Cyclic, mechanical fatigue testing of coated materials has increased with the wider use of thermal spray coatings on components experiencing fatigue loading. Fatigue testing of coated specimens presents all of the difficulties associated with fatigue testing of uncoated specimens and several difficulties that do not arise when testing uncoated specimens. A summary of fatigue test methods and test specimen geometries for both coated and uncoated specimens is presented. Issues of specimen standardization, geometry, substrate preparation, and post-spray surface finishing are discussed. Several specimen configurations are described.

Thick thermal spray coatings are used to repair worn parts during aircraft overhaul. The thermal spray coating is used to restore a part to its original dimensions. Characteristics of the as-applied coating that affect the performance of thermal sprayed parts are the residual stress in the coating, the tensile bond strength, the amount of porosity, oxides and impurities near the coating/substrate interface, and the hardness of the coating. An understanding of the relation of these coating characteristics to process variables such as the material used for the coating, spray process, spray angle, and thickness of the applied material is needed. In this paper, four thermal spray coatings, Ni5Al, Ni5Al-atomized, (NiCr)6Al, and Inco 718, on a substrate of Hastelloy X are investigated. These materials are applied using two different thermal spray application processes: plasma spray and High Velocity Oxy-Fuel (HVOF). Spray angles of 90° and 45° are used during spraying. The nominal thickness of the applied coatings ranges from 0.4 mm to 1.8 mm. The thermal spray coatings are evaluated in four types of tests. Residual stresses in the coatings and substrate are evaluated using the modified layer removal method. A tensile bond strength test is performed. Metallographic examination is used to determine the porosity and content of oxides and bond zone impurities (percent) of the applied materials. In addition, the hardness of the coating is measured. For the materials and conditions investigated, it is found that residual stress varies with each of the four process parameters. The bond strength for plasma sprayed coatings is related to the type of material and possibly to the coating thickness. The percent porosity varies with coating material, but, for Ni5Al, it does not depend on application process. Oxide content, as a percentage, varies with material and process, but not with spray angle and thickness. The percentage of impurities near the coating/substrate interface varies with process and, for the specimens that were coated using the HVOF process, with thickness. The hardness of the coating was found to vary with material and spray process. For three of the four coatings, hardness increases with thickness but, for Inco 718, hardness decreases as thickness increases.

Thermal spray coatings are subjected to mechanical loadings in many applications, and there is a need to evaluate the mechanical properties of these coatings. Mechanical properties of interest in the performance of thermal spray coatings include fatigue life, wear resistance, bond strength. Young's modulus, Poisson's ratio, and residual stresses. One property that has a large effect on the performance of thermal spray coated parts is the residual stress distribution in the thermal spray coating and in the substrate. Thus, it is important to have (1) a fundamentally sound method for evaluating residual stresses and (2) a written recommended procedure for applying the method. ASM International is not a standard writing organization. Yet, the increased use of thermal spray coatings and the need for documentation on methods for evaluating mechanical properties of thermal spray coatings have generated a need to prepare Recommended Practices. To meet this need, the ASM International Thermal Spray Society has formed three subcommittees to prepare Recommended Practices for thermal spray coatings. This paper describes a draft form of a Recommended Practice for evaluating residual stresses in thermal spray coatings. This Recommended Practice is being developed by the Subcommittee on "Evaluating of Mechanical Properties of Thermal Spray Coatings". The method, called the Modified Layer Removal Method, has been presented in several papers and has been used for a variety of different coatings. The paper describes the dimensions of the test specimen, the equipment needed, the procedure for removing layers, and the methods for collecting and interpreting the data to evaluate through thickness residual stresses. The Recommended Practice (RP) is in Draft form, but is presented to let the thermal spray community know about the RP effort and invite comments and volunteers to write other RP's.

In situ values of Young's modulus and Poisson's ratio for thermal spray coatings are needed to evaluate properties and characteristics of thermal spray coatings such as residual stresses, in-service stresses, bond strength, fracture toughness, and fatigue crack growth rates. It is important to have methods documented in detail so that people can follow the document and use the methods. Such a document requires more pages than are allowed in conference proceeding and journal papers. Thus, Recommended Practices and Standards describing these methods are needed. Currently, there is not a recommended practice or standard for evaluating Young's modulus and Poisson's ratio for thermal spray coatings. The ASM International Thermal Spray Society has recognized this need and formed a committee on Recommended Practices for Thermal Spray Coatings. This paper describes one of the recommended practices being written by the Mechanical Properties Evaluation Subcommittee of the Recommended Practices Committee. The specimen is a coated substrate in the form of a cantilever beam. The method is easy to use and inexpensive. The equipment needed is a vise or clamping fixture, strain gages, a strain indicator, a micrometer, a ruler, a hanger, and a set of weights. The specimen is easy to machine and spray. The loading is easy to apply and remains constant during readings. The method can be used to evaluate Young's modulus and Poisson's ratio in tension or compression. A description of the method, a verification, and a sensitivity analysis was done and published in Reference [1]. Some of the details of implementing the method and the data sheet are presented here.

