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.     

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。     

Online monitoring of laser remelting of plasma sprayed coatings to study the effect of cooling rate on residual stress and mechanical properties

This investigation deals with laser remelting of plasma sprayed alumina and chromia coatings. The time-temperature history of the laser remelted zone was recorded using an infrared pyrometer during the remelting operation. Cooling rates, under varying scanning speed, were determined from the time temperature curve. Surface morphology, microstructure, and phases of the laser treated and as-sprayed coatings were characterized using scanning electron microscopy, optical microscopy, X-ray diffraction, respectively. X-ray diffraction was also employed to measure the surface residual stress of the coatings. Inherent features of plasma sprayed coatings like porosity and inter-lamellar boundary were obliterated upon laser remelting. A columnar grain growth perpendicular to the laser scanning direction was observed. The range of roughness of the as-sprayed coatings reduced from 6 to 8?µm to 1–2?µm in the remelted layers. For both coatings, more than 90% reduction in porosity was found upon laser remelting. Surface residual stress of the as-sprayed alumina and chromia coatings was found to be tensile and compressive, respectively. Within the limits of the testing condition the tensile residual stress of the remelted layers increased by up to around 500% in the alumina coatings. In the chromia coating a decrease of compressive stress by up to around 80% was recorded. In the remelted layer the tensile nature of the stress showed a tendency to increase with an increase in the cooling rate. However, the state of stress of the as-sprayed layer, i.e., tensile or compressive, was retained in the remelted layer. The residual stress was found to decrease in the remelted layer with an increase in the degree of overlap of the remelted tracks.     

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%.     

Thermal shock resistance of thermal barrier coatings modified by selective laser remelting and alloying techniques

Aiming to improve the thermal shock resistance of thermal barrier coatings (TBCs), the plasma-sprayed 7YSZ TBCs were modified by selective laser remelting and selective laser alloying, respectively, in this study. A self-healing agent TiAl₃ was introduced into the 7YSZ TBCs by selective laser alloying to fill cracks during thermal cycling. The thermal shock experiments of the plasma-sprayed, laser-remelted, and laser-alloyed TBCs were conducted by a means of heating and water-quenching method. Results revealed that some segmented microcracks were distributed on the surface of the laser-remelted and the laser-alloyed zones, showing a dense columnar crystal structure. After thermal shock tests, the numbers of segmented microcracks on the laser-remelted coating increased, whereas, in the laser-alloyed condition, some irregular particles formed, leading to the decreased numbers of segmented microcracks. The laser-alloyed coating exhibited the best thermal shock resistance, followed by the laser-remelted condition, with the thermal shock lifetime 3.3 and 2.7 times higher than that of the as-sprayed coating, respectively. On the one hand, both columnar grains and segmented microcracks in the laser-treated zone could effectively improve the strain tolerance of coatings. On the other hand, the oxidation products of TiAl₃ under high-temperature condition could seal the microcracks to postpone the crack connection. Thus, the thermal shock resistance of the laser-treated coatings was significantly improved.     

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.     

Grain growth characteristics of plasma-sprayed nanostructured Al2O3-13wt.%TiO2 coatings during laser remelting

Nanoparticle growth during laser remelting was studied to provide references for optimizing technological parameters. The variations of nanoparticle sizes during laser remelting plasma-sprayed presetting of nanostructured Al₂O₃-13 wt%TiO₂ coating were investigated on the basis of the classical theories of grain growth and by combining the temperature courses of nanoparticles at different positions, which were obtained from laser remelting finite element numerical simulation of the temperature field. Moreover, the effects of initial nanoparticle size and different remelting positions on grain growth were analyzed. A laser remelting test was conducted on the corresponding plasma-sprayed nanostructured coating. Results show that the overall trend of nanoparticle growth in the laser remelting test is consistent with theoretical calculation results, indicating that the classical theory of grain growth can offer some theoretical guidance to understanding nanoparticle growth during laser remelting.     

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%.     

