A study on erosive wear behavior of HVOF sprayed nanostructured WC-CoCr coatings

WC-CoCr cermet coatings were deposited on stainless steel substrate using high-velocity oxy-fuel (HVOF) thermal spray process. The coatings were developed with two different thermal spray powders: one has WC grains of conventional micron size and the other is composed of nanosized (near-nanostructured) grains. HVOF spraying was assisted with in-flight particle temperature and velocity measurement system to control the process parameters that have resulted in quality coatings. Cavitation erosion testing was performed using a vibratory test apparatus based on ASTM standard G32-98. Surface morphology of powders and coatings was examined using the FESEM images, and phase identification was performed by XRD analysis. The erosion behavior of coatings and mechanism of material removal was discussed by examining the microstructure images of worn-out surfaces. WC-CoCr cermet coating deposited with nanosized WC grains exhibited higher cavitation erosion resistance as compared to conventional coating.     

Solid Particle Erosion Behavior of WC-CoCr Nanostructured Coating

In the present study, solid particle erosion resistance of high-velocity oxy-fuel (HVOF)-sprayed WC-CoCr coatings was evaluated. Erosion testing was conducted using alumina (Al₂O₃) powder as the erodent with three different impact angles (30, 60, and 90°) and impact velocity was kept constant. The coatings were deposited using two different powders; one was composed of conventional WC particles and second one contained nanoscale particles mixed with CoCr binder material. Erosion testing was carried out at room temperature using an air-jet erosion test setup. The effect of varying impact angles was studied and discussed with the help of scanning electron microscopy images of worn surfaces of coatings. The results showed that coating properties like microhardness and fracture toughness have a strong influence on the erosion behavior. During erosion testing, material was removed by fracturing and pullout of WC grains from the binder matrix. The morphology of the eroded surface also showed cutting, lip, and groove formation in the binder matrix caused by the repetitive impacts of erodent particles. It was observed that coating with nano-WC grains exhibited higher erosion resistance compared to conventional coating.     

Cavitation erosion of NiAl-bronze layers generated by friction surfacing

Friction surfacing is a solid-state process, which allows deposition welding at temperatures below the melting range. For this investigation coating layers of NiAl-bronze were deposited by friction surfacing on self-mating substrates, followed by microstructural characterisation. Further, cavitation tests were performed in order to investigate wear resistance. Cavitation erosion mechanisms were analysed by means of optical and electron microscopy. All coatings show incubation periods about twice as long as those of the substrate material, while their average rate of material loss is about one half of that of the substrate. The differences in cavitation erosion resistance are due to more ductile behaviour of the coatings, as well as corrosion increasing the wear of the as-cast material.     

Liquid impact erosion characteristics of martensitic stainless steel laser clad with Ni-based intermetallic composites and matrix composites

NiAl-Ni₃Al intermetallic composites (IC) and intermetallic matrix composites (IMC) with TiC and WC reinforcement were laser clad to increase the liquid impact erosion resistance of AISI 420 Martensitic stainless steel. Laser process parameter optimisation and pre- and post-heat treatment of the laser clad specimens were carried out to minimise porosity and sensitivity to crack formation. The coatings were characterised by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). The erosion resistance of the substrate material at a water droplet exit velocity of up to 150 m/s was improved from 116.9 to 838.7 min/mm³ for the nickel aluminide IC coating and from 855 to 1446 min/mm³ for the IMC coating with TiC and WC reinforcement. The pseudo-elasticity combined with the high work hardening ability was attributed to the excellent erosion resistance of nickel aluminide IC coatings. The IMC coatings with ceramic reinforcement extended significantly the initial resistance against liquid impact erosion. However, once damage occurred the erosion accelerated rapidly. No direct correlation could be established between the erosion resistance and the mechanical properties. The influence of hardness, elastic modulus, strain-hardening coefficient and the reversible penetration ratio on the erosion resistance was discussed.     

