Ductile ceramics?

This has been an often claim every so often for so many years, but realistically, i.e., practically speaking from a structural materials viewpoint, it is a "pipe dream"!  Admittedly Max-phases, which are part-ceramic, part-metallic in character, have some characteristics of a ductile material, but true structural ceramics, i.e., alumina, silicon nitride, silicon carbide (and possibly zirconia), display near-zero tensile ductility at ambient temperatures and so that any notion of ductility for these materials is absurd. The directional and strong bonding in most ceramics means that dislocations are narrow, the Peierls force is high, and thus mobile dislocations are a rarity, at least at temperatures below the ductile-brittle transition.

Now most structural designers want some degree of ductility - I refer to this as their "parachute"! - i.e., to ensure that their structures will not break catastrophically but will endure some degree of damage before failure, such as strain hardening in a tensile test or subcritical crack growth along an R-curve in a fracture test. The former is not practical in actual terms for a structural ceramic but the latter can definitely be achieved.  Elongated grain structures in alumina, silicon nitride and silicon carbide can induce grain bridging, an extrinsic toughening mechanism which means that the ceramic will not fracture catastrophically but will support some degree of stable cracking prior to failure. 

These toughening concepts have been utilized to some degree for automobile components, for example,for turbocharger wheels, engine valves, brake disks, etc.  but for aerospace applications, such as gas turbine components,, monolithic ceramics represent too much of a risk.  This addresses the second part of your question, with respect to ceramic-matrix composites. Ductile-particle reinforced ceramics are a topic of many papers in journals - I have written a few myself - but are practically irrelevant for cutting-edge structural applications.  However, fiber-reinforced ceramics are most definitely relevant and are currently flying in GE aircraft engines, and will be soon in Rolls-Royce engines as well. Here ductility is mimicked by reinforcing the ceramic - mainly SiC - with itself - i.e., SiC fibers.  Specifically, apparent ductility is now created because when the SiC matrix fails - at the limit of proportionality - the fibers still hold the material together unit they fail at the UTS - creating a stress-strain curve not unlike a ductile material.  The designer designs to the limit of proportionality, but has a "parachute" because catastrophic failure does not occur until a larger strain is reached when the fiber fail. 

This, to my mind, represents the practical embodiment of "ductile ceramics", but as you can see, it has nothing to do with ductility as we understand it in terms of traditional metallurgical concepts.

ROR

1. There are some ceramics which are ductile at ambient temperatures. Such ceramics, for example are single crystals MgO, SrTiO3, etc. They undergo plastic deformation and by dislocation motion slip lines are observed on the deformed specimens.

http://www.springer.com/cda/content/document/cda_downloaddocument/9783319044910-c2.pdf?SGWID=0-0-45-1454720-p176464531

 2. Ductile particle ceramic matrix composites are a group of materials comprising micro- or nano-scale metallic particles in a ceramic matrix. The most studied materials have an alumina matrix; nickel, iron, molybdenum, copper and silver are some of the more frequently used metals. In contrast to conventional cermets and composites containing an interconnected metallic phase, the particles are discrete.

https://core.ac.uk/download/pdf/8767807.pdf

Florian Glodeanu  thank you for your answer.

but my point is effect of residual stress in failure mode of ceramic matrix if these residual stress exceeding brittle-ductile transition pressure of the matrix (e.g -0.55 GPa for AlN and -1.6 GPa for alumina ) , assuming that the resulted residual stress exceeding these values would that leading to ductilization of the ceramic matrix itself ?

https://link.springer.com/article/10.1007%2FBF00550614

Article Evidence of Ductile (Alumina) and Brittle (Boron Carbide) Re...

Ductile ceramics?

This has been an often claim every so often for so many years, but realistically, i.e., practically speaking from a structural materials viewpoint, it is a "pipe dream"!  Admittedly Max-phases, which are part-ceramic, part-metallic in character, have some characteristics of a ductile material, but true structural ceramics, i.e., alumina, silicon nitride, silicon carbide (and possibly zirconia), display near-zero tensile ductility at ambient temperatures and so that any notion of ductility for these materials is absurd. The directional and strong bonding in most ceramics means that dislocations are narrow, the Peierls force is high, and thus mobile dislocations are a rarity, at least at temperatures below the ductile-brittle transition.

Now most structural designers want some degree of ductility - I refer to this as their "parachute"! - i.e., to ensure that their structures will not break catastrophically but will endure some degree of damage before failure, such as strain hardening in a tensile test or subcritical crack growth along an R-curve in a fracture test. The former is not practical in actual terms for a structural ceramic but the latter can definitely be achieved.  Elongated grain structures in alumina, silicon nitride and silicon carbide can induce grain bridging, an extrinsic toughening mechanism which means that the ceramic will not fracture catastrophically but will support some degree of stable cracking prior to failure. 

