SE417618B - HARD METAL BODY WITH DURABLE LAYER - Google Patents

Description

_7900529'l + 2 During the 1970s, the technology for coating cemented carbide with thin surface layers developed very rapidly. From experience, e.g. hard metal inserts with an (outermost) layer of alumina with a thickness between 1 and 10 μm have been shown to give good results in cutting machining of steel etc. The most important method for creating such, well-adhering layers is the so-called CVD (Chemical Vapor Deposition) method. In the mentioned method, however, there are certain disadvantages for the coated product. Thus, due to the fact that the reactants come from the gas phase, the layers become thickest in the edges and corners, because these places are better accessible to the flowing gas. From a cutability point of view, however, it would be more favorable if the layers were thicker at a certain distance from the edge into the chip side of the insert, where the chip breaking normally takes place, É and where the temperature maximum in the flow layer between the chip and tool is usually located, i.e. where the pit wear takes place.

Furthermore, some form of intermediate layer is usually required for an alumina layer produced by CVD technology to have satisfactory adhesion to the substrate for practical use. These intermediate layers could, for example, consist of titanium carbide, ferrous metal aluminate or of cemented carbide containing TiC-type gamma phase. However, such intermediate layers together with the alumina layer have a certain effect on the flexural strength of the cemented carbide, which can lead to lower strength values.

It has now surprisingly been found that it is possible to provide a cemented carbide substrate with a durable oxide layer without being bothered by the previously mentioned disadvantages to any great extent. According to the invention, the binder phase is supplied with one or more additive metals whose oxides have greater thermodynamic free formation energy than the oxides of the metals of the iron group, i.e. the metals that normally form the binder phase in the cemented carbide.

GGHOW a subsequent oxidation process (incl. Heat treatment), where one selectively oxidizes said additive metals and / or achieves that such metal and oxygen atoms are enriched at the surface of the substrate, a thin layer with extremely good adhesion and toughness properties can be achieved.

Although it is previously known per se to seek to achieve durable surface layers of cemented carbide through various types of oxidation processes, these have not led to layers with optimal wear resistance and adhesion conditions. 7900529-4 Thus, one has i.a. treated cemented carbide (with bonding phase of Co) in an oxidizing atmosphere at elevated temperature to obtain a surface layer of cobalt oxide (see, for example, U.S. Pat. No. 3,615,884).

Carbide substrates have also been treated, the surface of which has first been added elements such as Ti, Uf an oxidizing atmosphere to form a surface layer containing oxides such as Tu, and W 1 of Ti, Ta, Hf etc. while elements such as Co and N have largely become opaque. appeared (see, for example, U.S. Pat. No. 4,018,631). In this latter case, however, the additive metals, the oxides of which have greater thermodynamic free formation energy than the oxides of the metals of the iron group, have not been added as alloying elements to the binder metal. This means greatly reduced possibilities for creating surface layers with optimal wear properties. Furthermore, the known method involves the training of an intermediate layer with the disadvantages mentioned above.

A further significant increase in the wear resistance of the surface zone and thus a corresponding increase in the service life of the cemented carbide body according to the invention can be achieved by applying one or more separate surface layers consisting of some durable compound, such as for example a ceramic oxide, to the oxidized hard metal body. . Preferably, the separate surface layer should consist of the same oxide or oxides formed by oxidation and / or heat treatment of additive metal. In the present invention, a principle has not been used which has not been found in other proposals to influence the adhesion between two phases of a substantially different nature, e.g. metal against oxide.

It has often been necessary in connection with the development of e.g. fiber-reinforced composite materials to use various additives to the metallic phase to improve adhesion across phase boundaries.

The usual solution has been the addition of so-called "active" (reactive) metals (such as Gr, Ti, Zr, V) which have reacted with the surface layer of the oxide and which, atom by atom, have partially replaced the original metal atoms of the oxide (eg Al). If the reaction zone has the correct thickness, an optimal adhesion is obtained (a maximum in the shear strength values).

