Geckeler K.E., Nishide H. (Eds.) Advanced Nanomaterials

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17.4 Mechanism 559

17.4

Mechanism

To keep the structural integrity of ceramic, an efﬁ cient self - healing should occur.

This is possible if healing the surface cracks obeys the following conditions:

1. healing must be triggered by cracking;

2. healing must occur at high temperature [as structural ceramics are expected to

typically operate at high temperature ( ∼ 1273 K) in air] in the corrosion

atmosphere, such as air;

3. Strength of the healed zone must be superior to the base material.

Self - crack healing driven by the oxidation of silicon carbide (SiC) can be quali-

tatively understood to satisfy the requirements 1 and 2. Figure 17.2 shows the

schematic of the crack healing in the ceramics containing SiC particles heated at

high temperature in the presence of air Cracking allows the SiC particles located

on the crack walls to react with oxygen in the atmosphere resulting in healing.

The details of the valid conditions are discussed later (Section 17.6 ). Subsequently,

the crack is completely healed as oxidation progresses. As mentioned earlier, if

the three important conditions of achieving strong healed zone is satisﬁ ed, then

fracture initiation changes from the surface crack to the other ﬂ aws such as

embedded ﬂ aw. This behavior is well demonstrated in Figures 17.3 a and b [29] .

The following equation showing oxidation of SiC also supports the above

ﬁ ndings.

SiC O SiO CO+=+32

(17.3)

There exist two important features in the above mentioned process. One is the

increase in the volume of the condensed phase and the other is the generation of

the huge exothermic heat. Because the mole number of silicon is held constant

during the oxidation, the volume increase is found to be 80.1%. As the oxidation

Table 17.1 Categorization of self - healing ceramics.

Types Mechanism Triggered by

damage

Valid under

service condition

Strong healed part

First generation Re - sintering No No Yes

Second generation Oxidation of SiC

( < 5 vol% SiC)

Yes Yes No

Third generation Oxidation of SiC

( > 10 vol% SiC)

Yes Yes Yes

560 17 Self-healing of Surface Cracks in Structural Ceramics

progresses, the crack walls are covered with the formed oxide. Finally, the space

between the crack walls is completely ﬁ lled with the formed oxide. For the com-

plete inﬁ lling of the space between crack walls, it is necessary to contain more

than 10 vol% SiC (Section 17.5 ). Another important parameter for attaining the

complete inﬁ lling is the size of crack.

From Figure 17.4 [31] , one can ﬁ nd the critical crack size that can be completely

crack healed. As an example, the critical crack size of alumina/30 vol% SiC parti-

cles composite is 300 μ m. This value is the surface length of a semi - elliptical crack

with an aspect ratio (crack depth/half of surface length) of 0.9 introduced by

indentation method. Below this value, the crack - healed specimens exhibit the

same strength, because the space between crack walls is completely ﬁ lled with the

formed oxide and because the fracture initiates from an embedded ﬂ aw. When

SiC

SiO

(a) Before cracking (b) After cracking

Figure 17.2 Schematic illustration of crack - healing mechanism.

(a) (b)

Vickers

indentation

Vickers

indentation

Fracture

100 µm 100 µm

Figure 17.3 Fracture initiation of alumina/15 vol% 0.27 μ m

SiC particles composite: (a) as - cracked and (b) crack healed

at 1573 K for 1 h in the presence of air.

17.4 Mechanism 561

the value is above the critical crack size, the space between the cracks walls is too

large to be ﬁ lled with the formed oxide.

Alternatively, the reaction heat makes the formed oxide and the base material

to react or to once melt. The second low enthalpy change of the reaction can be

evaluated to be − 945 kJ using the thermochemical data [32] of the pure substance.

This phenomenon might lead to strong bonding between the reaction products

and crack walls. The crack - healing mechanism is clearly demonstrated by in situ

observation, as shown in Figure 17.5 .

