Thermally sprayed coatings of zinc, and in particular zinc-aluminium alloys, offer maximum corrosion protection for steel structures and reinforcing steel in concrete. They are primarily produced by arc or flame spraying. The surfaces of zinc and zinc alloy spray coatings can be protected by sealing top coats. This produces an optimum combination of passive and active corrosion protection and allows a service life of over 20 years. The development of new materials assumes intensive investigations. This paper provides an overview of the properties of thermally sprayed zinc and zinc-aluminium alloys as well as their microstructure and investigates the corrosion protection effect in tests and near-practical conditions.

The aim of the research project is to combine repair brazing with protective coating against hot-gas corrosion into a common integrated process. Both the braze-metal as well as the hot-gas corrosion protection coating is applied by means of thermal spraying. The material layout is to be realized as far as possible to the near net shape by using thermal spraying. The processes are to be performed in such a way that the brazing is integrated into the CVD diffusion annealing process as a transient liquid phase bonding (TLP bonding) process which, as a consequence, can then be eliminated as a separate processing step. The thermal spraying processes of atmospheric plasma spraying (APS), high velocity oxygen fuel spraying (HVOF) and cold gas spraying (CGS) are to be qualified for this purpose. Thus the project working hypothesis is to be able to transform thermal coating and joining processes into a common integrated hybrid process and, in doing so, obtain both high-quality and economic advantages. The importance of combining these processes lies in reducing the effort of grinding as well as economizing on the vacuum brazing, which is currently a separate process step, and consequently lowering the production costs.

The efficiency of aero-engines combustion chambers with thermal barrier coating (TBC) is improved when numerous cooling holes are laser drilled with inclined angles. However, during the laser drilling process, especially in the percussion mode, a detrimental crack can be generated at the TBC interface. Thus, each hole could be edged with a non-visible delaminated area underneath the ceramic top-coat. The present work is focused on the thorough study of the delamination induced by laser percussion drilling when interrupted drilling conditions are presented. Shallow angle drilling was applied on separated holes with 1 to 4 laser pulses respectively and various acute incident angles. Crack length was assessed by conventional metallographic preparation. A special experimental method was carried out in order to inspect the delaminated interface and the lateral edge of a semi-hole. This non-destructive assessment of the delamination of laser drilled TBC was complemented by a 3D imaging of a semi-hole using X-Ray microscopy. Results are presented with attention on both crack initiation and propagation during the laser percussion drilling of plasma-sprayed TBC.

To understand performance of thermal barrier coatings (TBCs) in various industrial applications of Siemens medium size gas turbines, effects of three types of thermal exposures i.e., high temperature isothermal exposure, thermal cycle fatigue (TCF) test, and burner rig test (BRT) on adhesion strength of an air plasma sprayed (APS) TBC have been studied and reported in this paper. It has been seen that the TBC adhesion strength is influenced by the type of thermal exposures differently. Together with a microscopic examination on TBC microstructures and fractography, a correlation between failure mechanisms and types of thermal exposures is discussed. In addition to the impact of various engine operation conditions on behavior of TBC, impacts of TBC surface roughness on turbine performance have also been evaluated. Surface profile and surface roughness on as-sprayed and polished TBC and cast metal (uncoated) have been measured and two different polishing methods have been compared. As a result, a requirement of TBC surface roughness and a preferable polishing method are suggested.

High temperature gas turbines require ceramic abradable coatings for sealing to ensure high efficiency. In this study, a novel method is proposed to deposit porous ceramic coatings through deposition of ceramic spray powder particles in the semi-molten state. The commercially available alumina (Al 2 O 3 ) powders were spheroidized and screened to a particle size range from 40 to 50 µm for spray deposition. Flame spraying was employed for coating deposition. During deposition, the substrate surface was kept at 500°C. The effect of melting degree of spray particles on coating microstructure was investigated by changing the flame power and spray distance. The porosity of flame-sprayed Al 2 O 3 coatings was estimated by image analysis on coating cross-sectional microstructures. The results showed that porous Al 2 O 3 coatings were successfully prepared with a porosity range up to 59% by flame spray. Moreover, spray parameters such as acetylene flow rate and spray distance have significant influences on the particle melting state, thus the microstructure and the porosity of the coating. With the decrease of acetylene flow rate and spray distance, the porosity of coatings increased due to the decrease of the melting degree of the sprayed particles. At a spray distance of 20 mm, when the acetylene flow rate was reduced from 400 to 200 L/h, the porosity increased from 37% to 59%. The results clearly demonstrated the feasibility to prepare porous abradable coatings of high porosity through surface-melted spray particles.

