WO1980001487A1

WO1980001487A1 - Cemented carbide body having a wear resistant surface layer

Info

Publication number: WO1980001487A1
Application number: PCT/SE1980/000015
Authority: WO
Inventors: M Mikus; J Lindstroem; L Aschan
Original assignee: Sandvik Ab; M Mikus; J Lindstroem; L Aschan
Priority date: 1979-01-22
Filing date: 1980-01-18
Publication date: 1980-07-24
Also published as: EP0028600A1; SE417618B; SE7900529L; JPS56500934A

Abstract

By applying very thin layers of wear resistant materials to the surface of a hard metal body, its life can be essentially improved used as a cutting tool in metal cutting. In order to obtain improvements of the properties by means of such layers there is needed a securing adhesion or adherence between layer and substrate. The invention relates to such a hard metal body, to which one or more additive metals have been added, which can selectively be oxidized and thereby form a wear resistant layer of improved properties of adherence and ductility.

Description

Cemented carbide body having a wear resistant surface layer

The present invention relates to sintered hard metal bodies provided with a thin, wear resistant surface layer and a method of making such bodies.

It is known that the life of hard metal bodies - such as cutting tools of cemented carbide containing at least one hard material besides binder phase - can be increased essentially by applying a surface layer of a still more wear-resistant material, for example aluminium oxide. The hard metal contains normally hard materials as carbides, carbonitrides and nitrides and possibly also borides, oxides, diamond and/or boron nitride, while the binder phase usually consists of one or more of the iron group metals as Co, Ni and Fe. In order to obtain improvements of the properties by means of such layers there is needed a securing adhesion or adherence between layer and substrate. In that way, flaking of the layer by normal stresses can be avoided.

During the 1970-ies the technique used for coating cemented carbide with thin surface layers has developed very rapidly. Among other things it has been found that hard metal cutting inserts having an (outermost) layer of aluminium oxide with a thickness between 1 and 10 /um have shown good results in cutting of steel. The most important method for obtaining well adherent layers is to use the CVD-technique (Chemical Vapor Deposition).

The mentioned method involves, however, certain disadvantages for the coated product. Thus, the layers will be thickest on the edges or the corners because the reactants come from the gaseous phase and said places are better available for the flowing gas. From cutting views, however, it would be more favourable if the layers were thicker at a certain distance from the cutting edge into the rake face of the insert, where the chip breaking is normally done and where the temperature maximum in the flow layer between chip and tool is usually situated, i.e. where the crater wear can be considerable. Furthermore, some kind of intermediate layer is usually necessary in practical use to give an aluminium oxide layer - prepared by CVD-technique - a satisfactory adherence to the substrate. Such intermediate layers have for example consisted of titanium carbide, aluminate of iron group metals or of cemented carbide containing gamma-phase of TiC-type. Such intermediate layers together with the alumina layer have a certain influence on the tensile rupture strength of the hard metal, however, which can lead to lower strength.

It has now been found, however, quite surprisingly, that a hard metal substrate can be provided with a wear resistant oxide layer without the earlier mentioned drawbacks being observed. According to the invention it is added to binder phase one or more metals whose oxides have greater thermodynamically free energy of formation than the oxides of the metals of the iron group, i.e. the metals normally making the binder phase of the cemented carbide. By a following oxidation process (including heat-treatment) in which the mentioned additive metals are oxidized and/or it is obtained that such metal and oxygen atoms are enriched at the surface of the substrate, a thin layer having excellent adherence and toughness properties can be obtained.

It is true that techniques are earlier known for production of wear resistant surface layers on hard metal by varying types of oxidation processes, but they have not given layers of optimal wear resistance and adherence conditions.

Consequently hard metal (with binder phase of Co) has been treated in an oxidizing atmosphere at enchanced temperature for a surface layer of cobalt oxide (e.g. the American patent No. 3.615.884) to be obtained. Hard metal substrates, whose surface first has been supplied by elements such as Ti, Ta, Hf and W, have also been treated in an oxidizing atmosphere and a surface layer containing oxides of Ti, Ta, Hf etc. has been formed, but elements such as Co and W as a whole been left unaffected (e.g. the American patent No. 4.018.631 or the corresponding German announcement No. 2.625.940). In the last case mentioned the additive metals, whose oxides have greater thermodynamically free energy of formation than the oxides of the metals of the iron group, have no t been added as alloying elements to the binder metal. This means strongly deteriorated possibilities of a production of surface layers of optimal wear qualities. Furthermore, the known method results in the production of an intermediate layer with the previously mentioned disadvantages. Such an intermediate layer also prevents, in spite of a subsequent heat treatment, the mentioned alloying elements from arrival at the surface, because the fraction of the binder phase is less in the surface layer than in the bulk of the substrate. Thus, the mentioned patent rather diverges from the present invention.

