Thermal Shock-Resistant Composite Materials

Abstract

A composite material green body film includes a ceramic matrix including zirconium oxide, at least one secondary phase dispersed in the ceramic matrix, and an optional chemical stabilizer. The composite material green body film can be used to make a sensor.

Claims

1. A composite material green body film, comprising: a ceramic matrix comprising zirconium oxide, the ceramic matrix forming at most 99 percent by weight of the composite material green body film, 90 to 99% of a total amount of the zirconium oxide being in the tetragonal and cubic phase; at least one secondary phase dispersed in the ceramic matrix, the at least one secondary phase forming 1 to 30 percent by weight of the composite material green body film, the at least one secondary phase comprising ZrSiO.sub.4; and 0 to less than 12 mol % of a chemical stabilizer based on a content of the zirconium oxide, the chemical stabilizer comprising one or more selected from MgO, CaO, CeO.sub.2, Gd.sub.2O.sub.3, Sm.sub.2O.sub.3, Er.sub.2O.sub.3, Y.sub.2O.sub.3, Yb.sub.2O.sub.3, and Sc.sub.2O.sub.3, the tetragonal and cubic phase of the zirconium oxide being chemically stabilized and optionally mechanically stabilized when the composite material green body film is sintered and comprises the chemical stabilizer, and the tetragonal and cubic phase of the zirconium oxide being mechanically stabilized when the composite material green body film is sintered and does not comprise the chemical stabilizer, wherein the zirconium oxide has an average microstructure grain size of 0.5 to 2.0 m, and a d50 microstructure grain size of the at least one secondary phase is less than or equal to a d50 microstructure grain size of the zirconium oxide, and wherein the composite material green body film has a thickness of 10 m to 1 mm.

2. The composite material green body film according to claim 1, wherein the zirconium oxide has an average microstructure grain size of 0.5 to 1.5 m.

3. The composite material green body film according to claim 1, wherein the total content of the chemical stabilizer is <10 mol % based on the content of the zirconium oxide.

4. The composite material green body film according to claim 1, wherein the zirconium oxide and/or the at least one secondary phase includes soluble components.

5. The composite material green body film according to claim 1, wherein the d50 microstructure grain size of the at least one secondary phase is 0.1 to 2.0 m.

6. The composite material green body film according to claim 1, wherein the at least one secondary phase further comprises one or more selected from SrAl.sub.2O.sub.4, SrAl.sub.12O.sub.19, LaAlO.sub.3, LaAl.sub.11O.sub.18, MgAl.sub.2O.sub.4, Al.sub.2O.sub.3, KAlSi.sub.3O.sub.8, and La(PO).sub.4.

7. A composite material produced by sintering the composite material green body film according to claim 1 at temperatures <1670 C.

8. A sensor comprising a sintered form of the composite material green body film according to claim 1.

9. The composite material green body film according to claim 1, wherein the at least one secondary phase forms 1-15 percent by weight of the composite material green body film.

10. The composite material green body film according to claim 1, wherein the at least one secondary phase forms 1-5 percent by weight of the composite material green body film.

11. The composite material green body film according to claim 1, wherein 95 to 99% of the total amount of the zirconium oxide is in the tetragonal and cubic phase.

12. The composite material green body film according to claim 1, wherein 98 to 99% of the total amount of the zirconium oxide is in the tetragonal and cubic phase.

13. The composite material green body film according to claim 1, wherein the total content of the chemical stabilizer is <5 mol % based on the content of the zirconium oxide.

14. The composite material green body film according to claim 1, wherein the d50 microstructure grain size of the at least one secondary phase is 0.1 to 1.5 m.

15. The composite material green body film according to claim 1, wherein the d50 microstructure grain size of the at least one secondary phase is 0.1 to 0.5 m.

16. The composite material green body film according to claim 6, wherein the at least one secondary phase further comprises one or more selected from SrAl.sub.2O.sub.4, SrAl.sub.12O.sub.19, and Al.sub.2O.sub.3.

17. An oxygen sensor comprising a sintered form of the composite material green body film according to claim 1.

18. A lambda sensor comprising a sintered form of the composite material green body film according to claim 1.

19. The composite material green body film according to claim 1 comprising the chemical stabilizer.

Description

[0010] In one preferred embodiment, the invention relates to a composite material comprising a ceramic matrix made of zirconium oxide and at least one secondary phase dispersed therein, characterized in that the matrix made of zirconium oxide makes up a portion of at most 99 percent by weight of the composite material, and in that the secondary phase makes up a portion of 1 to 30 percent by weight of composite material, preferably 1-15 percent by weight, particularly preferably 1-5 percent by weight, wherein the zirconium oxide, relative to the total zirconium oxide portion, is present, essentially, in the tetragonal and cubic phase, preferably at 90 to 99%, more preferably at 95 to 99%, and particularly preferably at 98 to 99%, and wherein the tetragonal and cubic phase of the zirconium oxide is chemically and/or mechanically stabilized.

[0011] Mechanical stabilization shall be construed to mean that the inventive addition of the secondary phases, which addition during the conversion of the tetragonal crystal structure to the monoclinic crystal structure of the zirconium oxide, compensates extension that occurs in that micro-movements/micro-shears are possible in an otherwise comparatively rigid ceramic structure without it being necessary for macroscopic tears to occur.

