Detonation Sprayed Coatings and their Tribological Performances

The Detonation Spray Coating (DSC) process is a unique variant among the wide choice of thermal spray processes. The typical functionalities of DSC coatings include wear and corrosion resistance, elevated temperature oxidation resistance, thermal barrier, insulative/conductive, abradable, lubricious surface, etc. Among the coatings for wear resistance, the cermet coatings based on WC–Co and Cr3C2–NiCr are the most popular materials of choice and contribute to bulk of the utilization by the industry towards wear resistance. Notwithstanding the above materials, alternative materials involving modifications in both hard and binder phases like TiMo (CN)–NiCo, WC-CrC-Ni, WC-Co-Cr, WC-Ni, Cr3C2-Ni, Cr3C2-Inconel, etc. exhibit great promise towards tribological applications under diverse wear modes. This chapter on the tribological characteristics of the detonation sprayed coatings provides a comprehensive overview on the characteristics of various cermet coatings generated at varied process conditions and its influence on the tribological properties under abrasive, sliding, and erosive wear modes.

Development of Functionally Graded Coating by Thermal Spray Deposition

Functionally Gradient Coatings (FGCs) are emerging materials with an improved service life and have a promising future for the production of (a) tailored components for applications subjected to large thermal gradients, (b) smart coating with improved corrosion and wear resistance, (c) improved fatigue wear, and (d) improved material structures for energy applications like batteries, fuel cells, etc. FGCs may be developed by physical/chemical vapor deposition, electro/electroless deposition, thermal spray deposition technique, etc. Thermal spraying refers to the technique or a group of techniques whereby molten or semi-molten droplets of materials are sprayed onto a solid substrate to develop the coating. In this chapter, detailed overviews of the development of functionally graded coating by thermal spray deposition techniques are presented. In addition, a few research results on the development of functionally graded coating for tribological and thermal barrier applications are presented.

Characterization of Mechanical Properties and the Abrasive Wear of Thermal Spray Coatings

Thermal spray is a generic term used to define a group of coating processes used to apply both metallic and non-metallic coatings. These coatings are usually defined by their hardness, strength, porosity, roughness, and wear resistance. In this chapter, the authors turn their attention to the mechanical and tribological properties of thermal spray coatings. The individual phase plays a very important role in determining the performance of the coating. However, evaluating the mechanical and tribological properties at a nano-level requires new test methods and their validation. In this chapter, elaborate discussion of some techniques to evaluate and analyze the mechanical and tribological properties of different thermal spray coatings is done. This chapter is intended to help the reader to firstly understand the basic principle and methods of characterization of thermal spray coatings using instrumented nanoindentation, nanoscratch, abrasive wear testing techniques, and secondly to get an idea of the recent techniques and review the research and development in the same field.

Solid Particle Erosion of Thermal Sprayed Coatings

Solid particle erosion is an important material degradation mechanism. Although various methods of coating are tried and used for protection against erosion, thermal sprayed coating for such purpose is the most widely used method. In this chapter, evolution of thermal sprayed coating, erosion testing methods, and erosive wear of thermal sprayed coatings are discussed extensively with emphasis on recent developments. It is generally found that erosion of thermal sprayed coatings depends on erosion test conditions, microstructural features, and mechanical properties of the coating materials. Most thermal sprayed coatings respond in brittle manner having maximum erosion rate at oblique impact and velocity exponent in excess of 3.0. Erosion rate is also dependent on thermal spraying techniques and post coating treatment. However, little work is done on dependence of erosion rate on coating techniques and coating conditions. Future direction of work is also reported.

Slurry Erosion Behavior of Thermal Spray Coatings

Slurry erosion is a degrading phenomenon usually observed in machineries dealing with particle-laden fluid such as hydro power plants, ship propellers, pump impellers, valves, and connecting pipes. The low erosion resistance of commonly employed structural materials prompts the use of different surface modification techniques. Among several types of surface modification techniques, thermal spraying has achieved a significant recognition worldwide due to its versatile nature. In this chapter, slurry erosion behavior of thermal sprayed coatings has been discussed with special emphasis on the contribution of different coating related parameter. It has been observed that microstructure play an important role in determining the slurry erosion performance of thermal spray coatings. Different microstructural features such as splat boundaries, pores, un-melted particles, and cracks are detrimental for the thermal spray coatings exposed to erosive environment. A parameter useful for identification of primary erosion mechanism for thermal sprayed coatings is also discussed.

