Improvement of flame spraying PEEK coating characteristics using lasers

Flame spraying is frequently used for PEEK coatings elaboration on metallic surfaces. However, this process has a certain number of limitations particularly considering the quality of the coatings like high porosity or low interfacial adherence. For that reason a thermal post-processing is often necessary. From all of the processes which can be used (flame, oven, etc.), laser can be an interesting technology. Then, the aim of this study is to analyse the dependence of the morphological structure (compactness) and the adherence of the flame sprayed PEEK coatings to the stainless steel (304L) and aluminium alloy (AA2017) substrates on laser parameters. Moreover, the influence of the laser beam wavelength (by using a Nd:YAG, CO₂ or diode lasers) on compactness of the coating was analysed. Whatever the laser used, it consists to estimate the most optimized parameters to achieve melting without burning the PEEK material.     

Phase constituents and mechanical properties of laser in-situ synthesized TiCN/TiN composite coating on Ti-6Al-4V

Laser in-situ synthesis technology at room temperature was applied to obtain TiCN/TiN composite coating. A pulsed Nd:YAG laser (wavelength 1064 nm) was used to melt the mixture of Ti and C powder. Pure nitrogen gas with a pressure of 0.4 MPa was introduced coaxially together with laser beam to the melting pool to react with Ti and C atoms and in-situ synthesize TiCN/TiN composite coating. The coating consists of TiC_0.3N_0.7, TiN and TiN_0.3, but the proportions of these three constituents vary with the laser power density. SEM results revealed that dendrites were oriented in accordance with the heat flow and a metallurgical bonding between the coating and the substrate was achieved. The in-situ synthesized TiCN/TiN composite coating, with a thickness of about 200 μm, increased the hardness and wear resistance compared to the bare Ti-6Al-4V substrate. A remarkable improvement of the average microhardness (3-4 times) and an enhancement of the wear resistance (10-11 times) are observed by laser in-situ synthesizing TiCN/TiN composite coating.     

Laser densification of organic coating: Effects of laser wavelength,operating parameters and substrate properties

Mechanical bonding and interface behaviour play a key role for any materials deposited on different substrates. Usually, a post-spray heat treatment is required to improve the coating morphology and to enhance mechanical properties of thermal-sprayed polymeric coating. The effects of YAG, CO₂ and diode laser radiations on as-sprayed PEEK coating deposited on stainless steel and aluminum substrates were investigated. The results revealed a good coating densification and interface behavior. A correlation between coating and substrate absorption coefficients, their thermophysical properties and laser operating parameters was shown. Besides, the finite element modeling based on IR temperature measurements during diode laser irradiation demonstrated that the densification of organic coating occurs above its melting point.     

Visible laser microfabrication of transparent plastic using Au nanoparticles-dispersed polymer film

A polymer film in which Au nanoparticles with average diameter of around 3 nm dispersed in ethylcellulose was applied to an absorber for laser microfabrication of a transparent plastic. Since the polymer film has a strong absorption at the wavelength of around 530 nm, it can be micromachined using focused low power Nd:YVO₄–SHG laser (CW, wavelength of 532 nm). When laser beam was irradiated on the polymer film coated on transparent substrate, the substrate under the polymer film which has no absorbance in the range of wavelength of laser beam was processed. A micropattern was clearly fabricated on transparent poly(methyl methacrylate) and polyethylene terephthalate using our polymer film. In poly(methyl methacrylate) substrate, the processed depth showed a maximum at the polymer film thickness of 15 μm under the condition of laser power of 23 mW. Finally, laser marking test on the transparent poly(methyl methacrylate) was demonstrated as an application of our system using the optimal polymer film thickness.     

Deposition Mechanism and Microstructure of Laser-Assisted Cold-Sprayed (LACS) Al-12 wt.%Si Coatings: Effects of Laser Power

Surface treatment is one of the most costly processes for treating metallic components against corrosion. Laser-assisted cold spray (LACS) has an opportunity to decrease those costs particularly in transportation systems, chemical industries, and renewable energy systems. This article highlights some of those potential applications. In the LACS process, a laser beam irradiates the substrate and the particles, thereby softening both of them. Consequently, the particles deform upon impact at the substrate and build up a coating. To circumvent the processing problems associated with cold-spray (CS) deposition of low-temperature, corrosion-resistant Al-12 wt.%Si coatings, a preliminary investigation detailing the effect of laser power on its LACS deposition mechanism and microstructural properties is presented. The deposition efficiency, the microstructure, and the microhardness of the LACS-deposited coatings produced by a 4.4-kW Nd:YAG laser system were evaluated. The outcome of this study shows that pore- and crack-free Al-12 wt.%Si coatings were deposited via softening by laser irradiation and adiabatic shearing phenomena at an optimum laser power of 2.5 kW.     

