Microstructure and Wear Behavior of Plasma‐Sprayed Nanostructured WC–Co Coatings

Atmospheric plasma spraying of WC–Co particles with standard gas mixtures (Ar–H₂) typically results in largely decarburized coatings with relatively low wear resistance. To fabricate cermet coatings with enhanced tribological properties, nanostructured WC–Co coatings were plasma sprayed using two different process gas mixtures. Phase composition and microstructure were investigated by X‐ray diffraction and scanning electron microscopy, respectively. Microhardness increased by increasing the amount of retained WC grains in coating microstructure. Friction and wear properties, measured under dry sliding conditions, strongly depended on the degree of decarburization. They were comparable to those of conventional coatings produced using identical conditions.     

Surface and cross–section characteristics and friction–wear properties of high velocity oxy fuel sprayed WC–12Co coating

A WC–12Co coating was sprayed on H13 hot work mould steel using a high velocity oxy fuel (HVOF). The surface and cross–section morphologies, chemical compositions, and phases of obtained coatings were analyzed using a field emission scanning electron microscope (FESEM), energy dispersive spectrometer (EDS), and X–ray diffraction (XRD), respectively. The friction–wear properties were investigated using a wear test, the wear mechanism of WC–12Co coating was also discussed. The results show that the WC–12Co coating primarily is composed of WC hard phase with high hardness and Co as a binder, which is evenly distributed on the coating surface, no atom–rich zones. There is no W₃O phase appearing in the HVOF spraying, showing that the WC–12Co coating has high oxidation resistance, the new phases of W₂C and C are produced due to the decarburization of WC. The coating thickness is ~200 μm, which is combined the substrate with the mechanical binding and local micro–metallurgical bonding. The average coefficient of friction (COF) of WC–12Co coating is 0.272, showing good friction performance, the wear mechanism is primarily abrasive wear, accompanied with fatigue wear.     

Microstructural Properties of Air Plasma Sprayed Coating Consisting of Tungsten Carbide and Yttria‐Stabilized Zirconia

Thermal Spraying technologies are proven to be capable of producing composite materials and structures. In the present work, an innovative composite coating was produced to achieve high wear and thermal resistant properties in a single‐step process using air plasma spraying (APS) technique. Tungsten carbide has shown high wear resistance and zirconia coatings exhibited excellent tribological and insulation properties. It is speculated that a composite material consisting of zirconia and tungsten carbide exhibits excellent thermomechanical properties. A powder mixture of 50wt% WC‐10wt% Ni (WC‐Ni) and 50wt% ZrO₂‐8wt% Y₂O₃ (YPSZ) was deposited on a low carbon steel substrate using APS technique. Important microstructural properties of WC‐Ni/YPSZ coating such as splat boundaries, pore and grain morphology, microcracks, phase composition, elemental distribution of coatings, and lattice parameters of the crystals were investigated using optical microscopy, scanning electron microscopy (SEM), energy dispersive X‐ray (EDS), and X‐ray diffractometry (XRD). A good adhesion was observed between different phases in tungsten carbide mixed with zirconia coatings. Decarburization process which occurred during APS process resulted in formation of tungsten hemi‐carbide (W₂C) phase in plasma sprayed samples. The calculated crystal size for APS‐deposited coating was smaller than those of feedstock powder.     

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。     

Thermal spraying of ceramics

In the last few years, the number of publications dealing with ceramic coatings has increased very rapidly. To present an overview of this field, a bibliografic review of the thermal sprayed ceramics is performed. Investigated and studied successively are the thermal spraying techniques (flame, plasma and detonation), the sprayed ceramics and the applications of these coatings. It appears that plasma spraying is the most common technique, that ceramic coatings are most often made of the metal oxides and their main application is for wear resistance.     

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.     

超音速等离子喷涂制备陶瓷涂层的研究进展

超音速等离子喷涂技术由于具有高温、高速的独特优点,且制备的陶瓷涂层结合强度和致密度高,孔隙率低,具有优良的耐磨损、耐腐蚀、抗氧化和热冲击性能,已成为一些发达国家竞相研究的热点.本文介绍了常用的陶瓷涂层材料,综述了超音速等离子喷涂技术及其制备陶瓷涂层的工艺特点,并对超音速等离子喷涂制备高性能陶瓷涂层的发展趋势进行了展望.     