0 100 200 300 400 500 600

200

400

600

800

1000

1200

Flexural strength (MPa)

Precrack size, 2c (µm)

Crack-healed strength

Cracked strength

Specimens fractured

from the crack-healed zone

Figure 17.4 Flexural strength of the crack - healed

alumina/30 vol% 0.27 μ m SiC particles composite as a

function of surface length of a semi - elliptical crack.

At high temperature in air

Figure 17.5 Photographs of in situ observation of crack

healing, in which alumina/15 vol% 0.27 μ m SiC particles

composite containing cracks with an indentation is heat -

treated at 1573 K in the presence of air.

562 17 Self-healing of Surface Cracks in Structural Ceramics

Unit: mm

5 Pin

45°

0.2

Figure 17.6 Dimensions of three - point bending and the bar specimens.

The phenomenon was observed while alumina/15 vol% 0.27 μ m SiC particles

composite containing an indentation crack was heat - treated at 1573 K in the pres-

ence of air. The features of the crack healing behavior are as follows: (i) the reaction

products like sweats appear from the cracks and surface as the reaction progresses;

(ii) cracks are perfectly covered and ﬁ lled with the reaction products; (iii) the reac-

tion products form with bubbling; and (iv) there are no changes in the indentation

ﬁ guration. From the observation, it is noted that the high temperature makes the

reaction products, as well as base material melting, and the bubble, including

carbon monoxide (CO) gas, forming strong crack - healed zone.

Furthermore, to estimate the strength of the healed zone in detail, it is necessary

to take account of the following issues, that is:

1. Effective volume should be so small that most fracture initiates at the crack -

healed zone.

2. The strain energy at failure should be so low that fracture initiation is identiﬁ ed

easily.

Ando and coworkers adapted a three - point bending method as shown in Figure

17.6 for fracture tests.

The span of the geometry is 14 mm less than that of Japan Industry Standard

(JIS) [33] . The crack - healed specimens had higher strength than the smooth mir-

ror - polished specimens (Section 17.6 ). Polishing was carried out according to JIS

standard [33] .

17.5

Composition and Structure

17.5.1

Composition

Most important factor to decide the self – crack - healing ability is the volume

fraction of SiC. As mentioned in Section 17.4 , it is necessary for achieving strong

17.5 Composition and Structure 563

crack - healed zone that the volume between crack walls is completely ﬁ lled with

the products formed by the crack - healing reaction. Therefore, there is lower limit

of the SiC volume fraction to endow with adequate self – cracking - healing ability.

Figure 17.7 shows the cracked and crack - healed strengths of alumina containing

various volume fractions of SiC particles, which has mean particle size of 0.27 μ m.

As a result, the crack - healed strength varies with SiC volume fraction and shows

a maximum at SiC volume fraction of 7.5, as shown in Figure 17.7 . From the

strength dispersion, the crack - healing ability cannot be estimated by the strength

recovery behavior alone. However, the fracture surface observations can reveal

whether the crack is completely healed. There are two kinds of fracture mode. One

is the fracture initiated from the crack - healed zone, as shown in Figure 17.8 a. This

means the formed oxide is not enough to heal the precrack completely. This frac-

ture mode is observed in some specimens of the alumina mixed with 7.5 vol% SiC

particles and all specimens that contain less than 5 vol% SiC particles. The other

is the fracture initiated outside the crack - healed zone. as shown in Figure 17.8 b.

This means that the formed oxide is enough to heal the cracks. The enough quan-

tity of the oxide is formed by the oxidation of more than 10 vol% SiC particles.

From the results, the ceramics have to contain at least 10 vol% SiC to produce

strong crack - healed zone.

17.5.2

S i C Figuration

The SiC ﬁ guration also affects the crack - healing ability. Especially, SiC whisker

that has high aspect ratio causes intrinsic change to the micromechanism of the

crack healing.

SiC volume fraction (vol%)

0 5 10 2015

200

400

600

800

1000

Flexural strength (MPa)

Crack-healed strength

(fractured outside precrack healed)

Cracked strength

Crack-healed strength

(fractured from precrack healed)

Figure 17.7 Crack healed and cracked strength of alumina - SiC

composites as a function of SiC volume fraction.

