Field of the invention

The present invention relates to the field of production of objects provided with a coating applied by the method of thermal spray.

Prior art

According to the prior art there are various components (objects) produced from solid materials, be they metallic or non-metallic. A surface coating may be applied to their surface in order to improve resistance of the components. To obtain the surface coating, a thermal spray method may be used.

Prior to application of the surface coating, the surface of the component requires technological treatment and provision of a defined topology. This provides defined surface parameters (in particular roughness, and/or cleanliness, degreasing) for proper adhering of the surface coating. The technological surface treatment is carried out in particular by blasting of an abrasive material (“sand blasting”). For example, AI2O3, SiO, or steel or cast iron chips are used as the abrasive material. The required material blasting may be technologically demanding because identical parameters need to be achieved across the whole component surface. This is in particular challenging in concave areas of the surface, semi-closed hollows, channels, and more. The blasting process is associated with energy consumption, generation of waste in the form of dust and used blasting media, as well as health risks for the blasting apparatus operator (dusty environment, noise). In addition, there is risk of adhering of residual particles of the blasting medium on the component surface, which remain embedded on the interface between the component and the coating after spraying. These particles increase the risk of developing of cracks in the coating or fatigue cracks of the component under mechanical load, as well as the risk of initiation of corrosion attack in the component material.

Alternative abrasion materials may include e.g., ice or dry ice where neither environment nor component surface contamination occurs. However, when compared to classic abrasion materials, their aggression is low (blasting grade not more than Sa 2 * ), which means that although there are no impurities (rust, paint) on the component surface, the surface roughness is too low for application of the thermal spray.

Another method for surface roughing includes application of a mechanical apparatus based on metal tips striking the component surface.

Another method for surface treatment prior to the coating is micro-machining using specially designed tools being typically used on convex machines (e.g., mill). The method is advantageous in improved control over the resulting surface structure and potential use also in places being inaccessible to other methods (e.g., rather long holes of small internal diameter).

Another method for surface treatment prior to thermal spray is laser-based micro-machining - laser surface texturing. To achieve the roughness needed, the laser beam uses in this case the parameters allowing material removal in a way similar to the surface cleaning or laser cutting. Geometry of the texture created in this way and related roughness depend on configuration of technological parameters and laser type used.

All the methods for component surface preparation for application of the coating require one more technological step between the component production and application of the coating. This makes the surface coated component production more expensive.

Therefore, the objection of the present invention is to provide a method for production of a component with surface coating that eliminates the drawbacks described above.

Summary of the invention

The present invention relates to a method for production of a component having a surface coating. The method described produces a component from a solid metallic material. The metallic material may be e.g., carbon steel, austenitic steel, tool steel, cobalt, nickel, aluminium, titanium alloys, etc. These are the materials being used for the additive manufacturing (3D print) according to prior art. Whereas the component is provided in one of the next production steps with the coating using the thermal spray technology, it is obvious that such a component material must be selected of which melting temperature is higher than the component surface temperature achieved during application of the coating while cooling. Generally speaking, the component material melting temperature should be higher than about 200 °C. The component is manufactured using the additive manufacturing according to the prior art. More specifically, involved is the selective laser melting of metallic powder (Selective Laser Melting - SLM) method. Sometimes, equivalent DLMS or DMLS is used instead of SLM (the terms differ in their order of words Direct Laser Metal Sintering (or in German Direkt Laser Metall Schmelzen), or Direct Metal Laser Sintering). This is the technology within the Laser Powder Bed Fusion (LPBF) category where thermal energy from the laser beam selectively fuses areas of the powder bed.

