GB2172282A - Toughened glass-ceramics - Google Patents

Abstract

A divitrifiable glass is provided with the composition: MgO 10-30 wt. % Al2O3 10-40 wt. % SiO2 30-55 wt. % ZrO2 8-13 wt. % the balance (if any) consisting of compatible glass-forming oxides in a proportion up to 10% by weight. The glass can be thermally divitrified and heat-treated to form a zirconia transformation-toughened glass ceramic, which can resist high temperatures (typically 900 DEG C), is of high strength (typically 600MPa flexural strength under 4 point bending) and a dielectric constant in the range 7 to 10.

Description

SPECIFICATION Toughened glass-ceramics The present invention relates to glass ceramics of the MgO-AI2O-SiO2 type.

Glass-ceramics offer considerable advantages in microelectronic applications. Up to the present, the principal ceramic materials used as, for example, substrates have been beryllia (BeO) and alumina (AI2O2). Although BeO offers some advantages, principally arising from its high thermal conductivity, it suffers from the disadvantage of toxicity, particularly in powder form.

This means that the attention which has to be paid to health and safety measures during production increases costs significantly and, furthermore, potential users are reluctant to specify it in their components. Awl203 has been predominant in this field and whilst a great deal of progress has been made in its use there are limitations to its potential use in certain areas.

These are: (a) Owing to strength and brittleness characteristics there are limitations to the size of substrate which can be made and handled in the thickness required. Many substrates are--0.6mm thick although thickness up to about imm are used in some applications. Currently (100mm)2 substrates are made and up to (150mm)2 are possible.

(b) The relatively high dielectric constant of At203(~10) renders it unsuitable for high frequency work (frequencies greater than about 20GHz).

For many applications, capability of the substrate to withstand high temperatures (--900"C) as, for example, in the firing of thick film conductor and resistor inks, is necessary. This restricts the use of low dielectric constant polymers. Another alternative material, fused silica, which has a dielectric constant of 3.5, has found use in substrate applications but because of its low strength finds limited use.

Thus a need exists for strong glass-ceramics with dielectric constants in the range 5 to 10.

U.K. patent specification No. 1,108,476 (in the name of The English Electric Company Limited) discloses surface-devitrified glass-ceramics containing ZnO, Al2O3 and SiO2 as their principal constituents and typically exhibiting dielectric constant in the range 6.5 to 8.1. Zirconium dioxide, in a proportion of up to 7.5 weight per cent, is also disclosed as one of three possible nucleation catalysts. Surface devitrification increases the strength by inducing compressive stresses in the surface of the glass-ceramic material and is utilised in the preparation of glassceramics of the MgO-AI703-SiO2 type.

A further strengthening mechanism which can occur in glass-ceramics involves the transformation of certain oxides, such as zirconia (ZrO), between structurally different forms of different density. Thus zirconia exists in a tetragonal form (which is stable above a temperature of about 1000"C) and a low-temperature monoclinic form of lower density. It has been found that in certain ceramics the high temperature tetragonal form can be retained to room temperature provided that the zirconia is present as fine discrete particles. The effect is that in advance of an approaching fracture front, under the influence of the resultant tensile stress, the tetragonal zirconia transforms to the stable monoclinic form.The consequent increase in volume produces compressive stresses in the surrounding media which serve to halt or slow down the advancing fracture front. The material thus exhibits higher strength and increased fracture toughness. The material is thus said to be zirconia transformation toughened. However it has not previously been considered feasible to strengthen glass-ceramics of the MgO-AI203-SiO2 system by incorporating zirconia because in general, alkaline earths such as MgO tend to stabilise ZrO2 in the cubic form rather than the tetragonal form.

We have unexpectedly found that high-strength glass-ceramics of the MgO-AI203-SiO2 type can be prepared if a high proportion of zirconium dioxide is included in the composition and converted to the tetragonal form by suitable heat treatment.

According to one aspect of the present invention, a devitrifiable glass has a composition within the range: MgO 10 to 30% by weight Altos 10 to 40% by weight SiO2 30 to 55% by weight ZrO2 8 to 13% by weight the balance (if any) consisting of compatible oxides in a proportion up to 10% by weight.

According to another aspect of the invention, a glass having a composition within the above range is thermally devitrified by being heated to a temperature in the range 500"C to 800"C for a period so as to nucleate the crystal phases and is then heated for a further period at a temperature between 800"C and 1100 C before being cooled to form a glass-ceramic with the zirconium dioxide substantially in its tetragonal form.

The limits on the ZrO2 content are set by the constraints that less than 8 weight per cent will not provide adequate nucleation to enable a glass-ceramic with a fine crystal structure to be developed, and greater than 13 weight per cent goes beyond the solubility limits of the zirconia.

We have found that a surface layer of different phase composition is formed during heattreatment so that glasses in accordance with the invention, when powdered and subsequently sintered (typically at a temperature between 900"C and 1150 C) form a body of glass-ceramic material in which the predominant crystal phases are those of the surface layers found on the bulk crystallized materials. The development of the high prportion of the surface phases results from nucleation on the surfaces of the powder particles. The resultant glass-ceramic has a lower thermal expansion than the equivalent bulk crystallized material and is capable of being applied as a coating onto a suitable substrate (e.g. alumina) and is also compatible with gold and copper thick film conductor tracks.

The invention will now be described by way of example only with reference to the accompanying tables I to Ill.

Batches to provide glasses of the required compositions were made up by weighing out and mixing raw materials as follows: MgO from magnesium oxide, magnesium carbonate AI2O3 aluminium oxide SiO2 quartz, sand, zircon ZrO2 zirconium dioxide, zircon ZnO zinc oxide The mixtures of raw materials were melted, preferably in platinum/rhodium alloy crucibles, at temperatures in the region of 1500-1650"C. When seed free and homogeneous, the glasses were shaped to desired shapes using conventional glass forming techniques.

