Dear Erik,

Strengthening ceramics using the same processes as glasses is possible.  Kirchner (1979) described quenching to strengthen Al2O3 and SiC with a 2-fold increase in strength for Al2O3 (Richerson 2006).  Kirchner also described increasing the strength of Al2O3 by applying glazes with a lower CTE.  Chemical ion exchange has also been described, for example in some dental ceramics (although these are usually porcelains).  Other methods, like flame polishing, increase strength by reducing or removing surface flaws.    

Marc

 Dear Erik Carton

We hardened many of our products with chemical agents such as phosphoric acid, mono aluminium phosphate and etc. The use of these hardners leads to hardening first the surface and later the bulk. With low temperature thermal treatment (under 350oC), the surface is hardened completely, as we can use these ceramics for wear-resistance  applications.

Dear Abbas,

What ceramic materials do you harden chemically? What about fully dense ceramics like Alumina and SiC (no porosity available).

We hardened Alumina and SiC based dense bodies and refractories. But these products had 2-8% apprent porosity after thermal treatment.

Dear Eric Carton,

To my mind the perfect composite material for your task may be created on the base of SiC ceramic with some contamination of free silicon. Such type of composite may be practically non porous and more stable both in the acids and in the thermohocycles in comparison with Alumina. The important value is the difference of the thermal expansion parameters of these considered materials.    

Dear Erik,

Strengthening ceramics using the same processes as glasses is possible.  Kirchner (1979) described quenching to strengthen Al2O3 and SiC with a 2-fold increase in strength for Al2O3 (Richerson 2006).  Kirchner also described increasing the strength of Al2O3 by applying glazes with a lower CTE.  Chemical ion exchange has also been described, for example in some dental ceramics (although these are usually porcelains).  Other methods, like flame polishing, increase strength by reducing or removing surface flaws.    

Marc

@ Marc: "Kirchner (1979) described quenching to strengthen Al2O3 and SiC with a 2-fold increase in strength for Al2O3 (Richerson 2006)." can you provide me with these articles or references?  

@ SK Brantov. I have been at ISMAN in Chernogolovka. The material you suggest is Silicium infiltrated SiC (reaction bonded SiC or Si-SiC) right? How to harden its surface thermally of using ion-exchange, do you have a recipe of such process, or references? 

Hi Erik, 

Kirchner, H.P., Strengthening of Ceramics, Marcel Dekker, NY, 1979

Richerson, D.W., Modern Ceramic Engineering, 2006, pg. 542

Marc

Dear Erik.

The proposed composite is Si-SiC material. The simplest way is an impregnaton of usual graphite felt with a molten silicon. You can see some data in my recent book "Carbon-based functional composites" on my personal page in the RG.

I know how Si-SiC is manyfactured, but like to know once it is made how to harden its surfaces

Chemical strengthening of glasses involves exchange of potassium ions for sodium ions. There is no exchange of silicon ions for alkali ions in this process. 

AL2O3 can be hardened (and actually it is hardened in some type of prosthetic applications) by substituting Al+3 with Cr+3 (practically putting Al2O3 in contact with Cr2O3 at high temperature, since the two present complete solubility over the entire compositional range).

Valter: That sounds interesting, what temperature/time is needed to get enough diffusion of Cr+3 into Al2O3? Can I use a Cr2O3 powder as Cr3+ source?

Do you know some references of articles that describe this process? 

Times of the order of some Hours at temperatures around sintering temperatures (say 1500 -1600 C) would be the starting choice. You can use Cr2O3 powders and your Al2O3 component will be pink at the end of the process, since Cr+3 ions is what gives the color to Ruby, which is, chemically,  Al2O3 with Cr+3 impurities.

Be aware that during the process you increase the grain size of alumina, exponentially increasing temperatures and linearly (at a given temperature) increasing time. Depending on the application this grain size increase might be detrimental: above around 30 microns of grain size you will have spontaneous cracking due to thermal expansion anysotropy of alumina.

Out of the top of my head I do not have references, since these are concepts of fundamental Ceramic Engineering.