The Journal of Thermal Spray Technology could not exist without the ongoing contributions of guest editors and reviewers, which ensure the highest quality of our journal. Manuscripts are reviewed for technical excellence, quality, and relevance by competent reviewers. The editors identify and select reviewers based on their demonstrable and recognized research expertise, professional experience, critical judgment and foresight, and forward thinking in their field of expertise. We express our deepest appreciation to these guest editors who have created special topical issues of JTST published in 2013, and to these reviewers, all of whom have completed manuscript reviews during 2013. Thank you all for giving generously of your valuable time and expertise.

The degree of bonding between particles within cold-sprayed deposits is of great importance as it affects their mechanical and physical properties. This article describes a method for characterizing the bonding between aluminum and copper particles following deposition by cold spraying. Aluminum and copper powders were blended in the ratio 1:1 by volume, deposited onto a copper substrate and subsequently heat treated at 400 °C for 15 min. An intermetallic layer formed along some regions of the aluminum-copper boundaries, believed to be where true metal to metal contact had occurred. In other regions, metal to metal contact was inhibited by the presence of oxide films. Image analysis was employed to measure the fraction of the aluminum-copper interface covered with intermetallic phases and to estimate intermetallic thicknesses. By increasing the primary gas pressure in the cold-spray process, an increase in the degree of inter-particle bond formation was observed.

The deposition of copper by cold gas dynamic spraying has attracted much interest in recent years due to the capability to deposit low-porosity oxide-free coatings. However, it is generally found that as-deposited copper has a signicantly greater hardness, and potentially lower ductility, than bulk material. In this article, copper was deposited by cold spraying using helium as the driving gas at both 298 and 523 K. Evidence is presented indicating that the material sprayed at the lower temperature exhibits a lower dislocation density throughout the grain structure than the material sprayed at the higher temperature. The low stacking fault energy of copper restricts recovery during annealing, and thus microstructural changes during annealing only proceed once recrystallization begins. The material sprayed at low temperature (with the low dislocation density) exhibited recrystallization at annealing temperatures as low as 373 K with a corresponding reduction in hardness. However, the copper sprayed with helium at 523 K was resistant to annealing at temperatures up to 473 K where the dislocations in the structure prevented recrystallization. However, at higher temperatures, recrystallization did proceed (with corresponding reductions in hardness). The fracture behavior of the copper that was cold sprayed with helium at 523 K, both in the as-sprayed condition and following annealing, was measured and explained in terms of the annealing mechanisms proposed.

Deposition of copper by cold gas dynamic spraying has attracted much interest in recent years because of the capability to deposit low porosity oxide free coatings. However, it is generally found that as-deposited copper has a significantly greater hardness, and potentially lower ductility, than bulk material. This paper will describe work undertaken to investigate the effect of annealing heat treatments on the structure and mechanical properties of freestanding cold sprayed copper. After de-bonding from substrates these tracks were annealed for one hour at a range of temperatures up to 600 °C. Optical microscopy, scanning electron microscopy and X-ray diffraction were all employed to examine the microstructure. The peak widths in XRD were analysed according to the Hall – Williamson method so that changes in grain size and microstrain (i.e. dislocation content) could be quantified. Mechanical behaviour of the deposits was studied by microhardness measurements and tensile testing. The influences of annealing on mechanical properties are rationalised in terms of microstructure evolution and its effect on strengthening and recrystallization mechanisms in metals. The softening behaviour of cold sprayed Cu is explained considering the low stacking fault energy of Cu and the possibility of dynamic recystallization occurring during spraying.

In the Cold Gas Dynamic Spray (CGDS) process, coatings are deposited by the virtue of the high particle velocity achieved by the use of converging-diverging (de Laval) nozzle along with suitable particle characteristics and process parameters. In this study copper coatings were deposited on aluminium substrates using helium as the accelerating gas. The influence of the CGDS conditions, primarily driving gas temperature and pressure, on the nature of the deposited coatings and the deposition efficiency of the process were investigated.The results indicate that it is possible to deposit copper coatings at a wide range of process conditions, with successful deposition being observed with the driving gas at room temperature and 11 bar pressure (a condition where the nozzle is still choked). However, the nature of the coatings is strongly dependent upon the processing conditions. With room temperature driving gas, an increase in pressure lead to an increase in deposition efficiency, and increase in substrate deformation and an increase in microhardness in the deposit due to higher levels of work hardening. The use of driving gas at temperatures as low as 473 K resulted in recrystallisation in the deposit and a decrease in tendency to debond due to stress relief during recrystallisation. Recrystallisation also manifested itself in reduced hardness. The sensitivity of the recrystallisation conditions to the traverse speed of the jet over the substrate indicated that these processes are initiated by the impingement of the hot gas jet onto the deposit following deposition and not by changes in velocity or temperature of the particles upon impact.