聚醚醚酮骨植入体生物改性研究进展

聚醚醚酮是一种热塑性材料,凭其良好的生物相容性和X射线可透射性,被广泛用于生物医学领域。而PEEK骨植入体与人体骨骼强度存在一定差距,与人体骨组织结合能力较差和抗菌性能等不足,使其在生物领域的应用受到了限制。为了使PEEK更好地应用于人体骨植入领域,获得优异生物性能的PEEK骨植入体已成为研究的重点。概述了PEEK的加工技术、力学性能、骨整合性能和抗菌性能的研究。在此基础上,重点综述了近年来提高PEEK骨植入体力学性能与生物性能的各种改性方法的研究进展。在力学性能方面,对PEEK常用的填充材料碳纤维进行了概述,由于PEEK与碳纤维界面结合强度影响其整体力学性能,重点介绍了提高其结合强度的改性方法。在骨整合性能方面,对钛、二氧化钛和羟基磷灰石涂层材料及喷涂方法进行了概述,以及对等离子喷涂、喷砂、激光蚀刻和浓硫酸刻蚀表面处理方法的优缺点进行了分析。在抗菌性能方面,银离子释放浓度过高时会导致细胞毒性,重点阐述了如何控制银离子释放速度的研究。最后展望了PEEK骨植入体加工和改性的未来发展方向。     

Effect of h-BN addition on the properties of nanostructured Al2O3-TiB2-TiN based coatings developed by combined SHS and laser surface alloying

A hard coating was obtained on AISI1025 steel substrate by the action of a high power laser beam on a powder mixture of Al, TiO₂ and h-BN pre-placed on the substrate surface. The precursor powder mixture underwent self-propagating high-temperature synthesis (SHS) at the high temperatures induced by the incident laser. The products of SHS were subsequently laser alloyed onto the substrate, whereby, a hard, nanostructured coating was formed comprising of Al₂O₃, TiB₂ and TiN. Excess h-BN in the precursor resulted in the presence of free h-BN in the coating. Microhardness and coefficient of friction (with WC-Co as counterbody) of the coating were found to reduce with increase in h-BN content in the precursor. It was possible to develop a coating with a property combination of high hardness, low wear rate and low friction coefficient.     

Influence of the Surface State on the Adherence of the Coating: Case of an Alumina Coating Plasma Sprayed on SiC Composites

In thermal spraying, adherence between the coating and the substrate appears as the fundamental point. To favor a good interaction between both, it is often necessary to clean and prepare the substrate surface. Conventionally, solvents and sand blasting are applied to remove the contaminants and increase the surface roughness for a mechanical anchorage. However, according to the substrate nature (ceramic) or the substrate morphology, it can be prejudicial to apply a mechanical treatment because of peeling of the surface or a decrease in the global properties. Then, to obtain an appropriate preparation, several techniques can be investigated, such as water jet, ice blasting, and heat treatment; as well, laser ablation can be an interesting technology to prepare the substrate surface. The aim of this work was to study the modifications induced by 10 ns single or cumulative pulses of a Q-switched Nd:YAG near-infrared laser and its influence on the interface adhesion. The case of an alumina coating sprayed on a ceramic matrix composite (CMC) was studied. In these conditions, the laser treatment seems favorable from the adherence viewpoint according to the mechanical effect (induced by a conelike structure) and the chemical effect.     

