Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
1998-11-05
2001-04-24
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S512000, C427S597000
Reexamination Certificate
active
06221175
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for the production of a ceramic layer on a metallic base material, to an apparatus for carrying out the method and to workpieces manufactured in accordance with the method. The method in accordance with the invention can be used, for example, as a surface technology in turbine wheels (for the protection of the blades or blade tips in gas and steam turbines or in Pelton wheels) or in diesel engines (pistons, cylinder heads, valves).
2. Description of the Prior Art
Metallic surfaces can in principle be substantially improved with respect to various properties by means of ceramic coatings, namely, for example, with respect to their resistance to wear and/or to corrosion. The coating can also be used at higher temperatures as a heat insulating layer and as protection against wear. Pairing a ceramic material with a metallic one is very difficult due to the great differences between these materials. It is an object of the invention to apply a technically usable ceramic layer to a metallic base material by means of a suitable method and with a suitable choice of materials.
SUMMARY OF THE INVENTION
The method for the manufacture of a ceramic layer on a metallic base material combines the following measures: The base material is preheated. Ceramic coating material is applied to a locally melted surface region of the base material. In this situation, the coating material is likewise melted. A metallurgical bonding zone is produced by means of an additional material which reacts with the coating material and which is additionally applied to the base material as an adhesion producing layer or is admixed with the base material as a component of the alloy.
Thanks to the bonding zone in accordance with the invention the layer adheres well to the base material. Materials must be selected for which a bonding zone or an intermediate layer of this kind forms. In the choice of the materials care must further be taken that the ceramic layer has a suitable microstructure and that only low residual tensions remain between the layer and the base material. In particular, the production of a layer should be possible which is intact, i.e. largely free of tears, and compact, i.e. largely free of pores. For special applications porous coatings can also be advantageous. The method in accordance with the invention enables the manufacture of ceramic coatings of which the layer thicknesses are at least 0.1 mm, with it being possible for the layer thicknesses to be considerably greater.
The energy which is required for the melting of the ceramic coating material and for the piece of surface of the base material to be melted is advantageously introduced by means of a laser beam. Ceramic materials are very good absorbers in the infrared range so that a coating material present in powder form is preheated when flying through the laser beam.
The base material, including the adhesion producing layer where appropriate, forms the substrate. The laser is directed at this substrate. The powder particles which fly through the laser beam and are thereby preheated encounter a melt puddle formed by the laser beam in an interaction zone on the substrate. In the presence of an adhesion producing layer, the additional material at the surface of the substrate is transported in the interaction zone through convection and diffusion partially into the base material and partially into the coating material, where it forms transition regions as a result of metallurgical reactions which mechanically stably bond the base material to the ceramic coating after the subsequent solidification of the melt puddle.
The base material is advantageously a ferrite steel with low carbon content. A Martensite formation is largely absent during the cooling of a steel of this kind.
A specific coating method will be explained in the following:
A mixture of Al
2
O
3
and ZrO
2
is used as a coating material, advantageously in a relationship of the two oxides for which a eutectic results (42% ZrO
2
by weight) so that the melting point of the layer takes on a minimum value (about 1900° C.). This mixture has a relatively large coefficient of thermal expansion for ceramic materials, namely about 8 10
−6
K
−1
. A steel is used as a base material, the coefficient of thermal expansion of which is relatively small in comparison with other metals, namely a superferritic steel (X1 CrNiMo, coefficient of thermal expansion: 10.5 10
−6
K
−1
, melting point 1485° C.; composition in percent by weight: 0.003 C, 28.35 Cr, 3.35 Ni, 2.37 Mo, 0.38 Si, 0.37 Mn, remainder Fe). It is important that the coefficient of thermal expansion of the superferritic steel is somewhat greater than that of the ceramic layer in order that the coating is under a compression force after the cooling down. Titanium (Ti) is chosen for the adhesion producing layer.
Titanium as an adhesion producing layer can be applied to the base material by means of PVD (Physical Vapor Deposition). The application of the coating must be carried out screened off from oxygen and nitrogen, i.e. in a protective gas (Ar).
When the substrate melts, Ti diffuses out of the adhesion producing layer into the base material and into the oxide incident at the interaction zone. Oxygen proceeds from the oxide into the melted adhesion producing layer and oxidizes Ti there, whereas zirconium oxide (ZrO
2
) is partially reduced in a boundary region of the ceramic coating material. Assimilations of the chemical compositions thereby result in a transition region between the substrate and the coating. This transition region forms a mechanically stable bonding zone.
In the adhesion producing layer the Ti can be partially or wholly replaced by Zr or Hf. Other alloys also come into consideration as base materials which have relatively low coefficients of thermal expansions and melting points which do not differ greatly from those of the ceramic layer. One can, for example, use a ferritic construction steel. In this case the adhesion producing layer must be relatively thick (several hundredths of a millimeter) so that a volume increase through Martensite-wise conversions acts to a reduced extent on the ceramic layer. MeCrAlY (with Me═Fe, Ni, Co) can be used as a material for the adhesion producing layer which can be applied by means of laser coating. It is ideal if the values for the coefficients of thermal expansion and the melting temperatures are located between the corresponding values for the base material and the ceramic coating in the bonding zone.
For a successful production of a coating which adheres to the base material in accordance with the invention as a result of a bonding zone it is necessary, in addition, that the substrate be preheated. The heat required for the preheating is advantageously introduced inductively by means of electromagnetic energy. Preheating is done to a temperature T
G
, with the absolute temperature T
G
being greater than about 50% of the melting temperature T
m
of the base material.
The coating manufactured in accordance with the invention is also resistant to thermal shock. Relatively large temperature gradients between the coating and the base material are possible without a breaking off of the ceramic layer arising.
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Patent Abstracts of Japan, vol. 013, No. 042 (C-564), Jan. 30, 1989 & JP 63 241154 A (Toshiba Corp), Oct. 6, 1988, Abstract.
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Bourban Stèwes
Hofmann Heinrich
Jansen Franz
Kurz Wilfried
Mari Daniele
Oltmans Andrew L.
Sheehan John
Sulzer Innotec AG
Townsend and Townsend / and Crew LLP
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