Properties of Cr3C2-NiCr cermet coating sprayed by high power plasma and high velocity oxy-fuel processes

The structure, hardness, and shear adhesion strength have been investigated for Cr₃C₂-NiCr cermet coatings sprayed onto a mild steel substrate by 200 kW high power plasma spraying (HPS) and high velocity oxy-fuel (HVOF) processes. Amorphous and supersaturated nickel phases form in both as-sprayed coatings. The hardness of the HVOF coating is higher than that of the HPS coating, because the HVOF coating contains more nonmelted Cr₃C₂ carbide particles. On heat treating at 873 K, the amorphous phase decomposes and the supersaturated nickel phase precipitates Cr₃C₂ carbides so that the hardness increases in the HPS coating. The hardness measured under a great load exhibits lower values compared with that measured with a small load because of cracks generated from the indentation. The ratio of the hardnesses measured with different loads can be regarded as an index indicating the coating ductility. The ductility of the HVOF coating is higher than that of the HPS coating. Adhesion strength of the HVOF coating was high compared with the HPS coating. The adhesion of the coatings is enhanced by heat treating at 1073 K, and that of the HVOF coating is over 350 MPa.     

Wear Resistance Improvement of Small Dimension Invar Massive Molds for CFRP Components

Invar alloy (Fe-36%Ni) is used in industrial applications that require high dimensional stability because of its exceptionally low thermal expansion coefficient. The purpose of this work is to improve the wear resistance of the molds in the production of carbon-fiber reinforced plastic (CFRP) components applying thermal spray coatings. Four different kinds of commercial powders were coated on an Invar substrate: ZrO₂-8Y₂O₃, Al₂O₃-13TiO₂, and Cr₂O₃ by air plasma spray (APS) and WC-CoCr by high-velocity oxygen fuel (HVOF). Metallographic microscopy observation and scanning electron microscopic analysis were carried out, microhardness and fracture toughness were evaluated using the microindentation method. Friction behavior and wear resistance were evaluated with pin-on-disk apparatus. Tungsten carbide coating had the lowest average coefficient of friction. Cermet and alumina-titania coatings showed the lowest wear mass loss. Among the APS ceramic coatings, alumina-titania exhibited the best wear behavior and the HVOF cermet coating exhibited the best behavior among all the coatings.     

Advances in High Velocity Suspension Flame Spraying (HVSFS)

Three suspensions, containing oxide nanoparticle single phases, (Al₂O₃, 3YSZ and Cr₂O₃) were prepared and sprayed using High Velocity Suspension Flame Spraying (HVSFS) technique. The coatings were characterized concerning their mechanical properties by means of nano indenter hardness measurements (all coatings) and ball on disk tribometry (Al₂O₃, 3YSZ).APS and HVOF sprayed Al₂O₃ coatings were characterized under same conditions for comparison. X-ray diffraction analysis was performed on HVSF sprayed Al₂O₃ and Cr₂O₃ coatings and a plasma-sprayed Cr₂O₃ coating for comparison. A Williamson-Hall line profile analysis was performed to estimate and compare crystallite size in the coatings.     

Wear,erosion and corrosion resistance of HVOF-sprayed WC and Cr3C2 based coatings for electrolytic hard chrome replacement

Thermally sprayed carbide-based coatings are nowadays extensively considered as an alternative to electrolytic hard chrome (EHC) coatings to reduce the environmental impact and the overall cost associated with EHC process. In this investigation, high-velocity oxy-fuel (HVOF) spray process was employed to prepare coatings using the traditional carbide powders namely the WC-10Co4Cr, the Cr₃C₂-25NiCr and a new type of mixed carbide powder WC-40Cr₃C₂-25NiCr. The Powder deposition rate, basic mechanical properties, abrasive wear, slurry erosion and corrosion resistance of the three coatings were then compared with the EHC coating. The results show that WC-10Co4Cr coating exhibited the highest hardness, abrasive wear and slurry erosion resistance followed by WC-40Cr₃C₂-25NiCr, EHC, and Cr₃C₂-25NiCr coating. The deposition efficiency of the powders as per hierarchy was found to be WC-40Cr₃C₂-25NiCr > WC-10Co4Cr > Cr₃C₂-25NiCr and all the HVOF sprayed coatings exhibited higher corrosion resistance than EHC coating. The highest powder deposition efficiency coupled with low density, acceptable tribo-corrosion performance, as well as low post processing cost makes the HVOF sprayed WC-40Cr₃C₂-25NiCr coating a potential candidate to replace the EHC coating.     

