Coating Technology

The prime objective of a coating is to achieve a combination of the best properties of the substrate and the coating, while diminishing their less favourable characteristics. In the case of platinum coatings, the aim is to retain the mechanical properties of the substrate and confer on it the beneficial environmental resistance provided by platinum. By coating with platinum, the poor corrosion resistance of the substrate is improved, while the high density and the cost are lowered relative to those of bulk platinum. Mechanically cladding a substrate with platinum goes part way towards this ideal, but the lack of an integral bond between the substrate and the cladding means that the two materials act independently. However, the intimate nature of A.C.T.^™ coatings enables them to develop synergistic characteristics with the substrate.

At high temperatures the use of coatings or claddings is potentially limited by their interaction with the substrate. This interaction is pre-dominantiy controlled by diffusion and can lead to rapid degradation of the coating, with significant loss of properties (5). The whole A.C.T.^™ process has been modelled by computer using available diffusion data (6) and standard equations. This enables predictions to be made of the likely behaviour of various combinations of materials, over chosen temperature ranges and times, although these can only serve as a general guide to the actual performance of the materials under service conditions.

One example of a computer modelled prediction of how nickel and chromium could be expected to diffuse through a platinum coating is shown in Figure 3. The prediction indicates that after only 24 hours at 1200°C there would be extensive diffusion of both base elements into a 10 μm platinum layer. Actual nickel-chromium substrates, plated with 15 and 25 μm of platinum, are shown in Figure 4 after partial immersion for 24 hours in molten glass at 1200°C. The platinum coatings have been degraded by the outward diffusion of nickel (indicated by the green coloration) and also by the effects of thermal stress, leading to total disruption in the case of the thinner coating. Thus experimental behaviour was comparable with the predictions of the model, which was subsequently used to develop more complex coating systems able to overcome the problem of diffusion. Following extensive laboratory experiments and trials of systems selected on the basis of predictions from the models, a complex coating system designed to overcome the effects both of diffusion and differential thermal expansion between substrate and coating has been developed.

The make-up of the coating system is dependant upon the substrate to be protected and the intended service environment. For example, to protect nickel-chromium type substrates the coating consists of several distinct layers, each of which has one or more functions:

The total coating thickness may be as low as 250 μm, but it can be substantially greater. Of this thickness, 150 to 200 μm is usually platinum or a platinum group metal, although this could be varied significantly depending on the specific service requirements.

Testing the Coatings

The A.C.T.^™ coating system was designed to enhance the surface properties of substrates with inherently good mechanical properties and/or thermal stabilities at high temperatures. Considerable laboratory testing of such coatings was undertaken before the work was extended to include field trials of protected nickel-chromium alloys. This work was performed in fore-hearths of major glass container companies in the U.K. and in Australia. With typical molten glass temperatures of between 1100 and 1200°C, substrate protection was achieved for up to the maximum test time of 4500 hours, Figure 5.

It was recognised that this technology was limited by the strength of the substrate material at high temperatures, which in the case of nickel-chromium alloys imposed an upper temperature limit of 1200 to 1250°C. This temperature maximum would limit the applications of the coated materials to only a fraction of their potential uses in the glass industry. Attention was therefore transferred to the development of protection for higher temperature substrates, namely the refractory ceramics.

A.C.T.^™ Coatings on Ceramics

The technology developed to provide protective coatings for nickel based alloys was extended further to allow its application to some of the many varieties of refractory ceramic substrates (7). Mullite tubes, closed at one end, were used as substrates in the first trials, which were performed at temperatures up to 1350°C. During this phase of laboratory testing the thickness of the platinum in the protective coating system was between 150 and 400 µm. Failure of the alumina crucibles used to contain the molten glass limited the duration of tests to about 500 hours at 1200°C, and to 100 hours at 1350°C.

Components coated using the A.C.T.^™ system were subjected to extensive investigation and analysis in tested and untested conditions. Only after this were field tests undertaken using A.C.T.^™ on tri-level thermocouple assemblies, see Figure 6. The early field tests were undertaken in a forehearth operated by a glass container manufacturer, and the first components were subjected to temperatures of up to 1200°C in molten flint, amber and green glasses, in succession. Sufficient confidence was gained in the performance of these coated tri-level thermocouples to allow their use as part of an integrated system for glass temperature control. Now in its eleventh month of use, the A.C.T.^™ coated assembly is still “problem free”.

An example of a mullite substrate protected by a 10 per cent rhodium-platinum A.C.T.^™ system is shown in Figure 7.

Following the first field tests, further components have been tested in forehearths at various glass plants located around the world. These trials have involved mullite substrates, and various thicknesses and compositions of platinum group metal A.C.T.^™ coatings have been used to provide the protection over different lengths of the component. All have performed extremely well, and have even shown some unexpected additional benefits. For example, one user has observed significant reductions in the “noise” associated with thermocouple readings, compared with that from conventional products having the same protected length.

Although much of the research and development work used mullite as a substrate, other refractories have also been successfully protected by A.C.T.^™ systems. These have ranged from high purity alumina to almost pure silica, including materials such as zircon-mullite and alumina-silicates. Work to extend the variety of substrates which can be protected by A.C.T.^™ systems is now being undertaken.

