Deposizione di rivestimenti in Al2O3 mediante High Velocity Suspension Flame Spraying (HVSFS): caratteristiche dei riporti ed effetto dei parametri operativi

The HVSFS (High Velocity Suspension Flame Spraying) technique is a novel thermal spray process which consists of a modification to a standard gas-fuelled HVOF system in order to enable feeding of the torch with a liquid suspension instead of a dry powder. The suspension consists of fine (micrometre- or nanometer-sized) particles dispersed in a liquid medium. This process enables the deposition of relatively thin (<100 μm) and dense ceramic coatings, which could not be obtained by standard thermal spraying, since the flowability of very fine powders in dry form is too poor to allow proper feeding. These flowability problems are overcome by employing a liquid medium as carrier. Using such fine particles, the average lamella size is reduced compared to conventional thermal spray coatings, which is beneficial for improved smoothness (lower surface toughness in as-deposited condition) and density (better packing, smaller pores) and allows the deposition of homogeneous layers of reduced thickness. The properties of a HVSFS-deposited coating are critically affected by the properties of the suspension and particularly by the nature of the solid phase (primary particle size distribution, agglomeration behaviour); however, systematic studies on this relationship have not been performed yet. In this research, HVSFS Al2O3 coatings were therefore produced using three different suspensions, each containing a different fine alumina powder, dispersed in a water + isopropanol mixture: a micrometric powder (d50 ≈ 2 μm) and two nanometric ones with identical average size (d50 ≈ 70 nm) but different agglomeration behaviour. All suspensions contained 20 wt.% of solid phase. Each suspension was sprayed using two distinct parameter sets, so that a total of 6 different coatings were obtained. The results showed that, irrespective of the process parameters, the suspension containing micrometric particles yielded harder (HV0.05 = 1200 - 1300 kg/mm2) and smoother (Ra ≈ 1.3 μm) coatings. It was noted that micron-sized particles have low tendency to agglomeration; therefore, when the solvent (water + isopropanol mixture) evaporates in the thermal spray gas jet, the suspension releases many individual micrometric particles of 1 - 2 μm diameter. These particles are fully melted at impact and, on account of their high impact velocity, they result in a dense and homogeneous stacking of well-flattened lamellae. Although these coatings also exhibit the largest tensile residual stresses (between 50 MPa and 100 MPa), their excellent cohesive strength endows them with superior sliding wear resistance (tested under "ball-on-disk" configuration against sintered alumina spheres) in comparison to the other HVSFS-deposited coatings and to conventional HVOF-sprayed Al2O3 coatings. As nanometre-sized particles are always agglomerated within the suspension, the resulting lamella size does not depend on the size of the (nanometric) primary particles, but on that of the agglomerates, which are released in the gas jet when the liquid phase evaporates. When agglomerates are sufficiently small (0.5 - 2.5 μm), quite dense coatings can be achieved; excessively large agglomerates, by contrast, tend to remain unmelted, giving rise to defective coatings and impairing the deposition efficiency. In conclusion, the HVSFS process was shown to be able to produce high-quality coatings, superior to those obtained by conventional thermal spraying techniques; however, proper control over the behaviour of the solid phase must be acquired; in particular, individual particles or agglomerates should ideally be comprised between 0.5 and 2.5 μm in order to optimize density, hardness, smoothness and deposition efficiency.