Mechanofused powders for thermal spray

Mechanofusion is a novel powder processing technique, in which raw powdered materials are fed into the apparatus, and through mechanical-thermal interactions, the powder characteristics are changed, e.g., shape, size, and composition distribution. Mechanofusion has been used in this study to produce intermetallic nickel aluminide compounds and composite powders as feedstock for vacuum plasma spraying. Significant effects of the mechanofusion production route on the powder characteristics and spray deposition features have been observed.     

Suspension and solution thermal spray coatings

The emerging methods of coating deposition by suspension and solution thermal spraying are described. The liquid suspensions of fine powders and liquid precursors are injected into flames and/or jets generated in the torches. The formulation and stability of suspensions as well as the methods of fine powders synthesis are briefly described. Typical solutions, being often the liquid organo-metallics are also briefly described. An important problem of injection of liquids into jets and flames is then presented. Two principal modes of injection, used at present, are outlined, i.e.: (i) atomization; and, (ii) injection of a continuous jet. Subsequently, the phenomena occurring in flames and plasma jets are discussed and the major differences to these occurring during conventional spraying are stressed up. The build up of coatings starting from the impact of fine particles on the substrate is described and typical microstructures of suspension and solution sprayed coatings are shown. Some properties of the sprayed coatings, including mechanical, electrical, chemical, and thermophysical ones are collected and presented. Finally, the emerging applications of coatings are shown and the possible future applications are discussed.     

Nanostructures in thermal spray coatings

The nature of the nanograins formed by high velocity oxy-fuel thermal spraying of (FeAl) milled powder has been investigated using transmission electron microscopy on cross-sectional thin foils. Equiaxed 3D nanometer crystallites are formed by recrystallization in the unmelted powder particles while 2D nanometer columnar grains are produced by rapid solidification within the fully molten splats.     

The thermal plasma processing of fine powders

Interest in the thermal plasma processing of fine powders has increased in recent years. Its potential for the production of advanced materials is highly recognized. This paper discusses research carried out in the plasma-processing laboratory at the University of Alabama to produce fine powders using transferred and non-transferred arc plasma systems. Fine powders of metal, ceramics, and composites were synthesized using plasma technology. For more information, contact Ramana G. Reddy, University of Alabama, Department of Metallurgical and Materials Engineering, Tuscaloosa, AL 35487-0202.     

热喷涂涂层的钎焊工艺

使用钎焊方法对热喷涂H08低碳钢涂层、4Cr13马氏体不锈钢涂层与20钢进行连接,研究了这些热喷涂涂层的钎焊工艺性、接头的力学性能以及接头的金相组织形貌.试验结果表明,锡焊可以很好地连接2种低碳钢潦层或将低碳钢涂层与低碳钢基材连接起来;银焊不但可以连接低碳钢涂层.还可连接低碳钢基体与马氏体不锈钢涂层;2种钎焊接头的抗拉结合强度都高于涂层层间结合强度.     

Turning of thick thermal spray coatings

This technical note looks at several attempts to machine parts coated with Ni5Al thermal spray. This coating is used in the overhaul and repair of gas turbine components. Machining the thermal sprays to achieve the dimensional tolerances and surface finish is needed. Turning tests were performed with small carbide inserts and with CBN. A study was made of tool performance and cutting process. In this way, tool life, wear mechanism, chip formation process, and actual roughness of turned parts were analyzed. In addition to the good performance of CBN inserts, some disadvantages of using coolant with CBN tools were detected and analyzed.     

