This study investigates the development of fatigue failure in steel specimens coated by various spraying methods with and without grit blasting. Commercial titanium powder was deposited on structural steel substrates by low-pressure and portable cold spray as well as plasma and warm spray. Coating samples were subjected to strain-controlled cyclic bending, while monitoring resonant frequency as a measure of accumulated damage. A change in frequency of 4 Hz was chosen as the test-stop with the corresponding cycle count serving as the main indicator of fatigue life. Test results are presented in the paper along with explanations of fatigue mechanisms and process-related factors.

Superior characteristics of the cold sprayed coating have led to many high-tech applications. Until recently, all these applications were carried out using ‘stationary’ systems only, while some applications such as in-situ repair of aircraft body/engine parts require a portable system. Recently a ‘Portable High Pressure Cold Spray System’ called KINETIKS 2000-2 has been developed. This system is capable of 400 C/20 bars nitrogen/helium jet, and produces dense coatings with clean interfaces of many materials. In order to establish the suitability of this process for producing aluminum alloy coatings for aerospace and other high tech industries, various aluminum alloys (CP-Al, HP-Al, 6061 Al, 7005 Al) coatings were produced over many substrate materials (2024 Al, 7005 Al, 4041 Steel, ZE41A Mg). Coatings were characterized using microstructure, bond strength, bend test, corrosion studies, etc. Microstructural study showed that dense coatings with about 2-4% porosity values were produced with clean and well bonded interfaces. Bond strength of these coatings varied between 20 to 35 MPa, Bend test results showed that the coatings have adequate strengths and could withstand severe strain conditions. Salt fog corrosion studies (ASTM B 117) showed that the coatings impart corrosion resistance to the substrates.

Cold spray produces superior coatings with many unique properties, which have led to many new applications of sprayed coatings. In a few applications, such as in-situ repair of aircraft body/engine parts, etc., a portable system is required. Though some portable low pressure (5 – 10 bars) systems are available, these systems have many limitations on coating materials and coating qualities. Recently a ‘Portable high pressure cold spray system’, called Kinetiks 2000, has been developed. Kinetiks 2000 system can operate at 400 °C max nitrogen temperature and 20 bars max gas pressure. A touch panel on the console is used to input and control the process parameters. A hand held gun with a filament heater and a gun body, mounted directly onto the heater, is used for manual spray. This system operates with two different powder hoppers. Coatings of many materials, including aluminum, copper, titanium, zinc, etc have been produced. Microstructural investigations of sprayed coatings have shown that good, strong, dense coatings with clean interfaces and strong bonding to various substrates can be produced. Experiments are continuing to quantify the process characteristics and record the properties of sprayed coatings.

This work shows that hydroxyapatite (HA) can be cold sprayed simultaneously with titanium to form thick biocompatible coatings without compromising the phase constituents of the bioceramic material. XRD analysis indicates that the phase composition of the HA in the deposit is identical to that of the powder. The work also shows that very dense Ti and Ti-HA composite coatings can be produced using sponge Ti powders and nitrogen process gas. The adhesion strength of the cold-sprayed Ti-HA exceeded the reported values of comparable plasma-sprayed coatings.

The footprint of a cold spray beam has narrow dimensions and a large particle density, which lead to fast growth of deposit on precise locations. Cold spray produces compressively stressed coatings which lead to well bonded ultrathick coatings over many substrates. Since cold spray produces dense coatings with wrought-like microstructure, it can be used to produce large bulk forms. Two different bulk forms were produced: (i) ultrathick coatings strongly bonded to substrates and (ii) free standing structures (FSS). Both forms were post machined to obtain net-shape or near-net-shape articles. Bulk forms up to 60 mm were produced with many powders including metals, alloys and composites. Well bonded ultrathick coatings were produced over many substrates including aluminum, copper, SS, etc. FSS, including pipes, plates, etc, were produced by fabrication of a mandrel, spray coating bulk forms and removal of the mandrel. Coatings in the as-sprayed condition were machined and fine features such as grooves incorporated to obtain net-shape articles.

