Cold-Spray Coatings

Cold gas dynamic spraying, or cold spraying (CS), is a solid-state coating process wherein powders in a carrier gas are accelerated toward a substrate. Under sufficiently high impact velocities, the powder particles deform plastically and adhere to the substrate. Metals, ceramics, polymers, and composites can be deposited using CS. Among currently available surface coating technologies, CS offers several advantages over thermal spraying, because it utilizes kinetic rather than thermal energy for deposition. This avoids residual stresses, oxidation, and undesirable chemical reactions. The intent to develop new material systems with enhanced properties that fulfill the required surface and interface functionalities for components with many applications has inspired CS investigations of many material combinations. The number of studies and patents on CS and CS-related technologies has increased exponentially in recent years, establishing much new information in a short time. In this chapter, the process of CS is discussed from mechanistic and technological perspectives, including its general operating parameters, current applications to specific material systems, and ongoing research increasing the scope of the technique. A critical discussion on developing CS technologies examines the microstructural bonding mechanisms utilized in variations on the process. Future investigations are suggested, particularly in quantitatively linking CS processing parameters to the behaviors of material systems during impact. This chapter briefly summarizes the rapidly expanding common knowledge on CS to assist researchers and engineers in future endeavors with this technology.

This chapter will present various cold spray applications developed by the US Army Research Laboratory (ARL) associated with the repair of parts from aircraft and ships by providing actual examples and their relevance toward the advancement and implementation of cold spray technology. It is not an all-inclusive by any means, but it serves to illustrate that cold spray (CS) can be used across very diverse industry sectors. It is important to note that most cold spray applications today are for dimensional restoration of parts that have experienced wear and/or corrosion in service. Significant points of interest associated with the development of these CS applications will be highlighted. Emphasis will be placed on important aspects of process development and optimization, testing and evaluation, and qualification and specifications. The methodology and essential technical data for the transition of cold spray technology for these select applications will be discussed and illustrated by actual case studies. The substrate materials represented will include aluminum, magnesium, and titanium. The focus application will involve the restoration of magnesium aerospace components where much data will be presented, while the remaining case studies cannot be so inclusive due to page restrictions.

Tremendous attention has been given to the cold spray process, even more today with the emergence of additive manufacturing, worldwide. Several inventions related to the cold spray technology have been patented for over a century and mostly since a couple of decades. But the cold spray technology knows a period of great innovations due to recent and current substantial explorations. Various technological solutions have been developed. The technical dimension, and particularly in terms of manufacturing method, has also always been a major genesis of progresses and novelties. This chapter is a technological survey of the cold spray additive manufacturing and reports variant methods and innovative contributions. Through an introduction section, the chapter begins by a depiction of cold spraying (CS) that addresses a review of distinctive deposition methods based on a capability variance. Then, a section reports a brief chronology of cold spraying towards the generic modern deposition method as it stands nowadays. A concise survey of material aspect is addressed, this showing the current capability of cold spraying prior to a brief overview of various deposit possibilities and applications. Stand-alone sections will be focused on modern variances of cold spraying, viz. the low-pressure cold spray method bringing new benefits and the very-low-pressure cold spray manufacturing for the development of advanced technological solutions. Since combination of cold spray with another process for various innovations represents a new added value, such approach merits to be highlighted among modern trends of the cold spray use. For that purpose, this book chapter also addresses an overview of hybrid combinations based on the cold spray technology, including novel hybrid variant for 3D submicron architecturation and laser-assisted cold spray method that enables positive thermal effect, viz. improved deposition and increased deposit features. Combining high-temperature deposition and high kinematic conditions, a new cold spray variant, denoted pulse gas dynamic spraying capable of thermal improvement is also reported. Together, all these achievements reveal a great flexibility of the cold spray additive manufacturing in terms of material possibilities, deposition methods and technological solutions.

