Glass-ceramic coatings for use on metal substrates

A glass-ceramic characterised in that it is capable of being fired repeatedly to temperatures of greater than 950 DEG C without undergoing thermal deformation, it exhibits a coefficient of thermal expansion (25-600 DEG C) of from 75 to 150 x 10 /C DEG , it is essentially free from alkali metal oxides and it comprises expressed in terms of weight percent on the oxide basis: from 10 to 60% BaO, from 5 to 30% B2O3, from 25 to 40% SiO2,from 0 to 15% Al2O3, from 0 to 15% CaO, from 10 to 35% MgO, from 0 to 16% ZnO and from 5 to 20% Al2O3 + CaO +ZnO is disclosed. More particular, the present invention relates to such a glass-ceramic in the from of a coating on a substrate, for example on a low carbon or titanium-stabilized stainless steel. In the case of the glass-ceramic in the form of a coating, a process for the production thereof characterised in that it comprises: (a) melting an appropriate batch; (b) cooling the melt to a glass and comminuting the glass to fine particles passing a No. 325 United States Standard Sieve (44 x 10@ m); (c) preparing a liquid slurry of the glass particles; (d) applying the slurry as a coating onto a desired substrate; and (e) firing the coating to a temperature of at least 850 DEG C substantially simultaneously to sinter the glass particles together into an integral, essentially non-porous coating and to cause extensive crystallization in situ to take place therein is also disclosed. Inter alia, the present materials have improved properties as regards the use thereof in the construction of circuit boards.