J. Am. Ceram. Soc., 91[4] 1218-1225 DOl:10.1l11.1551-29162008.02 c 2008 The American Ceramic urna Nondestructive measurement of the residual stress profile in Ceramic laminates Matteo Leoni, Matteo Ortolani, Massimo bertoldi, Vincenzo M. sglavo, and Paolo Scardi Department of Materials Engineering and Industrial Technologies, University of Trento, Mesiano, Trento 38100, Italy Millimeter-thick symmetric ceramic laminates, designed to The proposed design procedure points out the importance of ossess a specific through-thickness residual stress profile, the knowledge of the residual stresses within the material. At were produced by tape casting from blends of alumina, zircon- tempts have been made to validate the calculated data by using ia, and mullite powders. The residual stress profile was checked fluorescence piezo-spectroscopy on Cr+, which is always pres- nondestructively by X-ray energy-dispersive diffraction using ent as impurity in alumina. The ob d results could repro- synchrotron white-beam radiation Measurement of the average duce the step profile as obtained by the calculations, although stress on very small volumes(ca. 10 um along the specimen the intensity of stresses was always much lower than expected hickness) provided results in good agreement with the design The discrepancy was related to the fact that piezo-spectroscol crystalline phase composing the laminate allowed inferences to not within the material be made on stress partition and grain-grain coupling in the a nondestructive measurement of residual stresses within the laminate can be performed by using neutron or X-ray diffrac tion techniques. The limited thickness of the specimen and the need for a high spatial resolution are, however, a challenge for both neutrons and laboratory X-rays: the former cannot be L. Introduction focused to a very small size (a few micrometers size would be HE most important limitation to the use of ceramic materials required in practical cases), and the latter have a shallow pen- n structural applications is inherent in their scarce mechan tration in alumina and in ceramic materials in general. more- ical reliability. Their brittleness is directly related to the low over, traditional measurement techniques(sin" v, n-rotation varlous olve rotation or ated both during production and in service. The consequence is specimen(thus a change in the sampled volume)during data a scatter of strength data too large to allow safe design, unless acquisition. This is deleterious when a pointwise resolution statistical approaches embodying acceptable minimum failure sought, like in the case analyzed here. Synchrotron radiation in risk are used many efforts have been made in the last decades this case offers an optimal solution, as it couples the possibility to overcome such problems to work with a narrow probe and to tune the wavelength of the The fracture behavior of ceramics can be improved by using radiation to increase the penetration within the material. A well- the reinforcing action of grain anisotropy or second phases, by hickness residual stress mapping is therefore the promotion of crack shielding effects by phase transforma- ndently of the particular geometry of the tion or microcracking, and by introducing low-energy paths for rack propagation in porous- or within weak interlayers in lam In this work, residual stresses of three symmetric ceramic inates As an alternative laminated structures characterized laminates designed and produced according to the procedure by the presence of thin layers in residual compression alternated proposed by Sglavo and colleagues were measured by minimum failure stress(threshold stress)or fracture toughness CCLRC Daresbury Laboratory Synchrotron Radiation Source values as high as 17 MPa-m (DL-SRS). Results are compared with calculated profiles and Recently, Sglavo and colleagues -16 have proposed the pos- discussed in terms of the microstructure of the single ceramic sibility of improving the mechanical behavior of ceramic lami- laminae ates by introducing a residual stress profile originating from constituting layers. By controlling the development of residual Il. Experimental Procedure tresses in ceramic multilayers, materials characterized by high fracture resistance and limited strength scatter have been de- (1) Specimens gned and produced. A specific procedure has been devel- Symmetric ceramic laminates were produced from laminae obtained knowledge of elastic properties (Young's modulus and Poissons Alpha-alumina (A- 6SG, ALCOA Corp, Pittsburgh, PA) was ratio) and thermal expansion coefficients of the constituent ma- terials. Residual stresses are included in the apparent fracture mullite(KM101, KCM Corp, Nagoya, Japan) and yttria(3 strength once the size of the flaws is known. ion of the final toughness of the material allowing pred mol%)-stabilized zirconia(TZ-3YS. TOSOH Corp, Tokyo, good agreement has been observed between theoretical failure tresses and experimental mechanical strengt amina/zirconia composite layers were prepared. Such compos- Ites were labeled as AMy and AZy, where"A, ""M, "Z, "and C.H. Hsuch-contnibuting editor y stand for alumina, mullite, zirconia and the volume percent content of mullite or zirconia, respectively. For the productio of the laminates, rectangular cards around 50 mm x 50 mm 2007: approved October 12, 200 cut from different green laminae, stacked together, and thermo- author to whom correspondence should be addressed. e-mail: Matteo. Leoni unitnit compressed at 70 C under a pressure of 30 MPa for 15 min. Bars 218Nondestructive Measurement of the Residual Stress Profile in Ceramic Laminates Matteo Leoni,w Matteo Ortolani, Massimo Bertoldi, Vincenzo M. Sglavo, and Paolo Scardi Department of Materials Engineering and Industrial Technologies, University of Trento, Mesiano, Trento 38100, Italy Millimeter-thick symmetric ceramic laminates, designed to possess a specific through-thickness residual stress profile, were produced by tape casting from blends of alumina, zircon￾ia, and mullite powders. The residual stress profile