Glass-Ceramics Displaying Inherent Lubricity

Glass-ceramic articles are prepared through the controlled crystallization in situ of parent or precursor glass articles. Hence, the manufacture of glass-ceramic articles comprises three general steps. First, a batch of ingredients in the proper proportions (customarily including a nucleating agent) is compounded and melted. Second, the melt is simultaneously cooled to a glass essentially free from crystallization and an article of a desired geometry shaped therefrom. Third, the glass article is exposed to a heat treatment at temperatures above the transformation range thereof to cause the in situ growth of crystals. Commonly, this heat treatment is divided into two steps. Thus, the glass article will first be heated to a temperature within or slightly above the transformation range to develop nuclei therein, after which the article will be heated to a considerably higher temperature, frequently in excess of the softening point of the glass, to cause the growth of crystals on the nuclei.

Inasmuch as the crystallization process contemplates the essentially simultaneous growth of crystals on countless nuclei dispersed throughout the precursor glass article, the microstructure of a glass-ceramic article customarily consists of relatively uniformly-sized, fine-grained crystals homogenously distributed, but randomly oriented, within a residual glassy matrix. Conventionally, the crystal phase constitutes the predominant proportion, i.e., greater than 50% by volume, of a glass-ceramic article. Such high crystallinity normally leads to glass-ceramic articles exhibiting chemical and physical properties quite different from those of the precursor glass body and more closely resembling those of the crystal phase. Furthermore, the residual glassy matrix will be small in amount and of a very different composition from that of the parent glass, since the constituents making up the crystal phase will have been removed therefrom during the crystallization process. Finally, the presence of a residual glassy matrix renders the glass-ceramic article free of voids and non-porous.

U.S. Pat. No. 2,920,971, the basic patent in the area of glass-ceramics, provides an extensive discussion of the mechanism of crystal growth and the practical considerations involved in the manufacture of glass-ceramic articles. Reference is made to the patent for a general understanding that the crystal phases developed in glass-ceramic articles and the amount of such crystallinity are dependent upon the parent glass composition and the heat treatment parameters to which the parent glass is exposed.

The present invention is grounded in the discovery of a new family of glass-ceramic articles which manifests hydrophobic behavior and an oily or a slippery feel to the touch, while exhibiting a very low coefficient of friction and wear constant. Operable compositions for such articles consist essentially, by weight on the oxide basis, of about 12-40% MgO, 3-15% Al.sub.2 O.sub.3, 35-55% SiO.sub.2, and 5-15% F. Up to about 25% total of various modifying oxides can be incorporated in the base compositions. Such oxides include BaO, CaO, Fe.sub.2 O.sub.3, K.sub.2 O, MnO, Na.sub.2 O, PbO, Bi.sub.2 O.sub.3, and SrO. In general, modifying oxides which tend to produce mica, e.g., K.sub.2 O, Na.sub.2 O, SrO, and BaO, will preferably be held below about 5%. However, CaO and PbO may be present in individual amounts up to 20%, and MnO, Bi.sub.2 O.sub.3, and Fe.sub.2 O.sub.3 in individual amounts up to 15%. Network formers such as B.sub.2 O.sub.3 and/or P.sub.2 O.sub.5 may be present in amounts up to about 10%.

The property of lubricity appears to be associated with the humite mineral norbergite (Mg.sub.2 SiO.sub.4.MgF.sub.2), inasmuch as glass-ceramic articles containing such crystals as the major phase display that characteristic. Chondrodite (2Mg.sub.2 SiO.sub.4.MgF.sub.2) is, perhaps, the basic member of the humite minerals, but it is well-recognized that a number of ions can replace Mg.sup.+2 ions in the octahedral position of the humite and structurally-related olivine group minerals. The incorporation of such ions in the glass-ceramic composition can accentuate the quality of lubricity exhibited by the final product. Especially effective for that purpose are the ions Pb.sup.+2, Sr.sup.+2, Ca.sup.+2, Mn.sup.+2, and Fe.sup.+2.

The inclusion of a necleating agent in the compositions is not necessary, although the addition of such conventional nucleating agents as TiO.sub.2, ZrO.sub.2, and SnO.sub.2 does not appear to adversely affect the properties of the final product. Whereas the crystallization mechanism operating in the inventive compositions is not thoroughly understood, it is believed that the crystallization process involves two steps. First, fluoride-containing crystallites (probably MgF.sub.2 -sellaite) develop at temperatures about and somewhat above the transformation range of the parent glass. Second, upon further heating, the humite-type crystals grow on those nuclei.

The method of the invention comprises three general steps: (1) a batch of a predetermined composition is melted; (2) the melt is simultaneously cooled to a temperature at least below the transformation range of the glass and a glass article of a desired configuration shaped therefrom; and (3) the glass shape is heated to a temperature between about 850.degree.-1150.degree. C. for a sufficient length of time to achieve the in situ crystallization of norbergite-type crystals.

Because of the crystallization phenomenon is both time and temperature dependent, it can be appreciated that only brief dwell periods will be demanded where temperatures at the upper extreme of the crystallization range are employed; whereas, in the cooler extreme of the range, much longer exposure periods will be required. For example, as brief a period as 0.5 hour may be adequate at temperatures in the vicinity of 1150.degree. C., but as long as 24 hours may be needed at the coolest extreme of the range.

