Microstructure of vapor deposited coatings on curved substrates

Abstract

Thermal barrier coating systems consisting of a metallic bond coat and ceramic over layer are widely used to extend the life of gas turbine engine components. They are applied using either high-vacuum physical vapor deposition techniques in which vapor atoms rarely experience scattering collisions during propagation to a substrate, or by gas jet assisted (low-vacuum) vapor deposition techniques that utilize scattering from streamlines to enable non-line-of-sight deposition. Both approaches require substrate motion to coat a substrate of complex shape. Here, direct simulation Monte Carlo and kinetic Monte Carlo simulation methods are combined to simulate the deposition of a nickel coating over the concave and convex surfaces of a model airfoil, and the simulation results are compared with those from experimental depositions. The simulation method successfully predicted variations in coating thickness, columnar growth angle, and porosity during both stationary and substrate rotated deposition. It was then used to investigate a wide range of vapor deposition conditions spanning high-vacuum physical vapor deposition to low-vacuum gas jet assisted vapor deposition. The average coating thickness was found to increase initially with gas pressure reaching a maximum at a chamber pressure of 8-10 Pa, but the best coating thickness uniformity was achieved under high vacuum deposition conditions. However, high vacuum conditions increased the variation in the coatings pore volume fraction over the surface of the airfoil. The simulation approach was combined with an optimization algorithm and used to investigate novel deposition concepts to tailor the local coating thickness. V

Vapor deposition on doublet airfoil substrates: Coating thickness control

“…The coating thickness profiles on the exterior surfaces were similar to those deposited on rotated single airfoils; especially when the channel width was small. 34 Along the interior surfaces, the profiles were of similar shape, but exhibit larger variation between minimum and maximum values. The minimum deposition efficiencies along the interior concave and convex surfaces occurred in NLS regions where gas phase diffusion (by binary atom collision scattering) transverse to the streamlines was the dominant mechanism controlling deposition.…”

“…Both airfoils were identical in design to that used in previous studies of deposition on single airfoil substrates. 33,34 The geometry and dimensions of the 2D doublet substrate are shown in Fig. 2.…”

“…3,45 Experimental deposition conditions were similar to those used for deposition on a single airfoil. 34 A 70 kV/2.45 kW electron beam was used for all depositions. A substrate with a channel width of 16 mm was used, and the substrates were rotated at 6 rpm while held at a temperature of 423 K. This rate and temperature combination was selected to create a columnar microstructure with a constant morphology through the thickness of the coating.…”

“…This dependence of the pressure near the external surfaces of the airfoils was similar to that recently observed during deposition on a single airfoil substrate. 34 Figure 5 also shows streamlines corresponding to locally averaged carrier gas atom trajectories. The streamlines were calculated with an initially uniform spacing of 0.96 mm along a line perpendicular to the gas jet located 50 mm upstream of the substrate's leading edge.…”

Thickness distribution of sputtered films on curved substrates for adjustable x-ray optics

“…[40][41][42][43] Controlling thin film thickness uniformity and microstructure becomes more complicated when depositing onto curved substrates as the ROC of the substrate and the angle of the deposition also play a role. [43][44][45][46][47][48] The use of peaks are labeled with pseudocubic notation for the perovskite. In addition, the (111) Pt peak is labeled, and the * symbol indicates instrument-related x-ray peaks.…”