Furthermore, it is known, according to for example the Swedish patent No. 357.984 or the corresponding USA patent No, 3.837.896, to oxidise the surface of hard metal before it is covered with one or more wear resistant layers of carbide or nitride. It is further specified that such a carbide layer can be surface oxidized before an outer oxide layer is deposited. Also in this case any alloying element, which can be oxidized and form a coating, is not purposely added to the binder phase. Thus, the direct bonding between oxide surface layer and substrate, which is referred to in the present application, cannot be obtained.

Among further examples of the technical state the Swedish patent application 77067706 (or the corresponding Swedish or American patents Nos. 406.090 and 4.180.400, respectively) can be mentioned. According to the former one an addition of Ti, Zr and/or Hf as doping reagents for the precipitation of aluminium oxide from gas phase onto hard metal can result in a peculiar structure of the layer of aluminium oxide. Such doping reagents are added separately to the gas of coating and if a diffusion of different elements from the substrate will occur during the precipitation it only means a diffusion of trace elements from the outermost surface zone Just below the Al₂O₃ layer into this layer during the actual coating process. However, the known technique does not involve a diffusion from the inner part of the substrate up to the surface of the hard metal in a separate process before a possible coating by an oxide layer, which is the case in the present patent application. (of. continued text)

For further illustration of the known state of the technology the German patent No. 847.390 can be mentioned, which states oxidation of hard metal, but with a removal of the produced oxide surface layer and it is stated that such a treated hard metal body has shown improved performance of cutting. Any purpose of producing an oxide layer with improved adherence was not actual in the known case but on the contrary the adherence must be less good to lead to the desired result. Furthermore, the Swedish patent No. 97.484 can be mentioned which shows that it is earlier known to add finely pulverized aluminium to an original mixture of hard metal powder to react with the oxygen left in the mixture and the carbides and give a desired content of aluminium oxide in the material.

The purpose, however, was to try to produce a hard metal as ductile and porefree as possible and was neither dealing with the production of surface layers on the hard metal body nor the utilization of the wear resistance of the aluminium oxide.

Another essential increase of the wear resistance of the surface zone and a subsequent increase of the life of the hard metal body, according to the invention, can be obtained by applying one or more separate surface layers, consisting of a wear resistant compound, for example a ceramic oxide, on the oxide coated hard metal body. The separate surface layer should preferably consist of the same oxide and/or oxides, that is formed by oxidation and/or heat treatment of additive metal. In the present invention a principle, which has not been found in other purposes, of influencing the adherence between two phases of essentially different character, e.g. metal and oxide, has been utilized.

It has often been necessary in connection with development of for example fiber reinforced I composite materials to use different additives to the metallic phase to improve the adherence across the phase boundaries. The common solution has been the addition of so-called active (reactive) metals (such as Cr, Ti, Zr, V) which have reacted with the surface layer of the oxide and which, atom after atom, partly have substituted the original metal atoms (e.g. Al) of the oxide. If the reaction zone has got the right thickness an optimal adherence is obtained (a maximum of the shear strength). However, too high a concentration of additives or too strong a reaction (for example by too high a reaction temperature) results in brittleness of the oxide material or in the phase boundary.

There is a theory proposed that if one imagines the positions of the atoms in the surface layer of a solid, crystalline oxide (e.g. Al₂O₃), the big and negatively charged oxygen ions lie farthest out and give an effect of "screening off" the metal ions of the oxide so that they will have only a weak influence on the outermost part of the outer surface. This gives a relatively weak adherence to "usual" (non-reactive) metal atoms in adjacent metallic phase.