In case of production from a material susceptible to oxidation, the component may be favourably produced by the additive manufacturing method under protective atmosphere. Component surface oxidation is prevented thereby. In the additive manufacturing, the component surface is provided in a controlled and pre-defined way with a set of projecting elements individually attached to the component. The projecting elements are capable of increasing the coating adhesion onto the component surface. The increased adhesion means the coating will adhere to the component surface made of given material with the projecting elements with greater force than would be the case with a completely smooth surface. The improved adhesion is achieved first by higher component surface area provided with the projecting elements when compared to the smooth surface. Further, the projecting elements mechanically avoid shifting of the coating in particular in tangential direction in relation with the component surface. The shape of the projecting elements may be, e.g., round, oval, or there may be series of projecting ribs being substantially extending in parallel across the length of the component in given direction. The series of the projecting ribs may variously cross over. The component surface manufactured in this way is then directly coated by the thermal spray technology according to the prior art. The coating spraying material may be ceramic, metal, metal alloys, metallic ceramic, or other materials known for their ability to be applied by the thermal spray technology. The coating thickness should be at least three times as much the height of the projecting elements. The coating surface smoothness and sufficient coating resistance are secured thereby.

It may be favourable in some cases to produce the projecting elements having various parameters in various areas of the component surface by the additive manufacturing method in a controlled and pre-defined way. These various parameters include in particular various sizes and/or shapes of the projecting elements between various areas of the component surface. Thereby, various surface roughness may be achieved, which influences the parameters of the applied surface in various parts of the component surface in a controlled way.

The advantage of the described method is that the surface topology required for proper adhesion of the surface coating is achieved as early as in production of the component itself. Thereby, the coating may be applied directly on the component surface without any additional technological step. In addition, the reliability and reproducibility of the fusion between the component surface and the coating are improved owing to the controlled and pre-defined production method of the component surface with the set of projecting elements by the additive manufacturing method.

Description of drawings

The exemplary embodiment of the proposed technique is described with reference to the drawings, where

Fig. 1 illustrates a schematic component cross-section with the set of the projecting elements and the coating;

Fig. 2 illustrates a schematic component cross-section of another embodiment of the component with the set of the projecting elements and the coating;

Fig. 3 illustrates a SEM micrograph showing the component surface with the set of the projecting elements prior to application of the coating;

Fig. 4 illustrates a SEM micrograph showing the cross-section of the component from Fig. 3 with the coating already applied;

Fig. 5 illustrates a SEM micrograph showing the cross-section detail of the component with the coating applied from Fig. 4;

Fig. 6 illustrates a SEM micrograph showing the component surface with the set of the projecting elements different from the component in Fig. 3 prior to application of the coating;

Fig. 7 illustrates a SEM micrograph showing the cross-section of the component from Fig. 6 with the coating already applied;

Fig. 8 illustrates a SEM micrograph showing the cross-section detail of the component with the coating applied from Fig. 7.

Example embodiment of the invention

In this example, a component 1_ was manufactured with a surface coating 2. First, the component 1 was manufactured from a solid metallic material being the maraging steel of the following composition: 18% wt. Ni; 9% wt. Co; 4.85% wt. Mo; 0.7% wt. Ti; 0.1% wt. Al, max 0.03% wt. C, while the remaining up to 100% wt. is Fe. The component 1_ was prepared by an additive manufacturing method under protective atmosphere. The selective laser melting of metallic powder (Selective Laser Melting - SLM) method was used. The method uses the principle of direct sintering of metallic powder using the laser beam. In the additive manufacturing, the component 1_ surface is provided in a controlled and pre-defined way with a set of projecting elements 3 individually attached to the component 1 in order to increase coating 2 adhesion to the component 1 surface. Hence, the produced component had its surface topology characterized by the following parameters: Ra = 20 to 22pm, Rq = 25 to 28pm, Rt = 170 to 190pm, Rz = 130 to 150pm. The coating 2 from WC-10%Co4%Cr (% wt.) was then applied on the component surface using the high velocity oxy-fuel (HVOF) technology of thickness ranging from 200 to 350pm with adhesion over 80MPa.

In the case of selective laser fusion technology, the specific component 1_ surface topologies were achieved by a specific combination of the technological parameters (laser power, laser spot movement speed, laser traces overlapping, laser spot size, layer thickness, laser motion trajectory), wherein some of these parameters are variable for creation of a specific surface topology.

The exemplary embodiment is shown in Fig. 1 to Fig. 8.

List of reference numerals

1 - component

2 - coating

3 - projecting element