Effect of Coating Thickness on Interfacial Shear Behavior of Zirconia-Coated Sapphire Fibers in a Polycrystalline Alumina Matrix

ABSTRACTThe effect of zirconia (ZrO2) interfacial coatings on the interfacial shear behavior in sapphire reinforced alumina was examined in this study. Zirconia coatings of thicknesses ranging from 0.15 to 1.45 μm were applied to single crystal sapphire (Saphikon) fibers using a particulate loaded sol dipping technique. After calcining at 1100°C in air, the coated fibers were incorporated into a polycrystalline alumina matrix via hot pressing. Interfacial shear strength and sliding behavior of the coated fibers was examined using thin-slice indentation fiber pushout and pushback techniques. In all cases, debonding and sliding occurred at the interface between the fibers and the coating. The coatings exhibited a dense microstructure and led to a higher interfacial shear strength (> 240 MPa) and interfacial sliding stress (>75 MPa) relative to previous studies on the effect of a porous interphase on interfacial properties [1]. The interfacial shear strength decreased with increasing fiber coating thickness (from 389 ± 59 to 241 ± 43 MPa for 0.15 to 1.45 μm thick coatings, respectively). Sliding behavior exhibited load modulation with increasing displacement during fiber sliding which is characteristic of fiber roughnessinduced “stick-slip”. No effect of fiber coating thickness on the interfacial sliding stress was observed for single pushout or pushback events. Heat treatment at 1550°C in air coarsened the fiber surface roughness, resulting in significantly higher interfacial shear strengths (>30%) and interfacial sliding stresses (>60%) relative to the coated fibers in an as-hot-pressed condition. Interfacial sliding resistance decreased significantly after the first sliding cycle. Evidence of substantial “stick-slip” behavior was eliminated from the load displacement plots after one pushout/pushback cycle; however, the pushback plots exhibited evidence of fiber reseating, followed by a decreasing trend in load with increasing displacement of the fiber back to the original position. These results directly support the interphase fragmentation scenario proposed for interface fatigue in ceramic composites.The high interfacial shear strengths and sliding stresses measured in this study, as well as the potentially strength degrading surface reconstruction observed on the coated fibers after hot pressing and heat treatment, indicate that dense zirconia coatings are not suitable candidates for optimizing composite toughness and strength in the sapphire fiber reinforced alumina system.