Edmund F. Rybicki

Effects of Coating Thickness and Residual Stresses on the Bond Strength of ASTM C633-79 Thermal Spray Coating Test Specimens

Wire-arc-sprayed nickel-aluminum is widely used in the aircraft industry for dimensional restoration of worn parts and as a bond coat for thermal barrier coatings and other top coats. Some repair applications require thick coatings, which often result in lower bond strength. A mechanism being investigated to ex-plain this decrease in bond strength is the free edge effect, which includes both coating residual stresses and coating thickness. The layer-removal method was used to determine experimentally the residual stresses in wire-arc-sprayed nickel-aluminum coatings of different thicknesses. Bond strength evalu-ations were performed using an improved ASTM C 633-79 test specimen. Finite-element analysis and fracture mechanics were used to investigate the effects of coating thickness and residual stress state on coating bond strength.

The effect of coating residual stress on the fatigue life of thermal spray-coated steel and aluminum

Abstract The acceptance of thermal spray coatings in many applications depends on the effect of the coating on the fatigue performance of the coated part. One of the factors that influences the fatigue life of thermal spray-coated components is the residual stress in the coating. This study investigates the fatigue performance of tungsten carbide–cobalt (WC–Co) thermal spray coating systems. Bending fatigue tests of specimens with WC–Co coatings on both 4130 steel substrates and 6061 aluminum substrates were conducted. The through-thickness residual stress level in the thermal spray coatings was determined using the modified layer removal method. The effect of the residual stresses on the fatigue life of the coated specimens was analyzed. It was found that there is a direct relation between the residual stress in the coating and the fatigue life of the coated part. Fatigue life can be changed by a factor of ten due to the level of compressive residual stress in the coating.

Through-thickness residual stress evaluations for several industrial thermal spray coatings using a modified layer-removal method

Residual stresses are inherent in thermal spray coatings because the application process involves large temperature gradients in materials with different mechanical properties. In many cases, failure analysis of thermal spray coatings has indicated that residual stresses contribute to reduced service life. An estab-lished method for experimentally evaluating residual stresses involves monitoring deformations in a part as layers of material are removed. Although the method offers several advantages, applications are lim-ited to a single isotropic material and do not include coated materials. This paper describes a modified layer-removal method for evaluating through-thickness residual stress distributions in coated materials. The modification is validated by comparisons with three-dimensional finite-element analysis results. The modified layer-removal method was applied to determine through-thickness residual stress distributions for six industrial thermal spray coatings: stainless steel, aluminum, Ni-5A1, two tungsten carbides, and a ceramic thermal barrier coating. The modified method requires only ordinary resistance strain-gage measuring equipment and can be relatively insensitive to uncertainties in the mechanical properties of the coating material.

The effect of residual stress in HVOF tungsten carbide coatings on the fatigue life in bending of thermal spray coated aluminum

One factor that affects the suitability of tungsten carbide (WC) coatings for wear and corrosion control applications is the fatigue life of the coated part. Coatings, whether anodized or thermal spray coated, can reduce the fatigue life of a part compared to an uncoated part. This study compares the fatigue life of uncoated and thermal spray coated 6061 Al specimens. The relation between the residual stress level in the coating and the fatigue life of the specimen is investigated.Cyclic bending tests were performed on flat, cantilever beam specimens. Applied loads placed the coating in tension. Residual stress levels for each of the coating types were determined experimentally using the modified layer removal method.Test results show that the fatigue life of WC coated specimens is directly related to the level of compressive residual stress in the coating. In some cases, the fatigue life can be increased by a factor of 35 by increasing the compressive residual stress in the coating.

A Cantilever Beam Method for Evaluating Young's Modulus and Poisson's Ratio of Thermal Spray Coatings

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, fracture toughness, and fatigue crack growth rates. It is difficult to evaluate Young’s modulus and Poisson’s ratio of thermal spray coatings be-cause coatings are usually thin and attached to a thicker and much stiffer substrate. Under loading, the substrate restricts the coating from deforming. Since coatings are used while bonded to a substrate, it is desirable to have a procedure to evaluate Young’s modulus and Poisson’s ratio in situ.The cantilever beam method to evaluate the Young’s modulus and Poisson’s ratio of thermal spray coat-ings is presented. The method uses strain gages located on the coating and substrate surfaces. A series of increasing loads is applied to the end of the cantilever beam. The moment at the gaged section is calcu-lated. Using a laminated plate bending theory, the Young’s modulus and Poisson’s ratio are inferred based on a least squares fit of the equilibrium equations. The method is verified by comparing predicted values of Young’s modulus and Poisson’s ratio with reference values from a three-dimensional finite ele-ment analysis of the thermal spray coated cantilever beam. The sensitivity of the method is examined with respect to the accuracy of measured quantities such as strain gage readings, specimen dimensions, ap-plied bending moment, and substrate mechanical properties. The method is applied to evaluate the Young’s modulus and Poisson’s ratio of four thermal spray coatings of industrial importance.

