Coating processes – Spray coating utilizing flame or plasma heat – Inorganic carbon containing coating – not as steel
Reexamination Certificate
1998-09-11
2001-03-06
Bareford, Katherine A. (Department: 1762)
Coating processes
Spray coating utilizing flame or plasma heat
Inorganic carbon containing coating, not as steel
C427S453000, C427S454000, C427S455000, C427S456000, C427S554000, C427S570000, C427S576000
Reexamination Certificate
active
06197386
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method for applying a coating to a substrate by means of plasma spraying with the use of a laser beam, which is suitable for coating a very wide variety of substrates, in particular for improving the properties, such as for example hardness, wear resistance and corrosion resistance, and which can be achieved with high coating capacities.
U.S. Pat. No. 3,310,423 has disclosed a flame-spraying method in which a pulsed laser beam is to be used for assistance in order to improve or increase the adhesion of the coating and its density.
In that document, it is proposed to direct the beam of a laser, which is in temporally short pulses, into a spray jet, specifically in such a way that the particles are reheated before they strike the surface to be coated. The document furthermore proposes to increase the particle temperature to such an extent that the surface is softened by heat transfer from the particles and forms an alloy with the particle material. In the case of a multi-layer build-up, the intention is for the surface of the top layer to be partially melted and fused to the particles sprayed on.
However, with the performance parameters indicated, only a limited amount of energy can be made available to increase the temperature in pulsed operation of the laser, so that only relatively low coating rates can be achieved or the desired level of success cannot be achieved.
Moreover, it is impossible to influence other properties of the coating applied which are again desirable in different respects for a very wide variety of coating materials. For example, it is possible that during cooling phases, microstructural or crystalline structures which do not have the desired properties, or have them only to an insufficient extent, will be formed. When applying a coating which contains at least Al
2
O
3
, the &ggr; phase may be formed, the properties of which are far removed from those of the preferred &agr; phase. Similar effects also occur for metals or such alloys. Owing to its high melting point, Al
2
O
3
, for example, cannot be processed at all to give usable layers by flame spraying, even with the laser assistance proposed in that document.
SUMMARY OF THE INVENTION
Therefore, on the basis of the above, the object of the invention is to provide a method with which it is possible to apply coatings to a very wide variety of substrates at a high coating rate while providing the possibility of influencing in a controlled manner the layer structure which is being formed, in addition to achieving suitable adhesion and density of the layer.
According to the invention, this object is achieved by means of coating a substrate by means of plasma spraying, in which method a continuous laser beam is additionally used and is directed through a plasma spray jet and at least one of the laser power and the beam shaping is set in such a way that the temperature of the layer being applied is kept above temperatures found in a cooling curve. Advantageous configurations and refinements of the invention make it possible to achieve the features mentioned in the subordinate claims.
In the novel and inventive method, the coating material is sprayed onto the surface of the substrate to be coated in a known manner using a plasma torch. The plasma arc does not come into any contact with the workpiece surface and the calorific content of the spray jet is insufficient to initiate melting of the substrate surface so as to achieve a fusion bond and/or diffusion bond between coating and substrate. However, this is achieved in that a laser beam, which is preferably out of focus, is directed at least partially through the spray jet onto the surface of the substrate or the surface of a layer which has already been applied thereto, with a predeterminable interaction time. The energy of the laser beam is to be sufficiently great to initiate melting of the surface, it being preferable for the melting to reach a depth of approx. 0.1 mm. Moreover, it is desirable to influence the temperatures in such a way that the particles of coating material which are sprayed on become fused together, it being possible, if necessary, to add a suitable flux (e.g. boron or silicon) to the coating material.
The laser beam to be used should have a power which lies in the range from 2 to 15%, particularly advantageously in the range between 5 and 10%, of the power of the plasma arc.
In the method according to the invention, although the laser beam does penetrate through the spray jet, it only produces its full action on reaching the surface of the substrate to be coated or a layer which has already been applied thereto, so that melting of the surface in question is initiated and the spray particles which have already struck the said surface are held at the melting temperature in order to allow them to be fused together. In this process, the required beating provided by the laser beam takes place in a suitably locally limited region. If coating materials which at least contain metals are used, it is possible to obtain a metallurgical bond of high density, high adhesive strength and homogeneity.
Advantageously, the out-of-focus laser beam is directed at an angle of between 0 and 90° onto the surface, thus forming a focal spot which deviates from the circular focal spot. The angle between surface and laser beam should preferably lie between 10 and 80°. The angle between laser beam and spray jet should be between 0 and 90° and preferably between 0 and 45°.
Since the focal spot on the substrate surface does not cover the entire area of the plasma jet, despite being out-of-focus, the laser beam can be deflected using suitable means, such as for example mirrors, and this should, of course, additionally take into account the relative movement between spray jet and substrate. To do this, it is possible to use oscillating mirror systems or faceted mirrors, by means of which not only the focal spot geometry but also the intensity distribution can be influenced. However, the laser beam should be guided at least to some extent directly in the region of the layer which is being formed, although it can also be used for preheating and for maintaining the temperature.
Expediently, the beam should be shaped by one-dimensional or two-dimensional beam oscillation using oscillating plane mirrors. In the most simple case, this can take place without coupling to the laser-power control. As a result, it is possible to achieve a variably adjustable laser focal spot which assumes the form of a rectangle, for example. Taking into account the required beam intensities of several 10
3
W/cm
2
in this way it is possible to cover at least the entire coating track applied using the spray jet.
Irrespective of the way in which the beam is shaped, it may be advantageous, by means of a suitable combination of laser, plasma and spray parameters, to thermally affect the layer which is forming, solidifying and cooling in such a way that certain desired microstructural states and phases can be formed. This is achieved in that it is possible, in contrast to the unaffected, abrupt solidification and cooling curve, to have a controlled influence, depending on the combination of materials and desired layer properties, on a temperature rise, by maintaining a specific temperature in the layer or on the spatial and temporal temperature gradients during the cooling operation by means of the action of the laser beam.
Given a plasma torch power of 20 kW, it is possible, by way of example, to use a laser with a power of 1.5 kW, the focal spot of which is defocused to 5 to 10 mm and directed onto the surface. Lasers which have proven suitable are Nd-YAG lasers with a wavelength of 1.06 &mgr;m or diode lasers in this power range with wavelengths of 0.94 &mgr;m. However, it is also possible to employ other lasers, it being possible to take into account the absorption behavior of the surfaces in order to keep the efficiency as high as possible. If the laser beam parameters are configured suitably, the spray jet can be used to achieve a foca
Beyer Eckhard
Nowotny Steffen
Bareford Katherine A.
Barnes & Thornburg
Fraunhofer-Gesellschaft zur Forderung der Angewandten Forschung
LandOfFree
Method for applying a coating by means of plasma spraying... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for applying a coating by means of plasma spraying..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for applying a coating by means of plasma spraying... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2504967