Microstructure and Wear Resistance of Fe-Based Amorphous Metallic Coatings Prepared by HVOF Thermal Spraying

Amorphous metallic coatings with a composition of Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Y₂ were fabricated by means of high velocity oxygen fuel (HVOF) thermal spraying process. The microstructure and wear performance of the coatings were characterized simultaneously in this article. It is found that the coatings present a dense layered structure with the porosity below 1.5%. The coatings primarily consist of amorphous matrix and some precipitated nanocrystals, though a fraction of Fe-rich phases and oxide stringers also formed during deposited process. High thermal stability enables the amorphous coatings to work below 920 K temperature without crystallization. Depending on the structural advantage, the amorphous coatings exhibit high average microhardness of 997.3 HV_0.2, and excellent wear resistance during dry frictional wear process. The dominant wear mechanism of amorphous coating under this condition is fatigue wear, leading to partial or entire flaking off of the lamellae. In addition, the appearance of oxidative wear accelerates the failure of fatigue wear.     

Influence of feedstock powder characteristics and spray processes on microstructure and properties of WC-(W,Cr)2C-Ni hardmetal coatings

The composition WC-(W,Cr)₂C-Ni (commercial designations WC-‘CrC’-Ni, WC-Cr₃C₂-Ni and WC-NiCr) is unique among the WC-based materials used for the preparation of thermally sprayed hardmetal coatings. These coatings show a significantly higher oxidation resistance and high-temperature sliding wear resistance than WC-Co and WC-CoCr coatings do. Unlike WC-Co and Cr₃C₂-NiCr, WC-(W,Cr)₂C-Ni is not a simple binary hard phase-binder metal composite as it is composed of two hard phases: WC and (W,Cr)₂C. Surprisingly this composition has been poorly investigated in the past.In this paper coating microstructures and properties obtained from five commercial feedstock powders of different origins using two different liquid-fuelled high velocity oxy-fuel (HVOF) systems (K2 and JP-5000) were investigated. Additional experiments were performed with one powder using atmospheric and vacuum plasma spraying (APS and VPS, respectively). The microstructures and phase compositions of the powders and the coatings were studied. Focus was on the appearance, composition and distribution of the (W,Cr)₂C phase which might form or might change its Cr/W ratio during the spray process. The composition of the (W,Cr)₂C phase was estimated from the lattice parameters. Hardness HV0.3 was measured for all coatings. The density, Young's modulus and abrasion wear resistance of HVOF-sprayed coatings were studied.     

The use of Al-Al2O3 cold spray coatings to improve the surface properties of magnesium alloys

Pure Al and 6061 aluminium alloy based Al₂O₃ particle-reinforced composite coatings were produced on AZ91E substrates using cold spray. The strength of the coating/substrate interface in tension was found to be stronger than the coating itself. The coatings have corrosion resistance similar to that of bulk pure aluminium in both salt spray and electrochemical tests. The wear resistance of the coatings is significantly better than that of the AZ91 Mg substrate, but the significant result is that the wear rate of the coatings is several decades lower than that of various bulk Al alloys tested for comparison. The effect of post-spray heat treatment, the volume fraction of Al₂O₃ within the coating and of the type of Al powder used in the coatings on the corrosion and wear resistance was also discussed.     