Study of the Splat-Substrate Interface for a PEEK Coating Plasma-Sprayed onto Aluminum Substrates

The quality of coatings made using thermal spray processes depends greatly on the degree of adhesion between the substrate and its coating. Yet the bonding mechanisms between a substrate and coating are not well understood. In this study, polyetheretherketone (PEEK) powder was plasma-sprayed to form single splats on aluminum substrates, which had undergone various surface treatments, including boiled (BT), etched (E, ET), and polished (PT), all of which had also been thermally treated to remove water from the substrate surface, with the exception of one etched aluminum substrate. Scanning electron microscopy was used to give an overview of the surface and splat morphology. The splat-substrate interfaces were studied in detail using focused ion beam imaging and transmission electron microscopy, to characterize microstructural features within the splat-substrate interface, including inter/intrasplat pores, pores along the splat-substrate interface, level of contact between the splat and the substrate, etc. The results showed that the splat-substrate interface for the BT and the E substrate surface had poor level of contact, with a high number of small pores (<1 μm) along the splat-substrate interface for the BT splat-substrate interface, and the formation of a near-continuous crevice between the PEEK splat and the aluminum substrate for the E substrate surface. The presence of the fine needle-like network of oxide layer on the BT substrate surface may have restricted the flow of the molten PEEK on the aluminum substrate, and the possible presence of physisorbed and chemisorbed water on the E substrate surface may have reduced the level of contact between the PEEK and the aluminum substrate. In contrast, specimens which had undergone thermal treatment to minimize the presence of water on the substrate surface, such as the ET and PT substrate surface, exhibited high level of contact at the splat-substrate interface. The number of pores for the ET and the PT splat-substrate interfaces were substantially lower than of the BT and E splat-substrate interface.     

Ablation laser and heating laser combined to cold spraying

Cold spraying is particularly suitable for the elaboration of coatings sensitive to heat and oxidation. As spraying particles are not subjected to melting, the adhesion and formation of the coating is due to the kinetic energy transmitted to the particles by accelerating gas. Bonding mechanisms are not only strongly dependent on the particle velocity but also on the state of the substrate surface. The presence of surface pollutants inhibits the coating elaboration. Surface modifications are necessary to reach a high adhesion between the coating and the substrate. A laser ablation and a laser heat treatment are proposed to prepare the substrate prior to cold spraying. Ablation laser is used to eliminate adsorbed pollutant molecules and heating laser is employed to improve the contact between substrate and particles and coating substrate adherence. The bonding of aluminium coating on aluminium 2017 alloy was evaluated by tensile adhesion tests and demonstrated the strong influence of the laser treatments in comparison with conventional processes (grit blasting and degreasing).     

Laser-assisted laser ablation method for high-quality hydroxyapatite coating onto titanium substrate

Hydroxyapatite (HAp) coating on titanium (Ti) or Ti alloy implant materials is one of the important technologies for improving the bioactivity of their surface. We recently developed a new HAp coating method using two laser beams, laser-assisted laser ablation method (LALA method). In this method, two excimer lasers were used. One laser beam from KrF laser, the ablation laser, is used for ablation of a HAp target. The other beam from ArF laser, the assist laser, is used to irradiate a Ti substrate surface during formation of the HAp coating. The assist laser plays an important role in the formation of a crystalline HAp coating and improves the strength of adhesion to the Ti substrate.The coating quality varied with the timing of the assist laser irradiation. A coating deposited with a long assist laser delay contained a large amorphous component. High-quality coatings were obtained with delay time between 2 and 10 μs.Using the present method, we succeeded in fabricating thin (≤1 μm) HAp coatings with high crystallinity and high adhesion strength.     

激光微纳烧结处理对玻璃陶瓷涂层组织及腐蚀性能的影响

采用高温烧结、球磨破碎的方法制备出了SiO2玻璃与Cr2O3陶瓷包覆型玻璃陶瓷复合粉末.利用常规氧—乙炔火焰喷涂技术在45钢基材表面制备出了玻璃陶瓷保护涂层,并使用激光微纳烧结技术对热喷涂层进行二次处理.研究了激光微纳烧结对玻璃陶瓷涂层组织与性能的影响.结果表明,采用激光微纳烧结对玻璃陶瓷涂层进行二次处理,可提高涂层结构的致密性,使组织均匀化,减少涂层中的微孔和微裂纹;明显提高界面的结合强度和涂层的疏水性能与耐蚀性能.因此激光微纳烧结二次处理技术可以显著提高玻璃陶瓷涂层的综合性能.     

