This paper discusses the differences between high-velocity air fuel (HVAF) and classical kerosene-fired high-velocity oxygen fuel (KF-HVOF) spray processes and explains how they impact coating metallurgy and properties such as hardness, Young’s modulus, and wear resistance. The biggest differences are observed in cavitation erosion tests where HVAF coatings have a 50% greater impact on wear reduction. Potential advantages in the use of HVAF spraying for depositing thin, corrosion resistant WC-CoCr layers are also discussed.

Due to the superposed thermal and mechanical stress profile, thermo-mechanically coupled forming processes require tools and machine components which meet high demands. High forming forces and process temperatures in the contact zone between the tool and the workpiece limit the life span of these tools. A promising approach for protecting such tools is a combination of thermally sprayed coatings and physical vapor deposited layers. This coating system combines the characteristics of the individual layers and leads to superior mechanical, tribological as well as thermal properties under the mentioned coupled stresses. In this study thermally sprayed alumina (Al 2 O 3 ) and yttria-stabilized zirconia (ZrO 2 ) coatings were produced by atmospheric plasma spraying. Therefor different coating porosities were adjusted in order to varied the effect of thermal insulation for the substrate made of AISI H11 (1.2343). After the coating process the surface roughness of the thermal barrier coatings (TBC) were reduced by polishing process in preparation for the PVD top layer. Subsequently, wear and heat resistant hard TiAlSiN and CrAlSiN coatings were deposited on top of the polished TBCs by using magnetron sputtering process. As a reference the PVD coatings were also applied on a nitrided steel samples. Titanium and chromium interlayers were applied by PVD technique in different coating thicknesses (50 – 150 µm) between PVD and thermally sprayed coatings. Afterwards the influence of these metallic interlayers on coating adhesion of PVD coatings were analyzed by performing scratch tests. Hardness and young’s modulus of PV coatings were investigated by means of nanoindentation. The morphology and topography of the coatings were analyzed by scanning electron microscopy, light microscopy and optical three-dimensional surface analyzer. EDX analyses and X-ray diffraction were used to determine the chemical composition of the PVD coatings. Finally the wear resistant of the PVD top layers were determined at different temperatures (20°C, 500°) by using a high temperature tribometer.

The presence of defects such as voids, inter-lamellar porosities or cracks, provides a decrease of the effective thermal conductivity of plasma sprayed coatings as well as a decrease of the corresponding mechanical properties such as the Young’s modulus. In general, effective properties of thermal spray coatings are thus strongly different from that of the bulk material and have thus to be quantified to validate their in service performances. A complementary approach allowing understanding the relationships between the microstructure of a coating and its macro-properties is the use of Finite Element Modeling. The case of composite coatings is still more complicated due to the presence of different materials. In the present study, thermo-mechanical properties of a plasma sprayed composite coating were estimated by numerical modeling based on FEM. The applied method uses directly cross-sectional micrographs without simplification using a one-cell per pixel approach. Characteristics such as the thermal conductivity, the Young modulus, the Poisson ratio and the dilatation coefficient were considered. The selected example was an AlSi/polyester coating used as abradable seal in the aerospace industry.

Atmospheric plasma sprayed (APS) thermal barrier coatings (TBCs) with lamellar structure exhibit low thermal conductivity and low stiffness. However, high temperature exposure for certain long duration causes the sintering which heals two-dimensional (2D) inter-lamellar pores and intrasplat pores. Such sintering effect increases the stiffness and thermal conductivity of the coatings and consequently reduces the stability and durability of TBCs. It can be expected that large 2D pores with a wide opening is difficult to be eliminated. In this study, inter-lamellar 2D pores with large opening width were fabricated in the La2Zr2O7 (LZO) coatings through spraying LZO+SrO coatings and removing the SrO splats in the water. Then, the conventional LZO coating and the porous LZO coating were subjected to high temperature exposure in the air at 1300 °C for different durations. The microstructure evolution especially in terms of the shape and density of inter-lamellar 2D pores was examined. In addition, the change of thermo-physic properties and the mechanical properties of the coatings with increasing exposure duration were studied. Results show that the 2D pores in LZO coating created by those SrO splats inherit primarily large opening width from 200nm to about 1 µm which endows the LZO coating with high sustainability at high temperature environment. Under thermal exposure at 1300oC, it was found that 2D pores resulting from SrO splats are free from healing while conventional 2D inter-lamellar pores with small opening width formed during splat cooling became healed rapidly. Thus, thermal conductivity and Young's modulus of the conventional LZO coating increased rapidly, while unhealed 2D pores in the highly porous LZO coatings contributed to the low Young's modulus and low thermal conductivity of LZO coating with remarkably high stability. With addition of 30% SrO in spray powder, a LZO coating with a thermal conductivity of about 0.39 W.m-1.K-1 in the as-prepared state was obtained. The coating maintained a thermal conductivity of 0.57 W.m-1.K-1 even after 100 hours exposure at 1300°C. The present results indicated that high sintering-resistant thermal barrier coating can be fabricated though designing inter-lamellar 2D pores with large opening width in the coating by the present novel approach.