The Chapter “Low-Pressure Cold Spray” presents a description of basics of LPCS technology, bonding mechanisms, temperature effects and LPCS localization processes. To illustrate the main aspects of LPCS system parameters determination, the numerical simulation and experimental data are presented. The emphasis is placed on the proper development of the cold-sprayed metal matrix composite coatings and their structure and properties, which are of paramount importance to the success of LPCS. Processes such as bonding, hardening and softening of the coating materials during cold spraying and coating structure formation at following heat treatment (sintering) are dealt with from the theoretical as well as practical aspect.

Applications of LPCS technology occur for the cases where conventional thermal spraying technologies cannot be successfully used. The analysis of various components repair is presented in which the methodology and accompanying technical data for the transition of cold spray technology into the industry, and several case studies are presented. The case studies show the tremendous impact that LPCS has made for a few select applications, representing steels, magnesium, aluminium and composite materials, while attempting to include as much technical data substantiating the advantages and benefits gained by transition of the process.

Cold spray, originally developed as a coating technique, is now applied as an additive manufacturing process to fabricate freestanding components and restore damaged components. Having low working temperature, cold spray additive manufacturing (CSAM) and cold spray restoration (CSR) have the ability to retain the original feedstock properties and prevent adverse influence on the underlying substrate materials. These superior merits are not achievable through conventional high-temperature additive manufacturing processes, thereby CSAM and CSR are attracting great attentions from both scientific and industrial communities. During the past decade, investigations of CSAM and CSR have been widely conducted. Existing works mainly focused on applications, product properties, processing parameters optimization, spray strategy, and robot control. Despite having a large number of reports, a systematic summarization and review on these works has yet to be carried out. Therefore, in this chapter, the existing CSAM and CSR works were summarized and reviewed for the purpose of systematically introducing the progress of CSAM and CSR techniques.

Cold gas dynamic spraying (CGDS) is an emerging technique that involves the surface modification in order to provide enhanced surface properties on material substrates. Particles, with size in the range of 1–50 μm, are accelerated by a supersonic jet gas up to 1200 m/s and impact on the substrate surface. Under specific conditions, the metal powders undergo a severe plastic deformation and adhere to the substrate. In the last decades, the cold spraying of several materials, like copper, aluminium and iron, has been widely explored providing optimal processing windows for a wide range of material pairs. Titanium and its alloys are finding a widespread use in many strategic industries, namely, aeronautic and aerospace field, due to the lightweight, high corrosion resistance and compatibility with polymer-reinforced composites, as well as in the biomedical sector, due to their biocompatibility. However, the high cost of raw materials and the manufacturing issues put severe restrictions to their wider use. On the other hand, replacement of titanium bulk with multilayer material, consisting in a cold sprayed titanium coating on aluminium components, could be a promising alternative and an advantageous trade-off between the cost compression and the higher surface properties of titanium alloy. The present chapter deals with the analysis of the deposition of pure titanium coatings on aluminium alloy substrate by means of low-pressure cold gas spray technique and deals also with the study of the properties of multilayer material. A post-deposition process to further improve the properties of the coating itself was also analysed.

This chapter describes recent efforts to understand the effects that chemical and physical properties of metallic glass (MG) powder particles have on the synthesis of their cold-sprayed coatings. Understanding the mechanical response of MG is fundamental to evaluate the conditions at which their powder particles can be deposited by cold spray. The characteristics of the feedstock powders are evaluated and used to ascertain ideal cold spray parameters. This information is also used to model the deposition mechanism of metallic glasses in the cold spray process. FE analysis and simulation is used to identify the phenomena behind the formation of MG coatings (i.e., homogenous or inhomogeneous deformation). The model defined considers strain rate and temperature dependence of MGs under different conditions.

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.

