Tripole university

Material and Metallurgical Department

Glass-Ceramic "MME660"

Home work "5"

Application of Glass -Ceramics

Prepared by :Hana Jamhour

D. Nibile Abed alwahab

PRESENT APPLICATION OF GLASS-CERAMICS IN SCIENCE,

ENGINEERING AND MEDICINE

The unique properties of glass-ceramics render them suitable for applications not only

in the technical engineering fields but also in production of consumer goods (1). The

first commercially viable glass-ceramics were developed in the aerospace industry in

the late 1950s were used to manufacture randomes to allow the use of and to protect

radar equipment (internal antennae) in the nose of aircraft and rockets. According to

McMillan (1979), materials used for this type application must exhibit a very

homogenous and low dielectric constant and low coefficient of thermal expansion ,

low dielectric loss, high strength, and high abrasion resistance. These properties are

also demonstrated by sintered Al 2O3 (2). In many cases, the selection of galas-ceramic

for particular application will rest primarily on one type physical characteristic and for

this reason the applications have been classified into groups depending on weather the

over-riding requirement is concerned with mechanical, thermal, or electrical

properties. Naturally, there will be some overlap in this classification and in those

cases, where a combination of properties is required, the classification has been made

on the basis of the more important physical characteristic for the particular

application. Many glass-ceramics have been produced in university and industrial

laboratories, but only a few have so far found commercial applications(2). And The

wide variety of applications include electric range tops, wood stove windows,

telescope mirrors, cooking utensils, dinnerware, building facing materials, precision

electronic parts, fluid amplifiers, inkjet printer heads, dental prostheses, and many

more can be grouped according to their type of crystal structure as shown in Table

"1"(3).

Table "1" glass-ceramic by primary crystal type and application

Some applications of glass-ceramic utilizing mechanical properties

1. Bearings

Glass-ceramics have potentialities for the manufacture of bearing and, a part

from the high mechanical strength which is necessary for this application, the

materials have other desirable properties including good abrasion resistance and

the ability to take a very smooth surface finish. In latter respect glass-ceramics

are superior to conventional ceramics, since the best surface finish attainable

for a 95 per cent alumina ceramic is 8 to 9 micro-inches where as glass-

ceramics have polished to have surface finishes of 1 to 2 micro-inches. The use

of glass-ceramics for bearing would probably be confined to application where

the operating conditions were particularly stringent. Glass-ceramic bearing

surface would be resistant to attack by corrosive liquids (e.g.-sea-water) and

this might therefore simplify the construction of bearing in pumps for handing

such liquids. The high abrasion resistance of glass-ceramics compared with

certain metal might permit bearings to operate with minimum lubrication and

this would be an advantage where lubrication is difficult due to inaccessibility

of the bearing during operation or under conditions where normal lubricants

would not function satisfactorily due to a high ambient temperature or other

cause. Wherever possible, of course, glass-ceramic bearings would be

lubricated by liquid or gas films. It is technically feasible to produced various

type of bearing such as ball or roller bearings or journal bearings using glass-

ceramics and to use techniques which permit metal shafts or housing to be clad

in glass-ceramic. Insufficient experience has yet been obtained to determine

which designs or techniques will give optimum results. In some cases the best

results may be achieved by having only one bearing surface made of glass-

ceramic, the other bearing surface being of a suitable metal. This would be in

accordance with normal practice for journal bearing where it is usual to make

the rotor and stator components from dissimilar metals to achieve minimum

wear.

