers on a ductile iron with multicarbide by laser surface alloying

Multicarbide reinforced metal matrix composite (MMC) layers on a ductile iron (QT600-3) were fabricated by laser surface alloying (LSA) using two types of laser: a 5 kW continuous wave (CW) CO₂ laser and a 400 W pulsed Nd:YAG laser, respectively. The research indicated that LSA of the ductile iron with multicarbide reinforced MMC layers demonstrates sound alloying layers free of cracks and porosities. The microstructure, phase structure and wear properties of MMC layers were investigated by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD), as well as dry sliding wear testing. The microstructure of the alloyed layer is composed of pre-eutectic austenite, ledeburite, spherical TiC, Cr₇C₃ and Cr₂₃C₆ with various morphologies. TiC particles are dispersed uniformly in the upper region of MMC layers. The average hardness of LSA layers by CO₂ laser and pulsed Nd:YAG laser is 859 HV_0.2 and 727 HV_0.2, respectively. The dry sliding wear testing shows the wear resistance of ductile iron is significantly improved after LSA with multicarbide.