Laser Surface Engineering Research Articles

Special Issue on Laser Surface Engineering for Tribology

The realm of laser surface engineering has seen continual development, particularly within the realm of tribology, a field dedicated to the examination of friction, wear, and lubrication between interacting surfaces in relative motion [...]

The Resistance to Wear and Thermal Cracking of Laser Surface Engineered P20 Steel

This study reports the microstructure and surface properties of P20 steel processed by laser surface engineering (involving surface hardening and melting), which are carried out using a fiber laser with the maximum power of 2 kW. Ultrafine martensite laths with high boundary density are formed both in the laser surface hardened layer and in the melted layer. This dramatically improves the surface hardness of the P20 steel. However, the laser surface melted layer exhibits a relatively lower hardness than the laser surface hardened layer. It can be attributed to the remarkable autotempering effect and the vaporization of alloy elements in the melted layer. The wear resistance and thermal cracking resistance of the samples treated by laser surface engineering show a significant improvement compared with the as received material. The surface hardened layer exhibits superior wear and thermal cracking resistance due to its relatively high surface hardness and plastic deformation resistance, which can effectively suppress the formation of cracks during wear and thermal cracking tests.

Laser surface engineering of Ti–6Al–4V with TiO2/Al2O3 composite powder for improved wear resistance

Laser surface engineering of Ti–6Al–4V with TiO2/Al2O3 composite powder for improved wear resistance

Rapid W/SiC reaction induced by laser surface remelting within SiCf/Ti17 composites

Compared to alloys, metal-matrix composites experience more complicated microstructure evolution and interfacial reactions under extreme thermal conditions during laser additive manufacturing, welding-repairing and laser surface engineering. In this study, a cubic-shaped SiCf/Ti17 composite is heated by laser surface remelting and the rapid W-SiC reaction within the core-section of SiC fiber located in the molten pool were investigated. As fiber location close to the center of laser spot, severe reaction between W-core and SiC was observed. By applying SEM, XRD, nanoindentation and TEM, the reaction products are confirmed to be W2C and W5Si3. The W2C has ellipsoid shape with a P-31m space group ordered structure, while W5Si3 solidify between W2C resulting from its lower melting point. Because of the extreme temperature that melt W5Si3, the reaction products exhibit different morphology from those produced by solid-state diffusion such as sintering.

Formation of Nano- and Micro-Scale Surface Features Induced by Long-Range Femtosecond Filament Laser Ablation.

In this work, we study the characteristics of femtosecond-filament-laser–matter interactions and laser-induced periodic surface structures (LIPSS) at a beam-propagation distance up to 55 m. The quantification of the periodicity of filament-induced self-organized surface structures was accomplished by SEM and AFM measurements combined with the use of discrete two-dimensional fast Fourier transform (2D-FFT) analysis, at different filament propagation distances. The results show that the size of the nano-scale surface features increased with ongoing laser filament processing and, further, periodic ripples started to form in the ablation-spot center after irradiation with five spatially overlapping pulses. The effective number of irradiating filament pulses per spot area affected the developing surface texture, with the period of the low spatial frequency LIPSS reducing notably at a high pulse number. The high regularity of the filament-induced ripples was verified by the demonstration of the angle-of-incidence-dependent diffraction of sunlight. This work underlines the potential of long-range femtosecond filamentation for energy delivery at remote distances, with suppressed diffraction and long depth focus, which can be used in biomimetic laser surface engineering and remote-sensing applications.

An energy-efficient method of laser remanufacturing process

With the global energy shortages and the increasing concerns of environmental protection in various countries, energy consumption optimization in laser remanufacturing has attracted much attention. To address the problems of large energy consumption and high cost in the laser remanufacturing process, the energy consumption characteristics and mechanism of the laser remanufacturing process are analyzed in this work. Then an energy consumption model of the laser remanufacturing process is established. Moreover, an optimization model of laser remanufacturing process parameters with objectives of energy consumption, powder utilization rate, hardness and aspect ratio is formulated. A novel non-dominated sorting genetic algorithm-II (NSGA-II) based on adaptive crossover probability and multi-crossover operators is proposed to solve the model. In this way, the optimal process parameters are obtained by the improved Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS) method. Finally, some remanufacturing experiments are carried out on the LDM4030 laser remanufacturing equipment. The results show that the model and algorithm presented in this work can effectively reduce energy consumption, improve powder utilization rate and ensure the quality of cladding. The research results of this work can provide an effective way for an energy-efficient laser remanufacturing process and give reference to the energy consumption optimization of other laser surface engineering technologies, such as laser shock strengthening and laser cleaning.

