Revealing true porosity in WC-Co thermal spray coatings

The principles underlying composite material behavior during metallographic preparation of coating cross-sections are generally not well understood. This study of the effect of extended fine polishing on apparent porosity shows that adequate polishing times, using a fine abrasive (3 μm) and low force, are required to remove prior deformation in the section surface and to reveal the true porosity of the underlying composite material. Insufficient polishing times can result in considerable underestimation of porosity. A model is described which proposes that the deformation induced in the material during grinding and polishing, even at low applied force, results in smearing of material into voids that exist in the plane of the section.     

Wear and erosion behavior of plasma-sprayed WC-Co coatings

Wear mechanisms of air plasma-sprayed WC-12%Co coatings were studied by using a dry sand rubber wheel (DSRW) abrasive, ring-on-square adhesive wear, and alumina particle erosion tests. Coating properties such as intersplat cohesive strength, porosity, surface roughness, hardness, and retained carbide as well as microstructures were characterized to assess their relationship on wear performance. Porosity, hardness, surface roughness, and retained carbide of the coatings are not the principal factors affecting wear performance. Intersplat cohesive strength of coatings, measured by a simple bonding test, is the most significant factor that relates to the wear rate of thermal spray coatings.     

Parameter study of HP/HVOF deposited WC-Co coatings

The deposition parameters of WC-17% Co coatings produced using the JP-5000 liquid-fuel HP/HVOF system (Eutectic TAFA) were investigated with the initial purpose of parameter improvement and optimization. The coating microstructures, porosities, phase compositions, and abrasion resistance were characterized. Preliminary work using the Taguchi statistical experimental design method aimed at optimizing the spray parameters in terms of the microstructure and phase composition was unsuccessful. The variations in the measured properties were too small to be correlated with the spray parameters. Subsequent experiments showed this was primarily due to the fact that the properties, particularly the abrasion resistance, of the WC-Co coatings were not primarily influenced by variations in the spray parameters, but were more dependent on the powder composition, particle size range, and manufacturing route. Hence, the application of Taguchi techniques would have been more effective over a much wider parameter space than was originally used. This result is valuable because it suggests that this process is robust and can be used for WC-Co coatings without large investments in spray parameter optimization and control once the coating and powder type have been fixed.     

Sliding wear evaluation of hot isostatically pressed thermal spray ceramet coatings

The principal aim of this study was to compare the sliding wear performance of as-sprayed and Hot Isostatically Pressed (HIPed) thermal spray cermet (WC-12Co) coatings. Results indicate that HIPing technique can be successfully applied to post-treat thermal spray cermet coatings for improved sliding wear performance, not only in terms of coating wear, but also in terms of the total volume loss for test couples. WC-12Co coatings sprayed by a HVOF system were deposited on SUJ-2 bearing steel substrate and then encapsulated and HIPed at 850 °C for one hour. A high frequency reciprocating ball on plate rig was used to measure the sliding wear resistance of these coatings in dry conditions under steel and ceramic contact configurations at two different loads. Results are discussed in terms of coating microstructure, microhardness, fracture toughness and residual stress evaluations. Microstructural investigations indicate fundamental changes in grain morphology, whereas x-ray diffraction revealed beneficial transformations in phase composition of these coatings during the HIPing post treatment. The effects of these microstructural changes on the physical properties and wear resistance are discussed.     

X-ray diffraction measurement of residual stress in WC-Co thermally sprayed coatings onto metal substrates

Investigation of the residual stresses and microstructural properties associated with HVOF thermal spray coating of WC-17 wt% Co of same thickness on three substrates with coefficients of thermal expansion different to that of WC. The residual stresses were measured by X-ray diffraction sin²ψ techniques using CoKα radiation. The results indicated residual stresses that have different natures for the as-sprayed coatings despite using the same powder as feedstock. The magnitudes of the stresses in the as-sprayed condition are low.     

Characterization of thermal sprayed nanostructured WC-Co coatings derived from nanocrystalline WC-18wt.%Co powders

Nanostructured WC-Co coatings were synthesized using high velocity oxygen fuel (HVOF) thermal spray. The nanocrystalline feedstock powder with a nominal composition of WC-18 wt.%Co was prepared using the novel integrated mechanical and thermal activation (IMTA) process. The effects of HVOF thermal spray conditions and powder characteristics on the microstructure and mechanical properties of the as-sprayed WC-Co coatings were studied. It was found that the ratio of oxygen-to-hydrogen flow rate (ROHFR) and the starting powder microstructures had strong effects on decarburization of the nano-coatings. Decarburization was significantly suppressed at low ROHFR and with the presence of free carbon in the powder. The level of porosity in the coatings was correlated with the powder microstructure and spray process conditions. The coating sprayed at ROHFR=0.5 exhibited the highest microhardness value (HV_300g=1077), which is comparable to that of conventional coarse-grained coatings.     