Influence of WC size and HVOF process on erosion wear performance of WC-10Co4Cr coatings

In the present study, based on the velocity and temperature measurements of in-flight particles and parameter optimization, multimodal and conventional WC-10Co4Cr cermet coatings were sprayed by high velocity oxygen gas fuel spraying (HVOGF) and high velocity oxygen liquid fuel spraying (HVOLF). The coatings’ structure, porosity, microhardness and fracture toughness were investigated by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) etc. Furthermore, erosion resistance of the coatings to solid sand was tested, followed by the exploration of the material failure mechanisms. Results show that the WC-10Co4Cr coatings deposited by HVOLF are mainly composed of WC with no obvious decarburization and amorphous CoCr binder. The mechanical properties of the coatings deposited by HVOLF are much more superior to those deposited by HVOGF. Multimodal WC-10Co4Cr coating deposited by HVOLF possesses the highest microhardness and fracture toughness, the lowest porosity and the most excellent resistance to sand solid erosion wear, which was enhanced by 15 and 40% than that of HVOLF conventional coatings at 30° and 90° impact angles. The improvement is even greater in comparison with multimodal coating deposited by HVOGF. These results have provided important reference for WC-CoCr anti-erosion coating design and optimization of high velocity oxygen fuel (HVOF) process.     

Studies on erosion behaviour of plasma sprayed coatings on a Ni-based superalloy

In the present investigation NiCrAlY, Ni-20Cr and Ni₃Al metallic coatings were deposited on a Ni-based superalloy (18.5Fe-19Cr-0.15Cu-0.5Al-3.05Mo-0.18Mn-0.9Ti-0.18S-0.04C-5.13 (Ta + Cb)-balance Ni). NiCrAlY was used as bond coat in all the cases. Erosion studies were conducted on uncoated as well as plasma spray coated superalloy specimens at room temperature. The erosion experiments were carried out using an air-jet erosion test rig at a velocity of 40 m/s and impingement angles of 30 and 90°. Silica sand particles of size ranging between 150 and 212 μm were used as erodent. The coatings have been characterised by scanning electron microscope (SEM), optical microscope, microhardness tester and X-ray diffractometer (XRD). Scanning electron microscope (SEM), equipped with an energy dispersive X-ray analyser (EDAX) was used to analyse the eroded surfaces. Possible erosion mechanisms are discussed. The phases revealed by XRD of the coatings have shown the formation of solid solutions. Out of the three plasma sprayed coatings, the Ni₃Al coating gave the lowest erosion rate regardless of the impact angle, and the Ni-20Cr coating gave the highest erosion rate.     

Erosive wear of hardfaced Fe–Cr–C alloys at elevated temperature

Many engineering components are subjected to erosive wear at elevated temperature. As erosive wear at elevated temperature is governed by the synergistic effect of erosive wear and oxidation, it is possible to modify surfaces of the components in order to achieve improved performances. In view of the above, two different types of hardfacing alloys of Fe–Cr–C were designed incorporating Nb, Mo and B to ensure improved performances at elevated temperature. In order to achieve the above objective, mild steel was hardfaced with these alloys under optimised gas metal arc welding (GMAW) condition. The microstructures of the hardfaced coating was characterised with the help of optical microscopy (OM) and scanning electron microscopy (SEM). The mechanical properties of these coatings were obtained by means of micro indenter. Erosive wear of these coatings was evaluated for four different temperatures, for two different impact angles and at one impact velocity. The morphologies and the transverse sections of the worn surfaces are examined with SEM. The erosive wear of these coatings were compared with conventional M2 tool steel. Results indicate that erosion rate of these coatings increases with increase of test temperature and impact angles. Among various coatings, Fe–Cr–C coating containing higher amount of Nb, Mo and B exhibits best erosion resistance particularly at elevated temperature.     