[0012] Moreover, mechanical stabilization shall also be construed to mean that the tetragonal crystal phase of the zirconium oxide is stabilized by mechanical tensions in the overall microstructure. Different coefficients of thermal expansion of ZrO2 and secondary phase during cooling after the sintering process may lead to such tensions. According to the invention, the matrix made of zirconium oxide in the composite material may have a microstructure grain size of an average of 0.5 to 2.0 m, preferably an average of 0.5 to 1.5 m. The microstructure grain size is determined as the mean linear intercept distance by means of the linear intercept method according to EN 623-3 (2003-01).

[0013] The composite material according to the invention may include one or a plurality of chemical stabilizers. The chemical stabilizers are selected from the group comprising MgO, CaO, CeO.sub.2, Gd.sub.2O.sub.3, Sm.sub.2O.sub.3, Er.sub.2O.sub.3, Y.sub.2O.sub.3, Yb.sub.2O.sub.3, and Sc.sub.2O.sub.3, or mixtures thereof, wherein the total content of chemical stabilizers is <12 mol % relative to the zirconium oxide content, preferably <10 mol %, particularly preferably <5 mol %.

[0014] The composite material according to the invention may contain additional components, in particular the zirconium oxide and/or the secondary phase may contain soluble components. Soluble components may be, e.g., Cr, Fe, Mg, Ca, Ti, Si, Y, Ce, lanthanides, and/or V. These components may, for one thing, function as dye additives, and, for another thing, as sintering aids. As a rule, they are added as oxides.

[0015] The secondary phase of the inventive composite material may also include dispersoids that, due to their crystal structure, permit shear deformations at the microscopic level and/or improve the mechanical properties.

[0016] In one preferred embodiment, the microstructure grain sizes of the secondary phase are less than or equal in size to the microstructure grain sizes of the zirconium oxide, wherein the microstructure grain sizes (d50) are preferably 0.1 to 2.0 m, preferably 0.1 to 1.5 m, particularly preferably 0.1 to 0.5 m. The microstructure grain sizes may also be determined with the linear intersection method.

[0017] The secondary phase is in particular one or a plurality of the following compounds selected from the group comprising strontium aluminate (SrAl.sub.2O.sub.4 or SrAl.sub.12O.sub.19), lanthanum aluminate (LaAlO.sub.3 or LaAl.sub.11O.sub.18), spinel (MgAl.sub.2O.sub.4), aluminum oxide (Al.sub.2O.sub.3), zirconium silicate (ZrSiO.sub.4), K feldspar (KalSi.sub.3O.sub.8), and lanthanum phosphate (La(PO).sub.4), preferably strontium aluminate, aluminum oxide, and zirconium silicate, particularly preferably zirconium silicate.

[0018] In one particularly preferred embodiment, the inventive material may also have a tertiary phase. The latter may be selected from the compounds cited in the foregoing for the secondary phase.

[0019] According to the invention, the composite material may be produced using very different methods, preferably using film casting, extrusion, dry pressing, additive manufacturing methods, and/or other molding methods such as, for example, slip casting or injection molding.

[0020] To obtain the composite material, the raw material mixture is then sintered, wherein the sintering is performed at temperatures <1670 C., preferably <1530 C., particularly preferably <1400 C.

[0021] The inventive composite material is significantly better in its characteristic properties. In particular, the composite material preferably has a four-point flexural strength 600 MPa according to DIN EN 843-1 (version EN 843-1: 2006/edition 08-2008).

[0022] One feature for evaluating high performance ceramics is the flaw tolerance. The test is performed using the Indentation Strength Method described in detail in the literature (see R. F. Cook, Multi-Scale Effects in the Strength of Ceramics, J. Am. Ceram. Soc. 98 [1] 2933-2947 (2015) and P. Chantikul, G. R. Anstis, B. R. Lawn, D. B. Marshall, A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: II Strength Method, J. Am. Ceram. Soc. 64 [9], 539-543 (1981).

[0023] The established method and procedure was worked out using different materials. It was found that this test is very well reproducible and may be considered informative with respect to the technically relevant reinforcing mechanisms, for instance transformation toughening. The separation precision in terms of material variants is significantly more reliable than, for instance, measurements of fracture toughness using the SEVNB test.

[0024] In the test itself, 3 indentations having the same load on the traction side of bending bars are made within the upper rollers. Then the bending bars damaged in a defined manner are stored in water for at least three days (saturation of sub-critical fracture propagation). Then the residual strength is determined analogous to a normal bending test.

[0025] The damage tolerance or residual strength of the inventive composite material following HV5 indentation is >170 MPa, preferably >190 MPa, and in particular preferably >200 MPa. The damage tolerance or residual strength of the inventive composite material following HV20 indentation is >100 MPa, preferably >110 MPa, and particularly preferably >120 MPa.

[0026] In addition, the composite material according to the invention has electrical conductivity of 2.0 S/m at 850 C. and of 5.0 S/m at 1000 C., preferably 6.0 S/m at 1000 C.

[0027] The fracture penetration test according to a thermal shock treatment based on DIN EN 820-3 (version EN 820-3: 2004/edition 11.2004) has a particularly positive result with the inventive composite materials compared to the known material from the prior art. The thermal shock resistance was determined using quenching tests on fasted bending rods. At a temperature difference of 220 C., the inventive composite material did not exhibit any damage. In contrast, the material from the prior art showed fracture formation following thermal tensions.

[0028] The inventive composite material is preferably used in electrical engineering and sensors, in particular for producing oxygen sensors, particularly preferably as a precursor product or as a component of the lambda sensor in the form of unsintered ZrO.sub.2 films and is present in a lambda sensor following production as a component.