Thermal spray processes benefit from workpiece cooling to prevent overheating of the substrate and to retain metallurgical properties (e.g., temper). Cold-gas “plume quenching” is a plume-targeting cooling technique, where an argon curtain is directed laterally above the substrate surface to re-direct high temperature gases without impacting particle motion. However, there has been little investigation of its effect on the molten particles and the resulting coating properties. This study examined high- and medium- density tantalum and nickel coatings, fabricated by Controlled Atmosphere Plasma Spray with and without plume quenching on aluminum and titanium substrates. To compare the effect of plume quenching, the deposition efficiency was calculated through coating mass gain, and the coating density, stiffness, and adhesion were measured. The tantalum and nickel coatings were largely unaffected by plume quenching with respect to deposition efficiencies, coating density, adhesion, and stiffness. These results indicate that a plume quench could be used without affecting the coating properties for high- and medium-density metals while providing the benefit of substrate cooling that increases with higher plume quench gas flow rates.

In cold spray (CS) additive manufacturing process, micrometer scale particles accelerated through a supersonic nozzle are targeted on a surface with velocities in the rage of 300-1500 m/s in solid state. The impact energy of the particles leads them to deform plastically with high shear energy near the impact interface and adhere to the surface metallurgically, mechanically, and chemically. Using CS, deposition of metals, metal matrix composites, and polymers are achieved with high adhesive/cohesive strength and low porosity. Sensitivity of the CS additive manufacturing process to the variabilities in the process parameters are still being understood. Among the process parameters, particle morphology can have significant implications on drag forces, and therefore, on the particle impact velocity. This in turn affects the deposition efficiency (DE) and the quality of products. In this work, a new approach is introduced for computing DE by incorporating particle sphericity and its variation into one-dimensional numerical models. Size, sphericity, and the variability of size and sphericity of aluminum, copper, titanium, and tantalum particles are measured from static optical microscope images. The data is used for predicting impact velocity, temperature, and DE. The model results are then compared with particle velocity measurements.

This paper presents the results of two metals coatings, molybdenum and tantalum, prepared by Controlled Atmosphere Plasma Spray (CAPS) onto Al 6061 substrates that were thermal cycled to calculate the effective coating modulus. Traditional uniaxial tensile testing samples were prepared from thicker duplicate coatings for comparison, as well as to measure thermal expansion properties and oxygen and nitrogen content. The molybdenum samples cut from thicker coatings were un-able to be tensile tested due to their fragility. Thermal cycle testing of molybdenum on an Al 6061 substrate was found to have a modulus approximately 18 to 19% of literature values for bulk molybdenum using the bi-layer beam thermal cycling method. Additionally, non-linear modulus behaviour was observed in the molybdenum sample when enough thermal strain was induced to shift the coating from a compressive to tensile stress state. The tantalum coating was found to have a modulus approximately 42 to 46% of literature values for bulk tantalum using the bi-layer thermal cycling method. Traditional tensile testing measured a modulus approximately 44 to 46% of bulk, which shows good agreement between the two methods and supports that the bi-layer thermal cycling method is valid for plasma sprayed refractory metal coatings.

Industrialization of cold spray brings along the questions of cost and time efficiency of various spray procedures. In this work, high rate deposition of tantalum was studied by producing coating specimens where the powder to helium mass flow rate varied from 5% to 14%. Quasi-1D fluid simulations predict a minimal effect of increased powder stream loading on particle impact velocity and temperature over those ranges, but the cost varies substantially. The experimental specimens, examined by using optical micrographs, porosity measurements, and hardness tests, show no discernable differences in the deposited samples. The increased stream loading rate, however, helped reduce the time required for processing the same amount of tantalum by a factor of three using identical helium spray conditions.

Ceramic coatings are often applied to metallic substrates using a bond coat to promote adhesion. The corrosion environment between the substrate and ceramic layer can be very harsh due to the absence of dissolved oxygen, high concentrations of corrosive electrolytes, and galvanic and crevice corrosion mechanisms within the bond coat itself. This study assesses the performance of several bond coats in sulfuric acid, including plasma sprayed tantalum and HVOF sprayed NiCr, Hastelloy C, and Ultimet. The bond coats were deposited on Hastelloy substrates and covered with a Cr 2 O 3 topcoat. Electrochemical polarization and open circuit potential measurements were recorded and coating microstructures were examined before and after the corrosion tests. Plasma sprayed tantalum exhibited the best corrosion resistance at room temperature, followed by HVOF sprayed Hastelloy C.