NiCoCrAlYTa/WC-Co复合涂层的制备及摩擦学性能研究

目的 为了改善MCrAlY涂层的耐磨损性能,通过在NiCoCrAlYTa粉末中添加不同比例的硬质相WC-Co粉末（质量分数为25%、50%、75%）,将2种粉末充分地机械混合、振荡均匀后,采用超音速火焰喷涂（HVOF）技术,制备不同配比的NiCoCrAlYTa/WC-Co复合涂层。方法 利用SEM、XRD、EDS等分析了复合涂层的微观形貌、物相组成和元素分布规律等;研究该复合涂层的力学性能、摩擦学性能以及摩擦磨损机理等。结果 采用HVOF技术制备的Ni CoCrAlYTa/WC-Co复合涂层结构致密,各元素及物相分布均匀;硬质相WC-Co的添加提高了涂层的显微硬度,同时也可显著改善复合涂层的耐磨损性能;复合涂层的摩擦因数随着WC-Co含量的增加逐渐增大,而磨损率逐渐减小。当WC-Co的添加量为75%时,复合涂层的摩擦因数最大,约为0.84;磨损率最小,约为9.28×10^-6 mm³/（N·m）。结论 在金属基涂层中引入硬质相WC-Co可有效提高涂层的硬度,并且提升该涂层的耐磨损性能,为金属基涂层发挥优异的摩擦学性能提供理论基础。     

Mechanisms of fatigue failure in thermal spray coatings

The aim of this experimental study was to ascertain the fatigue failure modes of thermal spray coatings in rolling/sliding contact. These failure modes outline the design requirements of thermal spray coatings for high-stress tribological applications including impact and point or line contact loading. Recently, a number of scientific studies have addressed the fatigue performance and durability of thermal spray coatings in rolling/sliding contact, but investigations on the mechanisms of these failures are seldom reported. The understanding of such failure mechanisms is, however, critical in optimizing the generic design of these overlay coatings. This study takes a holistic approach to summarize the results of ongoing research on various cermet (WC-Co) and ceramic (Al₂O₃) coatings deposited by detonation gun (D-Gun), high-velocity oxyfuel (HVOF), and high-velocity plasma spraying (HVPS) techniques, in a range of coating thickness (20–250 μm) on various steel substrates to deliver an overview of the various competing failure modes. Results indicate four distinct modes of fatigue failure in thermal spray cermet and ceramic coatings: abrasion, delamination, bulk failure, and spalling. The influences of coating process, thickness, materials, properties of substrate materials, and prespray conditions on these fatigue failure modes are also discussed. A modified four-ball machine was used to investigate these failure modes under various tribological conditions of contact stress and lubrication regimes in conventional steel and hybrid ceramic contact configurations. Results are discussed in terms of pre- and post-test surface examination of rolling elements using scanning electron microscopy (SEM), electron probe microscopy analysis (EPMA), and surface interferometry, as well as subsurface observations using x-ray diffraction (XRD), residual stress analysis, and dye-penetrant investigations.     

Effects of carbide size and Co content on the microstructure and mechanical properties of HVOF-sprayed WC-Co coatings

Twelve commercially available WC-Co powders with different average WC grain sizes (0.2, 2, and 6 μm) and cobalt contents (8, 12, 17 and 25 wt.%) were sprayed on carbon steel substrates using High Velocity Oxy-Fuel (HVOF) spraying process. Hardness, Young's modulus, and fracture toughness of the coatings were measured. While the hardness and Young's modulus decreased with increasing cobalt content from 1600 to 1100 Hv and from 400 to 300 GPa respectively, the fracture toughness remained in the range from 4 to 6 MPam^1/2. The coatings with 2 μm carbide showed lower hardness than those deposited from 0.2 and 6 μm carbide. These measured mechanical properties were discussed with the help of microstructures of the coatings investigated by scanning electron microscopy, X-ray diffraction and chemical analysis. Finally, the hardness of the binder phase in these coatings was estimated to range from 1000 to 1300 Hv by applying the mixture rule for composites to the experimental data, demonstrating that such hardening of the binder phase is a key factor affecting the mechanical properties of the coatings.     