In the present case the principle of alloying a certain fraction of the corresponding metallic atoms of the oxide as additive material to the metallic phase has been used. In this way the above mentioned "screening off" effect can to a certain degree be compensated by metal - oxygen - bondings alike these of the oxide but emanating from the atoms of the alloy ing additive in the metallic phase.

Thus, in the present case the adherence in a transition zone between the hard metal and the surface layer can be facilitated by the creation of metal-oxygen-metal-bonds across the surface of the phase boundary without being dependent on a slow diffusion process (when a number of atoms of the additive metal, corresponding to those of the oxide, already are present in the surface zone of the binder metal as an alloying additive).

It can be mentioned that it is true that it is earlier known that it is created on the surface of a crystal of a metal oxide a layer which is enriched in oxygen atoms and which "screens off" the attraction power of the metal atoms on such metal atoms that are brought on top of the surface by for example a layer coating process. (Similar conditions are actual for the reverse process that an oxide layer is applied to a metallic substrate, e.g. hard metal). From this it is realized that the steps or measures which according to the invention have been done to bridge or obstruct these unfavourable conditions are of the greatest importance to satisfactory bonding between layer and substrate.

One of the additive metals, which oxide has greater thermodynamically free energy of formation than the oxides of the metals of the iron group, is Al. As the aluminium oxide according to earlier mentioned facts, is the probably most important wear resistant compound that is used in layer coating of hard metal, the following description will to a great extent be concentrated to Al but would be applicated to several other metals as e.g. Mg, Ti, Zr, Hf, Cr, V, Th and Y.

Another example of improved adherence according to the above mentioned principle is shown in figure 1. As a model system Ni-Al-alloys have been used, respectively pure Ni, on a substrate consisting of single crystal aluminium oxide. Drops of the different alloys have been melted and solidified on the substrate in vacuum (with an oxygen potential of about

10^-15 or somewhat lower). After the solidification the shear strength for removal of the drop has been measured; the shear strength is a measure of the adherence in the contact surface. It appears that an improved adherence is obtained for additions of 2-3 w/o Al to Ni.

When these experiences are applicated to hard metal, it has been shown suitable that in the present case alloy with a certain fraction Al in the binder phase of the hard metal by additions of an alloy with e.g. the composition CoAl as a powder. The amount of Al should be adjusted so that the binder phase is alloyed with 0.05 - 25 w/o and preferably 0.1 - 20 w/o aluminium (calculated for the amount of the binder phase). Often an Al-concentration between 0.5 - 15 w/o and preferably 1 - 10 w/o (calculated for the amount of binder phase) has been shown giving optimal results, (c.f . the earlier mentioned example) The production of hard metal bodies according to the invention can suitably be done in such a way that the hard metal after pressing is sintered in a normal atmosphere, after which the body is treated in an oxygen contented atmosphere at the end of the sintering process. At or after the sintering process a certain fraction of aluminium oxide, which creates spotwisely appearing agglomerates towards the surface in a non-oxidized matrix, can in this way be obtained by selective oxidation. A great part of the amount of aluminium and oxygen can diffuse out of the hard metal, however, and be enriched at the surface. In certain cases it could be more preferable sintering the hard metal in a normal way and thereafter heat treating the sintered bodies in an oxygen contented atmosphere at a temperature which is lower than the sintering temperature but is higher than 150°C. By using another or more sequences of heat treatment bigger amounts of aluminium can diffuse out of the binder metal to the surface of the hard metal body. If the treatment is done under protective gas in a furnace,the protεctive gas can preferably be regulated so that a certain vapour pressure of gases containing oxygen (oxygen partial pressure) will be obtained. In a suitable range of the size of this partial pressure ) with increasing treatment temperature

and treatment time, a thicker aluminium oxide layer can be formed. It has been shown that the best adherence of the layer often has been obtained at partial pressures which have corresponded to = 10 ^-15 - 10^{- 5}. Pa. For other oxygen containing

gases a very much higher partial pressure can as a rule be accepted. In certain cases it can be preferable doing the treatment in a gas mixture containing essential amounts of nitrogen.

