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.     

Solution precursor thermal spraying of gadolinium zirconate for thermal barrier coating

Gadolinium zirconate (GZ) is an attractive material for thermal barrier coatings (TBCs). However, a single layer GZ coating has poor thermal cycling life compared to Yttria Stabilized Zirconia (YSZ). In this study, Solution Precursor High Velocity Oxy-Fuel (SP-HVOF) thermal spray was used to produce a double layer GZ/YSZ TBC and compared the thermal cycling performance with the single layer YSZ TBC. The temperature behaviour of the solution precursor GZ was studied, and single splat tests were carried out to obtain an optimised spray parameter. In thermal cycling tests, the single-layer YSZ reached 20 % failure at 85 ± 5 cycles, whereas the double-layer GZ/YSZ was at 70 ± 15 cycles. The single-layer failed at the topcoat/TGO interface, whereas the double-layer failed at GZ/YSZ interface and topcoat/TGO interface. Moreover, Gd diffusion occurred near the GZ/YSZ interface, resulting in porosities in the GZ layer.     

Enhancement of oxidation and wear resistance of Fe-based amorphous coatings by surface modification of feedstock powders

Surface oxidation of the in-flight powders during the preparation of amorphous coatings in high velocity oxygen fuel process causes the formation of oxygen-rich intersplat regions. These regions are brittle in nature and can dramatically deteriorate the mechanical performance of the coatings. To solve this problem, the starting FeCrMoCBY amorphous feedstock powders were modified by electroless plating a thin layer of Ni–W–P amorphous phase. It was found that the covering of the Ni–W–P layer can significantly reduce the oxygen content in the resultant Fe-based amorphous coatings. The wear resistance of the coatings with and without the modification of Ni–W–P thin layer was comparatively studied by ball-on-disk wear tests against Si₃N₄ counterpart in air. It revealed that the wear of two types of coatings follows the same oxidation wear mechanism but the modified coating exhibits much better wear resistance due to the improved oxidation resistance.     

An isothermal oxidation behaviour of atmospheric plasma and high-velocity oxy-fuel sprayed nickel based coating

This paper describes the isothermal oxidation behaviour of NiCrBSiFe coatings on SS 316 L deposited by the atmospheric plasma (APS) and high-velocity oxy-fuel (HVOF) processes. As-sprayed coatings were oxidised isothermally in the air at 900 °C temperature for 1000 h. The thermogravimetric analysis was performed to measure the oxidation rate. The coating and oxide scale characterisation was carried out using SEM, EDAX, XRD, porosity analysis, and nanoindentation. The HVOF sprayed NiCrBSiFe coating shows better oxidation resistance than the APS coating, due to high density, less porosity, and formation of more protective oxide scale.     

Wet abrasive wear behavior of WC-based cermet coatings prepared by HVOF spraying

In this study, three kinds of WC-based cermet coatings including WC–CoCr coating, WC–Ni coating and WC–Cr₃C₂–Ni coating were prepared by the high-velocity oxygen-fuel (HVOF) spraying process. Scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and Vickers hardness tester were used to analyze the microstructure and mechanical properties of these coatings. The WC–CoCr coating presented the highest average microhardness of 1205 HV_0.3, and then followed by the WC–Cr₃C₂–Ni coating (1188 HV_0.3) and the WC–Ni coating (1105 HV_0.3). The abrasive wear behavior of the WC-based coatings under the conditions of different applied loads and sediment concentrations were studied by a wet sand-rubber wheel tester. The results indicated that the abrasive wear loss rates of all the coatings increased with the increment of applied load or sediment concentration. In addition, the coatings with higher microhardness appeared to have higher abrasive wear resistance. The abrasive wear resistance of the WC-based coatings was 4–90 times higher than that of AISI 304 stainless steel under the same testing condition. The abrasive wear mechanism of the WC-based coatings was deduced to be the extrusion and removal of binder phases, as well as the fragmentation and peel-off of hard phases.     