Modeling thermal conductivity of thermal spray coatings: comparing predictions to experiments

Thermal conductivity plays a critical role in the thermal transport of thermal-sprayed coatings. In this article, a combined image analysis and finite-element method approach is developed to assess thermal conductivity from high-resolution scanning electron microscopy images of the coating microstructure. Images are analyzed with a collection of image-processing algorithms to reveal the microscopic coating morphology. The processed digital image is used to generate a two-dimensional finite-element mesh in which pores, cracks, and the bulk coating material are identified. The effective thermal conductivity is then simulated using a commercial finite-element code. Results are presented for three coating material systems [yttriastabilized zirconia (YSZ), molybdenum, and NiAl], and the results are found to be in good agreement with the experimental values obtained using the laser flash method. The YSZ coatings are also annealed, and the analysis procedure was repeated to determine whether the technique can accurately assess changes in coating morphology. This article was originally published inBuilding on 100 Years of Success: Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, 2006.     

Design of a separation device used in detonation gun spraying system and its effects on the performance of WC-Co coatings

During the detonation gun (D-gun) spraying process, the detonation products showed an undesirable dispersion when they were spurted out from the nozzle of the barrel at high velocity and pressure. To resolve this problem, a separation device was designed and constructed in the detonation gun spraying system. The WC-Co coatings were synthesized by the D-gun spraying system with and without using a separation device, respectively. The results showed that the use of the separation device resulted in better properties of the D-gun sprayed WC-Co coatings, e.g., lower the surface roughness, lower the porosity, higher the microhardness, higher the elastic modulus, and higher the interfacial adhesive strength. Also, the tribological performance of the WC-Co coatings was improved. The relationship of surface roughness, microhardness, elastic modulus, adhesive strength, and wear resistance of the WC-Co coatings with porosity was discussed. At the same time, there is an inevitable disadvantage for using the separation device, i.e., the relatively lower effective utility rate of the feedstock powder. Therefore, the separation device is suitable to be applied in occasions of high-performance requirements where increased costs are acceptable.     

复合添加晶粒长大抑制剂对WC-Co复合粉烧结硬质合金的影响

在超细WC-Co复合粉中复合添加不同含量的晶粒长大抑制剂VC和Cr3C2,利用X射线衍射仪分析粉末及硬质合金的物相组成,通过扫描电镜和透射电镜观察硬质合金的形貌,研究不同含量的晶粒长大抑制剂对硬质合金的显微组织和力学性能影响规律。结果表明:随着晶粒长大抑制剂的增加,晶粒异常长大现象消失,晶粒细小均匀;同时发现在WC-Co复合粉中VC的增加可以快速提高硬质合金的硬度,而适量的Cr3C2可以有效提高硬质合金的横向断裂强度。     

Ti- 6Al-4V/hydroxyapatite composite coatings prepared by thermal spray techniques

The poor mechanical properties of hydroxyapatite (HA) can be enhanced by forming a composite with a bioinert and mechanically strong metal alloy such as Ti-6A1-4V. Biomedical composites composed of titanium alloys and HA can offer concomitant bioactive properties as well as good mechanical strength and toughness. This paper describes an attempt to improve coating mechanical properties by forming a composite composed of HA and Ti-6A1-4V. Several compositions (20, 33, and 80 wt % HA) were prepared. Subsequent examination of the plasma-sprayed coatings revealed alternating HA-rich and titanium-rich lamella microstructures. The HA-rich regions appeared porous as a result of poor interparticle adhesion, with the 80 wt% HA coatings having the highest porosity. Mechanical property analysis showed the 20 wt% HA coating to have the highest storage modulus (∼60 GPa). This coating also had the highest bond strength (≥20 MPa max). The coatings tended to exhibit increased bond strength at thicknesses less than or equal to 60 μm. The excellent bond strength of the Ti-6A1-4V/HA composite is caused by the superior interfacial bond between the Ti-6Al-4V-rich splats and the substrate. The encouraging development of this composite raises the possibility of its use as a bond coat for plasma-sprayed HA on titanium-alloy implants.     

Underwater plasma processing of stabilized zirconia for thermal barrier coatings

The influence of powder technology on the spraying process is growing. The use of powders of different morphologies results in a variety of coating properties. Plasma processing of ceramic composite powders underwater produces dense and spherical powders with excellent morphologies. Coatings of improved quality are produced by spraying these powders. This paper describes the concept of producing zirconia powders by a plasma process that is performed underwater. The most important parameters and standards are highlighted. Moreover, the influence of powder characteristics on coating properties will be described and some results presented.     