2-Miscellaneous applications utilizing mechanical properties

Other applications where the abrasion resistance and durability of glass-

ceramics might be utilized with advantage include thread guides and godet

wheels in textile processing equipment. Spinnerets for the extrusion of synthetic

fibers might also be made from glass-ceramics. It has been suggested that half-

tone and intaglio printing plates could be made from to the desired from. The

use of glass-ceramics as binders for abrasives in the manufacture of grinding

wheels is also a distinct possibility. In this case, the glass-ceramic would

replace the usual glass bond and for certain types of grinding wheel this could

enable improved performance to be achieved. Another proposed application,

where the requirement is primarily for good mechanical characteristics, is the

use of glass-ceramic parts for internal combustion engines. Glass-ceramic

crowns for pistons may lead to engines. Glass-ceramic crowns for pistons may

lead to engines with increased life, and this possibility is being studied in

Russia (galina,1962) and elsewhere. Glass-ceramics, because of their durability

and the fact that they can be readily produced in a variety of shapes and sizes,

may find applications in architecture, especially as cladding or curtain walling

for buildings.

Some applications of glass-ceramic utilizing thermal properties

1-Cooking Wear

The largest application so far realized for glass-ceramics has been in the

production of cooking ware. One of the chief requirements here is for good

thermal shock resistance because of the rapid temperature changes which can

occur when heat is applied to removed from the cooking vessels. Glass-

ceramics, having thermal expansion coefficients less than 15x10-7 and high

mechanical strengths, posses more than adequate thermal shock resistance. For

this application. The high mechanical strength compared with that of

borosilicate heat-resisting glass confers an additional advantage in reducing the

probability of accidental breakage due to mechanical shock. since the high

mechanical strength of glass-ceramic is inherent and is not dependent on the

existence of a surface compressive layer as is the case for toughened glass

cooking ware, the possibility of catastrophic failure due to accidental

overheating of the vessel is eliminated. The extremely hard and a abrasion-

resistant surface of the glass-ceramic coupled with a smooth finish renders it

very hygienic and easily cleaned. In this respect, the glass-ceramic ware can be

superior to glaze may sometimes occur after prolonged use, permitting

permanent staining to occur; with glass-ceramics, the smooth surface is a

characteristic of the material and is not dependent upon the application of a

separate glaze layer.

A related application of glass-ceramics is in the field of table ware. Here again,

the outstanding surface durability of glass-ceramics confers distinct advantages,

and special materials have been described for this application in British patent

no 869,315(1961). These materials are prepared by devitrifying glasses made

up from calcium phosphate, silica and alumina and it is claimed that the glass-

ceramics produced have the appearance and characteristics of bone-china. The

chief advantage of these materials over those traditionally used would be that

high-speed mass-production methods are available for the shaping of table ware

of exceptionally high strength is already beginning in the united states.

2- Sealing and Bonding Media

An application of glass-ceramics which depends upon their thermal

characteristics is as sealing media or thermo-setting comments for use in the

conduction of electronic tube of various design where it is necessary to make

vacuum- tight joints. These joints may be between glass or ceramic and metal,

or between two glass parts such as the face- plate and cone of cathode-ray tube .

Until fairly recently, the joints were made simply by fusing the glass locally so

that it flowed and sealed to the metal or to another glass component. There are

certain disadvantages associated with this method, however, since loss of

dimensional accuracy due to softening of the glass can occur and also the fairly

high temperatures involved may cause damage to the internal structure of

components of the electronic device. For this reason a technique developed

some years ago, known as solder glass sealing, has come into use. In this

process a very thin layer of a low- melting glass( often a lead-zinc-borate type)

is used as a jointing medium, and sealing between adjacent parts is achieved at

relatively low temperatures. The solder glass flows and" wets" the metal or

glass parts at temperatures where the main parts of the envelope are still rigid.

Thus deformation of the envelop, leading to loss of dimensional accuracy, can

be avoided. The disadvantage of solder glass sealing is that the seal produced

will not withstand reheating to very high temperatures because the solder glass

must be kept at temperatures well below its sealing temperature well below its

sealing temperature to avoid softening. If the solder glass were devitrified to

convert it into a glass-ceramic it would then be possible to heat the seal to a

considerably higher temperature and this would confer great technical

advantages because the attainment of a high vacuum is dependent upon being

able to outgas the electronic tube at the highest possible temperature.

