The present invention relates to a tray to be used when sintering powder metallurgical articles such as products of cemented carbides and cermets.

Cemented carbides and cermets are alloys mainly con¬ taining hard constituents based on carbides, nitrides and/or carbonitrides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and/or W in a binder essentially based on Co and/or Ni and/or Fe. They are normally produced by powder metal¬ lurgical methods comprising milling a powder mixture containing powders forming the hard constituents and binder phase, pressing and sintering.

After pressing, the compacted bodies contain about 50% by volume of open porosity. Fully dense products are then produced by liquid phase sintering at a temperature when the binder metal is in liquid state, which in prac¬ tice is normally in the 1300-1550°C range depending on composition. In the production of cemented carbides and cermets, almost all sintering processes are performed in indust¬ rial vacuum, i.e. from about 1 torr upwards and in the presence of various gas atmospheres such as mixtures of H2 plus CO, CC>2, Ar, N2, CH4 etc. and with the bodies to be sintered lying on graphite trays. In order to minimi¬ ze the influence of the graphite both in the contact with the bodies to be sintered and with the atmosphere within the furnace these trays are normally surface coa¬ ted, see e.g. H.Kolaska et al; Sintering - Technical and Basic Principles; Powder Metallurgy of Hardmetals, EPMA Lecture Series, European Powder Metallurgy Association, Shrewsbury, UK, 1995, p.6/9-6/10. In practice, an AI2O3 or ZrC>2 based coating is used applied by thermal spray¬ ing, generally plasma powder spraying. Thus, these coa- tings act as barrier layers preventing reaction from oc-

curring between the sintered body and the tray. Reaction can lead to many problems, i.e. carbon uptake from the trays and distortion of the sintered bodies.

It is essential that these graphite trays with bar- rier layers can be reused for as many sintering cycles as possible without deterioration of the layers occur¬ ring, i.e. to maintain inertness and high adhesion to the graphite tray. Normally, the necessity to regrind and recoat the trays is determined by the skilled opera- tor when the coating has been torn off to such an extent that it is easily seen by the naked eye of the operator.

The deterioration is due to metallurgical reactions taking place between the cemented carbide body and the tray leading to unloading problems caused by sticking. When removing such bodies adhering to the tray, cracks may be formed in the coating of the tray. Individual grains from the very surface of the coating or pieces of the coating may be torn off. The problem is especially severe when sintering large bodies of cemented carbides having comparatively large contents of binder phase, these conditions promoting adhesion.

It has now surprisingly been found that using a bar¬ rier layer based on yttria (Y2O3) ^tne life of the coated trays used for production sintering exhibits a signifi- cant increase compared to prior art coated trays before they have to be reground and recoated. This results in large production cost savings and an improved quality of the sintered products.

Fig. 1 shows in 3x magnification the contact surfa- ces of two cutting tool inserts with identical back¬ ground after normal production sintering at 1410 °C and sintered in the same sintering batch but on different trays.

According to the method of the present invention ce- mented carbide or cermet bodies are sintered on graphite

trays coated with a covering layer of Y2O3 or based on Y2O3 containing <20 wt-% Zrθ2, preferably <10 wt-% Zrθ2 , and most preferably <5 wt-% Zrθ2 , or corresponding volu¬ metric amount of other refractory oxides, e.g. AI2O3 or combinations thereof. Preferably, one or more interme¬ diate layers of a refractory metal such as Mo, W, Nb, Zr, Ta, preferably Mo or W or combinations thereof, and/or of a refractory oxide, preferably ZrC>2-based, is applied. The average thickness of these layers should each be >10 μm, preferably 20 - 300 μm, most preferably 50 - 150 μm.

The most important property when choosing material for the intermediate layer or layers is a strong adhe¬ sion both to the underlying graphite surface and to the top coating of yttria or yttria based compound. However, it is also important that these interlayer materials are thermally stable within the temperature range from room temperature up to working temperature, which normally is up to about 1550 °C. Further, they should not react with the adjacent materials in such a way that the functions of the latter are affected negatively.

The coating is applied by known powder-spraying me¬ thods, preferably plasma spraying, using a powder having a grain or agglomerate size where the main distribution is within the interval 10-100 μm, preferably 10-75 μm. In order to facilitate the spraying process regarding powder flow from the container and through the gun the powder is preferably agglomerated.

In an alternative embodiment the coating is formed by applying a slurry containing the powder considered and then dried and sintered. Chemical Vapour Deposition techniques can also be used.

Example 1

Cutting tool inserts were pressed from four diffe¬ rent cemented carbide powder mixtures . Two of them con¬ sisted of WC - 10 wt-% Co with high and low carbon con- tent, denoted material A-hc and A-lc resp., and in the remaining two an addition of 5 wt-% TiC was made by ex¬ changing the corresponding amount of WC. These, also being separated by a high and low carbon content, are denoted material B-hc and B-lc resp. The pressed inserts were placed on three different trays, namely:

Tray 1: Uncoated graphite tray. (Prior art.)

Tray 2 : Graphite tray with a plasma-sprayed coating of Zrθ2 ~ ^{20% γ} 2°3 (Metco 202), coating thickness about 60 - 100 μm. (Prior art.) Tray 3 : Graphite tray with a plasma-sprayed top coa¬ ting of Y2O3 / about 10-15 μm thick, and an intermediate layer of Mo, about 20-50 μm thick and also applied by plasma-spraying. (Invention)

Note that the coating thicknesses were measured on a metallographic section through the tray.

