Experimental validation of FE predicted fracture behaviour in thermally sprayed coatings

The fracture behaviour of a brittle coating on a substrate is not only governed by its intrinsic fracture strength, but also by a range of other parameters. In order to be able to understand (and thus predict) the fracture response, it is very important to accurately determine the key coating properties such as elastic modulus, residual stresses and fracture strength. In the present work, the fracture behaviour of three different high velocity oxy-fuel (HVOF) sprayed WC–17% Co coatings on Ti–6Al–4V substrates was studied using four-point bending with the acoustic emission technique. The key coating properties were determined experimentally and finite element (FE) models incorporating these experimentally measured properties were used to predict the cracking behaviour. It was found that the FE model was able to predict the differences in the fracture responses of the three coating types based upon differences in their mechanical properties, which in turn enabled the properties which dominate the fracture response to be identified. Moreover, the ability to predict the fracture behaviour of the coatings provided validation of the physical basis of the FE model.     

铜基体超音速火焰喷涂碳化物涂层界面研究

使用扫描电子显微镜及透射电子显微镜，对铜基体超音速火焰喷涂（HVOF）碳化物涂层的形成过程，组织结构和界面形态进行了研究。结果表明，HVOF涂层界面显微组织非常复杂，存在纳米晶，非晶和大量位错孢，非晶是粘结相在喷涂中受热，骤冷形成的，而涂层中的纳米晶来自半熔碳化物的破碎。     

Formation of an amorphous phase in thermally sprayed WC-Co

A WC-Co coating was sprayed by the high-velocity oxyfuel process using a feedstock of tungsten carbide clad with cobalt. The structure of the sprayed coating was characterized by x-ray diffraction (XRD), differential scanning calorimetry (DSC), and differential thermal analysis (DTA). It was found that an amorphous phase of Co-W-C ternary alloy observed as a large, broad peak in the XRD pattern can be formed in the as-sprayed WC-Co coating. The DSC, DTA, and XRD analyses revealed that the amorphous phase crystallized at a temperature of around 873 K to metallic cobalt, Co₆W₆C, and tungsten with appreciable precipitation of free carbon. The heat treatment of as-sprayed WC-Co coating at a high temperature of 1173 K suggests that annealing at a temperature higher than about 1104 K will promote the reaction of tungsten and cobalt with carbon to form the complex carbide C₀₆W₆C.     

The influence of gun transverse speed on electrochemical behaviour of thermally sprayed Cr3C2-NiCr coatings in 0.5 M H2SO4 solution

In the present study, different types of 75% Cr₃C₂-25% NiCr coatings were applied on a steel substrate by means of high velocity oxygen fuel spraying (HVOF), and studied using ac and dc electrochemical measurements in an aerated and unstirred 0.5 M H₂SO₄ solution. Structural characterization was determined before and after electrochemical tests. Differences between all sprayed systems are related to the gun transverse speed and number of deposited layers, which strongly affected the electrochemical characteristics of the coated steels. The coating obtained with a higher torch speed showed better resistance against corrosion. The electrochemical impedance results were analyzed using an equivalent circuit where porosity of the coatings and substrate oxidation were considered.     

AP40生物活性玻璃陶瓷HVOF喷涂层的结构和性能

采用高速火焰(HVOF)喷涂技术,在Ti6Al4v基体上制备了AP40玻璃陶瓷涂层.利用光学显微镜、SEM和XRD对喷涂层形貌、显微组织结构和相组成进行了研究.探讨了热处理工艺对涂层组织结构及其性能的影响.按德国DIN EN 582—1994标准进行涂层的拉伸强度试验.结果表明:HVOF喷涂AP40玻璃陶瓷涂层具有层状结构,含有少量孔隙和未熔化的颗粒;涂层主要由羟基磷灰石、氟磷灰石、硅灰石、方石英及部分玻璃相组成,喷涂工艺对涂层的孔隙率有较大的影响;合适的热处理工艺可提高涂层的结晶度,使涂层变得致密,使孔隙明显减少,使涂层结合强度得到明显提高.     

On the gas dynamics of HVOF thermal sprays

An experimental study of the gas-dynamic aspects of the high-velocity oxyfuel (HVOF) thermal spray process has been performed using commercially available HVOF equipment (Hobart-Tafa JP-5000, Ho-bart-Tafa Technologies, Inc., Concord, NH). Optical diagnostic techniques, including microsecond-expo-sure schlieren and shadowgraph imaging, were applied to visualize the hot supersonic jet produced by this equipment without particle injection. Rapid turbulent mixing of the jet with the surrounding atmos-phere was observed, which is an issue of concern in coating quality due to the possibility of oxidation of sprayed particles. This mixing appears to be a function of the ratio of densities of the hot jet and the cold atmosphere as well as a function of the velocity of the jet, rather than one of combustion-chamber pres-sure or barrel length. The supersonic core of the HVOF jet dissipates rapidly due to the mixing, so that the jet is no longer supersonic when it impinges on the target surface being sprayed. Secondary issues also observed in this study include strong jet-noise radiation from the HVOF plume and the entrainment and induced bulk motion of the surrounding air.     

