CS198372B1 - Vitreous crystalline material and process for preparing thereof - Google Patents

Abstract

Vitreous ceramic, having high bending strength and resistance to chemicals and low heat expansion, is prepd. by doping a glass compsn. contg. by wt. 45-70% SiO2, 15-30% Al2O3 and 5-15% MgO, with 5-15% NiO, pref. replaceable opt. completely, by 1-15% Fe oxide. A finely crystallised sheathing zone having smaller heat-expansion than core is obtd. by cooling melt to 600 degrees C, re-heating to 800-850 degrees C at 6-7 degrees C/min., pref. keeping 1-5 hrs. at 800-850 degrees C, then heating at 3-5 degrees C/min. to 950-1030 degrees C, and keeping at this temp. for =2 hrs., and cooling at 5-9 degrees C/min. to room temp.

Description

The invention relates to a vitreous crystalline mass having a partially crystallized core surrounded by a layer of lower extensibility, thereby creating a compressive stress in the surface layer.

The glassy crystalline masses are obtained by controlled crystallization of the starting glass, which is converted into a finely crystalline glass product with a controlled thermal treatment. This product differs in many physical and chemical properties as a rule advantageously from the starting glass. This is particularly evident in the mechanical strength, which can double twice as much as the silicate starting glass is ceramicized. However, this value is in many cases insufficient for practical purposes, so additional reinforcement of this material is desirable.

So far, silicate glasses and glassy materials have achieved only a small fraction of the theoretically expected bending strength. Since there is no possibility of absorbing concentrated energy at the tip of the crack, for example by plastic molding, in the glass compared to the metal, the crack runs unstoppably through the glass body at the same load.

The methods of consolidating the glassy material therefore have a dual purpose. Firstly, to make crack growth difficult. This can be achieved by eliminating fine crystals inside the glass as observed with glassy crystalline masses. Second, there is an effort to reduce or eliminate crack formation in advance. This is usually done by creating a permanent compressive stress on the surface, which must first be overcome if the crack is to expand.

To date, it has been known to produce this compressive stress in the surface by heat curing the glass body, i.e. by rapidly supercooling the surface portion. In the last . At the time, surface ion exchange methods have been developed which also lead to the desired pressure stress. However, the low basic strength of the glasses remains unchanged. Attempts to subject the reinforced vitreous crystalline mass to a similar treatment have been made recently. It was preferred either to exchange the ions as in the case of glass, or to strengthen them by forming a crystalline surface layer with less thermal expansion.

Glassy crystalline masses strengthened by surface ion exchange have the disadvantage that the chemical stability of the product is greatly reduced by the necessary incorporation of alkali. In this process, chemical strengthening means an additional and costly production process.

Attempts to solidify the alkali-free glassy crystalline mass with a less thermally expandable crystal shell resulted in a maximum strength of 280 MPa and coefficients of expansion greater than 100 at temperatures of 16-18 hours above 1000 ° C. K ' ⁷ K ¹ .

This tempering program is very expensive in terms of time and temperature. Furthermore, the unfavorable combination of high strength and high coefficient of expansion, which is the tendency observed with most glassy crystalline materials, is confirmed for many applications.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a glassy crystalline mass characterized by high flexural strength, low thermal expansion and economical heat treatment (i.e. low working temperature or short tempering time) and consisting of a finely crystalline envelope portion of relatively low thermal expansion. which surrounds the partially crystallized core of higher thermal expansion and thus is under compressive stress.

In this treatment, a (Ni, Mg) Al 2 O 4 spinel composition is formed in the core of the material, which together with a considerable remainder of the base glass represents a system of relatively higher thermal expansion. On the other hand, the surface layer 30 to 90 μΐη thick, which is formed at the same time, consists of finely divided crystals of the mixed phase with a high quartz content of relatively less thermal expansion, which creates the desired pressure effect.

The subject of the invention is a glassy crystalline mass with high flexural strength - 460 MPa, high chemical stability, ie hydrolytic and leaching class 1, and low thermal expansion - 46.10 ' ⁷ K ^-1 , composed of 45 to 70% by weight of SiO2, 15 to 30% by weight of Al ₂ O ₃ , 5 to 15% by weight of MgO, and 5 to 15% by weight of NiO and / or iron oxides, consisting of a Ni-containing core -Al-spinel and a surface layer of 30 to 90 μπι thickness, containing mixed crystals with a high content of quartz, which layer has a lower extensibility than the core.