Wear protection is one of the major applications of thermally sprayed hardmetal coatings. This paper presents the latest results of a systematic study on the influence of Cr3C2-NiCr feedstock powder characteristics on coating microstructures and economic parameters like deposition rate and deposition efficiency. Four commercial Cr3C2-NiCr powders with spherically shaped particles but different structural features were characterized and deposited by a liquid-fueled and a gas-fueled HVOF and a HVAF process. Deposition rates and efficiencies were determined; all coatings were analyzed in as-sprayed condition and selected samples were heat-treated at 800 °C in argon atmosphere. The effects of the feedstock powders and spray processes on the coating characteristics (microstructure, hardness, Young’s modulus and diffusion processes during heat treatment) were studied.

The manufacture of submicrometer-structured coatings by thermal spraying is subjected nowadays to increasing research efforts in order to obtain unique and often enhanced properties compared to conventional coatings. Injecting suspensions of submicron ceramic particles into the plasma jet or the flame enables to deposit finely-structured coatings. Such fine microstructures can be advantageous for applications in the field of thermal barrier coatings (TBCs) for gas turbines. Often, suspension plasma sprayed (SPS) TBCs show unique mechanical, thermal and optical properties compared to conventional atmospheric plasma sprayed (APS) TBCs. They have thus the potential of providing increased TBC performances under severe thermo-mechanical loading. Experimental results show the capability of SPS to obtain yttria-stabilized zirconia (YSZ) coatings with high density of vertical segmentation cracks, yielding high strain tolerance and low Young’s modulus, while the porosity is still large compared to APS segmented coatings. Besides, sintering behavior of complete TBC systems under a thermal gradient exposure is of high importance. The evolution of the coating microstructure during thermal cycling test at very high temperature (1400°C) in our burner rigs as well as under isothermal annealing and its effects on the coating properties such as Young’s modulus were investigated.

Twelve commercially available WC-Co powders with different average carbide 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 Characterization of the coatings showed that the average carbide sizes and carbide volume contents in the coatings were lower than those of feedstock powders. Hardness and fracture toughness of the coatings were investigated using indentation techniques. Young’s modulus was measured by an ultrasonic technique. The hardness and Young’s modulus decreased with increasing cobalt content, while fracture toughness slightly increased. The effect of carbide size on the hardness showed no specific trend. These behaviors were discussed with the help of microstructure observations of the coatings by scanning electron microscopy, X-ray diffraction and chemical analysis. Using an improved HVOF process with a gas shroud could result in less decomposition of the powder and higher fracture toughness.

Titanium/dicalcium silicate composite coatings with different ratios (weight ratios as Ca 2 SiO 4 : Ti = 3:7, 5:5, 7:3) were prepared by plasma spraying. Effects of titanium addition on coating properties, such as bonding strength, Young’s modulus and dissolution in simulated physiological environment, were studied. Results showed that the bonding strength between coating and Ti-6Al-4V substrate increased with increase of titanium content in the composite coatings. It was explained by the narrowed dissimilarity of thermal expansion coefficients between the coatings and substrates. Degradation of mechanical properties after immersion in simulated body fluid was also studied. The dissolution of dicalcium silicate in the composite coatings resulted in the decrease of bend strength and Young’s modulus of the coatings in the simulated physiological environment. The higher titanium content in the composite coatings, the stabler are the composite coatings in the physiological environment.

Thermal barrier coatings were produced using both Ar and N 2 as the primary plasma gas. Various aspects of the process and the coatings were investigated. It was found that higher in-flight particle temperatures could be produced using N 2 , but particle velocities were lower. Deposition efficiencies could be increased by a factor of two by using N 2 as compared to Ar. Coatings having similar values of porosity, hardness, Young’s modulus and thermal diffusivity could be produced using the two primary gases. The coatings exhibited similar changes (increased hardness, stiffness and thermal diffusivity) when heat-treated at 1400°C. The results point to the potential advantage, in terms of reduced powder consumption and increased production rate, of using N 2 as compared to Ar as the primary plasma gas for TBC deposition.