Mode I and Mixed Mode Energy Release Rate Values for Delamination of Graphite/Epoxy Test Specimens:

This study is part of a larger program directed at developing guidelines for preventing web-to-stiffener debonding in stiffened composite panels. An important ingredient needed to attain this goal is the identification of a criterion to predict debonding. The objective of this work is to evaluate the energy release rate as a criterion for predicting debonding. A combined experimental and finite element modelling approach was used. Comparisons of strains and displacements measured prior to debonding and the corresponding values calculated with the finite element models are presented to illustrate the capability of the model to represent the behavior of the test specimens. Energy release rate values asso ciated with debonding of two types of graphite/epoxy composite test specimens were eval uated. One test specimen was the Mode I double cantilever beam. The other was a mixed mode (Mode I and Mode II) cracked lap shear specimen. Test specimens were loaded in laboratory conditions until the onset of debonding was observed. Critical load levels and the geometric and material properties of the test specimens were input data for the finite element analysis which evaluated the Mode I and Mode II components of the energy release rate at the onset of debond propagation. The tests provided a range of ratios of Mode II to Mode I energy release rates from 0 to 5.0. Data are shown in a graph of G, versus G~ for both types of test specimens. A discussion of the energy release rate as a criterion for debonding is presented.

Sliding wear behavior of tungsten carbide thermal spray coatings for replacement of chromium electroplate in aircraft applications

Tungsten carbide (WC) thermal spray coatings have gained increased acceptance for commercial aircraft applications driven by the desire to replace chromium electroplate due to environmental and economic considerations. In order to confidently replace electroplated chrome with WC thermal spray coatings in aircraft applications, the coatings must demonstrate fatigue and wear characteristics as good as or better than those of electroplated chrome. Previous research in this area has shown that the fatigue life of the WC thermal spray coatings can be improved by inducing compressive residual stresses in the coating. This paper compares the wear characteristics of several types of WC thermal spray coatings with those of electroplated chrome in sliding wear tests using the “block-on-ring” procedures described in the ASTM G77 standard. Wear results are interpreted in terms of coating residual stresses and in terms of x-ray diffraction (XRD) and scanning electron microscope (SEM) analyses.

Residual stresses as a factor in the selection of tungsten carbide coatings for a jet engine application

Tungsten carbide thermal spray coatings are important to the aerospace industry for the mitigation of midspan damper wear on jet engine fan and compressor blades. However, in some cases the coating can fail due to spallation and cracking, and in other situations the fatigue life of a fan or compressor blade is reduced when a coating is applied. Coating failures can result in decreased engine performance and costly maintenance time. A comprehensive experimental research program was conducted to evaluate coating crack resistance in bending, low-cycle fatigue properties of the coating and substrate, coating performance in jet engine tests, and microstructures for a wide range of coating compositions and application processes. Coating residual stress distributions also were evaluated. Eleven coatings were ranked according to their performance relative to the other coatings in each evaluation category. Results from the bend and low-cycle fatigue evaluations were compared to the experimentally evaluated residual stresses. Comparisons of rankings indicate a strong correlation between performance and the residual stresses in the coatings. Results from the program were used to select a suitable coating system for final in-service use based on two important criteria: (1) the coating must not fail while in service, and (2) the coating must not induce crack propagation into the substrate of the midspan damper.

Erosion-Corrosion of a Carbon Steel Elbow in a Carbon Dioxide Environment

Abstract For many conditions, erosion-corrosion can produce higher wall penetration rates than erosion or corrosion alone. While flow velocity generally is believed to be an important factor, more ...

Generalization of the API RP 14E Guideline for Erosive Services

The commonly used practice for controlling sand erosion in gas and oil producing wells is to limit production velocities following the provisions of American Petroleum Inst. (API) RP14E. This guideline contains a procedure to calculate a threshold velocity, the flow velocity below which an allowable amount of erosion occurs. While providing the advantage of computational ease, the approach has some disadvantages. One is that, while many factors influence the erosion rate, APE RP14E includes only one factor, the density of the medium. Thus, such factors as flow geometry, type of metal, sand size, and Reynolds number are not accounted for. In this paper, the method is applied to calculate threshold velocities. The computational procedure allows an allowable amount of erosion to be specified in mils per year for elbows, tees, and direct impingement geometries. Threshold flow-stream velocities are calculated for carrier fluids of crude oil, water, and methane at elevated pressure. Resulting threshold velocities are presented for a range of sand sizes, pipe diameters, sand production rates, and methane pressures. Results show that threshold velocities for single-phase gas flows are much lower than those for single-phase liquid flows.