In this investigation, submicron YSZ particles suspended in ethanol were sprayed on stainless steel substrates using a single-electrode cascaded arc-plasma torch. Various coating morphologies were produced by changing the input power, gas flow rate, and injection angle of the gun. Coating thicknesses between 500 and 600 μm were achieved for all experimental runs and stress evolution was monitored using an in situ coating property (ICP) sensor that measures changes in substrate curvature. Based on the stress-modulus relationship, there can be four microstructural zones in the coatings: a porous feathery zone, a dense columnar zone, a dense vertically cracked (DVC) zone, and a zone with cracked and defective columnar structures. From ICP curvatures, it was found that the porous columnar structures are highly compliant, the dense columnar structures are stiff, and the DVC structures are dense and compliant. The dense columnar and DVC microstructures displayed a twin stage curvature change which clearly indicated the stress build up, onset of cracking, and stress relief after axial crack propagation.

Plasma-spray injection conditions have a major effect on particle properties and thus the characteristics of deposited coatings. In this investigation, opposing particle streams were injected into a plasma jet and the effect of different configurations and injection parameters was systematically studied. First- and second-order process maps were constructed from particle diagnostic data and in-situ beam curvature measurements, which served to guide optimization efforts.

Spray parameters play an important role on the microstructure and properties of plasma sprayed coatings. Parameters such as spray distance, plasma gas flow and current, raster speed and spray angle all can be varied. In this paper, an integrated study to investigate the effects and influences of spray angle on properties of yttria-stabilized zirconia coatings was carried out with spray angles of 60°, 75° and 90° (to the substrate surface). In situ coating property sensor (ICP) based on beam curvature measurements was used to measure the evolving stress and elastic moduli of the resultant coatings and combined with other characterization tools for thermophysical property and microstructure analysis, such as laser flash and scanning electron microscopy (SEM). The results indicate that the coating with 60° spray angle had the lowest thermal conductivity and more compliant structure. This study seeks to understand the mechanism for this effect and will provide important insight into parametric sensitivities on complex spray parts.

Adhesive strength of the plasma-sprayed thermal barrier coating (TBC) is one of the most important parameters which influence the reliability during service. In the past, numerous test methods were reported to measure the coating adhesion. However, most of them require careful and time consuming preparation. Consequently, limited information could be obtained to establish the relationship between the processing conditions and the adhesive property. To produce more measurements using a simpler procedure, the interfacial indentation test and the modified tensile adhesive test are examined. In this paper, the interfacial fracture toughness of the plasma-sprayed ZrO 2 coatings, deposited on Al substrates, were evaluated by these two tests. In order to study the effects of the powder injection, samples were sprayed with various carrier gas flow rates. The test results show a certain correlation between the melting index and the interfacial fracture toughness. In addition, variations between the results obtained from the two different methods are discussed.

Complexity in dynamics and mechanism of supersonic flame formation and effects of processing variables has made the understanding of interaction of particles and flame a difficult task. Lack of such understanding limits the possibilities of controlling the process to obtain desired in-flight particles temperature and velocity and consequent particles state. This problem is even more pronounced in TS systems with no dedicated decoupled temperature and velocity controlled regime. Different approaches based on total volume flow, back pressure and fuel to oxygen ratio have been examined to address the robustness of each approach to control the temperature and velocity. WC-CoCr material was used employing DJ-2600 torch. A guideline to control the in-flight particles temperature and velocity based on process variables is provided. A process map was developed to establish a correlation between process, in-flight particles state, microstructure, properties and performance.

An empirical model was built for the prediction of HVOF sprayed NiCr in-flight particle properties based on the spraying parameters. The employment of factorial design in process parameter development, allowed determination of the contribution of the key process variables, such as flame energy (combustion pressure and O 2 /F ratio), spray distance and feed rate, on the resultant particle velocities and surface temperatures. The significance of each parameter was used to compose a simple model which enabled the description of the particles’ in-flight properties. Particles with velocities ranging as much as 300 m/s and temperatures ranging up to 350 °C were used to produce a range of coatings on an in situ curvature sensor enabling the determination of evolving and residual stress. These diverse particle states combined with the effect of flame impingement on the substrate, resulted in coatings of similar thickness, but significantly different stress states. Real time evolving stresses during deposition were interrelated to particle in-flight properties and, consequently, to spraying parameters.

