A UTONOMIC S ELF -R EPAIRING G LASSY M ATERIALS
Daniel COILLOT a
, François O. MEAR a,*
, Renaud PODOR b
and Lionel MONTAGNE a a Unité de Catalyse et de Chimie du Solide, UMR 8181 CNRS-USTL-ENSCL, BP 108, Université des Sciences et
Technologies de Lille, F-59652 Villeneuve d’Ascq cedex b Institut de Chimie Séparative de Marcoule, UMR 5257 CEA-CNRS-UM2-ENSCM Site de Marcoule, BP 17171,
F-30207 Bagnols sur Cèze cedex
* Corresponding author
E-mail address: francois.mear@univ-lille1.fr (F. Méar)
Abstract
A new process that enables glassy materials to self-repair from mechanical damage is presented in this paper. Contrary to intrinsic self-healing, which involves overheating to enables crack healing by glass softening, our process is based on an extrinsic effect produced by vanadium boride (VB) particles dispersed within the glass matrix. Self-repairing is obtained through the oxidation of VB particles, and thus without the need to increase the operating temperature. VB healing agent was selected for its capacity to oxidize at lower temperature than the softening point of the glass. Thermogravimetric analyses indeed showed that VB oxidation is rapid and occurs below the glass Tg. Solid-state Nuclear Magnetic
Resonance indicated that VB is oxidized into V
2
O
5
and B
2
O
3
. A demonstration of the selfhealing effect was obtained by an in-situ experiment realized into an environmental Scanning
Electron Microscope. It is shown that a crack could be filled by the VB oxidation products.
Key-words
Glass / glass-ceramic; Self-healing / repairing; Environmental scanning electron microscopy;
Extrinsic / intrinsic

Introduction
Many efforts in research and development of smart materials are currently driven towards higher performance applications. In recent years, it has been realized that an alternative strategy, based on damage management, can be conducted to make materials stronger and more reliable. These materials have indeed a built-in capability to repair the damages that may occur during use. When damage through thermal, mechanical, chemical or other origin is formed, the material has the ability to heal and restore itself to its original set of properties.
Thus, self-healing polymers, metals, ceramics and their composites have attracted broad attention.
Self-healing concept has been developed in many application fields such as polymers for coatings, microelectronic packaging,
[1]
and medical uses,
[2]
concrete or cementitious structures,
[3]
and composites materials for aerospace applications.
[4,5]
A robust self-healing effect must have following characteristics:
[3] efficiency (recovers both mechanical and chemical properties); repeatable (ability to self-repair for multiple damage events; long-shelf life (remains operating during the service life that may span decades); reliable (consistent self-healing in a broad range of environment); pervasiveness (ready for activation when and where needed); and economical (economically feasible, for instance for the highly cost sensitive construction industry).
Most of the work in the development of self-healing materials has concentrated in the polymers, for which several self-healing mechanisms were reported. One of the earliest one is that of Dry
[6]
, who used glass capillaries to carry a liquid resin to the damaged region. This process was then extended to hollow glass fibres.
[7-9] Another self-repairing process is achieved by incorporating a microencapsulated healing agent and a catalytic chemical trigger within a polymer matrix.
[10-12]
Chen et al.
[13]
have developed a polymer composition that stays under a dynamic polymerisation-depolymerisation equilibrium, which ensures the reformation of broken bonds on heating. Finally, Hayes et al.
[14]
have developed a smart composite system which combines structural health monitoring with a self-healing resin.
Another active application field of self-healing concerns the ceramic matrix composites
(CMCs). For instance, carbon fiber–reinforced carbon matrix composites (C/C composites) are widely used as aeronautic and spatial structural materials because of their outstanding mechanical properties at high temperature. These C/C composites are protected with antioxidation coatings, but micro-cracks often occur owing to CTE difference between C/C substrate and coatings. Therefore, self-healing of the coating is essential to enable long-term