In the present case, where hard metal is used as a substrate, layers which at favourable conditions achieve thicknesses of several /um and which have a very good adherence to the substrate, can be obtained at relatively high temperature. The layers are somewhat thinner at the edges of the hard metal body, i.e. at the edge areas In the case of cutting plates, which is favourable from durability view in the field of cutting. This is due to the fact that the condition for cutting is a diffusion of aluminium outwards from the binder phase and that the amount of the alloying component aluminium is less near an edge due to the locally bigger ratio area/volume. The layers which have been obtained at lower temperatures, e.g. in the range of 400 - 800°C, are usually very thin, e.g. 0.2 - 1μm. However, they can suit well as a substrate for more layers, which normally are deposited by CVD-technique.

If the heat treatment in an oxygen containing gas is done at relatively high temperature, ca. 1300 - 1500°C, relatively thick ( > 1μm) Al₂O₃, layers can be obtained in the surface zone of the hard metal body. At heat treatment in an atmosphere containing essential amounts of nitrogen at a lower temperature, ca. 500 - 1100°C, the presence of aluminium nitride on the surface of the hard metal body could be observed. Further more at higher temperature, ca. 1300 - 1500°C, an enrichment of ɣ -phase in the surface zone was obtained.

After a connected treatment step in gas atmosphere of certain, low concentration of oxygen (at temperatures over 800°C but below 1500°C) it has been shown that the layer of aluminium nitri de has been changed and now mainly consists of Al₂O₃ besides small amounts of remaining aluminium nitride.

It has been shown that the ductility qualities of the coated product are good due to the lack of the otherwise necessary intermediate layer (for example of titanium carbide). The decrease of the ductility, which is to be expected due to the fact that a fraction of aluminium is left in the binder phase of the hard metal has surprisingly appeared to be almost neglible as long as the aluminium concentrations are moderate. Under certain conditions even a solution hardening can occur with an increased strength as consequence.

However, when the fraction of aluminium is increased so that according to the known phase diagrams (e.g. Co-Al and Ni-Al) ranges containing stable intermediate phases (of type CoAl) are approached, the increased adherence of the oxide layer is obstructed. Thus, an increased fraction of Al involves a discontinuous precipitation of the intermediate phase in the form of coarse particles which can decrease the ductility of the binder phase and make the hard metal brittle. Further, too strong an internal oxidation of the substrate in the zone nearest below the layer and inwards can be obtained, when the hard metal is heated in an oxidizing atmosphere.

It can be mentioned that it Is true that it is earlier known to add small amounts of Al to alloys, which are like hard metal, but this has been carried out for quite other purposes than to produce wear resistant, well adherent surface layers. (Note for example the German announcements (Offenlegungsschrift) 2.407.410 and 2.407.411 or the American patents Nos. 3.916.497 and 4.108.649). In these the precipitation hardening mechanisms, successfully applied in the field of heat resistant material on Ni-basis for turbine blades in gas turbines, namely by increasing the high temperature strength by precipitation of small particles of phase, corresponding to about Ni₃(Al,Ti) have been tried to be imitated. Thus, by addition of a little amount of Al (e.g. 2.4 - 9.2 w/o of the binder metal) to for instance a TiC-Ni-alloy the fraction of matrix metal(Ni) can be increased up to 50 - 60 volume% at the sacrifice of the fraction of carbide grains without too much decrease of the strength hardness and wear resistance of the matrix metal. In other words, the decrea se of hardness etc, caused by such a high fraction of relatively soft matrix metal, may partly be compensated by different hardening mechanisms for the matrix, where one possibility is provided by using the Ni₃, (Al,Ti)- precipitation. (The purpose of these actions, however, has, as mentioned, no connection with the present application which refers to the improvements of especially the adherence, which can be obtained for surface layers of hard metal.)

According to the invention it has been shown advantageous to alloy the binder phase at the hard metal body with yttrium, either as separate additive metal or in combination with other additive metals as for example aluminium. A suitable concentration has been shown to be 0.005 - 5 w/o yttrium, preferably 0.05 - 1.5%, calculated for the amount of binder phase. Good results have been obtained in tests performed with hard metal bodies, such as cutting plates, which have been heat treated in an oxygen contented atmosphere in which a wear resistant surface layer essentially consisting of yttrium oxide has been obtained. Further improvements of life due to increased wear resistance have been noticed for such cutting inserts as have been coated with a thin surface layer of yttrium oxide and/ /or aluminium oxide on top of the surface oxidized substrate.