Cold spray is a relatively easy and economical approach to deposit composite coatings with comparable or even better properties than other processing methods. This article summarized the published papers on cold-sprayed metal matrix composite coatings. After a brief introduction of the remarkable advantage of cold spray in fabricating composite and the building-up process of the cold-sprayed composite coating, the versatility of cold spray to produce a great wide range of applicable materials was classified according to the matrix materials, i.e., aluminum, copper, nickel, tungsten carbide, titanium, hydroxyapatite, and polymer. Then, some key issues concerning the content loss and fragmentation of the hard particles were put forward, and post-spray treatment techniques and powder preparations were thus concluded.

Cold spray is a remarkable coating technology that has wide-ranging applications, including additive manufacturing and repair of aerospace components. The primary class of materials used for cold spray is metal, but more recently, one finds researchers preparing metal matrix composites that are useful for tribological applications. This chapter will review briefly fundamentals of tribology and processing principles for deposition of metal matrix composites by cold spray. The primary focus will be a review of the tribology of composite coatings manufactured by cold spray that include ceramic reinforcements (e.g., Al₂O₃, WC) and solid lubricants (e.g., MoS₂, h-BN). A discussion of the future directions for metal matrix composites made by cold spray and their usefulness for tribological applications is also presented.

Cold spray coating deposition is an innovative thermal spray technique, which addresses some of the shortcomings of several traditional thermal spray processes. The cold spray method can be used for coating deposition at ambient temperature that leads to near-negligible phase transformation during the process, which indicates no particle melting. Therefore unlike other thermal spray techniques, the harmful reactions such as oxidation, nitriding, decarburizing, and other types of decomposition mechanism in general are avoided in this process. These attributes offer significant advantages and new possibilities. The cold spray is applicable to corrosion protection where absence of process-induced oxidation may offer improved performance. The coatings produced by typical thermal spray methods like atmospheric plasma spray (APS) and high velocity oxygen fuel (HVOF) spraying contain comparatively higher porosity and possible oxides, which may lead to a decline of corrosion resistance of the as-sprayed coatings. Efforts are going on for the development of a more comprehensive scientific basis for the cold spray process and a broader range of appropriate materials along with their applications for corrosion protection. This chapter presents a tutorial introduction to the process of cold spray coating. It includes the comparison of cold spray coating process with some popular thermal spray techniques. The studies from various researchers have been reported with regard to use of cold spray coatings for corrosion protection. These studies clearly signify that the cold spray coatings have already proved successful for erosion-corrosion protection. A thorough survey on various coating compositions deposited by cold spraying on different substrates along with their properties has also been detailed with emphasis on corrosion prevention. It has been concluded that the cold spray technology has great prospective for future research especially with reference to its application to actual industrial solutions.

This chapter presents the corrosion properties of cold-sprayed coatings. Cold spraying has shown its potential producing corrosion-resistant coatings for several operation conditions due to the fact that dense and protective metallic and composite coatings can be manufactured by using cold spray processes, examples are presented in Fig. 13.1. This enables the use of cold-sprayed coatings as corrosion barrier coatings. In addition to corrosion resistance, other advantages of cold-sprayed coatings are dealing with high strength, electrical conductivity and minimal or compressive residual stresses as well as repairing and additive manufacturing (Papyrin et al., Cold spray technology, 1st edn. Elsevier, printed in the Netherlands, 328 p, 2007; Champagne and Helfritch, Int Mater Rev 61(7):437–455, 2016; Champagne, The cold spray materials deposition process: fundamentals and applications. Woodhead Publishing Ltd., Cambridge, 362 p, 2007; Koivuluoto, Microstructural characteristics and corrosion properties of cold-sprayed coatings, Doctoral Thesis, Tampere University of Technology, Tampereen Yliopistopaino Oy, Tampere, 153, 2010; Villafuerte, Modern cold spray – materials, process, and applications. Springer International Publishing, p 429, 2015). Corrosion properties and behavior of the cold-sprayed coatings are increasingly reported in the literature during last years. Recently, review papers concerning corrosion properties of cold-sprayed coatings have been published by Bala et al. (Surf Eng 30(6):414–421, 2014), Koivuluoto and Vuoristo, (Surf Eng 30:404–414, 2014) and Hassani-Gangaraj et al. (Surf Eng 31(11):803–815, 2015). Furthermore, cold spraying can be used for corrosion protection as well as repairing of corrosion defects (Vardelle et al., J Therm Spray Technol 25(8):1376–1440, 2016).

