Cold gas dynamic spraying (CGDS) can be used to deposit oxygen sensitive materials, such as titanium, without significant chemical degradation of the powder and with minimal heating of the substrate. The process is thus believed to have potential for the deposition of corrosion resistant barrier coatings. However, to be effective a barrier coating must not allow ingress of a corrosive liquid and hence must have minimal interconnected porosity. Thus the aim of the present study was to investigate the effects of processing, including a post-spray annealing treatment, on the deposit meso- and microstructures and corrosion behavior. Commercially pure titanium powder was deposited using pre-heated nitrogen as main and powder carrier gas using a CGT Kinetiks 4000 system to produce coatings on stainless steel. Selected coatings were debonded from the substrate, and the resultant free standing deposits heat treated at 1050° C in vacuum for 60 minutes. Changes in microhardness were measured and correlated with microstructural changes. Optical microscopy, scanning electron microscopy, X-ray diffraction (XRD), helium pycnometry and mercury porosimetry were all employed to examine the microstructural characteristics of coatings and free standing deposits, before and after heat treatment. Their corrosion performance was also investigated using potentiodynamic polarization tests in 3.5 wt% NaCl. The influences of heat treatment and corrosion behavior will be analyzed and discussed in terms of pores structure evolution and microstructural changes.

The deposition of titanium on a titanium alloy substrate is being examined for potential use as a surface treatment for medical prostheses. A Ti6Al4V alloy was coated with pure titanium by cold gas dynamic spraying (CGDS). Coatings were deposited onto samples with two different surface preparation methods (as-received and grit blasted). The fatigue life of the as-received and grit blasted materials, both before and following coating, was measured with a rotating-bend fatigue rig. An 18% reduction in fatigue life was observed following the application of the coating to the as-received substrate, but no significant reduction was observed on its application to the grit blasted substrate. The reduction in fatigue life has been related to the substrate-coating interface properties and residual stress states.

High-temperature nickel-based superalloys such as 1N718 are widely used in gas turbine components as they remain stable at operating temperatures up to ~750°C. There is now a growing interest in the repair and refurbishment of such components using spray deposition techniques. Although many investigations have been carried out to study the effect of conventional processing on the microstructure and mechanical properties of 1N718, much less attention has been given to the alloy when sprayed to form a coating. The purpose of the present study was to investigate and compare 1N718 deposits produced by HVOF spraying and cold gas spray deposition. Optical microscopy, scanning electron microscopy and X-ray diffraction were employed to examine the microstructural evolution of the coatings and to compare the deposition behaviour of the two different processes. Particular attention was paid to porosity, oxide content and the formation of secondary intermetallic phases. Coating microhardness and bond strength were also measured. Results will be presented and discussed in the context of the different thermal histories of the powder particles in the two processes.

The application of thick HVOF coatings on metallic parts has been widely accepted as a solution to improve their wear properties. The adherence of these coatings to the substrate is strongly influenced by the residual stresses generated during the coating deposition process. In an HVOF spraying process, due to the relatively low processing temperature, significant peening stresses are generated during impact of semi-molten particles on the substrate. At present, FE models of residual stress generation for the HVOF process are not available due to the increased complexities in modelling the stresses generated due to the particle impact. In this work, deposition of an HVOF sprayed copper coating on a copper substrate is considered as an example system. An explicit finite element analysis is carried out to study the effect of particle impingement. The results from the analysis are subsequently used in a thermo- mechanical FE model to allow the development of the residual stresses in these coatings to be modelled.

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.

Over the past five years, interest in cold gas dynamic spraying (CGDS) has increased substantially. Considerable effort has been devoted to process development and optimization for low melting point metals such as copper and aluminium. This paper will describe work undertaken to expand the understanding of deposition of titanium by cold spray methods. CGDS deposits have been produced from commercially pure Ti. Using room temperature helium gas, a range of processing conditions, powder size ranges, substrates and substrate preparation methods have been employed to study their impact on deposition of powders. Scanning electron microscopy has been employed to examine deposit microstructures, and microhardness testing of deposits has been conducted. Samples for pull-off bond strength test have been prepared from a number of the more promising sets of parameters and adhesive strengths have been determined. Computational estimates of gas velocity and in-flight particle velocity have been made focusing specifically on the influence that these factors have on the process deposition efficiency. Differences will be discussed in terms of powder feedstock characteristics and the underlying physical and mechanical properties of the powders and substrates.

