ournal 1. Am Ceram Soc, s1 [4] 1004-12 (1998) Crack Deflection and Propagation in Layered silicon Nitride/Boron Nitride ceramics Desiderio Kovar, .t.tM. D. Thouless. *s and John W. Halloran",t Materials Science and Engineering Department and Mechanical Engineering and Applied Mechanics Departme University of Michigan, Ann Arbor, Michigan 48109- Crack deflection and the subsequent growth of delamina- tion that occur at weak interfaces or within weak interphases tion cracks can be a potent source of energy dissipation between the strong layers. Although several models exist that during the fracture of layered ceramics. In this study, mu predict the conditions under which crack deflection should oc- tilayered ceramics that consist of silicon nitride(Si3N4)lay- cur, -o there is not much experimental data on all-ceramic ers separated by boron nitride/silicon nitride(BN/Si3N) port these models. Even more importantly, it has interphases have been manufactured and tested. Flexural been shown that, in some layered materials, delamination tests reveal that the crack path is dependent on the cor cracks kink out of the interface after propagating on the inter- position of the interphase between the Si,N, layers. Experi- face only a short distance. 10, II The result of such crack kinking mental measurements of interfacial fracture resistance and is that not much energy is absorbed during the fracture of these that both crease as the Si3N, content in the interphase increases. crack deflection and propagation along interfaces is needed to However contrary to existing theories, high energy maximize the energy dissipation capabilities of layered ceram- erbil ption capacity has not been realized in materials that exhibit crack deflection but also have moderately high in- In this the mechanical rties of Si3 N/BN multi- terfacial fracture resistance. Significant energy absorption layered ceramics are investigated. The properties of the inter- has been measured only in materials with very low inter ase are adjusted by ing the composition of the bn in- facial fracture resistance values. A method of predicting the terphase between the SiN layers. The strength and energy critical value of the interfacial fracture resistance necessary absorption of multilayered ceramics are measured, and the to ensure a high energy-absorption capacity is presented. crack path is characterized as a function of the composition of IL. Fabrication of Specimens T HAS previously been shown that it is possible to fabricate layered ceramics that have high strength in combination with with 2 wt% alumina(Al2O3)(HC-HP DBM, Reynolds Metals flexure -Because they can be manufactured from commer- Co., Bauxite, AZ) and 6 wt% yttria (Y20, )(99.g%, Johnson in ethanol. The slurry was dried and then compounded using ar these materials can potentially be significantly lower than instrumented high-shear-rate mixer(Model PL-2000, C.W those for fiber-reinforced composites. Thus, layered ceramics Brabender, South Hackensack, N)with a thermoplastic co- can provide a low-cost alternative to fiber-reinforced compos- polymer binder that was composed of equal parts ethylene- ites when strength and energy absorption capabilities are lim- vinyl acetate (Elvax 470, E I. DuPont De Nemours and Co iting factors in the component design. Silicon nitride(Si,N4) layered ceramics with weak boron Union Carbide Chemicals and Plastics Co, Cleveland, OH) in a novel layered structure known as a fibrous monolithic nation oro 32 nitride(BN)interphases have been previously manufactured in of the polymer/ceramic blend was controlled a conventional two-dimensional layered structure, as well as through addition of a lubricant that consisted of a combi- heral oil (white mineral oil-heavy, Mallinckrodt eramic b Impressive properties were achieved for both struc- Chemicals, Paris, KY) and methoxy-polyethyleI tures, with strengths of>600 MPa and work-of-fracture(WOF) (MPEG 550, Union Carbide, Danbury, CT). The tot values of -8000 J/m2. 7 These properties, as well as high- solids content in the compounds was varied from 37% temperature strength and oxidation resistance, make this sys which allowed the viscosity of the compounds to be tem attractive for commercial applications a factor of 2 monolithic ceramics result from crack deflection and propaga- compounds were chopped into blocks of material -1 mmono Many of the advantages that ceramic laminates have over To mold the materials into sheets, the polymer-ceram on each side and pressed between heated metal platens coated with aluminum foil and a lubricant( Carbowax 400, Union Carbide) at a temperature of 150 C under a pressure of 2.8 MPa. The resulting sheets could be varied in thickness from 100 um to 800 um, depending on the viscosity of the com- unds and the pressure at which the sheet was pressed. For the rrent study, the thickness of th sheets was fixed at po aa k ca th nder that had dimensions of ngi mm sxe6s mnre cut into rectangles amic Society To introduce weak interfaces between the Si,Na layers, the surface of each sheet was coated with a slurry that contained BN. The composition of the BN layers was varied through the nical Engineering and Applied Mechanics Department. addition of Si Na to the BN slurry. The slurries were made 1004Crack Deflection and Propagation in Layered Silicon Nitride/Boron Nitride Ceramics Desiderio Kovar,*,†,‡ M. D. Thouless,*,§ and John W. Halloran*,† Materials Science and Engineering Department and Mechanical Engineering and Applied Mechanics Department, University of Michigan, Ann Arbor, Michigan 48109–2125 Crack deflection and the subsequent growth of delamina￾tion cracks can be a potent source of energy dissipation during the fracture of layered ceramics. In this study, mul￾tilayered ceramics that consist of silicon