Fracture behaviour of chemical vapour deposited and hot isostatically pressed zinc sulphide ceramics

Fracture behaviour of zinc sulphide ceramics prepared by chemical vapour deposition (CVD) followed by hot isostatic pressing (CVD + HIP) was investigated in terms of flexural strength (σf), plane-strain fracture toughness (KIc), even conditional fracture toughness (KIQ), R-curve behaviour (variation of total fracture energy release rate, Jc with crack extension, δ/δc) and fracture mode. The corresponding Knoop Hardness number (KHN) and its correlations to flexural strength (σf) are also evaluated and reported. The present study showed that the zinc sulphide (ZnS) ceramics processed by CVD exhibited higher fracture resistance compared to ZnS processed by CVD + HIP condition. This observation is principally attributed to higher grain size associated with post-CVD HIPing process. In both conditions, the ZnS materials exhibited conditional fracture toughness (KIQ) that decreased moderately with increased crack length due to the change in fracture mode form grossly tensile to predominant shear. A constantly rising R-curve behaviour was indicated in both the materials with significant increase in total fracture energy release rate (Jc with the normalised displacement (δ/δc), a parameter representing crack extension.

► Microstructural and crystallographic changes in HIPed zinc sulphide ceramics.
► ZnS materials exhibiting significantly rising R-curves with no signs of saturation.
► ZnS materials failing under 3-point bend loading by mixed fracture mode.
► The cleavage facet size and its extent were significantly higher in CVD + HIP ZnS.