A long-time sintering process was made at 1450 °C during 50 hours in an atmosphere of 10 mbar CO. After cooling down, the sintered bodies were tested regarding how strongly they adhered to the support by a metallur- gical reaction. The results obtained are shown in Table 1 in which 'n-r' stands for no reaction with the tray, 'c-r' for a certain reaction but no noticeable adherence and 's' for sticking with a very high adherence.

Table 1. Results of sintering test on various trays. Tray Materials

A-hc A-lc B-hc B-lc

1 (prior art) s s s s

2 (prior art) s s s s 3 (invention) n-r n-r c-r c-r

Thus, the graphite trays with a metallic coating of Mo followed by a top coating of Y2O3 give a far better performance than the other two trays tested.

Example 2

Cutting tool inserts were pressed from a cemented carbide powder mixture consisting of WC-6.5 wt-% Co-8.5 wt-% (TiC + NbC + TaC) . Sintering was performed at 1450°C and the cooling rate was chosen to obtain a bin- der phase enrichment in the surface region of the in¬ serts as known in the art. This treatment also results in the formation of a cobalt layer on the insert surfa¬ ce. These inserts, about 100 pcs on each tray, were sin¬ tered on the following trays, namely: Tray 1: Graphite tray with a plasma-sprayed coating of Zrθ2-20%Y2θ3, coating thickness about 60-100 μm. (Prior art)

Tray 2 : Graphite tray with a plasma-sprayed coating of Y2O3 / about 100-150 μm thick, and an intermediate layer of plasma-sprayed Mo, about 100-150 μm thick. (Invention)

Note that the coating thicknesses were measured using a micrometer screw gauge.

After a sintering according to standard practice, hold time at temperature about 60 minutes, the inserts sintered on Tray 1 exhibited excessive reaction between tray material and the inserts such that about 10 % of the inserts were difficult to remove from the trays without pieces of the tray material strongly adhering to them. In contrast, the inserts sintered on Tray 2 showed no tendency to react with the tray material . The inserts were easily removed.

Fig. 1 shows in 3x magnification the contact surfa¬ ces of two cutting tool inserts from this Example where the insert to the left was sintered on Tray 1 and the

one to the right on Tray 2. The former insert shows se¬ vere sticking of tray material while the insert to the right is essentially unaffected.

Microprobe analysis showed presence of yttrium in the contact surfaces of the inserts sintered on Tray 2.

Example 3

Cutting tool inserts, pressed from a cemented car¬ bide powder mixture consisting of WC-9.5 wt-% Co-35 wt-% (TiC + NbC + TaC) , were sintered in normal production at 1520°C on the following type of trays, namely:

Tray 1: Graphite tray with a plasma-sprayed coating of Zrθ2-20%Y2θ3, coating thickness about 60-100 μm. (Prior art) Tray 2: Graphite tray with a plasma-sprayed coating of Y2O3 , about 100-150 μm thick, and an intermediate layer of plasma-sprayed Mo, about 100-150 μm thick. (Invention)

At this sintering temperature and with the cemented carbide composition given, the life of Tray 1 was two to three sinterings after which the tray was rejected due to large amounts of its coating being removed, while Tray 2 gave a life in excess of twenty sinterings.

Example 4

Cutting tool inserts were pressed from a cemented carbide mixture consisting of WC-10 wt-% Co-6 wt-% (TiC + NbC + TaC) and having a low carbon content. The pressed inserts were placed on two different trays, 100 inserts on each, and sintered for 1 hr at 1410°C.

Tray 1: Graphite tray with a plasma-sprayed coating of Zrθ2 -20%Y2θ3 (Metco 202), coating thickness about 60-100 μm. (Prior art.)

Tray 2: Graphite tray coated with Zrθ2-20%Y2θ3 (Metco 202) as in tray 1 but having a top, plasma-

sprayed coating of Y2O3 , about 10-15 μm thick. (Invention)

The thicknesses of the coatings were evaluated on a metallographic section through the tray after the sinte- ring test.

After sintering the inserts on Tray 1 suffered from heavy sticking and could only be removed by force. About 60% of the inserts removed exhibited pieces of the tray coating adhering to the bottom surfaces and these adhe- sions could only be removed by using a grinding wheel .

The tray, Tray 1, showed the corresponding damage within its surface coating.

The inserts on Tray 2 , however, could easily be re¬ moved by simply turning the tray upside down.

Example 5

Cemented carbide bodies for mining tool applications were pressed from cemented carbide powder mixtures con¬ sisting of WC-12 wt-% Co. Such bodies can weigh up to 300 g per piece. Production sintering was performed on the following trays, namely:

Tray 1: Graphite tray with a plasma-sprayed coating of Zrθ2~20%Y2θ3, coating thickness about 60-100 μm. (Prior art) Tray 2 : Graphite tray with a plasma-sprayed coating of Y2O3 , about 100-150 μm thick, and an intermediate layer of plasma-sprayed Mo, about 100-150 μm thick. (Invention)

At a sintering temperature of 1450°C Tray 1 could be used only twice. The comparatively large amount of co¬ balt in the cemented carbide bodies and the large unit weight caused excessive reaction between the bodies and the tray material. At this temperature and with the same cemented carbide material and unit weight Tray 2 could be used for greater than six sinterings .