anti-oxidation protection. Kessler et al.
[15] have extended the polymer self-healing concept by incorporating a healing agent to the coatings, such as SiC, B
4
C, ZrB
2
,or HfB
2
.
[16-21] Selfhealing is activated when the healing agents come into contact with air and form oxides that heal the cracks.
Passive smart self-healing has been developed in cementitious composite by Li et al.
[3]
, based on the process developed by Dry
[8] in polymers. Superglue contained in hollow brittle glass fibres serves as the sealing/healing agent.
Self healing was also claimed to enable an increase of the operating duration of glass seals for
Solid Oxide Fuel Cells (SOFC). It was indeed shown that the SOFC lifetime was limited by cracks that form within the glass seal used to join electro-active ceramic parts to the metallic structure.
[ 22 ]
However, contrary to the conventional self-healing processes described above, here the self-healing effect was not obtained by the addition of a healing agent, but simply by heating the sealing glass above its softening temperature.
[23-27]
This effect was shown to operate also in glass-ceramics sealants, provided that the amount of residual glass is enough to enable softening and healing.
[25]
However, this operation needs to heat the SOFC assembly at a temperature above the operation temperature, which may induce degradation of the metallic or electroceramics parts of the fuel cell.
Self-healing can thus be obtained in two ways: extrinsic or intrinsic . Intrinsic self-healing is fully autonomic but requires an external constraint such as temperature increase. On the other hand, extrinsic self-healing involves healing agents encapsulated in the material, and their reactivity is activated by a constraint (mechanical, chemical or thermal). White et al.
[12] , who introduced major innovations in self-healing polymers materials, predicted that their approach would be extended to brittle ceramics in the future. We indeed report here the first application of his autonomic extrinsic self-healing concept to glasses and glass-ceramics. We show that an appropriate selection of the healing agent enables to self-repair crack that may form during high-temperature operation, but at a temperature below the softening point of the material.
Our process thus enables self-healing without risk of deformation or thermal degradation of the structure.
Our self-healing concept, derived from that developed for polymeric materials,
[12]
is illustrated on figure 1. Healing particles constituted of vanadium boride (VB) are dispersed within the glass or glass-ceramic matrix. The choice of VB will be justified on the basis of its reactivity and the thermal characteristics of the matrix. When a crack occurs on the surface of the sample (A) and propagates in the bulk (B), VB particles react in contact with oxygen contained in the atmosphere to produce a new glass that fills the crack and finally closes it

(C). This process can be operated on any glass or glass-ceramic composition; we describe in the following text an example of application to an aluminosilicate glass composition.
Figure 1 near here
Experimental Procedure
A glass of composition (%wt.) 48.59 BaO-31.75 SiO
2
-10.47.Al
2
O
3
-8.89 CaO was prepared from BaCO
3
, Al(OH)
3
, CaCO
3
, and SiO
2
. These precursors were weighted, mixed, calcined at
1100°C and then melted in a 90Pt-10Rh crucible at 1500°C for 2h. The glass was obtained by pouring the melt on a stainless steel plate. The glass was then milled into powder (particle size
< 10 µm) and mixed with vanadium boride (VB, 99.5% purity, particle size < 20 µm).
Healing particles should not modify the intrinsic properties of the initial material; hence VB addition was limited to less than 20 vol.%.
Thermal properties were measured in air from 25 to 1000°C with a 10 K.min
-1
heating rate using coupled differential thermal analysis (DTA) and thermogravimetry (TG) (Setaram
Setsys).
11 B MAS-NMR spectra were acquired at 256.8 MHz with an Avance II Bruker 800 MHz
(18.8 T) spectrometer, equipped with a 3.2 mm probe allowing for spinning frequencies in the range of 10-15 kHz. 256 transients separated by a relaxation delay of 2 sec. were necessary to obtain spectra with good signal to noise ratio. The 11 B chemical shifts were referenced using a solution H
3
BO
3
1M (
 iso
= -19.6 ppm) as an external reference.
51 V MAS-NMR experiments were performed with an Avance II Bruker 400 MHz spectrometer (9.4 T) at 105.19 MHz with a 2.5 mm probe operating at spinning frequencies of
20-30 kHz. The spectra were acquired using standard single pulse acquisition with a pulse length of 0.8 µs, a radiofrequency field of 45 kHz (measured on a liquid sample), and 2048 transients separated by a relaxation delay of 1 s. The chemical shifts were referred to the secondary reference V
2
O
5
(
 iso
= -609 ppm).
Dense glass + VB composites were fabricated by spark plasma sintering (SPS). It was carried out under vacuum at 700°C under a pressure of 50 MPa.
In situ Environmental Scanning
Electron Microscopy (ESEM) was used to observe crack healing. 5 x 5 x 3 mm samples were cut in the densified glass + VB composite using a diamond saw, and polished. Vickers indentation was performed with 2kg load on the sample surface. This technique allows the formation of four symmetrical radial cracks emanating from the four corners of the indenter