Comparative study on dry sliding wear and oxidation performance of HVOF and laser re-melted Al0.2CrFeNi(Co,Cu) alloys

Al_0.2CrFeNiCo and Al_0.2CrFeNiCu high entropy alloys were deposited with high velocity oxygen fuel (HVOF) on 316L substrate. Later, a laser re-melting (LR) process was applied to enhancing the coating microstructure. LR process effects on dry sliding wear and oxidation behaviors were investigated. The mixture of powders with free elements led to the formation of inner oxides in HVOF coatings. The oxide and porosity were eliminated using LR. After LR, FCC was the dominant phase in both alloys, while BCC, sigma and Cr₂O₃ phases were observed in Al_0.2CrFeNiCo alloy. The hardnesses of the Al_0.2CrFeNiCo and Al_0.2CrFeNiCu coatings after HVOF were HV 591 and HV 361, respectively. After LR, the hardnesses decreased to HV 259 and HV 270, respectively. Although HVOF coatings were most affected by increased load, they showed the highest wear resistance compared to other samples. The lowest wear resistance could be seen in the substrate. After the oxidation tests, HVOF coating layer was completely oxidized and also, the coating layer was delaminated from the substrate after 50 h oxidation due to its porous structure. LR coatings exhibited better oxidation performance. Al_0.2CrFeNiCo was dominantly composed of Cr₂O₃, exhibiting a slower-growing tendency at the end of the oxidation tests, while Al_0.2CrFeNiCu was composed of spinel phases.     

超音速喷涂纳米WC复合涂层与电镀铬层的组织及性能

采用超音速火焰喷涂(HVOF)技术和电镀技术分别制备了纳米WC-10Co-4Cr涂层及电镀铬层,通过扫描电镜(SEM)、X射线衍射仪(XRD)、显微硬度计、摩擦磨损试验机、电化学工作站、磨损腐蚀加速试验机等手段对比分析了这两种涂层的性能以及使用寿命。结果表明,纳米WC复合涂层具有远高于电镀铬层的显微硬度、耐磨损性能及结合强度等,并且该涂层具有较高的致密度,而耐腐蚀性能略低于电镀铬层但也远高于基材,磨损腐蚀加速试验结果显示该涂层的使用寿命高达电镀铬层的5倍以上。     

热处理对HVOF WC-17Co涂层组织结构及耐磨性能的影响

采用超音速火焰喷涂技术在42CrMo钢表面制备了硬质WC-17Co耐磨涂层,分别在500,700,900,1 100 ℃保温1 h进行热处理,进而研究了热处理温度对涂层微观组织、相组成、硬度以及耐磨性能的影响。实验结果表明：随着热处理温度的升高,W2C相逐渐减少而非晶态的Co发生再结晶生成Co3W3C和Co6W6C;硬度呈现先升高后降低的趋势,700 ℃热处理后,WC硬质相增多,硬度最高;喷涂态WC-17Co涂层的耐磨性较差,900 ℃热处理后,析出的Co6W6C细小且弥散均匀分布,涂层的磨损量最小、耐磨性最好。     

WC粒度对超音速火焰喷涂WC-10Co-4Cr涂层耐腐蚀性能的影响

采用制粒?高温快速烧结法制备两种分别含亚微米级和微米级WC粒径的WC-10Co-4Cr喷涂粉末,并用超音速火焰喷涂(HVOF)技术在45#钢基体上制备涂层;利用扫描电子显微镜和电化学工作站分别对涂层的显微形貌及耐腐蚀性能进行分析检测,探讨WC粒度对涂层耐腐蚀性能的影响和涂层的电化学腐蚀机理。研究结果表明：两种涂层组织致密,界面结合良好;含亚微米级WC粒径的涂层具有相对较低的孔隙率,使其涂层的耐腐蚀性能优于含微米级WC粒径的涂层。在3.5%NaCl溶液中涂层的硬质相WC和粘结相Co发生电偶腐蚀,且低电位的Co相优先腐蚀,导致WC颗粒脱落,出现凹坑及点蚀现象。     

工业汽轮机阀杆超音速火焰喷涂的研究

采用超音速火焰喷涂球形烧结态Cr3C2-25%NiCr复合粉末制备高温耐磨损涂层,分析了涂层的显微组织结构,测试了涂层的结合强度、硬度和耐磨性能,结果表明：涂层中以Cr3C2为主的颗粒增强相弥散分布在NiCr固溶体中,涂层的显微硬度比基体(2Cr12MoV)提高了3倍多,摩擦磨损性能也有明显提高。