This paper presents the development of a solid homogeneous Aluminum Titanium Alloy wire which can produce, using conventional twin wire arc spray systems, a Thermal Sprayed Anti-Slip coating that will be effective in a range of applications and environments. In addition to its anti-slip characteristic the coating will also provide a hard wearing coated surface that provides corrosion protection to steel substrates. It will be shown that the macroscopically textured coating is extremely resistant to wear and maintains its anti-slip performance for a relatively long period even in high pedestrian traffic areas as assessed using the BS7976-2 2002 Pendulum Test method. Vehicle traffic applications will also be discussed. This paper will also demonstrate that the Aluminum Titanium Alloy has the same corrosion resistance as pure aluminum due to hard intermetallic particles within the alloy being chemically inert allowing this coating to be confidently used in environments in which TSA (Thermal Sprayed Aluminum) coatings are traditionally specified and used. Additionally the properties of the coatings are presented and compared to the wire feedstock to confirm the sprayed layer retains the required elements and all areas are equally durable.

Thermal sprayed coatings are often used for high temperature applications and, per se, are subjected to transient temperature gradients during operation. The recurrent temperature changes generate stresses that damage the coating with time, and can even lead to its delamination. The most common methods to evaluate coating behavior under thermal cycling are furnace testing or burner rigs. Both approaches cannot match the conditions reached in service for several applications, in terms of the achievable heating rates for instance. As a consequence, a versatile and robust method to evaluate coating resistance to spalling under thermal cycles is still to be found. This paper presents the development of a thermal cycling rig where the heat input is provided by a laser. This rig allows easy testing of several samples jointly for heating rates as high as 55°C/s and for thousands of thermal cycles. Preliminary trials have allowed the development of different spalling criteria. Finally, it was found that SS430-based materials arc-sprayed on Al substrates exhibit higher delamination resistance (life) under rapid heating/cooling cycles than SS304 coatings on the same substrate. For such high heating rates, the thermal stresses generated in the coating would be more critical than the thermal mismatch at the interface coating/substrate.

Compressor abradables coming into operational contact with bare, un-tipped titanium alloy rotor blades over a wide range of incursion conditions require excellent cuttability in order to avoid blade tip damage by wear and over-heating. This is more easily achieved for low temperature systems that can make use of low shear strength aluminum matrices than for compressor abradables operating closer to the maximum allowable temperature of advanced titanium alloy blade materials. In this case the rotor path linings will have to incorporate higher temperature resistant Ni and Co alloy matrices. To that end the availability of abradable coatings capable of operating at up to 550°C while showing little thermal ageing effects and excellent abradability over their entire service life can influence the compressor blade material selection and therefore compressor weight and performance characteristics. This paper provides an overview of titanium blade friendly compressor abradable concepts. Particular emphasis will be placed on the abradability of in-service and next-generation coatings designed for use up to the temperature capability of Ti blade rotor materials and beyond. Candidate coatings are also screened for other performance criteria such as thermal cyclic resistance and ageing behaviour.

Thermal barrier coatings (TBCs) are widely used to insulate the combustion chamber of internal combustion engines to improve their performance efficiency, reduce pollution, and protect the metals from high temperature oxidation. In this work, a TBC coating composition of 80% zirconium oxide and 20% gadolinium oxide (GdPSZ) was prepared in the laboratory and plasma spray coated on the combustion chamber of single-cylinder diesel engines. An engine performance test was conducted for both the baseline (uncoated) engines and the coated engines. The coatings in the combustion chamber of the engines were found to be well adhered after 300 hours of rigorous testing. A significant reduction in smoke density was observed, especially at higher loads, for the coated engines. However, the coated engines exhibited a 2-6% reduction in volumetric efficiency and an increase in brake-specific fuel consumption compared to the uncoated engines. The results for other performance parameters are also discussed.

