Analytical and Experimental Study of Residual Stresses in Thermal Barrier Coatings Deposited on Gas Turbine Blades in Laboratory Dimensions by APS and HVOF Methods to Calculate the Optimal Thickness

The constant need to use gas turbines has led to the need to increase turbines' inlet temperature. When the temperature reaches a level higher than the material's tolerance, phenomena such as creep, changes in mechanical properties, oxidation, and corrosion occur at high speeds, which affects the life of the metal material. Nowadays, operation at high temperatures is made possible by proceedings such as cooling and thermal insulation by thermal barrier coatings (TBCs). The method of applying thermal barrier coatings on the turbine blade creates residual stresses. In this study, residual stresses in thermal barrier coatings applied by APS and HVOF methods are compared by Tsui–Clyne analytical model and XRD test. The analytical model results are in good agreement with the experimental results (between 2 and 8% error), and the HVOF spray method creates less residual stress than APS. In the end, an optimal thickness for the coating is calculated to minimize residual stress at the interface between the bond coat and top coat layers.

The Cavitation Resistance of WC-CoCr Cermet Coating Deposited by HVOF for Hydraulic Application

Thermal spray is a versatile technique for enhancing the cavitation resistance of hydraulic water passage components, especially in on-site repair situation. Based on the application in hydraulic components, a WCCoCr cermet coating was deposited by high velocity oxygen fuel spraying. The microstructure and hardness were characterized, and the cavitation was studied by ultrasonic vibratory system according to ASTM G32 standard. The coating shows superior anti-cavitation behavior in term of mass weight loss compared with the martensite stainless steel. The cavitation erosion mechanism is elaborated with wrinkles and craters observed on the worn surfaces, and correlated with the incubation and accelerating stages.

Thermal Sprayed Composite Coatings for Biomedical Implants: A Brief Review

The implant materials used currently in field of cardiovascular and orthopedics surgery dearth in osteoconductivity. Different surface modification techniques are used, developed and investigated over the years to enhance the osteoconductivity of biomaterials like metals, polymer and ceramics. Although implants made up of metals are strong mechanically but have low bonding ability due to bio-inert nature.To overcome the limitations and to accomplish the desired purpose, composite coatings consisting of bioactive are developed on the metallic biomaterials. In general bio-inert ceramics like yttria stabilized zirconia (ysz), titania, and alumina may be incorporated into hydroxyapatite (HA) matrix to develop composite coatings with improved mechanical properties over the years. The composite coatings developed by thermal spraying have shown promising approach to have good mechanical and biological properties in comparison with single-component and/or monolayer coatings. The strategy to use composite coatings is adopted widely by the professionals/scientists in the area of biomaterials for development and production of materials in order to repair and regeneration of the human tissue. In this article, commercially used thermal spraying techniques used for deposition of composite coatings for biomedical implants are discussed.

HVOF Sprayed Mullite Coatings for Use In Extreme Environments

Products used in industry and marine applications are exposed to extreme environments like high heat, humidity, acidic or alkaline or hyper saline environment, UV and IR radiation. Metals with good corrosion and oxidation resistance may be used but are restricted to Ni and Cr alloys, titanium and super alloys etc., which are costly and have their limitations. Hence ceramic coatings on low cost metals may be an answer to this problem. Ceramics are inherently chemically inert, high temperature resistant, corrosion and oxidation resistant. Flame spraying of ceramics is a good and reliable method for applying ceramic coatings on metallic substrates with good bond strength (> 80 MPa) and 1% porosity. In this work, HVOF technique is applied to obtain 100 microns thick mullite coatings on MS substrates with a NiCr bond coat. Mullite has a high oxidation and corrosion resistance. It is chemically inert. It has high temperature resistance even at 2000 C. These properties are ideal for industrial components exposed to salty environments. Characterization studies like XRD, SEM/EDS, Corrosion tests using polarization technique, coating thickness and surface roughness have been studied and reported. A corrosion rate of 1.55 mm/ year has been achieved in a sea water environment.