超声辅助电火花粉末沉积WC-Ni金属陶瓷涂层的微观结构及摩擦学性能

针对传统电火花沉积工艺中工具电极预制成本高、工艺复杂、材料选择范围受限等问题,提出了一种超声辅助电火花粉末沉积（Ultrasonic-assisted electro-spark powder deposition,UEPD）的新方法。利用UEPD工艺成功地在316L不锈钢基材上制备了WC-Ni金属陶瓷涂层。所制备的WC-Ni金属陶瓷涂层的厚度为89~159 μm,表面粗糙度约为3.672 μm,并且与基材呈现良好的冶金结合。超声振动的引入能够有效改善涂层的成形质量。涂层的微观组织主要由亚微米级细小枝晶组成,主要物相包括FeNi、Cr₃Ni、WC、W₂C、Cr₂₃C₆和Cr₃C₂等。这些细小的晶粒和强化相使金属陶瓷涂层的硬度明显增加,平均硬度达到980.68 HV,约为基材的4.1倍。摩擦磨损性能测试表明,金属陶瓷涂层的磨损率相比基材和不含WC的Ni基合金涂层分别降低了50.7%和37.7%,并且还表现出明显低于二者的摩擦因数。WC-Ni金属陶瓷涂层的主要磨损机理为疲劳磨损和磨粒磨损,其中高硬度表面和具有颗粒流润滑效果的磨屑层是金属陶瓷涂层实现高耐磨、低摩擦的主要原因。UEPD工艺相比于传统的电火花沉积工艺省却了复杂的工具电极预制过程,其工艺更简单,成本更低廉、材料选择更广泛,并且所制备的涂层也表现出良好的成形质量和性能。这为电火花沉积技术的发展提供了一种新的思路。     

Deposition of duplex Al2O3/TiN coatings on aluminum alloys for tribological applications using a combined microplasma oxidation (MPO) and arc ion plating (AIP)

Microplasma oxidation (MPO) has recently been studied as a cost-effective plasma electrolytic process to provide thick and hard ceramic coatings with excellent surface load-bearing capacity on aluminum alloys. However, for sliding wear applications, such ceramic coatings often exhibit relatively high friction coefficients against many counterface materials. Although coatings deposited by physical vapour deposition (PVD) techniques such as TiN coatings are well known for providing surfaces with a high hardness, in practice they often exhibit poor performance under mechanical loading, since the coatings are usually too thin to protect the substrate from the contact conditions. In this paper, these challenges were overcome by a duplex process of microplasma oxidation and arc ion plating (AIP), in which an alumina layer Al₂O₃ was deposited on an Al alloy substrate (using MPO as a pre-treatment process) for load support, and a TiN hard coatings were deposited (using AIP) on top of the Al₂O₃ layer for low friction coefficient. Microhardness measurements, pin-on-disc sliding wear tests, and antiwear tests using a Timken tester were performed to evaluate the mechanical and tribological properties. Scanning electron microscopy (SEM) was used to observe coating morphology, and to examine wear scars from pin-on-disc test. The research demonstrates that a hard and uniform TiN coating, with good adhesion and a low coefficient of friction, can successfully be deposited on top of an alumina intermediate layer to provide excellent load support. The investigations indicate that a duplex combination of MPO coating and TiN PVD coating represents a promising technique for surface modification of Al alloys for heavy surface load bearing application.     

The Solid Particle Erosion Behavior of Carbon/Carbon and Carbon/Phenolic Composite Used in Re-entry Vehicles

Several engineering components made of carbon-based heat-resistant composites are subjected to severe erosive wear. In view of the above, the solid particle erosion behavior of two and four dimensionally reinforced carbon/carbon (C/C) composites as well as that of carbon/phenolic (C/P) composite has been characterized at the ambient temperature. The investigated C/C composites have been produced through a liquid-phase infiltration method followed by hot isostatic pressing, while the C/P composite prepegs have been cured inside an autoclave. The erosion rates of these composites have been determined for two different impact angles and two different impact velocities using silica sand with average particle diameter of 200 μm. The morphologies of as-received and eroded surfaces of test specimens have been examined with the help of scanning electron microscopy to understand the mechanism of material removal. The erosion response, erosion efficiency, and erosion micromechanisms of these composites have been studied in detail. While the erosion resistance of the C/P composite is found to be superior to that of the investigated C/C composites, the four dimensionally reinforced C/C composite have shown the highest erosion efficiency. All the composites have exhibited a semi-ductile erosion response. Their mechanical properties have little correlation with the erosion rates.     

