Stress Relaxation of Compression Loaded Plasma‐Sprayed 7 Wt% Y₂O₃–ZrO₂ Stand‐Alone Coatings

Abstract

Stand-alone plasma-sprayed tubes of 7 wt% Y 2 O 3 -ZrO 2 made from the same starting powder but at two different sites were subject to stress-relaxation testing in axial compression at temperatures of 251, 10001, 10501, 11001, and 12001C and at an initial stress of 10-80 MPa. A time-dependent stress response was observed for both coatings at all temperatures. For example, a 20 MPa stress applied at 10501C relaxed to B3 MPa in 180 min. When the same initial stress was applied at 12001C, the coating fully relaxed in 32 min. For all experimental conditions evaluated, an initial fast stress-relaxation regime was observed (o10 min), followed by a slower second stress-relaxation regime at later times (410 min). Coatings with higher as-sprayed densities exhibited a lengthened fast relaxation regime as compared with less dense coatings. A Maxwell model was modified in order to provide an accurate fit to the experimental stress-relaxation curves. From scanning electron microscopy experiments and mechanical data, the mechanism for stress relaxation from 251C through 12001C, particularly during fast relaxation, was proposed to be the formation of cracks parallel with respect to the applied load. In addition to this mechanism, stress relaxation that occurred in specimens tested at 10001C through 12001C was proposed to be due to partial or complete closure of cracks oriented perpendicular to the applied stress.

Design of 7wt.% Y2O3–ZrO2/mullite plasma-sprayed composite coatings for increased creep resistance

“…Stress relaxation experiments on plasma-sprayed YSZ cylinders 12 have revealed deformation mechanisms that are likely similar to those experienced during creep and may provide insight into the current data. YSZ has been shown to relax uniaxial compressive stresses in two stages.…”

“…YSZ coatings were sprayed using a PST 1108 torch, and a Metco F4 torch for the mullite and composite coatings. Coatings of each material type were sectioned into 15-20 mm tall specimens using the procedure specified by Dickinson et al 12 The two faces of each specimen were machined parallel on a lathe via a diamond tool. The thickness of the coatings was ∼650 m, with the inner diameter set at 12.7 mm by the aluminum substrate.…”

“…8,11 Kokini and others have shown that plasma-sprayed coatings almost immediately begin to relax this stress. [11][12][13][14][15][16] On engine cooling, a tensile stress gradient through the thickness of the coating develops in proportion to the amount of in-service relaxation that occurred. This tensile gradient can lead to through thickness cracks in the coating, which can eventually cause spallation and failure of the coating.…”

The Influence of Transient Thermal Gradients and Substrate Constraint on Delamination of Thermal Barrier Coatings

“…This assumption is commonly invoked based on the fact that the TBC coating creeps at high temperatures. Recent data [6,7] on plasma-sprayed 7wt% 2 3 2 Y O -ZrO suggest that stresses in the range 20 80MPa  will undergo significant creep relaxation within minutes for temperatures above 1000C .…”

Effect of Pulsed Electrical Field on Deposition of YSZ Thin Films in an Aqueous Solution

“…Zirconia is critical for electrical property and thermal, mechanical, chemical stability, [1][2][3] and its thin films have attracted much attention for such applications as the miniaturization of solid oxide fuel cells (SOFCs), [4][5][6] thermal-shield or corrosion-resistant coatings, 7) buffer layers of electric devices, 8) optical coatings 9) and oxygen sensors. 10) Thin and dense films are needed for these applications.…”