Gee et al. Enhanced fracture toughness by ceramic laminate design Longitudinal cracks Channel cracks SiN /SiN-30 wt-%TiN abricated in EMPA. Switzerland SiaN,SiN 4 50 wt-%TN Longitudinal cracks Channel cracks SiN /Si N -70 wt-%TIN 100 wt-%TN Examples of severe transverse matrix cracking in Si3,Si3Na-TiN ceramic laminates bifurcation and edge cracking: hese tensile stresses are above a threshold limit as with the mechanism responsible for edge cracking, the stres (6) can drive pre-existing cracks across the layer and into adjacent compressive layers. This type of internal crack is important to note that ceramic laminate propagation has been termed tunnel or tensile cracking materials that are designed to exhibit bifurcation Ho and Suo have shown that for a given residual tensile toughening will inevitably demonstrate surface edge stress, there is a critical tensile layer thickness below cracking and associated problems. In addition, the which no tunnel or tensile cracking can occur, indepen- potential to use crack bifurcation as a toughening dent of the initial flaw size. Transverse cracking in the mechanism in laminate ceramics with layers consistin tensile layer can only occur if the layer thickness is above of intrinsically high fracture toughness material is a critical value given by extremely limited as a result of the coarse macrostruc ture indicated by equation(6). The critical compressive te- tof t layer thickness necessary to produce crack bifurcation increases as the square of the compressive layer material Figure 2 shows some examples of severe transverse ture toughness and is inversely roportional to the IIN biaxial compressive stress in the layer. Therefore, for a ceramic laminates. All of the laminates shown have material of fixed thickness, it is possible to increase nominally the same layer architecture. It can be clearly the compressive residual stress by reducing the low Cte observed that as the tin content is increased (i.e. as material layer thickness. However, for high fracture CTE mismatch is increased), more transverse or tunnel toughness composite materials, this will usually cracks are created owing to the increased tensile stress. reduce the layer thickness below the critical value for Materials designed to exhibit delamination, crack Threshold strength deflection or bifurcation (by controlling the residual Improved ceramic processing methods, which eliminate these mechanisms during sample cutting or machining. reduced flaw size and subsequent enhanced reliability of These restrictions and others also place considerable ceramic components. However, a statistical distribu- value on having well characterised materials as the tion in strength remains. Ideally, the distribution of trengths of a ceramic component should display a high Weibull modulus, or ideally threshold strength beha- viour(a strength below which there is zero probability of Tensile cracking failure). One possible route to obtaining threshold During laminate ceramic processing, large biaxial tensile strength behaviour is the controlled application of stresses can be generated in the high CTE material. If macroscopic stress distribution Advances in Applied Ceramics 2005 VOL 104 No 3bifurcation and edge cracking: tco K2 Ic 0: 17 1{n2 ð Þs2 1 (6) It is important to note that all ceramic laminate materials that are designed to exhibit bifurcation toughening will inevitably demonstrate surface edge cracking and associated problems. In addition, the potential to use crack bifurcation as a toughening mechanism in laminate ceramics with layers consisting of intrinsically high fracture toughness material is extremely limited as a result of the coarse macrostruc￾ture indicated by equation (6). The critical compressive layer thickness necessary to produce crack bifurcation increases as the square of the compressive layer material fracture toughness and is inversely proportional to the biaxial compressive stress in the layer. Therefore, for a material of fixed thickness, it is possible to increase the compressive residual stress by reducing the low CTE material layer thickness. However, for high fracture toughness composite materials, this will usually reduce the layer thickness below the critical value for crack bifurcation unless considerable care is taken. Materials designed to exhibit delamination, crack deflection or bifurcation (by controlling the residual stress patterns) have also shown a propensity to fail by these mechanisms during sample cutting or machining. These restrictions and others also place considerable value on having well characterised materials as the laminate components. Tensile cracking During laminate ceramic processing, large biaxial tensile stresses can be generated in the high CTE material. If these tensile stresses are above a threshold limit, as with the mechanism responsible for edge cracking, the stress can drive pre-existing cracks across the layer and into adjacent compressive layers. This type of internal crack propagation has been termed tunnel or tensile cracking. Ho and Suo have shown that for a given residual tensile stress, there is a critical tensile layer thickness below which no tunnel or tensile cracking can occur, indepen￾dent of the initial flaw size.17 Transverse cracking in the tensile layer can only occur if the layer thickness is above a critical value given by: tc~ 4K2 Ic ps2 21 x (7) Figure 2 shows some examples of severe transverse matrix cracking in a number of Si3N4/Si3N4–TiN ceramic laminates. All of the laminates shown have nominally the same layer architecture. It can be clearly observed that as the TiN content is increased (i.e. as CTE mismatch is increased), more transverse or tunnel cracks are created owing to the increased tensile stress. Threshold strength Improved ceramic processing methods, which eliminate heterogeneities from the constituent powder, result in reduced flaw size and subsequent enhanced reliability of ceramic components.18 However, a statistical distribu￾tion in strength remains. Ideally, the distribution of strengths of a ceramic component should display a high Weibull modulus, or ideally threshold strength beha￾viour (a strength below which there is zero probability of failure). One possible route to obtaining threshold strength behaviour is the controlled application of macroscopic stress distributions. 2 Examples of severe transverse matrix cracking in Si3N4/Si3N4–TiN ceramic laminates Gee et al. Enhanced fracture toughness by ceramic laminate design Advances in Applied Ceramics 2005 VOL 104 NO 3 105