A contribution to the surface analysis and characterisation of HVOF coatings for petrochemical application

The appropriate selection of bulk materials and coatings of valve components is an important factor for the economic success of oil and gas production activities in the petrochemical field. Materials and coatings are important because particle erosion and surface wear are associated to corrosion by hydrogen sulphide during oil and gas flow. The wear of high pressure valves of gas system will lead to pollution, safety problems and cost increases. The most common solution of these problems is the deposition of hard materials as tungsten carbide or chromium carbide by thermal spray. These coatings are deposited by high velocity oxygen fuel (HVOF) thermal spray process to obtain a very high hardness with excellent cohesion and adhesion. Tungsten carbide cobalt–chromium based coating, chromium carbide nickel–chromium coating as well as Inconel 625 have been adopted in the specifications of petrochemical companies and their behaviour and wear, erosion and corrosion properties are reported in the literature.

This paper addresses the experimental study, surface analysis and functional characterisation of HVOF coatings innovative for the specific application such as NiAl and composite material WC/intermetallic compounds containing Ni, Cr, Co and Mo. These coatings have been systematically submitted to corrosion and functional tests based on the determination of the behaviour of the coatings in H₂S and CO₂ atmosphere and to wear and erosion according to standard ASTM G75-95 (slurry test); material loss and surface damage have been determined; the coatings have been completely characterised from the point of view of the structure (morphology, porosity, hardness, wear) and of the surface properties by means of a prototype 3-dimensional (3-D) stylus micro-geometrical surface analysis system; their corrosion and functional behaviour have been compared with the behaviour of the above mentioned coatings.

The slurry test allows a clear discrimination among the performances of analysed coatings. Namely, WC/Mo compound, because of its carbide content, shows fairly good behaviour in an erosive environment and higher erosion resistance than Inconel 625 and NiAl; all the tested coatings show similar behaviour in a corrosive environment.     

Application of cold spraying for flux-free brazing of aluminium alloy 6060

In the present study, samples of aluminium alloy 6060 were coated by cold spraying with a powder of brazing alloy Al12Si. The influence of the process gas temperature on particle velocities and coating build-up was investigated. The coated samples were heat-treated in air and under argon atmosphere to investigate the wetting behaviour of the deposited Al12Si and the diffusion processes between Al12Si coatings and substrates. Coated samples were brazed flux-free under argon atmosphere by an induction heating system. The microstructure of the coated, heat-treated, and brazed samples was investigated. The shear strength of the brazed joints was determined. The results show that the brazing alloy Al12Si could be very well deposited on the substrate by cold spraying. The particle velocity increased with increasing process temperature. Correspondingly, the thickness of Al12Si coatings increased with increasing process temperature. The heat treatments showed that a very good metallurgical bond between the Al12Si coatings and the substrate could be realized by the deposition using cold spraying. The coated samples could be well brazed without fluxes. The coating thickness and overlap width influenced the shear strength of the brazed joints. The highest shear strength of brazed joints amounts to 80 MPa.     

Application of cold spraying for flux-free brazing of aluminium alloy 6060

In the present study, samples of aluminium alloy 6060 were coated by cold spraying with a powder of brazing alloy Al12Si. The influence of the process gas temperature on particle velocities and coating build-up was investigated. The coated samples were heat-treated in air and under argon atmosphere to investigate the wetting behaviour of the deposited Al12Si and the diffusion processes between Al12Si coatings and substrates. Coated samples were brazed flux-free under argon atmosphere by an induction heating system. The microstructure of the coated, heat-treated, and brazed samples was investigated. The shear strength of the brazed joints was determined. The results show that the brazing alloy Al12Si could be very well deposited on the substrate by cold spraying. The particle velocity increased with increasing process temperature. Correspondingly, the thickness of Al12Si coatings increased with increasing process temperature. The heat treatments showed that a very good metallurgical bond between the Al12Si coatings and the substrate could be realized by the deposition using cold spraying. The coated samples could be well brazed without fluxes. The coating thickness and overlap width influenced the shear strength of the brazed joints. The highest shear strength of brazed joints amounts to 80 MPa.     