The effects of powder loading on temperature distributions of yttria-stabilized zirconia particles during flight, which can be used as a melting status indicator, were investigated in atmospheric dc plasma spraying. Commercially available diagnostic systems were utilized to measure the state of in-flight particle parameters. As the powder feed rate was increased, the intensity of a peak related to the latent heat increased, suggesting the increase of semi molten particles. Interesting findings are that the deposition efficiency of the coating actually increased in some conditions at higher powder feed rates. This implies that higher molten degrees of particles do not always give higher deposition efficiencies. The loading effects also affected the result of diagnostics, which requires special care when the diagnostic condition is different from the actual spray conditions.

Pores and cracks have significant influence on the structural rigidity and mechanical behavior of the thermally sprayed materials. For some applications dense coatings are needed, while for others (e.g. thermal barriers) some level of porosity is desirable. Recent development in thermal spraying focuses on the tailored design of pores and cracks for specific applications. In this project, ceramic coatings with different level and morphology of pores and cracks were plasma and HVOF sprayed on titanium alloy substrates. Coating microstructures were observed using scanning electron microscopy. Mechanical behavior of the prepared coatings was evaluated using four-point bending test in terms of changing coating stiffness with increasing mechanical load both in compression and tension. Significant level of coating non-linearity and hysteresis were observed. Tests carried out for coatings with the same chemical composition but different microstructure proved strong dependency of coatings mechanical properties on pores and cracks morphology. Microstructure features relevant for the applied loading are discussed.

Particle melting is one of the key issues in air plasma spraying of high-temperature ceramics such as YSZ. This study aims to estimate the molten content of the spray stream from in-flight particle temperature measurements. Particle temperature distribution is delineated into particle states (unmolten, partially molten, completely molten) as a first approximation, which is then used to estimate the molten content in the spray stream. The estimated percentage of molten content is shown to correlate well with deposition efficiency measurements for a wide range of process conditions and feedstock characteristics. The use of this estimation technique for other materials and processes is also discussed.

Iron aluminides have been lately proposed as promising materials for wear applications. Many authors have focused their investigations on the friction behaviour of FeAl coatings emphasizing the role of this intermetallic as a new matrix to embed ceramic particles and replace for high temperature the extensively studied WC-Co cermet system. However, few works deal with the evaluation of the different tribological properties and their relationship with the coating microstructure. Thus, in the present study, the near stoichometric Fe40Al was successfully sprayed by means of HVOF using different spraying parameters and the tribological behaviour was assessed through solid particle erosion, abrasive and dry sliding tests. The wear mechanisms that took place in the produced coatings are discussed with regard to the obtained results. The friction coefficient versus sliding distance was obtained. In addition, isothermally treated samples in air were tested showing both lower friction coefficient and lower wear rate.

The coating stresses induced by thermal spray using a High Velocity Gas Fuel (HVOF) and Liquid Fuel (HVLF) gun and a High Velocity Plasma (HVP) gun with the high velocity nozzle are compared using a curvature based in-situ coating stress analysis approach that measures the deflection of a beam while a coating is applied to it. This novel diagnostic tool provides new insights into the internal stresses generated in a coating system during the actual application of the coating. Coatings were sprayed with three process guns and the same material feed stock that result in similar coating structures and properties. HVOF, HVLF and HVP processes induce similar particle energy states at high velocity regimes as measured with particle diagnostic tools during spraying but due to the differences in particle history are expected to result in different coating stresses. In some cases the actual measured stress conditions using the in-situ coating stress method were dramatically different. Analysis is presented to explain the reason for these surprising results. The understanding of these differences will lead to an improved methodology for mapping coating processes from one another along with a more in depth understanding of coating stresses buildup.