On the other hand, too high additives or too strong a reaction (eg due to too high a reaction temperature) lead to brittleness in the oxide material or in the phase boundary. 7900529-4 4 The theory has been established that, if one considers the atomic positions in the surface layer of a solid, crystalline oxide (eg Al 2 O 3), the large and negatively charged oxygen ions lie at the far end and give a "shielding effect" "so that the metal ions of the oxide have only a slight outward effect beyond the outer surface. This gives a relatively weak adhesion to "ordinary" (non-reactive) metal atoms in the adjacent metallic phase.

In the present case, the principle has been used to alloy into a certain proportion of the corresponding metallic atoms of the oxide as additive material into the metallic phase. In this way, the above-mentioned "shielding effect" can to a certain extent be compensated by means of metal-oxygen bonds similar to those of the oxide but starting from the alloy set atoms in the metallic phase.

Thus, in the present case, the adhesion in a transition zone between the cemented carbide and the oxide layer can be facilitated by the formation of metal-oxygen-metal bonds across the phase interface without being dependent on a slow diffusion process (when a number of additive metal atoms, corresponding to the oxide, already exist). in the surface zone of the binder metal as an alloying additive).

It can be mentioned that it is per se previously known that a layer enriched in oxygen atoms is formed at the outermost surface of a metal oxide crystal which "shields" the attractiveness of the metal atoms on such metal atoms which are applied on top of the surface by, for example, a coating process. (Similar conditions apply in the reverse process that an oxide layer is applied to a metallic substrate, eg hard metal). It can thus be understood that the steps or measures taken according to the invention to bridge or counteract these unfavorable conditions are of the greatest importance for satisfactory bonding between layers and substrates.

One of the additive metals whose oxide has greater thermodynamic free energy of formation than the oxides of the metals of the iron group is Al. Since alumina, as previously mentioned, is perhaps the most important durable compound used for the coating of cemented carbide, the following description will be largely concentrated on Al but could be applied to a variety of other metals such as Mg, Ti, Zr, Hf, Cr, V, Th and Y. 79005294 »It has been found suitable to alloy a certain proportion of Al into the binder phase of the cemented carbide by adding an alloy with, for example, the composition CoA1 in powder form. The amount of Al should be adjusted so that the binder phase is alloyed with 0.05 - 25% by weight of aluminum (calculated on the amount of binder phase). The production of cemented carbide bodies according to the invention can suitably take place in such a way that the cemented carbide after pressing is sintered in a normal atmosphere, after which the body is treated in an oxygen-containing atmosphere at the final stage of the sintering process. In connection with or after the sintering process, a certain proportion of alumina can thus be obtained by selective oxidation, which forms spot-on accumulations against the surface of a non-oxidized matrix. However, a large part of the amount of aluminum and oxygen can diffuse out of the cemented carbide and be enriched at the surface. In some cases it may be more advantageous to sinter the cemented carbide in the normal way and then heat treat the sintered bodies in an oxygen-containing atmosphere at a temperature below the sintering temperature but exceeding 150 ° C.

By resorting to one or more additional heat treatment steps, larger amounts of aluminum can be diffused out of the binder metal to the surface of the cemented carbide body. If the treatment takes place under shielding gas in an oven, you can suitably regulate the shielding gas so that you get a certain vapor pressure of gases containing oxygen (oxygen partial pressure).

Within a suitable range for the magnitude of this partial pressure (PO 2), an increasingly thicker alumina layer can be formed with increasing treatment temperature and treatment time. It has been found that the best adhesion of the layer is often obtained at partial pressure values corresponding to P02 = 10 '15 - 10 '5 Pa. For other oxygen-containing gases, however, significantly higher partial pressures can generally be accepted. In some cases it may be advantageous to allow the treatment to take place in a gas mixture containing significant amounts of nitrogen.