To vary the processing properties, the glassy crystalline composition of the invention may preferably contain traces of up to 3% by weight of boron oxide, traces of up to 4% by weight of calcium oxide and traces of up to 3% by weight of zinc oxide.

This surface layer is formed by the method of heating the glass starting body to 800 to 850 ° C and maintaining it at this temperature for 1 to 5 hours, then further heating to 950 ° C at a rate of 3 to ° C. min and after two hours of retention in this area, it is cooled to 800 ° C at a rate of 5 to 9 ° C / min and then to room temperature at a rate of 3 to 4 ° C / min, the surface determining layer being adjusted by changing the temperature program Thickness limits between 30 and 90 μτη while being applied to the core.

The glassy crystalline mass of the invention combines high flexural strength with high chemical stability and low thermal expansion.

Also, the process for producing the glassy crystalline material of the present invention has a number of technological advantages over the prior art, the most important of which is the cheaper production process.

The surface layer is formed in the last stage of the tempering process and adheres perfectly to the core with suitable temperature control. Its thickness determines the vulnerability of the sample and can be controlled by conducting temperature within these limits.

The coefficient of linear thermal expansion was measured on a Linsel dilatometer and was only with small variations for the temperature range of 100 to 400 ° C 44. K ' ⁷ K ¹ .

The measured object was cooled rods 3-4 mm long with a diameter of approx. 4 mm, which were ground on one face and polished on the other. The heating rate was 5 ° C / min.

To determine the leaching class, glass pieces with a defined total surface area of 10 to 20 cm ^{2 were} boiled in a silver container with a NaOH-Na ₂ CO ₃ solution under reflux. The surface mass loss in mg / dm ² was then determined. To determine the hydrolytic class, 2 g of glass meal of 0.315-0.5 mm in 50 ml of distilled water was heated to 100 ° C for 60 minutes. After cooling, 26 ml of the solution was titrated with 0.01 N hydrochloric acid to methyl red. Acid consumption was then a measure of hydrolytic stability.

According to these methods, samples are fermented with a maximum loss of 50 mg / cm ² unambiguously in hydrolytic class 1. They thus have excellent chemical stability. The bending strength was determined on 25 cooled and wiped bars with a diameter of 3.5 mm and a length of 60 millimeters. In order to obtain a uniformly damaged surface, a grain size of 0.06-0.10 mm was worked in a mixing drum at 50 rpm for 30 minutes. The bars were then broken in a three-point support at 50 mm cutting distance. The load rate was constant and was 2 N / s. The eventual oval of the bars was corrected in number. The flexural strength thus measured was 470 MPa. For comparison under the same conditions, the measured bars had a bending strength of 80 MPa. The prior art solidified glassy crystalline products either have low chemical stability due to the necessary alkali build-up in ion exchange, or have very high expansion coefficients of over 100 in the case of surface crystallization. 1O ⁷ K1).

The glassy crystalline mass according to the invention differs. . from the products described above by combining high flexural strength (460 MPa) with high chemical strength. stability (hydrolytic class 1, leaching class 1) and low thermal expansion (44. 10 ' ⁷ K ^-1 ). By the ability to adjust the thickness of the crystalline coating as desired, the applicability of the test rods can be controlled within predetermined limits.

Therefore, the production of the glassy crystalline materials of the invention has a number of technological advantages over the prior art.

Compared to the method of ion exchange hardening, a very expensive working process is eliminated. Compared to the production of other solid surface-coated glass ceramics, the tempering times are 6 hours at relatively lower temperatures of 800 to 1000 ° C.

Comparable good results are achieved by partial substitution of nickel oxide with iron oxide.

Examples

The glasses used for the exemplary embodiments were uniformly melted at temperatures of 1500 to 1550 ° C in a platinum crucible and drawn onto bars. Then heated to 600 ° C at a rate ^of up ^to 6 C / min and further processed as described in Table.

Cooling in Examples 3 to 5 was carried out such that it declined above 800 ° C. the temperature dropped continuously from 9 ° C / min to 4 ° C / min.

