Effect of cooling rate on residual stress and mechanical properties of laser remelted ceramic coating

Mild steel substrates were coated with commercially available alumina and chromia powders using the powder flame spraying process. The top layers of the flame sprayed coatings were remelted using a 2?kW fiber laser. Thermo-cycles of the laser remelting process were monitored on-line using an infrared pyrometer. Cooling rates were varied using different laser scanning speeds. Surface morphology, microstructure and phases of the laser treated and as-sprayed coatings were investigated using optical microscopy, scanning electron microscopy, X-ray diffraction and X-ray tomography. Surface residual stress of the as-sprayed and laser treated coatings was measured using X-ray diffraction. The inherent defects like porosity and inter-lamellar boundary diminish to a great extent upon laser remelting. Surface residual stress of the remelted coatings tends to increase with increase in cooling rate. Surface crack density of the laser treated coating was reduced appreciably when coatings were preheated prior to laser remelting.     

Research of laser remelting on the thermal-mechanical behaviors and heat treatment of yttria-stabilized zirconia coatings

In this study, the thermal and mechanical behaviors were investigated by simulating laser remelting of atmospheric plasma-sprayed yttria-stabilized zirconia coatings, and the molten depth and regions of stress concentration were compared between simulation and experiment. The heat  treatment process of the remelted coating was also simulated. The crack formation mechanism in the YSZ coating remelted by laser and the heat-treatment effect on residual stress were investigated. Results showed that the simulated results were consistent with the experimental measurements, and the residual thermal stress was the main cause of cracks formation. The coating remelted by a laser power of 1500 W and a scanning rate of 9 mm/s possessed less residual concentrated stress and segmented cracks. Heat treatment released concentrated stress, which was still accurate for the ceramic coating. If the coatings were slowly heated to demonstrate heat treatment after laser remelting, the cracks in the remelted layer decreased correspondingly.     

Understanding the role of in-flight particle temperature and velocity on the residual stress depth profile and other mechanical properties of atmospheric plasma sprayed Al2O3 coating

This report deals with the influence of particle temperature and velocity on the microstructure, mechanical properties and residual stress depth profile of plasma sprayed alumina coating. The coatings were produced by varying the particle temperature while maintaining a relatively fixed particle velocity, and vice versa. Residual stress profiles were acquired by measuring the stress in successive layers using X-ray Sin²ψ technique. A substantial increase in hardness and indentation modulus by 76% and 64%, respectively were observed with an increase in particle temperature. With an increase in velocity, coating porosity was found to decrease initially. However, at a high velocity, porosity again increased to a limited extent. The mechanical properties were found to depend strongly on porosity. An increase in particle temperature resulted in an increase in the tensile residual stress in the coatings. Moreover, partial recovery of the grit blasted compressive substrate occurred during spraying owing to an annealing effect.     

Ultrasonic cavitation erosion of as-sprayed and laser-remelted yttria stabilized zirconia coatings

Cavitation erosion resistance of 8 wt.% yttria stabilized zirconia has been investigated in specimens prepared by atmospheric plasma spraying and laser remelting post treatment. The results indicate that as-sprayed coatings involve defects such as primary cavities and initial micro cracks inside a particle and among the interfaces of particles. When the specimens are subjected to cavitation erosion, the micro cracks initiate and coalesce along with chip removals. Laser remelting produces a dense glazed layer with some cracks though the coatings. With the increasing of erosion time, large pieces are delaminated from coating-substrate interface leading to a significant mass loss. However, the resistance of laser remelted coatings to cavitation erosion is significantly improved when they are impregnated with epoxy by vacuum castable mounting. The relationship between cracks formed inside the laser remelted YSZ coatings and their damage mechanism under cavitation is discussed.     

激光重熔处理等离子喷涂陶瓷涂层的研究现状及展望

本文介绍了激光重熔等离子喷涂陶瓷涂层的研究进展,并对其进行了展望。激光重熔使等离子喷涂涂层致密性提高,涂层与基体的结合方式由机械结合为主改为冶金结合为主,层状组织变化为柱状组织;激光重熔使等离子喷涂涂层的热疲劳抗力、耐蚀性、耐磨性、抗高温氧化性等性能提高。指出了激光重熔等离子喷涂陶瓷涂层目前存在的问题,探讨了激光重熔等离子喷涂陶瓷涂层易产生裂纹,甚至发生涂层剥落等问题的原因,提出了激光重熔技术的研究方向。     

Characterization of Laser Remelted Plasma-Sprayed Mo Coating on AISI 1020 Steel

Plasma-sprayed molybdenum (Mo) coating was deposited on an AISI 1020 steel substrate. Laser remelting was used to eliminate the open pores and microcracks of the plasma-sprayed molybdenum coating. The quantitative investigation of porosity was carried out with the help of Biovis image analysis software. The microhardness was measured using a Vickers indenter. The influence of laser remelting on the wear volume loss of plasma-sprayed Mo was estimated by using a pin-on-disc wear test rig. The worn surface was characterized by scanning electron microscopy. The experimental results demonstrate that the porosity of the coating was decreased and microhardness was improved by laser remelting. The laser remelted plasma-sprayed Mo coating exhibits better wear resistance compared to the untreated plasma-sprayed Mo coating. It is concluded that laser remelting is a potential treatment for the plasma-sprayed coating. In this study, the laser remelted plasma-sprayed Mo coating exhibited of lowest porosity, higher hardness and better wear resistance.     