edge. A field emission gun environmental scanning electron microscope (model FEI
QUANTA 200 ESEM FEG) equipped with a 1000°C hot stage was used for in situ gaseous secondary electron imaging at high temperature (HT-ESEM images). The observations were performed under air at an operating pressure of 450 Pa. These conditions ensured both the presence of an oxidizing atmosphere near the sample and the signal amplification yielding to the secondary electron image formation using the specific gaseous secondary electron detector. Acceleration voltage was 20 kV. A heating rate of 30 K.min
-1
was applied up to
700°C. The sample was then maintained at this temperature for the observation of the healing effect.
Results and Discussions
DTA curves for glass and vanadium boride (VB) particles are reported on figure 2. The glass
Tg temperature is 750°C. The DTA curve relative to VB indicates that it oxidizes in the temperature range 400 (T1) to 700°C (T2). This reaction is also revealed on the TG curve by weight increase due to the reaction with oxygen. Figure 2 thus confirms that the oxidation of the VB particles occurs as required at a temperature lower than the Tg of the glass, meaning that the healing effect can be obtained without any deformation of the glass piece. This confirms that our process can be classified as an extrinsic healing effect.
Figure 2 near here
Since the healing effect must operate within a reasonable delay, the oxidation kinetic of VB was followed by the weight increase due to the reaction with oxygen, as shown on figure 3.
We observe that VB is oxidized after only 45 minutes of exposition to oxygen at 700°C.
In order to identify the oxidation products of VB, we used
51
V and
11
B solid-state NMR, the spectra are shown as an inset of figure 3. The
51
V NMR spectrum of VB before heat treatment exhibits a large set of spinning sidebands that is typical of its paramagnetic behaviour.
Nevertheless, the
51
V isotropic resonance of VB can be measured at δ iso
= 2836 ppm. While the duration of the treatment at 700°C increases, the signal assigned to VB decreases readily after 15 min, and almost disappears after 30 min, thus confirming the rapid oxidation of VB.
Meanwhile, a resonance at δ iso
= -610 ppm appears, which is assigned to V
2
O
5
[28]
. Similarly,
11
B NMR spectra confirms the rapid oxidation of VB (
11
B resonance at 57.9 ppm) into B
2
O
3
(-

15.2 ppm).
[29] We notice that no reaction between B
2
O
3
and V
2
O
5
is observed with NMR, as mentioned in the literature.
[30]
Figure 3 near here
Our results show that vanadium boride can be used as a healing agent for glasses or glassceramics with a Tg or softening temperature higher than 700°C. We will now demonstrate the efficiency of the self-healing by an in situ observation using environmental-SEM. Figure 4 shows photomicrographs recorded after 0, 15, 30 and 45 minutes. A VB particle is visible near the crack at t = 0 (arrow on Fig. 4). After 15 minutes heat treatment, the particle is partially oxidized, as evidenced by the modification of the shape of the particle. NMR analyses have indeed shown that VB is oxidized into B
2
O
3
and V
2
O
5
, which are already molten at 700°C. Their low viscosity enables spreading of the reaction products, which begin to fill in the crack. The blocking up of the crack is completed after 45 minutes run duration.
The crack is then fully healed by a melt composed of B
2
O
3
and V
2
O
5
.
Figure 4 near here
Conclusions
We report a new autonomic healing of cracks that may occur in glass. The healing effect is obtained by the oxidation of VB particles into low viscosity V
2
O
5
and B
2
O
3 oxides, as demonstrated with 51 V and 11 B NMR. The healing effect is demonstrated to operate on a rigid glass, i.e. below its softening temperature.
This enables to apply this self-repairing concept to systems that would not withstand overheating, for instance Solid Oxide Fuel Cells (SOFC) build with complex metallic manifolds and ceramic electro-active components. Other applications involve hightemperature enamelled reactors for the chemical industry, or structural glass-ceramic composites for aerospace and military application.

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F IGURE C APTIONS
Figure 1 : Concept of “extrinsic” self-healing in glassy-materials: active particles (healing agent) such as vanadium boride (VB) are embedded in a glassy-matrix. A.
Crack forms in the matrix wherever damage occurs; B.
Oxidation of the active particle located in the crack occurs by reaction with oxygen. C.
V
2
O
5
and B
2
O
3
produced by the oxidation of VB have a low viscosity at high temperature, which enables their spreading and leads to the crack healing.
Figure 2 : DTA and TG curves of the glass and VB particles. DTA shows that glass Tg is
736°C. The baseline shift at 850°C is due to glass softening, as indicated by dilatometry. DTA and TG curves show that VB oxidation begins at 400°C (T1) and is almost completed at
700°C (T2), although some residual oxidation still occurs above 700°C, probably owing to limitation of oxygen diffusion by V
2
O
5
and B
2
O
3
formation.
Figure 3 : Oxidation kinetic of vanadium boride particles, measured by the weight variation at
700°C under air. The curve shows the complete oxidation of VB after 45 minutes. The inset shows 51 V and 11 B NMR spectra of VB before the treatment, and after 15, 30, and 45 minutes exposition at 700°C. They confirm the complete oxidation of V into V
2
O
5
and B into B
2
O
3 after 45 minutes at 700°C.
Figure 4 : Environmental-SEM images of a glass + 20 vol.% VB composite. A crack (0.5
 m width, 15
 m long) was generated by Vickers indentation. An isothermal treatment at 700°C under air leads to the oxidation of VB and formation of V
2
O
5
and B
2
O
3
, which flow and fill the crack. ESEM images were taken after: A.
0 min.; B.
15 min.; C.
30 min.; and D.
45 min.

F IGURE 1

F IGURE 2

F IGURE 3

F IGURE 4