Functionalised polycrystalline boron nitride materials via laser surface engineering

Polycrystalline boron nitride cutting tools are widely used for hard-part steel turning due to their high wear resistance and long durability, however microstructurally different boron nitride composites are suited for different machining applications. For example, low cBN content grades (cBN<50%) are used in continuous and lightly interrupted cutting of automotive steels and in the machining of valve seat alloys; while high cBN content grades (cBN>85%) are designed to provide excellent abrasion and chip resistance and therefore are suitable for interrupted machining of grey and hard cast irons, hardened steel milling and in machining valve seat alloys where a longer tool life is required. In this paper, using a nanosecond fibre laser (1064-nm wavelength), surface engineering of polycrystalline of two microstructurally different boron nitride materials (50% and 90% cBN content) is proposed to functionalise the mechanical properties and enable to extend the applicability of a specific grade to a variety of machining applications. The materials’ response to different fluences, feed speeds and pulse durations was investigated and characterised through a combination of 3D white light interferometry, scanning electron microscopy (SEM) and micro-hardness measurements. 3D metrological data (Sa, Spk, Sku and Sq) of the samples were measured prior and post-process to provide an indication of the change in wear properties. Fractures between hard grains and binder and reduced surface integrity were highlighted by SEM analyses due to thermal stress caused by a difference in thermodynamic properties between grains and binder. Increased frequency and energy density caused local heating corroborated by larger Sa, Spk and Sq values but lower Sku values. Micro-hardness measurements revealed up to 20% increased hardness and 10% decreased root mean square height (Sq) with reduced feed speed and energy density per pulse. Improvements to the hardness and reductions in the surface roughness of both PCBN grades were observed, suggesting that the use of laser surface engineering could lead to improvements in performance in application in the automotive industry beyond the conventional application for the specific grade.

Surface defect detection and prediction in carbide cutting tools treated by lasers

Laser surface engineering of cutting tools is used to improve the performance of cutting processes via altering the material interaction between the tool surface and workpiece. Laser processing applied to cemented carbide cutting tools can induce various thermal and mechanical surface defects including porosity, splatter, cracks, balling, spherical pores, voids, and dissociation. Those defects could be detrimental to the integrity of the tool, therefore parametric optimization is crucial to limit and control possible post-processing defects. This study aimed to identify and classify surface defects in post laser processed carbides to better understand the relationship between parameters and resultant surface integrity. A region convolutional neural network (R-CNN) was trained for identification and classification of these surface defects using scanning electron microscopy images (SEM) as inputs. The R-CNN provided a quantitative analysis of each defect with an average accuracy of 91%. Using the data from the R-CNN matched with the laser parameters, a back propagation neural network (BPNN) was trained to act as a predictive network. The network predicts the number and proportion of defects when the tool grain size, roughness and laser parameters are entered. The accuracy of this predictive network was 96.6%. The effect of individual laser parameters on the surface integrity is estimated by this method, enabling the optimization of laser processing in cutting tools. For the first time this can be used to predict tool performance based on tool’s surface integrity.

Micro-Machining of Diamond, Sapphire and Fused Silica Glass Using a Pulsed Nano-Second Nd:YVO4 Laser

Optically transparent materials are being found in an ever-increasing array of technological applications within industries, such as automotive and communications. These industries are beginning to realize the importance of implementing surface engineering techniques to enhance the surface properties of materials. On account of the importance of surface engineering, this paper details the use of a relatively inexpensive diode-pumped solid state (DPSS) Nd:YVO4 laser to modify the surfaces of fused silica glass, diamond, and sapphire on a micrometre scale. Using threshold fluence analysis, it was identified that, for this particular laser system, the threshold fluence for diamond and sapphire ranged between 10 Jcm−2 and 35 Jcm−2 for a laser wavelength of 355 nm, dependent on the cumulative effects arising from the number of incident pulses. Through optical microscopy and scanning electron microscopy, it was found that the quality of processing resulting from the Nd:YVO4 laser varied with each of the materials. For fused silica glass, considerable cracking and deformation occurred. For sapphire, good quality features were produced, albeit with the formation of debris, indicating the requirement for post-processing to remove the observed debris. The diamond material gave rise to the best quality results, with extremely well defined micrometre features and minimal debris formation, comparative to alternative techniques such as femtosecond laser surface engineering.