The acoustic emission (AE) technique, when used with the four-point bend test, is potentially an effective method for characterization of the ductility and fracture behaviour of certain thermally sprayed coatings. Monitoring of AE during such tests on brittle coatings reveals that the rate of the occurrence of events with increasing load changes gradually over the duration of the test. In this work, finite element modelling (FEM) of a brittle coating on an elastic substrate in four-point bending is used to analyze the fracture behaviour of the coating and predict its AE behaviour. The results from the FE analysis show good agreement with experimental data from four-point bending of thermally sprayed WC-Co coatings.

It is widely known that during high velocity oxy-fuel (HVOF) spraying of tungsten carbide – cobalt (WC-Co) coatings, decomposition occurs resulting in the formation of W2C and a relatively brittle amorphous binder phase (along with other carbides and even metallic tungsten). Decomposition has generally been seen to be deleterious to the wear resistance of these coatings and, as such, there have been moves to reduce it. Since decomposition during spraying initiates with WC dissolution into the molten binder phase, strategies for its minimization have been based on reduction of particle temperatures and exposure times during spraying. Moves in spraying from gas-fuelled systems to liquid-fuelled systems have contributed towards these goals. This paper examines microstructural features and wear behaviour of WC-Co coatings deposited with both a liquid-fuelled and a gas-fuelled system. Contrary to expectation, it was found that the wear rate of the liquid-fuel sprayed coating was five to ten times higher than that of the gas-fuel sprayed coating. It was shown that whilst the degree of decomposition was limited during spraying with a liquid-fuelled system, the solid core of WC-Co suffers significant mechanical damage on impact as it is deposited, resulting in carbide fracture and size reduction and thus to the low observed wear resistance.

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.

Aluminium powder of 99.7wt% purity and in the nominal particle size range –75+15µm has been sprayed onto a range of substrates by cold gas dynamic spraying (cold spraying). The substrates examined include metals with a range of hardness, polymers and ceramics. The substrate surfaces had very low roughness before deposition of aluminium in an attempt to separate effects of mechanical bonding from other forms of bonding. The cross-sectional area of a single track of aluminium sprayed onto the substrate was taken as a measure of the ease of initiation of deposition, assuming that once a coating had begun to deposit onto a substrate, its growth would occur at a constant rate regardless of substrate type. It has been shown that initiation of deposition depends critically upon substrate type. For metals where initiation was not easy, then small aluminium particles were seen to be deposited preferentially to large ones (due to their higher impact velocities); these may have acted as an interlayer to promote further building of the coating. A number of phenomena have been observed following spraying onto various substrates, such as substrate melting, substrate and particle deformation and evidence for the formation of a metal-jet (akin to that seen in explosive welding). Such phenomena have been related to the processes occurring during impact of the particles on the substrate. Generally, initiation of aluminium deposition was seen to be poor for non-metallic materials (where no metallic bonding between the particle and substrate was possible) and for very soft metals (in the case of tin, melting of the substrate was observed). Metallic substrates harder than the aluminium particles generally promoted deposition, although deposition onto aluminium alloy was difficult due to the presence of a tenacious oxide layer. Initiation was seen to be rapid on hard metallic substrates, even when deformation of the substrate was not visible.