Ultrafine Particulate Dispersed High-Temperature Coatings by Hybrid Spray Process

Oxide dispersion strengthened alloys (ODS), although not commonly used in coating applications, have long been used for high-temperature structural applications due to their superior creep properties. In this paper, we present the design, synthesis, and characterization of a new class of functionally engineered high-temperature coatings in which ultrafine oxide particulates are dispersed in the matrix alloy to achieve superior creep resistance along with improved high-temperature corrosion and erosion resistance. These coatings were fabricated using a novel technique called “hybrid spray process”. Hybrid spray technique combines arc spray and high-velocity oxy fuel (HVOF) spray processes; the metallic matrix alloys are fused by the wire arcing component of the process, whereas the ultrafine particles are synthesized in-flight by the HVOF component from liquid precursors. These particulate dispersed high-temperature composite coatings were fabricated using liquid precursors for SiO₂, Cr₂O₃, Al₂O₃, and wire feed stock of 55/45 NiCr, in one step. The coatings were then characterized using electron microscopy (SEM/TEM) and thermogravimetric analysis (TGA). High-temperature erosion, oxidation, and corrosion performance of these coatings were also evaluated and compared with 304 stainless steel, arc sprayed NiCr coatings as well as Alloy 625 overlay cladding. The hybrid spray process produced dense coatings with uniform dispersion of the ultrafine oxide particles. Further, these coatings also demonstrated superior corrosion, erosion, and oxidation resistance; SiO₂ particulate dispersion being most effective in terms of high-temperature corrosion resistance.     

Characterization of Fe-Cr-B based coatings produced by HVOF and PTA processes

Two Fe-Cr-B based gas atomized powders, Armacor M and 16, were thermally sprayed on a low carbon steel substrate, using the HVOF (High Velocity Oxygen Fuel) process. Armacor M was also weld-surfaced with the PTA (Plasma Transferred Arc) process. The resultant deposits were subsequently characterized, using X-ray diffraction, scanning electron microscopy, and microhardness measurement. The effects of heat treatment were also studied for HVOF-sprayed coatings. The wear performance of the coatings was investigated by two-body abrasive wear tests. The results of microstructural analysis of as-sprayed deposits revealed oxide and boride phases such as Fe₃O₄ and Cr_1.65F_0.35B_0.96 in an α matrix for the HVOF-sprayed Armacor 16 coating, and only the boride phases (Cr_1.65F_0.35B_0.96 and Cr₂B) in an α matrix for the HVOFsprayed Armacor M coating. PTA weld-surfaced Armacor M coating contains needle-type long precipitates of Cr₂B) and Cr_1.65F_0.35B_0.96, in the α matrix. The hardness of the HVOF-sprayed Armacor 16 coating after heat treatment was substantially less than that of the as-sprayed coating due to the phase transformation from α to γ phase. Heat treatments of the HVOF-sprayed Armacor M coating did not produce changes in phase and its hardness decreased as compared to that of the as-sprayed coating. While HVOF-sprayed and PTA weld-surfaced Armacor M coatings have the same hardness, the latter shows better abrasive wear resistance because of the size and orientation of its boride phases. The broadening of the XRD patterns and the increase in hardness after wear testing suggest that the transformation from the crystalline to the amorphous structure occurred on the uppermost layer during wear testing.     

Superior performance of high-velocity oxyfuel-sprayed nanostructured TiO<Subscript>2</Subscript> in comparison to air plasma-sprayed conventional Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-13TiO<Subscript>2</Subscript>

Air plasma-sprayed conventional alumina-titania (Al₂O₃-13wt.%TiO₂) coatings have been used for many years in the thermal spray industry for antiwear applications, mainly in the paper, printing, and textile industries. This work proposes an alternative to the traditional air plasma spraying of conventional aluminatitania by high-velocity oxyfuel (HVOF) spraying of nanostructured titania (TiO₂). The microstructure, porosity, hardness (HV 300 g), crack propagation resistance, abrasion behavior (ASTM G65), and wear scar characteristics of these two types of coatings were analyzed and compared. The HVOF-sprayed nanostructured titania coating is nearly pore-free and exhibits higher wear resistance when compared with the air plasma-sprayed conventional alumina-titania coating. The nanozones in the nanostructured coating act as crack arresters, enhancing its toughness. By comparing the wear scar of both coatings (via SEM, stereoscope microscopy, and roughness measurements), it is observed that the wear scar of the HVOF-sprayed nanostructured titania is very smooth, indicating plastic deformation characteristics, whereas the wear scar of the air plasma-sprayed alumina-titania coating is very rough and fractured. This is considered to be an indication of a superior machinability of the nanostructured coating.     

