等离子喷涂Ni包WC陶瓷涂层激光重熔研究

利用X射线衍射、金相显微镜及显微硬度计研究了等离子喷涂Ni包WC陶瓷涂层激光重熔后的组织结构和硬度变化特征。结果表明:利用激光重熔Ni包WC陶瓷涂层,能够有效提高涂层的致密度,减少孔隙率,XRD显示WC相在激光重熔后分解为W2C相,而显微硬度稍有提高。     

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。     

激光熔覆法制备ZrO2-Y2O3涂层的显微组织与性能研究

采用预制粉末式激光熔覆法在钛合金(Ti-6Al-4V)表面制备了纳米ZrO2-8％ Y2O3涂层,利用X射线衍射仪(XRD)、扫描电镜(SEM)和光学显微镜(OM)对涂层的相组成及组织结构进行分析,同时分析了涂层的显微硬度分布情况.结果表明:在一定功率范围内,涂层气孔随激光功率增大而逐渐减少,裂纹随激光功率增大而增多;ZrO2陶瓷涂层与基体间结合良好,熔覆层微观组织结构主要以细小的树枝晶形态存在.在不同区域表现出不同的组织特征,靠近表面处晶体尺寸相对较小,呈柱状整齐排列在熔覆层表层区域;熔覆层中部主要包含块状晶及少量树枝晶;熔覆层底部主要为树枝晶,其一次品轴方向沿垂直于涂层截面方向生长;熔覆层主要由四方相(t相)ZrO2和立方相(c相)ZrO2组成;熔覆层平均硬度达到1000～ 1300 HV0.2,约为基材硬度的3.5倍.     

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.     

激光重熔等离子喷涂Al2O3陶瓷层的相转变

研究了锌铝基Al2O3陶瓷层激光重熔区内陶瓷相的转变。采用METCO 4MP等离子喷涂机对基体材料表面喷涂氧化铝(Al2O3的质量分数为99％)陶瓷层。采用上海光学精密机械研究所HJ-4工业用横流CO2激光器进行激光重熔。通过X射线衍射分析和透射电镜对Al2O3陶瓷的微观结构、成分和分布的研究，得到以下结论：在等离子喷涂过程中，喷涂层中陶瓷相发生。α-Al2O3→γ-Al2O3 α-Al2O3的转变。激光重熔后，试样表层的原α-Al2O3陶瓷相、γ-Al2O3陶瓷相均转变为δ-Al2O3陶瓷相，为体心四方结构．熔池区陶瓷相主要分布在表层，次表层和过渡区。     

等离子喷涂Cr2O3涂层显微硬度的工艺优化

应用均匀设计实验方法和测量数据的统计分析结果,系统地考察了喷枪电流、等离子气体流量、喷涂距离和送粉率等工艺参数对Cr2O3涂层显微硬度的影响,并运用逐步回归分析建立显微硬度随所考察工艺参数变化的数学模型;然后以所建立的数学模型为基础,对Cr2O3涂层显微硬度进行了工艺优化。结果表明：以优化工艺参数喷涂涂层的显微硬度与数学模型预测的最优值相当一致,均匀设计法可用于等离子喷涂涂层显微硬度的工艺优化。     

一水硬铝石等离子喷涂-激光重熔氧化铝陶瓷涂层的制备

以一水硬铝石为原料,利用等离子喷涂-激光重熔技术制备了氧化铝陶瓷涂层。扫描电子显微镜和X射线衍射分析表明：所制备涂层具有良好的显微结构。涂层显微硬度Hv可达12．14GPa。与一水硬铝石常规热分解直接生成α-Al2O3不同,涂层制备过程中存在过渡相γ-Al2O3,其原因在于等离子喷涂过程中一水硬铝石经历了高温高压及快速凝固过程。探讨了一水硬铝石热分解反应理论。     

等离子喷涂纳米二氧化锆涂层性能的研究

采用大气等离子喷涂技术,在镍基高温合金基体上制备了纳米氧化锆涂层.运用扫描电镜对涂层显微结构进行观察和分析,同时测试了涂层的显微硬度、结合强度扣热震性能.结果表明:纳米氧化锆涂层由完全融化区和部分融化区组成,孔隙率约为7%,显微硬度为655.81 HV,结合强度为54.37 MPa,在1 000℃下,可承受40次热循环,涂层无明显脱落现象.     