Micro-abrasion wear testing of triode plasma diffusion and duplex treated Ti–6Al–4V alloy

In this paper micro-abrasion wear testing is used to evaluate the wear resistance of triode plasma diffusion-treated, single-layered TiN-, CrAlN-, and WC/C-coated and duplex-diffusion and coated Ti–6Al–4V under uniform three-body rolling abrasion. Nanoindentation, Knoop microhardness, mechanical surface profilometry, optical microscopy, scanning electron microscopy and atomic force microscopy, were used to characterise the surfaces under investigation. Optimum testing conditions for rolling abrasion were established by varying the test parameters and resultant severity of contact. Very low normal loads and high volume fractions of particles in the abrasive slurry are necessary to obtain predictable and reproducible results. Relatively coarse SiC abrasive particles, having a mean diameter of around 3 μm, appear more suitable for micro-abrasion testing of the samples investigated, compared to finer Al₂O₃ particles. Problems associated with the measurement of the scar volume and subsequent calculation of the wear rate for hard coatings deposited on relatively soft metals like titanium are identified, and suitable testing and measurement techniques are suggested. Three-dimensional wear scar maps generated by mechanical stylus profilometry were used to measure the wear volumes. Under the test conditions used, wear coefficients can be determined from perforating and non-perforating tests, although perforating tests provide more consistent results. Triode plasma diffusion treatments, plasma-assisted (PA) PVD TiN and PAPVD CrAlN can reduce the specific wear rate of Ti–6Al–4V, while PACVD-based WC/C coatings do not provide suitable protection against abrasive wear. The combination of triode plasma oxynitriding diffusion treatments and PVD coatings to create duplex treatments can also lead to further reductions in the coating wear coefficient when compared to non-duplex coatings deposited on non-pretreated substrates.     

Sliding wear behavior of Fe-Al and Fe-Al/WC coatings prepared by high velocity arc spraying

A comparative study was carried out to investigate the microstructure and tribological behavior of Fe-Al and Fe-Al/WC iron aluminide based coatings against Si₃N₄ under dry sliding at room temperature using a pin-on-disc tribotester. The coatings were prepared by high velocity arc spraying (HVAS) and cored wires. The effect of normal load on friction coefficient and wear rate of the coatings was studied. The microstructure and the worn surfaces of the coatings were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersion spectroscope (EDS). The results showed that, the main phases in both coatings were iron aluminide (Fe₃Al and FeAl) and α. WC/W₂C particles were embedded in the matrix of the composite coating. With adding WC hard particles, the Fe-Al/WC composite coating exhibited higher wear-resistance than Fe-Al coating. But the friction coefficient of both coatings showed little difference. As the load increased, the friction coefficient decreases slightly due to a rise of friction contact temperature and larger areas of oxide film formation on the worn surface, which act as a solid lubricant. Increasing load causes the maximum shear stress occurring at the deeper position below the surface, thereby aggravating the wear. The coating surface is subjected to alternately tensile stress and compression stress during sliding, and the predominant wear mechanism of the coatings appears to be delamination.     

Solid particle erosion behaviour of Ti6Al4V alloy

Abstract

The effects of particle impingement angle, impingement velocity and erodent particle size on the erosion rate and surface morphology of the Ti6Al4V alloy have been investigated comprehensively in order to evaluate solid particle erosion behaviour of Ti6Al4V alloy. Samples were eroded in a specially designed sandblasting system under various parameters using alumina (Al₂O₃) erodent particles. Surface morphology investigations were examined by scanning electron microscope using various analysis and modes (energy dispersive X-ray analysis, elemental mapping and compositional contrast). Ti6Al4V alloy showed ductile behaviour with a maximum erosion rate at 30° impingement angle. Erosion rate of Ti6Al4V alloy increased with increases in velocity and decreased with increases in erodent particle size. Scanning electron microscopy investigations of eroded surfaces of Ti6Al4V alloy samples reveal the dominant erosion mechanism such as microploughing, microcutting and plastic deformation. Embedded erodent particles on the surfaces of Ti6Al4V alloy nearly at all particle impingement angles and velocities were clearly detected.     

