Direct and quantitative observation of the stress generation during HVOF spray is carried out by measuring the curvature of substrates in-situ during spraying. A high pressure HVOF gun is used to spray SUS316L, Hastelloy C and WC-12%Co powder onto SUS316L substrates. The observed curvature data indicate that there are 3 regimes of stress evolution during the HVOF spray: (1) generation of compressive stress on the substrate surface at the beginning of spraying, (2) stress buildup in the coating during spraying, and (3) superposition of stress due to the mismatch in the thermal expansivity between the coating and the substrate as the specimen cools down to the room temperature after fabrication. Compressive stress ranging from 70 to 400 MPa is observed in the second regime during the HVOF spray; the value depending on the powder materials and spray conditions. Microstructural observation reveals that a significant portion of the coatings consists of poorly molten particles. Beneath the coatings formed by the HVOF process, a thin layer of increased hardness exists within the substrate.

An analytical model has been developed to predict the residual stress distributions in thermal spray coatings on substrates of finite thickness. This is based on the concept of a misfit strain, caused by either the quenching of splats or by differential thermal contraction during cooling. During spraying, the coatings are asssumed to deposit on the substrate in a progressive (layer-by-layer) manner. Although the misfit strain ("the quenching strain") is the same for each successive incremental layer of deposit, this is imposed each time on a "substrate" of changing thickness. The final stress distribution will in general differ from that which would result if the coating were imposed on the substrate (with the same misfit strain) in a single operation. The model is straightforward to apply: for example, it can be implemented using a standard spreadsheet program. The required input data are the quenching strain (or stress), the spraying temperature, material properties and specimen dimensions. Comparisons have been made between the predictions from this model and from a numerical model for two plasma sprayed systems. Good agreement is observed. The effects of varying certain parameters, such as coating thickness, substrate thickness, coating stiffness, etc, are readily explored, so that the model provides a useful tool for controlling residual stress levels. Application of the model to determine the quenching stress, in conjunction with the use of a curvature monitoring technique, is briefly outlined. In addition, an analysis is made of the errors introduced by using Stoney's equation to deduce stress levels from curvature measurements.

A system, developed in the laboratory, allows to record in situ the deformation of a flat beam with a displacement sensor and so to analyse stress formation during spraying and upon cooling with fixed or rotating substrates. The beam is fixed onto a pair of knife edges by springs. The knife edges are disposed on a water-cooled rotating cylindrical substrate holder and the beam substrate (2 x 15 x 100 mm 3 ) is parallel to the holder axis. The torch is moved back and forth parallel to the holder axis and the beam temperature is recorded by a thermocouple spot welded to it and also by an IR pyrometer. The influence of beam temperature for a given torch/substrate velocity on the residual stresses is studied for alumina and zirconia coatings. With fixed substrates a sharp increase of the residual stresses related to coating microstructure exists for a transition temperature around 600°C. It seems to correspond to a columnar growth throughout the layered splats. The effect of the torch to substrate velocity and so the pass thickness is studied too.

In order to determine residual stresses in industrial plasmasprayed coatings, a rather simple apparatus, which monitors the curvature of a beam substrate during the deposition process, has been developed. The experimental set-up consists of a water-cooled rotating cylinder, holding an initially plane substrate, whose curvature is continuously measured using a contacting displacement sensor disposed into the cylinder. The combination of the plasma gun translation and the cylinder rotation allows to reach industrial spraying velocities. Liquid argon cryogenic system is used to control the substrate temperature from about 50°C to more than 300°C independently from the process velocity. A typical recording is analyzed thoroughly and a theoretical approach to residual stress calculation discussed. This method is applied to partially stabilized zirconia coatings performed onto stainless steel substrates for spraying temperatures between 80°C and 210°C.

