BATCH LOADING SYSTEM FOR CVD

Abstract: According to the present invention there is provided a method of coating cutting tool inserts by CVD-methods with a completely covering coating. During the coating step the inserts are resting on a support and are in contact with the support in only a few tiny spots and sufficient space is allowed around each insert to ascertain optimum coating conditions. The support is a peg, comprising a foot (A) and shoulders (B), said foot is attached to a carrier. In case the pegs serve also as support for a following carrier placed on top of the carrier, they further comprise a neck (C) and a head (D). This batch loading system allows a rational production in larger scale and fully automatic CVD loading.

Description: Batch loading system for CVD The present invention relates to a CVD-batch loading method used in connection with coating of cutting in- serts suitable for a rational production in larger scale allowing fully automatic CVD loading. Chemical vapour deposition (CVD) of wear resistant coatings on cemented carbide inserts has been an indus¬ trial practise for more than 20 years. The CVD-technique has extensively been discussed in the literature. In order to minimise the production costs for coated inserts large sized (10000 - 15000 inserts) CVD-batches are generally used. Rational loading and unloading of the inserts from the CVD-reactor is thus required. Transportation of the whole load or parts of the loads must be possible to be carried out safely particularly prior to the coating operation. Another problem related to mass volume CVD is to find an appropriate loading arrangement of the inserts that will allow a well controlled gas flow through the CVD-reactor in order to ensure high and even coating quality throughout the whole load. Yet another problem is to avoid contact marks on the inserts formed during the coating process in the zone between the inserts and their support. Contact marks on the inserts are not acceptable from a cosmetic point of view and also not since they may deteriorate the cutting performance of the coated inserts. There are many different batch loading systems or methods known in the art. In one loading arrangement the inserts are allowed to hang in slits in a tray. This method has the disad¬ vantage of leaving contact marks on the clearance faces of the inserts . The system is unstable and there is al- ways a potential risk during transportation that the in- serts fall over and completely rearrange the packing. In addition, all surfaces of an insert with this type of loading will not be exposed to the same amount of gas flow during the coating process. This may result in se- rious variation in the coating thickness and hence the quality of the coating. In another similar system the inserts hang between rods . This system suffers from the same drawbacks as those mentioned for the previous system. In the so called net loading arrangement the inserts are placed on woven metal nets. The load is built up by piling each metal net on top of .each other. When using this method: a) the gas supply to the regions between the inserts and the metal net wire may be insufficient b) care has to be taken during transportation to avoid that inserts slide together resulting in contact marks between individual inserts c) contact marks between the nets and the inserts are always formed. In yet another method the inserts are threaded up on a rod. Spacers between each insert are generally used to avoid contact between the inserts. The rods may be ori¬ ented horizontally or vertically in the CVD-reactor. The main drawbacks of this method are the very complicated and time consuming loading and unloading procedure. Con- tact marks between the spacer and the inserts are diffi¬ cult to avoid. This method can be applied to inserts having a central hole for clamping. In German patent DE 39 02 532 a loading system is disclosed in which the inserts are stacked on top of each other on a central pin possibly with a spacer bet¬ ween. As a result the inserts will be only partially coated. This method is aimed for plasma CVD and will not be fully applicable to ordinary CVD since the inserts will be "glued" or grown together during an ordinary CVD-process . The object of the present invention is to eliminate most of problems associated with the prior art loading systems. The batch loading system according to the present invention consist of a peg attached to a carrier. As a result of this method of stacking cutting tool inserts in a CVD-reactor complete, even coating coverage with minimal contact marks are obtained. Fig 1 shows an example of a peg according to the present invention in side view. The peg comprises a foot A and shoulders B and a neck C and a head D. The shoul¬ ders B have a cicular as well as non-circular cross-sec¬ tion such as square, triangular or rectangular but can also be in the form of wings. The foot A is preferably circular. Figs 2 - 7 show various embodiments of the present invention. P is peg, X carrier, I]_ insert 1, 12 insert 2, I3 insert 3, I4 insert 4 and F depression. According to the invention inserts with a central hole for clamping, which generally is the case for in¬ serts used for turning, is positioned on a peg P as shown in Fig 2. The insert rests on the peg shoulder B having such a shape that there is minimal contact bet¬ ween the peg and the insert and preferably so that con- tact occurs at only a few tiny points. This can be done by altering the shape of the shoulder (e.g. circular, rectangular, square or in the from of wings) depending upon the shape of the hole or the insert itself. Inside the hole H there is a wide passage around the neck to ensure even coating thickness of the inner walls of the hole. (The dimension of the hole H is important for a safe fixturing of the insert into its tool holder.) The function of the head D, which protrudes above the top surface of the insert I]_, is to serve as a support for the next carrier placed on top. In case the hole H is straight the thicker head acts to prevent the inserts from tilting from their horizontal position and touch the upper and/or the lower carrier surfaces. In some types of inserts the hole is wider on the top surface of the insert and then a peg without a head can be used. The peg P is attached to the carrier X. For inserts without a central hole a loading ar¬ rangement according to the invention is illustrated in Fig 3 for square inserts in which the pegs are placed between the inserts and generally supports at least two inserts. The peg shoulder shall have such a shape e.g. a sharp edge or tip that the contact area is minimised. In one embodiment a peg similar to Fig 1 is used as shown in Fig 4 in which the peg supports an insert only at the middle of the edge. This arrangement is prefe¬ rably used for single sided inserts. In an alternative embodiment which can be used for positive inserts a peg as shown in Fig 5 is used. In this case the peg supports an insert essentially in the middle of each clearance face. In yet an alternative embodiment for inserts with chipbreaker a peg as shown in Fig 6 is used. In this case the peg supports essentially in the bottom of the chipbreaker which has to deep enough so as to ensure maximum stability of the loading arrangement. Alterna¬ tively, plane inserts are provided with a depression between the corners to receive the shoulders of the pegs . In the various embodiments above the pegs serve also as support for a following carrier. In case such support is provided in other ways the neck C and head D can be excluded. In such case an alternative embodiment accor¬ ding to Fig 7 can be used. In this case inserts are pro¬ vided with a central hole (not shown) or depression F of a suitable depth and shape adapted to the peg. These embodiments are given as examples only. Other embodiments are, of course, possible within the gist of the invention. Pegs according to the invention are made of a re- fractory material such as ceramics, reinforced graphite, molybdenum preferably cemented carbide. The carrier may be a woven metal net of e.g. an iron alloy with a suitable mesh size or a tray of graphite or other refractory material with mounting holes drilled for the pegs . The peg foot is snapped into the metal net or pressed into the mounting holes in the tray. A desired number of pegs is mounted in each carrier in a regular pattern. The distance between the pegs as well as the height of the pegs is selected to give a desired free gas volume around each insert. A free volume around each insert of 1 - 5 mm is generally suitable. The carriers are piled on top of each other (preferably resting on the peg heads) to complete a full CVD load. When using a tray carrier and a vertical gas flow, holes are suitably drilled in the trays in the areas between the pegs. The size and positioning of these holes can be optimized so that the gas flow can be con¬ trolled and directed in almost any desired way through the pile of trays . When using horizontal or radial gas flows e g when employing the MTCVD (Moderate Temperature CVD) -technique the holes in the trays can optionally be omitted. The loading system of the presently claimed inven- tion allows the process gases to reach all essential surfaces of the inserts improving the coating thickness distribution, minimising discoloration due to irregular gas flow pattern and reducing coating time. The rigid and exact positioning of the inserts allows a safe transportation prior to and after the coating step and also an automatic loading system. By using a suitable cross-section of the shoulders, the only contact between the peg and the insert will occur at only a few, less than five, tiny points which are not critical for the performance of the coated inserts. In the foregoing description only very few specific details have been given by way of examples only. The reason hereto is that the exact conditions depend to a certain extent on the design of the equipment and the deposition conditions used. It is within the purview of the skilled artisan to adapt the method of the present invention with regard to the design and deposition con¬ ditions of his equipment.