Unprecedented hardness of polycrystalline diamond via laser surface engineering

The use of polycrystalline diamond (PCD) as a tool material for machining intractable alloys has increased due to their excellent abrasive and wear properties. In this study, for the first time, a novel low-energy/high-frequency laser shock peening technique (LE/HF-LSP) is proposed to control the mechanical and surface properties of PCD. This technique utilises laser energies below the ablation threshold (10–100 mJ), high frequencies (35–70 kHz) and short pulse durations (in the region of ns) and is applied to the target material with/without sacrificial coatings. The results of the samples processed using this novel technique without coatings revealed a 14% increase of micro-hardness in comparison to the commercially available PCDs. In addition, this technique showed a reduction in cobalt volume by 18.42% compared to the case using coatings increasing in the range of 16.4 to 66.3%. It was also observed that using vinyl & quartz as sacrificial coatings, improved the hardness from 150 GPa to 230 GPa. On the other hand, fluence above 102.29 J/cm2 processed with vinyl and soda-lime glass as ablative and transparent overlays showed softening of the material. A transition between tensile to compressive stress has been observed when processed with vinyl and quartz. Graphitisation and deformation of cobalt have been observed from the results obtained from XRD and Raman spectroscopy. The results showed a correlation between the surface modification, micro-hardness and phase changes for PCDs revealing a relationship between the phase transition and hardness to the laser fluence. It is believed that a suitable range of coatings with a variety of acoustic impedances can be effective in controlling the mechanical properties of PCD.

Effect of micro-segregation of alloying elements on the precipitation behaviour in laser surface engineered Alloy 718

Laser surface engineering has been employed to mimic the microstructural evolution during laser-based additive manufacturing of Alloy 718, specifically when the layers are successively fused and solidified. The micro-segregation of alloying elements in the fusion zone of laser surface melted Alloy 718 samples has been found to have a strong influence on the local precipitation behavior. The dendritic structure produced by rapid melting and solidification has shown three distinct zones – a) the core and b) the periphery of dendrites, and c) the inter-dendritic channel. While precipitation of γ’ [Ni3(Al, Ti, Nb)] dominates in the core, the periphery is decorated by γ” [Ni3(Nb, Mo)] and composite γ’/γ” precipitates, all in the austenite(γ) matrix. The inter-dendritic channel, rich in Nb, Mo, and C, contains discretized aggregates of Laves phase and carbides, which act as preferential nucleation sites for the equilibrium δ phase. The Nb enrichment in these channels also results in a high density of γ” precipitates on subsequent annealing. Microscopic examinations at different length scales using SEM, site specific TEM (both diffraction contrast and phase contrast) and Atom Probe Tomography (APT) have revealed several hitherto unknown features of precipitation processes in a compositionally inhomogeneous Alloy 718, which are invariably encountered in components produced by laser additive manufacturing. Results are rationalized in terms of quantitative elemental analyses, in nm scale, of micro-segregation of alloying elements in local regions by 3-D APT.

Short-Pulse Lasers: A Versatile Tool in Creating Novel Nano-/Micro-Structures and Compositional Analysis for Healthcare and Wellbeing Challenges.

Driven by flexibility, precision, repeatability and eco-friendliness, laser-based technologies have attracted great interest to engineer or to analyze materials in various fields including energy, environment, biology and medicine. A major advantage of laser processing relies on the ability to directly structure matter at different scales and to prepare novel materials with unique physical and chemical properties. It is also a contact-free approach that makes it possible to work in inert or reactive liquid or gaseous environment. This leads today to a unique opportunity for designing, fabricating and even analyzing novel complex bio-systems. To illustrate this potential, in this paper, we gather our recent research on four types of laser-based methods relevant for nano-/micro-scale applications. First, we present and discuss pulsed laser ablation in liquid, exploited today for synthetizing ultraclean “bare” nanoparticles attractive for medicine and tissue engineering applications. Second, we discuss robust methods for rapid surface and bulk machining (subtractive manufacturing) at different scales by laser ablation. Among them, the microsphere-assisted laser surface engineering is detailed for its appropriateness to design structured substrates with hierarchically periodic patterns at nano-/micro-scale without chemical treatments. Third, we address the laser-induced forward transfer, a technology based on direct laser printing, to transfer and assemble a multitude of materials (additive structuring), including biological moiety without alteration of functionality. Finally, the fourth method is about chemical analysis: we present the potential of laser-induced breakdown spectroscopy, providing a unique tool for contact-free and space-resolved elemental analysis of organic materials. Overall, we present and discuss the prospect and complementarity of emerging reliable laser technologies, to address challenges in materials’ preparation relevant for the development of innovative multi-scale and multi-material platforms for bio-applications.

