Characterization of Thermal Spray Coatings Used for Dimensional Restoration

2000 ◽  
Author(s):  
R.T.R. McGrann ◽  
J. Kim ◽  
J.R. Shadley ◽  
E.F. Rybicki ◽  
N.-G. Ingesten
Keyword(s):  
Residual Stress ◽  
Bond Strength ◽  
Thermal Spray ◽  
Thermal Spray Coatings ◽  
Tensile Bond Strength ◽  
Spray Angle ◽  
Metallographic Examination ◽  
Substrate Interface ◽  
Spray Coatings ◽  
Spray Process

Abstract 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.

Prediction of Residual Stress and Damage in Thermal Spray Coatings Using Hybrid Computational Approach

2018 ◽  
Author(s):  
Abba A. Abubakar ◽  
Abul Fazal M. Arif ◽  
Khaled S. Al-Athel ◽  
S. Sohail Akhtar
Keyword(s):  
Residual Stress ◽  
Thermal Spray ◽  
Point Cloud ◽  
Hybrid Approach ◽  
Computational Approach ◽  
Thermal Spray Coatings ◽  
Element Analysis ◽  
Influence Of Process Parameters ◽  
Spray Coatings ◽  
Spray Process

Due to the multilayered pattern of coating deposition, numerical prediction of residual stress and damage in thermal spray coatings (TSCs) has been challenging. Several numerical approaches previously used failed to capture essential aspects such as deposition stress build-up, presence of heterogeneities, and influence of process parameters. In the present study, a hybrid computational approach which combines “point cloud” (PC) and finite elements (FE) has been used to model the spray process as well as the evolution of residual stress and damage. Smooth particle hydrodynamics (SPH) is used to model multiple droplets deposition and associated deformation on PC. Then, several recent algorithms (for point cloud processing) are used to convert the deformed droplets (in form of PC) into FE domains (i.e. splats). The FE mesh of deposited splats is used for thermo-mechanical finite element analysis where the evolution of temperature, residual stress and damage is predicted on simulated coating microstructure. By comparing our numerical results with that of previous works, the hybrid approach has been found to be a viable tool for quantitative assessment of residual stresses and failure in TSCs.

Modeling Residual Stress Development in Thermal Spray Coatings: Current Status and Way Forward

2017 ◽  
Vol 26 (6) ◽  
pp. 1115-1145 ◽  
Author(s):  
Abba A. Abubakar ◽  
Abul Fazal M. Arif ◽  
Khaled S. Al-Athel ◽  
S. Sohail Akhtar ◽  
Javad Mostaghimi
Keyword(s):  
Residual Stress ◽  
Thermal Spray ◽  
Thermal Spray Coatings ◽  
Current Status ◽  
Stress Development ◽  
Spray Coatings

Fatigue Life in Bending and Coatings Residual Stress in Tungsten Carbide Thermal Spray Coatings

1997 ◽  
Author(s):  
R.T.R. McGrann ◽  
J.R. Shadley ◽  
E.F. Rybicki ◽  
D.J. Graving ◽  
B.E. Badger ◽  
...  
Keyword(s):  
Residual Stress ◽  
Fatigue Life ◽  
Tungsten Carbide ◽  
Thermal Spray ◽  
Thermal Spray Coatings ◽  
Chrome Plating ◽  
Spray Coatings ◽  
Wear Protection ◽  
Carbide Coatings ◽  
Flat Beam

Abstract 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.

Variation in Residual Stress Field During Fatigue Failure of Thermal Spray Coatings

2000 ◽  
Author(s):  
R. Ahmed ◽  
M. Hadfield ◽  
S. Tobe
Keyword(s):  
Residual Stress ◽  
Thermal Spray ◽  
Fatigue Failure ◽  
Contact Stress ◽  
Rolling Direction ◽  
Rolling Contact ◽  
Thermal Spray Coatings ◽  
Residual Stress Field ◽  
Stress Measurements ◽  
Spray Coatings

Abstract A non-destructive experimental approach was adapted to investigate the variations in residual stress fields within thermal spray coatings. WC-Co coatings produced by a HVOF technique were considered for concentrated rolling sliding contacts in this study. These coatings were produced in various thicknesses on various substrates. Residual stress measurements were made using an x-ray diffraction technique, along and across the rolling direction. A modified four-ball machine was used to conduct rolling contact fatigue tests under various tribological conditions of contact stress, lubrication and contact configuration. Residual stress measurements were made before and after the tribological tests. Failed rolling elements were analyzed using scanning electron microscopy, electron probe microscopy and surface interferometry. Results indicate that the magnitude of compressive residual stress attenuates during fatigue failure. The magnitude of attenuated residual stress was dependent upon the type of tribological failure. This attenuation of residual stress was attributed to the microcracking of coating under the influence of contact stress.

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