This paper investigated the role of particle injection on inflight particle behavior and its coupling effect on plasma plume in an external orthogonally injected air plasma spray system as well as effects of primary, secondary and carrier gases on in-flight particle status through both experiments and simulations. Diagnostic sensors such as In-flight Particle Pyrometer (IPP) and Spray Position Trajectory (SPT) have been used to obtain the plume characteristics and ensemble temperature, while DPV-2000 was used to measure the distributions of individual particle status such as temperature, velocity and size at the maximum particle flux point. Three-dimensional simulations have been carried out for the corresponding experimental conditions to examine the effects of in-flight particle heating on the plasma plume and in-flight characteristics at different spray distances. Both experiment and simulation results show that particle temperature and velocity will initially increase with plume angle and then decrease after reaching a maximum value for different combination of process parameters at the same plume angle. Theoretical analysis shows strong dependence of the plume angle on the velocity ratio of vertical component from the carrier gas to the horizontal one from primary and secondary gas at their respective nozzle exits. This study enables a better understanding of influence of plasma forming and stabilizing parameters on the particle in-flight characteristics.

There have been recent efforts to expand the thermal spraying capabilities for novel corrosion resistant coatings for metal bipolar plates were produced by thermal spraying for proton exchange membrane (PEM) fuel cell applications. Recently, substrate heated by plasma gun or by external laser beam has been proposed to enhance the mechanical and thermal properties of the coatings. Studies were found that with sufficient substrate heating, substrate melting may happen. When droplets solidified on a thin liquid layer on the top of the substrate, conditions will be similar to crystal growth and Epitaxy film growth will be possible. It is therefore possible that using substrate melting as tool to promote epi-layer growth using plasma spraying. Difficulty is how to control the substrate temperature to cause substrate melting during droplet solidification. In this study we will propose a new idea for better temperature control on the substrate. The capability of epitaxy growth using thermal spraying will be investigated. Molybdenum droplets impact on an Aluminum substrate will be studied. A splat formation model including undercooling, nucleation, and non-equilibrium solidification will be used to study the possibility of the substrate melting and grain size distribution.

The concept of ‘process maps’ has been utilized to study the fundamentals of process–structure–property relationships in high velocity oxygen fuel (HVOF) sprayed coatings. Ni- 20%Cr was chosen as a representing material of metallic alloys. In this paper, concurrent experiments including diagnostic studies, splat collection, and deposition of coatings were carried out to investigate the effects of fuel gas chemistry (fuel gas/oxygen ratio), total gas flow, and energy input on particle temperature (T) and velocity (V), and coating microstructure formation and properties. Coatings were deposited on an ‘in situ’ curvature monitoring sensor to study residual stress evolution. A strong influence of particle velocity on induced compressive stresses through peening effect is discussed. The complete tracking of the coating buildup history including residual stress evolution and temperature deposition, in addition to single splat analysis allows the interpretation of resultant coating microstructures and properties, and enables coating design with desired properties.

The microstructure of thermally sprayed ceramic coatings is characterized by the existence of various pores and microcracks. The porous microstructure makes coating desirable for thermal insulation, but this unique microstructural feature also gives rise to anelastic response under tension and compression loads. Detail investigations of curvature measurements of ceramic coated substrate indicate the coatings to exhibit anelastic behavior composed of nonlinear and hysteresis characteristics. In this paper, the mechanisms of such behaviors were studied from curvature-temperature measurements and finite element analysis through modeling the microstructure of yttria stabilized zirconia (YSZ) coating. Computational models contain numerous randomly distributed pores and microcracks with various sizes, aspect ratios, locations and orientations. The effects of such attributes of pores and microcracks on coating anelastic behavior were studied by simulations of curvature change during thermal cycles.

The general method of process maps to understand and control thermal spray processes has been applied to monitor the deposition of WC-Co by high velocity oxygen fuel (HVOF). A selected number of particle state conditions (velocity and temperature) has been performed to produce a variety of coatings. Microstructure, mechanical properties, and wear resistance were evaluated and compared. A second order process map for sliding wear, impacting particle erosion and abrasive wear control can be constructed from the process map to provide the limits within which the particle state can be changed to achieve a predefined coating specification. The mechanisms behind the wear resistance are discussed within the framework of wear maps –third order process map-in the context of analysis of inter splat de-bonding, mechanical properties of the coating, and delamination failure.