In the present case, where cemented carbide is used as the substrate, layers can be obtained at a relatively high temperature which, under favorable conditions, reach thicknesses of several .mu.m and which have a very good adhesion to the substrate. The layers are slightly thinner at the edges of the cemented carbide body, i.e. at the edge areas in the case of cutting plates, which is favorable from a durability point of view when it comes to cutting machining. 7900529-'4 '6 This is due to the fact that the formation of layers presupposes diffusion of aluminum outwards from the binder phase and that the supply of the alloy component aluminum is less close to an edge due to the greater surface / volume ratio locally. The layers obtained at lower temperatures, for example in the range 300-600 ° C, are usually very thin {for example 0.2-1 / μm. However, they can be well suited as a basis for additional layers, which are normally applied by means of CVD technology.

It has been found that the toughness properties of the coated product are good due to the lack of the otherwise necessary intermediate layer (for example of titanium carbide or gamma phase). The reduction in toughness that is to be expected due to a proportion of aluminum remaining in the binder phase of the cemented carbide has surprisingly proved to be almost negligible as long as the Al contents are moderate. Under certain conditions, t.o.m. a solution cure occurs with increased strength as a result.

However, when the proportion of aluminum is increased so that, according to the known phase diagrams (eg Co-Al and Ni-Al), areas containing stable intermediate phases (of the CoAl type) are approached, the improved adhesion of the oxide layer is counteracted. An increased proportion of Al thus leads to a discontinuous precipitation of the intermediate phase CoAl, in the form of coarse particles, which can reduce the toughness of the binder phase and make the cemented carbide brittle. Furthermore, one can get an excessive internal oxidation of the substrate in the zone immediately below the layer and inwards, when the cemented carbide is heated in an oxidizing atmosphere. It may be mentioned that it is per se previously known to add small amounts of A1 to alloys which resemble cemented carbide, but this has been done for completely different purposes than to provide durable, well-adhering surface layers. (See, for example, German Patent Nos. 2,407,410 and 2,407,411 or U.S. Patents 3,916,497 and 4,108,649). Attempts have been made to imitate the precipitation hardening mechanisms which have been successfully applied in the case of heat-resistant material on a nickel basis for e.g. turbine blades in gas turbines, namely to increase the high temperature strength by precipitation of small particles of Ja 'phase, corresponding to approximately Ni3 (Al, Ti). By adding a small amount (for example 2.4 - 9.2% by weight of the binder metal) Al to e.g. a TiC-Ni alloy, one can thus increase the proportion of matrix metal (Ni) up to 50-60% by volume »at the expense of the proportion of carbide grains without reducing the pour strength, hardness and abrasion resistance of the matrix metal too much. In other words, the decrease in hardness m.m. which such a high proportion of relatively soft matrix metal entails, can be partially compensated by various curing mechanisms for the same, whereby one possibility is to utilize the Ni3 (Al, Ti) precipitation.

It has been found advantageous to alloy the binder phase with yttrium, either as a separate additive metal or in combination with other additive metals such as, for example, aluminum. A suitable content has been found to be 0.005 to 5% by weight of yttrium, preferably 0.05 to 1.5% calculated on the amount of binder phase. Good results have been obtained in tests carried out with cemented carbide bodies, such as cutting plates, which have been heat-treated in an oxygen-containing atmosphere and thereby obtained a durable surface layer consisting essentially of yttrium oxide. Further improvements in service life due to increased wear resistance have been found in such inserts coated with a thin surface layer of yttrium oxide and / or aluminum oxide on top of the surface oxidized substrate.

Advantageous results have also been obtained with cemented carbide, where the binder phase has been alloyed with aluminum and yttrium in a total content of 0.05 ~ 10% by weight calculated on the amount of binder phase. The conditions during the oxidation, and / or the subsequent oxide coating, have been suitably adjusted so that the separate surface layer consists essentially of alumina and a proportion of yttrium oxide amounting to a maximum of 25 mol%, preferably not more than 10 mol%, calculated on the composition of the layer.