The preferred crystallization heat treatment practice contemplates a two-step schedule. Thus, the precursor glass article is initially heated to a temperature somewhat above the transformation range thereof, i.e. about 750.degree.-850.degree. C., and maintained therewithin for a period of time adequate to assure good nucleation. Subsequently, the temperature is raised to about 850.degree.-1150.degree. C. and held therewithin for a sufficient period of time to complete substantial crystal growth. A nucleation period of about 1-6 hours followed by a crystallization hold for about 1-8 hours has been adjudged to constitute the most preferred embodiment.

It will be appreciated that no specific dwell period as such at any one temperature is required within the nucleation and crystallization ranges. However, to insure the desired high crystallinity of very fine-grained crystals, the precursor article will be retained within the prescribed temperature ranges for the stated times.

U.S. Pat. No. 3,801,295 describes the production of glass-ceramic articles wherein synthetic fluormica constitutes the predominant crystal phase. The microstructures of the articles consist of very large (>150 microns), essentially two-dimensional crystals of high aspect ratio distributed throughout the glass matrix. The presence of such easily cleavable flakes imparts excellent machineability, high fracture energy, and good thermal shock damage resistance.

That patent mentions the development of norbergite and mullite as preceding the formation of the desired fluormica crystals. The crystalline assemblage of norbergite and mullite transforms to fluormica as the heat treatment of the precursor glass is continued such that the microstructure and properties of the final product are far removed from those of the present inventive articles.

Table I lists compositions, expressed in terms of parts by weight on the oxide basis, which are operable in the instant invention. Since the sum of the individual components of the several examples totals or closely approximates 100, the examples can reasonably be deemed to be recited in weight percent. Also, because it is not known with which cation(s) the fluorine is combined, it is simply recorded as MgF.sub.2, the batch ingredient employed to provide the fluoride content. It will be appreciated, of course, that other fluoride compounds would be equally useful as batch materials.

The batch ingredients, other than the fluoride component(s), can be any materials, either the oxides or other compounds, which, when melted together, will be converted into the desired oxide in the proper proportions. The batch ingredients were compounded, ballmilled together to assist in obtaining a homogeneous melt, and thereafter melted in closed platinum crucibles for about 4-6 hours in a furnace operating at about 1400.degree.-1450.degree. C. The melts were poured into steel molds to yield slabs about 6 .times. 6 inches .times. 1/2 inch and those slabs immediately transferred to an annealer operating at about 600.degree.-650.degree. C.

The loss of fluoride due to volatilization during the melting step is relatively low, ranging between about 10-25% by weight.

After annealing the glass slabs and inspecting them for glass quality, they were transferred to an electrically-fired furnace and there exposed to the heat treatment schedules listed in Table II. In each instance, the article was heated at about 200.degree.-300.degree. C./hour to the recited dwell temperatures. More rapid rates of heating are possible, particularly where formers or other physical supports are employed to inhibit deformation of the articles. However, that rate of heating avoided warpage or other body distortion throughout the entire operable composition range.

It should be observed that in these laboratory examples the glass slabs were cooled to room temperature to permit inspection of glass quality prior to being exposed to the crystallization step. However, that practice is not required for successful operability of the invention. Rather, the glass shapes can merely be cooled below the transformation range thereof and then immediately reheated to induce nucleation and crystallization.

Upon completion of the heat treatment schedules, the electric current to the furnace was cut off and the crystallized articles permitted to cool to room temperature within the furnace. This practice, undertaken as a matter of convenience only, has been termed "cooling at furnace rate". This mode of cooling has been estimated to be at a rate of about 200.degree.-300.degree. C./hour. The coefficient of thermal expansion for the crystallized articles over the range of 25.degree.-300.degree. C. averages between about 60-90 .times. 10.sup.-7 /.degree. C. Hence, more rapid rates of cooling are entirely feasible without hazarding cracking or spalling of the articles, particularly with relatively thin-walled articles.

Table II also records a visual description of the crystallized body, the crystal phases identified via X-ray diffraction analyses, listed in order of decreasing amount present, and measurements of static coefficient of friction and wear constants (x10.sup.-6 gram/gram second) as determined via a modification of ASTM Designation D3028-72. The static coefficients of friction were measured at ambient temperatures (about 21.degree.-24.degree. C.) and relative humidities (about 50-58%) utilizing a 50 gram load. The wear constants were also measured at ambient temperatures and relative humidities with a 50 gram load employing a sliding speed of 300 cm/second. In each test, two pieces of the product were rubbed against each other.

Comparison tests were run utilizing three different articles: (1) a borosilicate glass, Corning Code 7740, sold under the trademark PYREX.RTM. for laboratory glassware; (2) a glass-ceramic material, Corning Code 9608, sold under the trademark CORNING WARE.RTM. for culinary ware; and (3) the precursor glass body before it was crystallized into the glass-ceramic Corning Code 9608. The approximate analyses in weight percent of Corning Code 7740 and Corning Code 9608 are recited below:

The heat treated products were highly crystalline, i.e., greater than 50% by volume and, in the majority of instances, about 50-75%. The crystals were very fine-grained, essentially all having diameters less than 2 microns with a large majority having diameters less than 1 micron.

An examination of Table II points out the lubricity inherent in the inventive products plus, very importantly, a very low wear constant, which clearly indicates the excellent resistance to wear demonstrated thereby. Thus, the inventive products exhibit a resistance to wear of at least about two orders of magnitude better than the "conventional" glass-ceramic, and even greater when compared with glass.