ZrO 2 7Y 2 O 3 plasma sprayed coatings were applied on high temperature Ni-based alloys precoated with a thin, dense, stabilized zirconia coating produced by Physical Vapour Deposition. This contribution concerns with the experimental and numerical analysis of residual stresses in PVD/Plasma Sprayed TBC's systems after coating deposition and after high temperature testing, such as cyclic oxidation and rapid thermal cycling. The thermal residual stress developed in the plasma sprayed top coating during spraying was simulated by using an heat transfer FEM program and an elasto-plastic biaxial stress model which calculate the stress gradients in coating/substrate system. The residual stress distributions within the TBC undergoing thermal cycling is then calculated by a biaxial stress model, taking into account the residual stress due to the deposition technique of the PVD and plasma sprayed top coating and the presence of the growing oxide interlayer. The residual stress within the upper layers of the top coating was verified experimentally by X-ray Diffraction for the as-deposited and thermal cycled samples, and the stresses within PVD bond coating and oxide interlayer were measured by microRaman spectroscopy technique in cross-sectioned samples. The measurements are in good agreement with residual stress modelled results.

The present contribution concerns with a numerical modelling of the residual stress distribution within a multilayered coating system which consists of a functionally gradient material (FGM). The structure of the graded system is made of a ceramic layer and a metallic layer, where between them there is an interlayer which is a graded composite made of the metal and ceramic. The composition changes gradually from 0% ceramic to 100% ceramic. This graded interlayer was modelled as a serie of perfectly bonded finite thin layers, each having slightly different material properties. We analyse the FGM design in respect to thermal stress optimization (e.g. reduction the interfacial stresses). The case of a bilayer, thick ceramic coating on a metallic substrate and a graded thermal barrier coating (TBC) is considered. The effects on thermal residual stress gradients of the compositional profiles and graded interlayer thickness were studied. This FGM stress model enable us to calculate the thermal strain and stress distributions, which gives a contribute to a better understanding of the failure of a graded coating system and is, therefore, a potential tool for FGM stress optimization to improve the thermo-mechanical stability of multilayer graded structures such as high temperature ceramic coatings for use in thermal barrier applications.

Processing induced residual stresses play an important role in the performance of thermally sprayed coatings. Their precise determination is a key to influence the coating properties by modification of process variables and to understand the processing-property relationship. Among various methods for residual stress measurement, x-ray diffraction holds a specific position by being nondestructive, phase distinctive, localized and applicable for real parts. The sin 2 ψ method is commonly applied for bulk materials as well as coatings. However, the results are often reported without sufficient experimental details and the method is used in its simplified form without justification of certain assumptions. In this investigation, the sin 2 ψ x-ray diffraction method was used to measure residual macrostress in plasma sprayed Ni, NiCrAlY and ZrO 2 +8%Y 2 O 3 coatings. Reproducibility of the method was tested and the assumptions allowing its use are discussed and experimentally verified. For Ni coatings, a comparison with blind hole and neutron diffraction measurements is presented. The results are discussed with respect to processing, structure and properties of the coatings.

Neutron diffraction is a promising tool for the investigation of residual stresses in thermally sprayed coatings. In principle, the neutron diffraction method has several distinct advantages over other methods. 1) It is possible to perform triaxial stress analysis throughout the thickness of both the coating and the substrate without material removal. 2) The stress can be determined in all phases of a multi-phase coating. 3) Repeated measurements can be performed on mechanically or thermally fatigued specimens. 4) Stress concentrations and shape/edge effects in actual parts can be located. In this paper, these unique capabilities will be reviewed first. In the second part of the paper it will be shown how the analysis of these coatings differs from experimental analysis of bulk materials. Finally, the analysis of the stress distribution in plasma sprayed NiCrAlY and ZrO 2 +Y 2 O 3 thermal barrier coatings in as-sprayed and annealed states will be presented and discussed.

Residual stresses exert profound influence on the longevity of parts with thermal spray coatings. The distribution and value of the residual stresses depend on method of coating deposition, composition of the applied material, parameters of thermal spraying and methods of post-treatment. Therefore, the study of the influence of the various technological factors on the residual stresses in the plasma spray coatings is very important. Due to heterogeneity of the coating, residual stresses can be determined only by the experimentation by using new methods which take into consideration real values of elastic characteristics and density of elementary layers. Methods and formulas for the calculations of the residual stresses in coatings deposited on bars, rings, discs, cylinders are presented. Experimental results for the various thermal spray coatings are also shown. These results can be used for the optimization of coating deposition and would supplement the existing database.