Study of structure and corrosion resistance of Fe-based amorphous coatings prepared by HVAF and HVOF

Fe-based amorphous coatings with a composition of Fe_49.7Cr₁₈Mn_1.9Mo_7.4W_1.6B_15.2C_3.8Si_2.4 have been prepared on a mild steel substrate by High velocity air fuel (HVAF) and High velocity oxygen fuel (HVOF) processes. The microstructure and corrosion resistance in 3.5 wt.% NaCl solution of the coatings prepared by the two processes were comparatively studied. It was found that the two coatings exhibit dense structure with the porosity of 0.4% and compact bonding with the substrate. However, HVOF coating contains higher oxygen content than HVAF coating, resulting from the formation of significant oxide contours between the partially melted particles in HVOF process. Electrochemical polarization tests and electrochemical impedance spectroscopy (EIS) analysis indicate that the HVAF coating has better corrosion resistance than the HVOF coating. The preferential corrosion along the oxide contours thus providing efficient diffusion channels for electrolyte accounts for the poor corrosion resistance in HVOF coating. The present results demonstrate that HVAF with less cost can be a promising spray process to fabricate the Fe-based amorphous coating for industrial applications.     

温度对WC-WB-CoCr涂层高温摩擦磨损性能的影响

采用超音速火焰喷涂(HVOF)制备了WC-WB-CoCr涂层,研究了温度对WC-WB-CoCr涂层高温摩擦磨损性能的影响。通过SEM、XRD和显微硬度仪对涂层的微观组织、相结构和力学性能进行表征。通过摩擦磨损试验机和拉曼光谱仪研究了WC-WB-CoCr涂层的高温摩擦学性能和氧化产物,采用台阶仪扫描磨痕形貌并计算WC-WB-CoCr涂层的磨损率。结果表明：WC-WB-Co-Cr涂层主要由WC和CoW₂B₂组成,涂层结构致密,与基体结合紧密;随着磨损试验温度升高,涂层的摩擦系数从0.66降低到0.57,涂层的磨损率随着温度的升高而升高,但是其磨损率增长程度随着温度的升高而降低。在高温磨损过程中,磨痕表面的氧化膜主要由WO₃和CoWO₄组成,且CoWO₄比WO₃表现出更好的耐高温磨损性能。涂层的主要磨损机制为氧化磨损、疲劳磨损和粘着磨损。     

Fabrication and characterization of Al2O3-metal composite coating on steel plate with nonpressure combustion synthesis

Al2O3-metal composite coatings with different reactants and diluents were fabricated on mild steel plate with nonpressure combustion synthesis process. The coat-ings were characterized by means of X-ray diffraction, scanning electron microscopy, and energy-dispersive spec-trometry, respectively. Thermal shock tests were carried out to determine the bond strength of the coating with the steel substrate. The results indicate that the coating is composed of α-A1203, α-（Fe-Cr） and Al2SiO5 as the main phases. It is found that the coating with the diluents of Al2O3-SiO2 and transition layer of Al2O3-Cr presents the hi.ghest hardness of 2270 HV0.2 and the lowest porosity of 3.93 %. Owing to a metallurgical bond of the coating-to-substrate, the coating exhibits a good thermal shock resistance.     

Oxidation behaviour of CoNiCrAlY bond coats produced by plasma, HVOF and cold gas dynamic spraying

This paper examines and compares the microstructure and oxidation behaviour of CoNiCrAlY coatings manufactured by the APS, HVOF and CGDS deposition techniques. The coatings microstructural features were characterized by means of SEM and XRD analyses. Coating samples were then subjected to isothermal heat treatments at 1000 °C. Oxide growth rates were obtained from a series of mass gain measurements while oxide scale compositions were determined from SEM, XRD and EDS analyses. Results obtained in this study show that the as-sprayed CGDS and HVOF coatings exhibit similar microstructures, whereas the APS coating features high levels of visible defects and oxide content. Oxidation experiments revealed low oxide growth rates for both the CGDS and HVOF coatings as a result of low porosity and oxide content. The oxide scale on the CGDS and HVOF coatings after 100 h of oxidation were composed mainly of alumina without the presence of detrimental fast-growing mixed oxides. The presence of Cr₂O₃ and dispersed NiO was however also observed for the HVOF coating. As expected, the APS coating featured the onset of mixed oxides in the early stages of oxidation. From these results, it appears that potential improvements to the bond coat oxidation behaviour can be achieved using low-temperature processing methods such as CGDS.     