Characterization of thermal sprayed hydroxyapatite powders and coatings

Calcium phosphate materials such as hydroxyapatite (HA) have biocompatible properties that can promote osteogenesis or new bone formation. Thermal spraying is an economical and effective process for coating the hydroxyapatite onto metal. It has been reported that plasma spraying changes the degree of crystallinity as well as the phase composition of the HA. This article reports the preparation and characterization of HA powders and coatings by two thermal spray processes (plasma and combustion flame) and suggests that the state of the starting powder adversely affects the coating characteristics. The raw HA powders are synthesized through a chemical reaction involving calcium hydroxide and orthophosphoric acid. Phase analysis using an X- ray diffractometer revealed that the synthesized powder consists of predominantly the HA phase. Calcined and crushed HA powders of various size ranges were fed into the plasma jet to produce HA coatings on metallic substrates. In addition, some HA powders were sprayed into distilled water by plasma spraying and combustion flame spraying to study powder melting characteristics. Other samples were plasma sprayed onto a solid rotating target to study atomization and impact behavior. The morphology of the rapidly solidified powders and thermal sprayed coatings were examined by scanning electron microscopy (SEM). An X- ray sedimentation particle size analyzer, laser diffraction particle size analyzer, and image analyzer performed the particle size analysis. Preliminary results indicate that particle cohesion, size range, and thermal treatment in the plasma affect the phase and structure of the as- sprayed coating, and some post- spray treatment may be necessary to produce a dense and adherent coating with the desired biocompatible properties.     

Post-treatment of thermal spray coatings on magnesium

Magnesium alloys have a beneficial combination of high strength to weight ratio, good machinability and high recycling potential. Despite this, the application of magnesium still is behind that of other constructive materials mainly due to low wear and corrosion resistance. For more demanding applications, a large amount of surface treatment methods are developed to overcome this problem. Thermal spraying is an efficient and flexible method of coating deposition and is widely used for protection of different materials against corrosion and wear. Nevertheless, the bonding of thermal spray coatings on magnesium alloys is not sufficient, so the following post-treatment processes are needed. One of such possibilities is high energy beam treatment of thermally sprayed coatings. During the heat treatment of magnesium substrates with coating the remelting of coating and a thin surface layer of substrate occurs. Depending on the combination of applied coating system and treatment method, different processes can be realised in modified layers: the alloying of magnesium substrate with other elements to improve corrosion properties, redistribution of hard particles from composite coating and new phases formation during the processing to improve the wear resistance of magnesium alloys. In the present work some examples concerning the laser and electron beam treatment of aluminium based composite coatings as well as infra red irradiation of zinc based coatings are described. Coatings are deposited on magnesium substrates (AM20, AZ31, AZ91) by arc spraying with Zn, ZnAl4 and ZnAl15 solid wires and cored wires in aluminium core with powder filling containing different hard particles, such as boron, silicon and tungsten carbide or titanium oxide. Remelting of thermal spray coatings is carried out by means of continuous irradiation of СО₂-laser in nitrogen or argon atmosphere, electron beam in vacuum and focused tungsten halogen lamp line heater in atmosphere. Microstructure of sprayed coatings as well as that of modified surface layers is investigated by metallographic methods. Corrosion properties are estimated by electrochemical measurements. Abrasion wear resistance of the modified layers is determined by scratch test, corundum grinding disk test and Rubber wheel test. It is shown that all methods applied for processing of thermal spray coatings lead to formation of modified surface layers in magnesium substrate with improved wear and corrosion properties. Different mechanisms of microstructure formation such as redistribution of chemical composition of composite coating components, partial remelting of hard phase particles, and new phases formation are discussed. Electrochemical behaviour of modified surface layers is mostly improved due to alloying, homogenization of element distribution and strong decrease of as-sprayed coating porosity. Abrasion wear resistance of processed magnesium substrates strongly depends on the microstructure and usually is 5 to 20 times higher compared with base material.     

Sealing of thermal spray coatings by impregnation

Results from the sealing of porosity by impregnation show that below a certain wetting angle of the sealant, high penetration depths are achieved. However, only sealants with very low curing shrinkages can prevent the transport of electrolyte through the coating. Various sealant types and impregnation methods are discussed, and factors influencing impregnation and sealing ability of sealants are reviewed. Experimental results from the sealing of plasma-sprayed aluminum-oxide coatings are presented.     

Peel adhesion test for thermal spray coatings

A technique for determining the adhesion of a thermal spray coating was developed by modifying procedures commonly used to test adhesion by peeling. A coating is deposited on a metal foil that has been soldered to a massive copper block, which provides mechanical support and serves as a heat sink. Then the block, foil, and coating are glued to a stiff aluminum plate, after which the copper block is removed. The foil is peeled from the coating according to a procedure similar to the ASTM D 3167 peel test. This method causes a crack to propagate precisely along the coating/substrate interface in a stable fashion, with the movement of the crack tip controlled by the peeling speed. Sample preparation, test procedures, and in-itial results are discussed. The technique has been applied to testing the local variations in adhesion for plasma-sprayed Cr₂O₃ and a Ni-Mo-Al composite on a stainless steel foil. Based on these results, testing procedures are recommended and a peel test jig is specified.     

