Stock material or miscellaneous articles – All metal or with adjacent metals – Having composition – density – or hardness gradient
Patent
1987-01-15
1989-02-28
Rutledge, L. Dewayne
Stock material or miscellaneous articles
All metal or with adjacent metals
Having composition, density, or hardness gradient
428627, 428637, 428660, 427 34, 427405, 4274197, B32B 1504
Patent
active
048084875
DESCRIPTION:
BRIEF SUMMARY
The invention is related to a protective layer applied to a metallic support by a plasma spray process consisting of at least one metal adherence layer and a multi-layer outerlayer which include differing amounts of metallic and ceramic materials in their layers. Such protective layers can be applied to very different support substances. It is always the intention to increase the lifetime of the support substance in a particular application and/or to open up new areas of application for the support material. With the help of the protective layer, surfaces of workpieces have been successfully given other specific properties at definate positions. This broadens the range of use of workpieces and increases their resistance in daily use.
According to PCT-WO No. 82/01898 protective layers according to the type described in the opening paragraph are well-known which include an adherence layer made of NiCrAlY and multilayer outer-layer with amounts of ceramic oxide materials depending upon the layer, such as ZrO.sub.2 --Y.sub.2 O.sub.3, Al.sub.2 O.sub.3 or Ca.sub.2 SiO.sub.4. The total layer thickness given in the examples therein is between 0.8 mm and 2.5 mm, 0.5 to 8 mm according to the claims, alternatively, 2 to 7 mm. The adherence layer thickness is 0.1 mm, 0.15 mm or 0.2 mm.
Nowadays very different techniques are used for the coating of workpiece surfaces. Due to the high energy density in the plasma flame, plasma spraying has very quickly gained a leading role in coating technology. Practically all powderlike materials can be disposited as a layer with this coating technology under defined conditions on widely differing support materials. Mostly, it is the hard, tough high temperature resistant and corrosion resistant plasma sprayed layers which increase the endurance considerably of high quality machine tool parts in agressive environmental conditions. Nevertheless, the industrial application of the plasma spraying technique also has its physical application limits. In many cases the sprayed layer is not thick enough, its adhesion to the base metal being not sufficient. With reactive spray powders, the chemical composition in the sprayed layer is altered too much. Oxygen from the air can defuse relatively easily into the plasma flame having an oxidizing and thereby a disturbing effect.
The vacuum plasma spray technique (VPS technique) was developed with these points in mind. Its development with appropriate regard to the special requirements of this new technology, resulted in considerable improvements in the coating conditions and the layer properties in comparison to spraying in air. Thus, vacuum plasma spraying is a further development and improvement of the in-air plasma spraying process (APS process). The main difference is that the coating process is carried out in a vacuum chamber at below atmospheric pressure.
The well-known improvements of the coating conditions and layer properties of the VPS technique can be arranged in four groups:
1. Particle speed
The warming of the plasma gas in the electric arc and its expansion into the vacuum accelerates the gas atoms to more than three times the speed of sound. In comparison with in-air spraying, the beam speed is about two to three times higher in vacuum. Correspondingly, the spray powder particles which are injected inside the burner jet in the hot zone of the plasma beam are also quicker. Higher powder particle speeds result in denser sprayed layers and reduce significantly the residual porosity and the roughness of the surface.
2. Surface cleaning
With help of the transferred electric arc, the surface of the workpiece can be cleaned before coating with a sputter process. Gas contamination, moisture and oxide layers are removed. This results in a noticeable adhesion improvement of the sprayed layers, in particular, with smooth surfaces. The neutralization of free surface energy of the cleaned support by layer atoms brings a pure mechanical keying of the sprayed layer to the material of the support. Additionally, favourable conditions are produced for
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Plasmainvent AG, Im Oberleh 2
Rutledge L. Dewayne
Wyszomierski George
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