For compositions with Al contents (in the binder phase) in the range of about 0.2 - about 20% by weight, the product can be identified due to the residual content of aluminum and / or alumina in the substrate (matrix or binder phase of the cemented carbide).

If the cemented carbide bodies are not to be ground, the sintering can suitably be modified by an additional treatment step at 150 - 125000 so that most of the aluminum content in the surface zone diffuses out to the surface, oxidizes and forms a layer of alumina. Among the particular advantages obtained by the described methodology according to the invention it can be mentioned that the additive metal in the form of Al diffuses outwards the interface from the bonding phase, which principle is the right one from the bonding point of view (comparison can be made e.g. In cemented carbide, there is an X '-phase-type cubic carbide in the alloy, for which the bonding ability against an Al2O3 layer does not need to be appreciably improved, since E1Ö VíGUäffIYIiILg of A120 is obtained; be so that the surface of both sintered and in a subsequent work operation ground or mechanically surface-treated cemented carbide is coated by a very thin membrane of binder metal, in the now mentioned example (according to the invention) also containing aluminum.This membrane can cover practically all carbide grains , which would otherwise constitute the outer limiting surface of the core metal.

The following examples show in more detail the conditions under which cemented carbide bodies according to the invention have been produced and the results obtained in practical tests.

Example 1 Cemented carbide bodies of a sintered test alloy contained 12% cobalt (Co) and 86.2% tungsten carbide (WC) plus an addition of 1.8% Al (% by weight). After sintering, it could be observed by analysis in X-ray microanalyzer (microprobe) that the binder metal in the surface zone (between the WC grains) and in the rest of the matrix was mainly present as an Oo-rich phase with Al in solid solution. The cemented carbide bodies were ground to the plate type SPUN 120308, after which edge rounding was done by tumbling.

Thereafter, the samples of said alloy were heat treated at 55,000 for a period of 2 hours in an oxidizing atmosphere. Upon re-examination of the microprobe, it could be established that a proportion of the original dissolved Al amount closest to the surface has now been converted into cobalt matrix-insoluble, separated particles containing alumina, by so-called selective oxidation.

Such a sample was then coated in a CVD reactor with an Al 2 O 3 layer about 8 μm thick, in a manner previously known per se. As a comparison material, a cemented carbide alloy with a similar composition but without Al additive was treated in a corresponding manner. 7900529'4 <1 l In both cases gray to dark colored coatings of Alzuä are applied to the specimens. In a proximity tester, an impression (with a load of 295 N) was made by means of a conical diamond body in the surfaces of one of the respective test bodies. The Al-alloyed and heat-legged sample thickness showed no flaking near or between the hard net impressions, while the comparison material showed clear flaking of the dark Algüö coating; this could be seen with the naked eye as shiny spots due to the metallic luster of the underlying cemented carbide.

The flexural strength of the test rods had decreased by about 18%, which is normal for carbide coated with A12O5, if relatively thick layers have been deposited.

Example 2 To a test alloy containing öb.5 (weight) -ß WC, 5.9% Co and 8 ß cubic carbides was added 0.3 ß Al in the form of a Co-Al compound.

The powder mixture was pressed into cemented carbide type TNMM 160308. The sintering temperature was 155,000 and the time was 3 hours. Towards the end, the sintering atmosphere was oxidizing.

After cleaning, the material was added together with the corresponding comparative alloy (cemented carbide with 86 wt-A WC, 6% Co and the rest consisting of cubic carbides) in a CVD reactor and an approximately 5 .mu.m thick, refractory layer of Al 2 O 5 was applied.