A Thermo-metallurgical Model for Laser Surface Engineering Treatment of Nodular Cast Iron

A Thermo-metallurgical Model for Laser Surface Engineering Treatment of Nodular Cast Iron

Improved Field Emission Properties of Carbon Nanostructures by Laser Surface Engineering.

We herein present an alternative geometry of nanostructured carbon cathode capable of obtaining a low turn-on field, and both stable and high current densities. This cathode geometry consisted of a micro-hollow array on planar carbon nanostructures engineered by femtosecond laser. The micro-hollow geometry provides a larger edge area for achieving a lower turn-on field of 0.70 V/µm, a sustainable current of approximately 2 mA (about 112 mA/cm2) at an applied field of less than 2 V/µm. The electric field in the vicinity of the hollow array (rim edge) is enhanced due to the edge effect, that is key to improving field emission performance. The edge effect of the micro-hollow cathode is confirmed by numerical calculation. This new type of nanostructured carbon cathode geometry can be promisingly applied for high intensity and compact electron sources.

Laser deposition of wear-resistant titanium oxynitride/titanium composite coatings on Ti-6Al-4V alloy

TiOxNy/Ti composite coatings were deposited on Ti-6Al-4V via laser surface engineering. This process was conducted in the air using TiO2 powder as the starting material. The effect of laser power on the defect, microstructure, chemical composition and mechanical properties of the coatings were systematically investigated. The use of a low laser power resulted in the formation of numerous cracks and pores in the coatings, which reduced considerably with the increased laser power. The predominant phase in the coatings was TiOxNy, a product of the nitridation of titanium oxide in air. The concentrations of O and N in TiOxNy varied with laser power. Ti alloy from the partially melted base metal mixed into the ceramic melt, leading to the formation of α-Ti grains between TiOxNy dendrites. The microhardness and wear resistance of the composite coatings were significantly improved by factors up to 4.7 and 3.9 respectively, compared to the base metal.

Wear and corrosion behavior of P20 steel surface modified by gas nitriding with laser surface engineering

Gas nitriding and laser surface engineering are popular technologies to improve the surface performance of steels. This study evaluates the effect of duplex treatment of gas nitriding and laser surface engineering on surface hardness, wear and corrosion resistance of P20 plastic mold steel. It is found that gas nitriding at 550 °C for 20 h with nitrogen potential of 0.18 generates a nitrided layer without nitrides, which drastically improves the wear resistance. Simultaneously, the corrosion resistance is also marginal improved by the gas nitriding. Laser surface hardening further elevates the surface hardness and wear resistance of the nitrided layer due to the formation of martensite. Besides, a small amount of austenite retains in surface layer after laser surface hardening, which may lead to the galvanic attack at the phase interface between martensite and retained austenite and deteriorate the surface corrosion resistance. Laser surface melting results in the denitrification of nitrided layer and the formation of holes at the bottom of melted layer, thus reducing the surface hardness and wear resistance. Meanwhile, the denitrified surface layer also exhibits a degraded corrosion resistance owe to the decreasing nitrogen content and the formation of mixed structure of martensite and retained austenite.

