ETCHING PROCESS

Abstract: The present invention relates to a method of removing by etching the binder phase from the surface of a hard material consisting of hard constituents in a binder phase based on cobalt and/or nickel. By carrying out the etching process electrolytically in a mixture containing concentrated sulphuric acid and concentrated phosphoric acid at a volume ratio of (0,5-2):1 with a water content of less than 50 % at a temperature of 25-60 DEG C an even binder phase removal without deep penetration is obtained.

Description: Etching process The present invention relates to an etching process for the purpose of removing the binder phase from the surface of cemented carbide inserts before applying coa¬ tings on said surface. Coated cemented carbide inserts have now for many years been commercially available for chip cutting ma¬ chining of metals in the metal cutting industry. Such inserts are commonly made of a metal carbide, normally C, generally with addition of carbides of other metals such as Nb, Ti, Ta, etc and a metallic binder phase of cobalt. By depositing onto said inserts a thin layer of a wear resistant material such as TiC, TiN, AI2O3 etc separately or in combination it has been possible to in¬ crease the wear resistance at essentially maintained toughness. A still further improvement in properties has been obtained by subjecting the inserts to a binder phase enrichment in the surface below the coating, so called cobalt gradient. Binder phase enrichment can be accomplished, for instance, by sintering in vacuum with nitride addition as is disclosed in Swedish patent app¬ lication 8201930-8 or by controlled cooling as disclosed in EP-A-337696. Such inserts, however, often also have a thin layer of binder phase on their surface and someti¬ mes even with a layer of graphite thereon. The two lat¬ ter layers have a negative effect on the process when carrying out CVD- or PVD-deposition, which results in layers with inferior properties and insufficient adhe- rence. These layers must therefore be removed before carrying out the deposition process. It is possible to remove such cobalt- and possible graphite-layers mechanically by blasting. The blasting method is, however, difficult to control. The difficulty resides in the inability to control consistently the blasting depth with necessary accuracy, which leads to an increased scatter in the properties of the final pro¬ duct - the coated insert. Chemical or electrolytic methods could be used as alternatives for mechanical methods. US Patent 4,282,289 discloses a method of etching in a gaseous phase by using HCl in an initial phase of the coating process. In EP-A-337 696 there is proposed a wet chemical method of etching in nitric acid, hydrochloric acid, hydrofluoric acid, sulphuric acid and similar or electro-chemical methods. From JP 88-060279 it is known to use an al¬ kaline solution, NaOH, and from JP 88-060280 to use an acid solution. JP 88-053269 discloses etching in nitric acid prior to diamond deposition. There is one drawback with these methods, namely, that they are incapable of only removing the cobalt layer. They also result in deep penetration, particularly in areas close to the edge. The etching medium not only removes cobalt from the sur¬ face but also penetrates areas between the hard consti- tuent grains and as a result an undesired porosity bet¬ ween layer and substrate is obtained at the same time as the cobalt layer may partly remain in other areas of the insert. It is therefore an object of the present invention to provide an etching method which does not give a deep penetration effect. In Fig 1 there is shown in 1200 x magnification the structure of a cross-section of a surface zone of a ce¬ mented carbide insert after being subjected to electro- lytic etching according to prior art and Fig 2 shows the structure after electrolytic etching according to the invention. It has now surprisingly been found that electrolytic etching in a mixture containing concentrated sulphuric acid, H2SO4, and concentrated phosphoric acid, H 3 P04, gives the desired effect of cleanly and effectively re¬ moving the surface layer of binder metal and any gra¬ phite. By using such a mixture of acids the cobalt layer on the surface will be effectively removed whereas the cobalt in the channels between the hard constituent grains will not be etched away. The binder phase layers between the carbide grains, which are necessary for the strength of the cemented carbide are not affected. The method is even self-regulating. When the cobalt layer has disappeared the process stops which means that the length of the process time is not critical. The reason why the electrolyte according to the in¬ vention gives rise to such positive results is not com¬ pletely known. It is probably due to the viscosity in combination with the solubility of the salts formed. If, e.g., diluted sulphuric acid is used deep penetration is obtained. The electrolytic etching process is carried out in a manner known per se. Voltage, current density, time etc depend of the thickness of cobalt- and possible graphite layer, number of inserts, design of equipment and have to be found by experiment to obtain the best result. The electrolyte shall contain a mixture of commercially