Tanvir Hussain

Cold Spraying of Titanium: A Review of Bonding Mechanisms, Microstructure and Properties

Cold gas dynamic spraying (CGDS) is a relatively new branch of surface engineering that involves modification of the surface of substrates to provide specific engineering advantages, which the substrate alone cannot provide. Cold spraying, as a metal deposition technique, involves spraying of typically 10-40 μm particles which are accelerated by a propellant gas to 300-1200 m/s at a temperature well below the melting point of material, and upon impact deform and adhere to the substrate. The deposition process in cold spraying occurs in a solid state which results in reduced oxidation and absence of phase changes; whereas, in thermal spraying deposition occurs of molten or semi molten particles. Over the last decade the interest in cold spraying has increased substantially. Considerable effort has been invested in process developments and optimization of coatings like copper. However, bonding in cold spraying is still a matter of some debate. The most prevalent theory is that when a particle travels at a minimum required velocity the particle deforms at a very high strain rate upon impact and during this deformation thermal softening dominates over work hardening in impact zone and a material jet is produced. This material jet removes oxides from the surface of the materials and the metal-to-metal contact is established between the freshly exposed surfaces. However, precisely how this high strain rate deformation behaviour of material promotes bonding is still unclear and requires further investigations. This article provides a comprehensive review of the current theories of bonding in cold spraying based on numerical modelling of impact and experimental work. The numerical modelling of the impact section reviews adiabatic shear instability phenomena, critical velocity, critical particle diameter, window of deposition of particles, particle impact on various substrates and the role of adhesion and rebound energy. The review of the experimental section describes the shear lip formation, crater formation on the substrates, role of surface oxides, characterization of bond formation, role of substrate preparations, coating build up mechanisms and contributions of mechanical and metallurgical components in bonding. Cold spraying of copper and aluminium has been widely explored in the last decade, now it is of growing interest to the scientific and engineering communities to explore the potential of titanium and its alloys. Titanium and its alloys are widely utilized in many demanding environments such as aerospace, petrochemical, biomedical etc. Titanium components are very expensive to manufacture because of the costly extraction process of titanium and their difficult to machine properties. Therefore, additive manufacturing from powder and repair of titanium components are of great interest to the aerospace industry using technologies such as cold gas spraying. Titanium coating as a barrier layer has a great potential for corrosion resistant applications. Cold spraying has a great potential to produce oxygen-sensitive materials, such as titanium, without significant chemical degradation of the powder. In-flight oxidation of materials can be avoided to a great extent in cold spraying unlike thermal spraying. This review article provides a critical overview of deposition efficiency of titanium powder particles, critical velocity, bond strength, porosity, microhardness, microstructural features including microstrain and residual stress, mechanical properties reported by various research groups. A summary of the competitor warm sprayed titanium coating is also presented in this article.

Fireside issues in advanced power generation systems

Abstract The requirements to supply increasing quantities of electricity and simultaneously to reduce the environmental impact of its production are currently major issues for the power generation industry. Routes to meeting these challenges include the development and use of power plants with ever increasing efficiencies coupled with the use of both a wider range of fuels and technologies designed to minimise CO2 emissions. For fireside hot gas path components, issues of concern include deposition, erosion and corrosion in novel operating environments and increased operating temperatures. The novel operating environments will be produced both by the use of new fuel mixes and by the development of more complex gas pathways (e.g. in various oxyfired or gasification systems). Higher rates of deposition could significantly reduce heat transfer and increase the need for component cleaning. However, degradation of component surfaces has the potential to be life limiting, and so such effects need to be minimised. Materials and operational issues related to these objectives are reviewed.

Bonding between aluminium and copper in cold spraying: story of asymmetry

Abstract The bonding mechanism in cold spraying is still a matter of some debate, which requires further investigation. In the present work, aluminium powder was cold sprayed onto a copper substrate and copper powder was cold sprayed onto an aluminium substrate using the same process gas and spray parameters. Separate experiments were performed to produce thick (∼400 μm) coatings and isolated particle impacts. Deposits were characterised using scanning electron microscopy and image analysis. The coating–substrate interfacial bonding was assessed via a method in which, following a short heat treatment at 400°C, intermetallics grow at the interface where metal to metal contact has been established. In addition, the bond strength values of deposits were determined using a standard pull-off test. It was found that the copper particles deposited onto an aluminium substrate resulted in significant substrate deformation, whereas aluminium particles caused minimal deformation of the copper substrate. Furthermore, the former displayed a higher degree of metallurgical bonding at the coating/substrate interface in comparison with the latter. These results suggest that the removal of oxide films from the surfaces was greater when copper was the material being sprayed rather than aluminium. The impact behaviour of the two materials and the removal of oxide due to deformation at high strain rate are discussed with the aid of the Johnson–Cook plasticity model.