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。     

Microhardness and wear resistance of Al2O3-TiB2-TiC ceramic coatings on carbon steel fabricated by laser cladding

Al₂O₃-TiB₂-TiC ceramic coatings with high microhardness and wear resistance were fabricated on the surfaces of carbon steel substrates by laser cladding using different coating formulations. The microstructures of these ceramic coatings with the different coating formulations were investigated using X-ray diffraction, scanning electron microscopy, and energy dispersive spectrometer. The wear resistance and wear mechanism were analyzed using Vickers microhardness and sliding wear tests. The results showed that when the amount of independent Al₂O₃ was increased to 30%, the ceramic coatings had a favorable surface formation quality and strong metallurgical bond with the steel matrix. The cladding layer was uniformly and densely organized. The black massive Al₂O₃, white granular TiB₂, and TiC distributed on the Fe substrate significantly increased the microhardness and wear resistance. The laser cladding ceramic coating had many hard strengthening phases, and thus resisted the extrusion of rigid particles in frictional contact parts. Therefore, the wear process ended with a “cutting-off” loss mechanism.     

Microstructure,adhesion, and tribological properties of conventional plasma-sprayed coatings on steel substrate

A variety of metallic and oxide coatings were deposited under various conditions on 1020 mild steel substrate by conventional plasma spraying. The coating thickness, microhardness, cohesion and adhesion failure loads, friction coefficient, and abrasive wear resistance were evaluated. The coatings were classified as follows, in order of decreasing microhardness and wear resistance: alumina, chromia, 316 stainless steel, Ni-5% Al, elemental aluminum and aluminum-polyester. Wear resistance increased with increasing microhardness and decreasing friction coefficient. The microhardness and wear resistance of high-velocity oxy-fuel (HVOF) diamond jet (DJ)-sprayed aluminum were found to be superior to those of plasma-sprayed aluminum. Plasma or flame-sprayed metallic coatings adhered well to the substrate. The cohesion, adhesion, microhardness, and wear resistance of alumina coatings exceeded those of equally thick chromia coatings.     

激光重熔纳米晶镍镀层的研究

介绍了喷射电镀的基本原理,采用自行设计制造的喷射电镀设备制备纳米晶镍镀层,并对镀层进行激光重熔处理.重点研究了在直流电源和脉冲电源作用下,电流密度对镀层金属的微观结构的影响,以及采用激光重熔处理对直流纳米晶镍镀层形貌的影响;考察了金属基体、喷射电镀层以及激光重熔后的镀层的显微硬度的变化.研究结果表明:与基体金属相比,喷射电镀层的显微硬度有明显提高;经过激光重熔处理后镀层的显微硬度得到进一步提高.     

等离子喷涂工艺参数对B4C涂层性能的影响

本文研究了喷涂距离对大气等离子喷涂Ｂ４Ｃ涂层的沉积效率、气孔率、显微硬度和抗热震性能的影响,发现喷涂距离会严重影响Ｂ４Ｃ粉末在等离子体射流中的熔融状态,从而对这些性能产生显著影响。实验结果表明,通过优化工艺参数,尤其是选择适当的喷涂距离,可以在大气呀涂条件下制得同高压等离子喷涂相近所孔率和显微硬度的Ｂ４Ｃ涂层。     

Corrosive wear behavior of HVOF-sprayed micro-nano-structured Cr3C2–NiCr cermet coatings under aqueous media

A novel micro-nano-structured Cr₃C₂–NiCr cermet coating was prepared on 316L stainless steel by high-velocity oxygen fuel spraying technology (HVOF). Cermet coatings with different contents of micro-and nano-sized Cr₃C₂ particles as the hard phase and a NiCr alloy matrix as the bonding phase were prepared and characterized in terms of porosity, microhardness, and corrosive wear resistance in a 3.5% NaCl solution and artificial seawater. Compared to nanostructured coatings, micro-nano-structured coatings avoid decarburization and reduce nanoparticle agglomeration during the spray process, and mechanical and electrochemical properties were improved in comparison with those of conventional coatings. The micro-nano-structured Cr₃C₂–NiCr coating rendered low porosity (≤0.34%) and high microhardness (≥1105.0HV_0.3). The coating comprising 50% nano-sized Cr₃C₂ grains exhibited the best corrosive wear resistance owing to its densest microstructure and highest microhardness. Furthermore, compared to static corrosion, the dynamic corrosion of the coatings led to more severe mechanical wear, because corrosion destroyed the coating surface and ions promoted corrosion to invade coatings through the pores during corrosion wear.     

