FI71773C - Process for preparing titanium nitride coating with gas phase process. - Google Patents

Description

1 71773 METHOD OF PREPARING A TITANIUM NITRIDE COATING BY THE GAS PHASE METHOD

The invention relates to a process for producing a titanium nitride coating by a gas phase process. The method according to the invention can substantially improve the properties of the TiN coating.

Gas phase coating processes have recently become widespread. The coating can take place either physically in a vacuum, in which case PVD (physical vapor deposition) methods are used, or chemically at high temperature, in which case a CVD method (chemical vapor deposition) is used.

The PVD method can be used for coating in three different ways, which are sputtering, ion coating and high vacuum evaporation. An overview of the performance of these methods can be found, for example, in Technical Bulletin No. 29/82 of the Confederation of Finnish Metal Industry.

In the CVD process, a coating is formed from a gaseous feedstock by a chemical reaction. The method is able to form coatings that have both good corrosion resistance and good wear resistance. General information on the CVD method is also presented in the above-mentioned bulletin 29/82.

One of the favorable applications of coating technology has traditionally been tools, the properties of which can be substantially improved by coating. The titanium nitride coating, for example, increases the service life of cutting tools by 3-5 times compared to an uncoated high-speed steel tool. In molding and editing tools, the improvement is even greater in some circumstances. The coating also reduces the sticking between the blank and the tool and thus the need to grind the tool during production.

Although coatings have many good properties, they still have a number of detrimental features. For example, when coating with the CVD method, the dimensions of the part change and it is difficult to make dimensionally accurate products. In even more demanding applications, dimensional changes usually need to be considered in advance 71773. The working temperature of the CVD method is so high, up to 1000 ° C, that for example when coating tool steels, normal hardening treatment (hardening annealing, quenching and 2-3 emissions) is still required after coating.

When coated with the PVD method, on the other hand, it is possible to manufacture parts in such a way that the dimensions remain unchanged for practical applications. However, the process is more unstable and more difficult to control than the CVD process, which often results in uneven coating quality.

The application possibilities of the coatings are almost limitless.

New applications include the watch industry, whose TiN-coated "Gold Watches" are even better than genuine ones. However, development is slowed down by the fact that the properties of the coating films are not known with sufficient precision.

One solution for improving the ion coating method is presented in Finnish Patent Publication No. 66.656 (C 23 C 11/08), where it is proposed to improve the uniformity of the coating by a shade placed in front of the vapor source and by conducting gases from a multi-hole nozzle in the vicinity of the vapor source.

The approach according to the invention is another, because the basic knowledge of the method according to the invention has been obtained by carefully examining the structure of the coating, which has not been done thoroughly before. There are at least two factors behind the backwardness of structural research: process development has progressed rapidly through experimentation and the results have been so good that there has been no compelling need for detailed structural research; on the other hand, the study of typically a few microns thick films firmly attached to the base material is very difficult and requires the development of a new research technique.

Structural studies have shown that the PVD coating formed in plasma is formed according to a matastable equilibrium. In the metastable equilibrium are the nitrogen-containing Ti and the tetragonal TiN, which begin to form as the titanium nitrogen content rises to 48-50 at.%. In equilibrium, titanium can dissolve nitrogen up to 20 at.%.

3,71773

The equilibrium compounds are cubic TiN as well as Ti ^ N. With a high nitrogen content, the resulting metastable structure is very fine-grained, 50-100 Å, but not amorphous, as erroneously reported in the literature.

The metastable structure formed in the coating begins to decompose during the coating into different equilibrium structures (rt-Ti (20 at.% N), Ti ^ N and cubic TiN depending on the nitrogen content, if the temperature of the body to be coated is sufficiently high, i.e. above 500 degrees Celsius. very slow and does not lead to near equilibrium in normal coating.

Studies have shown that substantially better coatings now in use are obtained by adjusting the process so that the resulting coating has a nitrogen content of 30-60 at.%, Most preferably 35-40 at.%, And by decomposing the formed metastable, relatively soft o <-Ti in a separate heat treatment Ti2N or a mixture of Ti2N and TiN, depending on the nitrogen content.

The characteristic features of the method according to the invention are set out in the appended claims.

The coating made according to the invention is lead gray in color, its hardness is HK 7000, which is higher than the hardness values reliably measurable with a diamond tip (gold-colored TiN coatings HK 3000-4000) and its durability in working tests has been about 2-5 times that of the best commercial PVD method. compared to TiN coatings made. Due to the heat treatment, the impact toughness of the coated high-speed steel has also remained normal, and there has been no collapse-like decrease in the impact toughness of the high-speed steel caused by conventional TiN coating. The adhesion of the heat-treated coating in the base material is also good: In the Vickers tip scratch test, the adhesion of the normal TiN coating is 15-25 g (both PVD and CVD method) as a critical load when a value of 150 is obtained for the heat-treated Ti2N (+ TiN) coating. -200 g. The improvement in partial adhesion is due to the release of stresses in the titanium initrid well and the surface of the base material.

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The following is an example of a heat treatment according to the invention.

The finished part is coated by the PVD method. After coating, below the softening temperature of the steel, annealing is carried out for several hours, typically A to 12 hours, in a suitable shielding gas, for example argon. The body is cooled either in air or in a suitable extinguishing medium.

Alternatively, the coating and subsequent emission can also be combined with the normal heat treatment of the steel. In this case, for example with high-speed steel tools, the coating can be carried out after the second release. The heat treatment of the coating then takes place in connection with the third release. All conventional furnaces in which a shielding gas can normally be used are suitable for the treatment, e.g. retort or chamber furnaces.

The advantageous effects of the treatment described above are not limited to the coating of high-speed steel tools, but also when coating with a hard metal tool and a titanium nitride coating, the properties of the coating can be considerably improved by the heat treatment described above.

Claims (2)

5 PATENT CLAIMS 7177 3 A method for coating steel or carbide with titanium nitride by the gas phase method, characterized in that the coating process is controlled so that the nitrogen content of the coating becomes 30-60 at.%, Preferably 35-40 at.%, And the formed metastable t ^ -Ti is decomposed in a separate heat treatment depending on the nitrogen content, Ti 2 N or a mixture of Ti 2 Nm and TiN. A method according to claim 1, characterized in that the heat treatment of the steel base material is carried out below the softening temperature of the steel by annealing for several hours, typically 4-12 hours, in a suitable shielding gas and cooling in air or a suitable extinguishing medium.