Microscopy of fireside corrosion on superheater materials for oxy-fired pulverised fuel power plants

Abstract The current pressures for increased worldwide electricity supplies coupled with reduced environmental emissions, are leading to a revolution in the operating conditions within pulverised fuel fired boilers to improve their generating efficiencies. This paper reports results from a series of 1000 hour ‘deposit recoat’ laboratory tests that are assessing the effects of increasing heat exchanger surface temperatures (600, 650 and 700°C) on the fireside corrosion resulting from the combustion of a biomass/coal mix using oxy-firing (with hot flue gas recycle before desulfurisation). The results presented focus on two materials: a ferritic steel (T92) and an austenitic stainless steel (TP347HFG), to illustrate the effects of alloy Cr content. After exposure, the samples were examined using scanning electron microscopy/energy dispersive X-ray analysis to evaluate: surface morphological changes (formation of nodules and whiskers) on bare samples; microstructures of scale/deposit/metal in crosssections; and, elemental distributions. The performances of the samples were determined using dimensional metrology: pre-exposure contact metrology and post-exposure optical microscopy image analysis measurements. The metal loss data generated is being used to develop statistical models to predict the lifetimes of candidate materials for use in superheaters/reheaters in advanced power plants.

Comparison between oxidation of Fe–Cr–Al sputter coatings in air and air–HCl environments at 550°C

Abstract In biomass fired power plants the superheaters and reheaters are known to be particularly susceptible to chloride induced fireside corrosion. One approach to giving them longer lives is to develop new coatings that are resistant to this type of fireside corrosion damage. This paper reports the initial stages of such an approach using the combinatorial model alloy development method. Physical vapour deposition (PVD) using a two-target magnetron sputtering system (99·95 wt-%Cr and Fe–30 wt-%Al) has been used to obtain a range of coating compositions. The coatings were deposited onto an array of sapphire discs (10 mm diameter; 3 mm thick) placed in front of the targets. This resulted in a group of samples with coatings with a range of different Cr to Fe/Al ratios, which have been characterised using scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDX) and X-ray diffraction (XRD). Two groups of eleven coatings have been exposed at 550°C for up to 150 h in air and air–315 vppm HCl. Weight change data was gathered from these exposures after 50 and 150 h. After each exposure period, the surfaces of the oxidation/corrosion products were characterised using SEM/EDX and XRD. This analytical data has been used to identify the phases formed and the morphology of the scales generated. The best performing coatings from the mass change data were cross-sectioned to characterise the damage.

Fabrication and microstrain evolution of Al-TiB2 composite coating by cold spray deposition

This paper investigates the microstructure evolution of Al-TiB2 coatings prepared by cold spraying. In situ Al-TiB2 composite powders containing uniformly distributed titanium diboride (TiB2) particles with a size range of 5–100 nm in the Al matrix and Al/Al-TiB2 blended powders were used as the cold spray feedstock for coating fabrication on aluminium alloy substrates. The microstructures of the feedstock powders and as-deposited coatings were characterised using scanning electron microscopy with energy dispersive X-ray analysis and X-ray diffraction. Al/Al-TiB2 blended powder coatings, compromising closely packed powder particles, were sprayed to an approximate thickness of 500 µm. Al-TiB2 composite coatings (approximately 50 µm thick) were obtained retaining the microstructure of the composite powders being sprayed and no evidence of detrimental phase transformation was found. However, micro-cracks were found to exist in the Al-TiB2 coating due to the hardly deformable powder particles. Little or no microstrain was revealed in the as-sprayed Al-TiB2 coating, indicating that annealing may have occurred due to the localised adiabatic heating during the spraying process. It is demonstrated that it is possible to fabricate the Al-TiB2 composite coating by cold spray deposition but further improvements to eliminate coating cracking are required.