The steadily increasing requirements to the properties of thermal spray coatings have to lead to the development of new characterization tools, in particular for non-destructive testing. Laser acoustic surface waves relate to the most promising methods for cost-effective non-destructive testing. In this work seven HVOF-sprayed WC-based coatings were systematically studied by laser acoustic surface waves using the LAWave device. Due to short measurement and calculation times the coating can be easily multiply tested. Young`s modulus and densities of the coating were obtained by this method. The values of the Young's modulus were compared with those derived from a micro-indentation method using Vickers indents and were found to be in a good agreement. Moreover, Vickers hardness values of the coatings obtained by using different loads were compared and the phase composition was studied by X-ray diffraction. The coating porosity was determined by image analysis of optical micrographs of metallographic cross-sections. It is proposed that in the case of WC-based coatings changes in the theoretical density of the material composition due to phase transformations induced by the spray process (formation of W2C and solid solutions on their base) prevent a direct access to the porosity values.

Mechanical properties of thermal-sprayed ceramic coatings were investigated. Al 2 O 3 and Y 2 O 3 -stabilized ZrO 2 (YSZ) coatings were deposited on plate substrates. Stainless steel plates and aluminum plates, of different thermal expansion coefficients, were used as substrates. The coatings were prepared at two different thicknesses. During deposition of each sample, the history of substrate temperature was recorded. Four-point bending tests were carried out, while strains at the coating surface and the substrate surface were measured with strain gages. The apparent Young's modulus of the coating was determined using the composite beam theory. In addition, the rupture strain of the coating was measured by three-point bending test. The relationship between the results of these tests and the temperature of each substrate during deposition is discussed.

Nowadays the use of light weight materials increases rapidly. Owing to growing requirements regarding material properties and corresponding production costs new material designs and novel production concepts are needed. The low density of aluminium and its alloys is accompanied by lower Young’s modules and lower strengths compared to steel. These disadvantages regarding to stiffness and strength can be overcome by using a composite material consisting of aluminium and embedded endless reinforcing elements. In this work a novel technology based on the thermal spraying process to manufacture endless reinforcing elements for extrusion molding of Al-profiles will be discussed. A specific handling system for arc-spraying Al-alloys onto steel wires has been developed. The influence of the coatings materials and coating parameters on the subsequent extrusion moulding process has been studied.

It is generally known that plasma sprayed coatings exhibit rather a low strength thanks to their characteristic microstructure with porosity and microcracks. To determine the role of varying types of deformation in different parts of the coatings profile, 1.8 mm thick chromia coatings on a steel substrate were made using WSP plasma spray. Stress gradients were then measured "in-situ" on a four-point bending device during bend deformation of the coated beam using neutron diffraction. Only compressive loading in coatings increased the resulting stresses. From the plot of stresses vs. applied strain (linear dependence) the Young’s moduli of the substrate and the coating were determined. Both values agree well with those obtained by mechanical testing. In general it is believed that, in a simplified way, interlamellar voids (cohesion defects) roughly parallel to the substrate decrease the tensile strength in the direction perpendicular to the substrate, while intralamellar cracks, roughly perpendicular to the substrate, significantly affect the Young’s modulus of coating. It is supposed that the tensile deformation of coatings opens the cracks and no stress increase is observed. On the other hand the applied compressive deformation in the coating closes the intralamellar microcracks and the internal stresses increase, as suggested by obtained results.

Industry faces the continual challenges of operating in less space and accelerating line speed in order to increase productivity, improve quality, and reduce cost. These challenges are particularly evident in the film and papermaking markets. To maintain competitive products in the market place, it is essential to incorporate rollers that are; lightweight, high stiffness, and compact. To satisfy these requirements, rolls made of carbon fiber reinforced plastics (hereinafter referred to CFRP) have proven effective and have come into wide use in many industrial fields. CFRP rolls are the structural materials that were developed mainly to improve the Young's modulus of rolls. Guiding and transfer rollers frequently require surface features such as non-adhesion and wear resistance. The typical surface treatments to CFRP currently employed are; painting, plating, and rubber lining or the like, but these treatments have their limits in the manufacturing process and effectiveness. Consequently, we have developed the technology to provide rolls with the desired surface by spraying various metals, ceramics, or cermets, directly on CFRP substrate. These sprayed rolls have been intensely monitored in actual operations to insure high speed capability and longevity. In this presentation, we will talk about the rotary bending fatigue test of CFRP rolls on which the carbide cermet was plasma-sprayed and the quality stability of the roll with the two-layer composites that was confirmed based on the test results.