was checked nondestructively by X-ray energy-dispersive diffraction using synchrotron white-beam radiation. Measurement of the average stress on very small volumes (ca. 10 lm along the specimen thickness) provided results in good agreement with the design data. Moreover, the possibility of independently measuring each crystalline phase composing the laminate allowed inferences to be made on stress partition and grain–grain coupling in the laminas. I. Introduction THE most important limitation to the use of ceramic materials in structural applications is inherent in their scarce mechan￾ical reliability. Their brittleness is directly related to the low value of fracture toughness and to the presence of flaws gener￾ated both during production and in service. The consequence is a scatter of strength data too large to allow safe design, unless statistical approaches embodying acceptable minimum failure risk are used1 : many efforts have been made in the last decades to overcome such problems. The fracture behavior of ceramics can be improved by using the reinforcing action of grain anisotropy or second phases, by the promotion of crack shielding effects by phase transforma￾tion or microcracking,1 and by introducing low-energy paths for crack propagation in porous2 or within weak interlayers in lam￾inates.3–6 As an alternative, laminated structures characterized by the presence of thin layers in residual compression alternated with thicker layers in tension7–12 have been shown to possess a minimum failure stress (threshold stress) or fracture toughness values as high as 17 MPa
m1/2. Recently, Sglavo and colleagues13–16 have proposed the pos￾sibility of improving the mechanical behavior of ceramic lami￾nates by introducing a residual stress profile originating from differences in thermal expansion coefficients of the different constituting layers. By controlling the development of residual stresses in ceramic multilayers, materials characterized by high fracture resistance and limited strength scatter have been de￾signed and produced.13–16 A specific procedure has been devel￾oped to design symmetric laminate structures starting from the knowledge of elastic properties (Young’s modulus and Poisson’s ratio) and thermal expansion coefficients of the constituent ma￾terials. Residual stresses are included in the apparent fracture toughness of the material allowing prediction of the final strength once the size of the flaws is known.13–15 In most cases, good agreement has been observed between theoretical failure stresses and experimental mechanical strength. The proposed design procedure points out the importance of the knowledge of the residual stresses within the material. At￾tempts have been made to validate the calculated data by using fluorescence piezo-spectroscopy on Cr31, 17 which is always pres￾ent as impurity in alumina.18 The obtained results could repro￾duce the step profile as obtained by the calculations, although the intensity of stresses was always much lower than expected. The discrepancy was related to the fact that piezo-spectroscopy allows the measurement of stresses only on external surfaces and not within the material.19 A nondestructive measurement of residual stresses within the laminate can be performed by using neutron or X-ray diffrac￾tion techniques.20–22 The limited thickness of the specimen and the need for a high spatial resolution are, however, a challenge for both neutrons and laboratory X-rays: the former cannot be focused to a very small size (a few micrometers size would be required in practical cases), and the latter have a shallow pen￾etration in alumina and in ceramic materials in general. More￾over, traditional measurement techniques (sin2 c, Z-rotation, and the various modifications20,21) involve rotation or tilt of the specimen (thus a change in the sampled volume) during data acquisition. This is deleterious when a pointwise resolution is sought, like in the case analyzed here. Synchrotron radiation in this case offers an optimal solution, as it couples the possibility to work with a narrow probe and to tune the wavelength of the radiation to increase the penetration within the material. A well￾defined through-thickness residual stress mapping is therefore possible, independently of the particular geometry of the specimen under analysis. In this work, residual stresses of three symmetric ceramic laminates designed and produced according to the procedure proposed by Sglavo and colleagues13–17 were measured by synchrotron radiation X-ray diffraction on station 16.3 at the CCLRC Daresbury Laboratory Synchrotron Radiation Source (DL-SRS). Results are compared with calculated profiles and discussed in terms of the microstructure of the single ceramic laminae. II. Experimental Procedure (1) Specimens Symmetric ceramic laminates were produced from green laminae obtained by tape casting water-based slurries.13–15,17 Alpha-alumina (A-16SG, ALCOA Corp., Pittsburgh, PA) was considered as the fundamental starting material. High-purity mullite (KM101, KCM Corp., Nagoya, Japan) and yttria (3 mol%)-stabilized zirconia (TZ-3YS, TOSOH Corp., Tokyo, Japan) powders were chosen as the second phases. The exper￾imental procedure used to produce the green tapes is described in detail in previous works.13–15,17 Alumina/mullite and al￾umina/zirconia composite layers were prepared. Such compos￾ites were labeled as AMy and AZy, where ‘‘A,’’ ‘‘M,’’ ‘‘Z,’’ and ‘‘y’’ stand for alumina, mullite, zirconia and the volume percent content of mullite or zirconia, respectively. For the production of the laminates, rectangular cards around 50 mm 50 mm were cut from different green laminae, stacked together, and thermo￾compressed at 701C under a pressure of 30 MPa for 15 min. Bars C.-H. Hsueh—contributing editor w Author to whom correspondence should be addressed. e-mail: Matteo.Leoni@unitn.it Manuscript No. 22917. Received March 12, 2007; approved October 12, 2007. Journal J. Am. Ceram. Soc., 91 [4] 1218–1225 (2008) DOI: 10.1111/j.1551-2916.2008.02260.x r 2008 The American Ceramic Society 1218