Discontinuously reinforced aluminum (DRA) has many attractive mechanical and physical properties. Discrete hard particles such as B 4 C have been added to aluminum alloys, such as 6061, to add strength and stiffness for an improved performance. Traditionally, these particles are introduced in melt additions and extruded or made into preforms and cast. These ceramic particles are also added in aluminum through a powder metallurgy process resulting in extrusion or forging. Inhomogeneous distribution of these particles in casting results in inferior properties and also it is difficult to produce preforms with high volume fraction materials. While wrought properties can be achieved in the powder metallurgy approach, extrusion directionality of the process introduces anisotropic properties. Though thermal spray has been used to produce DRA coatings, it has been observed that the melting and re-solidification of the aluminum matrix material as well as reaction with potential substrates such as copper is of concern. Since cold spray does not melt the feedstock material, it is ideally suitable for producing DRA coatings. Boron carbide particles and 6061 aluminum alloy were used as reinforcing and matrix material respectively. Experimental studies were carried out to optimize the raw material characteristics, DRA powder preparation technique and spray parameters, and using these optimized parameters, dense and strong coatings and free-forms were produced. Proper selection of the particle size distribution of the matrix and reinforcing materials, wet blending them and subsequent drying yielded the required spray feedstock. Optimization of the spray parameters resulted in strong, dense and well-bonded coatings with acceptable deposition efficiency and deposition rate values. Post spray heat treatments were carried out to retrieve the ductility of the specimens. A series of mechanical property measurements were carried out on the as-sprayed and heat treated specimens. Results of these studies were analyzed to evolve a preliminary model of the DRA structures and establish the feasibility of cold spray production of DRA coatings and free-form structures.

Cold spray is an effective technique to deposit pure metallic coatings on a variety of substrates. 1100 series aluminum-based coating has been deposited on similar aluminum substrate by cold spray technique using helium and a mixture of helium and nitrogen as powder carrying gases. The microstructural features of the coating were analyzed using optical micrography, Scanning Electron Microscopy (SEM) and Microhardness depth profiles. The electrochemical behavior of the materials was studied using 0.5 N H2SO4 as electrolyte. Tafel extrapolation experiments were carried out in order to calculate the corrosion rate, corrosion potential and the Tafel slopes, whereas, potentiodynamic polarization experiments were performed to understand the passivation behavior of the material under the experimental conditions. The electrochemical studies validate the use of cold sprayed aluminum coating for corrosion protection applications. SEM characterization of the coatings was also performed after the electrochemical experiments.

Pratt & Whitney's upper stage rocket engine development program, designated "RL60", has incorporated cold-sprayed copper to improve the design and function of this new engine. Combustion chamber designs contain two stainless steel manifolds connected by a series of copper tubes. The manifold where the hydrogen fuel exits is located near the injector face. The combustion gases from the injector would cause over-heating of this manifold. Thick copper application was needed to actively cool this manifold by conducting the cold temperatures from the hydrogen fuel inside the copper tubes. Plating copper greater than 0.050-inch thick resulted in poor adhesion following a subsequent braze cycle and required 2 weeks to plate. Cold sprayed copper was attempted which has surpassed plated copper in its ability to adhere through this braze cycle and can be applied in a few hours. In addition, the hazardous chemicals associated with copper plating have now been eliminated.

ASB Industries is involved in taking the cold spray technology from research lab into industrial applications. Cold spray programs include building a human engineered spray system, design and development of nozzles and guns capable of reliable and reproducible operation for long duration spray, process optimization for producing quality coatings at reasonable deposition efficiency and deposition rate values and preparation and characterization of coatings of industrially relevant materials. These research activities have resulted in the development of a patented ‘Advanced Cold Spray System’ with enhanced features and performance characteristics. ASB is involved in a number R&D programs to develop engineered coatings for specific customers, and a few of these applications are in various stages of commercial adaptation.

The cold spray process was used to prepare nanostructured WC-Co coatings. The coating microstructural characteristics and phase composition were analyzed via optical microscopy, SEM and XRD. The morphology and microstructure of the nanostructured WC-Co powder were also analyzed by SEM and XRD. A 10µm thick coating was achieved. The results show that there is no degradation of the WC-Co powder during the cold spray process and well-bonded and phase-pure WC coating can be produced by the cold spray process.

Oxygen-free titanium coatings are at present produced using vacuum plasma spray systems. The cold spray process can produce titanium coatings in atmospheric air at reduced cost. Variations of the deposition efficiency as well as the coating characteristics with various process parameters were studied. Results show that the deposition efficiency drops drastically below a critical temperature. The parameters which affect the jet velocity; viz., the type of gas and the nozzle geometry have maximum effect on the process deposition efficiency. Sprayed coatings were porous and exhibited a low modulus and high hardness. Post processing of the coating by machining produced dense, strong and hard coatings. Key words: Cold-Spray, Titanium, Oxidation.

This paper compares the coating characteristics of two HVOF processes: air-cooled converging-nozzle Diamond Jet (DJ) spraying and Hybrid 2600 air/water-cooled converging/diverging technology. WC-Co coatings were deposited on steel substrates using different combinations of spray parameters, gas flow ratios, and cooling gas types. The coatings were then examined and tested to determine the extent to which microstructure, hardness, surface roughness, wear resistance, and deposition efficiency can be controlled. In addition to investigating process relationships, the paper also addresses the issues of practicality and cost.