Spray parameters play an important role on the microstructure and properties of plasma sprayed coatings. Parameters such as spray distance, plasma gas flow and current, raster speed and spray angle all can be varied. In this paper, an integrated study to investigate the effects and influences of spray angle on properties of yttria-stabilized zirconia coatings was carried out with spray angles of 60°, 75° and 90° (to the substrate surface). In situ coating property sensor (ICP) based on beam curvature measurements was used to measure the evolving stress and elastic moduli of the resultant coatings and combined with other characterization tools for thermophysical property and microstructure analysis, such as laser flash and scanning electron microscopy (SEM). The results indicate that the coating with 60° spray angle had the lowest thermal conductivity and more compliant structure. This study seeks to understand the mechanism for this effect and will provide important insight into parametric sensitivities on complex spray parts.

Surface treatment for sink rolls is an important issue due to the hostile environment in the zinc pot. Thermal sprayed WC-Co coating is one of the solutions to prolong the life of the rolls and to improve the quality of the galvanized steel sheets. In this study, a high velocity oxy-fuel (HVOF) spray gun was used to prepare nano-structured and conventional WC-Co coatings. After immersion in the zinc pot for six weeks, some coatings were seriously damaged. Cross-sectional microstructure and elemental analysis were performed to determine the corrosion morphology and mechanism. The results showed the nanostructured coating is less corrosion resistant than the conventional one in molten zinc. The molten zinc intrudes through cracks to attack the substrate which causes major destruction of the coating.

Gas turbine efficiency is of paramount importance in the modern carbon conscious global economy and the industry is always looking for ways to improve the efficiency of gas turbine engines. Gas bypass between the rotating turbine blade tip and the engine casing affects both the efficiency and the power output of an engine. An increase in this clearance of 125µm can result in an increase of 0.5% in specific fuel consumption. Abradable coatings have been designed to allow the turbine blade abrasive tip to cut a path into shroud abradable coating to improve the seal between the blade tip and the casing. A holistic approach to improving the abradable system – the abradable coating and the blade tip – is necessary. Better blade tips can result in use of denser, more erosion resistant abradables improving performance of the whole system. Current blade tips are limited as the matrix oxidizes at high temperature losing its ability to hold as well as protect the CBN particles. Improvement in blade tips – both in the cutting particles and the matrix which hold these particles – will therefore improve the abradable system performance, as well as allow the use of denser, more erosion resistant abradable materials. This paper represents efforts to improve the matrix oxidation resistance which holds CBN particles. The matrix is a low-aluminum MCrAlHf which is further aluminized to improve the oxidation resistance. New coatings being tested are enhanced aluminide coatings, platinum aluminide coatings and platinum chromide aluminide coatings. The results will be discussed in terms of matrix composition and microstructure as deposited and after static oxidation. The effect of matrix and its impact on the blade tip performance will also be reviewed.

This case study is the result of an investigation on HVOF Cr 3 C 2 -NiCr coating failure of on-off metal seated ball valve (MSBV) used in supercritical steam lines in a power plant and solution. HVOF Cr 3 C 2 -NiCr coating is used to protect thousands of MSBVs without incident. However, in this case the valves are challenged with exposure to rapid high pressure and temperature variation resulting in a unique situation where the coating experiences cracking and cohesive failure. A detailed investigation was undertaken by Velan, Kennametal Stellite, the National Research Council of Canada and the Ecole Polytechnique to determine possible failure cause and develop solutions in which highlights are presented.

Adhesive strength of the plasma-sprayed thermal barrier coating (TBC) is one of the most important parameters which influence the reliability during service. In the past, numerous test methods were reported to measure the coating adhesion. However, most of them require careful and time consuming preparation. Consequently, limited information could be obtained to establish the relationship between the processing conditions and the adhesive property. To produce more measurements using a simpler procedure, the interfacial indentation test and the modified tensile adhesive test are examined. In this paper, the interfacial fracture toughness of the plasma-sprayed ZrO 2 coatings, deposited on Al substrates, were evaluated by these two tests. In order to study the effects of the powder injection, samples were sprayed with various carrier gas flow rates. The test results show a certain correlation between the melting index and the interfacial fracture toughness. In addition, variations between the results obtained from the two different methods are discussed.