Residual stresses in HVOF-sprayed ceramic coatings

In this paper, the residual stress state of thermally sprayed ceramic coatings was examined by combining different experimental and analytical techniques, in order to provide a thorough characterisation of through-thickness stress profiles and a cross-verification of results. HVOF-sprayed ceramics, manufactured using commercial and nanostructured Al₂O₃ powders and commercial Cr₂O₃ powders, and atmospheric plasma-sprayed (APS) ceramics, manufactured using commercial Al₂O₃ and Cr₂O₃ powders, were investigated.The near-surface stress was measured by X-ray diffraction. The through-thickness profile and the intrinsic quenching stress were analytically computed by the Tsui-Clyne iterative model, using the X-ray measurement result as input, and results were validated by the substrate chemical removal method. Further verification was achieved by applying the in-situ curvature technique to the deposition of HVOF-sprayed Al₂O₃ coating.HVOF-sprayed Al₂O₃ coatings deposited using both conventional and nanostructured powders feature a similar, almost equibiaxial tensile stress on the top surface (116.5 MPa and 136.5 MPa, respectively) and a moderate through-thickness gradient (about 12 MPa and 20 MPa, respectively). Their intrinsic quenching stresses were analytically estimated to be 184 MPa and 205 MPa, respectively. APS Al₂O₃ possesses higher top surface stress (220 MPa) and quenching stress (311 MPa). However, it shows a less pronounced stress gradient (≈ 3 MPa) than HVOF-sprayed Al₂O₃-based coatings, because cracks, pores and weak lamella boundaries in the APS coating can accommodate the deformations induced by the bending moments arising both during coating deposition and during cooling.The model-derived quenching stress of the conventional HVOF Al₂O₃ coating was validated by the in-situ curvature measurement technique.Cr₂O₃-based coatings are significantly different. They display a lower residual stress in the near-surface region: 20 MPa in the APS coating, 27.5 MPa in the HVOF one. The HVOF coating also exhibits a very large stress gradient of ≈ 77 MPa. Machining and sliding processes (like polishing and dry sliding tribological testing) change their surface residual stresses to compressive ones.     

Short-term oxidation resistance and degradation of Cr2AlC coating on M38G superalloy at 900–1100 °C

High temperature oxidation behavior of the Cr₂AlC coating was investigated at 900–1100 °C. During the oxidation, a continuous Al₂O₃ scale formed, resulting in the improvement of the oxidation resistance of the substrate. Meanwhile, the oxidation induced depletion of Al within the Cr₂AlC coating resulted in the transformation of Cr₂AlC to Cr–C phases. Compared with bulk Cr₂AlC, the Cr₂AlC coating possessed similar oxidation behavior, but with higher oxidation rate. This is because a great number of columnar grain boundaries existed in the as-deposited coating, through which oxygen and nitrogen could diffuse inwardly, resulting in the internal oxidation and nitridation.     