脉冲激光重熔对YSZ涂层微观结构的影响

本文采用电子束物理气相沉积技术在Ni基单晶基体表面制备双层结构的热障涂层后,采用脉冲Nd∶YAG(钇铝石榴石晶体)激光对其进行表面改性处理,分别研究了脉冲宽度和脉冲频率对重熔钇稳定氧化锆涂层(yttria-stabilised-zirconia,YSZ)微观结构及孔隙率的影响。结果发现:激光重熔的YSZ陶瓷层会在快速冷却过程中形成裂纹等缺陷,激光束的脉冲宽度和频率会显著地影响到涂层微观结构,脉冲宽度的增加会使重熔陶瓷层表面紧密、孔隙减少,脉冲频率的增加会使重熔陶瓷层表面的孔隙率先减小后增大。     

Microstructure and bioactivity in SBF of the hydroxyapatite coatings plasma sprayed with a magnetic field

A 100 mT magnetic field was introduced during the plasma spraying of hydroxyapatite (HA) coating on Ti6Al4V substrate. The influence of the magnetic field on the microstructure, bonding strength, and bioactivity of the coating was investigated. Compared with as-sprayed coating, the coating sprayed under a magnetic field showed fewer porosity and microcracks and a significantly decreased absolute value of the residual stress. The bonding strength of the coating sprayed under a magnetic field was approximately 173.6 % greater than as-sprayed coating. With increasing time in simulated body fluid, the width of microcracks on the surface of both coatings first increased and then decreased. The widths of microcracks on the coating sprayed under a magnetic field were much smaller than as-sprayed coating. During spraying process, the magnetic field would enhance the wetting and flowing ability of the HA molten droplets, prolonging their cooling, which affects the microstructure and bioactivity of the coating.     

Residual Stress and Microstructural Evolution in Tantalum Oxide Coatings on Silicon Nitride

Due to its coefficient of thermal expansion (CTE) and phase stability up to 1360°C, tantalum oxide (Ta₂O₅) was identified and investigated as a candidate environmental barrier coating for silicon nitride-based ceramics. Ta₂O₅ coatings were plasma sprayed onto AS800, a silicon nitride ceramic from Honeywell International, and subjected to static and cyclic heat treatments up to 1200°C in air. Cross-sections from coated and uncoated substrates were polished and etched to reveal the effect of heat treatments on microstructure and grain size. As-sprayed coatings contained vertical cracks that healed after thermal exposure. Significant grain growth that was observed in the coatings led to microcracking due to the anisotropic CTE of Ta₂O₅. High-energy X-ray diffraction was used to determine the effect of heat treatment on residual stress and phases. The uncoated substrates were found to have a surface compressive layer before and after thermal cycling. Coating stresses in the as-sprayed state were found to be tensile, but became compressive after heat treatment. The microcracking and buckling that occurred in the heat-treated coatings led to stress relaxation after long heat treatments, but ultimately would be detrimental to the function of the coating as an environmental barrier by affording open pathways for volatile species to reach the underlying ceramic.     

Microstructure evolution and mechanical properties of thermally sprayed coating modified by laser remelting and injection with tungsten carbide

The forming quality of thermally sprayed coatings is often severely impacted by inherent defects, including porosity, microcracks, and mechanical bonding. The poor adhesive strength hinders the utilization of thermal spray technology when fabricating ceramic-reinforced metal matrix composite coatings (MMCCs). Thus, in this study, a negative defocus laser remelting and injection method (LRI) is introduced to modify a thermally sprayed coating with WC ceramics. The microstructure and mechanical property (microhardness, elastic modulus, and wear resistance) evolution of a LRI-modified WC reinforced composite coating is systematically characterized and compared with that for an as-sprayed coating. The LRI method is proven to improve the inherent defects of the initial coating and avoid severe reactions and dissolution of reinforced particles at high temperatures, and can be used to form a high-quality composite coating with a maximum strengthening effect of the ceramic particles. Compared with the initial coating, the elastic modulus and microhardness of the LRI coating are increased by 57.22% and 111.06%, respectively, whereas the abrasion rate is decreased by 54.33%.     