Within the glass industry this technology could be applied to the protection of a wide variety of components including the rotors for fibre glass production, bubbler tubes, electrodes, skimmer blocks, flow control devices, gobbing units, stirrers, orifice rings and other special delivery-forming equipment and furnace furniture. Commercialisation has now begun, primarily for thermocouple sheathing.

The Tri-Level Thermocouple Assembly Design

The protection of thermocouples is the starting point for the commercial application of A.C.T.^™ technology. This is particularly directed at the arduous service environments associated with the intrusive monitoring of glass temperatures either in forehearths or in melting furnaces (8, 9). One important sub-group of thermocouples is the tri-level thermocouple.

In this particular application the benefits resulting from the use of A.C.T.^™ on thermocouple assemblies in preference to conventional cladding include the ability to customise a standardised basic assembly design with:

The Financial Benefits of Using A.C.T.^™

In addition to the technical benefits resulting from the integration of the substrate and the protective metal coating, the A.C.T.^™ system also confers the potential for significant financial benefits to accrue from the more effective use of the platinum group metals. First, protective A.C.T.^™ coatings can be prepared at thicknesses down to 175 μm, which compares favourably with the lower limits for conventional cladding procedures, which are about 350 to 400 μm. Secondly, platinum group metal layers of 400 μm thick, or more, can easily be achieved by A.C.T.^™ over critical areas of components where enhanced durability or other specific properties are required for a particular application. Importantly, A.C.T.^™ technology enables the thickness of the platinum group metal coating, and thus the protection provided, to be matched to the various service conditions encountered at different parts of the same component.

Typical dimensions of A.C.T.^™ and conventional coatings used on a traditional design of tri-level thermocouple sheath are shown in the Table, while the cost benefits, see Figure 8, which may result from the more effective use of the platinum group metals can be deduced from the Table by a comparison of the relative weights of the metals used.

Gathering Balls

The transfer of molten glass into moulds can be accomplished by a number of techniques, with the selection being based on the volume and speed of the required transfers. At low rates a gathering or transfer ball is used (10); this utilises the rotation of the ball and the viscosity of the melt to achieve “carry over” of the glass. Traditionally iron balls were used – a natural extension of the blowing iron, but more recently ceramic balls were introduced, mainly to eliminate problems resulting from contamination of the glass with iron. These balls are available in a wide range of sizes and compositions but all have some basic disadvantages, such as being susceptible to corrosion, the severity of which depends on the composition of both the ball and the glass. This corrosion may result in the production of blisters and perhaps even stones in the glass. The gathering balls also tend to be susceptible to sudden failure due to thermal shock or thermal cycling, so they must be carefully preheated before use and maintained hot between campaigns. The operating regimes required, even when successful, cause considerable loss of production time.

Recent trials of gathering balls made of selected ceramic substrates clad with A.C.T.^™ platinum coatings, see Figure 9, have shown that considerable progress has been made in overcoming the problems discussed above. In addition to overcoming corrosion, gathering balls with coatings of 200 µm nominal thickness have demonstrated excellent glass transfer characteristics, have enabled start-up times to be minimised and have shown tolerance to repeated stop-start cycles.

To date trials have been performed in borosilicate glasses and in lead crystal, with operating temperatures in the range 1200 to 1400°C. To enable the gathering balls to be examined, trials have now been curtailed after several weeks of operation. An evaluation of the potential for using A.C.T.^™ on transfer balls is still being carried out, and the potential benefits of using platinum group metal alloys as alternatives to platinum are also being examined and assessed.

Other Components

One of the exciting aspects of the A.C.T.^™ coating technology is its versatility. It can be applied to a wide range of component shapes. Indeed the major benefits of the technology to users are expected to occur from the elimination or reduction of component failure, which otherwise results in major production downtime. Many of these components are complex in shape and therefore not amenable to cladding by the traditional techniques. On such shapes A.C.T.^™ will allow the utilisation of platinum group metals, and their use will be readily justified by the ensuing extension of component operating life and/or improvement in product quality.

A large component coated with 10 per cent rhodium-platinum alloy by A.C.T.^™ has been supplied to a major European glass manufacturer.

Summary and Conclusions

Despite their inherent disadvantages, notably brittleness, poor thermal shock resistance and poor resistance to molten glass corrosion, the refractory ceramics have for many years been the preferred materials of choice for equipment used in the glass industry. In many of their applications satisfactory performance is achieved by regular replacement. In other situations, platinum and its alloys, which are also accepted as essential materials for the fabrication of crucial components, are used to clad the refractory ceramics with a protective coating. The use of cladding is, however, limited by the complexity of the shape of the substrate which can be satisfactorily clad, and by the minimum thickness which can realistically be applied.

These restrictions can be overcome by A.C.T^™, so there will be opportunities for A.C.T.^™ platinum group metal coatings on ceramics to progress into areas of glass production technology where the cost of bulk platinum group metals has inhibited their use, or where the process of cladding is too unwieldy, or indeed technically non-feasible.

Platinum group metal A.C.T.^™ coatings have several advantages over cladding:

As in any industry, the glass manufacturers are not only looking to improve the quality of their product but also to achieve economies in production.

To these ends, the benefits which users of A.C.T.^™ coatings should derive will include improved reliability of the coated component, improved thermal shock resistance, reduced down times, and generally maintained or improved quality of the glass end product, with respect to both defects and to dimensional tolerances.