Effect of Chemical Compositions and Surface Morphologies of MCrAlY Coating on Its Isothermal Oxidation Behavior

Chemical composition and surface morphology of MCrAlY coatings are factors which influence the oxidation behavior and the thermal durability of thermal barrier coatings. In this study, Cold-sprayed Ni20Cr10AlY and Ni23Co20Cr8.5Al4.0Ta0.6Y coatings with polished surfaces were employed to study the effect of composition on the oxidation behavior. The cold-sprayed MCrAlY coatings at the as-sprayed and shot-peened surface conditions, along with the low pressure plasma-sprayed MCrAlY coating with sputters adhered weakly on the surface, were employed to investigate the effects of surface morphologies of MCrAlY coatings on their oxidation behavior. Cold-sprayed Ni20Cr10AlY coating exhibited a two-stage oxidation behavior and a higher TGO growth rate than that of the cold-sprayed Ni23Co20Cr8.5Al4.0Ta0.6Y coating at the rapid growth stage. After 10-h oxidation, the TGO on the as-cold-sprayed coating surface was mainly constituted by Al₂O₃, while the TGO on the coating surface attached with sputters was composed of Al₂O₃ and Cr/Ni-oxides. After 500-h oxidation, Cr₂O₃ and porous spinel appeared in the TGO on the surface of the as-cold-sprayed coatings with different compositions. The growth of Cr/Ni-oxides was attributed to the Al depletion. The content of spinel decreased on the cold-sprayed NiCrAlY with a shot-peened surface compared with the as-sprayed coating.     

Evaluation of Cyclic Oxidation and Hot Corrosion Behavior of HVOF-Sprayed WC-Co/NiCrAlY Coating

Corrosion of metallic structural materials at an elevated temperature in complex multicomponent gas environments are potential problems in many fossil energy systems, especially those using coal as a feedstock. Combating these problems involves a number of approaches, one of which is the use of protective coatings. The high velocity oxy fuel (HVOF) process has been used to deposit WC-Co/NiCrAlY composite powder on two types of Fe-based alloys. Thermocyclic oxidation behavior of coated alloys was investigated in the static air as well as in molten salt (Na₂SO₄-60%V₂O₅) environment at 700 °C for 50 cycles. The thermogravimetric technique was used to approximate the kinetics of oxidation. WC-Co/NiCrAlY coatings showed a lower oxidation rate in comparison to uncoated alloys. The oxidation resistance of WC-Co/NiCrAlY coatings can be ascribed to the oxide layer of Al₂O₃ and Cr₂O₃ formed on the outermost surface. Coated alloys extend a protective oxide scale composed of oxides of Ni and Cr that are known to impart resistance to the hot corrosion in the molten salt environment.     

Effect of a sputtered TiAlCr coating on the oxidation resistance of TiAl intermetallic compound

The effect of a sputtered TiAlCr coating on the oxidation resistance of TiAl intermetallic compound was investigated in static air. The bare TiAl alloy exhibited poor isothermal and cyclic-oxidation resistance at 800–1000°C due to the formation of TiO₂-base scales which tend to spall during cooling. A sputtered Ti-50Al-10Cr coating remarkably improved the oxidation resistance of TiAl, due to the formation of an adherent Al₂O₃ scale at 800–1000°C. After long-term oxidation (at 900°C for 1000 hr), TiAlCr coating still provided excellent protection for the TiAl alloy. Minor interdiffusion occurred due to the inward diffusion of Cr, while no Kirkendall voids were found at the coating/ substrate interface. In contrast, NiCrAlY and CoCrAlY coatings reacted extensively with the TiAl alloys. Moreover, the TiAlCr coating alloy is based on -TiAl and TiAlCr Laves phases, which may offer improved mechanical properties. The TiAlCr coating exhibited a better combination of oxidation resistance and substrate compatibility than conventional aluminide and MCrAlY coatings.     

Wear and corrosion properties of plasma sprayed AI2O3 and Cr2O3 coatings sealed by aluminum phosphates

Plasma-sprayed aluminum oxide (AI₂O₃) and chromium oxide (Cr₂O₃) coatings were sealed by aluminum phosphates. Phosphates were formed throughout the coating, down to the substrate, and were verified by scanning electron microscopy and hardness measurements. The sealing increased the hardness of the coatings by 200 to 300 Vickers hardness (HV) units. Abrasion and erosion wear resistances were increased by the sealing treatment. Sealing also substantially closed the open porosity, as shown in electro-chemical corrosion tests. The sealed structures had good resistance against corrosion during 30 days of immersion in both acidic and alkaline solutions with pH values from 0 to 10. No decrease in abrasion wear resistance was observed after immersion.