MONASH UNIVERSITY Department of Materials Engineering

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1 1 MONASH UNIVERSITY Department of Materials Engineering TRANSFORMATION TOUGHENING EXPERIMENT P48 AIM The aim of this experiment is to explore the features of transformation toughening in Magnesia Partially Stabilised Zirconia Ceramics (Mg-PSZ) compared to metals and typical sister ceramics. This experiment should leave the user with an appreciation of the capabilities of X-Ray Diffraction (XRD) and the mechanical properties obtained from Vickers indentation. REFERENCES Garvie R. C., Hannink R. H. & Pascoe R. T., Ceramic Steel? Nature 258, (25 December 1975); doi: /258703a0 Green D.J., Hannink R.H.J., Swain M.V., Transformation Toughening of Ceramics, CRC Press, Florida, Hill R. J., Reichert B. E., Measurement of Phase Abundance in Magnesia-Partially- Stabilized Zirconia by Rietveld Analysis of X-ray Diffraction Data, Journal of American Ceramic Society, 73 [10], (1990) Lutz E. H., Swain M. V., Stress-Strain Behavior of Alumina, Magnesia-Partially- Stabilized Zirconia, and Duplex Ceramics and Its Relevance for Flaw Resistance, KR- Curve Behavior, and Thermal Shock Behavior, Journal of American Ceramic Society, 75 [11], , (1992) Rahaman M. H., Ceramic Processing and Sintering, (2003), 2nd Edition, Marcel Dekker Inc. INTRODUCTION Materials selection is invariably a compromise of choices, which often include materials properties and processing cost. The dominance of metals is generally regarded as the result of the inherent brittleness of ceramics. This is observed and characterized in high strain rate/impact conditions, inherent flaws in processing and flaw sensitivity testing. In selection where ceramics dominate structures composed of bricks, concrete and tableware these loading conditions do not take place. Advanced ceramics tend to be applied in specialized requirements where the low fracture toughness is accepted for its typical high strength and refractoriness. However, transformation toughened ceramics are a significant and unique engineering material. The key to understanding the effects observed in transformation toughened ceramics is the transformation from tetragonal to monoclinic symmetry that the transforming particle undergoes when influenced by a stress field. In this experiment two types of magnesia-partially stabilized zirconia (Mg-PSZ) are subjected to different forms of applied stresses. The grades of Mg-PSZ are known as MS and TS which stand for maximum strength and thermal shock respectively. In this experiment we are unable to observe directly the particles taking part in the transformation but

2 2 Figures 1 and 2 outline the features of the microstructure important in understanding the processes taking place. Fig1. Tetragonal precipitates within a grain of Mg-PSZ In Figure 1, a micrograph recorded from an ion beam thinned specimen using a transmission electron microscope shows the shape and size of the tetragonal precipitates which will transform on the application of stress. a b Fig2. Optical micrographs of chemically etched surfaces of Mg-PSZ. The light phase decorating the grain boundary phase results from the decomposition reaction has been aged for (a) 2 hours; (b) aged for 16 hours

3 3 Figure 2 is an optical micrograph recorded from the same sample after it was chemically etched. Seen in this picture is the grain size, with the grains outlined by the decomposition region (the light coloured features which play an important role in determining the final properties of these materials). The phase diagram shown in Figure 3 shows the composition and heat treatment regions relevant to the samples under study in this experiment. Equipment Samples of 5 polished specimens (Alumina, Mg-PSZ Thermal Shock, Mg- PSZ Maximum Strength, glass and an copper specimen). X-ray diffraction spectra pre- and post-surface scratching Indentation apparatus Optical microscope fitted with Nomarski contrast Calculator USB memory stick

4 4 EXPERIMENTAL PROCEDURE (in class) X-ray diffraction (a) Determine from the x-ray diffraction data provided the region of interest over which scans should be run to obtain information on the transformation process. You will be determining the amount of transformation which occurs in the ceramics as the material transforms from tetragonal symmetry to monoclinic symmetry. (b) Without detailed knowledge of X-ray diffraction, a phase (distinctly unique structure and composition) can still be fingerprinted under most conditions. Label the peaks on the attached spectra showing at least two phases (4 marks). (c) Determine during the lab class the monoclinic content of the polished surface of the zirconia samples using the following equation to estimate the monoclinic content of the polished surface of the zirconia samples. (Ref. D.L. Porter and A.H. Heuer, J. Am. Ceram. Soc., 62(5-6)298, 1979) where V m %=volume percent of monoclinic zirconia = XRD peak intensity of m-zro2 ( deflection = XRD peak intensity of t-zro2 (111) deflection (d) This monoclinic content is known as PSM (polished surface monoclinic). (e) SiC paper has been used to scratch only one side of each of the zirconia specimens before x-raying. (f) Re-measure the monoclinic content of the damaged surfaces. These values will be given to you by your demonstrator. This value is known as the ground surfaces monoclinic GSM. Note: this is addressed in a discussion question. (g) Present your PSM results in a table pre-/post-scratching for MS and TS versions (4 marks). (h) XRD is not restricted to phase analysis. The Scherrer equation is a useful tool to identify crystallite size without using morphology microscopy (SEM) or microstructure (TEM). Larger crystallites with tens of thousands of parallel planes produce narrow peaks due to destructive interference at even minor deviations. As the crystallite becomes smaller, this does not occur and some constructive interference occurs, which becomes more intense as the size of the crystal decreases. Known as line broadening this can be used to determine crystallite size. While strictly applied to powder compacts, this session can be an exercise of its practice. Given below:

5 5 (i) The defined terms are d vol crystallite size (weighted by volume), K = 0.89, λ = Å and β = full width at half maximum (in radians 2θ) at angle θ. You should use the two peaks provided in the Appendix. Note your units (Å). Mechanical Properties (all specimens) (j) (k) Using the still polished sides of each sample, including the untouched specimen, indent the specimen using the indenting device provided. Three indentations in each will be sufficient. For the Mg-PSZ specimens, a 50 kg load should be used, for the Al 2 O 3 a 20 kg load, glass using the 1kg load and copper using a 10kg load. Using the optical microscope, record a micrograph of the various indentations and calculate the hardness and fracture toughness of each of the specimens. Begin with the Mg-PSZ specimens. Respective formulae are provided: P = Indentation Load (N) E = Young s Modulus (GPa) E a -SiO2 E Mg-PSZ E Alumina E Cu ~75GPa ~ 208GPa ~ 300GPa ~ 117GPa Vickers Indentor: 1. Turn power on at switch and ensure light(s) engage. 2. Put the specimen on the stage 3. Set the dial to 0000 and twist the entire head in or out to manually calibrate 4. Turn to the objective lens (anti-clockwise) to the working position if not already set. 5. Twist the captain s wheel to focus the sample 6. When focused, select load by manual moving the load selection knob located on the rear right side of equipment. 7. Check that the handle at the front bottom is in vertical position:

6 6 8. Turn the indentor into position (clockwise) until it clicks 9. Push the indentor button adjacent to the wheel. You should observe the indentor impress the sample, while the handle moves to a position 10. Count to ten, and turn this handle back to vertical: 11. Turn to the objective lens (anti-clockwise) and measure your indentation on axis normal to one another. 12. If more indentations are required, carefully move the sample one screen away and repeat from step Once completed, remove the specimen 14. Note your indentor units mm or microns (this will change your hardness and toughness values). 15. Turn power off. (l) Retrieve the five sets of micrographs (1 mark) of the indentation zones and describe with descriptive captions (1 mark) the zone observed. Explain your observations (1 mark) such as flaw divergence, path and size. Comment on any arrested flaws if any. DISCUSSION Q1: What is the most likely reason that there are no apparent peaks on the XRD spectra for the glass specimen? Why? (1+1 marks) Q2: Why is there a difference in m-zro 2 content between MS and TS specimens and where do you think this monoclinic zirconia resides in TS? (1 marks) Q3: How do TS and MS work in their specified applications? Discuss the differences between the two specimens in terms of the physical processes (behavior of salient microstructural features) and kinetics (if necessary) of the designed microstructure. Would either of the specimens have been strengthened by scratching? (2+4+2 marks) Q5: Explain whether either would be less susceptible to thermal shock fatigue (1+1+1 marks). Q6: Using the simplified Scherrer equation in part (h) and the two expanded peaks in the Appendix, show with working the average crystallite size of the initial t-zro 2 and transformed monoclinic zirconia crystals (1+1 marks). At your discretion, draw your line broadening as a Gaussian or Lorenzian fit for both peaks (1 mark) and give your answers in nm. Does it compare favorably to the micrographs (1 mark)? Q7: Discuss in terms of thermodynamics (free, strain and surface energy states) that occur when a flaw is introduced in transformation toughened Mg-PSZ (MS) ( marks). You may need to references for components of the transformation.

7 7 Q8: Is it pragmatically possible to back-transform i.e. go from the monoclinic to the tetragonal phase and would the damage to the sample be fully repaired by this back transformation (1 mark)? Q9: Refractories and crucibles are constructed of pure alumina due to its high thermal stability. It is a dominant structural material for these applications below 1600 C. However, zirconia composites have demonstrated superior fracture toughness and along with an additional 600 C thermal stability. List and argue three reasons why alumina and not zirconia is used (2+2+2 marks). Q10: The Griffith criterion generally applies well to brittle materials, although Irwin s method for crack extension resistance curve also known as an R-curve applies better to materials with plastic behavior. From your results describe the behavior of the propagating flaw on glass/alumina in contrast to Mg-PSZ (1+1 marks). Comment on their individual crack resistance as a function of flaw displacement from origin (1+1 marks). REPORT A report of ~2,000 words is required in typical scientific report format. This should include: 1. Summary/aim 2. Short introduction (1-2 pages including any helpful diagrams and labeled, relevant captions) 3. Experimental method (refer to this document) 4. Results - Attach a labeled spectra identifying 2 major peaks/phase (b) - Attached Scherrer spectra - Table of PSM (g) - Indentation section (l) 5. Discussion - Answers to questions 6. Conclusion 7. References a. You will likely require at least 5 primary references, either from recommended texts or primary journal articles. Total: ( = 70 marks) ASSESSMENT The experiment is worth 7% of the total mark for the subject. Please note the syllabus handout contains: (a) (b) The deadlines for the practical reports. The Departmental policy on late submissions

8 8 APPENDIX: Discussion: Q6 Phillips PW1120, 40kV 25mA, 1 min -1 Step size: 0.01

9 9 Results: Labeled spectra Phillips PW1120, 40kV 25mA, 2 min -1 Step size: 0.02, full scan