Effect of Target Coating Thickness on the Abrasive Waterjet Machining Response of 8YSZ Segmented Thermal Barrier Coatings

Segmented thermal barrier coatings (STBC) are a more strain-tolerant and erosion resistant alternative of the conventional porous thermal barrier coatings for hot gas components in gas turbines. Due to their relatively higher thermal conductivity and the ever-increasing turbine inlet temperature, there is a need for thicker STBCs. This paper presents an investigation of the microstructures and properties of STBCs with respect to coating thicknesses. Coatings with two significantly different thicknesses were fabricated under identical conditions and evaluated using standard metallographic methods. It was found that the microstructures for thin and thick coatings had subtle differences and hardness values decreased with in increasing thickness. When subjected to an abrasive waterjet machining process, the thicker STBCs showed a higher wear rate. A hypothesis is proposed to explain the observed thickness dependent changes and the microstructures and properties of STBCs.

An Investigation on Splat and Flattening Behavior of Thermally Sprayed Copper on A Rough Surface: A New Approach

All thermal spray coatings are finally deposited on a rough and active grit-blasted surface of the job. But, available literatures are reporting splat and flattening behavior on a polished surface. There is a gap in thermal spraying to understand actual solidification on a rough surface. Therefore, in the present work an attempt has been made to study of splat formation of thermally sprayed copper onto grit-blasted rough surface. An optimization study is done to collect rounded/semi-rounded disk like splats to set spray parameters. Optimized parameters were also tested at four different gun traverse speeds to fabricate thin Cu coatings (30-50µm).

Coatings of Carbide-Metal Systems (Cr3C2-NiCr and WC-Co-Cr) Deposited by High-Velocity Atmospheric Plasma Spraying from Specially Modified Fine-Grained Powders

In this article, the author poses and responds to a provocative question that has practically ceased to be asked in the field of thermal spraying of carbide coatings: Is the current worldwide trend of using only unmelted metal particles to form coatings (HVOF, HVAF and cold spray methods) correct, and is the deposition of completely melted powders (plasma spraying methods) really outdated? The results of high-speed plasma deposition of new carbide powders allowed the author to prove that only coatings from molten particles can solve the main problem of such coatings, namely the problem of permeability along the grain boundaries. Through the use of modern Axial III plasma torches combined with optimized fine-grained powders, it has been possible to create Cr3C2-NiCr as well as WC-Co-Cr gas-tight layers.

Sensitivity of Process Parameters in Atmospheric Plasma Spray Coating

Atmospheric plasma spraying (APS) is one of the most widely used thermal spraying technique which finds a lot of applications due to its versatility of spraying a wide range of materials from metallic to nonmetallic and hence more suitable for spraying of high melting point materials like refractory ceramics material, cermets etc. In recent era,any material can be used for plasma spraying on almost any type of substrate. Process parameters are the key factor that affects the formation of microstructures, bonding of coating with substrate and mechanical strength of coating. In this paper, the process parameters and their sensitivity towards the plasma modified structural elements are discussed.The microstructure of thermally sprayed coatings, which results from the solidification and sintering of the particles, frequently contain pores, oxides and cracks. The amount and distribution of these defects, as well as other coating properties as for instance thickness, hardness and bond strength, will be defined by the selected spray parameters. Therefore, the correct choice of the spray process as well as respective parameters (particle size, particle velocity, spray distance, plasma gun power, working pressure, substrate roughness, substrate temperature and so on) is very important for the deposition of good coatings and, consequently, to enlarge the useful life in service of the components.

ChroSiMol: A Completely Different Coating Against Corrosion and Wear

The paper deals with new layers of the systems Cr-Mo-Ni-Si and Cr-Mo-Co-Si bearing the common name "ChroSiMol". These layers are applied by means of atmospheric plasma spraying on metallic substrates of iron, nickel, cobalt, and titanium-based alloys and serve as corrosion, oxidation and wear protection for these metals at temperatures up to max. 900°C. The layer ChroSiMol is an acid-resistant layer and provides for its substrates a perfect protection against hot sulfuric acid and phosphoric acid in all concentrations. With a hardness of approx. 900 HV and very good anti-friction properties of the molybdenum-containing surface, these layers are optimally suited for use with friction pairs with corrosive loads.