Abstract:
A self-drilling screw composed of an austenitic or some other non-corrosive base material ( 1 ) is provided, having a shank ( 12 ), having a cutting or shaping drilling or threaded tip ( 16 ) arranged at one shank end and having a hard coating ( 3 ) applied at least to the drilling or threaded tip ( 16 ) by electrodeposition, this hard coating ( 3 ) having a layer containing a transition metal, for example a hard chromium layer. The shank ( 12 ) has a hard edge layer ( 2 ) at least under the hard coating ( 3 ). A topcoat ( 4 ) composed of a lubricating and heat-dissipating material, which preferably is metal, wax and/or lacquer, is applied to the hard coating ( 3 ).

Description:
BACKGROUND 
       [0001]    The invention relates to a self-drilling screw produced from an austenitic or other stainless base material, comprising a shank which has a cutting or forming boring tip or thread crest arranged on a shank end and a hard edge layer formed on the surface of the shank in the region of the boring tip or thread crest and at least in the region of a thread connecting thereto. 
         [0002]    DE 10 2012 009 400 B4, which relates to a patent also by the applicant, discloses a self-drilling screw formed in one piece. This known self-drilling screw has a simpler design than a two-part self-drilling screw in which the two parts are welded together, one of these two parts comprising the boring tip and the thread-forming region and, by way of example, consisting of carbon steel which has, in a final step, been hardened by induction or flame hardening. 
         [0003]    This known screw is only ever used once. A hard coating which is galvanically applied to the boring tip or thread crest and comprises a transition metal layer or chromium layer has been found to be stable for the single drilling process required for this purpose. Simple coating such as chromating or hard chromating or, more generally, galvanically applying the hard coating as a layer that contains a transition metal, is sufficient at least with suitable additives to be able to use the known self-drilling screw at least once. In this case, contrary to the opinion of a person skilled in the art, hard coatings of this type can be achieved without the eggshell effect occurring. The self-drilling screw according to the aforementioned document achieves, owing to the coating applied, the required drilling capacity and the required thread formation. 
         [0004]    A self-drilling screw of the type mentioned at the outset is known from U.S. Pat. No. 5,419,948 A. In order to harden said known self-drilling screw, it is chemically reacted, i.e. nitrated, with nitrogen trifluoride on its surface. As a result of the nitration, the known screw, which is stainless per se, produces an ultra-hard surface layer which is susceptible to rust and has to be removed again after the nitration, specifically in the region of the screw head. 
         [0005]    U.S. Pat. No. 4,802,807 A discloses a self-drilling screw for which corrosion resistance by galvanising, i.e. by galvanically applied zinc, is achieved. In addition, for this known self-drilling screw, an aluminium protective layer is produced on the zinc protective layer. The aluminium protective layer is applied to the galvanised screw by means of ion vapour precipitation technology. The composite protective layer thus produced is intended not only to improve corrosion resistance but also to reduce the torque when screwing in the self-drilling screw. Finally, a topcoat is applied to the self-drilling screw in order to prevent the aluminium protective coating from oxidising. 
       SUMMARY 
       [0006]    Proceeding from a self-drilling screw according to the above-cited document U.S. Pat. No. 5,419,948 A, the object of the invention is to improve the drilling capacity and the thread formation in a simple manner. 
         [0007]    Proceeding from this known self-drilling screw, the sought further improvement in the drilling capacity and the thread formation is achieved according to the invention in that a hard coating is galvanically applied at least to the hard edge layer of the boring tip or thread crest, which hard coating comprises a layer containing a transition metal, and in that a topcoat made of a lubricating and heat-dissipating material is applied to the hard coating. 
         [0008]    In the self-drilling screw according to the invention, the hard edge layer improves the adhesiveness of the hard coating galvanically applied thereto. In turn, the hard coating improves the adhesiveness of the topcoat applied thereto. Since the topcoat is produced from a lubricating and heat-dissipating material, it in turn improves the stability of the hard coating. This improvement in the stability of the hard coating leads to the sought improvement in the drilling capacity and the thread formation. The sought improvement in the drilling capacity and the thread formation is achieved for the self-drilling screw according to the invention, which is inherently stainless, without the self-drilling screw being placed in a state in which it is susceptible to rust that has to be removed again at least in part. 
         [0009]    The prevention of the eggshell effect mentioned at the outset is reliably achieved for the self-drilling screw according to the invention owing to the hard edge layer, which the shank has under the hard coating. 