In one process, described by S.A. claypool (1959), glasses of the lead-zinc-

borate type having approximate weight percentage compositions:

PbO:70-80,ZnO:10-15;B2O3:6.5-10

Together with certain minor constituents such as Al2O3and SiO2 are used. The

glasses are employed in the form of fine powder which is made up into a

suspension with an organic vehicle and binder. The surfaces of the components

to joined are coated with this suspension by dipping or spraying. The coatings

may be prefired at this stage to increase their strength but the temperature for

this is controlled to avoid premature devitrification. The precoated surfaces are

placed in contact and the temperature is raised to the sealing temperature which

is in the range 430oc to 450oc; this temperature is maintained for about 30

minutes. During this time, flow of the glass occurs to accomplish sealing and

the glass devitrifies so that it becomes more refractory. Seals of this type can be

heated to temperatures within 20oc or so of the sealing temperature.

The lead-zinc-borate compositions are suitable for sealing together glasses

having thermal expansion coefficients in the range 80x10-7to 120x10-7 and one

commercial application has been in the sealing of face-plate to cones in the

manufacture of color tevsion tubes. Other compositions suitable for sealing

together glasses or other materials having relatively low thermal expansion

coefficients in the range 30x10-7 to 50x10-7 are available. These glass-ceramic

compositions are based on zinc- borosilicate glasses having weight percentage

compositions in range ZnO:60-65B2O3:20-25;SiO2:10-15

Plus certain minor constituents.

Some applications of glass-ceramic utilizing electrical properties

1- Insulators

Although glass-ceramics have not yet been widely used for the manufacture of

insulators there is no doubt that they possess the characteristic required for this

application since they have high surface and volume resistivities and are

resistant to surface tracking under arcing conditions. Their high dielectric break

down and mechanical strengths, compared with those of normal electrical

porcelains, permit the use of thinner sections resulting in weight savings and

giving increased freedom to designer. The smooth surfaces of glass-ceramics

which do not required glazing constitute a most valuable feature, since

insulators which are required to operate under polluted conditions can easily be

cleaned to restore their insulating characteristics. In many cases, the glass-

ceramic would be used as a simple insulator in some required shape but very

often it is necessary to join the insulator to metal components. These

components may be required to act as conductors or as mechanical supports or

attachments. Examples of this type of insulator-metal construction include

hermetic or oil-light bushing for transformer and capacitors and also various

types of disc and pod insulators. Glass-ceramics possess a distinct advantage

for the manufacture of metal-insulator assemblies of this type since their

thermal expansion coefficients can be closely matched to those of suitable

metals. This ensures that stresses generated in the insulator during temperature

cycling will be low and also permits actual sealing of the insulating component

to the metal parts, thus giving more reliable joints than are obtainable with

cementing or other conventional techniques.

2-Capacitors

The production of dielectric layers giving high capacitance per unit volume is

achieved more easily with glass-ceramics than with conventional ceramic

material where the high permittivity ceramics can be produced by conventional

techniques, but the use of the glass-ceramic process offer certain advantages.

One of the principle advantages is that the glass-ceramic composition(special

glass-ceramics containing Ferro-electric crystal phases having high

permittivities),and in its glass state, can be drawn into a very thin film by a

continuous process, whereas the manufacture of very thin plate of conational

ceramics is extremely difficult than glass-ceramics.

The process for making a glass-ceramic capacitor comprises stacking alternate

layers of the thin glass sheet and of a conducting metal, heating the assembly to

soften the glass and to fuse the edges of the glass laminations together and

afterwards heat-treating the assembly to crystallize the Ferro-electric

compounds. The relatively low dielectric losses, high dielectric breakdown

strengths and good insulation resistance of suitable glass-ceramics of this type

are valuable additional characteristics for this application (2).