These toughening concepts have been utilized to some degree for automobile components, for example,for turbocharger wheels, engine valves, brake disks, etc.  but for aerospace applications, such as gas turbine components,, monolithic ceramics represent too much of a risk.  This addresses the second part of your question, with respect to ceramic-matrix composites. Ductile-particle reinforced ceramics are a topic of many papers in journals - I have written a few myself - but are practically irrelevant for cutting-edge structural applications.  However, fiber-reinforced ceramics are most definitely relevant and are currently flying in GE aircraft engines, and will be soon in Rolls-Royce engines as well. Here ductility is mimicked by reinforcing the ceramic - mainly SiC - with itself - i.e., SiC fibers.  Specifically, apparent ductility is now created because when the SiC matrix fails - at the limit of proportionality - the fibers still hold the material together unit they fail at the UTS - creating a stress-strain curve not unlike a ductile material.  The designer designs to the limit of proportionality, but has a "parachute" because catastrophic failure does not occur until a larger strain is reached when the fiber fail. 

This, to my mind, represents the practical embodiment of "ductile ceramics", but as you can see, it has nothing to do with ductility as we understand it in terms of traditional metallurgical concepts.

ROR

It is great  to hear, all these notes, however. there is available ductile ceramic, that induce yielding before failure.

That is cfcc, continuous fiber ceramic composite.

Ductility is gained fron slipping along interface.

So ductility will not gain from brittle ceramic, but from ductile reinforcement.

Ductility will be a syenergetic effect of reinforcement, even if particulate, or fiber.

Dr. Rabeeh:

That is exactly what I described in the last but one paragraph of my answer, although everyone should note that there is no actual yielding before fracture (although there is apparent ductility) and the reinforcements are anything but ductile - SiC is reinforced with SiC fibers and alumina is reinforced with alumina fibers - these are the ceramic-matrix composites of choice being used, or should I more accurately state, being about to be used, in gas-turbine engines by the commercial aviation industry.

ROR

Thank you prof. Ritchie

Four your fruitful wards. All the best

B.R.

The MAX phases exhibit some plastic deformation at room temperature

So, for all structural ceramics room temperature is (far) below their ductile-brittle transition temperature (DBTT), which is a function of pressure and strain-rate. Bringing the DBTT down to RT requires a lot of pressure (GPa range) and probably very low strain rates (creep).

There are ways to get such high pressure (even at RT) although not throughout the bulk of a ceramic. The surface (and some distance below) can be compressively loaded by things like quenching and shot peeling (sphere impacts, laser). Residual surface stresses up to 2 GPa have been obtained this way and may help to enhance the RT ductility of the ceramics (at least locally near the treated surfaces). 

Another class of ceramic may present an "pseudo-plastic" behaviour at room temperature, with apparent ductility before fracture: Ce-stabilized tetragonal zirconia / alumina composites. In these materials the pseudo-plastic behaviour comes from the tetragonal-to monoclinic transformation under stress of the zirconia phase. More details in:

-Towards long lasting zirconia-based composites for dental implants: Transformation induced plasticity and its consequence on ceramic reliability; Reveron, H., Fornabaio, M., Palmero, P., (...), Montanaro, L., Chevalier, J. ; 2017; Acta Biomaterialia 48, pp. 423-432

- Palmero, P.; Fornabaio, F.; Montanaro, L.; Reveron, H.; Esnouf, C.; Chevalier, C. Towards Long Lasting Zirconia-Based Composites for Dental Implants. Part I: Innovative Synthesis, Microstructural Characterization and In Vitro Stability. Biomaterials 2015, 50, 38–46.

- Nawa, M.; Kurizoe, N.; Okamoto, Y.; Ueno, A. Transformation-Induced Plastic Deformation in Ce-TZP/Alumina Nanocomposite Generated During Fatigue Tests at Room Temperature. J. Eur. Ceram. Soc. 2014, 34, 4337–4345.

-Tsai, J. ‐.F.; Chon, U.; Ramachandran, N.; Shetty, D. K. Transformation Plasticity and Toughening in CeO2 – Partially‐Stabilized Zirconia–Alumina (Ce‐TZP/Al2O3) Composites Doped With MnO. J. Am. Ceram. Soc. 1992, 75 (5), 1229–1238.

I saw an article in Nature Materials which claimed that the ZrO2 nanorods behaved like a ductile material and had exceptional mechanical properties.