RESISTENZA ALL USURA DI UN CERAMICO A STRUTTURA MULTILAMINARE. Goffredo de Portu

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1 RESISTENZA ALL USURA DI UN CERAMICO A STRUTTURA MULTILAMINARE Goffredo de Portu ISTEC- CNR Istituto di Scienza e Tecnologia dei Materiali Ceramici Consiglio Nazionale delle Ricerche Faenza

2 ACKNOWLEDGEMENT Stefano Guicciardi Edoardo Roncari Cesare Melandri Paola Pinasco Francesco Toschi Lorenzo Micele Giuseppe Pezzotti

3 Summary Why laminated composites Preparation of laminated composites Evaluation of residual stresses Wear behaviour

4 Why laminated composites The performances of wear-resistant materials are mainly related to the properties of thin surface layers Removal of material in engineering ceramics under sliding conditions is generally caused by the propagation of surface cracks resulting from tensile stresses in the wake of rubbing contact

5 An increase in apparent surface toughness should lead to an improvement in wear resistance Laminated structures can be designed to induce compressive residual stresses at the surface by combining the different thermophysical characteristics (i.e. thermal expansion and shrinkage on sintering) of the different materials used

6 Artistic view of a Functional Graded Material

7 Cross Section Micrograph of a Shell

8 Tape - Casting Apparatus

9 Ceramic sheets produced by tape-casting

10 Technique for preparing laminated composites

11 Diagram of samples preparation A AZ Lamination procedure Monolithic Alumina Laminated Alumina Laminated A/AZ Laminated A/AZ (MA) (AA) after sintering after machining

12 Warm Pressing Pistons Green Layers Heating Elements Thermocouple

13 Section of Al2O3/Al2O3-ZrO2 laminated composites

14 Microstructure of polished and thermally etched surface of the three materials: a) MA; b) AA; c) AZ; d) interface A/AZ a b 2 µ m 2 µ m c d 2 µ m 2 µ m

15 Tunneling Crack in Laminated Composite

16 Vickers Impression on Layer in Tension

17 Vickers Impression on Layer in Compression

18 Indentation model: relationship among K IC, indentation load and crack length P K Ic = χ 3 c 0 2 K Ic = toughness of the stress free material χ = dimensionless constant (experimentally determined) P = indentation load c 0 = crack length

19 Relationship among K IC, indentation load and crack length in presence of residual stress P K Ic = χ c Y σ res c 1 where: c 1 = crack length in the stressed material Y = 1.29 geometrical factor σ res = residual stress

20 Evaluation of residual stress in laminated composite 250 Applied load, N s res =-141 MPa á AAZ ò MA Crack length, µm

21 Stress Map in A/AZ Laminated Composite

22 Stress Profile in A/AZ Laminated Composite

23 Stress Map in A/2AZ Laminated Composite

24 Stress Profile in A/2AZ Laminated Composite

25 Measured hardness, calculated Young s modulus and surface toughness for the different materials MATERIAL HV (GPa) E (GPa) KIc (MPa m)* A/AZ 17.6 ± ± AA 16.4 ± ± MA 16.6 ± ± (3.61 ± 0.13 )

26 Distribution of stresses in sliding contacts Tension COMPRESSION

27 Schematic of the effects of compressive residual stresses in the surface

28 Ashby map with the points relative to the test conditions

29 Method inverted pin-on-disk tests on a Wazau tribometer Electric motor Torque sensor Disk Load cell Pin Dead weight

30 Pin-on-Disk Configuration 1.7 mm 30 mm Disk Load Ra=0.13 µm PIN Height = 16 mm Diameter = 5 mm Tip radius = 2.5 mm

31 Experimental Procedures Load: 50 N, 100 N, 150 N Sliding speed: 0.05 m/s, 0.10 m/s, 0.15 m/s Sliding distance: 15 km Temperature: 22 C Humidity: 70%

32 Mean values of the friction coefficients measured on the various materials for the different experimental conditions v=0.05 m/s 0.6 Friction coefficient v=0.10 m/s v=0.15 m/s A/AZ AA MA Applied load, N

33 Semi-Log plot of the disc specific wear of the various materials as function of the different experimental conditions Log 10 (disk specific wear), g km -1 N A/AZ AA MA v=0.05 m/s v=0.10 m/s v=0.15 m/s severe Applied load, N mild

34 Surface cracking of stress free alumina (MA) and alumina containing compressive residual stresses (A/AZ) 100 N, 0.05 m/s

35 Fracture of the surface with detachment of flakes (100 N and 0.05 m/s) AZ AA MA

36 Plastically deformed debris spread over the surface (150 N and 0.15 m/s) AZ AA MA

37 Fracture surfaces observed in the wear tracks Composite A/AZ and Monolithic MA 150 N; 0.15 m/s (5000 X)

38 CONCLUSIONS 1 Suitable design and processing can lead to the production of laminated ceramic composites with compressive residual stresses at the surface. The stresses can be measured also using the indentation model or raman spectroscopy. These stresses are responsible for an increase of hardness and apparent surface toughness. If the wear mechanism is microcracking, the wear resistance of ceramics can be improved and the transition from mild to severe wear retarded.

39 CONCLUSIONS 2 Friction coefficient of laminated composites is lower if compared with that exibited by the stress free materials. Laminated ceramic composites are attractive structures suitable for structural and tribological applications.

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