Some applications of glass-ceramic in medical

During the past 30–40 years there has been a major advance in the

development of medical materials and this has been in the innovation of

ceramic materials for skeletal repair and reconstruction. The materials

within this class of medical implant are often referred to as“Bioceramics”

and the expansion in their range of medical applications has been

characterised by a significant increase in the number of patents and

publications in the field and an ever increasing number of major

international conferences and themed meetings. Bioceramics are now used

in a number of different applications throughout the body. According to the

type of bioceramics used and their interaction with the host tissue, they can

be categorised as either “bioinert” or “bioactive” and the bioactive ceramics

may be resorbable or non-resorbable. The materials used include:

polycrystalline materials; glasses, glass ceramics and ceramic-filled

bioactive composites, and all these may be manufactured either in porous or

in dense form in bulk, as granules or in the form of coatings(4). Bioactive

glass-ceramics form in-situ a biologically active layer of hydroxycarbonate

apatite (the mineral phase of bone and teeth) that bonds to bone and teeth

and sometimes even to soft tissue. Moreover, load bearing applications

require excellent mechanical properties. Many products have reached

commercial success: Cerabone A-W (apatite wollastonite), Ceravital

(apatite-devitrite), BioveritI (mica–apatite), Bioverit II (mica) and I

lmaplant L1 and AP40. They have been used as granular fillers, artificial

vertebrae, scaffolds, iliac spacers, spinous spacers, intervertebral spacers,

middle-ear implants and as other types of small-bone replacements. Some

of their interesting properties are listed in Table "2".

Table 2. relevant properties of bioactive glass-ceramic

Cerabone developed by Tadashi Kokubo and produced by Nippon Electric

Glass Co. Ltd. is probably the most widely used bioactive glass-ceramic for

bone replacement. Numerous clinical trials have shown intergrowth

between this glass-ceramic and human bone. Tadashi informed us in 2009

that about 50,000 successful implants already have been made using

Cerabone. Bioverits are machineable glass-ceramics that are very useful,

because they can be easily modified during clinical procedures. Bioverit II

is especially good in this respect. A different type of highly bioactive glass-

ceramic was developed by Peitl etal in 1995. This is a low-density glass-

ceramic in the Na-Ca-Si-P-O system that has a Young’s modulus closer to

that of cortical bone and much higher bioactivity than previous bioactive

glass-ceramics. This particular combination of properties is desired for

several applications. This glass-ceramic is about 30 to 50 percent

crystalline, and its main phase is Na2O.2CaO.3SiO2. The first clinical trials

for middle-ear bone replacements in 30 patients yielded very positive

results. Table I summaries the main properties of some bioactive glass-

ceramics. A new glass-ceramic based on the same Na-Ca-Si-P-O system

(Biosilicate) but with some compositional modifications and greater than

99.5 percent crystallinity recently was developed by Zanotto and

colleagues. This glass-ceramic is as bioactive as the “gold standard”

bioglass 45S5 invented by LarryHench. Clinical tests of treatment with

Biosilicate powder for dentin hypersensitivity in 160 sensitive teeth

conducted by dentist Jessica Cavalle are shown in Fig. 1. After the first

treatment, one-third of the teeth lost their sensitivity. After six applications

of Biosilicate powder, 94 percent of the teeth were cured. This powdered

glass-ceramic also can be useful for making small sintered bones and

bioactive scaffolds, such as those shown in the studies of Enrica Verné

and Aldo Boccaccini and their colleagues. Another interesting class of

bioactive glass-ceramics is heat-generating bioactive or biocompatible

glass-ceramics intended for use for hyperthermic treatment of tumors. For

instance, in one study by Koichiro et al., glass plates of he chemical

composition CaO-SiO2- Fe2O3-B2O3-P2O5 were ceramized. The resulting

glass-ceramic containing magnetite and wollastonite crystals showed high-

saturation magnetization. This glass-ceramic formed a calcium- and

phosphorous-rich layer on its surface and tightly bonded with bone within

about eight weeks of implantation. The parent glass did not form the

calcium- and phosphorous-rich layer and did bond with bone at 25 weeks.

Under an external magnetic field, granules of this glassceramic filled in

rabbit tibias heated surrounding bone to more than 42°C and maintained

this temperature for 30 minutes. Since then, this promising route for tumor

treatment has been followed by several authors. Several other compositions

have been and are presently being tested in various laboratories(5).