Laser surface coating of Fe-Cr-Mo-Y-B-C bulk metallic glass composition on AISI 4140 steel

Fe-based bulk metallic glasses exhibit very high hardness, elastic modulus/limit and wear/corrosion resistance. In the present investigation, an attempt has been made to develop an amorphous coating with Fe₄₈Cr₁₅Mo₁₄Y₂C₁₅B₆ bulk metallic glass on AISI 4140 substrate by laser surface processing. Following coating, the microstructure and phase aggregate were analyzed by scanning electron microscope and X-ray diffraction, respectively. Microhardness and wear resistance were assessed using Vickers microhardness tester and ball-on-plate wear testing machine, respectively. The coating thickness varied directly with incident laser power and interaction time. Despite trials with wide range of process parameters, the present experiments were unable to retain complete amorphous surface microstructure after laser surface coating. Numerical prediction of the thermal profile and related parameters suggest that the cooling rate and thermal gradient experienced by the coated zone were fairly high. Yet failure to retain amorphous/glassy microstructure of an otherwise bulk metallic glassy alloy suggests that compositional changes (solute redistribution) within the coated zone and across the coating-substrate interface are responsible for nucleation and growth of crystalline phases from the melt. However, correlation between coating parameters and surface microstructure and properties allowed determination of the optimum conditions that ensured fine grained uniform microstructure with a significant improvement in hardness and wear resistance.     

Combination of coating and heat treatment processes

For a proper use of coated tools and components excellent coating properties as well as excellent substrate properties of the coating/substrate composite are necessary. A well known example is the load support of hard steel substrates for thin wear resistant coatings. Therefore coating processes must be combined with heat treatment processes of the substrates, whereby several practical processing constraints like order and compatibility of the applied treatment procedures must be considered. The combination of CVD-coating plus laser beam hardening is an example of a post coating heat treatment. Applications may be possible for large forming tools made of high alloyed tool steels, which nowadays are usually through hardened after CVD-coating. Laser beam hardening offers the possibility, to harden only the highly loaded edges of the tools. The advantages of this combination are short process time, less distortion and compressive residual stresses in the substrate surface. CVD-coatings can endure laser beam hardening with suitable parameters. The influence of CVD TiN-coated steel substrates and laser beam hardening parameters on microstructures and properties of coating/substrate compounds have been investigated. Special attention was paid to overlapping hardening zones, which may occur at the start and end points of a laser hardened track. In summary, the combination of coating and heat treatment processes show a high potential to produce coating/substrate compounds with excellent coating properties as well as excellent substrate properties.     

Refractory ceramic-composite coatings via laser surface engineering

Refractory ceramic coatings are desirable for their physical and chemical natures, which make the coatings suitable for many applications involving wear and corrosion environments. The synthesis of a refractory ceramic coating on a metallic substrate can be achieved using laser surface engineering. In this article, laser surface engineering is demonstrated for the synthesis of a TiB₂ coating on plain carbon steel. The predeposited layer of a TiB₂-based precursor in an organic vehicle treated with a laser produced a variety of microstructures within the modified surface that are unattainable with conventional coating methods. For more information, contact N.B. Dahotre, University of Tennessee Space Institute, Department of Materials Science and Engineering, Center for Laser Applications, Tullahoma, Tennessee 37388; (931) 393-7495; fax (931) 454-2271; e-mail ndahotre@utsi.edu.     

On the influence of different superficial laser texturing on the deposition of powders through cold spray process

Cold spray is a technique allowing the deposition of metallic powders on several different substrates. The deposition process is ruled by different factors, among these, of particular interest are the superficial roughness and texturing of the substrate. The aim of this research was to study the influence of different superficial texturing on the deposition process of aluminium powders on an AA 2024 T3 plate. Samples with different superficial texturing were produced by laser engraving through a Yb:YAG fibre laser, varying the laser treatment parameters. Aiming to highlight the influence of the laser treatment, the deposition process was carried out on all the samples under the same spraying condition. The cross-section of the interface between the substrate and the coating and the microstructure of the coating itself were observed to study the influence of the superficial texturing on the coating formation. It was found that the superficial texturing influences both the adhesion mechanism and the microstructure of the coating.     