Below 800 ° C it was cooled to room temperature at a rate of 3 ° C / min.

Products obtained. have a finely divided (Ni, Mg, Al) spinel core with less than 50% by volume and a surface layer of mixed quartz crystals with a high content of quartz.

They all have a light green appearance. Except for the relatively coarse surface crystallization in Example 1, the resulting crystal layers were formed from very small individuals.

This leads to the desired high strength for the materials of Examples 2 to 5. Influence of the tempering program. the thickness of the surface layer is also shown in the table. Linear thermal expansion and chemical stability remained practically constant for Examples 2-5. The total retention time above 810 ° C did not exceed 6 hours, including the heating and cooling periods.

According to the described properties of the glassy crystalline materials according to the invention, significant progress has been made in comparison with the prior art in terms of increasing strength with excellent chemical stability, low thermal expansion, and a rational process.

and

Table Examples

Thickness of the surface layer 1 40 μΓΠ - 61 μΓη Hydrolytic and leaching class rH rH rH Coefficient of thermal expansion K * ¹ r- 1 O τ — 1 irí co tx 1 o rH Γχ 1 o rH WHAT 1 ^! Flexural strength MPa 280 o Гх 440 Tempering program 4 ° C / min to 810 ° C 22 h at 810 ° C 3 ° C / min to 950 ° C 1 h at 950 ° C 3 ° C / min to room temperature 4 ° C / min to 800 ° C 5 h at 800 ° C 3 ° C / min to 950 ° C 1 h at 950 ° C 3 ° C / min to room temperature 4 ° C / min to 810 ° C 3 h at 810 ° C 4 ° C / min to 950 ° C 2 h at 950 ° C 4-9 ° C / min cooling to room temperature Weight composition% ™ O o ry O 00 9 55 <N Z CO CQ CO rH CO CO CD CD rH • ^0.1 OOZ — O OO 9 55 <! SZ r-o oo CD o CO rH rH as in Example 2 Example # rH CM what

Thickness of the surface layer WITH Ю with 4. cm CO Hydrolytic and leaching class rH T-H Coefficient of thermal expansion K ¹ 1 O rH CM чф 1 o rH WHAT? Flexural strength MPa 410 430 ------------------_ Tempering program 4 ° C / min to 810 ° C 3 h at 810 ° C 4 ° C / min to 950 ° C Cooled to room temperature for 1 h at 950 ° C 4-9 ° C / min 4 ° C / min to 830 ° C for 1.5 h at 830 ° C 4 ° C / min to 950 ° C for 1.5 h at 950 ° C 4 ° C / min to 1000 ° C 4-9 ° C / min. Cooled to room temperature Weight composition% as in Example 2 as in Example 2 1 Example C. ID

SUBJECT

Claims (4)

1. Glassy crystalline mass with high flexural strength - up to 460 MPa, high chemical stability, ie hydrolytic and leaching class 1, and low thermal expansion - 46.10 ⁷ K ^_1 , composed of 45 to 70% by weight of SiO2, 15 to 70 30 wt.% Of Al ₂ O 0, 5 to 15 wt.% MgO and 5 to 15 wt.% NiO or iron oxide, consisting of a core containing Ni-Al-spinel and a surface layer thickness of 30 up to 90 μΐη containing mixed quartz crystals with a high quartz content, the layer having a lower expansion than the core. 2. A glassy crystalline mass according to claim 1, characterized in that it contains traces of up to 3 in order to change the processing properties I INVENTION by weight boric oxide, traces up to 4% by weight calcium oxide and traces up to 3% by weight zinc oxide. 3. Process for producing a vitreous crystalline mass according to claims 1 and 2, characterized in that the glass starting body is heated to 800 to 850 [deg.] C. and maintained at this temperature for 1 to 5 hours, then further heated to 950 to 1030 [deg.] C. 3 to 5 ° C / min and after a maximum of two hours in this area, cool to 800 ° C at a rate of 5 to 9 ° C / min and then to room temperature at 3 to 5 ° C 4 ° C / min, wherein the strength-determining surface layer is adjusted by changing the tempering program within predetermined limits to a thickness of between 30 and 90 μΐη while adhering firmly to the core.