This paper reports the corrosion behaviour of coatings deposited by high velocity oxy-fuel (HVOF) spraying and representative boiler substrate alloys in simulated high temperature biomass combustion conditions. Four commercially available oxidation resistant Ni alloy coating materials were selected: NiCrBSiFe, alloy 718, alloy 625 and alloy C-276. These were sprayed onto P91 substrates using a JP5000 spray system. The corrosion performance of the coatings varied when tested at 525, 625 and 725°C in K 2 SO 4 - KCl and gaseous HCl-H 2 O-O 2 containing environments. Alloy 625, NiCrBSiFe and alloy 718 coatings performed better than alloy C-276 coating at 725°C, which had very little corrosion resistance resulting in degradation similar to uncoated P91. Alloy 625 coatings provided good protection from corrosion at 725°C, with the performance being comparable to wrought alloy 625, with significantly less attack of the substrate than uncoated P91. Alloy 625 performs best of these coating materials, with an overall ranking at 725°C as follows: alloy 625 > NiCrBSiFe > alloy 718 >> alloy C-276. Although alloy C-276 coatings performed poorly in the corrosion test environment at ~725°C, at lower temperatures (i.e. below the eutectic temperature of the salt mixture) it outperforms the other coating types studied.

Typically, standard alloys do not have the wear resistance properties necessary to combat the aggressive wear and corrosive conditions prevalent throughout the oil sands mining process. For production-critical components, it is common to apply tungsten carbide-based metal matrix composite (WC-MMC) overlays to extend equipment life and prevent unplanned outages. The performance of composite overlays is very much dependent on the wear-environment. This paper will discuss how the interactions between abrasive conditions and the mechanical and structural properties of the WC-MMCs are key in determining the resultant levels of performance. Such information can lead to a better selection of materials and subsequent extended component life.

Minimizing wear of mining components in the oil sands industry is key to increasing productivity and decreasing excessive maintenance costs. A significant amount of research has gone into the selection of appropriate materials for improved wear protection. Tungsten carbide overlays are applied to the most critical components, typically by plasma transferred arc welding (PTA-W). This study aims at investigating the effects of several commercial PTA torches in terms of overlay microstructure and performance. A commercial tungsten carbide-NiCrBSi metal matrix composite (MMC) was used as the overlay material. A variety of parameters were studied when comparing the torches; including heat input, powder delivery, and deposition pattern. The effect of the overlay microstructure was examined using optical, digital and electron microscopy. The overlay performance was gaged using dry sand abrasion testing (ASTM G65-04). The type of torch, powder delivery method or power source did not have any significant effect on the quality or performance of the overlay in terms of microstructure or abrasion resistance.

Turbine blades are generally protected by thermal barrier coatings (TBCs) against high temperature oxidation and corrosion. A novel method has been developed to prepare nanostructured Al 2 O 3 powders for thermal-spraying (atmospheric plasma spray) with high flowability. In this method, nano Al 2 O 3 powders are granulated and then heat treated at 200°C which become suitable to be used in thermal spraying equipment. The normal Al 2 O 3 and granulated nano Al 2 O 3 powders were sprayed separately by using an atmospheric plasma spray device onto a typical TBC consisting of a superalloy bond coat and an YSZ top coat. Then, TBC/ normal Al 2 O 3 and TBC/ nano Al 2 O 3 coatings were oxidized at 1000°C for 24h and allowed to form the thermally grown oxide (TGO) layer onto the bond coat (NiCrAlY layer). The flowability of the granulated nano Al 2 O 3 powders was studied by using a Hall flowmeter. The microstructural characterization showed that the granulated nano Al 2 O 3 powders had very high flowability. The increased apparent density and flowability of the granulated nano Al 2 O 3 powders had substantially reduced the micro-cracks and interconnected porosities in the coating in comparison with normal Al 2 O 3 coating. The thickness of the TGO layer in YSZ/ normal Al 2 O 3 coating was higher in comparison with YSZ/ nano Al 2 O 3 coating after oxidation. Thus by using nano Al 2 O 3 as a third layer, the thickness of the TGO layer and oxidized regions inside the bond coat decreased effectively which led to less mechanical stresses and can cause the improvement of TBC life.

According to a recent survey, the thermal spray market in Japan is approximately 100 billion yen. The market consists of three sectors, coating services (45 billion yen), in-house production (45 billion yen), and sales of thermal spray equipment and consumables (10 billion yen). This paper provides an overview of the coating services market broken out by application segments. It also discusses future trends.

This paper provides a summary of the status and potential of thermal spray activities in Korea. It describes how various market sectors in Korea are using spray coating technology and presents annual sales estimates and share of market data broken out by industry, spraying technology, and patents awarded. It also reviews recent advancements in the development and use of thermal spray technology in Korea along with anticipated future impacts.