This study investigates how Al and Ta diffusion affects the growth of surface oxides in NiCoCrAlYTa coatings and the interdiffusion that occurs between coatings and single-crystal substrates in high-temperature oxidation processes. HVOF-sprayed layers were tested at temperatures between 900 °C and 1100 °C and the corresponding oxidation behavior of Al and Ta was assessed. It was found that higher amounts of Al in MCrAlY coatings promote the selective oxidation of Al 2 O 3 and that the addition of Ta increases the stability of the γ’-Ni 3 (Al,Ta) phase.

This paper is the second part of a study on how Al and Ta diffusion affects the oxidation of NiCoCrAlYTa coatings. Thermodynamic and diffusion simulations of the coatings with different additions of Al and Ta predicted the development of a typical γ, γ’, ß-phase microstructure and suggested that the ß-NiAl phase was depleted as Al diffused into the substrate. The simulations also indicated that Ta could diffuse back to γ’-Ni 3 (Al,Ta) phase in the substrate with a γ’ depletion due to inward diffusion of Co and Cr from the HVOF-sprayed deposit. How this process impacts microstructure development is discussed in detail.

Due to high melting temperatures and excellent corrosion resistance of refractory metals, they are used for manufacturing parts working under extreme conditions. The formation of refractory metal coatings by thermal spraying is associated with two major challenges: 1) particles of materials having high melting temperatures should be heated to reach a semi-molten or a molten state; 2) oxidation of the metals should be prevented. In this work, the CCDS2000 detonation spray system was used for obtaining molybdenum and tantalum coatings. The coatings were deposited on steel substrates at O 2 /C 2 H 2 =1.1 and stand-off distances of 20 mm and 100 mm. The calculation of the particle temperatures and velocities were carried out to find the optimal spraying modes for Mo and Ta powders. No oxide phases were found in the coatings obtained by spraying of the Mo powder. In the Ta-based coatings, Ta 2 O 5 was found as a second phase. The hardness of the Mo coatings sprayed at 20 mm and 100 mm was 500 HV 300 and 625 HV 300 , respectively. The porosity of the Mo coatings was less than 0.5% for both stand-off distances. The hardness of the Ta-based coatings sprayed at 20 mm and 100 mm was 800 HV 300 and 1000 HV 300 , respectively. The porosity of these coatings was less than 1% for both stand-off distances. The bond strength of the Mo coatings determined by the pin test method was 92 and 126 MPa and that of the Ta-based coatings was 43 and 77 MPa, for coatings deposited at 20 and 100 mm, respectively.

A bio-implant is a medical device used to support, replace or enhance a biological structure. Bio-implants have incalculable importance for mankind due to their crucial applications in situations like heart diseases, joint replacements, bone fixation and bone replacements etc. The materials which are used to manufacture bio-implants are generally called biomaterials. Corrosion and biocompatibility of biomaterials are the two issues, which are needed to be addressed to enhance the life span of the bio-implants. Various strategies were used to enhance the performance of biomaterials; however surface modification and alloying are the two main strategies among them. To employ these two strategies; various ceramic, metallic and composite materials were utilized as coatings and alloying elements with different biomaterials. Among all these materials, Niobium and Tantalum metals are studied by various researchers and proved to improve the corrosion resistance as well as biocompatibility of the bioimplants. This article gives a systematic overview of research work carried out in the area of bio-implants by utilizing Niobium and Tantalum for the enhancement of corrosion resistance and biocompatibility.

In this paper, it is shown that mixing metal powders can lead to improvements in cold sprayability (i.e. increases in the deposition efficiency and decreases in the porosity) of the ‘matrix’ powder. This finding is illustrated with the following ‘binary’ mixtures: Ti – 6Al-4 V and commercial purity Ti; stainless steel and iron; spherical tantalum with irregular tantalum and two types of spherical tantalum with different characteristics. It is shown that a critical addition can significantly influence cold sprayability, but above this, there is little change in cold sprayability. Hardness differences are considered to be the first order influence, but differences in particle sizes and morphology may also be contributing factors.

The residual stress level in coatings is a main issue in controlling in-service deformation, spallation or cracking. Residual stress generation has been widely studied for plasma and HVOF sprayed coatings, but only scare data are available for cold sprayed coatings. This paper describes the measurement and analysis of residual stresses in tantalum cold sprayed coatings. Residual stress measurements were performed by the hole-drilling and curvature methods. The former provided a through-thickness residual stress profile in the coating while the latter was used to investigate the in-situ residual stress evolution during the deposition process. The results from both methods were consistent and showed compressive stress of 350 MPa for a tantalum coating deposited on a 3 mm thick copper substrate at 80°C.