Residual stresses in net-shaped plasma sprayed tubes was measured by X-ray microdiffraction, as a function of radial position in the sample. A tensile to compressive hoop stress profile was measured, ranging 200 MPa in tension at the outer diameter, to ~125 MPa at the inner. A force balance model was used to explain the evolution of stresses when incrementally adding layers to the pre-existent material.

Thermal barrier coatings are used in several industries to improve thermal efficiency, for example, of gas turbine engines. The performance and life of thermal barrier coated components depend on many factors. One important factor is the residual stresses in the coating and substrate. Residual stresses can be influenced by the parameters of the application process. Parameters affecting residual stresses include the condition of the substrate, the type of spray application process, and the prespray heat treatment of the substrate. Residual stresses can also change significantly during the life of a thermal barrier coated material. The goal of this work is to quantitatively evaluate the changes in residual stresses of the thermal barrier coating and the substrate during the stages of processing and during simulated in-service testing. Through-thickness residual stresses distributions of the coating and the substrate material were determined using a destructive laboratory method, called the "Modified Layer Removal Method." Thin thermal barrier coatings (less than 0.5 mm) were evaluated in this work. Residual stresses in thermal barrier coated specimens were evaluated at three stages of the processing history: (1) after grit blasting of the Hastelloy substrate, (2) after application of the bond coat, and (3) after spraying the top coat. The effect on residual stresses of substrate temperature during spraying is examined. Changes in the residual stresses for thin thermal barrier coatings are shown at selected stages during the processing history of the coated materials. Differences between residual stresses at the selected stages are identified and discussed. Changes to residual stress distribution due to in-service conditions are examined. The effect of bond coat oxidation is examined by long-term, high-temperature exposure. Also, residual stresses are evaluated for thick thermal barrier coatings after thermal shock testing.

Thermal barrier coatings are used in the aerospace industry for thermal insulation in hot sections of gas turbines. Improved coating reliability is a common goal among jet engine designers. In-service failures, such as coating cracking and spallation, result in decreased engine performance and costly maintenance time. A research program was conducted to evaluate residual stresses, microstructure, and thermal shock life of thermal barrier coatings produced from different powder types and spray parameters. Sixteen coatings were ranked according to their performance relative to the other coatings in each evaluation category. Comparisons of residual stresses, powder morphology, and microstructure to thermal shock life indicate a strong correlation to thermal barrier coating performance. Results from these evaluations will aid in the selection of an optimum thermal barrier coating system for turbine engine applications.

Tungsten carbide cobalt thermal spray coatings are used in the aircraft industry to reduce wear damage of lightweight metals such as titanium The performance and life of tungsten carbide (WC-Co) coated titanium materials depend on many factors. An important factor that has received increased attention in thermal spray research is the residual stresses in the coating and substrate. Residual stresses depend on the parameters of the application process. Parameters affecting residual stresses include the prespray treatment of the substrate material (grit blasting, shot peening) and the type of spray application process (HVOF, plasma arc) During the in-service life of a WC-Co coated material, residual stresses can change significantly. The goal of this work is to quantitatively evaluate the changes in residual stresses of the substrate and the WC-Co coating during various stages of processing. A destructive laboratory method, called the "Modified Layer Removal Method," was used to evaluate the through-thickness residual stresses of the WC-Co coating and the titanium substrate material. Residual stresses are determined for three conditions: (1) shot peened, (2) shot peened and grit blasted, and (3) shot-peened, grit blasted and thermal spray coated. The changes in the residual stresses are shown at selected stages during the processing history of the coated materials. Differences between residual stress levels at selected stages are identified and discussed. The effect of coating thickness and HVOF application process on the residual stress in the coating is also examined.