Laser surface modification of plasma sprayed CYSZ thermal barrier coatings

In this study, Inconel 738 LC superalloy coupons were first sprayed with a NiCoCrAlY bond coat and then with a ceria and yttria stabilized zirconia (CYSZ) top coat by air plasma spraying (APS). After that, the plasma sprayed CYSZ thermal barrier coatings (TBCs) were treated using a Nd:YAG pulsed laser. The effect of laser glazing on the microstructure of the coatings was investigated. The microstructures and surface topographies of both as-sprayed and laser glazed samples were investigated using field emission scanning electron microscope (FESEM) and atomic force microscope (AFM). The phases of the coatings were analyzed with X-ray diffractometry (XRD). The microstructural analysis results revealed that laser surface glazing of ceramic top coat reduced the surface roughness considerably, eliminated the surface porosities and produced a network of continuous cracks perpendicular to the surface. XRD patterns also showed that both as-sprayed and laser glazed top coats consisted of nonequibrium tetragonal (T′) phase.     

Influence of interface morphology of transition layer on the residual stresses of plasma sprayed ZrC-based coatings examined by finite element simulations

The influence of the interface morphology of the SiC transition layer, including the shape, roughness and curvature, on the residual stresses of plasma sprayed ZrC-based coatings was simulated and analyzed in the current work. The results indicated that when the valleys and peaks of the interface were removed from the sine wave interface of the transition layer, the maximum values of radial compressive stress and axial tensile stress in the coating have transferred from the peak to the flat valley zone. An abrupt shear stress was found at the peaks and valleys, and a stress gradient was observed at the interface, making it easy to induce cracks perpendicular to the interface. Tensile stress concentration was located at the edge of the coating sample, where a large stress gradient was generated, resulting in easy cracking at the edge of the coating. The transition layer interface of the sine wave with valley-only morphology reduced the residual stress and improved the bonding strength of the interface, which was considered to be the optimal interface. The radial compressive stress and axial tensile stress in the coating increased with increasing interfacial curvature, with more significant changes observed for the axial tensile stress. This will lead to easy cracking and spalling of the coating, which was confirmed by experimental results. When the curvature of the interface is in the range of [0.2,0.3], the coating and substrate have good interface bonding strength.     

Thermal analysis of plasma sprayed oxide coatings sealed with aluminium phosphate

Thermal analysis by thermogravimetry and differential scanning calorimetry was carried out for plasma-sprayed alumina and chromia coatings to study their stability after plasma spraying and for aluminum phosphate sealant to study phosphate reactions during the sealing heat treatment. Thermogravimetric analysis for alumina coating did not show any change in the coating due to the heat treatment, though the phase structure had changed from metastable γ-Al₂O₃ to stable α-Al₂O₃. In the chromia coating thermogravimetric analysis showed 1.7 wt.% weight increase due to the oxidation of the sprayed coating. During plasma-spray process the chromia coating had gained some under-stoichiometry or some of the chromia had decomposed into metallic chromium or other oxides. Thermal analysis for aluminum phosphate sealant showed weight loss of about 27 wt.%. This corresponds well to the formation of metaphosphates via dehydration of aluminum phosphate solution during the sealing heat treatment. Thermal analysis for the mixture of sealant and alumina coating showed slightly different behaviour than plain sealant. The sealant reacted with the alumina coating forming a crystalline phase, berlinite-type orthophosphate AlPO₄. Thermal analysis for the mixture of sealant and chromia coating showed nearly similar behaviour than plain sealant and no indications of the chemical reactions were detected.     

Microstructure and mechanical properties of flame-sprayed PEEK coating remelted by laser process

Poly-ether-ether-ketone (PEEK) is one of the high-performance thermoplastics. It is being increasingly used for many industrial applications due to its excellent properties. In this paper, a flame spraying technique is used to deposit PEEK coating on 304L stainless substrates. CO₂ and Nd:YAG laser treatments are chosen to remelt the as-sprayed polymer coating to get a dense coating. The microstructures of the as-sprayed and remelted coatings are characterized by SEM and XRD. The results show that both CO₂ and Nd:YAG lasers are suitable for densifying the PEEK coating on stainless substrate. However, the remelted coatings present different crystalline structure due to their laser processing parameters. Hardness measurements, tribological and scratch tests are conducted to characterize the mechanical properties of remelted coating. The coatings’ mechanical properties are correlated with their structures.     

