Mark F. Smith

Gas dynamic principles of cold spray

This paper presents an analytical model of the cold-spray process. By assuming a one-dimensional isentropic flow and constant gas properties, analytical equations are solved to predict the spray particle velocities. The solutions demonstrate the interaction between the numerous geometric and material properties. The analytical results allow determination of an optimal design for a cold-spray nozzle. The spray particle velocity is determined to be a strong function of the gas properties, particle material density, and size. It is also shown that the system performance is sensitive to the nozzle length, but not sensitive to the nozzle shape. Thus, it is often possible to use one nozzle design for a variety of operational conditions. Many of the results obtained in this article are also directly applicable to other thermal spray processes.

Impact of high velocity cold spray particles

This article presents experimental data and a computational model of the cold spray solid particle impact process. Copper particles impacting onto a polished stainless steel substrate were examined in this study. The high velocity impact causes significant plastic deformation of both the particle and the substrate, but no melting was observed. The plastic deformation exposes clean surfaces that, under the high impact pressures, result in significant bond strengths between the particle and substrate. Experimental measurements of the splat and crater sizes compare well with the numerical calculations. It was shown that the crater depth is significant and increases with impact velocity. However, the splat diameter is much less sensitive to the impact velocity. It was also shown that the geometric lengths of the splat and crater scale linearly with the diameter of the impacting particle. The results presented will allow a better understanding of the bonding process during cold spray.

Particle Velocity and Deposition Efficiency in the Cold Spray Process

Copper powder was sprayed by the cold gas-dynamic method. In-flight particle velocities were measured with a laser two-focus system as a function of process parameters such as gas temperature, gas pressure, and powder feed rate. Mean particle velocities were uniform in a relatively large volume within the plume and agreed with theoretical predictions. The presence of a substrate was found to have no significant effect on in-flight particle velocities prior to impact. Cold-spray deposition efficiencies were measured on aluminum substrates as a function of particle velocity and incident angle of the plume. Deposition efficiencies of up to 95% were achieved. The critical velocity for deposition was determined to be about 640 m/s for the system studied.

Oxidation in wire HVOF-sprayed steel

It is widely held that most oxidation in thermally sprayed coatings occurs on the surface of the droplet after it has flattened. Evidence in this paper suggests that, for the conditions studied here, oxidation of the top surface of flattened droplets is not the dominant oxidation mechanism. In this study, a mild steel wire (AISI 1025) was sprayed using a high-velocity oxy-fuel (HVOF) torch onto copper and aluminum substrates. Ion milling and Auger spectroscopy were used to examine the distribution of oxides within individual splats. Conventional metallographic analysis was also used to study oxide distributions within coatings that were sprayed under the same conditions. An analytical model for oxidation of the exposed surface of a splat is presented. Based on literature data, the model assumes that diffusion of iron through a solid FeO layer is the rate limiting factor in forming the oxide on the top surface of a splat. An FeO layer only a few nanometers thick is predicted to form on the splat surface as it cools. However, experimental evidence shows that the oxide layers are typically 100× thicker than the predicted value. These thick oxide layers are not always observed on the top surface of a splat. Indeed, in some instances the oxide layer is on the bottom, and the metal is on the top. The observed oxide distributions are more consistently explained if most of the oxide forms before the droplets impact the substrate.

The 2016 Thermal Spray Roadmap

Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications.

Current Problems in Plasma Spray Processing

This article summarizes eight contributions from a thermal spray conference that was held in late 1991 at Brookhaven National Laboratory, Upton, Long Island, New York. Plasma spray processing is discussed in terms of plasma-particle interactions, deposit formation dynamics, thermal properties of thermal barrier coatings, mechanical properties of coatings, feedstock materials, porosity, manufacture of intermetallic coatings, and synchrotron X-ray microtomographic methods for thermal spray materials. Each section is intended to present a concise statement of a specific practical and/or scientific problem. It then describes current work that is being performed to investigate this area, and finally suggests areas of research that may be fertile for future activity.

A comparison of techniques for the metallographic preparation of thermal sprayed samples

Metallographic preparation of thermal spray coated samples is often difficult because hard and soft materials, which normally require different polishing techniques, are commonly present in a single spraycoated sample. In addition, the microstructures of many spray- deposited materials make them prone to pull-out damage during cutting, grinding, and polishing operations. This study compares alternative metallographic techniques to prepare three common types of thermal sprayed coatings: (1) a plasma sprayed alumina-titania wear coating, (2) a plasma sprayed zirconia thermal barrier coating, and (3) a high-velocity oxy-fuel (HVOF) sprayed tungsten- carbide/cobalt (WC/Co) hard coating. Each coating was deposited onto a steel substrate and was prepared with metallographic protocols based on silicon carbide (SiC) papers, bonded diamond platens, and diamond slurries. Polishing with SiC papers generally produced edge rounding and significant pull- out, which increased the apparent porosity of the coatings. Polishing with bonded diamond platens produced less edge rounding, but some pull- out was still observed. Preparation by diamond slurry lapping consistently produced the best overall results. Porosity artifacts produced by polishing with SiC papers and bonded diamond platens also resulted in spuriously low hardness values for the WC/Co samples; however, hardness results for the two ceramic coatings were not affected by the polishing method.

Effect of chamber pressure on particle velocities in low pressure plasma spray deposition

Abstract A laser velocimeter has been used to measure spray particle velocities in a low pressure plasma spray system at chamber pressures ranging from 6.7 to 80 kPa (50 to 600 Torr). For Al2O3 spray powder with a mean diameter of 44 μm, peak particle velocities were of the order of 200 – 400 m s-1. The measured velocity distributions were strongly dependent upon spray chamber pressure, with the highest particle velocities at intermediate pressures of about 40 kPa (300 Torr). Particle velocities predicted with a simple analytical model are in reasonable agreement with experimental results close to the spray gun, where drag due to chamber gases can be neglected. This simple model also correctly predicts a particle velocity maximum at 45 kPa (340 Torr).

Thermal coating development for impulse drying

A plasma-sprayed coating has been developed for the heated surface of rolls used in a new energy-efficient paper drying process, known as“Impulse Drying,” which could save the US paper industry an estimated

Investigations into the plasma spray process

800 million annually in reduced energy costs. Because impulse drying rolls operate at substantially higher surface temperatures than conventional drying rolls, the thermal properties of the roll surface must be carefully tailored to control energy transfer to the paper and thus prevent sheet delamination or other undesirable effects. To meet this requirement, a plasma-sprayed thermal barrier coating has been developed to control thermal mass, heat transfer, and steam infiltration. A coated test platen significantly outperformed a comparable uncoated steel platen in preliminary experiments with a heavyweight grade of paper on a laboratory-scale impulse drying simulator. Based on these results, the coating was then tested on the roll of a pilot-scale impulse dryer. Compared to conventional wet pressing, linerboard that was impulse dried with the coated test roll showed marked improvements in water removal as well as improved physical properties, such as density and specific elastic modulus. The successful prototype coating design has three plasma-sprayed layers that are deposited sequentially: a nickel alloy bond coat, a thick, 17% porous zirconia thermal barrier, and a thin, 5 to 7% porous zirconia top coat.