The present invention relates to a thermal protective shield to be used for hot gases in machinery. Primarily, the invention refers to a flame protective shield to be usedin connection with the .combustion of solid,liquid or gaseous fuels. Other possible fields of application are e.g. in Sterling engines driven by solar energy. The invention also relates to a method of manuf cturing such a thermal protec- ti.ve shield and the use of the same in the special applica¬ tion constituted by a Sterling engine.

Thermal protective shields of different kinds are used in connection with combustion for various purposes. The neces- sity of special thermal protective shields appears only at the nighest temperatures occuring in connection with com¬ bustion. When developing new engine types and improving known types for achieving higher efficiency, there is also a desire to increase the working temperatures and thus the temperature of the materials used. In addition to the tempera¬ ture , a flame protective shield must resist the influence of combustion gases and, in certain applications, meet the requirements of mechanical strength. The term flame protec¬ tion does not imply that the shield should be directly ex- posed to a flame resulting from the combustion. In many ap¬ plications, the major part of the transfer of heat to the protective shield is constituted by radiation. In other ap¬ plications, the transfer of heat is entirely effected by radiation, e.g. in case of collecting solar energy.

A plurality of different materials for. heat shields have been tested previously and even found a certain use. Among these materials, various high temperatureresistant alloys could be mentioned, which alloys are usually based on nickel or cobolt. As far as these material have met the technological.require¬ ments, however, it has turned out that these materials are

OMP

expensive and in many cases difficult to obtain. Therefore, they are not suitable for mass production.

In trying to avoid the above-mentioned difficulties in con- nection with high temperature resistant alloys, materials consisting of several layers have been tested. A material, which is satisfactory from the mechanical point of view, has been coated on the side to be facing the combustion chamber or the like with a ceramic, high temperature resis- tant material. The coating is applied by thermal spraying, e.g. flame spraying or plasma spraying. As an example of coating materials zirconium oxide can be mentioned. .In con¬ nection with these multi-layer materials, however, problems have arisen, primarily due to the different thermal expan- sion coefficients of the substrate and the coating. This may result either in crack formations in the coating materi¬ al or in a tendency of the coating to loosen entirely or partly from the substrate. The problems have been increased in that, due to the high temperature in the combustion cham- ber and the relatively bad characteristics of the substrate at high temperatures,the coatings are necessarily made re¬ latively thick.

The present invention concerns a thermal protective material consisting of several layers of such a kind that the above- mentioned problems are avoided, whereas satisfactory mechani¬ cal and physical characteristics are obtained. According to the invention, the thermal protective shield comprises at least two layers, wherein the outer layer facing the hot gases in the combustion chamber or the like consists of an alloy of the irσn-chrome-aluminium-type. The latter is con¬ nected over a surface fully corresponding to the surface exposed to the combustion gases, to another material having sufficient mechanical characteristics at high temperatures to ^' meet the requirements of the thermal protective shield. The material is produced by connecting two shields so as to

OM

avoid enclosed oxides and pores being obtained to a great extent when coating by means of thermal spraying. By produ¬ cing the thermal protection out of metallic shields, which are connected to each other in several layers, one may also choose the thickness of the layer at will without any spe¬ cial problems.

The invention will be described further below with reference to an example concerning the use of a thermal shield in a Sterling engine. Fig. 1 shows schematically the combustion chamber in such an engine. The fuel may be in a solid, li¬ quid or gaseous state and is supplied through a channel (1) to the combustion chamber (2) . In the upper part of the com¬ bustion chamber there is a heat exchanger (3.) which may en- tirely enclose this part of the cylindrical combustion chamber. Furthermore, there are exhaust channels (4) and channels for the working medium (5) . The combustion cham¬ ber is surrounded by insulating material (6) . The thermal shield (7) is situated at the top of the combustion chamber and is kept in place by a pressing member (8) . In the shown example, the thermal protective shield consists of two ma¬ terials, there being a layer of iron-chrome—aluminium di¬ rectly facing the combustion chamber and having a thickness of a little more than 1 mm, this layer being united, over its entire surface, with a four mm thick layer of an alloy containing appr. 40% nickel (here and in the following % means per cent per weight) . The resistance requirements of the thermal shield are very high. The flame temperature may reach 2000 C, and the temperature of the gas adjacent to the thermal shield amounts to 1700 - 1800°C. The outer tempera¬ ture of the thermal shield is then 1300 - 1400°C. The com¬ bustion in the Sterling engine takes place with excess oxy¬ gen which secures the creation and maintainance of a pro¬ tective aluminum oxide layer on the surface of the thermal shield. A thermal shield with satisfactory characteristics should stand more than 1000 hours of operation under these

conditions.. his requirement is met quite well by a thermal shield according to the invention.

It is essential that the materials, of which the thermal shield is made, are well joined together all over the sur¬ face corresponding to the surface facing the combustion chamber. A loose combination of the same materials or e.g. joining the materials point by point will result in unsa¬ tisfactory characteristics as far as mechanical strength and life are concerned. The thermal shield in this example is provided, adjacent to the combustion chamber, with a plate of iron-chrome-aluminiumhaving the composition of appr. 22% chrome, appr. 5% aluminium and the rest mainly iron. Minor additions of other substances, e.g. yttrium or hafnium in the order of some tenth % may occur. Such additions and usual impurities, however, do not amount to more than approximately 2% in total. This plate has been joined by hot rolling with a plate composed of 20% chrome, 35% nickel, the rest mainly iron, even in this case containing usual impurities. It has turned out to be possible to join these materials by hot rolling provided that the joining surfaces are cleaned thoroughly before the rolling process and are protected from oxidation by means of a protective gas during the rolling operation.

Other material combinations and production methods in addi¬ tion to those mentioned in the above example may naturally be considered within the scope of the invention. In certain applications there could be a reason to use a material consis ting of more than two layers, and the combination of iron- chrome-aluminium on both outer sides and an intermediate layer of an iron-nickel-chrome alloy has turned out to be useful. It may also be desireable to cover the outer edge around the thermal shield so that the intermediate layer will be en- tirely enclosed. This can be achieved by covering the edges by means of welding or thermal spraying. It is also feasable

OM

to produce the material by cold-rolling. In all these cases, however, it is essential that the plate closest to the com¬ bustion chamber consists of an iron-chrome-aluminum alloy which, all over the surface corresponding to the surface facing the combustion chamber, is joined to another plate for obtaining a thermal protection of satisfactory mecha¬ nical characteristics.

WIP