Favourable results have also been obtained with hard metal, where the binder phase has been alloyed with aluminium and yttrium, total concentration 0.05 - 10 w/o, calculated for the amount of binder phase. The conditions at the oxidation and/or the following oxide coating have in this case preferably been adjusted so that the separate surface layer essentially has consisted of aluminium oxide and a fraction of yttrium oxide up to at most 25 mol-%, preferably max. 10 mol-%, calculated for the composition of the layer. For compositions with Al-concentrations (of the binder phase) within the range ca. 0.2 - ca. 20 w/o it is a fact that the product can be identified due to the residual concentration of aluminium and/or aluminium oxide In the substrate (the matrix or binder phase of the hard metal).

If the hard metal bodies shall not be ground the sintering can preferably be modified by an extra treatment step at 150-1250°C so that the main part of the aluminium concentration in the surface zone will diffuse out to the surface, will be oxidized and will create a layer of aluminium oxide.

Among the special advantages which are obtained with the method descripted according to the invention can be mentioned that the additive metal in the form of Al will diffuse outwards the boundary surface from the binder phase, which principle is the right one from bonding point of view.

Furthermore, it has also been shown to be normal that the surface of both sintered and in a following operation ground or mechanically surface treated hard metal is coated by a very thin film of binder metal, for this mentioned example (according to the invention) even containing aluminium. This film can cover practically all carbide grains, which otherwise should be the outer bondary surface of the hard metal.

By the following examples the conditions are more clearly evident under which hard metal bodies according to the invention have been able to be produced and the results which have been obtained in practical tests.

Examυle 1

Hard metal bodies of a sintered experiment alloy contained 12% cobalt (Co) and 86.2% tungsten carbide (WC) and an addition of 1.8% Al (w/o) . After the sintering it was observed by microprobe analysis that the binder metal in the surface zone (between the WC-grains) and in the rest of matrix mainly was a Co-rich phase with Al in solid solution. The hard metal bodies were ground to the plate type SPUN 120308, after which the Inserts were rounded off by tumbling.

Then the specimens of the mentioned alloy were heat treated at 550°C during 2 hrs in an oxidizing atmosphere. In a new microprobe analysis there could be observed, that a fraction of the originally soluted amount of Al nearest the surface now had been transformed to, in cobalt-matrix, non-soluble, precipitated particles containing aluminiumoxide, by so-called selective oxidation.

Such a specimen was afterwards coated in a CVD-reactor with an approximately 8 /um thick Al₂O₃-layer, in an earlier known way, per se. As a material of comparison a hard metal alloy of similar composition, but without additive Al, treated in a corresponding way, was used. In both cases grey to dark coloured coatings of Al₂O₃. were obtained on the specimens.

In a hardness testing apparatus indentations (with the load 295 N) were made on one of the surfaces of each specimen. The Al-alloyed and heat treated specimen showed no flaking near or between the hardness indentations, while the material of comparison showed distinct flaking of the dark Al₂O₃-coating; this could be observed by bare eyes as shining spots due to the metallic brightness of the underlying hard metal.

The tensile rupture strength of the specimens had decreased ca 18$, which is normal for Al₂O₃-coated hard metal, if relatively thick layers have been precipitated.

Example 2

To a test alloy containing 85,5 w/o WC, 5.9% Co and 8% cubic carbides 0.3% Al was added in the form of a Co-Al-compound. From the powder mixture hard metal inserts of type TNMM 160308 were pressed. The sintering temperature was 1550°C and the time 3 hrs. At the end of the process intervals the sintering at mosphere was weakly oxidizing.

After cleaning the material and the corresponding comparative alloy (hardmetal with 86 w/o WC, 6% Co and the rest consisting of cubic carbides) were put in a CVD-reactor and a refractory layer, around 3/um thick, of Al₂O₃, was applied.

Inserts of test- and comparison alloys were tested by turning tests In steel under the following conditions:

Wear resistance tests of steel cutting

Material: SIS 2541, KB = 290 Cutting data: Cutting speed 180 m/min

Feed 0.30 mm/rev.

Cutting depth 2.0 mm

Insert; type: TNMM 160308

Number of tested edges: 3/variant

Variant Life, min.