Laser patterned hydroxyapatite surfaces on AZ31B magnesium alloy for consumable implant applications

Currently, materials employed for use as consumable prosthetic devices possess uncontrolled degradation via corrosive action and inferior bio-integration characteristics. A material that is biocompatible and mechanically rigid is necessary for osseointegration. In an attempt to address these issues, a novel laser surface engineering approach was used to produce physically patterned hydroxyapatite coatings on the magnesium AZ31B alloy. Variations of two different laser processing parameters, namely laser power (800–1200 W) and laser track separation (0.15–0.9 mm), were explored in conjunction to determine the optimal processing combination. Biocompatibility was subsequently examined via contact angle measurements and performing in-vitro immersions in simulated body fluid. The surface characteristics, microstructure, and phase evolution of post-processed and post-immersed samples were examined using optical profilometry and scanning electron microscopy. It was confirmed that samples with hydrophilic contact angles correlated to higher degrees of biomineralization and minimal corrosion. These hydrophilic contact angles were realized in samples processed either with low laser power and a higher degree of track overlap or with high laser power and no laser track overlap.

Enhanced wear performance of laser machined tools in dry turning of hardened steels

Polycrystalline boron nitride cutting tools are currently used for precision machining of automotive components made of hardened steel due to their high wear resistance and long durability, however adhesion of workpiece to the cutting tool can reduce cutting tool lifetime. Laser surface engineering of polycrystalline boron nitride single point cutting tools using a nanosecond fibre laser is proposed to improve their wear properties and extend tool life in turning of continuous hardened steel components. The effect of different designs on the wear properties was investigated in turning tests under dry conditions and compared to benchmark cutting tools made of the same material. Condition monitoring of machining forces revealed a 20% reduction of cutting force and 30% reduction of feed force for tools with crosshatch design and grooves perpendicular to chip flow direction, when benchmarked to commercial cutting tools. The number of grooves in contact with the forming chip contributed to the creation of three distinguished interfacial secondary deformation areas. Texturing the chamfer proved to alter the dynamic of chip formation in the secondary deformation zone, while engineering the formation of laser-induced solid lubricant h-BN improved the heat dissipation rate in the secondary and tertiary deformation areas.To the best of authors’ knowledge, this paper reports for the first time the possibility of simultaneously engineer geometry and chemistry of PcBN cutting tools so to enhance their service life by up to 20%.

Femtosecond laser surface engineering of biopolymer ceramic scaffolds coated with ZnO by low temperature atomic layer deposition method

Surface femtosecond laser texturing of biopolymer and biopolymer/ceramic composites with subsequent ZnO film deposition on the samples by low temperature Atomic Layer Deposition (ALD) method was performed. In the current study, the deposition of ZnO layers was implemented at temperature 50 °C under pressure of 2 mbar. In order to investigate the effect of diverse ZnO thin films thickness, 100 or 500 repeating ALD cycles were applied. The samples were exposed to ultra-short laser pulses of 130 fs duration generated by a CPA (chirped pulse amplifier) Ti:Sapphire laser system. The artificial scaffolds were irradiated by varying the laser fluence (0.2 J/cm2, 0.41 J/cm2 and 2.07 J/cm2) and the number of applied laser pulses (N = 1, 2, 5 and 10). The morphology and chemical properties of the treated samples were evaluated by Scanning Electronic Microscopy, Energy-Dispersive X-ray Spectroscopy and X-ray photoelectron spectroscopy. By combining fs laser modification with low temperature ALD method, essentially improved bioactivity properties of hybrid organic–inorganic bone tissue scaffolds could be achieved, which is of great importance for future tissue engineering application of the samples.

Laser surface engineering of B4C/Fe nano composite coating on low carbon steel: Experimental coupled with computational approach

Laser surface engineering was successfully used to engineer a nano carbide composite coating on a low carbon steel substrate. Two powder precursors of B4C and B4C+ 4 wt% Fe were utilized with different laser powers and processing speeds. A smooth crack free coating layer was achieved with B4C+ 4 wt% Fe powder precursor. The phase evolution and microstructure characterization of the coating layer were conducted using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Microscopy observations revealed a fine dendritic microstructure consisting of α-Fe phase dendrites with nanosized B4C precipitates, while the interdendritic regions contained mainly Fe2B and Fe3C phases. The retention of B4C in the coating layer was observed only at high laser power with high scanning speed. This is due to the lower stability of B4C with respect to Fe2B and Fe3C phases as confirmed with density functional theory based thermodynamic calculations. The hardness increased nearly seven fold for the nano-composite coating compared to the steel substrate due to evolution of high hardness ceramic phases, in addition to the refinement of the microstructure. This laser coating has a high potential in advance engineering applications where a thick coating layer with high hardness is required.