Hybrid aerosol deposition (HAD) has been proposed recently as a new coating regime to deposit homogeneous ceramic coatings via the utilization of mesoplasma and solid particle deposition. This study will discuss the implementation of HAD for the deposition of alumina (Al 2 O 3 ) coatings on 304 stainless steel and aluminum substrates, and evaluation of the hardness and Young’s modulus using a nanoindentation method to clarify the through-thickness properties. Dense and uniform coatings with a nanocrystalline structure were fabricated on both substrate materials. The fabricated HAD coatings consisted of α-Al 2 O 3 phase. The hardness and Young’s modulus distributions along the through-thickness direction showed a significant difference across the coating-substrate interface and tended to show a slight decrease by 10-15% as the measured position went close the surface. Increasing the hardness and Young’s modulus on the substrate side near the interface is presumably related to the peeing effect of the substrate as well as the increase of interface roughness during the room temperature impact consolidation (RTIC) and deformation of the hard ceramic particles on the substrate. The decrease in the coating’s mechanical properties along the through-thickness direction is considered to be related to the particle deformation tendency during the coating build-up. At the beginning stage of the deposition, initial particles are impacting on a metallic substrate which is ductile enough to facile plastic deformation and the deposited layer can have an enough hammering effect by the subsequent impacting particles. The hardness and Young’s modulus in this location are 15.6 GPa and 246 GPa, respectively, and the highest through the thickness in case of the stainless steel substrate. However, the later particles are impacting on a hard ceramic surface (initially formed HAD Al 2 O 3 layers), which hardly undergo plastic deformation or led to less particle deformation. In addition, through-thickness measurements revealed that the deposited coatings on the stainless steel substrate showed higher hardness than deposited coatings on aluminum substrates. Thus, the stainless steel enhances the degree of deformation of the deposited particles, and the resulted smaller crystallite size and strain lead to increased hardness and modulus.

Deposition of ceramic coatings by spraying of fine powder particles is a promising solution to obtain uniform microstructure and improved properties, because it is expected to form small splats with reduced residual stress and pore size. Although direct injection of fine particles has many challenges such as poor rheological properties (e.g., low flow ability and agglomeration), and small momentum to inject and impact, it is still attractive and worth to try. From this point of view, we have improved feeding and injection technique of fine powder particles, which enables us to directly spray fine ceramic particles with a newly developed low power dc plasma torch. Preliminary results obtained with this system will be compared with those by other fine powder spray technology such as suspension plasma spray and hybrid aerosol deposition to show similarity and difference when the injection methods of the fine powder particles are different in this paper.

Suspension plasma spraying (SPS) process is showing promising improvements for the Thermal barrier coatings (TBCs) performance. This study will focus on the development of a low cost suspension plasma spray system with an axial feeding of yttria stabilized zirconia (YSZ) coatings. A unique twin-cathode plasma spray gun with three plasma torches; P-torch (reversed polarity: cathode nozzle and anode), and two N-torches (normal polarity: anode nozzle and cathode) was used. During operation, the electric power is supplied between P-torch anode and N-torches cathodes to maintain the plasma jet. The system requires only argon as plasma gas, since the enthalpy of plasma jet is enough high for melting ceramic powders, which makes it as cost reduction system. Moreover, the p-torch electrode is hollowed anode and therefore commercial dual tube nozzle was used for the axial feeding to increase the deposition efficiency. It was possible to fabricate dense and/or porous 8YSZ coatings with using only Ar plasma gas through using the twin cathode plasma spray system and the axial injection. The coating microstructure and porosity level can be controlled by appropriate changes in the solids amount in the suspension and the spraying parameters. To control the microstructure the main factor is controlling and adjusting the splat size.

Aluminum nitride (AlN) ceramics is characterized with its high thermal conductivity and chemical stability. However, it was impossible to fabricate AlN coatings by conventional thermal spray processes directly from AlN feedstock powder due to thermal decomposition of AlN during spraying. In the last decade we were apple to fabricate the AlN coatings through the reactive plasma spraying process (RPS) in the atmospheric ambient. This study describes the way to fabricate high thermal conductivity plasma sprayed AlN coatings. The thermal conductivity of the AlN coatings was investigated by laser flash method. The as sprayed coating had very low thermal conductivity (2.43 W/m.K), compared to the AlN value. It is attributed to presences of high oxide content (Al 5 O 6 N, γ-Al 2 O 3 and α-Al 2 O 3 phases), low density (2.32 g/cm 3 ) and high porosity in the plasma sprayed coating (about 22%). Besides that, although the N 2 gas flow improved the nitride content, the thermal conductivity decreased gradually. It is related to the further increase of the coating porosity and decreasing its density with the N 2 gas. The influence of the process parameters on the thermal conductivity was investigated and to fabricate high thermal conductivity AlN coating adjusting the oxide content, the coating porosity and microstructure are the main factors. Very high thermal conductivity (about 95 W/m.K) atmospheric plasma sprayed AlN coating was fabricated. The coating consists of mainly AlN phase (more than 95% AlN), very small amount of oxide phases, low porosity (about 3%) with a sintered microstructure (nicked-shape sintered particles).

Cold spraying is a promising candidate for the metallization of carbon fiber reinforced polymer (CFRP) composites, but it requires the use of a protective interlayer deposited by either flame or plasma spray. This study evaluates the effect of different surface treatments on interlayer adhesion strength. CFRP samples were treated mechanically, chemically, and thermally and then a copper layer was deposited by atmospheric plasma spraying. Adhesion strength was measured by pullout testing and the results are compared with surface roughness measurements, SEM fracture surface images, FIB bisections of single Cu splats, and XPS analysis of the chemical affinity between the polymer and copper.