Cold spray coatings have shown great corrosion protections due to the intrinsic dense character of the sprayed material. However, these types of coatings do not get wide application in low-temperature corrosion for their economical inferiority compared to other alternative coating technologies. For this reason, the other possible application for cold spray corrosion coatings needs to be developed, such as high-temperature oxidation resistance coatings on titanium-based alloys. The titanium-based alloys such as orthorhombic Ti₂AlNb and γ-TiAl-based alloys are considered as potential structural materials for high-temperature applications in aeroengines, but they still suffered from oxidation and environmental embrittlement at elevated temperatures. Coatings represent an effective way to solve the problems. However, the available coatings do not meet the requirements of titanium-based alloys for many reasons. In this chapter, the use of cold spray technology for preparing TiAl₃, TiAl₃/Al, TiAl₃/Al₂O₃, and TiAlSi coatings on titanium-based alloys and their behaviors at high temperatures are reviewed. This chapter is mainly divided into four parts, including the background of the study, the coating preparations, the coating characterization, and the high temperature performance of the coatings. The main results of the investigations showed that the TiAl₃ composite coatings prepared by cold spray exhibited great improvement for the oxidation of substrate alloys, which is different from those prepared by other available technologies. The coating protection and degradation mechanisms are analyzed. Comparison of TiAl₃ composite coatings prepared by cold spray and other technologies is also discussed.

Adhesion strength is often cited as the most important characteristic of cold spray coatings, especially in regard to repair applications. The bonded interface between coating and substrate is complex and calls into play many mechanical and chemical facets. Exploring this phenomenon has been of great interest to the scientific community in recent years. Adhesion research can be separated into two streams; one focuses on the mechanism of metallic bonding and the influence that cold spray parameters have on particle deformation, oxide removal, and the resulting particle/substrate interface. The other stream focuses on characterizing the influence of substrate preparation prior to deposition on adhesion. The experimental observations reported in this chapter are often supported with finite element modeling, as the high strain rates and very short impact times in cold spray processing make it difficult to observe local behavior. This chapter aims to summarize the current state of understanding with respect to the different adhesion mechanisms and the physics involved in the bonding process—with the goal of providing concise information that can be applied to the prediction and improvement of cold spray adhesion strength, particularly for structural repairs and coating applications.

In the current work, a series of coatings produced by cold spray using varying materials were studied with neutron diffraction. The through-thickness residual stress was evaluated in a number of substrate-coating systems, typically at a scale of several millimetres. By combining neutron stress and texture analysis, optical and electron microscopy and numerical simulations of the high strain rate deformation, a multi-scale analysis approach is presented herein to provide insights into the mechanism of residual stress accumulation in cold spray coatings and to help establish a solid framework to understand the mechanisms of splat/coating formation.