Measurement of imperfections in thermal spray coatings using synchrotron-computed microtomography

The determination of microstructural imperfections in thermal spray coatings using nondestructive synchrotron-computed microtomography (CMT) is described. This technique provides spatial resolutions in the 2-to 4-ώm range, enabling identification of cracks and voids. Comparisons with other conventional methods for identifying microstructural features in coatings are made. Work supported by the US Department of Energy under Contract No. DE-AC02-76CH00016.     

Combined pre-annealing and pre-oxidation treatment for the processing of thermal barrier coatings on NiCoCrAlY bond coatings

A combined pre-annealing and pre-oxidation treatment was developed for the processing of partially yttria stabilized (PYSZ) thermal barrier coatings (TBC) on top of NiCoCrAlY bond coatings (BC). To develop this pre-treatment, the influence of the oxygen potential during pre-annealing and pre-oxidation on the life span and failure mechanisms of the entire high temperature coating system upon thermal cycling was investigated. The results of this study showed that the service life of the coating system depended strongly on the composition and microstructure of the thermally grown oxide (TGO) after pre-oxidation. The longer life spans were obtained if the TGO thickened very slowly during thermal cycling due to a large α-Al₂O₃ grain size. Such a slow-growing TGO corresponded with a pre-treatment for which θ-Al₂O₃ was formed during pre-oxidation and for which the yttrium was located within a high density of pegs along the TGO/BC interface after pre-oxidation. If the yttrium was present on top of the TGO after pre-oxidation, a thick mixed alumina-zirconia layer formed upon thermal cycling. This mixed oxide layer contributed significantly to the total oxide layer thickness, resulting in short life spans. The formation of NiAl₂O₄ spinel in between the TBC and the α-Al₂O₃ should be avoided, since this can lead to premature failure along the spinel/α-Al₂O₃ interface.     

Investigation of splat curling up in thermal spray coatings

The curling up of the edges of splats of molten metal deposited on a cold substrate was investigated both experimentally and numerically. An analytical model, based on mismatch of thermal expansion between the splat and substrate, was developed to calculate the deformation of splats after curling up. The curling-up angle was measured from both millimeter-sized splats of aluminum alloy and bismuth and plasma-sprayed nickel particles. The curling-up angles were predicted using both the analytical model and a numerical code and were found to agree reasonably well with experimental measurements. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J.Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Finely grained nanometric and submicrometric coatings by thermal spraying: A review

Technology, microstructure and properties of nanostructured coatings obtained using different feedstock including: (i) powders composed of agglomerated nanocrystals; (ii) solutions; and (iii) suspensions are discussed. The methods of nanostructured coarse powders manufacturing are reviewed together with the problems related to formulation of solutions and suspensions. A particular attention is paid to the key problem at liquid feedstock spraying, namely to their delivery and injection into jets or flames. The physical and chemical phenomena occurring at flight of injected liquid droplet of solution and suspension are shown and related to the formation of coatings and their microstructure. Some microstructural, chemical, mechanical and electrical properties of coatings are collected and related to the operational processing parameters by regression equations derived from the design of spray experiments. Finally, the possible applications of nanostructured coatings are briefly discussed.     

爆炸喷涂WC-Co/CoMoCrSi复合涂层的组织与性能

为了提高CoMoCrSi涂层的结合强度和力学性能,采用爆炸喷涂技术制备了WC-12Co涂层作为过渡层的WC-Co/CoMoCrSi复合涂层,借助SEM和EDS等手段分析了涂层截面组织形貌和化学元素组成,采用显微硬度计、万能拉伸机及销盘式摩擦磨损试验机等研究了涂层的力学及摩擦磨损性能。结果表明：在氧燃充枪比为60%的喷涂参数下,制备WC-Co/CoMoCrSi复合涂层平均结合强度高达66 MPa,涂层截面组织致密、均匀,孔隙率小于0.6%,平均显微硬度为667 HV0.1,复合涂层摩擦因数0.53~0.56,具有优异的耐磨性能。     

A new approach to online thickness measurement of thermal spray coatings

In the past 10 years, significant progress has been made in the field of advanced sensors for particle and spray plume characterization. However, there are very few commercially available technologies for the online characterization of the as-deposited coatings. In particular, coating thickness is one of the most important parameters to monitor and control. Current methods such as destructive tests or direct mechanical measurements can cause significant production downtime. This article presents a novel approach that enables online, real-time, and noncontact measurement of individual spray pass thickness during deposition. Micron-level resolution was achieved on various coatings and substrate materials. The precision has been shown to be independent of surface roughness or thermal expansion. Results obtained on typical high-velocity oxyfuel and plasma-sprayed coatings are presented. Finally, current fields of application, technical limitations, and future developments are discussed. This article was originally published inBuilding on 100 Years of Success: Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.