Comparative study of the corrosion resistance of thermally sprayed ceramic coatings and their bulk ceramic counterparts

A comparative study of the corrosion properties of thermally sprayed ceramic coatings (Al₂O₃, Al₂O₃–TiO₂ with different ratios, mullite, and ZrSiO₄) and their sintered bulk ceramic counterparts was performed. The coatings were deposited on corrosion-resistant steel substrates using atmospheric plasma spraying (APS) and high velocity oxy-fuel (HVOF) spraying processes. The corrosion properties were investigated in 1 N solutions of NaOH and H₂SO₄ at 85 °C, respectively. The coating microstructures and phase compositions, as well as the corrosive environment were shown to have a strong effect on the corrosion resistance of the coatings. Al₂O₃–coatings were more sensitive to these factors than Al₂O₃–TiO₂ coatings were.The corrosion resistance of the bulk ceramics was superior to that of the thermally sprayed coatings. This is mainly because the coatings exhibited specific microstructure and contained amorphous and/or metastable phases not appearing in the bulk ceramics.     

Microstructural,mechanical and tribological properties of nanostructured YSZ coatings produced with different APS process parameters

Plasma sprayed ceramic coatings can be used in turbine engines as thermal barrier or abradable coatings, in order to improve the durability of the components as well as the efficiency. The presence of nanostructures, deriving from partial melting of agglomerated nanostructured particles, represents an interesting technological solution in order to improve their functional characteristics. In this work nanostructured yttria stabilized zirconia (YSZ) coatings were deposited by air plasma spraying (APS). The influence of the main process parameters on their microstructural, mechanical and tribological properties was investigated by scanning electron microscopy (SEM), indentation techniques at micro- and nano-scale and wear tests, respectively. Their porous microstructure was composed of well melted overlapped splats and partially melted nanostructured areas. This bimodal microstructure led to a bimodal distribution of the mechanical properties. An increase of plasma power and spraying distance was able to produce denser coatings, with lower content of embedded nanostructures, which exhibited higher elastic modulus and hardness as well as lower wear rate.     

Phase transition and interface evolution of Al2O3/ZrO2 particles in plasma-sprayed coatings

Alumina and zirconia ceramic particles exhibit high hardness and excellent wear resistance at high temperature, and hence are used as ceramic reinforcement phases in some plasma sprayed coatings. In this study, the interface evolution of a zirconia/alumina eutectic ceramic and the phase transition of zirconia in a plasma-sprayed coating were investigated. Scanning electron microscopy and transmission electron microscopy combined with focused-ion beam and energy dispersive X-ray were used to analyze the microstructure and composition of the ceramic interface. The results showed that the eutectic ceramic particles consisted of alumina (outer) and columnar zirconia (inner) before and after the plasma spraying process. The inner zirconia part showed the martensitic transformation of t-type zirconia to stripe-like m-type zirconia. After the plasma spraying, the interface between alumina and zirconia changed significantly, which formed a new oxide layer. The phase transition mechanism in the ceramic particle and oxide layer formation mechanism at the alumina/zirconia interface were investigated.     

Cracking induced tribological behavior changes for the HVOF WC-12Co cermet coatings

HVOF sprayed WC based cermet coatings have been widely used in industries as barriers against wear and hydrodynamic cavitation due to their high hardness and relatively high toughness. However, cracking of the coatings can occur during coating production or in service, which can reduce operational performances. It can be difficult to assess the performance impact due to cracks within the coating and as to whether the cracked coatings should be resprayed or removed from service. In this work, artificial cracks of different widths were introduced to liquid fuel HVOF sprayed WC-12Co coating through uniaxial tension of the coated steel substrate to assess the implications of such cracking. Tribological performances of the cracked coatings were examined using rubber wheel dry abrasion, ‘ball on disc’ sliding wear, and ultrasonic cavitation erosion. The results show that the crack deteriorates the abrasive wear resistance of the coating at the initial stage due to preferable mass loss at the cracks. However, after 30?min of abrasion, all the cracked coatings showed the same wear rate as compared to the non-cracked coating, with the abrasive wear resistance acting independent to the crack characteristics. Because the cracks could store wear debris and thus minimize the debris induced abrasion to the coating surface during sliding wear test, both improvement in wear resistance and reduction in coefficient of friction (COF) were detected in the cracked coatings. During the cavitation test, it was found that the mass loss of the specimen increased significantly (up to 75%)with crack width and density suggesting that the crack presence greatly deteriorated the cavitation resistance of the cermet coatings.     