High-velocity oxyfuel thermal spray coatings for biomedical applications

Plasma spraying is used to produce most commercially available bioceramic coatings for dental implants; however, these coatings still contain some inadequacies. Two types of coatings produced by the high- velocity oxyfuel (HVOF) combustion spray process using commercially available hydroxyapatite (HA) and fluorapatite (FA) powders sprayed onto titanium were characterized to determine whether this relatively new coating process can be applied to bioceramic coatings. Diffuse reflectance Fourier transform infrared (FTIR) spectroscopy, x- ray diffraction (XRD), and scanning electron microscopy (SEM) were used to characterize the composition, microstructure, and morphology of the coatings. The XRD and FTIR techniques revealed an apatitic structure for both HA and FA coatings. However, XRD patterns indicated some loss in crystallinity of the coatings due to the spraying process. Results from FTIR showed a loss in the intensity of the OH^∼ and F^∼ groups due to HVOF spraying; the phosphate groups, however, were still present. Analysis by SEM showed a coating morphology similar to that obtained with plasma spraying, with complete coverage of the titanium substrate. Interfacial SEM studies revealed an excellent coating-to-substrate apposition. These results indicate that with further optimization the HVOF thermal spray process may offer another method for producing bioceramic coatings.     

Resistance of Mo-bearing stainless steels and Mo-bearing stainless-steel coating to naphthenic acid corrosion and erosion-corrosion

Naphthenic acid corrosion (NAC) and erosion-corrosion (NAEC) behavior of Mo-bearing (0-7.0 wt.%) stainless steels have been investigated in laboratory to evaluate the essential role of Mo on their NAC and NAEC resistance. The NAC and NAEC resistance of a high-velocity-oxygen-fuel (HVOF) thermal sprayed Mo-bearing stainless-steel coating was also investigated in both laboratory and an oil refinery. It was found that increasing the Mo content remarkably enhanced the NAC and NAEC resistance of stainless steels. The Mo-rich areas or phases in the steels played an importance role in resisting the NAEC. The HVOF coating showed excellent NAC and NAEC resistance in laboratory tests. The 700-day field test in an oil refinery revealed that the coating can effectively prevent the carbon-steel substrate from erosion-corrosion during long-term exposure in oil-refining environment. The corresponding NAC and NAEC mechanisms of the stainless steels and the beneficial role of Mo were discussed by taking account of inherent susceptibility of metal elements to corrosion, possible effects of Mo on surface films, and Mo-induced change in microstructure and microhardness.     

超音速火焰喷涂涂层抗高温氧化和耐冲蚀性能

采用超音速火焰喷涂（HVOF）技术喷涂WC涂层，并对其抗高温氧化和耐冲蚀性能进行测定。试验结果显示，与基体1Cr12W1MoV比较，HVOF制备的WC-17Co、WC-12Co、NiCrBSi＋35WC涂层具有非常良好的抗高温氧化和耐冲蚀性能。其中WC-17Co涂层在任何冲蚀角度下均表现出优良的抗冲蚀能力，是一种理想的汽轮机高压部件防护涂层。     

Corrosion behavior of HVOF coated sheets

High velocity oxygen-fuel (HVOF) thermal spray coating finds application in industry due to its superior resistance to corrosion and thermal loading. In the HVOF process, the metallic powders at elevated temperature are sprayed at supersonic speed onto a substrate material. The powder granules sprayed impact onto each other, forming a mechanical bonding across the coating layer. In most of the cases, the distances among the particles (powder granules sprayed) are not the same, which in turn results in inhomogeneous structure across the coating layer. Moreover, the rate of oxidation of the powder granules during the spraying process varies. Consequently, the electrochemical response of the coating layer surfaces next to the base material and free to atmosphere differs. In the current study, the electrochemical response of a coating sheet formed during HVOF thermal spraying was investigated. NiCrMoNb alloy (similar to Inconel 625) wass used for the powder granules. Thermal spraying was carried out onto a smooth surface of stainless steel workpiece (without grid blasting), and later the coating layer was removed from the surface to obtain the coating sheet for the electrochemical tests. It was found that the corrosion rate of the smooth surface (surface next to the stainless steel surface before its removal) is considerably larger than that corresponding to the rough surface (free surface) of the coating sheet, and no specific patterns were observed for the pit sites.