Claims: Claims 1. Method of coating cutting tool inserts by CVD- methods with a completely covering coating during which coating step the inserts are resting on a support c h a r a c t e r i s e d in that said support is a peg attached to a carrier said peg comprising a foot A and shoulders B. 2. Method according to the preceding claim c h a r a c t e r i s e d in that said peg further com- prises a neck C and a head D serving as support for an¬ other carrier placed on top of said carrier. 3. Method according to any of the preceding claims c h a r a c t e r i s e d in that said peg passes through a hole in an insert whereby the insert rests on the shoulders portion. 4. Method according to any of claims 1 and 2 c h a r a c t e r i s e d in that the pegs are placed between the inserts. 5. Method according to claim 4 c h a r a c t e r i s e d in that the insert rests on said shoulders in the bottom of a chipbreaker. 6. Method according to claim 4 c h a r a c t e r i s e d in that the insert rests on said shoulders essentially in the middle of a clearance face. 7. Method according to claim 4 c h a r a c t e r i s e d in that the insert rests on said shoulders in the middle of an edge.

Inventors: LJUNGBERG BJOERN, NORRGRANN TOR, PAALSSON KJELL, LJUNGBERG, BJOERN, NORRGRANN, TOR, PAALSSON, KJELL
Applicants: SANDVIK AB
Application year: 1995
Main classification: C23C16
IPC codes: B23B27, C23C16