Contact Damage and Strength Degradation in Brittle/Quasi‐Plastic Silicon Nitride Bilayers

Abstract

A study is made of the damage resistance of silicon nitride bilayers consisting of a hard overlayer (coating) on a soft underlayer (substrate). The two layers are fabricated with different starting powders, to provide distinctive elongategrain microstructures, and are cosintered, to provide strong interfacial bonding and thus to minimize subsequent delamination. Contact testing with spherical indenters is used to characterize the damage response. The elasticplastic mismatch between the layers is sufficiently high as to produce distinctive damage modes in the two layers: predominantly cone cracking in the coating, and quasiplasticity in the substrate. However, the mismatch is also sufficiently low as to preclude secondary transverse cracks of the kind observed in other bilayer systems to initiate immediately beneath the contact at the coating/substrate interface and propagate upward within the coating. The dominant damage mode shifts from coating fracture to substrate quasi-plasticity with increasing contact load and decreasing coating thickness. Significantly, the presence of the soft underlayer inhibits growth of the coating cone cracks as the latter approach and intersect the interface. The underlayer also substantially diminishes strength losses from the contact-induced damage, especially in bilayers with thinner coatings. The implication is that bilayer structures with thin, hard coatings can preserve benefits from the inherent toughness of soft substrate materials, and at the same time afford surface protection (high wear resistance) to the underlayer.

Evaluation of Thermal Durability of Thermal Barrier Coating and Change in Mechanical Behavior

“…Microcracks are initiated in the damage zone, followed by ring cracks or accompanied by radial cracks according to different starting granules [15,23].…”

“…A Hertzian contact test with a spherical indenter characterizes elastic and the quasiplastic properties of thermal barrier coatings [13][14][15]. Instrumented indentation techniques such as nanoindentations can be used to investigate contact damage, fracture, and mechanical properties related to wear of thermal barrier coatings [16,17].…”

Properties of silicon nitride for aluminum melts prepared by nitrided pressureless sintering

“…[13][14][15][16][17] Indentation stress-strain curves were obtained from measurements of contact radius at each value of P and r, with varying the load from 30 to 4000 N. Then the indentation stress, p o = P/πa 2 , and indentation strain, a/r were calculated. 14,18 Strength degradation was evaluated by four-point flexural bending test after Hertzian indentation as shown in Fig.…”

“…Dark symbols represent symbols that broke at indentation sites. It is reported that the mode of contact damages change as the grain sizes of ceramic material become coarser and more heterogeneous, and consequently tougher, undergo- ing a "brittle-to-quasi-plastic" transition 13,14,16,17 : in coarse-␤ grain heterogeneous microstructures exhibit diffuse microdamage forms in the region of strong compression-shear stress beneath the contact point rather than ring or cone cracks. In heterogeneous microstructures, failure occurs from single shear fault/wing cracks within the damage zone as shown in Fig.…”

“…The contact damage response and strength degradation of the NPS Si 3 N 4 are evaluated using spherical indenter. [13][14][15][16][17][18] Thermal properties such as thermal expansion coefficient and thermal conductivity are also measured. Additionally, the strength degradations of the NPS by corrosion medium are also checked.…”