Tribological behavior of nanostructured high velocity oxy-fuel (HVOF) thermal sprayed WC-17NiCr coatings

In this research, the nanostructured WC-17NiCr cermet coatings were developed using the high velocity oxy-fuel (HVOF) thermal spraying processes on ACI CD4MCu cast duplex stainless steel substrates, widely used in pump industry for abrasive wear protection of surfaces. The coatings, sprayed by both robotic and manual methods, had two different fuel (methane) to oxygen ratios (FTOR), namely 0.68 and 0.62. Using different analytical and microstructural techniques, the microstructural characteristics of the powder particles and mechanical, microstructural, and tribological properties of the coatings were determined. Different morphologies were assigned to sprayable particles, namely spherical, apple, donut, irregular, and mixed. It was revealed that the rate of WC decarburization had increased with increasing the FTOR. In contrast, the scanning electron microscopy and image analyses showed that the lowest porosity percentage was obtained for the robotically-sprayed coating with 0.68 FTOR. The Vickers microhardness increased along with fracture toughness, which can be attributed to the effect of the ‘duplex structure’ associated with the particle outer coating of Co and is a novelty in the research. The pin-on-disk reciprocal sliding wear tests at various loadings had shown different wear rates in the coatings. It was inferred that the wear performance was improved with the microstructural homogeneity, hardness, and the fracture toughness in the coatings. In all coatings, lower coefficient of friction (COF) was observed at higher loads. Finally, the wear mechanisms involved in the wear processes were identified as deformation and removal of the binder, fracture and pullout of the carbide particles, and delamination and spallation of the splats.     

Studies on plasma sprayed bi-layered ceramic coating on bio-medical Ti-13Nb-13Zr alloy

Biomedical Ti alloys are prone to undergo degradation due to the combined effect of wear and corrosion. To overcome these problems, surface modification techniques are being used. In this paper, the biomedical Ti alloy Ti-13Nb-13Zr was plasma sprayed with nanostructured Al₂O₃-13 wt%TiO₂, yttria stabilized zirconia powders and bilayer containing alternate layers of the two coatings to improve the corrosion resistance and microhardness of the substrate. The plasma sprayed coatings were characterized by X-ray diffraction, scanning electron microscopy and Raman spectroscopy. The microstructure, microhardness and surface roughness of the coatings were investigated. The corrosion resistance of the coatings was studied in simulated body conditions. The results show improved corrosion resistance for the bilayered coating compared to the individual plasma sprayed coatings on biomedical Ti-13Nb-13Zr alloy substrate.     

Microstructure and wear properties of plasma‐sprayed aluminum–silicon–polyester coatings

Powder coatings, which are made by plasma‐spraying processes, are being used in industrial applications because of their wear resistance, chemical resistance, and high impact strength even at low service temperatures. These factors increase the importance of plastic and plastic‐based coatings in industrial applications. In this study, an aluminum–silicon–polyester‐based composite coating was applied by plasma‐spraying processes with and without an intermediate bond coat (Ni–Al). The effects of the coating thickness, intermediate bond coat, and processes parameters on the microstructure and wear properties of the coating were studied experimentally. The wear properties of the coatings were determined according to ball‐on‐disk procedure. The microstructures of the coating were examined by optical microscopy and scanning electron microscopy. The results indicated that the plasma‐spraying current and thickness had a strong influence on the wear resistance and microstructural properties of the aluminum–silicon–polyester coating. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3609–3614, 2006     

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%.     