Spraying TiN by a Combined laser and low-pressure plasma spray system

The formation of a TiN-Ti composite coating by thermal spraying of titanium powder with laser processing of the subsequent coating in a low-pressure N₂ atmosphere was examined. A low-pressure plasma spray system was used in combination with a CO₂ laser. First, the coating was plasma sprayed onto a mild steel substrate using a N₂ plasma jet and titanium powder in a controlled low-pressure N2 atmosphere. The coating was then irradiated with a CO₂ laser beam in a N₂ atmosphere, and the coating was heated with a N₂ plasma jet. The amount of TiN formed in the coating was characterized by X-ray diffraction analysis. The influence of plasma spraying conditions such as plasma power, flow of plasma operating gases, chamber pressure, and laser irradiating conditions on the formation of TiN was investigated. The effect of TiN formation in the titanium coating on Vickers hardness of the coatings was examined. It was evident that coating hardness increased with an increase in TiN content in the coating and that a TiN-Ti composite coating with a hardness of more than 1200 H V can be obtained with the use of laser irradiation processing.     

Corrosion Performance of Laser Posttreated Cold Sprayed Titanium Coatings

The recent development of cold spray technology has made possible the deposition of highly reactive, oxygen sensitive materials, such as titanium, without significant chemical reaction of the powder, modification of particle microstructure and with minimal heating of the substrate. However, the presence of interconnected pathways (microscale porosity) within the deposit limits the performance of the metallic coating as an effective barrier to corrosion and substrate attack by corrosive media is usually inevitable. The aim of the present study was to investigate the effects of processing, including a postspray laser treatment, on the deposit microstructure and corrosion behavior. Commercially pure titanium (CP Ti) was deposited onto a carbon steel substrate, using a commercial cold spray system (CGT^TM Kinetiks^? 4000) with preheated nitrogen as both the main process gas and the powder carrier gas. Selected coatings were given a surface melting treatment using a commercial 2 kW CO₂ laser (505 Trumpf DMD). The effect of postdeposition laser treatment on corrosion behavior was analyzed in terms of pore structure evolution and microstructural changes. Optical microscopy, scanning electron microscopy, and x-ray diffraction were employed to examine the microstructural characteristics of the coatings. Their corrosion performance was investigated using electrochemical methods in 3.5 wt.% NaCl (ASTM G5-94 (2004)). As-sprayed titanium coatings could not provide favorable protection to the carbon steel substrate in the aerated NaCl solution, whereas the coatings with laser-treated surfaces provided barrier-like properties.     

Differential positron annihilation spectroscopy in nondestructive testing of thin chromium coatings on copper

The differential positron annihilation spectroscopy (DPAS) is used for the nondestructive investigation of thin metallic coatings on metallic substrates when the defectiveness of the coating applied is judged from the difference between the spectra of the individual substrate and the substrate covered with the coating. Using the DPAS angular correlation curves of positron annihilation radiation (ACAR), the presence of nanoscale void clusters in the X-ray amorphous galvanic chromium coatings with a weight thickness of 9.15 mg/cm² (1.3 μm) is determined on the copper substrate. The ACAR curves of the coatings have a complex structure compared to those of the specimens of Cr₃C₂ and Cr₂₃C₆ carbide powders and involve a narrow peak with a half width FWHF = 3 mrad and an intensity about 2% assigned to void clusters with a size of 0.86 nm, which act as positron traps, in the coating.     

Laser surface alloying fabricated porous coating on NiTi shape memory alloy

Laser surface alloying technique was applied to fabricate a metallic porous coating on a solid NiTi shape memory alloy. By laser surface alloying a 40%TiH2-60%NiTi powder mixture on the surface of NiTi alloy using optimized laser process parameters, a porous but crack-free NiTi layer can be fabricated on the NiTi substrate. The porous coating is metallurgically bonded to the substrate NiTi alloy. The pores are uniformly distributed and are interconnected with each other in the coating. An average pore size of less than 10μm is achieved. The Ni content of the porous layer is much less than that of the original NiTi surface. The existence of the porous coating on the NiTi alloy causes a 37% reduction of the tensile strength and 55% reduction of the strain as compared with the NiTi alloy. Possible biomedical or other applications for this porous surface with good mechanical strength provided by the substrate are prospective.