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.     

粘结剂对超音速火焰喷涂碳化钨涂层磨粒磨损性能的影响

采用超音速火焰喷涂(HVOF)工艺在35钢基体上制备了WC-10Ni涂层和WC-12Co涂层,研究了镍、钴这两种粘结剂对WC涂层的显微硬度、摩擦系数和抗磨粒磨损性能的影响,采用扫描电子显微镜观察涂层磨损前后的表面形貌,探讨了WC涂层的磨粒磨损机理。结果表明,以HVOF方法制备的2种WC涂层均有较高的显微硬度,WC-10Ni涂层和WC-12Co涂层与SiC砂纸摩擦副之间的干摩擦系数相差不大。2种涂层在低载荷下均有较好的抗磨粒磨损性能,但在较高载荷下WC-12Co涂层的抗磨性明显优于WC-10Ni涂层。2种涂层的磨粒磨损形式主要为均匀磨耗磨损,磨损机理以微切削和微剥落为主。WC-12Co涂层的磨损表面损伤较轻微,综合性能优于WC-10Ni涂层。     

铁基电弧喷涂铝层的高频感应重熔工艺

利用高频感应重熔工艺对铁基电弧喷涂铝层进行处理。分析了工作电流、感应时间对涂层组织的影响,并做出了感应重熔过程中平均功率和温度的分布情况。结果表明,铁基电弧喷涂铝层的重熔先从界面开始,然后向表层推进,处理后的喷涂层与基体之间形成了铁铝金属间化合物,达到了冶金结合,当感应电流为300A,感应时间为35s时,所获得的重熔涂层质量最好。     

Effects of Mo addition on tribological performance of plasma-sprayed Ti–Si–C coatings

Ti–Si–C–Mo composite coatings were fabricated by plasma spraying using Ti, Si, graphite and Mo powders. The effect of Mo on microstructure and tribological performance of the Ti–Si–C coatings were investigated. The results showed that the Ti–Si–C coating consisted of TiC, Ti₃SiC₂, Ti₅Si₃, and residual graphite. The Ti–Si–C–Mo coatings consisted of TiC, Ti₃SiC₂, Ti₅Si₃, residual graphite, Mo and Mo₅Si₃ phases. With increasing Mo contents, the fractions of Mo and Mo₅Si₃ phases increased, and the fractions of Ti₃SiC₂ and Ti₅Si₃ phases decreased. All the coatings existed a typical lamellar structure. The addition of Mo enhanced the hardness and fracture toughness of Ti–Si–C coating by 16% and 52%, respectively. The coating porosity decreased by 57.6%. The wear resistance of the Ti–Si–C coating was also improved and the mass loss decreased by 83%. The wear mechanism of the Ti–Si–C–Mo coatings was the combination of abrasive wear, adhesive wear, and tribo-oxidation wear.     

Microstructure and properties of IN718/WC-12Co composite coating by laser cladding

To improve microhardness and tribological properties of IN718, WC-12Co particles were added to it by the laser cladding. This study investigated the effect of the content of WC-12Co on the microstructure, phase composition, microhardness, tribological properties and machinability of the composite coating. The results shows that WC-12Co can inhibit the growth of columnar grains and the (200) growth direction of γ-Ni, and refine the microstructure. The average microhardness of coating increases from 245.83HV_0.5 to 462.63HV_0.5 with the increase of WC-12Co content. The coating containing 30% wt. WC-12Co has the smallest wear loss, that is, the best wear resistance. However, the coating containing 20 wt% WC-12Co has the lowest COF (0.518), that is, the best antifriction capability. With the increase of WC-12Co content, the milling force increases and the instability of the cutting process is aggravated. Moreover, with the addition of WC-12Co, the wear mechanism changed from adhesive wear to abrasive wear and oxidation wear.