Cold spray technology is aimed to produce coatings thanks to the high energy impact of metallic and ceramic particles on similar and dissimilar substrates. The high energy is provided to the impacting particles by high-pressure-carrying gases capable of accelerating the coating material to very high speeds in the order of 1000 m/s. Cold spray temperatures are in the order of 0.6T_m leading the sprayed particles to severely plastic deform at the contact with the substrate or with already deposited coating. In all the technological available coating processes, the intrinsic required properties are mainly thickness, porosity, adhesion strength, deposition efficiency and surface finishing. Among the latter, porosity is the coating property most influencing the mechanical behaviour of the substrate-coating system. The low processing temperatures minimize or eliminate phase transformations and reduce porosity and residual stresses that normally characterize high-temperature thermal spray processes. To take advantage of the material properties of the feedstock powder, cold spraying was developed because phase transformations, chemical changes, residual stresses and porosity are minimized, if not eliminated, in these coatings due to the low processing temperatures and high velocities. Because of its non-combustive nature, cold spray process is of great interest for spraying temperature-sensitive materials such as Ni and Ti. Ni-based coatings are used in applications where wear resistance, combined with corrosion resistance, is required. Ti and its alloys are extensively used for aerospace components that work till elevated temperatures like airframe and jet engine components. The aim of the present chapter is to describe the processing parameters, feedstock properties and materials coupling leading to the coating’s porosity variation and its effect on the mechanical behaviour of the deposited coatings.

Cold spray is a promising innovative technology based on the effect of kinetic energy transferred to fine particles impacting on the substrate. The energy is provided by high-pressure carrying gases (Air, He, N2) heated at temperatures well below the melting point of the sprayed materials. The severe plastic deformation due to the high impact velocity of cold-sprayed particles leads to very interesting microstructural features and surface modifications allowing to obtain promising properties of the substrates in terms of fatigue resistance and crack behavior. In order to acquire long-term safety and reliability of materials modified by cold spray, the fatigue properties of coated substrates must be clarified. The present work is aimed to show the effect of various cold-sprayed particles (Al alloys, Ni, and Ni-based alloys) on the microstructural behavior of the coatings, on the coated substrates fatigue properties, and on the crack behavior of pre-notched and coated substrates. A literature review regarding cold spray fatigue properties and crack behavior is presented. The effect of cold spray processing parameters on fatigue properties was analyzed for different materials such as AA7075 sprayed on AA7075 substrate, AA2024 sprayed on AA7075 substrate, and AA2024 sprayed on AZ91 magnesium alloy substrate. They were analyzed those conditions leading to an increase in fatigue life. In the work paper, the possibility of repairing cracks, in aluminum panels, through cold spray is demonstrated. 2099 aluminum alloys with a surface 30° V notch were filled with 2198 and 7075 aluminum alloy powders via cold spray. The crack behavior of V-notched panels subjected to bending loading was studied by FEM and mechanical experiments. The simulations and the mechanical results showed a good behavior in terms of K factor reduction and crack nucleation and growth behavior of the repaired panels. The study was finalized to predict the failure initiation locus in the case of repaired panels subjected to bending loading and deformation. The way stress concentration was quantified, residual stress field and failure are affected by the mechanical properties of the sprayed materials and by the geometrical and mechanical properties of the interface with particular effect of the adhesion strength and of the local residual stresses. The coating benefits are also demonstrated for pure Ni and IN625 particles sprayed on various substrates. The crack behavior is, in this case, shown for 30°, 60°, and 90° notches filled with cold-sprayed particles; the analyses are conducted through FEM and experiments.

Cold spraying (CS) offers many advantages in front of the conventional thermal spraying processes and is becoming competitive in several industrial sectors. The biomedical industry is a quite well-established field, but still there are many challenges to solve where improvements in surface engineering can play a great role. The use of coatings in biomaterials has been fundamental on the improvement of mechanical as well as biological properties, thus, ameliorating human quality life. Studies about cold-sprayed coatings are emerging in orthopedics industry (internal fixation systems and prosthesis) as well as for antibacterial purposes (in body and touch external surfaces). These works are very new, and most deal with the improvement of biocompatibility and bioactivity of hard tissue replacement. Several combinations of substrate and coating materials are attempted, even trying to overcome any limitation on the spraying of ductile materials; biocompatible metallic materials, bioactive ceramics and polymers, and combinations have been successfully deposited by this method. Therefore, research on biocoatings is in constant development with the aim to produce implant surfaces that provide a balance between cell adhesion and low cytotoxicity, mechanical properties, and functionalization.