For simulating residual stresses in plasma sprayed multilayer systems a finite element and an analytic model are compared with measured results. The measured stresses which depend on substrate temperature of the plasma spray process are produced exactly by the finite element model. Mesh deformation and simulated stress distribution of the sample are studied.

The full potential of rolling element bearings operating in specialised conditions such as high speed and corrosive environments are realised using surface coatings. Tungsten Carbide coating by thermal spray HVOF and D-Gim processes are considered for these applications. An experimental approach using a modified four-ball machine simulates the tribological conditions within a rolling element bearing. The fatigue failure modes of the tungsten carbide coating in rolling contact with steel and silicon nitride are examined using conventional surface analysis techniques. The stress fields within the coating are examined using traditional contact theory and residual stress measurement by X-ray diffraction. The residual stress measurements of the pre-test coating, the contacting surface and the fatigue failures are described. Results of residual stress relating to orientation, failure depth, coating thickness are discussed along with the fatigue failure mode.

Residual stresses are inherent in thermal barrier coatings (TBC's) and can influence in-service performance and life of the coatings. Therefore, the effective design and processing of TBC's requires knowledge about residual stress generation and the effect of residual stresses on TEC life. Understanding residual stress generation and the effects on thermal barrier coating life are formidable tasks that have received little attention in the literature. This work addresses the first task. Specifically, the objectives of this work were to better understand how processing and post-processing residual stresses are generated in TBC's. The approach was to evaluate the effect of substrate temperature during processing and the effect of post-processing thermal cycling on the generation of coating residual stresses. Residual stress measurements were conducted using an experimental residual stress evaluation technique called the "Modified Layer Removal Method." Results showed residual stresses could be changed both by controlling the substrate temperature during processing and by thermal cycling after processing. Residual stresses in specimens with a higher substrate temperature during processing were found to be more compressive than residual stresses in specimens with a lower processing substrate temperature. Post-processing thermal cycling caused the residual stresses to become more compressive for specimens with both the higher and lower substrate processing temperatures. Residual stresses for one and ten post-processing thermal cycles were evaluated. For both substrate processing temperatures, the change in TBC compressive residual stresses for the first cycle was more than three times the total residual stress change that occurred in cycles two through ten. Interestingly, the increase in residual stresses in cycles two through ten for the higher substrate processing temperature was greater than that for the lower processing substrate temperature. In other words, based on results obtained here, compressive residual stresses generated during thermal cycling appear to depend on the existing processing residual stress. For these conditions, higher processing compressive residual stresses lead to higher post-processing changes in compressive stresses per thermal cycle.

A new instrument has been developed for measuring stresses due to particle quenching, temperature gradients during spraying, temperature fluctuations during coating formation and expansion mismatch between coating and substrate upon cooling. It records in situ and continuously the curvature of a substrate during spraying and upon cooling after spraying with a contacting displacement sensor. The substrate is fixed onto a pair of knife edges by springs. The knife edges are disposed on a water-cooled rotating cylindrical substrate holder and the substrate (2*15*100 mm 3 ) is parallel to the holder axis. The torch is moved back and forth parallel to the holder axis and the substrate temperature is recorded by a thermocouple spot welded to it. Examples of results with alumina coatings on steel substrates are presented.

Thermal barriers made up by a ceramic top coating and a metallic bond coating are subjected to thermal cycles in service. The thermal stresses vary during the cycles and the residual stresses change as a result of plastic flow and creep. The stress state in thermal barrier coatings during a thermal cycle has been examined with a finite element method using temperature dependent material data. The calculated results were verified by measurements of the residual stresses with the layer removal technique before and after cycling of specimens heated in furnace with air environment. According to the simulation of a thermal cycle to 700 ° C, using a finite element method, the bond coat is approximately stress free after 1 hour dwell time. Thus, the residual stresses after a thermal cycle is a result of thermal expansion mismatch and temperature drop.

The theory of residual stresses and of residual bending of plates with relatively thick sprayed coatings is summarized and some methods to minimize the bending are proposed. The particular case of an aluminium alloy substrate with a relatively thick plasma sprayed ceramic coating, with a large difference of thermal expansion coefficients, is discussed.