Patent Publication Number: US-6033735-A

Title: Method of coating cutting inserts

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of coating cutting tool inserts with a layer of an iron group metal. Inserts with a coating applied according to the invention are particularly suitable for brazing. 
     Cemented carbide inserts are generally attached to tool-holders by mechanical means if possible. In case of saw-blades, drills and circular cutters, the design does not permit mechanical clamps and/or similar mechanical attachments. In these cases the inserts have to be brazed to the toolholder. 
     When brazing cemented carbide with a low binder content there are problems with the wetting of the braze and therefore the inserts have to be coated with cobalt prior to the brazing procedure. For coating on industrial scale, this cobalt coating is generally made electrolytically. However, such coating generally has poor adherence. In order to improve the adhesion, the inserts are heat treated in a subsequent production step. This coating method is rather complex and expensive and the resultant coating adhesion is still not always satisfactory. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of this invention to avoid or alleviate the problems of the prior art. 
     It is further an object of this invention to provide a method of coating cutting tool inserts with a layer of an iron group metal. 
     It is an aspect of the invention to provide a method of coating metal composite bodies formed of carbides, nitrides, carbonitrides with a binder phase of Co and/or Ni at least partly with a layer of at least one iron group metal comprising the following steps: 
     dissolving and complex binding at least one salt of at least one iron group metal containing organic groups in at least one polar solvent with at least one complex former comprising functional groups of OH or NR 3 , where R═H or alkyl; 
     optionally adding a soluble carbon source and/or other soluble agents to improve the wetting properties into the solution; 
     applying the solution at least partly on said bodies by dipping, spraying or painting; 
     drying the bodies to evaporate the solvent; and 
     heat treating the dried bodies in inert and/or reducing atmosphere to obtain said bodies at least partly coated with said at least one iron group metal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows in 150× the surface structure of the coating of cemented carbide insert coated with the method of the present invention. 
     FIG. 2 shows in 1250× the microstructure and the thickness of the coating from the cross section of a T-shape crack in the coating. The network of the coating is observed clearly. The crack has been formed during the cooling step because of difference in thermal expansion coefficient between coating and cemented carbide. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
     It has now surprisingly been found that using a technique related to the SOL-GEL technique, cemented carbide inserts can be provided with a cobalt layer with improved bond to the tool. 
     According to the method of the present invention one or more metal salts of at least one iron group metal containing organic groups are dissolved and complex bound in at least one polar solvent with at least one complex former comprising functional groups in the form of OH or NR 3 , (R═H or alkyl). Optionally, a soluble carbon source is added to the solution which is applied onto the cemented carbide inserts. The solvent is evaporated and the coated inserts are heat treated in an inert and/or reducing atmosphere. As a result, coated cemented carbide inserts are obtained which can be brazed to a tool according to standard practice. 
     The process according to the invention comprises the following steps where Me═Co, Ni and/or Fe, preferably Co: 
     1. At least one Me-salt containing organic groups such as carbo-oxylates, acetyl-acetonates, nitrogen containing organic groups such as Schiff bases, preferably Me-acetates, is dissolved in at least one polar solvent such as ethanol, acetonitrile, dimetyl-formamide or dimethyl-sulfoxide and combinations of solvent such as methanol-ethanol and water-glycol, preferably methanol. Triethanolamine or other complex former, especially molecules containing more than two functional groups, i.e., OH or NR 3  with R═H or alkyl(0.1-2.0 mole complex former/mole metal, preferably about 0.5 mole complex former/mole metal) is added under stirring. 
     2. Optionally, sugar (C 12  H 22  O 11 ) or other soluble carbon source such as other types of carbohydrates and/or organic compounds which decompose under formation of carbon in the temperature range 100-500° C. in non-oxidizing atmosphere is added(0.1-2.0 mole C/mole metal, preferably about 0.5 mole C/mole metal), and the solution is heated to 40° C. in order to improve the solubility of the carbon source. The carbon is used to reduce the MeO formed in connection with heat treatment and to regulate the carbon-content in the coating layer. 
     3. The solution is applied at least onto the surface/surfaces to be brazed by dipping into the solution or by spraying or painting with the solution. 
     4. The coated inserts obtained in the preceding step are heat treated in nitrogen at about 700-1100° C. To achieve a full reduction, a holding temperature might be needed. The time of reduction (5-120 minutes) is influenced by process factors such as coating thickness and reduction temperature. Nitrogen is normally used but argon, hydrogen, NH 3 , CO and CO 2  (or mixtures between them) can be used whereby the composition and micro-structure of the coating can be modulated. 
     5. As a result of the heat treatment, cemented carbide inserts coated with Me are obtained which, e.g., can be brazed to a tool in the conventional way, however, with improved strength of the brazed joint. 
     The thickness of the final coating can be varied by varying the thickness of the initial coating. For brazing purposes a thickness of 0.1-0.5 μm is suitable. For other purposes, however, the coating can be thicker. 
     Because of the difference in thermal expansion, the coating generally shows cracks. These cracks however, do not affect the brazing properties of the coating. 
     The method according to this invention can be used to provide coatings also on Ti-based carbonitrides the so-called cermets, binderless carbide and ceramics. 
     In these applications the coating can be tailor-made to form a good wetting to the base material. In addition to or instead of the carbon source mentioned above, e.g., Ti can be added as soluble salt in the metal salt-solution to form a good adhesion to a Ti containing base material. 
     Most of the solvent can be recovered which is of great importance on an industrial production scale. 
     The invention is additionally illustrated in connection with the following Examples which are to be considered as illustrative of the present invention. It should be understood, however, that the invention is not limited to the specific details of the Examples. 
     EXAMPLE 1 
     134.89 g cobalt acetate-tetrahydrate (Co(C 2  H 3  O 2 ) 2 .4H 2  O) was dissolved in 800 ml methanol(CH 3  OH). 36.1 ml triethanol-amine ((C 2  H 5  O) 3  N (0.5 mole TEA/mole Co) was added during stirring and after that 7.724 g sugar (0.5 mole C/mole Co) was added. The solution was heated to about 40° C. in order to dissolve all the sugar added. About 100 cemented carbide, grade SANDVIK DC03, saw tooth inserts were dipped into the solution and dried in a drying cabinet at a temperature of about 70° C. 
     The inserts were placed onto net trays and heat treated in a furnace with nitrogen atmosphere. The heating rate was 10° C./min to 700° C., no holding temperature, cooling 10° C./min and finally completed with reduction in hydrogen, holding temperature 800° C. for 90 minutes. 
     As a result the cemented carbide inserts had been coated with a 0.3 μm coating of cobalt. 
     EXAMPLE 2 
     The inserts from Example 1 were brazed onto a saw blade according to the following: 
     
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Steel               DIN75Cr1                                              
Brazing material         Degussa 49 Cu                                    
Flux                                 Degussa Special H                    
Brazing temperature   690° C.                                      
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     As a reference, a saw blade was manufactured using the same materials, but the inserts had been coated with cobalt in the conventional way, i.e. by electrochemical deposition. The strength of the brazing joint was determined on both saw blades by pushing off the inserts in a compression tester, using a special fixture to support the steel blade in the interface between the brazing joint and the steel. The force needed to remove (push off) the inserts was measured with the following results: 
     
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                       Coating acc.                                       
               Conventional                                               
                        to the                                            
               coating       invention                                    
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Number of inserts                                                         
                 100       100                                            
Force N per mm.sup.2, mean                                                
                 246                  287                                 
standard dev      19                   11                                 
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     The inserts according to the invention show both higher mean value and lower spread in the force required to remove them than the inserts coated in the conventional way. 
     The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the invention.