This work reports research concerning the production of powder, suitable for reactive HVOF spraying, produced by mechanically alloying Ni(Cr), Ti and C elemental powder constituents. Powder mixing was achieved using a high-energy Uni-Ball II Mill and optimisation of the milling parameters are reported. The composition of the powder, at 50wt.%NiCr-40wt.%Ti-10wt.%C, was such that the application of heat has the potential to cause a SHS (Self propagating High Temperature Synthesis) reaction to take place. The utilisation of SHS reactions to produce TiC particles within metallic matrices is well known in bulk systems. However, this work describes carrying out this reaction in individual powder particles on exposure to the high temperature within the HVOF gun. The powder having undergone the SHS reaction during the spray process was deposited onto mild steel substrates to form a dense, coherent coating. The coatings thus formed were shown to contain nanoscale TiC in a Ni(Cr) matrix, indicating a SHS reaction had taken place. This TiC is much finer than that produced in conventional SHS reactions, which is typically ~5ìm. The percentage of TiC formed, and retained in the coating, was lower than expected from the constituent proportions and explanations for this observation are proposed. The microstructure of the coating is described and compared with a Ni(Cr)-TiC cermet coating sprayed using conventional SHS powder generated from reacted compacts which were crushed, sieved and classified to give sprayable feedstock powder.

This research examined the use of 4-point bend testing, with in-situ acoustic emission analysis, for the characterization of the deformation of WC-Co coatings sprayed by the high-velocity oxy-fuel method. Coatings were deposited from WC- 17wt.%Co powders using a Praxair JP5000 system. Two sets of gun operating parameters were employed to produce coatings with distinctly different structures. In the present studies the cracking behaviour of the coatings was investigated through analysis of the acoustic emission (AE) data. In particular, AE data recorded has shown clear differences between the two coating types, in terms of the critical strain level and amount of energy released during cracking. The critical strain levels for the different coatings were 0.32% and 0.6% respectively. Analysis of the coatings by scanning electron microscopy and X-ray diffraction has allowed correlations to be made between the principle microstructural features of the coatings and the mechanical behaviour under bending. The AE responses for these cermet coatings were highly reproducible.

This paper examines the effect of spraying conditions and WC grain size on the wear behavior of WC-Co coatings. Two powders, one with a fine grain fraction and one with a more conventional grain fraction, were applied using liquid and gas HVOF systems. All coatings were subjected to sliding wear tests against an aluminum oxide ball. A sintered WC-Co composite was also tested for comparison. The sintered composite structure (cermet) exhibited the highest wear resistance, while the conventional powder sprayed by means of a gas-operated burner produced the best coating. Paper includes a German-language abstract.

It has been shown that high velocity oxy-fuel (HVOF) thermal spray coatings with good wear resistance can be produced from Ni(Cr)-TiC powders manufactured by self-propagating high temperature synthesis (SHS) reactions. In the present work the process was expanded to include additions of Mo and W with the objective of modifying the carbide phase in an attempt to increase the wear resistance further. The effect of changing the matrix, i.e. substituting Fe for Ni, and changing the ceramic phase from TiC to TiB 2 was also examined. The feedstock powder and resultant coatings are characterised in terms of x-ray diffraction analysis and scanning electron microscopy while the coating properties are measured by microhardness and dry sand rubber wheel (DSRW) abrasive wear testing. The results show that Fe(Cr)-TiB 2 and Ni(Cr)-(W, Ti)C coatings have wear rates comparable to that of conventional Cr 3 C 2 -NiCr coatings produced from sintered and crushed powder, but further improvements are needed to achieve the wear resistance of WC-Co coatings.

In this paper, the production of NiCr-TiC powder by SHS, suitable for HVOF spraying, is discussed together with results on the microstructure and coating properties. Compacts for SHS were prepared by mixing elemental Ti and C with pre-alloyed Ni-20wt.% Cr powder to give an overall composition of 35wt.% NiCr and 65wt.% TiC. These were then ignited and a self-sustaining reaction proceeded to completion. Reacted compacts were crushed, sieved, and classified to give feedstock powders in size ranges of 10-45 µm and 45-75 µm. All powder was characterized prior to spraying based on particle size distribution, x-ray diffraction (XRD), and scanning electron microscopy (SEM/EDS). Thermal spraying was performed using both H2 and C3H6 as fuel gases in a UTP/Miller Thermal HVOF system. The resulting coatings were characterized by SEM and XRD analysis, and the microstructures correlated with powder size and spray conditions. Abrasive wear was determined by a modified 'dry sand rubber wheel' (DSRW) test and wear rates were measured. It has been found that wear rates comparable to those of HVOF sprayed WC-17wt% Co coatings can be achieved.