Robust Hydrophobic Surfaces from Suspension HVOF Thermal Sprayed Rare-Earth Oxide Ceramics Coatings

This study has presented an efficient coating method, namely suspension high velocity oxy-fuel (SHVOF) thermal spraying, to produce large super-hydrophobic ceramic surfaces with a unique micro- and nano-scale hierarchical structures to mimic natural super-hydrophobic surfaces. CeO2 was selected as coatings material, one of a group of rare-earth oxide (REO) ceramics that have recently been found to exhibit intrinsic hydrophobicity, even after exposure to high temperatures and abrasive wear. Robust hydrophobic REO ceramic surfaces were obtained from the deposition of thin CeO2 coatings (3–5 μm) using an aqueous suspension with a solid concentration of 30 wt.% sub-micron CeO2 particles (50–200 nm) on a selection of metallic substrates. It was found that the coatings’ hydrophobicity, microstructure, surface morphology, and deposition efficiency were all determined by the metallic substrates underneath. More importantly, it was demonstrated that the near super-hydrophobicity of SHVOF sprayed CeO2 coatings was achieved not only by the intrinsic hydrophobicity of REO but also their unique hierarchically structure. In addition, the coatings’ surface hydrophobicity was sensitive to the O/Ce ratio, which could explain the ‘delayed’ hydrophobicity of REO coatings.

Effect of Particle and Carbide Grain Sizes on a HVOAF WC-Co-Cr Coating for the Future Application on Internal Surfaces: Microstructure and Wear

The use of nanoscale WC grain or finer feedstock particles is a possible method of improving the performance of WC-Co-Cr coatings. Finer powders are being pursued for the development of coating internal surfaces, as less thermal energy is required to melt the finer powder compared to coarse powders, permitting spraying at smaller standoff distances. Three WC-10Co-4Cr coatings, with two different powder particle sizes and two different carbide grain sizes, were sprayed using a high velocity oxy-air fuel (HVOAF) thermal spray system developed by Castolin Eutectic-Monitor Coatings Ltd., UK. Powder and coating microstructures were characterized using XRD and SEM. Fracture toughness and dry sliding wear performance at three loads were investigated using a ball-on-disk tribometer with a WC-Co counterbody. It was found that the finer powder produced the coating with the highest microhardness, but its fracture toughness was reduced due to increased decarburization compared to the other powders. The sprayed nanostructured powder had the lowest microhardness and fracture toughness of all materials tested. Unlubricated sliding wear testing at the lowest load showed the nanostructured coating performed best; however, at the highest load this coating showed the highest specific wear rates with the other two powders performing to a similar, better standard.

Characteristics of Feedstock Materials

Cold spray is a process which relies on the kinetic energy of accelerated particles to impart high strain-rate deformation upon impact, which consequently leads to bonding. Proper selection of process parameters and appropriate powder characteristics are critical for exploiting the process capabilities fully so as to obtain the maximum possible process efficiency and desirable coating properties. A good cold-sprayed coating requires a good quality feedstock powder. It is already known that there exists a critical particle velocity, a material property, for the deposition of particle to occur, as explained in the previous chapters. For a given material, the critical velocity depends on the particle characteristics and the temperature achieved. The characteristics of the powders, such as morphology, size distribution, porosity and chemical purity, contribute to the overall coating properties. This chapter describes the properties of the feedstock required for cold spraying and their various manufacturing routes. The effect of particle size, morphology, oxide content and composition of the feedstock on the cold sprayability is also discussed in detail in this chapter. Cold spraying is increasingly used to deposit composite coatings, and this chapter summarises the characteristic of composite powder manufacturing and their cold sprayability.

Cold-Sprayed Metal Matrix Composite Coatings

Metal matrix composites (MMCs) combine high-hardness materials, such as ceramics, with ductile metal matrices. The resulting MMC material often has excellent strength and toughness due to the addition of hard reinforcing particles in the matrix. Typically, MMCs can be created by sintering, hot pressing, extrusion, pressure infiltration, reaction processing, or thermal spraying. However, most of these techniques are expensive and require high temperatures. Cold spraying is an innovative and cost-effective technique to manufacture of MMCs. Cold spraying relies on the plastic deformation of particles upon impact to fabricate coatings. Hard-facing particles used in MMCs typically do not plastically deform and can fracture upon high-velocity impact. It has been found that ceramic particle velocity, size, and morphology may have a significant influence on ceramic deposition efficiency. Cold spraying mechanically blended metal-ceramic powders can result in MMC ceramic content of approximately 30 vol.%, while cold spraying with composite powders can achieve ceramic content up to 52 vol.%. In addition, due to the high-velocity impact of the particles in cold spraying, ceramic powders in the feedstock provide additional enhancements to the properties of the deposited MMCs. It has been found that the addition of ceramic particles improves the deposition efficiency of the blended powders, increases pull-off bond strength, and increases the bonding within the coating. This chapter will highlight the research that has been conducted to improve the deposition efficiency of the ceramic particles and their impact on the cold-sprayed MMC material system.