nitride (Si3N4) lay￾ers separated by boron nitride/silicon nitride (BN/Si3N4) interphases have been manufactured and tested. Flexural tests reveal that the crack path is dependent on the com￾position of the interphase between the Si3N4 layers. Experi￾mental measurements of interfacial fracture resistance and frictional sliding resistance show that both quantities in￾crease as the Si3N4 content in the interphase increases. However, contrary to existing theories, high energy￾absorption capacity has not been realized in materials that exhibit crack deflection but also have moderately high in￾terfacial fracture resistance. Significant energy absorption has been measured only in materials with very low inter￾facial fracture resistance values. A method of predicting the critical value of the interfacial fracture resistance necessary to ensure a high energy-absorption capacity is presented. I. Introduction I T HAS previously been shown that it is possible to fabricate layered ceramics that have high strength in combination with the ability to absorb large amounts of energy when tested in flexure.1–4 Because they can be manufactured from commer￾cially available ceramic powders via conventional ceramic- and polymer-processing technology, the manufacturing costs for these materials5 can potentially be significantly lower than those for fiber-reinforced composites. Thus, layered ceramics can provide a low-cost alternative to fiber-reinforced compos￾ites when strength and energy absorption capabilities are lim￾iting factors in the component design. Silicon nitride (Si3N4) layered ceramics with weak boron nitride (BN) interphases have been previously manufactured in a conventional two-dimensional layered structure,4 as well as in a novel layered structure known as a fibrous monolithic ceramic.6 Impressive properties were achieved for both struc￾tures, with strengths of >600 MPa and work-of-fracture (WOF) values of ∼8000 J/m2 . 7 These properties, as well as high￾temperature strength and oxidation resistance, make this sys￾tem attractive for commercial applications. Many of the advantages that ceramic laminates have over monolithic ceramics result from crack deflection and propaga￾tion that occur at weak interfaces or within weak interphases between the strong layers. Although several models exist that predict the conditions under which crack deflection should oc￾cur,8–10 there is not much experimental data on all-ceramic systems to support these models. Even more importantly, it has been shown that, in some layered materials, delamination cracks kink out of the interface after propagating on the inter￾face only a short distance.10,11 The result of such crack kinking is that not much energy is absorbed during the fracture of these materials. Thus, an understanding of the factors that control crack deflection and propagation along interfaces is needed to maximize the energy dissipation capabilities of layered ceram￾ics. In this paper, the mechanical properties of Si3N4/BN multi￾layered ceramics are investigated. The properties of the inter￾phase are adjusted by varying the composition of the BN in￾terphase between the Si3N4 layers. The strength and energy absorption of multilayered ceramics are measured, and the crack path is characterized as a function of the composition of the interphase. II. Fabrication of Specimens Si3N4 powder (M-11, H. C. Starck, Newton, MA) was mixed with 2 wt% alumina (Al2O3) (HC-HP DBM, Reynolds Metals Co., Bauxite, AZ) and 6 wt% yttria (Y2O3) (99.9%, Johnson Matthey Electronics, Ward Hill, MA) and ball milled for 24 h in ethanol. The slurry was dried and then compounded using an instrumented high-shear-rate mixer (Model PL-2000, C. W. Brabender, South Hackensack, NJ) with a thermoplastic co￾polymer binder that was composed of equal parts ethylene– vinyl acetate (Elvax 470, E. I. DuPont De Nemours and Co. Wilmington, DE) and ethylene–ethyl acrylate (DPDA-6182, Union Carbide Chemicals and Plastics Co., Cleveland, OH). The viscosity of the polymer/ceramic blend was controlled through the addition of a lubricant that consisted of a combi￾nation of mineral oil (white mineral oil–heavy, Mallinckrodt Chemicals, Paris, KY) and methoxy-polyethylene glycol (MPEG 550, Union Carbide, Danbury, CT). The total ceramic solids content in the compounds was varied from 37% to 51%, which allowed the viscosity of the compounds to be varied by a factor of 2. To mold the materials into sheets, the polymer–ceramic compounds were chopped into blocks of material ∼1 mm long on each side and pressed between heated metal platens coated with aluminum foil and a lubricant (Carbowax 400, Union Carbide) at a temperature of 150°C under a pressure of 2.8 MPa. The resulting sheets could be varied in thickness from ∼100 mm to 800 mm, depending on the viscosity of the com￾pounds and the pressure at which the sheet was pressed. For the current study, the thickness of the green sheets was fixed at ∼200 mm. After molding, the sheets were cut into rectangles that had dimensions of 51 mm × 76 mm. To introduce weak interfaces between the Si3N4 layers, the surface of each sheet was coated with a slurry that contained BN. The composition of the BN layers was varied through the addition of Si3N4 to the BN slurry. The slurries were made F. W. Zok—contributing editor Manuscript No. 191231. Received February 4, 1997; approved July 21, 1997. Supported by DARPA, administered by the U.S. Office of Naval Research under Contract No. N0014-95-0302. *Member, American Ceramic Society. † Materials Science and Engineering Department. ‡ Current address: Mechanical Engineering Department, The University of Texas at Austin, Austin, TX 78712–1063. § Mechanical Engineering and Applied Mechanics Department. J. Am. Ceram. Soc., 81 [4] 1004–12 (1998) Journal 1004