A comparative study of tribological behavior of microarc oxidation and hard-anodized coatings

Microarc oxidation (MAO), a novel coating technique capable of depositing dense, hard ceramic composite coatings on aluminium and its alloys, has the potential to replace conventional hazardous anodizing techniques. However, the emergence of such a scenario depends strongly on the properties and performance of MAO coatings in comparison to hard-anodized coatings. In order to facilitate such a comparative investigation, a 6061 T6 aluminium alloy was employed as the substrate and the coatings were deposited through microarc oxidation (MAO) and hard anodizing techniques. The tribological performance of the coatings was evaluated using dry-sand abrasive wheel tests at different normal loads and solid-particle erosion wear tests at different particle velocities and impact angles by employing silica as erodent. The hard-anodized coatings reduced the abrasive wear rate of 6061 Al alloy by a factor of 2, while the MAO coatings reduced the wear rate by a factor of 12-30. Under erosion conditions, the overall wear rate of MAO coatings is identical to that of bare alloy, whereas the hard-anodized coatings exhibit 10 times higher erosion rate.     

Comparison in erosion behavior of iron-base coatings sprayed by three different arc-spray processes

The elevated temperature erosion behavior of three iron-base coatings sprayed by high velocity continuous combustion (HVCC) process was determined in a laboratory elevated temperature erosion tester and compared with the behavior of coatings sprayed using combustion arc-spray and standard arc-spray processes. Test conditions attempted to simulate erosion conditions found at the heat exchanger tubes in the coal-fired boilers. Two bed ashes A (highly erosive) and B (medium erosive bed ash) retrieved from two operating boilers were used as erodent materials. In addition, some other properties, such as microhardness, porosity, and oxidation rate, of coatings were also measured and compared.

Erosion test results indicated that the three iron-base coatings sprayed with HVCC process demonstrated higher erosion resistance than the ones sprayed with the other two arc-spray processes, when eroded by bed ash B. When eroded by high erosive ash (bed ash A), the three arc-sprayed iron-base coatings exhibited similar levels of erosion resistance, the ones sprayed with HVCC process being slightly better than those sprayed with the other two processes. The difference of erosion behavior between sprayed coatings produced by different processes closely correlated with their microstructure and the characteristics of ash materials rather than the microhardness of coatings. The erosion mechanism of coatings was also discussed and compared.     

等离子喷涂纳米Al_2O_3-13%TiO_2陶瓷涂层的组织结构与抗冲蚀性能

采用等离子喷涂方法分别制备了常规和纳米Al2O3-13%TiO2陶瓷涂层,用扫描电子显微镜分析了涂层的显微结构,并对涂层进行了抗冲蚀试验。结果表明:常规陶瓷涂层具有典型的片层状结构,但纳米陶瓷涂层片层状结构并不十分明显,且涂层裂纹数量明显减少;纳米陶瓷涂层中的显微结构的变化改善了涂层的韧性和结合性能;在冲蚀过程中,常规陶瓷涂层表面剥落严重,而纳米陶瓷涂层的冲蚀质量损失较小,抗冲蚀性能比常规陶瓷涂层提高了30%左右。     

Corrosion and erosion performance of HVOF/TiAlN PVD coatings and candidate materials for high pressure gate valve application

The main objective of this paper is to study the slurry erosion and corrosion behavior of WC10Co4Cr, Armcore ‘M’ Stellite 6 and 12 HVOF coatings, TiAlN PVD coating, selected steels, such as X20Cr13, 17Cr–4Ni pH steel and Ti6Al4V titanium alloy alongwith conventional hard weld deposits of Stellite 6 and 21. The slurry erosion studies were carried out at 60° angle of impingement for the velocities in the range of 15–20 m/s using mineral sand of −40 to +80 mesh. The corrosion studies were carried out as per ASTM B 117-73 for 100 h. During slurry erosion testing, WC10Co4Cr HVOF along with TiAlN PVD coating are found out, to be the best coating materials followed by HVOF coating of Armcore ‘M’ material. However, for corrosion, Ti6Al4V, Stellite 6 and 21 hard weld deposits and 17Cr–4Ni pH steel turned out to be the best materials followed by HVOF coating of Stellite 6 and 12. HVOF coatings of WC10Co4Cr and Armcore ‘M’ materials corroded significantly, however, TiAlN PVD coating corroded very badly even after 24 h of testing.