564 17 Self-healing of Surface Cracks in Structural Ceramics

Sato et al . [34] investigated the relation between SiC particle size and the

crack - healed strength in the case of mullite (3Al

2SiO

)/20 vol% SiC composite.

From the results shown in Figure 17.9 , they concluded that the crack healing

at 1573 K for 1 h in the presence of air causes 100 – 300 MPa strength enhancement

to all specimens, which shows a maximum with SiC particle size of 0.56 μ m.

Ceramics containing SiC whiskers also show the self – crack - healing ability, but

there are some differences between the crack - healing behaviors driven by oxida-

tions of the SiC whiskers and that of SiC particles. This difference arises from the

geometric relation between the SiC whiskers and the crack wall. The SiC whiskers

Precrack healed

30 µm

Embedded flaws

30 µm

(a) (b)

Figure 17.8 Fracture initiations of (a) alumina/7.5 vol% SiC

particles composite in which fracture initiates from the

precrack healed and (b) alumina/10 vol% SiC particles

composite in which fracture initiates from an

embedded ﬂ aws.

0.1 0.2 0.5 1.0 2.0

SiC particle size (µm)

500

600

700

800

900

1000

Flexural strength (MPa)

Crack-healed strength

Smooth strength

Figure 17.9 Variation in crack - healed strength with SiC

particle size in mullite containing 20 vol% SiC particles

composite.

17.5 Composition and Structure 565

stick out at the crack wall and bridge between the crack walls as illustrated in

Figure 17.10 .

Owing to this geometry, partial bondings between the crack walls can be formed

despite the small amount of the oxide formation. The partial bondings [31, 35] was

observed in the crack - healed zone of alumina/20 vol% SiC whiskers (diame-

ter = 0.8 – 1.0 μ m, length = 30 – 100 μ m). The partial bondings enhance the strength

recovery of the crack healing and at primary stage on large cracks, as shown in

Figure 17.11 [31] (cf. Figure 17.4 ). Both in the primary stage and on large crack,

the amount of the formed oxide is too less to completely ﬁ ll the crack. In this

situation, a large strength recovery could not be attained without the partial

Matrix

Crack

SiC whisker

Formed oxide

Figure 17.10 Schematic illustration of crack - healing mechanism by SiC whiskers.

0 100 200 300 400 500 600

200

400

600

800

1000

1200

Bending strength (MPa)

Precrack size, 2c (µm)

Crack-healed strength

Cracked strength

Specimens fractured

from the crack-healed zone

Figure 17.11 Flexural strength of the crack - healed

alumina/30 vol% SiC whiskers (diameter = 0.8 – 1.0 μ m,

length = 100 μ m) composite as a function of surface length

of a semi - elliptical crack.

566 17 Self-healing of Surface Cracks in Structural Ceramics

bonding. Therefore, composites with SiC whiskers do not only improve the frac-

ture roughness but also have the advantage on crack - healing ability. However, the

reliability of the crack - healed zone comprised by the partial bonding is inferior to

that of the crack - healed zone completely ﬁ lled with the formed oxide. Therefore,

composites containing SiC whiskers as well as SiC particles show the excellent

self – crack - healing ability.

17.5.3

Matrix

Since the self – crack - healing ability can only be seen in case of SiC composites,

there is no restriction in selecting the matrix. Ando and coworkers succeeded

to endow silicon nitride [24 – 27] , alumina [28 – 31, 35] , and mullite [21 – 23] with

the self – crack - healing ability. Also monolithic SiC [36, 37] has excellent self –

crack - healing ability.

These composites can be prepared from commercially available powders using

ball mill mixing and hot pressing techniques. Sintering additives does not show

any inﬂ uence on the crack - healing ability. Moreover, a further improvement in

mechanical properties can be obtained by employing the optimized sintering

conditions. For example, the entrapped SiC particles [28] presented in the alumina

matrix grains, when alumina containing 15 vol% SiC particles composite is hot

pressed at 1873 K for 4 h. The entrapped SiC particles can inhibit the glide defor-

mation of the alumina grains above 1273 K and this increases the temperature

limit for bending strength, as shown in Figure 17.12 [28] .