Surface roughness of thermal spray coatings made with off-normal spray angles

The formation of a thermal spray coating using an off-normal direction angle for the spray has been analyzed to identify the causes of the large surface roughness of the coating. In the analysis, the string method was used for modeling the formation of the coating. The method uses a string of equally spaced node points to define the shape of the coating surface and to track the change in this shape as the thermal spray mass is deposited. The method allows for the calculation of arbitrary shapes for the coating surface that may be very complex. The model simulates the stochastic deposition of a large number of thermal spray droplets. Experiments were carried out to obtain the data used in the model for the mass flux distribution on the target surface. The data show that when the thermal spray mass impinges on the target surface a large fraction of it, called overspray, splashes off the target and is redeposited with a small direction angle. This component of the deposited mass results in a large coating roughness.     

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.     

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.     

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.     

Characterization and processing of spray-dried zirconia powders for plasma spray application

The correlation between the performance of plasma spray coatings and feedstock powder properties is not fully understood. To demonstrate this correlation, eight spray-dried zirconia powders containing a mass fraction of 20% Y₂O₃ (yttria) were characterized with respect to their physical, bulk chemical, and surface chemical properties. The same powders were plasma spray deposited as coatings, and their relative performance was evaluated using a thermal rupture test developed by Pratt and Whitney. The specific powder properties studied were chemical composition, binder content, particle size distribution, powder morphology, interface chemistry, thermogravimetry, phase composition, and specific surface area. Among the characterization data, the binder-related properties of the powder correlated most strongly with the thermal rupture test data. Specifically, higher binder contents were associated with poor thermal rupture test performance.     

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.     

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.     

Superior thermal barrier coatings using solution precursor plasma spray

A novel process, solution precursor plasma spray (SPPS), is presented for depositing thermal barrier coatings (TBCs), in which aqueous chemical precursors are injected into a standard direct current plasma spray system. The resulting coatings microstructure has three unique features: (1) ultra fine splats (1 μm), (2) nanometer and micron-sized interconnected porosity, and (3) closely spaced, through-thickness cracks. Coatings over 3 mm thick can be readily deposited using the SPPS process. Coating durability is excellent, with SPPS coatings showing, in furnace cycling tests, 2.5 times the spallation life of air plasma coatings (APS) and 1.5 times the life of electron beam physical vapor deposited (EB-PVD) coatings. The conductivity of SPPS coatings is lower than EB-PVD coatings and higher than the best APS coatings. Manufacturing cost is expected to be similar to APS coatings and much lower than EB-PVD coatings. The SPPS deposition process includes droplet break-up and material arriving at the deposition surface in various physical states ranging from aqueous solution, gel phase, to fully-molten ceramic. The relation between the arrival state of the material and the microstructure is described.     

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.     

Powder/processing/structure relationships in WC-Co thermal spray coatings: A review of the published literature

Thermally sprayed coatings based on tungsten carbide are widely used but not yet fully understood, particularly with regard to the chemical, microstructural, and phase changes that occur during spraying and their influence on properties such as wear resistance. The available literature on thermally sprayed WC-Co coatings is considerable, but it is generally difficult to synthesize all of the findings to obtain a comprehensive understanding of the subject. This is due to the many different starting powders, spray system types, spray parameters, and other variables that influence the coating structures and cause difficulties when comparing results from different workers. The purpose of this review is to identify broad trends in the powder/processing/structure relationships of WC-Co coatings, classified according to powder type and spray method. Detailed comparisons of coating microstructures, powder phase compositions and coating phase compositions as reported by different researchers are given in tabular form and discussed. The emphasis is on the phase changes that occur during spraying. This review concerns only WC-12% Co and WC-17% Co coatings, and contrasts the coatings obtained from the cast and crushed, sintered and crushed, and agglomerated and densified powder types. Properties such as hardness, wear, or corrosion resistance are not reviewed here.