         [0010]    Topcoats made of a lubricating antifriction lacquer are already known, for example the lubricating lacquer F 417, according to technical product information available to the applicant from the company Tevier Öl-Fenkart GmbH, which is based in 6845 Hohenems, Austria. This antifriction lacquer, which contains MoS 2 , is pressure-resistant and is intended to achieve lifetime lubrication of general mechanical engineering components, automobile industry components and supplier industry components. Specifically, said antifriction lacquer can also be used for screws; however, the use of such antifriction lacquers for self-drilling screws is not known to the applicant. 
         [0011]    The topcoat that is applied to the hard coating in accordance with the invention and that is made of a lubricating and heat-dissipating material not only improves heat-dissipation but also reduces the heat produced during the screwing-in process of the self-drilling screw according to the invention by reducing friction, and therefore said topcoat not only dissipates heat but also reduces the amount of heat to be dissipated overall. 
         [0012]    The hard edge layer can be achieved in particular by selecting a suitable strain when the self-drilling screw is formed and before the hard coating is galvanically applied to the shank of the self-drilling screw. 
         [0013]    For the self-drilling screw according to the invention, the hard edge layer and the hard coating applied thereto as well as the topcoat applied to the hard coating ensure one-time drilling and the formation of the thread in steel along with optimal drilling capacity and optimal thread formation. 
         [0014]    For the self-drilling screw according to the invention, heat dissipation is ensured over a large surface area and owing to a special topology of the surface, which also encourages the adhesion of the topcoat. 
         [0015]    The characteristics of the hard coating and of the topcoat such as layer thickness, hardness, adhesiveness, antifriction properties, roughness, temperature resistance and thermal conductivity can each be optimised depending on the use. 
         [0016]    The description and claims advantageous embodiments of the invention. 
         [0017]    In an embodiment of the self-drilling screw according to the invention, the lubricating and heat-dissipating material comprises metal, wax and/or lacquer. As a result of this alone, the above-mentioned characteristics of the hard coating can be optimised depending on the use. 
         [0018]    In another embodiment of the self-drilling screw according to the invention, the hard coating, together with the topcoat, has a thermal conductivity of λ&gt;9 W/mK, as a result of which the sought optimisation in terms of drilling capacity and thread formation is achieved even more reliably. 
         [0019]    In another embodiment of the self-drilling screw according to the invention, the topcoat comprises a layer which intrinsically has differences in layer thickness of up to +/−60%, as a result of which the adhesion of the topcoat to the hard coating is ensured even more securely. 
         [0020]    In another embodiment of the self-drilling screw according to the invention, the topcoat comprises a layer which intrinsically has differences in layer thickness of up to +/−30%. These maximum differences in layer thickness are easier to achieve than for the previously mentioned embodiment of the self-drilling screw according to the invention, but also ensure sufficient adhesion of the topcoat to the hard coating and sufficient heat dissipation through the topcoat. 
         [0021]    In another embodiment of the self-drilling screw according to the invention, the topcoat is mixed with metal particles. These particles can, as is known per se, consist of MoS 2 , i.e. of at least lubricating particles. The inventors have identified that an improvement in the heat dissipation during the screwing-in process of the self-drilling screw according to the invention can also be achieved by means of such particles used per se for lubrication. 
         [0022]    However, an improvement in heat dissipation surprisingly also results from using only heat-dissipating particles made of Au, Ag, Cu or the like, which constitute another embodiment of the self-drilling screw according to the invention. As a general principle, most of the heat is given off via the cutting that results during the drilling process, lacquer and/or wax (having and not having metal particles or metal powder) still positively assisting this effect, however. 
         [0023]    In another embodiment of the self-drilling screw according to the invention, the base material has a hardness of from 100 to 300 HV0.1 and preferably from 150 to 200 HV0.1. A base material having such a hardness has proven to be particularly advantageous. 
         [0024]    In another embodiment of the self-drilling screw according to the invention, the hard edge layer has a hardness of from 300 to 600 HV0.1. A hard edge layer having such a hardness has proven to be particularly advantageous. 
         [0025]    In another embodiment of the self-drilling screw according to the invention, the hard coating has a serrated or beaded topography. It had been found that both types of topography ensure sufficient differences in thickness, which ensure sufficient adhesion of the topcoat. 
         [0026]    In another embodiment of the self-drilling screw according to the invention, the hard coating has a hardness of at least 700 HV0.1. A hard coating having such a hardness has proven to be advantageous. 
         [0027]    In another embodiment of the self-drilling screw according to the invention, the hard coating has a hardness of between 800 and 1400 HV0.1. A hard coating having such a hardness has proven to be particularly advantageous. 