400 800 1200 1600

200

400

600

800

1000

Alumina/15 vol% SiC composite

Monolithic alumina

Temperature (

°C)

Flexural strength (MPa)

Figure 17.12 Temperature dependence of the ﬂ exural strength

of alumina containing 15 vol% SiC particles, which are

entrapped in the alumina grins, compared with that of

monolithic sintered alumina.

17.6 Valid Conditions 567

17.6

Valid Conditions

17.6.1

Atmosphere

The annealing atmosphere has an outstanding inﬂ uence on the extent of the crack

healing and the resultant strength recovery, as shown in Figure 17.13 [29] .

From the ﬁ gure, it can be clearly seen that the presence of oxygen causes the

self - healing phenomenon, as the crack healing is driven by the oxidation of SiC.

However, the threshold oxygen partial pressure can be expected to be quite low.

Therefore, the self - crack healing must be valid in the atmosphere, except in deoxi-

dized conditions, for example, the atmosphere containing hydrogen. Also embed-

ded ﬂ aws cannot be healed, because the SiC particles present in the embedded

ﬂ aws cannot react with oxygen.

Annealings in vacuum, argon (Ar), and nitrogen (N

) result in a slight strength

recovery. Wu et al . [20] discussed this phenomenon to be the release of the tensile

residual stress at the indentation site. Furthermore, Fang et al . [38] used a satellite

indentation technique to show that, after 2 h at 1573 K, the degree of the annealing -

induced relaxation in the stress intensity factor of the residual stress at the indenta-

tion site was ∼ 26% for alumina/5 vol% SiC nanocomposite. Using this result,

one can predict that annealing in atmosphere without oxygen leads to 10%

strength recovery.

Recently, two large efforts [39, 40] made to know the quantitative inﬂ uence of

oxygen partial pressure on the self - healing behavior. From the results, the crack -

healing rate is signiﬁ cantly decreased with decreasing oxygen partial pressure, as

shown in Figure 17.14 , in which the bending strengths of the alumina/20 vol%

Air Vacuum Ar N

Atmosphere

200

400

600

1000

800

Flexural strength (MPa)

Cracked strength

Smooth strength

Crack-healed strength

(1573 K, 1 h)

Specimens fractured

from the crack-healed zone

Figure 17.13 Crack - healing behavior of alumina/15 vol%

0.27 μ m SiC particles composite under several atmospheres.

568 17 Self-healing of Surface Cracks in Structural Ceramics

SiC particles composite crack - healed under several oxygen pressure atmospheres.

Moreover, the detail of the kinetics will be discussed later (Section 17.10 ).

17.6.2

Temperature

The ceramic components are usually operated at high temperatures. The self -

healing relies on the oxidation of SiC, thereby leading to the self - crack healing.

Thus, it is important to know the valid temperature range for self - crack healing.

As the crack healing is induced by chemical reaction, the strength recovery rate

decreases exponentially with decreasing temperature. For example, Figure 17.15

[28] shows the relationship between the crack - healing temperature and the strength

recovery for alumina/15 vol% 0.27 μ m SiC parades composite.

In order to completely heal a semi - elliptical crack of 100 μ m in surface length

within 1 h, heating above 1573 K is required. In the similar way, heating above

1473 and 1273 K is needed in order to completely heal the surface crack within 10

and 300 h, respectively. The relation between the crack - healing temperature and

the strength recovery rate follows Arrhenius ’ equation.

Figure 17.16 [29] shows the Arrhenius plots on the crack healing of several

ceramics having the self – crack - healing ability, in which the crack - healing rate is

deﬁ ned as the inverse of the time when complete strength recovery is attained at

Figure 17.14 Bending strength of the specimens crack - healed

at 1573 K under several oxygen partial pressures as a function

of the healing time, with the bending strength of healed

smooth specimen and the as - cracked specimen.