等离子喷涂Cr3C2-NiCr涂层的Vickers硬度研究

研究了等离子喷涂Ｃｒ３Ｃ２－ＮｉＣｒ涂层的Ｖｉｃｋｅｒｓ硬度服从正态分布,对数正态分布和Ｗｅｉｂｕｌｌ分布的拟合优度,在统计分析的基础上,考察了压头载荷和测量位置对涂层Ｖｉｃｋｅｒｓ硬度的影响。     

Microstructure and oxidation resistance of a NiCrAlY/Al2O3-sprayed coating on Ti-19Al-10Nb-V alloy

A Ti₃Al (α₂) alloy (Ti-19Al-10Nb-3V, wt%) coated by air plasma spraying (APS) with Ni22Cr10AlY and Ni22Cr10AlY/Al₂O₃ has been tested against cyclic oxidation in air at 850 °C for 10 cycles of 10 h each. Effects of pre-oxidation and cyclic oxidation treatments, both in air, on the microstructure of the single α₂/Ni22Cr10AlY and double coated α₂/Ni22Cr10AlY/Al₂O₃ specimens were investigated by X-ray diffraction (XRD), SEM-EDS and Raman spectroscopy. Several distinct results appear through the cyclic oxidization of the pre-oxidized α₂/Ni22Cr10AlY, namely, elemental diffusion where Ti and Al diffuses from one side and Ni and Cr from the other side, oxygen is found to be present at the support-bond coat interface and a partial reaction of the superficial NiO with α-Al₂O₃ to form the non-protective NiAl₂O₄ takes place. Conversely, pre-oxidation treatment of the double coated α₂/Ni22Cr10AlY/Al₂O₃ specimens causes transient alumina polymorphs obtained by APS to convert to α-Al₂O₃ and hence to ensure the lowest cyclic oxidation rate seen by a k_p value of 2.62×10⁻¹² g² cm⁻⁴ s⁻¹. Good adhesion to support of all the coated specimens is recorded following the cyclic oxidization treatment.     

Phase composition,structural, and plasma erosion properties of ceramic coating prepared by suspension plasma spraying

Y₃Al₅O₁₂ (YAG), Y₂O₃, and Al₂O₃ ceramic coatings were manufactured to investigate the plasma erosion properties. The X‐Ray Diffraction (XRD) analysis confirmed that YAG coating was synthesized successfully by Y₂O₃ and Al₂O₃ mixture suspension using the plasma spraying method. Meanwhile, metastable phases were found in Y₂O₃ and Al₂O₃ coatings due to the quenching in cooling process of melted droplets. The coating surface morphology and microstructure of cross sections were characterized by SEM. The results reveal that coatings are composed by ultrafine splats and exhibit dense lamellar structure. The plasma erosion properties were evaluated at different etching test power under Ar/CF₄/O₂ plasma gas. The experimental results clarify that both of YAG and Y₂O₃ coatings show the better plasma erosion resistance than Al₂O₃ coatings. The formation of fluorination layer surface prevents the coatings from further erosion with plasma gas. Moreover, the etching rate of coatings depended on the fluorination and removing rate of fluoride layer.     

Microstructural evolution and fracture toughness of plasma sprayed CNT reinforced Yttria-stabilized Hafnia coating

Yttria (8 wt%)-stabilized hafnia (YSH) and carbon nanotubes (CNTs) (1 wt%) reinforced yttria-stabilized hafnia (YSHC) coatings were fabricated on alumina substrate using atmospheric plasma spray technique. Raman spectra confirmed the survival of CNTs in plasma sprayed YSHC coating and indicated about graphitization of CNTs. Whereas, the FE-SEM micrograph infers the presence of few 2-D graphene platelet-like structure in plasma sprayed YSHC coating. Addition of 1 wt% CNTs has significantly increased the densification of YSH coating from 86% to 92%, whereas average hardness and elastic modulus increased by ~57% and ~16%, respectively. A phenomenal increase of ~125% in relative fracture toughness was observed in YSHC coating, which is attributed to three major factors viz. (a) Enhanced densification (b) High fraction of fully melted regions and (c) Various toughening mechanisms, like CNTs pull out, CNTs braiding, graphene splat wrapping, CNTs anchoring.     

Role of Lamellae Morphology on the Microstructural Development and Mechanical Properties of Small-Particle Plasma-Sprayed Alumina

The influence of spray parameters on the microstructure and flexural strength of plasma-sprayed alumina was investigated. Coatings were applied using a small-particle plasma spray (SPPS) method, which is a recently patented process that allows submicrometer-sized powders to be sprayed. Using identical starting powders, coatings that were produced using two distinctly different spray conditions exhibited significant differences in both microstructure and strength. Scanning electron microscopy investigations of single lamellae (or splats) revealed that, for one spray condition, melted alumina particles will splash when they contact the substrate. The morphology of the splats that comprised the subsequent layers of the coating also were highly fragmented and thinner than lamellae formed under "nonsplashing" spray conditions. The surface roughness was strongly dependent on the morphology of the lamellae; increased roughness was noted for fragmented splats. Thick coatings that were comprised of splashed splats developed a unique microstructural feature that was responsible for the observed increase in roughness. These microstructural differences greatly influenced the flexure strength, which varied from 75 ± 21 MPa for the nonsplashing spray condition to 17 ± 2.4 MPa for the "splashing" condition.     