Figure " 1" micrographs of open and partially blocked dentin tubes Biosilicate glass-ceramic powder. RHS-results of a clinical study of dentin sensitivity level of 160 teeth: initial and after 1to 6 applications of Biosilicate.).

FUTURE OF GLASS-CERAMIC

at the present time advances are being made in the technology of glass-

ceramics and it is not likely that the pace of development will slacken in the

immediate future. An obvious field concerns the development of glass-

ceramics prepared from inexpensive raw materials. Some of the important

present-day glass-ceramics contain lithium oxide which is a relatively

expensive constituent and the development of glass-ceramics based on more

conventional raw materials would give economic advantages. In addition,

some of the present glass-ceramics require relatively high melting

tempratures for the preparation of the parent glasses and these tend to be

rather corrosive towards available furnace refractory materials, so that again

it would be advantageous if more conventional glass compositions could be

used. A further point is that most glass shaping machinery has been

developed for conventional glasses having viscosity- temperature

characteristics falling within a fairly narrow range. Clearly, glass-ceramic

compositions which exhibited these characteristics would be advantageous.

Further development of glass-ceramic compositions for specialized

application can be expected. Magnetic glass-ceramics containing be

developed and have the advantage that they could readily of formed into

fibers or thin films which would be extremely difficult with more

conventional ferrite ceramics. Another possibility is the development of

semiconducting glass-ceramics for the manufacture of resistors and other

devices. An extremely important field for future development concerns

surface crystallization of glasses. Materials of very high strength have

already been produced and the extension of technique to other types of glass

will certainly occur.one aim will be to use glasses prepared from

inexpensive raw materials and having good melting and working

characteristics. In other developments, the production of surface crystallized

materials which are completely transparent could provide a high-strength

replacement for conventional glass in many applications. Continued growth

of applications for glass-ceramics can be expected. In some of these

applications they may tend to displace conventional ceramics and glasses,

but it is equally likely that the outstanding characteristics of glass-ceramics

will enable them to be used in new fields so that the overall effect will be to

create an expansion in the use of ceramic or glass-type materials. Expansion

of the use of glass-ceramics for technical applications is likely to occur and

important field include vacuum tubes, miniature electronic components, high

voltage electrical insulation, special types of bearings and refractory coating

for the protection of metals. In additionto these rather specialized

applications, glass-ceramics will find increasing use in more general fields.

In building construction, for example, the durability and strength of these

materials could be of great value. It is even possible that load-bearing

structural members might be made from the high strength surface-

crystallized glasses with the advantage that they would be completely stable

against corrosion. One of the important uses for glass is in the manufacture

of containers (bottles,jars,etc.) and it is likely that the superior properties of

glass-ceramics will suggest their use in the field. One of the chief factors

here is an economic one, but if low-cost glass-ceramics are developed there

is no reason why they should not be widely applied. The high-strength

surface-crystallized materials may be more suitable for this application and

it should be possible to produce containers which are considerably lighter

and stronger than those being made at present. Glass-ceramics have already

secured an important place in the general field of materials technology and it

is clear that the extensive research and development efforts, which are being

applied in many countries, will ensure continued growth of the significance

of these materials(2).

Reference

1. Glass-ceramic technology /Second edition wolfram Hland george H. Beall/Copyright 2012 by The American Ceramic Society.

2. Glass-ceramics/PW McMillan/a cadmic press London New York/copyright second

printing 1964.

3.Hand Book of Ceramics, Glasses and Diamonds/ Charles. A. Harper. Editorin

chief /Copyright"1" 2001 by McGraw-Hill Companies/ http://books.google.com.ly.

4-Bioceramics: Past, present and for the future paper /science direct journal of European

ceramic society 2008 / Edited by S.M. Best a,∗, A.E. Porter b, E.S. Thian a, J. Huangc

5- Emerging ceramics and glass technology/October/November 2010/by Americ Society.