One candidate alternative to chrome plating and hard anodizing is a tungsten carbide (WC) coating applied by the High Velocity Oxy-Fuel (HVOF) process. HVOF WC coatings are currently being evaluated in many service life tests, including fatigue. The purpose of this paper is to compare the fatigue life of HVOF WC coated specimens with the fatigue life of hard anodized and bare aluminum specimens. This work examines WC thermal spray coatings as candidates for replacement of hard chrome plating and hard anodizing in aircraft and helicopter applications such as landing gear. In fatigue testing, the results showed an expected fatigue deficit for hard anodizing as compared to bare aluminum. Paper includes a German-language abstract.

Tungsten carbide thermal spray coatings have been used for more than twenty years in the commercial aircraft industry in applications such as turbine blade and flap-track wear surfaces. Additionally, the evaluation of tungsten carbide (WC) coatings to replace chrome plating in other aircraft applications has been underway for several years. For example, WC coatings applied by the high velocity, oxy-fuel (HVOF) process are being evaluated for use on aircraft landing gear parts. One factor that affects the suitability of WC coatings is the fatigue life of the coated part. This study compares the fatigue life of electrodeposited chrome plated specimens to the fatigue life of WC HVOF-sprayed specimens on aircraft landing gear alloys. Fatigue tests were run on cantilever flat beam specimens coated on one side and subjected to bending fatigue loads. Residual stress levels for the coatings were determined using the Modified Layer Removal Method on rectangular residual stress specimens processed with the flat beam specimens. Also, the Young's modulus and Poisson's ratio of the coating were determined using the Cantilever Beam Bending Method performed on beam specimens that were processed with the fatigue specimens and the residual stress specimens. Results indicate that certain levels of residual stress in the coating can enhance the fatigue life of the parts. The fatigue lives in bending tests of several WC coated specimens are compared with the fatigue life of chrome plated specimens.

The Young's modulus of the ceramic top coat of a plasma sprayed thermal barrier coating (TBC) has been reported to vary by as much as a factor of three with changes in processing parameters and by as much as a factor of four due to prolonged thermal exposure. Since the residual stress is expected to vary directly with the modulus of the ceramic layer, it follows that a change in modulus will change the residual stresses in the ceramic layer. The objective of this study was to evaluate the modulus of plasma sprayed coatings as a function of thermal cycle exposure and silica content of the ceramic. The study employed the Cantilever Beam Bending Method to examine Young's modulus for an yttria stabilized zirconia TBC applied by plasma spraying, for zero and ten thermal cycles and for silica contents of 0.1% and 1.0%. Results are discussed in terms of mechanisms that may affect modulus and the effect of modulus variations on residual stresses.

Tungsten caibide (WC) thermal spray coatings are being used for wear protection on selected components of aircraft. Tungsten carbide coatings are being used on aircraft flap tracks and fan and compressor blade mid-span dampers. However, a larger use of tungsten carbide coatings is being considered for other commercial aircraft applications where it would be used as a replacement for chrome plating. For instance, WC coatings are currently being tested on aircraft landing gear parts. One factor that affects the suitability of WC coatings for these applications is the fatigue life of the coated part. Coatings, whether chrome plating or thermal spray coating, can reduce the fatigue life of the part compared to an uncoated part. This study compares the fatigue life of uncoated 6061 aluminum specimens to the fatigue life of WC thermal sprayed coated 6061 aluminum specimens. The relation between the residual stress level in the coating and the fatigue life of the specimens is also investigated. Fatigue tests were run on cantilever flat beam specimens that were coated on one side. Specimens were cycled in bending so that the coatings experienced tensile fatigue stresses. Residual stress levels for each type of coating were determined using the Modified Layer Removal Method on specimens processed along with the cantilever flat beam specimens. Test results show that the fatigue life of the WC coated specimens is directly related to the level of compressive residual stress in the coating.

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.

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.

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.