Specific examples of the glass compositions made up are listed in Table I.

TABLE I

GLASS-CERAMIC COMPONENT (PROPORTIONS IN WEIGHT %) NO. SiO2 A1203 MgO Zero2 P205 ZnO 1 43.7 22.2 22.3 11.8 - 2 41.0 29.4 18.4 - 11.2 - - 3 39.7 33.7 15.7 10.9 - - 4 47.0 16.0 24.8 12.2 - - 5 43.0 21.9 21.9 11.6 1.6 - 6 45.4 15.5 20.8 11.8 - 6.5 The glass shapes were then subjected to heat-treatment schedules involving holding stages of firstly, 500"-800"C to provide the desired nucleation, and secondly, 800"C-1 1 000C to develop the required crystalline structure.

Specific examples of the heat-treatment schedules given to selected glasses are illustrated in Table II.

TABLE II

GLASS NO. HEAT-TREATMENT SCHEDULE 1 5500C/10 hrs + 8800C/10 hrs ~ 550 C/10 hrs + 880 C/10 hrs 1 550 C/10 hrs + 950 C/10 hrs 1 5800C/20 hrs + 10000C/20 hrs 2 5500C/20 hrs + 910 C/20 hrs 2 5800C/10 hrs + 10500C/10 hrs 3 5600C/20 hrs + 9O00C/10 hrs 4 650 C/12 hrs + 8600C/12 hrs 5 6000C/20 hrs 1 880 C/20 hrs 5 7700C/20 hrs 1 9000C/20 hrs 6 5500C11O hrs + 975 C/lO hrs The glass-ceramics of the present invention possess high strengths, flexural strengths under 4 point bending in the region of 600MPa having been measured as mean values on a set of samples (compared with the modulus of rupture of alumina of--300MPa). The dielectric constants of the glass-ceramics fall in the range 7-10 and the materials have high electrical restivities. Specific examples of properties of the glass-ceramics are given in Table III.

TABLE III 4 point Thermal Glass bend Expansion Ceramic strength Coefficient Dielectric No. Heat-Treatment (MPa) (10 7 "C l) Constant 1 5500C/20 hrs+910 C/20 hrs 600 70 8.9 2 560"C/20 hrs+910 C/20 hrs 250 77 8.7 3 560"C/20 hrs+910 C/10 hrs 300 68 8.6 4 560"C/20 hrs+880 C/20 hrs 370 82 8.8 5 580"C/20 hrs+880 C/20 hrs 480 71 8.8 6 550 C/10 hrs+975 C/10 hrs 280 101 9.6 The fine grained structures of the glass-ceramics enable good surface finishes to be obtained and the materials are compatible with thick film gold and copper conductor inks, which can be fired on at temperatures in the region of 900"C.

X-ray diffraction analysis has revealed that the MgO-AI203-SiO2 glass-ceramics of the present invention contain the following crystal phases; the phases developing being influenced by composition and heat-treatment given to the materials.

clino-enstatite ss-quartz ss-quartz solid solution monoclinic zirconia tetragonal zirconia The glass-ceramics are nucleated and crystallize throughout the bulk, but an interesting phenomenon found with some compositions is the develoment of a surface layer with differing proportions of the crystal phases. It has been shown that the surface layer formed has a lower expansion than the bulk and so induces compressive stresses in the surface with resultant increase in strength. However, it has been further found that if the surface crystal layer is removed by grinding, the residual bulk crystallized material is stili of high strength arising from the influence of tetragonal zirconia.

The materials of the present invention, when reduced to powder form and subsequently sintered, form strong glass-ceramic bodies with properties differing from those materials processed in bulk form. Specific examples of sintering schedules and properties of selected ma terials are shown in Table IV.

TABLE IV 4 point Thermal bend expansion strength coefficient Dielectric Glass No. Sintering Schedule (MPa) (10-70C-') constant 1 900"C/5 minutes 130 40 7.2 1 55O0C/lOhrs+10000C/lOhrs 165 77 7.2 4 550 C/1Ohrs+ 1000 C/1Ohrs 190 90 8.7 6 550 C/10hrs+ 1000 C/10hrs 220 102 8.5

Claims (6)

1. A devitrifiable glass having a composition within the range MgO 10-30% by weight Al2O3 10-40% by weight SiO2 30-55% by weight ZrO2 8-13% by weight the balance (if any) consisting of compatible oxides in a proportion up to 10% by weight.

2. A glass-ceramic made from glass according to Claim 1 wherein the zirconium dioxide in said glass-ceramic is substantially in a tetragonal crystalline form.

3. A body of glass-ceramic according to Claim 2 wherein the surface layer and the bulk contain different proportions of the crystal phases such that the coefficient of thermal expansion of the surface layer is lower than that of the bulk.

4. A method of forming a glass-ceramic wherein the zirconium dioxide is substantially in its tetragonal crystalline form comprising the steps of devitrification of a glass by heating to a temperature in the range 500 to 800"C for a period sufficient to nucleate the crystal phases, heating for a further period at a temperature in the range 800 to 1100 C, and cooling to form said glass-ceramic, said glass having a composition within the range: MgO 10-30% by weight Awl203 10-40% by weight SiO2 30-55% by weight ZrO2 8-13% by weight the balance (if any) consisting of compatible oxides in a proportion up to 10% by weight.

5. A method of forming a glass-ceramic with a coefficient of thermal expansion in the range 30 to 125X10 70C t comprising the steps of reducing to powder a glass according to Claim 1, and sintering said powder at a temperature in the range 800"C to 1150"C.

6. A method of forming a glass-ceramic substantially as hereinbefore described.