Effect of laser power on microstructure and tribological performance of WC−10Co4Cr coating

In order to enhance wear resistance of cold work molds, WC−10Co4Cr coating was fabricated on Cr12MoV steel by laser cladding. The morphologies, chemical compositions, and phases of obtained coatings were analyzed using a scanning electron microscopy (SEM), energy disperse spectroscopy, and X−ray diffraction, respectively. The effect of laser power on the tribological performance was analyzed using a ball−on−plate friction machine, and the wear mechanism was also discussed. The results show that the WC−10Co4Cr coating is composed of WC and Co₆W₆C phases, and the average hardness of coating cross−sections fabricated at the laser power of 1200, 1500, and 1800 W was 1296, 1375, and 1262 HV_0.5, respectively, in which that fabricated at the laser power of 1500 W is the highest among the three kinds of coatings. The average coefficients of friction of coatings fabricated at the laser power of 1200, 1500, and 1800 W are 0.61, 0.52, and 0.59, respectively; and the corresponding wear rates are 64.38, 35.38, and 123.92 μm³•N⁻¹•mm⁻¹, respectively, showing that the coating fabricated at the laser power of 1500 W has best friction reduction and wear resistance. The wear mechanism of WC−10Co4Cr coating is fatigue wear and abrasive wear, which is contributed to the increase of hard WC mass fraction.     

热喷涂陶瓷涂层的耐磨应用及涂层结构调控方法

因涂层材料适用范围广、基材适应性强、工艺灵活等特点,热喷涂陶瓷涂层作为一类新型耐磨涂层已经在很多领域获得成功应用。然而,现代工业发展对耐苛刻条件下严酷磨损的高性能耐磨涂层提出了越来越高的需求,如何通过材料?工艺的整体技术体系进行涂层结构的有效调控,成为涂层技术领域的重要研究课题之一。本文在简要介绍热喷涂陶瓷涂层作为耐磨涂层应用现状的基础上,提取出对涂层耐磨性具有普遍意义的层内扁平粒子间界面结合这一重要的涂层结构本质特征,明确了涂层内扁平粒子间界面强化的基本思路,阐述了基于界面同质强化和界面异质强化的两条思路进行层间结合界面强化的研究进展,以期为面向更高耐磨性能的热喷涂陶瓷涂层的材料选择、结构设计以及工艺优化提供有益参考。     

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。     

High-temperature oxidation and wear resistance of WC-Co based coatings with WB addition

A new type of WC-based coating with high oxidation- and wear-resistance at elevated temperature was fabricated by thermal spraying the pre-treated WC-Co powder doped with WB. Addition of WB led to in situ formation of WCoB, which acted as a substitute for Co in the powders and the resultant coatings. It was shown by thermal analysis that WCoB has obviously higher oxidation resistance at high temperatures than that of WC and Co. Thus, the oxidation of the WC-WCoB coating was mainly initiated from WC, rather than from Co in the conventional WC-Co coatings. Most of WCoB was preserved in the coating after high-temperature wear tests. Particularly, with an addition of 40 wt.% WB, the wear rates of the WC-Co coating were dramatically decreased by 90% and 77% at the room and elevated temperatures, respectively.     

A robust quasi-superhydrophobic ceria coating prepared using air-plasma spraying

Hydrophobic coatings that could survive in harsh environment have a wide range of applications from industry to houseware. However, the state-of-the-art polymer-based coatings cannot meet such requirements due to their low melting point and poor wear resistance. In this study, we reported a plasma sprayed ceramic coating made of ceria with exceptional hydrophobicity, high-temperature stability, and good wear resistance. The coating exhibited a water contact angle (WCA) up to 139°, due to the intrinsic hydrophobicity of ceria and unique surface morphology produced by plasma spraying. The WCA only slightly decreased to 131° after annealing at 773 K. In addition, the polished coating (WCA ~ 116°) was still more hydrophobic than the sintered bulk specimen (WCA ~ 95°) with the same composition and roughness, which can be attributed to the surface chemistry change induced by Ar⁺ ion bombing by plasma. It is believed that such robust hydrophobic coating should have great potential in engineering application.