According to the invention 21

Al-free hardmetal with Al₂O₃-layer 6.4 x) Al-free hardmetal without 5.2

x) Al₂O₃-layer started flaking after 1-2 min. testing time.

Example 3

For turning of cast iron the hard metal insert produced and treated according to example 2 showed better wear resistance than the corresponding product (Al-free hard metal), which had been coated with a 2μm thick layer of titanium carbide (TIC).

Wear resistance tests of cutting of cast iron

Material: SIS 140125, HB = 230

Cutting data: Cutting speed 350 m/min.

Feed 0.30 mm/rev.

Cutting depth 2.0 mm Insert type: SPUN 120308 Number of tested edges: 3/varIant

Variant Life, min.

According to the invention - 17.3

Al-free hardmetal, with TIC-layer 5.7

Example 4 For heat treatment tests of a hard metal alloy containing 0.3 w/o Al (93% WC, 6.7% Co) in an atmosphere of certain oxygen concentration at temperatures between 1000 and 1400°C the presence of a layer with a high concentration of refractory α-Al₂O₃, (to a depth of around 2/um from the surface) could be observed.

For cutting of cast iron under the same conditions as under example 3, inserts treated as above were compared with inserts of a normal hard metal quality (93% WC, 7% Co) and the following results were obtained:

Variants: Life, min.

According to the invention: 93WC, 6.7Co, 0.3 Al 11.6 Normal hard metal: 93 WC, 7 Co 1.7

Example 5

In heat treatment tests of a hard metal alloy containing 0.15 w/o Al, 5.9 % Co, 8% cubic carbides and 85.95 % WC, in an atmosphere consisting of a nitrogen-hydrogen mixture, at 1400°C for a time of 2 hrs and a following treatment at 1000°C in an oxidizing atmosphere for 15 minutes, a refractory layer (thickness about 1μm) mainly consisting of aluminium oxide and a residual amount of aluminium nitride was obtained. This layer was mainly In contact with the hard metal substrate by bordering on the surface zone (mainly consisting of cubic carbides) enriched in ɣ-phase.

Such a specimen was after that coated In a CVD-reactor with a ca. 7μm thick Al₂O₃-layer, in an earlier known way, per se. As comparative material a hard metal alloy of similar composition but without Al addition was used. In both cases coatings of Al₂O₃, on the specimens were obtained.

In wear resistance tests in connection with turning of cast iron (in a corresponding way to example 3) a considerably longer (85%) life of the specimen with Al -alloyed binder phase compared to the above mentioned comparative specimen, was observed.

Claims

1. Body consisting of a core or a substrate of sintered hard metal containing at least one hard material besides binder phase consisting of one or more of the metals of the iron group Fe, Co and Ni, where the substrate is equipped with a thin surface coating of higher wear resistance than that of the hard metal in the core, c h a r a c t e r i z e d in that the binder phase contains one or more additive metal(s) with greater affinity to oxygen than the metals of the iron group, and that the surface coating essentially is constituted by such oxide as is formed mainly by oxidation of additive metal.

2. Body according to claim 1, c h a r a c t e r i z e d in that one or more separate surface layers consisting of the same oxide or oxides as that or those which have been formed by oxidation of the additive metal (s), are applied to the hard metal body already supplied with oxide surface coating.

3. Body according to any of the previous claims, c h a r a c t e r i z e d In that the binder phase is alloyed with 0.05 - 25 w/o aluminium calculated on the amount of binder phase.

4. Body according to any of the previous claims, c h a r a c t e r i z e d in that the separate surface layer essentially consists of aluminium oxide,

5. Body according to any of the claims 1 and 2, c h a r a c t e r i z e d in that the binder phase is alloyed with 0.005 - 5 w/o yttrium, calculated on the amount of binder phase.

6. Body according to any of the claims 1, 2 and 5, c h a r a c t e r i z e d in that the separate surface layer essentially consists of yttrium oxide.

7. Body according to any of the previous claims, c h a r a c t e r i z e d in that the binder phase is alloyed with aluminium and yttrium in a total concentration of 0.05 - 10 w/o calculated on the amount of binder phase,

8. Body according to claim 7, c h a r a c t e r i z e d in, that the separate surface layer essentially consists of aluminium oxide and a fraction of yttrium oxide up to at most 25 mole-%, calculated on the composition of the layer.