Increasing Fracture Toughness and Transmittance of Transparent Ceramics using Functional Low-Thermal Expansion Coatings

Marc Rubat Du Merac,Mirko Ziegner,Martin Bram,Jürgen Malzbender,Marcin Rasinski,Olivier Guillon

doi:10.1038/s41598-018-33919-5

Abstract

Transparent polycrystalline ceramics have the potential to enable applications no other materials can, but to do so their strength and toughness must be improved. However, surface strengthening treatments like those used for glasses have so far remained elusive. Here for the first time, we report on engineering unprecedented surface compression, of the magnitude achieved for ion-exchange strengthened glasses (~750 MPa) in transparent ceramics. This was achieved by applying functional, low thermal-expansion yttria coatings onto yttria-stabilized zirconia substrates and thermally treating. In some instances, the treatment more than doubled the fracture toughness while simultaneously increasing light transmittance.

Highlights

Ceramics typically fail at existing stress-concentrating flaws near surfaces, where tensile loading stresses are usually highest, for example during bending[13,24]
We prove the feasibility of this novel concept by implementing high surface compression in transparent yttria-stabilized zirconia (YSZ) by applying functional, low-thermal expansion yttria thin-coatings followed by a thermal treatment
Cubic yttria-stabilized zirconia (YSZ) exhibits high transmission from the near-ultraviolet to the mid-infrared, a high refractive index, optical isotropy, and low emissivity combined with chemical resistance, low thermal conductivity, and high hardness, strength, and toughness[1,26]

Summary

Introduction

Ceramics typically fail at existing stress-concentrating flaws near surfaces, where tensile loading stresses are usually highest, for example during bending[13,24]. Surface compression can be generated by cooling two bonded materials, or material stoichiometries, with different thermal expansion from elevated temperature. Substituting values for a 750 nm thick Y2O3 coating bonded to an 8 YSZ substrate into Eqs 1–3, surface compression of ~850 MPa should be achievable. This would be of the same magnitude as stresses in ion-exchanged strengthened glass[31,32,33]. Post-polishing would add cost, complicate obtaining the desired thickness, and introduce surface flaws These constraints can be overcome by first making polished transparent substrates and subsequently applying coatings followed by thermal treatment

Methods

Results

Discussion

Conclusion