The Cold Gas Dynamic Spray Method (CGDS) requires high-kinetic energy particles to obtain dense coatings. The aim of this study is to characterize some properties of CGDS coatings and compare them to the bulk materials. Pure nickel and nickel-base alloys (NiCrAlY, CoNiCrAlY, and Hastelloy C) are sprayed on 316L steel substrates. Coating thickness is about 700 µm. The particle velocity at impact is measured using an imaging technique based on a fast-shutter CCD camera and a high-power diode laser. The original powder characterization involves particle size, density, chemical composition and hardness, and coating analysis includes micro hardness, nano hardness (performed in the first 60 µm of the coating), Young’s modulus determination and porosity level. The hardness test results make it possible to determine effects of this specific property on the sprayability with this process.

Fatigue behavior and Young’s modulus of plasma sprayed gray alumina on low-carbon steel substrates were investigated. The investigation of the properties of composite material “coating-substrate” included the measurements of microhardness profile, residual stress on the top of the coating and residual stress profile in substrate. Fatigue samples were periodically loaded as a cantilever beam on a special testing machine. Failed samples were observed in SEM to determine failure processes in the coating. The Young’s modulus of the coating was measured by the four-point bending method. Samples were tested both in tension and compression at low (300 N) and high (800 N) loads. Our experiments revealed that the average fatigue lives of coated specimens were nearly 2 times longer than those of the uncoated specimens. The Young’s modulus of the coating varied between 27 and 53 GPa with an average value of 43 GPa. Loading in tension caused decrease in Young’s modulus of the coating while loading in compression lead to increase in Young’s modulus. Increase in the lifetime of coated samples was likely due to compressive residual stresses in the substrate, originating from the spray process. Failure of the coating consisted of several processes, among which the most important are splat cracking, splat debonding and coalescence of cracks through the voids in the coating.

Plasma spraying is a fast and inexpensive process for fabricating YSZ electrolyte for SOFCs. In this investigation, free-standing plasma sprayed YSZ disks are treated by spark plasma sintering at different temperatures, soak times, and loading cycles. SEM examination shows that the lamellar microstructure of the as-sprayed zirconia is converted to a predominantly granular structure with no significant phase changes as per XRD analysis. Microhardness and laser flash diffusivity measurements show that the SPS treatments also improve YSZ layer density, tensile modulus, and thermal conductivity. Paper includes a German-language abstract.

Yttria stabilized zirconia particles are plasma sprayed on polished stainless steel substrate. Starting powders are fused and crushed powder, and hollow spherical powder. Four types of the splat morphology, which are splash, rugged, gravel mounted, and disk splats, are observed. Splash and disk splats are fully melted particles, but rugged and gravel mounted like splats are partially melted particles Gravel mounted like splat is observed from only hollow spherical powder, and disk splat is observed in the case of high substrate temperature. It is found that the ratio of splat morphology changes with spraying parameters. Porosity of the coating from fused and crushed powder is higher and Young's modulus of that is lower than that from hollow spherical powder. The ratio of rugged and gravel mounted splats affect porosity and Young's modulus. Adhesive strength increases with the increase in the ratio of disk splat. So, the coating properties are improved by controlling splat morphology. KEYWORDS: Splat Morphology, Partially Melted Particle, Disk Splat, Porosity, Young's Modulus, Adhesive Strength

Among candidate materials for plasma spraying titanates ATiO3, where A is an element from the alkaline earth group (11), were not systematically tested until today. This paper reports on plasma spraying of synthetic perovskite CaTiO3 and geikielite-perovskite system MgTiO3-CaTiO3. Perovskite CaTiO3 is well known as dielectric material and a basic component of complex dielectric ceramics. Since it is relatively chemically simple and inexpensive material it has been selected for the basic preliminary studies. Mixture of geikielite-perovskite MgTiO3-CaTiO3, with Mg:Ca ratio equal to 94:6, was chosen because its permittivity is independent of temperature. Plasma spraying was done with the water stabilized plasma gun WSP. Plasma spraying conditions were optimized using single splat observation for various substrates and varying substrate temperature. Standard experimental techniques were used for studying of microstructures, chemical and phase compositions and porosity of as-sprayed and annealed deposits. Mechanical properties such as Young’s modulus and microhardness were measured.

This paper aims to measure the residual stresses of plasma sprayed oxide ceramic deposits using the X-ray diffraction method and measure the Young’s Modulus in specially designed four-point bend test device. Aluminum oxide and chromium oxide coatings are made with a water stabilized plasma spray gun. The paper analyzes the microstructure and the phase composition. The X-ray diffraction method is used to determine residual stresses on the layer surfaces. The same method is used for the local measurement of surface tensions in a four-point bending device built into an X-ray diffractometer. This device also enables the force applied and the specimen deflection to be measured. Effective modules of elasticity are determined from the gradient from load to deflection during bending. The results of the tension measurements and the values calculated from the applied force are compared. Paper includes a German-language abstract.