By approaching the glass transition temperature it was possible to realize well quality metallic coatings on two different glasses using Cold Spray deposition, CS. A roughness is introduced on the glass surface and is proportional to the energy deposited. Using the thermal pressure memory effect of glass, Raman spectroscopy mapping allowed determining that the CS introduced a strong heterogeneity of the glass substrate characterized by zones with different cooling rate and residual stress. Using a simplified Eshelby's inclusion approach, it is demonstrated that the residual stress can be in first approximation explained by the introduction of local density fluctuation induced with high cooling rate of micrometric regions related with the impact of the deposited metallic particles.

A wide range of properties can be achieved in intermetallic coatings applied by gas detonation spraying (GDS). The properties of Fe-40at%Al GDS layers, however, may change when exposed to temperatures exceeding a threshold level. To characterize such changes, Fe-40at%Al GDS coatings were subjected to systematic dilatometric studies in which temperatures were cycled from room temperature to 1180 °C. The investigation revealed both irreversible and reversible phase transitions as described in the paper. Dilatometry measurements obtained from sintered samples made from the same powder are presented for comparison.

This study assesses the viability of using nitrogen instead of helium to cold spray NiCoCrAlTaY coatings onto single-crystal superalloy substrates. The process, though feasible, has a low deposition efficiency, leading to a high level of deformation that affects the microstructure of both the coating and substrate. SEM and TEM analysis revealed metallurgical and mechanical bonding at the interface and grain refinement in the coating. A fine grain structure that developed in the substrate after deposition was also observed possibly caused by dynamic recrystallization during the deposition process. Evidence of element segregation in the substrate, identifiable as zones with a deformed γ/γ’ structure, was found as well.

Strontium zirconate is a candidate material for thermal barrier coatings due to its high melting point, good sintering resistance, and high TCE. One drawback, however, is a phase transition that occurs below 1200 °C , although rare-earth element doping offers a way to suppress it. In this study, SrZrO 3 doped with two rare earth oxides, ytterbia and gadolinia, is deposited by solution precursor plasma spraying and the layers obtained are evaluated before and after heat treatment. The coatings are characterized by two phases, SrZrO 3 and t-ZrO 2 , with interpass boundary structure, nano and microscale porosity, and through-thickness vertical cracks. XRD analysis after heat treatment at 1400 °C for 360 h shows that the two phases are very stable due to the doping of rare-earth elements, which is also shown to reduce thermal conductivity in the as-sprayed deposits by nearly 35%.

Thermally sprayed coatings exhibit a lamellar structure with a bonding ratio less than 32% when a coating is deposited at ambient temperature. The lamellar bonding is one of the most important factors controlling the mechanical, thermal and electrical properties of the coatings. However, it is not clear why only limited lamellar bonding exists in a thermal spray coating even though many studies have been focused on the formation of bonding. In our previous study, it was found that there exists a critical deposition temperature for depositing ceramic splats to form the bonding with the underlying identical substrate, i.e., critical bonding temperature. Moreover, the critical bonding temperature is related to the interface temperature prior splat solidification which is determined by the glass transition temperature of splat material. In the present study, the critical bonding temperature and its relationship with interface temperature are used to understand the limited lamellar bonding ratio in a coating. A numerical simulation model involving heat transfer among depositing splat was proposed to establish the sufficient condition for liquid splat to form the bonding with the underlaying splats. The non-uniform splat thickness model was established to calculate theoretically the interface bonding formation. The calculation based on the model yielded a bonding ratio of 38.5% which agrees reasonably with the observed maximum interface bonding ratio.

In this study, aluminum is deposited by cold gas spraying on smooth and rough alumina substrates. SEM examination of single particle splats on polycrystalline Al 2 O 3 suggests that mechanical interlocking is the primary bonding mechanism for cold gas sprayed metals on rough ceramic surfaces. Coating cross-sections on monocrystalline, atomically smooth sapphire substrates were examined by means of high-resolution TEM, revealing nanosized Al grains close to the interface, which could be an indicator of recrystallisation induced by local heating and the deformation energy stored in the particle during impact.

In the present study, hydroxyapatite coatings were deposited on Ti-6Al-4V alloy substrate by high velocity oxy-fuel (HVOF) spray technique. The as sprayed HA powders and coatings were analyzed with the aim to reveal the melting state of HA powders and its influence on coating properties. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) were employed for the characterization of the starting powders and as-sprayed coatings. Differential scanning calorimetry (DSC) was performed to determine the recrystallization temperature of the amorphous phase in HVOF HA coating. Results show that different melting state of HA powders can be achieved through altering HA powder size and/or spray parameters. XRD result reveals that the as sprayed HA coating made from large powders with size of ~50 µm is composed of crystalline HA and very small amount of a-tricalcium phosphate (TCP). While the coatings deposited using fine powders around 30 μm demonstrated a lot of amorphous phase besides crystalline HA and small amount of a-TCP. The recrystallization temperature of the amorphous phase in HA coating is ~720°C. The adhesive strength of the HVOF sprayed HA coatings is ~31MPa and is largely dependent on the melting state of HA powders. This suggests that the fully melted state of the feedstock can result in the formation of amorphous phase, and simultaneously decrease the adhesive strength. It also suggests that the melted fraction of the powders is the most critical factor influencing the adhesive strength and phase composition of HVOF HA coatings. The partial melting state of HA powders is beneficial in terms of adhesive strength and crystallinity.

Experimental measurements have been carried out with the aim of investigating the residual stresses generated during plasma spray deposition of glass composite coatings. The research shows that the behaviour of these materials is fundamentally different from metals and ceramics. The quench stress in the glass composites can be eliminated by plasma-scanning. This is attributed to their low glass transition temperatures, which enable the stresses to be completely relaxed. The work also shows that the addition of alumina as a second phase allows the expansion mismatch between the coating and the steel substrate to be controlled. Control of the second-phase volume-fraction enables the residual stress in the composite coatings to be reduced to zero. Real-time measurements on deflection and temperature show that the dimensions of the substrate, plasma operating conditions and scanning rate have substantial effects on the temperature profiles within the deposits. Keywords: glass composite coatings, thermal stress, plasma spraying.

An investigation has been undertaken on the analysis of residual stress in glass coatings during plasma spraying. Theoretical analysis and in-situ experimental measurements show that the residual stresses in glass coatings are particularly sensitive to the heat input from the plasma flame, since this can raise the temperature to above the glass transition temperature. Control of the spraying parameters enables the quench stress of splats to be relaxed by the end of the spraying and the only significant remaining source of stress derives from the differential contraction between the coating and substrate during cooling. The analysis also shows that a stress transition occurs during cooling and that the sign of the final residual stress depends upon the expansion coefficient of the glass. The residual stresses are shown to govern the critical coating thickness for cracking and the coating adhesion.