available concentrated sulphuric- and phosphoric acids in volume ratio (0,5-2) :1, preferably (0,75-1,25) :l, most preferably (0,95-1,05) :1. The water content of the solution shall be <50 %, preferably <25 %, most prefe¬ rably <15 %. The etching is suitably carried out at a combination of time, current and exposed surface up to 150 As/cm 2 . However, in production scale operating at constant voltage is generally more convenient. The tem¬ perature of the electrolyte shall be 25-60°C. Precau¬ tions must be adhered to when carrying out the etching since explosive or health damaging gases and vapours might develop. After finalizing the etching process, the inserts shall be neutralized and cleaned, for instance, by rin¬ sing in alkaline baths followed by rinsing in water. Cleaning is suitably carried out by ultra-sonic means followed by drying. Still further improved results can be obtained in those cases where a graphite layer is present on top of the cobalt layer. By carrying out a light wet blasting or mechanical working before the etching process essen- tially only the graphite is removed. In this way those problems are avoided that might occur when insufficient electrical contact is obtained between inserts and cur¬ rent supply which leads to insufficient etching effect. In addition, difficulties with removal of graphite fla- kes during the etching process are avoided. Removing the binder phase by the method of this in¬ vention results in a substrate with a well defined hard surface, which can be used uncoated, but is very suitab¬ le for the deposition of a thin wear-resistant layer of a metal carbide, oxide, nitride or mixtures thereof, e.g., TiC, TiN, AI2O3, diamond etc. by CVD- and PVD-me- thods. A further advantage of using the present method is the reduced risk for decarburization of the substrate surface and eta phase formation associated therewith when carrying out deposition with CVD-methods. The de- carburized zone in certain cases gives negative effects on the cutting properties of the final product. The invention has been described above with refe¬ rence to binder phase enriched cemented carbide. The method can also be applied to coated or uncoated conven¬ tional cemented carbide, i.e., hard material based on carbides of W, Ti, Ta and/or Nb in a binder phase of co¬ balt as well as to other types of hard materials contai¬ ning hard constituents (carbides, nitrides, carbonitri- des etc) in a binder phase based on cobalt and/or nickel, such as titanium based carbonitride alloys usu¬ ally called cermets. Example 1 (prior art) Cemented carbide inserts of type CNMG120408-QM with a composition of in addition to WC, 5,5 % Co, 8,5 % Tie + TaC + NbC and sintered in such a way that they had a cobalt enrichment in the surface zone and with a cobalt layer of about 2 μm thickness and a graphite layer of about 2 μm thereon were subjected to electrolytic etching in diluted 10 % sulphuric acid. By applying 1-2 V voltage and 30-70 As/cm 2 weight losses of 55-130 mg/insert were obtained which resulted in etching away not only of cobalt from the surface but also in certain areas up to 30 μm in depth. Fig 1. Example 2 Cemented carbide inserts according to Example 1 were subjected to etching in a mixture of concentrated sulphuric acid and concentrated phosphoric acid in a vo¬ lume ratio of 1:1 at a temperature of about 50°C. In this case a weight loss of 10-14 mg/insert was obtained at an applied voltage 4,5-5 V and 100-140 As/cm 2 after the same time as in Example 1. The cobalt layer was re- moved from the surface without any deep penetration as is evident from Fig 2. Example 3 Example 2 was repeated with the difference that the inserts were initially subjected to a light wet blasting with 150 mesh AI2O3 at 1.2 bar pressure during 2 min in order to remove the graphite layer. At 35-40 As/cm 2 and 6 V a weight loss of about 5-8 mg/insert without any deep etching was obtained. Example 4 Example 2 was repeated with the difference that the etching was performed with an applied constant voltage of 15 V and 50-100 As/cm 2 . The weight loss was in this case 10-12 mg/insert without any deep etching. Example 5 Inserts of type TN G160408-QF of a titanium based carbonitride alloy with a binder phase of about 10 % co- bait and 5 % nickel which after the sintering had a bin¬ der phase layer of about 2 μm thickness on the surface were etched according to Example 2 but at 50-90 As/cm 2 and 6 V. The weight loss was 6-9 mg/insert. No deep pe¬ netration was observed.

Claims: Claims 1. A method of removing by etching the binder phase from the surface of hard material containing hard con¬ stituents in a binder phase based on cobalt and/or nickel, c h a r a c t e r i z e d in that the etching is carried out electrolytically in a mixture comprising concentrated sulphuric acid and concentrated phosphoric acid in the volume ratio of (0,5-2) :1 with a water con¬ tent of <50 % at a temperature of 25-60°C. 2. A method as defined in claim 1, c h a r a c t e r i z e d in that the material » initi¬ ally subjected to a light blasting or other mecianical treatment before carrying out the electrolytic etching.

Inventors: SVENSSON ROLF, SVENSSON, ROLF
Applicants: SANDVIK AB
Application year: 1992
Main classification: C25F3
IPC codes: C25F3, C25F5