Precursor plasma spray synthesis is an innovative and rapid method to make functional oxide ceramic coatings by starting from solution precursors and directly producing inorganic films. This emerging method, utilizes molecularly mixed precursor liquids, which essentially avoids the handling and selection of powders, opening up new avenues for developing compositionally complex functional oxide coatings. Precursor plasma spray also offers excellent opportunities in exploring the non-equilibrium phase evolution during plasma spraying of multi-component oxides from inorganic precursors. Although there have been efforts in this area since the 1980s and early 1990s with the goal of synthesizing nanoparticles, only recently has the work progressed in the area of functional systems. At the Center for Thermal Spray Research an integrated investigative strategy has been conducted to explore the benefits and limits of this synthesis strategy. Water and alcohol based sol/solution precursors derived from various chemical synthesis methods were used as feedstocks to deposit thin/thick films of spherical and nanostructured coatings of yttrium aluminum garnet (YAG), yttrium iron garnet (YIG), lanthanum strontium manganite (LSM) and Zr-substituted yttrium titanates, compositions of Y 2 O 3 -Al 2 O 3 and their microstructural space centered around stochiometric YAG. A detailed discussion of the salient features of RF induction plasma spraying (RFPPS) approach, results obtained in the investigations to develop various functional oxide coatings and process issues and challenges are presented.

Thermal conductivity plays a critical role in the thermal transport of thermal sprayed coatings. In this paper, a combined image analysis and finite element method approach is developed to assess thermal conductivity from high-resolution scanning electron microscopy (SEM) 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 meshing 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: yttria stabilized zirconia (YSZ), molybdenum and NiAl, and results are found to be in good agreement with experimental values, obtained using the laser flash method. YSZ coatings are also annealed and the analysis procedure repeated to determine if the technique could accurately assess changes in coating morphology.

Controlling particle state is important to not only achieve the required microstructure and properties in coatings but also to clearly isolate and understand the role of other clusters of variables (such as the various substrate and deposition conditions) on the aforementioned attributes. This is important to design coatings for high performance applications and in the ongoing efforts towards achieving prime reliance. This study examines the variabilities in particle state and explores a few strategies to control them for improved reproducibility with the aid of in-flight particle and plume sensors. The particle state can be controlled by controlling the torch parameters or by directly controlling the particle state itself via feedback from particle and plume sensors such as DPV2000 & TDS. There exist at least a few control protocols to control the particle state (predominantly temperature and velocity) with judicious choice of critical parameters. In the present case the particle state has been controlled by varying the critical torch parameters in a narrow range using 8% YSZ of angular morphology (fused and crushed) with 10-75 microns size distributions in conjunction with a N 2 -H 2 laminar (non-swirl) plasma. Two important results emerge. (1) The particle state resulting from averaged individual particle measurements (DPV 2000) is surprisingly stable with variabilities in T < 1% and variability in V of < 4%. Ensemble approaches yield a somewhat higher variability (5%). In spite of this the variability in basic coating attributes such as a thickness and weight is surprisingly large. (2) Applying a much simpler control strategy to only control the particle injection and hence the particle trajectory results in reduced variabilities in coating attributes.

Over the last decade there has been an explosion in terms of available tools for sensing the particle spray stream during thermal spray processes. This has led to considerable enhancement in our understanding of process reproducibility and process reliability. However, in spite of these advances, the linkage to coating properties has continued to be an enigma. This is partially due to the complex nature of the build-up process and the associated issues with measuring properties of these complex coatings. In this paper, we present an integrated strategy, one that combines process sensing, with process modeling and extracting coating properties in situ through the development of robust and advanced curvature based techniques. These techniques allow estimation of coating modulus, residual stress and non-linear response of thermal sprayed ceramic coatings all within minutes of the deposition process. Finally, the integrated strategy examines the role of process maps for control of the spray stream as well as design of thermal spray coatings. Examples of such studies for both MCrAlY and YSZ coatings will be presented.