激光功率对Fe基合金重熔热喷焊层组织和耐磨性的影响

利用金相显微镜和X射线衍射仪等分析手段,研究了Fe基合金热喷焊层在其他工艺参数相同的条件下,经不同激光功率重熔处理后组织结构的变化,并对其硬度和耐磨性进行了测定比较. 结果表明,Fe基合金热喷焊层的组织较粗大,经不同激光功率重熔处理后有不同程度的细化,激光功率越小,重熔热喷焊层组织越细小. Fe基合金重熔热喷焊层组织主要由g-(Fe,Ni)固溶体基体相和Cr7C3, Cr23C6及Cr2B等强化相组成,硬度和耐磨性较原始热喷焊层均有较大幅度提高,激光功率越大硬度和耐磨性越好,硬度提高约16%,耐磨性提高约55%.     

Modeling birefringence in SiO2 glass fiber using surface stress relaxation

The retardance of silica glass fibers was evaluated using photoelastic techniques. Here, surface birefringence in glass fibers is shown to be a consequence of surface stress relaxation for as-received fibers drawn from Suprasil II. The surface features of the birefringent fibers were compared to a model of the residual axial stress profile resulting from a diffusion-controlled surface stress relaxation. Additionally, a uniform birefringence in the fiber equivalent to a constant tensile stress was recognized and attributed to structural anisotropy produced during fiber drawing. The contribution of structural anisotropy to the observed birefringence remained constant as the surface features were successively etched away. Surface compressive stress generation was also observed, as retardance corresponding to a surface compressive stress was found to increase with applied tensile stress during short heat treatments. Significant features of the retardance profile in as-received silica glass fibers, with a thin surface compressive stress layer and compensating interior tensile stress, agreed with the residual stress profiles predicted by the surface stress relaxation model after correcting for this observed structural anisotropy.     

Residual Stresses in Machined MgO Crystals

The residual stresses introduced in MgO crystals by grinding on {100} surfaces in 〈100〉 directions were measured using photoelastic techniques. Grinding was conducted with two wheels; a 100-grit diamond wheel removed material by brittle fracture, and a 46-grit alumina wheel caused plastic flow and burnishing. Both wheels introduced a discrete, highly deformed layer adjacent the machined surface. In all cases the machined surfaces were under a residual tensile stress which became compressive within the deformed region. Beneath the deformed layer the residual stress patterns were distinctly different. In crystals ground with the alumina wheel the stresses became tensile again within 0.5 mm of the ground surface, whereas the subsurface stresses in crystals ground with the diamond wheel remained compressive to distances ≥1 mm. These residual stress distributions are discussed in terms of a simple model based on the superposition of mechanically and thermally induced stresses.     

Investigation of thermal residual stresses during laser ablation of tantalum carbide coated graphite substrates using micro-Raman spectroscopy and COMSOL multiphysics

Laser ablation of high-temperature ceramic coatings results in thermal residual stresses due to which the coatings fail by cracking and debonding. Hence, the measurement of such residual stresses during laser ablation process holds utmost importance from the view of performance of coatings in extreme conditions. The present research aims at investigating the effect of laser parameters such as laser pulse energy, scanning speed and line spacing on thermal residual stresses induced in tantalum carbide-coated graphite substrates. Residual stresses were measured using micro-Raman spectroscopy and correlated with Raman peak shifts. Transient thermal analysis was performed using COMSOL Multiphysics to model the single ablated track and residual stresses were reported at low, moderate and high pulse energy regimes. The results showed that the initial laser conditions caused higher tensile residual stresses. Moderate pulse energy regime comprised higher compressive residual stresses due to off centre overlapping of the laser pulses. Higher pulse energy (250 μJ), higher scanning speed (1000 mm/s) and moderate line spacing (20 μm) caused accumulation of tensile residual stresses during the final stage of laser ablation. The deviation of experimental residual stresses from COMSOL numerical model was attributed to unaccounted additional stresses induced during thermal spraying process and deformation potentials in the numerical model.     

激光重熔等离子喷涂NiCr-Cr3C2涂层微观结构和性能研究

利用扫描电镜(SEM)、能谱仪(EDS)、显微硬度计及金相分析软件,对等离子喷涂NiCr-Cr3C2涂层激光重熔前后的显微组织结构、硬度和孔隙率变化进行研究,并探讨不同扫描速度对激光重熔效果的影响.结果表明:利用激光重熔NiCr-Cr3C2陶瓷涂层,能够有效提高涂层的硬度和致密度,减少孔隙率;研究条件下1.5m/min的扫描速度时激光重熔效果最好.     

激光重熔NiCrAlY涂层研究

采用空气等离子喷涂技术(APS)将NiCrAlY粉末作为粘结层材料喷涂在IN718镍基合金上,再用5 kW CO2连续激光器对其进行激光重熔处理,利用扫描电镜(SEM)、能谱分析(EDX)和X射线衍射(XRD)等手段对等离子喷涂层和激光重熔层的微观组织与成分进行了比较分析.结果表明:应用优化激光重熔工艺进行重熔后,涂层...