The inserts of the test and comparison alloys were tested by turning tests in steel under the following conditions: Abrasion resistance test in steel processing Material: SIS 2541, HB = 290 Cutting data: Cutting speed 1n0 m / min Feed rate 0.30 mm / revolution Cutting depth 2.0 mm Cutting type: TNMh 160308 Number of tested edges : 3pcs / variant _79oos29-4 = <> 10 Variant Livs1änëd {'min. ""' According to the invention 21 0 Al-free nalrdlxletall with Al203 layer 6.4 n) Al-free cemented carbide without "5.2 Al 0 2 5-layer began to flake after 1-2 min, intervention time.

Example gel 3 'When casting in cast iron, the cemented carbide insert prepared and treated according to Example 2 showed better wear resistance than the corresponding product (Al-free cemented carbide) which coated with a 2 .mu.m thick layer of titanium carbide (TiC).

Durability test when machining cast iron Material: SIS 140125, HB = 230 Cutting data: Cutting speed 550 m / min Nlatnixlg 0.30 mm / Rotation Cutting depth 2.0 mm Cutting type: SPUN 120508 Number of tested edges: 3pcs / variant Variant Life, min.

According to the invention 17.3 Al-free cemented carbide, with TiC layer 5.7 Example gel 4 In heat treatment experiments with a cemented carbide alloy containing 0.3% by weight of Al (93% WC, 6.7% Co) in an atmosphere with a certain oxygen content at temperatures between 1000 and 1400 ° C, the presence of a layer with a high refractory content could be ascertained on the surface of the cemented carbide bodies: L - Al 2 O 3 (to a depth of about 2 .mu.m from the surface) - When machining cast iron under the same conditions as in Example 3, in which cutting plates, treated as above, compared with inserts of a normal carbide grade (93% WC, 7% Co) the following results were obtained: 11 Variants: According to the invention: 93 WC, 6.7 Go, 0.3 Al Normal carbide: 93 WC, 7 Co 790Ü529 ~ Å n Lifespan , min 11.6 1.7 ._, __: .- ^ _ '. ZI. .-

Claims (7)

1. A molding comprising a core or a substrate of sintered cemented carbide containing at least one hard blank and a binder phase consisting of one or more of the metals of the Iron Group Fe, Co and Ni, the substrate being intentionally provided with a thin coating with a higher wear resistance than that of the cemented carbide in the core, as indicated by the fact that the binder phase contains one or more additives with greater affinity for oxygen than the iron group's matallers, and that the coating is é 2 / um and essentially consists of such oxide as 1 is mainly formed by oxidation of additive metal. Molded body according to Claim 1, characterized in that one or more secondary surface layers consisting of the same oxide or oxides formed by oxidation of additive metal are applied to the hardened metal body provided with an oxidic coating. Molding according to one of the preceding claims, characterized in that the binder phase is alloyed with 0.05 - - 25 wt.% Of aluminum calculated on the amount of binder phase. N. Molding according to Claim 2, characterized in that the secondary surface layer consists essentially of alumina. A shaped body according to any one of claims 1 and 2, characterized in that the binder phase is alloyed with 0.005 - - * 5% by weight of yttrium, calculated on the amount of binder phase. A molding according to claim 5, characterized in that the secondary surface layer consists essentially of yttrium oxide. A shaped body according to claim 1, characterized in that the binder phase is alloyed with aluminum and yttrium 1 a total content of 0.05 - 10% by weight calculated on the amount of binder phase. 13 v9oos29 ~ 4 fi. Funnkx-upp according to, the requirement '{, k n n e t e c k n a d thereof, that; the; secondary-a ytsklknet. väscnLllgerx consists of alumina and an axle part ytir-iumoxld amounting to a maximum; 25 mel-i, calculated, on the composition of the layer. .'7900529'1 + Abstract By applying extremely thin layers of durable substances to the surface of a cemented carbide body, it is possible to substantially increase its service life when used as a cutting tool in metalworking. However, in order for such layers to lead to property improvements, a satisfactory adhesion or adhesion between the layer and the substrate is required. adhesion and toughness properties.