         [0028]    In another embodiment of the self-drilling screw according to the invention, the hard coating has a maximum thickness of 60 μm. This thickness of the hard coating has proven to be appropriate for ensuring drilling capacity and thread formation of the self-drilling screw and for the adhesion of the topcoat to the hard coating. 
         [0029]    The invention further provides the use of a self-drilling screw according to one or more features of the invention for producing a connection to at least one steel connection element through which the self-drilling screw is drilled, this use being particularly suitable when one of at least two components to be fastened to one another is exposed to weathering. The production of the self-drilling screw according to the invention from a stainless base material means that the part of the self-drilling screw which is exposed to weathering does not require additional rust protection as is the case for the conventional two-part self-drilling screw according to the German patent DE 10 2012 009 400 B4 which is mentioned at the outset, or as is the case for the self-drilling screw according to U.S. Pat. No. 4,802,807 A. The boring tip, that is to say the part of the conventional two-part self-drilling screw that is exposed to weathering, requires special measures for the corrosion resistance thereof because the boring tip consists of carbon steel so that it can be hardened. The thread-forming region of the conventional two-part screw consists of stainless steel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    Advantageous embodiments of the invention are described in greater detail in the following with reference to the drawings, in which: 
           [0031]      FIG. 1  shows, as a first embodiment of the invention, a metal self-drilling screw which is provided with a thread crest and, as a larger scale detail, a thread flank; 
           [0032]      FIG. 2  shows a boring tip of a second embodiment of the self-drilling screw according to the invention, together with a larger scale detail of a corner of the boring tip; 
           [0033]      FIG. 3  is an even larger scale detail of a part of the detail according to  FIG. 2  or  FIG. 3 ; 
           [0034]      FIG. 4  symbolically shows the heat dissipation from a thread flank or boring tip via a cutting; 
           [0035]      FIG. 5  shows, in the same view as  FIG. 4 , heat dissipation mainly over the surface of the thread flank or boring tip in a region adjacent to a cutting; 
           [0036]      FIG. 6  shows, as a first example of a use of the self-drilling screw according to the invention, the fastening of a corrugated metal sheet to a vertical spar; 
           [0037]      FIG. 7  shows, as a second example of a use of the self-drilling screw according to the invention, the fastening of a trapezoidal metal sheet to a Double T support; and 
           [0038]      FIG. 8  shows, as a third example of a use of the self-drilling screw according to the invention, a mutual fastening of two trapezoidal metal sheets. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]      FIG. 1  shows, as a first embodiment of the invention, a metal self-drilling screw  10  comprising a shank  12  which has a thread-forming region  14  and a thread crest  16 . The screw has a head, for example a conventional hexagonal head (not shown in  FIG. 1 , however). The metal self-drilling screw  10  is produced in a single piece from an austenitic or other stainless steel, which forms the base material of the screw.  FIG. 1  also shows, as a larger scale detail, a thread flank of the metal self-drilling screw  10 . In this detail, the base material of the screw is denoted by reference sign  1 . The shank  12  is designed so as to be harder, at the surface in the region of the drill tip  16  and at least in the region of a thread  15  connected thereto, than the remainder of the metal self-drilling screw  10 . This harder design of the surface is important in order for the metal self-drilling screw  10  to be able to perform its function, specifically to drill a hole using the thread crest  16  and subsequently to form a thread in the hole. The austenitic or other stainless base material  1  of the self-drilling screw  10  would specifically not be inherently suitable for this purpose. The base material would be too soft for this purpose. 
         [0040]    In the harder region, the metal self-drilling screw  10  is provided with a hard edge layer  2  and this in turn is provided with a hard coating  3 . The self-drilling screw  10  comprises a hard edge layer  2  at least in the region below the hard coating  3 . The hard coating  3  is galvanically applied as a thin layer, for example as a hard chromium layer, in particular according to German patent DE 25 02 284 C2, to the hard edge layer  2 . Instead of a hard chromium layer, a layer containing a transition metal can generally be selected, which layer can be galvanically applied. 
         [0041]    The hard edge layer  2  can be achieved in particular by selecting a suitable strain when the metal self-drilling screw  10  is formed and before the hard coating  3  is galvanically applied to the shank  12  and the thread crest  16  of the metal self-drilling screw. Aside from the hard coating  3 , the metal self-drilling screw  10  is produced in a conventional manner by a screw blank being mechanically shaped, i.e. rolled, and in the process being provided with the thread crest  16  and remaining thread of the metal self-drilling screw  10 . The larger the strain, the harder the hard edge layer  2  is formed. 