Preoxidation of bond coat in IN-738LC/NiCrAlY/LaMgAl11O19 thermal barrier coating system

The low thickness of thermally grown oxide (TGO) layer and presence of amorphous phase in the as-sprayed LaMgAl₁₁O₁₉ (LaMA) coating reduce the thermal cycling lifetime of thermal barrier coatings (TBCs). In the present study, the as-sprayed Ni-22Cr-10Al-1.0Y bond coat was preoxidized at 1060?°C to produce a continuous oxide scale prior to subsequent deposition of the ceramic top coat. The optimum time of peroxidation treatment and thickness of the continuous aluminum oxide layer were estimated 15?h and 2?µm respectively. The oxidized layer due to the preoxidation treatment of bond coating reduces the amorphous phase in as-sprayed LaMA coating and increases the microhardness of LaMA coating from approximately 600 to 900HV. Also, preoxidation of the NiCrAlY bond coating increases adhesion strength of the LaMA top coating, even slightly more than the adhesion strength of the as-spray 8YSZ coating. The LaMA coatings have a lower hardness in compared with the 8YSZ coating (~ 1010Hv), which results a better elastic behavior.     

Modification of thermal barrier coating architecture by in situ laser remelting

Yttria partially stabilized zirconia thermal barrier coatings (TBCs) are widely used to protect components of aero gas turbines against high heat fluxes and hence increase their properties by reducing their in-service temperature. However, these coatings degrade in service conditions.

Therefore, manufacturing TBC which present both low thermal conductivity and high life-time is a real challenge. Engineering the coating architecture by an adapted process is a prerequisite to modify TBC characteristics. In this study, laser remelting was combined to thermal spraying in order to modify the TBC properties.

In situ laser treatment (i) changes structure from lamellar to dendritic columnar; (ii) generates a pore architecture less sensitive to sintering, maintaining the TBC thermal and mechanical properties during thermal treatments at high temperatures; (iii) improves the thermal insulation properties of the TBC by decreasing its thermal conductivity of about 30%; (iv) decreases its permeability permitting to reduce oxidation and corrosion phenomena of the underneath layers and substrate; (v) increases the resistance to isothermal shocks (with the possibility to double the number of cycles); (vi) conducts to a metastable tetragonal phase more stable during thermal shocks; (vii) without modifying the elastic response of the deposit.     

Ferrochromium slag as a protective coating material against oxidation for caster rolls

In this study, ferrochromium (FeCr) slag, which is available as an industrial waste, is proposed as a protective surface coating material particularly for protection of continuous casting rolls against oxidation. FeCr slag was successfully deposited with atmospheric plasma spray (APS) method. Before the coating process, FeCr slag powder prepared in the particle size range of 5‐38 μm, was investigated using conventional characterization methods (XRD, SEM, TGA etc.). Thermal Barrier Coating (TBC) system was used as a basis for deposition processes. Accordingly, NiCoCrAlY (Amdry, ?45 + 5 μm) was firstly deposited as metallic bond coat layer onto the surface of AISI 420 substrate, and then FeCr slag layer was deposited as the top coating layer. After the deposition of FeCr slag powder, the resulting coating layer was found to have low porosity with a homogeneous microstructure. The deposited FeCr slag coatings were subjected to isothermal oxidation tests at different temperatures and test durations for determination of their oxidation behavior and upper operating temperature limits. The results obtained from this study indicate that FeCr slag can be considered as an alternative protective coating material for caster rolls which are subjected to high temperatures up to 800°C.     

Present status and future prospects of plasma sprayed multilayered thermal barrier coating systems

Thermal barrier coatings (TBCs) play a pivotal role in protecting the hot structures of modern turbine engines in aerospace as well as utility applications. To meet the increasing efficiency of gas turbine technology, worldwide research is focused on designing new architecture of TBCs. These TBCs are mainly fabricated by atmospheric plasma spraying (APS) as it is more economical over the electron beam physical vapor deposition (EB-PVD) technology. Notably, bi-layered, multi-layered and functionally graded TBC structures are recognized as favorable designs to obtain adequate coating performance and durability. In this regard, an attempt has been made in this article to highlight the structure, characteristics, limitations and future prospects of bi-layered, multi-layered and functionally graded TBC systems fabricated using plasma spraying and its allied techniques like suspension plasma spray (SPS), solution precursor plasma spray (SPPS) and plasma spray –physical vapor deposition (PS-PVD).