Tribology of Thermally Sprayed Coatings in the Al2O3-Cr2O3-TiO2 System

With the exception of ZrO2, the individual oxides and binary compositions in the system Al2O3-Cr2O3-TiO2 are the most important oxide materials for thermally sprayed coating solutions. Traditionally, these coatings are prepared by Atmospheric Plasma Spraying (APS), but processes such as Detonation Gun Spraying (DGS) and High Velocity Oxy-Fuel (HVOF) spraying can produce coatings with lower porosity and higher wear resistance. Traditionally, feedstock powders have been used for coating preparation. Recent developments have seen the emergence of suspensions as a new feedstock, but tribological properties of coatings prepared using suspensions have not yet been studied in detail. This chapter summarizes some important issues regarding wear protection applications of coatings in the Al2O3-Cr2O3-TiO2 system, the advantage of alloying the individual oxides, and the influence of different feedstocks and spray processes.

High Temperature Sliding Wear of Thermal Sprayed Coatings

In this chapter, various layers that are formed during sliding wear of thermal sprayed coatings at elevated temperature are discussed. Glazed layers are formed on the worn surfaces during elevated temperature sliding wear of thermal sprayed coatings. These layers have a characteristic physical appearance, mechanical properties, chemical compositions, and failure mechanisms. Wearing conditions, wearing material, and mating material influence formation and characteristics of glazed layers. Among these parameters, wearing material and mating material are most important. These glazed layers are divided into different types of layers, namely Transfer Layer (TL), Mechanically Mixed Layer (MML), Reaction Layer (RL), and Composite Layer (CL). The recent results on friction of thermal sprayed coatings at elevated temperature are rationalised in the light of different types of glazed layer formation.

Abrasion-Corrosion of Thermal Spray Coatings

WC-based thermal-spray and High Velocity Oxy-Fuel (HVOF) coatings are extensively used in a wide range of applications ranging from downhole drilling tools to gas turbine engines. WC-based thermal spray coatings offer improved wear resistance as a result of hard phases dispersed in binder-rich regions. However, the presence of hard and soft phases within the coating can also lead to the formation of micro-galvanic couplings in aqueous environments leading to some reduction in combined wear-corrosion resistance. Furthermore, the coating also responds differently to change in mechanical loading conditions. This chapter examines the wear-corrosion performance of thermal spray coatings in a range of wear, electrochemical, and wear-corrosion tests under varying contact conditions to develop models and establish relationships between wear mechanisms, wear rates, and environmental factors such as pH and applied load.

Composite Coatings Employing a Novel Hybrid Powder and Solution-Based Plasma Spray Technique for Tribological Applications

Solution Precursor Plasma Spray (SPPS) technique is an emergent technique which offers significant performance advantages with various functional coatings like YSZ, ZnFe2O4, LSM, metal-doped ZnO, etc. Apart from the above capabilities, efforts towards development of an innovative hybrid approach through simultaneous deposition of powder and solution resulted in coatings with unique microstructures. Some illustrative examples of the large variety of coatings that can be realized through the hybrid route combining the conventional plasma spray and SPPS techniques are discussed. The attractive prospects offered by hybrid technique for spraying nanocomposite coatings are specifically highlighted through a case study. Successful development of hybrid coatings using a Mo-alloy powder and a suitable oxide-forming solution precursor has been shown to exhibit improved sliding wear performance. The relationship between the splats, the ensuing coating microstructure at varied processing conditions, and its tribological behaviour of the coatings is comprehensively discussed.

Tribocorrosion of Thermal Sprayed Coatings

Triboelectrochemical response is concerned with different electrochemical-controlled wear experiments designed to understand tribocorrosion behaviour of materials and coatings. Tribocorrosion is defined as material degradation resulting from simultaneous action of wear and corrosion and it is found in many engineering applications, but the involved mechanisms are still only partially understood. In this chapter, a brief overview of tribocorrosion testing techniques is followed by issues which have helped us in gaining in-depth scientific knowledge of tribocorrosion. The overview is further substantiated by detailed studies and observations on tribocorrosion of thermal sprayed coatings in recent times.