         [0042]    The base material  1 , which is used in the metal self-drilling screw according to the invention, has a hardness of from 100 to 300 HV0.1 and preferably from 150 to 200 HV0.1. The hard edge layer  2  has a hardness of from 300 to 600 HV0.1. 
         [0043]    The hard coating  3  has a serrated or beaded topography and has a maximum thickness of 60 μm. The hard coating also has a hardness of at least 700 HV0.1 and preferably a hardness of between 800 and 1400 HV0.1. 
         [0044]      FIG. 2  shows a boring tip of a second embodiment of a self-drilling screw  17  shown in  FIG. 8 , which is provided with a boring tip  18  instead of a thread crest as is the case for the metal self-drilling screw  10  according to  FIG. 1 . A shank such as the shank  12  is attached to the top of said boring tip  18 , which shank has a thread such as the thread of the shank  12  in  FIG. 1  (now shown in  FIG. 2 , however). The boring tip  18  is integrally moulded on the shank (not shown in  FIG. 2 ) just as the thread crest  16  is integrally moulded on the shank  12 . Unlike the thread crest  16 , the boring tip  18  has a bit  20  having a cutting edge  22 . The above comments made in relation to the metal self-drilling screw  10  thus also apply to the self-drilling screw shown in  FIG. 2  only by its boring tip  18 .  FIG. 2  also shows a corner of the boring tip  18  in a larger-scale detail. This detail shows that the self-drilling screw according to  FIG. 2  has the hard edge layer  2  and the hard coating  3  on the base material  1  thereof. Both for the metal self-drilling screw  10  according to  FIG. 1  and for the self-drilling screw  17  according to  FIGS. 2 and 8 , a topcoat  4  made of a lubricating and heat-dissipating material is applied to the hard coating  3 . A suitable lubricating and heat-dissipating material comprises metal, wax and/or lacquer. The topcoat is preferably mixed with metal particles which comprise MoS 2 , Au, Ag or Cu. 
         [0045]      FIG. 3  is an even larger scale detail of a part of the detail according to  FIG. 2  or  FIG. 3 . In the embodiment shown in  FIG. 3 , the hard coating  3  has a jagged structure. The topcoat  4  applied to the hard coating  3  thus comprises a layer which intrinsically has differences in layer thickness. These differences in layer thickness are at most +/−60%, preferably a maximum of +/−30%. 
         [0046]    The hard coating  3  together with the topcoat  4  has a thermal conductivity of λ&gt;9 W/mK. 
         [0047]      FIG. 4  symbolically shows the heat dissipation from the thread flank  15  or the boring tip  18  via a cutting  26  in the event that a hole is drilled into a workpiece  28 , which hole is intended to be provided with threads. In  FIG. 4 , the workpiece  28  consists of a highly thermally conductive material, as a result of which the cutting  26  dissipates heat effectively, which heat is represented by a row of arrows  30  in  FIG. 4 . If the workpiece  28  consists of a poorly thermally conductive material or of a heat-insulating material, the heat resulting during the boring or -tapping process is introduced into the thread flank  15  or the boring tip  18  and is conveyed via the topcoat  4  to the outside, as is shown in  FIG. 5  by a row of arrows  30  and an arrow  32 . 
         [0048]      FIGS. 6 to 8  show the use of the self-drilling screw  17  for producing a connection to at least one steel connection element, through which the self-drilling screw is drilled. The connection element is shown in these figures as a corrugated metal sheet  34  or as a trapezoidal metal sheet  36 ,  42  and  44 .  FIG. 6  shows, as a first example of the use of the self-drilling screw  17 , the fastening of a corrugated metal sheet  34  to a vertical spar  38 .  FIG. 7  shows, as a second example of the use of the self-drilling screw  17 , the fastening of a trapezoidal metal sheet  36  to a Double T support  40 .  FIG. 8  shows, as a third example of the use of the self-drilling screw  17 , a mutual fastening of two trapezoidal metal sheets  42 ,  44 . The screws  17  are shown in  FIG. 6-8  having different heads. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           1  base material 
           2  hard edge layer 
           3  hard coating 
           4  topcoat 
           10  metal self-drilling screw 
           12  shank 
           14  thread-forming region 
           15  thread flank 
           16  thread crest 
           17  self-drilling screw 
           18  boring tip 
           20  bit 
           22  cutting edge 
           24  topcoat 
           26  cutting 
           28  workpiece 
           30  arrow 
           32  arrow 
           34  corrugated metal sheet 
           36  trapezoidal metal sheet 
           38  spar 
           40  double T support 
           42  trapezoidal metal sheet 
           44  trapezoidal metal sheet